Continuous Thermal Cycling Research Articles

Experimental study of erythritol – SiO2 phase change nanocapsules for medium temperature thermal storage

Erythritol holds significant potential for medium-temperature solar energy heat utilization. However, its phase change process is susceptible to issues such as leakage, and adhesion to the container. Nanoencapsulation is considered an effective method to mitigate these challenges. Until now, progress in erythritol nanocapsulation with excellent phase change properties has been limited. This study is dedicated to improving the phase change properties of erythritol nanoencapsulation. The authors innovatively synthesized erythritol nanocapsules with a silica shell using the Sol-gel method at ambient temperature. The nanocapsules were characterized to validate the encapsulation effect and to uncover the mechanism through which thermal cycling affects the thermal storage performance of nanocapsules. The results show that the nanocapsules are mostly fabricated at 46–70 nm in diameter, have a melting enthalpy of 294.3 J/g, and a thermal storage efficiency of 78.3 %. Continuous thermal cycling leads to aging degradation of erythritol and damage to the shell layer of the nanocapsules. These factors are the primary reasons for the reduction in the phase change enthalpy of nanocapsules. The nanocapsules studied in this research have broad applications in solar medium-temperature heat utilization, waste heat recovery, and various other fields.

In Vitro Wear of Glass-Ionomer Containing Restorative Materials.

Advertisements of glass-ionomer-containing restorative materials recommend suitability as load-bearing permanent or semi-permanent restorations. Historically, unacceptably high wear rates limit clinical indications of glass-ionomer-containing restorations in this regard. To compare the in vitro wear of contemporary glass-ionomer-containing dental materials commercially advertised for use in permanent dentition as load-bearing restorations in a chewing simulator. Resin composite was tested as a control. A resin-modified glass ionomer (Ionolux, VOCO gmbH), a high viscosity glass-ionomer hybrid system (Equia Forte HT with Equia Coat, GC America), and a bioactive ionic resin with reactive glass filler (Activa Bioactive Restorative, Pulpdent) were evaluated. Filtek Supreme Ultra (3M ESPE) is a visible light-activated resin composite that served as a control. Standardized flat disk-shaped specimens (n=12/group) were submitted to 500,000 cycles with continuous thermal cycling against steatite antagonists. Volumetric wear was measured at 1000, 10,000, 200,000, and 500,000 cycles. There was a statistically significant difference in mean volumetric wear for Activa Bioactive Restorative (p=0.0081, 95% CI: 0.3973, 0.4982) and Equia Forte HT (p<0.001, 95% CI: 1.2495, 1.8493), but no statistically significant difference in mean volumetric wear for Ionolux (p=0.6653) compared to control. Activa Bioactive Restorative wore approximately 60% less than, and Equia Forte HT twice more than Filtek Supreme Ultra on average, respectively. Compared to a resin composite, contemporary glass-ionomer-containing restorative materials advertised for use as load-bearing restorations display measurably variable in vitro wear rates.

Demonstration of Mg(NO3)2·6H2O-based composite phase change material for practical-scale medium-low temperature thermal energy storage

This work focuses on the industrialization-oriented investigation of a latent heat storage using a magnesium nitrate hydrate-based composite phase change material. Three aspects of studies are included: (i) characterizations on the thermal-physical properties of the composite, (ii) explorations on the supercooling stability of the composite within continuous thermal cycles, and (iii) evaluations on the thermal performance of a pilot-scale storage unit. The results indicate that the freshly-prepared composite has a fusion heat of 147.9 ± 0.5 J g−1, a storage capacity of 253.5 ± 0.9 J g−1 within 70–110 °C. The PCM presents no significant degradation after 100 thermal cycles. The material performs a better supercooling stability than pure magnesium nitrate hexahydrate. The designed latent heat storage unit can achieve stable charges and discharges with an effective heat storage density of 33.5 ± 2.6 kWh m−3, a thermal efficiency of 88.1 ± 9.6% and a thermal loss of around 10% during a daily operation. In addition, the storage unit performs a higher latent heat release concentration as discharging rate drops and its effective storage capacity within 80–85 °C is over 4.3 times of water storage. Thedensity of the device within a daily operation can be effectively improved by reducing the thickness of thermal insulation layer.

