Preferred Orientation Prepared by the Carbothermal Nitridation of

CRYSTAL GROWTH & DESIGN

Polycrystalline ZrN1-xCx Layers with (111) Preferred Orientation Prepared by the Carbothermal Nitridation of ZrO2 Ceramics Guanghua Liu,†,‡,§ Jiangtao Li,† Kexin Chen,*,‡ Heping Zhou,‡ C. Pereira,§ and J. Ferreira§ Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, and Department of Ceramics and Glass Engineering, UniVersity of AVeiro, CICECO, AVeiro 3810-193, Portugal

2009 VOL. 9, NO. 1 562–568

ReceiVed August 7, 2008; ReVised Manuscript ReceiVed October 7, 2008

ABSTRACT: Polycrystalline ZrN1-xCx layers have been prepared by the carbothermal nitridation of consolidated ZrO2 ceramics. Thermodynamic analysis indicates that the formation of ZrN1-xCx is favored by increasing the reaction temperature and the pressure of N2. The kinetics in the carbothermal nitridation is studied, and the overall reaction is considered to be controlled by the infiltration and diffusion of gaseous reactants. From the surface to the center, the sample shows a porosity gradient due to the difference in reaction degree. A strong (111) preferred orientation is observed in the as-synthesized ZrN1-xCx layers, which is enhanced at elevated temperatures and higher pressures of N2. It is proposed that the growth of ZrN1-xCx crystals takes place by the two-dimensional nucleation and growth mechanism. Introduction ZrN is an important transition metal nitride with a rock-salt structure. It has many good physical and chemical properties, including high melting point, superior hardness, excellent wear resistance, corrosion resistance, and gold color for decorative purpose.1 Because of these properties, ZrN is widely used in industry as a wear-resistant coating for steel tools, protective layer on vessels, and diffusion barrier in IC technology. ZrN coatings and films can be prepared by a variety of methods, such as chemical vapor deposition, reactive sputtering, ion beam sputtering, pulsed laser deposition, and plasma nitridation.2-5 The microstructure and physical properties of the ZrN films depend on the deposition method applied.6-9 The reduction and nitridation of ZrO2 is also a possible way to synthesize ZrN. Lerch and Wrba10 reported the formation of Zr(N,O,C) phases by the nitridation of pure monoclinic ZrO2 in a graphite resistance furnace. By the reduction and nitridation of ZrO2 in a N2 or NH3 atmosphere, ultrafine ZrN powders have also been prepared.11,12 In this paper, consolidated ZrO2 ceramics, instead of ZrO2 powders, are used as the precursor to synthesize ZrN by carbothermal nitridation. Polycrystalline ZrN1-xCx layers with (111) preferred orientation are prepared at the surface of ZrO2 ceramics. The effects of reaction temperature and the pressure of N2 on the formation of ZrN1-xCx are investigated. On the basis of thermodynamic analysis and the experimental results, the reaction kinetics in carbothermal nitridation is discussed. The growth mechanism of ZrN1-xCx crystals and the evolution of (111) preferred orientation are studied. Experimental Section

Figure 1. XRD pattern of the ZrO2 ceramics used as the precursor. Table 1. Properties of the ZrO2 Ceramics Used as the Precursor density (g/cm3)

relative density (%)

average grain size (µm)

thickness (mm)

5.79

∼96

0.81 ( 0.17

∼0.25

A carbothermal nitridation experiment was carried out using a special graphite crucible shown in Figure 2. Some carbon black powder was loaded in the lower part of the crucible, and the ZrO2 sample was put on a porous graphite plate. Then, the crucible was placed in a graphite resistance furnace and heat-treated at 1500-1700 °C for 2 h in a N2 atmosphere. The initial pressure of N2 in the furnace was in the range of 0.1-0.9 MPa. During the reaction process, the pressure in the furnace

The ZrO2 ceramics used in this study were prepared by tape casting and pressureless sintering from a partially stabilized ZrO2 powder (TZ3YS, Tosoh). Figure 1 shows the XRD pattern of the ZrO2 ceramics, where only the tetragonal phase is observed. More information about the ZrO2 ceramics is reported in Table 1. * Corresponding author. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Tsinghua University. § University of Aveiro.

Figure 2. A brief illustration of the graphite crucible used for carbothermal nitridation.

