Sikalidis C. (ed.) Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

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Combustion Synthesis of Ceramic Powders with Controlled Grain Morphologies

Fig. 26. TEM photographs of β-SiAlON nanorods with SAED pattern

From the above experimental results and discussion, an epitaxial nucleation and anisotropic

growth mechanism is proposed to explain the formation of β-SiAlON micropalings. This

mechanism includes two primary hypotheses: (1) epitaxial nucleation on side faces of coarse

prismatic crystals; (2) anisotropic growth of nanorods in [001] direction. By epitaxial

nucleation, a new crystal forms on a side face of a coarse prismatic crystal and then grows in

three orthogonal directions, as illustrated in Figure 27. If the growth rate in the preferred

direction (V

) is much higher than those in two lateral directions (V

and V

), the new

crystal undergoes an anisotropic growth and develop into a slim nanorod. With the

formation of more nanorods around the central coarse crystal, a micropaling is produced.

Fig. 27. A schematic illustration of the formation of β-SiAlON micropalings

5. Combustion synthesis of Ti-Al-C powders with lamellar grains

The lamellar ceramics in the Ti-Al-C ternary system have unique physical and mechanical

properties, such as high melting point, good thermal and electrical conductivity, and

Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

machinability by both electrical discharge method and conventional cutting tools [32-34].

Among these ceramics, Ti

AlC

and Ti

AlC are two typical materials that have been mostly

studied. For example, it is reported that Ti

AlC

exhibits room-temperature compressive

plasticity in contrast to normal brittle ceramics, and Ti

AlC shows excellent machinability.

Ti-Al-C ternary compounds can be synthesized by different methods, such as sintering,

mechanical alloying, and combustion synthesis.

From the raw materials of Ti, Al, carbon black, and TiC, both Ti

AlC

and Ti

AlC can be

prepared by combustion synthesis [35-38]. In the synthesized products, TiC is also present

other than the Ti-Al-C ternary compounds, as revealed by XRD patterns shown in Figure 28.

In the combustion synthesis of Ti-Al-C ternary carbides, two reaction stages are generally

involved:

Ti + C  TiC and Ti + Al  Ti-Al melt

TiC + Ti-Al melt  Ti

AlC

/Ti

AlC

In the formation of Ti

AlC

and Ti

AlC, TiC is involved as an intermediate product, which is

produced first and then reacts with Ti-Al melt to form ternary carbides. That is to say, the

ternary carbides are produced through a dissolution-precipitation process. Because the

combustion reaction occurs quickly, the dissolution of TiC is often incomplete in a limited

period, and some un-reacted TiC remains in final products.

Fig. 28. XRD patterns of Ti

AlC

and Ti

AlC powders prepared by combustion synthesis

Figure 29 shows the SEM images of Ti

AlC

and Ti

AlC powders prepared by combustion

synthesis, where lamellar grains are observed. More careful observation reveals that most

grains exhibit a terraced structure with parallel layers overlapped. This terraced

morphology is proposed to be caused by a two-dimensional nucleation and growth

mechanism. As illustrated in Figure 30, when a lamellar grain is precipitated from the Ti-Al-

C melt and grows larger than a critical size, it can act as a substrate for the nucleation of new

grains. The new grains then grow up and form a new layer (the second layer) on the

substrate. Similarly, when the second layer become enough large the third layer can form on

it. Finally, a terraced structure is produced by continuous stacking of parallel layers. In this

process, each layer undergoes a preferential growth, expanding quickly along radial (R)

directions in the basal plane but growing slowly in normal (N) direction.

