PTC Thermistor Research Articles

Potential barriers in PTC barium titanate observed by EFM

PTC thermistors based on doped barium titanate are ferroelectric materials that show commercial interest due to a large variation in electrical resistance with temperature. The PTC effect in ceramics based on semiconducting barium titanate is due to formation of electric potential barriers in grain boundaries, increasing the electrical resistance [1]. Barium titanate ceramics when doped shows an electric behavior, which consists of an abrupt increase of resistivity with the temperature, close to Curie temperature (Tc). This effect happens in barium titanate during the change of cubic to tetragonal phase when cooled to temperatures below 130 ◦C [2, 3]. The models more accepted attributes the PTC effect to the formation of potential barriers in grain boundaries, increasing the resistivity when temperatures above 130 ◦C are reached [4]. Being basically a grain boundaries effect the PTC behavior depends strongly on microstructure [2–5]. The electric properties of ceramic PTC can be observed through the nature of the grains boundaries, and the levels of energy involved in their process were not well explained [4, 5]. The formation of potential barriers on barium titanate doped with yttrium were investigated using electrostatic force microscopy (EFM) with electric field gradient applications and surface potential images shows the formation of potential along grain boundary at applied voltage of 10 V. Semiconducting barium titanate was obtained by mixing 0.3% of yttrium in the nitrate form with BaTiO3 powder (TAM ceramics). The mixing was made in aqueous solution and after drying, the powder was dispersed and pellets were obtained by compaction with a uniaxial press. Sintering was done at 1350 ◦C by 2 hr and grains were reveled by thermal attack at 1250 ◦C for 1 min. EFM images were obtained using an atomic force microscope (Nanoscope IIIa) operating in electrostatic force mode (EFM) using a silicon probe (NSC15). The images of surface potential were done with applications of 0, 4 and 10 V, in situ. Fig. 1a show the AFM topographic image of 0.3YBaTiO3 reveling microstructure where grains and grain boundaries are clearly defined. Fig. 1b show an EFM

Evaluation of compatibility of thick-film PTC thermistors and LTCC structures

The electrical and microstructural characteristics of 1 kΩ/sq. thick-film thermistors with high positive temperature coefficients of resistivity, i.e. PTC 5093 (Du Pont) fired either on “green” LTCC (low temperature cofired ceramics) substrates or buried within LTCC structures, were evaluated. The active phase (ruthenium oxide) in the PTC thermistors is not present in the dried films but is formed during firing due to the decomposition of the ruthenate phase. Because of interactions between a glassy LTCC material and thermistors electrical characteristic, i.e. sheet resistivities and noise indices of thermistors fired on the surface LTCC substrates, changed from values obtained on alumina substrates. The differences in the measured electrical parameters were attributed to the interactions between the thermistor layers and the glassy LTCC substrates. The inter-diffusion of oxides, mainly PbO and Al 2O 3, was confirmed by microanalysis. In the case of buried thermistors, presumably due to the incompatibility of both materials, the structure de-laminated during firing. Cracks between the buried PTC films and the LTCC substrates as well as cracks in PTC films resulted in high sheet resistivities and high noise indices.

Evaluation of a novel PTC thermistor for telecom overcurrent protection

This paper presents a novel PTC thermistor for telecom overcurrent protection. The proposed thermistor is composed of Mn(NO 3) 2-doped(Ba 0.7368Ca 0.1Sr 0.16Sb 0.0032)Ti 1.01O 3 (abbreviated BCST) and a small amount of SiO 2 and is fabricated using traditional ceramic processes. By integrating compositional modification, dopant addition and double annealing techniques, good static material properties are obtained, including a room resistance ( R 25 °C ) of 14.3 Ω, a resistivity jump ( ρ max/ ρ min) of 3.2 × 10 6, a temperature coefficient of resistance (TCR) of +20.4%/°C, and a maximum operating voltage ( V o) of 357.5 V rms. In order to evaluate the dynamic characteristics of the device when working in rated and trip conditions, two simulation circuits are designed in accordance with ITU-T recommendation K.30. In the simulation circuit for the rated test, the thermistor is required to pass 110 mA for 1 h with a 50 V dc supply. The results indicate that a stable operation cannot be maintained when the thermistor operates at ambient temperatures exceeding 45 °C. In the simulated trip test, the circuit assumes a 220 V ac power contact with a transition current of 1 A rms and the response time of the thermistor is measured under a range of ambient temperatures. It is found that the response time of the device is always less than 4 s over the temperature range of −10 to 80 °C. The present static and dynamic evaluation results demonstrate that the developed BCST thermistor satisfies the requirements of telecom overcurrent protection.

