全文获取类型
收费全文 | 53794篇 |
免费 | 8750篇 |
国内免费 | 5712篇 |
专业分类
化学 | 37733篇 |
晶体学 | 643篇 |
力学 | 3216篇 |
综合类 | 329篇 |
数学 | 6005篇 |
物理学 | 20330篇 |
出版年
2024年 | 243篇 |
2023年 | 1177篇 |
2022年 | 2147篇 |
2021年 | 2261篇 |
2020年 | 2337篇 |
2019年 | 2179篇 |
2018年 | 1859篇 |
2017年 | 1801篇 |
2016年 | 2618篇 |
2015年 | 2664篇 |
2014年 | 3142篇 |
2013年 | 3990篇 |
2012年 | 4883篇 |
2011年 | 4820篇 |
2010年 | 3351篇 |
2009年 | 3131篇 |
2008年 | 3377篇 |
2007年 | 2976篇 |
2006年 | 2675篇 |
2005年 | 2289篇 |
2004年 | 1748篇 |
2003年 | 1406篇 |
2002年 | 1283篇 |
2001年 | 1024篇 |
2000年 | 902篇 |
1999年 | 1048篇 |
1998年 | 865篇 |
1997年 | 810篇 |
1996年 | 797篇 |
1995年 | 729篇 |
1994年 | 615篇 |
1993年 | 523篇 |
1992年 | 431篇 |
1991年 | 394篇 |
1990年 | 333篇 |
1989年 | 242篇 |
1988年 | 178篇 |
1987年 | 163篇 |
1986年 | 164篇 |
1985年 | 143篇 |
1984年 | 89篇 |
1983年 | 97篇 |
1982年 | 59篇 |
1981年 | 42篇 |
1980年 | 32篇 |
1979年 | 21篇 |
1976年 | 22篇 |
1975年 | 25篇 |
1973年 | 18篇 |
1972年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
Conjugated linoleic acids (CLA) have been a subject of extensive investigation for their anticarcinogenic, hypolipidemic, antiatherosclerotic and immune-enhancing activities. Stability of CLA in foods has not received much attention by both academics and industry. Although CLA has shown many beneficial effects, its decomposition must be prevented when CLA in foods is processed, stored and transported. However, no study to date has addressed the stability of CLA in foods. The present study was carried out further to examine the stability of CLA in egg yolk during the storage and frying, using gas liquid chromatography (GLC) and silver ion high-performance liquid chromatography (Agt-HPLC). The eggs, containing 4.0% CLA per gramme of egg yolk, were 40s. Either storage for 6 months or frying for 40 s did not significantly change the composition of CLA in egg yolk. However, the degradation of CLA was statistically significant when the CLA components of egg yolk protected CLA from degradation. It is concluded that CLA is well preserved in egg before it is consumed. 相似文献
72.
Gengliang Yang Zhiwei Li Dexian Wang Zhefeng Zhang Erdong Liu Yi Chen 《Chromatographia》2002,56(7-8):515-518
Summary A high-performance liquid chromatographic method was developed for the chiral separation of a new anti-diabetic agent, N-(trans-4-isopropylcyclohexylcarbonyl)-D-phenylalanine,
and its L-enantiomer. The separation was performed on a Sumichiral OA-3300 column. Optimized mobile phase was 0.025 mol.L−1 ammonium acetate in methanol solution. UV detection was at 210 nm. Baseline chiral separation was obtained within 12 minutes.
The detection limits are 80 pg for the D-enantiomer and 120 pg for the L-enantiomer. RSD of the method was below 1% (n=5). 相似文献
73.
A new chromogenic reagent tribromocarboxyarsenazo has been synthesized and the chromogenic reaction of this reagent with zirconium studied. In the medium of 0.5 mol/1 HCl, a blue complex is formed by the reaction of zirconium with tribromocarboxyarsenazo. The maximum absorption of the complex is at 633 nm, and the apparent molar absorptivity is 4.8 x 10(4) 1/mol/cm. Beer's law is obeyed over the zirconium concentration range 0-18, mug/25 ml. The method has been applied to the determination of zirconium in aluminum, copper and magnesium alloys with satisfactory results. 相似文献
74.
Nitration of 2-bromo or 2-chlorotropone gave a new compound, 2,7-dinitrotropone: The halegen atom on the 2-position was displaced by the nitro group through an electrophilic substitution reaction. Nitration of 3,5,7-tribromotropolone gave 3,7-dibromo-5-nitropolone: the nitro group again displaced the bromine atom at position 5. 相似文献
75.
Stopped-flow was used to evaluate the methylation and reduction kinetics of the isolated alpha subunit of acetyl-Coenzyme A synthase from Moorella thermoacetica. This catalytically active subunit contains a novel Ni-X-Fe4S4 cluster and a putative unidentified n = 2 redox site called D. The D-site must be reduced for a methyl group to transfer from a corrinoid-iron-sulfur protein, a key step in the catalytic synthesis of acetyl-CoA. The Fe4S4 component of this cluster is also redox active, raising the possibility that it is the D-site or a portion thereof. Results presented demonstrate that the D-site reduces far faster than the Fe4S4 component, effectively eliminating this possibility. Rather, this component may alter catalytically important properties of the Ni center. The D-site is reduced through a pathway that probably does not involve the Fe4S4 component of this active-site cluster. 相似文献
76.
巯基葡聚糖凝胶分离富集光度法测定镉的研究 总被引:2,自引:0,他引:2
研究了在TritonX-100存在下,新显色剂邻羧基苯基重氮氨基偶氮苯(CDAA)与镉的显色体系,在pH10.6的Na2B4O7-NaOH体系中,显色反应灵敏度很高。络合物的最大吸收位于545nm处,其摩尔吸光系数为2.22×10^5L·mol^-1·cm^-1。镉含量在0 ̄12μg/25mL符合比耳定律。将该体系用于巯基葡聚糖凝胶分离富集-光度法测水样中微量镉,结果令人满意。 相似文献
77.
A new short synthesis of 3-substituted 5-amino-1-(chloromethyl)-1,2-dihydro-3H-benzo[e]indoles from Martius Yellow is disclosed. The key steps of the synthesis were three efficient regioselective reactions (iodination, 5-exo-trig aryl radical-alkene cyclization and carboxylation). 相似文献
78.
The UV and visible spectra of six model compounds were studied. These compounds can be considered as models of polyaniline in the reduced, cation radical, partially oxidized and oxidized forms. After treatment of a mixture of equal molar quantities of the reduced form (DPPD) and the oxidized form (QDIM) with acid, the following reaction was observed: DPPD(I)+QDIM(III)+2H+=2DPPD+(II) After similar treatment of the partially oxidized form (V) with acid, the radical cation salt formed. The UV and visible spectra of polyaniline in the reduced form, oxidized form and conductive form are similar to the spectra of DPPD, QDIM and DPPD+ or radical cation salt of V respectively. We propose that the polyaniline synthesized by chemical oxidation of aniline consists of oxidized and reduced repeat units. Upon protonation a redox reaction (or electron rearrangement) occurs and forms delocalized radical cations (polarons) in the polymer chain which are highly conductive. 相似文献
79.
对1993年6月至1996年6月因体弱多病,生长发育矮小等来诊的351例儿童作了血铅测查和临床全征分析,以期引起儿科临床医师重视。 相似文献
80.
Nitropyridines reacted with an excess of vinyl Grignard reagent to produce 4- or 6-azaindoles. Improved yields were obtained when a halogen atom was present at the position alpha to the nitrogen atom in the pyridine ring. 相似文献