High Thermal Expansion Coefficient Research Articles

Compressibility and thermal expansion of magnesium phosphates

Abstract. The ambient-temperature compressibility and room-pressure thermal expansion of two Mg3(PO4)2 polymorphs (farringtonite = Mg3(PO4)2-I, with 5- and 6-fold coordinated Mg, and chopinite = “Mg-sarcopside” = [6]Mg3(PO4)2-II), three Mg2PO4OH polymorphs (althausite, hydroxylwagnerite and ε-Mg2PO4OH, all with [5]Mg and [6]Mg) and phosphoellenbergerite ([6]Mg) were measured on synthetic powders using a synchrotron-based multi-anvil apparatus to 5.5 GPa and a laboratory high-temperature diffractometer, with whole-pattern fitting procedures. Bulk moduli range from 64.5 GPa for althausite to 88.4 GPa for hydroxylwagnerite, the high-pressure Mg2PO4OH polymorph. Chopinite, based on an olivine structure with ordered octahedral vacancies (K0 = 81.6 GPa), and phosphoellenbergerite, composed of chains of face-sharing octahedra (K0 = 86.4 GPa), are distinctly more compressible than their homeotypical silicate (127 and 133 GPa, respectively). The compressibility anisotropy is the highest for chopinite and the lowest for phosphoellenbergerite. First-order parameters of quadratic thermal expansions range from v1 = 2.19×10-5 K−1 for ε-Mg2PO4OH to v1 = 3.58×10-5 K−1 for althausite. Phosphates have higher thermal-expansion coefficients than the homeotypical silicates. Thermal anisotropy is the highest for farringtonite and the lowest for hydroxylwagnerite and chopinite. These results set the stage for a thermodynamic handling of phase-equilibrium data obtained up to 3 GPa and 1000 °C in the MgO–P2O5–H2O and MgO–Al2O3–P2O5–H2O systems.

A simple method for the preparation of cordierite ceramics with high strength and low thermal expansion coefficient

AbstractCordierite ceramics have great application prospects due to their very low coefficient of thermal expansion. However, the low purity of ceramics leads to their unsatisfactory fracture strength, which has always been a research focus. Herein, by applying external pressure to the green body of mixed raw materials and optimizing the synthesis temperature, high‐purity cordierite powder was prepared, and then it was used as the starting material for the fabrication of high‐strength cordierite ceramics. The optimal sample exhibited a flexural strength of 146 MPa, which is about 1.4 times that of the cordierite ceramics reported in the literature. This is attributed to applied external pressure increasing the contact area of raw materials, leading to a more thorough reaction, and the content of the α‐cordierite phase is 98.4 wt% in the sample synthesized at 1275°C. Concurrently, the samples possessed a thermal expansion coefficient of only 1.9 × 10−6°C−1 (room temperature to 800°C). In addition, the increase in columnar crystal volume fraction is also one of the reasons for the high strength and low thermal expansion coefficient of cordierite ceramics.

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.

Impact of cement content in cement bound materials on the reflection cracking performance of asphalt pavements

This study investigates the impact of cement contents in cement bound material (CBM) on asphalt pavement reflection cracking, considering two levels: 4.5 % (CBM1) and 5.4 % (CBM2). Mechanical properties of CBMs, including elastic modulus, rupture modulus, and thermal expansion, were evaluated. Simultaneously, properties of asphalt pavement layers (wearing course and base course) underwent assessment through dynamic modulus, flow number, and overlay tests. Resilient modulus tests were conducted for subbase and subgrade layers. Laboratory test outcomes served as inputs for a AASHTOWare Pavement ME Design program. The results indicated that despite CBM2's higher elastic modulus and rupture modulus, CBM1 outperformed in pavement design. Based on the outcomes from pavement design program CBM1 exhibited lower total transverse cracking (400.19 m/km) compared to CBM2 (403.38 m/km) after 6 years of service. The higher cement content in CBM2 was identified as a potential contributor to increased cracking, attributed to shrinkage during cement hydration process. Moreover, the higher cement content showed a higher coefficient of thermal expansion (CTE), where CBM1 displayed a CTE of 70 µm/°C, while CBM2 material had a CTE of 40 µm/°C. These findings emphasize the intricate relationship between cement content in CBM and pavement performance, crucial for effective design considerations.

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.

