首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3800篇
  免费   117篇
  国内免费   209篇
系统科学   118篇
丛书文集   112篇
教育与普及   131篇
理论与方法论   22篇
现状及发展   20篇
研究方法   6篇
综合类   3717篇
  2024年   17篇
  2023年   34篇
  2022年   58篇
  2021年   83篇
  2020年   38篇
  2019年   12篇
  2018年   20篇
  2017年   32篇
  2016年   44篇
  2015年   99篇
  2014年   110篇
  2013年   126篇
  2012年   120篇
  2011年   146篇
  2010年   156篇
  2009年   181篇
  2008年   197篇
  2007年   152篇
  2006年   128篇
  2005年   138篇
  2004年   103篇
  2003年   141篇
  2002年   181篇
  2001年   186篇
  2000年   141篇
  1999年   202篇
  1998年   170篇
  1997年   154篇
  1996年   140篇
  1995年   139篇
  1994年   141篇
  1993年   97篇
  1992年   90篇
  1991年   76篇
  1990年   75篇
  1989年   65篇
  1988年   53篇
  1987年   40篇
  1986年   20篇
  1985年   9篇
  1984年   2篇
  1981年   1篇
  1979年   2篇
  1975年   1篇
  1969年   1篇
  1968年   2篇
  1966年   1篇
  1965年   1篇
  1963年   1篇
排序方式: 共有4126条查询结果,搜索用时 15 毫秒
101.
采用气相色谱-质谱联用技术,首次对木果楝叶子和果实挥发油进行了成分分析,从木果楝叶子和果实分别鉴定出了33和36种化合物,分别占挥发油的99.11%和97.63%.两个部位中挥发油化学组成各有异同,但其主要成分均是倍半萜类化合物b-桉叶烯和γ-桉叶烯.本研究结果可为木果楝的综合开发利用提供理论依据.  相似文献   
102.
本文通过对植物学科2003、2004年度自由申请项目的同行评议结果随机抽样进行统计处理,并结合2005、2006年度3位专家的实际评议情况,初步探讨了3位专家评议结果与5位专家评议结果对比的变化规律及植物学科今后如何针对3位专家评议后的结果进行" X"(1或2位专家)选择的具体做法。  相似文献   
103.
分析了国内外许多学者近年来对不同国家或地区股票市场波动规律的研究,提出了股票市场存在普适性,并论述了普适性在收益的概率分布、波动率的变化规律和收益的多重分形分布中的存在,为进一步研究股票市场波动的动力学规律提供了依据.  相似文献   
104.
Novel bifunctional terbium complex-based nanoparticles were developed using a modified Stber method and a layer-by-layer assembly process. A magnetic core of Fe3O4 nanoparticles was coated with a silica shell to form the first layer. Then a ternary Tb3+ complex (TESPPA-Tb), which acted as a luminescent marker, was covalently bound to the silica surface by stable Si-O-Si bonds. The TESPPA monomer was synthesized by binding pyridine 2,6-dicarboxylic acid to 3-aminopropyltriethoxysilane, which was used as a ligand for coordination with the Tb3+ ions. An outer shell of silica was applied to the nanoparticles to allow for versatility with surface functionalization. The nanoparticles were characterized by X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, ultravioletvisible spectroscopy, vibration sample magnetometer, and photoluminescence spectroscopy. The bifunctional nanoparticles exhibited favorable superparamagnetic behavior and photoluminescence properties of Tb3+. These nanoparticles have potential applications in biolabeling, bioseparation, immunoassays, and pathogenic diagnosis.  相似文献   
105.
In an electromagnetic field, the morphology of a binary faceted-faceted (FF) Ni31Si12-Ni2Si eutectic microstructure and the alloy’s mechanical properties were investigated. Hardness experiments demonstrated that the solidified ingots were significantly strengthened, and the hardness was improved to 63.1 and 786.6 on the Rockwell hardness C and Vickers hardness scales, respectively. Tests of friction and wear in stirred FF eutectic alloys showed excellent anti-fatigue and anti-adhesion wear performance. Alloy changed from an anomalous microstructure to a refined quasi-regular structure, and there was an increase in the lamellar microstructure fraction. The formation process of the refined quasi-regular microstructure and the resulting mechanical properties were investigated.  相似文献   
106.
