1997

The dimensional changes that occur in the steel during heat treating is one of the most difficult problems facing the heat treater. While the influence of the alloy chemical composition, of the heat treatment cycle and of the shape and dimensions of the pieces on the deformations caused by heat treatments on the wrought steels are now a days well known, the presence of the porosity in the sintered steels arouses some inconvenients which influence directly the heat treatment effects.

It is known that solids with composition Na3Zr2Si2PO12 heated at 1200°C crystallize in the nasicon structure. This material shows a high ionic conductivity that represents an interesting improvement in the field of solid electrolytes. Our experimental results allow to establish for the first time that nasicon structures are stable along the compositional join Na3Zr2-x/4Si2-xP1+xO12 with x extending from 0 to 1.667. These structures are characterized by a Zr underoccupation of octahedral sites and a constant number of Na+ ions.

An investigation of the thermolysis of 3% yttria doped zirconia samples has been undertaken using the method of Controlled transformation Rate Thermal Analysis (CRTA) under a residual pressure of 4.5 x 10 4 mbar. Investigation of the water washed samples reveals a simple water loss. Thermolysis of the ethanol washed samples however, reveal a series of mechanisms without ethanol production. Several ethene related species are evolved during the course of the reaction indicating the catalytic nature of zirconia.

Two mullite powders, different in chemical and morphological characteristics, were compared with respect to their sinterability. For the development of a low-cost sintering step, MgO was added as sintering aid and its content was optimized for both mullites: the enhancement in sintering was imputed to a liquid phase formation. The two mullites presented a strongly different sinterability; the one gave rise to high density sintered bodies while the other, even after a grinding step which made its particle size distribution similar to that of the first, did not reach high final density.