Crystal pressure effect

Goto, T., Sato, J. Syono, Y. (1982) Shock-induced spin-pairing transition in Fe203 due to the pressure effect on the crystal field. In High-Pressure Research in Geophysics. (S. Akimoto M. H. Manghani, eds D. Reidel Publ. Co., Dortrecht), pp. 595-609. 

Mao, H.-K. Bell, P. M. (1972a) Optical and electrical behaviour of olivine and spinel (Fe2Si04) at high pressure. Interpretation of the pressure effect on the olivine absorption bands of natural fayalite to 20 kb. Crystal field stabilization of the olivine-spinel transition. Ann. Rept. Geophys. Lab., Yearb. 71, 520-8... 

Condensation via Terephthalic Acid. The salt from the acid and the diamine is formed easily in the aqueous phase (8). Crystallization is effected by isopropyl alcohol addition. Recrystallization is unnecessary when pure starting components are used. The aqueous solution of the salt is heated under pressure, the solution water is distilled off, the condensation begins, the temperature is increased, and the pressure is lowered. The reaction is complete when the desired viscosity is reached. 

Also, external parameters influencing the crystal field effects (besides the temperature-dependent dynamical behaviour) such as pressure or external electric fields have to be taken into account. 

In molecular crystals we assume to a first approximation that the molecules are not changed in their structure and chemical bond properties. Therefore the differences in NQR frequencies must be due to the crystal field effect. The following forces may be responsible for the stability of different crystal modifications at different temperatures or pressures ... 

Horsewill et al. (1994) examined the hydrostatic pressure effect on the proton transfer in crystals of a carboxylic acid dimer. Under a hydrostatic pressure, the distance of hydrogen bonds becomes shorter, and this is accompanied by a decrease in the potential barrier to proton transfer. The temperature dependence of the rate of the proton transfer turns out to be of a non-Arrhenius type. The influence of phonon-assisted tunnelling becomes evident as the external pressure increases, especially at lower temperatures. 

Samuels (1) has pointed out that plastic deformation occurs during abrasion of ionic salt crystals which are ordinarily considered brittle and that this has been attributed to a hydrostatic pressure effect. 

It was observed that ultrasound stimulation (ultrasonication) also accelerated the crystallization of the more stable polymorphs of CB (111). A fundamental study of the effects of ultrasonication on the polymorphic crystallization of PPP and LLL showed that several factors, such as pressure effect, shear flow, cavitation, and thermal energy caused by absorption of attenuated ultrasound wave, may play concurrent effects of ultrasonication. As a result, there are optimal conditions for temperature and duration of ultrasonication to increase the rate of crystallization and the occurrence of the more stable polymorphs (20). This effect was also observed in CB (111). 

C. Ultrasound (200 kHz, 300 W) was stimulated to a 250-mL sample of CB at 32.3°C during cooling before crystallization. Form II occurred without ultrasonication, whereas Form V was observed when ultrasonication was done for 3 seconds. Further ultrasonication for 9 seconds formed a mixture of Form II and Form V, and only Form II was observed by the ultrasonication for 15 seconds. It is assumed that there are conflicting effects by ultrasonication promotion of nucleation by pressure effect and retardation of nucleation by thermal energy caused by absorption of attenuated ultrasound wave. The former effect may prevail at the ultrasonication for 3 seconds. The temperature rise, however, of the sample caused by absorption of attenuated ultrasound wave was 2°C for 9 seconds and 3.9°C for 15 seconds, and the latter effect may result in the case of cooling from above the melting point of CB. 

Figure 3 Transitiometric investigation of the pressure effect on the isobaric thermal expansivities a. of gaseous, liquid, and solid substances (a) comparison between calculated ana experimental data of methane at 333 K (bl) quinoline as a simple fluid (b2) water as the associated liquid (cl) effect of temperature on medium-density polyethylene (MDPE) (c2) evolution of with the crystallinity (points represent the experimental data and the lines were obtained by the least squares fitting of data) (c3) comparison between the experimental a for three polyethylenes with different degrees of crystallinity and the predicted values for crystal and amorphous phases obtained by extrapolation from linear fitting of the experimental data...

The pressure effect on the melting/crystallization temperature of an MDPE sample, in the pressure range from 50 to 200 MPa, is illustrated in Figure 5. The isobaric temperature scans were performed at a rate of 0.833 mK s either in ascending or... 

Figure 5 Heat flow curves obtained on heating and cooling at a rate of0.833 mK s for the mercury-pressurized MDPE. The base lines were shifted for the sake of clarity on the pressure effect on the melting/crystallization temperatures...

Figure 6 Pressure effect on the temperature-induced and gas-assisted melting of tetracosane and PVDF (for more details and explanations, see refs. 26 and 29) (al) and (a2) heat rate evolution during fusion in the presence of supercritical CH and C 2 2 (VF), respectively (bl) fluid phase equilibria in the tetracosane/methane system and (b2) partial p—T phase diagram for the PVDF-VF and PVDF-N2 systems. Note the depression in the melting/crystallization temperatures in the pressure range up to 30 MPa for the tetracosane/CH and PVDF/VF systems...

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