Effect of Temperature Variation on the Characteristics of Microwave Power AlGaN/GaN MODFET

The prime motivation for developing the proposed model of AlGaN/GaN microwave power device is to demonstrate its inherent ability to operate at much higher temperature. An investigation of temperature model of a 1 μm gate AlGaN/GaN enhancement mode n-type modulation-doped field effect transistor (MODFET) is presented. An analytical temperature model based on modified charge control equations is developed. The proposed model handles higher voltages and show stable operation at higher temperatures. The investigated temperature range is from 100 °K–600 °K. The critical parameters of the proposed device are the maximum drain current (I_Dmax), the threshold voltage (V_th), the peak dc trans-conductance (g_m), and unity current gain cut-off frequency (f_T). The calculated values of f_T (10–70 GHz) at elevated temperature suggest that the operation of the proposed device has sufficiently high current handling capacity. The temperature effect on saturation current, cutoff frequency, and trans-conductance behavior predict the device behavior at elevated temperatures. The analysis and simulation results on the transport characteristics of the MODFET structure is compared with the previously measured experimental data at room temperature. The calculated critical parameters suggest that the proposed device could survive in extreme environments.     

Thermal annealing and channel composition influences on the electrical properties of transparent-TFTs based on Zn-In-SnO ternary compound: Experiment and modeling

In this study, we present a quantitative analysis of the electrical properties of a series of bottom- gate top-contact n-channel transparent thin film transistors (TFTs) based on zinc indium tin oxide (Zn-In-SnO) ternary compound with various ZnO content. In addition, the effect of annealing on the TFTs electrical properties was examined theoretically and experimentally. The obtained results revealed that the thermal annealing of fabricated devices in air atmosphere at 300°C has enhanced their performances; this behavior is well observed for all devices fabricated with different composition of Zn-In-SnO. TFTs having the lowest Zn content of 17.1% and annealed 300^°C showed the high electrical performances in term of drain current, saturation mobility, threshold voltage. For the total resistance modeling of the fabricated devices with various content of ZnO and that annealed 300 ^°C, grain boundary model based on Meyer–Neldel rule was applied. The obtained results revealed that the total resistance was increased with increasing ZnO content. Furthermore, an analytical model has been refined in order to reproduce the current-voltage relationships of the fabricated TFTs using the overall resistance obtained from the NMR–GBT model. The calculated results are in good agreement with the experimental measurements of all fabricated devices. The obtained performance of TFTs based on zinc indium tin oxide with low content of ZnO and annealed will be promising for application in the future backplane of flat panel displays.     

AlInGaAs垂直谐振腔顶面发射半导体激光器横向温度效应的解析热模型及其表征

依据增益导波垂直谐振腔顶面发射半导体激光器热区特点与室温连续波运行条件建立了一个优化的全解析热模型，对AlInGaAs/AlGaAs垂直谐振腔激光器的横向温度效应进行了详细的解析计算分析，其完全的解析表达展示了更加清晰的横向热场物理图像，对于器件内部热场变化规律的理论预期与实验结果完全符合. 该解析模型及其结果对于研究热稳态下的垂直谐振腔激光器热动力学特性，优化器件结构和控制激光器的阈值电流与功率温度饱和效应，特别是二维面阵器件中的横向热叠加效应提供了一个有用的工具. 关键词： 半导体面发射激光器 热效应 解析计算模型.     

Medium-temperature phase-change materials thermal characterization by the T-History method and differential scanning calorimetry

This paper presents a thermal characterization of salt mixtures applying the T-History method (THM) and the differential scanning calorimetry (DSC) techniques. By using water as a standard substance, the original THM was developed to analyze materials with melting points under 100°C. This is the first research that proposes to replace water by glycerin to characterize medium-melting-temperature phase-change materials (PCMs) (140–220°C). Moreover, the DSC technique was used to validate and compare the results obtained with the THM for each mixture. For instance, the system compound of 40% KNO₃–60% NaNO₃ was studied, and the enthalpy of fusion determined by THM and DSC differs by 2.1% between each method. The results given by the two methods for all mixtures showed that both techniques are complementary and present satisfactory agreement with the specialized literature.     

