Showing papers on "Thermal expansion published in 2022"

A review of band structure and material properties of transparent conducting and semiconducting oxides: Ga2O3, Al2O3, In2O3, ZnO, SnO2, CdO, NiO, CuO, and Sc2O3

Abstract: This Review highlights basic and transition metal conducting and semiconducting oxides. We discuss their material and electronic properties with an emphasis on the crystal, electronic, and band structures. The goal of this Review is to present a current compilation of material properties and to summarize possible uses and advantages in device applications. We discuss Ga2O3, Al2O3, In2O3, SnO2, ZnO, CdO, NiO, CuO, and Sc2O3. We outline the crystal structure of the oxides, and we present lattice parameters of the stable phases and a discussion of the metastable polymorphs. We highlight electrical properties such as bandgap energy, carrier mobility, effective carrier masses, dielectric constants, and electrical breakdown field. Based on literature availability, we review the temperature dependence of properties such as bandgap energy and carrier mobility among the oxides. Infrared and Raman modes are presented and discussed for each oxide providing insight into the phonon properties. The phonon properties also provide an explanation as to why some of the oxide parameters experience limitations due to phonon scattering such as carrier mobility. Thermal properties of interest include the coefficient of thermal expansion, Debye temperature, thermal diffusivity, specific heat, and thermal conductivity. Anisotropy is evident in the non-cubic oxides, and its impact on bandgap energy, carrier mobility, thermal conductivity, coefficient of thermal expansion, phonon modes, and carrier effective mass is discussed. Alloys, such as AlGaO, InGaO, (Al xIn yGa1− x− y)2O3, ZnGa2O4, ITO, and ScGaO, were included where relevant as they have the potential to allow for the improvement and alteration of certain properties. This Review provides a fundamental material perspective on the application space of semiconducting oxide-based devices in a variety of electronic and optoelectronic applications.

Hypocrystalline ceramic aerogels for thermal insulation at extreme conditions

Abstract: Thermal insulation under extreme conditions requires materials that can withstand complex thermomechanical stress and retain excellent thermal insulation properties at temperatures exceeding 1,000 degrees Celsius1-3. Ceramic aerogels are attractive thermal insulating materials; however, at very high temperatures, they often show considerably increased thermal conductivity and limited thermomechanical stability that can lead to catastrophic failure4-6. Here we report a multiscale design of hypocrystalline zircon nanofibrous aerogels with a zig-zag architecture that leads to exceptional thermomechanical stability and ultralow thermal conductivity at high temperatures. The aerogels show a near-zero Poisson's ratio (3.3 × 10-4) and a near-zero thermal expansion coefficient (1.2 × 10-7 per degree Celsius), which ensures excellent structural flexibility and thermomechanical properties. They show high thermal stability with ultralow strength degradation (less than 1 per cent) after sharp thermal shocks, and a high working temperature (up to 1,300 degrees Celsius). By deliberately entrapping residue carbon species in the constituent hypocrystalline zircon fibres, we substantially reduce the thermal radiation heat transfer and achieve one of the lowest high-temperature thermal conductivities among ceramic aerogels so far-104 milliwatts per metre per kelvin at 1,000 degrees Celsius. The combined thermomechanical and thermal insulating properties offer an attractive material system for robust thermal insulation under extreme conditions.

High-entropy perovskite RETa3O9 ceramics for high-temperature environmental/thermal barrier coatings

Abstract: Abstract Four high-entropy perovskite (HEP) RETa 3 O 9 samples were fabricated via a spark plasma sintering (SPS) method, and the corresponding thermophysical properties and underlying mechanisms were investigated for environmental/thermal barrier coating (E/TBC) applications. The prepared samples maintained low thermal conductivity (1.50 W·m −1 ·K −1 ), high hardness (10 GPa), and an appropriate Young’s modulus (180 GPa), while the fracture toughness increased to 2.5 MPa·m 1/2 . Nanoindentation results showed the HEP ceramics had excellent mechanical properties and good component homogeneity. We analysed the influence of different parameters (the disorder parameters of the electronegativity, ionic radius, and atomic mass, as well as the tolerance factor) of A-site atoms on the thermal conductivity. Enhanced thermal expansion coefficients, combined with a high melting point and extraordinary phase stability, expanded the applications of the HEP RETa 3 O 9 . The results of this study had motivated a follow-up study on tantalate high-entropy ceramics with desirable properties.

