Temperature Coefficient Research Articles

Crystal Structure, Infrared Reflection Spectrum, and Improved Microwave Dielectric Characteristics of Ba4Sm28/3Ti18O54 Ceramics via One-Step Reaction Sintering

High-k Ba4Sm28/3Ti18O54 ceramics with improved microwave dielectric characteristics were successfully fabricated using the one-step reaction sintering (RS) route. The sintering characteristics, microstructure, crystal structure, infrared reflection spectrum, and microwave dielectric characteristics of Ba4Sm28/3Ti18O54 ceramics prepared by the RS route were systematically investigated. Samples prepared by the RS route exhibited single-phase orthorhombic tungsten–bronze structure and dense microstructure at optimum sintering temperature. Compared with the conventional solid-state (CS) process, the Ba4Sm28/3Ti18O54 ceramics fabricated by the RS route presented a smaller temperature coefficient (TCF), a higher quality factor (Q × f), and a higher permittivity (εr). The improved microwave dielectric characteristics were highly dependent on the theoretical permittivity, atomic packing fraction, suppression of Ti3+, and Ti-site bond valence. Excellent combined microwave dielectric characteristics (TCF = −7.9 ppm/°C, Q × f = 9519 GHz, εr = 80.26) were achieved for Ba4Sm28/3Ti18O54 ceramics prepared by RS route sintered at 1400 °C, suggesting the RS route was a straightforward, economical and effective route to prepare high-performance Ba4Sm28/3Ti18O54 ceramics with promising application potential.

Theoretical investigation of the OH-initiated atmospheric degradation mechanism of CX2CHX (X = H, F, Cl) by advanced quantum chemical and transition state theory methods.

Halogenated olefins are anthropogenic compounds with many industrial applications but at the same time raising many environmental and health concerns. Gas-phase electrophilic addition of the OH radical to the olefinic CC bond represents the primary sink for these chemicals in the atmosphere, with the degree and type of halogenation playing a significant role in their overall reactivity. In this work, we present a theoretical investigation of the reaction mechanisms and kinetics for the reactions between the OH radical and CH2CH2 (ethylene, ETH), CF2CHF (trifluoroethylene, TFE) and CCl2CHCl (trichloroethylene, TCE), simulated by state-of-the-art protocols and methods, with the aim of providing a detailed interpretation of the available experimental results, as well as new data of relevance to tropospheric chemistry. Specifically, potential energy surfaces (PESs) are obtained using the jun-Cheap (jChS) composite scheme, whereas temperature and pressure dependent rate coefficients and product distributions in the 100-600 K temperature range are calculated within the Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) framework. The rates for barrierless channels are obtained from variable reaction coordinate-variational transition state theory (VRC-VTST) combined with the two transition state model. While the reactions with ETH and TFE proceed mainly via the formation of addition adducts at P = 1 atm and T = 298 K, the dominant channel for TCE is the Cl-elimination reaction. Global rate constants for the two halogenated olefins, TFE and TCE, are found to be pressure-independent, contrary to the case of ETH. The computed rate constants, as well as their temperature and pressure dependence, are in remarkable agreement with the available experimental data, and they are used to derive atmospheric lifetimes (τ) for both TFE and TCE as a function of altitude (h) in the atmosphere, by taking into account variations in the rate coefficients (k (T, P)) and [OH] concentration.

Dynamics of changes in the temperature coefficient of electrical conductivity of distilled water in conductometric cells during heating and cooling

Dynamics of changes in the temperature coefficient of electrical conductivity of distilled water in conductometric cells during heating and cooling

Low temperature coefficient of frequency AlN Lamb wave resonator using groove structure between interdigital transducers

