Changes In Thermal Conductivity Research Articles

Lattice thermal conductivity of defected tungsten evaluated by equilibrium molecular dynamics simulation

Exposed to high fluxes of helium particles and heat, tungsten divertor plates will suffer various forms of damage thus degrading its performance such as its thermal conductivity. In this paper, the equilibrium molecular dynamics method is employed to evaluate the lattice thermal conductivity (LTC) changes of the tungsten lattice containing one type of defects (dislocation loops, helium bubbles and impurity helium atoms). It is found that the LTC of tungsten is slightly reduced in the presence of dilute dislocation loops and vacuum bubbles, while a remarked reduction of the thermal conductivity is engendered by dilute helium particles. A tiny number of helium atoms as randomly-distributed tetrahedral interstitials (accounting for 0.2 % of the total number of atoms in the simulation cell), result in a drastic drop of the LTC to 15 % of that of the perfect tungsten lattice. The phonon density of states of the perfect and defected tungsten lattices are compared to identify the causes of the LTC reduction at the atomic level.

Revisiting thermal transport in single-layer graphene: On the applicability of thermal snapshot interatomic force constant extraction methodology for layered materials

The thermal conductivities of single-/bi-layer graphene and bulk-graphite are obtained using the Boltzmann transport equation (BTE) framework by accounting for three-phonon and four-phonon scatterings. For single-layer graphene, the thermal conductivity and interatomic force constants obtained using temperature-independent finite-difference, and temperature-dependent molecular dynamics-based approaches agree with each other. The use of the thermal snapshot approach to get temperature-dependent force constants results in a non-physical description of interatomic distances for single-layer graphene. The predicted thermal conductivity at room temperature using finite-difference based force constants is 800 W/m K, which is a severe under-prediction of experimentally measured values. For bi-layer graphene and bulk graphite, the thermal snapshot methodology is applicable and thermal conductivity changes by 25% and 5% with temperature-dependent force constants. The effect of four-phonon scattering is less than 10% on the predicted thermal conductivity of bi-layer graphene and graphite, and the obtained thermal conductivities using thermal snapshot methodology are in agreement with the literature. The limitation in the prediction of thermal conductivity of single-layer graphene via the BTE approach stems from non-accountability of temperature-dependence in finite-difference based force constants and non-physical description of interatomic bonds in thermal snapshot based force constants extraction for planar 2-atoms unitcell of single layer graphene.

A Theory on Increasing the Heat Transfer Performance of Building Wall

The target of traditional thermal conductivity of wall research is the spatial distribution form. In these studies, the change of thermal conductivity with temperature is neglected. Meanwhile, case studies are always used. This method needs large computation and it is hard to obtain the optimal result. In order to overcome the problems, a new approach has been put forward in this paper. Different from the traditional approach, the new approach solves an inverse problem under the concept of passive ideal energy-saving buildings to obtain the optimal distribution of heat ability with temperature on an external wall. The result for a typical summer day shows the heat ability distribution of a wall in summer is a staircase. It is similar to the heat pipe. It is also found that the optimal heat transfer property of the external wall is closer to the heat pipe when its heat capacity per square meter (ρcpL) is of extreme value. This study can provide guidance to researchers in building materials.

Optimization of Thermal Conductivity vs. Bulk Density of Steam-Exploded Loose-Fill Annual Lignocellulosics.

Lignocellulosic biomass (LCB)-based thermal insulation materials available in the market are more expensive than conventional ones and consist mainly of wood or agricultural bast fibers which are primarily used in construction and textile industries. Therefore, it is crucial to develop LCB-based thermal insulation materials from cheap and available raw materials. The study investigates new thermal insulation materials from locally available residues of annual plants like wheat straw, reeds and corn stalks. The treatment of raw materials was performed by mechanical crushing and defibration by steam explosion process. Optimization of thermal conductivity of the obtained loose-fill thermal insulation materials was investigated at different bulk density levels (30-45-60-75-90 kg m-3). The obtained thermal conductivity varies in range of 0.0401-0.0538 W m-1 K-1 depending on raw material, treatment mode and a target density. The changes of thermal conductivity depending on density were described by the second order polynomial models. In most cases, the optimal thermal conductivity was revealed for the materials with the density of 60 kg m-3. The obtained results suggest the adjustment of density to achieve an optimal thermal conductivity of LCB-based thermal insulation materials. The study also approves the suitability of used annual plants for further investigation towards sustainable LCB-based thermal insulation materials.

