Improvement of high-temperature oxidation resistance and strength in alumina-forming austenitic stainless steels

A new alumina-forming austenitic stainless steel with greatly improved high-temperature oxidation resistance and strength was developed via alloying 3.0 wt.% Al in the Fe-25Ni-18Cr based alloy. Continuous, stable and exclusive alumina scale was formed in either dry air or air with 10% water vapor mixed environment at 800 °C. The long-term high-temperature oxidation performance is appreciably enhanced which is associated with the high density of the B2-NiAl precipitation phase maintaining the Al₂O₃ surface layer. Moreover, when tested at 750 °C in dry air environment, the new steel showed high yield and fracture tensile strength of 310–335 and 480–500 MPa, respectively.     

Characterization of chromia scales formed in supercritical carbon dioxide

Abstract

Initial experimental work at 700°–800 °C is in progress to develop a lifetime model for supercritical CO₂ (sCO₂) compatibility for a 30-year lifetime of a >700 °C concentrated solar power system. Nickel-based alloys 282, 740H and 625 and Fe-based alloy 25 are being evaluated in 500-h cycles at 1 and 300 bar, and 10-h cycles in 1 bar industrial grade CO₂. The alloys showed similar low rates of oxidation in 1 and 300 bar CO₂ in 500-h cycles at 750 °C. However, in 10-h cycles, alloy 25 showed accelerated attack at 700° and 750 °C. Transmission electron microscopy scale cross-sections on alloy 25 after 1000 h at 700 °C in sCO₂ and in air only showed a small row of carbides beneath the scale in the former environment. Similar characterisation was performed on alloys 625 and 282 after sCO₂ exposure at 750 °C.     

有机朗肯循环模拟及涡旋式膨胀机的性能研究

近些年来,太阳能作为一种可再生能源受到了广泛的关注。其中利用太阳能集热器实现100℃以下高效的热量回收,是一种普遍且有效的太阳能利用方式。采用有机朗肯循环与100℃的低温热源相结合进行发电,目前也逐渐受到了研究人员的关注。考虑到膨胀机是有机朗肯循环的核心部件,本文选择了R600制冷剂作为ORC系统的工质,对其进行了计算以及热力学性能分析。同时搭建了利用压缩空气来驱动的涡旋式膨胀机性能研究的实验台。从ORC的理论分析得,当热源温度为78~97℃,环境温度为30℃,可以获得0.7~1kW的电量,效率为0.84~0.89。利用压缩空气模拟R600,当温度从75℃变化到95℃,对应的压力从0.8MPa变化到1.2MPa,膨胀机出口压力控制在0.28MPa,等熵效率维持在0.7左右。膨胀机的功电转化效率随着膨胀机理想输出功的增加而降低。     

Semi-empirical Density Estimations for Binary,Ternary and Multicomponent Alkali Nitrate–Nitrite Molten Salt Mixtures

For sensible thermal energy storage in Concentrating Solar Power (CSP) plants, a molten salt mixture of 60 wt% sodium nitrate (NaNO₃) and 40 wt% potassium nitrate (KNO₃), known as Solar Salt, is commonly utilized. The paper presents semi-empirical estimation results of the density of Solar Salt and alternative molten salt mixtures with low melting temperatures in a range from 70 °C to 140 °C. These mixtures are Hitec, HitecXL, LiNO₃–KNO₃–NaNO₃ and a multicomponent mixture. The paper shows that density values of mixtures can be closely predicted from single salt densities. The paper examines different estimation rules for mixtures. The quasilinear volumetric additivity rule (QVAR) is known for ternary reciprocal systems. For the first time, the presented work extends the QVAR to multicomponent mixtures. Temperature-dependent densities of selected salt mixtures of the system Ca,Li,K,Na//NO₂,NO₃ were estimated. Estimations are motivated by a fast and reliable method compared to time-consuming and error-prone measurements of several mixtures.     

Oxidation behavior of molten magnesium in air/HFC-134a atmospheres

The oxidation behavior of molten magnesium in the atmosphere of air containing HFC-134a has been investigated in the temperature range of 660–760 °C. The oxidation rates and kinetics have been measured by the weight gain method and oxidation products have been characterized by XRD, SEM, EDS, XPS and AES. The results show that the oxidation kinetics is complex which cannot be described by simple equations. The rate of oxidation of molten magnesium in air/HFC-134a covering gas mixtures varies with the concentration of HFC-134a and molten temperature. Increasing the concentration of HFC-134a and decreasing the temperature can slow down the oxidation rate of molten magnesium. The film formed +on the surface of molten magnesium is mainly composed of MgF₂, MgO and C. MgF₂ is predominant product at the top layer and decreased gradually with the depth while MgO and C remain almost constant with relatively low content. The mechanisms of the oxidation of molten magnesium in air containing HFC-134a have also been discussed based on the experimental results.     

