Decrease In Thermal Conductivity Research Articles

Recycle of ground granulated blast furnace slag and fly ash on eco-friendly brick production

Use of industrial by-products as waste-based materials in the building industry has recently received significant attention to develop eco-friendly building materials. This paper presents an experimental study on properties of bricks containing clay, ground granulated blast furnace slag (GGBS) and fly ash (FA) at 850, 950 and 1050 °C. Physical, mechanical, thermal tests and microstructural analyses were performed. Results show that an increase in firing temperature results in a decrease in porosity and an increase in the bulk density and compressive strength of bricks. Results also show that an increase in the FA content results in an increase in porosity and water absorption but a decrease in bulk density, compressive strength, and thermal conductivity of the bricks. GGBS content has much less significant influence on porosity, water absorption, bulk density and thermal conductivity of bricks, but an increase in GGBS content leads to a significant increase in compressive strength of bricks. Even the binary brick with 30% GGBS/10% FA exhibits nearly identical properties to the control brick. These highly promising findings suggest that replacement of clay with industrial by-products can provide eco-friendly bricks, the use of which would contribute toward reducing the environmental impact of abundant waste products and conserving non-renewable natural resources.

Effect of Xe bubbles on the thermal conductivity of UO2: Mechanisms and model establishment

The thermal conductivities of UO2 containing Xe bubbles were investigated using non-equilibrium molecular dynamics (MD) simulations as a function of porosity, bubble radius and the ratio of Xe atoms to U vacancies (Xe:VU). It was found that when the pores are filled with Xe atoms to form bubbles, the porosity and radius of the bubble both increase with the increase in Xe:VU. Based on the equilibrium porosity and bubble radius, a previous model for porous UO2 was used to predict the thermal conductivity. This model predicted only a slight change in the thermal conductivity with Xe:VU, which is induced by the changes in porosity and bubble radius. However, according to MD simulations, the thermal conductivity of UO2 containing Xe bubbles decreases obviously with Xe:VU. This indicates that in addition to porosity and bubble size effects, there exists another mechanism for the degeneration in UO2 thermal conductivity due to the presence of Xe bubbles. MD details revealed that many defects are produced on UO2/bubble interface, the concentration of which increases with Xe:VU. This leads to the increase in interfacial thermal resistance (ITR), thus resulting in the decrease in overall thermal conductivity. Based on the previous model for porous UO2, a model considering the above mechanism was established for UO2 containing Xe bubbles through phenomenological phonon hydrodynamics. In this new model, the increase in ITR is reflected by multiplying the thermal-drag force by a correction factor. The proposed model can be implemented in fuel performance codes to improve calculations of fuel thermal conductivity.

Aging effects on thermal conductivity of glass-forming liquids.

Thermal conductivity of a model glass-forming system in the liquid and glass states is studied using extensive numerical simulations. We show that near the glass transition temperature, where the structural relaxation time becomes very long, the measured thermal conductivity decreases with increasing age. Second, the thermal conductivity of the disordered solid obtained at low temperatures is found to depend on the cooling rate with which it was prepared. For the cooling rates accessible in simulations, lower cooling rates lead to lower thermal conductivity. Our analysis links this decrease of the thermal conductivity with increased exploration of lower-energy inherent structures of the underlying potential energy landscape. Further, we show that the lowering of conductivity for lower-energy inherent structures is related to the high-frequency harmonic modes associated with the inherent structure being less extended. Possible effects of considering relatively small systems and fast cooling rates in the simulations are discussed.

Enhancement of thermoelectric performance of In doped Bi2Te2.7Se0.3 compounds

Bulk samples of (Bi1-xInx)2 Te 2.7 Se0.3 (x = 0.00, 0.02, 0.04) were prepared by solid state reaction technique. Powder X-ray diffraction pattern confirms that the polycrystalline samples have hexagonal structure with spacegroup R3‾m. Surface morphology shows a reduction in porous behaviour of the material due to co-doping. Energy dispersive X ray analysis demonstrates the elements present in the sample. Electrical resistivity has shown a quasi-degenerate semiconducting behaviour. Hall effect and Seebeck coefficient confirmed that all the samples are n-type. There is a decrease in thermal conductivity with the variation in dopant concentration. The maximum ZT was found to be 0.6 at 350 K for the sample (Bi0.98In0.02)2 Te 2.7 Se0.3 which is about 5 times that of the pristine sample Bi2Te3.

