全文获取类型
收费全文 | 1713篇 |
免费 | 285篇 |
国内免费 | 39篇 |
专业分类
化学 | 214篇 |
晶体学 | 15篇 |
力学 | 17篇 |
综合类 | 1篇 |
数学 | 6篇 |
物理学 | 1784篇 |
出版年
2024年 | 5篇 |
2023年 | 14篇 |
2022年 | 25篇 |
2021年 | 30篇 |
2020年 | 29篇 |
2019年 | 22篇 |
2018年 | 29篇 |
2017年 | 62篇 |
2016年 | 66篇 |
2015年 | 46篇 |
2014年 | 94篇 |
2013年 | 70篇 |
2012年 | 63篇 |
2011年 | 117篇 |
2010年 | 89篇 |
2009年 | 89篇 |
2008年 | 122篇 |
2007年 | 124篇 |
2006年 | 101篇 |
2005年 | 81篇 |
2004年 | 77篇 |
2003年 | 60篇 |
2002年 | 102篇 |
2001年 | 55篇 |
2000年 | 51篇 |
1999年 | 35篇 |
1998年 | 34篇 |
1997年 | 40篇 |
1996年 | 34篇 |
1995年 | 35篇 |
1994年 | 15篇 |
1993年 | 14篇 |
1992年 | 11篇 |
1991年 | 16篇 |
1990年 | 13篇 |
1989年 | 8篇 |
1988年 | 13篇 |
1987年 | 20篇 |
1986年 | 18篇 |
1985年 | 20篇 |
1984年 | 19篇 |
1983年 | 16篇 |
1982年 | 22篇 |
1981年 | 12篇 |
1980年 | 9篇 |
1979年 | 1篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1973年 | 4篇 |
1970年 | 1篇 |
排序方式: 共有2037条查询结果,搜索用时 46 毫秒
81.
R. Giovanelli 《Il Nuovo Cimento D》1995,17(1):61-83
Summary A direct numerical analysis, based on the Lienard-Wiechert potentials, is performed in the present paper, aiming to describe
the relativistic interaction of the electrons composing a high-intensity beam (in Raman regime) both with each other and with
the fields of an FEL structure and of an external resonant travelling electromagnetic wave. The different accelerations, due
to the various forces acting on the charged particles, are seen to give different contributions to the total radiation field,
which are separately considered here. The angular and frequency distributions of the obtained radiation are compared with
the analytic ones deduced in the particular case of a single charge launched along the FEL structure. The interference effect
between the fields of many bunches is seen to cause the shrinkage of the resulting radiation beam. 相似文献
82.
The use of non-contact laser techniques for the generation of ultrasound has extended the limits of the application of traditional ultrasonic techniques. This paper focusses on the use of one such non-contact laser technique, known as ‘optical fibre array’, to generate shear and surface waves. The shear wave experimental directivity pattern results are presented and compared with the theoretical results of a single source and an array source. The experimental directivity results for the surface wave are also presented, and compared with the theoretical results. The data show that the array enhances signal generation in the forward direction for both shear and surface waves. The array gain is also discussed. The receiver for the directivity measurements was a contact piezoelectric transducer. 相似文献
83.
The present study examines the combined effects of chemical reactions taking place between a gas jet and molten metal, the cooling effect of the jet and the evaporation of metal, during a CO2 laser cutting process. A laminar boundary layer approach was used to develop a theoretical model for the oxygen gas jet laser cutting mechanism. An experiment was carried out to monitor the keyhole formation using a video recorder and detect the light emitted from the entrance and exist surfaces of the workpiece using a fibre-optic probe during the cutting process. The experimental study was extended to employ two different workpiece materials (stainless steel and mild steel) at two thicknesses, and varying oxygen assisting gas pressures. It is found that the theoretical model developed in the present study is valid for a cutting speed of about 30 mm s−1 and all jet velocities up to sonic, since the effect of shock is excluded in the model. 相似文献
84.
Operating characteristics of a Q-switched Nd3+: doped single-mode silica fibre with a mechanical chopper as a Q-switching device are presented. Peak powers as high as 16 W with pulse duration of 146 ns are demonstrated. Self-mode-locking in Nd3+:doped fibre is also reported. An output power of 312 W with a centre pulse duration of 4 ns is observed. 相似文献
85.
M. Dell'erba 《Optics & Laser Technology》1985,17(5):260-261
For the first time it has been possible to laser weld copper sheets up to 3 mm thick with a 2 kW cw CO2 laser. The main questions arising from the process are indicated. 相似文献
86.