10.1021/cg8008656 CCC: $40.75  2009 American Chemical Society Published on Web 12/02/2008

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Figure 4. Relationship between the reaction equillibrium constant (KR) and temperature for the reaction ZrO2(s) + 2C(s) + 1/2N2(g) ) ZrN(s) + 2CO(g).

Figure 3. XRD patterns of the samples prepared by carbothermal nitridation at different temperatures [P(N2) ) 0.1 MPa]. was kept constant, which was recorded by a manometer and adjusted automatically by an electromagnetic valve. Considering the presence of other gases during the reaction, the nominal pressure given by the manometer was the total pressure in the furnace, which was contributed not only by N2 but also by the other gases. The phase composition was identified by X-ray diffraction (XRD; D8-Advance, Bruker) using Cu KR radiation. The microstructure was examined by scanning electron microscopy (SEM; JSM-6460LV and JSM-7000F, JEOL) combined with energy dispersive spectroscopy (EDS; INCA, Oxford Instrument).

A. Thermodynamic Analysis on the Formation of ZrN. Figure 3 shows the XRD patterns of the products after carbothermal nitridation at different temperatures. At 1500 °C, no ZrN was obtained and the sample remained ZrO2. At 1600 °C, ZrN was produced, but the predominant phase was still ZrO2. When the temperature was further elevated to 1700 °C, ZrO2 almost disappeared and ZrN was synthesized as the major product. These results make it evident that the reaction temperature plays a key role in the formation of ZrN, which can be further explained by thermodynamic analysis. In the carbothermal nitridation, the synthesis of ZrN can be expressed by the reaction

(1)

According to the reported data,13 the Gibbs free energy change (∆G) of the above reaction can be calculated as

∆G ) 480 100 - 262.7T + RT ln KR

(2)

where KR is the reaction equilibrium constant. Assuming that the activity coefficients of solid substance equal to 1, the equilibrium constant can be further written as

KR ) [P(CO)]2/[P(N2)]1/2

temp (°C)

KR

P(N2) (MPa)

P(CO) (MPa)

1500 1525 1550 1575 1600

0.373 0.587 0.912 1.401 2.126

0.049 0.039 0.030 0.020 0.013

0.051 0.061 0.070 0.080 0.087

and K, the unit of P(CO) and P(N2) is Pθ (Pθ ) 105 Pa), KR is dimensionless, and R is the gas constant. At equilibrium state

∆G ) 0

(4)

Thus, the reaction equillibrium constant KR can be expressed as the function of temperature (T)

Results and Discussion

1 ZrO2(s) + 2C(s) + N2(g) ) ZrN(s) + 2CO(g) 2

Table 2. Equilibrium Pressures of N2 and CO at Different Temperatures for the Reaction ZrO2(s) + 2C(s) + 1/2N2(g) ) ZrN(s) + 2CO(g)

(3)

where P(CO) and P(N2) are the pressures of CO and N2, respectively. In eqs 2 and 3, the units of ∆G and T are J mol-1

ln KR ) 31.6 - 57774/T

(5)

The relationship between KR and T, according to eq 5, is plotted in Figure 4. The KR-T curve divides the refrence frame into two areas, where different phases are thermodynamically stable. It is clear that higher temperatures and smaller KR values favor the formation of ZrN. If KR is constant, increasing temperature will bring the reaction system from the ZrO2-stable area into the ZrN-stable one. In other words, ZrN can be synthesized more readily at higher temperatures, as revealed by the experimental results. In contrast to temperature, the actual pressures of N2 and CO are difficult to measure and KR values cannot be determined. In this case, only a rough estimation can be achieved by thermodynamic analysis. Ignoring the other gases that possibly exist, the sum of the pressures of N2 and CO is 0.1 MPa. By eqs 3 and 5, the equillibrium pressures of N2 and CO at different temperatures are calculated and reported in Table 2. The experimental results suggest that ZrN starts to form at a temperature between 1500 and 1600 °C. In this temperature range, the equillibrium pressure of N2 is always lower than that of CO, but with the same magnitude (0.01-0.1 MPa) as the latter. During the carbothermal nitridation of metallic oxides, carbides are also possible products other than nitrides. To evaluate the priority of the formation of ZrC and ZrN, the following reaction is considered:

