Evolution Of Thermal Conductivity Research Articles

Durability of vacuum insulation panels in the cavity of an insulating glass unit

The durability of vacuum insulating panels included in the cavity of insulating glass panels has been studied using a heat flow meter to monitor the thermal conductivity evolution upon accelerated aging. The study highlighted that, thanks to the high degree of protection of the inner cavity against moisture, the vacuum insulation panel maintains its thermal insulation performances even after 25 weeks of cycling from −20°C to 80°C and from 10% to 90% relative humidity. In comparison, unprotected vacuum insulation panels show a steady increase of thermal conductivity, which is setting them close to the end of their theoretical service lifetime. Comparing the slopes of the thermal conductivity evolution over time, the calculated degree of protection provided by such an assembly is superior to 10 in comparison to unprotected panels. This mode of protection could be useful to extend the lifetime of vacuum insulation panels in various building applications.

Effect of Surrogate Aggregates on the Thermal Conductivity of Concrete at Ambient and Elevated Temperatures

The accurate assessment of the thermal conductivity of concretes is an important part of building design in terms of thermal efficiency and thermal performance of materials at various temperatures. We present an experimental assessment of the thermal conductivity of five thermally insulated concrete specimens made using lightweight aggregates and glass bubbles in place of normal aggregates. Four different measurement methods are used to assess the reliability of the thermal data and to evaluate the effects of the various sensor types. The concrete specimens are also assessed at every 100°C during heating to ~800°C. Normal concrete is shown to have a thermal conductivity of ~2.25 W m−1 K−1. The surrogate aggregates effectively reduce the conductivity to ~1.25 W m−1 K−1 at room temperature. The aggregate size is shown not to affect thermal conduction: fine and coarse aggregates each lead to similar results. Surface contact methods of assessment tend to underestimate thermal conductivity, presumably owing to high thermal resistance between the transducers and the specimens. Thermogravimetric analysis shows that the stages of mass loss of the cement paste correspond to the evolution of thermal conductivity upon heating.

Thermal conductivity of graphite filled liquid crystal polymer composites and theoretical predictions

Abstract Composites of liquid crystal polymer (LCP) and powdered synthetic graphite were melt processed to explore the effect of the filler contents on the properties of these composites. Morphology and mechanical properties were evaluated to correlate the filler alignment with mechanical properties. The difference observed in Izod impact strength between the far and gate ends of these specimens stems from the difference in the morphology. Thermal expansion decreased as filler content increased for all composites, but the decrease at high filler loading was less significant. Measurement of thermal conductivity for the resulting composites shows very high value up to 28.3 W/m K that is 71-fold increase compared to the pure LCP. To our knowledge, this thermal conductivity value is the highest one for thermally conductive polymer composites without any filler modification among the many similar reports. Thermographic images for heat releasing behaviors show good agreements with the thermal conductivity results indicating the rapid temperature drops for these composites. Various models have been compared to predict the evolution of thermal conductivity of the composites and shows acceptable prediction to experimental results.

In suit synthesis and bonding of Cu to W–Cu composite by combustion synthesis and centrifugal infiltration

Abstract A novel method for the in suit synthesis and bonding of Cu to W–Cu composite was developed by combining combustion synthesis with centrifugal infiltration. Highly exothermic combustion reaction in Al–CuO system was utilized to realize high temperature and produce molten products. The application of high gravity field of 1000 G resulted in a rapidly phase separation of Cu melt and infiltration of W loosen powder. Composition and microstructure showed a layered variation along high gravity direction. Cu was bonded to W–Cu composite to form a Cu/W–Cu integral material. Columnar grain microstructure was observed on Cu layer due to a fast cooling rate produced along high gravity direction. Pores were found concentrated at the interface between Cu and W–Cu layer. The evolution of thermal conductivity and hardness along high gravity direction were investigated.

