全文获取类型
收费全文 | 2225篇 |
免费 | 606篇 |
国内免费 | 143篇 |
专业分类
化学 | 547篇 |
晶体学 | 20篇 |
力学 | 252篇 |
综合类 | 13篇 |
数学 | 76篇 |
物理学 | 2066篇 |
出版年
2024年 | 19篇 |
2023年 | 57篇 |
2022年 | 76篇 |
2021年 | 108篇 |
2020年 | 83篇 |
2019年 | 75篇 |
2018年 | 86篇 |
2017年 | 107篇 |
2016年 | 126篇 |
2015年 | 73篇 |
2014年 | 194篇 |
2013年 | 155篇 |
2012年 | 121篇 |
2011年 | 176篇 |
2010年 | 152篇 |
2009年 | 132篇 |
2008年 | 138篇 |
2007年 | 132篇 |
2006年 | 118篇 |
2005年 | 112篇 |
2004年 | 81篇 |
2003年 | 97篇 |
2002年 | 68篇 |
2001年 | 77篇 |
2000年 | 67篇 |
1999年 | 25篇 |
1998年 | 48篇 |
1997年 | 38篇 |
1996年 | 35篇 |
1995年 | 29篇 |
1994年 | 26篇 |
1993年 | 16篇 |
1992年 | 16篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 9篇 |
1988年 | 16篇 |
1987年 | 8篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 7篇 |
1983年 | 2篇 |
1982年 | 10篇 |
1981年 | 2篇 |
1978年 | 5篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1974年 | 3篇 |
1973年 | 5篇 |
1969年 | 1篇 |
排序方式: 共有2974条查询结果,搜索用时 0 毫秒
1.
Quantitative H–Al distances in acid sites of two zeolites with MFI and IFR framework topology were obtained by numerical simulation of 1H{27Al} rotational echo adiabatic passage double resonance (REAPDOR) experiments. A 27Al offset-dependent data set yields for each resolved 1H NMR line a corresponding nuclear electric quadrupole coupling constant of the neighboring 27Al site. This information is used for analyzing a second data set for on-resonance irradiation, where the dipolar evolution time (number of rotor cycles) was varied, to yield the 1H–27Al dipolar coupling constant. Numerical simulations indicate that the REAPDOR method does not depend significantly on the polar angles, defining the orientation of the electric field gradient tensor of 27Al with respect to the Al–H dipolar vector. In contrast, the transfer of populations in double resonance sequence is sensitive to these angles, and it can be thus used to measure them. 相似文献
2.
Two types of optical current transducers (OCTs) have a bulk Faraday sensor inserted into the gap of an iron core and a porcelain
insulator with optical fiber. The sensor consists of Bi12SiO20 (BSO) single crystal, a polarizer, and an analyzer. The OCTs satisfied the target performance requirement for fault location
and metering and demonstrated maintained performance at some power utilities in Japan and the US. We have developed a fault
location system that immediately detects the fault current with the OCTs, there by locating the fault section. The OCT can
easily replace the existing support insulators for the disconnecting switch without any modifications to structure height
or bus-bar. For metering requiring 0.3% class accuracy, use of a BSO with right optical rotatory power combined with BSO with
left optical rotatory power results in a Faraday sensor with improved temperature characteristics. The OCT demonstrated 0.3%
class accuracy for metering described in the current transformer Specifications of IEEE C57–13, 1993. 相似文献
3.
Arabinda Mallick 《Journal of luminescence》2006,118(2):165-172
Solvatochromic effects on the fluorescence behavior of 7-hydroxy-4-methyl-8-(4′-methyl-piperazine-1′ yl)methylcoumarin (HMMC) was studied in different solvents. The fluorescence of HMMC was found to be highly sensitive to both the polarity and the protic character of the solvent. Exploiting the polarity-sensitive fluorescence property of HMMC, its excited-state dipole moment has been determined. Fluorescence (Förster) resonance energy transfer (FRET) process from HMMC to a potent bioactive molecule 3-acetyl-4-oxo-6,7-dihydro-12 H indolo-[2,3-a] quinolizine (AODIQ) was studied. From the determined KSV and R0 values, it is argued that a long-range dipole-dipole interaction is operating for the energy transfer mechanism. The energy transfer efficiency (E) and the distance between the acceptor and the donor (r0) have been determined. 相似文献
4.
