Corresponding author: Vladimir A. Stepanov ( stepanov@iate.obninsk.ru ) Academic editor: Yury Kazansky
© 2021 Vladimir A. Stepanov, Pavel V. Demenkov, Olga V. Nikulina.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Stepanov VA, Demenkov PV, Nikulina OV (2021) Radiation hardening and optical properties of materials based on SiO2. Nuclear Energy and Technology 7(2): 145-150. https://doi.org/10.3897/nucet.7.69926
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Preliminary studies have shown that the optical absorption spectra of radiation-colored glasses correspond to the spectral behavior of the scattering losses of an optically inhomogeneous medium. The reasons for the same optical changes in glasses of different compositions are the radiation-induced electric charge separation in the structurally nano-inhomogeneous glass volume, polarization and formation of nanometer optical inhomogeneities.
The authors of this work prove that the radiation changes in the mechanical and optical properties of silicate glasses are of the same nature. The performed estimates indicate that the electric charge separation in the glasses occurs up to absorbed doses of about 1 MGy. The local electric charge separation due to the appearance of Coulomb forces leads to radiation hardening of the glasses. The estimated Coulomb hardening of the quartz glasses was ~ 107 Pa.
The theoretical results were experimentally confirmed by measuring the mechanical properties of the glasses under high intensity proton irradiation as well as by testing the mechanical strength of a composite material based on quartz glass. Under proton irradiation with a dose rate of 5×103 Gy/s (energy of 8 MeV) up to threshold doses of ~ (1 – 5) ×106 Gy in the KU-1 quartz glasses, the decrement of acoustic vibrations decreased due to Coulomb hardening. After gamma irradiation with 1.34×105 Gy, the tensile strength of the composite material based on quartz glass increased by up to 20 MPa. This value is in the range of estimates of Coulomb hardening of quartz glasses. It is also shown that ionizing radiation does not affect the elastic modulus of materials based on SiO2.
Silica glass, gamma irradiation, optical properties, acoustic measurement, radiation hardening
The use of glasses as components in the optical systems of radiation devices has a number of significant limitations associated with the decreasing spectral transparency range under irradiation. For example, the spectral transparency range of quartz glasses as a result of irradiation can narrow from 0.2–4.0 µm to 0.3–2 µm (
Figures
52%ZrF420%BaF24%LaF34%AlF320%NaF;
36%AlF312.3%BaF212.3%SrF212.3%CaF212.3%MgF212.3%YF32.5%LaF3.
After irradiation, the fundamental absorption edge is blurred and shifted toward longer wavelengths. Such changes in the optical absorption spectra of the glasses cannot be explained by the appearance, upon irradiation, of structural defects, i.e., centers of optical transitions. In this case, we have to assume that irradiation, regardless of the type of glass, forms a continuous series of color centers with a continuous spectrum of energy levels, at which optical transitions become possible in the entire visible and ultraviolet ranges. The identical nature of the radiation changes in the optical properties of glasses of various compositions is due to the common nature of their nanostructure inherent for all the glasses (
As early as in 1921, A.A. Lebedev suggested that silica glass has a cryptocrystalline structure, i.e., it consists of a large number of tiny crystallites. According to this hypothesis, the structure of the glasses can be considered as an inhomogeneous medium of microstructural formations, each of which is characterized by an ordered arrangement of atoms. Microinhomogeneity of the glasses was confirmed by various research methods (
Ionizing radiation leads to the electric charge separation due to the recharging of electron-hole traps and the appearance of optical inhomogeneities in the glass volume due to the polarization of the volume near the charged centers (
, (1)
where re is the classical radius of an electron (2.8·10–15 m); λ0 is the wavelength corresponding to the forbidden gap; ne is the concentration of valence electrons, in order of magnitude atomic concentration; nd is the concentration of domains active in the scattering. If we assume that all the absorbed energy of gamma radiation Eg is spent on the creation of charge carriers, then with an increase in the absorbed dose D in the absence of electron-hole annihilation, when all the free electrons are captured by the traps, the concentration of the domains active in scattering increases as
, (2)
where ρ is the density of the glass. Substituting (2) into (1), we obtain the dose dependence:
. (3)
The size of the light-scattering domains in accordance with this mechanism of radiation-induced optical losses in the glasses is determined by the short-range order correlation radius, which also corresponds to the size of the smallest structural elements, and the size of the regions of coherent scattering of X-ray radiation. Calculated from the optical spectra of irradiated glasses (Fig.
