全文获取类型
收费全文 | 23920篇 |
免费 | 2090篇 |
国内免费 | 1374篇 |
专业分类
电工技术 | 805篇 |
综合类 | 2997篇 |
化学工业 | 5028篇 |
金属工艺 | 2321篇 |
机械仪表 | 1109篇 |
建筑科学 | 1539篇 |
矿业工程 | 713篇 |
能源动力 | 864篇 |
轻工业 | 1753篇 |
水利工程 | 599篇 |
石油天然气 | 958篇 |
武器工业 | 135篇 |
无线电 | 1282篇 |
一般工业技术 | 2937篇 |
冶金工业 | 1304篇 |
原子能技术 | 278篇 |
自动化技术 | 2762篇 |
出版年
2024年 | 59篇 |
2023年 | 263篇 |
2022年 | 475篇 |
2021年 | 669篇 |
2020年 | 680篇 |
2019年 | 643篇 |
2018年 | 569篇 |
2017年 | 725篇 |
2016年 | 778篇 |
2015年 | 786篇 |
2014年 | 1255篇 |
2013年 | 1459篇 |
2012年 | 1641篇 |
2011年 | 1785篇 |
2010年 | 1274篇 |
2009年 | 1408篇 |
2008年 | 1264篇 |
2007年 | 1479篇 |
2006年 | 1472篇 |
2005年 | 1244篇 |
2004年 | 1070篇 |
2003年 | 967篇 |
2002年 | 805篇 |
2001年 | 642篇 |
2000年 | 590篇 |
1999年 | 540篇 |
1998年 | 411篇 |
1997年 | 372篇 |
1996年 | 339篇 |
1995年 | 314篇 |
1994年 | 313篇 |
1993年 | 207篇 |
1992年 | 179篇 |
1991年 | 166篇 |
1990年 | 136篇 |
1989年 | 101篇 |
1988年 | 83篇 |
1987年 | 49篇 |
1986年 | 31篇 |
1985年 | 30篇 |
1984年 | 28篇 |
1983年 | 22篇 |
1982年 | 23篇 |
1981年 | 9篇 |
1980年 | 4篇 |
1979年 | 10篇 |
1976年 | 2篇 |
1960年 | 1篇 |
1959年 | 2篇 |
1951年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
The high-temperature crystallization of poly(p-phenylene terephthalamide) (PPTA) from dilute organic solutions was achieved through the introduction of a non-solvent, or precipitating agent, at the desired crystallization temperature. The morphology and crystal structure were examined for crystals produced from PPTA polymer with two different molecular weights (Mw = 46000 and 3430 g mol−1), using transmission electron microscopy. For the high-molecular-weight polymer, ribbon-like crystals were produced, while the low-molecular-weight polymer yielded small needles or platelets. In both cases, electron diffraction showed that the Northolt allomorph was obtained. For the high-molecular-weight polymer, the molecular axes were parallel to the ribbon axes in a chain-extended type structure. A hypothesis for the orientation of the low-molecular-weight PPTA in the small platelets, is also given. 相似文献
62.
63.
水利部珠江水利委员会规划计划处 《中国水利》2007,(11):32-34
大藤峡水利枢纽是珠江流域西江干流上的控制性水利枢纽工程,在流域防洪及水资源方面具有不可替代的作用,同时具有发电、航运与灌溉等综合利用效益。国务院在对《珠江流域防洪规划》的批复中要求抓紧做好前期工作.积极促进大藤峡水利枢纽建设。本文对大藤峡水利枢纽及在珠江防洪与水资源综合利用中的地位与作用进行了简要的介绍。 相似文献
64.
A scheme to randomly generate dust points on the surface of a three-dimensional (3D) object is proposed. This scheme will potentially eliminate the bias of a density of points which are approximately equally spaced on the surface, as well as eliminate possible weighting requirements for areas of higher density. This method has the potential to reduce computational effort, as fewer points might be required to define the surface area. The global trial function vector can be assigned to these dust points and then entered into the complex variable boundary element method (CVBEM) commercial code. This scheme can be applied to each piece of a system that is comprised of a connected multiple geometry. A noticeable benefit of this approach is that the same problem can be studied several times, using a new random distribution of surface points each time. The smallest error can then be determined and that run selected. The Mathematica code used to generate the scheme is attached as an appendix. 相似文献
65.
