No abstract

Vacancies in graphite crystals can be rendered visible for electron microscopy by a technique which combines cleaving, etching, and decorating. The crystals are cleaved, and the vacant lattice sites in the exposed surface are attacked by a mixture of chlorine and oxygen, which expands these vacancies into monolayer loops of any desired diameter. These loops are decorated with gold to enhance their contrast. Numerous tests have established that the loop concentration is a quantitative measure of the vacancy concentration. The method is capable of determining vacancies at concentrations at least as low as 10—10 per carbon atom. The relative chemical reactivities of carbon atoms surrounding a vacancy, of carbon atoms at the edges of layers and of carbon atoms within layers have been measured.

Graphite contains varying concentrations of screw dislocations whose Burgers vector parallels the c axis. Single crystals of natural graphite contain very few such dislocations; furthermore, their Burgers vector always exceeds 450 angstroms. Pyrolytic graphites annealed above 3000 degrees C contain abundant screw dislocations, ranging from 10(6) to 5 x 10(8) per square centimeter in two different samples prepared by somewhat different methods. The Burgers vectors of these screws are predominantly 3.35 angstroms.

Sodium nitrate, potassium nitrate, and potassium chlorate have been exposed two to four weeks in the experimental thimble of the Argonne Heavy Water Pile. After exposure these salts evolve a gas when dissolved in water or when melted or heated to a crystal transition point. The gas yields are in sodium nitrate ca 0.3, in potassium nitrate ca 0.8, and in potassium chlorate 2.0–3.0 molecules gas/100 ev. For the nitrates this gas has been shown to be largely oxygen which is trapped during irradiation in the gaseous form in small pockets in the crystal. This conclusion is based on the paramagnetism of the irradiated crystals, on chemical analysis of the evolved gases, and on measurements of the amounts of gas liberated in crushing experiments. When irradiated crystals are heated below the melting point, the gas pockets grow and coalesce, this being accompanied by a decrease in crystal density. In irradiated nitrates, oxygen and nitrite ion are formed in equivalent amounts. Experiments with low energy x-rays indicate that most of the decomposition results from electronic ionization and excitation and not from elastic collision of the bombarding particles.

The reactions of NH2(X̄2B1) with the hydrocarbons methane (1), ethane (2) and propane (3) were investigated behind incident shock waves at temperatures between 1500 K and 2100 K. The decay of NH2 was measured by laser absorption. NH2 was formed by the thermal decomposition of hydrazine. The following rate constants were obtained for the temperature range mentioned: k1 = (1.2±0.6)·1013 exp(‐(63.4±3.4) kJ/mol/RT) cm3 mol−1 s−1 k2 = (9.7±0.3)·1012 exp(‐(48.0±1.8) kJ/mol/RT) cm3 mol−1 s−1 k3 = (1.7±0.4)·1013 exp(‐(44.6±2.0) kJ/mol/RT) cm3 mol−1 s−1. These measurements extend the results obtained at low temperatures by Demissy and Lesclaux (330 K–250 K) and by Hack et al. (400 K–1080 K) into the range of interest for combustion processes. The apparent energies of activation fit rather well into an Evans Polanyi plot. They correlate with the energies of activation of the corresponding reactions of OH(X2II)and those of CH3(X2A2) radicals.