全文获取类型
收费全文 | 10510篇 |
免费 | 1904篇 |
国内免费 | 1166篇 |
专业分类
化学 | 7455篇 |
晶体学 | 94篇 |
力学 | 623篇 |
综合类 | 62篇 |
数学 | 1179篇 |
物理学 | 4167篇 |
出版年
2024年 | 31篇 |
2023年 | 246篇 |
2022年 | 253篇 |
2021年 | 415篇 |
2020年 | 449篇 |
2019年 | 412篇 |
2018年 | 398篇 |
2017年 | 320篇 |
2016年 | 552篇 |
2015年 | 487篇 |
2014年 | 623篇 |
2013年 | 819篇 |
2012年 | 1028篇 |
2011年 | 999篇 |
2010年 | 675篇 |
2009年 | 661篇 |
2008年 | 683篇 |
2007年 | 589篇 |
2006年 | 547篇 |
2005年 | 442篇 |
2004年 | 346篇 |
2003年 | 259篇 |
2002年 | 238篇 |
2001年 | 192篇 |
2000年 | 213篇 |
1999年 | 252篇 |
1998年 | 200篇 |
1997年 | 184篇 |
1996年 | 200篇 |
1995年 | 155篇 |
1994年 | 132篇 |
1993年 | 93篇 |
1992年 | 108篇 |
1991年 | 60篇 |
1990年 | 76篇 |
1989年 | 56篇 |
1988年 | 27篇 |
1987年 | 28篇 |
1986年 | 39篇 |
1985年 | 39篇 |
1984年 | 14篇 |
1983年 | 18篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1957年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A nonlinear model for a steady flow in a deformable porous medium is considered. The flow is governed by the poroelasticity system consisting of an elasticity equation for the displacement of the porous medium and Darcy's equation for the pressure in the fluid. This poroelasticity system is nonlinear when the permeability in Darcy's equation is assumed to depend on the dilatation of the porous medium. Existence and uniqueness of a weak solution of this poroelasticity system is established under rather weak assumptions on the regularity of the data. Convergence of a finite element approximation is proved and verified through numerical experiments. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
992.
In this paper, we study a kind of system of second order quasilinear parabolic partial differential equation combined with algebra equations. Introducing a family of coupled forward–backward stochastic differential equations, and by virtue of some delicate analysis techniques, we give a probabilistic interpretation for it in the viscosity sense. 相似文献
993.
Zhiyong Li Jonathan Crook Galina Andreeva 《The Journal of the Operational Research Society》2014,65(3):466-479
Bankruptcy prediction is a key part in corporate credit risk management. Traditional bankruptcy prediction models employ financial ratios or market prices to predict bankruptcy or financial distress prior to its occurrence. We investigate the predictive accuracy of corporate efficiency measures along with standard financial ratios in predicting corporate distress in Chinese companies. Data Envelopment Analysis (DEA) is used to measure corporate efficiency. In contrast to previous applications of DEA in credit risk modelling where it was used to generate a single efficiency—Technical Efficiency (TE), we assume Variable Returns to Scale, and decompose TE into Pure Technical Efficiency and Scale Efficiency. These measures are introduced into Logistic Regression to predict the probability of distress, along with the level of Returns to Scale. Effects of efficiency variables are allowed to vary across industries through the use of interaction terms, while the financial ratios are assumed to have the same effects across all sectors. The results show that the predictive power of the model is improved by this corporate efficiency information. 相似文献
994.
995.
In this paper we propose a new method for estimating parameters in a single-index model under censoring based on the Beran estimator for the conditional distribution function. This, likelihood-based method is also a useful and simple tool used for bandwidth selection. Additionally, we perform an extensive simulation study comparing this new Beran-based approach with other existing method based on Kaplan–Meier integrals. Finally, we apply both methods to a primary biliary cirrhosis data set and propose a bootstrap test for the parameters. 相似文献
996.
