Poor Heat Conduction Research Articles

Thermally Conductive Electrically Insulative (TCEI) Materials for E-Motors

<div class="section abstract"><div class="htmlview paragraph">Engineering plastics are often used in electric components and systems. For these materials, many requirements such as electrical resistance, mechanical properties, temperature capability, flammability, thermal conductivity, moldability etc. are considered for material selection. In general plastics are good electric insulators (electrically safe) but poor conductors of heat (compared to metals). In electric motors, losses due to friction, windage losses, winding losses, iron losses etc. manifest as undesirable heat, resulting in temperature buildup inside the motor. So, choice of materials which offer high thermal conduction and electrical insulation can directly impact motor performance.</div><div class="htmlview paragraph">Material properties of engineering plastics are greatly influenced by the filler reinforcements used. Although unfilled polymers typically have a thermal conductivity of ~0.2 W/m/K, appropriate choice of fillers can increase this value many-fold. One of the challenges with injection molded plastics is that material properties, including thermal conductivity, are a function of material orientation (orthotropic). In the present work, TCEI materials have been developed for use in bobbins. Freudenberg Sealing Technologies, with its knowledge of filled polymeric systems has developed multiple TCEI grades with improved thermal conductivity compared to commercially available materials. Thermal conductivity measurements on these novel material grades have been conducted to quantify improvements. A finite element model has been developed to capture effect of material choice on temperature distribution within the system. The model incorporates varying material orientation within the bobbin and components within the motor and uses temperature dependent material properties for the analysis. The effect of varying thermal conductivity values on thermal performance and influence of an air gap are studied. Results show that materials with improved thermal conductivity show reduced bobbin and winding temperatures, offering significant performance benefits.</div></div>

Study on heat conduction and adsorption/desorption characteristic of MIL-101/few layer graphene composite

Aiming at the poor heat conduction performance of porous MIL-101 applied in adsorption cooling process, few layer graphene (FLG) was selected as a promising thermal conductive additive to enhance thermal conductivity of MIL-101. The factors which influencing thermal conductivity of MIL-101/FLG composites were investigated. Thermal conductive mechanism of FLG for MIL-101 was discussed. Adsorption/desorption characteristics of water on the MIL-101/FLG composites were determined. Results show a two-dimensional structure of FLG with no defects and high degree of order is beneficial to the improvement of thermal conductivity for MIL-101/FLG composites. At 30 °C and bulk density of 0.55 g/cm3, thermal conductivity of MIL-101/20%FLG and MIL-101/25%FLG composite is 6.5 and 11.3 times higher than that of MIL-101. Thermal conductivity of MIL-101/FLG composites is related to the alignment direction of FLG and test direction of heat flow, and the interfacial thermal resistance between MIL-101 and FLG. Adding FLG in the MIL-101/FLG composites does not affect adsorption/desorption mechanism of water on the MIL-101. The addition of FLG can strengthen mass transfer and thermal diffusion process on the surface of MIL-101. Adsorption rate constant of water on the MIL-101/25%FLG composite is 2.05 times higher than that of MIL-101. The desorption temperatures and desorption activation energies of water on MIL-101/20%FLG and MIL-101/25%FLG composites are lower than those of MIL-101. It can provide basic research for the development of new adsorption water chiller working pair (MIL-101/FLG–water) with high efficiency.

Fabrication and characterization of SMA film actuator array with bias spring for high-power MEMS tactile display

A MEMS-type SMA thick-film actuator (5 × 5 array) with a pitch of 2 mm was designed and fabricated for high-power vibrational stimuli to human skin. The deformable part (1.08 mm × 1.52 mm) of the SMA actuator was initially deformed to 320 μm to provide vibrational motion. The SMA actuator device was successfully fabricated without fractures in an SMA meandering actuator and microheater circuit when the SMA actuator was deflected to the initial displacement (320 μm). The surface temperature of the meandering-shaped SMA actuator successfully rose to about 65 °C, the shape recovery temperature, in a short time by high-pulse voltage heating (32 V) to the SMA. However, it was found that a cooling time longer than 99 ms was needed to completely cool down owing to poor heat conduction. Although the SMA actuator generated 38–40 mN of force by DC current heating (~72 mW), by periodic pulse-driving above 15 Hz, where human skin has high sensitivity, the generated force drastically decreased by 1/4 owing to a poor thermal response during the pulse-off time. The completed SMA tactile display with a bias-spring mechanism generated a displacement amplitude of 23 μm at a frequency of 1 Hz. However, the amplitude decreased to 10–13 μm at 15 Hz, which is slightly above the human perception limit

