Short-term Thermal Cycling Research Articles

Stability of the microstructure and elevated-temperature mechanical properties of additively manufactured Inconel 718 superalloy subjected to long-term in-service thermal cycling

The present study investigates the thermal stability of the additively manufactured Inconel 718 (IN718) under a long-term thermal cycling exposure, similar to what is encountered in aircraft turbine engines, up to 3000 h. Before thermal cycling, the IN718 parts were heat-treated using two conditions designated, respectively, 2.5H and 4H. The results show that, after the simulated in-service thermal cycling conditions, the 2.5H and 4H treated samples exhibited initially a slight increase in their tensile (TS) and yield strengths (YS) at 650 °C to reach their maximum values after 500 and 1000 h, respectively, and then gradually decreased with further thermal cycling time to reach their lowest values after 3000 h. The microstructural analysis and the X-ray diffraction (XRD) results revealed that a slight initial strength increase is mainly due to a larger precipitation of γ′′ and inter-granular δ-phase during a relatively short-term thermal cycling exposure, whereas the prolonged thermal cycling time resulted in an undesired phase transformation of the metastable γ′′ to intra-granular δ-phase which adversely affected the mechanical properties. However, comparing the tensile results of both treated conditions revealed that the 4H treatment effectively delayed the deteriorations of the material strength until after 2000h as compared to the 2.5H condition for which the strength deterioration occurred after 1000 h due to a slower phase transformation of γ′′ into δ-phase in the former condition. Considering the thermal stability during in-service exposure, it was found that the 4H treated samples demonstrate a higher strength stability than their 2.5H treated equivalents, since after a thermal cycling exposure for 3000 h, the TS and YS values of the former decreased by only 3.3 and 1.3% of the initial state, respectively, as compared to 5.3 and 3.8%, for the latter.

Component-Level Reliability Assessment of a Direct-Drive PMSG Wind Power Converter Considering Two Terms of Thermal Cycles and the Parameter Sensitivity Analysis

The availability and efficiency of a wind power system are highly affected by the failure or the unreliable operation of its power converter. This article investigates the failure rate and annual consumed lifetime for a 2-MW direct-drive permanent magnet synchronous generator based full-scale converter in a wind power generation system. The reliability assessment mainly focuses on the component level, namely, diodes and IGBTs, in both of the machine-side converter and the grid-side converter. Annual damages and power cycles for semiconductors are calculated separately under long-term thermal cycles (several seconds to hours) and short-term thermal cycles (dozens to hundreds of milliseconds). Experiments regarding thermal stress measuring for different semiconductors under short-term thermal cycles are affiliated. A comparison between different thermal cycles is given and discussed in detail. To ensure a visualized time-to-failure evaluation, a Monte Carlo method is used to generate the lifetime distributions and entire unreliability functions for power semiconductors. Final B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> and B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> lifetimes can easily be observed from the cumulative distribution functions. Moreover, different standard deviations are assumed for parameters in the Bayerer's lifetime model, and by using several parallel Monte Carlo algorithms, parameter sensitivity to the final lifetime evaluation is analyzed.

High temperature thermal storage materials with high energy density and conductivity

A metastable miscibility gap in the C-Al binary phase diagram has been exploited to produce macroscopically solid phase-change enhanced thermal energy storage materials. With 50% by volume of Al or Al-12.7%Si dispersed in a graphite matrix, the materials have thermal conductivity of ∼150 W/m K, energy densities of 0.9 and 1.1 MJ/L for ΔT = 100 °C and energy storage/delivery temperatures centred around 660 °C and 577 °C respectively. These characteristics are matched to both direct-steam and fluid-mediated concentrated solar thermal power systems using conventional Rankine cycle steam turbine-generator technology. Powder metallurgy processing combined with a low-temperature binder ensures a non-percolating inverse microstructure in which the phase change Al or Al-Si particles are securely encapsulated, thereby overcoming the usual containment problems associated with metallic phase change materials. The graphite and binder system used have been shown to be stable upon short-term thermal cycling or holding at the maximum expected operating temperature.

