共查询到20条相似文献,搜索用时 31 毫秒
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目的提高靶场毫米波海上目标RCS测量标定精度。方法针对海上目标的外场雷达目标特性(Radar Cross Section,RCS)测量系统动态标定问题,在给出了无源相对标校法的基础上,对毫米波RCS测量系统标定过程中云雾杂波、大气衰减、海面及地面杂波的影响进行详细分析。结果结合靶场外场实际测量,提出了衰减修正、合理选择标校气象条件等相应减小标定误差的措施。结论基于毫米波海上目标RCS测量标定的分析及其措施可极大地减小毫米波外场海面测量标校中气象环境、海面或地面杂波等对系统标定的影响,提高系统标定精度。 相似文献
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Dressel M 《Die Naturwissenschaften》2007,94(7):527-541
Low-dimensional organic conductors could establish themselves as model systems for the investigation of the physics in reduced
dimensions. In the metallic state of a one-dimensional solid, Fermi liquid theory breaks down and spin and charge degrees
of freedom become separated. But the metallic phase is not stable in one dimension: as the temperature is reduced, the electronic
charge and spin tend to arrange themselves in an ordered fashion due to strong correlations. The competition of the different
interactions is responsible for which broken-symmetry ground state is eventually realized in a specific compound and which
drives the system toward an insulating state. Here, we review the various ordering phenomena and how they can be identified
by optic and magnetic measurements. While the final results might look very similar in the case of a charge density wave and
a charge-ordered metal, for instance, the physical cause is completely different. When density waves form, a gap opens in
the density of states at the Fermi energy due to nesting of the one-dimension Fermi surface sheets. When a one-dimensional
metal becomes a charge-ordered Mott insulator, on the other hand, the short-range Coulomb repulsion localizes the charge on
the lattice sites and even causes certain charge patterns. We try to point out the similarities and conceptional differences
of these phenomena and give an example for each of them. Particular emphasis will be put on collective phenomena that are
inherently present as soon as ordering breaks the symmetry of the system. 相似文献
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Daniel Teixidor Thanongsak Thepsonthi Joaquim Ciurana Tugrul Özel 《Journal of Manufacturing Processes》2012,14(4):435-442
This paper reports an investigation into the effects of nanosecond laser processing parameters on the geometry of microchannels fabricated from polymethylmethacrylate (PMMA). The Nd:YAG solid-state pulsed laser has a wavelength of 1064 nm and a measured maximum power of 4.15 W. The laser processing parameters are varied in a scanning speed range of 400–800 pulses/mm, a pulse frequency range of 5–11 Hz, a Q-switch delay time range of 170–180 μs. Main effects plots and microchannel images are utilized to identify the effects of the process parameters for improving material removal rate and surface quality simultaneously for laser micromachining of microchannels in PMMA polymer. It is observed that channel width and depth decreased linearly with increasing Q-switch delay time (hence average power) and increased non-linearly with higher scanning rates and not much affected by the increase in pulse frequency. 相似文献
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铊,铊中毒及铊在生态系中迁移径迹 总被引:1,自引:1,他引:0
铊属于分散元素,独立矿物稀少,极难形成独立铊矿床。铊世界年产量约15吨,几乎都是从有色金属选冶产品中作为副产品顺便回收。我国铊资源比较丰富,按相同等级资源相比,我国铊储量居世界首位。铊的地球化学性质受其电子构型和地质地球化学作用制约,铊原子处于基态时的电子构型为6S26P1。铊有两个地球化学价态,正一价和正三价,在自然界多数呈正一价。铊具有低温成矿,亲硫和高温分散,亲石的双重地球化学性质。铊是有用元素,也是有毒元素。铊被广泛用于超导、电子、合金、光学、化工、玻璃和医药等工业。铊的毒性对哺乳动物比汞、隔、铅、铜、锌还强。铊对生物有毒害,高含量的铊对人体可导致不同程度的铊病,甚至死亡。由于铊矿床的开发利用,使采挖出的含铊岩矿石和冶炼矿渣堆积地表,在长期风化淋滤作用下,使释放的铊进入地表水体、土壤、植物、动物和人体生态链,故引起铊环境污染和铊中毒。 相似文献