Effect of fire flame exposure on basalt and carbon fiber reinforced concrete

Concrete in general is a poor conductor of heat, but it can suffer significant damage when exposed to fire. Investigation of concrete behaviour when exposed to fire has generated interest where most of the fire cases take place in buildings and tunnels. Incorporation of fiber in concrete can improve the mechanical performance as it acts as crack arresters, resisting the development of cracks, hence making the concrete composite matrix stronger. In this study, mechanical behaviour of basalt and carbon fiber reinforced concrete for different characteristic strengths at 28 days were investigated for its performance when exposed to fire flame. Fire flame test using methane gas was developed to simulate real fire phenomena. Concrete specimens were subjected to fire flame at the temperature of 1000 °C for a period of 90 minutes. Visual observation (colour change, cracking and spalling), loss of density and residual compressive strength of the concrete specimens were performed. The results obtained were compared with reference specimens. Test results showed that tested samples experienced cracking and spalling, reduction of mass and compressive strength. All samples experienced compressive strength loss except for G20 OPC which recovers 7.8 % compressive strength. Addition of carbon fiber showed an increased in strength before exposure compared to basalt fiber. Minor damages were observed for carbon fiber in concrete after fire exposure compared to basalt fiber.

CFD investigation of the feasibility of polymer-based microchannel heat sink as thermal solution

Abstract Microchannel heat sinks (MCHSs) are promising thermal solutions in miniaturized or compact devices. Lightweight aspect has been given huge emphasis in recent years. Metal-based materials are commonly used to fabricate MCHSs due to their high thermal conductivity. Consequently, MCHSs are heavy due to the high density of these materials albeit the small footprint of MCHSs. Polymer-based materials are interesting alternatives. Despite their poor thermal conductivity, lightweight feature attracts the interest of researchers. Heat transfer is a conjugate process of heat conduction and heat convection. Poor heat conductions aspect may be compensated through enhancement of heat convection aspects. Although polymer-based materials have been used in microscale heat transfer studies, their focus was not on their feasibility. The present study aims to evaluate the feasibility of polymer-based MCHSs as thermal solutions. The effect of thermal conductivity of fabrication materials, including polymer-based PDMS, PTFE, PDMS/MWCNT, and metal-based aluminum, on the thermal performance of MCHSs was investigated and compared at various inlet flow rate, fluid thermal conductivity, and microchannel ratio at different constant heat fluxes using three-dimensional CFD approach. Results showed that the thermal performance of MCHSs was greatly affected by the heat conduction aspect in which poor heat conduction limited the thermal performance improvement due to enhanced heat convection aspects. This suggests polymer-based materials have the potential for heat transfer applications through thermal conductivity enhancement. This was confirmed in the further analysis using a recently proposed high thermal conductivity polymer-based graphite/epoxy MCHS and a hybrid-based PDMS/aluminum MCHS.

Comparative Evaluation of Thermoelectric Energy Conversion Systems for Heat Recovery With and Without a Water-Cooling Thermal Energy Adjustment Structure

The stable exchange of energy and the effective conduction of heat are particularly critical to ensuring the stable operation of thermoelectric components in entire thermoelectric conversion processes. Poor heat exchange and conduction will cause thermal energy in a thermoelectric system to accumulate, leading to the consequence of burning the energy conversion modules. To solve these problems, a thermoelectric energy conversion system equipped with a water-cooling thermal energy adjustment structure settled on the surface of the heat source was proposed. The working temperature of the heat source, the output voltage, current, and power of the thermoelectric system with and without the structure were obtained and compared. Through comparative analysis, benefiting from the above structure, the temperature fluctuation of the heat sink surface of the heat source was found to drop from 10.36 K to 1.18 K, and the maximum output of the system increased from 255 to 290 mV through the process of input voltage from 1 to 6 V. Furthermore, in the process of gradually increasing the load from 0 to 180 ohms, the system achieves an increased output of 53.8 mV, 1.81 mA and 52.1 mV, 1.83 mA when the input voltage was 4 and 5 V, respectively. In conclusion, the application and design of the above structure obviously promotes the stability and output capacity during thermoelectric energy conversion. The combination of the water-cooling thermal energy adjustment structure with pulse-width modulation (PWM) or maximum power point tracking (MPPT) theory is worth further study.

