ZrB_2基超高温陶瓷复合材料的高温拉伸损伤行为

开展了SiC(20vol%)-石墨(15vol%)/ZrB2复合材料室温及高温拉伸性能实验,发现高温时复合材料的拉伸强度和弹性模量有所降低,并且具有明显的非线性特征。引入热损伤来表征弹性模量随温度的衰减规律,利用强度统计分析方法确定单向应力状态下材料的机械损伤演化方程,建立了材料在热力耦合条件下的高温拉伸损伤非线性本构模型。分析表明:随着温度的升高,SiC-石墨/ZrB2复合材料的热损伤和机械损伤不断增加,延性增强,且脆性-延性破坏转变温度范围为1 250~1 350℃。     

Metal on metal oxide nanowire Co-catalyzed Si photocathode for solar water splitting

We report a systematic study of Si|ZnO and Si|ZnO| metal photocathodes for effective photoelectrochemical cells and hydrogen generation. Both ZnO nanocrystalline thin films and vertical nanowire arrays were studied. Si|ZnO electrodes showed increased cathodic photocurrents due to improved charge separation by the formation of a p/n junction, and Si|ZnO:Al (n(+)-ZnO) and Si|ZnO(N(2)) (thin films prepared in N(2)/Ar gas) lead to a further increase in cathodic photocurrents. Si|ZnONW (nanowire array) photocathodes dramatically increased the photocurrents and thus photoelectrochemical conversion efficiency due to the enhanced light absorption and enlarged surface area. The ZnO film thickness and ZnO nanowire length were important to the enhancements. A thin metal coating on ZnO showed increased photocurrent due to a catalyzed hydrogen evolution reaction and Ni metal showed comparable catalytic activities to those of Pt and Pd. Moreover, photoelectrochemical instability of Si|ZnO electrodes was minimized by metal co-catalysts. Our results indicate that the metal and ZnO on p-type Si serve as co-catalysts for photoelectrochemical water splitting, which can provide a possible low-cost and scalable method to fabricate high efficiency photocathodes for practical applications in clean solar energy harvesting.     

超高温氧化物陶瓷激光增材制造技术与缺陷控制研究进展

超高温氧化物陶瓷具有高强度、高硬度、天然杰出的抗氧化和抗腐蚀性能,被认为是极端氧化腐蚀环境下长期服役的理想高温结构材料.激光增材制造(LAM)技术以其特有的高效、快速、无需模具、柔性制造等优点,成为近年来直接快速制备高性能复杂结构陶瓷构件最具潜力的近净成形技术.本文概述了陶瓷材料LAM技术的原理,重点阐述了高性能氧化物...     

Metal matrix composites for aeroengines

Abstract

Metal matrix composites (MMCs) will play a significant role in the future of gas turbine aeroengine development. This paper outlines the benefits and some of the potential applications for Al and Ti MMCs and discusses issues involved in the introduction of this relatively new class of composite materials into engine components. The potential for cost savings and performance improvements which may be achieved by the introduction of silicon carbide particulate reinforced Al alloys (Al–SiC_p MMC)are discussed, where the properties of the composite can be tailored to meet the requirements of a specific application. Some of the processing implications for Ti matrix composites (Ti MMCs) are explained for a range of component applications. Illustrations are given of how the manufacturing process can be controlled for a complex component. Finally, the influence of raw material, manufacturing, and component costs on the successful introduction of MMCs into aeroengines is discussed.     

Portland cement solutions for ultra-high temperature wellbore applications

Designing highly stable and low permeability high-strength oil well cement above 500 °F (>260 °C) is extremely difficult because the Portland cement undergoes strength retrogression starting at 230 °F (110 °C). Thus, fine crystalline silica stabilizer is required in the cement slurry design when cured until 400 °F (204.4 °C) to prevent this problem. However, the optimum particle size and the right practical dosage of silica in the cement slurry have not been clearly determined and studied for cement that will be subjected at such extremely high temperature (>500 °F), condition applicable for many ultra-deep geothermal and steam injection wells. Due to extreme heating conditions and tedious experimentation, there are only few published studies to date toward understanding the behavior of Portland cement used at that exceedingly high temperature; and thus, making the initial cement design highly challenging. Based on thorough experimental study, this paper presents new understanding and provides proper guidance to designing highly stable non-retrogressing and low steady permeability oil well cement for ultra-high temperature use. Moreover, this research carefully investigates the effect of the particle size of crystalline silica to the compressive strength, porosity and permeability of the cured cement. Finally, the substitution of crystalline silica with amorphous silica material in the ultra-high temperature cement is also exploited. Supplementary powder X-ray diffraction measurement identifies high temperature stable crystal phase Xonotlite (Ca₆Si₆O₁₇(OH)₂) in the silica-stabilized Portland cement. However, based from these current experimental results, it is observed that formation of Xonotlite crystal phase does not completely guarantee the longer term mechanical integrity of the cement sheath at ultra-high temperature, especially when the mixture contains amorphous-type silica in the blend.     

