全文获取类型
收费全文 | 54997篇 |
免费 | 5968篇 |
国内免费 | 4041篇 |
专业分类
化学 | 27918篇 |
晶体学 | 286篇 |
力学 | 2873篇 |
综合类 | 867篇 |
数学 | 10075篇 |
物理学 | 22987篇 |
出版年
2023年 | 532篇 |
2022年 | 916篇 |
2021年 | 1877篇 |
2020年 | 1510篇 |
2019年 | 1566篇 |
2018年 | 1244篇 |
2017年 | 1479篇 |
2016年 | 1803篇 |
2015年 | 1820篇 |
2014年 | 2552篇 |
2013年 | 4006篇 |
2012年 | 2959篇 |
2011年 | 3277篇 |
2010年 | 2696篇 |
2009年 | 3443篇 |
2008年 | 3478篇 |
2007年 | 3692篇 |
2006年 | 2953篇 |
2005年 | 2214篇 |
2004年 | 1953篇 |
2003年 | 1933篇 |
2002年 | 1760篇 |
2001年 | 1527篇 |
2000年 | 1238篇 |
1999年 | 1030篇 |
1998年 | 969篇 |
1997年 | 770篇 |
1996年 | 760篇 |
1995年 | 713篇 |
1994年 | 681篇 |
1993年 | 701篇 |
1992年 | 621篇 |
1991年 | 498篇 |
1990年 | 393篇 |
1989年 | 332篇 |
1988年 | 349篇 |
1987年 | 298篇 |
1986年 | 281篇 |
1985年 | 399篇 |
1984年 | 314篇 |
1983年 | 174篇 |
1982年 | 357篇 |
1981年 | 518篇 |
1980年 | 463篇 |
1979年 | 501篇 |
1978年 | 399篇 |
1977年 | 301篇 |
1976年 | 263篇 |
1974年 | 89篇 |
1973年 | 166篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
81.
82.
83.
Crisólogo Dolores-Flores Martha Iris Rivera-López Javier García-García 《International Journal of Mathematical Education in Science & Technology》2019,50(3):369-389
This paper reports the results of a research exploring the mathematical connections of pre-university students while they solving tasks which involving rates of change. We assume mathematical connections as a cognitive process through which a person finds real relationships between two or more ideas, concepts, definitions, theorems, procedures, representations or meanings or with other disciplines or the real-world. Four tasks were proposed to the 33 pre-university students that participated in this research; the central concept of the first task is the slope, the last three tasks contain concepts like velocity, speed and acceleration. Task-based interviews were conducted to collect data and later analysed with thematic analysis. Results showed most of the students made mathematical connections of the procedural type, the mathematical connections of the common features type are made in smaller quantities and the mathematical connection of the generalization type is scarcely made. Furthermore, students considered slope as a concept disconnected from velocity, speed and acceleration. 相似文献
84.
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
85.
Alberto Piloni Roberto Simonutti Martina H. Stenzel 《Journal of polymer science. Part A, Polymer chemistry》2019,57(18):1994-2001
Fluorine‐19 (19F)‐based contrast agents are increasingly used for magnetic resonance imaging. Conjugated to polymers, they provide an excellent quantitative imaging tool to detect the movement of the polymeric nanoparticles in vivo as there is no background signal in tissue. One of the challenges is the decline in signal intensity when the conjugated hydrophobic fluorinated functionalities aggregate. Therefore, a new fluorinated monomer was prepared from l ‐arginine that carries a 2,2,2‐trifluoroethyl functional group for imaging. The resulting monomer, 2,2,2‐trifluoroethylamide l ‐arginine methacrylamide (3FArgMA), was copolymerized with poly(ethylene glycol) methyl ether methacrylate (PEGMEMA), [2‐(2,3,4,6‐tetra‐O‐acetyl‐α‐d ‐mannopyranosyloxy)ethyl methacrylate or 1‐O‐methacryloyl‐2,3:4,5‐di‐O‐isopropylidene‐β‐d ‐fructopyranose, respectively, using poly(methyl methacrylate) macro‐reversible addition–fragmentation chain transfer polymerization agent. The resulting block copolymers, which varied in 3FArgMA content, were self‐assembled into micelles of hydrodynamic diameters from 25 to 60 nm. The permanently positively charged arginine functionality on the 3FArgMA displayed repulsive forces against aggregation enabling high spin–spin relaxation times (T2) in acidic as well as alkaline solutions. However, the longer poly(ethylene glycol) side functionality in PEGMEMA enabled better steric stabilization (T2~30 ms) while the short fructose side chain was not enough to maintain high T2 values, in particular when a higher 3FArgMA content was used. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1994–2001 相似文献
86.
