Showing papers on "Thermal expansion published in 2018"

Thermal and Chemical Expansion in Proton Ceramic Electrolytes and Compatible Electrodes

Abstract: This review paper focuses on the phenomenon of thermochemical expansion of two specific categories of conducting ceramics: Proton Conducting Ceramics (PCC) and Mixed Ionic-Electronic Conductors (MIEC) The theory of thermal expansion of ceramics is underlined from microscopic to macroscopic points of view while the chemical expansion is explained based on crystallography and defect chemistry Modelling methods are used to predict the thermochemical expansion of PCCs and MIECs with two examples: hydration of barium zirconate (BaZr1−xYxO3−δ) and oxidation/reduction of La1−xSrxCo02Fe08O3−δ While it is unusual for a review paper, we conducted experiments to evaluate the influence of the heating rate in determining expansion coefficients experimentally This was motivated by the discrepancy of some values in literature The conclusions are that the heating rate has little to no effect on the obtained values Models for the expansion coefficients of a composite material are presented and include the effect of porosity A set of data comprising thermal and chemical expansion coefficients has been gathered from the literature and presented here divided into two groups: protonic electrolytes and mixed ionic-electronic conductors Finally, the methods of mitigation of the thermal mismatch problem are discussed

A study of thermal expansion coefficients and microstructure during selective laser melting of Invar 36 and stainless steel 316L

Abstract: This paper presents an experimental study on the metallurgical issues associated with selective laser melting of Invar 36 and stainless steel 316 L and the resulting coefficient of thermal expansion. Invar 36 has been used in aircraft control systems, electronic devices, optical instruments, and medical instruments that are exposed to significant temperature changes. Stainless steel 316 L is commonly used for applications that require high corrosion resistance in the aerospace, medical, and nuclear industries. Both Invar 36 and stainless steel 316 L are weldable austenitic face-centered cubic crystal structures, but stainless steel 316 L may experience chromium evaporation and Invar 36 may experience weld cracking during the welding process. Various laser process parameters were tested based on a full factorial design of experiments. The microstructure, material composition, coefficient of thermal expansion, and magnetic dipole moment were measured for both materials. It was found that there exists a critical laser energy density for each material, EC, for which selective laser melting process is optimal for material properties. The critical laser energy density provides enough energy to induce stable melting, homogeneous microstructure and chemical composition, resulting in thermal expansion and magnetic properties in line with that expected for the wrought material. Below the critical energy, a lack of fusion due to insufficient melt tracks and discontinuous beads was observed. The melt track was also unstable above the critical energy due to vaporization and microsegregation of alloying elements. Both cases can generate stress risers and part flaws during manufacturing. These flaws could be avoided by finding the critical laser energy needed for each material. The critical laser energy density was determined to be 86.8 J/mm3 for Invar 36 and 104.2 J/mm3 for stainless steel 316 L.

Synthesis and thermophysical properties of RETa3O9 (RE = Ce, Nd, Sm, Eu, Gd, Dy, Er) as promising thermal barrier coatings

Abstract: Thermal barrier coatings (TBCs) are one of the most important materials in gas turbine to protect the high temperature components. RETa3O9 compounds have a defect-perovskite structure, indicating that they have low thermal conductivity, which is the critical property of TBCs. Herein, dense RETa3O9 bulk ceramics were fabricated via solid-state reaction. The crystal structure was characterized by X-ray diffraction (XRD) and Raman Spectroscope. Scanning electron microscope (SEM) was used to observe the microstructure. The thermal physics properties of RETa3O9 were studied systematically, including specific heat, thermal diffusivity, thermal conductivity, thermal expansion coefficients and high-temperature phase stability. The thermal conductivities of RETa3O9 are very low (1.33-2.37 W/m.K, 373-1073 K), which are much lower than YSZ and La2Zr2O7; and the thermal expansion coefficients range from 4.0×10-6 K−1 to 10.2×10-6 K−1 (1273 K), which is close to La2Zr2O7 and YSZ. According to the differential scanning calorimetry (DSC) curve there is not phase transition at the test temperature. Due to the high melting point and excellent high-temperature phase stability with these oxides, RETa3O9 ceramics were promising candidate materials for TBCs. This article is protected by copyright. All rights reserved.

Ultralow Thermal Conductivity and Ultrahigh Thermal Expansion of Single-Crystal Organic–Inorganic Hybrid Perovskite CH3NH3PbX3 (X = Cl, Br, I)

Abstract: Improving device lifetime and stability remains the stumbling block of the commercialization of hybrid perovskite-based devices (HPDs). Although extensive efforts have been paid, thermal property, one of the most crucial parameters in conventional solid-state electronic devices, has rarely been studied for HPDs. Here, we investigate the temperature-dependent ultralow thermal conductivity and ultrahigh thermal expansion of single-crystalline MAPbX3 (MA = CH3NH3), which are found distinct from traditional thin-film solar cells materials. Particularly, for MAPbI3, thermal conductivity is observed being only 0.3 W·m–1·K–1 and linear thermal expansion coefficient along [100] direction is as high as 57.8 × 10–6 K–1 (tetragonal) and much higher at the structural phase transition point. We attribute the ultralow thermal conductivity and ultrahigh thermal expansion to the weak chemical bonds associated with the soft perovskite materials. These unique properties can be very challenging for the multilayer device des...

