Sintering behavior and dielectric properties of pyrochlore Pb 2 FeWO 6.5
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Nobuyasu Mizutania) Department of Inorganic Materials, Tokyo Institute of Technology, Tokyo 152, Japan (Received 20 May 1993; accepted 16 September 1993)
Pyrochlore-type Pb2FeWO6.s is difficult to be sintered without applied pressure at temperatures lower than its decomposition temperature. Through hot pressing or hot isostatic pressing processes, densification of specimens is greatly enhanced; moreover, grain growth during sintering is effectively suppressed. Densified Pb 2 FeWO 65 exhibits paraelectric characteristic from —135° to —2 °C with low dielectric constants and low dissipation factors. The dielectric constants show rather weak dependence of temperature and frequency.
The lead-containing complex oxides with perovskite structure, such as Pb(Fe2/3W1/3)O3,1 Pb(Mg1/3Nb2/3)O3,2 and Pb(Fe1/2Nbi/2)O3,3 exhibit ferroelectric properties and high dielectric constants, and are used as the materials of multilayer capacitors. In the synthesis of these perovskite compounds, due to improper processing or uncompleted reaction, the residual pyrochlore compounds are easy to form with the perovskite compounds.4'5 The existence of the pyrochlore compounds was reported to have detrimental effects on the dielectric properties of the perovskite compounds.6'7 Therefore, new synthesis methods to obtain pyrochlore-free compounds have been researched.8-9 Despite the fact that the effect of pyrochlore in the pyrochlore-perovskite mixture has been investigated, the dielectric properties of the pyrochlore compound itself are not very well understood. In the formation process of Pb(Fe2/3W1/3)O3, a pyrochlore compound forms as an intermediate compound.10-11 The authors have tried to prepare its single phase through a two-stage precipitation technique, and verified that its chemical composition is Pb2FeWO6.5.12 In order to obtain this compound with good crystallinity, prolonged soaking at 650 °C for 30 days is necessary. When heated over 700 °C, it easily decomposes to Pb(Fe2/3Wi/3)O3 and PbWO 4 . For investigating the dielectric properties of Pb 2 FeWO 65 , the densified compacts for dielectric measurement are necessary. Therefore, in this study, the sintering processes of Pb 2 FeWO 65 powders were studied first. After densified Pb2FeWO6.s was obtained, its dielectric properties were investigated. Pure pyrochlore-type Pb 2 FeWO 65 powders were prepared from the precipitation method followed by
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J. Mater. Res., Vol. 9, No. 2, Feb 1994
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heating at 650 °C for 30 days.12 The synthesized powders were compacted into disks of 10 mm diameter under 98 MPa, then heated by three kinds of sintering processes: pressureless sintering, uniaxial hot pressing, and hot isostatic pressing. In pressureless sintering, specimens were heated at 650 °C for 2 h to 7 days. In hot pressing, compacted disks, which were wrapped in platinum foil, were heated at 650 °C under 49 MPa for 0.5 h to 12 h. In hot isostatic pressing, the platinumwrapped
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