Sintered films

Fig 2.9. The field dependence of electric conductivity of a thin sintered film (/) and a pressed ZnO sample (2) [37]... 

To dissociate molecules in an adsorbed layer of oxide, a spillover (photospillover) phenomenon can be used with prior activation of the surface of zinc oxide by particles (clusters) of Pt, Pd, Ni, etc. In the course of adsorption of molecular gases (especially H2, O2) or more complex molecules these particles emit (generate) active particles on the surface of substrate [12], which are capable, as we have already noted, to affect considerably the impurity conductivity even at minor concentrations. Thus, the semiconductor oxide activated by cluster particles of transition metals plays a double role of both activator and analyzer (sensor). The latter conclusion is proved by a large number of papers discussed in detail in review [13]. The papers cited maintain that the particles formed during the process of activation are fairly active as to their influence on the electrical properties of sensors made of semiconductor oxides in the form of thin sintered films. 

It is obvious that the isobars shown in Fig. 4 cannot be explained by adsorption. In the first place, the difference between the adsorption at —196° as obtained initially and the total sorption found at —196°, after obtaining the isobars, decreases little with increased temperature of sintering. Films sintered at 23° have about ten times the initial adsorption at — 196°C. of films sintered at 200° and about 175 times the... 

C. also decreases proportionally to the decrease in activity for hydrogenation of ethylene at room temperature. Thus both the relative chemisorption of CO at room temperature and the van der Waal s adsorption of krypton are reliable measurements for the surface available for hydrogenation of ethylene. The same is true for the fast adsorption of hydrogen at — 196°C., so that the latter presents a third criterion by which it is possible to determine the catalytically active surface of sintered films. 

R.K. Bordia and A. Jagota, Crack growth and damage in constrained sintering films. J. Am. Ceram. Soc., 76 (1993) 2475-2485. 

A remarkable effect was observed when sintered iron films were used. The multiple exchange process disappears and a normal distribution is obtained (see Pig. 7b). The use of sintered films also enabled a course of reaction to be followed (cyclohexene at 0°, cyclopentene at —35°) stepwise olefin exchange was observed, slightly more marked with cyclopentene than with cyclohexene, and the results bear a marked resemblance to those shown for the reactions of ethylene and of 1-butene with deuterium over nickel in Figs. 5 and 6. Sintering also removes the ability of iron films to catalyze the disproportionation of cyclohexene to cyclohexene and benzene, and for this reason it was... 

Electron diffraction photographs of sintered films prepared after thorough outgassing of the apparatus did not show nickel oxide lines, but this, as Dr. de Boer has suggested, does not necessarily mean that nickel oxide was not there. However, exposure of the films to oxygen poisoned the surface completely, and this too, indicates that oxide was not present. 

Arrhenius parameters and selected values of M are given in Table 6.2 in conjunction with Figure 6.3, the following comments can be made, (i) Sintered films of palladium and platinum again have low activities and show high activation energies this appears to be the only bimetallic system looked at. (ii) Certain... 

Variously structured nickel films also produced some very peculiar product distributions," as they did with ethane at 393-433 K on (111) and (100) oriented films gave propane-r/g and (usually) -d2 as the major products, but randomly orientated but sintered film at 400-435 K yielded propane-di and -d%, and an unsintered film at 273 K gave only the -d isomer. These results closely resemble those obtained with ethane, and lend stress to the importance of surface structure in deciding what intermediates are formed and how they react. The absence of results for single crystal surfaces of nickel is sorely felt. 

Figure 6. In an 1100 °C-sintered film (a) cross-sectional SEM through a relatively deep indent ...

Further evidence of substrate effect can be observed as below. Figure 8 shows representative SEM images of the indentation imprints and their cross-sections at indentation depths from 800 to 3600 nm in an 1100 °C-sintered film (tf 10 pm) obtained using the FIB/SEM instrument. The imprints for indentation depth lower than 800 nm (i.e. hmax f 8%) could not be identified with SEM because the residual depths were too shallow. To simplify the image acquisition process, no surface protection coating or impregnation was applied to this sample. 

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