Nuclear quantum effect with pure anharmonicity and the anomalous thermal expansion of silicon

Abstract: Despite the widespread use of silicon in modern technology, its peculiar thermal expansion is not well understood. Adapting harmonic phonons to the specific volume at temperature, the quasiharmonic approximation, has become accepted for simulating the thermal expansion, but has given ambiguous interpretations for microscopic mechanisms. To test atomistic mechanisms, we performed inelastic neutron scattering experiments from 100 K to 1,500 K on a single crystal of silicon to measure the changes in phonon frequencies. Our state-of-the-art ab initio calculations, which fully account for phonon anharmonicity and nuclear quantum effects, reproduced the measured shifts of individual phonons with temperature, whereas quasiharmonic shifts were mostly of the wrong sign. Surprisingly, the accepted quasiharmonic model was found to predict the thermal expansion owing to a large cancellation of contributions from individual phonons.

Zero Thermal Expansion in Magnetic and Metallic Tb(Co,Fe)2 Intermetallic Compounds

Abstract: Due to the advantage of invariable length with temperatures, zero thermal expansion (ZTE) materials are intriguing but very rare especially for the metals based compounds. Here, we report a ZTE in the magnetic intermetallic compounds of Tb(Co,Fe)2 over a wide temperature range (123–307 K). A negligible coefficient of thermal expansion (αl = 0.48 × 10–6 K–1) has been found in Tb(Co1.9Fe0.1). Tb(Co,Fe)2 exhibits ferrimagnetic structure, in which the moments of Tb and Co/Fe are antiparallel alignment along the c axis. The intriguing ZTE property of Tb(Co,Fe)2 is formed due to the balance between the negative contribution from the Tb magnetic moment induced spontaneous magnetostriction and the positive role from the normal lattice expansion. The present ZTE intermetallic compounds are also featured by the advantages of wide temperature range, high electrical conductivity, and relatively high thermal conductivity.

A coupled thermo-mechanical bond-based peridynamics for simulating thermal cracking in rocks

Abstract: A coupled thermo-mechanical bond-based peridynamical (TM-BB-PD) method is developed to simulate thermal cracking processes in rocks. The coupled thermo-mechanical model consists of two parts. In the first part, temperature distribution of the system is modeled based on the heat conduction equation. In the second part, the mechanical deformation caused by temperature change is calculated to investigate thermal fracture problems. The multi-rate explicit time integration scheme is proposed to overcome the multi-scale time problem in coupled thermo-mechanical systems. Two benchmark examples, i.e., steady-state heat conduction and transient heat conduction with deformation problem, are performed to illustrate the correctness and accuracy of the proposed coupled numerical method in dealing with thermo-mechanical problems. Moreover, two kinds of numerical convergence for peridynamics, i.e., m- and $$\delta $$ -convergences, are tested. The thermal cracking behaviors in rocks are also investigated using the proposed coupled numerical method. The present numerical results are in good agreement with the previous numerical and experimental data. Effects of PD material point distributions and nonlocal ratios on thermal cracking patterns are also studied. It can be found from the numerical results that thermal crack growth paths do not increases with changes of PD material point spacing when the nonlocal ratio is larger than 4. The present numerical results also indicate that thermal crack growth paths are slightly affected by the arrangements of PD material points. Moreover, influences of thermal expansion coefficients and inhomogeneous properties on thermal cracking patterns are investigated, and the corresponding thermal fracture mechanism is analyzed in simulations. Finally, a LdB granite specimen with a borehole in the heated experiment is taken as an application example to examine applicability and usefulness of the proposed numerical method. Numerical results are in good agreement with the previous experimental and numerical results. Meanwhile, it can be found from the numerical results that the coupled TM-BB-PD has the capacity to capture phenomena of temperature jumps across cracks, which cannot be captured in the previous numerical simulations.

Elastic modulus and coefficient of thermal expansion of piezoelectric Al1−xScxN (up to x = 0.41) thin films

Abstract: Aluminum scandium nitride (Al1−xScxN with x = 0–0.41) thin films were deposited by reactive pulsed-DC magnetron sputtering on Si(001) and Al2O3(0001) substrates. X-ray diffraction indicated high degree of c-axis orientation in all the films, and based on pole figure measurements, epitaxial relationship could be defined as [101¯0]AlScN//[112¯0]sapphire and (0001)AlScN//(0001)sapphire in films deposited on Al2O3. Piezoelectric coefficient increased up to d33 = 31.6 pC/N in Al0.59Sc0.41N, which is 550% higher than for AlN. The biaxial elastic modulus and the in-plane coefficient of thermal expansion (CTE) as a function of Sc concentration were determined by thermal cycling method: biaxial elastic modulus decreased from 535 GPa in pure AlN to 269 GPa in Al0.59Sc0.41N and CTE was 4.65 × 10−6 K−1 for AlN and 4.29 × 10−6 K−1 for Al0.59Sc0.41N. Additionally, we observed an increase in CTE from 4.18 × 10−6 K−1 at 65 °C to up to 6.38 × 10−6 K−1 at 375 °C for Al0.68Sc0.32N. The experimentally determined CTE and elastic modulus allow a more precise design of Al1−xScxN-based frequency filters which are used in mobile communications and are important parameters for the prediction of device performance at elevated temperatures.

