A study of tungsten trioxide (WO3) by differential thermal analysis and x-ray diffraction at temperatures up to 980°C showed a new tetragonal form to exist at temperatures of 740°C and higher. This form has a distorted ReO3 structure. There are two molecules in a unit cell of dimensions a0=5.25A, c0=3.91A. All atoms lie in the 2- or 4-fold special positions of the space group P4/nmm. The tungsten atoms are alternately displaced in opposite directions from the center of charge of the oxygen atoms so as to resemble the antiferroelectric structure proposed by Kittel.

The sintering of AI-Cu compacts made from powder mixtures has been studied by dilatometry and metallography in an attempt to elucidate the mechanism of dimensional changes occurring in this alloy system. Sintering kinetics are governed by the formation and spreading of eutectic liquid; this has been found to be influenced by the formation of intermetallic phases during heating to,.the eutectic temperature.Liquid phase sintering has made aluminium an attractive material for powder metallurgy processing. 1 -4 During formation of the liquid phase the thin but highly stable oxide film which is always present on the surface of aluminium particles is disrupted and good particle bonding is achieved by subsequent neck formation during which the liquid phase facilitates material transport. 5 ,6 Eutectic liquid can be formed by reaction of Al with elemental powders such· as Cu, Mg, Si, and Zn,7,8 which also convey improved strength directly or upon heat treatment. In the Inain, alloys of type AI-Cu-Mg (typically 4-6%Cu, up to 1%Mg) or AI-Mg-Cu (typically O' 5-1' 5%Mg, up to O' 5%Cu) are preferred in powder metallurgy.9During sintering, compacts of elemental powder mixtures of these alloy types exhibit a sudden swelling upon reaching and surpassing the eutectic temperature.The extent of this dimensional change depends on alloycomposition, particle size, sintering atmosphere, and temperature. lO The swelling is followed by a rapid contraction. lO • 11 These strong dimensional changes are undesirable in precision parts. It is the objective of this study to throw some light on their mechanism.Swelling of a similar type occurs in Fe-Cu alloys.12.13Its mechanism has been elucidated in terms of penetration of the melt into the grain boundaries of the iron particles and diffusion of the copper into the iron. The amount of swelling has been found to depend critically on the wetting conditions between the melt and the solid phase, which can be affected by carbon additions. A quantitative understanding of the swelling process and of the ways in which it can be controlled has been developed. 17 -19The swelling of AI-Cu compacts has been studied less extensively. On the basis of dilatometric measurements at high rates of heating, Esper and Leuze ascribed the swelling to the penetration of Al grain boundaries by the melt. lO Sundaresan and Ramakrishnan interpreted dilatometric observations on the sintering of Al-Sn and Al-Si-Cu-Ni alloys in terms of particle rearrangement and solutionreprecipitation. 20 * Liquid phase .• sintering is usually described in terms of three mechanisms which are dominant during successive stages of the process 12 ,16.(i) particle rearrangement with an almost linear time dependence of shrinkage (ii) dissolution and reprecipitation which removes material from the contact zones of particles, thereby diminishing the cente-to-centre distances. This stage produces shrinkage at a less than linear rate, and produces the shape adjustment of adjacent grains which is characteristic of liquid-phase sintered structure...

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The Mössbauer effect has been observed for Fe(57) atoms at the surface of eta Al(2)O(3). The Fe(57) is trivalent, and the quadrupole splitting found is consistent with a surface location. Anisotropy of thermal vibration relative to the surface is observed.