全文获取类型
收费全文 | 4698篇 |
免费 | 310篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 35篇 |
综合类 | 3篇 |
化学工业 | 1694篇 |
金属工艺 | 46篇 |
机械仪表 | 126篇 |
建筑科学 | 153篇 |
矿业工程 | 3篇 |
能源动力 | 117篇 |
轻工业 | 1259篇 |
水利工程 | 24篇 |
石油天然气 | 21篇 |
无线电 | 159篇 |
一般工业技术 | 665篇 |
冶金工业 | 194篇 |
原子能技术 | 16篇 |
自动化技术 | 497篇 |
出版年
2024年 | 15篇 |
2023年 | 64篇 |
2022年 | 296篇 |
2021年 | 340篇 |
2020年 | 173篇 |
2019年 | 188篇 |
2018年 | 191篇 |
2017年 | 215篇 |
2016年 | 206篇 |
2015年 | 143篇 |
2014年 | 243篇 |
2013年 | 363篇 |
2012年 | 331篇 |
2011年 | 358篇 |
2010年 | 233篇 |
2009年 | 255篇 |
2008年 | 231篇 |
2007年 | 219篇 |
2006年 | 135篇 |
2005年 | 128篇 |
2004年 | 99篇 |
2003年 | 79篇 |
2002年 | 67篇 |
2001年 | 51篇 |
2000年 | 41篇 |
1999年 | 32篇 |
1998年 | 63篇 |
1997年 | 71篇 |
1996年 | 43篇 |
1995年 | 32篇 |
1994年 | 12篇 |
1993年 | 14篇 |
1992年 | 9篇 |
1991年 | 12篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 4篇 |
1975年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有5012条查询结果,搜索用时 15 毫秒
71.
Andreas Lehner Cristina Rico García Thomas Strang 《Wireless Personal Communications》2013,69(4):1647-1669
In this paper we present the results of an empirical study investigating the performance of TETRA (TErrestrial Trunked RAdio) in a Vehicular Ad-hoc Network (VANET) for safety related railway traffic applications. The Short Data Service (SDS) of TETRA in the Direct Mode Operation (DMO) allows an infrastructure-less exchange of traffic relevant information between vehicles in range of the communication. The propagation channel in case of such a direct (base station free) communication between railway vehicles underlies specific effects, which are not equivalent to other well known terrestrial mobile scenarios. We will present measurements covering urban, suburban and rural environments along a regional railway network in the south of Bavaria. Beside different operational conditions such as front, rear, and flank approaches of trains, we investigated several topological scenarios on both, single and double track sections along the line. We will also discuss the observed characteristic changes in narrow band signal attenuation and Doppler spectra for passages through forests, hilly areas, stations and a tunnel. We determined statistics for the transmission delay of messages and the message erasure rates for single and multi user access on a single common carrier for different transmission intervals. Finally, the quality of service for the envisaged safety applications is assessed. 相似文献
72.
Marcos A. Simplicio Bruno T. de Oliveira Cintia B. Margi Paulo S.L.M. Barreto Tereza C.M.B. Carvalho Mats Näslund 《Ad hoc Networks》2013,11(3):1221-1236
Security is an important concern in any modern network. This also applies to Wireless Sensor Networks (WSNs), especially those used in applications that monitor sensitive information (e.g., health care applications). However, the highly constrained nature of sensors imposes a difficult challenge: their reduced availability of memory, processing power and energy hinders the deployment of many modern cryptographic algorithms considered secure. For this reason, the choice of the most memory-, processing- and energy-efficient security solutions is of vital importance in WSNs. To date, a number of extensive analyses comparing different encryption algorithms and key management schemes have been developed, while very little attention has been given to message authentication solutions. In this paper, aiming to close this gap, we identify cipher-based Message Authentication Codes (MACs) and Authenticated Encryption with Associated Data (AEAD) schemes suitable for WSNs and then evaluate their features and performance on a real platform (TelosB). As a result of this analysis, we identify the recommended choices depending on the characteristics of the target network and available hardware. 相似文献
73.
Biologically Inspired and Magnetically Recoverable Copper Porphyrinic Catalysts: A Greener Approach for Oxidation of Hydrocarbons with Molecular Oxygen 下载免费PDF全文
César A. Henriques Auguste Fernandes Liane M. Rossi M. Filipa Ribeiro Mário J. F. Calvete Mariette M. Pereira 《Advanced functional materials》2016,26(19):3359-3368
An efficient synthetic method for magnetically recoverable hybrid copper porphyrinic nanomaterials is reported. These functionalized magnetic materials prove to be efficient bioinspired oxidation catalysts of olefins and thiols, using molecular oxygen as oxidant, in total absence of reductants and solvents, with the highest TON (turnover number) yet achieved for this reaction (≈200 000). A comparative study between homogeneous and heterogeneous oxidation of cyclohexene is discussed, revealing the heterogeneous system to be the most promising concerning stability and reusability of the catalysts. The full characterization of the magnetic hybrid porphyrinic nanomaterials, by transmission electron microscopy, flame atomic absorption spectrometry, thermogravimetry, N2 sorption, and infrared spectroscopy, is also described. 相似文献
74.
Developing Smart Grids Based on GPRS and ZigBee Technologies Using Queueing Modeling–Based Optimization Algorithm 下载免费PDF全文
Gustavo Batista de Castro Souza Flávio Henrique Teles Vieira Cláudio Ribeiro Lima Getúlio Antero de Júnior Deus Marcelo Stehling de Castro Sérgio Granato de Araujo Thiago Lara Vasques 《ETRI Journal》2016,38(1):41-51
Smart metering systems have become widespread around the world. RF mesh communication systems have contributed to the creation of smarter and more reliable power systems. This paper presents an algorithm for positioning GPRS concentrators to attain delay constraints for a ZigBee‐based mesh network. The proposed algorithm determines the number and placement of concentrators using integer linear programming and a queueing model for the given mesh network. The solutions given by the proposed algorithm are validated by verifying the communication network performance through simulations. 相似文献
75.