The lithographically tunable and small size features of Lamb wave resonators (LWRs) take a bright future for their use in high frequency narrow band filters. Resonant frequency drift due to temperature variation has a large impact on the narrower passband and a temperature coefficient of frequency (TCF) closer to 0 can broaden the stable temperature range of the resonator. This paper proposes a method of etching grooves in the area of the piezoelectric layer not covered by the Interdigital transducer (IDT) electrodes to improve the temperature stability of the Lamb resonator. Through the finite element method and theoretical analysis, the phase velocity, group velocity, and TCF dispersion curve of different modes of the resonator with groove structure were determined. The reason for the change of TCF under different normalized thicknesses of AlN was explained through the conversion of the LWR from a contour mode resonator (CMR) to a Cross-Sectional Lamé Mode Resonator. Simulation and test have shown that etching grooves have the effect of reducing TCF by adjusting the electromechanical coupling coefficient. The test shows that 205 nm-depth grooves can lower the TCF of the S0 mode IDT-Open LWR from −13.3 to −5.1 ppm/°C and S1 mode from −36.8 to −9.0 ppm/°C, respectively. The TCF of S0 and S1 mode LWRs decreased by 61.7% and 75.5%, respectively, after 205 nm groove etching. The groove etching method greatly raises the temperature stability of the LWR, enabling Lamb wave filters to operate stably over a wider temperature range.

Cost‐efficiency potential of solar energy on a global scale: Case studies for Si solar modules with PERC and heterojunction structures

AbstractLevelized cost of electricity (LCOE) is a crucial metric for assessing the socio‐economic cost‐efficiency potential of various energy sources including solar photovoltaics. Nevertheless, accurate LCOE estimations for commercialized high‐efficiency Si solar modules with passivated emitter and rear cell (PERC) and silicon heterojunction (SHJ) structures have been lacking. In this study, we present the first global LCOE estimates for a PERC module (20% cell efficiency) and a SHJ module (23% cell efficiency), which have been derived by (i) performing rigorous energy‐yield calculations with full‐spectral and temperature‐dependent simulations that incorporate all essential meteorological effects and (ii) considering country‐specific capital costs and discount rates. Moreover, to determine the universal global LCOE, the LCOEs for three distinct installation capacities (100 MW for a utility, 500 kW for a commercial, and 5 kW for a residential system) have been unified by selecting an appropriate system size at each location based on a population density. We find that the LCOEs of both PERC and SHJ systems are below 3 cents/kWh in 2020 US dollar in many areas of China, Saudi Arabia, the United States, Australia, Chile, and Botswana, where the conditions of a high energy yield, low population density, low capital cost, and low country‐risk premium are satisfied simultaneously. In contrast, many European countries exhibit a moderate LCOE of 3~5 cents/kWh. Notably, Japan and Russia exhibit quite high LCOEs (6~10 cents/kWh) primarily due to significantly higher installation costs and moderate energy yields. Importantly, the global LCOEs of the PERC and SHJ modules are quite similar, with the SHJ module showing a slightly better cost performance in the regions near the equator due to its low temperature coefficient. Conversely, the PERC module demonstrates a cost advantage in the Northern Hemisphere due to a lower module cost.

Cost‐efficiency potential of solar energy on a global scale: Case studies for Si solar modules with PERC and heterojunction structures

AbstractLevelized cost of electricity (LCOE) is a crucial metric for assessing the socio‐economic cost‐efficiency potential of various energy sources including solar photovoltaics. Nevertheless, accurate LCOE estimations for commercialized high‐efficiency Si solar modules with passivated emitter and rear cell (PERC) and silicon heterojunction (SHJ) structures have been lacking. In this study, we present the first global LCOE estimates for a PERC module (20% cell efficiency) and a SHJ module (23% cell efficiency), which have been derived by (i) performing rigorous energy‐yield calculations with full‐spectral and temperature‐dependent simulations that incorporate all essential meteorological effects and (ii) considering country‐specific capital costs and discount rates. Moreover, to determine the universal global LCOE, the LCOEs for three distinct installation capacities (100 MW for a utility, 500 kW for a commercial, and 5 kW for a residential system) have been unified by selecting an appropriate system size at each location based on a population density. We find that the LCOEs of both PERC and SHJ systems are below 3 cents/kWh in 2020 US dollar in many areas of China, Saudi Arabia, the United States, Australia, Chile, and Botswana, where the conditions of a high energy yield, low population density, low capital cost, and low country‐risk premium are satisfied simultaneously. In contrast, many European countries exhibit a moderate LCOE of 3~5 cents/kWh. Notably, Japan and Russia exhibit quite high LCOEs (6~10 cents/kWh) primarily due to significantly higher installation costs and moderate energy yields. Importantly, the global LCOEs of the PERC and SHJ modules are quite similar, with the SHJ module showing a slightly better cost performance in the regions near the equator due to its low temperature coefficient. Conversely, the PERC module demonstrates a cost advantage in the Northern Hemisphere due to a lower module cost.

