Differential Thermal Expansion Research Articles

Analysis of the critical stresses in high-voltage composite winding insulations under thermal loads

Stator bars are the critical components in generators with respect to their lifespan. The winding insulation may be susceptible to damage under excessive alternating temperature loads incurred by a high number of start–stop cycles and heavy overloads. The differences in thermal expansion between the different components in the multi-layered winding insulation lead to thermal stresses. This work deals with the thermo-mechanical characterisation of the individual constituents of the winding insulation and the finite element model of a stator bar, based on the material data obtained from the tests that enable the identification of the thermal stresses leading to delamination over thermal cycles. The out-of-plane and the interfacial shear stress are found to be in the critical range with respect to the relatively low cohesive strength of the included individual mica layers.

Numerical investigation on the cooling-related mechanical properties of heated Australian Strathbogie granite using Discrete Element Method

A series of laboratory tests was carried out to study the cooling-related mechanical properties of Australian Strathbogie granite, which were heated from room temperature to various high temperatures (200, 400, 600 and 800°C) followed by two different cooling methods (exposed in air for slow cooling and put into water for rapid cooling). Using a two-dimensional discrete element code, UDEC, a grain-breakable model was firstly established, in which the grains were made breakable by connecting the centroid and midpoints of edges for each grain. Therefore, both the inter-granular and intra-granular fractures can be handled in the model for capturing the fracturing mechanism of rock under heating or cooling treatment. In the experiments and simulation, three types of granite with different grain sizes were considered, which are fine grain (FG), medium grain (MG) and coarse grain (CG). Besides, the effects of grain size homogeneity and mineral composition were also taken into consideration in the simulation. It was found that the strength and deformation characteristics from the numerical simulation had a good consistency with that from the experiments. For all types of granite, no significant variations in compressive strength and failure strain were found at lower heating temperature (below 400°C) under both slow and rapid cooling conditions. Compressive strength reduced sharply as heating temperature rising from 400 to 800°C, while failure strain exhibited the opposite trend and was more pronounced for rapid cooling. Fracturing analysis indicated that granite exhibited more fractures after heating and cooling treatments compared with that without cooling treatment. Compared with slow cooling, rapid cooling introduced more fractures, especially intra-granular fractures, and damaged rock more severely. CG was found to have more inter-granular fractures but fewer intra-granular fractures than FG and MG granite at all testing temperatures except for 200°C. Granite with heterogeneous grain size distribution exhibited lower strength and more intra-granular fractures than that with uniform grain size distribution after cooled from the same temperature. The increasing quartz content introduced higher mineral heterogeneity in granite during heating and cooling due to thermal expansion differences. Granite with higher quartz content displayed lower strength and more fractures than that with lower quartz content after heating and cooling treatments.

Adhesive-free joining and application for flexible devices - a review

Interface has been recognized for a long time as a part of materials such as surface and grain boundary in bulk in spite of the fact that interface gives decisive influences on the performance and durability of devices in practical use. Interface is also well known to have four principal performances and roles such as mechanical, electrical, magnetic and optical properties. Here, I will at first review the interface behavior with special emphasis on materials science and then introduce typical examples of case study. Mechanical behavior of interface can be attributed to joining of different materials in particularly at low temperature and intrinsic defects introduced in the process of joining caused by difference in thermal expansion. Defects thus introduced is known to result in work hardening of interface. Recently, an epoch-making method of joining to utilize water gas adsorption layer as an adhesive agent. This gas adsorption joining (GAJ) was demonstrated in cycloolefin polymer (COP) film and borosilicate glass system. Both surfaces were first modified by atmospheric pressure plasma, followed by close lamination to reaction distance and heated at about 100°C to result in the formation of a strong covalent bond for joining. Durability thus formed system was found to remain unchanged even after weathering test in 60°C, 95%RH for 2000 hrs. Joining mechanism can be explained in terms of an increase in density of functional molecules at the interface, followed by dehydration. Thickness of joining layer was confirmed by X-ray photoelectron spectroscopy and STEM and EELS to be 1∼2nm. Sealing performance of GAJ was examined by measuring Water Vapor Transmission Rate (WVTR) by MOCON system using joined PET films on which high barrier layer was coated. The WVTR of line sealing with 1 mm width and 1-2 nm thick was found to be better than 5 x 10−4 g/m2/day. Practical applications of thus developed GAJ for joining and sealing are now in progress for flexible film devices such as OLED display, solar cells, and healthcare devices. Finally, roles of interface in materials science in future will be critically discussed.

