全文获取类型
收费全文 | 73416篇 |
免费 | 10148篇 |
国内免费 | 6903篇 |
专业分类
化学 | 49254篇 |
晶体学 | 713篇 |
力学 | 4800篇 |
综合类 | 530篇 |
数学 | 8756篇 |
物理学 | 26414篇 |
出版年
2024年 | 232篇 |
2023年 | 1432篇 |
2022年 | 2342篇 |
2021年 | 2559篇 |
2020年 | 2603篇 |
2019年 | 2522篇 |
2018年 | 2274篇 |
2017年 | 2010篇 |
2016年 | 3085篇 |
2015年 | 3221篇 |
2014年 | 3799篇 |
2013年 | 5067篇 |
2012年 | 6140篇 |
2011年 | 6256篇 |
2010年 | 4250篇 |
2009年 | 4196篇 |
2008年 | 4398篇 |
2007年 | 3969篇 |
2006年 | 3720篇 |
2005年 | 3244篇 |
2004年 | 2584篇 |
2003年 | 1992篇 |
2002年 | 1777篇 |
2001年 | 1577篇 |
2000年 | 1455篇 |
1999年 | 1605篇 |
1998年 | 1365篇 |
1997年 | 1195篇 |
1996年 | 1252篇 |
1995年 | 1096篇 |
1994年 | 1034篇 |
1993年 | 880篇 |
1992年 | 782篇 |
1991年 | 681篇 |
1990年 | 570篇 |
1989年 | 489篇 |
1988年 | 371篇 |
1987年 | 361篇 |
1986年 | 304篇 |
1985年 | 309篇 |
1984年 | 210篇 |
1983年 | 181篇 |
1982年 | 150篇 |
1981年 | 115篇 |
1980年 | 107篇 |
1978年 | 79篇 |
1977年 | 100篇 |
1976年 | 68篇 |
1975年 | 75篇 |
1973年 | 63篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
本文对近20年来多复变函数的一个发展迅速的数学热门分支-逆紧映照作了一个回顾和整理。这是作者继续从事此方向研究的先声,也希望本文能为有志于此的研究者提供一些便利。本文从经典的结果开始,通过对逆紧映照在边界上的开拓及分支点的分布的讨论,详细地阐述了这些年来关于逆紧映照何时成为双全纯映照的若干结果。最后,对近年来关于逆紧映照另外的一些工作进行了简单的介绍。 相似文献
52.
53.
Perovskite-type lithium fast ion conductors of Li3xLa0.67−xScyTi1−2yNbyO3 system were prepared by solid state reaction. X-Ray powder diffraction shows that perovskite solid solution form in the ranges
of x=0.10, y≤0.10. AC impedance measurements indicate that the bulk conductivities and the total conductivities are of the
order of 10−4 S·cm−1 and 10−5 S·cm−1 at 25 °C respectively. The compositions have low bulk activation energies of about 17 kJ/mol in the temperature ranges of
298 – 523 K and total activation energies of about 37 kJ/mol in the temperature ranges of 298 – 523 K. 相似文献
54.
55.
This paper investigates mutual influence of duct and room acoustics in the whole fan-duct-plenum-room integrations. Applying the parametric design language of finite element software ANSYS (APDL), dimensional and positional influence on system acoustics has been studied. Models with different room dimensions, duct lengths, duct cross-sections, duct locations, duct discharges and duct elbow were constructed, and their characteristics were compared qualitatively. Results show that small rooms, short ducts, large duct cross-sections and bell mouth duct discharges help to increase room sound pressure levels (SPLs); SPLs in ducts and plenums are sensitive to duct dimensions and duct discharge types but insensitive to duct locations and room dimensions; duct elbows have relatively indistinct acoustic influence in each component. Based on the calculation results, a semi-experimental method was proposed for simply and approximately evaluating indoor acoustic spectra of fan-duct-plenum-room integrations, then an example was used to demonstrate the prediction process. Finally, by adopting several ideal models, sound field constitutions, duct and room wall admittances and duct end reflection were explored quantitatively. This study may give a detailed understanding of fan-duct-plenum-room acoustics for researchers, also it might provide a new, simple and approximate prediction method for professionals to evaluate and improve fan-ducted acoustics. 相似文献
56.
