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141.
X-ray absorption spectroscopy measurements at the Cr K-edge of a trichroic crystal of alexandrite BeAl(2)O(4):Cr(3+) for different orientations of the crystal with respect to the polarization and direction of the x-ray incident beam have been performed. Analysis of the experimental spectra with the help of first-principles calculations of x-ray absorption spectra allowed us to estimate the proportion of chromium Cr(3+) cations among the two different octahedral sites of the alexandrite structure (70% in the C(s) site-30% in the C(i) site). The methodology presented in this work opens up new possibilities in the field of mineralogy for the study of complex minerals containing several sites potentially occupied by several transition elements or for solid solutions. 相似文献
142.
Miedema PS Ngene P van der Eerden AM Weng TC Nordlund D Sokaras D Alonso-Mori R Juhin A de Jongh PE de Groot FM 《Physical chemistry chemical physics : PCCP》2012,14(16):5581-5587
X-Ray Raman Spectroscopy (XRS) is used to study the electronic properties of bulk lithium borohydride (LiBH(4)) and LiBH(4) in porous carbon nano-composites (LiBH(4)/C) during dehydrogenation. The lithium (Li), boron (B) and carbon (C) K-edges are studied and compared with calculations of the starting material and intermediate compounds. Comparison of the B and C K-edge XRS spectra of the as-prepared samples with rehydrogenated samples shows that the B and C electronic structure is largely regained after rehydrogenation. Both Li and C K-edge spectra show that during dehydrogenation, part of the Li intercalates into the porous carbon. This study shows that XRS in combination with calculations is a promising tool to study the electronic properties of nano-crystalline light-weight materials for energy storage. 相似文献
143.
Pérez N López-Calahorra F Labarta A Batlle X 《Physical chemistry chemical physics : PCCP》2011,13(43):19485-19489
The process of formation of magnetite nanoparticles has been investigated by liquid chromatography and mass spectroscopy in the liquid phase decomposition of either Fe(III) acetylacetonate with decanoic acid or Fe(III) decanoate. In both cases, the dissociation into radicals of the iron carboxylate bonds provides the reduction of the Fe(III) cations and the oxygen atoms required for the formation of the mixed-valence inverse spinel magnetite structure. A reaction mechanism is proposed. It is also shown that the reaction of free decanoic acid with the Fe(III) cations in solution promotes the growth of faceted particles at the reflux temperature of the solvent (ca. 280 °C), while, under the same conditions, the stepwise decomposition of the Fe(III) decanoate generates smaller and pseudo-spherical particles. The latter also yields faceted particles when the temperature is increased above that of the total decomposition of the salt. Magnetic measurements make evident that the reaction starting from Fe(III) acetylacetonate yields nanoparticles with higher magnetization and lower spin disorder, due to the improved regularity of the surface crystal structure. The starting conditions for the decarboxylation process thus affect the morphology and magnetic properties of the resulting nanoparticles. 相似文献
144.
Most of the analytical models found in the literature, to study the contact between cylindrical bodies, are based on the Hertz
pressure distribution. The major shortcomings associated with these cylindrical models concern their nonlinearity. Firstly,
the indentation is expressed as an implicit function of the contact force, thus a numerical iterative technique is required
to evaluate the contact force for a given indentation. In a dynamic analysis code, it is implied that at each integration
time step, the iterative process for the solution of the nonlinear equations has to be solved. Secondly, the current cylindrical
contact models include logarithmic functions, which impose mathematical and physical limitations on their application, particularly
for conformal contact conditions with lower clearance values. The validity domain of each contact model is identified in this
work with relation to the clearance value and material properties of the contacting cylinders. A comparative assessment of
the performance of each model is performed calculating the relative difference of each one in relation to Johnson’s model.
The results show that, in general, different models exhibit distinct behavior for both the internal and external contact between
cylinders. The load limit of each model and the restrictions on its application is identified using two simple examples of
mechanical engineering practice in which internal contacting cylinders are involved and analyzed to include: journal bearings
and roller chain drives. 相似文献
145.
