全文获取类型
收费全文 | 1297篇 |
免费 | 48篇 |
国内免费 | 10篇 |
专业分类
化学 | 805篇 |
晶体学 | 5篇 |
力学 | 28篇 |
数学 | 215篇 |
物理学 | 302篇 |
出版年
2022年 | 17篇 |
2021年 | 10篇 |
2020年 | 20篇 |
2019年 | 12篇 |
2018年 | 17篇 |
2017年 | 11篇 |
2016年 | 25篇 |
2015年 | 17篇 |
2014年 | 32篇 |
2013年 | 56篇 |
2012年 | 48篇 |
2011年 | 74篇 |
2010年 | 34篇 |
2009年 | 25篇 |
2008年 | 43篇 |
2007年 | 72篇 |
2006年 | 74篇 |
2005年 | 43篇 |
2004年 | 52篇 |
2003年 | 46篇 |
2002年 | 20篇 |
2001年 | 17篇 |
2000年 | 13篇 |
1999年 | 10篇 |
1997年 | 13篇 |
1996年 | 16篇 |
1994年 | 10篇 |
1993年 | 11篇 |
1991年 | 11篇 |
1990年 | 14篇 |
1989年 | 12篇 |
1988年 | 14篇 |
1987年 | 9篇 |
1986年 | 13篇 |
1985年 | 13篇 |
1984年 | 10篇 |
1983年 | 15篇 |
1982年 | 17篇 |
1980年 | 13篇 |
1979年 | 16篇 |
1978年 | 14篇 |
1977年 | 20篇 |
1976年 | 21篇 |
1975年 | 22篇 |
1974年 | 12篇 |
1973年 | 12篇 |
1932年 | 19篇 |
1928年 | 10篇 |
1927年 | 9篇 |
1893年 | 13篇 |
排序方式: 共有1355条查询结果,搜索用时 15 毫秒
31.
A simple modification of the West?? extraction procedure for methylmercury and its determination by gas chromatography (GC) is presented. The cysteine clean-up step has been modified, with use of cysteine-impregnated paper instead of cysteine solution. Methylmercury bromide is extracted from the sample into toluene and is selectively adsorbed on the cysteine paper. Interfering compounds are washed from the paper with toluene. The isolated methylmercury is set free with sulphuric acid containing bromide, extracted into benzene and determined by GC. The modification of the extraction procedure results in good recovery and reproducibility for various biological and environmental samples, good sensitivity with a detection limit of 0.1 ng/g, avoidance of difficulties arising from emulsion formation, cleaner chromatograms, and faster analysis. It is particularly suitable for determination of low levels of MeHg. 相似文献
32.
De Silva V Woznichak MM Burns KL Grant KB May SW 《Journal of the American Chemical Society》2004,126(8):2409-2413
The biological role of selenium is a subject of intense current interest, and the antioxidant activity of selenoenzymes is now known to be dependent upon redox cycling of selenium within their active sites. Exogenously supplied or metabolically generated organoselenium compounds, capable of propagating a selenium redox cycle, might therefore supplement natural cellular defenses against the oxidizing agents generated during metabolism. We now report evidence that selenium redox cycling can enhance the protective effects of organoselenium compounds against oxidant-induced DNA damage. Phenylaminoethyl selenides were found to protect plasmid DNA from peroxynitrite-mediated damage by scavenging this powerful cellular oxidant and forming phenylaminoethyl selenoxides as the sole selenium-containing products. The redox properties of these organoselenoxide compounds were investigated, and the first redox potentials of selenoxides in the literature are reported here. Rate constants were determined for the reactions of the selenoxides with cellular reductants such as glutathione (GSH). These kinetic data were then used in a MatLab simulation, which showed the feasibility of selenium redox cycling by GSH in the presence of the cellular oxidant, peroxynitrite. Experiments were then carried out in which peroxynitrite-mediated plasmid DNA nick formation in the presence or absence of organoselenium compounds and GSH was monitored. The results demonstrate that GSH-mediated redox cycling of selenium enhances the protective effects of phenylaminoethyl selenides against peroxynitrite-induced DNA damage. 相似文献
33.
