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Low temperature synthesis of nanocrystalline titanium nitride from a single-source precursor of titanium and nitrogen
Journal of Alloys and Compounds 486 (2009) 223–226
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Low temperature synthesis of nanocrystalline titanium nitride from a single-source precursor of titanium and nitrogen Meining Wu ∗ Oujiang College, Wenzhou University, Wenzhou, Zhejiang 325027, PR China
a r t i c l e
i n f o
Article history: Received 9 February 2009 Received in revised form 9 July 2009 Accepted 10 July 2009 Available online 18 July 2009 Keywords: Nanostructured materials Titanium nitride Chemical synthesis X-ray diffraction Thermal analysis
a b s t r a c t Nanocrystalline titanium nitride has been prepared via a convenient route from a single-source precursor of titanium and nitrogen (ammonium fluotitanate) in an autoclave at 650 ◦ C. X-ray powder diffraction patterns indicate that the product is cubic titanium nitride, and the cell constant is a = 4.235 Å. Transmission electron microscopy image shows that it consists of particles with an average size of about 40 nm in diameter. The product was also studied by BET and TGA. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Titanium nitride (TiN) is an important material due to its many superior properties, such as superior high-temperature strength, excellent corrosion and wear resistance, high melting point, extreme hardness, high chemical and thermal stability, and high electrical and thermal conductivity and gold color [1]. These attractive properties make it to be used as hard, protective coatings for cutting tools [2], as diffusion barriers in microelectronics [3], as crucibles in metal smelting, as an optical coating [4] and as a goldcolored surface for jewelry [5]. Because of the promising properties and extensive applications of TiN, it is meaningful to synthesize nanocrystalline TiN in a simple route at low temperature and with convenient manipulations. Conventionally, TiN has been synthesized by the reaction of titanium tetrachloride with ammonia, the direct nitridation of titanium metal or metal hydride with nitrogen [6,7], the reaction of nanosized titania nitrified in flowing ammonia gas [8], combustion synthesis [9], and solid state metathesis (SSM) routes [10]. However, most of these reactions involve processing temperatures higher than 1000 ◦ C and for extended time periods. Besides these methods, many other routes have been developed, such as mechanical milling [11], sol–gel process [12], solvothermal synthesis [13], metal-catalyzed reduction–nitridation (MCRN) route [14].
∗ Tel.: +86 577 88201581; fax: +86 577 86689508. E-mail address: [email protected]. 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.061
In this paper, we have developed a convenient route from a single-source precursor of titanium and nitrogen to synthesize nanocrystalline titanium nitride at low temperature. Nanocrystalline TiN has been synthesized by the reaction of metallic sodium with ammonium fluotitanate in an autoclave at 650 ◦ C. Compared to the previous methods, this simple synthesis route has a unique advantage, i.e., both titanium and nitrogen sources from a single cheap safe precursor (NH4 )2 TiF6 . And metallic sodium is used as a reductant in the synthesis process. The whole synthesis route is carried out in the sealed autoclave. Because the raw materials are very facile and inexpensive, and the synthesis temperature is relatively low, this synthesis route to nanocrystalline TiN may be developed into mass production. 2. Experimental In this research, all the manipulations were carried out in a dry glove box with flowing nitrogen gas. In a typical experiment, 0.010 mol (about 1.980 g) ammonium fluotitanate and 0.070 mol (about 1.609 g) metallic sodium were put into a stainless steel autoclave which had a capacity of some 50 ml. After sealing under argon atmosphere, the autoclave was heated from room temperature to 650 ◦ C within 1 h, and maintained at 650 ◦ C for 10 h, followed by cooling gradually to room temperature in the furnace. Under the help of ultrasound cleaner, the obtained product from the autoclave was washed with absolute ethanol to react with the excessive unreacted metallic sodium. Then it was washed several times with dilute HCl aqueous solution, and distilled water to remove impurities. Finally the product was washed with absolute ethanol to remove water. The final product was vacuum-dried at 60 ◦ C for 12 h. Black powders were obtained. The obtained sample was analyzed by powder X-ray diffraction (XRD) on a Rigaku Dmax-␥A X-ray diffractometer using Cu K␣ radiation (wavelength = 1.54178 Å). The operating voltage is 40 kV. 2 angle is from 25◦ to 85◦ . The morphology of TiN was observed from transmission electron microscopy (TEM)
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M. Wu / Journal of Alloys and Compounds 486 (2009) 223–226
Fig. 1. XRD patterns of (A) the sample and (B) TiN (JCPDS Card No. 38-1420).
