Showing papers on "Temperature coefficient published in 2022"

Design of a Sub-6 GHz Dielectric Resonator Antenna with Novel Temperature-Stabilized (Sm1-xBix)NbO4 (x = 0-0.15) Microwave Dielectric Ceramics.

Abstract: Microwave dielectric ceramics exhibiting a low dielectric constant (εr), high quality factor (Q × f), and thermal stability, specifically in an ultrawide temperature range (from -40 to +120 °C), have attracted much attention. In addition, the development of 5G communication has caused an urgent demand for electronic devices, such as dielectric resonant antennas. Hence, the feasibility of optimizing the dielectric properties of the SmNbO4 (SN) ceramics by substituting Bi3+ ions at the A site was studied. The permittivity principally hinges on the contribution of Sm/Bi-O to phonon absorption in the microwave range, while the reduced sintering temperature results in a smaller grain size and slightly lower Q × f value. The expanded and distorted crystal cell indicates that Bi3+ doping effectively regulates the temperature coefficient of resonant frequency (TCF) by adjusting the strains (causing the distorted monoclinic structure) of monoclinic fergusonite besides correlating with the permittivity. Moreover, a larger A-site radius facilitates the acquisition of near-zero TCF values. Notably, the (Sm0.875Bi0.125)NbO4 (SB0.125N) ceramic with εr ≈ 21.9, Q × f ≈ 38 300 GHz (at ∼8.0 GHz), and two different near-zero TCF values of -9.0 (from -40 to +60 °C) and -6.6 ppm/°C (from +60 to +120 °C), respectively, were obtained in the microwave band. A simultaneous increase in the phase transition temperature (Tc) and coefficients of thermal expansion (CTEs) by A-site substitution provides the possibility for promising thermal barrier coating (TBC) materials. Then, a cylindrical dielectric resonator antenna (CDRA) with a resonance at 4.86 GHz and bandwidth of 870 MHz was fabricated by the SB0.125N specimen. The exceptional performance shows that the SB0.125N material is a possible candidate for the sub-6 GHz antenna owing to the advantages of low loss and stable temperature.

Crystal structure, chemical bond characteristics, infrared reflection spectrum, and microwave dielectric properties of Nd ₂ (Zr <sub> 1− <i>x</i>

Abstract: Microwave dielectric ceramics (MWDCs) with low dielectric constant and low dielectric loss are desired in contemporary society, where the communication frequency is developing to high frequency (sub-6G). Herein, Nd₂(Zr_1−xTi_x)₃(MoO₄)₉ (NZ_1−xT_xM, x = 0.02–0.10) ceramics were prepared through a solid-phase process. According to X-ray diffraction (XRD) patterns, the ceramics could form a pure crystal structure with the R $\overline{3}$c (167) space group. The internal parameters affecting the properties of the ceramics were calculated and analyzed by employing Clausius–Mossotti relationship, Shannon’s rule, and Phillips–van Vechten–Levine (P–V–L) theory. Furthermore, theoretical dielectric loss of the ceramics was measured and analyzed by a Fourier transform infrared (IR) radiation spectrometer. Notably, when x = 0.08 and sintered at 700 ℃, optimal microwave dielectric properties of the ceramics were obtained, including a dielectric constant (ε_r) = 10.94, Q·f = 82,525 GHz (at 9.62 GHz), and near-zero resonant frequency temperature coefficient (τ_f) = −12.99 ppm/℃. This study not only obtained an MWDC with excellent properties but also deeply analyzed the effects of Ti⁴⁺ on the microwave dielectric properties and chemical bond characteristics of Nd₂Zr₃(MoO₄)₉ (NZM), which laid a solid foundation for the development of rare-earth molybdate MWDC system.

Improved electrical resistivity-temperature characteristics of insulating epoxy composites filled with polydopamine-coated ceramic particles with positive temperature coefficient

Abstract: Incorporating positive temperature coefficient (PTC) ceramic particles into polymer matrices provides a promising solution to suppress the distortion of DC electric field within polymeric insulation caused by the negative temperature coefficient (NTC) of electrical resistivity. However, along with the suppressed NTC effect comes the decrease of dielectric strength due to the agglomeration of PTC particles. Here, we addressed this issue in the epoxy resin system by coating polydopamine onto PTC particles to improve the compatibility with matrix, thus achieving a maintained DC breakdown strength. The improved evenness of PTC particles dispersion also provided sufficient interface to generate interface charge traps, which contributed to the increase of electrical resistivity at elevated temperature. Combined with temperature dependent charge carrier amount affected by PTC filler, NTC effect of epoxy composites was further mitigated and the maximum DC electric field was reduced by 49% within insulation assessed by the simulation of HVDC bushing. The method proposed in this paper is crucially important for the reliable and economic application of composite dielectrics under harsh conditions.

