Effects of α-Si3N4 and AlN addition on formation of α-SiAlON by combustion synthesis

Journal of Alloys and Compounds 509 (2011) 529–534

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Effects of ␣-Si3 N4 and AlN addition on formation of ␣-SiAlON by combustion synthesis C.L. Yeh ∗ , K.C. Sheng Department of Aerospace and Systems Engineering, Feng Chia University, 100 Wenhwa Road, Seatwen, Taichung 40724, Taiwan

a r t i c l e

i n f o

Article history: Received 7 June 2010 Received in revised form 15 September 2010 Accepted 18 September 2010 Available online 25 September 2010 Keywords: Ceramics Gas–solid reactions X-ray diffraction ␣-SiAlON Combustion synthesis

a b s t r a c t Preparation of Yb ␣-SiAlON was investigated by self-propagating high-temperature synthesis (SHS) from ␣-Si3 N4 - and ␣-Si3 N4 /AlN-diluted powder compacts under nitrogen of 2.17 MPa. For the AlN-free samples, the molar ratio of Si3 N4 /Si varies between 0.22 and 0.5. The starting stoichiometry of the AlN-added samples comprises a constant proportion of Si3 N4 /Si equal to 0.22, but a broad range of AlN/Al from 0.33 to 1.0. The self-sustaining combustion wave propagated in the spinning mode on account of highly diluted samples adopted in this study. The overall reaction exothermicity increases with Si3 N4 /Si ratio for the AlN-free samples, while decreases with AlN/Al ratio for the AlN-added powder compacts. As a result, the amount of unreacted Si left in the final product was significantly reduced and the formation of nearly single-phase Yb ␣-SiAlON was achieved in the sample with Si3 N4 /Si = 0.5. Moreover, the growth of elongated ␣-SiAlON grains was enhanced in the samples with high contents of Si3 N4 . In contrast, the nitridation of Si was only improved to a certain extent with the addition of AlN and no further improvement was attained by increasing the AlN content. Due to the lack of sufficient liquid phases during combustion and the weak reaction exothermicity, the samples with high contents of AlN were inclined to produce ␣-SiAlON grains in a fine equiaxed form. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Sialon has been considered as a promising high-temperature structural material and a superior tribological ceramic, owing to its excellent mechanical properties and chemical stability [1–5]. There are two well-known sialon phases, ␣- and ␤-SiAlON, which are solid solutions based upon structural modifications of ␣- and ␤-Si3 N4 , respectively. ␣-SiAlON can be described by the formula Mem/v Si12−(m+n) Alm+n On N16−n (Me stands for an interstitial metal ion of valence v) and ␤-SiAlON by Si6−z Alz Oz N8−z . In general, the ␣-SiAlON ceramics are in the form of equiaxed grains and feature high hardness, good wear and oxidation resistance, and excellent thermal shock resistance [4,5]. The ␤-SiAlON phase appears as elongated grains in the microstructure and is characterized by good fracture toughness [4,5]. The high hardness, fracture toughness, and thermal shock resistance make SiAlONs well suited to use in cutting tools. SiAlONs are an attractive low cost alternative to hot-pressed silicon nitride for machining grey cast iron for automotive applications. They have also replaced cemented carbide tools when machining nickel-based superalloys. With the merits of electrical insulation, low thermal

∗ Corresponding author. Tel.: +886 4 24517250x3963; fax: +886 4 24510862. E-mail address: [email protected] (C.L. Yeh). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.09.093