images taken with a Hitachi H-800 transmission electron microscope. X-ray photoelectron spectra (XPS) were recorded on a VGESCALAB MKII X-ray photoelectron spectrometer, using non-monochromatized Mg K␣ X-rays as the excitation source. The specific surface area of the sample was measured by BET method (Model ASAP 2000, Micromeritics, Norcross, GA). N2 is used as the absorptive gas. The average diameter of the powders (specific surface diameter) was estimated using the specific surface area. The thermogravimetric analysis was performed on a thermal analyzer (Model: Q600) from room temperature to 1000 ◦ C in flowing air at a rate of 10 ◦ C min−1 to study its thermal stability and oxidation behavior.
Fig. 2. TEM image and SED pattern (inserted) of the prepared TiN sample.
3. Results and discussion The composition and crystallinity of the prepared TiN sample are examined by XRD. The XRD patterns of the obtained products and TiN (JCPDS Card No. 38-1420) are shown in Fig. 1. There are five obvious diffraction peaks in the pattern A. And all these diffraction peaks ((1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2)) at different d-space can be indexed as cubic titanium nitride. The refinement gives the cell constants, a = 4.235 Å, which is consistent with the value reported in the literature (a = 4.241 Å) (JCPDS Card No. 38-1420), shown in the pattern B. No obvious evidences of impurities such as metallic titanium, titanium oxides, other titanium nitrides, can be found in this XRD pattern. The morphology of the prepared titanium nitride sample was investigated by transmission electron microscopy. The TEM image of the as-prepared TiN is shown in Fig. 2. In this figure, the sample shows that it consists of particles in the range of 30–50 nm in diameter. These particles exhibit slightly agglomerated particle morphology due to the ultrafine size of the sample. Fig. 2 also shows the selected area electron diffraction (SED) pattern (inserted) of crystalline TiN. And the SED pattern can also confirm the crystallinity of TiN, in which the diffraction rings diameters and intensities again correspond well to the cubic titanium nitride. These rings can be indexed as (1 1 1), (2 0 0) and (2 2 0). Further evidence for the formation of titanium nitride can be obtained from the XPS of the sample. Fig. 3 shows the results of the XPS spectra of the sample. They show that the sample surface consists of nitrogen, titanium, carbon and oxygen. The C1s and O1s peaks indicate that there exists a small amount of impurity elements such as C and O due to the adsorption of CO2 , H2 O and O2 on the surface of the sample or due to the surface oxidation. The N1s peak at 396.7 eV, Ti2p3/2 and Ti2p1/2 peaks at 455.6 and 460.9 eV indicate titanium nitride. Quantitative analysis gives the Ti:N molar ratio as 1:0.96, which closely agrees with the stoichiometric composition of TiN.
We studied the particle size of the sample by measuring the specific surface area of the powders with BET method. In our result, the sample has a value of 21.2 m2 /g. So the average diameter of the powders (specific surface diameter) is estimated according to the equation (d = 6/·S, where d is the estimated average diameter of the powders, is the density of the sample and S is the specific surface area of the powders) using the specific surface area on the assumption that the powder shape is spherical and the density of the powder is 5.22 g cm−3 . The estimated average diameter of the powders is about 54 nm which is much larger than the value observed from the TEM image, suggesting the slightly agglomeration of primary particles. The thermal stability of these nanosized TiN powders was investigated in flowing air under different temperatures. A typical TGA/DTA profile studied at temperatures below 1000 ◦ C in the flowing air is shown in Fig. 4. From the TGA curve, we can find that the weight gain of the sample has not changed significantly below 400 ◦ C. The onset of the oxidation of the TiN sample is found to begin at about 400 ◦ C due to the increase of the weight. So, the product has good thermal stability and oxidation resistance below 400 ◦ C. We know that titanium nitride (bulk material) is recognized as the potential candidate for high-temperature structural applications. But in this paper, the size of the prepared TiN is very small. Nanoparticles have huge specific surface areas. Therefore they also have large surface energy, which results in agglomeration. When some adjacent primary particles collide, they attach to each other and combine into a secondary one. Since the synthesis temperature is much lower (650 ◦ C), it is not easy for crystallite to grow bigger, and the size of TiN prepared via this route tends to be nanosized. But these crystallites can agglomerate together because of large surface energy, forming loosely structured particle. So, it can be oxidized by oxygen at relatively lower temperature than that of bulk material. But the sample prepared in this route has good thermal stability below 400 ◦ C.
