INCREASING THE INFILTRATION ABILITY OF ALUMINUM ALLOY FOR IMPREGNATION IN A NON-AUTOCLAVE METHOD

An increase in the efficiency of gas-free impregnation of a porous carbon-graphite frame to aluminum alloys by applying thin electroplated metal alloying coatings to the surface of the pores of a carbon-graphite sample by electrochemical method is investigated. It is shown that the preliminary deposition of such coatings makes it possible to obtain alloying with highly pure elements, increasing the filling of pores with matrix melt at impregnation temperature (800 ° C).

Effects of the Stacking Faults on the Electrical Resistance of Highly Ordered Graphite Bulk Samples

High anisotropy and the existence of two-dimensional highly conducting interfaces at stacking faults parallel to the graphene planes of the graphite structure influence, in a non-simple way, the transport properties of highly oriented graphite. We report two related effects on the electrical resistance of highly oriented pyrolytic as well as of natural graphite bulk samples, measured with the four points method in the temperature range 300 K ≤T≤ 410 K. A qualitative and quantitative change in the temperature dependence of the resistance was obtained by simply enlarging the electrodes and contacting the edges of the internal interfaces on the same sample. Additionally, at temperatures T≳350 K the resistance can change with time. We show that this temperature-dependent annealing effect is related to the stacking faults and can irreversibly change the absolute value of the resistance and its temperature dependence. A partial recovery is obtained after leaving the sample at normal conditions for several days. The overall results stress the importance of the electrodes location on a bulk graphite sample, the contribution of the stacking faults in the interpretation of the measured transport properties and the need of systematic studies on the influence of high temperature annealing on the interfaces properties.

Single step Production of graphite from organic Samples for Radiocarbon Measurements

We present a new low-cost, high-throughput method for converting many types of organic carbon samples into graphite for radiocarbon (14C) measurements by accelerator mass spectrometry (AMS). The method combines sample combustion and reduction to graphite into a single procedure. In the Single Step method, solid samples are placed directly into Pyrex containing zinc, titanium hydride and iron catalyst. The tube is evacuated, flame sealed, and placed in a muffle furnace for 7 hr. A variety of organic samples have been tested including oxalic acid, sucrose, wood, peat, collagen, humic acid, and contamination swipe samples. The method significantly reduces the time required to produce a graphite sample for 14C measurement, with analytical precision and accuracy approaching that of traditional two-step combustion and hydrogen reduction methods. The details and applicability of the method are presented.

Purification of Indonesian Natural Graphite by Acid Leaching Method as Nuclear Fuel Matrix: Physical Characterization

Graphite matrix in Pebble Bed Reactor (PBR) fuel has an important role not only as neutron moderator and structural material to protect nuclear fuel, but also as heat transfer media. Therefore, the graphite matrix must meet the criteria of physical and chemical properties specified for PBR fuel. This paper focuses on the purification of the Indonesian natural graphite by using hydrometallurgy method with acid treatments. The characteristic of the purified graphite was studied for its specification compliance as a candidate of fuel matrix for PBR type of High Temperature Gas Cooled Reactor (HTGR). Acid and acid mixtures such as HF, HNO3+H2SO4 and HF+HCl+H2SO4 were used for the purification process. Crystal structure examination by X-Ray Diffraction indicates that the graphite sample was 2H poly type with hexagonal crystal structure and lattice group of P 63 m c space group. It was observed that the graphite sample purified by HNO3+H2SO4 mixture had the closest resemblance to single crystalline graphite with a <d002> deviation of 0.94 when compared to perfect graphite crystal. The density of graphite decreases from 2.3273 g/cm3 (before acid treatment) to 2.1808; 2.2203 and 2.2752 g/cm3 after treatment with HF, HNO3+H2SO4 and HF+HCl+H2SO4, respectively. These results are close to the theoretical density value of 2.26 g/cm3. The surface area decreases from 10.346 m2/g to 6.177; 5.831 and 7.63 m2/g for the treated graphite with HF, HNO3+H2SO4 and HF+HCl+H2SO4 respectively. However, these values are still higher than that of nuclear grade graphite (i.e. between 4.80 and 5.55 m2/g). The average diameter size of graphite decreased from 29.65 μm (before treated acid) into 23.12 μm (after treated acid). The Indonesian natural graphite obtained from acid purification treatment is potential to be used as matrix material for PBR - HTGR fuel, but further treatment is necessary.

Microphysical properties of carbonaceous aerosol particles generated by laser ablation of a graphite target

In this work various microphysical properties including mass concentration, size distribution and morphology of aerosol particles generated by laser ablation of a high purity graphite sample were investigated in detail. Supplementary chemical analysis of the generated particles including microstructure investigation by high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy were also made. These measurements proved that the proposed method can be used to model the real atmospheric carbonaceous particulate under various climate relevant conditions regarding its specific properties investigated here. However, to introduce the presented methodology as a novel BC surrogate, further improvement and investigation including simplification in experimental setup, comprehensive analysis of thermochemical refractiveness and optical responses of the generated particles as well as comparative study with the presently available concurrent surrogates are required. The related results of these issues are planned to be demonstrated in other studies.

GEO-MIX-SELF calculations of the elastic properties of a textured graphite sample at different hydrostatic pressures

The recently developed GEO-MIX-SELF approximation (GMS) is applied to interpret the pressure dependence of the longitudinal ultrasonic wave velocities in a polycrystalline graphite sample that has already been investigated in a wide range of experimental contexts. Graphite single crystals have extremely anisotropic elastic properties, making this sample a challenging test to demonstrate the potential of the GMS method. GMS combines elements of well known self-consistent algorithms and of the geometric mean approximation. It is able to consider mixtures of different polycrystalline phases, each with its own nonspherical grain shape and preferred orientation (texture). Pores and `cracks', typical for bulk graphite, are modeled as phases with `empty' grains. The pressure dependence (up to 150 MPa) of the experimental wave velocities can be well explained using the known texture of the sample by fitting the shape parameters and volume fractions of the graphite grains, cracks and spherical pores. The pressure dependence of these parameters describes a reasonable scenario for the closing of the cracks and pores with increasing pressure.