全文获取类型
收费全文 | 158197篇 |
免费 | 25755篇 |
国内免费 | 16861篇 |
专业分类
化学 | 108119篇 |
晶体学 | 1739篇 |
力学 | 10415篇 |
综合类 | 986篇 |
数学 | 20662篇 |
物理学 | 58892篇 |
出版年
2024年 | 311篇 |
2023年 | 2895篇 |
2022年 | 3385篇 |
2021年 | 5114篇 |
2020年 | 6160篇 |
2019年 | 5883篇 |
2018年 | 5130篇 |
2017年 | 4940篇 |
2016年 | 7308篇 |
2015年 | 7120篇 |
2014年 | 8658篇 |
2013年 | 11398篇 |
2012年 | 14124篇 |
2011年 | 14726篇 |
2010年 | 10043篇 |
2009年 | 9494篇 |
2008年 | 10313篇 |
2007年 | 9343篇 |
2006年 | 8593篇 |
2005年 | 7315篇 |
2004年 | 5597篇 |
2003年 | 4560篇 |
2002年 | 4405篇 |
2001年 | 3558篇 |
2000年 | 3154篇 |
1999年 | 3454篇 |
1998年 | 2891篇 |
1997年 | 2787篇 |
1996年 | 2831篇 |
1995年 | 2400篇 |
1994年 | 2065篇 |
1993年 | 1676篇 |
1992年 | 1549篇 |
1991年 | 1389篇 |
1990年 | 1131篇 |
1989年 | 889篇 |
1988年 | 679篇 |
1987年 | 605篇 |
1986年 | 592篇 |
1985年 | 491篇 |
1984年 | 339篇 |
1983年 | 286篇 |
1982年 | 197篇 |
1981年 | 161篇 |
1980年 | 133篇 |
1979年 | 75篇 |
1977年 | 84篇 |
1976年 | 78篇 |
1975年 | 70篇 |
1973年 | 71篇 |
排序方式: 共有10000条查询结果,搜索用时 591 毫秒
911.
Zhengqian Luo Guoyong Sun Zhiping Cai Miaoling Si Qibo Li 《Optics Communications》2007,277(1):118-124
In this paper, continuous wave Yb3+-doped double-clad fiber lasers (DCFLs) with linear-cavity are investigated theoretically and numerically using the rate equations. Under the steady state conditions, the simplified analytic solutions of Yb3+-doped DCFLs under considering the scattering loss are deduced in the strongly pump condition. Compared with the known analytic solutions in published literatures, our analytic solutions are more accurate, especially, at higher reflectivity of output mirror. In addition, a fast and stable algorithm based on the Newton-Raphson method is proposed to simulate numerically Yb3+-doped DCFLs. The results by simplified analytic solutions are in good agreement with those by the numerical simulation. Moreover, we have performed the optimization of an Yb3+-doped DCFL using the simplified analytic solutions and the numerical simulations, respectively. 相似文献
912.
913.
In this paper, the authors present airflow field characteristics of human upper airway and soft palate movement attitude during
breathing. On the basis of the data taken from the spiral computerized tomography images of a healthy person and a patient
with Obstructive Sleep Apnea-Hypopnea Syndrome (OSAHS), three-dimensional models of upper airway cavity and soft palate are
reconstructed by the method of surface rendering. Numerical simulation is performed for airflow in the upper airway and displacement
of soft palate by fluid-structure interaction analysis. The reconstructed three-dimensional models precisely preserve the
original configuration of upper airways and soft palate. The results of the pressure and velocity distributions in the airflow
field are quantitatively determined, and the displacement of soft palate is presented. Pressure gradients of airway are lower
for the healthy person and the airflow distribution is quite uniform in the case of free breathing. However, the OSAHS patient
remarkably escalates both the pressure and velocity in the upper airway, and causes higher displacement of the soft palate.
The present study is useful in revealing pathogenesis and quantitative mutual relationship between configuration and function
of the upper airway as well as in diagnosing diseases related to anatomical structure and function of the upper airway.
The project supported by the National Natural Science Foundation of China (10672036, 10472025 and 10421002), the Natural Science
Foundation of Liaoning Province (20032109).
