全文获取类型
收费全文 | 517566篇 |
免费 | 7148篇 |
国内免费 | 2328篇 |
专业分类
化学 | 286943篇 |
晶体学 | 7070篇 |
力学 | 21367篇 |
综合类 | 68篇 |
数学 | 63541篇 |
物理学 | 148053篇 |
出版年
2020年 | 3918篇 |
2019年 | 4068篇 |
2018年 | 4875篇 |
2017年 | 4730篇 |
2016年 | 8037篇 |
2015年 | 6234篇 |
2014年 | 8275篇 |
2013年 | 23037篇 |
2012年 | 18909篇 |
2011年 | 22829篇 |
2010年 | 14966篇 |
2009年 | 14475篇 |
2008年 | 21020篇 |
2007年 | 21274篇 |
2006年 | 20295篇 |
2005年 | 18556篇 |
2004年 | 16617篇 |
2003年 | 14539篇 |
2002年 | 14232篇 |
2001年 | 15077篇 |
2000年 | 11630篇 |
1999年 | 9056篇 |
1998年 | 7348篇 |
1997年 | 7159篇 |
1996年 | 7361篇 |
1995年 | 6500篇 |
1994年 | 6397篇 |
1993年 | 6145篇 |
1992年 | 6793篇 |
1991年 | 6657篇 |
1990年 | 6248篇 |
1989年 | 6040篇 |
1988年 | 6167篇 |
1987年 | 5902篇 |
1986年 | 5747篇 |
1985年 | 8062篇 |
1984年 | 8212篇 |
1983年 | 6735篇 |
1982年 | 7346篇 |
1981年 | 7140篇 |
1980年 | 6750篇 |
1979年 | 7017篇 |
1978年 | 7187篇 |
1977年 | 7193篇 |
1976年 | 7237篇 |
1975年 | 6864篇 |
1974年 | 6843篇 |
1973年 | 7005篇 |
1972年 | 4618篇 |
1971年 | 3549篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Backman Ulrika Jokiniemi Jorma K. Auvinen Ari Lehtinen Kari E.J. 《Journal of nanoparticle research》2002,4(4):325-335
We have prepared spherical non-agglomerated silver nanoparticles by an evaporation–condensation–dilution/cooling technique. Silver was evaporated from a crucible in a tubular flow reactor. A porous tube diluter was used to quench the carrier gas at the outlet of the reactor to enhance the formation of small particles and to suppress agglomeration and other particle growth mechanisms. The number size distribution of the prepared particles was measured with a differential mobility analyser–condensation nucleus counter combination and the size and the shape of the particles were analysed with transmission electron microscope. The system was modelled using a sectional aerosol dynamics computer code to estimate the importance of different aerosol processes. In all conditions the particles obtained were non-agglomerated and spherical. The mean particle diameter varied from 4 to 10-nm depending on boundary conditions. From the modelling studies it can be concluded that the nucleation rate is the most important parameter controlling the final particle size. 相似文献
992.
L. V. Soboleva A. E. Voloshin V. A. Kirikov M. V. Biglova 《Crystallography Reports》2002,47(1):130-134
Based on the analysis of the K2O-P2O5-D2O solubility phase diagram, the optimum conditions of KD2PO4 crystallization—the compositions of mother solutions and the temperature range of crystallization—in the KH2PO4-D2O system have been determined. The technique of K(DxH1 ? x )2PO4 growth is developed. The DKDP single crystals with deuterium concentration up to 88 wt % are grown on DKDP seeds from KH2PO4 solutions in D2O by the method of temperature decrease. 相似文献
993.
Sonya V. Roberson Albert J. Fahey Amit Sehgal Alamgir Karim 《Applied Surface Science》2002,200(1-4):150-164
We present a simple method for chemical modification of chlorosilane self-assembled monolayers (SAMs) on Si surfaces by exposure to a gradient of UV-ozone radiation to create stable substrates with a range of contact angles (θH2O≈5–95°) and surface energies on a single substrate. These gradient energy substrates are developed to potentially generate libraries for combinatorial studies of thin film phenomenology, where a systematic variation of interfacial surface energy represents one of the significant parameters along one axis. The graded oxidation process presents a systematic variation of surface chemical composition. We have utilized contact angle measurements and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to investigate this variation for a series of ions, among which are SiCH3+, SiOH+ and COOH−. We show that the macroscopic measurements of surface free energy/contact angle correlate with the detailed analysis of surface chemistry (as assessed by ToF-SIMS) on these test substrates. 相似文献
994.
