High Temperature Thermistor Research Articles

Microstructural and electrical properties of La0.9B0.1AlO3 (B=Pr, Nd, Sm) ceramics obtained by rare-earth ion doping

This study used the solid-phase method to prepare thermosensitive perovskite ceramics, La0.9B0.1AlO3 (B = Pr, Nd, Sm). The XRD and EDS results indicated that all components presented a single phase and all elements were uniformly distributed, formed a good solid solution. The SEM results clearly illustrated that the doped elements had enhanced the surface morphology, resulting in more uniform grain formation. The TEM results showed that linear defects were present, which led to an uneven stress distribution; consequently, this enhanced the carrier transport. The alternating current (AC) impedance showed that the grain boundary resistance determined the overall complex impedance and exhibited a different conductive mechanism with increasing temperature, from polariton to ion conduction. With the introduction of Nd3+, the conductivity of La0.9Nd0.1AlO3 increased and its temperature range increased from 600 – 1400 °C to 100–1500 °C. In La0.9Sm0.1AlO3, the presence of Sm2+ enhances the aging stability by elevating the oxygen vacancy concentration and the aging drift rate (ΔR/R0) was found to be lower than 2 %. In addition, DFT-based calculations were performed to investigate the property changes in the crystal structure and energy band structure of pure and doped LaAlO3. This study demonstrated an effective way to improve the performance of high-temperature thermistor materials using different doping strategies.

Effect of heat treatment on performance of high-temperature thermistor LaCrO3

Effect of heat treatment on performance of high-temperature thermistor LaCrO3

Temperature-Dependent electrical and dielectric characteristics of lead germanate Pb5Ge1.5Sn1.5O11

A solid-state reaction was used to synthesize the tin modified lead germanate: Pb5Ge1.5Sn1.5O11 (PGSO). The composition was modified to enhance its dielectric and electrical properties. The PGSO sample was characterized by various analytical techniques, such as XRD for structural analysis and FTIR for additional structural information. The surface morphology and features of PGSO were examined by SEM, while its band gap was determined by UV–Visible-Diffuse Reflectance Spectroscopy absorbance spectra. The impedance, dielectric behaviour, AC conductivity, activation energy, and modulus of PGSO were also studied. The temperature dependence of the dielectric constant and AC conductivity showed an increase with increasing temperature. The electrical resistivity (R) decreased as the temperature increased from 100 °C to 300 °C, exhibiting a NTCR behaviour for PGSO sample, which could be applied as a high temperature thermistor and sensor, and could be used to fabricate a high sensitivity, low-cost thermistor. The circuit parameters such as bulk resistance (Rb), frequency factor (n), and bulk capacitance (Cb) were determined by fitting the Nyquist plot using an equivalent circuit (CQR)(CR).

High-entropy chromate (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)CrO3 for high-temperature NTC thermistors

This paper presents a novel rare earth-based chromate (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)CrO3 for high-temperature thermistor applications. X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy tests reveal that the material is a high-entropy ceramic with a perovskite structure, exhibiting a dense microstructure with a relative density of 97.6 %. Electrical analysis shows that (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)CrO3 exhibits high resistivity at high temperatures but with a material constant of 1,968 K, making it capable of meeting the resistance requirements for commercial thermistors over an ultrawide temperature range of 25 – 1,300 °C. AIMD simulations indicate the excellent structural stability of (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)CrO3 even at 1,300 °C, suggesting promising high-temperature aging characteristics. Further aging tests demonstrate that its resistance only drifted by 4.57 % after being subjected to 1,300 °C for 1,000 h. The above findings represent breakthroughs in terms of maximum operating temperature, applicable temperature range, aging performance and sintering densification for chromate-based high-temperature thermistor ceramics.

Enhanced Thermal Stability and Broad Temperature Range in High-Entropy (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)NbO4 Ceramics.

Next-generation high-temperature applications increasingly rely heavily on advanced thermistor materials with enhanced thermal stability and electrical performance. However, thus far, the great challenge of realizing high thermal stability and precision in a wide temperature range has become a key bottleneck restricting the high-temperature application. Here, we propose a high-entropy strategy to design novel high-temperature thermistor ceramics (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)NbO4. Differences in atomic size, mass, and electronegativity in this high-entropy system cause high lattice distortion, substantial grain boundaries, and high dislocation density. These enhance the charge carrier transport and reduce the grain boundary resistance, thus synergistically broadening the temperature range. Our samples maintain high precision and thermal stability over a wide temperature range from room temperature to 1523 K (ΔT = 1250 K) with an aging value as low as 0.42% after 1000 h at 1173 K, showing breakthrough progress in high-temperature thermistor ceramics. This study establishes an effective approach to enhancing the performance of high-temperature thermistor materials through high-entropy strategies.