66.
Influences of land use/cover on water quality in the upper and middle reaches of River Njoro, Kenya 总被引:1,自引:0,他引:1
William A. Shivoga Mucai Muchiri Samuel Kibichi Jethro Odanga Scott N. Miller Tracy J. Baldyga Eric M. Enanga Maina C. Gichaba 《Lakes & Reservoirs: Research and Management》2007,12(2):97-105
Data from 10 sampling sites along the River Njoro are used to examine the contribution of nutrients from upstream land uses draining each of the sampling sites. The data also are used to assess whether both the proportion of land uses and the size of the subwatersheds account for the variability in water quality in the River Njoro watershed. Geographical Information System analysis was used to determine the spatial distribution of land‐cover types and subwatersheds contributing run‐off to the sampling sites in the River Njoro. Standard Digital Elevation Model‐based routines were used to establish the watershed area contributing run‐off to each sampling site. Water and sediment samples were collected for chemical analysis, and the nutrient levels were related to the upstream land‐use types and the size of the subwatersheds. The mid‐stream portion of the River Njoro (near Egerton University) accounts for the highest nutrient contributions. The percentage contribution is magnified by additions from industrial, human settlements and agricultural land uses around the University. There is a significant decrease in nutrient levels downstream, however, indicating natural purification as the river flows through an area of large‐scale farming with intense, well‐preserved riparian and in‐stream vegetation. Steep slopes of the land upstream of Egerton University enhance erosion and nutrient losses from those subwatersheds. Mixed small‐scale agricultural and bare lands contribute over 55% of the phosphorus load to the upper and mid‐reaches of the River Njoro. The size of the subwatershed accounts for about 53% of the variability in the soluble phosphorus in the river. The land‐use subwatershed proportions are important for characterizing and modelling water quality in the River Njoro watershed. Upland land uses are as important as near‐stream land uses. We suggest that conservation of intact riparian corridor along the river and its tributaries contributes significantly to natural purification processes and recovery of the ecological integrity of the River Njoro ecosystem. 相似文献
67.
在氧化铝生产过程中,熟料折合比是烧结法一项重要的综合性指标,本文通过对熟料折合比的定义、计算公式的剖析,分析了生产过程中影响熟料折合比的主要因素,对有效降低熟料折合比的措施进行了探讨。 相似文献
68.
The viscometric, turbidimetric and potentiometric properties of copolymers of acrylamide (AM) with 2-acrylamido-2-methylpropanedimethylammonium chloride (AMPDAC) were studied in aqueous solutions. The AMPDAC polymers exhibit poor salt tolerance and large, negative viscosity/temperature coefficients. Furthermore, the polymers were found to be sensitive to changes in pH. The AMPDAC polymers undergo phase separation in the presence of dianions as a function of temperature and AMPDAC composition. 相似文献
69.
Songping Wu 《Materials Letters》2007,61(16):3526-3530
In this paper, non-agglomerated monodispersed ultra-fine copper metallic powders have been synthesized with chemical reduction method. Fine lead-free glass powders were also prepared by solid synthesis process. Thick film paste prepared by above-mentioned copper metallic powders and lead-free glass powders was applied as conductive paste of MLCC. Mixture of glass and zinc oxide give the thick film a high adhesion strength which is attributed to the rough interface from interfacial reaction between glass and chip, and a good densification. Diffusion of metal between copper thick film and nickel thick film is clear. Ni-Cu solid solution appears under high temperature firing. 相似文献
70.
外转子电机关键技术及解决措施 总被引:1,自引:0,他引:1
对外转子电机的关键技术和主要工艺问题进行了探讨分析,并提出了解决措施。 相似文献