High-density Micro- and Nano-Grain Size Ceramics. Transition from Open into Closed Pores. Part 1. Powder Preparation, Molding Mixture, Molding

Abstract

An explanation of processes occurring during production of high-density micro- and nanogranular ceramics without the use of external pressure is proposed on the basis of accumulated published data. It is well known that pore growth commences after the start of transition from open into closed pores, which begins with about 30% open porosity. It is necessary to maintain open pores to the maximum possible total density of sintered ceramics. This may be achieved by slowing down formation of local compaction areas (unequal density of samples) by various methods. Retention of open pores is facilitated by a decrease in shrinkage rate with which there is self-consistent local compaction and less dense surrounding zones. This regime can be implemented in different ways: by reducing the activity of powder particles (powder preliminary heat treatment, and preliminary low-temperature sintering, i.e., presintering), use of additives that slow down shrinkage (preparing transparent ceramics without external pressure), in the molding material preparation stages, workpiece molding, binder removal by adjusting the specimen heating rate (sintering with a controlled shrinkage rate), and prolonged sintering at a relatively low temperature in the second stage of sintering.