全文获取类型
收费全文 | 527篇 |
免费 | 21篇 |
专业分类
化学 | 278篇 |
晶体学 | 3篇 |
力学 | 15篇 |
数学 | 23篇 |
物理学 | 114篇 |
无线电 | 115篇 |
出版年
2023年 | 3篇 |
2022年 | 7篇 |
2021年 | 6篇 |
2020年 | 12篇 |
2019年 | 16篇 |
2018年 | 9篇 |
2017年 | 14篇 |
2016年 | 27篇 |
2015年 | 10篇 |
2014年 | 23篇 |
2013年 | 42篇 |
2012年 | 35篇 |
2011年 | 31篇 |
2010年 | 17篇 |
2009年 | 22篇 |
2008年 | 29篇 |
2007年 | 23篇 |
2006年 | 24篇 |
2005年 | 25篇 |
2004年 | 17篇 |
2003年 | 9篇 |
2002年 | 18篇 |
2001年 | 5篇 |
2000年 | 10篇 |
1999年 | 9篇 |
1998年 | 4篇 |
1997年 | 13篇 |
1996年 | 10篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1991年 | 8篇 |
1989年 | 4篇 |
1988年 | 7篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1966年 | 1篇 |
1965年 | 3篇 |
1936年 | 4篇 |
排序方式: 共有548条查询结果,搜索用时 234 毫秒
41.
Characterizing Superposition Arrival Processes in Packet Multiplexers for Voice and Data 总被引:10,自引:0,他引:10
This paper analyzes a model of a multiplexer for packetized voice and data. A major part of the analysis is devoted to characterizing the aggregate packet arrival process resulting from the superposition of separate voice streams. This is done via the index of dispersion for intervals (IDI), which describes the cumulative covariance among successive interarrival times. The IDI seems very promising as a measurement tool to characterize complex arrival processes. This paper also describes the delays experienced by voice and data packets in the multiplexer using relatively simple two-parameter approximations. 相似文献
42.
Wavelength Conversion Placement in WDM Mesh Optical Networks* 总被引:1,自引:0,他引:1
Wavelength conversion helps improve the performance of wavelength division multiplexed (WDM) optical networks that employ wavelength routing. In this paper, we address the problem of optimally placing a limited number of wavelength converters in mesh topologies. Two objective functions, namely, minimizing the average blocking probability and minimizing the maximum blocking probability over all routes, are considered. In the first part of the paper, we extend an earlier analytical model to compute the blocking probability on an arbitrary route in a mesh topology, given the traffic and locations of converters. We then propose heuristic algorithms to place wavelength converters, and evaluate the performance of the proposed heuristics using the analytical model. Results suggest that simple heuristics are sufficient to give near-optimal performance. 相似文献
43.
44.
An experiment is discussed in which CO can be excited up to energies of several electronvolts by the absorption of infrared radiation from a relatively low-power CO laser. Furthermore, experimental results are examined through kinetic modelling. In the experiment, the beam of an intracavity-chopped CO laser operating on all lines at 500 mW and containing a few milliwatts of the fundamental ν= 1→0 band component, is focused into an absorption cell containing a mixture of CO and Ar. The absorption of this infrared radiation is monitored by the optoacoustic effect. A second CO laser operating cw and capable of providing 8 W on all lines but not lasing on the ν= 1→0 band component, is then focused into the same volume in the absorption cell. With both lasers simultaneously focused into the absorption cell, strong fluorescence from the irradiated region is detected by a photomultiplier tube. Modulation of the signal intensity with time is observed, and indicates chemical destruction of the CO in the cell. An analysis and kinetic modelling calculation of this experiment shows that it is possible to excite CO up to high vibrational quantum numbers (ν40) at gas temperatures up to 800 K. by means of CO laser irradiation at the fundamental ν= 1→0 band component. One source responsible for the fluorescence signal observed in the experiment is identified as the 4th positive A 1Π→X 1Σ+ spontaneous emission. Although the present kinetic model does not incorporate the chemical processes that may lead to the production of additional fluorescing species such as C2, good agreement is obtained with the observed fluorescence signal characteristics. 相似文献
45.
In this paper, a novel high-frequency/high-power field-effect-transistor structure is presented to reduce gain degradation caused by common-source inductance. In this structure, the reduction in common-source inductance is achieved without the need for using very thin substrates or very complicated fabrication technology, such as vias under each source finger. Using detailed transmission-line modeling, it is shown that a significant reduction in common-source inductance and improvement in RF performance can be achieved even for moderately high values of source grounding via inductance. The new structure allows simpler fabrication technology and is expected to be particularly useful to reduce the cost and improve the performance of high-power microwave and millimeter-wave devices and circuits 相似文献
46.
