首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1585篇
  免费   81篇
  国内免费   3篇
化学   1422篇
晶体学   7篇
力学   8篇
数学   95篇
物理学   137篇
  2024年   3篇
  2023年   10篇
  2022年   63篇
  2021年   78篇
  2020年   47篇
  2019年   49篇
  2018年   35篇
  2017年   43篇
  2016年   74篇
  2015年   66篇
  2014年   64篇
  2013年   105篇
  2012年   96篇
  2011年   129篇
  2010年   84篇
  2009年   72篇
  2008年   109篇
  2007年   87篇
  2006年   59篇
  2005年   78篇
  2004年   58篇
  2003年   40篇
  2002年   32篇
  2001年   14篇
  2000年   18篇
  1999年   5篇
  1998年   11篇
  1997年   14篇
  1996年   11篇
  1995年   13篇
  1994年   8篇
  1993年   7篇
  1992年   6篇
  1991年   5篇
  1990年   6篇
  1989年   9篇
  1988年   10篇
  1987年   6篇
  1986年   2篇
  1985年   4篇
  1984年   2篇
  1983年   2篇
  1982年   2篇
  1981年   3篇
  1980年   6篇
  1979年   7篇
  1978年   5篇
  1977年   4篇
  1976年   2篇
  1973年   3篇
排序方式: 共有1669条查询结果,搜索用时 15 毫秒
951.
The complex of Fe(II) ions of general formula [Fe L 2](BF4)2 with triphenylamine-hydrazone ligand L has been synthesized and characterized. Oxidative electropolymerization of the complex proceeded smoothly on the working electrode producing a homogenous thin film of metallopolymer. The film thickness and morphology of the layer was investigated by microscopy techniques such as scanning electron microscopy and atomic force microscopy, and the composition of the film was confirmed by X-ray photoelectron spectroscopy analysis. It was found that fifty successive oxidation/reduction cycles resulted in a 120 nm thick film on the electrode surface. The metallopolymer was also characterized using cyclic voltammetry and spectroelectrochemical methods. The film was found to change its color from yellow to green-blue, exhibit high change in transmittance of 60 % at 770 nm, and possess good electrochemical stability during 375 cycles of switching of the potential between −0.1 V and +1.5 V, owing to the presence of metal ions that link two ligand molecules resulting in formation of highly cross-linked film. The switching times (coloration and bleaching) were calculated to be 34.2 s and 7.3 s, respectively. Coloration efficiency of the formed film of polymeric complex was found to be 144 cm2 C−1.  相似文献   
952.
953.
A new dichromium(III) cobalt(II) diphosphate(V) of the formula CoCr2(P2O7)2 was detected in the Co3Cr4(PO4)6–Cr(PO3)3 system. The new compound was obtained as a result of high-temperature solid-state reactions between CoCO3, Cr2O3 and (NH4)2HPO4 as well as between Cr(PO3)3 and Co3Cr4(PO4)6. CoCr2(P2O7)2 was characterized using XRD, DTA and IR methods. Results demonstrated that CoCr2(P2O7)2 crystallizes in the triclinic system and its unit cell parameters were calculated. Its infrared spectrum was presented. CoCr2(P2O7)2 melts incongruently at 1270±10 °C with a formation of solid α-CrPO4. The compound Co3Cr4(PO4)6, component of the system under study, was obtained for the first time as a pure phase. Its thermal stability was also investigated. Co3Cr4(PO4)6 is stable in air up to 1410 ± 20 °C.  相似文献   
954.
The redox chemistry of [Cp*Fe(η5-As5)] ( 1 , Cp*=η5-C5Me5) has been investigated by cyclic voltammetry, revealing a redox behavior similar to that of its lighter congener [Cp*Fe(η5-P5)]. However, the subsequent chemical reduction of 1 by KH led to the formation of a mixture of novel Asn scaffolds with n up to 18 that are stabilized only by [Cp*Fe] fragments. These include the arsenic-poor triple-decker complex [K(dme)2][{Cp*Fe(μ,η2:2-As2)}2] ( 2 ) and the arsenic-rich complexes [K(dme)3]2[(Cp*Fe)2(μ,η4:4-As10)] ( 3 ), [K(dme)2]2[(Cp*Fe)2(μ,η2:2:2:2-As14)] ( 4 ), and [K(dme)3]2[(Cp*Fe)444:3:3:2:2:1:1-As18)] ( 5 ). Compound 4 and the polyarsenide complex 5 are the largest anionic Asn ligand complexes reported thus far. Complexes 2 – 5 were characterized by single-crystal X-ray diffraction, 1H NMR spectroscopy, EPR spectroscopy ( 2 ), and mass spectrometry. Furthermore, DFT calculations showed that the intermediate [Cp*Fe(η5-As5)], which is presumably formed first, undergoes fast dimerization to the dianion [(Cp*Fe)2(μ,η4:4-As10)]2−.  相似文献   
955.
