Emergence of limiting electric fields and mechanical stresses in glasses under irradiation with an electron flux
Abstract Generated abstract
Electron irradiation of aluminophosphate glasses was studied to examine coloration, conductivity changes, electrical breakdown, and mechanical damage induced by accumulated charge. Specimens of two composition groups were irradiated with 1.25 MeV electrons, and the resulting optical absorption, discharge behavior, and temperature-dependent electrical conductivity were compared with unirradiated samples. Irradiation produced nonuniform coloration with visible absorption bands near 410 and 520 nm, increased conductivity that annealed out near 300°C, and, in high-resistivity glasses, discharges beginning at doses above about 40 Mrad with associated cracking or delamination. The results suggest that higher electrical resistance favors charge accumulation and limiting internal fields sufficient for breakdown, while radiation-induced colored and uncolored regions may also generate mechanical stresses capable of separating glass layers.
Full Text
Emergence of Limiting Electric Fields and Mechanical Stresses in Glasses under Irradiation by an Electron Flux
E. K. Zavadovskaya, V. A. Starodubtsev
We subjected two groups of aluminophosphate glasses of different composition to irradiation by an electron beam with a flux density of \(3.1 \times 10^{12}\) electrons/\(\text{cm}^2\) sec and an electron energy of \(1.25\) MeV:
1) \(10\%\,RO + 20\%\,Al_2O_3 + 70\%\,P_2O_5\) and \(25\%\,Al_2O_3 + 75\%\,P_2O_5\),
2) \(40\%\,RO + 5\%\,Al_2O_3 + 55\%\,P_2O_5\).
The molar percentage of the glass composition was determined from the synthesis. The glasses were melted from ChDA-grade reagents in quartz crucibles. Glass specimens measuring \(15 \times 15 \times 2\) mm had polished surfaces. During irradiation the specimens were placed on a grounded aluminum substrate and cooled with an air jet.
Upon irradiation all the glasses became colored. The intensity of coloration over the depth of the specimens is nonuniform. The layer of the most intense coloration has a thickness of approximately \(1.5\) mm. At comparable irradiation doses, the glasses of the second group of compositions acquire a stronger coloration. The optical absorption spectra of all irradiated glasses have two absorption bands in the visible region, with maxima at 410 nm and 520 nm. Similar absorption spectra are observed in phosphate glasses irradiated by \(\gamma\)-rays from \(Co^{60}\) [1].
In glasses of the first group of compositions, when irradiated with electrons at a dose of \(4 \cdot 10^7\) rad and higher, an electric discharge is observed. In glasses of the second group of compositions, no discharge was observed when the indicated irradiation dose was increased by more than a factor of six.
The discharge develops in the middle of the specimen, probably at the boundary between the intensely colored and uncolored parts of the volume, i.e., in the region of the greatest gradient.
The discharge observed in the glasses is shown in Figs. 1 and 2. The discharge has a branched form; melting of the glass at the breakdown site is not observed visually. In appearance, the discharge shown in Fig. 1 resembles a klydonogram for a positive electrode—a point; and in Fig. 2, for a negatively charged point. The discharge in the glasses is also observed after their prolonged storage (up to 2 weeks or more) upon touching the glass with a grounded metallic electrode.
The discharge that arises at irradiation doses higher than \(4 \times 10^7\) rad is accompanied by destruction of the specimens along the discharge tracks into separate pieces. Judging from the character of the discharge developing in the glasses, it apparently may be attributed to the electrical form of breakdown.
In some glass specimens, partial destruction occurred only in the weakly colored layer through which the discharge channel passed.
Fig. 1. Discharge traces in glass specimens of composition \(Al_2O_3 \cdot 3P_2O_5\), irradiated with a dose of \(74\) Mrad
Fig. 2. Discharge traces in glass specimens of composition \(10\%\ MgO + 20\%\ Al_2O_3 + 70\%\ P_2O_5\), irradiated with a dose of \(84\) Mrad
This layer separated by itself from the colored layer not affected by the discharge, without any external action. It is known that internal mechanical stresses arise in crystals colored by ionizing radiation [2]. It is probable that mechanical stresses also arise in irradiated glasses. Mechanical forces may exist between the colored regions of the specimens and the uncolored regions. When the magnitude of these forces reaches a value equal to the mechanical strength, delamination of the uncolored glass layer is observed.
