全文获取类型
收费全文 | 72026篇 |
免费 | 5627篇 |
国内免费 | 6950篇 |
专业分类
化学 | 27191篇 |
晶体学 | 300篇 |
力学 | 6361篇 |
综合类 | 1339篇 |
数学 | 27618篇 |
物理学 | 21794篇 |
出版年
2023年 | 577篇 |
2022年 | 883篇 |
2021年 | 1179篇 |
2020年 | 1351篇 |
2019年 | 1604篇 |
2018年 | 1348篇 |
2017年 | 1597篇 |
2016年 | 1822篇 |
2015年 | 1636篇 |
2014年 | 2678篇 |
2013年 | 4859篇 |
2012年 | 2900篇 |
2011年 | 3395篇 |
2010年 | 2898篇 |
2009年 | 4150篇 |
2008年 | 4533篇 |
2007年 | 4601篇 |
2006年 | 4279篇 |
2005年 | 3708篇 |
2004年 | 3370篇 |
2003年 | 3437篇 |
2002年 | 2997篇 |
2001年 | 2592篇 |
2000年 | 2472篇 |
1999年 | 2209篇 |
1998年 | 2076篇 |
1997年 | 1719篇 |
1996年 | 1532篇 |
1995年 | 1436篇 |
1994年 | 1336篇 |
1993年 | 1146篇 |
1992年 | 1108篇 |
1991年 | 866篇 |
1990年 | 704篇 |
1989年 | 615篇 |
1988年 | 561篇 |
1987年 | 437篇 |
1986年 | 383篇 |
1985年 | 468篇 |
1984年 | 447篇 |
1983年 | 207篇 |
1982年 | 354篇 |
1981年 | 423篇 |
1980年 | 318篇 |
1979年 | 310篇 |
1978年 | 231篇 |
1977年 | 218篇 |
1976年 | 161篇 |
1974年 | 109篇 |
1973年 | 109篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
101.
102.
103.
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
104.
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
105.
Nan Zhang Dr. Tohru Kawamoto Yong Jiang Dr. Akira Takahashi Dr. Manabu Ishizaki Miyuki Asai Prof. Masato Kurihara Prof. Zhenya Zhang Dr. Zhongfang Lei Dr. Durga Parajuli 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(23):5950-5958
Cobalt hexacyanoferrate of various compositions was prepared in flow mode and the role of the vacancy on the structure, thermogravimetric (TG) properties, and the adsorption efficiency was studied. The material, NayCo[Fe(CN)6]1−x ⋅ z H2O, with a minimum vacancy of x=0.014 to the highest x=0.47, was obtained. The TG-differential scanning calorimetry (DSC) profile showed a distinct influence of the vacancy on the water release temperature. Materials with x>0.35 showed a smooth release of water at a relatively lower temperature. However, for the materials with x<0.35, water release took place in multiple steps, suggesting the existence of various forms of water. The FTIR profiles supported the existence of free and bonded water molecules. However, the materials with multiple water peaks in the FTIR spectra showed a shift of the major XRD peaks when heated at 285 °C in N2 atmosphere. Regarding the effect of the vacancy on the adsorption behavior, for NH4, the adsorption was found to be proportional to the number of Na atoms in the material, confirming the ion-exchange process. On the contrary, the materials with low vacancy and high Na content showed nominal Cs adsorption capacity. Interestingly, the K adsorption capacity was found to be in between that of the other two ions. This means the ionic size decides the rate of placement into the interstitial sites. For larger ions like Cs, the ease of percolation via the vacancy decides the overall adsorption efficiency. 相似文献
106.
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
107.
108.
109.
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
110.
This paper deals with the Cauchy–Dirichlet problem for the fractional Cahn–Hilliard equation. The main results consist of global (in time) existence of weak solutions, characterization of parabolic smoothing effects (implying under proper condition eventual boundedness of trajectories), and convergence of each solution to a (single) equilibrium. In particular, to prove the convergence result, a variant of the so-called ?ojasiewicz–Simon inequality is provided for the fractional Dirichlet Laplacian and (possibly) non-analytic (but ) nonlinearities. 相似文献