Thermal Cycling Performance Research Articles

Thermal cycling performance of GYbZ/YSZ thermal barrier coatings with different microstructures based on finite element simulation

Three (Gd0.9Yb0.1)2Zr2O7/YSZ double ceramic layer (DCL) thermal barrier coatings (TBCs) with different microstructures were manufactured by electron-beam physical vapor deposition (EB-PVD). The thermal cycling behavior of the three TBCs was evaluated comparatively at 1150 ℃. The results showed that TBCs with small columnar grains exhibited the best thermal cycling performance among all the tested TBCs. The effect of microstructure on thermal cycling behavior, including crack evolution and failure mode was discussed. A finite element model tailored to the microstructural characteristics of the DCL coatings was established. Several in-plane stress concentration points appeared in the taller GYbZ column due to discontinuities in strain tolerance resulting from sudden structural or compositional changes. Analysis of the strain energy release rate variation for each concentration point suggested that the predicted delamination position and process aligned with experimental results.

Revealing the oxidation growth mechanism and crack evolution law of Si-HfO2/Yb2Si2O7/Yb2SiO5/high-entropy hafnate thermal/environmental barrier coatings during thermal cycling

The Si-HfO2/Yb2Si2O7/Yb2SiO5/(Dy0.2Ho0.2Er0.2Tm0.2Lu0.2)2Hf2O7 thermal/environmental barrier coating (T/EBC) protects ceramic matrix composites (CMCs) from turbine multi-corrosive media erosion. Challenges persist in the thermal cycling performance of multi-layered T/EBC, notably in understanding the oxidation of the Si-HfO2 bond coat, compatibility of its mixed thermally grown oxide (m-TGO) with adjacent layers, and the evolution of cracks caused by thermal cycling. Utilizing plasma spraying physical vapor deposition (PS-PVD), this T/EBC on CMC substrates withstands up to 200 hours at 1450 ℃ to 1550 ℃. The m-TGO oxidation follows a parabolic growth curve, with oxygen diffusion activation energies of 160.31 kJ/mol from 1450 ℃ to 1500 ℃, and 125.16 kJ/mol from 1500 ℃ to 1550 ℃. Thermo-mechanical calculations indicate that elastic strain energy accumulation causes interlaminar cracks between m-TGO and adjacent layers. Controlling mud crack density is key to preventing the stress attraction at the tips of bifurcated cracks, thereby avoiding the formation of interlaminar cracks.

Novel high-entropy (La0.35Gd0.35Y0.35Sm0.35Yb0.6)Zr2O7 thermal barrier coatings: Thermal cycling performance and failure behavior

The high-entropy ceramic (HEC) (La0.35Gd0.35Y0.35Sm0.35Yb0.6)Zr2O7 has emerged as a strong candidate for thermal barrier coatings (TBCs) due to its exceptional thermal properties. We investigated the thermal shock resistance of HEC–lanthanum zirconate (LZ)/yttria partially stabilized zirconia (YSZ) (i.e., the HEC coating) and LZ/YSZ (i.e., the LZ coating) double-ceramic-layer TBCs fabricated via plasma spraying. During thermal shock testing at 1100°C, the LZ coating failed after 60 ± 3 cycles, while the HEC coating had a considerably longer thermal cycling lifetime of 135 ± 5 cycles. The HEC coating’s superior performance is attributable to its enhanced oxygen barrier properties and inherent lattice disorder; the former reduced the growth rate of the thermally grown oxide, and the latter increased lattice defects and distortion. These factors helped improve the thermal expansion coefficient and reduced interlayer thermal stress accumulation. Furthermore, the higher fracture toughness of the HEC layer impeded crack propagation, collectively enhancing the TBCs’ thermomechanical shock resistance. These findings underscore the potential of (La0.35Gd0.35Y0.35Sm0.35Yb0.6)Zr2O7 as a robust material for extending the service life of TBCs in high-temperature environments.

