Positive Temperature Coefficient Of Resistivity Research Articles

Effect of donor concentration on the PTCR behavior of Y-doped BaTiO3–(Bi1/2Na1/2)TiO3 ceramics

Lead-free positive temperature coefficient of resistivity (PTCR) ceramics of 99 mol%BaTiO3–1 mol%(Bi1/2Na1/2)TiO3 (BBNT1) and 90 mol%BaTiO3–10 mol%(Bi1/2Na1/2)TiO3 (BBNT10) were prepared by the conventional solid state reaction method. The effect of donor concentration on the microstructure and PTCR behavior of the BBNT ceramics were investigated. The results show that all BBNT1 and BBNT10 ceramics have formed a single perovskite structure with tetragonal phase when sintered in air or N2. 0.2 mol% Y-doped BBNT1, sintered at 1,330 °C for 30 min in air, has room-temperature resistivity (ρ25) of ~60 Ω cm and resistivity jump [maximum resistivity (ρmax)/minimum resistivity (ρmin)] of ~4.2 orders of magnitude with Tc about 150 °C. 0.4 mol% Y-doped BBNT10, sintered at 1,250 °C for 3 h in N2, has ρ25 of ~100 Ω cm and resistivity jump of ~4.7 orders of magnitude with Tc about 180 °C. The BBNT10 has higher breakdown voltage of 200 V/mm (a.c.) than that of BBNT1, making these BBNT10 ceramics promising candidates for high temperature lead-free PTCR materials.

Metals and Superconductors: Molecular Analogs of Atomic Hydrogen

One for the half: recent work on atomic hydrogen and neutral radical heterocyclic molecules has moved the field of spin S=1/2 solids closer to the realization of synthetic intrinsic metals and superconductors. High-pressure experiments on hydrogen and chalcogenide-nitrogen molecules show evidence of metallic character. The molecular radicals exhibit a positive temperature coefficient of resistance under pressure-the classic signature of a metal.

Synthesis and electrical characteristics of (1 − x)BaTiO3–xK0.5Bi0.5TiO3 PTCR ceramics

Solid solutions of (1 − x)BaTiO3–xK0.5Bi0.5TiO3 were prepared by the solid state reaction technique. Samples were sintered in reducing atmosphere of N2/H2 in the temperature range 1100–1240 °C with subsequent oxidation at 700 °C. Phase composition and crystal structure were investigated by X-ray powder diffractometry (XRPD). It was shown that all samples of (1 − x)BaTiO3–xK0.5Bi0.5TiO3 (0 ≤ x < 0.4) exhibit a tetragonal structure at room temperature, the parameters a and c decrease with increasing x. With increasing x the Curie temperature of solid solutions (1 − x)BaTiO3–xK0.5Bi0.5TiO3 (0.1 ≤ x < 0.4) increases from 150 to 220 °C. The values of potential barriers at grain boundaries were calculated on the basis of Heywang model. With increasing x, the potential barrier at grain boundaries increases. It was shown that the grain size decreases with increasing the bismuth–potassium titanate content. The complex impedance results indicate that the grain boundary and the outer layer region, located between the boundary and the core of the grain, make a contribution to the positive temperature coefficient of resistance (PTCR) effect in (1 − x)BaTiO3–xK0.5Bi0.5TiO3 solid solutions. With increasing x ρmax increases due to an increase in potential barrier at grain boundaries.

Effects of different bismuth-based compounds additives on the positive temperature coefficient effect of BaTiO3 ceramics

As a candidate for lead-free positive temperature coefficient of resistivity (PTCR) ceramics, BaTiO3 (BT) ceramics with different Bi-based compounds (K0.5Bi0.5TiO3, BaBiO3, Bi4Ti3O12 and Bi2O3) were fabricated by a conventional solid state reaction. The effects of Bi-based compounds additives on the phase composition, microstructure and PTCR characteristics of BT ceramics were investigated with X-ray diffraction, scanning electron microscope, and resistivity-temperature measurement instrument. The results showed that the average grain size, the Curie temperature (Tc) and the room temperature resistivity (ρRT) of BT ceramics varied with the additives of Bi-based compounds. The minimum value of ρRT for BT ceramics with different Bi-based compounds could be obtained when the atom concentrations of bismuth were around 0.2%. The ceramic samples showed a moderate PTCR effect, with a resistance jump greater by 1.4–3 orders of magnitude and increased Tc about 120–132°C, along with a low resistance of 4–1500Ωcm.

