علم سرامیک

Chapter 1 82. Varshneya, A. K. Fundamentals of Inorganic Glasses; Academic Press: San Diego, CA, 1994, Chap. 20. 83. Glass: Science and Technology; Uhlmann, D. R., Kreidl, N. J. eds.; Academic Press: San Diego, CA, 1983, Vol. 1Vol. 2, Pt. 1. 84. McMillan, P. W. Glass Ceramics; 2nd ed.; Academic Press: New York, 1979. 85. Grossman, D. G. Concise Encyclopedia of Advanced Ceramic Materials; Brook, R. J. ed.; The MIT Press: Cambridge, Massachusetts, 1990, pp. 170–176. 86. Matijevic, E. Ultrastructure Processing of Ceramics, Glasses, and Composites; Hench, L. L., Ulrich, D. R. eds.; John Wiley: New York, 1984, pp. 334–352. 87. Zhou, Y. C.; Rahaman, M. N. J. Mater. Res. 1993, Vol. 8 (7), 1680. 88. Lange, F. F. J. Am. Ceram. Soc. 1989, Vol. 72 (1), 3. 89. Barringer, E. A.; Bowen, H. K. J. Am. Ceram. Soc. 1982, Vol. 85 (12), C-199; Am. Ceram. Soc. Bull. 1982, 61, 336. 90. Brook, R. J. Concise Encyclopedia of Advanced Ceramic Materials; Brook, R. J. ed.; The MIT Press: Cambridge, Massachusetts, 1990, pp. 1–8. 91. Chu, G. P. K. Ceramic Microstructures; Fulrath, R. M., Pask, J. A. eds.; John Wiley: New York, 1968, p. 833. 92. Dalgleish, B. J.; Evans, A. G. J. Am. Ceram. Soc. 1985, Vol. 68 (1), 44. 93. Yan, M. F.; Rhodes, W. W. Am. Ceram. Soc. Bull. 1984, Vol. 63 (12), 1484. 94. Schmutzler, H. I. J. Am. Ceram. Soc. 1994, Vol. 77, 721. 2 Synthesis of Powders 2.1 INTRODUCTION As outlined in Chapter 1, the characteristics of the powder have a remarkable effect on subsequent processing, such as consolidation of the powder into a green body and firing to produce the desired microstructure. As a result, powder synthesis is very important to the overall fabrication of ceramics. In this chapter we shall first define, in general terms, the desirable characteristics that a powder should possess for the production of successful ceramics and then consider some of the main methods used for the synthesis of ceramic powders. In practice, the choice of a powder preparation method will depend on the production cost and the capability of the method for achieving a certain set of desired characteristics. For convenience, we shall divide the powder synthesis methods into two categories: mechanical methods and chemical methods. Powder synthesis by chemical methods is an area of ceramic processing that has received a high degree of interest and has undergone considerable changes in the last 25 years. Further new developments in this area are expected in the future. 2.2 DESIRABLE POWDER CHARACTERISTICS Traditional ceramics generally must meet less specific property requirements than advanced ceramics. They can be chemically inhomogeneous and can have complex microstructures. Unlike the case of advanced ceramics, chemical reaction during firing is often a requirement. The starting materials for traditional ceramics therefore consist of mixtures of powders with a chosen reactivity. For example, the starting powders for an insulating porcelain can, typically, be a 49 50 Chapter 2 mixture of clay (
50 wt%), feldspar (
25 wt%), and silica (
25 wt%). Fine particle size is desirable for good chemical reactivity. The powders must also be chosen to give a reasonably high packing density that serves to limit the shrinkage and distortion of the body during firing. Clays form the major constituent and therefore provide the fine particle size constituent in the starting mixture for most traditional ceramics. Generally, low cost powder preparation methods are used for traditional ceramics. Advanced ceramics must meet very specific property requirements and therefore their chemical composition and microstructure must be well controlled. Careful attention must be paid to the quality of the starting powders. For advanced ceramics, the important powder characteristics are the size, size distribution, shape, state of agglomeration, chemical composition, and phase composition. The structure and chemistry of the surface are also important. The size, size distribution, shape, and state of agglomeration have an important influence on both the powder consolidation step and the microstructure of the fired body. A particle size greater than
1 m generally precludes the use of colloidal consolidation methods because the settling time of the particles is fairly short. The most profound effect of the particle size, however, is on the sintering. As we shall show later, the rate at which the body densifies increases strongly with a decrease in particle size. Normally, if other factors do not cause severe difficulties during firing, a particle size of less than
