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Synthesis of ZrW2O8 by quick cooling and measurement of negative thermal expansion of the sintered bodies
Journal of Alloys and Compounds 417 (2006) 187–189
Synthesis of ZrW2O8 by quick cooling and measurement of negative thermal expansion of the sintered bodies Shin Nishiyama ∗ , Tsukasa Hayashi, Takeo Hattori Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan Received 20 May 2005; accepted 11 July 2005 Available online 23 November 2005
Abstract ZrW2 O8 is known as a material showing negative thermal expansion. We prepared this compound from ZrO2 and WO3 powders by firing at 1200 ◦ C in air followed by cooling in a furnace, in air, in water or in liquid nitrogen in order to cool at various rates. As the cooling rate became slower, a larger proportion of ZrW2 O8 reacted decomposed to ZrO2 and WO3 . Samples cooled in a furnace showed positive thermal expansion, whereas other quenched samples showed negative expansion of about −4 × 10−3 from room temperature to 600 ◦ C. © 2005 Elsevier B.V. All rights reserved. Keywords: Liquid quenching; X-ray diffraction; Thermal analysis
1. Introduction Negative thermal expansive compounds are useful for producing composites that have no or very low thermal expansion. Some silicates such as eucriptite and spodumen [1,2] and some titanates [3] show negative expansion. However, negative thermal expansion in these materials is generally restricted to narrower temperature ranges and is anisotropic. In 1996, ZrW2 O8 was found to show isotropic negative thermal expansion from 0.3 to 1050 K [4], This material has the potential to make composites with no expansion and is expected to be used widely in the electronic and ceramic industries. ZrW2 O8 was first synthesized in 1959 by firing mixed powders of ZrO2 and WO3 at 1200 ◦ C encapsulated followed by quenching, and was found to have a cubic crystal structure that was unstable at room temperature [5]. According to a phase diagram of the ZrO2 –WO3 binary system determined by Chang et al. [6], ZrW2 O8 is stable at temperatures ranging from 1105 to 1257 ◦ C. Evans et al. [7] reported the isotropic negative thermal expansion of ZrW2 O8 at temperatures ranging from 0.3 to 1050 K (777 ◦ C). They considered that the origin of this negative expansion was transverse vibration of O ions that were located between Zr and W ions. However, although this material has
these excellent properties of expansion, it is hard to prepare since this material is unstable at temperatures lower than 777 ◦ C and WO3 is very volatile at high temperatures. Therefore, the encapsulation has been required in the process of the preparation. In the present work, ZrW2 O8 was prepared in an ordinary atmosphere, and the microstructure and thermal expansion were studied. 2. Experimental procedure As starting materials, zirconium oxide (special grade, Wako Pure Chemical) and tungsten oxide (99.9%, Wako Pure Chemical) were used. These powders were weighed and mixed in the molar ratio of Zr to W of 1/2, then pressed uniaxially to make disk-formed compacts of dimensions 6 × 4 mm. Those compacts were fired at 1200 ◦ C for 2 h in air followed by quenching in water. After drying, the samples were ground for 1 h and the powders obtained were again pressed into rectangular bars of 4 mm × 4 mm × 20 mm followed by cold pressing isostatically under 100 MPa. Then the compacts were sintered at 1200 ◦ C for 2 h in air and cooled under various conditions: in a furnace, in air (out of furnace), in water, or in liquid nitrogen. Densities of the sintered bodies and ground powders were measured using Archimedes’ method and pycnometry, respectively. They were investigated with X-ray powder diffraction (XRD). The microstructure of the sintered bodies was observed by scanning electron microscopy (SEM), Thermal expansion measurements from room temperature to 600 ◦ C were performed using a laser inter-ferometry type thermal expansion meter.
3. Results and discussion ∗
Corresponding author. Tel.: +81 43 290 3434; fax: +81 43 290 3039. E-mail address: [email protected] (S. Nishiyama).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.07.075
XRD patterns of samples are shown in Fig. 1. For samples quenched in water and in liquid nitrogen, the only crystal
188
S. Nishiyama et al. / Journal of Alloys and Compounds 417 (2006) 187–189 Table 1 Density and molar ratio of each specimen cooled under various conditions
Fig. 1. XRD patterns of samples cooled under various conditions.