M. Wu / Journal of Alloys and Compounds 486 (2009) 223–226
225
Fig. 4. TGA curve heated in flowing air for the sample.
As to the reaction mechanism, we consider that at the reaction temperature, (NH4 )2 TiF6 can decompose generating NH3 (gas), HF (gas) and TiF4 (gas). (NH4 )2 TiF6 = 2NH3 + 2HF + TiF4
(1)
So the pressure in the autoclave will be very large. At the reaction temperature, the freshly produced TiF4 , NH3 and metallic sodium could react with each other to produce TiN. 2NH3 + 2TiF4 + 8Na = 2TiN + 8NaF + 3H2
(2)
The high pressure in the autoclave would be helpful for reducing the reaction temperature and enhancing the reaction speed. The produced HF gas was strongly absorbed by the metallic sodium to form NaF and H2 . 2HF + 2Na = 2NaF + H2
(3)
Therefore, the total possible reaction might be expressed as follows: 2(NH4 )2 TiF6 + 12Na = 2TiN + 12NaF + 2NH3 + 5H2
(4)
The formation of cubic NaF was confirmed by the XRD analysis of the intermediate product (omitted here), which can be removed by washing with distilled water at all. In the process, the use of metallic Na is in favor of forming TiN at lower temperature, because the reaction of HF with Na is thermodynamically spontaneous and exothermic and the reaction heat could produce higher local reaction temperatures. 4. Conclusion
Fig. 3. XPS spectra of the TiN sample.
In summary, nanocrystalline TiN has been prepared via a convenient route by the reaction of metallic sodium with ammonium fluotitanate in an autoclave at 650 ◦ C. The product has the cubic titanium nitride structure. It consists of particles with an average size of about 40 nm. The product has good thermal stability and oxidation resistance below 400 ◦ C. This simple chemical synthesis route maybe provides a new method to prepare nanocrystalline titanium nitride at low temperature. Acknowledgment This work was supported by Department of Education of Zhejiang Province of China under Grant No. 20070546. Thanks are due to Dr. Jianhua Ma for his useful technical help and discussions.
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M. Wu / Journal of Alloys and Compounds 486 (2009) 223–226
References [1] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. [2] R. Buhl, H.K. Pulker, E. Moll, Thin Solid Films 80 (1981) 265–270. [3] G.I. Grigorov, K.G. Grigorov, M. Stoyanova, J.L. Vignes, J.P. Langeron, P. Denjean, Appl. Phys. A57 (1993) 195–197. [4] E. Velkonen, T. Karlsson, B. Karlsson, B.O. Johansson, Proc. SPIE 1983 Int. Conf. 401 (1983) 41. [5] B. Zega, M. Kornmann, J. Amiguet, Thin Solid Films 45 (1977) 577–582. [6] Z.A. Munir, J.B. Holt, J. Mater. Sci. 22 (1987) 710–714. [7] M. Eslamloo-Grami, Z.A. Munir, J. Am. Ceram. Soc. 73 (1990) 2222–2227.
