Quartz crystal growth

Quartz crystals, growth of, 14 93, 94 Quartz-pebble conglomerate uranium deposits, 17 520... 

Figure 3-26 Quartz crystal growth and diffusion profile in (a) a laboratory-fixed reference frame and (b) an interface-fixed reference frame. At a given time, a given kind of curve is used to outline the crystal shape and plot the concentration profile.

Hydrothermal crystallisation processes occur widely in nature and are responsible for the formation of many crystalline minerals. The most widely used commercial appHcation of hydrothermal crystallization is for the production of synthetic quartz (see Silica, synthetic quartz crystals). Piezoelectric quartz crystals weighing up to several pounds can be produced for use in electronic equipment. Hydrothermal crystallization takes place in near- or supercritical water solutions (see Supercritical fluids). Near and above the critical point of water, the viscosity (300-1400 mPa s(=cP) at 374°C) decreases significantly, allowing for relatively rapid diffusion and growth processes to occur. 

In an attempt to stimulate onshore production of synthetic quartz and piezoelectric devices in the 1970s, Brazil imposed an embargo on exports and ultimately raised the price several-fold for small quartz crystals used as the starting material for quartz growth. However, sources of suitable pure quartz were located in the United States and Canada, including vein and pegmatic deposits (1). Synthetic processes compatible with the natural U.S. quartz starting material from a variety of sources were developed, and U.S. production became relatively independent of imports (1). 

Effects of Rate Conditions. It is essential for commercial a-quartz crystals to have usable perfection growth at a high rate and at pressure and temperature conditions that allow economical equipment design. The dependence of rate on the process parameters has been studied (8,14) and may be summarized as follows. Growth rate depends on crystallographic direction the (0001) is one of the fastest directions. Because AS is approximately linear with AT, the growth rate is linear with AT. Growth rate has an Arrhenius equation dependence on the temperature in the crystallization zone ... 

Fig. 9. Quartz glass ampoule with AuTe I and Au. [From A. Rabenau, In "Crystal Growth an Introduction (P. Hartmann, ed.), Fig. 7.1., p. 201. North-Holland Publ. Co., Amsterdam, 1973.]...

Lincot D, Ortega-Borges R (1992) Chemical bath deposition of cadmium sulfide thin films. In situ growth and structural studies by Combined Quartz Crystal Microbalance and Electrochemical Impedance techniques. J Electrochem Soc 139 1880-1889... 

E)ven though it is evacuated, capsules have been known to explode because the quartz (metal) walls could not contain the internal vapor pressure of the material being grown as single crystal. Care must be exercised not to handle the hot ciy>sule before and after crystal growth. 

Jerkiewicz G, Vatankhah G, Lessard J, Soriaga MP, Park YS. 2004. Surface-oxide growth at platinum electrodes in aqueous H2SO4 Reexamination of its mecharusm through combined cyclic-voltammetry, electrochemical quartz-crystal nanobalance, and Auger electron spectroscopy measurements. Electrochim Acta 49 1451-1459. 

The monotonic increase of immobilized material vith the number of deposition cycles in the LbL technique is vhat allo vs control over film thickness on the nanometric scale. Eilm growth in LbL has been very well characterized by several complementary experimental techniques such as UV-visible spectroscopy [66, 67], quartz crystal microbalance (QCM) [68-70], X-ray [63] and neutron reflectometry [3], Fourier transform infrared spectroscopy (ETIR) [71], ellipsometry [68-70], cyclic voltammetry (CV) [67, 72], electrochemical impedance spectroscopy (EIS) [73], -potential [74] and so on. The complement of these techniques can be appreciated, for example, in the integrated charge in cyclic voltammetry experiments or the redox capacitance in EIS for redox PEMs The charge or redox capacitance is not necessarily that expected for the complete oxidation/reduction of all the redox-active groups that can be estimated by other techniques because of the experimental timescale and charge-transport limitations. 

Therefore, and to avoid possible reactions with the quartz wall, it was attempted to combine synthesis and crystal growth of actinide pnictides in a modified van Arkel process Actinide metal or carbide - the latter obtained by carboreduction of the oxide - are heated in the presence of the pnictogen and of the transporting agent at the... 

Crystallization and reactivity in two-dimensional (2D) and 3D crystals provide a simple route for mirror-symmetry breaking. Of particular importance are the processes of the self assembly of non-chiral molecules or a racemate that undergo fast racemization prior to crystallization, into a single crystal or small number of enantiomorphous crystals of the same handedness. Such spontaneous asymmetric transformation processes are particularly efficient in systems where the nucleation of the crystals is a slow event in comparison to the sequential step of crystal growth (Havinga, 1954 Penzien and Schmidt, 1969 Kirstein et al, 2000 Ribo et al 2001 Lauceri et al, 2002 De Feyter et al, 2001). The chiral crystals of quartz, which are composed from non-chiral Si02 molecules is an exemplary system that displays such phenomenon. 

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