1 m allows the achievement of high density within a reasonable time (e.g., a few hours). Whereas a powder with a wide distribution of particle sizes (sometimes referred to as a polydisperse powder) may lead to higher packing density in the green body, this benefit is usually vastly outweighed by difficulties in microstructural control during sintering. A common problem is that the large grains coarsen rapidly at the expense of the smaller grains, making the attainment of high density with controlled grain size impossible. Homogeneous packing of a narrow size distribution powder (i.e., a nearly monodisperse powder) generally allows greater control of the microstructure. A spherical or equiaxial shape is beneficial for controlling the uniformity of the packing. Agglomerates lead to heterogeneous packing in the green body which, in turn, leads to differential sintering during the firing stage. Differential sintering occurs when different regions of the body shrink at different rates. This can lead to serious problems such as the development of large pores and cracklike voids in the fired body (see Fig. 1.24). Furthermore, the rate at which the body densifies is roughly similar to that for a coarse-grained body with a particle size equivalent to that of the agglomerates. An agglomerated powder therefore has serious limitations for the fabrication of ceramics when high density coupled with a finegrained microstructure is desired. Agglomerates are classified into two types: soft agglomerates in which the particles are held together by weak van der Waals forces and hard agglomerates in which the particles are chemically bonded toSynthesis of Powders 51 gether by strong bridges. The ideal situation is the avoidance of agglomeration in the powder. However, in most cases this is not possible. In such cases, we would then prefer to have soft agglomerates rather than hard agglomerates. Soft agglomerates can be broken down relatively easily by mechanical methods (e.g., pressing or milling) or by dispersion in a liquid. Hard agglomerates cannot be easily broken down and therefore must be avoided or removed from the powder. Surface impurities may have a significant influence on the dispersion of the powder in a liquid, but the most serious effects of variations in chemical composition are encountered in the firing stage. Impurities may lead to the formation of a small amount of liquid phase at the sintering temperature, which causes selected growth of large individual grains (Fig. 1.23). In such a case, the achievement of a fine uniform grain size would be impossible. Chemical reactions between incompletely reacted phases can also be a source of problems. We would therefore like to have no chemical change in the powder during firing. For some materials, polymorphic transformation between different crystalline structures can also be a source of severe difficulties for microstructure control. Common examples are ZrO2, for which cracking is a severe problem on cooling, and - Al2O3, where the transformation to the  phase results in rapid grain growth and a severe retardation in the densification rate. To summarize, the desirable powder characteristics for the fabrication of advanced ceramics are listed in Table 2.1. 2.3 POWDER SYNTHESIS METHODS A variety of methods exist for the synthesis of ceramic powders. In this book, we divide them into two categories: mechanical methods and chemical methods. Mechanical methods are generally used to prepare powders of traditional ceramics from naturally occurring raw materials. Powder preparation by mechanical methods is a fairly mature area of ceramic processing in which the scope for new developments is rather small. However, in recent years, the preparation of fine TABLE 2.1 Desirable Powder Characteristics for Advanced Ceramics Powder characteristic Desired property Particle size Particle size distribution Particle shape State of agglomeration Chemical composition Phase composition Fine (
