全文获取类型
收费全文 | 32486篇 |
免费 | 780篇 |
国内免费 | 409篇 |
专业分类
化学 | 16168篇 |
晶体学 | 494篇 |
力学 | 1265篇 |
综合类 | 14篇 |
数学 | 4389篇 |
物理学 | 11345篇 |
出版年
2023年 | 113篇 |
2022年 | 482篇 |
2021年 | 717篇 |
2020年 | 678篇 |
2019年 | 827篇 |
2018年 | 1304篇 |
2017年 | 1341篇 |
2016年 | 1666篇 |
2015年 | 870篇 |
2014年 | 1490篇 |
2013年 | 2322篇 |
2012年 | 2162篇 |
2011年 | 2261篇 |
2010年 | 2185篇 |
2009年 | 2142篇 |
2008年 | 2099篇 |
2007年 | 2040篇 |
2006年 | 1781篇 |
2005年 | 1196篇 |
2004年 | 863篇 |
2003年 | 515篇 |
2002年 | 496篇 |
2001年 | 444篇 |
2000年 | 742篇 |
1999年 | 487篇 |
1998年 | 528篇 |
1997年 | 503篇 |
1996年 | 226篇 |
1995年 | 106篇 |
1994年 | 89篇 |
1993年 | 72篇 |
1992年 | 74篇 |
1991年 | 71篇 |
1990年 | 58篇 |
1989年 | 52篇 |
1988年 | 50篇 |
1987年 | 60篇 |
1986年 | 47篇 |
1985年 | 50篇 |
1984年 | 43篇 |
1983年 | 43篇 |
1982年 | 38篇 |
1981年 | 28篇 |
1980年 | 40篇 |
1979年 | 26篇 |
1978年 | 38篇 |
1977年 | 35篇 |
1976年 | 24篇 |
1974年 | 19篇 |
1968年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
Journal of Structural Chemistry - Thermodynamic characteristics are calculated for aqueous alkanolamine solutions that are obtained by substituting alkyl radicals for the protons in the amino group... 相似文献
32.
33.
34.
Dehghanpour-Sahron Ja’far Mahdavi-Amiri Nezam 《4OR: A Quarterly Journal of Operations Research》2021,19(4):473-499
4OR - Partitioning a given data-set into subsets based on similarity among the data is called clustering. Clustering is a major task in data mining and machine learning having many applications... 相似文献
35.
Muhammad Usman Khan Muhammad Yasir Mehboob Riaz Hussain Zainab Afzal Muhammad Khalid Muhammad Adnan 《International journal of quantum chemistry》2020,120(22):e26377
The development of organic electron acceptor materials is one of the key factors for realizing high-performance organic solar cells (OSCs). Nonfullerene electron acceptors, compared to traditional fullerene acceptor materials, have gained much impetus owing to their better optoelectronic tunabilities and lower cost, as well as higher stability. Therefore, 5 three-dimensional (3D) cross-shaped acceptor materials having a spirobifullerene core flanked with 2,1,3-benzothiadiazole are designed from a recently synthesized highly efficient acceptor molecule SF(BR) 4 and are investigated in detail with regard to their use as acceptor molecules in OSCs. The density functional theory (DFT) and time-dependent DFT (TDDFT) calculations have been performed for the estimation of frontier molecular orbital (FMO) analysis, density of states analysis, reorganization energies of electron and hole, dipole moment, open-circuit voltage, photo-physical characteristics, and transition density matrix analysis. In addition, the structure-property relationship is studied, and the influence of end-capped acceptor modifications on photovoltaic, photo-physical, and electronic properties of newly selected molecules ( H1-H5 ) is calculated and compared with reference ( R ) acceptor molecule SF(BR) 4 . The structural tailoring at terminals was found to effectively tune the FMO band gap, energy levels, absorption spectra, open-circuit voltage, reorganization energy, and binding energy value in selected molecules H1 to H5 . The 3D cross-shaped molecules H1 to H5 suppress the intermolecular aggregation in PTB7-Th blend, which leads to high efficiency of acceptor material H1 to H5 in OSCs. Consequently, better optoelectronic properties are achieved from designed molecules H1 to H5 . It is proposed that the conceptualized molecules are superior than highly efficient spirobifullerene core-based SF(BR) 4 acceptor molecules and, thus, are recommended to experiments for future developments of highly efficient solar cells. 相似文献
36.
In this study, stoichiometric protonation constants of L-tyrosine, L-cysteine, L-tryptophane, L-lysine, and L-histidine, and
their methyl and ethyl esters in water and ethanol–water mixtures of 30, 50, and 70% ethanol (v/v), were determined potentiometrically
using a combined pH electrode system calibrated as the concentration of hydrogen ion. Titrations were performed at 25∘C and the ionic strength of the medium was maintained at 0.10 mol⋅L−1 using sodium chloride. Protonation constants were calculated by using the BEST computer program. The effect of solvent composition
on the protonation constants is discussed. The log10 K2 values of esters generally decreased with increasing ethanol content. However, the log10 K1 values of the esters of L-tyrosine, L-cysteine, and L-tryptophane were found to increase with increasing ethanol content
in contrast those of L-lysine and L-histidine esters. 相似文献
37.
Planar laser-induced fluorescence visualisation is used to investigate nonuniformities in the flow of a hypersonic conical nozzle. Possible causes for the nonuniformity are outlined and investigated, and the problem is shown to be due to a small step at the nozzle throat. Entrainment of cold boundary layer gas is postulated as the cause of the signal nonuniformity.
PACS 47.80.Jk, 47.40.Ki, 47.60.+i 相似文献
38.
The semiclassical theory of proximity effects predicts a gap E g~?D/L 2 in the excitation spectrum of a long diffusive superconductor/normal-metal/superconductor (SNS) junction. Mesoscopic fluctuations lead to anomalously localized states in the normal part of the junction.As a result, a nonzero, yet exponentially small, density of states (DOS) appears at energies below E g. In the framework of the supermatrix nonlinear σ model, these prelocalized states are due to the instanton configurations with broken supersymmetry. The exact result for the DOS near the semiclassical threshold is found, provided the dimensionless conductance of the normal part G N is large. The case of poorly transparent interfaces between the normal and superconductive regions is also considered. In this limit, the total number of subgap states may be large. 相似文献
39.
Ya. V. Fattakhov M. F. Galyautdinov T. N. L’vova I. B. Khaibullin 《Technical Physics》1997,42(12):1457-1459
The first results regarding the formation of a two-dimensional periodic structure of local melting regions on a silicon surface
upon pulsed light irradiation are presented. The conditions are established, and the mechanism of the formation of such structures
is discussed.
Zh. Tekh. Fiz. 67, 97–99 (December 1997) 相似文献
40.
玻璃陶瓷材料中Tm3+离子红外到蓝色上转换发光 总被引:4,自引:0,他引:4
系统研究了PbF2+GeO2+WO3ⅩⅣTmF3玻璃陶瓷材料中,在近红外光(1.06μm)激发下,Tm3+离子的发光特性.实验中观测到Tm3+离子的两组峰值位置分别在20920cm-1和22173cm-1的蓝色上转换发光,并证实这两组上转换发光分别与吸收三个和四个光子有关,同时建立了上转换发光的模型.为了选择最佳掺杂浓度,详细地测量了Tm3+离子峰值为20920cm-1的蓝色上转换发光强度与TmF3浓度的关系. 相似文献