全文获取类型
收费全文 | 5349篇 |
免费 | 218篇 |
国内免费 | 222篇 |
专业分类
系统科学 | 240篇 |
丛书文集 | 191篇 |
教育与普及 | 154篇 |
理论与方法论 | 12篇 |
现状及发展 | 25篇 |
研究方法 | 12篇 |
综合类 | 5155篇 |
出版年
2024年 | 24篇 |
2023年 | 73篇 |
2022年 | 83篇 |
2021年 | 109篇 |
2020年 | 64篇 |
2019年 | 21篇 |
2018年 | 41篇 |
2017年 | 51篇 |
2016年 | 49篇 |
2015年 | 123篇 |
2014年 | 181篇 |
2013年 | 131篇 |
2012年 | 163篇 |
2011年 | 213篇 |
2010年 | 164篇 |
2009年 | 231篇 |
2008年 | 227篇 |
2007年 | 245篇 |
2006年 | 186篇 |
2005年 | 211篇 |
2004年 | 138篇 |
2003年 | 133篇 |
2002年 | 138篇 |
2001年 | 157篇 |
2000年 | 181篇 |
1999年 | 308篇 |
1998年 | 257篇 |
1997年 | 314篇 |
1996年 | 262篇 |
1995年 | 236篇 |
1994年 | 193篇 |
1993年 | 195篇 |
1992年 | 143篇 |
1991年 | 122篇 |
1990年 | 121篇 |
1989年 | 98篇 |
1988年 | 97篇 |
1987年 | 63篇 |
1986年 | 32篇 |
1985年 | 10篇 |
1984年 | 1篇 |
排序方式: 共有5789条查询结果,搜索用时 109 毫秒
681.
为了克服在晶场理论中利用微扰近似公式计算d~5(~6S)离子基态立方零场分裂参量的困难,本文用O_h 点群表象的强场方案,构造了d~5电子组态在六配位O_h 对称晶场中微扰哈密顿H′[=H_(el)+H_(?)+V_(CF)(O_h)]的完全能量矩阵.用它研究了d~5离子基态立方零场分裂参量与吸收光谱.我们发现,立方零场分裂参量a 对晶场参量Dq 的依赖关系应为:a(+|Dq|)>a(-|Dq|).由于Powell 等人的能量矩阵中Dq 和ξ_d 之间存在相对位相错误而导致零场分裂参量的计算困难,本文利用正确的d~5(O_h~(?))完全能量矩阵,统一地计算了几种六配位八面体O_h 的晶场中Fe~(3+)和Mn~(2+)离子的基态立方零场分裂和d-d跃迁吸收谱.数值分析中发现,旋轨耦合常数ξ_d 及晶场参量Dq 对立方零场分裂参量有重要影响,ξ_d 取值的差异正是Fe~(3+)和Mn~(2+)络离子立方零场分裂参量巨大差别的主要原因.由理论值与实验观测结果的一致性表明,利用正确的d~5(O_h~(?))完全能量矩阵分析,可以克服Low 等人断言的简单晶场理论计算的困难. 相似文献
682.
集成式液压系统油路块CAPP系统开发研究 总被引:2,自引:1,他引:1
在综合分析各种CAPP系统应用模式的特点及实际情况的基础上,提 出了液压集成油路块CAPP系统-JKCAPP的应用模式:以派生型为主、 检索型为辅、交互型为补充的实用型、多层次、综合应用模式.介绍了 JKCAPP系统的功能、特点、工作原理及各功能子模块.该成果已应用于液压 系统CAD/CAM一体化集成软件系统. 相似文献
683.
为了减少无线传感器网络节点的能量消耗,以延长无线传感器网络的寿命,首先研究了如何通过在无线传感器网络中构建小世界特性以延长其网络寿命,并着重分析了DAS(directedangulation tow ards the sink)方案.然后针对DAS方案中临近sink节点区域内的节点能量过快消耗的问题,提出了一种在无线传感器网络中构建小世界特性的新方案——菱形区域方案.该方案根据sensor节点到sink节点距离的大小,动态地改变菱形捷径端点搜索区域的大小,达到构建小世界特性并增强网络抗毁性的目的.理论证明了菱形区域方案在一定条件下较DAS方案既节省能量又能延长网络的寿命. 相似文献
684.
