Search Results (27)

In this paper, voxel-based Asymptotic Homogenization (AH) is employed to calculate the thermal expansion and thermal conductivity characteristics of lattice materials that have a Representative Volume Element (RVE) with non-orthogonal periodic bases. The non-orthogonal RVE of the cellular lattice is discretized using voxel elements (iso-parametric hexahedral element, on a cartesian grid). A homogenization framework is developed in python that uses a fast-nearest neighbor algorithm to approximate the (non-orthogonal) periodic boundary conditions of the discretized RVE. Validation studies are performed where results of the homogenized Thermal Expansion Coefficient (TEC) and thermal conduction performed in this paper are compared with results generated by commercially available software. These included comparison with the results for (a) bi-material unidirectional composite with orthogonal RVE cell envelope; (b) bi-material hexagon lattice with orthogonal cell envelope; (c) bi-material hexagon lattice with non-orthogonal cell envelope; and (d) bi-material square lattice. A novel approach of visualizing the contribution of each voxel towards the individual terms within the homogenized thermal conductivity matrix is presented, which is necessary to mitigate any potential errors arising from the numerical model. Additionally, the effect of the thermal expansion and thermal conductivity for bi-material hexagon lattice (orthogonal and non-orthogonal RVE cell envelope) are presented for varying internal cell angles and all permutations of material assignments for a relative density of 0.3. It is found that when comparing the non-orthogonal RVE with the Orthogonal RVE as a reference model, the numerical error due to approximating the periodic boundary condition for the non-orthogonal bi-material hexagon is generally less than 2% as the numerical error is pseudo-cyclically dependent on the discretization along the cartesian axis. Full article

High-entropy ceramics have gained wider attention due to their structural integrity and stability, which can be used in various functional applications. Especially, high-entropy oxides exhibit excellent thermal stability, particularly at high temperatures. Thermal barrier coating materials must demonstrate good thermal stability without any phase transformation or phase separation, which is critical in aerospace and energy conversion applications. To address this, we have prepared new high-entropy stannate pyrochlore oxide nanoparticles with the composition (Gd_0.2Nd_0.2La_0.2Pr_0.2Sm_0.2)₂Sn₂O₇ through a simple glycine-assisted sol–gel synthesis. The phase evolution was probed at different heat-treatment temperatures from 1000 °C to 1500 °C. Among the temperatures investigated, a single-phase pyrochlore oxide was formed from 1300 °C without any impurity or phase separation. The obtained nanoparticles were characterized using various techniques, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), nanoindentation, and dilatometry to investigate their physiochemical and mechanical properties. The Vickers hardness of high-entropy oxides is 4.2 ± 0.33 GPa, while a thermal expansion coefficient (TEC) of 8.7 × 10⁻⁶ K⁻¹ at 900 °C is calculated. The results show that the prepared high-entropy pyrochlore oxide can be a suitable candidate for thermal barrier coating. Full article

F-doped PrBa_0.8Sr_0.2Co₂O_5+δ−xF_x (PBSCF_x, x = 0, 0.025, 0.05, 0.075, 0.1) cathode powder was synthesized by the sol–gel method. X-ray diffraction results showed that all the samples doped with F exhibited a typical tetragonal perovskite structure without a heterophase. F doping can effectively reduce the thermal expansion coefficient (TEC) of the cathode materials, which decreased from 25.3699 × 10⁻⁶ K⁻¹ of PBSC to 23.5295 × 10⁻⁶ K⁻¹ of PBSCF_0.1. The area-specific resistance (ASR) of PBSCF_0.05 was 0.0207 Ω·cm² at 800 °C, with a conductivity of 1637.27 S·cm⁻¹ and maximum power density of 856.08 mW·cm⁻². Its performance had slight decay after 100 h at 800 °C. F doping significantly improved the electrochemical performance of this cathode material for solid oxide fuel cells (SOFCs). Full article

