Crystallite parameters

The physicochemical properties of carbonaceous materials can be altered in a predictable manner by different types of treatments. For example, heat treatment of soft carbons, depending on the temperature, leads to an increase in the crystallite parameters, La and Lc and a decrease in the d(0 0 2) spacing. Besides these physical changes in the carbon material, other properties such as the electrical conductivity and chemical reactivity are changed. A review of the electronic properties of graphite and other types of carbonaceous materials is presented by Spain [3],... 

The normalized peak-shape function PS introduced by equation (1) must be determined in order to figure out the dependence of PS on several crystallite parameters, such as average size of crystallites, misorientation of crystallites in the sample etc. These parameters lead to a broadening of reflections, which must be taken into account. 

X-Ray Diffraction Data. The QI components found in the products of hydrocarbon pyrolysis are generally brittle, infusible solids. Because of this appearance the QI are usually classified under the general heading of coke. However, to more clearly define the nature of the QI produced in this work, x-ray diffraction patterns were obtained. The crystallite parameters for a graphitic matrix, as defined in Table II, were calculated for the QI and are compared in Table II with the same parameters determined for a sample of /3-resins and for a sample of coke obtained from a test carbon anode. The x-ray diffraction pattern for the sample of /3-resins was not well defined, so the value of Lc could not be determined reliably. The values of the other /3-resin parameters are... 

Table II. Crystallite Parameters from X-ray Diffraction Patterns...

Crystallite parameters La, crystallite diameter in the plane of the a axis Lc, crystallite thickness in the C axis a, graphite unit cell dimension in the plane of the layers C, twice the interlayer spacing in the graphite matrix. 

Table II contains the micropore volume equations for model E. The first equation shows the value of the micropore volume for this structural unit, which, logically, will be equal to the volume of one micropore multiplied by the number of micropores in the structural unit (eq. 7). The mass of this unit is known (eq. 6) and, consequently, the specific micropore volume can be calculated. It can be observed that the specific micropore volume only depends on the pore width (w) and on the graphitic crystallite parameters (J and p), being independent of the assumed shape of the lamellas, consequently it can be accept a more real shape (discotic).

Mirrors of Ni, Fe, Cd, Hg deposited from the vapour on cold surfaces Crystallites. Parameters same Gen, Zelmanov and... 

Measurements conducted on samples, made of other grades of steel have shown that the shift of frequency characferistics of the applied signal are closely connected with sizes of crystallite grains and may be applied for the determination of parameters of the material structure. 

Draw ratio Density of the amorphous material da) (g/cm-" ) Amorphous orientation function fa) Crystallite length Oc) (nm) Long period (L) (nm) Degree of crystallinity (X=>) Substructure parameter (A) Axial elastic modulus ... 

The low electrical conductivity of PET fibers depends essentially on their chemical constituency, but also to the same extent on the fiber s fine structure. In one study [58], an attempt was made to elucidate the influence of some basic fine structure parameters on the electrical resistivity of PET fibers. The influence of crystallinity (jc) the average lateral crystallite size (A), the mean long period (L), and the overall orientation function (fo) have been considered. The results obtained are presented in the form of plots in Figs. 9-12. 

In the second group of models, the pc surface consists only of very small crystallites with a linear parameter y, whose sizes are comparable with the electrical double-layer parameters, i.e., with the effective Debye screening length in the bulk of the diffuse layer near the face j.262,263 In the case of such electrodes, inner layers at different monocrystalline areas are considered to be independent, but the diffuse layer is common for the entire surface of a pc electrode and depends on the average charge density <7pc = R ZjOjOj [Fig. 10(b)]. The capacitance Cj al is obtained by the equation... 

Differences in the parameters of the electron gas between fine crystallites and the compact metal or large crystals. 

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

Some other studies showed that the combination of the three polymorphs with reduced crystallite size and high surface area can lead to the best photocatalysts for 4-chlorophenol degradation [37], or that particles in the dimension range 25-40 nm give the best performances [38]. Therefore, many elements contribute to the final photocatalytic activity and sometimes the increased contribution of one parameter can compensate for the decrease of another one. For example, better photocatalytic activity can be obtained even if the surface area decreases, with a concomitant increase in the crystallinity of the sample, which finally results in a higher number of electron-hole pairs formed on the surface by UV illumination and in their increased lifetime (slower recombination) [39]. Better crystallinity can be obtained with the use of ionic liquids during the synthesis [39], with a consequent increase of activity. 

All of these parameters are not under control. To deliver a more uniform distribution of crystallites, specific nucleating agents are added whilst processing. It has also been shown that many of the desirable properties are the result of small regular spherulites (or nascent crystals). 

The thickness of the ordered crystalline regions, termed crystallite or lamellar thickness (Lc), is an important parameter for correlations with thermodynamic and physical properties. Lc and the distribution of lamellar thicknesses can be determined by different experimental methods, including thin-section TEM mentioned earlier, atomic force microscopy, small-angle X-ray scattering and analysis of the LAM in Raman spectroscopy. 

It should be clear that contrast and composition are by no means related to each other. Melting is changing only the composition parameter. Different thermal expansion of crystallites and amorphous matrix is (almost) only changing the contrast. 

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