Small Thermal Expansion Coefficient Research Articles

Study of freeze-thaw deterioration of saturated and unsaturated hydraulic concrete based on measured strain

In actual concrete water projects, areas such as the back surface, above-water surface, and even areas where the water level fluctuates are in the unsaturated freeze-thaw state. The complexity of freeze-thaw tests of unsaturated hydraulic concrete has thus far prevented a sound understanding of the mechanisms at work in freeze-thaw deterioration of unsaturated hydraulic concrete. Given this situation, this study first compares three different unsaturated hydraulic concrete freeze-thaw test processes (polyurea seal, paraffin seal, and vacuum-bag seal) and then uses the vacuum-bag seal method to implement freeze-thaw tests of unsaturated hydraulic concrete. Afterward, three test schemes (unsaturated sealed freeze-thaw, water-freeze-thaw without air-entraining agent, and water-freeze-thaw with air-entraining agents) are conducted and the mechanisms of freeze-thaw deterioration of saturated and unsaturated hydraulic concrete are evaluated based on the measured temperature and strain of the concrete specimens. The results reveal a continuous and irreversible deterioration leading to the freeze-thaw damage of unsaturated sealed freeze-thaw specimens and water-freeze-thaw specimens with and without an air-entraining agent. After 200 freeze-thaw cycles, the cumulative residual strain of the aforementioned three test schemes reach 47 με, 616 με, and 273 με, respectively. The thermal expansion coefficient of concrete remains relatively constant with increasing freeze-thaw cycles. Water-freeze-thaw specimens without air-entraining agents have a greater thermal expansion coefficient than those with air-entraining agents, and unsaturated sealed freeze-thaw specimens have the smallest thermal expansion coefficient. The frost heave coefficient increases with increasing freeze-thaw cycles, and water-freeze-thaw specimens without air-entraining agents have greater frost heave coefficients than those with air-entraining agents.

Superconductivity in Monolayer Carbon Allotropes with High Thermal Stability.

Intrinsic superconductivity is rarely discovered in sp2-hybridized monolayer carbon allotropes. Here we design a carbon monolayer configured of pentagon, heptagon, and hexagon rings with p2 plane group symmetry. Full-sp2 hybridization is proposed to favor thermal metastability on a low Gibbs free energy. The extremely small thermal expansion coefficient is predicted to the turn negative value to positive with elevating temperature. Carbon polygon structures remain intact at a high thermal temperature of 3,000 K. The high specific surface area is found to approach 2,700 m2/g, with O2-adsorption being advantageous over pristine graphene. We reveal electronic Fermi surfaces mediated by phonon modes of carbon out-of-plane vibrations. By calculating the Eliashberg equation, we evaluate intrinsic superconductivity with a large electron-phonon coupling coefficient. The superconducting transition temperature is estimated to reach 20 K under a high logarithmic average frequency. These first-principles calculations shall stimulate experimentalists' interest in exploring low-dimensional carbon superconductors with gas sensitivity.

Yttria substitution effect of tantalate aluminosilicate glasses with ultra‐high Young's modulus

AbstractWe report novel oxide glasses that are compatible with ultra‐high Young's modulus (∼150 GPa) and a small coefficient of thermal expansion (∼5.0 ppm/K). These glasses were prepared using a conventional melt‐quenching technique. The viscosity–temperature relation provides the reshaping temperature for the stiff and less dilated oxide glasses. The origin of the ultra‐high Young's modulus is the inclusion of Y2O3 as well as five‐ and six‐coordinated alumina oxide and Ta2O5, which has a large ionic packing ratio due to higher oxygen coordination numbers. On the other hand, the small thermal expansion coefficient originates from including Ta2O5 and SiO2 with large dissociation energy due to strong bonds.

The Numerical Simulations and Experimental Study of an 8-Inch SiC Single Crystal with Reduced BPD Density.

