Difference In Thermal Expansion Coefficients Research Articles

Growth and Luminescence of Polytypic InP on Epitaxial Graphene

AbstractVan der Waals epitaxy is an attractive alternative to direct heteroepitaxy where the forced coherency at the interface cannot sustain large differences in lattice parameters and thermal expansion coefficients between the substrate and the epilayer. Herein, the growth of monocrystalline InP on Ge and SiO2/Si substrates using graphene as an interfacial layer is demonstrated. Micrometer‐sized InP crystals are found to grow with interfaces of high crystalline quality and with different degrees of coalescence depending on the growth conditions. Some InP crystals exhibit a polytypic structure, consisting of alternating zinc‐blende and wurtzite phases, forming a type‐II homojunction with well (barrier) width of about 10 nm. The optical properties, investigated using room temperature nano‐cathodoluminescence, indicate the signatures of the direct optical transitions at 1.34 eV across the gap of the zinc‐blende phase and the indirect transitions at ≈1.31 eV originating from the alternating zinc‐blende and wurtzite phases. Additionally, the InP nanorods, found growing mainly on the graphene/SiO2/Si substrate, show optical transition across the gap of the wurtzite phase at ≈1.42 eV. This demonstration of InP growth on graphene and the correlative study between the structure and optical properties pave the way to develop hybrid structures for potential applications in integrated photonic and optoelectronic devices.

Temperature Effects on the Magnetization and Magnetoimpedance in Ferromagnetic Glass-Covered microwires

The effect of temperature on static and dynamic magnetization in Co-based amorphous microwires was investigated with the aim of potential applications in miniature temperature sensors. The wires of two compositions with different magnetostriction and Curie temperature in glass-cover and after removing the glass layer demonstrated very different temperature behaviour of the magnetization loops and magnetoimpedance. The mechanisms of the temperature effects are related to the residual stress distribution due to fast solidification, the difference in thermal expansion coefficient of metal and glass and the proximity to the Curie temperature. The interplay of these factors may result in a very strong temperature dependence of magnetoimpedance in a moderate temperature range (room temperature −373K). Such elements may be incorporated in various composite materials for a local temperature monitoring.

Design and Electro-Thermo-Mechanical Behavior Analysis of Au/Si₃N₄ Bimorph Microcantilevers for Static Mode Sensing.

This paper presents a design optimization method based on theoretical analysis and numerical calculations, using a commercial multi-physics solver (e.g., ANSYS and ESI CFD-ACE+), for a 3D continuous model, to analyze the bending characteristics of an electrically heated bimorph microcantilever. The results from the theoretical calculation and numerical analysis are compared with those measured using a CCD camera and magnification lenses for a chip level microcantilever array fabricated in this study. The bimorph microcantilevers are thermally actuated by joule heating generated by a 0.4 μm thin-film Au heater deposited on 0.6 μm Si3N4 microcantilevers. The initial deflections caused by residual stress resulting from the thermal bonding of two metallic layers with different coefficients of thermal expansion (CTEs) are additionally considered, to find the exact deflected position. The numerically calculated total deflections caused by electrical actuation show differences of 10%, on average, with experimental measurements in the operating current region (i.e., ~25 mA) to prevent deterioration by overheating. Bimorph microcantilevers are promising components for use in various MEMS (Micro-Electro-Mechanical System) sensing applications, and their deflection characteristics in static mode sensing are essential for detecting changes in thermal stress on the surface of microcantilevers.

Performance Evaluation of Nose Cap to Silica Tile Joint of RLV-TD under the Simulated Flight Environment using Plasma Wind Tunnel Facility

Indian Space Research Organisation, India has successfully flight tested the reusable launch vehicle through launching of a demonstration flight known as RLV-TD HEX mission. This mission has given a platform for exposing the thermal protection system to the real hypersonic flight thermal conditions and thereby validated the design. In this vehicle, the nose cap region is thermally protected by carbon–carbon followed by silica tiles with a gap in between them for thermal expansion. The gap is filled with silica fibre. Base material on which the C–C is placed is made of molybdenum. Silica tile with strain isolation pad is bonded to aluminium structure. These interfaces with a variety of materials are characterised with different coefficients of thermal expansion joined together. In order to evaluate and qualify this joint, model tests were carried out in Plasma Wind Tunnel facility under the simultaneous simulation of heat flux and shear levels as expected in flight. The thermal and flow parameters around the model are determined and made available for the thermal analysis using in-house CFD code. Two tests were carried out. The measured temperatures at different locations were benign in both these tests and the SiC coating on C–C and the interface were also intact. These tests essentially qualified the joint interface between C–C and molybdenum bracket and C–C to silica tile interface of RLV-TD.

