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排序方式: 共有471条查询结果,搜索用时 31 毫秒
21.
Liliana C. Tomé Susana C. M. Fernandes Denilson Silva Perez Patrizia Sadocco Armando J. D. Silvestre Carlos Pascoal Neto Isabel M. Marrucho Carmen S. R. Freire 《Cellulose (London, England)》2013,20(4):1807-1818
Thin nanocomposite films of thermoplastic starch, chitosan and cellulose nanofibers (bacterial cellulose or nanofibrillated cellulose) were prepared for the first time by solvent casting of water based suspensions of the three polysaccharides. The role of the different bioploymers on the final properties (thermal stability, transparency, mechanical performance and antimicrobial activity) of the films was related with their intrinsic features, contents and synergic effects resulting from the establishment of interactions between them. Thermoplastic starch displays an important role on the thermal stability of the films because it is the most stable polysaccharide; however it has a negative impact on the mechanical performance and transparency of the films. The addition of chitosan improves considerably the transparency (up to 50 % transmittance for 50 % of chitosan, in respect to the amount of starch), mechanical performance and antimicrobial properties (at least 25 % of chitosan and no more than 10 % of cellulose nanofibers are required to observe bacteriostatic or bactericidal activity) but decrease their thermal stability. The incorporation of cellulose nanofibers had the strongest positive impact on the mechanical properties of the materials (increments of up to 15 and 30 MPa on the Young′s modulus and Tensile strength, respectively, for films with 20 % of BC or NFC). Nonetheless, the impact in thermal stability and mechanical performance of the films, promoted by the addition of chitosan and cellulose nanofibres, respectively, was higher than the expected considering their percentage contents certainly because of the establishment of strong and complex interactions between the three polysaccharides. 相似文献
22.
M. C. Neves C. S. R. Freire B. F. O. Costa C. Pascoal Neto T. Trindade 《Cellulose (London, England)》2013,20(2):861-871
Cellulose/iron oxide hybrids were prepared by the controlled hydrolysis of FeC2O4 in the presence of vegetable and bacterial cellulose fibres as substrates. By varying the relative amount of FeC2O4 and NaOH, either hematite or magnetic iron oxides were grown at the cellulose fibres surfaces. This chemical strategy was used for the production of a number of materials, whose coloristic properties associated to their reinforcement role allowed their use as new hybrid pigments for thermoplastic starch (TPS) based products. The TPS reinforced materials were characterized by several techniques in order to evaluate: the morphology and the compatibility between the matrix and the fillers; the mechanical reinforcement effect of the cellulose/iron oxide pigments on TPS and the coloristic properties of the composites. All materials showed good dispersion and strong adhesion for the cellulose/iron oxide nanocomposites in the TPS matrix thus resulting in improved mechanical properties. 相似文献
23.
We study the dynamics of patterns exhibited by rule 52, a totalistic cellular automaton displaying intricate behaviors and wide regions of active/inactive synchronization patches. Systematic computer simulations involving 2(30) initial configurations reveal that all complexity in this automaton originates from random juxtaposition of a very small number of interfaces delimiting active/inactive patches. Such interfaces are studied with a sidewise spatial updating algorithm. This novel tool allows us to prove that the interfaces found empirically are the only interfaces possible for these periods, independently of the size of the automata. The spatial updating algorithm provides an alternative way to determine the dynamics of automata of arbitrary size, a way of taking into account the complexity of the connections in the lattice. 相似文献
24.
25.
