全文获取类型
收费全文 | 6431篇 |
免费 | 737篇 |
国内免费 | 505篇 |
专业分类
化学 | 3241篇 |
晶体学 | 74篇 |
力学 | 1255篇 |
综合类 | 43篇 |
数学 | 456篇 |
物理学 | 2604篇 |
出版年
2023年 | 57篇 |
2022年 | 97篇 |
2021年 | 152篇 |
2020年 | 241篇 |
2019年 | 150篇 |
2018年 | 167篇 |
2017年 | 202篇 |
2016年 | 238篇 |
2015年 | 199篇 |
2014年 | 338篇 |
2013年 | 394篇 |
2012年 | 299篇 |
2011年 | 400篇 |
2010年 | 344篇 |
2009年 | 437篇 |
2008年 | 401篇 |
2007年 | 466篇 |
2006年 | 383篇 |
2005年 | 349篇 |
2004年 | 376篇 |
2003年 | 278篇 |
2002年 | 231篇 |
2001年 | 194篇 |
2000年 | 176篇 |
1999年 | 165篇 |
1998年 | 121篇 |
1997年 | 96篇 |
1996年 | 77篇 |
1995年 | 65篇 |
1994年 | 53篇 |
1993年 | 36篇 |
1992年 | 27篇 |
1991年 | 40篇 |
1990年 | 28篇 |
1989年 | 14篇 |
1988年 | 25篇 |
1987年 | 18篇 |
1986年 | 21篇 |
1985年 | 16篇 |
1984年 | 19篇 |
1982年 | 23篇 |
1981年 | 22篇 |
1980年 | 38篇 |
1979年 | 43篇 |
1978年 | 30篇 |
1977年 | 25篇 |
1976年 | 25篇 |
1974年 | 11篇 |
1973年 | 28篇 |
1972年 | 14篇 |
排序方式: 共有7673条查询结果,搜索用时 46 毫秒
991.
Wei-Xiu Pang Hong-Ye Wu Jian-Jun Zhao 《Phosphorus, sulfur, and silicon and the related elements》2019,194(1-2):69-75
The anharmonic force field and spectroscopy constants for CH2PCl are determined using CCSD(T), VPT2, and density functional theory employing cc-pVQZ basis sets. The molecule structure, rotational spectroscopic constants, and vibrational wave numbers are compared with the available experimental data. Anharmonicity constants, vibration-rotation interaction constants and cubic force constants are predicted. Vibrational wave numbers and rotational constants for CD2PCl are also determined using the same levels. The isotopic shifts of vibrational wave numbers are remarkable by D atom substitution for 1-chlorophosphaethene. 相似文献
992.
Zhongqi He Rachel L. Sleighter Patrick G. Hatcher Shasha Liu Fengchang Wu Haixuan Zou O. Modesto Olanya 《Journal of mass spectrometry : JMS》2019,54(8):655-666
Soluble extractives in wood function to protect living trees from destructive agents and also contribute to wood color and fragrance. Some extractive components have biological activities with medical applications. They also play important roles in wood processing and related applications. To increase the knowledge of wood chemistry, maple and oak were extracted by water. Ultraviolet/visible (UV/vis) spectroscopy indicated the presence of a phenolic compound, resorcinol, in maple extractives having higher molecular mass and more aromatic components than oak extractives. Negative and positive electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT‐ICR‐MS) identified thousands of formulas in the two samples in the m/z range of 200 to 800. They mainly fall into the lignin‐like, carbohydrate‐like, and tannin‐like compound categories. The top 25 peaks (ie, formulas) with the highest relative magnitude in negative ESI represented nearly 50% of the summed total spectral magnitude of all formulas assigned in the maple and oak extractives. Furthermore, the base peak (ie, most abundant peak) accounted for about 14% of the total abundance in each wood sample. Literature comparisons identified 17 of 20 formulas in the top five peaks of the four spectra as specific bioactive compounds in trees and other plants, implying the potential to explore utilization of maple and oak extractives for functional and medicinal applications. The various profiling of the top 25 peaks from the two samples also suggested the possible application of FT‐ICR‐MS for detecting chemical markers useful in profiling and identification of wood types and sources. 相似文献
993.
994.
995.