High-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Ce2O7: A potential thermal barrier material with improved thermo-physical properties

Abstract: Abstract High-entropy oxides (HEOs) are widely researched as potential materials for thermal barrier coatings (TBCs). However, the relatively low thermal expansion coefficient (TEC) of those materials severely restricts their practical application. In order to improve the poor thermal expansion property and further reduce the thermal conductivity, high-entropy (La 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Gd 0.2 ) 2 Ce 2 O 7 is designed and synthesized in this work. The as-prepared multicomponent material is formed in a simple disordered fluorite structure due to the high-entropy stabilization effect. Notably, it exhibits a much higher TEC of approximately 12.0 × 10 −6 K −1 compared with those of other high-entropy oxides reported in the field of TBCs. Besides, it presents prominent thermal insulation behavior with a low intrinsic thermal conductivity of 0.92 W·m −1 ·K −1 at 1400 °C, which can be explained by the existence of high concentration oxygen vacancies and highly disordered arrangement of multicomponent cations in the unique high-entropy configuration. Through high-temperature in-situ X-ray diffraction (XRD) measurement, this material shows excellent phase stability up to 1400 °C. Benefiting from the solid solution strengthening effect, it shows a higher hardness of 8.72 GPa than the corresponding single component compounds. The superior thermo-physical performance above enables (La 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Gd 0.2 ) 2 Ce 2 O 7 a promising TBC material.

Oriented BN/Silicone rubber composite thermal interface materials with high out-of-plane thermal conductivity and flexibility

Abstract: Polymer composites with high out-of-plane thermal conductivity, flexibility, and low modulus have always been one of the ideal thermal interface materials. Using softer polymer composites can reduce thermal contact resistance and stress caused by the mismatch of the coefficient of thermal expansion, as well as heat can be quickly transferred due to its high out-of-plane thermal conductivity. Herein, we report a silicone rubber-based composites with high out-of-plane thermal conductivity and softness prepared by combining shear orientation and layer-by-layer stacking methods. The composites exhibit an out-of-plane thermal conductivity of 7.62 Wm-1K−1, while maintain the flexibility and high elastic recovery of silicone rubber. According to finite element simulation analysis, the increase in thermal conductivity comes from the orientation of the BN flakes and the formation of the thermal network. This work provides a simple method for the preparation of flexible thermal interface materials with high out-of-plane thermal conductivity for many potential applications.

High-entropy rare-earth zirconate ceramics with low thermal conductivity for advanced thermal-barrier coatings

Abstract: Abstract The high-entropy rare-earth zirconate ((La 0.2 Nd 0.2 Sm 0.2 Gd 0.2 Yb 0.2 ) 2 Zr 2 O 7 , 5RE 2 Zr 2 O 7 HEREZs) ceramics were successfully prepared by a new high-speed positive grinding strategy combined with solid-state reaction method. The microstructure, crystal structure, phase composition, and thermophysical and mechanical properties of the samples were systematically investigated through various methods. Results indicate that the samples have a single-phase defect fluorite-type crystal structure with excellent high-temperature thermal stability. The as-prepared samples also demonstrate low thermal conductivity (0.9–1.72 W·m −1 ·K −1 at 273–1273 K) and high coefficient of thermal expansion (CTE, 10.9 × 10 −6 K −1 at 1273 K), as well as outstanding mechanical properties including large Young’s modulus ( E = 186–257 GPa) and high fracture toughness ( K IC ). Furthermore, the formation possibility of the as-prepared samples was verified through the first-principles calculations, which suggested the feasibility to form the 5RE 2 Zr 2 O 7 HE-REZs in the thermodynamic direction. Therefore, in view of the excellent multifunctional properties exhibited by the as-prepared 5RE 2 Zr 2 O 7 HE-REZs, they have great potential applications in next-generation thermal-barrier coatings (TBCs).

Single-phase rare-earth high-entropy zirconates with superior thermal and mechanical properties

Abstract: Emerging of high-entropy ceramics has brought new opportunities for designing and optimizing materials with desired properties. In the present work, high-entropy rare-earth zirconates (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 and (Yb0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 are designed and synthesized. Both high-entropy ceramics exhibit a single pyrochlore structure with excellent phase stability at 1600 °C. In addition, the Yb-containing system possesses a high coefficient of thermal expansion (10.52 × 10−6 K-1, RT∼1500 °C) and low thermal conductivity (1.003 W·m-1 K-1, 1500 °C), as well as excellent sintering resistance. Particularly, the Yb-containing system has significantly improved fracture toughness (1.80 MPa·mm1/2) when compared to that of lanthanum zirconate (1.38 MPa·mm1/2), making it a promising material for thermal barrier coatings (TBCs) applications. The present work indicates that the high-entropy design can be applied for further optimization of the comprehensive properties of the TBCs materials.