Combustion Synthesis of Ceramic Powders with Controlled Grain Morphologies

Fig. 29. SEM images of (a) and (b) Ti

AlC

and (c)-(e) Ti

AlC powders prepared by

combustion synthesis

Fig. 30. A schematic illustration of the formation mechanism of the terraced structure in

Ti-Al-C ternary carbides prepared by combustion synthesis

Based on the two-dimensional nucleation and growth mechanism, the phase formation and

microstructure evolution can be further discussed in detail. At first, Ti

AlC

and Ti

AlC

grains are precipitated from the Ti-Al-C liquid matrix. The ternary carbide grains are

separately distributed in a continuous liquid and each grain is surrounded by liquid. At this

stage, the material supply for the growth of the lamellar layers is sufficient. With the

formation and growth of more ternary carbide grains, the volume proportion of liquid

greatly decreases. The ternary carbides become the major phase, and the liquid is not

continuous but separately located at the surface of the ternary carbide grains. In this case,

the nucleation and growth of new layers take place at the interface between the liquid and

the ternary carbide crystals, and the growth of the underlying layers with no contact with

liquid is not active. Finally, the liquid phase disappears by severe consumption and with the

Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

decrease in temperature, and the growth of the ternary carbide crystals stops, leaving a

terraced structure .

Figure 31 shows TEM images and SAED patterns of Ti

AlC and Ti

AlC

lamellar crystals

prepared by combustion synthesis. For both Ti

AlC and Ti

AlC

, the basal faces of the

lamellar crystals are parallel to (001) plane and the normal direction is parallel to the c-axis

in the hexagonal lattice. Similar result has been found in Ti

SiC

, which is another typical

ternary carbide with the same hexagonal structure as Ti

AlC

. It is reported that, the linking

modes of octahedral Ti

C units have a strong influence on the growth behavior and

morphology of Ti

SiC

grains [39]. In the direction along c-axis, the Ti

C octahedrons are

separated by Si atomic layers, and hence the growth rate is much lower that those in other

directions. Therefore, the ideal crystal morphology of Ti

SiC

should be a hexagonal prism

bounded with {110} and {001} faces. The above analysis on the growth kinetics of Ti

SiC

also applicable to Ti

AlC

and Ti

AlC. That is to say, during the growth of Ti

AlC

and

AlC lamellar crystals, the growth rate in the direction parallel to c-axis should be lower

than those in other directions. In this case, the (001) faces will be exposed most frequently in

final crystal shape, resulting in a lamellar morphology.

Fig. 31. TEM images and SAED patterns of (a) Ti

AlC and (b) Ti

AlC

lamellar crystals

6. Conclusions and perspectives

With the progress of theories and development of experimental skills, combustion synthesis

has shown an increasing importance in preparing ceramic materials. The unique non-

equilibrium reaction state in combustion synthesis offers an opportunity to control crystal

growth kinetics. By combustion synthesis, ceramics powders with various grain

morphologies can be prepared, including faceted, rod-like, and lamellar crystals. This

advantage of combustion synthesis is desirable for both basic research on crystal growth

and industrial applications of ceramic powders.

Besides the results presented in this chapter, other new findings have been recently reported

related with combustion synthesis of ceramic materials. For example, some nitride ceramic

Combustion Synthesis of Ceramic Powders with Controlled Grain Morphologies

powders were prepared by combustion synthesis in air instead of high-pressure N

atmosphere [40-44]. This enhances the advantage of combustion synthesis as a low-cost

technique and shows the possibility to further reduce the production cost of nitride

powders. In addition, combustion synthesis was carried out in a high-gravity field to

directly fabricate bulk ceramics through melt-casting. By this method, both single-phase

translucent ceramics and eutectic ceramic composites have been prepared [45-48]. These

new findings expand the field for the application of combustion synthesis and make this

versatile technique available for preparing more kinds of ceramic materials.

7. Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No.

50102002, 50932006, and 51002163).

The first author would like to thank Prof. Zhou Heping, Dr. Ge Zhenbin, and Prof. Guo

Junming for their great help and instructive discussions.

8. References

[1] Munir Z A, Umberto A T. Mater Sci Rep, 1989, 3:277.

[2] Merzhanov A G. Ceram Int, 1995, 21:371.

[3] Moore J J, Feng H J. Prog Mater Sci, 1995, 39:243.

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[5] Toth L E. Transition metal carbides and nitrides, Academic Press, New York, 1971.

[6] Liu G H, Chen K X, Zhou H P, Tian J J, Pereira C, Ferreira J M F. Cryst Growth Des, 2006,

6:2404.

[7] Pamplin B R. Crystal growth, Pergamon Press Ltd., U. K., 1980.

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Netherlands, 1973.

