Temperature Coefficient Research Articles

On the τf measurement of microwave dielectric ceramics

As a key parameter of microwave dielectric ceramics, the temperature coefficient of resonant frequency (τf) determines the temperature stability of microwave dielectric ceramics and relevant microwave devices. The τf of a microwave dielectric ceramic is typically calculated from the measured resonant frequencies of the system, consisting of the ceramic sample plus other dielectric elements, at two temperatures ∼60 – 70 °C apart. This measurement process assumes that τf is a constant; however, it is generally temperature-dependent based on both theoretical and experimental results. Furthermore, the as-measured τf is actually the τf of the dielectric resonator system, which is different from that of the sample itself. In this paper, these two overlooked problems during τf measurement are discussed in detail, and suggestions are given to avoid their possible misleading consequences and thus ensure the reliability of τf measurements.

Neutronic characteristic analysis of the GAMA microreactor

The GAMA Microreactor is a low-power nuclear power plant dedicated for remote area with a power output of 300 kWe. The primary characteristics of the GAMA Microreactor are its system compactness, design simplicity, and safe operation without moving parts. The fuel is uranium hydride powder, which works simultaneously as a moderator, contained within a stainless-steel vessel and surrounded by a graphite reflector. Liquid sodium-filled heat pipes are embedded in the fuel powder to extract fission-generated heat via passive coolant flow due to capillary action. As a part of the design process, neutronic parameters of the GAMA Microreactor have been determined using SCALE6.2 and OpenMC codes. Among the analyzed parameters are the excess reactivity as a function of burn-up, shutdown margin, control rod worth, temperature coefficient of reactivity (TCR), fuel density coefficient of reactivity (DCR), and reactivity coefficient due to hydrogen dissociation. From the calculation results, the GAMA Microreactor has a maximum excess reactivity of 0.049 at the operational temperature (700 °C) and 0.0788 at ambient temperature. The TCR value is negative (−2.267 pcm/°C) at the beginning of the cycle (BOL) and −4.9618 pcm/°C at the end of the cycle (EOC). Hydrogen dissociation imposes negative reactivity on the reactor both at the BOL (−0.2631 Δk/k-%dissociation) and EOC (−0.3524 Δk/k-%dissociation). Therefore, the GAMA Microreactor maintains its inherent safety characteristics over a considerably long operation time.

Improvement of thermal stability by co-introducing Dy and Co with dual-alloy method in NdFeB magnets

A dual-main-phase magnet with excellent thermal stability was prepared by co-mixing two types of magnetic powders, which was the conventional N55-grade powder without Dy elements, while the Co and Dy contents reached 50 wt% and 6 wt% respectively in another Co-rich powder. Studies showed that the magnetic properties along with the thermal stabilities of the magnets exhibited significant differences with the changes in the mixing ratio of the two powders. Especially, when mixing the two powders in equal proportions, a great remanence temperature coefficient of −0.0725 %/°C at 20–100 °C was obtained with a rather high Curie temperature of 581 °C, possessing its magnetic properties with Br = 12.58 kGs, Hcj = 15.9 kOe, and (BH)max = 36.88 MGOe. Note that as the proportion of Co-rich powder increased to 2/3, although the Curie temperature of the prepared magnets further increased to 622 °C, both the remanence temperature coefficient and the magnetic properties deteriorated significantly. The phase composition and microstructure analyses revealed that when the proportion of added Co-rich component exceeded 1/2, the RE(Fe,Co)2B phase transformed into the RE(Fe,Co)4B phase, and a significant amount of RE(Fe,Co)1.8B0.2 phase precipitated at the grain boundaries, leading to a deterioration in magnetic performance. In addition, dynamic domain evolution analyses under variable temperature conditions demonstrated the positive effect of the appropriate introduction of Co/Dy on the anti-demagnetization in high temperatures. This work provides insights and an experimental foundation for the development of high-performance magnets with high thermal stability, used in high-temperature environments.