1 m) Narrow or monodisperse Spherical or equiaxial No agglomeration or soft agglomerates High purity Single phase 52 Chapter 2 powders of some advanced ceramics by mechanical methods involving milling at high speeds has received a fair amount of interest. Chemical methods are generally used to prepare powders of advanced ceramics from synthetic materials or from naturally occurring raw materials that have undergone a considerable degree of chemical refinement. Some of the methods categorized as chemical involve a mechanical milling step as part of the process. The milling step is usually necessary for the breakdown of agglomerates and for the production of the desired physical characteristics of the powder such as average particle size and particle size distribution. Powder preparation by chemical methods is an area of ceramic processing that has seen several new developments in the past 25 years and further new developments are expected in the future. Table 2.2 provides a summary of the common powder preparation methods for ceramics. 2.4 POWDER PREPARATION BY MECHANICAL METHODS 2.4.1 Comminution The process in which small particles are produced by reducing the size of larger ones by mechanical forces is usually referred to as comminution. It involves operations such as crushing, grinding, and milling. For traditional, clay-based ceramics, machines such as jaw, gyratory, and cone crushers are used for coarse size reduction of the mined raw material, to produce particles in the size range of 0.1–1 mm. The equipment and the processes involved in the production of these coarse particles are well described elsewhere (1–3). Here we will assume that a stock of coarse particles (with sizes 1 mm) is available and consider the processes applicable to the subsequent size reduction to produce a fine powder. The most common way to achieve this size reduction is by milling. One or more of a variety of mills may be used, including high-compression roller mills, jet mills (also referred to as fluid energy mills), and ball mills (3,4). Ball mills are categorized into various types depending on the method used to impart motion to the balls (e.g., tumbling, vibration, and agitation). For the following discussion, we define the energy utilization of the comminution method as the ratio of the new surface area created to the total mechanical energy supplied. The rate of grinding is defined as the amount of new surface area created per unit mass of particles per unit time. Obviously, there is a connection between the two terms. A comminution method that has a high energy utilization will also have a high rate of grinding, so that the achievement of a given particle size will take a shorter time. For a given method, we will also want to understand how the rate of grinding depends on the various experimental factors. Synthesis of Powders 53 TABLE 2.2 Common Powder Preparation Methods for Ceramics Powder preparation method Advantages Disadvantages Mechanical Comminution Mechanochemical synthesis Chemical Solid-state reaction Decomposition, reaction between solids Liquid solutions Precipitation or coprecipitation; solvent vaporization (spray drying, spray pyrolysis, freeze drying); gel routes (sol–gel, Pechini, citrate gel, glycine nitrate) Nonaqueous liquid reaction Vapor-phase reaction Gas–solid reaction Gas–liquid reaction Reaction between gases Inexpensive, wide applicability Fine particle size, good for nonoxides, low temperature route Simple apparatus, inexpensive High purity, small particle size, composition control, chemical homogeneity High purity, small particle size Commonly inexpensive for large particle size High purity, small particle size High purity, small particle size, inexpensive for oxides Limited purity, limited homogeneity, large particle size Limited purity, limited homogeneity Agglomerated powder, limited homogeneity for multicomponent powders Expensive, poor for nonoxides, powder agglomeration commonly a problem Limited to nonoxides Commonly low purity, expensive for fine powders Expensive, limited applicability Expensive for nonoxides, agglomeration commonly a problem 54 Chapter 2 In the milling process, the particles experience mechanical stresses at their contact points due to compression, impact, or shear with the mill medium or with other particles. The mechanical stresses lead to elastic and inelastic deformation and, if the stress exceeds the ultimate strength of the particle, to fracture of the particles. The mechanical energy supplied to the particle is used not only to create new surfaces but also to produce other physical changes in the particle (e.g., inelastic deformation, increase in temperature, and lattice rearrangements within the particle). Changes in the chemical properties (especially the surface properties) can also occur, especially after prolonged milling or under very vigorous milling conditions. Consequently, the energy utilization of the process can be fairly low, ranging from 20% for milling produced by compression forces to 5% for milling by impact. Figure 2.1 summarizes the stress mechanisms and the range of particle sizes achieved with different types of mills for the production of fine powders. 2.4.1.1 High-Compression Roller Mills In the high-compression roller mill, the material is stressed between two rollers. In principle, the process is similar to a conventional roller mill, but the contact pressure is considerably higher (in the range of 100–300 MPa). The stock of coarse particles is comminuted and compacted. This process must therefore be used in conjunction with another milling process (e.g., ball milling) to produce FIGURE 2.1 Range of particle sizes reached with different types of mills. (From Ref. 4.) Synthesis of Powders 55 a powder. Although the process is unsuitable for the production of particle sizes below
