Papers by Author: Robert Freer

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Authors: Z. Ling, Colin Leach, Robert Freer
Authors: Hiroaki Takeda, Han Joong Sang, Takashi Tateishi, Spela Kunej, Colin Leach, Robert Freer, Takuya Hoshina, Takaaki Tsurumi
Abstract: BaTiO3–(Bi1/2K1/2)TiO3 (abbreviated as BT-BKT) solid solution ceramics, as a lead-free PTC (positive temperature coefficient of resistivity) thermistor material usable over 130°C, has been synthesized by sintering in N2 atmosphere and after annealing in air over 1200°C. In the BT-BKT ceramics with PTC property, the impedance/modulus spectroscopic plots have revealed that a third resistance-capacitance (RC) response besides grains and grain boundaries. Using the remote electron beam induced current (REBIC) configuration, imaging has revealed EBIC contrast consistent with the presence of negatively charged electrostatic grain boundary barriers in the BT-BKT semiconducting ceramics.
Authors: J.D. Russell, Colin Leach, Robert Freer
Authors: Ji Wei Fan, Robert Freer
Abstract: Dense tin oxide based ceramics are a new type of varistor materials. To further understand the electrical properties of SnO2 varistors doped with CoO, Nb2O5, and Cr2O3, the techniques of capacitancevoltage (C-V) measurement and deep level transient spectroscopy (DLTS) were used to investigate the electron traps in the SCN samples (doped with 1.0 mol% CoO and 0.05mol% Nb2O5) and SCNCr samples (doped with 1.0 mol% CoO, 0.05mol% Nb2O5 and 0.05mol% Cr2O3). Two electron traps were detected: trap T1 is located at Ec - 0.30 ± 0.01eV and trap T2 is located at Ec – 0.69 ± 0.03eV for both SCN and SCNCr samples. The variations in the donor density and trap density could be related to the addition of chromium oxide. The features of these traps are discussed based on the defect theory related to the SnO2 varistors.
Authors: F. Azough, Colin Leach, Robert Freer
Abstract: Ba(Me1/3Nb2/3)O3 (Me=Zn, Co, Ni and Mg) ceramics were prepared using the conventional mixed oxide route; additives included Al2O3, Ga2O3, SiO2, WO3, B2O3 and V2O5. Powders were mixed, milled for 18h, calcined at 1100°C, remilled pressed into pellets at 100 MPa, sintered in air at temperatures in the range 1350-1550°C and then cooled at 360C h–1 to 5°C h–1. Products were characterised in terms of phase analysis (X-ray diffraction), microstructure (SEM and TEM) and electrical properties (relative permittivity, εr, dielectric Q value and temperature coefficient of resonant frequency,τf). The Q values of the Ba(Me1/3Nb2/3)O3 ceramics depend on the degree of cation ordering and the additives. Slow cooling leads to 1:2 ordering of the B sites and enhanced dielectric Q values. For samples cooled at 5°C h–1 after sintering the Qxf values are in the range 28000 to 98000 GHz, and are in the sequence Ba(Ni1/3Nb2/3)O3, Ba(Co1/3Nb2/3)O3, Ba(Mg1/3Nb2/3)O3 and Ba(Zn1/3Nb2/3)O3. Additions of BaO-4WO3 or V2O5 yield higher Qxf values than Al2O3. Highly ordered Ba(Zn1/3Nb2/3)O3 has a relative permittivity of 39.4, but most other Ba(Me1/3Nb2/3)O3 ceramics exhibit εr of 31-32. The temperature coefficient of resonant frequency, τf, varies from –18 ppm/°C (Ba(Ni1/3Nb2/3)O3) to +32 ppm/°C (Ba(Mg1/3Nb2/3)O3); the sintering additives (Al2O3 and BaO-4WO3) change τf by typically 10-16 ppm/°C.
Authors: Z. Ling, Colin Leach, Robert Freer
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