Barium titanate resistivity

R. D. Roseman, Influence ofYttria and Zirconia on the Positive Temperature Coefficient of Resistance in Barium Titanate Ceramics, M.S. dissertation. University of Illinois, Urbana, Dl. 1991. 

This kind of microstructure also influences other kinds of conductors, especially those with positive (PTC) or negative (NTC) temperature coefficients of resistivity. For instance, PTC materials (Kulwicki 1981) have to be impurity-doped polycrystalline ferroelectrics, usually barium titanate (single crystals do not work) and depend on a ferroelectric-to-paraelectric transition in the dopant-rich grain boundaries, which lead to enormous increases in resistivity. Such a ceramic can be used to prevent temperature excursions (surges) in electronic devices. 

Positive temperature coefficient (PCT) thermistors are solids, usually consisting of barium titanate, BaTiOi, in which the electrical resistivity increases dramatically with temperature over a narrow range of temperatures (Fig. 3.38). These devices are used for protection against power, current, and thermal overloads. When turned on, the thermistor has a low resitivity that allows a high current to flow. This in turn heats the thermistor, and if the temperature rise is sufficiently high, the device switches abruptly to the high resisitvity state, which effectively switches off the current flow. 

Some examples of ceramic semiconductors are magnetite (Fe304), doped barium titanate and doped zinc oxide. By for instance doping zinc oxide with bismuth, cobalt, manganese or antimony, you can vary the resistance of zinc oxide. 

Another circumstance is when deterioration becomes apparent under fields in excess of 0.5 MVm-1 at temperatures above 85 °C, and occurs more rapidly the higher the field or the temperature. The fall in resistance has been observed in single crystals of rutile and barium titanate and so must be assumed to be a bulk rather than a grain boundary effect, although there is evidence that grain boundaries play a part in degradation processes in ceramics. 

There are many different kinds of acoustical probes including microphones [57-62], hydrophones, radiometers, and piezoelectric devices (most often small barium titanate transducers) [63-68], and the hot wire microphone (based on acousto-resistive effect) [63], Their resonance frequency is generally very different from that of the ultrasonic field under study. 

Typical fillers carbon fiber, glass fiber, graphite lubricant for wear resistant applications molybdenum sulfide, PTFE. antimony trioxide, barium titanate, clay, silica, aluminum nitride, smectite... 

Metals passivation in the resid FCC unit may also be accomplished through the addition of metal traps. Tin, barium titanate, strontium titanate, magnesium oxide, manganese oxide, and specialized zeolite types and contents have all been used for vanadium trapping. In addition, zeolites coated with alumina and catalyst particles coated with rare earth have also been applied to resist vanadium poisoning. Antimony, bismuth, and specialized active high crystalline aluminas have all been used successfully to counteract the negative effect of nickel in the FCC unit. 

Titanates are double oxides of the form MeTiOa or Me2Ti04. Barium titanate BaTiOa and its solid solution crystals with other titanates are especially well-known. BaTiOs crystallizes in the perovskite structure. Its technical importance results from its ferroelectric and associated piezoelectric properties, its high dielectric constant at room temperature, and the interesting semiconducting properties which it exhibits when doped [13]. The remarkable temperature dependence of the electrical resistance of such doped material (the temperature coefficient can be metal-like) is used to advantage in control and circuit devices. 

Miraceram Barium titanate ceramics with positive temperature coefficient of resistance, used to make heating elements for domestic appliances, which then need no separate overheating cutout. The heaters are made as honeycomb with high specific surface. 

PTCR. Positive temperature coefficient of resistance. Ceramics with PTCR s include barium titanates. 

Detavernier, C., Van Bael, M.K., Van den Rul, H and Mullens, J. (2009) Crystallization resistance of barium titanate zirconate ultrathin films from aqueous CSD a study of cause and effect. 

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