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Low-temperature synthesis of boron carbide powder from condensed boric acid–glycerin product
Materials Letters 65 (2011) 1839–1841
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Low-temperature synthesis of boron carbide powder from condensed boric acid–glycerin product Masaki Kakiage ⁎, Naoki Tahara, Ikuo Yanase, Hidehiko Kobayashi Department of Applied Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan
a r t i c l e
i n f o
Article history: Received 22 October 2010 Accepted 13 March 2011 Available online 17 March 2011 Keywords: Boron carbide (B4C) Glycerin Boric acid Precursor Low-temperature synthesis
a b s t r a c t Crystalline boron carbide (B4C) powder was synthesized by the carbothermal reduction of a condensed product formed from boric acid (H3BO3) and glycerin (C3H8O3). The condensed product was prepared by dehydration after directly mixing equimolar amounts of H3BO3 and glycerin, which was followed by pyrolysis in air to obtain a precursor powder from which the excess carbon had been eliminated. The prepared precursor powder had a bicontinuous boron oxide (B2O3)/carbon network structure. Crystalline B4C powder without residual carbon was successfully synthesized from this precursor powder by heating at 1250 °C for 5 h in an Ar flow. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Boron carbide (B4C) is an important nonoxide ceramic with attractive properties (low specific weight, excellent hardness, and high chemical stability), which is used as a cutting tool, an abrasive compound, and for sandblasting [1–3]. Moreover, B4C is utilized as a neutron-absorbing material because of its high boron content [1,2]. B4C powder is synthesized by several methods [1,4]; by a solidstate reaction from boron and carbon elements [5], by co-reduction in an autoclave using BBr3 and CCl4 as the reactants and metallic Na as the co-reductant [6], by the magnesiothermic reduction of boron oxide (B2O3) in the presence of carbon [7], by laser irradiation of boron nanoparticles dispersed in an organic solvent [8], and by the carbothermal reduction of B2O3 by graphite or petroleum coke [2]. Among these methods, the carbothermal reduction, for which the overall reaction is given by Eq. (1), is the commercial method for producing B4C owing to the low cost of the raw materials, and the process is performed at a high temperature of approximately 2000 °C. 2B2 O3 + 7 C→B4 C + 6CO
ð1Þ
However, this process has some problems, for example, the product contains a free-carbon residue and the process requires a high synthesis temperature. The carbothermal reduction of organic precursors with borate ester (B–O–C) bonds, for which polyols such as cellulose [9] and citric acid [10,11] are used as a carbon source, has recently been investigated as a ⁎ Corresponding author. E-mail address: [email protected] (M. Kakiage). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.03.046
low-temperature synthesis method for crystalline B4C powder. Wada et al. [12] reported that B4C was synthesized from a boric acid (H3BO3)glycerin precursor by heating at a relatively low temperature of 1300 °C for 4 h, but the product contained a large amount of residual carbon, which is a common disadvantage of B4C synthesis using organic precursors. We overcame this disadvantage by performing a pyrolysis process in air before the carbothermal reduction process: B4C crystalline powder containing little free carbon was synthesized from a condensed H3BO3-poly(vinyl alcohol) product, which was pyrolyzed at 600 °C for 2 h in air, followed by heat treatment at 1300 °C for 5 h in an Ar flow [13]. In this study, we attempt to further reduce the synthesis temperature of crystalline B4C powder without residual carbon from a condensed H3BO3–glycerin product that is based on both borate ester bond formation and a pyrolysis process in air. 2. Experimental 2.1. Synthesis of B4C powder H3BO3 (99.5%) and glycerin (C3H8O3, 99.0%), used as starting materials, were purchased from Wako Pure Chemical Industries, Ltd., Japan. These materials were used as received. A condensed product was prepared by dehydration at 150 °C after directly mixing equimolar amounts of H3BO3 and glycerin. The resulting product was a transparent glassy solid. This condensed product was placed in an alumina crucible and heated in air at 250 °C for 2 h then at 350 °C for 2 h, which was followed by pyrolysis in air at 450–650 °C for 2 h to obtain a precursor powder from which the excess carbon had been eliminated. The above steps were consecutively performed using a temperature-
1840
M. Kakiage et al. / Materials Letters 65 (2011) 1839–1841
4.5
programmed muffle furnace. The obtained black precursor powder was placed in a graphite boat after grinding with an agate mortar and heated at 1100–1300 °C for 5 h in an Ar flow (200 ml/min) at a heating rate of 10 °C/min.
