全文获取类型
收费全文 | 73142篇 |
免费 | 7677篇 |
国内免费 | 4089篇 |
专业分类
电工技术 | 4860篇 |
技术理论 | 3篇 |
综合类 | 5728篇 |
化学工业 | 11866篇 |
金属工艺 | 3991篇 |
机械仪表 | 4634篇 |
建筑科学 | 5888篇 |
矿业工程 | 2056篇 |
能源动力 | 2289篇 |
轻工业 | 5548篇 |
水利工程 | 1497篇 |
石油天然气 | 4154篇 |
武器工业 | 804篇 |
无线电 | 8532篇 |
一般工业技术 | 8522篇 |
冶金工业 | 2793篇 |
原子能技术 | 1045篇 |
自动化技术 | 10698篇 |
出版年
2024年 | 315篇 |
2023年 | 1292篇 |
2022年 | 2546篇 |
2021年 | 3640篇 |
2020年 | 2605篇 |
2019年 | 2179篇 |
2018年 | 2297篇 |
2017年 | 2559篇 |
2016年 | 2251篇 |
2015年 | 3153篇 |
2014年 | 3865篇 |
2013年 | 4486篇 |
2012年 | 4887篇 |
2011年 | 5288篇 |
2010年 | 4663篇 |
2009年 | 4411篇 |
2008年 | 4297篇 |
2007年 | 3900篇 |
2006年 | 3809篇 |
2005年 | 3201篇 |
2004年 | 2314篇 |
2003年 | 2345篇 |
2002年 | 2645篇 |
2001年 | 2311篇 |
2000年 | 1689篇 |
1999年 | 1634篇 |
1998年 | 1134篇 |
1997年 | 960篇 |
1996年 | 945篇 |
1995年 | 730篇 |
1994年 | 633篇 |
1993年 | 487篇 |
1992年 | 322篇 |
1991年 | 251篇 |
1990年 | 187篇 |
1989年 | 169篇 |
1988年 | 162篇 |
1987年 | 100篇 |
1986年 | 66篇 |
1985年 | 35篇 |
1984年 | 26篇 |
1983年 | 18篇 |
1982年 | 25篇 |
1981年 | 17篇 |
1980年 | 21篇 |
1979年 | 16篇 |
1972年 | 1篇 |
1971年 | 2篇 |
1959年 | 7篇 |
1951年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
81.
82.
Three-dimensional models, or pharmacophores, describing Euclidean constraints on the location on small molecules of functional
groups (like hydrophobic groups, hydrogen acceptors and donors, etc.), are often used in drug design to describe the medicinal
activity of potential drugs (or ‘ligands’). This medicinal activity is produced by interaction of the functional groups on
the ligand with a binding site on a target protein. In identifying structure-activity relations of this kind there are three
principal issues: (1) It is often difficult to “align” the ligands in order to identify common structural properties that
may be responsible for activity; (2) Ligands in solution can adopt different shapes (or `conformations’) arising from torsional
rotations about bonds. The 3-D molecular substructure is typically sought on one or more low-energy conformers; and (3) Pharmacophore
models must, ideally, predict medicinal activity on some quantitative scale. It has been shown that the logical representation
adopted by Inductive Logic Programming (ILP) naturally resolves many of the difficulties associated with the alignment and
multi-conformation issues. However, the predictions of models constructed by ILP have hitherto only been nominal, predicting
medicinal activity to be present or absent. In this paper, we investigate the construction of two kinds of quantitative pharmacophoric
models with ILP: (a) Models that predict the probability that a ligand is “active”; and (b) Models that predict the actual
medicinal activity of a ligand. Quantitative predictions are obtained by the utilising the following statistical procedures
as background knowledge: logistic regression and naive Bayes, for probability prediction; linear and kernel regression, for
activity prediction. The multi-conformation issue and, more generally, the relational representation used by ILP results in
some special difficulties in the use of any statistical procedure. We present the principal issues and some solutions. Specifically,
using data on the inhibition of the protease Thermolysin, we demonstrate that it is possible for an ILP program to construct
good quantitative structure-activity models. We also comment on the relationship of this work to other recent developments
in statistical relational learning.
Editors: Tamás Horváth and Akihiro Yamamoto 相似文献
83.
The estimation of the link capacity and its available bandwidth in an end-to-end path is crucial for network management, admission
control and flow control for adaptive applications. This paper introduces an estimation mechanism able to accurately estimate
the available bandwidth of all links in an end-to-end path, through its capacity and cross-traffic estimation. The estimation
procedures resort to the dispersions of packet pairs and trains using the concepts of ICMP Timestamp and Traceroute that,
efficiently used together, enable the measurement of the dispersions in all links in the path. These mechanisms were evaluated
through simulation experiments, where we analyzed the influence of the several network parameters on each estimation mechanism.
The results show that these estimation methods are able to accurately estimate both the capacity and the cross-traffic of
all links in a path with moderate length and with low overhead. 相似文献
84.
85.
水力脉动冲击钻井工具初步研究与试验 总被引:1,自引:0,他引:1
掘力脉动冲击钻井是将水力脉冲、机械冲击振动结合为一体提高钻井速度的一种方法,介绍了辽河油田研制的脉动冲击钻井工具的结构、工作原理及现场试验情况。在潜山混合花岗岩、砂岩,硬脆性泥岩等不同地层的现场试验表明:水力脉动工具工作平稳,未出现蹩跳钻现象,也没有出现脱、落、断、掉等问题,主体安全性、可靠性较好;在潜山混舍花岗岩、砂岩、硬脆性泥岩地层机械钻速提高幅度较大,但在部分泥岩地层效果不佳。试验中发现密封圈耐温性差、脉冲腔易磨损等问题,并据此提出了水力脉动冲击工具的下一步研究方向。 相似文献
86.
87.
研究了厚膜永磁阵列微致动器中的磁场分布,并研究了永磁阵列单元几何尺寸对微致动器电磁力的影响。结果表明,厚膜永磁阵列单元高宽比和磁体单元间隔对微致动器电磁力影响较大磁徕单元高宽比为0.7是一个比较合适的尺寸。 相似文献
88.
随着光电器件的广泛应用,微机械光开关成为核心光交换器件的主流。在研发过程中,其器件检测手段成为人们所关注的话题。本文介绍了一种新颖的测量平台,通过高幅值利用单片机控制脉冲频率的方法来选择器件。与当前同类方法相比,具有精度高、可靠性强、成本低、易操作等特点,具有广阔的使用前景。 相似文献
89.
90.