全文获取类型
收费全文 | 897442篇 |
免费 | 25319篇 |
国内免费 | 11005篇 |
专业分类
电工技术 | 25323篇 |
技术理论 | 24篇 |
综合类 | 14705篇 |
化学工业 | 137263篇 |
金属工艺 | 39431篇 |
机械仪表 | 37167篇 |
建筑科学 | 35105篇 |
矿业工程 | 9843篇 |
能源动力 | 23776篇 |
轻工业 | 72485篇 |
水利工程 | 9541篇 |
石油天然气 | 22743篇 |
武器工业 | 1674篇 |
无线电 | 110173篇 |
一般工业技术 | 156951篇 |
冶金工业 | 132479篇 |
原子能技术 | 12905篇 |
自动化技术 | 92178篇 |
出版年
2023年 | 4983篇 |
2022年 | 9136篇 |
2021年 | 13489篇 |
2020年 | 10185篇 |
2019年 | 9939篇 |
2018年 | 20297篇 |
2017年 | 21775篇 |
2016年 | 17562篇 |
2015年 | 14469篇 |
2014年 | 20024篇 |
2013年 | 41528篇 |
2012年 | 31151篇 |
2011年 | 43529篇 |
2010年 | 36552篇 |
2009年 | 37237篇 |
2008年 | 37760篇 |
2007年 | 39196篇 |
2006年 | 30426篇 |
2005年 | 30362篇 |
2004年 | 24835篇 |
2003年 | 23359篇 |
2002年 | 21167篇 |
2001年 | 20623篇 |
2000年 | 20069篇 |
1999年 | 21909篇 |
1998年 | 42475篇 |
1997年 | 30949篇 |
1996年 | 24864篇 |
1995年 | 19421篇 |
1994年 | 17146篇 |
1993年 | 15856篇 |
1992年 | 11797篇 |
1991年 | 11043篇 |
1990年 | 10310篇 |
1989年 | 9669篇 |
1988年 | 9226篇 |
1987年 | 7671篇 |
1986年 | 7525篇 |
1985年 | 8738篇 |
1984年 | 8101篇 |
1983年 | 7260篇 |
1982年 | 6793篇 |
1981年 | 6663篇 |
1980年 | 6304篇 |
1979年 | 6004篇 |
1978年 | 5556篇 |
1977年 | 6782篇 |
1976年 | 9178篇 |
1975年 | 4621篇 |
1974年 | 4474篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Isolating applied mathematics from computer science harms the profession and those who depend on it. 相似文献
22.
Improved homogenization of Ni in sintered steels through the use of Cr-containing prealloyed powders
The homogenization of Ni in powder metal (PM) steel compacts is usually difficult even after high-temperature sintering at
1250°C. An earlier study by the authors demonstrated that this problem can be alleviated through the addition of 0.5 wt pct
Cr in the form of stainless steel powders. To further improve the microstructure and mechanical properties of Ni-containing
PM steels and to understand the mechanisms, an attempt was made in this study using the Fe-3Cr-0.5Mo prealloyed powder as
the base material. The results showed that the distribution of the Ni additives was significantly improved. As a result, the
tensile strength of the Fe-3Cr-0.5Mo-4Ni-0.5C compact sintered at 1250°C reached 1323 MPa. The elongation was higher than
1 pct. These sinter-hardened properties, which were attained using a slow furnace cooling rate, were comparable to those of
the sinter-hardened alloys reported in the literature using accelerated cooling and were equivalent to those of the best quenched-and-tempered
alloys registered in the Metal Powder Industries Federation (MPIF) standards. These improvements were attributed to the positive
effect of Cr addition on alloy homogenization due to the reduction of the repelling effect between Ni and C, as was demonstrated
through the thermodynamic analysis using the Thermo-Calc program. 相似文献
23.
Based on the current spin density functional theory, a theoretical model of three vertically aligned semiconductor quantum dots is proposed and numerically studied. This quantum dot molecule (QDM) model is treated with realistic hard-wall confinement potential and external magnetic field in three-dimensional setting. Using the effective-mass approximation with band nonparabolicity, the many-body Hamiltonian results in a cubic eigenvalue problem from a finite difference discretization. A self-consistent algorithm for solving the Schrödinger-Poisson system by using the Jacobi-Davidson method and GMRES is given to illustrate the Kohn-Sham orbitals and energies of six electrons in the molecule with some magnetic fields. It is shown that the six electrons residing in the central dot at zero magnetic field can be changed to such that each dot contains two electrons with some feasible magnetic field. The Förster-Dexter resonant energy transfer may therefore be generated by two individual QDMs. This may motivate a new paradigm of Fermionic qubits for quantum computing in solid-state systems. 相似文献
24.
