首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   109906篇
  免费   7481篇
  国内免费   2850篇
电工技术   4343篇
技术理论   4篇
综合类   3731篇
化学工业   19120篇
金属工艺   5272篇
机械仪表   6685篇
建筑科学   5399篇
矿业工程   1801篇
能源动力   3766篇
轻工业   7777篇
水利工程   1411篇
石油天然气   3037篇
武器工业   554篇
无线电   15943篇
一般工业技术   17344篇
冶金工业   8456篇
原子能技术   1403篇
自动化技术   14191篇
  2024年   313篇
  2023年   1464篇
  2022年   2506篇
  2021年   3698篇
  2020年   2741篇
  2019年   2524篇
  2018年   3029篇
  2017年   3134篇
  2016年   3311篇
  2015年   3557篇
  2014年   4846篇
  2013年   6744篇
  2012年   6977篇
  2011年   7837篇
  2010年   6445篇
  2009年   6482篇
  2008年   6068篇
  2007年   5444篇
  2006年   5071篇
  2005年   4302篇
  2004年   3615篇
  2003年   3303篇
  2002年   3339篇
  2001年   2757篇
  2000年   2381篇
  1999年   2325篇
  1998年   2982篇
  1997年   2060篇
  1996年   1791篇
  1995年   1482篇
  1994年   1096篇
  1993年   995篇
  1992年   737篇
  1991年   662篇
  1990年   554篇
  1989年   486篇
  1988年   420篇
  1987年   323篇
  1986年   310篇
  1985年   258篇
  1984年   226篇
  1983年   168篇
  1982年   167篇
  1981年   139篇
  1980年   134篇
  1979年   112篇
  1978年   98篇
  1977年   130篇
  1976年   164篇
  1975年   87篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
Multi-H phase-coded modulation (MHPM) is a bandwidth-efficient modulation scheme which offers substantial coding gain over conventional digital modulation schemes. MHPM with asymmetric modulation indices corresponding to the bipolar data +1 and -1 is considered, and numerical results for the minimum Euclidean distances are provided. It is shown that performance improvements on the error probability over conventional MHPM are gained with essentially the same bandwidth and a very slight modification in implementation. The upper bounds on the error probabilities as functions of observation intervals and received Eb/N0 are also investigated in detail. It is concluded that the concept of asymmetric modulation indices for MHPM is attractive for bandwidth and power-efficient modulation  相似文献   
82.
Survival of rotavirus on lettuce, radishes, and carrots was studied to evaluate the potential of rotavirus transmission by vegetables irrigated with wastewater. The vegetables were contaminated with rotavirus SA-11 and stored at 4°C and room temperature in covered and uncovered containers to simulate post harvest conditions. Virus decay rates were greater on radishes and carrots than lettuce. Decay rates of rotavirus on lettuce, radish, and carrot ranged from ?0·057 to ?0·479 (log10 pfu/day). Rotavirus SA-11 survived on lettuce, radish, and carrot for 25 to 30 days at 4°C but at room temperature survival was very different for the various vegetables varying from 5 to 25 days. Greatest survival was always observed on the lettuce. These data suggest that rotaviruses can survive long enough on contaminated vegetables as to be transmitted by this vehicle.  相似文献   
83.
The structure of sintered nickel composite plaque is characterized by an irregular pore structure with a wide distribution of pore sizes as well as a l  相似文献   
84.
In this paper, a feasibility study of a randomized space vector switching scheme for three-level inverters is reported. Based on the traditional space vector concept for pulse width modulation (PWM) inverters, the sequence of the space vector is randomized in this scheme. The overall vector, resulting from the randomization, is mathematically identical to that of the traditional space vector. It has been found that the randomized scheme can effectively spread the switching noise over a wide frequency range and at the same time keep the steady-state link capacitor voltage balanced. The proposed scheme has been implemented in real time in a digital controller and its features have been experimentally confirmed  相似文献   
85.
86.
This study examined the predictive efficacy of 10 rationally derived, archivally coded variables for assessing reoffense risk among extrafamilial child molesters. Follow-up data on 111 child molesters who were discharged from the Massachusetts Treatment Center between 1960 and 1984 were used. Degree of sexual preoccupation with children, paraphilias, and number of prior sexual offenses predicted sexual recidivism. Juvenile and adult antisocial behavior, paraphilias, and low amount of contact with children predicted nonsexual victim-involved and violent recidivism. Area under the receiver-operating characteristic curves in all three analyses indicated discrimination that was substantially better than chance, ranging from .73 to .79. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
87.
The widening of interest and scope of property management and maintenance has attracted a greater diversity of professional disciplines into facilities management. Dr Quah, coordinator of CIB W70 on Management, Maintenance and Modernization of Building facilities, expresses her personal views on the current fascination with the subject.  相似文献   
88.
89.
二次型时频分析技术的开发与应用   总被引:1,自引:1,他引:0  
基于二次型的时频分析是高分辨率时频分析技术的有效方法之一,该项技术能够有效反映时变信号的频谱特性,具有分辨率高、能量集中和跟踪瞬时频率的特点。从时变信号的特点出发,阐述了二次型时频分析技术的基本方法原理,采用正演模型分析验证了该方法的有效性,并在实际地震资料中应用于识别物源方向、划分沉积相带、预测储层结构以及相对厚度等诸多方面,均取得了较好的效果。  相似文献   
90.
为了将高纯锗(HPGe)探测器用于快定时的物理实验中,本实验研究厂影响高纯锗探测器定时谱仪时间性能的各种因素,找出了获得谱仪最小时间分辨的各种最佳参数。并建立了分辨时间为4.19ns,能量分辨率为20keV,微分非线性好于±2.0%,计效率可承受120×10 ̄3s ̄(-1)的时间微分扰动角关联谱仪。利用此谱仪,以 ̄(204)Bi为探针核,研究了Bi系高温超导的微观机理,取得了较好的结果。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号