Low Thermal Conductivity Values Research Articles

A cotton waste reinforced composite for automotive applications: development and thermal characterization

Fibre-reinforced composites are largely used in automobiles and building materials in various forms. Thermal insulation and acoustic characteristics are being mainly considered in building construction and automotive applications to meet overall comfort conditions as well as fulfil energy efficiency requirements. It is critical to look for new and sustainable approaches to increasing the thermal characteristics of the composites. Integration of various wastes is one of the most sustainable and efficient methods for increasing the thermal characteristics of composites. The present work is aimed to investigate the thermal insulation performance of cotton waste-reinforced composites using both epoxy and modified epoxy using the C-THERM thermal conductivity analyzer according to ASTM C518 standards. A computational model of randomly dispersed epoxy matrix interphase with cotton fibers is developed and validated with experimental results. It was observed that the thermal conductivity is higher when the combination of 40% flake cotton waste and 60% matrix material is used, both for epoxy and modified epoxy. The thermal conductivity for epoxy composites is 0.5715 W/mK, while for modified epoxy composites it is 0.4935 W/mK. The results indicate that modified epoxy composites exhibit better thermal insulation properties, as they have lower thermal conductivity values compared to epoxy composites. The modified epoxy composite is considered the best for thermal insulation, can be utilised in various automotive applications, such as partitioning panels, automotive panels, brake pads, and similar applications.

Ultralow lattice thermal conductivity in K2AuBi and its thermoelectric properties

Thermoelectric materials are best known for their prowess to transform the environment’s waste heat into electricity. In an endeavor to explore new thermoelectric prospects, in the present study, we investigate K2AuBi using density functional theory-based first-principles simulations. From our simulations, we find an intrinsically low lattice thermal conductivity of 0.43 W m−1 K−1 at 300 K in K2AuBi. Based on our detailed analysis, we find the reasons for such a low value of lattice thermal conductivity as, low phonon group velocities, short phonon lifetimes, anharmonicity in the lattice vibrations, and significant mean square displacements of K and Au atoms. The large mean square displacements hint at weak bonding and anharmonicity in the lattice vibrations, favoring more phonons scattering. We also find that the vibrations of K-atoms can be related to rattlers, conducive to low lattice thermal conductivity. Our simulations predict a high value, ∼784 μV K−1, of Seebeck coefficient at 700 K on account of the large density of states in the vicinity of Fermi level. Combining our computed lattice thermal conductivity with electrical transport properties, we obtain a high figure of merit, ZT∼ 1.04, at 700 K in K2AuBi.

Unraveling the structural and thermoelectric properties of the Sn-doped filled skutterudite Smy(FexNi1-x)Sb11.5Sn0.5 (y = 0.17–0.34, x = 0.43–0.64)

A systematic study of the filled skutterudite system Smy(FexNi1-x)4Sb11.5Sn0.5 was carried out with the aim of investigating the effect of the partial substitution of Sb by Sn on the structural and thermoelectric properties of the material. The presence of Sn induces a shift of the p/n crossover toward lower values of x compared to the corresponding Sn-free system, as a consequence of the smaller number of electrons supplied. Moreover, a discontinuity at the p/n crossover is observed in the cell parameter and related structural features. The thermoelectric properties suggest lower thermal conductivity values in comparison to similar Sn-free skutterudite systems, resulting in higher ZT. This result highlights the significant role of Sn in creating new scattering centers able to affect the phonon transmission through the crystal lattice.

Thermoelectric properties of InGaN/GaN superlattices structure with high indium composition quantum dots

High indium composition InGaN is a promising material for thermoelectric harvesting application, which can work at high temperature and extreme environments. Due to the strong composition segregation, high indium composition InGaN material usually forms localized quantum dots, which advantageously enhances the thermoelectric (TE) properties. In this research, the two-dimensional InGaN/GaN superlattices (SLs) structured TE material with high In composition of 35% quantum dots is first grown and characterized. Using open-circuit voltage measurement, the Seebeck coefficient (S) exhibits a high value of −571 μV/K. Analysis indicates this relatively high S value is related to the increased density of electron states near the Fermi level induced by the reduced dimensionality, resulting in a power factor of 11.83 × 10−4W/m·K2. The dense boundary between InGaN quantum dots also increases the interface phonon scattering, thereby suppressing the heat transportation and leading to a low thermal conductivity (k) value of 19.9 W/m·K. As a result, a TE figure of merit (ZT) value of 0.025 is demonstrated in the sample. This work first clarifies the impact of embedded quantum dots in InGaN/GaN SLs structure on TE properties. It is very conductive for the design and fabrication of low-dimensional GaN based TE device.

