全文获取类型
收费全文 | 5474篇 |
免费 | 144篇 |
国内免费 | 8篇 |
专业分类
化学 | 3243篇 |
晶体学 | 152篇 |
力学 | 186篇 |
数学 | 453篇 |
物理学 | 1592篇 |
出版年
2023年 | 42篇 |
2022年 | 133篇 |
2021年 | 119篇 |
2020年 | 132篇 |
2019年 | 138篇 |
2018年 | 132篇 |
2017年 | 172篇 |
2016年 | 208篇 |
2015年 | 152篇 |
2014年 | 209篇 |
2013年 | 433篇 |
2012年 | 331篇 |
2011年 | 346篇 |
2010年 | 232篇 |
2009年 | 206篇 |
2008年 | 285篇 |
2007年 | 233篇 |
2006年 | 169篇 |
2005年 | 165篇 |
2004年 | 137篇 |
2003年 | 106篇 |
2002年 | 109篇 |
2001年 | 61篇 |
2000年 | 55篇 |
1999年 | 38篇 |
1998年 | 35篇 |
1997年 | 41篇 |
1996年 | 46篇 |
1995年 | 32篇 |
1994年 | 35篇 |
1993年 | 45篇 |
1992年 | 51篇 |
1991年 | 39篇 |
1990年 | 41篇 |
1989年 | 50篇 |
1988年 | 48篇 |
1987年 | 42篇 |
1986年 | 32篇 |
1985年 | 62篇 |
1984年 | 57篇 |
1983年 | 40篇 |
1982年 | 45篇 |
1981年 | 51篇 |
1980年 | 45篇 |
1979年 | 46篇 |
1978年 | 43篇 |
1977年 | 51篇 |
1976年 | 54篇 |
1975年 | 44篇 |
1974年 | 38篇 |
排序方式: 共有5626条查询结果,搜索用时 171 毫秒
991.
992.
Multiple pure tone noise prediction 总被引:1,自引:0,他引:1
This paper presents a fully numerical method for predicting multiple pure tones, also known as “Buzzsaw” noise. It consists of three steps that account for noise source generation, nonlinear acoustic propagation with hard as well as lined walls inside the nacelle, and linear acoustic propagation outside the engine. Noise generation is modeled by steady, part-annulus computational fluid dynamics (CFD) simulations. A linear superposition algorithm is used to construct full-annulus shock/pressure pattern just upstream of the fan from part-annulus CFD results. Nonlinear wave propagation is carried out inside the duct using a pseudo-two-dimensional solution of Burgers? equation. Scattering from nacelle lip as well as radiation to farfield is performed using the commercial solver ACTRAN/TM. The proposed prediction process is verified by comparing against full-annulus CFD simulations as well as against static engine test data for a typical high bypass ratio aircraft engine with hardwall as well as lined inlets. Comparisons are drawn against nacelle unsteady pressure transducer measurements at two axial locations as well as against near- and far-field microphone array measurements outside the duct. 相似文献
993.
In this paper we have presented long period fiber grating (LPFG) as temperature sensor. Temperature based sensors have found a number of applications in commercial and industrial fields. In LPFG based temperature sensors, they respond to shift in various peak resonant wavelengths corresponding to various attenuation bands of the transmission spectrum. Temperature effect on the various attenuation bands of a LPFG have been investigated to create a highly sensitive measurement device. The temperature sensitivities of various attenuation bands of a LPFG over the wavelength region of 1.1–1.7 μm, for a grating period of 280 μm period, are obtained by monitoring the wavelength shift of each peak resonant wavelength with temperature increment of 20 °C, ranging from 0 °C to 100 °C. 相似文献
994.
In this paper, the SRS-induced crosstalk has been evaluated in a SCM–WDM communication links at different modulation frequencies and transmission lengths for variety of fiber. Results show that SRS-induced crosstalk dominates at low frequency. As the dispersion and effective area of fiber (Aeff) decreases, initially the crosstalk remains high and then it decreases with increase in modulation frequency. The present work shows that out of five different types of fiber, standard single mode fiber (SMF) has minimum crosstalk (−78 to −38) dB, (−55 to −33) dB and (−46 to −34) dB at modulation frequencies, transmission lengths and optical powers. Dispersion compensation fiber (DCF) has maximum crosstalk (−60 to −12) dB, (−37 to −12) dB and (−27 to −12) dB at modulation frequencies and transmission lengths. 相似文献
995.
We report an analysis on the hardness behavior of glassy Se80?xTe20Snx alloy. The crucial thermo-mechanical parameters (micro-hardness, volume and formation energy of micro-voids and the modulus of elasticity) are examined. The results indicate that the thermo-mechanical parameters are changed significantly after incorporation of Sn in glassy Se80Te20 alloy. 相似文献
996.
