Bi-layer Functionally Gradiend thick film semiconducting butane sensors

National seminar on Advanced nanomaterials and its applications, Jadavpur University, 8-9 August, 08<

Introduction

Bi-layer functionally gradient thick film semiconducting butane sensors

S. Shrivastava1, R. Biswal2, S. Chakraborty3, I. Roy3, A. K. Halder3, A. Sen3,*

Abstract

Gas sensors based on metal oxide semiconductors like tin dioxide are widely used for the detection of toxic and combustible gases like carbon monoxide, methane and butane. One of the problems of such sensors is their lack of sensitivity, which to some extent, can be circumvented by using different catalysts. However, highly reactive volatile organic compounds (VOC) coming from different industrial and domestic products (e.g. paints, lacquers, varnishes etc) can play havoc on such sensors and can give rise to false alarms. Any attempt to adsorb such VOCs results in sorption of the detecting gases (e.g. butane) too. To get round the problem, bi-layer sensors have been developed. Such tin oxide based functionally gradient bi-layer sensors have different compositions at the top (high resistance in the order of M? range) and bottom layers (low resistance in the order of K?). Here, instead of adsorbing the VOCs, they are allowed to interact and are consumed on the top layer of the sensors and a combustible gas like butane being less reactive, penetrates the top layer and interacts with the bottom layer and the electrical signal generated at the bottom layer from the combustible gas is collected. Such functionally gradient sensors, being very reliable, can find applications in domestic, industrial and strategic sectors. The processing steps for the fabrication of such sensors are also simple and cost-effective.

1. Introduction:

With increasing world-wide distribution of gas sensors for different applications, the demand of sensors fulfilling specific standards is growing in leaps and bounds1-2. Incidentally, volatile organic compounds are posing as a menace for satisfactory sensor performance because they come out from many industrial and domestic products like paints, lacquers, varnishes, cosmetics and automobile exhausts and being highly reactive, they tend to interfere with the sensor operation. In this regard, a particular concern is to avoid VOC cross-sensitivity with the detecting gases as VOCs being highly reactive, can give rise to false alarm. So far, primarily two techniques have been tried to get around the problem. One of the techniques to avoid interference from unwanted vapours is to use charcoal filters3. Such sensors are satisfactory for detection of CO in presence of VOCs. The disadvantage of this method lies in the fact that charcoal filters also adsorb most of the combustible gases and hence cannot be used satisfactorily to detect combustible gases like methane, propane, CNG and LPG in presence of VOCs. The other technique is to use uncoated/coated (with Pt, Pd etc.) filters of Al2O3, SiO2, WO3 etc on SnO2 coatings4-5. Such protecting filters can, to some extent, check specifically alcohol cross-sensitivity. The second method also has drawbacks because VOCs, in general, are either not affected by such filters or, the overall sensitivity of the sensors towards the detecting gases dramatically goes down. In this study we prepare a functionally graded tin dioxide based composition for gas sensors capable of detecting combustible gases in presence of volatile organic compounds.

2. Experimental

A precursor powder (SnO2+0.25 Sb2O3+ 5Pd+0.1B) for the bottom layer was prepared by the following steps6-7. Firstly, reagent grade stannous chloride (SnCl2, 2H2O) was dissolved in 200 mL of hot distilled water containing 20 drops of HCl with continuous stirring. Secondly, reagent grade Sb2O3 was dissolved in 50 mL distilled water (at 80oC) containing 5 drops of HCl. In the next step, reagent grade PdCl2 was taken in 100 mL distilled water containing 10 drops of HNO3 and PdCl2 was slowly dissolved by heating the mixture at 80oC under constant stirring for 1 h. The three solutions were mixed and added to ammonia solution under sonication (ultrasonic processor, vibronics, 25 kHz, 250 W) and the pH of the solution has maintained at 9. The precipitate was centrifuged and dried at 100oC for 10 h. Then calculated amount boric acid solution was added into the precursor powder and again dried into a mortar pestle. Finally, the dries powder was calcined at 900oC for 2 h.
To prepare the precursor powder (SnO2+ 10 Pd+ Al2O3 (varying amount 10 to 30 wt% with respect to tin dioxide based composition)) for the top layer, again stannous chloride and PdCl2 were dissolved in distilled water following the above procedure. Reagent grade alumina powder was then mixed with the calcined powder in the different ratio by weight using an agate mortar and a pestle.
A thick paste of the powder formulation for the bottom coating was made by mixing the prepared powder for the bottom layer with dilute alumina gel and cured at 600oC for 1 h. A thick paste of the powder formulation for the top coating was made by mixing the prepared powder for the top layer with dilute alumina gel and cured at 500oC for 30 min. Gold electrodes and platinum lead wires were attached at the ends of the tubes (by curing at a higher temperature) before applying the paste. Kanthal heating coils were placed inside the tubes and the leads were bonded to nickel pins. The electrical resistance and butane (500 ppm) sensitivity of the coatings were measured at 350°C by using a digital multimeter (Solartron), a constant voltage/current source (Keithley 228A) and X–Y recorder (Yokogawa). All the fired samples were initially preheated at 350°C for 72 h to achieve the desired stability before the measurements.

3. Results and discussion

The X-ray diffractogram of calcined tin dioxide powder (SnO2+0.25Sb2O3+ 5Pd+ 0.1B) is depicted in figure 1, which indicates complete SnO2 phase formation after firing at 900°C. The crystallite size of the SnO2 based powder is calculated by scherrer formula and the value is around 27 nm.

From this spectrum a strong band associated with the anti-symmetric Sn–O–Sn stretching mode of the surface binding oxide can be observed apparently at 600 cm-1.
The percent response (S) of SnO2 based sensors in different gases at 350°C has been calculated by

S = (RA – RG)/RA × 100%

RA and RG being the sensor resistance in air and gas at the same temperature. The measurement temperature, 350°C, was selected for our studies because C4H10 sensitivity is maximum around this temperature.

It is shown that the double coated sensors (30 wt% Al2O3 doped) prepared in this way showed an average sensitivity of around 90% in 500 ppm butane at 350oC. Incidentally, the sensors show a low sensitivity of around 30% or even less when kept inside a container containing standard paint thinner (a source of conc. VOCs) or acetone or alcohol etc. whereas, under the same condition, the sensitivity in such conc. VOCs can be as high as 75%, when the sensor is singly coated, i.e., the top coating of the sensor is absent. Hence, by properly designing the electronic circuit, the double coated sensors can be made selective to the detecting gases even in the presence of VOCs. The basic mechanism besides that the top layer of the functionally gradient bi-layer sensor contains tin dioxide, palladium and alumina. Alumina raises the resistance of the top coating to the order of 10–100 MW at the operating temperature. Such increase in resistance can be explained by considering the electronic interaction between semiconducting tin dioxide grains in close contact with the Lewis acid sites (electron acceptor) of alumina grains. Whereas, the bottom coating is devoid of alumina and contains antimony resulting in the resistance value of the order of 10–100 kW at the operating temperature. The VOCs being highly reactive, interact with adsorbed oxygen on the top layer releasing free electrons. However, due to three orders of higher resistance of the top coating with respect to that of the bottom coating, the top coating always remains shunted to the bottom coating. Less reactive gases like methane penetrate the top layer and interacts with the bottom layer of adsorbed oxygen and the change in resistance is picked up by the electrical leads at the bottom.

4. Conclusions

By modifying the chemical compositions of the top and bottom layers, novel functionally gradient bi-layer tin dioxide based sensors have been developed, which show excellent sensitivity towards methane with negligible cross-sensitivity towards volatile organic compounds. The processing steps for the fabrication of such sensors are also simple and cost-effective.

References

1. W. Gopel and K.D. Schierbaum, SnO2 sensors Current Status and Future Prospects, Sens Actuators B, 26-27 (1995) 1-12.
2. G. Sberveglieri (ed.), Gas Sensors – Principle, Operation and Development, kluwer, Dordrecht (1992).
3. M. Schweizer-Berberich, S. Strathmann, U. Weimar, R. Sharma, A. Senbe, A Peyre-Lav and W. Gopel, Sens Actuators B 58 (1999) 318-324
4. K. Komatsu and S. Sakai, U.S. patents 4,592,967 (1986).
5. C.A. Papadopoulos, D.S. Valchos and J.N. Avaritsotis, Sens Actuators B, 32(1996) 61-69.
6. A. Banerjee, A. K. Haldar, J. Mondal, A. Sen, H. S. Maiti, Bull. Mater. Sci., 25 (6) (2002) 497.
7. M. Saha, A. Banerjee, A. K. Halder, J. Mondal, A. Sen, H. S. Maiti, Sens Actuators B79 (2001) 187.

Hydrogen and humidity sensing properties of C60 thin films

Hydrogen and humidity sensing properties of C60 thin films
G. Sberveglieri a,b, G. Faglia a, C. Perego a, P. Nelli al*, R.N. Marks ‘, T. Virgili ‘, C. Taliani ‘,
R. Zamboni ’
a Department of Chemistry and Physics for Materials, INFM, via Valotti 9, I-25133 Brescia, Italy
b Department of Physics, INFM, via Paradiso 12, I-44100 Ferrara, Italy
’ CNR-Institute of Molecular Spectroscopy, via P. Gobetti 101, I-40129 Bologna, Italy



Abstract

We discuss the sensing properties of C,a thin films towards reducing gases and humidity. C,a thin films were sublimed onto alumina substrates with platinum interdigitated contacts, and a platinum heating element was deposited on the substrate back face. The electrical responses of the C6a films towards 1000 ppm of hydrogen, ethyl alcohol and carbon monoxide were measured both in dry and humid air, in the temperature range between 50 and 300 “C. At 300 “C we observe a reversible increase of the current in the presence of hydrogen, while the water vapour causes a reversible decrease in the current. We suggest that the response to hydrogen may be due to a chemical reaction between the oxygen molecules trapped in the film and the hydrogen, reducing the concentration of oxygen-based electron traps in the CGO,
hence increasing the conductivity. The reduction in conductivity in the presence of humidity is probably due to the formation of extra trap states by water molecules in the film.

1. Introduction
In the past few years, fullerenes have attracted a great deal of attention due to their novel electronic properties. They are semiconducting molecular solids with an optical bandgap of
1.7 eV [ 11; when suitably doped (e.g. with alkali metals) they show metallic or super conducting properties. Cc0 can be sublimed in vacuum in the form of thin polycrystalline films, which have been used in devices such as field-effect transistors [ 2,3] and photodiodes [ 4-61.
CeO crystals are face-centred cubic ( f.c.c.) with large interstitial voids which allow the efficient diffusion of gases through them, and make this material an interesting candidate
for gas sensing applications. In this paper we describe the detection properties of C6,, thin films towards some reducing gases.

