Defects intrinsic

Intrinsic defects (or native or simply defects ) are imperfections in tire crystal itself, such as a vacancy (a missing host atom), a self-interstitial (an extra host atom in an otherwise perfect crystalline environment), an anti-site defect (in an AB compound, tliis means an atom of type A at a B site or vice versa) or any combination of such defects. Extrinsic defects (or impurities) are atoms different from host atoms, trapped in tire crystal. Some impurities are intentionally introduced because tliey provide charge carriers, reduce tlieir lifetime, prevent tire propagation of dislocations or are otlierwise needed or useful, but most impurities and defects are not desired and must be eliminated or at least controlled. 

The intrinsic defects include paramagnetic and diamagnetic species (24,27,28). The paramagnetic defects have received the most study because they are readily detectable by electron spin resonance (esr) spectrometry. Paramagnetic defects that have been identified by esr include the center, 6i the... 

In pure and stoichiometric compounds, intrinsic defects are formed for energetic reasons. Intrinsic ionic conduction, or creation of thermal vacancies by Frenkel, ie, vacancy plus interstitial lattice defects, or by Schottky, cation and anion vacancies, mechanisms can be expressed in terms of an equilibrium constant and, therefore, as a free energy for the formation of defects, If the ion is to jump into a normally occupied lattice site, a term for... 

In a tensile test on an un-notched sample of acrylic the fracture stress is recorded as 57 MN/m. Estimate Ae likely size of the intrinsic defects in Ae material. 

A certain grade of PMMA has a K value of 1.6 MN m and it is known that under cyclic stresses, cracks grow at a rate given by (2 x 10 A/( ). If the intrinsic defects in the material are 50 mm long, how many hours will the material last if it is subjected to a stress cycle of 0 to 10 MN/m at a frequency of 1 Hz. 

A very wide sheet of grp which is known to contain intrinsic defects 1 mm long, is subjected to a fluctuating stress which varies from 0 to 80 MN/m. How many cycles would the sheet be expected to withstand if it is made from (a) chopped strand mat (CSM) and (b) woven roving (WR) reinforcement. The crack growth parameters C and n, and the critical stress intensity factors, Kc, for these materials are... 

The defects of the matrix play an important role on luminescent performances in these materials. Taking into consideration the preparation process of these compounds with the solid-state reaction of mixtures of BaC03, H3BO3, and NH4H2PO4 at different molar ratio, non-equal evaporation during the sintering process of these powders is inevitable and thus results in the formation of intrinsic defects, such as cation and oxygen vacancies. Positional disorder of B and Vacant B (Vb)" have been reported in SrBPOs crystals on the basis of... 

It is the intrinsic defects that have the most interest for us, since they affect the chemical properties of the solid while extrinsic defects havelittle effect. Extrinsic defects are the proper study for those interested in the mechanics of solids, particularly metals. 

These, then, cire the set of possible defects for the Plane Net, and the following summarizes the types of intrinsic defects expected. Note that we have used the labelling V = vacancy i = interstitial M = cation site X = anion site and s = surface site. We have already stated that surface sites are special. Hence, they are included in our listing of intrinsic defects. 

Although the results eu e equal as far as utility is concerned, we shedl continue to use our symbolism, for reasons which will become clearer later on. The following Table is a comparison of defect symbolism, as used by prior Authors. Note that our symbolism most resembles that of Kroeger, but not in aU aspects. These prior authors also considered other intrinsic defects that we have not touched, namely interstices and the so-called "anti-structure" occupation. 

To see how these equations might be used, consider the following. First, suppose we want to be able to determine the number of intrinsic defects in a given solid. Since pairs of defects predominate in a given solid, the number of each type of intrinsic defect, Ni (M) or Ni (X), will equal each... 

Here, we have expressed the concentration as the ratio of defects to the number of M- atom sites (this has certain advantages as we will see). We can than rewrite the defect equilibria equations of Table 3-3 and 3-4 in terms of numbers of intrinsic defect concentrations, shown as follows ... 

Equation 3.6.10. given above shows that intrinsic defect concentrations will increase with increasing temperature and that they will be low for high Enthalpy of defect formation. This arises because the entropy effect is a positive exponential while the enthalpy effect is a negative exponential. Consider the following examples of various types of compounds and the natural defects which may occur (depending upon how the compounds were originally formed) ... 

TIO is cubic with the NaCl structure. A sample was annealed at 1300 °C. Density and X-ray measurements revealed that the intrinsic defects were Schottlqr in nature (Vxi + Vq) and that their concentration was 0.140. In... 

CeHot. From thermodsmamic measurements, it was found that the intrinsic defects were Anti-Frenkel in nature, i.e.- (H + Vh). An equilibrium constant was calculated as ... 

We have given defect-equations for edl nine types of defects, and the Equilibrium Constants thereby associated. However, calculation of these equilibria would require values in terms of energy at each site, values which are difficult to determine. A better method is to convert these EC equations to those involving numbers of each Qrpe of intrinsic defect, as a ratio to an intrinsic cation or einion. This would allow us to calculate the actual number of intrinsic defects present in the crystal, at a specified temperature. 

