Difference In Thermal Expansion Coefficients Research Articles

Experimental and Simulation Studies on Stable Polarity Reversal in Aged HVDC Mass-Impregnated Cables

Mass-impregnated (MI) cables have been used for many years as cables in high-voltage direct current (HVDC) systems. In line commutated converter (LCC) HVDC systems, polarity reversal for power flow control can induce significant electrical stress on MI cables. Furthermore, the mass oil and kraft paper comprising the impregnated insulation have significantly different coefficients of thermal expansion. Load fluctuations in the cable lead to expansion and contraction of the mass, creating pressure within the insulation and causing redistribution of the impregnant. During this process, shrinkage cavities can form within the butt gaps. Since the dielectric strength of the cavities is lower than that of the surrounding impregnation, cavitation phenomena in impregnated paper insulation are considered a factor in degrading insulation performance. Consequently, this study analyzes the electrical conductivity of thermally aged materials and investigates the transient electric field characteristics within the cable. Additionally, it closely analyzes the formation and dissolution of cavities in MI cables during polarity reversal based on a numerical model of pressure behavior in porous media. The conductivity of the impregnated paper indicates that it has excellent resistance to thermal degradation. Simulation results for various load conditions highlight that the interval of load-off time and the magnitude of internal pressure significantly influence the cavitation phenomenon. Lastly, the study proposes stable system operation methods to prevent cavitation in MI cables.

Fabrication of penetrating pores in epitaxial Ge-on-Si through preferential etching along threading dislocations

Epitaxial Ge-on-Si possesses a high density of threading dislocations (TDs) due to the lattice mismatch and difference in thermal expansion coefficient. By employing the lattice distortion at the TDs, we demonstrate that penetrating pores along TDs can be formed in both p- and n-type heteroepitaxial Ge layers through a preferential etching. It has been found that the preferential etching at TD sites takes place in the porosification process of Ge-on-Si samples and is independent of the doping type and concentration. The penetrating pores follow the path of the TD lines and can penetrate the entire Ge layer of 1.3 μm to further porosificate the Si substrate. The effects of anodic current density and total etching duration have been thoroughly investigated on forming penetrating pores at TD sites. The dissolution mechanism in the porosification process has been revealed by dissolution valence calculation and recorded potential curves. Our findings shed light on Ge perforation in both p- and n-type Ge-on-Si and show great potential in Si-based integrated photonics and microelectronics.

A novel bi-material tubular metamaterial with twist effects under thermal and uniaxial loads

In this work, a novel bi-material missing rib-type tetra-chiral tubular metamaterial (BMTTM) is designed by utilizing the base materials with different coefficients of thermal expansion (CTEs), and considering various material combinations. Counterintuitively, twist effect can be observed with temperature variation or uniaxial load for the BMTTM. The theoretical formations of the twist angle, Poisson's ratio (PR) and CTE are established and verified subsequently by numerical simulations. Theoretical and numerical results indicate that tailoring twist angle of the BMTTM can be achieved by manipulating the base material combination and the microstructural geometry. The BMTTM proffers a novel avenue to design temperature sensors, actuators, and satellite support tubes, serving in fluctuating temperature environments.

Efficient and environmentally friendly separation and recycling of cathode materials and current collectors for lithium-ion batteries by fast Joule heating

Spent Li-ion batteries(LIBs) cathodes possess high recycling value. To improve subsequent recovery efficiency and product purity, separating the cathode materials from the aluminum foil is critical. However, traditional separation methods are characterized by high energy consumption, low recycling efficiency, and environmental pollution. This research presents a novel method that involves injecting Joule heat directly into the aluminum foil in the air, resulting in the melting and slight thermal decomposition of the PVDF binder, which reduces its adhesive properties. Owing to the difference in thermal expansion coefficients between the binder and aluminum foil, an instantaneous thermal stress was generated at the interface of the cathode materials and aluminum foil, resulting in a peeling force. This method enabled a rapid and efficient separation of lithium iron phosphate (LFP) and ternary Li-ion (NCM) battery cathode materials. The optimal separation conditions, separation mechanism, and properties of the recovered products were investigated thoroughly using high-speed camera imaging, temperature rise calculations, and microscopic characterization. This chemical-free method avoids the generation of wastewater and gas emissions. Under optimal experimental parameters, the separation efficiency and purity of cathode material reached 99 % and 99.7 %, respectively. Additionally, this method preserves both the structural integrity of the aluminum foil and the crystalline structure of the cathode materials, offering a new pathway for sustainable recycling of end-of-life LIBs.

