Chemical cubic phases

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

Recently, many methods in the synthesis of c-BN films were studied, which include physical vapor deposition [1,5-7] (PVD) and chemical vapor deposition [1,8,9] (CVD, such as PECVD, HFCVD, MW-ECR-CVD). Most experiments have indicated that the commercial application of c-BN films depends on enhancement and improvement in the stability and repeatability of preparation process. Generally, ion energy flow density or ion bombardment was attributed to the essential factor with the influence in the c-BN formation [1,10-13]. However, it is noted that the discrepancy between PVD and CVD maybe result from the difference in the substrate temperature (Ts , ). Unfortunately, the role which Tjub plays on the growth of cubic phase in PVD was seldom investigated systemically. 

Figure 12.25 Phase diagram of didodecyldimethylammonium bromide (DDAB) in water and styrene at 20°C. The phases include an oil-rich isotropic phase L2, lamellar phases, and five distinct cubic strut phases, including the G, D, P, C(P), and an unknown phase C5. Above the cubic phases are regions of two- and three-phase coexistence. (From Strom and Anderson 1992, reprinted with permission from Langmuir 8 691. Copyright 1992, American Chemical Society.)...

The temperature dependences of the chemical shifts in a series of cuprous halides have been determined by Becker (1978). These results suggest that CuBr is the most ionic of these compounds and Cul is the most covalent. The Cu shifts of the low temperature (cubic) phases increase with temperature due to increasing vibrational overlap. At higher temperatures the Cu shifts become increasingly diamagnetic with temperature, reflecting the highly disordered state of the Cu under these conditions (Becker 1978). 

Figure 25. MAS-NMR spectrum of the cubic phase of a Cs-substituted leucite (CsAlSi206), obtained at 150°C. In the cubic phase Si and A1 ate disordered on one ciystallographic position. The Si chemical shift depends on the number of A1 in the adjacent framework sites (from zero to four), giving distinct peaks for the same crystallographic site, but with different local configurations. [Redrawn from data of Philhps and Kirkpatrick (1994).]...

Solid state 14N NMR measurements have been reported for ND4C1, (169) (CH3)4NMnCl (177) NH KSO. 12H20, (178) KCN, (179) NaCN (179) and Ba(N03)2. (180) These studies have yielded values for the 14N chemical shifts and quadrupole coupling constants and, where appropriate, the asymmetry parameter r in equation (35). By following the temperature dependence of the 14N relaxation time in the cubic phases of NaCN and KCN the activation energy of the reorientational motion of the cyanide ions is obtained. It is reported at 300 K to be 1-41 0-02 kcal mole-1 in NaCN and 0-50 0-25 kcal mole-1 in KCN. 079)... 

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