全文获取类型
收费全文 | 445347篇 |
免费 | 4106篇 |
国内免费 | 648篇 |
专业分类
电工技术 | 7552篇 |
综合类 | 340篇 |
化学工业 | 72019篇 |
金属工艺 | 21667篇 |
机械仪表 | 15357篇 |
建筑科学 | 9038篇 |
矿业工程 | 4519篇 |
能源动力 | 8489篇 |
轻工业 | 32643篇 |
水利工程 | 6032篇 |
石油天然气 | 15373篇 |
武器工业 | 55篇 |
无线电 | 41083篇 |
一般工业技术 | 96720篇 |
冶金工业 | 71776篇 |
原子能技术 | 14691篇 |
自动化技术 | 32747篇 |
出版年
2021年 | 4241篇 |
2019年 | 4094篇 |
2018年 | 7535篇 |
2017年 | 7778篇 |
2016年 | 8233篇 |
2015年 | 4640篇 |
2014年 | 8066篇 |
2013年 | 18411篇 |
2012年 | 11963篇 |
2011年 | 15429篇 |
2010年 | 12394篇 |
2009年 | 13736篇 |
2008年 | 14246篇 |
2007年 | 14098篇 |
2006年 | 11967篇 |
2005年 | 10497篇 |
2004年 | 10245篇 |
2003年 | 9919篇 |
2002年 | 9915篇 |
2001年 | 9632篇 |
2000年 | 9443篇 |
1999年 | 8854篇 |
1998年 | 18673篇 |
1997年 | 13820篇 |
1996年 | 10631篇 |
1995年 | 8474篇 |
1994年 | 7707篇 |
1993年 | 7723篇 |
1992年 | 6379篇 |
1991年 | 6198篇 |
1990年 | 6454篇 |
1989年 | 6264篇 |
1988年 | 6019篇 |
1987年 | 5632篇 |
1986年 | 5584篇 |
1985年 | 6140篇 |
1984年 | 5927篇 |
1983年 | 5566篇 |
1982年 | 5251篇 |
1981年 | 5347篇 |
1980年 | 5275篇 |
1979年 | 5383篇 |
1978年 | 5569篇 |
1977年 | 5882篇 |
1976年 | 7094篇 |
1975年 | 5031篇 |
1974年 | 5038篇 |
1973年 | 5153篇 |
1972年 | 4494篇 |
1971年 | 4143篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
On the basis of the analysis of the literature data and the data of our investigations, the role of iron and products of its oxidation in sulfide pulps is shown. The technological factors are noted, such as the pH value of the medium, concentration of modifiers, mineral composition, and points and the order of batching flotation reagents; which affect the redox processes with the participation of iron and the characteristics of separation of sulfide minerals during flotation. 相似文献
972.
Consideration was given to the characteristic polynomials with special affine uncertainty. For this family, the stability domain in the parameter space was shown to be a union of polyhedra. For continuous-time and discrete-time systems, a simple method was proposed to single out the stability domain and determine the stability radius for different norms of uncertainty. Efficiency of this method was corroborated by examples. 相似文献
973.
974.
Tiwari S.P. Namdas E.B. Ramgopal Rao V. Fichou D. Mhaisalkar S.G. 《Electron Device Letters, IEEE》2007,28(10):880-883
Solution-processed n-type organic field-effect transistors (OFETs) based on the fullerene derivative {6}-1-(3-(2- thienylethoxycarbonyl)-propyl)-{5}-l-phenyl-[5,6]-C61 (TEPP) and phenyl-C61-butyric acid methyl ester (PCBM) in a multiring source/drain structure are reported. Devices with TEPP show high electron mobility up to 7.8 x 10-2 cm2/Vs in the saturation regime for bottom-contact OFETs with Au S/D electrodes with a solution-processed fullerene derivative. The ON/OFF ratios reported in this letter, which are in the range of 105 -106, are among the highest values reported for such devices. This mobility is always higher compared to PCBM devices prepared in identical conditions. The mobility of TEPP and PCBM increased with increasing temperatures in the range of 100-300 K with activation energy of 78 and 113 meV, respectively, which suggests that the thermally activated hopping of electrons is dominant in TEPP. 相似文献
975.
We demonstrate, for the first time to our knowledge, that a fast coherent collision between two Kerr spatial solitons can give rise to a significant phase shift for both interacting beams. The maximal collision-induced phase shift ≈π rad takes place when the amplitudes of the solitons are equal (η1=η2) and the length of the interaction zone is comparable with a soliton phase period. Depending on the ratio η2/η1, and the collision angle between the solitons, the magnitude of the phase shift can be varied within a reasonable range, for example from 180° to 40°. The analysis of the effect performed by the finite-difference beam-propagation method has shown that it is insensitive to the initial phase difference between the incident beams (δi), even in the case when η 1≠η2. It has been demonstrated that the phenomenon can be used for all-optical three-soliton logic elements, which are capable of providing more than 3-dB signal amplification and possess bi -independent output characteristics 相似文献
976.