Structural Origins of the Anisotropic Thermal Expansion of BINOL Crystals

AbstractMeasurement of the unit cell of (R)-BINOL over a 200° temperature range (300–100 K) reveals an anisotropic contraction where the c-axis contracts ca. 2.3% compared to a ca. 0.45% contraction of the a and b axes, a ca. six-fold difference in linear thermal expansion coefficient. This contraction corresponds to a decrease in the helical pitch of the 31 screw axis in the [001] direction. The anisotropic nature of the contraction is rationalised by a thorough analysis of intermolecular contacts within the crystal and their impact on the conformation of the molecule and crystal packing. Graphical Abstract The crystal structure of (R)-BINOL exhibits a pronounced anisotropic thermal expansion.

Microstructure, hardness, and tribological properties of CoCrFeNiX (X = Mo, Ti, W) high entropy alloy coating by red-blue composite laser cladding on copper alloy

In this study, the problem of high reflectivity of copper alloy surface to traditional infrared laser beam was overcome by red-blue composite laser cladding technology. The problem of cracks and other defects caused by the large difference in thermal expansion coefficient between copper substrate and high entropy alloy coating was overcome by electroplating Ni intermediate layer on the surface of copper alloy. A high-entropy alloy coating with tight bonding and no obvious defects was successfully prepared on the surface of copper alloy. The phase composition, microstructure evolution, hardness and tribological properties of high-entropy alloy coatings after adding different elements were studied. The experimental results show that all the CoCrFeNi coatings are FCC phase. After the addition of Mo, Ti and W elements, σ phase rich in Mo and Cr, TiCo2-type Laves phase and Fe7W6-type μ phase appeared in the high entropy alloy coating, respectively. The average microhardness of CoCrFeNiX (X = Mo, Ti, W) high entropy alloy coatings reached 730 HV0.2, 1090 HV0.2 and 378 HV0.2, respectively. The wear rates of CoCrFeNiMo, CoCrFeNiTi and CoCrFeNiW high-entropy alloy coatings were 47.17 %, 90.56 % and 22.64 % lower than those of CoCrFeNi coating, respectively. The wear scars were analyzed by SEM and EDS, and the friction and wear behavior mechanism of CoCrFeNiX (X = Mo, Ti, W) high-entropy alloy coating was investigated. It is mainly composed of abrasive wear, adhesive wear and oxidation wear. The research results provide practical guidance for the preparation of high-entropy alloy coatings on copper surface, and lay a theoretical foundation for laser cladding such high-entropy alloy materials and substrate materials.

Strain-dependent grain boundary properties of n-type germanium layers

Polycrystalline Ge thin films have attracted considerable attention as potential materials for use in various electronic and optical devices. We recently developed a low-temperature solid-phase crystallization technology for a doped Ge layer and achieved the highest electron mobility in a polycrystalline Ge thin film. In this study, we investigated the effects of strain on the crystalline and electrical properties of n-type polycrystalline Ge layers. By inserting a GeOx interlayer directly under Ge and selecting substrates with different coefficients of thermal expansion, we modulated the strain in the polycrystalline Ge layer, ranging from approximately 0.6% (tensile) to − 0.8% (compressive). Compressive strain enlarged the grain size to 12 µm, but decreased the electron mobility. The temperature dependence of the electron mobility clarified that changes in the potential barrier height of the grain boundary caused this behavior. Furthermore, we revealed that the behavior of the grain boundary barrier height with respect to strain is opposite for the n- and p-types. This result strongly suggests that this phenomenon is due to the piezoelectric effect. These discoveries will provide guidelines for improving the performance of Ge devices and useful physical knowledge of various polycrystalline semiconductor thin films.