English text was polished by Yunming Chen. 相似文献
914.
The kinetic behaviour of aluminum naphthenate/benzene system in the process of gelation and degradation was systematically studied. The results obtained from the kinetic experiments indicated that the gelation stage was in accordance with a kinetic model of an auto-catalytic first-order reaction, and the degradation stage was a simple first-order reaction. The rate constants in these two stages at various temperaturs were measured. And the determined apparent activation energies for the gelation stage and for the degradation stage are 27.5±2.0 and 37.7±2.3 kJ/mol, respectively. 相似文献
915.
基于RBC模型的动态随机一般均衡的数值解 总被引:1,自引:0,他引:1
以一般均衡为框架,引入随机变量的动态随机一般均衡系统,由于模型的庞大和函数形式的复杂,往往不能给出明确的数值解.本文给出几种操作性强的、有效的寻求一般化模型数值解的方法,并通过介绍模型参数确定的办法,使得理论的模型有了现实的含义. 相似文献
916.
The article is concerned with oscillation of nonautonomous neutral dynamic delay equations on time scales. Sufficient conditions are established for the existence of bounded positive solutions and for oscillation of all solutions of this equation. Some results extend known results for difference equations when the time scale is the set Z of positive integers and for differential equations when the time scale is the set R of real numbers. 相似文献
917.
Approximation of signals from local averages 总被引:4,自引:0,他引:4
This work is concerned with approximation of a signal from local averages. It improves a result of Butzer and Lei [P.L. Butzer, J. Lei, Approximation of signals using measured sampled values and error analysis, Commun. Appl. Anal. 4 (2000) 245–255]. 相似文献
918.
Guosong Wu Xiaoqin Zeng Wenbin Ding Xingwu Guo Shoushan Yao 《Applied Surface Science》2006,252(20):7422-7429
Ceramic thin films have been widely used to protect the metal substrate as coatings in the past years. In order to improve the poor corrosion resistance of AZ31 magnesium alloy, the study in this paper used the electron beam evaporation method to prepare ceramic PVD films on its surface with TiO2 and Al2O3 as donors, respectively. Atomic force microscopy (AFM), scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS), Auger electron spectroscopy (AES) and X-ray diffraction (XRD) were used to investigate the surface morphology, composition and microstructure of the thin films. Both films deposited on AZ31 took on compact top surface morphologies and grew as amorphous structures on substrate. AES test not only showed that films compositions deviated the standard stoichiometric ratios, but also found that element Mg diffused into films and existed as magnesium oxide in the TiOx film as well as the AlOx film. In the electrochemical corrosion test, the AlOx coating on AZ31 exhibited the largest electrochemical impedance in a 3.5% NaCl solution. But it did not show better corrosion resistance than others for the poorer adhesion. Even if its thickness was small, the TiOx coating on AZ31 exhibited the best corrosion resistance in this study. According to the observation and analysis, the damage of these films on AZ31 in aggressive solutions was mainly due to the existence of pores, microcracks, vacancies and poor adhesion between coating and substrate. 相似文献
919.
Currently, self-organized magnetic array (SOMA) is a non-oriented media, with randomly distributed easy axis directions. We investigate the recording performance of SOMA under two different head types—Ring Head (RH) and single pole head (SPH). Our results show an interesting dependence of the recorded transition pattern on the field strength of RH, where a weak RH field generates longitudinal transition pattern and a strong RH field generates a perpendicular transition pattern in the media. The SPH only generates perpendicular transition patterns and has a higher signal-to-noise ratio (SNR) than that of the RH. 相似文献
920.
Gap length (GL) of reading head is one of the most critical parameters for high-density magnetic recording systems. A novel method is proposed for quantitative evaluation of the GL fluctuation among a batch of magneto-resistive/giant magneto-resistive heads with same structure design. The method works at head-gimbal assembly level and the evaluation can be done with any read/write analysis equipment. The testing process is based on harmonic analysis of the readback signal. The testing system consists of a selected reference head and sample heads for evaluation. A GL variation function is introduced for the evaluation of GL deviation between the reference head and sample head. This method proved to be easy for implementation and results suggest that variation of GL is considerable and has obvious effect on recording performance in high recording density systems. 相似文献