全文获取类型
收费全文 | 39375篇 |
免费 | 2124篇 |
国内免费 | 135篇 |
专业分类
电工技术 | 530篇 |
综合类 | 255篇 |
化学工业 | 7787篇 |
金属工艺 | 889篇 |
机械仪表 | 788篇 |
建筑科学 | 1444篇 |
矿业工程 | 137篇 |
能源动力 | 1383篇 |
轻工业 | 4867篇 |
水利工程 | 387篇 |
石油天然气 | 239篇 |
武器工业 | 6篇 |
无线电 | 3048篇 |
一般工业技术 | 6356篇 |
冶金工业 | 7842篇 |
原子能技术 | 304篇 |
自动化技术 | 5372篇 |
出版年
2023年 | 273篇 |
2022年 | 592篇 |
2021年 | 1111篇 |
2020年 | 810篇 |
2019年 | 926篇 |
2018年 | 1063篇 |
2017年 | 1114篇 |
2016年 | 1124篇 |
2015年 | 872篇 |
2014年 | 1324篇 |
2013年 | 2266篇 |
2012年 | 1856篇 |
2011年 | 2426篇 |
2010年 | 1733篇 |
2009年 | 1762篇 |
2008年 | 1695篇 |
2007年 | 1533篇 |
2006年 | 1241篇 |
2005年 | 1257篇 |
2004年 | 1264篇 |
2003年 | 1118篇 |
2002年 | 1073篇 |
2001年 | 858篇 |
2000年 | 692篇 |
1999年 | 683篇 |
1998年 | 2210篇 |
1997年 | 1464篇 |
1996年 | 1042篇 |
1995年 | 720篇 |
1994年 | 575篇 |
1993年 | 624篇 |
1992年 | 304篇 |
1991年 | 329篇 |
1990年 | 295篇 |
1989年 | 265篇 |
1988年 | 270篇 |
1987年 | 220篇 |
1986年 | 223篇 |
1985年 | 250篇 |
1984年 | 206篇 |
1983年 | 156篇 |
1982年 | 189篇 |
1981年 | 167篇 |
1980年 | 165篇 |
1979年 | 124篇 |
1978年 | 121篇 |
1977年 | 174篇 |
1976年 | 260篇 |
1975年 | 114篇 |
1973年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 359 毫秒
121.
In most used rotary valves in GM-type pulse-tube refrigerators the rotor makes heavy mechanical contact with the stator, so the valve is liable to wear, and large torques are needed. In this paper we will describe two types of valves, which have balanced forces on the rotor. In the first valve the rotor and the stator make no mechanical contact. The second type is a contact valve, like the classical valves, but the forces on the rotor are balanced in a different way. Therefore, these valves are less liable to wear, and the torques needed to rotate the valves are small. 相似文献
122.
Broersen P.M.T. de Waele S. Bos R. 《IEEE transactions on instrumentation and measurement》2004,53(4):981-986
Time series solutions for spectral analysis in missing data problems use reconstruction of the missing data, or a maximum likelihood approach that analyzes only the available measured data. Maximum likelihood estimation yields the most accurate spectra. An approximate maximum likelihood algorithm is presented that uses only previous observations falling in a finite interval to compute the likelihood, instead of all previous observations. The resulting nonlinear estimation algorithm requires no user-provided initial solution, is suited for order selection, and can give very accurate spectra even if less than 10% of the data remains. 相似文献
123.
Finite sample properties of ARMA order selection 总被引:3,自引:0,他引:3
Broersen P.M.T. de Waele S. 《IEEE transactions on instrumentation and measurement》2004,53(3):645-651
The cost of order selection is defined as the loss in model quality due to selection. It is the difference between the quality of the best of all available candidate models that have been estimated from a finite sample of N observations and the quality of the model that is actually selected. The order selection criterion itself has an influence on the cost because of the penalty factor for each additionally selected parameter. Also, the number of competitive candidate models for the selection is important. The number of candidates is, of itself, small for the nested and hierarchical autoregressive/moving average (ARMA) models. However, intentionally reducing the number of selection candidates can be beneficial in combined ARMA(p,q) models, where two separate model orders are involved: the AR order p and the MA order q. The selection cost can be diminished by creating a nested sequence of ARMA(r,r-1) models. Moreover, not evaluating every combination (p,q) of the orders considerably reduces the required computation time. The disadvantage may be that the true ARMA(p,q) model is no longer among the nested candidate models. However, in finite samples, this disadvantage is largely compensated for by the reduction in the cost of order selection by considering fewer candidates. Thus, the quality of the selected model remains acceptable with only hierarchically nested ARMA(r,r-1) models as candidates. 相似文献
124.
