全文获取类型
收费全文 | 33507篇 |
免费 | 2907篇 |
国内免费 | 2327篇 |
专业分类
化学 | 20080篇 |
晶体学 | 149篇 |
力学 | 653篇 |
综合类 | 713篇 |
数学 | 6992篇 |
物理学 | 10154篇 |
出版年
2023年 | 239篇 |
2022年 | 423篇 |
2021年 | 1299篇 |
2020年 | 831篇 |
2019年 | 874篇 |
2018年 | 627篇 |
2017年 | 716篇 |
2016年 | 952篇 |
2015年 | 1031篇 |
2014年 | 1376篇 |
2013年 | 2229篇 |
2012年 | 1514篇 |
2011年 | 1733篇 |
2010年 | 1592篇 |
2009年 | 2064篇 |
2008年 | 2160篇 |
2007年 | 2316篇 |
2006年 | 1801篇 |
2005年 | 1216篇 |
2004年 | 1089篇 |
2003年 | 1078篇 |
2002年 | 1015篇 |
2001年 | 1007篇 |
2000年 | 704篇 |
1999年 | 568篇 |
1998年 | 555篇 |
1997年 | 420篇 |
1996年 | 476篇 |
1995年 | 427篇 |
1994年 | 434篇 |
1993年 | 470篇 |
1992年 | 446篇 |
1991年 | 300篇 |
1990年 | 256篇 |
1989年 | 210篇 |
1988年 | 238篇 |
1987年 | 195篇 |
1986年 | 211篇 |
1985年 | 322篇 |
1984年 | 234篇 |
1983年 | 141篇 |
1982年 | 288篇 |
1981年 | 467篇 |
1980年 | 424篇 |
1979年 | 465篇 |
1978年 | 370篇 |
1977年 | 279篇 |
1976年 | 238篇 |
1974年 | 75篇 |
1973年 | 149篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
991.
This paper presents a novel approach for the automatic localization of pupil and iris. Pupil and iris are nearly circular regions, which are surrounded by sclera, eyelids and eyelashes. The localization of both pupil and iris is extremely important in any iris recognition system. In the proposed algorithm pupil is localized using Eccentricity based Bisection method which looks for the region that has the highest probability of having pupil. While iris localization is carried out in two steps. In the first step, iris image is directionally segmented and a noise free region (region of interest) is extracted. In the second step, angular lines in the region of interest are extracted and the edge points of iris outer boundary are found through the gradient of these lines. The proposed method is tested on CASIA ver 1.0 and MMU Iris databases. Experimental results show that this method is comparatively accurate. 相似文献
992.
Li-Shin Chang 《Applied Surface Science》2011,258(3):1279-1281
The performance of a Li-ion cell strongly depends on the solid-electrolyte interface (SEI) on electrodes. The depth distribution of composition in SEI is normally determined by means of X-ray Photoelectron Spectroscopy (XPS) via Ar ion sputtering. Recently, a new kind of ion gun using C60 ions as sputtering source was introduced. In this report, a comparison between the effects of these two kinds of ion guns on the quantification of Li(Ni,Co,Mn)O2 electrodes was made. It was found that the C60 ion gun is more suitable for analyzing the composition and chemical state of Li(Ni,Co,Mn)O2 electrode since that it causes lower chemical damage in the superficial layer. 相似文献
993.
Yanfeng Wang Li ZhengxianDu Jihong Hua YunfengWang Baoyun 《Applied Surface Science》2011,258(1):456-460
Materials’ surface service property could be enhanced by transition metal nitride hard coatings due to their high hardness, wear and high temperature oxidation resistance, but the higher friction coefficient (0.4-0.9) of which aroused terrible abrasion. In this work, quinternary (Ti,Al,Si,C)N hard coating 3-4 μm was synthesized at 300 °C using plasma enhanced magnetron sputtering system. It was found that the coating's columnar crystals structure was restrained obviously with the increase of C content and a non-columnar crystals growth mode was indicated at the C content of 33.5 at.%. Both the XRD and TEM showed that the (Ti,Al,Si,C)N hard coatings had unique nanocomposite structures composed of nanocrystalline and amorphous nc-(Ti,Al)(C,N)/nc-AlN/a-Si3N4/a-Si/a-C. However, the coatings were still super hard with the highest hardness of 41 GPa in spite of the carbon incorporation. That a-C could facilitate the graphitization process during the friction process which could improve the coating's tribological performance. Therefore, that nanocomposite (Ti,Al,Si,C)N coatings with higher hardness (>36 GPa) and a lower friction coefficient (<0.2) could be synthesized and enhance the tribological performance and surface properties profoundly. 相似文献
994.
