全文获取类型
收费全文 | 32261篇 |
免费 | 1630篇 |
国内免费 | 146篇 |
专业分类
化学 | 16541篇 |
晶体学 | 253篇 |
力学 | 733篇 |
数学 | 2393篇 |
物理学 | 5925篇 |
无线电 | 8192篇 |
出版年
2024年 | 50篇 |
2023年 | 347篇 |
2022年 | 459篇 |
2021年 | 823篇 |
2020年 | 643篇 |
2019年 | 703篇 |
2018年 | 625篇 |
2017年 | 585篇 |
2016年 | 1053篇 |
2015年 | 801篇 |
2014年 | 1194篇 |
2013年 | 1900篇 |
2012年 | 2093篇 |
2011年 | 2302篇 |
2010年 | 1524篇 |
2009年 | 1522篇 |
2008年 | 2145篇 |
2007年 | 1872篇 |
2006年 | 1876篇 |
2005年 | 1670篇 |
2004年 | 1492篇 |
2003年 | 1299篇 |
2002年 | 1168篇 |
2001年 | 780篇 |
2000年 | 726篇 |
1999年 | 555篇 |
1998年 | 408篇 |
1997年 | 380篇 |
1996年 | 403篇 |
1995年 | 278篇 |
1994年 | 267篇 |
1993年 | 274篇 |
1992年 | 216篇 |
1991年 | 187篇 |
1990年 | 176篇 |
1989年 | 137篇 |
1988年 | 108篇 |
1987年 | 99篇 |
1986年 | 64篇 |
1985年 | 92篇 |
1984年 | 66篇 |
1983年 | 62篇 |
1982年 | 75篇 |
1981年 | 79篇 |
1980年 | 46篇 |
1979年 | 51篇 |
1978年 | 56篇 |
1976年 | 45篇 |
1975年 | 45篇 |
1974年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
This research extends previous work with dynamic models to manage groundwater quality by using the consumptive nitrate use rate instead of the nitrate application rate. The analysis indicates that misspecification results in overestimation of economic benefits, and supra-optimum nitrogen fertilizer application rates and groundwater nitrate stocks at a steady state. 相似文献
72.
A family of infinite measure preserving odometers is presented which exhibit examples ofp-recurrent but notp+1-recurrent ergodic transformations for everyp>1. 相似文献
73.
Yong Nam Choi Shin Ae Kim Sung Kyu Kim Sung Baek Kim Chang-Hee Lee Pavel Mikula 《Pramana》2004,63(1):175-181
In a conventional diffractometer having single monochromator, only one position, parallel position, is used for the diffraction
experiment (i.e. detection) because the resolution property of the other one, anti-parallel position, is very poor. However,
a bent perfect crystal (BPC) monochromator at monochromatic focusing condition can provide a quite flat and equal resolution
property at both parallel and anti-parallel positions and thus one can have a chance to use both sides for the diffraction
experiment. From the data of the FWHM and the Δd/d measured on three diffraction geometries (symmetric, asymmetric compression and asymmetric expansion), we can conclude that
the simultaneous diffraction measurement in both parallel and anti-parallel positions can be achieved. 相似文献
74.
McIntyre H. Wendell D. Lin K.J. Kaushik P. Seshadri S. Wang A. Sundararaman V. Ping Wang Song Kim Hsu W.-J. Hee-Choul Park Levinsky G. Jiejun Lu Chirania M. Heald R. Lazar P. Dharmasena S. 《Solid-State Circuits, IEEE Journal of》2005,40(1):52-59
A 4-MB L2 data cache was implemented for a 64-bit 1.6-GHz SPARC(r) RISC microprocessor. Static sense amplifiers were used in the SRAM arrays and for global data repeaters, resulting in robust and flexible timing operation. Elimination of the global clock grid over the SRAM array saves power, enabled by combining the clock information with array select signals. Redundancy was implemented flexibly, with shift circuits outside the main data array for area efficiency. The chip integrates 315 million transistors and uses an 8-metal-layer 90-nm CMOS process. 相似文献
75.
The single fundamental mode output power of photonic crystal vertical cavity lasers is improved by varying sizes of oxide apertures and defect lasing apertures. A maximum output power of 3.1 mW in the fundamental mode has been achieved with a new fabrication process that involves only focused ion beam etching to create holes in selectively oxidised VCSELs. 相似文献
76.