Suzana Corte Real Maria de Nazareth Leal de Meirelles 《Microscopy research and technique》1991,18(2):192-196
Immunocytochemistry was used to investigate the localization of F1 ATPase in mitochondria of cryosections of adult mouse heart muscle cells. The initial aldehyde fixation was the only denaturation step for antigens. The fine structure was preserved with contrast enhancement as the sections were maintained hydrated, with the advantage that the entire procedure is completed in one working day. The reaction was highly specific, and entire mitochondria were labeled with the Protein A-gold complex. A new analytical technique, electron spectroscopic imaging (ESI), contributed to a better visualization of the localization of the F1 factor. 相似文献
125.
Kahler Christopher W.; Read Jennifer P.; Wood Mark D.; Palfai Tibor P. 《Canadian Metallurgical Quarterly》2003,17(3):226
Structural equation modeling was used to test the mediational role of social environmental selection on alcohol use in cross-sectional samples of 447 students from a rural state university and 421 students from an urban private university. Results showed that male gender, White ethnicity, and sensation seeking were uniquely associated with greater alcohol use. Mediational analyses indicated that socioenvironmental factors (i.e., Greek involvement, friends' approval of drinking/getting drunk) were positively associated with alcohol use and significantly accounted for parts of the effects of ethnicity and sensation seeking, but not gender, on alcohol use. Results suggest that White students and those high on sensation seeking may drink more heavily in college, in part because they select social environments in which alcohol use is encouraged. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
126.
Christopher M. Gourlay Arne K. Dahle Hans I. Laukli 《Metallurgical and Materials Transactions A》2004,35(9):2881-2891
Banded defects are often found in high-pressure die castings. These bands can contain segregation, porosity, and/or tears,
and changing casting conditions and alloy are known to change the position and make-up of the bands. Due to the complex, dynamic
nature of the high-pressure die-casting (HPDC) process, it is very difficult to study the effect of individual parameters
on band formation. In the work presented here, bands of segregation similar to those found in cold-chamber HPDC aluminum alloys
were found in laboratory gravity die castings. Samples were cast with a range of fraction solids from 0 to 0.3 and the effect
of die temperature and external solid fraction on segregation bands was investigated. The results are considered with reference
to the rheological properties of the filling semisolid metal and a formation mechanism for bands is proposed by considering
flow past a solidifying immobile wall layer. 相似文献
127.
van Baaren Rick B.; Maddux William W.; Chartrand Tanya L.; de Bouter Cris; van Knippenberg Ad 《Canadian Metallurgical Quarterly》2003,84(5):1093
The present studies demonstrated the moderation of self-construal orientation on mimicry. Recent research has indicated that an interdependent self-construal is associated with assimilation of the other to the self whereas an independent self-construal is associated with minimizing the influence of others on the self (H. R. Markus & S. Kitayama, 1991; D. Stapel & W. Koomen, 2001). Therefore, the authors hypothesized that an interdependent self-construal would be associated with more mimicry than an independent self-construal. When self-construal orientations were experimentally primed. as in Studies 1 and 2, independent self-construals produced less nonconscious mimicry than interdependent self-construals. When self-construals were examined as cultural differences with either a chronically dominant independent (Americans) or interdependent (Japanese) construal of the self, these results were replicated. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
128.
129.
Critical Shear Stress of Bimodal Sediment in Sand-Gravel Rivers 总被引:1,自引:0,他引:1
A new model for the critical shear stress and the transport of graded sediment is presented. The model is based on the size distribution of the bed surface and can be used to compute sediment transport rates in numerical simulations with an active layer model. This model makes a distinction between unimodal and bimodal sediments. It is assumed that all size fractions of unimodal sediments have the same critical shear stress while there is selective transport for the gravel fractions of bimodal sediments. A recently published laboratory transport data set is used to calibrate our model. 相似文献
130.