Al alloy metal matrix composites reinforced by WS2 inorganic nanomaterials

In the present work, advanced aluminium metal matrix composite reinforced by nanomaterials is reported. The materials used for the experiments are AA6061 aluminium alloy, compounded with inorganic nanotubes (INT) and fullerene-like (IF) nanoparticles of WS2. Aluminium metal matrix composite with different weight percentage (0.1–0.5wt%) of the INT and IF were prepared. Both the AA6061 aluminium and the aluminium metal matrix nanocomposites (MMCs) were re-melted by the stirring-casting method. Mechanical properties and microstructure analysis of the nano-MMCs were conducted.The measurements have shown that the hardness, yielding strength, ultimate tensile strength, and ductility were improved by up to 68% compared to the neat alloy for nanoparticles concentrations the range of 0.2wt%. Metallography microstructure analysis showed that increasing the WS2 INT or IF nanoparticles weight percentage in aluminium metal matrix composite, resulted in a more refined grains. The grain size of the composite matrix was reduced by up to 48.4% compared with that of neat AA6061 alloy ingot. Scanning electron microscope (SEM) analysis showed that addition of 0.5wt% of WS2 nanotubes into the Al matrix resulted in INT agglomeration which had deleterious effect on the tensile strength. The composite materials reinforcement mechanism was studied. This analysis showed that the improved mechanical properties of the metal nanocomposites can be predominantly attributed to the differences in thermal expansion coefficients of the WS2 INT (or WS2 IF nanoparticle) and the AA6061 alloy.

Active vibration control of a beam instrumented with MWCNT/epoxy nanocomposite sensor and PZT-5H actuator, robust to variations in temperature

Multi walled carbon nano tube (MWCNT)/epoxy nanocomposite strain sensor and piezoelectric actuator pair are used in active vibration control (AVC) of a smart cantilevered beam. Due to different coefficients of thermal expansion of MWCNT and epoxy, the resistance of carbon nanotube/epoxy based strain sensor changes nonlinearly with change in temperature. The temperature dependence of CNT based strain sensor is modelled by fitting a polynomial curve on the experimentally obtained characterization data of CNT/epoxy strain sensor. Using augmented constitutive equations of piezoelectricity, finite element model of smart cantilevered beam instrumented with collocated nanocomposite strain sensor and PZT-5H actuator is derived. Effect of temperature on PZT-5H actuator is also considered in derivation of the control law. Active vibration control of the first mode of a smart cantilevered beam instrumented with collocated CNT based strain sensor and PZT-5H actuator is performed at ambient temperature and at temperature slightly away from the ambient temperature for two cases: (1) without considering the temperature dependence of CNT composite strain sensor in Kalman observer and (2) considering the temperature dependence of CNT composite strain sensor in Kalman observer. Simulations are also performed to visualize the behaviour of smart beam when the beam is subjected to a temperature impulse. This paper suggests a novel AVC strategy using MWCNT/epoxy strain sensor and PZT-5H actuator patch that is robust to variations in temperature.

Effect of Wafer and Damaged Layer Thickness on Residual Stress and Bow of Free-Standing Gallium Nitride Wafers during Wafering Process

