全文获取类型
收费全文 | 39902篇 |
免费 | 3943篇 |
国内免费 | 4486篇 |
专业分类
化学 | 30034篇 |
晶体学 | 740篇 |
力学 | 2139篇 |
综合类 | 316篇 |
数学 | 5360篇 |
物理学 | 9742篇 |
出版年
2024年 | 51篇 |
2023年 | 422篇 |
2022年 | 682篇 |
2021年 | 1078篇 |
2020年 | 1246篇 |
2019年 | 1200篇 |
2018年 | 1084篇 |
2017年 | 1356篇 |
2016年 | 1504篇 |
2015年 | 1392篇 |
2014年 | 1749篇 |
2013年 | 2867篇 |
2012年 | 2232篇 |
2011年 | 2504篇 |
2010年 | 2053篇 |
2009年 | 2615篇 |
2008年 | 2464篇 |
2007年 | 2495篇 |
2006年 | 2204篇 |
2005年 | 1986篇 |
2004年 | 2036篇 |
2003年 | 1593篇 |
2002年 | 1982篇 |
2001年 | 1175篇 |
2000年 | 953篇 |
1999年 | 817篇 |
1998年 | 703篇 |
1997年 | 637篇 |
1996年 | 693篇 |
1995年 | 618篇 |
1994年 | 578篇 |
1993年 | 414篇 |
1992年 | 425篇 |
1991年 | 264篇 |
1990年 | 209篇 |
1989年 | 194篇 |
1988年 | 190篇 |
1987年 | 152篇 |
1986年 | 115篇 |
1985年 | 134篇 |
1984年 | 115篇 |
1983年 | 56篇 |
1982年 | 116篇 |
1981年 | 163篇 |
1980年 | 152篇 |
1979年 | 141篇 |
1978年 | 127篇 |
1977年 | 105篇 |
1976年 | 96篇 |
1973年 | 53篇 |
排序方式: 共有10000条查询结果,搜索用时 397 毫秒
991.
Smetanina O. F. Kuznetzova T. A. Denisenko V. A. Pivkin M. V. Khudyakova Yu. V. Gerasimenko A. V. Popov D. Yu. Il"in S. G. Elyakov G. B. 《Russian Chemical Bulletin》2001,50(12):2463-2465
A pentacyclic tritepenoid, 3-methoxyolean-18-ene (miliacin), was isolated for the first time from the marine fungus Chaetomium olivaceum. Its structure was determined on the basis of 2D NMR spectroscopy and X-ray diffraction data. 相似文献
992.
制备了不同摩尔比的甲基丙烯酸-磷脂酰乙醇甲基丙烯酰胺共聚物,研究了这种共聚物在水面的表面压力(π)-分子面积(A)曲线、共聚物制成的MOS LB膜的电容(C)-电压(V)特性及此LB膜的相变温度。 相似文献
993.
M. Paluch 《Colloid and polymer science》1998,276(7):648-652
The results of experimental studies of the adsorption at the solution/air interface from an aqueous mixture: 2,4,6-trimethylphenol–2,4,6-trichlorophenol
are presented. The surface properties of the above-mentioned mixture were studied by surface potential and surface tension
measurements. These measurements were carried out as a function of the concentration of 2,4,6-trimethylphenol aqueous solution
at a constant concentration of 2,4,6-trichlorophenol.
Using the results obtained and based on the Gibbs equation, Helmholtz formula and Motomura’s method the relative surface excesses
of adsorbed substances, effective dipole moments, surface molar fractions of solutes and miscibility of adsorbed films were
determined.
Received: 7 November 1997 Accepted: 26 February 1998 相似文献
994.
995.
S.D. Christian J. Grundnes P. Klaeboe C.J. Nielsen T. Woldbaek 《Journal of Molecular Structure》1976,34(1):33-45
The infrared spectra of 1,1,2-trichloroethane were recorded from 4000 to 50 cm?1 in the vapour and liquid states. Additional spectra above 200 cm?1 of the low temperature crystal and of two crystalline solids, prepared by compressing the sample in a diamond anvil cell, were obtained. Also, infrared spectra of the liquid and of the compound dissolved in CS2 were recorded at increased pressures. Raman spectral data of the liquid (including polarization measurements) and of the low temperature crystal were obtained.The fundamental frequencies for each of the two conformers, C1 and Cs, were assigned and the results checked by normal coordinate analysis. The same diagonal and off-diagonal force constants were employed for the two conformers and the force fields were derived by means of a least squares refinement, including data for chloroethane, 1,1-dichloroethane and various deuterated species. A standard deviation of 3 % was obtained in the final fit.From the changes in relative intensities for infrared bands belonging to the C1 and Cs conformers with increasing pressure, the volume differences (ΔV¯between the conformers were determined in CS2 solution and in the pure liquid. 相似文献
996.
