全文获取类型
收费全文 | 20498篇 |
免费 | 704篇 |
国内免费 | 21篇 |
专业分类
电工技术 | 215篇 |
综合类 | 20篇 |
化学工业 | 3766篇 |
金属工艺 | 322篇 |
机械仪表 | 404篇 |
建筑科学 | 1005篇 |
矿业工程 | 49篇 |
能源动力 | 501篇 |
轻工业 | 1675篇 |
水利工程 | 120篇 |
石油天然气 | 57篇 |
武器工业 | 1篇 |
无线电 | 1584篇 |
一般工业技术 | 3057篇 |
冶金工业 | 5416篇 |
原子能技术 | 188篇 |
自动化技术 | 2843篇 |
出版年
2023年 | 157篇 |
2022年 | 297篇 |
2021年 | 460篇 |
2020年 | 341篇 |
2019年 | 366篇 |
2018年 | 426篇 |
2017年 | 389篇 |
2016年 | 492篇 |
2015年 | 423篇 |
2014年 | 520篇 |
2013年 | 1072篇 |
2012年 | 856篇 |
2011年 | 1115篇 |
2010年 | 765篇 |
2009年 | 822篇 |
2008年 | 861篇 |
2007年 | 757篇 |
2006年 | 701篇 |
2005年 | 590篇 |
2004年 | 484篇 |
2003年 | 439篇 |
2002年 | 420篇 |
2001年 | 295篇 |
2000年 | 318篇 |
1999年 | 415篇 |
1998年 | 1465篇 |
1997年 | 882篇 |
1996年 | 685篇 |
1995年 | 434篇 |
1994年 | 396篇 |
1993年 | 408篇 |
1992年 | 195篇 |
1991年 | 177篇 |
1990年 | 160篇 |
1989年 | 156篇 |
1988年 | 145篇 |
1987年 | 136篇 |
1986年 | 128篇 |
1985年 | 151篇 |
1984年 | 133篇 |
1983年 | 122篇 |
1982年 | 103篇 |
1981年 | 115篇 |
1980年 | 108篇 |
1979年 | 99篇 |
1978年 | 96篇 |
1977年 | 176篇 |
1976年 | 313篇 |
1975年 | 83篇 |
1974年 | 72篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Martin J. Beckmann 《The Annals of Regional Science》1992,26(3):199-207
The question explored in this paper is whether the center attracts more spending by all agents in a spatial economy than do peripheral locations, and why this might be so. This is the macroeconomics pendant to the microeconomic concern with accessibility.This paper is the result of discussions on the subject of time and space in economics with my good friend, Tonu Puu. I dedicate it to the memory of another dear friend, Claude Ponsard. 相似文献
92.
H.W. Choi C.W. Jeon M.D. Dawson P.R. Edwards R.W. Martin 《Photonics Technology Letters, IEEE》2003,15(4):510-512
High-performance, two-dimensional arrays of parallel-addressed InGaN blue micro-light-emitting diodes (LEDs) with individual element diameters of 8, 12, and 20 /spl mu/m, respectively, and overall dimensions 490 /spl times/490 /spl mu/m, have been fabricated. In order to overcome the difficulty of interconnecting multiple device elements with sufficient step-height coverage for contact metallization, a novel scheme involving the etching of sloped-sidewalls has been developed. The devices have current-voltage (I-V) characteristics approaching those of broad-area reference LEDs fabricated from the same wafer, and give comparable (3-mW) light output in the forward direction to the reference LEDs, despite much lower active area. The external efficiencies of the micro-LED arrays improve as the dimensions of the individual elements are scaled down. This is attributed to scattering at the etched sidewalls of in-plane propagating photons into the forward direction. 相似文献
93.
94.
Signal classification through multifractal analysis and complex domain neural networks 总被引:1,自引:0,他引:1
W. Kinsner Vincent Cheung K. Cannons J. Pear T. Martin 《IEEE transactions on systems, man and cybernetics. Part C, Applications and reviews》2006,36(2):196-203
This paper describes a system capable of classifying stochastic self-affine nonstationary signals produced by nonlinear systems. The classification and the analysis of these signals are important because these are generated by many real-world processes. The first stage of the signal classification process entails the transformation of the signal into the multifractal dimension domain, through the computation of the variance fractal dimension trajectory (VFDT). Features can then be extracted from the VFDT using a Kohonen self-organizing feature map. The second stage involves the use of a complex domain neural network and a probabilistic neural network to determine the class of a signal based on these extracted features. The results of this paper show that these techniques can be successful in creating a classification system which can obtain correct classification rates of about 87% when performing classification of such signals without knowing the number of classes. 相似文献
95.