Zn_{1-x}Ni_xTe semiconductor nanocrystals in transparent glass for optoelectronic device applications

Doping glass with semiconductors, particularly with nanostructured semiconductors, has attracted attention due to the large optical absorption cross-sections of the latter. Based on this property, Ni^{2+} (5 wt%) doped phosphate glass and Zn_{1-x}Ni_xTe (x = 0.5, 1.0, 5.0 and 10.0 wt% of Ni^{2+}) nanocrystals (NCs) doped phosphate glasses (GCs) were prepared by fusion method and subsequent heat treatment. Influence of Ni^{2+} on structural, thermo-optical and third-order nonlinear optical properties have been analysed through various spectroscopic characterizations. The XRD pattern of the glass (G) exhibits the amorphous nature of the host material while GCs exhibit not only amorphous halo but also the presence of quantum dots (QDs) or nanocrystals (NCs) phases. TEM analysis of the studied GCs samples confirm the presence of quantum dots (QDs) and bulk NCs with an average diameter of approximately 4.2 {pm } 0.3 nm and 13.4 {pm } 0.2 nm, respectively. Several phosphate groups were observed and reported from Raman and FTIR-ATR spectra. The absorption band positions confirmed that Ni^{2+} ions resemble to the octahedral symmetry. The intensity of absorption band around 1352 nm (^3T{_1}(F) rightarrow^3A{_2}(F)) increased with the increase of Ni^{2+} in GCs which is an indicative of the ^{[6]}Ni^{2+} coordination. The emission properties such as emission cross-sections ({sigma }_{emi}) full width at half maxima (FWHM) for the ^1T{_2}(D) rightarrow^3T{_2}(F) (visible) and ^3A{_2}(F) rightarrow^3T{_1}(F) (near-infrared) emission transitions were reported. Among the glass-containing semiconductor nanocrystals (GCs), the emission cross-sections in GC4 sample (x = 10% of Ni^{2+}) are the largest for both the visible (11.88 times 10^{-18} cm^2) and infrared (0.98 times 10^{-20} cm^2) transitions. Thermal diffusivity (D), thermal conductivity (K) and temperature dependent optical path length change (ds/dT) were obtained through time-resolved thermal lens (TL) and thermal relaxation (TR) methods. The D and K parameters do not change significantly with increase of Ni^{2+} ions (0.5–5%) in GCs. Nonlinear-refractive index and nonlinear absorption of the studied samples were also obtained using femtosecond Z-scan technique. The increase of nonlinear absorption coefficient (beta) is observed from GC2 (2.53 {times } 10^{-10} cm/W) to GC4 (7.98 {times } 10^{-10} cm/W). The GC4, sample with 10 wt% of Ni^{2+}, showed the lowest ds/dT (1.22 times 10^{-6} K^{-1}) with good lasing (FOM and emission cross-sections) and nonlinear absorption properties suggesting that it can be a good candidate for visible-red emission light conversion in LED technology.

Tracing the Soil Water Flux in Variably Saturated Zones Using Temperature Data

AbstractThe coexistence of water, vapor, and solid phases and the spatiotemporal changes in thermal conductivities in response to variations in water content pose a challenge to the heat‐tracing method used to calculate water flux in variably saturated zones. In this study, a new approach, which adopted finite difference numerical schemes along with the limited‐memory Broyden–Fletcher–Goldfarb–Shanno bounded (L‐BFGS‐B) parameter optimization algorithm, was developed to inversely trace vertical soil water fluxes in variably saturated zones using temperature‐depth time‐series data. The newly developed method did not utilize hydraulic parameters, and only temperature time‐series data were used at the adjacent depths as boundary conditions. The soil water, vapor flux, and spatiotemporal variations in thermal conductivity at a given location were optimized by fitting the temperature time‐series at the target depth using the L‐BFGS‐B algorithm. Further, the method was verified through laboratory experiments under different saturated conditions that were conducted in two columns with saturated and free drainage bottom boundaries, and five sets of different flow rates with a relatively stable flow condition. The soil water flux calculated using the proposed method was consistent with the observed data, thus, verifying the reliability of the approach in applications with relatively stable flow rates. Furthermore, the new method was applied to a field site to estimate the time‐varying soil water flux using the measured temperature data. The trends of the calculated soil water flux at different depths were consistent with the field hydrologic conditions and observations. Thus, this study provides a novel computationally efficient and easy‐to‐implement tool to determine soil water flux using temperature measurements.

