TiO3 Positive Temperature Coefficient Research Articles

Potential Barrier Behavior of BaTiO3—(Bi0.5Na0.5)TiO3 Positive Temperature Coefficient of Resistivity Ceramic**Supported by the Special Foundation for Scientists of Guizhou Province under Grant Nos KY[2012]102 and TZJF-2011-10, the Foundation of Key Laboratory of Inorganic Function Material and Device, the Chinese Academy of Sciences under Grant No KLIFMD2012-02, and the National Natural Science Foundation of China under Grant No 51462030.

High-Curie-temperature (Tc) lead-free Y-doped 90 mol%BaTiO3—10 mol%(Bi0.5Na0.5)TiO3 ceramic with positive temperature coefficient of resistivity (PTCR) is prepared by the conventional solid state reaction in nitrogen atmosphere. The PTCR ceramic exhibits a room-temperature resistivity (ρ25) of ∼500Ω·cm and a high PTCR effect (maximum resistivity (ρmax)/minimum resistivity (ρmin)) of ∼4.5 orders of magnitude. A capacitance-voltage approach is first employed to calculate the potential barrier (Φ) of the grain boundary of PTCR ceramic above Tc. It is found that the potential barrier changes from 0.17 to 0.77 eV as the temperature increases from 180 to 220°C, which is very close to the predictions of the Heywang–Jonker model, suggesting that the capacitance-voltage method is valid to estimate the potential barrier of PTCR thermistor ceramics.

Impedance and dielectric spectroscopy analysis of high TC lead-free BaTiO3-(Bi1/2Na1/2)TiO3 positive temperature coefficient resistivity ceramics

92 mol%BaTiO3-8 mol% (Bi1/2Na1/2)TiO3 (BBNT8) lead-free positive temperature coefficient resistivity (PTCR) ceramics doped with Y2O3 are prepared by the solid state reaction method. The microstructure of the sample is observed by transmission electron microscope. Results show that the microstructure of the sample mainly consists of grain and grain boundary, and no obvious shell structure is found. The electrical properties of the sample are further analyzed using the impedance spectroscopy. It is found that the total resistance of BBNT8 is composed of the resistances of grain and grain boundary. The resistance of grain is low and slightly changes with temperature when the temperature is above the Curie temperature. The PTCR effect of the material is mainly dominated by the grain boundary resistance. As the temperature increases up to above the Curie temperature, the grain boundary permittivity decreases, leading to the increase of the potential barrier height and the resistivity of the grain boundary. As a result, the distinct PTCR effect takes place. Finally, the room-temperature resistivity of the BBNT8 is studied by measuring the permittivity-frequency characteristics.

Fabrication of high Tc BaTiO3–Bi0.5Na0.5TiO3 positive temperature coefficient of resistivity ceramics using the sol–gelmethod

BaTiO3–Bi0.5Na0.5TiO3 (BT–BNT) positive temperature coefficients of resistivity ceramics were successfully prepared by the sol–gel method. The synthesized fine and chemical homogeneous powders could decrease the volatilization of Bi2O3 and Na2O by lowering the sintering temperature. The effect of BNT content on Lg(ρmax/ρmin), the room temperature resistivity, the Curie temperature and the microstructure of the lead-free PTCR ceramics was studied. When BNT ≤4mol%, the PTCR ceramics sintered at 1250°C in air atmosphere displayed excellent PTC effect with a resistivity jump of 3–4 orders of magnitude. The room temperature resistivity and the Curie temperature of the sample with 5mol% BNT were 4.46×106Ωcm and 148°C, respectively. However, when the BNT content exceeded 5mol%, the ceramics displayed dielectric behavior.

Fabrication of high Tc lead free (1 − x)BaTiO3–xBi0.5K0.5TiO3 positive temperature coefficient of resistivity ceramics using reoxidation method

(1 – x)BaTiO3−xBi0.5K0.5TiO3 (abbreviated as BT–BKT, where x = 0, 0.1 and 0.2) ceramics were prepared by solid state reaction method. All ceramic samples were sintered in a pure N2 flow atmosphere, subsequently reoxidized at a temperature range of 800–1,100 °C in air for several hours. The influences of BKT content and reoxidation on the positive temperature coefficient of resistivity (PTCR) behavior of ceramic samples were investigated. BT–BKT ceramic samples sintered in N2 possessed relatively low room temperature resistivity (ρRT) and showed weak PTC effect. Through an appropriate reoxidation, the ceramic samples re-obtained PTC effect of almost three orders of magnitude. With the addition of BKT, the Curie temperature (Tc) was enhanced by ~50 °C than the pure BT ceramics.