PTC characteristics of (TiC/polyethylene) conductive composites in relation to their particle‐filled structures

AbstractTitanium carbide (TiC)/polyethylene (PE) composites have been prepared with various volume fractions of TiC by a melt bleeding method. Two types of TiC powder with their different particle diameters, 1.48 µm (TiC‐A) and ca. 100 µm (TiC‐B), were utilized in the present study. The electric resistivities (ρ) of the samples decreased gradually with increasing TiC volume fraction (ϕA); in the case of smaller particle size, the value of ρ at room temperature jumped down in the relatively narrow range of ϕA = 0.39 to 0.42, reaching as low as 0.1 Ωcm. Temperature dependencies of ρ were also measured to examine the possible application of the composite to polymer PTC thermistors. The resistivity showed positive temperature dependence and changed abruptly in the tenth order of its magnitude during the melting transition of PE. The electric current cutoff characteristics of the samples indicated excellent features in the overcurrent response despite their low resistivity values, as compared with those of other PTC materials. In parallel, the mechanism of conductivity of the composite was discussed in terms of the percolation theory. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 152(2): 1–9, 2005; Published online in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/eej.20115

Microstructures of BaTiO 3 based PTC thermistors with Ca, Sr and Pb additions

BaTiO 3 based PTC thermistors undergo a large and rapid increase in resistance just above the Curie temperature, T C. The resistance increase is predominantly associated with an increase in grain boundary barrier height, therefore it is important to gain information about the grain boundary character in these devices. In this study the microstructures of three different BaTiO 3 PTC thermistor formulations were compared by transmission electron microscopy. The samples differed in the A-site substitutions, giving the formulae: (1) BaTiO 3, (2) (Ba,Ca)TiO 3, (3) (Ba,Ca,Sr,Pb)TiO 3. No grain boundary films were observed, though rounding of the grains at the grain junctions was evidence of a liquid second phase being present during sintering. EDX measurements were made across grain boundaries but no significant segregation of elements to the boundaries was observed.

Thick-film temperature sensors on alumina and LTCC substrates

The electrical and microstructural characteristics of 1 kΩ/sq. thick-film thermistors with high positive or negative temperature coefficients of resistivity, i.e., PTC 5093 (Du Pont) and NTC 4993 (EMCA Remex), fired on alumina or co-fired on “green” low-temperature co-fired ceramic (LTCC) substrates, are compared. The active phase in both materials (the ruthenium oxide and the semiconducting spinel in the PTC and NTC thermistors, respectively) is not present in the dried films but is formed during firing. The differences in the measured electrical parameters (sheet resistivities, temperature dependence of resistivities and noise) of the thermistors, fired on alumina or on LTCC were attributed to the interactions between the thermistor layers and the glassy LTCC substrates. The inter-diffusion of oxides, mainly PbO and SiO 2, was confirmed by microanalysis.

Characteristics of porous BaTiO 3-based PTC thermistors fabricated by adding graphite powders

The crystalline structure, microstructure, and electrical properties of porous graphite-containing BaTiO 3-based PTC thermistors were studied. The sintered ceramics showed a tetragonal perovskite structure, irrespective of the added graphite content. The added graphite formed the interstitial solid solutions with Ba 0.999Sb 0.001TiO 3 and was distributed homogeneously in the sintered ceramics. The grain size of the as-sintered graphite-containing BaTiO 3-based ceramics was much larger than that of the as-sintered graphite-free ones. This is mainly due to the exothermic reactions of the graphites and oxygen molecules in the ceramics. The porosity of the as-sintered ceramics increased with increasing graphite content due to an enhanced evolution of CO and CO 2 gases associated with the exothermic reactions. It was found that the newly prepared porous graphite-containing BaTiO 3-based PTC thermistors showed a large PTCR effect.

PTC Characteristics of(TiC/ Polyethylene) Conductive Composites in Relation to their Particle-filled Structures

Titanium carbide (TiC)/polyethylene (PE) composites have been prepared with various volume fractions of TiC by a melt bleeding method. Two types of TiC powder with their different particle diameters, 1.48 µm (TiC-A) and ca. 100 µm (TiC-B), were utilized in the present study. The electric resistivities (ρ) of the samples decreased gradually with increasing TiC volume fraction (ϕA); in the case of smaller particle size, the value of ρ at room temperature jumped down in the relatively narrow range of ϕA = 0.39 to 0.42, reaching as low as 0.1 Ωcm. Temperature dependencies of ρ were also measured to examine the possible application of the composite to polymer PTC thermistors. The resistivity showed positive temperature dependence and changed abruptly in the tenth order of its magnitude during the melting transition of PE. The electric current cutoff characteristics of the samples indicated excellent features in the overcurrent response despite their low resistivity values, as compared with those of other PTC materials. In parallel, the mechanism of conductivity of the composite was discussed in terms of the percolation theory. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 152(2): 1–9, 2005; Published online in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/eej.20115