Achromatic terahertz quarter-wave retarder in reflection mode

A compact achromatic quarter-wave retarder (QWR) operating in reflection mode is designed for using in terahertz region. It is a composite device utilizing form birefringence and Fabry–Pérot (FP) interference. Under illumination of plane waves with incidence angle of 45°, from 1.8 THz to 2.8 THz, the QWR achieved only ±2° variation around 90° phase retardation, enlarging the working bandwidth of ordinary QWR greatly. An analytical model combining transmission-line (TL) theory with effective medium theory (EMT) is presented and results agree well with the time-consuming numerical calculation. The 38 μm thick construction is simple and easy for fabrication by the existing lithographic technique and a promising application in terahertz or other frequency region is believed.     

Study on the thermoelectric properties of porous Bi-Te films deposited using thermal evaporation on AAO template

The application of thermoelectric films is limited to retain the temperature gradient. In this study, the Bi-Te films are deposited on the AAO template with a pore size of 100 nm using thermal evaporation. The results show that the conductive types of the Bi-Te film are tuned by source temperature. The power factor of the p-type porous film decreases 36% by comparing to that of the p-type nonporous film (1020 μW/mK² at 250 °C). Meanwhile, the temperature difference in the porous device is maintained and is approximately 5.0 °C. Thus, the maximum output power is achieved in the porous device (about 25 pW), which is 5 times higher than that of the nonporous device. This provides a method to improve the conversion efficiency of thermoelectric film device by maintaining the temperature difference by using porous structure.     

Optimizing the EDFA gain for WDM lightwave system with temperature dependency

The gain-flattened erbium-doped fiber amplifier (EDFA) is a key device for wavelength division multiplexing (WDM) modern optical network systems. A flat spectral gain EDFA has been achieved by controlling the doped fiber length and the pump power. The purpose of this paper is to study the variation of gain flattening over the temperature range from ?20 to +60 °C. The results obtained here indicate that gain flatness increases as temperature increases.     

Thermal Decomposition Behavior of a Heterocyclic Aramid Fiber

Heterocyclic aramid fibers are one of the high-performance fibers with excellent mechanical and thermal properties. In this article, the thermal decomposition behaviors of a type of the fibers were studied in nitrogen and air by pyrolysis/gas chromatography–mass spectrometry (Py/GC-MS), thermal gravimetric analysis–differential thermal analysis/Fourier transform infrared spectroscopy (TGA-DTA/FTIR), and thermal gravimetric analysis–differential thermal analysis/mass spectrometry (TGA-DTA/MS). The results showed that under nitrogen atmosphere, the thermal decomposition mainly happened between 520°C and 580°C, the temperature of the maximum weight loss rate was 550°C, and the weight remaining at 800°C was 58%. HCN, NH₃, NO₂, NO, CO₂, CO, H₂O, and some other compounds containing benzene rings were detected by the TGA-DTA/FTIR. Among these released chemicals, the intensity of the absorption peak assigned to CO₂ was the strongest. These chemicals were also identified by the TGA-DTA/MS. The Py-GC/MS analysis revealed that the number of chromatographic peaks increased with the increase of temperature. Most of the pyrolysis products were produced between 550°C and 600°C, which represented the major pyrolysis process. Moreover, the detection of benzene ring containing compound fragments reflected the process of the molecular chain scission. In air atmosphere, the thermal decomposition mainly happened between 500°C and 680°C. The maximum weight loss rate was observed at 600°C, and almost 100% weight was lost at 900°C. NH₃, NO₂, CO₂, and H₂O were detected by the TGA-DTA/MS, and the ion current intensity of CO₂ was again the strongest with a strong oxidation reaction at around 670°C. It was speculated that the thermal decomposition began with the breaking of the bonds between PPTA (poly-p-phenylene terephthalamide) blocks and heterocyclic blocks at high temperature. Then, with the increase of temperature, the chemical bonds inside the PPTA blocks and heterocyclic blocks were broken. In this process, free radicals that led to restructuring and new breakages to produce micromolecular products were introduced.     

