全文获取类型
收费全文 | 262853篇 |
免费 | 3331篇 |
国内免费 | 825篇 |
专业分类
电工技术 | 5170篇 |
综合类 | 332篇 |
化学工业 | 37785篇 |
金属工艺 | 10304篇 |
机械仪表 | 8549篇 |
建筑科学 | 6153篇 |
矿业工程 | 1079篇 |
能源动力 | 6466篇 |
轻工业 | 23243篇 |
水利工程 | 2573篇 |
石油天然气 | 3935篇 |
武器工业 | 68篇 |
无线电 | 34825篇 |
一般工业技术 | 50303篇 |
冶金工业 | 47550篇 |
原子能技术 | 5430篇 |
自动化技术 | 23244篇 |
出版年
2021年 | 2157篇 |
2019年 | 2108篇 |
2018年 | 3573篇 |
2017年 | 3631篇 |
2016年 | 3816篇 |
2015年 | 2370篇 |
2014年 | 4124篇 |
2013年 | 11447篇 |
2012年 | 6681篇 |
2011年 | 8996篇 |
2010年 | 7087篇 |
2009年 | 7954篇 |
2008年 | 8752篇 |
2007年 | 8668篇 |
2006年 | 7846篇 |
2005年 | 7180篇 |
2004年 | 6919篇 |
2003年 | 6759篇 |
2002年 | 6394篇 |
2001年 | 6490篇 |
2000年 | 6198篇 |
1999年 | 6312篇 |
1998年 | 14573篇 |
1997年 | 10495篇 |
1996年 | 8230篇 |
1995年 | 6495篇 |
1994年 | 5762篇 |
1993年 | 5661篇 |
1992年 | 4563篇 |
1991年 | 4233篇 |
1990年 | 4060篇 |
1989年 | 3790篇 |
1988年 | 3640篇 |
1987年 | 3177篇 |
1986年 | 3072篇 |
1985年 | 3620篇 |
1984年 | 3387篇 |
1983年 | 3033篇 |
1982年 | 2860篇 |
1981年 | 2956篇 |
1980年 | 2765篇 |
1979年 | 2649篇 |
1978年 | 2504篇 |
1977年 | 2962篇 |
1976年 | 3602篇 |
1975年 | 2321篇 |
1974年 | 2313篇 |
1973年 | 2320篇 |
1972年 | 1850篇 |
1971年 | 1745篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
791.
A. D. Bulanov P. G. Sennikov V. A. Krylov T. G. Sorochkina L. A. Chuprov O. Yu. Chernova O. Yu. Troshin 《Inorganic Materials》2007,43(4):364-368
Using gas chromatography and high-resolution Fourier-transform IR spectroscopy, we have determined the concentrations of C1–C4 hydrocarbon impurities in isotopically unmodified silicon tetrafluoride before and after fine purification and in 28Si-enriched SiF4. The concentrations of C1–C4 hydrocarbon impurities in silicon tetrafluoride for SiH4 synthesis have been shown to correlate with those in the synthesized silane. 相似文献
792.
M.-F. Foulon T. Crepin X. Melique D. Lippens 《Microwaves, Antennas & Propagation, IET》2007,1(1):23-28
Co-planar strip (CPS) transmission lines, aimed at operating up to sub-millimetre wavelengths, were periodically loaded by lumped capacitances and inductances in series and shunt configuration, respectively. Under this condition, propagation on the line is backward with group and phase velocities of opposite signs. The transmission lines were fabricated onto a quartz substrate by e-beam patterning and thin film deposition, silicon nitride and gold layers, on a deep sub-micron scale (~200 nm). A direct evidence of left-handedness between 160 and 380 GHz was achieved via the tracking of phase advance in a differential phase experiment using electro-optic sampling. The comparison between modelling results by electromagnetic full wave circuit analysis and experiment was performed in the time and frequency domains. It shows a good agreement in the dispersion characteristics and ultra-fast variations in the transmitted signals. At last, we took benefit of a lumped element fitting approach in order to discuss of the frequency capability of this non-resonant approach 相似文献
793.
Open window buses without air-conditioning are a major mode of urban and inter-city transport in most countries. High occupancy
combined with hot and humid conditions makes travel in these buses quite uncomfortable. In this study air flow through a bus
has been studied that could be the basis for low cost and eco-friendly methods of increasing passenger comfort and possibly
reduce drag. The aerodynamics of such a road vehicle has not been studied as previous investigations have been confined to
vehicles with closed windows that present a smooth exterior to air flow. Using a 1:25 scaled Perspex model of an urban bus
in Delhi, flow visualization was performed in a water channel. The Reynolds numbers were one-tenth of a real bus moving at
10 m/s. Smoke and tuft visualizations were also performed on an urban bus at 40 km/h. Numerical simulations were performed
at the actual Reynolds number. Even though there were Reynolds number differences, the broad features were similar. Air enters
the bus from the rear windows, moves to the front (relative to the bus) and exits from the front windows. Inside air velocity
relative to the bus is about one-tenth of the free-stream velocity. The flow is highly three-dimensional and unsteady. 相似文献
794.