Microstructure and thermal-physical properties of hypereutectic Al-Ni alloys

The microstructure and thermal-physical properties of Al-xNi (x = 6–25 wt.%) alloys were investigated by experiments first. With the increase of Ni content, the volume fraction and size of the Al3Ni increase significantly, while the morphology has no change. The thermal expansion coefficient (CTE) and the thermal conductivity (TC) decrease at the same time. The CTE of Al-6 wt.%Ni alloy is 20.2 × 10−6/K at 100 °C, decreasing to 15.4 × 10−6/K for Al-25 wt.%Ni, which demonstrates favorable thermal expansion performance. The TC changes from 202.8 W/(m·K) to 103.9 W/(m·K). Besides, the structure and thermal properties of Al3Ni were studied by the first-principles calculation to provide relevant thermodynamic data. The expansion performance was analyzed at the same time, the linear CTE of Al3Ni is 13.82 × 10−6/K at 100 °C. Based on the experiments and first-principles calculations, the modified Turner and general Effective Medium Theory (GEMT) models were used to explain the relationships between microstructure and thermal expansion coefficient, thermal conductivity of Al-xNi alloys.

Effects of atmospheric gases and compression load on effective thermal conductivity of Li2TiO3 pebble bed

Tritium breeder material in the solid-type of breeding blanket concepts is usually used in a pebble bed form because of the efficiency and convenience of tritium extraction. The pebble bed is continuously heated by the exothermal reaction between neutron and lithium-6 during the operation of a fusion reactor. Thus, a compression pressure is applied on the pebble bed by the thermal expansion of pebbles and container structure. Therefore, the accurate thermal properties of the pebble bed are one of the important design parameters to ensure a function and stability of the breeding blanket. This study aims at the investigation of the change on effective thermal conductivity of Li2TiO3 pebble beds under the operation conditions of breeding blanket.The measurement of effective thermal conductivities of Li2TiO3 pebble beds has been performed by laser flash technique in several atmosphere gases, such as helium, nitrogen, and air, under the statically compression load controlled by specially design pneumatic device incorporated to the laser flash apparatus. Also, in the case of helium atmosphere, the effects of hydrogen concentration from 0.1 wt.% to 1.0 wt.% on the effective thermal conductivity were also investigated. The effective thermal conductivity of the Li2TiO3 pebble bed was strongly dependent on a kind of atmosphere gases, especially its thermal conductivity, in the relatively low temperature. However, the concentration of hydrogen in the helium atmosphere did not significantly affect the effective thermal conductivity of the pebble bed. Applied axial compression load has influenced on the effective thermal conductivity of the Li2TiO3 pebble bed, especially at high temperature. However, the obvious change of effective thermal conductivity with the value of compression load was not observed.

ВЛИЯНИЕ ВЫСОКОГО ДАВЛЕНИЯ НА ТЕРМОЭЛЕКТРИЧЕСКИЕ СВОЙСТВА ФУЛЛЕРЕНА С70

Research has been performed into baric dependences of thermo-EMF and thermal conductivity derivative of C70 crystalline fullerene at pressures up to 46 GPa and room temperature. Pressure were created in a chamber with conducting diamond anvils. Structural transformations were recorded by changes in thermo-EMF and thermal conductivity. The pressure ranges in which there is a significant change in the thermoelectric properties of fullerene C70 are determined. The nature of the baric dependences of thermo-EMF and thermal conductivity derivative indicates irreversible structural transformations at pressures of the order of 32 GPa, due to the destruction of the structure of fullerene molecules. At pressures above 32 GPa, the thermo-EMF values become close to zero. This may be due to the formation of graphite-like carbon.

