Significant Reduction In Thermal Conductivity Research Articles

3DP for sustainable built environment – Synthesis and thermomechanical characterization of composite materials based on local soil and date palm fiber waste

There is an increasing demand for sustainable construction materials to address the challenges posed by the environmental impact of the built environment (BE), which is driven by climate change, population growth, and urbanization. Conventional construction materials like cement contribute significantly to carbon emissions during production, transport, use, and disposal phases, and their poor thermal conductivity hinders efforts to maintain comfortable indoor environments. To address these challenges, there is an urgent need for innovative, locally sourced thermal insulation materials specifically designed for regions with extreme climates and high energy demands to maintain building comfort. This research explores synthesizing novel, environmentally friendly building materials using locally sourced date palm fiber waste and clay. The study also investigates the developed material's 3D printing (3DP) capabilities and optimizes its printing parameters with lab-scale prototypes. Additionally, thermo-mechanical characterization is conducted to assess its suitability for built environment applications. Different concentrations, from 1 to 5 wt% of date palm fiber to clay ratio (Dpf/C), were studied regarding microstructural, thermal, and mechanical properties, dimensional accuracy, and feasibility for 3DP of BE structures. Results demonstrated a significant reduction in thermal conductivity by 73%, achieving 0.244 W/mK, and an increase in compressive strength by 106%, reaching 10.9 MPa at 5 wt% Dpf/C. The promising thermomechanical properties of these composites and their suitability for 3DP support their use in real-world applications in the future's sustainable built environment. This scalable methodology can be adapted to regions with similar sustainable local and waste resources, advancing the circular economy.

Widening the frequency bandgap and reducing thermal conductivity in phononic crystals by tuning structural parameters using a novel computational simulation method

AbstractThis paper introduces and models a phononic structure based on single-crystal silicon, aiming to investigate the width of its frequency bandgap and the impact of key parameters on thermal conductivity. The modeled phononic crystal structure features a periodic arrangement of cylindrical holes in a silicon matrix. This research holds the potential to enhance thermal management performance of thermal metamaterials. Utilizing a 3D finite element method (FEM) model in COMSOL, we have computed phonon dispersion to estimate thermal conductivity. The study systematically has explored the influence of phononic crystal parameters—specifically, porosity, lattice constant, and thickness—along with their interactions on both thermal conductivity and frequency bandgap width.A comprehensive investigation of these parameters has been conducted for their optimization to achieve the maximum frequency bandgap width and minimum thermal conductivity using the response surface method model. Eigenfrequencies and wave vector parameters are extracted from the finite element model using a MATLAB script. Subsequently, thermal conductivity is calculated through the Callaway–Holland model, a simplification of the Boltzmann transport equation (BTE).Our results indicate that the frequency bandgap begins to form at approximately 43% porosity for a lattice constant and thickness of 100 nm each. Furthermore, adjusting the parameters led to a significant reduction in thermal conductivity, decreasing from 43.89 W m−1 K−1 to 0.39 W m−1 K−1. The novelty of our research lies in thermal conductivity control of phononic crystal metamaterials through their parameter variations, or a predictive method of thermal conductivity and its parameter sensitivity. This study advances the state of the art in phononic crystal metamaterial research, contributing to improved thermal management performance by enlarging frequency bandgaps.Overall, our findings deepen the understanding of how porosity, lattice constant, and thickness influence thermal conductivity and frequency bandgap width. They offer valuable insights into optimizing phononic crystal parameters, enhancing thermal management performance, and designing more efficient and effective phononic crystal structures.

