Search Results (350)

Thermal barrier coatings (TBCs) require materials with intrinsically low thermal conductivity and high grain-boundary thermal resistance to maximize the temperature gradient across the top coat. In this work, the effective thermal conductivity of more than 40 prospective TBC oxides belonging to seven structural families (YSZ/YSH, pyrochlores/fluorites A₂B₂O₇, defective fluorites A₃BO₇, fergusonite/monazite ABO₄, and perovskites ABO₃) was systematically deconvoluted into intrinsic grain thermal conductivity (k_grain) and grain-boundary (R_gb) contributions. It is shown that grain-boundary Kapitza resistance dominates heat transport in virtually all advanced oxides, contributing 60–90% to the total thermal resistance of polycrystalline samples. The lowest k_grain values (4–12 W m⁻¹ K⁻¹) are found for cerates and certain tantalates, while the highest R_gb (up to 7.2 × 10⁻⁶ m² K W⁻¹) are characteristic of high-entropy and heavily doped perovskites. Orthorhombically distorted SrCeO₃-based and high-entropy perovskites combine moderate k_grain (4.7–27.9 W m⁻¹ K⁻¹), high R_gb, and tunable thermal-expansion coefficients (10–13 × 10⁻⁶ K⁻¹), making them the most promising candidates for next-generation TBCs. These findings provide a rational basis for microstructure engineering and composition design aimed at maximizing the temperature drop across TBC layers while maintaining phase stability and CMAS resistance. Full article

The alloy with a chemical composition of 64 at. % Fe and 36 at. % Ni is known as Invar36 and is characterized by a coefficient of thermal expansion (CTE) less than 2 × 10⁻⁶ °C⁻¹ below Curie temperature (about 250 °C). The conventional method of obtaining Invar36 alloys consists of melting and casting, followed by a series of heat treatments. In recent years, research has focused on unconventional technologies for Invar36 preparation such as the sintering of Fe and Ni elemental powders. Also, Invar36 in powder form can be synthesized by mechanical alloying (MA). The aim of this paper is the characterization of Invar36 compacts obtained by conventional sintering of mechanically alloyed Fe and Ni elemental powders. MA was performed in a high-energy planetary ball mill (Ar atmosphere). Mechanically alloyed powders were densified by conventional sintering (simple and double). The sintering parameters used are those specific to the sintering of ferrous parts. After simple sintering, the relative density was 74%. Re-pressing and double sintering lead to an increase in the relative density to 78.6%. The microstructure of Invar36 compacts consists of two phases. The coefficient of thermal expansion (CTE) was determined for Invar36 compacts obtained by both simple and double sintering at 1120 °C in endogas. The CTE values of Invar36 simple sintered (α = 0.6 × 10⁻⁶ °C⁻¹) and double sintered (α = 0.5 × 10⁻⁶ °C⁻¹) are very low, up to 195 and 225 °C, respectively. HV_0.05 values of the Invar-ss sample are lower than the values of the Invar-ds sample. Thus, the HV_0.05 value in areas where the γ phase predominates increases from 203 to 218, while in areas where the α phase is predominant it increases from 257 to 271. The results of this study have potential applicability in obtaining Invar parts by sintering under the specific conditions used for ferrous parts, without requiring any modification of the production flow. Full article

The temperature sensitivity coefficient greatly affects the interior ballistic performance of propellant charges. Even under consistent loading conditions, variations in environmental temperature can lead to maximum chamber pressure fluctuations of 40–80 MPa, thereby compromising weapon efficiency and operational safety. In order to obtain a single-base propellant with a higher energy and lower temperature sensitivity coefficient, ultra-fine RDX particles were added into the single-base propellant. The difference in thermal expansion coefficients between RDX and the single-base propellant matrix leads to temperature-dependent microcracking. These microcracks increase the burning surface area at low temperatures, compensating for the reduced chemical reaction rate and thereby lowering the temperature sensitivity coefficient. A scanning electron microscope (SEM) was used to observe the inner structure of the single-base propellant with and without RDX particles. The thermal mechanical analysis (TMA) results, together with SEM observations, reveal that the interfaces between the propellant matrix and the RDX particles are temperature-dependent. As a result, the burning surface area of the modified single-base propellant varies with temperature, contributing to a reduced temperature sensitivity coefficient. Closed bomb tests were conducted to verify this inference, and the obtained curves and relevant quickness (RQ) values showed that the modified single-base propellant had stable burning behavior and lower temperature sensitivity. This study leverages the structural interactions between high-energy fillers and polymer matrices to provide a potential strategy for designing climate-resilient ammunition. Full article

