Crystalline, defined

Samples evacuated overnight at 350 C prior to argon adsorption. Percent crystallinity defined as argon adsorption capacity divided by the adsorption capacity of pure VPI-5. 

Crystallinity defines several features of polymers rigidity, fluidity, the resistance to diffusion of small molecules in the polymer, and degradation. 

Crystallinity Determination. The crystallinity of dried PVA hydrogels was evaluated by the density gradient tube method using CCU-toluene mixtures at 37°C (5). The crystallinity defined as the weight ratio of... 

Probably the most widely used technique for determining degrees of crystallinity is differential scanning calorimetry (see Thermal Analysis). This is due to the ready availability of such instrumentation, rapid turn around time, and apparent simplicity. The heat of fusion is measured experimentally and the weight-fraction degree of crystallinity defined as ... 

Linear crystallinity defined as Half-time of crystallization measured at 130°C. 

The three-dimensional synnnetry that is present in the bulk of a crystalline solid is abruptly lost at the surface. In order to minimize the surface energy, the themiodynamically stable surface atomic structures of many materials differ considerably from the structure of the bulk. These materials are still crystalline at the surface, in that one can define a two-dimensional surface unit cell parallel to the surface, but the atomic positions in the unit cell differ from those of the bulk structure. Such a change in the local structure at the surface is called a reconstruction. 

The first analytical tool to assess tire quality of a zeolite is powder x-ray diffraction. A collection of simulated powder XRD patterns of zeolites and some disordered intergrowths togetlier witli crystallographic data is available from tlie IZA [4o]. Phase purity and x-ray crystallinity, which is arbitrarily defined as tlie ratio of tlie intensity of... 

For tire purjDoses of tliis review, a nanocrystal is defined as a crystalline solid, witli feature sizes less tlian 50 nm, recovered as a purified powder from a chemical syntliesis and subsequently dissolved as isolated particles in an appropriate solvent. In many ways, tliis definition shares many features witli tliat of colloids , defined broadly as a particle tliat has some linear dimension between 1 and 1000 nm [1] tire study of nanocrystals may be drought of as a new kind of colloid science [2]. Much of die early work on colloidal metal and semiconductor particles stemmed from die photophysics and applications to electrochemistry. (See, for example, die excellent review by Henglein [3].) However, the definition of a colloid does not include any specification of die internal stmcture of die particle. Therein lies die cmcial distinction in nanocrystals, die interior crystalline stmcture is of overwhelming importance. Nanocrystals must tmly be little solids (figure C2.17.1), widi internal stmctures equivalent (or nearly equivalent) to drat of bulk materials. This is a necessary condition if size-dependent studies of nanometre-sized objects are to offer any insight into die behaviour of bulk solids. 

Figure C2.17.8. Powder x-ray diffraction (PXRD) from amoriDhous and nanocry stalline Ti02 nanocrystals. Powder x-ray diffraction is an important test for nanocrystal quality. In the top panel, nanoparticles of titania provide no crystalline reflections. These samples, while showing some evidence of crystallinity in TEM, have a major amoriDhous component. A similar reaction, perfonned with a crystallizing agent at high temperature, provides well defined reflections which allow the anatase phase to be clearly identified.

This group comprises substances of the tjrpe RCONHR and RCONR R", i.e., substituted amides of the aromatic series. They are all well-defined crystalline sohds, sparingly soluble in cold but, often, appreciably soluble hi hot water and moderately soluble in ether they are generally neutral or feebly basic in reaction. 

Suppose we define the rate of radial growth of the crystalline disks as r. Then disks originating from all nuclei within a distance rt of an arbitrary point, say, point X in Fig. 4.6a, will reach that point in an elapsed time t. If the average concentration of nuclei in the plane is N (per unit area), then the average number of fronts F which converge on x in tliis time interval is... 

The dissipation factor (the ratio of the energy dissipated to the energy stored per cycle) is affected by the frequency, temperature, crystallinity, and void content of the fabricated stmcture. At certain temperatures and frequencies, the crystalline and amorphous regions become resonant. Because of the molecular vibrations, appHed electrical energy is lost by internal friction within the polymer which results in an increase in the dissipation factor. The dissipation factor peaks for these resins correspond to well-defined transitions, but the magnitude of the variation is minor as compared to other polymers. The low temperature transition at —97° C causes the only meaningful dissipation factor peak. The dissipation factor has a maximum of 10 —10 Hz at RT at high crystallinity (93%) the peak at 10 —10 Hz is absent. 

When water activity is low, foods behave more like mbbery polymers than crystalline stmctures having defined domains of carbohydrates, Hpids, or proteins. Water may be trapped in these mbbery stmctures and be more or less active than predicted from equiUbrium measurements. As foods change temperature the mobiUty of the water may change. A plot of chemical activity vs temperature yields a curve having distinct discontinuities indicating phase... 

Fig. 2. Time—temperature—transformation (TTT) diagram where A represents the cooling curve necessary to bypass crystallization. The C-shaped curve separates the amorphous soHd region from the crystalline soHd region. Terms are defined ia text.

Silicon Epitaxy. A critical step ia IC fabricatioa is the epitaxial depositioa of sdicoa oa an iategrated circuit. Epitaxy is defined as a process whereby a thin crystalline film is grown on a crystalline substrate. Silicon epitaxy is used ia bipolar ICs to create a high resistivity layer oa a low resistivity substrate. Most epitaxial depositioas are doae either by chemical vapor depositioa (CVD) or by molecular beam epitaxy (MBE) (see Thin films). CVD is the mainstream process. 

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