Calcium-magnesium-alumina-silicate (CMAS) resistant high entropy ceramic (Y0.2Gd0.2Er0.2Yb0.2Lu0.2)2Zr2O7 for thermal barrier coatings

Abstract: A novel high-entropy material, (Y0.2Gd0.2Er0.2Yb0.2Lu0.2)2Zr2O7 was successfully synthesized by the solid state reaction method and spark plasma sintering, and investigated as a promising thermal barrier coating material. Rare-earth elements were distributed homogeneously in the pyrochlore structure. It was found that the prepared high-entropy ceramic maintains pyrochlore structure at the temperature up to 1600 °C, and it possesses a similar thermal expansion coefficient (10.2 × 10−6 K−1 at 25–900 °C) to that of YSZ, low thermal conductivity (< 0.9 W m−1 K−1 at 100–1000 °C) and good CMAS resistance (infiltration depth is 22 μm after annealed at 1300 °C for 24 h). The corrosion process was investigated, and RE elements distributing homogeneously in (Y0.2Gd0.2Er0.2Yb0.2Lu0.2)2Zr2O7 show different diffusion rates in CMAS. RE3+ with a larger radius (closer to Ca2+) is easier to react with CMAS to form an apatite phase.

Density-functional-theory predictions of mechanical behaviour and thermal properties as well as experimental hardness of the Ga-bilayer Mo2Ga2C

Abstract: Abstract Mo 2 Ga 2 C is a new MAX phase with a stacking Ga-bilayer as well as possible unusual properties. To understand this unique MAX phase structure and promote possible future applications, the structure, chemical bonding, and mechanical and thermodynamic properties of Mo 2 Ga 2 C were investigated by first-principles. Using the “bond stiffness” model, the strongest covalent bonding (1162 GPa) was formed between Mo and C atoms in Mo 2 Ga 2 C, while the weakest Ga-Ga (389 GPa) bonding was formed between two Ga-atomic layers, different from other typical MAX phases. The ratio of the bond stiffness of the weakest bond to the strongest bond (0.33) was lower than 1/2, indicating the high damage tolerance and fracture toughness of Mo 2 Ga 2 C, which was confirmed by indentation without any cracks. The high-temperature heat capacity and thermal expansion of Mo 2 Ga 2 C were calculated in the framework of quasi-harmonic approximation from 0 to 1300 K. Because of the metal-like electronic structure, the electronic excitation contribution became more significant with increasing temperature above 300 K.

Nanomechanical probing and strain tuning of the Curie temperature in suspended Cr2Ge2Te6-based heterostructures

Abstract: Abstract Two-dimensional magnetic materials with strong magnetostriction are attractive systems for realizing strain-tuning of the magnetization in spintronic and nanomagnetic devices. This requires an understanding of the magneto-mechanical coupling in these materials. In this work, we suspend thin Cr 2 Ge 2 Te 6 layers and their heterostructures, creating ferromagnetic nanomechanical membrane resonators. We probe their mechanical and magnetic properties as a function of temperature and strain by observing magneto-elastic signatures in the temperature-dependent resonance frequency near the Curie temperature, T C . We compensate for the negative thermal expansion coefficient of Cr 2 Ge 2 Te 6 by fabricating heterostructures with thin layers of WSe 2 and antiferromagnetic FePS 3 , which have positive thermal expansion coefficients. Thus we demonstrate the possibility of probing multiple magnetic phase transitions in a single heterostructure. Finally, we demonstrate a strain-induced enhancement of T C in a suspended Cr 2 Ge 2 Te 6 -based heterostructure by 2.5 ± 0.6 K by applying a strain of 0.026% via electrostatic force.

Vertical Alignment of Anisotropic Fillers Assisted by Expansion Flow in Polymer Composites

Abstract: Orientation control of anisotropic one-dimensional (1D) and two-dimensional (2D) materials in solutions is of great importance in many fields ranging from structural materials design, the thermal management, to energy storage. Achieving fine control of vertical alignment of anisotropic fillers (such as graphene, boron nitride (BN), and carbon fiber) remains challenging. This work presents a universal and scalable method for constructing vertically aligned structures of anisotropic fillers in composites assisted by the expansion flow (using 2D BN platelets as a proof-of-concept). BN platelets in the silicone gel strip are oriented in a curved shape that includes vertical alignment in the central area and horizontal alignment close to strip surfaces. Due to the vertical orientation of BN in the central area of strips, a through-plane thermal conductivity as high as 5.65 W m-1 K-1 was obtained, which can be further improved to 6.54 W m-1 K-1 by combining BN and pitch-based carbon fibers. The expansion-flow-assisted alignment can be extended to the manufacture of a variety of polymer composites filled with 1D and 2D materials, which can find wide applications in batteries, electronics, and energy storage devices.