Widening the temperature range and improving the linearity of CaCu3Ti4O12 ceramics for high-temperature NTC thermistor application through entropy engineering strategy

The entropy engineering strategy has been used for the first time to improve the grain boundary (GB) barrier of CaCu3Ti4O12 (CCTO) ceramics, in order to solve its nonlinear problem at around 300 ℃ and expand its application in the field of high-temperature thermistors. To achieve this goal, we prepared (CaBaEuZnSr)0.2Cu3Ti4O12 (CBEZSCT) high-entropy ceramics, and the GB spontaneous segregation properties were exploited to improve the GB resistance of CBEZSCT ceramics. The results show that the GB activation energy of CBEZSCT high-entropy ceramics (0.87 eV) is higher than that of CCTO ceramics (0.7 eV), which linearizes the resistance temperature curve and extends the application temperature limit from 300° to 750 °C. The proposed method of using a high entropy strategy to tune the GB resistance of CCTO to optimize its performance is highly effective, thus providing a new way of thinking about studying high-performance CCTO materials.

A novel LaAl1-xNbxO3 NTC ceramic with excellent stability for high-temperature thermistor

A series of LaAl1-xNbxO3 (0 ≤ x ≤ 0.100) samples were synthesized via the solid phase method. Characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) confirmed the formation of a solid solution in LaAl1-xNbxO3 (0 ≤ x ≤ 0.100) materials without the emergence of additional phases following Nb doping. Transmission electron microscopy (TEM) revealed distinct lattice fringes with a d value of 0.274 nm, corresponding to the (110) plane of LaAlO3 (PDF #82–0478). The introduction of Nb dopant caused distortions in the cell parameters of LaAlO3 due to ionic radius size effects, changes in Nb valence, fluctuations in oxygen vacancy concentration and Coulomb forces. Specifically, the lattice exhibited a shrinkage for 0 ≤ x ≤ 0.025, expansion for 0.025 <x ≤ 0.075, and subsequent shrinkage for 0.075 <x ≤ 0.100. Resistance measurements indicated that LaAl1-xNbxO3 (0 ≤ x ≤ 0.100) materials exhibited favorable negative temperature coefficient (NTC) characteristics in the temperature range of 600 °C–1400 °C. Hall measurements revealed that the materials functioned as P-type semiconductors with hole carriers, and Nb doping enhanced carrier concentration. Aging tests demonstrated that the aging drift rate of LaAl0.9Nb0.1O3 could be as low as 0.906% after aging it at 1000 °C for 500 h, a crucial parameter for practical applications of LaAl1-xNbxO3 (0 ≤ x ≤ 0.100) materials in high-temperature NTC applications.

New high temperature negative temperature coefficient thermistors in Mg0.5Mn0.5AlFeO4 ceramics

For high temperature thermistor applications, the MgAl2O4 exhibits poor conductivity and requires high sintering temperature for ceramic densification. In this study, we have reported on the preparation and characterization of spinel Mg0.5Mn0.5AlFeO4 ceramics by solid solution modification. The resultant ceramics exhibit a combination of high temperature stability and semi-conductivity, which are the characteristics of MgAl2O4 and MnFe2O4. The material has a moderate thermistor constant, which is in favor of application at high temperatures and a wide temperature range. The electrical conductivity is due to the small-polaron hopping in different valence states of manganese and iron ions. Our findings demonstrate a promising approach to extend the applicability of spinel ceramics for high temperature thermistors.

New linear high-temperature thermistors based on the xAl2O3-(1-x)CaMnO3 ceramics

It is very important for a thermistor to have linear electrical properties, which can effectively reduce the complexity of circuit design. Phase structure modulation through introducing a second phase is crucial to achieve high performance electronic ceramics. Herein, new linear high-temperature thermistors based on the xAl2O3-(1-x)CaMnO3 ceramics were synthesized by the solid phase reaction method. With increasing Al2O3 content, CaMn2O4 and CaAl2O4 phase were observed, which formed internal conducting network and induced linear electrical properties. The ceramics exhibited negative temperature coefficient (NTC) thermistor characteristics in the temperature range of 523 K–1173 K. Their resistance-temperature (R-T) relationship displayed a good linear characteristic, especially for x = 0.3, which showed the highest linearity (R2: 0.9984) in the temperature range of 523 K–1173 K, covering medium to high temperature region. The mechanism of linear electrical properties was explained by a parallel conductive model in combination with complex impedance analysis. This approach could be generalized for designing linear high-temperature thermistors with a wide temperature range.

