Technique For Measurement Of Thermal Conductivity Research Articles

Depth-dependent recovery of thermal conductivity after recrystallization of amorphous silicon

The depth-dependent recovery of silicon thermal conductivity was achieved after the recrystallization of silicon that had been partially amorphized due to ion implantation. Transmission electron microscopy revealed nanoscale amorphous pockets throughout a structurally distorted band of crystalline material. The minimum thermal conductivity of as-implanted composite material was 2.46 W m−1 K−1 and was found to be uniform through the partially amorphized region. X-ray diffraction measurements reveal 60% strain recovery of the crystalline regions after annealing at 450 °C for 30 min and almost full strain recovery and complete recrystallization after annealing at 700 °C for 30 min. In addition to strain recovery, the amorphous band thickness reduced from 240 to 180 nm after the 450 °C step with nanoscale recrystallization within the amorphous band. A novel depth-dependent thermal conductivity measurement technique correlated thermal conductivity with the structural changes, where, upon annealing, the low thermal conductivity region decreases with the distorted layer thickness reduction and the transformed material shows bulk-like thermal conductivity. Full recovery of bulk-like thermal conductivity in silicon was achieved after annealing at 700 °C for 30 min. After the 700 °C anneal, extended defects remain at the implant projected range, but not elsewhere in the layer. Previous results showed that high point-defect density led to reduced thermal conductivity, but here, we show that point defects can either reform into the lattice or evolve into extended defects, such as dislocation loops, and these very localized, low-density defects do not have a significant deleterious impact on thermal conductivity in silicon.

Anisotropic thermal conductivity in two-dimensional van der Waals crystals

Understanding thermal energy transport of crystalline materials, often highly dependent on their crystalline directions, is crucial for energy harvesting and thermal management applications. In this sense, anisotropy in thermal conductivity (<i>κ</i>), which is the unique characteristic of two-dimensional (2D) materials involving graphene and transition metal dichalcogenides (TMDs), has been attracting tremendous attention in terms of fundamental science and application-driven technology aspects. This distinctive heat transport behavior of 2D van der Waals (vdW) materials generally originates from their intrinsic crystal structures and associated lattice vibrations. Here, we thoroughly review and summarize the anisotropic thermal conductivity in 2D vdW crystals in two different categories: 1) in-plane vs. out-of-plane and 2) between two different in-plane directions. In addition, we introduce a range of thermal conductivity measurement techniques that can be employed for 2D vdW materials provided with their working principles, advantages, and limitations. Beyond their intrinsic anisotropic ratio, we conclude with perspectives on the extrinsic modulations of thermal conductivities, thereby maximizing it toward effective thermal management.

Thermal Conductivity Enhancement of Metal Oxide Nanofluids: A Critical Review.

Advancements in technology related to energy systems, such as heat exchangers, electronics, and batteries, are associated with the generation of high heat fluxes which requires appropriate thermal management. Presently, conventional thermal fluids have found limited application owing to low thermal conductivity (TC). The need for more efficient fluids has become apparent leading to the development of nanofluids as advanced thermal fluids. Nanofluid synthesis by suspending nano-size materials into conventional thermal fluids to improve thermal properties has been extensively studied. TC is a pivotal property to the utilization of nanofluids in various applications as it is strongly related to improved efficiency and thermal performance. Numerous studies have been conducted on the TC of nanofluids using diverse nanoparticles and base fluids. Different values of TC enhancement have been recorded which depend on various factors, such as nanoparticles size, shape and type, base fluid and surfactant type, temperature, etc. This paper attempts to conduct a state-of-the-art review of the TC enhancement of metal oxide nanofluids owing to the wide attention, chemical stability, low density, and oxidation resistance associated with this type of nanofluid. TC and TC enhancements of metal oxide nanofluids are presented and discussed herein. The influence of several parameters (temperature, volume/weight concentration, nano-size, sonication, shape, surfactants, base fluids, alignment, TC measurement techniques, and mixing ratio (for hybrid nanofluid)) on the TC of metal oil nanofluids have been reviewed. This paper serves as a frontier in the review of the effect of alignment, electric field, and green nanofluid on TC. In addition, the mechanisms/physics behind TC enhancement and techniques for TC measurement have been discussed. Results show that the TC enhancement of metal oxide nanofluids is affected by the aforementioned parameters with temperature and nanoparticle concentration contributing the most. TC of these nanofluids is observed to be actively enhanced using electric and magnetic fields with the former requiring more intense studies. The formulation of green nanofluids and base fluids as sustainable and future thermal fluids is recommended.