Evaluating the effect of Incorporating Chicken Feather Fibers on the Technological Properties of Eco-Friendly Compressed Earth Bricks

This paper investigates the feasibility of improving the performance of raw bricks by using chemically treated chicken feather fibers as a sustainable reinforcement material. Although numerous studies have already explored the use of chicken feathers in brick composites, this work is distinguished by the application of a novel chemical treatment involving formaldehyde and silane to enhance the fibers' compatibility with the clay matrix. The treated fibers were incorporated into raw brick samples at varying concentrations. The effects of fiber reinforcement on critical properties such as porosity, bulk density, compressive strength, and thermal conductivity were systematically analyzed. The results indicate that incorporating chemically treated chicken feather fibers reduces porosity and bulk density while increasing compressive strength within an optimal concentration range. Additionally, a significant reduction in thermal conductivity was observed, demonstrating the potential of these materials to improve thermal insulation in construction applications. This study highlights the promise of utilizing chicken feather waste, treated with formaldehyde and silane, as reinforcement in the production of eco-friendly bricks, with implications for sustainable construction practices.

The Effect of Fly Ash Additive on the Thermal Conductivity of Polystyrene Concrete

The use of fly ash in compositions as a substitute for a part of cement is economically favorable and ecologically feasible in connection with large accumulations of waste at the enterprises of the energy sector. In addition, the technology of cement production provides high-temperature treatment of mineral substances in kilns with significant emissions of carbon dioxide. One of the most effective directions of the utilization of fly ash is their use in concrete composites. The use of this material will provide the required temperature and humidity conditions in residential premises, solve the problem of “cold bridges” in structures, minimize heat losses of the structure, and increase the energy efficiency of buildings in general. At the same time, polystyrene concrete, due to its structural structure and the presence of thermally conductive concrete, has limited opportunities for thermal and physical–mechanical properties. To improve the operational properties of polystyrene concrete, it is proposed to use composite binders, including fly ash from the thermal power station of Astana. The main aim of this study is to develop compositions of polystyrene concrete with reduced thermal conductivity and improved physical and mechanical properties. The objectives of this study include the determination of characteristics of fly ash from Astana, formulation of polystyrene concrete mixtures with different proportions of fly ash, and evaluation of their thermal conductivity properties. These tasks are in line with the objectives of the ISO 50001 standard to improve energy efficiency and reduce environmental impact. The results showed that the addition of fly ash from Astana to polystyrene concrete leads to a marked reduction in thermal conductivity, contributing to improved energy efficiency of the building envelope. Optimal results were achieved by using 15% of Astana fly ash as an additive in polystyrene concrete, which led to a significant reduction in thermal conductivity of 51.47%. This reduction is in line with improving the energy efficiency of building materials, especially in cold climates.

Rattling effect in YbaCabSi7.5Al4.5O1.5N14.5 α‐SiAlON ceramics

AbstractWhen a small atom is induced into a larger lattice, new phonon scattering modes are aroused and thermal conductivity is significantly reduced by increased anharmonicity. This effect, called “rattling,” has been reported in pyrochlores and some cage compounds, but not in α‐SiAlON. In this study, we reveal that in (Yb+Ca) co‐doped α‐SiAlON ceramics (YbaCabSi7.5Al4.5O1.5N14.5, x = a/[a+b]), the rattling effect plays a crucial role in reducing thermal conductivity. Samples with different Yb/Ca ratios (x = 0, 0.1, 0.2, 0.3…, 1.0) were sintered by spark plasma sintering (SPS) at 1600°C and their thermal diffusivity/conductivity were measured by the laser‐flash method. We found that substituting Ca2+ with smaller Yb3+ cations in Ca‐α‐SiAlON caused significant reduction in thermal conductivity. On the contrary, substituting Yb3+ with larger Ca2+ cations in Yb‐α‐SiAlON caused only slight reduction in thermal conductivity. A lowest thermal conductivity of 3.3 W/(m·K) was achieved, when x = 0.3 rather than x = 0.5 or 0.7. Further analysis of phonon scattering intensity confirmed that both intrinsic and extrinsic scatterings are enhanced in the sample of x = 0.3. This study demonstrates that by utilization of the rattling effect, α‐SiAlON, well‐known for its excellent mechanical properties, can be tuned to exhibit low thermal conductivity comparable to that of La2Zr2O7 and 8YSZ.

Electro-Thermo-Magnetic Effect-Induced Large Thermoelectric Performance of Calcium Cobaltite Nanocomposites.

