Differential Thermal Expansion Effects Research Articles

High critical current solution derived YBa2Cu3O7 films grown on sapphire

Superconducting fault current limiters (SFCLs) are very attractive devices which require to increase its robustness against the destructive hot spots. The use of sapphire substrates to grow YBa2Cu3O7 (YBCO) films is a very attractive approach due to its high thermal conductivity. This article reports the growth of microcrack-free, epitaxial YBCO layers by chemical solution deposition (CSD) on Ce1−x Zr x O2 (CZO)/yttrium-stabilized zirconia/r-cut barely polished sapphire (BPS) substrates which can be produced in long lengths at low cost. The surface quality of the r-cut sapphire and its role on the epitaxy of CZO and YBCO layers is discussed. The issue of the microcrack generation in YBCO layers is investigated in relation to the film thickness, the growth process and the oxygenation annealing step. We demonstrate that microcracks formation is related to the in-plane tensile stress generated during the oxygenation step instead of the differential thermal expansion effects and thus it can be minimized through an adapted oxygenation process. We have shown that CSD growth of YBCO films with thicknesses up to 400 nm and attractive superconducting properties (J C ∼ 1.9 MA cm−2 at 77 K) can be achieved on BPS substrates which can be used at moderate cost for SFCL devices.

Linear creep of bonded FRP-strengthened metallic structures at warm service temperatures

Ambient cured epoxy adhesive is widely used for bonding fibre reinforced polymer (FRP) plates to metallic structures. The present paper examines a typical strengthening adhesive to investigate the effect of adhesive thermo-viscoelasticity. The response of the adhesive was determined using a series of tests using the multi-frequency scanning mode of a dynamic mechanical analyser (DMA). The thermo-mechanical properties of the adhesive were then characterised using time–temperature superposition parameters and a Prony series representation for generalised Maxwell creep. The adhesive response was in turn used within two finite element (FE) models to examine the effect of creep in the adhesive at warm temperatures (<100 °C) on the performance of a lab-scale carbon fibre-reinforced polymer (CFRP) plate strengthened steel beam and a real-scale CFRP plate strengthened cast-iron beam respectively. The study found that thermo-viscoelastic creep of the adhesive bonding layer causes an increase in the slip between the FRP and the structure, which could induce damage in the bonded joint and make the CFRP becomes less effective, potentially resulting in failure of the strengthening system during the long-term service. Differential thermal expansion effects can enhance the joint bonding stress and allow the plate to maintain its contribution to the moment capacity of the beam; however, this benefit could be lost when temperature decrease, and the additional irreversible damage caused by the increased joint stress could reduce the effectiveness of strengthening further..

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.

Thermal stress analysis of the planar SOFC bonded compliant seal design

A new materials joining approach known as the bonded compliant seal (BCS) has recently been developed for hermetically sealing the cell and window frame components in planar solid oxide fuel cell stacks. At the heart of the BCS is a thin deformable metal foil that is designed to decouple the effects of differential thermal expansion between the two components and thereby mitigate the generation of potentially destructive thermal stresses in the stack. While preliminary viability of the BCS design has been demonstrated in small-scale rotationally symmetric test specimens, issues concerning the scale-up of this seal to a size and shape that is prototypic of full-size stacks are addressed here. Finite element analysis was undertaken to investigate the magnitudes of thermally induced stress, strain, and part deflection in the cell, seal, and window frame components under uniform heating and cooling conditions. From the point of stress mitigation, particularly in the brittle ceramic cell, the initial BCS design appears to function quite well by accommodating the mismatch thermal strains as elastic and plastic strain within the sealing foil. However, the model predicts that some bowing will take place within the cell. While the amount of bowing is likely manageable through proper design of the adjacent interconnects, it is believed that a substantial portion of the predicted bow can be eliminated via minor adjustments to the various seal parameters.

