全文获取类型
收费全文 | 2993篇 |
免费 | 206篇 |
国内免费 | 155篇 |
专业分类
化学 | 2048篇 |
晶体学 | 20篇 |
力学 | 9篇 |
综合类 | 10篇 |
数学 | 23篇 |
物理学 | 1244篇 |
出版年
2023年 | 16篇 |
2022年 | 26篇 |
2021年 | 34篇 |
2020年 | 60篇 |
2019年 | 42篇 |
2018年 | 55篇 |
2017年 | 55篇 |
2016年 | 71篇 |
2015年 | 82篇 |
2014年 | 95篇 |
2013年 | 127篇 |
2012年 | 178篇 |
2011年 | 127篇 |
2010年 | 115篇 |
2009年 | 133篇 |
2008年 | 170篇 |
2007年 | 283篇 |
2006年 | 170篇 |
2005年 | 139篇 |
2004年 | 127篇 |
2003年 | 141篇 |
2002年 | 137篇 |
2001年 | 118篇 |
2000年 | 103篇 |
1999年 | 103篇 |
1998年 | 85篇 |
1997年 | 51篇 |
1996年 | 44篇 |
1995年 | 39篇 |
1994年 | 33篇 |
1993年 | 36篇 |
1992年 | 40篇 |
1991年 | 33篇 |
1990年 | 26篇 |
1989年 | 28篇 |
1988年 | 26篇 |
1987年 | 9篇 |
1986年 | 14篇 |
1985年 | 19篇 |
1984年 | 15篇 |
1983年 | 15篇 |
1982年 | 13篇 |
1981年 | 13篇 |
1980年 | 15篇 |
1979年 | 15篇 |
1978年 | 10篇 |
1976年 | 16篇 |
1975年 | 24篇 |
1974年 | 8篇 |
1973年 | 11篇 |
排序方式: 共有3354条查询结果,搜索用时 15 毫秒
101.
102.
利用合成的巯基含量为2.2~2.5%的高巯基粉状巯基棉,对水中超痕量汞进行富集测定。流速可达100mL·min~(-1),检出限为0.5×10~(-12),加标回收率为90~104%。 相似文献
103.
Structures of Bis(trifluoromethyl)halogeno and thiocyanato Mercurates, [Hg(CF3)2X]— (X = Br, I, SCN), and a Comparison of the Structural Parameters of the CF3 Groups [(18‐C‐6)K]2[Hg(CF3)2SCN]2 (1) and [P(CH3)(C6H5)3]2[Hg(CF3)2X]2 (X = Br (2) , I (3) ) are prepared and their crystal structures are determined. [(18‐C‐6)K]2[Hg(CF3)2SCN]2 (1) crystallizes in the monoclinic space group P21/c with Z = 2, [P(CH3)(C6H5)3]2[Hg(CF3)2Br]2 (2) in the monoclinic space group P21/n with Z = 2 and [P(CH3)(C6H5)3]2[Hg(CF3)2I]2 (3) in the triclinic space group P1¯ with Z = 1. In the solid state the three compounds form dimeric anions with planar Hg2X2 rings. The structural parameters of the Hg(CF3)2 units in the till now known bis(trifluoromethyl)halogeno mercurates are compared. In all compounds one nearly symmetric and one distorted CF3 group exist. The largest differences of the C—F bond lengths is found for [(18‐C‐6)K][Hg(CF3)2I]. This can be regarded as the experimental evidence for the properties of trifluoromethyl mercury compounds to act as excellent difluorocarbene sources in the presence of alkali iodides. 相似文献
104.
Blanca San Vicente de la RivaJosé M. Costa-Fernández Wei Jun JinRosario Pereiro Alfredo Sanz-Medel 《Analytica chimica acta》2002,455(2):179-186
A novel method has been developed for the sensitive determination of mercury in aqueous media by room temperature phosphorescence (RTP). The measurement principle is based on the energy transfer (ET) from a phosphor molecule (acting as a donor) to a Hg-sensitive dye (acceptor). To our acknowledgment this is the first RTP method for mercury measurement developed so far. α-Bromonaphthalene (BrN) was selected as the phosphorescent donor molecule (BrN can produce significant RTP emission in aqueous media in a β-cyclodextrin rigid microenvironment without deoxygenation).The absorption spectrum of the complex formed between mercury and the dithizone dye possesses a desirable spectral overlap with the RTP emission spectrum of the donor (BrN), giving rise to a nonradiative ET from the phosphor molecules to the mercury complex. An increase in the concentration of Hg(II) causes an increase on the concentration of the dithizone complex (acceptor) with the subsequent increase of the absorbance and, therefore, resulting in a decrease of the RTP emission. Both, RTP intensities and triplet lifetimes of the BrN decreased with increases on the Hg(II) concentration.Possible interferences present in natural waters, including different cations and anions, which could affect the analytical response, were evaluated and the analytical performance characteristics investigated. The use of phosphorescence measurements (low background noise signals) resulted in an improvement on the sensitivity of the Hg(II) detection higher than five times as compared to the molecular absorption spectrophotometric method for Hg(II) detection based on dithizone as Hg-indicator. A detection limit (D.L.) of 14 ng ml−1 of Hg(II) was obtained by RTP with a precision of ±4.8% for five replicates of 300 ng ml−1 of Hg(II). The usefulness of the method was successfully evaluated by the determination of Hg(II) in spiked natural water samples. 相似文献
105.