conductivity, and wear and thermal shock resistance, representative wear applications of SiAlONs are fixtures for positioning and transferring metal parts during processes such as induction heating or resistance welding. In addition, SiAlONs have been used in metal wire and tube drawing tools for non-ferrous metals such as copper and aluminum alloys. Fabrication of SiAlON ceramics has been accomplished by a variety of processing routes, such as hot pressing (HP) [6–8], hot isostatic pressing (HIP) [2,4], carbothermal reduction and nitridation (CRN) [9–11], liquid-phase sintering [12], and combustion synthesis [13–17]. For example, Shen et al. [7] obtained Yb-stabilized ␣-SiAlON with a wide range of compositions from the powder mixture of Si3 N4 , AlN, Al2 O3 , SiO2 , and Yb2 O3 under hot pressing at 1800 ◦ C and 25–32 MPa for 2 h. By using Si3 N4 , AlN, Al2 O3 , and Mex Oy (where Me = Sr, Ca, La, Ce, Nd, and Yb), Mandal [4] prepared various ␣-SiAlONs by HIP and observed the elongated morphology in the (Nd + Ca)- and (Ce + Ca)-doped products. The CRN technique often adopts Al2 O3 , SiO2 , and carbon black as the starting materials [9,10]. Ekström et al. [11] utilized the halloysite clay as a precursor along with Y2 O3 , carbon, and either SiO2 or Si to synthesize ␣SiAlON powders through CRN at atmospheric pressure. Fully dense and near net shaped components of SiAlON can be formed by HP and HIP if sintering aids such as Y2 O3 and Yb2 O3 are added to the compact. However, HP and HIP are time- and energy-consuming methods when compared with combustion synthesis.

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Table 1 Summary of starting compositions of AlN-free samples for combustion synthesis of Yb ␣-SiAlON (with m = 1.6 and n = 0.8). Samples

SN01 SN02 SN03 SN04 SN05 SN06

Compositions (mol%) Yb2 O3

Si

␣-Si3 N4

Al

2.73 2.80 2.86 2.95 3.04 3.16

59.77 57.61 55.51 52.95 49.73 45.58

12.89 14.39 15.85 17.64 19.88 22.78

24.61 25.20 25.78 26.46 27.35 28.48

Combustion synthesis in the mode of self-propagating hightemperature synthesis (SHS) takes advantage of the self-sustaining merit from highly exothermic reactions, and hence, has the benefits of low energy requirement, short processing time, and simple facilities [18–21]. The SHS technique can be applied in either solid–gas or solid–solid reaction systems and has produced a number of advanced materials, including borides, carbides, nitrides, carbonitrides, silicides, intermetallics, and composites on their bases [18–21]. Recently, Liu et al. [13–16] conducted a series of studies on combustion synthesis of ␣-SiAlON from different Mex Oy –Si–Si3 N4 –(SiO2 )–Al–(AlN) systems. According to Liu et al. [13], the rapid heating rate and short reaction time of combustion synthesis facilitate the growth of rod-like ␣-SiAlON crystals through a dissolution–precipitation mechanism assisted by a coexisting liquid phase. The starting composition and seeding crystal were shown to substantially affect the grain size and morphology of synthesized ␣-SiAlON [14]. The addition of NH4 F in a proper amount was beneficial to enhance the nitridation of Si and evolution of elongated ␣-SiAlON grains, but excessive NH4 F resulted in the formation of ␤-SiAlON [15]. Moreover, the use of Fe2 O3 as an additive in the reactant mixture was favorable for the growth of ␣-SiAlON whiskers with high aspect ratios [16]. The objective of this study is to investigate the effect of starting stoichiometry of the reactant compact on the formation of ␣-SiAlON through combustion synthesis in the SHS mode. Two nitrides, ␣-Si3 N4 and AlN, of different quantities were adopted in the reactant mixtures and their influence on the composition and morphology of the final product was studied. In addition, the dependence of combustion wave velocity and reaction temperature was explored on the molar ratios of Si3 N4 /Si and AlN/Al of the green samples. 2. Experimental methods of approach Yb-stabilized ␣-SiAlON, Ybm/3 Si12−(m+n) Alm+n On N16−n , with the composition of m = 1.6 and n = 0.8 is investigated in this study. Test samples of the first type were prepared from the powder mixture of Yb2 O3 (Strem Chemicals, 99.9% purity), Si (Strem Chemicals, ≤45 ␮m, 99% purity), ␣-Si3 N4 (Aldrich Chemical, ≤45 ␮m, 99% purity), and Al (Showa Chemical Co., 10 ␮m, 99.9% purity). Table 1 lists the starting compositions of the AlN-free samples labeled by SN01–SN06, within which the molar ratio of Si3 N4 /Si increases from 0.22 to 0.5. The second type of the sample incorporates AlN (Strem Chemicals, 99% purity) into the powder mixture. According to Table 2, the AlN-containing samples have the same mole fraction of ␣-Si3 N4 (with Si3 N4 /Si = 0.22), but different contents of AlN. The molar proportion of AlN/Al varies from 0.33 for the sample of SN01-A1 to 1.0 for SN01-A6. The reactant powders were dry mixed in a ball mill and then coldpressed into cylindrical samples with a diameter of 7 mm, a height of 12 mm, and a compaction density of 40% relative to the theoretical maximum density (TMD). The role of ␣-Si3 N4 and AlN played in the solid–gas combustion process is to suppress the melting of metallic reactants and to improve the degree of nitridation. When compared with the maximum molar proportions of Si3 N4 /Si and AlN/Al, respectively about 0.34 and 0.22 adopted by Liu et al. [13–16], the amounts of ␣Si3 N4 and AlN containing in the samples of this study are even larger. This aims to provide better understanding of the effects of ␣-Si3 N4 and AlN on combustion synthesis of ␣-SiAlON. Another advantage of using highly diluted samples is to keep the powder compact in shape during the SHS process for a comprehen-