Thermal properties of nano-sized polyethylene glycol confined in silica gels for latent heat storage

Polyethylene glycols (PEG, average molecular weight 2000, and 10,000) are embedded in silica gels (SG) with pore diameters of d=10–200nm. The PEG/SG composites were analyzed using differential scanning calorimetry (DSC), scanning electron microscope (SEM) and powder X-ray diffractions (XRD). Inside the nanopores, melting temperatures of the PEG and the depression of the melting points, have linear relation with the reverse pore size. The latent heat is reduced with the decreasing pore size. The composites remained no leakage of PEG above the melting point in pore fillingness of 80%. In a short-term thermal cycling, the composites display stable melting points and heat storage capacities. The diffractions of PEG in the pores show the same patterns with the bulk, revealing a same structure basis with good heat storage performance. The nano-sized PEG act as a series of new phase change materials obtained by adjusting the size.

Thermal Stability of the Eutectic Composition in NaCl-CaCl2-MgCl2 Ternary System Used for Thermal Energy Storage Applications

The ternary eutectic chloride salt (NaCl-CaCl2-MgCl2) was designed and prepared for thermal energy storage over 550°C in a concentration solar power system. The melting temperature and fusion enthalpy of the eutectic salt were measured experimentally using the Differential Scanning Calorimeter (DSC) technique which were determined to be 420.83°C and 201.50 J/g, respectively. The thermal stability including the short-term and the long-term thermal stability of the ternary system was investigated by the Thermo-gravimetric Analyzer (TGA) technique and the DSC technique. The thermal properties of the eutectic mixture kept stable during the short-term thermal cycling test. The weight loss of the eutectic salt was less than 5%, and the melting temperature and fusion enthalpy changed slightly as the temperature increased below 650°C in the long-term isothermal stability study. The thermal stability test revealed that the ternary eutectic system demonstrated excellent thermal stability below 650°C which was taken as the upper limit temperature of the ternary system. The eutectic composition was characterized by the X-Ray Diffraction (XRD) technique before and after thermal treatment to provide preliminary analysis on the mechanism of thermal instability. The results suggested that the ternary eutectic chloride salt was a potential candidate for applying in high-temperature thermal energy storage.

Thermal properties and thermal stability of the ternary eutectic salt NaCl-CaCl2-MgCl2 used in high-temperature thermal energy storage process

The ternary eutectic chloride salt (NaCl-CaCl2-MgCl2) was designed and prepared for thermal energy storage over 500°C in a concentrated solar power system. The thermal properties of the eutectic salt were measured experimentally using the Differential Scanning Calorimeter (DSC) technique, and the thermal stability including the long-term and short-term thermal stability of the ternary system was investigated by the Thermo-gravimetric Analyzer (TGA) technique and the DSC technique. The results indicated that the melting temperature and fusion enthalpy were 420.83°C and 201.50J/g, respectively, and the specific heat capacity was 1.49J/(g·°C). In the long-term isothermal stability study, the total weight loss of the eutectic salt was less than 5% below 650°C, and the melting temperature and fusion enthalpy changed slightly as the temperature increased. The ternary eutectic system demonstrated excellent thermal stability below 650°C, which was defined as the upper temperature limit of the ternary system. During the short-term thermal cycling test, the stability and recyclability of the ternary salt were verified in future during the working temperature, generally 50°C lower than the upper temperature limit. The eutectic composition before and after thermal treatment was characterized by the X-ray Diffraction (XRD) technique to analyze the mechanism of thermal instability. The mechanism of thermal instability turned out to be the thermal decomposition of the generated magnesium chloride hexahydrate at high temperature. It was proved that the ternary eutectic chloride salt was a potential candidate for high-temperature thermal energy storage applications up to 650°C.

Solid–solid phase transition of tris(hydroxymethyl)aminomethane in nanopores of silica gel and porous glass for thermal energy storage

To extend the use of phase change materials (PCMs) of those with good performance, tris(hydroxymethyl)aminomethane (THAM) is embedded in silica gels (SG) (pore diameter, d = 15–100 nm) and in controlled porous glasses (CPG) (nominal d = 12–100 nm). The THAM/SG and THAM/CPG composites are characterized by SEM and XRD. Latent heat storage performance of THAM/SG and THAM/CPG is investigated using differential scanning calorimetry, showing size dependence of phase transition temperature, enthalpy change and supercooling. THAM in the nanopores displays stable transition temperature and heat storage capacity in a short-term thermal cycling. The bulk and the nanosized THAM own the same diffraction patterns. The encapsulation of phase change material in nanoporous media is a promising way of tuning the thermal energy properties of those ideal PCMs for use in a wider range of temperature.