4-Me2NC6H4HgOH was prepared from 4-Me2NC6H4HgOAc. Full characterisation showed that it crystallises as discrete molecules, the first example of a true organomercury hydroxide in the solid state. The structures of 4-Me2NC6H4HgOAc and (4-Me2NC6H4)2Hg are also discussed. 相似文献
106.
Óscar López 《Tetrahedron》2005,61(38):9058-9069
An efficient method for the preparation of urea-bridged cyclodextrins using triphosgene in the isocyanation step in an aqueous two-phase system is reported. Per-O-acetylated glycopyranosylamines and 2-amino-2-deoxy-α and β-d-glucose were also transformed into the corresponding isocyanates using either an aqueous two-phase or an anhydrous dichloromethane medium, and converted in situ into ureas. An alternative method for the preparation of sugar-derived ureas consisting of desulfurization of sugar thioureas with mercury oxide is also presented. 相似文献
107.
J. Becnel C. Falgeust T. Cavalier K. Gauthreaux F. Landry M. Blanchard M.J. Beck J.N. Beck 《Microchemical Journal》2004,78(2):205-210
To determine if a correlation exists between mercury concentrations in lichens and wood from tree cores, tree core and lichen samples were taken from sites under mercury advisories and analyzed for mercury using a Bacharach 50B Mercury Analyzer System employing the Manual Cold Vapor Atomic Absorption EPA method 7471A. Wood from tree cores was dated using tree ring data. Wood samples from the cores were analyzed for various depths in the tree core (periods of time) to determine if variations occur throughout the cores. Preliminary data indicate that a strong correlation exists between mercury concentrations in tree core and lichen samples. Samples were analyzed for the 6-month period of summer 2003–winter 2004. A correlation coefficient of 0.882 was found between the tree core and lichen data sets. The derived correlations were used to estimate concentrations of mercury in tree rings for sites in lichen samples which were previously analyzed. These predicted values compared favorably to recently determined concentrations of Hg in tree cores. The strong correlation between lichen and tree core Hg concentrations suggests similar uptake mechanism for the two types of biota. 相似文献
108.
Present proteomics and biomedicine require sensitive analytical methods for all proteins. Recent progress in electrochemical analysis of peptides and proteins based on their intrinsic electroactivity is reviewed. Tyrosine and/or tryptophan‐containing proteins are oxidizable at carbon electrodes. At mercury electrodes all peptides and proteins (about 13 peptides and >25 proteins were tested) produce chronopotentiometric peak H at nanomolar concentrations. This peak is sensitive to changes in protein structure. Microliter sample volumes are sufficient for the analysis. Electrochemical methods can be used in studies of nucleic acid‐protein interactions and can be applied in biomedicine. Examples of such applications in neurogenerative diseases and cancer are presented. 相似文献
109.
《Electroanalysis》2004,16(16):1311-1317
The determination of some toxic metals by stripping chronopotentiometry with a supporting solution having an unconventional composition has been investigated with the aim of using such components in disposable measuring cells preservable in dry state and quite ready for use, only needing addition of a small volume of sample. The new supporting solution is prepared with a solid strong acid, p‐toluenesulfonic acid, in the place of the inorganic acids commonly used to improve the cation availability. The other components are, as usual, sodium chloride, which fixes the potential of the screen‐printed silver – silver chloride reference electrode, and mercury(II) chloride as the plating agent. This supporting solution has been tested in batch measurements with the mercury film glassy carbon electrode as well as with screen‐printed carbon‐ink electrodes, either with mercury film or bare. The physical shape of the mercury layer electrolytically deposited on screen‐printed carbon‐ink electrodes from a supporting solution containing 0.1 M p‐toluenesulfonic acid and 0.1 M sodium chloride has been investigated by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) microanalysis. In chronopotentiometric stripping p‐toluenesulfonic acid performs as well as the usual inorganic strong acids, particularly in terms of sensitivity. At 0.1 mol dm?3 it proved very suitable for the determination of toxic metals, in particular lead(II), at levels down to a few μg dm?3. The overall results appear promising and can open new avenues for preparing disposable cells for on‐field stripping chronopotentiometric determination of toxic metals. 相似文献
110.
Coordination compounds of mercury(II) chloride, bromide, cyanide and thiocyanate with N-methylnicotinamide, a potentially
bidentate ligand, have been prepared. The complexesisolated have 1∶1 (metal:ligand)stoichiometry. Molecular weight measurements
in molten camphor indicate that the mercury (II) chloride and bromide complexes are monomeric. Based on conductance values,
molecular weight determinations and infrared spectral data, it is inferred that in the solid state in all these complexes
the metal ion has a coordination number three and is bonded to the N-methylnicotinamide via its pyridine ring nitrogen, and
is terminally bonded to the halogen/pseudohalogens. 相似文献