全文获取类型
收费全文 | 9132篇 |
免费 | 482篇 |
国内免费 | 88篇 |
专业分类
电工技术 | 198篇 |
综合类 | 89篇 |
化学工业 | 1653篇 |
金属工艺 | 224篇 |
机械仪表 | 306篇 |
建筑科学 | 365篇 |
矿业工程 | 13篇 |
能源动力 | 290篇 |
轻工业 | 476篇 |
水利工程 | 39篇 |
石油天然气 | 45篇 |
武器工业 | 2篇 |
无线电 | 1816篇 |
一般工业技术 | 1659篇 |
冶金工业 | 1268篇 |
原子能技术 | 89篇 |
自动化技术 | 1170篇 |
出版年
2023年 | 101篇 |
2022年 | 140篇 |
2021年 | 197篇 |
2020年 | 153篇 |
2019年 | 141篇 |
2018年 | 204篇 |
2017年 | 196篇 |
2016年 | 200篇 |
2015年 | 201篇 |
2014年 | 302篇 |
2013年 | 562篇 |
2012年 | 471篇 |
2011年 | 506篇 |
2010年 | 399篇 |
2009年 | 467篇 |
2008年 | 466篇 |
2007年 | 425篇 |
2006年 | 386篇 |
2005年 | 329篇 |
2004年 | 309篇 |
2003年 | 314篇 |
2002年 | 266篇 |
2001年 | 237篇 |
2000年 | 215篇 |
1999年 | 203篇 |
1998年 | 491篇 |
1997年 | 328篇 |
1996年 | 223篇 |
1995年 | 149篇 |
1994年 | 129篇 |
1993年 | 151篇 |
1992年 | 85篇 |
1991年 | 80篇 |
1990年 | 72篇 |
1989年 | 50篇 |
1988年 | 53篇 |
1987年 | 54篇 |
1986年 | 61篇 |
1985年 | 35篇 |
1984年 | 35篇 |
1983年 | 24篇 |
1981年 | 41篇 |
1980年 | 22篇 |
1979年 | 21篇 |
1978年 | 28篇 |
1977年 | 42篇 |
1976年 | 44篇 |
1975年 | 13篇 |
1974年 | 13篇 |
1971年 | 12篇 |
排序方式: 共有9702条查询结果,搜索用时 31 毫秒
31.
Inertinite concentrates from three Australian bituminous coals were hydrogenated at various temperatures ranging from 350 to 475 °C in the presence of tetralin without any added catalyst. Both conversion yields and microscopic observations of the benzene-extracted hydrogenation residues have revealed that the hydrogenation of the inertinite macerals becomes significant only > 400 °C, whereas the dissolution-hydrogenation of the accompanying vitrinite macerals occurs mainly between 350 and 400 °C. The major reaction pathway for the inertinite particles in the hydrogenation process appears to be one of initial mild carbonization followed by hydrogenation. A simplified reaction scheme is proposed which describes the reaction pathway involved in the hydrogenation of inertinite. 相似文献
32.
33.
34.
35.
36.
Control of calcium hexaluminate grain morphology in in-situ toughened ceramic composites 总被引:2,自引:0,他引:2
The influence of processing conditions on the morphology of calcium hexaluminate (CA6) grains in Al2O3: 30 vol% CaO·6Al2O3 (CA6) ceramic composites was investigated. Specimens were prepared by in-situ reaction sintering using precursor powders of alumina, and either calcium carbonate or calcium oxide. In some samples, 1 vol% anorthite glass was added as a sintering aid. X-ray diffraction was used to study the phase development in the as-calcined and sintered states. The resultant microstructures were characterized using both scanning electron microscopy (SEM), and imaging secondary ion mass spectrometry (SIMS). It was found that the CA6 grains developed a platelike morphology when CaCO3 was used as the starting calcium-rich powder. In contrast, samples prepared using CaO resulted in equiaxed CA6 grains. This result was observed to be independent of the anorthite glass addition. The findings are rationalized in terms of distinct CA6 reaction mechanisms, resulting from differences in the reactivity of the powders during the early stages of calcining.Cement Nomenclature C
CaO
- A
Al2O3 相似文献
37.
