Activated interstitial model

Dariel (1973), on the basis of similar results, argued that it seemed highly implausible that the activated interstitial model should yield similar results in bcc y-La, S-Ce and jS-Pr considering that the model depended so critically upon the number of 4f electrons of the migrating atom and the possibility of their promotion to the conduction band. According to Dariel, the common diffusion behavior ensues from the particular properties of the bcc structure of the rare earth elements rather than from the electronic structure of the various diffusing species. 

The activated interstitial model which hinges on the relative ease of some f electron promotion in its initial form or on the possibility of d-f hydridization in its modified form seems to account for the main anomalous features observed in connection with self-diffusion in bcc e-Pu, S-Ce and possibly y-La and y-Yb. Beside these metals, however, anomalous diffusion has also been observed in /3-Ti, /3-Zr, /3-Hf, y-U and the rare earth metals -Pr, /3-Nd and P-Gd. The normal behavior of Eu (table 12.2), which among this latter group of metals is the only one having the bcc structure as its only allotropic form, stands out in marked contrast to the other bcc rare earth metals. It strongly supports Seeger s (1972) suggestion that anomalous diffusion in bcc metals is in some way connected to the phase transformation which precedes the bcc phase. 

In the course of the past decade several explanations have been put forward to account for the anomalous diffusion in bcc metals. The different models based on the presence of extrinsic vacancies (Kidson, 1963), on the temperature dependence of elastic constants (Aaronson and Shewmon, 1967), on dislocation enhanced diffusion (Peart and Askill, 1967) or on vacancy anharmonicity (Gilder and Lazarus, 1975) have been thoroughly discussed in the original and in the various review papers. Two additional models, the activated interstitial and, more recently, the w-embryo model, have been proposed as alternative explanations for anomalous bcc diffusion. These two models and their possible common interpretation on the basis of the Engel-Brewer electron correlation theory will be discussed in the next sub-section. 

Small solute atoms in the interstices between the larger host atoms in a relaxed metallic glass diffuse by the direct interstitial mechanism (see Section 8.1.4). The host atoms can be regarded as immobile. A classic example is the diffusion of H solute atoms in glassy Pd8oSi2o- For this system, a simplified model that retains the essential physics of a thermally activated diffusion process in disordered systems is used to interpret experimental measurements [20-22]. 

Here (3 is a function of vp, the number of sites active toward recombination in the recombination sphere. (In [110] the concentration and recombination volumes were expressed in units of the volume v0 of the unit cell, and n0 coincides with the fraction of sites or interstiatial sites occupied respectively by vacancies or interstitial atoms.) We note that, in the model being discussed, the cell itself in which a vacancy occurs is considered inactive with respect to recombination of an interstitial on it. 

Anti-tumor activity of bevacizumab has been reported in various preclinical animal models (primary and metastatic) with a broad array of tumor types [106, 107]. Clinical studies have further validated the focal role of VEGF in cancer. A single infusion of bevacizumab at 5 mg/kg in patients with primary and locally advanced adenocarcinoma of the rectum demonstrated direct and rapid antivascular effect in human tumors, with decreases in tumor perfusion, vascular volume, microvascular density, and interstitial pressure [108]. Clinical efficacy of bevacizumab in combination with 5-FU- and irinotecan-based regimens has been demonstrated in patients with metastatic colorectal cancer a significant improvement in overall survival time was observed compared with chemotherapy alone (20.3 versus 15.6 months for chemotherapy plus bevacizumab versus chemotherapy alone) [109]. 

Figure 5.3 shows the measured temperature dependence of the loss tangent of LaA103 single crystals and a fit employing the skm model plus a Debye relaxation term. The best fit was achieved for an activation energy of 31 meV, the estimated defect concentration is only 1016/cm3. It was suggested that aluminium atoms on interstitial lattice positions are responsible for the observed relaxation phenomena. This indicates, that the dielectric losses are extremely sensitive to very small concentrations of point defects [21],... 

The reason for this structure sensitivity is not known. Martin340 has concluded that the rate-determining step is the adsorption of ethane on an ensemble of 15 adjacent Ni atoms free of H2. On the other hand, Goodman338 suggests a model in which ethane is adsorbed at interstitial sites involving 6 Ni atoms in the (100) surface or 4 Ni atoms in the (111) surface. He proposes that the difference in activity between these two surfaces may be due either to electronic differences or to the fact that the spacing between the sites in the (100) surface is such as to encourage the dissociation of the C-C bond in ethane. The latter model, while attractive for ethane, would not readily... 

---