Reddish orange phosphorescence of some types of zinc borosilicate glasses activated with Mn2+ and/or Sm3+

The current research focuses on studying the photoluminescence and phosphorescence performance of some types of undoped zinc borosilicate glasses together with BaO or Bi2O3. The effects of addition transition metal oxide (MnO2) and/or rare—earth oxide (Sm2O3) on the behavior of mentioned characteristics were also studied. The glasses were prepared through the usual method of melt and annealing technique. X-ray diffraction (XRD) analysis proved the amorphous nature of the prepared glasses. The optical properties including the absorption in the UV–Visible NIR region and estimation of the optical band gap, Urbach energy, and refractive index have been studied. Both undoped zinc borosilicate and BaO glasses exhibit UV absorption and Bi2O3 doped glass exhibit an extended near—visible absorption. The characteristic absorption bands of MnO2—doped glasses in the visible region beside UV absorption were identified while the extended absorptions in the NIR region appeared for glasses containing Sm2O3. According to the progressive changes in the optical absorptions, variations in the optical parameters (Eopt, ∆E, n) were identified depending on the composition. The photoluminescence measurements revealed characteristic emissions of MnO2 and/or Sm2O3. The prepared glasses show good photoluminescence responses that gave the chance to estimated the phosphorescence performance of the prepared glasses. The phosphorescence measurements show different emissions related to reddish orange emission. The CIE chromaticity coordinates also indicated the appearance of a clear observable reddish orange color produced as a result of phosphorescence in the presence of MnO2 and/or Sm2O3 as a dopant in the glass matrix. The FTIR measurements represent the building structure of the studied glass network. Both tetrahedral BO4 and triangle BO3 beside SiO4 groups were shared as the basic building blocks of the glasses. Heavy metal ions Ba2+ and Bi3+ show characteristic changes in the FTIR absorption which reflect their effect on the building structure. The thermal properties of the prepared glasses have been identified by differential thermal analysis (DTA) and thermal expansion measurements. The thermal analysis was correlated to the change of heavy metal ion Ba2+ or Bi3+ in the glass network and indicating the crystallization conductance of the prepared glasses.

Mineral Composition, Pore Structure, and Mechanical Characteristics of Pyroxene Granite Exposed to Heat Treatments

In deep geoengineering, including geothermal development, deep mining, and nuclear waste geological disposal, high temperature significantly affects the mineral properties of rocks, thereby changing their porous and mechanical characteristics. This paper experimentally studied the changes in mineral composition, pore structure, and mechanical characteristics of pyroxene granite heated to high temperature (from 25 °C to 1200 °C). The results concluded that (1) the high-temperature effect can be roughly identified as three stages: 25–500 °C, 500–800 °C, 800–1200 °C. (2) Below 500 °C, the maximum diffracted intensities of the essential minerals are comparatively stable and the porous and mechanical characteristics of granite samples change slightly, mainly due to mineral dehydration and uncoordinated thermal expansion; additionally, the failure mechanism of granite is brittle. (3) In 500–800 °C, the diffraction angles of the minerals become wider, pyroxene and quartz undergo phase transitions, and the difference in thermal expansion among minerals reaches a peak; the rock porosity increases rapidly by 1.95 times, and the newly created pores caused by high heat treatment are mainly medium ones with radii between 1 μm and 10 μm; the P-wave velocity and the elastic modulus decrease by 62.5% and 34.6%, respectively, and the peak strain increases greatly by 105.7%, indicating the failure mode changes from brittle to quasi-brittle. (4) In 800–1200 °C, illite and quartz react chemically to produce mullite and the crystal state of the minerals deteriorate dramatically; the porous and mechanical parameters of granite samples all change significantly and the P-wave, the uniaxial compressive strength (UCS), and the elastic modulus decrease by 81.30%, 81.20%, and 92.52%, while the rock porosity and the shear-slip strain increase by 4.10 times and 11.37 times, respectively; the failure mechanism of granite samples transforms from quasi-brittle to plastic, which also was confirmed with scanning electron microscopy (SEM).