The basal plane dislocation (BPD) density is one of the most important defects affecting the application of SiC wafers. In this study, numerical simulations and corresponding experiments were conducted to investigate the influence of cooling processes, seed-bonding methods, and graphite crucible materials on the BPD density in an 8-inch N-type 4H-SiC single crystal grown by the physical vapor transport (PVT) method. The results showed that the BPD density could be effectively reduced by increasing the cooling rate, optimizing the seed-bonding method, and adopting a graphite crucible with a similar coefficient of thermal expansion as the SiC single crystal. The BPD density in the experiments showed that a high cooling rate reduced the BPD density from 4689 cm-2 to 2925 cm-2; optimization of the seed-bonding method decreased the BPD density to 1560 cm-2. The BPD density was further reduced to 704 cm-2 through the adoption of a graphite crucible with a smaller thermal expansion coefficient.

Influence of the TiN diffusion barrier on the leakage current and ferroelectricity in an Al-doped HfOx ferroelectric memristor and its application to neuromorphic computing.

The HfOx-based ferroelectric memristor is in the spotlight due to its complementary metal-oxide-semiconductor compatibility and scaling compared to existing perovskite-based ferroelectric memory. However, ferroelectric properties vary depending on the coefficient of thermal expansion of the top electrode, which is caused by strain engineering. When tungsten (W) with a small coefficient of thermal expansion is used as an electrode, the ferroelectric properties are improved, although the reliability is poor due to the diffusion of W atoms. Here, TiN can be used to prevent the diffusion of W. This metal nitride successfully suppresses the leakage current and induces a larger remanent polarization of 19.7 μC cm-2, a smaller coercive voltage of 9.26 V, and a faster switching speed. W/TiN/HAO/n+ Si can also exhibit multi-level characteristics and achieve a 10% read margin in 320 × 320 arrays. Ferroelectrics can also be applied to neuromorphic computing by imitating synaptic properties such as potentiation, depression, paired-pulse facilitation, and excitatory postsynaptic current. Using short-term plasticity, successful implementation in reservoir computing is also realized, achieving 95% classification accuracy. This paper shows promise for the use of memristors in artificial neural networks.

CFRP drive shaft damage identification and localization based on FBG sensing network and GWO-BP neural networks

Carbon Fiber Reinforced Plastic (CFRP) has the advantages of light weight, high strength, small coefficient of thermal expansion, good vibration damping performance, etc., the use of its production drive shaft can reduce weight, improve the intrinsic frequency, transmission efficiency and precision, and has been used in helicopters, heavy duty machine tools, wind turbines, ships and other high-end equipment. Once the damage occurs, it will cause adverse effects such as reduction of mechanical efficiency of the system, degradation of transmission system performance, and decrease of reliability and safety. The existing non-destructive testing technology for carbon fiber reinforced plastic has limitations such as inconvenient use, complex equipment, and no real-time online monitoring. To address this problem, this paper adopts Fiber Bragg Gratings (FBG) sensors to obtain real-time strain field data of CFRP drive shaft, and uses the linear and nonlinear mapping ability of Back Propagation Neural Network (BP neural network) to identify and locate the damage of CFRP drive shaft. At the same time, the GWO optimization algorithm is used to optimize the BP neural network to construct a CFRP drive shaft damage identification and localization system based on FBG sensing network and GWO-BP neural network. The experiments show that the CFRP drive shaft damage identification and localization system constructed in this paper can obtain the CFRP drive shaft strain field information in real time and accurately identify the damage and its location.

