On a etudie les possibilites de moulage par injection de SiC en employant differents additifs organiques, a partir de l'examen de la viscosite a haute temperature de differents melanges, c'est a dire en comparant les valeurs de viscosite, dans les memes conditions, ainsi que la dependance en temperature de la viscosite. On a aussi etudie le comportement durant le moulage par injection. Les composants organiques de base entrant dans toutes les formulations sont la paraffine et l'acide stearique, mais on peut aussi ajouter du polyethylene, du poly propylene, de la glycerine, et de l'huile de poisson. Les formulations contenant de la glycerine et du polyethylene ne sont pas fluides dans les conditions etudiees. Les formulations contenant du polypropylene ne peuvent pas etre mises en forme par moulage par injection a 200 °C, et meme a plus haute temperature. Les formulations contenant de l'huile de poisson donnent de bons resultats, mais les meilleurs resultats sont obtenus avec les formulations les plus simples, contenant de la paraffine et de l'acide stearique. On a etudie en detail la viscosite de trois de ces formulations, avec 85-89 % en masse de poudre ceramique (63-71 % en volume). On a etabli qu'il est non seulement possible, mais meme aise, de mettre en oeuvre des melanges contenant un maximum de 89 % en masse de poudre ceramique (71 % en volume), c'est a dire la formulation proche de la concentration critique de phase solide dans un melange pour moulage par injection.

In this work the quantitative relations between the properties of sintered fused silica (density, mechanical propel-ties, cristobalite content) and slip preparation parameters as well as sintering conditions (milling balls material, temperature and rime of sintering) were established Multifactorial experimental design ,cas used in ol der to obtain rite respective mathematical al models. The response surfaces of these models were analyzed, together with correlations between the compressive and tensile strength and cristobalite content. it was established that the maximum values of mechanical properties for a given material were obtained with 6-8 vol.% of cristobalite in sintered samples.

The investigations of the influence of particle size distribution in the fused silica slip on slip casting kinetics at elevated temperature were performed. Fused silica slips with and without coarse grain filler were rheologicaly investigated at different temperatures. It was established that the casting rate is proportional to the applied temperature and inversely proportional to the content of the coarse grain filler in the slip. Rheological behaviour of the used slips was also found to be very sensitive to the changes of temperature and grain size distribution.

Frittage et devitrification sont des processus simultanes lors du frittage au-dessus de 1100°C de silice fondue de qualite technique preparee par coulage de barbotine. Cela rend difficile l'obtention d'un materiau a faible porosite et faible teneur en cristobalite. L'article montre les posibilites du pressage chaud pour l'obtention de ce type de materiau. La densification intervient par un mecanisme d'ecoulement visqueux, du moins si la devitrification n'est pas trop prononcee.

An analysis of ceramic slip-casting in porous molds is given in this paper. A new process parameter, called the slip-casting coefficient, is introduced in order to indicate the progression of the slip-casting process, independent of mold shape. Material balances for the solid and liquid phases of ceramic slip, as well as the filtration equation (Carman-Kozeny), are the basis for establishing the mathematical model of slip-casting in porous molds.

Formation of the phosphate bonding in electrofused MgO by addition of ammonium and Mg phosphates was investigated and formation of amorphous compounds has been found. On heating these compounds undergo almost identical changes what leads to the conclusion that chemical properties of the phosphate anion play essential role in this bonding reaction which does not depend to the same extent on the present cation. In the present work zero shrinkage sintering was observed. However, porosity substructure considerably changed during sintering. Electron microprobe analyses have shown that phosphate compounds concentrate at the grain boundaries. Phosphorus is present in MgO even after heating at 1660°C but it is no more continuously arranged at the boundaries of periclase grains.

AbstractBei der Solvolyse der stereoisomeren trans‐ (Ib) und cis‐3β‐Acetoxy‐5,10‐seco‐1(10)‐cholesten‐5S‐yl‐p‐nitrobenzoate (IIIb), die aus den entsprechenden Carbinolen (Ia) bzw. (IIIa) hergestellt wurden, erhält man ausschließlich unter transannularer Reaktion das Cyclisierungsprodukt (II).

AbstractDa die Einführung einer Sauerstoff‐Funktion in den Lactonring durch chemische Methoden schwierig ist, wurde die Einwirkung von Bleitetraacetat auf Steroid‐lactone als Modellsubstanzen untersucht.

Assignment of configuration to the 1,10-double bond in the cis-trans isomeric 3β-acetoxy-5,10-seco-1,10-cholesten-5-ones has been achieved by means of NMR spectrometry. Some reactions of these new compounds have been studied and, as expected from conformational analysis, it was found that only the trans-isomer readily undergoes transannular cyclizations.

The lead tetraacetate oxidation of cholestan-1β-ol and cholestan-1α-ol in benzene solution did not afford cyclic ethers but resulted, in major part, in the opening of ring A with formation of 1,10-seco-aldehydes. In addition, cholestane, cholest-1-ene and cholestan-1-one were also obtained.