In this paper, we consider the following problem in the wireless ad hoc network: Given a set of paths between source and destination, how to divide the data flow among the paths and how to control the transmission rates, times, and powers of the individual links in order to maximize the operation time of the worst network node. If all nodes are of equal importance, the operation time of the worst node is also the lifetime of the network. By solving the problem, our aim is to investigate how the network lifetime is affected by link conditions such as the maximum transmission power of a node and the peak data rate of a link. For the purpose, we start from a system model that incorporates the carrier to interference ratio (CIR) into a variable data rate of a link. With this, we can develop an iterative algorithm for the lifetime maximization, which resembles to the distributed power control in cellular systems. Numerical examples on the iterative algorithm are included to illustrate the network lifetime as a function of the maximum transmission power and the peak data rate. 相似文献
77.
Man Young Kim Seung Wook Baek 《Journal of Quantitative Spectroscopy & Radiative Transfer》2005,90(3-4):377-388
Radiative heat transfer in an axisymmetric enclosure with absorbing, emitting, and scattering medium is studied here by using the different methods such as MDOM, FVM, and MFVM with emphasis on the treatment of angular derivative term, which appears in curvilinear coordinates due to angular redistribution. After final discretization equation for MFVM is introduced by using the step scheme and directional weights, the present approach is validated by applying it to three different benchmarking problems with absorbing, emitting, and scattering medium. All of the results presented here support its accuracy as well as moderate efficiency. Finally, the present approaches are applied to a truncated cone-shaped enclosure as a body-fitted geometry case. 相似文献
78.
D. Ohlberg J.J. Blackstock R. Ragan S. Kim R. Stanley Williams 《Applied Physics A: Materials Science & Processing》2005,80(6):1327-1334
A recently demonstrated [1] in-vacuo template-stripping process is applied to the study of platinum films stripped from ultra-flat silicon-oxide surfaces. Template-stripped (TS) Pt surfaces, prepared with a range of post-deposition annealing times prior to being stripped from the templating surface in an ultra-high vacuum (UHV) environment, are examined by UHV scanning tunneling microscopy (STM). These studies reveal that without post-deposition annealing, TS Pt surfaces are largely made up of poorly-ordered, granular nanostructures undesirable for many applications. The post-deposition annealing treatments explored in the study result in the emergence and continuous growth of large smooth crystallites. Issues with crystallite orientation relative to the TS surface and artefacts arising as a result of the epoxy used in the template-stripping process are presented and discussed in relation to optimizing the template-stripping procedure for specific applications such as self-assembled monolayer (SAM) formation for molecular electronics. PACS 68.37.Ef; 68.47.De; 68.55.Jk; 81.05.Bx; 81.15.Ef 相似文献
79.
Ju Yeon Woo Eun Hee Kim Tae Hoon Yoon Jae Chang Kim Byung Kyu Kim 《Liquid crystals》2007,34(9):1115-1120
Chiral dopants were added to the formulation of holographic polymer-dispersed liquid crystals and the effects studied in terms of grating formation dynamics, morphology, diffraction efficiency, contrast ratio and electro-optical properties of the films. A gradual increase of real-time diffraction efficiency, decrease of droplet size and increase of diffraction efficiency of the composite film were obtained with the addition and increasing content of chiral dopant, due to the increased viscosity of the liquid crystal (LC) doped with the chiral dopant leading to decreased droplet coalescence. The contrast ratio decreased with increasing content of chiral dopant due to the difficult orientation of LC molecules caused by the formation of a helical structure. Addition of a small amount of the chiral dopant increased the driving voltage slightly, whereas the decay time is decreased significantly as a result of the high twisting of the helical structure. 相似文献
80.
The dry etching characteristics of transparent and conductive indium-zinc oxide (IZO) films have been investigated using an inductively coupled high-density plasma. While the Cl2-based plasma mixture showed little enhancement over physical sputtering in a pure argon atmosphere, the CH4/H2/Ar chemistry produced an increase of the IZO etch rate. On the other hand, the surface morphology of IZO films after etching in Ar and Ar/Cl2 discharges is smooth, whereas that after etching in CH4/H2/Ar presents particle-like features resulting from the preferential desorption of In- and O-containing products. Etching in CH4/H2/Ar also produces formation of a Zn-rich surface layer, whose thickness (∼40 nm) is well-above the expected range of incident ions in the material (∼1 nm). Such alteration of the IZO layer after etching in CH4/H2/Ar plasmas is expected to have a significant impact on the transparent electrode properties in optoelectronic device fabrication. 相似文献