In vacuuo tribological investigations of metal, ceramic and hybrid interfaces for high-speed spacecraft bearing applications

New bearing technologies are being developed for use in space systems to meet future demands including longer lifetimes, higher speeds and affordability. Specifically, improvements are necessary for high speed control momentum gyroscopes as longterm operation can lead to a fluid starved condition through evaporation and/or creep. Over the course of the bearing lifetime, the bearing will progress from elastrohydrodynamic to mixed and boundary layer regimes that result in failure.In order to understand this failure scenario, this paper discusses the development of a UHV tribometer and test methodology to simulate a fluid starved condition on a laboratory scale in vacuum. Results are shown for metal-on-metal, hybrid (metal-ceramic) and ceramic material pairs using a multialkylated cyclopentane liquid lubricant. These results show that ceramic material pairs provide approximately three orders of magnitude longer life than both of the other systems through the reduction of fluid decomposition. Three regions were noted in tests run to failure that are associated with decreasing film thickness and increasing fluid decomposition. Ranking within each system can be understood based on contact angle data which gives insight into relative film thickness.     

Solid particle erosion of SiC-Al2OC ceramics

Erosion rates of SiC-Al₂OC ceramics, with Al₂OC content varying from 5 to 75wt%, were assessed using 240-grit alumina abrasive particles accelerated to a velocity estimated at 120msec^–1 and impacting the target at normal incidence. The target ceramics varied in hardness from 27.1 GPa for SiC-5wt% Al₂OC to 10.8 GPa for SiC-75wt% Al₂OC, but the fracture toughness was essentially independent of composition (K _lc ã 3.5 MPa m^1/2). The erosion weight loss varied linearly with the test duration for all the ceramics and the erosion rate decreased systematically with increasing target hardness; the hardness dependence of the erosion rate was, however, much greater than the predictions of the currently available erosion models.     

Colloidal Gold and Silver Triangular Nanoprisms

It is now well‐known that the size, shape, and composition of nanomaterials can dramatically affect their physical and chemical properties, and that technologies based on nanoscale materials have the potential to revolutionize fields ranging from catalysis to medicine. Among these materials, anisotropic particles are particularly interesting because the decreased symmetry of such particles often leads to new and unusual chemical and physical behavior. Within this class of particles, triangular Au and Ag nanoprisms stand out due to their structure‐ and environment‐dependent optical features, their anisotropic surface energetics, and the emergence of reliable synthetic methods for producing them in bulk quantities with control over their edge lengths and thickness. This Review will describe a variety of solution‐based methods for synthesizing Au and Ag triangular prismatic structures, and will address and discuss proposed mechanisms for their formation.

     

Toughening of Layered Ceramic Composites with Residual Surface Compression

Effects of macroscopic residual stresses on fracture toughness of multilayered ceramic laminates were studied analytically and experimentally. Stress intensities for edge cracks in three-layer, single-edge-notch-bend (SENB) specimens with stepwise varying residual stresses in the absence of the crack and superimposed bending were calculated as a function of the crack length by the method of weight function. The selected weight function and the method of calculation were validated by calculating stress intensities for edge cracks in SENB specimens without the residual stresses and obtaining agreement with the stress-intensity equation recommended in ASTM Standard E-399. The stress-intensity calculations for the three-layer laminates with the macroscopic residual stresses were used to define an apparent fracture toughness. The theoretical predictions of the apparent fracture toughness were verified by experiments on three-layer SENB specimens of polycrystalline alumina with 15 vol% of unstabilized zirconia dispersed in the outer layers and 15 vol% of fully stabilized zirconia dispersed in the inner layer. A residual compression of ∼400 MPa developed in the outer layers by the constrained transformation of the unstabilized zirconia from the tetragonal to the monoclinic phase enhanced the apparent fracture toughness to values of 30 MPa^.m^1/2 in a system where the intrinsic fracture toughness was only 5 to 7 MPa^.m^1/2.     

