Difference In Expansion Coefficients Research Articles

A Novel Approach of Utilizing Mechanically Flexible SiC Substrate to Grow Crack-Free AlN Bulk Crystal by Thermal Strain Relaxation Functionality

The fabrication of novel semiconductor seed crystals using hetero-epitaxial growth on substrates such as Si, sapphire, and SiC, which have been successfully grown to large diameter and high quality, is very attractive as a breakthrough technology. However, a critical issue in heteroepitaxial growth is the formation of cracks due to thermal stress caused by the difference in the thermal expansion coefficient between the substrate and the growth layer during the cooling process after growth. In this study, we propose a method to reduce thermal stress by using a "Flexible substrate," which is a substrate with mechanical flexibility enhanced by removing more than 80% of its volume with periodic through holes. Using this method, we obtained an AlN hetero-epitaxial growth layer with absolutely no cracks observed. This method is applicable not only for AlN on SiC but also for the fabrication of various new semiconductor materials.

Investigation of the effect of electric heating of expanded clay concrete mixture on the structure and properties of lightweight expanded clay concrete exposed to high temperatures

Objective. The aim of the study is to study the effect of preliminary electrical heating of the concrete mixture on the structure and properties of heat-resistant expanded clay concrete on a composite binder before drying at a temperature of 105°C. Method. It is based on domestic and foreign experience in studying the effect of preliminary electrical heating on the properties and structure of various concretes, including heat-resistant ones. Result. A laboratory stand for preliminary electrical heating of mixtures has been developed and tested. The influence of preliminary electric heating on the process of self-steaming of heat-resistant expanded clay concrete has been studied. A mode of preliminary electric heating of a heat-resistant expanded clay concrete mixture in time has been developed. Stress dependences are obtained due to the difference in the temperature coefficients of linear expansion of coarse and fine aggregates and the mortar part on the heating temperature, as well as the dependences of the ultimate strength of expanded clay concrete on the heating temperature of the expanded clay concrete mixture under various modes of preheating with a composite binder with a ratio of components: Portland cement 70%, mineral additive 30%. Conclusion. Based on the results obtained and the analysis of the studies carried out in the field of preliminary electrical heating of concrete mixtures, it can be concluded that the preliminary electrical heating of a mixture for heatresistant concrete with expanded clay filler based on an activated composite binder will make it possible to obtain concrete with higher physico-thermal and operational characteristics, as well as to reduce the technological process of manufacturing heat-resistant products, the drying period and bringing the thermal unit to the operating mode.

Investigation of the influence of mechanical and thermal effects on the level of stresses and deformations in three-dimensional microassemblies sealed with two types of compounds

Manufacturers of plastic packages use high silica filler epoxy molding compounds to reduce the difference in coefficient of thermal expansion between the die and the encapsulation material. However, in addition to the die, commutation substrate has a significant effect on the thermo-mechanical stresses level in the package. Three-dimensional integration makes it possible to combine several substrates, which can be made from various dielectrics, in a one microassembly with the help of a vertical volume commutation. This work shows that the use of capsulation materials with different thermal expansion coefficient in the outer and inner parts of the product can reduce the level of temperature stresses in such a structure. The dependences of the thermo-mechanical stress and deformation on the number of levels and the amount of filler in the external compound were established. This study was carried out by means of computer simula-tion for various designs of microassemblies capsulated with compounds characterized by differ-ent values of temperature and mechanical parameters. The dependences obtained in the article al-low us to establish optimal values of filler content in the external and internal compounds to en-sure minimal thermal and mechanical (under the influence of acceleration) deformation of microassemblies with different levels.

Wet-Etched Microchamber Array Digital PCR Chip for SARS-CoV-2 Virus and Ultra-Early Stage Lung Cancer Quantitative Detection.