[8] S. Kawano, J. Takahashi, S. Shimada, J. Am. Ceram. Soc. 86 (2003) 1609– 1611. [9] A.G. Merzhanov, Combustion and Plasma Synthesis of High-Temperature Materials, VCH Publishers, New York, 1990. [10] E.G. Gillan, R.B. Kaner, Inorg. Chem. 33 (1994) 5693–5700. [11] M.S. Eleskandarany, K. Sumiyama, K. Aoki, K. Suzuki, J. Mater. Res. 7 (1992) 888–893. [12] I.S. Kim, P.N. Kumta, J. Mater. Chem. 13 (2003) 2028–2035. [13] J.Q. Hu, Q.Y. Lu, K.B. Tang, S.H. Yu, Y.T. Qian, G.E. Zhou, X.M. Liu, J. Am. Ceram. Soc. 83 (2000) 430–432. [14] X.C. Ma, Z.D. Zhang, X.B. Li, S.T. Wang, Y. Du, F. Xu, Y.T. Qian, Chem. Lett. 33 (2004) 746–747.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Low temperature synthesis of nanocrystalline titanium nitride from a single-source precursor of titanium and nitrogen Meining Wu ∗ Oujiang College, Wenzhou University, Wenzhou, Zhejiang 325027, PR China
a r t i c l e
i n f o
Article history: Received 9 February 2009 Received in revised form 9 July 2009 Accepted 10 July 2009 Available online 18 July 2009 Keywords: Nanostructured materials Titanium nitride Chemical synthesis X-ray diffraction Thermal analysis
a b s t r a c t Nanocrystalline titanium nitride has been prepared via a convenient route from a single-source precursor of titanium and nitrogen (ammonium fluotitanate) in an autoclave at 650 ◦ C. X-ray powder diffraction patterns indicate that the product is cubic titanium nitride, and the cell constant is a = 4.235 Å. Transmission electron microscopy image shows that it consists of particles with an average size of about 40 nm in diameter. The product was also studied by BET and TGA. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Titanium nitride (TiN) is an important material due to its many superior properties, such as superior high-temperature strength, excellent corrosion and wear resistance, high melting point, extreme hardness, high chemical and thermal stability, and high electrical and thermal conductivity and gold color [1]. These attractive properties make it to be used as hard, protective coatings for cutting tools [2], as diffusion barriers in microelectronics [3], as crucibles in metal smelting, as an optical coating [4] and as a goldcolored surface for jewelry [5]. Because of the promising properties and extensive applications of TiN, it is meaningful to synthesize nanocrystalline TiN in a simple route at low temperature and with convenient manipulations. Conventionally, TiN has been synthesized by the reaction of titanium tetrachloride with ammonia, the direct nitridation of titanium metal or metal hydride with nitrogen [6,7], the reaction of nanosized titania nitrified in flowing ammonia gas [8], combustion synthesis [9], and solid state metathesis (SSM) routes [10]. However, most of these reactions involve processing temperatures higher than 1000 ◦ C and for extended time periods. Besides these methods, many other routes have been developed, such as mechanical milling [11], sol–gel process [12], solvothermal synthesis [13], metal-catalyzed reduction–nitridation (MCRN) route [14].
∗ Tel.: +86 577 88201581; fax: +86 577 86689508. E-mail address: [email protected]. 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.061
In this paper, we have developed a convenient route from a single-source precursor of titanium and nitrogen to synthesize nanocrystalline titanium nitride at low temperature. Nanocrystalline TiN has been synthesized by the reaction of metallic sodium with ammonium fluotitanate in an autoclave at 650 ◦ C. Compared to the previous methods, this simple synthesis route has a unique advantage, i.e., both titanium and nitrogen sources from a single cheap safe precursor (NH4 )2 TiF6 . And metallic sodium is used as a reductant in the synthesis process. The whole synthesis route is carried out in the sealed autoclave. Because the raw materials are very facile and inexpensive, and the synthesis temperature is relatively low, this synthesis route to nanocrystalline TiN may be developed into mass production. 2. Experimental In this research, all the manipulations were carried out in a dry glove box with flowing nitrogen gas. In a typical experiment, 0.010 mol (about 1.980 g) ammonium fluotitanate and 0.070 mol (about 1.609 g) metallic sodium were put into a stainless steel autoclave which had a capacity of some 50 ml. After sealing under argon atmosphere, the autoclave was heated from room temperature to 650 ◦ C within 1 h, and maintained at 650 ◦ C for 10 h, followed by cooling gradually to room temperature in the furnace. Under the help of ultrasound cleaner, the obtained product from the autoclave was washed with absolute ethanol to react with the excessive unreacted metallic sodium. Then it was washed several times with dilute HCl aqueous solution, and distilled water to remove impurities. Finally the product was washed with absolute ethanol to remove water. The final product was vacuum-dried at 60 ◦ C for 12 h. Black powders were obtained. The obtained sample was analyzed by powder X-ray diffraction (XRD) on a Rigaku Dmax-␥A X-ray diffractometer using Cu K␣ radiation (wavelength = 1.54178 Å). The operating voltage is 40 kV. 2 angle is from 25◦ to 85◦ . The morphology of TiN was observed from transmission electron microscopy (TEM)
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M. Wu / Journal of Alloys and Compounds 486 (2009) 223–226
Fig. 1. XRD patterns of (A) the sample and (B) TiN (JCPDS Card No. 38-1420).