E. Cattaruzza M. MardeganE. Trave G. BattaglinP. Calvelli F. EnrichiF. Gonella 《Applied Surface Science》2011,257(12):5434-5438
Glass layers for planar light waveguides prepared by Ag-Na ion exchange of different silicate glasses in molten salt baths are annealed and/or irradiated with a laser beam in the UV region, with different energy density values and total pulse numbers. The samples are mainly characterized by optical absorption spectroscopy, luminescence spectroscopy, and Rutherford backscattering spectrometry, in order to determine the role of irradiation parameters and of the host matrix structure in the aggregation phenomena. Photoluminescence spectroscopy gave information regarding the presence of Ag multimeric aggregates, the primal seeds for the growing (nano)crystals. The appearance of the plasmon resonance band in the optical absorption spectra proved the formation of Ag clusters and allowed the evolution steps of the clusterization process to be followed as a function of the energy deposited during the laser irradiation. 相似文献
995.
Agata Roguska Andrzej KudelskiMarcin Pisarek Magdalena OparaMaria Janik-Czachor 《Applied Surface Science》2011,257(19):8182-8189
Tubular arrays of TiO2 nanotubes (ranging in diameter from 40 to 110 nm) on a Ti substrate were used as a support for Ag, Au or Cu deposits obtained by the sputter deposition technique, where the amount of metal varied from 0.01 to 0.2 mg/cm2. Those composite supports were intended for surface-enhanced Raman scattering (SERS) investigations. Composite samples were studied with the aid of scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to reveal their characteristic morphological and chemical features. Raman spectra of pyridine (as a probe molecule) were measured at different cathodic potentials ranging from −0.2 down to −1.2 V after the pyridine had been adsorbed on the metal-covered TiO2 nanotube/Ti substrates. In addition, SERS spectra on a bulk standard activated Ag, Au and Cu substrates were also measured. The SERS activity of the composite samples was strongly dependent on the amount of metal deposit, e.g. at and above 0.06 mg Ag/cm2, the intensity of SERS signal was even higher than that for the Ag reference substrate. The high activity of these composites is mainly a result of their specific morphology. The high SERS sensitivity on the surface morphology of the substrate made it possible to monitor very small temporal changes in the Ag metal clusters. This rearrangement was not detectable with microscopic (SEM) or microanalytical (AES) methods. The SERS activity of Au or Cu clusters was distinctly lower than those of Ag. The spectral differences exhibited by the three kinds of composites as compared to the reference metal samples are discussed. 相似文献
996.
997.
998.
利用新研制的紧凑型线性变压器驱动源(LTD)脉冲功率源二极管产生的电子束源,开展了S波段长脉冲相对论速调管放大器(RKA)的初步实验研究。采用无箔空心阴极和0.9 T恒流源引导磁场从LTD二极管引出了电压600 kV、束流6 kA、脉宽150 ns的环形电子束,该电子束经过1个同轴输入腔和2个同轴调制腔的调制后,产生了幅度5 kA、脉宽110 ns的基波调制束流,采用该调制束流驱动同轴微波提取腔,辐射输出了峰值功率820 MW/110 ns的辐射微波,效率28%,增益36 dB。同时,开展了重复脉冲RKA和相位特性的实验研究,重复频率10 Hz运行时,辐射微波达到800 MW/100 ns,相位抖动小于 20°。 相似文献
999.
1000.
利用G3和CBS-QB3的理论方法研究CF3OH分子裂解成FCFO和HF,并考虑大气中双分子和氨气对CF3OH分子裂解的催化作用. 理论计算表明:由于在G3的理论水平下,计算的能垒为188.52 kJ/mol,所以CF3OH分子在大气条件下不可能发生单分子裂解;当氨气和双分子水被加入时,能垒都被降到25.08 kJ/mol,起了强的催化作用. 除此之外,应用过渡态理论对速率常数进行了计算,计算结果表明:氨气催化CF3OH分子的速率常数是单分子和双分子催化CF3OH分子裂解速率常数的109和105倍. 考虑到大气中这些物质的浓度,计算结果预测了氨气催化CF3OH分子裂解在大气中起到重要的作用. 相似文献