Coefficient Of Thermal Expansion Mismatch Research Articles

Preparation of carbon fiber/Mg-doped nano-hydroxyapatite composites under low temperature by pressureless sintering

In order to protect carbon fibers (CF) from oxidation damage during sintering process, rod-like Mg-doped nano-hydroxyapatite (Mg-nHA) with an increased thermal decomposition temperature and reduced sintering temperature was synthesized by hydrothermal method. The synthesized bone-like Mg-nHA with similar composition and morphology to bone apatite was used as the matrix to prepare CF reinforced Mg-nHA composites (CF/Mg-nHA) at a low temperature of 700 °C by pressureless sintering. The increase of temperature slightly influenced the growth of Mg-nHA prepared by hydrothermal method from 160 °C to 200 °C. The Mg-nHA were short and rod-like in structure with a length of approximate 100 nm. When doping 1% magnesium, the decomposition temperature of Mg-nHA increased by 100 °C compared with that of nHA. This can protect CF from oxidation damage which is often encountered when sintering CF reinforced hydroxyapatite composites at high temperature and enhance reinforcing effects of CF. The bending strength of CF/Mg-nHA with 1 wt% CF was 8.51 MPa, which increased by 19.5% compared with Mg-nHA. Alternatively, the rod-like Mg-nHA was prepared on the surface of CF by electrochemical deposition and Mg-nHA coated CF was used to reinforce Mg-nHA, the coefficient of thermal expansion mismatch between CF and HA matrix could be mitigated. The compressive strength of Mg-nHA coated CF reinforced Mg-nHA (CF/Mg-nHA/Mg-nHA) composites with 0.5% CF sintered at 800 °C were 41.3 ± 1.56 MPa, which was attributed to the improved strengthening effect of CF and the good interface between CF and Mg-nHA matrix.

The effect of the SiC content on the high duration erosion behavior of SiC/ZrB2– SiC/ZrB2 functionally gradient coating produced by shielding shrouded plasma spray method

In this research, the effect of the ZrB 2 middle layer and SiC Weight percentage on the erosion behavior of SiC/ZrB 2 – SiC/ZrB 2 functionally gradient coating were investigated. For this purpose, SiC gradient coating was prepared via the reactive melt infiltration method (RMI). Afterwards ZrB 2 –SiC layers containing 10, 20 and 30 wt% SiC and, ZrB 2 as the outer layer were applied on SiC coated graphite via solid shielding shrouded plasma spraying (SSPS). To investigate the erosion resistance of the coating, the specimens were subjected to oxy-acetylene and propane flame. The results showed that by applying the ZrB 2 –SiC layer between SiC and ZrB 2 coating, due to the gradual change of the coefficient of thermal expansion mismatch and reduction of thermal stresses, erosion resistance improves, so that the coating with 20 wt% SiC with mass and linear erosion rate, −0.072 × 10 −4 g.cm −2 .s −1 , 0.0166 μm s −1 respectively had the best erosion resistance under oxy propane flame. In the oxyacetylene flame test, a similar result was obtained to the oxy propane test and the SiC/ZrB 2 -20% wt. SiC/ZrB 2 coating had the lowest erosion rate.

Preparation and characterization of corundum ceramics doped with Fe2O3 and TiO2 for high temperature thermal storage

High-temperature thermal storage materials have received urgent attention for efficient thermal transfer in solar thermal power generation. Corundum ceramics doped with Fe2O3 and TiO2 were prepared via a pressureless sintering. A Fe2O3–TiO2 system with different Fe2O3/TiO2 ratios was applied to corundum ceramics. Phase composition, microstructural evolution, sintering properties, high temperature resistance and thermophysical properties were evaluated. The results indicated that Fe2O3 and TiO2 rendered the grains highly active and enhanced the bonding between grains due to existing stably in the lattice of corundum. In addition, decrease in the Fe2O3/TiO2 ratio led to a new phase of FeAlTiO5, which refined the grains. These effects gave the samples good sintering properties and thermal shock resistance, but the thermal expansion coefficient mismatch between FeAlTiO5 and corundum deteriorated the high-temperature (1300 °C) stability. Formula C1 (Fe2O3/TiO2 ratio of 9:1) sintered at 1600 °C had the optimum comprehensive properties, possessing a bending strength loss rate of 1.54% after 30 cycles of thermal shock (1100 °C-room temperature, air cooling) and a constant strength retention rate of approximately 71.34% after 90 h high-temperature cycle. The corresponding thermal conductivity and specific heat capacity were 18.81 W/(m·K) and 1.02 J/(g·K) at 25 °C, which was suitable as a high-temperature thermal storage material.

