Measurements of dc electrical conductivity and emf of single crystal and polycrystalline α‐Al2O3 doped, respectively, with 100 and 300 ppm yttrium show that yttrium, although isoelectronic with aluminum, is a donor. The electronic energy level of the donor is attributed to O2‐ next to Y3+, its level being forced up as a result of the large size of the yttrium ion. The donor activity of Y supports the view that the favorable effect of Y on the oxidation of super alloys is due to reduced diffusion of Al in the bulk through a reduced concentration of Ali… without a marked increase in the concentration of V…Al.
Annealing of acceptor dominated α-Al2O3 at Tsat = 1100–1500 °C in atmospheres containing hydrogen leads to dissolution of hydrogen, changing the material from p- to n-type when the concentration of hydrogen donors is larger than that of the acceptor impurities present. If oxygen equilibrium is maintained, the concentration of ionized donors depends on pH2 as well as pO2 (or pH2O). In measurements of dc conductivity in atmospheres not containing hydrogen, a contribution to the conductivity by mobile protons rapidly disappears as a result of polarization. Analysis of the rate of polarization leads to values for the concentration and mobility of mobile protons. In measurements in atmospheres with well-defined hydrogen and oxygen fugacities, protons, native ions, and electrons and holes contribute continuously to the conductivity. Transference numbers of the various species and the corresponding partial conductivities were determined by emf measurements. The amount of dissolved hydrogen increases with decreasing temperature of saturation, increasing hydrogen pressure, and increasing oxygen pressure. Attempts to find detailed defect models explaining the results were only partially successful.
Compensation of single‐crystal, acceptor‐dominated samples of α‐Al2O3 by hydrogen leads to samples showing conductivity by protons, electrons, holes, Al‴i, and V‴Al Values of the partial conductivities as a function of temperature are used to determine the parameters of native ionic and electronic disorder.
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