phase detected was ZrW2 O8 . The detected phases of the sample cooled in air were ZrW2 O8 , ZrO2 and WO3 , and for the sample cooled in the furnace, ZrO2 and WO3 were detected. These show that as the quenching rate became lower, a larger amount
Cooling conditions
Density of sintered bodies (Mgm−3 )
Density of powders (Mgm−3 )
Molar ratio of ZrW2 O8 (%)
In liquid nitrogen In water In air In a furnace
4.09 4.17 4.08 4.61
5.27 5.21 5.84 10.85
82.8 83.1 79.3 20.5
of ZrW2 O8 prepared at 1200 ◦ C decomposed to ZrO2 and WO3 . Table 1 shows the density of the sintered bodies, the density of the ground powders and the molar ratio of ZrW2 O8 in the powders, which were calculated from the data of the density and theoretical density of ZrW2 O8 (1.271 Mg/m3 ), ZrO2 (5.818 Mg/m3 ) and WO3 (14.598 Mg/m3 ). This table indicates that as the cooling rate decreases, a larger amount of ZrW2 O8 decomposes to ZrO2 and WO3 . The sample cooled in air showed a relatively low density, which was due to the pores generated during cooling. There was no apparent difference between the samples quenched in water and those in liquid nitrogen. From the calculated molar ratio, it was found that ZrO2 and WO3 existed. In XRD measurements, however, no peak of those compounds was detected. It is thought that ZrO2 and WO3 had formed an amorphous phase during quenching. On the other hand, the sample cooled in a furnace showed the existence of ZrW2 O8 from the calculation of densities, though XRD revealed only ZrO2 and WO3 . Thus, it seems that ZrW2 O8 prepared at 1200 ◦ C
Fig. 2. SEM photographs of samples cooled under various conditions.
S. Nishiyama et al. / Journal of Alloys and Compounds 417 (2006) 187–189
189
Fig. 3. Thermal expansion curves of samples cooled under various conditions.
decomposed partially to ZrO2 and WO3 and the residue formed amorphous phases. Fig. 2 shows the SEM fractographs of the samples. In the sample cooled in a furnace, grains over 10 m in size were destroyed and separated to form a fine texture. This indicates that ZrW2 O8 shrank in volume (theoretically by more than 1/9) when decomposed to ZrO2 and WO3 . The microstructure of the sample cooled in air consisted of large grains with some pores inside and other kinds of small grains, which seemed to be ZrO2 and WO3 . The samples quenched in water and in liquid nitrogen contained large grains with a secondary phase along the boundary of the grains and pores. Comparing the sample cooled in the furnace with the quenched ones, it was observed that the grains of quenched samples were more spread out in space, which suggests that the volume of ZrW2 O8 increase and expand into the grain boundaries. Fig. 3 shows the thermal expansion of those samples from room temperature to 600 ◦ C. The sample cooled in the furnace showed a positive thermal expansion curve, which indicated that most of the ZrW2 O8 in the sample synthesized at high temperature decomposed to ZrO2 and WO3 . Whereas, quenched samples showed negative thermal expansion curves regardless of quenching conditions. The expansion curves of the samples quenched in water and liquid nitrogen were nearly identical. However, the sample cooled in air out of the furnace had a little bit less negative expansion. This indicated that the amount of decomposed ZrO2 and WO3 affected expansion behavior. It is also shown that each expansion curve of quenched sample has a break of slope at around 200 ◦ C, which corresponds to the transition of space group from P21 3 to Pa3 reported by Evans et al [7].
4. Conclusion ZrW2 O8 was synthesized in an open system by quenching from 1200 ◦ C. The proportion of ZrW2 O8 in the sintered bodies increased as the quenching rate increased. Samples quenched both in water and in liquid nitrogen had only the ZrW2 O8 crystal phase, whereas the ZrW2 O8 phase in the samples cooled slowly in the furnace had almost decomposed to ZrO2 and WO3 . Density measurements and SEM photographs showed that even specimens obtained by quenching included the secondary phase which seemed to be an amorphous phase of ZrO2 or WO3 . Thermal expansion of the sample cooled slowly was positive, whereas the thermal expansion of all other samples, i.e., those cooled in air and quenched both in water and in liquid nitrogen, showed negative thermal expansion. References [1] W. Ostertag, G.R. Fischer, J.P. Williams, J. Am. Ceram. Soc. 51 (1968) 651–654. [2] H. Schultz, J. Am. Ceram. Soc. 57 (1974) 313–318. [3] W.R. Buessem, N.R. Thielke, R.V. Sarakauskas, Ceram. Age 60 (1952) 38–40. [4] T.A. Mary, J.S.O. Evans, T. Vogt, A.W. Sleight, Science 272 (1996) 90–92. [5] J. Graham, A.D. Wadsley, J.H. Weymouth, L.S. Williams, J. Am. Ceram. Soc. 42 (1959) 570. [6] L.L.Y. Chang, M.G. Scroger, B. Phillips, J. Am. Ceram. Soc. 50 (1967) 211–215. [7] J.S.O. Evans, T.A. Mary, T. Vogt, M.A. Subramanian, A.W. Sleight, Chem. Mater. 8 (1996) 2809–2823.