Structural properties of epitaxial SrTiO3 thin films grown by molecular beam epitaxy on Si(001)

This work reports on the structural properties of an epitaxial SrTiO3 (STO) layer grown by molecular beam epitaxy on a Si(001) substrate with a two step process. The study, which includes a complete characterization of large scale plane-view images of the STO layer, is based on a careful analysis of x-ray spectra and transmission electron microscopy images. The STO layer presents a good crystalline quality and a slight texturation related to the presence of extended defects. A thin Ti-rich amorphous silicate layer (thickness ≈1.3nm) is formed at the interface between the STO and the Si substrate, evidencing the thermodynamic instability of the STO/Si interface. The difference between the thermal expansion coefficients of Si and STO is shown to be at the origin of an increased in-plane lattice parameter (3.927Å) of the STO layer as compared to its bulk value (3.905Å). This effect of differential thermal expansion is expected to be responsible for the formation of at least part of the extended defects of the STO layer.

Salt-weathering simulations under hot desert conditions: agents of enlightenment or perpetuators of preconceptions?

Because of access difficulties and inhospitable environmental conditions, our understanding of rock weathering in hot deserts has been strongly influenced by laboratory simulations. However, a risk exists that results may come to owe more to experimental design than to actual environmental conditions experienced at the atmosphere/rock surface interface. This was especially true in early salt-weathering simulations that seemingly sought to reinforce the orthodoxy of a physically dominated weathering environment driven by extremely high absolute temperatures and large diurnal temperature ranges. However, improvements in environmental sensors over the last 20 years have established the complexity of desert climates and the importance of microclimatic controls and allowed considerable refinement of experimental design. This overview evaluates the implications of recent simulation studies and presents new information on possible fatigue effects of differential thermal expansion of salts and the salt weathering of test blocks under compressive loading. In doing so, it demonstrates how physical rock properties change during experimentation, the importance of short-term surface temperature fluctuations, distinctions between the weathering of unconfined debris and larger rock surfaces, the importance of moisture and salt applications that replicate their availability in deserts, the importance of rock thermal properties in controlling their weathering response in environments dominated by radiative heating and cooling, and the overall complexity of weathering regimes.

Tolerance rings [motor-overload protection

Designers are constantly facing the challenges of packing more and more functionality into ever-smaller spaces while improving power and performance. Mechanical overload protection for drive-train systems has long been desirable but, in the past, has required bulky and expensive slip-clutch mechanisms. Tolerance rings have traditionally been used to compensate for machining tolerances or differential thermal expansion effects, but developments in tolerance ring technology outlined in this article are enabling them to be more widely used as simple, low-cost, nondestructive overload-protection devices. The principles of tolerance-ring design are outlined here, along with an explanation of the factors affecting the resulting design parameters, such as assembly force, radial load capacity, spring compression, and slip torque. We also present examples of successful overload-protection applications from the appliance and automotive industries.

Welded Joints for Robotic, On‐Orbit Assembly of Space Structures

A preliminary design concept for a weldable joint for on-orbit assembly of large space structures is described. The joint was designed for ease of assembly, for structural efficiency, and to allow passage of fluid (for active cooling or other purposes) along the member through the joint. The members were assumed to consist of graphite/epoxy tubes which were bonded 2219-T87 aluminum alloy end fittings for welding on-orbit to nodes of the same alloy. A modified form of gas tungsten arc welding was assumed to be the welding process. The joint was designed for the thermal and structural loading associated with a 37 m diameter tetrahedral truss intended as an aerobrake for a mission to Mars. It was concluded that the assembly process could lock large loads into the truss members and that the assembly robot could be required to exert large forces while aligning pairs of nodes during assembly. It was also concluded that the connections between the composite struts and the aluminum fittings will be subjected to very high service stresses due to the effects of differential thermal expansion.

Residual stress distribution in carbon fibre/thermoplastic matrix pre-impregnated composite tapes

The distribution of residual strain in carbon fibre (P75) reinforced thermo-plastic (polyetheretherketone, PEEK) composites has been studied by monitoring the Raman frequencies of the embedded carbon fibres at the microscopic level. The average longitudinal strain in the fibre measured in a 100 mm 2 section of a P75/PEEK prepreg was found to be compressive and comparable in magnitude to residual strain values computed analytically. The combined effects of differential thermal expansion of the two materials, differences in fibre size and in the thickness of resin layer, are offered as likely explanations for the incurred variations in the Raman frequency values and the considerable spread of the fibre Raman frequency distribution in the prepreg tape. Finally, it is demonstrated that the previously reported high values of apparent compressive strain in similar composite tapes by means of Raman spectroscopy were artefacts of the experimental procedure.