Dynamics and thermal stability of the bypass polymerase, DinB homolog (Dbh).

The DinB homolog polymerase (Dbh) is a member of the Y-family of translesion DNA polymerases that can synthesize using a damaged DNA template. Since Dbh comes from the thermophilic archaeon Sulfolobus acidocaldarius, it is capable of functioning over a wide range of temperatures. Existing X-ray structures were determined at temperatures where the protein is least active. Here we use NMR and circular dichroism to understand how the structure and dynamics of Dbh are affected by temperature (2°C-65°C) and metal ion binding in solution. We measured hydrogen exchange protection factors, temperature coefficients, and chemical shift perturbations with and without magnesium and manganese. We report on regions of the protein that become more dynamic as the temperature is increased toward the functional temperature. Hydrogen exchange protection factors and temperature coefficients reveal that both the thumb and finger domains are very dynamic relative to the palm and little-finger (LF) domains. These trends remain true at high temperature with dynamics increasing as temperatures increase from 35°C to 50°C. Notably, NMR spectra show that the Dbh tertiary structure cold denatures beginning at 25°C and increases in denaturation as the temperature is lowered to 5°C with little change observed by CD. Above 35°C, chemical shift perturbation analysis in the presence and absence of magnesium and manganese reveals three ion binding sites, without DNA bound. In contrast, these bound metals are not apparent in any Dbh crystal structures of the protein without DNA. Two ion binding sites are confirmed to be near the active site, as reported in other Y-family polymerases, and we report a novel ion binding site in the LF domain. Thus, the solution-state structure of the Dbh polymerase is distinct from that of the solid-state structures and shows an unusually high cold denaturation temperature.

Corrosion, wear and machinability studies on scrap aluminium alloy wheel based metal matrix composite

Recycled materials, especially scrap aluminum alloy wheels with reinforcement castings, are gaining applications in various fields such as aerospace, naval, military, and automobile. In this research, Honda car alloy wheels are stir-cast with 5% alumina as reinforcement and subjected to corrosion, wear, and machinability studies. Corrosion studies were conducted to evaluate the corrosion current density (ICorr), corrosion potential (ECorr), and corrosion rate for the cast aluminum metal matrix composite. The wear studies with different loads, i.e., 50 N, 60 N, 70 N, and 80 N, were performed on the specimen and investigated the coefficient of friction, frictional force, and pin temperature. The sliding load has a significant effect on wear performance, and for the sliding load of 80 N, the wear recorded was 2574.83 µm. The electrochemical machinability studies with voltage, duty cycle, and concentration of citric acid electrolyte are performed on the machining rate and surface corrosion factor. Analysis of the electrochemical machining process with PROMETHEE-II shows that 7 V, 60% duty cycle, and 30 g/L electrolyte concentration are estimated to be the best combination for a higher machining rate and a lower surface corrosion factor. KEY WORDS: Alumina, Machining rate, Nyquist plot, Electrochemical machining, PROMETHEE-II Bull. Chem. Soc. Ethiop. 2024, 38(4), 1163-1175. DOI: https://dx.doi.org/10.4314/bcse.v38i4.27

Tuning εr and τf by the combined effects of rattling RE3+ and compressed Ca2+ at the A-site in microwave dielectric ceramics CaREAlO4 (RE = Eu, Ho, Er, Yb)

The well-densified CaREAlO4 (RE = Eu, Ho, Er, Yb) microwave dielectric ceramics with the K2NiF4 structure are fabricated at 1360–1500 °C. Crystal structures were investigated via XRD and HRTEM, and no phase transition occurred. The ceramics exhibit low εr of 17.9–19.3 and relatively high Q × f values of 25,110 - 48,310 GHz, and tunable τf values ranging from −1.1 ppm/°C to +18.4 ppm/°C and then to −20.4 ppm/°C. The variations in εr and τf were ascribed to the mutual effects of rattling RE3+ and compressed Ca2+ characterized by bond valence theory and temperature coefficient of ion polarizability (ταm). The Q×f values of CaREAlO4 ceramics positively correlate with structure stability and lattice energy, the main intrinsic factors affecting Q×f.