A numerical study on high-velocity impact behaviour of ceramic composite laminates

Currently, silicon carbide (SiC) is of enormous concern because of its high compressive strength,high thermal conductivity, high thermal shock resistance and small coefficient of thermal expansion. SiC has its applicability in armored vehicles, hypersonic armaments, and bullet-resistant armours in the future. The current focus is on understanding the behavior of SiC as used in laminated ceramic armours during the ballistic impact for its profound use in armoured vehicles. To achieve these objectives, computational simulations have been performed for the aforementioned materials using ANSYS/AUTODYN. It was concluded that when SiC was used as a front layer in laminated armours, it significantly increased the strength of the material against the hitting projectiles. The aforementioned composite material was even successful against the sniper gun projectile. The optimized thickness for the given configuration was found to be 41 mm with the consideration of approximately 50% depth of penetration. The modeled configuration was found to be approximately nine times stronger than the conventional laminated armours. The current study was compared with the previous experimental studies and the results were in good agreement. This agreement solely pertains to the pattern observed and not to individual values. It indicated the difference in % age DOP for the present research and previous studies with the increasing thickness of laminated armour. The trend observed was nearly similar to previous experimental studies. This showed that the DOP was reduced, as the thickness increased. Moreover, the previous studies showed that only ALON (aluminium-oxynitride) and sapphire had the required strength to act as a hard front-faced layer in laminated glass, but the current study improvised it to a Silicon carbide hard front-faced layer also.

Experimental Study on the Influence of Ultraviolet Laser Parameters on the Micro Machining Quality of Silicon Carbide Ceramic

Silicon carbide(SiC) ceramics have great application value in microelectronics and semiconductors due to their small dielectric constant, high thermal conductivity, and small thermal expansion coefficient. However, due to its high hardness and brittleness, defects such as stress concentration, microcracks, and an excessively large heat-affected zone are prone to occur in traditional machining methods. In this paper, the ultra-thin SiC surface was micro-grooved by ultraviolet(UV) laser. The effects and rules of various machining parameters (scanning speed, repetition frequency, pulse width, defocus amount) in UV laser micro-grooves machining on the surface quality, machining depth, and cross-sectional morphology of SiC were studied. The experimental results showed that the surface roughness of SiC micro-grooves decrease with the increase of scanning speed; The relatively small surface roughness (R a 0.72 μm) could be obtained at a scanning speed of 500 mm s−1 and a repetition frequency of 40 kHz; Under low pulse width of 5 ns, high repetition frequency of 50 kHz, and high scanning speed of 500 mm s−1, micro-grooves with a relative error of 2.3% in removal depth could be obtained; The taper of the micro-groove section first decreases and then increases with the defocus from negative to positive, and reached the minimum value of 5° when the defocus positive 0.6 mm.

Thermal expansion behavior of vanadium pernitride, CuAl2-type VN2, synthesized under high pressures

CuAl2-type VN2, which is synthesized under high pressure, is a recoverable material at ambient conditions and has a high bulk modulus. In this study, we investigated the thermal expansion behavior of CuAl2-type VN2 by low-temperature X-ray diffraction measurements between 109.3(5) K and 298.3(8) K. The axial thermal expansion coefficient of VN2 was determined to be αa = 2.7(9) × 10−6 K−1 and αc = 17.8(12) × 10−6 K−1 at 298.3(8) K, which has large anisotropy similar to that of compression behavior. It is found that the small coefficient of thermal expansion of the a-axis is due to the negative and positive effects on the a-axis length with increasing temperature of the bond angles and bond lengths of VN2, respectively. As a result, VN2 exhibits very large anisotropic thermal expansion behavior.

Preparation and Optical Properties of Nd:Bi2Ti2O7 Laser Crystal with Disordered Structure and Attractive Multiwavelength Emission Characteristics.

