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   6042篇
  免费   1000篇
  国内免费   1081篇
化学   6678篇
晶体学   135篇
力学   67篇
综合类   48篇
数学   7篇
物理学   485篇
无线电   703篇
  2024年   7篇
  2023年   77篇
  2022年   116篇
  2021年   202篇
  2020年   338篇
  2019年   226篇
  2018年   203篇
  2017年   206篇
  2016年   359篇
  2015年   353篇
  2014年   391篇
  2013年   606篇
  2012年   562篇
  2011年   386篇
  2010年   314篇
  2009年   310篇
  2008年   351篇
  2007年   359篇
  2006年   355篇
  2005年   377篇
  2004年   345篇
  2003年   301篇
  2002年   179篇
  2001年   154篇
  2000年   149篇
  1999年   111篇
  1998年   94篇
  1997年   116篇
  1996年   99篇
  1995年   88篇
  1994年   77篇
  1993年   46篇
  1992年   80篇
  1991年   43篇
  1990年   40篇
  1989年   31篇
  1988年   13篇
  1987年   10篇
  1986年   10篇
  1985年   9篇
  1984年   3篇
  1983年   4篇
  1982年   3篇
  1981年   3篇
  1980年   3篇
  1979年   2篇
  1977年   4篇
  1974年   1篇
  1973年   4篇
  1972年   2篇
排序方式: 共有8123条查询结果,搜索用时 234 毫秒
71.
1 INTRODUCTION Silver and its compounds have received much attention due to their current and potential applica- tions in many areas[1, 2]. As a metal with the highest electrical and thermal conductivities, silver was one of the most important noble metals used in electrical industries in the last century. Several decades ago, however, the development of silver application in electronic area seemed not so quick. The fear for some undesirable phenomena involving silver, like “electrochemi…  相似文献   
72.
Transmission electron microscopy (TEM) studies on bipyridine (bpy) containing block copolymer systems showed the formation of nanoscopic polymer–ion complexes through complexation with copper(I) ions which segregated to highly ordered columnar domains of mesoscopic dimensions. The domains, i.e. stacks of [(bpy)2Cu(I)] complex moieties could be visualized by complementary TEM techniques. First, electron energy loss spectra (EELS) showed the absorption edges of copper and nitrogen, which are specific for the bipyridine copper complexes. The element spectroscopic imaging (ESI) technique allowed the imaging of the net copper and net nitrogen distribution, and the coinciding pictures exhibited a microphase separated system in the case of a 3-block copolymer with complexed end segments. High resolution elastic bright field images showed interference lines with a line to line distance of about 8 A which could be related to the Cu–Cu distance in staggered Cu(I)–bipyridine complexes. Received: 6 August 1997 Accepted: 28 April 1998  相似文献   
73.
为了研究白血病骨髓移植病人全血微量元素铜的变化及其意义 ,用原子吸收光谱法检测了正常对照组与白血病骨髓移植患者预处理前及移植后骨髓空虚期全血微量元素铜的含量。结果表明 ,正常对照组全血铜浓度与白血病骨髓移植患者预处理前全血铜浓度差异无显著性 (P >0 0 5 ) ,而与骨髓空虚期全血铜浓度有显著性差异 (P <0 0 5 ) ,说明白血病缓解后血铜接近正常对照组水平 ,而白血病骨髓移植患者预处理后骨髓空虚期全血铜含量高于正常对照组 ,说明预处理影响微量元素铜的代谢 ,本文就其影响机理进行了研究  相似文献   
74.
Side self-oxidation of thiols was studied. It was found that these reactions in neutral and alkaline solutions are induced by impurities of variable-valence metals. The ability of transition metals to catalyze oxidation of thiols changes in the order Cu > Mn > Fe > Ni Co. The plot of the self-oxidation rate vs. pH passes through a maximum whose position on the pH scale depends on both the nature of metal and the structure of the thiol oxidized. For thiols having different structures, the kinetic orders in reactions catalyzed by copper ions differently vary with pH, which is apparently associated with the formation of complexes possessing different catalytic activity.  相似文献   
75.
