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   48378篇
  免费   4951篇
  国内免费   3126篇
电工技术   2621篇
技术理论   1篇
综合类   4606篇
化学工业   8365篇
金属工艺   3298篇
机械仪表   2116篇
建筑科学   4360篇
矿业工程   2185篇
能源动力   2075篇
轻工业   3631篇
水利工程   1510篇
石油天然气   2725篇
武器工业   635篇
无线电   4686篇
一般工业技术   4944篇
冶金工业   3365篇
原子能技术   1140篇
自动化技术   4192篇
  2024年   185篇
  2023年   622篇
  2022年   1230篇
  2021年   1481篇
  2020年   1661篇
  2019年   1386篇
  2018年   1421篇
  2017年   1778篇
  2016年   1948篇
  2015年   1959篇
  2014年   3034篇
  2013年   3238篇
  2012年   3594篇
  2011年   3706篇
  2010年   2834篇
  2009年   2792篇
  2008年   2536篇
  2007年   3128篇
  2006年   2651篇
  2005年   2312篇
  2004年   2002篇
  2003年   1725篇
  2002年   1499篇
  2001年   1301篇
  2000年   1106篇
  1999年   908篇
  1998年   768篇
  1997年   672篇
  1996年   566篇
  1995年   486篇
  1994年   392篇
  1993年   294篇
  1992年   284篇
  1991年   203篇
  1990年   177篇
  1989年   139篇
  1988年   96篇
  1987年   65篇
  1986年   58篇
  1985年   46篇
  1984年   38篇
  1983年   21篇
  1982年   20篇
  1981年   13篇
  1980年   19篇
  1979年   4篇
  1978年   8篇
  1975年   10篇
  1959年   8篇
  1951年   9篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
介绍了PHS网呼入成功率和短消息流程,依据短消息模型和性能统计结果分析了群发短信对PHS网呼入成功率的影响,并建议选择合理的时间进行短消息群发以减少对网络负荷的影响。  相似文献   
82.
软件无线电中采样率转换技术的实现及在3G中的应用   总被引:1,自引:0,他引:1  
季薇  杨震 《电子工程师》2004,30(9):17-20
目前移动通信标准繁多,且新旧体制混杂,很难做到统一.为了实现互连互通,目前比较可行的方法是采用软件无线电.采样率转换技术是实现互连互通的关键.文中针对第三代移动通信系统(3G)中不同通信体制的带宽、速率要求,根据分级变换的思想,对3G中的CDMA2000和TDSCDMA的采样率转换方案进行了设计,并对3G中的5种制式的采样率转换方案进行了分析比较.实验表明,这一方案可以在采样率变换的高效网络结构基础上进一步降低滤波运算量和存储量.  相似文献   
83.
As the surface properties of the drying materials are very important not only for the drying rate but also for the quality change during drying, the effects of surface concentration on the drying behavior of liquid foods (sugar solutions) were investigated by isothermal drying experiments and by numerical calculation experiments. The isothermal drying experiments with gelled sugar solution systems (sucrose and maltodextrin) were carried out at various relative humidity (RH) values (RH = 0 to 84%). Separate experiments were carried out for determination of the desorption isotherms.

The isothermal drying curves of sugar solutions at RH = 0 to 51% were very similar. Numerical simulations also showed that the drying curves of these sugars at the surface concentration = 0 and 0.1 are almost the same, although the concentration distributions are different.

When a small amount of gelatin was added to sugar solutions, the drying rate decreased remarkably as the gelatin might form a thin film (skin) near the surface, and consequently the retention of ethanol increased.  相似文献   
84.
This research reports age and gender differences in cardiac reactivity and subjective responses to the induction of autobiographical memories related to anger, fear, sadness, and happiness. Heart rate (HR) and subjective state were assessed at baseline and after the induction of each emotion in 113 individuals (61 men, 52 women; 66% European American, 34% African American) ranging in age from 15 to 88 years (M = 50.0; SD = 20.2). Cardiac reactivity was lower in older individuals; however, for anger and fear, these age effects were significantly more pronounced for the women than the men. There were no gender differences in subjective responses, however, suggesting that the lower cardiac reactivity found among older people is dependent on gender and the specific emotion assessed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
85.
