全文获取类型
收费全文 | 225200篇 |
免费 | 10143篇 |
国内免费 | 2130篇 |
专业分类
化学 | 143354篇 |
晶体学 | 2883篇 |
力学 | 8367篇 |
综合类 | 7篇 |
数学 | 23317篇 |
物理学 | 59545篇 |
出版年
2021年 | 1380篇 |
2020年 | 2631篇 |
2019年 | 3955篇 |
2018年 | 2523篇 |
2017年 | 2167篇 |
2016年 | 6186篇 |
2015年 | 5723篇 |
2014年 | 6274篇 |
2013年 | 12709篇 |
2012年 | 10265篇 |
2011年 | 11197篇 |
2010年 | 8238篇 |
2009年 | 8324篇 |
2008年 | 10666篇 |
2007年 | 10086篇 |
2006年 | 9373篇 |
2005年 | 9041篇 |
2004年 | 8082篇 |
2003年 | 6936篇 |
2002年 | 7454篇 |
2001年 | 7684篇 |
2000年 | 6124篇 |
1999年 | 4060篇 |
1998年 | 2863篇 |
1997年 | 2839篇 |
1996年 | 2846篇 |
1995年 | 2473篇 |
1994年 | 2391篇 |
1993年 | 2216篇 |
1992年 | 2533篇 |
1991年 | 2484篇 |
1990年 | 2200篇 |
1989年 | 2148篇 |
1988年 | 2179篇 |
1987年 | 2092篇 |
1986年 | 2001篇 |
1985年 | 2949篇 |
1984年 | 2930篇 |
1983年 | 2314篇 |
1982年 | 2521篇 |
1981年 | 2418篇 |
1980年 | 2363篇 |
1979年 | 2267篇 |
1978年 | 2338篇 |
1977年 | 2299篇 |
1976年 | 2211篇 |
1975年 | 2201篇 |
1974年 | 2077篇 |
1973年 | 2178篇 |
1972年 | 1230篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
A convergent-barrel (CB) cold spray nozzle was designed through numerical simulation. It was found that the main factors influencing significantly particle velocity and temperature include the length and diameter of the barrel section, the nature of the accelerating gas and its pressure and temperature, and the particle size. Particles can achieve a relatively low velocity but a high temperature under the same gas pressure using a CB nozzle compared to a convergent-divergent (CD) nozzle. The experiment results with Cu powder using the designed CB nozzle confirmed that particle deposition can be realized under a lower gas pressure with a CB nozzle. 相似文献
242.
243.
We apply a Bayesian approach to the problem of prediction in an unbalanced growth curve model using noninformative priors. Due to the complexity of the model, no analytic forms of the predictive densities are available. We propose both approximations and a prediction-oriented Metropolis-Hastings sampling algorithm for two types of prediction, namely the prediction of future observations for a new subject and the prediction of future values for a partially observed subject. They are illustrated and compared through real data and simulation studies. Two of the approximations compare favorably with the approximation in Fearn (1975, Biometrika, 62, 89–100) and are very comparable to the more accurate Rao-Blackwellization from Metropolis-Hastings sampling algorithm. 相似文献
244.
This report focuses on a theoretical study of the cobalt-cabon bond activation mechanism in Vitamin B12, its co enzyme complexes, and model compounds such as cobalamins (Cbl), cobaloximes and cobalt porphyrins. In particular, emphasis is placed on the nucleotide loop and its role in the base on and base off mechanism. 相似文献
245.
C.-S. Huang T. Li L. Wei Q.-S. Yan 《The European Physical Journal C - Particles and Fields》2002,23(1):195-199
It is shown that new sources of CP violation can be generated in models with more than one extra dimension. In the supersymmetric models on the space-time , where the radius moduli have auxiliary vacuum expectation values and the supersymmetry breaking is mediated by the Kaluza–Klein
states of the gauge supermultiplets, we analyze the gaugino masses and trilinear couplings for two scenarios and obtain the
result that there exist relative CP violating phases among the gaugino masses and trilinear couplings.
Received: 10 October 2001 / Published online: 20 December 2001 相似文献
246.