CaMnxZr(1-x)O3: A novel NTC thermo-sensitive ceramic for applications in a wide temperature range

The perovskite CaMnxZr(1-x)O3 (x = 0, 0.05, 0.10, 0.15) (CMZO) NTC (Negative Thermal Coefficient) thermo-sensitive ceramic were prepared by the solid phase method that can be used as high temperature thermistor materials in the range of 200–1200 °C.The XRD pattern showed that CMZO is a finite solid solution and that a second phase, CaMnO3, will be formed after exceeding the solid solution limit (i.e., at x = 0.1). The thermal constant B value was found to decrease from 19521 K to 9217–9535 K, with a corresponding decrease in activation energy, Ea, from 1.69 eV to 0.82 eV. The Nyquist plot indicates that the grain boundary resistance determines the overall complex impedance. Moreover, the aging drift rate (ΔR/R0) was found to be lower than 4.3%, which is an indication of an excellent high temperature stability of the CMZO ceramic material.

Enhanced linearity of CaCu3Ti4O12 by changing energy band structure induced by Fe3+ doping for high temperature thermistor application

Polycrystalline oxide materials exhibit semiconductor properties due to grain boundary (GB) and grain characteristics, which enrich the variety of applications. However, how to regulate the energy band structure of grains and the potential barriers at GBs through defect engineering is crucial to achieve a high performance electronic device. Herein, it is found that Fe3+ ions can change the grain energy band structure of CaCu3Ti4O12 (CCTO) materials, which enhances the linearization of the resistance–temperature curve (lnρ–1000/T) in the high temperature region. First principles calculation indicates that Fe3+ doping narrows the forbidden band and induces new impurity energy levels in the forbidden band, which matches the conclusion that the resistivity–temperature dependence of grains shifts toward the low-temperature region as derived from impedance spectroscopy. This shift results in no monotonic variation in grain resistivity within the application temperature region, thus enhancing the linearity of the lnρ–1000/T curve of CCTO materials in the high temperature region. In addition, Fe3+ ions can modulate the activation energy of CCTO materials in a wide range by changing the activation energy of GBs, which broadens the temperature range of CCTO. The significance of this work lies not only in achieving linearization of CCTO materials for high temperature thermistor application, but more importantly, the method presented here provides an avenue for the study of polycrystalline semiconductor materials.

Novel NTC ceramics based on pyrochlore-type A2Ti2O7 (A = Sm, Eu, Y) titanates for high-temperature thermistors

In this study, a series of novel negative temperature coefficient thermistor materials based on A2Ti2O7 (A = Sm, Eu, Y) titanate ceramics have been developed for high-temperature applications. The A2Ti2O7 (A = Sm, Eu, Y) ceramics showed cubic pyrochlore structures with dense morphology in X-ray diffraction and scanning electron microscopic analyses. These ceramics showed typical NTC characteristics in a wide temperature range of 673–1473 K, accompanied by resistance drift rates of 2.71%, 1.63%, and 4.95% for 400 h of ageing at 1173 K. The proposed ceramics have high resistivity and thermal constants B that ensure their better sensitivity at high temperatures and also show unique oxygen insensitivity. These properties outperform those of traditional high-temperature NTC materials composed of perovskite-type or scheelite-type ceramics, making A2Ti2O7 (A = Sm, Eu, Y) ceramics promising candidates for fabricating high-temperature NTC thermistors.

A novel NTC ceramic based on La2Zr2O7 for high-temperature thermistor

A novel negative temperature coefficient material based on lanthanum zirconate ceramics was proposed for high-temperature applications. This material was synthesized through a solid-state reaction by sintering at 1923 K for 10 h in air. The X-ray diffraction and scanning electron microscopy results confirmed that La2Zr2O7 ceramics exhibited a pyrochlore phase with a relative density of 98.2 %. The resistance–temperature characteristics of the material revealed that La2Zr2O7 ceramics exhibited an NTC feature within the broad temperature range of 973–1773 K in addition to maintaining high thermal constant B, and resistivity to ensure good sensitivity at high temperatures. These properties, along with high ceiling temperature, unique oxygen insensitivity, and excellent ageing coefficient of <0.7 % at 1773 K, render La2Zr2O7 ceramics a promising candidate as thermistor materials with high-temperature NTC.

