全文获取类型
收费全文 | 110770篇 |
免费 | 3475篇 |
国内免费 | 1336篇 |
专业分类
电工技术 | 2073篇 |
综合类 | 3326篇 |
化学工业 | 16194篇 |
金属工艺 | 6381篇 |
机械仪表 | 4887篇 |
建筑科学 | 3502篇 |
矿业工程 | 1158篇 |
能源动力 | 2135篇 |
轻工业 | 5632篇 |
水利工程 | 1590篇 |
石油天然气 | 1301篇 |
武器工业 | 116篇 |
无线电 | 13147篇 |
一般工业技术 | 20996篇 |
冶金工业 | 4276篇 |
原子能技术 | 519篇 |
自动化技术 | 28348篇 |
出版年
2024年 | 89篇 |
2023年 | 490篇 |
2022年 | 776篇 |
2021年 | 1301篇 |
2020年 | 961篇 |
2019年 | 824篇 |
2018年 | 15256篇 |
2017年 | 14184篇 |
2016年 | 10851篇 |
2015年 | 1710篇 |
2014年 | 1624篇 |
2013年 | 1954篇 |
2012年 | 4949篇 |
2011年 | 11359篇 |
2010年 | 9877篇 |
2009年 | 7179篇 |
2008年 | 8233篇 |
2007年 | 8944篇 |
2006年 | 1358篇 |
2005年 | 2252篇 |
2004年 | 1966篇 |
2003年 | 1965篇 |
2002年 | 1293篇 |
2001年 | 733篇 |
2000年 | 751篇 |
1999年 | 655篇 |
1998年 | 733篇 |
1997年 | 527篇 |
1996年 | 458篇 |
1995年 | 362篇 |
1994年 | 290篇 |
1993年 | 211篇 |
1992年 | 168篇 |
1991年 | 136篇 |
1990年 | 105篇 |
1989年 | 85篇 |
1988年 | 61篇 |
1987年 | 49篇 |
1986年 | 47篇 |
1985年 | 33篇 |
1968年 | 46篇 |
1967年 | 33篇 |
1966年 | 44篇 |
1965年 | 46篇 |
1959年 | 36篇 |
1958年 | 37篇 |
1957年 | 36篇 |
1956年 | 34篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 223 毫秒
141.
142.
143.
The hydrogen gas pick-up problem that can occur during Lost Foam Casting was investigated with reduced pressure tests and real castings.The initial hydrogen concentration of the melt and the contact time between melt and polystyrene had a main effect on the hydrogen gas pick-up of Al melt. The hydrogen gas pick-up of Al alloy depended also on pouring temperature and a proper metal front temperature gave the minimum hydrogen pick-up. At a low pouring temperature, the hydrogen went into the melt mainly from entrapped liquid product of polystyrene but at high pouring temperature it was by the gas as well as the liquid product. The mold flask evacuation down to 710 torr decreased the gas porosity down by around 0.4 vol%. The permeability of coating thickness had a great effect because it affects the filling time and the easy removal of liquid polystyrene. 相似文献
144.
LiFePO4 is a potential candidate for the cathode material of the lithium secondary batteries. A co-precipitation method was adopted to prepare LiFePO4 because it is simple and cheap. Nitrogen gas was needed to prevent oxidation of Fe2+ in the aqueous solution. The co-precipitated precursor shows the high reactivity with the reductive gas, and the single phase of LiFePO4 is successfully synthesized with the aid of carbon under less reductive conditions. LiFePO4 fine powder prepared by co-precipitation method shows high rate capability, impressive specific capacity and cycle property. 相似文献
145.