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1 ZrC(s) + N2(g) ) ZrN(s) + C(s) 2

(6)

The Gibbs free energy change (∆G) of this reaction is

∆G ) -183 400 + 87.5T - 1/2RT ln P(N2)

(7)

At the equillibrium state, the relationship between the pressure of N2 and temperature can be given as

ln P(N2) ) 21.05 - 44118/T

(8)

According to eq 8, the P(N2)-T curve is plotted in Figure 5. It can be seen that ZrN is more stable than ZrC at lower temperatures and with higher pressures of N2. The experimental parameters, viz., T, P(N2) coordinates, in this work are all located in the ZrN-stable area. The equillibrium pressures of N2 are calculated and reported in Table 3, which are much lower than the nominal pressure in the furnace (0.1-0.9 MPa). Although the actual pressure of N2 is lower than the nominal one, it is considered to be above the equilibrium values. From the above discussion, the formation of ZrN instead of ZrC is favorable under the reaction conditions here. The formation of ZrN instead of ZrC is verified by the experimental results. The samples prepared by carbothermal nitridation show the characteristic golden color of ZrN, in contrast to the black color of ZrC. Additionally, the lattice parameter of the product in each sample is in better accordance with that of ZrN than that of ZrC. As shown in Table 4, the lattice parameters exist in a narrow range of 0.4590-0.4595 nm, which are close to the data reported by Yamamura et al.11 but a little larger than the value given by PDF 35-0753.

Figure 6. SEM images of the samples prepared by carbothermal nitridation at 1700 °C with a N2 pressure of (a) 0.5 MPa and (b) 0.9 MPa in comparison with (c) the ZrO2 sample used as the precursor.

Figure 7. XRD pattern for the powder of the carbothermal nitridation product prepared at 1700 °C with a N2 pressure of 0.1 MPa.

Figure 5. Relationship between the equillibrium pressure of N2 and temperature for the reaction ZrC(s) + 1/2N2(g) ) ZrN(s) + C(s). Table 3. Equilibrium Pressures of N2 at Different Temperatures for the Reaction ZrC(s) + 1/2N2(g) ) ZrN(s) + C(s) temp (°C) P(N2) (MPa)

1500 0.0022

1550 0.0043

1600 0.0082

1650 0.015

1700 0.027

Table 4. Lattice Parameters of ZrN1-xCx Prepared by Carbothermal Nitridation under Different Reaction Conditions temp (°C) 1600 1700 1700 1700 ZrN (PDF 35-0753) ZrC (PDF 35-0784)

P(N2) (MPa)

a (nm)

0.1 0.1 0.5 0.9

0.4595 0.4592 0.4594 0.4591 0.4578 0.4693

Although the formation of ZrC is not favored, as discussed above, the possibility of the solution of C atoms in ZrN cannot be excluded. With the same rock-salt structure and close lattice parameters, ZrC and ZrN can form a continuous solid solution. In fact, the larger lattice parameters (compared with PDF 350753 for ZrN) observed here are probably caused by the incorporation of C atoms. If so, the concentration of C atoms dissolved in ZrN can be roughly estimated by Vegard’s law. According to Vegard’s law, the lattice parameter of ZrN1-xCx solid solution can be expressed as

a (nm) ) 0.4578 + 0.0115x

(9)

From this equation, the concentration of C atoms, viz., x value, is calculated to be 11-15%. Thus, strictly speaking, the as-synthesized product is a ZrN1-xCx solid solution. B. Reaction Kinetics in the Formation of ZrN1-xCx. The above thermodynamic analysis gives a general understanding on the formation of ZrN1-xCx, but it does not clarify the reaction kinetics. In principle, at least two reaction routes are possible for the synthesis of ZrN1-xCx. By the first route, a Zr-containing

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Figure 8. XRD patterns showing the splitting phenomenon of ZrN1-xCx peaks.

