Low Coefficient Of Thermal Expansion Research Articles

Microstructure, Thermal and Mechanical Properties of AlN-MgO Composites Prepared by Spark Plasma Sintering

AlN-MgO composites with different compositions were prepared by spark plasma sintering, and the effects of their composition on their microstructure, thermal properties, and mechanical properties were systemically investigated. MgO compositions in the AlN-MgO composites were controlled from 20 to 80 wt%. The results indicated that a phase transition did not occur during the sintering process, and different solid solutions were formed within the MgO and AlN lattices. The AlN-MgO composites exhibited finer-grain microstructures than those of the sintered pure AlN and MgO samples. Transmission electron microscopy analysis showed that both oxygen-rich, low-density grain boundaries and clean boundaries with spinel phases were present in the composites. The sintered pure AlN sample exhibited the highest thermal conductivity (53.2 W/mK) and lowest coefficient of thermal expansion (4.47 × 10<sup>-6</sup> /K) at 100 °C. And, the sintered pure MgO sample exhibited moderate thermal conductivity (39.7 W/mK) and a high coefficient of thermal expansion (13.05 × 10<sup>-6</sup> /K). With increasing MgO contents in the AlN-MgO composites, however, the thermal conductivity of the AlN-MgO composites decreased, from 33.3 to 14.9 W/mK, while their coefficient of thermal expansion generally increased, from 6.49×10<sup>-6</sup> to 10.73×10<sup>-6</sup> /K with increasing MgO content. The composite with an MgO content of 60 wt% exhibited the best mechanical properties overall. Thus, the composition and microstructure of AlN-MgO composites have a determining effect on their thermal and mechanical properties.

Computational investigation of the structural, elastic, electronic, and thermodynamic properties of chloroperovskites GaXCl3 (X = Be, Ca, or Sr) using DFT framework

In this study, we employed the pseudopotential plane wave approach to examine the influence of the X atom (X = Be, Ca, or Sr) on the physical properties of isostructural chloroperovskites GaXCl3. The GGA-PBEsol functional was employed to simulate the exchange–correlation interactions. The computed equilibrium lattice parameters exhibit a high level of concordance with the existing theoretical findings. The cohesion energy and enthalpy of formation were computed to verify the energetic stability of the materials under consideration. The determined values of the single-crystal elastic constants (Cij) indicate that GaBeCl3 remains mechanically stable up to a hydrostatic pressure of 18 GPa. Similarly, GaCaCl3 preserves its stability up to 5 GPa, while GaSrCl3 remains mechanically stable up to 1.25 GPa. The projected Cij values were used to estimate several elastic moduli and related properties, including the shear and bulk moduli, sound wave speeds, Young’s modulus, Poisson’s ratio, and Debye temperature. The energy band structures of the studied compounds, as predicted by the HSE06 functional, demonstrate their wide bandgap semiconductor nature. Specifically, GaBeCl3 demonstrates an indirect bandgap of 3.828 eV, while GaCaCl3 reveals an indirect bandgap of 4.612 eV and GaSrCl3 has an indirect bandgap of 4.405 eV. The quasiharmonic Debye approach was employed to examine various thermal parameters, including the temperature dependence of the unit cell volume, bulk modulus, expansion coefficient, Debye temperature, isochoric and isobar heat capacities, Grüneisen parameter, and entropy function. It has been shown that GaBeCl3 demonstrates a lower thermal expansion coefficient and a higher Debye temperature in comparison to GaCaCl3 and GaSrCl3.

