全文获取类型
收费全文 | 83764篇 |
免费 | 614篇 |
国内免费 | 402篇 |
专业分类
化学 | 28765篇 |
晶体学 | 808篇 |
力学 | 6833篇 |
数学 | 32484篇 |
物理学 | 15890篇 |
出版年
2023年 | 71篇 |
2022年 | 95篇 |
2021年 | 130篇 |
2020年 | 146篇 |
2019年 | 165篇 |
2018年 | 10531篇 |
2017年 | 10334篇 |
2016年 | 6309篇 |
2015年 | 1026篇 |
2014年 | 490篇 |
2013年 | 621篇 |
2012年 | 4290篇 |
2011年 | 11057篇 |
2010年 | 5881篇 |
2009年 | 6220篇 |
2008年 | 7002篇 |
2007年 | 9157篇 |
2006年 | 654篇 |
2005年 | 1702篇 |
2004年 | 1810篇 |
2003年 | 2217篇 |
2002年 | 1213篇 |
2001年 | 306篇 |
2000年 | 322篇 |
1999年 | 179篇 |
1998年 | 222篇 |
1997年 | 175篇 |
1996年 | 225篇 |
1995年 | 138篇 |
1994年 | 97篇 |
1993年 | 118篇 |
1992年 | 71篇 |
1991年 | 80篇 |
1990年 | 60篇 |
1989年 | 71篇 |
1988年 | 67篇 |
1987年 | 68篇 |
1986年 | 68篇 |
1985年 | 67篇 |
1984年 | 56篇 |
1983年 | 44篇 |
1982年 | 52篇 |
1981年 | 51篇 |
1980年 | 57篇 |
1979年 | 57篇 |
1978年 | 41篇 |
1914年 | 45篇 |
1912年 | 40篇 |
1909年 | 43篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
V. P. Silva Neto C. F. L. Vasconcelos M. R. M. L. Albuquerque A. G. D’Assunção 《Applied Physics A: Materials Science & Processing》2014,115(2):421-425
This work presents an analysis of the microstrip bandpass filter on metamaterial substrates. The filter is composed of two ring resonators with quarter-wavelength side-coupled sections. The filter input is provided, as well as the output port, using a quarter-wavelength side-coupled microstrip line section. Simulations by finite element method have been carried out to verify the effect of the metamaterial substrate properties on the filter performance, and to compare these results to those obtained considering isotropic substrate. 相似文献
122.
Background
The morphological development of neurons is a very complex process involving both genetic and environmental components. Mathematical modelling and numerical simulation are valuable tools in helping us unravel particular aspects of how individual neurons grow their characteristic morphologies and eventually form appropriate networks with each other.Methods
A variety of mathematical models that consider (1) neurite initiation (2) neurite elongation (3) axon pathfinding, and (4) neurite branching and dendritic shape formation are reviewed. The different mathematical techniques employed are also described.Results
Some comparison of modelling results with experimental data is made. A critique of different modelling techniques is given, leading to a proposal for a unified modelling environment for models of neuronal development.Conclusion
A unified mathematical and numerical simulation framework should lead to an expansion of work on models of neuronal development, as has occurred with compartmental models of neuronal electrical activity.123.
The local structure of the low-temperature ordered phase of the negative thermal expansion (NTE) material has been investigated by reverse Monte Carlo (RMC) modeling of neutron total scattering data. We obtain, for the first time, quantitative measurements of the extent to which the and polyhedra move as rigid units, and we show that these values are consistent with the predictions of rigid unit mode theory. We suggest that rigid unit modes are associated with the NTE. Our results do not support a recent interpretation of x-ray-absorption fine structure spectroscopy data in terms of a larger rigid structural component involving the Zr-O-W linkage. 相似文献
124.
We present a model for the kinetics of spontaneous membrane domain (raft) assembly that includes the effect of membrane recycling ubiquitous in living cells. We show that domains can have a broad power-law distribution with an average radius that scales with the 1/4 power of the domain lifetime when the line tension at the domain edges is large. For biologically reasonable recycling and diffusion rates, the average domain radius is in the tens of nm range, consistent with observations. This represents one possible link between signaling (involving rafts) and traffic (recycling) in cells. Finally, we present evidence that suggests that the average raft size may be the same for all scale-free recycling schemes. 相似文献
125.
