全文获取类型
收费全文 | 50560篇 |
免费 | 4623篇 |
国内免费 | 1931篇 |
专业分类
电工技术 | 3378篇 |
技术理论 | 3篇 |
综合类 | 5406篇 |
化学工业 | 5297篇 |
金属工艺 | 1771篇 |
机械仪表 | 2867篇 |
建筑科学 | 6439篇 |
矿业工程 | 2749篇 |
能源动力 | 1494篇 |
轻工业 | 2773篇 |
水利工程 | 1984篇 |
石油天然气 | 1919篇 |
武器工业 | 453篇 |
无线电 | 4143篇 |
一般工业技术 | 3262篇 |
冶金工业 | 2982篇 |
原子能技术 | 422篇 |
自动化技术 | 9772篇 |
出版年
2025年 | 306篇 |
2024年 | 789篇 |
2023年 | 787篇 |
2022年 | 1020篇 |
2021年 | 1195篇 |
2020年 | 1359篇 |
2019年 | 1224篇 |
2018年 | 1121篇 |
2017年 | 1438篇 |
2016年 | 1726篇 |
2015年 | 1839篇 |
2014年 | 2751篇 |
2013年 | 2487篇 |
2012年 | 3062篇 |
2011年 | 3497篇 |
2010年 | 2775篇 |
2009年 | 2944篇 |
2008年 | 2775篇 |
2007年 | 3260篇 |
2006年 | 3048篇 |
2005年 | 2597篇 |
2004年 | 2224篇 |
2003年 | 2032篇 |
2002年 | 1656篇 |
2001年 | 1461篇 |
2000年 | 1160篇 |
1999年 | 965篇 |
1998年 | 707篇 |
1997年 | 632篇 |
1996年 | 557篇 |
1995年 | 489篇 |
1994年 | 397篇 |
1993年 | 274篇 |
1992年 | 247篇 |
1991年 | 167篇 |
1990年 | 171篇 |
1989年 | 125篇 |
1988年 | 98篇 |
1987年 | 101篇 |
1986年 | 56篇 |
1985年 | 163篇 |
1984年 | 260篇 |
1983年 | 303篇 |
1982年 | 207篇 |
1981年 | 317篇 |
1980年 | 130篇 |
1979年 | 46篇 |
1978年 | 41篇 |
1975年 | 24篇 |
1974年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
Tony Maillet Jacques Barbier Jr. Daniel Duprez 《Applied catalysis. B, Environmental》1996,9(1-4):251-266
A 1% Pd catalyst (38% dispersion) was prepared by impregnating a γ-alumina with palladium acetylacetonate dissolved in acetone. The behaviour of this catalyst in oxidation and steam reforming (SR) of propane was investigated. Temperature-programmed reactions of C3H8 with O2 or with O2 + H2O were carried out with different stoichiometric ratios S(S =[O2]/5[C3H8]). The conversion profiles of C3H8 for the reaction carried out in substoichiometry of O2 (S < 1) showed two discrete domains of conversion: oxidation at temperatures below 350°C and SR at temperatures above 350°C. The presence of steam in the inlet gases is not necessary for SR to occur: there is sufficient water produced in the oxidation to form H2 and carbon oxides by this reaction. Contrary to what was observed with Pt, an apparent deactivation between 310 and 385°C could be observed with Pd in oxidation. This is due to a reduction of PdOx into Pd0, which is much less active than the oxide in propane oxidation. Steam added to the reactants inhibits oxidation while it prevents the reduction of PdOx into Pd0. Compared to Pt and to Rh, Pd has a higher thermal resistance: no deactivation occurred after treatment up to 700°C and limited deactivation after treatment up to 900°C, provided that the catalyst is maintained in an oxygen-rich atmosphere during the cooling. 相似文献
2.
With the growing size and complexity of power systems, system analysis—such as transients calculation—takes much time. Hence, fast calculation methods are required. Although parallel processing is a hopeful method, there have been difficulties in the parallel solution of linear equations which appear in power-flow calculations by the Newton-Raphson method. This paper aims at the fast calculation of the power-flow problem by means of parallel processing. In order to improve the suitability to the parallel solution of the differential equation in transients calculation, we assume the use of a direct-mapping parallel processing machine to map directly the network of a power system onto a network of processors. Under this assumption, we propose a new parallel-processing-oriented method in which the linear equation is solved by linear iterations between nodes with Aitken acceleration. We simulate the method on three model power systems and compare this Parallel Iterative Method (PIN) with a Parallel Direct Method (PDM) which uses the banded matrix according to the number of operations required. As a result, we can expect that PIM may solve linear equations faster than PDM with m processors, although the PIM might be inferior to the PDM with m × m processors, where m denotes the half-band width of the banded matrix. 相似文献
3.