Landsliding generated by thermomechanical interactions between rock columns and wedging blocks: Study case from the Larzac Plateau (Southern France)

The Larzac Plateau is delimited by vertical cliffs whose geometry is controlled by vertical joints. Cliff’s erosion involves landslides initiated by incremental enlargement of joints that progressively detach rock columns at very low velocities (1.2 mm/yr). We find that enlargement of joints is linked to intraseasonal thermal cycles ranging between 2-15 days in relation with dilation/contraction of rock blocks trapped inside the joints. The mechanism involves two successive stages in which blocks create a wedging and a ratcheting effect on the rock column. Wedging is associated with compressional forces acting on the rock column, resulting from temperature increase and dilation of the shallow rocks. Ratcheting is associated with downward displacement of blocks by gravity to a new equilibrium position, resulting from temperature decrease and contraction of shallow rocks. The displacement vector in a thermal cycle is split into a plastic and a thermal component; plastic displacements range between 10 — 200 μm according to the seasons, and are absorbed along a shear plane dipping ~40° beneath the rock column: they are largest during autumn and winter, minor during spring and negligible in summer. This deformation mechanism is termed thermomechanical creep as permanent deformations are associated to mechanical forces induced by short-term thermal cycles.

Solid-solid phase transition of (1-C14H29NH3)2ZnCl4 in nanopores of silica gel for thermal energy storage

Latent heat storage performance of a layered perovskite-type compound, 1-C14H29NH3)2ZnCl4 (C14Zn), embedded in a series of silica gel (SG) with pore sizes of d=15–200nm is investigated using differential scanning calorimetry (DSC), and powder X-ray diffractions (XRD). C14Zn in the nanopores of silica gel shows size-dependent phase transition temperature, enthalpy change and supercooling. They have a stable transition temperature and heat capacity at each size in a short-term thermal cycling. Similar X-ray diffraction patterns are observed for the nano-sized and the bulk C14Zn. The encapsulation of a phase change material in nanopores is a new way of tuning its thermal energy storage properties for a wider range of temperature regulation.

A Reliability Evaluation Model for the Power Devices Used in Power Converter Systems Considering the Effect of the Different Time Scales of the Wind Speed Profile

This paper presents a reliability assessment model for the power semiconductors used in wind turbine power converters. In this study, the thermal loadings at different timescales of wind speed are considered. First, in order to address the influence of long-term thermal cycling caused by variations in wind speed, the power converter operation state is partitioned into different phases in terms of average wind speed and wind turbulence. Therefore, the contributions can be considered separately. Then, in regards to the reliability assessment caused by short-term thermal cycling, the wind profile is converted to a wind speed distribution, and the contribution of different wind speeds to the final failure rate is accumulated. Finally, the reliability of an actual power converter semiconductor for a 2.5 MW wind turbine is assessed, and the failure rates induced by different timescale thermal behavior patterns are compared. The effects of various parameters such as cut-in, rated, cut-out wind speed on the failure rate of power devices are also analyzed based on the proposed model.

Role of Prostaglandins in Glaucoma: Design and Optimization

The present study was to find the role of prostaglandin (PG) analogue (Latanoprost) and an effort to develop its anti-glaucoma ophthalmic solution (using solubility agents via safe preservative system). To formulate the eye drops of Latanoprost wherein benzalkonium chloride (BKC; cause external disturbance of the corneal epithelium) was replaced with sodium perborate as the preservative using optimize concentration of surfactant’s for better aqueous solubility and evaluated its effectiveness in ocular drug delivery system. The results obtained during the optimization of process in conjunction with process parameters are showed maximum solubility and stability with Tween-80 (Polysorbate-80) in water (0.25% w/v) in phosphate buffer at pH 6.8. In the presence of phosphonic acid, sodium perborate (0.3 mg) has maximum stability at pH ranging from six to seven which gave optimum preservative action during the preservative efficacy test (PET). Due to the results of the assay of the drug and preservative content, and minimizes leaching of impurities Eto sterilized vials were used for storage as compared to gamma sterilized vials. Also, the stability study (short term excursion, freeze thaw and thermal cycling studies), there is no significant change has been observed. The formulation was stable enough with better shelf-life and potency, withstand in extreme temperature condition during shipment.

