Combined cycle power generation plants fired by low Btu gasified coal will be among the most efficient, environmentally acceptable means of coal-based power generation available. A major cost of producing clean gasified fuel from high sulfur coal is associated with H2S removal and conversion to sulfur. A new approach now under development to H2S removal is based on an immobilized HC03~7C032_ membrane through which facilitated transport of H2S occurs. The membrane has sufficiently high K2S permeability to be a candidate for economical industrial application. A membrane configuration has been devised which provides a degree of H2S/C02 selectivity not available with conventional hot carbonate scrubbers. Thus the prognosis for an economical, efficient membrane H2S scrubber is favorable, although much development has yet to be done to prove the viability of the concept.

The chemical vapor deposition of semiconductor silicon by pyrolysis of Sill4 and SiH.2CI~ often produces undesirable silicon powder in the reactor, owing to homogeneous nucleation in the gas phase. Experiments reported herein show the temperature:composition dependence of powdered silicon formation from SiH~C12-H~ mixtures. Classical nucleation theory has been applied to the St-H-C1 system to calculate nucleation rates for liquid or solid silicon from each silicon containing homolog. In general, these rates are dual valued, there being both a higher and a lower temperature associated with each. The lower temperature values compared favorably with literature values reported by Eversteijn and with experimental measurements reported here.

Oxygen present in cuprous iodide greatly increases the electrical conductivity. Copper films were formed with differing oxygen contents by copper vapor condensation at various oxygen pressures. After iodination to CuI the conductivity was found proportional to the 1/8.4 power of the oxygen pressure during condensation. Cuprous iodide with oxygen created conductors is a mixed valency anion semiconductor.

No abstract

of the surface in sufficient quantity to assure its involvement in the propagation of the reaction. The etch rates thus seem to be the result of a balance struck between the autocatalytic factors, which are explosive in nature, and some rate limiting mechanism. Since the rates are the result of the difference between these two large quantities, the system may be inherently unstable, and this may cause the measured etch rates to be nonreproducible. ConclusionsThe role of acetic acid in the etchant is primarily that of a diluent. As such it is able to provide control over the rate of the reaction by reducing the concentration of the kinetically important species. However, this function also can be performed with nitric acid as diluent in the compositions rich in nitric acid, or with hydrofluoric acid in the compositions rich in hydrofluoric acid. Excessive addition of acetic acid can cause the etch rates to become erratic and extremely critical with respect to small changes in composition. Water also can be used as a diluent, except that in the high hydrofluoric acid region the system is critical with respect to the addition of diluent.The kinetics in the acetic acid diluted system are governed by the same factors that operate in the CHEMICAL ETCHING OF SILICON 111water diluted system. Thus the reaction rates are determined by the rate of diffusion of the kinetically important species to the surface of the specimen. The autocatalytic factors that operate in the water diluted system play the same role in the acetic acid diluted system. The quantitative differences between the two systems can be explained on the basis of the difference in the basicity and dielectric constant of the two diluents.ABSTRACT Silicon tetraiodide is formed from commercial silicon and iodine in a fluidbed reactor. Impurities are rejected by solvent recrystallization followed by fractional distillation. High-purity silicon is formed by thermal decomposition of the iodide on a silicon surface. Silicon made this way has a resistivity over a 1000 ohm-cm. Reproducible results are obtained which suggest the product quality is controlled by back-contamination from the decomposition and crystal growing steps. Data from the decomposition of silicon tetraiodide at 1000~ and low pressure are correlated by the relation:where p ~ lb SiL vapor feed/hr/ft 2 heated surface and m = fraction of SiL~) converted to SLs>. It is postulated that this reaction reaches equilibrium.Iodide chemistry was developed by Van Arkel and de Boer (1) into a useful method for reducing the oxygen, nitrogen, or carbon content of such refractory materials as titanium, zirconium, hafnium, and silicon. Other impurities, aluminum, beryllium, boron, iron, and copper, are not rejected effectively by this simple iodide cycle and, as a result, further