Decrease In Thermal Conductivity Research Articles

Thermal conductivity of graphene/graphane/graphene heterostructure nanoribbons: Non-equilibrium molecular dynamics simulations

We investigate thermal conductivity of graphene/graphane/graphene heterostructure nanoribbons, which are constructed by substituting graphane nanoribbons into middle part of the armchair graphene nanoribbons, using non-equilibrium molecular dynamics. Effects of hydrogen atoms in middle graphane nanoribbons, H-termination on edge, width, and H coverage on thermal conductivity of the heterostructure nanoribbons were clarified. The thermal conductivity of the heterostructure nanoribbons are at least one order less than that of graphene nanoribbons. Reduction of thermal conductivity decreases, as width of the heterostructure nanoribbons and adsorption width increase. Reduction of thermal conductivity in graphene by hydrogen-functionalization can be used to control the thermal transportation properties. The graphene/graphane/graphene heterostructure nanoribbons may be candidate for high merit of figure thermoelectric conversion material with low thermal conductivity and high electric conductivity.

Mineral and textural transformations in mixtures of Al-rich and Al–K-rich clays with firing: Refractory potential of the fired products

The present work analyses a set of cylinders manufactured by pressing and fired from 1000 °C to 1270 °C, which are composed of pure Al-rich rocks (bauxite) and mixtures of bauxite with Al–K-rich clays. The aim of the study is to determine the mineral and textural transformations that take place in the mixtures with firing and their influence on the physical properties of the final products in order to evaluate their refractory potential by comparing them with various fired commercial bauxites. To this end, raw and fired samples were analysed by X-ray diffraction, X-ray fluorescence, and optical and field emission scanning electron microscopy, and significant physical properties (e.g. density, linear shrinkage, porosity, colour, water absorption, thermal conductivity and point load resistance) were determined in the fired cylinders. The raw bauxite was mainly formed by boehmite, kaolinite and hematite whereas the illite- and kaolinite-rich clay was formed by quartz, illite and kaolinite. Kaolinite, illite and boehmite are not detected at 1000 °C and, from this temperature upwards, mullite, corundum, γ-Al2O3, hercynite, ilmenite, cristobalite and vitreous phase are formed. Changes in physical properties with rising temperature are associated with the mineralogical changes. The increases in density and linear shrinkage are related to the formation of vitreous phase and the crystallization of mullite, and correlate with the decrease in porosity, water absorption and thermal conductivity. The changes in the colour of the samples are related to the hematite content at each temperature, whereas the point load resistance is greater with higher corundum content. Both the firing temperature and the clay content play an important role in the refractory potential of the mixtures, since samples mixed with illite- and kaolinite-rich clay present similar properties at lower temperatures to those of the fired commercial bauxites.

Thermoelectric Properties of PbSe Nanocomposites from Solution-Processed Building Blocks

High thermoelectric performance and low material cost are essential to the commercialization of thermoelectric modules. Microstructural design, size effects, and rational band engineering are among the keys to high thermoelectric performance, especially for nanocomposites. Here, we demonstrate a facile and versatile bottom-up approach to produce nanocomposites by heat consolidating solution-processed PbSe nanoparticles with a proper amount of Ag. In this PbSe-Ag system, a certain amount of Ag diffuses into the PbSe matrix and partially substituted Pb while other forms Ag2Se at the interphases. This increased the electrical conductivity and inversed the n-type behavior of PbSe to p-type, which is observed from the positive Seebeck coefficient for PbSe-Ag nanocomposites over the temperature range. Nonetheless, the decrease of total lattice thermal conductivity at high temperature is ascribed to the effective phonon at grain boundaries, Ag2Se nanodomain, and point defect. Overall, we obtained an optimized figure of merit, ZT = 0.97 at 723 K for 5 wt % Ag, a significant improvement of 8.8 times than the pure PbSe. Thus, this partially substituting host atom and the presence of a secondary phase provide another potential strategy to enhance the thermoelectric performance of PbSe, promoting its commercialization.

