Thermal Coefficient Of Resistance Research Articles

Silica/Epoxy Hybrid Encapsulation with High Heat-Resistance and Low Coefficient of Thermal Expansion

As a matrix resin of silica/epoxy hybrid encapsulation material with high heat resistance and low coefficient of thermal expansion, a tri-functional triglycidyl p-aminophenol (TGPAP) epoxy was successfully synthesized with the desired chemical structure by reacting epichlorohydrin with p-aminophenol using sodium hydroxide catalyst. In order to produce highly pure TGPAP with lower viscosity, a physical thin film vacuum distillation was carried out after synthesis, resulting in 97% purity with the viscosity of 700 cps at room temperature. Silica/TGPAP hybrid was fabricated using two kinds of ground fumed silica with different sizes of 20 μm and 9 μm. Glass transition temperature of the hybrid was found to be as high as 185 °C. Viscosity of the hybrid was 30,000 cps at room temperature before curing, implying good processability. When two different sizes of silica were used with 1 to 1 weight ratio, very low coefficient of thermal expansion of 22.79 ppm/°C was obtained. From thermal shock test, no cracks were observed even after 1,000 cycles of thermal shock between −40 °C and 125 °C when 67 wt% (45.8 vol%) of silica was involved in the hybrid. In conclusion, silica/TGPAP hybrid with improved processability, superior thermal shock resistance and low thermal expansion coefficient could be fabricated as electronic device encapsulation.

Low Operating Voltage Carbon-Graphene Hybrid E-textile for Temperature Sensing.

Graphene-coated polypropylene (PP) textile fibers are presented for their use as temperature sensors. These temperature sensors show a negative thermal coefficient of resistance (TCR) in a range between 30 and 45 °C with good sensitivity and reliability and can operate at voltages as low as 1 V. The analysis of the transient response of the temperature on resistance of different types of graphene produced by chemical vapor deposition (CVD) and shear exfoliation of graphite (SEG) shows that trilayer graphene (TLG) grown on copper by CVD displays better sensitivity due to the better thickness uniformity of the film and that carbon paste provides good contact for the measurements. Along with high sensitivity, TLG on PP shows not only the best response but also better transparency, mechanical stability, and washability compared to SEG. Temperature-dependent Raman analysis reveals that the temperature has no significant effect on the peak frequency of PP and expected effect on graphene in the demonstrated temperature range. The presented results demonstrate that these flexible, lightweight temperature sensors based on TLG with a negative TCR can be easily integrated in fabrics.

Analysis and research on the heating principle and causes of over-temperature of wind power converter

In view of the frequent over-temperature faults of ABB wind power converter in the process of using, this paper makes an in-depth study and detailed analysis on the over-temperature faults and solutions and effects of the converter through the analysis of the converter heating principle, technical transformation test, thermal simulation analysis, etc.It is found that there are three main reasons for the converter over-temperature fault which are high operating environment temperature, insufficient ventilation and increased thermal resistance coefficient. The three main causes of the over-temperature fault of the converter are analyzed and the technical modification scheme and operability analysis are put forward. In order to verify the technical modification effect, the operation temperature data of the technical modification unit and adjacent units were collected and compared with FLOTHERM software to simulate the heat change trend of the tower cylinder, and the influence of the scheme on the effect and other parts of the unit was judged. The results showed that the scheme could effectively solve the over-temperature problem of the converter and would not affect the normal operation of the wind turbine.

Numerical analysis of micro-channel heat sink using ethylene glycol based nanofluid in case of electronics cooling

As size of Integrated circuit is goes on decreasing and number of elements per chip increases, the heat dissipation per unit area of IC increases. Therefore, nanofluid forced convection cooling using an micro-channel heat sink is introduced as new research topic for efficient cooling of IC devices. In this manuscript, ethylene glycol base nanofluid has been studied by using five different oxide nanoparticles including Al2O3, TiO2, CuO, SiO2 and ZnO and varying their concentration from 0 to 10% volume fraction. Single phase model has been considered in simulation analysis using ANSYS Fluent 13.0 software and the results obtained from simulation have been validated with previous experimental and simulation work. Furthermore, several aspects of parameters such as local thermal resistance, pumping power and local heat transfer coefficient are presented in below manuscript. From the result it is analysed that ethylene glycol base nanofluid increases the heat transfer parameters very effectively. Further, as concentration of nanoparticle sis increases the convective heat transfer coefficient and pumping power increases. Also, CuO-EG nanofluid cooled electronics devices more efficiently compared to other nanofluid which are considered in this study.

