Expansion Coefficient Research Articles

Dimensional change and springback of spherical shell in cryogenic forming

Cryogenic forming has been developed to manufacture thin-walled curved aluminum alloy components, whose final dimensions are affected by cryogenic shrinkage and springback. Therefore, dimensional changes of spherical shell in cryogenic forming were studied theoretically and experimentally. The cryogenic forming process was discussed to elucidate the factors affecting the dimensional change. The stress distribution was analyzed to qualitatively reveal the springback behavior. Cryogenic dimensional measurement devices were built to quantitatively evaluate the dimensional changes in the forming processes of punch cooling, springback, and specimen restoration to room temperature. The temperature dependencies of the elastic modulus and expansion coefficient were modeled to quantitatively calculate the effect of thermal expansion and contraction on the dimensions of specimen and punch. The theoretical analysis results indicate that depth reduction and opening expansion are produced by cryogenic springback, determined by the radial stress, hoop stress, and bending moment in different deformation regions. The cryogenic springback in the biaxial tensile stress zone was reduced by 37.8 % owing to the increasing radial deformation and decreasing bending deformation. In contrast, cryogenic springback in the tensile-compressive stress zone increased by 30.8 %. The punch cooling shrinkage and specimen warming expansion in depth direction can reduce for the dimensional deviation caused by springback but cause the opposite effect in hoop direction. Expansion and shrinkage were effectively predicted using the proposed model, with an error of less than 16 %. The deformation region of the biaxial tensile stress can be enlarged by the significantly improved hardening ability at cryogenic temperature, which benefits enhancing deformation uniformity and further reduces springback. Therefore, cryogenic forming offers considerable potential for the precision manufacturing of aluminum alloy deep-cavity thin-walled components.

Utilizing porous dielectric material to enhance the performance of capacitive MEMS accelerometers for shaft monitoring

This research investigates the optimization of the performance of a capacitive-based microelectromechanical systems (MEMS) shaft monitoring accelerometer using a porous soft dielectric material with high polarization capability and low Young’s modulus. The nonlinear governing equations of a microbeam with a proof mass coupled with the squeeze motion equation of the polymeric dielectric material excited by shaft vibrations are extracted and solved in both the spatial and temporal domains. The studied model considers the stiffness, dissipative, and inertial effects of the polymeric material and incorporates porosity as a displacement-dependent variable, which introduces an additional nonlinearity. After discretizing the coupled nonlinear differential equations using the Galerkin method, the response in the frequency domain is obtained by applying an energy balance method, where the coefficients of the Fourier expansion of the response are found by arc-length continuation through a physical gradient descent learning-based approach method. The occurrence of secondary and primary resonances in different harmonic responses is studied for different harmonic acceleration inputs. The equivalent rigidity of the design is investigated for different shaft frequency magnitudes and different proof mass geometries. The effect of the initial porosity volume fraction of the elastomeric dielectric material on the dynamic response of the accelerometer is also examined. The sensor’s sensitivity is dependent on its geometry and properties and can vary based on the structure’s material parameters. Consequently, the selection of different geometries and materials may result in either an improvement or a deterioration of the sensor’s performance.

The Role of Alkyl Chains in the Thermoresponse of Supramolecular Network.

Supramolecular materials provide a pathway for achieving precise, highly ordered structures while exhibiting remarkable response to external stimuli, a characteristic not commonly found in covalently bonded materials. The design of self-assembled materials, where properties could be predicted/design from chemical nature of the individual building blocks, hinges upon ourability to relate macroscopic properties to individual building blocks - a feat which has thus far remained elusive. Here, a design approach is demonstrated to chemically engineer the thermal expansion coefficient of 2D supramolecular networks by over an order of magnitude (\boldmath 120 to \boldmath 1000 × 10-6K-1). This systematic study provides a clear pathway on how to carefully design the thermal expansion coefficient of a 2D molecular assembly. Specifically, a linear relation has been identified between the length of decorating alkyl chains and the thermal expansion coefficient. Counter-intuitively, the shorter the chains the larger is the thermal expansion coefficient. This precise control over thermo-mechanical properties marks a significant leap forward in the de-novo design of advanced 2D materials. The possibility to chemically engineer their thermo-mechanical properties holds promise for innovations in sensors, actuators, and responsive materials across diversefields.