Impact of atmospheric plasma spraying parameters on microstructure, mechanical properties and thermal cycling performance of YSZ coatings

Yttria-stabilized zirconia (YSZ) coatings are widely utilized thermal protective layers for metal and superalloy surfaces. These coatings are employed as thermal barrier coating (TBCs) to insulate metallic parts from higher thermal energy, thereby minimizing the cooling need for the topcoat ceramic layer. The choice of material is 7 wt% YSZ due to its exceptional capability to withstand challenging conditions characterized by extremely high temperatures and pressures, typically employed by the atmospheric plasma spraying (APS) in gas turbines, effectively extending their lifespan and improving performance. The deposition process of TBCs is significantly influenced by various spraying parameters, including gas flow rate and current (amp). These key parameters mutually play a significant role in controlling thermal stability, phase composition, and improved mechanical and microstructure properties. The aim of this research is to evaluate and contrast the impact of various spraying parameters on YSZ TBC performance. Accordingly, the influence of Hydrogen (H2), Argon (Ar) flow rate, and Current (amp) was investigated. Microstructure and elemental mapping studying was conducted using Field Emission Scanning Electron Microscopy FESEM/EDS and phase studying was examined with X-ray diffraction (XRD). Porosity was measured by IMAGE J software while hardness measurement was calculated by Vickers hardness tester. Additionally, thermal cycling tests were conducted between 700 °C and 1200 °C. The porous coatings exhibited poor performance, delaminating early during the tests. In contrast, dense coatings performed significantly better, enduring up to 140 cycles before failure.

Preparation and thermal properties investigation of pentaerythritol based phase change microcapsules for low and medium temperature thermal energy storage

As one of classic organic compounds, pentaerythritol (PE) has been reported to have two phase-change processes with one solid-solid transition and other solid-liquid transition. To utilize these two phase-change enthalpies and avoid the liquid leakage over heat storage process, this work for the first time proposes to encapsulate PE with use of silicon dioxide (SiO2) as a shell substance through a so-called so-gel technology. The microstructure characterization and thermoproperties as well as the cycling performance of the PE- SiO2 composite are detailedly investigated. The results showed that PE could be successfully covered by SiO2 to form a spherical core-shell structure. Such an encapsulation has a mean size of 0.3 μm and an encapsulated rate of over 77.8 %. Due to the involvement of two phase-transition stages, the latent heat of the PE- SiO2 composite reached to 255 kJ/kg, which is higher than other organic microencapsulations. In addition, the thermal conductivity is largely enhanced by adjusting the ratio of SiO2. For a given SiO2 ratio of 77.8 %, the thermal conductivity of PE-SiO2 composite is 0.93 W/m·K, 36.8 % higher than that of pristine PE. Apart from these, the results also indicated that the composite achieves excellent thermal cycling performance. After one hundred repeated heating-cooling cycles, the latent heat of the PE- SiO2 composite decreases by less than 5 %, demonstrating that this composite material has a large potential for application in the field of thermal energy storage at low and medium temperatures.

Life Cycle of LiFePO4 Batteries: Production, Recycling, and Market Trends.

Significant attention has focused on olivine-structured LiFePO4 (LFP) as a promising cathode active material (CAM) for lithium-ion batteries. This iron-based compound offers advantages over commonly used Co and Ni due to its lower toxicity abundance, and cost-effectiveness. Despite its current commercial use in energy storage technology, there remains a need for cost-effective production methods to create electrochemically active LiFePO4. Consequently, there is ongoing interest in developing innovative approaches for LiFePO4 production. While LFP batteries exhibit significant thermal stability, cycling performance, and environmental benefits, their growing adoption has increased battery disposal rates. Improper disposal practices for waste LFP batteries result in environmental degradation and the depletion of valuable resources. This review comprehensively examines diverse synthesis approaches for generating LFP powders, encompassing conventional methodologies alongside novel procedures. Furthermore, it conducts an in-depth assessment of the methodologies employed in recycling waste LFP batteries. Moreover, it emphasizes the importance of LFP cathode recycling and investigates pretreatment techniques to enhance understanding. Additionally, it provides valuable insights into the recycling process of used LFP batteries, aiming to raise awareness regarding the market for retired LFP batteries and advocate for the enduring sustainability of lithium-ion batteries.

