Fabrication and properties of extruded and sintered BeO

Abstract

This work describes the fabrication of BeO specimens of six compositions at three porosity levels with varying grain size, and shows the effects of composition and microstructure on the mechanical and physical properties. Three grades of BeO were used: UOX, a sulfate derived high purity BeO; HPA, a hydroxide derived high purity BeO; and AOX, a somewhat less pure grade of BeO. The UOX grade was also prepared with additions of 0.5 weight percent MgO or 3.0 weight percent ZrU 2 and the HPA with additions of 0.5 weight percent MgO. The impurities present, particle size by electron micrographs, X-ray crystallite size, and particle distribution of the BeO used are given. Impurity contents were also determined after sintering. All six compositions were prepared in the porosity range of 2 to 3 percent and having a nominal average grain size of 20 microns. Three of the six compositions, UOX 0.5 weight percent MgO, AOX and HPA were prepared in three porosity ranges (2 to 3 %, 7 to 8.5 %, and 12 to 14 %) and in as many as six nominal average grain sizes (5, 10, 20, 50, 80, and 100 microns). Most of the samples were fabricated by extrusion of a dispersion of a powder mixed with an organic plasticizer but for comparison purposes, some were prepared by isostatic pressing. The samples were dried, prefired in air at about 1000° C to remove most of the organic matter, and sintered in a hydrogen atmosphere. Methods such as adding an organic “filler” or varying the time and temperature of the sintering cycle were used to achieve a given porosity and grain size. The effect of time, temperature, and additives on the density and grain growth are shown in tables and charts. The UOX grade BeO was the only grade that showed evidence of preferred orientation of crystallites in the extruded and sintered rods; however, no evidence of preferred orientation was found when specimens of UOX grade BeO were prepared by isostatic pressing and sintering. Pores were studied by electron microscopy and were found to give reflections of the crystal habit. The effect of porosity, grain size, and temperature on elastic moduli, modulus of rupture, compressive creep, lattice parameters, and thermal expansion are given. The correlation between the elastic moduli and porosity were found to be statistically significant for the ranges stated and these results are expressed as empirical equations. In addition, increasing orientation of the C-axis along the longitudinal axis of the specimen causes the elastic moduli to increase. Modulus of rupture data are presented which show the effect of grain size, porosity, composition, and temperature. Compressive creep for several combinations of composition, grain size and porosity have been measured in dry air at 1200° C for 500 hours at loads from 1000 to 10 000 psi, and the data are correlated with the Nabarro-Herring relationship.