全文获取类型
收费全文 | 60005篇 |
免费 | 3017篇 |
国内免费 | 204篇 |
专业分类
电工技术 | 915篇 |
综合类 | 100篇 |
化学工业 | 11831篇 |
金属工艺 | 2214篇 |
机械仪表 | 3250篇 |
建筑科学 | 1318篇 |
矿业工程 | 53篇 |
能源动力 | 2353篇 |
轻工业 | 4277篇 |
水利工程 | 262篇 |
石油天然气 | 214篇 |
武器工业 | 1篇 |
无线电 | 10663篇 |
一般工业技术 | 11916篇 |
冶金工业 | 5988篇 |
原子能技术 | 680篇 |
自动化技术 | 7191篇 |
出版年
2023年 | 539篇 |
2022年 | 884篇 |
2021年 | 1488篇 |
2020年 | 1081篇 |
2019年 | 1110篇 |
2018年 | 1454篇 |
2017年 | 1470篇 |
2016年 | 1800篇 |
2015年 | 1414篇 |
2014年 | 2187篇 |
2013年 | 3728篇 |
2012年 | 3448篇 |
2011年 | 4232篇 |
2010年 | 3188篇 |
2009年 | 3385篇 |
2008年 | 3076篇 |
2007年 | 2618篇 |
2006年 | 2381篇 |
2005年 | 2065篇 |
2004年 | 1939篇 |
2003年 | 1797篇 |
2002年 | 1738篇 |
2001年 | 1366篇 |
2000年 | 1278篇 |
1999年 | 1239篇 |
1998年 | 2272篇 |
1997年 | 1483篇 |
1996年 | 1261篇 |
1995年 | 1004篇 |
1994年 | 744篇 |
1993年 | 696篇 |
1992年 | 514篇 |
1991年 | 508篇 |
1990年 | 435篇 |
1989年 | 431篇 |
1988年 | 339篇 |
1987年 | 289篇 |
1986年 | 270篇 |
1985年 | 241篇 |
1984年 | 209篇 |
1983年 | 159篇 |
1982年 | 160篇 |
1981年 | 140篇 |
1980年 | 133篇 |
1979年 | 109篇 |
1978年 | 95篇 |
1977年 | 126篇 |
1976年 | 163篇 |
1975年 | 80篇 |
1974年 | 74篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
161.
162.
163.
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
164.
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
165.
Junho Lee Hyungsoo Kim Joungho Kim 《Microwave and Wireless Components Letters, IEEE》2005,15(8):505-507
We experimentally demonstrated the great advantages of a high dielectric constant thin film electromagnetic bandgap (EBG) power distribution network (PDN) for the suppression of power/ground noises and radiated emissions in high-performance multilayer digital printed circuit boards (PCBs). Five-layer test PCBs were fabricated and their scattering parameters measured. The power plane noise and radiated emissions were measured, investigated and related to the PDN impedance. This successfully demonstrated that the bandgap of the EBG was extended more than three times, covering a range of hundreds of MHz using a 1-cm /spl times/ 1-cm EBG cell, the SSN was reduced from 170 mV to 10 mV and the radiated emission was suppressed by 22 dB because of the high dielectric constant thin film EBG power/ground network. 相似文献
166.
This paper looks at the concepts of entities, identities, identifiers and credentials, their definitions, and how they combine
to authenticate an identity to an account, or function, within a service. Credentials can be divided into several different
types on the basis of their use, origin and characteristics. In particular, types such as composite, dynamic and derived credentials
will be considered, and their applicability within an identity management system. Each credential must be managed through
its entire life cycle from initial registration, general use and suspension, to deletion. The notion of public (i.e. freely
distributable) and private (i.e. known only to you) aspects of a credential is explored. The bindings, by trusted third parties,
of identifiers and public credentials into permits and instruments of proof is explained along with the convenience, but greater
exposure, of local information. Several scenarios are analysed in terms of how different types of credentials are employed
during the authentication process. 相似文献
167.
Xiaomeng Shi Jian-Guo Ma Kiat Seng Yeo Manh Anh Do Erping Li 《Very Large Scale Integration (VLSI) Systems, IEEE Transactions on》2005,13(9):1060-1071
This paper investigates the properties of the on-wafer interconnects built in a 0.18-/spl mu/m CMOS technology for RF applications. A scalable equivalent circuit model is developed. The model parameters are extracted directly from the on-wafer measurements and formulated into empirical expressions. The expressions are in functions of the length and the width of the interconnects. The proposed model can be easily implemented into commercial RF circuit simulators. It provides a novel solution to include the frequency-variant characteristics into a circuit simulation. The silicon-verified accuracy is proved to be up to 25 GHz with an average error less than 2%. Additionally, equivalent circuit model for longer wires can be obtained by cascading smaller subsections together. The scalability of the propose model is demonstrated. 相似文献
168.
Dielectric reliability in Al2O3(2–3.1nm)–HfO2(3nm) stack capacitor with Metal–Insulator–Si(MIS) structure is investigated in this paper. We propose an optimized capacitor process through the Time–Dependent Dielectric Breakdown (TDDB) data under various process conditions. Furthermore, due to asymmetric current at both negative and positive voltage stress polarities, we show different lifetime extrapolation by a fluence–driven model. As a result, the maximum allowed operating voltage is projected to be 1.7V (failure rate 10ppm during 10year @ 85°C) for Data “0” retention lifetime. 相似文献
169.
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
170.
Grazing incidence reflection absorption Fourier transform infrared (GIRA-FTIR) spectroscopy was used to characterize the ferroelectric behavior of a thin poly(vinylidene fluoride–trifluoroethylene) P(VDF–TrFE) copolymer. The lab-built GIRA-FTIR apparatus allowed the heating and corona poling process to be carried out whilst collecting the GIRA spectra of the thin polymer film. The Curie transition from the ferroelectric to paraelectric phase was confirmed from the abrupt change in intensity of the 849 cm−1 band in the RA-FTIR spectrum. It was found that the Curie temperature dropped dramatically when the film thickness was lowered to below a certain critical value of approximately 100 nm. The switching of the CF2 dipoles in the ferroelectric crystals after applying the external electric field could be determined by monitoring the change in the 849 cm−1 band intensity. For the 600 nm thick P(VDF–TrFE) film, the switching of the dipoles appears to occur almost instantaneously, while the kinetics of dipole switching of the 75 nm thick film were significantly retarded. The repeated switchability of the CF2 dipoles upon the application of a bipolar cyclic electric field was also confirmed. The bistability of the film due to remnant polarization was also confirmed from the absorbance of the 849 cm−1 band after removing the applied voltage during corona poling. 相似文献