Effect of high temperature on the strength and thermal conductivity of glass fiber concrete

This research selected a water-cement ratio of 0.5 and a slump of 18 cm for an ACI mix design, and used 0.60 and 2.54 cm glass fibers of 0.5%, 1.0% and 1.5% of cement by weight, respectively, to discuss the effect of one hour high temperature (440, 500, 580, 800 and 1000 °C) on the compressive strength and thermal conductivity of concrete. The results show that the additional amount of glass fibers used for enhancing the compressive strength of the concrete should be appropriate. When short glass fiber is added to concrete, the flexural strength of concrete will be reduced when the amount of fiber is too large; when the length of glass fiber added to concrete is long enough, the flexural strength of concrete will increase with the increase of the amount of fiber. Under the effect of high temperature, the residual compressive strength of concrete increases with the amount of glass fibers added and increases with the glass fiber length. The thermal conductivity is lower than the concrete of the control group; the decrement increases with the amount of glass fibers added and increases with the glass fiber length. Temperatures of 400–500 °C have the greatest effect on the decrease of thermal conductivity. Temperatures of 500–580 °C have the second greatest effect on the decrease in thermal conductivity.

Incorporating hollow glass microsphere to cool asphalt pavement: Preliminary evaluation of asphalt mastic

In order to cool asphalt pavement by replacing limestone mineral filler (LMF) with hollow glass microsphere (HGM) in asphalt mixture, this work preliminarily evaluated the thermal, high-temperature rheological and fatigue properties of asphalt mastic. The microstructure, particle size distribution, chemical element composition and crystal structure of LMF and HGM were tested to characterize their physical and chemical properties. A series of tests, including scanning electron microscope, Fourier transform infrared, thermal conductivity, spectral reflectance, multiple stress creep recovery (MSCR) and linear amplitude sweep (LAS) tests were performed to investigate the influence of HGM on the performances of asphalt mastic. The results show that HGM/LMF and asphalt were physically blended, and some HGM particles were broken when preparing asphalt mastic. Completely replacing LMF with HGM in asphalt mastic resulted in a decrease of thermal conductivity by 40% and an increase of infrared reflectance by 60%. The MSCR test result shows that HGM negatively affected the anti-rutting performance of asphalt mastic, while the LAS test result shows that HGM could extend the fatigue life of asphalt mastic. The findings in this study indicate that HGM is a potential material for cooling asphalt pavement.

Molecular dynamics simulations of the effect of dislocations on the thermal conductivity of iron

Phonons contribute an appreciable proportion of the thermal conductivity of iron-based structural materials used in the nuclear industry. The decrease in thermal conductivity caused by defects such as dislocations will decrease the efficiency of nuclear reactors or lead to melting failure under transient heat flow. However, the phonon–dislocation scattering rate in iron is unknown, and the details of the scattering process have not been well studied. In this paper, the effect of dislocations on lattice thermal conductivity in pure iron is studied using molecular dynamics simulations. The temperature distribution in the neighborhood of the dislocation, the spectral heat flux, and the frequency-dependent phonon mean free paths are obtained. From a comparison with the results for a perfect crystal, we find that the dislocation can significantly decrease the lattice thermal conductivity. By using an average phonon group velocity, the phonon–dislocation scattering rate under a given dislocation density is obtained from the phonon mean free paths. Moreover, eigenmode analysis of a dislocation dipole model indicates that the phonons have a certain degree of localization, which reduces their ability to transport heat. Our study reveals the details of the phonon–dislocation scattering process and may help to interpret the reduced thermal conductivity caused by the dislocations that are generated during the service lives of iron-based structural materials.

Tuning the electronic, mechanical, thermal, and optical properties of tetrahexcarbon via hydrogenation

Recently, Tetrahexcarbon (TH-carbon), a new two-dimensional(2D) carbon allotrope, has been identified with an intrinsic direct bandgap, which makes it promising for practical applications in optoelectronic devices. Using first-principles calculations, we examined the possibility of manipulating the physical and chemical properties of TH-carbon sheet by controlled hydrogenation. We systematically studied pristine TH-carbon and its hydrogenated derivatives with various configurations such as single- and double-sided hydrogenation. Our study revealed their stability in energetic, dynamic, thermal, and mechanical aspects. Depending on the hydrogen coverage and configurations, we observed the tunability of the phononic and electronic bandgap, and the direct-indirect-direct bandgap transitions. These results suggest the plausibility of modulating its electronic properties by hydrogenation. The heat transport in TH-carbon is anisotropic. A significant decrease in thermal conductivity was observed in the fully hydrogenated TH-carbon. The thermal conductivity in TH-carbon can be controlled by the sp3 C–H low conduction domains. A notable increase in specific heat capacity was observed in hydrogenated derivatives of TH-carbon, which would make them useful in nanoscale engineering of thermal transport. The hydrogenation was found to reduce the in-plane stiffness and Young’s modulus, but increase the ultimate strength. These findings would provide important guidelines for practical applications of TH-carbon.

