Growth and Morphology of Magnesium Oxide Whiskers

Magnesium oxide crystals and whiskers were grown by vapor phase reactions between MgO and tungsten, hydrogen, and carbon. The large number of morphologies obtained are described in terms of two, three, and fourfold growth symmetry and are related to the growth conditions by current heterogeneous nucleation theory. Threefold (or 〈111〉) growth occurred principally with MgO + H ₂ reactions, and in all experiments 〈100〉 and 〈110〉 growth occurred under closely similar conditions of temperature and vapor supersaturation. Observations of growth steps suggest that {100} surfaces are favored only at the highest vapor supersaturation. Tensile strength measurements of whiskers with l/d ratios of about 300 show increasing strength with decreasing cross section. A value of 8.13 × 10 ⁴ psi was observed for a whisker 9.76μ in diameter. The results are of particular interest to the formation of refractory oxides crystallizing in the simple cubic NaCl structure.