Thermal and mechanical properties of mullitic substrates for low-cost solar cells obtained by dry-pressing

A study has been made of the possibility of producing ceramic substrates for low-cost solar cells by means of the simple technology of moulding by dry-pressing. Special attention was taken to avoid adding costly materials (organic and inorganic) to the basic commercial kaolin. It emerged that the only treatment necessary was the precalcination of approximately half the powder at temperatures of 1200 to 1350° C. The various characterization techniques showed that, in spite of the simplicity of the technologies utilized, after a final firing at 1600° C, one can obtain finished products with the same thermal expansion coefficient as silicon (from ambient temperature to 800° C) and with mechanical characteristics that make the products perfectly suitable for resistance to the thermal shock, resulting from contact with molten silicon (the lower resistance to bending compared to products made with sophisticated production techniques being perfectly well compensated for by the greater elasticity, which is due to a considerably lower Young's modulus value). Even if this methodology has been applied on a laboratory scale, it is quite easy to automate it for industrial scale production.     

Oxidation and fatigue crack propagation in the range of low stress intensity factor in relation to the microstructure in P122 Cr–Mo steel

Fatigue strength and life of weldment at high temperatures are important for the materials in power plants. The fatigue crack growth rate is accelerated by oxidation. Similarly, the high-temperature fatigue life is influenced by oxidation. The base metal, the weld metal and the heat-affected zone (HAZ) of the P122 (Cr–Mo steel) weldment were oxidized between 600 °C and 700 °C for up to 500 h in air, and their oxidation behavior was examined. The oxidation resistance increased in the order of HAZ, base metal and weld metal. The scales were mainly Fe₂O₃. Fatigue tests were performed to measure the fatigue crack growth rate in the range of low stress intensity factor, and the results are discussed from the viewpoint of different microstructures and oxidation.     

Oxidation behaviour of TiAlYN/CrN and CrAlYN/CrN nanoscale multilayer coatings with Al2O3 topcoat deposited on γ-TiAl alloys

Abstract

TiAlYN/CrN and CrAlYN/CrN nanoscale multilayer coatings were deposited on γ-TiAl specimens using magnetron sputtering techniques. The nitride layers were manufactured by unbalanced magnetron sputtering (UBM) and high power impulse magnetron sputtering (HIPIMS). The CrAlYN/CrN coatings had an oxy-nitride overcoat. On some of the coated samples an additional alumina topcoat was deposited. The oxidation behaviour of the different coatings was investigated at 750 and 850°C performing quasi-isothermal oxidation tests in laboratory air. Mass change data were measured during exposure up to failure or the maximum exposure length of 2500 h. When exposed to air at 750°C, the Ti-based nitride films exhibited higher oxidation resistance than the Ti – 45Al –8Nb substrate material. The alumina topcoat enhanced the oxidation protection of this coating system, acting as diffusion barrier to oxygen penetration. At 850°C, the TiAlYN/CrN films exhibited poor stability and rapidly oxidised, and therefore were not applicable for long-term protective coatings on γ-TiAl alloys. The beneficial effect of the additional Al₂O₃ layer was less pronounced at this exposure temperature. The Cr-based nitride films exhibited high oxidation resistance during exposure at 850°C. HIPIMS deposition improved the oxidation behaviour of the CrAlYN/CrN nanoscale multilayer coatings in comparison to UBM coatings. For these coatings, the decomposed and partially oxidised nitride films were an effective barrier to oxygen inward diffusion. The alumina topcoat did not significantly increase the oxidation resistance of the γ-TiAl alloy coated with Cr-based nitride films.     

Ionic liquids for electrochemical energy storage devices applications

Ionic liquids, defined here as room-temperature molten salts, composed mainly of organic cations and (in)organic anions ions that may undergo almost unlimited structural variations with melting points below 100?°C. They offer a unique series of physical and chemical properties that make them extreme important candidates for several energy applications, especially for clean and sustainable energy storage and conversion materials and devices. Ionic liquids exhibit high thermal and electrochemical stability coupled with low volatility, create the possibility of designing appropriate electrolytes for different type batteries and supercapacitors. Herein, varieties of ionic liquids applications are reviewed on their utilization as electrolytes for Li-ion batteries, Na-ion batteries, Li-O₂(air) batteries, Li-Sulfur (Li-S) batteries, supercapacitors and as precursors to prepare and modify the electrode materials, meanwhile, some important research results in recent years are specially introduced, and the perspective on novel application of ionic liquids is also discussed.     