Enhanced thermoelectric performance of Mo nanoparticle decorated n-type Bi2Te2.7Se0.3 powder composites

Although thermoelectric energy conversion technology has received much attention because of its noiseless and economical operations, a low figure of merit (ZT) values of thermoelectric materials has prevented a wider variety of practical applications. Recently thermoelectric composite structures with metal or oxide nanoparticles have been widely investigated to enhance the ZT values by increasing the power factor and decreasing the thermal conductivity simultaneously. However, the difficulties in distributing the nanoparticles in the thermoelectric material matrix homogeneously have been the main problem. Herein we fabricated a thermoelectric nanocomposite structure by decorating the Mo nanoparticles on the surface of Bi2Te2.7Se0.3 powders. The Mo nanoparticles were uniformly coated on the Bi2Te2.7Se0.3 powder surface based on the sacrificial organic supporter composite consisting of Mo-coated glucose. The Mo nanoparticle/Bi2Te2.7Se0.3 composites exhibited a simultaneous decrease of thermal conductivity and electrical resistivity due to an Anderson localization effect. Therefore, the Mo nanoparticle composite structure showed enhanced ZT values by 1.31 times compared to that of bare Bi2Te2.7Se0.3 powders exhibiting a ZT value of 0.67 at 373 K.

Use of Textile Waste as an Addition in the elaboration of an Ecological Concrete Block

The textile industry has grown significantly in recent years, reaching a global fiber production of 53 million tons which 12 % are recycled; Construction sector has been using more and more recycled materials from different industrial sources, to apply them in their constructions and to reduce CO2 emissions and final energy consumption. The present study aims to study the behavior of concrete blocks of f´c = 210 kg/cm2 adding polyester textile waste with 3 %, 6 %, 9 %, 12 % and 15 %; void content, compressive strength and thermal conductivity decrease, and water absorption, acoustic insulation and unit price increase by 3 %, 34 % and 16 % compared to conventional concrete block.

Thermal and Transport Properties of Molten Chloride Salts with Polarization Effect on Microstructure

Molten chloride salt is recognized as a promising heat transfer and storage medium in concentrating solar power in recent years, but there is a serious lack for thermal property data of molten chloride salts. In this work, local structures and thermal properties for molten chloride salt—including NaCl, MgCl2, and ZnCl2—were precisely simulated by Born–Mayer–Huggins (BMH) potential in a rigid ion model (RIM) and a polarizable ion model (PIM). Compared with experimental data, distances between cations, densities, and heat capacities of molten chloride slats calculated from PIM agree remarkably better than those from RIM. The polarization effect brings an extra contribution to screen large repulsive Coulombic interaction of cation–cation, and then it makes shorter distance between cations, larger density and lower heat capacity. For NaCl, MgCl2, and ZnCl2, PIM simulation deviations of distances between cations are respectively 3.8%, 3.7%, and 0.3%. The deviations of density and heat capacity for NaCl between PIM simulation and experiments are only 0.6% and 2.2%, and those for MgCl2 and ZnCl2 are 0.7–10.7%. As the temperature rises, the distance between cations increases and the structure turns into loose state, so the density and thermal conductivity decrease, while the ionic self-diffusion coefficient increases, which also agree well with the experimental results.

Recycling waste concretes as fine aggregate and fly ash as binder in production of thermal insulating foam concretes

This paper presents the first investigation on foam concretes with fly ash and fine recycled concrete aggregate (RFA). Foam concretes were produced considering 0, 25, 50, 75 and 100% RFA ratio instead of virgin sand. Tests were conducted to assess their porosity, density, compressive strength, dynamic elastic modulus, absorption, ultrasound and thermal characteristics. Scanning electron microscopy analyzes were performed for investigating the microstructure of concretes. Findings show that incorporating RFA results in slight rise in porosity and absorption while decrease in dynamic elastic modulus, ultrasound and thermal conductivity. Observations present an expectancy toward production of light-weight construction material contributing to significant environmental benefits.