Ana Fortuna Joana Sousa Gilberto Alves Amílcar Falcão Patrício Soares-da-Silva 《Analytical and bioanalytical chemistry》2010,397(4):1605-1615
For the first time, a simple, selective and accurate high-performance liquid chromatography method with ultraviolet detection
was developed and validated to quantify simultaneously three structurally related antiepileptic drugs; carbamazepine, oxcarbazepine,
and the recently launched eslicarbazepine acetate and their main metabolites, carbamazepine-10,11-epoxide, 10,11-trans-dihydroxy-10,11-dihydro-carbamazepine, and licarbazepine. The method involves a solid-phase extraction and a reverse-phase
C18 column with 5 cm length. The mobile phase consisting of water, methanol, and acetonitrile in the ratio 64:30:6 was selected
as the best one and pumped at 1 mL/min at 40 °C. The use of this recent column and an aqueous mobile phase instead of buffers
gives several advantages over the method herein developed; namely the fact that the chromatographic analysis takes only 9 min.
The method was validated according to the guidelines of the Food and Drug Administration, showing to be accurate (bias within
±12%), precise (coefficient variation <9%), selective and linear (r
2 > 0.997) over the concentration range of 0.05–30 μg/mL for carbamazepine; 0.05–20 μg/mL for oxcarbazepine; 0.15–4 μg/mL for
eslicarbazepine acetate; 0.1–30 μg/mL for carbamazepine-10,11-epoxide; 0.1–10 μg/mL for 10,11-trans-dihydroxy-10,11-dihydro-carbamazepine, and 0.1–60 μg/mL for licarbazepine. It was also shown that this method can adequately
be used for the therapeutic drug monitoring of the considered antiepileptic drugs, carbamazepine, oxcarbazepine, eslicarazepine
acetate, and their metabolites. 相似文献
146.
Romão RI Ferreira Q Morgado J Martinho JM Gonçalves da Silva AM 《Langmuir : the ACS journal of surfaces and colloids》2010,26(22):17165-17177
Phase separation and interactions in mixed monolayers of dipalmitoylphosphatidylglycerol (DPPG) with the rhodamine B end-labeled double-hydrophilic block copolymer (DHBC), poly(N,N-dimethylacrylamide)-block-poly(N,N-diethylacrylamide) (RhB-PDMA(207)-b-PDEA(177)), was studied at the air-water interface. Surface pressure versus area isotherms indicate that both components behave almost independently. Brewster angle microscopy (BAM) images show a random distribution of liquid condensed (LC) domains of DPPG in an apparent homogeneous matrix of DHBC, excluding the macroscopic phase separation. The laser scanning confocal fluorescence microscopy (LSCFM) of the rhodamine dye at the end of the PDMA chain showed how the DHBC is distributed in Langmuir-Blodgett (LB) mixed monolayers. The high spatial resolution of atomic force microscopy (AFM) combined with the LCSFM images indicates that DHBC incorporates in the expanded phase of DPPG to form mixed domains, being excluded from the condensed regions. Upon compression, nanosized LC domains of DPPG nucleate inside the mixed domains corralled in the nanopatterning of pure DHBC. The negatively charged polar group of DPPG inhibits rhodamine aggregation, while the long polymer chains promote the formation of corralled nanodomains of DPPG in two dimensions. 相似文献
147.
Maximum likelihood estimation in a partially observed stratified regression model with censored data
Amélie Detais Jean-François Dupuy 《Annals of the Institute of Statistical Mathematics》2011,63(6):1183-1206
The stratified proportional intensity model generalizes Cox’s proportional intensity model by allowing different groups of
the population under study to have distinct baseline intensity functions. In this article, we consider the problem of estimation
in this model when the variable indicating the stratum is unobserved for some individuals in the studied sample. In this setting,
we construct nonparametric maximum likelihood estimators for the parameters of the stratified model and we establish their
consistency and asymptotic normality. Consistent estimators for the limiting variances are also obtained. 相似文献
148.
149.
150.
Unprecedented Trinuclear AgI Complex with 2,4,6‐Tris(2‐pyrimidyl)‐1,3,5‐triazine as an Efficient Catalyst for the Aziridination of Olefins 下载免费PDF全文
Dr. Damir A. Safin Dr. Amélie Pialat Dr. Ilia Korobkov Prof. Muralee Murugesu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(16):6144-6149
An unprecedented trinuclear heteroleptic AgI complex was isolated using a stable multidentate 2,4,6‐tris(2‐pyrimidyl)‐1,3,5‐triazine (TPymT) ligand. The obtained compound is an efficient catalyst for the direct aziridination of terminal olefins. 相似文献