Review of melt casting of dense ceramics and glasses by high gravity combustion synthesis

Abstract

High gravity combustion synthesis is a recently reported technique to prepare dense ceramics and glasses by melt casting instead of conventional powder sintering. This technique combines strong exothermic chemical reactions with a high gravity field, and offers an efficient and furnace free way for rapid production of bulk ceramic and glass materials. This article reviews major results on melt casting of dense ceramics and glasses by high gravity combustion synthesis. Several ceramic and glass materials prepared by high gravity combustion synthesis are firstly presented as examples, including single phase ceramics, multiphase eutectic or composite ceramics, glasses and glass–ceramics. Then, the reaction kinetics in high gravity combustion synthesis are discussed in detail, with an emphasis on phase separation, solidification and microstructure evolution. Finally, a conclusion is drawn with a perspective on further development and application of high gravity combustion synthesis.

Keywords

High gravity combustion synthesis Ceramics Glass Phase separation Solidification Microstructure

Introduction

Polycrystalline ceramics are usually fabricated by sintering from powders in contrast to metals and alloys that are generally produced by casting from melts. The melt casting of ceramics involves two problems. First, it is difficult to prepare stable and homogeneous ceramic melts, because most ceramic materials have extremely high melting points (e.g. >2000°C for Al₂O₃, ZrO₂ and MgAl₂O₄, and >3000°C for TiC). For melting the refractory ceramic materials, conventional resistance heating is often insufficient and more powerful heating media are required, such as high frequency induction, lamp mirror furnaces and lasers. Another problem in the melt casting of ceramics is exaggerated grain growth during solidification from hot melts, which will impair their mechanical strength. The grain growth can be limited by increasing cooling rates, incorporating proper secondary phases, or in multiphase eutectic ceramics.

The melt solidification of ceramic materials can be realised by unidirectional solidification approaches, such as the Bridgman method, Czochralski method and floating zone method.1 The Bridgman method is suitable for preparing large samples, where the ingot size is limited only by crucibles. The Czochralski method also requires crucibles, but direct contact between crucibles and growing materials is avoided. In the floating zone method, crucibles are not necessary and a small volume of sample is melted by high frequency induction or lasers. All these methods require an external heating source and complex facilities despite their difference in technical details.

Recently, a furnace free technique for melt casting of ceramic materials has been developed, which is called high gravity combustion synthesis.2^–4 Combustion synthesis, which is also known as self-propagating high temperature synthesis (SHS), is a method to produce inorganic materials from exothermic combustion reactions. This method was discovered about 40 years ago by Merzhanov and Borovinskaya5 during their study on the phenomenon of solid flame. Since then, a great diversity of inorganic materials has been produced by combustion synthesis,6^–10 including many ceramic powders.11^–16 In high gravity combustion synthesis, combustion synthesis is combined with a high gravity field, where highly exothermic combustion reactions are used to produce ceramic melts and a high gravity field is applied to promote the separation between the ceramic melts and other concomitant phases (e.g. metallic melts and gas bubbles). In high gravity combustion synthesis, the heat energy for melting ceramic materials is provided by combustion reactions, and an external heating source is not necessary. At the same time, combustion reactions proceed very quickly and usually finish in several seconds, which is helpful to reduce the total processing time. In this way, high gravity combustion synthesis offers an efficient way to produce bulk ceramics with a short processing time and low energy consumption. In addition to ceramics, many other inorganic materials have also been prepared by high gravity combustion synthesis, including glasses, glass ceramics, alloys, intermetallics and cermets.17^–21

This article gives a brief review on melt casting of ceramics and glasses by high gravity combustion synthesis. After a short description of the experimental procedure (the section on ‘Experimental procedure’), several ceramic and glass materials prepared by high gravity combustion synthesis are presented as examples (the section on ‘Ceramic and glass materials prepared by high gravity combustion synthesis’) to show the feasibility of the method. Then, the reaction mechanism in high gravity combustion synthesis is discussed, where the kinetics in phase separation and microstructure evolution are studied (the sections on ‘Phase separation in high gravity combustion synthesis’ and on ‘Solidification and microstructure evolution in high gravity combustion synthesis’), because these two processes strongly affect the microstructure and properties of final products. Finally, conclusions are drawn with a perspective on further development and application of high gravity combustion synthesis.

Experimental procedure

The melt casting of ceramics and glasses by high gravity combustion synthesis is generally realised by carrying out combustion reactions in a high gravity field induced by a centrifugal effect. A typical experimental procedure for high gravity combustion synthesis is described as follows.

The reactant powder is cold pressed into a round compact and loaded in a graphite crucible. The crucible is coated by carbon felt and placed into a steel vessel mounted on a Ni based superalloy rotator (Fig. 1). The rotator is placed in a closed reaction chamber. The chamber is evacuated to a vacuum of <100 Pa, and then the rotator is started. From high speed rotation, a high gravity field is induced by the centrifugal effect. The strength of the high gravity field can be modulated by controlling the rotation frequency and evaluated by a high gravity factor of g′/g, where g′ and g mean the centrifugal acceleration and normal gravitational acceleration respectively. When a designated high gravity factor is reached, a combustion reaction is triggered by electrically heating a tungsten coil near the sample. In the combustion reaction, a large amount of heat energy is created and the products are melted. The ceramic and metallic melts are separated in a high gravity field because of their density difference. After cooling and solidification, bulk ceramic samples and metal ingots are obtained. The ceramic samples are machined and polished for later characterisations and tests.

Figure 1

Illustration of facilities for high gravity combustion synthesis and prepared samples:

Ceramic and glass materials prepared by high gravity combustion synthesis

For the melt casting of ceramics by high gravity combustion synthesis, there is a precondition that the adiabatic temperature of combustion reactions must exceed the melting point of the ceramic materials. This precondition can be readily satisfied by using strong exothermic combustion reactions. For example, adiabatic temperatures in combustion synthesis of some ceramic materials from the aluminothermic reaction of 2Al+3NiO = Al₂O₃+3Ni are calculated (Table 1), which are much higher than the melting or eutectic points of the ceramic materials.

Table 1

Calculated phase assemblages and adiabatic temperatures in high gravity combustion synthesis of ceramic materials with different chemical compositions and melting/eutectic points*

Sample	Nominal chemical composition/mol.-%	T_m/e/°C	Calculated phase assemblage/vol.-%	T_ad/°C
Al₂O₃	Al₂O₃	2054	Al₂O₃	2884
MgAl₂O₄	50Al₂O₃+50MgO	2135	MgAl₂O₄	2476
YAG	62·5Al₂O₃+37·5Y₂O₃	1950	YAG	2593
AY-1	80Al₂O₃+20Y₂O₃	1825	41Al₂O₃+59YAG	2859
AY-2	95Al₂O₃+5Y₂O₃	1825	84Al₂O₃+16YAG	2884
AZ	62Al₂O₃+37ZrO₂+1Y₂O₃	1860	67Al₂O₃+33ZrO₂	2678
AYZ	65Al₂O₃+16Y₂O₃+19ZrO₂	1715	37Al₂O₃+49YAG+14ZrO₂	2678

*T_m/e: melting or eutectic point; T_ad: adiabatic temperature.