Temperature induced strengthening in TiNiCuNb shape memory alloy

Mechanical behavior of two-phase (Ti54Ni34Cu12)90Nb10 alloy with B2–B19–B19’ martensitic transformation types was studied by tensile test in wide temperature range. A temperature-dependent stress diagram was obtained based on experimental data, from which the influence of temperature on the different critical stresses was determined. It was shown that the increasing in testing temperature leads to the increasing in dislocation yield of martensite and ultimate fracture stresses, while the dislocation yielding stresses of austenite are stabilized. It was proposed that the phase hardening caused by difference in thermal expansion coefficients between B19 and β-Nb phases may be a possible reason for the temperature induced strengthening. High dislocation density near B19/β-Nb interface supports this assumption.

Tensile-strained Ge1−x Sn x layers on Si(001) substrate by solid-phase epitaxy featuring seed layer introduction

Herein, we examined the seed layer-induced solid-phase epitaxy (SPE) of Ge1−x Sn x layers on Si(001) substrate toward their in-plane strain control. We sequentially deposited crystallized Ge1−x Sn x seed layers at 360 °C with thicknesses of 2 and 4 nm and amorphous Ge1−x Sn x layers at 80 °C. First, it was found that the thicker Ge1−x Sn x seed layer effectively promotes the Ge1−x Sn x epitaxial growth, whereas polycrystalline Ge1−x Sn x is likely to be formed for the no seed layer case. Then, we discussed the possible role of the seed layer in SPE promotion of the Ge1−x Sn x layer on Si(001). Finally, we demonstrated that the in-plane strain ε in the SPE-Ge1−x Sn x layer can be controlled by the annealing temperature according to the thermal expansion coefficient difference between the SPE-Ge1−x Sn x layer and Si substrate. In this study, we succeeded in forming tensile-strained Ge and Ge1−x Sn x layers on Si(001) substrate by 500 °C annealing; the ε values were 0.20% and 0.16%, respectively.

Texturing to Dramatically Increase Thermal Deformation of Bilayer Film and Application to Actuator

The effect of adding texture to increase the deformation of a bilayer actuator by a factor of over 100 was demonstrated. A bilayer actuator is driven by the difference in thermal deformation. Therefore, two films or plates with a large difference in thermal expansion coefficient are required. The newly developed textured film actuator (TFA) obtained large deformation even in a case involving a combination of two films with a small difference in thermal expansion coefficient. The TFA was fabricated by transferring the texture when laminating two PI films with different coefficients of thermal expansion. A TFA with thin‐film electrodes was heated by applying electric power. Thereby, it unfolded from a curled state. The maximum generated force and curvature of the TFA were 7.4 mN and 0.19 mm−1, respectively, at 0.38 W. The manufacturing process for the TFA is simple and convenient, as it does not require adhesives or special materials. In thermal drive bilayer actuator fabrication, the range of material selection is expanded substantially. In addition, large deformations are generated with less energy. Therefore, the energy consumption is likely to be reduced. © 2024 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Mathematical modeling and optimization of vacuum diffusion bonding parameters for predicting and enhancing the strength of dissimilar IN-718/MSS-410 joints using RSM for power generation applications

