全文获取类型
收费全文 | 546510篇 |
免费 | 6344篇 |
国内免费 | 1417篇 |
专业分类
电工技术 | 10218篇 |
综合类 | 709篇 |
化学工业 | 84353篇 |
金属工艺 | 20440篇 |
机械仪表 | 15810篇 |
建筑科学 | 13643篇 |
矿业工程 | 2534篇 |
能源动力 | 14135篇 |
轻工业 | 50891篇 |
水利工程 | 5316篇 |
石油天然气 | 9800篇 |
武器工业 | 22篇 |
无线电 | 64195篇 |
一般工业技术 | 105081篇 |
冶金工业 | 100258篇 |
原子能技术 | 11689篇 |
自动化技术 | 45177篇 |
出版年
2021年 | 4039篇 |
2019年 | 3786篇 |
2018年 | 6664篇 |
2017年 | 6506篇 |
2016年 | 6879篇 |
2015年 | 4669篇 |
2014年 | 7988篇 |
2013年 | 24243篇 |
2012年 | 12948篇 |
2011年 | 17770篇 |
2010年 | 14284篇 |
2009年 | 16262篇 |
2008年 | 16926篇 |
2007年 | 16792篇 |
2006年 | 14894篇 |
2005年 | 13801篇 |
2004年 | 13271篇 |
2003年 | 12836篇 |
2002年 | 12772篇 |
2001年 | 12762篇 |
2000年 | 12112篇 |
1999年 | 12477篇 |
1998年 | 29642篇 |
1997年 | 21800篇 |
1996年 | 17061篇 |
1995年 | 13092篇 |
1994年 | 11881篇 |
1993年 | 11592篇 |
1992年 | 8875篇 |
1991年 | 8591篇 |
1990年 | 8388篇 |
1989年 | 8254篇 |
1988年 | 8082篇 |
1987年 | 6856篇 |
1986年 | 6943篇 |
1985年 | 8061篇 |
1984年 | 7516篇 |
1983年 | 7004篇 |
1982年 | 6447篇 |
1981年 | 6744篇 |
1980年 | 6333篇 |
1979年 | 6238篇 |
1978年 | 6081篇 |
1977年 | 7129篇 |
1976年 | 9218篇 |
1975年 | 5492篇 |
1974年 | 5192篇 |
1973年 | 5248篇 |
1972年 | 4455篇 |
1971年 | 4040篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
This paper presents a new computerized procedure for dealing with the design of horizontal ground heat exchangers (HGHE). The computer program is based on the transient model of coupled nonlinear partial differential equations governing heat and mass flow in soils. The model is two-dimensional and delineates the operation of ground heat storage with the HGHE and such phenomena as freezing/thawing and drying/rewetting of soil moisture. Comprehensive climatological data, such as ambient temperature, solar radiation, wind velocity, rainfall, snowfall, snow characterstics, and water vapour pressure is used to simulate conditions at the ground surface over any required length of time. The package can be applied to any geographical location by changing climatic and soil data input. The designer has the possibility of selecting any of 12 types of soils from sand to clay, 12 commercial heat pumps, nine different configurations of the HGHE, 16 plastic pipes for ground coils, and 13 ground coil fluids. The program, however, does not calculate the length of the HGHE but it evaluates the thermodynamic performance of a ground heat pump system and provides comprehensive data on thermal and hydraulic conditions in ground heat storage. The length of the ground heat exchanger is obtained from a line source theory model or from site dimensions and pipe spacing. Computed results for ground heat exchanger operation correlate fairly well with experimental data. Simulation of temperature and moisture content in the ground for natural conditions (no heat extraction/deposition) showed a fair agreement with field data. The entire computer program is user-friendly, interactive, menu-driven, and written in FORTRAN 77. 相似文献
952.
A cryo-specimen storage system for low-temperature scanning electron microscopy (LTSEM) specimens is described, which: liberates multi-specimen experiments from sampling restrictions imposed by the rate at which LTSEM specimens can be examined in the SEM; provides security against experiment loss resulting from breakdown of the SEM or cryo-system; enables collection of specimens in the field or in laboratories remote from the SEM laboratory; and facilitates international air transport of LTSEM specimens. The components of the system, which has a capacity of 98 stub-mounted specimens, are readily made in a laboratory workshop. The details of the design may be altered to suit particular specimen types or experimental approaches. 相似文献
953.
A precoding scheme for noise whitening on intersymbol interference (ISI) channels is presented. This scheme is compatible with trellis-coded modulation and, unlike Tomlinson precoding, allows constellation shaping. It can be used with almost any shaping scheme, including the optimal SVQ shaping, as opposed to trellis precoding, which can only be used with trellis shaping. The implementation complexity of this scheme is minimal-only three times that of the noise prediction filter, hence effective noise whitening can be achieved by using a high-order predictor 相似文献
954.