Producer estimates of prevalence and perceived importance of lameness in dairy herds with tiestalls,freestalls, and automated milking systems

Lameness is one of the most important welfare and productivity concerns in the dairy industry. Our objectives were to obtain producers' estimates of its prevalence and their perceptions of lameness, and to investigate how producers monitor lameness in tiestall (TS), freestall with milking parlor (FS), and automated milking system (AMS) herds. Forty focal cows per farm in 237 Canadian dairy herds were scored for lameness by trained researchers. On the same day, the producers completed a questionnaire. Mean herd-level prevalence of lameness estimated by producers was 9.0% (±0.9%; ±SE), whereas the researchers observed a mean prevalence of 22.2% (±0.9%). Correlation between producer- and researcher-estimated lameness prevalence was low (r = 0.19) and mean researcher prevalence was 1.6, 1.8, and 4.1 times higher in AMS, FS, and TS farms, respectively. A total of 48% of producers thought lameness was a moderate or major problem in their herds (TS = 34%; AMS =53%; FS = 59%). One third of producers considered lameness the highest ranked health problem they were trying to control, whereas two-thirds of producers (TS = 43%; AMS = 63%; FS = 71%) stated that they had made management changes to deal with lameness in the past 2 yr. Almost all producers (98%) stated they routinely check cows to identify new cases of lameness; however, 40% of producers did not keep records of lameness (AMS = 24%; FS = 23%; TS = 60%). A majority (69%) of producers treated lame cows themselves immediately after detection, whereas 13% relied on hoof-trimmer or veterinarians to plan treatment. Producers are aware of lameness as an issue in dairy herds and almost all monitor lameness as part of their daily routine. However, producers underestimate lameness prevalence, which highlights that lameness detection continues to be difficult in in all housing systems, especially in TS herds. Training to improve detection, record keeping, identification of farm-specific risk factors, and treatment planning for lame cows is likely to help decrease lameness prevalence.     

Using nacelle‐based wind speed observations to improve power curve modeling for wind power forecasting

Wind power forecasting for projection times of 0–48 h can have a particular value in facilitating the integration of wind power into power systems. Accurate observations of the wind speed received by wind turbines are important inputs for some of the most useful methods for making such forecasts. In particular, they are used to derive power curves relating wind speeds to wind power production. By using power curve modeling, this paper compares two types of wind speed observations typically available at wind farms: the wind speed and wind direction measurements at the nacelles of the wind turbines and those at one or more on‐site meteorological masts (met masts). For the three Australian wind farms studied in this project, the results favor the nacelle‐based observations despite the inherent interference from the nacelle and the blades and despite calibration corrections to the met mast observations. This trend was found to be stronger for wind farm sites with more complex terrain. In addition, a numerical weather prediction (NWP) system was used to show that, for the wind farms studied, smaller single time‐series forecast errors can be achieved with the average wind speed from the nacelle‐based observations. This suggests that the nacelle‐average observations are more representative of the wind behavior predicted by an NWP system than the met mast observations. Also, when using an NWP system to predict wind farm power production, it suggests the use of a wind farm power curve based on nacelle‐average observations instead of met mast observations. Further, it suggests that historical and real‐time nacelle‐average observations should be calculated for large wind farms and used in wind power forecasting. Copyright © 2011 John Wiley & Sons, Ltd.     

Human axillary extracts: Analysis of compounds from samples which influence menstrual timing

Previous studies have shown that menstrual cycle length and fertility are affected by a regular pattern of either: (1) intimate contact with men; or (2) contact with extracts from male axillary secretions; and (3) the axillary extracts of women. Experiments utilizing axillary extracts employed a controlled, double-blind research design using either extract or placebo. The study reported here employed quantitative gas chromatography-mass spectrometry (GC-MS) techniques to determine the concentration of several steroids in the male and female axillary secretion extracts. The steroids examined were: androsterone sulfate, dehydroepiandrosterone sulfate, and the volatile steroid androstenol. The results suggest the following: (1) The concentration of androstenol produced by secretion donors varied during the collection period. Males appear to produce more androstenol at certain times; women's secretions show a menstrual variation in androstenol; the highest concentrations of this compound appear to be produced in the midfollicular phase, prior to ovulation. (2) The concentration of dehydroepiandrosterone sulfate is significantly greater in men than in women. (3) In addition to the steroids, a series of aliphatic acids which range from C₂ to C₁₈ in chain length were found. The more volatile members of this series may contribute to the odor of the secretions.     