Thermal buoyancy driven flows inside a differentially heated enclosure with porous fins of multiple morphologies attached to the hot wall

The application of porous media for electronic cooling process, instead of solid one, generally strengthens heat conduction while weakens convection insignificantly at high Darcy number. In the present work, effects of morphology and topology of porous fins on the laminar natural convection heat transfer were investigated in a differentially heated enclosure. Volume averaged Darcy-Forchheimer model was applied to solve the transport process within the porous media while the Navier-Stokes equations were employed within pure fluid region.Relevant governing parameters, including thermal Rayleigh number, Darcy number, thermal conductivity of the porous matrix, designs of porous fins, are sensitively varied to identify their effects and roles on the natural convection flows. Depending on thousands of numerical data, the correlation has been developed for all designs of porous fins. Numerical results illustrate that the adding porous fins with excellent permeability and heat conduction contribute to the remarkable heat transfer enhancement while the adding fins, acting like solid ones and having poor heat conduction, could result in an increase of thermal resistance and the deterioration of heat transfer. Numerical results further show that there exists an optimal design of porous fins to achieve the best performance of heat transfer if some conditions were satisfied. Overall, this study could benefit the electronic cooling by the installation of porous-alike materials.

A Note for Probabilistic Model of Polymer Crystallization in Temperature Gradients

A polymer crystallization kinetics model is the most important way to characterize the crystallization rate of polymers. Because polymers are poor heat conductors, the cooling of thick-walled shapes results in temperature gradients. Piorkowska (Piorkowska, E. J. Appl. Polym. Sci., 2002, 86: 1351–1362.) derived the probabilistic analytical model of polymer crystallization in temperature gradients based on the Avrami equation. However, there are some misunderstandings when using this model. Here, isotactic polypropylene (iPP) is chosen as a model polymer and its crystallization is studied in a temperature gradient field. Based on the results of the Monte Carlo method, the probabilistic model methodology is discussed. The results show that when the product has a large temperature gradient and a large temperature difference, the probabilistic model cannot be used directly; instead, it is necessary to use the average probabilistic model. This means that the sample should be divided into several smaller parts and the probabilistic model used separately for each small part. The values are then averaged to obtain the mean conversion degree of the melt into spherulites for the whole product. The effects of the division number are also discussed. The goal of the present paper is to better understand the polymer crystallization kinetics model in terms of temperature gradients.

Shape-stabilized phase change materials of polyolefin/wax blends and their composites

The continuous growth of greenhouse gas emission and rising costs of fossil fuels are the major driving force behind high rate of research on effective utilization of energy. The storage of energy through different innovative capacitors and otherwise are some of the trending research. In this review, more about polyolefin/wax blend composites are discussed and explored as a potential system of energy. Phase changes and effect of each component in polyolefin/wax blend composites and eventual energy storage are discussed. Latent heat storage system through phase change materials (PCMs) remained effective way of storing thermal energy. This promoted some applications including solar energy utilization, waste heat recovery and air-conditioned building. There are disadvantages such as poor heat conductors, in which heat transport amongst the components of the phase change material is relatively slow in polyolefin/wax blend composites. In most cases, based on the chemical constituents of the polyolefins and phase change materials, shape-stabilized PCM materials are naturally flammable which limits their applications. This review summarizes the thermal conductivity, mechanical, flammability and thermal properties based on polyolefins/PCMs blends and their composites and highlights possible recommendations.

