Lamellar reflections characteristics

Table I. Characteristics that describe the lamellar reflections (Figures 2-8)...

The parameters in Table I fully account for the various features of the lamellar reflections. Many of these have been used in the past. We have introduced two additional parameters, misorientation of the lamellar stacks P and ellipticity e. These parameters can be derived from a least-squares fit to 2-D data, which is best carried out in elliptical coordinates. The ellipticity is clearly seen in the vs. tan < (Figure 8). Brandt and Ruland have found a similar characteristics in their SAXS pattern from deformed microdomain structures of block copolymers. As we pointed out in our earlier publication, a wide variety of SAS patterns found in the literature can be simulated in elliptical coordinates. 

The effect of water on lamellar distances was first investigated. In the HDTABr/pentanol/water system (Ww/lTs variable) the Bragg reflections characteristic of d are shifted toward lower wave vectors (angles). In liquid crystalline systems a minimum of 1 mol HzO/surfactant is necessary for the hydration of the polar head groups. As water content (1F ) increases, d... 

As discussed in the section B of this chapter, the sample with a very low molecular weight is predominantly composed of a lamellar crystalline region, with a minor amount of interfacial region, and no liquidlike interzonal region at room temperature, as can be schematically depicted in either Fig. 10 (B) or (C). The interfacial region comprises relatively short methylene sequences with very limited mobility that are excluded from the crystalline region. This characteristic feature of the phase structure is also reflected in the temperature dependence of the NMR spectrum. 

Figure 2 depicts the variation of Scanning Electron Micrographs with crystallization time at different temperatures. At the beginning of the crystallization, the texture of obtained solid phase can be described as an agglomeration of fibrous (Fig. 2a, 2b, 2d). The 110 and 200 reflections are not present in the XRD pattern at this moment. When those reflections appear, the morphology characteristic of MCM-41 described by Tanev et al [6] or Elder et al [12] is detected (Fig,2e, 2f, 2h, 2j). Crystals have variable size and form and very porous surface. Finally when the triphasic mixture composed of hexagonal MCM-41, lamellar MCM-50 and amorphous phase is detected (Fig. 2i, 2k, 21), crystals with a sandy-rose like structure and spheric grains are clearly observed. The sandy-rose crystals belong to the MCM-50 lamellar structure [13] and the spheric grains correspond to the amorphous silica phase. The presence of MCM-50 and amorphous silica is proved by XRD patterns.

Fig. 5 Bright-field and dark-field imaging (A) BF image of lamellar y/ 2 titanium aluminide (B) corresponding SAD pattern (see Fig. 4) (C) and (D) DF images of the reflections marked in (B). Each of these reflections is characteristic for one twin variant of tetragonal y-TiAl appearing with high intensity in the corresponding DF image. (View this art in color at www.dekker.com.)...

Among the many varieties of mesoporous materials several stand out as most extensively studied. The initial 3 structures [3,4] remain dominant although MCM-50 has attracted least attention and found little practical value. It has an unstable structure collapsing upon calcination but still exhibits considerable microporosity, which is intriguing and possibly deserving closer attention. However, if a post treatment by adding a reactive silica source, e.g. TEOS, to the as-synthesized MCM-50 is done prior to air calculations, then a uniform mesopore structure (after calcination to remove the surfactant) with retention of the lamellar-like XRD is observed. MCM-41 and MCM-48 type structures and SBA-15 are considered here as representative of the entire class and to reflect the expected range of characteristics and behavior. 

The most often found types of mixed oxides are perovskites (RBO3), K2NiF4-type oxides (R2BO4), R 1 B C>2 +1 ( = 2 or 3), lamellar perovskites, pyrochlores, spinels, and oxide solid solutions. Perovskite oxides are, by far, the most commonly used oxides. Therefore, this chapter will reflect this situation by putting more emphasis on this type of materials. Within this section the structure, preparation methods and general characteristics of the mixed oxides will be discussed. Note that R stands for rare-earth elements while A includes all types of elements. 

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