Experimental inquisition of heat pipe: performance evaluation for different fluids

ABSTRACT A comprehensive study for testing of cylindrical heat pipe has been carried out with a number of experiments. Characteristics and performance of heat pipe were evaluated individually with water, methanol, and acetone at different inclination angles (θ). Results obtained show that heat pipe charged with acetone has the smallest thermal resistance and highest heat transfer coefficient compared to other working fluids, hence identified as the best working fluid for the prepared heat pipes. Also, the best angle of inclination for the working heat pipe is 45° as the thermal performance of the heat pipe is optimal at this angle.

Heating and cooling capacity of phase change material coupled with screen mesh wick heat pipe for thermal energy storage applications

The thermal performance of a phase change material (PCM) heat pipe system is experimentally analysed using acetone as heat pipe fluid in a heat load range of 10-50 W at different flow rates of the condenser coolant. The evaporator of the heat pipe is enclosed in a chamber which filled with a PCM or water. Heat inputs are applied at the evaporator of the heat pipe through the PCM or water. In this study, the heat retention as well as cooling time of the PCM-water are estimated at different heat loads and flow rates of condenser coolant. Similarly, the thermal resistance, evaporator and condenser heat transfer coefficients are also estimated at different heat loads. It is observed that the PCM takes more time during heating and cooling cycles to reach the steady-state temperatures and the temperature values reached during heating are also higher for PCM compared to water. The use of PCM enhances the thermal storage capacity and shows a maximum enhancement of 200% in heat retention time compared to water at 50 W. Moreover, a maximum enhancement of 63.6% is observed in the steady-state temperature of the PCM compared to water. Similarly thermal resistance, evaporator wall temperature and heat transfer coefficients of the heat pipe also vary for PCM and water. The experimental results indicate that PCM or water can be used in this combined system depending upon requirement of thermal storage or electronics cooling.

Multiscale Disordered Porous Fibers for Self-Sensing and Self-Cooling Integrated Smart Sportswear.

Smart clothing has demonstrated potential applications in a wide range of wearable fields for human body monitoring and self-adaption. However, current wearable sensors often suffer from not seamlessly integrating with normal clothing, restricting sensing ability, and a negative wearing experience. Here, integrated smart clothing is fabricated by employing multiscale disordered porous elastic fibers as sensing units, which show the capability of inherently autonomous self-sensing (i.e., strain and temperature sensing) and self-cooling. The multiscale disordered porous structure of the fibers contributes to the high transparency of mid-infrared human body radiation and backscatter of visible light, which allows the microenvironment temperature between the skin and clothing to drop at least ∼2.5 °C compared with cotton fabrics. After the capillary-assisted adsorption of graphene inks, the modified porous fibers could also possess real-time strain and temperature-sensing capacities with a high gauge factor and thermal coefficient of resistance. As a proof of concept, the integrated smart sportswear achieved the measuring of body temperature, the tracking of large-scale limb movements, and the collection of subtle human physiological signals, along with the intrinsic self-cooling ability.

Inverse-Coefficient Problem of Heat Transfer in Layered Nanostructures

The rapid development of electronics is leading to the creation and use of small electronic components, including nanoelements of a complex layered structure. The search for effective methods for cooling electronic systems dictates the need for the development of methods for the numerical analysis of heat transfer in nanostructures. A characteristic feature of the energy transfer in such systems is the dominant role of contact thermal resistance at interlayer interfaces. Since the contact resistance depends on a number of factors associated with the manufacturing technology of heterostructures, it is of great importance to determine the corresponding coefficients from the results of temperature measurements. The purpose of this paper is to evaluate the possibility of reconstructing the thermal resistance coefficients at the interfaces between layers by solving the inverse problem of heat transfer. The complex of algorithms includes two major blocks: a block for solving the direct heat transfer problem in a layered nanostructure and an optimization block for solving the inverse problem. The direct problem is formulated in an algebraic (finite difference) form under the assumption of a constant temperature within each layer due to their small thickness. The inverse problem is solved in the extreme formulation and optimized using zero-order methods that do not require calculating the derivatives of the optimized function. As a basic optimization algorithm, the Nelder–Mead method is used in combination with random restarts to search for the global minimum. The results of identifying the contact thermal resistance coefficients obtained in the context of a quasi-real experiment are presented. The accuracy of the solution of the identification problem is estimated as a function of the number of layers in the heterostructure and the measurement error. The results are planned to be used in the new technique of multiscale modeling of thermal regimes of the electronic component base of the microwave range, when identifying the thermal conductivity coefficients of the heterostructure.