Conductive mode imaging of thermistor grain boundaries

PTC thermistors undergo a rapid increase in grain boundary resistivity, covering several orders of magnitude, at temperatures just above the Curie temperature, T C, which is associated with a ferroelectric to paraelectric phase transformation. In this study, hot-stage conductive mode scanning electron microscopy has been used to investigate the characteristics of individual thermistor grain boundaries over a range of temperatures around T C. Using the remote electron beam induced current (REBIC) configuration, imaging has revealed EBIC contrast consistent with the presence of negatively charged electrostatic grain boundary barriers for the first time in a commercial thermistor. Not all grain boundaries within the thermistor were found to be EBIC active and the EBIC contrast was only observed at temperatures above T C. EBSP analysis of grain boundary crystallography indicated that the EBIC active grain boundaries were predominantly high angle.

Analysis on the aging characteristics of PTCR of donor-doped barium titanate

The resistance versus time curves of room temperature unload ageing, high temperature unload ageing and high temperature continuous load test ageing of PTC thermistors are measured. At the initial aging stage under both high temperature unload ageing and high temperature continuous load test ageing, the resistance decreases first, and then increases, which are different from that of the room temperature aging. Such abnormal phenomena exist in the initial ageing stage and are attributed to buffering of the unstable mechanical stress and desorption of the absorbed oxygen molecules under the thermal action. R–T characteristic curves of aged sample are different from that of oxidized and reduced samples. A new mechanism is proposed to interpret the aging characteristics of the donor-doped BaTiO 3 PTC thermistor.

Influences of the electroless nickel electrode on the electrical characteristics of BaTiO3-based PTCR ceramics

This paper investigated the influences of the microstructure of electroless nickel electrode on the a.c. shock characteristics of BaTiO 3 -based PTCR ceramics and the degradation and failure of PTC thermistors with electroless nickel electrode. In the experiments, we coated metal nickel electrode on the surface of PTCR ceramics samples by employing the different electroless plating baths and analyzed the microstructure of electroless nickel electrodes by SEM. The experimental results indicated that the various baths made the microstructure of nickel electrodes different. The nickel electrodes from acid bath possessed the densified stacked structure, which resulted in being cracked when being shocked by 220 V, 3 A a.c. power; owing to having sparse branch-and-tendril structure, those from alkaline bath had no effects on the ceramics samples being shocked. The work came to the conclusion that the microstructure of electroless nickel electrodes varied the thermal stress in the body of PTCR ceramics. In addition, the a.c. shock characteristics of BaTiO 3 -based PTCR ceramics were related to the content of phosphorus in the electroless nickel-phosphorus alloy and thickness of the nickel electrodes. According to the experiments, the thickness of the electroless nickel alloy should be less than 1 μm and the phosphorus content, 4 wt.% in the alloy.

A miniature PTC thermistor based sensor element fabricated by tape casting technique

Miniature Positive Temperature Coefficient thermistors measuring around 4 mm×4 mm×0.3 mm have been prepared from semiconducting barium titanate by tape casting technique. Their performance as sensor elements for sensing of temperature and related parameters is far superior due to their miniature size and therefore very low thermal mass. These elements have considerably high signal-to-noise ratio and their response is also very fast.

β-Conductivity contrast at barium titanate thermistor grain boundaries

Barium titanate based PTC thermistor materials have been examined using the conductive mode of a scanning electron microscope. Under an applied voltage bias, at temperatures above the Curie temperature, bright and dark lines which are due to β-conductivity appear coincident with grain boundaries. The mechanisms of contrast formation are discussed. Such contrast can be used to evaluate the distribution of this type of active grain boundary.

A new smart material-multilayer PTC thermistor

New multilayer PTC thermistors, barium titanate-based ceramics, and polymer V/sub 2/O/sub 3/-based composites, have been studied. These thermistors are also passively smart materials with a self-repair mechanism by nonlinear temperature dependent resistance (R(T)) and voltage dependent current (I(V)) behavior. The laminated ceramic and composite thermistors possess many good features, such as small size and new characteristics of lower resistivity and larger current. It is the first time that such multilayer PTC thermistors have been reported. The lamination technology and electrical behavior of a multilayer positive temperature coefficient (MLPTC) thermistor were investigated. >

ChemInform Abstract: Preparation of Porous BaTiO3 PTC Thermistors by Adding Graphite Porosifiers.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

PTC Effect and Its Mechanism in a Novel Thermistor Material —— Hot-pressed SiC/SiO 2 Multiphase Ceramic

