全文获取类型
收费全文 | 160152篇 |
免费 | 7279篇 |
国内免费 | 6242篇 |
专业分类
化学 | 75160篇 |
晶体学 | 1573篇 |
力学 | 11452篇 |
综合类 | 366篇 |
数学 | 39458篇 |
物理学 | 45664篇 |
出版年
2025年 | 111篇 |
2024年 | 1043篇 |
2023年 | 1994篇 |
2022年 | 2988篇 |
2021年 | 3477篇 |
2020年 | 3985篇 |
2019年 | 3636篇 |
2018年 | 12403篇 |
2017年 | 12030篇 |
2016年 | 9387篇 |
2015年 | 4428篇 |
2014年 | 4570篇 |
2013年 | 5705篇 |
2012年 | 10397篇 |
2011年 | 16826篇 |
2010年 | 9936篇 |
2009年 | 10033篇 |
2008年 | 10953篇 |
2007年 | 12576篇 |
2006年 | 3976篇 |
2005年 | 4429篇 |
2004年 | 3954篇 |
2003年 | 3838篇 |
2002年 | 2739篇 |
2001年 | 1742篇 |
2000年 | 1659篇 |
1999年 | 1693篇 |
1998年 | 1509篇 |
1997年 | 1439篇 |
1996年 | 1487篇 |
1995年 | 1176篇 |
1994年 | 1006篇 |
1993年 | 871篇 |
1992年 | 745篇 |
1991年 | 656篇 |
1990年 | 567篇 |
1989年 | 463篇 |
1988年 | 405篇 |
1987年 | 344篇 |
1986年 | 305篇 |
1985年 | 252篇 |
1984年 | 197篇 |
1983年 | 144篇 |
1982年 | 131篇 |
1981年 | 111篇 |
1980年 | 121篇 |
1979年 | 77篇 |
1978年 | 67篇 |
1977年 | 59篇 |
1973年 | 53篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
In many-valued logic the decision of functional completeness is a basic and important problem, and the thorough solution to this problem depends on determining all maximal closed sets in the set of many-valued logic functions. It includes three famous problems, i.e., to determine all maximal closed sets in the set of the total, of the partial and of the unary many-valued logic functions, respectively. The first two problems have been completely solved ([1], [2], [8]), and the solution to the third problem boils down to determining all maximal subgroups in the k-degree symmetric group Sk, which is an open problem in the finite group theory. In this paper, all maximal closed sets in the set of unary p-valued logic functions are determined, where p is a prime. Mathematics Subject Classification: 03B50, 20B35. 相似文献
32.
33.
34.
When the cations bound to purple membrane are removed it turns blue, and when this blue membrane is irradiated its color changes to pink. Irradiation of pink membrane leads to the reformation of blue membrane. We have determined that the quantum efficiency for the formation of pink membrane from deionized blue membrane is 1.6 ± 0.6 ± 10 4 at 0o C, pH 5.0. We also found that the quantum efficiency for the back photoconversion, i.e. the formation of blue membrane from pink membrane, is 8.8 ± 1.6 ± 10-3 at 0o C, 55 times greater than that of the forward photoconversion reaction. The extinction coefficients of the pink membrane and blue membrane were determined to be 44 500 ± 670 cm-1 M-1 at 491 nm and 54 760 ± 830 cm-1 M -1 at 603 nm, respectively, assuming light-adapted purple membrane is 63 000 cm-1 M -1 at 568 nm. The quantum efficiency for forming pink membrane from blue membrane is much lower than that for forming the photointermediate of the blue membrane's photocycle. Their relationship is similar to that of light-adaptation and photocycle of the dark-adapted purple membrane. 相似文献
35.
采用火焰原子吸收法对30例男性型秃发患者及38例毛发健康男性对照者发中宏量元素K、Na、Ca、Mg的含量进行了测定。结果表明,男性型秃发患者发K含量明显高于对照组,具有统计学上的显著差异(P〈0.01)。发Na、Ca、Mg含量在两组间未见显著差异(P〉0.05)。本文提示,体内某些宏量元素变化对男性型秃发可能产生一定影响。 相似文献
36.