Thermal, microstructural, wettability and mechanical properties of Sn1.0Ag0.5Cu composite solder modified with Ti nanoparticles

In this study, highly active Ti nanoparticles (Ti NPs) were added to a low silver Sn-1.0Ag-0.5Cu (SAC105) solder. The thermal, microstructural, interfacial, wettability and mechanical properties of SAC105-xTi NPs (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5 wt%) solders were investigated. The results showed that adding Ti NPs had little effect on the melting point and melting point range of SAC105 solder alloy. The solder alloy containing 0.2 wt% Ti NPs had the lowest coefficient of thermal expansion (CTE) of 21.1 × 10−6 m/°C. The SAC105–0.4 Ti NPs solder has a contact angle of 24.3°, 7.57° lower than the SAC105 solder. However, Ti NPs deteriorate the surface of the solder joint as they oxidize to produce TiO2 during the soldering process, resulting in a higher number of small solder balls precipitating from the surface of the joint. In addition, EPMA showed that Ti NPs adsorbed on the Cu6Sn5 grains, leading to changes in the morphology of the Cu6Sn5 grains and a decrease in the thickness of the IMC layer. The addition of Ti NPs had little effect on the mechanical properties of solder alloys. Excessive Ti NPs will agglomerate, leading to a sharp decrease in the shear strength of the solder joints.

High temperature structure and vibrational properties of ScXO4 (X= V5+ and P5+): A comparative study

ScVO4 and ScPO4 represent the zircon (xenotime) type structures with smallest trivalent cations, and that enable them to host both transition metal and rare-earth ions for applied optical materials. Thus, their crystal chemistry and thermophysical properties becomes relevance for their application in non-ambient conditions. In this report, high temperature crystal chemistry and vibrational properties of ScVO4 and ScPO4, as observed from in situ high temperature powder XRD and Raman spectroscopic studies, are reported. The comparative analyses of the results indicate that, though both are isostructural, they show drastically different thermal expansion behavior. In case of ScVO4, the c-axis shows significantly larger expansion compared to a-axis, while in ScPO4 the thermal expansion along and a and c-axes are more or less similar. At ambient condition, the thermal expansion anisotropy in ScPO4 and ScVO4 are 1.02 and 3.97, respectively. Additionally, ScPO4 shows relatively lower coefficient of volume thermal expansion compared to ScVO4, (αv = 23.64 × 10−6 K−1 for ScPO4 and 26.09 × 10−6 K−1 for ScVO4), and is contributed by the expansion of ScO8 units in their structures. The thermal expansion coefficients of ScO8 unit in ScPO4 and ScVO4 are 36.3 × 10−6 K−1 and 39.1 × 10−6 K−1, respectively. Temperature evolution of Raman modes indicates weakening of all the modes, except a symmetric stretching mode, with increasing temperature. The anharmonic analyses of the Raman modes indicate that implicit contributions in ScVO4 and ScPO4 are appreciably higher than the explicit contributions, and hence the changes in mode wavenumbers with volume play dominating role in governing their thermal expansion behaviors. Further, it is concluded that ScPO4 is characterized by more or less like rigid unit cell compared to ScVO4.

Ultralow-Permittivity and Temperature-Stable Ba1-xCaxMg2Al6Si9O30 Dielectric Ceramics for C-Band Patch Antenna Applications.

Ca-substituted Ba1-xCaxMg2Al6Si9O30 ceramics were prepared to explore the relationships among their crystal structural parameters, phase compositions, dielectric properties, and coefficients of thermal expansion and applications in C-band antenna. The maximum solubility of Ba1-xCaxMg2Al6Si9O30 was located at x = 0.25, and Ba1-xCaxMg2Al6Si9O30 ceramics (0 ≤ x ≤ 0.25) crystallized in the space group P6/mcc. In Ba1-xCaxMg2Al6Si9O30 single-phase ceramics, εr was dominated by ionic polarizability and "rattling effects" of Ba2+ and Al(2)3+; Q × f was controlled by the roundness of [Si4Al2O18] inner rings and total lattice energy; and τf was affected by the bond valence of Si/Al(1)-O(1). Notably, the low average coefficients of thermal expansion (2.668 ppm/°C) at -150 °C ≤ T ≤ 850 °C and near-zero coefficients of thermal expansion (1.254 ppm/°C) at -150 °C ≤ T ≤ 260 °C were achieved for the Ba1-xCaxMg2Al6Si9O30 (x = 0.1) ceramic. Optimum microwave and terahertz dielectric properties were obtained for the Ba1-xCaxMg2Al6Si9O30 (x = 0.1) ceramic with εr = 5.80, Q × f = 31,174 at 13.99 GHz, τf = -7.10 ppm/°C, and εr = 5.71-5.85 at 0.2 THz ≤ f ≤ 1.0 THz. Also, the Ba1-xCaxMg2Al6Si9O30 (x = 0.1) ceramic substrate had been designed as a C-band patch antenna with a high simulated radiation efficiency (87.76%) and gain (6.30 dBi) at 7.70 GHz (|S11| = -38.41 dB).

