Predicting the Thermodynamic Stabilities and Oxidation Resistances of Silicide Cermets

Available information on the thermodynamic stabilities of transition metal silicides is summarized and is used to demonstrate the calculation of the stability of silicide-metal mixtures. The cause of the outstanding oxidation resistance of certain silicides is analyzed, and the results are used to predict high oxidation resistance for several untested disilicides. Thermodynamic and structural arguments are applied to the prediction of compositions of thermodynamically stable and oxidation-resistant cermets.     

Phase evolution and thermal stability of novel high-entropy (Mo0.2Nb0.2Ta0.2V0.2W0.2)Si2 ceramics

Owing to the high melting points and high-temperature stability, transition-metal disilicides are potential components for aerospace, automotive, and industrial engineering applications. However, unwanted oxidation known as PEST oxidation severely limits their application owing to the formation of volatile transition metal oxides, especially in the temperature range of 500–1000 °C. To overcome this problem, a new class of high-entropy disilicides, (Mo_0.2Nb_0.2Ta_0.2V_0.2W_0.2)Si₂, was selected by first-principles calculations and then successfully fabricated using a hot-pressing sintering technique. Furthermore, the phase evolution, thermal expansion behavior, thermal conductivity, and oxidation behavior were systematically investigated. Compared with MoSi₂, (Mo_0.2Nb_0.2Ta_0.2V_0.2W_0.2)Si₂ possessed a lower thermal conductivity (10.9–14.7 W·m^?1·K^?1) at 25–1000 °C, higher thermal expansion coefficients (8.6 ± 1.3^–6 K^–1) at 50–1200 °C, and especially an excellent thermal stability at 500–1000 °C owing to slow diffusion and selective oxidation. This work provides a strong foundation for the synthesis and application of high-entropy disilicides.     

“Refractory Metal,RM – Wrap”: A tailorable,pressure-less joining technology

The RM-wrap (RM = Refractory Metal) is a pressure-less, versatile and tailorable joining process: it consists of wrapping Si foils inside a refractory metal wrap (i.e., Mo, Nb, Ta) in order to prevent molten silicon from leaking outside the joined region and infiltrating the facing materials during the joining process.RM-wrap (RM = Mo, Nb, Ta) has been successfully applied to join C/SiC composites in this work: optimized joining treatment consisted of heating to 1450?°C with a heating rate of 1000?°C/h followed by a dwell time of 5?min in a non-reactive environment of Argon flow.The joints were characterized by morphological analysis and lap shear tests at room temperature and 1000?°C.Microscopical analysis revealed an in-situ formed composite joint consisting of a silicon matrix reinforced with silicides of the refractory metals. Joining material exhibited continuous and cracked free bonding with C/SiC irrespective of composite fibre orientation.Joints lap shear strength values at 1000?°C were higher than at room temperature, probably due to the brittle to ductile transition (BTDT) of silicon and silicides.Vickers microhardness on refractory metal disilicides measured inside the joints showed a trend similar to their mechanical strength, with higher lap shear strength and hardness for Mo-Wrap and lower for Ta-wrap joints.     

Joining of SiC,alumina, and mullite by the Refractory Metal—Wrap pressure-less process

The aim of this work was to discuss the suitability of the joining process called “RM-Wrap” (RM = Refractory Metals, ie, Mo, Nb, Ta, Zr) as a pressure-less and tailorable technique to join several different ceramics such as SiC, alumina, and mullite (3Al₂O₃.2SiO₂). In the RM-Wrap joining technique the refractory metal foil is used as a wrap containing one or more silicon foils. It is performed at 1450°C, under flowing argon, and the resulting joining materials are in situ formed composites made of refractory metal disilicides (MoSi₂, NbSi₂, TaSi₂, or ZrSi₂) embedded in a silicon-rich matrix; their coefficient of thermal expansion has been calculated and the Laser Flash Method was used to measure the thermal diffusivity of one of them (MoSi₂/Si) in 25°C-1000°C range, then to calculate its thermal conductivity. All the obtained joints are uniform, continuous, and crack free. Some preliminary oxidation tests were carried out on all joints at 1100°C, 6 hours in air, giving unchanged morphology of the interface and the joining materials itself; the joint strength of RM-Wrap joined SiC was measured at room temperature using three different mechanical tests: (a) single lap (SL), (b) single lap off-set (SLO) and (c) torsion on hourglass-shaped samples (THG) (on Mo-wrap joined SiC).     

