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91.
目的 设计针对L波段射频前端敏感器件的强电磁脉冲防护模块,利用瞬态电压抑制二极管、气体放电管和发夹型微带带通滤波器进行联合仿真设计。方法 瞬态电压抑制二极管具有快速响应时间,气体放电管具有高功率容量,而微带带通滤波器可分离噪声信号,并保留有效信号。通过结合这些器件进行防护设计,可以充分发挥各自优势,提高系统稳定性和强电磁脉冲防护能力。结果 该模块设计工作频段为1.3~ 1.7 GHz,带内插入损耗小于1.5 dB,在70 dBm功率注入的情况下,防护效果可以达到42.5 dB,具有良好的防护效果。结论 通过分析研究不同器件的特性和优化设计,实现了对L波段频谱的防护,具有重要的实用价值和广阔的应用前景。 相似文献
92.
板式橡胶支座作为海蚀环境中桥梁结构体系中的重要支撑构件,其摩擦滑移与橡胶老化会影响桥梁结构整体的抗震性能。为了探究海蚀环境下考虑摩擦滑移的桥梁板式橡胶支座老化剪切性能,开展了板式橡胶支座的摩擦滑移试验,并采用有限元方法,分析了橡胶老化对各形状系数的板式橡胶支座的剪切性能和摩擦滑移的影响。 结果表明:①支座的摩擦滑移起始位移随着老化时间的增长和形状系数的增大而不断变大;②随着老化时间的增加,支座通过摩擦滑移消耗的能量也会逐渐增多;③支座的等效黏滞阻尼比随着老化时间和形状系数的增加而变大。海蚀环境会加速板式橡胶支座的老化并影响其剪切性能,因此在海蚀较为严重的地区进行桥梁抗震设计时应考虑支座摩擦滑移及老化对桥梁地震动力响应的影响。 相似文献
93.
Nadiah P. Kristensen Wei Wei Seah Kwek Yan Chong Yi Shuen Yeoh Tak Fung Laura M. Berman Hui Zhen Tan Ryan A. Chisholm 《Conservation biology》2020,34(5):1229-1240
Extinction is a key issue in the assessment of global biodiversity. However, many extinction rate measures do not account for species that went extinct before they could be discovered. The highly developed island city–state of Singapore has one of the best-documented tropical floras in the world. This allowed us to estimate the total rate of floristic extinctions in Singapore since 1822 after accounting for sampling effort and crypto extinctions by collating herbaria records. Our database comprised 34,224 specimens from 2076 native species, of which 464 species (22%) were considered nationally extinct. We assumed that undiscovered species had the same annual per-species extinction rates as discovered species and that no undiscovered species remained extant. With classical and Bayesian algorithms, we estimated that 304 (95% confidence interval, 213–414) and 412 (95% credible interval, 313–534) additional species went extinct before they could be discovered, respectively; corresponding total extinction rate estimates were 32% and 35% (range 30–38%). We detected violations of our 2 assumptions that could cause our extinction estimates, particularly the absolute numbers, to be biased downward. Thus, our estimates should be treated as lower bounds. Our results illustrate the possible magnitudes of plant extirpations that can be expected in the tropics as development continues. 相似文献
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96.
We report a child with a subarachnoid cyst with hydrocephalus following a mid-trimester amniocentesis. Although fetal trauma is a rare complication, it is important to stress the need to perform amniocentesis under ultrasound control. Children with neurological disease or convulsions of unknown origin in early infancy who are born to mothers who have had midtrimester amniocentesis should have a CT scan as part of the investigations to exclude this rare but nevertheless important complication. 相似文献
97.
水土化学体系中钙镁对土体结构强度贡献的试验研究 总被引:5,自引:0,他引:5
城市区域的土体环境一般是处在常态即常温、常压、缓变异的水化学环境中,在这种环境中,影响土体稳定的要素-土体中的化学组份扮演着重要的角色,而其对诸如地面沉降、土洞的产生都有重要影响。土体稳定受其结构强度控制,钙镁是土体结构构成中重要组成部分,理论与实践分析表明,在常态水环境下,其对结构往往起着控制作用,在常态水环境中,土结构的破裂面并不是通过颗粒本身,而是通过颗粒、颗粒集合体与集合体之间粒间连结。设计的常态水环境下的化学组份变异与CO2分压增高水环境下的室内模拟试验,揭示了化学组份钙镁对土体结构的贡献与对宏观强度的作用。 相似文献
98.
土地利用改造规划的多因子空间分析 总被引:4,自引:0,他引:4
以福清市土地利用分布为例 ,在完成土地利用状况和土地适宜性参评因子空间数据库基础上 ,集成土地适宜性综合指数模型和地块紧凑度模型 ,开发了基于地理信息系统的土地适宜性评价和土地利用控制决策支持系统。通过土地适宜性多因子空间分析 ,模拟大农业 (耕地、林地、园地 )各目标用地条件下各地块土地使用适宜度 ,并结合土地利用现状 ,得到单目标土地改造方案 ;再经模型优化和综合比较得出多目标土地利用改造规划可选方案。计算结果表明 ,福清市土地资源中开发改造为新耕地的潜力极有限 ,而用于开发改造新林地和园地的土地尚有226 80km2 和36 34km2 ,为福清市土地利用改造规划和总体规划的制定和调整提供了定量依据。 相似文献
99.
The consumption of heavy metals is detrimental to human health and most countries restrict the concentration of metals such as lead (Pb) in food and beverages. Recent tests have detected high Pb concentrations in certain commercial brands of tea leaves and this finding has raised concerns for both producers and consumers. To investigate what factors may be contributing to the increase in Pb accumulation in the tea leaves we collected tea leaves and soils from tea producing areas and analyzed them for Pb concentration, pH and organic matter content. The result showed the Pb concentration of 47% investigated tea leaves samples was beyond 2 mg kg(-1), the permissible levels given by China. The total Pb concentration in the surface and subsurface soil layers averaged 36.4 and 32.2 mg kg(-1), respectively which fall below of the 60 mg kg(-1) limit provided for organic tea gardens in China. The pH of the tea garden soils was severely acidic with the lowest pH of 3.37. Soils under older tea gardens tended to have a lower pH and a higher Pb bioavailability which was defined as the amount of lead extracted by CaCl2 solution than those under younger tea gardens. We found that the concentration of bioavailable Pb and the percentage of bioavailable Pb (bioavailable Pb relative to total Pb concentration) were positively correlated with soil H+ activity and soil organic matter content, and the organic matter accumulation contribute more effects on Pb bioavailability in these two factors. We conclude that soil acidification and organic matter accumulation could contribute to increasing Pb bioavailability in soil and that these could increase Pb uptake and accumulation in the tea leaves. 相似文献
100.
An unconventional approach to studying the reaction kinetics of the Fenton's oxidation of 2,4-dichlorophenoxyacetic acid 总被引:1,自引:0,他引:1
The Fenton's oxidation kinetics of herbicide 2,4-D at various [Fe(II)] and [H(2)O(2)] combinations was investigated and modelled through an unconventional approach. The reaction kinetics of 2,4-D degradation demonstrated a two-stage pattern of decay, where a very fast reactive stage was followed by a retardation stage due to the depletion of oxidants and to the competitive side-reactions of the intermediates (including 2,4-dichlorophenol, chlorohydroquinone and 2,6-dichlororesorcinol). A model characterized by two newly established constants, the initial decay rate and the maximum oxidative capacity, was proposed and proven capable of describing the two-stage process, which cannot easily be described by conventional first- or second-order kinetics approaches. 相似文献