Use of expanded polystyrene in developing solid brick masonry units

This paper presents the results of an experimental program aimed at using expanded polystyrene (EPS) wastes in developing lightweight mortar and masonry solid brick units of adequate mechanical and thermal characteristics for use in building and construction applications. Five dosages of EPS were used to partially substitute the sand in the mix, namely; 0, 10, 15, 20 and 26 kg/m3. The results showed that the density, compressive strength, tensile strength, static modulus of elasticity, and coefficient of thermal conductivity decrease with the increase of the EPS dosage in the mix. The results showed also that the proposed EPS solid bricks possess adequate strength for non-loadbearing applications have superior thermal properties.

Enhanced Thermoelectric Performance of Bi2Te2.7Se0.3/Bi2S3 Synthesized by Anion Exchange Method

Decoupling the electric conductivity, Seebeck coefficient, and thermal conductivity is a core issue to improve the properties of thermoelectric materials by tuning the electron and phonon transport properties. Herein, Bi2Te2.7Se0.3/Bi2S3 heterostructure is successfully synthesized by a simple anion exchange method with Bi2Te2.7Se0.3 as the starting material. The Bi2Te2.7Se0.3/Bi2S3 heterointerfaces can achieve an effective phonon scattering and significant decrease in thermal conductivity, while maintaining a high power factor, resulting in an enhanced figure of merit (ZT). By controlling the concentration of Bi2Te2.7Se0.3/Bi2S3 heterointerfaces, the carrier‐transporting and phonon‐scattering behaviors can be simultaneously optimized. A maximum ZT of 0.7 is obtained at 473 K, which is 23% higher than that of pure Bi2Te2.7Se0.3. In addition, the average ZT at 300–548 K increases from 0.49 to 0.61, which is 25% higher than that of pure Bi2Te2.7Se0.3.

Incorporation of Wooden Furniture Wastes in Fired Clay Bricks for Improved Thermal Insulation: A Feasability Study

The main objective of this study is to improve thermal insulation of fired clay bricks through addition of wooden furniture waste. The effect of the nature and the amount (5 or 10 wt%) of wooden furniture waste on the properties of the final porous fired brick prepared by extrusion was assessed though measurements of mass loss, drying and sintering shrinkage, porosity, bulk density and thermal conductivity. Results showed that incorporation of wooden furniture waste into the clay mixture yielded increase of porosity after firing and hence decrease of thermal conductivity. For example, addition of 10 wt% wood waste yielded a pore volume fraction of 47% with a conductivity of 0.42 W m−1 K−1. Moreover, due to orientation of the clay platelets, the anisotropy in thermal conductivity was significant, ranging from a ratio of 1.1–1.5.

Use of a smartphone thermometer to monitor thermal conductivity changes in diabetic foot ulcers: a pilot study.

To investigate the dependability of a thermal gadget connecting to a smartphone. Comparing the exact temperature of damaged tissue with adjacent parts of the limb and to evaluate the changes in thermal conductivity of hard-to-heal wounds in patients with a diabetic foot ulcer (DFU). Potential candidates were divided into three groups and selected from different hospitals in Tehran. Group 1 contained patients with inflammation, Group 2 were patients with vascular complications, and Group 3 were patients who were managing to control their glucose levels to an acceptable level, according to their medical records (i.e. 'healthy cases'). All thermal images were taken without any external stimulus, in room temperature conditions after 15 minutes' rest. All medical records were confirmed by therapeutic supervisors. Moreover, the tissue conditions in patients were considered in the computational part of the study. The temperatures of the ulcer and adjacent tissues were observed and compared. The collected data were used in a suggested model for human tissues and the method of calculation in this study was trial and error. In this study, patients in Group 2 were considered in the computational section of the study. Temperature difference between the wounds and adjacent tissues for the big toe in three patients in Group 2 was 2.2ºC for the healthier candidate and almost 6.9ºC in the worst case. By comparing the thermal conductivity of normal and damaged tissues, a significant reduction in thermal conductivity was observed for the candidate with the worst status of big toe by about 84.3%. For the other two candidates in this category, it was almost 68.86% and 20.47%. The variation in thermal conductivity represents the change in tissue properties. Thermal conductivity can be applied for early DFU detection. This data may allow introduction of the smartphone thermometer as an authentic and alternative apparatus that is beneficial in diabetic clinics as well as self-assessment by patients. Moreover, due to the decrease in thermal conductivity, this study suggests using intelligent thermal sheets in vulnerable parts of the diabetic foot.

