全文获取类型
收费全文 | 57468篇 |
免费 | 7358篇 |
国内免费 | 3460篇 |
学科分类
工业技术 | 68286篇 |
出版年
2024年 | 290篇 |
2023年 | 979篇 |
2022年 | 1686篇 |
2021年 | 1901篇 |
2020年 | 2106篇 |
2019年 | 1844篇 |
2018年 | 1699篇 |
2017年 | 2007篇 |
2016年 | 2142篇 |
2015年 | 2261篇 |
2014年 | 3150篇 |
2013年 | 3850篇 |
2012年 | 3885篇 |
2011年 | 4235篇 |
2010年 | 3174篇 |
2009年 | 3294篇 |
2008年 | 3196篇 |
2007年 | 3691篇 |
2006年 | 3338篇 |
2005年 | 3011篇 |
2004年 | 2464篇 |
2003年 | 2200篇 |
2002年 | 1831篇 |
2001年 | 1507篇 |
2000年 | 1361篇 |
1999年 | 1073篇 |
1998年 | 969篇 |
1997年 | 862篇 |
1996年 | 738篇 |
1995年 | 641篇 |
1994年 | 518篇 |
1993年 | 417篇 |
1992年 | 382篇 |
1991年 | 296篇 |
1990年 | 285篇 |
1989年 | 230篇 |
1988年 | 171篇 |
1987年 | 117篇 |
1986年 | 101篇 |
1985年 | 74篇 |
1984年 | 82篇 |
1983年 | 46篇 |
1982年 | 40篇 |
1981年 | 20篇 |
1980年 | 24篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1966年 | 7篇 |
1959年 | 18篇 |
1951年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
1.
《Journal of the European Ceramic Society》2020,40(8):3348-3353
Large transduction coefficient () is difficult to obtain in piezoelectric ceramics because these two parameters show opposite trends with compositional modifications. Herein, the Pb(Zr0.53Ti0.47)O3 ceramic powders were calcinated under different temperatures (A:830 °C, B:860 °C, and C:890 °C), and then mixed together according to different weight ratios (1A:1B:1C, 1A:2B:1C, 1A:2B:3C and 3A:2B:1C) for ceramics preparation. Both d33 and g33 are improved successfully, and the transduction coefficient with the weight ratio of 1A:2B:3C reaches up to 17,500 × 10−15 m2/N, which is 60 % higher than that with the powders calcinated under 830 °C, and at least twice those of commercial PZT-4, PZT-5A and PZT-8 ceramics. The improved transduction coefficient is owing to the enhanced piezoelectric constant and spontaneous polarization resulted from the increased grain size, relative density and the fraction of tetragonal phase. These results indicate that this is a simple but effective way to tailor the transduction coefficient in piezoelectric ceramics. 相似文献
2.
3.
Gas/liquid mass transfer has been investigated using a stirred vessel gas/liquid contactor using non‐Newtonian media and carbon dioxide as absorbent and gas phase, respectively. The volumetric mass transfer coefficients at different operational variables have been determined. Non‐Newtonian media (liquid phase) were prepared as aqueous solutions of sodium carboxymethyl cellulose salt. The influence of the rheological properties, polymer concentration, stirring rate, and gas flow rate on mass transfer was studied for these liquid phases. Kinematic viscosity and density experimental data were used to calculate the average molecular weight corresponding to the polymer employed. The Ostwald model has been used to fit the rheological behavior of aqueous solutions of the polymer employed as absorbent phase. Reasonably good agreement was found between the predictions of the proposed models and the experimental data of mass transfer coefficients. 相似文献
4.
讨论了输出终端带不等式约束情形的多输入多输出系统,说明了这些约束可以通过修改输入柔化系数而使其得以满足,在柔化系数的选择范围确定过程中,为了避免矩阵求逆,采用利用已知数据辨识参数的方法来确定柔化系数,从而使得问题简化。降低了计算量且加快了收敛速度。 相似文献
5.
为了提高非稳定渗流计算的计算精度 ,通过数学分析得到了水位传导系数随时间变化时 ,一类非稳定渗流井流模型的半解析解 ,并进行了计算比较 相似文献
6.
Ph. D. Candidate:Chen Xue-fengState Key Laboratory of Offshore Coastal Engineering Dalian University of Technology Dalian China e-mail: cxfeng @ student dlut. edu.cn 《水动力学研究与进展(B辑)》2003,15(6)
The reflection coefficient of perforated caissons and the total horizontal forces acting on them were experimentally and numerically analyzed and discussed when wave propagates normally. To consider the viscosity effect of fluid and nonlinear action of waves on structures, the VOF (Volume Of Fluid) method combined with the k-ε turbulence model was used to simulate the interaction between waves and structures. Governing equations were solved with the finite difference method. Through 2D experimental study in the wave flume, the empirical relationship between the reflection coefficient of perforated caissons and the main affecting factors were obtained from the experimental data using the least square method. Also the correlation between the ratio of the total horizontal force acting on perforated caisson and the force acting on solid caisson and the main affecting factors were regressed from the experimental data. 相似文献
7.
