Vacancy behavior

In order for the trapping rate of vacancies by solute atoms to be important with respect to the thermal release in determining the bounded vacancy concentration, vs 5i T [17], which in the present case gives T 524 K. This means that the trapping effect is more dominant in stage 1 than in stage 2. 

during this stage transport by the association-dissociation reaction and by migration of complexes is also to be expected. Another effect connected with vacancy behavior, which influences the effective activation energy for migration and hence the ordering rate, will be considered below. 

First-order kinetics were assumed for the elimination of vacancy supersaturation 5, which assumes that elimination takes place on fixed sinks. The characteristic process frequency factor is given by kom = Pvi Om, where Pv is the effective sink density. For a dislocation density J = lO cmcm , as expected in the present material and using the conventional relation Pv = 27r6 5/ln(rs/rc) in which Tg is the average distance between dislocations, Vc the capture radius of the dislocation and b the atom jump distance, pv = 4.3 x 10 at is obtained. This value, in conjunction with the values calculated for uom, gives kom = 6.7 x 10, 13 X 10 and 2.3 x 10 s for 19, 13 and 6.5% Al. Furthermore, under nonisother-mal conditions, S = exp(—/com i(-E m)- Curves for S and Xi, plotted against T are shown in Fig. 12. For illustration purposes only, an 5 curve was plotted for a deformed material with 6 = 10 cmcm, which corresponds to pv = 10 at . For such a specimen, it can be inferred that stage 1 should go to completion at a lower temperature. 

Comparison of both x, and S curves, confirms that stage 1 effectively corresponds to an SRO process assisted by excess vacancies, because in all cases it goes to completion just as the vacancy supersaturation vanishes. Moreover, both curves exhibit symmetrical kinetic paths. It is then inferred, necessarily, that during stage 2 the ordering process must be completed, assisted by equilibrium vacancies and by vacancy complexes which were not yet eliminated during stage 1. 

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Diffusion in NijAl has been studied by few investigators - in particular Chou and Chou (1985) and Hoshino et al. (1988) -and has been reviewed and discussed with respect to mechanisms and defects (Bakker, 1984 Wever et al., 1989 Stoloff, 1989). The constitutional defects are antistructure atoms on both sides of stoichiometry, i.e. Al on Ni sites and Ni on Al sites, and the concentration of constitutional, i.e. ather-mal, vacancies is very small. The vacancy content of 6 x 10 at the melting temperature and the vacancy formation enthalpy of 1.60 eV correspond to the respective values for Ni, i.e. the vacancy behavior of NijAI is similar to that of pure metals (Schaefer et al., 1992). The diffusion of Ni in NijAl is not very different from that in pure Ni and at high temperatures it is insensitive to deviations from stoichiometry. The diffusion of Al in NijAl is less well studied because a tracer is not readily available. Defects may interact with dissolved third elements which affects diffusion. In particular vacancies interact with B which is needed for ductilization , and this leads to a complex dependence of the Ni diffusion coefficient on the Al and B content of NijAl (Hoshino etal., 1988). Data for the diffusion of the third elements, Co, Cr, or Ti, in Nij Al are available (Minamino etal., 1992). 

Abstract. The Thermal Analsysis (TA) applies a great variety of techniques suitable for determining the thermophysical properties of solids. Here after a wide and detailed review on more conventional methods of differential thermal analysis (DTA) and differential scanning calorimetry (DSC) to study non-equilibrated materials, the experimental results obtained from Short-Range Ordering (SRO) in a Cu-Al alloys is presented and discussed. The kinetic parameters and laws in these materials are deeply discussed focusing attention also to vacancy behavior and effects of quenching conditions. 

The diffusion coefficient corresponding to the measured values of /ch (D = kn/4nRn, is the reaction diameter, supposed to be equal to 2 A) equals 2.7 x 10 cm s at 4.2K and 1.9K. The self-diffusion in H2 crystals at 11-14 K is thermally activated with = 0.4 kcal/mol [Weinhaus and Meyer 1972]. At T < 11 K self-diffusion in the H2 crystal involves tunneling of a molecule from the lattice node to the vacancy, formation of the latter requiring 0.22 kcal/mol [Silvera 1980], so that the Arrhenius behavior is preserved. Were the mechanism of diffusion of the H atom the same, the diffusion coefficient at 1.9 K would be ten orders smaller than that at 4.2 K, while the measured values coincide. The diffusion coefficient of the D atoms in the D2 crystal is also the same for 1.9 and 4.2 K. It is 4 orders of magnitude smaller (3 x 10 cm /s) than the diffusion coefficient for H in H2 [Lee et al. 1987]. 