Operation of micro tubular solid oxide fuel cells integrated with propane reformers

This paper presents the operating results of micro tubular solid oxide fuel cells (MT-SOFCs) integrated with propane catalytic partial oxidation (CPOX) reformers. The cells combined with the propane CPOX reformers successfully survived 1000 continuous thermal cycles totaling 1922 h of operation with only a 0.47 mW power loss per cycle, as well as surviving 100 extreme thermal cycles with a maximum ramp rate of 1000 °C.min−1 without any power loss. This excellent thermal shock resistance is due to both the well matched coefficient of thermal expansion (CTE) among all of the cells’ layers as well as the homogenous anode structure present in all of the cells. Additionally, the cells operated on propane for more than 1500 continuous hours with an average degradation rate of 0.067 mW h−1 (0.58%/1000 h). This degradation was attributed to the sintering of the nickel in the anode, degradation of the current collection and the reformer. The fact that the cells showed no sign of delamination, cracking or coking after these tests also proves the successful integration of cell and CPOX reformer. Overall, the 3D printed MT-SOFCs with integrated CPOX reformers exhibited a breakthrough in terms of cell thermal cycling and long-term stability which will significantly advance the development of portable SOFCs systems.

Effect of hole modulation on the electrical transport properties and thermal stability of single crystal α-Cu2Se prepared by Bi-flux method

In this study, α-Cu2Se single crystal with space group C2/c was prepared by the Bi-flux method according to the stoichiometric ratio of Cu2Se(1+x)Bi3 (x = 0, 0.01, 0.02, 0.03). All samples involve hole conduction and carrier concentration increases with the increase of Se content. The valence of Bi atom is +3 in the sample, and the solubility limit of Bi atom is approximately 0.012% in Cu2Se. The crystal cell volume of all samples is larger than that of standard α-Cu2Se for the larger ion radius of Bi3+ than Cu+. In addition, the crystal cell volume of the samples decreases with the increase of Se content because of more Cu vacancies caused by excessive Se. The transmission electron microscopy image and the corresponding area electron diffraction pattern show that the prepared samples have a complete crystal structure. Eventually, the single crystal α-Cu2Se1.01 sample obtains the highest power factor of 1.53 × 10−3 W m−1 K−2 at 690 K. Thermogravimetric-differential thermal analysis shows that no weight loss occurred below the melting point. After three continuous thermal cycles, the value of power factor drops down to ∼0.04 × 10−3 W m−1 K−2 at 660 K. In comparison with polycrystalline sample prepared by hot pressing, the thermal stability is improved obviously.

Cyclic Supercooling of Liquid Bismuth Drops during Solidification

The laws of changing the presolidification supercooling of bismuth drops during continuous thermal cycling were studied by thermal analysis in the temperature–time coordinates. A sawtooth cyclic dependence of the supercooling on the sequence of heating and cooling cycles, which is described by a Fourier function, is detected. An expression is derived for the energy of coagulation of nuclei, and it is used to find a relation between the experimental and calculated supercoolings. The obtained results are interpreted in terms of a cluster-coagulation solidification model and the anomalous properties of bismuth in melting and solidification.