Nicolai Haydn Matthew Nicol Sandro Vaienti Licheng Zhang 《Journal of statistical physics》2013,153(5):864-887
Suppose B i :=B(p,r i ) are nested balls of radius r i about a point p in a dynamical system (T,X,μ). The question of whether T i x∈B i infinitely often (i.o.) for μ a.e. x is often called the shrinking target problem. In many dynamical settings it has been shown that if $E_{n}:=\sum_{i=1}^{n} \mu(B_{i})$ diverges then there is a quantitative rate of entry and $\lim_{n\to\infty} \frac{1}{E_{n}} \sum_{j=1}^{n} 1_{B_{i}} (T^{i} x) \to1$ for μ a.e. x∈X. This is a self-norming type of strong law of large numbers. We establish self-norming central limit theorems (CLT) of the form $\lim_{ n\to\infty} \frac{1}{a_{n}} \sum_{i=1}^{n} [1_{B_{i}} (T^{i} x)-\mu(B_{i})] \to N(0,1)$ (in distribution) for a variety of hyperbolic and non-uniformly hyperbolic dynamical systems, the normalization constants are $a^{2}_{n} \sim E [\sum_{i=1}^{n} 1_{B_{i}} (T^{i} x)-\mu(B_{i})]^{2}$ . Dynamical systems to which our results apply include smooth expanding maps of the interval, Rychlik type maps, Gibbs-Markov maps, rational maps and, in higher dimensions, piecewise expanding maps. For such central limit theorems the main difficulty is to prove that the non-stationary variance has a limit in probability. 相似文献
126.
Non-additivity effects in coupled dynamic-stochastic systems are investigated. It is shown that there is a mapping of the
replica approach to disordered systems with finite replica indexn on Tsallis non-extensive statistics, if the average thermodynamic entropy of the dynamic subsystem differs from the information
entropy for the probability distribution in the stochastic subsystem. The entropic indexq is determined by the entropy difference ΔS. In the case of incomplete information, the entropic indexq=1−n is shown to be related to the degree of lost information. 相似文献
127.
Combustion phenomena are of high scientific and technological interest, in particular for energy generation and transportation
systems. Direct Numerical Simulations (DNS) have become an essential and well established research tool to investigate the
structure of turbulent flames, since they do not rely on any approximate turbulence models. In this work two complementary
DNS codes are employed to investigate different types of fuels and flame configurations. The code is π3 is a 3-dimensional DNS code using a low-Mach number approximation. Chemistry is described through a tabulation, using two
coordinates to enter a database constructed for example with 29 species and 141 reactions for methane combustion. It is used
here to investigate the growth of a turbulent premixed flame in a methane-air mixture (Case 1). The second code,Sider is an explicit three-dimensional DNS code solving the fully compressible reactive Navier-Stokes equations, where the chemical
processes are computed using a complete reaction scheme, taking into account accurate diffusion properties. It is used here
to compute a hydrogen/air turbulent diffusion flame (Case 2), considering 9 chemical species and 38 chemical reactions. 相似文献
128.
Robert M Caudle Andrew J Mannes Jason Keller Federico M Perez Shelby K Suckow John K Neubert 《BMC neuroscience》2007,8(1):30
Background
Several investigators have coupled toxins to neuropeptides for the purpose of lesioning specific neurons in the central nervous system. By producing deficits in function these toxin conjugates have yielded valuable information about the role of these cells. In an effort to specifically stimulate cells rather than kill them we have conjugated the neuropeptide substance P to the catalytic subunit of cholera toxin (SP-CTA). This conjugate should be taken up selectively by neurokinin receptor expressing neurons resulting in enhanced adenylate cyclase activity and neuronal firing. 相似文献129.
130.
Wang Zi-Jian Yu Yong-Ji Zhai Rui-Zhi Chen Xin-Yu Wu Chun-Ting Jin Guang-Yong 《Journal of Russian Laser Research》2016,37(1):97-101
We report a narrow pulse width optical parametric oscillator based on periodically poled MgO:LiNbO3 (PPMgLN) with a high repetition rate under quasi-phase matched conditions. When the maximum pumping power of the 1,064-nm laser was 14.57 W, the acousto-optical (A-O) Q-switch repetition rate was 100 kHz, and the PPMgLN crystal grating period was 29.5 μm. A 1,474-nm signal light output power of 4.21 W and a 3,828 nm idler light output power of 1.547 W were obtained, corresponding to a pulse width of 9.52 ns and 9.65 ns, respectively. The overall optical–optical conversion efficiency was 39.5%. Additionally, by changing the temperature from 25°C to 150°C, a tunable signal wavelength of 1,474–1,499 nm and idler wavelength at 3,676–3,828 nm of the output laser were achieved. 相似文献