Preparation of Al[sbnd]Si microcapsules with high latent heat and durability via control of shell thickness

Microencapsulation of the AlSi alloy is an effective approach to overcome the challenges of corrosion and oxidation during high temperature operational service when used as phase change heat storage material. Research to date has shown that the prepared microcapsules was not able retain the high latent heat of AlSi alloy and achieve excellent thermal cycling performance at the same time. In this work, AlSi alloy microcapsules with adjustable shell thickness were successfully prepared through hydrothermal reaction, in situ polymerization and heat treatment at different oxygen contents. The phase composition and microstructure of the microcapsules were characterized in detail by XRD and electron microscopy. The thermal performances were tested through thermal cycling and thermal analysis. The results demonstrated that the outer protective shell layer of microcapsules was composed of dense α-Al2O3 and the thickness was adjustable from 372 to 1504 nm by controlling the oxygen content during heat treatment. Meanwhile, the regulation mechanism of shell thickness was analyzed. According to thermal analysis, the latent heat of microcapsules could achieve 450 J·g−1, and the latent heat retention was 100 % after 5000 thermal cycles from 500 °C to 650 °C. This work provides new method of adjusting the shell thickness of AlSi microcapsules to achieve an ultra-high thermal performance in terms of latent heat and retention, which promotes the development of applications for metal-based heat storage materials.

Exploring key factors of radiative cooling performance of n-octadecane@SiO2 MEPCMs

With the development of passive daytime radiative cooling, various dielectric scatterers have been utilized. Among these, n-octadecane@SiO2 microencapsulated phase change materials (MEPCMs) have garnered significant attention for their effective radiative cooling, as well as unique temperature control and thermal energy storage capabilities. However, due to incomplete optical data for phase change materials (PCMs), past simulations used simplified parameters, leading to simulation-experiment mismatches. This study used simulation and experimentation to explore how particle size and shell thickness affect the radiative cooling of n-octadecane@SiO2 MEPCMs, leading to the production of n-octadecane@SiO2 MEPCMs with superior cooling capacity. To enhance simulation accuracy, we used a two-thickness inversion method to determine the complex refractive index of n-octadecane across 0.25–2.5 μm wavelengths. The simulation showed that particles of 0.25–0.4 μm radius scatter better with <50 nm shell, while those of 0.4–0.75 μm radius benefit from <100 nm shell for improved scattering. Finally, we designed n-octadecane@SiO2 MEPCMs with an average particle size of 0.66 μm and a shell thickness of 30 nm. This particle size can cause strong Mie scattering effect to reflect sunlight and SiO2 itself has a high mid-infrared emissivity. Thus n-octadecane@SiO2 MEPCMs show a high solar reflectivity of 94.93 % and a high mid-infrared emissivity of 94.62 %. In addition, it not only exhibited a satisfactory temperature control capacity under a latent heat capacity of over 154.4 J/g, but also achieved an excellent thermal cycling performance. This work expands the potential applications of MEPCMs and radiative cooling in energy conservation and environmental protection.

Design, analysis, and operation optimisation of a shipborne absorption refrigeration–ice thermal storage system based on waste-heat utilization

The integration of waste-driven absorption refrigeration technology into marine applications is recognized as a promising low-carbon solution. Nonetheless, its practical application does not inherently outperform electric compression technology, primarily due to the absorption system’s inherent thermal inertia and limited adjustability. This characteristic renders the system less responsive to the dynamic refrigeration requirements typically encountered on board vessels. In this study, a shipborne ammonia absorption refrigeration–ice storage system is proposed to balance the cooling load between supply and demand. To evaluate the efficiency of the integrated system for ice storage and cooling during ship sailing, key operational variables including refrigeration temperature, cooling water temperature, and generation temperature are analyzed to assess the thermal cycle performance of the system. A typical shipping route is considered for cooling operation optimisation of the absorption refrigeration–ice storage system. This paper explores four typical cooling-supply strategies and determines the operation modes for the ice storage period and the cooling period based on considerations of energy consumption, energy utilization, and economic factors. The final optimisation results indicate that the combined system has significant advantages based on energy consumption and operation cost. In comparison to compression and absorption refrigeration systems, the proposed system exhibits 52.4% and 8.31% reductions in energy consumption, respectively. Additionally, the operation costs are decreased by 12.27% and 27.41%, respectively, and the life cycle costs are lowered by 11.51% and 32.35%, respectively. The findings of this combined system can provide a technical reference for the design and operation of a ship waste-heat refrigeration system.

