Interfacial Thermal Stresses Research Articles

Predicted Thermal Stress in a Multileg Thermoelectric Module (TEM) Design

A physically meaningful analytical (mathematical) model is developed for the prediction of the interfacial shearing thermal stress in an assembly comprised of two identical components, which are subjected to different temperatures. The bonding system is comprised of a plurality of identical columnlike supports located at equal distances (spaces) from each other. The model is developed in application to a thermoelectric module (TEM) design where bonding is provided by multiple thermoelectric material supports (legs). We show that thinner (dimension in the horizontal direction) and longer (dimension in the vertical direction) TEM legs could result in a significant stress relief, and that such a relief could be achieved even if shorter legs are employed, as long as they are thin and the spacing between them is significant. It is imperative, of course, that if thin legs are employed for lower stresses, there is still enough interfacial “real estate,” so that the adhesive strength of the assembly is not compromised. On the other hand, owing to a lower stress level in an assembly with thin legs and large spacing, assurance of its interfacial strength is less of a challenge than for a conventional assembly with stiff, thick, and closely positioned legs. We show also that the thermal stresses not only in conventional TEM designs (using Be2Te3 as the thermoelectric material, and Sn-Sb solder), but also in the future high-power (and high operating temperatures) TEM design (using Si or SiGe as the thermoelectric material and Gold100 as the appropriate solder), might be low enough, so that the short- and long-term reliability of the TEM structure could still be assured. We have found, however, that thin-and-long legs should be considered for lower stresses, but not to an extent that appreciable bending deformations of the legs become possible. Future work will include, but might not be limited to, the finite-element computations and to experimental evaluations (e.g., shear-off testing) of the stress-at-failure for the TEMs of interest.

Graded YSZ/Al2O3 hot corrosion resistant coating with enhanced thermal shock resistance

In order to reduce the interfacial thermal stress, a novel functionally graded YSZ/Al2O3 nanocoating (YSZ/Al2O3 FGC) which is hot corrosion resistant was developed with the sols of ZrO2, Y2O3 and AlOOH in this paper. SEM and XRD analyses indicated that the YSZ/Al2O3 FGC was about 4.2 μm in thickness and composed of t′-YSZ and γ-Al2O3 nanoparticles. Hot corrosion tests showed that the YSZ/Al2O3 FGC remarkably restrained the infiltration of the molten NaVO3 salt into the YSZ substrate. A thermal shock test at 700 °C showed that the thermal shock resistance of the YSZ/Al2O3 FGC was enhanced greatly compared with that of the Al2O3 coating. The improvement of the YSZ/Al2O3 FGC in the thermal cycle lifetime could be attributed to its compositionally graded structure, which decreased the thermal stress due to the thermal expansion coefficient mismatch between the YSZ substrate and the Al2O3 coating.

Design Key Points for High Power LED Encapsulation

LED performance was significantly improved because of the improvement of encapsulation design and materials properties. The design key points were reviewed in point view of optical, electrical, thermal, and reliability consideration. It was concluded that the packaging design should be simultaneously implemented with the chip design, integrally considering the optics, electrics, thermal, and reliability together. The interfacial thermal resistance and stress from packaging also play critical roles for the optical efficiency and reliability of packaged LED device.

Maintenance of highly interfacial shear strength by diblock copolymer between carbon fiber and epoxy resin in hostile environment

The environmental resistance properties of carbon fiber (CF), with various surface modifications, reinforcing epoxy resin composites have been studied by a microbond test. The results of cooling–heating cycling between −40 and 95 °C indicate that the introduction of the flexible poly(n-butyl acrylate) (PnBA) blocks into the interface can effectively decrease the interfacial degradation rate, induced by interfacial thermal stress. After 50 cooling–heating cycles, the interfacial shear strength between CF and epoxy resin was still as high as 32.69 ± 2.13 MPa. The results of hygrothermal treatment by immersing the composites in hot water show that assembly morphology of the diblock copolymer hydroxyl-terminated poly(n-butyl acrylate-b-glycidyl methacrylate) (OH-PnBA-b-GMA) at the interface can decrease the interfacial water absorption and thus increase the hygrothermal resistance of the composite. Besides, the length of PnBA block in the diblock copolymer influenced the interfacial properties of the composite in a hygrothermal environment.