Environmental barrier coatings (EBCs) are widely used to protect ceramic matrix composites (CMCs, SiC_f/SiC, and Al₂O_3f/Al₂O₃), and they should have low thermal expansion coefficients (TECs) matching the CMCs and excellent mechanical properties to prolong their lifetime. Current EBC materials have disadvantages of phase transitions and insufficient mechanical properties, which affect their working temperatures and lifetime. It is necessary to develop new oxide EBCs. Ytterbium tantalate (YbTaO₄) is a stable and novel EBC material, and we have improved the mechanical properties and TECs of Yb_1−xAl_xTaO₄ (x = 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) ceramics by replacing Yb with Al. XRD, SEM, and EDS are used to verify the crystal and microstructures, and nano-indentation is used to measure the modulus and hardness when changes in TECs are measured within a thermal expansion device. The results show that the phase structure of Yb_1−xAl_xTaO₄ (x = 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) is stable at 25–1400 °C within air atmosphere, and their high-temperature TECs (6.4–8.9 × 10⁻⁶ K⁻¹, 1400 °C) are effectively regulated by introductions of different contents of Al, which enlarge their engineering applications for SiC_f/SiC and Al₂O_3f/Al₂O₃ CMCs. The evolutions of TECs are analyzed from structural characteristics and phase compositions, and the increased TECs make Yb_1−xAl_xTaO₄ potential EBCs for Al₂O₃ matrixes. Due to the high bonding strength of Al–O bonds, hardness, as well as Young’s modulus, are enhanced with the increasing Al content, with Yb_1−xAl_xTaO₄ (x = 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) having a nano-hardness of 3.7–12.8 GPa and a Young’s modulus of 100.9–236.6 GPa. The TECs of YbTaO₄ are successfully regulated to expand their applications, and they match those of Al₂O₃ and SiC matrixes, as well as displaying improved mechanical properties. This work promotes applications of YbTaO₄ as potential EBCs and provides a new way to regulate the TECs of tantalates. Full article

Boron carbide plays a crucial role in various extreme environment applications, including thermal barrier coatings, aerospace applications, and neutron absorbers, because of its high thermal and chemical stability. In this study, the temperature-dependent elastic stiffness constants, thermal expansion coefficient, Helmholtz free energy, entropy, and heat capacity at a constant volume ($C_v$) of rhombohedral B₄C have been predicted using a quasi-harmonic approach. A combination of volume-dependent first-principles calculations (density functional theory) and first-principles phonon calculations in the supercell framework has been performed. Good agreement between the elastic constants and structural parameters from static calculations is observed. The calculated thermodynamic properties from phonon calculations show trends that align with the literature. As the temperature rises, the predicted free energy follows a decreasing trend, while entropy and $C_v$ follow increasing trends with temperature. Comparisons between the predicted room temperature thermal expansion coefficient (TEC) ($7.54\times {10}^{-6}$ K⁻¹) and bulk modulus (228 GPa) from the quasi-harmonic approach and literature results from experiments and models are performed, revealing that the calculated TEC and bulk modulus fall within the established range from the limited set of data from the literature (TEC = 5.73–9.50 ×${10}^{-6}$ K⁻¹, B = 221–246 GPa). Temperature-dependent $C_{ij}s$ are predicted, enabling stress analysis at elevated temperatures. Overall, the outcomes of this study can be used when performing mechanical and thermal stress analysis (e.g., space shielding applications) and optimizing the design of boron carbide materials for elevated temperature applications. Full article

The oxygen vacancy formation behavior and electrochemical and thermal properties of Ba_0.5Sr_0.5Fe_1−xMn_xO_3−δ (BSFMnx, x = 0–0.15) cathode materials were investigated. For thermogravimetric analysis, the weight decreased from 1.98% (x = 0) to 1.81% (x = 0.15) in the 400–950 °C range, which was due to oxygen loss from the lattice. The average oxidation state of the B-site increased, the O_ads/O_lat ratio decreased, and the binding energy of the O_lat peak increased with Mn doping. These results indicate that Mn doping increases the strength of the metal–oxygen bond and decreases the amount of oxygen vacancies in the lattice. The electrical conductivity of BSFMnx increased with the temperature due to the thermally activated small-polaron hopping mechanism showing a maximum value of 10.4 S cm⁻¹ (x = 0.15) at 450 °C. The area-specific resistance of BSFMn0.15 was 0.14 Ω cm² at 700 °C and the thermal expansion coefficient (TEC) gradually decreased to 12.7 × 10⁻⁶ K⁻¹, which is similar to that of Ce_0.8Sm_0.2O₂ (SDC) (12.2 × 10⁻⁶ K⁻¹). Mn doping increased the metal–oxygen bonding energy, which reduced the oxygen reduction reaction activity but improved the electrical conductivity and thermal stability with SDC. Full article