Yuanyi Cao Shaolei Wang Haiyan Mou Haiyan Xu Chifei Wu 《Journal of Crystal Growth》2010,312(10):1747-1750
Crystal growth of a hindered phenol compound, tetrakis [methylene-3-(3-5-ditert-butyl-4-hydroxy phenyl) propionyloxy] methane (trade name AO-60), was successfully recorded by optical microscopy (OM) equipped with a hot stage. The morphology of AO-60 crystals, grown at 100 °C from amorphous state, appeared in the form of tetragonal-sloped step growth. Further study using scanning electron microscopy (SEM) and atomic force microscopy (AFM) experimentally demonstrated that AO-60 crystals had a hopper-like morphology, which had occurred rarely in the condition of organic compound crystals but predominantly rather to inorganic compound crystals in the reported literatures. The morphological features observed on the crystal surfaces suggested step growth and hopper growth mechanism. Besides, a raising around the edge of the AO-60 hopper crystal was also experimentally characterized for the first time. 相似文献
997.
In this report, we describe the noncatalytic and template-free synthesis of zinc nitride (Zn3N2) novel microstructures with hollow interiors via simple nitridation reaction of zinc powder at optimum temperature of 600° C for 120 min in ammonia gas environment under atmospheric pressure. Hollow microstructures obtained were mostly of spherical shape with diameters in the range 8–35 μm and with open mouth on the surface. The growth mechanism has been proposed for the elucidation of hollow structures formation. Crystal structure and phase purity of the product were investigated by X-ray diffraction (XRD) characterization and energy dispersive X-ray spectroscopy (EDS) analysis confirmed the chemical composition of the product. Morphology of the as-prepared product was investigated using scanning electron microscopy (SEM). Ultraviolet–visible–near infrared (UV–vis–NIR) spectrophotometry was used to study the transmittance behaviour of zinc nitride microstructures and thereby an indirect optical band gap of 2.81 eV was calculated using Davis–Mott model. Room temperature photoluminescence (PL) studies exhibited two prominent peaks of the product; one very strong peak near band edge UV emission (395 nm) and other comparatively suppressed and broad peak at orange luminescence emission (670 nm). 相似文献
998.
Shuti Li Jun Su Guanghan Fan Chao Liu Jianxing Cao Yian Yin 《Journal of Crystal Growth》2010,312(21):3101-3104
We have investigated the growth of magnesium-doped GaP (GaP:Mg) layers on GaN by metalorganic chemical vapor deposition. The hole carrier concentration increased linearly from 0.8×1018 to 4.2×1018 cm−3 as the Bis(cyclopentadienyl) magnesium (Cp2Mg) mole flow rate increased from 1.2×10−7 to 3.6×10−7 mol/min. However, the hole carrier concentration decreased when the CP2Mg mole flow rate was further increased. The double crystal X-ray diffraction (DCXRD) rocking curves showed that the GaP:Mg layers were single crystalline at low CP2Mg molar flow. However, the GaP:Mg layers became polycrystalline if the CP2Mg molar flow was too high. The decrease in hole carrier concentration at high CP2Mg molar flow was due to crystal quality deterioration of the GaP layer, which also resulted in the low hole mobility of the GaP:Mg layer. 相似文献
999.
S.R. Xu Y. Hao J.C. Zhang Y.R. Cao X.W. Zhou L.A. Yang X.X. Ou K. Chen W. Mao 《Journal of Crystal Growth》2010,312(23):3521-3524
We have investigated the unintentional impurities, oxygen and carbon, in GaN films grown on c-plane, r-plane as well as m-plane sapphire by metal-organic chemical vapor deposition. The GaN layer was analyzed by secondary ion mass spectroscopy. The different trend of the incorporation of oxygen and carbon has been explained in the polar (0 0 0 1), nonpolar (1 1 2¯ 0) and semipolar (1 1 2¯ 2) GaN by a combination of the atom bonding structure and the origin direction of the impurities. Furthermore, it has been found that there is a stronger yellow luminescence (YL) in GaN with higher concentration of carbon, suggesting that C-involved defects are originally responsible for the YL. 相似文献
1000.