全文获取类型
收费全文 | 2867篇 |
免费 | 19篇 |
国内免费 | 2篇 |
专业分类
电工技术 | 19篇 |
综合类 | 5篇 |
化学工业 | 161篇 |
金属工艺 | 8篇 |
机械仪表 | 17篇 |
建筑科学 | 42篇 |
矿业工程 | 2篇 |
能源动力 | 28篇 |
轻工业 | 74篇 |
水利工程 | 1篇 |
石油天然气 | 7篇 |
无线电 | 123篇 |
一般工业技术 | 87篇 |
冶金工业 | 2215篇 |
原子能技术 | 25篇 |
自动化技术 | 74篇 |
出版年
2021年 | 18篇 |
2020年 | 7篇 |
2019年 | 4篇 |
2018年 | 11篇 |
2017年 | 14篇 |
2016年 | 11篇 |
2015年 | 5篇 |
2014年 | 18篇 |
2013年 | 23篇 |
2012年 | 16篇 |
2011年 | 32篇 |
2010年 | 19篇 |
2009年 | 31篇 |
2008年 | 30篇 |
2007年 | 35篇 |
2006年 | 30篇 |
2005年 | 25篇 |
2004年 | 32篇 |
2003年 | 24篇 |
2002年 | 32篇 |
2001年 | 20篇 |
2000年 | 25篇 |
1999年 | 84篇 |
1998年 | 683篇 |
1997年 | 413篇 |
1996年 | 248篇 |
1995年 | 160篇 |
1994年 | 118篇 |
1993年 | 121篇 |
1992年 | 41篇 |
1991年 | 40篇 |
1990年 | 37篇 |
1989年 | 30篇 |
1988年 | 28篇 |
1987年 | 32篇 |
1986年 | 37篇 |
1985年 | 32篇 |
1984年 | 8篇 |
1983年 | 11篇 |
1982年 | 14篇 |
1981年 | 24篇 |
1980年 | 20篇 |
1979年 | 8篇 |
1978年 | 11篇 |
1977年 | 62篇 |
1976年 | 124篇 |
1975年 | 6篇 |
1974年 | 4篇 |
1970年 | 6篇 |
1969年 | 3篇 |
排序方式: 共有2888条查询结果,搜索用时 15 毫秒
1.
2.
J Brake TJ Walsh CE Benton JN Petitte R Meijerhof G Pe?alva 《Canadian Metallurgical Quarterly》1997,76(1):144-151
The relative importance of fat and lean tissue mass in determining bone mineral mass among postmenopausal women was examined in this 1-year longitudinal study. Fifty postmenopausal Caucasian women entered the study; 45 of them completed a 1-year follow-up. Dual-energy X-ray absorptiometry was employed for measuring total and regional bone mineral density (BMD) and bone mineral content (BMC), fat tissue mass (FTM), lean tissue mass (LTM), and body weight. Results from linear regression analysis using the cross-sectional data (n = 50) of the study indicated that LTM explained a larger percentage of variation in bone mineral mass than did FTM. FTM and LTM were found to be moderately correlated (r = 0.55); when FTM was entered in the same predicting regression models, LTM was a significant predictor (p < 0.05) of the total and regional BMC, but not BMD. The percent FTM (and inversely %LTM) was correlated with BMD and BMC, but significant correlation was primarily found only for total body BMD (or BMC). Weight was the best predictor of total body BMD and BMC. Longitudinally (n = 45), annual changes in both FTM and weight were significantly associated with annual changes in regional BMD after adjustment for initial bone mineral values (p < 0.05). We conclude that bone mineral mass is more closely related to LTM than to FTM, while annual changes in regional BMD are more closely correlated with changes in FTM in healthy postmenopausal women. Meanwhile, increased body weight is significantly associated with increased bone mineral mass. 相似文献
3.
DC Reynolds CE Leak KK Bajaj CE Stutz RL Jones KR Evans PW Yu WM Theis 《Canadian Metallurgical Quarterly》1989,40(9):6210-6217
4.
5.
In this article we survey a number of predeployed secure key distribution (PSKD) schemes proposed in the technical literature. We also propose a new time-based PSKD (TPSKD), which operates under the assumption of loose time synchronization, and discuss the performance of the scheme. Since the TPSKD scheme uses time information, which would typically already be available in sensor nodes, the cost of the scheme?s implementation is low. 相似文献
6.