Single-walled carbon nanotube (SWCNT) is a promising candidate for strengthening nanocomposite. As the matrix of nanocomposite, a single crystal of copper is designed to be in-plane auxetic along the crystal orientation [1 1 0]. In that way, the nanocomposite could also be auxetic when enhanced by (7, 2) a single-walled carbon nanotube with relatively small in-plane Poisson’s ratio. A series of molecular dynamics (MD) models of the nanocomposite metamaterial are then established to study mechanical behaviors of the nanocomposite. In the modelling, the gap between copper and SWCNT is determined following the principle of crystal stability. The enhanced effect for different content and temperature in different directions is discussed in detail. This study provides a complete set of mechanical parameters of nanocomposite including thermal expansion coefficients (TECs) from 300 K to 800 K for five weight fractions, which is essential for a wide range of applications of auxetic nanocomposites in the future. Full article

We presented a theoretical study for the structural, mechanical, and thermophysical properties of the precipitates in 2xxx series aluminum alloy by applying the widely used density functional theory of Perdew-Burke-Ernzerhof (PBE). The results indicated that the most thermodynamically stable structure refers to the Al₃Zr phase in regardless of its different polymorphs, while the formation enthalpy of Al₅Cu₂Mg₈Si₆ is only -0.02 eV (close to zero) indicating its metastable nature. The universal anisotropy index of A^U follows the trend of: Al₂Cu > Al₂CuMg ≈ Al₃Zr_D0₂₂ ≈ Al₂₀Cu₂Mn₃ > Al₃Fe ≈ Al₆Mn > Al₃Zr_D0₂₃ ≈ Al₃Zr_L1₂ > Al₇Cu₂Fe > Al₃Fe₂Si. The thermal expansion coefficients (TECs) were calculated based on Quasi harmonic approximation (QHA); Al₂CuMg shows the highest linear thermal expansion coefficient (LTEC), followed by Al₃Fe, Al₂Cu, Al₃Zr_L1₂ and others, while Al₃Zr_D0₂₂ is the lowest one. The calculated data of three Al₃Zr polymorphs follow the order of L1₂ > D0₂₃ > D0₂₂, all of them show much lower LTEC than Al substance. For multi-phase aluminum alloys, when the expansion coefficient of the precipitates is quite different from the matrix, it may cause a relatively large internal stress, or even produce cracks under actual service conditions. Therefore, it is necessary to discuss the heat misfit degree during the material design. The discrepancy between a-Al and Al₂CuMg is the smallest, which may decrease the heat misfit degree between them and improve the thermal shock resistant behaviors. Full article

Cu-content La_1−xSr_xNi_1−yCu_yO_3−δ perovskites with A-site strontium doping have been tuned as cobalt-free cathode materials for high-performance anode-supported SOFCs, working at an intermediate-temperature range. All obtained oxides belong to the R-3c trigonal system, and phase transitions from the R-3c space group to a Pm-3m simple perovskite have been observed by HT-XRD studies. The substitution of lanthanum with strontium lowers the phase transition temperature, while increasing the thermal expansion coefficient (TEC) and oxygen non-stoichiometry δ of the studied materials. The thermal expansion is anisotropic, and TEC values are similar to commonly used solid electrolytes (e.g., 14.1 × 10⁻⁶ K⁻¹ for La_0.95Sr_0.05Ni_0.5Cu_0.5O_3−δ). The oxygen content of investigated compounds has been determined as a function of temperature. All studied materials are chemically compatible with GDC-10 but react with LSGM and 8YSZ electrolytes. The anode-supported SOFC with a La_0.95Sr_0.05Ni_0.5Cu_0.5O_3−δ cathode presents an excellent power density of 445 mW·cm⁻² at 650 °C in humidified H₂. The results indicate that La_1−xSr_xNi_1−yCu_yO_3−δ perovskites with strontium doping at the A-site can be qualified as promising cathode candidates for anode-supported SOFCs, yielding promising electrochemical performance in the intermediate-temperature range. Full article

