Thermal Lifetime Research Articles

Effect of irregular microcracks on the hot corrosion behavior and thermal shock resistance of YSZ thermal barrier coatings

The durability and service performance of thermal barrier coatings (TBCs) is influenced by its microstructure. One type of yttria stabilized zirconia coating containing irregular microcracks (mc-YSZ) was prepared by atmospheric plasma spraying (APS) method in this study, and the differences in hot corrosion behavior and thermal shock resistance between the conventional YSZ coating and the mc-YSZ coating were investigated. Results showed that the presence of irregular microcracks facilitated the penetration of corrosive salt (V2O5 and Na2SO4) and accelerated the hot corrosion reaction. The deflection effect of irregular microcracks in the early stage of hot corrosion could suppress the propagation of cracks generated by corrosion; but irregular microcracks would merge with corrosion cracks in the later stages of corrosion and aggravate the failure of the coatings. Thermal shock tests showed no significant difference of the thermal shock lifetime between YSZ coating and mc-YSZ coating, which proved that this irregular microcrack could not effectively improve the strain tolerance. The present study provides a reference for the structure design of thermal barrier coatings: when designing the structure of the coating, the appearance of that irregular microcracks should be avoided.

Thermal conductivity of intercalation, conversion, and alloying lithium-ion battery electrode materials as function of their state of charge

Upon insertion and extraction of lithium, materials important for electrochemical energy storage can undergo changes in thermal conductivity (Λ) and elastic modulus (M). These changes are attributed to evolution of the intrinsic thermal carrier lifetime and interatomic bonding strength associated with structural transitions of electrode materials with varying degrees of reversibility. Using in situ time-domain thermoreflectance (TDTR) and picosecond acoustics, we systemically study Λ and M of conversion, intercalation and alloying electrode materials during cycling. The intercalation V2O5 and TiO2 exhibit non-monotonic reversible Λ and M switching up to a factor of 1.8 (Λ) and 1.5 (M) as a function of lithium content. The conversion Fe2O3 and NiO undergo irreversible decays in Λ and M upon the first lithiation. The alloying Sb shows the largest and partially reversible order of the magnitude switching in Λ between the delithiated (18 W m−1 K−1) and lithiated states (<1Wm−1K−1). The irreversible Λ is attributed to structural degradation and pulverization resulting from substantial volume changes during cycling. These findings provide new understandings of the thermal and mechanical property evolution of electrode materials during cycling of importance for battery design, and also point to pathways for forming materials with thermally switchable properties.

Numerical and Experimental Investigations of the Thermal Fatigue Lifetime of CBGA Packages

A thermal fatigue lifetime prediction model of ceramic ball grid array (CBGA) packages is proposed based on the Darveaux model. A finite element model of the CBGA packages is established, and the Anand model is used to describe the viscoplasticity of the CBGA solder. The average viscoplastic strain energy density increment ΔWave of the CBGA packages is obtained using a finite element simulation, and the influence of different structural parameters on the ΔWave is analyzed. A simplified analytical model of the ΔWave is established using the simulation data. The thermal fatigue lifetime of CBGA packages is obtained from a thermal cycling test. The Darveaux lifetime prediction model is modified based on the thermal fatigue lifetime obtained from the experiment and the corresponding ΔWave. A validation test is conducted to verify the accuracy of the thermal fatigue lifetime prediction model of the CBGA packages. This proposed model can be used in engineering to evaluate the lifetime of CBGA packages.

Modelling of Wind Turbine Operation for Enhanced Power Electronics Reliability

Enhancing power electronics (PE) converter reliability is crucial for ensuring a reliable operation of current and future operating Wind Turbines (WTs). Achieving high reliability of variable speed WT PE systems requires careful consideration of their operation, and particularly their thermal cycling. This paper presents a methodology for evaluating and reconsidering operational strategies of WTs with relation to the thermal loading and lifetime consumption of the converter. The methodology is applied to compare control strategies for the WT generator and evaluate their impact on the converter reliability by observation of the thermal cycles and by calculating the resultant lifetime consumption of those stress cycles. The thermal stress on both the Machine Side Converter (MSC) and the Grid Side Converter (GSC) is examined and compared. It is shown that the least reliable of the three evaluated control strategies is the one that tracks the power curve below rated speed most closely. This paper suggests that dynamic transients associated with the WT control largely influence the IGBT module wear-out and their modelling needs to be prioritized for lifetime studies. These dynamic transients are captured by the improved model whose value is confirmed for the comparisons in the case study of the paper.