S. Dzhumanov P.J. Baimatov Sh.T. Inoyatov Sh.S. Djumanov A.G. Gulyamov 《Physics letters. A》2019,383(12):1330-1335
The formation of the optical polaron and bipolaron in two-dimensional (2D) systems is studied in the intermediate electron–phonon coupling regime. The total energies of the 2D polaron and bipolaron are calculated by using the Buimistrov–Pekar method of canonical transformations. The obtained results are compared with other existing results obtained by using the Feynman path integral method and the modified Lee–Low–Pines unitary transformation method. It is shown that the electron–phonon correlation significantly reduces the total energy of the 2D polaron in comparison with the energy of the strong coupling (adiabatic) polaron. It is found that the polaron formation in 2D systems is possible when the electron–phonon coupling constant α is greater than the critical value , which is much lower than a critical value of the electron–phonon coupling constant α in three-dimensional (3D) systems. The critical values of the Fröhlich coupling constant α and the ratio (where and are the high frequency and static dielectric constants, respectively), which determine the bipolaron stability region in 2D systems, are calculated numerically. It is interesting for application to the layered cuprate superconductors that the (bi)polarons are formed more easily in quasi-2D regions than in the bulk. It is argued that the high- cuprate superconductivity can exist above the bulk superconducting transition temperature as the persisting superfluidity of polaronic (bosonic) Cooper pairs and large bipolarons at quasi-2D grain boundaries or in the CuO2 layers above . 相似文献
87.
Oxidative dehydrogenation (ODH) of n-octane was carried out over a vanadium–magnesium oxide catalyst in a continuous flow fixed bed reactor. The catalyst was characterized by ICP–OES, powder XRD and SEM. The catalytic tests were carried out at different gas hourly space velocities (GHSVs), viz. 4000, 6000, 8000, and 10,000 h?1. The best selectivity for octenes was obtained at the GHSV of 8000 h?1, while that for C8 aromatics was attained at the GHSV of 6000 h?1 at high temperatures (500 and 550 °C). The catalytic testing at the GHSV of 10,000 h?1 showed the lowest activity, while that at the GHSV of 4000 h?1 consistently showed the lowest ODH selectivity. Generally, the best ODH performance was obtained by the catalytic testing at the GHSVs of 6000 and 8000 h?1. No phasic changes were observed after the catalytic testing. 相似文献
88.
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
89.
In this paper, we propose a sufficient and necessary condition for the boundedness of all the solutions for the equation with the critical situation that on g and p, where , is periodic and is bounded. 相似文献
90.
在结构可靠性分析中,引入含可调参数的转换函数能对传统的最大熵方法进行改进,获得更高的失效概率预测精度。但是,此可调参数的最佳取值很难确定。针对这一问题,引入概率守恒方程,从功能函数转换前后所得概率密度函数出发,建立其最大熵值的变化关系,给出转换前后最大熵值之差的理论形式。通过对三种典型单调非线性转换函数开展算例研究,发现功能函数转换前后的最大熵值之差与转换函数的最佳可调参数值有关。改变可调参数值驱使最大熵值之差变化的同时,改进最大熵方法能遍历到更好的失效概率估计值。 相似文献