10 m, it has two significant advantages. First, the energy utilization is fairly good because the mechanical energy supplied to the rollers goes directly into comminuting the particles. For the production of the same size of particles from a stock of coarse particles, the use of a high-energy roller mill in conjunction with a ball mill is more efficient than the use of a ball mill alone. A second advantage is that since only a small amount of material makes contact with the rolls, the wear can be fairly low (e.g., much lower than in ball milling). 2.4.1.2 Jet Mills Jet mills are manufactured in a variety of designs. Generally, the operation consists of the interaction of one or more streams of high-speed gas bearing the stock of coarse particles with another high speed stream. Comminution occurs by particle–particle collisions. In some designs, comminution is achieved by collisions between the particles in the high speed stream and a wall (fixed or movable) within the mill. The milled particles leave the mill in the emergent fluid stream and are usually collected in a cyclone chamber outside the mill. The gas for the high-speed stream is usually compressed air, but inert gases such as nitrogen or argon may be used to reduce oxidation of certain nonoxide materials (e.g., Si). The average particle size and the particle size distribution of the milled powder depend on a number of factors, including the size, size distribution, hardness and elasticity of the feed particles, the pressure at which the gas is injected, the dimensions of the milling chamber, and the use of particle classification in conjunction with the milling. Multiple gas inlet nozzles are incorporated into some jet mill designs in order to provide multiple collisions between the particles, thereby enhancing the comminution process. In some cases the flow of the particles in the high-speed gas stream can be utilized for their classification in the milling chamber. The feed particles remain in the grinding zone until they are reduced to a sufficiently fine size and then are removed from the milling chamber. An advantage of jet mills is that when combined with a particle classification device, they provide a rapid method for the production of a powder with a narrow size distribution for particle sizes down to
1 m. A further advantage is that for some designs, the particles do not come into contact with the surfaces of the milling chamber, so contamination is not a problem. 2.4.1.3 Ball Mills The high-compression roller mills and jet mills just described achieve comminution without the use of grinding media. For mills that incorporate grinding media (balls or rods), comminution occurs by compression, impact, and shear (friction) between the moving grinding media and the particles. Rod mills are not suitable for the production of fine powders, whereas ball milling can be used to produce 56 Chapter 2 particle sizes from
10 m to as low as a fraction of a micrometer. Ball milling is suitable for wet or dry milling. Ball milling is a fairly complex process that does not lend itself easily to rigorous theoretical analysis. The rate of grinding depends on a number of factors, including themill parameters, the properties of the grinding media, and the properties of the particles to be ground (3). Generally, ball mills that run at low speeds contain large balls because most of the mechanical energy supplied to the particle is in the form of potential energy. Those mills that run at high speeds contain small balls because, in this case, most of the energy supplied to the particle is in the form of kinetic energy. For a given size of grinding medium, since the mass is proportional to the density, the grinding medium should consist of materials with as high a density as possible. In practice, the choice of the grinding medium is usually limited by cost. The size of the grinding medium is an important