Role of hemp fiber addition on thermal stability, heat insulation, air permeability and cellular structural features of rigid polyurethane foam

In this current study, rigid polyurethane foams (RPUFs) composites were prepared using different percentage (3, 6, 9 and 12%) of the hemp fibers via one-shut one-step polymerization method. The influences of the hemp fiber addition on the RPUFs were investigated meticulously by means of Fourier-transform infrared spectroscopy (FTIR), thermo gravimetric analysis (TGA), thermal conductivity measurement and scanning electron microscopy (SEM) techniques by evaluating the alternations in the chemical structures of the component, thermal stability, apparent density, insulation performance and cellular topology of the produced samples. The structural analysis revealed that there existed the strong secondary chemical bonds between the functional groups belonging to the components and, depending on that, the improvement in the thermal stability of the foam samples was recorded accompanied by the formation of the better interfacial adhesion. Furthermore, thermal conductivity values of the hemp fiber-loaded RPUFs were observed to increase regularly with the increasing of the content level of the hemp fibers. This was explained by enhancement in the bulk phase conduction level depending of the apparent density rising, reduction in CO2concentration inside cells as well as the formation of the distorted cellular structures. The obtained air permeability results displayed that the hemp fibers incorporated successfully with RPUF structure, which provides the occurrence of the novel micro barriers and pathways limiting the passage of the air throughout the matrix. The taken scanning electron microscopy images also indicated that the cellular morphology and dimensional stability of the produced foams affected negatively by the hemp fiber addition. At high contents, the wrinkled, non-uniform and irregular cellular structures were observed with ruptured and collapsed walls and struts.

Review: 3-$$\omega$$ Technique for Thermal Conductivity Measurement—Contemporary and Advancement in Its Methodology

Review: 3-$$\omega$$ Technique for Thermal Conductivity Measurement—Contemporary and Advancement in Its Methodology

Thermal Conductivity of Nanoporous Materials: Where Is the Limit?

Nowadays, our society is facing problems related to energy availability. Owing to the energy savings that insulators provide, the search for effective insulating materials is a focus of interest. Since the current insulators do not meet the increasingly strict requirements, developing materials with a greater insulating capacity is needed. Until now, several nanoporous materials have been considered as superinsulators achieving thermal conductivities below that of the air 26 mW/(m K), like nanocellular PMMA/TPU, silica aerogels, and polyurethane aerogels reaching 24.8, 10, and 12 mW/(m K), respectively. In the search for the minimum thermal conductivity, still undiscovered, the first step is understanding heat transfer in nanoporous materials. The main features leading to superinsulation are low density, nanopores, and solid interruptions hindering the phonon transfer. The second crucial condition is obtaining reliable thermal conductivity measurement techniques. This review summarizes these techniques, and data in the literature regarding the structure and thermal conductivity of two nanoporous materials, nanocellular polymers and aerogels. The key conclusion of this analysis specifies that only steady-state methods provide a reliable value for thermal conductivity of superinsulators. Finally, a theoretical discussion is performed providing a detailed background to further explore the lower limit of superinsulation to develop more efficient materials.