A phenomenological model of conduction in Ho-doped barium titanate sensor

PTCR effect in doped barium titanate (BaTiO3) sensors originates from the grain boundary resistance and is attributed to the existence of potential barriers at these boundaries. Height of the potential barrier at the grain boundary is a function of the dielectric constant which is strongly temperature dependent. The height of the barriers between the grains has been investigated in semiconducting BaTiO3 samples prepared by doping with 0.3mol% Ho2O3 using the conventional ceramic methods. The results show a Positive Temperature Coefficient of Resistance (PTCR) for temperatures above the Curie temperature (TC) followed by a region of Negative Temperature Coefficient of Resistance (NTCR). The dielectric properties of the samples were studied in the temperature interval of 300–570K using capacitance measurements at 1.2kHz from which the permittivity and the barrier height were determined. It was found that the barrier height increases linearly in a narrow range of temperature where the PTCR effect is observed beyond which it continues to increase slowly. The barrier height does not become constant but varies from 0.06eV to 0.18eV over a temperature interval of 410–570K. A phenomenological model based on Thermionic Emission (TE) and Thermally Assisted Field Emission (TFE) mechanism has been proposed to account for the NTCR behavior.

Influence of sintering conditions on the electrical properties and the PTCR effect of the multilayer Ba1.005(Ti1−xNbx)O3 ceramics with Ni internal electrode

The effects of the Nb-dopant content and the sintering conditions on the electrical properties and the positive temperature coefficient of resistance (PTCR) effect of Ba1.005(Ti1−xNbx)O3 (BTN) ceramics were investigated, which were sintered at 1,070–1,220 °C for 0.5–6 h in a reducing atmosphere and then re-oxidized at 600–750 °C for 1 h. The results indicated that both the sintering temperature and sintering time affected the electrical properties and the PTCR effect of the multilayer BTN samples, whose room-temperature (RT) resistance first reduced and then increased as a function of the donor–doped concentration at all sintering temperature; moreover, the higher the sintering temperature was, the lower the critical dopant concentration. The BTN ceramics showed a remarkable PTCR effect, with a resistance jump greater by 3.3 orders of magnitude, along with a low RT resistance of 0.3 Ω at a low reoxidated temperature of 600 °C after sintering in a reducing atmosphere.

Effect of reoxidation on positive temperature coefficient of resistance behavior for BaTiO3-K0.5Bi0.5TiO3 ceramics

As a positive temperature coefficient of resistivity (PTCR) material, (1-x)BaTiO3-xK0.5Bi0.5TiO3 (BT-KBT, 0.05≦ x ≦0.15) ceramics without any donor doping were prepared by a conventional oxide mixing method. All samples were sintered in an Ar atmosphere at 1280 ~ 1350°C, subsequently, reoxidized at 800 ~ 1100°C in a gas mixture (99 %Ar–1 %O2). The PTCR behavior of BT-KBT ceramics were investigated in terms of KBT content, reoxidation temperature and time. The results showed that the BT-KBT ceramics exhibited an abrupt increase in their resistivity near the Curie temperature (Tc) after annealing in gas mixture, Tc of 0.9BT-0.1KBT ceramic was shifted to a higher temperature (~150°C). Furthermore, the room-temperature resistivity (ρRT) of ceramic samples sintered in Ar and reoxidized in a gas mixture decreased to 102 Ω·cm. The jump in resistivity (maximum resistivity [ρmax]/minimum resistivity [ρmin]) was enhanced by three orders of magnitude through a suitable reduction–reoxidation method without sacrificing the ρRT.

Time-dependent changes in morphology and resistance of W/TiN/Ti metal lines upon applying voltage pulses

Transient current profiles, measured while applying voltage pulses to W/TiN/Ti metal lines, showed two different stages. In the first stage, the transient current decreased due to both structural changes associated with Ti/W diffusion and the positive temperature coefficient of resistance of polycrystalline W/TiN/Ti. In the second stage, the transient current remained approximately constant due to the effect of morphological and microstructural changes counter-balanced by the thermal expansion of W near the melting temperature and the latent heat of fusion of W. Asymmetric electromigration and symmetric thermomigration fluxes resulted in the asymmetric metal line failure location near the anode.