Synthesis of ZrW2O8 by quick cooling and measurement of negative thermal expansion of the sintered bodies Shin Nishiyama ∗ , Tsukasa Hayashi, Takeo Hattori Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan Received 20 May 2005; accepted 11 July 2005 Available online 23 November 2005
Abstract ZrW2 O8 is known as a material showing negative thermal expansion. We prepared this compound from ZrO2 and WO3 powders by firing at 1200 ◦ C in air followed by cooling in a furnace, in air, in water or in liquid nitrogen in order to cool at various rates. As the cooling rate became slower, a larger proportion of ZrW2 O8 reacted decomposed to ZrO2 and WO3 . Samples cooled in a furnace showed positive thermal expansion, whereas other quenched samples showed negative expansion of about −4 × 10−3 from room temperature to 600 ◦ C. © 2005 Elsevier B.V. All rights reserved. Keywords: Liquid quenching; X-ray diffraction; Thermal analysis
1. Introduction Negative thermal expansive compounds are useful for producing composites that have no or very low thermal expansion. Some silicates such as eucriptite and spodumen [1,2] and some titanates [3] show negative expansion. However, negative thermal expansion in these materials is generally restricted to narrower temperature ranges and is anisotropic. In 1996, ZrW2 O8 was found to show isotropic negative thermal expansion from 0.3 to 1050 K [4], This material has the potential to make composites with no expansion and is expected to be used widely in the electronic and ceramic industries. ZrW2 O8 was first synthesized in 1959 by firing mixed powders of ZrO2 and WO3 at 1200 ◦ C encapsulated followed by quenching, and was found to have a cubic crystal structure that was unstable at room temperature [5]. According to a phase diagram of the ZrO2 –WO3 binary system determined by Chang et al. [6], ZrW2 O8 is stable at temperatures ranging from 1105 to 1257 ◦ C. Evans et al. [7] reported the isotropic negative thermal expansion of ZrW2 O8 at temperatures ranging from 0.3 to 1050 K (777 ◦ C). They considered that the origin of this negative expansion was transverse vibration of O ions that were located between Zr and W ions. However, although this material has
these excellent properties of expansion, it is hard to prepare since this material is unstable at temperatures lower than 777 ◦ C and WO3 is very volatile at high temperatures. Therefore, the encapsulation has been required in the process of the preparation. In the present work, ZrW2 O8 was prepared in an ordinary atmosphere, and the microstructure and thermal expansion were studied. 2. Experimental procedure As starting materials, zirconium oxide (special grade, Wako Pure Chemical) and tungsten oxide (99.9%, Wako Pure Chemical) were used. These powders were weighed and mixed in the molar ratio of Zr to W of 1/2, then pressed uniaxially to make disk-formed compacts of dimensions 6 × 4 mm. Those compacts were fired at 1200 ◦ C for 2 h in air followed by quenching in water. After drying, the samples were ground for 1 h and the powders obtained were again pressed into rectangular bars of 4 mm × 4 mm × 20 mm followed by cold pressing isostatically under 100 MPa. Then the compacts were sintered at 1200 ◦ C for 2 h in air and cooled under various conditions: in a furnace, in air (out of furnace), in water, or in liquid nitrogen. Densities of the sintered bodies and ground powders were measured using Archimedes’ method and pycnometry, respectively. They were investigated with X-ray powder diffraction (XRD). The microstructure of the sintered bodies was observed by scanning electron microscopy (SEM), Thermal expansion measurements from room temperature to 600 ◦ C were performed using a laser inter-ferometry type thermal expansion meter.
3. Results and discussion ∗
Corresponding author. Tel.: +81 43 290 3434; fax: +81 43 290 3039. E-mail address: [email protected] (S. Nishiyama).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.07.075
XRD patterns of samples are shown in Fig. 1. For samples quenched in water and in liquid nitrogen, the only crystal
188
S. Nishiyama et al. / Journal of Alloys and Compounds 417 (2006) 187–189 Table 1 Density and molar ratio of each specimen cooled under various conditions
Fig. 1. XRD patterns of samples cooled under various conditions.