sive diagnosis of the combustion characteristics, including the combustion wave velocity, reaction temperature, and propagation mode of self-sustaining combustion. The SHS experiments were conducted in a stainless-steel windowed combustion chamber under a nitrogen pressure of 2.17 MPa. The nitrogen gas used in this study has a purity of 99.999%. The sample holder was equipped with a 600 W cartridge heater used to raise the initial temperature of the sample prior to ignition. In this study, a preheating temperature of 150 ◦ C was required to assure self-sustaining combustion for the samples of both types. In addition, a compacted pellet (2 mm in height and 7 mm in diameter) made up of the powder blend of titanium (Ti) and carbon black (C) (with a molar ratio Ti:C = 1:1) was placed on the top of the test specimen to serve as an ignition enhancer which was triggered by a heated tungsten coil. Details of the experimental setup and measurement approach were reported elsewhere [22]. The microstructure of synthesized products was examined under a scanning electron microscope (Hitachi S-3000N), and phase composition was analyzed by an X-ray diffractometer (Shimadzu XRD-6000) with CuK␣ radiation.

3. Results and discussion 3.1. Observation of combustion characteristics Three recorded combustion sequences illustrating the entire SHS processes associated with formation of ␣-SiAlON from the powder compacts of different starting compositions are displayed in Fig. 1(a)–(c), all of which feature one or two localized reaction zones propagating spirally along the sample in a self-sustaining manner. Instead of forming a planar combustion front spreading longitudinally, the presence of a spinning combustion wave implies that the synthesis reaction is not sufficiently exothermic [23]. According to Ivleva and Merzhanov [24], once the heat flux liberated from self-sustaining combustion is no longer adequate to maintain the steady propagation of a planar front, the combustion front forms one or several localized reaction zones. Therefore, the dilution of powder compacts with nitride phases of high concentrations could be responsible for the spinning combustion zone observed in this study. As also shown in Fig. 1(a), solid–gas combustion of the sample SN01 (with Si3 N4 /Si = 0.22) in nitrogen is accompanied by apparent shrinkage of the burned product, suggestive of the combustion temperature exceeding the melting point of the metallic component. The liquid melt tends to agglomerate and reduce the permeability of the powder compact, which is unfavorable for the formation of ␣SiAlON because the infiltration of nitrogen is hindered. It was found that the addition of a larger amount of Si3 N4 or the introduction of both Si3 N4 and AlN in the powder compacts was able to weaken the agglomeration of liquid melts and to prevent the samples from significant shrinkage. As revealed in Fig. 1(b), a lesser extent of contraction is observed for the sample of SN05 (with Si3 N4 /Si = 0.4) than that of SN01 shown in Fig. 1(a). Consequently, the penetration of nitrogen is feasible and subsequent nitridation is enhanced. Likewise, Fig. 1(c) shows that melting of the AlN-added sample SN01-A5 is considerably reduced in comparison to that of the sample SN01. However, the combustion wave of Fig. 1(c) appears to be slower than those of Fig. 1(a) and (b), due probably to the lowerTable 2 Summary of starting compositions of AlN-added samples for combustion synthesis of Yb ␣-SiAlON (with m = 1.6 and n = 0.8). Samples

