全文获取类型
收费全文 | 102523篇 |
免费 | 8029篇 |
国内免费 | 9103篇 |
专业分类
化学 | 48163篇 |
晶体学 | 2299篇 |
力学 | 3639篇 |
综合类 | 455篇 |
数学 | 18071篇 |
物理学 | 47028篇 |
出版年
2023年 | 357篇 |
2022年 | 422篇 |
2021年 | 757篇 |
2020年 | 1176篇 |
2019年 | 2409篇 |
2018年 | 1541篇 |
2017年 | 2045篇 |
2016年 | 1323篇 |
2015年 | 1207篇 |
2014年 | 2068篇 |
2013年 | 3780篇 |
2012年 | 2724篇 |
2011年 | 3330篇 |
2010年 | 3433篇 |
2009年 | 8156篇 |
2008年 | 9400篇 |
2007年 | 7960篇 |
2006年 | 7286篇 |
2005年 | 5263篇 |
2004年 | 5038篇 |
2003年 | 5042篇 |
2002年 | 6059篇 |
2001年 | 4174篇 |
2000年 | 3965篇 |
1999年 | 3626篇 |
1998年 | 3056篇 |
1997年 | 2302篇 |
1996年 | 2024篇 |
1995年 | 2546篇 |
1994年 | 2373篇 |
1993年 | 1811篇 |
1992年 | 1317篇 |
1991年 | 980篇 |
1990年 | 777篇 |
1989年 | 688篇 |
1988年 | 631篇 |
1987年 | 443篇 |
1985年 | 978篇 |
1984年 | 643篇 |
1983年 | 501篇 |
1982年 | 667篇 |
1981年 | 815篇 |
1980年 | 731篇 |
1979年 | 585篇 |
1978年 | 589篇 |
1977年 | 544篇 |
1976年 | 547篇 |
1975年 | 322篇 |
1974年 | 358篇 |
1973年 | 467篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
21.
Wenxiong Chen & Ruobing Zhang 《数学研究》2020,53(4):436-470
In this paper, we will explore the geometric effects of conformally covariant
operators and the induced nonlinear curvature equations in certain nonlocal nature.
Mainly, we will prove some regularity and rigidity results for the distributional solutions to those equations. 相似文献
22.
Ziya CAN Büra KESKN Ayem ARDA Erol ERA Mustafa Reat APAK 《Turkish Journal of Chemistry》2020,44(5):1366
Excessive amounts of reactive oxygen species (ROS), unless counterbalanced by antioxidants, can cause cellular damage under oxidative stress conditions; therefore, antioxidative defenses against ROS must be measured. With the development of nanotechnology, nanoparticles have found numerous applications in science, health, and industries. Magnetite nanoparticles (Fe 3 O 4 :MNPs) have attracted attention because of their peroxidase-like activity. In this study, hydroxyl radicals (•OH) generated by MNPs-catalyzed degradation of H 2 O 2 converted the N,N-dimethyl-p-phenylenediamine (DMPD) probe into its colored DMPD•+ radical cation, which gave an absorbance maximum at λ = 553 nm. In the presence of antioxidants, •OH was partly scavenged by antioxidants and produced less DMPD• + , causing a decrease in the 553 nm-absorbance. Antioxidant concentrations were calculated with the aid of absorbance differences between the reference and sample solutions. The linear working ranges and trolox equivalent antioxidant capacity coefficients of different classes of antioxidants were determined by applying the developed method. In addition, binary and ternary mixtures of antioxidants were tested to observe the additivity of absorbances of mixture constituents. The method was applied to real samples such as orange juice and green tea. Student t-test, F tests, and the Spearman’s rank correlation coefficient were used for statistical comparisons. 相似文献
23.
24.
Hazardous 4-nitrophenol (4-NP) has created serious threats to humans and the environment; therefore, it is highly desirable to develop a facile and practical method for the monitoring of 4-NP in environment and food. Here, a fluorescence method based on modified polyethyleneimine-capped carbon dots (mPEI-CDs) was developed for sensitive and selective determination of 4-NP in water, fruit, and vegetable samples. First, highly fluorescent mPEI-CDs (quantum yield about 40.3%) were easily synthesized via a one-step hydrothermal method by using novel acetic anhydride modified polyethyleneimine (mPEI) and citric acid as precursors. Compared to the unmodified PEI-CDs, the acetic anhydride mPEI-CDs exhibited excellent fluorescent stability in a wider pH range of 4.0–9.0. Under pH 8.0, a selective determination of 4-NP was achieved based on the inner filter effect (IFE) mechanism. After optimization, good linear relationships between fluorescence intensity function (F0-F)/F0 and the concentration of 4-NP were obtained in ranges of 0.5–10 and 10–100 μM, respectively, while efficiently avoiding the interferences from two other nitrophenol isomers, possible coexisting metal cations and anions in samples. Finally, the proposed approach was successfully applied for the determination of 4-NP in water, honey, strawberry, and tomato samples. 相似文献
25.
In this work, a vanillin complex is immobilized onto MCM-41 and characterized by FT-IR, X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, thermogravimetric analysis, and BET techniques. This supported Schiff base complex was found to be an efficient and recoverable catalyst for the chemoselective oxidation of sulfides into sulfoxides and thiols into their corresponding disulfides (using hydrogen peroxide as a green oxidant) and also a suitable catalyst for the preparation of 2,3-dihydroquinazolin-4(1H)-one derivatives in water at 90°C. Using this protocol, we show that a variety of disulfides, sulfoxides, and 2,3-dihydroquinazolin-4(1H)-one derivatives can be synthesized in green conditions. The catalyst can be recovered and recycled for further reactions without appreciable loss of catalytic performance. 相似文献
26.
27.
Existence and Uniqueness of Solution for a Class of Nonlinear Degenerate Elliptic Equations 下载免费PDF全文
Albo Carlos Cavalheiro 《分析论及其应用》2020,36(1):69-88
In this work we are interested in the existence and uniqueness of solutions for the Navier problem associated to the degenerate nonlinear elliptic equations■,in the setting of the weighted Sobolev spaces. 相似文献
28.
29.
Weiliang Xiao & Xuhuan Zhou 《数学研究》2020,53(3):316-328
We study a kind of generalized porous medium equation with fractional Laplacian and abstract pressure term. For a large class of equations corresponding to the form: $u_t+\nu \Lambda^{\beta}u=\nabla\cdot(u\nabla Pu)$, we get their local well-posedness in Fourier-Besov spaces for large initial data. If the initial data is small, then the solution becomes global. Furthermore, we prove a blowup criterion for the solutions. 相似文献
30.
Recent Progress in the $L_p$ Theory for Elliptic and Parabolic Equations with Discontinuous Coefficients 下载免费PDF全文
Hongjie Dong 《分析论及其应用》2020,36(2):161-199
In this paper, we review some results over the last 10-15 years on elliptic and parabolic equations with discontinuous coefficients. We begin with an approach given by N. V. Krylov to parabolic equations in the whole space with $\rm{VMO}_x$ coefficients. We then discuss some subsequent development including elliptic and parabolic equations with coefficients which are allowed to be merely measurable in one or two space directions, weighted $L_p$estimates with Muckenhoupt ($A_p$) weights, non-local elliptic and parabolic equations, as well as fully nonlinear elliptic and parabolic equations. 相似文献