全文获取类型
收费全文 | 21331篇 |
免费 | 2567篇 |
国内免费 | 1147篇 |
专业分类
化学 | 20827篇 |
晶体学 | 96篇 |
力学 | 141篇 |
综合类 | 15篇 |
数学 | 41篇 |
物理学 | 3925篇 |
出版年
2024年 | 35篇 |
2023年 | 267篇 |
2022年 | 655篇 |
2021年 | 710篇 |
2020年 | 1277篇 |
2019年 | 1000篇 |
2018年 | 807篇 |
2017年 | 736篇 |
2016年 | 1355篇 |
2015年 | 1162篇 |
2014年 | 1251篇 |
2013年 | 1434篇 |
2012年 | 1066篇 |
2011年 | 1347篇 |
2010年 | 1099篇 |
2009年 | 1239篇 |
2008年 | 1144篇 |
2007年 | 1160篇 |
2006年 | 1043篇 |
2005年 | 856篇 |
2004年 | 871篇 |
2003年 | 687篇 |
2002年 | 1328篇 |
2001年 | 381篇 |
2000年 | 262篇 |
1999年 | 270篇 |
1998年 | 317篇 |
1997年 | 220篇 |
1996年 | 152篇 |
1995年 | 170篇 |
1994年 | 111篇 |
1993年 | 78篇 |
1992年 | 72篇 |
1991年 | 69篇 |
1990年 | 56篇 |
1989年 | 36篇 |
1988年 | 46篇 |
1987年 | 36篇 |
1986年 | 75篇 |
1985年 | 17篇 |
1984年 | 12篇 |
1983年 | 11篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1975年 | 9篇 |
1973年 | 15篇 |
1972年 | 12篇 |
1970年 | 8篇 |
1966年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
221.
K. Mohammed Jasim 《Optics and Lasers in Engineering》2011,49(7):785-792
The present paper reports an experimental investigation based on X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and wavelength dispersive spectroscopy (WDS) analyses of phases formed after laser sealing of plasma sprayed coatings of 8.5 wt% yttria partially stabilized zirconia (YPSZ). X-ray diffraction and X-ray step-scanning analyses showed that the plasma sprayed and sealed coatings consisted mainly of t′ phase with a very small amount of monoclinic phase (m phase) in the plasma sprayed coatings. It was also found that the small amounts of m and cubic phases (c phase) present in the sealed coatings were dependent on laser processing specific energies (specific energy is equal to laser power/traverse speed x beam diameter). It was also found that rhombohedral (r) phase formed after laser sealing of coatings at higher specific energies. A direct relationship between c/a ratio of transformable tetragonal phase (t phase) produced by thermal treatment of sealed coatings and nontransformable tetragonal phase (t′ phase) and the amount of Yttria was obtained. A new equation was derived, which describes the relationship between Yttria concentration and the c/a ratio of tetragonal phases (t or t′). 相似文献
222.
Wear resistance of reactive plasma sprayed TiB2-TiC0.3N0.7 based composite coatings and the as-sprayed coating with laser surface treatment was investigated using plate-on-plate tests. Wear tests were performed at different normal loads and sliding speeds under dry sliding conditions in air. The surface morphologies of counterparts against as-sprayed and laser remelted coatings were investigated. The microstructure and chemical composition of wear debris and coatings were studied using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively. The results show that the wear resistance of the laser remelted coating is improved significantly due to their increased microhardness and reduced flaws. The primary wear mechanism of the remelted coating is oxidation wear and its minor wear mechanisms are grain abrasion and fatigue failure during the course of wear test. In contrast, the primary wear mechanism of the as-sprayed coating is grain abrasion at the low sliding speed (370 rpm) and fatigue failure at the high sliding speed (549 rpm). The oxidation wear mechanism is a minor contributor for the as-sprayed coating. 相似文献
223.
以单周期信号为例,数值模拟并讨论了注入频率和注入光场波动强度对主从式半导体激光器同步性能的影响。分别通过直接调制和外部调制两种方式获得单周期信号,并采用改进的参数——相似指数评价系统的同步性能。研究结果表明:当注入强度足够大时,系统实现注入锁定同步,外部调制的相似指数大于内部调制;注入强度恒定时,同步品质几乎不受调制指数的影响,而受调制频率的影响;由于内部、外部调制引起主激光器的输出反相,两种调制方式下同步品质随调制频率的变化趋势也相反。 相似文献
224.
P. Karvonen T. Sonoda I. D. Moore J. Billowes A. Jokinen T. Kessler H. Penttilä A. Popov B. Tordoff J. Äystö 《The European physical journal. Special topics》2007,150(1):283-284
The ion guide technique was developed in Jyv?skyl? during the early 1980's. In the ion guide the reaction recoil products
are stopped and thermalized in high purity helium gas where they remain ionic due to high ionization potential of helium atoms.