It is possible to estimate the saturation dose, at which the entire glass volume is divided into optical scattering regions:
. (4)
This expression for silica glasses gives an estimate of Dd ~ (0.7–2)×106 Gy, which is in full agreement with the experimental values of the saturation doses of the radiation color (
The studies (
In the quartz glasses, an increase in the radioluminescence intensity with the radiation dose was also associated with radiation-induced charge separation (
, (5)
where C = εε0S/d; S is the area of the plates; d is the distance between them. Pressure arises between the plates due to Coulomb attraction
,
and, if Q = ρsS, then
. (6)
The minus sign means compression.
In the Al2O3:Cr3+ ceramics, under proton ionizing irradiation, up to 1×10–1 electron-hole traps associated with Cr3+ are charged. If we assume that, in the quartz glasses under irradiation at the domain boundaries, the maximum percentage of charged traps is the same, and their number is on the order of the concentration of Si-O bonds, then we obtain an upper estimate of 1×108 Pa for the Coulomb hardening. This value, however, is an order of magnitude higher than the yield strength and even the tensile strength (for some glasses). At the domain size d = 1 nm (e.g., in the KU-1 glasses), up to 30 SiO2 formula units are located in the surface layer of the same thickness. This gives a lower estimate for the number of charged traps of about 3% and a lower estimate for hardening up to 1×107 Pa, which is apparently close to real values.
The electric charge in the glasses is separated, in accordance with (4), up to doses of the order of 1 MGy. The dynamics of radiation hardening during irradiation should be the same as the dynamics of optical degradation. To experimentally verify this statement, the mechanical properties of the glasses were measured directly in the process of powerful ionizing proton irradiation using the method of a composite piezoelectric vibrator. The resonance method described in (
The resonance methods are based on the analysis of steady-state forced vibrations of samples in the form of rods or plates. Figure
To study changes in the acoustomechanical properties of the samples under powerful irradiation, the composite piezoelectric vibrator was located at the outlet of the ion wire of the EGP-10M light ion accelerator (SSC RF-IPPE) so that a proton beam 1 cm in diameter fell into the center of the sample (see Fig.
The decrement of acoustic vibrations and Young’s modulus of the KU-1 glasses were measured under proton irradiation with energy of 8 MeV and a dose rate of 5×103 Gy/s. The KU-1 glass samples were plates 28.2×3×0.4 mm. The measurements were carried out at a relative deformation of the sample ~ 1×10–6. As Figure
The decrement behavior under proton irradiation is associated with the changes in microplasticity and is determined by the formation of stress-strain regions in the glass. A drop in the decrement at the initial stage indicates a decrease in microplasticity due to hardening. The regions with Coulomb hardening arising in the process of ionizing radiation block the motion of defects-carriers of plasticity. A decrease in the decrement of acoustic vibrations is associated with an increase in the number of such regions in the glass. At an irradiation dose of up to ~ 1×106 Gy, in accordance with (4), the hardened micro-regions fill the entire volume, which leads to the completion of the decrement reduction process.
An estimate of Coulomb hardening in irradiated silicate glasses gives values of the order of tens of megapascals at absorbed doses close to the saturation dose (4) up to 1 MGy. This hardening leads to an increase in the strength of materials based on SiO2 glasses. OPPE Tekhnologiya, using vacuum and contact molding methods, produces such an inorganic composite material (CAPB brand) for aerospace purposes (
The strength of the composite is determined by the strength of the quartz filler. An increase in strength from 10 MPa can be detected in mechanical tests of materials, even taking into account the inevitable scatter of the measurement results. To carry out the tests, 30 CAPB samples (20×10×2) mm, were made, half of which were irradiated with a 60Co gamma source. The mechanical tests of the material were carried out in accordance with GOST 4651-82 on an LFM-50 installation; the elastic modulus was calculated in accordance with GOST 9550-81.
The results of mechanical tests are shown in Figs
It has been shown that the radiation-induced changes in the mechanical and optical properties of silica glasses are of the same nature and are associated with radiation-induced separation of the electric charge on structural nanoinhomogeneities. The performed estimates indicate that the electric charge is separated in the glasses up to absorbed doses of about 1 MGy. The local electric charge separation leads to radiation hardening of glasses due to the appearance of Coulomb forces. The estimated Coulomb hardening of the quartz glasses was ~ 1×107 Pa.
The estimates were experimentally confirmed by measuring the mechanical properties of glasses under high intensity proton irradiation as well as by testing the mechanical strength of a composite material based on quartz glass. During proton irradiation with an energy of 8 MeV and a dose rate of 5×103 Gy/s, in the KU-1 quartz glasses, the decrement of acoustic vibrations (hardening) decreased up to doses of ~ (1–5)×106 Gy. After gamma irradiation of 1.34∙105 Gy, the tensile strength of the CAPB composite based on a fabric of silica fibers increased by up to 20 MPa, and this is in the range of calculations of Coulomb hardening of the quartz glass. It is also shown that ionizing radiation does not affect the elastic modulus of materials based on SiO2.