By high gravity combustion synthesis, a variety of ceramic and glass materials have been prepared, including single phase ceramics,4,22^–27 multiphase eutectic or composite ceramics,2,3,28^–43 glasses,17, 44, 45 and glass–ceramics.17, 46 As examples, some ceramic and glass materials prepared by high gravity combustion synthesis are discussed as follows.

Single phase ceramics

Al₂O₃

Al₂O₃ ceramics can be prepared by high gravity combustion synthesis from the aluminothermic reaction of 2Al+3NiO = Al₂O₃+3Ni. For this reaction, the adiabatic temperature reaches the boiling point of Ni (2884°C). In high gravity combustion synthesis, the vaporisation of Ni should be avoided, because it will increase the porosity in the samples. To reduce the reaction temperature and depress the vaporisation of Ni, Al₂O₃ powder is usually added as diluent into the (2Al+3NiO) thermite. For the reaction of 2Al+3NiO+xAl₂O₃ = (1+x)Al₂O₃+3Ni, the adiabatic temperature decreases with increasing x value (Fig. 2). When too much diluent is used (x = 1·6), the adiabatic temperature becomes lower than the melting point of Al₂O₃ (2054°C). Considering the influence of heat dissipation, an appropriate content of diluent is recommended as 0·2≤x≤0·6 in actual synthesis experiments.

Figure 2

Effect of proportion of Al₂O₃ diluent on adiabatic temperature of reaction 2Al+3NiO = Al₂O₃+3Ni (Ref. 56)

High gravity combustion synthesis of Al₂O₃ was carried out under different high gravity factors from 200 to 800.4, 22 It was found that the aluminothermic reaction was fully finished and no Al or NiO remained in the product. In all the samples, pure α-Al₂O₃ was obtained without Ni inclusions, implying a complete phase separation between the Al₂O₃ ceramic melt and the Ni metallic melt. The density of the synthesised Al₂O₃ samples increased with increasing high gravity factors, and reached 3·7 g cm⁻³ (93% theoretical density) under a high gravity factor of 800 (Fig. 3).

Figure 3

Dependence of density of Al₂O₃ samples on high gravity factor (g′/g)

MgAl₂O₄

MgAl₂O₄ ceramics were prepared by high gravity combustion synthesis from the reaction of 2Al+3NiO+MgO = MgAl₂O₄+3Ni.26 The synthesised samples consisted of major MgAl₂O₄ phase and minor Ni, and the content of Ni decreased with increasing high gravity factors. In the sample prepared with g′/g = 900, only a trace of Ni was present and the sample was nearly pure MgAl₂O₄, but in the sample prepared with g′/g = 1, much more Ni was observed (Fig. 4). This indicates that the high gravity field has an evident effect to promote the phase separation. The microstructure of the samples was also affected by the high gravity field, where both the porosity and pore size were reduced by increasing the high gravity factor. In the sample prepared with g′/g = 1, large pores with a size of >100 μm and Ni particles of ∼20 μm existed, and in the sample prepared with g′/g = 900, the average pore size was ∼1 μm and less Ni particles were found (Fig. 5). The relative density of the sample prepared with g′/g = 900 reached 92%.

Figure 4

X-ray diffraction patterns of MgAl₂O₄ samples prepared by high gravity combustion synthesis under different high gravity accelerations26

Figure 5

Images (SEM) of MgAl₂O₄ samples prepared under high gravity factor g′/g of a 1, b 200 and c 90026

In high gravity combustion synthesis of MgAl₂O₄ ceramics, the density of samples can be further improved by adding glass powders. The glass powders will melt at a temperature much below the melting point of MgAl₂O₄, and can feed the shrinkage cavities during solidification of MgAl₂O₄. As an example, translucent MgAl₂O₄ ceramics were prepared by high gravity combustion synthesis (g′/g = 900) with the addition of 5% Si–Na–Ca–Mg–O glass (Fig. 6).

Figure 6

Photo of translucent MgAl₂O₄ ceramics prepared by high gravity combustion synthesis (sample thickness is 0·8 mm)26

Y₃Al₅O₁₂

Y₃Al₅O₁₂ (YAG) ceramics were prepared by high gravity combustion synthesis from the reaction of 10Al+15NiO+3Y₂O₃ = 2Y₃Al₅O₁₂+15Ni.23^–25 The density of the synthesised YAG samples increased with increasing high gravity factors, and translucent YAG ceramics with a relative density of ∼98% were produced with g′/g = 800 (Fig. 7). In high gravity combustion synthesis of YAG ceramics, the density and microstructure homogeneity of samples can be improved by applying a mechanical pressure. It was also found that, the addition of some glass was helpful to reduce the porosity and grain size in the YAG ceramics prepared by high gravity combustion synthesis.

Figure 7

Dependence of relative density of YAG samples on high gravity factor (g′/g)

Multiphase eutectic or composite ceramics

In addition to single phase ceramics, multiphase eutectic or composite ceramics have also been prepared by high gravity combustion synthesis. Compared with single phase ceramics, multiphase eutectic ceramics have lower melting temperatures and thus provide longer time for phase separation and removal of gas bubbles. In this way, the eutectic ceramics prepared by high gravity combustion synthesis generally exhibit lower porosities than the single phase ceramics. On the other hand, multiphase ceramics consist of two or more different phases and can offer more opportunity for tailoring the microstructure. For example, in multiphase eutectic ceramics prepared by high gravity combustion synthesis, ultrafine eutectic structures with grain sizes on a submicrometre scale are often observed in contrast to the coarse crystals in single phase ceramics.

Al₂O₃–ZrO₂ is the first and also the most investigated example of eutectic ceramics prepared by high gravity combustion synthesis.2,28^–34 The synthesised Al₂O₃–ZrO₂ eutectic ceramics were composed of Al₂O₃, t-ZrO₂ and m-ZrO₂ (Fig. 8), and the maximum relative density reached 99·8%. The Al₂O₃–ZrO₂ ceramics showed a typical eutectic microstructure with submicrometre ZrO₂ fibres or lamellas embedded in the Al₂O₃ matrix (Fig. 9). The Al₂O₃–ZrO₂ eutectic ceramics exhibited excellent mechanical properties, where the hardness, flexural strength and fracture toughness were 22·8 GPa, 1568 MPa and 14·8 MPa m^1/2 respectively.30

Figure 8

X-ray diffraction pattern of Al₂O₃–ZrO₂ binary eutectic ceramics prepared by high gravity combustion synthesis56

Figure 9

Images (SEM) of Al₂O₃–ZrO₂ binary eutectic ceramics prepared by high gravity combustion synthesis56:

Al₂O₃–YAG–ZrO₂ ternary eutectic ceramics was also fabricated by high gravity combustion synthesis.35^–37 The maximum relative density of the Al₂O₃–YAG–ZrO₂ ceramics was 99·3%. In the Al₂O₃–YAG–ZrO₂ ceramics, the eutectic microstructure was characterised by an interpenetrating network of the three phases with a submicrometre interphase spacing (Fig. 10). The hardness and fracture toughness of the Al₂O₃–YAG–ZrO₂ ternary eutectic ceramics were 17·82 GPa and 5·51MPa m^1/2 respectively.35

Figure 10

Image (SEM) of Al₂O₃–YAG–ZrO₂ ternary eutectic ceramics prepared by high gravity combustion synthesis56

Besides the oxide ceramics, non-oxide TiC–TiB₂ multiphase composite ceramics were prepared by high gravity combustion synthesis.38^–43 The relative density of the TiC–TiB₂ ceramics reached 99·2%. In the TiC–TiB₂ ceramics, fine TiB₂ platelets were embedded in the TiC matrix. The hardness, flexural strength and fracture toughness of the TiC–TiB₂ ceramics were 28·5 GPa, 750±25 MPa and 6·2±0·5 MPa m^1/2 respectively.38 The hardness of the TiC–TiB₂ composite ceramics produced by high gravity combustion synthesis was evidently higher than that of TiC–TiB₂ materials prepared by powder metallurgy approaches.

