全文获取类型
收费全文 | 19183篇 |
免费 | 3327篇 |
国内免费 | 2292篇 |
专业分类
化学 | 13517篇 |
晶体学 | 177篇 |
力学 | 1105篇 |
综合类 | 118篇 |
数学 | 2717篇 |
物理学 | 7168篇 |
出版年
2024年 | 58篇 |
2023年 | 412篇 |
2022年 | 538篇 |
2021年 | 646篇 |
2020年 | 844篇 |
2019年 | 757篇 |
2018年 | 671篇 |
2017年 | 628篇 |
2016年 | 960篇 |
2015年 | 901篇 |
2014年 | 1054篇 |
2013年 | 1426篇 |
2012年 | 1664篇 |
2011年 | 1753篇 |
2010年 | 1229篇 |
2009年 | 1100篇 |
2008年 | 1249篇 |
2007年 | 1105篇 |
2006年 | 980篇 |
2005年 | 883篇 |
2004年 | 704篇 |
2003年 | 614篇 |
2002年 | 683篇 |
2001年 | 542篇 |
2000年 | 437篇 |
1999年 | 429篇 |
1998年 | 342篇 |
1997年 | 311篇 |
1996年 | 330篇 |
1995年 | 291篇 |
1994年 | 232篇 |
1993年 | 163篇 |
1992年 | 165篇 |
1991年 | 158篇 |
1990年 | 123篇 |
1989年 | 94篇 |
1988年 | 54篇 |
1987年 | 56篇 |
1986年 | 71篇 |
1985年 | 47篇 |
1984年 | 32篇 |
1983年 | 26篇 |
1982年 | 15篇 |
1981年 | 11篇 |
1980年 | 4篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1969年 | 1篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
951.
Su-Wen Chen Bo-Long Guo Yu-Long Wang Yuan Li Li-Juan Song 《Journal of Radioanalytical and Nuclear Chemistry》2013,295(2):1435-1442
The mesoporous silicas (MCM-41 and MCM-48) are synthesized by hydrothermal method, which are characterized by XRD and BET techniques. The application of mesoporous silicas for the sorption of U(VI) from aqueous solution are studied by using batch technique under ambient condition. The effects of contact time, solid-to-liquid ratio (m/V), solution pH, ionic strength and temperature are determined, and the results indicate that the sorption of U(VI) to MCM-41 or MCM-48 are strongly dependent on pH values but independent of ionic strength. Compared with Langmuir model, the sorption isotherms can be simulated by Freundlich model well according to the high relative coefficients. The parameters for Langmuir and Freundlich sorption isotherms are calculated from the temperature at 298, 318 and 338 K, respectively, and the results suggest that the sorption of U(VI) on MCM-41 or MCM-48 is a spontaneous and exothermic process. In contrast to its sorption capacity for U(VI), MCM-48 is a suitable material for the preconcentration of U(VI) from large volumes of aqueous solutions. 相似文献
952.
利用微型流化床动力学分析仪研究了两相流条件下无烟煤粉的燃烧反应机理和动力学特征,并与热重法所得结果进行比较分析。结果表明,当温度大于850℃时,煤粉燃烧机理发生了变化,燃烧气态产物的生成比例也随之改变;当气速大于0.10 m/s时,气体扩散限制基本被消除,煤粉燃烧反应速率主要受界面化学反应控制;煤粉燃烧反应速率随着氧气分压的增大呈幂函数形式增长,且氧气分压对煤粉静置燃烧的影响更加显著。煤氧两相流燃烧的表观活化能与静置燃烧相比降低了49 kJ/mol,相同温度条件下两相流燃烧的界面化学反应阻力也明显小于热重法测试结果。 相似文献
953.
We report on fast-switching and high-efficient optical beam steering based on a polymerisable liquid crystal polarisation grating (PG) in combination with ferroelectric liquid crystal (FLC) phase shutter. The PG was fabricated in a convenient single-step holographic exposure process using photo-sensitive azo-dye material as alignment layer for liquid crystal (LC) director. A binary electro-optical FLC was employed for circular polarisation selection, which enables the electro-tunable steering of the combined system. The efficiency of 95.7% with 82 μs switching time is obtained for 1064 nm laser. This work provides a versatile candidate for non-mechanical beam steering devices. 相似文献
954.
