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   81117篇
  免费   15815篇
  国内免费   7296篇
化学   73060篇
晶体学   1000篇
力学   3060篇
综合类   499篇
数学   7718篇
物理学   18891篇
  2024年   94篇
  2023年   798篇
  2022年   1293篇
  2021年   1669篇
  2020年   2767篇
  2019年   3956篇
  2018年   2348篇
  2017年   2014篇
  2016年   5134篇
  2015年   5275篇
  2014年   5591篇
  2013年   7074篇
  2012年   6839篇
  2011年   6499篇
  2010年   5783篇
  2009年   5519篇
  2008年   5432篇
  2007年   4563篇
  2006年   4061篇
  2005年   3901篇
  2004年   3207篇
  2003年   2836篇
  2002年   3613篇
  2001年   2597篇
  2000年   2293篇
  1999年   1387篇
  1998年   886篇
  1997年   783篇
  1996年   785篇
  1995年   685篇
  1994年   604篇
  1993年   542篇
  1992年   515篇
  1991年   441篇
  1990年   385篇
  1989年   301篇
  1988年   260篇
  1987年   194篇
  1986年   205篇
  1985年   229篇
  1984年   140篇
  1983年   97篇
  1982年   93篇
  1981年   76篇
  1980年   61篇
  1979年   65篇
  1978年   41篇
  1976年   37篇
  1974年   42篇
  1973年   46篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
991.
Limited land and insufficient technicians to operate a wastewater treatment system are main restrictions for many factories. Therefore, an ideal wastewater treatment method for a small or land-limited factory must possess merits such as high performance efficiency, high organic loading rate, little odor, simple operation, easy maintenance, and little land required (simultaneously). An entrapment technique to immobilize mixed microorganisms to treat organic wastewater, which was developed in the present work, possesses these characteristics. This project was done on a laboratory scale. The microorganisms were activated sludge (an undefined mixture of microorganisms obtained directly from a domestic wastewater treatment plan) and the mixed microorganisms were immobilized in cellulose triacetate by means of an entrapment technique to treat organic wastewater from food industry. After wastewater was treated by this system, the SCOD (soluble COD) removal efficiency of 81% evaluated samples exceeded 80% in 1.5 ± 0.9 g SCOD/L/d of the volumetric loading rate and 7–10 h for the hydraulic retention time. This wastewater treatment method can be applied to other organic industrial wastewater.  相似文献   
992.
铁电极上硫脲及衍生物的缓蚀作用研究   总被引:7,自引:1,他引:7  
用极化曲线方法测定了硫酸溶液中硫脲及其衍生物的浓度变化对铁腐蚀电化学行为的影响,获得了腐蚀电位下缓蚀剂对阴极和阳极过程的作用系数,用EHMO方法对硫脲及其衍生物的结构参数进行了量化计算,分析表明,缓蚀剂对阴极和阳极过程作用系数的大小与这些参量有密切关系。  相似文献   
993.
A convenient synthetic pathway to 2-aryl-5,6-dihydro-4H-benzo[f][1,2,4]triazolo[1,5-α]azepine derivatives 7 was developed. The synthesis was based on the cycloaddition of the 1,2,3,4-tetrahydronaphthalene a-acetoxy azo compounds 3 with Ar-CN in the presence of AlCl3 and the consecutive ring enlargement.  相似文献   
994.
A cobalt hexacyanoferrate (CoHCF) nanoparticle (size ca. 60 nm) chemically modified electrode (CME) was fabricated and the electrochemical behavior of hemoglobin (Hb) at this nanosized CoHCF CME was studied. In comparison with a bare glassy carbon electrode (GCE) and a general CoHCF CME electrodeposited in a traditional manner, the present nanosized CoHCF CME performed efficiently electrocatalytic reduction for Hb with relatively high sensitivity, stability, and longlife, Combined with liquid chromatography (LC), the nanosized CoHCF CME was used as the electrochemical detector of Hb in the established flow injection analysis-electrochemical determination (FIA-ECD) system. The peak current was a linear function of concentrations in the range from 2.5×10^-8 to 5.0×10^-6mol/L for Hb, with detection limit of 1.4×10^-8 mol/L. The FIA-ECD system has been successfully applied to assess the Hb content of clinic blood samples with advantages of sensitiveness, speediness, easy control and small sample-consumption.  相似文献   
995.
