Reduction Of Thermal Stress Research Articles

Pressureless joining of silicon carbide using Ti3SiC2 MAX phase at 1500oC

Silicon carbide ceramics were pressureless joined by using Ti foil and Si powder, which generated Ti3SiC2 phase as the interlayer. The phase composition, microstructure and the shear strength were investigated. It was revealed that the SiC assembly after joining at 1500oC showed the high shear strength of 76.2 ± 19.5 MPa. The secondary phase of TiC within the interlayer was expected to contribute to densification improvement of Ti3SiC2 phase and thermal stress reduction.

Finite Control Set Model Predictive Control with Secondary Problem Formulation for Power Loss and Thermal Stress Reductions

Thermal stress has been identified as one of the major failure causes in power modules. Generated from the power loss, thermal stress accelerates the degradation of semiconductor devices and downgrades the system reliability. This article presents a finite control set model predictive control (FCS-MPC) oriented to reduce the power loss over the mission profile and relieve the thermal stress in power modules. Conventional control approaches including the switching frequency regulation, the reactive power injection, and the dc-bus voltage adaption show an effective progress. However, the increased control loops and complicated modulation schemes limit the system performance and practical implementation. In the proposed FCS-MPC, a secondary problem formulation is defined to reduce the power loss for the thermal stress reduction in power modules. It is simply integrated with the primary problem formulation in order to achieve the power flow control and power loss reduction simultaneously. An energy-based loss model is proposed for the loss prediction. The impact of the weightings between primary and secondary problem formulations is investigated and a most efficient weighting curve with a weighting-zones strategy is presented to design the proposed FCS-MPC. The proposed FCS-MPC is validated in the simulations and experiments. A 2.5-kW grid-tied inverter prototype is developed for the hardware testing and validation.

Thermo-optic durability of cool roof membranes: Effect of shape stabilized phase change material inclusion on building energy efficiency

Cool roofs represent an acknowledged passive cooling technique aimed at reducing the amount of solar radiation absorbed by buildings and producing indoor overheating, particularly, in summer conditions. Cool roofs owe their unique behavior to improved thermo-optic performances which, however, have been shown to deteriorate when exposed to intense atmospheric weathering. In this context, the authors produced a shape stabilized composite with improved heat storage performance, by adding 15, 25, or 35 weight percentage of non-encapsulated phase change materials (PCMs) to the original blend of a liquid waterproof-polyurethane-based cool membrane. The behavior of such composite material, when exposed to accelerated temperature, humidity, and UV radiation cycles by means of standardized long-term weathering tests (QUV test), is investigated. The final aim of the study is to clarify if the PCM inclusion could help the membrane to better behave during the course of the time, because of thermal stress reduction. In order to do so, controlled atmospheric forcing and surface temperature continuous monitoring are used to investigate the degradation of the membrane produced by the imposed weathering stress. Results show that the introduction of 25% PCM in weight optimizes the superficial finishing characteristics of the prototype, allowing to maintain a more stable thermo-optic behavior, reducing both the thermal-induced degradation and the leakage phenomenon.

Thermal and desiccation constraints drive territory preference in fiddler crabs

Territory quality is often critical for reproductive success, survival and ultimately fitness. Holding a good quality territory can make a male more attractive to females but can also have several other advantages. It can decrease the effects of environmental stressors, for example, by reducing the need for behavioural thermoregulation or decreasing the risk of predation. We found that, in the fiddler crab Austruca mjoebergi, male territories differed in the shaded mangrove and unshaded open areas of the habitat. The lower temperatures and wetter sediments of the mangrove areas resulted in lower desiccation rates allowing males to spend longer periods of time on the surface than they did in the sun-exposed open areas. Males lived at higher densities in the mangrove areas, and preferentially selected shaded territories when given a choice. Male fights to attain/retain territories in the mangrove areas were longer than those in the open areas. Because females do not preferentially mate with males with territories in shaded mangrove habitats, the advantages to males in maintaining these territories is likely a reduction in thermal and/or desiccation stress.