Tuning thermal expansion behavior and surface roughness of tubular Al2O3 substrates for fabricating high-performance carbon molecular sieving membranes for H2 separation

Tubular alumina substrates have been widely used as supporting membranes in gas separation. Owing to the demand for supporting membranes with a dense or ultra-micropore texture, the quality/quantity control of substrates is required to prevent the formation of defects due to rough surfaces, high curvature, and high difference in thermal expansion between the polymer precursor and the alumina substrate. This study proposes a new strategy to modify the pore texture, surface properties, and thermal expansion coefficient of the substrate by filling it with TiO2 nanoparticles and using the grinding/polishing method. The effect of CMS preparation conditions, including coating cycles and pyrolysis temperature on the microstructure of the carbon matrix is also discussed. A tubular CMS membrane with excellent permselectivity toward H2/CO, H2/N2, and O2/N2 (77.52, 162.94, and 13.87, respectively), and permeabilities of 55.9 barrer and 6.49 barrer are obtained for H2 and O2, respectively.

Finite element simulations of thermal residual stresses in realistic 3D WC-Co microstructures

During cooling of WC-Co cemented carbides from sintering temperatures, large internal residual stresses are obtained due to the difference in thermal expansion of WC and Co. The residual stresses depend on the properties of the microstructure, such as volume fractions and morphology, as well as the thermomechanical properties of the constituents. These residual stresses affect the engineering performance and strength of the hardmetal.In this paper, we present finite element simulations of the cooling of realistic synthetic WC-Co microstructures from sintering temperatures to room temperature. The microstructures are generated in voxel grids using a newly developed tool CCBuilder. Finite element analyses are conducted to obtain effective macroscopic thermomechanical properties and thermal residual stresses of WC-Co microstructures in a free sintered specimen. The analyses are performed within the framework of multi-scale computational homogenization using periodic boundary conditions. In the analyses individual anisotropic elastic and thermal properties of WC-grains are accounted for.An excellent agreement between numerically predicted and experimental results for the macroscopic thermomechanical properties for WC-Co is obtained. In addition, the predicted residual stresses in the microstructure after cooling are in good agreement with experimental data. The influence from different constitutive model assumptions (elasticity, elastoplasticity) for the Co-phase on the residual stresses is studied. Finally, the proposed modelling framework is used to study the influence of WC-grain geometry on the residual stress field and our results show a substantial dependency of the residual stresses on the detailed morphology of the microstructure.In conclusion, we demonstrate the power of the presented modelling framework using realistic synthetic 3D microstructures with periodic boundary conditions together with multi-scale computational homogenization. In the future it may be open up strategies for optimal material design of microstructures of WC-Co cemented carbides.

대면적 패널 패키징의 휨 해석

Panel Level Packaging (PLP) is a next-generation semiconductor packaging technology which has productivity and cost competitiveness. However, warpage issue is needed to be solved for successful product which occurs during packaging process. Warpage is occurred by a coefficient of thermal expansion (CTE) difference between each material, and this warpage should be minimized. In this study, warpage issue of the PLP induced by molding process and detaching process is investigated. Warpage simulation and optimization are carried out by the ANSYS Workbench. In order to figure out which constituent material critically affects warpage problem, single element is adjusted in the range of properties. After then, the selected influential elements are adjusted to figure out optimized values for meeting the process requirement of warpage.