Thermal analysis and pack level design of battery thermal management system with liquid cooling for electric vehicles

In this paper, a comparative study for structural design of battery thermal management system is presented for electric vehicles. A thermal model for the pouch battery pack with liquid cooling is developed for thermal analysis of various pack designs. Typical battery pack with fin-cooling structure is set as a reference design, and thermal behavior of the battery pack is examined in the aspect of cooling performance and temperature uniformity. Numerical results indicate that poor heat conductivity from the bottom of the cell stack to the cooling plate is one of the major barriers to the efficient heat dissipation and asymmetric design of fin-cell arrangement have negative effect on the temperature uniformity of the battery pack. To improve the performance of the thermal management system, various structural designs are suggested and evaluated based on plural criteria. A new structural design for the large-scale battery pack is suggested to enhance the cooling performance and temperature uniformity of the battery pack minimizing the increase in system volume, weight, and pressure drop.

Morphological Monte Carlo Simulation for Crystallization of Isotactic Polypropylene in a Temperature Gradient

Polymers are poor heat conductors, so the cooling of thick-walled shapes results in temperature gradients. Here, isotactic polypropylene (iPP) is chosen as a model polymer for the study of polymer crystallization in a temperature gradient field. The morphological Monte Carlo algorithm is applied, combined with the radius growth model, to predict the growth of spherulites. Through comparison of the two numerical solutions, analytical solution and experimental data, the validity of the morphological Monte Carlo algorithm is demonstrated. In addition, the roles of central temperature, temperature gradient for the evolution of spherulites, and the conversion degree of the melt into spherulites are considered. The results of the study show that increases in central temperature and temperature gradient can increase the anisotropy of spherulites. Isothermal crystallization and crystallization in a temperature gradient field are compared, and the differences are considered. Results show that when the central temperature is below 125 °C, and when the temperature gradients are less than 15 K/mm and 27 K/mm, the differences in the conversion degree of the melt into spherulites are less than 2% and 5%, respectively. Therefore, crystallization under such temperature gradient conditions can be simplified as isothermal crystallization.

Ultra-high performance wearable thermoelectric coolers with less materials

Thermoelectric coolers are attracting significant attention for replacing age-old cooling and refrigeration devices. Localized cooling by wearable thermoelectric coolers will decrease the usage of traditional systems, thereby reducing global warming and providing savings on energy costs. Since human skin as well as ambient air is a poor conductor of heat, wearable thermoelectric coolers operate under huge thermally resistive environment. The external thermal resistances greatly influence thermoelectric material behavior, device design, and device performance, which presents a fundamental challenge in achieving high efficiency for on-body applications. Here, we examine the combined effect of heat source/sink thermal resistances and thermoelectric material properties on thermoelectric cooler performance. Efficient thermoelectric coolers demonstrated here can cool the human skin up to 8.2 °C below the ambient temperature (170% higher cooling than commercial modules). Cost-benefit analysis shows that cooling over material volume for our optimized thermoelectric cooler is 500% higher than that of the commercial modules.

Characteristics of chemical reaction and convective boundary conditions in Powell-Eyring nanofluid flow along a radiative Riga plate

A typical base fluid such as water, oil or glycol is poor conductor of heat due to deficient thermo-physical properties. This deficiency is normally addressed by saturation of thermally strong conductive metallic nanoparticles such as Fe, Ti, Hg, Cu, Au into the base fluid resulting a stronger thermal conductivity, electric conductivity, heat and mass flux of the so formulated nanofluid. Nanoparticles having a diameter size less than 100 nano-meter are preferred in this formulation because these nano-sized particles stay suspended into the base fluid for a longer time-period. This communication aims to investigate the salient features of a nanofluid flow along a radiative Riga plate using Powell-Eyring model and convective boundary conditions. The flow model involves the effect of first order chemical reaction as well as the Brownian motion diffusion and Thermophoresis effects. Governing PDEs are transformed into ODEs using suitable transformations. HAM is applied for convergent series solutions to the boundary value problem. Impact of various parameters including Brownian motion, Thermophoresis, modified Hartman number, Lewis and Prandtl number on flow profiles is analyzed graphically. Parameters of physical interest like Skin-friction, Nusselt and Sherwood numbers are illustrated through numerical data. Effect of modified Hartman number is significant on flow profiles due to involvement of Riga plate. An efficiency is achieved in fluid flow and heat-mass flux through suspension of nanoparticles in base fluid.

