全文获取类型
收费全文 | 61284篇 |
免费 | 10800篇 |
国内免费 | 7965篇 |
专业分类
化学 | 31378篇 |
晶体学 | 615篇 |
力学 | 2738篇 |
综合类 | 439篇 |
数学 | 4904篇 |
物理学 | 17301篇 |
无线电 | 22674篇 |
出版年
2024年 | 263篇 |
2023年 | 1392篇 |
2022年 | 1993篇 |
2021年 | 2384篇 |
2020年 | 2367篇 |
2019年 | 2248篇 |
2018年 | 2009篇 |
2017年 | 2178篇 |
2016年 | 2669篇 |
2015年 | 3218篇 |
2014年 | 3843篇 |
2013年 | 4740篇 |
2012年 | 5451篇 |
2011年 | 5465篇 |
2010年 | 4408篇 |
2009年 | 4424篇 |
2008年 | 4695篇 |
2007年 | 4087篇 |
2006年 | 3813篇 |
2005年 | 3030篇 |
2004年 | 2294篇 |
2003年 | 1898篇 |
2002年 | 1877篇 |
2001年 | 1540篇 |
2000年 | 1329篇 |
1999年 | 1062篇 |
1998年 | 809篇 |
1997年 | 722篇 |
1996年 | 677篇 |
1995年 | 558篇 |
1994年 | 526篇 |
1993年 | 407篇 |
1992年 | 333篇 |
1991年 | 258篇 |
1990年 | 237篇 |
1989年 | 139篇 |
1988年 | 132篇 |
1987年 | 100篇 |
1986年 | 91篇 |
1985年 | 77篇 |
1984年 | 40篇 |
1983年 | 39篇 |
1982年 | 22篇 |
1981年 | 24篇 |
1980年 | 11篇 |
1979年 | 22篇 |
1977年 | 11篇 |
1974年 | 9篇 |
1957年 | 8篇 |
1925年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Suppose μ is a Radon measure on ℝ
d
, which may be non doubling. The only condition assumed on μ is a growth condition, namely, there is a constant C0>0 such that for all x∈supp(μ) and r>0, μ(B(x, r))⪯C0rn, where 0<n⪯d. We prove T1 theorem for non doubling measures with weak kernel conditions. Our approach yields new results
for kernels satisfying weakened regularity conditions, while recovering previously known Tolsa’s results. We also prove T1
theorem for Besov spaces on nonhomogeneous spaces with weak kernel conditions given in [7]. 相似文献
152.
Inspired by the recent work [HHM03], we prove two stability results for compact Riemannian manifolds with nonzero parallel spinors. Our first result says that Ricci flat metrics which also admit nonzero parallel spinors are stable (in the direction of changes in conformal structures) as the critical points of the total scalar curvature functional. Our second result, which is a local version of the first one, shows that any metric of positive scalar curvature cannot lie too close to a metric with nonzero parallel spinor. We also prove a rigidity result for special holonomy metrics. In the case of SU(m) holonomy, the rigidity result implies that scalar flat deformations of Calabi-Yau metric must be Calabi-Yau. Finally we explore the connection with a positive mass theorem of [D03], which presents another approach to proving these stability and rigidity results. Dedicated to Jeff Cheeger for his sixtieth birthday 相似文献
153.
154.
155.
数据通信网综合网管的设计分析 总被引:3,自引:0,他引:3
结合TMN电信管理网的体系结构.对数据通信网综合网管的结构和功能进行了分析。 相似文献
156.
软件无线电中采样率转换技术的实现及在3G中的应用 总被引:1,自引:0,他引:1
目前移动通信标准繁多,且新旧体制混杂,很难做到统一.为了实现互连互通,目前比较可行的方法是采用软件无线电.采样率转换技术是实现互连互通的关键.文中针对第三代移动通信系统(3G)中不同通信体制的带宽、速率要求,根据分级变换的思想,对3G中的CDMA2000和TDSCDMA的采样率转换方案进行了设计,并对3G中的5种制式的采样率转换方案进行了分析比较.实验表明,这一方案可以在采样率变换的高效网络结构基础上进一步降低滤波运算量和存储量. 相似文献
157.
Deposition of Hydrogen-Free Silicon Nitride Thin Films by Microwave ECR plasma Enhanced Magnetron Sputtering at Room Temperature
下载免费PDF全文
![点击此处可从《中国物理快报》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Hydrogen-free silicon nitride (SiNx) films were deposited at room temperature by microwave electron cyclotron resonance (MW-ECR) plasma enhanced unbalance magnetron sputtering system. Both Fourier-transform infrared spectroscopy and x-ray photoelectron spectroscopy are used to study the bonding type and the change of bonding structures of the silicon nitride films. The results indicate that the chemical structure and composition of SiNx films deposited by this technique depend strongly on the N2 flow rates, the stoichiometric SiNx film, which has the highest hardness of 22.9 GPa, could be obtained at lower N2 flow rate of 4 sccm. 相似文献
158.
We investigate the origin of ultraviolet (UV) emission from Mg0.12 Zn0.88 O alloy thin films with a wurtzite structure fabricated on c-plane Al2O3 substrates by plasma assisted molecular beam epitaxy. At room temperature, the absorption edge and UV emission band of the Mg0.12Zn0.88O film shift to high-energy side compared with ZnO films. Temperature dependence of the photoluminescence spectra shows that the UV emission is composed of free exciton and neutral donor bound exciton emissions. Two-step dissociation processes of the UV emission are observed with the increasing temperature. The thermal quenching mechanism is attributed to the dissociation of the free exciton from the neutral donor bound exciton in the low temperature region and the dissociation of free electron and hole from the free exciton in the high temperature region. 相似文献
159.
160.
Jianxin Wei Mark R Pickering Michael R Frater John F Arnold John A Boman Wenjun Zeng 《IEEE transactions on image processing》2005,14(8):1033-1042
It is well known that tile-boundary artifacts occur in wavelet-based lossy image coding. However, until now, their cause has not been understood well. In this paper, we show that boundary artifacts are an inescapable consequence of the usual methods used to choose tile size and the type of symmetric extension employed in a wavelet-based image decomposition system. This paper presents a novel method for reducing these tile-boundary artifacts. The method employs odd tile sizes (2N + 1 samples) rather than the conventional even tile sizes (2N samples). It is shown that, for the same bit rate, an image compressed using an odd tile length low-pass first (OTLPF) convention has significantly less boundary artifacts than an image compressed using even tile sizes. The OTLPF convention can also be incorporated into the JPEG 2000 image compression algorithm using extensions defined in Part 2 of this standard. 相似文献