Thermal Degradation Kinetics and Lifetime Prediction of Cellulose Biomass Cryogels Reinforced by its Pyrolysis Waste

Degradation kinetics is an important tool in order to understand and improve energy conversion and the final application of a material. Cellulose cryogels (CC) are a new class of materials that can be reinforced by several types of particle, including biochar. Apart from it, degradation kinetics and lifetime prediction of biomass cellulose cryogels reinforced by cellulose pyrolysis waste (BC) has been investigated using TG techniques and iso-conversional model free methods. Additionally, the same study was applied to cellulose cryogels reinforced by graphene nanoplatelets (NPG) to compare the behavior of a filler from waste (BC) and a noble filler (NPG). Furthermore, the influence of the addition of the fillers into the cellulose biomass were evaluated in terms of thermal stability and crystallinity. BC and GNP led to higher values of activation energies (Ea) calculated from model-free isoconversional methods and all samples degraded in two-steps. Finally, lifetime prediction was successfully applied and the CC cryogel became more stable over time, maintaining almost 80% of the mass for 1 year exposed at 180 °C. The results of this study shown that only cellulose biomass cryogels are more suitable to produce thermal insulators due to it higher thermal stability.

Comparative investigation of the effect of composition and porosity gradient on thermo-mechanical properties of functionally graded thick thermal barrier coatings deposited by atmospheric plasma spraying

The present contribution aims to propose a modified thick thermal barrier coating (TTBC) to maximize the durability of convectional TTBC while maintaining the thermal insulation capacity by applying graded chemical composition layers and a porosity gradient to the ceramic top coat. To this end, a TTBC, a functionally graded TTBC (FGTTBC) with a structure consisting of five layers, and two modified FGTTBCs with a porous and porosity gradient top coat were designed. Resultant coatings were assessed for their microstructure, mechanical properties (hardness, elastic modulus, fracture toughness, and bonding strength), thermal shock lifetime, and thermal insulation capacity. The microhardness of the as-sprayed coatings was evaluated via the indentation method and the bonding strength testing was carried out according to ASTM C633-01 standard. Their thermal shock lifetime and thermal insulation capacity were also tested in a furnace at 1000 °C. The results indicated that the use of gradual NiCrAlY layers improved the fracture toughness of the coating to 8.07 MPa m0.5. The highest and lowest elastic modulus was determined to be in the 100 wt% NiCrAlY and porous 100 wt% YSZ layers, respectively. The FGTTBC with porosity gradient has better interface stability than the porous one due to its higher bonding strength (22.3 MPa compared to 19 MPa). The porous FGTTBC failed after 140 cycles, which is superior to TTBC after 50 cycles and conventional FGTTBC after 104 cycles. Ultimately, porous FGTTBC provides greater thermal insulation capacity than TTBC and reduces surface temperature by 91 °C.

Influence of different flux-materials on phase structure, morphology, photoluminescence, thermal stability, and cathodoluminescence in Ba2La0.9Eu0.1SbO6 phosphors

Recently, flux-materials have attracted considerable attention for improving the photoluminescence (PL) properties of luminescent materials. Herein, the phase structure, morphology, PL property, thermal stability, and cathodoluminescence (CL) property of Ba2La0.9Eu0.1SbO6 phosphors with and without the flux-materials of NH4F, C6H8O7, and H3BO3 were investigated and compared. At first, these mentioned flux-materials would barely affect the crystal structure through analyzing the X-ray diffraction patterns and Rietveld refinements. However, the morphology, PL intensity, thermal stability, lifetime, and CL performance were implicated by adding the flux-materials. Especially, the C6H8O7 flux-material could obviously improve the emission intensity and CL property of Ba2La0.9Eu0.1SbO6 phosphors with respect to the other samples. In addition, these flux-materials could enhance thermal stability and lifetime. In terms of quantum yield, the H3BO3 flux-material has the best optimization capabilities. Briefly, this work would provide a deep insight into a kind of suitable flux-materials to optimize luminescent materials in the synthesized processes.