[9] Padture N P, Lawn B R. J Am Ceram Soc, 1994, 77:2518.

[10] She J H, Jiang D L. Ceram Eng, 1998, 227:3.

[11] Nagano T, Kaneko K, Zhan G D, Mitomo M. J Am Ceram Soc, 2000, 83:2497.

[12] Peng H, Salamon D, Bill J, Rixecher G, Burghard Z, Aldinger F, Shen Z J. Adv Eng Mater,

2007, 9:303.

[13] Liu G H, Yang K, Li J T, Du J S, Hou X Y. J Phys Chem C, 2008, 112:6285.

[14] Pampuch R, Stobierski L, Lis J, Rczka M. Mater Res Bull, 1987, 22:1225.

[15] Gorovenko V I, Knyazik V A, Shteinberg A S. Ceram Int, 1993, 19:129.

[16] Narayan J, Raghunathan R, Chowdhurry R, Jagannadham K. J Appl Phys, 1994, 75:7252.

[17] Jack K H. J Mater Sci, 1976, 11:1135.

[18] Ekstrom T, Nygren M. J Am Ceram Soc, 1992, 75:259.

[19] Chen I W, Rosenflanz A. Nature, 1997, 389:701.

[20] Shen Z J, Zhao Z, Peng H, Nygren M. Nature, 2002, 417:266.

[21] Liu G H, Chen K X, Zhou H P, Ferreira J. J Mater Res, 2004, 19:3408.

[22] Liu G H, Chen K X, Zhou H P, Ning X S, Ferreira J. J Eur Ceram Soc, 2005, 25:3361.

[23] Liu G H, Chen K X, Zhou H P, Pereira C, Ferreira J. J Am Ceram Soc, 2006, 89:364.

[24] Liu G H, Chen K X, Zhou H P, Ren K G, Li J T, Pereira C, Ferreira J. Scripta Mater, 2006,

55:935.

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[25] Liu G H, Pereira C, Chen K X, Zhou H P, Ning X S, Ferreira J. J Alloy Comp, 2008,

454:476.

[26] N Shibata, S Pennycook, T Gosnell, G Painter, W Shelton, P Becher. Nature, 2004,

428:730.

[27] A Ziegler, J Idrobo, M Cinibulk, C Kisielowski, N Browning, R Ritchie1. Science, 2004,

306:1768.

[28] M Kramer, M Hoffman, G Petzow. J Am Ceram Soc, 1993, 76: 2778.

[29] M Kitayama, K Hirao, M Toriyama, S Kanzaki. Acta Mater, 1998, 46:6541.

[30] I W Chen, A Davenport, L Wang. Acta Mater, 2003, 51:1691.

[31] M Kramer, D Wittmuss, H Kuppers, M Hoffmann, G Petzow. J Cryst Growth, 1994,

140:157.

[32] M Barsoum, T Elraghy. J Am Ceram Soc, 1996, 79:1953.

[33] M Barsoum, D Brodkin, T Elraghy. Scr Metall Mater, 1997, 36:535.

[34] Y C Zhou, Z M Sun. Phys Rev B, 2000, 61:12570.

[35] W C Lee, S L Chung. J Am Ceram Soc, 1997, 80:53.

[36] A G Zhou, C A Wang, Y Huang. Mater Sci Eng A, 2003, 352:333.

[37] Z B Ge, K X Chen, J M Guo, H P Zhou, J Ferreira. J Eur Ceram Soc, 2003, 23:567.

[38] Liu G H, Chen K X, Zhou H P, Guo J M, Ren K G, Ferreira J. Mater Lett, 2007, 61:779.

[39] K Tang, C Wang, Y Huang, Q Zan. J Cryst Growth, 2001, 222:130.

[40] Liu G H, Chen K X, Zhou H P, Ren K G, Jin H B, Ferreira J. J Am Ceram Soc, 2007,

90:2918.

[41] Liu G H, Chen K X, Zhou H P, Li J T, Pereira C, Ferreira J. Mater Res Bull, 2007, 42:989.

[42] Li J T, Mei L, Yang Y, Lin Z M. J Am Ceram Soc, 2009, 92:636.

