全文获取类型
收费全文 | 30724篇 |
免费 | 3074篇 |
国内免费 | 2253篇 |
学科分类
工业技术 | 36051篇 |
出版年
2024年 | 67篇 |
2023年 | 482篇 |
2022年 | 700篇 |
2021年 | 940篇 |
2020年 | 1018篇 |
2019年 | 946篇 |
2018年 | 808篇 |
2017年 | 1051篇 |
2016年 | 1057篇 |
2015年 | 1093篇 |
2014年 | 1617篇 |
2013年 | 1884篇 |
2012年 | 2141篇 |
2011年 | 2333篇 |
2010年 | 1698篇 |
2009年 | 1756篇 |
2008年 | 1701篇 |
2007年 | 1898篇 |
2006年 | 1928篇 |
2005年 | 1555篇 |
2004年 | 1320篇 |
2003年 | 1242篇 |
2002年 | 988篇 |
2001年 | 905篇 |
2000年 | 848篇 |
1999年 | 702篇 |
1998年 | 567篇 |
1997年 | 524篇 |
1996年 | 410篇 |
1995年 | 389篇 |
1994年 | 286篇 |
1993年 | 256篇 |
1992年 | 229篇 |
1991年 | 177篇 |
1990年 | 140篇 |
1989年 | 134篇 |
1988年 | 94篇 |
1987年 | 42篇 |
1986年 | 21篇 |
1985年 | 15篇 |
1984年 | 21篇 |
1983年 | 15篇 |
1982年 | 12篇 |
1981年 | 14篇 |
1980年 | 3篇 |
1979年 | 11篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1959年 | 5篇 |
1951年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
32.
Although Mg alloy possesses high specific strength, low density, and good biocompatibility, poor corrosion resistance hinders its further applications. In the present study, an innovative protective layer against corrosion was prepared on the AZ31 Mg alloy via alkali pretreatment followed by vanillic acid treatment. The alkali pretreatment supplied –OH for the AZ31 Mg alloy surface to react with vanillic acid. The vanillic acid treatment played a crucial role in enhancing the corrosion resistance due to the excellent ability to act as a barrier and retard aqueous solution penetration, which effectively isolated the underlying Mg alloy from the corrosive environment. The corrosion current density of alkali and vanillic acid-treated Mg alloy (AZ31V) almost showed two orders of magnitude lower values in comparison with that of the AZ31 Mg alloy, and the corrosion potential of AZ31V Mg alloy increased from −1.41 to −1.25 V. The immersion tests proved that there was no occurrence of severe corrosion. Hence, the alkali pretreatment and vanillic acid treatment may represent a promising method to improve the corrosion resistance of Mg alloy. 相似文献
33.
The effect of the acidic hairy layer length on the interdiffusion of polymer between particles and as a consequence on the mechanical properties of the films produced from waterborne coatings has been studied. In order to isolate this effect, latexes with the same particle diameter and molecular weight but stabilized with poly(acrylic acid)-block-poly(butyl acrylate) (PAA-b-PBA) block copolymers of controlled and different lengths were prepared. Tensile strength measurements showed at the macroscopic level that the presence of AA chains in the particle surface reduced the mechanical properties of the films dried at room temperature, being its effect worse the longer the AA chain length. Higher annealing temperatures erased the negative effect of the acidic hairy layer on mechanical properties. The neutralization with NaOH instead of with NH4OH also led to worse mechanical properties. These macroscopic results were supported by Fluorescence Resonance Energy Transfer (FRET) experiments that showed that at the microscopic level, the extent of interdiffusion occurred slower when the AA chains in the particles surface increased, the annealing temperature was lower and when NaOH was used as neutralizing agent instead of NH4OH. 相似文献
34.
35.
36.
37.
38.
湘中海相浅层湘冷1井酸压工艺研究及应用 总被引:2,自引:0,他引:2
湘冷1井具有气层埋藏浅(400m-500m)、纵向上裂隙发育的特点,属低孔、低渗的含泥质灰岩致密储层。大量室内实验和研究采用“前置液酸压+闭合酸化”工艺技术,并优选了相应降阻酸和闭合酸配方。通过酸压实现了该井工业油气流的突破,取得了明显的增产效果,为新区海相浅层气的开发探索出了一套成熟的改造增产措施。 相似文献
39.
水驱后的残余油膜将贴于孔隙壁面,为壁面边界层流体,处于三相界面的包围和共同作用下。目前的调研结果显示,油膜的组成及力学特性沿孔壁方向上是变化的。在化学驱过程中油膜不仅受到驱油剂的驱替作用。还受驱油剂/油界面张力、油/固体界面张力及驱油剂/固体界面张力的阻碍作用。由于油膜组成、力学特性及相间作用的复杂性.以往对油膜驱替机理的研究都是从微观实验的角度对现象作观察和定性解释。本文根据水驱后残余油膜在油藏孔隙中存在的特点,提出了油膜驱替的简化模型。基于油膜驱替的微观实验,采用数值方法从定量的角度分别计算了不同黏弹性驱油剂在不同的驱油剂/原油界面张力作用下对残余油膜的驱替效率。考察了驱油剂的流变性、驱油剂/油界面张力、驱替速度等因素对驱替不同厚度油膜的影响规律。进一步探讨了驱油剂/原油界面特性和流变性在驱替残余油中的综合作用。研究结果表明,驱油剂对油膜的驱替始于驱油剂/油界面,要同时克服驱油剂/油界面约束力和油膜的屈服应力。降低界面张力、增强驱油剂的黏弹性、增加驱替速度,有利于提高驱替残余油膜的驱替效率。适当增加驱油刑的黏弹性可以放宽驱替薄油膜时对超低界面张力的要求。图13参9。 相似文献
40.
Barbara M. Johnston Peter R. Johnston 《International journal for numerical methods in engineering》2003,56(4):589-607
Accurate numerical evaluation of integrals arising in the boundary element method is fundamental to achieving useful results via this solution technique. In this paper, a number of techniques are considered to evaluate the weakly singular integrals which arise in the solution of Laplace's equation in three dimensions and Poisson's equation in two dimensions. Both are two‐dimensional weakly singular integrals and are evaluated using (in a product fashion) methods which have recently been used for evaluating one‐dimensional weakly singular integrals arising in the boundary element method. The methods used are based on various polynomial transformations of conventional Gaussian quadrature points where the transformation polynomial has zero Jacobian at the singular point. Methods which split the region of integration into sub‐regions are considered as well as non‐splitting methods. In particular, the newly introduced and highly accurate generalized composite subtraction of singularity and non‐linear transformation approach (GSSNT) is applied to various two‐dimensional weakly singular integrals. A study of the different methods reveals complex relationships between transformation orders, position of the singular point, integration kernel and basis function. It is concluded that the GSSNT method gives the best overall results for the two‐dimensional weakly singular integrals studied. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献