Oleg V. Mikhailov Marina A. Kazymova Tatyana A. Shumilova Galina A. Chmutova Svetlana E. Solovieva 《Transition Metal Chemistry》2005,30(3):299-304
The complexing process proceeding in the NiII–thiocarbohydrazide (H2N–H–NC(=S)–NH–NH2)–propanone triple system in EtOH solution and nickel(II)hexacyanoferrate(II) gelatin-immobilized matrix has been studied. It has been found that in the first case, template synthesis leading, as a minimum, to formation of three coordination compounds of NiII with (N,N,S,S)-donor tetradentate ligands having NiL1, NiL2 and NiL3compositions where L1 is 4,6,6-trimethyl-2,3,7,8-tetraazanonen-3-di(thiohydrazide)-1,9, L2 is 4,6,6,12-tetrametyl-1,9-dithio-2,3,7,8,10,11-hexaazatridekadien-3,11-hydrazide-1 and L3 is 2,8,10,10,16-pentamethyl-5,13-dithio-3,4,6,7,11,12,14,15-octaazaheptadekatrien-2,7,15 is observed, whereas in the gelatin-immobilized matrix, a complexing process in the system considered does not occur. 相似文献
37.
在非水溶剂(二甲基亚砜(DMSO)及N,N_二甲基甲酰胺(DMF))中的伏安曲线.呈现三对电流峰,表明存在三个可逆或准可逆酞菁配体的单电子转移过程,而在水溶液中则不呈现准可逆行为,且波形改变很大.此外,还比较了四磺酸基酞菁锌的水溶液在自然光和红光(600~700nm)照射下的循环伏安曲线,其氧化还原峰的数目和峰电位基本不变,但在红光照射下比自然光照射下的峰电流明显增大. 相似文献
38.
The Pd-catalyzed annulation of arynes by 2-halobiaryls and related vinylic halides provides a very efficient, high yielding synthesis of polycyclic aromatic and heteroaromatic hydrocarbons. This process appears to involve the catalytic, stepwise coupling of two very reactive substrates, an aryne and an organopalladium species, to generate excellent yields of cross-coupled products. 相似文献
39.
A new method based on hollow fiber-protected liquid-phase microextraction (LPME) was developed for the simultaneous determination
of atrazine, desethyl atrazine and desisopropyl atrazine in environmental water samples. In LPME, analytes were extracted
into 1-octanol immobilized in the micropores of a poly(vinylidene fluoride) porous hollow fiber membrane, and back extracted
into the acceptor (4 M HCl) filled in the lumen of the hollow fiber. After LPME, the analytes trapped in the acceptor were
analyzed with high-performance liquid chromatography after neutralization. The effect of extraction factors such as sample
pH, acceptor pH, salinity, extraction time, stirring rate, and humic acid were studied. Under the optimized conditions, the
limits of detection and relative standard deviations were respectively in the range of 0.5–1.0 μg L−1 and 3.9–4.7% (n = 5). The proposed method was applied to determine atrazine, desethyl atrazine and desisopropyl atrazine in wastewater and
groundwater samples. The three analytes were below the limits of detection, but good relative spiked recoveries over 90.1
± 5.9% at 5 μg L−1 spiked level were obtained. 相似文献
40.
Dynamic behaviour of river colloidal and dissolved organic matter through cross-flow ultrafiltration system 总被引:1,自引:0,他引:1
Through cross-flow filtration (CFF) with a 1-kDa regenerated cellulose Pellicon 2 module, the ultrafiltration characteristics of river organic matter from Longford Stream, UK, were investigated. The concentration of organic carbon (OC) in the retentate in the Longford Stream samples increased substantially with the concentration factor (cf), reaching approximately 40 mg/L at cf 15. The results of dissolved organic carbon (DOC) and colloidal organic carbon (COC) analysis, tracking the isolation of colloids from river waters, show that 2 mg/L of COC was present in those samples and good OC mass balance (77-101%) was achieved. Fluorescence measurements were carried out for the investigation of retentate and permeate behaviour of coloured dissolved organic materials (CDOM). The concentrations of CDOM in both the retentate and permeate increased with increasing cf, although CDOM were significantly more concentrated in the retentate. The permeation model expressing the correlation between log[CDOM] in the permeate and logcf was able to describe the permeation behaviour of CDOM in the river water with regression coefficients (r(2)) of 0.94 and 0.98. Dry weight analysis indicated that the levels of organic colloidal particles were from 49 to 71%, and between 29 and 51% of colloidal particles present were inorganic. COC as a percentage of DOC was found to be 10-16% for Longford Stream samples. 相似文献