Bowtie design for thermal expansion mimicry and mitigation

A geometric bowtie design for the purpose of mimicking (matching) and/or mitigating (reducing) the thermal expansion effects between two laterally opposing structures is investigated. A bowtie design is that of two opposing overlapping triangles with farthest sides parallel. This design embodies thermal centering concepts which utilize the relationships of the trigonometric identity known as the law of sines for a triangle from which beneficial thermal properties can be engineered. For validation. The bowtie design is embodied using two separated Zerodur plane mirrors, symmetrically opposed, and each seated in Maxwell-style kinematic couplings of spheres and vee-groves. The vee-grooves form a wheel-like pattern on an aluminum baseplate. The intersection of the vee-grooves is the common overlapping thermal center of both mirror mounts. The refractive-index corrected interferometric measurement of the displacement between these two mirrors shows that even during non-isothermal non-equilibrium transitions and states, the nominal ∼90 μm thermally induced (13 °C) expansion of an aluminum baseplate can be mitigated, being reduced more than 98 % to ∼1.4 μm by mimicking the thermal expansion properties of the mirrors’ structural material. Engineering thermally-invariant kinematic bowtie designs using materials having higher thermal expansion coefficients is also described.

Ultra-sensitive fiber-optic temperature sensor based on UV glue-based FPI and Vernier effect

In this paper, compact cascaded Fabry-Perot interferometers (FPI) for fiber-optic temperature sensors are proposed and verified. The sensors are prepared by combining the properties of temperature-sensitive material ultraviolet glue (UV glue) and the basic principle of the traditional Vernier effect (TVE), first-order harmonic Vernier effect (HVE), and second-order HVE. The sensing cavity FPIs is made up of UV glue filling the gap between two single-mode fibers (SMF) with flat cut ends. The reference cavity FPIr is composed of SMF-capillary-SMF structure. The temperature sensitivity of the FPIs is up to 1.01 nm/°C by the high thermal expansion coefficient of the UV glue. By changing the free spectral range (FSR) of the FPIr, high detection sensitivities achieved are −10.14 nm/°C, 15.22 nm/°C, and 22.24 nm/°C, corresponding the situation of TVE, first-order HVE, and second-order HVE, respectively. The experimental results indicate that temperature sensors based on UV glue possess a simple and compact structure and strong resistance to electromagnetic interference, making them suitable for temperature measurement in various environments.

Structural evolution and self-destructive behavior of Mo/Ti transition layers during free-standing diamond-film preparation

Graphite-based metal transition-layer substrates hold great potential for preparing large-area diamond films and achieving crack-free release. In this study, a five-inch crack-free free-standing diamond film was prepared on a graphite substrate with a sacrificial Mo/Ti double-layer transition layer using direct-current arc plasma jet chemical vapor deposition (DC jet CVD). The interfacial bonding and diffusion behavior between the diamond and the transition layers were investigated. The effect of abundant metal particles and elemental diffusion between layers on film adhesion and stress release are discussed and analyzed. The surface of Ti layer prepared by the multi-arc ion-plating contains numerous metal particles that provide a template for the fine-crystalline Mo layer. Metal particles are encapsulated and embedded in the diamond and act as anchors that ensure the stability of the substrate–diamond bond during high-temperature deposition. After deposition, those metal particles act as weak points and play crucial stress-release roles. Mutual elemental diffusion occurs between the Ti, Mo, and diamond layers. Ti penetrates the Mo layer to form a continuous 140-nm-thick thin layer during the initial deposition period, facilitating diamond nucleation. Carbon from the plasma and diamond diffuses into the metal transition layer. Thermal stress generated between the diamond and the graphite substrate is released by destroying the metal transition layer with high thermal expansion coefficient and low elastic modulus during cooling from the deposition temperature. Cracks rapidly propagate along the interface between destroyable transition layers and graphite substrate, which results in the liftoff of a fully crack-free diamond film.