Different designs of ion guide exist for light-ion induced fusion reactions, for heavy-ion induced fusion and for proton induced
fission. Although the IGISOL method is fast and universal it is chemically unselective and in many cases relatively inefficient.
In order to address these deficiencies in the technique, a new laser ion source project, FURIOS (Fast Universal Resonant laser
Ion
Source), commenced in 2004. In addition, resonance ionization spectroscopy has been tested off – line within a sextupole ion
beam
guide. 相似文献
225.
226.
研究了光电干扰对测距和跟踪测量激光雷达干扰效果的评估方法.在深入分析激光雷达光电干扰机理的基础上,提出了相应的干扰效果评估准则.对激光测距机的距离欺骗干扰效果采用测距误差-干扰成功率准则或准测率准则定量评估,对测距机的激光盲干扰效果依据盲激光对测距准确度、测距功能的影响程度进行评估.对于跟踪测量激光雷达,根据实施干扰后其跟踪误差是否超出正常跟踪准确度的三倍判定一次干扰是否有效,再依据多次试验的干扰成功率的大小评估干扰效果. 相似文献
227.
Scott C. Herndon David D. Nelson Jr Mark S. Zahniser 《Journal of Quantitative Spectroscopy & Radiative Transfer》2005,90(2):207-216
The spectral line strengths in the v2 band of H2CO (segments spanning 1720-) have been determined relative to two sets of spectral line groups in the v1 and v5 band, using tunable diode laser spectroscopy. Simultaneous detection using a dual-diode instrument with a absorption cell was employed to assure identical H2CO column density for the two spectral regions. The results in the selected regions of this study are in good agreement with the line positions and the relative intensities specified in an unpublished complete line listing for the v2 band prepared by Linda Brown (see full text for reference). Based upon measurements of individual groups of spectral lines in the P, Q and R branches, the absolute band strength has been determined to be . 相似文献
228.
K.D.G.I. Jayawardena Charles OpokuJames Fryar S. Ravi P. SilvaSimon J. Henley 《Applied Surface Science》2011,257(12):5274-5277
The synthesis of ZnO nanocrystals is reported using a hydrothermal chemical growth technique combined with 248 nm nanosecond excimer laser heating at fluences in the range 0-390 mJ cm−2. The effect of laser heating in controlling the morphology of the nanocrystals is investigated using optical spectroscopy and electron microscopy characterization. Laser heating is shown to allow control of the crystal morphology from nanoparticles to nanorods as well as to modify the size distributions. The results indicate that not only does the laser accelerate the growth of nanocrystals, but can also produce crystals with a narrow size distribution possibly via photothermal size selection. An initial study of electrical conduction properties of ZnO nanocrystal thin films is also discussed. 相似文献
229.
B.Y. Man S.C. XuC. Yang M. LiuS.Z. Jiang Y.Y. MaC.S. Chen A.H. LiuX.G. Gao C.C. WangB. Hu 《Applied Surface Science》2011,257(15):6321-6325
Single-phase β-FeSi2 films on silicon (1 0 0) were fabricated by pulse laser deposition. The structure and crystal quality of the samples were characterized by X-ray diffraction and Fourier transform infrared spectroscopy. The field scanning electron microscopy showed that the film thickness increases with the increasing of the laser fluence. Moreover, atomic force microscopy observations revealed the changes of surface properties with different laser fluence. Based upon all experimental results, it is found that 7 J/cm2 is the most favorable for the formation of β-FeSi2 thin films. 相似文献
230.
Surface texturing (for example, producing micro dimples on the surface) of mechanical parts has a great potential to improve the surface tribological properties. Surface texturing through nanosecond laser ablation has many associated advantages and hence has drawn lots of attentions. However, the produced micro dimple bottom (if through laser spot scanning) is often very rough, which may harm the surface tribological properties. In this paper, a two-step laser surface texturing process is proposed and studied, where a relatively high-fluence laser ablation step (which is to create dimples) is followed by a low-fluence laser-induced melting, melted material flow and re-solidification step (which is to smooth the ablated dimple bottom surface). The study shows that the two-step laser surface texturing process can produce dimples with very smooth bottom surfaces. The effects of laser pulse duration and scan speed in Step 2 on the dimple bottom surface morphology and roughness have also been investigated, and some very interesting physical phenomena have been found, which have been rarely reported before in literature. Some hypothesized explanations are given for the observed effects, which require future work to completely understand their underlying mechanisms. 相似文献