Showing papers on "Temperature coefficient published in 2023"

Microwave Dielectric Properties of Li3TiO3F Oxyfluorides Ceramics

Abstract: Using a solid-state reaction strategy, nominal Li3TiO3F oxyfluorides ceramics were fabricated, and its sintering behavior, microstructure, phase assemblages, as well as microwave dielectric performances were all investigated. The main phase of Li3TiO3F with cubic structures accompanied with small amounts of the LiF or Li2TiO3 secondary phase was identified by XRD analysis. SEM analysis showed that a uniform and dense microstructure was obtained for 750 °C-sintered samples. The dielectric constant (εr) and quality factor (Q × f) were found to be strongly correlated with porosity and grain size distribution, whereas the temperature coefficient of resonance frequency (τf) was mainly dominated by the phase assemblages. In particular, the 750 °C-sintered Li3TiO3F samples exhibited good microwave dielectric performances: εr = 18, Q × f = 57,300 GHz (under 9.2 GHz), τf = −43.0 ppm/°C.

Phase transitions and microwave dielectric behaviors of the (Bi _1‐x Li _0.5x Y _0.5x )(V _1‐x Mo _x )O ₄

Abstract: Microwave dielectric ceramics with intrinsic low sintering temperatures are potential candidates for low temperature co-fired ceramics technology. In the present work, the (Li0.5Y0.5)MoO4 ceramic with tetragonal scheelite structures was selected to improve microwave dielectric properties of BiVO4 ceramics. As proved by X-ray diffraction (XRD) results, scheelite structured solid-solution ceramics were formed with x value ≤0.1 in the (Bi1−xLi0.5xY0.5x)(V1−xMox)O4. In situ XRD results further confirmed that the addition of (Li0.5Y0.5)MoO4 also lowered transition temperature from distorted monoclinic to tetragonal scheelite structure. When x value increased further, zircon phase was detected by XRD. Room and high-temperature Raman spectra also supported the XRD results. Differences of thermal expansion coefficients of both monoclinic and tetragonal scheelite phases lead to an abnormality at phase transition temperature. Good microwave dielectric properties with permittivity above 70 and Qf (Q = quality factor = 1/dielectric loss and f = frequency) value above 8000 GHz were obtained in the (Bi1−xLi0.5xY0.5x)(V1−xMox)O4 solid-solution ceramics with x value ≤0.1 sintered below 800°C. However, permittivity peak values at phase transition temperatures lead to large positive or negative temperature coefficient of resonant frequency, and this needs to be modified via composite technologies in the future.

Thermal sensing capability and current–voltage–temperature characteristics in Pt/n-GaP/Al/Ti Schottky diodes

Abstract: We have discussed the thermal sensing capability under a constant current level and current versus voltage ( I–V) traces by measuring the temperature of high series resistance Pt/ n-GaP/Al/Ti Schottky structures in the 100−320 K range. The Rs values of 35 Ω and 4.50 × 103 kΩ for the device have been determined from I–V traces at 320 and 100 K, respectively. The thermal sensing ( V–T) curves are expected to give a straight line at each current level. However, the V–T curves have deviated upward from linearity due to the high Rs value of the device after a certain temperature. The deviation point from linearity in V–T traces shifts to higher temperatures with an increase in bias voltage and current level. Thereby, the straight-line interval portion of the V–T curve has become too small with an increase in the current value. The thermal sensing coefficient α changed from 2.49 mV/K at 10 μA to 3.21 mV/K at 0.50 nA. Therefore, it has been concluded that the Pt/ n-GaP/Al/Ti Schottky barrier (SB) is preferable for thermal sensor applications at the small current levels of 0.50, 1.0, 2.0, and 10.0 nA with high sensitivity up to a minimum temperature of 100 K. From I–V curves, [Formula: see text] and ideality factor values have ranged from 1.200 eV and 1.066 at 320 K to 0.854 eV and 1.705 at 100 K. It has been reported in the literature that the large SB height leads to a better temperature response.

Novel high-entropy microwave dielectric ceramics Sr(La0.2Nd0.2Sm0.2Eu0.2Gd0.2)AlO4 with excellent temperature stability and mechanical properties

Abstract: Novel high-entropy Sr(La0.2Nd0.2Sm0.2Eu0.2Gd0.2)AlO4 ceramics with a layered perovskite structure have been prepared via the standard solid-state reaction method. The design of high-entropy improves the bond valence and subsequently optimizes the large negative temperature coefficient of resonant frequency (τf = −32 ppm/°C) of the simple SrLaAlO4 ceramics. Excellent temperature stability (τf = −6 ppm/°C) together with a relative permittivity (εr) of 18.6 and a quality factor (Qf = 14,509 GHz) are obtained in Sr(La0.2Nd0.2Sm0.2Eu0.2Gd0.2)AlO4 ceramics sintered at 1475 °C. It indicates that the present ceramics have great application prospects in passive microwave components such as resonators and filters. Meanwhile, significant improvements in compressive strength and strain are achieved, which are 1040 MPa and 15.7% for Sr(La0.2Nd0.2Sm0.2Eu0.2Gd0.2)AlO4 compared to 583 MPa and 12% in SrLaAlO4. The enhanced mechanical properties originate from the dislocation strengthening mechanism as the intertwining of interlayer lattices is revealed from the high-resolution transmission electronic micrographs.

