全文获取类型
收费全文 | 18547篇 |
免费 | 2689篇 |
国内免费 | 6396篇 |
专业分类
化学 | 12883篇 |
晶体学 | 578篇 |
力学 | 1384篇 |
综合类 | 759篇 |
数学 | 2998篇 |
物理学 | 9030篇 |
出版年
2024年 | 12篇 |
2023年 | 88篇 |
2022年 | 350篇 |
2021年 | 340篇 |
2020年 | 355篇 |
2019年 | 362篇 |
2018年 | 388篇 |
2017年 | 537篇 |
2016年 | 375篇 |
2015年 | 596篇 |
2014年 | 700篇 |
2013年 | 1014篇 |
2012年 | 933篇 |
2011年 | 2248篇 |
2010年 | 1592篇 |
2009年 | 1698篇 |
2008年 | 1269篇 |
2007年 | 1241篇 |
2006年 | 1192篇 |
2005年 | 1261篇 |
2004年 | 1896篇 |
2003年 | 1219篇 |
2002年 | 1191篇 |
2001年 | 1107篇 |
2000年 | 821篇 |
1999年 | 606篇 |
1998年 | 463篇 |
1997年 | 313篇 |
1996年 | 231篇 |
1995年 | 233篇 |
1994年 | 206篇 |
1993年 | 558篇 |
1992年 | 575篇 |
1991年 | 358篇 |
1990年 | 375篇 |
1989年 | 367篇 |
1988年 | 131篇 |
1987年 | 65篇 |
1986年 | 72篇 |
1985年 | 56篇 |
1984年 | 56篇 |
1983年 | 43篇 |
1982年 | 35篇 |
1981年 | 23篇 |
1980年 | 9篇 |
1979年 | 20篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1966年 | 4篇 |
1959年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
991.
The enlargement of the emitting aperture is usually one of the important methods of increasing vertical- cavity surface-emitting laser (VCSEL) optical output power. However, in a VCSEL with a larger aperture, the inhomogeneity in the injected current often causes inhomogeneous or even no emission. To solve this problem and to increase VCSEL output power, as well as to improve its thermal characteristics, we develop a new type of injected VCSEL with a larger aperture and a reticular electrode, where the conventional circular injection electrode of the P side is turned into a reticular one, and the heat sink is on the N side. The tests of the new VCSEL show an improvement in homogeneity in not only the injected current but also the emission intensity. The optical output power is also considerably increased, and the device optoelectronic performance is improved. 相似文献
992.
WANG Bo LIU ZhengWei HAN YunKun LEI ZhenXin LUO YangPing & HAN ZhanWen National Astronomical Observatories/Yunnan Observatory Chinese Academy of Sciences Kunming China Key Laboratory for the Structure Evolution of Celestial Objects Graduate University of Chinese Academy of Sciences Beijing 《中国科学:物理学 力学 天文学(英文版)》2010,(3)
Type Ia supernovae (SNe Ia) play an important role in diverse areas of astrophysics, from the chemical evolution of galaxies to observational cosmology. However, the nature of the progenitors of SNe Ia is still unclear. In this paper, according to a detailed binary population synthesis study, we obtained SN Ia birthrates and delay times from different progenitor models, and compared them with observations. We find that the Galactic SN Ia birthrate from the double-degenerate (DD) model is close to those infe... 相似文献
993.
LIU HaiFeng LIU HaiYan & SONG WeiDong State Key Laboratory of Explosion Science Technology Beijing Institute of Technology Beijing China School of Civil Hydraulic Engineering Ningxia University Yinchuan School of Science 《中国科学:物理学 力学 天文学(英文版)》2010,(2)
This paper takes concrete as a four-phase composite made of the intact matrix and three mutually perpendicular groups of penny-shaped micro-cracks. The intact matrix is assumed to be elastic,homogeneous and isotropic,and the micro-cracks are penny-shaped. Combined with the failure mechanism of concrete subjected to impact loading,a dynamic constitutive model for concrete is developed based on Mori-Tanaka's average stress concept and Eshelby's equivalent inclusion theory. Experimental results show that concr... 相似文献
994.
讨论了两列光波发生干涉现象的理想化条件和在实验中能否观察到干涉现象发生的操作性条件,推导了各种情况下的理论公式。从工程应用的角度出发,研究了检测仪器的响应时间T和光强分辨能力对观察光的干涉现象的影响,提出了观察光的干涉现象的第四个必要条件,对在各种情况下应用光的干涉现象有一定的指导作用。 相似文献
995.
按氢分类的分子电性距离矢量(Hydrogen-association classified molecular electronegafivity-distance vector,H-MEDV)是由4种类型原子间的相互作用得到的一种描述分子二维结构的拓扑描述子.本文根据H-MEDV来研究创新构型抗肿瘤药物德氮吡格(TNBG)及其衍生物活性与其结构之间的关系,利用多元线性回归和逐步回归,建立的4变量模型的相关系数R=0.862.对筛选后建立的模型用留一法交互检验的结果为Rcv=0.713,结果表明模型具有较好的预测能力和稳定性. 相似文献
996.
997.
High-spin states in nucleus 139Pm have been studied using the reaction 116Cd(27Al,4n)139Pm.Two dipole cascades have been found.Spin and parity assignments were based on the Directional Correlation of Oriented Nuclei(DCO) ratios and systematic behavior in neighboring odd-proton nuclei.The level structures of 139Pm are compared with those of the N =78 isotone 141Eu in which two dipole bands have been confirmed as magnetic rotational bands.The close similarity between them suggests that the dipole bands in 139... 相似文献
998.
999.
两个均匀分布总体标准差比的区间估计 总被引:1,自引:0,他引:1
考虑取两个均匀分布总体样本数不等时,两个均匀分布总体标准差比的估计,给出了两个均匀分布总体标准差比的区间估计. 相似文献
1000.
Coherent X-ray microscopy has advanced towards higher-energy, more brilliant sources over the past decade since its demonstrations, and many advancements have been made towards optimizing this imaging technique. Here we present both the experimental instrument for obtaining diffraction patterns and the primary reconstruction of yeast cell 2D projection. In addition, the characteristics of the existing optics at BL29XUL of SPring-8 Facility and the method of image reconstruction are discussed. 相似文献