Neil McN. Alford

Sintered alumina with low dielectric loss

Low dielectric loss materials are required for applications in radio‐frequency and microwave communications. Aluminium is the second most abundant element in the Earth’s crust and aluminium oxide (alumina) is one of the commonest ceramics. Single crystals of aluminium oxide, i.e., sapphire, possess one of the lowest dielectric losses of any material. Polycrystalline alumina has a higher loss due to extrinsic factors. The dielectric loss of sintered alumina is studied in an attempt to determine the causes of extrinsic loss. Impurities are shown to play an important role, but the microstructure also is a key factor. High‐purity aluminas, sintered to near theoretical density, are found to display very low loss, tan δ=2.7×10−5 at 10 GHz. Doping alumina with titanium dioxide was found to reduce the tan δ=2×10−5.

Microwave dielectric loss in oxides: Theory and experiment

We present a model that provides a description of the microwave dielectric loss in oxides. The dielectric loss (tan δ) in single crystal and polycrystalline MgO and Al2O3 is measured over the temperature range 70–300 K. We are able to model the dielectric loss in terms of a two-phonon difference model. There are two key parameters in this model: The third derivative, φ3, of the lattice potential and the linewidth, γ, of the thermal phonons. In polycrystalline samples, rather than considering the different mechanisms of extrinsic loss, it is assumed that the main effect of extrinsic factors is a modification of the linewidth of the thermal phonons. By varying γ(T), it is shown that the model can describe the loss in both single crystals and polycrystallines materials. In single crystal and polycrystalline MgO, we use γ as a fitting parameter. In single crystal and polycrystalline Al2O3, we obtain γ(T) by Raman spectroscopy. The theory gives the right order of magnitude of the measured loss.

Ultralow loss polycrystalline alumina

Polycrystalline alumina with extremely low microwave dielectric loss is reported with properties analogous to a theoretical ensemble of randomly oriented, single crystal sapphire grains. By avoiding deleterious impurities and by careful control of microstructure, we show that grain boundaries in aluminum oxide have only a limited influence on the dielectric loss. A method of measuring the electric permittivity and loss tangent of low-loss microwave ceramic dielectrics is reported. The electrical parameters such as relative permittivity and loss tangent are extracted using the radial mode matching technique. The measured values for ultralow loss polycrystalline aluminum oxide agree well with theoretical values modelled on an ensemble of randomly oriented anisotropic single crystal sapphire grains.

Addendum: ‘‘Low noise YBa2Cu3O7−x‐SrTiO3‐YBa2Cu3O7−x multilayers for improved superconducting magnetometers’’ [Appl. Phys. Lett. 66, 373 (1995)]

The improvement in performance of superconducting magnetometers is discussed. It is demonstrated that the use of multilayer devices in a flip‐chip arrangement, enables one to select the lowest noise SQUID from several made on a single substrate. (AIP)

Improved SrTiO3 thin films using oxygen relaxation technique

The oxygen relaxation technique for thin-film growth is explored. In this technique, films are deposited as nanoscale multilayered structures. After the deposition of each layer, the deposition process is stopped, the sample is cooled down slowly in situ in an oxygen-rich atmosphere to a temperature of 250°C, then it is heated up to the set deposition temperature for growth of the next layer. The results show that by using the oxygen relaxation technique, it is possible to improve the electrical properties and the microstructure of SrTiO3 films keeping the surface smooth and free from pinholes and pores. The oxygen relaxation technique is more efficient for smaller film thickness repetition.

High-capacity superconducting current leads of Y1Ba2Cu3O7−x

We have fabricated and measured a high-capacity superconducting current lead composed of a Y1Ba2Cu3O7−x cylinder, 20 cm long and 0.9 cm2 cross section. A steady-state, d.c., critical current of 225 A at a temperature of 77 K was measured in this sample, using a voltage criterion of 2×10−7 V/cm (p = 8×10−10 ohm-cm). This current was limited by the currentinduced, self magnetic field. To our knowledge this is the largest d.c. critical current so far reported in a Y1Ba2Cu3O7−x sample and demonstrates the possibility of using hightemperature superconducting HTS materials for current leads to low-temperature superconducting LTS magnets or in power distribution systems.

Enhanced electrical properties of ferroelectric thin films by ultraviolet radiation

Ferroelectric films in the paraelectric phase exhibit two undesirable properties: hysteresis in the voltage-capacitance characteristics and a significant relaxation time of the capacitance. Our experiments show that suppression of both of these is achieved by using UV radiation with wavelengths corresponding to the material forbidden gap. Experimentally we also observed UV radiation induced modulation of thin film permittivity without an applied electric field. The observed phenomena are believed to have the same origin: UV light generates nonequilibrium charge carriers that screen out local electric field induced by defects and interfaces inside ferroelectric thin films and change films effective dielectric properties.

Evaluation of Residual Stress in Thin Ferroelectric Films Using Grazing Incident X-Ray Diffraction

A comparative study of three methods for thin film, in-plane residual strain evaluation is presented. Thin films of SrTiO3 and BaxSr(1 - x)TiO3 deposited using pulsed laser deposition on LaAlO3 and MgO substrates were investigated. It is shown that the most precise way to evaluate thin film, in-plane residual strain is by using Grazing Incidence X-ray diffraction.

Sintering Behaviour of BaxSr1-xTiO3

This paper deals with the sintering behaviour of Barium Strontium Titanate (BST). Two methods were examined; method 1: BaTiO3 + SrTiO3 source powders; method 2: BaCO3 + SrCO3 + TiO2 source powders. Commercially available 50 nm BST (x = 0.60) powder was used to investigate the effect of particle size on activation energy (Ea). Discs were sintered in a Netzsch 402C Dilatometer and Ea for the powders were obtained by applying the Arrhenius expression. The results indicate powders made from both methods display a varying trend in Ea over a range of x. The commercial powder yielded a higher Ea compared with powder made from method 1.

Single- and Multi-target Pulsed Laser Deposition of Thin BSTO films: Preparation, Microstructure and Electrical Properties

A comparative study of microstructure and electrical properties of Ba x Sr 1-x TiO 3 films made by single- and multi-target pulsed laser deposition was carried out. The films were epitaxially grown on both LaAlO 3 and MgO substrates. The structural properties of all samples were investigated using X-ray diffraction and Raman spectroscopy. The elemental composition of the samples was investigated using energy dispersive X-ray analysis. For electrical properties examination, a simple capacitor structure was patterned on the film surface. Thin films made using both methods exhibit similar structural and electrical properties; however the samples made by a multi-target method underwent phase transition in a broader temperature region. The results prove the possibility of using the multi-target pulse laser deposition as a more flexible method for engineering thin film stoichometry.