Poly crystallinity levels

The drop of the voltammetric crurent is associated with Pt surface oxidation, and the drop on the negative-going mn is due to Reaction (12.9) (surface poisoning by CO) and the Tafehan kinetics of Reaction (12.8). Further, the shift between curves in Fig. 12.13a and b indicates that in the potential range between 0.5 and 0.6 V, methanol oxidation occms with zero or low level atop CO smface intermediate. The amplitudes on Fig. 12.13 on both scans nearly equal to each other indicate a high level of preferential (111) crystallographic orientation of the poly crystalline Pt surface used for this work, as inferred from data in [Adzic et al., 1982]. 

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. 

The block diagram in the last section shows the spectrometer with the CAMAC modules identified. Several different experiments requiring different pulse sequences can be performed easily with such a system. A moderately complicated example is a spin-lattice relaxation time measurement in the time domain on a poly crystalline intermetallic sample containing 1=3/2 nuclei. Since a non-cubic 1=3/2 system has unequally spaced levels, special techniques must be used for relaxation time measurements (see III.C.3.) and we adopt the procedure of Avogadro and Rigamonti (1973) to initialize the populations before the magnetization recovery. 

Lastly, in the following discussion the term structure connotes molecular structure at the subcrystal level and includes both electronic bonding, as discussed in Chapter 5, and the spatial, dynamic, and energetic aspects of atomic configuration, as discussed in Chapters 3 and 4. Macrostructure is used to describe the nature of poly crystalline samples down to the level of individual crystals and their internal and external surfaces. 

Note that there are currently very few techniques to make wholly microporous silicon (see handbook chapter Microporous Silicon ) where the average pore diameter is under 2 nm. For virtually all top-down techniques, the porous silicon created is poly crystalline. For some bottom-up techniques such as sputtering/dealloying (Fukatani et al. 2005), electrodeposition (Krishnamurthy et al. 2011), or sodiothermic reduction (Wang et al. 2013), it is reported to be amorphous. Choice of fabrication technique for both mesoporous and macroporous silicon is very much dictated by application area, which in turn has differing requirements on porosity levels, pore morphology, skeleton purity, physical form, cost, and volume. 

The curve B of Fig. 4.20 represents a DTA trace of poly(ethylene tereph-thalate) which was quenched rapidly from the melt to very low temperatures before analysis. (DSC cell as in Fig. 4.3D, 10 mg sample in N2 at atmospheric pressure, heating rate 20 K/min.) Under such conditions, poly(ethylene terephthalate) remains amorphous on cooling i.e., it does not have enough time to crystallize, and thus it freezes to a glass. At point (1), at about 348 K, the increase in heat capacity due to the glass transition can be detected. At point (2), at 418 K, crystallization occurs with an exotherm. Note that after crystallization the base line drops towards the crystalline level. 

Fig. 9.20 Plots of crystallinity levels as functions of molecular weight, (a) Linear polyethylene fractions (34) (b) poly(ethylene oxide) fractions (49). Pseudoequilibrium level of crystallinity that is attained A crystallinity levels at which deviations occur from theory Goler-Sachs Avrami o. Dashed curve represents ratio of crystallinity level at which deviation occurs to that actually attained.

Fig. 11,18 Plot of normalized crystallinity level of poly(butylene succinate) in blends with poly(vinylidene fluoride) as a function of log time. Composition poly(butylene succinate)/poly(vinylidene fluoride) O 100/0 80/20 V 60/40 T 40/60. (Data from (35))...

Fig. 13.28 Schematic representation of superposed isotherms of poly(dimethyl siloxane) and its mixtures with toluene. The normalized crystallinity level is plotted against log time. The curves are arbitrarily shifted along the log t axis for clarity. Curve (1), V2 = 1.00 curve (2), V2 = 0.79 curve (3), V2 = 0.59 curve (4), 1)2 = 0.42 curve (5), V2 = 0.32. (FromFeio etal. (78))...

The second route toward increased crystalline level is the addition of a plasticizer. This enhances the polymer chain mobility and thus increases the crystallite growth rate. In the context of a non-isothermal crystallization, plasticization also plays a role by decreasing the Tg thus widening the crystallization temperature window [8]. Plasticizers such as Citrate esters, triacetine and poly(ethylene glycol) have been used in PLA as a mean to increase its ductility. It has been reported that addition of plasticizers also decreased the cold crystallization temperature [9]. 

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