全文获取类型
收费全文 | 13356篇 |
免费 | 2597篇 |
国内免费 | 1335篇 |
专业分类
化学 | 5249篇 |
晶体学 | 115篇 |
力学 | 1214篇 |
综合类 | 148篇 |
数学 | 776篇 |
物理学 | 9786篇 |
出版年
2024年 | 34篇 |
2023年 | 138篇 |
2022年 | 375篇 |
2021年 | 359篇 |
2020年 | 390篇 |
2019年 | 411篇 |
2018年 | 405篇 |
2017年 | 459篇 |
2016年 | 564篇 |
2015年 | 518篇 |
2014年 | 829篇 |
2013年 | 1073篇 |
2012年 | 821篇 |
2011年 | 985篇 |
2010年 | 701篇 |
2009年 | 861篇 |
2008年 | 873篇 |
2007年 | 922篇 |
2006年 | 844篇 |
2005年 | 720篇 |
2004年 | 578篇 |
2003年 | 533篇 |
2002年 | 553篇 |
2001年 | 528篇 |
2000年 | 435篇 |
1999年 | 331篇 |
1998年 | 299篇 |
1997年 | 246篇 |
1996年 | 196篇 |
1995年 | 197篇 |
1994年 | 153篇 |
1993年 | 144篇 |
1992年 | 113篇 |
1991年 | 110篇 |
1990年 | 77篇 |
1989年 | 61篇 |
1988年 | 57篇 |
1987年 | 50篇 |
1986年 | 42篇 |
1985年 | 54篇 |
1984年 | 50篇 |
1983年 | 24篇 |
1982年 | 34篇 |
1981年 | 27篇 |
1980年 | 25篇 |
1979年 | 17篇 |
1978年 | 14篇 |
1977年 | 12篇 |
1976年 | 14篇 |
1973年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
M. V. Kartikeyan E. Borie M. Thumm 《International Journal of Infrared and Millimeter Waves》2007,28(8):611-619
A design concept of a 250 GHz, ≈ 50 W, CW, second harmonic gyrotron for spectroscopy will be presented in this paper. Mode
competition, in particular with regard to competition with first harmonic modes, and mode selection procedures are carefully
investigated by considering various candidate modes and the TE6,2 and TE6,3 modes are chosen as probable operating modes. Cold cavity and self-consistent calculations of power and efficiency are carried
out to determine optimum parameters for these two modes. The results indicate that an output power of well over 50 W, CW can
be achieved. 相似文献
992.
Earlier we reported an ion current jump which was observed at a fixed negative biased disc potential in the 6.4GHz ECR ion source at VECC,Kolkata.In a recent experiment with neon ions,we measured the time spectra of the ion current and observed the presence of a burst frequency in the kilohertz range.This frequency shows a correlated jump with the ion current jump described above.Another interesting feature is that the observed burst frequency shows a good linear correlation with the extracted ion current.The higher the ion current,the higher is the burst frequency.This means that current per burst is a constant factor;when there are more number of bursts,the current also increases. 相似文献
993.
This study presents the electromagnetic wave propagation through the frequency-dispersive and lossy double-negative slab embedded
between two different semi-infinite media. The double-negative slab is realized by using two models, the Lorentz and Drude
medium models. The properties and the required equations for the frequency-dispersive and lossy double-negative slab, the
Lorentz medium and Drude medium are given in detail. After the construction of the problem, the reflection and transmission
coefficients are derived for both TE and TM waves. Then, the reflected, transmitted and loss powers are determined using these
coefficients. Finally, in the numerical results, the mentioned powers for TE and TM waves are computed and illustrated as
a function of the incidence angle, the frequency and the slab thickness when the damping frequency changes. 相似文献
994.
Optical spectroscopy measurements of the congruent LiNbO3 (LN) single crystals, weakly doped with Er (0.1–0.3 wt%) and Er (0.3 wt%) and Yb (0.5 wt%), have been carried out. The shape of the optical absorption and additional absorption bands registered after γ-irradiation suggests the presence of Er3+ ion pairs. EPR investigations were performed for LN single crystal doped with Er (0.1 wt%). Unusual behaviour of the temperature dependence of the intensity and linewidth of the main EPR line—corresponding to the fine transition of evenEr3+—ions, is reported. The main EPR line appears to be a superposition of several paramagnetic centres originating from isolated evenEr3+ ions and evenEr3+–evenEr3+ pairs of ions. In low temperature region (below 20 K), the main EPR signal is dominated by signals arising from evenEr3+–evenEr3+ pairs of ions. The inverse intensity of the EPR line at low temperature region fulfils the Curie–Weiss law and enabled to determine the Curie–Weiss constant Θ=1.5±0.5 K. The positive sign of Θ suggests that the ferromagnetic interactions arise in the system of evenEr3+–evenEr3+ ion pairs in LN. Our results suggest that the distribution of Er ions in congruent LN is not homogeneous and Er impurity ions can form clusters in host lattice even in the case of weak erbium doping. 相似文献
995.
