Vapor Technique Sublimation

FIGURE 29.18 Modified subiimation process for SiC singie crystais. 

SiC wafers produced from the boules are very expensive. A single SiC wafer 30 mm in diameter and 0.3 mm thick can cost upward of 3000. By comparison, an Si wafer of this size costs less than 20. 

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The enthalpies of phase transition, such as fusion (Aa,s/f), vaporization (AvapH), sublimation (Asut,//), and solution (As n//), are usually regarded as thermophysical properties, because they referto processes where no intramolecular bonds are cleaved or formed. As such, a detailed discussion of the experimental methods (or the estimation procedures) to determine them is outside the scope of the present book. Nevertheless, some of the techniques addressed in part II can be used for that purpose. For instance, differential scanning calorimetry is often applied to measure A us// and, less frequently, AmpH and AsubH. Many of the reported Asu, // data have been determined with Calvet microcalorimeters (see chapter 9) and from vapor pressure against temperature data obtained with Knudsen cells [35-38]. Reaction-solution calorimetry is the main source of AsinH values. All these auxiliary values are very important because they are frequently required to calculate gas-phase reaction enthalpies and to derive information on the strengths of chemical bonds (see chapter 5)—one of the main goals of molecular energetics. It is thus appropriate to make a brief review of the subject in this introduction. 

Butadiene d4 complexes were obtained (i) from [NbCU(dmpe)2] and magnesium butadiene (equation 87) j706 (ii) by dimerization of ethylene using alkylidenes (equation 88) 707 or (iii) by metal vapor techniques (equation 89), which yielded sublimable methylallyl derivatives.70 Compound (62) could not be prepared by Na/Hg reduction of (22) in the presence of butadiene. Compound (63) is also accessible from [TaH2ClL4] (Scheme 9). 

Depending on the wavelength of radiation used, irradiation of Co2(CO)g produces either CO dissociation (at 250 nm) or cleavage into Co(CO)4 radicals (at 360 nm). The radical Co(CO)4 (2) itself has been detected by its Raman, infrared, UV-vis, and EPR see Electron Paramagnetic Resonance) spectra. It can be found by EPR when (1) is heated and sublimed on a 77 K cold finger in the EPR cavity, or it can be generated in a matrix at low temperature either by photolysis of (1) or by the metal vapor technique see Metal Vapor Synthesis of Transition Metal Compounds). 

We have already acknowledged our intent to use relevant estimation approaches to enthalpies of vaporization and sublimation to maximize the usefulness of the data available. That dienes and polyenes have multiple double bonds that are potentially hydrogenatable to the totally saturated aliphatic or alicyclic hydrocarbons allows the employment of two other assumptions. The first assumption argues that the enthalpy of hydrogenation, AHn measured in a nonpolar solvent is essentially equal to that which would be obtained in the gas phase. The second assumption, implicitly employing the first, legitimizes the use of estimation techniques and even molecular mechanics to derive the enthalpy of formation of the totally saturated species. From this last number, the enthalpy of formation of the unsaturated diene or polyene of interest can be derived by equation 3 and simple arithmetic. 

While all vapor techniques depend on a condensation step, the direct vaporization techniques begin with the desired composition and evaporate or sublimate the material. This is straightforward and allows a diversity of compositions to be used, but excessively high temperatures are demanded to vaporize refractory ceramics. Various heating methods used, including dc arcs, dc plasmas, rf plasmas, and electron beam heating. These techniques are not popular for large-scale production or routine laboratory powder preparation. 

DSC is often used in conjunction with TA to determine if a reaction is endothermic, such as melting, vaporization and sublimation, or exothermic, such as oxidative degradation. It is also used to determine the glass transition temperature of polymers. Liquids and solids can be analyzed by both methods of thermal analysis. The sample size is usually limited to 10-20 mg. Thermal analysis can be used to characterize the physical and chemical properties of a system under conditions that simulate real world applications. It is not simply a sample composition technique. Much of the data interpretation is empirical in nature and more than one thermal method may be required to fully understand the chemical and physical reactions occurring in a sample. Condensation of volatile reaction products on the sample support system of a TA can give rise to anomalous weight changes. 

A vapor deposition technique (sublimation) seems to be a good alternative [141,151-153], especially for those studies where uniformity and purity of the film is extremely important, e.g., in conductivity [141,153] or spectroscopic [151] studies. Results by Nishizawa et al. [153] show that the density of the vapor deposited Ceo films corresponds to ca. 84% of a close-packed single crystal. Films of comparable quality can also be obtained by electrochemical deposition techniques, as can pure fiillerene and reduced fiillerene films. Films grown electrochemical ly are more uniform than the solution-cast films. However, they are also composed of small crystallites as shown for (Ru(bpy)3 +) (Qo )2 (bpy = bipyridine) [154] and mixed tetraphenylpho-sphonium fulleride (Ceo) bromide salt [155]. The known electrochemical methods [156,157] are based on a potentiostatic electrolysis of a C o" solution in the presence of a suitable background electrolyte to produce a more or less reduced species that deposits on the electrode as the salt of the supporting electrolyte cation. 50 films are produced in acetonitrile/TBAC104 solution [156,157] upon reoxidation of and... 

Vapor pressures of phases in these systems were measured by the Knudsen effusion technique. Use of mass spectrometer-target collection apparatus to perform thermodynamic studies is discussed. The prominent sublimation reactions for these phases below 2000 K was shown to involve formation of elemental plutonium vapor. Thermodynamic properties determined in this study were correlated with corresponding values obtained from theoretical predictions and from previous measurements on analogous intermetallics. 

The synthesis of black, pyrite type of silicon diphosphide was first accomplished at high pressure1 (15-50 kb.). It has since been prepared by vapor-transport techniques.2 3 A low-pressure, red form of SiP2 is produced by sublimation from 900-500°C. in a... 

R.B. Cundall et al, "Vapor Pressure Measurements on Some Organic High Explosives , J-ChemSoc, Faraday Trans I, 74 (6), 1339—45 (1978) CA 89, 181933 (1978) [Equilibrium vap press were detd for various expls by the Knudson cell technique. The data for HMX follows the Clausius-Clapeyron eqtn. The values detd for the const A and B in the eqtn, log10p = A—(B/T), plus the std enthalpy, entropy and Gibbs energy of sublimation from the authors calcns are presented in Table 7 ... 

A general characteristic of all the data shown in Figures 4 and 5 is the relatively slow initial response time and even slower return to baseline as the concentration of the test vapor was reduced. Because of the exponential dilution of vapor flowing to the sensor, the concentration dropped to 8.5% of its initial value after 25 minutes and to 0.3% of the initial value by the end of an hour. The sublimed films obviously do not equilibrate rapidly with the test vapors. This is a major reason for the interest in being able to deposit ultra-thin, ordered films reproducibly by the L-B technique. 

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