全文获取类型
收费全文 | 85583篇 |
免费 | 192篇 |
国内免费 | 88篇 |
专业分类
电工技术 | 1882篇 |
综合类 | 94篇 |
化学工业 | 10965篇 |
金属工艺 | 7526篇 |
机械仪表 | 6261篇 |
建筑科学 | 1524篇 |
矿业工程 | 912篇 |
能源动力 | 776篇 |
轻工业 | 1379篇 |
水利工程 | 1293篇 |
石油天然气 | 2168篇 |
武器工业 | 1篇 |
无线电 | 8023篇 |
一般工业技术 | 22437篇 |
冶金工业 | 6898篇 |
原子能技术 | 1431篇 |
自动化技术 | 12293篇 |
出版年
2018年 | 9354篇 |
2017年 | 10032篇 |
2016年 | 5301篇 |
2015年 | 651篇 |
2014年 | 218篇 |
2013年 | 214篇 |
2012年 | 2993篇 |
2011年 | 9232篇 |
2010年 | 8863篇 |
2009年 | 7550篇 |
2008年 | 7281篇 |
2007年 | 9510篇 |
2006年 | 192篇 |
2005年 | 4068篇 |
2004年 | 2307篇 |
2003年 | 1923篇 |
2002年 | 809篇 |
2001年 | 175篇 |
2000年 | 258篇 |
1999年 | 90篇 |
1998年 | 92篇 |
1997年 | 83篇 |
1993年 | 85篇 |
1992年 | 95篇 |
1991年 | 109篇 |
1989年 | 92篇 |
1988年 | 118篇 |
1987年 | 96篇 |
1986年 | 85篇 |
1980年 | 83篇 |
1979年 | 100篇 |
1978年 | 119篇 |
1976年 | 93篇 |
1975年 | 97篇 |
1974年 | 110篇 |
1973年 | 106篇 |
1972年 | 109篇 |
1969年 | 124篇 |
1968年 | 177篇 |
1967年 | 166篇 |
1966年 | 183篇 |
1965年 | 165篇 |
1964年 | 92篇 |
1961年 | 93篇 |
1960年 | 106篇 |
1958年 | 87篇 |
1957年 | 131篇 |
1956年 | 123篇 |
1955年 | 139篇 |
1954年 | 153篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
61.
Effect of doping of carbon nanotubes by magnetic transition metal atoms has been considered in this paper. In the case of
semiconducting tubes, it was found that the system has zero magnetization, whereas in metallic tubes the valence electrons
of the tube screen the magnetization of the dopants: the coupling to the tube is usually antiferromagnetic (except for Cr). 相似文献
62.
Starting from a microscopic Hamiltonian defined on a semi-infinite cubic lattice, and employing a mean-field approximation, the surface parameters relevant for wetting in confined ternary mixtures are derived. These are found in terms of the microscopic coupling constants, and yield a physical interpretation of their origins. In comparison with the standard expression for the surface free-energy density, several new terms arising from the derivation are identified. The influence of the surface parameters on a predicted unbinding transition in a mixture of oil, water, and amphiphile demonstrate that existing results are robust to the addition of the extra surface terms. 相似文献
63.
P. G. Muzykov Y. I. Khlebnikov S. V. Regula Y. Gao T. S. Sudarshan 《Journal of Electronic Materials》2003,32(6):505-510
To establish fast, nondestructive, and inexpensive methods for resistivity measurements of SiC wafers, different resistivity-measurement
techniques were tested for characterization of semi-insulating SiC wafers, namely, the four-point probe method with removable
graphite contacts, the van der Pauw method with annealed metal and diffused contacts, the current-voltage (I-V) technique,
and the contactless resistivity-measurement method. Comparison of different techniques is presented. The resistivity values
of the semi-insulating SiC wafer measured using different techniques agree fairly well. As a result, application of removable
graphite contacts is proposed for fast and nondestructive resistivity measurement of SiC wafers using the four-point probe
method. High-temperature van der Pauw and room-temperature Hall characterization for the tested semi-insulating SiC wafer
was also obtained and reported in this work. 相似文献
64.
The main objective of this present study was to evaluate, for a standard mortar, the drying effect on its mechanical behaviour. Numerous uniaxial compression tests were thus performed with loading-unloading cycles. They were carried out on different samples previously preserved under various conditions of conservation: preserved from desiccation, air drying and rapid drying at 60°C. The obtained results showed significant influences of these conditions on the material behaviour (increase in strength, decrease in Young's modulus and Poisson's ratio) and the necessity of taking into account the coupling effects between mechanical—poromechanical behaviours and drying. 相似文献
65.
A mechanism of soldering of an aluminum alloy die casting to a steel die is proposed. A soldering critical temperature is
postulated, at which iron begins to react with aluminum to form an aluminum-rich liquid phase and solid intermetallic compounds.
