全文获取类型
收费全文 | 84996篇 |
免费 | 7933篇 |
国内免费 | 4704篇 |
专业分类
电工技术 | 6274篇 |
技术理论 | 4篇 |
综合类 | 6054篇 |
化学工业 | 12535篇 |
金属工艺 | 5300篇 |
机械仪表 | 6093篇 |
建筑科学 | 6522篇 |
矿业工程 | 2726篇 |
能源动力 | 2534篇 |
轻工业 | 5126篇 |
水利工程 | 1742篇 |
石油天然气 | 4491篇 |
武器工业 | 888篇 |
无线电 | 10003篇 |
一般工业技术 | 9720篇 |
冶金工业 | 4316篇 |
原子能技术 | 1004篇 |
自动化技术 | 12301篇 |
出版年
2024年 | 386篇 |
2023年 | 1337篇 |
2022年 | 2557篇 |
2021年 | 3690篇 |
2020年 | 2695篇 |
2019年 | 2317篇 |
2018年 | 2568篇 |
2017年 | 2715篇 |
2016年 | 2506篇 |
2015年 | 3588篇 |
2014年 | 4570篇 |
2013年 | 5354篇 |
2012年 | 6047篇 |
2011年 | 6610篇 |
2010年 | 5689篇 |
2009年 | 5453篇 |
2008年 | 5577篇 |
2007年 | 5248篇 |
2006年 | 5075篇 |
2005年 | 4244篇 |
2004年 | 3105篇 |
2003年 | 2494篇 |
2002年 | 2300篇 |
2001年 | 1972篇 |
2000年 | 1773篇 |
1999年 | 1563篇 |
1998年 | 1176篇 |
1997年 | 978篇 |
1996年 | 872篇 |
1995年 | 674篇 |
1994年 | 544篇 |
1993年 | 355篇 |
1992年 | 310篇 |
1991年 | 247篇 |
1990年 | 211篇 |
1989年 | 195篇 |
1988年 | 120篇 |
1987年 | 102篇 |
1986年 | 66篇 |
1985年 | 54篇 |
1984年 | 43篇 |
1983年 | 41篇 |
1982年 | 32篇 |
1981年 | 25篇 |
1980年 | 33篇 |
1979年 | 23篇 |
1978年 | 15篇 |
1976年 | 18篇 |
1975年 | 10篇 |
1973年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Wenhui MA Gang XIE Hua WANGFaculty of Materials Metallurgical Engineering Kunming University of Science Technology Kunming China Manuscript received January in revised form May 《材料科学技术学报》2002,18(3):286-287
The thermal expansion behavior of La1-xSrxMn1-yCoyO3-δ (x=0.2-0.4, y=0.1-0.3) perovskites in air has been investigated. The average linear thermal expansion coefficients increased with increasing Sr content up to 40 mole fraction or Co content up to 30 mole fraction. The expansion is generally attributed to an increase in the average cation radius as some of the cations in the perovskite are reduced in valence when oxygen ions are removed from the structure. 相似文献
42.
43.
The Gd substituting effects for La in La0.67Ca0.33MnO3 has been studied .With increasing the substituting amount of Gd,the phase transition temperature of metal-isolator for the samples decreases,the corresponding peak resistivity increases,the Curie temperature decreases monotonically.The substitution of La-Ca-Mn-O with 11% Gd for La improved the magnetoresistance ratio by an order of magnitude.The effects of substituting Gd can be explained in terms of the lattice effects.An irreversible MR behaviour was observed in Gd-substituting compounds.This effect became marked when the substituting amount of Gd was greater than 7%.A maximum irreversible increment of MR ratio as large as 91% was obtained when Gd substituting amount was 11%. 相似文献
44.
45.
46.
Zhihui Zhang Bing Q. Han Kyung H. Chung Enrique J. Lavernia 《Metallurgical and Materials Transactions A》2006,37(7):2265-2273
The current study aims to provide fundamental insight into the behavior of microstructures containing grain sizes that span
multiple length scales. A commercial 5083 Al alloy was selected as the material of interest to facilitate comparison with
recently published data. The materials studied here were prepared via the thermal consolidation of powders that were cryomilled
for different times (i.e., 0, 2, 4, and 8 hours). Following consolidation, the resultant microstructure was characterized by an equiaxed grain morphology
with a size distribution centered around 200∼300 nm. Dispersed among the 200- to 300-nm grains were coarse-grained regions
or ligaments with a grain size ranging from 600 nm to 2 μm. The occurrence of coarse-grained regions is rationalized on the
basis of recrystallization or subgrain coarsening, whereas the occurrence of equiaxed fine regions is proposed to be a result
of continuous grain growth. Two types of microstructures were selected for study, containing coarse-grained volumes of approximately
28 pct and 43 pct that corresponded to an ultimate tensile strength (UTS) of 566 MPa and 535 MPa, and a fracture strain of
3.2 pct and 3.5 pct, respectively. The observed ductility and the relevant toughening mechanisms were discussed in light of
the presence of multiple length scales. 相似文献
47.
48.
Bing Q. Han Farghalli A. Mohamed Enrique J. Lavernia 《Metallurgical and Materials Transactions A》2003,34(1):71-83
In the present study, the mechanical properties of Fe processed via severe plastic deformation (equal-channel angular pressing (ECAP)) at room temperature were investigated for the first time.
The grain size of annealed Fe, with an initial grain size of about 200 μm, was reduced drastically during ECAP. After eight passes, the grain size reaches 200 to 400 nm, as documented by means of
transmission electron microscopy (TEM). The value of microhardness during pressing increases 3 times over that of the starting
material after the first pass and increases slightly during subsequent pressing for higher-purity Fe. Examination of the value
of microhardness after eight passes as a function of post-ECAP annealing temperature shows a transition from recovery to recrystallization,
an observation that resembles the behavior reported for heavily deformed metals and alloys. The tensile and compression behaviors
were examined. In tension, a drop in the engineering stress-engineering strain curve beyond maximum load was observed both
in the annealed Fe and the ECAP Fe. This drop is related to the neck deformation. The fracture surface, examined by scanning
electron microscopy (SEM), shows vein patterns, which is different from the dimples found on the fracture surface of annealed
Fe. In compression, an initial strain-hardening region followed by a no-strain-hardening region was observed in the ECAP Fe.
The yield strength in tension of the ECAP Fe was observed to be higher than that in compression. The strengthening mechanisms
and softening behavior are discussed. 相似文献
49.
人本理念,是在对当代社会发展过程中人的主体地位和作用日益突出的反思中,提出的一种发展理念,是一种对人在社会发展中的主体作用与地位的肯定.教育科学中的人本理念,关键是要以学生为本,坚持"一切为了学生、为了一切学生、为了学生一切". 相似文献
50.
H. Tsukamoto T.D. Boone J. Han J.M. Woodall 《Photonics Technology Letters, IEEE》2005,17(7):1411-1413
We present a novel optical switching technique utilizing emission packet positioning of semiconductor heterostructure. A modulation-doped p-AlGaAs-GaAs heterostructure is employed to control spontaneous emission packet positioning with electric fields. Emission packets generated by optical input signals are brought over 150 /spl mu/m with electric fields, so the output fibers can detect the emission intensity as signals. The first-order analysis indicates that the drift velocity of minority electrons in GaAs limits the detectable maximum data rate and nanoseconds timescale signal routing operation at 20 Gb/s is possible at an electron drift velocity of 2/spl times/10/sup 7/ cm/s. 相似文献