求解二维结构-声耦合问题的一种直接方法 总被引:1,自引:1,他引:0
本文基于传递矩阵法(TMM)和虚拟边界元法(VBEM),提出了一种求解在谐激励作用下二维结构-声耦合问题的直接法。文中对任意形状的二维弹性环建立了一阶非齐次运动微分方程组,便于用齐次扩容精细积分法求解,对于含有任意形状孔穴的无穷域流体介质的Helmholtz外问题,采用复数形式的Burton-Miller型组合层势法建立了虚拟边界元方程,保证了声压在全波数域内存在唯一解。根据叠加原理并结合最小二乘法,提出了一种耦合方程的直接解法,由于该方法不存在迭代过程,因而具有较高的计算精度和效率。文中给出了二个典型弹性环在集中谐激励力作用下声辐射算例,计算结果表明本文方法较通常采用的混合FE/BE法更为有效。 相似文献
8.
L. Sheeney‐Haj‐Ichia S. Pogorelova Y. Gofer I. Willner 《Advanced functional materials》2004,14(5):416-424
Three different configurations of Au‐nanoparticle/CdS‐nanoparticle arrays are organized on Au/quartz electrodes for enhanced photocurrent generation. In one configuration, Au‐nanoparticles are covalently linked to the electrode and the CdS‐nanoparticles are covalently linked to the bare Au‐nanoparticle assembly. The resulting photocurrent, φ = 7.5 %, is ca. 9‐fold higher than the photocurrent originating from a CdS‐nanoparticle layer that lacks the Au‐nanoparticles, φ = 0.8 %. The enhanced photocurrent in the Au/CdS nanoparticle array is attributed to effective charge separation of the electron–hole pair by the injection of conduction‐band electrons from the CdS‐ to the Au‐nanoparticles. Two other configurations involving electrostatically stabilized bipyridinium‐crosslinked Au/CdS or CdS/Au nanoparticle arrays were assembled on the Au/quartz crystal. The photocurrent quantum yields in the two systems are φ = 10 % and φ = 5 %, respectively. The photocurrents in control systems that include electrostatically bridged Au/CdS or CdS/Au nanoparticles by oligocationic units that lack electron‐acceptor units are substantially lower than the values observed in the analogous bipyridinium‐bridged systems. The enhanced photocurrents in the bipyridinium‐crosslinked systems is attributed to the stepwise electron transfer of conduction‐band electrons to the Au‐nanoparticles by the bipyridinium relay bridge, a process that stabilizes the electron–hole pair against recombination and leads to effective charge separation. 相似文献
9.
An experimental technique together with a numerical model is proposed with which the diffusion coefficient and the sorption isotherm of water in paint can be measured. Inside a closed vessel, paint films are on stainless-steel plates. Water is present as water vapour in the air and in the paint. After blowing dry or wet air through the vessel for some time, the situation moves to a new equilibrium. The relative humidity of the air inside the vessel is measured as a function of time. From fitting the theoretical/numerical model against the experimental values, follow the diffusion coefficient and the sorption isotherm of water in the paint. The results show large scattering. When the independently measured sorption isotherm is used as an input parameter in the model, the fitting procedure gives much smaller scattering for the diffusion coefficient. 相似文献
10.
M. Ganschow C. Hellriegel E. Kneuper M. Wark C. Thiel G. Schulz‐Ekloff C. Bruchle D. Whrle 《Advanced functional materials》2004,14(3):269-276
Dye‐loaded AlPO4‐5 single crystals were prepared by microwave‐assisted hydrothermal synthesis from a batch, containing a mixture of three chromophores (Coumarin 40, Rhodamine BE50, and Oxazine 1) differing in their absorption range, molecular dimensions, and solubilities. Confocal fluorescence images reveal a spatial separation effect of the dye molecules, where the slimmer, more‐soluble dye molecule (Coumarin 40) is uniformly distributed in the body of the single crystal, and the bulky and/or less‐soluble ones (Rhodamine BE50, Oxazine 1) are situated in distinct domains. Visible spectra show good panchromatic absorption of visible light. Fluorescence lifetime measurements indicate the presence of an energy transfer cascade of the entirely fixed dye molecules from Coumarin 40 to Oxazine 1. The transfer mechanism is predominantly radiative. 相似文献