High Temperature Cycling Research Articles

Characterizing and improving the performance of molten-salt-steam heat exchangers in concentrating solar power plants

Shell-and-tube heat exchangers (HXs) for steam generation from molten salts in concentrating solar power (CSP) plants experience thermal fatigue due to significant temperature gradients and inherent transient operation. Molten salt-steam HX design lifespans exceed actual lifespans, and, as a consequence, designers overpredict plant profitability and operators neglect appropriate prescriptions to optimize these lifetimes. This study refines HX lifespan estimates with data benchmarked against thermal-fluid mechanical modeling of stress and accumulated fatigue. Reduced-order thermal models of the molten salt-steam, shell-and-tube evaporator and superheater predict transient temperature profiles along the two HXs salt-steam flow paths. The modeled evaporator and superheater temperature profiles enable assessment of cyclic stresses within the HX tubesheets, where molten-salt HX failures are most common. Evaporator and superheater performance data from a current 110 MWelec commercial CSP plant provide a basis for validating the reduced-order HX models. HX life predictions derived from stochastic failure distributions serve as inputs for simulating and optimizing existing plant operations. The impact of the updated lifespans on overall plant revenue depends on operating scenarios. This study suggests that typical ramping rates for a CSP plant with a high-temperature Rankine cycle result in an evaporator and superheater life of approximately 10 and 25 years, respectively, compared to the design target of 30 years. Reduced HX lifespans decrease operational plant revenue on average by 4.6-5.1%. Furthermore, there may be as many as four HX replacements over the 30-year lifetime of the plant; and, purchase agreement loss due to failure to meet contractual production requirements can have ramifications that include the risk of bankruptcy.

Revolutionizing multifunctional electrolyte additive design and synthesis for high-voltage nickel-rich batteries in diverse climates

The high-voltage LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode presents a promising avenue for advancing the energy density of lithium-ion batteries. Nevertheless, current limitations primarily arise from electrolyte systems unable to meet stringent criteria, including stable performance under elevated temperatures and efficient discharge at low temperatures. In this study, we address these challenges by designing a molecular structure incorporating boron oxygen functional groups, phenyl-, and fluorine moieties to simultaneously enhance ion conductivity and thermal stability under extreme voltages. A multifunctional electrolyte additive, 3, 4, 5-trifluorophenylboronic anhydride (TFPBA), has been successfully developed via synthesis. TFPBA plays a pivotal role in promoting the formation of a hierarchical cathode electrolyte interphase (CEI) through its preferential decomposition and oxidation processes. This CEI features a mechanically stable inner layer enriched with fluorophenyl- groups, crucial for high-temperature cycling, alongside an outer layer concentrated with BxOyFz complexes that significantly improve Li+ conductivity, ensuring superior performance under low-temperature conditions and at elevated rates. Moreover, TFPBA acts as an HF scavenger within the electrolyte, mitigating corrosive effects on the cathode surface, thereby curtailing CEI reformation and the transition metal dissolution. The Li/NCM622 half cells with TFPBA exhibit impressive capacity retention rates of 92.58%, 80.09%, and 81.77% after extensive cycling at temperatures of −20 °C, 25 °C, and 55 °C, respectively. Moreover, NCM622/graphite full cells with TFPBA achieve a remarkable capacity retention of 97.97% after 150 cycles at room temperature. Our findings highlight the exceptional electrochemical performance of NCM622 cathodes at elevated voltages across a wide temperature range, providing valuable insights for the development of advanced electrolytes in high-demand applications.