Evolution of thermal conductivity of dysprosia stabilised thermal barrier coating systems during heat treatment

Dysprosia stabilised zirconia coatings offer a potential reduction in thermal heat transfer for thermal barrier coating systems with the added benefit of being producible with existing equipment and spray knowledge. However, there is little information on the long term performance of such systems relative to the standard coatings. Whilst a low thermal conductivity is important for a gas turbine; sintering resistance is important to maintain properties over the lifetime of a component.In this study, four dysprosia stabilised zirconia coatings are compared with a standard yttria stabilised coating in present industrial use.Samples were exposed to isothermal furnace conditions at 1150°C from 5 to 200h to observe the sintering resistance of the coating systems. Tests carried out include microstructural analysis with SEM, thermal conductivity measurements using laser flash analysis and porosity changes monitored using image analysis.

Weak Bonding Effect on the Ultralow Thermal Conductivity of Germanium Nanodot Arrays in Silicon

ABSTRACTThe thermoelectric figure of merit ZT depends on the thermal conductivity inverse. uperlattices with periodic thin layers were studied to obtain ZT > 1 due to phonon confinement between their layers. Unfortunately, their synthesis with ZT higher than 1 is hazardous due to lattice mismatches forming dislocations and cracks. Nanowires with low dimensionalities were also proposed. However, as the superlattices, they decrease the thermal conductivity in only one propagation direction. In experiments, these one-dimensional insulating materials usually fail to beat the lowest limit of amorphous Si (+/- 1 W/m/K). In this theoretical study, three-dimensional Ge quantum dot arrays in Si are proposed to obtain an extreme thermal-conductivity reduction. Two decrease effects are shown from a molecular supercrystal model. First, low phonon group velocities are computed by lattice dynamics. Second, near-field scattering is exalted assuming weak interface bonding. This prediction can lead to a significant ZT increase. Indeed, a thermalconductivity global minimum λ* = 0.009 W/m/K is predicted for a Si/Ge supercrystal with nanodot spacing of +/- 30 nm and Ge concentration of +/- 12.5 Ge at.%. This ultralow λ* is computed at 300 K assuming that all Ge nanodots are weakly bonded and scatter the phonons at the Si-Ge interfaces in the geometrical limit. Thermal conductivity evolution is analyzed with respect to the weakly-bonded Ge nanodot density.

Layer and size dependence of thermal conductivity in multilayer graphene nanoribbons

Using nonequilibrium molecular dynamics method (NEMD), we have found that the thermal conductivity of multilayer graphene nanoribbons monotonously decreases with the increase of the number of layers which can be attributed to the phonon resonance effect of out-of-plane phonon modes. The reduction of thermal conductivity is proportional to the layer size, which is caused by the increase of phonon resonance. The results clearly show the dimensional evolution of thermal conductivity from quasi-one dimension to higher dimensions in graphene nanoribbons.

Predicting Thermal Conductivity Evolution of Polycrystalline Materials Under Irradiation Using Multiscale Approach

A multiscale methodology was developed to predict the evolution of thermal conductivity of polycrystalline fuel under irradiation. At the mesoscale level, a phase field model was used to predict the evolution of gas bubble microstructure. Generation of gas atoms and vacancies was taken into consideration. Gas bubbles were predicted to form, grow, and coalesce around grain boundary (GB) areas. On the macroscopic scale, a statistical continuum mechanics model was applied to predict the anisotropic thermal conductivity evolution during irradiation. Microstructures predicted by the phase field model were fed into the statistical continuum mechanics model to predict properties and behavior. A decrease of thermal conductivity during irradiation was demonstrated. The influence of irradiation flux, the exposure time, and the grain microstructure were investigated. If the initial GB microstructure was isotropic, the thermal conductivity under irradiation would be similarly isotropic. If the initial GB configuration was anisotropic, anisotropy of thermal conductivity would intensify under irradiation as gas bubbles coalesce around GB areas. The prediction of microstructure and property evolution of polycrystalline materials under irradiation by bridging two models in different scales were demonstrated successfully. This approach provides a deep understanding from a basic scientific viewpoint.