Analysis of interfacial thermal stresses for damaged structures: new theoretical model

The use of the composites for the reinforcement of the beams and structures metallic and non-metallic, is one of the recent methods of the rehabilitation by composites, what permits to replace the classic techniques of the assembly based on soldering, bonding, etc. But this technique gives stresses concentrations to the level of the zone of the assembly. The methods of rehabilitation by composites permit to prolong the life span of these structures under a reduced exploitation cost and with less pollution to the environment. In this article, an original survey on the stresses between the structure and the composite (fiber reinforced polymer [FRP]) was finalized, taking account, as well, of the mechanical and thermal loadings, as well as the effects of deformation shear lag. This originality puts in evidence a new theoretical model which takes into account the thermal effect of the stresses concentration that has not been treated seriously by the previous studies.

Interfacial Thermal Stresses in Fiber Reinforced Composites

This article presents an analytic solution for the two-dimensional thermoelastic problem of interfacial thermal stresses in fiber reinforced composites. The solution is derived by superposing the solution for single fiber embedded in matrix successively. Study examples demonstrate the elegance and robustness of the present approach. Analysis results show that the effect of a single fiber is localized and there is an optimal elastic mismatch of fiber and matrix where the interfacial hoop stress is insensitive to the fiber packing arrangement and the fiber volume fraction.

Influence of Cobalt Additive on Mechanical Properties and Residual Stress of Al<sub>2</sub>O<sub>3</sub>-TiC Ceramic Cutting Tool Material

Cobalt additive was added into the Al2O3-TiC composite to improve the mechanical properties. The microstructure and fracture mode of composite tool materials Al2O3-TiC and Al2O3-TiC-Co were investigated, emphasizing on the difference of thermal residual stress distribution with added cobalt. The interfacial thermal stress and the stress distribution in each phase were worked out according to thermoelasticity theory. The results demonstrated that cobalt could strengthen the interfacial bonding strength of ceramic grains and influence the residual stress distribution in Al2O3 matrix, which change the fracture mode and lead to the flexural and irregular crack propagation path.

Room vs. temperature studies of model composites: modes of failure of carbon fibre/epoxy interfaces

Laser Raman spectroscopy (LRS), transmission polarised optical microscopy (TPOM) and scanning electron microscopy (SEM) have been used to characterise the interface of model single-fibre composites. The composites consisted of single-carbon fibres embedded in epoxy resin. Local stress measurements as a function of applied strain were performed using LRS at both room temperature (RT) and 60 °C. Consecutively, the coupons were strained to failure and field emission SEM was used to study the fracture surfaces. In a parallel study, identical systems were subjected to incremental tension and fracture events were recorded as a function of applied strain. At RT, TPOM was used to provide additional insight in the local stress transfer. The stress transfer was found to depend on the combined effect of interfacial chemistry and thermal stresses. Thus, in the case of sized fibres, there is a distinct change in the interfacial failure mode at high temperature, whereas in the case of unsized fibres, the stress transfer is dominated by thermal stresses: at high temperature it is weak, due to the relief of the thermal stress field.

