Temperature gradient method

In this high pressure process large c-BN crystals should be grown. The key to obtain larger c-BN crystals of high quality is to control the super-saturation of BN in the solvent. A temperature gradient method with exact regulation of the temperature has been developed (Fig. 11) [156, 178]. 

There are two possibilities for the regulation of the temperature gradient [179]. On the one hand a vertical reactor can be used, in which a temperature gradient exists. On the other hand a molybdenum disc with a hole in the middle can be put between the h-BN and the flux, and by varying the hole s diameter the temperature gradient can be regulated. 

It was possible to grow c-BN single crystals up to 2 mm in size, without using seeding crystals [178]. Process parameters were 1500-1750 °C and 

with a h-BN source and Li3BN2 or Ba3B2N4 as solvents. Crystal quality of the c-BN was best in the lower temperature range where c-BN nucleation is rare and crystal growth slow. The reaction time for the larger c-BN crystals was up to 80 h. 

Using the temperature gradient method it was also possible to deposit c-BN on various substrates, e.g., on diamond crystals or CVD-diamond sheets [156, 180]. 

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The influence of polymorphism on hardness can be illustrated using the case of PP. i-PP can crystallize into several polymorphic forms. Microhardness measurements have been carried out on samples prepared by the temperature-gradient method of Fujiwara Asano (Fujiwara, 1968 Asano Fujiwara, 1978), which allows a distinct separation of the a (monoclinic) and (hexagonal) phases within the sample. Results show that H decreases from 111 MPa to 90 MPa when passing from the a to the )8 phase (Balta Calleja et al, 1988). The H decrease is partially connected with a crystallinity decrease from 0.72 to 0.62. However, according to eq. (4.7) the microhardness of each phase depends not only on crystallinity but also on the crystal microhardness. He, itself. Thus, we can write for both crystalline phases ... 

Fig. 9. Temperature-composition isobaric phase diagram for the fully hydrated dipal-mitoylphosphatidylcholine/dipalmitoyl-phosphatidylethanolamine system constructed using the temperature gradient method. The notation used is that of Luzzati [8] and is as follows Lc, lamellar crystalline (also referred to as the subgel phase) L f, lamellar gel phase with hydrocarbon chains tilted with respect to the bilayer normal P)el, ripple phase L. lamellar liquid crystal phase. Insert bold line in graph as indicated...

Fig. 1.14. Experimental determination of the saturation temperature of a wine by the temperature gradient method (Wurdig et al., 1982). (a) Example of a wine that is not highly supersaturated, in which no induced crystallization occurs after the addition of tartrate crystals at low temperature, (b) Example of a highly supersaturated wine, in which induced crystallization occurs immediately after the addition of calcium potassium tartrate crystals...

J. Jang and J. Bae, Formation of polyaniline nanorod/hquid crystalline epoxy composite nanowires using a temperature-gradient method, Adv. Funct. Mater., 15,1877-1882 (2005). 

MAO Mao, H., Li, C., Zhang, Y., Bergbreiter, D.E., and Cremer, P.S., Measuring LCSTs by novel temperature gradient methods Evidence for intermolecrrlar interactiorrs in mixed polymer solutions, J. Amer. Chem. Soc., 125, 2850, 2003. 

The tray temperature gradient method requires only one steady-state simulation at design conditions, while the shift in material balance method requires two simulations with different product stream compositions. The maximum tray temperature gradient of 7.6% of 17.7°C or 1.35 C/tray occurs at stages 15 and 16 as shown in Figure 4.1. These observations show... 

Two principal light scattering techniques have been used to determine the MWD of polymers. The first of these is the temperature gradient method. This method has been applied to the determination of the MWD of PE [151,152]. In this method a solution of PE in a-chloronaphthalene solvent and a non-solvent containing a very low concentration of PE is slowly cooled. 

In accordance with the experience gained with diamond synthesis, we can think of two solvent methods the so-called high-pressure film method (224) and the temperature difference (or temperature gradient) method (225,226). Both methods provide supersaturation of B and N atoms in a solvent from which BN crystals are precipitated. 

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