PTC Resistor Research Articles

Fabrication process for inexpensive, biocompatible and transparent PCBs. Application to a flow meter

The requirements of the Printed Circuit Boards (PCBs) are gradually more demanding due to the advances on the technology of many fields. Some of these requirements are biocompatibility and transparency for biological/biomedical applications. In this paper, the fabrication of inexpensive, biocompatible and transparent substrate for rapid prototyping is reported. This substrate is intended to be used as PCB because of their similarities. It is composed of polymethylmetacrylate (PMMA) as substrate, aluminum (Al) as conductive material and a light cure acrylic biocompatible adhesive to perform the bonding between the aluminum and the substrate. In addition, a solder mask layer of the same adhesive is performed to cover the aluminum tracks. The process is easy to perform and inexpensive if taking into account the used materials and the facilities. In this respect, a conventional UV light source is needed. Taking into account these requirements, a rapid prototyping of a PCB-based flow meter is developed as an example of application. This flow meter is fabricated using the proposed PCB substrate. This device is intended to be integrated on transparent and rapid prototyping lab on chip devices for biomedical applications. This integration implies the compatibility of this flow meter with optical sensing of biological samples. The flow sensor is composed of a microheater with a range of temperature from room temperature up to 100°C. The fabricated microheater is used as PTC resistor to develop the highly integrated flow sensor. The proposed device has a sensitivity of 450μV/(μL/min) and a normalized sensitivity of 450ppm/(μL/min) for a driven current of 0.6A. The range of flow rate of the fabricated device lies between 0 and 450μL/min.

Heat development at the microscale in ceramic components - thermography and complementary tools

In many ceramic components for electric applications, small defects in the microstructure may cause an anomalous current flow. Especially in components exhibiting nonlinear electro-thermal properties, such as varistors, PTC or NTC resistors, inhomogeneities lead to high concentrations of current and localized heat generation due to positive feedback effects. While the integral behavior of those components is often nearly not affected by these defects, the local load at these hot spot regions is considerably high and can even lead to the complete failure of the device. In this work micro-analytical tools are combined to identify the location and type of defects which may cause leakage currents, overheat or other reasons for inhomogeneous current distributions in functional ceramic components. Lock-in thermography is used to localize dominating current paths. This is even possible if the heat sources are in the micrometer range or less. Additionally, micro-sectioning techniques (e.g. Focus Ion Beam) are utilized to expose such regions to investigate features of the microstructure and causes for the enhanced current flows. Examples are shown in PTC resistor components as well as in LTCC based modules.

A characterization of thick-film PTC resistors

Thick-film PTC resistors (5093, Du Pont, 1 kΩ/sq.) with a high, linear and positive TCR were fired at temperatures between 750 and 950 °C. The development of the resistors’ conductive phase and microstructure was investigated by X-ray powder-diffraction analysis and by scanning electron microscopy, respectively. Temperature coefficients of resistivity, sheet resistivities and noise indices were measured as a function of firing temperature. The 5093 resistor material is based on ruthenate, which decomposes during firing at temperatures over 800 °C into RuO 2. As the needle-like RuO 2 crystals form, the sheet resistivities decrease from very high values to a nominal resistivity of around 1 kΩ/sq. At firing temperatures higher than 850 °C the volume of the single-crystal RuO 2 grains increases and therefore their number in a given volume of thick-film PTC layer decreases. The network of “needles ” starts to break, leading to increased sheet resistivities and increased noise indices.

Polymer Nanocomposites and Their Applications

Recent developments ofpolymer nanocomposites were reviewed in terms Offobrlcotlon, mechonlcal ond barrier properties, thermal stobiiity, flame retardoncy and otherfundionol properties. PolymerlC/oy nonocomposltes can befabricated through In situ polymerlsatlon, solution blending, and melt compounding. The high asped rotio and huge surface area of clay layers are responsible for the enhanced properties at low clay loading. These nanacomposltes are being used in automotive ond packoglng applications. Other polymer nanocomposltes with electrical, magnetic or optical properties were also discussed briefly. Their applications include eledrochemical display devices, eledrical and magnetiC shields, wave absorption, eledronic packaging, PTC resistors, photonic devices, etc.