Measurement and modelling of the thermochemical expansion of polymer composites

The thermochemical expansion of two very similar polymer composites has been measured from approximately 30°C to 1500°C. Expansion measurements were obtained at heating rates of 5, 10, 20 and 50°C min⁻¹. One material exhibited a net expansion while the other exhibited a net contraction. Both types of behaviour were successfully modelled to approximately 1040°C using a previously proposed expansion model. The constants in the model were determined using the experimental expansion data and non-linear estimation.Expansion behaviour calculated using the model is compared with the observed expansion behaviour.     

Metal matrix composites for industrial application

The present status of Metal Matrix Composite materials Research & Development (R&D) and industrial exploitation is summarised. Metal Matrix Composites (MMC) are defined and described and their value to the designer is discussed in the context of their technical strengths and weaknesses. The driving force for the present high level of investment in MMC R&D worldwide is discussed and the economic advantages of collaborative development initiatives are analysed. The present UK industrial collaborative MMC R&D programme is described and set in the context of the scope and objectives of major development programmes in the US, Japan and Europe.     

Selective liquid phase deposition of silicon oxide at low temperature for nanometer-scale structures

Selective liquid phase deposition to overcome the thermal stress that occurs on photoresist patterns during semiconductor device fabrication was investigated. Silicon oxide films were selectively deposited on silicon substrates at temperatures under 50 °C without damaging the photoresist patterns. The deposited SiO₂ films had a uniform surface, good step coverage, and excellent chemical stability. A line-and-space structure of silicon oxide at the nanometer scale (≤ 100 nm) was successfully produced by plasma downstream ashing and selective liquid phase deposition. This method can therefore be used to fabricate unique tools for nanometer-scale devices.     

Stability of high temperature chemical vapor deposited silicon based structures on metals for solar conversion

Highly crystallized silicon layers were grown on metal sheets at high temperature (950 degrees C) by thermal CVD from silane. An intermediate buffer layer was mandatory to prevent interdiffusion and silicide formation but also to compensate lattice parameters and thermal expansion coefficients mismatches between metal and silicon and ideally transfer some crystalline properties (grain size, texture) from the substrate to the silicon layer. After a thermodynamic study, aluminum nitride or titanium nitride diffusion barrier layers were selected and processed by CVD. The structure and the interfaces stabilities of these silicon/nitride/metal stacks were studied by field effect gun scanning and transmission electron microscopy, X-ray diffraction, Raman and energy dispersive X-ray spectroscopy. As a result, TiN deposited by CVD appears to be an efficient material as a buffer layer between steel and silicon.     

Working fluids for high temperature sorption cycles

Sorption cycles could be used in industrial processes to lift waste heat available at temperatures from ≈ 100 to > 200°C. Three different organic working pairs have been tested within these boundary conditions. Each pair has hexafluoroisopropanol as refrigerant. Quinoline and n-methylpyrrolidone are used as absorbents. Vapour-liquid equilibrium measurements in the range 0.1–20 bar† and 25–250°C as well as caloric measurements, estimates of specific volumes and thermal stability tests are reported. It is shown that due to stability problems only one of the working pairs can be used for the whole temperature range considered. The technical data for a conventional and an inverse absorption heat pump cycle are calculated, figures are presented and conclusions drawn for both applications.     

Characterization and stability analysis of oil‐based copper oxide nanofluids for medium temperature solar collectors

Thermal oils are widely used as heat transfer fluids in medium temperature applications. Addition of small amounts of nanoparticles in such fluids can significantly improve their thermophysical properties. This paper presents experimental investigation of an oil‐based nanofluids prepared by dispersing different concentrations (0.25 wt%–1.0 wt%) of copper oxide nanoparticles in Therminol‐55 oil using two‐step method. Shear mixing and ultrasonication were used for uniform distribution and de‐agglomeration of nanoparticles to enhance the stability of the suspensions. The effect of nanoparticles concentrations on thermophysical properties of the nanofluids was analysed by measuring thermal conductivity, dynamic viscosity, effective density and specific heat capacity at different temperatures (25 °C–130 °C). Thermal conductivity exhibited increasing trend with rising temperature and increase in nanoparticles loading. A significant decrease in dynamic viscosity and effective density against increasing temperature makes it suitable for medium temperature applications. Nano‐oils with improved thermal properties are expected to increase the efficiency of concentrating solar thermal collectors.     

Metal oxide nanoarchitectures for environmental sensing

Metal oxide materials are widely used for gas sensing. Capable of operating at elevated temperatures and in harsh environments, they are mechanically robust and relatively inexpensive and offer exquisite sensing capabilities, the performance of which is dependent upon the nanoscale morphology. In this paper we first review different routes for the fabrication of metal oxide nanoarchitectures useful to sensing applications, including mesoporous thin films, nanowires, and nanotubes. Two sensor test cases are then presented. The first case examines the use of highly uniform nanoporous Al2O3 for humidity sensing; we find that such materials can be successfully used as a wide-range humidity sensor. The second test case examines the use of TiO2 nanotubes for hydrogen sensing. Going from a nitrogen atmosphere to one containing 1000 ppm of hydrogen, at 290 degrees C, 22-nm-diameter titania nanotubes demonstrate a 10(4) change in measured resistance with no measurement hysteresis.     