Tailoring the Lattice Thermal Conductivity of Al‐Incorporated Ag2Se for Near Room Temperature Waste Heat Recovery

AbstractAg2Se is categorized as a typical ′phonon‐liquid‐electron‐crystal (PLEC)′material, exhibiting an extremely low . In this study, Al substituted Ag2Se samples were synthesized by mechanical alloying process followed by hot press densification technique. The Al substituted sample shows an exceptionally high mobility of 2360 cm2 V−1 S−1 at room temperature and a maximum power factor of 1442 μWm−1 K−2 at 383 K. The phonon scattering induced by various crystal imperfections due to Al substitution helped to obtain a very low lattice thermal conductivity value of 0.3 W/mK. A high figure of merit (zT) of 0.4 at 383 K was obtained as a result of the simultaneous effect of boosting the electrical transport properties and decreasing the thermal conductivity. This present work summarizes that the Al substitution is highly significant in improving the thermoelectric properties of Ag2Se.

Thermal properties and anti-corrosion behavior of multicomponent (Dy1/5Er1/5Tm1/5Yb1/5Y1/5)2SiO5 monosilicate against water vapor attack at high temperature

Materials with a thermal expansion coefficient compatible with silicon carbide based ceramic matrix composites and low thermal conductivity at higher temperatures are crucial for developing environmental barrier coatings (EBCs) for aero-engines. Lately, high entropy silicate engineering has been a promising approach to develop optimized EBC materials. The solid solution reaction method was adopted for the fabrication of high entropy multicomponent (Dy1/5Er1/5Tm1/5Yb1/5Y1/5)2SiO5 or (5RE1/5)2SiO5 monosilicate. Scanning electron microscopy and X-ray diffraction show the homogenous distribution and a single-phase solution of (5RE1/5)2SiO5 monosilicate. Differential scanning calorimetry testing was done at 1450 °C which shows no exo/endothermic reaction has occurred. The value of the coefficient of thermal expansion (6 ppm/°C) up to temperature 1500 °C and low thermal conductivity values for (5RE1/5)2SiO5 monosilicate were observed (1.1–1.4 W/m°C) in the temperature range between 25 and 1500 °C. The water vapor corrosion resistance of (5RE1/5)2SiO5 monosilicate further confirms that the material is suitable for EBC applications due to the synergistic effect of different rare earth radii and stable crystal structure with a negligible weight loss value of 0.0005 mg/cm2 after 150 h corrosion time under 90 % H2O and 10 % O2 at 1300 °C. The results of this work suggest that (5RE1/5)2SiO5 monosilicate can be used as a promising candidate for environmental barrier coatings.

Enhancing the thermoelectric performance of Sr0.6La0.4Nb2O6-δ-based ceramics through composite effects

Donor-doped SrNb2O6-based materials are regarded as highly promising candidates for high-temperature thermoelectric applications. The Sr0.6La0.4Nb2O6-δ/x wt% Ti (x = 1, 5, 10, 15) composite ceramics thermoelectric materials were prepared and the mechanism for enhancing their thermoelectric properties was investigated. The experimental results demonstrate that during sintering, nano-additive titanium powder undergoes oxidation to form TiO2. The inclusion of a secondary phase ultimately leads to successful reduction in electrical resistivity within the composite oxides. Due to crystal defects, complex structure and phonon scattering at grain boundaries, the samples consistently exhibit low thermal conductivity values below 3.0 W m−1K−1. Among them, the sample doped with 10 wt% Ti shows the highest PF value (470.0 μW/mK2 at 1073 K), demonstrating a significant increase in power factor of 280 % compared to Sr0.6La0.4Nb2O6 without the Ti composite. Consequently, the Sr0.6La0.4Nb2O6-δ/10 wt% Ti composite material achieved a maximum ZT value of 0.20, representing a fifty-three percent enhancement compared to the undoped sample.