Influence of Yttria content in YSZ: An evaluation of water-based SPS coatings and spark plasma sintered pellets

The inherent phase instability of the state-of-the-art 7–8 mol% partially stabilised Yttria-Stabilised Zirconia (YO1.5, 8 YSZ, tetragonal) under a molten CMAS attack lacks the technological readiness needed to increase the gas inlet temperatures for more thermally efficient gas turbine engines. In this study, the concentration of Yttria in YSZ was systemically increased to impart CMAS resistance and their applicability via emerging Suspension Plasma Sprayed (SPS) coatings and Spark Plasma Sintered (SpPS) pellets under simulated conditions was evaluated. The fully stabilised higher Yttria YSZ compositions (21.4 mol% and 50.2 mol%, cubic) severely restricted the CMAS infiltration and interaction areas, with the later composition forming an apatite layer and the trans-granular cracking and chipping of YSZ into layers reduced with an increase in Yttria content. However, their application into water-based SPS coatings resulted in a complete coating failure following the high-temperature CMAS tests. During Furnace Cycling Test (FCT) tests, the 8 YSZ coating survived 34 thermal cycles compared to just one on the 50.2 mol% YSZ coatings. The premature delamination of the higher Yttria coating seems to have been caused by the spallation associated with horizontal cracks within the coating. The YSZ composition could be modified into CMAS-resistant chemistry; however, their applicability as a functioning TBC with inherent microstructural features, such as the Dense Vertically Cracked (DVC) coating strategy examined in this study, requires new coating design strategies that impart sustainable thermal cycling performance.

Preparation and thermal cycling performance of Ce-doped YSZ thermal barrier coatings by a novel cathode plasma electrolytic deposition

The preparation and realization of highly reliable thermal barrier coatings (TBCs) on the internal surface of complex cavities is a bottleneck problem. Traditional coating methods are difficult to deposit and fulfil service requirements. In this study, a novel structure of YSZ and Ce-doped YSZ (CeYSZ) TBCs was prepared by a newly cathode plasma electrolytic deposition (CPED) technology. The results suggested that both the CPED-YSZ and CPED-CeYSZ coatings exhibited porous cross-linked dendritic structures. Because Ce doping significantly improved the phase stability of tetragonal structure and enhanced thermal insulation temperature during a burner-rig test at 1300 °C, resulting in a threefold increase in the thermal cycling life of the CPED-CeYSZ coating compared to the CPED-YSZ coating. This work provided a new strategy for tailoring multiple rare-earth principal components of high-performance TBCs and depositing them on the inner surface of complex cavities with high aspect ratios.

Investigation of the effects of rare earth element doping on the thermophysical properties of Gd2Zr2O7 by first principles

Gd2Zr2O7 is one of the most promising new generation of ceramic materials for high temperature thermal barrier coatings. However, it still has great potential of improving thermal and mechanical properties and contributing tobetter thermal cycling performance of the coating. Doping rare earth elements in Gd2Zr2O7 is an effective method to enhance its thermal and mechanical properties. In this paper, the first principles is used to calculate the thermophysical properties of doped Gd2Zr2O7 with 16 rare earth elements, including modulus of elasticity, Debye temperature, minimum high-temperature thermal conductivity, high temperature thermal expansion coefficient, and fracture toughness, by which all doped Gd2Zr2O7 ceramics were ranked using the entropy method. The results show that doping with Pr, Sm, Yb, and Nd showed superior comprehensive performance. Specially, (Gd0.875Pr0.125)2Zr2O7 has the lowest Young's modulus of 244 MPa, Pugh ratio of 0.53, Debye temperature of 523.4 K, thermal conductivity of 1.27 W/(m·K), and the largest coefficient of thermal expansion of 9.19 × 10−6 K−1, and (Gd0.875Ho0.125)2Zr2O7 has the highest fracture toughness of 2.21 MPa·m1/2.