Synthesis and characterization of Copper Oxide based nanostructure by wet chemical route

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

Synthesis and characterization of Copper Oxide based nanostructure by wet chemical route
S.Das and K.K.Chattopadhyay
Department Of Nano Science And Technology Jadavpur University
Different CuO nanostructues on copper substrate were synthesized successfully using a simple wet chemical method at room temperature by varying pH and reaction time of the solution. The composition, morphology, and growth conditions of the nanostructures were characterized by XRD, AFM, SEM, and EDAX. UV-Visiable spectroscopy measurement indicates that the CuO nanostructures shows reflectivity about 25 -12 % in the wavelength of 400-1500 nm with a direct band gap lying in the range of 1.37-1.47 eV. FT-IR analyses of nanostructured CuO thin films have been performed for the films deposited on Cu substrates. FT-IR spectra of the CuO thin films shows all the bands are due to the absorption peaks of Cu-O, at 600, 421cm-1.Cu-OH shows at 3647, 3428 and at 789 cm-1 and one bond of carbon dioxide at 2334 cm-1.field emission shows that electrons are emitted by cold field emission process and turn on current is 12.2 V/ μm for the self assembled nanorods of diameter lying in the range 180–200 nm with electrode distance 130 μm.
Corresponding author: kalyan_chattopadhyay@yahoo.com

High Rectification Ratio in Nanostructured Organic-Inorganic Photodiodes

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

High Rectification Ratio in Nanostructured Organic-Inorganic Photodiodes
Santanu Karan and Biswanath Mallik*
Department of Spectroscopy, Indian Association for the Cultivation of Science,
2A & 2B, Raja S. C. Mullick Road, Jadavpur, Kolkata-700 032, India.
High quality organic-Inorganic heterojunction photodiodes based on nanostructured copper (II) phthalocyanine (CuPc) and intrinsic zinc oxide (i-ZnO) have been fabricated. The i-ZnO thin films/ layers were grown by RF magnetron sputtering on clean ITO coated glass substrate. These films have been characterized by optical absorption and Field Emission Scanning Electron Microscopy (FESEM). CuPc thin films deposited at room temperature on i-ZnO have exhibited change in their surface morphology with the post-deposition annealing temperature under normal atmosphere. The electrical dark and photoconductivity of ITO/i-ZnO/CuPc/Au sandwich structure have been measured under various photoexcitation intensities using Xenon light source. The devices have shown excellent reproducibility of their electrical characteristics and high rectification ratio. The rectification ratio is nearly 831 calculated above the threshold voltage at room temperature. The effects of annealing temperature on the surface morphology and rectification ratio have been discussed. -1.5-1.0-0.50.00.51.01.50246810111315171678 0.00.51.01.50.00.51.01.52.0876541Total Current (
mA/cm2 )
Biased Voltage (Volt)Total Current (mA/cm2 ) Biased Voltage (Volt)
(a)
(b)
(c)
(d)
Fig. 1 (a) FESEM images of the CuPc thin filmdeposited at room temperature on i-ZnOsurface. Inset shows the morphology of i-ZnOsurface deposited on ITO. FESEM images of theCuPc thin film annealed at 100 (b), 200 (c), and250 oC (d), respectively. Insets show the surfaceat higher magnification.
Fig.2 The I-V characteristics for the devices under dark and different intensity of photoexcitation: The curves represent for (1) Dark, (2) 5, (3) 10, (4) 15, (5) 30, (6) 45, (7) 70 and (8) 100 mW/cm2, respectively. The inset refers to the magnified view in forward
After deposition of CuPc thin films on i-ZnO layer at room temperature, the deposited thin films were annealed at various temperatures. The FESEM images of the CuPc thin films annealed at different temperatures are shown in Figure 1. Figure 1a shows the FESEM image of as deposited CuPc film at room temperature. Most of the particles are almost spherical in shape and the average size is nearly 30-40 nm. The inset of Figure 1a shows the morphology of i-ZnO deposited on ITO. Figure 1b shows the surface morphology of the CuPc film annealed at 100 oC. Clearly, the aggregation of the CuPc nanoparticles is observed. Figure 1c shows the surface morphology of the CuPc film annealed at 200 oC. Here the film has been found to be formed uniformly with aggregated nanoparticles and some nanorod like structure. Figure 1d represents comparatively smoother surface for a CuPc film annealed at 250 oC. The surface shows the compactness of the nanorod like structures. Inset of Figure 1d shows the nanorod composed of small nanoparticles of size below 10 nm. The high tendency of the self-ordering of phthalocyanine molecules could be one of the main reasons for the change in film morphology depending on the annealing condition. The I-V characteristics for Pc 250 devices under different intensity of photoexcitation are shown in Figure 2. From this figure it has been found that all the devices responded to photoexcitation and gave rise to photocurrent. The RR under different intensity of photoexcitation has been calculated from Figure 2 and is listed in Table 1.
RR for the Devices
Intensity (mW/cm2 )
Pc 30
Pc 100
Pc 200
Pc 250
0
5
6.16
7.51
14.37
44.03
1.28
2.90
831.54
5.27
10
5.01
36.10
2.63
9.42
15
5.99
29.48
2.55
12.90
30
7.17
27.28
3.11
30.37
45
7.78
25.28
3.41
54.01
70
8.35
23.34
3.50
126.37
100
11.73
21.24
3.12
356.23 -1.6-1.2-0.8-0.40.0-32-24-16-8087654321Total Current (
μA/cm2 )
Biased Voltage (Volt)
TABLE 1: Rectification Ratio (RR)Calculated above the Threshold Voltage ofthe Devices under Different Intensity ofPhotoexcitation.
Fig. 3 Reverse biased I-V characteristics in dark and under photoexcitation of different intensities for the device Pc 250. The curves represent for (1) Dark, (2) 5, (3) 10, (4) 15, (5) 30, (6) 45, (7) 70 and (8) 100 mW/cm2, respectively. Inset shows the photovoltaic performance of the devices.
Typical I-V characteristics of the photodetector at reverse bias from the devices, under 5-100 mW/cm2 photoexcitation range are shown in Figure 3. I–V response under dark condition is also shown in the figure. The plot shows that upon increasing intensity of photoexcitation, the amplitude of device current at any voltage increases. At higher photoexcitation intensities, in addition to more photo generated excitons and hence more dissociated carriers, higher carrier mobility due to increase in the density of carriers may be the cause of photocurrent. At any particular photoexcitation intensity, the magnitude
of increase is also more at higher bias amplitudes. Field dependent dissociation of excitons at the interfaces can result in higher photocurrent at higher bias amplitudes. Higher bias may also pull out more carriers out of the device and leaves space for more exciton dissociation. At lower bias, charge confinement may hinder exciton dissociation.
In conclusion, for the growth of CuPc films on i-ZnO by vacuum evaporation, surface morphology of the thin films are influenced very strongly on the post-deposition annealing temperature. Good rectification and photosensitivity was observed for the devices. Higher rectification ratio was observed for the device having CuPc layer annealed at 250 oC. For the device having CuPc layer annealed at 200 oC the value of ISC is higher and RR is very small. Above the annealing temperature of 200 oC, i.e. when β-phase occurs, the value of ISC again becomes less and RR becomes higher. The results regarding effects of annealing temperature on CuPc deposited on i-ZnO may be extended to structures involving the growth of multilayer photodiodes.
Corresponing author: spbm@mahendra.iacs.res.in;

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Synthesis of nanocrystalline undoped tetragonal and cubic zirconia using poly-acrylamide as gel and matrix