795.
X. Biquard O. Proux J. Cibert D. Ferrand H. Mariette R. Giraud B. Barbara 《Journal of Superconductivity and Novel Magnetism》2003,16(1):127-129
Mn has been incorporated in epilayers of the large-gap semiconductor GaN grown by molecular beam epitaxy using a nitrogen plasma cell. Detailed extended X-ray absorption fine structure (EXAFS) studies of a Ga0.98Mn0.02N epilayer confirm that the Mn atoms substitute the Ga atoms, with an increase by 2.7% of the distance to the nearest nitrogen atoms. Near-edge spectroscopy results tend to indicate that the valence state of Mn is slightly higher than 3+, while EXAFS analysis suggests an electron transfer to the N neighbors. 相似文献
796.
797.
J. R. Díaz-Estrada E. Camps L. Escobar-Alarcón J. A. Ascencio 《Journal of Materials Science》2007,42(4):1360-1368
Deposition of Ti was carried out by laser ablation onto hydroxyapatite porous discs in an Ar atmosphere. Ti nanoparticles
were deposited onto HAp surface in order to modify its roughness and morphology as it is observed by scanning electron microscopy
(SEM) and scanning probe microscopy (SPM). A homogeneous distribution of Ti over the disc surface was corroborated by elemental
mapping. A comparison of the hydroxyapatite hardness before and after deposition was performed using SPM nanoindentation.
Transmission Electron Microscopy (TEM) showed that the Ti nanoparticles obtained were covered by an oxygen shell. It is shown
that surface modifications of the covered HAp by Ti result in better mechanical properties, reducing the possible damage to
the HAp ceramic by friction or impacts as it often happens in meniscus, bone junctions and the inclusion of prosthesis for
human treatments. 相似文献
798.
Fresh niobium hydroxide was first precipitated from NbF5 solution using an aqueous ammonium hydroxide under basic conditions. Then a simple procedure of mixing lithium and niobium hydroxides together and heating at a low temperature (400 °C) produced pure ultrafine single phase LiNbO3 (LN). In the literature, this is the lowest temperature so far reported on the formation of LN. The phase content and lattice parameters are determined by X-ray diffraction (XRD). The average particle size and morphology were studied by transmission electron microscopy (TEM). 相似文献
799.
D. Swenson 《Journal of Materials Science: Materials in Electronics》2007,18(1-3):39-54
Most lead-free solders comprise tin (Sn) as the majority component, and nominally pure β-Sn is the majority phase in the microstructure
of these solders. It is well established that nucleation of β-Sn from Sn-base liquid alloys is generally difficult. Delays
in the onset of β-Sn formation have a profound effect upon the microstructural development of solidified Sn-base alloys. Utilizing
stable and metastable phase diagrams, along with solidification principles, the effects of inhibited β-Sn nucleation on microstructural
development are discussed, employing the widely studied Sn–Ag–Cu (SAC) alloy as a model system. This analysis shows that the
main effect of suppressed β-Sn nucleation on near-eutectic SAC solders is to increase the number and/or volume fraction of
primary or primary-like microconstituents, while simultaneously decreasing the volume fraction of eutectic microconstituent.
General strategies are outlined for avoiding unwanted microconstituent development in these materials, including the use of
metastable phase diagrams for selecting alloy compositions, employment of inoculants to promote β-Sn nucleation, and utilization
of high cooling rates to limit solid phase growth. Finally, areas for future research on the development of inoculated Sn-base
solder alloys are outlined. 相似文献
800.
Squeeze-film effects of perforated plates for small amplitude vibration are analyzed through modified Reynolds equation (MRE).
The analytical analysis reckons in most important influential factors: compressibility of the air, border effects, and the
resistance caused by vertical air flow passing through perforated holes. It is found that consideration of air compressibility
is necessary for high operating frequency and small ratio of the plate width to the attenuation length. The analytical results
presented in this paper agree with ANSYS simulation results better than that under the air incompressibility assumption. The
analytical analysis can be used to estimate the squeeze-film effects causing damping and stiffness added to the system. Since
the value of Reynolds number involved in this paper is low (< 1), inertial effects are neglected. 相似文献