Mapping Thermal Expansion Coefficients in Freestanding 2D Materials at the Nanometer Scale.

Abstract: Two-dimensional materials, including graphene, transition metal dichalcogenides and their heterostructures, exhibit great potential for a variety of applications, such as transistors, spintronics, and photovoltaics. While the miniaturization offers remarkable improvements in electrical performance, heat dissipation and thermal mismatch can be a problem in designing electronic devices based on two-dimensional materials. Quantifying the thermal expansion coefficient of 2D materials requires temperature measurements at nanometer scale. Here, we introduce a novel nanometer-scale thermometry approach to measure temperature and quantify the thermal expansion coefficients in 2D materials based on scanning transmission electron microscopy combined with electron energy-loss spectroscopy to determine the energy shift of the plasmon resonance peak of 2D materials as a function of sample temperature. By combining these measurements with first-principles modeling, the thermal expansion coefficients (TECs) of single-layer and freestanding graphene and bulk, as well as monolayer MoS_{2}, MoSe_{2}, WS_{2}, or WSe_{2}, are directly determined and mapped.

Effect of replacement of Al 2 O 3 by Y 2 O 3 on the structure and properties of alkali-free boro-aluminosilicate glass

Abstract: Alkali-free boro-aluminosilicate glasses were prepared by the melt quenching method. The effect of Al2O3 replaced by different contents of Y2O3 on the structure and various properties of alkali-free boro-aluminosilicate glasses was studied. The changes of glass structure with varying components were investigated by the Fourier transform infrared spectra. Simultaneously, the relationship between the glass structure and the coefficient of thermal expansion, density, vickers hardness, bending strength, glass transmittance and chemical stability was discussed. The results show that the coefficient of thermal expansion and the density of glass increase with the increase of Y2O3 content. While, the chemical stability of the glass is reduced. The vickers hardness and bending strength decrease first and then increase with the Y2O3 content increasing. All the glasses have excellent optical transmittance, reaching more than 85%.

Thermal Properties of SiOC Glasses and Glass Ceramics at Elevated Temperatures

Abstract: In the present study, the effect of the chemical and phase composition on the thermal properties of silicon oxide carbides (SiOC) has been investigated. Dense monolithic SiOC materials with various carbon contents were prepared and characterized with respect to their thermal expansion, as well as thermal conductivity. SiOC glass has been shown to exhibit low thermal expansion (e.g., ca. 3.2 × 10−6 K−1 for a SiOC sample free of segregated carbon) and thermal conductivity (ca. 1.5 W/(m∙K)). Furthermore, it has been observed that the phase separation, which typically occurs in SiOC exposed to temperatures beyond 1000–1200 °C, leads to a decrease of the thermal expansion (i.e., to 1.83 × 10−6 K−1 for the sample above); whereas the thermal conductivity increases upon phase separation (i.e., to ca. 1.7 W/(m∙K) for the sample mentioned above). Upon adjusting the amount of segregated carbon content in SiOC, its thermal expansion can be tuned; thus, SiOC glass ceramics with carbon contents larger than 10–15 vol % exhibit similar coefficients of thermal expansion to that of the SiOC glass. Increasing the carbon and SiC content in the studied SiOC glass ceramics leads to an increase in their thermal conductivity: SiOC with relatively large carbon and silicon carbides (SiC) volume fractions (i.e., 12–15 and 20–30 vol %, respectively) were shown to possess thermal conductivities in the range from 1.8 to 2.7 W/(m∙K).

Routes to program thermal expansion in three-dimensional lattice metamaterials built from tetrahedral building blocks

Abstract: Thermal expansion can be problematic in manifold applications that require thermal stability, yet it can also be purposely exploited to meet specific directional requirements of thermal deformation. Opportunities to tailor thermal expansion in architected materials exist, but design options that are stiff and provide full directional authority on thermal expansion are currently limited by the structural characteristics of existing concepts. In this work, we report routes to systematically engineer thermally responsive lattice materials that are built from dual-material tetrahedral units that are stiff and strong. Drawing from concepts of vector analysis, crystallography, and tessellation, a scheme is presented for three-dimensional lattices to program desired magnitude and spatial directionality, such as unidirectional, transverse isotropic, or isotropic, of thermal expansion. Demonstrations on thermal expansion and mechanical properties include theoretical, computational, and experimental studies of nine representative concepts, from tetrahedral building blocks to compound unit cells that can tessellate structurally efficient lattices with tunable magnitude and prescribed directionality of thermal expansion.