Azobenzene/Tetraethyl Ammonium Photochromic Potassium Channel Blockers: Scope and Limitations for Design of Para-Substituted Derivatives with Specific Absorption Band Maxima and Thermal Isomerization Rate

Azobenzene/tetraethyl ammonium photochromic ligands (ATPLs) are photoactive compounds with a large variety of photopharmacological applications such as nociception control or vision restoration. Absorption band maximum and lifetime of the less stable isomer are important characteristics that determine the applicability of ATPLs. Substituents allow to adjust these characteristics in a range limited by the azobenzene/tetraethyl ammonium scaffold. The aim of the current study is to find the scope and limitations for the design of ATPLs with specific spectral and kinetic properties by introducing para substituents with different electronic effects. To perform this task we synthesized ATPLs with various electron acceptor and electron donor functional groups and studied their spectral and kinetic properties using flash photolysis and conventional spectroscopy techniques as well as quantum chemical modeling. As a result, we obtained diagrams that describe correlations between spectral and kinetic properties of ATPLs (absorption maxima of E and Z isomers of ATPLs, the thermal lifetime of their Z form) and both the electronic effect of substituents described by Hammett constants and structural parameters obtained from quantum chemical calculations. The provided results can be used for the design of ATPLs with properties that are optimal for photopharmacological applications.

Phase stability, thermal shock behavior and CMAS corrosion resistance of Yb2O3-Y2O3 co-stabilized zirconia thermal barrier coatings prepared by atmospheric plasma spraying

Rare-earth element doping has been extensively employed as one of the effective methods to enhance the durability of thermal barrier coatings (TBCs). In present study, ytterbia and yttria co-stabilized zirconia (YbYSZ) coatings and corresponding double-ceramic-layer (DCL) coatings comprised of a dense layer were produced by atmospheric plasma spraying. And the service capability including phase stability, thermal shock behavior and CMAS resistance were systemically evaluated. After heat treatment at 1500 °C for various durations, less monoclinic phase can be found in YbYSZ specimen, indicating excellent phase stability at high temperature. In the thermal shock test, the conventional YbYSZ coatings show a longest thermal shock lifetime, which increases by ~10% as compared with that of conventional yttria-stabilized ZrO 2 (YSZ) coatings. CMAS corrosion resistance of DCL coatings was studied using an isothermal corrosion test at 1300 °C. The results show that YbYSZ systems with a dense layer performed a high resistibility to CMAS corrosion, and less damage can be observed in coatings as well as smaller infiltration depth within the same time. • A double-ceramic-layer (DCL) coating comprised of dense layer was prepared. • The YbYSZ material exhibits higher resistance to high temperature. • The YbYSZ coatings have a longest thermal shock lifetime comparatively. • The DCL-YbYSZ coatings show enhanced CMAS resistance due to high chemical inertness.

Photophysical properties of highly green luminescent Tb(III) complexes

This article reports four Tb(III) green luminescent complexes based on the ligand 4-oxochromene-3-carbaldehyde and other synergistic ligands. Sharp peaks in the X-ray diffractogram confirms the crystalline property of the complexes. Four characteristic emission peaks of Tb(III) ion at ~499 nm, 545 nm, 586 nm, and 621 nm, assignable to 5D4→7F6,5,4,3 transitions, respectively, were obtained in photoluminescence spectra when irradiated at 345 nm. Bright green emission by the complexes was due to the dominant 5D4→7F5 peak at 545 nm. Ternary complexes exhibited improved thermal stability, emission intensities, quantum yield, and lifetime compared to the binary complex. Our investigations demonstrated that the prepared complexes might be practically useful in display devices, lighting systems, OLEDs, and liquid lasers due to their fascinating photometric properties.