Drug resistance presents a challenge to the treatment of cancer patients. Many studies have focused on cell-autonomous mechanisms of drug resistance. By contrast, we proposed that the tumour micro-environment confers innate resistance to therapy. Here we developed a co-culture system to systematically assay the ability of 23 stromal cell types to influence the innate resistance of 45 cancer cell lines to 35 anticancer drugs. We found that stroma-mediated resistance is common, particularly to targeted agents. We characterized further the stroma-mediated resistance of BRAF-mutant melanoma to RAF inhibitors because most patients with this type of cancer show some degree of innate resistance. Proteomic analysis showed that stromal cell secretion of hepatocyte growth factor (HGF) resulted in activation of the HGF receptor MET, reactivation of the mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3-OH kinase (PI(3)K)-AKT signalling pathways, and immediate resistance to RAF inhibition. Immunohistochemistry experiments confirmed stromal cell expression of HGF in patients with BRAF-mutant melanoma and showed a significant correlation between HGF expression by stromal cells and innate resistance to RAF inhibitor treatment. Dual inhibition of RAF and either HGF or MET resulted in reversal of drug resistance, suggesting RAF plus HGF or MET inhibitory combination therapy as a potential therapeutic strategy for BRAF-mutant melanoma. A similar resistance mechanism was uncovered in a subset of BRAF-mutant colorectal and glioblastoma cell lines. More generally, this study indicates that the systematic dissection of interactions between tumours and their micro-environment can uncover important mechanisms underlying drug resistance.  相似文献   
107.
戈塘矿区层滑构造研究   总被引:7,自引:0,他引:7  
戈塘金矿存在多层次层滑动构造系统,其主滑面位于茅口灰岩之上的岩溶不整合面上,矿区内不同构造层的变形型式,变形强度差异很大,变形不协调,强硬岩层以脆性变形为主,软弱层则以塑性变形的为特征,对矿区内层滑构造的研究可以从一个侧面揭示戈塘金矿的成因并进而指导找矿。  相似文献   
108.
 关于冬种黑麦草(Lolium multiflroum Lam.)使后作水稻增产的机理,以往的研究着重于冬种黑麦草对稻田土壤理化和生物学性状的改善方面。本研究旨在证明黑麦草根茬腐解物中存在促水稻生长活性物质的的假设。实验结果表明,黑麦草根茬腐解液可显著地促进水稻幼苗的生长,水稻的根、苗和植株的干物质质量分别比对照增加41.9%,42.6%和42.3%。腐解液的乙酸乙酯和正丁醇萃取物对水稻幼苗的生长也有显著的促进作用。对腐解液的有机成分进行初步分离,发现了4个具促水稻生长活性的洗脱组分。由此可以推断,黑麦草根茬腐解物中确实存在能刺激水稻生长的活性物质。  相似文献   
109.
The control law design for a near-space hypersonic vehicle (NHV) is highly challenging due to its inherent nonlinearity, plant uncertainties and sensitivity to disturbances. This paper presents a novel functional link network (FLN) control method for an NHV with dynamical thrust and parameter uncertainties. The approach devises a new partially-feedback-functional-link-network (PFFLN) adaptive law and combines it with the nonlinear generalized predictive control (NGPC) algorithm. The PFFLN is employed for approximating uncertainties in flight. Its weights are online tuned based on Lyapunov stability theorem for the first time. The learning process does not need any offline training phase. Additionally, a robust controller with an adaptive gain is designed to offset the approximation error. Finally, simulation results show a satisfactory performance for the NHV attitude tracking, and also illustrate the controller's robustness.  相似文献   
110.
多点地质统计建模方法通过训练图像获取空间结构和相关性统计特征,重建结构复杂的储层地质模型,为提高油气预测效果服务。不同建模参数的选择会直接影响计算效率和模拟结果,因此选择合适的建模参数至关重要。鉴于传统参数灵敏度分析方法的不足,提出一种基于模式均熵的多点地质统计建模参数优选方法,采用Hsim相似度对随机模型和训练图像的模式均熵差异进行量化分析。以多点地质统计建模参数——样板尺寸为例,计算建模参数集和训练图像的空间及结构特征相似度,建立基于模式均熵差异的空间相关性评价指标和建模参数的拟合曲线,将相关性评价曲线趋于平稳的拐点所对应参数值作为最优参数。实验结果表明,相比传统参数优选方法,基于单点熵代替两点熵进行平均熵值计算的新方法可以准确客观地优选出多点地质统计建模算法的参数。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号