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691.
Russo J Tavares JM Teixeira PI Telo da Gama MM Sciortino F 《Physical review letters》2011,106(8):085703
We introduce a microscopic model for particles with dissimilar patches which displays an unconventional "pinched" phase diagram, similar to the one predicted by Tlusty and Safran in the context of dipolar fluids [Science 290, 1328 (2000)]. The model-based on two types of patch interactions, which account, respectively, for chaining and branching of the self-assembled networks-is studied both numerically via Monte Carlo simulations and theoretically via first-order perturbation theory. The dense phase is rich in junctions, while the less-dense phase is rich in chain ends. The model provides a reference system for a deep understanding of the competition between condensation and self-assembly into equilibrium-polymer chains. 相似文献
692.
Teixeira JM Ventura J Araujo JP Sousa JB Wisniowski P Cardoso S Freitas PP 《Physical review letters》2011,106(19):196601
We report an inelastic electron tunneling spectroscopy study on MgO magnetic junctions with thin barriers (0.85-1.35 nm). Inelastic electron tunneling spectroscopy reveals resonant electronic trapping within the barrier for voltages V>0.15 V. These trapping features are associated with defects in the barrier crystalline structure, as confirmed by high-resolution transmission electron microscopy. Such defects are responsible for resonant tunneling due to energy levels that are formed in the barrier. A model was applied to determine the average location and energy level of the traps, indicating that they are mostly located in the middle of the MgO barrier, in accordance with the high-resolution transmission electron microscopy data and trap-assisted tunneling conductance theory. Evidence of the influence of trapping on the voltage dependence of tunnel magnetoresistance is shown. 相似文献
693.
Rodolfo Teixeira de Souza Ervin Kaminski Lenzi Luiz Roberto Evangelista 《Liquid crystals》2013,40(12):1559-1568
The influence of surface viscosity and anchoring energy on the reorientation process of a nematic liquid crystal cell is theoretically investigated. The cell is a slab of thickness, d, whose limiting surfaces are characterised by different anchoring strengths and present easy directions parallel to the bounding surfaces, changing with time due to some external action. The exact space-time profile of the director angle is obtained by means of integral transform techniques and a Green function approach. From this formalism, the time dependence of the optical path difference is exactly determined and its behaviour is analysed in connection with the presence of surface viscosity and different anchoring energies. The problem is also exactly solved in the presence of a constant electric field. It is shown that the compatibility problem between the time derivative of the director field on the surface and in the bulk can be avoided. 相似文献
694.
Ana Carolina Corrêa Eliangela de Morais Teixeira Vitor Brait Carmona Kelcilene Bruna Ricardo Teodoro Cauê Ribeiro Luiz Henrique Capparelli Mattoso José Manoel Marconcini 《Cellulose (London, England)》2014,21(1):311-322
Nanocomposites of polyamides with cellulose whiskers are difficult to obtain by conventional processing of extrusion and injection molding because of the low thermal stability of the cellulosic nanostructures and the relatively high processing temperature of polyamides, which is higher than the temperature of thermal degradation of cellulose whiskers. Thus, in this study cellulose whiskers were coated with polyamide 6 (PA6) in order to increase their thermal stability and prevent the formation of agglomerates. This coating on cellulose whiskers allows their application to obtain nanocomposites with polyamides, whose processing temperatures are relatively high, around 250 °C. Cellulose whiskers (CWs) were obtained from cotton fibers by acid hydrolysis. The freeze-dried CWs were coated with PA6 by dispersing them in formic acid; PA6 was solubilized in this suspension. The cellulose-coated whiskers (CCWs) were characterized by X-ray diffraction, differential scanning calorimetry (DSC), thermogravimetry (TG), scanning electron microscopy (SEM-FEG) and infrared spectroscopy. SEM-FEG and TG results showed that the PA6 coating on CWs prevented high agglomeration of dried CWs and promoted an increase in their thermal stability from 180 to 280 °C, allowing the use of CCWs to obtain nanocomposites with PA6 using conventional processing routes, such as extrusion and injection molding, at appropriate processing temperatures. In this way, 1 wt% CCWs was used to prepare nanocomposites with PA6. The PA6 + 1CW nanocomposites were compared to neat PA6 without CWs. The samples were characterized by tensile tests and DSC, and the results showed that the PA6 coating on CWs was effective in raising the thermal stability of CWs, improving the dispersion of CWs in the matrix of PA6, resulting in a 45 % increase in the elastic modulus of the nanocomposite with only 1 wt% of coated cellulose whiskers in comparison to neat PA6. 相似文献
695.