Laser crystals with multiwavelength emission characteristics are potential light sources for terahertz radiation. Herein, the pure and Nd-doped Bi2Ti2O7 (BTO) laser crystals with sizes up to 16 × 13 × 5 mm3 were successfully grown using the flux method in the KF-B2O3-CaBi4Ti4O15 growth system. The crystal structure, ideal morphology, chemical, mechanical, and thermal properties, optical transmission and Raman spectra, refractive index, absorption, and fluorescence spectra, as well as fluorescence lifetimes, were systematically studied. Besides, the spectral parameters of Nd3+ ions in the BTO crystal were systematically calculated based on the Judd-Ofelt theory. The Nd:BTO crystal has a wide transmittance range (0.44-7.30 μm), a small coefficient of thermal expansion (5.80 × 10-6 K-1), and a large absorption full width at half-maximum (fwhm) (31.2 nm) at around ∼804 nm, making it more potential for use in high-power laser systems. Moreover, fluorescence spectra show four emission peaks at 1054, 1062, 1104, and 1112 nm. The strong multiwavelength emission property makes Nd:BTO a promising laser crystal, serving as a potential light source for terahertz radiation.

Tribological behaviors of tantalum prepared by SPS under different loads

PurposeTantalum is a kind of metal material with moderate hardness, high ductility, small thermal expansion coefficient, excellent corrosion resistance and outstanding biocompatibility. The purpose of this study is that its tribological performance could be tested and analyzed so as to use it in different fields.Design/methodology/approachThe friction resistance of a-Ta under dry friction conditions was tested at different roads. The relationships between load and friction coefficient, wear rate and two-dimensional shape of wear scars were studied.FindingsThe stable Ta2O5 film with lubrication effect was generated in the process of friction. And, the larger the test load, the more Ta2O5 would be generated.Originality/valueThis work lays a theoretical foundation for tantalum as an excellent wear-resistant material.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-02-2023-0047/

Effects of electrode materials on the performance of Zr0.75Hf0.25O2-based ferroelectric thin films via post deposition annealing

In this work, the effects of top electrode (TE) and bottom electrode (BE) on the ferroelectric properties of zirconia-based Zr0.75Hf0.25O2 (ZHO) thin films annealed by post-deposition annealing (PDA) are investigated in detail. Among W/ZHO/BE capacitors (BE = W, Cr or TiN), W/ZHO/W delivered the highest ferroelectric remanent polarization and the best endurance performance, revealing that the BE with a smaller coefficient of thermal expansion (CTE) plays a vital role in enhancing the ferroelectricity of fluorite-structure ZHO. For TE/ZHO/W structures (TE = W, Pt, Ni, TaN or TiN), the stability of TE metals seems to have a larger impact on the performance over their CTE values. This work provides a guideline to modulate and optimize the ferroelectric performance of PDA-treated ZHO-based thin films.

Origin of improved average power factor and mechanical properties of SnTe with high-dose Bi2Te3 alloying

Chemical bond and defect engineering have profound impact on energy band and crystal structure of materials, which can adjust physicochemical and mechanical properties of materials. BiSn and VSn are simultaneously designed in SnTe material system via the (SnTe)x(Bi2Te3) alloying form, which was realized by vacuum melting and spark plasma sintering technology. Both point defects increase band gap and decrease ΔEL−Σ, which improves obviously Seebeck coefficient of SnTe. The high-efficiency improvement in the middle temperature zone makes average power factor of (SnTe)24(Bi2Te3) achieves 1656 μWm-1K−2(323K–773K). BiSn induces strong coupling hybridization between Te-5s and Te-5p, and forms the lower bonding state. The lower bonding state and the larger -IpCOHP increase bond strengthen, which result in the smaller thermal expansion coefficient and the higher hardness. Bonding evolution makes thermal expansion coefficient of SnTe reduce by 27%, and Vickers hardness increase by 44%. Higher average power factor, lower thermal expansion coefficient and higher hardness can improve output power density and service life, which provides strategies for exploiting high-power thermoelectric devices with suitable mechanical properties.

