全文获取类型
收费全文 | 284346篇 |
免费 | 3806篇 |
国内免费 | 857篇 |
专业分类
电工技术 | 5433篇 |
综合类 | 344篇 |
化学工业 | 42640篇 |
金属工艺 | 10731篇 |
机械仪表 | 8909篇 |
建筑科学 | 7176篇 |
矿业工程 | 1400篇 |
能源动力 | 6997篇 |
轻工业 | 24386篇 |
水利工程 | 2766篇 |
石油天然气 | 4146篇 |
武器工业 | 68篇 |
无线电 | 36303篇 |
一般工业技术 | 53711篇 |
冶金工业 | 51887篇 |
原子能技术 | 5662篇 |
自动化技术 | 26450篇 |
出版年
2021年 | 2420篇 |
2019年 | 2348篇 |
2018年 | 3848篇 |
2017年 | 3867篇 |
2016年 | 4177篇 |
2015年 | 2688篇 |
2014年 | 4607篇 |
2013年 | 12842篇 |
2012年 | 7411篇 |
2011年 | 9944篇 |
2010年 | 7812篇 |
2009年 | 8751篇 |
2008年 | 9716篇 |
2007年 | 9571篇 |
2006年 | 8683篇 |
2005年 | 7939篇 |
2004年 | 7553篇 |
2003年 | 7385篇 |
2002年 | 7003篇 |
2001年 | 6911篇 |
2000年 | 6521篇 |
1999年 | 6649篇 |
1998年 | 14621篇 |
1997年 | 10647篇 |
1996年 | 8534篇 |
1995年 | 6810篇 |
1994年 | 6118篇 |
1993年 | 5960篇 |
1992年 | 4931篇 |
1991年 | 4452篇 |
1990年 | 4418篇 |
1989年 | 4115篇 |
1988年 | 3922篇 |
1987年 | 3522篇 |
1986年 | 3378篇 |
1985年 | 4022篇 |
1984年 | 3813篇 |
1983年 | 3401篇 |
1982年 | 3190篇 |
1981年 | 3218篇 |
1980年 | 3032篇 |
1979年 | 2973篇 |
1978年 | 2775篇 |
1977年 | 3158篇 |
1976年 | 3748篇 |
1975年 | 2532篇 |
1974年 | 2511篇 |
1973年 | 2513篇 |
1972年 | 1997篇 |
1971年 | 1842篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
SAR-retrieved wind in polar regions-comparison with in situ data and atmospheric model output 总被引:1,自引:0,他引:1
Furevik B.R. Johannessen O.M. Sandvik A.D. 《Geoscience and Remote Sensing, IEEE Transactions on》2002,40(8):1720-1732
European remote sensing (ERS) satellites synthetic aperture radar (SAR) wind retrievals using CMOD-IFR2 are, for the first time, retrieved in the marginal ice zone (MIZ) and in Arctic coastal areas and compared with in situ observations from reseach vessels (RVs) and output from a high-resolution atmospheric model. The root mean squares (rms) of the comparisons were 1.6 m s/sup -1/ and 2 m s/sup -1/, respectively. The spatial variation of the SAR wind fields established a decrease in wind speed close to the ice edge for the late summer situations where the wind was along the ice edge with the ice to the left. This decrease is believed to be due to changes in atmospheric stability, possibly through development of an internal boundary layer caused by the cold ice cover and melt water. Lower wind speed near the ice edge is confirmed by the atmospheric model and the in situ observations. Furthermore, good results are obtained from SAR wind retrieval in leads when compared with model output during a cold-air outbreak. Routine measurements in the MIZ are useful for estimating the wind stress, and therefore SAR may play an important role in this region. Finally, the identification of a jet out from Hinlopen Strait in the Svalbard region and low wind wakes along the coast in the SAR-retrieved wind field is confirmed by in situ observations as the RV moves through the region. The jet is also confirmed by the atmospheric model, which is able to reproduce the situation. 相似文献
102.
A robust classification procedure based on mixture classifiers and nonparametric weighted feature extraction 总被引:1,自引:0,他引:1
Bor-Chen Kuo Landgrebe D.A. 《Geoscience and Remote Sensing, IEEE Transactions on》2002,40(11):2486-2494
There are many factors to consider in carrying out a hyperspectral data classification. Perhaps chief among them are class training sample size, dimensionality, and distribution separability. The intent of this study is to design a classification procedure that is robust and maximally effective, but which provides the analyst with significant assists, thus simplifying the analyst's task. The result is a quadratic mixture classifier based on Mixed-LOOC2 regularized discriminant analysis and nonparametric weighted feature extraction. This procedure has the advantage of providing improved classification accuracy compared to typical previous methods but requires minimal need to consider the factors mentioned above. Experimental results demonstrating these properties are presented. 相似文献
103.