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

Synthesis of nanocrystalline undoped tetragonal and cubic zirconia using poly-acrylamide as gel and matrix
J. Ghose* and Subir Roy#,
Chemistry Department. IIT Kharagpur, PIN-721302, India
*Guest faculty, School of Material Science & Tech. for M.Tech., Jadavpur University, India
# Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad-500058.
Abstract:Nanocrystalline zirconia with tetragonal and cubic structure has been prepared from ZrO2-polyacrylamide gel and by precipitation of zirconia in polyacrylamide matrix respectively. X-ray diffraction results of the samples annealed at high temperature show that the tetragonal and cubic form, obtained from the gel, are fairly stable in air upto 1173K and partially stable in inert atmosphere, upto 1273K. The stability at such high temperatures is due to the presence of oxygen vacancies in zirconia sample, incorporated in the decomposition of polymer.
Key Words: Tetragonal zirconia ; cubic zirconia; chemical synthesis; poly-acryl amide; gel
I. Introduction
Pure crystalline zirconia exists in three crystalline polymorphs: monoclinic (m), tetragonal (t) and cubic(c). The room temperature stable form is monoclinic [1]. The metastable tetragonal and cubic phases have important technological applications although the phases are unquenchable [2]. Tetragonal zirconia has become very important engineering ceramic material due to t→m phase transition and this phase transition is associated with transformation toughening [3]. Apart from its important mechanical properties the tetragonal phase of zirconia has attracted considerable attraction due to its application in some acid catalyzed reactions [4-6]. The cubic phase has drawn considerable attention for its application in automobile industry as an oxygen sensor [7] and in the fabrication of fuel cells [8].
Thus for wide application it is essential to stabilize the tetragonal and cubic phases, study their thermal stability and the factors that govern the phase transitions. Over the last two decades a large amount of work has been devoted to stabilize tetragonal and cubic phase of zirconia in pure form [9-18] and by doping with some oxides [19-22]. These reports have shown that the metastable phase in undoped nanocrystalline zirconia is tetragonal in most of the studies. The thermal stability of metastable tetragonal phases in doped zirconia is upto 1723K, although the tetragonal phase in pure nanocrystalline zirconia is thermally stable only upto 773K [9-12]. The applications of polymer as matrix for distribution of nanoclusters and capping agents in the synthesis of nanostructured ceramic oxides and sulphides have been studied by a number of researchers [23-26]. The
present work includes the synthesis of metastable nanocrystalline tetragonal and cubic zirconia using sulphonated poly-acrylamide in two different forms i.e. as polymer gel and polymer matrix. The same precursor chemicals were used with slight alteration of synthesis chemistry i.e the difference in the sequence of addition of the chemical reagents to obtain different metastable phases of zirconia. Attention has also been paid to study the interaction between the metal ions and the polymer both in matrix and gel. Moreover, it is interesting to note that the metastable phases of undoped ZrO2 are stable up to 1173K in air. * Corresponding Author, e mail: j_ghose@hotmail.com
2
II. Experimentals
ZrOCl2.8H2O(98%) was used as the source of zirconium. Three solutions were used in the two different under mentioned techniques for the synthesis of tetragonal and cubic zirconia: aqueous ZrOCl2 solution (5 wt%), aqueous sulphonated polyacryl amide (SPAM) solution (2 wt%), and aqueous ammonia solution (5 wt%). The details of the synthesis procedure are given in the scheme-I and scheme-II of figure 1. X-ray diffraction analyses of the heat-treated samples were carried out with a Phillips X-ray diffraction unit (model-PW1710) using CuK∝(λ =1.5418A0) radiation with a Ni filter. Crystallite sizes of the zirconia samples were measured from XRD line broadening analyses. Thermal analyses of the dried gel were carried out with Shimadzu Thermal Analyzer DT-40 between 298K and 1173K in air. Infrared spectra of all the samples were recorded in KBr medium in the range 4000-400cm-1 for polymer gel and 1000-400cm-1 for annealed ZrO2 samples with a Perkin Elmer spectrophotometer. TEM microstructure analyses were carried out using a transmission electron microscope (Model JEM 200 CX) operating at a voltage of 120KV.
Air-annealing
Air-annealing
Sample B
Metastable cubic zirconia (Scheme-I)
Cl – free precipitate
Filtering and washing with de-ionized water
Sample A
Precipitation of hydrated zirconium oxide in presence of SPAM at constant pH of 9.0
Zirconium oxy-chloride aqueous solution (5 wt%) from a burette
Ammonia solution (5 vol %) from a burette
SPAM aqueous ammoniacal solution (2 wt%)
Oven-drying
and calcination at 873K
Oven-drying and decomposition at 873K
Metastable tetragonal zirconia (Scheme-II)
Zirconia -SPAM gel
Ammonia solution (5%)
Cl- free precipitated dispersed inde-ionizedwater
Filtering and washing withde-ionizedwater
Precipitate of zirconium-SPAM coordinated complex
Zirconium oxy-chloride aqueous solution (5 wt%)
SPAM aqueous solution (2 wt%)
Figure1. Flow diagram for synthesis of metastable cubic and tetragonal zirconia
III. Results and Discussion
III.A. Structural characterization by XRD
Fig.2 shows the X-ray diffraction patterns of the decomposed gel at 873K (as- burnt powder-Sample-B) (Fig.2a), as-burnt powder annealed for 6 hours at 873K(Fig.2b), 973K(Fig.2c), 1073K(Fig.2d), 1173K(Fig.2e), and 1273K(Fig2f) respectively. The diffraction patterns show all the samples are tetragonal, except the 1273K-annealed sample. The samples were characterized as tetragonal by the tetragonal splitting of the
3
peaks at different values of 2θ [27]. This splitting of the peaks is not observed for the sample annealed at 873K but the crystal symmetry in this sample and even the local order in the as-burnt amorphous phase may be assumed to be tetragonal as per the assumption made by Bokhimi et al. [17]. The sample annealed at 1273K shows lines of a monoclinic phase. XRD pattern of zirconia sample A annealed up to 1073K showed no splitting of the peaks, which indicated the presence of cubic phase [24]. The 1173 K annealed sample-A shows a small amount of monoclinic phase along with the cubic as the major phase, which transforms to monoclinic phase completely at 1273K[24].
TEM bright field micrograph of ZrO2 (Sample B) annealed at 873K is given in figure 3. The micrograph shows the fine spherical features of the particles with round grain junctions. The average particle size calculated manually from the micrograph is 20nm and it was observed from TEM analysis that the average particle size does not change significantly for the sample annealed at 1173K although the crystallite size showed marked changes between 1073 and 1173K(Table1).
Variation of the crystallite size and lattice parameters with annealing temperature of the zirconia samples is given in table I. From the table it is evident that the crystallite size increases slowly between 873K and 1073K and then size increases rapidly between 1073K and 1273K. The lattice parameters of the annealed tetragonal samples increases slightly with annealing temperature (Table I)
Figure 2. XRD patterns of (a) gel decomposed at 873K and decomposed powder annealed for six hours in air at (b) 873K, (c) 973K, (d) 1073K, (e) 1173K and (f) 1273K respectively.
Figure 3. Transmission electron micrographs of tetragonal ZrO2 sample annealed for six hours at 873K in air.
III.B. Chemical reactions between the polymer and the metal ions
Sulphonic acid group is incorporated in polyacrylamide chain by chemical reaction deliberately to increases the hydrophilic character and thus increasing solubility of the polyacrylamide in water which is otherwise having poor solubility in water. When
4
Zr4+ ions are added to the sulphonated polyacrylamide (SPAM) in presence of ammonium hydroxide, the metal ions react with OH- ions forming hydrated oxides of zirconium in preference to the SPAM. There is no chemical interaction between the polymer and the metal ions in presence of the OH- ions. The polymer plays the function of the matrix and minimizes the agglomeration of hydrated zirconium oxide(Fig.1, scheme-I). In contrast, when the metal ions are added to water solution of SPAM in absence of ammonium hydroxide, the former form complex with the polymer. In aqueous medium sulfonate groups of the polymer form complexes with metal lions. The essential feature of the complexes is that the metal ion coordinates only with one polymer ligand group [28]. The zirconium ions attract the hydrophilic polar groups of SPAM towards itself so strongly that the remaining hydrophobic chain of SPAM separates out of the water and discrete precipitate forms which can be separated out by filtering (Fig.1, scheme-II). The complex may be represented as [Zr(SPAM)x]4+. The coordination number of the metal ion determines the value of ‘x’. The addition of ammonium hydroxide to the complex precipitate will start the competition between the OH- ions and the polar linkages of SPAM for room in the inner coordination sphere. OH- ion being smaller in size and a stronger ligand penetrates through the inner coordination sphere of the metal-SPAM complex pushing the polar groups of SPAM to the outer coordination sphere of the complex. The complex may be represented as [Zr(OH-)x(SPAM)y]4+ where OH- ion satisfies both the primary valences and the coordination number. SPAM satisfies only the coordination number. Moreover, addition of OH- ions results crosslinking among the polymer chains. The crosslinking of poly acrylamide by Zr4+ at high pH has also been reported earlier [29]. This chemical reaction brings remarkable physical changes of the [Zr(SPAM)x]4+ precipitate in the ammonia solution. The precipitate dissolves in ammonia solution and forms progressively continuous gel.
Table I. Details of thermal treatment, phase and lattice parameters of the annealed
tetragonal ZrO2 samples
Lattice parameters (±.003Ǻ)
Annealing
temperature(K)
in air/argon
Annealing
time
(hour)
Crystallite size(nm)
Phase(s)
a
b
c
β
(degree)
873
6
10.3
t
- -
973
6
11.2
t
5.0961 5.1750
1073
6
12.5
t
5.1006 5.1869
1173
6
19.0
t
5.1080 5.1990
1273
6
37.5
m
5.1670 5.2200 5.3370 99.290
1273
1
35.5
m
5.1705 5.2521 5.3405 98.740
1273(argon)
1
-
m(major) +t
5.1892 5.2290 5.3243 99.030
5
Chemical reactions:
Scheme –I: Zr4+ + NH4OH + SPAM+ H2O ZrO(OH)2 + SPAM(matrix)+ H2O
Scheme-II: Zr4++ SPAM [Zr(SPAM)x]4+
[Zr(SPAM)x]4++ NH4OH +H2O [Zr(OH-)x(SPAM)y] 4+ + H2O
Thus Zr4+ encounters two different chemical environments in the two different schemes of reactions and crystallizes as two different metastable forms. These two different chemical environments have been created only by the difference in time of addition of ammonia. This slight alteration in synthesis chemistry has been proved enough to change the crystal structure of the finally crystallized ZrO2.
III.C. Thermal stability of tetragonal ZrO2 in inert atmosphere
The effect of annealing in inert atmosphere on the gel derived zirconia samples was studied by annealing the decomposed gel sample at, 1273K, in argon, for one hour. The results show that the tetragonal phase is partially retained when the samples are annealed in argon for one hour (Fig.4b). In contrast, the air-annealed sample transforms completely to the monoclinic phase in one hour (Fig.4a). These results clearly show that the annealing in inert atmosphere affects the stability of the tetragonal phase of zirconia. It appears that during the decomposition of zirconia-polymer gel, a reducing atmosphere is generated due to burning of carbonaceous parts of the gel in air, which is able to generate
Figure 4. XRD patterns of the calcined ZrO2 powder annealed for one hour at 1273K (a) in air, (b) in argon.
some oxygen vacancies in the sample. These oxygen vacancies are responsible for the stability of the metastable tetragonal phase in the zirconia samples up to 1173K. In argon, at 1273K some of the oxygen vacancies are stabilized and hence in argon annealed samples, tetragonal zirconia is found to be partially stable upto 1273K. Air annealing at 1273K, however, removes the oxygen vacancies from the samples and hence the sample undergoes complete t→m phase transition. Several theories have been put forward to explain the stability of the metastable phases in zirconia. Garvie [30] realized the tetragonal monoclinic phase transition as the particle size effect. Garvie’s hypothesis of particle size effect on the phase transition has been questioned by several researchers [12,31-33]. Murase and Kato [31] suggested that there is no clear relationship between
6
the particle size and the t→m phase transition. Mitsuhasi et al.[12] assumed that anion impurities and domain structure have an effect on phase stability. Literature shows that the monoclinic phase can be prepared in the form of very fine particles of about 6 nm size [32]. The role of oxygen vacancies to stabilize the metastable phase in zirconia was also proposed by some researchers [32,33].
The present experimental results strongly supports the role of oxygen vacancy to stabilize the metastable tetragonal phase up to 1173K.
IV. Conclusions
Tetragonal ZrO2 of average particle size ~20 nm has been synthesized ZrO2-polymer gel. IR spectra of ZrO2-polymer gel indicate strong electrostatic interaction between the metal ion and the polar linkages of polymer. Tetragonal phase of zirconia can be stabilized up to 1173K without any foreign stabilizing agent. The stability of pure tetragonal zirconia, synthesized from the zirconia-polymer gel, is due to the presence of oxygen vacancies. The oxygen vacancies are formed in the samples as a result of the reducing atmosphere created during decomposition of the gel. The partial retention of the tetragonal phase at 1273K, in argon, is due to partial stabilization of the oxygen vacancies in argon atmosphere.
Acknowledgement
The authors wish to thank DST, Government of India, for financial support.
References
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2. Masahiro Yoshimura, Am. Ceram. Soc. Bull., 67(12) 1950-55(1988).
3. Berkby, R. Steven Key, Eng. Mater. 122, 527(1996).
4. Michael D. Rhodes, Konstantin A.Pokrovski and Alxis T. Bell., J. Catal., 233[1] 210-20(2005).
5. Marco Faticanti, Nicola Cioffi, Sergio De Rossi, Nicoletta Ditaranto, Pieroporta, Luigia Sabbatini and Teresa Bleve-Zacheo, Appl. Catal. B: Environmental, 60[1]273-82(2005).
6. W.Stichert, F.Schüth, S, Kuba and H.Knözinger, ,J. Catal.,198, 277(2001).
7. E.C. Subbarao, Ferroelectrics 102 , 267(1990).
8. D. Susnik, J. Holc, M. Hrovat, S. Zupancic, J. Mater. Sci. Lett. 16 , 1118(1997).
9. R.C. Garvie , J. Phys. Chem., 82[2], 218-24(1978).
10. E. Tani, M. Yoshimura and S. Somiya, J. Am. Ceram. Soc., 66[1] 11-14(1983).
11. Hai-Yang Zhu, Bin Liu, Ming-Mn- Shen, Yan Kong, Xi Hong, Yu-Hai Hu, Wei-Ping Ding, Lin Dong and Xi Chen., Mater. Lett., 58(25) 3107-10(2004).
12. T. Mitsuhashi, and M. Ichihara and U.Tatsuke, J. Am. Ceram. Soc., 57, 97-101(1974).
13. E. Djurado,P. Bouvier, and G. Lucazeau , J. Solid State Chem.,149[2] 399-407(2000).
14. R.D. Purohit, S. Saha and A.K. Tyagi, Mat. Sc.& Eng.B, 130,57-60,(2006).
15. M.Rezaei, S.M. Alavi, S. Sahebdelfar and Zi-Feng Yan, Powder Tech.,168 [2],59-63(2006).
16. M. Bhagwat and V. Ramaswamy, Mat. Res. Bull., 39[11],1627(2004).
7
17. X. Bokhimi, A. Morales, O. Novaro, M. Portilla, T. Lópezà, F. Tzompantzià and R. Gómezà , J. Solid. State. Chem., 135, 28-35(1998).
18. Jin Joo, taekyung, Young Woon Kim, Hyun Min park, Fanxin Wu, Jin Z. Zhang, J. Am. Chem. soc., 125[1] 6553-6557 (2003).
19. T. J. Chung, H.S. Song, G.H.Kim and D.Y.Kim, J. Am. Ceram. Soc., 80, 2607-12(1997).
20. Xin M. Wang, Gordon Lorimer and Ping Xiao, J. Am. Ceram. Soc., 88(4)809-816(2005).
21. Chih-Wei Kuo, Yueh_Hsun Lee, Kuan- Zong Fung and Moo-Chin Wang, J. Non. Cryst. Solid., 351[4] 304-11(2005).
22. F. Bondioli, C. Leonelli and T. Manfredini, J. Am. Ceram. Soc. 88[3] 633(2005).
23. A. Chevreau, B. Phillips, B.G. Higgins and S.H. Risbud, J. Mater. Chem., 6, 1643 (1996).
24. S.Roy and J.Ghose, Mat. Res. Bull., 35, 1195-1203(2000).
25. S.Roy and J.Ghose, J. Appl. Phys.,87(9) 6226-28(2000).
26. S.Roy and J.Ghose, J. Mag. Mag. Mater., 307, 32-37(2006).
27. Gerald Katz, J. Am. Ceram. Soc., 54 [1-12] 531(1971).
28. Alexander T. Dembo, Kirill A. Dembo, Vladimir V. Volkov, Alexander I. Kokorin, Alexei A. Lyubimov, Eleonora V. Shtykova, Serguei G. Starodoubtsev, and Alexei R. Khokhlov, Langmuir, 19, 7845-7851(2003).
29. Karine Sigale, and Aziz Omari , J Appl Polym Sci., 64 1067–1072, (1997).
30. T. Hirata, E. Asari and M. Kitajima, J. Solid State Chem.,110, 201-207(1994).
31. C. M. Phillippi, and K.S. Mazdiyashi, J. Am. Ceram. Soc., 54[5] 254(1971).
32. Hao Zhang, Yulong Liu, Ke Zhu, Gueigu Siu, Yonghong Xiong and Caoshui Xiong, J. Phys.: Condens. Matter 11, 2035–2042(1999).
33. R.C. Garvie ,J. Phys. Chem., 69[4] 1238-43(1965).