A differential resonator design using a bossed structure for applications in mechanical sensors

Theory and experimental results are presented of a differential resonator design employing a bossed structure for applications in mechanical sensors. The effects of residual strain, temperature and mechanical load on the resonance frequency are investigated. Mismatches in the resonators are accounted for in the analysis, resulting in a predicted temperature dependence of the offset and of the sensitivity. Experimental data obtained from a macroscopic brass model, mounted on a steel bar and applied as a force sensor, are given. Compared to a design employing a single resonator, the measurements indicate a doubling in force sensitivity and a reduction of both the intrinsic temperature dependence and of the differential thermal expansion effects. The results of this research are directly applicable to micromachined structures in silicon.

Thermal, mechanical and chemical effects in the degradation of the plasma-deposited α-SiC:H passivation layer in a multilayer thin-film device

Amorphous, hydrogenated silicon carbide (α-SiC:H) deposited by a plasma-enhanced-chemical-vapour-deposition (PECVD) process has been used as the topmost passivation layer in a multilayer thin-film device, namely, a thermal ink-jet printhead. Normal operations of the device involve rapid heating of the multilayer structure at several kHz, repetitively forming vapour bubbles that grow and collapse in an otherwise liquid environment. During such operations, the α-SiC:H layer is subjected to a variety of thermal, mechanical and chemical stresses that are detrimental to its integrity. It is found that thermally activated failures may occur when the multilayer structure is driven for extended periods of time or by pulses of excessive magnitude or duration, or when the α-SiC:H film is so thick as to store excessive elastic energy due to large differential thermal expansion effects. It is also found that severe mechanical (namely, cavitation) effects associated with bubble collapse cause early device failures if the surface of the device is exposed to an unlimited liquid reservoir; however, such adverse effects are greatly mitigated by the presence of a nozzle plate in the vicinity of the top surface. Finally, it is shown that high pH and various chemicals tend to etch the α-SiC:H thin film through an oxidation-dissolution process, removing the passivating material rapidly and leading to device failures.

High-stability bimorph scanning tunneling microscope

A novel scanning tunneling microscope (STM) is described which is constructed almost entirely of aluminum and uses bimorph piezoelectric disks as the x,y,z drive elements. The design uses a simple, rugged tripod configuration for the fine motion drive arms. Coarse motion of the sample, which is mounted on an aluminum holder, is achieved by pushing or pulling, with a piezoelectric louse. Differential thermal expansion effects are avoided by design, to first order, and the resulting drift is ≤0.5 Å/min after only a short warm-up period. It is easy to build and operate, and has good immunity to mechanical vibrations. The frequency response is flat to 5 kHz. The x,y,z drive sensitivity is 8 Å/V and the range is ≥12000 Å. Some preliminary experimental results are presented, including atomic resolution images in air, of graphite and NbSe2.

Stress analysis of encapsulated fine-line aluminum interconnect

Mechanical stresses are known to cause hillock formation and creep voids in aluminum based interconnect on integrated circuits. While the effects of differential thermal expansion between an aluminum film and a silicon substrate are well known, the effects on an encapsulated narrow aluminum line are not. We report here an analysis of the stresses generated in such a line upon cooling from 400 °C after passivation deposition. At 25 °C the principal stresses in the aluminum are all tensile with magnitudes several times the yield strength. The aluminum need not fail, but a high driving force for creep remains. High stresses also are generated in the passivation which could lead to cracking.

Dielectric Breakdown in Polyethylene at Elevated Temperatures

A two-terminal dielectric test cell for the study of the dielectric properties of viscous polymers above their softening temperature was used to perform 60 Hz breakdown measurements in polyethylene between 23 and 150°C. Molded specimens of polyethylene with embedded electrodes of parallel-plane configurations were employed. The interelectrode spacing was kept constant over the entire temperature range by suitable compensation of differential thermal expansion effects in the test cell. Low density (0.919 g/cm3) polyethylene was used for this study. The results show a marked decrease of the breakdown strength as a function of temperature between room temperature and the crystalline melting temperature, Tm. Above Tm the breakdown strength appears to be only weakly dependent on temperature. The results are analyzed with the help of Weibull statistics. The temperature dependence of the breakdown strength is discussed in the light of Artbauer's theory of free volume breakdown.