Results of Thermal Tests of Recirculation Cooling Installations

The article presents the results from thermal tests of some recirculation installations for cooling air in nuclear power plant premises, including the volume under the containment. The cooling effect in such installations is produced by pumping water through its heat-transfer tubes. Air from the cooled room is blown by a fan through a bundle of transversely finned tubes and is removed to the same room after having been cooled. The finning of tubes used in the tested installations was made of Grade 08Kh18N10T and Grade 08Kh18N10 stainless steels or Grade AD1 aluminum. Steel fins were attached to the tube over their entire length by means of high-frequency welding. Aluminum fins were extruded on a lathe from the external tube sheath into which a steel tube had preliminarily been placed. Although the fin extrusion operation was accompanied by pressing the sheath inner part to the steel tube, tight contact between them over the entire surface was not fully achieved. In view of this, the air gap’s thermal resistance coefficient was introduced in calculating the heat transfer between the heat-transferring media. The air gap average thickness was determined from the test results taking into account the gap variation with temperature due to different linear expansion coefficients of steel and aluminum. These tests were mainly aimed at checking if the obtained thermal characteristics were consistent with the values calculated according to the standard recommendations with introduction, if necessary, of modifications to those recommendations.

Printed Strain Gauge on 3D and Low-Melting Point Plastic Surface by Aerosol Jet Printing and Photonic Curing.

Printing sensors and electronics directly on the objects is very attractive for producing smart devices, but it is still a challenge. Indeed, in some applications, the substrate that supports the printed electronics could be non-planar or the thermal curing of the functional inks could damage temperature-sensitive substrates such as plastics, fabric or paper. In this paper, we propose a new method for manufacturing silver-based strain sensors with arbitrary and custom geometries directly on plastic objects with curvilinear surfaces: (1) the silver lines are deposited by aerosol jet printing, which can print on non-planar or 3D surfaces; (2) photonic sintering quickly cures the deposited layer, avoiding the overheating of the substrate. To validate the manufacturing process, we printed strain gauges with conventional geometry on polyvinyl chloride (PVC) conduits. The entire manufacturing process, included sensor wiring and optional encapsulation, is performed at room temperature, compatible with the plastic surface. At the end of the process, the measured thickness of the printed sensor was 8.72 μm on average, the volume resistivity was evaluated 40 μΩ∙cm, and the thermal coefficient resistance was measured 0.150 %/°C. The average resistance was (71 ± 7) Ω and the gauge factor was found to be 2.42 on average.

Light Clay and Straw Bale-Based Building Technologies

The article deals with issues related to light clay and straw bale in modern housing. It presents the characteristics of clay and straw, as building materials. Attention is paid to the possibility of using these raw materials for building walls in single-family houses. In addition, the paper presents a thermal analysis for this type of walls and the resulting conclusions, which allow for the treatment of straw bale and light clay as a viable alternative to commonly used technologies for erecting buildings. According to the PN-EN ISO 6946: 2008 standard, thermal resistance and heat transfer coefficient were calculated for selected vertical partitions. Other natural building technologies have been omitted.

Size effects in the resistivity of graphene nanoribbons

The electrical resistivity of single-layer graphene nanoribbons has been studied experimentally for ribbon widths from 16 to 320 nm and is shown to validate the expected quantum scattering model for conduction through confined graphene structures. The experimental findings are that the resistivity follows a more dramatic trend than that seen for metallic nanowires of similar dimensions, due to a combination of the nature of the charge carriers in this 2D material, surface scattering from the edges, bandgap related effects and shifts in the Fermi level due to edge effects. We show that the charge neutrality point switches polarity below a ribbon width of around 50 nm, and that at this point, the thermal coefficient of resistance is a maximum. The majority doping type therefore can be controlled by altering ribbon width below 100 nm. We also demonstrate that an alumina passivation layer has a significant effect on the mean free path of the charge carriers within the graphene, which can be probed directly via measurements of the width-dependent resistivity.

