全文获取类型
收费全文 | 30335篇 |
免费 | 2931篇 |
国内免费 | 1275篇 |
专业分类
电工技术 | 1754篇 |
综合类 | 1523篇 |
化学工业 | 5200篇 |
金属工艺 | 1864篇 |
机械仪表 | 2088篇 |
建筑科学 | 1804篇 |
矿业工程 | 794篇 |
能源动力 | 1131篇 |
轻工业 | 2290篇 |
水利工程 | 455篇 |
石油天然气 | 1329篇 |
武器工业 | 179篇 |
无线电 | 3986篇 |
一般工业技术 | 4750篇 |
冶金工业 | 1559篇 |
原子能技术 | 296篇 |
自动化技术 | 3539篇 |
出版年
2024年 | 127篇 |
2023年 | 566篇 |
2022年 | 998篇 |
2021年 | 1418篇 |
2020年 | 1065篇 |
2019年 | 917篇 |
2018年 | 1051篇 |
2017年 | 1035篇 |
2016年 | 1085篇 |
2015年 | 1215篇 |
2014年 | 1586篇 |
2013年 | 1883篇 |
2012年 | 2003篇 |
2011年 | 2250篇 |
2010年 | 1959篇 |
2009年 | 1836篇 |
2008年 | 1719篇 |
2007年 | 1472篇 |
2006年 | 1358篇 |
2005年 | 1251篇 |
2004年 | 919篇 |
2003年 | 903篇 |
2002年 | 873篇 |
2001年 | 771篇 |
2000年 | 654篇 |
1999年 | 636篇 |
1998年 | 582篇 |
1997年 | 460篇 |
1996年 | 394篇 |
1995年 | 340篇 |
1994年 | 273篇 |
1993年 | 187篇 |
1992年 | 153篇 |
1991年 | 100篇 |
1990年 | 97篇 |
1989年 | 84篇 |
1988年 | 55篇 |
1987年 | 40篇 |
1986年 | 36篇 |
1985年 | 26篇 |
1984年 | 27篇 |
1983年 | 19篇 |
1982年 | 15篇 |
1981年 | 27篇 |
1980年 | 15篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1976年 | 12篇 |
1973年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
含氮化合物对NiW体系催化剂芳烃加氢性能的影响 总被引:2,自引:0,他引:2
以γ-Al2O3、B改性的γ-Al2O3、F改性的γ-Al2O3、SiO2-Al2O3为载体制备Ni、W含量相同的四种催化剂,通过程序升温还原表征考察活性金属与不同载体的相互作用。利用氮含量不同、四氢萘含量相同的四种原料考察含氮化合物对同种NiW体系催化剂四氢萘加氢的影响以及对活性金属与载体相互作用不同的催化剂四氢萘加氢的影响。结果表明,以γ-Al2O3或SiO2-Al2O3载体制备的催化剂的金属组分与载体相互作用较强,B或F改性的γ-Al2O3能显著削弱活性金属与载体的相互作用;含氮化合物对四氢萘加氢具有强烈的抑制作用,使四氢萘加氢反应的表观活化能增加;在实验研究的四种催化剂中,金属组分与载体相互作用较弱的催化剂受含氮化合物的抑制较强。 相似文献
42.
43.
This paper addresses the problem of power control in a multihop wireless network supporting multicast traffic. We face the problem of forwarding packet traffic to multicast group members while meeting constraints on the signal-to-interference-plus-noise ratio (SINR) at the intended receivers. First, we present a distributed algorithm which, given the set of multicast senders and their corresponding receivers, provides an optimal solution when it exists, which minimizes the total transmit power. When no optimal solution can be found for the given set of multicast senders and receivers, we introduce a distributed, joint scheduling and power control algorithm which eliminates the weak connections and tries to maximize the number of successful multicast transmissions. The algorithm allows the other senders to solve the power control problem and minimize the total transmit power. We show that our distributed algorithm converges to the optimal solution when it exists, and performs close to centralized, heuristic algorithms that have been proposed to address the joint scheduling and power control problem. 相似文献
44.
Saehoon Ju Hyeongdong Kim Hyung-Hoon Kim 《Microwave and Wireless Components Letters, IEEE》2003,13(9):405-407
This letter presents a numerical dispersion relation for the two-dimensional (2-D) finite-difference time-domain method based on the alternating-direction implicit time-marching scheme (2-D ADI-FDTD). The proposed analytical relation for 2-D ADI-FDTD is compared with those relations in the previous works. Through numerical tests, the dispersion equation of this work was shown as correct one for 2-D ADI-FDTD. 相似文献
45.
