Cascades recrystallization

Hsu et al. [1.121] observed recrystallization on the recombinant CD4-IgG with a cryomicroscope cooled to -60 °C by a cascade of four Peltier modules. The observation cell can also be evacuated for freeze drying studies. 

The specimen temperature during irradiation can be controlled between -250°C and 800°C. This allows the damage accumulation and simultaneous thermally-assisted recrystallization processes to be studied. At low temperatures (-250°C), no thermally-assisted annealing can occur in the specimen, and the measured amorphization dose depends only on the cascade structure and evolution. At higher temperatures, thermally-assisted annealing at cascade boundaries occurs, and the amorphization dose increases. 

In the ion beam experiments, the beam-current density is measured and converted to a flux, ion/cm s. The fluence (in ions/cm experienced by the specimen is the flux multiplied by time. In many papers, the measured fluence is converted to an ion dose in units of displacements per atom (dpa). This is essentially a measure of the actual amount of damage (i.e. number of atomic displacements) that results from cascade formation, but it does not consider subsequent relaxation or recrystallization events. The fluence-to-dose... 

Second, the simple models used for predicting the fa vs. D relationship have not considered the kinetics of defect recovery for recrystallization of the amorphous domains. Experiments have shown that defect recovery or cascade annealing can be substantial at room temperature, or even lower temperatures (Wang and Weber 1999, Bench et al. 2000). As an example, two recent studies of zircon can be used to support either the double-overlap (Weber 1990) or direct impact (Rios et al. 2000) models. 

Other metals, such as W, do not present this tendency of the vacancies to cluster, and at the end of the collisional cascade most of the vacancies are isolated. Such observation can be clearly seen in Fig. 5 where we present the result of those defects produced in W by a 30 keV W atom. This lack of vacancy clustering is observed for energies as high as 100 keV, where already sub-cascade formation occurs. The level of clustering in a system has been attributed to the rate of recrystallization during the thermal spike [50]. However, an exhaustive study for fee and bcc systems regarding this issue has not been done to date. 

AIBN in boiling benzene, the radical cascade products 15a and 15b were obtained in a ratio of 3.4 1 and in 36% combined yield. A similar reaction with Et3B at -78 °C in toluene afforded 15a and 15b in a ratio of 37 1 in 46% yield. Recrystallization of the obtained mixture gave the pure isomer 15a (Scheme 11). 

Transformation from the crystalline to amorphous state can be initiated by either homogeneous point defect accumulation or heterogeneous displacement cascade collisions (Trinkaus 1997). The prime requisite for this is the high mobility of radiation defects. However, in the case of oxide ceramics, the mobility of the radiation defect is very low so that thermally activated recrystallization occurs at high rate compared to the defect production (Motta 1990, Weber et al. 1994). Hence, all of the specimens remained crystalline for the investigated irradiation conditions as confirmed from the XRD studies except for the Li implanted MgO films. 

Fig. 6.7 Annealing of cascade-induced amorphous pockets by molecular dynamics, a State immediately after the cascade, b After Ins annealing at 1300K the amorphous material has recrystallized and selfinterstitials and vacancy clusters are left behind. The vacancies and their clusters induce tensile stress in the neighboring atoms and these are shown as blue spheres. The self-interstitials are shown as red and gray spheres [48]...

A substitutional configuration was suggested for a third site = —0.07(3) mm/s) on the basis of the Fej + V Fcj model and from theoretical calculations. This configuration is attributed to Fe atoms that find themselves outside the amorphized collision cascade region, or in a recrystallized part of it. 

Figure 9.1.7. Crosscurrent cascade for recrystallization via cooling crystallization.

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