consideration. Small grinding media are generally better than large ones. For a given volume, the number of balls increases inversely as the cube of the radius. Assuming that the rate of grinding depends on the number of contact points between the balls and the powder and that the number of contact points, in turn, depends on the surface area of the balls, then the rate of grinding will increase inversely as the radius of the balls. However, the balls cannot be too small since they must impart sufficient mechanical energy to the particles to cause fracture. The rate of grinding also depends on the particle size. The rate decreases with decreasing particle size and, as the particles become fairly fine (e.g., about 1 m to a few micrometers), it becomes more and more difficult to achieve further reduction in size. A practical grinding limit is approached (Fig. 2.2). This limit depends on several factors. An important factor is the increased tendency for the particles to agglomerate with decreasing particle size. A physical equilibrium is therefore set up between the agglomeration and comminution processes. Another factor is the decreased probability for the occurrence of a comminution event with decreasing particle size. Finally, the probability of a flaw with a given size existing in the particle decreases with decreasing particle size, i.e., the particle becomes stronger. The reduction of the limiting particle size may be achieved by wet milling as opposed to dry milling (Fig. 2.2), by using a dispersing agent during wet milling (Fig. 2.3), and by performing the milling in stages (3). For staged milling, as the particles get finer, they are transferred to another compartment of the mill or to another mill operating with smaller balls. A disadvantage of ball milling is that wear of the grinding medium can be fairly high. For advanced ceramics, as discussed before, the presence of impurities in the powder is a serious concern. The best solution is to use balls with the same composition as the powder itself. However, this is only possible in very few cases and even for these, at fairly great expense. Another solution is to use a grinding medium that is chemically inert at the firing temperature of the body (e.g., ZrO2 Synthesis of Powders 57 FIGURE 2.2 Particle size versus grinding time for ball milling. (From Ref. 3.) FIGURE 2.3 Effect of Flotigham P, an organic dispersing agent, on grinding of quartzite and limestone in a rod mill. (From Ref. 5.) 58 Chapter 2 balls) or can be removed from the powder by washing (e.g., steel balls). A common problem is the use of porcelain balls or low-purity Al2O3 balls that wear easily and introduce a fair amount of SiO2 into the powder. Silicate liquids normally form at the firing temperature and make microstructural control very difficult. A list of grinding balls available commercially and the approximate density of each is given in Table 2.3. Tumbling ball mills, usually referred to simply as ball mills, consist of a slowly rotating horizontal cylinder that is partly filled with grinding balls and the particles to be ground. In addition to the factors discussed above, the speed of rotation of the mill is an important variable since it influences the trajectory of the balls and the mechanical energy supplied to the powder. Defining the critical speed of rotation as the speed required to just take the balls to the apex of revolution (i.e., to the top of the mill where the centrifugal force just balances the force of gravity), we find that the critical speed (in revolutions per unit time) is equal to (g/a)1/2/(2 ), where a is the radius of the mill and g is the acceleration due to gravity. In practice, ball mills are operated at
75% of the critical speed so that the balls do not reach the top of the mill (Fig. 2.4). As we outlined earlier, the ball milling process does not lend itself easily to rigorous theoretical analysis. We therefore have to be satisfied with empirical relationships. One such empirical relationship is Rate of milling ≈ Aa d r m 1/2 ρ (2.1) where A is numerical constant that is specific to the mill being used and the powder being milled, a is the radius of the mill,  is the density of the balls, d TABLE 2.3 Commercially Available Grinding Media for Ball Milling Grinding media Density (g/cm3) Porcelain Silicon nitride Silicon carbide Alumina Lower than 95% purity Greater than 99% purity Zirconia MgO stabilized High purity Y2O3 stabilized Steel Tungsten carbide 2.3 3.1 3.1 3.4–3.6 3.9 5.5 6.0 7.7 14.5 Synthesis of Powders 59 FIGURE 2.4 Schematic of a ball mill in cataracting motion. (From Ref. 4.) is the particle size of the powder, and r is the radius of the balls. According to Eq. (2.1), the rate decreases with decreasing particle size; however, this holds up to a certain point since, as discussed earlier, a practical grinding limit is reached after a certain time. The variation of the rate of grinding with the radius of the balls must also be taken with caution; the balls will not possess sufficient energy to cause fracture of the particles if they are too small. In themilling process, the objective is to have the balls fall onto the particles at the bottom of the mill rather than onto the mill liner itself. For a mill operating at