The dissimilar welding of Inconel 718 (IN-718) alloy and AISI 410 martensitic stainless steel (MSS-410) is crucial in advanced gas turbines, and ultra-supercritical power plants to meet the demands of different operating conditions and lower the cost. However, the dissimilar fusion welding of IN-718/MSS-410 is challenging due to the differences in thermal expansion coefficient, physical and mechanical properties of base metals. In this study, the solid-state vacuum diffusion bonding (VDB) technology is employed to develop the dissimilar IN-718/MSS-410 joints. The aim of this study is to find the optimal combination of VDB parameters such as diffusion bonding pressure-DBP (MPa), diffusion bonding temperature-DBT (°C) and diffusion bonding time-DBt (min) for enhancing the strength of IN-718/MSS-410 joints. The response surface methodology (RSM) was integrated for designing the experimental matrix. The strength performance of VDB joints was evaluated by conducting the lap shear strength (LSS) and bonding strength (BS) tests. The mathematical LSS and BS predicting models were established using regression analysis and verified employing the variance analysis. The microstructural features were analyzed using optical and scanning electron microscopy (SEM). The X-ray diffractometer (XRD) was employed to identify the phases evolution in the joint interface. The experimental results revealed that the IN-718/MSS-410 joints diffusion bonded using the DBP of 14 MPa, DBT of 960 °C and DBt of 90 min exhibited the greater LSS of 280 MPa and BS of 373 MPa. The prediction models accurately predicted the LSS and BS of IN-718/MSS-410 joints within 2 % error at 95 % confidence. It is primarily concerned with developing the optimal bonding width with the fewest possible embrittlement implications and better joining interface coalescence. According to variance analysis, the DBt was the most significant parameter influencing the LSS and BS of joints followed by the DBP and DBT.

An Optimized Device Structure with a Highly Stable Process Using Ferroelectric Memory in 3D NAND Flash Memory Applications

In this paper, we propose an optimized device structure with a highly stable process that addresses threshold voltage shift issues in the String-Select-Line (SSL) and Ground-Select-Line (GSL) gates using ferroelectric memory in 3D NAND flash memory applications. The proposed device utilizes nickel (Ni) instead of tungsten (W) for the GSL and SSL gates, enabling optimized polarization properties during the annealing process and leveraging the disparity in thermal expansion coefficients. Notably, the difference in thermal expansion coefficient from tungsten (W), employed in other Word Line (WL) gates, allows effective control over polarization properties. To validate the proposed structure, we fabricated and measured a Metal–Ferroelectric–Insulator–Silicon (MFIS) capacitor utilizing Hafnium–Zirconium Oxide (HZO) material. The measurement results indicate that a change in the upper metal layer results in a more than fivefold increase in the variance of polarization characteristics between the WL gates (responsible for the memory function) and the SSL and GSL gates dedicated to channel control. In addition, process simulation was conducted using the same device structure, confirming the application of tensile stress to the HZO thin film in the case of a W electrode and compressive stress in the case of a Ni electrode. Furthermore, applying this controlled polarization characteristic parameter to the 3D NAND flash memory structure revealed a reduction in the threshold voltage shift of the control gate from a previous change of 2.6 V or more to 0.05 V, facilitating stable control.

Electrical properties of strained off-stoichiometric Cu–Cr–O delafossite thin films

Off-stoichiometric Cu–Cr–O delafossite thin films with different thicknesses were grown by metal organic chemical vapor deposition on substrates with different coefficients of thermal expansion. Seebeck thermoelectric coefficient and resistivity measurements were performed on the range of 300–850 K. A qualitative change in the temperature-dependence of the resistivity is observed at the temperature corresponding to the deposition process, where the transition from tensile to compressive strain takes place. Arrhenius plots reveal different slopes in these two thermal ranges. The fact that the shift is more pronounced for the thinner films might indicate the induced strain plays a role in changing electrical behaviour. Furthermore, changes below 0.1% in electrical mobility were measured when the strain is induced by mechanical bending.

The Effect of Fe on the Mechanical Properties of Stellite 6

Stellite 6 is a Co-based alloy with excellent wear and corrosion resistance, resulting in it frequently being used as a hard-facing material. The main defect that occurs in the hard-facing process is the peeling of the hard-facing layer, which is caused by thermal stress due to differences in thermal expansion coefficients. This peeling phenomenon can be prevented by using a material with an intermediate coefficient of thermal expansion as a buffer layer. Component surfaces are strengthened through a technique called hard-facing, which increases the components’ resistance to wear. In order to increase the wear and corrosion resistance of metal components, a high-hardness substance is usually coated onto their surface. Grinding is carried out to smooth the surface after the material is welded onto the base component. Small pieces, also referred to as “scrap”, are frequently produced during this surface machining procedure. In the hard-facing process, scrap containing a mixture of base material and Stellite 6 is generated during material processing. If this scrap can be used as a buffer layer, the cost of raw materials can be reduced through the recycling of resources, and the overall energy consumption for recycling can be reduced because the process of separating Stellite 6 from the scrap is eliminated. In this study, the physical properties of Stellite 6 with Fe added were explored to determine whether the scrap produced could be used as a buffer layer. When iron was added to the mixture, the amount of carbide decreased, in addition to the wear resistance and hardness decreasing. Regarding the thermal expansion coefficient, the thermal expansion coefficient of Stellite 6 was found to be 15.879 μm/m·°C at 300 °C, and that of Stellite 6 with 40% Fe added was found to be 14.313 μm /m·°C at 300 °C. The thermal expansion coefficient of carbon steel is reported to be 12.8 μm /m·°C at 300 °C; thus, Stellite 6 with 40% Fe can be used as a buffer layer for Stellite 6 and carbon steel because of its suitable thermal expansion coefficient.

