Crystallites, introduction

In this section, we will describe the principal features of the main methods and the information that the Rietveld analysis generally delivers, especially related to the investigations on poor crystallites and multiphase compounds. This introduction is not exhaustive there are hundreds of articles that explore practically all of the aspects of this kind of analysis and now, 40 years after the first article [43], new improvements and results are still being published. 

First, the breadth of the XRD peaks of p-MgH increases with increasing milling time. As discussed in Sect. 1.4.3, this is related to the formation of crystallites (nanograins) within the powder particles, which may be accompanied by the introduction of lattice strains. Table 2.8 lists nanograin size and lattice strain of P-MgH as a fnnction of milling time estimated from the procedure described in Sect. 1.4.3. 

Figure 1.67 Specific volume as a function of temperature on cooling from the melt for a polymer that tends to crystallize. Region A is liquid, B liquid with elastic response, C supercooled liquid, D glass, E crystallites in a supercooled liquid matrix, F crystallites in a glassy matrix, and G completely crystalline. Paths ABCD, ABEF, and ABG represent fast, intermediate, and very slow cooling rates, respectively. From K. M. Ralls, T. H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

The structure of crystalline polymers may be significantly modified by the introduction of fillers. All aspects of the structure change on filling, crystallite and spherulite size, as well as crystallinity, are altered as an effect of nucleation [9]. A typical example is the extremely strong nucleation effect of talc in polypropylene [10,11], which is demonstrated also in Fig. 2. Nucleating effect is characterized by the peak temperature of crystallization, which increases significantly on the addition of the filler. Elastomer modified PP blends are shown as a comparison crystallization temperature decreases in this case. Talc also nucleates polyamides. Increasing crystallization temperature leads to an increase in lamella thickness and crystallinity, while the size of the spherulites decreases on... 

On the supposition that the total number of unit cells keeps invariable and no aluminum atoms are lost during the boronation, the composition of unit cell and the population of vacancies can be estimated as listed in composition of unit cell (I) in Table 2. It can be seen that the vacancies occupy about 30-50% of total T sites after the boronation. However, it should be noted that the population of vacancies thus obtained by chemical analysis is only a bulk average result. The composition on the surface of crystallites is actually different from that in the bulk because the dissolution of silicon starts first from the outer surface, so that the vacancies on the surface are much more than those in the interior of crystallites. Such a large number of vacancies on the surface will result in corrosion and dissolution of the surface parts of crystal particles. Therefore, the number of unit cells in the sample after the boronation is actually less than that before the boronation, whereas boron atoms in each unit cell should be more than those shown in composition of unit cell (1) in Table 2. On the other hand, if all the 64 T sites are occupied by silicon and trivalent atoms, we can give another set of compositions as shown in composition of unit cell (II) in Table 2. The real composition of a unit cell should be between these two sets of compositions, that is, the 64 T sites are neither occupied completely nor vacated so severely that the collapse of the framework occurs. It can also be seen that the introduction of boron atoms is so limited that there are no more than 1.5 atoms per unit cell even though the repeated boronation is performed. 

The porous volumes measured by N2 adsorption are listed in Table 3. After the boronation, the total porous volumes (Vt) of the samples increase, corresponding to the increase of benzene adsorption capacity mentioned above. This should be resulted from the following aspects (1) The average mass of zeolite crystallite decrease and the number of crystal particles in unit weight of sample increases after the boronation owing to a limited introduction of trivalent atoms and Na+cations as counterions, as well as a severe dissolution of silicon. Thus, the total porous volume (mL/g) and the adsorption capacity increase. (2) The transformation of pore size occurs during the boronation. As shown in Table 3, the mesoporous volumes increase and the microporous volumes decrease after the boronation, meaning that some micropores are developed into mesopores due to the removal of silicon from the framework. This is also one of the important reasons why the total porous volumes as well as the adsorption capacities increase after the boronation. 

Overall, it can be concluded that zeolites, and more specifically MFI, are adequate catalysts for oligomerization of short chain olefins to produce gasoline and even diesel range fuels. Selectivity and catalyst life is strongly dependent on parameters such as crystallite size, Si/Al ratio, and poisoning of external surface sites. The introduction of some metals (Ni) can be helpful. 

A higher density7 sol—gel abrasive, produced by the introduction of seed crystallites formed by wet-milling with high alumina media or by introduction of submicrometer a-alumina particles, was patented (28) and designated Norton SG. The micro structure of this abrasive consists of submicrometer a-alumina crystals (Fig. 1) and its bulk density7 approaches that of fused alumina. Norton SG has proven to be an exceptional performer in coated and bonded abrasive products it was awarded the 1989 ASM Engineering Materials Achievement Award (29). 

The introduction of other metals into the lattice through alloy formation introduces alien crystallites that will also impede slip in steel crystals. In particular, if the alloying agent is carbon, hard crystallites of iron carbide may form, changing the microstructure to impede slip. By comparison, austenite usually does not contain iron carbide, and is quite susceptible to slip. 

The subsequent decrease of H in Fig. 6.5 back to 25 MPa again at the highest deformations (s = 60-80%) can be explained by a partial destruction of crystallites due to the pulling out of hard segments from their crystallites. This mechanism has been demonstrated by SAXS and WAXS (Stribeckefa/., 1997) measurements. The introduction of defects in the crystallites in this way may induce a decrease in their value. 

When the shapes of crystallites are isotropic, random distribution of their orientations is not a problem and deviations from an ideal sample are usually negligible. However, quite often the shapes are anisotropic, e.g. platelet-like or needle-like and this results in the introduction of distinctly non-random crystallite orientations due to natural preferences in packing of the anisotropic particles. The non-random particle orientation is called preferred orientation and it may cause considerable distortions of the scattered intensity. 

Lorentz Correction for Highly Collimated Beams. The rotational correction should be used if the powder sample is rotated within the beam in the single crystal sense, i.e. all crystallites should complete their rotation within the beam. Should the beam be collimated to dimensions below those of the sample containment then this further reduces the rotational impact on the cumulative Lorentz factor. A term Rl can be introduced to quantify the rotational Lorentz factor from 0 for no rotational element to 1 for full rotation of all crystallites within the beam (Figure 14.12). The introduction of this factor leads to ... 

This leads to values of 0 greater than 90° and the metal does not wet the substrate well. In a hydrogen atmosphere, the chemisorption of the gas on the surface of the metal decreases the value of aCg. However, in an oxidizing atmosphere, because of the oxidation of the metal, aCg and acs become much smaller than in the above mentioned cases and the contact angle 0 can decrease substantially. The interaction energy between the two solid phases in contact plays a major role in this decrease and also in many of the phenomena described in the Introduction and the Experimental parts. The interaction energy and the interfacial free energy between the crystallite and substrate are related via the expression (2,j5) ... 

Modifications of Cu electrode have been reported with introduction of oxygen, sulfur, or chlorine atoms to the surface in order to improve the product selectivity. 2.222 jj jg confirmed whether or not these atoms still remain on the electrode surface during electrolytic reduction of CO2. These atoms would be removed from the surface by cathodic polarization during the electrolysis, and newly reduced Cu surface may be composed of crystallites with favorable atomic configurations for preferred selectivity. 

Identity of the angles of diffraction maximums and equality of their hemispheres show that introduction of XLY does not change interplane distances of polymer crystallites and their sizes. 

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