Understanding the role of defects in influencing the thermoelectric properties of SnSe

As compared to single crystals, polycrystalline SnSe shows a considerable decline in its ZT value. Optimization of carrier concentration by the way of chemical doping is useful but creates point defect and vacancies that are often overlooked. Here we study polycrystalline Sn0.95M0.05Se (M = Co, Ni, In) with an aim to understand the role of defects. The overall crystal structure and microstructure of SnSe is not much affected with substitution as evident from X-ray diffraction and scanning electron microscopy study. Rietveld refinement confirms the single phase nature of the all compositions and provides unit cell parameters. Analysis of the stoichiometry reveals the presence of cation vacancies. Optical spectroscopy indicates a degradation of the in-direct gap and Urbach band tail-width fitting confirms the presence of localized states within the gap. Electrical resistivity and Seebeck coefficient are adversely affected by defects, but thermal conductivity decreases by almost 50% of SnSe value.

Uncovering design principles for amorphous-like heat conduction using two-channel lattice dynamics

The physics of heat conduction puts practical limits on many technological fields such as energy production, storage, and conversion. It is now widely appreciated that the phonon-gas model does not describe the full vibrational spectrum in amorphous materials, since this picture likely breaks down at higher frequencies. A two-channel heat conduction model, which uses harmonic vibrational states and lattice dynamics as a basis, has recently been shown to capture both crystal-like (phonon-gas channel) and amorphous-like (diffuson channel) heat conduction. While materials design principles for the phonon-gas channel are well established, similar understanding and control of the diffuson channel is lacking. In this work, in order to uncover design principles for the diffuson channel, we study structurally-complex crystalline Yb14 (Mn,Mg)Sb11, a champion thermoelectric material above 800 K, experimentally using inelastic neutron scattering and computationally using the two-channel lattice dynamical approach. Our results show that the diffuson channel indeed dominates in Yb14MnSb11 above 300 K. More importantly, we demonstrate a method for the rational design of amorphous-like heat conduction by considering the energetic proximity phonon modes and modifying them through chemical means. We show that increasing (decreasing) the mass on the Sb-site decreases (increases) the energy of these modes such that there is greater (smaller) overlap with Yb-dominated modes resulting in a higher (lower) thermal conductivity. This design strategy is exactly opposite of what is expected when the phonon-gas channel and/or common analytical models for the diffuson channel are considered, since in both cases an increase in atomic mass commonly leads to a decrease in thermal conductivity. This work demonstrates how two-channel lattice dynamics can not only quantitatively predict the relative importance of the phonon-gas and diffuson channels, but also lead to rational design strategies in materials where the diffuson channel is important.

Performance of mortars with PET

The massive amount of plastic production inevitably leads to huge levels of pollution and, consequently, is threatening the environment and people’s safety. To reduce plastic pollution, research on new ways of reutilization of plastic waste is an open issue nowadays. The main purpose of this work is to evaluate the feasibility of the incorporation of ground polyethylene terephthalate (PET) waste in mortars by comparing the values obtained of mechanical resistance (after 28 days), capillarity water absorption coefficients and thermal conductivity coefficients in an experimental program with the standard values. Mortars prototypes produced at ratios of 1:4 and 1:3 in weight, with partial replacement of the aggregate by PET residuals in different percentages (0, 5 and 10%) were considered. The obtained results show a decrease in mechanical resistance of mortars which was inversely proportional to the use of PET residues in the mixture. An increase in resistance of water absorption by capillary action and a decrease in thermal conductivity was also identified. These results indicate that the replacement of natural aggregates by PET residues in mortars is an interesting option to be considered for the production of a new and environmentally friendly material.

Segregation of NiTe2 and NbTe2 in p-Type Thermoelectric Bi0.5Sb1.5Te3 Alloys for Carrier Energy Filtering Effect by Melt Spinning

