全文获取类型
收费全文 | 238824篇 |
免费 | 22516篇 |
国内免费 | 12327篇 |
专业分类
电工技术 | 57577篇 |
技术理论 | 14篇 |
综合类 | 19819篇 |
化学工业 | 23757篇 |
金属工艺 | 13356篇 |
机械仪表 | 9922篇 |
建筑科学 | 13206篇 |
矿业工程 | 6456篇 |
能源动力 | 11774篇 |
轻工业 | 12612篇 |
水利工程 | 6080篇 |
石油天然气 | 9006篇 |
武器工业 | 1945篇 |
无线电 | 24124篇 |
一般工业技术 | 22625篇 |
冶金工业 | 8919篇 |
原子能技术 | 5026篇 |
自动化技术 | 27449篇 |
出版年
2024年 | 772篇 |
2023年 | 2242篇 |
2022年 | 4389篇 |
2021年 | 5362篇 |
2020年 | 6271篇 |
2019年 | 5319篇 |
2018年 | 5106篇 |
2017年 | 6356篇 |
2016年 | 7378篇 |
2015年 | 8534篇 |
2014年 | 14416篇 |
2013年 | 13257篇 |
2012年 | 15909篇 |
2011年 | 17191篇 |
2010年 | 12890篇 |
2009年 | 14507篇 |
2008年 | 14060篇 |
2007年 | 16311篇 |
2006年 | 14719篇 |
2005年 | 12224篇 |
2004年 | 10242篇 |
2003年 | 9199篇 |
2002年 | 8187篇 |
2001年 | 6724篇 |
2000年 | 6402篇 |
1999年 | 5434篇 |
1998年 | 4191篇 |
1997年 | 3677篇 |
1996年 | 3771篇 |
1995年 | 3563篇 |
1994年 | 3182篇 |
1993年 | 1967篇 |
1992年 | 1909篇 |
1991年 | 1311篇 |
1990年 | 990篇 |
1989年 | 868篇 |
1988年 | 776篇 |
1987年 | 481篇 |
1986年 | 297篇 |
1985年 | 451篇 |
1984年 | 499篇 |
1983年 | 493篇 |
1982年 | 399篇 |
1981年 | 451篇 |
1980年 | 308篇 |
1979年 | 130篇 |
1978年 | 131篇 |
1977年 | 81篇 |
1976年 | 50篇 |
1975年 | 63篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
131.
A metal matrix composite has been obtained by a novel synthesis route, reacting Al3Ti and graphite at 1000 °C for about 1 min after ball-milling and compaction. The resulting composite is made of an aluminium matrix reinforced by nanometer sized TiC particles (average diameter 70 nm). The average TiC/Al ratio is 34.6 wt.% (22.3 vol.%). The microstructure consists of an intimate mixture of two domains, an unreinforced domain made of the Al solid solution with a low TiC reinforcement content, and a reinforced domain. This composite exhibits uncommon mechanical properties with regard to previous micrometer sized Al–TiC composites and to its high reinforcement volume fraction, with a Young’s modulus of ∼110 GPa, an ultimate tensile strength of about 500 MPa and a maximum elongation of 6%. 相似文献
132.
This paper deals with the investigation of the effect of hygrothermal conditions on the bending of nanoplates using Levy type solution model employing the state-space concept. The nanoplates are assumed to be subjected to a hygrothermal environment. The two-unknown function plate theory is used to derive the governing differential equations on the basis of Eringen's nonlocal elasticity theory. The governing equations contain the small scale effect as well as hygrothermal and mechanical effects. These equations are converted into a set of first-order linear ordinary differential equations with constant coefficients. Analytical solution of bending response for nanoplates under combinations of simply supported, clamped and free boundary conditions is obtained. Comparison of the results with those being in the open literature is made. The influences played by small scale parameter, temperature rise, the degree of moisture concentration, boundary conditions, plate aspect ratio and side-to-thickness ratio are studied. 相似文献
133.
