全文获取类型
收费全文 | 47946篇 |
免费 | 7743篇 |
国内免费 | 5249篇 |
专业分类
化学 | 20933篇 |
晶体学 | 910篇 |
力学 | 4473篇 |
综合类 | 730篇 |
数学 | 12198篇 |
物理学 | 21694篇 |
出版年
2023年 | 516篇 |
2022年 | 1108篇 |
2021年 | 1222篇 |
2020年 | 1433篇 |
2019年 | 1368篇 |
2018年 | 1293篇 |
2017年 | 1688篇 |
2016年 | 1990篇 |
2015年 | 1700篇 |
2014年 | 2549篇 |
2013年 | 4201篇 |
2012年 | 2844篇 |
2011年 | 3164篇 |
2010年 | 2794篇 |
2009年 | 3045篇 |
2008年 | 3043篇 |
2007年 | 3018篇 |
2006年 | 2744篇 |
2005年 | 2401篇 |
2004年 | 2214篇 |
2003年 | 1958篇 |
2002年 | 1796篇 |
2001年 | 1459篇 |
2000年 | 1424篇 |
1999年 | 1177篇 |
1998年 | 1076篇 |
1997年 | 861篇 |
1996年 | 754篇 |
1995年 | 597篇 |
1994年 | 548篇 |
1993年 | 415篇 |
1992年 | 376篇 |
1991年 | 276篇 |
1990年 | 219篇 |
1989年 | 246篇 |
1988年 | 189篇 |
1987年 | 159篇 |
1986年 | 152篇 |
1985年 | 545篇 |
1984年 | 400篇 |
1983年 | 285篇 |
1982年 | 306篇 |
1981年 | 215篇 |
1980年 | 175篇 |
1979年 | 183篇 |
1978年 | 116篇 |
1977年 | 111篇 |
1976年 | 108篇 |
1974年 | 124篇 |
1973年 | 149篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
1.
2.
3.
An efficient edge based data structure has been developed in order to implement an unstructured vertex based finite volume algorithm for the Reynolds-averaged Navier–Stokes equations on hybrid meshes. In the present approach, the data structure is tailored to meet the requirements of the vertex based algorithm by considering data access patterns and cache efficiency. The required data are packed and allocated in a way that they are close to each other in the physical memory. Therefore, the proposed data structure increases cache performance and improves computation time. As a result, the explicit flow solver indicates a significant speed up compared to other open-source solvers in terms of CPU time. A fully implicit version has also been implemented based on the PETSc library in order to improve the robustness of the algorithm. The resulting algebraic equations due to the compressible Navier–Stokes and the one equation Spalart–Allmaras turbulence equations are solved in a monolithic manner using the restricted additive Schwarz preconditioner combined with the FGMRES Krylov subspace algorithm. In order to further improve the computational accuracy, the multiscale metric based anisotropic mesh refinement library PyAMG is used for mesh adaptation. The numerical algorithm is validated for the classical benchmark problems such as the transonic turbulent flow around a supercritical RAE2822 airfoil and DLR-F6 wing-body-nacelle-pylon configuration. The efficiency of the data structure is demonstrated by achieving up to an order of magnitude speed up in CPU times. 相似文献
4.
We investigate the possibility of phantom crossing in the dark energy sector and the solution for the Hubble tension between early and late universe observations. We use robust combinations of different cosmological observations, namely the Cosmic Microwave Background (CMB), local measurement of Hubble constant (), Baryon Acoustic Oscillation (BAO) and SnIa for this purpose. For a combination of CMB+BAO data that is related to early universe physics, phantom crossing in the dark energy sector was confirmed at a 95% confidence level and we obtained the constraint km/s/Mpc at a 68% confidence level, which is in perfect agreement with the local measurement by Riess et al. We show that constraints from different combinations of data are consistent with each other and all of them are consistent with phantom crossing in the dark energy sector. For the combination of all data considered, we obtained the constraint km/s/Mpc at a 68% confidence level and the phantom crossing happening at the scale factor at a 68% confidence level. 相似文献
6.
Chemical bonding in representative astrophysically relevant neutral,cation, and anion HCnH chains 下载免费PDF全文
Ioan Baldea 《中国物理 B》2022,31(12):123101-123101
Most existing studies assign a polyynic and cumulenic character of chemical bonding in carbon-based chains relying on values of the bond lengths. Building on our recent work, in this paper we add further evidence on the limitations of such an analysis and demonstrate the significant insight gained via natural bond analysis. Presently reported results include atomic charges, natural bond order and valence indices obtained from ab initio computations for representative members of the astrophysically relevant neutral and charged HC2k/2k+1H chain family. They unravel a series of counter-intuitive aspects and/or help naive intuition in properly understanding microscopic processes, e.g., electron removal from or electron attachment to a neutral chain. Demonstrating that the Wiberg indices adequately quantify the chemical bonding structure of the HC2k/2k+1H chains—while the often heavily advertised Mayer indices do not—represents an important message conveyed by the present study. 相似文献
7.
8.
G. Peruginelli 《代数通讯》2018,46(11):4724-4738
We classify the maximal subrings of the ring of n×n matrices over a finite field, and show that these subrings may be divided into three types. We also describe all of the maximal subrings of a finite semisimple ring, and categorize them into two classes. As an application of these results, we calculate the covering number of a finite semisimple ring. 相似文献
9.
Dr. Antonio M. Rodríguez Dr. Pilar Prieto Prof. Antonio de la Hoz Prof. Ángel Díaz-Ortiz D. Raúl Martín Prof. José I. García 《ChemistryOpen》2015,4(3):308-317
The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves. For this purpose, a computational study was carried out on a variety of reactions, which have been reported to be improved under microwave irradiation. This comprises six types of reactions. The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate. In addition to this, in most cases, slower reacting systems observe a much greater improvement under microwave irradiation. Furthermore, for these reactions, the presence of a polar component in the reaction (solvent, reagent, susceptor, etc.) is necessary for strong coupling with the electromagnetic radiation. We also quantified that an activation energy of 20–30 kcal mol−1 and a polarity (μ) between 7–20 D of the species involved in the process is required to obtain significant improvements under microwave irradiation. 相似文献
10.
Close‐Packed Two‐Dimensional Silver Nanoparticle Arrays: Quadrupolar and Dipolar Surface Plasmon Resonance Coupling 下载免费PDF全文
Sukang Yun Soonchang Hong Jesus A. I. Acapulco Jr. Ho Young Jang Songyi Ham Kyungeun Lee Prof. Seong Kyu Kim Prof. Sungho Park 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(16):6165-6172
Silver nanoparticles (NPs) ranging in size from 40 to 100 nm were prepared in high yield by using an improved seed‐mediated method. The homogeneous Ag NPs were used as building blocks for 2D assembled Ag NP arrays by using an oil/water interface. A close‐packed 2D array of Ag NPs was fabricated by using packing molecules (3‐mercaptopropyltrimethoxysilane) to control the interparticle spacing. The homogeneous 2D Ag NP array exhibited a strong quadrupolar cooperative plasmon mode resonance and a dipolar red‐shift relative to individual Ag NPs suspended in solution. A well‐arranged 2D Ag NP array was embedded in polydimethylsiloxane film and, with biaxial stretching to control the interparticle distance, concomitant variations of the quadrupolar and dipolar couplings were observed. As the interparticle distance increased, the intensity of the quadrupolar cooperative plasmon mode resonance decreased and dipolar coupling completely disappeared. The local electric field of the 2D Ag NP array was calculated by using finite difference time domain simulation and qualitatively showed agreement with the experimental measurements. 相似文献