We report a novel design of chamber-based digital polymerase chain reaction (cdPCR) chip structure. Using a wet etching process and silicon-glass bonding, the chamber size can be adjusted independently of the process and more feasibly in a normal lab. In addition, the structure of the chip is optimized through hydrodynamic computer simulations to eliminate dead space when the sample is injected into the chip. The samples will be distributed to each separated microchambers for an isolated reaction based on Poisson distribution. Due to the difference in expansion coefficients, isolation of the sample in the microchambers by the oil phase on top ensures homogeneity and independence of the sample in the microchambers. The prepared microarray cdPCR chip enables high-throughput and high-sensitivity quantitative measurement of the SARS-CoV-2 virus gene and the mutant lung cancer gene. We applied the chip for the detection of different concentrations of the mix containing the open reading frame 1ab (ORF1ab) gene, the most specific and conservative gene region of the SARS-CoV-2 virus. In addition to this, we also successfully detected the fluorescence of the epidermal growth factor receptor (EGFR) mutant gene in independent microchambers. At a throughput of 46 200 microchambers, solution mixtures containing both genes were successfully tested quantitatively, with a detection limit of 10 copies/μL. Importantly, the chips are individually inexpensive and easy to industrialize. In addition, the microarray can provide a unified solution for other viral sequences, cancer marker assay development, and point-of-care testing (POCT).

Experimental Study of Adobe Masonry and its Adherence with Reinforced Concrete Confinement Elements

The constructions with adobe masonry confined with concrete elements, experience a separation between them, mainly due to the inherent physical and chemical characteristics of both materials that are not very compatible with each other; adobe, a raw masonry with a high clay content, undergoes changes in its shape in the presence of humidity and temperature variation, which affects its adherence to concrete elements, compromising the confinement, function and stability of the walls. The different coefficients of expansion and contraction between concrete and adobe, added to their physical properties, make it difficult to achieve adequate adherence between these two materials. In the present study, 6 mixtures of earth (adobe) were designed, with different granulometry proportions to measure the adherence between adobe and concrete, identifying the most suitable one with the purpose of using it as a bonding material in walls for houses, from this study, with the best response being the sample MA-3. The test methodology was based on standards applicable to concrete and annealed brick masonry, as there is no available one for this material. Thirty-six adobe specimens adhered to concrete were tested, to which a normal load was gradually applied to their cross section, to a piece of adobe between two pieces of concrete, recording the ultimate horizontal shear stress between both materials. The scope is considered the result of the test of adobe pieces adhered to concrete subjected to horizontal shear force, considering different granulometry in the composition of the adobe mixture. This work provides a starting point for the standardization of the test and the justification of the need for an additional element that contributes to the confinement.

Room temperature direct bonding of diamond and InGaP in atmospheric air

A new technique of diamond and InGaP room temperature bonding in atmospheric air is reported. Diamond substrate cleaned with H2SO4/H2O2 mixture solution is bonded to InGaP exposed after removing the GaAs layer by the H2SO4/H2O2/H2O mixture solution. The bonding interface is free from interfacial voids and mechanical cracks. An atomic intermixing layer with a thickness of about 8 nm is formed at the bonding interface, which is composed of C, In, Ga, P, and O atoms. After annealing at 400 °C, no exfoliation occurred along the bonding interface. An increase of about 2 nm in the thickness of the atomic intermixing layer is observed, which plays a role in alleviating the thermal stress caused by the difference of the thermal expansion coefficient between diamond and InGaP. The bonding interface demonstrates high thermal stability to device fabrication processes. This bonding method has a large potential for bonding large diameter diamond and semiconductor materials.

Cracking of Copper Brazed Steel Joints Due to Precipitation of MnS upon Cooling

The process of brazing of different steel grades by pure liquid copper foil was studied to reveal the critical conditions when cracks do or do not appear in the braze upon cooling without any external load. Steel grades C45 (S 0.030 wt.%, no Mn and no Cr), S103 (Mn 0.25 wt.% and S 0.020 wt.% with no Cr), CK60 (0.75 wt.% Mn, 0.07 wt.% S and 0.15 wt.% Cr) and EN 1.4034 (Cr 12 wt.%, Mn 1.0 wt.% and S 0.035 wt.%) are studied under identical conditions using copper foils of 70-microns-thick. The samples were held above the melting point of copper at 1100 °C under high vacuum for different time durations (between 180 and 3600 s) and then cooled spontaneously. The joints were found cracked during cooling after a certain critical holding time. This critical holding time for cracking was found to decrease with increasing the Mn content and the S content of steel. It is observed that cracking is due to the precipitation of a critical amount of MnS phase upon cooling. The MnS/Cu interface is the weakest interface in the joint (with adhesion ensured only by van-der-Waals bonds), which is broken/separated upon cooling due to difference in heat expansion coefficients of the sulfide and copper phases. Higher is the Mn and S content, shorter is the probable time required for crack to appear in the joint. The braze integrity diagram is constructed as function of solubility product of MnS in steel and holding time showing a stable crack-free technological region and an unstable technological region with high probability of crack formation.