Additive laser metal deposition onto silicon

By employing selective laser melting (SLM), we demonstrate how Sn3Ag4Ti alloy can robustly bond to silicon via additive manufacturing. With this technology, heat removal devices (e.g., vapor chamber evaporators, heat pipes, micro-channels) can be directly printed onto the electronic package without using thermal interface materials. This has the advantage of keeping the current microprocessor about 10 °C cooler by eliminating two thermal interface materials. This reduces operating temperature, saving power and reducing electronic-waste. The bonding of common metal alloys used in additive manufacturing onto silicon is relatively weak and generally possesses high contact angles (poor wetting and interfacial strength). By using the proper interlayer material, wettability and reactivity with the silicon substrate increase drastically. Unlike conventional dissimilar material brazing that can take tens of minutes to form a strong bond, this study demonstrates how this kinetic limitation can be overcome to form a good bond in sub-milliseconds via intense laser heating. The mechanism for rapid bonding lies in using an alloy that can form a strong intermetallic bond to the substrate at a low temperature, and exposing the sample multiple times to give sufficient diffusion time for a strong bond. Bonding of Sn3Ag4Ti to silicon occurs through the formation of a thin (∼μm) titanium-silicide interfacial layer that makes the silicon wettable to the Sn3Ag4Ti. These printed parts are mechanically resistant to thermal cycling, with no mechanical failures visible after over a week of continuous thermal cycling (−40 °C and 130 °C).

Transient post-glacial processes on Mars: Geomorphologic evidence for a paraglacial period

On Earth a transitional phase between glacial and interglacial periods is referred to as the paraglacial period. This period immediately postdates glacial retreat and is characterized by ice removal, glacial unloading, and the exposure of steep slopes and large sediment stores. These responses led to the development of a suite of morphologic units (e.g., talus cones, gullies, sackungen, and polygons) which, when observed together, are indicative of the paraglacial period. A similar period of transitional climate and deglaciation is identified on Mars in the Late Amazonian, characterized by the association of features in a glaciated 10.6 km diameter mid-latitude crater. This crater contains concentric crater fill (CCF) formed by debris-covered glaciers, as well as a suite of stratigraphically younger geomorphic units (e.g., spatulate depressions, washboard terrain, gullies, and polygonal terrain) that are all indicative of the local environmental response to deglaciation. These features are interpreted to represent a geologically recent martian paraglacial period within this crater. The morphology and relative stratigraphic relationships among these paraglacial features are described in order to assess the processes operating during deglaciation and to document the recent history of glaciation on Mars: spatulate depressions formed by the differential sublimation of pure glacial ice near the base of the crater wall; subsequently, due to the loss of basal support and steepened slopes, remnant ice on the crater wall began to flow downhill, and formed transverse crevasses that created washboard terrain. Continuous thermal cycling of sediment-mantled ice on crater walls created fractures that formed polygonal terrain. During this time and after, gullies formed by the transport of sediment downslope from crater rim alcoves. Analyses of modeled obliquity variations suggest that the paraglacial period could have operated within the last ∼5 Myr and may still be ongoing, suggesting that the current martian paraglacial period is much longer in duration than typical paraglacial periods on Earth. Understanding the nature and sequence of paraglacial activity can help to identify variations in climate in recent Mars history.

Microstructural Evolution of SAC305 Solder Joints in Wafer Level Chip-Scale Packaging (WLCSP) with Continuous and Interrupted Accelerated Thermal Cycling

Four high-strain design wafer level chip scale packages were given accelerated thermal cycling with a 10°C/min ramp rate and 10 min hold times between 0°C and 100°C to examine the effects of continuous and interrupted thermal cycling on the number of cycles to failure. The interruptions given two of the samples were the result of periodic examinations using electron backscattered pattern mapping, leading to room temperature aging of 30 days–2.5 years after increments of about 100 cycles at several stages of the cycling history. The continuous thermal cycling resulted in solder joints with a much larger degree of recrystallization, whereas the interrupted thermal cycling tests led to much less recrystallization, which was more localized near the package side, and the crack was more localized near the interface and had less branching. The failure mode for both conditions was still the same, with cracks nucleating along the high angle grain boundaries formed during recrystallization. In conditions where there were few recrystallized grains, recovery led to formation of subgrains that strengthened the solder, and the higher strength led to a larger driving force for crack growth through the solder, leading to failure after less than half of the cycles in the continuous accelerated thermal cycling condition. This work shows that there is a critical point where sufficient strain energy accumulation will trigger recrystallization, but this point depends on the rate of strain accumulation in each cycle and various recovery processes, which further depends on local crystal orientations, stress state evolution, and specific activated slip and twinning systems.

