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   38992篇
  免费   4711篇
  国内免费   3720篇
化学   20886篇
晶体学   363篇
力学   1406篇
综合类   234篇
数学   3542篇
物理学   9036篇
无线电   11956篇
  2024年   82篇
  2023年   704篇
  2022年   846篇
  2021年   1198篇
  2020年   1178篇
  2019年   1183篇
  2018年   1012篇
  2017年   1032篇
  2016年   1405篇
  2015年   1513篇
  2014年   1806篇
  2013年   2395篇
  2012年   2871篇
  2011年   3086篇
  2010年   2324篇
  2009年   2361篇
  2008年   2570篇
  2007年   2439篇
  2006年   2199篇
  2005年   1935篇
  2004年   1517篇
  2003年   1265篇
  2002年   1238篇
  2001年   1031篇
  2000年   894篇
  1999年   827篇
  1998年   723篇
  1997年   662篇
  1996年   671篇
  1995年   561篇
  1994年   522篇
  1993年   473篇
  1992年   446篇
  1991年   375篇
  1990年   322篇
  1989年   239篇
  1988年   207篇
  1987年   144篇
  1986年   164篇
  1985年   176篇
  1984年   117篇
  1983年   75篇
  1982年   81篇
  1981年   59篇
  1980年   58篇
  1979年   62篇
  1978年   49篇
  1976年   46篇
  1974年   48篇
  1973年   47篇
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
31.
Itaconate (ITA) is an emerging powerhouse of innate immunity with therapeutic potential that is limited in its ability to be administered in a soluble form. A library of polyester materials that incorporate ITA into polymer backbones resulting in materials with inherent immunoregulatory behavior is developed. Harnessing hydrolytic degradation release from polyester backbones, ITA polymers result in the mechanism specific immunoregulatory properties on macrophage polarization in vitro. In a functional assay, the polymer-released ITA inhibits bacterial growth on acetate. Translation to an in vivo model of biomaterial associated inflammation, intraperitoneal injection of ITA polymers demonstrate a rapid resolution of inflammation in comparison to a control polymer silicone, demonstrating the value of sustained biomimetic presentation of ITA.  相似文献   
32.
概述了中国PCB产品必须走出低价的市场竞争的困境,坚决走自主创新来发展中国PCB工业的道路。  相似文献   
33.
城域网的建设是今后光纤网络建设的一个热点,根据城域网的特点和系统技术要求选择光纤光缆是网络设计的重要组成部分。本根据城域网的特点和需求,对市场上现有的光纤光缆做了详细的分析和探讨,提出了城域网规划中光纤光缆选择的原则和具体的建议。  相似文献   
34.
从做大到做强是中国PCB工业的唯一出路.而做强中国PCB工业的主体是中国的PCB企业(集团).只有大多数的中国PCB企业做大并做强了,中国PCB工业才能称得上是PCB强国.  相似文献   
35.
文章针对使用USB存储设备造成的内部泄密问题,提出了一种USB存储设备访问控制方案,该方案对用户的身份认证基于安全强度很高的椭圆曲线数字签名算法,同时使用过滤器驱动程序实现了USB存储设备的读写控制,因而此方案具有较好的安全性、实用性和通用性。  相似文献   
36.
s5935是AMCC公司生产的PCI接口芯片 ,可实现直通 (PASS-THRU)、邮箱 (MAILBOX)、FI FO/DMA三种工作方式。文章简要介绍了这三种工作方式 ,并给出了s5935的WDM驱动编程。  相似文献   
37.
Personal telecommunication radiation safety and the precautionary principle   总被引:4,自引:0,他引:4  
The approach to risk management for RF and microwave radiation has been to establish guidelines for maximum permissible levels of exposure. These guidelines are based on evaluations of the relevant scientific literature and, with safety margins to account for uncertainties and gaps in scientific knowledge, offer a significant level of protection against established hazards of RF and microwave radiation for short-term exposures. One can argue, on this basis, that the precautionary principle - specifically, safety factors -have been used to guide the making and promulgation of existing permissible-exposure standards for RF and microwave radiation. Therefore, discussions on the appropriateness of the precautionary principle for RF radiation may be "much ado about nothing". Clearly, this approach to "pre caution" neither has stifled technological development, nor has it helped to enforce the acceptance of zero risk, but it provided a basis for action when science was unable to give a clear answer.  相似文献   
38.
This article presents a graph-theoretic method for constructing low-density parity-check (LDPC) codes from connected graphs without the requirement of large girth. This method is based on finding a set of paths in a connected graph, which satisfies the constraint that any two paths in the set are either disjoint or cross each other at one and only one vertex. Two trellis-based algorithms for finding these paths are devised. Good LDPC codes of practical lengths are constructed and they perform well with iterative decoding.  相似文献   
39.
A new hybrid fuzzy controller for direct torque control (DTC) induction motor drives is presented in this paper. The newly developed hybrid fuzzy control law consists of proportional-integral (PI) control at steady state, PI-type fuzzy logic control at transient state, and a simple switching mechanism between steady and transient states, to achieve satisfied performance under steady and transient conditions. The features of the presented new hybrid fuzzy controller are highlighted by comparing the performance of various control approaches, including PI control, PI-type fuzzy logic control (FLC), proportional-derivative (PD) type FLC, and combination of PD-type FLC and I control, for DTC-based induction motor drives. The pros and cons of these controllers are demonstrated by intensive experimental results. It is shown that the presented induction motor drive is with fast tracking capability, less steady state error, and robust to load disturbance while not resorting to complicated control method or adaptive tuning mechanism. Experimental results derived from a test system are presented confirming the above-mentioned claims.  相似文献   
40.
A new and interesting Pd-oxide-Al/sub 0.3/Ga/sub 0.7/As MOS hydrogen sensor has been fabricated and studied. The steady-state and transient responses with different hydrogen concentrations has been measured at various temperatures. Based on the large Schottky barrier height and presence of oxide layer, the studied device exhibits a high hydrogen detection sensitivity and wide temperature operating regime. The studied device exhibits the low-leakage current and obvious current changes when exposed to hydrogen-contained gas. Even at room temperature, a very high hydrogen detection sensitivity of 155.9 is obtained when a 9090 ppm H/sub 2//air gas is introduced. Furthermore, when exposed to hydrogen-contained gas at 95/spl deg/C, both the forward and reverse currents are substantially increased with increased hydrogen concentration. In other words, the studied device can be used as a hydrogen sensor under the applied bidirectional bias. Under the applied voltage of 0.35 V and 9090 ppm H/sub 2//air hydrogen ambient, a fast adsorption response time about 10 s is found. The transient and steady-state characteristics of hydrogen adsorption are also investigated.  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