In recent years, free-standing gallium nitride (FS-GaN) wafers were researched due to high demand for power device, high brightness LED (HB-LED) and laser diode (LD). The GaN wafers grown by hydride vapor phase epitaxy (HVPE) have been used for the substrate of power device, HB-LED and LD because they demonstrated lower dislocation density (DD) than that of GaN wafers grown by metal-organic chemical vapor deposition (MOCVD). However, they still have residual stress and wafer bow, which lower the device productivity in mass production. To overcome these issues, the control of crystallographic quality and wafer bow are the key technologies. The crystallographic quality of the GaN wafer is mostly determined at the growth process. The bow of the wafer is determined by the difference in thermal expansion coefficient (TEC) between sapphire substrate and GaN, and it can be controlled at the wafering process. In this study, therefore, we investigated how the bow of the FS-GaN wafer is changed during wafering process. In particular, the effect of the abrasive size on the damaged layer thickness during lapping process was investigated and residual stress was calculated theoretically. Two inch GaN wafers were fabricated on (0001) sapphire substrates by HVPE and separated by the laser lift-off (LLO) process. The bow of as-grown GaN wafer after LLO process was ranged from -300 to -100 um, which was determined by the residual thermal stress. The GaN wafer bow is changed during the wafering process, especially the lapping process. From the Stoney’s formula, it is notable that the thickness of substrate (tS) and sub-surface damaged layer (tSSD) can change the wafer bow. Fig. 1 shows the wafer bow (black triangle), residual stress (blue square) of experimented GaN wafer and wafer bow calculated by the Stoney’s formula (red triangle). It was confirmed that the wafer bow was reduced as the thickness of GaN wafer reduced. In particular, the bow was inverted from concave to convex at the thickness of around 180 um. As the lapping process progressed, tS is reduced and tSSD is generated, which led to the change of bow to convex. Fig. 2 shows the sub-surface damaged layer of the GaN wafer after lapping with 2-um diamond abrasives. It exhibits that the crystalline GaN layer was damaged by the diamond abrasives and changed to polycrystalline phase at the top surface. In addition, it indicates that the microcracks propagated into the wafer under the polycrystalline layer. Total thickness of polycrystalline layer and microcracks, tSSD, was around 1.09 um. Thus, the tSSD should be removed for achieving the damage-free GaN wafer. In our presentation, we report how we achieved zero-bow FS-GaN wafer shown in Fig. 3, and how to control residual stress and optimize wafering process. * This work was financially supported by the Brain Korea 21 Plus Program in 2017. Figure 1

A review of typical thermal fatigue failure models for solder joints of electronic components

For electronic components, cyclic plastic strain makes it easier to accumulate fatigue damage than elastic strain. When the solder joints undertake thermal expansion or cold contraction, different thermal strain of the electronic component and its corresponding substrate is caused by the different coefficient of thermal expansion of the electronic component and its corresponding substrate, leading to the phenomenon of stress concentration. So repeatedly, cracks began to sprout and gradually extend [1]. In this paper, the typical thermal fatigue failure models of solder joints of electronic components are classified and the methods of obtaining the parameters in the model are summarized based on domestic and foreign literature research.

Suitable sealants for cracked aluminized austenitic steels and their oxidation behaviour

Aluminizing of steels may result in the appearance of tensile cracks from the coating surface to the substrate due to the difference in thermal expansion coefficients. The cracks often need to be sealed to make the coatings impervious to chemical attack and to limit their mechanical degradation. In this work, dipping of the cracked aluminide coatings elaborated on IN-800HT in 0.5, 1 and 3.75mol·L−1 concentrations of H3PO4 in acetone and increasing the dipping time are investigated. The three concentrations resulted in the formation of P-O compounds and of Al2O3. However, the 0.5mol·L−1 allowed better wetting of the surface. Further reduction of the concentration from 0.5mol·L−1 to 0.2mol·L−1 brought about the complete sealing of both large and fine cracks. The efficiency of the sealing process with 0.2mol·L−1 and 0.5mol·L−1 was thereafter evaluated by cyclic oxidation in air at 650°C for >1000cycles. The overall microstructure and composition of the aluminide coatings remained identical after sealing and the oxidation behaviour comparable to the untreated counterparts. The oxide scale formed on the untreated samples appeared thicker compared to the sealed ones. The latter were covered by a thin layer of AlPO4 which evolved towards a thin protective alumina scale. However, for the sealed samples, some minor cracks were detected after oxidation but they appeared completely healed with Al2O3 and no propagation of them beyond the additive layer/interdiffusion zone was noticed.