Heat transfer augmentation in a heating surface covered by variable-shape or double-layer porous media subjected to water jet impingement

In this study, a cooling scheme for high-power and highly-integrated electronic devices is proposed in which the heating wall is covered with a variable-shape or double-layer porous medium. The porous media impacted by water jet impinging is covered over the heating surface, wherein heat from bottom wall to porous matrix is transmitted by thermal conduction and dissipated by convection generated by water jet impinging in porous layer. A combination of Brinkman–Darcy–Forchheimer model with energy equations in local thermal equilibrium is employed to describe the hydrothermal transport details in porous matrix, and the k-ε model is used to interpret water jet impinging. The impacts of porous shapes, porosity distributions, and thickness ratios among lower and upper layers with various porosities in gradient variation configuration on heat dissipation capacity are investigated numerically. The numerical calculations are well verified by published experiments. Results show that rectangular porous media covered heating surface is more sensitive to changes in jet velocity, while the mode with triangular porous media results in lower surface temperature and higher heat transfer coefficient at stagnation point. Besides, the rise ratio of heat transfer coefficient of porous configuration with gradient increase in porosity from 0.6 to 0.8 is 11.84% higher than that with gradient decrease in porosity from 0.9 to 0.6. Finally, the optimized shape, porosity arrangement and thickness ratio, jointly affect the heat transport. The results obtained can be used to generalize and apply the coupling of jet impingement with porous media above the heating surface to augment heat transfer.

Study on cold start of fuel cell stack combined with liquid heating and fuel cell self-starting

Fuel cell vehicles have been paid more and more attention, while the low temperature cold start capability has become the main reason that hinders their large-scale commercialization, especially for the fuel cell bus. Generally, liquid heating (positive temperature coefficient thermistor (PTC) as the heat source) and fuel cell self-starting heating are the common cold start methods. However, these methods can bring about low start speed and high energy consumption. Therefore, a new cold start method combined with liquid heating and fuel cell self-starting is proposed to solve the problems. Besides, phase change material (PCM) is introduced to reduce energy consumption further. The PCM can absorb the waste heat of the fuel cell stack. Then the heat can be used for stack cold start by circulating liquid. In this study, a fuel cell bus cold start system simulating model is built. Based on this model, the fuel cell temperature and energy consumption during the cold start are analyzed. The results show that the method combined with liquid heating and fuel cell self-starting is superior to the liquid heating method. The cold start time and PTC consumption of the PTC-PCM-self-starting heating mode are 19.0 % and 19.2 % less than that of PTC-PCM heating mode. As for the combined heating method, dropping the self-starting temperature can not only increase the cold start speed but also reduce the proportion of electric heating. For every 1 °C decrease, the start-up time is shortened by 1 % on average, and the waste heat utilization rate is increased by 0.5 %. Although increasing the self-starting current can increase energy consumption, it will save more cold start time and reduce the proportion of electric heating. For every 1 A increase, the start-up time is shortened by 0.3 % on average, and the waste heat utilization rate is increased by 0.1 %. The PTC-PCM-self-starting heating method provides a way for fuel cell stack to start quickly and energy-efficiently in low temperature environment.