This study presents unique polymeric materials applicable to plastic substrates for use in flexible-display devices that overcome the trade-off between low linear coefficients of thermal expansion (CTE) and low thickness-direction birefringence (Δn_th) while combining a very high T_g, sufficiently high thermal stability, excellent optical transparency, good solubility, and minimum-required ductility. Polyimide (PI) films obtained from 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) with 2,2′-bis(trifluoromethyl)benzidine (TFMB) under different conditions resulted in widely varying CTE values and provided a clear CTE–Δn_th correlation, which can be regarded as a virtual lower boundary in the CTE–Δn_th relationship for various PI systems. The pristine CBDA/TFMB and CpODA/TFMB (CpODA = norbornane-2-spiro-α-cyclopentanone-α′-spiro-2″-norbornane-5,5″,6,6″-tetracarboxylic dianhydride) systems were modified using numerous specifically designed monomers, i.e., a vertical-alignment-type liquid-crystalline diamine and cardo-type and spiro-type monomers. However, it was very challenging to overcome the trade-off between low CTE and low Δn_th, that is, to significantly exceed this lower boundary by modifying the pristine systems, while ensuring other target properties. One of the keys to achieving the present goal was compatibility with chemical imidization or one-pot polymerization processes (i.e., high solubility of the PIs), because these processes were more advantageous in reducing CTE and enhancing film transparency than the conventional two-step process. The modifications using phenyl-substituted xanthene-pendant 2,7-diaminofluorene and fluorene-pendant 2,3,6,7-xanthenetetracarboxylic dianhydride exhibited a prominent effect on overcoming the trade-off without the help of any fillers, while combining other excellent target properties. Polarized FT-IR difference spectra measured at varying incidence angles suggested that these side groups, which are connected perpendicularly to the PI main chains, align in the Z-direction, rationalizing the observed prominent effect. Thus, unique high-temperature transparent materials applicable to plastic substrates were successfully obtained in this study. Full article

While carbon fiber-reinforced polymer (CFRP) composites are widely utilized in aerospace applications due to their exceptional specific strength and stiffness, they are inevitably subjected to impact loads during service, which can easily induce internal damage such as delamination. To mitigate these issues, this study investigates the low-velocity impact behavior of an SMA-reinforced CFRP U-shaped structure, emphasizing the critical role of curing-induced residual stresses. A numerical model incorporating the thermal-mechanical manufacturing history was developed and validated against experimental data. Results indicate that while embedded superelastic SMA wires effectively suppress crack propagation and enhance energy absorption, neglecting residual stresses leads to a significant overestimation of structural rigidity and peak loads. Due to the coefficient of thermal expansion mismatch between the SMA wires and the resin matrix, the SMA-CFRP system exhibits higher sensitivity to initial internal stresses than pure CFRP. By accounting for the residual stress field, the relative error in predicted peak force and absorbed energy for the SMA-CFRP model was reduced from 9.3% to 3.5% and 18.9% to 7.8%, respectively. These findings demonstrate that residual stress lowers the failure threshold and is essential for capturing the synergistic effects of SMA phase transformation and matrix damage, providing a more accurate reconstruction of the structural energy balance. Full article

Hafnium oxide thin films have been extensively investigated for high-speed and low-power memory applications. Herein, we investigated the influence of oxygen vacancies and external stress on the ferroelectric characteristics of Al-doped HfO₂ (HfAlO). Compared with HfAlO with 14% oxygen vacancies, films with 21% oxygen vacancies could lower the polarization switching barrier and increase the fraction of the ferroelectric phase. Furthermore, significant external stress promotes ferroelectric phase formation, thereby enhancing ferroelectric characteristics. The remanent polarization achieved with W electrodes (2Pr = 38 µC/cm²) is about 18 times that of Au electrodes, owing to the lower thermal expansion coefficient of W electrodes. Density functional theory calculations and finite element analysis provide theoretical insights corroborating the experimental results, helping to pave the way for developing hafnium-based materials for next-generation in-memory computing applications. Full article