Analysis of Thermal and Physical (Mechanical) Properties of Local Fruits and Vegetables in Wukari, Taraba State, Nigeria

In this research,physical methods have been used to analyzed fruits: black berry (Vitex Doriana), Shea butter (Vitellaria Paradixa), Bush Fig (Ficussur), Wild Custard Apple (Annona Senegalensis); Vegetables: Drumstick tree (Moringa oleifera), Zobo drink or Roselle (Hibiscus Sabdariffa) and Beans leave (Phaseolus vulgaris). The pH, density (ρ) , thermal conductivity (K), specific heat capacity (c), latent heat capacity (Lf ), thermal diffusivity (Td), moisture content( MC ), and the total solid (Ts ) were determined. The average mechanical properties of the samples are: pH (5.48), density (435.06Kg/m3), K (0.53JS m3), c (3.60J/kgK), Lhf (2585.61J/Kg), Td (3.89m2s-1), MC (76.77 %) and Ts is (23.25%). The following samples have the highest properties among others: Shea butter has pH (6.81), Wild Custard Apple has ρ (600Kg/M3), beans leave has K (0.5642 JS M3), beans leave has c (3.79J/kg K), beans leave has Lf (282840.00.J/kg), black berry Td (5.8802m2s-1), beans leave has Mc (84.43 %) and bush Fig has Ts (34.00%). The beans leave has the 50% highest property while other samples have the other 50% of the properties. The pH were acidic, K were lower than that of water, Td were higher than that of water, c and Lf were very high, the TS were high and MC were very high which make the sample perishable. To preserve these samples, their moisture contents have to be reduced. A lot of heat will be required to heat or cool these samples, because they are poor conductors of heat.

Environment Friendly Machining of Inconel 625 under Nano-Fluid Minimum Quantity Lubrication (NMQL)

Superalloy Inconel 625 although having many industrial applications owing to its high strength, exhibits poor machinability because of its sticky nature and poor heat conductivity. To improve its machinability, use of cutting fluids becomes necessary to remove heat and provide lubrication in the cutting region. However, harmful effects of cutting fluids on environment and operator health restrict their application. Several efforts have been carried out to replace or minimize the quantity of conventional cutting fluids used in machining to strive for green machining and economizing machining operations. Nano-fluid minimum quantity lubrication (NMQL) technique has evolved as best alternative to flood conditions cooling /lubrication especially for machining of alloys like Inconel. This paper experimentally investigates the suitability of NMQL (carbon nanotube; CNT in vegetable oil) in machining of Inconel 625. The objective was to minimize tool wear and surface roughness under different machining conditions. Tool performance in NMQL was also compared with that under dry and flood conditions. The results revealed superiority of NMQL in terms of better tool life and improved surface finish over dry machining and nearly equivalent performance to wet (flood) machining thus provides the way forward for sustainable and environmental friendly machining.

Numerical investigation of thermal and optical performance of window units filled with nanoparticle enhanced PCM

Filling window units with phase change material (PCM) improves the thermal performance of windows, but on the other hand it has a deteriorative effect on the optical performance due to poor heat conductivity of PCM. A novel method to tackle this drawback of PCM is to disperse nanoparticles in the PCM. In this study, a model was developed to evaluate the thermal and optical performances of window units filled with nanoparticle enhanced PCM (NePCM). The effect of different types of nanoparticles, volume fractions of nanoparticles and sizes of nanoparticles on the thermal and optical performances of windows such as temperature, heat flux, solar transmittance, absorptance and reflectance were numerically investigated and compared with the referent case (i.e. pure PCM). The results showed that the optical and thermal performances of window units filled with nanoparticle dispersed paraffin wax are improved compared to that of with pure paraffin. However, the improvement is nearly the same regardless of nanoparticle type. The effect of volume fraction and size of nanoparticle is significant during the sunset and sunrise periods. Considering both thermal and optical performances of window units, it is recommended to disperse CuO nanoparticles with the volume fraction of below 1% and nanoparticle size of below 15 nm in PCM.

