2.1. Natural occurrence
Alumina is the naturally occurring crystalline form of aluminum oxide. Rubies and sapphires are gem-quality forms of corundum with their characteristic colors due to trace impurities in the corundum structure.
2.2. Chemical Perporties OF Alumina (Al2O3)
Molar mass:- 101.96 g/mol
Density:- 3.97 g cm−3, solid
Melting point:- 2054 °C
Boiling point:- 2980 °C [1]
Solubility:- insoluble in water
Structure Coordination :- octahedral
geometry
Thermo chemistry
Std enthalpy of
formation ΔfHo298:- −1675.7 kJ mol−1
Standard molar
entropy So298:- 50.92 J mol−1 K−1
Flash point:- non-flammable
Thermodynamic data:- Phase behaviour solid, liquid, gas
2.3. Stoichiometric alumina particles (r = 1.5)
Al2O3
2.4. Physical Properties
Aluminum oxide is an electrical insulator but has a relatively high thermal conductivity (40 W/m K). In its most commonly occurring crystalline form, called corundum or α-aluminum oxide, its hardness makes it suitable for use as an abrasive and as a component in cutting tools. Aluminum oxide is responsible for metallic aluminum’s resistance to weathering. Metallic aluminum is very reactive with atmospheric oxygen, and a thin passivation layer of alumina quickly forms on any exposed aluminum surface. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodizing. A number of alloys, such as aluminum bronzes, exploit this property by including a proportion of aluminum in the alloy to enhance corrosion resistance. The alumina generated by anodizing is typically amorphous, but discharge assisted oxidation processes such as plasma electrolytic oxidation result in a significant proportion of crystalline alumina in the coating, enhancing its hardness.
Aluminum oxide was taken off the United States Environmental Protection Agency's chemicals lists in 1988. Aluminum oxide is on EPA's TRI list if it is a fibrous form.
2.5. Crystal structure:-
The most common form of crystalline alumina, α-aluminum oxide, is known as corundum. Corundum has a trigonal Bravais lattice with a space group of R-3c (number 167 in the International Tables). Each unit cell contains six formula units of aluminum oxide. The oxygen ions nearly form a hexagonal close-packed structure with aluminum ions filling two-thirds of the octahedral interstices.
3. METHODS OF SYNTHESIS OF NANO ALUMINA:-
There are several methods to synthesize nano-alumina, and these are categorized into physical and chemical methods.
3.1. Physical Methods:-
1. Milling
2. Laser ablation
3. Flame spray
4. Thermal decomposition in plasma.
3.2. Chemical Methods:-
1. Sol–gel processing,
2. Solution combustion
3. Decomposition and
4. Vapour deposition
5. Milling
5.1. High energy ball milling for nanoparticle synthesis
High energy ball milling of powder particles as a method for materials synthesis has been developed as an industrial process to successfully produce new alloys and phase mixtures in 1970’s. This powder metallurgical process allows the preparation of alloys and composites, which cannot be synthesized via conventional routes. In nanomaterials research, this top down technique is well used to fine-tune the grain sizes of the materials in nanoscales.
5.2. Milling devices
1. Tumbler mills
2. Attrition mills
3. Shaker mills
4. Vibratory mills
5. Planetary mills
6. Fritsch planetary micro mill
5.3. Discussion
Here we discuss milling of nano material by Fritsch planetary micro mill, ‘pulverisette 7’. (Here grinding bowls rotates on their own axis while simultaneously rotating through an arc around the central axis. The grinding balls and the material in the grinding bowl are thus acted upon by the centrifugal forces, which constantly change in direction and intensity resulting in efficient, fast grinding process. The grinding bowl and the supporting disc rotate in opposite directions, so that the centrifugal forces alternatively act in the same and opposite directions. This results in, as a frictional effect, the grinding balls running along the inner wall of the grinding bowl, and impact effect, the balls impacting against the opposite wall of the grinding bowl. The energy thus created by impact is many times higher than for traditional mills. This results in excellent grinding performance and considerably shorter grinding times. Atmospheric contamination can be minimized by sealing the vial with flexible ‘O’ ring after the powder has been loaded. If a milling medium-a fluid (usually an organic fluid) is used, contamination by the milling tools can be prevented and also it minimizes the wear. A few parameters exists in high energy ball milling which on changing, we can produce a wide range of fine particles with different sizes and consequently with different physical properties. These parameters are
(1) Type of mill
(2) Milling atmosphere
(3) Milling media
(4) Intensity of milling
(5) Ball to powder weight ratio (BPR)
(6) Milling time and
(7) Milling temperature
The reduction in grain size is accomplished by the kinetic energy transfer from balls to powder. Since the kinetic energy of the balls is a function of their mass and velocity, dense materials are preferred like steel or tungsten carbide. Other materials used as balls are agate, sintered corundum, zirconium dioxide, Teflon, chrome nickel, silicon nitride etc. In order to prevent excessive abrasion, the hardness of the grinding bowl used and of the grinding balls must be higher than that of the materials used. Normally grinding bowls and grinding balls of the same material should be chosen. In this work tungsten carbide vial and balls (with density~14.75 g/cm3) are used to mill ferrite system and steel vial and balls (with density~7.85 g/cm3) are used to mill aluminates systems.