Figure 9. XRD patterns of the samples prepared by carbothermal nitridation at 1700 °C with different N2 pressures. Table 5. Preferred Orientation Factors (F) of the Polycrystalline ZrN1-xCx Layers Prepared under Different Reaction Conditions temp (°C) 1600 1700 1700 1700 ZrN (PDF35-0753) a

P(N2) (MPa)

Fa

0.1 0.1 0.5 0.9

0.49 0.70 0.92 0.97 0.48

F ) I(111)/(I(111) + I(200) + I(220)).

vapor is produced and then reacts with N2 and C-containing gases to form ZrN like a CVD process. By the second route, ZrN is produced by the reaction between solid ZrO2 and N2 and C-containing gases. If ZrN1-xCx is synthesized via a CVD process, it is expected to deposit not only on the ZrO2 sample but also on the graphite plate and the walls of the graphite crucible (Figure 2). But in fact, no golden layer was observed on the graphite plate or on the crucible walls. On the other hand, no evidence of the existence of any available Zr-containing vapor has been found. In the Zr-O binary phase diagram,14 no liquid phase is present below 1855 °C and no Zr-containing vapor is mentioned. In the literature on the carbothermal

nitridation of ZrO2, Zr-containing vapor was not reported, either.10,11 In this way, the synthesis of ZrN1-xCx is thought to take place by the second route, viz., the reaction between solid ZrO2 and gaseous species. Figure 6 shows the SEM images of the fracture surface of the carbothermally nitridized samples. From the surface to the center, the porosity gradually decreases and the pores change from continuous to isolated ones. Because consolidated ZrO2 ceramics with a relative density of ∼96% are used as the precursor (Figure 6c), the porous structure should be produced during the reaction. In the phase transition from ZrO2 to ZrN1xCx, a shrinkage in volume is expected. According to the densities of ZrO2 and ZrN, this shrinkage is roughly estimated to be 32%. For this reason, many pores were created after the carbothermal nitridation. Considering the gradient porous structure, it is proposed that the reaction starts at the surface and then proceeds toward the center of the ZrO2 sample. As a result, a difference in reaction degree exists between the surface and the center. In order to verify this proposition, a sample prepared by carbothermal nitridation at 1700 °C was pulverized into powder to perform XRD analysis. The XRD pattern of this powder is shown in Figure 7, in which strong diffraction peaks of ZrO2 are observed. In the XRD pattern of the same sample but in a block form (Figure 3), however, the peaks of ZrO2 are very weak. By these experimental results, the difference in reaction degree is confirmed. The overall carbothermal nitridation reaction can be briefly divided into two steps. At first, ZrO2 is reduced to ZrOx (x < 2) with a remarkable loss of oxygen, and then ZrOx is further reduced and nitridized to form ZrN1-xCx. During this reaction process, some transient phases such as Zr(N,O,C) may be produced. In the XRD pattern of the sample prepared at 1600 °C (Figure 3), the splitting of the ZrN1-xCx peaks is noticed. To reveal the splitting phenomenon more clearly, three strong peaks are separately plotted in Figure 8. On the left side of each peak of ZrN1-xCx, another peak can be observed, as indicated by arrows. According to this result, it is proposed that another unknown phase exists with the same cubic structure as ZrN1xCx but a slightly larger lattice parameter. If so, the lattice parameter of this unknown phase can be calculated by XRD to be a ) 0.4623 nm, which is close to the lattice parameters of Zr(N,O,C) phases reported by Lerch and Wrba.10 In this case, it seems reasonable to identify the unknown phase as a Zr(N,O,C) compound. In the carbothermal nitridation of ZrO2, the Zr(N,O,C) phase can be regarded as an intermediate product, which will transform into ZrN1-xCx by further reduction and nitridation at a higher temperature. For this reason, the Zr(N,O,C) phase is not found in the samples prepared at 1700 °C.

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Figure 10. SEM images of the samples prepared by carbothermal nitridation at 1700 °C: (a-c) P(N2) ) 0.1 MPa, (d) P(N2) ) 0.5 MPa.

Figure 11. SEM images showing the terraced shape of the Zr1-xCx crystals prepared at 1700 °C with a N2 pressure of 0.1 MPa.

