全文获取类型
收费全文 | 403878篇 |
免费 | 5387篇 |
国内免费 | 1363篇 |
专业分类
电工技术 | 7580篇 |
综合类 | 317篇 |
化学工业 | 59997篇 |
金属工艺 | 16437篇 |
机械仪表 | 13500篇 |
建筑科学 | 8396篇 |
矿业工程 | 2154篇 |
能源动力 | 11552篇 |
轻工业 | 28493篇 |
水利工程 | 4268篇 |
石油天然气 | 7914篇 |
武器工业 | 17篇 |
无线电 | 49359篇 |
一般工业技术 | 83114篇 |
冶金工业 | 75116篇 |
原子能技术 | 9184篇 |
自动化技术 | 33230篇 |
出版年
2022年 | 2569篇 |
2021年 | 3901篇 |
2020年 | 2960篇 |
2019年 | 3815篇 |
2018年 | 6507篇 |
2017年 | 6494篇 |
2016年 | 6944篇 |
2015年 | 4338篇 |
2014年 | 7165篇 |
2013年 | 20080篇 |
2012年 | 11428篇 |
2011年 | 15315篇 |
2010年 | 12188篇 |
2009年 | 13743篇 |
2008年 | 14081篇 |
2007年 | 13858篇 |
2006年 | 12445篇 |
2005年 | 11150篇 |
2004年 | 10527篇 |
2003年 | 10439篇 |
2002年 | 9844篇 |
2001年 | 9753篇 |
2000年 | 9147篇 |
1999年 | 9534篇 |
1998年 | 24383篇 |
1997年 | 16732篇 |
1996年 | 12821篇 |
1995年 | 9534篇 |
1994年 | 8372篇 |
1993年 | 8395篇 |
1992年 | 6060篇 |
1991年 | 5702篇 |
1990年 | 5683篇 |
1989年 | 5323篇 |
1988年 | 5048篇 |
1987年 | 4425篇 |
1986年 | 4304篇 |
1985年 | 4816篇 |
1984年 | 4423篇 |
1983年 | 3999篇 |
1982年 | 3655篇 |
1981年 | 3734篇 |
1980年 | 3430篇 |
1979年 | 3326篇 |
1978年 | 3336篇 |
1977年 | 3755篇 |
1976年 | 4852篇 |
1975年 | 2850篇 |
1974年 | 2666篇 |
1973年 | 2732篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
V. Vassilev S. Thijs P. L. Segura P. Wambacq P. Leroux G. Groeseneken M. I. Natarajan H. E. Maes M. Steyaert 《Microelectronics Reliability》2005,45(2):255-268
This paper describes an approach to design ESD protection for integrated low noise amplifier (LNA) circuits used in narrowband transceiver front-ends. The RF constraints on the implementation of ESD protection devices are relaxed by co-designing the RF and the ESD blocks, considering them as one single circuit to optimise. The method is applied for the design of 0.25 μm CMOS LNA. Circuit protection levels higher than 3 kV HBM stress are achieved using conventional highly capacitive ggNMOS snapback devices. The methodology can be extended to other RF-CMOS circuits requiring ESD protection by merging the ESD devices in the functionality of the corresponding matching blocks. 相似文献
952.
The effects of fluidized bed heat treatment on the microstructural and mechanical properties of Al-Si-Cu-Mg cast alloys, namely,
354 and 319, were studied. The heating rate in fluidized beds (FBs) is greatervis-à-vis conventional electrical resistance furnaces (CFs). The high heating rate in FBs increases the kinetics of metallurgical phenomena
such as Si fragmentation and spherodization during solution heat treatment, as well as the precipitation rate of phases such
as Al5Cu2Mg8Si6 and Al2Cu during aging. It is observed that the dissolution rate of phases such as Mg2Si and Al5Cu2Mg8Si6 takes place very rapidly. The solution heat treatment of 319 alloy using FB results in complete dissolution of Mg2Si and Al5Cu2Mg8Si6 particles within 45 minutes. However, for phases such as Al2Cu and Ferich intermetallics, the dissolution rate is relatively slow. Even on prolonged solution heat treatment for 6 hours,
these phases do not dissolve completely. It is observed that incomplete dissolution of the Al2Cu phase does not significantly affect tensile properties of T4-treated alloys. The optimum solution heat-treatment time in
FB for both 354 and 319 alloys is 45 minutes at 527 °C and 493 °C, respectively. Thermal analysis shows an exothermic peak
owing to recrystallization and coarsening of eutectic grains during solution heat treatment. The high heating rate in FB causes
this transformation to take place at a lower temperature than in CF. It is observed that the nucleation rate of Al5Cu2Mg8Si6 during aging in FB is greater than using CF. Thermal analysis of samples during the ramp-up stage while aging using FB did
not show any phase transformation, while those using CF show two endothermic transformations, which are most likely due to
the dissolution of GP zones or the co-cluster of solutes. Aging at 200 °C results in a greater number density of precipitates
than those at 240 °C. The tensile strength of samples aged at 200 °C is greater than those aged at 240 °C, because the amount
of precipitates formed at 200 °C is greater than that at 240 °C. The total heat-treatment time for T6 temper is less than
2 hours in FBs, which is a significant reduction in heat-treatment time, as well as energy consumption. 相似文献
953.