Hao HX Xie Y Zhang Y Charlat O Oster E Avello M Lei H Mickanin C Liu D Ruffner H Mao X Ma Q Zamponi R Bouwmeester T Finan PM Kirschner MW Porter JA Serluca FC Cong F 《Nature》2012,485(7397):195-200
R-spondin proteins strongly potentiate Wnt signalling and function as stem-cell growth factors. Despite the biological and therapeutic significance, the molecular mechanism of R-spondin action remains unclear. Here we show that the cell-surface transmembrane E3 ubiquitin ligase zinc and ring finger 3 (ZNRF3) and its homologue ring finger 43 (RNF43) are negative feedback regulators of Wnt signalling. ZNRF3 is associated with the Wnt receptor complex, and inhibits Wnt signalling by promoting the turnover of frizzled and LRP6. Inhibition of ZNRF3 enhances Wnt/β-catenin signalling and disrupts Wnt/planar cell polarity signalling in vivo. Notably, R-spondin mimics ZNRF3 inhibition by increasing the membrane level of Wnt receptors. Mechanistically, R-spondin interacts with the extracellular domain of ZNRF3 and induces the association between ZNRF3 and LGR4, which results in membrane clearance of ZNRF3. These data suggest that R-spondin enhances Wnt signalling by inhibiting ZNRF3. Our study provides new mechanistic insights into the regulation of Wnt receptor turnover, and reveals ZNRF3 as a tractable target for therapeutic exploration. 相似文献
685.
介绍了 LDO线性稳压器的系统组成原理,分析了系统的电源电压抑制比(PSRR)以及噪声与电路结构的关系,在此基础上,对LDO的核心电路模块进行了设计,并基于0.5μm标准CMOS工艺,运用Cadence平台进行了模拟仿真和验证.测试结果表明:该LDO的PSRR最低约为-45 dB@1 MHz,最高约为-75 dB@217 Hz;输出电压噪声在10 Hz频率以下约为0.78 μV(P-P),在10 Hz至l00 kHz频率范围内约为0.1 μV(RMS),能满足低噪声和高PSRR应用的要求. 相似文献
686.
Hydrodynamic process of Tibetan Plateau lake revealed by grain size: Case study of Pumayum Co 总被引:3,自引:0,他引:3
JianTing Ju LiPing Zhu JinLiang Feng JunBo Wang Yong Wang ManPing Xie Ping Peng XiaoLin Zhen XinMiao Lü 《科学通报(英文版)》2012,57(19):2433-2441
Under the background of global warming,some lakes on the Tibetan Plateau(TP) are potentially sensitive to temperature change.With a case study of Pumayum Co,where glacier meltwater is important to supply(we call this a glacier-fed lake hereafter),we analyze the sensitivity of lake sediment grain size to temperature change.This is done by resolving the modern hydrodynamic process,coupled with comparison of paleoclimatic proxies.According to the spatial distributions of parameters,percentage of grain size and the grain size frequency distribution curve,hydrodynamic processes are analyzed.Five clastic sedimentation types are thereby discriminated.In the open lake area,suspended load transport is the main transport agent.Grain Size Trend Analysis(GSTA),a sediment dynamics model,reveals a trend toward eastward transport.This indicates that the largest and glacier-fed river,the Jiaqu River,influences the entire lake(not just the subsurface alluvial fan),and that lake sediment grain size may serve as a temperature indicator.Time series comparison between grain size of a short core from the central lake and meteorological data confirms this temperature indication,which in turn shows reliability of the method discriminating the hydrodynamic process.This case study will improve the ability of paleoclimatic reconstruction using lake sediment in glacier-fed lakes on the Tibetan Plateau. 相似文献
687.