K.P. Pipe R.J. Ram 《Photonics Technology Letters, IEEE》2003,15(4):504-506
By measuring the total energy flow from an optical device, we can develop new design strategies for thermal stabilization. Here we present a comprehensive model for heat exchange between a semiconductor laser diode and its environment that includes the mechanisms of conduction, convection, and radiation. We perform quantitative measurements of these processes for several devices, deriving parameters such as a laser's heat transfer coefficient, and then demonstrate the feasibility of thermal probing for the nondestructive wafer-scale characterization of optical devices. 相似文献
25.
Experimental studies of the bond response of three-wire strands and some influencing parameters 总被引:5,自引:0,他引:5
R. Gustavson 《Materials and Structures》2004,37(2):96-106
The bond behavior of prestressing strands is of great importance for the capacity of precast prestressed concrete structures. In the present study, the bond behavior of three-wire strands, and some influencing parameters, were examined by means of steel encased pull-through and push-in tests. The three mechanisms: adhesion, friction and other mechanical actions were found to be present at the strand-concrete interface at different slip values. The results from the experiments showed that the micro roughness of the strand surface strongly affected the initial bond response of the strand, that is the adhesion in the interface. The maximum bond capacity of indented three-wire strands was found to be directly connected to the geometric properties of the strand indents. The influence of the concrete strength on the bond capacity of the strand was hard to interpret. However, the density of the concrete matrix was found to be a better parameter for determine the influence of the concrete rather than the compressive strength. 相似文献
26.
Note on B-splines, wavelet scaling functions, and Gabor frames 总被引:3,自引:0,他引:3
Grochenig K. Janssen A.J.E.M. Kaiblinger N. Pfander G.E. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(12):3318-3320
Let g be a continuous, compactly supported function on such that the integer translates of g constitute a partition of unity. We show that the Gabor system (g,a,b), with window g and time-shift and frequency-shift parameters a,b>0 has no lower frame bound larger than 0 if b=2,3,... and a>0. In particular, (g,a,b) is not a Gabor frame if g is a continuous, compactly supported wavelet scaling function and if b=2,3,... and a>0. We give an example for our result for the case that g=B/sub 1/, the triangle function supported by [-1,1], by showing pictures of the canonical dual corresponding to (g,a,b) where ab=1/4 and b crosses the lines N=2,3,. 相似文献
27.
Supported metal catalysts, particularly noble metals supported on SiO2, have attracted considerable attention due to the importance of the silica–metal interface in heterogeneous catalysis and in electronic device fabrication. Several important issues, e.g., the stability of the metal–oxide interface at working temperatures and pressures, are not well-understood. In this review, the present status of our understanding of the metal–silica interface is reviewed. Recent results of model studies in our laboratories on Pd/SiO2/Mo(1 1 2) using LEED, AES and STM are reported. In this work, epitaxial, ultrathin, well-ordered SiO2 films were grown on a Mo(1 1 2) substrate to circumvent complications that frequently arise from the silica–silicon interface present in silica thin films grown on silicon. 相似文献
28.
Shunkang Liu 《Journal of Infrared, Millimeter and Terahertz Waves》2003,24(5):629-638
The pill-box and block RF windows for millimeter wave (MMW) tubes are presented. The pill-box window is suitable for broadband MMW tubes. And the block window can be used for high power tubes in short MMW. The equivalent circuits for two windows are given. To reach better match characteristics in wider operating bandwidth, the optimum design methods for pill-box and block window in MMW tubes are described in this paper. The testing results show that the theoretic computation is fast and useful accuracy. The design methods possess references value to designer for MMW tubes. 相似文献
29.
The In-site of Ba2In2O5 with Brownmillerite structure was partially substituted for Ce4+ ions in order to examine the doping effect on the order-disorder transition. Ba2In2 – x
Ce
x
O5 + x/2 (x = 0.1, 0.2, 0.3, 0.5, 1.0, and 1.5) were prepared by solid state reaction. X-Ray diffraction analyses of these powder samples demonstrated that Ba2In2 – x
Ce
x
O5 + x/2 (x = 0.1 and 0.2) possesses Brownmillerite structure. With increasing content of Ce4+ ion the crystal system of Ba2In2 – x
Ce
x
O5 + x/2 (x = 0.3, 0.5, and 1.0) changed to cubic perovskite structure above the order-disorder transition temperature of Ba2In2O5. Arrhenius plots of the electrical conductivities of Ba2In2 – x
Ce
x
O5 + x/2 (x = 0.2, 0.3, and 1.0) exhibited no discontinuity. These compounds had high transference numbers of oxide ion above 973 k. 相似文献
30.
The basic methods of verifying continuous automatic belt weighers are described. A comparative analysis of these methods on the basis of experimental studies is made and ways of implementing the results in industry are recommended. 相似文献