全文获取类型
收费全文 | 49785篇 |
免费 | 4299篇 |
国内免费 | 2021篇 |
专业分类
电工技术 | 2536篇 |
技术理论 | 2篇 |
综合类 | 2507篇 |
化学工业 | 9094篇 |
金属工艺 | 2863篇 |
机械仪表 | 3326篇 |
建筑科学 | 3100篇 |
矿业工程 | 1296篇 |
能源动力 | 1501篇 |
轻工业 | 3276篇 |
水利工程 | 661篇 |
石油天然气 | 2480篇 |
武器工业 | 337篇 |
无线电 | 6390篇 |
一般工业技术 | 7167篇 |
冶金工业 | 2777篇 |
原子能技术 | 569篇 |
自动化技术 | 6223篇 |
出版年
2024年 | 249篇 |
2023年 | 945篇 |
2022年 | 1606篇 |
2021年 | 2320篇 |
2020年 | 1707篇 |
2019年 | 1584篇 |
2018年 | 1703篇 |
2017年 | 1719篇 |
2016年 | 1666篇 |
2015年 | 2059篇 |
2014年 | 2541篇 |
2013年 | 3050篇 |
2012年 | 3131篇 |
2011年 | 3597篇 |
2010年 | 2789篇 |
2009年 | 2818篇 |
2008年 | 2771篇 |
2007年 | 2440篇 |
2006年 | 2533篇 |
2005年 | 2042篇 |
2004年 | 1527篇 |
2003年 | 1380篇 |
2002年 | 1275篇 |
2001年 | 1113篇 |
2000年 | 1057篇 |
1999年 | 1093篇 |
1998年 | 998篇 |
1997年 | 801篇 |
1996年 | 710篇 |
1995年 | 586篇 |
1994年 | 455篇 |
1993年 | 323篇 |
1992年 | 256篇 |
1991年 | 226篇 |
1990年 | 195篇 |
1989年 | 180篇 |
1988年 | 116篇 |
1987年 | 111篇 |
1986年 | 71篇 |
1985年 | 59篇 |
1984年 | 51篇 |
1983年 | 43篇 |
1982年 | 33篇 |
1981年 | 25篇 |
1980年 | 28篇 |
1979年 | 19篇 |
1978年 | 17篇 |
1976年 | 20篇 |
1975年 | 12篇 |
1973年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Quasi-static and dynamic crushing behaviors of aluminum and steel tubes with a cutout 总被引:3,自引:0,他引:3
Tubular members are commonly used as an energy absorber in engineering structures and many such members have a cutout. In this study, the crushing behaviors of tubes with a cutout are characterized and the effects of cutout on the energy absorption capabilities of these tubes are quantified. Systematic parametric studies were carried out to study the effect of material properties, including yield and ultimate strength of material, strain rate effect, location of cutout, tube length and impact speed on the crushing behaviors and energy absorption capacity of aluminum and steel tubes. First, a numerical model was constructed with a commercial explicit finite element code. It will be first proven that the numerical simulation can produce sufficiently accurate results in an economic manner. Subsequently, the crushing behavior of aluminum and steel tubes with a cutout was experimentally characterized and their energy absorption capacity was evaluated in terms of mean crushing force, peak crushing force and specific energy absorption (SEA). Tubes of various lengths with a cutout located at different locations, subject to both quasi-static and dynamic impact loadings were considered. For steel tubes, the numerical simulation investigated the influence of the strain rate effect and variation in strain hardening ratio of the material. Empirical equations describing the mean and peak crushing forces of aluminum and steel tubes with a cutout were developed using linear and nonlinear regression methods applied to the results obtained from the numerical and experimental studies. 相似文献
52.
53.
54.
55.
56.
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
57.
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
58.
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
59.
Plasmas play a critical role in depositing thin films or etching fine patterns while manufacturing integrated circuits. A new model for plasma diagnosis is presented. This was accomplished by linking atomic force microscopy (AFM) to plasma parameters using a neural network. Experimental AFM data were collected during the etching of silicon oxynitride films in C2F6 inductively coupled plasma. Surface roughness of etched patterns was characterized by means of discrete wavelet transformation. This led to the construction of three vertical (type I), diagonal (type II), and horizontal (type III) wavelet coefficient-based models. The performance of diagnosis models was evaluated in terms of the prediction and recognition accuracies. Both accuracies were optimized as a function of the number of hidden neurons. Comparisons revealed that the type I model yielded the largest recognition and the smallest prediction error. This was demonstrated even under stricter monitoring conditions. More improved diagnosis is expected by enhancing AFM resolution. 相似文献
60.
采用SE 3 0为固定液进行色谱柱分离 ,用外标定量法测定泥土中的微量六氯环戊二烯 ,结果最小检知量为 0 .18× 10 - 6 ,相关系数为 0 .99992。 相似文献