images taken with a Hitachi H-800 transmission electron microscope. X-ray photoelectron spectra (XPS) were recorded on a VGESCALAB MKII X-ray photoelectron spectrometer, using non-monochromatized Mg K␣ X-rays as the excitation source. The specific surface area of the sample was measured by BET method (Model ASAP 2000, Micromeritics, Norcross, GA). N2 is used as the absorptive gas. The average diameter of the powders (specific surface diameter) was estimated using the specific surface area. The thermogravimetric analysis was performed on a thermal analyzer (Model: Q600) from room temperature to 1000 ◦ C in flowing air at a rate of 10 ◦ C min−1 to study its thermal stability and oxidation behavior.
Fig. 2. TEM image and SED pattern (inserted) of the prepared TiN sample.
3. Results and discussion The composition and crystallinity of the prepared TiN sample are examined by XRD. The XRD patterns of the obtained products and TiN (JCPDS Card No. 38-1420) are shown in Fig. 1. There are five obvious diffraction peaks in the pattern A. And all these diffraction peaks ((1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2)) at different d-space can be indexed as cubic titanium nitride. The refinement gives the cell constants, a = 4.235 Å, which is consistent with the value reported in the literature (a = 4.241 Å) (JCPDS Card No. 38-1420), shown in the pattern B. No obvious evidences of impurities such as metallic titanium, titanium oxides, other titanium nitrides, can be found in this XRD pattern. The morphology of the prepared titanium nitride sample was investigated by transmission electron microscopy. The TEM image of the as-prepared TiN is shown in Fig. 2. In this figure, the sample shows that it consists of particles in the range of 30–50 nm in diameter. These particles exhibit slightly agglomerated particle morphology due to the ultrafine size of the sample. Fig. 2 also shows the selected area electron diffraction (SED) pattern (inserted) of crystalline TiN. And the SED pattern can also confirm the crystallinity of TiN, in which the diffraction rings diameters and intensities again correspond well to the cubic titanium nitride. These rings can be indexed as (1 1 1), (2 0 0) and (2 2 0). Further evidence for the formation of titanium nitride can be obtained from the XPS of the sample. Fig. 3 shows the results of the XPS spectra of the sample. They show that the sample surface consists of nitrogen, titanium, carbon and oxygen. The C1s and O1s peaks indicate that there exists a small amount of impurity elements such as C and O due to the adsorption of CO2 , H2 O and O2 on the surface of the sample or due to the surface oxidation. The N1s peak at 396.7 eV, Ti2p3/2 and Ti2p1/2 peaks at 455.6 and 460.9 eV indicate titanium nitride. Quantitative analysis gives the Ti:N molar ratio as 1:0.96, which closely agrees with the stoichiometric composition of TiN.
We studied the particle size of the sample by measuring the specific surface area of the powders with BET method. In our result, the sample has a value of 21.2 m2 /g. So the average diameter of the powders (specific surface diameter) is estimated according to the equation (d = 6/·S, where d is the estimated average diameter of the powders, is the density of the sample and S is the specific surface area of the powders) using the specific surface area on the assumption that the powder shape is spherical and the density of the powder is 5.22 g cm−3 . The estimated average diameter of the powders is about 54 nm which is much larger than the value observed from the TEM image, suggesting the slightly agglomeration of primary particles. The thermal stability of these nanosized TiN powders was investigated in flowing air under different temperatures. A typical TGA/DTA profile studied at temperatures below 1000 ◦ C in the flowing air is shown in Fig. 4. From the TGA curve, we can find that the weight gain of the sample has not changed significantly below 400 ◦ C. The onset of the oxidation of the TiN sample is found to begin at about 400 ◦ C due to the increase of the weight. So, the product has good thermal stability and oxidation resistance below 400 ◦ C. We know that titanium nitride (bulk material) is recognized as the potential candidate for high-temperature structural applications. But in this paper, the size of the prepared TiN is very small. Nanoparticles have huge specific surface areas. Therefore they also have large surface energy, which results in agglomeration. When some adjacent primary particles collide, they attach to each other and combine into a secondary one. Since the synthesis temperature is much lower (650 ◦ C), it is not easy for crystallite to grow bigger, and the size of TiN prepared via this route tends to be nanosized. But these crystallites can agglomerate together because of large surface energy, forming loosely structured particle. So, it can be oxidized by oxygen at relatively lower temperature than that of bulk material. But the sample prepared in this route has good thermal stability below 400 ◦ C.
