全文获取类型
收费全文 | 27785篇 |
免费 | 1328篇 |
国内免费 | 436篇 |
专业分类
化学 | 14759篇 |
晶体学 | 132篇 |
力学 | 1858篇 |
数学 | 5573篇 |
物理学 | 7227篇 |
出版年
2024年 | 27篇 |
2023年 | 229篇 |
2022年 | 296篇 |
2021年 | 322篇 |
2020年 | 1089篇 |
2019年 | 520篇 |
2018年 | 1887篇 |
2017年 | 1915篇 |
2016年 | 1603篇 |
2015年 | 1059篇 |
2014年 | 1557篇 |
2013年 | 3070篇 |
2012年 | 2138篇 |
2011年 | 2342篇 |
2010年 | 1863篇 |
2009年 | 1741篇 |
2008年 | 1375篇 |
2007年 | 1210篇 |
2006年 | 885篇 |
2005年 | 689篇 |
2004年 | 520篇 |
2003年 | 483篇 |
2002年 | 389篇 |
2001年 | 509篇 |
2000年 | 221篇 |
1999年 | 453篇 |
1998年 | 242篇 |
1997年 | 181篇 |
1996年 | 128篇 |
1995年 | 124篇 |
1994年 | 74篇 |
1993年 | 64篇 |
1992年 | 65篇 |
1991年 | 50篇 |
1990年 | 19篇 |
1987年 | 4篇 |
1986年 | 9篇 |
1985年 | 4篇 |
1984年 | 8篇 |
1982年 | 8篇 |
1981年 | 12篇 |
1980年 | 18篇 |
1979年 | 33篇 |
1978年 | 24篇 |
1977年 | 16篇 |
1976年 | 25篇 |
1975年 | 13篇 |
1974年 | 8篇 |
1973年 | 10篇 |
1972年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
The global epidemic owing to COVID-19 has generated awareness to ensuring best practices for avoiding the microorganism spread. Indeed, because of the increase in infections caused by bacteria and viruses such as SARS-CoV-2, the global demand for antimicrobial materials is growing. New technologies by using polymeric systems are of great interest. Virus transmission by contaminated surfaces leads to the spread of infectious diseases, so antimicrobial coatings are significant in this regard. Moreover, antimicrobial food packaging is beneficial to prevent the spread of microorganisms during food processing and transportation. Furthermore, antimicrobial textiles show an effective role. We aim to provide a review of prepared antimicrobial polymeric materials for use in coating, food packaging, and textile during the COVID-19 pandemic and after pandemic. 相似文献
32.
Zinc polyoxometalate on activated carbon: an efficient catalyst for selective oxidation of alcohols with hydrogen peroxide 下载免费PDF全文
[PW11ZnO39]5? was immobilized on activated carbon and characterized using Fourier transform infrared, X‐ray diffraction, Brunauer–Emmett–Teller and elemental analysis techniques. Effective oxidation of various alcohols with hydrogen peroxide was performed in the presence of this catalyst. Easy separation of the catalyst from the reaction mixture, cheapness, high activity and selectivity, stability as well as retained activity in subsequent catalytic cycles make this supported catalyst suitable for small‐scale synthesis. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
34.
The present study was designed to evaluate the contents of different antioxidants compounds and their antioxidant activities in Jalopeno peppers (Capsicum annuum) cultivars (El Dorido, Grande, Tula, Sayula and El Rey) extracts. Free radical scavenging activity of Grande was recorded as high as 87% followed by El Dorido (83%). Results of reducing power (Fe3+ to Fe2+) showed that Grande (0.85%) and El Dorido (0.81%) fruit extract absorbance value were close to synthetic antioxidant BHT (0. 97%) obtained at100 μg/mL. The results showed that total phenolic content of El Dorido and Grande were significantly higher compared to other Jalapeno pepper. Results indicated strong and positive correlation between antioxidant activity and carotenoids content (r = 0.75), vitamin C (r = 0.78) and total capsaicinoids (r = 0.84), respectively. The results of the antioxidant activity assays showed that the El Dorido and Grande had strongest antioxidant activity compared to other peppers cultivars in this study. 相似文献
35.
36.
37.
38.
In view of immense importance of silylenes and the fact that their properties undergo significant changes on substitution with halogens, here, we have used B3LYP/6-311++G** level of theory to access the effects of 1–4 halogens (X = F, Cl, Br, and I) on four unprecedented sets of cyclopentasilylene-2,4-dienes; with the following formulas: SiC4H3X ( 1 X ), SiC4H2X2 ( 2 X ), SiC4HX3 ( 3 X ), and SiC4X4 ( 4 X ). In going down from F to I, the singlet (s)-triplet (t) energy gap (ΔEs-t, a possible indication of stability), and band gap (ΔEH-L) decrease while nucleophilicity (N), chemical potential (μ), and proton affinity (PA) increase. The overall order of N, μ, and PA for each X is 2 X > 1 X > 3 X > 4 X . Precedence of 2 X over 1 X is attributed to the symmetric cross conjugation in the former. The highest and lowest N are shown by 2 I and 4 F . The trend of divalent angle () for each X is 4 X > 1 X > 3 X > 2 X . The results show that in going from electron withdrawing groups (EWGs) to electron donating groups (EDGs), the ΔEs-t and ΔEH-L decrease while N, μ, and PA increase. Also, rather high N of our scrutinized silylenes may suggest new promising ligands in organometallic chemistry. 相似文献
39.
《Solid State Ionics》2006,177(26-32):2687-2690
High pressure conductivity measurements have been carried out on P(EO)20LiBETI from 295 K to 368 K. The decrease of electrical conductivity with pressure is larger in the partially crystalline phase (low temperature and low pressure or high temperature and high pressure) than in the fully amorphous phase (high temperature and low pressure). It is found that if the phase transition is approached from the crystalline phase (decreasing pressure), the pressure of the phase transition varies from 0 to 0.23 GPa as the temperature increases from 336 K to 358 K. The shift of the phase transition temperature with pressure is approximately the same as the shift of the glass transition temperature with pressure for pure PEO. This can be understood in terms of the defect diffusion model. If the material is above 336 K and is in the fully amorphous phase, after pressure is increased above the critical pressure, the material remains in the amorphous phase for extended periods of time before transforming to the partially crystalline phase. This is reminiscent of a super-pressed state but may be an indication of slow crystallization kinetics. 相似文献
40.
A simple, precise and accurate method is proposed for rapid determination of trace amounts of hydroxylamine based on the reaction of hydroxylamine with iodate in acidic media. The reaction of neutral red by the produced nitrite ion was used to monitor the reaction spectrophotometrically at 525 nm by a fixed time method. Hydroxylamine in the range of 0.0400-1.200 microg mL(-1) could be determined. The relative standard deviation for 10 determinations of 0.500 microg mL(-1) hydroxylamine was 1.81% and the limit of detection was 0.010 microg mL(-1). The proposed method was applied to the determination of hydroxylamine in water samples with satisfactory results. 相似文献