全文获取类型
收费全文 | 12026篇 |
免费 | 1285篇 |
国内免费 | 1066篇 |
专业分类
化学 | 7452篇 |
晶体学 | 70篇 |
力学 | 847篇 |
综合类 | 107篇 |
数学 | 567篇 |
物理学 | 1258篇 |
无线电 | 4076篇 |
出版年
2024年 | 44篇 |
2023年 | 176篇 |
2022年 | 362篇 |
2021年 | 324篇 |
2020年 | 473篇 |
2019年 | 396篇 |
2018年 | 419篇 |
2017年 | 653篇 |
2016年 | 708篇 |
2015年 | 661篇 |
2014年 | 818篇 |
2013年 | 810篇 |
2012年 | 918篇 |
2011年 | 888篇 |
2010年 | 682篇 |
2009年 | 647篇 |
2008年 | 628篇 |
2007年 | 715篇 |
2006年 | 620篇 |
2005年 | 517篇 |
2004年 | 462篇 |
2003年 | 373篇 |
2002年 | 339篇 |
2001年 | 277篇 |
2000年 | 251篇 |
1999年 | 196篇 |
1998年 | 178篇 |
1997年 | 169篇 |
1996年 | 176篇 |
1995年 | 128篇 |
1994年 | 104篇 |
1993年 | 36篇 |
1992年 | 25篇 |
1991年 | 24篇 |
1990年 | 32篇 |
1989年 | 25篇 |
1988年 | 16篇 |
1987年 | 12篇 |
1986年 | 14篇 |
1985年 | 14篇 |
1984年 | 14篇 |
1983年 | 10篇 |
1982年 | 11篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 7篇 |
1978年 | 2篇 |
1976年 | 3篇 |
1959年 | 4篇 |
1957年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 14 毫秒
11.
本文重点研究基于包错误率测量的链路自适应算法,分析了包错误率与信道参数的关系,提出了一种有别于固定门限值方案的优化门限值方案。仿真结果表明,优化门限值方案的网络吞吐量要大于固定门限值方案和基于信干比估计的链路自适应算法,最大增幅可达12Mbit/s。在系统引入重传机制后,网络吞吐量将有所损失,但最大不超过1Mbit/s,相比较服务质量的提高是值得的。需指出的是,虽然分析和仿真均以HIPERLAN/2为例,但不失一般性,它们同样适合于WPAN/WLAN等类似系统。 相似文献
12.
基于有限状态的全周期序列混沌映射的研究 总被引:1,自引:1,他引:0
在有限长度的数字计算机中,混沌迭代序列是一个有限周期序列,其相关性能远远低于理想序列。基于skew tent映射,提出一种离散化的有限状态混沌映射,它允许精确的数值计算,没有舍入误差,通过选择合适的参数,映射能产生全周期长度的序列。仿真实验表明,归一化的序列平均周期对计算机精度不敏感,生成的二进制序列具有良好的相关性能。 相似文献
13.
Navid Ehsan Mingyan Liu Roderick J. Ragland 《International Journal of Communication Systems》2003,16(6):513-534
Performance enhancing proxies (PEPs) are widely used to improve the performance of TCP over high delay‐bandwidth product links and links with high error probability. In this paper we analyse the performance of using TCP connection splitting in combination with web caching via traces obtained from a commercial satellite system. We examine the resulting performance gain under different scenarios, including the effect of caching, congestion, random loss and file sizes. We show, via analysing our measurements, that the performance gain from using splitting is highly sensitive to random losses and the number of simultaneous connections, and that such sensitivity is alleviated by caching. On the other hand, the use of a splitting proxy enhances the value of web caching in that cache hits result in much more significant performance improvement over cache misses when TCP splitting is used. We also compare the performance of using different versions of HTTP in such a system. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
14.
Sensitivity analysis is a mathematical tool, first developed for optimization methods, which aim is to characterize a system response through the variations of its output parameters following modifications imposed on the input parameters of the system. Such an analysis may quickly become laborious when the thermal model under consideration is complex or the number of input parameters is high. In this paper, we develop a mathematical model to analyse the heat exchanges in four different types of solar air collectors. When building this thermal model we show that for each collector, at quasi-steady state, the energy balance equations of the components of the collector cascade into a single first-order non-linear differential equation that is able to predict the thermal behaviour of the collector. Our heat transfer model clearly demonstrates the existence of an important dimensionless parameter, referred to as the thermal performance factor of the collector, that compares the useful thermal energy which can be extracted from the heater to the overall thermal losses of that collector for a given set of input parameters. A sensitivity analysis of our thermal model has been performed for the most significant input parameters such as the incident solar irradiation, the inlet fluid temperature, the air mass flow rate, the depth of the fluid channel, the number and nature of the transparent covers in order to measure the impact of each of these parameters on our model. An important result which can be drawn from this study is that the heat transfer model developed is robust enough to be used for thermal design studies of most known flat plate solar air heaters, but also of flat plate solar water collectors and linear solar concentrators. 相似文献
15.
16.
托卡马克工程试验混合堆等离子体性能的等值线图分析 总被引:3,自引:3,他引:0
盛光昭 《核聚变与等离子体物理》1989,9(1):29-36
本文简要叙述托卡马克工程试验混合堆等离子体概念设计的物理基础,对等离子体性能进行了等值线图(Plasma Operation Contour)分析。根据工程试验混合堆的要求,得出一组等离子体参数。 相似文献
17.
18.
介绍了PHS网呼入成功率和短消息流程,依据短消息模型和性能统计结果分析了群发短信对PHS网呼入成功率的影响,并建议选择合理的时间进行短消息群发以减少对网络负荷的影响。 相似文献
19.
Solving large scale linear systems efficiently plays an important role in a petroleum reservoir simulator, and the key part is how to choose an effective parallel preconditioner. Properly choosing a good preconditioner has been beyond the pure algebraic field. An integrated preconditioner should include such components as physical background, characteristics of PDE mathematical model, nonlinear solving method, linear 相似文献
20.
Christos Bouras Agisilaos Konidaris 《International Journal of Communication Systems》2005,18(2):119-142
Redundant data transfers over the Web, can be mainly attributed to the repeated transfers of unchanged data. Web caches and Web proxies are some of the solutions that have been proposed, to deal with the issue of redundant data transfers. In this paper we focus on the efficient estimation and reduction of redundant data transfers over the Web. We first prove that a vast amount of redundant data is transferred in Web pages that are considered to carry fresh data. We show this by following an approach based on Web page fragmentation and manipulation. Web pages are broken down to fragments, based on specific criteria. We then deal with these fragments as independent constructors of the Web page and study their change patterns independently and in the context of the whole Web page. After the fragmentation process, we propose solutions for dealing with redundant data transfers. This paper has been based on our previous work on ‘Web Components’ but also on related work by other researchers. It utilises a proxy based, client/server architecture, and imposes changes to the algorithms executed on the Proxy server and on clients. We show that our proposed solution can considerably reduce the amount of redundant data transferred on the Web. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献