Glasses

Besides polycrystalline ceramics, non-crystalline glasses can also be produced by high gravity combustion synthesis. As an example, Y₂O₃–Al₂O₃–SiO₂ (YAS) glasses were prepared by high gravity combustion synthesis.17, 44, 45 The density and hardness of the YAS glasses decreased with increasing content of SiO₂, and increased with increasing high gravity factors (Fig. 11).

Figure 11

Dependence of density and hardness of YAS glasses on high gravity factor (g′/g) (Ref. 44)

In high gravity combustion synthesis of YAS glasses, the reaction temperature plays an important role and the combustion reaction should be carefully modulated. If a weakly exothermic combustion reaction is used with a lower reaction temperature, the lifetime of the YAS melt will be short or even the product cannot be fully melted. In this case, it is impossible to produce pure and dense samples. On the other hand, if the combustion reaction is too strong, explosion can take place and no bulk samples will be obtained because of splashing.

In high gravity combustion synthesis of YAS glasses, the reaction temperature can be raised by preheating. Preheating is helpful to prolong the lifetime of the YAS melt and improves the phase separation and removal of gas bubbles. With the assistance of preheating, transparent YAS glasses were prepared by high gravity combustion synthesis (Fig. 12). The glasses showed a maximum transmittance of 54% in visible to near infrared region and hardness of 8·1–8·5 GPa.45 If the preheating temperature was too high, however, splashing took place and the transparency of the glasses decreased.

Figure 12

Transmittance curve of transparent YAS glass prepared by high gravity combustion synthesis (sample thickness is 1·40 mm)45

Glass–ceramics

High gravity combustion synthesis is also effective in preparing glass–ceramics.17, 46 By high gravity combustion synthesis, glass–ceramics with different chemical compositions have been prepared, including Al₂O₃–SiO₂–ZrO₂ (ASZ), Al₂O₃–SiO₂–Y₂O₃ (ASY) and Al₂O₃–La₂O₃–ZrO₂ (ALZ). The chemical compositions, phase assemblages and properties of the glass–ceramics are summarised in Table 2.

Table 2

Chemical compositions, phase assemblages and properties of glass–ceramics prepared by high gravity combustion synthesis

Sample	Chemical composition/mol.-%	Phase assemblage	Density/g cm⁻³	Hardness/GPa
ASZ-1	50Al₂O₃–26SiO₂–24ZrO₂	ZrO₂+mullite+Al₂O₃+glass	3·32	12·8
ASZ-2	30Al₂O₃–50SiO₂–20ZrO₂	ZrO₂+mullite+glass	3·15	12·1
ASY-1	46Al₂O₃–36SiO₂–18Y₂O₃	YAG+Al₂O₃+glass	3·61	8·5
ASY-2	35Al₂O₃–55SiO₂–10Y₂O₃	Glass+mullite	3·20	8·2
ASYZ	30Al₂O₃–42SiO₂–18Y₂O₃–10ZrO₂	Glass+ZrO₂+YAG+Al₂O₃	3·73	10·1
ASYL	32Al₂O₃–45SiO₂–19Y₂O₃–4La₂O₃	Glass+YAG	3·67	11·3
ALZ	50Al₂O₃–25La₂O₃–25ZrO₂	LaAlO₃+LaAl₁₁O₁₈+ZrO₂+glass	3·97	9·2
ASLZ-1	50Al₂O₃–10SiO₂–20La₂O₃–20ZrO₂	LaAlO₃+LaAl₁₁O₁₈+ZrO₂ +glass	4·35	8·8
ASLZ-2	35Al₂O₃–45SiO₂–10La₂O₃–10ZrO₂	Glass+LaAlO₃+ZrO₂+LaAl₁₁O₁₈	3·86	8·9

In the ASZ system, two samples of ASZ-1 and ASZ-2 were prepared. The chemical composition of ASZ-1 agreed with the ternary eutectic point in the Al₂O₃–SiO₂–ZrO₂ phase diagram, and the composition of ASZ-2 was rich in SiO₂. Three crystalline phases of ZrO₂, mullite and Al₂O₃ were produced in ASZ-1, which was consistent with the prediction from the phase diagram. Besides the crystalline phases, amorphous glass also existed in the sample, which was revealed by X-ray diffraction analysis. The formation of glass was probably caused by the fast cooling in high gravity combustion synthesis. In ASZ-2 with more SiO₂, only crystalline ZrO₂ and mullite were observed with no occurrence of Al₂O₃, and more glass was produced. The two ASZ samples showed different microstructures (Fig. 13). In ASZ-1, both primary crystallites and eutectic structure were found. The primary crystallites consist of coarse mullite and Al₂O₃ grains, and the eutectic structure was characterised by submicrometre ZrO₂ fibres embedded in the mullite/Al₂O₃ matrix. Such eutectic structure was not found in ASZ-2, where small ZrO₂ particles were distributed at the boundaries of large mullite grains. The different microstructures of the two samples were attributed to different contents of Al₂O₃ in their chemical compositions.

Figure 13

Images (SEM) and EDS spectra of ASZ glass–ceramics prepared by high gravity combustion synthesis:

In the ASY system, four samples were prepared, including ASY-1 rich in Al₂O₃, ASY-2 rich in SiO₂, Al₂O₃–SiO₂–Y₂O₃–ZrO₂ (ASYZ) with the addition of ZrO₂ and Al₂O₃–SiO₂–Y₂O₃–La₂O₃ (ASYL) with the addition of La₂O₃. A glass–ceramic composed of glass and crystalline YAG and Al₂O₃ was produced in ASY-1, where polyhedron YAG and plate-like Al₂O₃ grains were embedded in an ASY glass matrix (Fig. 14). No cracks or cavities were found at the interface between the crystalline phases and the glass matrix. The sample ASY-2 consisted of glass and mullite as large plate-like grains or smaller faceted crystals. In ASYZ, the major crystalline phase was ZrO₂, accompanied by minor YAG and Al₂O₃. In ASYL, only one crystalline phase of YAG was observed, and no La containing crystalline phase was produced.

Figure 14

Images (SEM) and EDS spectra of ASY glass–ceramics:

In the ALZ system, three samples were prepared. Al₂O₃–La₂O₃–ZrO₂had a ternary eutectic composition, and ASLZ-1 and ASLZ-2 had compositions with SiO₂. Glass–ceramics were produced in all the three samples. Similar phase assemblage was observed in ALZ and ASLZ-1, where LaAlO₃ was the predominant crystalline phase besides minor LaAl₁₁O₁₈ and ZrO₂. In ASLZ-2 with 45%SiO₂, more glass was produced.