Exploring the effects of finite size and indenter shape on the contact behavior of functionally graded thermoelectric materials

The performance enhancement of functionally graded thermoelectric (FGTE) devices is significantly influenced by contact studies of the FGTE materials. It is unclear how the finite thickness and the punch geometry influence the FGTE materials’ contact behaviors. This paper investigates the frictionless contact problem between three types of rigid punches (flat, triangular, and cylindrical) and the FGTE strip with finite thickness. The electric-thermo-elastic parameters of the FGTE strip vary in the thickness direction according to an exponential function. Based on the Fourier integral transform and the transformation matrix method, the problem is transformed into the numerical solution of three sets of singular integral equations. The presence of singular features on either side of the punch demands the adoption of specific collocation strategies. The distribution of the normal current density, the normal energy flux, and the normal contact stress is obtained by adjusting multiple electric-thermo-elastic parameters. The contact stresses in the case of punches with varying shapes can be effectively controlled by modulating the coefficient of thermal expansion and the strip thickness, whereas the effect of the electrical conductivity, the shear modulus, and the thermoelectric load on these stresses depends on whether they are increased or decreased.

Spectral expansion methods for prediction uncertainty quantification in systems biology

Uncertainty is ubiquitous in biological systems. For example, since gene expression is intrinsically governed by noise, nature shows a fascinating degree of variability. If we want to use a model to predict the behaviour of such an intrinsically stochastic system, we have to cope with the fact that the model parameters are never exactly known, but vary according to some distribution. A key question is then to determine how the uncertainties in the parameters affect the model outcome. Knowing the latter uncertainties is crucial when a model is used for, e.g., experimental design, optimisation, or decision-making. To establish how parameter and model prediction uncertainties are related, Monte Carlo approaches could be used. Then, the model is evaluated for a huge number of parameters sets, drawn from the multivariate parameter distribution. However, when model solutions are computationally expensive this approach is intractable. To overcome this problem, so-called spectral expansion (SE) methods have been developed to quantify prediction uncertainty within a probabilistic framework. Such SE methods have a basis in, e.g., computational mathematics, engineering, physics, and fluid dynamics, and, to a lesser extent, systems biology. The computational costs of SE schemes mainly stem from the calculation of the expansion coefficients. Furthermore, SE effectively leads to a surrogate model which captures the dependence of the model on the uncertainty parameters, but is much simpler to execute compared to the original model. In this paper, we present an innovative scheme for the calculation of the expansion coefficients. It guarantees that the model has to be evaluated only a restricted number of times. Especially for models of high complexity this may be a huge computational advantage. By applying the scheme to a variety of examples we show its power, especially in challenging situations where solutions slowly converge due to high computational costs, bifurcations, and discontinuities.

Internal radiation effect on semiconductor β-Ga2O3 crystals grown by the VB Method and anisotropic thermal stress