Bayesian optimization with active learning of design constraints using an entropy-based approach

The design of alloys for use in gas turbine engine blades is a complex task that involves balancing multiple objectives and constraints. Candidate alloys must be ductile at room temperature and retain their yield strength at high temperatures, as well as possess low density, high thermal conductivity, narrow solidification range, high solidus temperature, and a small linear thermal expansion coefficient. Traditional Integrated Computational Materials Engineering (ICME) methods are not sufficient for exploring combinatorially-vast alloy design spaces, optimizing for multiple objectives, nor ensuring that multiple constraints are met. In this work, we propose an approach for solving a constrained multi-objective materials design problem over a large composition space, specifically focusing on the Mo-Nb-Ti-V-W system as a representative Multi-Principal Element Alloy (MPEA) for potential use in next-generation gas turbine blades. Our approach is able to learn and adapt to unknown constraints in the design space, making decisions about the best course of action at each stage of the process. As a result, we identify 21 Pareto-optimal alloys that satisfy all constraints. Our proposed framework is significantly more efficient and faster than a brute force approach.

Investigation on the temperature coefficient of frequency performance of LiNbO3 on quartz and glass surface acoustic wave resonators

High impedance ratio (Z-ratio), low temperature coefficient of frequency (TCF) and spurious free characteristic are three main requirements for surface acoustic wave (SAW) resonators. The reported LiTaO3/Quartz (Qz) hetero acoustic layer (HAL) resonator can satisfy them, but the bandwidth is too narrow. LiNbO3 (LN) is suitable for wideband resonators, but the TCF is too large due to its high coupling factor. In this work, we study wideband HAL SAW resonators combining LN and three kinds of substrates with positive TCF and small coefficient of thermal expansion, including Qz, ABC-G and ZERO glasses. By simulation, the Euler angles of (0°, 101°, 0°) for LN, (0°, 160°, 90°) for Qz, and the 0.1–0.2λ (λ is the wavelength) LN thickness are used. Results show LN/Qz, LN/ABC-G and LN/ZERO exhibit Z-ratio of 82.7 dB, 76.2 dB and 76.2 dB, respectively. LN/ABC-G shows the lowest TCF of −61 ppm C−1 at resonance frequency and −62 ppm C−1 at anti-resonance frequency.

Melt‐quenched oxide glasses with ultrahigh Young's modulus and small thermal expansion coefficient

AbstractThe well‐known Makishima–Mackenzie relationship, consisting of two terms of the dense packing structure and dissociation energy regarding bonding in constituent oxides, enables fabricating oxide glasses with ultrahigh Young's modulus (∼140 GPa) and a small coefficient of thermal expansion (CTE) (∼4 ppm/K). The effects of increasing MgO and Ta2O5 contents in an MgO–Ta2O5–Al2O3–SiO2–B2O3 glass system using a conventional melt‐quenching method are revealed. The essential oxides of Al2O3 and Ta2O5 are primarily suitable for dense packing structures dominated by a large coordination number of oxygens. The substitution of CaO by MgO results in high dissociation energy when the glass composition falls in the peraluminous regime (Al2O3/[MgO + CaO] > 1). A small CTE is realized by increasing the molar ratio of Al2O3/MgO. According to magic‐angle spinning‐nuclear magnetic resonance spectra, mechanically and thermally functional oxide glasses depend on their structures. These findings facilitate the development of glass substrate applications without thermal dilatation.

Effects of graphene on the mechanical properties of AlSi10Mg melted by SLM

Graphene(GNPs) aluminum matrix composites have been widely used in transportation, aerospace, energy and power fields due to their high strength, good wear resistance and small thermal expansion coefficient. In this paper, AlSi10Mg-(0,0.5,1.0,2.0,4.0wt.%)graphene composite powders were prepared by vacuum mechanical milling, and then high quality composites were fabricated by selective laser melting(SLM). The SLM process of the composites was optimized, the microstructure and mechanical properties of AlSi10Mg-(0,0.5,1.0,2.0,4.0wt.%) graphene composites were analyzed. The results of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy(EDS) and X-ray diffraction(XRD) showed, with the addition of graphene, the microstructure and mechanical properties of the samples are greatly affected. This benign change in the microscopic level has brought the improvement of mechanical properties to the sample.