Photoluminescence enhancement of chemical vapor-deposited MoSe2 monolayers

The transition metal dichalcogenide (TMDC) materials have attracted a great interest owing to their superior features. The chemical vapor deposition (CVD) is very feasible to synthesis of TMDC materials. The thermal expansion coefficient difference between TMDC materials and substrate in the CVD synthesis can cause a strain, resulting in non-radiative recombination and PL decrement for these materials. Therefore, in this work, we studied acetone and isopropanol (IPA) treatment on the photoluminescence (PL) properties of CVD-grown MoSe2 monolayers. Firstly, monolayer MoSe2 flakes on SiO2/Si substrates were synthesized via the CVD method by optimizing synthesis parameters. The Raman and PL measurements were taken after acetone and IPA treatments were applied to grown samples. The results indicated an important PL enhancement was seen for acetone treatment. For IPA treatment, there was a PL peak position shifting with decreasing intensity due to its possible structural damage. The acetone, IPA, and transfer processes caused the releasing strain on MoSe2 by breaking the strong interaction between MoSe2 and substrate. For the whole treatment, PL peaks shift to blue with about 80 meV. As a result of the present study, acetone treatment was found as an easy and quick way to enhance radiative emission of CVD-grown MoSe2.

Y1/6Yb5/6TaO4/8YSZ composite ceramics with enhanced mechanical and thermal properties

AbstractCeramic thermal barrier coatings (TBCs) with high toughness and low thermal conductivity are important for developing aero engines with high thrust‐to‐weight ratio and thermal efficiency. In this work, the properties of tantalate Y1/6Yb5/6TaO4 (YYT) were optimized by introducing 8 wt.% yttria‐stabilized zirconia (8YSZ), synthesized using spark plasma sintering, as a second‐phase ceramic. The highest toughness (3.57 MPa m1/2) of the YYT/8YSZ composite ceramics was due to multiple toughening mechanisms. The underlying mechanisms of these processes were revealed based on the microstructures and residual stress of the YYT/8YSZ composite ceramics. A shift from an intergranular fracture to a transgranular fracture was caused by the difference between soft‐grain YYT and hard‐grain 8YSZ, which consumed the fracture energy. Furthermore, the formation of microcracks, crack deflection, bridging, and branching were analyzed based on the differences in Young's moduli and thermal expansion coefficients of YYT and 8YSZ. Internal defects and weak connections between the YYT and 8YSZ grains reduced the thermal conductivity of the composites until it reached its lowest value at 1.4 W m−1 K−1. The studied toughening mechanisms could be applied to various composites. Moreover, herein, we provide a new strategy to design and synthesize TBCs with enhanced performance.

Strain accumulation and relaxation on crack formation in epitaxial AlN film on Si (111) substrate

The formation of cracks is often observed in the epitaxial growth of ultrawide-bandgap aluminum nitride (AlN) semiconductor films on economical and versatile silicon (Si) substrates due to the significant differences in in-plane lattice parameters and thermal expansion coefficients between the film and the substrate, which hampers the development of template, buffer layer, and device structure with a relatively thick AlN layer for devices. The present study aims to elucidate the conditions of crack formation through a simple but comprehensive estimation of strain energy accumulation and relaxation by lattice strain, misfit dislocation density, and crack formation. Strain energy in the epitaxial film from lattice and thermal mismatches is evaluated by an elastic strain equation tailored to the epitaxy of the hexagonal crystal structure. The effects of temperature, thickness, and dislocation density on the lattice and dislocation strain energies of the film are also considered. Finally, the comparison in the changes in the total strain energy and cleavage energy with decreasing temperature shows that cleavage energy is higher than strain energy if the film is thinner than 400 nm but becomes lower than the strain energy if the film is thicker than 400 nm during cooldown, suggesting the crack formation, which matches well with experimental observations.

