Positive Temperature Coefficient Thermistors Research Articles

Fabrication of Lead-Free Semiconducting Ceramics Using BaTiO<sub>3</sub>-(Bi<sub>1/2</sub>Na<sub>1/2</sub>)TiO<sub>3</sub> System by Sintering in Air Atmosphere

BaTiO3–(Bi1/2Na1/2)TiO3(abbreviated as BT-BNT) solid solution ceramics, as a lead-free PTC (Positive Temperature Coefficient of resistivity) thermistor material usable over 130°C, were synthesized in air atmosphere. By conventional processes, La-doping was not effective in giving semiconducting property to the BT-BNT ceramics because of volatilization of sodium and bismuth during sintering. By preventing the volatilization by lowering the sintering temperature using very fine TiO2powder and by adding calcium oxide to calcined BT-BNT powders, we obtained BT-BNT semiconducting ceramics which shows a resistivity change near Curie temperature.

Characterization of Grain Boundaries of Lead-Free Semiconducting Ceramics Using BaTiO<sub>3</sub>-(Bi<sub>1/2</sub>K<sub>1/2</sub>)TiO<sub>3</sub> System

BaTiO3–(Bi1/2K1/2)TiO3 (abbreviated as BT-BKT) solid solution ceramics, as a lead-free PTC (positive temperature coefficient of resistivity) thermistor material usable over 130°C, has been synthesized by sintering in N2 atmosphere and after annealing in air over 1200°C. In the BT-BKT ceramics with PTC property, the impedance/modulus spectroscopic plots have revealed that a third resistance-capacitance (RC) response besides grains and grain boundaries. 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 in the BT-BKT semiconducting ceramics.

Ti/NiCu/Ag and Al/NiCu/Ag thin film deposited on positive temperature coefficient (PTC) thermistor by direct current magnetron sputtering

Metallization of positive temperature coefficient (PTC) thermistors deposited by direct current magnetron sputtering has been investigated. Two kinds of multilayer thin films, namely Ti/NiCu/Ag and Al/NiCu/Ag, are proposed as electrode materials. Conventional NiCr/Ag thin film serves as reference electrode in the experiment. Experimental results show that the newly proposed electrodes exhibit good performance including low ohmic contact, firm adhesion, fine solderability and stability. The surface resistances at room temperature of Ti/NiCu/Ag and Al/NiCu/Ag PTC range from 4.3 to 6.2 Ω, and they change little in moisture condition. The average tensile strength is 6.3 Mpa, which provides firm adhesion between electrode and substrate. At solderability test, the new electrodes can withstand the soakage of lead-free solder at 350 °C for 5 s and the properties remain stable after high temperature treatment.

Integrated current–time characteristic measurement system for multichannel positive temperature coefficient thermistors

The current–time characteristic plays a crucial role in assessing the actual working performance of a positive temperature coefficient (PTC) thermistor. However, the existing measurement instruments have serious limitations and hardly adapt to the actual production. This paper presents a state-of-the-art measurement system for the current–time characteristic of the PTC thermistor. Multiple updated measurement methods, including pulsed dry circuit testing, RMS digital computation, oversampling and decimation, are proposed to design this system. Several advanced techniques, specifically CAN bus, mixed signal SoC and VI, are introduced to implement this system. As a result, high accuracy class 0.1 is achieved. The current dynamic range is extended to 104. The efficiency is increased by more than 12 times. In short, this highly integrated and cost-effective system reaches commercial grade for actual production. The practical application demonstrates that this system is quite suitable for batch production.

Fabrication of lead-free semiconducting ceramics using a BaTiO3-(Bi1/2Na1/2)TiO3 system by adding CaO

BaTiO3–(Bi1/2Na1/2)TiO3 (abbreviated as BT–BNT) solid solution semiconducting ceramics, as a candidate for a lead-free PTC (Positive Temperature Coefficient of resistivity) thermistor usable over 130°C, were synthesized by adding CaO into the calcined powder. The CaO added BT–BNT ceramics showed semiconducting behavior with the resistivity at room temperature ρRT = 103 Ω cm and a resistivity change near Curie temperature. Chemical composition analysis suggested that Ca2+ acted as a donor by substituting for Na+ in the BT–BNT ceramics.

ChemInform Abstract: Effects of Yttrium Doping on the Grain and Grain-Boundary Resistivities of BaTiO3 for Positive Temperature Coefficient (PTC) Thermistors.