Physical Properties of (TiC/Polypropylene) Conductive Composites in Relation to Their Particle-filled Structures

Titanium carbide (TiC) ⁄ acid-modified polypropylene (m-PP) composite is one of the candidate as the PTC resistor stable in relatively high temperatures, which is utilized for a self-recoverable element protecting from over currents in many kinds of electrical circuits. In the present study, the m-PP-based composites containing various volume fraction (φ2) of TiC have been prepared; then, their viscoelastic characteristics were investigated in comparison with their electric conductivity as a function of temperature. Melting behaviors of the samples were also examined and discussed in terms of their morphological features. Alternatively, the changes in the storage moduli at room temperature with various φ2 values of the samples could be expressed by a modified Kerner equation with the maximum φ2 value of 0.69. When the value of φ2 exceeded up to be 0.36, the resistivities of the sample decreased steeply with increasing φ2 values. The filling structure of TiC⁄m-PP Composites is expected to be in random packing systems.

Temperature sensors made by combinations of some standard thick film materials

Thick film resistors consist basically of a conductor phase, a glass phase and an organic vehicle, which burns out during high temperature processing. In most contemporary resistor compositions the conductive phase is either RuO2 or ruthenates. In addition, some other oxides are added either as temperature coefficient of resistivity (TCR) modifiers or modifiers of the temperature coefficient of expansion (TEC) of glass phase [1–3]. Most thick film resistor materials are designed for printing and firing on alumina substrates. The main requirements for their characteristics are stability, relatively narrow tolerances of sheet resistivities, and a low temperature coefficient of resistivity (TCR). During firing the resistors are at the highest temperature (typically 850◦C) a relatively short time (typically 10 min). All the constituents of the resistor material react with each other, with the conductor termination and also with the substrate [4–6]. Due to the short time at firing temperature the reactions do not reach equilibrium so that the characteristics of fired materials are, to an extent, a compromise as the consequence of this frozen unequilibrium. If thick film resistors, intended for firing on alumina substrates, interact with other materials, for example with multilayer dielectrics or NTC materials (thick film resistors with high negative and nonlinear TCRs) this could influence the resistors’ characteristics, in most cases for the worse. This is probably due to the interaction between different materials although some authors ascribe this to a different TEC [6–9]. As already mentioned the TCR of thick film resistors is aimed to be as low as possible. However, some combinations of thick film materials have been reported to result in characteristics, which allow them to be used for some temperature sensor applications (7, 10, 11). To make a useful sensor the dependence of resistivity vs. temperature ought to be high enough (high TCR) and reasonably linear. Some combinations of resistor materials and multilayer dielectrics or NTC resistors resulted in these desired characteristics. This indicates a possible use of standard thick film pastes for making fairly useful and inexpensive temperature sensors. Note, however, that data reported in this letter are not the result of a systematic search for such combinations and that the mechanisms responsible for such “behavior” were not investigated. The thick film materials, which were used in different combinations, are listed in Table I. The nominal sheet resistivities of the resistors and NTC thermistors are 1 and 10 kohm/sq. For comparison, the PTC resistor (high positive linear dependence of resistivity vs. temperature) and the platinum based conductor are also included, but note that these two materials were not fired in combination with other materials. Thick film resistors (sheet resistivities 1 kohm/sq. and 10 kohm/sq.) are part of the Du Pont HS-80 resistor series. However, the materials denoted 80× 1 and 80× 9 (for example, 1 kohm/sq. resistors 8031 and 8029) differ in the conductive phase used as well as in the fired resistor microstructure. In order to observe the microstructure the fired resistors were mounted in epoxy in a cross-sectional orientation and then polished using standard metallographic techniques. Prior to analysis in the scanning electron microscope (SEM), the sample was coated with carbon to provide electrical conductivity and to avoid charging effects. A JEOL JSM 5800 SEM equipped with an energy dispersive X-ray analyser (EDS) was used for overall microstructural and compositional analysis.