Finite element analysis of the effect of TiC or graphite modified composite fillers on the thermal residual stress of AMB ceramic substrates

Abstract Ag–Cu–Ti–In + X (X = TiC, graphite) composite fillers are developed to improve the comprehensive mechanical performance of active metal brazing (AMB) ceramic substrates (AlN/filler/Cu). This paper reports the effect of TiC or graphite modified composite fillers on the thermal residual stress of AMB ceramic substrates. Models based on force and moment balances and thermomechanical finite element analysis are applied to estimate the distributions of residual stresses. Results indicate that the graphite modified composite filler provides better thermal residual stress reduction on AMB ceramic substrates compared to the TiC modified composite filler. Furthermore, submodels are established to investigate the mechanism of the influence of TiC or graphite particles on the residual stress of AMB ceramic substrates. The stress distribution trends obtained from the submodels are consistent with those obtained from the corresponding global simulations, that is, stress is concentrated on the ceramic side. This further proves the rationality of adopting the submodels. It is indicated that softer graphite particles can be appropriately deformed like a pore structure to decrease stress. For TiC particles, higher hardness and stronger load transfer ability can alleviate the deformation of a metal layer, and the compressive stress concentration transfers to the particles.

Induced seismicity in geologic carbon storage

Abstract. Geologic carbon storage, as well as other geo-energy applications, such as geothermal energy, seasonal natural gas storage and subsurface energy storage imply fluid injection and/or extraction that causes changes in rock stress field and may induce (micro)seismicity. If felt, seismicity has a negative effect on public perception and may jeopardize wellbore stability and damage infrastructure. Thus, induced earthquakes should be minimized to successfully deploy geo-energies. However, numerous processes may trigger induced seismicity, which contribute to making it complex and translates into a limited forecast ability of current predictive models. We review the triggering mechanisms of induced seismicity. Specifically, we analyze (1) the impact of pore pressure evolution and the effect that properties of the injected fluid have on fracture and/or fault stability; (2) non-isothermal effects caused by the fact that the injected fluid usually reaches the injection formation at a lower temperature than that of the rock, inducing rock contraction, thermal stress reduction and stress redistribution around the cooled region; (3) local stress changes induced when low-permeability faults cross the injection formation, which may reduce their stability and eventually cause fault reactivation; (4) stress transfer caused by seismic or aseismic slip; and (5) geochemical effects, which may be especially relevant in carbonate-containing formations. We also review characterization techniques developed by the authors to reduce the uncertainty in rock properties and subsurface heterogeneity both for the screening of injection sites and for the operation of projects. Based on the review, we propose a methodology based on proper site characterization, monitoring and pressure management to minimize induced seismicity.

Effects of the superheater layout on the thermal performance of a central cavity receiver

For a water/steam solar cavity receiver, the superheater usually works at a very high temperature. It is necessary to consider the installation location of the superheater inside the cavity for its safe operation. In the present work, the effects of the superheater layouts on the thermal performance of the receiver were numerically investigated. A computational model was proposed to solve both the heat transfer and the thermal stress. With this model, the heat losses of the receiver and the temperature distributions were obtained under different superheater locations. Besides, the thermal elasticity theory was employed to quantify the thermal stress in the superheater. The results showed that the heat flux of the superheater apparently drops when it is moved from the center to the upper zone of the cavity. Consequently, the maximum wall temperature of the superheater is remarkably reduced from 740℃ to 470℃, resulting in the reduction of thermal stress by 50% approximately. In addition, due to the cavity effect, the thermal efficiency of the receiver changes slightly as the superheater location varies. Moreover, with the incident power rising, the boiler and superheater panels can absorb more solar radiation simultaneously, since they are installed together inside the cavity. It will lead to the evaporation rate and flowrate of superheated steam increasing. Therefore, the outlet temperature of superheated steam has noobviouschange even though the incident power varies significantly.