A thermal balance method for measuring thermal conductivity by compensation of electric cooling or heating based on thermoelectric modules

To achieve the accuracy and convenience of thermal conductivity measurement, a thermal balance method for measuring thermal conductivity by compensation of electric cooling or heating based on thermoelectric modules is proposed in this paper. In this method, the thermoelectric module is regarded as the compensation unit to achieve the thermal balance between the compensation and test sides. Using the balance principle, the sample's thermal conductivity to be measured can be obtained by knowing the thermal conductivity of the compensation side. According to theoretical and experimental research, the compensation model is divided into self-supply energy and energy supply modes, and the law of thermal conductivity changes in thermoelectric modules in the two supply modes is summarized. To verify the feasibility of the thermal balance method, the thermal conductivity of alumina ceramics and polymethyl methacrylate (PMMA) materials with large differences in thermal conductivity were measured. The experiment proved that the thermal conductivity of alumina ceramic is 19.23 W/mK, slightly smaller than the standard value. The average thermal conductivity of PMMA is 0.21 W/mK, consistent with standard values. Taking the experiment conditions and error into account, the data are effective, which verified the feasibility of this method.

Thermal conductivity of metallic nanoparticle using surface effect and kapitza resistance

To estimate the size dependence of thermal conductivity of metallic nanoparticles, different theoretical models and experimental techniques have been used by different groups of researchers. But it still requires an accurate model that supports the experimental results at low dimensions. A model is proposed to predict the change in thermal conductivity of metallic nanoparticles with size by extending the bond-energy model assuming core shell structure. In present work, we have taken metallic nanoparticles as samples for the verification of proposed model. It is observed that the coefficient of thermal conductivity of nanoparticles decreases by lowering the size of nanoparticles. The variation in thermal conductivity of metallic nanoparticles with their shape and size can be successfully explained by introducing the theory including the surface effect and kapitza resistance. It is found that the computed value has a deviation of maximum ±5% with experimental results which is in the permissible range of experimental measurement.

Development of monitoring system for the muon rotating target at J-PARC using an infrared camera

An infrared camera was installed in the proton beamline upstream of the muon-producing rotating target for real-time two-dimensional temperature imaging of the target. For the first time in a muon production facility, temperature profiles of the target during high-intensity proton beam irradiation were obtained. The observed temperatures were similar to the evaluated values. A moving image of the target was successfully taken during the rotation. Accumulation of moving image data is expected to provide information on the thermal conductivity change due to radiation damage, and is expected to make a significant contribution to the development of accelerators and nuclear materials.

Study on the Thermal Characteristics of Pavement Materials Using Polymer Binder (Polymer-Binder-Used Mixture)

Asphalt pavement accounts for ~87% of the overall pavement in Korea. However, owing to the black surface and extensive cover area, the ambient temperature rapidly increases, thereby contributing to the urban heat island effect. Therefore, pavement materials with modified porosity and pavement surface conditions have been developed; however, studies on the heat-related characteristic values of such pavement materials are insufficient. In this study, the thermal characteristics of pavement materials were evaluated with regard to the urban heat island effect. Test specimens such as WC-2 and PA-13, i.e., polymer-binder-used mixture, were compressed using a Gyratory Compactor. The thermal characteristics, i.e., thermal conductivity, specific heat capacity, thermal diffusivity, and thermal emissivity, were studied, and a heat accumulation test was performed. The water-permeable polyurethane concrete mixture (polymer-binder-used mixture) exhibited ~4% lower thermal emissivity than the conventional asphalt mixture, and according to the analysis of the change in thermal conductivity based on heat accumulation, it was confirmed that the rate of the heat energy transfer effect was ~20% slower than that of the asphalt mixture. The study results provide essential information for the analysis and design of energy harvesting and freeze-thaw pavement-related systems.