Geometrically engineered mass sensing CMOS-MEMS resonators for temperature compensation via folded anchors

We present, for the first time, a single-layer plate resonator with specific folded anchors capable of reducing the temperature coefficient of frequency (TCF) by two orders of magnitude compared to straight beams. Folded anchors provide differential strain through either compressive or tensile thermal stress depending on the anchor design and, in turn, allow for positive and negative temperature-induced frequency change components. With a nominal 2 MHz frequency, we experimentally report a frequency shift of 2500 ppm over a temperature span from 0ºC to 90ºC using an optimal anchor folding aspect ratio. The frequency-temperature curve exhibits a turnover point at 27ºC granting a unique opportunity to, in a future application, complement this passive strategy with an active micro-oven for negligible TCF. The proposed approach main advantage relies on its mechanical design-level implementation exploiting a geometrical engineering strategy than can be directly extended to other topologies. This technique reveals as particularly suited for ultra-sensitive mass sensing applications based on microelectromechanical systems (MEMS) resonators where single-layer resonators are optimal thanks to their minimum footprint, attaining minimum mass structures. Results demonstrate that adoption of the proposed folded anchors strategy decreases effectively the thermally induced frequency fluctuations within the system intrinsic noise level.

Investigation of the Positive Temperature Coefficient Resistivity of Nb-Doped Ba0.55Sr0.45TiO3 Ceramics

The demands of low-Curie-temperature (~−10 °C) positive temperature coefficient (PTC) thermistors are increasing in advanced precision integrated circuits and other industries. In this paper, the Nb-doped Ba0.55Sr0.45TiO3(BST)-based PTC resistivity materials are reported. The effects of the sintering process, especially the cooling rate on the PTC properties of the material, are investigated. The results indicate that the Ba0.55Sr0.45Ti0.9985Nb0.0015O3 composition of the prepared PTC ceramics demonstrates promising PTC characteristics. These include a Curie temperature as low as −13 °C, a high temperature coefficient of 0.296 at −3.4 °C, a large enough resistivity change of 3.1 over a narrow phase transition temperature range of approximately 38 °C, and moderate resistivity below the Curie temperature. Such properties suggest that the Ba0.55Sr0.45Ti0.9985Nb0.0015O3 ceramics are likely suitable for use in thermal management systems designed for low-temperature control.

Strain and Temperature Sensing Based on Different Temperature Coefficients fs-FBG Arrays for Intelligent Buoyancy Materials.

The temperature and strain fields monitoring during the preparation process of buoyancy materials, as well as the health status after molding, are important for mastering the mechanical properties of buoyancy materials and ensuring the safety of operators and equipment. This paper proposes a short and high-density femtosecond fiber Bragg grating (fs-FBG) array based on different temperature coefficients fibers. By optimizing the parameters of femtosecond laser point-by-point writing technology, high-performance fs-FBG arrays with millimeter level gating length and millimeter level spatial resolution were prepared on two types of fibers. These were successfully embedded in buoyancy materials to achieve in-situ online monitoring of the curing process and after molding. The experimental results show that the fs-FBG array sensor has good anti-chirp performance and achieves online monitoring of millimeter-level spatial resolution. Intelligent buoyancy materials can provide real-time feedback on the health status of equipment in harsh underwater environments. The system can achieve temperature monitoring with an accuracy of 0.56 °C and deformation monitoring with sub-millimeter accuracy; the error is in the order of micrometers, which is of great significance in the field of deep-sea exploration.

