全文获取类型
收费全文 | 45733篇 |
免费 | 5393篇 |
国内免费 | 3305篇 |
专业分类
电工技术 | 11208篇 |
技术理论 | 6篇 |
综合类 | 4287篇 |
化学工业 | 1004篇 |
金属工艺 | 703篇 |
机械仪表 | 3166篇 |
建筑科学 | 1875篇 |
矿业工程 | 1100篇 |
能源动力 | 1028篇 |
轻工业 | 662篇 |
水利工程 | 835篇 |
石油天然气 | 833篇 |
武器工业 | 397篇 |
无线电 | 9433篇 |
一般工业技术 | 2009篇 |
冶金工业 | 528篇 |
原子能技术 | 232篇 |
自动化技术 | 15125篇 |
出版年
2024年 | 149篇 |
2023年 | 531篇 |
2022年 | 989篇 |
2021年 | 1383篇 |
2020年 | 1478篇 |
2019年 | 1079篇 |
2018年 | 1018篇 |
2017年 | 1376篇 |
2016年 | 1524篇 |
2015年 | 1872篇 |
2014年 | 3754篇 |
2013年 | 3026篇 |
2012年 | 4237篇 |
2011年 | 4431篇 |
2010年 | 3210篇 |
2009年 | 3212篇 |
2008年 | 3098篇 |
2007年 | 3501篇 |
2006年 | 2992篇 |
2005年 | 2495篇 |
2004年 | 2007篇 |
2003年 | 1675篇 |
2002年 | 1121篇 |
2001年 | 943篇 |
2000年 | 724篇 |
1999年 | 534篇 |
1998年 | 395篇 |
1997年 | 319篇 |
1996年 | 275篇 |
1995年 | 205篇 |
1994年 | 180篇 |
1993年 | 132篇 |
1992年 | 105篇 |
1991年 | 68篇 |
1990年 | 69篇 |
1989年 | 69篇 |
1988年 | 37篇 |
1987年 | 19篇 |
1986年 | 18篇 |
1985年 | 35篇 |
1984年 | 38篇 |
1983年 | 30篇 |
1982年 | 31篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1976年 | 4篇 |
1975年 | 2篇 |
1974年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
1.
Bernette Maria Oosterlaken Heiner Friedrich Gijsbertus de With 《Microscopy research and technique》2022,85(1):412-417
Transmission electron microscopy (TEM) is an important analysis technique to visualize (bio)macromolecules and their assemblies, including collagen fibers. Many protocols for TEM sample preparation of collagen involve one or more washing steps to remove excess salts from the dispersion that could hamper analysis when dried on a TEM grid. Such protocols are not standardized and washing times as well as washing solvents vary from procedure to procedure, with each research group typically having their own protocol. Here, we investigate the influence of washing with water, ethanol, but also methanol and 2-propanol, for both mineralized and unmineralized collagen samples via a protocol based on centrifugation. Washing with water maintains the hydrated collagen structure and the characteristic banding pattern can be clearly observed. Conversely, washing with ethanol results in dehydration of the fibrils, often leading to aggregation of the fibers and a less obvious banding pattern, already within 1 min of ethanol exposure. As we show, this process is fully reversible. Similar observations were made for methanol and propanol. Based on these results, a standardized washing protocol for collagenous samples is proposed. 相似文献
2.
5G系统将移动通信服务从移动电话、移动宽带和大规模机器通信扩展到新的应用领域,即所谓对通信服务有特殊要求的垂直领域。对使能未来工厂的5G能力进行了全面的分析总结,包括弹性网络架构、灵活频谱、超可靠低时延通信、时间敏感网络、安全和定位,而弹性网络架构又包括对网络切片、非公共网络、5G局域网和边缘计算的支持。希望从广度到深度,对相关的理论及技术应用做透彻、全面的梳理,对其挑战做清晰的总结,从而为相关研究和工程技术人员提供借鉴。 相似文献
3.