Uncovering the Recent Progress of CNC-Derived Chirality Nanomaterials: Structure and Functions.

Cellulose nanocrystal (CNC), as a renewable resource, with excellent mechanical performance, low thermal expansion coefficient, and unique optical performance, is becoming a novel candidate for the development of smart material. Herein, the recent progress of CNC-based chirality nanomaterials is uncovered, mainly covering structure regulations and function design. Undergoing a simple evaporation process, the cellulose nanorods can spontaneously assemble into chiral nematic films, accompanied by a vivid structural color. Various film structure-controlling strategies, including assembly means, physical modulation, additive engineering, surface modification, geometric structure regulation, and external field optimization, are summarized in this work. The intrinsic correlation between structure and performance is emphasized. Next, the applications of CNC-based nanomaterials is systematically reviewed. Layer-by-layer stacking structure and unique optical activity endow the nanomaterials with wide applications in the mineralization, bone regeneration, and synthesis of mesoporous materials. Besides, the vivid structural color broadens the functions in anti-counterfeiting engineering, synthesis of the shape-memory and self-healing materials. Finally, the challenges for the CNC-based nanomaterials are proposed.

Superior oxidation resistance of a Y-Hf co-doped Al18Co30Cr10Fe10Ni32 eutectic high-entropy alloy at 1100–1300 °C

We report a study on the oxidation behavior of Al18Co30Cr10Fe10Ni32 eutectic high-entropy alloy co-doped with reactive elements (RE) Y and Hf at 1100–1300 °C. The highly stable eutectic microstructure of γ’/β phases within this alloy contributes to a low coefficient of thermal expansion, thus a low residual stress in the Al2O3 scale. The spinel with a low thickness is formed at the top of Al2O3 scale at 1100 °C due to the low Al diffusion coefficient of γ’-phase and low aspect ratio of β-phase in maze-like eutectic region. As the oxidation temperature increases, the oxidation products transition to exclusive Al2O3 at 1200 °C and 1300 °C, facilitated by the increased Al diffusion coefficient. The Al2O3 scale exhibits a dominated columnar grain microstructure at 1100–1300 °C, suggesting that the inward O diffusion plays more critical role than Al diffusion and thus leads to the low oxidation rate. Besides, the segregation of S to scale/metal interface is inhibited by the uniform RE distribution in the alloy resulting from the fine eutectic microstructure in the alloy. Overall, this alloy exhibits low residual stress in the oxide scale, slow oxidation rate, and the lack of interfacial sulfur segregation, thus contributing to its superior oxidation resistance. The strong oxidation resistance of Y-Hf co-doped Al18Co30Cr10Fe10Ni32 eutectic high-entropy alloy makes it a potential candidate for advanced oxidation-resistant alloy or coating applications.

