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The BiVO4 additive was found effective for low-temperature firing of ZnNb2 O6 polycrystalline ceramics below 950°C in air without a serious degradation in their microwave dielectric properties. Dense BiVO4 -doped ZnNb2 O6 samples of a relative sintered density over 95% could be prepared even at 925°C. An optimally processed specimen exhibited excellent microwave dielectric properties of Q · f = 55000 GHz, ɛr = 26, and τf =−57 ppm/°C. With increasing BiVO4 addition up to 20 mol% relative to ZnNb2 O6 , while the quality factor Q · f was gradually decreased, the relative dielectric constant, ɛr , was linearly increased and the temperature coefficient of resonant frequency, τf , was slightly increased. The variations in Q · f and ɛr are surely attributable to the residual BiVO4 in the ZnNb2 O6 matrix. An unexpected slight increase in τf is probably due to the formation of the Bi(V,Nb)O4 -type solid solution. 相似文献
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采用称重法测得Cr18Ni30Mo2Al3Nb合金在不同温度下的高温氧化动力学曲线。结果表明,该合金的氧化曲线遵循抛物线氧化规律,具有优良的抗氧化性能。利用扫描电镜、X射线衍射的方法对氧化膜表面形貌及结构进行研究,该合金在3个温度下氧化膜完整致密,700℃氧化膜主要由Fe和Cr的混合氧化物(Fe0.6Cr0.4)2O3和少量Al的氧化物组成;800℃氧化膜主要是Al和Cr的混合氧化物(Al0.9Cr0.1)2O3和少量Al2O3及少量Fe的氧化物;900℃氧化膜主要是(Al0.9Cr0.1)2O3和Al的氧化物,还含有少量Fe(Cr,Al)2O4和MnFe2O4。 相似文献
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Lamellar boundary alignment of DS-processed TiAl−W alloys by a solidification procedure 总被引:1,自引:0,他引:1
In-Soo Jung Myung-Hoon Oh No-Jin Park K. Sharvan Kumar Dang-Moon Wee 《Metals and Materials International》2007,13(6):455-462
In this study, a β solidification procedure was used to align the lamellae in a Ti-47Al-2W (at.%) alloy parallel to the growth
direction. The Bridgman technique and the floating zone process were used for directional solidification. The mechanical properties
of the directionally solidified alloy were evaluated in tension at room temperature and at 800°C. At a growth rate of 30 mm/h
(with the floating zone approach), the lamellae were well aligned parallel to the growth direction. The aligned lamellae yielded
excellent room temperature tensile ductility. The tensile yield strength at 800°C was similar to that at room temperature.
The orientation of the γ lamellar laths in the directionally solidified ingots, which were manufactured by means of a floating
zone process, was identified with the aid of electron backscattered diffraction analysis. On the basis of this analysis, the
preferred growth direction of the bcc-β dendrites that formed at high temperatures close to the melting point was inferred
to be [001]β at a growth rate of 30 mm/h and [111]β at a growth rate of 90 mm/h. 相似文献
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Segment-based streaming media proxy: modeling and optimization 总被引:2,自引:0,他引:2
Researchers often use segment-based proxy caching strategies to deliver streaming media by partially caching media objects. The existing strategies mainly consider increasing the byte hit ratio and/or reducing the client perceived startup latency (denoted by the metric delayed startup ratio). However, these efforts do not guarantee continuous media delivery because the to-be-viewed object segments may not be cached in the proxy when they are demanded. The potential consequence is playback jitter at the client side due to proxy delay in fetching the uncached segments, which we call proxy jitter. Thus, for the best interests of clients, a correct model for streaming proxy system design should aim to minimize proxy jitter subject to reducing the delayed startup ratio and increasing the byte hit ratio. However, we have observed two major pairs of conflicting interests inherent in this model: (1) one between improving the byte hit ratio and reducing proxy jitter, and (2) the other between improving the byte hit ratio and reducing the delayed startup ratio. In this study, we first propose and analyze prefetching methods for in-time prefetching of uncached segments, which provides insights into the first pair of conflicting interests. Second, to address the second pair of the conflicting interests, we build a general model to analyze the performance tradeoff between the second pair of conflicting performance objectives. Finally, considering our main objective of minimizing proxy jitter and optimizing the two tradeoffs, we propose a new streaming proxy system called Hyper Proxy. Synthetic and real workloads are used to evaluate our system. The performance results show that Hyper Proxy generates minimum proxy jitter with a low delayed startup ratio and a small decrease of byte hit ratio compared with existing schemes. 相似文献
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