The formation of secondary phases of NiTe2 and NbTe2 in p-type Bi0.5Sb1.5Te3 thermoelectric alloys was investigated through in situ phase separation by using the melt spinning process. Adding stoichiometric Ni, Nb, and Te in a solid-state synthesis process of Bi0.5Sb1.5Te3, followed by rapid solidification by melt spinning, successfully segregated NiTe2 and NbTe2 in the Bi0.5Sb1.5Te3 matrix. Since heterointerfaces of Bi0.5Sb1.5Te3 with NiTe2 and NbTe2 form potential barriers of 0.26 and 0.08 eV, respectively, a low energy carrier filtering effect can be expected; higher Seebeck coefficients and power factors were achieved for Bi0.5Sb1.5Te3(NiTe2)0.01 (250 μV/K and 3.15 mW/mK2), compared to those of Bi0.5Sb1.5Te3 (240 μV/K and 2.69 mW/mK2). However, there was no power factor increase for NbTe2 segregated samples. The decrease in thermal conductivity was seen due to the possible additional phonon scattering by the phase segregations. Consequently, zT at room temperature was enhanced to 0.98 and 0.94 for Bi0.5Sb1.5Te3(NiTe2)0.01 and Bi0.5Sb1.5Te3(NbTe2)0.01, respectively, compared to 0.79 for Bi0.5Sb1.5Te3. The carrier filtering effect induced by NiTe2 segregations with an interface potential barrier of 0.26 eV effectively increased the Seebeck coefficient and power factor, thus improving the zT of p-type Bi0.5Sb1.5Te3, while the interface potential barrier of 0.08 eV of NbTe2 segregation appeared to be too small to induce an effective carrier filtering effect.

Innovation and characterization of a new multifunctional light weight low cost construction material from foamed rubber blend

New amendment in vulcanization process was used to prepare foamed rubber with enhanced physical properties other than expected and desirable thermal insulation properties. In this work the influence of foaming agent loaded NR/ SBR on thermal conductivity, density, water absorption swelling and mechanical properties was investigated. Different concentrations of foaming agent (2, 4, 6, 8, and 10 phr) were added to rubber matrix. The morphology of selected samples was characterized using scan electron microscope (SEM). An experimental investigation was carried out to obtain low cost multifunctional construction material with desirable mechanical and thermal insulation properties. The addition of foaming agent led to a decrease in thermal conductivity and density values until the concentration of 8phr after that their values slightly changed. It was also observed that mechanical properties of blends were improved with the addition of foaming agent. The optimum concentration was found to be 8phr for all measurements

Effects of Nucleating Agent on the Thermal Conductivity and Creep Strain Behavior of Rigid Polyurethane Foams Blown by an Environment-Friendly Foaming Agent

The effects of liquid-type additive, methoxynonafluorobutane, on the morphology, thermal conductivity, creep strain, and mechanical strength of polyurethane (PU) foams with an environment-friendly foaming agent, hyrdofluoroolefin, were investigated. The methoxynonafluorobutane under the trade name of NOVEC likely acted as a nucleating agent during the formation of PU foams. When NOVEC was added in the amount of 3 parts per hundred polyol by weight (php), the cell size of the foam was minimal, and the cell size distribution was relatively uniform. At the NOVEC content of 3.0 php, the thermal conductivity of the PU foams was also minimal. This decrease in thermal conductivity was due to the smaller cell size of the foams lowering their thermal conductivity. At 3.0 php NOVEC content, the creep strain of the PU foams was minimal being 0.29% at 1,000 h. At 3.0 php NOVEC, the estimated creep strain of the PU foam exhibited the lowest creep strain of 3.47% at 50 years. As a result, at 3.0 php NOVEC content, the cell wall was relatively less fractured, resulting in a small deformation of the PU foam. These results suggest that the main factors in improving the thermal insulation properties and stability of the PU foams are smaller cell size and narrow cell size distribution.

Utilization of ground waste glass cullet in the industrial production of precast concrete elements

In the study, a field trial was carried out to evaluate the applicability of ground waste glass cullet as a binder component, in the industrial production of precast concrete elements. The influence of glass cullet addition on the properties of fresh and hardened concretes was compared to siliceous fly ash. Moreover, the effects of a partial substitution of the natural aggregate with recycled concrete and expanded clay aggregate were determined. The compressive strength of concretes, in which 25% of cement was substituted with ground waste glass cullet, was even 20% higher compared to elements with an analogous addition of siliceous fly ash. Over 56 days, no drops in strength related to the harmful effects of the alkali-silica reaction were observed. Replacing half of the natural aggregate with graded recycled concrete aggregate, allowed to lower the thermal conductivity of the walls by approx. 10%, with no negative impact on the compressive strength of precast elements. However, a decrease in the consistency of fresh mix was observed after 30 minutes. Replacing the 2÷8 mm gravel with the same fraction of expanded clay resulted in an approx. 18% decrease in thermal conductivity, while maintaining satisfactory mechanical properties. It was concluded that ground waste glass cullet can be successfully used as a fly ash replacement in the production of precast concrete elements.

