全文获取类型
收费全文 | 25091篇 |
免费 | 1183篇 |
国内免费 | 4741篇 |
专业分类
化学 | 28672篇 |
晶体学 | 298篇 |
力学 | 52篇 |
综合类 | 233篇 |
数学 | 17篇 |
物理学 | 1743篇 |
出版年
2024年 | 27篇 |
2023年 | 238篇 |
2022年 | 583篇 |
2021年 | 740篇 |
2020年 | 737篇 |
2019年 | 755篇 |
2018年 | 655篇 |
2017年 | 799篇 |
2016年 | 958篇 |
2015年 | 894篇 |
2014年 | 1039篇 |
2013年 | 2291篇 |
2012年 | 1798篇 |
2011年 | 1397篇 |
2010年 | 1222篇 |
2009年 | 1454篇 |
2008年 | 1529篇 |
2007年 | 1688篇 |
2006年 | 1591篇 |
2005年 | 1478篇 |
2004年 | 1343篇 |
2003年 | 1085篇 |
2002年 | 1027篇 |
2001年 | 691篇 |
2000年 | 673篇 |
1999年 | 547篇 |
1998年 | 447篇 |
1997年 | 471篇 |
1996年 | 427篇 |
1995年 | 390篇 |
1994年 | 340篇 |
1993年 | 293篇 |
1992年 | 284篇 |
1991年 | 208篇 |
1990年 | 159篇 |
1989年 | 175篇 |
1988年 | 124篇 |
1987年 | 85篇 |
1986年 | 60篇 |
1985年 | 51篇 |
1984年 | 59篇 |
1983年 | 30篇 |
1982年 | 42篇 |
1981年 | 29篇 |
1980年 | 26篇 |
1979年 | 20篇 |
1978年 | 22篇 |
1977年 | 5篇 |
1973年 | 5篇 |
1968年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
851.
无烟煤氧化制苯多羧酸 总被引:3,自引:2,他引:3
对阳泉无烟煤进行了碱-氧气氧化研究。发现反应温度、碱的用量、反应时间以及氧初压是影响和控制水溶酸产率和反应速度的重要因素。最佳氧化条件为:温度260℃,碱煤比3.28:1,反应时间2h,氧初压5.5MPa。在最佳氧化条件下水溶酸的产率高达66.0%,其中67.4%为苯多羧酸。并采用红外光谱和气相色谱对反应产物进行了定性和定量分析。 相似文献
852.
Miyuki Narita Junya Itoh Tohru Kikuchi Fumio Hamada 《Journal of inclusion phenomena and macrocyclic chemistry》2002,42(1-2):107-114
-Cyclodextrin dimer linked with ethylenediamine at the upper rim of the cyclodextrin has been synthesized and then modified with two dansyl moieties inthe presence of N,N'-dicyclohexylcarbodiimide. The sensing ability and bindingproperty of the title compound were investigated for steroids and terpenoids. Thefluorescence intensity of this dimer was decreased when a host–guest complex was formed. The value I/I0, where I0 and I are fluorescence intensitiesin the absence and presence of a guest and I is I0- I, was used as a parameter of sensitivity. This host exhibited a much higher sensitivity and selective molecular recognition ability for bile acids such as ursodeoxycholic acid andchenodeoxycholic acid and terpenoids such as (-)-borneol than the dansyl-modifiedcyclodextrins reported previously including -cyclodextrin dimer. The behaviors of the appended moieties of the host during the formation of host–guest complexes were studied using induced circular dichroism (ICD) and fluorescence spectra. The ICD intensityof this dimer was decreased on accommodation of a guest and this spectral pattern of the title dimer was opposite to that of bis dansyl-modified -cyclodextrin monomer. Theguest-induced variations in the fluorescence and ICD intensities suggest that this dimer formed a 1 : 1 host–guest complex and the appended moieties act as a hydrophobic cap. 相似文献
853.