[43] Mei L, Li J T. Acta Mater, 2008, 56:3543.

[44] Yang Y, Lin Z M, Li J T. J Eur Ceram Soc, 2009, 29:175.

[45] Z Zhao, L Zhang, Y Song, W Wang. Scripta Mater, 2008, 58:207.

[46] R Liang, J Pei, J T Li, K X Chen. J Am Ceram Soc, 2009, 92:549.

[47] J Pei, J T Li, G H Liu, K X Chen. J Alloy Comp, 2009, 476:854.

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Molten Salt Synthesis of Ceramic Powders

Toshio Kimura

Keio University

Japan

1. Introduction

Molten salt synthesis, one of the methods of preparing ceramic powders, involves the use of

a molten salt as the medium for preparing complex oxides from their constituent materials

(oxides and carbonates). Ceramic powders are prepared from solid, liquid, and gas phases

by various methods (Rahaman, 2003). For large-scale commercial production, ceramic

powders are fabricated mainly from the solid phase by a conventional powder metallurgical

method. Molten salt synthesis is a modification of the powder metallurgical method. Salt

with a low melting point is added to the reactants and heated above the melting point of the

salt. The molten salt acts as the solvent.

Molten salts have been used as additives to enhance the rates of solid state reactions for a

long time. The amount of salt is small, typically a few percent of the total weight. In contrast,

in molten salt synthesis, a large amount of salt is used as the solvent to control powder

characteristics (size, shape, etc.). In this sense, molten salt synthesis is different from the flux

method, which uses the salt as an additive to enhance the reaction rate.

Typical examples of salts used in molten salt synthesis are chlorides and sulfates. In many

cases, eutectic mixtures of salts are used to lower the liquid formation temperature. The

melting points of NaCl and KCl are 801°C and 770°C, respectively, and that of 0.5NaCl–

0.5KCl (eutectic composition) is 650°C. For example, 0.635Li

–0.365Na

is the most

commonly used salt among sulfates because of its low melting temperature, which is 594°C,

whereas that of Na

–K

is 823°C. The solubilities of oxides in molten salts vary

greatly from less than 1 x 10

–10

mole fraction to more than 0.5 mole fraction, typically 1×10

–3

1×10

–7

mole fraction (Arendt et al., 1979). In many cases, the formation reaction occurs in the

presence of solid reactant particles. In this sense, molten salt is somewhat different from

ordinary solvents, which dissolve all reactant particles and the product particles precipitate

from a homogeneous liquid phase.

Generally, a complex oxide powder is prepared from reactants by the following procedure.

A mixture of the reactants and salt is heated above the melting temperature of the salt. At

the heating temperature, the salt melts and the product particles form. The characteristics of

the product powder are controlled by selecting the temperature and duration of the heating.

Then, the reacted mass is cooled to room temperature and washed with an appropriate

solvent (typically, water) to remove the salt. The complex oxide powder is obtained after

drying. The procedure is the same as that of a conventional powder metallurgical method

and is easily scaled up for the fabrication of large quantities of materials.

The use of molten salt is a common method to grow single crystals from solution (Elwell &

Scheel, 1975). In this method, the reactant materials are completely dissolved in molten salt

Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

to obtain a uniform liquid. Upon cooling, solid particles nucleate homogeneously in the

liquid phase (“homogeneously” means that a nucleus forms somewhere in the liquid phase).

A single crystal with a large size can be obtained by limiting the number of nuclei formed

during cooling. Therefore, a salt having a high solubility of the reactant materials is

required. Conversely, in molten salt synthesis, a large number of nuclei are necessary to

obtain powder particles with an appropriate size, typically from a few tenths to about ten

micrometers. The surfaces of the reactant particles are utilized as the nucleation sites. In

other words, the product particles nucleate heterogeneously on the surfaces of the reactant

particles. Therefore, high solubility for all reactants is not desirable. The control of the

cooling rate is very important in the single-crystal growth, because the cooling rate

determines the number of nuclei and the size of the product crystals. The cooling rate gives

a minor influence on the particle sizes in the molten salt synthesis, because a vast number of

particles are already present before cooling and the materials dissolved in the molten salt

precipitate on the surfaces of the already existing solid particles.

