Cooling crystal orientation

SIMS, and SNMS in rare cases, such as for HgCdJTei samples or some polymers, the sample structure can be modified by the incident ion beam. These effects can often be eliminated or minimized by limitii the total number of particles incident on the sample, increasing the analytical area, or by cooling the sample. Also, if channeling of the ion beam occurs in a crystal sample, this must be included in the data analysis or serious inaccuracies can result. To avoid unwanted channelii, samples are often manipulated during the analysis to present an average or random crystal orientation. 

A Ni(lll) single crystal, oriented to within 0.2° of the (111) plane, is mounted on a manipulator which rotates it 360° around an axis parallel to its surface and translates it in three mutually perpendicular directions while maintaining the ultrahigh vacuum conditions in the main chamber. The crystal can be cooled to 8 K by contact with a liquid He reservoir and can be heated to 1400 K. A chromel-constantan thermocouple is spot-welded to the crystal for temperature measurements. The procedure for cleaning the crystal by Ar ion sputtering, oxidation and reduction has been discussed previously (ref. 5). 

In 1959/1960, Le Blanc [9] and Kepler [10] investigated the transient photoconductivity of high-purity anthracene crystals. The charge carriers were activated by a UV pulse and their mobihties were measured (see Sect. 8.4.1). Both for holes and for electrons, they found values between 0.3cm /Vs and 3cm /Vs at room temperature. These were dependent on the crystal orientation relative to the electric field and increased on cooling. Their results are still vahd today (see Table 8.2 in Sect. 8.5.2). 

Polarized IR spectroscopy was employed by Wu et al. to investigate crystallinity, the formation of different crystal orientations and the crystal texture of syndiotactic PS (sPS) crystallized inside AAO hard templates while in contact with a bulk reservoir of the same polymer [101]. The 6-polymorph was obtained by cooling from the melt to 260 °C and crystallizing at this temperature for 2 h, while heating amorphous samples quenched from the molten state to 240 °C and heating to this temperature for 2h resulted in the formation of the or-polymorph. A comparison of the areas of peaks... 

Fig. 52. Inverse magnetic susceptibility of UAs versus temperature as a function of crystal orientation, measured on field-cooled samples.

Terephthahc acid (TA) or dimethyl terephthalate [120-61 -6] (DMT) reacts with ethyleae glycol (2G) to form bis(2-hydroxyethyl) terephthalate [959-26-2] (BHET) which is coadeasatioa polymerized to PET with the elimination of 2G. Moltea polymer is extmded through a die (spinneret) forming filaments that are solidified by air cooling. Combinations of stress, strain, and thermal treatments are appHed to the filaments to orient and crystallize the molecular chains. These steps develop the fiber properties required for specific uses. The two general physical forms of PET fibers are continuous filament and cut staple. 

Flow processes iaside the spinneret are governed by shear viscosity and shear rate. PET is a non-Newtonian elastic fluid. Spinning filament tension and molecular orientation depend on polymer temperature and viscosity, spinneret capillary diameter and length, spin speed, rate of filament cooling, inertia, and air drag (69,70). These variables combine to attenuate the fiber and orient and sometimes crystallize the molecular chains (71). 

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