Amorphous lead iron tungstate prepared by twin-roller quenching
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D. A. Payne Department of Materials Science and Engineering and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received 16 April 1990; accepted 22 June 1990)
Amorphous lead iron tungstate was prepared from the melt by twin-roller quenching. The rapidly solidified material was characterized in terms of density, x-ray diffraction, differential thermal analysis, and electrical properties. Results are given for the amorphous-crystalline transformation.
The ability to form glasses is almost a universal property of condensable matter.1 Materials in the molten state can be rapidly quenched into the amorphous state by high heat extraction rates. Thus the glass-forming range can be greatly extended without the addition of traditional network formers. As a result, many new amorphous materials have been prepared with interesting and valuable properties. For example, high values of ionic conductivity2 and dielectric constant2 4; unusual pyroelectric,4 electrochromic,5 and magnetic properties6; far ir transmission7; anisotropy in grain orientation on crystallization,8 etc., have been reported. Lead iron tungstate, Pb(Fe2/3W1/3)O3 (PFW), is a dielectric material with a complex perovskite structure, which was first synthesized in the late 1950s.9 Since that time several studies have been published on the fabrication of multilayer capacitors with high values of dielectric constant and with reduced processing temperatures for densification.1011 However, nothing has been reported on the properties of PFW in the amorphous state, even though novel processing routes can be envisioned. In this paper, we report on the preparation and properties of amorphous PFW, and the crystallization behavior on heat treatment. Amorphous PFW was prepared by rapid solidification from the melt with high heat-extraction rates. No network forming aids were used. A twin-roller quenching method was used.12 The purpose was to obtain an understanding of the properties of PFW in both the amorphous and crystalline states, before and after heat treatment. Starting materials were analytical grade reagent chemicals of PbO, Fe2O3, and WO3. The fine powders were carefully weighed according to formulation requirements and blended in de-ionized water in a polyethylene jar with resin media. In order to reduce the possibility of lead loss at high temperatures, the mixed oxides were slowly heated to 900 °C and reacted for J. Mater. Res., Vol. 5, No. 10, Oct 1990
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3-4 h. The product was crushed and size reduced, and analyzed by x-ray diffraction (XRD), before using as a stock material in melting experiments. Figure 1 illustrates the equipment used, which was comprised of a vertical tube furnace and a twin-roller quencher. The rollers were made from stainless steel for rapid heat-extraction rates and for rigidity against deformation during quenching. The maximum rotation speed was 3000 rpm. Presynthesized powders were placed in a platinum crucible and heated to 150-
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