Рассеяние фононов на парамагнитных ионах европия и самария в феррите висмута

The temperature dependence of the thermal diffusivity and thermal conductivity of the multiferroics BiFeO3, Bi0.90Sm0.10FeO3 and Bi0.90Еu0.10FeO3 is studied. It was found that the substitution of bismuth ions by the rare-earth europium and samarium ions in bismuth ferrite leads to a decrease in phonon thermal conductivity in a wide temperature range. It was established that the decrease in the thermal conductivity of bismuth ferrite is due to resonance scattering of phonons at paramagnetic levels of europium and samarium ions.

Neutron irradiation sensitivity of thermal conductivity for Al2O3 nanofluids

In this work, the thermal conductivity of Al2O3 nanofluids has been investigated for the sensitivity towards neutron irradiation. The solution combustion method has been used for the synthesis of Al2O3 nanoparticles that have been used for the preparation of the nanofluids. Prepared nanofluids have been neutron-irradiated for 7 and 14 days. Dynamic Light Scattering, Scanning Electron Microscopy, and Ultraviolet-Visible Spectroscopy have been used to ascertain the change in properties before and after neutron-irradiation. Thermal conductivity has been measured for un-irradiated and neutron-irradiated nanofluids at 30 °C using a KD2 pro thermal properties analyzer. The decrease in thermal conductivity has been observed after neutron-irradiation that further decreases with increased duration of exposure and concentration of nanoparticles. 5 and 10% decrease in thermal conductivity has been recorded after 7 and 14 days of neutron irradiation for change in concentration from 0 to 2 volume percent. Neutron-irradiation sensitivity analysis revealed that heat transfer characteristics are sensitive at higher concentrations and during initial exposure of neutron-irradiations.

Application of experimental setup for the thermal conductivity measurement for searching novel environmental material solutions used in construction

The presented work includes experimental data on the infl uence of lightweight filler on the thermal conductivity of λ gypsum. In this research, cenospheres were used. Cenospheres were added in the amount of 10% in respect to the mass of gypsum. To measure thermal conductivity, a non-stationary method was used based on the “hot wire method”. A decrease of thermal conductivity and density with added lightweight filler was observed because of microstructure modifications of the gypsum product. Additional microscopic images of the fractures of the modifi ed gypsum were made.

Analysis of heat and mass transfer mechanism during thermal energy storage and temperature regulation

To strengthen the heat and mass transfer capacity and improve the temperature regulation rate, potential storage is taken as the research object in this research to study the heat energy storage of the battery in the low temperature environment. Lattice Boltzmann method is adopted to study the heat energy storage influence mechanism of the temperature regulation system of the low temperature phase-change materials. In addition, the influence of different physical parameters (thermal conductivity and latent heat of phase change) on the thermal insulation of the system in the process of temperature control is revealed. The results show that the mechanism of heat and mass transfer in the process of heat storage and temperature control is related to the different physical properties of phase change materials. The decrease of thermal conductivity and the increase of latent heat of phase change materials will greatly increase the effect of heat energy storage. Therefore, under the action of phase change latent heat, phase change material can effectively extend the holding time of the battery in the low temperature environment.

Морфология и термоэлектрические свойства керамики твердых растворов γ-Gd-=SUB=-x-=/SUB=-Dy-=SUB=-1-x-=/SUB=-S-=SUB=-1.5-y-=/SUB=-

The temperature dependences of Seebeck coefficient, electrical conductivity (Т=300–873K), thermal conductivity and figure of merit (Т=300–770K) of polycrystalline samples of solid solutions based on gadolinium and dysprosium sulfides with γ-GdxDy1-xS1.49 (x=0.1, 0.2, 0.3, 0.4) composition were studied. It was established that samples morphological features, namely, the specific surface area of crystallites causing a change in the number of deformation centers determines the thermal conductivity of -GdxDy1-xS1.49, and it was found that there is an anomalous decrease in thermal conductivity for composition with х = 0.2. For this composition the lowest values of Seebeck coefficient -371 mkV/K at 873K, electrical resistivity 880 mkΩ∙m at 873K and thermal conductivity 0.68 ± 0.03 W/m·K at 770K were obtained, in this case the thermoelectric figure of merit reaches ZT = 0.23.