AsTe3: A novel crystalline semiconductor with ultralow thermal conductivity obtained by congruent crystallization from parent glass

The synthesis of novel narrow-band-gap semiconductors with ultralow thermal conductivity opens a pathway to the design of functional materials with high thermoelectric performance or with interesting optical sensitivity in the infrared range. Here, we report on the discovery of the novel crystalline binary AsTe3 (c-AsTe3) prepared by spark plasma sintering (SPS) from full and congruent crystallization of amorphous AsTe3 (a-AsTe3) previously prepared by twin roller quenching. X-ray diffraction suggests that the structure of c-AsTe3 can be described as a superstructure of elementary Te with a specific distribution of As and Te atoms. More specifically, it appears as an intergrowth of a Te subunit (3 atoms) and As2Te5 subunit (6 As + 15 Te atoms) separated with interlayer spaces. The optical transmittance measured on both crystalline and amorphous AsTe3 indicates a maximum transmittance of 22 % over the infrared range 10–25 µm. Transport properties measurements, performed between 5 and 375 K, reveal that AsTe3 behaves as a lightly doped, p-type semiconductor. The complex crystal structure combined with a small-grain-size microstructure of the sample yields extremely low lattice thermal conductivity values of 0.35 W m−1 K−1 near 300 K. This poor ability to conduct heat is the main property that gives rise to an estimated dimensionless thermoelectric figure of merit ZT of ∼0.3 at 375 K. These findings show that the recrystallization of amorphous phases by SPS provides an effective approach for stabilizing novel phases with interesting functional properties.

Ultrahigh average zT realized in polycrystalline SnSe0.95 materials through Sn stabilizing and carrier modulation

The average zT determines the conversion efficiency, and the power factor plays an important role in average zT value. However, the inadequate electrical conductivity of SnSe materials seriously limits its application. Herein, the TaCl5-doped in polycrystalline SnSe0.95 materials synthesized using the melting method and combined with spark plasma sintering technology achieves a zT value of 1.64 at 773 K and a record zTave of 0.62 from 323 K to 773 K. The electrical conductivity increases due to the released electron carrier induced by effective TaCl5 doping. According to the DFT calculation, the energy band of TaCl5-doped samples is narrowed, which can enhance the electron transport. Besides, the Seebeck coefficient is maintained at an elevated level as a result of the incorporation of the heavy element Ta. Due to the significantly enhanced electrical conductivity and maintained high Seebeck coefficient, the power factor reaches to 622 μW·m−1·K−2 at 773 K for the SnSe0.95 + 1.75% (in mass) TaCl5 sample, which is almost 21 times higher than that of the pristine sample. Simultaneously, a high average power factor value of 334 μW·m−1·K−2 for the SnSe0.95 + 1.75% (in mass) TaCl5 sample from 323 to 773 K was obtained. It is surprisingly found that the Ta element plays another important role to improve the stability of SnSe0.95 by forming Ta2Sn3 and removing the low melting point Sn, which usually existed in n-type SnSe samples, resulting in the decreased lattice thermal conductivity. A low lattice thermal conductivity value of 0.24 W·m−1·K−1 was also obtained for the SnSe0.95 + 2.0% (in mass) TaCl5 sample at 773 K due to the multiscale defects. Consequently, the SnSe0.95 + 2.0% (in mass) TaCl5 sample obtains a peak zT value of 1.64 at 773 K and a record zTave of 0.62 from 323 to 773 K, and the theoretically calculated conversion efficiency reaches 11.2%, it can be utilized for power generation and/or cooling at a broad temperature range. This strategy of introducing high-valence halides with heavy element can optimize the thermoelectric performance for other material systems.

The uniqueness of flexible and mouldable thermal insulation materials in thermal protection systems—A comprehensive review

The uniqueness of flexible and mouldable thermal insulation materials in thermal protection systems—A comprehensive review

Study The Thermal Conductivity of Polymethyl Methacrylate Reinforcing with Cotton, Hair, and Burlap

This study includes the manufacture of two sets of composite materials consisting of polymethyl methacrylate reinforced with natural fibers. The first (R) group included eight layers of cotton with one layer of hair in the middle, and the second (O) group included six layers of cotton with three layers of burlap in the middle. The thermal conductivity of the two groups was checked using a Lee disc to identify the thermal conductivity (K), of group R 0.0771 w/m.c, and of group O 0.0754 w/m.c. These values are considered good as the material has good thermal insulation due to its low thermal conductivity values and is therefore suitable for use in prosthetic limbs as it can preserve the body temperature from external influences.