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Synthesis Of Graphene And Some Graphene Based Composite Materials

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

Synthesis Of Graphene And Some Graphene Based Composite Materials
Arpan Bhatacharyya and K. K. Chattopadhyay
Department Of Nano Science And Technology
Jadavpur University
Abstract:
Graphene sheets—one-atom-thick two-dimensional layers of sp2-bonded carbon—are predicted to have a range of unusual properties. Their thermal conductivity and mechanical stiffness may rival the remarkable in-plane values for graphite their fracture strength should be comparable to that of carbon nanotubes for similar types of defects; and recent studies have shown that individual graphene sheets have extraordinary electronic transport properties. One possible route to harnessing these properties for applications would be to incorporate graphene sheets in a composite material. The manufacturing of such composites requires not only that graphene sheets be produced on a sufficient scale but that they also be incorporated, and homogeneously distributed, into various matrices(Polymers like Poly Vinyl Alcohol or Poly Aniline). Here in this project a general approach for the preparation of graphene-polymer composites via complete exfoliation of graphite via chemical route using aqueous medium in presence of a proper surfactant and non aqueous dispersing medium(DMF). Finally the aqueous exfoliated graphene sheets were deposited on a gold coated glass substrate via electrophoretic deposition and the morphological studies of all the samples were performed. The measurement of electrical properties manifested the fact that the resistance of graphene based PVA composite decreases with increase of temperature and the percolation threshold also lie in between 1- 2%(w/w) of the filler.
Our bottom-up chemical approach of tuning the graphene sheet properties provides a path to a broad new class of graphene based materials and their use in a variety of applications.

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Nanocrystalline Low Temperature Mullite Formation by Sol-Gel Technique.

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

Nanocrystalline Low Temperature Mullite Formation by Sol-Gel Technique.
Biswajoy Bagchi, Dr. Sukhen Das.
Physics Department, Jadavpur University Kolkata 700032
Abstract.
Mullite, or porcelainite, is a rare clay mineral with the chemical composition Al6Si2O13, a form of aluminium silicate. It is produced artificially during various melting and firing processes, and is used as a refractory. Mullite research is gaining importance because of the excellent properties it exhibits such as high thermal and chemical stability, high creep resistance, low thermal coefficient of expansion and good dielectric properties.
Synthesis of mullite by sol-gel technique using suitable precursors is advantageous due to the fact that the reactants are mixed at molecular level ensuring chemical homogeneity.
Transition metals have been shown to have a mineralizing effect on the transformation of Kaolinite to Mullite ceramics with accelerated decrease of surface area and pore volume. These transition metals distort the local ligand symmetry(Jahn-Teller effect) which results in accelerated phase transformation in substrates i.e. at a lower temperature.
Mullite formation by conventional methods require a temperature of over 10000C with reaction completing at around 16000C. In our work we have synthesized nanocrystalline mullite at a temperature as low as 6000C by using sol-gel technique. Particle size have been calculated to be within 13-30 n.m.. This drastic lowering of the mullitization temperature is a direct consequence of a doping agent in the form of a transition metal.This metal actually forms a complex with the reactant and by distorting the local ligand symmetry brings down the temperature of mullitization.
Corresponding author: biswajoy_ju@rediffmail.com

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Bias dependent dielectric relaxation dynamics of electrically tuned large-scale aligned zinc oxide nanorods in nematic liquid crystal host

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

Bias dependent dielectric relaxation dynamics of electrically tuned large-scale aligned zinc oxide nanorods in nematic liquid crystal host
P. Nayek,1 S. Ghosh,1 S. Karan,1 T. Pal Majumder,2 and S. K. Roy1, a)
1Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata – 700032, India
2Department of Physics, University of Kalyani, Kalyani 741235, West Bengal, India
Abstract
It was observed that mixed ZnO nanorods and twisted nematic (TN) liquid crystals (LC) show highly ordered molecular system. The observed molecular relaxation arose due to reorientation of long molecular axis of the order of 400 kHz for pure ZLI-1636 while such relaxation was shifted to a lower frequency for mixture. By using the proposed equation, )exp(CVDBBEBff−−=−αand )exp(CVsDBBsEB−−=−ασσ, the obtained equivalent activation energy was increased remarkably by the introduction of ZnO nanorods in pure LC and a strong long range interaction is established in 0.01% ZnO nanorods mixed LC system.
________________________________________________________________________
corresponding author:

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Langmuir Blodgett and self assembly methods to fabricate Ovalbumin–Phospolipids thin film with minimal protein aggregation