Reaction-induced phase separation (RIPS) of miscible blends of poly(?-caprolactone) (PCL) and an epoxy resin based on poly(propylene oxide) (PPO) was used to prepare thermoplastic vulcanizates (TPVs) with fine rubber dispersions. Scanning electron microscopy (SEM) confirmed the formation of cross-linked rubber particles dispersed in the thermoplastic matrix at PCL contents ≥20 wt%. The morphology development during phase separation was studied by optical microscopy (OM) and time-resolved small-angle light scattering (SALS). It was shown that higher curing temperatures lead to a decrease in rubber particle size, but at the same time lead to an increase in the extent of particle connectivity. In some cases, gelation of the PPO-rich phase limits full structure development, which leads to extensive connectivity between the dispersed rubber particles and a strong deterioration in tensile properties. 相似文献
96.
Harris Grant T.; Rice Marnie E.; Quinsey Vernon L.; Lalumière Martin L.; Boer Douglas; Lang Carol 《Canadian Metallurgical Quarterly》2003,15(3):413
Four actuarial instruments for the prediction of violent and sexual reoffending (the Violence Risk Appraisal Guide [VRAG], Sex Offender Risk Appraisal Guide [SORAG], Rapid Risk Assessment for Sex Offender Recidivism [RRASOR], and Static-99) were evaluated in 4 samples of sex offenders (N = 396). Although all 4 instruments predicted violent (including sexual) recidivism and recidivism known to be sexually motivated, areas under the receiver operating characteristic (ROC) were consistently higher for the VRAG and the SORAG. The instruments performed better when there were fewer missing items and follow-up time was fixed, with an ROC area up to .84 for the VRAG, for example, under such favorable conditions. Predictive accuracy was higher for child molesters than for rapists, especially for the Static-99 and the RRASOR. Consistent with past research, survival analyses revealed that those offenders high in both psychopathy and sexual deviance were an especially high-risk group. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
97.
Jan Paca Eva Klapkova Martin Halecky Kim Jones Carlos Riccardo Soccol 《Clean Technologies and Environmental Policy》2007,9(1):69-74
The removal of a mixture of painting solvents from waste air using a biofiltration process was evaluated in this project.
The pollutants removed included hydrophobic (aromatic hydrocarbons) and hydrophilic (water soluble ketones and esters) compounds.
A trickle bed reactor with a defined immobilized mixed culture on polyethylene Pall rings was utilized in this biodegradation
study. The removal efficiencies (RE) of the individual groups of pollutants during loading experiments were determined. An
increase of the aromatic hydrocarbons loading resulted in a drop of their REAROM with no effect on the RE value of ketones. The overloading of ketones caused a rapid drop in REAROM and a small drop in REKET. To achieve a restoration of the biocatalyst degradation properties after the increase in loading, an addition of phosphate
to the aqueous medium was implemented which successfully restored the removal efficiency. 相似文献
98.
99.
Norman Fenton Martin Neil William Marsh Peter Hearty Łukasz Radliński Paul Krause 《Empirical Software Engineering》2008,13(5):499-537
Standard practice in building models in software engineering normally involves three steps: collecting domain knowledge (previous
results, expert knowledge); building a skeleton of the model based on step 1 including as yet unknown parameters; estimating
the model parameters using historical data. Our experience shows that it is extremely difficult to obtain reliable data of
the required granularity, or of the required volume with which we could later generalize our conclusions. Therefore, in searching
for a method for building a model we cannot consider methods requiring large volumes of data. This paper discusses an experiment
to develop a causal model (Bayesian net) for predicting the number of residual defects that are likely to be found during
independent testing or operational usage. The approach supports (1) and (2), does not require (3), yet still makes accurate
defect predictions (an R
2 of 0.93 between predicted and actual defects). Since our method does not require detailed domain knowledge it can be applied
very early in the process life cycle. The model incorporates a set of quantitative and qualitative factors describing a project
and its development process, which are inputs to the model. The model variables, as well as the relationships between them,
were identified as part of a major collaborative project. A dataset, elicited from 31 completed software projects in the consumer
electronics industry, was gathered using a questionnaire distributed to managers of recent projects. We used this dataset
to validate the model by analyzing several popular evaluation measures (R
2, measures based on the relative error and Pred). The validation results also confirm the need for using the qualitative factors
in the model. The dataset may be of interest to other researchers evaluating models with similar aims. Based on some typical
scenarios we demonstrate how the model can be used for better decision support in operational environments. We also performed
sensitivity analysis in which we identified the most influential variables on the number of residual defects. This showed
that the project size, scale of distributed communication and the project complexity cause the most of variation in number
of defects in our model. We make both the dataset and causal model available for research use. 相似文献
100.