SN01-A1 SN01-A2 SN01-A3 SN01-A4 SN01-A5 SN01-A6

Compositions (mol%) Yb2 O3

Si

␣-Si3 N4

Al

AlN

2.73 2.73 2.73 2.73 2.73 2.73

59.75 59.75 59.75 59.75 59.75 59.75

12.89 12.89 12.89 12.89 12.89 12.89

18.45 17.58 16.40 14.77 13.25 12.30

6.16 7.03 8.21 9.84 11.36 12.30

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Fig. 1. Typical SHS sequences illustrating spiral propagation of combustion waves along powder compacts with different starting stoichiometries: (a) SN01, (b) SN05, and (c) SN01-A5.

3.2. Measurement of flame-front propagation velocity and combustion temperature The average combustion wave velocity (Vf ) in the axial direction was determined from the recorded combustion sequence and reported in Figs. 2 and 3 correspondingly with respect to the initial contents of Si3 N4 and AlN. For the powder compacts without AlN addition, Fig. 2 shows that the reaction front velocity increases from 1.1 mm/s for the sample of Si3 N4 /Si = 0.22 with increasing Si3 N4 content and approaches an asymptotic value of about 1.36 mm/s for the samples containing Si3 N4 /Si = 0.4 and 0.5. The increase of combustion wave velocity is attributed to an increase in the magnitude of reaction enthalpies liberated by enhanced nitridation of Si and Al. As the content of Si3 N4 is further increased, however, the dilution effect of Si3 N4 compensates the increase of reaction exothermicity. Therefore, no further increase in the reaction front velocity was observed.

1.5

Flame-Front Velocity, V f (mm/s)

ing of reaction exothermicity caused by excessive dilution in the sample.

1.4

Yb2O3-Si-Si3N4-Al Powder Compacts (without AlN addition)

1.3

1.2

1.1

1.0 0.15

Nitrogen Pressure: 2.17 MPa Sample Density: 40% TMD Preheating Temperature: 150 ºC

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

Molar Ratio of Si3N4/Si in Powder Mixture Fig. 2. Effect of molar ratio of Si3 N4 /Si on flame-front propagation velocity of AlNfree reactant compacts.

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Flame-Front Velocity, V f (mm/s)

1.4 Yb2O3-Si-Si3N4-Al-AlN Powder Compacts (with AlN addition)

1.2

1.0

0.8

0.6

Nitrogen Pressure: 2.17 MPa Sample Density: 40% TMD Preheating Temperature: 150 ºC

0.4 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Molar Ratio of AlN/Al in Powder Mixture Fig. 3. Effect of molar ratio of AlN/Al on flame-front propagation velocity of AlNadded reactant compacts with a constant proportion of Si3 N4 /Si = 0.22.