AVALIAÇÃO NUMÉRICA DA DISTRIBUIÇÃO DE TEMPERATURA EM CILINDROS DE LAMINAÇÃO

In hot rolling processes occur changes in the profile of the rolling mill rolls (expansion and contraction) and constant wear due to mechanical stress and continuous thermal cycles of heating/cooling caused by contact rolled material- working roll and the cooling system by water jets in their surface, decreasing their lifetime. This paper presents a computational model to simulate the thermal performance of rolling mill rolls. The model was developed using the finite volume method for a transient two-dimensional system and allows calculating the temperature distribution of the rolling mill rolls under various conditions of service. Here it is investigated the influence of flow rate and temperature of the cooling water on the temperature distribution. The results show that the water temperature has greater influence than the water flow to control the surface temperature of the cylinders

Thermomechanical Simulation of the Solar One Thermocline Storage Tank

The growing interest in large-scale solar power production has led to a renewed exploration of thermal storage technologies. In a thermocline storage system, heat transfer fluid (HTF) from the collection field is simultaneously stored at both excited and dead thermal states inside a single tank by exploiting buoyancy forces. A granulated porous medium included in the tank provides additional thermal mass for storage and reduces the volume of HTF required. While the thermocline tank offers a low-cost storage option, thermal ratcheting of the tank wall (generated by reorientation of the granular material from continuous thermal cycling) poses a significant design concern. A comprehensive simulation of the 170 MWht thermocline tank used in conjunction with the Solar One pilot plant is performed with a multidimensional two-temperature computational fluid dynamics model to investigate ratcheting potential. In operation from 1982 to 1986, this tank was subject to extensive instrumentation, including multiple strain gages along the tank wall to monitor hoop stress. Temperature profiles along the wall material are extracted from the simulation results to compute hoop stress via finite element models and compared with the original gage data. While the strain gages experienced large uncertainty, the maximum predicted hoop stress agrees to within 6.8% of the maximum stress recorded by the most reliable strain gages.

An Investigation on the Cracking of Pallet Side Walls at a Sinter Plant

A failure analysis on the cracking of pallet side walls of a sintering machine in an integrated steel plant is presented. The pallets moving at a constant speed carry the base mix for sintering and enter an ignition hood furnace (temperature ≈ 1150°C) at a regular interval of time. The pallet side walls of a sintering machine are therefore subjected to continuous thermal cycling. The material of the pallet side wall is spheroidal graphite (SG) cast iron. Ten cracked side walls are collected and analyzed. The failure investigation involves field visit, visual observation of the cracked side walls, fractography, chemical analysis, microstructural characterization, tensile and impact tests. Most of the cracks are observed between the bolt–hole locations of the lower side walls; bolt–hole locations act as obstructions to thermal movement of the casting. The chemical analysis shows higher level of sulfur while the materials must be of higher purity for SG iron. Fractography shows predominantly intergranular fracture. Examinations of microstructures at the cross sections of the samples show the presence of primarily intergranular cracks. Matrix structure reveals pearlite along with ferrite surrounding the embedded graphite nodules. The amount of pearlite in the matrix is measured around 30–35% whereas predominantly ferrite matrix is desirable at the elevated temperature application. Determinations of tensile and impact properties exhibit low values of elongation (10%) and impact energy (7 J), respectively, indicating poor toughness properties of the casting. The presence of pearlite and lower amount of graphite nodules deteriorate the thermal conductivity of the casting, thereby generating more thermal stress. The analyses show that the pallet side walls start cracking under cyclic high thermal stress due to embrittlement because of improper material.