Design and Development of Apparatus to Measure Thermal Conductivity of Liquid

- Thermal conductivity is the fundamental material property for characterizing the heat transfer. The measurement of thermal conductivity include the measurement of the heat flux and temperature difference. This study aims to design, fabricate and development a reliable instrument to measure the thermal conductivity. The apparatus is completely different form of thermal conductivity apparatus which has been widely applied in normal laboratory. A literature survey concerning available experiment technique for thermal conductivity measurement was conducted. The level of reliability that based on the accuracy and consistent will be proven by comparing the experimental value with theoretical value. The results obtained certified the aim of work which was to develop the thermal conductivity measurement apparatus suitable for data collection and experimental experience in economic way. Key Words: Thermal conductivity, heat flux

Study on energy storage performance of thermally enhanced composite phase change material of calcium nitrate tetrahydrate

In this study, a layer of nano-alumina film was coated on the EG surface and the matrix material was then mixed with molten calcium nitrate tetrahydrate salt to form a CPCM (MEG-CN4W). The prepared materials were characterized by Fourier transform infrared spectroscopy, X-ray diffractometer, contact angle measurement, thermal conductivity measurement and other techniques. When MEG has a mass ratio of 8% ~ 10%, the average phase transition enthalpy value of MEG-CN4W is 123 J/g, the phase transition temperature is 42.7 °C, and the peak temperature is 49.7 °C. The average activation energy of this CPCM is lower than that of CN4W. The thermal conductivity is 13 times higher than CN4W. On this basis, the cyclic thermal stability analysis of CPCM was carried out. MEG-CN4W was found to be stably and highly adsorbed on the expanded graphite surface after 200 heat storage-release cycles without liquid leakage. The phase change temperature and enthalpy value remained nearly unchanged, indicating that the prepared CPCM has potential application prospects.

Mechanical and Thermal Behavior of Fibrous Carbon Materials.

The ability of various commercial fibrous carbon materials to withstand stress and conduct heat has been evaluated through experimental and analytical studies. The combined effects of different micro/macro-structural characteristics were discussed and compared. Large differences in mechanical behavior were observed between the different groups or subgroups of fibrous materials, due to the different types of fibers and the mechanical and/or chemical bonds between them. The application of the Mooney–Rivlin model made it possible to determine the elastic modulus of soft felts, with a few exceptions, which were studied in-depth. The possible use of two different mechanical test methods allowed a comparison of the results in terms of elastic modulus obtained under different deformation regimes. The effective thermal conductivity of the same fibrous materials was also studied and found to be much lower than that of a single carbon fiber due to the high porosity, and varied with the bulk density and the fiber organization involving more or less thermal contact resistances. The thermal conductivity of most materials is highly anisotropic, with higher values in the direction of preferential fiber orientation. Finally, the combination of compression and transient thermal conductivity measurement techniques allowed the heat conduction properties of the commercial fibrous carbons to be investigated experimentally when compressed. It was observed that thermal conductivity is strongly affected under compression, especially perpendicular to the main fiber orientation.

Thermal and Electrical behaviors of Boron Nitride/Epoxy reinforced polymer matrix composite—A review

AbstractIn electronic and electric instruments, heat dissipation is one of the major issues that degrade performance and efficiency. Polymer material has high specific strength and modulus, and it is mainly used in electrical insulation materials due to easy processability and light in weight with great flexibility but has low thermal conductivity. This paper examines the recent developments in highly thermal conductive epoxy‐based polymer composites. First, the composite materials, with applications, are briefly introduced. Second, the thermally conductive polymer types with their thermal conductivity are summarized. Third, the thermal and electric properties of different boron nitride (BN) with epoxy‐filled composite are listed. It also focuses on enhancement in thermal conductivity and electrical insulation, which are mainly influenced by the microstructure and amount of filler content. Some highly thermal conductive BN polymer based composites are reviewed. Fourth, various thermal conductivity measurement techniques are listed with their drawbacks. Finally, the review gives current scenario on thermal conductive polymer materials and developments needed with challenges in this area.

Thermal conductivity measurements in nanosheets via bolometric effect

Thermal conductivity measurement techniques for materials with nanoscale dimensions require fabrication of very complicated devices or their applicability is limited to a class of materials. Discovery of new methods with high thermal sensitivity are required for the widespread use of thermal conductivity measurements in characterizing materials’ properties. We propose and demonstrate a simple non-destructive method with superior thermal sensitivity to measure the in-plane thermal conductivity of nanosheets and nanowires using the bolometric effect. The method utilizes laser beam heating to create a temperature gradient, as small as a fraction of a Kelvin, over the suspended section of the nanomaterial with electrical contacts. Local temperature rise due to the laser irradiation alters the electrical resistance of the device, which can be measured precisely. This resistance change is then used to extract the temperature profile along the nanomaterial using thermal conductivity as a fitting parameter. We measured the thermal conductivity of V2O3 nanosheets to validate the applicability of the method and found an excellent agreement with the literature. Further, we measured the thermal conductivity of metallic 2H-TaS2 for the first time and performed ab initio calculations to support our measurements. Finally, we discussed the applicability of the method on semiconducting nanosheets and performed measurements on WS2 and MoS2 thin flakes.