Fourier transform infrared (FT-IR) spectra of the starting materials and condensed product were measured using a Shimadzu IRPrestige-21 spectrometer. The measurements were performed by a transmission method using KBr pellets, each containing a sample. X-ray diffraction (XRD) measurements were performed using a powder X-ray diffractometer (Rigaku RAD-C) with monochromatized CuKα radiation. Scanning electron microscopy (SEM) observation of the precursor powder was conducted with a Hitachi S-4100 field-emission scanning electron microscope operated at 15.0 kV. A B2O3 content in the precursor powder was determined as H3BO3 by the titration of mannitol– H3BO3 complex with sodium hydroxide solution using phenolphthalein as an indicator. The B2O3 component was recovered from the precursor powder by washing in hot water. 3. Results and discussion The formation of borate ester (B–O–C) bonds in a condensed product, which is a characteristic feature of the synthesis of B4C from an organic precursor, leads to the homogeneous blending of H3BO3 and glycerin, i.e., the boron and carbon sources, at the microscale. The bonding state of the prepared condensed product was first investigated using FT-IR measurements. Fig. 1 shows the FT-IR spectra of H3BO3, glycerin, and the condensed product. The absorption peak at 1455 cm− 1 for H3BO3 (Fig. 1(a)) was assigned to the B–O stretching vibration mode [14] and that at 1041 cm− 1 for glycerin (Fig. 1(b)) is the C–O stretching vibration mode; these peaks were also observed for the condensed product (Fig. 1(c)). Note that O–H stretching bands are clearly observed at approximately 3300 cm− 1 for the raw materials, H3BO3 and glycerin (Fig. 1(a) and (b)), whereas they are relatively weaker for the condensed product (Fig. 1(c)). This means that B–O–C bonds are formed in the prepared condensed product across the whole sample, thus the mixing of equimolar amounts of H3BO3 and glycerin is suitable for the preparation of the homogeneous condensed product.
1455 νB-O
3.5
3.0
2.5
2.0 400
450
500
550
600
700
Fig. 2. Change in C/B2O3 ratio of precursor powders prepared by pyrolysis at 450–650 °C for 2 h in air. The B2O3 component was recovered from the precursor powder by washing in hot water and determined as H3BO3 by the titration. The dotted line indicates the stoichiometric C/B2O3 ratio of 3.5.
The carbon content in the condensed product prepared from suitable equimolar mixing (C/B2O3 = 6.0) is obviously higher than that required for the carbothermal reduction process given by Eq. (1) (C/B2O3 = 3.5). Therefore, we performed a pyrolysis process in air before carbothermal reduction, which was effective for eliminating excess carbon [13]. Change in the C/B2O3 ratio, estimated from the titration, of the precursor powders prepared by pyrolysis at 450– 650 °C for 2 h in air is summarized in Fig. 2. The XRD pattern of the precursor powder exhibited B2O3 peaks and an amorphous halo of carbon regardless of pyrolysis temperature (not shown); all precursor powders consisted of B2O3 and amorphous carbon. The C/B2O3 ratio of the precursor powder was controlled by the pyrolysis temperature and the precursor powder with stoichiometric composition for the carbothermal reduction process (C/B2O3 = 3.5) was obtained at 550 °C. Consequently, the pyrolysis temperature at 550 °C should preferably be used owing to its stoichiometric C/B2O3 ratio. Fig. 3 shows the XRD patterns of products obtained by pyrolysis at 550 °C for 2 h in air and heat treatment at 1100–1300 °C for 5 h in an Ar flow. Peaks corresponding to a rhombohedral B4C crystal were recognized at 1150 °C and above, indicating the existence of crystalline B4C. Moreover, only B4C peaks appeared at 1250 °C, meaning that crystalline B4C powder without residual carbon can be synthesized at 1250 °C, which is the lowest reported synthesis
B4C B2O3 C
(b)
(a)
650
Pyrolysis Temperature / oC
Intensity / arb. unit
νC-O 1041
3350 νO-H νO-H 3208
Absorbance / arb. unit
3292 νO-H
(c)
C/B2O3 Ratio
2.2. Measurements
4.0
1300oC
1250oC 1200oC 1150oC 1100oC
10 4000
3000
2000
1000
400
20
30
40
50
60
70
80
2θ / °
Wavenumber /cm-1 Fig. 1. FT-IR spectra of (a) H3BO3, (b) glycerin, and (c) condensed product.
