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991.
Silicon is by far the most important semiconductor material in the microelectronic industry mostly due to the high quality of the Si/SiO2 interface. Consequently, applications requiring chemical functionalization of Si substrates have focused on molecular grafting of SiO2 surfaces. Unfortunately, there are practical problems affecting homogeneity and stability of many organic layers grafted on SiO2, such as silanes and phosphonates, related to polymerization and hydrolysis of Si–O–Si and Si–O–P bonds. These issues have stimulated efforts in grafting functional molecules on oxide-free Si surfaces, mostly with wet chemical processes. This review focuses therefore directly on wet chemical surface functionalization of oxide-free Si surfaces, starting from H-terminated Si surfaces. The main preparation methods of oxide-free H-terminated Si and their stability are first summarized. Functionalization is then classified into indirect substitution of H-termination by functional organic molecules, such as hydrosilylation, and direct substitution by other atoms (e.g. halogens) or small functional groups (e.g. OH, NH2) that can be used for further reaction. An emphasis is placed on a recently discovered method to produce a nanopattern of functional groups on otherwise oxide-free, H-terminated and atomically flat Si(1 1 1) surfaces. Such model surfaces are particularly interesting because they make it possible to derive fundamental knowledge of surface chemical reactions. 相似文献
992.
Jesús Ferrando‐Soria Dr. Rafael Ruiz‐García Dr. Joan Cano Dr. Salah‐Eddine Stiriba Julia Vallejo Dr. Isabel Castro Prof. Dr. Miguel Julve Prof. Dr. Francesc Lloret Prof. Dr. Pedro Amorós Dr. Jorge Pasán Prof. Dr. Catalina Ruiz‐Pérez Dr. Yves Journaux Dr. Emilio Pardo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(6):1608-1617
The concept of “molecular magnetic sponges” was introduced for the first time in 1999 by the creative imagination of the late Olivier Kahn. It refers to the exotic spongelike behavior of certain molecule‐based materials that undergo a dramatic change of their magnetic properties upon reversible dehydration/rehydration processes. Here we report a unique example of a manganese(II)–copper(II) mixed‐metal–organic framework of formula [Na(H2O)4]4[Mn4{Cu2(mpba)2(H2O)4}3]? 56.5 H2O ( 1 ) (mpba=N,N′‐1,3‐phenylenebis(oxamate)). Compound 1 possesses a 3D MnII4CuII6 pillared layer structure with mixed square and octagonal pores of approximate dimensions 1.2×1.2 nm and 2.1×3.0 nm, respectively, hosting a large amount of crystallization H2O molecules and hydrated NaI countercations as guests. It reversibly switches from a crystalline hydrated phase with long‐range ferromagnetic ordering at a rather high critical temperature (Tc) of 22.5 K to an amorphous dehydrated phase with Tc as low as 2.3 K, which is accompanied by a breathing‐type dynamic effect involving a large crystal volume (ca. 45 %) and color changes after water desorption/adsorption. The combination of both the open‐framework structure and the spongelike optical, mechanical, and magnetic switching behavior in this new class of oxamato‐based porous magnets offers fascinating possibilities in designing multifunctional materials for host–guest molecular sensing. 相似文献
993.
Carine Maaliki Dr. Christine Lepetit Dr. Yves Canac Dr. Christian Bijani Carine Duhayon Prof. Remi Chauvin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(25):7705-7714
Two types of imidazoliophosphane with additional electron‐withdrawing substituents, such as alkoxy or imidazolio groups, are experimentally described and theoretically studied. Diethyl N,N′‐2,4,6‐methyl(phenyl)imidazoliophosphonite is shown to retain a P‐coordinating ability toward a {RhCl(cod)} (cod=cycloocta‐1,5‐diene) center, thus competing with the cleavage of the labile C? P bond. Derivatives of N,N′‐phenylene‐bridged diimidazolylphenylphosphane were isolated in good yield. Whereas the dicationic phosphane proved to be inert in the presence of [{RhCl(cod)}2], the monocationic counterpart was shown to retain the P‐coordinating ability toward a {RhCl(cod)} center, thus competing with the N‐coordinating ability of the nonmethylated imidazolyl substituent. The ethyl phosphinite version of the dication, thus possessing an extremely electron‐poor PIII center, was also characterized. According to the difference between the calculated homolytic and heterolytic dissociation energies, the N2C???P bond of imidazoliophosphanes with aryl, amino, or alkoxy substituents on the P atom is shown to be of dative nature. The P‐coordinating properties of imidazoliophosphanes with various combinations of phenyl or ethoxy substituents on the P atom and those of six diimidazolophosphane derivatives with zero, one, or two methylium substituents on the N atom, were analyzed by comparison of the corresponding HOMOs and LUMOs and by calculation of the IR C?O stretching frequencies of their [RhCl(CO)2] complexes. Comparison of the νCO values allows the family of the electron‐poor Im+PRR′ (Im=imidazolyl) potential ligands to be ranked in the following order versus (R,R′): P(OEt)3<(Ph,Ph)<(Ph,OEt)<(OEt,OEt)<PF3<(Ph,Im)<(Ph,Im+)<(OEt,Im+). The (Ph,Im) representative is therefore the least electron‐donating phosphane for which coordinating behavior toward a RhI center has been experimentally evidenced to date. Ultimate applications in catalysis could be envisaged. 相似文献
994.
