Isotropic Graphite Research Articles

The hardness of irradiated graphite

The Vickers hardness of a near isotropic reactor graphite has been measured before and after neutron irradiation of various doses. The hardness of the irradiated specimens has also been measured after annealing at successively higher temperatures. An attempt is made to interpret the results in the light of present knowledge of irradiation damage in graphite.

The mechanical properties of reactor graphite

The following mechanical properties of three types of isotropic reactor graphite have been measured, before and after fast neutron irradiation at 150°C in DIDO: Young's modulus, Poisson's ratio, stress-strain curves in tension and compression, shear strength, uniaxial tensile and compressive strength, triaxial compressive strength and Vicker's hardness. The propagation of cracks in these graphites has been examined microscopically and measurements made of the work of fracture. The accumulation of irradiation damage has been followed by measuring changes in linear dimensions of the specimens and of pyrolytic graphite samples which show that the graphite is heavily damaged. The experimental data have been compared with theories of the mechanical properties of unirradiated and irradiated graphite. A model in which large pores act as “Griffith” flaws is compatible with the data on irradiated graphite, but in unirradiated graphite, plastic flow modifies the failure in compression.

The elastic constants of reactor graphites

Abstract The complete set of elastic constants’ has been determined for reactor Grade A graphite and also for a nearly isotropic graphite of similar density and crystallite size. The specimens used were irradiated in a hollow fuel element in the materials testing reactor BEPO under conditions of good temperature and neutron flux stability at 25°C and were re-measured periodically until all the elastic moduli had ceased to increase. The experimental values were then compared with theoretical values computed from current elastic constant values for single crystals and the anisotropy function describing the distribution of crystallite orientations in a block. The cases of uniform stress and uniform strain were considered, and the role of the porosity identified.

The dimensional changes produced in graphite by absorption of bromine

It is well known that, in general, polycrystalline graphites have substantially lower volume coefficients of thermal expansion than the crystallites of which they are composed. It has previously been suggested that this is because microporosity, orientated parallel to the layer planes, absorbs part of the c-axis expansion of the crystallites. If this is true then the volume expansion of polycrystalline graphite when it absorbs bromine should also be lower than that of the crystallites and there should be a correlation between linear thermal expansion and initial linear growth per unit bromine concentration. The dimensional changes of a wide variety of polycrystalline graphites, when they absorb bromine, have been investigated and the correlation between initial growth per unit concentration of bromine and thermal expansion coefficient has been demonstrated. It has also been observed that when the strain of the crystallites in the c-axis direction becomes large the orientated microporosity fills up. When this happens the volume growth of the polycrystalline graphites per unit concentration of bromine approaches that of the crystallites. At even higher crystallite strains in the c-axis direction, anisotropic polycrystalline materials tend to expand faster than the crystallites and this is considered to be caused by the generation of additional porosity. This effect is less marked in the isotropic polycrystalline graphites. The relation between growth due to bromination and dimensional changes produced by fast neutron irradiation is discussed. Data on the effect of graphitization temperature on the subsequent dimensional changes produced by bromination are also described.

The Thermal Dependence of the Elastic Moduli of Polycrystalline Graphite

Young's modulus and the shear modulus of grade SA-25 molded graphite and grade AUF extruded graphite were measured as functions of temperature in the range from 25°C to 2000°C. In the case of the extruded material, variations of the two moduli with position and direction relative to the axis of extrusion were also investigated. The moduli were found to increase with temperature, a behavior in contrast with that exhibited by most materials. Maxima in the low frequency shear modulus values and increases in the internal friction of both graphites observed in the range from 1550–1750°C may possibly be interpreted in terms of grain boundary relaxation. The bulk modulus and Poisson's ratio at room temperature were calculated from the data for the nominally isotropic SA-25 graphite to be 6×1010 dynes/cm2 and 0.27, respectively.

A Search for Preferentially Directed Electron Velocities in Crystalline Graphite with the Multicrystal Spectrograph

The author's theory of the broadening of the Compton line as a Doppler effect of electron velocities is briefly reviewed and it is pointed out that only the component velocity along a direction which nearly bisects the angle between primary and scattered x-ray beams should be effective in broadening the line. Crystalline Ceylon graphite possesses properties which lend hope to the belief that a class of weakly bound or structure electrons in this crystal might have momenta restricted uniquely to parallelism with one plane in the crystal: the (0001) plane. A composite scatterer was built up out of blocks consisting of the crystal flakes all orientated with their (0001) planes in mutual parallelism and the blocks in turn were so orientated that the normals to these planes bisected the angle formed by the primary and scattered x-ray beams. If the electron momenta are orientated parallel to the plane of the graphite flakes one should expect the contribution of such electrons to the shifted scattering to give a sharp line or peak superposed on the broader structure caused by the remaining isotropically distributed momenta. Details of the experimental set-up are described and the spectrum obtained from scattering by the Ceylon graphite scatterer is compared with the spectrum from an isotropic Acheson graphite scatterer. The breadth and structure of the shifted line proves to be quite identical in the two cases and the conclusion is drawn that if a class of electrons having selectively orientated momenta exists in the crystal-line graphite it constitutes less than 5 percent of the total number. The bearing of this result on related questions is discussed.