Crystallite size Crystallization

Like excessively small crystallite size, crystal structure defects also increase the half-value width of a reflection. Such defects make the coherent scattering domains smaller. The amount of broadening depends on the type of defect, because in general newly generated scattering domains remain coherent with one another across the new disruptions. 

Table 9 shows structural properties for solution-crystallized polyethylene. Of particular interest are the relationships between crystallite size, crystallization temperature, and the stability of the crystals formed. 

Titania photocatalyst is used for air and water purification, photo-splitting of water to produce hydrogen, odor control and disinfectant. Crystal structure and crystallite size of titania particles are one of the most important factors that affect on the photoactivity. Photoactivity of anatase is higher than that of rutile, and increases with crystallite size [1]. Therefore, to increase photoactivity, it is desirable to find a route for the synthrais of the pure anatase titania with large crystallite size. 

In this work, flame spray pyrolysis was applied to the synthesis of titania particles to control the crystal structure and crystallite size and compared with the particle prepared by the conventional spray pyrolysis... 

To prepare the charge generation material of photoreceptor used in xerography, the crude VOPc synthesized at 150 °C for 4 h in the microwave synthesis was acid-treated, and then recrystallized. As shown in Fig. 4, the amorphous VOPc can be obtainol from crude VOPc by acid-treatment and the fine crystal VOPc can he obtained fixim amorphous VOPc by recrystallization. From XRD results, it can be calculated that the crystallite size of fine crystal VOPc is about 18 nm. As shown in Fig. 5, the fine crystal VOPc is well dispersed with uniform size. It indicates that this fine crystal VOPC can be probably used as the chaige generation material of photoreceptor. Thus, further research will be required to measure the electrophotographic properties of fine crystal VOPc. 

X-Ray diffraction has an important limitation Clear diffraction peaks are only observed when the sample possesses sufficient long-range order. The advantage of this limitation is that the width (or rather the shape) of diffraction peaks carries information on the dimensions of the reflecting planes. Diffraction lines from perfect crystals are very narrow, see for example the (111) and (200) reflections of large palladium particles in Fig. 4.5. For crystallite sizes below 100 nm, however, line broadening occurs due to incomplete destructive interference in scattering directions where the X-rays are out of phase. The two XRD patterns of supported Pd catalysts in Fig. 4.5 show that the reflections of palladium are much broader than those of the reference. The Scherrer formula relates crystal size to line width ... 

Studies performed on CdS [282, 283] have revealed the importance of the microstructure, i.e., crystal structure, crystallite size, and geometrical surface area, in both the control of band structure and the concentration and mobility of charges, in relation to the photocatalytic performance of the photocatalyst. It has been shown also that the solubility product of CdS colloids prepared from acetate buffer aqueous solutions of suitable precursors increases from 7.2x 10 for large particles to about 10 for small (< 2.5 nm) particle colloids, this increase invoking a positive shift on the cathodic corrosion potential [284]. 

The majority of crystallites observed were 3 or 4 nm In size. In Figure 3, a bar graph Illustrates the size range distribution and a comparison of mass variation for the 3 and 4 nm crystallite sizes. Although only thirty analyses were oiade, overall visual analysis confirmed the presence of hundreds of 3 to 4 nm platinum crystals with negligible numbers less or greater than these dimensions. It appears that slight variations In crystallite diameter and thickness have resulted In a fairly uniform number of platinum atoms per crystallite for the majority of the crystallites analyzed. In order to normalize count rates, the decrease In the field emission Intensity was taken Into account. 

The most important characteristic of the magnesium oxide powder used in these cements is its reactivity (Glasson, 1963). Magnesium oxide needs to be calcined to reduce this, otherwise the cement pastes are too reactive to allow for placement. Surface area and crystal size are important and relate to the calcination temperature (Eubank, 1951 Harper, 1967 Sorrell Armstrong, 1976 Matkovic et ai, 1977). The lower reactivity of calcined magnesium oxide relates to a lower surface area and a larger crystallite size. 

The valence band structure of very small metal crystallites is expected to differ from that of an infinite crystal for a number of reasons (a) with a ratio of surface to bulk atoms approaching unity (ca. 2 nm diameter), the potential seen by the nearly free valence electrons will be very different from the periodic potential of an infinite crystal (b) surface states, if they exist, would be expected to dominate the electronic density of states (DOS) (c) the electronic DOS of very small metal crystallites on a support surface will be affected by the metal-support interactions. It is essential to determine at what crystallite size (or number of atoms per crystallite) the electronic density of sates begins to depart from that of the infinite crystal, as the material state of the catalyst particle can affect changes in the surface thermodynamics which may control the catalysis and electro-catalysis of heterogeneous reactions as well as the physical properties of the catalyst particle [26]. 

The crystalline lamellar thickness Dc obtained by StrobPs method is initially 1.4 nm and grows to about 2.0 nm, which is roughly equal to the crystallite size in the chain direction of 2.8 nm estimated from the wide-angle X-ray diffraction (WAXD) [7]. Interestingly, the persistence length /p = 1.45 nm just before crystallization measured by SANS (also see Fig. 11) [9,10] is almost equal to the crystal thickness. 

At the crystal corners C9,10 atoms are located their number is six, irrespective of the crystal size. The number of C79 atoms, occurring along the edges, varies linearly with the crystallite size. The faces are built up of C93 atoms, whose number is a quadratic function of the crystallite size. Hence, the surface of a very large octahedron consists almost exclusively of C93 atoms, whereas in a very small crystallite it is made up to a substantial degree of C9,10 atoms. One may reasonably assume that somewhere in between these extremes there will be a region of crystallite diameters where a considerable fraction of the surface atoms are C79 atoms. For quantitative confirmation we refer to Fig. 2, in which the quantities N(Cpj q r " ) /Ns for the various atom types are plotted versus die. (In this and other similar plots the lines drawn through the points merely serve as an illustration, and do not refer to any actual crystals.) The main conclusions that can be drawn from Fig. 2 are ... 

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