Highly flexible TPU/SWCNTs composite-based temperature sensors with linear negative temperature coefficient effect and photo-thermal effect

Abstract: Conductive polymer composites (CPCs) based flexible temperature sensors are highly desirable for electronic skins (e-skins) due to their flexibility, good processability, and lightweight. However, it is still a challenge to fabricate a flexible CPC-based temperature sensor with linear negative temperature coefficient (NTC) effect because CPCs normally exhibit non-monotonic dependence on temperature. Herein, we prepare the flexible thermoplastic polyurethane (TPU)/single-walled carbon nanotubes (SWCNTs) composites by the facile method of solution blending and thermal annealing. The as-prepared composites exhibit a monotonic and linear NTC effect in the temperature range of 30–100 °C, which can be designed into highly flexible and sensitive temperature sensors. The as-prepared sensor can achieve respiratory monitoring, cellphone charging time monitoring and non-contact temperature detection, attributing to its high accuracy (0.1 °C), excellent reproducibility and high reliability (Deformation and heating rate have no effect on the thermal response.). Moreover, the sensors also show resistance response to infrared radiation owing to the excellent photo-thermal effect of SWCNTs. The integrated linear NTC effect and photo-thermal effect endow the sensors with tremendous potentials in e-skins and wearable electronics.

Self-sensing shape memory polymer composites reinforced with functional textiles

Abstract: In this work, thermally reduced graphene oxide coated glass fabric (TRGO-GF) was used as a sensing element and reinforcement to manufacture thermo-responsive polyurethane-based self-sensing shape memory polymer composite (SSMPC). It was demonstrated that the proposed novel functional SSMPC was able to detect changes in temperature and shape recovery at the same time, providing its utility in a number of applications. The temperature sensing capability and the relationship between temperature and electrical resistance of manufactured SSMPC was demonstrated by inducing heating and cooling cycles using a heating film attached to the sample. It was shown that the proposed SSMPC was able to detect any changes in temperature taking place within the composite. The SSMPC showed a negative temperature coefficient (NTC) of resistance of −14.80 × 10−3 (°C)−1 and −7.25 × 10−3 (°C)−1 for testing regime I (30–40 °C) and testing regime II (40–50 °C) respectively. It was demonstrated through experiments that the SSMPC was able to detect the recovery ratio and rate of recovery during the thermally induced shape recovery process. It was also revealed that the extent and rate of shape recovery can be manipulated via controlling the thermal stimulus.

Impact of Co2+ on the spectral, optoelectrical, and dielectric properties of Mg0.25Ni0.25Cu0.5−xCoxFe1.97La0.03O4 ferrites prepared via sol–gel auto-combustion route

Abstract: Spinel ferrites are attaining huge importance in the modern era, due to their incredible properties, which means they are widely used in various fields. Therefore, in this paper a cost-effective and environmentally friendly preparation of Mg0.25Ni0.25Cu0.5−xCoxFe1.97La0.03O4 (x = 0.0, 0.125, 0.25, 0.375, and 0.5) ferrites via the sol–gel auto-combustion method were carried out. How the structural and functional properties varied with the Cu to Co ratio was studied. It was found that the crystallite size (D) was reduced from 47.2 to 27.6 nm as the Co2+ increased from x = 0.0 to 0.5. While the two major absorption bands including the higher frequency band (υ1) and lower frequency band (υ2) lie in the range of 573.80–538.65 cm−1 and 470.37–405.65 cm−1 belong to the spinel matrix. Five active Raman modes were found in the range of wave number 200–800 cm−1 corresponds to the sublattices of the spinel structure. The optical bandgap increased from 0.85 eV to 1.33 eV for x = 0.0 to x = 0.25 and then observed optical band gap was 1.15 eV and 1.46 eV for x = 0.375 to x = 0.5. The resistivity (ρ) was minimum for Co2+ concentration x = 0.25 at low temperature, while at high temperature the resistivity (ρ) was maximum for Co2+ doping x = 0.25. The temperature coefficient of resistance percentage (TCR %) was −2.66%/K at 513 K for x = 0.25, as shown in the graphical abstract. At lower frequencies, the impedance was observed maximum for Co2+ doping x = 0.125 and minimum for x = 0.375. These findings indicate that the as-prepared ferrites are a potential candidate for optoelectrical and bolometric devices.