During the carbothermal nitridation, ZrN1-xCx was produced at the sample surface and the central part remained ZrO2. Nevertheless, the lattice structure of the ZrO2 phase had slightly changed, which is revealed by XRD analysis. In the XRD pattern of the ZrO2 ceramics before carbothermal nitridation (Figure 1), the (002) or (112) peak can be readily distinguished from

the (110) or (200) one. In the XRD patterns for the samples after carbothermal nitridation (Figures 3 and 7), however, each pair of neighboring peaks [(002) and (110), (112) and (200)] overlap with each other and even merge to form one peak. It has been reported that the tetragonal structure of ZrO2 can be modified by the enrichment of Y3+ cations or by the substitution

Polycrystalline ZrN1-xCx Layers

Figure 12. A schematic illustration of the two-dimensional nucleation and growth of the ZrN1-xCx crystals.

of O by N or C.15-17 By EDS analysis, the concentration of Y [defined as Y/(Y + Zr) in molar ratio] in the remained ZrO2 phase was determined to be 4.4%. For comparison, EDS analysis was also carried out for the starting 3YZ powder and the precursor ZrO2 ceramics before carbothermal nitridation, giving a Y concentration of 5.6% and 5.5% (close to the nominal value of 5.8%), respectively. According to these results, after the carbothermal nitridation, the concentration of Y in ZrO2 did not increase but decreased. Because no Y element was detected by EDS in the newly formed ZrN1-xCx phase, the loss of Y is probably caused by the formation of some Y-containing volatile species in the strongly reductive atmosphere. In this case, the change in the lattice structure of ZrO2 should be attributed not to the enrichment of Y but to the substitution of O by N and C. This proposition is verified by EDS results showing the presence of N and C in the remaining ZrO2 phase. C. Evolution of (111) Preferred Orientation and Crystal Growth of ZrN1-xCx. In the as-synthesized ZrN1-xCx layers, a remarkable (111) preferred orientation is observed. Figure 9 shows the XRD patterns of the samples prepared at 1700 °C, where the (111) peak is strong but the (200) and (220) peaks are very weak. In order to evaluate the degree of preferred orientation, a factor is defined as F ) I(111)/(I(111) + I(200) + I(220)), where I(111), I(200), and I(220) are the integrated intensity of the (111), (200), (220) peaks, respectively. The F-values of the ZrN1-xCx samples are calculated and reported in Table 5. The F-values of the ZrN1-xCx crystals synthesized here are much larger than the data in PDF 35-0753. It is also found that the (111) preferred orientation is enhanced by increasing temperatures and the pressures of N2. The preferred orientation has been widely observed and studied in ZrN films. Larijani et al.4 reported that in ZrN thin films prepared by ion beam sputtering, the (111) preferred orientation is strengthened by increasing the N2/(N2 + Ar) ratio. Rizzo et al.9 found that in ZrN thin films deposited by radiofrequency reactive magnetron sputtering, the (111) orientation forms at a higher deposition rate and (100) orientation occurs at a lower deposition rate. These reports are consistent with the results obtained in this study, despite the different methods applied for the synthesis of ZrN. The (111) preferred orientation of ZrN1-xCx crystals is also revealed by SEM observation. As shown in Figure 10, the ZrN1-xCx crystals have an equilateral triangle morphology. Because ZrN1-xCx has a cubic rock-salt structure, this 3-fold

