BaTiO3-based Positive Temperature Coefficient Research Articles

Temperature-dependent resistivity performance of Mn/Y-doped Ba1-xSrxTiO3 compositions with potential thermal control applications

Doped BaTiO3 ceramics exhibit an attractive application prospect in the adaptive thermal control of electronic devices in spacecraft that originate from its remarkable positive temperature coefficient (PTC) characteristics. However, the Curie temperature of most current BaTiO3-based PTC materials is much higher than the normal operating temperature range of electronic devices. In this work, we successfully synthesized Ba1-xSrxTiO3 ceramics with a room temperature Curie point. The crystal structure, surface morphology and temperature dependence of resistivity are investigated. The Curie temperature where the crystal structure of the composition changes from a paraelectric phase to a ferroelectric phase is adjusted by increasing the doping level(x). In the temperature range 18–120 °C, the variation amplitude of resistivity exceeds 104, and the positive temperature coefficient effect is as high as 10.7%/°C. The potential thermal control properties were discussed based on the experimental and theoretical analysis. The heating power of compositions can be automatically changed by varying the operating temperature. At the same initial heating power, the equilibrium temperature of the controlled equipment using the PTC heating element is lower than that when adopting an ordinary heater. Moreover, the effect of thermal control becomes more prominent as the resistivity-temperature coefficient increases.

Electrode Interface Polarization in BaTiO<sub>3</sub>-Based PTC Ceramics

Electrodes play a vital role on the electrical properties of positive temperature coefficient (PTC) ceramics. An ohmic contract between ceramics and electrodes is necessary for the PTC effect. In this work, silver mixed aluminium electrode and pure silver electrode were pasted on BaTiO3-based PTC ceramics, which results in an ohmic contact and non-ohmic contact, respectively. Impedance spectroscopy and dielectric and conductivity properties was investigated at different temperature for the two contacts. Small difference of electrical properties was found between the two contacts above the Curie temperature. Below the Curie temperature, however, carriers could pass through the interface of ohmic contract but gather on the interface of non-ohmic contact. The latter resulted in a space charge polarization, which increased low-frequency dielectric permittivity.

Research of fuel temperature control in fuel pipeline of diesel engine using positive temperature coefficient material

As fuel temperature increases, both its viscosity and surface tension decrease, and this is helpful to improve fuel atomization and then better combustion and emission performances of engine. Based on the self-regulated temperature property of positive temperature coefficient material, this article used a positive temperature coefficient material as electric heating element to heat diesel fuel in fuel pipeline of diesel engine. A kind of BaTiO3-based positive temperature coefficient material, with the Curie temperature of 230°C and rated voltage of 24 V, was developed, and its micrograph and element compositions were also analyzed. By the fuel pipeline wrapped in six positive temperature coefficient ceramics, its resistivity–temperature and heating characteristics were tested on a fuel pump bench. The experiments showed that in this installation, the surface temperature of six positive temperature coefficient ceramics rose to the equilibrium temperature only for 100 s at rated voltage. In rated power supply for six positive temperature coefficient ceramics, the temperature of injection fuel improved for 21°C–27°C within 100 s, and then could keep constant. Using positive temperature coefficient material to heat diesel in fuel pipeline of diesel engine, the injection mass per cycle had little change, approximately 0.3%/°C. This study provides a beneficial reference for improving atomization of high-viscosity liquids by employing positive temperature coefficient material without any control methods.

Preparation and effects of glass-coatings on BaTiO3-based PTC thermistors

Glasses in the Na2O–K2O–K2SiF6–ZnO–Al2O3–B2O3–SiO2 system were investigated as potential intermediate temperature sealing materials to protect BaTiO3-based positive temperature coefficient (PTC) thermistors. The influence of compositional changes on the crystallization tendency and sintering behavior of the glass was reported. Additionally, the electrical properties of PTC thermistors with glass-coatings were compared with those without glass-coatings. The results showed that the formulation (in wt%), 3.1Na2O–9.4K2O–10.6K2SiF6–4.3ZnO–4.3Al2O3–20.3B2O3–48SiO2 (G2), performed better on thermal properties, had a good glass formability within a relatively wide range of sintering temperature and was compatible with the PTC thermistors (no cracks or pinholes). Furthermore, the glass layer did not affect the characteristics of the PTC thermistors too much.