In the glasses we studied, electrical conductivity was measured before and after electron irradiation. The measurements were carried out in vacuum, in the temperature range from \(50^\circ C\) to \(350^\circ C\), at a heating rate of \(3^\circ C\) per minute and an electric-field strength of \(E = 200\ \text{V/cm}\). Measuring and guard platinum electrodes were deposited on the specimens by cathodic sputtering.
Properties of the investigated glasses
Table 1
| No. | I group: RO | I group: Dose, Mrad | I group: \(\rho,\ \Omega\cdot\text{cm}\) | I group: \(\mu,\ \text{cm}^{-1}\) | I group: \(\dfrac{\sigma_{\text{irr}}}{\sigma_{\text{unirr}}}\) max | II group: RO | II group: Dose, Mrad | II group: \(\rho,\ \Omega\cdot\text{cm}\) | II group: \(\mu,\ \text{cm}^{-1}\) | II group: \(\dfrac{\sigma_{\text{irr}}}{\sigma_{\text{unirr}}}\) max |
|---|---|---|---|---|---|---|---|---|---|---|
| 3 | MgO | 48 | \(4\cdot10^{15}\) | 0.97 | 700 | MgO | 190 | \(1.8\cdot10^{15}\) | 1.21 | 73 |
| 4 | CaO | 42 | \(1.7\cdot10^{15}\) | 2.20 | 138 | CaO | 170 | \(6.1\cdot10^{13}\) | 8.10 | 3.0 |
| 5 | SrO | 41 | \(2\cdot10^{14}\) | 3.68 | 50 | SrO | 220 | \(5.6\cdot10^{13}\) | 9.15 | 2.3 |
| 6 | BaO | 44 | \(1.4\cdot10^{15}\) | 2.84 | 250 | — | — | — | — | — |
| 7 | — | 37 | \(4\cdot10^{15}\) | 0.22 | 12,000 | — | — | — | — | — |
Notes.
- \(\mu\) is the absorption coefficient in the maximum band with \(\lambda = 520\ \text{nm}\).
- The value \(\rho\) was calculated for the temperature at which
\((\sigma_{\text{irr}}-\sigma_{\text{unirr}})/\sigma_{\text{unirr}}=\max\). - In row 7 the data are given for glass of composition \(Al_2O_3\cdot 3P_2O_5\).
Figure 3 shows the dependence of conductivity on temperature for strontium aluminophosphate glasses of the first group of compositions (curve 1—for irradiated glasses, curve 2—for unirradiated glasses). A similar character of the temperature dependence of electrical conductivity is observed for all the glasses studied.
Irradiation of glasses with an electron beam causes an increase in electrical conductivity and a deviation from the linear character of the dependence of the logarithm of electrical conductivity on temperature.
Upon heating the specimens during measurement, an increase is observed in the difference between the conductivity values of the glasses before irradiation and after irradiation; this difference reaches its greatest value at temperatures of 160°C–200°C. With further heating of the irradiated specimens, their electrical conductivity decreases and, at a temperature of 300°C, assumes a value equal to the electrical conductivity of unirradiated glasses of the same compositions. The effects induced by irradiation in the glasses are annealed out during the measurement: the glasses become completely decolorized, and the course of the conductivity upon repeated measurement coincides with the course obtained for the same specimens before irradiation.
Fig. 3. Dependence of the conductivity of glasses of composition 10% SrO + 20% Al₂O₃ + 70% P₂O₅ on temperature. Curve 1—for specimens irradiated with a dose of 41 Mrad; curve 2—for unirradiated specimens.
Table 1 gives, for glasses of the two groups studied, the values of the specific electrical resistivity of unirradiated specimens, the values of the absorption coefficient at $\lambda = 520$ nm for irradiated specimens, and the magnitudes of the greatest deviation of the conductivity of irradiated glasses from the conductivity of the same specimens at the same temperature before irradiation.
Comparison of the data shows that glasses of the first group of compositions are characterized by a larger value of specific electrical resistivity, a greater change in conductivity after irradiation, and lower colorability under irradiation in comparison with glasses of the second group.
The high resistance of glasses of the first group of compositions causes the accumulation of a larger charge, producing electric fields sufficient for breakdown and contributing to the change in the conductivity of irradiated glasses.
LITERATURE
- N. V. Berbat, V. P. Danilov. Optiko-mekhanicheskaya promyshlennost’, 5, 43 (1959).
- Primak W. et al., Phys. Rev., 98, 1708 (1955).