Inclusion stresses in a two-phase alloy deformed to a plastic strain of 1%

Abstract An X-ray technique has been used to measure the strains in silicon particles in a 7Si, 0·43Mg aluminium casting alloy during pure bending. Following the work of Shibata and Ono (1978), three components of the inclusion stress have been successfully identified and measured w functions of the applied strain. These components are the misfit stress, occurring because of differential thermal expansion effects, the inhomogeneity stress, occurring because of differences in elastic moduli, and the plasticity stress, which occurs when the matrix flows plastically around the elastic inclusion. The point at which the inclusions fracture has been identified and a calculation made of their fracture strength.

The effect of differential thermal expansion on residual stresses in a welded connection

Two tests were conducted on a welded joint between a forging and a pipe adaptor, to investigate the effect of differential thermal expansion in the various parts of the joint, on the residual stress. The first test was carried out with the weld in the approved manufacturing stress relieved condition. The second test was carried out following normalizing and extended stress relief heat treatments. Strain gauge measurements were made on the heat affected zones and at various positions across the weld as the forging was heated and cooled through approximately 250° C. The apparent strain outputs of the gauges used at each location were determined from gauges attached to ‘stress free islands’ machined in the weld at the required postions Since the tests could only be carried out over a 250° C range the results were linearly extrapolated to cover the range of stress relief temperatures. It is shown that differential expansion can have significant effects on residual weld stresses as the structures are temperature cycled.

Growth Effects on Stress in Nickel Films

Intrinsic stress dam of 2000-Å Ni films evaporated by an electron beam gun onto Ni substrates has been obtained as a function of temperature. The choice of the substrate material has reduced differential thermal expansion effects, to less than 108 dyn/cm2 for a total stress of 109–1010 dyn/cm2. This figure includes possible temperature gradient effects through the thickness of the substrate. The cantilevered beam technique, with deflection determined by a modified microbalance, was used to measure the stress. External circuitry was added to the microbalance to determine the Young's modulus of each substrate in vacuum at temperature preceeding the stress measurement. The stress in the initial 800 Å of film growth is very sensitive to the cleaning technique used on the substrate. The instataneous stress appears constant with increasing thickness above 800 Å at constant temperature, and it decreases slowly with temperature above 100 °C.

Some interfacial problems in metal matrix?carbon fibre composites

Metals reinforced with continuous graphite (Type I) fibres are prone to compatibility and oxidation problems when they are subjected to high temperature treatments. To prevent such interactions, the feasibility of using evaporated barrier layers has been examined in a nickel matrix. A number of techniques, e.g., tensile testing, thermal-balance determinations and microscopy have been employed to test the usefulness of this approach. Results have shown that 1 µm thick metal coatings either react vigorously with the fibre or allow diffusion of oxygen or carbon to take place sufficiently quickly so as to delay fibre degradation by only a short time interval. Whilst chemically stable nonmetallic coatings tend to prevent diffusion of both carbon and oxygen, they are extremely susceptible to brittle cracking due to differential thermal expansion effects. In this respect, zirconium carbide and boron nitride seem to be affected to a lesser degree than other carbides, nitrides and oxides, and can provide protection over a limited temperature range.

A high-temperature oscillating-disk viscometer

The paper describes a new, high-precision, oscillating-disk viscometer in which it is possible to make absolute determinations of the viscosity of gases at atmospheric pressure in a temperature range 20–950°C, approximately. The effect of differential thermal expansion on the spacing between the disk and the fixed plates as well as that of high internal damping in the wire, both of which precluded the use of this method at high temperatures in the past, has been eliminated by making all such parts, including the suspension strand, of fused quartz. This was made possible by a novel method of drawing the strand. Estimates of precision and tests of reproducibility suggest that the measurements are accurate to ±0.1%. Preliminary results on pure nitrogen up to 500°C are presented. Excellent agreement with the most reliable determinations, which are available up to 75°C, is obtained, thus inspiring confidence in the results at higher temperatures.