全文获取类型
收费全文 | 5182篇 |
免费 | 394篇 |
国内免费 | 161篇 |
专业分类
电工技术 | 96篇 |
综合类 | 305篇 |
化学工业 | 1645篇 |
金属工艺 | 196篇 |
机械仪表 | 91篇 |
建筑科学 | 258篇 |
矿业工程 | 121篇 |
能源动力 | 192篇 |
轻工业 | 1545篇 |
水利工程 | 72篇 |
石油天然气 | 162篇 |
武器工业 | 7篇 |
无线电 | 164篇 |
一般工业技术 | 457篇 |
冶金工业 | 149篇 |
原子能技术 | 86篇 |
自动化技术 | 191篇 |
出版年
2024年 | 18篇 |
2023年 | 63篇 |
2022年 | 117篇 |
2021年 | 127篇 |
2020年 | 149篇 |
2019年 | 165篇 |
2018年 | 146篇 |
2017年 | 195篇 |
2016年 | 149篇 |
2015年 | 195篇 |
2014年 | 251篇 |
2013年 | 383篇 |
2012年 | 349篇 |
2011年 | 414篇 |
2010年 | 325篇 |
2009年 | 324篇 |
2008年 | 272篇 |
2007年 | 342篇 |
2006年 | 263篇 |
2005年 | 224篇 |
2004年 | 197篇 |
2003年 | 190篇 |
2002年 | 161篇 |
2001年 | 83篇 |
2000年 | 94篇 |
1999年 | 82篇 |
1998年 | 68篇 |
1997年 | 50篇 |
1996年 | 47篇 |
1995年 | 47篇 |
1994年 | 31篇 |
1993年 | 33篇 |
1992年 | 36篇 |
1991年 | 26篇 |
1990年 | 21篇 |
1989年 | 16篇 |
1988年 | 18篇 |
1987年 | 16篇 |
1986年 | 5篇 |
1985年 | 15篇 |
1984年 | 13篇 |
1983年 | 4篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1951年 | 1篇 |
排序方式: 共有5737条查询结果,搜索用时 15 毫秒
41.
42.
An efficient method to monitor changes in soil inorganic N content during crop growth would be a useful means to guide N fertilization
to ensure high yields and low N losses to the environment. In this study, soil apparent electrical conductivity (ECa) measured by the widely used conductivity meter EM38 was tested as an indirect measurement of available N in spring barley
during two cropping seasons at two sites with morainic loam in SE Norway. The experiment was constructed to maximize soil
variation. In spite of the ȁ8noiseȁ9 caused by the soil heterogeneity, concentrations of inorganic N (cNinorg) or NO3-N were most strongly correlated with ECa in both years and at both locations (with one exception). The measurements of ECa reflected well the temporal variation in inorganic N content (Ninorg), and a ranking of the treatments based on ECa fitted very well with a ranking based on Ninorg at the first three sampling times after fertilizing. The best subset of sensor variables (i.e. variables which can be measured
ȁ8on-the-goȁ9 by sensor techniques in the field) described 27–69% (average 47%) of the variation in topsoil cNinorg. When expanding the regression models to include pH as well, the degree of explanation increased significantly. In conclusion,
the method of using ECa appears to be quite robust in terms of detecting relative differences in cNinorg, whereas a determination of absolute levels of cNinorg with the method is unreliable. 相似文献
43.
MODELING AND PREDICTIVE CONTROL OF MIMO NONLINEAR SYSTEMS USING WIENER-LAGUERRE MODELS 总被引:1,自引:0,他引:1
Prabirkumar Saha S. H. Krishnan V. S. R. Rao Sachin C. Patwardhan 《Chemical Engineering Communications》2004,191(8):1083-1119
In this work, a Weiner-type nonlinear black box model was developed for capturing dynamics of open loop stable MIMO nonlinear systems with deterministic inputs. The linear dynamic component of the model was parameterized using orthogonal Laguerre filters while the nonlinear state output map was constructed either using quadratic polynomial functions or artificial neural networks. The properties of the resulting model, such as open loop stability and steady-state behavior, are discussed in detail. The identified Weiner-Laguerre model was further used to formulate a nonlinear model predictive control (NMPC) scheme. The efficacy of the proposed modeling and control scheme was demonstrated using two benchmark control problems: (a) a simulation study involving control of a continuously operated fermenter at its optimum (singular) operating point and (b) experimental verification involving control of pH at the critical point of a neutralization process. It was observed that the proposed Weiner-Laguerre model is able to capture both the dynamic and steady-state characteristics of the continuous fermenter as well as the neutralization process reasonably accurately over wide operating ranges. The proposed NMPC scheme achieved a smooth transition from a suboptimal operating point to the optimum (singular) operating point of the fermenter without causing large variation in manipulated inputs. The proposed NMPC scheme was also found to be robust in the face of moderate perturbation in the unmeasured disturbances. In the case of experimental verification using the neutralization process, the proposed control scheme was found to achieve much faster transition to a set point close to the critical point when compared to a conventional gain-scheduled PID controller. 相似文献
44.