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symmetric feature should be assigned to (111) planes. From the cross-section SEM images (Figure 11), the ZrN1-xCx crystals show a terraced shape consisting of many parallel layers. The thickness of each layer ranges from 3 to 9 nm and the size of the smallest triangular islands is below 30 nm. According to the terraced shape observed by SEM, the ZrN1-xCx crystals are proposed to undergo a two-dimensional nucleation and growth process. As discussed before, the carbothermal nitridation of ZrO2 takes place with the loss of O and the incorporation of N and C. The nucleation of ZrN1-xCx is thought to originate at the sample surface, viz., the solid-gas interface, because at the surface the concentration of N and C species is higher and the escape of O can occur more readily. By nucleation, fine ZrN1-xCx crystals are created as separate islands. These islands then expand by a lateral growth until they joined together to form a continuous layer. On the basis of this layer, new ZrN1-xCx islands can be created by epitaxial nucleation and then another new layer will be produced analogously. By this means, the ZrN1-xCx crystals grow into three-dimensional blocks with a terraced shape. In the two-dimensional nucleation and growth process, the orientation of initial ZrN1-xCx nuclei will be inherited owing to the epitaxial nucleation, and the as-grown coarse crystals should have the same orientation with the fine nuclei. This implies that the (111) preferred orientation of ZrN1-xCx occurs at the early nucleation stage. In other words, the nucleation of (111)-oriented ZrN1-xCx crystals is favorable under the reaction conditions here. This is consistent with the triangular shape of the ZrN1-xCx islands. A schematic illustration of the nucleation and growth of ZrN1-xCx is shown in Figure 12. Isolated ZrN1-xCx crystals with (111) orientation are created first, which then undergo a lateral growth along the directions normal to their edges. By this lateral growth, the small crystals start to contact with each other and combine to form larger ones. At last, a continuous layer is produced in the XOY plane. On this layer, new small crystals can nucleate and growth to form more layers. By repeating this process, a terraced structure consisting of many parallel layers is finally produced. That is to say, the crystal growth of ZrN1-xCx in the Z direction is realized by continuous formation of new layers. By integrating the lateral growth in the XOY plane with the vertical growth in the Z direction, the three-dimensional growth of ZrN1-xCx crystals is achieved. Conclusion Polycrystalline ZrN1-xCx has been prepared by the carbothermal nitridation of consolidated ZrO2 ceramics in a N2 atmosphere. The formation of ZrN1-xCx is dependent on the reaction temperature, and ZrN1-xCx can be synthesized above 1600 °C. The carbothermal nitridation reaction is controlled by the infiltration and diffusion of gaseous reactants. ZrN1-xCx is produced at the surface of the sample and the center part remains ZrO2. This difference in reaction degree leads to a gradient porous structure. A strong (111) preferred orientation is observed in the ZrN1-xCx layers, which can be strengthened by raising the temperature and the pressure of N2. It is proposed that the growth of ZrN1-xCx crystals takes place by the two-dimensional nucleation and growth mechanism.

References (1) Toth, L. E. Transition Metal Carbides and Nitrides; Academic Press: New York, 1971.

568 Crystal Growth & Design, Vol. 9, No. 1, 2009 (2) Fix, R.; Gordon, R. G.; Hoffman, D. M. Chem. Mater. 1991, 3, 1138– 1148. (3) Yanagisawa, H.; Shinkai, S.; Sasaki, K.; Sakurai, J.; Abe, Y.; Sakai, A.; Zaima, S. J. Cryst. Growth 2006, 297, 80–86. (4) Larijani, M.; Tabrizi, N.; Norouzian, S.; Jafari, A.; Lahouti, S.; Hosseini, H.; Afshari, N. Vacuum 2006, 81, 550–555. (5) Pichon, L.; Straboni, A.; Girardeau, T.; Drouet, M. J. Appl. Phys. 2000, 87, 925. (6) Huang, J.; Ho, C.; Yu, G. Mater. Chem. Phys. 2007, 102, 31–38. (7) Niu, E.; Li, L.; Lv, G.; Chen, H.; Feng, W.; Fan, S.; Yang, S.; Yang, X. Mater. Sci. Eng., A 2007, 460-461, 135–139. (8) Ma, C.; Huang, J.; Chen, H. Surf. Coat. Technol. 2000, 133-134, 289–294. (9) Rizzo, A.; Signore, M. A.; Riccardis, M. F.; Capodieci, L.; Dimaio, D.; Nocco, T. Thin Solid Films 2007, 515, 6665–6671.

Liu et al. (10) Lerch, M.; Wrba, J. J. Mater. Sci. Lett. 1996, 15, 378–380. (11) Yamamura, H.; Emoto, S.; Mori, T. J. Ceram. Soc. Jpn. 1998, 106, 650–653. (12) Fu, B.; Gao, L. J. Am. Ceram. Soc. 2004, 87, 696–698. (13) Cheng, L. Z.; Zhang, Y. H. Physical Chemistry (in Chinese); Shanghai Science and Technology Publishing House: Shanghai, China, 1997. (14) Abriata, J.; Garces, J.; Versaci, R. Bull. Alloy Phase Diagrams 1986, 7, 116–124. (15) Lamas, D. G.; Reca, N. E. J. Mater. Sci. 2000, 35, 5563–5567. (16) Yashima, M.; Kakihana, M.; Yoshimura, M. Solid State Ionics 1996, 86-88, 1131–1149. (17) Cheng, Y.; Thompson, D. P. J. Am. Ceram. Soc. 1991, 74, 1135–1138.

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