全文获取类型
收费全文 | 725496篇 |
免费 | 18932篇 |
国内免费 | 7553篇 |
专业分类
电工技术 | 19058篇 |
技术理论 | 9篇 |
综合类 | 9301篇 |
化学工业 | 109112篇 |
金属工艺 | 33602篇 |
机械仪表 | 28135篇 |
建筑科学 | 23247篇 |
矿业工程 | 6544篇 |
能源动力 | 18449篇 |
轻工业 | 57079篇 |
水利工程 | 8982篇 |
石油天然气 | 18391篇 |
武器工业 | 1084篇 |
无线电 | 80452篇 |
一般工业技术 | 136979篇 |
冶金工业 | 116256篇 |
原子能技术 | 14653篇 |
自动化技术 | 70648篇 |
出版年
2022年 | 7614篇 |
2021年 | 10865篇 |
2020年 | 7938篇 |
2019年 | 8300篇 |
2018年 | 17908篇 |
2017年 | 18538篇 |
2016年 | 15704篇 |
2015年 | 11276篇 |
2014年 | 16088篇 |
2013年 | 32345篇 |
2012年 | 25347篇 |
2011年 | 35325篇 |
2010年 | 29456篇 |
2009年 | 30352篇 |
2008年 | 30503篇 |
2007年 | 31183篇 |
2006年 | 22406篇 |
2005年 | 22477篇 |
2004年 | 19452篇 |
2003年 | 18972篇 |
2002年 | 17582篇 |
2001年 | 16916篇 |
2000年 | 15434篇 |
1999年 | 15709篇 |
1998年 | 35174篇 |
1997年 | 24864篇 |
1996年 | 19248篇 |
1995年 | 14615篇 |
1994年 | 12617篇 |
1993年 | 12071篇 |
1992年 | 8940篇 |
1991年 | 8326篇 |
1990年 | 8129篇 |
1989年 | 7792篇 |
1988年 | 7149篇 |
1987年 | 6380篇 |
1986年 | 6190篇 |
1985年 | 6767篇 |
1984年 | 6373篇 |
1983年 | 5616篇 |
1982年 | 5339篇 |
1981年 | 5426篇 |
1980年 | 5146篇 |
1979年 | 5107篇 |
1978年 | 4934篇 |
1977年 | 5821篇 |
1976年 | 7470篇 |
1975年 | 4279篇 |
1974年 | 4033篇 |
1973年 | 4142篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
WCDMA作为第三代移动通信系统的主流标准,能够提供多种类型的多媒体服务。为了实现各种用户平面数据的传输,系统会提前通过控制平面对所需的无线资源进行分配。由于配置过程是通过接口之间发送控制平面应用协议消息来实现的,因此能否正确有效地对信令消息进行编解码成为保证系统稳定工作的关键因素。3GPP标准中,ASN.1语言被用来描述接口信令消息。首先简要介绍了WCDMA系统结构以及主要接口协议,重点描述了ASN.1存在的意义及其编解码规则,最后给出了基于ASN.1的开发环境下应用层网络协议的开发流程。 相似文献
72.
The Boundary Element Method (BEM) incorporating the Embedded Cell Approach (ECA) has been used to analyse the effects of constituent material properties, fibre spatial distribution and microcrack damage on the localised behaviour of transversely fractured, unidirectional fibre-reinforced composites. Three specific composites, i.e., glass fibre reinforced polyester, carbon fibre reinforced epoxy and a glass-carbon hybrid, are considered. The geometrical structures examined were perfectly periodic, uniformly spaced fibre arrangements in square and hexagonal embedded cells. In addition, numerical simulations were also conducted using embedded cells containing randomly distributed fibres. The models involve both elastic fibres and matrix, with the interfaces between the different phases being fully bonded. The results indicate that the constituent material properties (two phase composite) and spatial distribution have a significant effect on the localised stress distributions around the primary crack tip. However, the strain energy release rate associated with crack propagation is predominantly influenced by the material composition. The three-phase hybrid composite exhibited an apparent intermediate fracture toughness value, compared to the all-glass and all-carbon models. Furthermore, the strain energy release rate for the macrocrack lowers as it enters a zone of localised damage (microcracking). The presence of microcracks relaxes the stress field, which can result in a significant reduction in the energetics of the primary crack. 相似文献
73.