Five compounds, terephthalic acid mono-[2-(4-carboxy-phenoxycarbonyl)-vinyl] ester (1), (E)-3-(5′-hydroperoxy-2,2′-dihydroxy[1,1′-biphenyl]-4-yl)-2-propenoic acid (2), 3,4,5-trihydroxybenzoic acid (3), succinic acid (or butanedioic acid) (4), and 2,3,4,5,6-pentahydroxybenzoic acid (5), were isolated from Phyllanthus urinaria. The structures of these compounds were elucidated by means of spectral techniques including IR, MS, and 1D/2D NMR. 1 and 2 are new compounds.__________Published in Khimiya Prirodnykh Soedinenii, No. 1, pp. 14–17, January–February, 2005.  相似文献   
996.
刘文成  金巨广 《分析化学》1992,20(5):532-534
本文描述了一个倾斜交叉、差分式激光热透镜光谱法。采用本方法可消除较高的溶剂空白的影响。文章讨论了调制频率和泵浦光功率对热透镜信号的影响,给出了检测Cu~(2+)的工作曲线,讨论了影响方法灵敏度的诸因素,提出了可能的改进方案。  相似文献   
997.
The effects of Manganese(Mn)incorporation on a precipitated iron-based Fischer-Tropsch synthesis(FTS)catalyst were investigated using N_2 physical adsorption,air differential thermal analysis (DTA),H_2 temperature-programmed reduction(TPR),and M(?)ssbauer spectroscopy.The FTS perfor- mances of the catalysts were tested in a slurry phase reactor.The characterization results indicated that Mn increased the surface area of the catalyst,and improved the dispersion ofα-Fe_2O_3 and reduced its crystallite size as a result of the high dispersion effect of Mn and the Fe-Mn interaction.The Fe-Mn inter- action also suppressed the reduction ofα-Fe_2O_3 to Fe_3O_4,stabilized the FeO phase,and(or)decreased the carburization degree of the catalysts in the H_2 and syngas reduction processes.In addition,incorporated Mn decreased the initial catalyst activity,but improved the catalyst stability because Mn restrained the reoxidation of iron carbides to Fe_3O_4,and improved further carburization of the catalysts.Manganese suppressed the formation of CH_4 and increased the selectivity to light olefins(C_(2-4)~=),but it had little effect on the selectivities to heavy(C_(5 )) hydrocarbons.All these results indicated that the strong Fe-Mn interaction suppressed the chemisorptive effect of the Mn as an electronic promoter,to some extent,in the precipitated iron-manganese catalyst system.  相似文献   
998.
Polynuclear mixed‐valent nickel oxide and nickel hexacyanoferrate hybrid film was prepared on glassy carbon electrode by multiple scan cyclic voltammetry. The film growth was monitored using electrochemical quartz crystal microbalance (EQCM). The cyclic voltammogram of the nickel hexacyanoferrate film is characterized by single redox couple whereas nickel oxide/nickel hexacyanoferrate hybrid film exhibits two redox couples. Cyclic voltammetric features suggest that the charge transfer process in both films resembles that of surface‐confined redox species. In stronger basic solution (pH ≥9), nickel hexacyanoferrate film was gradually converted into nickel oxide film during potentiodynamic cycling. The peak potential of nickel oxide redox couple moved into more negative side with increasing pH of contacting solution whereas the peak potential of nickel hexacyanoferrate redox couple remains the same. Electrocatalytic behavior of hybrid film coated electrodes toward ascorbic acid, hydrazine and hydroxylamine was investigated using cyclic voltammetry technique. Analytical application of nickel oxide/nickel hexacyanoferrate hybrid film electrode was tested in amperometry and flow injection analysis.  相似文献   
999.
Treatment of heterocyclic β‐ketonitriles 1a,b with hydrazine hydrate and phenylhydrazine afforded the hydrazine derivatives 2a‐d which cyclized in PPA into pyrrolo[3,4‐c]pyrazoles 3a‐d. Reaction of 1a,b with cyanoacetohydrazide furnished the cyanoacetyl pyrrolo[3,4‐c]pyrazoles 4a,b. The hydrazine 2c reacted with β‐diketone and β‐ketoesters to afford pyrazolyl‐pyrrolines 5‐7. Also the later hydrazine reacted with some D‐aldoses and aceteophenone to give the corresponding hydrazones 10‐12 and hydrazine carboxamide derivatives 15a,b respectively.  相似文献   
1000.
本研究采用直接曝热技术,分别以氢气和甲烷为反应气,测定了10个血卟啉衍生物和13个金属血卟啉衍生物的正、负离子解吸化学电离质谱,探讨了各种化学电离质谱法在结构测定中的应用。  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