The main objective of the present work is to modify the traditional mapping method for the simulation of distributive mixing of multiphase flows in geometries involving moving parts such as, internal mixers or twin-screw extruders without a limitation on their geometrical periodicity. The periodicity condition, limits the results of traditional mapping method to tracking mapping mesh between specific discrete time intervals or distances for that geometry is repeated, hence, result is only for fixed orientation of rotors. Imaginary domain method is introduced to track mapping mesh from one state to the next free of geometrical periodicity limitations. In this work the method is introduced and its applicability and accuracy is discussed in details. A two-dimensional (2D) simulation of mixing of two Newtonian fluids with different viscosities in an intermeshing internal mixer is used as a test case study. In this example the key issues of ability to predict mixing state in details for all orientations of rotors is presented. To reduce diffusion errors of mapping method in the boundaries of the rotors, mapping mesh refinement technique that relies upon one single reference mesh is also presented.  相似文献   
86.
This paper investigates the effects of processing and equipment parameters of a ribbon blender (i.e. loading method of lubricant, fill level, blade speed and blade design) on magnesium stearate homogeneity. A core sampling technique is used to obtain at least one hundred samples per sampling event, which are extracted throughout the blender and yield a thorough characterization of the entire bed. The results presented here can be used as a guideline to develop appropriate blending processes and characterization protocols for ribbon blenders.  相似文献   
87.
涡轮钻具由前苏联研制出来以后,得到了迅速发展,目前已发展到减速器涡轮钻具阶段。现在,俄罗斯西西伯利亚地区普遍使用减速器涡轮钻具钻井,而减速器涡轮钻具驱动PDC钻头的使用还是空白。采用俄罗斯生产的几种不同型号的减速器涡轮钻具,在西西伯利亚进行了驱动PDC钻头钻井的阶段性试验。试验结果表明,在优选PDC钻头结构的情况下,用减速器涡轮钻具驱动PDC钻头,钻头进尺和机械钻速都较牙轮钻头有显著的提高,经济和社会效益显著。介绍了试验中使用的3种俄罗斯减速器涡轮钻具(TPM-195,TPⅢ-195和TPO3—195)的结构特点和主要技术参数。  相似文献   
88.
新型刮刀钻头的设计及现场试验研究   总被引:2,自引:1,他引:1  
在理论分析的基础上,综合PDC钻头和刮刀钻头的优点,设计了一种新型的刮刀钻头,并进行了现场试验。现场试验结果表明,该钻头运转平稳,采用低钻压、高转速,钻进速度快,井眼规则通畅,有较明显的防斜作用,该种钻头的推广应用对提高钻井工程的整体效益具有十分重要的意义。  相似文献   
89.
边坡支护中多种技术措施的综合应用   总被引:1,自引:0,他引:1  
原艳 《山西建筑》2007,33(5):120-121
通过工程实例,介绍了锚杆喷射混凝土支护技术与高压喷射混凝土搅拌桩支护技术在工程中的综合利用,同时也验证了该技术的可行性及经济性。  相似文献   
90.
Industry widely uses rotary valves and blow tanks for the pneumatic conveying of products, each having their pros and cons depending on the specific application. This article shows the differing results obtained when low-velocity conveying a product through a common pipeline using both a drop-through rotary valve and a bottom-discharge blow tank feeder. A number of issues arise in the rotary valve system, the main one being rotary valve air leakage. A blow tank system, on the other hand, does not leak, as it is an enclosed system. The experimental results show dramatic differences in product throughput. Further exploration leads to a novel modification being made to the rotary valve system in an attempt to increase its capacity. The result of this modification shows a slight increase in output tonnage, but still significantly less than that obtained from the blow tank.  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