Utilization of carbonized water hyacinth for effective encapsulation and thermal conductivity enhancement of phase change energy storage materials

In this work, a biochar carrier was developed to adsorb phase change materials and enhance thermal conductivity. Six kinds of modified carbonized water hyacinth were prepared by freeze drying (or vacuum drying) combined with high-temperature carbonization. The pore volume and specific surface area (SSA) of FC900 are the highest, reaching 0.3153 cm3/g and 479.966 m2/g, which are 32% higher than those of VC900. The load rates of FC900 and VC900 on lauric acid-myristic acid-paraffin section (LMPS) are 75% and 65% respectively. Meanwhile, the phase transition enthalpy of LMPS/FC900 is 104.36 J/g, which is 20.75% higher than that of LMPS/VC900, and the phase transition temperatures are 25.93 and 26.77 °C respectively. After 500 times of heat storage and release experiments, LMPS/FC900 maintains a stable phase change peak, which has good thermal performance stability. Notably, the thermal conductivity of LMPS/FC900 is 0.4168 W/(m·K), which is 56.93% higher than that of LMPS. Otherwise, under the constant temperature environment of 50 °C, the heating time of LMPS/FC900 is 11.89% longer than that of LMPS/VC900, which has more excellent temperature control performance. This study concludes that the carbonization temperature of 900 °C and freeze-drying measures are the best preparation process for carbonized water hyacinth, and LMPS/FC900 has good thermal conductivity and phase change energy storage performance.

Studies on Myristyl Alcohol based microencapsulated phase change material for thermal energy storage applications

ABSTRACT In this paper, a novel Myristyl Alcohol (MAL) phase change material (PCM) microencapsulated with calcium carbonate (CaCO3) shell through a self-assembly process is developed. Despite having strong thermal energy storage capabilities, pure MAL PCM has low thermal conductivity and phase change leakage issues, which limit its usefulness for thermal energy storage applications. To avoid leakage and improve thermal conductivity, pure MAL PCM is encapsulated with a CaCO3 shell for different core/shell mass ratios. The morphology, chemical structure, and crystalline structure of the microencapsulated phase change material (MAL-MEPCM) samples are analyzed by the SEM, FTIR, and XRD. The SEM morphology reveals that the generated microcapsules have spherical, needle-shaped, and floral forms. The DSC thermograms reveal the latent heat (melting) values of 223.36 and 155.40 J/g with an encapsulation efficiency of 69.42% for pure MAL PCM and MAL-MEPCM, the maximum core/shell mass ratio. The TGA thermograms confirm good thermal stability. Even after 200 thermal cycles, the samples have consistent chemical stability and phase change properties, as evidenced by FTIR and DSC results. The prepared MAL-MEPCM samples have higher thermal conductivity than the pure MAL PCM and thus enhance thermal energy storage performance.

発泡プラスチック系断熱材の表面被覆層が長期断熱性能に及ぼす影響

It is known that the thermal conductivity of insulation foams increases with time due to the emission of the blowing agents from foams and the transfer of the air into them. In this paper, the aging of thermal conductivity in phenolic foam is discussed with measured data and numerical analysis. Measurement of specimens with different thicknesses and surface material conditions are conducted and the aging characteristics are examined. Using the estimation equations of thermal conductivity change, the effective diffusion coefficient and mass transfer coefficient of the blowing agents is obtained by comparing the simulation results and the measured value.

Corrosiveness of Li2TiO3 ceramic tritium breeder to structural RAFM steel: Effects on microstructure and thermal conductivity