M. Wu / Journal of Alloys and Compounds 486 (2009) 223–226
225
Fig. 4. TGA curve heated in flowing air for the sample.
As to the reaction mechanism, we consider that at the reaction temperature, (NH4 )2 TiF6 can decompose generating NH3 (gas), HF (gas) and TiF4 (gas). (NH4 )2 TiF6 = 2NH3 + 2HF + TiF4
(1)
So the pressure in the autoclave will be very large. At the reaction temperature, the freshly produced TiF4 , NH3 and metallic sodium could react with each other to produce TiN. 2NH3 + 2TiF4 + 8Na = 2TiN + 8NaF + 3H2
(2)
The high pressure in the autoclave would be helpful for reducing the reaction temperature and enhancing the reaction speed. The produced HF gas was strongly absorbed by the metallic sodium to form NaF and H2 . 2HF + 2Na = 2NaF + H2
(3)
Therefore, the total possible reaction might be expressed as follows: 2(NH4 )2 TiF6 + 12Na = 2TiN + 12NaF + 2NH3 + 5H2
(4)
The formation of cubic NaF was confirmed by the XRD analysis of the intermediate product (omitted here), which can be removed by washing with distilled water at all. In the process, the use of metallic Na is in favor of forming TiN at lower temperature, because the reaction of HF with Na is thermodynamically spontaneous and exothermic and the reaction heat could produce higher local reaction temperatures. 4. Conclusion
Fig. 3. XPS spectra of the TiN sample.
In summary, nanocrystalline TiN has been prepared via a convenient route by the reaction of metallic sodium with ammonium fluotitanate in an autoclave at 650 ◦ C. The product has the cubic titanium nitride structure. It consists of particles with an average size of about 40 nm. The product has good thermal stability and oxidation resistance below 400 ◦ C. This simple chemical synthesis route maybe provides a new method to prepare nanocrystalline titanium nitride at low temperature. Acknowledgment This work was supported by Department of Education of Zhejiang Province of China under Grant No. 20070546. Thanks are due to Dr. Jianhua Ma for his useful technical help and discussions.
226
M. Wu / Journal of Alloys and Compounds 486 (2009) 223–226
References [1] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. [2] R. Buhl, H.K. Pulker, E. Moll, Thin Solid Films 80 (1981) 265–270. [3] G.I. Grigorov, K.G. Grigorov, M. Stoyanova, J.L. Vignes, J.P. Langeron, P. Denjean, Appl. Phys. A57 (1993) 195–197. [4] E. Velkonen, T. Karlsson, B. Karlsson, B.O. Johansson, Proc. SPIE 1983 Int. Conf. 401 (1983) 41. [5] B. Zega, M. Kornmann, J. Amiguet, Thin Solid Films 45 (1977) 577–582. [6] Z.A. Munir, J.B. Holt, J. Mater. Sci. 22 (1987) 710–714. [7] M. Eslamloo-Grami, Z.A. Munir, J. Am. Ceram. Soc. 73 (1990) 2222–2227.
[8] S. Kawano, J. Takahashi, S. Shimada, J. Am. Ceram. Soc. 86 (2003) 1609– 1611. [9] A.G. Merzhanov, Combustion and Plasma Synthesis of High-Temperature Materials, VCH Publishers, New York, 1990. [10] E.G. Gillan, R.B. Kaner, Inorg. Chem. 33 (1994) 5693–5700. [11] M.S. Eleskandarany, K. Sumiyama, K. Aoki, K. Suzuki, J. Mater. Res. 7 (1992) 888–893. [12] I.S. Kim, P.N. Kumta, J. Mater. Chem. 13 (2003) 2028–2035. [13] J.Q. Hu, Q.Y. Lu, K.B. Tang, S.H. Yu, Y.T. Qian, G.E. Zhou, X.M. Liu, J. Am. Ceram. Soc. 83 (2000) 430–432. [14] X.C. Ma, Z.D. Zhang, X.B. Li, S.T. Wang, Y. Du, F. Xu, Y.T. Qian, Chem. Lett. 33 (2004) 746–747.