Gerrit Verspui

Growth of SiC whiskers in the system SiO2-C-H2nucleated by iron

Abstract A study has been made of the nucleation, growth and morphology of α- and β-SiC whiskers obtained from silica and carbon in hydrogen between 1200 and 1300°C by a VLS process with iron as agent. On polycrystalline substrates - the solid starting materials or an inert third material - β-whiskers are obtained mainly grown in the [111] direction; on basal {0001} and prismatic {10 1 0} faces of α-SiC single crystals in addition α-[0001] and α-[1 1 0] whiskers respectively grow epitaxially. The growth rate is in the order of 1 mm/hr, irrespective of polytype and growth direction. The rate depends on the relative position of C, SiO2 and substrate; in a given arrangement the rate increases with increasing temperature with an activation energy of about 85 kcal/mole. As to be expected for a VLS process in which the supply of vapour is rate-limiting, the rate is higher for whiskers grown from relatively larger droplets. The β-[111] whiskers often show a fluctuation in diameter, whereas the α-whiskers are always smooth. For smooth whiskers the ratio of the diameter of the Fe-Si-C alloy droplet at the tip and the diameter of the whiskers is about 2, and for knotted whiskers generally larger. These ratios are associated with minima in the Gibbs free surface energy at the vapour-liquid-solid interface. It is discussed how instabilities in the liquid can give rise to knottiness of β-[111] whiskers in a system where solid SiC grows from a gas phase with Si/C ⪡ 1 via a liquid phase with Si/C ⪢ 1.

Phase transformations, habit changes and crystal growth in SiC

Abstract The phase transitions and habit changes of SiC crystals of the polytypes 2H, 3C, 4H and 15R have been studied in the conditions of a semiclosed SiC/C system, by annealing in argon at 1 atm pressure. In the temperature range of investigation (1500–2830 °C) all types transform to tabular crystals of the 6H structure. The transition of 2H whiskers into 6H crystals takes place via the metastable 3C modification. The experimental data are discussed in terms of models assuming either in situ volume transitions or surface mechanisms. The observed transition rates of normal crystals of 3C, 4H and 15R to 6H are considerably lower than the rate of the 2H→3C transformation of whiskers. From Arrhenius plots of the transition rates activation energies were derived which, within the limits of experimental accuracy, are equal (≈ 140 kcal/mole), and identicall with the activation energies of evaporation and of crystal growth by recrystallization of 6H. It is concluded that both for the transformations, where near-surface transport takes place, and for evaporation one and the same molecular step is rate controlling.

Columnar growth of carbon

Abstract It has been found that in the deposition of carbon by pyrolysis of carbon monoxide above 1800°C, a columnar growth of carbon is induced by β-SiC crystals, showing rotation twinning and stacking faults on {111}. The microstructure of the carbon columns can be visualized as being stacked from parallel layers of carbon atoms in a hexagonal graphite network, bent into a cone mantle with a top angle of about 141°. The successive layers are randomly rotated with respect to each other over angles being multiples of 60 or 30° divided by a prime number. The columns are thought to be generated on defects, by the topology of twinned β-SiC crystals and blocking of the sides by C-O bonds.

Lanthanum-stimulated high-temperature whisker growth of α-SiC

Abstract A study has been made of the growth and morphology of α-SiC whiskers obtained by the effect of lanthanum on silicon carbide crystals growing into a cavity in a SiC mass, heated in an inert ambient of 1 atm pressure, at temperatures between 2300 and 2600 °C. The morphology and the growth rates of the ribbonlike 6H- whiskers are compared with the corresponding properties of the almost equilateral hexagon-shaped plane- parallel platelets of 6H SiC obtained in the same conditions, except for the presence of lanthanum. The addition of lanthanum causes the growth of thinner elongated crystals with an increased growth rate in the longitudinal direction and a decreased rate of growth in width. For platelets and ribbons the activation energies associated with the increase of the growth rates with increasing temperature are the same (∼ 130 kcal/mole) for the two growth directions within experimental accuracy. Platelets and ribbons are both bounded by large-area basal faces {0001} and first-order prismatic and pyramidal side faces {10 i l }; the tips of whiskers interrupted in their growth are rounded and show both first-order faces {10 i l } and second order faces {11 2 l }. The effect of lanthanum is understood as an adsorption-blocking effect on the {10 1 l } side faces favouring ao. {11 2 l } faces of rapid growth at the tip of whiskers.

GROWTH AND TRANFORMATION OF SiC WHISKERS

The structure and habit of crystals of silicon carbide strongly depend on the procedure used for their production. In different growth conditions characterized by the type of silicon and carbon source, the temperature, and the presence of impurities, an extensive series of different modifications (polytypes) has been obtained, while the habits may vary from lamellar to acicular. The polytypes, e.g., cubic (β) 3C and hexagonal (α) 2H, 4H, 6H, and 15R, may all be described as stacking variants of hexagonal close-packed double layers [1, 2].