Promoted aging precipitation behavior of TC4/GZ151K magnesium matrix composites

The precipitation behavior of TC4/GZ151K composite (Ti–6Al–4V reinforced Mg-15Gd-1Zn-0.4Zr) after solution and aging heat treatment (T6-treated) has been investigated. Quantitative measurements of precipitates revealed that nano-β′ phase in T6-treated TC4/GZ151K composite was denser and thinner than that in T6-treated GZ151K matrix, generating higher ultimate tensile strength of composite than that of matrix (410 MPa vs. 373 MPa). it was disclosed by geometric phase analysis that there were more geometrically necessary dislocations in solution-treated composite than that in the matrix, which was caused by different coefficient of thermal expansion between TC4 and Mg. High-density dislocations are beneficial to precipitate fine and dense β′ phase in the composite while low-density dislocations result in coarse and loose β′ phase in the matrix after aging process. This work confirmed the accelerating effect of TC4 particles on Mg-matrix's aging precipitation, which was of great significance to regulate the precipitation behavior of Mg alloy and produce high strength-ductile Mg materials.

Large‐Scale Synthesis of Flexible Cermet Interdigital Electrodes with Stable Ceramic‐Metal Contact for Fire‐Resistant Pressure Tactile Sensors

AbstractCermet has broad applications in electronics, but achieving stable ceramic‐metal contact has always been a challenge, particularly at hot temperature and large deformation conditions, due to the significant differences in thermal expansion coefficient and stiffness of ceramics and metals. Here, stress matching and strong interphase bonding at the heterogenous ceramic‐metal contact are realized by using flexible ceramics, leading to stable applications in flexible fire‐resistant pressure tactile sensors. Cu/Au nanolayers are deposited first onto the toughened flexible SiO2 nanofibers with a chemical plating method to form sensitive interdigital electrode sensors, then use an in situ thermal reaction strategy to form conformal and interlocked Au3Cu and CuSiO3 interphases, which enhance the mechanical thermal stability of the ceramic‐metal contact. With deep learning technique, the as‐fabricated intelligent pressure tactile gloves can accurately recognize the human body parts in low‐visible burning environments and show a long‐term stability over 1000 cycles with a high sensitivity of 112.18 kPa−1, which is important for developing intelligent firefighting.

THERMAL SHOCK BEHAVIOR OF LANTHANUM ZIRCONATE-BASED THERMAL BARRIER COATINGS

In this study, La2Zr2O7, Yb2O3 and Gd2O3-doped La[Formula: see text](Gd, Yb)[Formula: see text]Zr2O7 thermal barrier coatings have been manufactured from the crystalline and amorphous feedstock by plasma spraying. The thickness of the bond coat is 100±25 [Formula: see text]m and the thickness of the top coat is 250±25 [Formula: see text]m. Thermal shock tests were carried out in furnace temperature of 1200∘C. The thermal shock tests indicate that lifetime of the La[Formula: see text]Gd[Formula: see text]Zr2O7 coating has 55 cycles which is the highest thermal shock lifetime compared to other coatings in this study. The type of feedstock, amorphous or crystalline, strongly affects the thermal shock lifetime of La2Zr2O7 coatings. Coating damage after thermal shock tests was investigated using scanning electron microscope to observe the microstructure and X-ray diffraction techniques to analyze the phase composition. The failure of coatings is mainly in consequence of the accumulation of stress between bond coat–ceramic coat interface due to thermal expansion coefficient differences and thermally grown oxide layer formation during thermal shock cycling tests.