全文获取类型
收费全文 | 249144篇 |
免费 | 18654篇 |
国内免费 | 10901篇 |
专业分类
电工技术 | 14985篇 |
技术理论 | 4篇 |
综合类 | 16322篇 |
化学工业 | 34527篇 |
金属工艺 | 15028篇 |
机械仪表 | 14418篇 |
建筑科学 | 16315篇 |
矿业工程 | 6078篇 |
能源动力 | 6160篇 |
轻工业 | 16153篇 |
水利工程 | 5465篇 |
石油天然气 | 8541篇 |
武器工业 | 2008篇 |
无线电 | 29407篇 |
一般工业技术 | 32580篇 |
冶金工业 | 9052篇 |
原子能技术 | 2178篇 |
自动化技术 | 49478篇 |
出版年
2024年 | 723篇 |
2023年 | 2594篇 |
2022年 | 5355篇 |
2021年 | 7542篇 |
2020年 | 5599篇 |
2019年 | 4327篇 |
2018年 | 18856篇 |
2017年 | 18685篇 |
2016年 | 14593篇 |
2015年 | 8105篇 |
2014年 | 9724篇 |
2013年 | 11469篇 |
2012年 | 16323篇 |
2011年 | 22961篇 |
2010年 | 20886篇 |
2009年 | 17473篇 |
2008年 | 18525篇 |
2007年 | 18873篇 |
2006年 | 10284篇 |
2005年 | 9140篇 |
2004年 | 6638篇 |
2003年 | 5849篇 |
2002年 | 5033篇 |
2001年 | 4072篇 |
2000年 | 3347篇 |
1999年 | 2564篇 |
1998年 | 1639篇 |
1997年 | 1328篇 |
1996年 | 1255篇 |
1995年 | 1044篇 |
1994年 | 855篇 |
1993年 | 529篇 |
1992年 | 433篇 |
1991年 | 325篇 |
1990年 | 246篇 |
1989年 | 211篇 |
1988年 | 154篇 |
1987年 | 97篇 |
1986年 | 87篇 |
1984年 | 37篇 |
1981年 | 40篇 |
1980年 | 41篇 |
1968年 | 50篇 |
1967年 | 37篇 |
1966年 | 46篇 |
1965年 | 45篇 |
1959年 | 45篇 |
1958年 | 38篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 937 毫秒
121.
122.
123.
The molecular design of short peptides to achieve a tailor-made functional architecture has attracted attention during the past decade but remains challenging as a result of insufficient understanding of the relationship between peptide sequence and assembled supramolecular structures. We report a hybrid-resolution model to computationally explore the sequence–structure relationship of self-assembly for tripeptides containing only phenylalanine and isoleucine. We found that all these tripeptides have a tendency to assemble into nanofibers composed of laterally associated filaments. Molecular arrangements within the assemblies are diverse and vary depending on the sequences. This structural diversity originates from (1) distinct conformations of peptide building blocks that lead to different surface geometries of the filaments and (2) unique sidechain arrangements at the filament interfaces for each sequence. Many conformations are available for tripeptides in solution, but only an extended β-strand and another resembling a right-handed turn are observed in assemblies. It was found that the sequence dependence of these conformations and the packing of resulting filaments are determined by multiple competing noncovalent forces, with hydrophobic interactions involving Phe being particularly important. The sequence pattern for each type of assembly conformation and packing has been identified. These results highlight the importance of the interplay between conformation, molecular packing, and sequences for determining detailed nanostructures of peptides and provide a detailed insight to support a more precise design of peptide-based nanomaterials. 相似文献
124.
Jialong Tian M. Babar Shahzad Wei Wang Lichang Yin Zhouhua Jiang Ke Yang 《材料科学技术学报》2018,34(9):1671-1675
The effect of Co addition on the formation of Ni-Ti clusters in maraging stainless steel was studied by three dimensional atom probe (3DAP) and first-principles calculation. The cluster analysis based on the maximum separation approach showed an increase in size but a decrease in density of Ni-Ti clusters with increasing the Co content. The first-principles calculation indicated weaker Co-Ni (Co-Ti) interactions than Co-Ti (Fe-Ti) interactions, which should be the essential reason for the change of distribution characteristics of Ni-Ti clusters in bcc Fe caused by Co addition. 相似文献
125.
