Amorphous neutron irradiation

Irradiation by fast neutrons causes a densification of vitreous silica that reaches a maximum value of 2.26 g/cm3, ie, an increase of approximately 3%, after a dose of 1 x 1020 neutrons per square centimeter. Doses of up to 2 x 1020 n/cm2 do not further affect this density value (190). Quartz, tridymite, and cristobalite attain the same density after heavy neutron irradiation, which means a density decrease of 14.7% for quartz and 0.26% for cristobalite (191). The resulting glass-like material is the same in each case, and shows no x-ray diffraction pattern but has identical density, thermal expansion (192), and elastic properties (193). Other properties are also affected, ie, the heat capacity is lower than that of vitreous silica (194), the thermal conductivity increases by a factor of two (195), and the refractive index, increases to 1.4690 (196). The new phase is called amorphous silica M, after metamict, a word used to designate mineral disordered by radiation in the geological past (197). 

Many minerals are unstable when exposed to the beam in the electron microscope and transform from a crystalline to an amorphous (or glassy) state at a rate that varies widely from one mineral to another and as a function of a number of variables such as beam intensity, accelerating voltage, temperature, and even impurity content. A similar transformation can be induced by neutron irradiation, and TEM has been used to... 

The rate at which electron radiation damage occurred in neutron-irradiated specimens also varied from one variety of quartz to another. Whereas synthetic quartz (dose Dq) became amorphous after a few seconds of exposure to a focused electron beam, amethyst quartz (dose < Do) was more stable in the electron beam than the sample before neutron irradiation. 

Micro-Raman spectroscopy was used to characterise 4H-SiC layers grown from a variety of precursor systems.381 FTIR data were able to characterise hydrogenated amorphous silicon nitride films with embedded nanoparticles. Oxidation leads to the appearance of an Si O feature at 1070 cm 1,382 Raman spectra were used to determine the degree of micro-crystallinity of pc Si I I layers, using the intensity ratio of bands at 520 cm-1 and 480 cm-1.383 IR and Raman spectra were used to determine the effects of neutron irradiation on a-SiC H films.384 A range of a-SiCx I I and a-SiCxNy H films were studied using IR spectroscopy 385 similar experiments were carried out on a-Si i xGcx Il,F films.386... 

It should also be noted that crystalline solids can also be made amorphous by irradiation with neutrons or ions, or by massive plastic deformation. The structure (Large and Bicknell (1967), Moss era/. (1971)),... 

Cubic fluorite-structure (Fm3m) zirconia-based solid solution, (Zr,ACT,REE)02 x, exhibi ts significant compositional flexibility to incorporate high concentrations of Pu, neutron absorbers, and impurities contained in Pu-bearing wastes (Gong et al. 1999). The phase has excellent radiation stability. No amorphization was observed under ion irradiation at room temperature to a dose corresponding to 200 dpa, and at 20 K to a dose of 25 dpa. Irradiation with I+ and Sr+ up to 300 dpa produced defect clusters in Y-stabilized zirconia, but did not cause amorphization. Amorphization... 

The fa vs. D curves generated from the various models have been compared with the limited amount of experimental data to infer which type of model, e.g. direct impact vs. double overlap, is most applicable. As shown in Figure 7, the measured dependence of fa on D for Cm-doped Ca2Nd8(Si04)602 is consistent with the predicted curve for the direct impact model for ion-irradiated zircon, the measured dependence offa on D is consistent with the double-overlap model SrTiOs, the defect accumulation model (Weber 2000). It has been generally accepted that for heavy ion irradiations, especially at low temperatures, the amorphization process in ceramic materials is the result of amorphization within the cascade. For lighter ions (or electrons and neutrons) or at high temperatures, the dominant mechanism is considered to be cascade-overlap or defect accumulation. 

Silica M. an amorphous silica formed when cither amorphous or crystalline silicas are irradiated with high speed neutrons. The density of amorphous silica... 

The number of dpa is increasing with increasing fluence. For a depth of 7 pm, the number of dpa increases from 0.02 at lO ions/cm to 0.2 dpa at lO ions/cm and about 2 dpa at 10 ions/cm. Under neutron or ion irradiations, the disorder increases with the irradiation fluence to reach amorphous state as soon as the fluence exceeds an amorphization threshold. For monocrystalline 6H-SiC this amorphization threshold was determined to be in the 0.2- 0.5 dpa range at room temperature. 

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