G. A. Miller 《The European Physical Journal A - Hadrons and Nuclei》2007,31(4):578-584
The discovery more than twenty years ago, by the EMC Collaboration, that the deep-inelastic-scattering DIS structure functions
are influenced by the nuclear environment stunned the nuclear physics community and brought quarks and gluons into the field
with great impact. A great length of time has passed, but despite a semi-infinite number of papers on the subject, there is
no explanation that is universally accepted. Many models (related in one way or another to QCD) have been successful in reproducing
data for deep inelastic scattering on nuclear targets, but fewer have described both the DIS and nuclear Drell-Yan experiments.
Although there are some positive indications, no model has been used to predict correctly and unambiguously new independent
phenomena. We review the history and discuss the best experimental prospects for future discovery. 相似文献
996.
M. Sainath K. Venkataramaniah P. C. Sood 《The European Physical Journal A - Hadrons and Nuclei》2007,31(2):135-140
Low-lying two-quasiparticle bandhead energies for the Z = 99 odd-odd nucleus 254Es are evaluated using a simple phenomenological model with the inclusion of the residual p-n interaction. Configurations of the intrinsic levels directly fed in 254Es from the parent 258Md are discussed in the light of this model. Our analysis predicts the occurrence of ten K≤5 bandheads in 254Es with excitation energies E
x≤300keV. Structures of these as yet unidentified low-lying intrinsic levels and their expected locations are discussed in
the light of available experimental information. 相似文献
997.
The effect of light noise on a Neurospora circadian clock system in the steady states is investigated. It is found that the circadian oscillations could be induced
by light noise, leading to various resonance phenomena including internal signal stochastic resonance (ISSR) and ISSR without
tuning in the system. The strength of ISSR could be significantly reinforced with the decrease of the distance of the control
parameter to the Hopf bifurcation point of the system. The fundamental frequency of noise-induced circadian oscillations almost
does not change with the increment of light noise intensity, which implies that the Neurospora system could sustain intrinsic circadian rhythms. In addition, the ISSR and ISSR without tuning could be both amplified,
suppressed or destroyed by tuning the frequency or amplitude of external signal. 相似文献
998.
The mechanical design of the target module of an accelerator driven subcritical nuclear reactor system (ADSS) calls for an
analysis of the related thermal-hydraulic issues because of large amount of heat deposition in the spallation region during
the course of nuclear interactions with the molten lead bismuth eutectic (LBE) target. The LBE also should carry the entire
heat generated as a consequence of the spallation reaction. The problem of heat removal by the LBE is a challenging thermal-hydraulic
issue. For this, one has to examine the flows of low Prandtl number fluids (LBE) in a complex ADSS geometry. In this study,
the equations governing the laminar flow and thermal energy are solved numerically using the streamline upwind Petrov-Galerkin
(SUPG) finite element (FE) method. The target systems with a straight and a nozzle guide have been considered. The principal
purpose of the analysis is to trace the flow and temperature distribution and thereby to check the suitability of the flow
guide in avoiding the recirculation or stagnation zones in the flow space that may lead to hot spots.
相似文献
999.
The Geant4 CHIPS model simulates both decay and nuclear capture of negative muons. In hadron level models the muon is captured
by a proton, p(μ,νμ)n , and the neutron transfers to the nucleus only 5MeV, which is not enough to split a nucleon from the nucleus, while the
measured spectra of emitted nucleons reach 80MeV. In CHIPS, which considers asymptotically free quark-partons, the muon can
be captured by a quark, u(μ,νμ)d , and transfers 52MeV to the nucleus. This capture mechanism fits the main part of the nucleon spectra, while the high-energy
part of the spectra is not fitted. A precise fit of the high-energy part of the nucleon spectra can be made if the muon decay
μ → dˉνμ is taken into account. 相似文献
1000.
J. Blomgren 《Pramana》2007,68(2):269-278
One of the outstanding new developments in the field of partitioning and transmutation (P&T) concerns accelerator-driven systems
(ADS) which consist of a combination of a high-power, high-energy accelerator, a spallation target for neutron production
and a sub-critical reactor core. The development of the commercial critical reactors of today motivated a large effort on
nuclear data up to about 20 MeV, and presently several million data points can be found in various data libraries. At higher
energies, data are scarce or even non-existent. With the development of nuclear techniques based on neutrons at higher energies,
nowadays there is a need also for higher-energy nuclear data. To provide alternative to this lack of data, a wide program
on neutron-induced data related to ADS for P&T is running at the 20–180 MeV neutron beam facility at ‘The Svedberg Laboratory’
(TSL), Uppsala. The programme encompasses studies of elastic scattering, inelastic neutron production, i.e., (n, xn′) reactions, light-ion production, fission and production of heavy residues. Recent results are presented and future program
of development is outlined.
相似文献