Effect of high-energy ball-milling on the spark plasma sinterability of ZrB2 with transition metal disilicides

A wide suite of powder mixtures of ZrB₂ with five different additions of transition metal disilicides (MeSi₂; 5, 17.5, or 30 vol% MoSi₂; 17.5 vol% TaSi₂; or 17.5 vol% ZrSi₂) were prepared by high-energy ball-milling (HEBM) for different times, and their optimal densification temperatures were evaluated by dilatometric spark-plasma-sintering (SPS) tests to compare their sinterability. SPS densification tests at the so-determined optimal densification temperatures were also performed in selected cases. It was found that HEBM progressively enhanced the sinterability, especially once the ZrB₂ particle sizes were refined to the ultrafine range or below (<250 nm). It was also observed that the sinterability was enhanced (i) with the greater addition of MeSi₂, and (ii) with the lower refractoriness of the MeSi₂. Nonetheless, under long-time HEBM, the addition of harder, more refractory MeSi₂ or of softer, less refractory MeSi₂ is equally effective in terms of sinterability, both enabling low-temperature densification (SPS at ∼1200 °C).     

Densification of ZrB2-based composites and their mechanical and physical properties: A review

This study reviews densification behaviour, mechanical properties, thermal, and electrical conductivities of the ZrB₂ ceramics and ZrB₂-based composites. Hot-pressing is the most commonly used densification method for the ZrB₂-based ceramics in historic studies. Recently, pressureless sintering, reactive hot pressing, and spark plasma sintering are being developed. Compositions with added carbides and disilicides displayed significant improvement of densification and made pressureless sintering possible at ≤2000 °C. Reactive hot-pressing allows in situ synthesizing and densifying of ZrB₂-based composites. Spark plasma sintering displays a potential and attractive way to densify the ZrB₂ ceramics and ZrB₂-based composites without any additive. Young's modulus can be described by a mixture rule and it decreased with porosity. Fracture toughness displayed in the ZrB₂-based composites is in the range of 2–6 MPa m^1/2. Fine-grained ZrB₂ ceramics had strengths of a few hundred MPa, which increased with the additions of SiC and MoSi₂. The small second phase size and uniform distribution led to higher strengths. The addition of nano-sized SiC particles imparts a better oxidation resistance and improves the strength of post-oxidized ZrB₂-based ceramics. In addition, the ZrB₂-based composites showed high thermal and electrical conductivities, which decreased with temperature. These conductivities are sensitive to composition, microstructure and intergranular phase. The unique combinations of mechanical and physical properties make the ZrB₂-based composites attractive candidates for high-temperature thermomechanical structural applications.     

Blending,chilling, and tempering of margarines and shortenings

The preparation of the milk and fat phase of margarine, which is a milk-in-fat emulsion, is discussed. Refined and deodorized fats are blended to meet constraints in regard to price, solid fat index, taste stability, and crystallization behavior. The milk phase is prepared according to specifications and concerning protein content, salt concentration, and other ingredients, and adjusted to the pH. The flavored fat and the milk are then dosed in the proper ratio and processed by means of scraped coolers in which the mixture is emulsified, cooled, and crystallized. High speed machines pack the product in tubs or wrappers. The processing of shortenings is roughly the same, but no water phase is used. Occasionally, tempering, which is a hold of several hours at elevated temperature, is applied to improve the plasticity of the product, its creaming properties, and overall baking performance.     

镍钴铜铁和稀土混合废料中各元素的分离提纯

综合使用溶剂萃取法和化学沉淀法,研究了从镍钴铜铁和稀土混合废料中分离各主要有价金属元素的工艺条件。通过前处理,得到有价金属溶液,调节pH值为2.0沉淀稀土元素,在pH值为1.0时用Acorga M5640萃取分离铜,再在pH值等于5.0时用Cyanex 272萃取钴,剩余水相溶液用来制取镍皂,从而使镍钴铜铁和稀土元素逐步得以分离提纯处理,达到处理含镍钴铜铁和稀土元素的混合废料,回收有用物质,控制污染,保护环境的目的。     

Characterization of crystalline cellulose in biomass: Basic principles, applications, and limitations of XRD, NMR, IR, Raman, and SFG

Cellulose is among the most important and abundant biopolymers in biosphere. It is the main structural component of a vast number of plants that carries vital functions for plant growth. Cellulose-based materials have been used in a variety of human activities ranging from papers and fabrics to engineering applications including production of biofuels. However, our understanding of the cellulose structure in its native form is quite limited because the current experimental methods often require separation or purification processes and provide only partial information of the cellulose structure. This paper aims at providing a brief background of the cellulose structure and reviewing the basic principles, capabilities and limitations of the cellulose characterization methods that are widely used by engineers dealing with biomass. The analytical techniques covered in this paper include x-ray diffraction, nuclear magnetic resonance, and vibrational spectroscopy (infrared, Raman, and sum-frequency-generation). The scope of the paper is restricted to the application of these techniques to the structural analysis of cellulose.     