Investigations of SiC VDMOSFET With Floating Island Structure Based on TCAD

Using TCAD simulations, the silicon carbide metal–oxide–semiconductor field-effect transistor with p-type floating islands (SiC FLIMOSFET) is systematically investigated in this paper. The doping concentration (NFLI), length (L), and position (D1) of floating islands are optimized according to breakdown voltage (BV), electric field distribution, and on-resistance. The results show that $\text{N}_{\textsf {FLI}} = \textsf {1} \times \textsf {10}^{\textsf {17}}$ cm $^{\textsf {-3}}, \textsf {L}= \textsf {2.5}\,\,\mu \text{m}$ , and $\textsf {D1}=9.0\,\,\mu \text{m}$ are superior values for FLI structure considering tradeoff between BV and on-resistance. With the same BV capacity, the on-resistance of SiC FLIMOSFET is decrease by 32% comparing to the conventional SiC VDMOSFET. Besides, the dynamic property shows 16.5% reduction of FoM $\text{R}_{\textsf {on}}\cdot \textsf {Q}_{\textsf {GD}}$ in the SiC FLIMOSFET. Significantly, comparing to the conventional structure, the electro-thermal simulation indicates that the SiC FLIMOSFET has a higher robustness under short-circuit condition owing to the reduction of thermal stress in SiC/SiO2 interface. All the results show that the SiC FLIMOSFET has a good potential in SiC power device.

Using 3D-printed tungsten to optimize liquid metal divertor targets for flow and thermal stresses

Liquid metal divertors aim to provide a more robust alternative to conventional tungsten divertors. However, they still require a solid substrate to confine the liquid metal. This work proposes a novel design philosophy for liquid metal divertor targets, which allows for a two orders of magnitude reduction of thermal stresses compared to the state-of-the-art monoblock designs. The main principle is based on a 3D-printed tungsten structure, which has low connectedness in the direction perpendicular to the thermal gradient, and as a result also short length scales. This allows for thermal expansion. Voids in the structure are filled with liquid lithium which can conduct heat and reduce the surface temperature via vapor shielding, further suppressing thermal stresses. To demonstrate the effectiveness of this design strategy, an existing liquid metal concept is re-designed, fabricated, and tested on the linear plasma device Magnum-PSI. The thermo-mechanical finite element method analysis of the improved design matches the temperature response during the experiments, and indicates that thermal stresses are two orders of magnitude lower than in the conventional monoblock designs. The relaxation of the strength requirement allows for much larger failure margins and consequently for many new design possibilities.

Effects of deciduous shade trees on surface temperature and pedestrian thermal stress during summer and autumn.

In a context of urban warming, the effects of trees on outdoor thermal stress are important even during the increasingly hot autumn season. This study examines the effects of a deciduous tree species (Tilia x europaea L) on surface temperature over different ground materials and in turn on human thermal comfort, with a particular focus on tree shade variation due to leaf fall. Grass, asphalt, and gravel-covered ground surfaces, both sun-exposed and under the Tilia, were monitored in Florence, Italy, during the summer (2014) and autumn (2017) seasons. The Index of Thermal Stress (ITS) was used to gauge the micrometeorological effects of the changing tree canopy, with tree defoliation quantified by the Plant Area Index. On clear summer days, thermal discomfort was especially pronounced over exposed asphalt, and even more intense above exposed gravel due to its higher reflectivity-while shaded surfaces showed large reductions in thermal stress. Even though incoming solar radiation decreases over the course of the fall season, the direct radiation under the gradually defoliating tree canopy actually increases. Due to this diminished shading effect, the differences in surface temperature between exposed and shaded asphalt shrink dramatically from about 20 to 3°C. However, since ambient conditions become milder as the season progresses, the Tilia demonstrated a double benefit in terms of ITS: providing thermal comfort under its full canopy at the beginning of autumn and maintaining comfort even as its canopy thins out. At the same time, tree species with earlier defoliation may be unable to replicate such benefits.

