Atomic size of irradiated advanced materials evaluation
Izida Kh. Abdukadirova ( )
Institute of Nuclear Physics, Academy of Sciences of Uzbekistan. BOX 702132, Tashkent, Ulugbel, Uzbekistan.
The optical, mechanicel and dielectrical properties of some advanced dielectrics materials for opto- and microelectronics crystals corundum (samples 1) and quartz (samples 2) irradiation by neutrons are investigated, a comparison is made with the properties of the samples befor irradiation. We investigated plates with the some history, pure and geometry. The plates were irradiated with different neutron doses in the channels of a WWR-SM reactor. We measured the electrical parameter - dielectrical permeability ($\epsilon$) at 1 kHz of neotron-irradiated plates. The $\epsilon(F)$ dependence was found to be nonmonotonic, with a kink near dose $F = 4 \times 10^{19} cm^{-2}$ (for samples 2) and $6 \times 10^{19} cm^{-2}$ (for samples 1). Using these data and dose dependence of the refractive index ($n$), we calculatede the total ($\alpha_1$) and electronic ($\alpha_2$) polarisabilities of the crystals from the Clausius-Mossoti and Lorentz formulas. It is established that the $\alpha_1(F)$ and $\alpha_2(F)$ curves exhibit similar behavior, that at the initial stage ( $F=10^{18} - 10^{19} cm^{-2}$ ) - table 1 of irradiation these parameters changes little; a noticeable increase them occurs at the second stage (for $F > 10^{19} cm^{-2}$). It is of particular interest to stady the effect of doses, which the $\alpha_1(F)$ and $\alpha_2(F)$ curves exhibit similar behaviour, with an upturn around a dose $4 \times 10^{19} cm^{-2}$, when a rapid phase transition occurs in samples 2. Using the available data on the temperature dependence of the dielectrical permeability and refractive index for the crystal, we evaluated these parameters as a function of temperature. Analysis of the $\alpha_1(F)$ and the $\alpha_1(T)$ curves a correlation between change of the polarizabilities of crystals at the thermally and radiation treatments (table 1). It is revealed of the equality significance them at the certain dose $F_c$ and temperature transition $T_c$ indicate that a state similar to the high-temperature modification of quartz is formed near $F_c$. This is confirmed by X-ray difraction data. $\alpha$-quartz lattice parameters ( c, a ) nonlinear change and intensity relations of reflections 33(-)01 and 3(-)301 responsible for phase transition visulation have been established. It was found that the relation of intensity this reflections $Y(F)=1$ and $c(F)=5,460 \AA$ near $F_c$ as well as the temperature of $T_c$. These relations testify about formation at $F_c$ finishing of state like high-temperature modificatiion of quartz. Our data demonstrates that properties of $\alpha$-quartz varies nonlinearly' with the degree of structure modification, slowly at the first stage of irradiation with fast neutrons and much more rapidly at doses more what $10^{19} cm^{-2}$. Approximation of radiation-induced changes in parameters polarozabilities at the second stage yields the exponential relation. It is established that the critical exponent less what a one in our case. Proceeding from approximation products and the steam chink theory, a real mechanism of radiation-induced phase transition in the $\alpha$-quartz is proposed.
Table 1.The dose and temperature dependence of microparameters polarisabilities in the quartz.
$F, cm^{-2}$ $\alpha_1, \AA^3$$T. {}^oC$ $\alpha_2, \AA^3$
0 4.86 20 4.85
$1\times 10^{18}$ 4,86 100 4,85
$1\times 10^{19}$ 4,85 200 4,86
$3\times 10^{19}$ 4,86 300 4,89
$5\times 10^{19}$ 4,89 500 4,94
$7\times 10^{19}$ 4,99 700 4,99
On the basis of measurements of the characteristics the value of microparameters polarisability before and after irradiation a samples 1 was calculated. Using these data dose dependence of these microcharacteristics polarizability of crystals was evaluated. It was established that a noticeable the variation of atomic and electronic undersistems of crystals may take place a dose at which a rapid damage of structure occurs in corundum, however no the phase transition. Using these data of polarisability,we also evaluated from the Braun formula a value of atomic size both crystals ( $\alpha_1$ and $\alpha_2$ ). Analysis of the dose dependence $\alpha_1(F)$ and $\alpha_2(F)$ is found that reveals radiation stability atomic size of studied crystals at high irradiation doses
( $\alpha_1(F)$ ) in a channels of reactor. The experimental dependence of atomic size for irradiated dielectrics materials is of especial interest in theoretical aspects at the atomic scale materials design.