The hardness of the glass–ceramics prepared by high gravity combustion synthesis ranged from 8·2 to 12·8 GPa. The maximum hardness was observed for the sample ASZ-1, which can be attributed to the eutectic microstructure with fine ZrO₂ fibres embedded in a mullite matrix. Among the three systems of glass–ceramics, the ASZ samples may be more desirable for most structural applications, because they have a higher hardness/density ratio.

Phase separation in high gravity combustion synthesis

High gravity combustion synthesis is a complex process involving a series of physical and chemical variations such as combustion reaction, phase transformation (melting and solidification), and phase separation (separation between ceramic and metallic melts and removal of gas bubbles from the melts). The variations take place in a short time and overlap with each other. In this case, it is difficult to give a universal model that can exactly describe all the physical processes and combustion reactions in high gravity combustion synthesis, especially when different reaction systems are involved. Nevertheless, a simplified analysis of the synthesis process revealing essential steps there is still helpful for optimising the processing parameters and improving the properties of products.

High gravity combustion synthesis can be simply divided into three steps:

combustion reaction and melting of products

phase separation

iii

solidification.

The first step is very short, because most combustion reactions take place quickly. For example, in combustion synthesis of Y₃Al₅O₁₂ from the reaction 10Al+15NiO+3Y₂O₃ = 2Y₃Al₅O₁₂+15Ni, the propagation velocity of combustion wave under normal gravity was measured to be 28–100 mm s⁻¹ (varying with the particle size of reactants). In a high gravity field, the velocity will be enhanced. It was reported that, for the (2Al+3FeO) thermite with 18·5%Al₂O₃ dilute, the burning rate in a high gravity field of g′/g = 895 was ∼6 times that under normal gravity.47 According to this result, the burning rate in combustion synthesis of Y₃Al₅O₁₂ under high gravity of g′/g = 900 can reach 600 mm s⁻¹. In this case, for a sample with a height of 50 mm, the combustion reaction will finish in 0·1 s. In such a short period, the progress in phase separation or solidification can be neglected. That is to say, the first step of combustion reaction has little overlap with the later two steps.

In high gravity combustion synthesis, the second step of phase separation plays a key role in determining both purity and density of the products. In phase separation, two processes are included, i.e. separation between ceramic and metallic melts, and removal of gas bubbles from the melts. A complete separation between ceramic and metallic melts guarantees a high purity of the ceramic products, and the removal of gas bubbles is necessary to produce highly dense bulk ceramics.

For the separation between ceramic and metallic phases, there are three possible ways:

settling of metallic liquid droplets in ceramic melts

rising of ceramic liquid droplets in metallic melts

iii

rising of ceramic solid particles in metallic melts.

In high gravity combustion synthesis of Al₂O₃ based ceramics from the reaction of 2Al+3NiO = Al₂O₃+3Ni, the first way is most likely to be operative because of two reasons. First, in the combustion synthesised products, ceramic phases have higher volume fractions (60% for Al₂O₃ and 74% for Y₃Al₅O₁₂) than Ni, and thus ceramic phases should be present as continuous matrix with discrete Ni particles dispersed there. Second, from experimental results, Ni particles were usually observed in the ceramic samples, while few ceramic grains were found in the Ni ingots (Fig. 15).

Figure 15

Images (SEM) of a YAG ceramics and b Ni ingot prepared by high gravity combustion synthesis48

During the settling of Ni droplets in ceramic melts in a high gravity field, the stable settling velocity can be calculated according to the Stokes law.48 On a settling Ni droplet, three forces are considered, including high gravity G′, buoyant force F and viscous resistance f, as shown in Fig. 16. These forces are expressed as (1) (2) (3) where ρ₁ and ρ₂ mean the densities of liquid Ni and the ceramic melt, g′ means the high gravity acceleration, R means the radius of the Ni droplet, η means the viscosity of the ceramic melt and V means the settling velocity of the Ni droplet.

Figure 16

a settling of Ni particles in ceramic melts; b rising of gas bubbles in ceramic or metallic melts

When G′ = F+f, the Ni droplet reaches the maximum settling velocity, which is called stable settling velocity V_s. From equations (1)–(3), the stable settling velocity can be deduced as (4) From equation (4), the stable settling velocity shows a linear dependence on the high gravity acceleration. With the data in Table 3,49^–54 the stable settling velocity of Ni droplets and the time for a displacement of 1 cm (t_1cm) under different high gravity factors (g′/g) are calculated and plotted in Fig. 17. It is evident that a high gravity field increases the settling velocity and reduces the time for phase separation.

Figure 17

Dependence of stable settling velocity of Ni droplets (R = 10 μm) in ceramic melts on high gravity factor (g′/g)48

Table 3

Physical properties of Ni and some ceramic melts

Melts	Density	Viscosity	Surface energy	Reference
Melts	ρ/g cm⁻³	η/Pa s	γ/N m⁻¹	Reference
Ni	7·9	0·0489	1·778	49
Al₂O₃	3·008	0·04	0·64	50, 51
YAG	3·688	0·046	0·781	52
YAS	3·25	0·08	0·3	53, 54

Similarly, the stable rising velocity of gas bubbles in ceramic or metallic melts is written as (5) where ρ₂ means the density of the melt, ρ₃ means the density of gas in the bubbles, and R means the radius of the bubbles. Considering ρ₃/ρ₂<10⁻³, equation (5) can be simplified as (6) From equation (6), the stable rising velocity of gas bubbles and the time for a displacement of 1 cm under different high gravity factors (g′/g) are calculated and plotted in Fig. 18. By increasing the high gravity factor, the rising velocity is enhanced. The rising velocity is also affected by the viscosity of the melt, and a higher rising velocity is expected in Ni melt than in the ceramic melts. The bubble size has a strong influence on the rising velocity, and large bubbles will rise more quickly than smaller ones (Fig. 19). For this reason, large pores were usually observed in the upper part of the ceramic samples and small pores in the lower part.

Figure 18

Dependence of stable rising velocity of gas bubbles (R = 10 μm) in ceramic and Ni melts on high gravity factor (g′/g)48

Figure 19

Dependence of stable rising velocity of gas bubbles in ceramic and Ni melts on bubble size (g′/g = 1000)48

Because the rising velocity of the bubbles is related to their size, it is necessary to investigate the critical bubble size. The Gibbs free energy change for homogeneous nucleation of a bubble in a melt can be expressed as55 (7) where R means the radius of the bubble, γ means the surface energy of the melt and P means the pressure in the bubble. The pressure in the bubble is the sum of gas pressure above the melt, static pressure in the melt and capillary force (8) where P₀ means the gas pressure above the melt, H means the depth of the bubble in the melt. Thus, ΔG can be further written as (9) On the condition of d(ΔG)/dR = 0, the critical size R_c can be derived as (10) Because high gravity combustion synthesis is usually carried out in vacuum, P₀ is much smaller than the other two terms and can be omitted. Thus, R_c can be written as (11) From equation (11), the critical bubble size is inversely proportional to the high gravity factor. With increasing the high gravity factor, the critical bubble size will decrease, which facilitates the nucleation of bubbles and reduces the porosity of final ceramic products.