Gallium oxide crystals are semitransparent semiconductors with good optical and electrical properties, which allow their use for several technological applications. During the growth process of β-Ga2O3 crystals, internal radiation plays a crucial role that affects the growth process and then the crystal quality. In this work, the effect of the melt and the crystal transparency on the vertical Bridgman growth of β-Ga2O3 oxide is thoroughly studied. Using a global 2D/3D finite element model, temperature, melt flow, melt-crystal interface, and three-dimensional anisotropic thermal stress are computed at different growth stages. At each stage, four cases are considered, namely, opaque melt and crystal, semi-transparent melt and opaque crystal, semitransparent crystal and opaque melt, and finally semitransparent melt and crystal. The role of internal radiation in each case at different growth stages is then highlighted separately and then coupled together. It was found that the melt-crystal interface is shifted from a convex shape at the early stage to a nearly plane and then to a concave shape at the last stage. The melt flow is then changed from two rolls pattern at the beginning to a single-roll structure at the last stage. Thermal stress of the as-grown ingot is decreased during the growth due to the decrease of temperature non-linearities. Internal radiation inside the crystal acts to increase the melt-crystal interface convexity at the early and middle stages of the growth process and leads to a decrease in its concavity at the final stage. However, the melt transparency leads to the opposite effects, i.e., it decreases the interface convexity at the early stage and increases the interface concavity at the final stage. As a result, for semitransparent crystal and melt, the interface is between the two previous cases. The calculated thermal stresses are found to be more affected by the transparency of the crystal than the melt as the absorption coefficient of β-Ga2O3 crystal is smaller than that of the melt. At all stages, the thermal stresses are found to be larger for the opaque case due to the increase of temperature non-linearities in the crystal. Large values are found at the bottom and the lower part of the periphery. Furthermore, internal radiation inside the melt plays a major role during the early growth stage due to the large liquid volume. It reduces the melt flow intensity close to the free surface where the shear stress is combined with the buoyant force and leads to the flattening of the interface decreasing then the radial temperature gradients, which leads to small attenuation of the thermal stress. At the final stage, the transparency of the melt plays the opposite role due to the increase of the interface concavity which leads to an increase in both of the temperature gradients and the thermal stress inside the crystal. Due to the crystal anisotropy, and especially to the large thermal expansion coefficient in the [010] direction, the 3D thermal stress values are found to be larger in this direction, especially in the bottom part of the ingot. Comparisons with available experimental and numerical works are provided in this paper.

Effect of filer content on the thermal characteristics of underfill materials for Ball-Grid-Array component package

High-density integration and fast processing speed in the semiconductor industry have increased heat generation in electronic devices. Underfill materials, known for their high thermal conductivity and low coefficient of thermal expansion (CTE), offer a potential solution to dissipate heat and improve device reliability. In this study, we investigate the effect of filler content on the thermal reliability of underfill materials for ball grid array (BGA) component packaging. Mainly, we investigate the thermal conductivity, CTE, and mechanical properties of different filler contents of Al2O3 and Al2O3–BN hybrid underfills to facilitate heat dissipation and improve device reliability. The thermal conductivity of the underfill materials was evaluated by measuring the surface temperatures of underfill molded flip-chip light-emitting diodes (FCLEDs). The mechanical properties and thermo-mechanical reliability of the underfill materials were evaluated via a three-point bending test of the underfill packaged BGA components after the thermal shock test. The results showed that optimizing underfill properties based on specific application environments is crucial for obtaining enhanced thermal reliability and mechanical properties of underfill packaged BGA components.

Enhancing the strength and plasticity of Kovar alloy without sacrificing thermal expansion properties

Enhancing the strengths of most constant-expansion alloys, including Kovar alloys, typically results in reduced plasticities and increased coefficients of thermal expansion (CTEs). In this study, we systematically investigated the effects of carbon on the microstructure, mechanical properties, and CTE of a 4J29 Kovar alloy prepared via metal injection molding (MIM). When the carbon content was in the appropriate range, the strength and plasticity could be improved without compromising the CTE. MIM samples prepared by adding 1000 ppm graphite and oxalic acid for degreasing exhibited increases of 28.2 % and 17.3 % in tensile strength and elongation, respectively, compared to those of samples with low carbon contents of 46 ppm. The average CTE remained stable at 4.92 × 10⁻⁶ ℃–1 (25–400 ℃). The reduced number of pores at the grain boundaries was the main cause of the improvement in plasticity. The low CTE is attributed to the combined effect of the grain boundary volume fraction and porosity. This study provides significant theoretical and practical guidance for use in producing high-performance 4J29 Kovar alloys that may be used in complex components with constant-expansion properties.