A high‐precision voltage reference with a curvature‐compensated bandgap for fluorescence detection

SummaryFluorescent optical fiber temperature sensors require accurate online temperature monitoring in hazardous environments with strong electromagnetic fields, high voltages, flammability, or explosiveness. This imposes stringent requirements on the temperature coefficient stability of the bandgap reference (BGR) circuit. To address these challenges, this paper proposes a high‐order curvature compensation bandgap reference (HCC_BGR) circuit fabricated using a 0.18‐μm bipolar‐CMOS‐DMOS (BCD) process. A traditional first‐order bandgap reference (TRA_BGR) circuit is also fabricated for comparison. Experimental results demonstrate that the proposed HCC_BGR circuit generates a stable 1.22‐V reference voltage with a low‐temperature coefficient of 5.56 ppm/°C from −20°C to 85°C. Compared to the TRA_BGR circuit, the HCC_BGR reduces the temperature coefficient by 3.07 times. Furthermore, the low‐dropout regulator (LDO) using the proposed HCC_BGR exhibits excellent line sensitivity of 1.52%/V from 3.4 to 5 V.

In situ high-temperature transmission electron microscopy investigation of dynamic element migration for Sm-Co-Fe-Cu-Zr sintered magnets

The investigation delved into the dynamic migration of elements and the evolution of interface structures within the Sm(CobalFe0.09Cu0.09Zr0.03)7.2 magnet as temperature increased, employing high-temperature transmission electron microscopy. Notably, the Cu distribution exhibited temperature susceptibility. Elevating the temperature from room temperature (RT) to 400 °C led to a notable expansion in the distribution width of Cu, ranging from 28.5 to 50.8 nm. Simultaneously, the average Cu content at the cell boundaries decreased from 14.4 to 11.6 at.%. According to the high-temperature in-situ experimental results of the transmission electron microscopy thin-film samples, the derived diffusion coefficients, D300°Cand D400°C, were determined as 0.0525 nm2/s and 1.414 nm2/s, respectively. The significantly higher D400°C compared to D300°C underscores the considerable influence of temperature on Cu element diffusion behavior. The simulation results demonstrate that the trend of magnetic properties variation at different temperatures closely aligns with the observed trend. Subsequent calculation of the temperature coefficient of intrinsic coercivity (β) reveals that |β300–400°C| is notably higher than |βRT-300°C|. This suggests a substantial impact of Cu diffusion-induced deterioration on the β. Moreover, micromagnetic simulation results delineated the reverse magnetization process for magnets at varying temperatures, revealing that the presence of the Cu diffusion region diminished the pinning effect of the cell boundary phase. Furthermore, a wider Cu diffusion region facilitated easier magnetic moment reversal at the cell boundary phase, resulting in a marked deterioration of intrinsic coercivity. This elucidates that the formation of the Cu diffusion region stands as one of the primary reasons for the sharp decline in magnetic properties of Sm2Co17-type magnets under high-temperature conditions.

Influence of positive temperature coefficient and battery aging on external short circuit for 18,650-type battery

Influence of positive temperature coefficient and battery aging on external short circuit for 18,650-type battery

Electrical actuation of single-crystal diamond MEMS resonators at high temperatures

Achieving efficient low-voltage actuation of microelectromechanical system (MEMS) resonators in high-temperature environments poses a difficult topic due to the thermal interference and the risk of high-temperature failure. In this work, the single-crystal diamond (SCD) resonators fabricated through the ion implantation-assisted lift-off (IAL) technique exhibit a SCD-on-SCD cantilever structure. We propose an electrical actuation system based on the electrostatic effect specifically designed for SCD MEMS resonators with a low radio-frequency amplitude of ∼ 100 mV. The SCD resonators demonstrate stable and efficient actuation across a wide temperature range, from room temperature to 500 °C. Importantly, the actuation voltage exhibits little impact on the resonance frequency and the Q factor of the resonator. The SCD resonator showcases exceptional thermal stability in resonance frequency, with a low temperature coefficient of frequency (TCF) below −12 ppm/°C up to 500 °C. The developed actuation scheme holds tremendous potential as a robust platform for realizing SCD MEMS devices, particularly in applications requiring high integration at high temperatures.

Development of new materials for electrothermal metals using data driven and machine learning.

