Alan W. Searcy

Activation Energy for the Sublimation of Gallium Nitride

Gallium nitride was found to sublime congruently from a torsion—effusion cell when the ratio of orifice area to sample area was about 1/30 and incongruently to yield nitrogen gas and liquid gallium when this ratio was about 1/100 or less. A mass‐spectrometer investigation revealed no measurable concentrations of gallium nitride vapor molecules. The heat of activation for the reaction 2GaN(s)=2Ga(1)+N2(g) was calculated to be 39 kcal at 1300°K from the temperature dependence of the effusion data.The rate of the reaction 2GaN(s)=2Ga(g)+N2(g) was measured by a torsion—Langmuir method. From the temperature dependence of sublimation the heat of activation for this reaction was calculated to be ΔH1300‡=218.6 kcal compared to 173 kcal for the equilibrium reaction, and the entropy of activation was calculated to be 74.3 cal/deg.

Use of the Langmuir method for kinetic studies of decomposition reactions: calcite (CaCO3)

The Langmuir method for measurement of vapour pressures has been tested for use in studies of decomposition reactions. The isothermal weight loss in vacuum from cleavage (10text-decoration:overline11) planes of calcite (CaCO3) single crystals was measured continuously at temperatures from 934 to 1013 K. The rate was constant until approximately 80% of a 1 mm slice had decomposed. The apparent activation enthalpy was 205 kJ (49 kcal) mol–1. Micrographic examination showed an approximately 30 µm thick layer, probably a metastable form of calcium oxide, separating the calcite from the growing layer of oriented stable calcium oxide. The 30 µm layer yielded a single X-ray diffraction peak which was displaced slightly from the strongest (220) peak of the oriented normal calcium oxide. Lower apparent activation enthalpies measured in previous studies of calcite decomposition in inert atmospheres are suggested to result either from partial diffusion control of the process or from catalysis of the direct formation of normal calcium oxide by carbon dioxide or a component of the system atmosphere. The ratio of the measured decomposition rate in vacuum to the maximum rate, which can be calculated from the Hertz–Knudsen–Langmuir equation, is suggested to be a useful parameter in correlating and predicting decomposition reaction rates.

Calcium oxides of high reactivity

A RECENT study of the kinetics of decomposition of calcite (CaCO3) single crystals in vacuo showed that if the reaction was interrupted before completion a 30-µm layer of a poorly crystalline material was present between the undecomposed CaCO3 and a layer of normal polycrystalline CaO1. It was hypothesised that the material of this 30-µm layer is a metastable form of CaO that transforms irreversibly to the stable polycrystalline oxide when the accumulated strain exceeds a critical level. If so, the free energy of formation of the metastable oxide from the stable oxide must lie in the range between zero and +31,500−21 TJ mol−1 and might well lie at the positive end of this range2. Such a metastable oxide should be chemically more reactive than the stable oxide. If the hypothesis is true, then the principal product of decomposition in vacuo of calcite particles of diameter < 30 µm should be the metastable oxide. We present here evidence that this inferrence is correct, and we demonstrate that not only this oxide, but also a highly crystalline oxide which is produced when large calcite crystals are decomposed in vacuo, reacts more vigourously with water than does the product of calcite decomposition in air or nitrogen.

VAPOR PRESSURE OF SILICON AND THE DISSOCIATION PRESSURE OF SILICON CARBIDE

The vapor pressure of silicon and the dissociation pressure of silicon carbide have been obtained from total weight loss experiments with Knudsen effusion cells. Combination of the measured data with known entropies yields at 298°K for the heat of sublimation of silicon to silicon atoms 108.4±3 kcal and for the heat of the reaction SiC(s)=Si(g)+C(s) 126.0±3 kcal. From the pressure studies and from phase equilibria for the condensed phase silicon‐carbon system, the heats of formation for both the cubic and hexagonal modifications of silicon carbide are concluded to be −15.0±2 kcal.

The Variation of Vaporization Rates with Orientation for Basal Planes of Zinc Oxide and Cadmium Sulfide

The metal (0001) basal faces of hexagonal (wurtzite) modifications of both zinc oxide and cadmium sulfide are shown to vaporize at higher rates than the nonmetal (0001) faces. The difference in rates has been measured for cadmium sulfide as a function of temperature. Surface morphologies also differ between opposite basal faces of both kinds of crystals. The data suggest that desorption may be rate limiting.

The rate and activation enthalpy of decomposition of CaCO3

The heat balance during steady state decomposition of CaCO3 single crystals in vacuum is analyzed. It is shown that, contrary to the contention of others, the rate of decomposition of CaCO3 can be measured under conditions which make the slowest chemical step of the process, rather than heat transfer or gas phase diffusion, rate limiting. From new experimental measurements and measurements of previous investigators the apparent enthalpy of activation for CaCO3 decomposition (when the CO2 background pressure is negligible) is found to be 50 ± 3 kcal (calculated in terms of pressures equivalent to the CO2 decomposition flux densities), compared to 40.5 kcal for the equilibrium decomposition reaction. The rates of decomposition of single crystals which are measured in different studies differ by as much as a factor of 4, but average only about 10-5 times the flux densities that are theoretically achievable for the reaction.

Microstructural evolution during the decomposition of Mg(OH)2

Abstract The microstructural evolution during the decomposition of Mg(OH) 2 is characterized by transmission electron microscopy. The reaction is pseudomorphic and topotactic with an orientation relationship of parallel close-packed planes and directions. High-resolution micrographs show directly the small size and nearly cubic shape of the resulting MgO crystallites and their oriented stacking. Optical diffraction reveals a relatively narrow size distribution of pores and crystallites. The microstructure is discussed in terms of alternative reaction mechanisms.

Vapor Species of the Barium‐Oxygen System

Mass‐spectrometric and vacuum‐thermobalance studies of barium oxide between 1365° and 1917°K yield for the equilibrium pressure of BaO gas, logPatm = − 22610 / T + 7.90.The enthalpy for the reaction BaO(a) = BaO(g) was calculated to be ΔH°298 = 110.9 ± 2.8 kcal / mole by the second‐law method. The third‐law date are consistent with this value if a 3Π state is assumed to be at or near (within a few hundred cm−1) the ground state of BaO(g).The heat of sublimation of the dimer Ba2O2(g) was found from the mass‐spectrometer study to be about 142 kcal/mole and the heat of the reaction BaO(g) + Ba(g) = Ba2O(g) was found to be − 73 ± 15 kcal/mole at 1800°K. No reaction of barium oxide at 1800°K with O2(g) at pressures up to 10−3 atm was detected.

Prediction of Isomeric Form and Bond Angles for Covalent Molecules and Ions

An electrostatic model for prediction of bond angles in covalent molecules is extended to molecules and ions whose central atoms are surrounded by five or six valence electron pairs of which at least one pair is a lone pair. The stable isomers are the isomers which provide the lone pair electrons with the maximum possible cone of space adjacent to the central atom. Angular deflections from symmetrical coordination positions in these molecules are in the directions predicted by the model, but the observed deflections are usually less than those expected on the basis of behavior in simpler molecules.

Anomalous Temperature Dependence for a Partial Vapor Pressure

In a limited temperature range the partial pressure of gallium subsulfide (Ga2S) above gallium sesquisulfide (Ga2S3) increases when the temperature is decreased. The anomaly in the partial pressure is caused by changes with temperature in the equilibrium compositions of two solid phases that coexist at 1228� � 3�K. At this temperature the solids differ in sulfur content by 0.4 atomic percent sulfur.