全文获取类型
收费全文 | 412003篇 |
免费 | 6552篇 |
国内免费 | 1383篇 |
专业分类
电工技术 | 7704篇 |
综合类 | 349篇 |
化学工业 | 61993篇 |
金属工艺 | 14284篇 |
机械仪表 | 11871篇 |
建筑科学 | 11636篇 |
矿业工程 | 869篇 |
能源动力 | 11117篇 |
轻工业 | 43798篇 |
水利工程 | 3312篇 |
石油天然气 | 1835篇 |
武器工业 | 13篇 |
无线电 | 55388篇 |
一般工业技术 | 76612篇 |
冶金工业 | 74953篇 |
原子能技术 | 5571篇 |
自动化技术 | 38633篇 |
出版年
2021年 | 2666篇 |
2019年 | 2435篇 |
2018年 | 3838篇 |
2017年 | 3903篇 |
2016年 | 4144篇 |
2015年 | 3426篇 |
2014年 | 5714篇 |
2013年 | 19279篇 |
2012年 | 10203篇 |
2011年 | 14680篇 |
2010年 | 11283篇 |
2009年 | 12884篇 |
2008年 | 13914篇 |
2007年 | 14286篇 |
2006年 | 12777篇 |
2005年 | 11962篇 |
2004年 | 11574篇 |
2003年 | 11279篇 |
2002年 | 10913篇 |
2001年 | 11131篇 |
2000年 | 10271篇 |
1999年 | 10769篇 |
1998年 | 24011篇 |
1997年 | 17588篇 |
1996年 | 13903篇 |
1995年 | 10979篇 |
1994年 | 9771篇 |
1993年 | 9354篇 |
1992年 | 7121篇 |
1991年 | 6790篇 |
1990年 | 6397篇 |
1989年 | 6108篇 |
1988年 | 5860篇 |
1987年 | 4885篇 |
1986年 | 4834篇 |
1985年 | 5978篇 |
1984年 | 5599篇 |
1983年 | 4832篇 |
1982年 | 4496篇 |
1981年 | 4447篇 |
1980年 | 4208篇 |
1979年 | 4080篇 |
1978年 | 3771篇 |
1977年 | 4548篇 |
1976年 | 6097篇 |
1975年 | 3115篇 |
1974年 | 3012篇 |
1973年 | 2890篇 |
1972年 | 2318篇 |
1971年 | 2003篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
A method is presented and analyzed for determining the impulse response and frequency response of a system using an asymmetrical stimulus signal. The considered signal is a pulse with a rapid first transition, followed by a nearly constant region, and then followed by a second transition that is much slower than the first. The advantage of such a signal is that unlike a symmetric pulse, its Fourier transform has no zeros. This allows one to divide by the Fourier transform in the data analysis. It is shown that such a pulse retains the advantages of using an impulse or steplike stimulus. Error estimates similar to those previously published for impulse and steplike stimuli are given for the asymmetric stimulus 相似文献
962.
Residual stress as measured quantity in quality assurance The x‐ray method is used in the Volkswagen Automotive Group at different world‐wide locations as a standard procedure to determine residual stress. As an important measured quantity in the quality assurance residual stresses are specified in drawings of different components (e.g. gears, coil springs). Measurements are carried out during quality assurance, production, damage analyse, technical development department and design control of components from suppliers. An optimum efficiency can be achieved, if it exists a unique correlation between residual stress values and loading capacity of components, and if the processes to introduce residual stress can be optimized using residual stress measurements during the production. 相似文献
963.
Evans Jonathan St. B. T.; Handley Simon J.; Over David E. 《Canadian Metallurgical Quarterly》2003,29(2):321
The authors report 3 experiments in which participants were invited to judge the probability of statements of the form if p then q given frequency information about the cases pq, p?q, ?pq, and ?p?q (where ? = not). Three hypotheses were compared: (a) that people equate the probability with that of the material conditional, 1 - P(p?q); (b) that people assign the conditional probability, P(q/p); and (c) that people assign the conjunctive probability P(pq). The experimental evidence allowed rejection of the 1st hypothesis but provided some support for the 2nd and 3rd hypotheses. Individual difference analyses showed that half of the participants used conditional probability and that most of the remaining participants used conjunctive probability as the basis of their judgments. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
964.
Fe–Co bimetallic metals supported on CaCO3 support have been prepared using three different methods for the preparation of the catalysts; wet impregnation (IMP), deposition–precipitation (DP) and reverse micelle impregnation (RM). By careful choice and optimization of CVD reaction conditions and catalyst preparation procedures, we have tailored the nanofabrication of CNTs using 5 wt.% Fe–Co/CaCO3. High yields of ‘clean’ MWNTs with near 100% selectivity and excellent reproducibility were obtained (1215% and 835% yields for the IMP and DP methods respectively). The RM technique enabled more precise control over the size of the CNTs and a narrower diameter distribution (3–5 nm i.d. and 10–15 nm o.d.) was obtained. 相似文献
965.