From the above analysis, the effects of processing parameters on phase separation in high gravity combustion synthesis can be discussed. First, the high gravity factor has an evident influence on phase separation. The phase separation can be accelerated by increasing the high gravity factor. Second, phase separation depends on the viscosity of the ceramic melt, and a lower viscosity facilitates phase separation. Third, the size of Ni droplets or gas bubbles affects their settling or rising velocity, and a larger size is favourable for phase separation. Finally, the extent of phase separation is connected with the lifetime of the melt, and a longer lifetime is desired for complete phase separation.

In high gravity combustion synthesis, the phase separation can be improved by optimisation of processing parameters. An increased high gravity factor will accelerate the phase separation and escape of gas bubbles from the melt. By preheating the reactant powders, the reaction temperature can be elevated, resulting in longer lifetime and lower viscosity of the melt and improving phase separation. Employing a mechanical pressure on the melt is also helpful for phase separation, because the pressure can reduce the critical bubble size and shrinkage cavities during solidification. Another way to promote phase separation is to reduce the formation temperature and viscosity of the melt by tailoring its chemical composition, where both the lifetime of the melt and phase separation velocity will be enhanced.

Solidification and microstructure evolution in high gravity combustion synthesis

In high gravity combustion synthesis, the microstructure of products forms during the solidification step. The kinetics in solidification and microstructure evolution strongly depends on the melt composition and cooling condition.56 In this section, solidification and microstructure evolution in high gravity combustion synthesis of ceramic materials is discussed, where both single phase and multiphase eutectic ceramics are included.

Single phase ceramics

During the solidification of ceramic melts, the driving force for phase transformation and growth rate of solid phases are closely related to the undercooling degree. The solidification kinetics and crystal growth rates are affected by the thermal gradient in the ceramic melts. In high gravity combustion synthesis, the solidification of ceramic melts takes place in a graphite crucible. Because of heat dissipation caused by the crucible, a thermal gradient occurs in the radial direction, where the temperature decreases from the centre to the side. In this case, solidification initiates at the side and then proceeds into the centre.

In most single phase ceramics (e.g. Al₂O₃, MgAl₂O₄ and YAG) prepared by high gravity combustion synthesis, large dendrites composed of faceted crystals were observed (Fig. 20). This can be explained by the Jackson's criterion on interface structures and crystal growth modes in solidification of melts. According to the Jackson's criterion,57 the interface structure is evaluated by an interface roughness parameter defined as α = AΔS_m/R, where ΔS_m is the melting entropy, R is the gas constant and A is a crystallographic parameter and usually estimated to be 0·5–1. In a solidification process, if α<2, the liquid/solid interface is atomically rough and non-faceted interface will be produced. If α>2, the interface is atomically smooth and faceted interface is expected. For Al₂O₃, MgAl₂O₄, and YAG ceramics, the condition of α>2 is applicable, judging from their ΔS_m/R values (Table 4). In this way, faceted crystals were produced in the ceramics.

Figure 20

Large dendrites consisting of faceted crystals observed in Al₂O₃ ceramics prepared by high gravity combustion synthesis:

Table 4

Melting entropies (ΔS_m) and ΔS_m/R values of several ceramic materials*

Ceramic materials	ΔS_m/J mol⁻¹ K⁻¹)	ΔS_m/R
Al₂O₃	47·72	5·74
YAG	122·38	14·72
MgAl₂O₄	81·64	9·82
ZrO₂	29·51	3·55

*R: gas constant.

At the surface of some Al₂O₃ samples, bright layers with a thickness of ∼0·5 mm were found. From SEM observation, the layers consisted of small flakes with different orientations, and each flake was composed of orderly arranged faceted crystals (Fig. 21). Such strongly textured structure can be attributed to two reasons. One is the intrinsic faceted growth character of Al₂O₃, and the other is particular solidification condition at the melt surface. In high gravity combustion synthesis, high frequency rotation causes a fast gas flow at the melt surface, resulting in fast cooling, solidification and crystal growth. At the melt surface, crystal growth is not constrained by the crucible, and thus the Al₂O₃ crystals can grow freely via parallel continuous growth. In this growth process, separate primary nuclei with random orientations are firstly precipitated in the melt. Then, based on each primary nucleus, new crystals are formed by epitaxial nucleation and grow parallel to the basis. By continuous nucleation and parallel growth, an array of faceted crystals with the same orientation is finally produced and recognised as a flake on a macroscopic scale.

Figure 21

Images (SEM) showing strongly textured microstructure of Al₂O₃ ceramics prepared by high gravity combustion synthesis56

Multiphase eutectic ceramics

Al₂O₃–ZrO₂

In the Al₂O₃–ZrO₂ binary eutectic ceramics prepared by high gravity combustion synthesis, typical eutectic microstructure was produced (Fig. 9). The sample consisted of colonies with an average size of >10 μm, and in each colony eutectic structure on a submicrometre scale was observed. In the centre of each colony the eutectic structure is unaffected, but at the boundary of the colonies the eutectic structure became irregular. At the same time, the interphase spacing in the centre of a colony was smaller than that at the boundaries of the colonies.

During the solidification of eutectics, the interphase spacing is determined by the balance of two opposite energy contributions. On the one hand, transverse diffusion of solutes must occur over greater distance for thicker lamellae or fibres, therefore, favouring smaller interphase spacings. On the other hand, the interfacial energy associated to interphase boundaries will increase as the width of lamellae or fibres decreases, thus favouring larger interphase spacings. In general, the interphase spacing is related with undercooling degree and growth rate, which can be written as where λ is the interphase spacing, ΔT is the undercooling degree, V is the growth rate, C₁ and C₂ are constants.58 From the experimental results, a greater undercooling degree is expected in the centre than that at the boundaries of the colonies in the Al₂O₃–ZrO₂ eutectic ceramics.

According to the model proposed by Hunt and Jackson,58 the microstructure of binary eutectics depends on the melting entropies of the components. If the large volume component has a high melting entropy (α>2) and the minor one has a small melting entropy (α<2), a ‘complex regular’ microstructure will be produced, showing many features of the lamellar or fibrous eutectic microstructure. In the Al₂O₃–ZrO₂ eutectic ceramics with a phase composition of 67%Al₂O₃+33%ZrO₂ in volume fraction, the large volume component of Al₂O₃ has a higher melting entropy of ΔS_m/R = 5·74, and the minor ZrO₂ has a lower melting entropy of ΔS_m/R = 3·55. In this case, the eutectic ceramics showed a complex microstructure, where both fibrous and lamellar features were found (Fig. 9). In the fibrous structure, ZrO₂ fibers with a width of 100–200 nm were homogeneously embedded in a continuous Al₂O₃ matrix.

The complex microstructure in the Al₂O₃–ZrO₂ eutectic ceramics is connected with their phase compositions. It was reported that,59 for binary eutectics, if the surface energies are isotropic or with no significant anisotropy, the fibrous structure is stable when the volume fraction of the minor component is <32%, and when the fraction is >32% the lamellar structure dominates. In the Al₂O₃–ZrO₂ eutectic ceramics prepared by high gravity combustion synthesis, the volume fraction of the minor component (ZrO₂) was 33%, which is almost the critical point for the transition between the two structures. In this case, a complex microstructure was obtained as a mixture of fibrous and lamellar structures.