In the initial stage of milling, a fast decrease of grain size occurs which slows down after extended milling. Once the minimum steady state grain size is reached, further refinement ceases. Initially the kinetic energy transfer leads to the production of an array of dislocations. This is accompanied by atomic level strains. At a certain strain level, these dislocations annihilate and recombine to form small angle grain boundaries separating the individual grains. Thus sub grains are formed with reduced grain size. During further milling, this process extends throughout the entire sample. To maintain this reduction in size, the material must experience very high stresses. But extended milling could not able to maintain the high stresses and hence reduction of grain size is limited in extended milling. The two other parameters which also cause this limit to grain size reduction are the local temperature developed due to ball collisions and the overall temperature in the vial. Temperature rise arises from balls to balls, balls to powder and balls to wall collisions.
The impact speed and the impulsive load of the grinding balls are the two key parameters, which determine the kinetic energy transfer. The impulsive load of grinding balls is given by,
F = mv/t ---------- (1)
Where ‘m’ is the ball mass,
‘v’ the ball velocity and
‘t’ the ball-vial contact time
5.4. Limitation
Contamination of material occurs at the time of powdering by ball milling.
6. Laser Ablation
Pulsed laser ablation provides a means of depositing thin coatings, of a wide range of target materials, on a wide range of substrates, at room temperature. Despite its versatility and wide applicability, however, many aspects of the detailed chemical physics underlying the ablation process are still far from completely understood. The process is often envisaged as a sequence of steps, initiated by the laser radiation interacting with the solid target, absorption of energy and localised heating of the surface, and subsequent material evaporation. The properties and composition of the resulting ablation plume may evolve, both as a result of collisions between particles in the plume and through plume-laser radiation interactions. Finally the plume impinges on the substrate to be coated; incident material may be accommodated, rebound back into the gas phase, or induce surface modification (via sputtering, compaction, sub-implantation, etc.). Such a separation has conceptual appeal but, inevitably, is somewhat over-simplistic. Furthermore, the laser-target interactions will be sensitively dependent both on the nature and condition of the target material, and on the laser pulse parameters (wavelength, intensity, fluence, pulse duration, etc.). Subsequent laser-plume interactions will also be dependent on the properties of the laser radiation, while the evolution and propagation of the plume will also be sensitive to collisions and thus to the quality of the vacuum under which the ablation is conducted and/or the presence of any background gas. Obviously, the ultimate composition and velocity distribution (or distributions, in the case of a multi-component ablation plume) of the ejected material is likely to be reflected in the detailed characteristics of any deposited film.
We use excimer laser radiation to ablate a range of prototypical target materials e.g. elemental materials like graphite, CVD diamond, Cu and Al, two component systems like ZnO and LiF, and various polymeric materials, under vacuum and in the presence of lower pressures of background gas (He, Ar, H2, N2, O2).
Figure 1. Schematic diagram of an apparatus for PLA of a solid target with deposition on an on-axis mounted substrate
Current activities are focussed on two aspects of pulsed laser ablation and deposition. Fundamentals of the ablation process are investigated by wavelength, spatially and temporally resolved optical emission spectroscopy, by ion probe measurements and through use of a purpose designed and built quadrupole mass spectrometer designed to allow measurement of the nascent kinetic energy distributions of mass selected neutrals and charged particles within the plume of ablated material.
A selection of i-CCD images of the emitting species arising in the 248 nm nanosecond pulsed laser ablation of a graphite target in a vacuum: (a) C+ ions (b) C neutrals (c)-(e) particulates. (a), (b) and (d) are accumulated images of 200 laser pulses, while (c) and (e) are single pulse events.