Eduardo M. R. Fairbairn Iuri A. Ferreira Guilherme C. Cordeiro Marcos M. Silvoso Romildo D. Toledo Filho Fernando L. B. Ribeiro 《Materials and Structures》2010,43(8):1061-1074
Cement production gives rise to CO2 emissions generated by the calcination of CaCO3 and by the combustion of fossil fuels, being responsible for about 5% of the global CO2 emissions. These emissions can be substantially reduced if cement replacement materials are used. In this paper two residual ashes that can be used as mineral additions are considered: sugar cane bagasse ash and rice husk ash. A case study of the construction of a dam with a blended material composed by cement and these two ashes is presented, indicating the potentiality of its use for civil engineering applications. The analyses were performed using experimental and numerical tools developed on the basis of a thermo-chemo-mechanical model. This model considers the coupling, within the theory of thermodynamics, of the several phenomena that intervene in the hydration process, namely, exothermicity, thermo-activation, chemo-plasticity, evolution of thermal and mechanical properties with the hydration reaction, which includes creep and relaxation. 相似文献
76.
Alexandre Antunes Ribeiro Rodrigo Fernando Costa Marques Antônio Carlos Guastaldi João Sinézio de Carvalho Campos 《Journal of Materials Science》2009,44(15):4056-4061
Many techniques have been used to coat metallic substrate with bioceramics. The aim of this study was to study the physical-chemical
characteristics of polyvinylidene fluoride (α-PVDF)/hydroxyapatite (HA) composite coating, obtained by casting method, on
commercially pure titanium (α-CP Ti) substrate surface modified by laser beam irradiation. The preparation of coating was
done for mixing α-PVDF pellets shape dissolved in dimethylacetamide (DMA) with HA/DMA emulsion. The mixture was poured onto
the α-CP Ti sample and left to dry in an oven. CP Ti plates were coated with α-PVDF/HA composite film, in proportions of 100/00
and 60/40 in weight, and characterized by particle size analysis, scanning electron microscopy, energy dispersive spectroscopy
(EDS), X-ray diffractometry, thickness measurement and contact angle. Uniform coating with a small thickness variation along
the coated surface was successfully obtained. 相似文献
77.
78.
Diogo Ribeiro Jorge Leite Nuno Pinto Rui Calada 《The Structural Design of Tall and Special Buildings》2019,28(11)
This article describes the experimental evaluation of the dynamic effects induced by wind on a high‐rise telecommunications tower based on a permanent monitoring system. Monte da Virgem telecommunications tower is located near the city of Porto (Portugal), and its structure consists in a reinforced concrete shaft and a steel mast, with a total height of 177 m. The monitoring system includes accelerometers, anemometers, and a meteorological station, allowing the characterization of the maximum accelerations of the structure and wind regimes during a period of 6 months. The analysis of the results enabled identifying specific events, denominated as critical events, for which the dynamic response of the tower under wind actions appears significantly amplified due to wind aeroelastic instability phenomena in the steel mast. The automatic identification of the critical events was based on the application to the acceleration's records of an autoregressive model and estimation of its optimal order number based on a singular value decomposition. The results proved the robustness and efficiency of the proposed technique in identifying the number, duration, and maximum amplitude of accelerations associated to the critical events, envisaging its potential integration in structural health monitoring systems. 相似文献
79.
Iris Papa Cristina Alvarez Vronique Verrez-Bagnis Joël Fleurence Yves Benyamin 《Journal of the science of food and agriculture》1996,72(1):63-70
α-Actinin release and its degradation from myofibrils Z-line were studied in post mortem white dorsal muscle from bass and sea trout stored at 4°C and 10°C. Using α-actinin specific antibodies, we show that this protein is rapidly released within the first 24 h for the two species, and reaches a plateau within 4 days. Proteolysis take place very rapidly in bass muscle yielding 80 and 40 kDa fragments from α-actinin as major bands of proteolysis. Sea trout muscle is more resistant, and muscle stored at 4°C is not significantly α-actinin degraded even 10 days after death. In the case of sea trout muscle stored at 10°C, an increasing quantity of 80 and 40 kDa fragment can be observed after the third day. These results show that release and proteolysis of α-actinin are time- and temperature-dependent processes that take place at the early stages of fish storage. Furthermore, we observed that proteolysis of α-actinin seems to be dependent on fish species. In both species studied, the early release of α-actinin comes before the degradation of released molecules, and appears as a biphasic process throughout the disorganisation of post mortem muscle in fish cold-stored above 0°C. 相似文献
80.
An improved colorimetric method, based on the well-known reaction between amylose and iodine, is described. Native starch is dissolved in ureadimethylsulfoxide and the resulting solution defatted with ethanol. An aliquot of the lipid-free solution is then reacted with iodine and the absorbance of the blue-coloured amylose-iodine complex measured, thus determining the iodine-binding capacity of starch (Blue Value). A collaborative study was conducted, according to official guidelines, to determine the method's repeatability and reproducibility: 8 participants were each sent 8 starch samples from different botanic origin for duplicate analysis. Statistical evaluation of the results gave good precision results, with an average repeatability relative standard deviation of 2.1% and an average reproducibility relative standard deviation of 5.6%. The method is, therefore, suitable to determine precisely the amylose content of native starch. 相似文献