Calculation of the Thermal Parameters of Boron Silicide by Differential Scanning Calorimetry

The DSC, TG, DTA, and DTG analyses of boron silicides, which are widely applied in radiation material science and nuclear technology, have been performed depending on the thermal treatment rate. The kinetic parameters (energy, enthalpy, oxidation reaction rate, heat capacity, and activation energy) of effects occurring in the thermal treatment of boron silicides of 99.5% purity within a temperature range of 25–900°C at a rate of 5–20°C/min have been established. It has been established that the phase transition typical for silicides with its central peak at a temperature 572 ± 5°C can exist in boron silicides depending on the thermal treatment rate. In TG and DTG spectra, this appears as an oxidation thermal effect at T ≥ 660°C with a change increase in mass of nearly 9%.     

Measurement of thermal conductivity of fluids using 3-<Emphasis Type="Italic">ω</Emphasis> method in a suspended micro wire

A thermally controlled, compact device employing the 3-ω technique, used to measure the thermal conductivity of fluids, is designed, developed, and presented in this paper. The 3-ω method, which analyzes temperature oscillations data in the frequency domain, requires a microscopic sample and extremely low heating power. The functionality is derived from the approximate solutions of temperature oscillations of a line heater based on the infinite line-heater model over an empirically and analytically chosen range of frequencies. The method is devoid of errors related to transient measurements, fluid thermal stratification and mobility errors, which pose difficulties in other methods. A platinum (99.99% pure) wire of 50 μm diameter and a length of 30 mm, suspended in a sample volume of 25 μl of the test fluid, serves simultaneously as the heater and thermometer. Structure-wise, the device is designed to support measurements over a range of temperatures and fluid pressures providing modularity and flexibility to the instrument. The device is successfully employed to measure the thermal conductivity of de-ionized water for temperatures between 15 and 35°C with an accuracy of ±1.2% inmeasurement.     

复合抛物面聚光器作为可见光通信光学天线的设计研究与性能分析

针对室内可见光通信的特点, 选择复合抛物面聚光器作为可见光通信系统光学天线, 介绍了复合抛物面聚光器的几何结构和光学特性, 利用光学仿真软件 TracePro对复合抛物面聚光器进行了设计、建模与仿真. 通过对不同光源条件下复合抛物面聚光器聚光特性的仿真发现: 在光源为朗伯辐射模型时复合抛物面聚光器的聚光性能更好, 且视场角越小增益越高; 但接收端与光源的相对位置对小视场复合抛物面聚光器的实际增益有明显影响, 在仿真条件下, 视场角为10°的复合抛物面聚光器实际增益为22.88, 比理论值降低了31%. 在此基础上, 在一个5 m×5 m×3 m的房间中对采用复合抛物面聚光器为光学天线的室内可见光通信系统进行了建模, 分别得到了直射链路和非直射链路下房间内各个位置的光功率分布. 仿真结果表明, 采用一个视场角为60°的复合抛物面聚光器为光学天线, 两种链路下平均接收功率分别提高了4.29 dBm和4.77 dBm, 非直射链路比直射链路的平均接收功率提高了11.2%.     

Thermal mode-switching of a passively Q-switched microchip Nd:YAG laser

In the present article, the thermal distribution inside the gain medium of a passively Q-switched microchip laser is modeled and simulated for actual practical values associated with an available microchip laser constructed in our laboratory. The effects of the non-uniform heat distribution on the spectral properties of the output laser beam have been investigated and simulated with the variation of diode-pump power and pulse repetition rate. It is observed that the gain bandwidth as well as Optical Path Difference (OPD) values of the propagating pulses are significantly decreased, while the Nd:YAG chip is cooled down to a certain value. The validity of the utilized model is checked by setting and characterizing the spectral properties of a fabricated laboratory microchip laser under different heating conditions. It is verified that when the temperature of the gain material is changed by an electronically controlled Peltier device, the spectrum of the output laser beam can be switched between single- and dual-mode situations. This physical character has shown good agreement between the presented model and obtained experimental results.     