Doublet doped titanate ferroelectric system for capacitors and NTC thermistor applications

A simple negative temperature coefficient (NTC) thermistor Ba0.85Sr0.15Ti0.85Zr0.15O3 system is elaborated using a conventional solid-state reaction route. The structural analysis confirms that the elaborated pseudo-cubic system, which is indexed in the P4mm space group, exhibits one single phase. The dielectric properties and the electrical resistivity data are investigated over a large temperature range from 413 K to 673 K. In this temperature interval, the Steinhart–Hart equations are employed to confirm that Ba0.85Sr0.15Ti0.85Zr0.15O3 is a promising system for sensor application. Accordingly, the thermistor constant (β=10154 K), the sensitivity factor (-5.5 %/K≤α≤-2 %/K), the activation energy (Ea=0.875 eV), and the stability factor (SF=2.27) parameters are calculated to approve that Ba0.85Sr0.15Ti0.85Zr0.15O3 exhibits an NTC-thermistor behavior. The temperature dependence of the electrical resistivity confirms that Ba0.85Sr0.15Ti0.85Zr0.15O3 reveals a semiconductor behavior. The dielectric investigation shows that Ba0.85Sr0.15Ti0.85Zr0.15O3 exhibits a motivating dielectric properties (elevated dielectric constant value (105<ε’<106) and low dielectric losses (mostly < 0.007) with almost frequency independence between 20 Hz to 500 kHz). The colossal dielectric response of Ba0.85Sr0.15Ti0.85Zr0.15O3 is attributed to the internal barrier layer capacitor (IBLC) and the surface barrier layer capacitor (SBLC) effects. Such response suggests the suitability of Ba0.85Sr0.15Ti0.85Zr0.15O3 ceramic for high temperature capacitor applications.

Electrical, Optical and Thermal Properties of Ge-Si-Sn-O Thin Films.

This work evaluates the electrical, optical and thermal properties of Sn-doped GexSi1-xOy thin films for use as microbolometer sensing materials. The films were prepared using a combination of a radio frequency (RF) magnetron and direct current (DC) sputtering using a Kurt J Leskar Proline PVD-75 series sputtering machine. Thin films were deposited in an O2+Ar environment at a chamber pressure of 4 mTorr. The thicknesses of the thin films were varied between 300 nm-1.2 µm by varying the deposition time. The morphology and microstructure of thin films were investigated by atomic force microscope (AFM) imaging and X-ray diffraction (XRD), while the atomic composition was determined using the energy dispersive spectroscopy (EDS) function of a scanning electron microscope. The thin film with an atomic composition of Ge0.45Si0.05Sn0.15O0.35 was found to be amorphous. We used the Arrhenius relationship to determine the activation energy as well as temperature coefficient of resistance of the thin films, which were found to be 0.2529 eV and -3.26%/K, respectively. The noise voltage power spectral density (PSD) of the film was analyzed using a Primarius-9812DX noise analyzer using frequencies ranging from 2 Hz to 10 kHz. The noise voltage PSD of the film was found to be 1.76 × 10-11 V2/Hz and 2.78 × 10-14 V2/Hz at 2 Hz and 1KHz frequencies, respectively. The optical constants were determined using the ellipsometry reflection data of samples using an RC2 and infrared (IR) VASE Mark-II ellipsometer from J A Woollam. Absorption, transmission and reflection data for a wavelength range of 900 nm-5000 nm were also determined. We also determined the optical constant values such as the real and imaginary parts of refractive index (n and k, respectively) and real and imaginary part of permittivity (ε1 and ε2, respectively) for wavelength ranges between 193 nm to 35 µm. An optical band gap of 1.03 eV was determined from absorption data and using Tauc's equation. In addition, the thermal conductivity of the film was analyzed using a Linseis thin film analyzer employing the 3ω method. The thermal conductivity of a 780 nm thick film was found to be 0.38 Wm-1K-1 at 300 K. From the data, the Ge-Si-Sn-O alloy was found to be a promising material for use as a sensing material for microbolometers.