The title compound, {[Cu(2,2'-bipy)(C7H4O5S)(H2O)2](H2O}n (2,2'-bipy = 2,2'- bipyridine), was synthesized by the hydrothermal reaction of Cu(NO3)2(3H2O, 2,2'-bipyridine and 2-sulphobenzoic acid, and structurally characterized by single-crystal X-ray diffraction analysis. The crystal is of triclinic, space group P with a = 9.21(3), b = 10.17(3), c = 10.77(3) (A), α = 77.017(16), β = 89.80(8), γ = 68.46(7)°, V = 911(5) (A)3, Z = 2, (D/s)max = 0.001, Mr = 473.94, Dc = 1.728 g/cm3, μ(MoKα) = 1.365 mm-1, F(000) = 486, the final R = 0.0246 and wR = 0.0628 for 3809 observed reflections with I > 2σ(I). The mononuclear crystal structure extends into a two-dimensional net- work via hydrogen-bonding interactions and a three-dimensional framework is further formed by means of π-π stacking interactions.  相似文献   
76.
Inclusion of tert-butylbenzeneinto the l-D channel of the titlehost matrix results in a-[CuL2]1/3(tert-butylbenzene)inclusion compound(trigonal, space groupR =3;a = 24.495(3),c = 10.510(2) Å,V = 5461(1) Å3,Z = 9;R = 0.049). The observed guest–host mole ratio of 1/3 was confirmed by X-ray diffraction and arises from the stoichiometric filling of the larger wide segments of the channel. This contrasts with the benzene inclusion compound studied previously, where both the larger and smaller wide segments were occupied to give a 2/3 guest–host stoichiometry. A comparison of these two structures explains the experimental fact that the guest–host mole ratios for inclusions of the title host lie between values of 1/3 and 2/3for 20 different benzene derivatives.  相似文献   
77.
The transition metal and alkaline earth metal heterobinuclear complex[CuMg(TS)(H2O)·H2O],where TS is the schiff base N,N′-bis(3-carboxylsalidene)trimethylenediamine, was synthesised and its crystal structure was determined by the single-crystal X-ray diffraction method at room temperature. It crystallizes in the monoclinic system, space group P21/n. The lattice parameters are a=12.041(2)?, b=14.736(3)?, c=12.062(2)?, β=118.00(3)°, Z=4 with R1=0.0364. The thermal property and EPR studies of the complex according to crystal structure are also investigated. CCDC: 182408.  相似文献   
78.
在常压和250℃下,考察了涂层TiO2改性铜催化剂上甲醇脱氢生成甲酸甲酯的活性和选择性以及催化剂的稳定性。结果表明,涂层TiO2明显提高了铜催化剂的活性和稳定性。在所考察的铜催化剂中,以Cu/TiO2/AlO3的催化活性最高,甚至高于离子交换法制备的Cu(ex)/SiO2催化剂。XPS分析表明,Cu/TiO2/AlO3中存在着两种化学环境不同的CU(0),即负载在r-ALO3和TiO2-Al2O3  相似文献   
79.
以铜离子为模板的褐藻酸凝胶对铜离子的选择性富集研究   总被引:2,自引:0,他引:2  
邬建敏  王永尧  杨晨磊 《分析化学》2002,30(12):1414-1417
以铜离子为模板,制备了褐藻酸凝胶(Cu-alginate),通过吸附实验及过柱渗滤实验,试验了该吸附剂对铜离子的选择性吸附性能,并且采用该吸附剂富集了自来水中微量的Cu^2 ,结合原子吸收法测定了水样中Cu^2 的含量。结果表明:该吸附剂对Cu^2 有较高的选择性吸附性能,显著优于一些化学合成铜离子模板缩聚物及非铜模板褐藻酸凝胶(如Ca-alginate),非重金属离子(如K^ 、Na^ 、Ca^2 )及某些重金属离子(如Ni^2 、Cd^2 )等对Cu^2 的吸附均不产生明显干扰。用该吸附剂富集水中的微量铜离子,回收率可达97.7%。  相似文献   
80.
为观察二硫化碳(CS2)对机体铜,锌水平的影响,对某化纤厂35名接触CS2工人和41名非接触工人的血铜,锌值进行调查,比较,并进行了动物染毒实验。SD大鼠吸入CS2浓度分别为5和50mg/m^3,每日5h,每周6d,连续6个月,于染毒2,4,6个月时测定血浆铜,锌值及染毒6个月时肝脏铜,锌含量。结果显示,CS2接触组工人血铜,锌水平较对照组显著降低(P<0.05)。在CS2染毒6个月时,50mg/m^3 CS2染毒组血浆及肝脏铜水平均较对照组及5mg/m^3 CS2染毒组显著降低(P<0.05)。可见接触CS2能干扰机体铜,锌代谢,导致体内铜,锌含量减少。  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