全文获取类型
收费全文 | 12519篇 |
免费 | 1963篇 |
国内免费 | 1692篇 |
专业分类
化学 | 9498篇 |
晶体学 | 184篇 |
力学 | 687篇 |
综合类 | 112篇 |
数学 | 1335篇 |
物理学 | 4358篇 |
出版年
2024年 | 41篇 |
2023年 | 285篇 |
2022年 | 479篇 |
2021年 | 496篇 |
2020年 | 626篇 |
2019年 | 606篇 |
2018年 | 459篇 |
2017年 | 458篇 |
2016年 | 583篇 |
2015年 | 615篇 |
2014年 | 674篇 |
2013年 | 959篇 |
2012年 | 1010篇 |
2011年 | 1099篇 |
2010年 | 778篇 |
2009年 | 704篇 |
2008年 | 827篇 |
2007年 | 654篇 |
2006年 | 610篇 |
2005年 | 541篇 |
2004年 | 433篇 |
2003年 | 350篇 |
2002年 | 381篇 |
2001年 | 298篇 |
2000年 | 281篇 |
1999年 | 232篇 |
1998年 | 189篇 |
1997年 | 184篇 |
1996年 | 187篇 |
1995年 | 166篇 |
1994年 | 168篇 |
1993年 | 131篇 |
1992年 | 99篇 |
1991年 | 82篇 |
1990年 | 85篇 |
1989年 | 74篇 |
1988年 | 53篇 |
1987年 | 49篇 |
1986年 | 45篇 |
1985年 | 57篇 |
1984年 | 32篇 |
1983年 | 17篇 |
1982年 | 17篇 |
1981年 | 20篇 |
1980年 | 11篇 |
1979年 | 6篇 |
1977年 | 3篇 |
1973年 | 3篇 |
1959年 | 2篇 |
1957年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
53.
54.
江汉平原上空QTEC变化特征的统计分析 总被引:1,自引:0,他引:1
提出了一种观测台网用多接收机性能一致性的标定方法,并利用江汉平原GPS接收台网的观测数据,分析了该地区上空QTEC的日变化、季节变化、年变化和逐年变化的统计特征.标定结果表明,不同接收机之间的仪器偏差是从双频GPS卫星信标推算QTEC的主要误差来源之一,必须校正.统计分析证实,江汉平原上空QTEC的日变化特征同时受电离层和等离子层控制,其年变化以半年周期为主,峰值和谷值分别出现在两分点和两至点附近,QTEC值的逐年变化从太阳活动峰年到低年逐年下降,与以太阳黑子数为代表的太阳活动密切相关. 相似文献
55.
提出了利用气象资料对MODIS影像进行大气校正的一种快速实用方法.读取MODIS前两个通道250min的反射率,采用常用的黑色目标扣除法(Dark-object subtraction)计算天空程辐射,通过卫星过境时刻的气象能见度和水汽资料计算大气透过率,最后推导出目标物的反射率.通过比较两个邻近日期MODIS影像校正前后的归一化植被指数(NDVI)值,得出利用本文提出的方法对MODIS进行大气校正所得到的NDVI值更接近实际情况.结果表明:利用气象能见度和水汽压可以对MODIS数据进行快速有效的大气校正. 相似文献
56.
Leong‐Huat Gan Palaniswamy Ravi Bao Wei Mao Kam‐Chiu Tam 《Journal of polymer science. Part A, Polymer chemistry》2003,41(17):2688-2695
Polymerization of 2‐(diethylamino)ethyl methacrylate (DEAEMA) via homogeneous atom transfer radical polymerization under various reaction conditions is described. The effects of the initiators and solvents were examined. With 1,1,4,7,10,10‐hexamethyl triethylenetetramine/copper(I) chloride/p‐toluenesulfonyl chloride as the ligand/catalyst/initiator system in methanol, poly(DEAEMA) with a polydispersity index as low as 1.07 was synthesized. Kinetic studies demonstrated the polymerization was very well controlled and exhibited the living characteristic of the process. Well‐defined block copolymers of DEAEMA and tert‐butyl methacrylate (tBMA) were successfully synthesized. The copolymers could be synthesized with equally good results by starting with either p(DEAEMA) or p(tBMA) as the macroinitiators. However, only the macroinitiators terminated with chlorine should be used. The corresponding macroinitiators with bromine as a transferable group did not yield well‐defined copolymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2688–2695, 2003 相似文献
57.
In this paper, by the use of a fixed point theorem, many new necessary and sufficient conditions for the existence of positive solutions in C[0,1]∩C1[0,1]∩C2(0,1) or C[0,1]∩C2(0,1) are presented for singular superlinear and sublinear second-order boundary value problems. Singularities at t=0, t=1 will be discussed. 相似文献
58.
In this paper, multi-objective models for designing 3D trajectory of horizontal wells are developed in a fuzzy environment. Here, the objectives of minimizing the length of the trajectory and the error of entry target point are fuzzy in nature. Some parameters, such as initial value, end value, lower bound and upper bound of the curvature radius, tool-face angle and the are length of each curve section, are also assumed to be vague and imprecise. The impreciseness in the above objectives have been expressed by fuzzy linear membership functions and that in the above parameters by triangular fuzzy numbers. Models have been solved by the fuzzy non-linear programming method based on Zimmermann [1] and Lee and Li [2]. Models are applied to practical design of the horizontal wells. Numerical results illustrate the accuracy and efficiency of the fuzzy models. 相似文献
59.
Positron-lifetime experiments have been carried out on two undoped n-type liquid encapsulated Czochralski (LEC)-grown InP samples with different stoichiometric compositions in the temperature range 10-300 K. For temperatures below 120 K for P-rich InP and 100 K for In-rich InP, the positron average lifetime began to increase rapidly and then leveled off, which was associated with the charge state change of hydrogen indium vacancy complexes from (VInH4)+ to (VInH4)0. This phenomenon was more obvious in P-rich samples that have a higher concentration of VInH4. The transformation temperature of approximately 120 K suggests that the complex VInH4 is a donor defect and that the ionization energy is about 0.01 eV. The ionization of neutral VInH4 accounted for the decrease of the positron average lifetime when the sample was illuminated with a photon energy of 1.32 eV at 70 K. These results provide evidence for hydrogen complex defects in undoped LEC InP. 相似文献
60.