A new phenomenon accompanying the electrolysis of salts has been observed, whereby an ordered composite is formed under certain conditions (electrolyte composition, current density). The composite comprises an inorganic salt crystal penetrated by metal nano- and microwires. This phenomenon can serve as a basis for the technology of oriented and ordered nanowire array synthesis. 相似文献
954.
A framework of virtual prototyping environment for the design and analysis of mechanical mechanism with clearance 总被引:7,自引:0,他引:7
Mechanical mechanisms with clearance abound in technological fields such as robotics, machine and steam turbine industry. The dynamic performance of these mechanisms in working condition can be achieved provided that the effects of the different on the mechanisms (such as mechanism dynamics, technological effects and thermal behaviour) are thoroughly understood. Virtual prototyping provides an integration of multi-domain dynamic simulation for the design and analysis process. In the current paper, a framework of virtual prototyping environment for the design and analysis of mechanical mechanism with clearance is presented. A case study with such a simulation strategy is studied for the optimum design and analysis of the technological parameter. 相似文献
955.
S. V. Ivanova 《Materials Science》2007,43(5):718-724
We have established that technological defects (cracks) in zirconium articles can develop in the course of long-term storage
prior to their mounting in a reactor, which is caused by the action of residual stresses and hydrogen. We have considered
the conditions of after-operation storage of zirconium articles of the fuel core of thermal reactors in water of the cooling
ponds of reactors and in spent-fuel storehouses. The possibility of subsequent hydrogenation of these articles during their
storage has been shown. Finally, we have studied the effect of hydrogen, absorbed in the course of storage, on the crack resistance
of zirconium articles and the development of defects in them due to delayed hydride cracking.
__________
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 5, pp. 105–109, September–October, 2007. 相似文献
956.
The paper proposes a novel method of forecasting short-term electricity price based on a two-stage hybrid network of self-organised map (SOM) and support-vector machine (SVM). In the first stage, a SOM network is applied to cluster the input-data set into several subsets in an unsupervised manner. Then, a group of SVMs is used to fit the training data of each subset in the second stage in a supervised way. With the trained network, one can predict straightforwardly the next-day hourly electricity prices. To confirm its effectiveness, the proposed model has been trained and tested on the data of historical energy prices from the New England electricity market. 相似文献
957.
We report on the use of pulsed plasma-enhanced chemical vapor deposition (P-PECVD) technique and show that “state-of-the-art” amorphous silicon (a-Si:H) materials and solar cells can be produced at a deposition rate of up to 15 Å/s using a modulation frequency in the range 1–100 kHz. The approach has also been developed to deposit materials and devices onto large area, 30 cm×40 cm, substrates with thickness uniformity (<5%), and gas utilization rate (>25%). We have developed a new “hot wire” chemical vapor deposition (HWCVD) method and report that our new filament material, graphite, has so far shown no appreciable degradation even after deposition of 500 μm of amorphous silicon. We report that this technique can produce “state-of-the-art” a-Si:H and that a solar cell of p/i/n configuration exhibited an initial efficiency approaching 9%. The use of microcrystalline silicon (μc-Si) materials to produce low-cost stable solar cells is gaining considerable attention. We show that both of these techniques can produce thin film μc-Si, dependent on process conditions, with 1 1 1 and/or 2 2 0 orientations and with a grain size of approx. 500 A. Inclusion of these types of materials into a solar cell configuration will be discussed. 相似文献
958.
The paper is devoted to the effect of compressibility of the avalanche snow impacting an obstacle. Compression shocks generated by obstacle cause high pressure peaks at first instants of impact. That is why the account of compressibility is essential for the understanding of measurements and the design of structures. The main problem in calculation compression shocks in avalanches is to formulate an equation of state for moving snow in impact. Two different types of equations of state are proposed depending on the type of the avalanche (low-density and high-density flows). The approach is not totally new. It was earlier proposed mainly in Russian literature. Here a brief review of the previous work is given with discussion of some gaps in it. The theory is reformulated and further developed to account thermodynamical equations. The simplest case of a normal compression shock in an avalanche flow is studied. Examples of estimations of pressure and density behind a shock are given. It is important to emphasize that the Mach number plays an important role in the theory of compressible flows so it should be taken into account (together with the Froude number) in calculation and modelling an avalanche impact pressure. 相似文献
959.
X-ray diffraction analysis shows that the transformation during heat treatment from amorphous to crystalline phases of bismuth-borate glass samples takes place in sequences. After a short heat treatment, 5 min at 550 °C, a layered structure with a preferred orientation of crystallites on the surface is observed. After a long heat treatment, 8 h at the same temperature, normal polycrystalline bulk samples are obtained. 相似文献
960.
The effect of varying normalising and hardening temperatures on the prior austenite grain size in a low alloy Cr–Mo–Ni–V steel
has been examined. An initial relative insensitivity of grain size to increasing austenitising temperature was observed followed
by a sudden growth of grains at approximately 1000 °C. A detailed study of the precipitates in the steel showed the presence
of a bimodal size distribution of vanadium carbides. The grain size increase is attributed to a decrease in volume fraction
and an increase in size of V4C3 particles with increasing temperature. 相似文献