Porous polyimide (PI) films with low dielectric constants and excellent thermal properties have been a pressing demand for the next generation of high-performance, miniature, and ultrathin microelectronic devices. A series of novel porous PI films containing fluorenyl-adamantane groups were prepared successfully via thermolysis of poly(ethylene glycol) (PEG) added in the PI matrix. The cross-sectional morphologies of porous PI films showed closed pores with diameters ranging from 135 to 158 nm, which were uniform and regular in shape without interconnectivity. These porous PI films exhibited excellent thermal properties with a glass-transition temperature at 376 °C whereas the 5% weight loss temperature in air excess of 405 °C due to enhanced rigidity afforded by fluorenyl-adamantane groups. Accompanied by thermolysis content of PEG increasing from 0 to 20 wt %, the density of porous PI films decreased, and the corresponding porosity grew significantly from 0 to 11.48%. Depending on porosity, the dielectric constant and dielectric loss of porous PI films significantly declined from 2.89 to 2.37 and from 0.050 to 0.021, respectively. These excellent properties benefit the as-prepared porous PI films for application as interlayer dielectrics, integrated circuit chips, or multichip modules in microelectronic fields. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47313. 相似文献
134.
ABSTRACTThe RF output power dissipated per unit area is calculated using Runge-Kutta method for the high-moderate-moderate-high (n+-n-p-p+) doping profile of double drift region (DDR)-based impact avalanche transit time (IMPATT) diode by taking different substrate at Ka band. Those substrates are silicon, gallium arsenide, germanium, wurtzite gallium nitride, indium phosphide and 4H-silicon carbide. A comparative study regarding power dissipation ability by the IMPATT using different material is being presented thereby modelling the DDR IMPATT diode in a one-dimensional structure. The IMPATT based on 4H-SiC element has highest power density in the order of 1010 Wm?2 and the Si-based counterpart has lowest power density of order 106 Wm?2 throughout the Ka band. So, 4H-SiC-based IMPATT should be preferable over others for the power density preference based application. This result will be helpful to estimate the power density of the IMPATT for any doping profile and to select the proper element for the optimum design of the IMPATT as far as power density is concerned in the Ka band. Also, we have focused on variation of power density with different junction temperatures and modelled the heat sink with analysis of thermal resistances. 相似文献
135.
高浓度聚合物驱提高采收率方法实验研究 总被引:3,自引:0,他引:3
为探索一种新的提高原油采收率的方法,在外形尺寸为4.5 cm×4.5 cm×30 cm、气测渗透率0.9~1.0μm2、变异系数为0.72的二维纵向非均质人造岩心上,模拟大庆油田油藏流体性质及温度条件,研究了高浓度聚合物(HPAM)驱注入时机、聚合物相对分子质量、聚合物段塞体积及段塞组合对驱油效果的影响。通过注入大分子量、高浓度聚合物,结合合理的注入方式,在化学剂成本与三元复合驱相当的情况下,采收率比水驱提高20个百分点以上,接近或超过三元复合驱的水平。实验表明,采用高浓度聚合物驱油是一种较好的提高原油采收率的方法。 相似文献
136.
北斗三星无源定位技术 总被引:6,自引:2,他引:4
介绍了北斗双星定位系统的特点、功能、系统组成和工作原理,说明了北斗有源定位方式在应用方面的局限性。针对北斗有源定位方式不能无线电静默,和人们对具有无线电隐蔽性的卫星定位的需求,详细介绍一种北斗三星无源定位技术:包括工作原理、实现方法、定位精度分析和目前达到的定位精度。阐述了北斗三星无源定位技术的优点和应用形势。 相似文献
137.
138.
鄂尔多斯盆地三叠系延长组层序地层与生储盖组合特征 总被引:14,自引:2,他引:12
鄂尔多斯盆地三叠系延长组地层层序划分方案较多。在V ail经典层序地层学理论指导下,综合利用露头、岩芯、钻井、地震和古生物等资料,通过古水深分析和层序界面的识别与划分,认为将鄂尔多斯盆地三叠系延长组划分为7个三级层序更为合理,指出层序4水进体系域(生)+层序5低位体系域(储)+层序5水进体系域(盖)和层序4水进体系域(生)+层序4低位体系域(储)+层序4水进体系域(盖)是相对较佳的2套生储盖组合,并指出了鄂尔多斯盆地延长组最主要的2套生储盖勘探目的层。 相似文献
139.
多带OFDM-UWB系统峰均功率比降低方法研究 总被引:1,自引:0,他引:1
针对多带OFDM—UWB信号存在高峰均功率比的问题,提出了利用扩展与交织降低系统信号峰均功率比的方法。该方法通过对传输数据进行正交扩展与交织,使得进入多载波调制的数据趋于高斯分布,减小了传输数据自相关函数的旁瓣峰值,降低了OFDM-UWB信号的峰均功率比。由于采用正交矩阵进行扩展,扩展前后的数据传输速率保持不变。仿真结果表明,扩展与交织可以有效地降低信号峰均功率比2-5dB左右。同时该方法还具有抗窄带干扰的鲁棒性。 相似文献
140.