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

Langmuir Blodgett and self assembly methods to fabricate Ovalbumin–Phospolipids thin film with minimal protein aggregation
Tapanendu Kamilya, Prabir Pal, Mrityunjoy Mahato and G. B. Talapatra*
Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India
The fabrication of protein as well as protein-lipid mixed films on appropriate substrates is the subject that has received considerable attention in recent years due to their prospective biotechnological and biomedical applications and devices fabrication. A foremost challenge is to immobilize the protein molecules on appropriate substrate without aggregates. Self-assembly (SA) and Langmuir-Blodgett (LB) methods are typical techniques to immobilize biomaterials on to solid surfaces. Our main aim is to prepare a protein- lipid mixed film with minimal aggregation of protein by using LB and SA technique.
We have used zwitterionic (DPPC) [1-2], cationic ODA [3], anionic SA [4] lipids and a protein OVA to prepare a protein lipid mixed film by LB technique using a computerized, Teflon-bar-barrier type LB trough (model 2000C, Apex Instruments Co. India). Pressure-area isotherm, surface pressure growth kinetics, fluorescence studies show that DPPC-OVA mixed film is better to prepare a protein-lipid mixed film by LB technique. FE-SEM analysis shows the squeezing out of protein from the monolayer at higher surface pressure. In situ compressibility analysis is used to transfer a film with almost smooth surface morphology below the squeezing out point. Moreover, nanometer to micrometer aggregates were found in case of ODA-OVA [Fig.1], SA-OVA [Fig. 2] mixed film. DPPC-OVA mixed film shows the aggregates with 60-80 nm in dimension [Fig. 3]. Therefore, we have controlled the aggregation and squeezing out of protein in protein-lipid mixed film by using appropriate zwitterionic lipid [Fig.4]. Next our aim is to minimize the aggregation of protein in protein lipid mixed film.
To fulfill our aim we have done self assembly of OVA on prefabricated DPPC LB film. It shows nanometer to micrometer aggregates with unfolding of OVA, studied by
FE-SEM [Fig.5], CD and FTIR spectroscopy. Hence, this method is not good to prepare a protein –lipid mixed film. LB method is far better than this technique.
Finally, we invented a simple but novel technique to prepare a protein-lipid mixed film. This is self-assembly of DPPC-OVA vesicle on hydrophilic glass substrate. FE-SEM analysis shows that distribution of whitish small globules through out the film [Fig.6]. The dimensions of these globules are in range of 8-10 nm in diameter, similar to the dimension of OVA monomer (7 nm x 4.5 nm x.5 nm) [5]. Therefore, these granules are expected to be the individual OVA monomeric structure and they preserve their individual structure in vesicle matrix. Here, DPPC may form membrane by fusion of vesicle on hydrophilic glass slide. CD and FTIR analysis of this film show that the increment of α-helix due to hydrophobic interaction in vesicle matrix preserves the folding of the OVA molecules and prevents the formation of larger aggregates [6]. Immobilization of protein on vesicle and subsequent self-assembly on hydrophilic substrate may be useful for the preparation of protein-lipid mixed film with a minimum aggregation of protein.
Fig. 2
Fig. 6
Fig. 4
Fig. 5
Fig. 3
Fig. 1
References: [1] T. Kamilya, P. Pal, G. B. Talapatra, J. Phys. Chem. B 11, 1199, 2007 [2] T. Kamilya, P. Pal, G. B. Talapatra, Asian J. Physics. (In press) [3] T. Kamilya, P. Pal, G. B. Talapatra, J. Colloid Interface Sci. 315, 464, 2007 [4] T. Kamilya, P. Pal, G. B. Talapatra, Colloids Surf. B: Biointerfaces 58, 137, 2007. [5] Protein data bank [6] T. Kamilya, P. Pal, M. Mahato, G. B. Talapatra, J. Nanosci. Nanotech. (Communicated). Acknowledgement: We thank DST, Government of India (Project No. - SR/S2/CMP-0051/2006) for partial financial support. MM also thanks to CSIR, Government of India for providing the CSIR-NET fellowship.

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Facile synthesis of palladium nanoparticles in Y-zeolite matrix: An efficient catalyst for Heck coupling reaction

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

Facile synthesis of palladium nanoparticles in Y-zeolite matrix: An efficient catalyst for Heck coupling reaction
Buddhadeb Dutta, Sreyashi Jana, Rajesh Bera, Subratanath Koner*
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
_____________________________________________________________
Abstract
A new facile and reliable route for synthesis of nano-sized Pd(0) particles onto zeolitic matrix has been developed. [Pd(NH3)4]2+ ions have been first immobilized in NaY matrix by ion exchange, then on reaction with hydrazine hydrate [Pd(NH3)4]2+ ions were directly reduced to nano-sized Pd(0) particles almost instantaneously. Transmission electron microscopic analysis reveals that well separated nano-sized (10-20 nm) palladium(0) particles have been formed on the surface of the zeolite crystals. X-ray powder diffraction analysis supports that the particles are highly dispersed in the zeolite matrix. The prepared nano-composite is found to be highly catalytically active in a variety of Heck cross-coupling reactions.
Figure 1. Transmission electron micrographs showing Pd nanoparticle in NaY matrix.

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SIMULATION OF THERMOMAGNETIC CONVECTION IN A RECTANGULAR CAVITY

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

SIMULATION OF THERMOMAGNETIC CONVECTION IN A RECTANGULAR CAVITY
Sumanta Banerjee1, Achintya Mukhopadhyay1, Swarnendu Sen1, Ranjan Ganguly2
1 Mechanical Engineering Department, Jadavpur University, Kolkata, 700032, India
2 Power Engineering Department, Jadavpur University, Kolkata, 700098, India
INTRODUCTION
Studies on thermal advection enhancement in a shallow rectangular cavity filled with an electrically-conducting fluid by applying a transverse external magnetic field [1] discusses the growing interest in problems of magnetic field-fluid interactions. This is primarily due to the numerous industrial processes in which these interactions assume prominence. For example, unavoidable convection movements during the manufacturing of crystals can be dampened with the help of a magnetic field [2].
The motivation for the present work is in addressing the problem of heat dissipation in electronic circuits and/or MEMS devices of high component densities [3]. With progressive miniaturization of circuits, the increasing heat flux densities adversely affect the performance, life and reliability of the devices, in addition to augmenting signal noise [4]. The junction temperature of a semiconductor device should, therefore, be kept below the safe operating temperature specified by the manufacturer. Under a situation where the heat sinks are to be designed to dissipate a constant value of heat energy under all working conditions, the thermal load among the components should be optimally distributed to prevent the maximum temperature on any device from exceeding a certain prescribed value.
The global objective is, thus, to maximize heat transfer density or minimize hot-spot temperatures when the total heat generation rate, circuit-board layout and/or other constraints are specified [5]. In generic configurations (e.g. in sealed enclosures housing PCBs), this amount to, at least partly, in investigating the influence of the aspect ratio behind the evolution of the flow field inside the cavity. This is evident from the significant volume of analytical [6], numerical [7,8,9] and/or experimental work [10,11] in the domain of natural/magnetothermal convection studies in confined fluid-filled enclosures of different configurations, under a variety of thermal boundary conditions and/or fluid rheology. However, the area of thermomagnetic convection analysis in shallow rectangular cavities (with more than one wall DHS modeling power-dissipating components) remains particularly unexplored, although such a study is an essential pre-requisite to thermal analyses of micro-electronics and Nano-CMOS chips [4, 12].
The transport of momentum and energy in miniaturized devices is, in general, diffusion dominated and requires very long transport time scales [13].By using ferrofluids and manipulating the flow pattern by external magnetic fields, thermomagnetic convection proves to be a viable alternative to enhance convection in these devices. This is due to the fact that the flow fields established inside a cavity by the ferrofluid depends both on the field gradient as well as the local fluid susceptibility gradients set up by the presence of heat sources [14].
In the present paper, numerical investigation of steady-state, thermomagnetic heat transfer is carried out in a shallow rectangular enclosure, where the cavity-width is twice the height. The effect of gravity has been neglected. Two discrete heat sources are flush-mounted on the bottom wall, representing heat-generating electronic components. A line dipole placed below the bottom wall sets up a two-dimensional, non-uniform magnetic field. Under the constraint where the overall heat energy supplied by the heaters is a constant and the dipole is placed symmetrically halfway along the cavity length, the study analyzes the evolution of the flow fields (depicted through streamfunction, heatline and isotherm plots [15]) for two representative cases of unequal heater lengths and strengths. The results are contrasted against those obtained for a square cavity configuration, keeping all other factors identical. The comparative study shows that the aspect ratio plays a central role in thermal energy transport and temperature field evolution within the cavity.
1
MATHEMATICAL MODEL
Figure 1(a) illustrates the geometry and the boundary conditions of the chosen physical configuration. The top wall and the non-heated portions of the bottom wall are adiabatic. The sidewalls are maintained isothermally cold at TCT=, providing the heat sinks. The height and the breadth of the shallow cavity are Wand respectively. The cavity extends to infinity in the third dimension such that the resulting flow field is two-dimensional. The finite-sized heaters are of lengths and, and their flux strengths are respectively qW21L2L1′′ and2q′′. The dimensionless length 1ε of the left heater is kept fixed at a value of 0.2, while the normalized right heater length 2ε is varied. The centerlines of the heaters are symmetrically placed (about the vertical mid-plane) and are separated by length such that S0.1=WS. All boundaries satisfy the no-slip velocity conditions. The line dipole is placed adjacent to the bottom wall, halfway along the enclosure length at and at Wx=04W0y= below its inner surface.
Fig. 1(a): Schematic of physical system
Fig. 1(b): Field configuration for symmetric dipole placement
The magnetic field inside the ferrofluid medium can be expressed in the form [14]: B
􀁇
()

−+=ϕϕϕχμerermBrmˆcosˆsin1220􀁇. (1)
The r−ϕ coordinate system (with respective unit vectors e and) is depicted in Fig. 1(a). The centerline of the coil is considered as the virtual line-origin of the dipole. Figure 1(b) depicts the imposed normalized magnetic field configuration and the corresponding lines of force. The contours are circular in nature, with centers at the virtual origin of the dipole. rˆϕeˆ􀁇
The assumed magnetic field and H (which inside a magnetic medium is ()[]mBHχμ+=10􀁇􀁇) conform to Maxwell’s equation in static form [16]. Here, 0μ is the permeability (= 4π × 10-7 N/A2)
The governing steady state, two-dimensional continuity, momentum, and energy equations are: 0=∂∂+∂∂yvxu, (2)
2
()xyxCHyTHxTHxHTTyuxuxpyuvxuu