Ultrafine α-alumina (α-Al2O3) powders have been successfully produced by laser ablation (aluminum target, Nd:YAG laser, 1.06 μm, 7 ns, filter-target distance 6 cm, oxygen flow rate 1 l/min, pressure 200 Torr, fluence 16 J/cm^2). The structural properties of the Al2O3 powders have been studied by X-ray diffraction, transmission electron microscopy and Brunauer, Emmet, Teller analysis. The behavior of luminous plume in background oxygen has been investigated by streak and fast photography, and emission spectroscopy. Stoichiometric α-Al2O3 powder has been obtained at an oxygen pressure of 200 Torr, with a diameter of 11.8 nm and a specific surface area of 160 m^2/g. At lower oxygen pressures the average diameter decreased to 6.1 nm, and the surface particle area increased to over 300 m^2/g, but the powder composition was altered by the presence of some unreacted aluminum. The measurements on plume expansion revealed initial plume velocities of 0.04–0.18 cm/μs, slowing down with pressure and distance. Emission spectroscopy indicates the presence of both neutral and ionized species, the most prominent lines being Al(I) 394.4 and 396.1 nm, Al(I) 358.7 nm, Al(III) 360.1 and 361.2 nm and Al(II) 466.7 nm; in time, only Al(I) 394.4 and 396.1 nm lines persisted for more than 2 μs. Much weaker lines could be observed for O(II) ions. α-Al2O3 (0.9 g) powders have been obtained for 1 kWh (laser output energy), with a powder collection efficiency of 75%.
7. Flame Spray
Flame processes are by far the most widely used due to its cost-effectiveness and process versatility for controlled production of nanoparticles. In flame reactors, the energy of the flame is used to drive chemical reactions of precursors producing clusters which further grow to nanoparticles by surface growth and/or coagulation and coalescence at high temperature. Generally, there are two different routes to obtain nanoparticles, namely Flame Spray Pyrolysis and Flame Spray Hydrolysis.
Flame Spray Pyrolysis is a flame process with gas phase precursors. In the synthesis of Al2O3 nanoparticles, an anhydrous aluminum chloride powder is vaporized and injected by an inert gas (e.g. nitrogen) into an oxy-ethylene flame. The combustion process occurs at temperature > 2000°C, where the aluminum chloride salt decomposes into hydrogen chloride (HCl) and aluminum oxide (Al2O3). This method could produce 10-30 nm as sprayed Al2O3 particle and 40-70 nm as calcined Al2O3 particle.
Meanwhile Flame Spray Hydrolysis method use liquid precursor to start the process with. The liquid precursor feed is supplied by a syringe pump and atomized with oxygen resulting in a fine spray. The evaporation and ignition of the spray is initiated by a smaller flame ring, which emerge from the centre of the nozzle. The combustion process will evaporate the liquid, and gas phase reaction will occur subsequently. Condensation of the vapor will result in small nano-size particle deposited in the chamber. Cerium oxide nanoparticles have been successfully synthesized, which can be used for solid oxide fuel cell.
8. Thermal decomposition in plasma
The present invention relates to a method for producing an α-alumina particulate. More particularly, the present invention relates to a method for producing an α-alumina particulate to provide a small amount of α-alumina particulate having necking.α-alumina is alumina [Al2O.sub.3] in which crystal phase is α, and widely used as a raw material for producing a sintered body such as a translucent tube. As the method for producing α-alumina, there are known methods in which water is removed from an aqueous mixture prepared by dispersing an aluminum hydrolysate and a seed crystal in water to obtain a powder mixture containing an aluminum hydrolysate and a seed crystal, and the powder mixture is calcined. (A. Krell, NanoStructured Materials, Vol. 11, 1141 (1999)).However, in the method described herein, obtained α-alumina has a large amount of particulate having necking, and it is difficult to produce a dense sintered body.The present inventors have investigated a method for producing an α-alumina particulate and resultantly completed the present invention.
Namely, the present invention provides a method for producing an α-alumina particulate comprising steps of (Ia) and (Ib), or a step of (II) described below:
(Ia) removing water from a mixture containing water, a seed crystal and a hydrolysate obtained by hydrolysis of an aluminum compound under conditions of a pH of 5 or less and a temperature of 60° C. or less,(Ib) calcining the resulted powder,
(II) calcining a mixed powder containing 75-1 wt % of an α-alumina precursor (in terms of Al2O.sub.3) and 25-99 wt % of a seed crystal (in terms of oxide of metal component).
Introduction
The sol-gel process is a versatile solution process for making ceramic and glass materials. In general, the sol-gel process involves the transition of a system from a liquid "sol" (mostly colloidal) into a solid "gel" phase. Applying the sol-gel process, it is possible to fabricate ceramic or glass materials in a wide variety of forms: ultra-fine or spherical shaped powders, thin film coatings, ceramic fibers, microporous inorganic membranes, monolithic ceramics and glasses, or extremely porous aerogel materials. An overview of the sol-gel process is presented in a simple graphic work below.
