全文获取类型
收费全文 | 93785篇 |
免费 | 11838篇 |
国内免费 | 9007篇 |
专业分类
化学 | 47676篇 |
晶体学 | 667篇 |
力学 | 4757篇 |
综合类 | 543篇 |
数学 | 8594篇 |
物理学 | 26047篇 |
无线电 | 26346篇 |
出版年
2024年 | 380篇 |
2023年 | 2148篇 |
2022年 | 2697篇 |
2021年 | 3387篇 |
2020年 | 3218篇 |
2019年 | 3029篇 |
2018年 | 2663篇 |
2017年 | 2493篇 |
2016年 | 3654篇 |
2015年 | 3942篇 |
2014年 | 4707篇 |
2013年 | 6123篇 |
2012年 | 7266篇 |
2011年 | 7415篇 |
2010年 | 5358篇 |
2009年 | 5382篇 |
2008年 | 5655篇 |
2007年 | 5174篇 |
2006年 | 5002篇 |
2005年 | 4385篇 |
2004年 | 3320篇 |
2003年 | 2706篇 |
2002年 | 2395篇 |
2001年 | 2116篇 |
2000年 | 2060篇 |
1999年 | 2226篇 |
1998年 | 1965篇 |
1997年 | 1693篇 |
1996年 | 1700篇 |
1995年 | 1519篇 |
1994年 | 1357篇 |
1993年 | 1197篇 |
1992年 | 1062篇 |
1991年 | 899篇 |
1990年 | 727篇 |
1989年 | 590篇 |
1988年 | 473篇 |
1987年 | 397篇 |
1986年 | 345篇 |
1985年 | 335篇 |
1984年 | 235篇 |
1983年 | 203篇 |
1982年 | 169篇 |
1981年 | 125篇 |
1980年 | 91篇 |
1979年 | 63篇 |
1978年 | 62篇 |
1976年 | 60篇 |
1975年 | 61篇 |
1973年 | 67篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A new branch-and-bound algorithm is presented to solve the two-sided assembly line balancing problem of type 1 (TALB-1). First, a pair of two directly facing station is defined as a position, and then the two-sided assembly line (TAL) is relaxed to a one-sided assembly line (OAL). Some new lower bound on positions are computed, and dominance rules and reduction rules for the one-sided assembly line balancing problem of type 1 (OALB-1) are extended and incorporated into a station-oriented assignment procedure for the TALB-1 problem. Finally, the tests are carried out on a well-known benchmark set of problem instances, and experimental results demonstrate that the proposed procedure is efficient. 相似文献
992.
993.
Keh-Yi Lee Chia-Chen Tsai Tsong-Cheng Weng Yi-Lin Kuo Chih-Wei Kao Kuei-Yuan Chen Yu-Jen Lin 《Fiber and Integrated Optics》2006,25(1):29-40
There are several key factors that affect the transmission characteristics of the 90° (L-shaped) bent photonic crystal waveguides. The first factor is the direct coupling efficiency from the incident lights into the waveguides. The second one is the bandgap deviations of the photonic crystals. And the third factor is the optical reflections in the bent corners. In this article, we compare three types of L-shaped bent photonic crystal waveguides. One is the original type, which has an abrupt right-angle bend. Another is an improved 90° bend with a 45°-mirror. The other is an L-shaped bent photonic crystal waveguide with a 45°-transitional section. We investigate their respective frequency responses and observe the improvements in the total transmission efficiencies provided by the latter types. 相似文献
994.
J. Chen H. Mizuno H. Kawano A. Miyawaki K. Midorikawa 《Applied physics. B, Lasers and optics》2006,85(1):45-48
We experimentally demonstrated two-photon pumping of random lasers using picosecond and nanosecond pump lasers. The picosecond laser pumping experiment was performed with 400 ps laser pulses at 770 nm, and the gain media was a Coumarin 480D dye solution doped with TiO2 nanoparticles. Onset of laser action was observed at a pump laser pulse energy below 500 μJ. The nanosecond laser pumping experiment was performed with 7 ns laser pulses at 1064 nm, and the gain media was a Rhodamine 640 dye solution doped with TiO2 nanoparticles. Onset of laser action was observed at a pump laser energy ∼18 mJ. Our results suggest that there exists an optimal pulse duration of the pumping laser in two-photon pumped random lasing that leads to minimum photodamage of the gain media and still keeps a high pumping efficiency. PACS 33.50.Dq; 42.55.Mv; 42.55.Zz 相似文献
995.