We have also shown that the intrinsic defects can become ionized to form species not found in nature. 

The higher-purity compound may undergo soUd state reactions somewhat different than those considered "normcd for the compound. If we reflect but a moment, we realize that this is what we might expect to occur as we obtain compounds (crystals) containing far fewer intrinsic defects. 

Given ksh = 3 x 10-3 for CaQ2, calculate the number of intrinsic defects present in this crystal. If CaCl2 is face-centered cubic, use the same equilibrium constant to calculate the intrinsic Frenkel, Anti-Frenkel and Interstial defects expected in this crystal. 

This presentation is presented for those who wish to examine the mathematics of both non-stoichiometric intrinsic- defect compounds and the ionization of defects in both stoichiometric and nonstoichiometric compounds as represented by ... 

Herein is shown how 6 changes from negative to positive at the higher pressure ratios. For a hypotheticcd MXs compound (S = 1), which contains Anti-Freiikel defects, to obtain a 0.7% deviation from the stoichiometric composition requires a pressure increase of some 5000 fold, when the original intrinsic defect concentration is 10". However, if it is lO, only a two-fold increase in pressure is needed to cause the same effect on a deviation of 0.7%, i.e.- 8 = 0.007 in MXs 6. ... 

Although we win not treat the other types of pairs of defects, it is well to note that similar equations can also be derived for the other intrinsic defects. What we have really shown is that external reactants can cause further changes in the non-stoichiometry of the soUd. Let us now consider ionized defects. It should be clear that an external gaseous factor has a major effect upon defect formation. The equations given above are very complicated and represent more closely what actually happens in the real world of defect formation in crystals. 

Note that we have defined "interface angle" as the angle between the growing crystal and the residual melt. Rate of pulling also ciffects the quality of the crystal as well as the actual number of intrinsic defects which may appear in the final crystal. In the upper left of 6.4.7., aflat-... 

It has been determined that there is a distribution coefficient for the impurities between crystal and melt which favors the melt. We can see how this arises when we reflect that impurities tend to cause formation of intrinsic defects within the crystal and lattice strain as a result of their presence. In the melt, no such restriction applies. Actually, each impurity has its own distribution coefficient. However, one can apply an average value to better approximate the behavior of the majority of impurities. 

The intrinsic defects found in single crystals include vacancies. 

Lattice Vacancies and Interstitials Defects such as lattice vacancies and interstitials fall into two main categories intrinsic defects, which are present in pure crystal at thermodynamic equilibrium, and extrinsic defects, which are created when a foreign atom is inserted into the lattice. 

Intrinsic Defects The simplest crystalline defects involve single or pairs of atoms or ions and are therefore known as point defects. Two main types of point defect have been identified Schottky defects and Frenkel defects. A Schottky defect consists of a pair of vacant sites a cation vacancy and an anion vacancy. A Schottky defect is... 

Nonstoichiometric Compounds Intrinsic defects are stoichiometric defects (i.e., they do not involve any change in overall composition). Defects can also be nonstoichiometric. In the case of extrinsic defects where the host crystal is doped with aliovalent impurities, the solid so formed is a nonstoichiometric compound because the ratio of the atomic components is no longer the simple integer. There is also... 

The kinds of substitution mechanisms that may be relevant to super-low concentration elements such as Pa involve intrinsic defects, such as lattice vacancies or interstitials. Vacancy defects can potentially provide a low energy mechanism for heterovalent cation substitution, in that they remove or minimise the need for additional charge balancing substitutions. Formation of a vacancy per se is energetically unfavourable (e.g., Purton et al. 1997), and the trace element must rely instead on the thermal defect concentration in the mineral of interest, at the conditions of interest. Extended defects, such as dislocations or grain boundaries, may also play a key role, but as these are essentially non-equilibrium features, they will not be considered further here. 

Fluxes of iron from the plasma towards BM and other tissues can be quantified by ferrokinetic studies, using 59Fe and sophisticated computer models (Ricketts et ah, 1975 Ricketts and Cavill, 1978 Barosi et ah, 1978 Stefanelli et ah, 1980). Plasma iron turnover (PIT), erythroid iron turnover (EIT), non-erythroid iron turnover (NEIT), marrow iron turnover (MIT), and tissue iron turnover (TIT) could be calculated in many disorders of iron metabolism and in all kinds of anaemias. Iron is rapidly cleared from the plasma in iron deficiency and in haemolytic anaemias. If more iron is needed for erythropoiesis, more transferrin receptors (TfR) are expressed on erythroblasts, resulting in an increased flux of iron from intestinal mucosal cells towards the plasma. In haemolytic anaemias MPS, and subsequently hepatocytes, are overloaded. In hereditary haemochromatosis too much iron is absorbed by an intrinsic defect of gut mucosal cells. As this iron is not needed for erythropoiesis,... 

Irradiation of high surface area silica has produced several well-defined paramagnetic centers, one of which appears to be an intrinsic defect in the... 

Since ID conjugated chains are interrupted by such intrinsic defects (the Pople-Wamsley defects) or eventually impurities the... 

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