Microstructures and mechanical properties of Cf/LAS composite and Ti60 alloy joints brazed with TiZrNiCu + Cf mixed powders

AbstractIn this paper, carbon fiber reinforced lithium aluminosilicate (LAS) glass‐ceramics matrix composites (Cf/LAS composites) are joined to Ti60 alloy using TiZrNiCu + Cf mixed powders by proper process parameters. The carbon fibers distribute uniformly in the brazing interlayer and react with Ti, Zr elements in the brazing alloy to form (Ti, Zr)C thin reactive layers, which are between the carbon fibers and the Ti, Zr elements. The effect of Cf content on the mechanical properties and microstructure of brazed joints are investigated. The microstructure of brazed joints varied obviously with the increasing of Cf content. The thickness of reactive layer between interlayer and Cf/LAS composites and Ti solid solution (Ti (s.s)) decrease gradually, and the volume of eutectic structure (Ti(s,s) + (Ti,Zr)2(Ni,Cu)) decrease gradually. The obtained brazed joints exhibit a maximum shear strength of 73.5 MPa at room temperature using TiZrNiCu + 0.3 wt% Cf mixed powders. The enhanced shear strength can be attributed to the reduction in thermal stress and the reinforcing effect originated from the carbon fiber addition.

Studying thermal stresses of a solar absorber in single and two-phase regimes and effects of various coatings on the absorber

In the solar parabolic trough collectors, functioning based on direct steam generation system, heat transfer system and appropriate thermal stress are indicated as two most important thermal parameters. High amount of the critical heat flux creates an extensive application range for the system so heat transfer systems can function without being subjected to high thermal stresses. In this regard, by using a coat on the external surface of the absorber thermal stresses can be reduces in single-phase, sub-cooled, boiling flow regimes and even at the critical heat flux point. In this study, by considering a numerical model of heat transfer, the input heat flux of the absorber which is considered to be a boundary condition for the absorber’s external surface is calculated. Energy conservation is assessed for three different zones of the fluid flow, single-phase, sub-cooled and boiling. Obtaining the temperature profile of the absorber’s surface from the beginning of the fluid flow to the critical heat flux point, leads to calculation of thermal stresses, which has been accomplished by finite element modeling of Misses and Tresca Model, and these two criteria have been compared. Mentioned analyses are performed and compared for coated and uncoated tubes. Three different coats (zinc, chromium and tungsten) are considered. It is shown that zinc is much more preferred than the two other coats, due to 39% and 83% reduction of thermal stress at the onset of critical thermal flux in both Mises and Tresca, respectively.

Analysis and Comparison of Spatial-Temporal Entropy Variability of Tehran City Microclimate Based on Climate Change Scenarios.

Urban microclimate patterns can play a great role for the allocation and management of cooling and heating energy sources, urban design and architecture, and urban heat island control. Therefore, the present study intends to investigate the variability of spatial and temporal entropy of the Effective Temperature index (ET) for the two basic periods (1971–2010) and the future (2011–2050) in Tehran to determine how the variability degree of the entropy values of the abovementioned bioclimatic would be, based on global warming and future climate change. ArcGIS software and geostatistical methods were used to show the Spatial and Temporal variations of the microclimate pattern in Tehran. However, due to global warming the temperature difference between the different areas of the study has declined, which is believed to reduce the abnormalities and more orderly between the data spatially and over time. It is observed that the lowest values of the Shannon entropy occurred in the last two decades, from 2030 to 2040, and the other in 2040–2050. Because, based on global warming, dominant areas have increased temperature, and the difference in temperature is reduced daily and the temperature difference between the zones of different areas is lower. The results of this study show a decrease in the coefficient of the Shannon entropy of effective temperature for future decades in Tehran. This can be due to the reduction of temperature differences between different regions. However, based on the urban-climate perspective, there is no positive view of this process. Because reducing the urban temperature difference means reducing the local pressure difference as well as reducing local winds. This is a factor that can effective, though limited, in the movement of stagnant urban air and reduction of thermal budget and thermal stress of the city.

