全文获取类型
收费全文 | 6823篇 |
免费 | 232篇 |
国内免费 | 21篇 |
专业分类
化学 | 4468篇 |
晶体学 | 68篇 |
力学 | 126篇 |
数学 | 749篇 |
物理学 | 1665篇 |
出版年
2023年 | 55篇 |
2022年 | 145篇 |
2021年 | 198篇 |
2020年 | 162篇 |
2019年 | 172篇 |
2018年 | 141篇 |
2017年 | 136篇 |
2016年 | 244篇 |
2015年 | 218篇 |
2014年 | 220篇 |
2013年 | 420篇 |
2012年 | 437篇 |
2011年 | 526篇 |
2010年 | 332篇 |
2009年 | 349篇 |
2008年 | 453篇 |
2007年 | 390篇 |
2006年 | 344篇 |
2005年 | 294篇 |
2004年 | 277篇 |
2003年 | 187篇 |
2002年 | 175篇 |
2001年 | 120篇 |
2000年 | 118篇 |
1999年 | 87篇 |
1998年 | 63篇 |
1997年 | 68篇 |
1996年 | 65篇 |
1995年 | 66篇 |
1994年 | 65篇 |
1993年 | 53篇 |
1992年 | 67篇 |
1991年 | 42篇 |
1990年 | 32篇 |
1989年 | 24篇 |
1988年 | 29篇 |
1987年 | 17篇 |
1986年 | 20篇 |
1985年 | 25篇 |
1984年 | 27篇 |
1983年 | 14篇 |
1982年 | 36篇 |
1981年 | 20篇 |
1980年 | 22篇 |
1979年 | 9篇 |
1978年 | 11篇 |
1977年 | 17篇 |
1976年 | 14篇 |
1975年 | 10篇 |
1974年 | 17篇 |
排序方式: 共有7076条查询结果,搜索用时 453 毫秒
151.
L. L. M. Santos J. A. Osso S. G. Araujo 《Journal of Radioanalytical and Nuclear Chemistry》1995,200(3):227-232
The development of an automated water target for the production of18F is described. The system was fully fested and shown to be reliable and secure. The chemical separation of18F was carried out using an anionexchange resin and K2CO3 as eluent. The18F production yields were, on average, (4.81±0.42) MBq/Ah and the specific activity was higher than 6.623×105 MBq/mmol. Heat transfer measurements and calculations were made. 相似文献
152.
Micellar catalysis of the reaction of 2,4-dinitrofluorobenzene with phenoxide and thiophenoxide ions
H. Chaimovich A. Blanco L. Chayet L.M. Costa P.M. Monteiro C.A. Bunton C. Paik 《Tetrahedron》1975,31(9):1139-1143
The reactions of 2,4-dinitrofluorobenzene with phenoxide and thiophenoxide ion in water are strongly catalyzed by micelles of cetyltrimethylammonium bromide (CTABr) by factors of 230 and 1100 respectively. Nonionic micelles of Brij weakly catalyze the reaction with thiophenoxide ion. Spectral measurements show that phenoxide, and especially thiophenoxide, ions interact strongly with micelles of CTABr which also markedly change the acid dissociation of phenol under given buffer conditions. 相似文献
153.
Margarida Maria Correia dos Santos Sheila Alves Maria de Lurdes Simões Gonçalves 《Electroanalysis》2007,19(22):2351-2361
The kinetics of formation and dissociation of Ni(II) complexes with oxalic and citric acids was studied by cyclic voltammetry in model solutions of xylem sap of Q. ilex (the dominant tree growing on serpentine soils of Northeast Portugal) using representative concentrations, pH and ionic strength. The role of magnesium on complex formation was analyzed from solutions where Mg is present at concentration levels found in the xylem sap of Q. ilex growing on both nonserpentine and serpentine soils. Kinetics studies were also done in diluted solutions of real xylem sap samples, spiked with increasing amounts of magnesium. The values obtained for the apparent rate constants were those anticipated by the proposed model. To test the validity of the methodology and mechanisms, formation rate constants, kf (M?1 s?1) of Ni(II) complexes with citrate and oxalate were evaluated that compare with the values from Eigen mechanism. 相似文献
154.
Preconcentration Systems Using Polyurethane Foam/Me-BDBD for Determination of Copper in Food Samples
Valfredo Azevedo Lemos Daniel Rodrigues Vieira Cleber Galvão Novaes Marcelo Eça Rocha Moacy Selis Santos Regina Terumi Yamaki 《Mikrochimica acta》2006,153(3-4):193-201
A new reagent, 6-[2′-(6′-methyl-benzothiazolylazo)]-1,2-dihydroxy-3,5-benzenedisulfonic acid (Me-BDBD), was synthesised and
used in on-line and off-line systems for copper preconcentration by solid-phase extraction. Spectrophotometry and Flame Atomic
Absorption Spectrometry (FAAS) were the detection techniques. Polyurethane foam loaded with Me-BDBD packed in a minicolumn
was used as sorbent in both systems. The spectral characteristics of Me-BDBD were investigated. The optimum pH values for
maximum sorption of the metal are between 7.0 and 8.5. Copper was desorbed with 0.05 and 0.50 mol L−1 hydrochloric acid solutions, for on-line and off-line systems, respectively. The effects of several foreign substances on
the adsorption of copper are reported. The enrichment factors obtained were 7 (on-line) and 26 (off-line) for the systems.
