全文获取类型
收费全文 | 31793篇 |
免费 | 2913篇 |
国内免费 | 2436篇 |
专业分类
电工技术 | 716篇 |
综合类 | 2377篇 |
化学工业 | 8115篇 |
金属工艺 | 3213篇 |
机械仪表 | 1444篇 |
建筑科学 | 1215篇 |
矿业工程 | 1040篇 |
能源动力 | 638篇 |
轻工业 | 2114篇 |
水利工程 | 408篇 |
石油天然气 | 800篇 |
武器工业 | 393篇 |
无线电 | 4739篇 |
一般工业技术 | 5743篇 |
冶金工业 | 2088篇 |
原子能技术 | 397篇 |
自动化技术 | 1702篇 |
出版年
2024年 | 120篇 |
2023年 | 500篇 |
2022年 | 810篇 |
2021年 | 963篇 |
2020年 | 1049篇 |
2019年 | 992篇 |
2018年 | 931篇 |
2017年 | 1213篇 |
2016年 | 1142篇 |
2015年 | 1125篇 |
2014年 | 1671篇 |
2013年 | 2077篇 |
2012年 | 2067篇 |
2011年 | 2265篇 |
2010年 | 1546篇 |
2009年 | 1878篇 |
2008年 | 1638篇 |
2007年 | 1950篇 |
2006年 | 1990篇 |
2005年 | 1708篇 |
2004年 | 1410篇 |
2003年 | 1199篇 |
2002年 | 1080篇 |
2001年 | 863篇 |
2000年 | 743篇 |
1999年 | 711篇 |
1998年 | 539篇 |
1997年 | 486篇 |
1996年 | 378篇 |
1995年 | 343篇 |
1994年 | 331篇 |
1993年 | 263篇 |
1992年 | 216篇 |
1991年 | 196篇 |
1990年 | 164篇 |
1989年 | 122篇 |
1988年 | 90篇 |
1987年 | 54篇 |
1986年 | 43篇 |
1985年 | 49篇 |
1984年 | 36篇 |
1983年 | 28篇 |
1982年 | 32篇 |
1981年 | 13篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1978年 | 8篇 |
1964年 | 7篇 |
1959年 | 11篇 |
1951年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
J. Gegner 《Materialwissenschaft und Werkstofftechnik》2003,34(3):290-297
Chemical Composition and Microstructure of Polymer‐Derived Glasses and Ceramics in the Si–C–O System. Part 2: Characterization of microstructure formation by means of high‐resolution transmission electron microscopy and selected area diffraction Liquid or solid silicone resins represent the economically most interesting class of organic precursors for the pyrolytic production of glass and ceramics materials on silicon basis. As dense, dimensionally stable components can be cost‐effectively achieved by admixing reactive filler powders, chemical composition and microstructure development of the polymer‐derived residues must be exactly known during thermal decomposition. Thus, in the present work, glasses and ceramics produced by pyrolysis of the model precursor polymethylsiloxane at temperatures from 525 to 1550 °C are investigated. In part 1, by means of analytical electron microscopy, the bonding state of silicon was determined on a nanometre scale and the phase separation of the metastable Si–C–O matrix into SiO2, C and SiC was proved. The in‐situ crystallization could be considerably accelerated by adding fine‐grained powder of inert fillers, such as Al2O3 or SiC, which permits effective process control. In part 2, the microstructure is characterized by high‐resolution transmission electron microscopy and selected area diffraction. Turbostratic carbon and cubic β‐SiC precipitate as crystallization products. Theses phases are embedded in an amorphous matrix. Inert fillers reduce the crystallization temperature by several hundred °C. In this case, the polymer‐derived Si–C–O material acts as a binding agent between the powder particles. Reaction layer formation does not occur. On the investigated pyrolysis conditions, no crystallization of SiO2 was observed. 相似文献
2.
Banded textures produced in a thermotropic liquid crystal polymer by shearing between glass slides are examined by using both transmission electron and polarized light microscopy. The periodic variation in director orientation about the shear axis, as measured by light microscopy, is shown to be distinctly different from that indicated by electron diffraction. Measurements of birefringence and observation of Zernicke phase contrast indicate periodic variations in optical properties of the polymer, in step with the bands. Such effects are accounted for in terms of a synchronous rotation of the planar aromatic groups about the molecular chain axes. Evidence for an out-of-plane component of molecular orientation is also presented. 相似文献
3.