Understanding and Control of Stress at Si-SiO<sub>2</sub> Interface

This paper presents the results of the investigation of stress relaxation at the Si-SiO2 interface using electron paramagnetic resonance (EPR) spectra, scanning electron microscopy (SEM) image technique, deflection analysis, X-ray photoelectron spectroscopy (XPS) and C-V characterisation of metal oxide semiconductor (MOS) structures. It has been shown, on the basis of EPR, XPS, C-V, and deflection data, that the mechanism of stress relaxation depends on the oxidation conditions: temperature, cooling rate and oxide thickness. In the Si-SiO2-Si3N4 system the stress relaxation occurs due to the difference in the thermal expansion coefficient of SiO2 and Si3N4 material. With an appropriate choice of oxidation conditions compressive stresses in SiO2 and tensile stresses in Si are almost equal and stress can be reduced considerably at the interface.

Evaluation of Cyclic Hot Corrosion Resistance of Plasma-Sprayed Composite Coating in Na2SO4-60%V2O5 Molten Salt Environment

The hot corrosion resistance of air plasma-sprayed 8YSZ-Al2O3-multiwall carbon nanotubes (MWCNTs) has been analyzed and compared to that of 8YSZ coating. The feedstock constituents were blended to produce 8YSZ-20% alumina-0%MWCNT (type II), 8YSZ-19% alumina-1%MWCNT (type III), and 8YSZ-17% alumina-3%MWCNT (type IV) feedstock. After the cyclic hot corrosion (cHC) test in Na2SO4-60%V2O5 molten salt, the morphology of coatings was analyzed using EDS, SEM, and XRD while the coating cross-section properties were determined by a nanoindentation test and the infiltration resistance of the topcoats was estimated from the Weibull modulus (m) of the bond coat (BC) properties. The formation of YVO4 and phase transformation of zirconia from tetragonal to monoclinic were found as important degrading mechanisms of the coatings. The difference in coefficient of thermal expansion also proved to be detrimental to the performance of the coating. The zirconia monoclinic phase content for the 8YSZ(type I), type II, type III, and type IV coatings was around 86, 80, 92, and 89%, respectively, after the cyclic hot corrosion test. The Weibull analysis of the Young’s modulus of the bond coat showed that the type IV coating exhibited the highest heterogeneity in Young’s modulus. Finally, the addition of MWCNT was found to be detrimental in the cyclic hot corrosion test as it led to the cracking of the coatings.

Mechanical Reliability of Photovoltaic Cells under Cyclic Thermal Loading

Metallic coatings placed on solar cell should retain their structural integrity over the life-span of the devices in order to ensure their reliable functioning. One critical component of such a life-assessment exercise is based on their response to the cyclic thermal stresses generated due to the temperature fluctuation, which is inevitable during regular operation of a solar cell and the difference in the thermal expansion coefficients of metal coatings and Si. Here, we have studied the impact of accelerated thermal cycling on the integrity of the semiconductor–metal layer in a commercial monocrystalline Si based photovoltaic solar cell comprising Ag finger-lining and Al backside coating. We observed that, compared to Si-Ag interface, the Al-Si interface was significantly weaker, wherein cracks easily nucleated and grew during thermal cycling between − 40°C and 90°C. The experimental results were augmented with finite element method (FEM), including extended-FEM (XFEM), simulations using geometry based on the actual microstructure of various metal-Si interfaces in the solar cell module. FEM-based simulations suggest excessive stress concentration at the interface of Al-Si eutectic-Al layers due to the irregular wavy nature of this interface. XFEM results indicate the critical role of the interfacial adhesion strength and roughness of the eutectic-Al interface on the crack growth and its propagation path. Based on the obtained results, a discussion on the fabrication of solar cell modules resistant to thermal stress induced structural damage is presented.