Platinum Disulfide (PtS₂) and Silicon Pyramids: Efficient 2D/3D Heterojunction Tunneling and Breakdown Diodes

Abstract: p–n junctions constructed from the group-10 TMDCs, or namely, transition metal dichalcogenides with an intrinsic layered structure, are not considerably reported. This study presents a mechanical exfoliation-based technique to prepare PtS2/Si pyramid p–n junctions for an investigation of tunneling and breakdown diodes. The demonstrated p–n diode exhibited a high rectifying performance reaching a rectification ratio (If/Ir) of ∼7.2 × 104 at zero gate bias with an ideality factor of ∼1.5. The Zener tunneling was observed at a low reverse bias region of breakdown voltage (from −6 to −1.0 V) at various temperatures (50 to 300 K), and it was a negative coefficient of temperature. Conversely, for the greater breakdown voltage regime (−15 to −11 V), the breakdown voltage increased with the increased temperature (200 to 300 K), indicating a positive coefficient of temperature. Therefore, this phenomenon was attributed to the avalanche breakdown. The p–n junctions displayed photovoltaic characteristics under the illumination of visible light (500 nm), such as a high responsivity (Rph) and a photo gain (G) of 11.88 A/W and 67.10, respectively. The maximum values for both the open-circuit voltage (VOC) and the short-circuit current (ISC) were observed to be 0.45 V and 10 μA, respectively, at an input intensity of light of 70.32 mW/cm2. The outcomes of this study suggest that PtS2/ Si pyramid p–n junctions may be employed in numerous optoelectronic devices including photovoltaic cells, Zener tunneling diodes, avalanche breakdown diodes, and photodetectors.

Highly sensitive flexible strain and temperature sensors using solution processed graphene palladium nanocomposite

Abstract: This work reports highly sensitive, flexible, and fast response strain sensors for load cell application and a temperature sensor with a tunable temperature coefficient of resistance (TCR). The proposed sensors are fabricated with solution-processed reduced graphene oxide (rGO) and Palladium (Pd) nanocomposite. The strain sensor is encapsulated with a Polydimethylsiloxane (PDMS) substrate to ensure flexibility and moisture protection. The strain sensor's performance was studied; the gauge factor was calculated as ~22 to ~198 in the strain range of 0.05–0.625%. The sensor exhibited high durability beyond 1000 cycles, fast response (~39 ms). A binocular type of load cell was designed and simulated using COMSOL Multiphysics software to study the strain profiles under applied load conditions. The designed load cell was tested with a micro universal test machine (UTM) for different loads. Its response was stable, linear, and repeatable with a sensitivity of 23.52 mV/V/Kg and a resolution of 0.0085 Kg/mV at an excitation of 5 V. Additionally, the nanocomposite was investigated for temperature response with different ratios of rGO-Pd and tuned the TCR of the sensor. The temperature sensor displayed good sensitivity and linearity with both negative temperature coefficient (NTC) and positive temperature coefficient (PTC) depending on rGO and Pd nanoparticles (NPs) composition.

Design and Fabrication of a Satellite Communication Dielectric Resonator Antenna with Novel Low Loss and Temperature-Stabilized (Sm1–xCax) (Nb1–xMox)O4 (x = 0.15–0.7) Microwave Ceramics

Abstract: Phase transition–structure–dielectric properties in microwave band correlations were determined for the (Sm1–xCax) (Nb1–xMox)O4 (SNCMo@x) system. X-ray and Raman analyses along with selected-area electron diffraction indicated that SNCMo@x (0.15 ≤ x < 0.375) ceramics crystallize in the I2/a space group (monoclinic fergusonite), whereas the I41/a space group (tetragonal scheelite) best describes SNCMo@x (0.375 ≤ x ≤ 0.7), suggesting that the increased ionic radius of the A-site effectively contributed to the ferroelastic phase transition and ensures the stability of the scheelite phase. The SNCMo@x ceramic materials exhibit composition-dependent permittivity (εr) with a distribution between 12.0 and 17.7. The distortion and deformation of the [BO] polyhedra should be responsible for the shift from negative to positive temperature coefficient of resonant frequency (TCF) and the irregular behavior of the quality factor (Q × f). An optimum microwave dielectric performance was achieved for SNCMo@0.18 (εr ∼ 17.1, Q × f ∼ 52, 800 GHz at ∼8.80 GHz, and TCF ∼ −1.4 ppm/°C). This work demonstrates the important role of simultaneous substitution of A/B cations on [BO] polyhedral distortion and deformation in RENbO4 materials and its significant effect on the microwave dielectric properties. Also, the SNCMo@0.18 ceramic has been designed as a cylindrical dielectric resonator antenna with a high simulated radiation efficiency (97.1%) and gain (5.96 dBi) at the center frequency (7.75 GHz), indicating its promising application in X-band satellite communication (7.62–7.89 GHz) because of its adjustable permittivity, low loss, and good temperature stability.