Resistance-temperature characteristics of a new high-temperature thermistor ceramics of Mn-doping Ba–Ca–Zr–Ti–O system

Resistance-temperature characteristics of a new high-temperature thermistor ceramics of Mn-doping Ba–Ca–Zr–Ti–O system

New high temperature NTC thermistors based on BaSm2Ti4O12 ceramics

In our research it is firstly found that BaSm2Ti4O12 (BSTO) ceramics only show negative temperature coefficient (NTC) effect in wide temperature range. The effects of sintering temperature on the microstructure and electrical properties of BSTO have been investigated. The relative density of as-sintered ceramics increases at first and then decreases with increasing sintering temperature. In the temperature of 400-850℃, all samples show a linear relationship between the natural logarithm of resistivity and the reciprocal of temperature, indicating the NTC characteristics. The electrical resistivity decreases with increasing sintering temperatures. However, the activation energy Ea400/850℃ and material constant B400/850℃ decrease first and then increase, which are attributed to the variation of oxygen vacancy. The value of resistivity ρ400℃, Ea400/850℃ and B400/850℃ is in the range of 4.8×107–1.43×108Ω•cm, 1.33–1.41 eV and 15300–16400 K, respectively. This material has broad prospects in the field of high-sensitivity high-temperature thermistors research.

SrCexNbxWO4+4x: A novel NTC ceramic for high-temperature thermistor with a wide temperature range

A series of novel negative temperature coefficient thermistor materials based on SrCexNbxWO4+4x (0.1 ≤ x ≤ 0.8) ceramics was synthesized by the solid-state route. X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy revealed that the SrCexNbxWO4+4x ceramics were composed of scheelite structural solid solution based on the SrWO4 phase and a CeNbO4.08 secondary phase that exhibited a fergusonite structure. The resistance-temperature analysis demonstrated that the resistivity ρ800 of the SrCexNbxWO4+4x ceramics decreased from 8.25 ± 0.08 × 106 to 2.52 ± 0.11 × 102 with the increase of x value, along with a decrease in the thermal constant B from 11,102 ± 97–4137 ± 37 K. It was observed that the SrCexNbxWO4+4x ceramics exhibited high resistivity and excellent aging characteristics as well as suitable B values at elevated temperatures, making it a promising candidate that can be used for the fabrication of high-temperature NTC thermistors with a wide operating temperature range.

New negative temperature coefficient ceramics in Ca0.9Y0.1MoO4–CeNbO4 system

The Ca0.9Y0.1CeNbMoO8 compounds are the solid solutions of Ca0.9Y0.1MoO4 and CeNbO4, and have a potential in the high temperature thermistor application. A concept has been proposed to adjust the negative temperature coefficient (NTC) properties of these thermistors by increasing the content of CeNbO4. The structure of as-sintered Ca0.9Y0.1MoO4–xCeNbO4 ceramics is a single Ca0.9Y0.1MoO4 phase at x < 2.5, while the CeNbO4 phase begins to appear at x ≥ 2.5. The thermistor samples have remarkable NTC characteristics in the temperature of 50–700 °C. The resistivity, β300/500℃ constant and activation energy all decrease with increasing CeNbO4. The increase of Ce3+ content proved by XPS analysis is the reason for the decrease of the resistivity. The β300/500℃ and Ea values of the NTC thermistors are in the range of 5768–6583 K, 0.497–0.568 eV, respectively. After annealing at 500 °C in air for 200 h, the coefficient of aging (ΔR/R) is less than 2%, suggesting the materials show excellent stability.

Development of a Stable High-Temperature Diamond Thermistor Using Enhanced Supporting Designs

Diamond is an excellent material for a high-temperature thermistor given its superior material properties. Despite its long history of the development, diamond thermistor is not widely used for commercial sensors because it is difficult to maintain optimum conditions for the diamond thermistor to operate at high temperature ranges in a compact size. In this paper, we demonstrate methods to improve the practicality and the feasibility of the diamond thermistor, focusing on the supporting components. The diamond thermistor design, where the diamond is sandwiched between contact pads, is tested up to 700 °C and being characterized by the Steinhart–Hart equations. For temperatures higher than 700 °C, the design of the thermistor is improved by encasing it in a metal sheath tube that protects and seals the diamond thermistor. Our supporting components provide oxygen free environment and a sturdy structure for the sensor components in a compact size. The encased diamond thermistor operates up to 880 °C and shows stable performance in a cycling test between 880 °C and the room temperature. An additional long-term stability test is performed at various temperatures for 10-hour durations. The tests show that the metal sheath tube design enables the diamond thermistor to be used as a practical temperature sensor at high-temperature ranges.