As attractive thermoelectric oxides, Ca3Co4O9-based materials have been intensively studied for their applications in recent years. However, their thermoelectric performance is enormously limited due to the contradiction of electrical resistivity and thermal conductivity. Herein, BaFe12O19 nanospheres were introduced into the Ca3Co4O9 matrix. The metallic Ag, ferrites, and matrix phase survived together, and a high density of nanoscale BaFe12O19 precipitation was observed. The reduction of work function could lead to band bending and form an interface potential due to the electro-thermo-magnetic effect contributing to the hole migration. As a result, a huge ZT value of 0.51 for the 8 wt % BaFe12O19/Ca3Co4O9 nanocomposites was obtained at 1073 K, accompanied by a low electrical resistivity of 6.7 mΩ·cm and a high Seebeck coefficient of 217.5 μV/K. In addition, a significant reduction of thermal conductivity (1.11 W/(m·K)) occurred, which was due to the nanoscale ferromagnetic phase effectively scattering the mid- and short-wavelength heat-carrying phonons. The synergistic enhancement of thermoelectric performance confirmed that the electro-thermo-magnetic effect is an effective way to solve the challenging problem of performance deterioration in oxide thermoelectric materials.

Utilizing bio-based and industrial waste aggregates to improve mechanical properties and thermal insulation in lightweight foamed macro polypropylene fibre-reinforced concrete

This research intends to examine the potential of incorporating waste materials in place of coarse aggregates to augment the mechanical characteristics and thermal insulation of lightweight foamed concrete (LWFC), along with the inclusion of polypropylene (PP) fibres. The PP fibre exhibits excellent tensile strength and enhancement on thermal insulation properties of cementitious materials. In this study, eco-friendly bio-based oil palm shell (OPS) and robust solid polyurethane (SPU) aggregates derived from industrial waste were utilized as substitutes for conventional aggregates. Additionally, silica fume was partially incorporated as supplementary cementitious materials. The addition of PP fibre impacted the workability of the LWFC with the inverted slump value decreased by 1.7 %–17.5 %. The inclusion of PP fibre proportion increased from 0 % to 0.5 % revealed a positive result in mechanical properties and thermal insulation performance. The OPSSPU/0.5 showed the significantly increment of 40.4 % compressive strength, 102.9 % of splitting tensile strength, and 70.8 % flexural strength, respectively. The optimum result was obtained in the OPSSPU/0.5 mixture, which exhibited 3.74 MPa residual compressive strength. Furthermore, utilizing PP fibres brought about a significant reduction in thermal conductivity in the environmentally sustainable LWFC, leading to improvements ranging from 0.2 % to 16.3 % when compared to the control composition. Thus, these findings are essential for encouraging the adoption of such practices in the construction industry and contributing to the reduction of environmental impacts associated with sustainable concrete production.

Synergistic effect of fly ash and polyvinyl alcohol fibers in improving stability, rheology, and mechanical properties of 3D printable foam concrete

Foam concrete, a lightweight cellular cementitious mixture has gained significant attention globally due to its multifunctional properties. Employing such multifunctional material to additive manufacturing helps in reducing the material wastage, time for construction and cost of production. However, there are various challenges encountered in employing such material with conflicting attributes in 3D concrete printing technology. Therefore, it is important to address these challenging requirements (i.e. stability along with printability). In this study, fly ash and polyvinyl alcohol fibers are employed as sand replacement and reinforcing material respectively to improve stability, rheology and mechanical characteristics of 3D printable foam concrete. Initially, printability of the mixture is achieved through moderation of slump and slump flow. Further, these mixtures are studied for fresh density, rheological properties and buildability. Essential hardened properties like oven dry density, compressive strength, flexural strength, bond strength, and thermal conductivity are studied. Later, microstructural studies are conducted to analyze the variations in hardened properties. Results show that replacement of sand with fly ash has significant influence (more than 100%) in improving the printability (for both 1300 and 1000 kg/m3 densities), however it has negative impact on the stability of foam. Stability of foam concrete could be counteracted by adding the fibers which forms networks with the foam bubbles and reduced the breakage and coalescence. Nonetheless, addition of fly ash increased the mechanical performance and thermal conductivity, but the dry density is increased due to bubble breakage. Addition of fibers reduced dry density with significant reduction in thermal conductivity and minimal compromise in mechanical performance.