Effects of different tunnel layers on retention characteristics for Pd-nanocrystal-based nonvolatile memory

Thin Pd metal cannot be separated well into Pd nanocrystals (NCs) on 4-nm-thick SiO2/Si, but the Pd nanocrystals about 5nm in diameter can be produced very easily on 4-nm-thick HfAlO/Si at room temperature. Owing to a huge thermal stress in SiO2 film during Pd nanocrystal formation at 500^oC; it results in a large number of leakage current paths and shallow traps in SiO2 film, which leads to a worse retention characteristics. From some material and electrical analyses, these induced defects mainly result from the interfacial reaction and thermal stress due to different coefficients of thermal expansion between Pd and tunnel layer during Pd annealing. However, fewer leakage current paths and shallow traps in HfAlO film result in a better retention characteristic. The degradation of retention characteristic is more obvious with annealing temperature, and the final memory window is 0.95V for 900^oC annealed Pd NCs nonvolatile memory with HfAlO tunnel oxide.

Release of interfacial thermal stress and accompanying improvement of interfacial adhesion in carbon fiber reinforced epoxy resin composites: Induced by diblock copolymers

In this work, diblock copolymer Hydroxyl-Terminated poly (n-butylacrylate)-b-poly (glycidyl methacrylate) (OH-PnBA-b-PGMA) was synthesized by atom transfer radical polymerization (ATRP) and was then introduced into the interface between carbon fiber and epoxy resin. Micro-Raman spectroscopy and microbond test were employed to study the influence of grafted polymers on the interfacial properties. From the Micro-Raman spectroscopy results, the interfacial thermal stress in carbon fiber/epoxy resin micro-composite decreases from 546.9MPa to 451.9MPa due to the grafting of OH-PnBA180-b-PGMA70 on the carbon fiber. Meanwhile, the interfacial shear strength (IFSS) value increases rapidly from 29.8MPa to 52.3MPa, measured by microbond test. Therefore, it can be concluded that such a diblock copolymer can effectively both release the thermal stress and improve the interfacial adhesion. Moreover, it proves that the length of PnBA block has great influence on the interfacial properties of carbon fiber/epoxy composite.

Bi-Layered Model of Interfacial Thermal Stresses with the Effect of Different Temperatures in the Layers

An interfacial shearing and peeling stress model is proposed to account for different uniform temperatures and thickness wise linear temperature gradients in the layers. This upgraded model can be viewed as a more generic form to determine interfacial stresses under different temperature conditions in a bi-layered assembly. The selected shearing and peeling stress results are presented for the case of die and die attach as commonly seen in electronic packaging. The obtained results can be useful in interfacial stress evaluations and physical design of bi-material assemblies, which are used in microelectronics and photonic applications.

Misfit as a characteristic of the level of interfacial stresses in single-crystal nickel superalloys

A method for determining the level of interfacial stresses in single-crystal nickel superalloys and the results of its determination are presented. The splitting of the (h00) X-ray diffraction peaks in the X-ray diffraction patterns of the γ and γ′ phases that is caused by interfacial thermal stresses is studied.

In situ synthesis of TiB whisker reinforcements in the joints of Al2O3/TC4 during brazing

Al2O3 ceramic has been successfully joined to Ti–6Al–4V alloy with Ag–Cu–Ti–B mixed powder. The TiB whiskers in the brazing layer were in situ synthesized during brazing. The effects of B content in reactant on the phase composition, microstructure and shear strength of the joints were investigated using SEM, EDS, and shear test. Results indicate that B content in the filler has a great impact upon the microstructure of the joints via exerting an influence on the volume fraction of in situ synthesized TiB whiskers. When the TiB content is 40vol.%, the shear strength reaches the maximum value of 77.9MPa. The higher content of TiB (≥40vol.%) depresses the shear strength of the joints due to the interfacial thermal stress cannot be relaxed. Reaction phases (Ti3Cu2AlO, Ti2Cu, Ti2(Cu, Al), Ti(Cu, Al) and Ti3Al) appear in the joint, moreover, as the volume fraction of TiB increase, Ag (s.s) and Ti(Cu, Al) distribute more uniform and fine in the brazing layer, as well as TiB whiskers mainly distribute in them. Eventually, Ti3Cu2AlO, TiB and TiB2 firstly generate based on the thermodynamic analysis, and in excessive Ti circumstances, TiB whiskers remain in the brazing alloy.