Jacob D. Teeter Dr. Percy Zahl Mohammad Mehdi Pour Dr. Paulo S. Costa Prof. Axel Enders Prof. Alexander Sinitskii 《Chemphyschem》2019,20(18):2281-2285
We report the on-surface synthesis and spectroscopic study of laterally extended chevron graphene nanoribbons (GNRs) and compare them with the established chevron GNRs, emphasizing the consistency of bandgap reduction of semiconducting GNRs with increased width. The laterally extended chevron GNRs grown on Au(111) exhibit a bandgap of about 2.2 eV, which is considerably smaller than the values reported for chevron GNRs in similar studies. 相似文献
996.
Valentina Laghezza Masci Anna‐Rita Taddei Thomas Courant Ozgül Tezgel Fabrice Navarro Franco Giorgi Denis Mariolle Anna‐Maria Fausto Isabelle Texier 《Macromolecular bioscience》2019,19(5)
Curcumin‐loaded collagen cryostructurates have been devised for wound healing applications. Curcumin displays strong antioxidant, antiseptic, and anti‐inflammatory properties, while collagen is acknowledged for promoting cell adhesion, migration and differentiation. However, when curcumin is loaded directly into collagen hydrogels, it forms large molecular aggregates and clogs the matrix pores. A double‐encapsulation strategy is therefore developed by loading curcumin into lipid nanoparticles (LNP), and embedding these particles inside collagen scaffolds. The resulting collagen/LNP cryostructurates have an optimal fibrous structure with ≈100 µm average pore size for sustaining cell migration. Results show that collagen is structurally unaltered and that nanoparticles are homogeneously distributed amidst collagen fibers. Hydrogels soaked in saline buffer release about 20 to 30% of their nanoparticles content within 24 h, while achieved 100% release after 25 days. When exposed to NIH 3T3 fibroblasts, these hydrogels provide a satisfactory scaffold for cell interaction as early as 4 h after seeding, with no cytotoxic counter effect. These positive features make the collagen/lipid cryostructurates a promising material for further use in wound healing. 相似文献
997.
Theodore W. Burkhardt 《Journal of statistical physics》2008,133(2):217-230
We consider a particle which moves on the x axis and is subject to a constant force, such as gravity, plus a random force in the form of Gaussian white noise. We analyze
the statistics of first arrival at point x
1 of a particle which starts at x
0 with velocity v
0. The probability that the particle has not yet arrived at x
1 after a time t, the mean time of first arrival, and the velocity distribution at first arrival are all considered. We also study the statistics
of the first return of the particle to its starting point. Finally, we point out that the extreme-value statistics of the
particle and the first-passage statistics are closely related, and we derive the distribution of the maximum displacement
m=max
t
[x(t)]. 相似文献
998.
The quadratic, cubic and semi-diagonal quartic force field of propene has been calculated at the MP2 level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quintuple-ζ quality and a core correlation correction. The experimental mass-dependent rm structures are also determined and their accuracy is discussed. The use of isolated CH stretching frequencies is shown to be a good method to determine CH bond length. 相似文献
999.
The quadratic, cubic and semi-diagonal quartic force field of ethyl cyanide has been calculated at the B3LYP level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quadruple-ζ quality and a core correlation correction. The empirical structures are also determined and their accuracy is discussed. The potential barrier V3 hindering internal rotation of the methyl group has been calculated from 23 rotational transitions of CH3CH2C15N which were found split into doublets, giving V3 = 3074(27) cal mol−1. 相似文献
1000.
The evolution of gradient force pattern induced by an annular phase distribution plate is numerically investigated in this paper. The phase plate, which may alter the wavefront phase of incident Gaussian beam with tunable topological charge, consists of two concentric portions, one center circle portion and one annular portion. Numerical simulations show that the proposed plate can induce the tunable gradient force on the particles in the focal region. By adjusting the geometrical parameters or changing the topological charge of the phase-shifting plate, some novel trap patterns may occur, such as triangle shape trap, quadrangle shape trap, pentagon shape trap, hexagon shape trap, and the shapes of optical traps change very considerably. Therefore, the phase plate may be very advantageous for constructing tunable optical traps. The method is more versatile in that it allows precise control of the parameters and has the possibility of generating specific patterns of optical vortices. The gradient force pattern focal of intensity distribution depends on both the annular width and the topological charge. 相似文献