75% of its critical speed, this occurs for dry milling for a quantity of balls filling
50% of the mill volume and for a charge of particles filling
25% of the mill volume. For wet milling, a useful guide is for the balls occupying
50% of the mill volume and the slurry
40% of the mill volume with the solids content of the slurry equal to
25–40%. Wet ball milling has an advantage over dry milling in that its energy utilization is somewhat higher (by
10–20%). A further advantage, as we have mentioned earlier, is the ability to produce a higher fraction of finer particles. Disadvantages of wet milling are the increased wear of the grinding media, the need for drying of the powder after milling, and contamination of the powder by the adsorbed vehicle. Vibratory ball mills or vibro-mills consist of a drum, almost filled with a well-packed arrangement of grinding media and the charge of particles, that is vibrated fairly rapidly (10–20 Hz) in three dimensions. The grinding medium, usually cylindrical in shape, occupies more than 90% of the mill volume. The amplitude of the vibrations is controlled so as not to disrupt the well-packed 60 Chapter 2 arrangement of the grinding media. The three-dimensional motion helps in the distribution of the charge of particles and, in the case of wet milling, to minimize segregation of the particles in the slurry. The fairly rapid vibratory motion produces an impact energy that is much greater than the energy supplied to the particles in a tumbling ball mill. Vibratory ball mills therefore provide a much more rapid comminution process compared to the tumbling ball mills. They are also more energy efficient than tumbling ball mills. Agitated ball mills, also referred to as attrition mills or stirred media mills, differ from tumbling ballmills in that themilling chamber does not rotate. Instead, the stock of particles and the grinding medium are stirred rather vigorously with a stirrer rotating continuously at frequencies of 1–10 Hz. The grinding chamber is aligned either vertically or horizontally (Fig. 2.5) with the stirrer located at the center of the chamber. The grinding media consist of small spheres (
0.2–10 mm) that make up
60–90% of the available volume of the mill. Although it can be used for dry milling, most agitated ball milling is carried out with slurries. Most agitated ball milling is also carried out continuously, with the slurry of particles to be milled fed in at one end and the milled product removed at the other end. For milling where the agitation is fairly intense, considerable heat is produced, and a means of cooling the milling chamber is required. Agitated ball mills have a distinct advantage over tumbling ball mills and vibratory ball mills in that the energy utilization is significantly higher. They also have the ability to handle a higher solids content in the slurry to be milled. Furthermore, as we have discussed earlier, the use of fine grinding media improves the rate of milling. The high efficiency of the process coupled with the short duration required for milling means that contamination of the milled powder is less serious than in the case for tumbling ball mills or vibratory ball mills. Contamination in agitated ball milling can be further reduced by lining the mill FIGURE 2.5 Schematic of an agitated ball mill. (From Ref. 4.) Synthesis of Powders 61 chamber with a ceramic material or a plastic and by using ceramic stirrers and grinding media. 