Efficiently high energy storage density in Ba2+ ion modified K0·5Na0·5NbO3 (KNN) ferroelectric ceramics for super capacitors

In the present work, Perovskites BaxK0.5-xNa0.5NbO3 (x = 0.00, 0.05, 0.10, 0.15, 0.20) were synthesized via solid-state synthesis technique. XRD analysis confirmed the successful phase formation of the synthesized perovskites. Further, Rietveld refinement revealed the presence of coexisting orthorhombic and tetragonal phases. High-resolution transmission electron microscopy (HRTEM) was employed to explore the lattice fringes and SAED patterns at nano scale level of prepared samples. The morphology was investigated using a field emission scanning electron microscope (FESEM). Dielectric measurements indicated that both the dielectric constant (ε′) and loss tangent (tan δ) exhibited significant dispersion at low frequencies and high temperatures. Notably, the Ba-doped perovskite samples demonstrated enhanced dielectric properties compared to the undoped KNN sample, with Ba0·1K0·4Na0·5NbO3 (x = 0.10) showing optimal dielectric performance, characterized by a dielectric constant (ɛ՛⁓4000) and a tanẟ ⁓7.23 at 100 Hz frequency and room temperature.AC conductivity is observed to increase with increasing both frequency and temperature. DC conductivity exhibited positive temperature coefficient indicating the semiconductor-like behavior of the prepared samples. The ferroelectric behavior was confirmed through P-E hysteresis curves and Ba0·1K0·4Na0·5NbO3 sample revealed the highest energy storage density among all prepared samples making it a suitable candidate for energy storage applications.

A DFT insight of the electronic, thermodynamic, and thermoelectric properties of RuO2

In this study, we explore the structural, electronic, thermodynamic, and thermoelectric properties of RuO2 using density functional theory. The derived equilibrium structural parameters agree with other theoretical and experimental results. The widely used modified Becke–Johnson (mBJ-GGA) potential is adopted for accurate electronic band gap estimation. To incorporate the effect of the extended orbital of the Ru atom, spin-orbit coupling has been used in combination with the mBJ potential. The investigation of electronic properties revealed an indirect semi-conducting nature with a band gap along the W-L symmetry. The calculated band gaps are 1.685 and 1.658 eV from mBJ and mBJ + SOC, respectively. The dynamical stability is tested and verified by calculating the phonon dispersion curve. We have employed the quasiharmonic approximation-based Gibbs2 package to determine the pressure and temperature-dependent thermodynamical parameters, such as cell volume, Debye temperature, heat capacity, entropy, and thermal expansion coefficient. This study uses the BoltzTraP simulation algorithm to determine the thermoelectric parameters such as the Seebeck coefficient, electrical conductivity, and thermal conductivity.

Mitigation of Humidity Interference by Graphene Derivatives for Efficient Temperature Sensors without Encapsulation

AbstractTemperature monitoring and regulation are essential in various environments, including modern industry and living and storage spaces. The growing demand for temperature sensors calls for affordable, efficient, interference‐resistant, and eco‐friendly solutions. The challenge of humidity interference in constructing temperature sensors often leads to compromising on the dynamic sensor properties in particular due to the need for encapsulation. To this end, this study introduces a temperature sensor leveraging a carefully designed graphene derivative to mitigate the humidity interference. The material, synthesize through scalable fluorographene chemistry with benzylamine, is optimized in order to enhance its properties, which led to achieving peak efficiency with a minimal humidity impact. The sensor demonstrated full functionality across a temperature range from 10 to 90 °C, with a temperature coefficient of resistivity 8.63 × 10−3 K−1, which is more than twice as high as that of conventional platinum thermometers. Remarkably, the sensor exhibited only a 2% change in resistance when exposed to relative humidity in the range of 20 to 70%. Notably, the sensor continues to give a consistent performance even after six months, which proved its stability. The presented device holds promise for evolving into a fully printed, cost‐effective and reliable next‐generation temperature sensors.