For the powder compacts diluted with both AlN and ␣-Si3 N4 , Fig. 3 indicates that the propagation rate of combustion decreases slightly when the proportion of AlN/Al increases from 0.33 to 0.67, beyond which an appreciable drop in the combustion velocity is noticed. Besides, the flame-front velocities of the samples with AlN/Al = 0.33–0.67 are comparable to that of the AlN-free sample having an identical ratio of Si3 N4 /Si (SN01). This implies that the enthalpy increase resulting from improved nitridation is nearly diluted by the addition of AlN. When the molar concentration of AlN relative to Al increases up to 0.85 and 1.0, the dilution effect of AlN overwhelms the contribution of enthalpy increase, thus resulting in a decrease in the overall reaction exothermicity and an obvious decline in the combustion front velocity. Fig. 4 depicts four combustion temperature profiles recorded from the powder compacts with different starting stoichiometries. The abrupt rise in temperature signifies arrival of the combustion wave. The peak value corresponds to the flame-front temperature. It is interesting to note that some temperature curves exhibit two spikes accounting for the spinning motion of a couple of localized reaction spots. As revealed in Fig. 4, a higher reaction front temperature of 1220 ◦ C is detected for the sample SN06 (profile #1) than the sample SN01 (profile #2) which has a peak temperature of 1055 ◦ C. This confirms an increase in the overall reaction exothermicity with increasing Si3 N4 content.

Fig. 5. XRD patterns of products synthesized from powder compacts with different Si3 N4 /Si ratios in starting stoichiometries: (a) SN01, (b) SN02, (c) SN04, and (d) SN06.

For the AlN-added samples with AlN/Al ratios of 0.4 and 0.85 (profiles #3 and #4 in Fig. 4), the peak combustion temperature decreases approximately from 1100 to 1000 ◦ C with increasing AlN content. This suggests a reduction in the reaction exothermicity. It is useful to note that the variations of combustion temperature with molar ratios of Si3 N4 /Si and AlN/Al for both types of the samples are in a manner consistent with those of combustion wave velocity.

Combustion Temperature (ºC)

1400 40% TMD and T p = 150 oC N2: 2.17 MPa

#1

1200

#3

#2

#4

1000 800 600 Stoichiometry #1 SN06 #2 SN01

400

#3 #4

SN01-A2 SN01-A5

200 0

2

4

6

8

10

12

14

16

18

Time (s) Fig. 4. Combustion temperature profiles of reactant compacts with different molar ratios of Si3 N4 /Si and AlN/Al for SHS formation of Yb ␣-SiAlON.

Fig. 6. XRD patterns of products synthesized from powder compacts with different AlN/Al ratios in starting stoichiometries: (a) SN01-A2, (b) SN01-A4, and (c) SN01-A6.

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Si3 N4 /Si = 0.5, Fig. 5(d) reveals that the residual Si is trivial in the final product. The phase composition of the products obtained from AlNadded samples is presented in Fig. 6(a)–(c). When compared with the sample SN01 of Fig. 5(a), the remnant Si was reduced in the final product of the AlN-added sample SN01-A2 in Fig. 6(a). However, as shown in Fig. 6(b) and (c), no further improvement in the nitridation of Si was achieved by increasing the content of AlN. This could be partially due to the weak reaction exothermicity of the powder compacts diluted with high contents of AlN. In part, this might be because of a lesser amount of the liquid melt existing in the reactant compact during combustion. As proposed by Liu et al. [14], the formation of ␣-SiAlON is assisted by the co-existing liquid phase. According to the measured combustion temperature, molten Al is the major liquid phase for the AlN-added compacts. The increase of AlN means a decrease in Al for the reactant compacts, which is detrimental for the evolution of ␣-SiAlON. Due to the high contents of Al (24.61–28.48 mol%) in the AlNfree samples of this study, the melting of Al during combustion led to densification of the final products. As a result, for the AlN-free samples the relative density of the synthesized product is in the range from about 50% to 70% and increases with decreasing ␣-Si3 N4 content. For the AlN-added samples, however, the degree of sample shrinkage was noticeably reduced, and therefore, the relative density of the resultant product was around 45%. Fig. 7(a) shows the SEM micrograph of as-synthesized Yb ␣SiAlON from the sample SN01. Two types of the ␣-SiAlON grains are observed in Fig. 7(a): equiaxed crystals of about 2 ␮m and elongated grains with high aspect ratios. It was found that ␣-SiAlON in the form of elongated grains became dominant in the products from the samples with higher contents of Si3 N4 ; for instance, the samples SN05 and SN06. As illustrated in Fig. 7(b), the majority of ␣-SiAlON synthesized from the sample SN06 is elongated or whisker-like. This is most likely due to the increase of Si3 N4 in the sample, leading to a higher reaction temperature that is favorable for the development of the elongated morphology [25]. Another reason could be that the existence of a larger amount of molten Al for the AlN-free samples also facilitates the growth of ␣-SiAlON whiskers through a dissolution–precipitation mechanism [13]. On the other hand, as displayed in Fig. 7(c), fine isotropic gains of ␣-SiAlON about 1 ␮m were synthesized from the AlN-added sample SN01-A6. Formation of equiaxed crystals is a combined consequence of the weak reaction exothermicity and lack of the sufficient liquid phase in the sample comprising a large quantity of AlN.