Gaunt AJ May I Helliwell M Richardson S 《Journal of the American Chemical Society》2002,124(45):13350-13351
The reaction between PW9O349- and NpO2+ has yielded the first structurally characterized neptunyl(V) polyoxometalate complex, [Na2(NpO2)2(A-PW9O34)2]14-. This complex is isostructural with the uranyl(VI) analogue, and there is also spectroscopic evidence for its existence in solution. The complex is readily extracted into toluene, and this may have significance in the sequestering and/or separation of the neptunyl ion in terms of nuclear waste management. 相似文献
34.
The electrochemical and spectroelectrochemical properties of niobium(V) and the Nb(3)O(2)(SO(4))(6)(H(2)O)(3)(5-) cluster in sulfuric acid and methanesulfonic acid were investigated using cyclic voltammetry, constant potential electrolysis, and spectroelectrochemistry. These chemical systems were suitable to probe the formation of "Nb(3)O(2)" core trinuclear clusters. In 9 M H(2)SO(4) the cluster Nb(3)O(2)(SO(4))(6)(H(2)O)(3)(5-) exhibited a reversible 1-electron reduction peak at E(pc) = -1.30 V vs Hg/Hg(2)SO(4) electrode, as well as a 4-electron irreversible oxidation peak at E(pa) = -0.45 V. Controlled potential reduction at E = -1.40 V produced the green Nb(3.33+) cluster anion Nb(3)O(2)(SO(4))(6)(H(2)O)(3)(6-). In 12 M H(2)SO(4) Nb(V) displayed two reduction peaks at E(pc) = -1.15 V and E(pc) = -1.30 V. It was determined that the first process involves a quasi-reversible 2-electron reduction. After reduction of Nb(V) to Nb(III) the following chemical step involves formation of [Nb(III)](2) dimer, which further reacts with Nb(V) to produce the Nb(3)O(2)(SO(4))(6(H(2)O)(3)(5-) cluster (ECC process). The second reduction peak at E(pc) = -1.30 V corresponds to further 2-electron reduction of Nb(III) to Nb(I). The electrogenerated Nb(I) species also chemically reacts with starting material Nb(V) to produce additional [Nb(III)](2). In 5 M H(2)SO(4), the rate of the second chemical step in the ECC process is relatively slower and reduction of Nb(V) at E = -1.45 V/-1.2 V produces a mixture of Nb(3)O(2)(SO(4))(6)(H(2)O)(3)(5-) and [Nb(III)](2) dimer. [Nb(III)](2) can be selectively oxidized by two 2-electron steps at E = -0.65 V to Nb(V). However, if the oxidation is performed at E = -0.86 V, the product is Nb(3)O(2)(SO(4))(6)(H(2)O)(3)(5-). A double potential pulse electrolysis waveform was developed to direct the reduction of Nb(V) toward selective formation of the Nb(3)O(2)(SO(4))(6)(H(2)O)(3)(5-) cluster. Proper application of dc-voltage pulses alternating between E(1) = -1.45 V and E(2) = -0.86 V yields only the target trinuclear cluster. Analogous double potential pulse electrolysis of Nb(V) in methanesulfonic acid generates the "Nb(3)O(2)" core cluster Nb(3)O(2)(CH(3)SO(3))(6)(H(2)O)(3)(+). 相似文献
35.
Solutions of selenium(IV) standards with different acid matrices were stored in containers constructed of boosilicate glass, conventional polyethylene, and fluorinated ethylenepropylene (teflon FEP). After 50 days of storage in FEP, there were highly significant losses of Se(IV) from standards in either 5% HCl/5% H2SO4, or 5% H2SO4. Increasing the hydrochloric acid concentration, e.g., 15% HCl/5% H2SO4, greatly reduced this loss. Addition of selenium-75 (selenate-free) indicated that the losses did not result from physical adsorption onto container surfaces. It is shown that the losses were caused by oxidation of Se(IV) to Se(VI). 相似文献
36.