Synthesis,characteristic, and application of new flame retardant containing phosphorus,nitrogen, and silicon

In this study, the novel halogen‐free flame retardants (PSiN, A and B), which contain phosphorus, nitrogen, and silicon, have been synthesized. The structure of PSiN‐A is characterized by fourier transform infrared spectroscopy and proton nuclear magnetic resonance, and its thermal property is studied through thermo gravimetric analysis (TGA). PSiN‐A and B were blended with polypropylene(PP) to obtain PP/PSiN composites. The flame‐retardant properties of PP/PSiN composites are estimated by Limiting Oxygen Index (LOI) values, and their degradation behaviors are investigated through TGA, under nitrogen from room temperature to 800°C. The fire performance of PP is improved by PSiN(A or B): the LOI value of PP/PSiN‐A reach 26.0 vol% and the char yield is at 27 wt% at 800°C. The phosphorus in PSiN provides possibility for the PP blends to form char, and the silicon improves the thermal stability of char. The active energies of PSiN and PP are calculated through the method of Horowitz–Metzger. POLYM. ENG. SCI. 46:344–350, 2006. © 2006 Society of Plastics Engineers     

Handling,shipping, and storage of oilseeds

Conclusion The rapid crop movement has been discussed, also the problem of deterioration in storage, and some of the procedures and equipment which the crusher may use to solve his problems. In a few weeks the oil seeds will start their flow to the mills. No one season is entirely typical of another, and the crusher will soon be faced with new problems relative to unloading, storage allocation, and segregation day by day and hour by hour. Each problem is demanding of solution and cannot be deferred or referred. The success of the mill’s year-round operation may depend in large measure upon the initiative, hard work, imagination, and judgment with which an unloading season is handled. The crushing industry is looking forward to this season and to every season with an unique interest that is hard to describe. The interest is so much a part of processors that oil mill men could never, and would never want to lose it.     

Continuous solvent extraction of sunflower,seed, groundnuts,palmkernels, rapeseed,and copra

In principle, the oil milling process is straightforward. The wide variety of equipment in use reflects, however, a multiparametric process. The main limiting parameters are kind of seed, oil content, particle strength, particle structure, temperature, moisture, particle size, residence time, mechanical forces, etc. Additionally, the seed specific parameters vary not only between kinds of seed but also for the same seed, due to different conditions in climate, soil, and harvesting. Therefore, equipment design must be widely based on statistical averages of the limiting parameters. In other words, exact precalculation of the single technological steps is impossible. As a consequence, for more than 5,000 years, the oil milling process has been in the stage of being optimized. The acutal technical standard is very satisfying, but there is still a strong need for further research and development. Currently, as in the past, the oil milling process seems to be not so much a science as an art.     

Stable silyl, germyl, and stannyl cations, radicals, and anions: heavy versions of carbocations, carbon radicals, and carbanions

The rapidly growing chemistry of the cations, radicals, and anions based on the group 14 elements heavier than carbon (Si, Ge, Sn, and Pb) is one of the most important organometallic fields. Recent developments in this research area moved such species from the class of short-lived reactive intermediates to the class of easily accessible, isolable, and fully characterizable compounds. In this Account, we deal with the major accomplishments in the field of the stable representatives of "heavy" cations, radicals, and anions.     

Separation and quantitation of nitrocellulose,nitroglycerin, diethylphthalate,and centralite in double-base powder

A method is described which makes possible a direct determination of nitrocellulose in double-base powder and powder paste after separation from low-molecular components by gel permeation chromatography and subsequent integration of the nitrocellulose peak. It is also possible to determine nitroglycerin and stabilizers in the same sample by collecting the components in the low-molecular peak and subsequent injection into a reversed-phase chromatography system. The procedure is exemplified on a powder paste containing nitrocellulose, nitroglycerin, centralite I, and diethylphthalate. The whole quantitation procedure will take less than an hour.     

