全文获取类型
收费全文 | 38349篇 |
免费 | 5152篇 |
国内免费 | 3401篇 |
专业分类
化学 | 13665篇 |
晶体学 | 288篇 |
力学 | 668篇 |
综合类 | 236篇 |
数学 | 1682篇 |
物理学 | 13556篇 |
无线电 | 16807篇 |
出版年
2024年 | 170篇 |
2023年 | 571篇 |
2022年 | 1059篇 |
2021年 | 1260篇 |
2020年 | 1224篇 |
2019年 | 1069篇 |
2018年 | 958篇 |
2017年 | 1420篇 |
2016年 | 1585篇 |
2015年 | 1798篇 |
2014年 | 2480篇 |
2013年 | 2722篇 |
2012年 | 2918篇 |
2011年 | 2579篇 |
2010年 | 2031篇 |
2009年 | 2307篇 |
2008年 | 2384篇 |
2007年 | 2633篇 |
2006年 | 2200篇 |
2005年 | 2043篇 |
2004年 | 1739篇 |
2003年 | 1489篇 |
2002年 | 1159篇 |
2001年 | 961篇 |
2000年 | 886篇 |
1999年 | 803篇 |
1998年 | 682篇 |
1997年 | 636篇 |
1996年 | 553篇 |
1995年 | 450篇 |
1994年 | 344篇 |
1993年 | 324篇 |
1992年 | 292篇 |
1991年 | 225篇 |
1990年 | 164篇 |
1989年 | 158篇 |
1988年 | 138篇 |
1987年 | 82篇 |
1986年 | 65篇 |
1985年 | 58篇 |
1984年 | 53篇 |
1983年 | 25篇 |
1982年 | 54篇 |
1981年 | 38篇 |
1980年 | 18篇 |
1979年 | 29篇 |
1978年 | 12篇 |
1977年 | 10篇 |
1974年 | 11篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
102.
103.
二硒化钼的层间相互作用强,单层结构具有更低的带隙和更好的稳定性.由于独特的光学性质和优异的电学性能受到研究人员的广泛关注.本文基于密度泛函理论的第一原理,计算和分析了在双轴拉伸压缩应变条件下单层MoSe2能带结构,拉曼光谱和声子谱的变化规律以及性质产生的原因.在拉伸压缩应变作用下,直接带隙转变为间接带隙.当拉伸应变达到12%时,材料发生半导体-金属相变.当压缩应变达到6%时,声子谱中开始出现虚频率,表明结构开始变得不稳定. 相似文献
104.
航天器对接相对位置姿态自主测量方法研究 总被引:2,自引:0,他引:2
航天器间相对位置姿态的准确测量是实现自主自动对接的先决条件。将三圆标志靶安装在目标航天器上;追踪航天器用数码相机对靶拍照。设计了亚像素处理方法实时处理数字照片,可精确测定圆心像点位置和特征直径的像长;用其作为基本特征参数,基于透视投影原理,建立了求圆心物空间坐标的公式,求出靶面的平面方程,可用来求两航天器间的相对位置姿态参数和实时标定相机的可变参数。计算机仿真结果表明,测距为2m时,测圆心像点位置的误差小于0.03个像素,测距误差小于1mm,测姿态角的误差小于0.01°。该方法也适用于无人直升飞机定点着陆时自主测量其相对位姿参数。 相似文献
105.
Hui Ma Lulu Xiao Dongchen Xu Yingrui Geng Xuesong Liu Yong Chen Yongjiang Wu 《Molecules (Basel, Switzerland)》2022,27(9)
Quality control methods of current traditional Chinese medicine (TCM) preparation is time-consuming and difficult to assess in terms of overall efficiency of the drug. A non-destructive rapid near-infrared spectroscopy detection system for key chemical components and biological activity of Lanqin oral solution (LOS), one of the best-selling TCM formulations, was established for comprehensive quality evaluation. Near infrared spectral scanning was carried out on 101 batches of commercial LOS under the penetrated vial state and traditional state. RAW 264.7 cells were cultured to detect the anti-inflammatory ability of LOS, and the reference concentrations of epigoitrin, geniposide, and baicalin were obtained by HPLC. The quantitative models were optimized by three kinds of variable selection methods. The correlation coefficients of prediction value of the models were greater than 0.94. The system also passed the external validation. The performance of the non-invasive models was similar to the traditional models. The established non-destructive system can be applied to the rapid quality inspection of LOS to avoid unqualified drugs from entering the market and ensure drug effectiveness. The biological activity index of LOS was introduced and predicted by NIRs for the first time, which provides a new idea about the quality control of TCM formulations. 相似文献
106.
Effects of Organic Acids on the Release of Fruity Esters in Water: An Insight at the Molecular Level
It is well known that organic acids (OAs) could affect the flavour of fruit juices and beverages. However, the molecular mechanism of aroma release is still unclear. In this study, the effects of citric acid (CA), L-(-)-malic acid (MA) and L-lactic acid (LA) on the release of six selected esters and their sensory perception were investigated by means of HS-GC-MS analyses and odour detection threshold determination, respectively. Meanwhile, the density functional theory (DFT) calculation was employed to explore the interaction modes between esters and OAs. HS-GC-MS analyses showed that the concentration and the type of OAs regulated the release of esters. The results were basically consistent with the detection threshold change of those esters. The DFT calculation suggested that the main intermolecular interaction was hydrogen bonds, and several esters could form a ternary ring structure with OAs through hydrogen bonds. The interactions can induce the different release behaviours of esters in OAs water solution. The number of carboxyl functional groups in OAs and the spatial conformation of esters appeared to influence the magnitude of the interaction. The above results demonstrated the mechanism of OAs affecting the release of esters and indicated a possible flavour control way by using different OAs and OA concentrations. 相似文献
107.
108.
The 235U(n, f) cross-section has been measured after the installation of a liquid nitrogen cooled methane moderator at GELINA. The results are compared with previous data, and the influence of the cross section shape on the Westcott gf-factor is discussed. 相似文献
109.
Due to the complexity of the scene, target detection in forward-looking infrared (FLIR) imagery is a challenging problem, especially for occluded target. The main contribution of this paper is to propose an indirect detection method for improving the recognition probability and effectiveness of target detection method in FLIR image sequences under complex conditions. The proposed method mainly includes four steps: preparation of forward-looking reference image of landmark, extraction of the real-time scene image, template matching and target location, in which some key technologies are proposed, such as perspective transformation used to solve projective problems, position prediction for improving real-time performance, and target location used for identifying the target’s position. Experimental results are shown to demonstrate the robustness and efficiency of proposed method in FLIR image sequences. 相似文献
110.
We present a region of interest (ROI) generation method specialized for nighttime pedestrian detection using far-infrared (FIR) images. Because pedestrians typically appear brighter than background in FIR images, previous research efforts primarily attempted to extract ROIs based on the intensity threshold. However this approach has problems resulting from the intensity variances of pedestrians due to their clothing and, especially in urban scenarios, and other heat sources that emit more heat than the pedestrians. In this paper, we propose a novel ROI generation method that is based on combining image segments instead of using the intensity threshold. In order to minimize dependence on brightness, we utilize the low-frequency characteristics of FIR images. As a result, our proposed method generates a small number of ROIs at an acceptable miss rate and the generated ROIs provide advantages for classification because the pedestrians are satisfactorily arranged within a bounding box. Experiments conducted indicate that our proposed method performs reliably in urban scenarios. 相似文献