A number of selenium and tellurium compounds of the presently discussed metals show a quite different behavior from the Fe-S system. Iron and selenium form two compounds FeSe with a broad stoichiometry range and FeSe2 with a much narrower composition field. Below 400 the non-stoichiometric Fei xSe exists by creation of iron vacancies and can have compositions lying between FeySes and Fe3Se4. At low temperatures there exist two phases an a (PbO type) and a f) (NiAs type) phase. The crystal sUiicture of the diselenide, FeSe2, is an orthorhombic, C18 (marcasite) type. In the Fe-Te system, the defect NiAs structure is found at a composition close to FeTei.s, as about one-third of the Fe atoms are missing. At compositions around FeTe the behavior is complex, and the f)-phase has the PbO structure (like FeSe) but with additional metal atoms (i.e., FeuTe). 

On the right are the t5rpes of point defects that could occur for the same sized atoms in the lattice. That is, given an array of atoms in a three dimensional lattice, only these two types of lattice point defects could occur where the size of the atoms are the same. The term vacancy is self-explanatory but self-interstitial means that one atom has slipped into a space between the rows of atoms (ions). In a lattice where the atoms are all of the same size, such behavior is energetically very difficult unless a severe disruption of the lattice occurs (usually a "line-defect" results. This behavior is quite common in certain types of homogeneous solids. In a like manner, if the metal-atom were to have become misplaced in the lattice cuid were to have occupied one of the interstitial... 

Therefore, a bifUnctional mechanistic scheme, including the participation of both the metal (via the adsorption of CO) and the support (via the formation of "oxygen vacancies" which are active sites for the H2O dissociative adsorption) seems quite relevant to explain the specific behavior, for the NO r uction in the presence of water, of samples containing Zr02 Such active sites would be located at the met -support interface and are linked to the redox properties of the support... 

The possible active species are OH radicals, the photo-produced holes (h+) as suggested by Draper and Fox [9], the surface oxygen vacancies or anions (02 ) suggested by Lu et al. [4], and chlorine radicals (Cl ) when chloroolefins (e. g. TCE) are present [1-3, 5, 6]. We may anticipate several possible behaviors for plots of photocatal5dic rate vs. kinetic variable ... 

A group of scientists have studied current transients in biased M-O-M structures.271,300 The general behavior of such a system may be described by classic theoretical work.268,302 However, the specific behavior of current transients in anodic oxides made it necessary to develop a special model for nonsteady current flow applicable to this case. Aris and Lewis have put forward an assumption that current transients in anodic oxides are due to carrier trapping and release in the two systems of localized states (shallow and deep traps) associated with oxygen vacancies and/or incorporated impurities.301 This approach was further supported by others,271,279 and it generally resembles the oxide band structure theoretically modeled by Parkhutik and Shershulskii62 (see. Fig. 37). 

The activity of the transition metals, especially for the chemisorption of molecular hydrogen and in hydrogenation reactions has been correlated, in the past, with the existence of partially filled d bands. Many alloy studies were prompted by the expectation that catalytic activity would change abruptly once these vacancies were filled by alloying with a group IB metal. Examples of such behavior have been collected together for the Pd-Au system (1). It is to be expected also that various complications might superimpose on the simple activity patterns observed for primitive... 

The vacancy coverage, 9V, which is initially equal to 0.075, rapidly decreases during the initial period of NO exposure but then very slowly increases. This behavior can be attributed to the following factors. The first is that 0V in equilibrium with 0.10 atm of H2 is larger than 0V in equilibrium with 0.0028 atm of NO. Calculating the equilibrium constants for H2 and NO adsorption and desorption of these gases, given in Table I, one... 

The simplest way to account for composition variation is to include point defect populations into the crystal. This can involve substitution, the incorporation of unbalanced populations of vacancies or by the addition of extra interstitial atoms. This approach has a great advantage in that it allows a crystallographic model to be easily constructed and the formalism of defect reaction equations employed to analyze the situation (Section 1.11). The following sections give examples of this behavior. 

Zinc oxide is normally an w-type semiconductor with a narrow stoichiometry range. For many years it was believed that this electronic behavior was due to the presence of Zn (Zn+) interstitials, but it is now apparent that the defect structure of this simple oxide is more complicated. The main point defects that can be considered to exist are vacancies, V0 and VZn, interstitials, Oj and Zn, and antisite defects, 0Zn and Zno-Each of these can show various charge states and can occupy several different... 

A cation vacancy will be opposite to this in behavior. Removal of a neutral metal atom from a material will involve removal of a cation plus the correct number of electrons, which are taken from the valence band. Cation vacancies will therefore be represented as acceptor levels situated near to the valence band together with an equivalent number of holes in the band. These materials are p-type semiconductors. 

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