Multifunctional basalt fiber reinforced polymer composites with in-situ damage self-sensing and temperature-sensitive behavior

In this work, a multifunctional basalt fiber reinforced polymer composite (BFRP) with damage self-sensing and temperature-sensitive behavior was developed. Firstly, carbon nanotubes (CNTs) were deposited on basalt fibers (BF) by electrostatic self-assembly. Then, strong shear forces were applied during melt mixing with polyarylene ether nitrile (PEN) to construct a structure of CNTs surrounding the BF (CSB). This CSB structure endowed the composite with a low percolation threshold (0.82 wt% CNTs), a high conductivity (1.53 × 10-2 S/m at 1.12 wt% CNTs) and enhanced mechanical properties. The combination of resistance and acoustic emission real-time measurements confirmed that the composite achieved an excellent damage self-sensing capability with a maximum gauge factor (GF) of 7.68. Moreover, the composite exhibited a temperature-sensitive behavior with a temperature coefficient of resistance (TCR) of 0.542%/°C. This study provides a significant guidance for the development of multifunctional BFRP with damage self-sensing and temperature sensitive behavior.

Enhancement of the photothermoelectric effect through synergistic modulation of multiple parameters

The performance of photothermoelectric materials is severely limited by various intrinsic parameters that are difficult to simultaneously modulate, thereby hindering their applications in the fields of photodetectors, solar cells, thermoelectric therapy and so on. In this study, we have prepared suspended films composed of carbon nanotube (CNT) and molybdenum disulfide quantum dots (MoS2 QDs), which demonstrate a remarkable maximum photothermal conversion efficiency of 2201 K·mm2·W−1. This outstanding performance is attributed to the formation of a heterojunction between CNT and MoS2 QDs. Furthermore, the light absorption, temperature coefficient of resistance, and Seebeck coefficient of this suspended film are simultaneously modulated in a direction conducive to enhancing the photothermoelectric effect. Consequently, we have fabricated CNT-CNT/MoS2 QDs p-n junction photodetectors to showcase the improved photoresponse performance. This study presents an innovative approach for modulating multiple intrinsic parameters of photothermoelectric materials, which offers valuable insights for enhancing the performance of corresponding photothermoelectric devices.

Electrical Property Enhancement of Non-Heat-Treatable Wrought Aluminum Alloys Using Graphene Additives

With growing efforts of electrification, aluminum’s role as a light-weight conductor material has become increasingly prominent. There is a critical need to improve the electrical performance of aluminum at room temperature and high operating temperatures. In this study, the effect of graphene nanoparticle additives on the electrical performance of a non-heat treatable alloy were (AA3003) explored. Graphene’s unusual structure and electronic properties were used to improve AA3003 properties. Here, the effects of graphene on the evolution of electrical properties and microstructural features have been explored on lab scale hot extruded AA3003-graphene composites. Hot pressing schedules and extrusion temperatures were varied to investigate changes in intermetallic dispersion characteristics in the presence of dispersed graphene. We measured a reduction of 10.3% in the temperature coefficient of resistance in the AA3003 sample with 0.05wt.% graphene extruded at 400 °C, along with a maximum increase of 1.1% in electrical conductivity at 20 °C. Increasing the hot-pressing times up to 8hours was also found to consistently increase the electrical conductivity, due to increased precipitation of intermetallic phases. Despite being a non-heat treatable alloy, AA3003 displays interesting precipitation dynamics and grain recrystallization trends that can be modulated with varying levels of heat treatment, graphene concentrations, and hot extrusion process parameters.

Simulation and verification of cylinder head’s temperature field for marine low-speed engine

Abstract The high load capacity of marine low-speed engines poses a challenge to the thermal load of the cylinder head and temperature field simulation can guide its structural design and optimization. The 3-dimensional models of the fluid domain for the cooling water and the solid domain for the cylinder head are established and meshed, the fluid domain model is simulated to research the heat transfer pattern by Fluent software, and the surface temperature and heat transfer coefficient of the cooling water is calculated. The thermal boundary conditions of the gas in the cylinder are simulated through the analysis of the engine working process. The temperature of the cylinder head is calculated based on the fluid-structure interaction model with the thermal boundary conditions by Workbench software. The validation of the model is carried out by comparing the simulated and measured temperature data in the heated inner wall of the cylinder head. The research results show that compared with the measured value, the error of simulation results is small, which shows that the model and method are reasonable. The temperature in the inner wall of the combustion chamber is low, which can provide possibilities to appropriately improve the power density of the low-speed engine and reduce the risk of low-temperature corrosion.

