Differential Thermal Expansion Research Articles

Mesoscopic Elastic Distortions in GaAs Quantum Dot Heterostructures.

Quantum devices formed in high-electron-mobility semiconductor heterostructures provide a route through which quantum mechanical effects can be exploited on length scales accessible to lithography and integrated electronics. The electrostatic definition of quantum dots in semiconductor heterostructure devices intrinsically involves the lithographic fabrication of intricate patterns of metallic electrodes. The formation of metal/semiconductor interfaces, growth processes associated with polycrystalline metallic layers, and differential thermal expansion produce elastic distortion in the active areas of quantum devices. Understanding and controlling these distortions present a significant challenge in quantum device development. We report synchrotron X-ray nanodiffraction measurements combined with dynamical X-ray diffraction modeling that reveal lattice tilts with a depth-averaged value up to 0.04° and strain on the order of 10-4 in the two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. Elastic distortions in GaAs/AlGaAs heterostructures modify the potential energy landscape in the 2DEG due to the generation of a deformation potential and an electric field through the piezoelectric effect. The stress induced by metal electrodes directly impacts the ability to control the positions of the potential minima where quantum dots form and the coupling between neighboring quantum dots.

Light-Induced Buckles Localized by Polymeric Inks Printed on Bilayer Films.

Buckling instabilities generate microscale features in thin films in a facile manner. Buckles can form, for example, by heating a metal/polymer film stack on a rigid substrate. Thermal expansion differences of the individual layers generate compressive stress that causes the metal to buckle over the entire surface. The ability to dictate and confine the location of buckle formation can enable patterns with more than one length scale, including hierarchical patterns. Here, sacrificial "ink" patterned on top of the film stack localizes the buckles via two mechanisms. First, stiff inks suppress buckles such that only the non-inked regions buckle in response to infrared light. The metal in the non-inked regions absorbs the infrared light and thus gets sufficiently hot to induce buckles. Second, soft inks that absorb light get hot faster than the non-inked regions and promote buckling when exposed to visible light. The exposed metal in the non-inked regions reflects the light and thus never get sufficiently hot to induce buckles. This second method works on glass substrates, but not silicon substrates, due to the superior thermal insulation of glass. The patterned ink can be removed, leaving behind hierarchical patterns consisting of regions of buckles among non-buckled regions.

Uniaxial Strain Induced Critical Current Degradation of Ag-Sheathed Bi-2212 Round Wire

The critical current degradation of Bi-2212 Ag-sheathed round wire subjected to uniaxial strain was studied at 4.2 K in 14 T background field. The strains applied on the sample are both tension and compression. The additional tensile strain caused by the difference in thermal expansion between the Bi-2212 round wire and Ti–6Al–4V alloy spring was calculated. The results showed that a drastic degradation of the critical current occurred when the intrinsic strain exceeded around 0.5% in tensile direction. For the compressive strain, the degradation of critical current was almost linear but more gradual than tensile strain. The intention is to use these results as a basis for Bi-2212 conductor and superconducting coil design.

Radial Flow Paths for Oil in Mass Impregnated HVDC Subsea Cables

During temperature cycling the radial flow of oil, which is caused by differences in thermal expansion between the oil and paper, may lead to cavity formation in mass impregnated HVDC cables. Partial discharge activity in these voids is an important failure mechanism in this type of cable, but the governing processes are not fully established.This paper contributes to flow modeling through a literature survey and a comparison between two alternative flow patterns in the insulation. Only one detailed model of radial flow in this kind of insulation has been found in the literature. The model, which is based on porous media flow in a homogeneous insulation with constant porosity, and its suitability as a starting point for further modeling is reviewed.Two possible flow paths, radially outwards through the paper or between paper tapes from one layer to the next, have been considered. Based on previously unpublished experimental results, the flow resistance R of the two alternatives has been estimated. This quantity is found to be of the same order of magnitude in both cases, meaning that there probably will be flow both through and between the paper tapes.

