No abstract

The contribution of plastic flow to overall densification of a powder compact during hot-pressing was analyzed by incorporating the creep characteristics of materials at high temperatures into an equation applicable to hot-pressing conditions. When the particles are assumed to be spherical and when the effective stress acting at the points of contact under axial loading conditions is taken into consideration, the final form of the equation is:where CY, and p are geometric constants which can be calculated from the packing geometry, A and n are material constants, D is the relative density of the compact, and R is the radius of the spheres at any stage of deformation in arbitrary units. Computerized plots of D vs t were obtained for several materials. Experimental verification of these plots using hotpressing data for Pb2% Sb, Ni, and A1203 spheres and coarse irregular Al,O, particles showed that the experimental data fitted the theoretical prediction for orthorhombic packing well. A large deviation with respect to the initial packing density was encountered with very fine irregular ALO, powder, although its densification behavior with time was similar to that of the coarse particles.

[reaction: see text] The intramolecular Mannich reaction of delta-amino beta-keto esters with aldehydes and ketones is a new methodology for the synthesis of polysubstituted piperidines and is illustrated by the concise asymmetric synthesis of the dendrobate alkaloid (+)-241D and its C-4 epimer.

Zn-proline (5 mol%) performs as a novel water-soluble and recyclable Lewis acid catalyst for the selective synthesis of 1,2-disubstituted benzimidazoles from wide range of substituted o-phenylenediamines and aldehydes in moderate to excellent isolated yields (42-92%) using water as solvent at ambient temperature.

No abstract