Differential Thermal Contraction Research Articles

Green recycling of end-of-life photovoltaic modules via Deep-Eutectic solvents

Sustainable End-of-life (EOL) photovoltaic (PV) modules recycling is essential for achieving resource conservation and alleviating environmental issues. Ethylene vinyle acetate (EVA) adhesive films separation is a prerequisite for the realization of valuable component recovery of PV modules. However, the conventional wet chemical separation processes are usually accompanied by the challenges of low separation efficiency, high toxicity and difficult multiple recycling. Herein, a green, biodegradable, low-cost and recyclable deep eutectic solvent (DES) is firstly developed to efficiently separate EVA films in EOL PV modules. The high temperature stability and the acidic characteristic of DES can facilitate the separation of EVA film between component layers. At 175 ℃, the warping effect caused by the differential thermal contraction between the module component layers creates favorable conditions for the efficient reaction and separation by the DES. Additionally, the acidic medium reacts with the aluminum electrode layer of PV cells, which further enhances the separation efficiency of EVA films. The EVA separation rate reached 100%, the separation rate of the EVA containing aluminum (Al) up to 98.4% after 12.5 h at 175 ℃. Moreover, the developed DES exhibits excellent recyclable performance, the EVA separation rate remained 100% after 10 cycles. Compared to the traditional solvent toluene, DES has advantages in terms of economics, environmental impact, and efficiency. This strategy of design a green, biodegradable, low-cost and recyclable solvent to replace traditional solvents, and provides a promising route to efficiently separate EVA films in EOL PV modules.

Experimental observation of cavity-free ice-free isochoric vitrification via combined pressure measurements and photon counting x-ray computed tomography

Isochoric (constant-volume or volumetrically confined) vitrification has shown potential as an alternative cryopreservation-by-vitrification technique, but the complex processes at play within the chamber are yet poorly characterized, and recent investigations have prompted significant debate around whether a truly isochoric vitrification process (in which the liquid remains completely confined by solid boundaries) is indeed feasible. Based on a recent thermomechanical simulation of a high-concentration Me2SO solution, Solanki and Rabin (Cryobiology, 2023, 111, 9–15.) argue that isochoric vitrification is not feasible, because differential thermal contraction of the solution and container will necessarily drive generation of a cavity, corrupting the rigid confinement of the liquid. Here, we provide direct experimental evidence to the contrary, demonstrating cavity-free isochoric vitrification of a ∼3.5 M vitrification solution by combined isochoric pressure measurement (IPM) and photon-counting x-ray computed tomography (PC-CT). We hypothesize that the absence of a cavity is due to the minimal thermal contraction of the solution, which we support with additional volumetric analysis of the PC-CT reconstructions. In total, this study provides experimental evidence both demonstrating the feasibility of isochoric vitrification and highlighting the potential of designing vitrification solutions that exhibit minimal thermal contraction.

Hybrid Analytical-Numerical Modeling of Surface Geometry Evolution and Deposition Integrity in a Multi-Track Laser-Directed Energy Deposition Process

Abstract Modeling multitrack laser-directed energy deposition (LDED) is different from single-track deposition. There is a temporal variation in the deposition geometry and integrity in a multitrack deposition, which is not well understood. This article employs an analytical model for power attenuation and powder catchment in the melt pool in conjunction with a robust fully coupled metallurgical-thermomechanical finite element (FE) model iteratively to simulate the multitrack deposition. The novel hybrid analytical–numerical approach incorporates the effect of preexisting tracks on melt pool formation, powder catchment, geometry evolution, dilution, residual stress, and defect generation. CPM 9V steel powder was deposited on the H13 tool steel substrate for validating the model. The deposition height is found to be a function of the track sequence but reaches a steady-state height after a finite number of tracks. The height variation determines the waviness of the deposited surface and, therefore, the effective layer height. The inter-track spacing (I) plays a vital role in steady-state height evolution. A larger value of I facilitates faster convergence to the steady-state height but increases the surface waviness. The FE model incorporates the effects of differential thermal contraction, volume dilation, and transformation-induced plasticity. It predicts the deposition geometry and integrity as a function of inter-track spacing and powder feed rate. The insufficient remelting of the substrate or the preceding track can induce defects. A method to predict and mitigate these defects has also been presented in this article.