The effect of the aggregation state of Pluronic copolymer (PEO100–PPO65–PEO100, F127) and the concentration of hydrophilic modified ibuprofen (Ibuprofen–PEG800, IP800) on the interaction between F127 and IP800 was systematically investigated by nuclear magnetic resonance, dynamic light scatter (DLS), surface tension, and freeze-fractured transmission electron microscopy. In the solution of F127 unimers (5 °C), F127 unimers tended to wrap around IP800 micelles, and the binding model of F127 unimers to IP800 micelles transferred from wrapping around to partly threading through with increasing IP800 concentration. The latter binding model was straightly confirmed by nuclear Overhauser enhancement spectroscopy. As the aggregation state of F127 is in the beginning of the micellization (20 °C), the addition of IP800 significantly promoted the micellization of F127 to form the F127/IP800 complex with F127 micelles as the skeleton called the F127–micelle complex. The sudden decrease of the size obtained from DLS stemmed from the disruption of the F127–micelle complex and accompanying rehydration of PPO which is weaker compared with refs. The amount of IP800 to disintegrate the F127–micelle complex increased in the F127–micelle-dominated solution (40 °C) compared to that at 20 °C. 相似文献
955.
Ming Hu Yansu Guo Huifang Chen Weisong Duan Chunyan Li 《Applied biochemistry and biotechnology》2013,170(2):301-319
Alternative splicing is a common strategy for creating functional diversities of proteins. While conventional identification of splice variants generally targets individual genes in amyotrophic lateral sclerosis, we present a novel exon-centric array that allows genome-wide identification of splice variants and concurrently provides analysis of gene expression. Compare 1 was asymptomatic SOD1G93A transgenic mice with nontransgenic littermates; compare 2 was symptomatic with asymptomatic transgenic mice. RT-PCR was performed to validate. Pathway and GO analysis were performed on abnormal genes. These findings could guide us to demonstrated the potential influence of mutant human CuZn-SOD1 and of splicing regulation in pathological processes. 相似文献
956.
Ayat A. Bozeya Abeer F. Al-Bawab Stig E. Friberg Lingling Ge Guo Rong 《Colloid and polymer science》2013,291(8):1933-1938
The system water–benzene–ethanol was used to illustrate the complexity of spontaneous emulsification, when water-poor emulsions are brought in contact with water. In the first case, an O/W emulsion located close to the plait point in the system was used. The aqueous phase in the emulsion was incompatible with water, and a strong spontaneous emulsification to an O/W between the two liquids took place in the water layer close to the interface between layers. In the second case, a W/O emulsion, also close to the plait point, was brought in contact with water. Now, the spontaneous emulsification between the water and the oil phase of the original emulsion to an O/W emulsion also took place in the water layer forming a distinct emulsion layer beneath the interface. 相似文献
957.
Wei Hu Lili Ma Changsheng Guo Jian Sha Xingwang Zhu 《International journal of environmental analytical chemistry》2013,93(6):698-713
An analytical method was developed for the simultaneous extraction and determination of 18 fluoroquinolones (FQs), tetracyclines (TCs) and sulfonamides (SAs) antibiotics from soils using solid phase extraction and liquid chromatography-tandem mass spectrometry. The soils were extracted by different solvents with the help of mechanical shaking and ultrasonic treatment at 59?kHz, followed by a strong anion exchange (SAX) cartridge to clean up soil samples and a hydrophilic lipophilic balance (HLB) cartridge as enrichment. The method was evaluated by testing the following variables: extraction solvents, the type of SPE cartridges, solvent volumes, initial spiking levels and soil types (silty clay loam and clay loam soils). The soil extraction method was validated using these two types of soils, representing two typical agricultural soils in northern China. For 2?g soil, the extraction steps with the mixture of potassium phosphate buffer and acetonitrile (ACN) (1/1, v/v, pH 3.2) provided satisfactory recoveries. In the clay loam soil, the recoveries of all the compounds were from 56% to 89% at the spiking level of 50?µg?kg?1 soil, and from 69% to 97% at the spiking level of 200?µg?kg?1 soil, respectively. Recoveries in silty clay loam soil were similar to that in clay loam. The method was successfully employed using soil samples collected from a farmland and afforestion area irrigated with sewage in northern China. The result indicates that trace antibiotics in sewage may accumulate in soil irrigated by river water containing sewage. 相似文献
958.