Lightweight Biobased Polyurethane Composites Derived from Liquefied Polyol Reinforced by Biomass Sources with High Mechanical Property and Enhanced Fire-Resistance Performance.

Lightweight biobased insulation polyurethane (BPU) composite foams with high fire-resistance efficiency are interested in building effective energy and low environmental impact today. This study focuses on manufacturing lightweight BPU from liquefied bamboo polyols and biomass resources, including rice husk and wood flour. Then, they are combined with three flame retardant (FR) additives, such as aluminum diethyl phosphinate, aluminum trihydroxide, and diammonium phosphate, to improve their fire resistance performance. The physicochemical properties, microstructure, thermal stability, mechanical properties, and flame-retardant properties of the BPU composites are characterized to optimize their compromise properties. The results showed that composites with optimized FRs achieved UL94 V-0 and those with nonoptimized FRs reached UL94 HB. The limiting oxygen index exhibited that the fire resistance of BPU composites could increase up to 21-37% within FR additives. In addition, the thermal stability of BPU composites was significantly improved in a temperature range of 300-700 °C and the compressive strength of the BPU composites was also enhanced with the presence of FRs. The scanning electron microscopy observation showed an influence of FRs on the morphology and cell size of the BPU composites. The bio-PU-derived samples in this study showed significantly low thermal conductivity values, demonstrating their remarkable thermal insulation effectiveness.

Chemical-free thermal-acoustic panels from agricultural waste for sustainable building materials

To address the pressing need for sustainable building materials, this study introduced an innovative and eco-friendly approach to manufacturing thermal-acoustic panels, utilizing agricultural waste with rice straw as the primary material. Paper pulp (PP) and Persea kurzii (PK) were used as non-chemical binders at ratios of 50:50, 60:40, 70:30, and 80:20. After mixing, all the samples were subjected to heat-free hydraulic compression at 5 bars to evaluate their physical, mechanical, thermal, and acoustic properties. Increasing the proportion of the binder directly impacted panel density and flexural strength while also inversely affecting porosity. The PK binder had a low thermal conductivity value of 0.040 W/mK, proving it was a good thermal insulator with a high sound absorption coefficient, especially at higher frequencies. The RSPP-4 panel had the highest noise reduction coefficient (0.51) and absorbed low frequencies, suggesting its potential for noise reduction. Microscopic analysis provided further insight into panel surface characteristics. PP exhibited a smooth surface with a continuous fiber weave that did not obscure the pores, while PK consisted of particles. The correlation between surface characteristics and acoustic performance, especially at high frequencies, underscored the intricate balance between material properties. Research results can be applied in the construction industry to develop sustainable building materials that offer superior thermal and acoustic properties. These thermal-acoustic panels can effectively utilize agricultural waste and show potential as environmentally friendly construction materials to enhance indoor comfort and acoustics in various building environments.

Opto-electronic and thermophysical characteristics of A2TlAgF6 (A = Rb, Cs) for green technology applications.

Lead-free double perovskites are unique materials for transport and optoelectronic applications that use clean resources to generate energy. Using first-principle computations, this study thoroughly investigates the structural, thermoelectric, and optical attributes of A2TlAgF6 (A = Rb, Cs). Tolerance factor and formation energy estimates are used to verify that these materials exist in the cubic phase. Elastic constants with high melting temperature values are ductile when evaluated for mechanical stability using the Born stability criterion. The optical absorption band is adjusted from 2 to 4 eV via band gaps of 1.88 and 1.99 eV, as indicated by band structures. Analysis of optical properties reveals perfect absorption in the visible spectrum, whole polarization, and low optical loss. Furthermore, thermoelectric properties are assessed at 300, 500, and 700 K in the range of -0.5 to 3 eV for chemical potential (μ). The materials exhibit significant improvements in the Figure of Merit scale due to their elevated electrical conductivity, Seebeck coefficient, and extremely low thermal conductivity values.