∂∂+∂∂−∂∂−−

∂∂+∂∂+∂∂−=∂∂+∂∂∗ρβχμβρχμνρρρ200200222221, (3) ()yyxCHyTHxTHyHTTyvxvypyvvxvu

∂∂+∂∂−∂∂−−

∂∂+∂∂+∂∂−=∂∂+∂∂∗ρβχμβρχμνρρρ200200222221, (4)
and 

∂∂+∂∂=∂∂+∂∂2222yTxTyTvxTuα. (5)
Here, is the effective pressure, defined as ∗p()2200Hppχμ−=∗, where is the absolute pressure. The last term in the momentu equations (Eq. (3) and Eq. (4)) contains the Kelvin body force (KBF) per unit volume p
m ()BMf􀁇􀁇􀁇􀁇∇⋅=that a magnetic fluid experiences in a spatially non-uniform magnetic field [17].The magnetic Rayleigh number, denoted by, refers to the dimensionless group mRa)2WNuRH(20TmρναβρΔ=LHmax,θmax,θ0χμ[17] and is referenced with respect to the flux strength of the left heater. The heater Nusselt numbers (,) and their maximum non-dimensional temperatures ((,)) are defined as in [14]. LHavg,RHavgNu,
The working fluid chosen is a representative ferrofluid continuum considered at a reference temperature of 300K, and having the following properties: density ρ = 1180 kg/m3, specific heat C= 4180 J/kg-K, kinematic viscosity pν= 5.93×10-6 m2/s, Pr = 49.6, reference magnetic susceptibility = 0.1 and compressibility coefficient 0,mχρβ = 5.6 × 10-4 /K [17].
NUMERICAL PROCEDURE
The coupled mass, momentum and energy equations are solved by a finite volume method using SIMPLER algorithm developed by Patankar [18]. The set of algebraic equations are solved sequentially by TDMA (Tri-Diagonal Matrix Algorithm). The power-law differencing scheme by Patankar is used for the formulation of the convection-diffusion terms in the equations. The pertinent variables ()TpV,,􀁇 and the fluid properties (thermophysical and magnetic) are described using a staggered grid arrangement. Solution is obtained by progressive minimization of the mass residual. The computation is terminated when the root mean square value of the residuals get below10. The non-uniform grid, which is required to resolve the sharp gradients near the walls and close to the location of the magnetic dipole, is based on a sinusoidal mesh size distribution. The code is validated by comparing the simulations for buoyancy-driven convection with the benchmark results of de Vahl Davis [19] for thermogravitational convection, and with the results of Ganguly et al. [17] for thermomagnetic convection. 11−
RESULTS AND DISCUSSIONS
A numerical study already carried out by the present group shows that pertinent geometrical parameters (the length and/or strength ratio of heaters) can be suitably adjusted to achieve certain desired operating conditions [Error! Bookmark not defined.]. The study shows that, at higher values of , the fraction of heat energy dissipated through a cold sidewall is roughly proportional to the fraction of the total heat energy pumped in by the heat source placed adjacent to the wall (when the dipole is centrally placed). For the same overall energy input, the present study takes up the chosen mRa
3
system (Fig. 1(a)) to investigate the influence of aspect ratio on heat load division (between the sidewalls) under steady-state advection conditions.
Temperature non-uniformity inside the cavity due to the presence of heat sources produces non-uniformmχ values within the cavity. The KBF experienced by a ferrofluid element increases with a rise in temperature. The resultant force is primarily directed along()HH􀁇􀁇􀁇⋅∇, or towards the virtual origin of the dipole [17]. Due to the combined effects of non-uniform temperature and magnetic fields, the resulting steady-state thermomagnetic convection depends on such factors as the selection of the heater sizes, magnitudes of their flux strengths, and dipole placements.
In this paper, the following representative case (leading to asymmetry in wall heat transfer) is taken up for discussion:
• For symmetric placement of the dipole (external field distribution as in Fig. 1b) and heaters of the same flux strength (=1.0), the heater length ratiorq′′rε is taken as 0.5.
For central placement of the line dipole, the base situation corresponds to =rq′′rε=1.0 and . This all-symmetrical situation serves as the global standard for the present study, with reference to which any representative asymmetric situation is considered. 5107×≈mRa
Figures 2a.1, 2b.1 and 2c.1 depict the streamline, heatline and isotherm plots for rε=0.5 in the convection-dominated regime of . This value of ensures that the total heat input for the present case is the same as the base case of [14]. 5103.9×≈mRamRa5107×≈mRa
As the heaters are of different sizes, they pump in unequal amounts of energy. The resulting convection rolls are, therefore, not symmetric. The dominant roll is formed over the left heat source (Fig. 2a.1), as the latter contributes to %67≈ of the total energy input. A smaller roll sits atop the right heater (the weaker source). The major portion of heat (%64≈) is dissipated through the cold wall adjacent to the left source (see Fig. 2b.1), with the larger vortex affecting bulk convection. The larger number of heatlines terminating on the left wall is a visual evidence of the same. The overall advection of heat through the right wall is comparatively poor (%36≈). The contours-layout depict the coupled action of the stronger left vortex and the weaker right vortex, serving to dissipate a portion of the thermal energy pumped in by the left heater through the right wall (Fig. 2b.1). This explains why the heat advection through the right wall is %3≈ higher than the contribution of the right heater in the overall energy input. The warping of the corresponding isotherm contours (Fig. 2c.1) roughly corresponds to the predominance of the clockwise circulation. The conductive heat flux is higher at the left wall than at the right, as evident by the denser packing of isotherms at the left wall. The isotherm contours are packed more densely over the left heat source, which is stronger of the two.
The distribution of heat load among the sidewalls of the present physical system (rectangular cavity with ) is seen to be markedly different from an analogous situation studied for a square cavity () [14]. Figures 2a.2, 2b.2 and 2c.2 depict the streamline, heatline and isotherm plots for the square cavity. The flow-field inside the latter evolves in such a way that the heat dissipation through the right wall remains at 0.2=AR.1=AR0%33≈ for rε=0.5 for all regimes of heat transfer.
Table 1: Comparison of heat transfer parameters between 0.2=AR and 0.1=AR
AR
LHavgNu,
RHavgNu,
LWf
RWf
LHmax,θ
RHmax,θ
2.0
7.28
8.74
0.64
0.36
0.155
0.121
1.0
14.96
18.14
0.67
0.33
7.53E-02
5.81E-02
Table 1 clearly brings out the influence of the aspect ratio in heat energy distribution among the sinks in a convection-dominant regime. The energy dissipated through available heat sinks, for a given boundary condition and/or external field setup, need to be estimated to determine the safe operational
4
conditions for a device. The functional dependence of flow-field evolution on the aspect ratio warrants further study.
Fig. 2a.1
Fig. 2a.2
Fig. 2b.1
Fig. 2b.2
Fig. 2c.1
Fig. 2c.2
CONCLUSIONS
Heat transfer characterization is done in a convection-dominated regime for two values of the cavity aspect ratio, for a representative asymmetry situation of rq′′=1.0 and rε=0.5. The total heat input to the cavity is maintained constant. The flow-field evolutions are visualized through streamline, heatline and isotherm plots, and the influence of cavity geometry in transport of thermal energy borne out through the heatline plots. The values of pertinent heat transfer parameters tabulated in Table 1 quantitatively bears out the influence of aspect ratio in advection in rectangular enclosures.
5
6
ACKNOWLEDGEMENTS
This work was partially supported by the Centre for Nano Science and Technology, Jadavpur University under the UGC Scheme of University with Potential for Excellence. We acknowledge Council of Scientific and Industrial Research (CSIR), Government of India for the financial support provided to the first author.
REFERENCES
1 S. Alchaar, P. Vasseur, and E. Bilgen, Natural convection heat transfer in a rectangular enclosure with a transverse magnetic field, Journal of Heat Transfer, vol.117, pp. 668-673, 1995.
2 C. Vives, and C. Perry, Effects of magnetically dampened convection during the controlled solidification of metals and alloys, Int. J. Heat Mass Transfer, vol.30, pp. 479-496, 1987
3 S. P. Gurrum, S. K. Suman, Y. K. Joshi, and A. G. Fedorov, Thermal Issues in next-generation integrated circuits, IEEE Transactions, vol. 4, pp. 709-714, 2004
4. R. Remsburg, Thermal design of electronic equipment, chap.1, CRC Press LLC, Boca Raton, Florida, 2001
5. A. K. Da Silva, S. Lorente, and A. Bejan, Optimal distribution of discrete heat sources on a wall with natural convection, Int. J. Heat and Mass Transfer, vol.47, pp.203-214, 2004
6 M. Prud’homme, and H. Bougherara, Weakly non linear convection in a shallow horizontal cavity under uniform cross-fluxed heat fluxes, Int. J. Heat and Mass Transfer, vol. 48, pp. 2278-2289, 2005
7 O. Polat, and E. Bilgen, Natural convection and conduction heat transfer in open shallow cavities with bounding walls, Heat Mass Transfer, vol. 41, pp. 931-939, 2005
8 N. B. Cheikh, B. B. Beya, and T. Lili, Aspect ratio effect on natural convection flow in a cavity submitted to a periodical temperature boundary, Journal of Heat Transfer, vol.129, pp. 1060-1068, 2007
9 D. Zablockis, V. Frishfelds, and E. Blums, Numerical investigation of thermomagnetic convection in a heated cylinder under the magnetic field of a solenoid, J. Phys.: Condens. Matter, vol. 20 (2008) 204134 (5pp)
10 E. Blums, A. Mezulis, and G. Kronkalns, Magnetoconvective heat transfer from a cylinder from a cylinder under the influence of a nonuniform magnetic field, J. Phys.: Condens. Matter, vol. 20 (2008) 204128 (5pp)
11 D. S. R. Ramambason, and P. Vasseur, Influence of a magnetic field on natural convection in a shallow porous enclosure saturated with a binary fluid, Acta Mechanica, vol. 191, pp. 21-35, 2007
12. F. P. Incropera, Convection heat transfer in electronic equipment cooling, J. Heat Transfer, vol.110, pp.1097-1111, 1988
13 H. J. Kim, and A. Beskok, Quantification of chaotic strength and mixing in a microfluidic system, J. Micromech. Microeng. vol. 17, pp. 2197-2210, 2007
14 S. Banerjee, A. Mukhopadhyay, S. Sen, and R. Ganguly, Optimizing thermomagnetic convection for electronics cooling, Numerical Heat Transfer, Part A, vol. 53, pp.1231- 1255, 2008
15. S. Kimura, and A. Bejan, The “heatline” visualization of convective heat transfer, J. Heat Transfer, vol.105, pp.916-919, 1983.
16 R. E. Rosensweig, Ferrohydrodynamics, pp.1-4, Dover Publications Inc., New York, 1997
17 R. Ganguly, S. Sen, and I.K. Puri, Thermomagnetic convection in a square enclosure using a line dipole, Physics of Fluids, vol.16, pp.2228-2236, 2004.
18. S. V. Patankar, Numerical Heat Transfer and Fluid Flow, Hemisphere, New York, 1980.
19. G. de Vahl Davis, Natural Convection of air in a square cavity: A benchmark numerical solution, Int. J. Num. Methods Fluids, vol.3, pp.249-264, 1983.

