全文获取类型
收费全文 | 283篇 |
免费 | 24篇 |
国内免费 | 5篇 |
专业分类
化学 | 112篇 |
晶体学 | 3篇 |
力学 | 31篇 |
综合类 | 4篇 |
数学 | 62篇 |
物理学 | 100篇 |
出版年
2023年 | 3篇 |
2022年 | 4篇 |
2021年 | 3篇 |
2020年 | 3篇 |
2019年 | 3篇 |
2018年 | 5篇 |
2017年 | 6篇 |
2016年 | 3篇 |
2015年 | 12篇 |
2014年 | 11篇 |
2013年 | 16篇 |
2012年 | 11篇 |
2011年 | 17篇 |
2010年 | 9篇 |
2009年 | 19篇 |
2008年 | 24篇 |
2007年 | 18篇 |
2006年 | 11篇 |
2005年 | 10篇 |
2004年 | 14篇 |
2003年 | 18篇 |
2002年 | 12篇 |
2001年 | 8篇 |
2000年 | 8篇 |
1999年 | 10篇 |
1998年 | 4篇 |
1997年 | 10篇 |
1996年 | 5篇 |
1995年 | 4篇 |
1994年 | 10篇 |
1993年 | 1篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有312条查询结果,搜索用时 31 毫秒
181.
In the literature, performance analyses of numerous single server queues are done by analyzing the embedded Markov renewal
processes at departures. In this paper, we characterize the departure processes for a large class of such queueing systems.
Results obtained include the Laplace–Stieltjes transform (LST) of the stationary distribution function of interdeparture times
and recursive formula for {cn ≡ the covariance between interdeparture times of lag n}. Departure processes of queues are difficult to characterize and for queues other than M/G/1 this is the first time that
{cn} can be computed through an explicit recursive formula. With this formula, we can calculate {cn} very quickly, which provides deeper insight into the correlation structure of the departure process compared to the previous
research. Numerical examples show that increasing server irregularity (i.e., the randomness of the service time distribution)
destroys the short-range dependence of interdeparture times, while increasing system load strengthens both the short-range
and the long-range dependence of interdeparture times. These findings show that the correlation structure of the departure
process is greatly affected by server regularity and system load. Our results can also be applied to the performance analysis
of a series of queues. We give an application to the performance analysis of a series of queues, and the results appear to
be accurate.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
182.
183.
Konrad Terpilowski 《Applied Surface Science》2010,256(17):5475-5481
Advancing and receding contact angles of water, formamide, glycerol and diiodometane were measured on the two polymers; EGDMA (dimethacrylate of ethylene glycol) and TRIM (trimethacrylate-1,1,1-trihydroksymethylopropane) which were polymerized next to glass, silanized glass, stainless steel, mica and silicon surfaces as the matrices. Then from the contact angle hystereses (CAH) and van Oss, Good, Chaudhury (LWAB) approaches the apparent surface free energies were evaluated. The measured contact angles not only depend solely on the polymer chemical structure but also, to some extent, on the solid matrix next to whose surface the sample has polymerized. Surface free energy of the polymer samples calculated from the LWAB approach shows that they interact mainly by dispersive forces. The apparent surface free energy of the polymers calculated from the diiodomethane contact angles hysteresis is practically the same irrespective of the kind of the matrix used. Therefore it can be concluded that the observed weak polar interactions in the surface free energy of the samples depend on the polymer surface preparation. The AFM images show that the obtained surfaces are of different roughness. The RMS values of roughness range between 3.7-90.2 nm for EDGMA, and 5.3-124.5 nm for TRIM. However, as reported in literature, rather protrusions bigger than 1 μm may significantly affect the contact angles, especially the receding ones. 相似文献
184.
Advancing and receding contact angles of water, formamide, glycerol and diiodomethane were measured on polypropylene (PP) and polycarbonate (PC) sample surfaces which solidified at Teflon, glass or stainless steel as matrix surfaces. Then from the contact angle hystereses (CAH) the apparent free energies of the surfaces were evaluated. The original PP surface is practically nonpolar, possessing small electron donor interaction (), as determined from the advancing contact angles of these liquids. It may result from impurities of the polymerization process. However, it increases up to 8-10 mJ/m2 for PP surfaces contacted with the solids. The PC surfaces both original and modified show practically the same . No electron acceptor interaction is found on the surfaces.The of modified PP and PC surfaces depend on the kind of probe liquid and contacted solid surface. The modified PP values determined from CAH of polar liquids are greater than that of original surface and they increase in the sequence: Teflon, glass, stainless steel surface, at which they solidified. No clear dependence is observed between and dielectric constant or dipole moment of the polar probe liquids. The changes in of the polymer surfaces are due to the polymer nature and changes in its surface structure caused by the structure and force field of the contacting solid. It has been confirmed by AFM images. 相似文献
185.
