全文获取类型
收费全文 | 13949篇 |
免费 | 1698篇 |
国内免费 | 905篇 |
专业分类
化学 | 2181篇 |
晶体学 | 60篇 |
力学 | 1334篇 |
综合类 | 119篇 |
数学 | 4557篇 |
物理学 | 4292篇 |
无线电 | 4009篇 |
出版年
2024年 | 24篇 |
2023年 | 127篇 |
2022年 | 201篇 |
2021年 | 263篇 |
2020年 | 329篇 |
2019年 | 326篇 |
2018年 | 298篇 |
2017年 | 444篇 |
2016年 | 484篇 |
2015年 | 462篇 |
2014年 | 854篇 |
2013年 | 1290篇 |
2012年 | 782篇 |
2011年 | 862篇 |
2010年 | 733篇 |
2009年 | 811篇 |
2008年 | 853篇 |
2007年 | 888篇 |
2006年 | 794篇 |
2005年 | 743篇 |
2004年 | 734篇 |
2003年 | 588篇 |
2002年 | 536篇 |
2001年 | 454篇 |
2000年 | 393篇 |
1999年 | 354篇 |
1998年 | 318篇 |
1997年 | 249篇 |
1996年 | 224篇 |
1995年 | 171篇 |
1994年 | 159篇 |
1993年 | 123篇 |
1992年 | 94篇 |
1991年 | 91篇 |
1990年 | 58篇 |
1989年 | 45篇 |
1988年 | 38篇 |
1987年 | 30篇 |
1986年 | 18篇 |
1985年 | 42篇 |
1984年 | 37篇 |
1983年 | 20篇 |
1982年 | 30篇 |
1981年 | 41篇 |
1980年 | 30篇 |
1979年 | 19篇 |
1978年 | 15篇 |
1977年 | 22篇 |
1976年 | 21篇 |
1974年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
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
3.
《Physics letters. A》2019,383(23):2784-2788
By modifying the conventional one-electron hopping behavior, we study effects of an occupation-dependent hopping on the ground state of the half-filled one-dimensional pair-hopping model. At weak coupling, the use of bosonization and renormalization-group analysis techniques helps to derive the phase diagram. Such unusual hopping is shown to drive a spin-gap transition and to introduce a new region where the triplet superconducting instability dominates for positively small pair-hopping interaction. 相似文献
4.
《Discrete Mathematics》2021,344(12):112589
Let be the set of positive integers. For a nonempty set A of integers and every integer u, denote by the number of with such that . For a sequence S of positive integers, let be the counting function of S. The set is called a perfect difference set if for every positive integer u. In 2008, Cilleruelo and Nathanson (2008) [4] constructed dense perfect difference sets from dense Sidon sets. In this paper, as a main result, we prove that: let be an increasing function satisfying for any positive integer n, then for every Sidon set B and every function , there exists a set such that for every positive integer u and for all . 相似文献
5.
Yeong‐Tarng Shieh Pei‐Yu Tai Chih‐Chia Cheng 《Journal of polymer science. Part A, Polymer chemistry》2019,57(21):2149-2156
Poly(methyl methacrylate) (PMMA) nanoparticles with a sensitive CO2‐responsive hydrophilic/hydrophobic surface that confers controlled dispersion and aggregation in water were prepared by emulsion polymerization at 50 °C under CO2 bubbling using amphiphilic diblock copolymers of 2‐dimethylaminoethyl methacrylate (DMAEMA) and N‐isopropyl acrylamide (NIPAAm) as an emulsifier. The amphiphilicity of the hydrophobic–hydrophilic diblock copolymer at 50 °C was triggered by CO2 bubbling in water and enabled the copolymer to serve as an emulsifier. The resulting PMMA nanoparticles were spherical, approximately 100 nm in diameter and exhibited sensitive CO2/N2‐responsive dispersion/aggregation in water. Using copolymers with a longer PNIPAAm block length as an emulsifier resulted in smaller particles. A higher concentration of copolymer emulsifier led to particles with a stickier surface. Given its simple preparation and reversible CO2‐triggered amphiphilic behavior, this newly developed block copolymer emulsifier offers a highly efficient route toward the fabrication of sensitive CO2‐stimuli responsive polymeric nanoparticle dispersions. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2149–2156 相似文献
6.
Markus Bödenler Ludovic de Rochefort P. James Ross Nicolas Chanet Geneviève Guillot Gareth R. Davies 《Molecular physics》2019,117(7-8):832-848
ABSTRACTFast field-cycling (FFC) nuclear magnetic resonance relaxometry is a well-established method to determine the relaxation rates as a function of magnetic field strength. This so-called nuclear magnetic relaxation dispersion gives insight into the underlying molecular dynamics of a wide range of complex systems and has gained interest especially in the characterisation of biological tissues and diseases. The combination of FFC techniques with magnetic resonance imaging (MRI) offers a high potential for new types of image contrast more specific to pathological molecular dynamics. This article reviews the progress in FFC-MRI over the last decade and gives an overview of the hardware systems currently in operation. We discuss limitations and error correction strategies specific to FFC-MRI such as field stability and homogeneity, signal-to-noise ratio, eddy currents and acquisition time. We also report potential applications with impact in biology and medicine. Finally, we discuss the challenges and future applications in transferring the underlying molecular dynamics into novel types of image contrast by exploiting the dispersive properties of biological tissue or MRI contrast agents. 相似文献
7.
8.
论文比较详细地阐述了涉密网主机系统所面临的安全隐患,提出了涉密网主机安全防护所要解决的几个技术问题,并就可能实现的技术方法进行了探讨和比较。 相似文献
9.
国产风机盘管空调系统常见问题分析 总被引:2,自引:1,他引:1
分析了国产风机盘管使用中常见的问题,指出机组因风量不足、冷量风量匹配不合理以及风机盘管某些设备故障,导致了风机盘管精度低、舒适性差、能耗高等一系列问题。指出应引起空调设计和运行维护人员的注意。 相似文献
10.
提出在县市级广电宽带数据网络的建设过程中,要着眼业务发展,突出高扩展性,构建基于标准、扩展灵活的主干网,实现网络建设的可持续性发展,并结合实际,从核心设备、端口、中继带宽、网络业务4个方面对扩展能力进行了阐述. 相似文献