Let {Z u = ((εu, i, j))p×n} be random matrices where {εu, i, j} are independently distributed. Suppose {A i }, {B i } are non-random matrices of order p × p and n × n respectively. Consider all p × p random matrix polynomials \(P = \prod\nolimits_{i = 1}^{k_l } {\left( {n^{ - 1} A_{t_i } Z_{j_i } B_{s_i } Z_{j_i }^* } \right)A_{t_{k_l + 1} } }\). We show that under appropriate conditions on the above matrices, the elements of the non-commutative *-probability space Span {P} with state p?1ETr converge. As a by-product, we also show that the limiting spectral distribution of any self-adjoint polynomial in Span{P} exists almost surely.  相似文献   
956.
The health benefits of green tea are associated with its high catechin content. In scientific studies, green tea is often prepared with deionized water. However, casual consumers will simply use their local tap water, which differs in alkalinity and mineral content depending on the region. To assess the effect of water hardness on catechin and caffeine content, green tea infusions were prepared with synthetic freshwater in five different hardness levels, a sodium bicarbonate solution, a mineral salt solution, and deionized water. HPLC analysis was performed with a superficially porous pentafluorophenyl column. As water hardness increased, total catechin yield decreased. This was mostly due to the autoxidation of epigallocatechin (EGC) and epigallocatechin gallate (EGCG). Epicatechin (EC), epicatechin gallate (ECG), and caffeine showed greater chemical stability. Autoxidation was promoted by alkaline conditions and resulted in the browning of the green tea infusions. High levels of alkaline sodium bicarbonate found in hard water can render some tap waters unsuitable for green tea preparation.  相似文献   
957.
Epilobium angustifolium L. is applied as an antiseptic agent in the treatment of skin diseases. However, there is a lack of information on human skin penetration of active ingredients with antioxidative potential. It seems crucial because bacterial infections of skin and subcutaneous tissue are common and partly depend on oxidative stress. Therefore, we evaluated in vitro human skin penetration of fireweed ethanol-water extracts (FEEs) by determining antioxidant activity of these extracts before and after penetration study using 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and Folin–Ciocalteu methods. Microbiological tests of extracts were done. The qualitative and quantitative evaluation was performed using gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC-UV) methods. The in vitro human skin penetration using the Franz diffusion chamber was assessed. The high antioxidant activity of FEEs was found. Gallic acid (GA), chlorogenic acid (ChA), 3,4-dihydroxybenzoic acid (3,4-DHB), 4-hydroxybenzoic acid (4-HB), and caffeic acid (CA) were identified in the extracts. The antibacterial activities were found against Serratia lutea, S. marcescens, Bacillus subtilis, B. pseudomycoides, and B. thuringiensis and next Enterococcus faecalis, E. faecium, Streptococcus pneumoniae, Pseudomonas aeruginosa, and P. fluorescens strains. In vitro penetration studies showed the penetration of some phenolic acids and their accumulation in the skin. Our results confirm the importance of skin penetration studies to guarantee the efficacy of formulations containing E. angustifolium extracts.  相似文献   
958.
The study compared the content of eight phenolic acids and four flavonoids and the antioxidant activity of six Polish varietal honeys. An attempt was also made to determine the correlations between the antioxidant parameters of the honeys and their polyphenol profile using principal component analysis. Total phenolic content (TPC), total flavonoid content (TFC), antioxidant activity (ABTS) and reduction capacity (FRAP) were determined spectrophotometrically, and the phenolic compounds were determined using high-performance liquid chromatography (HPLC). The buckwheat honeys showed the strongest antioxidant activity, most likely because they had the highest concentrations of total phenols, total flavonoids, p-hydroxybenzoic acid, caffeic acid, p-coumaric acid, vanillic acid and chrysin. The principal component analysis (PCA) of the data showed significant relationships between the botanic origin of the honey, the total content of phenolic compounds and flavonoids and the antioxidant activity of the six Polish varietal honeys. The strongest, significant correlations were shown for parameters of antioxidant activity and TPC, TFC, p-hydroxybenzoic acid, caffeic acid and p-coumaric acid. Analysis of four principal components (explaining 86.9% of the total variance), as a classification tool, confirmed the distinctiveness of the Polish honeys in terms of their antioxidant activity and content of phenolic compounds.  相似文献   
959.
Plasma Chemistry and Plasma Processing - Hydroxyapatite (HA) coatings were deposited on high purity titanium substrate by atmospheric plasma spraying (APS) with high electric energy input....  相似文献   
960.
Phytochelatins (PCs) play an important role in the detoxification and homeostasis processes in plants, since xenobiotics complexed by sulfhydryl groups are less toxic than the free ions. This paper describes studies related to PCs synthesis in Sinapis alba L. exposed to As and Tl salts, and in particular to the identification of the PC complexes that are formed in white mustard tissues. To reach the goal, two analytical methods were applied – high-performance liquid chromatography with fluorescent detection analysis after derivatisation of thiol compounds and high-performance liquid chromatography with electrospray ionization mass spectrometry (HPLC ESI MS). The study confirmed the synthesis of PCs in all plant organs under the influence of As salt. Moreover two As-PC complexes were identified: As-PC3 and As-PC4. While in the case of Tl – although that element strongly influenced the growth of Sinapis alba L. and to a great extent was transported to above-ground organs – no PC complexes or free PCs were found in plants exposed to Tl salt. Although both elements exist in mustard plant tissues at their trivalent oxidation state, showing high affinity to thiol groups, a defence mechanism associated with the synthesis of PC occurred only in case of arsenic.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号