The microstructure of eutectic ceramics also depends on the growth rate. When the growth rate is below a critical value, planar growth front is stable, and with increasing growth rate shallow cells will be formed. The microstructure transition from coupled to cellular and then to shallow cells with increasing growth rate was observed in Al₂O₃–ZrO₂ eutectics grown by the floating zone method.60 In the Al₂O₃–ZrO₂ eutectic ceramics prepared by high gravity combustion synthesis, the discrepancy in growth rates at different parts of the samples was probably responsible for the diversity of microstructures.

Al₂O₃–YAG

Al₂O₃–YAG is another example of binary eutectic ceramics prepared by high gravity combustion synthesis. The microstructure of the Al₂O₃–YAG ceramics was affected by their chemical compositions. For example, two Al₂O₃–YAG samples were prepared with a eutectic composition (AY-1) and a hypoeutectic composition (AY-2) respectively. Both the samples are composed of Al₂O₃ and YAG, but with different volume fractions (Table 1). In the two samples, different microstructures were observed (Fig. 22), where AY-1 showed a typical eutectic structure with interphase spacings on a submicrometre scale, but AY-2 consisted of large Al₂O₃ crystals and minor eutectics at the boundaries of the large crystals.

Figure 22

Images (SEM) of Al₂O₃–YAG binary eutectic ceramics prepared by high gravity combustion synthesis:

The different microstructures in the two Al₂O₃–YAG samples can be attributed to their different compositions and solidification paths. From the binary phase diagram (Fig. 23), for AY-1 with a eutectic composition, the eutectic reaction of ‘liquid→YAG+Al₂O₃’ will take place during the whole solidification process, resulting in a homogeneous eutectic structure. For AY-2 with a hypoeutectic composition, during solidification primary Al₂O₃ crystals will be precipitated first. The precipitation of Al₂O₃ causes an enrichment of Y₂O₃ in the melt, and the melt composition varies along the liquidus line (AE line in Fig. 23). When the composition of the melt reaches the eutectic point, eutectic reaction begins. Finally, the microstructure is composed of primary Al₂O₃ crystals and YAG–Al₂O₃ eutectics at the boundaries of the primary crystals.

Figure 23

Schematic phase diagram of Al₂O₃–Y₃Al₅O₁₂ binary system56

Al₂O₃–YAG–ZrO₂

Besides binary eutectic systems, Al₂O₃–YAG–ZrO₂ ternary eutectic ceramics were also prepared by high gravity combustion synthesis. The Al₂O₃–YAG–ZrO₂ eutectic ceramics consisted of YAG, Al₂O₃ and t-ZrO₂, with volume fractions of 49, 37 and 14% respectively (Table 1).

In the YAG–Al₂O₃–ZrO₂ ternary eutectic system, YAG and Al₂O₃ have higher melting entropies and ZrO₂ has a lower one. According to the model proposed by Hunt and Jackson,58, 59 Al₂O₃ and ZrO₂ will form lamellar and fibrous structures respectively. Such prediction was verified by SEM observation (Fig. 24). In the Al₂O₃–YAG–ZrO₂ eutectic ceramics, colonies with a size of up to 100 μm were produced, and in the colonies lamellar and three-dimensional interpenetrating structures were observed. The eutectic structure was mostly composed of YAG and Al₂O₃ crystals with interphase spacing of <1 μm, and ultrafine ZrO₂ fibres or particles were distributed at the interface between YAG and Al₂O₃. Similar microstructure was also observed in Al₂O₃–YAG–ZrO₂ eutectics prepared by other methods.61^–63

Figure 24

Images (SEM) showing eutectic microstructures in Al₂O₃–YAG–ZrO₂ ternary eutectic ceramics prepared by high gravity combustion synthesis:

In the solidification of multiphase eutectic ceramics, thermal residual stress is usually involved because of different thermal expansion coefficients of the components. The magnitude of the residual stress depends on the thermal expansion mismatch, cooling rate and topology of the eutectics. For Al₂O₃–YAG–ZrO₂ ternary eutectics grown by the floating zone method, the compressive residual stress in Al₂O₃ was measured to be 160–300 MPa.63 In the YAG–Al₂O₃–ZrO₂ eutectic ceramics prepared by high gravity combustion synthesis, no cracks were observed at the interfaces between different phases (Figs. 10 and 24), which suggests a strong interfacial bonding there.

Conclusion

High gravity combustion synthesis is a recently developed technique to prepare dense ceramics and glasses by melt casting instead of conventional powder sintering. By this technique, a diversity of ceramic and glass materials have been produced (Table 5). The phase assemblage and microstructure of the products can be manipulated from optimisation of starting compositions and processing parameters. High gravity combustion synthesis combines exothermic combustion reactions with a high gravity field and offers an efficient way for preparing bulk ceramic and glass materials.

Table 5

Examples of ceramic and glass materials prepared by high gravity combustion synthesis

Materials	Compositions	Reference
Ceramics	Al₂O₃	4, 22
	MgAl₂O₄	26
	Y₃Al₅O₁₂	24, 25
	Al₂O₃–Y₃Al₅O₁₂	56
	Al₂O₃–ZrO₂	2, 28–34
	Al₂O₃– Y₃Al₅O₁₂–ZrO₂	35–37
	TiC–TiB₂	38
Glasses	Y₂O₃–Al₂O₃–SiO₂	17, 44, 45
Glass–ceramics	Al₂O₃–SiO₂–Y₂O₃	17, 46
	Al₂O₃–SiO₂–ZrO₂	46
	Al₂O₃–La₂O₃–ZrO₂	46

High gravity combustion synthesis is a complex process involving a series of physical variations and chemical reactions, and can be simply divided into three steps: combustion reaction and melting of products; phase separation; and solidification. The first step is very short because combustion reactions take place quickly. The second step plays a key role in determining both purity and density of the products, where the extent of phase separation is affected by the high gravity factor, the lifetime and viscosity of the ceramic melt, and the size of Ni droplets or gas bubbles. An increased high gravity factor, longer lifetime and lower viscosity of the ceramic melt, and larger size of Ni droplets or gas bubbles are favourable to promote the phase separation. In the third step of solidification, the microstructure of the products is formed. The kinetics in solidification and microstructure evolution depends on the melt composition and cooling condition. Faceted crystals were produced in single phase ceramics of Al₂O₃, YAG and MgAl₂O₄ because of their high melting entropies. In the multiphase eutectic ceramics, various eutectic structures were obtained, such as lamellar, fibrous and three-dimensional interpenetrating frameworks, with interphase spacings on a submicrometre scale. In the eutectic ceramics, no cracks were found at the interface between different phases, indicating a strong interfacial bonding.

In addition to ceramics and glasses, more kinds of materials can be prepared by high gravity combustion synthesis, including refractory alloys, intermetallics, cermets, and functionally graded materials. By proper modifications in the processing such as preheating and employing a mechanical pressure, the quality of the products can be further improved. With a short processing time and lower energy consumption, high gravity combustion synthesis provides a furnace free approach to rapid production of inorganic bulk materials.

Footnotes

Acknowledgements

This work is supported by National Natural Science Foundation of China (grant nos. 50932006 and 51002163) and Beijing Natural Science Foundation (grant no. 2112043).

References

LLorca

, Orera

: ‘Directionally solidified eutectic ceramic oxides’, Prog. Mater. Sci., 2006, 51, 711–809.

Zhao

, Zhang

, Zheng

, Bai

, Zhang

, Xu

: ‘Microstructures and mechanical properties of Al₂O₃/ZrO₂ composite produced by combustion synthesis’, Scr. Mater., 2005, 53, 995–1000.