Xiaodong Wen Shengchun Yang Haizhu Zhang Xia Zhao Jie Guo 《International journal of environmental analytical chemistry》2013,93(12):1243-1253
As a new developed instrument, a portable tungsten coil electrothermal atomic absorption spectrometer (W-coil ET-AAS) was first coupled with surfactant assisted dispersive liquid–liquid microextraction (SA-DLLME) to improve its analytical performance and expand its applications in this work. SA-DLLME was very simple, rapid and the extraction efficiency was considerably improved by the effect of surfactant, which was suitable to be coupled with the portable instrument in field analysis. After SA-DLLME, concentrated chromium in organic phase was directly determined on W-coil atomiser. The influence factors relevant to SA-DLLME and instrumental conditions were studied systematically. Under the optimal conditions, the limit of detection (LOD) for Cr(VI) was 0.016 µg L?1, with sensitivity enhancement factor (EF) of 107. The relative standard deviation (RSD) for seven replicate measurements of 0.5 µg L?1 of Cr(VI) was 4.6%. The recoveries for the spiked samples were in the acceptable range of 96.8–104%. The rapid, simple and high effective method greatly improved the sensitivity of this portable spectrometer for the determination of Cr(VI) and was applied to the analysis of ultra-trace Cr(VI) in real and certified water samples with satisfactory results. 相似文献
959.
Three oxovanadium complexes, namely [VO(NOSAA)(bpy)] (1) (NOSAA = 2-hydroxy-5-nitrosalicylidene anthranilic acid, bpy = 2,2′-bipyridyl), [VO(NOSAA)(4,4′-dimebpy)] (2) (4,4′-dimebpy = 4,4′-dimethyl-2,2′- bipyridyl), and [VO(NOSAA)(phen)] (3) (phen = 1,10-phenanthroline), have been prepared and characterized. The binding modes and strengths of these complexes with calf thymus DNA (CT-DNA) were studied using various techniques. The chemical nuclease activities and photocleavage reactions of the complexes were also tested. All three complexes interact with CT-DNA through intercalative modes, and complex 3 possesses the largest binding affinity. All three complexes can efficiently cleave pBR322 DNA upon irradiation or under physiological conditions in the presence of H2O2, and complex 3 has the best cleaving ability. In vitro experimental results showed that the three complexes are cytotoxic against myeloma (Ag8.653) and gliomas (U251) cell lines and complex 3 again showed the highest efficacy. 相似文献
960.
Yuming Guo Feifei Wang Jie Zhang Lin Yang Xiaoman Shi Qilong Fang Xiaoming Ma 《Research on Chemical Intermediates》2013,39(6):2407-2415
Using three different amino acids (AAs) as organic matrices, including the highly nonpolar hydrophobic l-valine, the positively charged l-arginine and the less polar uncharged l-serine, calcium carbonate (CaCO3) with different morphologies and polymorphs were synthesized by a facile gas diffusion reaction based on biomimetic strategy. Compared with the control cubic calcite obtained in the absence of AAs, the product from l-valine was cubic calcite aggregates assembled by nano-platelets. The product from l-arginine was spherical vaterite aggregates assembled by spherical nanoparticles. The product from l-serine was the mixture of cubic calcite and spherical vaterite. The structures and properties of the side chains of the AAs exerted the significant effects on the nucleation and growth of the CaCO3. The formation mechanisms of the CaCO3 in the presence of AAs are preliminarily discussed. The results suggest that the polymorphs and morphologies of the inorganic nanomaterials might be easily adjusted through the careful selection of the organic matrices. 相似文献