Tensile Fracture Behavior of Carbon Fiber/Al Composites Produced by the Liquid Extrusion Infiltration Technique

The liquid extrusion infiltration technique was used to fabricate two 2D CF/Al composites with T700 and T300 carbon fiber fabrics. Microstructure examination demonstrated their different macroscopic infiltration morphology, but the infiltration effect was perfect. The coefficient of thermal expansion of those two composites was shown to be much lower than that of the matrix alloy below 673 K (400°C). At a temperature higher than 673 K (400°C), this value for a T300 CF/Al composite was equivalent to only 22.6%, which was much smaller than that of the matrix alloy. The ultimate tensile strength of T700 CF/Al and T300 CF/Al composites was improved by factors of 2.007 and 2.467 than that of the matrix alloy, respectively. The differences in the thermal expansion coefficient and ultimate tensile strength stem from carbon fiber volume fractions and its grades of the two different fabrics. The fracture morphology of tensile specimens was also influenced by the fiber orientation in fabrics. A T300 CF/Al composite displayed better tensile and thermal expansion properties than a T700 CF/Al composite, which were prepared and designed for further industrial applications.

Microstructure and mechanical properties of dissimilar friction stir welded type 304 austenitic stainless steel to Q235 low carbon steel

In this work, friction stir welding (FSW) was used to weld dissimilar metals (Type 304 stainless steel (SS304) and Q235 low carbon steel). The microstructure, interfacial characteristics, residual stress distribution and mechanical properties of the joint were investigated. The results show that FSW results in grain refinement in the stir zone (SZ) and thermal-mechanical affected zone (TMAZ) in SS304 side. Discontinuous dynamic recrystallization and twinning-induced dynamic recrystallization are the main grain refinement mechanisms. Dynamic recovery occurs in the heat-affected zone (HAZ). On the Q235 steel side, the acicular ferrite and pearlite are generated in the SZ, which is due to its peak temperature of up to Ac3. Additionally, the amount of acicular ferrite decreases in the TMAZ. Continuous and discontinuous dynamic recrystallization is the main recrystallization mechanism in the two regions. The interfacial bonding mechanism of FSW joint consists of both mechanical and metallurgical bonding. In addition, the difference in expansion coefficient and microstructure between the two steels results in the formation of residual compressive stress in the SZ. Grain refinement and acicular ferrite are responsible for the microhardness distribution in the whole joint. The tensile strength of FSW joint is 493 MPa, which is more than that of Q235 steel by around 4%. However, the elongation is 17%, which shows a decrease of around 50%. Furthermore, the fracture surface shows ductile fracture with dimples.

Relief Formation on the Free Surface of Metallic Polycrystals upon Recrystallization Annealing

The mechanism of the formation of a complicated relief first found on the recrystallized tungsten surface in studies performed by A.I. Belayeva and coauthors is described in this work. Its essence is the deformation of boundaries and near-boundary volumes of crystallites (grains) in the near-surface polycrystalline-metal layer upon cooling after high-temperature holding that is caused by the difference in the ultimate strength and thermal expansion coefficients of boundaries and bodies of grains. In the near-surface layer upon cooling of the metal, due to the difference in the size reduction of grain boundaries and grain bodies, cracks (grooves) arise at boundaries on whose banks identical ridges are formed. The bending of the boundary makes the profile of grooves asymmetric and creates rather shallow creases in front of ridges. The surface relief impairs the service properties of metals but its formation can be prevented, if articles become single-crystalline owing to recrystallization.