Accurate measurement of nanofluid thermal conductivity by use of a polysaccharide stabilising agent

Measuring the thermal conductivity of low viscosity fluids such as aqueous nanofluids is challenging due to the formation of convection currents. In the current work, a modification of the transient hot-wire thermal conductivity measurement technique was investigated to address this problem. The polysaccharide agar was used as a gelling agent to prevent the formation of convection currents, thereby enabling measurement of thermal conductivity. The experimental method was validated by comparison of experimentally measured thermal conductivity values with published reference values over a range of temperatures for two reference fluids stabilised by agar: water and an ethylene glycol/water solution. The precision of thermal conductivity measurements was found to be significantly improved by use of this gelling agent. These findings indicate that agar, or a similar gelling agent, can be used to enable accurate measurement of the thermal conductivity of aqueous fluids. This measurement technique was utilised to accurately measure the thermal conductivity enhancements of copper and alumina aqueous nanofluids with low nanoparticle concentrations, over a range of temperatures. The thermal conductivities of these nanofluids were found to be within ±2% of those predicted by the Maxwell model.

A review of the application of carbon materials in solar thermal energy storage

Graphitic materials can potentially mitigate the issue of low thermal conductivity in phase change materials (PCM) when used in solar thermal energy storage. However, carbon can form an exceedingly wide variety of allotropes which are difficult to distinguish. This study has examined an extensive range of energy storage carbon composites including: synthetic and natural graphite, graphitic fibres, graphitic foams, expanded graphite, graphite nano-platelets, graphene, carbon nanotubes and amorphous carbons. The thermal energy storage media covered include conventional low temperature materials such as paraffins, alcohols, fatty acids and numerous others, as well as high temperature salts. In addition, a wide range of both steady and unsteady state thermal conductivity measurement techniques are represented.Based on the collated results it is evident that particulate additives are limited, potentially due to contact resistance, to improving PCM thermal conductivities by a factor of less than ten. On the other hand, matrix materials like compressed expanded graphite and graphitic foams are capable of achieving enhancements in excess of 10,000%. Compressed expanded graphite composites appear to be the most economically favourable option. Nanomaterials do not perform as anticipated and indications are that costs must drop by more than three orders of magnitude before they become attractive. It has been demonstrated that it may be possible to predict composite thermal conductivity for all carbon materials using a single model, if an additional structural parameter could be measured.

Note: Improvement of the 3ω thermal conductivity measurement technique for its application at the nanoscale.

Conventional techniques for thermal conductivity measurements can lead to unreliable results when applied to nanostructures because heaters and temperature sensors needed for the measurement cannot have a negligible size and therefore perturb the result. In this paper, we focus on the 3ω technique, applied to the evaluation of the thermal conductivity of suspended silicon nanoribbons. We introduce a numerical approach based on the finite element solution of the electrical and thermal transport equations and compare its results with those of conventional methods. We show that with our approach we achieve an excellent fit of the experimental data, in particular, for nanostructured materials.

Novel thermal efficiency-based model for determination of thermal conductivity of membrane distillation membranes

Development and selection of membranes for membrane distillation (MD) could be accelerated if all performance-determining characteristics of the membrane could be obtained during MD operation without the need to recur to specialized or cumbersome porosity or thermal conductivity measurement techniques. By redefining the thermal efficiency, the Schofield method could be adapted to describe the flux without prior knowledge of membrane porosity, thickness, or thermal conductivity. A total of 17 commercially available membranes were analyzed in terms of flux and thermal efficiency to assess their suitability for application in MD. The model described the flux with an average %RMSE of 4.5%, which was in the same range as the standard deviation of the measured flux. The redefinition of the thermal efficiency also enabled MD to be used as a novel thermal conductivity measurement device for thin porous hydrophobic films that cannot be measured with the conventional laser flash diffusivity technique.

