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101.
[structure: see text] The relative and absolute stereochemistry of the structurally unique 24-membered myxobacterial macrolides archazolid A and B, highly potent vacuolar-type ATPase (V-ATPase) inhibitors in vitro and in vivo, was determined on the basis of a combination of extensive high-field NMR studies, including J-based configuration analysis, molecular modeling, and chemical methods. 相似文献
102.
Hellman H Laitinen RS Kaila L Jalonen J Hietapelto V Jokela J Sarpola A Rämö J 《Journal of mass spectrometry : JMS》2006,41(11):1421-1429
The hydrolysis reactions of FeCl3 in 0.1 mM aqueous solutions were monitored by electrospray ionisation time-of-flight mass spectrometry (ESI-ToF-MS). In contrast to the other ionisation techniques, electrospray ionisation provides information on the composition of the elemental cores even in water and salt cluster ions. Therefore, the technique facilitates detection of the bonding of the chloride ions in the oxo-hydroxo-iron cores. A variety of mononuclear and polynuclear iron-oxohydroxo-chloride complexes were characterised from the cationic and anionic mass spectra of iron(III) solutions. Oxidation and reduction reactions of iron, as well as competition between OH- and Cl- ions within one iron core, were also observed. 相似文献
103.
Hyppänen I Hölsä J Kankare J Lastusaari M Pihlgren L Soukka T 《Journal of fluorescence》2008,18(6):1029-1034
The up-converting ZrO2:Yb3+,Er3+ nanomaterials were prepared with the combustion and sol–gel methods. FT-IR spectroscopy was used for analyzing the impurities.
The crystal structures were characterized with X-ray powder diffraction and the mean crystallite sizes were estimated with
the Scherrer formula. Up-conversion luminescence measurements were made at room temperature with IR-laser excitation at 977 nm.
The IR spectra revealed the conventional and OH− impurities for the combustion synthesis products. The structure of the ZrO2:Yb3+, Er3+ nanomaterials was cubic except for the minor monoclinic and tetragonal impurities obtained with the sol–gel method. The materials
showed red (650–700 nm) and green (520–560 nm) up-conversion luminescence due to the 4F9/2→4I15/2 and (2H11/2, 4S3/2)→4I15/2 transitions of Er3+, respectively. The products obtained with the combustion synthesis exhibited the most intense luminescence intensity and
showed considerable afterglow. 相似文献
104.
Luminescence emission and uv-excitation properties of LaOBr: Tb3+, LaOBr: Ce3+, and LaOBr: Tb3+, Ce3+ phosphors were studied. The visible emission spectra of La0.995Tb0.005OBr consists of5D3,4 →7F3–6 transitions in the wavelength range of 410–630 nm. The excitation of the Tb3+ ion gives a broad 4f → 5d transition band at 254 nm and weaker4f → 4f transition lines above 300 nm. The uv-excitation and emission of La0.995Ce0.005OBr at 290, 315, 355 (excitation), and 440 nm (emission) originate from transitions between the 4f-ground state and the four crystal field components of the5d2D excited state. The sensitization of Tb3+ luminescence in LaOBr with Ce3+ at varying concentrations is described and discussed. With increasing Ce3+ concentration the 5D3 →7F transitions of Tb3+ quench totally and the5D4 →7F transitions begin to quench gradually. The excitation spectrum of the5D4 →7F5 transition of Tb3+ consists of four bands due to Tb3+ and Ce3+, of which the three Ce3+ bands increase in intensity and the Tb3+ band decreases as the Ce3+ concentration is increased. 相似文献
105.
Jorma Hölsä Marco Kirm Mika Lastusaari Janne Niittykoski Pavel Novák 《Journal of luminescence》2009,129(12):1560-1563
The electronic structures of the distrontium magnesium disilicate (Sr2MgSi2O7(:Eu2+)) materials were studied by a combined experimental and theoretical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structures were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands and the band gap energy of the material as well as the position of the Eu2+ 4f ground state were calculated. The calculated band gap energy (6.7 eV) agrees well with the experimental value of 7.1 eV. The valence band consists mainly of the oxygen states and the bottom of the conduction band of the Sr states. The calculated occupied 4f ground state of Eu2+ lies in the energy gap of the host though the position depends strongly on the Coulomb repulsion strength. The position of the 4f ground state with respect to the valence and conduction bands is discussed using the theoretical and experimental evidence available. 相似文献
106.
