Transient Thermal Gradients in Barium Titanate Positive Temperature Coefficient (PTC) Thermistors

Barium titanate positive temperature coefficient (PTC) ceramic disk thermistors can suffer major mechanical damage if inhomogeneous heating occurs under voltage. The steady-state and transient temperature distributions for thin disk samples (radius of 5 mm, thickness of 2 mm) have been studied with an infrared microscope, using a spatial resolution of 35 µm. The transient temperature distribution is observed to be particularly sensitive to the electrical boundary conditions during the initial heating period after application of a voltage. Small variations in electrode symmetry can lead to axial asymmetric thermal gradients up to 25 K/mm across the entire rim when an ac voltage of 100 V is applied. A finite-difference model in two dimensions, based on solution of the heat equation with a local-temperature-dependent Joule heating-source term, has been developed to describe the axial and radial transient temperature distributions in the cylindrical geometry. The predictions reveal current concentration at the edge of an electrode when the metal layer coverage is slightly smaller than that of the opposite face. This phenomenon results in stronger localized heating in a ring that initiates the thermal gradient.     

Negative Temperature Coefficient Resistance (NTCR) Ceramic Thermistors: An Industrial Perspective

Monitoring and control of temperature is of paramount importance in every part of our daily life. Temperature sensors are ubiquitous not only in domestic and industrial activities but also in laboratory and medical procedures. An assortment of temperature sensors is commercially available for such purposes. They range from metallic thermocouples to resistive temperature detectors and semiconductive ceramics, showing a negative temperature coefficient of resistance (NTCR). NTCR ceramic sensors occupy a respected market position, because they afford the best sensitivity and accuracy at the lowest price. Despite the enormous commercial success of NTCR thermistors, this area of advanced functional ceramics has not been recently reviewed. Nearly 100 years elapsed between the first report of NTCR behavior and the fabrication of NTCR devices. The manufacture of the first NTCR ceramic thermistors was problematic, as often the devices suffered from poor stability and nonreproducibility. Before NTCR ceramics could be seriously considered for mass production of thermistors, it was necessary to devote a large amount of R&D effort to study the nature of their semiconductivity and understand the influence of impurities/dopants and heat treatments on their electrical characteristics, particularly in their time dependence resistivity (aging). Simultaneously, from a technological viewpoint it was important to develop methods enabling reliable and permanent electrical contacts, and design suitable housing for ceramics, in order to preserve their electrical properties under conditions of variable oxygen partial pressure and humidity. These topics are reviewed in this article from an industrial perspective. Examples of common applications of NTCR thermistors and future challenges are also outlined.     

Long-Term Operation of Positive Temperature Coefficient Resistors

The effect of various atmospheres on the long-term operation of positive temperature coefficient resistors was studied. In most cases where degradation of properties occurred, the operable mechanism appeared to be volatilization of oxygen from grain boundaries due to reduction of the partial pressure of oxygen in the atmosphere around the sample. The effect of CCl₂F₂, however, was much more pronounced and was attributed to a chemical mechanism.     

Effects of Yttrium Doping on the Grain and Grain-Boundary Resistivities of BaTiO3 for Positive Temperature Coefficient Thermistors

Several positive temperature coefficient (PTC) thermistors made of barium titanate with an excess of titania and containing additives such as yttria, and eventually silica, have been prepared following two different routes. The electrical properties of the ceramic samples have been studied at room temperature, i.e., below the transition temperature, using complex impedance spectroscopy. The latter proved to be very useful to measure separately the grain and grain-boundary resistivities which have been followed as a function of the yttrium concentration. They behave very similarly and go through a minimum for the same composition. From both electrical resistivity measurements and local chemical analysis, it is inferred that the average dopant concentration in the grains is lower than the nominal content in the starting powders. An overall interpretation is given, emphasizing the importance of liquid-phase sintering.     

Piezoresistivity in Semiconducting Positive Temperature Coefficient Ceramics

The experimental and theoretical aspects of piezoresistivity in semiconducting, positive temperature coefficient (PTC) of resistivity ceramics will be reviewed. Emphasis will be placed on their potential application in sensor technology. Future material and modeling challenges are highlighted.     

