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   4508篇
  免费   233篇
  国内免费   196篇
化学   2416篇
晶体学   6篇
力学   667篇
综合类   44篇
数学   953篇
物理学   851篇
  2024年   11篇
  2023年   46篇
  2022年   73篇
  2021年   78篇
  2020年   100篇
  2019年   105篇
  2018年   74篇
  2017年   106篇
  2016年   127篇
  2015年   116篇
  2014年   169篇
  2013年   296篇
  2012年   186篇
  2011年   223篇
  2010年   207篇
  2009年   241篇
  2008年   253篇
  2007年   294篇
  2006年   222篇
  2005年   224篇
  2004年   220篇
  2003年   181篇
  2002年   189篇
  2001年   140篇
  2000年   153篇
  1999年   122篇
  1998年   103篇
  1997年   123篇
  1996年   75篇
  1995年   97篇
  1994年   80篇
  1993年   42篇
  1992年   53篇
  1991年   50篇
  1990年   41篇
  1989年   29篇
  1988年   19篇
  1987年   8篇
  1986年   4篇
  1985年   10篇
  1984年   6篇
  1983年   7篇
  1982年   8篇
  1981年   4篇
  1980年   4篇
  1979年   3篇
  1978年   4篇
  1977年   4篇
  1976年   2篇
  1973年   2篇
排序方式: 共有4937条查询结果,搜索用时 15 毫秒
91.
老年性白内障与微量元素   总被引:1,自引:0,他引:1  
参阅有关老年性白内障的文献.归纳其病因有四种学说:(1)老化学说;(2)醌体学说;(3)微量元素学说;(4)中医学说.按病因学说不同.用药各异.目前治疗老年性白内障药物有滴眼剂及内服剂,其中有一种新药名为除障灵.它是根据中医学说及微量元素学说,筛选中药制成,它含有丰富的锌、硒、铜、镁等微量元素.应用除障灵治疗老年性白内障患者102例(204只眼),观察6个月至2年,有效率达95%,开创了治疗老年性白内障的新方法.  相似文献   
92.
Preparation and Characterization of Nano-ZnFe2O4/TiO2 Films   总被引:1,自引:0,他引:1       下载免费PDF全文
The nano-ZnFe2O4/TiO2 films possess the functions of desulfurization and degradation for organic pollutants. The sols of ZnFe2O4/TiO2 were prepared by sol-gel method and coated on glass and porous ceramic by vertical coating and dipping-lift processes, respectively, and the samples were obtained after drying and sintering. The composition, appearance, absorption spectrum of the films, and the influence of the film on porous ceramic performances were analyzed using SEM, AFM, UV-Vis spectrometer, and mercury porosimeter, respectively, to determine the operation parameters of the multifunction porous ceramic elements for gas-purification.  相似文献   
93.
Crystal Structures of KNdTe4, RbPrTe4, and RbNdTe4 — Investigations concerning the Thermal Stability of KNdTe4 as well as some Remarks concerning Additional Representatives of the Composition ALnTe4 (A = K, Rb, Cs and Ln = Rare Earth Metal) Of the compounds ALnQ4 (A = Na, K, Rb, Cs; Ln = Lanthanoid; Q = S, Se and Te) the crystal structures of the three new tellurides KNdTe4, RbPrTe4 and RbNdTe4 were determined by X‐ray single‐crystal structure analysis and of the three additional new ones KCeTe4, KPrTe4 and CsNdTe4 by X‐ray powder diffraction experiments. All six new compounds are isotypic with KCeSe4. Characteristic for the crystal structure of the compounds mentioned above are layers built from (Q2)2— dumbbells in form of 4.32.4.3 nets with embedded cations A+ and Ln3+ between them, which are coordinated eightfold in form of square‐shaped antiprisms by Q ions. The distances Te‐Te within the dumbbells were found to be 277.8(2) pm for all investigated tellurides. By combination of X‐ray diffraction and DTA measurements it was shown that the compound KNdTe4 is metastable at ambient temperature with a limited existence range between the temperatures 260 and 498 °C.  相似文献   
94.
Using electron spectroscopy, X-ray photoelectron spectroscopy, and ESR spectroscopy, the blue forms of bisphthalocyanines of rare-earth elements were found to have structures of sandwich-type complexes with isoelectronic phthalocyanine ligands linked with tetravalent metal ions, [Pc2–Ln4+Pc2–]0. A comparative spectral and electrochemical study of the blue and green forms oftert-butyl-substituted bisphthalocyanines was carried out for almost the whole series of rare-earth metals. Lutetium octa(perfluoro-tert-butyl)- and perchlorobisphthalocyanines were synthesized for the first time.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 425–430, March, 1995.This study was financially supported by the Russian Foundation for Basic Research (grant No. 94-03-08903).  相似文献   
95.