Viscoplastic characteristics of thermally grown chromia films obtained from in situ 2D synchrotron X-ray diffraction

This work takes place in the general context of a better understanding of materials degradation mechanisms in severe environments. More particularly, its purpose is to correlate microstructural elements to growth stress magnitude evolution and stress release mechanisms for chromia thin films thermally grown on model NiCr alloys. X-ray elastic strains have then been measured in situ, as the chromium oxides develop and evolve, by using synchrotron X-ray diffraction, coupled with an induction furnace. 2D diffraction patterns were continuously recorded by applying different temperature variation procedures between 1000°C and 700 °C. Thermal stresses imposed on Cr2O3/NiCr system by abruptly reducing the sample temperature, i.e. by exploiting the thermal expansion difference between oxide and substrate, showed noticeable subsequent stress relaxation by creep. The main results obtained from these experiments were the kinetic of the growth stress from the isothermal measurements, and the study of the stress release mechanism after the temperature jumps to lower values. In complement, the oxide microstructure development during the course of oxidation was also investigated from both the peaks intensity and peaks width evolution. The creep coefficients could be estimated for the first time for chromia under the thin film form. It was proposed that the stress release by diffusion creep in the chromia films is very likely governed by a mechanism involving diffusion of oxygen ions at the chromia grain boundaries.

Development of Corrosion‐Resistant Pressure Sensor with Semiconductor Strain Sensor

SUMMARYIn automotive and industrial fields, pressure sensors are a key component for precisely controlling the mechanical systems. Conventional micro electro mechanical system (MEMS)‐based pressure sensors have an advantage in noise resistance, because both strain gauges and control circuits are integrated in one chip. However, the MEMS‐based pressure sensors are generally fabricated on an Si substrate, and have a low stability against various active gases. Thus, we have newly proposed a pressure sensor which consists of an Si‐based strain sensor set on a stainless steel diaphragm with a high stability against the active gases. The key technology is that a Koval plate is inserted between the strain sensor and the stainless steel diaphragm, for preventing the breakage or the delamination of the strain sensor at the bonding interface due to a difference in thermal expansion. Structure of the sensor including the shape and the size of Koval plate and the assembly position of the strain sensor were designed using structural simulation and experiments.Eventually, the 2.8 mm wide and 0.17 mm thick Koval beam was bridged on the stainless steel diaphragm for efficiently transmitting the diaphragm deformation to the strain sensor. The strain sensor was assembled at the edge of Koval beam with a glass bonding technique. Consequently, the developed pressure sensor has achieved a small dispersion of less than 1% F. S. in a temperature range from 0 °C to 85 °C.

Quasi-dynamic and thermal analysis of a diesel engine piston under variable compression

Due to ever increasing demand for high speeds of automobiles and high gas loads, the piston of an IC engine is subjected to large thermal and structural stresses. The authors have therefore embarked upon evaluating the stresses induced in the piston and also study the effect of compression ratio on it. An experimental setup consisting of a computerized variable compression ratio diesel engine test rig with necessary instrumentation was established. A comprehensive load history in terms of pressure vs. crank angle(p-Θ) diagram and (pressure vs. gas volume), speed, fuel and air consumption rates, turning moment etc., were obtained at compression ratios of 16.5,17.5 and 18.5 from the experimentation. Part one is heat transfer analysis for evaluating the thermal boundary conditions like heat transfer coefficients and heat flux. Further the mean temperature of the working fluid is also obtained from thermodynamic analysis. Part two is dynamic analysis by using the equations of equilibrium for slider-crank mechanism. Part three is stress analysis using finite element method coupled field at critical angles of crank rotation where either inertia forces or wrist pin forces are maximum. The results in the form of von-mises stresses and deformation are presented. Further, factor of safety is evaluated. It was observed that increasing compression ratio has an appreciable effect on the stresses. It was interestingly observed that the mean temperature of gases is higher at compression ratio of 17.5. Stresses developed due to differential thermal expansion in the piston form a significant part of the total stresses.

