Studies on dielectric properties of Al–Zr oxide composites sintered by thermal plasma

Abstract

High-density sintered product of Al_x–Zr_100−x (x=0, 10, 20,…100) composites were produced using low-cost extended arc thermal Plasma heating (EATPH) source. Dielectric properties of these samples were studied as a function of different frequencies and at different temperatures. Particle sizes and density of the plasma-sintered products were controlled by controlling the plasma power, sintering time and plasmagen gas flow rate during sintering. Morphology and structure of sintered Al–Zr samples were examined by SEM and X-ray diffraction (XRD) to understand the plasma-sintered induced dielectric properties. It is noted that frequency- and temperature-dependent dielectric constant and loss factors are the consequence of polarization of dipole moment, which varied with the variation of plasma-sintered particle size. It is also observed that surface charge polarization along with dipole screening play crucial role at higher frequencies and at higher temperature. Dipole pinning and depinning mechanism are proposed to explain this complicated dielectric behavior.

Introduction

Al–Zr composites of fine monoclinic/tetragonal zirconia particles dispersed in an alumina matrix have long been recognized to yield toughened Ceramics [1]. Al–Zr composites are of interest for various structural applications [2], [3], [4]. Finer grain size in this system result in higher fracture toughness [5], [6] and strength [7], [8], [9]. Composites of Al–Zr system required high-temperature sintering to densify and are ideal materials to examine as possible candidate for low-temperature compaction. The metastable structure of the rapid solidification processed (RSP) materials may not necessarily be retained after conventional densification, which typically involve sintering at high temperature. Many advanced sintering techniques are being used for rapid sintering of high temperature ceramic materials including the use of laser, microwave and plasma source. However, using plasmas heating source, samples are heated to high temperature within very short period to promote rapid densification with restrained grain growth [10], [11]. Both DC and RF plasma system are used for sintering in a thermally equilibrium state where electron and ion temperature are identical. Condition satisfying this is termed as thermal plasma, which has a number of advantages as compared to conventional heating technique. This includes the potential for rapid heating, lower energy requirement, uniform grain size with no runway grain growth, etc. Most of the work on thermal plasma reported to date has focus on the relationship between density and temperature. It is desirable to achieve maximum density with minimum grain growth within minimum sintering time and power consumption. We have prepared a series of oxide materials with Al_x–Zr_100−x (x=0, 10, 20,…100) and attempts were taken to sintered using extended arc thermal plasma heating (EATPH) source. However, to our knowledge, no work in the area of sintering using EATPH has been reported except our group [12], [13]. Further to it, authors are not aware of any report regarding the dielectric properties of these plasma-sintered materials using EATPH. The aim of our paper is to study the dielectric property of highly dense sintered materials prepared by thermal plasma heating and compared with conventional sintered sample.