有机PTC材料稳定性研究进展

介绍了导电粒子填充有机PTC材料稳定性的主要影响因素,指出要提高有机PTC材料的稳定性,在本质上必须控制导电粒子的分散程度与分布状态。综述了近年来国内外通过基体选择、导电粒子处理及交联等多种方法来提高有机PTC材料稳定性方面的工作及其最新进展,为PTC材料提高稳定性及实现产业化提供了指导。     

BaBiO3掺杂BaFe0.4Sn0.6O3负温度系数热敏电阻的电学性能

用传统固态反应法制备多晶(1-x)BaFe0.4Sn0.6O3-xBaBiO3(0.05相似文献   

Depletion-layer Dielectric Properties of Positive Temperature Coefficient of Resistance Barium Titanate

For pure and impurity-added positive temperature coefficient of resistance (PTCR) barium titanate ceramic samples, a −11°C shift of the Curie point at the grain-boundary/depletion-layer region was observed. This result is obtained by fitting the PTCR grain-boundary resistance and capacitance data to a theory which combines a double-depletion-layer model with the Devonshire thermodynamic theory of barium titanate. The parameters used in the fitting are obtained from independent experiments. The shift of the Curie point is believed to result from the grain-boundary clamp ing effect near the cubic-tetragonal phase transition point.     

HDPE/PVDF/CB复合体系PTC性能的研究

以HDPE/PVDF共混物为基体 ,研究了两种不同类型炭黑 (CB)的用量 ,以及HDPE/PVDF配比对复合物的正电阻温度系数 (PTC)的影响。结果表明 ,HDPE/PVDF/CB三元复合体系具有很强的PTC特性 ,可用于制作电致发热稳定性良好、自限温度 ( 85± 5 )℃、具有商业用途的自控温伴热带     

填充型多相聚合物导电复合材料的PTC效应

以聚丙烯和低密度聚乙烯共混物为基体,用碳黑为填充材料制备了复合导电材料,导电性能的测试表明多相复合体系的渗滤阈值低于两相复合体系的渗滤阈值。对复合材料PTC效应的分析以及对材料的热性能测试结果表明碳黑在共混体系中的分布。同时探讨了体系碳黑含量的变化对PTC效应的影响。     

Fabrication and Electrical Properties of Mn–Ni–Co–Cu–Si Oxides Negative Temperature Coefficient Thermistors

The microstructure and electrical properties of Mn–Ni–Co–Cu–Si oxides negative temperature coefficient (NTC) thermistors were studied. The as-sintered (Mn_1.62Ni_0.72Co_{0.57− x} Cu _x Si_0.09)O₄ (0≤ x ≤0.12) and (Mn_1.2Ni_0.78Co_{0.87− x} Cu_0.15Si _x )O₄ (0≤ x ≤ 0.15) ceramics showed the solid solutions of Mn–Ni–Co–Cu–Si oxides with a cubic spinel structure. The addition of SiO₂ led to an increase in the temperature coefficient of resistivity. This demonstrates that the SiO₂ addition is desirable for developing highly sensitive NTC thermistors. In addition, the resistivity and the temperature coefficient of resistivity for (Mn_1.62Ni_0.72Co_{0.57− x} Cu _x Si_0.09)O₄ and (Mn_1.2Ni_0.78Co_{0.87− x} Cu_0.15Si _x )O₄ NTC thermistors were controlled by changing the composition.     

Factors and Mechanisms Affecting the Positive Temperature Coefficient of Resistivity of Barium Titanate

The chemistry of semiconducting lanthanum-doped barium titanate was examined to determine the factors critical to the positive temperature coefficient of resistivity exhibited by this material. This phenomenon was sensitive to oxidation under certain nonequilibrium conditions. Inhomogeneity is thought to result from such conditions and to produce regions of different resistivity throughout the ceramic body. The existence of high resistivity layers was demonstrated by analysis of dispersions in ac resistance at audio frequencies. Association of these layers with crystallite surfaces, i.e. grain boundaries, was demonstrated by resistivity measurements of semiconducting single crystals before and after oxygen treatments.     