Design Guideline for New Generation of High-Temperature Guarded Hot Plate

This paper complements the existing measurement standards and literature for High-Temperature Guarded Hot-Plates (HTGHPs) by addressing specific issues relating to thermal conductivity measurements of technical insulation at high temperatures, with examples focusing on the designs for measuring thin insulation. The sensitivity studies include the effect of temperature measurements, plate flatness and centre-guard gap design and imbalance. A new configuration of centre-guard gap with triangular shape cross-section has been optimised to obtain the same thermal resistance across the gap as the 2-mm wide gap with rectangular shape cross-section that has been used in the HTGPHs at NPL and LNE. Recommendations haven been made on the selections of heater plate materials, high-temperature high-emissivity coatings and miniature temperature sensors. For the first time, thermal stress analysis method has been applied to the field of HTGHPs, in order to estimate the effect of differential thermal expansion on the flatness of thin rigid specimens.

A unified materials approach to mitigating optical nonlinearities in optical fiber. II. B. The optical fiber, material additivity and the nonlinear coefficients

AbstractThe purpose of this paper, Part IIB in the trilogy [Int J Appl Glass Sci. 2018 (not available); Int J Appl Glass Sci. 2018 (not available); Int J Appl Glass Sci. 2018 (not available)], is to describe the continuum models employed to deduce the effects of differential thermal expansion in the clad optical fiber in addition to the nonlinear optical behaviors at high intensities of light. In this continuation of Part IIA, more of the state‐of‐the‐art in W‐S‐based continuum material models are reviewed here with specific examples provided based on canonical material systems suggested from the findings of Part I and treated in detail in Part III. Part IIB pays particular attention to the optical fiber and its nonlinearities.

Heat switch technology for cryogenic thermal management

Systematic review is given of development of novel heat switches at cryogenic temperatures that alternatively provide high thermal connection or ideal thermal isolation to the cold mass. These cryogenic heat switches are widely applied in a variety of unique superconducting systems and critical space applications. The following types of heat switch devices are discussed: 1) magnetic levitation suspension, 2) shape memory alloys, 3) differential thermal expansion, 4) helium or hydrogen gap-gap, 5) superconducting, 6) piezoelectric, 7) cryogenic diode, 8) magneto-resistive, and 9) mechanical demountable connections. Advantages and limitations of different cryogenic heat switches are examined along with the outlook for future thermal management solutions in materials and cryogenic designs.

Nanostructure and nanomaterial characterization, growth mechanisms, and applications

Abstract Nanostructures are playing significant roles in the development of new functions and the enhancement of the existing functions of industrial devices such as sensors, transistors, diodes, lithium-ion batteries, and photovoltaic cells, due to their piezoelectricity, biocompatibility, and pyroelectrical characteristics. This research focused on the review of the characteristics of different nanostructures and nanomaterials such as ZnO, ZnS, MoS2, GO, TiO2, SnO2, and Fe2O3, their fabrication techniques, growth mechanisms, and applications. The factors affecting the growth mechanisms, the crystallographic natures, growth models of anisotropic nanostructures, and growth of nanocrystals, were also highlighted in this research. The existence of lattice mismatch, differential thermal expansion, and high deposition temperature, have affected uniform deposition of nanoparticles on substrates and caused heteroepitaxy, which has resulted in defective nanostructures. Although heteroepitaxy has negatively affected the characteristics and potential applications of nanostructures, it has also opened new research frontiers for potential new applications of nanostructures. The use of nanostructures for gas sensing is attributed to the high specific area, change of resistance on exposure to gases, and high photoconduction abilities, while the photon-carrier collection abilities and anti-reflectance qualities are vital for solar photovoltaic cells. Nanostructures have also been used as coating pigments to prevent corrosion of facilities, reduce urban heat islands and energy consumptions, due to the near infrared (NIR) reflective characteristics.