The role of the molten salts is (1) to increase the reaction rate and lower the reaction

temperature; (2) to increase the degree of homogeneity (the distribution of constituent

elements in the solid solution); (3) to control particle size; (4) to control particle shape; and

(5) to control the agglomeration state. The major purpose of employing molten salt synthesis

is (1) to prepare powders for sintering and (2) to prepare anisometric particles. In sintering

of powders, a good sintered compact is obtained from a powder with grains of

submicrometer size and a low degree of agglomeration (Rahaman, 2003). Recently, textured

ceramics are prepared by the templated grain growth method, in which anisometric

particles with sizes from several to a few tens micrometers are required (Kimura, 2006;

Messing et. al., 2004; Tani & Kimura, 2006). The formation of aggregates must be avoided to

form green compacts by tape-casting.

The requirements on the salt are that they are stable, readily available, inexpensive, and

easily washed away with water. A low melting temperature is desirable, and the eutectic

composition or the composition at the minimum liquidus temperature is often used. Other

requirements are that they have a low vapor pressure at the heating temperature and do not

cause undesirable reactions with either the reactants or the product. One example of

unsuitable salt is LiCl, which is used for the preparation of LiFe

(Wickham, 1971). LiCl

accelerates the reaction between Fe

and Li

to yield LiFe

, but it is hygroscopic and

volatile at the reaction temperature. Furthermore, it is subject to hydrolysis; Li

O produced

by hydrolysis reacts with LiFe

and converts it to LiFeO

This chapter deals with (1) the phenomena occurring during synthesis; (2) the reaction rate;

(3) the characteristics of powders with a special emphasis on particle morphology; and (4)

the reaction of salt with the reactants and product. This chapter excludes (1) the salts as a

reaction promoter; (2) the formation of simple oxides (MO); and (3) the salts as a reactant,

such as the formation of LiCoO

and LiMn

using Li salts (LiCl, LiNO

, etc.), which is an

important application of molten salt synthesis to lithium ion batteries (Han et al., 2003).

Furthermore, this chapter does not include nitrate salts, because the reaction mechanisms

are different from those of the chlorides and sulfates (Afanasiev & Geantet, 1998).

2. Fundamentals of molten salt synthesis

2.1 Preparation procedure

2.1.1 Formulation

Figure 1 shows the flowchart of the preparation procedure. A reaction batch contains the

reactants and the salt. The salt is selected based on the desired powder characteristics. The

Molten Salt Synthesis of Ceramic Powders

relation between the properties of the salt and the powder characteristics is described in this

review. Sometimes, a surfactant is added to prepare nano-sized powders (Mao et al., 2003).

In general, the mixing ratio of reactants is stoichiometric, i.e., two moles of Bi

and three

moles of TiO

are mixed for Bi

. Care must be paid when carbonates are used as

reactants. The solubility of oxides in the molten salt is generally low but that of carbonates is

high, resulting in deviations from stoichiometry in the product phase. In the preparation of

, the Ba/Ti ratio of 6/17 in the reactant mixture results in the inclusion of

particles in the product and that of 6/15 is employed to obtain nearly single-

phase Ba

(Kimura, et al., 2005). When KNbO

is prepared from K

and K

using KCl, a K

/ K

ratio of 1.2 or greater is desirable (Saito & Takao, 2007).

A typical amount of salt is 80-120 wt% of the reactant mixture. The amount is determined by

the requirement that there is adequate salt to substantially fill the interstices of the reactant

particles and to coat the reactant surfaces. In some systems, the amount of salt influences the

product particle size (Yoon et al. 1998). When the amount of salt is too small, the effect of the

liquid phase is not fully expected. An extremely large amount causes two problems. One is

the separation of the reactant particles by sedimentation (Kimura et al., 1980; Wickham,

1971). When the reactant particles have different densities and sizes, they have different

sedimentation rates, resulting in the separation of the reactant particles and a reduction in

the reaction rate. Another problem is related to difficulties during treatment. When the

amount of salt is excessive, the interstices of the reactant particles can never hold all of the

molten salt, and a fairly large amount of the molten salt oozes from a gob of reactants.

