Purified graphite

Moderators. Neutrons are most effectively slowed by collisions with nuclei of about the same mass. Thus the best moderators are those light atoms which do not capture neutrons. These are H, " He, Be and C. Of these He, being a gas, is insufficiently dense and Be is expensive and toxic, so the common moderators are highly purified graphite or the more expensive heavy water. In spite of its neutron-absorbing properties, which as mentioned above must be offset by using enriched fuel, ordinary water is also used because of its cheapness and excellent neutronmoderating ability. 

Highly purified graphite types (> 99..S% C) only obtainable by additional chemical refining ... 

Experiments have shown that small amounts of certain metals can accelerate the rate of char combustion (4 9). A number of anions and cations have been shown to accelerate the combustion of carbons at concentrations of 10 to 1000 ppm. Table II shows the relative influence on the combustion rate of various salts added as solutions to purified graphite. Relatively small amounts of metals can accelerate the rate of combustion by many orders of magnitude. To effectively catalyze the combustion rate of coal, the metal which accelerates the rate must be distributed on nearly the molecular level, and be present in sufficient concentration to accelerate the rate. The range of relative acceleration of the combustion rate by different metals is shown in Figure 3. These estimates are made... 

Figure 2. Combustion rates of highly purified graphite. Reproduced with permission from reference 4. Copyright 1972 Butterworth.

Nisel son (394, 395) has concluded that any separation process based on differences in volatility should be carried out within the temperature range 440°-475°C. Vapor pressure measurements at the triple point only lead to a separation coefficient of 1.7 (415). On the other hand, small quantities of zirconium-free hafnium have been prepared by gas chromatographic techniques on a synthetic, highly purified graphite column at 295°C (543). 

The growth rate (r) has been measured as a function of temperature ( from 1173 to 1400K) and residence time (from 0.2 to 5s). The measurements were performed in situ with a microbalance. The purified graphite substrates were hung in the isothermal section by a molybdenum wire, 100pm in diameter, to the microbalance beam. No radial temperature dependence has been detected. The growth rate here is the mass gain measured by means of the microbalance per unit time and substrate area in the steady state. 

The active material of the positive electrode (1) consisting of mercury oxide HgO and 5-15% fine purified graphite is pressed into the nickel-plated steel case (6). Zinc powder (2) is pressed into the steel cover (4) and amalgamated. The separator (3) consists of several layers of a special alkali-resistant filter paper. The separator and the powdered zinc electrode are impregnated with a 40% KOH solution saturated with zincate. After assembly the battery is sealed by bending the edges of the case (6). The terminals (case and cover) are insulated by a mbber or plastic spacer (5). 

The thermodynamic parameters of adsorption of nitrophenols on highly purified graphite from buffered aqueous solution have been tabulated by Kozlov and Glushchenko. The adsorption isotherms were determined at 293, 308, and 323 K by a chromatographic technique. AG ds. is the quantity defined in equation (56), and the differential enthalpy was calculated from [3 In C2/3(1/T)]r = q/R (see ref. 3 p. 68). It is not stated how c was calculated from the observed adsorption. They argue that because nitrophenols are essentially dissociated when they adsorb at pH 6.5 the entropy of adsorption includes contributions arising from an entropy of ionization and entropy of hydration. In another paper Avramenko, Glushchenko, and Kozlov have studied adsorption at two pH s (1.60 and 6.50) of a series of nitrophenols from... 

In addition to leading to stored energy, irradiation of graphite results in activation of impurities. Residual, low levels of the chlorine used to purify graphite are converted to the long-lived chlorine-36. The research programme referred to previously has also addresses the chlorine-36 content of graphite. 

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