New WC-Cu thermal barriers for fusion applications: High temperature mechanical behaviour

The combination of tungsten carbide and copper as a thermal barrier could effectively reduce the thermal mismatch between tungsten and copper alloy, which are proposed as base armour and heat sink, respectively, in the divertor of future fusion reactors. Furthermore, since the optimum operating temperature windows for these divertor materials do not overlap, a compatible thermal barrier interlayer between them is required to guarantee a smooth thermal transition, which in addition may mitigate radiation damage.The aim of this work is to study the thermo-mechanical properties of WC-Cu cermets fabricated by hot pressing. Focus is placed on the temperature effect and composition dependence, as the volume fraction of copper varies from 25 to 50 and 75 vol%. To explore this behaviour, fracture experiments are performed within a temperature range from room temperature to 800 °C under vacuum. In addition, elastic modulus and thermal expansion coefficient are estimated from these tests.Results reveal a strong dependence of the performance on temperature and on the volume fraction of copper and, surprisingly, a slight percent of Cu (25 vol%) can effectively reduce the large difference in thermal expansion between tungsten and copper alloy, which is a critical point for in service applications. The thermal performance of these materials, together with their mechanical properties could indeed reduce the heat transfer from the PFM to the underlying element while supporting the high thermal stresses of the joint. Thus, the presence of these cermets could allow the reactor to operate above the ductile to brittle transition temperature of tungsten, without compromising the underlying materials.

Dichotomy between in-plane magnetic susceptibility and resistivity anisotropies in extremely strained BaFe2As2

High-temperature superconductivity in the Fe-based materials emerges when the antiferromagnetism of the parent compounds is suppressed by either doping or pressure. Closely connected to the antiferromagnetic state are entangled orbital, lattice, and nematic degrees of freedom, and one of the major goals in this field has been to determine the hierarchy of these interactions. Here we present the direct measurements and the calculations of the in-plane uniform magnetic susceptibility anisotropy of BaFe2As2, which help in determining the above hierarchy. The magnetization measurements are made possible by utilizing a simple method for applying a large symmetry-breaking strain, based on differential thermal expansion. In strong contrast to the large resistivity anisotropy above the antiferromagnetic transition at TN, the anisotropy of the in-plane magnetic susceptibility develops largely below TN. Our results imply that lattice and orbital degrees of freedom play a subdominant role in these materials.

Modeling thermal and irradiation-induced swelling effects on the integrity of Ti3SiC2/SiC joints

Previously, results for CVD-SiC joined by a solid state displacement reaction to form a dual-phase SiC/MAX phase joint subsequently irradiated at 800 °C to 5 dpa indicated some cracking in the joint. This paper elucidates the cracking origin by developing a model that accounts for differential thermal expansion and irradiation-induced swelling between the substrate and joint materials by using a continuum damage mechanics approach with support from micromechanical modeling. Damage accumulation in joined specimens irradiated at four temperatures (300 °C, 400 °C, 500 °C and 800 °C) is analyzed. We assume the experimental irradiation dose is sufficient to cause saturation swelling in SiC. The analyses indicate that the SiC/MAX joint survives irradiation-induced swelling at all the irradiation temperatures considered. The joint experiences only minor damage when heated to and irradiated at 800 °C as well as cooling to room temperature. The prediction agrees with the experimental findings available for this case. However, the joint heated to 300 °C suffers severe damage during irradiation-induced swelling at this temperature, and additional damage after cooling to room temperature. Irradiation at 400 °C and subsequent cooling to room temperature produced similar damage to the irradiation 300 °C case, but to a lesser extent. The joint heated to 500 °C and irradiated at this temperature suffered only very minor damage, but further moderate damage occurred after cooling to room temperature.