R. H. Howell P. A. Sterne M. J. Fluss J. H. Kaiser K. Kitazawa H. Kojima 《Journal of Superconductivity and Novel Magnetism》1994,7(1):169-173
High-statistics (>4 × 108 counts), room-temperature measurements of the electron-positron momentum density of La2?x Sr x CuO4 have been performed for samples with Sr concentrations of x=0.0, 0.1, 0.13, and 0.2. These spectra have been analyzed in conjunction with theoretical calculations of the electron-positron momentum density. The metallic samples show features consistent with the presence of a Fermi surface, but its evolution with increasing Sr concentration does not follow the predictions of band theory. These results may indicate the effects of electron-electron correlation on the electron momentum distribution in the Cu-O plane. 相似文献
955.
Aspergillus sydowii MG49 produces a 30-kDa exosplitting xylobiohydrolase during growth on xylan. A specific chemical modification and substrate protection analysis of purified xylanase provided evidence that tryptophan and carboxy and amino groups are present at the catalytic site of this enzyme. Thermal inactivation of the xylanase occurs because of irreversible polymolecular aggregation, which is slower in the presence of glycerol. 相似文献
956.
A wideband low-noise pseudomorphic HEMT MMIC variable-gain amplifier has been designed and fabricated. The amplifier has a nominal gain of 13 dB across the band 2-20 GHz, with gain flatness better than ±0.4 dB. The noise figure is less than 3 dB across the band 6-16 GHz. An on-chip temperature-sensing diode is used to provide a linear temperature correction which has been used to reduce the gain variation of the amplifier by a factor of 2 across the temperature range -50°C to +95°C 相似文献
957.
Yayla G. Krishnamoorthy A.V. Marsden G.C. Esener S.C. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1994,82(11):1749-1762
We report the implementation of a prototype three-dimensional (3D) optoelectronic neural network that combines free-space optical interconnects with silicon-VLSI-based optoelectronic circuits. The prototype system consists of a 16-node input, 4-neuron hidden, and a single-neuron output layer, where the denser input-to-hidden-layer connections are optical. The input layer uses PLZT light modulators to generate optical outputs which are distributed over an optoelectronic neural network chip through space-invariant holographic optical interconnects. Optical interconnections provide negligible fan-out delay and allow compact, purely on-chip electronic H-tree type fan-in structure. The small prototype system achieves a measured 8-bit electronic fan-in precision and a calculated maximum speed of 640 million interconnections per second. The system was tested using synaptic weights learned off system and was shown to distinguish any vertical line from any horizontal one in an image of 4×4 pixels. New, more efficient light detector and small-area analog synapse circuits and denser optoelectronic neuron layouts are proposed to scale up the system. A high-speed, feed-forward optoelectronic synapse implementation density of up to 104/cm2 seems feasible using new synapse design. A scaling analysis of the system shows that the optically interconnected neural network implementation can provide higher fan-in speed and lower power consumption characteristics than a purely electronic, crossbar-based neural network implementation 相似文献
958.
959.
Sodium tolerance in yeast is disrupted by mutations in calcineurin, a Ca2+/calmodulin-dependent protein phosphatase, which is required for modulation of Na+ uptake and efflux mechanisms. Five Na+-tolerant mutants were isolated by selecting for suppressors of calcineurin mutations, and mapped to the PMA1 gene, encoding the plasma membrane H+-ATPase. One mutant, pma1-alpha4, which has the single amino acid change Glu367 --> Lys at a highly conserved site within the catalytic domain of the ATPase, was analyzed in detail to determine the mechanism of Na+ tolerance. After exposure to Na+ in the culture medium, 22Na influx in the pma1 mutant was reduced 2-fold relative to control, consistent with a similar decrease in ATPase activity. Efflux of 22Na from intact cells was relatively unchanged in the pma1 mutant. However, selective permeabilization of the plasma membrane revealed that mutant cells retained up to 80% of intracellular Na+ within a slowly exchanging pool. We show that NHX1, a novel gene homologous to the mammalian NHE family of Na+/H+ exchangers, is required for Na+ sequestration in yeast and contributes to the Na+-tolerant phenotype of pma1-alpha4. 相似文献
960.
M Ramirez-Salomon R Soler-Bientz R de la Garza-González CM Palacios-Garza 《Canadian Metallurgical Quarterly》1997,23(9):586-587
It is important for endodontic instruments to have a low fracture rate. If a fracture does occur, it would be desirable to have the ability to bypass the broken segment and complete the root canal treatment. One hundred sixty-two root canals in 52 maxillary and mandibular first molars were cleaned and shaped with Lightspeed instruments by three endodontists in their private practices. The canals were instrumented using the technique recommended by the manufacturer. All canals were instrumented to at least a size 45 at the working length. Six instruments separated during treatment. All six had been used more times than recommended by the manufacturer. Five of the six were easily bypassed and treatment completed. 相似文献