Ruzene DS Silva DP Vicente AA Gonçalves AR Teixeira JA 《Applied biochemistry and biotechnology》2008,147(1-3):85-96
The effects of alkaline treatments of the wheat straw with sodium hydroxide were investigated. The optimal condition for extraction
of hemicelluloses was found to be with 0.50 mol/l sodium hydroxide at 55 °C for 2 h. This resulted in the release of 17.3%
of hemicellulose (% dry starting material), corresponding to the dissolution of 49.3% of the original hemicellulose. The yields
were determined by gravimetric analysis and expressed as a proportion of the starting material. Chemical composition and physico-chemical
properties of the samples of hemicelluloses were elucidated by a combination of sugar analyses, Fourier transform infrared
(FTIR), and thermal analysis. The results showed that the treatments were very effective on the extraction of hemicelluloses
from wheat straw and that the extraction intensity (expressed in terms of alkali concentration) had a great influence on the
yield and chemical features of the hemicelluloses. The FTIR analysis revealed typical signal pattern for the hemicellulosic
fraction in the 1,200–1,000 cm−1 region. Bands between 1,166 and 1,000 cm−1 are typical of xylans. 相似文献
696.
Graminha AE Batista AA Mendes IC Teixeira LR Beraldo H 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2008,69(4):1277-1282
[RuCl(L)(PPh(3))(2)] complexes with 2-benzoylpyridine- and 2-pyridineformamide-derived thiosemicarbazones (HL) were obtained and fully characterized. The complexes form cis-trans isomers. The cis isomer is disfavored by the sterical effect of two bulky groups close to each other whereas the trans isomer is disfavored by the electronic effect of competition of two phosphorous for pi-bonding d orbitals of the metal. Our results suggest that, although both factors may be operating simultaneously, in CH(2)Cl(2) solution the balance of these counterpoising effects favors the formation of the trans isomer. 相似文献
697.
Barbosa LC Pereira UA Martinazzo AP Maltha CR Teixeira RR Melo Ede C 《Molecules (Basel, Switzerland)》2008,13(8):1864-1874
The concentration and the chemical composition of the essential oils obtained from different samples of Cymbopogon citratus were evaluated. Among the 12 samples investigated (11 dried leaf samples and fresh plant leaves), seven presented essential oil concentrations within the threshold established by the Brazilian legislation. The moisture content was also determined and the majority of the samples presented humidity contents near 12%. The GC and GC/MS analyses of the essential oils led to identification of 22 compounds, with neral and geranial as the two major components. The total percentage of these two compounds varied within the investigated sample oils from 40.7% to 75.4%. In addition, a considerable variation in the chemical composition of the analyzed samples was observed. The process of grinding the leaves significantly decreased (by up to 68%) the essential oil content, as well as the percentage of myrcene in the oils. 相似文献
698.
Growing concern about the contamination of wastewaters by antibiotics demands fast but sensitive analytical methodologies, for the screening of a large number of samples. The purpose of this work was to develop a simple methodology, using direct injection of the samples, by HPLC with diode array detection (DAD), for a multiresidue analysis of five antibiotics of different classes. Wastewater from an urban water treatment plant was selected as a model to study possible coelution of interfering compounds. The linearity interval ranged from 40 to 400 microg/L for amoxicillin (Amox), metronidazole (Metro), cefazolin (Cefa), and chloramphenicol (Chloram) and from 20 to 200 microg/L for sulfamethoxazole (Sulfa), with LODs lower than 14 microg/L. Repeatability, expressed by the CV of six repeated injections, ranged from 1 to 8%, while the intermediate precision varied between 2 and 11%. The recovery ranged from 90 to 109%. This method enables the fast screening of a large number of samples, with an expanded uncertainty in the 1-22% range. The advantage of the proposed method is to significantly reduce the number of samples to be analyzed by more complex methods. 相似文献
699.
Cottenye N Teixeira F Ponche A Reiter G Anselme K Meier W Ploux L Vebert-Nardin C 《Macromolecular bioscience》2008,8(12):1161-1172
Oligonucleotide model surfaces allowing independent variation of topography and chemical composition were designed to study the adhesion and biofilm growth of E.coli. Surfaces were produced by covalent binding of oligonucleotides and immobilization of nucleotide-based vesicles. Their properties were confirmed through a combination of fluorescence microscopy, XPS, ellipsometry, AFM and wettability studies at each step of the process. These surfaces were then used to study the response of three different strains of E.coli quantified in a static biofilm growth mode. This study led to convincing evidence that oligonucleotide-modified surfaces, independent of the topographical feature used in this study, enhanced curli expression without an increase in the number of adherent bacteria. 相似文献
700.
We develop a variational theory to study the free boundary regularity problem for elliptic operators: Lu=Dj(aij(x)Diu)+biui+c(x)u=0 in {u>0}, 〈aij(x)∇u,∇u〉=2 on ∂{u>0}. We use a singular perturbation framework to approximate this free boundary problem by regularizing ones of the form: Luε=βε(uε), where βε is a suitable approximation of Dirac delta function δ0. A useful variational characterization to solutions of the above approximating problem is established and used to obtain important geometric properties that enable regularity of the free boundary. This theory has been developed in connection to a very recent line of research as an effort to study existence and regularity theory for free boundary problems with gradient dependence upon the penalization. 相似文献