하중 조건이 지반의 열전도도에 미치는 영향: 입자 스케일에서의 연구

지반 물질의 열전도도는 경험식이 제안하는 단위 중량, 간극률 등의 영향 인자 이외에도 하중조건에 따라 크게 좌우된다. 본 논문에서는 개별요소법에 의해 생성된 입자상 지반재료의 열 전달 특성을 열 네트워크 모델로 해석하여 하중이 열전도도에 미치는 영향을 평가하였다. 하중의 변화에 의한 개별 입자들간의 접촉수 및 간극률, 간극수의 전도도에 따른 열전도도를 산출하여 영향 요소들간의 관계를 분석하였다. 전도도의 변화 양상은 전단강성도 분석과 유사하게 열전달 방향 및 하중 크기에 따른 멱함수 형태로 회귀분석이 가능하였다. 해석 결과 하중에 따른 입자간 접촉 면적의 증가 및 간극수의 전도도가 전체 입자상 물질의 열 흐름에 큰 영향을 미침을 알 수 있었다. 열전도도의 이방성은 하중 방향에 의해 좌우되며 입자 스케일에서의 매커니즘이 열 흐름을 좌우하는 중요한 인자임을 보였다. The stress condition mainly dominates the thermal conductivity of soils whereas governing factors such as unit weight and porosity suggested by empirical correlations are still valid. The 3D thermal network model enables evaluation of the stress-dependent thermal conductivity of particulate materials generated by discrete element method (DEM). The relationship among dominant factors is analyzed based on the coordination number and porosity determined by stress condition and thermal conductivity of pore fluid. Results show that the variation of thermal conductivity is strongly attributed to the enlargement of inter-particle contact area by loading history and pore fluid conductivity. This study highlights that the anisotropic evolution of thermal conductivity depends on the directional load and that the particle-scale mechanism mainly dictates the heat transfer in soils.

Effects of V doping in La 0.67Ca 0.33MnO 3: Resistivity, magnetization, thermoelectric power and thermal conductivity

The V doped La 0.67Ca 0.33Mn 1− x V x O 3 (0 ≤ x ≤ 0.08) samples have been investigated by measuring the resistivity, magnetization, thermoelectric power and thermal conductivity. The V doping lowers the metal–insulator and magnetic transition temperature and enhances the thermoelectric power. The evolution of thermal conductivity with temperature exhibits a similar characteristics as reported previously. For the x = 0.04 sample, the transport behavior somewhat appears to be anomalous, and it is suggested to result from the competing interaction among the multiple effects due to the V doping.

Thermal conductivity and heat capacity of the relaxor ferroelectric[PbMg1/3Nb2/3O3]1−x[PbTiO3]x

The evolution of thermal conductivity and heat capacity for the series of ${[{\text{PbMg}}_{1/3}{\text{Nb}}_{2/3}{\text{O}}_{3}]}_{1\ensuremath{-}x}{[{\text{PbTiO}}_{3}]}_{x}$ $(\text{PMN-}{\text{PT}}_{x})$ single crystals with $0\ensuremath{\le}x\ensuremath{\le}1$ are reported. PMN shows typical glasslike behavior in both measurements, which is attributed to the presence of polar nanoregions. With PT doping up to the region of morphotropic phase boundary (MPB), the plateau in thermal conductivity is progressively suppressed. This is reminiscent of partially crystallized glasses and suggests enhanced thermal boundary resistance from ferroelectric domains. The linear temperature contribution and large ${C}_{p}/{T}^{3}$ peak in heat capacity remain nearly constant for PT doping up to the MPB but are rapidly suppressed for larger $x$. These results demonstrate that thermal properties are intimately coupled to the local structures in relaxor ferroelectric systems.

Thermal Conductivity Evolution of Saturated Clay under Consolidation Process

This paper presents the results of a study on the thermal conductivity of a soft saturated clay (Bangkok clay) carried out in relation to an investigation into thermal ground improvement using prefabricated vertical drains. The thermal conductivity of clay specimens was measured, at different porosities and temperature levels, using a simple nondestructive steady-state test method. In addition, a theoretical mixture model to simulate the evolution of thermal conductivity of saturated fine-grained soils has been introduced. It is formulated in terms of thermal conductivity and volume fraction of each soil phase (solid and water), and a morphological parameter controlled by the soil fabric condition. The proposed model has been validated against thermal conductivity results reported in the literature and results obtained from the present investigation. Reasonable agreement has been obtained between the predicted and measured thermal conductivity values.

