全文获取类型
收费全文 | 17262篇 |
免费 | 1606篇 |
国内免费 | 1090篇 |
专业分类
电工技术 | 608篇 |
综合类 | 1761篇 |
化学工业 | 1257篇 |
金属工艺 | 700篇 |
机械仪表 | 1067篇 |
建筑科学 | 4416篇 |
矿业工程 | 1295篇 |
能源动力 | 663篇 |
轻工业 | 276篇 |
水利工程 | 758篇 |
石油天然气 | 3015篇 |
武器工业 | 193篇 |
无线电 | 520篇 |
一般工业技术 | 2145篇 |
冶金工业 | 407篇 |
原子能技术 | 141篇 |
自动化技术 | 736篇 |
出版年
2024年 | 45篇 |
2023年 | 204篇 |
2022年 | 402篇 |
2021年 | 513篇 |
2020年 | 577篇 |
2019年 | 489篇 |
2018年 | 447篇 |
2017年 | 565篇 |
2016年 | 696篇 |
2015年 | 648篇 |
2014年 | 1063篇 |
2013年 | 1045篇 |
2012年 | 1486篇 |
2011年 | 1575篇 |
2010年 | 1139篇 |
2009年 | 1132篇 |
2008年 | 968篇 |
2007年 | 1060篇 |
2006年 | 970篇 |
2005年 | 871篇 |
2004年 | 644篇 |
2003年 | 569篇 |
2002年 | 509篇 |
2001年 | 393篇 |
2000年 | 372篇 |
1999年 | 292篇 |
1998年 | 272篇 |
1997年 | 198篇 |
1996年 | 187篇 |
1995年 | 147篇 |
1994年 | 134篇 |
1993年 | 63篇 |
1992年 | 50篇 |
1991年 | 35篇 |
1990年 | 38篇 |
1989年 | 23篇 |
1988年 | 22篇 |
1987年 | 21篇 |
1986年 | 15篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 9篇 |
1980年 | 11篇 |
1979年 | 6篇 |
1959年 | 4篇 |
1956年 | 2篇 |
1955年 | 2篇 |
1951年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Woong-Kee Loh 《计算机、材料和连续体(英文)》2021,66(2):1251-1267
Many database applications currently deal with objects in a metric space. Examples of such objects include unstructured multimedia objects and points of interest (POIs) in a road network. The M-tree is a dynamic index structure that facilitates an efficient search for objects in a metric space. Studies have been conducted on the bulk loading of large datasets in an M-tree. However, because previous algorithms involve excessive distance computations and disk accesses, they perform poorly in terms of their index construction and search capability. This study proposes two efficient M-tree bulk loading algorithms. Our algorithms minimize the number of distance computations and disk accesses using FastMap and a space-filling curve, thereby significantly improving the index construction and search performance. Our second algorithm is an extension of the first, and it incorporates a partitioning clustering technique and flexible node architecture to further improve the search performance. Through the use of various synthetic and real-world datasets, the experimental results demonstrated that our algorithms improved the index construction performance by up to three orders of magnitude and the search performance by up to 20.3 times over the previous algorithm. 相似文献
2.
水平井压裂技术在低渗透及非常规储层中得到了广泛应用,压裂后水平井的产能预测关系到油田开发方案的制定,因此,国内外学者对压后渗流模型的建立和求解方法做出了不懈的努力。本文详细回顾了国内外学者所建压裂水平井产能预测模型及求解方法,指出了不同模型和求解方法的优缺点,并展望了压裂水平井产能模型的发展方向。 相似文献
3.
《Geotextiles and Geomembranes》2020,48(6):897-911
Dynamic responses of the geosynthetic-encased stone column (GESC) supported embankment under traffic loads have become a hot topic. This study investigates the responses of GESC improved ground under vertical cyclic loading. A series of laboratory tests in a designed model test tank have been carried out with different loading parameters (varied loading amplitudes and frequencies), different column dimensions (varied encasement lengths and column diameters). In the tests, the soil-column stress distribution, accumulated settlement of loading plate, excess pore water pressure in the surrounding soil and lateral bulging of the stone column are monitored. Experimental results indicate that the vertical stress on the stone column increases with the increment of encasement length, and decreases with the increment of column diameter, loading amplitude and loading frequency. The increasing stress on the surrounding soil leads to a greater accumulated settlement of the loading plate and excess pore water pressure, while the increasing stress on the column leads to larger lateral bulging of the column. Excess pore water pressure dissipates effectively through vertical and horizontal drainage channels provided by the stone column and the sand bed. The geosynthetic encasement prevents the clay from obstructing the drainage channel by filtration and guarantees the drainage effect. 相似文献
4.
