Difficulties in measuring electrical conductivities in highly insulating materials: Radiation-induced electrical degrada
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asurement of the electrical conductivity in high-resistance insulators is made difficult by previously unrecognized limits in the electric guarding technique. Electron irradiation experiments on single-crystalline Al2O3, performed for studying the effects of irradiation on the electrical conductivity, revealed that radiation-induced electrical degradation effects in ceramic insulators, previously reported to occur after electron, ion, and neutron irradiation, are an artifact. Electrical conductivity is induced in insulating materials during ionizing irradiations as a result of the creation of electron-hole pairs. The effect is called radiation-induced conductivity (RIC).1 RIC requires a continuous production of electron-hole pairs. Hence RIC ceases when the radiation is terminated, and after some transient time the insulating property of the material is restored. As a result of fast particle irradiations, an additional, but permanent, rise in conductivity was reported, termed radiation-induced electrical degradation (RIED). Researchers have claimed that RIED occurs when the irradiations are performed with the specimen exposed to an electric field (>30 V/mm for single-crystal Al2O3) and at elevated temperature.2,3 In contrast to RIC, RIED is considered highly detrimental for the use of insulator materials under irradiation, since the observed conductivity increases were cumulative. In single- and polycrystalline aluminium oxide, RIED was reported after electron,4 –9 ion,10 and neutron11 irradiations. Electrical resistivity losses up to ten orders of magnitude were reported. Later, however, the existence of RIED was questioned.12 Because of the high importance of RIED for the use of insulating materials in future fusion reactors, many experiments have since been carried out resulting in arguments for and against RIED.13– 42 As a compromise between the contradictory positions, it was suggested that the exact brand of the insulating material might determine whether RIED occurs or not, since the conductivity of insulators is sensitively dependent on the type and concentration of impurity atoms.43 It was also suggested that the existence of RIED might depend on the type of irradiating particle.43 Particularly for electron irradiations, where the majority of results in favor of RIED exists,4 –9,13,14,16–18 a clarifying experiment was missing. In the present investigation, an electron irradiation experiment in search of RIED was performed on exactly the same specimen material and close to the same irradiation conditions for which RIED had been repeatedly observed and reported.5,8,9,13,14 (The experiment was 2280
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J. Mater. Res., Vol. 15, No. 11, Nov 2000 Downloaded: 17 Mar 2015
performed following the suggestions of an International Energy Agency (IEA) workshop on radiation effects in ceramic insulators.43) The specimen material was part of an Al2O3 single crystal (Union Carbide UV grade), provided by Dr. E.R. Hodgson, to either verify or disprove the RIED results that were obtained on another
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