Predicting the CTE & hardness for directed energy deposition of functionally graded WC-SS316L Coatings

One of the key components in the design of wear resistant coatings for directed energy deposition is the analysis of the thermo-mechanical properties within the substrate-coating interface. The coefficient of thermal expansion (CTE) and the mismatch are crucial due to their significant influence on the structural integrity and operational performance of the coating. In this study, a four-layered functionally graded tungsten carbide (WC)-316L stainless steel composite coating was designed using a rule-of-mixtures approach for manufacture by directed energy deposition. Two theoretical models were employed to predict the CTE and micro-hardness as a function of increasing volume fraction of WC particles. According to the graded concentrations predicted from the linear models, the four-layer coating exhibits a good hardness with minimal CTE mismatch.

Crack propagation in low dislocation density quantum dot lasers epitaxially grown on Si

Low threading dislocation density in epitaxial lasers on Si is required for high performance and robust devices for silicon photonic integrated circuits. However, as the threading dislocation density is further reduced, a point is reached where it is energetically favorable for cracking to occur in the layers after cooldown to room temperature due to the thermal expansion coefficient mismatch between the film and the substrate. This can be solved in most cases by increasing the optical confinement and reducing the total layer thickness. We combine models of dislocation motion (controlling plastic relaxation) and thin film channel cracking to describe the impact of dislocation density and cooling rate, which addresses a well-known and previously unsolved problem in heteroepitaxial growth for optoelectronic and electronic devices. Agreement between predictions and experiments illustrates that the model is effective in identifying critical dislocation densities, film thickness, and cooling rates that avoid cracking. We believe that this work is the first attempt to merge the fracture mechanism and dislocation-mediated plastic relaxation in semiconductor films to solve a practical problem in optoelectronic materials.

Achieving Reliable CoSb3 based thermoelectric joints with low contact resistivity using a high-entropy alloy diffusion barrier layer

Skutterudite (SKD) thermoelectric materials have high conversion efficiency, great mechanical properties, and economical practicability in the medium temperature range (500–550 °C). They need to be joined with metal electrodes to form a thermoelectric power generation device during application. However, high contact resistivity, severe element diffusion, and large coefficient of thermal expansion mismatch are main obstacles for their applications. To address these issues, a FeCoNiCrMo high-entropy alloy diffusion barrier layer was designed and prepared using an arc smelting method in this paper. Effect of heating temperatures on the microstructure and properties of the bonded joints were investigated. The maximum shear strength was 21.6 Mpa and the corresponding reaction layer thickness, contact resistivity were 3.77 μm, 1.8 μΩcm2 respectively at 600 °C, 40 MPa, 10 min. Shear strength dropped down to 18.8 MPa and the contact resistivity increased to 4.2 μΩ cm2 after aging for 640 h. Numerical model was established and it predicted that the contact resistivity would keep lower than 6.5 μΩ cm2 (300 h, 100 days) and 11 μΩ cm2 (8760 h, 1 year) and the reaction layer thickness would not exceed 25 μm (2400 h, 100 days) and 45 μm (8760 h, 1 year).