On the microwave sintering technique applied for enhancing the properties of PTC resistors and zno varistors

The benefit of using microwave (or millimeter-wave) as sintering source is demonstrated on the ZnO varistors and (Pb0.6Sr0.4)TiO3 PTC resistors. For ZnO materials, large nonlinearity (α =43.0) in accompanied with low leakage current density (J1=5.8 × 10−6 A/cm2) has been achieved by millimeter-wave (24 GHz) sintering at 1100°C (with 120°C/min heating rate). Such kind of marvelous varistor characteristics can not be reached for the samples, which were either microwave (2.45 GHz) sintered or conventionally sintered. For (Sr0.4Pb0.6) TiO3 SPT materials, the sintering temperature needed in microwave sintering process is generally ∼150°C lower than and soaking time required is generally 80% shorter than those commonly used in the conventional sintering process. Unique single high-Tc characteristics in p-T curves were obtained for microwave sintered SPT materials. It is attributed to the formation of the “core-shell” microstructure.

Novel medium voltage fault current limiter based on polymer PTC resistors

For limitation and interruption of short circuit currents in medium voltage (MV) applications, fuses and circuit breakers are widely used today. Fuses exhibit excellent current limitation, but work only for a single shot. Circuit breakers are multi-operation devices, but allow several periods of the short circuit current to pass. Based on new PTC materials (positive temperature coefficient of resistance), we propose a new current limiting device for MV applications, which offers the chance to combine the advantages of fuses with the advantages of circuit breakers: a fast current limitation with a repetitive operation. The active material consists of conducting particles in a polymeric matrix. Its challenging property is a change on resistivity by eight to ten orders of magnitude upon temperature increase. This allows the limitation of available short circuit currents in a stable insulating state for a long period of time. The device consists of a series connection of PTC resistors with varistor elements connected in parallel, which serve for a perfect voltage distribution. Using this principle, repetitive current limitation of prospective currents of 4 to 14 kA within about 1 ms could be demonstrated at circuit voltages up to 12 kV.

Repetitive current limiter based on polymer PTC resistor

Until now, one shot devices as fuses have been used for the limitation and interruption of short-circuit currents. A repetitive current limiter (CL) has been developed using a polymer composite as the functional part of the device. The CL is able to limit and interrupt a fault current flowing through it by a fast increase of its resistance by eight to ten orders of magnitude. Short-circuit tests were carried out on an AC generator circuit with a prospective test current of 16 kA/sub RMS/, U=220 V/sub RMS/, which was limited to 3 kA in peak. Also further short-circuit tests were performed using a capacitor battery with variable prospective currents between 0.7 and 78 kA/sub RMS/. The limitation of the short-circuit current by the CL is caused by the thermal expansion of the matrix of the polymer composite, which leads to a forced separation of the conducting filler particles. This fast motion in the sub-ms range with amplitudes on a /spl mu/m-scale is monitored by using a fast laser distance sensor. The necessary mechanical expansion of the polymer composite for a limitation of the current to a negligible value is 40 /spl mu/m within 700 /spl mu/s.

Single‐Grain Boundaries in PTC Resistors

Thin semiconducting barium titanate ceramic bars consisting of single grains joined together in series have been prepared, and the positive temperature coefficient of resistivity (PTCR) characteristics of strictly single‐grain boundaries in the materials were investigated. The resistivity (R)‐temperature (T) characteristics obtained for the present samples can be classified into typically three categories: (1) normal type PTCR characteristics, similar to those observed in usual ceramic samples, (2) saw‐tooth type PTCR characteristics, characterized by an abrupt increase in resistivity by more than three orders of magnitude at the Curie point, immediately follwoed by a monotonous decrease in it, and (3) flat type R–T characteristics, with substantially little or no resistivity jump. Of these R–T characteristics, normal type PTCR characteristics were the most frequently observed (about 60%; a total of 65 samples were examined). Flat type R–T characteristics were least frequently (about 10%) observed. Single boundaries with these three types of PTCR characteristics exhibited essentially the same ferroelectric capacitance–temperature characteristic; this demonstrates that the temperature dependence of the dielectric constant above the Curie point was not responsible for the PTCR anomalies. Single boundaries with normal and saw‐tooth type PTCR characteristics showed significantly nonlinear current‐voltage characteristics above the Curie point, which may be interpreted to be caused by a current strongly affected by traps (or surface acceptor states) present at the grain boundaries.

