J. S. Thorp

Optical absorption in current-blackened yttria-stabilized zirconia

The nature of the blackening caused by high-temperature electrolysis of 8 and 12 mol % yttria-stabilized zirconia single crystals has been investigated by studying the optical absorption between 260 and 700 nm. In the early stages the band edge remained at 271 nm, the crystal became red and the transmission showed a broad minimum centred at 480 nm; in the later stages the crystal became black and the transmission was reduced drastically over the whole visible range. The low transmission was shown, by integrating sphere measurements, to be due to absorbing centres. These were attributed to the formation of vacancy aggregates which ultimately produced colloidal metallic particles.

Electron spin resonance in single crystal yttria stabilized zirconia

Electron spin resonance has been observed at 35 GHz in 8 and 12 mol % yttria stabilized zirconia single crystals before and after blackening by current passage. Measurements were made between 293 and 77K. In both as-grown and blackened crystals the spectra showed anisotropic lines (type A) characterized by g‖=2.003±0.002 and g⊥=1.880 ±0.002 with respect to a [1 1 1] symmetry axis and a broader, slightly anisotropic line (type B) centred near g=1.993. In current blackened crystals a weak isotropic line (type C) was also found near g=1.986. The type A lines are attributed to a charged complex formed by an electron trapped at an oxygen vacancy and associated with an yttrium ion. Tentative models are suggested to explain the type-B and type-C lines.

The elastic constants and interatomic binding in yttria-stabilised zirconia

Zirconia (ZrO2) can be forced into a cubic fluorite structure by the addition of more than 7 mol % yttria (Y2O3). The elastic stiffness constants of cubic zirconia single crystals containing 8 and 12 mol % yttria have been determined between 77 and 300° K by an ultrasonic pulse echo technique. Elastic constants are almost temperature-independent and at 77° K are for the 8 and 12 mol % respectively: C11=2.04, C12=0.87, C44=1.58; C11=2.23, C12=0.973, C44=1.54 (units: 1012 dynes cm−2). Compressibilities and elastic compliances are also presented. The data provide a basis for discussion of crystalline stability and the nature of the interatomic forces. The decreasing cubic lattice stability as the yttria content is reduced is demonstrated quantitatively. An ionic model closely characterises the binding forces. The repulsive energy is about 12% of the Madelung attractive energy. Debye temperatures calculated from the elastic constant data extrapolated to 0° K are 595 and 604° K for the 8 and 12 mol % respectively.

The dielectric constants of current-blackened single crystral yttria-stabilized zirconia

The dielectric constants of 8 mol % single crystal yttria-stabilized zirconia have been measured, using both radio-frequency and microwave methods, before and after blackening by high temperature electrolysis. For as-grown material ε′ = 38±4 and tan δ = 0.004. After electrolysis both the dielectric constant and loss tangent of the current-blackened material increased; this experimental data has been compared with theoretical predictions for composite dielectrics. The dielectric behaviour in the current-blackened material was consistent with the formation of metallic particles; these initially appeared to be spherical but elongated as electrolysis proceeded.

The temperature dependence of permittivity in MgO and Fe-MgO single crystals

The permittivities (ε′) of undoped MgO and iron-doped Fe-MgO single crystals have been measured over the temperature range 20 to 650‡ C for frequencies between 500 Hz and 50 kHz. From 25 to around 200‡ C the temperature dependence of ε′ fits well with Havingas formula and the value of [(ε′ − 1) (ε′ + 2)]−1 (∂ε′/∂T) = 1.02 × 10−2K−1 found for undoped MgO agrees closely with data published for lower temperature ranges; this increases considerably with the addition of iron, rising to 2.85×10−5K−1 for MgO single crystals doped with 12 900 p.p.m. iron. Above 200‡ C the permittivity changes much more rapidly than the Havinga formula predicts, the variation being greater in iron-doped specimens. The frequency dependence ofε′ is also temperature-dependent; below 200‡ Cε′ follows [ε′(Ω) − ε′∞] ∫Ω(n−1) withn=0.98±0.02 for all samples, but above 200‡ C the value ofε′ falls more rapidly with frequency than would be expected from this law. The effect is more pronounced for MgO with 12 900 p.p.m. iron. The results are discussed in terms of a contribution to the measured permittivity arising from temperature-enhanced conductivity.

Field-dependent spin-lattice relaxation of Cr3+ in Al2O3

The variation of spin-lattice relaxation time (T1) with magnetic field (B) for the Cr3+ ion in ruby has been measured under non-Kramers conditions for fields between 0.3 and 2.6 tesla, corresponding to microwave frequencies in the range 9 to 71 GHz. Below about 0.8 tesla there is a slow variation, approximately asT1, ∝B−0.4; at higher fields the variation becomes more rapid, approachingT1, ∝B−2.0 at 2.6 tesla. This behaviour is explained in terms of the non-linear divergence of the energy levels with field.

Size dependent magnetometric demagnetisation tensors for single domain particles

Abstract This paper examines the conventional assumption that the demagnetising energy of a ferromagnetic single domain particle can be calculated by using the magnetometric demagnetisation tensor of a similarly shaped macroscopic sample ignoring lattice structure. It is shown that, in the case of ellipsoids and cuboids with elongations between 0.1 and 10, the errors introduced by the assumption are less than about 2% provided that the particles contain at least 1000 dipoles, but a small size dependence on the number of dipoles exists. A novel method has been used to calculate the magnetometric demagnetisation tensors.

Light-induced ESR centres in single crystal rutile

Abstract Electron spin resonance studies have been made on Verneuil-grown rutile single crystals, which were doped with a variety of transition gorup ions. Measurements were made at 9 GHz, both before and after UV irradiation, at temperatures in the range from 4.2 to 300 K. UV irradiation had two effects: (a) to affect the relative intensities of esr lines due to species already present, (b) to generate new esr spectra. Both effects are interpreted as representing a redistribution of charge amongst trapping centres. Seven UV generated centres have been identified and characterised in terms of their spin Hamiltonian parameters. Isochronal annealing techniques have been used to determine the ionisation temperatures of the traps. Observation of interactions and charge transfers between centres during isochronal annealing was used to determine the polarity and type of each centre.

ESR linewidth mechanisms in Ni2+/MgO

Abstract The electron spin resonance linewidth of Ni2+ in single crystal MgO was examined theoretically and experimentally at 9 GHz for a Ni2+ concentration of 1400 ppm. The experimental peak-to-peak linewidth for the δMs = ±1 transition had a value of about 6 mT over the temperature range 180 to 20 K and was polar angle dependent. The calculated dipolar linewidth exceeded that observed by a factor of about five; the ratio of moments M 1 4 4 /M 1 2 2 derived from experimental data was 1.34; the lineshape was Lorentzian and independent of polar angle. The data suggested that Ni2+ entered the lattice substitutionally, occupying magnesium sites, and that the linewidth for the δMs = ±1 transition was determined by exchange narrowing together with inhomogeneous broadening due to internal strain.

Magnesio-ferrite formation in heat-treated Fe/MgO powders

Preuve directe de cette formation par mesure de susceptibilite magnetique en fonction de la duree du traitement thermique et de la concentration initiale de Fe 3+ dans MgO