A new method for investigating the impact of temperature on in-situ reservoir properties using high-temperature AFM

Accurately evaluating the influence of temperature variations is crucial for geothermal engineering. Existing conventional testing methods are limited in providing detailed characterizations of reservoir micro properties at high temperatures. This study employed variable-temperature AFM testing techniques to examine sandstone and limestone under in-situ temperatures (25-200°C). The study analyzed the mechanisms behind the temperature effects on rock properties and established theoretical models for porosity and permeability variations with elevated temperatures based on these mechanisms. The research revealed that sandstone and limestone exhibit similar temperature effects at high temperatures. The pore fractal dimension of sandstone and limestone does not change with temperature change. As the temperature increases, sandstone and limestone experience mean roughness (Ra) and porosity reductions. The impact of high temperature was more pronounced on sandstone compared to limestone, with sandstone showing a decrease of 2.8 % and 0.59 % in mean roughness (Ra) and AFM porosity, respectively, from 25 °C to 200 °C, while limestone exhibited reductions of 1.4 % and 0.085 %, respectively. With increasing temperature, sandstone transformed some pores with equivalent diameters of 1-2.5 μm to pores smaller than 1μm, while limestone did not show significant changes in pore structure. The temperature effects on sandstone and limestone were attributed to mineral expansion. Sandstone contains a higher proportion of minerals with a high coefficient of thermal expansion, such as quartz. In contrast, limestone primarily comprises minerals with lower thermal expansion coefficients, such as calcite, resulting in a more significant temperature impact on sandstone than on limestone. The theoretical models based on the mechanisms for porosity and permeability at high temperatures exhibited good predictive performance for sandstone and are more suitable for reservoirs predominantly composed of minerals with high coefficients of thermal expansion. The variation of porosity and permeability at high temperatures have different impacts on geothermal reservoirs. Temperature effects on porosity and permeability should be considered in geothermal engineering.

Influence of doping elements on thermo-physical properties of co-doped strontium zirconate for potential thermal barrier coatings

Strontium zirconate is a promising material for thermal barrier coatings due to some excellent properties. New four co-doped strontium zirconate with lower thermal conductivity and higher thermal expansion coefficient were synthesized in this study. Sr(Zr0.9Y0.05Yb0.05)O2.95 (SZYYb) with the smallest difference in ionic radius and mass has lowest thermal diffusion coefficients mainly due to the effect of extrinsic phonon scattering mechanisms, including lattice distortion and grain size. The electronic structures of four co-doped SrZrO3 were analyzed by first-principles calculations. The findings suggested that the difference in bond strength caused by different doping affected the differences in the lattice constant, which led to the highest thermal expansion coefficient of SZYYb.

Metal matrix composites for sustainable products: A review on current development

In recent decades, with the increase in demand for lightweight and high-strength materials for engineering applications, metal matrix composites (MMCs) are found to be a better replacement for conventional materials owing to their excellent characteristics such as high strength-to-weight ratio, high strength and stiffness, high thermal conductivity and low coefficient of thermal expansion. MMCs have been used in various applications such as automobile parts, aerospace and aircraft parts, jet engines, satellites, missiles, military, heavy constructions, NASA space shuttle, bridges, biomedical applications (i.e., medical devices, implants and surgical instruments) and so on. Extensive research has been carried out on the performance of MMCs for the development of sustainable products, which motivated us to review the current development in the processing, properties and applications of these composites. This work presents a systematic review of the mechanical properties (tensile strength and modulus, flexural strength and modulus, impact strength and hardness) of MMCs. Further, it comprises the processing techniques, strengthening mechanism and applications of MMCs along with the recommendations for future work and challenges. The mechanical performances of MMCs are found to be highly influenced by the properties of reinforcement and matrices, interfacial bonding, dispersion of particles into matrix, shape and size of particles, percentage content of particles and processing techniques. This review study suggests that MMCs have great potential to efficiently fulfill the present and future demands.

Polyimide Films Based on β-Cyclodextrin Polyrotaxane with Low Dielectric and Excellent Comprehensive Performance.

In order to prepare polyimide (PI) films with a low dielectric constant and excellent comprehensive performance, a two-step method was employed in this study to integrate β-cyclodextrin into a semi-aromatic fluorine-containing polyimide ternary system. By introducing trifluoromethyl groups to reduce the dielectric constant, the dielectric constant was further reduced to 2.55 at 10 MHz. Simultaneously, the film exhibited noteworthy thermal stability (a glass transition temperature exceeding 300 °C) and a high coefficient of thermal expansion. The material also demonstrated outstanding mechanical properties, boasting a strength of 122 MPa and a modulus of 2.2 GPa, along with high optical transparency (transmittance reaching up to 89% at 450 nm). Moreover, the inherent high transparency of colorless polyimide (CPI) combined with good stretchability contributed to the attainment of a low dielectric constant. This strategic approach not only opens up new opportunities for novel electroactive polymers but also holds potential applications in flexible displays, circuit printing, and chip packaging.

Damage Monitoring of Regularly Arrayed Short-Fiber-Reinforced Composite Laminates under Tensile Load Based on Acoustic Emission Technology.

