A simple test for thermomechanical evaluation of ceramic fibers

A simple bend stress relaxation (BSR) test was developed to measure the creep related properties of ceramic fibers and whiskers. The test was applied to a variety of commercial and developmental Si based fibers to demonstrate capabilities and to evaluate the relative creep resistance of the fibers at 1200 to 1400 C. The implications of these results and the advantages of the BSR test over typical tensile creep tests are discussed

Stock Market Yields and the Pricing of Municipal Bonds

This paper proposes an alternative to the traditional model for explaining the spread between taxable and tax-exempt bond yields. This alternative model is a special case of a general class of clientele models of portfolio choice and asset market equilibrium. In particular, we consider a setting with two types of investors, a taxable investor and a tax-exempt investor, who hold specialized bond portfolios. The tax-exempt investor holds only taxable bonds, and the taxable investor holds only tax-exempt bonds. Both investors hold equity, and the taxable and tax-exempt bond markets are linked through the equilibrium conditions governing equity holding and bond holding for each type of investor. In contrast to the traditional model, this alternative model has the potential to explain the small observed spread between taxable and tax-exempt yields. In addition, this model predicts that the yield spread between taxable and tax-exempt bonds should be an increasing function of the dividend yield on corporate stocks. Although the substantial changes in the tax code during the last four decades complicate the testing of this model, we find some support for the predicted relationship between the equity dividend yield and the yield spread between taxable and tax-exempt bonds.

The Roughness Properties of Small Ice-Bearing Craters at the South Pole of the Moon: Implications for Accessing Fresh Water Ice in Future Surface Operations

The lunar poles provide a fascinating thermal environment capable of cold-trapping water ice on geologic timescales [1]. While there have been many observations indicating the presence of water ice at the lunar surface [e.g., 24], it is still not clear when this ice was delivered to the Moon. The timing of volatile dep-osition provides important constraints on the origin of lunar ice because different delivery mechanisms have been active at different times throughout lunar history. We previously found that some small (<10 km) cra-ters at the south pole of the Moon have morphologies suggestive of relatively young ages, on the basis of crisp crater rims [5]. These craters are too small to date with robust cratering statistics [5], but the possibility of ice in young craters is intriguing because it suggests that there is some recent and perhaps ongoing mechanism that is delivering or redistributing water to polar cold traps. Therefore, understanding if these small, ice-bear-ing craters are indeed young is essential in understand-ing the age and source of volatiles on the Moon. Here we take a new approach to understand the ages of these small polar cold traps: analyzing the roughness properties of small ice-bearing craters. It is well under-stood that impact crater properties (e.g., morphology, rock abundance, and roughness) evolve with time due to a variety of geologic and space-weathering processes [611]. Topographic roughness is a measurement of the local deviation from the mean topography, providing a measurement of surface texture, and is a powerful tool for evaluating surface evolution over geologic time [e.g., 1114]. In this study we analyze the roughness of southern lunar craters (40S90S) from all geologic eras, and determine how the roughness of small (<10 km) ice-bearing craters compare. We discuss the implications of the ages of ice-bearing south polar craters, and potential strategies for accessing fresh ice on the Moon

Internal Flow Physics of a Fluidic Oscillator in the Transition Regime

An experimental investigation of the underlying flow physics of a dual jet interaction fluidic oscillator has been conducted in the transition regime for a Reynolds number of 1680. The transition regime is defined as a narrow range of flow rates between two other operating modes of the fluidic oscillator. Particle image velocimetry (PIV) was used with refractive index matching sodium iodide solution to minimize reflections from the actuator geometry and obtain detailed internal velocity fields. PIV results showed that the inter-action of the two internal jets and the resultant vortices are responsible for the oscillation mechanism in the transition regime. Two side vortices sustain their existence throughout the oscillation period by altering their size, shape and strength, and a dome vortex is created twice each oscillation period (once from each jet). The dome vortex plays a key role in the kinetic energy transfer mechanism inside the oscillator by means of jet bifurcations. The primary oscillation mechanism in the transition regime is that each internal jet’s connection with the exiting jet is cut completely by the dome vortex in every period. This is in contrast to the low flow rate oscillation mechanism in which the oscillations are created by continuous collisions of the jets. Furthermore, the internal jets were observed to energize the side vortex on the opposite side of the chamber – a phenomenon which was not observed in the low flow rate regimeDFG, 200291049, SFB 1029: TurbIn - Signifikante Wirkungsgradsteigerung durch gezielte, interagierende Verbrennungs- und Strömungsinstationaritäten in Gasturbine