The tunable coefficient of thermal expansion (CTE) and Poisson’s ratio (PR) properties of metamaterials help address issues caused by drastic temperature variations and external loads. In this work, we propose a novel bimaterial thermal expansion and PR integrated tunable 2D metamaterial structure. Under certain parameter constraints, the structure based on an Al alloy/low carbon steel (LCS) combination demonstrates a wide tunability, with the CTE ranging from −47 to 28 ppm/°C and the PR varying from −14.8 to 7.3. A general thermoelastic equation is adopted to establish the relationship between temperature, external force, and displacement, which is then assembled into a theoretical model. Through theoretical analysis and numerical simulations, the underlying mechanisms of the proposed 2D metamaterial structure’s CTE, PR, and their relationship with geometric parameters and elastic modulus ratios are revealed. CTE and PR experiments are conducted to validate the theoretical modeling. Finally, the coupling relationship between CTE and PR is revealed. Full article

This study addresses the challenge of thermal accumulation and low efficiency in conventional ground heat exchangers for building heating and cooling applications. A novel direct-expansion CO₂ borehole heat exchanger (BHE) backfilled with well water is proposed to enhance heat transfer and mitigate soil thermal imbalance. A dynamic thermal resistance-capacity model (TRCM) coupling CO₂ phase change with natural convection in well water is developed and validated against full-scale field experiments (135 m depth), with prediction errors below 5% under cooling conditions (MAPE 2.29%, RMSE 2.49%). Quantitative analysis reveals that natural convection in well water enhances overall heat transfer by 14.9% compared to soil-backfilled systems, despite intensifying thermal short-circuiting. Two practical enhancement strategies for building energy efficiency are proposed: (1) adding insulation to the rising pipe, which increases the heat transfer rate by up to 35.1%; and (2) implementing artificial well-water circulation, which achieves up to 50.5% enhancement, with an equivalent coefficient of performance (COP) reaching 52.5 under intermittent operation. The proposed system and the parametric analysis of these strategies offer effective solutions for improving the energy performance of ground-source heat pumps in buildings, contributing to reduced operational energy consumption and enhanced system reliability. Full article

Efficient heat dissipation is crucial for semiconductor devices; however, conventional thermal management materials often cannot meet practical demands because of inadequate thermal conductivity and mismatched coefficients of thermal expansion with semiconductor materials. In this study, we develop a synergistic process integrating magnetron sputtering and annealing to fabricate a composition-controllable Cr/Cr₃C₂ composite interlayer on diamond surfaces. By regulating the annealing temperature from 700 to 1100 °C, three key parameters of the Cr/Cr₃C₂ composite interlayer can be tailored: the thickness varies from ~200 to 800 nm, the Cr/Cr₃C₂ fraction is adjustable, and the surface roughness ranges from 33.3 to 61.6 nm. In the current research, the sample that was annealed at 900 °C for 2 h exhibited the highest coating uniformity, with carbide coverage exceeding 98% and no discernible porosity. This optimized annealing process produces an interlayer with robust coverage, moderate thickness (~300 nm), and low surface roughness (Ra = 33.3 nm), thereby markedly enhancing interfacial bonding and thermal-transport performance. The resulting composite achieves a maximum thermal conductivity of 605.27 W·m⁻¹·K⁻¹, corresponding to 211% of the experimentally measured value for the uncoated sample. Analyses combining the diffusion mismatch model and experimentation indicate that the enhancement originates from improved phonon spectral matching and increased interfacial adhesion energy. This work provides processing guidance for precise interface engineering in high-thermal-conductivity diamond/copper composites. Full article