Sung‐Fu Hsu Tzong‐Ming Wu Chien‐Shiun Liao 《Journal of Polymer Science.Polymer Physics》2006,44(23):3337-3347
Poly(3‐hydroxybutyrate) (PHB)/layered double hydroxides (LDHs) nanocomposites were prepared by mixing PHB and poly(ethylene glycol) phosphonates (PEOPAs)‐modified LDH (PMLDH) in chloroform solution. Both X‐ray diffraction data and TEM micrographs of PHB/PMLDH nanocomposites indicate that the PMLDHs are randomly dispersed and exfoliated into the PHB matrix. In this study, the effect of PMLDH on the isothermal crystallization behavior of PHB was investigated using a differential scanning calorimeter (DSC) and polarized optical microscopy. Isothermal crystallization results of PHB/PMLDH nanocomposites show that the addition of 2 wt % PMLDH into PHB induced more heterogeneous nucleation in the crystallization significantly increasing the crystallization rate and reducing their activation energy. By adding more PMLDH into the PHB probably causes more steric hindrance of the diffusion of PHB, reducing the transportation ability of polymer chains during crystallization, thus increasing the activation energy. The correlation among crystallization kinetics, melting behavior and crystalline structure of PHB/PMLDH nanocomposites can also be discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3337–3347, 2006 相似文献
247.
Barotropic FRW cosmologies are presented from the standpoint of nonrelativistic supersymmetry. First, we reduce the barotropic FRW system of differential equations to simple harmonic oscillator differential equations. Employing the factorization procedure, the solutions of the latter equations are divided into the two classes of bosonic (nonsingular) and fermionic (singular) cosmological solutions. We next introduce a coupling parameter denoted by K between the two classes of solutions and obtain barotropic cosmologies with dissipative features acting on the scale factors and spatial curvature of the universe. The K-extended FRW equations in comoving time are presented in explicit form in the low coupling regime. The standard barotropic FRW cosmologies correspond to the dissipationless limit K = 0. 相似文献
248.
Apurba Bhattacharya Nitin C. Patel Michael Peddicord Luca Parlanti John A. Grosso 《Tetrahedron letters》2006,47(30):5341-5343
An efficient, one-pot, phase transfer N-amination technology was developed. The protocol utilizes chloramine, an inexpensive and safe electrophilic aminating agent potentially viable for commercial manufacturing. 相似文献
249.
Direct measurement of fluid velocity gradients at a wall by PIV image processing with stereo reconstruction 总被引:1,自引:0,他引:1
Velocity gradient is typically estimated in Particle Image Velocimetry (PIV) by differentiating a measured velocity field,
which amplifies noise in the measured velocities. If gradients near a boundary are sought, such noise is usually greater than
in bulk fluid, because of small tracer displacement, uncertainty in the effective positions of velocity vectors, intense deformation
of tracer patterns, and laser reflection. We consider here a modified form of the Particle Image Distortion (PID) method todirectly calculate velocity gradients at a fixed wall, and refer it as “PIV/IG” (“Interface Gradiometry”). Results from synthetic
2D PIV images suggest our method achieves higher SNR and accuracy than velocity differentiation. Also, we have developed a
procedure to reconstruct three-dimensional velocity gradient at a fixed wall the two non-zero components from PIV/IG data
obtained in stereo views; these equations simplify considerably thanks to the no-slip condition. Experimental data from the
bottom wall of turbulent open channel flow appear to suffer from a form of pixel locking. As with standard PIV, this underlines
the importance of adequate tracer diameter in the images, sufficient seeding density, and of dynamic range of the camera sensor. 相似文献
250.
We provide an approximate analysis of the transient sojourn time for a processor sharing queue with time varying arrival and
service rates, where the load can vary over time, including periods of overload. Using the same asymptotic technique as uniform
acceleration as demonstrated in [12] and [13], we obtain fluid and diffusion limits for the sojourn time of the Mt/Mt/1 processor-sharing queue. Our analysis is enabled by the introduction of a “virtual customer” which differs from the notion
of a “tagged customer” in that the former has no effect on the processing time of the other customers in the system. Our analysis
generalizes to non-exponential service and interarrival times, when the fluid and diffusion limits for the queueing process
are known. 相似文献