Fig. 3. XRD patterns of products obtained by heat treatment of precursor powder at 1100–1300 °C for 5 h in Ar flow. The precursor was pyrolyzed at 550 °C for 2 h in air.
M. Kakiage et al. / Materials Letters 65 (2011) 1839–1841
(a)
1 μm
(b)
1841
powder pyrolyzed at 550 °C for 2 h in air. The B2O3 component can be removed by washing the precursor powder in hot water, thus leaving the carbon component, as shown in Fig. 4(b), where the characteristic carbon network structure with nano-order spacing can be recognized. This distinctive structure is similar to macroporous materials with a three-dimensional bicontinuous morphology developed by a phase separation in a polymerization-derived sol–gel system [15], implying that a phase separation is induced by spinodal decomposition and the transitional bicontinuous structure of spinodal decomposition is fixed as a permanent morphology by carbonization during the pyrolysis process in air in this study. A bicontinuous B2O3/carbon network structure provides a markedly increased contact domain. This structure increases the reactivity and the number of nucleation sites of B4C, enabling a lowering of the synthesis temperature of B4C. Note that this bicontinuous network structure is generated in the condensed H3BO3–glycerin and maintained in the precursor powder, meaning that the stabilization process, i.e., the intermediate carbonization, is important. These results demonstrate that the lower synthesis temperature is due to the homogenization of the initial condensed product, which includes B– O–C bonds, and the subsequent intermediate carbonization at a suitable temperature. This approach is a promising methodology for the lowtemperature synthesis of B4C crystalline powder containing less residual carbon from an organic precursor using a carbothermal reduction process. 4. Conclusions
1 μm
B4C crystalline powder was synthesized from a condensed H3BO3– glycerin product by a carbothermal reduction process. Borate ester (B–O–C) bonds were formed in the condensed product. The formation of crystalline B4C started at 1150 °C, and crystalline B4C powder without residual carbon was synthesized by heating at 1250 °C, which is the lowest temperature reported for the synthesis of B4C, for 5 h.
Fig. 4. SEM images of precursor powder pyrolyzed at 550 °C for 2 h in air (a) before and (b) after removal of B2O3 by washing in hot water.
References
temperature of B4C using a carbothermal reduction process. The synthesized B4C crystalline powder consisted of equiaxed grains with a small particle size (D50 = 1.1 μm), which is smaller than those synthesized by general carbothermal reduction. These results demonstrate that a significant lowering of the synthesis temperature is achieved by the synthesis of B4C from the condensed H3BO3–glycerin product using this methodology. This lowering of the synthesis temperature of B4C in this study is explained in terms of the enlargement of the contact domain between B2O3 and carbon components and the carbonization (stabilization) by intermediate heating in air. Fig. 4(a) shows a SEM image of the precursor
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Thevenot F. J Eur Ceram Soc 1990;6:205–25. Jung CH, Lee MJ, Kim CJ. Mater Lett 2004;58:609–14. Alizadeh A, Taheri-Nassaj E, Ehsani N. J Eur Ceram Soc 2004;24:3227–34. Suri AK, Subramanian C, Sonber JK, Murthy TSRC. Int Mater Rev 2010;55:4–40. Benton ST, Masters DR. US Patent 3914371. Shi L, Gu Y, Chen L, Qian Y, Yang Z, Ma J. Solid State Commun 2003;128:5–7. Mohanty RM, Balasubramanian K, Seshadri SK. J Alloy Compd 2007;441:85–93. Ishikawa Y, Feng Q, Koshizaki N. Appl Phys A 2010;99:797–803. Sudoh A, Konno H, Habazaki H, Kiyono H. Tanso 2007;226:8–12. Sinha A, Mahata T, Sharma BP. J Nucl Mater 2002;301:165–9. Khanra AK. Bull Mater Sci 2007;30:93–6. Wada H, Kuroda K, Kato C. Yogyo Kyokai Shi 1986;94:71–5. Yanase I, Ogawara R, Kobayashi H. Mater Lett 2009;63:91–3. Pistorius CWFT. J Chem Phys 1959;31:1454–7. Nakanishi K. J Porous Mat 1997;4:67–112.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Low-temperature synthesis of boron carbide powder from condensed boric acid–glycerin product Masaki Kakiage ⁎, Naoki Tahara, Ikuo Yanase, Hidehiko Kobayashi Department of Applied Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan
a r t i c l e
i n f o