Mayank Kashyap 《Powder Technology》2008,183(3):441-453
The effect of electric field on the hydrodynamics of nanoparticles was studied in a fluidized rectangular bed, with electrodes attached to two parallel walls. It was shown that the electric field of the order of 3 times the gravity markedly decreased the bed expansion and increased the solids volume fraction of nanoparticles fluidized by air. In these experiments, a light diode assembly was utilized to infer the local solids volume fractions within a rectangular bed of 10 nm silica particles. These experimental measurements yielded a two dimensional solids volume fraction distribution within the rectangular bed. The experimental results provided some new insights into the distribution of solids within the bed. The agglomerate diameters were computed using a momentum balance with the drag given by the Ergun equation and the empirical Richardson-Zaki method. Both methods yielded agglomerate diameters of the order of 100 μm and showed dependence on the strength of the electric field. The electric field decreased the granular temperature of the nanoparticles. 相似文献
4.
M. Jayasankar 《Materials Letters》2007,61(3):790-793
Formation of aluminium titanate (AT) has been achieved at low temperature through sol-gel process using boehmite and titanium hydroxide as precursors by controlling the particle size at nanoscale followed by in-situ peptisation. The formations of AT phase, particle size distributions, sintering and thermal expansion characteristics, and microstructural features have been reported. DTA and XRD analysis have been performed to confirm the formation of AT. A 94% relative density was obtained for aluminium titanate sintered at 1550 °C with controlled grain size in the range of 2-3 μm. 相似文献
5.
Alena Orlov 《Materials Science and Engineering: A》1996,220(1-2):117-122
The present work studies the collection of experimental data from which Raj and Pharr (Mater. Sci. Eng., 81 (1986) 217) deduced a universal empirical dependence of the subgrain size on the applied stress. In accord with their result and some theoretical predictions the normalized subgrain size ds/b was ssumed to be proportional to G/σ (G is the shear modulus, b the Burgers vector length, σ the applied stress). The evaluated factor of proportionality K1, having the value within the interval from 0.76 to 180 in the inspected data sets, was discussed from the point of view of various factors which can influence the experimental data. 相似文献
6.
ListofSymbol B———Buoyancy ,m·s- 2 ; c———Concentrationofsoluteelement ; Cμ———Turbulentconstant; D———Diffusivityofsoluteelement ,m2 ·s- 1 ; fl,fs———Liquidandsolidfraction ; fμ———Turbulentcoefficient ; h———Enthalpy ,J·kg- 1 ; k———Turbulentkineticenergy ,m2 ·s- 2 ; kp———Equilibriumpartitioncoefficient; Kp———Permeabilityofmushyzone ,m2 ; K0 ———Permeabilitycoefficient; p———Pressure ,Pa ; Pr———Prandtlnumber ;… 相似文献
7.
J. Lub W.P.M. Nijssen R.T. Wegh J.P.A. Vogels A. Ferrer 《Advanced functional materials》2005,15(12):1961-1972
This paper describes the synthesis of photoisomerizable derivatives of isosorbide. These derivatives contain a stilbene or cinnamate moiety and can therefore be used as photoisomerizable chiral compounds in cholesteric liquid‐crystalline mixtures. The reflection wavelength of cholesteric layers made from these mixtures is increased by UV irradiation due to the fact that the Z‐isomers of these derivatives exhibit a lower helical twisting power than the corresponding E‐isomers. The cinnamate derivatives are very suitable for use in cholesteric color filters that find application in liquid‐crystal displays. 相似文献
8.
Optical absorption and photoluminescence of Ca3(VO4)2 single crystal grown by a floating-zone technique and containing Nd3+ ions were investigated. High absorption coefficients and broadening of most absorption bands are present at 300 K, while substructures in some of the same bands can be evidenced at 12 K. Most features of measured spectra are characteristic of random occupation of more than a single Ca2+ site by the Nd3+ ion and of distortions provoked by different charge compensation mechanisms involving oxygen vacancies promotion in the crystal lattice. Nd3+ optical properties were studied by using the Judd-Ofelt theory to calculate the spectral parameters relevant for laser applications. 相似文献
9.
By using scanning polarization force microscopy,the deliquescence process and the atomic steps on the cleavage surface of CaCO3 in air were studied in situ.Under an exposure to medium umidity(-57%),the sloiw step movement has been observed. 相似文献
10.
StudyonFactorsAffectingtheStructureofHighSpeedSteelIngotProducedbyESRLiZhengbang;CheXiangqianAbstract:Theinfluenceofthemetalp... 相似文献