全文获取类型
收费全文 | 1536篇 |
免费 | 191篇 |
国内免费 | 159篇 |
专业分类
化学 | 568篇 |
晶体学 | 11篇 |
力学 | 308篇 |
综合类 | 19篇 |
数学 | 119篇 |
物理学 | 861篇 |
出版年
2023年 | 38篇 |
2022年 | 56篇 |
2021年 | 81篇 |
2020年 | 87篇 |
2019年 | 32篇 |
2018年 | 58篇 |
2017年 | 53篇 |
2016年 | 88篇 |
2015年 | 60篇 |
2014年 | 108篇 |
2013年 | 100篇 |
2012年 | 79篇 |
2011年 | 86篇 |
2010年 | 60篇 |
2009年 | 74篇 |
2008年 | 86篇 |
2007年 | 70篇 |
2006年 | 87篇 |
2005年 | 72篇 |
2004年 | 68篇 |
2003年 | 57篇 |
2002年 | 61篇 |
2001年 | 37篇 |
2000年 | 36篇 |
1999年 | 29篇 |
1998年 | 27篇 |
1997年 | 41篇 |
1996年 | 27篇 |
1995年 | 26篇 |
1994年 | 21篇 |
1993年 | 16篇 |
1992年 | 8篇 |
1991年 | 7篇 |
1990年 | 12篇 |
1989年 | 3篇 |
1988年 | 7篇 |
1987年 | 2篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1979年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有1886条查询结果,搜索用时 31 毫秒
91.
ABSTRACTNew fluorene-containing copolymer was prepared via the catalytic dehydrocoupling reaction of 9,9-dipropargylfluorene and 4,4′-diphenyl-2,6-dibromo-dithieno-[3,2-b:2′,3′-d]silole in 44% yield. The resulting polymer was completely soluble in common organic solvents. The thermal behaviors and optical properties of the resulting polymer were measured and discussed. The chemical structure of polymer was characterized by NMR, IR, and UV-visible spectroscopies. The polymer showed characteristic wide UV-visible absorption band and blue PL maximum value at 440 nm, which is corresponding photon energy of 2.82 eV. The quantum yield (6.44%) of poly(DPF-DTS) was found to be 5 times higher than that of poly(DPF-bithiophene)). 相似文献
92.
N. Shanta Singh R.S. Ningthoujam S. Dorendrajit Singh B. Viswanadh N. Manoj R.K. Vatsa 《Journal of luminescence》2010,130(12):2452-2459
Highly crystalline spherical nanoparticles of MVO4:Tm3+ (M=Gd, Y) having a size of 20–45 nm were prepared using ethylene glycol as both capping agent and reaction medium. X-ray diffraction study shows linear decrease in the unit cell volume with an increase in Tm3+ concentrations in MVO4 (M=Gd, Y; Tm3+=0, 2, 5, 7, 10, 15, 20, 40, 60, 80 and 100 at%), indicative of quantitative substitution of Gd3+/Y3+ lattice sites by Tm3+ ions in MVO4. Blue light emission at 475 nm is observed after excitation at 310 nm due to energy transfer from VO43− absorption band to Tm3+. Emission intensity and average decay lifetime increase with an increase in heat treatment from 500 to 900 °C. This has been attributed to an extent of reduction in non-radiative process arising from surface. The emission intensity of Tm3+ in GdVO4 host is higher than that in YVO4 and thus the former host is better. Quantum yield increases with increase in heat-treatment temperature. This material will be the alternative blue light emitter. 相似文献
93.
94.
95.
96.
以中温热解煤焦油为原料,对其性质进行了分析,其中,350℃重质馏分中胶质含量30.88%,沥青质含量37.27%,四氢呋喃不溶物3.36%,属于常规固定床加氢裂化难以直接处理的馏分。合成了一种Mo系超分散催化剂,采用FT-IR、XPS、XRD、SEM和TEM等对催化剂进行了表征,催化剂中含有Mo=O和Mo-S特征结构,活性金属的硫化率为84.34%,在体系中具有优良的分散性,在反应体系内原位分解为超分散MoS_2颗粒;在0.25 t/d连续装置上进行了热解重油悬浮床加氢裂化实验研究,考察了反应条件对产物分布情况和结焦率的影响,得出适宜的反应条件为19 MPa,440℃,催化剂的添加量为300 mg/kg;此条件下石脑油收率24.47%,柴油馏分收率49.71%,结焦率1.32%。 相似文献
97.