Thin films composed of irregular micro-block arrays of closely packed CdS nanoparticles for enhanced photoelectrochemical performance

CdS is a very important semiconductor, and various micro-/nano-structured forms of CdS have been fabricated with the aim of improving its photoelectrochemical performance. We report here for the first time the preparation of a CdS film consisting of irregular micro-block arrays of closely packed CdS nanoparticles. It performs outstandingly well as a photoanode because it possesses the advantages of both arrays and nanoparticles. This CdS film is prepared simply by a combination of reaction and assembly at the gas/liquid interface (RAG/L) with successive ionic layer adsorption and reaction (SILAR), requiring no templates or expensive equipment. In this approach, the nanopores in the film of loosely aggregated CdS nanoparticles produced by RAG/L are filled by CdS nanoparticles via SILAR, forming a compact CdS film. Network micro-cracks form in the compact CdS film due to calcination caused by differential thermal expansion compared with the substrate, and these cut the CdS film into irregular micro-block arrays. This micro-/nano-structure in the prepared CdS film improves its capacity for visible light absorption, facilitates the generation/separation of excited charges, and enhances mass transfer. In an alkaline solution of methanol, the prepared CdS film exhibits the highest saturation photocurrent density (6.5mAcm−2) ever reported on CdS-based photoanodes under visible light illumination.

Thermal shock fragmentation of Mg silicates within scoriaceous micrometeorites reveal hydrated asteroidal sources

Scoriaceous micrometeorites are highly vesicular extraterrestrial dust particles that have experienced partial melting during atmospheric entry. We report the occurrence of clusters of anhedral relict forsterite crystals within these particles that testify to in situ fragmentation. The absence of similar clusters within unmelted micrometeorites suggests that fragmentation occurs during atmospheric entry rather than by parent body shock reprocessing. Clusters of broken forsterite crystals are shown to form as a result of fracturing owing to thermal stress developed during entry heating and require thermal gradients of >200 K µm–1 in order for differential thermal expansion to exceed the critical shear strength of olivine. Thermal gradients of this magnitude significantly exceed those resulting from thermal conduction and require the endothermic decomposition of phyllosilicates. Fragmented relict forsterite within scoriaceous micrometeorites, therefore, indicate that the precursor grains were similar to CI and CM2 chondrites and retained phyllosilicate prior to atmospheric entry and thus were not dehydrated on the parent asteroid by shock or thermal metamorphism. Explosive fragmentation of hydrous asteroids during collisions, therefore, does not significantly bias the interplanetary dust population.

Sintering and properties of borosilicate glass/Li-Na-K-feldspar composites for electronic applications

Abstract Glass/ceramic composites are considered as one of the most important materials for electronic applications. In the present work, an attempt for using Li-Na-K containing feldspar in addition to borosilicate glass to fabricate glass/ceramic composites having good properties was conducted. Firstly, the glass frit was prepared and grinded by high energy ball mill to get nano powder. Then, five designed batches of glass/feldspar composites containing 10, 20, 30, 40 and 50 wt% feldspar were mixed, pressed and sintered at 850 °C. Bulk density and apparent porosity of sintered specimens were determined by Archimedes method. Identification of the formed phases was examined by X-ray diffraction (XRD) analysis. Microstructure of sintered bodies was examined by scanning electron microscope. Microhardness of sintered samples was determined using Vickers indentation technique whereas the fracture toughness was determined by Indentation Fracture (IF) method. The dielectric constant of sintered composites was measured at 1 MHz. The thermal expansion coefficient (TEC) was also measured in the temperature range from room temperature to 1000 °C. The results revealed that Li-Na-K feldspar and borosilicate glass were successfully used to fabricate composites with good electrical properties and thermal expansion suitable for electronic applications. The inhibition of cristobalite amount and the formation of beta spodumene in addition to the quartz were responsible for the improvement of the electrical and thermal properties. The formed amount of beta spodumene was the ideal amount after which the higher amount can cause crack in the body due to the volume change occurred by the differences in thermal expansion of alpha and beta spodumene.