Influence of heat treatment on microstructure and thermo-physical properties of Al-50wt%Si alloy fabricated by selective laser melting

Herein, we investigate the influence of heat treatment on microstructure and thermo-physical properties of Al-50wt%Si alloy fabricated by selective laser melting (SLM). Microstructure characterizations reveal that SLM and heat-treated Al-50wt%Si alloys are all characterized by the primary Si and eutectic structure consisting of dominant Al matrix and tiny eutectic Si. However, the size of primary Si monotonously increases with increasing the heat treatment holding time at 550 °C. In addition, the SLM Al-50wt%Si alloy displays a highest coefficient of thermal expansion (CTE) among all examined alloys. As the heat treatment holding time increases, the Si precipitates from the Al(Si)-supersaturated solid solution, leading to the formation of a skeleton-like Si phase, which effectively alleviates the thermal expansion. Furthermore, the SLM Al-50wt%Si alloy exhibits a lowest coefficient of thermal conductivity (CTC), since the significantly increased size of primary Si with increasing the heat treatment holding time leads to a substantial reduction in the number of grain boundaries, which severely restricts the scattering of heat transfer phonons. Moreover, theoretical models are employed to calculate the CTE and CTC. Results indicate that the Schapery lower model provides a better prediction of CTE, and the actual CTC values of Al-50wt%Si alloys after heat treatment are closer to theoretical values. These findings can provide new insights into designing Al matrix composites with low CTE and high CTC through microstructural optimization.

Study on fs-laser machining of optical waveguides and cavities in ULE® glass

The potential of ultrafast laser machining for the design of integrated optical devices in ULE® glass, a material known for its low coefficient of thermal expansion (CTE), is addressed. This was done through laser direct writing and characterization of optical waveguides and through the fabrication of 3D cavities inside the glass by following laser irradiation with chemical etching. Type I optical waveguides were produced and their internal loss mechanisms at 1550 nm were studied. Coupling losses lower than 0.2 dB cm−1 were obtained within a wide processing window. However, propagation loss lower than 4.2–4.3 dB cm−1 could not be realized, unlike in other glasses, due to laser-induced photodarkening. Selective-induced etching was observed over a large processing window and found to be maximum when irradiating the glass with a fs-laser beam linearly polarised orthogonally to the scanning direction, akin to what is observed in fused silica laser-machined microfluidic channels. In fact, the etching selectivity and surface roughness of laser-machined ULE® glass was found to be similar to that of fused silica, allowing some of the already reported microfluidic and optofluidic devices to be replicated in this low CTE glass. An example of a 3D cavity with planar-spherically convex interfaces is given. Due to the thermal properties of ULE® glass, these cavities can be employed as interferometers for wavelength and/or temperature referencing.

Dependence of the M choice on the properties of Hf(M)SiCN (M = Nb, Ta, Ti, Zr) ceramic coatings: ab-initio and experimental study

The HfMSiCN coatings (M = Nb, Ta, Ti, Zr) are researched using ab-initio theoretical method and double glow plasma surface alloying. The aim is to enhance the high temperature stability and wear resistance of these coatings, alongside optimizing mechanical and thermodynamic properties. Firstly, the calculated mechanical properties for Hf0.5M0.5CN correlate with those experimental measurement for HfMSiCN. Secondly, the thermodynamic parameters exhibit that the Hf0.5Zr0.5CN has the lowest thermal expansion coefficient and higher bulk modulus at high-temperatures. Moreover, the wear rate of HfZrSiCN at 500 ℃ is less than 60% of its wear rate at 300 ℃. The superior wear resistance of HfZrSiCN coating is attributed to a friction-induced composite layer including a dense amorphous nanocomposite surface layer and a nano-friction glaze layer supported by nanograins.