Self-powered ferroelectric NTC thermistor based on bismuth titanate

The exploration of multifunctional perovskite nanostructures for flexible sensor/energy harvesting applications is of great interest. Herein, highly crystalline ferroelectric Bi4Ti3O12 nanoparticles (NPs) were brush-coated onto flexible Kapton film without using costly instrumental techniques or polymer templates. Moreover, flexible ferroelectric high-temperature thermistors (FHTTs) having high thermal sensitivity (β(100/260) = 6515 K) and a broad working temperature range (25–260 °C) were developed based on bismuth-layered perovskites. Polarization (P) vs. Electric field (E) (P–E) loops of the orthorhombic phase of Bi4Ti3O12 NPs were measured at different frequencies under poled and unpoled conditions. The Bi4Ti3O12 NPs exhibited a multi-fold improvement of remnant polarization (Pr ≈ 6 μC/cm2) at the coercive field (Ec ≈ 90 kV/cm) over that previously reported. Lastly, the prototype of a self-powered-FHTT was fabricated, which could be used as an efficient environmental temperature warning/monitoring system free of complex battery sources. The Bi4Ti3O12 NPs/PVDF composite film-based piezoelectric nanogenerator (BPCF-PNG) used an eco-friendly power source (generating 12.7 mW/m2 at 300 MΩ upon 4 N applied force) to drive the FHTT. The brush-coating method to prepare flexible ferroelectric films is highly suitable for large-scale fabrication, and is also an eco-friendly and time-efficient process. The detailed investigation reported herein is a novel approach to next-generation ferroelectric-based sensors, memristors, and energy harvesters.

Temperature and Spallation Sensors Based on Oxide and Oxide/Silicide Composites for High-Temperature System Monitoring

The applications for high-temperature, wireless sensor systems are numerous for the energy and manufacturing industries, especially for monitoring the health and process conditions of refractory brick and active components within extreme conditions. Current sensor technologies in these types of applications are primarily based on thermocouples inserted into the reactors via open access ports through the refractory, which typically leads to rapid corrosion by gaseous or molten reactants. There are very few refractory metals and ceramics that are capable of withstanding such extreme and variable conditions, which include high temperature, high pressure, various pO2 levels and corrosive conditions (molten inorganics or reactive gasses). Therefore, the common materials used in thermocouples, thermistors, pressure sensors, and strain gauges are not appropriate for long-term use in these applications. In this work, the high-temperature stability and electrical properties of conductive ceramics based on silicide/oxide and semiconducting oxides were investigated up to 1400ºC. These composites were patterned onto ceramic substrates and embedded within refractory brick to form embedded thermistors, thermocouples and spallation sensor architectures. In addition, sensor designs that include capacitive and inductor elements for RF passive wireless communication were also evaluated. The presentation will describe sensor design, fabrication processes, and testing protocols up to 1400ºC to assess sensor performance. After sensor testing, the composition and microstructure of the sensors were characterized by XRD, XPS and SEM. An example of the performance of a thick-film high temperature thermistor sensor produced in this work is shown in Figure 1. The sensor material in the embedded thermistor consisted of a patterned electrically conducting oxide buried within a laminate yttrium-stabilized zirconia (YSZ) substrate. The oxide sensor composition was initially printed on green YSZ substrates, laminated into a preform, and then sintered to 1400°C. The sintered preforms were embedded into chromia-based refractory bricks. The cast bricks were then sintered to 1450°C for 10 hrs in air. After sintering, the bricks were trimmed to expose the sensor material. Platinum wire, platinum mesh, and additional sensor ink was used to make connection to the sensor embedded inside a brick. Prior to testing, the assembled leads were heat treated to 1400°C to pre-sinter the sensor to wire connection. To collect data for Figure 1, the sensor to wire connection was placed at the same level as high temperature refractory for the furnace door with additional insulation to mimic typical high temperature furnace refractory configuration. With this testing arrangement, the leads connection was exposed to only about 500°C maximum temperature over the course of the test. With the leads outside the hot zone, the resistance drift during the temperature hold was recorded from 9.7 to 9.1 ohms over 20 hours. Acknowledgements: The authors would like to acknowledge the financial support of DOE NETL under contracts DE-FE0012383 and DE-FE0026171. The authors would also like to acknowledge West Virginia University Shared Research Facilities for support through materials characterization. Figure 1. Resistance and temperature profile of conductive oxide based thermistor co-sintered inside ceramic substrate and imbedded in a chromia-based brick. The brick was tested to 1350°C. Figure 1