The proposed procedures allowed the determination of copper with detection limits of 3.4 and 1.4 μg L−1 (0.85 and 0.35 μg per gram of sample) for on-line and off-line systems, respectively. The precision of the procedures was
also calculated: 3.2 (on-line) and 1.9% (off-line). The validation of the procedures was carried out by analysis of certified
reference material. The copper contents in corn and rice flour and black tea samples were determined by applying the proposed
procedures. 相似文献
155.
The reaction between copper (II) and azide has been studied spectrophotometrically at four wavelengths, at 25 degrees , and ionic strength 4.00M (sodium perchlorate). The formation constants beta(2) and beta(3) found are 2.90 +/- 0.08 x 10(4) and 3.02 +/- 0.07 x 10(6) respectively. The results obtained from potentiometric measurements with a solid-state electrode disagree with those calculated from the spectrophotometric data. Causes of the discrepancy are discussed. 相似文献
156.
S. Moreira A. E. S. Vives O. L. A. D. Zucchi E. F. O. de Jesus V. F. Nascimento Filho S. M. B. Brienza 《Journal of Radioanalytical and Nuclear Chemistry》2006,270(1):167-171
Summary In this study the concentrations of P, S, Cl, K, Ca, Mn, Fe, Zn and Br in twenty-nine brands of national and international
beers were determined by synchrotron radiation total reflection X-ray fluorescence analysis (SR-TXRF). The results were compared
with the limits established by the Brazilian legislation and the nutritional values established by National Agricultural Library
(NAL, USA). The measurements were performed at the X-Ray Fluorescence Beamline at Brazilian National Synchrotron Light Laboratory,
in Campinas, S?o Paulo, Brazil, using a polychromatic beam for excitation. A small volume of 5 ml of beers containing an internal
standard used to correct geometry effects was analyzed without pretreatment. The measuring time was 100 seconds and the detection
limits obtained varied from 1 mg . l-1 for Mn and Fe to 15 mg . l-1 for P. 相似文献
157.
158.
Van Hoof F Van Wiele P Bruchet A Schmitz I Bobeldiji I Sacher F Ventura F Marti I Morecos Do Monte MH Sa Da Costa M 《Journal of AOAC International》2001,84(5):1420-1429
As part of a project funded by the European Commission (EC) for the development and evaluation of multiresidue methods for analysis of drinking and related waters, 15 European laboratories evaluated a method using styrene-divinylbenzene co-polymer solid-phase extraction followed by gas chromatography/mass spectrometry. The main aim of the study was to evaluate whether the method meets the requirements of EC Directive 98/83 in terms of accuracy, precision, and detection limit for 22 pesticides according to the following requirements: limit of detection, < or = 0.025 microg/L; accuracy, expressed as recovery between 75 and 125%; and precision, expressed as repeatability relative standard deviation of the method of < 12.5% and as reproducibility relative standard deviation of the method of < 25%. Analyses for unknown concentrations were performed with fortified commercial bottled and tap waters. All laboratories were able to achieve detection limits of 0.01 microg/L for all pesticides except dimethoate and desisopropylatrazine (0.02 microg/L). The criteria for repeatability were met for all compounds except trifluralin, dimethoate, and lindane in bottled water and chlorpyrifos, dimethoate, and lindane in tap water. The criteria for reproducibility were met for all compounds except trifluralin, dimethoate, and lindane in bottled water and pendimethalin, chlorpyrifos, dimethoate, terbutryn, and lindane in tap water. In terms of accuracy, the method meets the requirements for all pesticides in both matrixes, except for lindane in bottled water and lindane and chlorpyrifos in tap water. 相似文献
159.
The thermal degradation under vacuum of poly(2,6-dimethoxycarbonyl-1,6-heptadiene), a polymer which contains cyclic structural units, has been investigated. The analysis of the degradation products has shown that depolymerisation (depropagation along the polymer chain with formation of diene monomer) occurs extensively, together with other degradation reactions. A mechanism is proposed to account for the degradation products which have been identified. 相似文献
160.
Fluorescence of the single tryptophan of cutinase: temperature and pH effect on protein conformation and dynamics 总被引:1,自引:0,他引:1
Martinho JM Santos AM Fedorov A Baptista RP Taipa MA Cabral JM 《Photochemistry and photobiology》2003,78(1):15-22
The cutinase from Fusarium solani pisi is an enzyme with a single L-tryptophan (Trp) involved in a hydrogen bond with an alanine (Ala) residue and located close to a cystine formed by a disulfide bridge between two cysteine (Cys) residues. The Cys strongly quenches the fluorescence of Trp by both static and dynamic quenching mechanisms. The Trp fluorescence intensity increases by about fourfold on protein melting because of the disruption of the Ala-Trp hydrogen bond that releases the Trp from the vicinity of the cystine residue. The Trp forms charge-transfer complexes with the disulfide bridge, which is disrupted by UV light irradiation of the protein. This results in a 10-fold increase of the Trp fluorescence quantum yield because of the suppression of the static quenching by the cystine residue. The Trp fluorescence anisotropy decays are similar to those in other proteins and were interpreted in terms of the wobbling-in-cone model. The long relaxation time is attributed to the Brownian rotational correlation time of the protein as a whole below the protein-melting temperature and to protein-backbone dynamics above it. The short relaxation time is related to the local motion of the Trp, whose mobility increases on protein denaturation. 相似文献