Gas-gap heat switches with negative room temperature conductor separation and their application to ultra-low temperature platforms

Controlling the thermal conductance between different temperature stages in a cryogenic system is a common problem. Often the desire is to achieve a high conductance during a pre-cooling phase, and a low conductance during a running phase. Here, we describe a novel method for constructing gas-gap heat switches: the design exploits the difference in thermal contraction of two materials to obtain a narrow (μm scale), tuneable, gas-gap at low temperature.We find that the measured temperature-dependent thermal conductance is in reasonable agreement with a simple thermal model and the switches have a sufficiently low ‘off’ conductance to be compatible with dilution refrigerator systems operating below 5 mK. A ‘remote’ sorption pump also allows the system to operate at elevated temperatures >30 K.We demonstrate the application of these switches for rapid sample exchange on cryogen-free dilution refrigerator systems, where a sample loaded from room temperature can be cooled to below 20 mK in under 3.5 hours.

Numerical analysis of delamination degradation of epoxy-impregnated superconducting coils wound with REBCO tapes caused by thermal stress

REBa2CuOy (REBCO) superconducting wires are expected to be used in coil applications owing to their excellent magnetic field properties. However, their weak delamination strength may deteriorate the superconducting characteristics of an epoxy-impregnated REBCO coil during cooling. In general, degradation is unlikely to occur if the outer-to-inner diameter ratio is less than 1.4; however, degradation has been reported in single pancake coils with an inner diameter of 560 mm and an outer diameter of 670 mm. A thermal stress analysis study is conducted to investigate the cause of this problem. Considering the non-uniformity of the epoxy resin thickness at the top and bottom of the coil, coil warping during cooling is simulated. When an FRP reinforcement plate is attached to the coil to suppress warpage, a high radial stress generated in the coil owing to the thermal contraction difference between the coil and FRP plate. The maximum local radial stress exerting in the delamination direction is 30 MPa, which is believed to degrade the superconducting characteristics of the REBCO single pancake coils.

Controlling superconductivity of CeIrIn5 microstructures by substrate selection

Superconductor/metal interfaces are usually fabricated in heterostructures that join these dissimilar materials. A conceptually different approach has recently exploited the strain sensitivity of heavy-fermion superconductors, selectively transforming regions of the crystal into the metallic state by strain gradients. The strain is generated by differential thermal contraction between the sample and the substrate. Here, we present an improved finite-element model that reliably predicts the superconducting transition temperature in CeIrIn5 even in complex structures. Different substrates are employed to tailor the strain field into the desired shapes. Using this approach, both highly complex and strained as well as strain-free microstructures are fabricated to validate the model. This enables a high degree of control over the microscopic strain fields and forms the basis for more advanced structuring of superconductors as in Josephson junctions yet also finds natural use cases in any material class in which a modulation of the physical properties on a chip is desirable.

On the Influence of Alloy Composition on the Additive Manufacturability of Ni-Based Superalloys

The susceptibility of nickel-based superalloys to processing-induced crack formation during laser powder-bed additive manufacturing is studied. Twelve different alloys—some of existing (heritage) type but also other newly-designed ones—are considered. A strong inter-dependence of alloy composition and processability is demonstrated. Stereological procedures are developed to enable the two dominant defect types found—solidification cracks and solid-state ductility dip cracks—to be distinguished and quantified. Differential scanning calorimetry, creep stress relaxation tests at 1000 °C and measurements of tensile ductility at 800 °C are used to interpret the effects of alloy composition. A model for solid-state cracking is proposed, based on an incapacity to relax the thermal stress arising from constrained differential thermal contraction; its development is supported by experimental measurements using a constrained bar cooling test. A modified solidification cracking criterion is proposed based upon solidification range but including also a contribution from the stress relaxation effect. This work provides fundamental insights into the role of composition on the additive manufacturability of these materials.