Oozed molten salt never acts as a solvent. Furthermore, the molten salt adheres to the wall

of the crucible and changes to a hard lump after solidification. The dissolution of hard salt in

the lumps is quite laborious.

Fig. 1. Preparation procedure in molten salt synthesis.

When all the reactants are dissolved in the molten salt, another problem arises. It is related

to the particle size. When Bi

is prepared from Bi

and WO

using NaCl-KCl, two

kinds of platelike particles are obtained depending on the amount of salt and the heating

temperature (Kimura & Yamaguchi, 1982). The first kind consists of particles with a

diameter of several micrometers, and the second kind of particles has a diameter of about

100 μm. When the amount of salt is small and the heating temperature is low, the system is

located in the solid-liquid two-phase region of the phase diagram. The Bi

particles

nucleate heterogeneously on the surfaces of the reactants, and a great number of them form.

Conversely, when the amount of salt is large and the heating temperature is high, the

system is located in the liquid single-phase region. The Bi

particles homogeneously

nucleate in the liquid phase. Because the cooling rate is relatively high compared to that

employed in the single-crystal growth, many nuclei form but the number of nuclei is less

Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

than that nucleated heterogeneously on the surfaces of the reactants. Thus, quite large-sized

particles are obtained.

2.1.2 Heat treatment

The mixture of reactants and salt is put in a covered or sealed crucible and heated in a

furnace. A platinum crucible is used in the laboratory experiment. Alternatively, alumina

and zirconia crucibles may be used if the chemical interaction between the crucible and the

reactants and product is negligible. The heating conditions such as temperature and duration

are determined by the desired powder characteristics. In general, the rate of material transport

is increased with an increase in the heating temperature. At the same time, the salt evaporation

increases as well. The heating duration is determined by the reaction rate and the size and

shape of the product particles. Typical conditions are temperatures between 800°C and 1100°C

with durations between 30 and 60 min. In a particular system, the heating rate influences the

size of the product particles (Yoon et al., 1998).

After heating, the product mass is washed with an appropriate solvent to remove the salt.

Ordinarily, this is water, which means that water-soluble salts are typically used in molten

salt synthesis. The solubilities of chlorides and sulfates are generally high and washing with

water two or three times seems sufficient to remove all the salt. Nevertheless, the ions from

the dissolved salt may adsorb on the surfaces of the product particles, and, then, repeated

washing is necessary. The chloride ions are sometimes detected by an Ag

solution even

after ten times of washing. To desorb ions efficiently, the use of hot, instead of cold, water is

recommended. After washing, the supernatant water is decanted and the remaining powder

is dried. When the formation of hard agglomerates needs to be avoided, the powder is

rinsed with a solvent with low surface tension, such as acetone, before drying.

2.2 Formation of product particles

2.2.1 Two stages of particle formation

The product particles are formed in two stages, which are the reaction and particle-growth

stages (Kimura & Yamaguchi, 1983). In the reaction stage, the product particles are formed

in the presence of solid reactant particles. The reactant particles dissolve in the molten salt

and product particles form. When all the reactant particles are consumed, the particle-

growth stage starts. There are solid product particles and molten salt in the system. The

product particles have a particle size distribution, and the large particles increase their size

by Ostwald ripening (Rahaman, 2003); particles smaller than a critical size dissolve in the

molten salt and precipitate on the surfaces of particles larger than the critical size. The

supersaturation with respect to the product compound is different in the two stages.

2.2.2 Supersaturation

The supersaturation is determined by the concentration of the reactants in the molten salt.

When the product (P) is formed by a reaction between reactants A and B (A + B → P), the

solubility of P determines the equilibrium concentration of A and B in the molten salt, i.e.,

[A]

and [B]

, respectively. The supersaturation is given by ([A][B]/[A]

[B]

−1), where [A]

and [B] are the actual concentration of A and B, respectively, in the molten salt. In the

reaction stage, the solid reactant particles are present, and [A] and [B] are equal to their

solubilities [A]* and [B]*, respectively. Figure 2 shows the relation between the molar free