Poly crystalline order

Distinct quadrupole shifts do occur as well in magnetically split spectra of single-crystals, poly crystalline powder or frozen solution samples. In all three cases, the line shifts obey the simple first-order expression at high-field condition. 

In a poly crystalline mixture of solid reactants, we might expect the particle size to be of the order of 10 pm even careful and persistent grinding will only reduce the particle size to around 0.1 pm. Diffusion during a ceramic reaction is therefore taking place across anywhere between 100 and 10,000 unit cells. Various ingenious methods, some physical and some chemical, have been pioneered to bring the components of the reaction either into more intimate contact, or into contact at an atomic level, and so reduce this diffusion path in doing so, the reactions can often take place at lower temperatures. 

See for example Benoit, H., Freund, L. and Spach, G. in Fasman, G. ed. Poly-alpha-amino acids . Vol. 1, p. 105, Dekker, 1967 and Samulski, E. T. Liquid crystalline order in polypeptides, in A. Blumstein, ed., Liquid Crystalline Order in Polymers . Academic Press, New York, NY 1978... 

Real substances often deviate from the idealized models employed in simulation studies. For instance, many complex fluids, whether natural or synthetic in origin, comprise mixtures of similar rather than identical constituents. Similarly, crystalline phases usually exhibit a finite concentration of defects that disturb the otherwise perfect crystalline order. The presence of imperfections can significantly alter phase behavior with respect to the idealized case. If one is to realize the goal of obtaining quantitatively accurate simulation data for real substances, the effects of imperfections must be incorporated. In this section we consider the state-of-the-art in dealing with two kinds of imperfection, poly-dispersity and point defects in crystals. 

Since compound layers are poly crystalline, grain boundary diffusion may to some extent affect both their morphology and growth kinetics. Grain boundary diffusivities of the components of any compound are known to be a few orders of magnitude higher than appropriate lattice ones. 

As charges are injected in the chains, electrical neutrality is maintained by counterion insertion in the polymer matrix. In this way conducting polymers resemble intercalation compounds, with the difference that usually the counterions arrange randomly. In some cases, however (more especially in poly acetylene, but also in poly aniline), ordering of the counterions, or stages, has been observed. The ion insertion can thus be compatible with a crystalline structure. It has been observed, however, that the crystallinity is degradated on cycling (see Chapter 12, Section II.C). 

The solution of pam-arormtic polyamides occupy a special place among the systems forming liquid crystalline order. Two typical representatives of this class of polymers turn out to be best studied owing to their practical importance. These are poly-(p-benzamide) (PBA)... 

In a poly crystalline powder, all combinations of 0 and ([) are present. For each transition, the angular dependent term varies from 2 (0 = 0) to -1 (0 = k/2), which spreads the intensity of the satellite transitions over a very wide frequency range, of the order of Cq (Fig. 4b). As a practical matter, the satellite transitions (and therefore much of the total signal intensity) are lost in the baseline and most NMR focuses on the central transition (Fig. 4c-d). In principal, the values of Cq and q can be determined from the width and shape of the full powder spectrum. But in most cases, commercial NMR spectrometers cannot measure the entire spectrum. However, a few exceptions are reviewed below. 

Sadron, C. Gallot, B. Makromo1. Chem. 1973, 164, 301 Gallot, B. "Liquid Crystalline Order in Poly-mers," Blumstein, A, Ed. Academic New York,... 

In the form of amorphous silieon, thin flexible silieon films can be fabricated as well. Because of the absence of a erystal lattiee, eleetronic performance of amorphous silicon is multiple orders of magnitude worse than the performance of (poly)crystalline silicon, but still high enough for applications like TFT backplanes. However, processing temperatures required to obtain a-Si films of this high performance are not compatible with flexible substrates either. 

The close similarity between adsorption of CO on tungsten and molybdenum is shown by the work of Jackson and Hooker (82) in a LEED study on the Mo(llO) face when they observed a complex ordered structure formed by heating the crystal to 1300°K in the presence of CO. This surface structure shows a marked resemblance to that obtained by May and Germer on W(llO) (71) after annealing the crystal to between 600-900°K. On molybdenum the removal of the additional structure was not observed until about 1800°K, whereas on W(llO) CO was observed to desorb at 1100°K. This result is puzzling in the light of the flash filament studies and calorimetric heats of adsorption on polycrystalline samples which indicate a weaker adsorption process for CO on poly crystalline molybdenum compared with tungsten. 

---