The liquid joins the die with the casting upon solidification. The critical temperature is determined by the elements in both
the casting alloy and the die material and is equal to the solidus temperature of the resulting alloy. The critical temperature
is used to predict the onset of die soldering, and the local liquid fraction is related to the soldering tendency. Experiments
have been carried out to validate the concept and to determine the critical temperature for die soldering in an iron-aluminum
system. Thermodynamic calculations are used to determine the critical temperature and soldering tendency for the cases of
pure aluminum and a 380 alloy in a steel mold. Factors affecting the soldering tendency are discussed, and methods for reducing
die soldering are suggested. 相似文献
66.
D. G. Pavel’ev N. V. Demarina Yu. I. Koshurinov A. P. Vasil’ev E. S. Semenova A. E. Zhukov V. M. Ustinov 《Semiconductors》2004,38(9):1105-1110
Characteristics of ohmic InGaAs contacts in planar diodes based on semiconductor superlattices with a small-area active region
(1–10 μm2) are studied. The diodes were formed on the basis of short (18 or 30 periods) heavily doped (1018 cm−3) GaAs/AlAs superlattices with a miniband width of 24.4 meV. The reduced resistance of the ohmic contact was equal to 2×10−7 Ω cm2 at room temperature. It is shown that the properties of fabricated planar diodes make it possible to use these diodes later
on in semiconductor devices that operate in the terahertz frequency region in a wide temperature range (4–300 K).
__________
Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 9, 2004, pp. 1141–1146.
Original Russian Text Copyright ? 2004 by Pavel’ev, Demarina, Koshurinov, Vasil’ev, Semenova, Zhukov, Ustinov. 相似文献
67.
A system composed of two heavy holes located in a two-dimensional (2D) quantum well (QW) and bound via mediation of an electron in a neighboring 2D QW is considered. Using a simple qualitative trial wave function, the ground-state energy of this kind of X+ trion is determined in the infinite-hole-mass approximation as a function of the QW spacing. Coordinate dependence of the effective potential binding the holes to each other is calculated for different values of QW spacing. In the adiabatic approximation, a set of dependences describing the X+ trion binding energy as a function of the electron mass to the hole mass ratio is obtained. Several estimates for the trion binding energy in GaAs-and ZnSe-based double-QW heterostructures are given. 相似文献
68.
Polycrystalline Cu(In,Ga)Se2 (CIGS) films with various ratios of Cu, In, and Ga were grown by codeposition of all elements in vacuum. The X-ray diffraction
study showed that the films are single-phase and possess a chalcopyrite structure with predominant [112] orientation. The
films exhibited a mirror smooth surface and had a close-packed structure composed of crystallites with clear faceting and
a transverse size of 0.1–0.3 μm. Related surface barrier structures of the (In,Ag)/Cu(In,Ga)Se2 type were obtained and their spectra of the quantum efficiency of photoconversion were studied. The obtained structures can
be used for optimization of the CIGS film technology. 相似文献
69.
Examination of the wreckage of a light aircraft revealed that approximately 20 cm was missing from one tip of the aluminum
alloy propeller. Fractographic and metallographic examination of the remaining portion of the propeller revealed extensive
grain-boundary separation in the vicinity of the fracture, and grain edges and corners rounded by corrosion on the fracture
surface. Energy dispersive X-ray analysis (EDXA) revealed fluorine on, and in the vicinity of, the fracture surface. In the
ensuing litigation, it was asserted that the crash occurred because the propeller fractured in flight as the result of intergranular
attack caused by the use of a fluorine-bearing cleaner. 相似文献
70.
Ting Gang Zhu Uttiya Chowdhury Michael M. Wong Jonathan C. Denyszyn Russell D. Dupuis 《Journal of Electronic Materials》2002,31(5):406-410
In this paper, we report the study of the electrical characteristics of GaN and AlGaN vertical p-i-n junctions and Schottky
rectifiers grown on both sapphire and SiC substrates by metal-organic chemical-vapor deposition. For GaN p-i-n rectifiers
grown on SiC with a relatively thin “i” region of 2 μm, a breakdown voltage over 400 V, and forward voltage as low as 4.5
V at 100 A/cm2 are exhibited for a 60-μm-diameter device. A GaN Schottky diode with a 2-μm-thick undoped layer exhibits a blocking voltage
in excess of ∼230 V at a reverse-leakage current density below 1 mA/cm2, and a forward-voltage drop of 3.5 V at a current density of 100 A/cm2. It has been found that with the same device structure and process approach, the leakage current of a device grown on a SiC
substrate is much lower than a device grown on a sapphire substrate. The use of Mg ion implantation for p-guard rings as planar-edge
terminations in mesageometry GaN Schottky rectifiers has also been studied. 相似文献