Origin of the Positive Temperature Coefficient of Resistivity Anomaly in the ZnO-NiO System

An anomalous positive temperature coefficient of resistivity (PTCR) was investigated in the ZnO-NiO system. It was found that the ZnOSS (Zn_0.97Ni_0.03O) and NiOSS (Ni_0.6Zn_0.4O) constituent phases of that system exhibit negative temperature coefficient of resistivity (NTCR) character, while their combination shows a PTCR effect with a maximum at 400°C, which coincides with a large difference in the coefficient of linear thermal expansion between the ZnO_SS and NiO_SS phases at that temperature. On the basis of the brick wall model microstructure, the PTCR anomaly of this system can be explained. The magnitude of the PTCR effect is governed by the difference in resistivity of the two constituent phases at the temperature where the maximum of the PTCR anomaly occurs. The predicted temperature dependence of the resistance, R(T) , of a model microstructure consisting of constituent phases with different grain sizes agrees well with the experimental R(T) of the prepared composite ceramics.     

Positive Temperature Coefficient of Resistivity Effect in Undoped, Atmospherically Reduced Barium Titanate

A sharp positive temperature coefficient of resistivity effect for undoped, fluorinated BaTiO_3−x samples was observed. The qualitative agreement between the behavior of BaTiO_3−x and La-doped BaTiO₃, with respect to the trap density at the boundaries, suggests that in the absence of counterdopants such as Mn, the main origin of surface acceptor states is chemisorbed gases. Impedance analysis indicates that, under the conditions of the present work, fluorine seems not to diffuse in significant amounts into the lattice. The treatment of the fluorinated samples in inert/reducing atmospheres did not markedly decrease the resistivity jump, suggesting that fluorine might be chemisorbed more strongly than oxygen. The composition of two different liquid phases, produced by excess titania and silica additions, did not have an important effect on the resulting properties.     

Positive Temperature Coefficient of Resistivity Effect in Highly Donor–Doped Barium Titanate

BaTiO₃ ceramics doped with different La concentrations (0–12 mol%) were prepared by sintering under the reducing conditions of a nitrogen atmosphere containing 1% hydrogen. The critical donor concentration that causes blocking of the exaggerated grain growth was observed to be ∼10 mol% La. The samples, which were semiconducting after sintering under reducing conditions, were subsequently reoxidized by annealing in air to induce the positive temperature coefficient of resistivity (PTCR) effect. After reoxidation at 1150°C a noticeable PTCR effect was observed in the samples doped with La concentrations as high as 2.5 mol%. The room-temperature resistivity after reoxidation was found to increase with increasing donor concentration due to an increase in the thickness of the insulating layers at the grain boundaries. TEM analysis showed that reoxidation of the samples caused precipitation of the Ti-rich compound Ba₆Ti₁₇O₄₀ inside the doped BaTiO₃-matrix grains.     

炭黑/聚烯烃导电复合材料PTC效应的研究

对炭黑/聚烯烃导电复合材料PTC效应做了较全面的论述,介绍了此类材料的导电机理和一些新的理论观点,总结了影响其PTC效应的主要因素及目前国内外研究这种新型功能材料的典型方法。文中详细讨论了工艺条件、基体与导电填料的性质、频率等方面对材料性能、PTC效应的影响,对研究、开发和应用高性能的PTC复合材料有重要的参考价值。     

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

The resistivity of Ba_{(1– y )(1– x )}Sr _y (1– x )La _x TiO₃ceramics 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.     

高抗冲聚苯乙烯/炭黑导电复合材料 PTC效应的研究

采用SEM、XRP、DSC和体积膨胀等手段对非结晶形HIPS／CB（聚苯乙烯／炭黑）导电复合材料的导电行为进行分析。实验结果表明，HIPS／CB导电复合材料的渗流区域较窄，炭黑含量为30～40g时，电阻发生急剧降低，在渗流阀值附近导电复合材料的电阻率变化最大，跃迁强度为2.2个数量级；HIPS／CB导电复合材料的PTC起始转变温度在85℃左右，与HIPS的玻璃化转变温度接近，HIPS作为非晶材料其体积膨胀较弱，并不足以单独解释PTC效益。     

聚乙烯／炭黑型PTC材料的稳定性研究

通过加入抗氧剂、交联剂及相容剂,采用熔融共混后挤出成型制样,对聚乙烯／炭黑体系正温度系数（ＰＴＣ）效应的稳定性进行了研究,为研究和开发高性能的聚合物ＰＴＣ材料提供了经验。