Heat transfer augmentation in a heating surface covered by variable-shape or double-layer porous media subjected to water jet impingement

In this study, a cooling scheme for high-power and highly-integrated electronic devices is proposed in which the heating wall is covered with a variable-shape or double-layer porous medium. The porous media impacted by water jet impinging is covered over the heating surface, wherein heat from bottom wall to porous matrix is transmitted by thermal conduction and dissipated by convection generated by water jet impinging in porous layer. A combination of Brinkman–Darcy–Forchheimer model with energy equations in local thermal equilibrium is employed to describe the hydrothermal transport details in porous matrix, and the k-ε model is used to interpret water jet impinging. The impacts of porous shapes, porosity distributions, and thickness ratios among lower and upper layers with various porosities in gradient variation configuration on heat dissipation capacity are investigated numerically. The numerical calculations are well verified by published experiments. Results show that rectangular porous media covered heating surface is more sensitive to changes in jet velocity, while the mode with triangular porous media results in lower surface temperature and higher heat transfer coefficient at stagnation point. Besides, the rise ratio of heat transfer coefficient of porous configuration with gradient increase in porosity from 0.6 to 0.8 is 11.84% higher than that with gradient decrease in porosity from 0.9 to 0.6. Finally, the optimized shape, porosity arrangement and thickness ratio, jointly affect the heat transport. The results obtained can be used to generalize and apply the coupling of jet impingement with porous media above the heating surface to augment heat transfer.

Exploring the structural characteristics and electrical conductivity of MnTiO3 across ferroelectric and paraelectric phases

In this study, we present the ceramic's structural, dielectric, and impedance characteristics obtained through the solid-state reaction method synthesis. Initial structural analysis was conducted utilizing X-ray diffraction at room temperature. The XRD results show that MnTiO3 exhibits rhombohedral symmetry. SEM images of MnTiO3 show the presence of both cubic and large agglomerated grains. This demonstrates the effectiveness of the dielectric HN model in controlling the mechanism. The alternating current (ac) conductivity, when examined in relation to frequency, displays a remarkable adherence to the Johnscher's power law. The change in electrical modulus and the width at half maximum (FWHM) of the curve indicate that the as-prepared sample exhibits a non-Debye relaxation mechanism. Both the temperature data and the frequency dependence of the impedance were used to analyze the electrical conductivity of the sample, which showed a negative temperature coefficient resistance (NTCR).

Study on Cold Start of Fuel Cell Stack Combined with Liquid Heating and Fuel Cell Self-starting

Fuel cell vehicles have been paid more and more attention, while the low temperature cold start capability has become the main reason that hinders their large-scale commercialization, especially for the fuel cell bus. Generally, liquid heating (positive temperature coefficient thermistor (PTC) as the heat source) and fuel cell self-starting heating are the common cold start methods. However, these methods can bring about low start speed and high energy consumption. Therefore, a new cold start method combined with liquid heating and fuel cell self-starting is proposed to solve the problems. Besides, phase change material (PCM) is introduced to reduce energy consumption further. The PCM can absorb the waste heat of the fuel cell stack. Then the heat can be used for stack cold start by circulating liquid. In this study, a fuel cell bus cold start system simulating model is built. Based on this model, the fuel cell temperature and energy consumption during the cold start are analyzed. The results show that the method combined with liquid heating and fuel cell self-starting is superior to the liquid heating method. The cold start time and PTC consumption of the PTC-PCM-self-starting heating mode are 19.0% and 19.2% less than that of PTC-PCM heating mode. As for the combined heating method, dropping the self-starting temperature can not only increase the cold start speed but also reduce the proportion of electric heating. For every 1 °C decrease, the start-up time is shortened by 1% on average, and the waste heat utilization rate is increased by 0.5%. Although increasing the self-starting current can increase energy consumption, it will save more cold start time and reduce the proportion of electric heating. For every 1 A increase, the start-up time is shortened by 0.3% on average, and the waste heat utilization rate is increased by 0.1%. The PTC-PCM-self-starting heating method provides a way for fuel cell stack to start quickly and energy-efficiently in low temperature environment.