OS0910 渦巻きばね型アクチュエータを用いたSMA熱エンジンの作製と出力特性

The fatigue life of conventional heat-engines using shape memory alloy (SMA) wire is too short (less than 10^5 cycles). Therefore, SMA heat engines do not come into practical use. This study proposes the new SMA heat engine using SMA spiral spring actuators expected to have a longer fatigue life. In this study, the new SMA heat engine has been built and the engine performance has been investigated. The SMA spiral springs are fabricated from the SMA ribbon by heat-treatment. The chemical composition of the SMA ribbon is Ti-49.9at%Ni. The dimension of the SMA ribbon is 1000mm in length, 6.8mm in breadth and 0.15mm in thickness. The SMA heat engine is driven by hot (353K) and cold (293K) water. It is confirmed that the SMA heat engine can continue to work by continuous thermal cycling. The maximum output power of the engine is 139.4mW.

Interfacial Effects During Thermal Cycling of Cu-Filled Through-Silicon Vias (TSV)

Large shear stresses may develop at interfaces between dissimilar materials during thermal excursions when there is a significant difference in their coefficients of thermal expansion. The shear stress may cause interfaces to slide via diffusional process, thereby accommodating the relative dimensional changes between the two materials. This phenomenon presents a significant reliability issue in three-dimensional (3-D) interconnect structures involving through-silicon vias (TSVs), which are subjected not only to continuous thermal cycling but also to large electric current densities during service. This paper reports experimental evidence of interfacial sliding between Cu and Si in Cu-filled TSVs during thermal cycling conditions, and in the presence of electric current. Two different thermal cycling conditions were used: (i) small ΔT thermal cycling (−25°C to 135°C) and (ii) large ΔT thermal cycling (25°C to 425°C). Prior to thermal cycling, a few Cu-filled TSV samples were annealed for 30 min at 425°C. Cu intruded inside Si in nonannealed samples during small ΔT thermal cycling, whereas protrusion of Cu relative to Si occurred during all other thermal excursions. Application of electric current biased the net displacement of the Cu in the direction of electron flow, leading to enhanced protrusion (or intrusion) of Cu relative to the thermal cycling only (i.e., without electric current) condition. A simple one-dimensional analytical model and associated numerical simulations are utilized to rationalize the experimental observations.

Comparison of IGBT short-circuit failure “ohmic mode”: Epoxy molded package versus silicone gel module for new fail-safe and interruptible power converters

An epoxy molded package is compared with a silicone gel module with IGBTs chips in short-circuit failure modes with respect to critical energy, I2Tmelting and explosion energy capabilities. Special importance was attached to “ohmic mode” assessment and ageing of the failed chips. The molded technology yields a very low and stable Rsc (<10mΩ) as a “residual ohmic value” of the dies in low energy short-circuit failure, which is analysed through a complete reverse. Continuous thermal cycling tests over a medium time duration (>1000h) also exhibit an acceptable drift of the Rsc property (<20%). The silicone gel module clearly exhibits an unstable Rsc value due to damage of the “free moving” wire-bonding on the chips. The authors show that the paralleled wires connections and the multiple parallel melting pits allow a sort of active redundancy and a possible on-state operation. All these results are used for the design of new and original failsafe converters. These topologies use only one paralleled safety leg that is spontaneously and directly connected in series with the failed devices, through the low Rsc value of the failed chips, without any additional complexity or extra cost.

Characterization of melting and solidification in a real-scale PCM–air heat exchanger: Experimental results and empirical model

This paper describes the experimental studies carried out to test thermal cycling of a real-scale PCM–air heat exchanger at ambient temperatures. To achieve this goal an experimental setup previously designed and used for testing real-scale prototypes of PCM–air heat exchangers is modified. The PCM used is commercially available, organic, and paraffin based. The total energy exchanged during melting and solidification, as well as the time elapsed until total melting/solidification are determined from the power curves experimentally obtained. The influence of the inlet air temperature and air flow is studied, and results show that the continuous thermal cycling of the unit is a repetitive process: running experiments with similar conditions leads to the same thermal behavior, no degradation in the PCM properties is noticed. Pressure drop is measured for different air flows. Depending on the inlet air temperature, full solidification of the PCM could be achieved in less than 3 h for an 8 °C temperature difference between the inlet air and the average phase change of the PCM. Average thermal powers of up to 4.5 kW and 3.5 kW for 1 h are obtained for melting and solidification stages, respectively. An empirical model is developed from the experimental results, which could be a useful designing tool for applications that use such technology: green housing, curing and drying processes, plant production, HVAC, and free-cooling.