After adopting a combined approach of data-driven methods and machine learning, the prediction of material performance and the optimization of composition design can significantly reduce the development time of materials at a lower cost. In this research, we employed four machine learning algorithms, including linear regression, ridge regression, support vector regression, and backpropagation neural networks, to develop predictive models for the electrical performance data of titanium alloys. Our focus was on two key objectives: resistivity and the temperature coefficient of resistance (TCR). Subsequently, leveraging the results of feature selection, we conducted an analysis to discern the impact of alloying elements on these two electrical properties.The prediction results indicate that for the resistivity data prediction task, the radial basis function kernel-based support vector machine model performs the best, with a correlation coefficient above 0.995 and a percentage error within 2%, demonstrating high predictive capability. For the TCR data prediction task, the best-performing model is a backpropagation neural network with two hidden layers, also with a correlation coefficient above 0.995 and a percentage error within 3%, demonstrating good generalization ability. The feature selection results using random forest and Xgboost indicate that Al and Zr have a significant positive effect on resistivity, while Al, Zr, and V have a significant negative effect on TCR. The conclusion of the composition optimization design suggests that to achieve both high resistivity and TCR, it is recommended to set the Al content in the range of 1.5% to 2% and the Zr content in the range of 2.5% to 3%.

Effect of Deposition Pressure and Temperature on Tungsten Thin-Film Heater for Phase-Change Switch Applications.

Tungsten (W) film is increasingly utilized in various microheater applications due to its numerous advantages. These advantages include a high melting point, positive constant temperature coefficient of resistance (TCR), good mechanical stability, and compatibility with semiconductor processes. In this paper, deposition parameters for enhancing the properties of W film were investigated, and an optimized microheater was fabricated. It was found that the deposition temperature and pressure can modify the TCR to be negative or positive and the crystalline phase of W films to be alpha phases or mixed with beta phases. A W film deposited under 650 °C with a pressure of 1 pa has a positive TCR and pure alpha phase crystalline structure. We applied this optimized W film as a microheater in an RF phase-change switch (RFPCS), and the maximum voltage of the optimized W microheater increased by at least 48% in this work. By optimizing the microheater, the phase-change switch can be successfully actuated in both on and off states, demonstrated by the Raman results of the phase-change material. A voltage pulse of 20 V/200 ns was enough to turn the switch off with MΩ, and 11 V/3 μs could turn the switch on with 138 Ω. The optimized microheater and device can cycle 500 times without failure. The insertion loss and isolation of the device at 20 GHz was 1.0 dB and 22 dB.

Electrothermal Instabilities in Barium-Titanate-Based Ceramics

An electrothermal analysis for barium-titanate-based ceramics is presented, combining the Heywang–Jonker model for the electric resistivity with a heat dissipation mechanism based on natural convection and radiation in a one-dimensional model on the device level with voltage as the control parameter. Both positive-temperature-coefficient (PTC) and negative temperature coefficient (NTC) effects are accounted for through the double Schottky barriers at the grain boundaries of the material. The problem formulated in this way admits uniform and non-uniform multiple-steady-state solutions that do not depend on the external circuit. The numerical bifurcation analysis reveals that the PTC effect gives rise to several multiplicites above the Curie point, whereas the NTC effect is responsible for the thermal runaway (temperature blowup). The thermal runaway phenomenon as a potential thermal shock could be among the possible reasons for the observed thermomechanical failures (delamination fracture). The theoretical results for the NTC regime and the thermal runaway are in agreement with the experimental flash sintering results obtained for barium titanate, and 3% and 8% yttria-stabilized zirconia.

Design of a Negative Temperature Coefficient Temperature Measurement System Based on a Resistance Ratio Model.