Measurement of Thermal Expansion Coefficient of MgZnY Alloys with Synchronized Long-Period Stacking Ordered Phase

We measured the thermal expansion coefficients of 18R-LPSO phase and α-Mg phase in a Mg97Zn1Y2 alloy polycrystal. This was achieved by using a Gandolfi camera, which was attached on a high precision diffractometer at SPring-8 BL40XU beamline. By using this system, fine diffraction data were obtained from a Mg97Zn1Y2 polycrystal at 6 different temperatures between 90 and 450 K. We succeeded to determine cell parameters of 18R-LPSO phase and α-Mg phase separately in the Mg97Zn1Y2 alloy polycrystal. The thermal expansion coefficients were determined from the refined cell parameters. The differences of the thermal expansion coefficients of 18R-LPSO phase and α-Mg phase in the Mg97Zn1Y2 alloy were much smaller than those of single-phase 18R-LPSO and α-Mg polycrystals.

Case Study of the Contact Failure of an Oil Temperature Sensor of an Automotive Transmission

In this study, we investigated the cause of the contact failure phenomenon of an oil temperature sensor whose signal error occurred intermittently as the vehicle in which it was installed was driving. The electrical contact failed sensor was disassembled and the contact surfaces observed with a scanning electron microscope (SEM). In this present oil temperature sensor, the difference in the thermal expansion coefficient of the male-female terminal and vibration during driving which arose when the vehicle was driven caused fretting corrosion at the contact area. The fretting corrosion caused an intermittent signal error. It is judged that the lead wire of the flexible thin plate is structurally vulnerable to vibration during driving, which further causes fretting of the contact point. Therefore, it is necessary to design an additional structural supplement to fix the flexible terminal of the sensor system.

Effects of oxide isolation layer on magnetic properties of L10 FePt film grown on Si substrate

Magnetic force microscope (MFM) is a powerful tool to subtly detect the stray field distribution of magnetic film or particles on a sub-micrometer scale. Due to its huge uniaxial magnetocrystalline anisotropy (Ku~7107 erg cm-3) and high Currie temperature (TC~500℃), FePt alloy in an L10 phase is expected to be coated on the MFM tip to display high coercive force (Hc) and to improve the magnetic stability and MFM resolution. A grain size of~3 nm will be enough to overcome the super paramagnetism. However, the growing fresh FePt films must experience a high temperature annealing (exceeding 700℃) in order to transform their structures thoroughly from a soft A1 phase into the desired hard L10 phase. This brings the risk of diffusion between FePt coating layer and the underneath Si cantilever. Several admixtures have been attempted by other researchers to obtain granular films with FePt grains separated by oxides, with the purpose to prevent the diffusion from happening between FePt and Si. But apparently, it will be very difficult to fabricate a separated FePt grain exactly on the top of MFM tip. This is a critical factor to affect the MFM resolution. And discussion about the influence of the interface diffusion is avoided in most of published papers. Alternatively, some oxide isolation layers with higher melting temperature can be useful for separating the top FePt film from the bottom Si crystal. In this paper, MgO and SiO2 are selected as isolation layers, deposited by magnetron sputtering. Subsequently, the FePt films are deposited at 400℃ and annealed at different temperatures (500℃ to 800℃) for 2 h. The experimental results indicate that the diffusion between FePt and Si substrate always occurs in the absence of any isolation layer, leading to a reluctant maximum Hc of~5 kOe for 50 nm FePt film. However, the coercive force could remarkably exceed 10 kOe if an isolation layer is used. In the case of MgO, a maximum Hc of~12.4 kOe for 50 nm FePt could be stably measured. However, the annealing temperature must be lower than 600℃ to hold back the occurrence of brittle cracks in isolation layer. Because of the smaller lattice mismatch and expansion coefficient difference between SiO2 isolation layer and Si substrate, the highest annealing temperature could exceed 800℃ when replacing MgO with SiO2. The Hc of FePt film could be adjusted in a range from~5 kOe to~15 kOe by changing the annealing temperature. These findings greatly benefit the fabrication of FePt-based MFM tips with high Hc. And it is expected to be able to effectively enhance the resolution of MFM image.