Thermal Cycling Performance of Heterogeneous Solder Ball Joints with Sn-3.0Ag-0.5Cu and Sn-0.75Cu(-Ni,Bi) Solder Ball Composition for BGA Package

Sn-3.0Ag-0.5Cu (SAC305) is one of the most commonly used Pb-free solder composition nowadays. A minor alloying element addition can improve mechanical properties and thermal reliability of the solder joint. Minor amount of Bithmuth (Bi) or Nickel (Ni) addition can improve mechanical properties of solder by solid solution strengthening, or ensure the structural stability of the intermetalllic compounds. Also, low melting temperature solder with Bithmuth can reduce the cost of soldering process and keep components from deterioration. In this study, reliability of heterogeneous and homogeneous solder joint in the ball-grid-array (BGA) package solder joint were investigated. Using Sn-3.0Ag-0.5Cu (SAC305) solder paste and two kinds of solder balls, we have investigated which combination of solder joint improves the thermo-mechanical reliability. SAC305 and SCN (Sn-0.75Cu-Ni,Bi) balls were used for homogeneous and hetrogeneous joints. Solder balls were attached in BGA package. To examine the solder joint reliability, thermal cycling test was conducted after reflow soldering. Shear strength measurement and cross-sectional analysis of the BGA package solder joints were carried out before and after 2000 cycles of the test. The shear strength of homogeneous solder joint decreased to 68.2% and heterogeneous decreased to 43.5%. The cross-sectional analysis using scanning electorn microscopy (SEM) and electorn back-scattered diffraction (EBSD) showed the (Ni,Cu)&lt;sub&gt;6&lt;/sub&gt;Sn&lt;sub&gt;5&lt;/sub&gt; and Cu&lt;sub&gt;6&lt;/sub&gt;Sn&lt;sub&gt;5&lt;/sub&gt; IMCs which were generated in the solder joint, and the crack propagated longer in the homogeneous solder joint of SAC305 after the thermal cycling test. IPFM showed the grain refinement effect of the SCN solder joint after thermal cycling. The minor addition of Bi had played a role as dislocation pinning site and became to equiaxied grain, so that the lifetime of heterogeneous SCN solder joint was superior than that of the SAC305.

Long-Term Oxidation Studies on Porous Stainless Steel 430L Substrate Relevant to Its Application in Metal-Supported SOFC

Metal-supported solid oxide fuel cells (MS-SOFCs) can be used in portable mobile power generators due to their excellent thermal cycling performance, low cost, and strong mechanical strength. The selection and lifetime of the support material are crucial factors that affect the cell’s performance and long-term stability. The oxidizability of porous 430L stainless steel in a dry air atmosphere at 800 °C was systematically studied and reported for up to 1500 h. The aim was to investigate the lifetime of porous stainless steel as a support skeleton in a symmetric MS-SOFC. The substrates were characterized and analyzed using scanning electron microscopy, energy spectroscopy, and X-ray diffractometry after different periods of oxidation. The analysis indicated that the porous substrate’s surface oxides, under dry air conditions, consisted primarily of Fe2O3 and Cr2O3, with small amounts of Fe3O4 and MnCr2O4 spinel. The long-term oxidation process can be divided into two stages with distinct characteristics. However, the oxide flaking phenomenon occurred after 1500 h of exposure. The estimated service life of the stainless steel was consistent with the experimental results, which were around 1500 h. This estimation was based on the measured weight gain and thickness data.