The effect of changing the combustion chamber head structure on turbine blades under thermal shock test

When the high-pressure turbine guide blade works in the aero-engine, it is in the complex gas-heat environment of the combustion chamber and the turbine. The temperature distribution and strength of the blade affect the reliability and life of the aero-engine. In order to more accurately analyze the complex flow field aero-thermal parameters between the aero-engine combustion chamber and the turbine, this paper considers the influence of the non-uniform aero-thermal parameters of the combustion chamber outlet on the turbine. Taking the first-stage guide vane of an aero-engine turbine as the research object, the high and low temperature cycle thermal shock test was carried out to obtain the crack location and strength of the turbine guide vane, which provided a certain reference for improving the design of the turbine guide vane. Through the test, it is found that after 750 thermal shock cycles, fatigue cracks appear in four concentrated areas of the blade, which are distributed in the leading edge and trailing edge of the blade respectively. This shows that the temperature at the leading edge and trailing edge of the blade is higher, the heat load is higher, and the risk of damage and ablation is also the highest. Based on the experiment, the cross-component model of the combustion chamber and the blade is established. On this basis, four kinds of swirler structures at the head of the combustion chamber are designed. ANSYS FLUENT software is used to study the unsteady parameters such as the surface temperature of the guide vane under the influence of strong swirling flow. The numerical results show that when the rotation direction of the cyclone is counterclockwise, the temperature of the guide vane wall is more uniform. Compared with the clockwise rotation of the cyclone, the temperature is reduced by about 5 %, and the installation angle of the 50° cyclone blade is better than the installation angle of the 45° blade.

Influence of the high temperature impact resistance of cutting tool coatings on their cutting performance

Metal cutting is a highly dynamic process, where the loading condition of tool coating is very complex. Excellent static mechanical properties of tool coatings are not enough to meet the dynamic cutting environment, including impact load, cyclic load and so on. The high-frequency and high-temperature cycle impact test device based on ultrasonic vibration was developed in this study. The device performed the high-frequency impact on coating samples through high-frequency reciprocating motion of ultrasonic horn, whose impact frequency could reach 20 kHz. In addition, a high-temperature module was added to perform high-frequency fatigue test at high temperature. Cutting experiments of titanium alloy and impact tests with different temperatures and loads were carried out for the bilayer TiSiN/TiAlN coating. The results of cutting experiments and impact tests were compared and analyzed. The relationship between impact test and cutting experiment was proposed to characterize and evaluate the cutting performance of coated cutting tool by using the high-temperature impact tests.

Low-Cost Magnesium-Based Thermoelectric Materials: Progress, Challenges, and Enhancements.

Magnesium-based thermoelectric (TE) materials have attracted considerable interest due to their high ZT values, coupled with their low cost, widespread availability, nontoxicity, and low density. In this review, we provide a succinct overview of the advances and strategies pertaining to the development of Mg-based materials aimed at enhancing their performance. Following this, we delve into the major challenges posed by the severe working conditions, such as high temperature and thermal cycling, which adversely impact the behavior and long-term stability of the TE modules. Challenges include issues like the lack of mechanical strength, chemical instability, and unreliable contact. Subsequently, we focus on the key methodologies aimed at addressing these challenges to facilitate the broader application of the TE modules. These include boosting the mechanical strength, especially the toughness, through grain refining and additions of second phases. Furthermore, strategies targeted at enhancing the chemical stability through coatings and modifying the microstructure, as well as improving the contact design and materials, are discussed. In the end, we highlight the perspectives for boosting the practical applications of Mg-based TE materials in the future.

Seepage characteristics and mechanism of double damage basalt fiber reinforced concrete under three-dimensional variable stress states

Concrete experiences a three-dimensional variable stress state during its service life. To explore the permeability evolution and mechanism under such conditions, a permeability test was conducted on double damage basalt fiber concrete subjected to loading and unloading of twice confining pressure. Pore structure and microstructure of specimens were analyzed before and after permeability test. The findings reveal that the reduction in permeability after primary cycle of confining pressure loading and unloading is significantly higher than that after secondary cycle, and residual deformation is also more pronounced after primary cycle. Both high temperature and salt freeze-thaw cycles induce initial damage to specimens, consequently altering internal pore structure and subsequently impacting initial permeability. Notably, initial permeability decreases in the case of 10th salt freeze-thaw cycles, while it increases under other conditions. The greater the initial extent of damage to the specimen, the higher the permeability damage coefficient and the permeability damage rate. As the confining pressure decreases, the attenuated permeability increases, with the permeability decay rate reaching a significant 88.53 %. Plastic deformation of pores during loading and unloading process cannot be fully reversed, leading to the development of macropores and mesopores into smaller pore sizes, resulting in a remarkable 87.17 % increase in the proportion of micropores. Microscopic observations reveal smaller pores within the matrix, shorter crack lengths, and narrower crack widths in the specimen. The application of confining pressure reduces permeability channels within specimens, ultimately causing a decline in permeability.