A comprehensive analytical-computational model of laser directed energy deposition to predict deposition geometry and integrity for sustainable repair

Laser directed energy deposition (DED) is an innovative additive manufacturing technology with tremendous potential for remanufacturing and repairing critical components. For sustainable repair, it is necessary to control the deposition geometry and integrity in terms of residual stresses and dilution. Obtuse contact angles and inadequate dilution can lead to inter-track porosity and cracks at the edge of the deposition-substrate interface. In addition, the fatigue life of the restored part is compromised if tensile residual stresses are induced in the deposited layer. A comprehensive modeling approach presented in this paper integrates analytical formulations for the laser-powder interaction and the powder entrapment in the melt pool, with the finite element models for determining the melt pool characteristics and the residual stresses. This model captures the physics of the key phenomena in DED, namely, power attenuation due to laser-particle interaction, melt-pool formation, powder catchment in the melt pool, and the residual stress evolution due to differential thermal contraction and metallurgical transformations. The model predictions have been experimentally validated for residual stresses, dilutions, catchment efficiencies, powder flux, and deposition geometries for crucible particle metallurgy (CPM 9V) steel powder on H13 tool steel. CPM 9V is a preferred material for repairing H-13 molds. Extensive simulations have been carried out using the comprehensive analytical-computational model to develop data-driven expressions for deposition geometry, normalized dilution, and residual stress as a function of process parameters (laser power, scan speed, and powder feed rate). For identifying the preferred deposition regime, these relations are employed to bifurcate the entire operating space into obtuse and acute contact angle, insufficient and sufficient dilution, tensile and compressive residual stress. Higher Pv/m˙ values yield depositions with the desired acute contact angles and higher specific energies (P/vd) induce favorable compressive longitudinal residual stress in the depositions.

Micro-mechanical modelling of low temperature-induced micro-damage initiation in asphalt concrete based on cohesive zone model

Asphalt pavement is subjected to cyclic temperature variations during its service life owing to changes in daily and seasonal climatic conditions. These variations tend to accumulate thermally induced distress leading to initiation and evolution of micro-cracks. The effect of cyclic thermal variations as well as thermal incompatibility of mastic and aggregates is of major significance for understanding the behavior of thermally induced damage in pavements. Thermal stress is developed due to differential contraction of mastic relative to aggregates in asphalt concrete at low temperatures. In this paper, low temperature micro-damage initiation in asphalt concrete due to differential thermal contraction is modelled using 2D micro-mechanical volume element. Cohesive zone model (CZM) is adopted to simulate low temperature damage initiation at the mastic-aggregate interface (adhesive failure) within the mixtures. A cycle of cooling and heating is applied in the micro-mechanical model in order to capture the effect of thermal damage initiation on the overall stiffness modulus of the mixtures. The results from the model reveal a reduction in stiffness modulus (as compared to the values at similar temperatures within a cycle) after the temperature of −40 °C is reached within the applied cyclic cooling and heating. The effects of aggregate gradation and binder grade are also monitored by considering four cases of mixtures formed from a combination of two different gradations and two different mastics. Results of the micro-mechanical modelling are also compared with experimental observations of comparable mixture types.

Structural Manipulation of Phase Transitions by Self‐Induced Strain in Geometrically Confined Thin Films

AbstractStrain engineering is a well‐known method often used to tune material properties in thin films. The most studied sources of strain are lattice mismatch and differential thermal contraction between the substrate and film. However, in materials which undergo a structural phase transition (SPT), a third and often overlooked source of strain may play a very significant role. If the substrate confines the area of the film, the SPT may induce stress which changes the evolution of the transition. This is a 2D analog of the isochoric phase transition between water and ice, where the freezing point drops below 0 °C. To illustrate this, the prototypical Mott insulator V2O3 which has an SPT coupled to a metal–insulator transition is used to show how self‐induced strain can drastically alter structural and electronic properties. This effect provides an elegant approach for mapping the phase diagram of the SPT and the transitions coupled to it. Moreover, the magnitude of self‐straining is tunable by modifying the substrate morphology. This effect may be important for numerous materials which exhibit an SPT and are subjected to geometrical constraints.