Adaptive Composite Materials with Novel Architectures

Today, there is a strong push to improve the thermal management of electronic components in order to increase the performance and the reliability of electronic devices. Up to now, most of the heat sinks are mainly made of Copper that presents a good thermal conductivity (TC) but a coefficient of thermal expansion (CTE) much higher than the ceramic of the DBC (direct bonding Copper). It induces interfacial thermal stresses and indeed it decreases the reliability of the global electronic system. Therefore, there is a strong need for the development of novel heat dissipation material having low CTE combined with high TC. Carbon fibres reinforced copper matrix offers a good compromise between thermo mechanical properties (i.e. CTE) and medium TC. In order to increase surface TC, pure Copper can be added on the top surface and/or on the bottom one of the composite heat sink playing the role of heat spreader for hot spots linked with the Si components. The fabrication technique of these materials is based on powder metallurgy technique. The thermal properties of adaptive materials, TC and CTE, have been measured for different Copper thicknesses and architectures ([C/Cu], [Cu – C/Cu] and [Cu – C/Cu – Cu]). Simulation of the TC and CTE have been performed and compared to the experimental results.

Nonlinear temperature characteristic of thermal expansion of Grf/Mg composites

Graphite fiber reinforced magnesium matrix(Grf/Mg) composites were fabricated by squeeze casting technology. M40 graphite fibers were reinforced to AZ91D and ZM6, their thermal expansion behaviors of M40/AZ91D and M40/ZM6 composites in the temperature range from 20 to 490 °C were investigated. The results show that the interfacial species and thermal stress have significant influence on the thermal expansion behavior of the composites. Simultaneously, the longitudinal coefficient of thermal expansion of Grf/Mg composites are affected by the thermal stress, interfacial species and yield strength of matrix alloy, it also decreases with increasing temperature and descending rate of longitudinal coefficient of thermal expansion(CTEs) of Grf/Mg composites changed in different temperature ranges. In terms of different descending rates, the curve of coefficient of thermal expansion vs temperature can be divided into three stages. The matrix alloys M40/AZ91D and M40/ZM6 yield at 170 and 155°C in the thermal expansion, respectively.

Micromechanical modeling of interface damage of metal matrix composites subjected to off-axis loading

A three dimensional micromechanics based analytical model is presented to investigate the effects of initiation and propagation of interface damage on the elastoplastic behavior of unidirectional SiC/Ti metal matrix composites (MMCs) subjected to off-axis loading. Manufacturing process thermal residual stress (RS) is also included in the model. The selected representative volume element (RVE) consists of an r × c unit cells in which a quarter of the fiber is surrounded by matrix sub-cells. The constant compliance interface (CCI) model is modified to model interfacial de-bonding and the successive approximation method together with Von-Mises yield criterion is used to obtain elastic–plastic behavior. Dominance mode of damage including fiber fracture, interfacial de-bonding and matrix yielding and ultimate tensile strength of the SiC/Ti MMC are predicted for various loading directions. The effects of thermal residual stress and fiber volume fraction (FVF) on the stress–strain response of the SiC/Ti MMC are studied. Results revealed that for more realistic predictions both interface damage and thermal residual stress effects should be considered in the analysis. The contribution of interfacial de-bonding and thermal residual stress in the overall behavior of the material is also investigated. Comparison between results of the presented model shows very good agreement with finite element micromechanical analysis and experiment for various off-axis angles.

