全文获取类型
收费全文 | 367篇 |
免费 | 50篇 |
国内免费 | 27篇 |
专业分类
化学 | 189篇 |
晶体学 | 5篇 |
力学 | 23篇 |
综合类 | 3篇 |
数学 | 26篇 |
物理学 | 198篇 |
出版年
2023年 | 1篇 |
2022年 | 6篇 |
2021年 | 5篇 |
2020年 | 9篇 |
2019年 | 8篇 |
2018年 | 10篇 |
2017年 | 14篇 |
2016年 | 18篇 |
2015年 | 14篇 |
2014年 | 26篇 |
2013年 | 29篇 |
2012年 | 26篇 |
2011年 | 22篇 |
2010年 | 23篇 |
2009年 | 21篇 |
2008年 | 27篇 |
2007年 | 16篇 |
2006年 | 27篇 |
2005年 | 20篇 |
2004年 | 19篇 |
2003年 | 15篇 |
2002年 | 18篇 |
2001年 | 17篇 |
2000年 | 7篇 |
1999年 | 7篇 |
1998年 | 10篇 |
1997年 | 3篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1985年 | 1篇 |
排序方式: 共有444条查询结果,搜索用时 31 毫秒
71.
Most dynamic analyses of planar curved beams found in the literature are carried out based on a curved beam model which assumes that the neutral axis coincides with the centroidal axis of the curved beam. This assumption leads to governing equations of motion which are relatively simple with analysis results that have acceptable accuracy for shallow curved beams. However, when a curved beam is not shallow and/or its cross section is not doubly symmetric, the offset distance between the neutral and centroidal axes may be large enough to influence the in-plane dynamics of the curved beam even for small motion. In this paper, the validity of this underlying assumption for modeling a linear curved beam is examined. To this end, two sets of equations of motion governing the in-plane dynamics of a planar curved beam are derived, in a consistent manner for comparison, based on the linear strain-displacement relations and Hamilton’s principle. The first set of equations is derived from the displacement components measured with reference to the neutral axis of the curved beam while the second set is derived with respect to the centroidal axis of the cross section. The curved beam is considered extensional and the effects of rotary inertia and radial shear deformation are included. In addition to the curvature parameter that characterizes the wave motion for both curved beam models, an eccentricity parameter is introduced in the first model to account for the offset between the neutral and centroidal axes. The dynamic behavior predicted by each curved beam model is compared in terms of the dispersion relations, frequency spectra, cutoff frequencies, natural frequencies and modeshapes, and frequency responses. In order to ensure that the comparison is accurate, the wave propagation technique is applied to obtain exact wave solutions. It is shown that, when the curvature parameter is not small, the underlying assumption has a substantial impact on the accuracy of the linear dynamic analysis of a curved beam. 相似文献
72.
回转体三维扫描形貌测量及其应用 总被引:1,自引:0,他引:1
本文提出一种测量三维形貌的扫描系统,该系统可被用于测量回转体的三维形貌。激光光刀被投射在被测物体表面,由于被测物体表面的形貌而改变了光刀的弯曲方式,摄像机将变形后的图像采集到计算机中。当被测物体放置在转台上旋转一周时,相应的一系列光刀变形的图片被采集到计算机中,这一系列图片包含了物体表面形貌的全部信息。通过连续扫描采集系列图片并计算,最终可以获得整个被测物体的三维形貌。当被测物体直径在一百毫米左右尺寸范围内的测量灵敏度可到0.01mm,其它尺寸范围要更换成像系统,才能达到相应的灵敏度。本文给出了两个较为复杂模型的三维形貌测量结果。该结果表明,本系统在对回转体表面形貌的测量方面具有一定的技术难度和广泛的工程需求,本文的研究具有重要的参考价值。 相似文献
73.
74.
航天时间延迟积分CCD相机摆扫成像快速几何校正设计与分析 总被引:2,自引:1,他引:1
为实现在轨卫星机动成像条带的快速准确拼接和应用,针对高分辨力卫星在轨推摆扫相结合的机动成像模式,提出了一种光线追迹像移匹配数学建模方法。通过对卫星机动过程中的摆角变化、地球表面面型的影响分析,利用摆扫过程中姿态对地指向不断改变导致的交轨方向速度失配量和成像变形量,补偿相机像面各点处的像移量从而进行快速几何校正。利用小卫星姿态控制系统全物理仿真平台对成像进行了仿真分析。分析结果表明,成像过程中随着扫描角的增加,像面上的像移量增大,成像变形情况也变严重。利用均方误差分析仿真成像与实验成像质量,仿真成像与实验成像相差-0.000011左右,较好地满足地面卫星相机成像仿真需求。对不同扫描角下存在速度失配的图像进行快速几何校正,该校正后图像的均方误差变小。该校正方法具有效率高,便于拼接应用的优点。 相似文献
75.
76.