K. Y. Sze W. K. Chan T. H. H. Pian 《International journal for numerical methods in engineering》2002,55(7):853-878
This paper presents eight‐node solid‐shell elements for geometric non‐linear analysis of elastic shells. To subdue shear, trapezoidal and thickness locking, the assumed natural strain method and an ad hoc modified generalized laminate stiffness matrix are employed. A selectively reduced integrated element is formulated with its membrane and bending shear strain components taken to be constant and equal to the ones evaluated at the element centroid. With the generalized stresses arising from the modified generalized laminate stiffness matrix assumed to be independent from the ones obtained from the displacement, an extended Hellinger–Reissner functional can be derived. By choosing the assumed generalized stresses similar to the assumed stresses of a previous solid element, a hybrid‐stress solid‐shell element is formulated. Commonly employed geometric non‐linear homogeneous and laminated shell problems are attempted and our results are close to those of other state‐of‐the‐art elements. Moreover, the hybrid‐stress element converges more readily than the selectively reduced integrated element in all benchmark problems. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
38.
As technology evolves, it is now feasible to implement sophisticated adaptive routing schemes on networks which support different kinds of services with heterogeneous bandwidth characteristics. Adaptive routing can increase the network throughput by routing calls to less congested paths. It can also be used to bypass transmission facility failures. In this paper, we analyze and compare two adaptive routing schemes. The first is called theMaximum mean time to blocking (MTB) routing which is based on themean time to blocking measure of a link. This measure captures the traffic rates, bandwidth characteristic and link capacity information and reflects more accurately the congestion status of different paths. The second is theM
2 routing, which is a modification of the least loaded routing (LLR). Aggregation of link status information can significantly reduce signalling traffic. We show in this paper that with properly designed aggregation, the aggregatedM
2 and MTB routings can have performance that approach that of the non-aggregated schemes. The use of complete sharing and restricted access policies together with trunk reservation control in multirate loss networks are also studied. 相似文献
39.
We have analyzed theoretically the effects of interdiffusion on the gain, differential gain, linewidth enhancement factor, and the injection current density of In0.2Ga0.8As-GaAs and Al0.3Ga0.7As-GaAs quantum-well (QW) lasers. We have calculated the electron and hole subband structures including the effects of valence band mixing and strains. The optical gain is then calculated using the density matrix approach. Our results show that the gain spectrum can be blue-shifted without an enormous increase in the injected current density. Imposing an upper limit (416 A·cm-2) on the injection current density for a typical laser structure, we find that the InGaAs-GaAs and AlGaAs-GaAs QW lasers can be blue-shifted by 24 and 54 mn, respectively. Our theoretical results compare well with the tuning ranges of 53 and 66 meV found for AlGaAs-GaAs QWs in some experiments. This indicates that the interdiffusion technique is useful for the tuning of laser operation wavelength for multiwavelength applications 相似文献
40.
Kwok Kee Chan Rao S.K. Morin G.A. Tang M.Q. 《Antennas and Propagation, IEEE Transactions on》1997,45(8):1277-1285
A new method of analysis for the radiation characteristics of dielectric lens antennas with arbitrary inner and outer surfaces is presented. The analysis is based on representing the feed illumination by a contiguous set of ray tubes and including the effects of surface reflections and ray divergence. Radiation patterns and the antenna gain are then computed by evaluating the closed-form expressions developed for the Kirchhoff's integral of the aperture fields. The validity of the analysis method has been demonstrated by comparing the computations with measured results of two different spherical lenses and a shaped lens configurations. The analysis method presented takes into account some of the practical aspects associated with lens design such as surface zoning to reduce the mass and surface matching to minimize the reflection loss 相似文献