Zhan

, Chen

, Liu

, Zhong

, Zhou

: ‘Combustion synthesis of Ti(C,N)–TiB₂ composite powders under super gravity conditions’, Rare Met. Mater. Eng., 2007, 36, 49–51.

Pei

, Li

, Liu

, Chen

: ‘Fabrication of bulk Al₂O₃ by combustion synthesis melt-casting under ultra-high gravity’, J. Alloy Compd, 2009, 476, 854–858.

Merzhanov

, Borovinskaya

: ‘Self-propagating high-temperature synthesis of refractory inorganic compounds’, Dokl. Akad. Nauk. SSSR, 1972, 204, 366–369.

Munir

, Anselmi-Tamburini

: ‘Self-propagating exothermic reactions: the synthesis of high-temperature materials by combustion’, Mater. Sci. Rep., 1989, 3, 277–365.

Moore

, Feng

: ‘Combustion synthesis of advanced materials’, Prog. Mater. Sci., 1995, 39, 243–316.

Lackner

, ed (ed): ‘Combustion synthesis: Novel routes to novel materials’; 2010, Oak Park, IL, Bentham Science Publishers.

Liu

, Li

, Chen

: ‘Combustion synthesis of ceramic powders with controlled grain morphologies’, in ‘Advances in ceramics: synthesis and characterization, processing and specific applications’, (ed. C. Sikalidis), 49–74; 2011, InTech.

10.

Morsi

: ‘The diversity of combustion synthesis processing: a review’, J. Mater. Sci., 2012, 47, 68–92.

11.

Liu

, Chen

, Tian

, Zhou

, Pereira

, Ferreira

: ‘Fast shape evolution of TiN micro-crystals in combustion synthesis’, Cryst. Growth Des., 2006, 6, 2404–2411.

12.

Liu

, Chen

, Zhou

, Ferreira

: ‘Study on the anisotropic growth of rod-like Yb α-SiAlON crystals prepared by combustion synthesis’, J. Mater. Res., 2004, 19, 3408–3413.

13.

Liu

, Li

: ‘Effect of reaction atmosphere on combustion synthesis of Ti₂AlN’, Adv. Appl. Ceram., 2012, 111, 196–201.

14.

Yeh

, Chen

: ‘Combustion synthesis of vanadium carbonitride from V–C powder compacts under nitrogen pressure’, Ceram. Int., 2007, 33, 365–371.

15.

Liu

, Yang

, Li

, Du

, Hou

: ‘Combustion synthesis of nanosized β-SiC powder on a large scale’, J. Phys. Chem. C, 2008, 112, 6285–6292.

16.

Liu

, Li

: ‘Effects of Si source and high gravity on combustion synthesis of Ti₃SiC₂ in air’, Adv. Appl. Ceram., 2012, 111, 208–213.

17.

Liu

, Li

, He

: ‘Melt-casting of Si–Al–Y–O glasses and glass–ceramics by combustion synthesis under high gravity’, J. Non-Cryst. Solids, 2011, 357, 1764–1767.

18.

Song

, Li

, Pei

, Liu

: ‘Preparation of large-scale textural microstructure Fe-based metallic composite by centrifugal-SHS method’, Mater. Heat Treat., 2008, 37, 4–10.

19.

Mai

, Fang

, Liu

, Chen

, He

, Li

: ‘Preparation of W–Ni graded alloy by combustion synthesis melt-casting under ultra-high gravity’, Mater. Lett., 2011, 65, 3496–3498.

20.

Mai

, Liu

, He

, Li

: ‘Preparation of Fe₃Al intermetallics by combustion synthesis melt-casting under ultra-high gravity’, Mater. Manuf. Proc., 2012, 27, 486–489.

21.

Liu

, Li

, Chen

: ‘Combustion synthesis of porous TiC/Ni composites under high gravity’,. Adv. Appl Ceram., 2012, 111, 404–407.

22.

Pei

, Liand

, Song

: ‘Mechanism of faceted morphology of bulk Al₂O₃ fabricated by combustion synthesis melt casting under ultrahigh gravity’, J. Chin. Ceram. Soc., 2009, 37, 1177–1181.

23.

Song

, Kim

, Lee

, Li

, Pei

: ‘Predicting the adiabatic temperature of transparent Y₃Al₅O₁₂ prepared via combustion synthesis under ultra-high gravity’, Mater. Trans., 2010, 51, 2230–2235.

24.

Liu

, Li

, Guo

, Ning

, Chen

: ‘Melt-casting of YAG ceramics by combustion synthesis under high gravity with the addition of glass’, J. Alloy Compd, 2011, 509, 213–215.

25.

Guo

, Yang

, Liu

, Yang

, Lin

, Li

: ‘Fabrication of YAG transparent ceramic by the SHS quick densification technique under ultrahigh gravity field’, J. Funct. Mater., 2011, 42, 1719–1726.

26.

Liu

, Li

, Yang

: ‘Melt-casting of translucent MgAl₂O₄ ceramics by combustion synthesis under high gravity’, Mater. Manuf. Proc., 2012, 27, 689–693.

27.

Liu

, Li

, Yang

: ‘Preparing bulk ceramics by high-gravity combustion synthesis’, Key Eng. Mater., 2012, 512–515, 350–353.

28.

Zhao

, Zhang

, Song

, Wang

, Wu

: ‘Microstructures and properties of rapidly solidified Y₂O₃ doped Al₂O₃/ZrO₂ composites prepared by combustion synthesis’, Scr. Mater., 2006, 55, 819–822.

29.

Zhang

, Zhao

, Zheng

, Bai

, Wu

: ‘Microstructures and multi-toughening of ZrO₂ nano-micron fibers self-toughening Al₂O₃ ceramics from in situ growth in the melts produced by combustion synthesis’, Key Eng. Mater., 2007, 336–338, 2257–2259.

30.

Zhao

, Zhang

, Song

, Wang

: ‘Al₂O₃/ZrO₂ (Y₂O₃) self-growing composites prepared by combustion synthesis under high gravity’, Scr. Mater., 2008, 58, 207–210.

31.

Zhao

, Zhang

, Song

, Wang

: ‘Microstructures, properties and in-situ toughening of rapidly solidified Al₂O₃/YSZ composite ceramics prepared by combustion synthesis’, Key Eng. Mater., 2008, 368–372, 771–774.

32.

Zhang

, Zhao

, Song

, Wang

, Liu

: ‘Al₂O₃/ZrO₂/Y₂O₃ prepared by combustion synthesis under high gravity’, Int. J. Modern Phys. B, 2009, 23, 1148–1153.

33.

Mei

, Mai

, Li

, Chen

: ‘Fabrication of nanostructure Al₂O₃/ZrO₂(Y₂O₃) eutectic by combustion synthesis melt-casting under ultra-high gravity’, Mater. Lett., 2010, 64, 68–70.

34.

, Zhao

, Zhang

, Huang

, Pan

: ‘Large-bulk solidified Al₂O₃–ZrO₂–Y₂O₃ prepared by self-pressure assisting combustion synthesis under high gravity’, Adv. Mater. Res., 2011, 177, 394–397.

35.

