全文获取类型
收费全文 | 238173篇 |
免费 | 19227篇 |
国内免费 | 9653篇 |
专业分类
电工技术 | 13148篇 |
技术理论 | 25篇 |
综合类 | 14931篇 |
化学工业 | 40273篇 |
金属工艺 | 13701篇 |
机械仪表 | 15159篇 |
建筑科学 | 18333篇 |
矿业工程 | 7806篇 |
能源动力 | 6753篇 |
轻工业 | 14347篇 |
水利工程 | 3807篇 |
石油天然气 | 17011篇 |
武器工业 | 1767篇 |
无线电 | 26371篇 |
一般工业技术 | 28479篇 |
冶金工业 | 13365篇 |
原子能技术 | 2318篇 |
自动化技术 | 29459篇 |
出版年
2024年 | 1043篇 |
2023年 | 3957篇 |
2022年 | 6838篇 |
2021年 | 9498篇 |
2020年 | 7410篇 |
2019年 | 6272篇 |
2018年 | 7018篇 |
2017年 | 7912篇 |
2016年 | 7109篇 |
2015年 | 9279篇 |
2014年 | 11795篇 |
2013年 | 13868篇 |
2012年 | 14934篇 |
2011年 | 16142篇 |
2010年 | 14089篇 |
2009年 | 13352篇 |
2008年 | 12944篇 |
2007年 | 12419篇 |
2006年 | 12833篇 |
2005年 | 11158篇 |
2004年 | 7617篇 |
2003年 | 6550篇 |
2002年 | 5890篇 |
2001年 | 5264篇 |
2000年 | 5682篇 |
1999年 | 6633篇 |
1998年 | 5563篇 |
1997年 | 4545篇 |
1996年 | 4290篇 |
1995年 | 3582篇 |
1994年 | 2889篇 |
1993年 | 2011篇 |
1992年 | 1559篇 |
1991年 | 1251篇 |
1990年 | 952篇 |
1989年 | 758篇 |
1988年 | 549篇 |
1987年 | 345篇 |
1986年 | 287篇 |
1985年 | 202篇 |
1984年 | 144篇 |
1983年 | 114篇 |
1982年 | 127篇 |
1981年 | 100篇 |
1980年 | 74篇 |
1979年 | 37篇 |
1978年 | 26篇 |
1977年 | 22篇 |
1976年 | 36篇 |
1973年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
用波前相因子判断法,将球面波照明物体的自由空间菲涅耳衍射光场分布与分数傅里叶逆变换的标准频谱分布进行位相比较,提供了广义条件下光学分数傅里叶逆变换的无透镜模式,给出其光学实现基本单元参量选择的判定法则,计算机模拟实验证明了结论的可靠与可行。 相似文献
992.
993.
求通信网生存能力的一种分裂二叉树算法 总被引:4,自引:1,他引:3
本文研究了布尔代数方法在通信网生存能力分析中的应用,提出了一种求布尔代数式不交化变换的分裂二叉树算法,与现有的同类算法相比,该算法具有结构清晰、执行效率高、易于实现等优点。 相似文献
994.
995.
高速宽带取样-滤波数字化系统中高斯滤波器的设计 总被引:2,自引:0,他引:2
本文基于频域逼近的方法提出了高斯滤波器的设计理论,得出了带宽归一化的各阶高斯滤波器的通用计算公式。并对高斯滤波器的数字化时间窗口、回复时间参数等进行了计算。 相似文献
996.
在2001-2002年度中,澳大利亚Internet用户已超过总人口的50%。用户订购业务的类型有较大程度的变化,按月、季度和年度订购Internet业务的用户数量不断增长,免费订房、按小时付费和按容量接入的用户在减少。网上商业交易和政府业务在增加,政府积极实施公共Internet接入项目。澳大利亚各州和地区的ISP及相应的接入点数量逐渐减少,通信基础设施则进一步改善。 相似文献
997.
YAG激光熔覆的研究现状与发展趋势 总被引:1,自引:1,他引:0
本文介绍了YAG激光熔覆的特点,并与CO2激光进行了比较,显示了YAG激光熔覆的优势和潜力。叙述了YAG激光熔覆的研究进展及其应用前景。 相似文献
998.
Shunkang Liu 《Journal of Infrared, Millimeter and Terahertz Waves》2002,23(6):899-906
The efficiency enhancement methods for helix and coupled cavity traveling wave tube (TWT) in millimeter wave (MMW) are described. The interaction efficiency has been reached 14.56% by dynamic velocity taper in Ka-band TWT. Efficiency improvement has been carried out by used copper helix for broadband MMW TWT. Owing to a ferruleless coupled cavity TWT used phase-adjusted taper, the interaction efficiency has been increased from 9.6 to 22.6%. Finally, enhancing collector efficiency used multistage depressed collectors is discussed in the paper. These concept and technology of the approach are very useful for tube designer. 相似文献
999.
Provision of Quality‐of‐Service (QoS) guarantees is an important and challenging issue in the design of integrated‐services
packet networks. Call admission control is an integral part of the challenge and is closely related to other aspects of networks
such as service models, scheduling disciplines, traffic characterization and QoS specification. In this paper we provide a
theoretical framework within which call admission control schemes with multiple statistical QoS guarantees can be constructed
for the Generalized Processor Sharing (GPS) scheduling discipline. Using this framework, we present several admission control
schemes for both session‐based and class‐based service models. The theoretical framework is based on recent results in the
statistical analysis of the GPS scheduling discipline and the theory of effective bandwidths. Both optimal schemes and suboptimal
schemes requiring less computational effort are studied under these service models. The QoS metric considered is loss probability.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
1000.
Ke-Shun Liu 《Journal of the American Oil Chemists' Society》1994,71(11):1179-1187
Theoretically, preparation of fatty acid methyl esters (FAMEs) deals with reversible chemical reactions in a complex system.
Methodologically, there are numerous ways, generally characterized by the type of catalysts used and steps involved. Although
there are more than a half dozen common catalysts, the majority fall into either acidic (HCl, H2SO4 and BF3) or alkaline types (NaOCH3, KOH and NaOH), with each having its own catalytic capability and application limitations. In terms of steps, many conventional
methods, including those officially recognized, consist of drying, digestion, extraction, purification, alkaline hydrolysis,
transmethylation/methylation and postreaction work-up. Although these methods are capable of providing reliable estimates
if some precautions are taken, they are cumbersome, time-consuming and cost-inefficient. A new approach has been to transmethylate
lipidsin situ. Due to its simplicity, high sensitivity, comparable reliability and capability to determine total fatty acids, the method
of direct transmethylation is finding a unique place in lipid determination. Regardless of which method is used, quantitative
methylation requires chemists to take precautions at every step involved, particularly during FAME formation and subsequent
recovery steps. Evidently, there is an urgent need for more systematic studies, guided by the chemical principle of reactions
involved and physicochemical properties of regents and end products, into factors affecting these steps. Hopefully, this will
lead to an improved method, which measures lipid composition in biological materials not only with high accuracy but also
with high efficiency and minimum costs. 相似文献