Numerical simulations and laboratory measurements have been used to illuminate the interaction of a moving shock wave impacting
on metallic grids at various shock strengths and grid solidities. The experimental work was carried out in a large scale shock
tube facility while computational work simulated the flow field with a time-dependent inviscid and a time-dependent viscous
model. The pressure drop measured across the grids is a result of two phenomena which are associated with the impact of the
shock on the metallic grids. First are the reflection and refraction of the incoming shock on the grid itself. This appears
to be the main inviscid mechanism associated with the reduction of the strength of the transmitted shock. Second, viscous
phenomena are present during the reflection and refraction of the wave as well as during the passage of the induced flow of
the air through the grid. The experimental data of pressure drop across the grid obtained in the present investigation are
compared with those obtained from computations. The numerical results slightly overpredict the experimental data of relative
pressure drop which increases substantially with grid solidity at fixed flow Mach numbers. The processes of shock reflection
and refraction are continuous and they can be extended in duration by using thicker grids that will result in lower compression
rates of the structural loading and increase the viscous losses associated with these phenomena which will further attenuate
the impacting shock. Preliminary theoretical analysis suggests that the use of a graded porosity/solidity material will result
in higher pressure drop than a constant porosity/solidity material and thus provide effective blast mitigation.
相似文献
47.
Zinah Rafea Ahmed Shamala K. Subramaniam Zuriati Ahmad Zukarnain Mohamed Othman 《Wireless Personal Communications》2014,77(3):2271-2289
Resource scheduling in Long Term Evolution (LTE) is an open and rising issue. It has an enormous impact on the entire system performance. Due to the nature of LTE system, the scheduler has to be designed carefully. It has to overcome many challenges such as limited processing time and the high dynamic behavior. This paper proposes a novel scheduling policy for the MAC layer in LTE called the Best Minimum Summation (BMS). The main aim of this scheduling policy is to achieve high performance with low complexity. Three sub-schedulers have been developed. Each one of these schedulers deals with scheduling table in different dimension. The first one operates on the scheduling table through the user dimension (BMS.UE); while the second one operates on the scheduling table through the resource block dimension (BMS.RB). The third scheduler operates on the scheduling table correlating both of these dimensions (BMS.2D). All of the proposed solutions were intensively evaluated in a system level simulator. Three performance metrics were used which are throughput, error rate and fairness. The results have shown that the ability of the BMS.UE scheduler to outperform other existing schedulers of LTE. 相似文献
48.
Wireless networks play a very important role in today’s modern world, convincingly surpassing the wired infrastructure in
terms of popularity. Hence, it is important to ensure that services which access wired networks should also be accessible
using a wireless network without any performance degradation. One of the most common variants in wireless communications is
the Wireless Mesh Network (WMNs). WMNs exploit multi-hop wireless communications between wireless access points. Hence, the
effective bandwidth decreases as the number of hops increases in a WMN, thus increasing latency and resulting in reduced performance.
This may be due to spatial contention, multipath fading, interference or inefficient queuing mechanisms etc. Here we take
queuing mechanisms into consideration and study the QMMN algorithm (Queue Management for Multi-hop Networks) which tends to
improve throughput, fairness and reduce global synchronization problems. Based on our study, we implement a modified version
of the QMMN algorithm, otherwise called the Enhanced QMMN (EQMMN) algorithm. EQMMN can be considered an effective algorithm
which solves the problem of fairness between flows (either responsive or unresponsive) and eventually improves TCP throughput
at wireless access points. Our experimental results prove that EQMMN algorithms have better performance characteristics such
as throughput (TCP) and fairness index compared to QMMN algorithms. 相似文献
49.
Proceedings - Mathematical Sciences - Benzophenone and paranitrotoluene have been studied in the solid and fused states. Their solutions in methyl alcohol have also been investigated and the... 相似文献
50.
Felsen L.B. Subramaniam N. Arichandran K. 《Antennas and Propagation, IEEE Transactions on》1990,38(8):1273-1279
The relation between Green's functions appropriate to closed and open shells is explored. The problem of constructing a rigorous reference solution for multiple reflected-ray fields traversing a section of a curved layer is addressed by exploiting the connection between traveling-wave (ray-type) and standing-wave (angular harmonic) fields in the closed layer. Utilizing Poisson summation, the discrete superposition of standing wave fields is converted into a discrete superposition of spectral integrals whose asymptotic evaluation generates traveling waves. It is possible to group together the spectral integrals that correspond to the collection of ray fields with N reflections inside the cavity, and to express this sum alternatively, and exactly, in angular harmonic form. The quality of the asymptotic (i.e. ray) approximation obtained from the spectral integrals can be checked against numerical data from the harmonic series. This problem strategy is implemented for the two-dimensional canonical configuration comprising a circular cylindrical homogeneous dielectric layer excited by an axially directed electric line current source located in the interior cavity surrounded by the layer 相似文献