Heat protective properties of NiCrAlY/Al2O3 gradient ceramic coating fabricated by plasma spraying and slurry spraying

Al2O3 non-gradient ceramic coating (marked as 1# coating) and NiCrAlY/Al2O3 gradient ceramic coating (marked as 2# coating) were prepared with the plasma spraying and slurry spraying methods, respectively. The coating specimens were characterized by scanning electron microscopy and X-ray diffractometry. The high temperature oxidation kinetics curves of the coatings were examined; the bonding strength between the coatings and the metal substrates was studied, and analyses were conducted on the thermal shock resistance and the failure mechanism of the coatings. The results show that spraying of Al2O3 ceramic slurries can effectively fill the cracks generated during the process of plasma spraying of NiCrAlY coatings, and metallurgical bonds form between the coatings and the metal substrates. α-Al2O3 and rutile TiO2 are the main crystalline phase structures of the temperature resistant ceramic coatings. The oxidation rate constant of 2# coating was 1.323×10−4mg2cm−4h−1, which shows the best resistance to high temperature oxidation. The effect of high-temperature melting of the Al metal in the bonding layer was found to be beneficial to the bonding strength. Under the same temperature conditions, 2# coating had a higher bonding strength than 1# coating, and when the oxidation time was 96h, its bonding strength reached up to 26.9MPa. The thermal expansion coefficient of the NiCrAlY bonding coating fell between those of the metal substrate and ceramic top coating, thus reducing the thermal expansion coefficient difference between the two and decreasing the rate of propagation of the cracks to improve the thermal shock resistance of the coating. An integrated study indicated that 2# coating had a better heat protective performance for the metal substrates.

Design methodology for porous composites with tunable thermal expansion produced by multi-material topology optimization and additive manufacturing

Abstract To realize negative thermal expansion (NTE), porous composites made of two materials with different coefficients of thermal expansion are being actively researched. NTE can be realized by taking advantage of the thermal deformation mechanisms of a composite material's internal geometry. However, in addition to negative thermal expansion, materials with anisotropic and large positive thermal expansion are also desirable for various applications. Also, additive manufacturing provides new ways to fabricate composites by layering multiple materials at arbitrary points in three-dimensional space. In this study, we developed a design methodology for porous composites, which showed defined thermal expansion characteristics, including negative and positive thermal expansion as well as isotropic and anisotropic thermal expansion. Our approach was tested based on the fabrication of a multi-material photopolymer by additive manufacturing. The internal geometries required to produce such characteristics were designed by topology optimization, which is the most effective structural optimization method for realizing macroscopic inward deformation and for maintaining stiffness. The designed structures were converted to three-dimensional models and fabricated by multi-material photopolymer additive manufacturing. Using laser scanning dilatometry, we measured the thermal expansion of these specimens, revealing well-ordered thermal expansion, from anisotropic positive thermal expansion to anisotropic negative thermal expansion, over a wide range of about −3 × 10−4 K−1 to 1 × 10−3 K−1.

Interfacial Area between Hetero‐Epitaxial γ‐Al2O3 and Silicon

The interfacial region between hetero‐epitaxial γ‐Al2O3 and Si (111) substrates is studied in detail. The purpose is to address many open questions regarding the growth of γ‐Al2O3 grown on Si, such as the atomic stacking orders, strain relaxation modes, and observed thin‐film qualities. The cross‐sectional atomic stacking order is directly evidenced to be a cubic spinel structure, with a possible in‐plane stacking order proposed. A 1.5 nm defect‐rich transition layer is found at the interface, in which the lattice structure transitions from Si to γ‐Al2O3 arrangement. The thin‐film quality, in terms of crystallinity and low film roughness, is observed to improve with increasing thickness up to ≈8 nm. For thicknesses above 8 nm, grain boundaries are observed along with the appearance of pinholes, due to the large lattice constant and thermal expansion coefficient differences between γ‐Al2O3 and Si. Polycrystalline islands form in these pinholes and gradually replace the initial layer‐by‐layer growth of monocrystalline γ‐Al2O3, leading to a mainly polycrystalline material at large thicknesses. The insights gained on the hetero‐epitaxy of γ‐Al2O3 on Si will be useful for future work looking to exploit this hetero‐epitaxial materials system.

