1997 TMS Annual Meeting: Monday Session

ADVANCES IN ELECTRICALLY CONDUCTIVE MATERIALS: Session I

Session Chairperson: S. Jin, Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974

8:30 am INVITED

ELECTRODE MATERIALS FOR LITHIUM BATTERIES AND FUEL CELLS: A. Manthiram, Center for Materials Science and Engineering, ETC 9.104, The University of Texas at Austin, Austin, TX 78712

Electrically conducting metal oxides find potential applications in electrochemical systems such as batteries and fuel cells. For example, several lithium insertion compounds are used as cathodes in rechargeable lithium batteries. However, the battery performance and cyclability of the cathodes are controlled partly by the microstructure and morphology, which in turn are influenced by the methods of synthesis and processing used. A novel approach involving the reduction of aqueous metallate solutions with aqueous alkali metal borohydrides at ambient temperatures to obtain amorphous or nanocrystalline transition metal oxide electrodes will be presented. The electrode performance of, for example, amorphous or nanocrystalline VO2, CrO2, MoO2, and LiMn2O4 will be discussed. In addition, the design and synthesis of perovskite-based metal oxides that exhibit mixed electronic and ionic conductivity will be presented. These mixed conductors are attractive not only as electrode materials for solid oxide fuel cells but also for oxygen separation membranes and catalysts.

9:00 am INVITED

EPITAXIAL THIN FILMS AND HETEROSTRUCTURES OF METALLIC OXIDES FOR DEVICE APPLICATIONS: C.B. Eom, Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708

For many electronic device applications, it is necessary to have epitaxial growth of metallic oxide thin films in a single heterostructure. We have grown epitaxial thin films of Sr1-xCaxRuO3 and La6.4Sr1.6Cu8O20 in-situ by 90 degree off-axis sputtering. These metallic oxides are pseudo-cubic perovskites, which could be ideal electrodes for ferroelectric devices, and normal metal barriers for SNS junctions in integrated superconducting devices. However, the properties of epitaxial thin films of the metallic oxides may be quite different from the corresponding bulk materials because of the existence of strain and cation disorder in the films. We have observed a strain stabilized metal-insulator transition in epitaxial Sr1-xCaxRuO3 thin films deposited on (100) SrTiO3 substrates that differed in crystalline quality. The growth mechanisms of the films and their correlation with the crystalline quality of the substrates is discussed. We have also grown epitaxial planar SNS heterostructures YBCO/La6.4Sr1.6Cu8O20/YBCO. We will discuss the microstructures, junction properties and the origin of the interface resistance between YBCO and La6.4Sr1.6Cu8O20.

9:30 am INVITED

FUNDAMENTAL INVESTIGATIONS OF CADMIUM STANNATE THIN FILMS: T.J. Coutts, W.P. Mulligan, X. Wu, National Renewable Energy Laboratory, Golden, CO 80215

We are investigating a variety of novel transparent conductive oxides, including cadmium stannate (Cd2SnO4), cadmium indate (CdIn2O4), zinc stannate (Zn2SnO4 and ZnSnO3), and zinc indium oxide (Zn2In2O5). To date, the cadmium stannate films have shown the highest conductivity, with resistivities as low as 1.3X10-4 ohm-cm. Films have been fabricated with sheet resistances as low 2 ohm/sq., while maintaining approximately 80% transmittance across the visible and near infrared. The conductivity of the films is high because of both high electron concentration and relatively high electron mobility. One possible explanation for the high mobility is that the electron effective mass is unusually low. Direct measurement of the effective mass by cyclotron resonance is not feasible due to the extremely short electron scattering times characteristic of these highly defective materials. Instead, we have estimated effective mass by Drude modelling of optical reflectance and transmittance data, and by analysis of a combination of electron transport measurements, including resistivity, Hall effect, thermopower, and transverse Nernst-Ettingshausen effect. Cadmium stannate films with a range of carrier concentrations were fabricated, and their optical and transport properties were measured. We will present the results of our comparative analysis of carrier effective mass by these two independent methods. Possible conduction band curvature, and carrier scattering time and mechanisms will also be reported. The implications for design of novel transparent conductive oxides will be discussed.