Antti Voss Niko Hänninen Mohammad Pour-Ghaz Marko Vauhkonen Aku Seppänen 《Materials and Structures》2018,51(3):68
The development of visualizing tools to monitor unsaturated moisture flow in cement-based materials is of great importance, as most degradation processes in cement-based materials are connected to and take place in the presence moisture. This paper investigates the ability of electrical capacitance tomography (ECT) to image two-dimensional (2D) unsaturated moisture flow in cement-based materials. In ECT, the electrical permittivity distribution within an object is reconstructed based on measured capacitances between electrodes attached on the object’s surface. In a series of experiments, mortar specimens with and without discrete cracks were imaged with ECT during a 2D moisture ingress. The results show that ECT is able to monitor the evolution of the moisture flow, and to approximate the shape and position of the moisture front. These findings indicate that ECT is a viable method for monitoring and visualizing 2D unsaturated moisture flow in cement-based materials in the presence and absence of discrete cracks. 相似文献
126.
Yejun Wu Bowen Yan Juan Zhou Huizhang Lian Xiaojun Yu Jianxin Zhao Hao Zhang Wei Chen Daming Fan 《Journal of food science》2020,85(10):3282-3292
127.
An agriglass composition containing different oxides acts as a slow release for macro and micro nutrients and was chosen to improve maize yield under most important abiotic stresses which affecting agriculture development; salinity and drought. A field experiment was performed in salt affected soil (EC =?7.5 dSm??1) by using different water deficit rates (I1 = 100, I2 = 85 and I3 = 70% of maize water requirements). Irrigation levels were located in main plots. Every main-plot divided into six sub-plots contained glassy fertilizer treatments [F1 = 55 kg fed?1 with 1/2 mm diameter of agriglass (fed. =?4200 m2), F2 = 55 kg fed?1 with 1 mm diameter, F3 = 80 kg fed?1 with 1/2 mm diameter, F4 = 80 kg fed?1 with 1 mm diameter, F5 = Recommendations of Ministry of Agriculture and F6 = control]. The experimental results demonstrated that, ears, straw, grains and biological yields increased with increasing both water and agriglass rates. Application of agriglass as a slow release fertilizer improved yield more than mineral fertilizer. Some growth parameters, water use efficiency (IWUE), macronutrients concentration and their relations were included. Other studies on residual effect of agriglass and the annual application rates to withstand salinity and drought stress by strategic crops are required. 相似文献
128.
Ahmed Obaid M. Alzahrani M. Sh. Abdel-wahab Meshari Alayash M. S. Aida 《Journal of Materials Science: Materials in Electronics》2018,29(19):16317-16324
In this work, p-NiO/n-ZnO heterostructures were successfully prepared at room temperature using RF sputtering technique. The influence of ZnO layer thickness on the performance of the heterojunction was investigated. The deposited ZnO layers have a hexagonal Wurtzite structure with preferable growth orientations along (002) and (103) for thinner films. Increasing the thickness results in more crystallographic orientation randomness. The current–voltage measurements of the realized heterojunctions showed a clear rectifying behavior. The measured ideality factor varies from 2.5 to 1.6 according to the thickness of ZnO layer. The series resistance of the device is enlarged with increasing ZnO thickness. The deduced parameters from the I–V characteristics suggest that 200 nm is the optimal thickness of the ZnO layer according to our experimental conditions. We attribute the relatively better performance of this thickness to achieving reasonable compensation between serial resistance and ideality factor. The best heterojunction was tested and successfully used as a UV detector. 相似文献
129.
Video transmission over IEEE 802.11e wireless networks still shows poor performance for large bandwidth demand and frequently changed environments. Thus, several enhancements of IEEE 802.11e were proposed. On the other hand, big frames and simultaneous sending of adjacent frames always cause packet dropping for buffer overflow. In the past, we proposed an IEEE 802.11e enhancement named DFAA and a content aware mechanism to solve the above problems. The motivation of this paper is to find a proper way to integrate these two mechanisms. A DFAA enhancement (DFAA-E) is proposed to make up the insufficiency of content aware mechanism. Experiments results show that the combination of DFAA-E and content aware mechanism improves the video decoded quality greatly. And its performance can be further enhanced by selecting the suitable settings of certain parameters. 相似文献