2.4.2 Mechanochemical Synthesis In comminution, our interest lies mainly in achieving certain physical characteristics, such as particle size and particle size distribution. However, the exploitation of chemical changes during milling for the preparation of powders has received some interest in recent years. Grinding enhances the chemical reactivity of powders. Rupture of the bonds during particle fracture results in surfaces with unsatisfied valences. This, combined with the high surface area favors reaction between mixed particles or between the particles and their surroundings. Powder preparation by high-energy ball milling of elemental mixtures is referred to by various terms, including mechanochemical synthesis, mechanosynthesis, mechanical driven synthesis, mechanical alloying, and high energy milling.While no term has received widespread acceptance, we shall use the term mechanochemical synthesis in this book. The method has attracted significant interest in the last 20 years or so for the production of powders of metals and alloys (6–8). While less attention has been paid to inorganic systems, the method has been investigated for the preparation of a variety of powders, including oxides, carbides, nitrides, borides, and silicides (9–12). Mechanochemical synthesis can be carried out in small mills, such as the Spex mill, for synthesizing a few grams of powder or in attrition mills for larger quantities. In the Spex mill, a cylindrical vial containing the milling balls and the charge of particles undergoes large amplitude vibrations in three dimensions at a frequency of
20 Hz. The charge occupies
20% of the volume of the vial, and the amount of milling media (in the form of balls 5–10 mm in diameter) makes up 2–10 times the mass of the charge. The milling is normally carried out for a few tens of hours for the set of conditions indicated here. The method therefore involves high-intensity vibratory milling for very extended periods. An advantage of mechanochemical synthesis is the ease of preparation of powders that can otherwise be difficult to produce, such as those of the silicides and carbides. For example, most metal carbides are formed by the reaction between metals or metal hydrides and carbon at high temperatures (in some cases as high as 2000C). Furthermore, some carbides and silicides have a narrow compositional range that is difficult to produce by other methods. A disadvantage is the incorporation of impurities from the mill and milling medium into the powder. The mechanism of mechanochemical synthesis is not clear. One possibility is the occurrence of the reaction by a solid-state diffusion mechanism. Since diffusion is thermally activated, this would require a significant lowering of the activation energy, a considerable increase in the temperature existing in the mill, 62 Chapter 2 or some combination of the two. While considerable heating of the mill occurs, the temperature is significantly lower than that required for a true solid-state mechanism. A second possibility is that the reaction occurs by local melting during the milling process. While melting of the particles may accompany highly exothermic reactions, as outlined for the next mechanism, the evidence for compound formation by local melting is unclear. A third possibility is the occurrence of the reaction by a form of selfpropagating process at high temperature. In highly exothermic reactions, such as the formation of molybdenum and titanium silicides from their elemental mixtures, the heat that is liberated is often sufficient to sustain the reaction (13,14). However, for the reaction to first occur, a source of energy must be available to raise the adiabatic temperature of the system to that required for it to become self-sustaining. The surface energy of the very fine powders prior to extensive reaction is quite enormous. For example, the average particle sizes of the Mo and Si powders prior to extensive formation of MoSi2 have been reported as
20 nm and
10 nm, respectively (13). The surface energy of the particles alone is estimated as 5–10% of the heat of formation of MoSi2 (131.9 kJ/mol at 298 K). This high surface energy, coupled with the stored strain energy (predominantly in theMo particles), may provide such a source of energy for sustaining the reaction. A critical step for the formation reaction in mechanochemical synthesis appears to be the generation of a fine enough particle size so that the available surface and strain energy is sufficient to make the reaction self-sustaining. Experimentally, the reaction for MoSi2 and other silicides shows features that are characteristic of a self-propagating process. As shown in Fig. 2.6, following an induction period, the reaction occurs quite abruptly. It is likely that after a small portion of the elemental powders react during the milling process, the heat liberated by the reaction ignites the unreacted portion until the bulk of the elemental powders is converted to the product. It is not clear whether the formation of a liquid or the melting of the product occurs during the rapid reaction. Immediately following the reaction, the product is highly agglomerated. For MoSi2, the agglomerate size was found to be