Stability analysis of Rayleigh–Bénard–Marangoni convection in fluids with cross-zero expansion coefficient

Cross-zero expansion coefficient Rayleigh–Bénard–Marangoni (CRBM) convection refers to the convective phenomenon where thermal convection with stratified positive and negative expansion coefficients in a liquid layer is coupled with the Marangoni convection. In the Bénard convection, fluids with a cross-zero expansion coefficient contain a neutral expansion layer where the expansion coefficient (α) is zero, and the local buoyancy-driven convection is coupled with the Marangoni convection, leading to unique flow instability phenomena. This paper uses linear stability theory to analyze the CRBM convection in a horizontal liquid layer under a vertical temperature gradient and performs numerical calculations for fluids under different Bond numbers (Bd) in both bottom-heated and bottom-cooled models, obtaining the critical destabilization conditions and modes. In the bottom-heated model, different combinations of buoyancy instability mechanism (BIM), tension instability mechanism, and coupled instability mechanism (CIM) appear depending on the dimensionless temperature for the neutral expansion layer (Tα0) and the Bd. In the bottom-cooled model, two mechanisms occur according to the variation of Tα0: BIM and CIM.

Improved thermal shock reliability of Ag paste sintered joint by adjusted coefficient of thermal expansion with Ag-Si atomized particles addition

In this study, two kinds of atomized Ag-20 wt%Si and Ag-50 wt%Si particles are fabricated and added to Ag paste to fabricate composite pastes to achieve a low coefficient of thermal expansion (CTE), and their performance stability in SiC power module is evaluated under thermal shock from −50 to 250 °C. Four kinds of composite Ag pastes using the two kinds of atomized Ag-Si addition with 10 % and 30 % were obtained, AS20-10, AS50-10, AS20-30 and AS50-30, respectively. The results show that the CTE reduction intensifies with increasing Ag-Si atomized particle content. The retention ratio of shear strength of AS20-30 sintered joint after 1000 cycles was 102.3 %, which means the shear strength did not decrease during so harsh thermal shock test. The mechanism was discussed in detail. In addition, the properties such as thermal conductivity and electrical conductivity for the composite pastes were also evaluated to gain a more comprehensive understanding for the new materials especially for the design of SiC power modules with a long-time life. Comprehensively considering the material parameters of the composite pastes and the reliability of the sintered joints under thermal shock, the optimized composite pastes as die attach material in power electronics are obtained. The findings should be inspiring in developing novel die attach materials for high reliable interconnections in SiC power modules.

Synergetic effect of diamond particle size on thermal expansion of Cu-B/diamond composite

Diamond particles reinforced Cu matrix (Cu/diamond) composites have promising applications for heat dissipation of high-power electronic devices because of their high thermal conductivity and suitable coefficient of thermal expansion. The effect of diamond particle size on thermal conductivity has been addressed; however, the effect of diamond particle size on thermal expansion still needs to be clarified. In this study, Cu-B/diamond composites with various diamond particle sizes ranging from 66 μm to 701 μm were fabricated to assess the impact of diamond particle size on the thermal expansion behavior. The composites exhibit low and adjustable coefficient of thermal expansion (CTE) values of 4.58–6.63 × 10−6 K−1, which align with 4–8 × 10−6 K−1 of widely employed semiconductors. The CTE of the Cu-B/diamond composites first decreases and then increases with increasing diamond particle size, which arises from a synergetic effect of interfacial bonding strength and matrix strengthening effect. As the diamond particle size is smaller than 272 μm, the interfacial bonding strength rises with increasing particle size, enabling the diamond particles to restrain the expansion of the Cu matrix more effectively and to reduce the CTE. As the diamond particle size exceeds 272 μm, the dislocation density in the Cu matrix continually decreases with increasing particle size, reducing the strength increment in the Cu matrix and increasing the CTE. The effect of thermal cycling on the thermal expansion of the Cu-B/diamond composites was also investigated, and all the composites show an increase in the CTE after 100 thermal cycles. Notably, the composite with 272 μm diamond particle size exhibits the lowest CTE increment. It shows a CTE value of 5.29 × 10−6 K−1 after thermal cycling, still compatible with the semiconductors for electronic packaging applications.

Achieving ZT > 1 in Cu and Ga Co-doped Ag6Ge10P12 with Superior Mechanical Performance and Its Fundamental Physical Properties toward Practical Thermoelectric Device Applications.

