全文获取类型
收费全文 | 261820篇 |
免费 | 2494篇 |
国内免费 | 990篇 |
专业分类
化学 | 122185篇 |
晶体学 | 3848篇 |
力学 | 14042篇 |
综合类 | 3篇 |
数学 | 53420篇 |
物理学 | 71806篇 |
出版年
2020年 | 1546篇 |
2019年 | 1711篇 |
2018年 | 12256篇 |
2017年 | 12156篇 |
2016年 | 9316篇 |
2015年 | 2983篇 |
2014年 | 3656篇 |
2013年 | 8604篇 |
2012年 | 10020篇 |
2011年 | 17841篇 |
2010年 | 10863篇 |
2009年 | 11224篇 |
2008年 | 13668篇 |
2007年 | 15682篇 |
2006年 | 7093篇 |
2005年 | 7622篇 |
2004年 | 7335篇 |
2003年 | 7147篇 |
2002年 | 5996篇 |
2001年 | 5621篇 |
2000年 | 4319篇 |
1999年 | 3250篇 |
1998年 | 2809篇 |
1997年 | 2721篇 |
1996年 | 2671篇 |
1995年 | 2393篇 |
1994年 | 2323篇 |
1993年 | 2248篇 |
1992年 | 2512篇 |
1991年 | 2453篇 |
1990年 | 2388篇 |
1989年 | 2301篇 |
1988年 | 2313篇 |
1987年 | 2321篇 |
1986年 | 2191篇 |
1985年 | 2926篇 |
1984年 | 2946篇 |
1983年 | 2595篇 |
1982年 | 2778篇 |
1981年 | 2542篇 |
1980年 | 2584篇 |
1979年 | 2688篇 |
1978年 | 2720篇 |
1977年 | 2640篇 |
1976年 | 2652篇 |
1975年 | 2532篇 |
1974年 | 2489篇 |
1973年 | 2677篇 |
1972年 | 1706篇 |
1971年 | 1424篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
A general framework is developed to treat inverse problems with parameters that are random fields. It involves a sampling
method that exploits the sensitivity derivatives of the control variable with respect to the random parameters. As the sensitivity
derivatives are computed only at the mean values of the relevant parameters, the related extra cost of the present method
is a fraction of the total cost of the Monte Carlo method. The effectiveness of the method is demonstrated on an example problem
governed by the Burgers equation with random viscosity. It is specifically shown that this method is two orders of magnitude
more efficient compared to the conventional Monte Carlo method. In other words, for a given number of samples, the present
method yields two orders of magnitude higher accuracy than its conventional counterpart. 相似文献
912.
The Planar Laser-Induced Fluorescence (PLIF) technique enables measurement of the local degree of deviation in an arbitrary
plane inside the mixing vessel. This is achieved by injecting a mixture of an inert dye and a reacting fluorescent into the
vessel. The inert dye serves as a tracer for the macromixing. The fluorescent characteristics of the reacting dye change while
undergoing a fast chemical reaction with the vessel content and it therefore shows the micromixing indirectly. The concentration
fields of the dyes are measured simultaneously. For that a laser beam is expanded into a thin light sheet which illuminates
an arbitrary plane in the mixing vessel, exciting the fluorescent dye in this area. The emitted light is detected by a CCD-camera
which is positioned vertical to the measurement plane. The fluorescent light passes through two optical filters which are
suitable for separating the fluorescent light of the two dyes. The light is then projected on half of the camera chip each
so that the same display window is detected twice, and thus the local concentration of the two dyes can be measured simultaneously.
Low Reynolds number measurements are performed in a mixing vessel equipped with a Rushton turbine. The lamellar structure
is clearly resolved. Areas of micromixing are detected by calculating the local degree of deviation from the concentration
fields. These areas are mainly found in the boundary layer of the lamellas. 相似文献
913.
Yiran He 《Journal of Global Optimization》2007,39(3):419-426
The existence of global error bound for convex inclusion problems is discussed in this paper, including pointwise global error
bound and uniform global error bound. The existence of uniform global error bound has been carefully studied in Burke and
Tseng (SIAM J. Optim. 6(2), 265–282, 1996) which unifies and extends many existing results. Our results on the uniform global
error bound (see Theorem 3.2) generalize Theorem 9 in Burke and Tseng (1996) by weakening the constraint qualification and
by widening the varying range of the parameter. As an application, the existence of global error bound for convex multifunctions
is also discussed. 相似文献
914.