100 m, made up of primary particles of
0.3 m in diameter (Fig. 2.7). 2.5 POWDER SYNTHESIS BY CHEMICAL METHODS A wide range of chemical methods exist for the synthesis of ceramic powders and several reviews of the subject are available in the ceramic literature (15–20). For convenience, we will consider the methods in three fairly broad categories: (1) solid-state reactions, (2) synthesis from liquid solutions, and (3) vapor-phase reactions. Synthesis of Powders 63 FIGURE 2.6 X-ray diffraction pattern of a stoichiometric mixture of Mo and Si powders after milling for (a) 3 h 12 min and (b) 3 h 13 min showing a fairly abrupt formation of MoSi2. (From Ref. 13.) FIGURE 2.7 TEM image of MoSi2 in the sample milled for 3 h 13 min showing three particles separated by Mo (arrow). (From Ref. 13.) 64 Chapter 2 2.5.1 Solid-State Reactions Chemical decomposition reactions, in which a solid reactant is heated to produce a new solid plus a gas, are commonly used for the production of powders of simple oxides from carbonates, hydroxides, nitrates, sulfates, acetates, oxalates, alkoxides, and other metal salts. An example is the decomposition of calcium carbonate (calcite) to produce calcium oxide and carbon dioxide gas: CaCO s CaO s CO g 3 2 ( )→ ( )+ ( ) (2.2) Chemical reactions between solid starting materials, usually in the form of mixed powders, are common for the production of powders of complex oxides such as titanates, ferrites, and silicates. The reactants normally consist of simple oxides, carbonates, nitrates, sulfates, oxalates, or acetates. An example is the reaction between zinc oxide and alumina to produce zinc aluminate: ZnO(s)+ AlO (s)→ ZnAlO (s) 2 3 2 4 (2.3) These methods, involving decomposition of solids or chemical reaction between solids are referred to in the ceramic literature as calcination. 2.5.1.1 Decomposition Because of the industrial and scientific interest, a large body of literature exists on the principles, kinetics, and chemistry of decomposition reactions. Several comprehensive texts or reviews are available on the subject (21–23). The most widely studied systems are CaCO3, MgCO3, and Mg(OH)2. We will focus on the basic thermodynamics, reaction kinetics and mechanism, and process parameters pertinent to the production of powders. Considering the thermodynamics, for the decomposition of CaCO3 defined by Eq. (2.2), the standard heat (enthalpy) of reaction at 298K, Ho R, is 44.3 kcal/ mol (24). The reaction is strongly endothermic (i.e., Ho R is positive), which is typical for most decomposition reactions. This means that heat must be supplied to the reactant to sustain the decomposition. The Gibbs free energy change associated with any reaction is given by: ΔG ΔG RT K R R = o+ ln (2.4) where Go R is the free-energy change for the reaction when the reactants are in their standard state, R is the gas constant, T is the absolute temperature, and K is the equilibrium constant for the reaction. For the reaction defined by Eq. (2.2), K a a a p CaO CO CaCO CO = 2= 3 2 (2.5) Synthesis of Powders 65 where aCaO and aCaCO3 are the activities of the pure solids CaO and CaCO3, respectively, taken to be unity, and aCO2 is the activity of CO2, taken to be the partial pressure of the gas. At equilibrium, GR = 0, and combining Eqs. (2.4) and (2.5), we get ΔG RT p R o CO = − ln 2 (2.6) The standard free energy for the decomposition of CaCO3,MgCO3, and Mg(OH)2 is plotted in Fig. 2.8, along with the equilibrium partial pressure of the gas for each of the reactions (25). Assuming that the compounds become unstable when the partial pressure of the gaseous product above the solid equals the partial pressure of the gas in the surrounding atmosphere, we can use Fig. 2.8 to determine the temperatures at which the compounds become unstable when heated in air. For example, CaCO3 becomes unstable above
810K, MgCO3 above 480K, and depending on the relative humidity, Mg(OH)2 becomes unstable above 445–465K. Furthermore, acetates, sulfates, oxalates, and nitrates have essentially zero partial pressure of the product gas in the ambient atmosphere so they are predicted to be unstable. The fact that these compounds are observed to be stable at much higher temperatures indicates that their decomposition is controlled by kinetic factors and not by thermodynamics. Kinetic investigations of decomposition reactions