Failure behavior of SiC/SiC with BSAS-based EBC in gas combustion environment

To promote the application of SiC/SiC in hot-end components in aero-engines, the thermal shock behavior of SiC/SiC and SiC/SiC-EBC were investigated in gas combustion environment between 450 °C and 1200 °C. The tensile strength change, microstructure evolution and thermal cycling performance of samples after 200, 400, 1000 and 2000 cycles were studied respectively to reveal the damage and failure mechanism. Cracking damage induced by thermal stress was the main reason for performance degradation of SiC/SiC after initial thermal cycles, then oxidation/corrosion damage dominated with increasing cycles. The oxidation/corrosion damage to SiC/SiC was effectively inhibited by EBC with almost no reduction in strength. However, the poor crack resistance of SiC/SiC was the main factor affecting its thermal shock performance. Improving the crack resistance or self-healing ability of SiC/SiC and combining it with EBC is considered to be an effective strategy to increase its service lifetime.

Development and investigation of salt based composite phase change material containing diatomite as skeleton substance by cold sintering technology for medium and high temperature thermal energy storage

This work concerns the development of a nitrate salt based form-stable composite phase change material by cold sintering technology with the employment of diatomite as skeleton substance. Such fabrication strategy has never before been reported in the literature and we demonstrate for the first time the feasibility of salt-diatomite composite production through a cold sintering process by means of sodium hydroxide solution as sintering additive. The microstructural characteristics and development mechanism, chemical and physical compatibility, mechanical strength, phase change behaviour, thermal and shape stability of the composite are investigated to reveal the structure formation and densification mechanism of salt and diatomite. The results show that the composite could be sintered at a temperature of 180 °C, far lower than that required in the traditional hot sintering approach. Due to the congruent solubility of the salt and diatomite in aqueous alkali, a rigid and dense structure induced by a so-called dissolution-precipitation process could be generated in the composite, which provides solution to accommodate the salt, endowing the composite with splendid capacity to maintain the structure stability and restrain the liquid leakage over the phase change process. An excellent chemical and physical compatibility has been achieved between the salt and other components, and over 60 wt% salt could be encapsulated in the composite by which a melting temperature around 278 °C and an energy storage density over 650 kJ/kg at a temperature range of 50–600 °C are attained. The results also reveal that the composite exhibits preeminent mechanical strength and excellent thermal cycling performance. For a given cycling temperature range of 50–500 °C, a stable macroscopic shape with a compressive strength over 210 MPa could be achieved in the composite even after 100 times of thermal cycles.

Composite high-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Ce2O7 thermal barrier coatings with enhanced thermal cycling performance

The emerging high-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Ce2O7 (LECO) has been verified as a potential thermal barrier material. To ensure accurate thermal cycling life prediction of LECO-based coating, double ceramic layered coatings LECO-8YSZ (8YSZ: 8 mol% yttrium stabilized zirconia) (Y-LECO) and composite (25 wt% 8YSZ + 75 wt% LECO)-8YSZ (Y-YLECO) were prepared via atmospheric plasma spraying (APS). In Y-YLECO, the toughening of 8YSZ in LECO effectively impeded the driving force of crack propagation and slowed down the delamination rate of the coating. Thus, the thermal cycling life of Y-YLECO is about 30 % higher than that of Y-LECO. Besides, both coatings exhibited mild delamination behavior. The exfoliation only appeared at the LECO-based coating with tiny flake-like delamination occurring on the surface. Such spallation pattern contributed to alleviating the stress concentration and preventing the further propagation of vertical cracks towards substrate. Moreover, the addition of 8YSZ did not sacrifice the thermal insulation performance of the composite coating, while it was 0.75 W·m−1·K−1 at 1000 °C for 25 wt% 8YSZ + 75 wt% LECO coating, close to the single-component LECO coating (0.67 W·m−1·K−1). The component design strategy could be a new pathway to further enhance the thermal protection performance of the high-entropy coatings.