Morphology of strain-induced crystallization of natural rubber. Part II. X-Ray studies on cross-linked vulcanizates

The results of wide- and small-angle X-ray studies on bulk vulcanizates of natural rubber are described. At room temperature no discrete X-ray peaks occur at small angles, even for highly strained (700%) specimens for which a significant degree of crystallinity is present at room temperature. Below the isotropic melting point of the network, discrete small-angle X-ray scattering develops for both unstrained and strained specimens. The Bragg periodicity corresponds closely to the longitudinal lamellar spacing from electron micrographs of thin films under corresponding conditions. The X-ray long period exhibits significant thermal reversibility. Upon heating, following initial crystallization at ?25°C, the long period continuously increases from about 140 Å at ?25°C to about 250 Å at 17°C. After heating, progressive cooling from room temperature results in continuous decreases in long period and at least 80% of the initial increase on heating is recovered. These results in conjunction with electron microscopy data (Part I) strongly suggest that thermally reversible changes in physical lamellar periodicity occur for natural rubber and are discussed in detail in light of current concepts concerning long period reversibility in bulk polymers.     

Thermal activation of current in an inhomogeneous Schottky diode with a Gaussian distribution of barrier height

This paper investigates the thermal activation behaviour of current in an inhomogeneous Schottky diode with a Gaussian distribution of barrier height by numerical simulation. The analytical Gaussian distribution model predicted that the I--V--T curves may intersect with the possibility of the negative thermal activation of current, but may be contradictory to the thermionic emission mechanism in a Schottky diode. It shows that the cause of the unphysical phenomenon is related to the incorrect calculation of current across very low barriers. It proposes that junction voltage V_j, excluding the voltage drop across series resistance from the external bias, is a crucial parameter for correct calculation of the current across very low barriers. For correctly employing the thermionic emission model, V_j needs to be smaller than the barrier height Ф. With proper scheme of series resistance connection where the condition of V_j > Ф is guaranteed, I--V--T curves of an inhomogeneous Schottky diode with a Gaussian distribution of barrier height have been simulated, which demonstrate normal thermal activation. Although the calculated results exclude the intersecting possibility of I--V--T curves with an assumption of temperature-independent series resistance, it shows that the intersecting is possible when the series resistance has a positive temperature coefficient. Finally, the comparison of our numerical and analytical results indicates that the analytical Gaussian distribution model is valid and accurate in analysing I--V--T curves only for small barrier height inhomogeneity.     

Temperature dependence of the hyperfine quadrupolar interaction in K2ZrF6

The thermal dependence of the hyperfine quadrupolar interaction of¹⁸¹Ta at Zr sites in K₂ZrF₆ has been studied by TDPAC between RT and 511 °C. Four phases of ever increasing symmetry have been found in the range RT-437 °C. The possible existence of a healing mechanism with an activation energy of 0.187±0.040 eV is discussed for the lattice recovery in the range 270 °C–335 °C. The results at temperatures above 437 °C can be understood assuming a thermal decomposition of the compound.     

Solid state photoelectrochemical cells utilising graphite thin films counter electrode

This paper reports the use of graphite thin films as a counter electrode of a solid state photoelectrochemical cells of ITO/TiO₂/PVC-LiClO₄/graphite. The photoelectrochemical cells material was a screen-printed layer of titanium dioxide onto an ITO-covered glass substrate which was used as a working electrode of the device. The solid electrolyte used was PVC-LiClO₄ that was prepared by solution casting technique. The graphite films which serve as a counter electrode were prepared onto glass substrate by electron beam evaporation technique at substrate temperatures variation of 25, 50, 100, 150 and 200 °C. The dependence of sheet resistance and surface morphology of the graphite films on substrate temperature were studied. The films deposited at 25 °C shows the smoothest surface morphology and the smallest grain size. Bigger grain size, rougher surface morphology of graphite film counter electrode. The current-voltage characteristics of four devices utilising the graphite counter electrode with different substrate temperature in dark as well as under illumination of 100 mWcm⁻² light from a tungsten halogen lamp were recorded at room temperature and at 50 °C, respectively. It was found that the photovoltaic parameters of the device such as short-circuit current density, J_sc and open-circuit voltage, V_oc increases with the decreasing average grain size of the graphite counter electrode. The device whose graphite film counter electrode was deposited onto the glass substrate at 25 °C gave the highest J_sc of 0.32 μA/cm² and V_oc of 117 mV, respectively.     