Preparation,Sedimentation, and Agglomeration of Nanofluids

Research on nanofluids has increased significantly in the last decade due to the distinctive potential in many applications such as enhanced heat transfer processes, drug delivery systems, and membrane manufacturing processes. The study of dispersion behavior of nanoparticles in liquids is a topic of keen interest towards the preparation of stable nanofluids. A comprehensive review on the recent progress in preparation and stability of nanofluids is presented. Settling of nanoparticles is the only predicament towards preparation of stable nanofluids which takes place due to formation of agglomerates. The settling behavior of nanoparticles in nanofluids and techniques to stop agglomeration are described. Nanofluid preparation techniques, assessment method of stability, and methods to reduce agglomeration are discussed.     

Abrasion,fatigue, and smearing of rubber

The abrasion of rubber on tracks without sharp-pointed asperities is dominated by fatigue failure. This is demonstrated by the effect of certain antioxidants on the abrasion of natural rubber compounds in air, which largely disappears in nitrogen. The experimental results obtained in air can be obscured by smearing which seems to originate from the abrasion process itself. In addition, the relative humidity of the surrounding atmosphere has a great, and little-understood influence on the rate of abrasion. The bearing of these findings on the interpretation of road test results is discussed.     

Breaking, making, and twisting of chemical bonds in gas, liquid, and nanocavities

In this Account, we recount on our studies of 1'-hydroxy-2'-acetonaphthone (HAN, a proton transfer prototype molecule) in gas, solutions, and nanocavities. The internal H-bond photoreaction in HAN leads to a keto type structure, and following its formation, an internal twisting motion gives birth to keto rotamers. Theory, temperature, and solvent effects on its photodynamics show the involvement of efficient radiationless processes in both keto structures. When HAN is caged in a cyclodextrin nanocavity, the spectroscopy, photodynamics, and issues of twisting motion are strongly affected and could be tuned: a behavior relevant to those of many chemical and biological systems.     

Bioaerosol Science,Technology, and Engineering: Past,Present, and Future

Poor air hygiene as a result of bioaerosol contamination has caused diverse forms of adverse health effects and diseases. In addition, global biosecurity is threatened by purposeful use of biowarfare agents and the vulnerability of people to the infectious agents. Accordingly, developments in high-volume biosampling, including aerosol-to-hydrosol techniques with low cut-off size, real-time bioaerosol detection, adequate biological quantification, and exposure control, as well as the investigation of the link between disease outcome and bioaerosol exposure, are current areas of bioaerosol research. Although milestone progress has been achieved both in bioaerosol sampling and analysis techniques since late 1800s, compared to atmospheric chemistry the bioaerosol field is still understudied. This is partially because of the lack of both bioaerosol scientists and multidisciplinary collaboration. It is becoming necessary to develop a pool of scientists with different expertise, e.g., bioaerosol scientists, environmental engineers, biomedical engineers, epidemiologists, microbiologists, chemists, physicists, as well as researchers in other engineering fields, in mitigating bioaerosol-related adverse health effects, eliminating diseases, and preventing and controlling epidemic outbreaks. This work is conducted to broadly review current state-of-the-art sciences and technologies in the bioaerosol field. In tackling the challenges ahead, the review also provides perspectives for bioaerosol research needs, and further reminds bioaerosol scientists of those existing technologies in other fields that can be leveraged. In view of the past, forward-looking hypotheses and revolutional perspectives are needed to be formed in order to allow the bioaerosol research have major impacts in the academic community in this new millennium.     

Shear flow rheological properties,fiber damage,and mastication characteristics of aramid-, glass-, and cellulose-fiber-reinforced polystyrene melts

An experimental study of the viscosity and principal normal stress difference of a polystyrene melt filled with aramid (Kevlar), glass, and cellulose fibers is reported. The influence of loading level and mastication on the rheological properties is discussed. The effects of mixing and mastication on fiber damage are considered. Glass fibers break down rapidly to very small aspect ratios, while aramid shows a “kinked” structure, with kinks occurring every 100 μm. A mechanism is proposed for fiber breakage based on buckling during rotation in shear flow. It is found that addition of fibers increases the viscosity in the same manner as a reduction in temperature, and data may be superposed by reduced plotting. This indicates that the viscosity increase is due solely to enhanced viscous dissipation in the matrix and not to interparticle forces as is the case with smaller particles. The principal normal stress difference increases at fixed shear stress with fiber loading. The extent of increase depends upon fiber loading, aspect ratio, and modulus.