High thermoelectric properties in polycrystalline SnSe materials realized by rare earth halide Co-doping

SnSe materials have garnered significant interest due to the intrinsic low thermal conductivity. However, its limited electrical conductivity hinders their practical implementation. This study achieved a high ZT value of 1.40 at 773 K in the samples of n-type polycrystalline SnSe0.95 + x wt% SmCl3 + y wt% ErCl3 (x = 0, 0.75, 1.0, 1.25, and 1.5; y = 0, 0.0675, 0.125, and 0.25) since (SmCl3, ErCl3) co-doping decoupled the electrical-thermal transport properties. First, the released electron carrier concentration due to SmCl3 doping enhanced the electrical transport properties. The SnSe0.95 + 1.25 wt% SmCl3 sample exhibited a high power factor of 563 μWm−1K−2 at 773 K. Then, the lattice thermal conductivity markedly decreased with further ErCl3 doping due to the high-density dislocations and coherent boundary, resulting in effective the phonon scattering. Additionally, the special microtopography was conducive to the decreased lattice thermal conductivity. Therefore, the SnSe0.95 + 1.25 wt% SmCl3 + 0.125 wt% ErCl3 sample achieved a low lattice thermal conductivity value of 0.28 Wm−1K−1 at 773 K. Simultaneously, the (SmCl3, ErCl3) co-doped samples maintained the electrical transport properties than the SmCl3-doped samples. Consequently, the SnSe0.95 + 1.25 wt% SmCl3 + 0.125 wt% ErCl3 sample obtained a peak ZT value of 1.40 at 773 K and a competitive average ZT value of 0.37 from 323 to 773 K, and a theoretically calculated conversion efficiency reached 7.2%, which can be applied in the deep space for power generation. This strategy can be utilized to optimize the thermoelectric performance of other thermoelectrical material systems.

FABRICATION OF THERMAL BIO-INSULATOR FROM OIL PALM TRUNK FIBER: ANALYSIS OF THERMAL, PHYSICAL AND MECHANICAL PROPERTIES

The majority of air conditioning systems, including both cooling and heating systems, consume a significant amount of electrical energy as a result of their high electrical consumption and prolonged periods of operation. The use of thermal insulation materials in the building can help conserve electrical energy used for room conditioning systems. Natural fibers are used as an alternative in the production of thermal insulation, which is commonly referred to as bio-insulators. The utilization of oil palm trunk (OPT) fiber as the primary material for thermal insulation shows promise. This study aims to determine the specific attributes of OPT fiberboard that make it suitable for use as a thermal bio-insulator. The features examined encompass physical, mechanical, thermal, and fire-resistant attributes. The OPT fiber underwent a treatment process involving boiling at a temperature of 80℃ for a duration of 30 minutes. The fiberboard is manufactured using epoxy adhesive and calcium carbonate additive, and then printed using the hand lay-up process and cold-compaction technique. The physical characteristics of fiberboard indicate that there is a direct relationship between its density and water absorption. Testing revealed that fiberboard has a low thermal conductivity and high heat capacity value. By including calcium carbonate, the burning time of the fiberboard was tested and seen to decrease, indicating a delay in the fiberboard burning process, as evidenced by the extended flame suppression time. The density of OPT fiberboard varies between 0.48 and 0.70 gr/cm3. The absorbency of water is inversely related to its density. Water absorption capacity generally rises with decreased density. The obtained heat capacity value is 1.28-2.38 J⁄(g℃). The mechanical value ranges from 1.00 to 3.55 MPa. The incorporation of calcium carbonate significantly impacts the thermal and mechanical characteristics of the fiberboard. The produced OPT fiberboard satisfies the requirements for good thermal, physical, and mechanical characteristics, making it a suitable bio-insulation material for buildings.

Comparative Thermal Insulation Nature of Ca2FeMnO6−δ and Sr2FeMnO6−δ

In this study, we investigate the utility of Ca2FeMnO6-δ and Sr2FeMnO6-δ as materials with low thermal conductivity, finding potential applications in thermoelectrics, electronics, solar devices, and gas turbines for land and aerospace use. These compounds, characterized as oxygen-deficient perovskites, feature distinct vacancy arrangements. Ca2FeMnO6-δ adopts a brownmillerite-type orthorhombic structure with ordered vacancy arrangement, while Sr2FeMnO6-δ adopts a perovskite cubic structure with disordered vacancy distribution. Notably, both compounds exhibit remarkably low thermal conductivity, measuring below 0.50 Wm−1K−1. This places them among the materials with the lowest thermal conductivity reported for perovskites. The observed low thermal conductivity is attributed to oxygen vacancies and phonon scattering. Interestingly as SEM images show the smaller grain size, our findings suggest that creating vacancies and lowering the grain size or increasing the grain boundaries play a crucial role in achieving such low thermal conductivity values. This characteristic enhances the potential of these materials for applications where efficient heat dissipation, safety, and equipment longevity are paramount.

