全文获取类型
收费全文 | 51455篇 |
免费 | 12521篇 |
国内免费 | 5415篇 |
专业分类
化学 | 53442篇 |
晶体学 | 828篇 |
力学 | 1366篇 |
综合类 | 387篇 |
数学 | 4173篇 |
物理学 | 9195篇 |
出版年
2024年 | 43篇 |
2023年 | 308篇 |
2022年 | 617篇 |
2021年 | 772篇 |
2020年 | 1884篇 |
2019年 | 3215篇 |
2018年 | 1675篇 |
2017年 | 1390篇 |
2016年 | 4064篇 |
2015年 | 4379篇 |
2014年 | 4447篇 |
2013年 | 5376篇 |
2012年 | 4403篇 |
2011年 | 3721篇 |
2010年 | 4156篇 |
2009年 | 4093篇 |
2008年 | 3698篇 |
2007年 | 3014篇 |
2006年 | 2542篇 |
2005年 | 2559篇 |
2004年 | 2171篇 |
2003年 | 1914篇 |
2002年 | 2674篇 |
2001年 | 1971篇 |
2000年 | 1774篇 |
1999年 | 715篇 |
1998年 | 283篇 |
1997年 | 197篇 |
1996年 | 178篇 |
1995年 | 162篇 |
1994年 | 161篇 |
1993年 | 111篇 |
1992年 | 93篇 |
1991年 | 98篇 |
1990年 | 74篇 |
1989年 | 53篇 |
1988年 | 49篇 |
1987年 | 53篇 |
1986年 | 32篇 |
1985年 | 48篇 |
1984年 | 21篇 |
1983年 | 31篇 |
1982年 | 23篇 |
1981年 | 23篇 |
1980年 | 17篇 |
1979年 | 17篇 |
1978年 | 14篇 |
1976年 | 9篇 |
1971年 | 9篇 |
1965年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
ZHONG Hui WAN Mingxi JIANG Yifeng WANG Supin 《声学学报:英文版》2006,25(2):104-115
The present study was to investigate the differential imaging method for detecting HIFU (High Intensity Focused Ultrasound)-induced lesions and the estimation of variation of attenuation for lesion evaluation with log spectral difference algorithm. Experiment results of bovine muscle and liver in vitro were acquired. Several algorithms for lesion detection - Absolute Difference (AD), Sum Absolute Differences (SAD) and Sum Squared Differences (SSD) - were analyzed with several window sizes and threshold values. Then three attenuation parameters were compared to evaluate the degree of tissue damage. It was found that variation of the mean attenuation △α was an effective parameter to evaluate lesions. 相似文献
72.
基于同步辐射加速器的康普顿背散射γ射线源(Ⅰ)产生MeV量级γ光子的数值计算 总被引:1,自引:0,他引:1
提出在筹建的上海同步辐射装置上建造一条MeV量级γ射线束及应用站,采用μm波长的红外(或远红外)激光与储存环中3.5GeV电子束进行康普顿背散射,从而获得能区为1—25MeV的康普顿背散射γ光子束,该光子束具有高强度、高极化度(线和圆极化)、准单色、方向性好的优点,可以广泛地应用于核物理和核天体物理基础研究及相关的应用研究领域.介绍了康普顿背散射的基本原理,并结合储存环参数给出了光子束性能的数值计算结果. 相似文献
73.
74.
75.
76.
A graph G is N2‐locally connected if for every vertex ν in G, the edges not incident with ν but having at least one end adjacent to ν in G induce a connected graph. In 1990, Ryjá?ek conjectured that every 3‐connected N2‐locally connected claw‐free graph is Hamiltonian. This conjecture is proved in this note. © 2004 Wiley Periodicals, Inc. J Graph Theory 48: 142–146, 2005 相似文献
77.
It is widely accepted that next-generation networks will provide guaranteed services, in contrast to the “best effort” approach today. We study and analyze queueing policies for network switches that support the QoS (Quality of Service) feature. One realization of the QoS feature is that packets are not necessarily all equal, with some having higher priorities than the others. We model this situation by assigning an intrinsic value to each packet. In this paper we are concerned with three different queueing policies: the nonpreemptive model, the FIFO preemptive model, and the bounded delay model. We concentrate on the situation where the incoming traffic overloads the queue, resulting in packet loss. The objective is to maximize the total value of packets transmitted by the queueing policy. The difficulty lies in the unpredictable nature of the future packet arrivals. We analyze the performance of the online queueing policies via competitive analysis, providing upper and lower bounds for the competitive ratios. We develop practical yet sophisticated online algorithms (queueing policies) for the three queueing models. The algorithms in many cases have provably optimal worst-case bounds. For the nonpreemptive model, we devise an optimal online algorithm for the common 2-value model. We provide a tight logarithmic bound for the general nonpreemptive model. For the FIFO preemptive model, we improve the general lower bound to 1.414, while showing a tight bound of 1.434 for the special case of queue size 2. We prove that the bounded delay model with uniform delay 2 is equivalent to a modified FIFO preemptive model with queue size 2. We then give improved upper and lower bounds on the 2-uniform bounded delay model. We also show an improved lower bound of 1.618 for the 2-variable bounded delay model, matching the previously known upper bound. 相似文献
78.
79.
Ching‐Yuan Cheng Kuan‐Jiuh Lin 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(8):m363-m365
In the title compound, catena‐poly[lithium‐μ3‐ethylenediphosphonato], [Li(C2H7O6P2)]n, the supramolecular monoclinic (C2/c) structure consists of one‐dimensional lithium chains [Li⋯Li = 2.7036 (8) Å] that are embedded within ethylenediphosphonate anions linked by strong symmetric hydrogen bonds [O⋯O = 2.473 (3) Å]. The Li atoms and the H atom in the symmetric hydrogen bond reside on twofold rotation axes and there is an inversion center at the mid‐point of the C—C bond of the ethylenediphosphonate ligand. 相似文献
80.
The inhomogeneous Dirichlet problems concerning divergence form elliptic equations are studied. Optimal regularity requirements on the coefficients and domains for the W1,p theory, 1 < p < ∞, are obtained. The principal coefficients are supposed to be in the John‐Nirenberg space with small BMO seminorms. The domain is a Reifenberg domain. These conditions for the W1,p theory not only weaken the requirements on the coefficients but also lead to a more general geometric condition on the domains. In fact, these domains might have fractal dimensions. © 2004 Wiley Periodicals, Inc. 相似文献