Polyimides with ultra-low coefficient of thermal expansion derived from diamine containing bisbenzimidazole and bisamide

To provide polyimide (PI) with high heat resistance and ultra-low coefficient of thermal expansion (CTE) for use in flexible display substrates, a novel diamine with bisbenzimidazole and bisamide groups, namely N,N'-(1H,1'H-[5,5'-bibenzo[d]imidazole]-2,2'-diyl)bis(4-aminobenzamide) (BZBA), was designed and successfully synthesized. Several poly(benzimidazole-amide-imide) (PBIAI) films were prepared by thermal imidization with 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 4,4'-oxydiphthalic anhydride (ODPA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), and 4,4'-(hexafluoroisopropylindene)-diphthalic anhydride (6FDA), respectively. Among them, BZBA-BPDA has a high glass transition temperatures ( Tg = 345 °C) while achieving an extremely low coefficients of thermal expansion (CTE = 1.9 ppm K−1), meeting the processing requirements of polymer flexible substrates in OLED devices. The effects of different dianhydrides on the performance of PBIAIs were compared, providing a meaningful reference for further adjusting the PI molecular structure to meet specific requirements for industrial polymer films.

Synthesis and Characterization of an Alicyclic Acid Anhydride Containing four Stereoisomers and the Derived Polyamideimides using Diisocyanates

AbstractAn alicyclic acid anhydride BOTA, containing four stereoisomers, racemic (1S,2R,3R,4R,5R)‐bicyclo[2.2.2]octane‐2,3,5‐tricarboxylic acid‐2,3‐anhydride (7a+7a’) and racemic (1S,2S,3S,4R,5R)‐bicyclo[2.2.2]octane‐2,3,5‐tricarboxylic acid‐2,3‐anhydride (7b+7b’), was synthesized using phthalic anhydride as the starting material. The chemical structures of the products in each step were fully characterized by NMR spectroscopy, GC‐MS, elemental analysis, and single‐crystal X‐ray diffraction. The polycondensation process involving BOTA and several diisocyanates was applied to prepare PAIs. Furthermore, PAI‐Xs were prepared using methylene diphenyl diisocyanate (MDI) and various mole ratios of BOTA and 4,4'‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA). PAI‐Xs exhibited good solubility in organic solvents such as DMSO, NMP, DMAc, GBL, DMF, and m‐cresol at room temperature. They can form flexible films with low coefficients of thermal expansion (CTE) (51‐70 ppm/°C) and high glass transition temperatures (Tg) (222‐272 °C). They also demonstrated high transparency with transmittance at 500 nm (T500nm) ranging from 77.2‐82.5%. BOTA can be a significant building block in PAI synthesis, offering an alternative to trimellitic anhydride (TMA).This article is protected by copyright. All rights reserved

Fluorinated Polydopamine Shell Decorated Fillers in Polytetrafluoroethylene Composite for Achieving Highly Reduced Coefficient of Thermal Expansion.

The noticeable difference in the coefficient of thermal expansion (CTE) for polytetrafluoroethylene (PTFE) coatings and copper substrates is a major challenge for thermal debonding of the copper-clad laminate (CCL) in high-frequency communications. Theoretically, ceramic fillers with low CTEs in the coating can effectively reduce the gap, and there remains a trade-off between the dispersibility of fillers and the interfacial interactions with the polymeric matrix. Here, we propose a novel approach to prepare a pentafluorobenzoyl chloride (PFBC)-modified polydopamine (PDA) shell on silica particles by using amidation. Such modified particles perform excellent dispersion and exhibit diminished interfacial gaps in the PTFE matrix, which highly reduces CTE to 77 ppm/°C, accounting for only 48.1% of the neat coating. Moreover, the composite exhibits enhanced mechanical strength and toughness, and consequently suppresses thermal debonding in CCL under high-temperature conditions. Therefore, results present a promising potential for its use in the next-generation CCL of high-frequency communication devices.