高分辨等离子体质谱法直接测定高纯镓中的痕量元素   总被引:9,自引:0,他引:9  
谢华林  聂西度  唐有根 《分析化学》2006,34(11):1570-1574
建立高分辨电感耦合等离子体质谱法(HR-ICP-MS)测定高纯镓样品中Be、Mg、A l、S i、Ti、V、Cr、Mn、Fe、Co、N i、Cu、Zn、Ge、As、Mo、Ag、Cd、In、Sb、Ba、Pb、B i等痕量元素的方法。样品用HNO3 HC l经微波消解后,试液直接进样用HR-ICP-MS法同时测定上述元素,在高分辨质谱测量模式下避免了大量的质谱干扰,详细地研究了HC l和高纯镓所产生的基体效应,以Sc、Rh、Tl作为内标元素校正了基体效应,讨论和确定了实验的最佳测定条件。结果表明,23种痕量元素的检出限在0.001~0.21μg/L之间;回收率在89.8%~111.6%之间,相对标准偏差(RSD)小于3.3%。  相似文献   
96.
本工作用 ICP-AES 法测定稀土-6063变形铝合金中 La、Ce、Pr、Nd、Sm、Gd、Fe、Mg、Ti、Zn,Cu 和 Mn,本方法不用化学分离,可一次同时测定这些元素,并做了 ICP光源参数、基体量变化、酸度(HCl)变化、稀土总量变化、共存元素 Fe 和 Mg 量的变化对被测元素的影响。本方法速度快、准确度高(除 Cu 稍差)。回收率91.0~110.0%,相对标准偏差±1.4~8.0%。  相似文献   
97.
NIST maintains a portfolio of more than 1300 standard reference materials (SRM), more than a third of these relating to measurements in the biological and environmental fields. As part of the continuous renewal and replacement efforts, a set of new marine sediments has been recently developed covering organic and inorganic determinations. This paper describes the steps taken in sample preparation, homogeneity assay, and analytical characterization and certification with specific emphasis on SRM 2702 inorganics in marine sediment. Neutron activation analysis showed the SRM to be highly homogeneous, opening the possibility for use with solid sampling techniques. The certificate provides certified mass fraction values for 25 elements, reference values for eight elements, and information values for 11 elements, covering most of the priority pollutants with small uncertainties of only several percent relative. The values were obtained by combining results from different laboratories and techniques using a Bayesian statistical model. An intercomparison carried out in field laboratories with the material before certification illustrates a high commutability of this SRM.Electronic Supplementary Material Supplementary material is available in the online version of this article at  相似文献   
98.
基于主成分分析的方法,研究了中药的相似性。主成分综合得分越近,中药就越相似。  相似文献   
99.
苏州荠苧总黄酮与微量元素含量的测定   总被引:1,自引:0,他引:1  
采用分光光度法测定了苏州荠芋的总黄酮含量;采用了等离子发射光谱法测定了其元素含量。结果表明,苏州荠芋总黄酮的含量分别为55.31mg/g(7月采收)、65.00mg/g(8月采收);含K、Ca、Mg等多种常量元素;含Cu、Fe、Mn、Zn、Cr、Co等多种人体必需的微量元素,其中Fe含量最高;而有害重金属元素Hg未检出,Pb和AS的含量均较低(小于药典规定量)。可见8月采收的苏荠芋总黄酮含量较高,苏州荠芋含多种人体必需的微量元素,有害重金属含量低,具有较大的开发价值。  相似文献   
100.
Abstract

Herein the bioaccessibility of Mn, Cu, Zn, Cd, and Pb, selected from essential and toxic elements, was determined in commercially sold vegetable purees intended for infant and toddler consumption. Chemical fractionation studies using water, acetone, diethyl ether, chloroform:methanol, and n-hexane were employed to predict the importance of the protein and lipid parts of a matrix to assess the bioaccessibility data. In addition, in-vitro gastrointestinal digestion was performed to determine the bioaccessibility of the elements using a five level, three factor central composite design (CCD) to maximize the elemental solubility. The total elemental concentrations in all of the fractions were determined by inductively coupled plasma – mass spectrometry (ICP-MS). Based on the consumption of one jar of vegetable/rice-based baby food, Zn was 1.3% of the recommended dietary allowance and Mn was 4.2% of the adequate intake level, while Cu was almost 100% of the adequate intake level. Additionally, Pb was always below the detection limit and Cd was sometimes under the detection limit for the percent bioaccessibility. However, in some samples, Cd was as high as 80% of the tolerable weekly intake level depending on the body weight.  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