全文获取类型
收费全文 | 30437篇 |
免费 | 3131篇 |
国内免费 | 2370篇 |
专业分类
电工技术 | 1558篇 |
综合类 | 2738篇 |
化学工业 | 3310篇 |
金属工艺 | 4193篇 |
机械仪表 | 2213篇 |
建筑科学 | 2350篇 |
矿业工程 | 2152篇 |
能源动力 | 890篇 |
轻工业 | 1277篇 |
水利工程 | 959篇 |
石油天然气 | 3449篇 |
武器工业 | 382篇 |
无线电 | 1314篇 |
一般工业技术 | 5799篇 |
冶金工业 | 1237篇 |
原子能技术 | 287篇 |
自动化技术 | 1830篇 |
出版年
2024年 | 100篇 |
2023年 | 408篇 |
2022年 | 836篇 |
2021年 | 965篇 |
2020年 | 1066篇 |
2019年 | 924篇 |
2018年 | 918篇 |
2017年 | 1124篇 |
2016年 | 1179篇 |
2015年 | 1233篇 |
2014年 | 1711篇 |
2013年 | 1917篇 |
2012年 | 2095篇 |
2011年 | 2280篇 |
2010年 | 1739篇 |
2009年 | 1752篇 |
2008年 | 1542篇 |
2007年 | 1908篇 |
2006年 | 1762篇 |
2005年 | 1534篇 |
2004年 | 1260篇 |
2003年 | 1106篇 |
2002年 | 1002篇 |
2001年 | 831篇 |
2000年 | 744篇 |
1999年 | 669篇 |
1998年 | 525篇 |
1997年 | 543篇 |
1996年 | 421篇 |
1995年 | 341篇 |
1994年 | 296篇 |
1993年 | 208篇 |
1992年 | 189篇 |
1991年 | 169篇 |
1990年 | 196篇 |
1989年 | 133篇 |
1988年 | 76篇 |
1987年 | 31篇 |
1986年 | 40篇 |
1985年 | 30篇 |
1984年 | 27篇 |
1983年 | 34篇 |
1982年 | 30篇 |
1981年 | 4篇 |
1980年 | 16篇 |
1979年 | 8篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1975年 | 4篇 |
1959年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Abstract We present molecular dynamics simulations examining the effect of the interatomic potential on steady‐state mode I crack propagation in a two‐dimensional triangular lattice as a function of applied strain. The interatomic potential is the Morse potential whose failure strain exhibits linear variation with its exponential parameter. The limiting crack speed is defined to be the steady‐state crack velocity observed at the onset of instability in steady‐state crack propagation leading to dislocation nucleation or crack branching. For all systems studied, the limiting crack speed is observed to be less than one third the Rayleigh wave speed. The fastest crack propagation in these ideal systems is associated with a material's long‐wavelength elastic properties being dominated by the strength of the nearest‐neighbor bond. 相似文献
992.
《Ceramics International》2017,43(8):6268-6275
The frequency dependent dielectric and AC conductivity properties of different concentrations of Pr3+ doped Sr0.5Ca0.5TiO3 ceramics were investigated for the frequency range 100 Hz to 2 MHz at different temperatures. The morphology of the prepared samples was analyzed by using Field-Emission Scanning Electron Microscope images. The value of dielectric constant and dielectric loss decreases with increase in the frequency of the applied signal in all the samples. Also, the value of dielectric constant and dielectric loss decreased with doping of different concentrations of Pr3+ ions. The conductivity of the samples obeys Jonscher's power-law and shows a decrease with increasing doping concentration of Pr3+ ions. The higher value of real and imaginary part of impedance at lower frequency indicates the space charge polarization of the material and its absence at higher frequencies was confirmed from the low value of impedance at higher frequency region. The Cole- Cole parameters of the samples were calculated and the semi-circle observed indicates a single relaxation process and can be modeled by an equivalent parallel RC circuit. At lower frequency region, the value of dielectric constant and dissipation factor increases with increase in the temperature. Also the value of conductivity increases with temperature at high frequency region, due to the enhanced mobility of charge carriers. 相似文献
993.
《Virtual and Physical Prototyping》2013,8(4):275-285
Porous scaffolds are biocompatible and bioactive temporary substrates. They should present appropriated microstructure, mechanical properties and surface properties for guiding bone tissue regeneration. In this work, scaffolds of Poly(3-hydroxybutyrate) (PHB) were printed by Selective Laser Sintering (SLS). The effect of scan spacing (SS) and powder layer thickness (PLT) on the morphology, mechanical properties and dimensional deviations related to the digital model of sintered scaffolds was evaluated. Curling was observed in the first built layers of scaffolds, mainly for scaffolds printed with the lowest PLT. Besides designed pores, the scaffolds also presented micropores derived from the incomplete sinterisation of PHB particles. This morphology can be advantageous for bone regeneration. The variation of PLT caused a higher difference between the values of scaffold mechanical properties than the variation of SS. The scaffolds, except the one printed with the highest PLT or SS, showed mechanical properties within the lower range of human trabecular bone. 相似文献
994.