Forced hydrolysis
The simplest method for the generation of uniformly sized colloidal metal oxide is based on forced hydrolysis of metal salt solution. It is well known that most polyvalent cations readily hydrolyzed, and that deportanation of coordinated water molecules is greatly accelerated with increasing temperature. Since hydrolysis products are intermediate to precipitation of metal oxides, increasing temperature results in an increasing amount of deprotonated molecules. When the concentration far exceeds the solubility, nucleation of metal oxides occurs. In principle, to produce such metal oxide colloids, one just needs to age hydrolyzed metal solutions at elevated temperatures. It becomes obvious that hydrolysis reaction should proceed rapidly and produce an abrupt supersaturation to ensure a burst of nucleation, resulting in the formation of small particles.
Mechanisms
The starting materials used in the preparation of the "sol" are usually inorganic metal salts or metal organic compounds such as metal alkoxides. In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymeration reactions to form a colloidal suspension, or a "sol". Further processing of the "sol" enables one to make ceramic materials in different forms. Thin films can be produced on a piece of substrate by spin-coating or dip-coating. When the "sol" is cast into a mold, a wet "gel" will form. With further drying and heat-treatment, the "gel" is converted into dense ceramic or glass articles. If the liquid in a wet "gel" is removed under a supercritical condition, a highly porous and extremely low density material called "aerogel" is obtained. As the viscosity of a "sol" is adjusted into a proper viscosity range, ceramic fibers can be drawn from the "sol". Ultra-fine and uniform ceramic powders are formed by precipitation, spray pyrolysis, or emulsion techniques.This process occurs in liquid solution of organometallic precursors (TMOS, TEOS, Zr(IV)-Propoxide, Ti(IV)-Butoxide,etc.), which, by means of hydrolysis and condensation reactions, lead to the formation of a new phase (SOL).
M-O-R + H2O M-OH + R-OH (hydrolysis)M-OH + HO-M M-O-M + H2O (water condensation)
M-O-R + HO-M M-O-M + R-OH (alcohol condensation)(M=Si,Zr,Ti)
The SOL is made of solid particles of a diameter of few hundred of nm suspended in a liquid phase. Then the particles condense in a new phase (GEL) in which a solid macromolecule is immersed in a liquid phase (solvent). Drying the GEL by means of low temperature treatments (25-100 C), it is possible to obtain porous solid matrices (XEROGELs). The fundamental property of the sol-gel process is that it is possible to generate ceramic material at a temperature close to room temperature.
Synthesis of Nano Alpha-Alumina by Sol-Gel Process:-
Introduction
The Al2O3 exists in several distinct crystallographic phases (such as β,γ,δ,α-Al2O3), and it can undergo a variety of transitions until the most stable corundum structure α-Al2O3 forms at high temperature. α-Al2O3 is an easily obtainable material that has a high melting point, high abrasion resistance and high electrical insulation and as therefore a mechanically strong and chemically stable compound. The technology for fabricating α-Al2O3 is well known. During the 20th century Al2O3 was applied in a wide variety of industries. α-Al2O3 is a ceramic material of industrial importance, due to its promising structural, chemical and morphological properties. For example, Al2O3 has been applied as an essential material in thermal tools, insulation materials, abrasion materials, cutting tools, sparking plugs, integrated circuits (ICs), artificial teeth, high pressure sodium lamp, catalysts, and compound materials. The method for synthesizing artificial Al2O3 powder was developed in 1881. α-Al2O3 powder are obtaind by calcinations of boehmite or gibbsite purified from bauxite. Traditional method of fabricating α-Al2O3 includes direct sintering of the transitional alumina phases. This method requires very high temperature, which inevitably results in a considerable degree of particle coarsening with small surface area. Several wet chemical methods have been employed to synthesize the α-Al2O3 at low temperature. Bell et al. has reported a hydrothermal method for the fabrication of α-Al2O3, using 1,4-butanediol and has demonstrated how to control the particles size and morphology.4 Additionally, the concept of seeding with a-alumina has also been employed in order to enhance the kinetics of transformation and to achieve the synthesis of a-alumina at a lower temperature. A fraction of ultrafine alumina crystallites, as seed materials, and the introduction of dopant (e.g. MnO2) were tried by Messing