Jie Xue Liang Chen Li Zhou Zhifeng Jia Yanping Wang Xinyuan Zhu Deyue Yan 《Journal of Polymer Science.Polymer Physics》2006,44(15):2050-2057
α‐Cyclodextrin (α‐CD) has been complexed with various poly(ethylene glycol) (PEG) derivatives in aqueous solution. It has been found that the end groups of PEG derivatives affect the complexation kinetics greatly, but have only a little influence on the thermodynamic behavior. By increasing the hydrophobicity of end groups, the complexation speeds up rapidly. On the other hand, the bulky end groups slow down the threading of polymeric guests into the cavity of CD. By changing the hydrophobicity and the size of end groups, the complexation rate can be adjusted in the range of several orders of magnitudes, which should be quite useful in the design of new supramolecular systems. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2050–2057, 2006 相似文献
57.
Ying Zhang Gang Hu Shigang Chen H.A. Cerdeira 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,27(3):381-384
A method of controlling global stochasticity in Hamiltonian systems by applying nonlinear perturbation is proposed. With the
well-known standard map we demonstrate that this control method can convert global stochasticity into regular motion in a
wide chaotic region for arbitrary initial condition, in which the control signal remains very weak after a few kicks. The
system in which chaos has been controlled approximates to the original Hamiltonian system, and this approach appears robust
against small external noise. The mechanism underlying this high control efficiency is intuitively explained.
Received 15 January 2002 Published online 6 June 2002 相似文献
58.
Backstepping design is proposed for synchronization of Genesio chaotic system. Firstly, the control problem for the chaos synchronization of nominal Genesio systems without unknown parameters is considered. Next, an adaptive backstepping control law is derived to make the error signals between drive Genesio system and response Genesio system with an uncertain parameter asymptotically synchronized. Finally, the approach is extended to the synchronization problem for the system with three unknown parameters. The stability analysis in this article is proved by using a well-known Lyapunov stability theorem. Note that the approach provided here needs only a single controller to realize the synchronization. Two numerical simulations are presented to show the effectiveness of the proposed chaos synchronization scheme. 相似文献
59.
Fengfu Fu Liangjun Xu Wei Ye Yiquan Chen Mingyu Jiang Xueqin Xu 《中国颗粒学报》2006,4(6):323-326
Different-sized aerosols were collected by an Andersen air sampler to observe the detailed morphology of the black carbon (BC) aerosols which were separated chemically from the other accompanying aerosols, using a Scanning Electron Microscope equipped with an Energy Dispersive X-ray Spectrometer (SEM-EDX). The results indicate that most BC aerosols are spherical particles of about 50 nm in diameter and with a homogeneous surface. Results also show that these particles aggregate with other aerosols or with themselves to form larger agglomerates in the micrometer range. The shape of these 50-nm BC spherical particles was found to be very similar to that of BC particles released from petroleum-powered vehicular internal combustion engines. These spherical BC particles were shown to be different from the previously reported fullerenes found using Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight Mass Spectrometry (MALDI-TOF-MS). 相似文献
60.
We have prepared composite magnetic core–shell particles using the process of soap-free emulsion polymerization and the co-precipitation method. The shell of the synthesized composite sphere is cobalt ferrite (CoFe2O4) nanoparticles and the core consists of poly(styrene-co-methacrylic acid) polymer. The mean crystallite sizes of the coated CoFe2O4 nanoparticles were controlled in the range of 2.4–6.7 nm by the concentration of [NH4+] and heated temperature. The magnetic properties of the core–shell spherical particles can go from superparamagnetic to ferromagnetic behavior depending on the crystalline sizes of CoFe2O4. 相似文献