Modeling of shallow, horizontal, unsaturated, ground-based heat exchangers with consideration of dry zone formation

High fluid temperatures in ground-based heat exchangers (GHXs) during the cooling season may result in a decrease in thermal conductivity of adjacent backfill (λbackfill), potentially causing degradation in the performance efficiency of the GHX system. In this study, numerical modeling was performed using the SVOffice™ finite-element software to evaluate coupled heat and moisture transfer around two GHXs. Constant-temperature boundary conditions of 35 ºC were applied to the GHX surfaces, and thermal properties of the backfill were controlled for comparisons. For estimate typical hydraulic and thermal properties for the modeling, laboratory tests and literature review were performed. Modeling results indicate that coupled heat and moisture transfer occurs rapidly near the GHX involving a dry zone formation when λbackfill decreases. A boundary between dry and wet zones where soil thermal properties rapidly change was observed around 50% GHX temperature dissipated (T50), and accordingly T50 was used to optimize the pipe configuration. Coupled heat and moisture transfer increased when the GHX configurations were optimized with consideration of dry zone formation. These results imply that thermally enhanced, engineered backfill and optimized configurations can enhance GHX system efficiency.

Thermal conductivity of lightweight foamed concrete filled fly ash

Wall is the wider component of high-rise building to exposed the sunshine than roof. The building wall in tropical countries receives the solar heat all day and it makes temperature rise of its room and becomes less comfortable. Therefore, a wall with good thermal insulation is needed for buildings in tropical countries. Lightweight foamed concrete (LFC) can be used as a wall brick substitution as well as thermal insulator. The experimental research about LFC was investigated using FA and foam content to the thermal conductivity of LFC. The investigations were performed through the measurement of thermal conductivity, material density, and observation to the morphology of LFC surface. The LFC samples were varied in volume fraction of foam and FA in mortar. The result shows that the increase in volume fraction of foam in all variations of FA content decreases its thermal conductivity. In contrast, the increase of FA content in LFC results in an increase in thermal conductivity until 45 wt% of FA content, then it decreases as the addition of FA. Thus, the optimum thermal conductivity of the LFC was obtained by FA of 45 wt%. The relationship between thermal conductivity and LFC density shows that any increase in LFC density resulted in an increase in thermal conductivity. Otherwise, any decrease in LFC density resulted in a decrease in thermal conductivity.

Heat transport behavior of bicrystal ZnO containing tilt grain boundary

Zinc oxide (ZnO), as a conventional semiconductor material, has excellent characteristics, such as piezoelectricity, photoelectricity, gas sensitivity, etc. With the improvement of nanopreparation technology, different types of nanostructrued ZnO compounds have appeared and their heat conductions have become a main research topic in nanodevices. In order to study the effects of grain boundary on the thermal properties of materials of this kind, bicrystal ZnO containing small-angle and high-angle grain boundaries are constructed by the embedded dislocation line and coincidence site lattice method. The variation of grain boundary energy with tilt angle is studied by the non-equilibrium molecular dynamics simulation. In addition, the dislocation density is calculated by using the Frank-Bilby formula. Our results show that the grain boundary energy and dislocation density increase with the increase of tilt angle in a small-angle region, and they tend to be stable in a high-angle region. The tilt angle of 36.86° is defined as the transition angle. The trend of the Kapitza resistance is the same as that of the grain boundary energy and satisfies the theoretical value from the extended Read-Shockley model. Furthermore, it is found that both the Kapitza resistance and thermal conductivity have a significant size effect. When the sample length is between 23.2 nm and 92.6 nm, the Kapitza resistance decreases sharply with the increase of the length and then tends to be stable. The thermal conductivity of the sample increases with length increasing, but is always less than that of the single crystal. At the same time, temperature is an important factor affecting the heat transport properties. The Kapitza resistance and thermal conductivity decrease with temperature increasing. At different temperatures, the Kapitza resistance of 38.94° grain boundary sample is greater than that of 5.45° grain boundary sample. In order to further explore the influence mechanism of grain boundary angle on heat conduction, the phonon state density of 5.45° and 38.94° grain boundary sample are calculated. The results indicate that the high-angle grain boundary has stronger scattering for acoustic branch phonons and the peak frequency becomes lower, whereas the optical branch ones have almost no effect on the heat conduction.