《Ceramics International》2020,46(15):24213-24224
We report an experimental approach, designed based on the recent findings that domain switching in ferroelectric ceramics can be separated into three regimes during antiparallel electric field loading, to investigate the influence of domain switching process on the electrical fatigue behavior of ferroelectrics. Uniaxial compressive stress (−2 MPã -100 MPa) and thermal loading (20 °C–150 °C) were used to tune the domain switching process. Under the same loading condition, the bipolar electrical fatigue behavior of soft lead zirconate titanate ceramics was systematically characterized. The amplitude and frequency of the applied electric field are 2 kV/mm and 10 Hz, respectively. By analyzing the evolution of the domain switching process, combined with the measured polarization and strain response, as well as the cracks observed on the surface of the specimen, it is found that the fatigue of ferroelectric ceramics was mainly related to the domain switching process near the coercive electric field: the regime 2 defined in this paper. The underlying mechanism was further discussed by considering the interplay between the domain switching process with the main factors affecting the electrical fatigue of ferroelectrics, namely defect redistribution, charge carrier injection, and crack initiation. 相似文献
5.
《Geotextiles and Geomembranes》2021,49(5):1256-1269
This study investigates the seismic performance of geosynthetic-reinforced modular block retaining walls backfilled with cohesive, fine grained clay-sand soil mixture. Shaking table tests were performed for three ½ scaled (wall height 190 cm) and ¼ scaled model walls to investigate the effects of backfill type, the influence of reinforcement length and reinforcement stiffness effects. The El Centro and Kobe earthquake records of varying amplitudes were used as base acceleration. Displacement of the front wall, accelerations at different locations, strains on the reinforcements, and the visual observations of the facing and the backfill surface were used to evaluate the seismic performance of model walls. The model walls were subjected to rigorous shaking and the walls did not exhibit any stability problems or signs of impending failure. The maximum deformations observed on the models with cohesive backfill was less than half of the deformation of the sand model. The load transfers between the geogrid and cohesive soil was comparable to that of sand and hence the needed reinforcement length was similar as well. As a result; the model walls with cohesive backfills performed within acceptable limits under seismic loading conditions when compared with granular backfilled counterparts. 相似文献
6.
7.
8.
高温高压下甲烷在油里的溶解度大,水包油乳化钻井液中存在油相,钻井作业中钻井液静置期间如起下钻、工具故障抢修等时间过长,储层中的甲烷气体会扩散到井眼里。在钻高含硫化氢及二氧化碳气井时,高温高压条件下天然气更有可能到达与油完全互溶的状态。文章将储层内的水平段井眼的气体扩散区域划分为4个区,外滤饼区、内滤饼区、滤液滞留区、滤液未污染区,根据外滤饼区、内滤饼区、滤液滞留区不同计算参数条件,计算预测了在使用水包油乳化钻井液情况下,储层中天然气通过扩散穿过3个区后,进入储层内井眼段环空导致的环空含气质量浓度变化,对环空内含气质量浓度随扩散时间、含油浓度因素的变化进行了定量计算。用伽略金有限元方法求解,计算表明,气体扩散量随钻井液静置时间延长而增加,水基钻井液加入油后与不加油的钻井液相比增大了气体扩散量。 相似文献
9.
大位移井钻柱振动规律研究与应用 总被引:5,自引:0,他引:5
在大位移井钻井工程中钻柱受着各种振动,当各种振动共振时,钻柱承受着非常大的动载,很容易受损,同时,钻柱的振动影响着井壁的稳定,严重威胁着井下的作业安全。以大位移井近井底的钻柱为研究对象,分析了钻柱振动行为所遵循的规律。采用线性分析法,获得了大位移井钻柱横向振动和扭转振动的频谱方程。同时获得了钻柱各种共振频率的分布。经分析表明:大位移井钻柱的横向振动和扭转振动的共振频率与实际钻井转速非常接近,应合理选择转速以减小钻柱共振的发生。文章的分析研究为大位移井钻柱动态行为的研究提供了理论依据。 相似文献
10.