Characterization of manganese-doped BaTiO3–(Bi1∕2Na1∕2)TiO3 positive temperature coefficient of resistivity ceramics using impedance spectroscopy

Small amount of manganese (Mn)-doped BaTiO3–(Bi1∕2Na1∕2)TiO3 (BT-BNT) positive temperature coefficient of resistivity ceramics are investigated by impedance analyses. The impedance/modulus spectroscopic plots reveal that a third resistance-capacitance (RC) response besides grains and grain boundaries is exhibited in the Mn-free BT-BNT ceramic with 4mol% BNT, but is not observed in the Mn-doped samples. The third RC element can be attributed to a barium vacancy-rich layer in the outer grain region. The evidence of impedance spectroscopy indicates that the highly Bi donor is partially compensated by the Mn acceptor and the predominant charge compensation defect shifts from barium vacancies to electrons with doping small amount of Mn dopant.

High Tc lead-free BaTiO3–(Bi1∕2Na1∕2)TiO3 positive temperature coefficient of resistivity ceramics with electrically heterogeneous structure

A distinct positive temperature coefficient of resistivity effect has been observed in BaTiO3–(Bi1∕2Na1∕2)TiO3 (BT-BNT) ceramics sintered in a N2 flow with low O2 concentration. With the addition of BNT, the samples exhibit resistivity jumps of ∼103–105 starting at ∼190–210°C. X-ray diffraction results indicate that the BNT phase and BT phase formed a solid solution during sintering. An electrically heterogeneous structure, consisting of the grain interiors, outer grain shells, and grain boundaries, is revealed by the complex impedance analyses. The observed dc resistivity jump is attributed to the rapid resistivity rise in both grain boundaries and grain shells.

Calculations and modeling of grain-boundary acceptor states for (Ba,Pb)TiO3 positive temperature coefficient ceramics

The electrical properties of high-Curie-point positive temperature coefficient of resistance ceramics of composition Ba0.497Pb0.5La0.003Ca0.005Ti0.995O3 prepared from commercial BaTiO3 powder were studied. The acceptor-state energy and acceptor-state density were determined from the calculated ρmax vs Tmax plot, where ρmax and Tmax are the resistivity and temperature corresponding to the maximum in resistivity. These two parameters, combined with the effect of nonvanishing spontaneous polarization below the Curie point, were treated quantitatively to demonstrate the dependence of measured resistivity ρdc on the measured permittivity εm. The obtained values of potential barrier height and acceptor-state density based on the procedure above were found to be in excellent agreement with those extracted from the slope in the Arrhenius plots of resistivity versus 1/(Tεm), where T is the absolute temperature. Also, the deviation of the measured dielectric constant εm from the Curie–Weiss law’s prediction was explained by the narrowing of the grain-boundary barrier layers.

Complex Impedance Analysis of Nb-Doped (Ba0.6Sr0.4)TiO3 PTC (Positive Temperature Coefficient) Thermistors

Frequency-dependent complex impedances were measured in Nb-doped (Ba0.6Sr0.4)TiO3 PTC (positive temperature coefficient) ceramics. The complex impedance data were analyzed in Cole-Cole-type equivalent circuits by the computer fitting method in order to determine individual resistance and capacitance values of the bulk grain and grain boundary. The PTC effect was found to have originated from the temperature-sensitive behavior of the grain boundary resistance. The temperature dependences of dielectric constants of both the bulk grain and grain boundary calculated on the basis of the two-layer model agreed closely to those predicted by Heywang's model. The width and potential height of the grain boundary layer were estimated to be 0.26 µm and 0.1 eV, respectively, in the temperature range below -20°C. The estimated donor density of 6×1018 cm-3 was much lower than calculated from the amount of Nb2O5 added.4