An experimental apparatus and related procedures for the determination of the effective thermal conductivity of sintered powder-metal
plates saturated with distilled water at temperatures in the range 20–150°C are discussed. The apparatus and procedures are
applied to two samples of sintered powder-metal plates, one made of nickel 200 and the other of stainless steel 316, with
porosities of 28.10 and 46.45%, respectively, and each of nominal dimensions 127 mm × 127 mm × 6.35 mm. The experimental
results are compared with corresponding predictions yielded by several correlations available in the literature. The correlations
based on experimental data for packed beds of spherical particles and also porous plates made of cold-pressed (but not sintered)
particles of angular shapes do not apply well to sintered powder-metal plates. A new correlation, which is based on extensions
of ideas contained in earlier works and provides improved predictions, is proposed. 相似文献
915.
In this work preliminary results of amorphous zirconium crystallization using ion beam pulses are presented. Energetic argon- and oxygen-ion beams generated by a plasma focus device were used to promote crystallization on amorphous ZrO2-2.5 mol% Y2O3 film deposited by chemical solution deposition onto silica glass substrate. The films were burnt at 370 °C for 1 h in normal atmosphere previous to plasma irradiation. The irradiation was performed by means of successive pulses of ion beams. The evolution of the surface morphology and crystallization was followed by AFM and X-rays diffraction in a grazing incidence asymmetric Bragg geometry (GIAB), respectively. Argon-irradiated films showed highly nucleated cubic zirconia after 10 pulses. On the other hand, oxygen-irradiated films showed a delayed and less extensive cubic nucleation, but a more ordered structure and well-defined grains. 相似文献
916.
Grafov B. M. Davydov A. D. Damaskin B. B. Lazorenko-Manevich R. M. 《Russian Journal of Electrochemistry》2003,39(4):439-440
Russian Journal of Electrochemistry - 相似文献
917.
Uzi Vishne 《Archiv der Mathematik》2003,80(2):119-122
We prove that assuming enough roots of unity in the base field, a central simple algebra of exponent 2 which is split by a dihedral group, is also split by certain abelian groups. 相似文献
918.
L. G. Smirnova Yu. B. Grunin S. V. Krasil'nikova M. A. Zaverkina D. R. Bakieva E. V. Smirnov 《Colloid Journal》2003,65(6):778-781
The pulsed NMR, isopiestic and pycnometric methods are applied to investigate the physicochemical properties of softwood [GOST (State Standard) 9571-89] and hardwood [GOST 28172-89] celluloses, as well as flax cellulose [GOST 10330-76, 9394-76], both in the initial state and subjected to chemical treatment. Possible changes occurring in the supramolecular structure of a polymer are analyzed. The structural and sorption characteristics of the investigated samples are calculated. Their water absorption capacities are determined. 相似文献
919.
S. D. Gunapala S. V. Bandara J. K. Liu S. B. Rafol J. M. Mumolo C. A. Shott R. Jones J. Woolaway II J. M. Fastenau A. K. Liu M. Jhabvala K. K. Choi 《Infrared Physics & Technology》2003,44(5-6):411-425
A 9 μm cutoff 640 × 512 pixel hand-held quantum well infrared photodetector (QWIP) camera has been demonstrated with excellent imagery. A noise equivalent differential temperature (NEDT) of 10.6 mK is expected at a 65 K operating temperature with f/2 optics at a 300 K background. This focal plane array has shown background limited performance at a 72 K operating temperature with the same optics and background conditions. In this paper, we discuss the development of this very sensitive long-wavelength infrared camera based on a GaAs/AlGaAs QWIP focal plane array and its performance in quantum efficiency, NEDT, uniformity, and operability. In the second section of this paper, we discuss the first demonstration of a monolithic spatially separated four-band 640 × 512 pixel QWIP focal plane array and its performance. The four spectral bands cover 4–5.5, 8.5–10, 10–12, and 13.5–15 μm spectral regions with 640 × 128 pixels in each band. In the last section, we discuss the array performance of a 640 × 512 pixel broad-band (10–16 μm full-width at half-maximum) QWIP focal plane. 相似文献
920.
We consider toy cosmological models in which a classical, homogeneous, spinor field provides a dominant or sub-dominant contribution to the energy-momentum tensor of a flat Friedmann-Robertson-Walker universe. We find that, if such a field were to exist, appropriate choices of the spinor self-interaction would generate a rich variety of behaviors, quite different from their widely studied scalar field counterparts. We first discuss solutions that incorporate a stage of cosmic inflation and estimate the primordial spectrum of density perturbations seeded during such a stage. Inflation driven by a spinor field turns out to be unappealing as it leads to a blue spectrum of perturbations and requires considerable fine-tuning of parameters. We next find that, for simple, quartic spinor self-interactions, non-singular cyclic cosmologies exist with reasonable parameter choices. These solutions might eventually be incorporated into a successful past- and future-eternal cosmological model free of singularities. In an Appendix, we discuss the classical treatment of spinors and argue that certain quantum systems might be approximated in terms of such fields. 相似文献