Oriented External Electric Field Tuning of Unsubstituted Azoheteroarene Thermal Isomerization Half-Lives.

Azoheteroarenes are relatively new photoswitchable compounds, where one of the phenyl rings of an azobenzene molecule is replaced by a heteroaromatic five-membered ring. Recent findings on methylated azoheteroarenes show that these photoswitches have potential in various optically addressable applications. The thermal stability of molecular switches is one of the primary factors considered in the design process. For molecular memory or energy storage devices, long thermal relaxation times are required. However, inducing a short thermal isomerization lifetime is required to release stored energy or as an alternative to photoswitching to avoid overlapping absorption spectra that reduce switching fidelity. In this study, we investigate how oriented external electric fields can be used to tune the thermal isomerization properties of three unsubstituted heteroaryl azo compounds-azoimidazole, azopyrazole, and azopyrrole. We show that favorable electric field orientations can increase the thermal half-life of studied molecules by as much as 60 times or reduce it from tens of days to seconds, compared to their half-life values in the field-free environment. A deeper understanding of the relationship between structure and kinetic properties provides insight as to how molecular switches can be designed for their electric field response in switching applications.

Enhancing Thermal Performance and Lifetime Cycles of Li-ion Battery in Electric Vehicles

Hybrid energy storage system has essential priority in Electric Vehicle applications. Therefore, the design of an appropriate power sharing algorithm among energy storage components is necessary to improve battery thermal performance and provide extra extension of battery lifetime cycles. This paper presents an analytical study on the effect of using wavelet decomposition-based power sharing algorithm to force the high frequency component to be fed by the supercapacitor and accordingly reduces the thermal stress on the battery. The proposed approach was investigated by applying it on electric vehicle model in ADVISOR Tool/MATLAB using different driving profiles such as Urban Dynamometer Driving Schedule profile, Highway Fuel Economy Test, New York City Cycle, Los Angeles 1992 and new European driving cycle. The results declare that by using proposed power sharing algorithm, the working temperature of lithium battery decreases significantly while battery lifetime cycles increase, apparently. For urban dynamometer driving schedule, the operating temperature of lithium battery is improved much at maximum decomposition levels reaching to only 25.6 °C compared to 35 °C. In addition, the battery lifetime cycles increased from 2213 to 2585 cycles. Neural Networks pattern recognition tool is also applied to classify the driving cycle to the nearest reference cycles chosen to represent the different driving conditions which help to detect the appropriate wavelet decomposition level, achieving better battery thermal performance and battery lifetime cycles.

Developing Cost-Effective Ultrathin Reliable High Power White LED Emitters

A packaging approach for cost-effective and ultrathin high-power phosphor-based white light-emitting-diode (WLED) emitters with outstanding reliability is presented. By removing the encapsulant in the conventional method, the new approach employs a single silicone-phosphor thin layer instead of multiple phosphor layers in chip-scale package (CSP) WLED for both encapsulations, as well as wavelength conversion. It is demonstrated that the presented WLEDs exhibit superior optical and thermal performance and lifetime durability. Accordingly, the present work offers a high reliability, cost-effective alternative approach for ultrathin high power WLED emitters with significant advantages over the conventional packaging structure and chip-scale packaging methods.