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目的研究提高飞行器结构地面试验有效性的途径。方法计算同一被试件结构在飞行状态和地面试验状态下的有限元模型,测量地面试验状态下的模态以验证有限元模型的正确性;计算各特征点(也可以是遥测点)在天地状态下的响应,用机器学习法获取各特征点的映射关系模型;基于该模型由飞行点响应(或遥测数据)确定出地面试验件对应点的响应,并用载荷反求法得到它们的等效载荷;最终确定施加在试验系统上的载荷。结果以细长体结构为例,所得到由其组成试验系统的有限元模型与实测模型之间的固有频率最大相对误差为6.76%,利用映射关系模型预测出对应点在飞行状态下的振动响应。确定了飞行状态下结构响应的特征点,由地面试验系统所对应的响应点反推出应施加的载荷为60 N。结论利用天地数值计算-地面试验验证联合法,无需在地面试验状态下刻意模拟飞行状态的边界条件,确定出所需要施加的载荷,从而提高了飞行器地面试验的有效性。 相似文献
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Mechanism of NO reduction with non-thermal plasma 总被引:4,自引:2,他引:2
Non-thermal plasma has been proved to be an effective and competitive technology for removing NO in flue gas since 1970. In this paper, the NO reduction mechanism of the non-thermal plasma reaction in NO/N2/O2 system was investigated using the method of spectral analysis and quantum chemistry. By the establishment of NO reduction and gas discharge plasma emission spectrum measuring system, the NO reduction results, gas discharge emission spectra of NO/N2/O2 and pure N2 were obtained, and then the model of molecular orbit of N2 either in ground state or its excited state was worked out using the method of molecular orbit Ab initio in SelfConsistent Field(SCF). It was found that NO reduction in NO/N2 gas discharge plasma was achieved mainly through a series of fast elementary reactions and the N(E6) at excited state was the base for NO reduction. 相似文献
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High-aspect-ratio microholes have many industrial applications, but are difficult to produce. Femtosecond (fs) and nanosecond (ns) laser ablation may produce potential manufacturing solutions. However, the laser-induced plasma–microhole sidewall interaction has not been well understood for laser ablation of deep microholes, which may significantly affect the hole size and/or quality. This interaction has been investigated in this paper. Due to the huge challenges involved in direct experimental observations, a physics-based model is applied as the research tool, which has been verified by measurements from literatures on laser ablation of flat targets (without deep microholes) that are relatively easy to perform. The study shows that under the same laser pulse fluence, the fs laser-induced plasma generates larger transient peak heat flux to the sidewall than the ns laser pulse. However, the high-heat-flux region moves up very rapidly in the hole, and hence the sidewall temperature is not significantly raised and sidewall melting does not occur under the studied conditions. On the other hand, for the ns laser pulse, the induced plasma maintains a relatively high heat flux to the sidewall near the hole bottom for a much longer time, which yields obvious sidewall melting and surface vaporization. The results are consistent with the previously observed sidewall surface morphology for microholes drilled by fs and ns laser pulses in air with the same pulse energy. 相似文献
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目的改进现阶段地面指控站对临近空间高超声速飞行器的通信和测控效果。方法采用计算流体动力学技术模拟钝锥体在不同高超声速飞行条件下的头身部绕流流场特性,并运用相关公式建立钝锥等离子体鞘套电磁模型。然后运用改进的移位算子时域有限差分方法计算电磁波经过等离子体鞘套的传播特性。结果计算结果表明,电磁波在等离子体鞘套内的传播特性在频率处于最大等离子体频率附近时发生转折,电磁波通过等离子体鞘套的透射效果与飞行工况和入射波条件密切相关。相对其他入射角度,当电磁波垂直于飞行体物面入射到等离子体鞘套尾部时的电磁波衰减较小。飞行高度增加或飞行速度减小时,等离子体鞘套对电磁波的衰减效应将减弱。结论该研究可为有效测控临近空间高超声速飞行器、缓解再入通讯中断现象提供一定的技术突破方法。 相似文献