Debdutta Lahiri Surinder M. Sharma Ashok K. Verma B. Vishwanadh G. K. Dey Gerhard Schumacher Tobias Scherb Heinrich Riesemeier Uwe Reinholz Martin Radtke S. Banerjee 《Journal of synchrotron radiation》2014,21(6):1296-1304
Short‐range order has been investigated in Zr69.5Cu12Ni11Al7.5 and Zr41.5Ti41.5Ni17 metallic glasses using X‐ray absorption spectroscopy and ab initio molecular dynamics simulations. While both of these alloys are good glass formers, there is a difference in their glass‐forming abilities (Zr41.5Ti41.5Ni17 > Zr69.5Cu12Ni11Al7.5). This difference is explained by inciting the relative importance of strong chemical order, icosahedral content, cluster symmetry and configuration diversity. 相似文献
997.
Mitya Reznikov Anshul Sharma Torsten Hegmann 《Particle & Particle Systems Characterization》2014,31(2):257-265
Ink‐jet printing of monolayer‐capped gold nanoparticles is introduced as a versatile and highly efficient means to pattern the alignment of nematic liquid crystals. Any homeotropic alignment patterns can be created quickly ranging in size from 30 μm (850 dpi) to several square inches, with high accuracy that does not deteriorate with time. Depending on the alignment underlayer, intermediate configurations between homeotropic and homogeneous are also feasible. Nematic liquid crystals with both positive and negative dielectric anisotropy can be switched by applying a DC or AC electric field in the printed vertical domains with the substrate configuration determining the electro‐optic response. 相似文献
998.
In the present work, we have studied the structural, dielectric, and electrical properties of a series of nanosized $\mathrm{ZnAl}_{2-2x}\mathrm{Y}_{2x}\mathrm{O}_{4}$ ( $x = 0.00$ , 0.01, 0.02, 0.03, 0.04, 0.05, 0.07, and 0.10) system prepared by chemical coprecipitation method. Powder X-ray diffraction (XRD) was carried out to study the influence of $\mathrm{Y}^{3+}$ substitution on the crystal structure of these samples. High Resolution Transmission Electron Microscopy (HRTEM) images reveal the nanocrystalline nature of the samples. The Fourier Transform Infrared (FTIR) spectra confirmed the preference of $\mathrm{Y}^{3+}$ ions at the octahedral B site. The variation of dielectric constant and loss tangent (1 kHz to 1 MHz) at room temperature for all the samples show the normal behavior of spinel compounds. AC conductivity study reveals that the conduction is due to small polaron hopping. The electrical modulus analysis shows that nanocrystalline $\mathrm{ZnAl}_{2-2x}\mathrm{Y}_{2x}\mathrm{O}_{4}$ system exhibits non-Debye-type relaxation. The DC electrical resistivity measured in the temperature range 303–373 K was found to increase with temperature and yttrium content. 相似文献
999.
Vinay Kumar A. K. Bedyal J. Sharma V. Kumar O. M. Ntwaeaborwa H. C. Swart 《Applied Physics A: Materials Science & Processing》2014,116(4):1785-1792
In the present work, red-emitting Ca2V2O7:xEu3+ (x = 0.5–6.0 mol%) nanophosphors, in the form of powders, were synthesized by the citrate-gel combustion method using metal nitrates as precursors and citric acid as fuel. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy, photoluminescence (PL) and cathodoluminescence (CL) spectroscopy were used to study the structure, morphology and spectral properties of the samples. The chemical compositions and electronic states of the powders were analyzed with X-ray photoelectron spectroscopy. The average crystallite sizes estimated using the XRD data were found to be in the range of 30–45 nm, and were cross verified by TEM. The lattice parameters determined by the POWD program were approximated as a = 7.242 Å, b = 6.674 Å, c = 6.932 Å and V = 291.24 Å3, respectively. Under UV (395 nm) (PL) and electron (CL) excitation, the nanophosphors show characteristic emission from the Eu3+ ion (5D0 → 7Fj, j = 1–5) with the main peaks at 612 and 616 nm. The maximum emission intensity was recorded from the sample with an Eu3+ concentration of 4 mol% and a critical energy distance of 19.084 Å between the donor and the acceptor. Above this concentration, there was a reduction in the intensity due to dipole–dipole induced concentration quenching effects. The potential applications of this phosphor as a high color-purity phosphor in light-emitting diodes are evaluated. 相似文献
1000.
Poulami Ghosh Ashwini Kumar Sharma 《Applied Physics A: Materials Science & Processing》2014,116(4):1877-1884
We report on the growth of highly c-axis-oriented ZnO nanostructures by pulsed laser deposition technique without using any catalyst. The full-width-at-half-maximum of (002) peak decreased with an increase in substrate temperature. However, a dip at 150 °C is attributed to the contribution from both the small- and large-size particles. FE-SEM images show that the increase in substrate temperature results in the formation of larger particles. Photoluminescence emission is observed both from near band edge as well as defect-related states for all the nanostructures. The presence of E 2(low) and E 2(high) Raman mode intensity and respective increase in the intensity with substrate temperature indicates better crystallinity. Both PL and Raman spectra indicate that A 1(LO) mode may arise due to the defect related to interstitial zinc. 相似文献