Article history: Received 22 October 2010 Accepted 13 March 2011 Available online 17 March 2011 Keywords: Boron carbide (B4C) Glycerin Boric acid Precursor Low-temperature synthesis
a b s t r a c t Crystalline boron carbide (B4C) powder was synthesized by the carbothermal reduction of a condensed product formed from boric acid (H3BO3) and glycerin (C3H8O3). The condensed product was prepared by dehydration after directly mixing equimolar amounts of H3BO3 and glycerin, which was followed by pyrolysis in air to obtain a precursor powder from which the excess carbon had been eliminated. The prepared precursor powder had a bicontinuous boron oxide (B2O3)/carbon network structure. Crystalline B4C powder without residual carbon was successfully synthesized from this precursor powder by heating at 1250 °C for 5 h in an Ar flow. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Boron carbide (B4C) is an important nonoxide ceramic with attractive properties (low specific weight, excellent hardness, and high chemical stability), which is used as a cutting tool, an abrasive compound, and for sandblasting [1–3]. Moreover, B4C is utilized as a neutron-absorbing material because of its high boron content [1,2]. B4C powder is synthesized by several methods [1,4]; by a solidstate reaction from boron and carbon elements [5], by co-reduction in an autoclave using BBr3 and CCl4 as the reactants and metallic Na as the co-reductant [6], by the magnesiothermic reduction of boron oxide (B2O3) in the presence of carbon [7], by laser irradiation of boron nanoparticles dispersed in an organic solvent [8], and by the carbothermal reduction of B2O3 by graphite or petroleum coke [2]. Among these methods, the carbothermal reduction, for which the overall reaction is given by Eq. (1), is the commercial method for producing B4C owing to the low cost of the raw materials, and the process is performed at a high temperature of approximately 2000 °C. 2B2 O3 + 7 C→B4 C + 6CO
ð1Þ
However, this process has some problems, for example, the product contains a free-carbon residue and the process requires a high synthesis temperature. The carbothermal reduction of organic precursors with borate ester (B–O–C) bonds, for which polyols such as cellulose [9] and citric acid [10,11] are used as a carbon source, has recently been investigated as a ⁎ Corresponding author. E-mail address: [email protected] (M. Kakiage). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.03.046
low-temperature synthesis method for crystalline B4C powder. Wada et al. [12] reported that B4C was synthesized from a boric acid (H3BO3)glycerin precursor by heating at a relatively low temperature of 1300 °C for 4 h, but the product contained a large amount of residual carbon, which is a common disadvantage of B4C synthesis using organic precursors. We overcame this disadvantage by performing a pyrolysis process in air before the carbothermal reduction process: B4C crystalline powder containing little free carbon was synthesized from a condensed H3BO3-poly(vinyl alcohol) product, which was pyrolyzed at 600 °C for 2 h in air, followed by heat treatment at 1300 °C for 5 h in an Ar flow [13]. In this study, we attempt to further reduce the synthesis temperature of crystalline B4C powder without residual carbon from a condensed H3BO3–glycerin product that is based on both borate ester bond formation and a pyrolysis process in air. 2. Experimental 2.1. Synthesis of B4C powder H3BO3 (99.5%) and glycerin (C3H8O3, 99.0%), used as starting materials, were purchased from Wako Pure Chemical Industries, Ltd., Japan. These materials were used as received. A condensed product was prepared by dehydration at 150 °C after directly mixing equimolar amounts of H3BO3 and glycerin. The resulting product was a transparent glassy solid. This condensed product was placed in an alumina crucible and heated in air at 250 °C for 2 h then at 350 °C for 2 h, which was followed by pyrolysis in air at 450–650 °C for 2 h to obtain a precursor powder from which the excess carbon had been eliminated. The above steps were consecutively performed using a temperature-
1840
M. Kakiage et al. / Materials Letters 65 (2011) 1839–1841
4.5
programmed muffle furnace. The obtained black precursor powder was placed in a graphite boat after grinding with an agate mortar and heated at 1100–1300 °C for 5 h in an Ar flow (200 ml/min) at a heating rate of 10 °C/min.
