全文获取类型
收费全文 | 83949篇 |
免费 | 18045篇 |
国内免费 | 2171篇 |
专业分类
电工技术 | 3776篇 |
技术理论 | 4篇 |
综合类 | 4180篇 |
化学工业 | 23928篇 |
金属工艺 | 3086篇 |
机械仪表 | 3954篇 |
建筑科学 | 5172篇 |
矿业工程 | 1352篇 |
能源动力 | 2087篇 |
轻工业 | 10908篇 |
水利工程 | 1338篇 |
石油天然气 | 2242篇 |
武器工业 | 490篇 |
无线电 | 11354篇 |
一般工业技术 | 15984篇 |
冶金工业 | 2865篇 |
原子能技术 | 522篇 |
自动化技术 | 10923篇 |
出版年
2024年 | 70篇 |
2023年 | 412篇 |
2022年 | 963篇 |
2021年 | 1623篇 |
2020年 | 2475篇 |
2019年 | 3979篇 |
2018年 | 4043篇 |
2017年 | 4506篇 |
2016年 | 4938篇 |
2015年 | 5644篇 |
2014年 | 6147篇 |
2013年 | 7750篇 |
2012年 | 6007篇 |
2011年 | 6004篇 |
2010年 | 6084篇 |
2009年 | 5819篇 |
2008年 | 5380篇 |
2007年 | 5083篇 |
2006年 | 4811篇 |
2005年 | 3941篇 |
2004年 | 3234篇 |
2003年 | 2820篇 |
2002年 | 2782篇 |
2001年 | 2492篇 |
2000年 | 2050篇 |
1999年 | 1330篇 |
1998年 | 765篇 |
1997年 | 638篇 |
1996年 | 489篇 |
1995年 | 379篇 |
1994年 | 327篇 |
1993年 | 246篇 |
1992年 | 171篇 |
1991年 | 173篇 |
1990年 | 95篇 |
1989年 | 95篇 |
1988年 | 79篇 |
1987年 | 42篇 |
1986年 | 43篇 |
1985年 | 29篇 |
1984年 | 19篇 |
1983年 | 35篇 |
1982年 | 22篇 |
1981年 | 16篇 |
1980年 | 16篇 |
1979年 | 15篇 |
1977年 | 14篇 |
1976年 | 25篇 |
1975年 | 7篇 |
1974年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
131.
A survey of quality of service in IEEE 802.11 networks 总被引:9,自引:0,他引:9
Developed as a simple and cost-effective wireless technology for best effort services, IEEE 802.11 has gained popularity at an unprecedented rate. However, due to the lack of built-in quality of service support, IEEE 802.11 experiences serious challenges in meeting the demands of multimedia services and applications. This article surveys 802.11 QoS schemes, including service differentiation in the MAC layer, admission control and bandwidth reservation in MAC and higher layers, and link adaptation in the physical layer, designed to meet these challenges by providing the necessary enhancements for the required QoS. Furthermore, the article addresses issues that arise when end-to-end QoS has to be guaranteed in today's pervasive heterogeneous wired-cum-wireless networks. Among these challenges, protocol interoperability, multihop scheduling, full mobility support, and seamless vertical handoff among multiple mobile/wireless interfaces are specifically addressed. 相似文献
132.
133.
L. Vu‐Quoc V. Srinivas Y. Zhai 《International journal for numerical methods in engineering》2003,58(3):397-461
We establish a systematic methodology to design and analyse electromagnetic components such as advanced multilayer ceramic capacitors (MLCCs) using the finite element (FE) method. We employ a coupled formulation to compute the interaction between the electric and magnetic fields. Unlike a linear distribution of current assumed in the circuit model, an accurate electrostatic solution to model the entire advanced MLCCs (4 × 4 × 27 = 432 cells) is presented. The FE solution is used to compute the lumped parameters for a range of frequencies. These lumped parameters are then used to compute the parasitic elements of the MLCCs. We introduce two algorithms to efficiently analyse the behaviour of a capacitor with changing frequency. The lower frequency (much below the self‐resonant frequency of the capacitor) algorithm separates the effect of the electric and magnetic fields and reduces the computational effort required to solve the FE problem, whereas, the high‐frequency algorithm couples the effect between the electric and the magnetic fields. We use these algorithms in conjunction with a new multiple scale technique to effectively determine the small values of R, L and C in MLCCs. The formulation, the implementation, and the numerical results demonstrate the efficacy of the present FE formulation and establish a systematic methodology to design and analyse advanced electromagnetic components. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
134.
Gas/liquid mass transfer has been investigated using a stirred vessel gas/liquid contactor using non‐Newtonian media and carbon dioxide as absorbent and gas phase, respectively. The volumetric mass transfer coefficients at different operational variables have been determined. Non‐Newtonian media (liquid phase) were prepared as aqueous solutions of sodium carboxymethyl cellulose salt. The influence of the rheological properties, polymer concentration, stirring rate, and gas flow rate on mass transfer was studied for these liquid phases. Kinematic viscosity and density experimental data were used to calculate the average molecular weight corresponding to the polymer employed. The Ostwald model has been used to fit the rheological behavior of aqueous solutions of the polymer employed as absorbent phase. Reasonably good agreement was found between the predictions of the proposed models and the experimental data of mass transfer coefficients. 相似文献
135.
136.
137.
In advancement of Pyrosil®‐technology a new kind of precursor delivery was developed, build and tested on real substrates. A Lab‐demonstrator was build to demonstrate the resources of the technology. 相似文献
138.
G. Casalbore‐Miceli M.C. Gallazzi S. Zecchin N. Camaioni A. Geri C. Bertarelli 《Advanced functional materials》2003,13(4):307-312
Voltammetric and spectrophotometric measurements of poly(3,3″‐dipentoxy‐3′‐dicyanoethenyl‐2,2′:5′,2″‐terthiophene) (polyCN) films, in connection with other experimental evidence, reveal a normal oxidative, but a peculiar reductive behavior consisting of trapping of the negative charge during the cathodic scan. Another interesting property of polyCN films is the tendency to form strong intramolecular and intermolecular associations, probably charge‐transfer (CT) complexes. These properties could account for the fact that the photovoltaic performance does not improve when polyCN is blended with a polythiophene donor. 相似文献
139.
Nucleation and growth mechanisms and kinetics of crystals of an amino acid salt were investigated in a methanol‐water system by measuring and evaluating the induction time as a function of the supersaturation ratio and temperature in batch salting out crystallization experiments. Discrimination between the possible crystallization mechanisms, and estimation of the kinetic parameters were carried out using nonlinear parameter identification. The results concerning the growth mechanism obtained were checked additionally by measuring the induction time as a function of number density of seed crystals. 相似文献
140.
Conducting poly(aniline‐co‐o‐anisidine) (PAS) films with different ratios of aniline units in the polymer chain were prepared by oxidative polymerization of different molar ratios of aniline and o‐anisidine in 1 M HCl using cyclic voltammetry. Due to the much higher reactivity of o‐anisidine, the structure and properties of PASs were found to be dominated by the o‐anisidine units. The polymerization of poly‐o‐anisidine and PASs followed zero‐order kinetics with respect to formation of the polymer (film thickness) and the autocatalytic polymerization of aniline was completely inhibited. In contrast to polyaniline, a decrease in the polymerization temperature was found to increase the amount of copolymer formed and its redox charge. The presence of aniline units in PASs led to a pronounced increase in the molecular weight and conductivity, and a decrease in the solubility in organic solvents. Repetitive charging/discharging cycles showed that PASs resist degradation more than polyaniline. Copyright © 2003 Society of Chemical Industry 相似文献