Investigating the physical properties of Li2AgGaX6 (X = Cl, Br, I) double perovskites using ab initio calculations

The double perovskites have emerged as focal point of exploration and innovation in energy harvesting applications. In this paper, we elucidate the mechanical, optical, and transport properties of Li2AgGaX6 (X = Cl, Br, I) through the application of DFT-based simulations by employing the WIEN2K software. The calculation of formation energy has been conducted to assess thermodynamic stability. The band structures of studied halides doped double perovskites have reported values of direct band gaps. These findings have implications for a variety of optoelectronic and transport applications due to their diverse characteristics. The peaks of the absorption band are shifted toward the lower frequency spectrum, and their width increases as we shift from Cl to I. Furthermore, thermoelectric attributes such as the Seebeck coefficient (S), power factor (σS2), and figure of merit (ZT) have been evaluated across the thermal spectrum range (200–600) K Low value of thermal conductivity and substantial ZT at standard temperature (300 K) indicate their profound significance in thermoelectric developments.

Poly(vinylidene fluoride) Aerogels with α, β, and γ Crystalline Forms: Correlating Physicochemical Properties with Polymorphic Structures.

Strategic customization of crystalline forms of poly(vinylidene fluoride) (PVDF) aerogels is of great importance for a variety of applications, from energy harvesters to thermal and acoustic insulation. Here, we report sustainable strategies to prepare crystalline pure α, β, and γ forms of PVDF aerogels from their respective gels using a solvent exchange strategy with green solvents, followed by a freeze-drying technique. The crucial aspect of this process was the meticulous choice of appropriate solvents to enable the formation of thermoreversible gels of PVDF by crystallization-induced gelation. Depending on the polymer-solvent interactions, the chain conformation of PVDF can be modulated to obtain gels and aerogels with specific crystalline structures. The crystalline pure α-form and piezoelectric β-form aerogels were readily obtained by using cyclohexanone and γ-butyrolactone as gelation solvents. On the other hand, the γ-form aerogel was obtained using a binary solvent system consisting of dimethylacetamide and water. These aerogels with distinct crystalline structures exhibit different morphologies, mechanical properties, hydrophobicities, acoustic properties, and electrical properties. Measurement of thermal conductivity for these aerogels showed exceptionally low thermal conductivity values of ∼0.040 ± 0.003 W m-1 K-1 irrespective of their crystal structures. Our results showcase the fabrication approaches that enable PVDF aerogels with varied physicochemical properties for multifunctional applications.

Enhancement of thermoelectric properties in Sr0.7Ba0.3Nb2O6−δ-based ceramics via nano-sized Ti as additive

A series of Sr0.7Ba0.3Nb2O6−δ/x wt. % Ti (x = 1, 3, 5, and 10) composite ceramic thermoelectric materials were prepared, and the mechanism for improving their thermoelectric properties was explored. The experimental results demonstrate that nano-additive titanium powder undergoes oxidation to form TiO2 during sintering. However, under annealing in a reducing atmosphere, oxidation reactions further deplete the lattice oxygen, leading to an increased generation of oxygen vacancies and enhanced carrier concentration, ultimately leading to successful resistivity reduction. The samples consistently exhibit low thermal conductivity values below 2.0 W m−1 K−1 due to crystal defects, complex structure, and phonon scattering at the grain boundaries. The sample doped with 5 wt. %. Ti exhibits the lowest resistivity and highest PF value (409.3 μW/m K2 at 1073 K). Consequently, the figure of merit of Sr0.7Ba0.3Nb2O6−δ with 5 wt. % Ti attains its maximum value of 0.30 at 1073 K, representing a 50% increase compared to that of the undoped sample Sr0.7Ba0.3Nb2O6−δ (0.20 at 1073 K).