Thermal conductivity enhancement of electrically insulating syndiotactic poly(styrene) matrix for diphasic conductive polymer composites

AbstractIn co‐continuous conductive polymer composites (CPCs) designed for heat generation by the Joule effect, thermal conductivity km of the electrically insulating matrix is the limiting factor to obtain good heat dissipation. To prevent the establishment of non‐desirable high temperature gradient in heating elements, it is necessary to enhance the thermal conductivity of the composite, kc, independently from its electrical conductivity, σ (already optimized). Several adequate fillers for this purpose have been selected, boron nitride (BN), talc (Mg3Si4O10 (OH)2), aluminum nitride (AlN) and aluminum oxide (Al2O3) particles, and their impact on thermal properties and syndiotactic polystyrene phase transition temperatures have been studies. Lewis and Nielson, Cheng and Vachon, Agari and Uno models were used to predict the evolution of thermal conductivity with filler content and were found to describe correctly thermal conductivity of CPCs after the determination of ϕm the maximum packing fraction, A = f(D/l) the shape factor, C1 and C2 parameters function of respectively modifications of crystals and ability of the filler particles to associate into chains. These models show that whatever the filler and unlike electrical conductivity, thermal conductivity does not go through a sharp insulator/conductor (percolation threshold). Only BN shows a real exponential increase of conductivity over 20% v/v filler. Consequently, in best conditions introducing 30% v/v of BN allows the thermal conductivity to be multiplied by six. BN particles were also found to shift the CPCs glass transition temperature (Tg), the non‐isothermal crystallization temperature (Tc,n) and the melting temperature Tm. Copyright © 2006 John Wiley & Sons, Ltd.

Experimental study and modelling approaches for the thermal conductivity evolution of hydrating cement paste

Thermal stresses due to hydration heat release within concrete structures can lead to early age cracking particularly in cases of massive structures. In order to prevent, minimise and avoid this, a good knowledge of the evolution of cement-based material heat transfer properties at early stages of hydration is essential. In order to address the problem of the evolution of the thermal conductivity of fresh cement paste, two models developed by the authors are discussed: first, the ‘apparent’ thermal conductivity is calculated, assuming the fresh cement paste to be a multiphase material, which makes it possible to consider the material chemical evolution. The second modelling approach is based on the theory of heat and mass transfer within non-saturated porous media. The second modelling results appear to fit the experimental results better.

Experimental study and modelling approaches for the thermal conductivity evolution of hydrating cement paste

Experimental study and modelling approaches for the thermal conductivity evolution of hydrating cement paste

Effective Thermal Conductivity Evolution as a Function of Temperature and Humidity, During Freezing of a High-Porosity Model Food

The modelling of heat transfer within materials with high porosity is complicated by the evaporation–condensation phenomenon. The aim of this work is to develop a model of apparent thermal conductivity in these products. The effective thermal conductivity of a porous food model (sponge) having 0–60% moisture content and 0.92–0.94 porosity was measured by a line-source heat probe system in the range –35–25°C. Two predictive models of the effective thermal conductivity of porous food were developed (Krischer and Maxwell models). The effective thermal conductivity predicted by the Krischer model was in good agreement with the experimental data. Also, it was shown that the model, including the effect of the evaporation–condensation phenomenon in addition to heat conduction, was useful in predicting the effective thermal conductivity of foodstuffs.

Thermal Conductivity of Whey Protein Films Undergoing Swelling: Measurement by Dynamic Gauging

Films of whey protein were cleaned from stainless steel surfaces by treatment with alkaline solutions flowing through a duct. The films were monitored by the simultaneous use of a heat flux sensor and a fluid dynamic gauge, backed up by a protein assay. This combinationof techniques yields directly, for the first time, a quantitative account of the variation of a structure-sensitive property of the films, namely thermal conductivity, as their structures evolve during cleaning, particularly during the swelling stage of the cleaning process. For raw protein films—that is, for films that have not been subjected to ageing by steaming—the conditions studied fall in the ranges: NaOH concentrations, 0.3–2.0 wt%; bulk fluid temperatures, 20–50°C; and flow velocities, 0.03–0.30 ms −1 , corresponding to Reynolds numbers of 500–10,000. These resulted in swelling ratios of up to 4.4, for films which initially consisted of ca. 86% water, by volume. Thermal conductivities varied linearly with time during the swelling stage, from initial values of 0.26 Wm −1 K −1 to values at maximum swelling time of up to 0.43 Wm −1 K −1 . These latter values are substantially lower than one might expect for gels consisting mainly of water and differ substantially from estimates reported in the literature.A brief study is also reported of the effect of deposit ageing, which was simulated by holding plates in a steaming environment at 100°C for prolonged periods. In such cases, it proved possible to follow the evolution of thermal conductivity over the static and decay stages of the cleaning process.

