全文获取类型
收费全文 | 486625篇 |
免费 | 6908篇 |
国内免费 | 2745篇 |
专业分类
电工技术 | 8890篇 |
技术理论 | 5篇 |
综合类 | 3535篇 |
化学工业 | 75709篇 |
金属工艺 | 21807篇 |
机械仪表 | 14901篇 |
建筑科学 | 12423篇 |
矿业工程 | 2973篇 |
能源动力 | 10613篇 |
轻工业 | 41994篇 |
水利工程 | 5462篇 |
石油天然气 | 8848篇 |
武器工业 | 245篇 |
无线电 | 52922篇 |
一般工业技术 | 91585篇 |
冶金工业 | 77928篇 |
原子能技术 | 9221篇 |
自动化技术 | 57217篇 |
出版年
2021年 | 3534篇 |
2019年 | 3111篇 |
2018年 | 18942篇 |
2017年 | 17818篇 |
2016年 | 15036篇 |
2015年 | 4669篇 |
2014年 | 6931篇 |
2013年 | 17970篇 |
2012年 | 14278篇 |
2011年 | 24190篇 |
2010年 | 20028篇 |
2009年 | 18658篇 |
2008年 | 20540篇 |
2007年 | 21652篇 |
2006年 | 12555篇 |
2005年 | 12254篇 |
2004年 | 11282篇 |
2003年 | 10685篇 |
2002年 | 10098篇 |
2001年 | 9749篇 |
2000年 | 9308篇 |
1999年 | 9277篇 |
1998年 | 21598篇 |
1997年 | 15756篇 |
1996年 | 12095篇 |
1995年 | 9174篇 |
1994年 | 8140篇 |
1993年 | 7966篇 |
1992年 | 6125篇 |
1991年 | 5907篇 |
1990年 | 5840篇 |
1989年 | 5836篇 |
1988年 | 5526篇 |
1987年 | 4833篇 |
1986年 | 4774篇 |
1985年 | 5442篇 |
1984年 | 5143篇 |
1983年 | 4791篇 |
1982年 | 4290篇 |
1981年 | 4547篇 |
1980年 | 4165篇 |
1979年 | 4348篇 |
1978年 | 4223篇 |
1977年 | 4652篇 |
1976年 | 6015篇 |
1975年 | 3757篇 |
1974年 | 3574篇 |
1973年 | 3675篇 |
1972年 | 3105篇 |
1971年 | 2829篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
Y.K. Su H.C. Wang C.L. Lin W.B. Chen S.M. Chen 《Photonics Technology Letters, IEEE》2003,15(10):1345-1347
The brightness of AlGaInP light emitting diodes (LEDs) has been raised by a factor of 1.12 at 20 mA by sulfide passivation. Meanwhile, the sulfide also can decrease leakage current of AlGaInP LEDs at -2 V to nearly one thousandth of that in the as-fabricated device. The possible causes for the brightness increase of AlGaInP LEDs after sulfide treatment including surface roughness, reduction of Fresnel loss, and effective injection of carriers were demonstrated. 相似文献
72.
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
73.
Stability analysis of the finite difference time domain scheme containing macromodels is presented. It is shown that for a stable macromodel, the stability of the combined scheme depends on the field interpolation at the macromodel boundary. The maximal allowable time step is shown to be much larger than for subgridding. 相似文献
74.
A new method has been developed for compressing the matrices that occur in most integral-equation-based computer programs. This method is easy to interface with existing computer programs, and allows them to run significantly faster and with significantly less memory. This method applies not only to electromagnetic and acoustic computation, but also to most programs involving a Green's function or any integral equation with a kernel having some smoothness properties. Our numerical computations, running on a high-end personal computer, have achieved compression ratios of fifty times, and compressed inversion of the matrices fifty times faster than by previous methods. For larger problems, solved on high-performance computers, these ratios would improve to about one thousand to one for larger moment method problems. 相似文献
75.
