全文获取类型
收费全文 | 41852篇 |
免费 | 5381篇 |
国内免费 | 4563篇 |
专业分类
化学 | 22973篇 |
晶体学 | 439篇 |
力学 | 1301篇 |
综合类 | 351篇 |
数学 | 3262篇 |
物理学 | 9960篇 |
无线电 | 13510篇 |
出版年
2024年 | 127篇 |
2023年 | 746篇 |
2022年 | 1040篇 |
2021年 | 1306篇 |
2020年 | 1303篇 |
2019年 | 1357篇 |
2018年 | 1154篇 |
2017年 | 1078篇 |
2016年 | 1675篇 |
2015年 | 1812篇 |
2014年 | 2089篇 |
2013年 | 2865篇 |
2012年 | 3180篇 |
2011年 | 3267篇 |
2010年 | 2559篇 |
2009年 | 2513篇 |
2008年 | 2906篇 |
2007年 | 2622篇 |
2006年 | 2424篇 |
2005年 | 2146篇 |
2004年 | 1879篇 |
2003年 | 1656篇 |
2002年 | 1768篇 |
2001年 | 1470篇 |
2000年 | 1125篇 |
1999年 | 887篇 |
1998年 | 605篇 |
1997年 | 584篇 |
1996年 | 553篇 |
1995年 | 468篇 |
1994年 | 389篇 |
1993年 | 364篇 |
1992年 | 305篇 |
1991年 | 250篇 |
1990年 | 234篇 |
1989年 | 200篇 |
1988年 | 132篇 |
1987年 | 112篇 |
1986年 | 97篇 |
1985年 | 86篇 |
1984年 | 56篇 |
1983年 | 42篇 |
1982年 | 37篇 |
1981年 | 50篇 |
1980年 | 33篇 |
1979年 | 25篇 |
1978年 | 29篇 |
1977年 | 32篇 |
1976年 | 30篇 |
1974年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Buffer analysis has traditionally relied heavily on the use of traffic models with short range (exponentially decaying) autocorrelation, e.g. Poisson and Markov modulated Poisson processes. Recent literature has suggested the presence of traffic with slowly (hyperbolically) decaying autocorrelation functions; these sources collectively and individually seem to be exhibiting the phenomenon of long range dependence (LRD). Furthermore, it has been claimed that this sort of traffic may exhibit behaviour, when buffered, that is advantageous in that a smaller buffer may be required than would be the case with non-LRD traffic. This phenomenon has been named the crossover effect, and the authors quantify its relevance to ATM networking, using simulation studies of a homogenous mix of chaotic sources. These show that there is a crossover effect, but the key finding is that this effect is much nearer to a buffer length of zero than was predicted by previous theoretical studies, actually occurring at a value so low that it has no practical significance 相似文献
32.
Gunapala S.D. Bundara S.V. Liu J.K. Winn Hong Mani Sundaram Maker P.D. Muller R.E. Shott C.A. Carralejo R. 《Electron Devices, IEEE Transactions on》1998,45(9):1890-1895
A 9-μm cutoff 640×486 snap-shot quantum well infrared photodetector (QWIP) camera has been demonstrated. The performance of this QWIP camera is reported including indoor and outdoor imaging. The noise equivalent differential temperature (NEΔT) of 36 mK has been achieved at 300 K background with f/2 optics. This is in good agreement with expected focal plane array sensitivity due to the practical limitations on charge handling capacity of the multiplexer, read noise, bias voltage, and operating temperature 相似文献
33.
本文叙述了智能网的业务管理系统所实现的3种功能:业务管理功能,网络管理功能和业务管理访问功能,还介绍了业务管理和网络管理之间的关系,SMS的集中式和分布式两种体系结构及业务管理功能的设计方法和进一步研究方向。 相似文献
34.
钢管内径光电检测装置 总被引:3,自引:0,他引:3
介绍了一种检测细长钢管内径的光电检测装置。该装置利用线阵CCD作传感器,采用8031单片机作为主控制。 相似文献
35.
Longlan Cui Hong Xu Ping He Keiko Sumitomo Yoshinori Yamaguchi Hongchen Gu 《Journal of polymer science. Part A, Polymer chemistry》2007,45(22):5285-5295
A hybrid emulsion polymerization was formulated for synthesizing Fe3O4/polystyrene composite latex. This system, containing binary droplets that are magnetic (Mag)‐droplets with a diameter of 100–200 nm and styrene (St)‐droplets with a diameter of 3–4 μm, was obtained by mixing Mag‐miniemulsion and St‐macroemulsion. With extremely low surfactants concentration (?critical micelle concentration, CMC), the nucleated loci are selectively controlled in the Mag‐droplets, as the result of smaller droplet size and larger surface ratio. Both water‐soluble potassium persulfate (KPS) and oil‐soluble 2,2′‐azobis(2‐isobutyronitrile) was adopted to initiate the polymerization. In the presence of KPS, magnetic polystyrene latices with particles size of 60–200 nm, narrow size distribution, and high magnetite content (86 wt % measured by TGA) were attained successfully. The synthesized magnetic Fe3O4/polystyrene latices assembled into well‐ordered hexagonal structure in the surface of a carbon supported copper grid. The influence of various parameters on various aspects of the as‐synthesized Fe3O4/polystyrene was investigated in detail: type of initiator on composite morphology, feed ratio of Mag‐miniemulsion and St‐macroemulsion on magnetite content, and hydrophobic agent or amount of surfactant on size and size distribution. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5285–5295, 2007 相似文献
36.
37.
Zhen‐Feng Chen Hong‐Li Zhou Hong Liang Yan Li Ren‐Gen Xiong Xiao‐Zeng You 《应用有机金属化学》2003,17(11):883-884
The centrosymmetric binuclear structure of [Pb2(H‐Norf)2(ONO2)4]shows the geometry around each lead(II) atom to be distorted trigonal bipyramidal with Pb–O distances ranging from 2.357(3) to 2.769(4) Å. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
38.
Chao-Chi Hong Jenn-Gwo Hwu 《Electron Device Letters, IEEE》2003,24(6):408-410
The current-voltage (I-V) characteristics of metal-oxide-semiconductor tunneling diodes distributed over a 3-in Si wafer were analyzed to investigate the stress distribution on the wafer. Generally, the substrate injection saturation current (J/sub sat/) decreases as the gate injection leakage current (J/sub g/) increases, the latter being dominated by oxide thickness via a trap related mechanism. A universal curve to fit all analyzed data was found and it is suggested that devices with extremely high (low) J/sub sat/ at a given J/sub g/ should be located in areas of the silicon lattice with relatively high external compressive (tensile) stress because of the stress-induced bandgap variation effect. The mapped locations of the highly stressed devices on a 3-in [100] Si wafer correspond to the patterns of slips caused by thermal stress during rapid thermal processing, as described in previous reports. 相似文献
39.
40.
Small‐Angle Neutron Scattering has been performed from poly(ethylene oxide) in deuterated water at temperature ranging from 10 to 80 °C. A simple fitting model was used to obtain a correlation length and a Porod exponent. The correlation length L characterizes the average distance between entanglements in the semidilute region and is proportional to the individual coil sizes in the dilute region. L was found to increase with temperature in the semidilute region but it decreases with temperature in the dilute region. This decrease is the precursor to the single‐chain collapse which applies to very dilute polymer solutions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2196–2200, 2007 相似文献