Evaporated films

Cadmium Sulfide Photoconductor. CdS photoconductive films are prepared by both evaporation of bulk CdS and settHng of fine CdS powder from aqueous or organic suspension foUowed by sintering (60,61). The evaporated CdS is deposited to a thickness from 100 to 600 nm on ceramic substates. The evaporated films are polycrystaUine and are heated to 250°C in oxygen at low pressure to increase photosensitivity. Copper or silver may be diffused into the films to lower the resistivity and reduce contact rectification and noise. The copper acceptor energy level is within 0.1 eV of the valence band edge. Sulfide vacancies produce donor levels and cadmium vacancies produce deep acceptor levels. 

Berylha ceramic parts ate frequendy used in electronic and microelectronic apphcations requiting thermal dissipation (see Ceramics as ELECTRICAL materials). Berylha substrates are commonly metallized using refractory metallizations such as molybdenum—manganese or using evaporated films of chromium, titanium, and nickel—chromium alloys. Semiconductor devices and integrated circuits (qv) can be bonded by such metallization for removal of heat. 

Figure 2 Surface EXAFS spectra above the Pd L edge for a 1.5 monolayer evaporated film of Pd on Sid 11) and for bulk palladium ailicide, Pd2Si and metallic Pd.

Emulsion paints These materials are water based and cure by coalescence as the water evaporates. Films formed... 

Fig. 10. Coefficient of H atom recombination on Ni-Cu alloy catalysts as a function of the alloy composition, at 20°C. A, on Ni-Cu foils (59), O, on Ni-Cu evaporated films af ter their previous homogenization at 400°C (65,65a) d, on Ni-Cu foils after a multiple hydrogen absorption-desorption treatment (64a).

A reliable determination of the adsorbent temperature is obviously a crucial requirement of the thermal desorption method. Thermocouples are mostly used to this end. With disks and foils, a thermocouple can be spot-welded onto the back or edge of the sample. Thermocouples can be attached also to ribbons as well as to the wall of the vessel containing an evaporated film. With powdered adsorbents, thermocouples are located in the layer of the sample. The adsorbents in the form of filaments and ribbons are frequently used simultaneously as resistance thermometers, switched... 

The structure of a CVD material can be classified into three major types which are shown schematically in Fig. 2.13. In Zone (A), the structure consists of columnar grains which are capped by a domelike top. In Zone (B), the structure is also columnar but more faceted and angular. In Zone (C), it consists of fine accost grains. Examples of these structures are shown in Fig. 2.14. " This is the CVD equivalent of the structural model for vacuum-evaporated films first introduced by Movchan and Demshishin.P l... 

It was found in experiments that electric conductivity of lower oxide film 2 decreased in time. On the contrary, electric conductivity of upper sensor film / increased. This peculiarity can be attributed to transfer of superstechiometric atoms of zinc from the lower (evaporating) film to the upper (sensor) film. In other words, a double control was realized in this case. 

It follows from calculations, that intensity of flux of zinc atoms incident upon the sensor film in these experiments amounted to 10 - 10 atom/cm, on the average. It is seen from Fig. 4.11, that all experimental points depicting in arbitrary units the rates of increase (sensor) or decrease (evaporating film) of electric conductivity can be well approximated by the linear dependence in a (In d - - ) plot. 

Evaporated films are, of course, not practical catalysts. Their use as model catalysts is however justified by the insight which such work may give toward an understanding of catalytic reaction mechanisms. 

An initial state of high surface purity may be achieved with evaporated films using relatively straightforward techniques, and it is the elimination of initial surface contamination as a significant experimental variable which makes evaporated films desirable as model catalysts compared to bulk supported catalysts. 

Recently, ultrathin evaporated films have been used as models for dispersed supported metal catalysts, the main object being the preparation of a catalyst where surface cleanliness and crystallite size and structure could be better controlled than in conventional supported catalysts. In ultrathin films of this type, an average metal density on the substrate equivalent to >0.02 monolayers has been used. The apparatus for this technique is shown schematically in Fig. 8 (27). It was designed to permit use under UHV conditions, and to avoid depositing the working film on top of an outgassing film. ... 

Evaporated film catalysts are virtually always used with a static gas phase, and with reactant gas pressures less than about 100 Torr. One thus relies upon gaseous diffusion and convection for transport to the catalyst surface. However, provided one is dealing with reaction times of the order of minutes to tens of minutes, gas phase transport has but a negligible effect on the reaction, provided none of the reaction volume is separated from the film by small bore tubulation. Beeck et al. (77) in fact originally used an all-glass magnetically coupled turbine for gas circulation, but this is only... 

Platinum is an important example of a metal where, even on an uncontaminated surface such as is offered by an evaporated film, there is a strong tendency for only one C—C bond to be ruptured in any particular reacting molecule. On this basis, one may express the distribution of reaction products in terms of relative C—C bond rupture probabilities. Some data of this sort are contained in Table XI for thick and ultrathin film catalysts, and for comparison there are included some data for reactions on a silica-supported catalyst containing 0.8% platinum. These data all refer to reactions carried out in the presence of a large excess of hydrogen, although the results of Kikuchi et al. (128) indicate that on platinum catalysts the position of C—C bond rupture (in n-pentane) is very little dependent on hydrogen pressure. The data in Table XI show that, on the whole, the 0.8% platinum/silica catalyst used by Matsumoto et al. (110) was inter-... 

Anderson and Kemball (35) examined the reaction between gaseous deuterium and benzene catalyzed by evaporated films of iron, nickel, palladium, silver, tungsten, and platinum. The order of reactivity (estimated from the temperature at which the addition reaction achieved an initial rate of 1% per minute for a 10 mg film at certain specified reactant... 

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