E. A. Gurieva P. P. Konstantinov L. V. Prokof D. A. Pshenaĭ-Severin M. I. Fedorov Yu. I. Ravich 《Semiconductors》2006,40(7):763-767
The coefficients of thermopower and electrical and thermal conductivity in the PbTe0.8Se0.1 S 0.1 solid solution with electron concentration (4.6–54) × 1018 cm?3 are studied in the range of 85–300 K (and in some cases up to 700 K). The temperature dependences of electrical and thermal conductivity indicate that the low-temperature electron and phonon scattering initiated by the off-center impurity of sulfur exists. The temperature dependences of the electronic and lattice components of thermal conductivity are calculated in the approximation of a parabolic spectrum and electron scattering by acoustic phonons and neutral substitutional impurities. The lattice thermal conductivity is found to have a feature in the form of a shallow minimum in the range of 85–250 K. A similar feature, while not so clearly pronounced, is found to exist also in Pb1?x SnxTe1?x Sex alloys (x≥0.15) with an off-center tin impurity. An analysis of the possible origins of this effect suggests that, at low temperatures, the Lorentz numbers L of the materials under study are smaller than the L0 numbers employed which correspond to the above scattering mechanisms. The cause of the decrease in L is related to electron scattering at two-level systems, a mechanism whose effect grows with increasing electron energy. An analysis of experimental data obtained at high temperatures, as well as on undoped samples with the lowest possible carrier concentrations, yields the values of L for samples with different electron densities. The minimum value L/L0 = 0.75 is obtained for a lightly doped sample at ~130 K. 相似文献
76.
The emerging software defined radio technologies will be an enabler for a new generation of dynamic wireless systems. It will also open up the possibility of allocating frequencies in a, more dynamic way than today. From an intersystem-interference point of view, this can cause unforeseen problems to occur due to the increased complexity in such applications. In such applications, a measure indicating whether or not a frequency band is possible to use from an electromagnetic interference point of view, must be found. A simple approach is to use the measured total average interference power within the receiver band. Since the interference impact on modern digital communication systems from an interference signal does not only depend on the power but also on the actual waveform of the interference signal, some kind of quality measure of the average-power approach would be convenient to use. In this paper, we introduce a simple quality measure of the average-power approach so that a rough adjustment for the interference-waveform properties can be done. 相似文献
77.
Reekmans S. De Maeyer J. Rombouts P. Weyten L. 《IEEE transactions on circuits and systems. I, Regular papers》2006,53(12):2529-2538
Quadrature sigma-delta analog-to-digital converters require a feedback path for both the I and the Q parts of the complex feedback signal. If two separate multibit feedback digital-to-analog converters (DACs) are used, mismatch among the unit DAC elements leads to additional mismatch noise in the output spectrum as well as an I/Q imbalance. This paper proposes new quadrature bandpass (QBP) mismatch shaping techniques. In our approach, the I and Q DACs are merged into one complex DAC, which leads to near-perfect I/Q balance. To select the unit DAC elements of the complex multibit DAC, the well-known butterfly shuffler and tree structure are generalized towards a complex structure, and necessary constraints for their correct functioning are derived. Next, a very efficient first-order QBP shaper implementation is proposed. Finally, the newly presented complex structures are simulated to prove their effectiveness and are compared with each other with respect to performance 相似文献
78.
The effect of spin splitting caused by structural asymmetry (Rashba’s contribution) and bulk asymmetry (Dresselhaus’s contribution) on the magnetoconductance of two-dimensional structures with high mobility of charge carriers is studied. The theory of weak localization with regard to both of the contributions is developed. The theory is valid in the entire region of classically low magnetic fields for arbitrary relations between the frequencies of spin precession and elastic collisions. The suppression of the correction for antilocalization is demonstrated in the case of equal contributions of structural anisotropy and bulk anisotropy to the spin splitting. The effect of the contribution, cubic in the wave vector, to the spin splitting on the quantum magnetoresistance is studied. 相似文献
79.
In self-organizing ad hoc networks, all the networking functions rely on the contribution of the participants. As a basic example, nodes have to forward packets for each other in order to enable multihop communication. In recent years, incentive mechanisms have been proposed to give nodes incentive to cooperate, especially in packet forwarding. However, the need for these mechanisms was not formally justified. In this paper, we address the problem of whether cooperation can exist without incentive mechanisms. We propose a model,based on game theory and graph theory to investigate equilibrium conditions of packet forwarding strategies. We prove theorems about the equilibrium conditions for both cooperative and noncooperative strategies. We perform simulations to estimate the probability that the conditions for a cooperative equilibrium hold in randomly generated network scenarios.. As the problem is involved, we deliberately restrict ourselves to a static configuration. We conclude that in static ad hoc networks where the relationships between the nodes are likely to be stab le-cooperation needs to be encouraged. 相似文献
80.
This paper provides an overview of the main aspects of modern fluorescence microscopy. It covers the principles of fluorescence and highlights the key discoveries in the history of fluorescence microscopy. The paper also discusses the optics of fluorescence microscopes and examines the various types of detectors. It also discusses the signal and image processing challenges in fluorescence microscopy and highlights some of the present developments and future trends in the field. 相似文献