Unified deep learning network for enhanced accuracy in predicting thermal conductivity of bilayer graphene, hexagonal boron nitride, and their heterostructures

In this research, we utilized density functional theory (DFT) computations to perform ab initio molecular dynamics simulations and static calculations on graphene, hexagonal boron nitride, and their heterostructures, subjecting them to strains, perturbations, twist angles, and defects. The gathered energy, force, and virial information informed the creation of a training set comprising 1253 structures. Employing the Neural Evolutionary Potential framework integrated into Graphics Processing Units Molecular Dynamics, we fitted a machine learning potential (MLP) that closely mirrored the DFT potential energy surface. Rigorous validation of lattice constants and phonon dispersion relations confirmed the precision and dependability of the MLP, establishing a solid foundation for subsequent thermal transport investigations. A further analysis of the impact of twist angles uncovered a significant reduction in thermal conductivity, particularly notable in heterostructures with a decline exceeding 35%. The reduction in thermal conductivity primarily stems from the twist angle-induced softening of phonon modes and the accompanying increase in phonon scattering rates, which intensifies anharmonic interactions among phonons. Our study underscores the efficacy of the MLP in delineating the thermal transport attributes of two-dimensional materials and their heterostructures, while also elucidating the micro-mechanisms behind the influence of the twist angle on thermal conductivity, offering fresh perspectives for the design of advanced thermal management materials.

A nanoscale perspective of the coexistence of multidimensional defects in the AgCuTe system

AgCuTe, a p-type medium-temperature thermoelectric material, exhibits great potential as a candidate for converting waste heat into electricity owing to its low thermal conductivity and high thermoelectric performance. However, an unstable phase structure of the material system has hindered the direct observation of various point defects contacting with high performance, despite their predicted possibilities solely based on first-principles calculations. Here, we present the novel observation of Te interstitial atoms together with other point defects, planar defects, and nanoscale atomic aggregation for the first time in the Zn-doped and Te non-stoichiometric AgCuTe system, employing a nanoscale perspective. Through our investigation, we disclose the remarkable coexistence of multidimensional defects, thereby resolving the previously conflicting opinions regarding point defects in this system. The effective control of these defects is crucial for achieving high thermoelectric performance. The ZnCu point defects regulate carrier concentration and Seebeck coefficient by influencing phase proportion and carrier effective mass. While maintaining the material's inherently large power factor (PF), thermal conductivity is effectively reduced through grain refinement and multiscale defect scattering. As a result, the dimensionless thermoelectric figure of merit (ZT) of approximately ∼ 1.16 is achieved at 773 K. By altering Te content, a coexistence of diverse defects is introduced, leading to intricate microstructures and phase structures. Consequently, a significant reduction in thermal conductivity is observed. The coexistence of multidimensional defects, as well as their impact on microstructure and thermoelectric transport properties, holds significant implications for the performance control of functional materials.

Substrate-induced bonding asymmetry leading to strong phonon anharmonicity and largely reduced thermal conductivity of monolayer XS2 (X = Mo and W) from first-principles calculations