Investigation on the sustainable use of different sizes of sawdust aggregates in eco-friendly foam concretes: Physico-mechanical, thermal insulation and durability characteristics

The extensive mining of natural resources, such as sand, for the construction industry, poses a hazard to the environment. To preserve the environment, the researcher must thus take into account the use of industrial byproducts rather than natural materials. The use of sawdust (SD) produced by the wood industry in foam concretes (FC) has been investigated in this study. This study examined the physical, mechanical, thermal, microstructural and durability characteristics of FC containing SD as a replacement of silica sand (SS) in the rates of 0, 25, 50 and 100 %. Two different SD sizes of 0–4 mm (SD4) and 0–1 mm (SD1) were used in the fabrication of FC mixtures. Protein-based foam agent was used to create FC mixtures. Fourteen mixtures with a 0.6 water/binder (w/b) ratio were produced at two different cement contents of 400 kg/m3 and 500 kg/m3. The fresh, mechanical, physical, and sorptivity characteristics of the FC mixtures were investigated in relation to the impacts of cement dosage, SD size, and SD replacement levels of SS. Durability of FC against high temperature, sulfate attack and freeze-thaw cycles was assessed. Costs and CO2 emissions from the blends were also assessed. The outcomes revealed that a 25 %SD1 integrated FC mixture with a cement content of 500 kg/m3 generated the highest 28-day compressive strength of 3.56 MPa with a 20 % strength loss as per the Ref.500 mixture. 17 % lower thermal conductivity was gained with the inclusion of 100 %SD4 as per the Ref.400 mixture. The mixture with 50 %SD4 and 400 kg/m3 cement dosage exhibited the best resistance to high temperature. The mixture with 25 %SD1 and 400 kg/m3 cement dosage showed the best frost resistance. SD incorporated mixtures showed better durability performance than the reference mixtures.

Functionally graded bi-material interface for Porcelain Veneered Zirconia dental crowns: A study using viscoelastic finite element analysis

ObjectivesDuring the manufacturing of Porcelain Veneered Zirconia (PVZ) dental crowns, the veneer-core system undergoes high-temperature firing cycles and gets fused together which is then, under a controlled setting, cooled down to room temperature. During this cooling process, the mismatch in thermal properties between zirconia and porcelain leads to the development of transient and residual thermal stresses within the crown. These thermal stresses are inherent to the PVZ dental crown systems and render the crown structure weak, acting as a precursor to veneer chipping, fracture, and delamination. In this study, the introduction of an intermediate functionally graded material (FGM) layer at the bi-material interface is investigated as a potentially viable alternative for providing a smoother transition of properties between zirconia and porcelain in a PVZ crown system. MethodsAnatomically correct 3D crown models were developed for this study, with and without the FGM layer modeled at the bi-material interface. A viscoelastic finite element model was developed and validated for an anatomically correct bilayer PVZ crown system which was then used for predicting residual and transient stresses in the bilayer PVZ crown. Subsequently, the viscoelastic finite element model was further extended for the analysis of graded sublayers within the FGM layer, and this extended model was used for predicting the residual and transient stresses in the functionally graded PVZ crown, with an FGM layer at the bi-material interface. ResultsThe study showed that the introduction of an FGM layer at the bi-material interface has the potential to reduce the effects from transient and residual stresses within the PVZ crown system relative to a bilayer PVZ crown structure. Furthermore, the study revealed that the FGM layer causes stress redistribution to alleviate the stress concentration at the interfacial surface between porcelain and zirconia which can potentially enhance the durability of the PVZ crowns towards interfacial debonding or fracture. SignificanceThus, the use of an FGM layer at the bi-material interface shows a good prospect for enhancing the longevity of the PVZ dental crown restorations by alleviating the abrupt thermal property difference and relaxing thermal stresses.