Wood-Composite Boards with a Low Coefficient of Linear Thermal Expansion

Wood-Composite Boards with a Low Coefficient of Linear Thermal Expansion

Preparation and properties of novolac epoxy resins containing polycyclic aromatic hydrocarbons

AbstractTo meet the demands of high‐density and miniaturized integrated circuits, electronic packaging materials need to have excellent properties. One approach to enhance the performance is to design novel structures. In this study, two novolac oligomers containing naphthalene Naphthalene novolac oligomer (NPF) and biphenyl Biphenyl novolac oligomer (LPF) were synthesized and then reacted with epichlorohydrin to synthesize novel novolac epoxy resins containing polycyclic aromatic hydrocarbons (NPF‐EP and LPF‐EP). Their chemical structures were characterized by Fourier transform infrared and 1H nuclear magnetic resonance spectra. Tetrahydromethyl‐1,3‐isobenzofurandione (MeTHPA) was used as curing agent to prepare cured NPF‐EP/MeTHPA and LPF‐EP/MeTHPA thermosets. Both NPF‐EP/MeTHPA and LPF‐EP/MeTHPA exhibited better thermal and mechanical properties compared with novolac epoxy resin (PF‐EP/MeTHPA). In particular, NPF‐EP/MeTHPA presented the highest glass transition temperature of 193°C and LPF‐EP/MeTHPA had excellent mechanical properties, with tensile and flexural strengths of 94 and 151 MPa, respectively. Additionally, the cured NPF‐EP/MeTHPA and LPF‐EP/MeTHPA also exhibit a lower dielectric constant and dielectric loss, and low coefficient of thermal expansion compared with cured PF‐EP/MeTHPA. This investigation provides valuable insights for the manufacturing of high‐performance novolac epoxy resins for application in electronic packaging materials.

Synthesis of colorless polyimides derived from benzimidazole diamines with bis‐amide structures

AbstractTo make the colorless polyimides (CPIs) suitable for the application of flexible display technology, the polyimide (PI) films need to have both good optical properties and thermal properties. To get the wanted properties, 2‐(2‐methyl‐4‐(2‐methyl‐4‐aminobenzamido)phenyl)‐5‐(2‐methyl‐4‐aminobenzamido)‐N‐phenylbenzimidazol (5a) and 2‐(2‐methyl‐4‐(2‐trifluoromethyl‐4‐aminobenzamido)phenyl)‐5‐(2‐trifluoromethyl‐4‐aminobenzamido)‐N‐phenylbenzimidazol (5b) were synthesized, which both contained benzimidazole and bis‐amide structures. Two kinds of PI films were polymerized with cyclobutane‐1,2,3,4‐tetracarboxylic dianhydride (CBDA). With the contribution of side groups (CH3 or CF3), the resulting PI films presented relatively superior optical properties (transmittance at 400 nm was 59% and 68% respectively) and low coefficient of thermal expansion (CTE = 22 and 25 ppm K−1). These good properties suggest that these PI films have the potential for flexible display usage.

Effect of Y2O3 doping on thermophysical properties and grain growth rate of lanthanum zirconate

Notwithstanding the impressive phase stability and low thermal conductivity exhibited by lanthanum zirconate (LZ), its low coefficient of thermal expansion is deemed a paramount limitation. In the present study, (La1-xYx)2Zr2O7 with x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, as new TBCs, were synthesized by reverse co-precipitation and calcination method. The XRD and Raman analyses elucidated that the substitution of Y3+ ions occurred exclusively at the La3+ positions. In addition, in accordance with FESEM and DTA findings, the incorporation of Y2O3 has induced a reduction in particle size and an enhancement in crystallization resistance. The evaluation of the coefficient of thermal expansion (CTE) demonstrated a notable improvement following the introduction of Y2O3. Specifically, the CTE of LZ increased from 9.34×10-6 oC-1 to 10.5×10-6 oC-1 in the case of (La0.5Y0.5)2Zr2O7 at 1100oC. Investigations of phase stability following a 50 h heat treatment at 1300oC indicated that Y2O3 had no impact on phase stability. All compounds, similar to LZ, exhibited excellent phase stability. Furthermore, the addition of Y2O3 significantly amplified the grain boundary diffusion mechanism during heat treatment, leading to an accelerated grain growth rate. The grain growth rate increased from 1.7 nm/h at LZ to 2.2 nm/h in the case of (La0.6Y0.4)2Zr2O7.