Applying Selective Laser Melting to Join Al and Fe: An Investigation of Dissimilar Materials

Combining aluminum and steel is a major goal of automobile manufacturers and other industries because the hybrid material reduces the weight of components. However, differences in chemical properties, thermal expansion, and physical characteristics of aluminum and steel are barriers to achieving this goal. In this article, selective laser melting (SLM), which is widely used in industrial fields, was applied to join dissimilar materials by printing aluminum on a steel substrate. Defects of joining during the SLM process, characteristics of the intermetallic reaction layer, and the effects of the process parameters were investigated. The analysis indicates that flake behavior could affect the quality of joining. The phases of the intermetallic layer found in this study were in agreement with other research, but the morphology of the layer was much different. A formula to estimate the join quality in terms of density energy is proposed. The results indicate that the SLM process is a promising method to manufacture a hybrid material.

Preparation of novel reticulated porous ceramics with hierarchical pore structures

Al2O3-ZrO2 reticulated porous ceramics (RPCs) with hierarchical pore structures were prepared by vacuum infiltration. Al(OH)3 was applied to provide the pore structure. Vacuum infiltration using an alumina/aluminum hydroxide slurry via a pre-sintering cycle was performed to fill up the hollow struts generated due to burnout of the polyurethane foam template and the coating on the Al2O3-ZrO2 ceramic struts. The results showed that as the Al(OH)3 content in the infiltration slurry was increased from 5% to 13%, the pore size distribution of the ceramic struts widened and the major pore size decreased. In addition, a compressive residual stress developed within the multilayered struts because of the difference in thermal expansion of the Al(OH)3-Al2O3 coating layer and Al2O3-ZrO2 ceramic struts. Moreover, with increasing Al(OH)3 content in the infiltration slurry, the porosity of the ceramic struts gradually increased, resulting in a decrease in Young's modulus and thermal conductivity, which influenced the compressive residual stress and thermal stress within the ceramic struts. Furthermore, the potential of the fabricated RPCs as catalyst supports was demonstrated.

Getting the most out of muscles

Artificial Muscles Materials that convert electrical, chemical, or thermal energy into a shape change can be used to form artificial muscles. Such materials include bimetallic strips or host-guest materials or coiled fibers or yarns (see the Perspective by Tawfick and Tang). Kanik et al. developed a polymer bimorph structure from an elastomer and a semicrystalline polymer where the difference in thermal expansion enabled thermally actuated artificial muscles. Iterative cold stretching of clad fibers could be used to tailor the dimensions and mechanical response, making it simple to produce hundreds of meters of coiled fibers. Mu et al. describe carbon nanotube yarns in which the volume-changing material is placed as a sheath outside the twisted or coiled fiber. This configuration can double the work capacity of tensile muscles. Yuan et al. produced polymer fiber torsional actuators with the ability to store energy that could be recovered on heating. Twisting mechanical deformation was applied to the fibers above the glass transition temperature and then stored via rapid quenching. Science , this issue p. [145][1], p. [150][2], p. [155][3]; see also p. [125][4] [1]: /lookup/doi/10.1126/science.aaw2502 [2]: /lookup/doi/10.1126/science.aaw2403 [3]: /lookup/doi/10.1126/science.aaw3722 [4]: /lookup/doi/10.1126/science.aax7304

Passive solar tracker based in the differential thermal expansion of vertical strips

This paper presents a multiple axis passive solar tracking concept that takes advantage of the thermal expansion induced length variation of a material when exposed to sunshine. The differential expansion of three vertical thin flat strips with different orientations is amplified by a lever system to enable tracking of the apparent motion of the sun in the sky. A passive solar tracking system prototype supporting a photovoltaic (PV) module was built and tested. The model and experimental results show that the tracking system can correctly follow the azimuthal motion during the day. The tracking of solar elevation is less pronounced. The use of this tracking system leads to a 28% increase in PV generation during the testing period.