Creep and stress relaxation modeling of polycrystalline ceramic fibers

A variety of high performance polycrystalline ceramic fibers are currently being considered as reinforcement for high temperature ceramic matrix composites. However, under mechanical loading about 800 C, these fibers display creep related instabilities which can result in detrimental changes in composite dimensions, strength, and internal stress distributions. As a first step toward understanding these effects, this study examines the validity of a mechanism-based empirical model which describes primary stage tensile creep and stress relaxation of polycrystalline ceramic fibers as independent functions of time, temperature, and applied stress or strain. To verify these functional dependencies, a simple bend test is used to measure stress relaxation for four types of commercial ceramic fibers for which direct tensile creep data are available. These fibers include both nonoxide (SCS-6, Nicalon) and oxide (PRD-166, FP) compositions. The results of the Bend Stress Relaxation (BSR) test not only confirm the stress, time, and temperature dependencies predicted by the model, but also allow measurement of model empirical parameters for the four fiber types. In addition, comparison of model tensile creep predictions based on the BSR test results with the literature data show good agreement, supporting both the predictive capability of the model and the use of the BSR text as a simple method for parameter determination for other fibers

A Neutron Scattering Study of the Structure of Poly(dimethylsiloxane)-Stabilized Poly(methyl methacrylate) (PDMS–PMMA) Latexes in Dodecane

Hard-sphere particles in nonpolar solvents are an essential tool for colloid scientists. Sterically stabilized poly(methyl methacrylate) (PMMA) particles have long been used as the exemplary hard-sphere system. However, neither the particles themselves nor the poly(12-hydroxystearic acid) (PHSA) stabilizer necessary to prevent aggregation in nonpolar solvents are commercially available. To counter this, several alternatives have been proposed. In recent years, there has been an increased interest in poly(dimethylsiloxane) (PDMS) stabilizers as a commercially available alternative to PHSA, yet the structure of particles made in this way is not as well understood as those produced using PHSA. In this work, we employ small-angle neutron scattering to determine the internal structure of PDMS-stabilized PMMA particles, synthesized with and without an additional crosslinking agent. We report data consistent with a homogeneous PMMA core with a linearly decaying PDMS shell. The thickness of the shell was in excess of 50 nm, thicker than the PHSA layer typically used to stabilize PMMA but consistent with reports of the layer thickness for similar molecular weight PDMS at planar surfaces. We also show that the amount of the hydrogenous material in the particle core of the crosslinked particles notably exceeds the amount of added ethylene glycol dimethacrylate crosslinker, suggesting some entrapment of the PDMS stabilizer in the PMMA matrix

Survival and Movement of Adult Rainbow Trout During Winter and Spring in the Henrys Fork of the Snake River

Discharge downstream from Island Park Dam on the Henrys Fork of the Snake River in Idaho is reduced each winter to facilitate storage of irrigation water. The effect this has on survival and movement of adult rainbow trout (Oncorhynchus mykiss) in this area is unknown. Additionally, fish movement during the spring has not been evaluated but may affect population estimates conducted in the tailwater monitoring area downstream from Island Park Dam prior to opening of fishing season. Therefore, we used radio telemetry to evaluate winter survival and movement of 61 adult rainbow trout in the Henrys Fork downstream from Island Park Dam under low and extremely low early winter flow conditions. Spring movement was also evaluated to asses whether the population estimates conducted in the monitoring area each spring represent fish from downstream adjacent reaches of the river, and how emigration between mark and recapture periods may affect the population estimate. Survival of radio-tagged trout was nearly 100 percent during early winter under both low and extremely low flow conditions and winter movement did not differ between the two years. Few radio-tagged rainbow trout from downriver were present in the monitoring reach during the time when the population estimate is normally conducted, indicating that large fluctuations in fish numbers in downstream reaches would likely be undetected based on population estimates conducted in the monitoring area. To remedy this, establishing a separate, regular population monitoring area in downstream reaches is recommended. We determined emigration from the monitoring reach between mark and recapture to have a minimal effect on the population estimate. However, we noted that all radio-tagged trout moving out of the monitoring reach during May moved into a short section of river between the monitoring reach and Island Park Dam, presumably to spawn. Therefore, emigration could be largely eliminated by extending the monitoring reach upstream to the dam

Fluorescence of thermal control coatings on S0069 and A0114

Many of the thermal control surfaces exposed to the space environment during the 5.8 year LDEF mission experienced changes in fluorescence. All of the thermal control coatings flown on LDEF experiments S0069 and A0114 were characterized for fluorescence under ambient conditions. Some of the black coatings, having protective overcoats, appear bright yellow under ultraviolet exposure. Urethane based coatings exhibited emission spectra shifts toward longer wavelengths in the visible range. Zinc oxide pigment based coatings experienced a quenching of fluorescence, while zinc orthotitanate pigment based and other ceramic type coatings had no measurable fluorescence