995.
《International Journal of Hydrogen Energy》2021,46(80):39665-39674
The behaviors and kinetics of hydrogen evolution reaction (HER) on pure aluminum with passive film in the presence and absence of chloride ion are quantitatively investigated by using the tip generation/substrate collection mode of scanning electrochemical microscopy (SECM) with dual Al/Pt ultramicroelectrode (UME) as tip electrode and Pt UME as substrate electrode. The standard rate constants k0 and transfer coefficients αH of HER in ClO4−- and Cl−-containing solution are 6.9 × 10−7 cm/s and 0.22, 7.1 × 10−6 cm/s and 0.19, respectively. Results show that the kinetic of HER is slow and the destruction of Cl− on passive film can significantly promote the HER on Al surface. Moreover, these αH far less than commonly used 0.5 in corrosion research, can explain the great difference between theoretical Tafel slopes and experimental results. Besides, the existence of current plateau in Al electrode explains the large difference in corrosion potential during parallel testing. 相似文献
996.
In digital communication systems, typical methodologies in determining loop parameters of the digital phase-locked loop (DPLL) are based on the mapping transformation from the analog domain to the digital domain. However, such transform based algorithms are relatively complicated and not straightforward, and they also cause the problem that loop parameters are affected by the pre-detection integration time greatly. To solve these issues, an effective direct method of determining loop parameters of the second-order DPLL in the z-domain is proposed in this paper. Through ascertaining specific positions of the closed-loop system function's poles inside the right-hand side of the z-plane's unit circle, unknown parameters are calculated directly and flexibly in this method, which enables the DPLL to acquire good low-pass filtering characteristic and system stability. This novel method not only reduces the complexity of solving the parameters, but also eliminates the effect of the pre-detection integration time on loop parameters. Simulation results are provided to confirm the feasibility of the proposed method and to show that the DPLL obtained by this method achieves the similar tracking performance to the discretized PLL. 相似文献
997.
《Journal of Nuclear Science and Technology》2013,50(7):423-435
Various combinations of biaxial stress were applied on five batches of recrystallized zircaloy-2 fuel cladding tubes with different textures; elongation in both axial and circumferential directions of the specimen was measured continuously up to 5% plastic deformation. The anisotropic theory of plasticity proposed by Hill was applied to the resulting data, and anisotropy constants were obtained through the two media of plastic strain loci and plastic strain ratios. Comparison of the results obtained with the two methods proved that the plastic strain loci provide data that are more effective in predicting quantitatively the plastic deformation behavior of the zircaloy-2 tubes. The anisotropy constants change their value with progress of plastic deformation, and judicious application of the effective stress and effective strain obtained on anisotropic materials will permit the relationship between stress and strain under various biaxialities of stresses to be approximated by the work hardening law. The test specimens used in the plastic deformation experiments were then stressed to fracture under the same combination of biaxial stress as in the proceeding experiments, and the deformation in the fractured part was measured. The result proved that the tilt angle of the c-axis which serves as the index of texture is related to fracture ductility under biaxial stress. Based on this relationship, it was concluded that material with a tilt angle ranging from 10° to 15° is the most suitable for fuel cladding tubes, from the viewpoint of fracture ductility, at least in the case of unirradiated material. 相似文献
998.
999.
针对文献[1]中提出的CHNN图像边缘检测算法缺乏足够的参数来调节边缘检测的灵敏度以及检测结果图像边缘过宽的缺陷,提出一种改进的CHNN方法,称之为Weighted CHNN(加权的CHNN,简称WCHNN)方法。该方法在CHNN神经网络元的n个连接上施加权值,可以通过各种局部搜索、优化算法,使用指定的样本输入、样本输出等方法来训练该WCHNN网络从而确定各权值,使得WCHNN在保留了CHNN的优点的同时,还可以根据不同的样本输入输出图像来调节边缘检测的灵敏度,从而提高检测结果质量并避免检测结果中出现边缘过宽的情况。实验结果表明,训练后的WCHNN网络,比起CHNN有着更低的边缘检测错误率,并可检出原来CHNN方法漏检的边缘。 相似文献
1000.
露天矿开采过程中边坡变形破裂过程一直是岩土工程领域研究的难点,尤其考虑含火烧煤层露天矿开采过程中的边坡稳定使得研究问题更加复杂。采用FEPG数值模拟的研究方法,对露天矿某边坡KT3剖面进行了破裂过程分析,得到了含火烧煤层露天矿边坡变形破裂规律,研究结果表明,边坡火烧砂岩的顶部首先出现拉张裂缝,边坡以压剪破坏为主,压剪破坏又促使拉张破坏,二者相互影响,最终导致边坡发生失稳破坏。 相似文献