The conditions for realization of the regime of self-oscillations of the intensity of a continuously pumped solid-state laser have been qualitatively analyzed using a system of kinetic equations. It has been established that, in the case where a thin semiconductor layer absorbing light at frequencies close to the lasing frequency is placed into the cavity of such a laser, self-oscillations of its radiation intensity arise as a result of the amplitude-phase self-modulation of the laser light field.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 1, pp. 136–139, January–February, 2005. 相似文献
87.
Scott C. Herndon David D. Nelson Jr Mark S. Zahniser 《Journal of Quantitative Spectroscopy & Radiative Transfer》2005,90(2):207-216
The spectral line strengths in the v2 band of H2CO (segments spanning 1720-) have been determined relative to two sets of spectral line groups in the v1 and v5 band, using tunable diode laser spectroscopy. Simultaneous detection using a dual-diode instrument with a absorption cell was employed to assure identical H2CO column density for the two spectral regions. The results in the selected regions of this study are in good agreement with the line positions and the relative intensities specified in an unpublished complete line listing for the v2 band prepared by Linda Brown (see full text for reference). Based upon measurements of individual groups of spectral lines in the P, Q and R branches, the absolute band strength has been determined to be . 相似文献
88.
Todd B. Sauke Joseph F. Becker 《Journal of Quantitative Spectroscopy & Radiative Transfer》2005,91(4):453-484
The instrument response function (IRF) of a spectrometer limits the accuracy of measured spectroscopic parameters by broadening recorded spectral lines/features. We describe methods to model the effects of the IRF on spectral data, to minimize the IRF widths, and to measure the resulting width of the spectrometer IRF. We have modeled the IRF of our Tunable Diode Laser Spectrometer as a Voigt function. A real-time method of eliminating the effects of low-frequency spectrometer drift has been implemented and has resulted in a substantial reduction in the width of the IRF, its residual Gaussian component reduced from about to about . An accurate measurement of the IRF Gaussian width utilizes a computationally simple method making use of the spectral dependence of the RMS noise of each signal-averaged data point. Various noise sources affecting the spectrometer (preamp/detector noise, laser AM noise, and laser FM noise) are identified and separately quantified by use of the same method. The IRF Gaussian-width measurement can be automatically applied to each measured spectrum of an experimental data set. A related method is discussed which allows accurate determination of the spectral dependence of statistical noise appropriate for use in quantitative Chi-square fitting of absorption spectra. We explore simple, efficient numerical processes which can dramatically enhance the quality and usefulness of acquired spectral data, improving the ability to apply TDL spectroscopy to high-precision, quantitative measurements and the determination of detailed spectroscopic lineshape parameters. This paper provides a guide for interested readers to implement these developments in their own spectrometers. 相似文献
89.
Harward CN Baren RE Parrish ME 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2004,60(14):6861-3429
A technique has been developed for the determination of molecular parameters, including infrared absorption line positions, strengths, and nitrogen-broadened half-widths for 1,3-butadiene (C(4)H(6)) and propylene (C(3)H(6)). The parameters for these two molecules are required for quantitation using Tunable Diode Laser Absorption Spectroscopy (TDLAS). These molecules have populations of highly overlapping infrared absorption lines in their room temperature spectra. The technique reported here provides a procedure for estimating the molecular parameters for these overlapping absorption lines from quantitative reference spectra taken with the TDLAS instrument at different pressures and concentrations. The system was developed for the quantitation of gaseous constituents in a single puff of cigarette smoke and this paper will describe the procedure and some of the factors that influence the accuracy of quantitation for 1,3-butadiene, including the approach taken to minimize the adverse effects of the absorption due to propylene in the same spectral region. 相似文献
90.
Baren RE Parrish ME Shafer KH Harward CN Shi Q Nelson DD McManus JB Zahniser MS 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2004,60(14):3437-3447
A compact, fast response, infrared spectrometer using four pulsed quantum cascade (QC) lasers has been applied to the analysis of gases in mainstream (MS) and sidestream (SS) cigarette smoke. QC lasers have many advantages over the traditional lead-salt tunable diode lasers, including near room temperature operation with thermoelectric cooling and single mode operation with improved long-term stability. The new instrument uses two 36 m, 0.3 l multiple pass absorption gas cells to obtain a time response of 0.1s for the MS smoke system and 0.4s for the SS smoke system. The concentrations of ammonia, ethylene, nitric oxide, and carbon dioxide for three different reference cigarettes were measured simultaneously in MS and SS smoke. A data rate of 20Hz provides sufficient resolution to determine the concentration profiles during each 2s puff in the MS smoke. Concentration profiles before, during and after the puffs also have been observed for these smoke constituents in SS smoke. Also, simultaneous measurements of CO(2) from a non-dispersive infrared (NDIR) analyzer are obtained for both MS and SS smoke. In addition, during this work, nitrous oxide was detected in both the MS and SS smoke for all reference cigarettes studied. 相似文献