4. Conclusions Fig. 7. SEM micrographs illustrating fracture surfaces of products synthesized from powder compacts of different starting stoichiometries: (a) SN01, (b) SN06, and (c) SN01-A6.

3.3. Composition and morphology analysis of combustion products Typical XRD patterns of the products synthesized from AlN-free powder compacts with different contents of Si3 N4 are presented in Fig. 5(a)–(d), within which ␣-SiAlON is identified as the dominant phase along with different amounts of residual Si. The presence of a large quantity of Si in Fig. 5(a) is mainly caused by the significant sample melting that impedes the penetration of nitrogen. With the increase of the molar ratio of Si3 N4 /Si, it is evident that the amount of elemental Si left unreacted is considerably reduced, indicative of a substantial improvement in the nitridation of metallic elements and subsequent formation of ␣-SiAlON. For the sample SN06 with

Fabrication of Yb-stabilized ␣-SiAlON (with m = 1.6 and n = 0.8) was conducted by the SHS process under nitrogen of 2.17 MPa. Two types of the powder compacts were prepared from raw materials including Yb2 O3 , Si, Al, ␣-Si3 N4 , and AlN. The molar ratio of Si3 N4 /Si varies between 0.22 and 0.5 in the samples containing no AlN. For the AlN-added samples, the proportion of Si3 N4 /Si keeps fixed at 0.22 but the AlN/Al ratio ranges from 0.33 to 1.0. Experimental evidences indicated that Si3 N4 and AlN contributed two opposing effects to the overall reaction exothermicity. They prevent the reactant compact from losing its permeability, which enhances the nitridation of metallic elements and increases the reaction enthalpy. On the contrary, they are diluents to the combustion process and tend to decrease the reaction temperature. For the AlN-free powder compacts, the combustion temperature as well as reaction front velocity increased with Si3 N4 content and approached asymptotic values as Si3 N4 /Si = 0.4. This is attributed to an increase in the overall reaction exothermicity. Based upon the

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XRD analysis, ␣-SiAlON was identified as the major phase in the final products and the residual Si was substantially reduced with the increase of Si3 N4 in the samples. The as-synthesized ␣-SiAlON exhibits two morphologies: equiaxed and elongated grains. The elongated grains with high aspect ratios dominate in the products obtained from the samples with Si3 N4 /Si = 0.4 and 0.5. On the other hand, the overall reaction exothermicity decreased with increasing AlN/Al ratio for the AlN-added samples, causing a decrease in the combustion temperature and flame propagation velocity and a restrained improvement in the nitridation of Si. Moreover, because of a lesser amount of the liquid phase present during combustion, ␣-SiAlON in the form of fine equiaxed gains was produced when the reactant compacts with AlN/Al = 0.85 and 1.0 were adopted. Acknowledgement This research was sponsored by the National Science Council of Taiwan, ROC, under the grant of NSC 98-2221-E-035-065-MY2. References [1] G.Z. Cao, R. Metselaar, Chem. Mater. 3 (1991) 242–252. [2] T. Ekström, M. Nygren, J. Am. Ceram. Soc. 75 (2) (1992) 259–276. [3] I.W. Chen, A. Rosenflanz, Nature 389 (1997) 701–704.

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