全文获取类型
收费全文 | 5409篇 |
免费 | 773篇 |
国内免费 | 620篇 |
专业分类
化学 | 1909篇 |
晶体学 | 84篇 |
力学 | 776篇 |
综合类 | 97篇 |
数学 | 1230篇 |
物理学 | 2706篇 |
出版年
2024年 | 9篇 |
2023年 | 51篇 |
2022年 | 74篇 |
2021年 | 131篇 |
2020年 | 107篇 |
2019年 | 104篇 |
2018年 | 98篇 |
2017年 | 161篇 |
2016年 | 216篇 |
2015年 | 165篇 |
2014年 | 275篇 |
2013年 | 406篇 |
2012年 | 309篇 |
2011年 | 392篇 |
2010年 | 336篇 |
2009年 | 357篇 |
2008年 | 366篇 |
2007年 | 401篇 |
2006年 | 332篇 |
2005年 | 332篇 |
2004年 | 260篇 |
2003年 | 248篇 |
2002年 | 240篇 |
2001年 | 174篇 |
2000年 | 171篇 |
1999年 | 143篇 |
1998年 | 115篇 |
1997年 | 109篇 |
1996年 | 79篇 |
1995年 | 84篇 |
1994年 | 85篇 |
1993年 | 78篇 |
1992年 | 55篇 |
1991年 | 52篇 |
1990年 | 37篇 |
1989年 | 32篇 |
1988年 | 26篇 |
1987年 | 17篇 |
1986年 | 19篇 |
1985年 | 23篇 |
1984年 | 19篇 |
1983年 | 8篇 |
1982年 | 27篇 |
1981年 | 12篇 |
1980年 | 18篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1976年 | 11篇 |
1974年 | 4篇 |
排序方式: 共有6802条查询结果,搜索用时 15 毫秒
941.
针对大气垂直方向上消光系数分布不均匀难以用传统方法直接测量垂直能见度的问题,提出了一种基于激光雷达探测垂直能见度的计算方法。根据大气辐射传输基本原理,借助于辐射传输方程,推导出了垂直能见度的计算公式;然后利用激光雷达原理方程和Klett算法反演出大气垂直方向上的消光系数分布,基于此提出了垂直能见度的迭代算法。最后,利用灰色模型GM(1,1)和批统计算法,对激光雷达反演得到的后向散射系数进行了评估,给出了误差置信区间为(0.760±0.339)×10^-4(srad·km)^-1。结果表明,该方法是一种特别有效的计算垂直能见度的方法,符合探测的基本需求,且误差小精度高。 相似文献
942.
Anneli Kruve Riin Rebane Karin Kipper Maarja-Liisa Oldekop Hanno Evard Koit Herodes Pekka Ravio Ivo Leito 《Analytica chimica acta》2015
This is the part II of a tutorial review intending to give an overview of the state of the art of method validation in liquid chromatography mass spectrometry (LC–MS) and discuss specific issues that arise with MS (and MS–MS) detection in LC (as opposed to the “conventional” detectors). The Part II starts with briefly introducing the main quantitation methods and then addresses the performance related to quantification: linearity of signal, sensitivity, precision, trueness, accuracy, stability and measurement uncertainty. The last section is devoted to practical considerations in validation. With every performance characteristic its essence and terminology are addressed, the current status of treating it is reviewed and recommendations are given, how to handle it, specifically in the case of LC–MS methods. 相似文献
943.
944.
针对RC梁开裂荷载计算方法尚未统一的现状,首先,结合18根RC梁试验数据对比了已有的6种RC梁开裂弯矩计算公式,发现开裂弯矩理论计算值与试验值的偏差大小和混凝土强度有关;然后,通过提出塑性变形发展程度系数k,推导新的RC梁开裂弯矩计算公式,并进一步基于k值和塑性影响系数计算值γk进行改进;最后,选取12根RC试验梁验证改进公式的准确性,证明改进公式的计算值与试验值吻合更好且偏于安全。 相似文献
945.
Tengzhou Ma Ban Xuan Dong Jonathan W. Onorato Jens Niklas Oleg Poluektov Christine K. Luscombe Shrayesh N. Patel 《Journal of polymer science. Part A, Polymer chemistry》2021,59(22):2797-2808
Molecular doping of conjugated polymers (CPs) plays a vital role in optimizing organic electronic and energy applications. For the case of organic thermoelectrics, it is commonly believed that doping CPs with a strong dopant could result in higher conductivity (σ) and thus better power factor (PF). Herein, by investigating thermoelectric performance of a polar side-chain bearing CP, poly(3-(methoxyethoxyethoxy)thiophene) (P3MEET), vapor doped with fluorinated-derivative of tetracyanoquinodimethane FnTCNQ (n = 1, 2, 4), we show that using strong dopants can in fact have detrimental effects on the thermoelectric performance of CPs. Despite possessing higher electron affinity, doping P3MEET with F4TCNQ only results in a σ (27.0 S/cm) comparable to samples doped with other two weaker dopants F2TCNQ and F1TCNQ (26.4 and 20.1 S/cm). Interestingly, F4TCNQ-doped samples display a marked reduction in the Seebeck coefficient (α) compared to F1TCNQ- and F2TCNQ-doped samples from 42 to 13 μV/K, leading to an undesirable suppression of the PF. Structural characterizations coupled with Kang-Snyder modeling of the α–σ relation show that the reduction of α in F4TCNQ-doped P3MEET samples originates from the generation of low mobility carrier within P3MEET's amorphous domain. Our results demonstrate that factors such as dopant distribution and doping efficiency within the crystalline and amorphous domains of CPs should play a crucial role in advancing rational design for organic thermoelectrics. 相似文献
946.
947.
948.
949.
Assume that we have iid observations on the random vector X = (X
,...,X
) following a multivariate normal distribution N
(,) where both R
and (p.d.) are unknown. Let
denote the multiple correlation coefficient between X
and (X
,...,X
). The parameter =
, called the multiple coefficient of determination, indicates the proportion of variability in X
explained by its best linear fit based on (X
,..., X
). In this paper we consider the point estimation of under the ordinary squared error loss function. The usual estimators (MLE, UMVUE) have complicated risk expressions and hence it is quite difficult to get exact decision-theoretic results. We therefore follow the asymptotic decision theoretic approach (as done by Ghosh and Sinha (1981, Ann. Statist., 9, 1334-1338)) and study Second Order Admissibility of various estimators including the usual ones. 相似文献
950.