Enhancing redox stability through metal substitution in nickel ferrite for chemical looping hydrogen production via water splitting

NiFe2O4 oxygen carrier has tremendous potential for application in chemical looping hydrogen production via water splitting. However, it exhibits poor high-temperature cycling stability. The current study investigates the impact of substituting other elements (Cu, Ce, La or K) for Fe in NiFe2O4 on the hydrogen production performance during the chemical looping water splitting process. No crystalline phase of oxides for Fe, La or Ni has been detected in the XRD patterns, indicating that substituting element has been successfully doped into the spinel structure of NiFe2O4. After the substitution with the other elements such as K, La, and Ce, the hydrogen yield of the oxygen carrier has been improved, with the highest hydrogen yield observed for the NiLa0.02Fe1.98O4 oxygen carrier. Through optimized experiments, the optimum substitution amount of La is 10%, resulting in the formation of NiLa0.2Fe1.8O4. With the substitution of 10% Fe by La, the hydrogen production performance of the fresh oxygen carrier is increased from 170 mL/kg to 297 mL/kg, which can be explained by various characterization results. Lattice distortion and an obvious increase in oxygen vacancy content are observed in NiLa0.2Fe1.8O4, with the reduction peak shifting to the low-temperature region. In addition, compared to NiFe2O4, NiLa0.2Fe1.8O4 exhibits better cycling stability in 30 cycles. Finally, the changes of structure and morphology for NiFe2O4, NiLa0.2Fe1.8O4 before and after 30 cycles have been comparatively analyzed using XRD, XPS, SEM, BET, and H2-TPR.

Comparison Study of Cascaded Organic Rankine Cycles with Single and Dual Working Fluids for Waste Heat Recovery

This study compares thermodynamics, economics, and environmental performance of cascaded ORCs operated under a single and dual fluids. In the single fluid cascaded ORC, toluene, benzene, acetone and cyclopentane are run in high and low temperature cycles, whereas in dual fluid cascaded ORC, toluene, benzene, acetone and cyclopentane are run in high temperature cycle and R601a in the low temperature cycle. The analysis compares variations in expander inlet temperature and condensation temperature. Thermodynamic performance involved net power output (Pnet) and thermal efficiency (ηth), while economic indicators included net present value (NPV) and levelized cost of electricity (LCOE). In environmental performance, the annual reduction in carbon dioxide emission (CO2-eq) is assessed. The findings revealed that dual fluid cascaded ORC generated the highest Pnet of 1245.11 kW while single fluid cascaded ORC reached 1170.27 kW. The dual fluid cascaded ORC showed the significant increase in Pnet (%DPnet) for about 43% at the lowest expander inlet temperature (500 K). In terms of ηth, dual fluid cascaded ORC attained 37.23 % while single fluid cascaded ORC reached 33.25%. It is further found that acetone+R601a performed well in dual fluid cascaded ORC, resulting in the highest Pnet and allowing system’s NPV to turn positive sooner than other fluids. Furthermore, cyclopentane+R601a had the lowest LCOE of 0.0158 US$/kWh, which is 1.1% lower compared to the single fluid cascaded ORC and competitive in the Thai electricity market. In environmental saving, dual fluid cascaded ORC reduced about 144.96 tCO2-eq/year, and outperformed single fluid cascaded ORC by roughly 6.39%.