For the Shanghai Synchrotron Radiation Facility (SSRF) and future Shanghai Free‐Electron Laser projects, ground vibration is an important factor and, in order to attenuate it, the construction of a deep tunnel is under consideration. This paper concentrates on the investigation of ground vibration at different underground levels down to 60 m below surface, in order to understand the effect of vibration attenuation with depth. The effect of traffic is also studied using a 10 ton truck, with ground motion compared in different directions. Finally, a summary and some suggestions on these two projects are given. 相似文献
996.
D. P. Chiang C. H. Peng J. K. Mei I. M. Jiang S. C. Lin Y. C. Chen H. T. Liu Y. F. Chen W. S. Tse 《Journal of Raman spectroscopy : JRS》2008,39(3):344-348
Raman studies of crystalline iron perchlorate hexahydrate (Fe(ClO4)2·6H2O) in the region of lattice and anion internal modes were carried out in the temperature range 80–385 K. The temperature‐dependent Raman results are consistent with those from previous works, showing that two phase transitions occur around 336 and 245 K. The transition at 336 K may be considered as an order–disorder transformation, while the one at 245 K is associated with the configurational disorder of the tetrahedron of the perchlorate ions. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
997.
Composite coatings mainly containing titanium carbides and borides were produced by laser surface alloying of Ti-6Al-4V with graphite and boron mixed powders. The test results show that the coatings have higher hardness (1600-1700 HV0.1) and are more resistant to wear than the as-received sample. Laser scanning speed and the content of alloying elements (weight ratio of graphite to boron) have an effect on both the microstructure and the wear resistance of the coatings. TEM results show that strip titanium carbides and borides grow alternately and thus restrain the formation of coarse needle-like TiB and dendritic TiC crystals produced by laser alloying of titanium alloys with boron and graphite separately. 相似文献
998.
A tapered fiber with a depressed-index outer ring is fabricated and dispersion engineered to generate a widely tunable (1250-1650 nm) fundamental-mode leakage loss with a high cutoff slope (-1.2 dB/nm) and a high attenuation for stop band (>50 dB) by modification of both waveguide and material dispersions. The higher cutoff slope is achieved with a larger cross angle between the two refractive index dispersion curves of the tapered fiber and surrounding optical liquids through the use of depressed-index outer ring structures in double-cladding fibers. 相似文献
999.
Zhongwei Jiang Wenxin Wang Hanchao Gao Linshen Liu Hong Chen Junming Zhou 《Applied Surface Science》2008,254(16):5241-5246
Low-temperature step-graded high indium content InAlAs (In% = 0.75) metamorphic buffer layers with reverse step layer grown on GaAs substrate by molecular beam epitaxy are investigated in this paper. The composition and the strain relaxation of the top InAlAs layer are determined by high-resolution triple-axis X-ray diffraction measurements, which show that the top InAlAs layer is nearly fully relaxed and the growth parameters for these samples have little influence on the strain relaxation ratio. Surface morphology is observed by reflection high-energy electron diffraction pattern and atomic force microscopy. The surface morphology is found to depend strongly on both the growth temperature and the As flux. Compared with other samples, the sample growth under the optimized conditions has the smallest value of root mean square surface roughness. Furthermore, the ω − 2θ and ω scans of the triple-axis X-ray diffraction and transmission electron microscopy result also show the sample grown under the optimized conditions has good crystalline quality. 相似文献
1000.
Xiaoying Yang Zhuohan Zhang Zunfeng Liu Yanfeng Ma Rongcun Yang Yongsheng Chen 《Journal of nanoparticle research》2008,10(5):815-822
Multi-functionalized single walled carbon nanotubes (SWNTs) were prepared and applied as tumor cell targeting biological transporters.
A positive charge was introduced on SWNTs to get high loading efficiency of fluorescein (FAM) labeled short double strands
DNA (20 base pairs). The SWNTs were encapsulated with the folic acid modified phospholipids for active targeting into tumor
cell. The tumor cell-targeting properties of these multi-functionalized SWNTs were investigated by active targeting into mouse
ovarian surface epithelial cells. The experimental results show that these multi-functionalized SWNTs have good tumor cell
targeting property. 相似文献