Effects of Deviation from Stoichiometry

In the previous section, we have found that a very small Li deficiency results in the 

The effects of an excess and a deficiency of lithium on the electrochemical properties are also very different. They are shown in Fig. 7.20. 

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The softening at high temperatures is related to thermally activated creep processes which are the subject of the next section. Deviations from stoichiometry produce constitutional defects which enhance diffusion and lead to softening at high temperatures, whereas at low temperatures these defects are immobile and act as strengthening deformation obstacles. These different effects of deviations from stoichiometry at low and high temperatures were studied in detail for binary NiAl (Vandervoort etal., 1966). 

Apart from the effect of deviations from stoichiometry. Fig. 23 shows a marked dependence of the creep resistance for the ternary B2 phases (Ni,Fe)Al on the Ni Fe ratio. It can be shown for (Ni,Fe)Al that this composition dependence results from the composition dependence of the diffusion coefficient, which can be estimated by using data from Cheng and Dayananda (1979) and Moyer and Dayananda (1976) (see Rudy, 1986 Rudy and Sauthoff, 1986). The maximum in the creep resistance of (Ni,Fe)Al is directly related to the minimum in the diffusion coefficient. It has to be noted that the analysis of creep data with respect to diffusion coefficients poses problems because, apart from the scarcity of data, the diffusion coefficients in Eqs. (2) and (3) are effective ones depending on the particular creep process which determines the coupling of partial... 

It is seen in Table I that the effects of deviation from stoichiometry on the kinetics of the reaction between rutile and strontium carbonate are very pronounced. On the basis of the applicability of the nucleus growth models it appears that increasing the defect concentration lowers the frequency factor as well as the activation energy for diffusion of the rate-con-... 

Maeda Y, Ariyoshi K, Kawai T, Sekiya T, Ohzuku T (2009) Effect of deviation from Ni/Mn stoichiometry in Li[Nii/2Mn3/2]04 upon rechargeable capacity at 4.7 V in nonaqueous lithium cells. J Ceram Soc Jpn 117 1216-1220... 

Effect of indium on pa-operties of SnTe with different degree of deviation from stoichiometry. Izo. AN SSSR. Neorgan. Mater., Vol. 19, No. 4, (March, 1983), pp. 573-577. 

This result then leads us to a plot of the effect of partial pressure of Bt2 on the deviation from stoichiometry, 6, for the AgBr crystal. (This work is due to Greenwood - 1968). 

In comparison to the research in n-type oxide semiconductors, little work has been done on the development of p-type TCOs. The effective p-type doping in TCOs is often compensated due to their intrinsic oxide structural tolerance to oxygen vacancies and metal interstitials. Recently, significant developments have been reported about ZnO, CuA102, and Cu2Sr02 as true p-type oxide semiconductors. The ZnO exhibits unipolarity or asymmetry in its ability to be doped n-type or p-type. ZnO is naturally an n-type oxide semiconductor because of a deviation from stoichiometry due to the presence of intrinsic defects such as Zn interstitials and oxygen vacancies. A p-type ZnO, doped with As or N as a shallow acceptor and codoped with Ga or Zn as a donor, has been recently reported. However, the origin of the p-type conductivity and the effect of structural defects on n-type to p-type conversion in ZnO films are not completely understood. 

The production of self-activated CaW04 is more difficult than that of MgW04. Small deviations from stoichiometry in the composition have a marked effect on the efficiency. Also, traces of Pb2 + shift the emission peak from 415 nm to 439 nm (0.0006 mol % Pb2+). To bind excess CaO in the production process, a small proportion of sulfate is used. The precipitated CaW04 is washed until chloride-free, dried,... 

The effects of bioethanol use on NOx emissions are not consistent between different vehicles and studies. Larsen et al. [37] provided an overview of available studies, and concluded that bioethanol use can lead to either increases or decreases in NOx emissions during tests, depending on the experimental conditions. This is consistent with the non-linear behaviour of the emission-control system in petrol vehicles, where the smallest deviations from stoichiometry greatly affect NOx emissions. Leaner mixtures lead to an increase in NOx emissions, and richer mixtures lead to a decrease in NOx emissions. This erratic behaviour suggests that any direct fuel effects are masked by the ability of the fuelling system to maintain stoichiometry when changing from petrol to bioethanol blends in the different vehicles tested. 

W.K. Chen, N.L. Peterson. Effect of the deviation from stoichiometry on cation self-diffusion and isotope effect in wiistite, Fei.xO // J.Phys.Chem.Solids.- 1975.- V.36, NO.10.-P.1097-1103. 

Effect of Partial Pressure of Br2Gas on Deviation from Stoichiometry of the AgBr Crystal... 

Figure 23 shows the effect of composition variations, i.e. deviations from stoichiometry and variations in the Ni Fe ratio, on the creep resistance of (Ni,Fe)Al. It can be seen that the creep resistance decreases with decreasing Al content, i.e. increasing deviation from stoichiometry. As already noted in the preceding section, deviations from stoichiometry introduce constitutional disorder, i.e. point defects which increase the diffusion coefficient... 

The sensitivity of p to deviations from stoichiometry and to the heat-treatment procedure has also been demonstrated by van Daal et al. (1975). This effect can explain some discrepancies between their data and those of Brodsky and Bridger (1973). 

Besides absorption of NO, YBa2Cu307 x has been shown to be very effective in the catalytic decomposition of NO to N2 and O2. This decomposition is related to the amount of absorbed NO, and hence to the deviation from stoichiometry. This is, in fact, one of the first examples of the direct decomposition of NO in Nj and Oj. ... 

CujS exhibits mixed conductivity, with Oqj+ 0.2 S/cm at 420°C. The electronic condnctivity is contributed by electrons and holes. For CujS eqniUbrated with copper, = 0.16 S/cm. As the mobility of the electrons is expected to be at least an order of magnitude larger than that of the ions, we conclude that n, p A/ . Yokota also fotmd that this class ( n, p A/ ) fits the experimental data. However, for T < 100°C, Allen and Bnhks find that the class /r = A/j fits their experimental data on CujS. This indicates that at elevated temperatnies thermally excited ionic defects dominate. However, thermal excitation of defect pairs is not effective at low temperatures T < 100°C), and one kind of ionic defect (copper vacancies) is formed by deviation from stoichiometry being accompartied by electronic defects (holes). Mixed condnctivity is observed also in Cuj xSe. Direct measurement of p and (n < p) shows that p N. Copper phosphates with the NASICON or allrrarrdite type stmcture exhibit mixed condnctivity with a wide range of ratios 0/0. ... 

It was revealed that the introduction of cations with variable valence (In, Cu) leads to a specific effect caused by the presence of NS vacancies and consisting in the recharging of cations under changing concentration of impurity atoms or deviation from stoichiometry. 

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