Thermal sublimation

The limiting temperature for graphite use in fusion systems is defined by tliermal sublimation (--1500-2000°C). However, a process which is very similar to thermal sublimation (in cause and in effect) appears to define the current temperature limit. This phenomenon, which is known as radiation enhanced sublimation (RES), is not clearly understood but dominates above a temperature of about 1000°C and increases exponentially with increasing temperatme. 

Astonishingly, the same ARUPS spectra have been observed for ex situ grown TTF-TCNQ thin films (Rojas et al, 2001). The hlms were obtained by thermal sublimation in HV ( 10 mbar) on cleaved KCl(lOO) substrates and consisted of highly oriented and strongly textured rectangular-shaped microcrystals as shown in the TMAFM image of Fig. 6.9. The molecular afc-planes are parallel to the substrate surface and the microcrystals are oriented with their a- and fc-axis parallel to the [110] and [110] substrate directions, respectively, due to the cubic symmetry of the substrates. ARUPS spectra taken on the as-received hlms, measured along the substrate equivalent [100] direchons at T 100 K, are shown in Fig. 6.10. 

The coating systems that can be used to coat an inorganic material on the surface of edible products have been adapted from the biomedical and electronics fields. They include (a) sputtering or analogous thermal sublimation, (b) electron beam, and (c) plasma deposition but their application in the food industry is still marginal. 

The processes of distillation and sublimation are closely related. There are three ways in which the vaporisation of a thermally stable substance may take place on heating —... 

Ablative materials are classified according to dominant ablation mechanism. There are three groups subliming or melting ablators, charring ablators, and intumescent ablators. Figure 4 shows the physical zones of each. Because of the basic thermal and physical differences, the classes of ablative materials are used in different types of appHcations. 

The thermal protection system of the space shutde is composed mainly of subliming or melting ablators that are used below their fusion or vaporization reaction temperatures (42). In addition to the carbon-carbon systems discussed above, a flexible reusable surface insulation composed of Nomex felt substrate, a Du Pont polyamide fiber material, is used on a large portion of the upper surface. High and low temperature reusable surface insulation composed of siHca-based low density tiles are used on the bottom surface of the vehicle, which sees a more severe reentry heating environment than does the upper surface of the vehicle (43). 

Thermal Theory. The thermal approach to flame retardancy can function in two ways. Eirst, the heat input from a source may be dissipated by an endothermic change in the retardant such as by fusion or sublimation. Alternatively, the heat suppUed from the source maybe conducted away from the fibers so rapidly that the fabric never reaches combustion temperature. 

Iron(III) bromide [10031-26-2], FeBr, is obtained by reaction of iron or inon(II) bromide with bromine at 170—200°C. The material is purified by sublimation ia a bromine atmosphere. The stmcture of inoa(III) bromide is analogous to that of inon(III) chloride. FeBr is less stable thermally than FeCl, as would be expected from the observation that Br is a stronger reductant than CF. Dissociation to inon(II) bromide and bromine is complete at ca 200°C. The hygroscopic, dark red, rhombic crystals of inon(III) bromide are readily soluble ia water, alcohol, ether, and acetic acid and are slightly soluble ia Hquid ammonia. Several hydrated species and a large number of adducts are known. Solutions of inon(III) bromide decompose to inon(II) bromide and bromine on boiling. Iron(III) bromide is used as a catalyst for the bromination of aromatic compounds. 

Metallocenes. Bis(cydopentadienyl)ii on oi feiiocene [102-54-5] Fe(C -H )2, is an aic and thermally stable orange solid that sublimes above... 

Chemical products are produced from technical-grade oxide in two very different ways. Molybdenum trioxide can be purified by a sublimation process because molybdenum trioxide has an appreciable vapor pressure above 650°C, a temperature at which most impurities have very low volatiUty. The alternative process uses wet chemical methods in which the molybdenum oxide is dissolved in ammonium hydroxide, leaving the gangue impurities behind. An ammonium molybdate is crystallized from the resulting solution. The ammonium molybdate can be used either directly or thermally decomposed to produce the pure oxide, MoO. ... 

A high quahty version of a dye-sublimation printer has been developed specifically for color proofing. This device uses a laser writing head, rather than the typical thermal printhead, to produce higher resolution images. The device is capable of tme halftones, providing an accurate rendition of a printed page. It is, however, expensive both in equipment and materials cost. 

Thermal Printing. Thermal printing is a generic name for methods that mark paper or other media with text and pictures by imagewise heating of special-purpose consumable media. Common technologies are direct thermal thermal, ie, wax, transfer and dye-sublimation, ie, diffusion, transfer. Properties and preferred appHcations are diverse, but apparatus and processes are similar (87—89). 

Transfer occurs by sublimation, condensation, and diffusion (101). Printhead thermal dissipation causes donor dye to travel to the surface of the donor ribbon and convert directiy to a gas. Colorant puffs immediately strike the nearby receptor and soak in, assisted by residual printhead heat. 

Dye sublimation requires more heat dissipation and a longer (>10 ms) heating period to make a dark mark than does thermal transfer. Carehil manipulation of heating time and temperature can proportion mark size and dye content to cover a wide density range (0 to ca 2 optical density). 

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