Positive Temperature Coefficient Of Electrical Resistivity Research Articles

(Ba0.55Sr0.45)1-xLaxTi1.01O3-Bi0.5Na0.5TiO3 Positive Temperature Coefficient Resistivity Ceramics with Low Curie Temperature (~-15 °C).

Positive temperature coefficient of electrical resistivity (PTCR) materials with low Curie temperature have been paid increasing attention lately. In this study, PTCR materials with a Curie temperature of approximately -15 °C were investigated by La3+ doping Ba0.55Sr0.45TiO3 ceramics. It could be expected to meet the requirements of thermal management systems for low-temperature control. In addition, a trace amount of Bi0.5Na0.5TiO3 (BNT) was employed to improve the resistivity and the PTCR performance. A significant PTCR effect was achieved with a high resistivity jump of nearly four orders of magnitude, a high temperature coefficient of ~28.76%/°C, and a narrow transition temperature span of 22 °C in the (Ba0.55Sr0.45)0.99875La0.00125Ti1.01O3-0.0025Bi0.5Na0.5TiO3 ceramics. The PTCR enhancement mechanism of BNT is discussed.

The jump-like PTCR effect in a Ni-BaTiO3 magnetic composite

The ceramic composites BaTiO3@Ni, composed of a conducting nickel and a highly resistive ferroelectric BaTiO3 phase, exhibiting a positive temperature coefficient of electrical resistivity (PTCR), are prepared using a conventional ceramic method at less than 1000 °C. The BaTiO3@Ni composites show a jump-like PTCR effect in electrical resistivity by about eight orders of magnitude (ρmax/ρmin ≈ 108) with ρmin < 1 Ωcm. This is the first example of a standard PTCR element with a relatively high magnetization. The grain relocations (a microstructural rearrangement) on a microscopic scale in the composite are filmed using a transmission electron microscope while heating through the ferroelectric phase transition (Curie temperature), which induces the PTCR anomaly. The resistivity anomaly of the composite is explained in terms of the ferroelectric phase transition-assisted anomaly in the electrical resistivity.

Microstructure and temperature coefficient of resistivity for ZnO ceramics doped with Al 2O 3

The ZnO doped with Al 2O 3 showed the minimum electrical resistivity at an Al 2O 3 content of 0.25%. It was found that the ZnO doped with Al 2O 3 exhibited the resistance-temperature characteristic reversion from negative temperature coefficient of electrical resistivity (NTCR) to positive temperature coefficient of electrical resistivity (PTCR) at temperatures of about 450 and 250 °C for ceramics sintered at 1400 and 1300 °C, respectively. Both NTCR and PTCR features were weakened for samples with lower Al 2O 3 content or sintered at a higher temperature. Microstructure studies exposed that the densification process was significantly depressed when the Al 2O 3 addition levels were over about 1%.

Positive Temperature Coefficient of Electrical Resistivity below 150 K in Barium Strontium Titanate

The resistivity of Ba(1–y)(1–x)Sry(1–x)LaxTiO3ceramics with x= 0.0025 and y= 0.25, 0.5, 0.75, and 0.9 was measured between 50 and 400 K. A resistivity anomaly corresponding to the positive temperature coefficient of electrical resistivity (PTCR) effect was observed for all compositions. The onset temperature decreased from 320 K (y= 0.25) to 70 K (y= 0.9). The extent of the PTCR effect was significantly enhanced for the strontium‐rich composition and reached ∼8 orders of magnitude for y= 0.9. These results strongly suggested the possibility to fabricate PTCR devices based on (Sr,Ba)TiO3 ceramics for application at cryogenic temperatures.

Composite ceramics with a positive temperature coefficient of electrical resistivity effect

Composite ceramics with compositions within the ZnO–NiO–TiO2, ZnO–MgO, and ZnO–Ln2O3 (Ln = Nd, Sm) systems were found to exhibit an anomalous positive temperature coefficient of electrical resistivity (PTCR) effect. The investigations revealed, that in all cases when the PTCR effect was identified, the composite ceramics were found to be composed of phases with different electrical resistivities and linear thermal expansion coefficients. Thermal mismatch between the phases in the composites leads to a disconnection of the grains of the low resistivity constituent phase on account of the high thermal expansion of the other high resistivity constituent phase, resulting in a PTCR effect.

Investigation of the PTCR effect in ZnO–NiO two-phase ceramics

A positive temperature coefficient of electrical resistivity (PTCR) in ZnO–NiO ceramics was observed and an attempt to explain its origin was made. According to electrical measurements, supported by dilatometric and microstructural investigations, the percolation principle was used for this purpose. A series of samples with different ZnO ss and NiO ss contents were prepared. The electrical resistivity of the samples was measured at different temperatures. ZnO ss represents a low ohmic and NiO ss represents a high ohmic phase. Monophase samples exhibited a strictly negative temperature coefficient of electrical resistivity (NTCR), while in some two-phase samples PTCR behaviour was observed. Results of electrical, dilatometric measurements and microstructural investigations indicated a possible explanation of PTCR behaviour on the basis of the percolation model.

