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981.
Summary Coagulation and reversal of charge effects of freshly prepared and heated solutions of hafnium tetrachloride have been studied as a function of the pH using aged silver halide sols and solsin statu nascendi. It was shown that the critical coagulation concentration and the critical stabilization concentration (due to charge reversal) increased with increasing pH. These observations have been related to the hydrolysis of the hafnium ion. At pH values above 4 essentially the entire amount of hafnium is present in the form of the neutral soluble, species Hf(OH)4. This accounts for the inability of the hafnium solutions to reverse the charge of the sols at higher pH values. The adsorption measurements carried out with the aid of the radioactive isotope181Hf showed that the neutral hydrolyzed species are strongly adsorbed on negatively charged silver iodide particles. The adsorbed amounts of hafnium on a AgI sol are considerably larger than in the case of charged hydrolyzed ions (such as hydrolyzed thorium ions) on a similar sol. This is explained by the ability of the neutral hafnium species, Hf(OH)4. to form a close-packed adsorbed layer. The results confirm previous findings that the enhanced adsorption of hydrolyzed ions is caused by the presence of the hydroxyl group, whereas the ionic charge plays a negligible role in this case.
Supported by the U.S. Army Research Office (Durham), Grant No. DA-ARO (D)-31-124-G 656.
Part of a PhD Thesis byL. J. Stryker, supported by a NASA Traineeship. 相似文献
Zusammenfassung Die Koagulations- und Umladungserscheinungen von frisch dargestellten und durch Erwärmung gealterten Lösungen von Hafniumchlorid wurden an Silberhalogenid-Solen in der Abhängigkeit vom pH untersucht. Es wurde festgestellt, daß die kritische Koagulationskonzentration und die kritische Stabilisationskonzentration (die Umladungsgrenze) mit steigendem pH höher werden. Diese Beobachtungen wurden durch die Hydrolyse des Hafnium-Ions erklärt. Wenn pH>4 ist, bestcht nahezu die ganze Menge von Hafnium als neutrale, gelöste Hf(OH)4 Moleküle. Damit wird es erklärt, daß die Silberhalogenid-Teilchen bei höheren pH-Werten durch Hafniumsalze nicht umgeladen werden können. Die mittels des radioaktiven Isotopen181Hf durchgeführten Messungen zeigten, daß die neutralen Hf(OH)4 — Moleküle stark an den negativ geladenen Silberjodid-Teilchen adsorbiert sind. Die adsorbierte Menge per Mol von AgI ist beträchtlich größer als die Menge der geladenen, hydrolysierten Ionen (wie z. B. ThOH3+), die an dem gleichen Sol bestimmt wurde. Dieser Effekt ist leicht erklärt, da man mit den neutralen adsorbierten Molekülen eine fest gepackte Adsorptionsschicht bilden kann. Diese Resultate bestätigen die früheren Befindungen, daß die höhere Adsorptionsfähigkeit der hydrolysierten Ionen durch die Hydroxylgruppe verursacht wird, während die Ionenladung eine zu vernachlässigende Rolle spielt.
Supported by the U.S. Army Research Office (Durham), Grant No. DA-ARO (D)-31-124-G 656.
Part of a PhD Thesis byL. J. Stryker, supported by a NASA Traineeship. 相似文献
982.
František Kvasnička Rudolf Ševčík Michal Voldřich Jana Krátká 《Central European Journal of Chemistry》2004,2(3):417-424
A simple and rapid capillary zone electrophoresis (CZE) method for the determination of aristolochic acid (AA) in dietary
supplements and selected herbs is described. A clear separation of AA from other sample constituents was achieved within 5
minutes without any sample clean up. A mixture of 20 mM-morpholinethanesulphonic acid+10 mM-BisTrisPropane+0.2% hydroxyethylcelullose
in 10% methanol serves as a background electrolyte. The linearity, accuracy, intra-assay and detection limit of the developed
method are 200–6000 ng/mL, 95–103%, 3.5%, and 50 ng/ml, respectively. Ease of use, sufficient sensitivity and low running
cost are the most important attributes of the CZE method. The proposed CZE method was compared with HPLC. 相似文献
983.
Robert Pietrzak Mieczysław Kozłowski Helena Wachowska Jan Yperman 《Central European Journal of Chemistry》2004,2(2):278-289
Soluble products obtained from the oxidation of four types of coal, each characterised by different degree of coalification
and different degree of sulphur content, are studied. The coals are oxidised with peracetic acid (PAA) and nitric acid. Analyses
are performed by Atmospheric Pressure-Temperature Programmed Reduction (AP-TPR) and Fourier Transform Infrared Spectroscopy
(FTIR). The soluble products contain much more sulphur than the insoluble products of oxidation. The products obtained from
the reaction with HNO3 contain higher amounts of inorganic sulphur compounds, while those obtained from the reaction with PAA are characterised
by an increased content of organic sulphur species. 相似文献
984.
Melinda Fekete Pál Kolonits Ngo Thi Hien Lajos Novák 《Central European Journal of Chemistry》2005,3(4):792-802
New tryptamine 5, 7 and β-carboline derivatives 3 were preparated by palladium-catalyzed coupling reaction of 5-bromotryptamine 1 with aryl boronic acids.
Dedicated to Professor András Lipták on his 70th birthday. 相似文献
985.