The effect of SiO2 addition and measurement temperature on the high-field electrical behavior of BaTiO3-based positive temperature coefficient thermistors

The effects of SiO2 addition, in the range 0–3 at. %, on the high field electrical characteristics of positive temperature coefficient of resistance (PTC) thermistors have been characterized at several temperatures above the ferroelectric transition temperature TC. It was found that decreases in SiO2 content and increases in measurement temperature both decreased the field dependency of the current-voltage response. The electrical data were analyzed in terms of existing current-flow models and found to exhibit reasonable agreement only with the model based on diffusion limited transport across a double Schottky barrier. The maximum value of voltage drop per grain boundary for fit to the diffusion limited model Vmax increased with increasing measurement temperature within the PTC region, but leveled off beyond the temperature of maximum resistance. This behavior directly parallels the variation of electrostatic barrier height with temperature. A reduction in Vmax with increasing SiO2 content was observed and attributed to a concomitant reduction in electrostatic barrier height.

Effects of Bi 1/2Na 1/2TiO 3 on the Curie temperature and the PTC effects of BaTiO 3-based positive temperature coefficient ceramics

In order to prepare lead-free BaTiO 3-based PTC with middle Curie point, the incorporation of Bi 1/2Na 1/2TiO 3 (abbreviated to NBT) into BaTiO 3-based ceramics was investigated on samples containing 0.1, 0.5, 1, 1.5, 1.6, 1.8, 2 mol% of NBT. It was found that for all samples containing up to 1.5 mol%, the Curie temperature was obviously improved with the increasing of NBT content, and the T C raised to 150 °C from 97 °C just only 1.5 mol% NBT added. Because A O bonds were weakened, which induced that Ti O bonds were strengthened correspondingly, and the actions between Ti 4+ (a departure of center) and its near O 2− were so strong that Ti 4+ could not resume its seat except that it was under the higher temperature, hence the Curie temperature was improved. At the same time, the PTC effects were also meliorated which attributed to vast electron capture centers formed by Bi 2O 3 acting with MnO. However, when the amount of NBT exceeded 1.5 mol%, the samples were dielectric material and had no PTC effects, but it was worth noting that the ρ– T curve of the samples sintered in reducing atmosphere was linear.

Evolution of low sigma grain boundaries in PTC thermistors during sintering

BaTiO 3-based positive temperature coefficient (PTC) thermistors undergo a large and rapid increase in grain boundary resistivity at temperatures just above the Curie temperature, T c. Ample evidence exists for variability in the magnitude and form of the resistivity increase between different grain boundaries, and it has been noted that many of the grain boundaries showing a weak PTC effect have low Σ values when indexed using coincidence site lattice notation. It has also been reported in the literature that, in undoped BaTiO 3, there is a strong preference for the formation of Σ = 3 grain boundaries in the microstructure, with many more present than would be expected by chance. In the current study, the formation and retention of low Σ grain boundaries in a PTC thermistor based on doped BaTiO 3 have been characterised through interrupted sintering experiments. Electron backscatter pattern (EBSP) analysis was used to establish grain boundary misorientation distributions in a series of samples prepared during an interrupted sintering study. A significant proportion of Σ = 3, 5 and 9 boundaries was observed, with Σ = 3 boundaries being systematically preferred over other low Σ boundaries. Since Σ = 3, 5 and 9 grain boundaries are believed to be PTC inactive, their presence in significant numbers in the microstructure is likely to be deleterious to the overall performance of a thermistor, particularly during transient loading. An increase in the proportion of Σ = 3 twin boundaries was noted with sintering time, however, the proportion of Σ = 3 grain boundaries remained fairly constant, although occurring with a higher frequency than would be expected in a random population. The proportion of Σ = 5 and 9 boundaries also remained approximately constant during the sintering process, indicating that the density of low Σ boundaries in the microstructure is fixed at an early stage of sintering and is not affected significantly by grain growth.

Experimental study on the mechanism of BaTiO 3-based PTC–CO gas sensor

The n-type semiconducting BaTiO 3-based positive temperature coefficient of resistivity (PTCR) ceramics is a potential CO gas sensing material with its high temperature NTCR region. The impedance spectroscopy was employed to analyze the resistance evolution of grain and grain boundary in atmospheres for the PTCR ceramics with various Curie point. The XPS studies were carried out at the temperatures from room temperature to 340 °C to understand the sensing characterization of BaTiO 3-based PTCR ceramics. It is indicated that the sensing mechanism for the PTC–CO gas sensor is due to oxygen desorption at the grain boundary or an increase in charge carrier density and related chemical defects, which are based on the fluctuation of the potential barrier of the grain boundary.