Thermoelectric materials have properties such as direct conversion of waste heat into electricity and environmental friendliness, which make them promising for renewable energy utilization, however, the lower conversion efficiency limits the large-scale application of thermoelectric materials. Monolayer transition metal dichalcogenides (TMDs) are candidates for thermoelectric materials due to their high power factor, but their higher thermal conductivity (TC) limits the further improvement of the figure of merit (ZT). Here, we employ first-principles calculations and solve the Boltzmann transport equation (BTE) to compare the lattice TC of free-standing monolayer XS2 (X = Mo and W) and monolayer XS2 supported on SiC (0001) and ZnO (0001) substrates. We find that the lattice TC of monolayer XS2 supported on SiC (0001) and ZnO (0001) substrates are sharply reduced. Especially supported on the ZnO (0001) substrate, the lattice TC of XS2 reduce by 65 %–70 %, which is expected to improve the thermoelectric properties of monolayer XS2. In addition, we report the underlying cause of this significant reduction in TC. Firstly, the substrate drives softening of the acoustic branch, which leads to a decrease of phonon group velocity in the low-frequency and an enhancement of the three-phonon scattering. Then, the asymmetric bonding of XS2 is induced by the substrates, which enhance phonon anharmonicity and reduce the phonon lifetime. Our results demonstrate that the lattice TC of monolayer XS2 can be tuned by adjusting the substrate, which has certain reference values for the thermoelectric design of XS2.

Significant reduction of the lattice thermal conductivity in antifluorites via a split-anion approach

Searching for thermoelectrics with low lattice thermal conductivity ${\ensuremath{\kappa}}_{l}$ is crucial to address needs for waste heat recovery, but few light element dominated materials provide a low ${\ensuremath{\kappa}}_{l}$. Herein, we propose an effective strategy, i.e., a split-anion approach, to reduce the ${\ensuremath{\kappa}}_{l}$ of the antifluorite. After introducing the split-anion approach, the stable quasiantifluorite ${\mathrm{Li}}_{4}\mathrm{OSe}$ is obtained, and ${\ensuremath{\kappa}}_{l}$ is reduced by $\ensuremath{\sim}90%$ compared to the antifluorite ${\mathrm{Li}}_{2}\mathrm{Se}$. The ${\ensuremath{\kappa}}_{l}$ value of ${\mathrm{Li}}_{4}\mathrm{OSe}$ reaches as low as 0.49--1.75 W ${\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$, the range of thermoelectric applications, despite numerous light Li atoms and a large mass mismatch among the constituent atoms. This significant reduction of the thermal conductivity mainly results from the soft phonon branches and large optical bandwidth induced by lattice symmetry breaking after the split-anion approach. These findings offer ample scope for tuning the phonon band structure and thermal transport by reducing lattice symmetry like in the split-anion approach, leading to ultralow ${\ensuremath{\kappa}}_{l}$ of materials.

Ag doping effect on electronic and thermoelectric properties of SrTiO3 (0 0 1) surface

The search for sustainable energy sources has been a global demand in recent decades. In this context, thermoelectric materials (TE) have been an interesting alternative for this purpose and constitute a relevant research topic for increasing the efficiency of sustainable energy production. In this sense, computational simulation of the functionalization of SrTiO3 is proposed via Ag-doping on the (001) surface. The results show the emergence of a midgap trap state, leading to a band gap energy decrease, an increase in dielectric constants and the effective mass electron. The Ag-doping generates a decrease in electrical conductivity and an increase in the figure of merit, which are associated with a significant reduction in thermal conductivity (∼5 times), indicating that the material can be used with TE at high temperatures. The Ag-SrTiO3 (001) surface is promising for developing new thermoelectric devices.

Coupling of electronic transport and defect engineering substantially enhances the thermoelectric performance of p-type TiCoSb HH alloy

Further advancements in thermoelectric technology rely on the capacity to control both electrical and thermal transport properties simultaneously. Although TiCoSb-based half-Heusler compounds are promising for mid-range-temperature thermoelectric applications owing to their high Seebeck coefficient and good electrical conductivity, their high thermal conductivity has been so far the main issue to overcome. Here, we show that a combined approach of tuning the electronic properties and defect engineering enhances the thermoelectric performance of p-type TiCoSb-based compounds. By alloying on the Co and Ti sites with Fe and Zr, respectively, an overall increase in the peak ZT value of up to ∼90% at 823 K is achieved in Ti0.8Zr0.2Co0.85Fe0.15Sb. This enhancement is directly tied to the more pronounced metallic nature of transport upon Fe alloying combined with a significant reduction in thermal conductivity due to mass and strain field fluctuations driven by the substitution of Zr for Ti, as evidenced by the Debye-Callaway model. Further adjusting the hole concentration with aliovalent Sn doping leads to an additional increase in ZT, eventually leading to a peak value of ∼0.54 at 823 K in Ti0.8Zr0.2Co0.85Fe0.15Sb0.96Sn0.04, which is 224% higher than in TiCo0.85Fe0.15Sb, and the highest value reported so far in Hf-free p-type TiCoSb based HH alloys.