and Gouvea, respect ively, in order to reduce the temperature of transformation of stable alumina.3,5 Sharma et al. has crystallized the α-Al2O3,which has a median particle size 60 nm, by hydrothermal method using a surface modifier.6 Nevertheless, the colloidal method, using sol-gel processing, is another chemical approach that provides an alternative route to synthesis of α-Al2O3. In order to utilize chemical process for synthesis of nano sized powder, the control over thermodynamics of interfaces with in the reacting system has to be required as these fine particulate system tend to
minimize their surface energy either by growing to large particles or agglomeration. One approach is to modify the surface of growing particles during precipitation process in a way that growth reaction can take place but
a ‘‘growing together’’ is prevented. The surface free energy of these particulate can reduced to an appropriate level by using surface active compound (so called ‘‘surface modifier’’) which interact with the generated particle surface. The synthesis of pure alumina materials via organic or inorganic sol-gel method has been extensively studied over the past three decades. Yoldas has carried out investigations on the hydrolysis of aluminum.iso propoxide in the presence of various inorganicand organic acids as peptides.7,8 The use of chelating agents has also been employed to control the hydrolysis and condensation rates during the sol-gel processing of alumina with desired physicochemical properties.A
number of papers discuss the effect of pH during thesol-gel process on pore size,por e volume,particle size,and surface area of SiO2, Y2O3,TiO 2.10,11 Rao and Ji have discussed the role of pH during the sol-gel processing of alumina. However,they did not mention the variation in particle size of the alumina with pH change.9,12 Nevertheless,the surface area of the alumina reported in their investigations was <56>0.1 μm for α-Al2O3 at 1100-1250˚C. So it is difficult to process nano α-Al2O3 powders which is less than 100 nm by conventional method. Aluminum chloride hex hydrate may be hydrolyzed to produce the sol, AlCl3 3H2 Of AlOH3 3HCl DH ¼ 15:71 _ 108 kj=mol ð1Þ Reaction (2) shows that Al powder reacts with HCl to produce aluminum chloride and hydrogen gas. Therefore, Al can be used as a source of AlCl3 to produce the sol containing Al(OH)3 nano particles. 2Al þ 6HClf22AlCl3 þ 3H2 DH ¼ _3:23 _ 1014 kj=mol ð2Þ. Finally the hydroxides groups produced in reaction (1) aggregate together to form the gel. The amount of acid in the mixture must be precisely controlled. Table 1 shows that the molar ratio of HCl/H2O should be fixed at about 0.18 to form a sol (pH=3.2). The appearance of sol would be opaque or translucent when the ratio of HCl/H2O is lower than 0.18 due to insufficient amount of acid for dissolving of aluminum powder. Adding more acid than 0.18 would result in a decrease of the gelation time which may be attributed to the catalytic effect. As can be seen in Table 1, the gelation time decreases by increasing aluminum powder due to increasing the rate of hydrolysis and condensation reactions. In addition, aluminum powder is cheaper than aluminum chloride hex hydrate. Therefore, it is desirable to increase Al/AlCl3•6H2O ratio. However, increasing the amount of aluminum 75 content in the sol decreases transparency due to insufficient solubility of aluminum in 76 the sol. It seems that the 1.81 molar ratio of Al/AlCl3•6H2O is an optimum ratio. 77 As can be seen in Fig. 1, θ-Al2O3 and η-Al2O3 phases are detected in the gel heat 78 treated at 600 °C. By increasing the heat treatment temperature to 900 °C, the intensity 79 of their corresponding peaks increases. Some peaks of α-Al2O3 appear at 1000 °C. 80 Transformation of the transitional alumina phases, θ-Al2O3 and η-Al2O3, to the final 81 stable α-Al2O3 takes place approximately at about 1100 °C. Usually α-Al2O3 crystallizes around 1200 °C, however, in the present study the lower crystallization temperature of α-Al2O3 could be related to the fine crystalline size and higher specific surface area. As shown in Fig. 2, the most weight loss (40%) occurs at lower than 300 °C due to evaporation of volatile components. The weight loss of the gel is less than 5% from 300 °C to 600 °C. So the volatilization does not occur obviously. From 600 °C to 700 °C, a 86 weight loss occurs due to volatilization of chlorine which corresponds to an endothermic reaction. However, there was an exothermic reaction due to crystallization 88 of transition phases of alumina in this range. From 1000 °C to 1200 °C, an exothermic reaction occurs due to transformation of transition phases of alumina to α-alumina as shown in Fig. 3, most of the particles heat treated at 1200 °C are in the range of 32–100 nm and spherical in shape. As can be seen, the particles began to sinter and agglomerate together in this temperature. This powder can be used as initial powder for 93 fabrication of bulk alumina with nano microstructure.