Performance of space-time codes for a large number of antennas 总被引:1,自引:0,他引:1
Biglieri E. Taricco G. Tulino A. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2002,48(7):1794-1803
We study the asymptotic behavior of space-time codes when the number of transmit and receive antennas grows to infinity. Specifically, we determine the behavior of pairwise error probabilities with maximum-likelihood (ML) decoding and with three types of receiver interfaces: the ML interface, the linear zero-forcing (ZF) interface, and the linear minimum-mean-square-error (MMSE) interface. Two situations are studied: when the number of receiving antennas grows to infinity while the number of transmitting antennas is finite, and when both numbers grow to infinity but their ratio remains constant. We show that with ML or linear interfaces the asymptotic performance of space-time codes is determined by the Euclidean distances between codewords. Moreover, with the two linear interfaces examined here the number r of receive antennas must be much larger than the number t of transmit antennas to avoid a sizeable loss of performance; on the other hand, when r ≫ t, the performance of these linear interfaces comes close to that of ML. The dependence of error probabilities on Euclidean distance is valid for intermediate signal-to-noise ratios (SNRs) even when the number of antennas is small. Simulations validate our theoretical findings, and show how asymptotic results may be substantially valid even in a nonasymptotic regime: thus, even for few antennas, off-the-shelf codes may outperform space-time codes designed ad hoc 相似文献
977.
Metal Science and Heat Treatment - 相似文献
978.
Georgii V. Samsonov 《Powder Metallurgy and Metal Ceramics》2002,41(9-10):537-547
The fundamental approaches to creation inorganic substances and materials are discused on the basis of electron structure of substances, formation of hierarchic structure and macroscopic properties of materials under various external effects that are realized in different production processes. The problems in designing materials and materials science as a whole are considered. 相似文献
979.
Dr. V. Recina Dr. D. Lundström B. Karlsson 《Metallurgical and Materials Transactions A》2002,33(9):2869-2881
The influence of chemical composition on the microstructure of the γ-titanium aluminide alloy Ti-48Al-2W-0.5Si (at. pct) and the accompanying tensile, low-cycle fatigue, and creep properties
has been evaluated. The study showed that small variations in chemical composition and casting procedures resulted in considerable
variations in the microstructure, yielding vastly different mechanical properties. Low contents of aluminum and tungsten led
to a coarse-grained lamellar (γ/α
2) microstructure with high creep resistance. A composition close to the nominal one produced a duplex (γ+γ/α
2) structure with favorable strength, ductility, and low-cycle fatigue properties. By controlling the solidification and cooling
rates at casting, a pseudoduplex (PS-DP) microstructure with a unique combination of high strength and high fatigue and creep
resistance can be obtained. These unique properties can be explained by the diffuse boundaries between the relatively small
γ grains and the neighboring lamellar colonies, combined with semicoherent interfaces between the γ and α
2 phases. At tensile and low-cycle fatigue loading, these boundaries act like high-angle boundaries, producing a virtually
fine-grained material promoting strength, whereas at creep loading, grain-boundary sliding is hindered in the semicoherent
interfaces leading to high creep resistance. 相似文献
980.
S. Abiraman H. K. Varma T. V. Kumari P. R. Umashankar Annie John 《Bulletin of Materials Science》2002,25(5):419-429
This study investigates quantitatively and qualitatively the sol-gel derived bioactive glass-ceramic system (BGS)—apatite-wollastonite
(AW) type granules in the size range of 0.5–1 mm, as an effective graft material for bone augmentation and restoration. Scanning
electron micrographs (SEM) of the sintered granules revealed the rough material surface with micropores in the range 10–30
μm. X-ray diffraction (XRD) pattern of the granules revealed the presence of crystalline phases of the hydroxyapatite and
wollastonite, and the functional groups of the silicate and phosphates were identified by Fourier transform infrared spectroscopy
(FT-IR). Thein vitro cell culture studies with L929 mouse fibroblast cell line showed very few cells adhered on the BGS disc after 24 h. This
could be due to the highly reactive surface of the disc concomitant with the crystallization but not due to the cytotoxicity
of the material, since the cellular viability (MTT assay) with the material was 80‰ Cytotoxicity and cytocompatibility studies
proved that the material was non-toxic and biocompatible. After 12 weeks of implantation of the BGS granules in the tibia
bone of New Zealand white rabbits, the granules were found to be well osteointegrated, as observed in the radiographs. Angiogram
with barium sulphate and Indian ink after 12 weeks showed the presence of microcapillaries in the vicinity of the implant
site implicating high vascularity. Gross observation of the implant site did not show any inflammation or necrosis. SEM of
the implanted site after 24 weeks revealed good osteointegration of the material with the newly formed bone and host bone.
New bone was also observed within the material, which was degrading. Histological evaluation of the bone healing with the
BGS granules in the tibial defect at all time intervals was without inflammation or fibrous tissue encapsulation. After 2
weeks the new bone was observed as a trabeculae network around the granules, and by 6 weeks the defect was completely closed
with immature woven bone. By 12 weeks mature woven bone was observed, and new immature woven bone was seen within the cracks
of the granules. After 24 weeks the defect was completely healed with lamellar bone and the size of the granules decreased.
Histomorphometrically the area percentage of new bone formed was 67.77% after 12 weeks and 63.37% after 24 weeks. Less bone
formation after 24 weeks was due to an increased implant surface area contributed by the material degradation and active bone
remodeling. The osteostimulative and osteoconductive potential of the BGS granules was established by tetracycline labelling
of the mineralizing areas by 2 and 6 weeks. This sol-gel derived BGS granules proved to be bioactive and resorbable which
in turn encouraged active bone formation. 相似文献