Effect of residual strains on the static strength of dissimilar single lap adhesive joints

ABSTRACT Adhesive joints usually experience different thermal conditions not only during their service life but even also when the joints are stored, and no external force is applied. One of the main concerns for such a condition is the residual strains/stresses induced into the adhesive layer due to the cyclic ambient temperature. In this research, the effects of residual strains on the static strength of dissimilar single lap adhesive joints (SLJ) are experimentally investigated. The thermal cycles were applied in an aluminum/carbon fiber-reinforced polymer (Al/CFRP) adhesively bonded SLJ with different adhesive thicknesses. Residual strains due to the differential thermal contraction of the two materials were measured inside the adhesive layer using the digital image correlation (DIC) technique. It was shown that for the studied conditions, the residual strains increase by thermal cycles, leading to a decrease in residual static strength of the joints. A 2D finite element thermal analysis was also performed and the numerical data were compared with the experimental results.

Thermomechanical Fatigue Behavior of Spray-Deposited SiCp/Al-Si Composite Applied in the High-Speed Railway Brake Disc

The thermomechanical fatigue (TMF) behaviors of spray-deposited SiCp-reinforced Al-Si alloy were investigated in terms of the size of Si particles and the Si content. Thermomechanical fatigue experiments were conducted in the temperature range of 150-400°C. The cyclic response behavior indicated that the continuous cyclic softening was exhibited for all materials, and the increase in SiC particles size and Si content aggravated the softening degree, which was attributed to dislocation generation due to differential thermal contraction at the Al matrix/Si phase interface or Al matrix/SiC particle interface. Meanwhile, the TMF life and stress amplitude of SiCp/Al-7Si composites were greater than those of Al-7Si alloy, and increased with the increasing SiC particle size, which was associated with “load sharing” of the direct strengthening mechanism. The stress amplitude of 4.5μmSiCp/Al-Si composite increased as the Si content increased; however, the influence of Si content on the TMF life was not so significant. The TMF failure mechanism revealed that the crack mainly initiated at the agglomeration of small-particulate SiC and the breakage of large-particulate SiC, and the broken primary Si and the exfoliated eutectic Si accelerated the crack propagation.

Probing strain modulation in a gate-defined one-dimensional electron system

Gate patterning on semiconductors is routinely used to electrostatically restrict electron movement into reduced dimensions. At cryogenic temperatures, where most studies are carried out, differential thermal contraction between the patterned gate and the semiconductor often lead to an appreciable strain modulation. The impact of such modulated strain to the conductive channel buried in a semiconductor has long been recognized, but measuring its magnitude and variation is rather challenging. Here we present a way to measure that modulation in a gate-defined GaAs-based one-dimensional channel by applying resistively-detected NMR (RDNMR) with in-situ electrons coupled to quadrupole nuclei. The detected strain magnitude, deduced from the quadrupole-split resonance, varies spatially on the order of $10^{-4}$, which is consistent with the predicted variation based on an elastic strain model. We estimate the initial lateral strain $\epsilon_{xx}$ developed at the interface to be about $3.5 \times 10^{-3}$.