To facilitate the development of low-cost LTCC substrate materials and the high-value utilization of industrial tailings, α-cordierite glass-ceramics with varying Na₂O additions were prepared from perlite tailings as the main raw material via the melt-quenching method followed by sintering-induced crystallization. The synergistic effects of sintering temperature and Na₂O addition on the parent glass structure, crystallization behavior, and properties were systematically investigated. The results demonstrated that the addition of Na₂O effectively depolymerized the degree of network polymerization of the parent glass, altered the crystallization pathway of cordierite crystal, and promoted the densification of glass-ceramics at lower sintering temperature. The calculations of crystallization kinetics revealed that the crystallization process of α-cordierite was mainly dominated by three-dimensional bulk growth, and its nucleation mechanism changed from “site saturation” to “continuous nucleation” with the increase of Na₂O addition. The α-cordierite glass-ceramics sintered at 850 °C with 0.6 wt.% Na₂O addition exhibited the optimal comprehensive properties, including low dielectric constant (5.82 @ 10 MHz) and dielectric loss (1.80 × 10⁻² @ 10 MHz), high flexural strength (147.3 MPa), a Vickers hardness (9.01 GPa), and suitable coefficient of thermal expansion (2.96 × 10⁻⁶ K⁻¹, close to Si). The glass-ceramics are expected to be an ideal candidate for low-cost LTCC substrate materials. Full article

The effects of Fe non-stoichiometry on crystal structure, microstructural evolution, and thermoelectric transport properties were systematically investigated in bornite (Cu₅Fe_1+yS₄; −0.06 ≤ y ≤ 0.06) synthesized by mechanical alloying followed by hot pressing. X-ray diffraction analysis confirmed the formation of a single-phase orthorhombic bornite structure over the entire composition range. Anisotropic lattice distortion was observed with increasing Fe non-stoichiometry, manifested as contraction along the a-axis and expansion along the b- and c-axes, with a non-linear dependence on composition. Crystallite sizes estimated from Lorentzian peak fitting increased from 64.1 nm for the stoichiometric composition to 70.6–76.3 nm for Fe-deficient samples and 73.2–90.9 nm for Fe-excess samples. Hall-effect measurements revealed p-type semiconducting behavior for the stoichiometric composition, degenerate p-type transport with increased hole concentration under Fe-deficient conditions, and a transition to n-type behavior with reduced carrier mobility under Fe-excess conditions. While Fe-deficient samples retained high electrical conductivity and positive Seebeck coefficients, Fe-excess samples exhibited negative Seebeck coefficients at low temperatures with sign reversal at elevated temperatures. As a consequence, the power factor of Fe-deficient samples was enhanced by approximately 20–30% relative to the stoichiometric composition. In addition, the total thermal conductivity remained below 0.8 W·m⁻¹·K⁻¹ for all samples, and Fe non-stoichiometry effectively suppressed lattice thermal conductivity. Consequently, the Cu₅Fe_0.94S₄ composition achieved a maximum dimensionless figure of merit of ZT = 0.61 at 673 K, representing a performance enhancement of approximately 30–70% compared with the stoichiometric composition (ZT = 0.36 at 673 K and 0.47 at 723 K). Full article

Polymer–ceramic hybrid composites are emerging as attractive candidates for lightweight, corrosion-resistant absorber components in solar thermal collectors; however, their adoption is constrained by the intrinsically low thermal conductivity of polymers, processing-induced anisotropic heat transport, interfacial thermal resistance at tube/laminate joints, and durability challenges under outdoor exposure. This review provides a collector-centered synthesis of polymer–ceramic hybrid materials, emphasizing the translation of composite properties into collector-level outcomes rather than conductivity enhancement alone. A structure–property–performance mapping approach is presented to connect directional thermal conductivity ((k_in-plane), (k_perp)), thermal diffusivity, heat capacity, coefficient of thermal expansion, and service temperature with collector performance parameters such as heat removal effectiveness, overall heat losses, and stagnation behavior. Ceramic fillers (e.g., boron nitride, aluminum nitride, silicon carbide, alumina) are examined for stable conduction-network formation, coating compatibility, and long-term reliability, while carbon fillers (graphite, graphene nanoplatelets, carbon nanotubes) are evaluated for combined heat spreading and solar absorption benefits, with attention to emissivity penalties. Hybrid ceramic–carbon architectures and multilayer absorber designs are identified as the most promising routes to balance thermal transport, optical selectivity (high solar absorptance and low thermal emittance), manufacturability, and durability under UV, humidity, and thermal cycling. Full article