Carbon nanotubes on fluorine-doped tin oxide for fabrication of dye-sensitized solar cells at low temperature condition

The multi-walled carbon nanotubes (MWCNTs), electrophoretically deposited on fluorine-doped tin oxide (FTO), were employed as charge-collecting channels in the TiO2 photoelectrode of dye-sensitized solar cells (DSSCs) fabricated at 200 degrees C. The CNT-networks at the conducting substrate increased the charge collection efficiency of the porous TiO2 film, while the short circuit current increased up to ca. 43% under optimized condition. However, the significant decrease in the open-circuit voltage (Voc) up to ca. 132 mV resulted in the failure of the overall cell efficiency improvement. Findings reveal that the transfer process for the back electron is mainly responsible for the significant Voc drop when the MWCNTs were deposited at the electron-collecting substrate of the photoelectrode. The study demonstrates that electrophoretic deposition of MWCNTs on charge collecting substrate would be applicable to introduce an effective charge-collecting channel for the fabrication of flexible DSSCs under low temperature sintering condition.     

Metal oxide mesocrystals and mesoporous single crystals: synthesis,properties and applications in solar energy conversion

Metal oxide mesocrystals (MCs) and mesoporous single crystals (MSCs) exhibit superior carrier transport ability,high specific surface area,shortened photo-carrier diffusion lengths to interfaces and enhanced absorbance of the incident sunlight.These advanced features make metal oxide MCs and MSCs be a promising candidate material in photocatalysis,photoelectrocatalysis,dye sensitized solar cells (DSSCs) and perovskite solar cells (PSCs).Recently,remarkable advances of applying metal oxide MCs and MSCs in these areas have been achieved.Therefore,it is extremely important to deeply understand the influence of the unique properties of metal oxide MCs and MSCs on solar energy conversion systems.Herein,we presented a brief introduction on the synthesis and carrier transfer behavior of metal oxide MCs and MSCs.Then,the rational structure design and modification of metal oxide MCs and MSCs for photocatalysis,photoelectrocatalysis,DSSCs and PSCs are systematically discussed.Finally,the perspectives on extending the application of metal oxide MCs and MSCs are addressed.     

Natural refrigerant-based subcritical and transcritical cycles for high temperature heating

Theoretical analyses of subcritical/transcritical heat pumps using four natural refrigerants, carbon dioxide, ammonia, propane and isobutane have been carried out for high temperature heating applications at different heating outlet temperatures and heat sources using computer models. The compressor discharge pressures have been optimized for transcritical and subcritical (with near critical operation of condenser) cycles. Results show that for subcritical heat pumps, use of subcooling is efficient for heating applications with a gliding temperature. Results also show that although propane yields better coefficient of performance (COP) in low temperature heating applications, ammonia performs the best in high temperature heating applications. Finally, design aspects of major components of all the four heat pumps for high temperature heating have been discussed, particularly with reference to suitability of available lubricants to the newly evolved operating conditions.     

Single crystalline oxide fibres for heat-resistant composites

A brief review of the methods of fabrication of single crystalline oxide fibres is presented. It is shown that the internal crystallisation method (ICM), that is crystallisation of the fibres in continuous channels in an auxiliary matrix (molybdenum carcass), yields high-productivity rate, which provides a base for the development of fabrication technology of oxide fibres as reinforcement for composites for high and very high use temperatures. The method has been used to produce a family of fibres including sapphire, aluminium–yttrium garnet, mullite as well as some oxide eutectics. Microstructure, strength and high-temperature creep of the fibres are discussed with the emphasis on creep properties. Results of creep tests of composites with matrices based on TiAl, nickel superalloys and oxides are also presented. It is shown that special microstructure of composites with brittle matrices, intermetallic and oxides, yields quasi-ductile behaviour of the composites.     

Research and development of ultra-high temperature materials in Japan

Abstract

Gas turbines for aircraft engine and power generation are typical fields of application of high temperature materials. Nickel-based superalloys are excellent and most useful materials for these applications and have been well developed especially with the outstanding progress of jet engines. In future, from a view point of global environmental problems, there will be strong demands for special materials for high temperature and high efficiency gas turbines for power generation. However, the temperature capability of the superalloys will saturate to some limit because of their melting points which are lower than 1400°C. So-called ultra-high temperature materials, such as intermetallic compounds, refractory metals and alloys, ceramics, and various composite materials are expected to surpass the temperature capability of the superalloys, although these materials have several problems such as difficulty of processing, lack of ductility and toughness or the poor resistance to oxidation and hot-corrosion. In this paper, present and future prospects of R&D of these ultra-high temperature materials have been briefly reviewed.