Role of MWCNTs Loading in Designing Self-Sensing and Self-Heating Structural Elements

Abstract: This work proposes nanocomposites with carbon nanotubes characterized by self-sensing and self-heating properties. Recently, a growing interest in these two properties has been found in many industrial sectors, especially in the aerospace and automotive fields. While the self-sensing function allows diagnosing the presence of micro-damage in the material thanks to the detection of residual resistance, the self-heating function is exploited to properly tune the heating performance in terms of the heating rate and final temperature values. An electrical percolation value of around 0.5% by weight of carbon nanotubes was found by electrical characterization. The AC conductivity of the nanocomposites, in the range of 100 Hz to 1 MHz, evidences that beyond a CNTs amount of 0.5% wt/wt, they are characterized by a purely resistive behavior. The self-sensing analysis displayed a gauge factor value of 4.1. The solid thermal stability up to 300 °C makes the material suitable as a heating element at high temperatures. SEM investigations and temperature maps evidence a good dispersion of the conductive filler in the epoxy matrix and, consequently, good isotropy in heat distribution. As regards the trend of electrical resistance by varying the temperature, the electro-thermal investigation has shown the presence of both Positive Temperature Coefficient (PTC) and Negative Temperature Coefficient (NTC) behaviors with a predominance of NTC as soon as the temperature becomes closer to the glass transition temperature of the epoxy resin.

Enhanced reproducibility of positive temperature coefficient effect of TPO/HDPE blends via elastic crosslinking

Abstract: Positive temperature coefficient (PTC) materials usually have NTC effect, and the effect is poor. In this paper, conductive carbon black (CB) and amorphous elastomer (TPO) were introduced into high-density polyethylene (HDPE) by melting mixing and hot pressing methods and treated with peroxide crosslinking to improve the PTC properties of conductive composites. The results show that the percolation threshold reduces from 24 wt% to 22 wt% for crosslinked elastic networks. At the same time, the room temperature resistance of the composite material does not show monotonicity after three cycles, which has good reproducibility and eliminates the NTC effect of the composite material. This work provides both a simple way to produce high-performance PTC materials and a new idea to improve the stability of PTC materials.

Body Diode of 1.2kV SiC MOSFET: Unipolar and Bipolar Operation

Abstract: In this work, we investigate the Body Diode (BD) of a 40mOhm, 1.2kV SiC MOSFETs. We performed DC measurements and switching measurements, at Room Temperature (RT) and High Temperature (HT) (T=175°C), together with TCAD simulation and calibration. In switching measurements, we focused on the low side (LS) switch turn-on event, i.e., the BD turn-off event. We demonstrated that unipolar and bipolar BD can both be achieved with different VGS. With VGS=-5V, BD conducts in bipolar mode with carriers being injected via pn junction. This is rather well known in literature and is well characterized by the Negative Temperature Coefficient (NTC) of BD VF. Switching with BD VGS=-5V show strong reverse recovery-temperature dependent that caused by the augmented minority carrier injection at high temperature. Unipolar BD is achieved by channel conduction at VGS=0V and it is well characterized by BD VF - Positive Temperature Coefficient (PTC). Thanks to the unipolar nature, turn-on switching with BD VGS=0V show no reverse recovery-temperature dependent.

A double perovskite BiLaCoMnO6: synthesis, microstructural, dielectric and optical properties

Abstract: ABSTRACT In this paper, synthesis (solid-state reaction) and characterization (XRD, SEM, EDX, RAMAN, UV & LCR) of a double perovskite: BiLaCoMnO6 (BLCMO) are reported. The structural analysis suggests monoclinic crystal symmetry with average crystallite size = 79.6 nm and lattice strain = 0.00149 respectively. The analysis of the scanning electron microscopy (SEM) micrograph suggests the uniform distribution of grains. The analysis of energy-dispersive X-ray spectroscopy (EDX) spectrum suggests the presence of all constituent elements (Bi, La, Co, Mn & O) in both weight and atomic percentage. The study of the Raman spectrum provides information about the atomic vibration of all the constituent elements. The study of the UV visible spectrum provides of bandgap energy of 2.06 eV and impedance spectroscopy suggests a negative temperature coefficient of resistance (NTCR) character, while the modulus study predicts the presence of a non-Debye type of relaxation. The study of temperature-dependent resistance supports NTC thermistor applications.

Independent Heating Performances in the Sub-Zero Environment of MWCNT/PDMS Composite with Low Electron-Tunneling Energy

Abstract: The structural stability of various structures (railroads, bridges, buildings, etc.) is lowered due to freezing because of the decreasing outside temperature in winter. To prevent damage from freezing, a technology for de-icing has been developed using an electric-heating composite. For this purpose, a highly electrically conductive composite film with multi-wall carbon nanotubes (MWCNTs) uniformly dispersed in a polydimethylsiloxane (PDMS) matrix through a three-roll process was fabricated by shearing the MWCNT/PDMS paste, through a two-roll process. The electrical conductivity and the activation energy of the composite were 326.5 S/m and 8.0 meV at 5.82 Vol% of MWCNTs, respectively. The dependence of the electric-heating performance (heating rate and temperature change) on the applied voltage and environmental temperature (from −20 °C to 20 °C) was evaluated. The heating rate and effective-heat-transfer characteristics were observed to decrease as the applied voltage increased, while they showed the opposite tendency when the environmental temperature was at sub-zero temperatures. Nevertheless, the overall heating performance (heating rate and temperature change) was maintained with little significant difference in the considered external-temperature range. The unique heating behaviors can result from the low activation energy and the negative-temperature (T) coefficient of resistance (R) (NTCR, dR/dT < 0) of the MWCNT/PDMS composite.