High-temperature high cycle fatigue performance of laser powder bed fusion fabricated Hastelloy X: Study into the microstructure and oxidation effects

High-temperature fatigue properties are critical for laser powder bed fusion (LPBF) fabricated Nickel-based superalloy Hastelloy-X (HX) used in aero engines. In this study, superior high-temperature high-cycle fatigue properties of LPBF HX were achieved. The detailed investigations show that the grain boundary oxidation promoted by the different deformation modes between neighboring grains, despite the reduced oxygen-related damage in the coarser LPBF HX microstructures, are the potential causes for the inter-granular fatigue crack initiation and the subsequent crack coalescence. In the meantime, the crack propagation could be hindered by the refined carbides and annealing twins with Σ3 misorientation (60°/〈111〉) in LPBF HX. Furthermore, the recrystallized grains formed ahead of the crack tip due to the severe deformation within the plastic zone and high testing temperature have different crystallographic orientations, which leads to the crack propagation path changes and increases the fatigue crack propagation resistance.

Microstructure and phase evolution of Fe-20Ni-20W foams during high-temperature redox cycling

Freeze-cast Fe-20Ni-20 W (at%) foams suitable for high-temperature redox cycling show excellent microstructural stability and regenerative porosity formation, but they exhibit slower reduction kinetics as compared to previously studied Fe-25 W foams. The Fe-20Ni-20 W lamellar foams, after initial hydrogen reduction, consist of a two-phase mixture of µ-Fe7W6 and γ-Fe(Ni,W), with significant microporosity due to the sintering inhibition of W. During oxidation by steam at 800 °C, these phases are oxidized to a three-phase mixture of (Fe,Ni)WO4, Fe3O4, and γ-Ni(Fe); upon subsequent reduction by H2, the foams return to their initial composition. The chemical vapor transport reduction of FeWO4 results in the formation of submicron pores during each reduction half-cycle which accelerate subsequent reaction and limit sintering in the reduced state. The reduction of mixed oxide is relatively sluggish, which is likely due to the increased stability of the (Fe,Ni)WO4 phase brought on by the substitution of Ni in Fe sites in FeWO4.

Energy, exergy and economical analysis of N2O based cascade refrigeration system for ultralow temperature cooling applications using different eco-friendly refrigerants in high temperature cycle

Implementing the Montreal Protocol has limited the use of conventional refrigerants and has led industries to find optimal or economical eco-friendly refrigerants to reduce environmental hazards. The present study analyzes an N2O-based cascade refrigeration system for achieving ultralow cooling temperatures using different eco-friendly refrigerants with low GWP and ODP in a high-temperature cycle. 3E (Energy, Exergy, and Economy) analysis of a cascade refrigeration system using eco-friendly refrigerants such as R290, R1270, RE170, R600, R600a, HFE7100, and HFE7000 in the upper cycle and N2O in the lower cycle was conducted. The R744A/R600 pair performed best among the other refrigerant pairs due to its lower discharge pressure and compression ratio. The refrigerant pair R744A/R600 results in a higher cascade cycle COP of approximately 0.87, with 48 % exergetic efficiency, while the refrigerant pair R41/HFE7100 has the lowest COP (0.68) and lowest exergetic efficiency (42 %). The minimal operational cost varies from 73006 to 76907 USD per annum for refrigerant pair R744A/R600, while for the other pairs, the cost varies from 98625 USD to 114674 USD. The R744A/R600 refrigerant pair provides COP values that are 1.38–5.38 % greater than the others, with a 1.01–3.45 % variation in exergy efficiency.