Weakly-Emergent Strain-Dependent Properties of High Field Superconductors

All superconductors in high field magnets operating above 12 T are brittle and subjected to large strains because of the differential thermal contraction between component parts on cool-down and the large Lorentz forces produced in operation. The continuous scientific requirement for higher magnetic fields in superconducting energy-efficient magnets means we must understand and control the high sensitivity of critical current density Jc to strain ε. Here we present very detailed Jc(B, θ, T, ε) measurements on a high temperature superconductor (HTS), a (Rare−Earth)Ba2Cu3O7−δ (REBCO) coated conductor, and a low temperature superconductor (LTS), a Nb3Sn wire, that include the very widely observed inverted parabolic strain dependence for Jc(ε). The canonical explanation for the parabolic strain dependence of Jc in LTS wires attributes it to an angular average of an underlying intrinsic parabolic single crystal response. It assigns optimal superconducting critical parameters to the unstrained state which implies that Jc(ε) should reach its peak value at a single strain (ε = εpeak), independent of field B, and temperature T. However, consistent with a new analysis, the high field measurements reported here provide a clear signature for weakly-emergent behaviour, namely εpeak is markedly B, (field angle θ for the HTS) and T dependent in both materials. The strain dependence of Jc in these materials is termed weakly-emergent because it is not qualitatively similar to the strain dependence of Jc of any of their underlying component parts, but is amenable to calculation. We conclude that Jc(ε) is an emergent property in both REBCO and Nb3Sn conductors and that for the LTS Nb3Sn conductor, the emergent behaviour is not consistent with the long-standing canonical explanation for Jc(ε).

Design of Strain-Limited Bi-2223 Insert Coils for High-Field Magnets

High field solenoid coils made with HTS conductors are limited primarily by total strain and then by critical current. A detailed accounting of all the sources of mechanical strain on a coil is an essential part of the design. Those sources include winding tension, bending about the coil axis, bending to form the layer transition, compression from overbanding, differential thermal contraction, magnetic hoop strain in the radial and axial directions, and thermal expansion from quenches. A design example for a high-field insert coil made with Sumitomo Type HT-NX Bi-2223 conductor is presented, with each of the relevant strains computed explicitly.

Preliminary Design of the Recombination Dipole for Future Circular Collider

The Future Circular Collider (FCC) project aims at producing a conceptual design of a post-Large Hadron Collider (LHC) particle collider, able to reach higher beam energy with higher luminosity both for proton-proton and electron-electron configurations. In the proton-proton configuration, the FCC must be able to circulate 50 GeV protons in a 100 km tunnel. A key role in this project is played by the R&D on superconducting magnets, which must be able to produce magnetic fields significantly higher than in the LHC. Beside the bending dipoles, which must produce a 16 T field, also the multiplets in the interaction regions represent a technological challenge: as an example, the recombination dipoles are expected to produce a field of 10 T and therefore Nb 3 Sn must be used. In this contribution, the preliminary design of a double aperture recombination dipole for the FCC will be presented in details. It features a two layers, cosine-theta design, in order to produce the required field of 10 T in the bore. The magnetic field has the same direction in both apertures, generating a not negligible crosstalk which is minimized using asymmetric coils. The magnetic design will be described, focusing on the main features of the Nb 3 Sn conductor. The winding configuration and the iron yoke shape have been optimized to achieve a suitable field quality. A solution for the mechanical design will also be presented: the necessary prestress will be given by the so-called “bladder and keys” concept, which avoids the collaring and allows to obtain, thanks to differential thermal contraction of the components, most of the prestress after the cool down, when the Nb 3 Sn is less brittle. The proposed solution fulfills all the standard requirements both from the magnetic point of view, i.e., field quality and current margin, and from the mechanical one, with a viable construction schema and reasonably low stress on conductor and support structures, during all the magnet operations.

Mechanical Utility Structure for Testing High Field Superconducting Dipole Magnets

The U.S. Magnet Development Program (MDP) collaboration is designing a utility mechanical structure for testing various high-field superconducting dipole coils. The design uses a shell-based structure concept, which allows applying preload in two steps: during a room temperature assembly and during a cool down to a cryogenic temperature. The structure is designed to accommodate various coil designs-including Nb3Sn Cosine Theta (CT) and Canted CT magnets as well as hybrid magnets with high temperature superconductor cables. Superconducting coils, enclosed by bolted pads to form an octagonal coil pack, will be inserted into a reusable yoke-shell subassembly and precisely preloaded during the assembly using a bladder-and-key technology. Due to a differential thermal contraction between an external aluminum shell and the magnet core, the coil preload increases during the cool down up to level required for 17-T excitation. Such a reusable structure will serve as a testing fixture supporting goals of the MDP program, decreasing cost and simplifying coil performance testing at different preload levels. We present a finite-element analysis of the structure preloading various coil designs and examine the predicted coil stress at each step of the magnet assembly and excitation.