The performances of two Laser Doppler Velocimetry (LDV) systems adapted for measuring the acoustic particle velocities are assessed in enclosed sound field. This assessment is performed by comparing the acoustic velocities measured by means of LDV to reference acoustic velocities estimated from sound pressure measurements. The two LDV systems are based on a single optical bench which delivers an optical signal called Doppler signal. The Doppler signal, which is frequency modulated, is analyzed by means of two signal processing systems, the BSA (Burst Spectrum Analyser from Dantec) on the one hand, and a system specifically developed for the estimation of the acoustic velocity on the other hand. Once the experimental setup has been optimized for minimizing the errors made on the reference velocities, the assessment is performed and shows that both systems can measure the acoustic velocity in enclosed field in two the frequency ranges [0-4 kHz] and [0-2 kHz] respectively for acoustic velocity amplitudes of 10 mm/s and 1 mm/s. 相似文献
5.
6.
7.
The determination of the sensitivity of the acoustical characteristics of vibrating systems with respect to the variation of the design parameters predicting these characteristics is a necessary and important step of the acoustic design and optimization process. Acoustic design sensitivity analysis includes the computation and evaluation of the sensitivity information required for this procedure. In this study, a boundary element code performing the sensitivity analysis of the acoustic pressure by using the matrix sensitivities with respect to different design variables has been developed. The effect of the precision of boundary element discretization on the acoustic pressure sensitivity is examined via this code. The formulation is applied to a multi-source system and the dimension sensitivity analysis of near field pressures of two-dilating-spherical source is performed. The last application is devoted to a real sound source: a washing machine sitting on the floor. Sensitivity of the field pressures to the machine’s dimensions (size), surface velocity and frequency is examined on the bases of the boundary element model of the machine and half-space condition. The impacts of these variables are compared; and a limiting speed for the machine responding both the acoustical and operational requirements is determined. 相似文献
8.
A. K. Ghosh A. D. Williams J. M. Zucker J. L. Mathews N. Spinhirne 《Experimental Mechanics》2008,48(2):139-152
In nature, shape and structure evolve from the struggle for better performance. Often, biological structures combine multiple
beneficial properties, making research into mimicking them very complex. Presented here is a summary of observations from
a series of experiments performed on a material that closely resembles the human skull bone’s cancellous structure under acoustic
loads. Transmission loss through flat and curved open-cell polyurethane foam samples is observed using air and water as the
two interstitial fluids. Reduction in strength and stiffness caused by porosity can be recovered partially by filling the
interstitial pores with a fluid. The test findings demonstrate the influence of the interstitial fluid on the mechanical characteristics
of a porous structure in a quantitative manner. It is also demonstrated that the transmission loss does not depend only on
the mass per unit area of the structure as predicted by acoustic mass law. Current tests also demonstrate that the transmission
loss is more sensitive to the interstitial fluid than the shape and support conditions of the structures. Test observations
thus support the concepts of “moisture-sensitivity of biological design” and the “law of hierarchy in natural design”. 相似文献
9.
It is of engineering importance to locate low altitude moving targets with acoustic methods due to the blindness of the traditional radar detecting. In this paper, an algorithm for locating low altitude moving targets is put forward based on a five-element planar acoustic sensor array. The method is realized through estimations of the sensor-to-sensor time delays of the source signal generated by the low altitude moving target. The angle and range estimation performance of the proposed method are analyzed, respectively, both in theoretical and numerical sense. 相似文献
10.
S. A. Chaplygin 《Regular and Chaotic Dynamics》2007,12(2):219-232
This text presents an English translation of the significant paper [6] on vortex dynamics published by the outstanding Russian
scientist S. A. Chaplygin, which seems to have almost escaped the attention of later investigators in this field. Although
it was published more than a century ago, in our opinion it is still interesting and valuable.
__________
Originally published in: Trudy otdeleniya fizicheskikh nauk imperatorskogo Moskovskogo obshchestva lyubitelei estestvoznaniya, antropologii i etnografii
(Transactions of the Physical Section of Moscow Society of Friends of Natural Sciences, Anthropology and Ethnography), 1903, Vol. 11, No. 2, pp. 11–14. Translated from Russian by S. Ramodanov; edited by D. Blackmore; commented by V. V. Meleshko
(Department of Theoretical and Applied Mechanics, Faculty of Machanics and Mathematics, Kiev National Taras Shevchenko University,
01030 Kiev, Ukraine. E-mail: meleshko@univ.kiev.ua) and G.J.F. van Heijst (Fluid Dynamics Laboratory, Faculty of Applied Physics,
Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. E-mail: g.j.f.v.heijst@tue.nl). 相似文献