We present and contrast two distinct ways of including extremal black holes in a Lorentzian Hamiltonian quantization of spherically symmetric Einstein-Maxwell theory. First, we formulate the classical Hamiltonian dynamics with boundary conditions appropriate for extremal black holes only. The Hamiltonian contains no surface term at the internal infinity, for reasons related to the vanishing of the extremal hole surface gravity, and quantization yields a vanishing black hole entropy. Second, we give a Hamiltonian quantization that incorporates extremal black holes as a limiting case of nonextremal ones, and examine the classical limit in terms of wave packets. The spreading of the packets, even the ones centered about extremal black holes, is consistent with continuity of the entropy in the extremal limit, and thus with the Bekenstein-Hawking entropy even for the extremal holes. The discussion takes place throughout within Lorentz-signature spacetimes. 相似文献
107.
A novel intramolecular cyclization reaction is reported, in which various 1,2-bis(amidinohydrazone)s are converted into 3-amino-1,2,4-triazine or its substituted analogues. The reaction is easily carried out at microscale as well as ordinary scale level by heating the appropriate bis(amidinohydrazone) to roughly 200°C in an inert atmosphere or in vacuo. No solvents or other reagents are needed. The products are easily isolated, being sublimed off from the starting material and forming single crystals. A microscale procedure employing glass ampoules as the reaction vessel is described. The reaction is rapid and offers a facile way of preparing large series of substituted 3-amino-1,2,4-triazines individually in crystalline form for screening purposes. Because large numbers of bis(amidinohydrazone)s have been synthesized for biochemistry and drug development and libraries exist, the present method may be more practical than combinatorial approaches for the production of 3-amino-1,2,4-triazines, as it allows simultaneous individual syntheses. As the reaction is intramolecular, it is probable that the method can also be used also for mixtures of bis(amidinohydrazone)s to produce several triazines in one ampoule. The structure of the products was confirmed by mass, NMR and IR spectroscopy, elemental analyses and comparison with authentic samples. The reaction was studied using five bis(amidinohydrazone)s of aliphatic glyoxals, phenylglyoxal bis(amidinohydrazone) as well as the bis(amidinohydrazone) of 1,2-cyclohexanedione as starting materials. From the compound mentioned last a bicyclic product was obtained. In the case of the unsymmetric starting material ethylglyoxal bis(amidinohydrazone), both possible isomers were formed. The safety of heating the nitrogen-rich starting materials in large-scale syntheses has not been verified. 相似文献
108.
Veli Hietaniemi Auli Kostamo Jorma Kumpulainen Juha-Matti Pihlava 《Fresenius' Journal of Analytical Chemistry》1995,352(1-2):40-42
Strawberry and cabbage candidate reference materials (RMs) have been prepared by our laboratory during 1992–1993. Fresh materials have been homogenized and spiked with known concentrations of pesticides representing compounds commonly used for strawberries and cabbages. The candidate RMs have been then freeze-dried, homogenized, bottled under a stream of nitrogen and stored frozen at –20°C. Homogeneity and stability of the candidate RMs have been studied during the storage period. Homogeneity of the candidate RMs for 5 g samples has been within 10%. Homogeneity and stability studies have been carried out using the conventional multiresidue method of Luke et al. [1]. Instrumental analysis have been performed by gas chromatography mass spectrometry (GC-MS) employing selected ion monitoring (SIM). 相似文献
109.
Summary The analytical quality control program employed by the ongoing FAO study on 14 trace elements in nationally representative staple foods of European countries is presented. The analytical quality control used is based on: 1) precautions taken to avoid trace element contaminations or losses during sampling and sample handling; and, 2) on methods used to guarantee that the actual determinations yield correct results. The precautions are presented. A number of certified biological reference materials (RMs) were used to validate the analytical methods employed. The following staple food RMs were also prepared: wheat flour, potato powder, animal muscle (pork) and milk powder. They were tested for homogeneity and subjected to an interlaboratory comparison study on the basis of which recommended values for trace element concentrations were defined. Further, the mean relative standard deviation for the 95% confidence limits of the medians in all RMs was below 5% for Ca, Mg and Zn; below 10% for Mn; below 15% for Fe, Cu and Se; and below 25% for Mo and Ni. These RMs were used to control the analytical quality of the trace element determinations in the actual samples. It is concluded that important contaminations were avoided in sampling and sample handling and that use of the RMs described was necessary to guarantee the analytical quality of the results. 相似文献
110.
Bounds for various functions of the eigenvalues of a Hermitian matrix A, based on the traces of A and A2, are improved. A technique is presented whereby these bounds can be improved by combining them with other bounds. In particular, the diagonal of A, in conjunction with majorization, is used to improve the bounds. These bounds all require O(n2) multiplications. 相似文献