试验证明GC-ECD及GC-MS/MS两种方法均能用于粮谷中残留除草剂灭草松的测定。GC-MS/MS法可同时进行定性和定量分析,测定结果的可靠性较高。GC-ECD法具有较高的灵敏度,用于定量分析的效果较好。提出一种可与上述两种方法接用的试样预处理方法。10 g试样用甲醇提取其中的测定组分,再用正己烷萃取除去脂肪。用稀盐酸调节水相的酸度至pH≤3, 用二氯甲烷将灭草松萃取至有机相。在45℃减压干燥除去溶剂,所得浓缩的残渣溶于0.2 mL甲醇中并进行甲烷化和纯化(用固相萃取法),所得残留灭草松溶于1 mL正己烷中供GC-ECD或 GC-MS/MS测定。用两种方法进行粮谷试样的分析,测定结果的RSD值依次为2.3%-4.7%和 3.5%-6.7%。用标准加入法测得两方法的回收率依次为77%-98%和80%-102%,方法的检出限为0.01 mg·kg-1。 相似文献
997.
热脱附/冷阱聚焦/气相色谱/质谱法卷烟香气分析 总被引:1,自引:0,他引:1
利用热脱附装置(TDS)对烟丝粉末进行快速升温,模拟卷烟燃吸过程中蒸馏区的部分反应,用冷阱收集所脱附的物质,以GC/MS对进行定性分析.成功的分离出了有机酸、饱和烃、芳香烃、烯烃以及含氮、氧杂原子的化合物等系列超过100种有机化合物,并且还发现了典型的存在于环境中的化学污染物——邻苯二甲酸二丁酯. 相似文献
998.
CexTi1-xO2复合氧化物的结构及负载CuO对NO还原性能研究 总被引:4,自引:1,他引:4
采用共沉淀法制备了不同摩尔比(x=0,0.1,0.2~0.9,1.0)的CexTi1-xO2复合氧化物,考察了CuO/CexTi1-xO2对NO+CO反应的活性,并用BET,TPR和XRD等技术对各试样进行了表征。结果表明,试样的结构和还原特性随焙烧温度变化而变化。XRD检测表明,x值从0.1增加到0.5时,650℃焙烧的试样已形成了CeTi2O6物相,且主要以无定形状态存在;试样经800℃焙烧后晶化完全;x>0.6时,一些TiO2已经进入了CeO2晶格,形成了Ce Ti固溶体。催化剂活性评价表明CuO/CexTi1-xO2(650℃)对NO+CO反应具有较好的活性,其活性随x值变化而变化。TPR及XRD结果表明CuO与CeTi2O6之间存在很强的相互作用,CeTi2O6物相的形成使CuO的还原峰温由380℃提前到200℃,而CuO的存在又促进CeTi2O6的还原峰温从600℃提前至200~300℃。 相似文献
999.
C. V. Rode M. M. Telkar R. Jaganathan R. V. Chaudhari 《Journal of molecular catalysis. A, Chemical》2003,200(1-2):279-290
Liquid phase hydrogenation of styrene oxide using 1% Pd/C and NaOH as a promoter was found to give selectively β-phenethyl alcohol (PEA) under very mild conditions (313–333 K; 0.68–5.5 MPa). The kinetics of this system was investigated by collecting initial rate data in a batch slurry reactor. Rate of hydrogenation was found to decrease beyond a certain concentration of both hydrogen (>3 MPa) and styrene oxide (>0.5 kmol/m3). A Langmuir–Hinshelwood type rate equation has been proposed based on the initial rate data in the kinetic regime. The model predictions agree very well with the experimentally observed concentration–time data indicating the applicability of the proposed rate model. 相似文献
1000.
The acylation of 2-methoxynaphthalene with acetic anhydride was carried out over zeolite beta catalysts having various Si/Al
ratios. It was found that the reaction performance is strongly dependent on the Si/Al ratio of the catalyst. The catalytic
reaction features were elucidated in terms of the acidic properties of the catalysts.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献