Fabrication of (Ba,Pb)TiO 3 -based tapes with positive temperature coefficients of resistivity by the oxidation of malle
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Fabrication of (Ba,Pb)TiO3-based tapes with positive temperature coefficients of resistivity by the oxidation of malleable, metal-bearing precursors (the volume identical metal oxidation process) Seyed M. Allameh and Kenneth H. Sandhage Department of Materials Science and Engineering, Ohio State University, Columbus, Ohio 43210 (Received 23 April 1999; accepted 2 September 1999)
The feasibility of producing (Ba,Pb)TiO3-based thermistor tapes by the oxidation of malleable, metal-bearing precursors has been demonstrated. Intimate Ba–Pb–Ti– TiO2-bearing powder mixtures, produced by high-energy vibratory milling, were packed within a fugitive metal can and then compacted and formed into tapes of uniform thickness by cold drawing and rolling. The tape-shaped precursors were oxidized and converted into (Ba,Pb)TiO3-based tapes with a series of heat treatments at 艋1120 °C. With proper control of thermal treatments and chemical additions (Sb2O3 + MnO2 dopants), positive-temperature-coefficient-of-resistivity thermistors were produced that exhibited significant increases in resistivity commencing at temperatures 艌350 °C.
I. INTRODUCTION
At the Curie temperature (TC), donor-doped BaTiO3based ceramics can exhibit dramatic increases in resistivity with increasing temperature.1–5 This so-called positive temperature coefficient of resistivity (PTCR) effect can be utilized in a wide range of applications, including self-regulating heaters (e.g., for hair dryers and curlers, food warmers, heaters in trains and cars, and ovens for electronic components), sensors (e.g., of temperature, fluid level, or fluid flow), and current limiters (e.g., for fuses, motor starters for refrigeration and air conditioning compressors, and degaussers for computer displays and televisions).6–9 Effective donor dopants to BaTiO3 include Sb2O3, Y2O3, and Nb2O5, and lanthanide series oxides such as La2O3, CeO2, Sm2O3, Gd2O3, and Ho2O3.2,4,7,10,12–18 By introducing a lesser amount of certain acceptor dopants (such as MnO2) along with the donor dopant, the PTCR effect can be further enhanced.11,15,19–22 In addition, partial substitution of lead for the barium in BaTiO3 can be used to shift TC (and, hence, the PTCR effect) to higher temperatures.23–27 For example, Kuwabara and Kumamoto23 and Kuwabara et al.24 have synthesized porous Sb2O3-doped (Ba,Pb)TiO3 thermistors that possessed TC values in excess of 350 °C and that exhibited resistivity changes at TC of 3 to ≈5 orders of magnitude. A conventional method for fabricating a shaped BaTiO3-based thermistor is to blend powder of the desired composition with a malleable organic binder and then form the mixture (e.g., by tape casting, pressing, J. Mater. Res., Vol. 14, No. 11, Nov 1999
extrusion, etc.) into a green body of desired shape.28,29 After forming, the organic material is removed by pyrolysis. Incomplete or nonuniform organic pyrolysis can result in flawed components (e.g., retained carbon, shape distortion).28–31 At the very least, the porous ceramic bo
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