45.
本工作是在非离子凝胶的分子热力学模型基础,引入Donnan平衡项和大分子离子链的静电排斥项对凝胶自由能的贡献,建立了一个新的离子凝胶的分子热力学模型。该模型在解释离子凝胶的PH敏感性及其各种影响因素,以及预测PH溶胀曲线方面都取得了较好的效果。 相似文献
46.
Jeewoong Kim 《Cement and Concrete Research》2003,33(5):621-627
The ethylenediaminetetraacetic acid (EDTA) titration method limits the measurement of calcium concentration to 50 mg/l in water. However, various acids and salt solutions are used in the investigation of the durability of concrete, and the adaptability of the EDTA titration method to determine the calcium in these solutions must be investigated. Hence, the purpose of this study was to investigate the interfering effects of hydrochloric acid, sulfuric acid, sodium chloride and sodium sulfate on measuring the calcium in the aqueous leaching solutions using the EDTA titration method. The calcium standard solutions were prepared using CaCl2 with initial pH from −0.8 to 7 and calcium concentration up to 160 mg/l. Sulfuric acid and hydrochloric acid were used to prepare calcium solutions with lower pH. Salt solutions with 3% NaCl and 3% Na2SO4 were used to evaluate the interfering effects of salt elements such as sulfate and sodium on the EDTA titration. Results indicated that the EDTA titration method was reliable in measuring the calcium up to 160 mg/l, the maximum concentration investigated in this study, in hydrochloric acid with pH higher than zero and 3% sodium chloride solution. Sulfuric acid with pH higher than zero and 3% sodium sulfate solutions showed 3% to 4% less calcium in the solutions. Acid solutions with pH less than zero showed interference with calcium measurement. 相似文献
47.
48.
49.
In membrane filtration, solution environment factors such as pH and solvent density are important in controlling the filtration
rate and the rejection of the particles and/or the macromolecules. The filtration rate and the rejection in membrane filtration
have been investigated from physicochemical aspects. It was shown that the properties of the filter cake formed on the membrane
surface play a vital role in determining the filtration rate in mem-brane filtration. It was clearly demonstrated that such
filtration behaviors as the filtration rate and the rejection are highly dependent on the electrical nature of the particles
and/or the macromolecules. Furthermore, it was shown that the solvent density ρ has a large effect on the steady filtration
rate in upward ultrafiltration. 相似文献
50.
By means of contact angle measurements on dry layers of electrostatically neutral dextran with pure water (pH 6.1), water acidified with HCl (to pH 1.94) and water made alkaline with NaOH (to pH 12.8), it could be shown that there was essentially no change as a function of pH in the ratio of γ+/γ- of water as compared with the aqueous acid and alkaline solutions. (Here γ+ is the Lewis acid parameter of the polar surface tension component of water and γ- is its Lewis base parameter). In contrast, with contact angles measured with the same liquids on negatively charged clean glass, a significant decrease in contact angle was observed with water at pH 12.8, which was caused by the fact that at this alkaline pH an increase in surface hydrophilicity took place. This is because surfaces that have a given surface electrical potential at neutral pH generally acquire an even higher surface potential under more alkaline conditions which, concomitantly, also gives rise to an increase in surface hydrophilicity, and thus to lower contact angles with water. Finally, contact angles with acid water, pure water, and alkaline water, deposited on hydrophobic Parafilm surfaces, were exactly the same. 相似文献