854.
855.
研究了水溶液中双环[2,2,1]-2,5庚二烯-2,3-二羧酸在吖和吖啶橙的敏化作用下的光诱导价键异构化反应。量子效率分别为0,33(AY)和0,31(AO)。 相似文献
856.
Summary The adsorption behaviour of 48 metal ions on DEAE-cellulose layers has been investigated in aqueous hydrobromic and hydriodic acid media. RF values are given as a function of the hydrobromic and hydriodic acid concentration over the ranges 0.01–6 mol dm–3 and 0.01–3 mol dm–3, respectively, and are compared with those obtained with Avicel SF. RF spectra are comparatively simple in both media, reflecting the strong affinity of the bromide and iodide ions to the DEAE-cellulose phase. Pd, Pt, Re, Au and Hg are distributed chromatographically in either system, while most other metal ions exhibit rather extreme RF values of near unity or zero. Therefore, the selectivity of the systems is particularly high for Pd, Pt, Re, Au and Hg, providing the possibility of their excellent selective separations. 相似文献
857.
N. E. Golantsov A. V. Karchava M. A. Yurovskaya 《Chemistry of Heterocyclic Compounds》2006,42(8):1021-1024
Methods of obtaining ketones and enamino esters have been developed on the basis of reactions of 1-(1-phenylethyl)indole-2-carbonitrile
with organomagnesium and organozinc compounds. Removal of a benzyl group from the indole nitrogen atom by the Grignard reagent
has been discovered.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1179–1183, August, 2006. 相似文献
858.
亚氨基二乙酸—硅胶键合相的合成及其在核苷酸分析中的应用 总被引:2,自引:1,他引:2
以大孔微球硅胶(30nm,8μm)为基体,经γ-氨丙基三乙氧基硅烷活化与间隔臂氯代环氧丙烷反应后,再与螯合剂亚氨基二乙酸键合,后者与铜离子(Ⅱ)螯合后,构成定位金属离子亲和色谱固定相。用磷酸盐缓冲溶液(pH=7.0)作流动相,经紫外吸收检测(260nm)对核苷酸混合物进行了高效液相色谱分析,取得较满意的结果。 相似文献
859.
XIE Qing-Ji PANG Xiao-Yong NIE Li-Hua YAO Shou-Zhuo Department of Chemistry Chemical Engineering Hunan University Changsha Hunan China 《中国化学》1994,12(5):425-431
Piezoelectric transmission spectroelectrochemistry (PTSEC), i.e., the combination of electrochemistry and spectroelectrochemistry (SEC) with electrochemical quartz crystal microbalance (EQCM) technique is reported by using a normal piezoelectric quartz crystal (PQC) as an optically-transparent electrode (OTE). A theoretical relationship between the PQC response and the spectroelectrochemical response is derived and used to estimate the apparent molar absorptivity of the absorbing species deposited on the OTE on the PQC surface. The complex of copper with aspartic acid is used to test this new PQC-SEC technique. Results show that the combination of three such diverse techniques provides a very useful methodology for studying electrode processes and electrode surface characteristics in situ. 相似文献
860.
A glassy carbon electrode was modified with an electropolymerized film of 1‐naphthylamine in aqueous solution. The electrocatalytic properties of this modified electrode (ME) were investigated using ascorbic acid (AA) as probe molecule. The electrochemical behavior of AA in buffer solution was examined by voltammetry and amperometry. The results showed that the ME exhibited good electrocatalytic activity towards the oxidation of AA, as a consequence, it can be used as amperometric sensor of this analyte in a flow injection system with good sensitivity. Calibration curves were linear over the concentration range 0.05–1.50 mM with a phosphate buffer solution pH 3 as the carrier, the detection limit was 1 ppm (S/N=3). The methods were applied to the determination of AA in beverages and pharmaceutical products. A good correlation with a reference method was attained. 相似文献