Liang

, Pei

, Li

, Chen

: ‘Fabrication of Al₂O₃/YAG/ZrO₂ ternary eutectic by combustion synthesis melt-casting under ultra-high gravity’, J. Am. Ceram. Soc., 2009, 92, 549–552.

36.

Pei

, Li

, Liang

, Chen

: ‘Rapid fabrication of bulk graded Al₂O₃/YAG/YSZ eutectics by combustion synthesis under ultra-high-gravity field’, Ceram. Int., 2009, 35, 3269–3273.

37.

Pei

, Yang

, Mei

, Qiu

, Li

: ‘Fabrication of Al₂O₃/YAG/YSZ eutectic ceramic by combustion synthesis melt casting under ultrahigh gravity field’, J. Chin. Ceram. Soc., 2009, 37, 1645–1648.

38.

Zhao

, Zhang

, Song

, Wang

, Liu

: ‘Microstructures and properties of large bulk solidified TiC–TiB₂ composites prepared by combustion synthesis under high gravity’, Scr. Mater., 2009, 61, 281–284.

39.

, Yang

, Zhao

, Zhang

, Pan

, Wang

: ‘Ultra-high-hard composites of TiC–TiB₂ prepared by combustion synthesis under high gravity’, Key Eng. Mater., 2010, 434–435, 13–16.

40.

Liu

, Li

, Ning

, Chen

: ‘Combustion synthesis of TiC_xN_1−x–TiB₂ ceramic composites in a high-gravity field’, Mater. Res. Bull., 2011, 46, 958–961.

41.

Liu

, Li

, Chen

, Ning

: ‘Self-propagating high-temperature synthesis (SHS) of Ti-containing ceramic composites under ultra-high gravity’, Rare Met. Mater. Eng., 2011, 40, 262–264.

42.

Gao

, Zhao

, Zhang

, Pan

, Huang

: ‘Fine-grained TiC– (Ti,W)C matrix ceramics prepared by combustion synthesis under high gravity’, Powder Metall. Indust., 2011, 21, 36–40.

43.

Huang

, Zhang

, Zhao

, Pan

, Su

: ‘TiB₂–(Ti,W)C eutectic composite ceramics prepared by combustion synthesis under high gravity’, Adv Mater. Res., 2011, 177, 386–389.

44.

, Liu

, Li

, Wu

, Yang

, Guo

, Chen

: ‘Preparation of Y₂O₃–Al₂O₃–SiO₂ glasses by combustion synthesis melt-casting under high gravity’, Mater. Res. Bull., 2011, 46, 1035–1038.

45.

Yang

, Liu

, Li

, Guo

, Xu

: ‘Preparation of transparent Y₂O₃–Al₂O₃–SiO₂ glasses by high-gravity combustion synthesis with heating assistance’, J. Am. Ceram. Soc., 2012, 95, 1799–1802.

46.

Liu

, Li

, He

: ‘Fast fabrication of glass–ceramics by high-gravity combustion synthesis’, Adv. Appl. Ceram., 2011, 110, 394–399.

47.

Serkov

, Maksimov

, Merzhanov

: ‘Combustion of condensed systems in a mass-force field’, Combust. Explos. Shock Wave, 1968, 4, 349–352.

48.

Liu

, Li

, Chen

: ‘Phase separation in melt-casting of ceramic materials by high-gravity combustion synthesis’, Mater. Chem. Phys., 2012, 133, 661–667.

49.

Takaimchi

, Roderick

: ‘The physical properties of liquid metals’; 1988, New York, Oxford University Press Inc.

50.

Blomquist

, Fink

, Leibowitz

: ‘Viscosity of molten alumina’, Am. Ceram. Soc. Bull., 1978, 57, 522–524.

51.

Paradis

, Ishikawa

: ‘Surface tension and viscosity measurements of liquid and undercooled alumina by containerless techniques’, Jpn J. Appl. Phys., 2005, 44, 5082–5085.

52.

Fratello

, Brandle

: ‘Physical properties of a Y₃Al₅O₁₂ melt’, J. Cryst. Growth, 1993, 128, 1006–1010.

53.

Hyatt

, Day

: ‘Glass properties in the yttria–alumina–silica system’, J. Am. Ceram. Soc., 1987, 70, 283–287.

54.

Saito

, Kai

, Furusho

, Nakashima

, Mori

, Shimizu

: ‘Properties of nitrogen-containing yttria–alumina–silica melts and glasses’, J. Am. Ceram. Soc., 2003, 86, 711–716.

55.

Zhang

, Li

: ‘Study on bubble nucleation in liquid metal’, Acta Phys. Sin., 2007, 56, 4864–4871.

56.

Liu

, Li

, Chen

, Jiang

, Song

: ‘Microstructure evolution of ceramic materials during solidification from melts in high-gravity combustion synthesis’, Mater. Res. Bull., 2012, 47, 222–234.

57.

Laudise

, Carruthers

, Jackson

: ‘Crystal growth’, Ann. Rev. Mater. Sci., 1971, 1, 253–288.

58.

Hunt

, Jackson

: ‘Binary eutectic solidification’, Trans. AIME, 1966, 236, 843–852.

59.

Jackson

, Hunt

: ‘Lamellar and rod eutectic growth’, Trans. AIME, 1966, 236, 1129–1142.

60.

Pena

, Merino

, Harlan

, Larrea

, Fuente

, Orera

: ‘Microstructure of Y₂O₃ doped Al₂O₃–ZrO₂ eutectics grown by the laser floating zone method’, J. Eur. Ceram. Soc., 2002, 22, 2595–2602.

61.

Lee

, Yoshikawa

, Fukuda

, Waku

: ‘Growth and characterization of Al₂O₃/Y₃Al₅O₁₂/ZrO₂ ternary eutectic fibers’, J. Cryst. Growth, 2001, 231, 115–120.

62.

Lee

, Yoshikawa

, Murayama

, Hanada

, Fukuda

: ‘Microstructure and mechanical properties of Al₂O₃/Y₃Al₅O₁₂/ZrO₂ ternary eutectic crystals grown by the micro-pulling-down method’, J. Eur. Ceram. Soc., 2005, 25, 1411–1417.

63.

Pena

, Larsson

, Merino

, Francisco

, Orera

, LLorca

, Pastor

, Martin

, Segurado

: ‘Processing, microstructure and mechanical properties of directionally-solidified Al₂O₃–Y₃Al₅O₁₂–ZrO₂ ternary eutectics’, J. Eur. Ceram. Soc., 2006, 26, 3113–3121.

Review of melt casting of dense ceramics and glasses by high gravity combustion synthesis

Abstract

Keywords

Introduction

Experimental procedure

Ceramic and glass materials prepared by high gravity combustion synthesis

Single phase ceramics

Al2O3

MgAl2O4

Y3Al5O12

Multiphase eutectic or composite ceramics

Glasses

Glass–ceramics

Phase separation in high gravity combustion synthesis

Solidification and microstructure evolution in high gravity combustion synthesis

Single phase ceramics

Multiphase eutectic ceramics

Al2O3–ZrO2

Al2O3–YAG

Al2O3–YAG–ZrO2

Conclusion

Footnotes

Acknowledgements

References

Al₂O₃

MgAl₂O₄

Y₃Al₅O₁₂

Al₂O₃–ZrO₂

Al₂O₃–YAG

Al₂O₃–YAG–ZrO₂