Research on the manufacture of laminated BaTiO 3-based thermistor by roll forming

New laminated BaTiO 3-based positive temperature coefficient thermistors prepared by roll forming have been studied. Through changing the factors that influence the roll forming process, thick film positive temperature coefficient (PTC) BaTiO 3 green sheets with good plasticity, uniformity, compactness and smoothness were obtained. After the films sintered at high temperature in the air and electrodes screen printed, laminated PTC thermistors, which possess many good features, such as small size and lower room resistivity and larger current flux, can be obtained. It is the first time that such laminated PTC thermistors are reported. The lamination technology and electrical behavior of PTC thermistors were also investigated.

Preparation of BaTiO 3-based chip thermistors by gelcasting approach

Gelcasting has been developed as a novel ceramic forming method for complex-shaped advanced materials of near-net-shape. In this work, the gelcasting process has been further developed to fabricate BaTiO 3-based positive temperature coefficient chip thermistor. Special attentions were given to the effect of atmosphere type on gelcasting behavior of ceramics and the effect of the presence of glycerol and monomers in slurries on the flexibility and mechanical strength of gelcast green sheets. Compared with roll formed thermistors, gelcast ones exhibit stronger PTC effect.

The kinetics of initial stage in sintering process of BaTiO 3-based PTCR ceramics and its computer simulation

In the present paper, the research on the sintering mechanism of BaTiO 3-based Positive Temperature Coefficient of Resistance (PTCR) semiconductive ceramics was carried out. According to the mechanisms of surface diffuse and volume diffuse, the comprehension acting mechanism was suggested, the kinetic equation was proposed at the initial stage of sintering, and the initial stage of sintering process was simulated by a computer. We obtained the object illustrations about the relationships between the grain growth of the initial stage of sintering on BaTiO 3-based PTCR ceramics and sintering temperatures, as well as the particle sizes of raw materials.

A structural model of Cr/(Ba,Pb)TiO 3 positive temperature coefficient composite

The over-current protector is one of the main applications of the positive temperature coefficient (PTC) thermistor. Low room-temperature resistance and a PTC effect are required for the use of the over-current protection. As a result, lowering the room-temperature resistivity of PTC materials becomes very important. From a Japanese patent, the method of adding metal to BaTiO3-based PTC ceramics to form composites has shown good results. But in recent publications, few papers were related to this area. Furthermore, in the limited literature, the resistance–temperature curve of the material expressed a negative temperature coefficient (NTC) effect when metal was present. In the present work, chromium (Cr), was added to (Ba,Pb)TiO3 ceramics to form PTC composite with higher Curie temperature (TC = 180°C). Under a given composition and method, the prepared composite had low room-temperature resistivity (p = 1.33Ωcm) and PTC effect (Pmax/Pmax = 10). From the experimental results obtained, a structural model of the composite is proposed. The co-function of metal and ceramics, and sintering atmosphere factor on the PTC effect are discussed in this model. By employing this model, the resistance–temperature properties of the composites can be explained satisfactorily.

BaTiO3-based positive temperature coefficient of resistivity ceramics with low resistivities prepared by the oxalate method

Positive temperature coefficient of resistivity (PTCR) ceramics with low resistivities at room temperature were obtained by using oxalate-derived barium titanate powders. The average room-temperature resistivity of the PTCR ceramics was 4Ω cm, and the magnitude of their PTCR jump was around four orders with a voltage proof of more than 50 Vmm−1. These PTCR properties are significantly influenced by the calcination temperature of the starting materials and by the resultant properties of the ceramic bodies. The microstructure of such-PTCR ceramics with a low room-temperature resistivity produced in this study was found to be rather heterogeneous. Complex impedance measurements revealed that the resistivity of the present PTCR materials was determined predominantly by the grain-boundary resistance even at room temperature.

Low temperature fired positive temperature coefficient resistors

Through the addition of BN as a sintering aid, Y-doped BaTiO3-based positive temperature coefficient resistors (PTCR) can be fabricated at temperatures below 1200‡C, which is much lower than is required when conventional sintering aids such as silica or A12O3, SiO2, and TiO2 (AST) are used. In comparison with the PTCR containing SiO2, the low-temperature-fired thermistor shows an enhanced PTC effect and a much higher temperature of resistivity maximum. Electrical properties of the BN-added BaTiO3, (Ba,Sr)TiO3, and (Ba,Pb)TiO3 ceramics were discussed.