Study of Thermal Stress Fluctuations at the Die-Attach Solder Interface Using the Finite Element Method

Solder joints in electronic packages are frequently exposed to thermal cycling in both real-life applications and accelerated thermal cycling tests. Cyclic temperature leads the solder joints to be subjected to cyclic mechanical loading and often accelerates the cracking failure of the solder joints. The cause of stress generated in thermal cycling is usually attributed to the coefficients of thermal expansion (CTE) mismatch of the assembly materials. In a die-attach structure consisting of multiple layers of materials, the effect of their CTE mismatch on the thermal stress at a critical location can be very complex. In this study, we investigated the influence of different materials in a die-attach structure on the stress at the chip–solder interface with the finite element method. The die-attach structure included a SiC chip, a SAC solder layer and a DBC substrate. Three models covering different modeling scopes (i.e., model I, chip–solder layer; model II, chip–solder layer and copper layer; and model III, chip–solder layer and DBC substrate) were developed. The 25–150 °C cyclic temperature loading was applied to the die-attach structure, and the change of stress at the chip–solder interface was calculated. The results of model I showed that the chip–solder CTE mismatch, as the only stress source, led to a periodic and monotonic stress change in the temperature cycling. Compared to the stress curve of model I, an extra stress recovery peak appeared in both model II and model III during the ramp-up of temperature. It was demonstrated that the CTE mismatch between the solder and copper layer (or DBC substrate) not only affected the maximum stress at the chip–solder interface, but also caused the stress recovery peak. Thus, the combined effect of assembly materials in the die-attach structure should be considered when exploring the joint thermal stresses.

Rational Design of Two-Layer Fe-Doped PrBa0.8Ca0.2Co2O6−δ Double Perovskite Oxides for High-Performance Fuel Cell Cathodes

A solid oxide fuel cell (SOFC) offers an attractive route to convert chemical energy into electrical energy; however, its commercial application is often limited by the large mismatch in thermal expansion coefficients (TECs) between the cathode and electrolyte and the insufficient activity of cathodes. Fe-doped PrBa0.8Ca0.2Co2O6−δ (PBCC) are promising cathode materials due to their high conductivity and excellent oxygen-reduction activity, in which Fe doping in PBCC can decrease the TEC to achieve better compatibility with the electrolyte, but excessive Fe will inhibit the oxygen kinetics. Here, we construct a two-layer PBCC cathode (TLC), which consists of a high Fe content layer with good thermal compatibility to electrolytes and a low Fe content layer with high ionic and electronic conductivity. Compared to homogeneous PBCC electrodes, the cell with TLC has a lower activation energy and a higher peak power density (1259.3 mW cm–2 at 700 °C). Meanwhile, it exhibits admirable stability up to 200 h in constant potential mode (0.8 V), and importantly, no discernible degradation is observed in further stability tests in the constant current mode (444 mA cm–2) for 200 h. The results reveal that the TLC based on double perovskite Fe-doping isostructural oxide is a promising candidate electrode for SOFC.

Stress reduction and wafer bow accommodation for the fabrication of thin film lithium niobate on oxidized silicon

Sub-micrometer-thick lithium niobate on an insulator is a promising integrated photonic platform that provides optical field confinement and optical nonlinearity useful for state-of-the-art electro-optic modulators and wavelength converters. The fabrication of lithium niobate on insulator on a silicon substrate through ion slicing is advantageous for electronic-photonic integration but is challenging because of debonding and cracking due to the thermal expansion coefficient mismatch between silicon and lithium niobate. In this work, the fabrication of thin film lithium niobate on insulator on a silicon handle wafer is achieved, informed by structural modeling, and facilitated by accommodating for dissimilar wafer bows using a bonding apparatus. Structural finite element analysis of strain energy and stress, due to thermal expansion coefficient mismatch at elevated temperatures, is conducted. High strain energies and stresses that result in debonding and cracking, respectively, are studied through modeling and reduced by selecting optimized substrate thicknesses followed by an experimental technique to bond substrates with dissimilar bows. A lithium niobate thin film with a thickness of 800 nm is successfully transferred to an oxidized silicon wafer with a root mean square surface roughness of 5.6 nm.