Stress Study on CrN Thin Films with Different Thicknesses on Stainless Steel

The reliability of a substrate curvature-based stress measurement method for CrN thin films on substrate with fluctuant surface was discussed. The stress error led by the ignorance of substrate thermal deformation was studied. Results showed that this error could be as large as several hundred MPa under general deposition conditions. Stress in the CrN thin films with different thicknesses ranging from 110 to 330 nm on stainless steel was studied by this method, in comparison with conventional results on silicon wafer. The thin films’ morphology and structure were investigated and related to the film stress. A significant result of the comparison is that stress evolution in the thin films on steel obviously differs from that on silicon wafer, not only because the two substrates have different coefficients of thermal expansion, which provokes thermal stress, but also the considerable discrepancy in the thin films’ grain coarsening rate and structure that induce different intrinsic stresses.

Numerical investigation of residual stresses in high temperature ZrO2 insulation coatings on Ag tape substrate for magnet technologies

In this research, the numerical investigation of residual stresses was carried out in high temperature ZrO2 insulation coatings on Ag tape substrate for magnet technologies. ZrO2 coatings were fabricated on Ag substrate using a reel-to-reel sol–gel system. The microstructural observation of ZrO2 coatings was performed through an environmental scanning electron microscope (ESEM). ESEM micrograph revealed that ZrO2 coatings with crack had mosaic structure which is suitable for high temperature insulation fields. The thermal stresses in the each layer were determined by the means of the lamination theory for both elastic solution and the finite element method (FEM) for elastoplastic solution in the temperature range of 298 to 50 K in liquid helium. The residual stresses happened throughout the production are neglected because of gradually cooling process. As a result of this, all of the residual stresses at the 298 K are accepted to be zero. Difference of thermal expansion coefficients and elasticity modulus cause the thermal stresses in the layers.

English

The main objective of this paper is to develop a method to predict the crack length and crack growth rate in unidirectional carbon fiber/epoxy composite as a function of temperature variation. In order to measure the crack length and crack growth rate, a new analytical method by applying experimental procedure data is developed. By the novel analytical method, relations are achieved to predict the crack length and crack growth rate in unidirectional carbon fiber/epoxy composite. Finally, by using these relations, comparison between crack length and crack growth rate between two thermal fatigue experiments is done. The results have shown that the crack growth rate in bidirectional carbon fiber/epoxy composite is very higher that in unidirectional carbon fiber/epoxy composite. Investigations regarding the achieved results have shown that the higher crack growth rate is due to the mismatch between the coefficients of thermal expansion in two directions of bidirectional carbon fiber/epoxy composite. The two different coefficients of thermal expansion may cause stress concentration in the inter-laminar plies of the composite and therefore may cause higher crack growth rate. Key words: Crack growth rate, thermal fatigue, temperature variation, carbon fiber, epoxy, composite material.

RETRACTED ARTICLE: Solder Selection for Reflowing Large Ceramic Substrates During PCB Assembly

Large substrates are prone to warpage issues during reflow due to Coefficient of Thermal Expansion (CTE) mismatch with Printed Circuit Board (PCB) material subsequently affecting solder joint quality and reliability in the form of cracks and interface dislocations. This paper describes best solder paste alloy for reflowing larger dimension ceramic substrates during PCB assembly process to improve solder joint quality by minimizing CTE differences. Experiments have been carried out using various commercially available solder alloys for reflowing larger substrates (80 mm × 80 mm). Solder joint quality was inspected using scanning electron microscope and X-ray for multiple reflow runs using different alloy combinations. Results showed Bismuth-based solder (Bi58/Sn42) to have minimal cracking and improved CTE over other solder alloys. Advantages include reduced costs and improved reliability for applicable devices.

Enhanced movement of CNT-based actuators by a three-Layered structure with controlled resistivity

Due to their unique movement and the lack of motoric parts, flexible actuators have recently been attracting considerable attention in regards to fabrication of soft robots. One of the most known flexible actuators are electro-thermal actuators (ETAs), based on Carbon Nanotubes (CNT). These are bi-layered actuators, triggered electrically, and capable of preforming actuation because of the different coefficient of thermal expansions (CTEs) of the layers. The main disadvantage of these actuators is their limited movement ability, therefore, significant efforts are being invested in improving the actuation of CNT-based actuators. A typical CNT-based actuator is composed of a CNT layer on a polyimide substrate. In this study we show how the deflection of the CNT-based actuators was improved by decreasing the resistance of the CNT layer. We also present, for the first time, a novel tri-layer structured actuator that enables extremely large movements, with a record value deflection of 300°, using a simple, low-cost fabrication method without any orientation of the layers. This was achieved by adding a third layer of a photopolymeriazble polymer on top of the two typical bilayer CNT Kapton device. We further modeled the effect of various parameters of the third layer, such as the Young's modulus and the thickness, on the actuation, supporting the experimental data.

