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171.
Trimethylene carbonate (TMC) was copolymerized with D ,L ‐lactide (DLLA) or with ε‐caprolactone (CL), and the degradation of melt‐pressed solid copolymer films in phosphate‐buffered saline at pH 7.4 and 37 °C was followed for a period of over two years. The parent homopolymers were used as reference materials. The degradation profile of TMC‐DLLA‐ and TMC‐CL based copolymers was similar and was best described by autocatalyzed bulk hydrolysis, preferentially of ester bonds. The hydrolysis rates varied by two orders of magnitude, depending on polymer composition and physical characteristics under the degradation conditions. TMC‐DLLA copolymers degraded faster than the parent homopolymers. The copolymers lost their tensile strength in less than five months, after which mass loss occurred. Copolymers with 50 or 80 mol‐% of TMC underwent total degradation in eleven months. For TMC‐CL copolymers, a slow and gradual decrease in molecular weight and deterioration of the mechanical performance was observed. These copolymers maintained suitable mechanical properties for seventeen months or longer. Chain scission in the semicrystalline copolymers resulted in an increase in crystallinity. In comparison with the CL homopolymer, the introduction of a small amount of TMC (10 mol‐%) significantly reduced the increase in crystallinity during degradation. Poly(TMC) specimens were dimensionally stable and showed a negligible decrease in molecular weight. A 60% decrease in the initial tensile strength of the polymer samples was observed after two years.
172.
Stueber D Orendt AM Facelli JC Parry RW Grant DM 《Solid state nuclear magnetic resonance》2002,22(1):29-49
The principal values of the 13C chemical shift tensors in potassium carbonate (K2CO3), trithiocarbonate (K2CS3), bicarbonate (KHCO3), methylcarbonate (KO2COCH3), S-methyl-monothiocarbonate (KO2CSCH3), O-methyl-monothiocarbonate (KOSCOCH3), S-methyl-dithiocarbonate (KOSCSCH3), and O-methyl-dithiocarbonate (KS2COCH3), were measured in solid-state nuclear magnetic resonance experiments. Chemical shift tensor calculations on the corresponding isolated anions were used to assign the chemical shift tensor orientations in the molecular frames of all anions. The correlation between experimental and calculated principal values improves significantly when the calculations are performed on isolated anions with proton-optimized X-ray geometries rather than on isolated anions with fully optimized geometries. Further considerable improvement in the correlation is achieved by utilizing the embedded ion method, which was recently developed to include electrostatic crystal potentials in chemical shift tensor calculations on ionic compounds. Similarities and differences in the chemical shift tensor orientations and principal values of the trigonal sp2 carbon atoms in the carbonate and thiocarbonate anions are compared with those known for condensed polyaromatic hydrocarbons. 相似文献
173.
Several representative examples are given of the successful application of negative staining across the holes of holey carbon support films using 5% (w/v) ammonium molybdate solution containing trehalose. The inclusion of 0.1% (w/v) trehalose is considered to be most satisfactory, although good data have also been obtained in the presence of 0.01 and 1.0% (w/v) trehalose. The examples given fall into the following groups: protein molecules in the absence of polyethylene glycol (PEG), protein molecules in the presence of PEG (Mr 1000), lipoproteins, lipids and membranes, filaments and tubules, viruses in the absence of PEG, viruses in the presence of PEG, aqueous polymer solutions, and finally for comparison purposes, four unstained samples studied in the presence of trehalose alone. In all these cases, and many others not documented here, successful spreading of the sample across holes has been achieved, with the sample embedded within a thin film of air-dried ammonium molybdate+trehalose. These specimens can be rapidly produced and provide an alternative to negatively stained specimens on carbon support films. Specimen stability in the electron bean is good and such specimens can usually generate superior negatively stained TEM images without flattening and adsorption artefacts. The formation of 2-D arrays/crystals of protein molecules and viruses, suspended across holes in the presence of ammonium molbybdate+trehalose, and trehalose alone, is also demonstrated. 相似文献
174.
Peter Hilton Dirk Scevenels 《Proceedings of the American Mathematical Society》1999,127(11):3433-3438
We prove that there is a bijection between the Mislin genus of a circle bundle over a certain nilpotent base space , which is constructed from a nilpotent group of a certain specified type, and the Mislin genus of itself.
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177.
We present an experimental realization of Hardy's thought experiment [Phys. Rev. Lett. 68, 2981 (1992)], using photons. The experiment consists of a pair of Mach-Zehnder interferometers that interact through photon bunching at a beam splitter. A striking contradiction is created between the predictions of quantum mechanics and local hidden variables. The contradiction relies on nonmaximally entangled position states of two particles. A Clauser-Horne-type inequality is derived and violated. 相似文献
178.
Nonlinear photonic crystals can be used to provide phase matching for frequency conversion in optically isotropic materials. The phase-matching mechanism proposed here is a combination of form birefringence and phase velocity dispersion in a periodic structure. Since the phase matching relies on the geometry of the photonic crystal, it becomes possible to use highly nonlinear materials. This is illustrated considering a one-dimensional periodic Al0.4Ga0.6As/air structure for the generation of 1.5 microm light. We show that phase-matching conditions used in schemes to create entangled photon pairs can be achieved in photonic crystals. 相似文献
179.
180.
Zahn D 《Physical review letters》2004,93(22):227801
Insight into the boiling of water is obtained from molecular dynamics simulations. The process is initiated by the spontaneous formation of small vacuum cavities in liquid water. By themselves, these defects are very short lived. If, however, several cavities occur at close distances, they are likely to merge into larger vacuum holes. At the liquid-vapor interfaces, single or small groups of water molecules tend to leave the liquid surface. Once the system is propagated beyond the transition state, these evaporation events outnumber the competing reintegration into the hydrogen-bonded network. 相似文献