Six months of data from the YSCAT94 experiment conducted at the CCIW WAVES research platform on Lake Ontario, Canada, are analyzed to evaluate a compound probability model. YSCAT was an ultrawideband small footprint (≈1 m) microwave scatterometer that operated at frequencies of 2-18 GHz, incidence angles from 0° to 60°, both h-pol and v-pol, and which tracked the wind using simultaneous weather measurements. The probability distribution function of the measured instantaneous backscattered amplitude (p(a)) is compared to theoretical distributions developed from-the composite model and a simple wave spectrum. Model parameters of the resulting Rayleigh/generalized lognormal distribution probability density function (pdf) (C, a1 , and a2) are derived directly from the data and are found to demonstrate relationships with wind speed, incidence angle, and radar frequency 相似文献
104.
This study contrasted six subscales of the Brief Psychiatric Rating Scale (BPRS) to determine their sensitivity to psychosocial treatment outcome. An expanded version of the BPRS was administered to 216 clients on admission to a day program. The subscale measuring hostility and suspiciousness discriminated at intake clients who were therapeutically discharged from clients who did not complete the program and predicted discharge status after the investigators controlled for the effects of demographic variables. Significant reductions in scores were obtained on five subscales for a subset of clients to whom the BPRS was readministered before discharge. The results support the use of the expanded BPRS as an evaluative tool in psychosocial rehabilitation programs. 相似文献
105.
O Doherty M Booth N Waran C Salthouse D Cuddeford 《Canadian Metallurgical Quarterly》1997,141(23):589-592
The energy expenditure and heart rate of five Shetland ponies were measured during transport and compared with the values recorded while they were at rest and walking. Secondary aims of the study were to investigate whether there was any correlation between heart rate and energy expenditure and whether different orientations affected the values to different degrees. The measurements were recorded at one-minute intervals while the ponies were at rest, walking and being transported in four different orientations, on journeys lasting 30 minutes. There were significant increases (P < 0.001) in both heart rate and energy expenditure during transport which were similar to those associated with walking, and there was a strong correlation between the two measurements. There was no conclusive evidence that orientation affected the measurements, but it appeared that rear facing transport might have had the least effect. 相似文献
106.
107.
Purified cell-envelope polyphosphatase as well as polyphoshatase activities of cytosol and isolated vacuoles, of nuclei and mitochondria of the yeast Saccharomyces cerevisiae were compared. The polyphosphatases of cell envelope and cytosol are similar, the polyphosphatases of nuclei, vacuoles and mitochondria differ in their kinetic properties, substrate specificity, requirements in divalent cations and in some effector actions both from these and from each other. 相似文献
108.
109.
This paper is concerned with partially observed stochastic optimal control problems when nonlinearities enter the dynamics of the unobservable state and the observations as gradients of potential functions. Explicit representations for the information state are derived in terms of a finite number of sufficient statistics. Consequently, the partially observed problem is recast as one of complete information with a new state generated by a modified version of the Kalman filter. When the terminal cost is quadratic in the unobservable state and includes the integral of the nonlinearities, the optimal control laws are explicitly computed, similar to linear-exponential-quadratic-Gaussian (LEQG) and linear-quadratic-Gaussian (LQG) tracking problems. The results are applicable to filtering and control of Hamiltonian systems 相似文献
110.
Lauren P.D. Nandhakumar N. 《IEEE transactions on pattern analysis and machine intelligence》1997,19(5):417-430
The ability to determine the viewing parameters of objects from their projections has enabled well established tomographic techniques to be employed in the 3D reconstruction of objects from images obtained via modalities where the orientation of the objects cannot be controlled. A method is described for the determination of the viewing parameters of randomly acquired projections of asymmetric objects. It extends upon the common lines algorithm by determining the relative orientation of projections from the location of lines of intersection among the Fourier transforms of the projections in 3D Fourier space. A new technique for finding the lines of intersection in the presence of translational displacement, and for subsequently finding the translational displacement, is presented. The complete algorithm is described and its efficacy is demonstrated using real data. A new technique for dealing with noise is also discussed 相似文献