Thermal Cycling Stability of Ti-Zr-Nb-Al Shape Memory Alloy

Shape memory alloys, as a novel class of intelligent materials, possess substantial application potential in the field of petroleum engineering at elevated temperatures. However, the complex downhole environment necessitates the alloy to endure alternating cycles of cold and heat, demanding both a high phase transformation temperature and excellent thermal cycling stability. This study focuses on a quaternary Ti-Zr-Nb-Al shape memory alloy, examining the thermal cycling stability of the Ti-20Zr-10Nb-xAl (x=1, 2, 3, 4 at%) alloys after subjecting them to 50 thermal cycles. Results indicate that the alloy composition, post 50 cycles, transitions from a single martensite α'' -phase to a duplex microstructure comprising α''-phase and β-phase. Changes in Al content did not significantly affect the phase composition of the alloy in its thermally cycled state. As Al content increases, both the tensile strength and shape memory effect of the alloy initially improve before declining. In terms of shape memory properties, the Ti-20Zr-10Nb-2Al alloy sample exhibits the most favorable thermal cycling stability, characterized by a tensile strength of 692 MPa, an elongation of 6%, a shape memory effect of 0.96% under 4% pre-strain, a shape memory recovery rate of 38%, and a Vickers microhardness of 381 HV.

Mesoscopic precursor anomaly characteristics of red sandstone under uniaxial compression after cyclic high-temperatures

The deformation and failure characteristics of rock caused by periodic changes of temperature are not only significantly different from that under normal temperature, but also significantly different from that produced by a single high-temperature event. Here, using a three-dimensional digital image correlation (3D-DIC) integrated test system, mechanical and micro-damage tests of red sandstone are performed under cycles of high-temperature and natural-cooling with a temperature range of 25°C to 300°C and the number of cycles varying between 0 and 25. Macroscopic, mesoscopic and microscopic characteristics such as rock strength, deformation, strain field and fracture morphology are examined and their evolution with the number of heating-cooling cycles is discussed. Using the time-history evolution of the differentiation rate of the precursor anomaly (En), the characteristic coefficient (K) and the damage variable (DF), three kinds of characteristic time points are put forward to quantify the incubation and development time of precursor abnormal characteristics and are used to divide the whole process of deformation and failure. These time points are the precursor abnormal differentiation time (PADT) tE, the precursor abnormal startup time (PAST) tK, and the precursor abnormal failure time (PAFT) tD. Test results show that: (1) The action of cyclic high-temperature natural-cooling not only shows the strengthening effect of temperature, but also shows the strengthening effect of a certain number of cycles. (2) The evolution sequence of the three precursor abnormal characteristic moments is PADT, PAST and PAFT in turn, and this sequence can intuitively describe the whole complex process of rock deformation and failure, and there are wave peaks in the evolution curve with the number of high-temperature cycle. (3) The effect of high temperature (i.e., 300°C) cyclic heating on rock properties is similar to the threshold effect of a single high-temperature heating event.

Lattice modulation improving surface passivation of LiMnPO4 for stable cycling at high temperatures

The undesirable capacity degradation of LiMnPO4 upon cycling at high temperatures is a challenge to its practical application. Herein, a lattice doping strategy is adopted to improve the high-temperature cycling stability of LiMnPO4, and the comparative study reveals that Al3+ doping into LiMnPO4 in a form of Li0.98Al0.02MnPO4 is highly beneficial to the cycling performance of LiMnPO4 and the capacity retention can be significantly improved from 67.4 % to 93.4 % after 100 cycles at 1C at 60 °C, because Al3+ doping can effectively reduce passivation products deposition on the cathode and manganese dissolution in the electrolyte, which thus improve the cathode/electrolyte interface and stabilize the structure of LiMnPO4 at high temperatures.

Plastic zone and surrounding strain analysis in a lined cavern for compressed air energy storage subjected to cyclic high internal gas pressure and temperature

In the context of compressed air energy storage (CAES) technology, the strategic excavation of hard rock caverns is pivotal for overcoming geological challenges and advancing CAES deployment. This study adopts a circular profile for the cavern and investigates the impact of variables such as cavern diameter and depth. Considering the influence of cyclic internal gas pressure, temperature fluctuations, and potential leakage on the cavern, a thermo-hydro-mechanical coupled numerical model is developed using Comsol software. This model, grounded in classic elastoplastic constitutive relations, aligns with the conditions of a proposed project. It facilitates the computation of the plastic zone and surrounding strain for both the cavern lining and adjacent rock, post-excavation and throughout a typical ‘charging-storage-discharging-storage’ operational cycle. Through detailed analysis, the study delineates the variations and characteristics of the plastic zone and surrounding strain, thereby providing a solid foundation for a comprehensive examination of cavern stability, and the design of linings and sealing mechanisms.