Carine Maaliki Dr. Christine Lepetit Carine Duhayon Dr. Yves Canac Prof. Remi Chauvin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(50):16153-16160
Carbene→chalcogenophosphenium adducts, which correspond to an intermolecular stabilization mode of the so far elusive, free oxo‐ and thiooxophosphenium species [R2P+ = X] (X=O, S) by imidazolylidene (NHC) and diaminocyclopropenylidene (BAC) donors, have been isolated and fully characterized. The dative character of the R2C:→P+(X)Ph2 bond was confirmed experimentally by nucleophilic displacement of the carbene donor with a chloride ion and by an exchange reaction of the NHC ligand of the NHC:→P+(O)Ph2 adduct with an independently prepared BAC ligand, thereby giving the BAC:→P+(O)Ph2 adduct. This dative character was further characterized by the DFT‐calculated preference of carbene→chalcogenophosphenium systems for a heterolytic dissociation mode over a homolytic one. 相似文献
995.
For Trypanosoma brucei, a parasite responsible for African sleeping sickness, carbohydrate metabolism is the only source of ATP, and glycolytic enzymes are localized within membrane-bound organelles called glycosomes. Hexokinase, the first enzyme of the glycolytic pathway, was chosen as a target for selective drug design. We have cloned and sequenced the hexokinase gene of T. brucei. In parallel, we have synthesized several inhibitors. Kinetic analysis revealed differences in the binding mode of these compounds toward yeast and T. brucei hexokinases, while the m-bromophenyl glucosamide was found to be selective for T. brucei. The modeled structure of T. brucei hexokinase-inhibitor complex (using the crystal structure of the Schistosoma mansoni hexokinase as a template) allows us to propose a mode of action of this inhibitor for the trypanosome hexokinase and to account for the observed selectivity. 相似文献
996.
Bryan Gardiner Khan Jakee Yves Lucet 《Computational Optimization and Applications》2014,58(1):249-272
Piecewise linear-quadratic (PLQ) functions are an important class of functions in convex analysis since the result of most convex operators applied to a PLQ function is a PLQ function. We modify a recent algorithm for computing the convex (Legendre-Fenchel) conjugate of convex PLQ functions of two variables, to compute its partial conjugate i.e. the conjugate with respect to one of the variables. The structure of the original algorithm is preserved including its time complexity (linear time with some approximation and log-linear time without approximation). Applying twice the partial conjugate (and a variable switching operator) recovers the full conjugate. We present our partial conjugate algorithm, which is more flexible and simpler than the original full conjugate algorithm. We emphasize the difference with the full conjugate algorithm and illustrate results by computing partial conjugates, partial Moreau envelopes, and partial proximal averages. 相似文献
997.
A new and very fast method of bootstrap for sampling without replacement from a finite population is proposed. This method can be used to estimate the variance in sampling with unequal inclusion probabilities and does not require artificial populations or utilization of bootstrap weights. The bootstrap samples are directly selected from the original sample. The bootstrap procedure contains two steps: in the first step, units are selected once with Poisson sampling using the same inclusion probabilities as the original design. In the second step, amongst the non-selected units, half of the units are randomly selected twice. This procedure enables us to efficiently estimate the variance. A set of simulations show the advantages of this new resampling method. 相似文献
998.
We develop a martingale-based decomposition for a general class of quadratic forms of Markov chains, which resembles the well-known Hoeffding decomposition of U-statistics of i.i.d. data up to a reminder term. To illustrate the applicability of our results, we discuss how this decomposition may be used to studying the large-sample properties of certain statistics in two problems: (i) we examine the asymptotic behavior of lag-window estimators in time series, and (ii) we derive an asymptotic linear representation and limiting distribution of U-statistics with varying kernels in time series. We also discuss simplified examples of interest in statistics and econometrics. 相似文献
999.
Ioan Baltog Mihaela Baibarac Serge Lefrant Jean Yves Mevellec 《Journal of Raman spectroscopy : JRS》2011,42(3):303-312
The capability of anti‐Stokes/Stokes Raman spectroscopy to evaluate chemical interactions at the interface of a conducting polymer/carbon nanotubes is demonstrated. Electrochemical polymerisation of the monomer 3,4‐ethylenedioxythiophene (EDOT) on a Au support covered with a single‐walled carbon nanotube (SWNT) film immersed in a LiClO4/CH3CN solution was carried out. At the resonant optical excitation, which occurs when the energy of the exciting light coincides with the energy of an electronic transition, poly(3,4‐ethylenedioxythiophene) (PEDOT) deposited electrochemically as a thin film of nanometric thickness on a rough Au support presents an abnormally intense anti‐Stokes Raman spectrum. The additional increase in Raman intensity in the anti‐Stokes branch observed when PEDOT is deposited on SWNTs is interpreted as resulting from the excitation of plasmons in the metallic nanotubes. A covalent functionalisation of SWNTs with PEDOT both in un‐doped and doped states takes place when the electropolymerisation of EDOT, with stopping at +1.6 V versus Ag/Ag+, is performed on a SWNT film deposited on a Au plate. The presence of PEDOT covalently functionalised SWNTs is rationalised by (1) a downshift by a few wavenumbers of the polymer Raman line associated with the symmetric C C stretching mode and (2) an upshift of the radial breathing modes of SWNTs, both variations revealing an interaction between SWNTs and the conjugated polymer. Raman studies performed at different excitation wavelengths indicate that the resonant optical excitation is the key condition to observe the abnormal anti‐Stokes Raman effect. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
1000.
The synthesis of a novel class of fluorescent nucleosides featuring 2-aryl-3-hydroxychromones (3-HC) as base analogues is described. Nucleoside 1a bearing the 2-thienyl-3-HC nucleobase was prepared using sequential aryl-aldol condensation/cycloetherification or a Friedel-Crafts glycosylation as key steps. The synthesis of the triazolyl derivative 1b was achieved using a convergent 1,3-dipolar cycloaddition strategy. Fluorescence studies show that 3-HC-thienyl-nucleoside 1a displays high sensitivity of its dual emission to polarity changes and therefore is highly promising for nucleic acid labelling. 相似文献