Glide of threading dislocations in (In)AlGaAs on Si induced by carrier recombination: Characteristics, mitigation, and filtering

III-V optoelectronics grown epitaxially on Si substrates have large networks of dislocations due to a lattice constant mismatch between the device layers and the substrate. Recombination-enhanced dislocation glide (REDG) allows these dislocations to move and increase in length during device operation, which degrades performance. In this paper, we study REDG dynamics of threading dislocations in situ in (In)AlGaAs double heterostructures grown on Si substrates using scanning electron microscopy cathodoluminescence. The driving force for REDG arises due to the coefficient of thermal expansion differences between Si and the III-V layers leading to large residual strains in the films. Tracking of threading dislocations as moving dark spot defects reveals glide characteristics that vary based on the nature of the dislocation. Remarkably, the alloying of a few atom percent of indium using metamorphic structures arrests threading dislocation glide by more than two orders of magnitude. Finally, we present REDG-based filtering as a pathway to reducing the threading dislocation density in select areas, removing a large fraction of the mobile dislocations. Together, these techniques will enable the understanding of dislocation–dislocation and carrier–dislocation interactions that have so far remained elusive during device operation, leading to reliable III-V integrated optoelectronics on silicon.

Mesoscale approach to numerical modelling of thermo-mechanical behaviour of concrete at high temperature

This article presents a lattice-type numerical modelling method for analysing the thermo-mechanical behaviour of concrete subjected to compressive stresses at high temperature. The concrete is considered as a two-dimensional mesoscale structure consisting of three material phases: aggregates, cement paste and interfacial transition zones (ITZ). Subjected to a high temperature, concrete might be damaged due to the differences in thermal expansion between these components. The damage results in the initiation and propagation of cracks that affect the thermal conductivity of the material and thus the heat transfer within the structure. A modified damage-elasticity model, coupled with a nonlinear heat transfer equation taking into account the damage induced thermal conductivity change, is implemented in the lattice framework to describe the thermo-mechanical behaviour of concrete. Comparison of the current study to experimental and numerical results reported in the literature indicates the significance of the proposed model.

Automatic compensation of thermal drift of laser beam through thermal balancing based on different linear expansions of metals

Abstract We present a technique for compensating laser beam up-shifting induced by the hot air with temperature-gradient in a 2D-scaning laser radar for monitoring the intruder in the railway system, in which the laser beam is scanning at a height only a few centimeters above the railway tracks so that any object with a size of 5 cm × 5 cm × 5 cm or larger can be detected. It is found that when the laser beam is traveling in the hot air with a temperature gradient, the direction of the laser can be up-shifted. The up-shifted laser beam may miss the obstacle lies on the railway track and fail to detect and report the accidence and it may eventually result in a disaster. The compensation of the upward shifting is necessary to insure the normal operation of the radar system and it can be done by using the difference in linear thermal expansion between metals, which produce an additional elongation when the temperature changes. The temperature dependent elongation is used to drive a rotational stage, which rotates the laser in the opposite direction against the up-shifting. The rotation of the stage compensates the change of angle of the laser beam induced by the temperature gradient of the air. A prototype device is constructed to test the idea. Without thermal compensation, the laser thermal shift can reach up to 15 mm when temperature changes 14 °C from 30 °C to 44 °C above the hot surface after the laser beam travels 30 m and such a shift can be reduced to about 8 mm at the same distance when the compensator system is used. The experimental data agrees very well with the theoretical calculation.

Effect of thermal cycling frequency on the durability of Yb-Gd-Y-based thermal barrier coatings

The effects of thermal cycling frequency and buffer layer on the crack generation and thermal fatigue behaviors of Yb–Gd–Y-stabilized zirconia (YGYZ)-based thermal barrier coatings (TBCs) were investigated through thermally graded mechanical fatigue (TGMF) test. TGMF tests with low- (period of 10 min) and high-frequency (period of 2 min) cycling were performed at 1100 °C with a 60 MPa tensile load. Different cycling frequencies in TGMF test generate two kinds of crack propagation modes. The sample with low-frequency cycling condition shows penetration cracks in the YGYZ top coat, and multiple narrow vertical cracks are generated in high-frequency cycling. To enhance the thermomechanical properties, different buffer layers were introduced into the TBC systems, which were deposited with the regular (RP) or high-purity 8 wt% yttria stabilized zirconia (HP-YSZ) feedstock. The purity of the feedstock powder used for preparing the buffer layer affected the fracture behavior, showing a better thermal durability for the TBCs with the HP-YSZ in both frequency test conditions. A finite element model is developed, which takes creep effect into account due to thermal cycling. The model shows the high stresses at the interfaces between different layers due to differential thermal expansion. The failure mechanisms of YGYZ-based TBCs in TGMF test are also proposed. The vertical cracks are preferentially created, and then the vertical and horizontal cracks will be propagated when the vertical cracks are impeded by pores and micro-cracks.