‘Temporary Plasticiser’: A novel solution to fabricate 3D printed patient-centred cardiovascular ‘Polypill’ architectures

Hypertension and dyslipidaemia are modifiable risk factors associated with cardiovascular diseases (CVDs) and often require a complex therapeutic regimen. The administration of several medicines is commonly associated with poor levels of adherence among patients, to which World Health Organisation (WHO) proposed a fixed-dose combination unit (polypill) as a strategy to improve adherence. In this work, we demonstrate the fabrication of patient-specific polypills for the treatment of CVDs by fused deposition modelling (FDM) 3D printing and introduce a novel solution to meet critical quality attributes. The construction of poly(vinyl alcohol) (PVA)-based polypills containing four model drugs (lisinopril dihydrate, indapamide, rosuvastatin calcium and amlodipine besylate) was revealed for the first time. The impact of tablet architecture was explored using multi-layered and unimatrix structures. The novel approach of using distilled water as a ‘temporary co-plasticiser’ is reported and was found to significantly lower the extruding (90 °C) and 3D printing (150 °C) temperatures from 170 °C and 210 °C respectively, with consequent reduction in thermal stress to the chemicals. XRD indicated that lisinopril dihydrate and amlodipine besylate maintained their crystalline form while indapamide and rosuvastatin calcium were essentially in amorphous form in the PVA tablets. From the multilayer polypills, the release profile of each drug was dependent on its position in the multilayer. In addition to the multilayer architecture offering a higher flexibility in dose titration and a more adaptive solution to meet the expectations of patient-centred therapy, we identify that it also allows orchestrating the release of drugs of different physicochemical characteristics. Adopting such an approach opens up a pathway towards low-cost multidrug delivery systems such as tablets, stents or implants for wider range of globally approved actives.

Structural-Technological Methods for Reduction of Thermal Stress in Grinding

A study has been made of temperature fields occurring in a workpiece in the process of its flat grinding. The author has obtained relations making it possible to calculate the rate of heating and cooling of the machined workpiece at any point of it with account of the machining regime and of the intensity of the heat flux and the time of its action, and also the dependence of the stresses in the workpiece on the time of action of the heat source and its temperature. It has been established that by changing the structural parameters of the cutting surface of the wheel and the relation between the velocity of motion of the workpiece, the velocity of motion of the wheel, and the grinding depth, one can control the time of action of the heat flux on the workpiece and reduce thermal stress of the process.

Dynamic performance and stress analysis of the steam generator of parabolic trough solar power plants

The thermal stress on thick-walled components, such as tubesheets and steam drums, limits both the temperature ramp-up rates and the temperature differences between outer and inner walls. The cyclic operation of concentrating solar power plants may lead to fatigue damage. For these reasons, a stress analysis of the steam generator is required to assure its lifetime.A methodology is presented for the thermo-mechanical analysis of the steam generator for a parabolic trough power plant. This methodology consists of coupling transient thermodynamic and stress models of the heat exchangers in order to calculate the stress. Besides the heat exchanger models, a transient model for a TEMA H heat exchanger is proposed. Finite element simulations are carried out to calculate the deviations of the simplified analytical models. In this way, a powerful tool that allows the analysis and optimization of the steam generator operation is proposed.The results suggest that U-tube tubesheets are exposed to high thermal stresses on the no-tube-lane zone, especially in the reheater. The steam generator start-up can be accomplished in around 45 min using 36.4 MWhth. Furthermore, the TEMA X evaporator presents a thermal stress reduction of 35% compared to the kettle evaporator.

Thermal stress reduction of quasi-Z source inverter drive by model predictive control

The quasi-Z source inverter (qZSI) is one of the most promising power electronics converter topologies suitable for traction applications. The thermal stress in power modules is one of the most important causes of their failure, which is not considered during the control of qZSI drive. Thus, this paper proposes a thermal stress reduction scheme for qZSI drive, which utilizes finite control set model predictive control. The cost function is designed by including terms of switching counts, number of cycles to failure and inverter constraints in addition to the currents and capacitor voltage. This allows one to achieve the minimum thermal stress for the best possible overall performance of qZSI drive. The approach is proved using simulation and experimental results.