Effect of melt temperature on the oxidation behavior of AZ91D magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres

The oxidation behaviors of AZ91D magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres at temperatures between 660 °C and 760 °C have been studied. The experimental results show that with the increase of melt temperature, the oxidation rate of molten AZ91D magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres increased and the oxidation kinetics changed from parabolic law to linear law. On the other hand, the amount of MgF₂ in the oxide film formed on AZ91D decreased, and the amount of MgO increased. The effect of melt temperature on the oxidation behaviors is primarily related to the relative content of MgO and MgF₂ in the film, as well as the diffusion rate and the evaporation rate of magnesium through the film.     

Microstructure and oxidation behaviors of near-α Ti-6.5Al-4Sn-4Zr-0.5Mo-based alloys with Ir addition

The microstructure and oxidation behaviors of near α-Ti-based alloys with small amount of iridium (Ir) additions were investigated. The microstructure of both Ir-free and Ir-containing alloys was observed to consist of α + β Widmanstätten colonies. The β lamellae gradually became continuous with increasing Ir additions since Ir acted as a β-stabilizer in the alloys. Isothermal oxidation test indicated that Ir addition reduced the oxidation resistance at 650 °C; while at 750 °C, the adherence of thermally grown oxides was enhanced, and a thin Al₂O₃-enriched layer on the oxide scale was promoted in the Ir-containing alloy, which suggests that Ir addition was effective in improving oxidation resistance of near-α-based alloys at 750 °C.     

Influence of aging temperature,time, and environment on thermo-oxidative behavior of PMR-15: nanomechanical characterization

A series of PMR-15 resin specimens were isothermally aged at 288, 316, and 343 °C over a range of time. For PMR-15 aged at 288 °C, the samples were also subjected to different aging environments including: ambient air, dry air, inert (argon), and pressurized air (0.414 MPa). Nanoindentation was performed to characterize localized mechanical properties as well as the development and growth of the oxidative layer. The measured increase in stiffness in the specimen surface oxidation layer is a manifestation of the chemical changes in the polymer occurring during oxidation. The average elastic modulus in the oxidized region is relatively insensitive to variations in aging temperature, time, and the environments. The thickness of the oxidative layer is observed to increase in the early stages of oxidation and the oxidation process eventually approaches an auto-retardation state. Aging under elevated pressure increases the thickness growth rate of the oxidation layer, while there is no significant difference in growth rate for specimens aged in dry air versus those aged in ambient air. It is shown that the measured average thickness of the oxidation layer and the transition region determined by the nanoindenter is in good agreement with optical microscopy measurements for all conditions considered.     

Protective coatings on orthorhombic Ti2AlNb alloys

Abstract

The oxidation behaviour of an orthorhombic Ti–22Al–25Nb alloy, bare and with protective coatings, was investigated at 750°C in air under quasi-isothermal and thermal cycling conditions. As found by post-oxidation analysis, the uncoated substrate material was severely degraded by formation of spalling oxide scales and ingress of oxygen and nitrogen causing nitride precipitation, internal oxidation and interstitial embrittlement. Metallic Ti–51Al–12Cr coatings as well as nitride coatings based on Ti–Al–Cr–Y–N, either monolithically grown or with superlattice structure, provided an effective diffusion barrier against oxygen. The excellent oxidation resistance of the TiAlCr coatings was associated with the ternary Ti(Al,Cr)₂ Laves phase promoting the formation of a protective alumina scale. The different intermetallic phases formed in the interdiffusion zone caused neither cracking nor spallation of the protective coating. Both, monolithically grown TiAlCrYN and superlattice TiAlYN/CrN coatings, exhibited slow, but nearly linear oxidation kinetics at 750°C in air. In the subsurface region of the substrate a niobium rich phase and the α₂-phase formed. At the coating/substrate interface pores and a thin, fine-grained TiN layer were found.     

Synthesis and Processing of Doped Hg1Ba2Ca2Cu3O y Superconductors

A series of quenching experiments were conducted to understand the sequence of reactions that occur during the synthesis of doped Hg1223, (Hg, A)Ba₂Ca₂Cu₃O _y , A = Re, Bi, and Pb (HgA1223). The formation and decomposition of the intermediate phases during the high-temperature reaction were followed as a function of temperature. HgA1223 phase forms over a wide range of temperatures, 750–950°C, 750–880°C, and 840–880°C for A = Re, Pb, and Bi, respectively. At T<750°C, HgA1212 phase forms for A = Re and Pb. Based on the results of quenching experiments, heat treatment conditions were optimized for the synthesis of pure HgA1223 phase using commercial BaCaCuO precursor powders. A reduced-temperature annealing stage after the high-temperature reaction helps in grain growth and improves the microstructural characteristics of HgA1223 samples. Control of Hg pressure during the reaction is crucial for achieving phase purity, grain growth, and texture in the final products. A novel approach for the control of Hg pressure during the synthesis of HgA1223, which consists of using CaHgO₂ as an external Hg source, is reported. HgA1223 samples synthesized using the new synthesis protocol exhibit improved microstructural and superconducting properties.     