Yousuke SEYAMA 《Proceedings of the Japan Academy. Series B, Physical and biological sciences》2012,88(8):462-469
Pancreatic cancer was found by an abdominal CT scan in a medical doctor, the author of this article, before the appearance of any symptoms. After considering all the imaging findings including the CT, MRI, and PET, a diagnosis was made. He was admitted in the University of Tokyo Hospital on the 14th day after the CT finding. On the 18th day, the operation was successfully performed, and no tumor invading to adjacent tissue was seen. On the 29th day, 11th day after the operation, he left the hospital with a drain still in place to excrete abdominal exudation. The remaining drain was finally removed on the 60th day, and the treatment by the surgeon was completed. The chronological events that occurred during these 60 days are described in diary form. There is nothing superior to early detection and early treatment in the fight against cancer. He recommends everybody to receive periodical medical examinations before praying for good luck. 相似文献
77.
T. W. Cornelius A. Davydok V. L. R. Jacques R. Grifone T. Schülli M.‐I. Richard G. Beutier M. Verdier T. H. Metzger U. Pietsch O. Thomas 《Journal of synchrotron radiation》2012,19(5):688-694
Mechanical deformation of a SiGe island epitaxically grown on Si(001) was studied by a specially adapted atomic force microscope and nanofocused X‐ray diffraction. The deformation was monitored during in situ mechanical loading by recording three‐dimensional reciprocal‐space maps around a selected Bragg peak. Scanning the energy of the incident beam instead of rocking the sample allowed the safe and reliable measurement of the reciprocal‐space maps without removal of the mechanical load. The crystal truncation rods originating from the island side facets rotate to steeper angles with increasing mechanical load. Simulations of the displacement field and the intensity distribution, based on the finite‐element method, reveal that the change in orientation of the side facets of about 25° corresponds to an applied pressure of 2–3 GPa on the island top plane. 相似文献
78.
Jing Li Yinzhu Jiang Tianyu Ma Cong Lu Yongbing Xu Deren Yang Mi Yan 《Physica B: Condensed Matter》2012,407(24):4783-4786
γ′-Fe4N thin films were grown on MgO-buffered Si (1 0 0) by pulsed laser deposition technique. Different crystallographic orientations and in-plane magnetic anisotropies were achieved by varying the growth temperature of the MgO buffer layer. When the MgO buffer layer was grown at room temperature, the γ′-Fe4N film shows isotropic in-plane magnetic properties without obvious texture; while in-plane magnetic anisotropy was recorded for the γ′-Fe4N films deposited on a 600 °C-grown-MgO buffer due to the formation of a (1 0 0)-oriented biaxial texture. Such a difference in in-plane magnetic anisotropy is attributed to the epitaxial growth of γ′-Fe4N film on an MgO buffer with relaxed strain when the MgO layer was grown at a high temperature of 600 °C. 相似文献
79.
建立了直接进样电喷雾串联质谱测定草鱼肌肉组织中磷脂的方法.以Bligh Dyer法提取总脂质,采用流动注射泵直接进样的方式将样品导人电喷雾离子源,利用串联三重四级杆质谱的母离子扫描和中性丢失扫描功能,通过扫描磷脂的特征性子离子或中性质量丢失实现对磷脂酰胆碱、磷脂酰乙醇胺、磷脂酰肌醇、磷脂酰丝氨酸、磷脂酰甘油和磷脂酸六类磷脂的源内分离和鉴定.结果显示,在-定的浓度范围内,磷脂的浓度与磷脂直接进样电喷雾电离后形成准分子离子的响应值呈现良好的线性关系,回收率(67.1%~96.6%)和精密度可以满足生物样品分析的要求.采用本方法测定了草鱼肌肉组织中磷脂酰胆碱、磷脂酰乙醇胺、磷脂酰肌醇和磷脂酰丝氨酸4类磷脂的分子种及含量.本方法前处理简单,定性和定量分析快速准确,可以应用于其它生物样本脂质组学中磷脂的分析. 相似文献
80.
Liang Li Ningning Yuan Ping Wang Yiqun Wu Yinglin Song Zhimin Chen Chunying He 《Journal of Physical Organic Chemistry》2012,25(10):872-877
Three‐photon absorption (3PA) properties of symmetric‐type carbazole derivatives show great potential for application in light‐activated therapy and optical limiting. A novel symmetrical carbazole derivative (abbreviated as POCP) with end‐groups of 1,10‐phenanthroline rings as the donor moieties, chained via carbon–nitrogen (C = N) double bond, has been synthetized and its three photon absorption properties has been also determined by using a Q‐switched Nd: YAG laser pumped with 30 ps pulses at 1064 nm in dimethylformamide. The measurement of 3PA cross‐section of this compound is performed by open aperture Z‐scan and σ3PA is 481 × 10–78 cm6 ? s2/photon2 for the transition S0 → S1. The influence of the molecular structure of this compound on three‐photon absorption cross‐sections is discussed micromechanically by Austin model 1 and Zerner's Intermediate Neglect of Differential Overlap/S method. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献