Influence of Lampang pottery stone: local materials in Thailand on C-Q-F ratio, key properties, mullite formation and glaze-body fit of vitreous ceramic sanitary ware

This study was aimed to investigate the key properties, mullite formation and glaze-body fit of sanitary ware bodies, by using pottery stone from Lampang, Thailand as a substitute for kaolin, feldspar, and silica. Seven samples of formulated bodies were prepared by wet milling, formed by plaster mold casting, and fired at 1175 °C, 1200 °C, and 1225 °C. Testings were carried out in accordance with ASTM C373-88, ASTM C326-09, ASTM C689-09, and ASTM C372-94. Phase analysis and microstructure of selected formulations were analyzed by using XRD and SEM. For the formulation having 37.7-20.7-41.6 of C-Q-F ratio and pottery stone in substitute for 100% silica, after firing at 1200 °C showed 9.27% total shrinkage, 0.17% water absorption, and 60 MPa flexure stress. The crystal structure resulted in higher amount of mullite. The linear thermal expansion and thermal expansion coefficient were lower to 0.34 and 7.07 × 10−6/°C, respectively. The difference of linear thermal expansion between the formulated body and commercial white-opaque glaze was − 0.02 at 500 °C. The shrinkage difference of body and glaze showed that the glaze-body fit was under slightly amount of compression. As a result, the formulation with the substitute for 100% silica could be vitrificated at lower temperature and indicated positive trends towards lower dunting during firing.

Improving the interface adherence at sealings in solid oxide cell stacks

Abstract

Avoiding sealing failure of flanged connection for tubes made of dissimilar materials subjected to elevated temperature

Carbonic composite materials and ceramics appear to be excellent structural materials for parts subjected to very high temperatures in molten salt reactors (MSRs), in which the reactor core outlet temperature is normally above 700 °C. Because of the high temperature, there are major challenges in the sealing of flanged connections for tubes made of alloys and nonmetallic materials. In this study, an improved method for sealing bolted flange connections for tubes made of dissimilar materials at high temperature is analyzed. The study focuses on the compensation mechanism for the difference in thermal expansion between the bolts and the flanges. An angle is introduced for the sealing surface in the flanged connection to provide effective sealing. The arctangent of the angle is the ratio of the thickness between the theoretical core of the sealing surface and the outside end face of the flange to the distances between the axis of the flanged joint and the theoretical core of the sealing surface of the flange; the sealing surface of the flange, which is made of the same material as the fastening assemblies, faces the fastening assemblies. To ensure effective sealing, the frictional coefficient between the two sealing surfaces should not exceed the tangent of the angle. This result does not agree well with the solution given by previous researchers. Further, in the modified flanged connection, the compression of each bolt in the clamped condition is increased to maintain the compaction force unchanged without increasing the number of bolts on the flanged joint.

Параметры зонной структуры тонких пленок Bi-=SUB=-1-x-=/SUB=-Sb-=SUB=-x-=/SUB=- (0≤ x≤ 0.15) на подложках с различным температурным расширением

The report presents the positions of the conductance and valence band extremes in relation to the chemical potential of the thin bismuth–antimony films (from 0 to 15 at% Sb) on substrates with different thermal expansion. The results are based on the galvanomagnetic properties study of thermal deposited thin films. A significant increase in the concentration of charge carriers in films on substrates with a large thermal expansion was found. The results of calculating the valence band and the conduction band positions at 77 K, depending on the thermal expansion coefficient of the substrate used, are presented. The thin films plane deformation caused by the difference in the film and substrate materials thermal expansion leads to a change in the positions of the conduction band and the valence band of the films relative to their positions in a single crystal with corresponding composition