TheSPoT: Thermal Stress-Aware Power and Temperature Management for Multiprocessor Systems-on-Chip

Thermal stress including temperature gradients in time and space, as well as thermal cycling, influences lifetime reliability and performance of modern multiprocessor systems-on-chip (MPSoCs). Conventional power and temperature management techniques considering the peak temperature/power consumption do not provide a comprehensive solution to avoid high spatial and temporal thermal variations. This work presents TheSPoT, a novel multilevel thermal stress-aware power and thermal management approach for MPSoCs. At the top level, core consolidation and deconsolidation is performed based on peak temperature, thermal stress, and power consumption constraints. These constraints are also used at the next level, where operating frequencies are determined. At this level, we obtain optimal core frequencies by solving a convex optimization problem. However, thereafter, to reduce the runtime overhead in large MPSoCs, we alternatively propose to use a fast heuristic algorithm. The efficacy of the proposed approaches in reducing the thermal cycles and temporal/spatial temperature gradients is evaluated by comparing the results with the state-of-the-art methods. The evaluation performed on 4-core, 8-core, and 16-core MPSoCs, using PARSEC benchmarks, reveals a considerable reduction in thermal stress. For the 8-core MPSoC case study, on average, for the proposed heuristic(optimal) approach, the mean time to failure improved by 47(35)% compared to the state-of-the-art techniques with only 6(4)% performance degradation. Also, our simulations show that TheSPoT is more efficient in thermal stress reduction when more heterogeneous workloads are used.

Reduction of Thermal Stress in Copper TSV due to Annealing by Low TEC Copper

Introduction Copper through-silicon via (TSV) is the key technology used in 3D packaging. However, the large mismatch in thermal expansion coefficient (TEC) between copper and silicon generates the thermomechanical stress inside and surround the TSV during fabrication process. This thermomechanical stress changes the carrier mobility1 and causes extrusion of copper2. In this study, thermomechanical stress in copper TSV, as well as copper extrusion were reduced using low TEC copper. Moreover, conventional copper TSV was also studied as a comparison. Results and Discussion Fig. 1 shows TEC measurement results of conventional copper and copper electrodeposited with different concentrations of 2M5S leveler. The expansion length of conventional copper (line a) is linear with temperature. Copper pipes electrodeposited with 2M5S leveler (line b, c, and d) start to shrink at 120oC. The reason for the shrinkage of copper is the diffusion of carbon out of copper lattices3. Furthermore, at a 2M5S leveler concentration of 1 ppm, the expansion length of electrodeposited copper is lowest. Therefore, 1 ppm of 2M5S leveler is used for TSV filling for the next experiments.Fig. 2 illustrates the cross-sectional views of copper TSVs after annealing at 500oC. Fig. 2a and 2b are conventional copper TSV. Fig. 2c is low TEC copper TSV (TSV filled with 1 ppm of 2M5S leveler). In Fig. 2a, cracks occur at the vias bottom. Wang found that the stress at the via bottom was very high by simulation using ANSYS4. That large stress is the cause of the cracks at the bottom of the TSV. In Fig. 2b, the extrusion height of conventional copper is about 1.5 µm. In Fig. 2c of low TEC copper TSV, no cracks are observed at the vias bottom. Moreover, the copper extrusion height is only 0.3 µm. Therefore, the stress as well as the extrusion of copper in TSV have been reduced by using low TEC copper. References S. E. Thompson, G. Sun, Y. S. Choi, and T. Nishida, IEEE Trans. Electron Devices, 53, 1010–1020 (2006).A. Heryanto et al., J. Electron. Mater., 41, 2533–2542 (2012).V. Q. Dinh and K. Kondo, ECS J. Solid State Sci. Technol., 6, P566–P569 (2017).J.-S. Hwang, S.-H. Seo, and W.-J. Lee, J. Electron. Packag., 138, 31006 (2016). Figure 1

Reduction of Thermal Stress in Copper TSV due to Annealing by Low TEC Copper

Copper through-silicon via (TSV) is the key technology used in 3D packaging. During the fabrication process, copper TSV is exposed to a high temperature between 400°C and 600°C. For conventional copper TSV, we observed the cracks at the bottom of TSV after annealing at 500°C and a very high copper extrusion height, which destroys the overlying layers above the TSV. We also succeeded to prevent cracks at the bottom of the TSV and reduce the copper extrusion height by using the low thermal expansion coefficient (TEC) copper.