In this work, the stability of Sr₂(FeMo)O_6−δ-type perovskites was tailored by the substitution of Mo with Ti. Redox stable Sr₂Fe_1.4Ti_xMo_0.6−xO_6−δ (x = 0.1, 0.2 and 0.3) perovskites were successfully obtained and evaluated as potential electrode materials for SOFCs. The crystal structure as a function of temperature, microstructure, redox stability, and thermal expansion properties in reducing and oxidizing atmospheres, oxygen content change, and transport properties in air and reducing conditions, as well as chemical stability and compatibility towards typical electrolytes have been systematically studied. All Sr₂Fe_1.4Ti_xMo_0.6−xO_6−δ compounds exhibit a regular crystal structure with Pm-3m space group, showing excellent stability in oxidizing and reducing conditions. The increase of Ti-doping content in materials increases the thermal expansion coefficient (TEC), oxygen content change, and electrical conductivity in air, while it decreases the conductivity in reducing condition. All three materials are stable and compatible with studied electrolytes. Interestingly, redox stable Sr₂Fe_1.4Ti_0.1Mo_0.5O_6−δ, possessing 1 μm grain size, low TEC (15.3 × 10⁻⁶ K⁻¹), large oxygen content change of 0.72 mol·mol⁻¹ between 30 and 900 °C, satisfactory conductivity of 4.1–7.3 S·cm⁻¹ in 5% H₂ at 600–800 °C, and good transport coefficients D and k, could be considered as a potential anode material for SOFCs, and are thus of great interest for further studies. Full article

This paper presents a novel method based on a dual-cavity fiber Fabry–Pérot interferometer (DCFFPI) for simultaneously measuring the thermo-optic coefficient (TOC) and thermal expansion coefficient (TEC) of a polymer. The polymer is, by nature, highly responsive to temperature (T) in that its size (length, L) and refractive index (RI, n) are highly dependent on the thermal effect. When the optical length of the polymer cavity changes with T, it is difficult to distinguish whether there is a change in L or n, or both. The variation rates of L and n with a change in T were the TOC and TEC, respectively. Therefore, there was a cross-sensitivity between TOC and TEC in the polymer-based interferometer. The proposed DCFFPI, which cascades a polymer and an air cavity, can solve the above problem. The expansion of the polymer cavity is equal to the compression of the air cavity with the increase in T. By analyzing the individual optical spectra of the polymer and air cavities, the parameters of TOC and TEC can be determined at the same time. The simultaneous measurement of TOC and TEC with small measured deviations of 6 × 10⁻⁶ (°C⁻¹) and 3.67 × 10⁻⁵ (°C⁻¹) for the polymer NOA61 and 7 × 10⁻⁶ (°C⁻¹) and 1.46 × 10⁻⁴ (°C⁻¹) for the NOA65 can be achieved. Experimental results regarding the measured accuracy for the class of adhesive-based polymer are presented to demonstrate the feasibility and verify the usefulness of the proposed DCFFPI. Full article

The thermal expansion coefficient (TEC) of suspended two-dimensional (2D) nanomaterials is usually negative due to their ability for large out-of-plane deflection as the temperature increases. The presence of a substrate can nonetheless restrict the flexibility of 2D materials and significantly change their dimensional change by temperature. In this short communication, the thermal expansion coefficients of suspended and supported four popular 2D structures of graphene, phagraphene, C₃N and BC₃ monolayers is systematically investigated. For this purpose, we conduct molecular dynamics simulation, in which the atomic interactions are defined by highly accurate machine learning interatomic potentials. The obtained results show that by increasing the strength of the van der Waals interactions between the monolayer and the substrate, from 2 meV to 8 meV, the TEC for graphene and phagraphene increases from a negative value to a positive one; while the negative value for the C₃N and BC₃ structures is still retained. Analysis of molecular dynamics trajectories reveals that the substrate can significantly reduce the formation of out-of-plane wrinkles and consequently affect the value of TEC. The obtained results provide useful vision on the role of substrate on the complex thermal expansion responses of 2D materials. Full article