Luis Palomo Rafael Rodríguez Samara Medina Emilio Quio Juan Casado Flix Freire Francisco J. Ramírez 《Angewandte Chemie (International ed. in English)》2020,59(23):9080-9087
Poly(phenylacetylene)s are a family of helical polymers constituted by conjugated double bonds. Raman spectra of these polymers show a structural fingerprint of the polyene backbone which, in combination with its helical orientation, makes them good candidates to be studied by Raman optical activity (ROA). Four different well‐known poly(phenylacetylene)s adopting different scaffolds and ten different helical senses have been prepared. Raman and ROA spectra were recorded and allowed to establish ROA‐spectrum/helical‐sense relationships: a left/right‐handed orientation of the polyene backbone (Mhelix/Phelix) produces a triplet of positive/negative ROA bands. Raman and ROA spectra of each polymer exhibited the same profile, and the sign of the ROA spectrum was opposite to the lowest‐energy electronic circular dichroism (ECD) band, indicating a resonance effect. Resonance ROA appears then as an indicator of the helical sense of poly(phenylacetylene)s, especially for those with an extra Cotton band in the ECD spectrum, where a wrong helical sense is assigned based on ECD, while ROA alerts of this misassignment. 相似文献
26.
A simple transformation of the eight-carbon side chain of a natural spirostan sapogenin into the cephalostatin north 1 spiroketal moiety is described. This methodology, based on an intramolecular hydrogen abstraction reaction promoted by alkoxy radicals, permits the synthesis of C-22 and C-25 stereoisomers of the dioxaspiro[4.4]nonane cephalostatin ring system. The acid-catalyzed isomerization of the spirocenter in the different isomers is studied. [reaction: see text] 相似文献
27.
Osvaldo Pessoa Jr. Olival Freire Jr. Alexis De Greiff 《Physics in Perspective (PIP)》2008,10(2):138-162
In 1966 the Brazilian physicist Klaus Tausk (b. 1927) circulated a preprint from the International Centre for Theoretical
Physics in Trieste, Italy, criticizing Adriana Daneri,Angelo Loinger,and Giovanni Maria Prosperi’s theory of 1962 on the measurement
problem in quantum mechanics. A heated controversy ensued between two opposing camps within the orthodox interpretation of
quantum theory, represented by Léon Rosenfeld and Eugene P.Wigner. The controversy went well beyond the strictly scientific
issues, however, reflecting philosophical and political commitments within the context of the Cold War, the relationship between
science in developed and Third World countries, the importance of social skills, and personal idiosyncrasies. 相似文献
28.
29.
D.?Bazeia F.?A.?BritoEmail author W.?Freire R.?F.?Ribeiro 《The European Physical Journal C - Particles and Fields》2005,40(4):531-537
We investigate confinement from new global defect structures in three spatial dimensions. The global defects arise in models described by a single real scalar field, governed by special scalar potentials. They appear as electrically, magnetically or dyonically charged structures. We show that they induce confinement, when they are solutions of effective QCD-like field theories in which the vacua are regarded as color dielectric media with an antiscreening property. As expected, in three spatial dimensions the monopole-like global defects generate the Coulomb potential as part of several confining potentials.Received: 25 August 2004, Published online: 16 March 2005PACS:
11.27. + d, 12.39.-x 相似文献
30.
Influence of velocity in nanoscale friction processes 总被引:3,自引:0,他引:3
R. Prioli A.M.F. Rivas F.L. Freire Jr. A.O. Caride 《Applied Physics A: Materials Science & Processing》2003,76(4):565-569
Force-microscopy images of boric acid crystals were obtained experimentally and simulated with the use of a two-dimensional
mechanical model. An analysis of the stick and slip movement of the microscope tip shows that the energy-dissipation mechanism
is strongly influenced by the non-linear dynamics of the sliding system. The contributions of stick and viscous forces on
the energy dissipation (or friction forces) are studied as a function of the relative scanning velocity. At low relative velocities,
the stick forces are shown to be responsible for the energy dissipation. This energy is velocity-dependent, due to the coupling
between the two degrees of freedom of the sliding system. As the scanning velocity increases the stick forces are damped;
the viscous force is then predominant in the energy-dissipation process.
Received: 30 October 2001 / Accepted: 17 May 2002 / Published online: 22 November 2002
RID="*"
ID="*"Corresponding author. Fax: +55-21/2295-9397, E-mail: prioli@vdg.fis.puc-rio.br 相似文献