Recently, phosphorus-based compounds have emerged as potential candidates for thermoelectric materials. One of the key challenges facing this field is to achieve ZT > 1, which is the benchmark for thermoelectric device applications. In this study, it is demonstrated that the thermoelectric performance of environmentally friendly Ag6Ge10P12 is enhanced by co-doping Cu and Ga. The mechanical properties, coefficient of linear thermal expansion, work function, and compatibility factor are comprehensively clarified to provide guidelines for reliable device applications. The peak and average dimensionless figures of merit of Ag5.85Cu0.15Ge9.875Ga0.125P12 reach 1.04 at 723 K and 0.63 at 300-723 K, respectively, which are the highest values for phosphorus-based thermoelectric materials. The Young's modulus, Vickers microhardness, fracture toughness, and compressive strength of Ag5.85Cu0.15Ge9.875Ga0.125P12 are 132 GPa, 589, 1.23 MPa m1/2, and 219 MPa, respectively, which are superior to those of typical state-of-the-art thermoelectric materials. The remarkable thermoelectric and mechanical performance of Ag5.85Cu0.15Ge9.875Ga0.125P12 mean that it is a promising candidate for medium-temperature thermoelectric conversion. Ti, V, Rh, and Pt are suitable for electrodes without exfoliation under thermal expansion and with ohmic contacts to Ag5.85Cu0.15Ge9.875Ga0.125P12 in terms of the coefficient of linear thermal expansion and work function. Considering that the compatibility factor of Ag5.85Cu0.15Ge9.875Ga0.125P12 is approximately 2.8, half-Heusler, skutterudite, and magnesium silicide-stannide compounds are suitable n-type thermoelectric counterpart materials in thermoelectric devices. These insights will lead to the development of phosphorus-based thermoelectric materials toward practical thermoelectric device applications.

Enhance Oil Recovery by Carbonized Water Flooding in Tight Oil Reservoirs

The low permeability of tight sandstone reservoirs limits the application of water flooding to enhance oil recovery. Due to the properties of CO2, people improve crude oil recovery by carbonizing water flooding. However, many studies have not prioritized determining the concentration of carbonated water before comparing the recovery rates of carbonated water flooding and pure water flooding after water flooding. This article takes the Chang 6 oil reservoir as the research object, and uses a combination of nuclear magnetic resonance testing and displacement experiments to conduct experiments on optimizing the concentration of carbonated water. Based on this, experiments on water flooding and carbon water flooding after water flooding to improve crude oil recovery are conducted to compare and analyze the oil enhancement potential of carbonated water flooding. The experimental results show that the physical properties, pore throat structure and seepage capacity of sandstone samples of type I, II and III decrease in turn. With the increase of carbonated water concentration, the expansion coefficient, viscosity reduction rate, contact angle reduction rate and core permeability loss rate of crude oil increase. The damage rate of 0.4mmol/cm3 carbonated water solution (17.6g CO2: 1L water) to the core permeability is only 0.98%, which is the optimal experimental concentration. During the water flooding process, crude oil in pores of different scales in Type I rock cores is utilized, and the degree of utilization in large pores is relatively high. Continuing to use carbonated water to drive oil in the rock cores after water flooding increased the recovery rate of crude oil by 15.24%; The degree of crude oil utilization in small pores of Tpye II core after carbonization and water flooding increased by 29.4%, and the final recovery rate of crude oil increased by 11.35% compared to the core after water flooding; The physical properties of Tpye III core are poor, with a small proportion of large pores, resulting in a 9.04% increase in crude oil recovery rate. Compared with pure water flooding, the recovery rate and utilization degree of large and small pores in the three types of rock cores after carbonization water flooding have been improved to varying degrees. Among them, the utilization degree of crude oil in large pores is the most significant, which increases the recovery rate of crude oil in the rock core after displacement. This study can provide a theoretical basis for improving crude oil recovery by carbonizing water flooding.