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PREPARATION OF MULTIWALLED CARBON NANOTUBES BY A NOVEL CATALYST ROUTE AND ITS FIELD EMISSION PROPERTY

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

PREPARATION OF MULTIWALLED CARBON NANOTUBES BY A NOVEL CATALYST ROUTE AND ITS FIELD EMISSION PROPERTY
D. BANERJEE a, K.K. Chattopadhyay a,b,*
a Thin Film & Nanoscience Laboratory, Department of Physics,
b Nanoscience and Technology Center
Jadavpur University, Kolkata 700 032, India
E-mail: kalian_chattopadhya @yahoo.com
Abstract
The possibilities of low macroscopic field emission from carbon based film such as amorphous carbon, diamond like carbon, carbon nanotube and nanofiber has made them the candidate of extreme interest during last decade. Specially since the discovery of carbon nanotube by Iijima apart from field emission it shows its importance in a number of other scientific field such as drug delivery agent, adsorption agent, energy storage device etc. There are number of techniques for the preparation of carbon nanotubes. Here in this paper preparation of carbon nanotube by Plasma Enhanced Chemical Vapour Deposition [PECVD] through an easy catalyst method has been reported. The as prepared carbon nanotubes show an excellent field emission property with turn on field as low as 1.96V/μm and effective work function 0.039eV. The straight line nature of F-N plot tells that the electrons are emitted mainly due to field emission process. The turn on field varying in the range between 4.24 V/μm -1.96 V/μm with inter electrode distance varying between 150μm -220μm is mainly due to variation of local nanostructure. The field enhancement factor, current density and dependence of effective work function on interelectrode distance have been calculated. This simple catalyst method may helps producing carbon nano tube in large scale.
Corresponding author:
Diptonil Banerjee.
E-mail: nilju82 @gmail.com

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Intense blue-violet photoluminescence from nanocrystalline silicon in silicon-oxide matrix prepared by mechanical milling

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

Intense blue-violet photoluminescence from nanocrystalline silicon in silicon-oxide matrix prepared by mechanical milling
Samata Sarkar
School of Materials Science and Engineering, Bengal engineering and Science University, Shibpur, Howrah: 711103, India
Email: samata_online@rediffmail.com
Abstract
Silicon is the building block of most of the modern solid state electronic devices like ICs, transistors, photovoltaic solar cells, sensing devices etc. The promotion of silicon from being the basic electronic material for microelectronic devices to a potential light emitter has emerged as a consequence of the possibility to reduce its dimensionality by different techniques [1,2]. Quantum confinement of photo excited carriers that yields a band gap widening and an increased radiative transition rate has been cited as the most probable reason for visible light emission of Si nanostructures with dimensions less than 7 nm [1-4]. The present study involved synthesis of Si nanocrystals (Si-nc) by mechanical milling of crystalline (-c) Si followed by chemical oxidation to reduce the dimension of the nanocrystals to the desired level. Colloidal solutions of oxidized Si-ncs were formed in isopropanol and intense room temperature photoluminescence (PL) detectable with unaided eye was observed. Ball milled silicon nanocrystals at different milling hours (25, 50, 75 and 100 h) in toluene medium were studied. Chemical and thermal treatment to promote oxidation of these nanocrystals was carried out for further size reduction. XRD spectrum of milled Si nanocrystals at different stages of milling time revealed that the least size (~40 nm) was obtained after 75 h of milling. However as-milled samples did not show any visible PL but intense visible PL was observed from colloidal suspension of the chemically treated and annealed nc-Si prepared by mechanical milling. The PL band peaks were obtained at 3.14, 3.11, 2.93 and 2.79 eV under UV excitation. Phase contrast AFM and HRTEM investigations of the light emitting Si nanocrystals revealed the existence of Si crystallites with dimensions < 5 nm embedded in an oxide matrix. Some of the particles formed a core-shell structure with a c-Si core surrounded by an amorphous (a-) Si and Si oxide shell. The thickness of the oxide layer and the diameter of the core crystalline region were measured by AFM line profile analyses. It is proposed that invasive oxidation takes place at the interface of the nc-Si and amorphous silicon oxide leading to the formation of large number of oxide and defect related states. The origin of violet blue PL is discussed in relation to the oxide related surface states, non-stoichiometric sub-oxides and defect related states. Initial investigation in this route has been successful and encouraging since visible blue-violet PL with PL band peaks at around 400 nm has been observed from the colloidal suspension Si nanocrystals. This preparation route is novel and can be a very good substitute for expensive techniques like plasma enhanced chemical vapour deposition for the preparation of Si quantum dots.
References
[1] L. T. Canham, Appl. Phys. Lett. 57, 1045 (1990).
[2] V. Lehmann and U. Gosele, Appl. Phys. Lett. 58, 856 (1991).
[3] B. Delley and E.F. Steigmeier, Phys. Rev. B 47, 1397 (1992).
[4] M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan and C. Delerue, Phys. Rev. Lett. 82, 197 (1999)

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Infrared and Raman Spectra of Nano-Bio molecules: An algebraic approach

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

Infrared and Raman Spectra of Nano-Bio molecules: An algebraic approach
Srinivasa Rao Karumuri1*, Nirmal Kumar Sarkar2, Joydeep Choudhury1 and Ramendu Bhattacharjee1
1 Condenser Matter Theory Research Group, Department of Physics, Assam University, Silchar, India
2 Department of Physics, Karimganj College, Karimganj-788710, India.
Corresponding author E-mail: srinivasa_karumuri@rediffmail.com
The spectroscopy is a branch of physics which deals with the interaction of electromagnetic radiation with matter. It is presently going through an exciting time of renewed interest, which is being fueled by the rapid development of sophisticated experimental approaches. Recent developments of powerful lasers to create complex excitations, thus allows one to study the highly excited levels with unprecedented resolution. With one of the new theoretical approach i.e Lie algebraic approach, we have study the vibrational spectra of Metalloporphyrins for both stretching and bending bands. Since vibrational Spectroscopy is one of the most powerful tools to gain insight into the nature of chemical bonds and geometrical structure of molecules, infrared (IR) and Raman spectra of metalloporphyrins. Using U(2) algebraic mode Hamiltonian is
H = E0 + AiCi + 1ni=Σnij〈ΣAij Cij + nij〈Σλij Mij
Where Ai, Aij and λij are the algebraic parameters which varies from molecule to molecule and Ci , Cij and Mij are algebraic operators [1,2,3,4]. The vibrational energy levels are calculated using algebraic mode Hamiltonian and the results compared with the experimental values. The results obtained by this model are in good accuracy with the observed data.
Keywords: Vibrational Spectra, Lie algebra, Metalloporphyrins.
References:
1. Srinivasa Rao Karumuri, Nirmal Kumar Sarkar, Joydeep Choudhury and Ramendu Bhattacharjee, J. Environ. Res & Develop, 2008(In Press).
2. Srinivasa Rao Karumuri, Nirmal Kumar Sarkar, Joydeep Choudhury and Ramendu Bhattacharjee, Mol.Phys, 2008(In Press).
3. Nirmal Kumar Sarkar, Srinivasa Rao Karumuri, Joydeep Choudhury and Ramendu Bhattacharjee, Mol.Phys, 106,693(2008)
4. Joydeep Choudhury, Srinivasa Rao Karumuri, Nirmal Kumar Sarkar and Ramendu Bhattacharjee, Pramana. J. Phys, 2008(In Press).

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Enhancement technique in the measurement of ac susceptibility for studying magnetic properties of nano materials

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

Enhancement technique in the measurement of ac susceptibility for studying magnetic properties of nano materials
Dr. S. Mukherjee,
Lecurer, Department of Physics, The University of Burdwan, Burdwan
Abstract :
The AC susceptibility enhancement method is the study of AC susceptibility in the presence of a second biasing field (pulsed DC or another AC field of frequency incommensurate with the measuring one). For bulk samples having a multi-domain structure, the application of a second biasing field causes an extra perturbation of the domain walls and thereby, a general loosening of the domain pinning. Thus the sample state is changed to one in which the domains are far more free to follow the applied field compared to the previous case. If from the complicated magnetization versus time curve thus obtained, only the Fourier coefficient of the measuring sine wave is computed (or measured, as with a lock-in amplifier with its reference driven by the same signal) the coefficient χ is found to increase; this is enhancement. [1]
For nano-sized ferromagnetic particles, the magnetic anisotropy energy (responsible for keeping the magnetization oriented in certain directions) is comparable to the thermal energy (kT ).When this happens, the particles become super-paramagnetic above TB ; as thermal fluctuations randomly flip the magnetization direction between parallel and anti-parallel orientations. They exhibit single-domain ferromagnetic behaviour below the blocking temperature TB and show hysteresis. In the super-paramagnetic state, he moment of each particle freely rotates, so a collection of particles acts like a paramagnet where the constituent moments are ferromagnetic particles (rather than atomic moments as in a normal paramagnet). The imaginary component of the ac susceptibility peaks at TB. Since TB depends on the measurement frequency, the peak in χ" vs. T occurs at different temperatures for different frequencies. The nature of the barrier potential can be probed with enhancement technique in the measurement of ac susceptibility.
Reference [1] S. Mukherjee, R. Ranganathan, A. Chakravarti and S. Sil; Journal of Magnetism and
magnetic materials Volume 224, Issue 3, 2001, Pages 210-220

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Comparison of Heat Transfer Enhancement due to Laminar Flow of Newtonian and non-Newtonian Nanofluid through Two Isothermally Heated Parallel Plates

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

Comparison of Heat Transfer Enhancement due to Laminar Flow of Newtonian and non-Newtonian Nanofluid through Two Isothermally Heated Parallel Plates
Apurba Kumar Santraa*, Swarnendu Senb and Niladri Chakrabortya
a Department of Power Engineering, Jadavpur University, Salt Lake Campus, Block – LB, Plot-8, Sector – III, Salt Lake, Kolkata – 700 098, India.
b Department of Mechanical Engineering, Jadavpur University, Kolkata- 700 032, India.
* Correspondent author, e_mail: aksantra@pe.jusl.ac.in
Ph: +91 33 2335 5813/ 5215 ; Fax: +91 33 2335 7254
Abstract
Heat transfer augmentation due to laminar flow of copper-water nanofluid in a two-dimensional horizontal rectangular duct has been studied numerically. The top and bottom walls are two symmetric heat sources, which are kept at constant temperature. The governing transport equations have been discretized using a finite volume approach and have been solved iteratively The correlation proposed by Patel et al. has been used to determine the effective thermal conductivity of nanofluid, which is a function of particle diameter as well as temperature. Nanofluid has been considered Newtonian as well as non-Newtonian. For Newtonian nanofluid Brinkman viscosity model has been used, while for non-Newtonian nanofluid Ostwald-de Waele model has been used. Study for a wide range of Reynolds number (Re= 5 to 1500) and solid volume fraction (0.00 ≤φ≤ 0.050) shows that the heat transfer augmentation is almost same for both the cases The rate of heat transfer increases with the increase in flow as well as increase in solid volume fraction of the nanofluid. The wall shear stress is much higher for low Re for non-Newtonian fluid and it increases with φ. But for higher Re it decreases with φ. For Newtonian nanofluid there is almost no change in wall shear stress with φ.
Keywords: Nanofluid; Rectangular Duct; Laminar Flow; Newtonian; Non-Newtonian