The oxidation behaviour of LnSiAlON (Ln=Y, La) glasses was studied at different temperatures (990-1150 °C) and under different water vapour pressures (360-2690 Pa). These results were also compared with those obtained under O2, N2/H2O or O2/H2O mixtures. When glasses are treated under a N2/H2O mixture, optical and SEM observations show porous scales. Transformations of the reaction rate data and a kinetic model show that there is only one limiting process occurring during oxidation. This rate limiting step is the progress of the chemical reaction at the internal interface. Determination of the pressure law dependence and thermodynamics calculations of water vapour molecules dissociation at the investigated temperatures allow us to suggest that the mechanism of oxidation corresponds to decomposition of water molecules on the oxynitride glass surface. 相似文献
995.
The pressure dependence of the experimental 7Li NMR spectra is reported for first stage lithium graphite (LiC6) intercalation compound at temperatures T = 232 and 293 K. This experiment together with the presented point charge model calculation of the 7Li quadrupole coupling constant allows an unambiguous determination of the sign of e2qQ/h which is negative: at p = 1 bar and T = 232 K. The averaged location of the electrons transferred from the Li intercalant to the graphite layers, as estimated in this study, is in excellent agreement with earlier theoretical energy-band calculations. The compressibility of LiC6 in the c-direction is predicted to be kc = 1.7 × 10-12cm2dyn-1, it agrees with estimates derived from the available phonon dispersion relations. 相似文献
996.
To date, ten experimental measurements of the asymmetry parameter have been made for the positron decay of52Mn. The Fermi to Gamow-Teller mixing ratioy can be deduced from such measurements and this quantity is important because the time-reversal violating amplitude is proportional toy/(1+y2). Our theoretical calculation using the Collective Model yieldsy=? 3.5×10?4 for the deformation parameterβ=0.1 andy=? 4.6×10?4 forβ=0.2. Such small values ofy are consistent with time-reversal invarisance. 相似文献
997.
S. M. J. Kelly 《Optics Communications》1989,70(6):495-501
The effect of incorporating an empty external cavity into the feedback loop of a mode-locked laser is investigated. Particular attention is paid to the response of the mode-locking as the external cavity is detuned by a fraction of a wavelength from the round trip transit time associated with the laser. The results for two regimes of operation are presented. In the first case a series of short lived transients develop whereas the other case leads to a persistent intermittent Q-switching scenario, where mode-locking fails. The role of coupling between the cavities and the laser's memory time in relation to the results is discussed. A mechanism for the improved mode-locking observed with non-linear external cavities is suggested. 相似文献
998.
L. Galeone C. Mastroserio M. Montrone 《Numerical Methods for Partial Differential Equations》1989,5(2):79-86
This paper presents the study of the numerical solution of a reaction-diffusion system involving a reaction term of integral type arising from biological models. By means of a monotone approach we introduce upper and lower solutions and then we show the existence and the asymptotic behavior of nonnegative numerical solutions. To this end, we require the positivity of the numerical scheme and so we can use some properties of positive and M-matrices. Finally we give some sufficient conditions to verify the asymptotic stability of the numerical solution. 相似文献
999.
This paper presents the development and laboratory evaluation of a PM10/2.5/1.0 trichotomous sampling inlet that consists of two main parts: a previously designed PM10 size‐selective inlet part and a PM2.5/1.0 two‐stage virtual impactor, which was newly fabricated and attached serially to the PM10 size selective inlet part. Particles are collected in three locations through the trichotomous sampling inlet to provide for not only particle concentration measurements of PM10, PM2.5 and PM1.0, but also those of PM2.5–10 and PM1.0–2.5. 相似文献
1000.
The efficiency of suppression of the generation of a photon echo response depending on the mutual spatial orientation of gradients of external nonumform electric fields acting on a resonant medium is studied. The possibility of creating an associative memory where the mutual orientation of the gradients of external electric fields is an associative key is discussed. 相似文献