Estimation of the position and effect of sterility genes is an important problem to be solved to understand the sterility mechanism in remote hybridization in plants.In this study,a maximum likelihood (ML) method was used for estimation of the position and effect of sterility genes that exhibit epistasis in an F 2 population using the distorted segregation of markers.The ML solutions for recombination fraction and viability were obtained via an expectation-maximization algorithm.The results of Monte Carlo simulations showed that the estimates of recombination fraction and viability were consistent with their true values.The bias and standard deviation of parameters indicated that a larger sample size,closer linkage and lower viability of sterile genes led to better estimates of the parameters involved.A subset of marker data of the F 2 population derived from a single cross between the rice japonica cultivar Nipponbare and the indica cultivar Kasalath was analyzed using this method.Eight sterility genes were identified on chromosomes 1,3,6,8 and 10,and significant epistasis was detected among four pairs of sterility genes. 相似文献
688.
Eocene high grade metamorphism and crustal anatexis in the North Himalaya Gneiss Domes,Southern Tibet 总被引:13,自引:0,他引:13
Determination of the timing and geochemical nature of early metamorphic and anatectic events in the Himalayan orogen may provide key insights into the physical and chemical behavior of lower crustal materials during the early stage of tectonic evolution in large-scale collisional belts.The Yardoi gneiss dome is the easternmost dome of the North Himalayan Gneiss Domes(NHGD),and contains three types of amphibolites with distinct mineral assemblage,elemental and radiogenic isotope geochemistry,as well as various types of gneisses.SHRIMP zircon U/Pb analyses on the garnet amphibolite and garnet-bearing biotite granitic gneiss yield ages of nearly peak metamorphism at 45.0±1.0 Ma and 47.6±1.8 Ma,respectively,which are 2 to 4 Ma older than the age for partial melting in migmatitic garnet amphibolite(43.5±1.3 Ma).Available data have demonstrated that ultra-high pressure metamorphism in the Tethyan Himalaya occurred at ~55 Ma,and high amphibolite facies to granulite facies metamorphism at 45 to 47 Ma.In addition,partial melting at thickened crustal conditions occurred at 43.5±1.3 Ma,which led to the formation of high Sr/Y ratios two-mica granites.The high-grade metamorphic rocks in the NHGD may represent the subducted front of the Indian continental lithosphere.In large collisional belts,fertile components in crustal materials could melt and form granitic melts with relatively high Na/K and Sr/Y ratios under thickened crustal conditions,significantly different from those formed by decompressional melting during rapid exhumation. 相似文献
689.
Improvement of the energy density and power density of the lithium-ion batteries is
urgently required with the rapid development of electric vehicles and portable electronic devices.
The spinel LiMn2O4 is one of the most promising cathode materials due to its low cost, nontoxicity,
and improved safety compared with commercial LiCoO2. Developing nanostructured electrode
materials represents one of the most attractive strategies to dramatically enhance battery
performance, such as capacity, rate capability and cycling life. Currently, extensive efforts have
been devoted to developing nanostructured LiMn2O4 and LiMn2O4/carbon nanocomposites to
further improve the rate capability of lithium-ion batteries for high-power applications. In this
paper, recent progress in developing nanostructured LiMn2O4 and LiMn2O4/carbon nanocomposites
is reviewed, and the benefits to the electrochemical performance of LiMn2O4-based cathodes
by using these electrode materials are also discussed. 相似文献
690.
影子银行是金融深化和金融创新的产物,它的生存及发展与传统商业银行存在着密不可分的关系。在影子银行提供信用、期限及流动性转换服务过程中,其自身固有的不完善的信用风险转移和高杠杆率缺陷,对金融体系宏观微观环境产生了深刻影响。由于发展阶段的差异,影子银行发展对西藏地区金融发展具有一定的积极意义,可有效降低该地区"金融弱化"现象,需予以适度引导和审慎监管。 相似文献