A single-sided method based on transient plane source technique for thermal conductivity measurement of liquids

It is well acknowledged that the onset of natural convection have significant impacts on transient thermal conductivity measurement of liquids. Aiming at this drawback, a single-sided horizontal method based on the transient hot disk technique is proposed to perform more accurate measurements of liquids by suppressing natural convection during the tests. Theoretical solutions of this proposed single-sided method are deduced, and numerical simulations are applied to obtain quantified effects of natural convections. Some experiments are performed to verify the numerical settings. Water, glycol and methanol are analyzed numerically using both the traditional vertical orientation method and the single-sided horizontal method. Both temperature profiles and velocity profiles are recorded. It is found that the viscosity and the coefficient of thermal expansion of liquid samples are the key factors for the onset of natural convection. For single-sided horizontal method, the temperature boundary is 2mm thinner and the maximum velocity is an order lower when testing water compared with the results from vertical method. Besides, the average temperature of the sensor recorded in single-sided horizontal method fits better with the standard value for all tested liquid samples. All of these analyses show that the proposed single-sided horizontal method can effectively suppress the onset of natural convection during the measurements.

Review on Thermal Conductivity of Nanofluids

Among the thermo physical properties of the nano fluids thermal conductivity is the key property which depends on the pertinent parameters of nanoparticles material, volume fraction, size, aspect ratio, base fluid thermo physical properties, temperature, and surfactant. Over last decades, numerous works have reported the higher thermal conductivity of nano fluids than that of the conventional heat transfer fluids with the reason of random motion of nanoparticles and a number of numerical and theoretical models have been proposed. In this article, the current state of knowledge on the several thermal conductivity measurement techniques employed by the researchers along with the factors affecting the thermal conductivity of nano fluids have been presented. This review leads to some directions for future research in nano fluids.

Probing technique for localized thermal conductivity measurement

A low-cost and non-destructive measurement technique based on the combination of a temperature sensor and a heater integrated in a very sharp tip is proposed for the determination of thermal conductivity of planar materials. The thermal sensor is fabricated by means of microtechnology technique. Associated to the system, an analytical thermal model is developed to express the measured Seebeck voltage as a function of the material thermal resistance. A numerical analysis based on COMSOL MultiphysicsⓇ is then developed to extract the thermal conductivity from the thermal resistance. To validate the approach proposed, experiments on planar dielectric materials and metals are conducted. Precision around 0.1 W m−1 K−1 for thermal conductivities lower than 2 W m−1 K−1 is obtained.

Comparative thermal conductivity measurement of chemical vapor deposition grown graphene supported on substrate

We develop a comparative thermal conductivity measurement technique based on miniature differential thermocouples and measure the temperature dependence of the thermal conductivity of chemical vapor deposition grown graphene supported on glass or SiO2 substrate in the temperature range of 180–420 K. For all 28 investigated samples, the obtained room temperature values of the thermal conductivity are bounded between 1800 and 2200 W/m K. Compared with other techniques, we find that a significant improvement in accuracy of our measurements is because of the absence of chemical doping/contamination from micro-fabrication procedures. Our results are confirmed using scanning thermal microscopy.

Thermal conductivity measurement at micrometer length scales based on a temperature-balance method

We have developed a novel thermal conductivity measurement technique that is applicable to micro- and nanoscale structures. The methodology integrates a device consisting of two identical microheaters with thin-filmed platinum heating elements and integrated resistance temperature devices (RTDs) on trenched, thermally isolated silicon substrates. The platinum RTDs used for temperature measurement were calibrated by monitoring the melting points of three metallic microspheres placed on the device as its power was increased. We validated our measurement technique by measuring thermal conductivities of several micrometer-sized metallic wire standards with known values. A multiphysical analysis based on a 3D finite element method has been conducted to simulate both the electric and thermal behaviors of the microheaters. These simulations describing the heating and heat flow in the device yield similar temperature profiles near the microheaters obtained in the experimental system.