Cycle stability of sorption materials and composites for the use in heat pumps and cooling machines

Thermally driven adsorption chillers and heat pumps are a very promising approach toward an efficient use of energy as well as an effective climate protection through reduced CO 2 emission of conventional heating and cooling devices. With regard to current market entrance of this technology, this paper presents results on the stability of current available materials like silica gels and zeolites, recently developed materials like aluminophosphates (AlPO) and silica-aluminophosphates (SAPO) and novel materials like metal organic frameworks (MOF) under hydrothermal treatment. Seven materials as powders or granules as well as three composite have been analyzed under continuous thermal cycling in a water vapour atmosphere in order to evaluate their suitability for the use in a periodically working heat pump with water as working fluid. The stability of powders has been analyzed in-situ by thermogravimetry in a first stage short-cycle test. In case of the composite, made up of an active sorption material and a support structure, a cycling-test rig has been developed in order to realize a life-cycle stress. The need for a first stage short-cycle test is demonstrated impressively by the dramatic loss of 40% in sorption capacity of a Cu-BTC sample within the first 15 cycles.

Efficiency Study of a Commercial Thermoelectric Power Generator (TEG) Under Thermal Cycling

Thermoelectric generators (TEGs) make use of the Seebeck effect in semiconductors for the direct conversion of heat to electrical energy. The possible use of a device consisting of numerous TEG modules for waste heat recovery from an internal combustion (IC) engine could considerably help worldwide efforts towards energy saving. However, commercially available TEGs operate at temperatures much lower than the actual operating temperature range in the exhaust pipe of an automobile, which could cause structural failure of the thermoelectric elements. Furthermore, continuous thermal cycling could lead to reduced efficiency and lifetime of the TEG. In this work we investigate the long-term performance and stability of a commercially available TEG under temperature and power cycling. The module was subjected to sequential hot-side heating (at 200°C) and cooling for long times (3000 h) in order to measure changes in the TEG’s performance. A reduction in Seebeck coefficient and an increase in resistivity were observed. Alternating-current (AC) impedance measurements and scanning electron microscope (SEM) observations were performed on the module, and results are presented and discussed.

Aluminium based metal matrix composites for improved elevated temperature performance

The thermal performance of six commercially available particulate reinforced metal matrix composites (MMCs) has been investigated, including the effects of continuous thermal exposure and thermal cycling on ambient and elevated temperature tensile properties, toughness, and fatigue. Additionally, accelerated creep tests and thermal cycling tests under load were carried out. Some aspects of the performance of these MMCs were very promising, particularly strength up to 200°C and good retained strength after extended exposure at up to 260°C, while resistance to creep and thermal cycling under load was generally not as good. The results were used as a basisfor selected tests on some development MMCs aimed at improvements to particular aspects of thermal performance. The use of 2618 type aluminium alloys as a matrix for powder route SiC reinforced MMCs, in place of 2124, had no significant effect, but initial results suggest that the use of ultrafine oxides as reinforcement in aluminium alloys may improve strength above 200°C.

Process modelling applied to 6082-T6 aluminium weldments—I. Reaction kinetics

The first part of this investigation deals with the development of an overall process model for the microstructural stability of 6082-T6 aluminium alloys at elevated temperatures. The model is composed of elements derived from thermodynamics, kinetic theory and simple dislocation mechanics, and allows predictions of the hardness distribution after reheating and subsequent natural ageing of the base metal for a wide range of operational conditions (including multi-step isothermal heat treatments and continuous thermal cycles). Applications of the model to prediction of strength losses in the heat affected zone (HAZ) of fusion welds are described in an accompanying paper (Part II).