以天然物质石斛为原料,一步水热法合成高荧光量子产率的氮掺杂碳点(NCDs),通过透射电子显微镜(TEM)、X射线光电子能谱(XPS)、傅里叶变换红外光谱(FT-IR)、紫外-可见光吸收图谱(UV-Vis)及荧光光谱(PL)对合成的NCDs进行表征。 实验结果显示合成的NCDs发强烈的蓝色荧光,呈现为球形或准球形,均匀分散,尺寸范围在1~5 nm;其表面含有丰富的COOH、OH和NH2等水溶性基团,最佳激发和发射波长分别为350和435 nm,且具有良好的发光稳定性。 通过测定,合成的NCDs的荧光量子产率高达29.19%。在pH=7.4的缓冲溶液中测定不同物质对NCDs的荧光影响,相同条件下发现只有阿莫西林能够对NCDs荧光进行明显猝灭,表明合成的NCDs可选择性的识别阿莫西林,通过NCDs的荧光强度变化构建一种可灵敏检测阿莫西林的传感器,检测线性范围为2.6~30 μmol/L,检出限为0.15 μmol/L。 相似文献
98.
An experimental study on yield stress of water-in-heavy crude oil emulsions has been carried out by using a HAAKE RS6000 Rheometer with a vane-type rotor. Several factors such as oil volume fraction, shear rate, temperature, and emulsifying agent on the yield stress of emulsions were investigated. Zero shear viscosity of heavy crude oil was 6000 mPas at 30°C, with a density 955 kg/m3. This study shows that the yield stress increases linearly with the increasing shear rate, and displays an exponential decay with increasing the temperature and oil volume fraction. Although the addition of emulsifying agent enhanced the stability of the emulsion, to some extent it also increased the yield stress, especially for the emulsions with high oil volume fractions. Therefore, to reduce the start-up force for the pipeline transport of water-in-heavy crude oil emulsions, the starting rate should be decreased, temperature increased, or oil volume fraction increased. These results are helpful to improve the transportation of water-in-heavy crude oil in pipeline. 相似文献
99.
This article discusses the effect of water fraction on the rheological properties of waxy crude oil emulsions including gel point, yield stress, viscosity, and thixotropy. The experimental results reveal that the rheological behaviors of the w/o emulsion samples all intensify with the increase of water volume fraction within 60%. Of more significance is that a correlation for w/o emulsions between yield stress and water volume fraction is put forward with an average relative error of 6.75%. In addition, some mainstream viscosity prediction models of w/o emulsions are evaluated, and Elgibaly model is the best-fit for the emulsions in this study. 相似文献
100.
TiO2-mediated photocatalysis is widely used in a variety of applications and products in the environmental and energy fields, including photoelectrochemical conversion, self-cleaning surfaces, and especially water purification systems. The dimensionality of the structure of a TiO2 material can affect its properties, functions, and more specifically, its photocatalytic performance. In this work, the photocatalytic inactivation of Gram-negative Escherichia coli using three photocatalysts, differing in their structure and other characteristics, was studied in a batch reactor under UVA light. The aim was to establish the disinfection efficiency of solid TiO2 compared with that of suspended catalysts, widely considered as reference cases for photocatalytic water disinfection. The bacterial inactivation profiles obtained showed that: (1) the photoinactivation was exclusively related to the quantity of photons retained per unit of treated volume, irrespective of the characteristics of the photocatalyst and the emitted light flux densities; (2) across the whole UV light range studied, each of the photocatalytic solids was able to achieve more than 2 log bacterial inactivation with less than 2 h UV irradiation; (3) none of the used catalysts achieved a total bacterial disinfection during the treatment time. For each of the catalysts the quantum yield has been assessed in terms of disinfection efficiency, the 2D material showed almost the same performance as those of suspended catalysts. This catalyst is promising for supported photocatalysis applications. 相似文献