Different Effects of Mg and Si Doping on the Thermal Transport of Gallium Nitride

Mg and Si as the typical dopants for p- and n-type gallium nitride (GaN), respectively, are widely used in GaN-based photoelectric devices. The thermal transport properties play a key role in the thermal stability and lifetime of photoelectric devices, which are of significant urgency to be studied, especially for the Mg- and Si-doped GaN. In this paper, the thermal conductivities of Mg- and Si-doped GaN were investigated based on first-principles calculations and phonon Boltzmann transport equation. The thermal conductivities of Mg-doped GaN are found to be 5.11 and 4.77 W/mK for in-plane and cross-plane directions, respectively. While for the Si-doped GaN, the thermal conductivity reaches the smaller value, which are 0.41 and 0.51 W/mK for in-plane and cross-plane directions, respectively. The decrease in thermal conductivity of Mg-doped GaN is attributed to the combined effect of low group velocities of optical phonon branches and small phonon relaxation time. In contrast, the sharp decrease of the thermal conductivity of Si-doped GaN is mainly attributed to the extremely small phonon relaxation time. Besides, the contribution of acoustic and optical phonon modes to the thermal conductivity has changed after GaN being doped with Mg and Si. Further analysis from the orbital projected electronic density of states and the electron localization function indicates that the strong polarization of Mg-N and Si-N bonds and the distortion of the local structures together lead to the low thermal conductivity. Our results would provide important information for the thermal management of GaN-based photoelectric devices.

BEHAVIOR OF APSED TBCS SUBJECTED TO COOLING MEDIA IN THERMAL CYCLE SYSTEMS

In this study, first, yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) with CoNiCrAlY bond coats both were fabricated on IN718 superalloy substrate, using air plasma spraying (APS) techniques. In order to understand the actual microstructural morphological behavior, phase stability, and thermal cycling behaviors of 8YSZ TBCs were characterized by field emission spectroscopy equipped scanning electron microscopy (FESEM), differential scanning calorimetry (DSC) and thermal cycling test. DSC results confirmed the basic reason of selecting the high cycling temperature. The crack initiation and propagation under thermal stresses due to temperature differences from maximum temperature of 1100∘C to cooling medium; here, air cooling and water quenching are used. Arising from either edges or corners it was observed serious problem which is restraining thermal cycling lifetime of TBCs, indicating failure mechanism is independent from operating parameters of thermal cycling test. However, in comparison to water-cooled thermal fatigue life, air-cooled thermal fatigue lifetime was 2.36 times better.

Thermal shock and fatigue lifetime characteristics of ceramic Nd:YAG.

The thermal shock and fatigue lifetime of a ceramic Nd:YAG laser material were analyzed. A new method is proposed to observe the effects induced by the intense temperature difference between the central portion and the surface of the material using a probe beam. The thermal effects of the intensity variations in the probe beam on a 2 at. % ceramic Nd:YAG were measured. The temperature difference and stress were high when the pump power suddenly increased or decreased compared to the thermal equilibrium. The thermal shock was the highest when the pumping started. The thermal shock resistance parameter of the laser material was 0.57 kW/m at a pump power of 14.9 W and zz stress component of 160.8 MPa. With 14.9 W pump power, the fatigue lifetime of the laser material was 238 cycles until it was damaged.

Thermal Analysis and Creep Lifetime Prediction Based on the Effectiveness of Thermal Barrier Coating on a Gas Turbine Combustor Liner Using Coupled CFD and FEM Simulation

Thermal barrier coating (TBC) plays a vital role in the gas turbine combustor liner (CL) to mitigate the internal heat transfer from combustion gas to the CL and enhance the parent material lifetime of the CL. This present study examined the thermal analysis and creep lifetime prediction based on three different TBC thicknesses, 400, 800, and 1200 μm, coated on the inner CL using the coupled computational fluid dynamics/finite element method. The simulation method was divided into three models to minimize the amount of computational work involved. The Eddy Dissipation Model was used in the first model to simulate premixed methane-air combustion, and the wall temperature of the inner CL was obtained. The conjugate heat transfer simulation on the external cooling flows from the rib turbulator, impingement jet, and cross flow, and the wall temperature of the outer CL was obtained in the second model. The thermal analysis was carried out in the third model using three different TBC thicknesses and incorporating the wall data from the first and second model. The effect of increasing TBC thickness shows that the TBC surface temperature was increased. Thereby, the inner CL metal temperature was decreased due to the TBC thickness as well as the material properties of Yttria Stabilized Zirconia, which has low thermal conductivity and a high thermal expansion coefficient. With the increase in TBC thickness, the average temperature difference between the TBC surface and the inner metal surface increased. In contrast, the average temperature difference between the inner and outer metal surfaces remained nearly constant. The von Mises equivalent stress, based on the material property and thermal expansion coefficient, was determined and used to find the creep lifetime of the CL using the Larson–Miller rupture curve for all TBC thickness cases in order to analyze the thermo-structure. Except in the C-channel, the increasing TBC thickness was found to effectively increase the CL lifespan. Furthermore, the case without TBC was compared with the damaged CL with cracks due to thermal stress, which was prevented by increasing TBC thickness shown in this present study.