Performance Test and Thermal Insulation Effect Analysis of Basalt-Fiber Concrete.

This paper examines the feasibility of applying inorganic thermal-insulating concrete in high geothermal roadways in underground coal mines. This innovative material is based on a mixture of ceramsite, glazed hollow beads, cement, and natural sand, enhanced with varying degrees of basalt fibers. Fibers were used as a partial substitute in the mixture, in the following volumes: 0% (reference specimen), 5%, 10%, 15%, and 20%. Their compressive strength, permeability resistance, and thermal conductivity were studied. A high content of fibers tends to entangle into clumps during mixing, resulting in a significant reduction in the mechanical properties of compressive strength. The appropriate amount of fiber content can improve impermeability, and the permeability height of 5% fiber concrete was reduced by 22.5%. Experiments on thermal behavior showed that an increase of basalt fibers leads to a significant reduction in thermal conductivity. For concrete containing 20% fiber, the thermal conductivity for the reference specimen (0%) in the wet state was reduced from 0.385 W/(m∙°C) to 0.098 W/(m∙°C). There was a slight increase in thermal conductivity when the temperature increased from 30 °C to 60 °C. Despite the reduced mechanical strength, the resulting concrete is well-suited for use in the insulation of underground roadways, as numerical simulations showed that insulating concrete with optimal fiber content (15%) can reduce the average temperature of the wind flow in a high ground temperature roadway of 100 m in length in a mine by 0.3 °C. The final cost-benefit analysis showed that insulating concrete has more economic benefits and broad development prospects when applied to high geothermal roadway cooling projects.

Effect of synthesis duration on heat and charge transport in polycrystalline CuCr1-xMgxO2

Magnesium-doped polycrystalline ceramic samples of cooper chromite (I) have been prepared by solid phase synthesis. Phase composition and crystal structure of synthesis have been investigated by X-ray diffraction. Microstructure of samples has been investigated by scanning electron microscopy. Thermal conductivity and electrical conductivity have been measured in the temperature range 78<T<320 K. Significant reduction of thermal conductivity with an increase of synthesis duration have been observed. This effect was explained by formation of small amount of MgCr2O4 and Cr2O3 and CuO crystallites operating as effective phonon scatters. Formation of the MgCr2O4 phase is observed in X-ray diffraction patterns and SEM images of the samples with Mg content higher than 3 at. %. Formation of a small amount of Cr2O3 or CuO phase could be due to deviation of precursor’s content from stoichiometry. Obtained results open a perspective of thermoelectric figure of merit enhancement for copper chromite-based material.

Low‐temperature synthesis of high‐entropy (Hf0.2Ti0.2Mo0.2Ta0.2Nb0.2)B2 powders combined with theoretical forecast of its elastic and thermal properties

AbstractA theoretical calculation combined with experiment was used to study high‐entropy (Hf0.2Ti0.2Mo0.2Ta0.2Nb0.2)B2 (HEB‐HfTiMoTaNb). The theoretical calculation suggested HEB‐HfTiMoTaNb could be stable over a wide temperature range. Then, a novel solvothermal/molten salt‐assisted borothermal reduction method was proposed to efficiently pre‐disperse transitional metal atoms in a precursor and synthesize (Hf0.2Ti0.2Mo0.2Ta0.2Nb0.2)B2 nanoscale powders at 1573 K for 6 h, which is nearly 300 K lower than previous reports. The characterization results indicated that the as‐synthesized nanoscale HEB‐HfTiMoTaNb powder was hexagonal single‐phase with homogeneous elements distribution and uniform size, and the oxygen content of the particles is 0.97 wt%. Simultaneously, the mechanical properties, anisotropic nature, and thermal properties of HEB‐HfTiMoTaNb were investigated by density functional theory (DFT) calculations. The Cannikin's law was adopted to explain the improvement of comprehensive mechanical properties. In addition, a significant reduction of thermal conductivity was observed for HEB‐HfTiMoTaNb and it only was 1/15 of the value of HfB2. This work suggests a reliable technique for synthesis of nanosized HEB powders and discovery of high‐entropy materials under the guidance of first‐principle theory.