Effects of interfacial residual stress on mechanical behavior of SiCf/SiC composites

Layer-structured interphase, existing between reinforcing fiber and ceramics matrix, is an indispensable constituent for fiber-reinforced ceramic composites due to its determinant role in the mechanical behavior of the composites. However, the interphase may suffer high residual stress because of the mismatch of thermal expansion coefficients in the constituents, and this can exert significant influence on the mechanical behavior of the composites. Here, the residual stress in the boron nitride (BN) interphase of continuous SiC fiber-reinforced SiC composites was measured using a micro-Raman spectrometer. The effects of the residual stress on the mechanical behavior of the composites were investigated by correlating the residual stress with the mechanical properties of the composites. The results indicate that the residual stress increases from 26.5 to 82.6 MPa in tension as the fabrication temperature of the composites rises from 1500 to 1650 °C. Moreover, the increasing tensile residual stress leads to significant variation of tensile strain, tensile strength, and fiber/matrix debonding mode of the composites. The sublayer slipping of the interphase caused by the residual stress should be responsible for the transformation of the mechanical behavior. This work can offer important guidance for residual stress adjustment in fiber-reinforced ceramic composites.

Property evolution of geopolymer composites with SiC whiskers loaded with BN coating at elevated temperatures

This article investigates the key interface properties of geopolymers reinforced by SiC whiskers loaded with BN coating (BN/SiCw) and the insight of mechanical performance and microstructures of the resulting composites. The interesting diverse strength evolutions of geopolymer composites calcinated at various temperatures suggest a result of rivalries of multiple factors including BN coating, geopolymer matrix transformations, SiC whiskers. A positive effect on whisker pull-out phenomenon has been achieved via the chemical bonding between SiC whisker (SiCw) and BN coating and the rough physical interface between BN coating and geopolymers; the addition of SiCw and BN/SiCw promotes the polycondensation reaction of geopolymers and the addition of the latter is superior. Through changing the pore structure characteristics of the composites, the SiCw in composites can release their excessive vapor pressure in the closed pores which is the main reason attributing to the strength loss of geopolymers at 900 °C. In addition, BN coating can make up for the mismatch of thermal expansion coefficient between SiCw and geopolymers, increasing the strength of the composites by 30.9% at 900 °C, whereas whisker pull-out and crack deflection are the main toughening effect for BN/SiCw.

Bending induced mechanical exfoliation of graphene interlayers in a through thickness Al-GNP functionally graded composite fabricated via novel single-step FSP approach

A novel step grooving approach is proposed for the reinforcement incorporation before FSP, enabling the creation of through-thickness composition gradient with a higher volume fraction of reinforcement near the surface and relatively lower volume fraction in the core by using a single step of FSP. The overall tensile strength of Al-GNP functionally graded composite (FGC) is increased by ∼52% compared to pure aluminium. The localized Young's modulus and yield strength of the core region is increased by ∼12% and ∼54% compared to the surface region of the fabricated composite, respectively. The maximum strain in the core region is also decreased by ∼38% depicting its higher stiffness over the surface region. Contrastingly, the microhardness of the surface region is increased by ∼18% compared to the core region of the fabricated composite. Thus, a combination of higher surface hardness and excellent core tensile strength is achieved with the proposed methodology. The bending of graphene interlayers leading to the mechanical exfoliation of GNP increased the load sharing capability of the Al/GNP interface. The load sharing through interface and coefficient of thermal expansion mismatch between Al and GNP are identified as the dominant strengthening mechanisms in the fabricated composite.

Five‐layer reverse tape casting of IT‐SOFC

AbstractTape casting is a well‐established method for manufacturing thin ceramic layers with controllable thickness and porosity. This study investigates the potential of 10Sc1CeSZ material for the electrolyte and anode layers for intermediate‐temperature solid oxide fuel cells (IT‐SOFC) in an anode‐supported cell (ASC) geometry. In order to use La0.6Sr0.4Co0.2Fe0.8 Oxide (LSCF) cathode material, a Gd0.2Ce0.8 Oxide (GDC) barrier layer is needed; however, thermal expansion coefficient mismatch results in delamination of the GDC from the electrolyte during high temperature sintering when fabricated by conventional tape casting procedures. For the first time, ASCs have been manufactured by a five‐layer tape casting technique; barrier layer, novel composite layer, electrolyte, anode functional layer, and anode substrate. Ni‐ScCeSZ composite cells were tested between 650 and 800°C in H2:N2 fuel (85% H2) on the anode and air on the cathode to yield a maximum power density of .46 W/cm2. These results demonstrate the feasibility of this new five‐layer tape casting technique to produce IT‐SOFC.