Influence of high gas pressure on combustion synthesis of the solid–solid reaction of NiAl compound

We show the advantage of using high gas pressure in the improvement of combustion synthesis conditions for NiAl compound synthesis. Experiments on high pressure of argon between 10 and 500 MPa showed that the exothermicity of Ni–Al reactions was considerably reduced, with a complete reaction giving NiAl compound. Thermodynamic and kinetic studies were carried out. The results indicate that pressure has relatively little influence on heat capacity or on the enthalpy of the reaction. We show that high pressure lowers the exothermicity of the reaction primarily by increasing the thermal conductivity of the sample. We propose an evolution of thermal conductivity as a function of pressure in the range 10–500 MPa. The main interest of this study is a pressure–heating rate diagram developed for self-propagating high-temperature synthesis (SHS) of NiAl.

Behavior of High Performance Concrete Under High Temperature (60-450°C) for Surface Long-Term Storage: Thermo-Hydro-Mechanical Residual Properties

ABSTRACTAfter various thermal treatments (up to 450°C), residual thermo-hydro-mechanical (T-H-M) properties of two OPC high performance concretes (HPC) were analyzed in the context of surface long-term storage. Materials were prepared with silico-calcareous aggregates (standard HPC) and hematite aggregates (heavy HPC). The initial microstructural (porosity ≈10%) and transport (gas permeability ≈10-19 m2) properties are similar for both concretes. As far as the mechanical aspect is concerned, heavy HPC shows a higher compressive strength and elastic modulus than standard HPC (78 and 63 MPa, 81 and 49 GPa, respectively). Heavy HPC is also characterized by a higher thermal conductivity (7.3 W m-1 K-1 compared to 2.7 W m-1 K-1 for standard concrete). Results analysis show that thermo-hydro-mechanical damages are smaller for heavy HPC. Between 60 and 250°C, the elastic modulus and the compressive strength of standard HPC decrease by 40% and 16%, respectively. For heavy HPC, these parameters respectively decrease by 10% and 4%. A similar trend was observed for thermal conductivity evolution. Gas permeability and porosity data confirm the good behavior of heavy HPC. As a conclusion, hematite HPC seems to provide more interesting T-H-M residual properties than standard HPC. Limited thermal expansion and thermal gradients induced by hematite are probably responsible of this behavior.

The Next Generation Blowing Agents: From One Single Product to a Product Range

In the context of the HCFC-141b phase out, the two candidates HFC-365mfc and HFC-245fa have been selected over the last few years, as reported in previous papers. Since our last presentation, a lot of work has been performed to increase our knowledge of these new products and to study their behavior in various applications. Although HFC-365mfc and HFC-245fa are promising candidates, a single HFC blowing agent cannot match all the outstanding characteristics of traditional fluorocarbon blowing agents: right boiling point, non flammability, cost effectiveness..., to name but a few. It is very unlikely that a single blowing agent will cover all the future needs of the polyurethane industry. Such a trend is already seen today with at least seven physical blowing agents finding commercial applications in the different sectors of the industry (HCFC-141b, HCFC-22/HCFC-142b, HFC-134a, cyclopentane, pentane or isopentane). The main reason why HFC blowing agents will be used in the future is their ability to give foams with optimized thermal conductivity. However, because of possible condensation phenomena at low temperature for relatively high boiling blowing agents, the rating of different blowing agents is a function of temperature. We have now measured foam thermal conductivity over the temperature range between -20°C and +45°C for several blowing agents, including HFC-245fa, HFC-365mfc, pentane isomers and blends thereof. Included in this study are the newly discovered low boiling azeotropes of HFC-365mfc with all three pentane isomers. In this paper, we will especially show that the evolution of thermal conductivity with temperature can be a very good selection criteria to find the most appropriate blend for a specific application.