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An investigation has been carried out on the mechanism of sintering of metallic particles. Single layers of spherical copper particles have been sintered under varying conditions of time and temperature in a dry hydrogen atmosphere. Analysis of the rate of interface contact between the particles has shown metallic sintering to be a diffusion process, in agreement with the theory of Kuczynski. The heat of activation of self-diffusion of copper has been calculated and found to be 55,000 cal./mole. This is in close agreement with Kuczynski and also values obtained by other workers using the radioactive tracer method.

The kinetics of the zirconium'UOn reaction was studied using crystal-bar zirconium and fused UOn or UOn high'fired in hydrogen. The reaction was studied with (1) solid zirconium cylinders and UOy powder, (2) compacted mixtures of zirconium and UOn powders, and (3) sandwich' type elements. Experiments performed with sandwich-type elements yielded the most useful data both from a practical and a fundamental point of view. Exploratory experiments designed to give a wider range of experimental materials were unsuccessful in producing substoichiometric UO^. Results obtained from sandwich-type experiments showed that slow reactions at PSO and 1100 F formed reaction zones less than 0.0002 in, thick in 65 days, A measurable reaction took place at 1300 F, while at 2000 F, the reaction was very rapid. The UOn seems to be reduced to its elements, uranium and oxygen, which diffuse simultaneously, but not with equal speed, into the zirconium. Reactants heated at 1300 c and slowly cooled to room temperature, were found to have formed alpha zirconium and alpha uranium adjacent to the UOn, Deeper in the original zirconium region the epsilon uraniumzirconium intermetallic phase was found dispersed in a zirconium-oxygen solid solution. Elements slowly cooled from 2000 F contained alpha zirconium and alpha uranium at the UOn-metal interface. A continuous stained band having the alpha uranium lattice structure and containing some zirconium was observed beneath the two-phase region. Oxygen had diffused beyond the uranium-containing region, forming the hard zirconium-oxygen solid solution. On the basis of limited data obtained, it has been estimated that penetration of oxygen through the 0.030-in.-thick zirconium jacket walls probably would cause their destructive embrittlement in about 300 days at 1300'*F or 2-1/2 days at 2000^. The reaction between Zircaloy 2 and UOn at 1300 F appears to be somewhat less rapid than that between unalloyed zirconium and UOn. The phases formed by this reaction have not been identified.

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The rate of Ieaction of oxygen with are-melted and rolled mdide thoriunl has been found to obey the parabolic rate law in the temperature range of 850 ~ to 1415~ at 1 arm pressure The rate constant can be expressed as k = 55 X 107 e -62,s~176

The sesquiearbide of uranium, U2Ca, has been found to exist as a stable compound below about 1800~ At higher temperatures, it decomposes into UC and UC> It has not been produced directly from the melt but only by heating a mixture of UC and UC~ between 1250 ~ and t800~ A slight amount of stressing, even as little as that incidental to handling a specimen, is necessary to initiate the reaction. An experimentally determined density of 12.7 g/cm 3 agrees very well with a density of 12.88 g/cm 3, calculated from x-ray data. The crystal structure of the compound had been found to be body-centered cubic, having a space group symmetry /713d and a unit-cell dimension of a0 = 8.088 =~ 0.001 A. INTRODU CTIONIn 1896, Moissan (i) first prepared a compound of uranium and carbon to which he assigned the formula U2Ca. Careful chemical analysis and metallographic observation of uranium carbide, prepared by others (2, 3) by the same method, indicated the correct formula to be UC2.With the advent of the Manhattan District Proiect, there was renewed interest in the uraniumcarbon system. Rundle, Snow, and co-workers at the Iowa State College studied the system and proposed a tentative constitution diagram (4, 5). They concluded that uranium sesquiearbide does not exist at ordinary temperatures even in a metastable state, but that at very high temperature (probably above 2000~ it "must indeed exist." This was inferred from microscopic and x-ray investigations. Quenched samples gave a Widmanstiitten structure of UC and UC> Powder x-ray diffraction patterns showed "single crystal" reflections. However, the reflections were due to both UC and UC2 arranged so that their crystal axes were mutually parallel to the main edges of the apparently single crystal.In the course of an investigation of the uraniumcarbon system in the compositional range of the carbides UC and UC2, it has been found that U~Ca can be formed from a mixture of UC and UC2. This report describes the preparation and properties of the sesquiearbide. PREPARATIONThe alloys used in this investigation were made by arc melting under an argon atmosphere in an enclosed water-jacketed copper crucible using a movable graphite electrode. The melting stock, which Manuscript received March 1, 1951. This paper prepared for delivery before the Philadelphia Meeting, May 4 to 8, 1952. The work reported herein was done under Contract W-7405-eng-92 for the United States Atomic Energy Commission.consisted of high-carbon uranium-carbon master alloy (10-12 weight per cent carbon) mixed with pure uranium chips, was charged continuously into the furnace during the melting operation. The original ingot was crushed and remelted at least once to achieve greater homogeneity. Pieces of these ingots were then given various heat treatments.The sesquicarbide of uranium was first observed by metallographie means in a specimen containing about 7.5 weight per cent carbon. The specimen had been heated at 1500~ for one hour in vacuum and cooled in the furnace, with the power off, at the cooling rate of the ...

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Reactions designed to reduce the oxygen content of UO2 below stoichiometric composition are described. No evidence was found for the existence of substoichiometric U02-The reaction of metal with UOj + ^ yields a phase described in the literature as uranium monoxide, UO. However, the reaction proceeds at a low rate and the reaction products are contaminated with a large amount of U02-The reaction of UC with UOj is described: this reaction is capable of producing a single-phase material of composition ^^x^Ux ^'-^'^^ ^y *«*'»•»« «•« vacuum at 1600 to 1800 C.