Fourier transform infrared (FT-IR) spectra of the starting materials and condensed product were measured using a Shimadzu IRPrestige-21 spectrometer. The measurements were performed by a transmission method using KBr pellets, each containing a sample. X-ray diffraction (XRD) measurements were performed using a powder X-ray diffractometer (Rigaku RAD-C) with monochromatized CuKα radiation. Scanning electron microscopy (SEM) observation of the precursor powder was conducted with a Hitachi S-4100 field-emission scanning electron microscope operated at 15.0 kV. A B2O3 content in the precursor powder was determined as H3BO3 by the titration of mannitol– H3BO3 complex with sodium hydroxide solution using phenolphthalein as an indicator. The B2O3 component was recovered from the precursor powder by washing in hot water. 3. Results and discussion The formation of borate ester (B–O–C) bonds in a condensed product, which is a characteristic feature of the synthesis of B4C from an organic precursor, leads to the homogeneous blending of H3BO3 and glycerin, i.e., the boron and carbon sources, at the microscale. The bonding state of the prepared condensed product was first investigated using FT-IR measurements. Fig. 1 shows the FT-IR spectra of H3BO3, glycerin, and the condensed product. The absorption peak at 1455 cm− 1 for H3BO3 (Fig. 1(a)) was assigned to the B–O stretching vibration mode [14] and that at 1041 cm− 1 for glycerin (Fig. 1(b)) is the C–O stretching vibration mode; these peaks were also observed for the condensed product (Fig. 1(c)). Note that O–H stretching bands are clearly observed at approximately 3300 cm− 1 for the raw materials, H3BO3 and glycerin (Fig. 1(a) and (b)), whereas they are relatively weaker for the condensed product (Fig. 1(c)). This means that B–O–C bonds are formed in the prepared condensed product across the whole sample, thus the mixing of equimolar amounts of H3BO3 and glycerin is suitable for the preparation of the homogeneous condensed product.
1455 νB-O
3.5
3.0
2.5
2.0 400
450
500
550
600
700
Fig. 2. Change in C/B2O3 ratio of precursor powders prepared by pyrolysis at 450–650 °C for 2 h in air. The B2O3 component was recovered from the precursor powder by washing in hot water and determined as H3BO3 by the titration. The dotted line indicates the stoichiometric C/B2O3 ratio of 3.5.
The carbon content in the condensed product prepared from suitable equimolar mixing (C/B2O3 = 6.0) is obviously higher than that required for the carbothermal reduction process given by Eq. (1) (C/B2O3 = 3.5). Therefore, we performed a pyrolysis process in air before carbothermal reduction, which was effective for eliminating excess carbon [13]. Change in the C/B2O3 ratio, estimated from the titration, of the precursor powders prepared by pyrolysis at 450– 650 °C for 2 h in air is summarized in Fig. 2. The XRD pattern of the precursor powder exhibited B2O3 peaks and an amorphous halo of carbon regardless of pyrolysis temperature (not shown); all precursor powders consisted of B2O3 and amorphous carbon. The C/B2O3 ratio of the precursor powder was controlled by the pyrolysis temperature and the precursor powder with stoichiometric composition for the carbothermal reduction process (C/B2O3 = 3.5) was obtained at 550 °C. Consequently, the pyrolysis temperature at 550 °C should preferably be used owing to its stoichiometric C/B2O3 ratio. Fig. 3 shows the XRD patterns of products obtained by pyrolysis at 550 °C for 2 h in air and heat treatment at 1100–1300 °C for 5 h in an Ar flow. Peaks corresponding to a rhombohedral B4C crystal were recognized at 1150 °C and above, indicating the existence of crystalline B4C. Moreover, only B4C peaks appeared at 1250 °C, meaning that crystalline B4C powder without residual carbon can be synthesized at 1250 °C, which is the lowest reported synthesis
B4C B2O3 C
(b)
(a)
650
Pyrolysis Temperature / oC
Intensity / arb. unit
νC-O 1041
3350 νO-H νO-H 3208
Absorbance / arb. unit
3292 νO-H
(c)
C/B2O3 Ratio
2.2. Measurements
4.0
1300oC
1250oC 1200oC 1150oC 1100oC
10 4000
3000
2000
1000
400
20
30
40
50
60
70
80
2θ / °
Wavenumber /cm-1 Fig. 1. FT-IR spectra of (a) H3BO3, (b) glycerin, and (c) condensed product.