Microstructure evolution and micromechanical behavior of solvent-modified Cu–Ag composite sintered joints for power electronics packaging at high temperatures

With the increased deployment of power modules in demanding conditions, sintering materials, especially composite sintering materials, have raised growing interest due to their cost-effectiveness and suitability. Therefore, this study explores the viability of Cu–Ag composite sintering material, focusing on solvent influence through microstructure and mechanical behavior analysis. Micron-sized particle-based Cu–Ag composite pastes were designed and compared using eight solvents (four epoxy-free and four epoxy-added) based on fluidity and thermal stability. The sintered joints' performance, assessed through shear strength analysis, showed comparable values to pure silver sintering for both epoxy-free and epoxy-added samples. Optimized samples from each solvent system underwent reliability analysis, demonstrating that Cu–Ag joints with epoxy resin exhibited significantly higher shear strength after high-temperature storage and thermal cycling tests. Micromorphology and elemental composition analysis revealed differences in aging mechanisms, primarily attributed to variations in porosity due to oxide formation and pore filling by epoxy resin under different solvent systems. Further nanoindentation characterization of micromechanical properties, including hardness, modulus, and creep properties, during high-temperature aging, established constitutive models for insights into reliability evolution. In conclusion, the optimized epoxy-added Cu–Ag sintered joints proposed in this study demonstrated exceptional reliability and acceptable micromechanical properties, presenting a promising option for high-temperature power packaging.

Comprehensive analysis on suitability of alkali metals with high enthalpy characteristics for space nuclear Rankine cycle

The compatibility between working fluids and high-temperature thermal systems is essential for improving system performance, stability, and safety. This article focuses on exploring the relative adaptability of alkali metals in nuclear high-temperature Rankine systems under three operating conditions: different output powers, different core powers, and different mass flow, evaluating from three dimensions: thermal, economic, and weight. The study finds that the cesium and potassium cycles represent systems for low and high turbine inlet temperatures, respectively. At a turbine inlet temperature of 1100 K, the cesium cycle can enhance thermal efficiency by 32.14 %, while with the turbine inlet temperature increased to 1500 K, the potassium cycle can improve efficiency by 28.19 %. Under varying output power and core power conditions, the high-temperature potassium cycle demonstrates more significant performance advantages. Compared to the cesium cycle, both economic and weight performances are seen an increase of approximately 20 %. Under varying mass flow conditions, the cesium cycle has a significant weight advantage. The mass flow increases from 0.5 to 2.5 kg/s, and the proportion of specific weight for the cesium cycle compared to the potassium cycle decreases from 54.90 % to 12.93 %. However, the output power of the potassium cycle is ten times that of the cesium cycle with the same mass flow. This research offers valuable insights for selecting optimal working fluids in high-temperature thermal systems considering operating conditions and mission requirements.

Tungsten Strongly Inhibits Sintering of Porous Iron During High-Temperature Redox Cycling.

Freeze-cast Fe-25W (at%) lamellar foams show excellent resistance to degradation at 800°C during steam-hydrogen redox cycling between the metallic and oxide states, with fast reaction kinetics maintained up to at least 100 redox cycles with full Fe utilization. This very high stability stems from the sintering inhibition of W combined with the freeze-cast architecture and the chemical vapor transport (CVT) mechanism of reduction. These three factors create a hierarchical porosity in the foam, consisting of i) macroscopic elongated channels, ii) micro-scale sintering inhibition pores, and iii) submicron CVT pores. Microstructural characterization via SEM and EDS is combined with in situ XRD to fully explore the phase evolution and microstructural impact of W on Fe during redox cycling. Comparison with tapped Fe-25W (at%) powder beds reveals that the freeze-cast channels and lamellae are not critical to the performance of the material.