Gelled Na2HPO4 · 12H2O with amylose-g-sodium acrylate: heat storage performance, heat capacity and heat of fusion

A novel gelling method was studied to stabilize phase change material Na2HPO4 · 12H2O with amylose grafted sodium acrylate. Gelled Na2HPO4 · 12H2O shows stable heat storage performance prepared at optimized conditions: 2.7mass/mass% sodium acrylate, 0.4 mass/mass% amylose, 0.05–0.09 mass/mass% N, N′-methylenebisacrylamide, 0.05–0.09 mass/mass% K2S2O8 and Na2SO3 (mass ratio 1:1), at 50 °C. Na2HPO4 · 12H2O was dispersed in gel network as tiny crystals less than 0.1 mm. Melting points were in the range 35.4 ± 2 °C. Short-term thermal cycling proves the effectiveness of the novel method for eliminating phase separation in the gelled salt. Adiabatic calorimetric measurement of heat capacities shows two phase transitions, which correspond to melting of Na2HPO4 · 12H2O and freezable bond water in gel, respectively. Heat of fusion of pure Na2HPO4 · 12H2O was determined as 260.9 J g−1. Distribution of extra water is: free water:freezable water:nonfreezing water = 0:0.85:0.15.

A novel gelling method for stabilization of phase change material Na 2HPO 4·12H 2O with sodium alginate grafted sodium acrylate

To stabilize phase change material, disodium hydrogen phosphate dodecahydrates (Na 2HPO 4·12H 2O), a novel gelling method by polymerizing sodium alginate grafted sodium acrylate in its molten salt was investigated. The optimum synthesis conditions to obtain product with stable heat storage performance were 2.8% (w/w) sodium acrylate, 0.3–0.4% (w/w) sodium alginate, 0.1–0.19% (w/w) N, N′-methylenebisacrylamide and 0.06% (w/w) K 2S 2O 8 and Na 2SO 3, at 50 °C. Na 2HPO 4·12H 2O was dispersed in the as-synthesized gel network as tiny crystals less than 0.1 mm. Melting points were in the range 35.4 ± 2 °C. Short-term thermal cycling proves the effectiveness of the novel gelling method for eliminating phase separation in the gelled salt.

Material Degradation during Isothermal Aging and Thermal Cycling of Hybrid Mica Seal with Ag Interlayer under SOFC Exposure Conditions

Hybrid phlogopite mica seals with silver interlayers were evaluated in terms of material degradation in a combined isothermal aging and thermal cycling test. The hybrid mica was composed of a phlogopite mica paper sandwiched between two foils. The hybrid micas were first aged at for in a low hydrogen fuel , followed by short-term thermal cycling between and . The combined test was repeated three times for a total of aging at and 119 thermal cycles. The results of high-temperature leak rates showed very good thermal stability and thermal cycle stability with leak rates of . The hybrid mica was also tested in a high-water-content fuel and demonstrated similar leakage during the isothermal aging and the subsequent thermal cycles. Postmortem analyses showed no extensive reaction of Ag with phlogopite mica as well as the mica degradations. The issues of volatility of Ag and mica in the solid oxide fuel cell (SOFC) environments were discussed. Simple models were simulated to estimate the effect of leakage on the open-circuit voltage and the total fuel loss at various SOFC stack sizes. The results showed very minute fuel loss for the current hybrid micas and demonstrated it to be a good candidate for SOFC sealing.

Material degradation during isothermal ageing and thermal cycling of hybrid mica seals under solid oxide fuel cell exposure conditions

Hybrid phlogopite mica seals with glass interlayers were evaluated in terms of materials degradation in a combined ageing and thermal cycling test. Three glass interlayers were investigated: a standard Ba–Ca–Al silicate glass (G18), a modified Ba–Ca–Al silicate glass with a nucleation agent added (G18m), and a borosilicate glass (G6). The hybrid phlogopite mica seals were aged at 800 °C for ∼500 to ∼1000 h in moist, dilute hydrogen fuel, and then subjected to short-term thermal cycling between ∼100 and 800 °C. Seals with G18 and G18m glass interlayers showed very poor thermal cycle stability after isothermal ageing. Seals with borosilicate glass interlayers showed very good thermal cycle stability with constant, low leakage (<0.01 sccm cm −1 at 0.2 psi) after ageing for 508 h followed by 56 thermal cycles. Post-mortem analyses showed degradation of the mica due to interaction with the interlayer glasses. Leak paths were identified and correlated to the measured leak rates.