The Dielectric Loss of Single Crystal and Polycrystalline TiO 2
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XIAORU WANG, ALAN TEMPLETON, STUART J. PENN AND NEIL MCN. ALFORD SEEIE, South Bank University, 103 Borough Road, London SE1 OAA ABSTRACT The dielectric loss of single crystal and polycrystalline TiO 2 has been studied. In polycrystalline TiO2 the dielectric loss is determined by both the microstructure and by the oxygen stoichiometry. Experiments have been carried out to determine the influence of both the microstructure (particularly porosity) and the oxygen stoichiometry. The TiO2 powder has been doped with partially stabilised zirconia, an oxygen ion conductor, in order to modify the oxygen stoichiometry. Sintered discs have been examined for loss as a function zirconia doping, pore volume and as a function of temperature. The behaviour of the doped and undoped titania powders is significantly different. Since many microwave dielectric materials contain Ti eg BaTi-O, Ba-Nd-Ti-O, (Ba-RE-Ti-O, RE=Rare Earth), Zr-Sn-Ti-O etc it is essential to understand the role of the titanium, particularly as it can exist in mixed valence states, and the role of oxygen and its influence on the dielectric loss. INTRODUCTION For dielectric resonator applications there are three main requirements. The first is that the dielectric loss should be very low. The second is that the dielectric constant should be high so as to aid miniaturisation and the third is that the Tcf should be controlled (usually to be as near zero as possible). The achievement of all three in one material is a formidable problem. In this paper, the base material of many dielectric resonator ceramics, titanium oxide, is chosen as a model material and studied in order to shed light on the dependence of the dielectric loss on factors such as pore volume and oxygen stoichiometry. Ti - containing ceramic materials have found uses as dielectric resonator materials'. In order to sinter the material, temperatures between 1000°C-1500'C are required. This can cause the TiO2 to lose oxygen producing a high concentration of oxygen vacancies. One of the main issues affecting the loss tangent is the non-stoichiometry believed to be the result of oxygen vacancies. TiO2 itself can have a high Q at a high dielectric constant but as a dielectric resonator material it is unsatisfactory because of the magnitude of the temperature coefficient of the resonant frequency (Tcf) which is influenced by changes in dimension and changes in the dielectric constant due to thermal effects. The pioneering work of O'Bryan 2'3 ' 4 demonstrated that Ti bearing ceramics, notably barium titanate could be prepared with compositions in which the Tcf could be controlled. O'Bryan also found that the phase composition in Ba 2Ti902o was strongly influenced by the oxygen stoichiometry. EXPERIMENT High purity TiO2 powder was used. (Pikem, UK). The size of the powder was determined by laser diffraction using a Coulter LS230 and was found to be 2-3.tm. The powder was formed into discs by uniaxial pressing at 100MPa in a 13mm diameter stainless steel die. The discs were sintered at temperatures ranging
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