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Microstructure and electrical properties of lead free PTCR ceramics

Lead free positive temperature coefficient of resistivity (PTCR) ceramics based on BaTiO3–(Bi1/2Na1/2)TiO3 (BT–BNT) solid solution were prepared by a conventional solid state reaction method using high purity reagents. BaTiO3 based positive temperature coefficient thermistors doped with BNT showed typical positive temperature coefficient effect. As the elements for achieving semiconducting grains, at a low dopant concentration the grain size is influenced significantly by the donor concentration because of a significant dopant drag on the boundary mobility. Temperature dependence of electrical resistivity was measured and a typical PTCR effect was confirmed in the solid solutions up to 3·0 mol.-% addition of BNT. Especially 1·0 mol.-% BNT added sample indicated superior PTCR properties by optimising the preparation conditions. At high BNT content (≥0·01), the room temperature resistivity increased again with increasing BNT content. A maximum of ρmax/ρmin ratio was obtained at the sintering temperature equal to 1280°C at a given content of BNT. Electrical properties and crystal structure were investigated in order to elucidate the origin of the PTCR effect in this system. The differential scanning calorimetry spectra of this system indicated the transition to cubic from tetragonal crystal structure and the transition temperature increased up to ∼ 170°C with the addition of BNT. Also, the microstructure was observed by SEM technique. In the present study, it was found that the lead free PTCR ceramics in BT–BNT system has extremely similar properties to the conventional lead contained one. Research and development in the field of electroceramics is driven by technology and device applications.

New non-linear electrical-thermal switching of carbon nanoparticles/silane coupling agent reinforced phenolic resin nanocomposites

This paper evaluates the use of a new multifunctional conducting polymer containing phenolic resin reinforced by carbon black (CB) nanoparticles modified by silane coupling agent to produce positive temperature coefficient (PTC) thermistors and switching current. The percolation threshold of the conducting composites at room temperature was found to be as low as 4 wt% of CB. Temperature dependent electrical characterisation of phenolic resin/CB nanocomposites is performed. The composites exhibit PTC properties with ρmax/ρmin value as great as 104. Electrical parameters such as charge carriers type, drift mobility, concentration of charge carriers, activation and hopping energy are verified with CB content. The current-voltage curves of the nanocomposites change from linear to non-linear behaviour and the switching current is observed within the non-linear regime.

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.

Modeling the effect of SiO2 additions and cooling rate on the electrical behavior of donor-acceptor codoped positive temperature coefficient thermistors

The resistivity-temperature [ρ(T)] characteristics of two series of (Ba,Ca)TiO3 based positive temperature coefficient (PTC) of resistance thermistors, prepared with different levels of SiO2 addition and cooling rates, have been analyzed in the context of the double Schottky barrier model. A significant improvement in fit, compared to earlier studies, was achieved between the modeled and experimental ρ(T) behavior over the full temperature range of the PTC transition through the incorporation of experimentally obtained permittivity data, coupled with an optimized acceptor energy spread (0.40±0.05eV). The permittivity below the Curie temperature (TC) increased slowly when either the SiO2 content or cooling rate was increased, but above TC the permittivities were almost unaffected. The bulk resistivity and average grain size were affected only slightly by increases in SiO2 addition or cooling rate, but the gradient of the ρ(T) curve and peak resistivity gradually decreased due to a reduction in surface state density from 1.1×1014to5.9×1013cm−2 at an acceptor energy depth of 1.57–1.60eV. A reduction in room temperature resistivity with increasing SiO2 content and cooling rate was consistent with a concomitant reduction in the ratio of residual surface charge unit density to permittivity.

Modeling the resistance-temperature characteristic of a positive temperature coefficient thermistor, using experimentally determined permittivity data

The resistivity-temperature [ρ(T)] and current-voltage [j(V)] characteristics of a (Ba,Ca)TiO3 based donor/acceptor codoped positive temperature coefficient of resistivity thermistor have been analyzed in the context of the double Schottky barrier (DSB) model, using experimentally measured permittivity data. The best fit between the modeled and experimental ρ(T) behavior occurred for a broad acceptor energy fluctuation width (Δ=0.35eV). Modeling the ρ(T) behavior below the Curie temperature required a nonvanishing surface charge, consistent with a residual potential barrier. j(V) characteristics showed satisfactory agreement with the DSB model at low applied voltages per grain boundary (Vgb⩽0.13–0.22eV).

Theoretical Aspects of PTC Thermistors

The discovery of ferroelectric barium titanate (BaTiO₃) in 1942 began the present era of dielectrics-based electronic ceramics. Ferroelectric barium titanate has a high dielectric constant and after the recognition of BaTiO₃ as a new ferroelectric compound, various attractive electrical properties have been extensively studied and reported. Since then, pioneering work on valence-compensated semiconduction led to the discovery of the positive temperature coefficient (PTC) of the resistance effect found in doped BaTiO₃. Significant progress has since followed with respect to understanding the PTC phenomena, advancing materials capabilities, and developing devices for sensor and switching applications. In this paper, the theoretical aspects of the various PTC models are discussed and the future trends of practical applications for PTC devices are briefly mentioned.