Micromachined piezoresistive inclinometer with oscillator-based integrated interface circuit and temperature readout

In this paper a dual-chip system for inclination measurement is presented. It consists of a MEMS (microelectromechanical system) piezoresistive accelerometer manufactured in silicon bulk micromachining and a CMOS (complementary metal oxide semiconductor) ASIC (application specific integrated circuit) interface designed for resistive-bridge sensors. The sensor is composed of a seismic mass symmetrically suspended by means of four flexure beams that integrate two piezoresistors each to detect the applied static acceleration, which is related to inclination with respect to the gravity vector. The ASIC interface is based on a relaxation oscillator where the frequency and the duty cycle of a rectangular-wave output signal are related to the fractional bridge imbalance and the overall bridge resistance of the sensor, respectively. The latter is a function of temperature; therefore the sensing element itself can be advantageously used to derive information for its own thermal compensation. DC current excitation of the sensor makes the configuration unaffected by wire resistances and parasitic capacitances. Therefore, a modular system results where the sensor can be placed remotely from the electronics without suffering accuracy degradation. The inclination measurement system has been characterized as a function of the applied inclination angle at different temperatures. At room temperature, the experimental sensitivity of the system results in about 148 Hz/g, which corresponds to an angular sensitivity around zero inclination angle of about 2.58 Hz deg−1. This is in agreement with finite element method simulations. The measured output fluctuations at constant temperature determine an equivalent resolution of about 0.1° at midrange. In the temperature range of 25–65 °C the system sensitivity decreases by about 10%, which is less than the variation due to the microsensor alone thanks to thermal compensation provided by the current excitation of the bridge and the positive temperature coefficient of resistance of the piezoresistors.

The structure and PTCR effects of Mg-doped ZnTiO3 ceramics

Zinc titanate doped with magnesium was produced by a conventional solid state reaction technique using metal oxides. The X-ray powder diffraction pattern showed a single phase of ZnTiO3. The magnesium can replace the zinc ions and form a solid solution in the ZnTiO3 phase, and the intensities of the ZnTiO3 peaks decreased with increasing addition of Mg. The materials thus produced are semiconductors, with a positive temperature coefficient of resistivity. Increased addition of magnesium decreases the electrical resistivity. In addition, the sintered density and c/a ratio also rise with the increasing addition of Mg, while the open porosity decreases. Moreover, the Curie temperature (Tc) of the (Zn1−xMgx)TiO3 materials can be shifted to a higher temperature by increasing the Mg2+ concentration.

PTCR and Thermoelectric Properties of Isovalent Co-Doped BaTiO3 Nanoceramics

Positive temperature coefficient resistance (PTCR) materials of Gd and Cr co-doped BaTiO3 ceramics were prepared by well known solid state reaction method. Temperature dependent electrical studies were performed in the temperature range from 300 K to 673 K. Curie temperature of the samples are around 345 K to 385 K. Resistivity was maximum at low temperature and decreases as the temperature increases confirming the semiconducting behavior and PTCR property of the synthesized nano-sized compounds. Thermoelectric properties demonstrated n-type semiconductor characteristics from room temperature to 673 K.

Polyaniline films doped with ladder-type sulfonated polyphenylsilsesquioxane and unusual dependence of their electrical conductivity on temperature

We report on conducting polyaniline (PANI) films doped with a thermally stable polymer dopant, linear ladder-type sulfonated polyphenylsilsesquioxane (S-PPSQ). The PANI/S-PPSQ films exhibited high-temperature stable electrical conductivity. With a heat treatment at 120 °C, the electrical conductivity of conventional PANI films doped with the molecular dopant camphorsulfuric acid (CSA) rapidly decreased to half of its initial value, while that of PANI/S-PPSQ films were relatively unaffected. Moreover, PANI/S-PPSQ films with high electrical conductivity exhibited unusual negative temperature coefficient of resistance (TCR) values above room temperature, in contrast to conventional PANI/CSA films that exhibited positive TCR values. By using a solution blend of PANI/S-PPSQ and PANI/CSA with negative and positive TCR values, respectively, we successfully demonstrated control of TCR behavior. Therefore, PANI/S-PPSQ materials are very promising for organic electronic applications.

