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MATERIALS RESEARCH

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High-temperature creep of polycrystalline BaTiO3 E. T. Park Energy Technology Division, Argonne National Laboratory, Argonne, Illinois 60439, and Mechanical, Materials, and Aerospace Engineering Department, Illinois Institute of Technology, Chicago, Illinois 60616

P. Nash Mechanical, Materials, and Aerospace Engineering Department, Illinois Institute of Technology, Chicago, Illinois 60616

J. Wolfenstine Army Materials Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783

K. C. Goretta and J. L. Routbort Energy Technology Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received 2 February 1998; accepted 28 May 1998)

Compressive creep of dense BaTiO3 having linear-intercept grain sizes of 19.3–52.4 mm was investigated at 1200–1300 ±C by varying the oxygen partial pressure from 102 to 105 Pa in both constant-stress and constant-crosshead-velocity modes. Microstructures of the deformed materials were examined by scanning and transmission electron microscopy. The stress exponent was ø1, the grain-size dependence was ø1yd 2 , and the activation energy was ø720 kJymole. These parameters, combined with the microstructural observations (particularly grain displacement and absence of deformation-induced dislocations), indicated that the dominant deformation mechanism was grain-boundary sliding accommodated by lattice cation diffusion. Because of the absence of an oxygen partial pressure dependence, diffusion was probably controlled extrinsically.

I. INTRODUCTION

Barium titanate (BaTiO3 ) is an important technical ceramic which finds use as a capacitor material. It is chemically and mechanically very stable, and, more importantly, exhibits ferroelectric properties up to 120 ±C.1 BaTiO3 has a perovskite-type structure and its ferroelectric properties have been extensively investigated.1 Surprisingly, however, little is known about its diffusional properties, which could be important for the fabrication of BaTiO3 ceramics. Oxygen self-diffusion has been measured in dense polycrystalline and singlecrystal BaTiO3 .2 At 900–1100 ±C, activation energy was reported to be 48 kJymole. Self-diffusion of Ba has been measured in 80% dense polycrystalline BaTiO3 , with 131 Ba used as a radioactive tracer.3 The diffusion coefficient was equal to 0.8 exp(2[372 kJymole]yRT ) cm2ys. No data exist for Ti diffusion in BaTiO3 . High-temperature deformation mechanisms most commonly observed in ceramics are controlled by diffusion of the slowest-moving species along the fastest path, e.g., Nabarro-Herring, Coble, grain-boundary sliding, and dislocation climb creep.4,5 An investigation of high-temperature creep is an alternative way to measure the diffusion coefficient of the rate-controlling species.4,5 However, only two investigations have been made of BaTiO3 creep. The first6 measured compressive creep of 0.45 mm grain-sized BaTiO3 between 1150 J. Mater. Res., Vol. 14, No. 2, Feb 1999

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and 1250 ±C and found that the mat