Nowadays, additive manufacturing (AM) is a powerful way to make complex-shaped components for airspace engineering from nickel-based superalloys. So, while nickel-based superalloys could easily be we L-DED in sheet-metal thicknesses, they suffered from strain-age cracking and solidification during AM or in the post-weld aging treatment. This is attributed to the fact that besides the limitation of γ′- phase forming elements (Al and Ti), as they form by AM very rapidly and reduce ductility, the majority of the superalloys contain carbide-forming elements such as Cr, Mo, and W. The precipitation of carbides, which is very effective in strengthening, develops cracks in the heat-affected zone (HAZ) during AM. The difference in isochoric heat capacities and the thermal expansion coefficient (TEC) at the phase boundary leads to the appearance of dangerous local destruction energy. If the area of the interfacial interface is sufficiently extended, then the accumulation of this energy reaches a level sufficient for a crack formation. We have proposed a crack initiation criterion (CIC) for assessing the dangerous level of fracture energy. The CIC was derived from an estimate of the local energy balance from the heat transfer equation for the two-phase area. Practical approbation of the criterion was carried out after L-DED of samples from Ni50Cr33W4.5Mo2.8TiAlNb (EP648) alloy powder with an increased carbon content based on the study of the chemical composition near the crack formed during solidification. Using the proposed criterion provides an opportunity to give the rank to carbide-forming elements according to the degree of their influence on the fracture energy. Thus, the release of aluminum carbide turned out to be 5.48 times more dangerous than the release of titanium carbide and more than 5 times more dangerous than the release of tungsten carbide and molybdenum. Full article

A continuous series of solid solutions (Bi_1.5Mg_0.75−xFe_xTa_1.5O_7±Δ (x = 0–0.75)) with the pyrochlore structure were synthesized with the solid-phase method. It was shown that iron, like magnesium, is concentrated in the structure in the octahedral position of tantalum. Doping with iron atoms led to an increase in the upper limit of the thermal stability interval of magnesium-containing pyrochlore from 1050 °C (x = 0) up to a temperature of 1140 °C (x = 1). The unit cell constant a and thermal expansion coefficient (TEC) increase uniformly slightly from 10.5018 Å up to 10.5761 Å and from 3.6 up to 9.3 × 10⁻⁶ °C⁻¹ in the temperature range 30–1100 °C. The effect of iron(III) ions on the thermal stability and thermal expansion of solid solutions was revealed. It has been established that the thermal stability of iron-containing solid solutions correlates with the unit cell parameter, and the lower the parameter, the more stable the compound. The TEC value, on the contrary, is inversely proportional to the cell constant. Full article

Samarium, gadolinium, and yttrium co-doped ceria (Ce_0.8Sm_0.16Y_0.03Gd_0.01O_1.9, CSYG) and BaIn_0.3Ti_0.7O_2.85 (BIT07) powders were prepared by sol-gel and solid-state reaction methods, respectively. CSYG-BIT07 composite materials were obtained by mechanically mixing the two powders in different ratios and calcining at 1300 °C for 5 h. Samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), as well as electrical properties and thermal expansion coefficient (TEC) measurements. A series of CSYG-BIT07 composite materials with relative densities higher than 95% were fabricated by sintering at 1300 °C for 5 h. The performance of the CSYG-BIT07 composite electrolyte was found to be related to the content of BIT07. The CSYG-15% BIT07 composite exhibited high oxide ion conductivity (σ_800°C = 0.0126 S·cm⁻¹ at 800 °C), moderate thermal expansion (TEC = 9.13 × 10⁻⁶/K between room temperature and 800 °C), and low electrical activation energy (E_a = 0.89 eV). These preliminary results indicate that the CSYG-BIT07 material is a promising electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Full article