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   3395篇
  免费   365篇
  国内免费   484篇
化学   1928篇
晶体学   12篇
力学   64篇
综合类   36篇
数学   81篇
物理学   851篇
无线电   1272篇
  2024年   16篇
  2023年   74篇
  2022年   151篇
  2021年   193篇
  2020年   187篇
  2019年   104篇
  2018年   87篇
  2017年   140篇
  2016年   166篇
  2015年   183篇
  2014年   203篇
  2013年   278篇
  2012年   233篇
  2011年   242篇
  2010年   154篇
  2009年   177篇
  2008年   206篇
  2007年   202篇
  2006年   190篇
  2005年   180篇
  2004年   128篇
  2003年   131篇
  2002年   101篇
  2001年   79篇
  2000年   78篇
  1999年   58篇
  1998年   39篇
  1997年   33篇
  1996年   27篇
  1995年   41篇
  1994年   34篇
  1993年   17篇
  1992年   24篇
  1991年   8篇
  1990年   12篇
  1989年   9篇
  1988年   16篇
  1987年   6篇
  1986年   6篇
  1985年   7篇
  1984年   10篇
  1983年   4篇
  1982年   4篇
  1981年   2篇
  1980年   1篇
  1979年   3篇
排序方式: 共有4244条查询结果,搜索用时 15 毫秒
31.
以芳香基三唑类杀菌剂三唑酮为先导物设计并合成了5个含N,N-二烷基二硫代氨基甲酸酯的芳香三唑类化合物, 通过元素分析、红外光谱、核磁共振氢谱和质谱对其结构进行了表征. 用X射线单晶衍射测定了[α-(4-甲氧基苯甲酰基)-2-(1,2,4-三唑-1-基)]乙基-N,N-二甲基二硫代氨基甲酸酯的晶体结构, 晶体属于三斜晶系, 空间群, 晶胞参数为: a=0.73482(15) nm, b=1.1051(2) nm, c=1.1209(2) nm, α=90.32(3)°, β=101.97(3)°, γ=105.13(3)°, V=0.8578(3) nm3, Z=2, Dc=1.357 g/cm3, F(000)=368, µ=0.324 mm-1. 生物测试结果显示这5种有机化合物都具有杀菌性和植物生长调节活性  相似文献   
32.
Nickel(II) complexes, having the general composition Ni(L)2X2, have been synthesized [where L: isopropyl methyl ketone semicarbazone (LLA), isopropyl methyl ketone thiosemicarbazone (LLB), 4-aminoacetophenone semicarbazone (LLC) and 4-aminoacetophenone thiosemicarbazone (LLD) and X=Cl-, 1/2SO(4)2-]. All the Ni(II) complexes reported here have been characterized by elemental analyses, magnetic moments, IR, electronic and mass spectral studies. All the complexes were found to have magnetic moments corresponding to two unpaired electrons. The possible geometries of the complexes were assigned on the basis of electronic and infrared spectral studies. Newly synthesized ligand and its nickel(II) complexes have been screened against different bacterial and fungal growth.  相似文献   
33.
A series of new 1,2,4-substituted hexahydropyridazines was prepared in order to evaluate qualitatively and quantitatively the role and influence of substituents on the biological activity of this system. An X-ray structural investigation of 4-hydroxyimino-2-methyl-1-phenethylhexahydropyridazine was performed. The compounds considered exhibit moderate local anesthetic, antihistaminic, and anticonvulsive activity.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 539–541, March, 1993.  相似文献   
34.
Metallic implants are widely used in orthopaedic surgery but metal release has been reported by several authors. High metallic ion concentration in surrounding tissues may play a major role in therapeutic failure. We have investigated in vivo and in vitro two biological glasses (BVA and BVH) used as coatings of metal implants. Physicochemical characterization was made by several complementary methods, particularly particle induced x‐ray emission (PIXE) and energy‐dispersive x‐ray spectroscopy (EDXS) associated with electronic microscopy. Analyses clearly show the differences of behaviour between both glasses. The BVA glass is bioactive, i.e. it develops a direct chemical bond between prosthesis and bone by the formation of a calcium phosphate layer at its surface. These structural and compositional modifications are caused by hydrolysis of the glass. After its dissolution BVA glass is replaced by bone, which ensures better integration of the implant in the bone site. The BVH glass is not bioactive and is used as a cement to isolate the metal implant from the biological environment, but the coating disaggregates with implantation time and glass grains migrate through the bony lacuna network. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   
35.
