Sintered polycrystalline adsorbents

Above we have considered the possibility of existence of various types of polycrystalline semiconductor adsorbents differing in the character of contact of specific microcrystals. Let us consider the effect of this difference on adsorption and electrophysical properties of adsorbents in more detail and, which is more important, address the item of the mechanisms of the change of electrophysical properties of semiconductor caused by its interaction with gaseous phiise. 

Following the widely cited paper by Hatson [18] let us depict the cross-section of a portion of a sintered sample as is shown in Fig. 2.5. The shaded area shows the domain of spatial charge. 

The contacts of the third type (see Fig. 2.2, a) which are interfaces or contact areas separate microcrystals are equivalent (this has been shown in preceding Section) to the double Shottky barrier or, to put it more correctly, to the isotype heterotransitions [22, 29]. As it has been shown in Section 1.10 in detail, the energy of activation of electric conductivity of the material with dominant fraction of contacts of this type is dependent on the heights of intercrystalline barriers. The change in electric conductivity due to effects of various external effects (adsorption in particular) is related to the height of these barriers. 

In real polycrystalline materials there are, presumably, all three above types of contacts present. It is assumed that the issue of dominance of a specific type of contacts is related to the technology of manufacturing of a sample and to its subsequent treatment [17, 24, 28, 30]. Moreover, there are various manufacturing techniques enabling one to obtain a sample with dominating type of contacts using the material of another type [30]. 

We should point out that up to now we have considered only polycrystals characterized by an a priori surface area depleted in principal charge carriers. For instance, chemisorption of acceptor particles which is accompanied by transition-free electrons from conductivity band to adsorption induced SS is described in this case in terms of the theory of depleted layer [31]. This model is applicable fairly well to describe properties of zinc oxide which is oxidized in air and is characterized by the content of surface adjacent layers which is close to the stoichiometric one [30]. 

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The effect of adsorption on electric conductivity of sintered polycrystalline adsorbents... 

We consider problems related to electrophysical properties of sintered polycrystalline oxides as well as their adsorption changes. We also analyze the difference in adsorption induced changes of electrophysical characteristics of stoichiometric and non-stoichiometric partially reduced oxide adsorbents. 

Thus, the whole complex of existing experimental data indicates that the major part of polycrystalline contacts in vacuum sintered polycrystalline oxides are provided by bridges of open type. Moreover, the vacuum sintering at moderate temperatures 300 - 350°C leads to formation of a unified pattern (see Fig. 2.4, b) which cannot be disjoint into specific microcrystals and connecting bridges [37, 40]. The structure of adsorbents obtained presents a complex intertwining of branches of various thickness. 

We used polycrystalline films of ZnO and Sn02 as adsorbents. The films were deposited from the water suspension of respective oxides on quartz substrates. These substrates contained initially sintered contacts made of platinum paste. The gap between contacts was of about lO" cm. All samples were initially heated in air during one hour at T 500 C. We used purified molecular oxygen an acceptor particle gas. H and Zn atoms as well as molecules of CO were used as donor particles. We monitored both the kinetics of the change of ohmic electric conductivity and the tangent of inclination angle of pre-relaxation VAC caused by adsorption of above gases and the dependence of stationary values of characteristics in question as functions of concentrations of active particles. 

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