| Densification of Undoped Yttria by Means of Flash-sintering |
| Hidehiro YOSHIDA |
| National Institute for Materials Science |
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Keywords: Y2O3, Flash-sintering, Field-assisted sintering, Electrical conductivity, Microstructure
Yttria (Y2O3) ceramics have special chemical and physical properties such as high resistance to halogen-plasma corrosion and thermal stability. At the same time they are difficult to sinter; conventional sintering requires very high temperatures, typically >1400¡ÆC, and a vacuum or hydrogen atmosphere. New techniques that use electrical fields have shown that ceramics can be sintered quickly at low temperatures. Collectively these methods are called field-assisted sintering techniques (FAST). Recently, it has been demonstrated that nanocrystalline, yttria stabilized tetragonal zirconia (Y-TZP) can be sintered at 850¡ÆC within 5 seconds under an electric field of 120 V/cm [1]. This phenomenon is called FLASH-sintering. The nature of flash-sintering, where densification occurs in just a few seconds under a threshold condition of temperature and applied field, is fundamentally different from FAST [2]. The flash-sintering can also be characterized by nonlinear increase of electric conductivity during sintering experiments. In the present study we demonstrate that undoped Y2O3 can be sintered nearly instantaneously to full density under DC applied fields of more than 500 V/cm [3]. For instance, the flash-sintering preceded by gradually accelerated field-assisted sintering occurs at furnace temperature of 1133¡ÆC under a DC applied field of 500 V/cm. In the Y2O3, FAST is followed by flash-sintering. The microstructure of the flash-sintered specimens indicated that the densification was accompanied by rapid grain growth. The single-phase nature of the flash-sintered Y2O3 was confirmed by high resolution transmission electron microscopy (HRTEM). It is postulated that a combination of Joule heating and defect generation triggered by the flash event leads to the higher rates of diffusion and densification.
References:
[1] M. Cologna, B. Rashkova, R. Raj, J. Am. Ceram. Soc., 93 (2010) 3556.
[2] R. Raj, M. Cologna, J.S.C. Francis, J. Am. Ceram. Soc., 94 (2011) 1941.
[3] H. Yoshida, Y. Sakka, T. Yamamoto, J.-M. Lebrun, R. Raj, J. Eur. Ceram. Soc., 34 (2014) 991.
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| Acknowledgements : The authors thank Dr. John S.C. Francis for his assistance with the sintering experiments. This work was financially supported by a Grant-in-Aid for Scientific Research on Innovative Areas 2505 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Lebrun and Raj were supported in this work by the Office of Naval Research under the guidance of Dr. Lawrence Kabacoff, under grant no. N00014-12-1-0710 |
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