Effect of ZnO/Li2O ratio on the structure and properties of Li2O-ZnO-SiO2-P2O5 glass-ceramics sealants

In order to meet the sealing temperature of copper alloy below 1000 °C, a kind of Li2O-ZnO-SiO2-P2O5-(B2O3+Al2O3+K2O) (LZS) glass-ceramics with high expansion coefficient and high resistivity was synthesized via the melting method and then crystallized through controlled cooling. The network structure, thermal properties, crystal phase transition, thermal expansion coefficient, and high-temperature resistivity have been investigated. As the mass ratio of ZnO/Li2O increases, the network structure of the LZS glass is strengthened, leading to an increase in the glass transition and crystallization temperature. Moreover, the predominant crystal phases in LZS glass-ceramics transit from Li2SiO3 to Zn[Zn0.1Li0.6Si0.3]SiO4 and subsequently to Zn2SiO4. At a ZnO/Li2O ratio of 3, a cristobalite phase with a high expansion coefficient emerges. The change in crystal phase results in a variation in the coefficient of thermal expansion (CTE) of glass-ceramics, ranging from 13.2 to 11.2 × 10−6 K−1. At a ZnO/Li2O ratio of 3, the softening temperature of glass-ceramics is minimized. Moreover, as the ZnO/Li2O mass ratio increases, the resistance of crystallized glass decreases first and then increases. The maximum resistivity is achieved at a ZnO/Li2O mass ratio of 5. By optimizing the mass ratio of ZnO/Li2O, it is determined that when the mass ratio of ZnO/Li2O is 3, the CTE is 12.4 × 10−6 K−1, and the softening temperature (Ts) is 679.8 °C, which is the best material for sealing applications.

Study on the electromagnetic-thermal-mechanical coupling damage of concrete under microwave irradiation

The paper aims to explore the thermal response of three-dimensional random aggregate concrete model under microwave irradiation. The multi-field distribution in concrete by coupling of the electromagnetics, heat transfer and solid mechanics in COMSOL Multiphysics software under microwave irradiation was investigated. The results shown that the different in dielectric properties and thermal expansion coefficients between aggregate and mortar resulted in the generation of temperature gradients and stress concentration at interface. In addition, due to the larger expansion coefficient of aggregate compared to mortar, stress concentration was formed at interface, promoting the damage of concrete. Microwave power exhibited a notable influence on thermal accumulation and thermal stress. Higher microwave power correlating to higher growth rates of temperature and stress. The Von Mises stress in mortar was significantly greater than that in aggregate, as energy absorption reached a certain threshold, damage occurred in mortar. In addition, microwave frequency significantly influenced the distribution of electric field within the cavity, and also the energy conversion ratio of aggregate within concrete. The study could provide a comprehensive understanding of heating effects of microwave radiation on concrete, providing the mechanical basis for microwave-assisted concrete recovery.

Employing Young's modulus and Debye temperature to calculate the elastic, thermodynamic and thermophysical properties of titanium oxynitrides.

The values of the shear v s and longitudinal v l wave velocities were calculated for 14 selected titanium oxynitrides TiN x O y using the known values of Young's modulus and Debye temperature. The errors Δ of the calculations did not exceed ±0.01%. It turned out that some TiN x O y samples are able to compete with artificial diamonds in terms of v l values and can potentially be used in acoustic resonators for intelligent chemical and biochemical sensors. A number of elastic, thermodynamic and thermophysical quantities were calculated, and graphical dependencies between them were plotted. The established correlations were used to develop two algorithms for predicting the properties of TiN x O y alloys based on a single experimental parameter, namely the X-ray coefficient of thermal expansion or pycnometric density. The highest accuracy was shown by the method based on the experimental density, which allowed to estimate, with acceptable errors, the values of the shear v s and mean v m wave velocities (Δ = ±(1-5)%), the minimum thermal conductivity λ min within the framework of the Cahill‒Pohl model (Δ = ±(0-3)%), the isobaric C p and isochoric C V heat capacities (Δ < 1%); while the known experimental methods and alternative models for determining these quantities are characterized by wider error intervals: Δ(v s) = ±(1-10)%, Δ(λ) = ±(1-10)% and Δ(C p) = ±(1-3)%.

