全文获取类型
收费全文 | 211篇 |
免费 | 13篇 |
国内免费 | 17篇 |
专业分类
化学 | 56篇 |
力学 | 10篇 |
数学 | 72篇 |
物理学 | 103篇 |
出版年
2023年 | 4篇 |
2022年 | 4篇 |
2021年 | 6篇 |
2020年 | 4篇 |
2019年 | 2篇 |
2018年 | 2篇 |
2017年 | 9篇 |
2016年 | 8篇 |
2015年 | 7篇 |
2014年 | 15篇 |
2013年 | 26篇 |
2012年 | 13篇 |
2011年 | 10篇 |
2010年 | 11篇 |
2009年 | 11篇 |
2008年 | 9篇 |
2007年 | 8篇 |
2006年 | 9篇 |
2005年 | 8篇 |
2004年 | 9篇 |
2003年 | 9篇 |
2002年 | 8篇 |
2001年 | 5篇 |
2000年 | 1篇 |
1999年 | 5篇 |
1998年 | 5篇 |
1997年 | 10篇 |
1996年 | 2篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1984年 | 2篇 |
1983年 | 1篇 |
排序方式: 共有241条查询结果,搜索用时 328 毫秒
51.
52.
53.
使用掺铒光纤放大器的光纤CATV系统载噪比的研究 总被引:1,自引:0,他引:1
本文全面分析了加入掺铒光纤放大器的CATV系统噪音,推导出了这类CATV系统的载噪比的计算公式。 相似文献
54.
Data envelopment analysis (DEA) is defined based on observed units and by finding the distance of each unit to the border of estimated production possibility set (PPS). The convexity is one of the underlying assumptions of the PPS. This paper shows some difficulties of using standard DEA models in the presence of input-ratios and/or output-ratios. The paper defines a new convexity assumption when data includes a ratio variable. Then it proposes a series of modified DEA models which are capable to rectify this problem. 相似文献
55.
This paper demontrates a novel configuration of a linear cavity erbium-doped fiber laser using fiber loop mirrors to effectively suppress the noise generated by the amplified spontaneous emission (ASE). A tunable filter is deployed in each fiber loop mirror to create a narrow passband for the selected mode. The fiber laser produces high background suppression ratio (BSR) of more than 75 dB from 1525 to 1565 nm at the output coupling of 95%. There is no residual ASE detected at the output to verify the effectiveness of the noise filtering in the proposed fiber laser. Low threshold powers are obtained in the range of 2.5–3 mW and the peak power variation is smaller than 1.6 dB. 相似文献
56.
57.
This article shows that we obtained blood plasma of healthy persons and patients with leukemia, got absorption spectrum of them by spectral photometer. After analyzing these absorption spectra by the origin software, we found absorbance (Abs) of the malignant plasma were more than Abs of the healthy plasma at 414 nm, and we also found significant difference between the malignant plasma and the healthy plasma when Abs of some characteristic peaks at 414 nm, 279 nm were rationed or fitted. It is important for doctors to diagnose patients with leukemia quickly. 相似文献
58.
Abstract The best graft ratio of the light-curable N-vinylpyrrolidone (NVP) modified terpolymers (LC NVPM TPs) with the molar ratio of 8:2:1 (acrylic acid:itaconic acid:NVP) and the optimal formulation for this light-curable glass-ionomer cement, based on the best graft ratio, were determined. Statistical models were utilized to predict the optimal formulations. The terpolymer was prepared using a free-radical polymerization reaction. The LC NVPM TPs were produced by grafting 2-isocyanatoethyl methacrylate (IEM) onto the terpolymer. Cements were formed by both light-curing and the reaction with glass particles. Compressive strength was used as the basic screening property to find the optimal formulation. Diametral tensile and flexural strengths were also used to evaluate the mechanical properties. The strength values were recorded on the specimens conditioned in distilled water at 37&C for 24 hours or 7 days. The best graft ratio for IEM in this system was 15% of the terpolymer by a molar ratio. The optimal formulation was found to be at the weight ratio of 55:15:30 [LC NVPM TP:2-hydroxyethyl methacrylate (HEMA): H2O]. Stress-strain curves showed that a relatively high amount of water in the formulation led to higher elastic modulus and proportional limit and lower malleability, whereas a relatively high amount of HEMA gave the opposite results. The light-curable NVP modified glass-ionomer cements showed statistically significantly higher values in compressive, diametral tensile, and flexural strengths than the commercial VitremerTM. 相似文献
59.
Abstract Copolymers of 3-methoxy-4-acryloyloxybenzal phenylimine and methyl methacrylate with different feed ratios are synthesized in ethyl methyl ketone using benzoyl peroxide as a free radical initiator at 70 ± 1°CC. The polymers were characterized by IR and 1H-NMR spectroscopic techniques. Copolymer compositions were determined by lH-NMR analysis of the polymers. The monomer reactivity ratios were determined by the application of conventional linearization methods such as Fineman-Rose and Kelen-Tüdös. The molecular weights Mn and Mw of the polymers and the poly-dispersity index were determined by gel permeation chroma-tography. The intrinsic viscosities and the thermal properties of the homo-and copolymers are also discussed. 相似文献
60.
Abstract Starch plastic sheets were prepared by extrusion processing of mixtures of granular high-amylopectin and high-amylose starches in the presence of glycerol and water as plasticizers. Amylose content varied between 0 and 70% (w/w). Structural characterization and determination of the mechanical properties of the sheets were performed after aging the materials between 40–65% relative humidity for 2 and 35 weeks and at 90% relative humidity for two weeks. The materials were semicrystalline and viscoelastic. The materials were described as complex heterogeneous multiphase materials. They consisted of amorphous and crystalline phases of amylose and amylopectin as well as granular structures and domains of amylose, amylopectin and amylose-amylopectin helices. Single-helical type crystallinity was formed solely by amylose directly after processing while B-type crystallinity was rapidly formed in amylose-rich materials and slowly during aging of amylopectin-rich materials. The stress-strain and stress-relaxation properties were related to differences in amylose content, degree of crystallization and water content. The amorphous amylopectin rich materials were flexible and soft but showed an increase in stiffness and a decrease in elongation due to crystallization. Amylopectin-rich materials showed unfavorable relaxation, shrinkage and cracking during aging. The materials rich in amylopectin were sensitive to water content while the amylose-rich materials were not sensitive to water in the range of 9–13% (w/w). Stress-strain relaxation behaviors of the materials were dependent on starch structure and on experimental conditions such as strain rate and extension by which the ratio of elastic and viscous response were varied. An increase in relaxation times was found with increasing amylose content and water content for the materials with solely amylose crystallinity. 相似文献