All-SiC Fiber-Optic Sensor Based on Direct Wafer Bonding for High Temperature Pressure Sensing

This paper presents an all-SiC fiber-optic Fabry-Perot (FP) pressure sensor based on the hydrophilic direct bonding technology for the applications in the harsh environment. The operating principle, fabrication, interface characteristics, and pressure response test of the proposed all-SiC pressure sensor are discussed. The FP cavity is formed by hermetically direct bonding of two-layer SiC wafers, including a thinned SiC diaphragm and a SiC wafer with an etched cavity. White light interference is used for the detection and demodulation of the sensor pressure signals. Experimental results demonstrate the sensing capabilities for the pressure range up to 800 kPa. The all-SiC structure without any intermediate layer can avoid the sensor failure caused by the thermal expansion coefficient mismatch and therefore has a great potential for pressure measurement in high temperature environments.

Mathematical Modeling of Survivability Function for Thermoelectric Module

Developing an optimized reliability model for thermoelectric module at the stress where the probability of module to functions without abruptive failure is a challenging aspect. One of the major reasons is the mismatch of thermal expansion coefficient, which has severe effects on segmented moduli compared to unsegmented moduli. The likelihood of a thermoelectric module to survive at certain level of thermo-mechanical stresses varies by varying number of component (layers) in thermoelectric leg. On another hand, selection of an adequate distribution model to predict reliability and sustainability of the thermoelectric module requires development of new optimized stress-strength-based model. In this paper the predictive reliability model for high temperature segmented module is derived from parametric Lognormal mean residual life and nonparametric Lognormal-kernel survival function to measure probability of module to survive at certain thermo-mechanical stress. A comprehensive comparative discussion has been done to illustrate the maximum likelihood based on Bayesian nonparametric lognormal-Kernel inference method regarding to Monte Carlo simulation, Weibull’s distribution, and Lognormal mean residual life for various shapes for the survival function. It has been demonstrated that nonparametric lognormal-kernel survival function has high ratio of probability to predict the survival of module at higher discrete thermo-mechanical stress data.

Thermal stress reduction strategy for high-temperature power electronics with Ag sintering

Thermal stress caused by the coefficients of thermal expansion (CTE) mismatch of materials is the main failure mechanism in power electronic packaging. High junction temperature (Tj) would induce larger thermal stress and accelerate failing process. In this paper, two packaging strategies to reduce thermal stress of power module for high-temperature operation were presented based on organic-free sintered Ag bonding and wireless packaging structuring: reduce elastic modulus of sintered Ag layer by changing sintering parameters; and reduce the CTE mismatch by changing the ribbon material. Finite-element analysis (FEA) simulations confirmed that these strategies can effectively reduce the maximum and average thermal stress in power electronics packaging. Furthermore, by combining these two strategies, high-temperature power cycling test indicated that the wireless packaging with Cu30Mo70 ribbon and 5 MPa sintering pressure exhibited excellent durability and reliability in the temperature swing at the junction (ΔTj∆Tj) over 150 K.

Passive Athermal Optical Design Method Considering Thermal-Induced Surface Deformation

Temperature variation not only results in changes in refractive index, radius, thickness, and air space, but also leads to surface deformation due to the mismatch in thermal expansion coefficients between glass and mechanical materials. However, existing passive athermal optical design methods cannot optimize thermal-induced surface deformation, and optimization methods usually focus on structural optimization or thermal control rather than optical optimization. Here, we investigate the deterioration in image quality caused by thermal-induced surface deformation and propose a passive athermal optical design method to reduce deterioration. To this end, MATLAB was utilized to jointly call finite element analysis (FEA) software (COMSOL) and optical design software (Code V) to realize the data exchange of an optical–mechanical–thermal integrated analysis for iterative optical optimization. This process makes automatic iterative optimization possible by transforming parametric FEA results into Zernike coefficients in each iteration of optimization. The theoretical and design examples indicate that our method can effectively reduce the degradation in image quality with surface deformation. Our method provides an optical optimization approach for optical designers to work on a passive athermal optical design by considering thermal-induced surface deformation.