Design of overlay coated region with hardfacing, transition and damage diminution layers for the reduction of damages of hot forging tools

Hot forging dies are damaged by the mixed failure modes including wear, thermal cracks, impact cracks, corrosion cracks, etc.. In order to extend the service life of the hot forging die, the diminution technology of the mixed failure modes should be developed. The aim of this paper is to design an overlay coated region consisting of a Hardfacing layer (HL), a Transition layer (TL) and a Damage diminution layer (DDL) to reduce damage of hot forging tools. The HL and the DDL are contrived to improve the wear resistance and the ductility of the overlay coated region, respectively. The TL is adopted to prevent thermal fatigue induced by the difference in the thermal expansion coefficients between successive layers. In order to estimate design alternatives of the overlay coated region, the effects of the thickness and the assignment of the DDL on stress-strain distributions in the overlay coated region are investigated via Finite element analyses (FEAs). The influence of the assignment of the DDL on thermal behaviors of the overlay coated region is investigated through thermal fatigue experiments. From the results of the FEAs and the thermal fatigue experiments, an appropriate design of the overlay coated region is determined.

Influence of Structural Components on Strength Properties of Silicate Stone Casting Materials during Controlled Crystallization

Study and description of positive effect influence of Cr2O3 catalyst in controlled crystallization technology for production of stone casting silicate materials are provided. The introduction of heat treatment method that influences the forming of the structural components of produced material. The basis of manufacturing stone casting silicate materials is controlled crystallization of silicate melt that is organized in such a way that nucleation occurs within the melt and the final product has a fine grain structure. The structural components of silicate stone casting materials and their influence on strength properties are determined. The strength of stone casting silicate materials is the reduction in the difference in the temperature coefficients of linear expansion of crystal phases and residual glass phase.

(Invited) III-V / Si Integration for Photonics

In the last decade a large progress took place in the development of a Si-based fabrication technology platform for integrated photonics, however, mainly for passive devices, such as filters, waveguides, splitters and combiners. The integration of active devices remains very challenging. The talk will start with a brief review of different epitaxial approaches for III-V material growth on Si before a new approach will be presented, which may combine high Si process compatibility with full III-V material functionality. Currently, active photonic devices on a silicon platform are obtained by bonding techniques, either on a single device chip or a full III-V wafer level. However, in this case the majority of III-V material is wasted by substrate removal. Direct epitaxy on silicon overcomes these problems but suffers in most cases from the lattice mismatch and the differences in expansion coefficients. This can be partially accommodated by thick relaxation layers and using quantum dot active regions to reduce the sensitivity against threading dislocations or using complex material compositions, like Ga(N,As,P), which allow lattice matching. However, the III-V layers have to be processed completely separate from the silicon. A new approach based on III-V-Si nanocomposite is under development, which may overcome most of those problems. This approach is based on the growth of III-V quantum dots (QDs) directly on silicon. Based on a GaAs/InAs core-shell geometry high quality optical emission could be obtained. By overgrowing InAs QDs with silicon a defect free single crystal planar silicon layer can be formed with embedded InAs nano clusters. Transmission electron microscope investigations show that the nanoclusters fully relax by closed loop interface dislocations but without initiating threading dislocations in the silicon matrix.

Influence of a Cr2O3 glass coating on enhancing the oxidation resistance of 20MnSiNb structural steel

Abstract In this paper, a glass protective coating containing chromium oxide deposited on a high-quality 20MnSiNb structural steel was prepared by a slurry-spraying technique at high temperature. Compared with a bare sample, the effect of the coating on the steel was investigated at a temperature range from 700 °C to 1150 °C in the air. The experimental results showed the coating had significant oxidation resistance and effective descaling ability for 20MnSiNb structure steel. The coating could enhance protective effect by 86% after heating for 60 min at 1050 °C compared with the bare sample. The formation of a dense chromium and silicon compound film on the 20MnSiNb structural steel prevented the diffusion of oxygen and iron ions. Due to the difference of the thermal expansion coefficient between the coating and the steel, the coating spalled during the cooling process, which enhanced the descaling ability significantly. The sample was characterized by XRD, SEM-EDS and TG-DTA/DSC to clarify the possible protective mechanism, which was the synergy between the glass coating and the protective film formed by the migration of chromium, preventing the diffusion of oxygen and iron ions. Because of the low cost and easy handling method, the coating has potentially applicable for reheating process of 20MnSiNb structural steel.