全文获取类型
收费全文 | 39083篇 |
免费 | 4743篇 |
国内免费 | 3556篇 |
专业分类
化学 | 20523篇 |
晶体学 | 284篇 |
力学 | 1944篇 |
综合类 | 273篇 |
数学 | 3280篇 |
物理学 | 10635篇 |
无线电 | 10443篇 |
出版年
2024年 | 147篇 |
2023年 | 907篇 |
2022年 | 999篇 |
2021年 | 1372篇 |
2020年 | 1358篇 |
2019年 | 1277篇 |
2018年 | 1098篇 |
2017年 | 1044篇 |
2016年 | 1506篇 |
2015年 | 1595篇 |
2014年 | 1936篇 |
2013年 | 2552篇 |
2012年 | 3094篇 |
2011年 | 3124篇 |
2010年 | 2118篇 |
2009年 | 2146篇 |
2008年 | 2273篇 |
2007年 | 2149篇 |
2006年 | 2002篇 |
2005年 | 1722篇 |
2004年 | 1246篇 |
2003年 | 1134篇 |
2002年 | 1012篇 |
2001年 | 805篇 |
2000年 | 859篇 |
1999年 | 940篇 |
1998年 | 843篇 |
1997年 | 773篇 |
1996年 | 869篇 |
1995年 | 690篇 |
1994年 | 632篇 |
1993年 | 538篇 |
1992年 | 505篇 |
1991年 | 409篇 |
1990年 | 323篇 |
1989年 | 239篇 |
1988年 | 186篇 |
1987年 | 159篇 |
1986年 | 156篇 |
1985年 | 131篇 |
1984年 | 100篇 |
1983年 | 67篇 |
1982年 | 46篇 |
1981年 | 38篇 |
1980年 | 21篇 |
1978年 | 23篇 |
1977年 | 25篇 |
1976年 | 21篇 |
1975年 | 34篇 |
1974年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
无线数据通信网发展趋势 总被引:1,自引:0,他引:1
无线数据通信具有两大发展趋势,即低速的广域移动数据网及高速的局域无线数据网。本文介绍了其技术特点、应用情况和市场前景,分析了无线数据通信传输技术、网络结构和媒质接入控制等问题。 相似文献
102.
Chih-Yao Huang Ming-Jer Chen 《Electron Devices, IEEE Transactions on》1994,41(10):1806-1810
This work reports the development of design model for n-well guard rings in a CMOS process utilizing a low-doped epitaxial layer on a highly doped substrate. The validity of the model has been judged by a wide range of experimental data measured from the fabricated n-well guard ring structures with guard ring width as parameter. From the model developed, guidelines have been drawn to minimize the guard ring width while critically suppressing the amount of electrons escaping from the guard ring 相似文献
103.
Tahui Wang Chimoon Huang Chou P.C. Chung S.S.-S. Tse-En Chang 《Electron Devices, IEEE Transactions on》1994,41(9):1618-1622
A two-dimensional numerical simulation including a new interface state generation model has been developed to study the performance variation of a LDD MOSFET after a dc voltage stress. The spatial distribution of hot carrier induced interface states is calculated with a breaking silicon-hydrogen bond model. Mobility degradation and reduction of conduction charge due to interface traps are considered. A 0.6 μm LDD MOSFET was fabricated. The drain current degradation and the substrate current variation after a stress were characterized to compare the simulation. A reduction of the substrate current at Vg ≃0.5 Vd in a stressed device was observed from both the measurement and the simulation. Our study reveals that the reduction is attributed to a distance between a maximum channel electric field and generated interface states 相似文献
104.
The implementation of time-domain diakoptics in the FDTD method 总被引:1,自引:0,他引:1
The time-domain diakoptics is implemented in the finite-difference time-domain (FDTD) method with two types of connecting interfaces: i) directional interface (TLM-type), and ii) total-field interface (FDTD-type). The FDTD-type interface provides a more efficient way to realize time-domain diakoptics than TLM, especially for device optimization problems. To emulate the TLM-type interface in FDTD, two novel algorithms are developed in this paper. One is to implement an ultra-wideband absorbing boundary on the excitation plane during excitation. The other is to separate directional waves without the knowledge of incident waves. For a large circuit with cascaded modules, sequential and parallel algorithms are provided to connect them. With these connecting algorithms, time-domain diakoptics is one candidate method to realize modular and parallel computation in FDTD simulations. The validity of these algorithms is confirmed by comparison with simulated results from Microwave SPICE 相似文献
105.