Preparation and properties evaluation of high-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)MgAl11O19 for advanced thermal barrier coating

LaMgAl11O19 (LMA) with magnetoplumbite structure has emerged as a promising candidate for thermal barrier coating (TBC). However, the inherent high thermal conductivity and relatively low thermal expansion coefficient (TEC) of LMA impose limitations on its further application. In this work, a novel high-entropy hexaluminate (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)MgAl11O19 (HE-LMA) has been designed and prepared to overcome these obstacles. HE-LMA with homogeneous chemical composition distribution possesses excellent phase stability up to 1600ºC. Moreover, the TEC of HE-LMA (9.22 × 10-6K-1 at 1300ºC) is larger than that of LMA. The high-entropy strategy effectively decreases the thermal conductivity of HE-LMA (from 3.27W/(m∙K) at room temperature to 2.19W/(m∙K) at 1000ºC) in comparison to that of LMA (3.63~2.62W/(m∙K)). Furthermore, HE-LMA demonstrates higher microhardness and fracture toughness due to the lattice distortion. These results indicate that (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)MgAl11O19 has the potential to be applied as a TBC.

First-principles study on the structure, mechanical and thermodynamic properties of (Ti, Hf, Nb, Ta)C high-entropy carbide ceramics

High-entropy carbide ceramics (HECCs) have demonstrated extraordinary properties and broad application potential. However, the action mechanisms of metallic elements on the properties have not been understood in depth. In this study, the structural stability and the mechanical and thermodynamic properties of (Ti, Hf, Nb, Ta)C HECCs across a broad composition spectrum were systematically analyzed based on the density functional theory and Debye–Grüneisen model. The crystal structure was constructed and validated through experimental testing. The mechanical properties such as elastic moduli, ductility, hardness, and Poisson's and Pugh's ratios were calculated. Furthermore, the temperature dependence of bulk moduli, thermal expansion coefficients, and heat capacities of the HECCs were investigated. To interpret the elastic and toughening mechanisms of non-equiatomic HECCs, the electronic structures were investigated. The results indicate that the HECCs with a single solid solution phase of the FCC rock-salt structure are thermodynamically stable. The HECCs containing higher Nb and Ta contents have greater strength and stiffness, and those containing higher Ti and Hf contents exhibit greater hardness and brittleness than the equiatomic HECC. In addition, non-equiatomic HECCs containing higher Nb and Ta contents have larger bulk moduli and lower linear thermal expansion coefficients, which can be attributed to the stronger covalent interaction among metallic elements and carbon. This study is expected to have an impact on designing ideal HECCs for high-temperature applications under extreme environments.

Elucidation of role of carbon fibers on joining of metal to composite

Weight reduction in the various application fields has propelled to the forefront in investigations of combining metal with polymer. Different from other studies, this work focused on exploring the effect of carbon fibers on the thermal joining of metal to composite. Three plastics with different fiber contents including pure polyether-ether-ketone (PEEK), short carbon fibers reinforced PEEK (SCF/PEEK), and continuous reinforced PEEK (CF/PEEK) were chosen to join to the 6061aluminum alloy using a fiber laser. Results showed the thermal transfer was fostered along the carbon fiber orientation due to a higher thermal conductivity, decreasing the heat accumulation at the interface. However, the existence of carbon fibers also hindered the spreading of plastics on the aluminum alloy surface. This led to a smaller adhesion width and lower tensile-shear force of joints. In addition, the tensile-shear strength of joints prepared with carbon fibers was enhanced owing to lower thermal expansion coefficient, which improved the thermal shrinkage and reduced the stress concentration at the interface. Ultimately, the role of carbon fibers during laser joining of aluminum alloy to plastics was explained, clarifying the bonding mechanism of two materials.