Carbon-fiber-reinforced polymer (CFRP) composites are widely used in lightweight structures because of their high specific strength, specific modulus, and low coefficient of thermal expansion. Additionally, the unidirectionally arrayed chopped strand (UACS) laminates have excellent mechanical properties and flowability, making them suitable for fabricating structures with complex geometry. In this paper, the damage process of UACS quasi-isotropic laminates under tensile load was tested using acoustic emission detection technology. The mechanical properties and damage failure mechanism of UACS laminates were studied combined with finite element calculation. By comparing and analyzing the characteristic parameters of acoustic emission signals such as amplitude, relative energy, and impact event, it is found that acoustic emission behavior can accurately describe the damage evolution of specimens during loading. The results show that the high-amplitude signals representing fiber fracture in continuous fiber laminates are concentrated in the last 41%, while in UACS laminates they are concentrated in the last 30%. In UACS laminates, more of the damage is caused by matrix cracks and delamination with medium- and low-amplitude signals, which indicates that UACS laminates have a good suppression effect on damage propagation. The stress-strain curves obtained from finite element analysis agree well with the experiment results, showing the same damage sequence, which confirms that the model described in this research is reliable.

Failure analysis on the abnormal cracking of Si3N4 ceramic substrates for SiC power modules in new energy vehicles

Si3N4 ceramic with good thermal conductivity, high mechanical strength and low thermal expansion coefficient is an advanced thermal management material which has been applied to new energy vehicles. At present, with the rapid development of the electric driving technology for new energy vehicles, Si3N4 ceramic has been gradually used in SiC power modules so as to efficiently achieve the functions of heat dissipation, insulation and voltage resistance. Due to its important role in the power conversion process, the reliability of Si3N4 ceramic was vital, and any factor that might lead to its premature failure must be attached importance to. In this paper, a case about abnormal cracking of Si3N4 ceramic substrates during the high and low temperature cycling tests was reported. To solve this problem, various characterization methods were conducted to analyze the density, thermal conductivity, flexural strength, phase structure, morphology characteristics. Finally, through comprehensive analysis, the root cause that resulted in abnormal cracking of Si3N4 ceramic substrates was determined and meanwhile corresponding countermeasures were also proposed. Hopefully, the achievements of this paper will have positive significance for upgrading the quality of Si3N4 ceramic substrates and improving the service reliability of power modules.

Enhancement of mechanical and physical properties of Cu–Ni composites by various contents of Y2O3 reinforcement

The increasing demand for materials possessing enhanced mechanical strength, high thermal conductivity, and excellent electrical properties has grown significantly. Cu-matrix composites, especially Cu– Ni, present a promising candidate to fulfill these demands. In this study, Cu–Ni composites were successfully synthesized using powder metallurgy with various additions (0–1.5 wt%) of Yttria (Y2O3)-reinforcement aiming to enhance their mechanical, thermal, and electrical properties. The microstructural investigations demonstrated a uniform distribution of Y2O3 particles and a slight increase in porosity of the Cu–Ni matrix. The Cu–Ni composites with 1.5 wt% Y2O3 showed the presence of Cu2NiZn intermetallic compounds, potentially harming their physical and mechanical properties. Y2O3-reinforcement significantly increased the hardness and led to a moderate rise in the yield and ultimate compressive strengths. The results indicated that the Cu–Ni matrix without Y2O3-reinforcement had the highest coefficient of thermal expansion, which decreased with the addition of Y2O3, potentially leading to improved thermal properties of Cu–Ni composites. This study puts an emphasis on the importance of Y2O3 particles dispersion and on the extent of porosity in enhancing the thermal and mechanical properties of Cu–Ni composites.

Theoretical study of the interfacial force‐field and thermodynamic properties for HMX‐Estane mixture explosives

AbstractA transferable force‐field of HMX/Estane interface is derived by first‐principles calculation and least square optimization. Based on the force‐field, the plastic bonded explosive consists of HMX and Estane is simulated by molecular dynamics. The polycrystal structure is obtained, and a set of thermodynamic properties are calculated, including the heat capacity, thermal expansion coefficients, bulk modulus, elastic constants and Hugoniot curve. We find that the mixture explosive has higher thermal expansion coefficient than single crystal, because the interface is debonding at high temperature condition.