Modeling of the effects of different substrate materials on the residual thermal stresses in the aluminum nitride crystal grown by sublimation

A three-dimensional numerical finite element modeling method is applied to compare interfacial residual thermal stress distribution in AlN single crystals grown by using different substrates such as silicon carbide, boron nitride, tungsten, tantalum carbide, and niobium carbide. A dimensionless coordinate system is used which reduces the numbers of computations and hence simplifies the stress analysis. All components of the stress distribution, both in the film and in the substrate, including the normal stress along the growth direction as well as in-plane normal stresses and shear stresses are fully investigated. This information about the stress distribution provides insight into understanding and controlling the AlN single crystal growth by the sublimation technique. The normal stress in the film at the interface along the growth direction and the shear stresses are zero except at the edges, whereas in-plane stresses are nonzero. The in-plane stresses are compressive when TaC and NbC substrates are used. A small compressive stress might be beneficial in prohibiting crack growth in the film. The compressive stress in the AlN is lower for the TaC substrate than that for the NbC. Tensile in-plane stresses are formed in the AlN for 6H-SiC, BN, and W substrates. This tensile stress in the film is detrimental as it will assist in the crack growth. The stress concentration at the edges of the AlN film at the interface is compressive in nature when TaC and NbC are used as a substrate. This causes the film to bend downward (i.e., convex shape) and assist it to adhere to the substrate. The AlN film curves upward or in a concave shape when SiC, BN, and W substrates are used since the stress concentration at the edges of the AlN film is tensile at the interface and this may cause detachment of the film from the substrate.

Micromechanics of interfacial thermal stresses in fiber reinforced composites

This paper develops a generic 2D micromechanical model that deal with the interfacial thermal stresses in fiber reinforced composites with randomly packed fibers of different sizes and thermoelastic properties by successive superposition of the solution of a single fiber embedded in a matrix. This approach reduces the complicated micromechanical model to a system of linear algebraic equations using the perturbation technique. The accuracy of current approach is verified by comparing it with the existing analytical solution. The micromechanics model is then applied to analyze periodically and randomly packing fiber reinforced composites. Analysis results show that the effect of a single fiber is localized and there is an optimal elastic mismatch of fiber and matrix where the interfacial hoop stress is insensitive to the fiber packing arrangement and the fiber volume fraction.

Analysis for Thermal Conductance Effect on the Interfacial Thermal Stresses of Anisotropic Composites

The present work applies the regularized boundary integral equations that are newly developed to treat the thermoelastic field in thin anisotropic media. For the anisotropic thermal field, a direct domain mapping technique is applied with a unique interface condition that considers the heat conductance relation. By incorporating the heat conductance effect, the paper investigates how interfacial thermal stresses between generally anisotropic materials vary with the heat conductance coefficient. Accounting for the thermal conductance effect, the paper presents the complete algorithm for computing the thermal as well as the subsequent elastic fields on interfaces between dissimilarly adjoined anisotropic composites.

Elastoplastic damage micromechanics for elliptical fiber composites with progressive partial fiber debonding and thermal residual stresses

Incorporating the interfacial damage and thermal residual stresses, an elasto-plastic damage formulation is proposed to predict the overall transverse mechanical behavior of continuous elliptical-fiber reinforced ductile matrix composites within the framework of micromechanics and homogenization. Based on the concept of equivalent inclusion and taking the progressive interfacial debonding angle into consideration, partially debonded fibers are replaced by equivalent orthotropic, perfectly bonded fibers. Three interfacial damage modes are considered. The Weibull's probabilistic function is adopted to describe the varying probability of progressive partial fiber debonding. The effective elastic moduli of four-phase composites, composed of a ductile matrix and randomly located yet unidirectionally aligned fibers are derived by a micromechanical formulation. Thermal residual stresses are taken into account through the concept of thermal eigenstrain to investigate the effects of the manufacturing processinduced residual stresses. Employing the micromechanical approximation, the overall stress-strain responses and the effective yield function are formulated with the thermal eigenstrain. When comparing with the available experimental data, significant effects of thermal residual stresses are discussed. Moreover, the effects of the interfacial strengths and the cross- sectional shapes of fibers on the mechanical behaviors of composites are systematically investigated.