全文获取类型
收费全文 | 1528篇 |
免费 | 75篇 |
国内免费 | 13篇 |
专业分类
电工技术 | 38篇 |
综合类 | 1篇 |
化学工业 | 314篇 |
金属工艺 | 25篇 |
机械仪表 | 77篇 |
建筑科学 | 73篇 |
矿业工程 | 7篇 |
能源动力 | 55篇 |
轻工业 | 137篇 |
水利工程 | 14篇 |
石油天然气 | 5篇 |
无线电 | 208篇 |
一般工业技术 | 280篇 |
冶金工业 | 191篇 |
原子能技术 | 4篇 |
自动化技术 | 187篇 |
出版年
2023年 | 15篇 |
2022年 | 21篇 |
2021年 | 45篇 |
2020年 | 23篇 |
2019年 | 25篇 |
2018年 | 35篇 |
2017年 | 36篇 |
2016年 | 42篇 |
2015年 | 42篇 |
2014年 | 57篇 |
2013年 | 94篇 |
2012年 | 95篇 |
2011年 | 115篇 |
2010年 | 80篇 |
2009年 | 96篇 |
2008年 | 71篇 |
2007年 | 75篇 |
2006年 | 56篇 |
2005年 | 45篇 |
2004年 | 56篇 |
2003年 | 49篇 |
2002年 | 40篇 |
2001年 | 35篇 |
2000年 | 31篇 |
1999年 | 32篇 |
1998年 | 49篇 |
1997年 | 40篇 |
1996年 | 37篇 |
1995年 | 33篇 |
1994年 | 21篇 |
1993年 | 18篇 |
1992年 | 4篇 |
1991年 | 12篇 |
1990年 | 11篇 |
1989年 | 20篇 |
1988年 | 11篇 |
1987年 | 6篇 |
1986年 | 2篇 |
1985年 | 8篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1976年 | 1篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1970年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有1616条查询结果,搜索用时 3 毫秒
71.
It is well known that the dynamic response rate of the Severinghaus-type CO2 electrode is improved significantly in some cases by addition of the enzyme carbonic anhydrase to the electrode assembly. Hysteresis in the response rate also is reduced. Experimental data and modelling results indicate that catalysis of the CO2 hydration reaction in the bulk of the bicarbonate layer (the Nernst film) is not responsible for the improved response behavior, Evidence is presented to show that catalysis in the electrostatic double layer region at the glass electrode surface is a possible explanation. This proposed phenomenon may have widespread implications for the optimal design of analytical devices, commercial processes involving electrochemical phenomena, and may also provide insight into electrobiologi-cal processes. 相似文献
72.
CH4/CO2 reforming over La2NiO4 and 10%NiO/CeO2–La2O3 catalysts under the condition of supersonic jet expansion was studied via direct monitoring of the reactants and products using the sensitive technique of cavity ring-down spectroscopy. Vibration–rotational absorption lines of CH4, H2O, CO2 and CO molecules were recorded in the near infrared spectral region. Our results indicated that La2NiO4 is superior to 10%NiO/CeO2–La2O3 in performance. In addition, we observed enhanced reverse-water-gas-shift reaction at augmented reaction temperature. The formation of reaction intermediates was also investigated by means of time-of-flight mass spectrometry and there was the detection of CHx+, OH+ and H+ species. 相似文献
73.
Limited past studies have indicated that Si3 N4 doped with Sc2 O3 may exhibit high-temperature mechanical properties superior to Si3 N4 systems with various other oxide sintering additives. High-temperature deformation of this system was studied by characterizing the microstructures before and after deformation. It was found that elements of the additive, Sc and O, exist in small amounts at thin grain boundary layers and within secondary phases at triple and multiple grain boundary junctions. The secondary phase is devitrified as crystalline Sc2 Si2 O7 . Deformation of the samples was dominated by cavitation rather than dislocation processes. Thus, the excellent deformation resistance of the samples at high temperature can be attributed to the high refractories and enhanced crystallization of a secondary phase. 相似文献
74.
This paper deals with the broadband noise due to the interaction between convected turbulent gusts and a rectilinear cascade
of flat plates bounded by two parallel walls. An analytic formulation for the acoustic power spectrum due to this turbulence-cascade
interaction is derived, which can be used to assess the effects of the span-wise wavenumber components of ingesting turbulent
gust on the overall acoustic power spectrum. This three-dimensional theory is based on the two-dimensional theory of Cheong
et al. (2006, 2009). The three-dimensional model is shown to provide a close fit to the measured spectrum of rotor-stator
interaction. The predictions using this three-dimensional model are also compared with those using the previous two-dimensional
model by Cheong et al. (2006; 2009). Through this comparison, it is found that the contributions to the acoustic power of
the span-wise wavenumber components of incident turbulent gust are increased as the frequency is augmented, which is mainly
due to three-dimensional dispersion-relation characteristics of acoustic waves. This implies that the number of incident turbulent
gust modes directly involved in generating cut-on acoustic waves increases as the frequency increases. Therefore, in the lower
frequency range, three-dimensional acoustic power is less than its corresponding two-dimensional one, whereas, as the frequency
increases, the three-dimensional acoustic power spectrum closely follows those of two-dimension. The formulation is also used
to make a parametric study about the effects on the power spectrum of the blade number, stagger angle, gap-chord ratio, and
Mach number. 相似文献
75.
