XRD Study of the Influence of Heat Treatment on the Microstructure of Sol-Gel Processed ZrO 2 -Y 2 O 3 Ceramics
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Mat. Res. Soc. Symp. Proc. Vol. 398 01996 Materials Research Society
EXPERIMENT Sample Preparation Yttria stabilized zirconia (YSZ) amorphous powders containing 5 and 10 mol % of Y 2 0 3 were prepared by controlled hydrolysis of zirconium propoxide (Fluka) and yttrium isopropoxide 9 (Aldrich). The hydrolysis reactions took place at room temperature in dry atmosphere. The dried powders were calcined at 800'C in air and then uniaxially pressed (70 MPa). The green compacts were sintered at 1400'C and 1550 0C for 2 h in air. The sintered body 5YSZ was superplastically forged in a 1185 INSTRON machine under 5 MPa uniaxial pressure at 1400'C. The 1OYSZ sample sintered at 1550'C could not be superplactically forged and it was annealed without pressing at 1600'C for 24 h instead. The samples studied will be referred to as 5C, 5L, 5H, 5LP, 10C, 10L, 10H and 10HA, where 5 and 10 mean nominal yttria composition, C indicates calcined powder, L and H compact body sintered at 1400'C and 1550'C, respectively. P and A are used to indicate, respectively, superplastically forged and free annealed samples. Whole Profile Fitting (WPF) Diffraction patterns were recorded using a PHILIPS PW 1830 diffractometer. In all cases the intensity measurements were made at room temperature with a CuKI (X,,i = 1.5406 A, X,,2= 1.5443 A) and a graphite monochromator. Data were collected over the 20 range 20' to 900, at intervals of 0.015' using a step-counting time of 1 s. A silica pattern, recorded in the same experimental conditions was used as a reference for microstructure analysis. Data were fitted with the Fullprof'° program. This program allows a multiphase analysis using the structural information and lattice parameters for each phase. The peak profile was chosen by minimizing a function for the full-width half maximum (FWHM) peak with using a Marquardt alorithm". The structural parameters of all zirconia polymorphs existing at room temperature (mZrO 2, t-ZrO 2, t'-ZrO 2 and c-ZrO 2) were included in the line profile calculation. The values given in the literature' 2,13"14 were taken as starting values. A Pseudo-Voigt15 (Thompson-Cox-Hastings) function is often used to approximate the Lorentzian-Gaussian convolution product. This is made using a simple addition of the two functions, independently obtaining the Gaussian and Lorentzian contributions in the calculated pattern. This function was used as shape profile function, because it is very useful to make an estimation of the average particle size. This value is related to the Lorentzian component. The FWHM values, Hk-L, are given by (1): X (1) Hk-L -=cosO X being a parameter that represents the contribution of the grain size to the integral width. The average particle size,
, can be estimated from 180K(2)
7r(x - x o) K being Scherrer's constant; X , the wave length of the radiation used; and X., a diffraction pattern parameter obtained from a standard silica sample.
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RESULTS To analyze the temperature dependence of the structural parameters and bulk composition of 5YSZ and 1OYSZ
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