Bhatt AI May I Volkovich VA Collison D Helliwell M Polovov IB Lewin RG 《Inorganic chemistry》2005,44(14):4934-4940
The reduction of selected lanthanide cations to the zerovalent state in the room-temperature ionic liquid [Me3N(n)Bu][TFSI] is reported (where TFSI = bistriflimide, [N(SO2CF3)2]-). The lanthanide cations were introduced to the melt as the TFSI hydrate complexes [Ln(TFSI)3(H2O)3] (where Ln = La(III), Sm(III) or Eu(III)). The lanthanum compound [La(TFSI)3(H2O)3] has been crystallographically characterized, revealing the first structurally characterized f-element TFSI complex. The lanthanide in all three complexes was shown to be reducible to the metallic state in [Me3N(n)Bu][TFSI]. For both the Eu and Sm complexes, reduction to the metallic state was achieved via divalent species, and there was an additional observation of the electrodeposition of Eu metal. 相似文献
37.
A summary is given about hitherto studied and applied versions of sample pretreatment and separation techniques of89Sr and90Sr from fission and activation products in plate-out experiments for security studies of graphite moderated gas cooled high temperature pebble bed reactors. The latest and most succesfully applied version with a combined double-column system is described and its performance and results are discussed in detail. 相似文献
38.
Subpicosecond pulse generation has been examined in synchronously pumped mode-locked ring dye laser systems. These include hybrid and composite absorber/gain media arrangements as well as a simple synchronous cavity. The shortest pulses recorded were 0.3 ps for the hybrid system, and this has been shown to be critically dependent on the positioning of the absorber jet in the centre of the cavity to better than 50 m. Stable operation for subpicosecond pulse generation has been achieved in the ring configuration with greater wavelength tunability and higher average power conversion efficiency than with conventional cavity arrangements. 相似文献
39.
A protocol has been developed for the reliable titration of aqueous sulfite solutions which minimizes photodecomposition effects.
This procedure has been used to measure the protonation constants of the sulfite ion in aqueous solution by glass electrode
potentiometry at 25.0‡C and ionic strengths (I) from 0.1 to 5.0M in NaCI media and atI = 1.0M in KC1 and Me4NCl media. These measurements provided evidence of weak but significant ionpairing between SO2/3 -and Na+ with a formation constant of logK
Na = 0.431 in 1.0M Me4NCl. This was in very good agreement with the value logK
Na = 0.410 measured directly by Na+ ion-selective electrode potentiometry. Evidence is also presented for an extremely weak association of K+ and SO
2
3
-with logK
k
= 0.22 in 1.0M Me4NCl. 相似文献
40.
Kremser L Brückner A Heger A Grunert T Buchacher A Josic D Allmaier G Rizzi A 《Electrophoresis》2003,24(24):4282-4290
The isoforms distribution of the glycoprotein antithrombin III (ATIII) derived from human plasma was investigated by means of isoelectric focusing (IEF) in polyacrylamide gels with immobilized pH gradients (IPG) and two-dimensional gel electrophoresis (2-DE) as well as capillary electrophoretic methods. It turned out that the presence of high concentrations of chaotropics (urea, thiourea) and zwitterionic detergents (3-[(3-cholamidepropyl)dimethylammonio]-1-propanesulfonate (CHAPS)) was decisive for attaining good resolution of the protein isoforms. Resolution by IPG-IEF was obtained with excellent reproducibility and pI differences down to 0.01 pH units could be distinguished. ATIII-alpha and ATIII-beta-fractions preseparated by heparin affinity chromatography showed an analogous but shifted spot pattern consisting each of one major and three minor isoforms. The main isoforms of ATIII-alpha and ATIII-beta exhibit pI values of 5.18 and 5.32, respectively, both values determined in the presence of high concentrations of urea. The pI difference of 0.14 pH units correspond to the effect of two sialic acids absent in ATIII-beta. The formation and occurrence of ATIII dimers and trimers turned out to be dependent on the sample preparation. The results obtained by 2-DE were compared with those of capillary zone electrophoresis (CZE) and capillary IEF (CIEF). Quantitative analysis regarding the CZE separated isoforms of plasma derived ATIII yielded a content of about 70% ATIII-alpha main isoform and about 6.6% of ATIII-beta. The pI values of ATIII determined by CIEF with internal calibration were in fair agreement with the pI values of the main isoforms achieved with 2-DE. 相似文献