into a 16-part hyperfine structure, indicating either a low concentration of V" ions and/or their existence in an unsymmetrical field. The signal for V4+ in K-ALO, consisted of a single peak, indicating a higher concentration of V" or its existence in a symmetrical field.In view of the atmospheres used in the preparations and the concentrations of V determined by analysis, it seems likely that concentration of V" is responsible for the difference between these spectra. The role of V4+ in orthorhombic S(3)-ALO, was indicated by further study of conditions favoring its formation. In air it was obtained from S(2) between 1000" and llOO°C, as described, but a sample of y-ALO, containing 10 wt% V fired at 750°C in 10%-H2-in-N1 gas and then at 800°C in Or yielded S(3)-AI20, directly.The structure of K-ALO, was studied by Brindley and Choe,' who reported the existence of two modifications obtained by heating fine-grained gibbsite. Saalfeld' observed only one type, corresponding to the form rather less frequently observed by Brindley and Choe, in the electron microscope. The present K-ALO:, gave an X-ray powder diffraction pattern which could be indexed on the larger cell given by Brindley and Choe (Table I ) and contained several lines which could not be indexed on the smaller cell found by Saalfeld, i.e. the 211, 207, 430, and 208 reflections. Samples of K-ALO~ were also prepared from a very pure fine-grained gibbsite (Na,O content 4 ppm) by the fluxing action of 5% V20, in 0, or air at 1100°C for 1 h. The relative intensities of the X-ray powder reflections were the same for this product as for that obtained from y-ALO, as described. In addition, no change in the relative intensities, e.g. of the 208 and 604 planes in particular, was observed for further samples prepared at 900" to 1080°C in the presence of V4+. No evidence was obtained, therefore, for the existence of ~-Al,0, with the smaller cell, which was present in the poorly crystalline material obtained from gibbsite. However, electron diffraction examination of these samples revealed some patterns which could be indexed on both possible a,, values, whereas others could be indexed only on the larger value. It cannot be concluded, however, that a mixture was present.Although the role of V,O, as a flux, with an effect depending on amount and temperature, is fairly straightforward, the role of the V4' ion is not. Its action is apparently required in the early stages of firing if the poorly crystalline y-Al,O,, is to be "directed" into the K-form. which could be obtained from Alumina as a Ceramic Material. Compiled and edited by W. H. Gitzen. The American Ceramic Society, Inc., Columbus, OH, 1970. 2 P. H. Rookshy; pp. 354-92 in X-Ray Identification and Crystal Structures of Clav Minerals. Edited bv G. Brown. Mineraloeical Society, London, 1961. minum Hydroxide Gels,"

The kinetics of mullite growth from kaolinite and halloysite at high temperatures were studied by quantitative X-ray diffraction analysis. The evidence in both cases is consistent with a nucleation-controlled model.

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HE sensitivity of the inversion temperature of cristobalite to T mechanical restraint in artificial mixtures,' and the quantitative applicability of the Clausius-Clapeyron equation to this phenomenon'+! have been reported. This note reports the detection of internal stresses in commercial ceramics containing cristobalite.In studies on cristobalites occurring in ceramic products, the inversion temperature of the cristobalite phase in each of 10 samples was estimated in two ways: (1) the temperature of the midpoint of the inversion "step" constructed on the thermal expansion curve obtained from a bar cut from the ceramica; (2) the DTA peak temperature obtained with a sample of ground ceramic. Both data were obtained from rising temperature tests. The results are given and significant differences between the corresponding data are noted in Table I.These differences have been confirmed by experiments on a mixture of cristobalite and plaster of Paris and on a synthetic mixture of cristobalite with a ceramic body which forms no cristobalite when fired. Bars were prepared from each mixture and prefired to 500" and 900°C, respectively, to confer sufficient strength for thermal expansion tests. Thermal expansion and DTA data were interpreted as described in the foregoing, and the two inversion temperatures for each material agreed within +3"C (Table I). This is a check on the techniques and shows that the differences for the ceramic products are real.It is apparent that the only difference between each pair of samples examined is the light grinding given the sample for DTA. The hypothesis is here put forward that the low inversion temperatures observed for cristobalites in ceramic materials are due to the mechanical restraint imposed on the cristobalite phase by the matrix. It is suggested that the dispersion of the materials by grinding is sufficient to relieve the stresses, so that a higher inversion temperature is given by DTA data than by thermal expansion.Harder firing of the clay-cristobalite mixture, with resulting glass formation, can be expected to lead t o greater restraint of the cristobalite phase by the glassy matrix. If the above hypothesis is correct, the synthetic bar should have a lower inversion temperature, as shown by thermal expansion, as it is fired harder. Accordingly, the bar fired a t 900°C was fired successively at 1150" and 1200°C for 1 hr and a thermal expansion curve was obtained after each firing. The results, given in Fig. 1 and Table I, demonstrate a marked lowering of the mean inversion temperature. The DTA inversion temperature of the material fired at 1200°C agrees, within experimental error, with the inversion temperature of the original cristobalite, showing that the restraint is removable and is in accord with the elastic stress hypothesis postulated above. The total expansion appears to show a maximum, with firing temperature, presumably because of the opposing effects of increasing density and dissolution of cristobalite as the temperature increases.