Failure Assessments for MQXF Magnet Support Structure With a Graded Approach

The high luminosity large hadron collider (LHC) upgrade requires new quadrupoles, MQXF, to replace the present LHC inner triplet magnets. The MQXFA magnet is the first prototype that has a 150-mm aperture and uses Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconducting technology in a 4.2-m magnetic length structure. The support structure design of the MQXFA magnet is based on the bladder-and-key technology, where a relatively low pre-stress at room temperature is increased to the final preload targets during the cool-down by the differential thermal contraction of the various components. The magnet support structure components experience different load levels from pre-load to cool-down and excitation. Consequently, a few parts experience high stresses that may cause localized plastic deformations or internal fracture development. The concept presented in this paper for the failure assessment of support structures integrates nonlinear finite-element (FE) analysis with detailed sub-models and fracture mechanics into an advanced engineering tool. The nonlinear FE solutions enable estimations of the structural response to the given loads, and the advanced fracture analysis with failure assessment diagram assesses the structure safety index of results obtained from the FE model. The paper describes how the MQXFA end-shell segments are being optimized based on the failure analyses.

High Speed Spin Testing of Reinforced 2212 Coils for High Field NMR Magnets.

Bi-2212 superconductors have very good performance in field, and recent developments by Solid Materials Solutions (SMS) of Chelmsford, MA to mechanically reinforce this material will help realize the potential of this material for these highfield (> 1 GHz-class) NMR magnets. While the strength of these materials can be tested using a conventional tensile test, it is difficult-to-impossible to test coils in the high-field environment required to impose the large Lorentz stresses on the superconductor, as the available warm bore for high-field magnets is usually too small to test typical NMR insert coils, which typically have either a 60 or 80-mm winding diameter. Since it is important to test the coils-and not just wire-in the high-stress environment, as such factors as differential thermal contraction (between mandrel, wire, insulation and epoxy) and stress-concentrations (due to layer-to-layer crossover, for example) only can be tested in coil form, the objective of this study is to simulate the high-field magnet environment by spinning these coils at very high speed (up to 100,000 rpm) using the spin test facilities of Barbour-Stockwell (BSI) in Woburn, MA. By spinning coils wound on a 60-mm diameter mandrel at a speed of 100,000 rpm, the hoop stress is ~700 MPa, which is sufficient to exceed the yield strength of the reinforced Bi-2212 conductor. This paper summarizes the early stage status of this 3-year, NIH-funded project.

Stress Analysis of Winding Process, Cooling Down, and Excitation in a 10.7 T REBCO HTS Magnet

A 25.7-T, 32-mm cold bore HTS/LTS superconducting magnet has been fabricated and tested at Institute of Electrical Engineering, Chinese Academy of Sciences (IEE, CAS). The REBCO HTS insert experienced large stress state while it was energized up to the target field, which is harmful for the protection of the HTS insert. In this paper, to understand the stress distributions in the HTS insert clearly, a simulation model was proposed, in which the analysis was divided into three stages, i.e., winding process, cooling down, and excitation. The effects of winding tension, band wound over the HTS insert were considered while the HTS insert is fabricated. The residual thermal stress caused by the thermal contraction differences of the coil, mandrel, and band was analyzed during the cooling down process. The final stress state was compared with the stress induced only by the electromagnetic force, which shows a large difference. The experiments conducted after the HTS insert quenched at 25.7 T proves that is not mechanical damage in the HTS magnet and the analysis results are believable.