A heat balance integral solution of the thermistor problem with a modified electrical conductivity

A key driver of thermistor response is a thermally-dependent electrical conductivity. Several variants have been proposed in the literature and it is the purpose of this paper to explore the applicability of a bulk model by way of using the heat balance integral approach to predict the response of a positive temperature coefficient thermistor. Conclusions are presented regarding the sensitivity of the response to electrical conductivity.

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.

Microstructure and Electrical Properties of Microwave-Sintered PTC Thermistors

BaTiO3—based positive temperature coefficient (PTC) thermistors have been prepared by microwave sintering. The sintering process was carried out in a 2.45 GHz multimode microwave tube furnace without any susceptor. The heating behavior of the PTC material, influence of processing conditions on microstructure, and electrical properties of the sintered samples were investigated. The sintering temperature range varied from 1275 °C to 1350 °C. The optimum density and microstructure were achieved between 1300 °C to 1325 °C. Samples were also microwave sintered at 1300 °C for various soaking times from 10 min to 90 min. It was found that soaking between 30 min to 60 min generated a uniform microstructure. Cooling rate varied from 7.5 °C/min to 2 °C/min. The study showed that the cooling rate has an influence on PTC effect but not on microstructure. Slower cooling rate is essential for achieving better PTC effect. Compared to conventional method, the microwave-processed samples exhibited higher density and better PTC effect. The relative density of microwave sample was 93.5%, compared to 90.2% of the conventional sample. The R25, αp ß, Tc, and Rmax of the microwave sintered samples are 17.6 Ω, 15.8, 6.7 × 105, 88.0 °C, and 11.5 MΩ, respectively, compared to 12.1 Ω, 14.6, 1.1 × 105, 88.1 °C, and 1.3 MΩ, respectively, for the conventional sample.

Finite element approaches to the PTC thermistor problem

In this paper, subdomain collocation (SC) and Petrov–Galerkin (PG) methods based on spline finite elements have been applied to the one-dimensional positive temperature coefficient (PTC) thermistor problem with a temperature dependent ramp function electrical conductivity to obtain the predicted temperature distributions and the locations of the free boundaries in the steady-state case. The numerical results obtained by the present methods have been compared with the exact solution and also those obtained by earlier authors. A Fourier stability analysis of each method is investigated.

Numerical solutions of the thermistor problem by spline finite elements

This paper presents approximate steady-state solutions of a one-dimensional positive temperature coefficient (PTC) thermistor problem, having a step function electrical conductivity that is a highly non-linear function of the temperature, using subdomain collocation and Petrov–Galerkin methods based on spline finite elements. The resulting system of ordinary differential equations is solved by the usual Crank–Nicolson finite difference method using a variant of Thomas algorithm. It is shown that the numerical solutions obtained by the present methods exhibit the correct physical characteristics of the problem and, they are in very good agreement with the exact solution. A Fourier stability analysis of each method is also investigated.

Finite element solution of the thermistor problem with a ramp electrical conductivity

This paper presents approximate steady-state solutions of a one-dimensional positive temperature coefficient thermistor problem with a ramp function electrical conductivity using the Galerkin cubic B-spline finite element method. The numerical results obtained by the present method have been compared with the exact one and also those obtained by earlier authors, and are found to be in very good agreement with each other. Further, it is shown that the numerical solutions satisfy the correct physical phenomena of the problem.

Direct measurement of electrical and compositional inhomogeneities in PTC thermistors

Positive temperature coefficient (PTC) thermistors, based on donor doped barium titanate, show a large, reproducible increase in grain boundary resistance over a small temperature interval just above the Curie temperature ( T C). T C can be increased by additions of lead, and reduced by adding strontium. In recent times thermistors have been prepared containing additions of both lead and strontium along with other dopants to modify device performance. However, the distribution of these dopants is rarely homogeneous, giving rise to local differences in the functional properties of these devices, which affect the overall behaviour. Electron microprobe analysis of a commercial thermistor disc has revealed a depletion of lead, together with calcium and strontium enrichment in a zone ∼300 μm from the surface of the sintered pellet concurrent with a small decrease in grain size. Local resistance-temperature ( R– T) measurements have also demonstrated that T C is decreased and the pre-switching resistance increased in the near surface region.

A B-spline finite element method for the thermistor problem with the modified electrical conductivity

In this paper, approximate steady-state solutions of a one-dimensional positive temperature coefficient thermistor problem with a modified step function electrical conductivity are obtained by using the Galerkin cubic B-spline finite element method. It is shown that the computational results obtained by the method display the correct physical characteristics of the problem, and they are found to be in very good agreement with the exact solution. It is also shown that the numerical solution exhibits the expected convergence to the exact one as the mesh size is refined. Further a Fourier stability analysis of the method is investigated.