Positive temperature coefficient of resistance effect in Na2Ti6O13-doped 0.94BaTiO3–0.06(Bi0.5Na0.5)TiO3 ceramics

The positive temperature coefficient of resistance (PTCR) characteristics of Na2Ti6O13 (NT)-doped 0.94BaTiO3–0.06(Bi0.5Na0.5)TiO3 (BBNT) ceramics were investigated in order to evaluate the effect of NT as a new additive for lead-free PTCR thermistor application. The BBNT ceramic sintered at 1325°C exhibited a relatively high Curie temperature (TC) of 158°C while its PTCR characteristic was not satisfactory for thermistor application. However, doping with NT significantly influenced the PTCR behavior of BBNT ceramic. It is considered that NT was responsible for grain growth of the BBNT by forming a liquid phase during sintering due to its low melting temperature of 1300°C. The grain growth resulted in the enhanced PTCR characteristics of BBNT ceramic. In particular, 0.1 mol% NT doped BBNT ceramic exhibited excellent PTCR performance of low resistivity at room temperature (1.6×102 Ω cm), resistivity increase near TC (1.28×104) and high TC of 158°C, suitable for lead-free PTCR thermistor application.

Effect of Ni Electrode on the Characteristics of BaTiO&lt;sub&gt;3&lt;/sub&gt; Based PTCR Ceramics

In order to achieve cost-effective inner electrodes for the multilayer BaTiO3-based ceramics having a positive temperature coefficient of resistivity (PTCR), we fabricated Ni paste based on Ni powder and investigated the effect of Ni electrode on the performance of semiconducting BaTiO3 ceramics. We adjusted the particle size of Ni powder (0.2μm, 0.6μm and 1μm) and incorporated them as the electrodes into both single-layer and laminated BaTiO3 PTCR devices. The device samples were sintered at 1200oC for 30min in reducing atmosphere consisting of N2 and H2 (97:3 by volume ratio), and went through a post-sintering in-air heat treatment at 700-900oC in air which is necessary for the PTCR effect. The results indicate that Ni powder with lager particle size are more stable against post-sintering heat treatment, and the heating temperature needs to be optimized to overcome the trade-off between ohmic behaviors of Ni electrodes and the PTCR effect of BaTiO3-based semiconducting ceramics.

Fabrication of Lead-Free and High T&lt;sub&gt;c&lt;/sub&gt; Positive Temperature Coefficient of Resistivity Ceramics Sintered in Air

As a lead-free positive temperature coefficient of resistivity (PTCR) material, [Ba0.95-x (K0.5Bi0.5)0.05Cax]1-yNbyO3 system was prepared by the conventional solid-state reaction method. All samples sintered in air at 1300°C possess PTC characteristics as well as semi-conductivity characteristics, especially they show high Tc(130°C~160°C) value and the jump of the resistivity (maximum resistivity ρmax / minimum resistivity ρmin ) is four orders of magnitude. Samples with the composition of 0.3mol% Nb5+ have low room-temperature resistivity (ρ25°C) of ~103Ω.cm.

The Influence of Ba-Ti Ratio on the Electrical Properties and Microstructures of the BaM-0.007Sm0.007TiO3 Based Ceramics Fired in Reducing Atmosphere

Electrical properties, positive temperature coefficient of resistivity (PTCR), and microstructures of (Bam-0.007Sm0.007)TiO3(BST) with different Ba-site/Ti-site (A/B) ratio sintered in a reducing atmosphere and reoxidized in air are investigated. The results reveal that the room temperature resistivity of the semiconducting BST ceramics first decreases and then increases with increasing of A/B ratio (m), particularly when m is equal to 1.006, the semiconducting BST ceramics which have been sintered in a reducing atmosphere and reoxidized at 800°C exhibit significant PTCR effect with a resistance jumping ratio of 3 orders magnitude, and achieve a lower room temperature resisitivity of 80.8 Ω∙cm, in addition, the grain size distribution of the Ti-excess specimens is much better than that of the Ba-excess ones.

Insulator-metal transition in GeTe/Sb2Te3 multilayer induced by grain growth and interface barrier

Unlike its two components, the temperature coefficient of resistivity (TCR) of GeTe/Sb2Te3 multilayer (ML) increases from negative to positive on annealing, indicating an insulator-metal transition (IMT). Impedance spectroscopy measurements demonstrate that the grain boundary resistance (negative TCR) determines the total resistance of initial ML. As grain grows, which is confirmed by x-ray diffraction, scanning electron microscope, and optical reflectivity measurements, the contribution of grain resistance (positive TCR) increases gradually to the leading part and finally accomplishes the IMT in a sufficiently crystallized film. Furthermore, the artificially introduced interfaces form additional potential barrier in ML and also modulate its IMT behavior.

