Face Of Graphite Research Articles

The effect of anisotropy on emissivity

The correlation of structural anisotropy with normal spectral emissivity has been investigated experimentally and theoretically. Measurements include data on the two major crystallographic faces of pyrolytic graphite, pyrolytic boron nitride and single crystal beryllium. The results were obtained at temperatures near 800°C over a wavelength range from visible to near i.r. (0.60 to 11 μ) in both purified hydrogen and argon atmospheres. Pyrolytic graphite C-faces deposited at higher temperatures exhibit generally lower normal spectral emissivity (0.60 to 11 μ) corresponding to an increase in basal plane absorption as the deposition temperature increases. A further increase in basal plane absorption upon extreme annealing results in the lowest normal spectral emissivity of C-face pyrolytic graphite. This highly annealed C-face pyrolytic graphite exhibits a normal spectral emissivity dispersion which is close to the theoretically predicted curve of C-face single crystal graphite. Pyrolytic boron nitride in its lattice absorption region ( λ ≳ 2.60 μ) exhibits a normal spectral emissivity of the A-face which is greater than that of the C-face. This corresponds to the greater absorption in the basal plane than in the prismatic plane. Single crystal beryllium in its free-carrier intraband absorption region ( λ ≳ 2.50 μ) exhibits a normal spectral emissivity of the A-face, (1010), which is greater than that of the C-face, (0001). Thus, absorption is greater in the basal plane ( E → ⊥ c- axis ) than in the prismatic plane ( E → ⊥ c- axis ) corresponding to an inverse relationship of normal spectral emissivity to absorption in the intraband region.

Dependence of heat capacity of adsorbate on surface coverage on the basis of BET theory assumptions

From a consideration of elementary quasi-chemical equilibria between adsorption complexes, an equation has been obtained within the framework of the BET theory for the dependence of the average molar and differential heat capacities of an adsorbed substance on the relative vapor pressure. Approximate dependences of the average molar heat capacities of benzene adsorbed by a basal graphite face on the adsorption value have been calculated using this equation and the independently determined values for heat of adsorption and heat capacity at low surface coverage. The results are compared with direct calorimetric measurements of the heat capacity of adsorbed benzene.

The Physics and Chemistry of Interstellar Grains. I. Crystal Growth in Space

view Abstract Citations (2) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Physics and Chemistry of Interstellar Grains. I. Crystal Growth in Space Donn, Bertram Abstract Experimental and theoretical developments since about 1945 in (1) nucleation of condensing vapors, (2) mechanism of crystal growth, (3) chemical reactions at low temperatures including surface reactions of free radicals, and (4) radiation chemistry of solids, now permits much more reliable analyses of grains to be carried out than were previously possible. We consider the simplest case of growth of pure crystals without reactions. Molecular impact frequencies, surface lifetimes and diffusion rates are calculated. Growth rates and shapes are estimated according to which of the two crystal growth mechanisms is appropriate, (1) trapping with little mobility, producing nearly spherical particles, or (2) dislocation trapping yielding nonspherical grains. Process 2 requires a significant diffusion distance. At 500K all abundant metals except aluminum, iron, and nickel would diffuse and tend to develop irregular particles. At 20~K process (1) would apply to all metals. Molecules of hydrogen with carbon, oxygen, and nitrogen would probably diffuse. At 5 K only molecules more volatile than CO2 undergo diffusion. ~latelets permit sufficient mass for water, ammonia, methane, or graphite grains to produce interstellar extinction. At 20~K the expected surface diffusion of hydrogen compounds is about 1 mm in 10 yr. Carbon atoms on the hexagonal graphite face appear to be similar. Trapping occurs on crystal edges, and platelets should tend to form. A small number of screw dislocations would lead to highly irregular whisker arrays. Exponential growth rates occur. For platelets with N (H) = 1/cm3, r reaches I0-~ cm in about 3x108 yr or 3x107 yr if N (H) =10. Inclusion of chemical reactions, heterogeneous composition, and radiation chemical effects should make growth less favorable. Also recent extinction measurements in the ultraviolet cannot be explained by uniform grains. Consideration should be given to extinction by Platt-type grains. Publication: The Astronomical Journal Pub Date: 1965 DOI: 10.1086/109645 Bibcode: 1965AJ.....70S.320D full text sources ADS |

The adsorption energies of CO2, SO2, (CH3)2CO and (C2H5)2O on graphite

1. The adsorption energies of isolated molecules of different structure have been calculated, i.e., molecules of CO2, SO2, (CH3)2CO, and (C2H52O, on the surface of the basal face of graphite. The two terms in the potential of the electro-kinetic (dispersion) forces, the potential of the electro-static induction forces and the potential of the repulsive forces were calculated and were found to be close to the values of the differential heats of adsorption on graphitized carbon blacks, measured at small coverages of the surface. 2. The calculated mutual interaction energies of adsyrbate molecules for molecules of CO2, SO2, (CH32CO and (C2H52O in a dense monolayer for different packings with consideration of the energy of electro-kinetic, electrostatic-dipolar, electrostatic quadrupolar and repulsive interaction forces, are found to be in agreement with the experimental values of the rise in the heat of adsorption with coverage of the monolayer.

The adsorption energies of water, alcohols, ammonia and methylamine on graphite

1. The adsorption energy of water, normal and isomeric alcohols, ammonia and methylamine molecules on the surface of the basal face of graphite has been calculated. In the calculation, the two terms in the potential of the electro-kinetic (dispersion) forces and the potential of the electrostatic (induced) attractive forces have been considered and the potential of the repulsive forces has been calculated in the form of its relation with the distance. 2. The values of the adsorption energies of isolated molecules of water, alcohols, ammonia and methylamine are significantly less than the measured heat of adsorption. Calculation of the energy of association of these molecules in the adsorption layer and using the energy of formation of hydrogen bonds, led to agreement between the calculated values of the adsorption energy and the heats of adsorption.