Thermal transport in solar-reflecting and infrared-transparent polyethylene aerogels

Polyethylene aerogels (PEAs) present a unique combination of optical (solar-reflecting but infrared-transparent) and thermal properties that are ideally suited for applications requiring infrared-transparent thermal insulation such as atmospheric radiative cooling and condensation-free radiant cooling. As the material's thermal resistance plays a large role in these systems’ performance, better understanding of thermal transport within the material is crucial to guide future material development. In this work, we characterize thermal transport in PEAs, elucidating contributions of different heat transfer mechanisms and identifying pathways to further improve their thermal insulation performance. We first present a theoretical model that separates thermal transport via solid conduction through the polyethylene backbone, gaseous conduction within the porous structure and radiative transfer through the semi-transparent material. We then fabricated PEA samples of densities ranging from 12.0 kg/m³ to 82.2 kg/m³ and experimentally characterized their thermal conductivity using a custom-built guarded-hot-plate thermal conductivity setup, with pressures ranging from vacuum to 1 atm, in three gas environments (nitrogen, argon and carbon dioxide), and with low and high emissivity boundaries. The experimental and modeling results indicate that conduction in the gas phase and radiative transfer (for the high emissivity boundary case) dominate heat transfer through the material. We find that reducing the pore size and gas pressure, using lower thermal conductivity gases, adding infrared opacifiers and maintaining low density could all provide significant reduction in thermal conductivity.

Graphene addition improved figure of merit in SnTe prepared by the rapid hybrid microwave solid-state method

We successfully synthesised SnTe-based powders (SnTe, Sn0.95Bi0.05Te, and SnTe with graphene addition) by a hybrid microwave solid-state method. This demonstrated comparable thermoelectric performance to the conventional heating method but had low energy consumption and rapid synthesis. Graphene addition to SnTe materials resulted in significant reduction of thermal conductivity. The SnTe with 5 wt% graphene exhibited a reduction in overall thermal conductivity from ∼10 W m−1 K−1 for SnTe to ∼2 W m−1 K−1 at 325 K and showed a moderate power factor. The Debye model was used to explain the origin of the effects of graphene on lattice thermal conductivity. The dimensionless figure of merit was increased by five times, from 0.07 for SnTe to 0.35 for SnTe with 5 wt% graphene. Our results demonstrated an effective method and additive material to synthesise and enhance the thermoelectric properties of SnTe materials.

Energy and seismic drawbacks of masonry: a unified retrofitting solution

All over the world, a large part of existing buildings is not adequate to satisfy the safety requirement and the thermal comfort criteria. For this reason, the interest in structural and energy retrofitting systems has steadily grown in the last decades. In this scenario, an innovative thermal resistant geopolymer mortar has been developed and used for Inorganic Matrix Composite (IMC) systems aimed to a combined seismic and energy new retrofitting technique. The geopolymer-based IMC is able to ensure competitive mechanical properties with respect to the traditional lime-based IMCs and, at the same time, a significant reduction in thermal conductivity. In this paper, an experimental program is reported considering small-scaled masonry panels with double-side IMC-retrofitting and determining both the in-plane shear strength and the thermal resistance. The experimental shear tests are aimed to compare the mechanical performance of the geopolymer innovative systems with those of the traditional lime-based ones. Moreover, the thermal resistance gain of the innovative solutions was measured and compared with traditional systems. The results evidenced the effectiveness of the proposed technique that significantly improved the performances of masonry walls from both the thermal and the mechanical point of view.