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Gas-Liquid reactive crystallization for the synthesis of CaCO3 nanocrystals

ADVANCED NANOMATERIALS AND ITS APPLICATIONS
8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University

Gas-Liquid reactive crystallization for the synthesis of CaCO3 nanocrystals
Suraj Varma1*, Pao-Chi Chen2, G. Unnikrishnan1
1Polymer Science and Technology Laboratory, National Institute of Technology Calicut, Kerala, India
2Department of Chemical and Materials Engineering, Lunghwa University of Science and Technology, Taiwan
*E-Mail: surajvarma@rediffmail.com
Calcium carbonate crystals were synthesized using Ca(OH)2- CO2-H2O reactive crystallization system inside a bubble column apparatus. Effect of dispersion agents on the morphology and particle size of precipitated crystals was studied using X-ray diffraction and BET surface area analysis. Citric acid, sodium metaphosphate and polyacrylic acid were used as the dispersion agents. Effect of dispersion agent concentration was also taken into account. Particle size of the crystals was found to decrease in presence of dispersion agents. Scanning electron micrographs and X-ray diffraction studies revealed that the morphology of the precipitated crystals is influenced by the presence of dispersion agents.
Key Words : Reactive Crystallization, dispersion agents.
1. Introduction
Sparingly soluble compounds are often produced by a precipitation process. The characteristics such as the surface area, the particle size and the morphology are influenced by the presence of dispersion agents [1]. An understanding of the effects of dispersion agents on the properties is crucial in the application of these particles. Small concentrations of these agents have a profound influence on the crystal growth kinetics [2]. Calcium carbonate crystals have found applications ranging from their use in biomaterials to reinforcements for high polymers. CaCO3 exists in three polymorphic forms- calcite, aragonite and vaterite. Calcite has a rhombic structure, Aragonite has a demtritic and Vaterite has a spherical morphology. Westin and Rasmuson [3] found that citric acid promotes the formation of small rhombic calcite crystals at all feed times in the semi batch mode. J. Sellami et al [4] used citrate ions as additives in the pH ranging from 8 to 10 in order to modify the influence of citrate concentration on the formation of calcium carbonate polymorphs. They also analyzed the phase equilibria involved in the precipitation reactions. Adsorption of CO2 gas which is a green house gas and converting them to a useful end product is always important in the present situation regarding environmental pollution. In the present work we analyze the effect of citrate ion. Sodium meta phosphate ion and PAA on the precipitation chemistry and
morphology of CaCO3 crystals in a gas-liquid reactive crystallization system in a bubble column. The prepared nanocrystals were characterized by means of XRD, SEM and BET surface area analysis.
2. Experimental
2.1 Materials
Several chemicals were used in this work such as Ca(OH)2, Citric acid monohydrate, sodium metaphosphate and PAA. All the chemicals were reagent grade and were supplied by Nacalai Tesque, INC, Japan.
2.2Preparation of CaCO3
Calcium Carbonate crystals were synthesized by means of gas-liquid reactive crystallization system using a bubble column. 700 ml of slaked lime slurry is introduced into the bubble column and CO2 gas (30%) was introduced from the bottom of the column. A pH electrode, CO2 analyzer and a thermometer were introduced into the system to follow pH, CO2 pressure and temperature respectively. Completion of the reaction is indicated by the lowering of solution pH to the acidic range. The crystals formed were filtered, washed and dried at 100 degree centigrade in an oven for 12 hours. Then the sample is powdered well and used for the further characterization. In the next set of experiments slaked lime slurries are prepared with the addition of citric acid, sodium meta phosphate and PAA solutions having varying concentrations. In all the experiments the composition of CO2 gas was fixed at 30%.
2.4 BET surface area
The surface areas of the prepared crystals were found out by nitrogen gas adsorption method using BET 201-APCW surface area analyzer.
2.5 X-ray diffraction studies
The approximate morphology of the precipitates were determined using X-ray diffraction pattern obtained from RIGAKU X-RAY DIFFRACTOMETER D/MAX 2200.
2.6 Morphological Studies
Dimensional and morphological inferences were also drawn out using electron microscopic technique. A JEOL scanning electron microscope was used for that purpose.
3. RESULTS AND DISCUSSION
3.1 Particle size and BET surface area
Table 1 shows the effect of dispersion agents on the particle size and surface area of calcium carbonate crystals. It is very much clear that, particle size drastically decrease by the presence of dispersion agents. As the concentration of the dispersion agents increases, its dispersion capacity also increases as evident from the results. Citrate ions proved to be a better dispersion agent as compared to the monophosphate ions. According to Sellami et al, inorder to study the transformation kinetics of calcium carbonate precipitation in presence of citrate ions, the knowledge of CaCO3 solubility in citrate medium is of utmost importance.
A drastic decreese in particle size resulted by the addition of citrate ions from 130 nm to about 40nm. Generally speaking, dispersion agents absorb at the solid-liquid interphase and form a layer of molecular membrane to hinder inter-contacy between particles. When we use a high polymer surfactant like PAA as dispersion agent, it is difficult to get powders with uniform particle size unlike in presence of citric acid or sodium metaphosphate.
3.2 XRD analysis
All the peaks obtained for a standard calcite variety are also can be seen here which indicates the precipitate contains more of calcite polymorphs. According to Vodovic and Kraj, the zeta potential of vaterite is positive in the pH range of 8-10. Then the citrate ions reduce the positive charge of vaterite because of their adsorption on the positively charged sites. At higher concentrations, zeta potential of vaterite is significantly reduced and the precipitation of vaterite is inhibited. The other polymorph arogonite is likely to precipitate only at higher operating temperatures. This explains the precipitation of calcite compared to other polymorphs.
3.3 Morphological Studies
Generally nucleation mechanism depends on the supersaturation of the solution. Low supersaturation nucleation is predominently hetrogeneous while at high supersaturation, homogeneous nucleation becomes prominent. Different types of anionic surfactants at higher concentration led to the various morphology and crystal phases. The surfactants with negatively charged head groups turn out to be very effective in controlling the morphology of inorganic materials. Surfactants can influence one or several crystallization steps and can control the formation of various crystal phases which are not easily formed under neutral environments. Morphology of precipitated CaCO3 are shown in the figures (12-14). As reported by previous researchers, the presence of citrate ions produces only rhomohedral calcite polymorph while the presence of MEA enhances the precipitation of spherical polymorph of calcium carbonate.
Conclusion
Calcium Carbonate crystals were prepared via gas-liquid reactive crystallization using a bubble column . It is found that the presence of dispersion agents can control the morphology and dimensions of precipitated crystals. The concentrations of dispersion agents are also limiting factors. The anionic surfactants inhibit the precipitation of the vaterite calcium carbonate and promote calcite precipitation.
References
1. Amico P., Daniele. P. G., Rigano. C and Sammartano. S, “Stability of calcium and magnesium citrate complexes in aqueous solution”, Annali di Chimica, 1(1982)72.
2. Daniele. P. G., De Robertis. A., De Stefano. C., Gianguzza. A., and Sammartano. S., “Studies of Polyfunctional O-ligands. Formation thermodynamics of simple and alkali metal complexes with citrate at different ionic strengths in aqueous solution”, J. Chem. Research, M (1990)2320.
3. Karl-Johan Westen and Ake. C. Rasmun, “Precipitation of calcium carbonate in the presence of citrate and EDTA”, Desalination, 159(2003)107.
4. J. Sellami and E. Plasari, “Citrate ion effect on the precipitation of calcium carbonate polymorphs”, 15th Ind. Cry. Serrento, Italy (2002).
5. G. F. Versteeg, L. A. J. van Dijck, W. P. M. van Swaaji, “On the kinetics between CO2 and alkanolamines both in aqueous and non-aqueous solutions. I. Primary ans secondary amines”, Chem. Eng. Sci, 43(1988)573.
6. Mitsutaka Kitumura, Haruo Konno, Atsunari Yasui and Hirokatsu Masuoka, “Controlling factors and mechanism of reactive crystallization of calcium carbonate polymorphs from calcium hydroxide suspensions”, J. Cyst. Growth, 236(2002)323.
7. Pao-Chi Chen, K. L. Kou, H. K. Tai, S. L. Jin, C. L. Lye and C. Y. Lin, “Removal of carbon dioxide by reactive crystallisation in a scrubber-kinetics of barium carbonate crystals”, J. Cryst. Growth, 237-239(2002)2166.
8. L. Xiang, Y. Wen, Q. Wang and Y. Jin, “Synthesis of dispersive CaCO3 in the presence of MgCl2”, Materials Chemistry and Physics, 98(2006)236.
9. Sergej Knez and Ciril Pohar, “The magnetic field influence on the polymorph composition of CaCO3 precipitated from carbonized aqueous solutions”, Journal of Colloid and Interface Science, 281(2005)377.
10. Guohua Wu, Yujun Wang, Shenlin Zhu and Jiading Wang, “Preparation of ultrafine calcium carbonate particles with micropore dispersion method”, Powder Technology, 172(2007)82.
Figures:
0E+04E-48E-41E-3Concentration of citric acid (M)04080120160Particle Dimensionsparticle sizes in nmBET surface areas in meter square per gram0E+04E-48E-41E-3Concentration of sodium metaphosphate04080120Particle dimensionsparticle sizes in nmBET surface areas in meter square per gram
Figure 1: effect of dispersion agents on the particle size and surface area of the precipitated crystals. 203040506070802θIntensity
4
3
2
1
Figure 2: XRD diagrams of some of the precipitated crystals.
Figure 3: Scanning electron micrographs of calcium carbonate crystals produced in presence of different dispersion agents.

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