Xiongyu Wu Xiaohong Liu Guangce Jiang Yongcheng Lin Willy Chan L. L. P. Vrijmoed 《Chemistry of Natural Compounds》2005,41(1):27-29
A novel metabolite 1, named xyloketal G, was isolated from cultures of marine derived mangrove fungus Xylaria sp. 2508. Its structure was elucidated by analysis of spectroscopic data.__________Published in Khimiya Prirodnykh Soedinenii, No. 1, pp. 22–23, January–February, 2005. 相似文献
986.
Marie-Noelle Avettand-Fènoël N. David G. Reumont J. -M. Fiorani M. Vilasi P. Perrot 《Journal of Thermal Analysis and Calorimetry》2007,90(2):329-332
The Fe-Sn-Zn system is of interest because Sn is one element added to the Zn galvanizing bath to overcome the drawbacks due
to the presence of Si in semi-killed steels. This work has been undertaken with the aim to understand the tin effect on the
microstructure and the layers growth in batch galvanized coatings on low alloyed steels.
Various experimental techniques such as metallography, scanning electron microscopy (SEM) coupled with X-ray energy dispersive
spectroscopy (EDX) are used in order to characterize the microstructure and the properties of such coatings elaborated in
a zinc bath enriched with tin.
Solidification phenomena and layers growth mechanisms during galvanization are explained by means of the ternary phase diagram
Fe-Sn-Zn at 450°C. The Calphad method allows to obtain this phase diagram from the three optimized binary phase diagrams Fe-Sn,
Fe-Zn and Sn-Zn and some experimental data inside the ternary Fe-Sn-Zn system. 相似文献
987.
New low-lying isomeric structures of Ar(n)HF clusters are reported for n=6-13. They were determined using simulated annealing and evolutionary programming, for pairwise additive intermolecular potential energy surfaces. New global minima were found for the clusters with n=7, 10, 11. The new lowest-energy structure of Ar(7)HF and several new local minima for n=6, 7 clusters have the HF bound on a threefold surface site, consistent with the recent spectroscopic data for Ar(n)HF clusters in helium nanodroplets. A new type of low-energy local minima were determined for n=9-13 clusters. 相似文献
988.
Pilar Jiménez Maria Victoria Roux JiŘi Kulhánek Otto Exner 《Structural chemistry》1996,7(5-6):375-381
The energy of combustion of 2,5-dimethoxybenzoic acid has been determined using a static bomb calorimeter. The vapor pressures of the compound have been measured over a 18 K temperature interval by the Knudsen effusion technique. Heat capacity measurements betweenT=270 K andT=338 K were carried out by DSC. From these experimental results the standard molar enthalpies of combustion, sublimation, and formation in the crystalline and gaseous state at the temperature 298.15 K have been derived. With this compound, the series of mono- and dimethoxy-benzoic acids have been completed. Theirf H
m
o
values were expressed by an additive relationship, taking into account the number of methoxy groups and the number of all 1,2 interactions: an accuracy of 3.3 kJ·mol–1 was achieved. In an alternative approach the substituent effect of the methoxy groups was evaluated within the framework of isodesmic reactions. The effect of disubstitution was referred to mono derivatives and the excess energy—the so-called buttressing effect—was evaluated (2–24 kJ· mol–1 for individual bis derivatives). These values were explained in terms of the conformation of the methoxy group around the Car-O bond. 相似文献
989.
The oxidation of iron(II) with tert-butyl hydroperoxide was investigated in the absence of oxygen in water, methanol, and the dichloromethane—methanol solvent
mixture (φr = 2:1). The oxidation rate depends on solvent polarity; measured in the presence of SCN− at constant 0.8 mmol dm−3 HCl, the rate constant increases with the polarity decrease passing from water and methanol to the dichloromethane—methanol
solvent mixture. Further, in non-aqueous solutions at this acid concentration the rate constant was higher than the rate constant
in the presence of Cl− only. The oxidation rate measured in the [FeCl]2+ complex in dichloromethane—methanol was slow in acidic medium and increased by decreasing the acid concentration. Approaching
the physiological pH conditions the rate constant attained the value of an order of magnitude of 103 dm3 mol−1 s−1, while very little alteration of stoichiometry of the oxidation reaction was observed. The rate constant measured in the
presence of Cl− strongly depends on electrolyte concentration at concentrations less than 0.5 mmol dm−3 HCl, both in MeOH and the solvent mixture. Based on these results, a possible mechanism of the influence of solvent, acidity,
and ligand type on the rate constant is discussed. We assume that the oxidation proceeds by an inner-sphere mechanism considering
that the breakdown of the successor inner-sphere complex forming reactive alkoxyl radicals is probably the rate-limiting step.
Presented at the 20th International Conference on the Coordination and Bioinorganic Chemistry organized by the Slovak Chemical
Society, Slovak University of Technology, Comenius University, and the Slovak Academy of Sciences, Smolenice Castle, 5–10
June 2005. 相似文献
990.
The method reported for the determination of calcium in silicate rocks involves titration with ethyleneglycol-bis(2-aminoethylether)tetraacetic acid (EGTA) to a potentiometric end-point. A Crytur calcium-selective electrode is used; the selectivity constants (Kca,x) are less than 10-3 for Mg, Ba, Sr, K and Na. The method can be used for calcium contents of 2% or higher (and exceptionally for lower contents), in the presence of up to 60% Mg, 33% AI, 17% Fe, 5% Ba, 5% Ti, and 2.5% Mn. Al, Fe and Ti are masked with sulphosalicylic acid, barium is precipitated as its sulphate, and manganese is bound as its cyanide complex. If the magnesium content is less than that of calcium, EDTA can be used as titrant, magnesium being masked with acetylacetone. 相似文献