Young Suk Kim Sung Soo Kim Sang Chul Kwon Yong Moo Cheong Kyung Soo Im 《Metallurgical and Materials Transactions A》2002,33(3):919-925
The objectives of this study are to systematically investigate the delayed hydride cracking (DHC) velocity and the threshold-stress
intensity factor, K
IH
, of a Zr-2.5Nb pressure tube as a function of orientation and elucidate the cause of this anistropic DHC behavior. The DHC
velocity as a function of orientation was determined using flattened cantilever beam specimens with 60 ppm H while the threshold-stress
intensity factor K
IH
, was evaluated as a function of orientation on the curved compact-tension (CT) and cantilever-beam (CB) specimens charged
with hydrogen to 200 ppm H. To infer a difference in a stress gradient ahead of the crack tip as a function of orientation,
tensile tests were conducted at temperatures ranging from room temperature (RT) to 560 °C using small tensile specimens of
2-mm-gage length taken from three directions of the tube. A textural change was investigated by comparing the inverse pole
figures before and after DHC while the {10
7} pole figures were constructed to find out the growth pattern of the DHC crack as a function of orientation. Faster DHC
velocity and lower K
IH
were obtained over temperatures of 170 °C to 270 °C, when the DHC crack grew in the longitudinal direction of the Zr-2.5Nb
pressure tube. The strain hardening after yielding and the extent of the textural change accompanied by DHC were higher in
the longitudinal direction of the tube, suggesting a higher stress gradient ahead of the crack tip. Thus, the anisotropic
DHC behavior of a Zr-2.5Nb pressure tube is discussed based on the stress gradient ahead of the crack tip governed by strain-hardening
rate after yielding and a change in texture accompanied by DHC, and the distribution of the {10
7} hydride habit planes.
This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals
and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following
ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic
Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. 相似文献
76.
Lo VC Chen ZJ 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2002,49(7):980-986
The role of space charge induced in a ferroelectric thin film and the presence of Schottky barriers at the two electrode/film interfaces are studied by numerical simulation using Landau-Khalatnikov theory. In this work, the whole film is considered as the stacking of dipolar layers, each of which contains multilayers of perovskite cells. In the presence of a local electric field, the double-well thermodynamic potential of each layer is modified in an asymmetric manner. The local electric field distribution is determined both by the space charge and the boundary conditions imposed by the Schottky barrier heights. Asymmetric and skewed hysteresis loops are generated 相似文献
77.
A microscope slide acting as a passive waveguide was coated by three separate poly(vinyl alcohol) films that were doped with Coumarin 460, Disodium Fluorescein, and Rhodamine 640 perchlorate. On collinear pumping by a nitrogen laser, these dyes furnished primary red-green-blue laser emissions that were collected and waveguided by the microscope slide but exited from both ends. Frosting the waveguide exit introduced light scattering at the glass-air interface and spatially overlaid the red-green-blue laser emissions that emerged as a uniform white-light beam. 相似文献
78.
Cheong Hee Park Author Vitae 《Pattern recognition》2005,38(4):495-503
In this paper, we present a new approach for fingerprint classification based on discrete Fourier transform (DFT) and nonlinear discriminant analysis. Utilizing the DFT and directional filters, a reliable and efficient directional image is constructed from each fingerprint image, and then nonlinear discriminant analysis is applied to the constructed directional images, reducing the dimension dramatically and extracting the discriminant features. The proposed method explores the capability of DFT and directional filtering in dealing with low-quality images and the effectiveness of nonlinear feature extraction method in fingerprint classification. Experimental results demonstrates competitive performance compared with other published results. 相似文献
79.
Text classification without negative examples revisit 总被引:1,自引:0,他引:1
Gabriel Pui Cheong Fung Yu J.X. Hongjun Lu Yu P.S. 《Knowledge and Data Engineering, IEEE Transactions on》2006,18(1):6-20
Traditionally, building a classifier requires two sets of examples: positive examples and negative examples. This paper studies the problem of building a text classifier using positive examples (P) and unlabeled examples (U). The unlabeled examples are mixed with both positive and negative examples. Since no negative example is given explicitly, the task of building a reliable text classifier becomes far more challenging. Simply treating all of the unlabeled examples as negative examples and building a classifier thereafter is undoubtedly a poor approach to tackling this problem. Generally speaking, most of the studies solved this problem by a two-step heuristic: first, extract negative examples (N) from U. Second, build a classifier based on P and N. Surprisingly, most studies did not try to extract positive examples from U. Intuitively, enlarging P by P' (positive examples extracted from U) and building a classifier thereafter should enhance the effectiveness of the classifier. Throughout our study, we find that extracting P' is very difficult. A document in U that possesses the features exhibited in P does not necessarily mean that it is a positive example, and vice versa. The very large size of and very high diversity in U also contribute to the difficulties of extracting P'. In this paper, we propose a labeling heuristic called PNLH to tackle this problem. PNLH aims at extracting high quality positive examples and negative examples from U and can be used on top of any existing classifiers. Extensive experiments based on several benchmarks are conducted. The results indicated that PNLH is highly feasible, especially in the situation where |P| is extremely small. 相似文献
80.
Weijia Jia Gaochao Xu Wei Zhao Pui-On Au 《Journal of Network and Systems Management》2002,10(4):417-438
A novel efficient and dynamic multicast routing protocol based on anycast routing techniques is presented. The contributions of the protocol differ from well-known shared-tree systems in two aspects: (1) Off-tree anycast routing: The nodes in the shared tree are formed into a virtual anycast group and multicast sources use anycast routing to select a better path from the source to one router in the group to achieve short delay and fault-tolerance. (2) On-tree dynamic routing: The shared-tree approach is extended with capability of alternative path selections. If a node becomes absent from the shared tree, some predefined backup path(s) is (are) used to bypass the node and enable dynamic multicast routing to continue. The protocol requires only the routers near the faulty node to be reconfigured, thus reducing the runtime overhead as compared with global reconfiguration. The simulation data demonstrates the efficiency of our routing protocol. 相似文献