Effect of microstructure of a phase separated sodium-borosilicate glass on mechanical properties

Abstract A glass with the mol% composition 60 SiO 2 ·37 B 2 O 3 ·30 Na 2 O was thermally treated at temperatures in the range from 520 to 680 °C for 1–80 h. This led to liquid/liquid phase separation and a droplet phase enriched in Na 2 O and B 2 O 3 was formed, while the matrix phase was enriched in SiO 2 . The phase separated glass showed a droplet size distribution which after thermal treatment at comparatively low temperatures showed a lognormal distribution, while higher temperatures resulted in a Brailsford and Wynblatt distribution, which should be expected for Ostwald ripening. The glasses showed two glass transition temperatures and the two phases possessed two different coefficients of thermal expansion. Since the droplet phase has a higher coefficient of thermal expansion, around the droplet radial tensile stresses are formed, while the tangential stresses are compressive. While the Young's Modulus was hardly affected by the phase separation, the hardness was notably smaller for phase separated glasses. The 4-point bending strength increased with the temperature of thermal treatment from 40 MPa for the not treated glass to 115 MPa for a glass thermally treated at 640 °C for 20 h. The stresses formed during cooling were estimated from the elastic properties, the coefficients of thermal expansion of the two glassy phases formed, and the glass transition temperature of the matrix phase as a function of the temperature of thermal treatment. The stresses increase with the temperature of thermal treatment up to a maximum at 620 °C and then decreases rapidly. This is in rough agreement with the 4-point bending strength-high stresses lead to high strength.

Coupled thermo-structural analysis of a bimetallic strip using the absolute nodal coordinate formulation

A bimetallic strip consists of two different metal pieces that are bonded together. Due to the different coefficients of thermal expansion, exposing the strip to temperature induces thermal stresses that cause the structure to bend. Most often, incremental finite-element methods that introduce element nodal coordinates have been successfully applied to analyze the thermally induced vibrations in such systems. The exposure of these bimetallic strips to high temperatures results in large deflections and deformations, where the effects of the rigid-body motion and large rotations must be taken into account. For classic, non-isoparametric elements such as beams and plates the incremental methods do not result in zero strains under arbitrary, rigid-body motion. Therefore, in this paper a new model of a bimetallic strip is proposed based on a coupled thermo-structural analysis using the absolute nodal coordinate formulation. The applied, non-incremental, absolute nodal coordinate formulation uses a set of global displacements and slopes so that the beam and the plate elements can be treated as isoparametric elements. In order to simulate the bimetallic strip’s dynamic response, the formulation of the shear-deformable beam element had to be extended with thermally induced stresses. This made it possible to model the coupled thermo-structural problem and to represent the connectivity constraints at the interface between the two strips of metal. The proposed formulation was verified by comparing the responses using a general-purpose finite-element software.

Residual stress induced mechanical property enhancement in steel encapsulated light metal matrix composites

Macro hybridized systems consisting of steel encapsulated metal matrix composites (MMCs) were produced with the goal of creating a low cost/light weight system with enhanced mechanical properties. The systems exhibit the high strength and modulus commonly expected from steels and high specific stiffness and low density observed in MMCs. The material combination also works to alleviate the high density of steels and the poor ductility of the MMCs. Furthermore, a coefficient of thermal expansion (CTE) mismatch induced residual compressive stress method is utilized to improve the ductility of the MMCs. Systems consisting of an A36 or 304 stainless steel shell with an Al-SiC or Al-Al2O3 shell are evaluated via neutron diffraction to quantify bulk residual stresses. The analysis shows variation in the measured strain due to steel thickness, difference in CTE between materials, and position within the composite. Improvements in ductility and yield stress are a result of these strains.