C. N. Haas B. F. Severin D. Roy R. S. Engelbrecht A. Lalchandani S. Farooq 《Water research》1985,19(3):323-329
This paper summarizes studies on the presence of acid-fast and yeast organisms in wastewater and water treatment plants and in surface water. These organisms were found to satisfy three of Bonde's criteria for indicator organisms: presence whenever pathogens are likely to be present; resistance at least equal to that of pathogens; and lack of regrowth in the post-treatment environment. This, plus prior data, indicates that these organisms are at least as acceptable as indicators of disinfection efficiency than the coliform group. 相似文献
7.
Awareness of the construction environment can be improved by automatic three-dimensional (3D) sensing and modeling of job sites in real time. Commercially available 3D modeling approaches based on range scanning techniques are capable of modeling static objects only, and thus cannot model dynamic objects in real time in an environment comprised of moving humans, equipment, and materials. Emerging prototype video range cameras offer an alternative by facilitating affordable, wide field of view, dynamic object tracking at frame rates better than 1?Hz (real time). This paper describes a methodology to model, detect, and track the position of static and moving objects in real time, based on data obtained from video range cameras. Experiments with this technology have produced results that indicate that video rate 3D data acquisition and analysis of construction environments can support effective modeling, detection, and tracking of project resources. This approach to job site awareness has inherent value and broad application. In combination with effective management practices and other sensing techniques, this technology has the potential to significantly improve safety on construction job sites. 相似文献
8.
Summary The laminar flow near an infinite plane wall perpendicular to a line sink of constant strength is investigated in the limit of large Reynolds numbers. Self-similarity requires that fluid is issuing from the boundary layer. The inviscid flow outside the boundary layer is governed by the Euler equations. A one-parametric set of solutions to the Euler equations with appropriate boundary conditions is given. Uniqueness of the inviscid flow solution is obtained from matching with the boundary layer expansion. The solution of the boundary-layer equations is given both in closed form and numerically. It is found that at the edge of the boundary layer the vorticity decays algebraically.Dedicated to Prof. Dr. Dr. h. c. Franz Ziegler on the occasion of his 60th birthday 相似文献
9.
Laura M. Haas Michael J. Carey Miron Livny Amit Shukla 《The VLDB Journal The International Journal on Very Large Data Bases》1997,6(3):241-256
In this paper, we re-examine the results of prior work on methods for computing ad hoc joins. We develop a detailed cost model for predicting join algorithm performance, and we use the model to develop cost formulas
for the major ad hoc join methods found in the relational database literature. We show that various pieces of “common wisdom” about join algorithm
performance fail to hold up when analyzed carefully, and we use our detailed cost model to derive op
timal buffer allocation schemes for each of the join methods examined here. We show that optimizing their buffer allocations
can lead to large performance improvements, e.g., as much as a 400% improvement in some cases. We also validate our cost model's
predictions by measuring an actual implementation of each join algorithm considered. The results of this work should be directly
useful to implementors of relational query optimizers and query processing systems.
Edited by M. Adiba. Received May 1993 / Accepted April 1996 相似文献
10.
Minimum onset latency (Lmin) of single- and multiple-unit responses were mapped in the primary auditory cortex (AI) of barbiturate-anesthetized cats. Contralateral Lmin for multiple units was non-homogeneously distributed along the dorso-ventral/isofrequency axis of the AI. Responses with shorter latencies were more often located in the central, more sharply tuned region while longer latencies were more frequently encountered in the dorsal and ventral portions of the AI. For single units, a large scatter of Lmin values was found throughout the extent of the AI including cortical depth. The relationship between Lmin and previously reported spectral, intensity and temporal parameters was analyzed and revealed statistically significant correlations between minimum onset latency and the following response properties in some but not all studied animals: sharpness of tuning of a frequency response area 10 dB above threshold, broadband transient response, strongest response level, monotonicity of rate/level functions, dynamic range, and preferred frequency modulation sweep direction. This analysis suggests that Lmin is determined by several independent factors and that the prediction of Lmin based on relationships with other spectral and temporal response properties is inherently weak. The spatial distribution and the functional relationship between these response parameters may provide an important aspect of the time-based cortical representation of specific features in the animal's natural environment. 相似文献