154 CERAMICS INTERNATIONAL, Vol. 11, “. 4. 1985

[AS] are studied. The formation of Ca2(NH4)2(S04)3r CazPzO, ELEVATED TEMPERATURE INSTRUMENTED CHARPY and Ca(PO& is observed between 200% and 300%; at IMPACT OF A SINTERED SILICON CARBIDE 400% CaSd4 appears. From 500 to 700°C, Ca(PO& reacts with Ca SO4 and with HAP and gives P-CazPzO,. Lastly, from 700% to 1000°C, p-Ca2Pz07 reacts with HAP and with CaS04 and gives p-Ca3(PO& [P-TCP]; from 1000 to 1 loo%, P-TCP and CaS04 react and form a sulfate ion containing calcium phosphosulfate, the structure of which is P-TCP.

COMPLETE RHEOLOGICAL CURVES FOR CLAY DISPERSIONS AND PASTES AND THEIR ASSOCIATED ENERGETICS

H.C. Chandan, L. Hermansson, H. Abe and R.C. Bradt

A commercially available, densely sintered alpha silicon car- bide was tested in air from room temperature to 1700% us- ing a modified instrumented pendulum unit and standard Charpy size test specimens. The resistance heated silicon carbide specimens exhibited only elastic behavior, even at 1700%. A compliance analysis of the test revealed a linear elastic decrease of the absorbed impact energy and also a decrease of the fracture stress of the silicon carbide with in- creasing temperature.

N. N. Krugliski and V. Ya. Kruglitskaya

The mechanism of structural formation within ceramic systems is determined by the chemical, physical, crystal chemical mineralogical, surface and adsorption properties of the raw materials. Clay minerals (layer silicates) are most common. In processing and formation the ceramic systems are inherent in coagulation set pattern of space-thixotropic nets of certain strength. Their rheological curves are the scientific base for optimization of constructive peculiarities of production equipment taking account of energetic expen- diture, in technology. In this connection the curves of clay suspensions having different structure are obtained, the yield and-viscosity points are calculated and peculiarities of the contact interactions within them are stated basing on realization power of dispersed structures.

TEMPERATUREDEPENDENCEOFELECTRICAL RESISTIVITY AND DIELECTRIC CHARACTERISTICS OF POLED AND BaZrOl MODIFIED BaTi03 CERAMICS

Morsy M. Abou Sekkina, M.K. El-Nimr and E. W. ABD Allah

Samples of pure BaTiOj, pure BaZr03 and intermediates containing from 5 up to 30% BaZrOs were carefully prepared by the usual ceramic procedure followed by X-ray diffraction analysis to ensure the complete reaction. Well sintered and translucent ceramic bodies were obtained. Measurements of dielectric constant (E), dielectric loss (tan 6) and ac. resistivi- ty (e) were undertaken as a function of temperature up to 250% and at various frequencies of lo-50 Kc/s before and after polarization (800 V). The effect of poling field is discuss- ed on the basis of the presence of spontaneous polarization in BaTi03 lattice. Finally, it was established that the introduc- tion of Zr“’ ions in BaTiOJ lattice occurs in three steps: first- ly, filling of Ba’+ vacancies, secondly solid solution forma- tion from BaZr03 in BaTi lattice and thirdly, occupying in- terstitial sites in BaTiO? lattice.

INFLUENCE OF CERAMIC PROCESSING ON DIELECTRIC PROPERTIES OF PEROVSKITE TYPE COMPOUND: Pb(Mg,,3 Nbz,J)OJ

M. Lejeune and J.P. Boilot

Pb (Mgr,, Nb2,j)Ox perovskite type compound which can be sintered at low temperatures has a ligh dielectric permit- tivity and so can be used as multilayer ceramic capacitors. In this study we show that ceramic processes resulting in dif- ferent mixtures of phases and different microstructures strongly influence the dielectric properties (dielectric permit- tivity, dissipation factor and resistivity). Only a careful characterization in each step of the process allow the op- timization of ceramics.

PRE-EUTECTIC DENSIFICATION IN MgF2-CaF2

S.C. Hu and L.C. De Jonghe

Increased densification rates were found as much as 200°C below the eutectic temperature (980%) for MgF2 containing small amounts of CaF2. Constant heating rate and costant temperature sintering data, as well as microstructural developments indicated that solid state grain boundary transport rates had been enhanced by the eutectic forming additive. The effect saturated at about 1 wtl% CaF2. The results suggest that densification of ceramic powders could be favorably affected without a substantial increase in the grain growth rate, by the addition of small amounts of eutec- tic forming additives, and sintering below the eutectic temperature.

DEVELOPMENT OF PHOSPHATE GLASS-CERAMICS FOR BONE IMPLANTS

F. Pernot, P. Balder, F. Bonnel, J. Zarzycki and P. Rabischong

A HIGH TEMPERATURE TERNARY PHASE IN THE SYSTEM MgO-A120j-Zr02

P. Reynen, A. Firatli, D. von Mallinckrodt, H. BoeB

Short communication. perties of the materials were studied: - textural properties: porosity,

Porous glass-ceramics were prepared by the controlled

interconnection pore size distribution; -

crystallization of various phosphate MfoarnBb glasses. The

mechanical properties: fracture bending stress, Young’s

crystalline phases were detected and the following pro-

STABLE AND METASTABLE PHASE EQUILIBRIA AND REACTIONS IN THE Si02-aA1203 SYSTEM

Joseph A. Pask

modulus. It is shown that these properties depend not only on the composition of the base glasses, but also upon the grain size of the foaming agent.

Procedures in the preparation of specimens in this system are critical due to the difficulty of nucleating aAl when the silicate liquid is not saturated with A1201 even though it may be supersaturated relative to crystalline aA120j, and the ease with which mullite growing from an alumino-silicate melt dur- ing cooling accomodates an excess of A1203. Metastable phase compositions and microstructures occur commonly. As a result, misinterpretations of experimental data have oc- curred in published reports. Stable and metastable phase equilibria diagrams in the aA1203-Si02 system are presented.

MECHANICAL PROCESSING OF BENTONITES

Jill Bulandr and Jaroslav Dudek

Investigations into dry grinding and pneumatic processing of three bentonite types, aimed at improving their quality, were carried out. Grinding tests indicated that changes in ben- tonite grain size have little effect on its quality, unless it is complemented with rearrangements in smectite texture. Common nonclay admixtures of bentonite were shown to in-