Doping engineering for high-temperature broadband high emissivity and low thermal conductivity of ytterbium chromate-based ceramics

Developing highly reliable infrared radiation materials with broadband high emissivity at high temperatures, low thermal conductivity, and excellent thermal stability is highly desirable for aerospace thermal protection applications. However, it remains a huge challenge to take into account infrared radiation heat dissipation and blocking heat transfer through low thermal conduction simultaneously. In this work, we reported a broadband high emissivity Ca2+-doped YbCrO3 ceramic with emissivity above 0.89 at room temperature across the entire range of wavelength (1–14 μm). This doping strategy leads to the introduction of impurity energy levels into the band gap of YbCrO3, which increases the possibility of light absorption to promote electron transition, improving the emissivity of the near-infrared band (1–3 μm). Simultaneously, un-equivalent doping induces electron exchange between chromate ceramic ions, which complicates the electronic structure (producing lattice distortion and extra multi-mode vibrations) and reduces the band gap width, thus boosting the emissivity in the mid-infrared band (3–14 μm). More importantly, (Yb0.8Ca0.2)CrO3 presents a high emissivity (0.76) at an elevated temperature of 1200 °C, together with low thermal conductivity (2.5 W m−1 K−1 at 1000 °C) due to strong phonon scattering. Moreover, the doping-dominating phase stabilization effect contributes to impressive thermal stability (stable at 1300 °C for 50 h) and a high coefficient of thermal expansion (9.0–9.5 × 10−6 K−1), which makes it suitable for long-term high-temperature thermal protection application. All these merits render the development of thermally stable high-temperature infrared radiation ceramic materials core competitive.

Configurational entropy of cerate ceramics regulating by multicomponent rare earth for enhanced thermophysical properties

Cerate is regarded as a promising material for the next-generation of thermal barrier coating due to its high coefficient of thermal expansion (CTE) and low thermal conductivity. However, its thermal contraction within the temperature range of 473-673 K poses a significant challenge to its widespread application. The aim of this work is to improve the thermal contraction by regulating the configurational entropy of cerate ceramics. Six kinds of rare earth doped cerate ceramics with different configurational entropy were prepared. The results indicated that the phases of the as-prepared ceramics are all fluorite structures. With the increase of configurational entropy, the problem of thermal contraction is improved continuously. More importantly, the as-prepared (La1/8Sm1/8Yb1/8Y1/8Er1/8Eu1/8Nd1/8Gd1/8)2Ce2O7 (8RC) high entropy cerate ceramic demonstrates a near elimination of thermal contraction in the 473-673 K temperature range, and accompanied by high CTE, low thermal conductivity and good thermal stability, suggesting its potential application in the field of thermal barrier coating material.

Defect‐Induced Rashba Effect in Metal Halide Perovskite Thin Films Deposited on a Flexible Substrate

AbstractMetal halide perovskites (MHPs) exhibit strong spin‐orbit coupling and structural inversion asymmetry, making them ideal for testbeds of Rashba effects and for use in spin‐related optoelectronic and spintronic applications. However, the Rashba band splitting is mainly observed in bulk MHPs so far and is rarely observed in thin films. In this study, a direct observation of Rashba band splitting in MHP thin films at room temperature is reported. By employing a silicone rubber substrate with a high thermal expansion coefficient, the out‐of‐plane tensile strain significantly reduces the activation energy of ion migration in these thin films and leads to a considerable amount of point defects. These point defects disrupt the inversion symmetry and ultimately give rise to the Rashba band splitting. These findings open up new possibilities for manipulating Rashba effects in MHP thin films through device engineering.

Phase compositions and thermophysical performances for (Sm1-xYbx)3TaO7 compounds

The (Sm1-xYbx)3TaO7 compounds were prepared using high-temperature calcining technology to evaluate the impact of Yb2O3 on the crystal-type and thermal-physical performances for Sm3TaO7. Final analytical conclusions show that the synthesised compounds have a sole pyrochlore or fluorite crystal structure, and the lattice variation from pyrochlore to fluorite is found with an increase in Yb2O3 fraction. Owing to the phonon scattering caused by substituting cation and disorder atomic arrangement, the heat insulation ability of (Sm1-xYbx)3TaO7 oxides is improved, which meets the requirements for thermal barrier coatings. The synthesised (Sm1-xYbx)3TaO7 oxides also present relatively high linear thermal expansion coefficients owing to the combined influence of ionic radius and crystal lattice energy. The addition of Yb2O3 also improves the crystal constancy until 1200 °C.