Fig. 3. XRD patterns of products obtained by heat treatment of precursor powder at 1100–1300 °C for 5 h in Ar flow. The precursor was pyrolyzed at 550 °C for 2 h in air.
M. Kakiage et al. / Materials Letters 65 (2011) 1839–1841
(a)
1 μm
(b)
1841
powder pyrolyzed at 550 °C for 2 h in air. The B2O3 component can be removed by washing the precursor powder in hot water, thus leaving the carbon component, as shown in Fig. 4(b), where the characteristic carbon network structure with nano-order spacing can be recognized. This distinctive structure is similar to macroporous materials with a three-dimensional bicontinuous morphology developed by a phase separation in a polymerization-derived sol–gel system [15], implying that a phase separation is induced by spinodal decomposition and the transitional bicontinuous structure of spinodal decomposition is fixed as a permanent morphology by carbonization during the pyrolysis process in air in this study. A bicontinuous B2O3/carbon network structure provides a markedly increased contact domain. This structure increases the reactivity and the number of nucleation sites of B4C, enabling a lowering of the synthesis temperature of B4C. Note that this bicontinuous network structure is generated in the condensed H3BO3–glycerin and maintained in the precursor powder, meaning that the stabilization process, i.e., the intermediate carbonization, is important. These results demonstrate that the lower synthesis temperature is due to the homogenization of the initial condensed product, which includes B– O–C bonds, and the subsequent intermediate carbonization at a suitable temperature. This approach is a promising methodology for the lowtemperature synthesis of B4C crystalline powder containing less residual carbon from an organic precursor using a carbothermal reduction process. 4. Conclusions
1 μm
B4C crystalline powder was synthesized from a condensed H3BO3– glycerin product by a carbothermal reduction process. Borate ester (B–O–C) bonds were formed in the condensed product. The formation of crystalline B4C started at 1150 °C, and crystalline B4C powder without residual carbon was synthesized by heating at 1250 °C, which is the lowest temperature reported for the synthesis of B4C, for 5 h.
Fig. 4. SEM images of precursor powder pyrolyzed at 550 °C for 2 h in air (a) before and (b) after removal of B2O3 by washing in hot water.
References
temperature of B4C using a carbothermal reduction process. The synthesized B4C crystalline powder consisted of equiaxed grains with a small particle size (D50 = 1.1 μm), which is smaller than those synthesized by general carbothermal reduction. These results demonstrate that a significant lowering of the synthesis temperature is achieved by the synthesis of B4C from the condensed H3BO3–glycerin product using this methodology. This lowering of the synthesis temperature of B4C in this study is explained in terms of the enlargement of the contact domain between B2O3 and carbon components and the carbonization (stabilization) by intermediate heating in air. Fig. 4(a) shows a SEM image of the precursor
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Thevenot F. J Eur Ceram Soc 1990;6:205–25. Jung CH, Lee MJ, Kim CJ. Mater Lett 2004;58:609–14. Alizadeh A, Taheri-Nassaj E, Ehsani N. J Eur Ceram Soc 2004;24:3227–34. Suri AK, Subramanian C, Sonber JK, Murthy TSRC. Int Mater Rev 2010;55:4–40. Benton ST, Masters DR. US Patent 3914371. Shi L, Gu Y, Chen L, Qian Y, Yang Z, Ma J. Solid State Commun 2003;128:5–7. Mohanty RM, Balasubramanian K, Seshadri SK. J Alloy Compd 2007;441:85–93. Ishikawa Y, Feng Q, Koshizaki N. Appl Phys A 2010;99:797–803. Sudoh A, Konno H, Habazaki H, Kiyono H. Tanso 2007;226:8–12. Sinha A, Mahata T, Sharma BP. J Nucl Mater 2002;301:165–9. Khanra AK. Bull Mater Sci 2007;30:93–6. Wada H, Kuroda K, Kato C. Yogyo Kyokai Shi 1986;94:71–5. Yanase I, Ogawara R, Kobayashi H. Mater Lett 2009;63:91–3. Pistorius CWFT. J Chem Phys 1959;31:1454–7. Nakanishi K. J Porous Mat 1997;4:67–112.