Preparation and Mechanical Properties of NaCl Ceramics

Abstract

NaCl powder specimens composed of equally sized (approximately 1.8μm), cubic single crystals were prepared by additions of acetone to saturated NaCl solution at -21°C under various preparation conditions. Grain growth during storage of the powder specimens was found to be greatly influenced by moisture in the ambient atmosphere. Storage of the specimens in a furnace at 400°C prevented grain growth. Compacts of the powder specimens produced by cold pressing (CP) and cold isostatic pressing (CIP) were sintered, or the powders were directly hot-pressed (HP). Relative densities (RD) of the resulting sintered bodies depended on the compaction or sintering conditions. From these sintered bodies, appropriate equations relating bending strength with porosity and grain size were obtained. The high-temperature bending strength increased up to 300°C and then decreased at higher temperatures. Deflections observed in the bending tests incerased with sintering temperature, that is, with the increase in grain size. Observed tensile strength values (5-10MPa) were smaller than observed bending strength values (8-20MPa). Measured values of the Weibull coefficient and fracture toughness were m=5.34 and K_1C=1.05MPa·m^1/2, respectively. Single crystal specimens obtained from melts by the Czochralski method were also subjected to bending tests. These specimens showed large deflections above 200°C, and strain-hardening phenomena were also observed. Tension tests for single crystal whiskers prepared by condensation of vaporized NaCl in a flowing air conducted along the [100] direction, and showed that the strength incresed with the whisker cross sectional area up to a maximum value and then decreased. The highest stregth value was obtained in vacuum with lower values being obtained in dry air and moist air because of the effects of oxygen and moisture, respectively.