During the 1997 winter season, shipborne polarimetric backscatter measurements of Great Lakes (freshwater) ice types using the Jet Propulsion Laboratory C-band scatterometer, together with surface-based ice physical characterization measurements and environmental parameters, were acquired concurrently with Earth Resource Satellite 2 (ERS-2) and RADARSAT Synthetic Aperture Radar (SAR) data. This polarimetric data set, composed of over 20 variations of different ice types measured at incident angles from 0° to 60° for all polarizations, was processed to radar cross-section to establish a library of signatures (look-up table) for different ice types. The library is used in the computer classification of calibrated satellite SAR data. Computer analysis of ERS-2 and RADARSAT ScanSAR images of Great Lakes ice cover using a supervised classification technique indicates that different ice types in the ice cover can be identified and mapped, and that wind speed and direction can have an influence on the classification of water as ice based on single frequency, single polarization data. Once satellite SAR data are classified into ice types, the ice map provides important and necessary input for environmental protection and management, ice control and ice breaking operations, and ice forecasting and modeling efforts. 相似文献
4.
The 226Ra and 228Ra content of the lichens Umbilicaria cylindrica, U. deusta, U. murina and U. hirsuta has been determined as a function of the growth altitude above sea level, based on data derived from material collected from localities in southwest Poland. These data are interpreted as graphs and formulae to show the influence of environmental factors, particularly altitude, on the concentration of 226Ra and 228Ra, and the coefficients of accumulation calculated. 相似文献
5.
龙潭拱坝坝高90m,砼上坝运输是砼施工过程中的中间环节,控制进度的关键,必须因时因地制宜地选择适当的运输机械和运输方案,使砼运输能做到保质、保量、及时和经济,通过论证比较,结合本工程现实情况,砼上坝运输不采用缆机而采用门机方案. 相似文献
6.
A stepwise degradation of a kerogen concentrate of Green River oil shale was effected by oxidation with perchloric acid of varying boiling point and concentration. Dissolution of the kerogen concentrate occurred between 160 and 170°C. Several model compounds were also oxidized with perchloric acid. The model compounds were polyethylene, graphite, nylon, and polyethylene oxide. The results indicated that the kerogen was similar to polyethylene oxide. The results of this study and previous work were used to present a model for the structure of the kerogen of Green River oil shale. 相似文献
7.
A novel current source inverter system interconnected to the single‐phase grid is proposed. It has the same construction as the conventional three‐phase current source inverter that is interconnected to the single‐phase three‐wire distribution system. Though the proposed circuit has no output transformer, it can be equivalently performed as the single‐phase double cascade inverter by diverting the pole transformer in the utility system. By controlling the appropriate scheme, the output currents can be obtained as the five‐level waveforms and their distortions can be decreased sufficiently. It is applied to the interactive electric energy storage system with batteries and the basic discharging characteristics are discussed experimentally. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 150(2): 50–61, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10380 相似文献
8.
Krystal M. Lee 《Energy》1985,10(7):785-791
Low temperature corrosion from flue gases is a common problem in energy recovery systems. We present a simple computation procedure to estimate the dewpoint of combustion gases, which is the limiting design temperature to prevent corrosion. If the composition of fuel oil and the percentage of CO2 or O2 in the flue gases are known, the air ratio, percentage of H2O, p.p.m. of SO3 in the flue gas, and the dewpoint can be estimated with an error of 1.9%. The results indicate that the dewpoint is high because the combustion conditions are not appropriately adjusted, even if low sulfur fuel oil is used. We conclude that both the fuel quality and the combustion conditions are controlling variables in the design and operation of energy recovery systems. 相似文献
9.
L.Z. Gandelsman E.I. Mostoslavskaya L.A. Khomenko L.M. Yagupolskii 《Dyes and Pigments》1981,2(4):279-284
A number of new azo disperse dyes with polyfluoroalkyl groups attached to the nitrogen atom of the coupling component have been prepared. The influence of polyfluoroalkyl groups on the colour and properties of azo dyes has been studied. The replacement of hydrogen atoms of the methyl group of the coupling component by polyfluoroalkyl radicals causes a hypsochromic shift of absorption maximum in a neutral solution and deepening of the dye colour in an acid solution. All tested dyes showed a high lightfastness on acetate and polyamide fibres. 相似文献
10.
The performance of an experimental pilot-scale electrochemical reactor using a rotating cylindrical electrode equipped with wiper blades is described. Data obtained from monopolar depositing and bipolar stripping—depositing of copper from dilute aqueous electrolytes are presented and certain economic aspects of metal recovery are discussed. 相似文献