The effects of gallia additions on the sintering behavior of gadolinia-doped ceria were systematically investigated from the following aspects: the variation in sintered density, the variation in grain size, and the existing forms of Ga2O3 in CeO2.Sintered density increased with increasing Ga2O3 content up to 5 mol.% and then it decreased with further addition of Ga2O3. Grain size also increased with increasing Ga2O3 content up to 5 mol.% and then decreased with further addition of Ga2O3. Decrease in grain size was caused by a pinning effect of Ga2O3 precipitation at grain boundaries. Lattice constant decreased with increasing Ga2O3 content up to 5 mol.%. This decrease will be due to the substitution of smaller Ga3+ ions for Ce4+ ions in the CeO2 structure. According to the results obtained from scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses, the solubility limit of Ga2O3 in Ce0.8Gd0.2O1.9 ceramics can be estimated to be nearly 5 mol.%. The addition of Ga2O3 up to the solubility limit was found to promote the sintering properties of Gd2O3-doped CeO2. 相似文献
146.
A practical resource management method that can significantly reduce cochannel interference (CCI) and improve spectrum utilisation in FH-OFDMA packet-based cellular networks is presented. The proposed method seeks an effective combination of dynamic resource allocation with fractional coding and bit loading to respectively minimise CCI and maximise system throughput for a desired performance. 相似文献
147.
A simple mathematical analysis on the effect of sand in Cr(VI) reduction using zero valent iron 总被引:1,自引:0,他引:1
A simple mathematical model was proposed to analyze the enhancement of Cr(VI) reduction when sand materials are added to the
zero valent iron (ZVI). Natural decay of Cr(VI) in a control experiment was analyzed by using a zero-order decay reaction.
Adsorption kinetics of Cr(VI) to sand was modeled as a first-order reversible process, and the reduction rate by ZVI was treated
as a first-order reaction. Natural decay of Cr(VI) was also included in other experiments, i.e., the adsorption to sand, the
reduction by ZVI, and both adsorption and reduction when sand and ZVI are present together. The model parameters were estimated
by fitting the solution of each model to the corresponding experimental data. To observe the effect of sand addition to ZVI,
both adsorption and reduction rate models were considered simultaneously including the natural decay. The solution of the
combined model was fitted to the experimental data to determine the first-order adsorption and reduction rate constants when
sand as well as ZVI is present. The first-order reduction rate constant in the presence of sand was about 35 times higher
than that with ZVI only. 相似文献
148.
M. K. Bakhadyrkhanov O. É. Sattarov Kh. M. Iliev K. S. Ayupov Tuérdi Umaier 《Semiconductors》2005,39(7):789-791
It is experimentally ascertained that light stimulates the negative magnetoresistance observed in a high electric field in silicon doped with boron and manganese. The optimum conditions (the electric field, temperature, illumination, and resistivity of the material) for observation of the largest magnitude of negative magnetoresistance in (Si:B):Mn are determined. The dependence of the negative magnetoresistance on the concentration of compensating impurity is established. 相似文献
149.
Seungsoo Kim Hyunchol Shin 《Microwave and Wireless Components Letters, IEEE》2008,18(10):701-703
A wideband complementary metal oxide semiconductor (CMOS) semidynamic frequency divide-by-3 covering more than two octave bandwidths is presented. The wideband operation without requiring a quadrature signal source is realized by employing a three-stage RC polyphase filter. The transfer function analysis on Type-II two- and three-stage polyphase filters is performed to provide analytic solutions of the peak phase error and peak attenuation. Implemented in 0.18 mum CMOS, the divide-by-3 operates over the input frequency range between 0.6 and 2.7 GHz while dissipating 15 mA from a 1.8 V supply. 相似文献
150.
The stress intensity factors for a short crack partially penetrating an inclusion of arbitrary shape
Some approximate solutions for predicting the stress intensity factor of a short crack penetrating an inclusion of arbitrary
shape have been developed under mode I and mode II loading conditions. The derivation of the fundamental formula is based
on the transformation toughening theory. The transformation strains in the inclusion are induced by the crack-tip field and
remotely applied stresses, and approximately evaluated by the Eshelby equivalent inclusion theory. As validated by detailed
finite element (FE) analyses, the developed solutions have good accuracy for different inclusion shape and for a wide range
of modulus ratio between inclusion and matrix material. 相似文献