АКТИВНЫЙ МЕТОД ТЕПЛОВОГО КОНТРОЛЯ ТЕПЛОПРОВОДНОСТИ СТРОИТЕЛЬНЫХ МАТЕРИАЛОВ И ИЗДЕЛИЙ

Thermal control refers to non-destructive testing methods. There are passive and active thermal non-destructive testing. With passive thermal control, the test object is characterized by a temperature field formed during its operation. With active thermal control, an additional source of thermal stimulation of the controlled object is used. Thermal control is widely used in various sectors of construction, energy, engineering and transport. The paper proposes a variant of active thermal non-destructive control of thermal conductivity coefficient of building materials and products on the example of a fragment of a building structure made of silicate bricks. The controlled object is subjected to thermal stimulation by an external source of thermal energy until the fixed thermal regime. Thermography of the test object surfaces is performed. The average values of surfaces temperature or individual sections of controlled object are calculated. The heat equation determines a controlled parameter - the heat coefficient of the object under control. The thermal resistance (heat transfer resistance) of the controlled object is calculated with a known coefficient of thermal conductivity. The heat transfer coefficient is calculated with a known coefficient of thermal resistance (heat transfer resistance). The method is implemented in the laboratory. It can be used in field and operating conditions for accurate and rapid determination of the key thermal properties of building materials and products.

Performance assessment of K-type, E-type and J-type coaxial thermocouples on the solar light beam for short duration transient measurements

The estimation of transient surface heat flux from transient temperatures background using fast response thermal sensors is one of the most prevalent technique used for short duration measurements where the heat transfer rate changes rapidly such as in the applications of internal combustion engine, gas turbine engine, high-speed flight test, aerodynamics re-entry vehicle, etc. The coaxial thermocouples are quite beneficial in these engineering applications due to having a very quick response time in the order of a millisecond or less (~0.1 ms). Moreover, these coaxial thermocouples are more cost-effective because they can be contrived in-house. As they have coaxial geometry, it can be directly executed in any type of surface geometry. This work deals with the fabrication of K-type, E-type and J-type coaxial thermocouples and a unique type of calibration technique has experimented in the laboratory. These fabricated coaxial thermocouples are calibrated statically using oil-bath based technique for performance analysis and calculate the thermal coefficient resistance (TCR) & sensitivity (S). A new type of experimental arrangement is devised by applying a known value of step heat load on the junction of the coaxial thermocouple. The concentrated sunlight beam has been focused on the junction of the coaxial thermocouple and the variation in transient temperature has been recorded. Subsequently, their transient heat flux has been estimated using one-dimensional heat conduction modeling for the semi-infinite body. The numerical analysis carried out using commercial software Ansys 15.0 and analytical evaluation is found to be a reasonable covenant between experimental results. This analysis states that these handmade coaxial thermocouples are suitable to measure the surface heat flux in the sunlight based application for short duration transient measurements.

Self-Regulating Electrically Conductive Materials Based on Polyethylene Compositions with UHMWPE and Carbon Black

Electrically conductive composites based on high density polyethylene (HDPE) / ultrahigh molecular weight polyethylene (UHMWPE) blends filled with carbon black were studied. The work is a part of the research of electrically conductive materials for the manufacture of self-regulating polymer heaters. In this work, the authors investigated composites based on HDPE/UHMWPE (molecular mass of ~ 7 million) blends filled with carbon black (average particle size ~ 20 nm). The goal of the work was to obtain a self-regulating electrically conductive polymer material with optimal thermoelectric characteristics and high heat resistance. It was shown that the effect of adding UHMWPE to the HDPE/carbon black composites on the thermoelectric behavior of the resulting material was similar to cross-linking. This reduced the undesirable effect of the negative thermal coefficient (NTC) of the electrical resistance. In addition, the heat resistance of the material at elevated temperatures was increased. This makes it possible to exclude the radiation or chemical cross-linking in the manufacture of self-regulating polymer heating elements. The rheological, mechanical and thermoelectric properties of HDPE/carbon black composites modified with UHMWPE were also studied. It was found that a diffusion interphase layer with a reduced fluidity in the melt of HDPE/UHMWPE blends was formed. It was established that self-regulating composites containing 30–40% of UHMWPE had the best operational properties. We can recommend the methods of extrusion and injection molding for the processing of HDPE/carbon black composites mixed with 30% UHMWPE, and the pressing method in case of UHMWPE content of 40%. The ability of the composites to be molded is sharply reduced at higher contents of UHMWPE.