Understanding the Origin of the Particularly Small and Anisotropic Thermal Expansion of MOF‐74

AbstractMetal–organic frameworks often display large positive or negative thermal expansion coefficients. MOF‐74, a material envisioned for many applications, shows a different behavior. Temperature‐dependent X‐ray diffraction reveals particularly small negative (positive) thermal expansion coefficients perpendicular (parallel) to the hexagonally arranged pores. These trends are explained by combining density‐functional theory calculations with the Grüneisen theory of thermal expansion, which allows tracing back thermal expansion to contributions of individual phonons. On the macroscopic level, the small thermal expansion coefficients arise from compensation effects caused by the large coupling between perpendicular stress and strain and by the small magnitudes of the mean Grüneisen tensor elements, 〈γ〉. These provide information on how strains impact phonon frequencies. To understand the small value of 〈〉, the individual mode contributions are analyzed using the corresponding atomic motions. This reveals that only the lowest frequency modes up to ≈3 THz provide non‐negligible contributions, such that 〈〉 drops sharply at higher temperatures. These considerations reveal, how the details of the anharmonic properties of specific phonon bands determine the magnitude and sign of thermal expansion in a prototypical material like MOF‐74. Beyond that, the authors also discuss, how the choice of the theoretical methodology impacts the obtained results.

Fabrication of thermal conductivity enhanced polymer composites with three‐dimensional networks based on natural cotton

AbstractFunctional fillers prepared by facile natural materials have attracted much attention for their superior engineering performances and promise to replace the traditional, expensive, and environmentally unfriendly fillers. Herein, carbon fiber aerogel (CFA) was prepared by an annealing process of natural cotton. Silicon carbide (SiC)@CFA with antihygroscopic and thermal stability was further obtained by in‐situ growth via template‐assisted chemical vapor deposition method. After that, CFA/Epoxy composites and SiC@CFA/Epoxy composites with enhanced thermal conductivity at low‐loading fraction were prepared by vacuum‐assisted impregnation of epoxy resin. The SiC@CFA/Epoxy composites have higher thermal conductivity (0.404 W/[m·K] at 1.85 wt% filler at room temperature), higher resistivity, smaller coefficient of thermal expansion, and higher glass transition temperature than CFA/Epoxy composites. Furthermore, the thermal conductivity of the pure epoxy resin and the composites were tested from 150 to 370 K, and the ratio of the thermal conductivity of the composites to that of the pure epoxy resin is greater at low temperatures than at room temperature.

The new stable phases of Fe2Pd crystal alloy and their properties

This work presents a combination of particle swarm optimization crystal structure search method and first-principles calculations to investigate new stable crystal structures of Fe2Pd. Three structurally stable tetragonal phases (with space groups I4/mmm, P4/mmm, P4/nmm) and a structurally stable cubic phase (C15 Laves phase (Fd3¯m)) are predicted at 0 GPa. The enthalpy of formation reveals that these four phases are thermodynamically stable in the ferromagnetic state, and the tetragonal P4/nmm phase has the highest stability. Calculated phonon spectra indicates that the remaining three predicted structures are dynamically stable except for I4/mmm structure. From the elastic constant, all of the predicted structures are mechanically stable phases. Among these studied structures, the three tetragonal phases with similar values of bulk modulus have high uniaxial magneto-crystalline anisotropy, high thermal expansion coefficient, large bulk modulus, and good ductility. However, the cubic Fd3¯m phase is characterized by low magneto-crystalline anisotropy, small thermal expansion coefficient, high shear and Young’s modulus, and brittleness. Finally, the high-pressure stability of these four structures is investigated. The theoretical prediction indicates a structural transition from tetragonal P4/nmm phase to ductile Fd3¯m structure at 26.8 GPa, while the I4/mmm is the most stable above 80 GPa.