全文获取类型
收费全文 | 3096篇 |
免费 | 575篇 |
国内免费 | 115篇 |
专业分类
化学 | 489篇 |
晶体学 | 11篇 |
力学 | 170篇 |
综合类 | 38篇 |
数学 | 727篇 |
物理学 | 2351篇 |
出版年
2024年 | 6篇 |
2023年 | 49篇 |
2022年 | 76篇 |
2021年 | 74篇 |
2020年 | 84篇 |
2019年 | 59篇 |
2018年 | 63篇 |
2017年 | 105篇 |
2016年 | 123篇 |
2015年 | 107篇 |
2014年 | 199篇 |
2013年 | 256篇 |
2012年 | 188篇 |
2011年 | 198篇 |
2010年 | 160篇 |
2009年 | 184篇 |
2008年 | 194篇 |
2007年 | 206篇 |
2006年 | 189篇 |
2005年 | 150篇 |
2004年 | 160篇 |
2003年 | 109篇 |
2002年 | 84篇 |
2001年 | 101篇 |
2000年 | 92篇 |
1999年 | 89篇 |
1998年 | 71篇 |
1997年 | 75篇 |
1996年 | 55篇 |
1995年 | 62篇 |
1994年 | 35篇 |
1993年 | 36篇 |
1992年 | 25篇 |
1991年 | 15篇 |
1990年 | 18篇 |
1989年 | 13篇 |
1988年 | 17篇 |
1987年 | 17篇 |
1986年 | 5篇 |
1985年 | 2篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1977年 | 2篇 |
1973年 | 2篇 |
1972年 | 3篇 |
1969年 | 1篇 |
排序方式: 共有3786条查询结果,搜索用时 203 毫秒
991.
Liang-Qi Gui Zu-Yin Zhang Wei Guo 《International Journal of Infrared and Millimeter Waves》2005,26(5):715-723
System gain and receiver noise are two main impact factors of total power millimeter wave (MMW) radiometer output. In this paper, a new method of using software to compensate system gain and receiver noise fluctuation is introduced. This method can keep the long-term stability and accuracy of the radiometer. At the same time, only one matching load is used as the reference source, which can prevent the trouble caused by using two cold and hot calibration sources in ground-based or airborne radiometer. The preliminary experiment proves the efficiency of the method. 相似文献
992.
Seongha?KimEmail author Jaehoon?Lee Yonggyoo?Kim Shinhee?Won Yun-je?Oh Tae?Won?Oh Jichai?Jeong 《Optical and Quantum Electronics》2005,37(6):545-559
We investigate the transmission performance of N × 40 Gbps carrier-suppressed return-to-zero (CS-RZ) and duobinary CS-RZ (DCS-RZ) modulated wavelength division multiplexed (WDM) signals over the standard single-mode fiber (SSMF) based on non-ideal Mach-Zehnder (MZ) modulators through numerical simulations. In addition to that, the impact on receiver margin related to the residual chirp due to an asymmetry ratio of modulators as well as the effect of the applied chirp of modulators has been studied. As the asymmetry ratio of modulators is increased, dispersion penalties are increased asymmetrically at around the zero dispersion wavelength for the CS-RZ format. The DCS-RZ modulation format has a symmetric behavior due to different characteristics of the residual chirp of modulators. The receiver margin for the DCS-RZ modulation format is larger than the CS-RZ modulation format above the asymmetry ratio of 0.82 of modulators (extinction ratio of 20 dB) with the optimal ΔλDCF. By controlling the negative/positive applied chirp of the first/second modulator, dispersion penalties can be reduced for both the CS-RZ and DCS-RZ formats. In the DCS-RZ format compared to the CS-RZ format, the effect of the applied chirp of the first modulator is more dominant than that of the second modulator. The simulation results show that the receiver margin is limited by the asymmetry ratio of modulators as well as a deviated wavelength from the zero dispersion wavelength of dispersion compensating fibers (DCF) in order to be fully post-compensated. Dispersion penalties can be reduced with appropriate chirp parameters of two non-ideal modulators. 相似文献
993.