Yu. V. Baldokhin V. V. Vavilova P. Ya. Kolotyrkin Yu. K. Kovneristyi N. A. Palii A. S. Solomatin 《Inorganic Materials》2003,39(6):562-567
Mössbauer effect measurements and physicochemical analysis demonstrate that annealing of amorphous Fe–P–Mn alloys leads to the formation of a nanocrystalline structure. 相似文献
74.
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
75.
As already documented in the literature, fractal image encoding is a family of techniques that achieves a good compromise between compression and perceived quality by exploiting the self-similarities present in an image. Furthermore, because of its compactness and stability, the fractal approach can be used to produce a unique signature, thus obtaining a practical image indexing system. Since fractal-based indexing systems are able to deal with the images in compressed form, they are suitable for use with large databases. We propose a system called FIRE, which is then proven to be invariant under three classes of pixel intensity transformations and under geometrical isometries such as rotations by multiples of /spl pi//2 and reflections. This property makes the system robust with respect to a large class of image transformations that can happen in practical applications: the images can be retrieved even in the presence of illumination and/or color alterations. Additionally, the experimental results show the effectiveness of FIRE in terms of both compression and retrieval accuracy. 相似文献
76.
Performance limitation due to statistical Raman crosstalk in a WDM system with multiple-wavelength bidirectionally pumped Raman amplification 总被引:1,自引:0,他引:1
A general theoretical model of statistical Raman crosstalk and its impact on system performance in a multiwavelength bidirectionally pumped Raman fiber amplifier (RFA) is developed for the first time, where we have taken modulation statistics, dispersion-induced pulse walk-off and signal-induced pump depletion into account. Two kinds of statistical Raman crosstalk, from signal-induced forward-pump depletion and from signal-signal Raman interaction, are included in one model. Formulas for normalized Raman crosstalk, Raman crosstalk-induced relative intensity noise spectral density, and its variance and system performance impact in terms of Q penalty are presented for both a single-span system and a dispersion-compensated multispan wavelength-division-multiplexed (WDM) link. Based on these formulas, we numerically investigate the impact of Raman crosstalk on system performance in a three-wavelengths bidirectionally pumped 40 /spl times/ 40-Gb/s WDM system for various fiber types. In addition, Raman crosstalk in a four-wavelength bidirectionally pumped RFA was experimentally measured. The results agree well with our theory. 相似文献
77.
G. I. Glazov A. M. Garaiev R. V. Timerkhanov 《Chemistry and Technology of Fuels and Oils》2003,39(1-2):26-53
A series of small–capacity units has recently been constructed in regions of oil field development and crude oil and gas condensate production to satisfy the requirements for such petroleum products as naphtha, diesel fuel, kerosene, and boiler fuel and to reduce costs for delivery of these products. There are almost no data in the technical literature, particularly in periodicals, on the construction and operation of small–capacity units. We attempt to generalize the experience of Orgeneftekhimzavody Trust in this area. We hope that this experience will be useful to specialists in the development and management of small–capacity plants. 相似文献
78.
The interference issues related to ultrawideband (UWB) radio pose tight restrictions on the maximum data rate of UWB radio telecommunication systems. A possible solution is to reduce the required signal to interference ratio (SIR) that gives satisfactory performance to the UWB system. In this letter, we propose coded M-ary UWB radio communication systems. Two classes of convolutional codes, namely, low-rate superorthogonal codes and high-rate punctured codes are considered for this purpose. Simulation results on the bit error rate of the proposed system indicates that the system is capable to work in lower SIR's and therefore supports higher data transmission rates in a real interference environment compared to the previously proposed UWB communication systems. 相似文献
79.
Reducible rank codes and their applications to cryptography 总被引:2,自引:0,他引:2
Gabidulin E.M. Ourivski A.V. Honary B. Ammar B. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(12):3289-3293
We present a new family of so-called reducible rank codes which are a generalization of rank product codes . This family includes maximal rank distance (MRD) codes for lengths n>N in the field F/sub N/. We give methods for encoding and decoding reducible rank codes. A public key cryptosystem based on these codes and on the idea of a column scrambler is proposed. The column scrambler "mixes" columns of a generator (parity-check) matrix of a code. It makes the system more resistant to structural attacks such as Gibson's attacks. Possible attacks on the system are thoroughly studied. The system is found to be secure against known attacks for public keys of about 16 kbits and greater. 相似文献
80.