In this paper, a temperature measurement system with NTC (Negative Temperature Coefficient) thermistors was designed. An MCU (Micro Control Unit) primarily operates by converting the voltage value collected by an ADC (Analog-to-Digital Converter) into the resistance value. The temperature value is then calculated, and a DAC (Digital-to-Analog Converter) outputs a current of 4 to 20 mA that is linearly related to the temperature value. The nonlinear characteristics of NTC thermistors pose a challenging problem. The nonlinear characteristics of NTC thermistors were to a great extent solved by using a resistance ratio model. The high precision of the NTC thermistor is obtained by fitting it with the Hoge equation. The results of actual measurements suggest that each module works properly, and the temperature measurement accuracy of 0.067 °C in the range from -40 °C to 120 °C has been achieved. The uncertainty of the output current is analyzed and calculated with the uncertainty of 0.0014 mA. This type of system has broad potential applications in industry fields such as the petrochemical industry.

A 2.41 ppm/°C bandgap voltage reference with second‐order curvature compensation

SummaryA current‐mode (CM) bandgap voltage reference (BGR) with second‐order curvature compensation is presented in this paper. The proposed CM BGR offers a simple compensation structure that requires only six additional transistors compared with traditional BGR designs, making it an attractive solution for low temperature coefficient (TC) voltage references. Proportional to absolute temperature (PTAT) current digital‐to‐analog converters (DACs) are used to suppress the effect of process variation and device mismatch on TC. The compensation signal is generated using the voltage difference of two bipolar junction transistor (BJT) emitter–bases whose currents are the sum and difference of the PTAT current and the zero to absolute temperature (ZTAT) current, respectively. The proposed CM BGR is designed with 0.18‐μm BIPOLAR CMOS DMOS process. A TC is 2.41 ppm/°C over a wide temperature range of −40°C to 150°C. The linear sensitivity of the supply voltage from 1.2 to 2 V is 0.027%/V. With an active area of 0.0721 mm2, it consumes 68 μA at room temperature. The integrated output noise from 0.1 to 10 Hz is 21.9 μV.

Wireless Temperature Measurement for Curved Surfaces Based on AlN Surface Acoustic Wave Resonators.

In this paper, we propose a novel method for temperature measurement using surface acoustic wave (SAW) temperature sensors on curved or irregular surfaces. We integrate SAW resonators onto flexible printed circuit boards (FPCBs) to ensure better conformity of the temperature sensor with the surface of the object under test. Compared to traditional rigid PCBs, FPCBs offer greater dynamic flexibility, lighter weight, and thinner thickness, which make them an ideal choice for making SAW devices working for temperature measurements under curved surfaces. We design a temperature sensor array consisting of three devices with different operating frequencies to measure the temperature at multiple points on the surface of the object. To distinguish between different target points in the sensor array, each sensor operates at a different frequency, and the operating frequency bands do not overlap. This differentiation is achieved using Frequency Division Multiple Access (FDMA) technology. Experimental results indicate that the frequency temperature coefficients of these sensors are -30.248 ppm/°C, -30.195 ppm/°C, and -30.115 ppm/°C, respectively. In addition, the sensor array enables wireless communication via antenna and transceiver circuits. This innovation heralds enhanced adaptability and applicability for SAW temperature sensor applications.

Study on the Fabrication and Acoustic Properties of Near-Stoichiometric Lithium Tantalate Crystal Surface Acoustic Wave Filters

Near-stoichiometric lithium tantalate (NSLT) wafers with different Li contents were prepared by vapour transfer equilibrium (VTE) method and fabricated into surface acoustic wave filters. The temperature coefficient of frequency, insertion loss, and bandwidth of the surface acoustic wave filters were tested using a special chip test bench and a network analyzer. The results show that the temperature coefficient of frequency shows a trend of first decreasing and then increasing with the increase in Li content, and the temperature stability of the surface acoustic wave filters is best when the Li content is 49.75%. It is also found that the surface acoustic wave filter fabricated from NSLT wafers has 21.18% lower temperature coefficient of frequency, 7.3% lower insertion loss, and 2.8% lower bandwidth than those fabricated from congruent lithium tantalate wafers. Therefore, NSLT crystals are more suitable for applications in acoustic devices, providing a new idea for performance enhancement of 5G communication devices.