Vertically Aligned Few-Layered Graphene-Based Non-Cryogenic Bolometer

In this study, we report the photoresponse of vertically aligned few-layered graphene (VAG) upon infra-red (IR) irradiation at room temperature. Four probe measurements showed the current–voltage (I–V) characteristic of electrical switching during pulsed IR irradiation. The photoresponse reported here for VAG was significantly higher than that reported for carbon nanotube (CNT) samples. Our investigation shows that such a photoresponse arose solely from the bolometric effect, where the conductivity changed with temperature. The resistance magnitude of the VAGs increased ~two fold for each 6 °C increase in temperature. Also, the Thermal Coefficient of Resistance (TCR) in this region was ~11%/K, which is the highest TCR value reported for any carbon nanomaterial.

Application of computational tools in alloy design

Abstract

Electrophysical properties of granular film alloys

The results of experimental researches of the structure and phase state and electrophysical properties of the double-component film systems based on Co and Cu or Ag, Fe and Cu, in which the granular solid solutions (s.s.) are presented. The formation of granular films was carried out as a layer condensation of superfine layers (d ≈ 0.5–3 nm) with subsequent annealing up to 850 K, and simultaneous condensation of separate components. With a total thickness of films up to 60 nm, the average size of Co magnetic granules is 3–7 nm. Parameters of the fcc lattice of a granular s.s. is a = 0.408–0.409 nm (based on Ag and Co) and a = 0.358 nm (based on Cu and Co). In multilayer films based on α-Fe and Cu formed s.s. based on α-Fe (at 1.5<dFe<2 nm) or s.s. based on Cu (at dFe<1.5 nm) with lattice parameters which are very close to the α-Fe or Cu parameter. Studies of electrophysical properties indicate that the resistivity depends to a large extent on the total concentration of the magnetic component and the temperature of the measurement. The value ρ in thermostabilized films is (1–4) × 10−7 Ω × m, and the value of the thermal coefficient of resistance β≈(0.5–1.5) × 10−3 K−1 does not depend from the samples thickness, temperature and the concentration of the magnetic component.

Investigation of Thermal Performance of Cylindrical Heat Pipe Using Silver Nanofluid. Part I: Experimental Evaluation

Cylindrical heat pipes are widely used in various applications to transfer the heat effectively and efficiently. Besides parameters of heat pipe, its thermal performance mainly depends on the characteristics of working fluid also. Recently, various nanofluids have been considered as the most promising working fluid in different heat transfer applications. In the present paper thermal performance of cylindrical heat pipe using silver nanofluid as working fluid has been experimentally evaluated. Extensive experiments have been performed on cylindrical copper heat pipe having two-layer screen mesh filled with silver nanofluid in filling ratio of 40% with a vacuum of 10−3 torr. The thermal performance of heat pipe has been evaluated in terms of thermal resistance, thermal conductivity and overall heat transfer coefficient by varying parameters such as size of nanoparticle, concentration of nanoparticle, angle of inclination of heat pipe, heat load on heat pipe, average evaporator temperature and average condenser temperature to propose the best combination of parameters. The results indicate that the cylindrical heat pipe kept at 45° inclination angle and filled with silver nanofluid of 35 nm size of silver nanoparticle in 0.3% volume concentration yields higher thermal performance when it is operated at 120–140 W heat input. Thus, the cylindrical heat pipe under study is suitable for applications which require operation of heat pipe at some angles, such as cooling of high-wattage LED street lights.

Transient heat flux measurement analysis from coaxial thermocouples at convective based step heat load

The measurement of surface heating rate is an imperative parameter in the force convection ground-based facility for short duration investigation due to the heat transfer rate is changing rapidly. The coaxial thermocouples are suitable to measure the transient heat flux in fast varying heat transfer application because of having fast response time in the range microseconds or less. In this addition, the K-type, E-type, and J-type of coaxial thermocouples are contrived as well as the thermal coefficient resistance (TCR) and sensitivity (S) has been calculated from oil-bath based technique. These handmade coaxial thermocouples are exposed in a forced convection flow facility having three different input step heat loads as well as their transient heat fluxes are estimated using one-dimensional heat conduction modeling for the semi-infinite body. The numerical simulation has also been carried out with the analogous experimental parameters using ANSYS-FLUENT v.15.0 and compared with the outcome of the experimental approach and it is found that the average value of the transient temperatures having 0.3% error and surface heat flux recovered from this temperature is 10%. This study reveals the measuring ability of these handmade coaxial thermocouples at low temperature and low velocity on short duration transient measurements.