Pd-Ge based ohmic contact to n-GaAs with a TiW diffusion barrier was investigated. Electrical analysis as well as Auger electron
spectroscopy and the scanning electron microscopy were used to study the contact after it was subjected to different furnace
and rapid thermal annealing and different aging steps. All analyses show that TiW can act as a good barrier metal for the
Au/Ge/Pd/n-GaAs contact system. A value of 1.45 × 10−6 Ω-cm2 for the specific contact resistance was obtained for the Au/TiW/Ge/Pd/n-GaAs contact after it was rapid thermally annealed
at 425°C for 90 s. It can withstand a thermal aging at 350°C for 40 h with its ρc increasing to 2.94 × 10−6Ω-cm2 and for an aging at 410°C for 40 h with its ρc increasing to 1.38 × 10−5 Ω-cm2. 相似文献
106.
The polarization-dependent absorption and emission spectra of the 4I13/2-4I15/2 transition (λ~1.5 μm) in single crystal bulk Er:LiNbO3 have been measured. Low-temperature (10 K) measurements of the Stark split energy levels of these two manifolds indicate at least two Er3+ sites. McCumber theory is applied to determine the Er:LiNbO3 absorption and emission cross sections. These values are used to calculate the gain characteristics of Er:LiNbO3 channel waveguides. Calculations indicate that a gain of 10 dB is achievable in a waveguide of several centimeters using ~20-mW pump power 相似文献
107.
108.
Xiao Huang Bishwa Ranjan Nayak Tao Lu Lowe 《Journal of polymer science. Part A, Polymer chemistry》2004,42(20):5054-5066
A series of novel multifunctional hydrogels that combined the merits of both thermoresponsive and biodegradable polymeric materials were designed, synthesized, and characterized. The hydrogels were copolymeric networks composed of N‐isopropylacrylamide (NIPAAM) as a thermoresponsive component, poly(L‐lactic acid) (PLLA) as a hydrolytically degradable and hydrophobic component, and dextran as an enzymatically degradable and hydrophilic component. The chemical structures of the hydrogels were characterized by an attenuated total reflection–Fourier transform infrared spectroscopy (ATR–FTIR) technique. The hydrogels were thermoresponsive, showing a lower critical solution temperature (LCST) at approximately 32 °C, and their swelling properties strongly depended on temperature changes, the balance of the hydrophilic/hydrophobic components, and the degradation of the PLLA component. The degradation of the hydrogels caused by hydrolytic cleavage of ester bonds in the PLLA component was faster at 25 °C below the LCST than at 37 °C above the LCST, determined by the ATR–FTIR technique. Due to their multifunctional properties, the designed hydrogels show great potential for biomedical applications, including drug delivery and tissue engineering. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5054–5066, 2004 相似文献
109.
In this paper we propose a new protocol called time-space label switching protocol (TSL-SP) in optical burst switching (OBS), and define the terms time-space label (TSL) and time-space routing (TSR). An important concept of response time is introduced in the time label mechanism. The TSL-SP is a new technology that can quickly and efficiently forward data with a label on the optical networks. A two-dimension label switched path (TD-LSP) can be set up, that is maintained and deleted by the TSL-SP. For clearly illuminating the operation principles of the TSL-SP, we propose a new approach of orthogonal time-space coordinates in which the vertical coordinate is the space label and the horizontal coordinate is the time label. The proposed TD-LSP can dramatically reduce the routing failure probability and greatly improve the link network efficiency compared with other signaling protocols. Moreover, we define the time-space label control plane that can achieve the higher efficiency. When the TSL-SP is applied to networks, switching performance can be improved by two orders compared to the switching performance with the conventional OBS signaling protocols. The fundamental goal of TSL-SP is to band the signaling and routing functions together closely. Also, the TSL-SP can reduce the complexity of the network, support automatic service offering, and provide traffic engineering. 相似文献
110.