B.?F.?Kuntsevich A.?N.?PisarchikEmail author V.?K.?Kononenko 《Optical and Quantum Electronics》2005,37(7):675-693
A detailed numerical study of dynamical behavior of a semiconductor laser under current modulation and cavity detuning has been performed on the base of four different models of the active medium which take into account direct transitions between ground subbands, transitions with no k-selection rule between ground subbands and contribution of excited subbands in each of the above-mentioned cases. We have shown that different nonlinear regimes (period doubling, chaos, generalized bistability) can be obtained either with cavity detuning from the gain band maximum or near the laser threshold.It has been established that the shape and principal peculiarities of amplitude detuning characteristics are determined by the relation between the current modulation frequency and maximum resonance frequency of the laser.PACS numbers: 05.45.Pq, 42.55.Px, 42.65.Sf 相似文献
994.
Jianji?Dong Xinliang?ZhangEmail author Zhong?Jiang Dexiu?Luang 《Optical and Quantum Electronics》2005,37(11):1011-1023
We analyze the all-optical wavelength converter (AOWC)-based on cross-gain modulation (XGM) in a single-port-coupled (SPC)
semiconductor optical amplifier (SOA). A comprehensive dynamic model is developed by considering longitudinal variations of
the carrier density, the residual rear-facet reflectivity of the SOA and the wide-band spontaneous noise emission. The numerical
simulations for the novel wavelength conversion at 10 Gbit/s are presented based on the model. The extinction ratio (ER),
conversion efficiency and pattern effect of the SPC-SOA-based wavelength converters are investigated, respectively. Compared
with the traditional scheme of the double-portcoupled (DPC) SOA, the SPC-SOA scheme has better performance. We have obtained
that the ER is higher than 10 dB with the pump wavelength turned over 15nm from experiments. The experimental results are
in agreement with the simulation results. 相似文献
995.
** Email: tim_marchant{at}uow.edu.au*** Email: n.smyth{at}ed.ac.uk A 1D partial differential equation (pde) describing the flowof magma in the Earth's mantle is considered, this equationallowing for compaction and distension of the surrounding matrixdue to the magma. The equation has periodic travelling wavesolutions, one limit of which is a solitary wave, called a magmon.Modulation equations for the magma equation are derived andare found to be either hyperbolic or of mixed hyperbolic/elliptictype, depending on the specific values of the wave number, meanheight and amplitude of the underlying modulated wave. The periodicwave train is stable in the hyperbolic case and unstable inthe mixed case. Solutions of the modulation equations are foundfor an initial-boundary value problem on the semi-infinite line,these solutions representing the influx of magma from a largereservoir. The modulation solutions are found to consist ofa full or partial undular bore. Excellent agreement with numericalsolutions of the governing pde is obtained, except in the limitwhere the wave train becomes a train of magmons. An alternativeapproximation based on the assumption that the wave train isa series of uniform magmons is also derived and is found tobe superior to modulation theory in this limit. 相似文献
996.
We present numerical investigations of ultrafast femtosecond (with time duration of 100 fs at 1/e intensity point) all-optical modulation of a pump-probe wave arrangement by using nondegenerate two-photon absorption (TPA),
namely cross absorption, inside silicon-on-insulator (SOI) optical waveguides. Our results show that when a pump pulse with
femtosecond duration and a continuous probe wave are co-propagating along the SOI, the probe wave can be modulated inversely
by the ultrafast pump pulse, whose modulation depth depends strongly on the system parameters such as the waveguide length,
the peak power and initial chirp of the pump wave, the group velocity dispersion (GVD), etc.; this means that the modulation
depth can be improved by an appropriate increase of the waveguide length, the pump peak, and the initial chirp, in addition,
which has a larger value for the probe wavelength in the normal dispersion regime compared with the case of abnormal dispersion
when the center wavelength of the pump wave is located at the zero-dispersion wavelength. 相似文献
997.