As functional materials, PTC thermistor ceramics characterized by a drastic increase in resistivity at its transition temperature have been widely used in high-technology areas such as temperature measurement and indication, temperature control, protective switches and so on. At present, the PTC thermistors are fabricated by means of doping BaTiO_3 and their PTC effects result from the existence of acceptor state or electronic trap on grain boundary. But the fabrication process of

Preparation of Porous BaTiO3 PTC Thermistors by Adding Graphite Porosifiers

Using an ordinary ceramic processing technique, a new method of preparing porous BaTiO3, PTC thermistors is introduced. Adding proper graphite powders into calcined BaTiO3, powder can increase porosity and enhance the PTC effect of the sintered sample. When the graphite addition is about 1.0 wt%, the resisivity of samples from low‐purity raw materials decreases to 20 Σ·cm at room temperature, while the PTC resistivity ratio is over 105 and 1–2 orders higher than that of samples without the porosifier. The porosifier has increased the reproducibility of resistivity from 60% up to 90%. In addition, the porous thermistor has a fast response to overcurrent and shows some improvement in heat resistance. With the aid of the Heywang model and the Kuwabara conclusion, the influence of graphite on grain surfaces is discussed. The experimental results show that this method is a useful technique to prepare good PTC thermistors from low‐purity raw materials.

On the mechanism of delamination fracture of BaTiO3-based PTC thermistors

Delamination fracture of positive temperature coefficient PTC thermistors is a mode of thermal shock failure which manifests itself by cracking of the ceramic along a plane approximately parallel to the electrodes. This mode of failure is observed in high-power switching applications. It results from the build-up of thermal stresses whose amplitude is governed by a large number of geometrical, electrical and thermomechanical parameters. An experimental procedure is proposed for the quantitative evaluation of the thermal shock resistance of PTC thermistors. This resistance is characterized by a 'fracture voltage' defined as the voltage causing an increase by more than 10% of the room-temperature resistance after less than 25 switching cycles. The crack trajectory during the successive cycles is revealed by microscopic observation of the fracture surfaces. These experimental results are interpreted by comparison with the temperature and stress distributions computed using a newly developed three dimensional model. This modelling allows evaluation of the relative importance of the parameters affecting failure. The cracking mechanism can be partly elucidated on the basis of the computations of the maps of the normal stresses arising in homogeneous and non-homogeneous ceramics.

Domain structures in donor-doped barium titanate ceramics

The effects of cooling rate and soaking time on the positive temperature coefficient of resistivity (PTCR) of donor-doped barium titanate ceramics have been well studied. Daniels and Wernicke presented a model which predicted that an increasing grain boundary thickness led to a continuous transition from a PTC thermistor to an intergranular capacitor. Common in grain boundary potential layer models of PTC behavior, is the assumption that the polarization vector is parallel to the domain boundary (180° domains). Haanstra and Ihrig observed only 90° domains (perpendicular polarization vectors in neighboring domains) in these materials. They did not observe any intergranular second phase in their microstructures, which had grain boundary thicknesses of 2 - 10 nm. They discarded the potential barrier model proposed by Hoffmann in which a 500 am thick second phase layer (possibly Ti-rich) was assumed to be present at the grain boundary. In the present investigation, the domain structures and grain boundary thicknesses in BaTiO3 PTC thermistor, and the intergranular capacitor specimens were studied.

Sensors, Actuators, and Smart Materials

The field of electronic ceramics includes components as diverse as Mn-Zn ferrites, lead zirconate titanate transducers, ZnO varistors, and Al2O3 or cordierite-based packages for integrated circuits. An overview of electroceramics is given in Figure 1, which depicts schematically the types of electronic and ionic phenomena of interest. Many of the materials pictured utilize some bulk property of the ceramic, such as the presence of ferroelectric of ferrimagnetic dipoles, electronic or ionic conductivity, or a phase transition to provide a useful function. In other materials, extraordinary responses can be engineered by suitably tailoring the thickness and properties of a grain boundary phase. Varistors, barrier layer capacitors, and PTC thermistors, for example, all rely on the preparation of semiconducting grains and insulating grain boundaries to create properties that cannot be found in single crystals. In a similar way, some of the porous ceramics utilized in humidity sensors rely on surface properties rather than on a bulk response to detect changes in the ambient humidity (Kulwicki, 1992). Finally, in materials for insulators or substrates, all of the otherwise interesting phenomena must be eliminated. With the exception of the last category, all of the phenomena depicted in Figure 1 can be utilized in ceramic sensors. While sensing and actuating materials are currently a fraction of the overall multibillion-dollar electroceramics market, the percentage is expected to grow as more and more devices are made “smart.”As discussed in the introduction to this issue, smart materials are capable of sensing a change in the environment and responding in a useful way.