Discoloration performance and mechanism research of a novel thermochromic energy storage material

The combination of phase change materials and thermochromic materials can realize the purpose of changing color while storing energy, so as to play the role of temperature regulation and color warning. In this paper, a thermochromic energy storage material (TESM) was prepared by using crystal violet lactone (CVL) and cresol red (CSR) as color former and color developer each other, taking octadecanol (OD) as the solvent and the energy storage material. The phase transition of OD induces ring-opened and ring-closed of lactone in both CVL and CSR, further leading to better discoloration. The influence of different proportions on the discoloration effect was studied. The thermal properties and functional groups of the samples were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and Fourier transform infrared spectrometer (FT-IR). The discoloration and re-coloration mechanism were explored. The results showed that the color of the prepared TESM can reversibly change between yellow-green (at room temperature) and red (at high temperature). The discoloration is attributed to the change of the intermolecular force between OD and CSR caused by the solid-liquid phase transformation of OD. When m(CVL): m(CSR): m(OD) = 3:1:500 (B3), the color difference of the prepared TESM is 38.81 NBS and the melting enthalpy is 219.8 J/g. Moreover, the prepared TESM shows good thermal stability and thermal cycling performance, and it has broad application prospects in the field of temperature alert and control.

Experiment and Numerical Simulation on Thermal Cycling Performance of YSZ-Based Sealing Coatings with “Brick-Mud” Layered Structure

The failure of premature thermal cycling spalling off is the bottleneck problem currently faced by yttrium oxide partially stabilized zirconia (YSZ) ceramic-based sealing coatings. Studies on the thermal cycling performance of coatings with “brick-mud” structures were carried out by experimental and simulation methods in this paper. The results showed that, as the thickness of “mud” layer increased, the bonding strength of the “brick-mud” structure coatings gradually decreased. When the thickness of the “mud” layer was about 3 μm and 10 μm, the thermal cycling lives of the T1 and T2 coatings were improved by 90.0% and 135.7%, respectively, compared with conventional coating (T0 coating), while that of the T3 coating (containing thick “mud” layers of about 20 μm) was decreased by 81.4%. The stress field of M2 “mud” layers with different thicknesses was subjected to a comprehensive effect by thermal mismatch stress and pores in “brick” layer. Compared with the medium and thick “mud” layers, the thin “mud” layer sustained obvious larger σ22 max and σ12 max, indicating its potential for the preferential initiation of transverse microcracks. In addition, the thin “mud” layer withstood the largest σ11 max and had the strongest potential for longitudinal crack growth. Both transverse and longitudinal cracking could consume energy during thermal cycling and reduce the stress concentration at the top coating/bond coating interface. These were the main reasons for the improvements in the thermal cycling performances of the T1 and T2 coatings. The degree of crack deflection and the capacity of energy dissipation in the “mud” layer increased significantly with its thickness. However, the propagation length of transverse cracks also gradually increased in the meantime. Especially when the “mud” layer was 20 μm, the length of the transverse cracks increased rapidly. Thus, early interlayer delamination failure occurred in the T3 coating during thermal cycling.

Cooling performance and optimization of a thermal management system based on CO2 heat pump for electric vehicles

Efficient integrated thermal management scheme is crucial for improving the driving range, thermal comfort, and safety of electric vehicles. Compared with the synthetic refrigerant heat pump indirect cooling/heating on the vehicle electrical system, the natural refrigerant CO2 heat pump direct thermal controlling integrated into the thermal management system has advantages in the thermal cycle performance and environmental friendliness. The present study proposed a thermal management system based on CO2 heat hump with multiple working modes for enhancing vehicle energy utilization coefficient. The system thermal performance under the World Light Vehicle Test Cycle was investigated via energy and exergy analyses. The operational control strategies of the thermal management system were optimized for the thermal controls of the cabin, battery and motor. The results show that the minimum exergy destruction of the thermal system occurs at the optimal discharge pressure corresponding to the highest coefficient of performance. Ensuring two-phase latent heat transfer for battery cooling is the most effective measure to maintain battery temperature uniformity via dynamic collaborative regulation of expansion valves’ openings and compressor speed. Compared with constant expansion valve’s opening, the coefficient of performance of the system can be improved by a maximum of 10% with the saturated CO2 vapor at the outlet of battery cooling plate. Due to the low thermal capacitance and minimal thermal inertia of driving motor, its cooling performance is significantly influenced by the heat generation rate and refrigerant flow. Optimal motor cooling efficiency is achieved when controlling the refrigerant quality to 0.85 at the cooling passage outlet. Under the World Light Vehicle Test Cycle, the thermal management system effectively regulated the temperatures across different operating modes while operating at optimal discharge pressures.