Experimental and numerical investigation of seepage and heat transfer in rough single fracture for thermal reservoir

Seepage characteristics and heat transfer efficiency in rough fracture are indispensable to evaluate thermal reservoir lifetime and production performance. This study presents the seepage experiment, experiment and simulation on convective heat transfer in rough fracture for two artificial specimens. The repeatable artificial specimens are prepared by cement mortar. The roughness on the fracture surface is manufactured based on JRC profiles and 3D printing technology. The effects of fracture surface property (including proppant) and specimen temperature on seepage characteristics under confining pressure of 30 MPa are obtained. The convective heat transfer performance is evaluated considering roughness, multiple flow rates and specimen temperatures. Moreover, the effects of roughness on temperature distribution, local heat transfer and flow velocity in two specimens are analyzed and discussed. The main results indicate that the proppant and higher specimen temperature can improve hydraulic aperture and be conducive to seepage in rough fracture. The higher specimen temperature can strengthen thermal convection and conduction. The higher flow rate can extract more heat to develop thermal breakthrough. Furthermore, the anisotropy of roughness can affect heat transfer efficiency by reducing effective heat transfer area. The bulge and groove on rough fracture can interfere with temperature distribution and local heat transfer by controlling retention time of fluid.

Performance improvement of a photovoltaic-thermal system using a wavy-strip insert with and without nanofluid

The influence of the wavy-strip insert on photovoltaic-thermal (PVT) system performance was studied. To this end, computational fluid dynamics were used to model the 3-D PVT system and wavy-strip insert. The effect of the wavy-strip insert in a given total mass flow rate was examined on different numbers of the tube (N), which is in the range of 5–25. A rise in N has a direct correlation by an increase in PVT efficiencies; meanwhile, for N > 16 based on the ASHRAE method, heat removal factor and the overall heat-loss coefficient tend to be almost constant. Moreover, the effect of the wavy-strip insert is more pronounced in a lower number of tubes, so that for N = 5, the heat removal factor improved by 18%, and the overall heat-loss coefficient reduced by 3.5%. The PV plate temperature distribution is a crucial issue that can lead to serious thermal stress and lifetime degradation. A detailed study on the PV plate temperature distribution demonstrates that using the wavy-strip insert considerably reduces the temperature gradient. For N = 16, the optimum performance of the PVT module was obtained, and hence the effect of different Reynolds numbers ranging from 600 to 1600 was examined for it. It has been found that the use of insert in considered Reynolds number range will cause the thermal and electrical efficiencies to improve by 6.92–8.64% and 2.01–2.45% compared to a typical PVT system, respectively. In entire cases, the pump power consumption compared to the PV cells’ electrical output power is negligible. A comparative study on the performance improvement of this method and other commonly used methods was carried out. It has shown that this method can become competitive with other methods to improve the PVT performance; meanwhile, it can tackle with substantial problems of previous common methods. Lastly, for optimum configuration of the studied PVT system, further improvement is achieved by dispersing Al2O3 nanoparticles into the working fluid. The results elucidate that the integrated PVT system/Al2O3-water based nanofluid with a wavy-strip compared to the typical PVT system leads to improve electrical and thermal efficiencies by 3.5% and 12.06%, respectively.

On specimens choice for thermal lifetime assessment of low voltage electrical machines insulation

On specimens choice for thermal lifetime assessment of low voltage electrical machines insulation