Fabrication and thermoelectric properties of Si micro/nanobelts

<sec>Currently, low-dimensional materials are a hot spot in the field of thermoelectric research, because the thermoelectric properties will be significantly improved after the low-dimensionalization of bulk materials. In a bulk material, its thermoelectric figure of merit <i>ZT</i> value cannot be increased by changing a single parameter, because the parameters of the material are interrelated to each other, which is not conducive to the research of internal factors and thus limiting the efficiency of thermoelectric material, but thermoelectric material on a micro-nano scale is more flexible to adjust its thermoelectric figure of merit <i>ZT</i> value. There are many different kinds of methods of implementing the low-dimensionalization of bulk materials. In this paper, size-controllable Si micro/nanobelts are prepared based on semiconductor micromachining and focused ion beam (FIB) technology, and the thermoelectric properties of Si micro/nanobelts of different sizes are comprehensively studied by the micro-suspension structure method.</sec><sec>In this experiment, we find that the conductivity of doped Si micro/nanobelt is significantly better than that of bulk Si material, that as the width of the Si micro/nanobelt decreases, the thermal conductivity of the material decreases significantly, from 148 W/(m·K) of bulk silicon to 17.75 W/(m·K) of 800 nm wide Si micro-nanobelt, that the Seebeck coefficient of the material is lower than that of the corresponding bulkmaterials. The decrease of thermal conductivity is mainly due to the boundary effect caused by the size reduction, which leads the phonon boundary scattering to increase, and thus significantly inhibiting the behavior of phonon transmission in the Si material, thereby further affecting the transmission and conversion of thermal energy in the material. At 373 K, the maximum <i>ZT</i> value of the 800 nm wide Si micro/nanobelt reaches ～0.056, which is about 6 times larger than that of bulk silicon. And as the width of the Si micronanobelt is further reduced, the thermoelectric figure of merit <i>ZT</i> value will be further improved, making Si material an effective thermoelectric material. The FIB processing technology provides a new preparation scheme for improving the thermoelectric performances of Si materials in the future, and this manufacturing technology can also be applied to the low-dimensional preparation of other materials.</sec>

Influence of Sisal fiber’s treatment on the kinetics of hydration, morphological and thermophysical properties of the composite cementitious mortar

Currently, cement-based bio-composite is a relevant concept for researchers in the building. However, these researches highlighted some handicaps. Plant fibers are acting as a retarder in the setting time of the cement. In this study, Sisal fiber (SF) (4% by mass of cement) was subjected to different treatments to improve bio-composites hydration kinetics (KH) “tested by isotherm calorimetry”. The treatment slowed down both alkaline hydrolysis and mineralization of fiber cell walls by promoting the hydration of cement. This result was coherent with morphological properties. In fact, the images obtained by scanning electron microscopy (SEM) showed a tinier calcium layer around the (SF) treated with NaOH and Paraffin oil on the adhesion surface. The Fourier transform infrared spectroscopy (FTIR) test revealed a disparity in the peaks of the absorption strips of CaCO3 and Ca(OH)2 and thus cement hydration. In addition, the tests results showed a decrease in thermal conductivity (λ) and volumetric heat capacity (ρ.CV) after treatment of (SF). Resistance (RTh) and thermal diffusivity (α) slightly increased with treated fiber. Considering that, the bio-mortar with treated Sisal fiber can be promising material from an insulation point of view.