曹克广  王忠卫  赵信歧  方涛 《有机化学》2003,23(12):1411-1415
合成了九个以噻吩为母体环的含三唑环的化合物——2,3-二取代-4-(1H-1, 2,4.三唑-1-基)-5-苯基氨基噻吩(2a-2i),并测定了2a的晶体结构.晶体为三斜 晶系,P-1空间群,晶胞参数为:a=0.79816(15)nm,b=1.00259(13)nm,c=1. 4478(4)nm,a=100.326(16)°,β=94.69(2)°,r=106.083(9)°,V=1.0845 (4)nm~3,z=2,D_c=1.396 g/cm~3.初步生物活性表明所有目标化合物杀菌活 性较低,有一定的植物生长调节活性.  相似文献   
36.
针对相关滤波目标跟踪中由于目标遮挡、出视野引起的跟踪失败等问题,以背景感知相关滤波(background aware correlation filters, BACF)算法为基础,提出一种抗遮挡优化算法。首先特征提取部分融合方向梯度直方图(histogram of oriented gradient, HOG)特征与颜色名称(color name, CN)特征;其次提出层级遮挡优化算法,在目标跟踪过程中根据平均峰值相关能量(average peak to correlation energy,APCE)和最大响应峰值判断目标外观是否发生较大变化,并通过巴氏距离进一步计算目标相邻滤波器模板特征相似程度,以此判断造成目标外观变化的真正原因,进而决定是否更新模板;针对目标出视野问题,提出模板筛查策略,跟踪过程中每隔K帧对滤波器模板进行置信度考查,若目标丢失,则生成新的模板并对目标进行全局搜索重新抓取目标。通过在目标跟踪数据集OTB2015上测试,优化算法精确度与成功率分别为83.0%与78.8%,有效提高了算法性能。  相似文献   
37.
It is shown that biological-natural-selection-like behavior can occur, as a general type of time evolution, in a statistical system where detailed balance is violated owing to the presence of metastable energy states. A model of a non-equilibrium phase transition corresponding to the spontaneous origin of self-reproduction in the system is suggested. After a phase transition, the system passes from one quasistationary distribution of self-reproducing subsystems to another, with an increase in the total organization, as long as the growth of the energy flow through the system or a reduction of energy dissipation in the system is possible. The entropy production is calculated for this process in terms of selective values of Eigen's theory for self-organization in autocatalytic systems. Correspondence of the extremal principle of Eigen's theory with the criterion of evolution in Prigogine's thermodynamics is established.  相似文献   
38.
Summary New quinidine metabolites, including 10,11-dihydrodiol quinidine N-oxide, 10,11-dihydrodiol quinidine and their glucuronides, were found in human urine. A quinidine monitoring HPLC method including these metabolites, is proposed by the direct injection of body fluid samples onto the precolumn for deproteinization followed by reverse phase separation in the analytical column with a column switching technique. The recovery of spiked quinidine and its metabolites in plasma was quantitative (98–102%) with good reproducibility (C.V.: 1.6–4.0%). Several clinical samples such as whole blood and urine were analyzed by the present method.  相似文献   
39.
A tetrabromo derivative of arglabin was synthesized stereoselectively. The molecular structure of the new compound was established using PMR and 13C NMR spectra and an x-ray structure analysis. Its antifungal activity was demonstrated. __________ Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 253–254, May–June, 2006.  相似文献   
40.
韩亮  李正名  张云  郭维明 《有机化学》2006,26(2):242-246
N-脂肪酰基乙醇胺(NAE)作为植物体内的一种内源物质, 在调节植物生长方面起着重要作用. 为了弥补其分子结构中长的脂肪链所带来的溶解性能以及在植物体内传导性能的缺陷, 我们将N-硬脂酰乙醇胺(NAE18)引入氮芥磷酸酯中, 合成了一系列标题化合物. 在合成工作中发现: 在NAE18与氮芥芳基磷酰氯的反应过程中, 4-二甲氨基吡啶(DMAP)起着关键性的催化作用. 在不加DMAP的相同实验条件下, 反应不能进行. 对所合成的标题化合物进行了植物生长调节和杀菌活性的测定, 初步生测结果表明: 经过结构修饰后, 大多数目标化合物的活性相对NAE18有所增强, 但有关生物活性仍有待进一步研究.  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