研究奇摄动积分微分方程组边值问题εy"=f(x,y,Ty,ε)y′++g(x,y,Ty,ε);y(0,ε)=A(ε),y(1,ε)=B(ε)其中y、g、A和B均为n维向量函数,f是n×n矩阵函数,(Ty)(x)=∫xK(x,s,y(sε),ε)ds在一定假设条件下,利用对角化技巧和逐步逼近法证明解的存在,并给出解的直到0(εN+1)的渐近展开式. 相似文献
998.
H. C. P. Berbee C. G. E. Boender A. H. G. Rinnooy Ran C. L. Scheffer R. L. Smith J. Telgen 《Mathematical Programming》1987,37(2):184-207
Two probabilistic hit-and-run algorithms are presented to detect nonredundant constraints in a full dimensional system of
linear inequalities. The algorithms proceed by generating a random sequence of interior points whose limiting distribution
is uniform, and by searching for a nonredundant constraint in the direction of a random vector from each point in the sequence.
In the hypersphere directions algorithm the direction vector is drawn from a uniform distribution on a hypersphere. In the
computationally superior coordinate directions algorithm a search is carried out along one of the coordinate vectors. The
algorithms are terminated through the use of a Bayesian stopping rule. Computational experience with the algorithms and the
stopping rule will be reported. 相似文献
999.
B. Kafková Z. Bosáková E. Tesařová J. Suchánková P. Coufal K. Ŝtulík 《Chromatographia》2002,56(7-8):445-447
Summary Triethylamine is often added to mobile phases in reversed-phase liquid chromatography for dynamic deactivation of free silanol
groups of the stationary phase. It has been observed that eluents composed of methanol and triethylamine generate two system
peaks in chromatograms obtained with LiChrosorb RP-select B stationary phase, whose retention times correspond to the dead
time and to the retention time of triethylamine. It has been demonstrated that the system peaks can be positive or negative
depending on the experimental conditions and may be incorrectly interpreted as peaks corresponding to sample components. An
approach is outlined to unambiguous identify these system peaks in chromatograms of practical samples. 相似文献
1000.
This paper considers both classical and minimax problems of optimal control which arise in the study of aeroassisted, coplanar orbital transfer. The maneuver considered involves the coplanar transfer from a high planetary orbit to a low planetary orbit. An example is the HEO-to-LEO transfer of a spacecraft, where HEO denotes high Earth orbit and LEO denotes low Earth orbit. In particular, HEO can be GEO, a geosynchronous Earth orbit.The basic idea is to employ the hybrid combination of propulsive maneuvers in space and aerodynamic maneuvers in the sensible atmosphere. Hence, this type of flight is also called synergetic space flight. With reference to the atmospheric part of the maneuver, trajectory control is achieved by means of lift modulation. The presence of upper and lower bounds on the lift coefficient is considered.Within the framework of classical optimal control, the following problems are studied: (P1) minimize the energy required for orbital transfer; (P2) minimize the time integral of the heating rate; (P3) minimize the time of flight during the atmospheric portion of the trajectory; (P4) maximize the time of flight during the atmospheric portion of the trajectory; (P5) minimize the time integral of the square of the path inclination; and (P6) minimize the sum of the squares of the entry and exit path inclinations. Problems (P1) through (P6) are Bolza problems of optimal control.Within the framework of minimax optimal control, the following problems are studied: (Q1) minimize the peak heating rate; (Q2) minimize the peak dynamic pressure; and (Q3) minimize the peak altitude drop. Problems (Q1) through (Q3) are Chebyshev problems of optimal control, which can be converted into Bolza problems by suitable transformations.Numerical solutions for Problems (P1)–(P6) and Problems (Q1)–(Q3) are obtained by means of the sequential gradient-restoration algorithm for optimal control problems. The engineering implications of these solutions are discussed. In particular, the merits of nearly-grazing trajectories are considered.This research was supported by the Jet Propulsion Laboratory, Contract No. 956415. The authors are indebted to Dr. K. D. Mease, Jet Propulsion Laboratory, for helpful discussions. This paper is a condensation of the investigation reported in Ref. 1. 相似文献