A Study on the Behavior of Single- and Twin-Gasketed Flange Joint Under External Bending Load

Gasketed flange joints are widely used in pressure vessels and piping systems. They are subjected to bending load due to differential thermal expansion, wind load, self-weight, etc., in addition to assembly and internal fluid load. Most of the flange designs are based on equivalent pressure method to include the effect of external bending loads. The behavior of gasketed flange joint is complex due to the nonlinear hysteretic behavior of gasket material and contact interfaces between joint members. It becomes more complex when the joint is subjected to bending load at elevated temperatures. In the present work, performance of a flange joint has been studied under internal pressure and external bending load at elevated temperatures. A 3D finite element model is developed, considering the nonlinearities in the joint due to gasket material and contact between its members along with their temperature-dependent material properties. The performance of joint under different bolt preloads, internal fluid pressures, and temperatures is studied. Flange joint with two gaskets (twin-gasketed flange joint, TGJ) placed concentric is also analyzed. The results from finite element analysis (FEA) are validated using four-point bending test on gasketed flange joint. The sealing and strength criteria are considered to determine the maximum allowable bending moment at different internal fluid temperatures, for both single- and twin-gasketed flange joints with spiral wound gasket. Twin gasket is able to withstand higher bending moment without leakage compared to single gasket. Results show that the allowable load on flange joint depends on operating temperature and gasket configuration.

Strains in Thermally Growing Cr<sub>2</sub>O<sub>3</sub> Films Measured <i>In Situ</i> Using Synchrotron X-Rays

This work takes place in the general context of a better understanding of materials degradation mechanisms in extreme environments. In particular, the aim of the present study was to correlate microstructural elements to growth stress magnitude evolution and stress release mechanisms for thermally grown chromia thin films on NiCr alloys. Strains in thermally grown oxides have been measured in-situ, as the oxides develop and evolve. Data have been acquired from oxides grown for different high temperatures evolutions on NiCr model alloys that form Cr2O3. Using synchrotron X-Ray at the ESRF (Beamline BM02) coupled with an induction furnace, Debye-Scherrer diffraction patterns from the oxidizing specimen were recorded in air at temperature between 700-1000°C and during cooling. The distortion of the diffraction rings was analyzed to yield the in-plane strain. Thermal stresses imposed on Cr2O3/NiCr by abruptly reducing the sample temperature for a period of time, exploiting the thermal expansion difference between oxide and substrate, showed noticeable subsequent stress relaxation by creep. Such a mechanism was monitored using time-dependent in situ measurements of strain relaxation in the oxide. The main results obtained from these experiments are the kinetic of the growth stress from the isothermal measurements (isothermal plateau), and the study of the stress release mechanism after the low-temperature jumps. In complement, the oxide microstructure development during the course of oxidation is also investigated from both the peaks intensity and width evolution. In all cases, the steady stage growth strain was relatively low and compressive. Different degrees of relaxation were also found on cooling depending on scale microstructure through the variation of the initial oxidation conditions. Results are compared with other reports of residual stresses evolution in Cr2O3 scales.

유한요소 해석을 통한 열 차폐 코팅의 열구배 피로 특성 평가

A gas turbine is a power plant unit that converts thermal energy into rotational energy by rotating a blade using hightemperature and high-pressure combustion gas. A gas turbine blade is directly exposed to a high-temperature flame. Various studies have aimed to improve the durability of the blade in harsh conditions. One proposes coating the blade with a thermal barrier to protect it from the flame, using a ceramic material with better thermal insulation. Another proposes using internal cooling, by creating an air flow path inside the blade to lower its temperature. Because both these techniques create a thermal gradient in the cross section of the blade, they amplify the difference in thermal expansion, thereby producing thermal stress in the blade and the thermal barrier coating. This study investigates the internal cooling effect on thermal gradient fatigue by using finite element analysis.