An RLC interconnect analyzable crosstalk model considering self-heating effect

According to the thermal profile of actual multilevel interconnects, in this paper we propose a temperature distribution model of multilevel interconnects and derive an analytical crosstalk model for the distributed resistance-inductance-capacitance (RLC) interconnect considering effect of thermal profile. According to the 65-nm complementary metal-oxide semiconductor (CMOS) process, we compare the proposed RLC analytical crosstalk model with the Hspice simulation results for different interconnect coupling conditions and the absolute error is within 6.5%. The computed results of the proposed analytical crosstalk model show that RCL crosstalk decreases with the increase of current density and increases with the increase of insulator thickness. This analytical crosstalk model can be applied to the electronic design automation (EDA) and the design optimization for nanometer CMOS integrated circuits.     

Thermal Performance of a Capillary Pumped Loop for Automotive Cooling

The design and test results for a capillary pumped loop (CPL) for thermal management of up to 210 W at the source and heat transfer over a distance of 1 m are discussed. The design configuration of the CPL evaporator consists of an internally grooved aluminum evaporator, 31.70-mm outer diameter and 500-mm long, fitted with a porous ultra-high molecular weight polyethylene wick, 8- to 15-μm pore radius, and 38% porous volume. Heat was transferred using a stainless steel tube of 4.5-mm internal diameter for vapor and liquid lines. High-grade acetone (99.99% pure) was used as the heat transfer fluid inside the loop. In the tests, thermal characteristics of the CPL were specifically studied with respect to the temperature control capability using an active thermal device on the reservoir and to the start-up process through pressure priming of the capillary evaporator. The loop was able to start-up successfully at both low and high heat loads, although proper priming of the wick structure before start-up was necessary to attain low evaporator temperatures during steady-state operation. While maintaining constant reservoir temperature through active means, the loop was able to control evaporator temperature within 55 ± 3°C, even with changing input heat from 30 to 210 W. Total thermal resistance from the evaporator surface to the surroundings was 0.19° to 1.15° C/W with the minimum value achieved at the maximum heat load of 210 W. This study is intended to illustrate the thermal potential of the CPL as an effective temperature control device in automotive applications.     

Numerical and Experimental Study on the Device Geometry Dependence of Performance of Heterjunction Phototransistors

Heterojunction phototransistors(HPTs)with scaling emitters have a higher optical gain compared to HPTs with normal emitters.However,to quantitativel.y describe the relationship between the emitter-absorber area ratio(A_e/A_a)and the performance of HPTs,and to find the optimum value of A_e/A_a for the geometric structure design,we develop an analytical model for the optical gain of HPTs.Moreover,five devices with different A_e/A_a are fabricated to verify the numerical analysis result.As is expected,the measurement result is in good agreement with the analysis model,both of them confirmed that devices with a smaller A_e/A_a exhibit higher optical gain.The device with area ratio of 0.0625 has the highest optical gain,which is two orders of magnitude larger than that of the device with area ratio of 1 at 3 V.However,the dark current of the device with the area ratio of 0.0625 is forty times higher than that of the device with the area ratio of 1.By calculating the signal-to-noise ratios(SNRs) of the devices,the optimal value of Ae/Aa can be obtained to be 0.16.The device with the area ratio of0.16 has the maximum SNR.This result can be used for future design principles for high performance HPTs.     

Thermographic diagnosis system and imaging algorithm by distributed thermal data using single infrared sensor

An infrared diagnosis device provides two-dimensional images and patient-oriented results that can be easily understood by the inspection target by using infrared cameras. However, this device has disadvantages such as large size, high price, and inconvenient maintenance. In this regard, this study has proposed a small diagnosis device for body heat using a single infrared sensor and implementing an infrared detection system using a single infrared sensor and an algorithm that represents thermography using the obtained data on the temperature of the point source. The developed system had a temperature resolution of 0.1 °C and reproducibility of ±0.1 °C. The accuracy was 90.39% at the error bound of ±0 °C and 99.98% at that of ±0.1 °C. To evaluate the proposed algorithm and system, the infrared images of the camera method were compared. To verify the device’s clinical applicability, thermal images with clinical meaning were obtained from a patient who had lesions.