Oxidation behavior and effect of oxidation on tensile properties of Ti60 alloy

Oxidation behavior of a near-alpha Ti60 titanium alloy was investigated in the temperature range of 600–750 °C for up to 100 h exposure. The results showed that the oxidation kinetics of Ti60 alloy followed parabolic kinetics below 700 °C but parabolic-linear kinetics above 700 °C. The total activation energy was calculated to be 256 kJ/mol over the whole temperature range. The oxidation products were TiO₂ after thermal exposure at 700 °C for 100 h, but a mixture of TiO₂ and a small amount of Al₂O₃ for the specimens oxidized at 750 °C for 50 h. The grain boundaries were preferred sites for oxidation products formation during the oxidation. The tensile tests were performed at room temperature for specimens before and after oxidation. Both of the strength and ductility decreased for the specimens with oxide scale. However, both of them increased when the oxide scale was removed before testing.     

Experiments on the transition from the steady to the oscillatory marangoni convection of a floating-zone under various cold wall temperatures and various ambient air temperature effects

The transition from the steady to the oscillatory Marangoni convection of a floating-zone under various cold wall temperatures and various ambient air temperature effects have been investigated experimentally by heating the sample from above (opposite direction of Marangoni convection and buoyant forces). The heat transfer takes place mainly through conduction as well as the natural convection of the air around the cylindrical liquid bridge. The ambient airflow in the present work is varied by varying the cold wall temperature and ambient air temperature. In this study, the transition from the steady to the oscillatory Marangoni convection flow of a high Prandtl number fluid in a floating half-zone is visualized by means of the already proven method of the “light-cut-technique”. The test fluid zone is held in ambient air at +4 °C, +10 °C, +16 °C, +23 °C, and +28 °C. The onset of oscillations, the oscillation level, and oscillation pattern are investigated under various conditions. It is found that the critical temperature difference (ΔT_Cr) varies substantially when the cold wall temperature and the ambient air temperature are varied.     

Characterization of the protective surface films formed on molten magnesium in air/HFC-134a atmospheres

The surface films formed on molten magnesium in an air/HFC-134a gas mixture at 700 °C were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Auger electron spectroscopy (AES). The results showed that there was a protective film on molten magnesium surface, which can prevent molten magnesium from oxidation and ignition. The surface film contained primarily four elements: magnesium, fluorine, oxygen and carbon, and was composed of MgF₂, MgO and C. The film properties depended on the HFC-134a concentration in the gaseous mixture and exposure time. The thickness of the film formed after exposure to air containing 0.5% HFC-134a for 10 min was about 1–2 μm.     

High-temperature microstructural evolution and quantification for alloys IN740 and IN740H: comparative study

In ultra-supercritical power plants, Ni-base alloys are candidate materials for long-term, high-temperature applications, operating at temperatures and pressures as high as 750°C and 35?MPa. Alloy IN740 and its modification, alloy IN740H, are considered for such applications. Their microstructural evolution, at 750°C for times ranging between 3000 and 5000 hours, has been investigated by means of scanning electron microscopy, electron back-scattered diffraction, energy dispersive X-ray analysis and phase quantification. All phases were identified and quantified allowing comparison between the two microstructures, their evolution and stability. Particular attention was paid to γ′, η and G phases. The results are used within a broader investigation aimed at improving and further developing a predictive creep model based on continuous damage mechanics.     

A multilayer MoSi2-SiC-B coating to protect SiC-coated carbon/carbon composites against oxidation

To protect carbon/carbon (C/C) composites against oxidation, a multilayer MoSi₂-SiC-B coating was prepared on the SiC-coated C/C composites by a simple and low-cost slurry method. The phase, microstructure and element distribution of the as-received coating were analyzed using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The as-received coating could effectively protect C/C composites against oxidation at 850 °C in air for 100 h without mass loss, which exhibits better oxidation protective ability than the multilayer MoSi₂-SiC coating prepared by the same method. At intermediate temperature (850 °C), the excellent oxidation protective ability of the coating is mainly attributed to the formation of the molten B₂O₃ for sealing the microcracks and preventing oxygen from attacking the C/C substrate.     

Prevention of icing by freezing point depressant systems

A freezing point depression system (dimethyl sulfoxide-Tullanox-water-polyvinyl alcohol) has been developed which prevents icing at a cold surface at moderate temperatures below 0°C in ambient air of 12.8°C and 70% relative humidity for several days. No drainage of liquids occurs during this time; the increase in weight is moderate (20–30%). The system is non-toxic and not inflammable. The system is easily applied to surfaces and is economical.