Structural steel will be in long-term contact with ceramic tritium breeder at a high temperature in a solid tritium breeding blanket. The maximum temperature in the tritium breeding zone would reach nearly 1173 K. At this high temperature, a large amount of lithium oxide would evaporate and deposit on the structural materials, causing the overall corrosion of the structural materials. Corrosiveness of Li2TiO3 ceramic tritium breeder to reduced activation ferritic-martensitic steel CLF-1 was investigated in this study. The experiment was carried out at 1123 K under He + 0.1 vol% H2 atmosphere for up to 400 h. The formation of non-uniform corrosion layers with a Fe-rich outer layer and a Cr-rich inner layer were found on the surface of CLF-1 steel. Secondary ion mass spectrometry (SIMS) test at contact and non-contact areas were performed to demonstrate evaporation of lithium compound. The corrosion layers were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. The non-contact area with tritium breeding pebbles were corroded indicating a large amount of lithium oxide on the non-contact zone, resulting in serious overall corrosion of structural materials. LiFe5O8 was found as the main phase on the surface of the corroded samples by X-ray diffraction (XRD). The thermal conductivity of LiFe5O8 (∼0.8 W/m/K, 300 K) calculated by first principles phonon Boltzmann equation (PBE) is significantly lower than the original CLF-1 (∼23 W/m/K, 300 K). Almost no change of thermal conductivity for experimental results was found after corrosion, which is due to non-uniform corrosion layer presenting thinner layer at some areas, as well as the hole, crack and peeling of corrosion layer indicated by SEM results. However, in the long-term service of blanket structural material, it is predicted that a much thicker corrosion layer may form on the surface of RAFM steels thus bring a risk to the heat transfer of blanket.

Dynamic Covalent Bonds in Vitrimers Enable 1.0 W/(m K) Intrinsic Thermal Conductivity

Polymers are under increasing demand as thermal management materials for electronic devices such as integrated circuits and electrical machines. However, the intrinsic thermal conductivity of polymers is typically low, around 0.2 W/(m K). Although crystallinity is qualitatively known to have a positive correlation with thermal conductivity, the quantitative relationship is unclear because, in most cases, changes in crystallinity are accompanied by differences in the chemical structure of the polymer. In this work, vitrimers with a fixed chemical structure and slow crystallization kinetics are investigated to reveal the relationships between crystallinity and various physical properties relevant to heat transport. As slow crystallization occurs over the span of one week, the physical properties of the vitrimers also evolve. Changes in thermal conductivity are dramatic from 0.10 to 1.0 W/(m K). Quantitative relationships among crystallinity, thermal conductivity, speed of sound, and chain conformation are elucidated by a combination of in situ measurements.

A thermal conductivity switch via the reversible 2H-1T′ phase transition in monolayer MoTe2

The two-dimensional (2D) material-based thermal switch is attracting attention due to its novel applications, such as energy conversion and thermal management, in nanoscale devices. In this paper, we observed that the reversible 2H–1T′ phase transition in MoTe2 is associated with about a fourfold/tenfold change in thermal conductivity along the X/Y direction by using first-principles calculations. This phenomenon can be profoundly understood by comparing the Mo–Te bonding strength between the two phases. The 2H-MoTe2 has one stronger bonding type, while 1T′-MoTe2 has three weaker types of bonds, suggesting bonding inhomogeneity in 1T′-MoTe2. Meanwhile, the bonding inhomogeneity can induce more scattering of vibration modes. The weaker bonding indicates a softer structure, resulting in lower phonon group velocity, a shorter phonon relaxation lifetime and larger Grüneisen constants. The impact caused by the 2H to 1T′ phase transition in MoTe2 hinders the propagation of phonons, thereby reducing thermal conductivity. Our study describes the possibility for the provision of the MoTe2-based controllable and reversible thermal switch device.

Response of vibrational properties and thermal conductivity of perovskites to pressure

We study the response of SrTiO3 and KTaO3's vibrational properties and thermal conductivity Λ to pressurization. Our goal is to improve the understanding of the relationship between crystal structure, vibrational dynamics, and thermal conductivity in perovskites. We measure the thermal conductivity of SrTiO3 and KTaO3 up to 28 GPa by time-domain thermoreflectance. We also perform Raman scattering and stimulated Brillouin scattering measurements of SrTiO3 and KTaO3 to characterize changes in vibrational dynamics with pressure. The thermal conductivity of SrTiO3 increases under pressure with a slope comparable to that of other perovskites whose thermal conductivity has been measured versus pressure. Alternatively, the thermal conductivity of KTaO3 has a stronger pressure dependence than that of other materials with similar crystal structure. We correlate pressure-induced changes in Raman and Brillouin spectra with pressure-induced changes in thermal conductivity. We show that pressure-induced changes in phonon lifetimes dominate the pressure dependence of thermal conductivity. This study provides benchmark knowledge of why Λ depends on pressure and improves understanding of structure/thermal-property relationships.