10:00 am BREAK

10:20 am INVITED

ELECTRICALLY ACTIVE POLYMERS: M.E. Galvin, Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974

It has been known for two decades that polymers with extended conjugation could be rendered highly conductive by doping, that is, by oxidizing or reducing the polymer chain. The commercialization of these polymers has, however, been limited by their poor chemical stability in the doped form. More recently these polymers have shown promise when used in their undoped forms in LEDs, light emitting diodes, or in thin film transistors. This talk will cover the recent developments in these two areas and describe the scientific challenges which still remain.

10:50 am INVITED

NOVEL ANISOTROPIC CONDUCTORS THROUGH THE DESIGN OF COMPOSITE STRUCTURES: S. Jin, Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974

Anisotropic electrical conductivity with many orders of magnitude change along different directions can be obtained by the design of novel, composite structures. Materials with such characteristics are useful for a variety of electronic applications including ultra-high density circuit interconnections, 3-D packaging, and solder-free interconnection. Optically transparent but electrically conductive medium can also be made via composite route. Novel composite structures with vertically aligned metal elements in ceramic or polymer sheets will be described. Materials fabrication process, electrical properties and reliability issues will also be discussed.

11:20 am

OPTICAL STUDIES FOR THE CHARACTERIZATION OF CONDUCTING POLYMERS: L.M. Abrantes, CECUL, Dept. Qumica, FCUL, Bloco C1-5 piso, 1700 Lisboa, Portugal; J.P. Correia, INETI, Dept. de Energias Renovaveis, 1699 Lisboa Codex, Portugal

The properties of conducting polymers have stimulated much interest in many fields of research and considerable effort has been directed towards reliable characterization of the envisaged properties for commercial applications. Apart from polyacetylene (PA) the major existing conducting polymers can be electrochemically synthesized and this encouraged the use of conventional electrochemical methods coupled to other techniques to study the electropolymerization and insulator/conductor conversion of these new materials. The purpose of this paper is to give an overview of the ability of optical methods to provide useful information on the properties of electronically conducting polymers. Three techniques will be focused and their contribution illustrated considering different systems: 1. The principles of the Probe Beam Deflection (also known as Mirage Effect) and respective data for polyaniline (PANI) systems discussed. 2. Recent applications of Photocurrent Spectroscopy are reviewed and photoeffects displayed by poly-3-methylthiophene (P3MeTh) analysed. 3. The information on structural changes during electropolymerization and doping processes, which can be provided by ellipsometry evidenced by the study of PANI, P3MeTh and polypyrrole.

11:40 am

HYDROSTATICALLY EXTRUDED COPPER-NIOBIUM SPARK ERODED POWDER: M.A. Hill, J.F. Bingert, F.E. Spada, A.E. Berkowitz, S.A. Bingert, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Center for Magnetic Recording Research, University of California at San Diego, La Jolla, CA 92093

High strength, high conductivity copper-niobium wire is a promising conductor wire for use in pulsed high field magnets. Due to the high melting point of niobium, it is difficult to produce an ingot with a fine, homogeneous structure. One possibility for reducing the scale of the cast structure, improving the homogeneity of the ingot, and enhancing the strength at a given level of deformation strain is to use rapid solidification techniques, such as powder processing, to fabricate the Cu-Nb ingot. Both melt spinning and gas atomization of Cu-Nb are difficult due to problems associated with thermal shock of components, such as crucibles and stopper rods. Spark erosion provides a method for powder production without the use of components which may crack under thermal stresses. Furthermore, in the spark erosion process quench rates exceeding 106 K/s from temperatures above 104 K promote formation of fine particles with homogeneous composition. Spark eroded Cu-10 vol % Nb powder has been consolidated by cold pressing and subsequently hydrostatically extruded to form 2.5 cm diameter rod with a yield strength of 500 MPa and a conductivity of 85% IACS at a reduction of =2.6.