Effects of p-type conductive properties of triaxial strain-regulated ZnO (S, Se, Te) system

P-type ZnO materials are a difficult problem worldwide. The strain caused by doping, substrate and sample lattice constant mismatch, and thermal expansion coefficient mismatch is often ignored. First principle calculation is used to select the appropriate strain range and solve this problem. The effect equivalent to the actual strain is simulated to study the conductive properties and the mechanism of a doping system. Studies have shown that under unstrained conditions, the binding energies of undoped ZnO and all doped systems are negative, and the stability is relatively high. The formation energy of all doped systems increases, and their stability decreases with the increase in tensile strain or compressive strain. This study shows for the first time that when the compressive strain is −5%, the hole mobility of the same doping systems of Zn36SO35, Zn36SeO35, and Zn36TeO35 is relatively maximum. The difference between the hole concentration of the Zn36MO35 (M = S, Se, Te) systems is extremely small regardless of tensile strain or compressive strain. When the compressive strain is −5%, the hole conductivity of the Zn36SO35 system is the best. These findings can serve as guide in the experimental design and preparation of new p-type ZnO conductive functional materials.

Fabrication of GaN/Diamond Heterointerface and Interfacial Chemical Bonding State for Highly Efficient Device Design

The direct integration of gallium nitride (GaN) and diamond holds much promise for high-power devices. However, it is a big challenge to grow GaN on diamond due to the large lattice and thermal-expansion coefficient mismatch between GaN and diamond. In this work, the fabrication of a GaN/diamond heterointerface is successfully achieved by a surface activated bonding (SAB) method at room temperature. A small compressive stress exists in the GaN/diamond heterointerface, which is significantly smaller than that of the GaN-on-diamond structure with a transition layer formed by crystal growth. A 5.3 nm-thick intermediate layer composed of amorphous carbon and diamond is formed at the as-bonded heterointerface. Ga and N atoms are distributed in the intermediate layer by diffusion during the bonding process. Both the thickness and the sp2 -bonded carbon ratio of the intermediate layer decrease as the annealing temperature increases, which indicates that the amorphous carbon is directly converted into diamond after annealing. The diamond of the intermediate layer acts as a seed crystal. After annealing at 1000 °C, the thickness of the intermediate layer is decreased to approximately 1.5 nm, where lattice fringes of the diamond (220) plane are observed.

Negative Thermal Expansion HfV2O7 Nanostructures for Alleviation of Thermal Stress in Nanocomposite Coatings.

A primary mode of failure of thin-film coatings is the mismatch in thermal expansion coefficients of the substrate and the coating, which results in accumulation of interfacial stresses and ultimately in film delamination. While much attention has been devoted to modulation of interfacial bonding to mitigate delamination, current strategies are constrained in their generalizability and have had limited success in imbuing resistance to prolonged thermal cycling. We demonstrate here the incorporation of rigid thermal expansion compensators within polymeric films as a generalizable strategy for minimizing thermal mismatch with the substrate. Nanostructures of the isotropic negative thermal expansion (NTE) material HfV2O7 have been prepared based on the reaction of nanoparticulate precursors. The NTE behavior, derived from transverse oxygen displacement within the cubic structure, has been examined using temperature-variant powder X-ray diffraction, Raman spectroscopy, electron microscopy, and selected-area electron diffraction measurements. HfV2O7 initially crystallizes in a 3 × 3 × 3 superlattice but undergoes phase transformations to stabilize a cubic structure that exhibits strong and isotropic NTE with a coefficient of thermal expansion (CTE) = -6.7 × 10-6 °C-1 across an extended temperature range of 130-700 °C. Incorporation of HfV2O7 in a high-temperature thermoset polybenzimidazole enables the reduction of compressive stress by 67.3% for a relatively small loading of 26.6 vol % HfV2O7. Based on a composite model, we demonstrate that HfV2O7 can reduce the thermal expansion coefficient of polymer nanocomposite films, even at low volume fractions, as a result of its substantially higher elastic modulus compared to the continuous polymer matrix. By changing the volume fraction of HfV2O7, the overall coefficients of thermal expansion of the film can be tuned to match a range of substrates, thereby mitigating thermal stresses and resolving a fundamental challenge for high-temperature composites and nanocomposite coatings.