ON THE CONTINUOUS TRANSITION FROM PTC EFFECT TO GBBL CAPACITOR FOR BaTiO3 SEMICONDUCTING CERAMICS——APPLICATION OF THE GRAIN BOUNDARY BARRIER MODEL

The quantitative analyses for our proposed potential-barrier model are given in this paper. The results quantitatively explain how the semiconductiong BaTiO3 ceramics transforms from PTC effect to GBBL capacitor. The designs of PTC resistors and GBBL capacitors from this model are made, and the design properties are much better than that of present experimental data. The mathods improving the properties of relevant devices are discussed. The model provides a theoretical basis for the design, manufacture and property improvement of semiconducting BaTiO3 ceramic devices.

Influence of Potassium on Preparation and Performance of PTC Resistors

Different amounts of K2CO3 were added to (Ba,Sr)TiO3‐based PTCR (positive temperature coefficient of resistance) ceramics to investigate their influence on the microstructural and electrical properties. Experimental results showed that the incorporation of K acted as an A‐site acceptor‐type dopant. In addition to enhancing discontinuous grain growth, the increase of K2CO3 was found to raise the room‐temperature resistivity which was dominated by grain‐boundary resistance rather than grain resistance. By adjusting to a suitable amount of donor dopant, the inherent contamination of K in raw material can be compensated to achieve a high‐quality PTC resistor.

Analysis of combustible gases in air with calorimetric gas sensors based on semiconducting BaTiO 3 ceramics

This paper presents a novel method to analyse a mixture of combustible gases in air by using electrically self-heated ceramic PTC resistors of semiconducting BaTiO 3 as microcalorimeters. The temperature dependence of the heat of a heterogeneous catalytic oxidation exhibits a transition state from the kinetic to the diffusion-controlled region at a certain temperature, which is characteristic for the combustible gas and the catalyst used. This characteristic temperature dependence of the heat of reaction can be used to identify one combustible gas in air. Moreover, the variation of the temperature dependence with the composition of a mixture of two combustible gases in air offers the possibility of determining the particular concentrations of the two gases by using PTC-microcalorimeters in an isothermal mode of operation.

Complex-plane impedance analysis for semiconducting barium titanate

Antimony doped semiconducting barium titanate used as PTC resistor has been studied by complex-plane impedance analysis. An equivalent circuit for the transport process has been proposed and the circuit parameters have been determined. The effect of electrode materials on the PTC effect has also been investigated. The variation of grain boundary resistance with cooling rate is clearly depicted by the impedance analysis.

Application principles and operating limitations of PTC resistors

The abrupt resistance increase of semicond ucting barium titante, together with its relatively easy manufacturability, permit some novel applications as a thermal power device. Some typical application modes and their limitations are discussed.

Anomalous temperature resistance characteristic of highly acceptor doped BaTiO3PTC resistors

Abstract The resistance increase above the Curie temperature in n-doped BaTiO3 ceramics is due to blocking layers. These layers are due to acceptor states at grain boundaries. When increasing the acceptor concentration an additional resistance maximum occurs below the Curie temperature, which finally prevails the original resistance increase. This maximum can be explained by the normal theory if the nonlinearity of the dielectric properties inside the blocking layers is taken into account.

Degaussing Circuits for Color Television Receivers

In 1967 the PTC resistor was introduced to the American TV industry for color television degaussing. This new component caused comparisons of performance and economics to be made with existing techniques. The PTC degausser offered sufficient advantages so that by 1971 it was used in 50% of color televisions made in this country. After a general review of degaussing this paper explains the circuitry, advantages, anddisa disadvantages of various degaussing techniques.

Thermistors as Circuit Elements in Low-Frequency Circuits

The properties of a polarizing NTC, as well as a PTC resistor, that arise from the fact that thermistors act as a negative resistance to low frequency signals, have been investigated and the impedances were deduced both theoretically and by experiment. The properties of the thermistors as a conductor and a capacitor were investigated and the conditions for oscillating at low frequencies were given. In addition, a phase-advance network and an integrating network were proposed.

Indirectly Heated PTC Resistor for Measuring Air Flow

An indirectly heated PTC resistor is described which was developed for sensitive measurements of low pressures and of high air velocities. The basic circuit which has provided high sensitivity in such measurements and its applićation to the determination of parameters of an electrical analogue for heat exchangers is discussed in the second part of the paper.

Indirectly Heated PTC Resistor for Measuring Air Flow

An indirectly heated PTC resistor is described which was developed for sensitive measurements of low pressures and of high air velocities. The basic circuit which has provided high sensitivity in such measurements is discussed in the last part of the paper.