Preparation and properties of recycled fine powder foam concrete

With the rapid development of the construction industry brought by the accelerated process of urbanization, the problem of random stacking and landfill of waste concrete can be seen everywhere, which seriously affects the environmental health. The secondary utilization of waste concrete as recycled aggregate has become a research hotspot. This paper mainly studies the influence of the cementing material system of recycled fine powder foam concrete on the water absorption performance of recycled fine powder foam concrete, and explores the feasibility of using recycled fine powder to prepare foam concrete. The experiment also used recycled fine powder instead of some cement to prepare foam concrete. The effects of the amount of recycled fine powder, the amount of hydrogen peroxide and the ratio of water to binder on the compressive strength, water absorption, thermal conductivity and microstructure of the foam concrete were studied. The results showed that the water absorption rate of recycled fine powder foam concrete increased with the increase of red brick powder content, but the water absorption rate gradually increased. When the content of hydrogen peroxide is 6%, the ratio of water to binder has a greater influence on the thermal conductivity of foam concrete. When the amount of hydrogen peroxide is higher, the influence of water binder ratio on thermal conductivity decreases.

Влияние мультиплетных уровней Sm-=SUP=-3+-=/SUP=- и Eu-=SUP=-3+-=/SUP=- на теплоемкость мультиферроика BiFeO-=SUB=-3-=/SUB=-

The temperature dependence of the heat capacity of the multiferroics BiFeO3, Bi0.90Sm0.10FeO3, and Bi0.90Eu0.10FeO3 has been studied. It was found that the substitution of europium and samarium ions for bismuth ions in bismuth ferrite leads to the appearance of an additional heat capacity component due to transitions of 4f - electrons of rare earth ions to higher levels of the multiplet. A connection is established between the decrease in phonon thermal conductivity and the Schottky effect for the specific heat.

First principles study of structure and property of Nb<sup>5+</sup>-doped SrTiO<sub>3</sub>

The modification of SrTiO<sub>3</sub> materials by doping Nb<sup>5+</sup> ions in B-site is studied through using the first-principles method to calculate the electronic structure, optical properties, mechanical properties and thermal properties at different Nb<sup>5+</sup> doping concentrations. The calculation results show that as the doping content of Nb<sup>5+</sup> increases, the lattice parameters increase. After being doped with Nb<sup>5+</sup>, SrTiO<sub>3</sub> changes from an indirect band gap compound into a direct band gap compound. Doping Nb<sup>5+</sup> can reduce the reflection coefficient, absorption coefficient, and energy loss of SrTiO<sub>3 </sub>material, which can be used to modify its optical properties. Additionally, the brittleness of SrTiO<sub>3 </sub>material is improved through doping Nb<sup>5+</sup>. As the doping content of Nb<sup>5+ </sup>increases, the elastic modulus of the material hardly changes, the shear modulus and Young's modulus decrease, the Poisson's ratio increases, and the Debye temperature decreases, and both the lattice thermal conductivity and the theoretical minimum lattice thermal conductivity decrease as well.

Thermoelectric properties of Ni/Ge-multilayer-laminated silicon

We investigate the thermoelectric properties of Ni/Ge-multilayer-laminated silicon thermoelectric devices based on the number of Ni/Ge layers from single layer to 50 layers. For the Ni/Ge-multilayer lamination, Ni and Ge thin layers are alternately deposited on a bulk silicon substrate using RF sputtering at room temperature. The Seebeck coefficient improves and the thermal conductivity decreases compared with the bulk silicon thermoelectric device as the number of Ni/Ge layers increases. A seven Ni/Ge-multilayer-laminated thermoelectric device indicates a Seebeck coefficient of – 260 μV/K and a thermal conductivity of 56 W/m∙K at 510 K without electrical conductivity deterioration; subsequently, a thermoelectric power factor of 5.6 mW/m∙K2 and zT of 0.05 are achieved at 510 K. The improvement in the Seebeck coefficient in the multilayer devices is attributed to the electron filtering effect due to the Schottky barriers at the Ni/Ge interfaces. It is speculated that the acoustic phonon impedance mismatch at the Si(Al)/Ni interface reduces/saturates the thermal conductivity. The thermoelectric results indicate the potential of using a Ni/Ge-multilayer structure for silicon-based thermoelectric devices.