Direct ink writing of a graphene/CNT/silicone composite strain sensor with a near-zero temperature coefficient of resistance

Abstract: Using a facile direct ink writing technique, highly stretchable graphene nanoplatelet (GNP)/carbon nanotube (CNT)/silicone elastomer (GCE) fiber-shaped strain sensors are successfully prepared with a near-zero temperature coefficient of resistance.

Highly Sensitive Flexible Temperature Sensor Made Using PEDOT:PSS/PANI

Abstract: This work proposes a design, fabrication, and characterization of flexible temperature sensors using temperature-sensitive materials, which are applied using a drop coating method. Temperature-sensitive materials were fabricated by mixing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) with polyaniline (PEDOT:PSS/PANI). The temperature coefficient of resistance reached −0.803%/°C. Moreover, the detection resolution reached 0.1 °C and had a fast response time of 200 ms. In addition, the sensor can sense spatial temperatures. The sensor has the advantages of low cost, a simple preparation process, and the potential to be used in medical rehabilitation and early disease prevention.

Low sintering temperature, temperature-stable scheelite structured Bi[V1-x (Fe1/3W 2/3)x]O4 microwave dielectric ceramics

Abstract: Low sintering temperature Bi[V1−x(Fe1/3W2/3)x]O4 (BVFWx) (0.02 ≤ x ≤ 0.08) ceramics have been prepared by solid-state reaction. Compositions with 0.02 ≤ x ≤ 0.08 were scheelite-structure, but distortion of [BO4] tetrahedra decreased with increase in x. Although the crystal class remained monoclinic, the phase transition temperature (TC) decreased from 255 ℃ (BV) to 51 ℃ (BVFW0.08), suggesting that higher concentrations of (Fe1/3W2/3) would result in the stabilization of tetragonal scheelite at room temperature. The decrease in the [BO4] distortion additionally resulted in a large reduction in the magnitude of the temperature coefficient of resonant frequency (TCF) with near-zero values obtained by sintering BVFW0.08:2BVFW0.06 at 780 °C to give TCF ≈ −5.9 ppm / °C, relative permittivity εr ≈ 77.4, and microwave quality factor, Q × f ≈ 8100 GHz (@ ~ 4.3 GHz). These promising microwave dielectric properties coupled with the low sintering temperature suggest that BVFWx ceramics are of interest in low-temperature cofired ceramic (LTCC) technology, and for monolithic resonator and filter applications.

Investigation of Temperature Sensing Capabilities of GaN/SiC and GaN/Sapphire Surface Acoustic Wave Devices

Abstract: This paper proposes high sensitivity temperature sensors based on single port surface acoustic wave (SAW) devices with GHz resonance frequencies, developed on GaN/SiC and GaN/Sapphire, which permit wide range, accurate temperature determinations. In contrast with GaN/Si SAW based temperature sensors, SiC and Sapphire substrates enable the proper functionality of these devices up to 500°C (773 K), as the high resistivity Si substrate becomes conductive at temperatures exceeding 250°C (523 K) due to the relative low bandgap (and high intrinsic carrier concentrations). Low temperature measurements were carried out using a cryostat between -266°C (7 K) and room temperature (RT) while the high temperature measurements are made on a modified RF probe station. A polynomial fit was used below RT and a linear approximation was evidenced between RT and 500°C (773 K). The structures were simulated at different selected temperatures based on a method that couples Finite Element Method (FEM) and Coupling of Modes (COM). The measured temperature coefficient of frequency (TCF) is about 46 ppm/°C for GaN/SiC SAWs and reaches values of 96 ppm/°C for GaN/Sapphire SAW in the temperature range 25 – 500°C (298 K – 773 K).

Direct Ink Writing of Graphene/Cnt/Silicone Composite Strain Sensor with Near-Zero Temperature Coefficient of Resistance

Abstract: Developing wearable strain sensors with zero temperature coefficient of resistance (TCR), which is crucial to overcome the problem of temperature disturbance, has been scarcely studied. Herein, highly stretchable graphene nanoplatelet...