全文获取类型
收费全文 | 52638篇 |
免费 | 4300篇 |
国内免费 | 3918篇 |
专业分类
化学 | 20903篇 |
晶体学 | 466篇 |
力学 | 4004篇 |
综合类 | 548篇 |
数学 | 12021篇 |
物理学 | 22914篇 |
出版年
2024年 | 37篇 |
2023年 | 469篇 |
2022年 | 802篇 |
2021年 | 1104篇 |
2020年 | 1221篇 |
2019年 | 1227篇 |
2018年 | 1109篇 |
2017年 | 1319篇 |
2016年 | 1462篇 |
2015年 | 1203篇 |
2014年 | 1878篇 |
2013年 | 3457篇 |
2012年 | 2186篇 |
2011年 | 2429篇 |
2010年 | 1964篇 |
2009年 | 3134篇 |
2008年 | 3403篇 |
2007年 | 3805篇 |
2006年 | 3262篇 |
2005年 | 2557篇 |
2004年 | 2167篇 |
2003年 | 2449篇 |
2002年 | 2806篇 |
2001年 | 2021篇 |
2000年 | 1920篇 |
1999年 | 1611篇 |
1998年 | 1598篇 |
1997年 | 926篇 |
1996年 | 858篇 |
1995年 | 771篇 |
1994年 | 811篇 |
1993年 | 603篇 |
1992年 | 692篇 |
1991年 | 458篇 |
1990年 | 419篇 |
1989年 | 326篇 |
1988年 | 287篇 |
1987年 | 282篇 |
1986年 | 247篇 |
1985年 | 226篇 |
1984年 | 232篇 |
1983年 | 134篇 |
1982年 | 184篇 |
1981年 | 176篇 |
1980年 | 112篇 |
1979年 | 137篇 |
1978年 | 104篇 |
1977年 | 87篇 |
1976年 | 42篇 |
1973年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
研究了基于G-N模型与三项延迟模型的热弹性波的传播特征.在各种广义热弹性理论中,热位移概念的引入使得G-N模型具有独特性质而引起广泛的关注和应用.基于G-N模型,最近一个三相延迟模型被提出.主要研究了这两种模型下第一和第二声波(耦合热弹性波)的色散和衰减特性,分析了温度场和位移场在两种模型下的振幅比和相位差.结果发现三相延迟模型更为通用和灵活,应得到更多的关注与应用. 相似文献
22.
23.
The health monitoring has been studied to ensure integrity of design of engine structure by detection, quantification, and prediction of damages. Early detection of faults may allow the downtime of maintenance to be rescheduled, thus preventing sudden shutdown of machines. In cylinder pressure developed, vibrations and noise emissions data provide a rich source of information about condition of engines. Monitoring of vibrations and noise emissions are novel non-intrusive methodologies for which positioning of various transducers are important issue. The presented work shows applicability of these diagnosis methodologies adopted in case of diesel engines. The effects of changing various fuel injection parameters was analyzed. Scope of using non-intrusive technique has been analyzed by changing locations of microphone. Novelty of this worklies in exploring signal processing methods for various locations around the engine test set up. Various frequency ranges of contributing noise and vibration sources were identified. Time-Frequency analysis showed the onset of various cyclic. Based on the identification of various frequency bands, it is possible to device suitable filters in order to extract more information. 相似文献
24.
25.
26.
27.
《Arabian Journal of Chemistry》2022,15(7):103915
There is a growing attention to the bio and renewable energies due to fast depletion of fossil fuels as well as the global warming problem. Here, we developed a modeling and simulation method by means of artificial intelligence (AI) for prediction of the bioenergy production from vegetable bean oil. AI methods are well known for prediction of complex and nonlinear process. Three distinct Adaptive Boosted models including Huber regression, LASSO, and Support Vector Regression (SVR) as well as artificial neural network (ANN) were applied in this study to predict actual yield of Fatty acid methyl esters (FAME) production. All boosted utilizing the Adaptive boosting algorithm. The important influencing parameters on the biodiesel production such as the catalyst loading (CAO/Ag, wt%) and methanol to oil (Soybean oil) molar ratio were selected as the input variables of models while the yield of FAME production was selected as output. Model hyper-parameters were tuned to maintain generality while improving prediction accuracy. The models were evaluated using three distinct metrics Mean Absolute Error (MAE), Root Mean Square Error (RMSE), and R2. Error rates of 8.16780E-01, 4.43895E-01, 2.06692E + 00, and 3.92713 E-01 were obtained with the MAE metric for boosted Huber, SVR, LASSO and ANN models. On the other hand, the RMSE error of these models were about 1.092E-02, 1.015E-02, 2.669E-02, and 1.01174E-02, respectively. Finally, the R-square score were calculated for boosted Huber, boosted SVR, and boosted LASSO as 0.976, 0.990, 0.872, and 0.99702, respectively. Therefore, it can be concluded that although the boosted SVR and ANN models were better models for prediction of process efficiency in terms of error, but all algorithms had high accuracy. The optimum yield of 83.77% and 81.60% for biodiesel production were observed at optimum operating values from boosted SVR and ANN models, respectively. 相似文献
28.
29.
二维材料MXene纳米片由于具有较大的比表面积和较高的电子迁移率而受到广泛的关注。本文采用基于密度泛函理论的第一性原理计算,对单层MXene纳米片Ti2N电磁特性的过渡金属(Sc、V、Zr)掺杂效应进行了系统研究。结果表明,所有过渡金属掺杂体系结合能均为负值,结构均稳定;其中Ti2N-Sc体系的形成能为-2.242 eV,结构更易形成,且保持稳定;掺杂后Ti2N-Sc、Ti2N-Zr体系磁矩增大;此外,Ti2N-Sc体系中保留了较高的自旋极化率,达到84.9%,可预测该体系在自旋电子学中具有潜在的应用价值。 相似文献
30.
This work is concerned with the extension of the Jacobi spectral Galerkin method to a class of nonlinear fractional pantograph differential equations. First, the fractional differential equation is converted to a nonlinear Volterra integral equation with weakly singular kernel. Second, we analyze the existence and uniqueness of solutions for the obtained integral equation. Then, the Galerkin method is used for solving the equivalent integral equation. The error estimates for the proposed method are also investigated. Finally, illustrative examples are presented to confirm our theoretical analysis. 相似文献