厚屏频率选择表面(FSS) 能克服双层或多层薄屏FSS级联引起的结构复杂和中心频率透过率容易降低的不利影响, 并且在拓展FSS带宽方面具有优势, 这使其在曲面隐身雷达天线罩应用方面有潜在的价值. 但是, 当电磁波以大范围角度入射时, 中心频率可能会随入射角度的变化而漂移, 而且在大角度入射时不易实现极化稳定性. 本文针对此问题, 设计了一种新Y形单元厚屏FSS, 利用模式匹配法对此结构进行分析计算, 并研究TE, TM波入射时角度变化及大角度入射时极化变化对通带带宽、 中心频率及其透过率的影响. 仿真实验结果表明: 新Y形单元厚屏FSS的中心频率在TE, TM波入射时均能实现角度稳定性和大角度入射时的极化稳定性. 这为FSS在曲面隐身雷达天线罩上的应用提供了一种借鉴. 相似文献
186.
In this paper, we consider a multi-user cognitive radio network (CRN) equipped with an intelligent reflecting surface (IRS). We examine the network performance by evaluating the fairness of the secondary system, which is satisfying the minimum required signal to interference and noise ratio (SINR) for each secondary user (SU). The minimum SINR of the SUs is maximized by joint optimization of the beamforming vector and three-dimensional beamforming (3DBF) angles at the secondary base station (SBS) and also the phase shifts of the IRS elements. This optimization problem is highly non-convex. To solve this problem, we utilize Dinkelbach’s algorithm along with an alternating optimization (AO) approach to achieve some sub-problems. Accordingly, by further applying a semi-definite relaxation method, we convert these sub-problems to equivalent convex forms and find a solution. Furthermore, analytically we propose an algorithm for optimizing 3DBF angles at the SBS. Through numerical results, the improvement of the sum SINR of the secondary system using the proposed method is illustrated. Moreover, it is shown that as the number of reflecting elements of IRS increases, the sum SINR significantly augments while satisfying fairness. Also, the convergence of the proposed algorithm is verified utilizing numerical results. 相似文献
187.
A novel four-legged loaded element thick-screen frequency selective surface with a stable performance 下载免费PDF全文
<正>A thick-screen frequency selective surface(FSS) has not only a broad bandwidth but also the advantages of overcoming the multilayer FSS shortcoming of complex structure and low transmittance of centre frequency due to the cascade of FSSs,and this means it could potentially be applied in a stealth curved streamlined radome.However,there is an unsteadiness of centre frequency in a wide range of incident angles and another unsteadiness of polarization in a big incident angle.In order to solve these problems,in this paper we provide a novel four-legged loaded element thick-screen FSS.The structure is analysed and simulated using the mode matching method and moment method.The centre frequency,the transmittance of centre frequency,and bandwidth of the structure are investigated when some parameters including the polarization at a big incident angle and the incident angles of TE & TM waves are changed. The novel four-legged loaded element thick-screen FSS has better transmission properties with a better steadiness of polarization and incident angle independence.The novel structure of the four-legged loaded element thick-screen FSS provides a valuable reference for their application in a stealth curved streamlined radome. 相似文献
188.
189.
190.
D. F. Abzalilov 《Journal of Applied Mechanics and Technical Physics》2008,49(6):978-984
A numerical-analytical solution of an inverse boundary-value problem of aerohydrodynamics is obtained for a two-element airfoil
in the full formulation, based on the velocity distribution defined on the sought airfoil contours in a range of angles of
attack. It is demonstrated that flow separation does not occur in the entire range considered for a specified non-separated
velocity distribution on the upper surfaces at the maximum angle of attack and on the lower surface at the minimum angle of
attack. An example of constructing a sectional airfoil is given; verification of the results obtained is performed with the
use of the Fluent software package.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 6, pp. 107–114, November–December, 2008. 相似文献