全文获取类型
收费全文 | 331530篇 |
免费 | 5962篇 |
国内免费 | 3017篇 |
专业分类
化学 | 165099篇 |
晶体学 | 4670篇 |
力学 | 16601篇 |
综合类 | 128篇 |
数学 | 50008篇 |
物理学 | 104003篇 |
出版年
2021年 | 2227篇 |
2020年 | 2479篇 |
2019年 | 2642篇 |
2018年 | 11591篇 |
2017年 | 12303篇 |
2016年 | 8485篇 |
2015年 | 4079篇 |
2014年 | 4724篇 |
2013年 | 10375篇 |
2012年 | 13363篇 |
2011年 | 22762篇 |
2010年 | 14405篇 |
2009年 | 14258篇 |
2008年 | 19903篇 |
2007年 | 23169篇 |
2006年 | 9497篇 |
2005年 | 14662篇 |
2004年 | 10679篇 |
2003年 | 9732篇 |
2002年 | 7730篇 |
2001年 | 7160篇 |
2000年 | 5625篇 |
1999年 | 4076篇 |
1998年 | 3351篇 |
1997年 | 3306篇 |
1996年 | 3462篇 |
1995年 | 2957篇 |
1994年 | 3009篇 |
1993年 | 2869篇 |
1992年 | 3029篇 |
1991年 | 3117篇 |
1990年 | 2895篇 |
1989年 | 2790篇 |
1988年 | 2732篇 |
1987年 | 2650篇 |
1986年 | 2662篇 |
1985年 | 3435篇 |
1984年 | 3515篇 |
1983年 | 2929篇 |
1982年 | 3164篇 |
1981年 | 2927篇 |
1980年 | 2720篇 |
1979年 | 2905篇 |
1978年 | 3124篇 |
1977年 | 3073篇 |
1976年 | 3138篇 |
1975年 | 2862篇 |
1974年 | 2967篇 |
1973年 | 3061篇 |
1972年 | 2361篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Paired single residue‐transposed Lys‐N and Lys‐C digestions for label‐free identification of N‐terminal and C‐terminal MS/MS peptide product ions: ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry and tandem mass spectrometry for peptide de novo sequencing 下载免费PDF全文
2.
The influence of different referencing methods on the accuracy of δ13C value measurement of ethanol fuel by gas chromatography/combustion/isotope ratio mass spectrometry 下载免费PDF全文
3.
4.
Nanocomposite hydrogel consisting of Na‐montmorillonite with enhanced mechanical properties 下载免费PDF全文
Ting Li Shuangfei Xiang Piming Ma Huiyu Bai Weifu Dong Mingqing Chen 《Journal of Polymer Science.Polymer Physics》2015,53(14):1020-1026
A new kind of nanocomposite (NC) hydrogel with Na‐montmorillonite (MMT) is presented in this article. The NC hydrogels were synthesized by free radical copolymerization of acrylamide and (3‐acrylamidopropyl) trimethylammonium chloride (ATC) in the presence of MMT and N,N′‐methylene‐bis‐acrylamide used as chemical cross‐linker. Due to the cation‐exchange reaction between MMT and ATC (cationic monomer) during the synthesis of NC hydrogels, MMT platelets were considered chemical “plane” cross‐linkers, different from “point” cross‐linkers. With increasing amount of MMT, the crosslinking degree enhanced, causing a decrease of the swelling degree at equilibrium. Investigations of mechanical properties indicated that NC hydrogels exhibited enhanced strength and toughness, which resulted from chemical interaction between exfoliated MMT platelets and polymer chains in hydrogels. Dynamic shear measurements showed that both storage modulus and loss modulus increased with increasing MMT content. The idea described here provided a new route to prepare hydrogels with high mechanical properties by using alternative natural Na‐MMT. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1020–1026 相似文献
5.
6.
7.
8.
Experimental and Theoretical Studies on the Rearrangement of 2‐Oxoazepane α,α‐Amino Acids into 2′‐Oxopiperidine β2,3,3‐Amino Acids: An Example of Intramolecular Catalysis 下载免费PDF全文
Dr. Diego Núñez‐Villanueva Dr. M. Ángeles Bonache Laura Lozano Dr. Lourdes Infantes Prof. José Elguero Prof. Ibon Alkorta Prof. M. Teresa García‐López Dr. Rosario González‐Muñiz Dr. Mercedes Martín‐Martínez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(6):2489-2500
Enantiopure β‐amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4‐carboxy‐2‐oxoazepane α,α‐amino acids to lead to 2′‐oxopiperidine‐containing β2,3,3‐amino acids, upon basic or acid hydrolysis of the 2‐oxoazepane α,α‐amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six‐membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4‐carboxylic acid substituted 2‐oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2‐oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis. 相似文献
9.
V. A. Bespalov V. M. Glazov E. A. Il’ichev Yu. A. Klimov S. V. Kuklev A. E. Kuleshov R. M. Nabiev G. N. Petrukhin B. G. Potapov G. S. Rychkov D. S. Sokolov V. V. Fandeev E. A. Fetisov S. S. Yakushov 《Technical Physics》2015,60(4):553-560
The results of investigation of optical image detectors designed for the largest problem, near-VUV, range of the spectrum are presented. The possibility of using a dual-stage image detection system to appreciably lower the sensitivity threshold and make computer data processing feasible is considered. The integration of a UV module into a wideband image detector is studied. 相似文献
10.
We consider conformal gravity as a gauge natural theory. We study its conservation laws and superpotentials. We also consider the Mannheim and Kazanas spherically symmetric vacuum solution and discuss conserved quantities associated to conformal and diffeomorphism symmetries. 相似文献