Achieving enhanced strength-ductility synergy in Mg-6Zn-1Mn alloy by introducing deformable Ti particles

The Mg-based composites have been considered as an efficacious method to enhance the strength of Mg matrix. How to concurrently augment the strength and ductility of the Mg matrix composites remains a pivotal concern. In this work, Mg-6Zn-1Mn (ZM61) composites, reinforced with varying contents of deformable Ti particles, were successfully manufactured employing a method of semi-solid stirring coupled with ultrasonic treatment, subsequently followed by hot extrusion. The outcomes demonstrated marked enhancement in strength and ductility of composites. The interfacial reaction between Ti and Mg matrix minimized the precipitation of MgZn2 and α-Mn in the matrix. Ti particles boosted the dynamic recrystallization (DRX) behavior effectively. The TEM characterizations revealed that MgZn2 nucleated on the Ti particle and Mn diffusion layer was formed in the Ti particle. The 2Ti/ZM61 composite achieved superior comprehensive mechanical properties, marked by the YS of 288 MPa, the UTS of 383 MPa, and the elongation to 22.1%. The augmented strength was primarily owning to the grain boundary strengthening and the mismatch coefficient of thermal expansion (CTE) between Ti and Mg. The synergistic deformation of Ti particles was the main reason of improving the ductility.

The influence of the elemental valence on the thermophysical properties of high entropy (La0.2Sm0.2Eu0.2Yb0.2Y0.2)2(Ce0.5Zr0.5)2O7 coatings for thermal barrier application

Commercial yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are no longer sufficient for the stringent demands in the high operating temperature and excellent thermophysical properties of the high-performance gas turbines and aviation engines. High entropy ceramics, distinguished by their superior thermophysical properties, have risen as a potential substitute for TBCs. However, the realm of atmospheric plasma spraying (APS) in the creation of these advanced TBCs remains relatively unexplored. In this work, three distinct high-entropy (La0.2Sm0.2Eu0.2Yb0.2Y0.2)2(Ce0.5Zr0.5)2O7 coatings were developed through the meticulous alteration of spraying power. The effects of spraying power on the microstructure, valence states of constituent elements and thermophysical properties of the coatings were deeply investigated. The results indicated that all the as-sprayed coatings exhibit a defect fluorite structure. Spraying power does have a significant impact on the thermophysical properties of the coatings. With the decrease in spraying power, the thermal conductivity of the coatings shows a progressive reduction, whereas the coefficient of thermal expansion (CTE) exhibits a gradual increase. This trend is closely related to valence states of constituent elements in the coatings. The resulting coating exhibits a low thermal conductivity of 0.54 W⋅m-1⋅K-1, a high CTE of 11.22×10-6·K-1 and a superior phase stability at elevated temperatures.

Simulation of thermal stress predictions for NiTi coating on a stainless-steel substrate using thermal plasma spraying

Thermal stresses arise during the thermal plasma spraying of composite coatings, influenced by the differing thermophysical properties of the substrate and coating materials. This study focuses on a thick coating with a top NiTi layer and a base stainless steel (AISI 304) substrate, along with an epoxy carrier layer. Initially, the substrate is stress-free at the deposition temperature of 1400 °C. Once the coating is applied, the carrier layer activates at 150°C, introducing thermal stresses before cooling to room temperature (20 °C). Using COMSOL Multiphysics, we examine stress distribution within the assembly at both 150 °C and room temperature. The model treats the thick plate as a two-dimensional solid, with layers assumed to be isotropic and linear elastic. The higher coefficient of thermal expansion in the substrate (17.3 × 10⁻⁶) compared to the coating (11 × 10⁻⁶) results in tensile stresses in the substrate and compressive stresses in the coating.

Tailoring thermal expansion of β-Cu2V2O7 with improved mechanical properties by incorporation of Li+Fe3+

βCu2V2O7 is known to have a negative thermal expansion (NTE) in a wide temperature range but its ceramics bulk is very loose and porous. Herein, a strategy for sintering densely packed ceramics body with tailored coefficients of thermal expansion (CTE) and improved mechanical properties is presented, where Cu2+ is partially substituted by the combination of Li+ and Fe3+ according to the formula (LiFe)0.5xCu2-xV2O7. The results show that the CTEs can be tuned from negative to near-zero while the elasticity modulus and compressive strength are 2 and 15 times higher for x = 1.0 than those of β-Cu2V2O7, respectively. The reduced NTE is attributed to the stiffening and bond angle lessening of V-O1-V linkages while the elevated mechanical properties are ascribed to the densely packed nature of the samples.