摘 要:为了提高码索引调制(code index modulation,CIM)系统的传输效率,提出了一种具有更低复杂度的单输入单输出(single input single output,SISO)的广义正交码索引调制(generalized orthogonal code index modulation,GQCIM)系统。CIM 系统使用扩频码和星座符号传输信息,但只能激活两个扩频码索引和一个调制符号。而 GQCIM 系统以一种新颖的方式克服了只激活一个调制符号的限制,同时充分利用了调制符号的正交性,增加扩频码索引以传输更多的额外信息位,提高了系统的传输效率。此外,分析了GQCIM系统的理论性能,推导了误码率性能的上界。通过蒙特卡罗仿真验证了GQCIM系统的性能,对比发现GQCIM系统的理论和仿真性能一致。而且在相同的传输效率下,结果显示GQCIM系统的性能优于同样具有正交性的调制系统,如广义码索引调制(generalized code index modulation,GCIM)系统、CIM系统、码索引调制-正交空间调制(code index modulation aided quadrature spatial modulation,CIM-QSM)系统、码索引调制-正交空间调制(code index modulation aided spatial modulation,CIM-SM)系统、脉冲索引调制(pulse index modulation,PIM)系统。 相似文献
4.
互联网通讯采取标准化模式主要以TCP/IP协议为载体,通讯的优越特性体现在同时具备便捷性与开放性,为办公提供很大的便利,但基于网络系统也会入侵病毒、也会给信息数据与办公体系安全性造成威胁,直接影响企业综合稳定发展。据此,为保障办公工作的顺利开展,本文对计算机网络办公自动化及安全策略进行详细分析。 相似文献
5.
分析研究目前通信工程网络安全问题,提出几点解决问题的对策,旨在为提升通信工程网络安全性提供一定的帮助,以此来促使通信工程网络系统安全性的提升。 相似文献
6.
Michael Lommel Vera Froese Moritz Sieber Marvin Jentzsch Tim Bierewirtz Ümit Hasirci Tim Rese Josef Seefeldt Sebastian Schimek Ulrich Kertzscher Christian Oliver Paschereit 《Indoor air》2021,31(6):1860-1873
The SARS-CoV-2 pandemic has created a great demand for a better understanding of the spread of viruses in indoor environments. A novel measurement system consisting of one portable aerosol-emitting mannequin (emitter) and a number of portable aerosol-absorbing mannequins (recipients) was developed that can measure the spread of aerosols and droplets that potentially contain infectious viruses. The emission of the virus from a human is simulated by using tracer particles solved in water. The recipients inhale the aerosols and droplets and quantify the level of solved tracer particles in their artificial lungs simultaneously over time. The mobile system can be arranged in a large variety of spreading scenarios in indoor environments and allows for quantification of the infection probability due to airborne virus spreading. This study shows the accuracy of the new measurement system and its ability to compare aerosol reduction measures such as regular ventilation or the use of a room air purifier. 相似文献
7.
8.
Smartphones are being used and relied on by people more than ever before. The open connectivity brings with it great convenience and leads to a variety of risks that cannot be overlooked. Smartphone vendors, security policy designers, and security application providers have put a variety of practical efforts to secure smartphones, and researchers have conducted extensive research on threat sources, security techniques, and user security behaviors. Regrettably, smartphone users do not pay enough attention to mobile security, making many efforts futile. This study identifies this gap between technology affordance and user requirements, and attempts to investigate the asymmetric perceptions toward security features between developers and users, between users and users, as well as between different security features. These asymmetric perceptions include perceptions of quality, perceptions of importance, and perceptions of satisfaction. After scoping the range of smartphone security features, this study conducts an improved Kano-based method and exhaustively analyzes the 245 collected samples using correspondence analysis and importance satisfaction analysis. The 14 security features of the smartphone are divided into four Kano quality types and the perceived quality differences between developers and users are compared. Correspondence analysis is utilized to capture the relationship between the perceived importance of security features across different groups of respondents, and results of importance-satisfaction analysis provide the basis for the developmental path and resource reallocation strategy of security features. This article offers new insights for researchers as well as practitioners of smartphone security. 相似文献
9.
10.
This paper presents a fast distance relay for series compensated transmission lines based on the R–L differential-equation algorithm using the theory of equal transfer process of transmission lines. The measuring distances based on the proposed algorithm can fast approach the actual value of fault distance when a fault occurs in front of the series capacitor. When a fault occurs behind of the series capacitor, the fault loop, including the series capacitor, does not match the R–L transmission line model, so the measuring distances fluctuate severely. Based on this, the relative position of the fault with respect to the series capacitor can be judged effectively according to the fluctuation range of the measuring distances, and the accurate fault location can be obtained fast. A variety of PSCAD/EMTDC simulation tests show that the new relay has fast operating speed and high accuracy when applied to the long series compensated transmission lines. 相似文献