用小型散热器对DirectFET封装功率MOSFET进行双面散热

本文探讨了可以用于生产厂封装的器件的各种热管理方案,并且引用了国际整流器公司的测试数据,来评估使用小型散热器改善这些器件散热性能的优点,并研究了在进行通电循环的情况下,在器件上直接安装散热器对器件可靠性的影响。     

Processing of Dense ζ‐Ta4C3−x by Reaction Sintering of Ta and TaC Powder Mixture

Tantalum carbides are commonly processed by hot pressing, canned hot‐isostatic‐pressing, or spark plasma sintering because of their high melting temperatures and low diffusivities. This paper reports processing of dense ζ‐Ta₄C_3?x by reaction sintering of a Ta and TaC powder mixture (C/Ta atomic ratio = 0.66). ζ‐Ta₄C_3?x is of interest due to its rhombohedral (trigonal) crystal structure that may be characterized as a polytype with both face‐centered‐cubic and hexagonal‐close‐packed Ta stacking sequences interrupted by stacking faults and missing carbon layers. This structure leads to easy cleaving on the basal planes and high fracture toughness. A key step in processing is the hydrogenation of the Ta powder to produce β‐TaH_x, a hard and brittle phase that enables efficient comminution during milling and production of small, equiaxed Ta particles that can be packed to high green density with the TaC powder. Studies of phase evolution by quantitative X‐ray diffraction during sintering revealed several intermediate reactions: (1) decomposition of β‐TaH_x to Ta; (2) diffusion of C from γ‐TaC to Ta leading to the formation of α‐Ta₂C_y' with the kinetics described by the Avrami equation with an exponent, n = 0.5, and an activation energy of 219 kJ/mole; (3) equilibration of α‐Ta₂C_y' and γ‐TaC_0.78 phases; and (4) formation of ζ‐Ta₄C_2.56 from the equilibrated α‐Ta₂C and γ‐TaC_0.78 phases with the kinetics characterized by a higher Avrami exponent (n ≈ 3) and higher activation energy (1007 kJ/mole). The sintered material contained ~0.86 weight fraction ζ‐Ta₄C_2.56 and ~0.14 weight fraction γ‐TaC_0.78 phases. The microstructure showed evidence of nucleation and growth of the ζ‐Ta₄C_2.56 phase in both the α‐Ta₂C and γ‐TaC_0.78 parent phases with distinct difference in the morphology due to the different number of variants of the habit plane.     

Corrosion reactions in molten sulphates

The redox potential in sulphate melts is controlled by the partial pressures of oxygen and SO₃ in equilibrium with the melt. The rate for the reduction of oxygen is limited by the solubility. Higher rates are observed for the reduction of SO₃ which is identified as the principal oxidizing species. Equilibrium potentials for reversible metal-metal ion couples for the principal components of superalloys are negative relative to the redox potential for the melt. The anodic oxidation of the metal therefore proceeds irreversibly in conjunction with the cathodic reduction of SO₃ An oxide layer is formed on the surface of the metal when the metal ion concentration at the surface, combined with the oxide ion concentration in the melt, which is related to the partial pressure of SO₃, exceeds the solubility limit for the oxide. The corrosion behaviour will depend on the mass transport processes through this oxide layer. Temperature gradients through the molten sulphate, and oxide ion concentration gradients established as a consequence of the corrosion reaction, may influence the morphology of the corrosion product. The gradient between the oxygen chemical potential at the outer surface of the molten sulphate, defined by the oxygen partial pressure in the gas and the oxygen chemical potential at the metal/metal oxide interface, defined by the dissociation equilibrium for the metal oxide, combined with the transport of SO₃ through the molten sulphate, increases the sulphur chemical potential at the metal surface and leads to the formation of the metal sulphide. Stress in the protective oxide layer caused by the growth of the sulphide phase at the interface between the metal and the oxide will eventually fracture the oxide and cause accelerated corrosion.