J. Pujara M. A. Siddiqui Z. Liu P. Bjegovic S. S. Takagaki P. Y. Li S. Ramaswamy 《Drying Technology》2008,26(3):341-348
This is part II of a study reported earlier on a method to characterize the air flow and water removal characteristics during vacuum dewatering. This article presents experimental data and analysis of results from the use of a cyclically actuated vacuum dewatering device for removing moisture from wetted porous materials such as paper with the intermittent application of vacuum and accompanying air flow though the material. Results presented include sheet moisture content as a function of residence time and hence water removal rate under a variety of process conditions. Also, experimental results on air flow through the wet porous structure and hence the role and importance of air flow during vacuum dewatering are presented. Vacuum dewatering process conditions include exit solids content between 11 and 20% solid under applied vacuum conditions of 13.5 to 67.7 kPa (4 to 20 in. Hg). Regression analysis indicated that the exit sheet moisture content exhibited a nonlinear relationship with residence time with exit solids reaching a plateau after a certain residence time. Final moisture content correlated linearly with the average overall flow rate of air through the paper sample and the basis weight of the material. 相似文献
966.
Biodiesel has become an attractive diesel fuel substitute due to its environmental benefits since it can be made from renewable resource. However, the high costs surrounding biodiesel production remains the main problem in making it competitive in the fuel market either as a blend or as a neat fuel. More than 80% of the production cost is associated with the feedstock itself and consequently, efforts are focused on developing technologies capable of using lower-cost feedstocks, such as recycled cooking oils and wastes from animal or vegetable oil processing operations. 相似文献
967.
Ravi Joshi Jörg Engstler P. Kesavan Nair Prathap Haridoss Jörg J. Schneider 《Diamond and Related Materials》2008,17(6):913-919
Carbon nanotubes (CNTs) have been synthesized using the arc discharge method with a rotating graphite disc as the cathode. Arcing was carried out in open air and without the use of catalysts. The current density was maintained constant through out the experiment, while, the rate of rotation of the cathode and atmosphere under which arcing was carried out were changed during experimentation. Characterization of the samples produced indicates that rotation of the cathode has a significant impact on the quality and yield of the process. It is proposed that rotation of the cathode drags plasma formed between two electrodes away from high temperature region. This results in a sudden quenching of the reactive plasma. The time available for nucleation and growth phenomena is significantly reduced and thus leads to the formation of highly graphitic multi walled CNTs (yield 60%) and traces of double walled CNTs. 相似文献
968.
Ezequiel R. Soulé Guillermo E. Eliçabe Roberto J. J. Williams 《Polymer Bulletin》2008,60(2-3):417-423
An unexpected experimental finding is reported where the primary morphology developed during polymerization-induced
phase separation in a rubber-modified thermoplastic practically disappears at high conversions. This process
was evidenced by light scattering (LS) and scanning electron microscopy (SEM) for a particular composition
of solutions of polyisobutylene oligomers (PIB) in isobornylmethacrylate (IBoMA), during the free-radical
polymerization of the monomer. The primary phase separation produced a dispersion of domains rich in PIB
containing significant amounts of the monomer (IBoMA). Polymerization of the monomer in these domains occurred
at high overall conversions producing the filling of dispersed domains with a PIBoMA-PIB blend that could
not be distinguished from the matrix. Under these conditions the final material had the appearance of a
homogeneous blend. 相似文献
969.
This is the first time an extensive investigation has been carried out regarding the effects of riser exit geometry on pressure drop and solid behaviour inside the Internal Circulating Fluidized Bed (ICFB) riser, using different riser exit geometries at several operating conditions.The Radioactive Particle-Tracking (RPT) technique was used for solid concentration measurements and solid residence time distribution at the exit zone. Experiments were conducted using Geldart B particles, in the gas superficial velocity range of 4 to 10 m/s. Axial solid hold-up, solid residence time distribution in the exit zone, and the reflux ratio factor km, (defined earlier by [E.H. Van der Meer, R.B. Thorpe, J.F. Davidson, Flow patterns in the square cross-section riser of a circulating fluidized bed and the effect of riser exit design, Chem. Eng. Sc. 55 (19) (2000) 4079-4099]), were the main criteria used to investigate the impact of gas-solid separator devices implemented at the ICFB riser exit.Solid residence time distribution results and axial solid hold-up profiles provided clear evidence that the separator device at the riser exit strongly influences the hydrodynamic structure of the ICFB riser. The V-shaped riser exit geometry was found to be the optimum of all the configurations studied. 相似文献
970.
Y.M. Wong W.P. Kang J.L. Davidson J.H. Huang D.V. Kerns 《Diamond and Related Materials》2008,17(4-5):552-555
A novel integrated vacuum field emission (VFE) differential amplifier (diff-amp) utilizing carbon nanotube (CNT) emitters has been developed. A dual-mask microfabrication process was employed to achieve a VFE diff-amp by integrating identical CNT VFE transistors with built-in split gates and integrated anodes. The identical pair of triode amplifiers was well-matched in their device characteristics. The measured ac small-signal characteristics of the diff-amp showed a common-mode-rejection ratio (CMRR) of ~ 320 (~ 50 dB). The proposed analytical model of the CMRR was verified to be in good agreement with the experimental data. The successful implementation of the CNT diff-amp demonstrates a new way to achieve temperature and radiation tolerant VFE integrated microelectronics. 相似文献