§ 4.3.2 Sintering of Lunar and Asteroidal Minerals
Sintering is a simple process whereby bulk basalt or a particular mineral or set of minerals in powder form are heated to a high temperature less than the melting point, whereby the particles bond to each other, producing a porous (on a microscopic scale) material. The material usually shrinks significantly, and often the sintering process occurs in a die with a compaction pressure. The vacuum in space generally helps this process. The heat can come from either direct solar energy and/or microwave. Microwave heating allows quicker uniform heating.
The result is a fairly low density material which can be cut and shaped fairly easily, can hold small loads in compression, and provides good thermal insulation, but cannot take much stress in tension and is brittle.
Sintering allows production of parts without melting and liquid casting processes, i.e., dealing with only powder or fine sand.
Examples of everyday items produced by sintering include sanitaryware (sinks, toilets, tubs) and some kinds of pottery.
Pletka gives an excellent analysis of the properties of lunar materials as they relate to various sintering methods, and performed laboratory analysis on sintered MLS-1 (Minnesota lunar simulant 1). Desai et al. ((1993, 1992, 1992a, 1992b) developed a "vacuum triaxial device" which was used to produced sintered samples from ALS (Arizona lunar simulant), performing tests to help determine stress-strain-strength behavior of materials under the expected loads and environmental conditions of space, including such properties as deformation moduli, strength parameters, elastic, plastic and creep response, fracture toughness, ductility, flexural response, and dynamic and impact response. Meek et al. used microwaves to heat three different lunar simulants approximating the returned soils of Apollos 11, 15 and 16, each to a variety of temperatures, and the resultant products were tested for compressive strength, Young's modulus, percent strain to failure, thermal shock, hardness and density.
Simonds (1988) proposed a small solar powered sintering plant for producing various products as well as oxygen and a little hydrogen (liberated by the heating process). Battelle produced a report for Lockheed on a lunar sintered regolith production facility which was very well received by the community.
The NASA Johnson Space Center has a technical paper online on sintering.