Research for waterborne polyurethane/composites with heat transfer performance: a review

Waterborne polyurethanes (WPUs) have been pushing toward more multifunctional directions developing which is due to the more extensively utilized in the world. Obviously, electronic devices are becoming more miniaturization and intelligentialize, accompanying many problems (e.g., heat dissipating and heat aging) to resolve. Organic polymers are to be good substitute for metal substrate used in devices because of their excellent performances. However, due to the most of polymers are poor conductor of heat, so many researchers utilize the hybrid method to enhance thermal conductivity of composites, through the pathway formed by fillers connection to build heat networks for phonon conduction. In this study, we do some reviews of WPU/composites on heat conducting. We put a focus on the enhancement of thermal conductivity through the compatibility and feasibility of hybrid methods, which includes the physical and chemical modification with non-covalent and covalent bond interactions. Plus, referring to laboratory previous study of heat conduction, on which pure WPU possessed with short-range, long-range ordering and microcrystal structures can facilitate the phonon conduction with the improvement of thermal conductivity.

Reliability challenge of ESD protection: From planner SOI MOSFET to SOI FinFET

Implementation of Electrostatic Discharge (ESD) protection in Silicon on Insulator (SOI) technology is a challenge because of the inherent properties of poor heat conductor and heat trapping. In this paper, a novel device as ESD clamp is proposed as Fix-Base SOI FinFET clamp which addresses the troublesome problem of floating base. Moreover, its manufacturing process is compatible to the normal SOI process flow well. Finally, a detailed discussion including current density and thermal distribution are presented with the technique of 3D TCAD simulation.

Die-Attach Voiding Reduction in Gold Alloy Solder Preforms

Abstract The need for high-temperature solders is growing as RF and power semiconductor devices continue to get smaller, with power density increasing both as a consequence of the shrink and as a result of increased power ratings. AuSn20 eutectic solder (Indalloy®182) has been the workhorse for high-temperature, high-reliability, small die-attach applications for many years; however, as junction temperatures (Tj) increase, the gold-tin eutectic is beginning to reach its limit of utility. Higher temperatures cause increased thermal fatigue, and even delamination is seen at the solder joints. The next option for RF and power semiconductor manufacturers needing these higher temperatures is either AuGe12 (Indalloy®183) or AuSi3.2 (Indalloy®184) eutectic alloy (see Table I).Table 1.Key properties of Au-based eutectic alloys. Over the years, many customers have tried AuGe12 and the feedback has been that the alloy has poor solderability, which manifests as large voids in the bond. Voids are poor conductors of heat, which create hot spots, and are the primary cause of premature failures.

Laser Ablation of Sub-10 nm Silver Nanoparticles

Laser ablation of silver nanoparticles (NPs) was studied with laser post-ionization (LPI) time-of-flight mass spectrometry (TOF MS). Silver NPs containing ∼15 000 Ag atoms (4 nm radius) were deposited by soft landing (energy 3 eV/atom) onto indium tin oxide (ITO)/glass substrates. Laser ablation was performed using frequency-doubled Ti:sapphire nanosecond pulsed laser irradiation at three different wavelengths (371, 401, and 421 nm), whereas for post-ionization, pulses from an F2 laser were used. Laser fluences and time delay dependencies of Ag and In signals were obtained. Using these data, the temperature of the desorption source as well as its time duration were calculated. It was found that the peak temperature of NPs was above their melting point and they cooled down slowly, with temperature decay time of several hundreds of nanoseconds. This anomalous behavior was explained based on a model where the semiconducting ITO substrate is initially transparent to the desorption laser radiation but starts to adsorb it due to the temperature increase arising from heat exchange with NPs. Poor heat conduction in the ITO film creates conditions for long-lived hot spots on the surface and initiates further optical damage of the substrate. No difference in the ablation process due to plasmon resonance was detected, likely due to thermal expansion and melting of NPs during laser irradiation, which then broadens the plasmon absorption band enough to cover all wavelengths used. These results clearly demonstrate that the process of NP interaction with laser radiation is governed not only by initial optical and thermophysical parameters of NPs and the surrounding media, but also by their alteration due to temperature increases during the irradiation process.