46.
标准化的目的是“获得最佳秩序和社会效益”,而最佳秩序是企业进行高效率生产和管理的前提条件。能够更及时准确的对企业的生产标准、技术标准、安全标准、管理标准进行监督和管理工作;有助于更好的督促企业在每个环节都建立起互相适应、配套的标准体系,使企业生产活动和经营管理活动井然有序,避免混乱;能够进一步加强对认证及认证咨询工作的质量,提高认证企业质量体系运行的有效性。为此我们提出了建设《哈尔滨市企业产品执行标准数据库管理系统》,对企业的产品执行标准进行登记注册管理,这将有助于组织、监督标准实施工作;有助于组织制订地方标准;推行采用国际标准和国外先进标准;有助于协调指导企业产品标准的制订并管理其备案;组织重要标准的宣传贯彻,监督检查重要标准的实施情况。 相似文献
47.
To determine three‐dimensional fiber orientation states in injection‐molded short‐fiber composites, a confocal laser scanning microscope (CLSM) is used. Since the CLSM optically sections the specimen, more than two images of the cross sections on and below the surface of the composite can be obtained. Three‐dimensional fiber orientation states can be determined by using geometric parameters of fiber images obtained from two parallel cross sections. For experiments, carbon‐fiber‐reinforced polystyrene is examined by the CLSM and geometric parameters of fibers on each cross‐sectional plane are measured by an image analysis. In order to describe fiber orientation states compactly, orientation tensors are determined at different positions of the prepared specimen. Three‐dimensional orientation states are obtained without any difficulty by determining the out‐of‐plane angles utilizing fiber images on two parallel planes acquired by the CLSM. Orientation states are different at different positions and show the shell–core structure along the thickness of the specimen. Fiber orientation tensors are predicted by a numerical analysis and the numerically predicted orientation states show good agreement with measured ones. However, some differences are found at the end of cavity. They may result from the fountain flow effects, which are not considered in the numerical analysis. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 500–509, 2003 相似文献
48.
Boundary element analysis of thermal stress intensity factors for interface griffith and cusp cracks
Kang Yong Lee
Woon Cheon Baik
《Engineering Fracture Mechanics》1994,47(6):909-918The thermal stress intensity factors for interface cracks of Griffith and symmetric lip cusp types under vertical uniform heat flow in a finite body are calculated by the boundary element method. The boundary conditions on the crack surfaces are insulated or fixed to constant temperature. The relationship between the stress intensity factors and the displacements on the nodal point of a crack-tip element is derived. The numerical values of the thermal stress intensity factors for an interface Griffith crack in an infinite body are compared with the previous solutions. The thermal stress intensity factors for a symmetric lip cusp interface crack in a finite body are calculated with respect to various effective crack lengths, configuration parameters, material property ratios and the thermal boundary conditions on the crack surfaces. Under the same outer boundary conditions, there are no appreciable differences in the distribution of thermal stress intensity factors with respect to each material property. However, the effect of crack surface thermal boundary conditions on the thermal stress intensity factors is considerable. 相似文献
49.
介绍了一种日常参数智能控制器的研制及研制过程中的几个重要问题的解决方法。特别是用于城市照明及美化城市的彩灯的开、关时间智能控制方面,给出了依据当地绝对时间、经度、纬度进行昼夜时间计算的方法,使之在不接光电传感器情况下可跟踪季节的昼夜变化。控制系统采用89C2051单片机,配有时钟日历芯片DS12887,系统体积小、功能强、可靠性高。 相似文献
50.
Ki Hyun Kim Yong Hoon Kang Byoungho Lee 《Photonics Technology Letters, IEEE》1997,9(12):1610-1612
A photorefractive volume hologram was recorded and probed using light diffracted from a tapered optical fiber as a reference beam. A single-mode fiber (SMF) was chemically etched and tapered to give a complicated beam pattern, and it is shown that the tapered optical fiber can be utilized to increase the storage density of the volume hologram. Spatial selectivity of the volume hologram with this method was increased by two times compared to the normal SMF referencing, which is due to the fact that the complicated beam pattern has little correlation with its shifted version 相似文献