Metal matrix composites composed of high-purity aluminum and Du Pont PRD-166 continuous zirconia-stabilized polycrystalline
alumina fibers are fabricated by liquid metal infiltration using three different casting procedures. The microstructure of
the composites is analyzed using optical and electron microscopy, including analytical electron microscopy. It is found that
discrete faceted particles of ZrAl3 form at the interface and grow into the matrix of samples processed above the melting point of the matrix for 13 minutes
or more. The formation of this compound is in agreement with thermodynamic stability calculations. It is also found that there
is a reaction between solid aluminum and the fibers at 913 K, yielding a reaction product which has the same morphology as
that observed with molten aluminum. When the fibers are infiltrated with an initial preform temperature below the metal melting
point and a solidifination time below 1 minute, no reaction products were visible in the composite using the scanning electron
microscope (SEM). This leads to the conclusion that aluminum matrix composites can be cast with no apparent interfacial reaction
product using these fibers provided that adequate processing parameters are chosen. 相似文献
The production of willow in short rotation is expected to result in low nitrate leaching losses as is the case with other permanent crops. However, there is a risk of leaching of nitrate during establishment when the plant cover is limited. Nitrate leaching was followed for three years from the establishment in 1993 of willow (Salix viminalis) at two sites, a coarse sand at Jyndevad and a loamy sand at Foulum. Two levels of nitrogen, 0 or 75 kg N/ha in NPK, were applied annually, though only 38 N/ha was applied in the first year.
Leaching was calculated from usually weekly measurements of the nitrate concentration in soil solution and relating with the amount of percolation based on measured precipitation and changes in the soil water content over the sampling periods. Mean leaching for the three periods 1993–94, 1994–95 and 1995–96 amounted to respectively 142, 61 and 0 kg N/ha at Foulum and to 130, 9 and 4 kg N/ha at Jyndevad. The high leaching in the first year was caused by an unusually high content of mineral N in the soil at the start of the experiment. The absence of any leaching 1995–96 at Foulum was due to very low precipitation and a consequent lack of percolation. For the first period (1993–94) the fertilized treatment increased leaching by 32 kg N/ha compared to no fertilizer as a mean of the two sites, while the difference for the following two periods was reduced to 1–2 kg N/ha. It is concluded that application of nitrogen should be avoided in the year of planting of willows, while in the following years75 kg N/ha can be given without risk of increased leaching. 相似文献
This article demonstrates that carrier concentrations in bismuth telluride films can be controlled through annealing in controlled
vapor pressures of tellurium. For the bismuth telluride source with a small excess of tellurium, all the films reached a steady
state carrier concentration of 4 × 1019 carriers/cm3 with Seebeck coefficients of −170 μV K−1. For temperatures below 300°C and for film thicknesses of 0.4 μm or less, the rate-limiting step in reaching a steady state for the carrier concentration appeared to be the mass transport
of tellurium through the gas phase. At higher temperatures, with the resulting higher pressures of tellurium or for thicker
films, it was expected that mass transport through the solid would become rate limiting. The mobility also changed with annealing,
but at a rate different from that of the carrier concentration, perhaps as a consequence of the non-equilibrium concentration
of defects trapped in the films studied by the low temperature synthesis approach. 相似文献
Young’s modulus of particulate metal matrix composites with volume fractions of reinforcement ranging from 40% to 60% is measured with the Impulse Excitation Technique. Results are compared with predictions of four common micromechanical mean-field models. Good agreement is obtained with the Torquato identical hard spheres (TIHS) and the generalized self-consistent (GSC) model. 相似文献
Solution-reprecipitation during liquid phase sintering can lead to gravity-aligned gradients in the amount of refractory phase
as a result of the interaction between gravitational forces and capillary forces. We provide an anlaysis of this mechanism
for gradient formation and show that for most important engineering materials, solution-reprecipitation does not cause substantial
gravity-induced gradients. This conclusion is in agreement with published data for tungsten heavy alloy materials containing
volume fractions of solids greater than about 0.7 at the sintering temperature. Macrostructural gradients in liquid phase
sintered materials have been reported in the literature; however, these materials contain sufficient liquid at the sintering
temperature that solid grains settle within the liquid, perhaps contributing to the observed gradients. 相似文献
Infiltration by a pure matrix in the presence of preform deformation and partial matrix solidification is analyzed using a
bounding approach for the preform rheology where solid metal is present. It is found, using parameters for the infiltration
of short alumina fiber preforms by aluminum, that the two bounds are close in comparison to other factors of uncertainty in
the prediction of infiltration rate. Using this approach, preform compression is shown to exert a significant influence on
the infiltration rate for the system explored; in particular, the analysis shows the existence of an optimal value of applied
pressure. Simplifications in the analysis are also presented, which yield fairly accurate results while easing their computation
significantly. 相似文献
Serotonin (5-hydroxytryptamine, 5-HT) may play an important role in the pathogenesis of schizophrenia. Previous studies suggested that the efficacy of atypical neuroleptic drugs (e.g., risperidone and clozapine) on negative symptoms may be related to the 5-HT2a receptor. Although association studies between MspI polymorphism (T102C) and the 5-HT2a receptor gene and schizophrenia have been reported, their results are still controversial. The aim of this study was to examine the association between T102C polymorphism of the 5-HT2a receptor gene and schizophrenia as well as the association between the polymorphism and negative symptoms in a Japanese population (106 patients with schizophrenia and 109 healthy controls). No significant positive associations were observed. Our results suggest that the 5-HT2a receptor gene is not involved in the pathogenesis of schizophrenia or negative symptoms. 相似文献
Aligned nickel wire bundles 4 to 5 cm in length and up to 2.5 cm in diameter are infiltrated by pure aluminum using gas-assisted
infiltration, to produce samples having global compositions in the range of 40 to 76 at. pct nickel. As-reacted sample macrostructures
feature a large single-phase region of NiAl independent of applied pressure and nickel preform volume fractions in the ranges
of 0.15 to 0.3 MPa and 0.23 to 0.44, respectively. These macrostructures also contain, in some cases, either an aluminum-rich
zone on top, or nickel-rich phases at the bottom of the sample, depending on the global stoichiometry of the material. Two
high-speed digital cameras are used to record the infiltration and reaction processes. The acquired images show that infiltration
and reaction are decoupled in time. Infiltration proceeds initially under partial preform saturation, at a rate on the order
of 50 cm·s−1. The sample then self-heats for 2 to 3 seconds due to exothermic reaction between aluminum and nickel. Thereafter, a spontaneous
combustion front ignites due to local melting of nickel and propagates at an average rate on the order of 5 cm·s−1. Macrostructures and process kinetics are explained by the combined influences of capillarity during infiltration, heat evolved
during the reaction, and gravity. 相似文献