全文获取类型
收费全文 | 214篇 |
免费 | 50篇 |
国内免费 | 41篇 |
专业分类
化学 | 106篇 |
晶体学 | 1篇 |
力学 | 15篇 |
综合类 | 1篇 |
数学 | 42篇 |
物理学 | 140篇 |
出版年
2021年 | 3篇 |
2020年 | 3篇 |
2018年 | 2篇 |
2017年 | 2篇 |
2016年 | 5篇 |
2015年 | 3篇 |
2014年 | 6篇 |
2013年 | 27篇 |
2012年 | 2篇 |
2011年 | 13篇 |
2010年 | 9篇 |
2009年 | 8篇 |
2008年 | 9篇 |
2007年 | 13篇 |
2006年 | 15篇 |
2005年 | 9篇 |
2004年 | 8篇 |
2003年 | 12篇 |
2002年 | 30篇 |
2001年 | 13篇 |
2000年 | 17篇 |
1999年 | 4篇 |
1998年 | 6篇 |
1997年 | 7篇 |
1996年 | 10篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 4篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1978年 | 4篇 |
1976年 | 1篇 |
1972年 | 2篇 |
1970年 | 1篇 |
1967年 | 1篇 |
1966年 | 1篇 |
1965年 | 5篇 |
1964年 | 2篇 |
1963年 | 3篇 |
1959年 | 3篇 |
1957年 | 1篇 |
1955年 | 1篇 |
1948年 | 3篇 |
1947年 | 3篇 |
排序方式: 共有305条查询结果,搜索用时 15 毫秒
71.
PARK K. H. LEE J. K. HAN J. H. CHO H. S. JANG D. H. PARK C. S. PYUN K. E. JEONG JICHAI 《Optical and Quantum Electronics》1998,30(1):23-31
We report the effects of external optical feedback on the power penalty of commercial distributed feedback laser diode (DFB-LD) modules for 2.5Gbs–1 optical transmission systems. External optical feedback presented to the DFB-LD modules causes the excitation of external cavity modes, resulting in increased relative intensity noise (RIN) and intensity noise ripples at low frequency region below 500MHz. For a 10–10 bit error rate (BER), the minimum power penalty is as much as 1.25dB for a feedback ratio of –8.8dB. An excess power penalty of 0.5dB per 3dB increase in the feedback ratio was also empirically obtained. We suggest that optical isolators in 2.5Gbs–1 DFB-LD modules used in conventional optical transmission systems or WDM systems must have a peak isolation ratio of better than 54.5dB, instead of the previously recommended 30dB, for negligible power penalty induced by external optical feedback. 相似文献
72.
73.
计算O-2在晶体场中出现的Jahn-Teller效应,得到具有电矩的2pπ3s3d0杂化原子轨道。BO6八面体共价性质决定了ABO3晶体的各种极化相。理论给出Ps比值、钛位移大小和热力学参数ξ′,ζ′均接近于实验结果。统一解释了钛酸钡的三个相转变。相变条件给出七种晶体中氧的屏蔽常数平均值与Slater的值一致。 相似文献
74.
75.
76.
77.
Based on the moving least-squares (MLS) approach, an efficient meshless method is employed to generate the displacement functions for vibration analysis of elastic bodies. The equation of motion is established by following the standard procedure and the boundary conditions are imposed by applying penalty functions. As the displacement functions are expressed in terms of weight functions, the accuracy will depend on the parameters of the weight functions. Therefore, a parametric study is carried out to determine the best values for these parameters. To demonstrate the accuracy, modal analyses of the beams and plates with different boundaries have been carried out. In addition, the responses of these structures under dynamic excitation have been analyzed. The examples include simply supported beams subjected to sudden excitations and simply supported plates subjected to initial displacements. 相似文献
78.
Free vibration of a spinning flexible disk-spindle system supported by ball bearing and flexible shaft is analyzed by using Hamilton's principle, FEM and substructure synthesis. The spinning disk is described by using the Kirchhoff plate theory and von Karman non-linear strain. The rotating spindle and stationary shaft are modelled by Rayleigh beam and Euler beam respectively. Using Hamilton's principle and including the rigid body translation and tilting motion, partial differential equations of motion of the spinning flexible disk and spindle are derived consistently to satisfy the geometric compatibility in the internal boundary between substructures. FEM is used to discretize the derived governing equations, and substructure synthesis is introduced to assemble each component of the disk-spindle-bearing-shaft system. The developed method is applied to the spindle system of a computer hard disk drive with three disks, and modal testing is performed to verify the simulation results. The simulation result agrees very well with the experimental one. This research investigates critical design parameters in an HDD spindle system, i.e., the non-linearity of a spinning disk and the flexibility and boundary condition of a stationary shaft, to predict the free vibration characteristics accurately. The proposed method may be effectively applied to predict the vibration characteristics of a spinning flexible disk-spindle system supported by ball bearing and flexible shaft in the various forms of computer storage device, i.e., FDD, CD, HDD and DVD. 相似文献
79.
PREDICTION OF NON-CAVITATING UNDERWATER PROPELLER NOISE 总被引:2,自引:0,他引:2
Non-cavitation noise of underwater propeller is numerically investigated. The main purpose is to analyze non-cavitation noise in various operating conditions with different configurations. The noise is predicted using time-domain acoustic analogy and boundary element method. The flow field is analyzed with potential-based panel method, and then the time-dependent pressure data are used as the input for Ffowcs Williams-Hawkings formulation to predict the farfield acoustics. Boundary integral equation method is also considered to investigate the effect of ducted propeller. Sound deflection and scattering effect on the duct is considered with the BEM. The governing equations are based on the assumption that all acoustic pressure is linear. A scattering approach is applied in which the acoustic pressure field is split into the known incident component and the unknown scattered component. Noise prediction results are presented for single propeller and ducted propeller in non-uniform flow conditions similar to real situation. The investigation reveals that the effect of a duct on the acoustic performance propeller is small in the far field under non-cavitating situations since the noise directivities of single and ducted propellers are almost the same. Only the high order BPFs are influenced by the existence of the duct. 相似文献
80.
光纤近红外光谱法在中草药分析中的应用——甘草中甘草酸含量的测定 总被引:7,自引:0,他引:7
提出光纤近红外光谱技术在线和快速检测中药甘草中有效成分甘草酸含量的方法。对含甘草酸浓度在0.94%~3.06%内的甘草,根据其在10 000~4 000 cm-1的近红外吸收光谱, 采用偏最小二乘算法(PLS)和主成分回归算法(PCR)建立了校正模型,比较了光谱不同前处理方法对校正结果的影响,当采用数据标准化并求一阶导前处理时可较好提取中药复杂体系的信息。当采用PLS算法时,校正集相关系数为0.958,校正集标准偏差(SEC)为0.179,验证集标准偏差(SEP)为0.197,PLS算法优于PCR算法。该方法快速和简便,适合于中药有效成分在线监控。 相似文献