Investigation of a PTCR effect in the Zn-Ti-Ni-O system

An anomalous positive temperature coefficient of electrical resistivity (PTCR) was investigated in three phase ceramics within the Zn-Ti-Ni-O system. The ceramics studied consisted of randomly dispersed constituent phases ZnO ss, NiO ss, and (Zn,Ni) 2TiO 4. It was found that although the pure constituent phases exhibit a NTC character, the multiphase ceramics exhibit a PTCR effect, with a maximum resistivity in the ϱ-T dependence at about 300 °C. The PTCR effect observed was about two orders of magnitude.

Resistivity in weakly disordered suicides induced by ion implantation

The temperature coefficient of electrical resistivity (TCR) of ion implanted suicide films are measured by van der Pauw method. The TCR tends to become small and even changes sign at certain temperatures as the implanted ion energy increases. The experimental data are much pertinent to the theoretical fitting with weak localization at temperature below 90K. However at 90K < T < 160K, the experiment is prevailed by electron phonon interaction in the presence of disorder. The retention of positive TCR of the ion-implanted titanium suicide at a rather high residual resistivity implies an enhancement of the localization effect due to the screening of the electron-phonon interaction by the d-state hybridization of the transition metals.

The positive temperature coefficient of resistivity of Ianthanum-doped Ba0.8Sr0.2TiO3 ceramics as a function of barium excess

It has been known since 1955 that semiconducting barium titanate exhibits a positive temperature coefficient of electrical resistivity (PTCR) near the ferroelectric Curie temperature. PTCR ceramics were used to fabricate a liquid level sensor [1], thermal controller, current limiter, current stabilizer, temperature compensator, television degausser, etc. [2, 3]. In spite of the marvellous development in the application of PTCR ceramics, many things about the PTCR effect still remain to be clarified, such as the shape of the resistivity-temperature characteristics below the Curie point, the grain size dependence of the effect, the exact role of transition elements normally used for enhancing it, etc. [4]. Kuwabara [5] has shown that the stoichiometry of the starting barium titanate powders (excess TiO2 or BaO) was very important in producing small grain-sized ceramics which can exhibit large PTCR effects. However, only a qualitative microstrucrural description was provided to interpret effect of grain size on the PTCR effect. In the present study, a more quantitative investigation of the microstructural influence on the PTCR effect in lanthanum-doped Ba0.8 Sr0.2 TiO3 with different amounts of BaO excess is reported. Commercially obtained BaTiO3 (HPBT-1, Fuji Titanium Industry Co. Ltd, Japan) was mixed with the required amount of Sr(OH)2"8H20 and La(NO3)2 • 6H20 and different amounts of BaCO3 to produce a low Curie temperature of semiconducting barium titanate powders with different BaO excesses. The raw material mixtures were mechanically milled in deionized water. The milled slurry was dried and calcined at 1100 ° C for 1 h. The calcined powders were then ball milled, dried again and sieved through a 140 mesh screen to produce the starting semiconducting powders with compositions of Ba0.sSr0.zTiO3 + 0.2mo1% La203 + xmol% BaO (x = 0, 1, 1.5, 2, 2.73 and 3.5). Disks were pressed and sintered at 1400 ° C for 1 h in a programmable furnace. A cooling rate of 150 ° C h~ was used in this study. These disks, after sintering, were about 8.8 mm in diameter and 2 mm in thickness. The average intercept length of the grains was determined by lineal analysis of the as-sintered specimens' surfaces observed by scanning electron microscopy (SEM). The resistance-temperature characteristics were measured by using a two-probe method with an indium-gallium (60:40) electrode applied to the opposite faces of the fired disks. A voltage of 1 V was applied to specimens. The addition of BaO at least up to 3.5 mol % seems to favour the PTCR effect, as shown in Fig. 1. It shows that an abrupt increase in the maximum to minimum ratio of resistivity as the amount of BaO added reaches 2 mol %. These data correlate well with direct microscopic observation (Fig. 2) and gt'ain size measurement (Fig. 3). When the average grain size decreased from 2.73#m (1.5tool % BaO excess) to 1.23/~m (3.5 mol % BaO excess), the maximum to minimum ratio of resistivity increased from 1.7 to 5, but all the relevant samples had almost the same relative sintered densities (Fig. 4) which have also been found to significantly affect the magnitude of the effect [6]. A phase diagram for the BaO-TiO2 system reported by Rase and Roy [7] has shown that a second phase, Ba2TiO4, appeared for the BaO excess BaTiO3 samples. The presence of BazTiO4 as a second phase for samples containing >/0.1 tool % excess BaO has been further confirmed by direct microscopical examination [8]. This second phase which was expected to form in our BaO-excess samples would play an important role in grain inhibition. The effect of the addition of small amounts of BaO on grain growth is evident from the SEM photographs (Fig. 2). The effect of the BaO addition on PTCR is quantitatively described in terms of inhibition of grain growth. However, the fact that barium titanate ceramics with Ba-rich compositions can exhibit large PTCR effects may be contrary to the model proposed by Daniels et al. [9], in which the existence of barium vacancies near the grain boundaries was assumed to be inevitable for the occurrence of the PTCR effect. On the other hand, Heywang [10] explained the PTCR characteristics in terms of the temperature dependent Schottky-type grain boundary potential barrier.