phase detected was ZrW2 O8 . The detected phases of the sample cooled in air were ZrW2 O8 , ZrO2 and WO3 , and for the sample cooled in the furnace, ZrO2 and WO3 were detected. These show that as the quenching rate became lower, a larger amount
Cooling conditions
Density of sintered bodies (Mgm−3 )
Density of powders (Mgm−3 )
Molar ratio of ZrW2 O8 (%)
In liquid nitrogen In water In air In a furnace
4.09 4.17 4.08 4.61
5.27 5.21 5.84 10.85
82.8 83.1 79.3 20.5
of ZrW2 O8 prepared at 1200 ◦ C decomposed to ZrO2 and WO3 . Table 1 shows the density of the sintered bodies, the density of the ground powders and the molar ratio of ZrW2 O8 in the powders, which were calculated from the data of the density and theoretical density of ZrW2 O8 (1.271 Mg/m3 ), ZrO2 (5.818 Mg/m3 ) and WO3 (14.598 Mg/m3 ). This table indicates that as the cooling rate decreases, a larger amount of ZrW2 O8 decomposes to ZrO2 and WO3 . The sample cooled in air showed a relatively low density, which was due to the pores generated during cooling. There was no apparent difference between the samples quenched in water and those in liquid nitrogen. From the calculated molar ratio, it was found that ZrO2 and WO3 existed. In XRD measurements, however, no peak of those compounds was detected. It is thought that ZrO2 and WO3 had formed an amorphous phase during quenching. On the other hand, the sample cooled in a furnace showed the existence of ZrW2 O8 from the calculation of densities, though XRD revealed only ZrO2 and WO3 . Thus, it seems that ZrW2 O8 prepared at 1200 ◦ C
Fig. 2. SEM photographs of samples cooled under various conditions.
S. Nishiyama et al. / Journal of Alloys and Compounds 417 (2006) 187–189
189
Fig. 3. Thermal expansion curves of samples cooled under various conditions.
decomposed partially to ZrO2 and WO3 and the residue formed amorphous phases. Fig. 2 shows the SEM fractographs of the samples. In the sample cooled in a furnace, grains over 10 m in size were destroyed and separated to form a fine texture. This indicates that ZrW2 O8 shrank in volume (theoretically by more than 1/9) when decomposed to ZrO2 and WO3 . The microstructure of the sample cooled in air consisted of large grains with some pores inside and other kinds of small grains, which seemed to be ZrO2 and WO3 . The samples quenched in water and in liquid nitrogen contained large grains with a secondary phase along the boundary of the grains and pores. Comparing the sample cooled in the furnace with the quenched ones, it was observed that the grains of quenched samples were more spread out in space, which suggests that the volume of ZrW2 O8 increase and expand into the grain boundaries. Fig. 3 shows the thermal expansion of those samples from room temperature to 600 ◦ C. The sample cooled in the furnace showed a positive thermal expansion curve, which indicated that most of the ZrW2 O8 in the sample synthesized at high temperature decomposed to ZrO2 and WO3 . Whereas, quenched samples showed negative thermal expansion curves regardless of quenching conditions. The expansion curves of the samples quenched in water and liquid nitrogen were nearly identical. However, the sample cooled in air out of the furnace had a little bit less negative expansion. This indicated that the amount of decomposed ZrO2 and WO3 affected expansion behavior. It is also shown that each expansion curve of quenched sample has a break of slope at around 200 ◦ C, which corresponds to the transition of space group from P21 3 to Pa3 reported by Evans et al [7].
4. Conclusion ZrW2 O8 was synthesized in an open system by quenching from 1200 ◦ C. The proportion of ZrW2 O8 in the sintered bodies increased as the quenching rate increased. Samples quenched both in water and in liquid nitrogen had only the ZrW2 O8 crystal phase, whereas the ZrW2 O8 phase in the samples cooled slowly in the furnace had almost decomposed to ZrO2 and WO3 . Density measurements and SEM photographs showed that even specimens obtained by quenching included the secondary phase which seemed to be an amorphous phase of ZrO2 or WO3 . Thermal expansion of the sample cooled slowly was positive, whereas the thermal expansion of all other samples, i.e., those cooled in air and quenched both in water and in liquid nitrogen, showed negative thermal expansion. References [1] W. Ostertag, G.R. Fischer, J.P. Williams, J. Am. Ceram. Soc. 51 (1968) 651–654. [2] H. Schultz, J. Am. Ceram. Soc. 57 (1974) 313–318. [3] W.R. Buessem, N.R. Thielke, R.V. Sarakauskas, Ceram. Age 60 (1952) 38–40. [4] T.A. Mary, J.S.O. Evans, T. Vogt, A.W. Sleight, Science 272 (1996) 90–92. [5] J. Graham, A.D. Wadsley, J.H. Weymouth, L.S. Williams, J. Am. Ceram. Soc. 42 (1959) 570. [6] L.L.Y. Chang, M.G. Scroger, B. Phillips, J. Am. Ceram. Soc. 50 (1967) 211–215. [7] J.S.O. Evans, T.A. Mary, T. Vogt, M.A. Subramanian, A.W. Sleight, Chem. Mater. 8 (1996) 2809–2823.