全文获取类型
收费全文 | 55840篇 |
免费 | 4882篇 |
国内免费 | 6718篇 |
专业分类
化学 | 41792篇 |
晶体学 | 412篇 |
力学 | 1502篇 |
综合类 | 796篇 |
数学 | 9849篇 |
物理学 | 13089篇 |
出版年
2023年 | 587篇 |
2022年 | 925篇 |
2021年 | 1984篇 |
2020年 | 1697篇 |
2019年 | 1648篇 |
2018年 | 1315篇 |
2017年 | 1493篇 |
2016年 | 1811篇 |
2015年 | 1840篇 |
2014年 | 2230篇 |
2013年 | 3868篇 |
2012年 | 2881篇 |
2011年 | 2966篇 |
2010年 | 2708篇 |
2009年 | 3292篇 |
2008年 | 3443篇 |
2007年 | 3669篇 |
2006年 | 2947篇 |
2005年 | 2207篇 |
2004年 | 2081篇 |
2003年 | 1965篇 |
2002年 | 4338篇 |
2001年 | 1937篇 |
2000年 | 1415篇 |
1999年 | 1012篇 |
1998年 | 993篇 |
1997年 | 760篇 |
1996年 | 816篇 |
1995年 | 778篇 |
1994年 | 664篇 |
1993年 | 713篇 |
1992年 | 605篇 |
1991年 | 456篇 |
1990年 | 364篇 |
1989年 | 307篇 |
1988年 | 309篇 |
1987年 | 252篇 |
1986年 | 252篇 |
1985年 | 365篇 |
1984年 | 261篇 |
1983年 | 160篇 |
1982年 | 315篇 |
1981年 | 500篇 |
1980年 | 445篇 |
1979年 | 485篇 |
1978年 | 381篇 |
1977年 | 285篇 |
1976年 | 249篇 |
1974年 | 79篇 |
1973年 | 157篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
Liang‐An Chen Alan R. Lear Pin Gao M. Kevin Brown 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(32):11072-11076
A method for the construction of boron‐substituted quaternary carbons or diarylquaternary carbons by arylboration of highly substituted alkenylarenes is presented. A wide range of alkenes and arylbromides can participate in this reaction thus allowing for a diverse assortment of products to be prepared. In addition, a solvent dependent regiodivergent arylboration of 1,2‐disubstituted alkenylarenes is presented, thus greatly increasing the scope of products that can be accessed. 相似文献
93.
Given a graph we are interested in studying the symmetric matrices associated to with a fixed number of negative eigenvalues. For this class of matrices we focus on the maximum possible nullity. For trees this parameter has already been studied and plenty of applications are known. In this work we derive a formula for the maximum nullity and completely describe its behavior as a function of the number of negative eigenvalues. In addition, we also carefully describe the matrices associated with trees that attain this maximum nullity. The analysis is then extended to the more general class of unicyclic graphs. Further our work is applied to re-describing all possible partial inertias associated with trees, and is employed to study an instance of the inverse eigenvalue problem for certain trees. 相似文献
94.
95.
The radius of spatial analyticity for solutions of the KdV equation is studied. It is shown that the analyticity radius does not decay faster than as time t goes to infinity. This improves the works of Selberg and da Silva (2017) [30] and Tesfahun (2017) [34]. Our strategy mainly relies on a higher order almost conservation law in Gevrey spaces, which is inspired by the I-method. 相似文献
96.
Shuqi Wang Tianyu Cai Huixiang Liu Aijuan Yang Jie Xing 《Journal of separation science》2019,42(21):3330-3339
The traditional antimalarial herb Artemisia annua L., from which artemisinin is isolated, is widely used in endemic regions. It has been suggested that artemisinin activity can be enhanced by flavonoids in A. annua; however, how fast and how long the flavonoids are present in the body remains unknown. In the present study, a rapid and sensitive liquid chromatography with tandem mass spectrometry method was developed and validated for the simultaneous determination of three major flavonoids components, i.e. chrysosplenol D, chrysoplenetin, and artemetin and their glucuronidated metabolites in rats after oral administrations of A. annua extracts at a therapeutic ultra‐low dose. The concentration of the intact form was determined directly, and the concentration of the glucuronidated form was assayed in the form of flavonoids aglycones, after treatment with β‐glucuronidase/sulfatase. The method was linear in the range of 0.5–300.0 ng/mL for chrysoplenetin and artemetin, and 2–600 ng/mL for chrysosplenol D. All the validation data conformed to the acceptance requirements. The study revealed a significantly higher exposure of the flavonoid constituents in conjugated forms in rats, with only trace intact from. Multiple oral doses of A. annua extracts led to a decreased plasma concentration levels for three flavonoids. 相似文献
97.
Preparation,structure elucidation,and antioxidant activity of new bis(thiosemicarbazone) derivatives
Hasan YAKAN 《Turkish Journal of Chemistry》2020,44(4):1085
Schiff-base–bearing new bis(thiosemicarbazone) derivatives were prepared from terephthalaldehyde and various thiosemicarbazides. FT–IR, 1H NMR, 13C NMR, and UV–Vis spectroscopic methods and elemental analysis were used to elucidate the identification of the synthesized molecules. The in vitro antioxidant activity of the synthesized compounds was analysed with the 1,1-diphenyl-2-picryl hydrazyl free-radical–trapping process. The synthesized compounds exhibited lower antioxidant activity than the standard ascorbic acid. IC50 values of the synthesized molecules measured from 3.81 ± 0.01 to 29.05 ± 0.11 μM. Among the synthesized compounds, compound 3 had the best antioxidant activity. Moreover, this study explained the structure–activity relationship of the synthesized molecules with different substituents in radical trapping reactions. 相似文献
98.
We introduce a quantum key distribution protocol using mean multi-kings’ problem. Using this protocol, a sender can share a bit sequence as a secret key with receivers. We consider a relation between information gain by an eavesdropper and disturbance contained in legitimate users’ information. In BB84 protocol, such relation is known as the so-called information disturbance theorem. We focus on a setting that the sender and two receivers try to share bit sequences and the eavesdropper tries to extract information by interacting legitimate users’ systems and an ancilla system. We derive trade-off inequalities between distinguishability of quantum states corresponding to the bit sequence for the eavesdropper and error probability of the bit sequence shared with the legitimate users. Our inequalities show that eavesdropper’s extracting information regarding the secret keys inevitably induces disturbing the states and increasing the error probability. 相似文献
99.
Ultrasound-assisted approach was successfully applied for the synthesis of mayenite from calcium and aluminum hydroxides and then subsequently impregnated with Ni by the wet impregnation method. The synthesis was performed with a 13 mm probe-type ultrasound, operating under an acoustic power of 30.5 W and a frequency of 20 kHz. Ultrasound application was studied in detail from a 3k experimental design, where the variables studied were ultrasound time (10–50 min) and calcination temperature (900–1200 °C). Ultrasound promoted an effective dispersion of the precursors in a short time of 10 min leading to a high conversion to mayenite after calcination at 1200 °C. Ultrasound treatment also had a positive effect on Ni impregnation, increasing the dispersion of the metal in the support and leading to a stronger interaction of nickel-containing species with mayenite support. The use of ultrasound application has proved to be attractive both for catalyst properties and for facilitating catalyst synthesis. 相似文献