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目的 提高脉冲激光发射模块的电磁兼容性。方法 基于腔体的电磁屏蔽机理,使用HFSS软件,建立脉冲激光发射模块腔体模型,以电磁敏感度、电磁干扰2个方面对腔体电磁屏蔽效能进行仿真分析。结果 设计的双层屏蔽腔体在电磁波辐照频率为1~100 kHz时,屏蔽效能达到28 dB;电磁波辐照频率为0.2~18 GHz时,屏蔽效能达到47 dB。结论 当电磁波辐照频率在1~100 kHz时,腔体的屏蔽效能随频率的增加而增大。辐射源外部激励时,双层屏蔽腔体使用外层铁内层铜屏蔽效能较高。电磁波辐照频率在0.2~18 GHz时,腔体的屏蔽效能随频率的增大而减小,且发生了谐振效应。当腔体厚度相等时,双层屏蔽的屏蔽效能高于单层屏蔽。使用电缆连接器代替孔洞进行信号传输时,腔体屏蔽效能增加。 相似文献
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依埋深波源在弹性半空间的传播原理,提出了地铁波源引起的地面振动实用计算公式和传递系数。对地铁波源的P波、S波引起的地面面波叠加,得出地面振动计算公式。实用计算公式与数个地铁工程运行所致的地面振动现场实测作了比较,能满足工程精度。 相似文献
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同步辐射XRF和XANES研究重金属污染环境中小羽藓体内硫元素的生物指示作用 总被引:3,自引:1,他引:2
在实验室将小羽藓(Haplocladium)暴露于不同浓度的铅、铁、铬重金属环境下进行培育,分别应用同步辐射X射线荧光(SRXRF)方法测定小羽藓植株硫元素的含量和X射线吸收近边结构(XANES)分析不同价态的硫所占的相对含量.结果表明,暴露于铅、铁下的小羽藓植株内硫的含量明显增加,铅、铁浓度分别为400 mg/L和200 mg/L时,硫元素含量下降.培养周期为15 d时,小羽藓在100 mg/L铅胁迫下,低价硫由对照组的17.8%升高到23.6%,而同时以硫酸盐形式存在的硫由对照组的56.3%下降到51.2%.在400 mg/L铅胁迫时,低价硫含量增加到24.8%,硫酸盐中的硫所占的比例下降到48.4%.小羽藓植株内的胱氨酸、半胱氨酸、甲硫氨酸和谷胱甘肽中所含低价态硫的总相对含量增加,以硫酸盐态存在的硫相对含量明显下降.研究表明,重金属污染环境导致小羽藓硫吸收同化过程中硫元素含量和价态变化特征具有一定的生物指示作用. 相似文献
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《Atmospheric Environment. Part A. General Topics》1992,26(9):1635-1642
An aerial spray Agricultural Dispersal (AGDISP) model was tested against quantitative field data. The microbial pesticide Bacillus thuringiensis (Bt) was sprayed as fine spray from a helicopted over a flat site in various meteorological conditions. Droplet deposition on evenly spaced Kromekote cards, 0.15 m above the ground, was measured with image analysis equipment. Six complete data sets out of the 12 trials were selected for data comparison. A set of statistical parameters suggested by the American Meteorological Society and other authors was applied for comparisons of the model prediction with the ground deposit data. The results indicated that AGDISP tended to overpredict the average volume deposition by a factor of two. The sensitivity test of the AGDISP model to the input wind direction showed that the model may not be sensitive to variations in wind direction within 10 degrees relative to aircraft flight path. 相似文献
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建立了纤维素滤膜采样、超声波提取、二苯碳酰二肼分光光度法测定环境空气PM2.5中六价铬的方法.采样前滤膜用硝酸溶液净化以去除空白,碳酸氢钠溶液浸渍以降低六价铬的转化.稳定性试验表明常温下样品保存期不超过2天,冷冻条件下可保存10天.样品用碳酸氢钠溶液和超声波提取优于热水提取.浓度低于1 mg/L的铁、镁、钼和钒不干扰六价铬分析.方法简单灵敏,检出限0.1ng/m3,实际样品平行测量相对偏差为5.9%~18%,加标回收率为76%~92%,可应用于环境空气中六价铬的测定. 相似文献
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Roland Fuß Bernhard RuthRolf Schilling Hagen ScherbJean Charles Munch 《Agriculture, ecosystems & environment》2011,144(1):61-68
Nitrous oxide (N2O) emissions from soil are characterized by strong emission pulses. Although several mechanisms are known to create them, pulses are difficult to predict. Currently there is no established systematic way to identify pulses from long-term static chamber measurement results. In this study we suggest a simple algorithm for pulse identification. The algorithm was applied on time series of N2O and carbon dioxide (CO2) fluxes from a field study on the long-term impact of fertilization and tillage practice. Between 4 and 9% of N2O values were pulse values; 20-60% of total emission was emitted as pulses. Minimum tillage resulted in more pulses than plowing. In contrast, long-term averages of N2O losses from nitrogen (N) fertilizer were similar (3-4%) for all management practices. N2O emissions per crop yield for increased fertilization practice were double the values for reduced fertilization practice independent of tillage practice. CO2 emission pulses were scarce and there was no significant effect of management practice on CO2 pulse probability. 相似文献