High-Temperature Electrical Insulation Behavior of Alumina Films Prepared at Room Temperature by Aerosol Deposition and
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High-Temperature Electrical Insulation Behavior of Alumina Films Prepared at Room Temperature by Aerosol Deposition and Influence of Annealing Process and Powder Impurities Michael Schubert1 Ralf Moos1
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Nico Leupold1 • Jo¨rg Exner1 • Jaroslaw Kita1
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Submitted: 18 December 2017 / in revised form: 27 March 2018 Ó ASM International 2018
Abstract Alumina (Al2O3) is a widely used material for highly insulating films due to its very low electrical conductivity, even at high temperatures. Typically, alumina films have to be sintered far above 1200 °C, which precludes the coating of lower melting substrates. The aerosol deposition method (ADM), however, is a promising method to manufacture ceramic films at room temperature directly from the ceramic raw powder. In this work, alumina films were deposited by ADM on a three-electrode setup with guard ring and the electrical conductivity was measured between 400 and 900 °C by direct current measurements according to ASTM D257 or IEC 60093. The effects of film annealing and of zirconia impurities in the powder on the electrical conductivity were investigated. The conductivity values of the ADM films correlate well with literature data and can even be improved by annealing at 900 °C from 4.5 9 10-12 S/cm before annealing up to 5.6 9 10-13 S/cm after annealing (measured at 400 °C). The influence of zirconia impurities is very low as the conductivity is only slightly elevated. The ADM-processed films show a very good insulation behavior represented by an even lower electrical conductivity than conventional alumina substrates as they are commercially available for thick-film technology. Keywords aerosol deposition method (ADM) annealing guard ring impurities insulation behavior roomtemperature impact consolidation (RTIC) vacuum kinetic spraying & Ralf Moos [email protected] 1
Department of Functional Materials, University of Bayreuth, Universita¨tsstraße 30, 95440 Bayreuth, Germany
Introduction Polycrystalline alumina (Al2O3) is the most frequently used ceramic material considering the volume of production. Many alumina phases can be synthesized depending on temperature. The most common one that is used for various applications is the thermodynamically stable a-phase (corundum) with a hexagonal crystal lattice. This makes it a material with excellent mechanical properties, e.g., high hardness, high Young’s Modulus and bending strength. It is also often used for its high melting point of 2053 °C, its chemical inertness and excellent electrical insulation properties. It has a low relative permittivity of 8.8 at 1 MHz and a high electrical resistivity above 1015 Xm (or a conductivity below 10-15 1/Xm) (Ref 1, 2). Typical applications are crucibles, insulators, or bearings. Owing to their excellent electrical insulation behavior, especially at high temperatures, Al2O3 films that are processed in ceramic HTCC technology (high-temperature co-fired ceramics) are used in planar lambda probes to form an insulating layer between the zirco
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