SiC-ZrB2복합체의 특성에 미치는 SPS의 압력영향

The SiC-ZrB₂composites were produced by subjecting a 40:60 vol.% mixture of zirconium diboride(ZrB₂powder and β-silicon carbide (SiC) matrix to spark plasma sintering(SPS). Sintering was carried out for 60sec at 1400℃ (designation as TP145 and TP146), 1500℃(designation as TP155 and TP156) and uniaxial pressure 50MPa , 60MP under argon atmosphere. The physical, electrical, and mechanical properties of the SiC-ZrB₂composites were examined. The relative density of TP145, TP146, TP155 and TP156 were 94.75%, 94.13%, 97.88% and 95.80%, respectively. Reactions between β-SiC and ZrB₂ were not observed via x-ray diffraction (hereafter, XRD) analysis. The flexural strength, 306.23MPa of TP156 was higher than that, 279.42MPa of TP146 at room temperature, but lower than that, 392.30MPa of TP155. The properties of a SiC-ZrB₂ composites through SPS under argon atmosphere were positive temperature coefficient resistance (hereafter, PTCR) in the range from 25℃ to 500℃. The electrical resistivities of TP145, TP146, TP155 and TP156 were 6.75×10^-4, 7.22×10^-4, 6.17×10^-4 and 6.71×10^-4Ω·cm at 25℃, respectively. The densification of a SiC-ZrB₂ composite through hot pressing depend on the sintering temperature and pressure. However, it is convinced that the densification of a SiC-ZrB₂ composite do not depend on sintering pressure under SPS.

MnO2가 도핑된 무연 High Tc(&gt;165℃) BaTiO3-(Bi0.5Na0.5)TiO3세라믹의 PTCR 특성 향상

0.935Ba<TEX>$TiO_3$</TEX>-0.065(<TEX>$Bi_{0.5}Na_{0.5}$</TEX>)<TEX>$TiO_3+xmol%MnO_2$</TEX> (BBNTM-x) ceramics with <TEX>$0{\leq}x{\leq}0.05$</TEX> were fabricated with muffled sintering by a modified synthesis process. Their microstructure and enhanced positive temperature coefficient of resistivity (PTCR) characteristics were systematically investigated in order to obtain lead-free high TC PTCR thermistors. All specimens showed a perovskite structure with a tetragonal symmetry and no secondary phase was observed. Grain growth was achieved when the doped MnO2 was increased above 0.02 mol%. This is due to the effect of positive Mn ion doping as an acceptor compensating a Ba vacancy occurred by the higher donor dopant concentration of <TEX>$Bi^{3+}$</TEX> ion. Especially, enhanced PTCR characteristics of the extremely low <TEX>${\rho}_{RT}$</TEX> of <TEX>$9\;{\Omega}{\cdot}cm$</TEX>, PTCR jump of <TEX>$5.1{\times}10^3$</TEX>, <TEX>${\alpha}$</TEX> of 15.5%/<TEX>$^{\circ}C$</TEX> and high <TEX>$T_C$</TEX> of <TEX>$167^{\circ}C$</TEX> were achieved for the BBNTM-0.04 ceramics.

Oxygen tracer diffusion in donor doped barium titanate

Oxygen exchange with the ambient atmosphere and oxygen diffusion are assumed to play a decisive role in the re-oxidation process of positive temperature coefficient (PTC) resistors based on donor doped barium titanate. 18O tracer experiments with subsequent time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements were thus carried out to investigate the oxygen diffusion properties of donor doped barium titanate. Fast grain boundary diffusion was found at temperatures between 750 °C and 900 °C. Moreover, evidence is given for a position dependent diffusion coefficient close to the surface. The secondary phase developing during the production process is shown to be Ti-rich and hardly any oxygen tracer exchange with this secondary phase could be observed. This suggests that grain boundary diffusion does not take place via such secondary phases. Rather, evidence of diffusion along an oxygen vacancy enriched space charge region is found.