全文获取类型
收费全文 | 178496篇 |
免费 | 2339篇 |
国内免费 | 601篇 |
专业分类
化学 | 99549篇 |
晶体学 | 1916篇 |
力学 | 7292篇 |
综合类 | 8篇 |
数学 | 20554篇 |
物理学 | 52117篇 |
出版年
2020年 | 1425篇 |
2019年 | 1474篇 |
2018年 | 1744篇 |
2017年 | 1732篇 |
2016年 | 3040篇 |
2015年 | 2146篇 |
2014年 | 3058篇 |
2013年 | 7659篇 |
2012年 | 6265篇 |
2011年 | 7857篇 |
2010年 | 5186篇 |
2009年 | 5075篇 |
2008年 | 7086篇 |
2007年 | 7150篇 |
2006年 | 6855篇 |
2005年 | 6287篇 |
2004年 | 5656篇 |
2003年 | 4898篇 |
2002年 | 4864篇 |
2001年 | 4766篇 |
2000年 | 3704篇 |
1999年 | 2953篇 |
1998年 | 2608篇 |
1997年 | 2595篇 |
1996年 | 2362篇 |
1995年 | 2245篇 |
1994年 | 2262篇 |
1993年 | 2061篇 |
1992年 | 2305篇 |
1991年 | 2423篇 |
1990年 | 2190篇 |
1989年 | 2188篇 |
1988年 | 2060篇 |
1987年 | 1912篇 |
1986年 | 1887篇 |
1985年 | 2487篇 |
1984年 | 2651篇 |
1983年 | 2158篇 |
1982年 | 2374篇 |
1981年 | 2228篇 |
1980年 | 2175篇 |
1979年 | 2266篇 |
1978年 | 2460篇 |
1977年 | 2317篇 |
1976年 | 2303篇 |
1975年 | 2187篇 |
1974年 | 2234篇 |
1973年 | 2202篇 |
1972年 | 1447篇 |
1971年 | 1289篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
991.
The determination of doxazosin by high-performance liquid chromatography with fluorescence detection is described. Propanolol was used as the internal standard. Plasma samples were treated with methanol to precipitate the proteins. Doxazosin was isolated with C18 reversed-phase extraction columns. The determination limit is 1 ng/ml of plasma, while the extraction columns can be reused frequently. The method is applied to clinical trial samples. 相似文献
992.
Karl F. Canter 《Hyperfine Interactions》1992,73(1-2):159-171
The present status of low energy positron diffraction (LEPD) and positron reemission microscopy (PRM) is reviewed in the context
of unanswered questions regarding the elastic scattering of positrons in the 1–300 eV energy range from atoms in solids and
at solid surfaces. Recent LEPD studies yield an agreement between theoretical and experimental diffraction intensities for
semiconductors that has never been equalled in electron diffraction studies. This situation is discussed in terms of the repulsive
interaction between the positron and the embedded atomic potential and the lack of exchange with the nonspherically distributed
valence electrons. The scattering of re-emitted positrons in PRM from atoms chemisorbed or physisorbed on the re-emitting
surface has not yet received the same theoretical attention as scattering from embedded atoms in LEPD. Possible ways in which
positron scattering from overlying atomic structures manifests itself in PRM as well as positron re-emission holography are
discussed, both from the practical viewpoint of observing these structures and in the context of fundamental questions regarding
the positron re-emission process itself. 相似文献
993.
994.
995.
Electron energy distribution functions (eedf) in CO2 laser discharge (He—CO2—N2—CO mixture) have been calculated by solving the Boltzmann equation in the presence of given concentrations of excited (vibrational and electronic) states. The results show a well structured eedf as a result of second kind collisions coming from metastable electronic states of N2 and He as well as a strong dependence of rate coefficients for CO2 dissociation and for the ionization of the different species. 相似文献
996.
M. F. Gorodnii 《Mathematical Notes》1992,51(4):338-341
Translated from Matematicheskie Zametki, Vol. 51, No. 4, pp. 17–22, April, 1992. 相似文献
997.
F. F. Éfendiev 《Mathematical Notes》1992,51(2):204-207
Translated from Matematicheskie Zametki, Vol. 51, No. 2, pp. 139–143, February, 1992. 相似文献
998.
J. Peter H. Pabon 《Journal of voice》1991,5(3)
A phonetogram is a plot of the dynamic range of the voice as a function of fundamental frequency. Traditionally, the phonetogram only records the sound-pressure level (SPL) of the threshold of phonation and the upper limit in SPL the voice can reach with sustained phonation as a function of the fundamental frequency (F0). In this study, the phonetogram is extended by including acoustic voice-quality parameters. Three additional parameters are tested: jitter, shimmer, and crest factor. For each individual voice, the variation in the three parameters is evaluated over the entire phonetogram area. By averaging individual phonetograms, standard or norm phonetograms are obtained revealing general patterns in voice-quality variation. These patterns reflect the complex relations between F0, SPL, and the acoustic voice-quality parameters just mentioned. Jitter and shimmer distributions over the phonetogram show that large variations in perturbation values can be expected when production conditions are altered. Highest perturbation values are found for the low F0 and low SPL phonations. For all voices, a gradual decrease of the crest factor is found with increasing F0, reflecting the degrading of spectral complexity with F0. The crest-factor parameter can also be used to mark those areas in the phonetogram where the fundamental dominates the waveform and where flow control is the main SPL regulating mechanism in voice production. The strong quality variations within the phonetogram stress the need for accurate control of F0 and SPL in objective voice-quality measurement. 相似文献
999.
Principles and recent developments in ultrasound contrast agents 总被引:6,自引:1,他引:5
The behaviour of gas bubbles and gas encapsulated spheres as echographic contrast agents is reviewed. Compared with rigid spheres, gas bubbles are superior scattering agents and they offer a number of useful properties which can be exploited in a variety of ways. The analysis of their velocity of sound, back-scatter intensity, second harmonic emission and resonant frequency opens up new perspectives in the development of contrast agents for echocardiographic research with potential clinical applications. 相似文献
1000.
Summary Oblique propagating magnetohydrodynamic waves with various wave forms and amplitudes are observed both at the Earth's foreshock
and at comets. The possibility of interpreting some observational results in terms of nonlinear evolution of one- and two-dimensional
hydromagnetic waves is investigated. For this purpose both analytical and numerical techniques are employed. It is found that
an initial monochromatic wave changes its polarization giving origin to magnetosonic shocks and rotational discontinuities;
the time evolution of density-magnetic-field correlation is studied, as a function of the plasma parameters and of the propagation
angle. In the two-dimensional case both a transverse instability and a self-focusing effect may take place. Moreover, a two-dimensional
magnetosonic solution is found, in which the density fluctuations are driven by the total pressure fluctuation as in a one-dimensional
simple wave. These theoretical predictions compare well with the features observed in the solar-wind waves.
Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990. 相似文献