The Crystallographic Forms

Thin films of metals, alloys and compounds of a few micrometres thickness, which play an important part in microelectronics, can be prepared by the condensation of atomic species on an inert substrate from a gaseous phase. The source of the atoms is, in the simplest circumstances, a sample of the collision-free evaporated beam originating from an elementary substance, or a number of elementary substances, which is formed in vacuum. The condensing surface is selected and held at a pre-determined temperature, so as to affect the crystallographic form of the condensate. If this surface is at room temperature, a polycrystalline film is usually formed. As the temperature of the surface is increased the deposit crystal size increases, and can be made practically monocrystalline at elevated temperatures. The degree of crystallinity which has been achieved can be determined by electron diffraction, while other properties such as surface morphology and dislocation structure can be established by electron microscopy. 

Polymeric precursors have been developed which are stable at room temperature and when polymerized convert to ceramics in high yield. Such precursors may be synthesized by reaction of a vinylsilane, vinylmethylsilane, acetylene silane, or acetylene alkyl silane with a borane or a borane amine derivative The reactants are mixed in an inert atmosphere, either neat or in an aprotic solvent like acetonitrile, tetrahydrofuran, or a hydrocarbon, or in a mixture of such solvents. The reaction mixture is heated for 0.1 to 120 h at 90-170°C. The solvent, if any, is then removed. The polymer is pyrolyzed in argon or N2 at SOO-ISOO C for 1 h. BN and other ceramics such as B4C, or SipB QNs compounds can be made, where p, q, r, and s have various numerical values. To date, no measurements of the crystallographic form of the resultant BN compound have been made. Other precursors convertible to BN include poly (2-vinylpentaborane) oligomers. ... 

The surface properties of various phases corresponding to the same chemical formula can substantially differ from one crystallographic form to another. This is well known, and the information about the crystallographic form of the adsorbents is specified in many publications on adsorption, especially when the compound of interest forms more than one phase showing a sufficient stability at the experimental conditions. Since the present survey is devoted to adsorption studies performed at room temperatme and atmospheric pressure the phases existing only at very high pressures or at very high (or very low) temperatures are not considered. 

Faculte de medecine , included Bouchardat, Bussy, and Dumas he expounded his system of classification, and since that date these principles have been developed with much sagacity by M. Dumas . Baudimont explains again his theory of numerical (or chemical) types, based on the natural order of numbers, i, 2, 3, 4,. .., and mechanical types, the first being indicated by the chemical formulae, and the mechanical by the crystallographic form. He is far from clear, but seems to have been a formidable man. 

In Fig. 57 the WAXS diffraction patterns for NaOH-extracted broom fibers, SEP broom fibers, and pure cellulose in the crystallographic form I are reported. 

From these graphics the following can be observed Both extracted fibers are in the crystallographic form I of the cellulose. Both spectra show the 22.6° (2 ) reflection related to the (002) crystallographic plane and the... 

It was later determined that the crystallographic form cellulose I was irreversibly chang to the form cellulose II. The unit cell of cellulose II also contained two cellobiose units with axes a = 0.814 nm, b = 1.03 nm, c = 0.914 nm, and fi = 62° (angle between a and c axes). The fibrillar cellulose II is less ordered than fibrillar cellulose I and is about 60% crystalline. This 1840s development, now called mercerization, is probably the most important process for the modification of natural cellulose, particularly cotton cellulosic textile fibers, to current date. Lowe later confirmed these results and showed the effects of mercerization on cotton fibers while under tension. 

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