TaC/amorphous-carbon (TaC/a-C) composite coatings with varied a-C contents were deposited on graphite by dual-target magnetron sputtering to mitigate the thermal-expansion mismatch that commonly triggers cracking and spallation in TaC coatings on carbon substrates during rapid thermal cycling. However, existing TaC–C (often termed “free carbon”) approaches rarely identify the carbon’s structural state and spatial distribution explicitly, and a clear correlation between carbon fraction, thermal-shock-driven microstructural evolution, and cyclic damage remains insufficiently established. Increasing the a-C fraction progressively refines the TaC grain structure and introduces an a-C phase along grain boundaries, thereby lowering the effective coefficient of thermal expansion (CTE) and improving compatibility with the graphite substrate. Under laser thermal cycling, coatings with higher a-C contents exhibit markedly enhanced resistance to cracking and spallation. After 15 cycles, the high-a-C (~28.99 at.%) coating remains free of through-thickness cracks, maintains its thickness, and retains a single-phase TaC structure without detectable Ta₂C, whereas the low-a-C coating shows severe thinning, through-cracks, and partial TaC → Ta₂C transformation. Microstructural observations indicate that the a-C phase forms a compliant, stress-relaxing boundary network and promotes a porous, mechanically interlocked TaC architecture, synergistically redistributing thermal stresses and deflecting crack propagation. Full article

The transition to sustainable thermosetting resins is frequently hindered by the trade-off between high bio-based content and processability. This study reports a novel strategy in developing a highly bio-based, styrene-free unsaturated polyester resin (UPR) by leveraging maltodextrin-derived mixed esters dissolved in dimethyl itaconate (DMI). Unlike conventional polysaccharide-based systems that suffer from extreme viscosity, our functionalized prepolymer–DMI system achieves a low-viscosity curing solution without requiring petroleum-derived diluents such as styrene. Fourier-transform infrared spectroscopy confirmed the formation of a robust crosslinked network via the complete consumption of C=C bonds. Consequently, the cured resin exhibits exceptional thermal and mechanical performance, outperforming many existing bio-based analogs: a glass transition temperature (T_g) reaching 141 °C, a decomposition onset near 250 °C, and superior dimensional stability with a linear thermal expansion coefficient as low as 77 ppm/°C. Demonstrating a fully renewable, easy-to-process formulation with a flexural strength of 44 MPa, this work provides a design template for the next generation of high-performance, eco-friendly industrial thermosets. Full article

Liquid crystalline poly(ester imide)s (LCPEIs) were synthesized by solution polymerization from 4-hydroxybenzoic acid (4-HBA), 6-hydroxy-2-naphthoic acid (HNA) and N-(3-carboxyphenyl)-4-hydroxyphthalimide (3-CHP), with the capping groups of benzocyclobutene (BCB)-containing compounds (BCB-HP for phenolic hydroxyl group and BCB-CP for aromatic carboxylic acid). Subsequent cross-linking of the BCB capping groups upon hot pressing afforded the cured LCPEI films. Optimal properties of these films were achieved by adjusting the capping BCB-HP/BCB-CP contents.These LCPEIs showed favorable thermal properties with a relatively high glass transition temperature (T_g, 137–167 °C) and low melting temperature (T_m, 186–194 °C). With the increase in BCB capping content, the tensile modulus, tensile strength, and coefficient of thermal expansion (CTE) exhibited a non-linear tendency of first decreasing and then increasing. LCPEI-3.0 (3 mol% BCB) showed optimal performance: a relatively low CTE (20 × 10⁻⁶ K⁻¹), a relatively high storage modulus (2.55 GPa), a moderate tensile modulus (2.65 GPa), a relatively low dielectric constant (Dk = 3.17) with low dielectric loss (Df = 0.0034) at 10 GHz, and excellent hydrophobicity (water contact angle = 133°). This improvement embodies an effective strategy to combine advantages of polyester, polyimide, and benzocyclobutene to achieve favorable and excellent comprehensive properties for convenient processability and practical application prospects. Full article