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21.
Nils Siegbahn Mats-Olle Maånsson Klaus Mosbach 《Applied biochemistry and biotechnology》1987,12(2):91-105
A soluble, bifunctional enzyme complex has been prepared by crosslinking lactate dehydrogenase and alcohol dehydrogenase with
glutaraldehyde. The crosslinking was performed on a solid phase while the active sites of alcohol dehydrogenase and lactate
dehydrogenase were held adjacent to one another with the aid of a bis-NAD analog. Subsequently, the enzyme complex was released
from the solid phase. The soluble enzyme complex was then purified by using NAD-Sepharose as an affinity adsorbent. Based
on gel filtration experiments, the complex was estimated to consist of one of each dehydrogenase.
By using a third enzyme, lipoamide dehydrogenase, which competes with lactate dehydrogenase for NADH produced by alcohol dehydrogenase,
the effect of site-to-site orientation was studied. It was found that about 83% of the NADH produced by alcohol dehydrogenase
was oxidized by site-to-site oriented lactate dehydrogenase compared to a figure of only about 61% obtained in an identical
system of separate enzymes. This indicates that given two alternative routes, the preference for the one to lactate dehydrogenase
over the one to lipoamide dehydrogenase is enhanced when lactate dehydrogenase and alcohol dehydrogenase are site-to-site
oriented. 相似文献
22.
René Androsch 《European Polymer Journal》2007,43(1):93-108
The melt-crystallization of an oligo[(R)-3-hydroxybutyrate] with five repeating units has been analyzed using standard and temperature-modulated calorimetry, optical microscopy, and atomic force microscopy. Specimens of different crystallinity and supermolecular structure were generated by variation of the rate of cooling of a quiescent melt, or by variation of the temperature of isothermal crystallization. Completely amorphous samples can be obtained by cooling of the melt at a rate of 40 K min−1, or faster, to a temperature lower than the glass transition. The crystallinity depends on the crystallization temperature. The maximum enthalpy-based crystallinity of about 40-45% is obtained by crystallization at temperatures lower than the temperature of the maximum crystallization rate, which is between 310 and 320 K. Analysis of the apparent heat capacity in metastable structural equilibrium reveals reversible melting at temperatures between 320 and 370 K by observation of an excess heat capacity above the level of the vibrational heat capacity, i.e., in the temperature range of irreversible reorganization and melting. The reversible melting is discussed in the context of coupling of the crystalline and amorphous phases, and compared to earlier studies on oligoethylene and oligo(oxyethylene). The presence of crystals causes formation of a rigid amorphous fraction of about 30% at a crystallinity of 40%. Optical and atomic force microscopy reveal spherulitic crystallization. At relatively high crystallization temperature, and in the early stage of the crystallization process, dendrites are observed which finally yield spherulites of decreased perfection. Larger spherulites of higher perfection grow at relatively low crystallization temperature, as deduced from the appearance of the Maltese cross, and the regularity of banding. The band spacing is less than 5 μm, as is accurately determined by atomic force microscopy. The temperature dependence of the spherulitic growth rate is in accord with the calorimetric analysis of the crystallization rate. 相似文献
23.
Two hydrophobic vinyl saccharide monomers based on D ‐glucose and D ‐fructose were polymerized by employing the reversible addition‐fragmentation transfer (RAFT) miniemulsion polymerization technique to prepare well‐designed glycopolymers. Three dithiobenzoate‐RAFT agents [S?C(Ph)S? R], 1‐phenylethyl dithiobenzoate (PED), 2‐phenylprop‐2‐yl dithiobenzoate (PPD), and 2‐cyanoprop‐2‐yl dithiobenzoate (CPD), were used to control the growth of polymer chains. The best results were obtained in the presence of the PPD‐RAFT agent and the formed polymers have polydispersity index's (PDI) lower than 1.15. Under adequate miniemulsion polymerization conditions, a glycopolymer with PDI of 1.1 and molecular weight of 5 × 104 g/mol has been successfully synthesized in a short reaction time of 100 min. Furthermore, some block copolymers containing saccharide segment with butyl or methyl methacrylate were prepared. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
24.
Chau‐Hui Wang Ging‐Ho Hsiue 《Journal of polymer science. Part A, Polymer chemistry》2002,40(8):1112-1121
A novel series of temperature‐ and pH‐sensitive hydrogels based on poly(2‐ethyl‐2‐oxazoline) and three‐arm poly(D,L ‐lactide) were synthesized via photocopolymerization. For the creation of polymeric networks, two types of macromers terminated with methacrylate groups were prepared: poly(2‐ethyl‐2‐oxazoline) dimethacrylate and three‐arm poly(D,L ‐lactide) trimethacrylate. The chemical structures were analyzed with 1H NMR and Fourier transform infrared techniques. The thermal behaviors, morphologies, and swelling properties were measured for the characterization of the polymeric networks. All the poly(2‐ethyl‐2‐oxazoline)/three‐arm poly(D,L ‐lactide)hydrogels provided high water retention capacity and exhibited reversible swelling–shrinking behavior in response to temperature and pH variations. The hydrogels with higher poly(2‐ethyl‐2‐oxazoline) dimethacrylate contents were more effective in raising the swelling ratio and temperature and pH sensitivity. However, higher contents of three‐arm poly(D,L ‐lactide) trimethacrylate produced larger particles and pore sizes in the hydrogels. This study effectively proves that this unique combination of water swellability and biodegradability provides hydrogels with a much wider range of applications in biomedical fields. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1112–1121, 2002 相似文献
25.
We have developed a time-reversible rigid-body (rRB) molecular dynamics algorithm in the isothermal-isobaric (NPT) ensemble. The algorithm is an extension of rigid-body dynamics [Matubayasi and Nakahara, J Chem Phys 1999, 110, 3291] to the NPT ensemble on the basis of non-Hamiltonian statistical mechanics [Martyna, G. J. et al., J Chem Phys 1994, 101, 4177]. A series of MD simulations of water as well as fully hydrated lipid bilayer systems have been undertaken to investigate the accuracy and efficiency of the algorithm. The rRB algorithm was shown to be superior to the state-of-the-art constraint-dynamics algorithm SHAKE/RATTLE/ROLL, with respect to computational efficiency. However, it was revealed that both algorithms produced accurate trajectories of molecules in the NPT as well as NVT ensembles, as long as a reasonably short time step was used. A couple of multiple time-step (MTS) integration schemes were also examined. The advantage of the rRB algorithm for computational efficiency increased when the MD simulation was carried out using MTS on parallel processing computer systems; total computer time for MTS-MD of a lipid bilayer using 64 processors was reduced by about 40% using rRB instead of SHAKE/RATTLE/ROLL. 相似文献
26.
Michael J. Monteiro 《Journal of polymer science. Part A, Polymer chemistry》2005,43(15):3189-3204
Living radical polymerization has allowed complex polymer architectures to be synthesized in bulk, solution, and water. The most versatile of these techniques is reversible addition–fragmentation chain transfer (RAFT), which allows a wide range of functional and nonfunctional polymers to be made with predictable molecular weight distributions (MWDs), ranging from very narrow to quite broad. The great complexity of the RAFT mechanism and how the kinetic parameters affect the rate of polymerization and MWD are not obvious. Therefore, the aim of this article is to provide useful insights into the important kinetic parameters that control the rate of polymerization and the evolution of the MWD with conversion. We discuss how a change in the chain‐transfer constant can affect the evolution of the MWD. It is shown how we can, in principle, use only one RAFT agent to obtain a polymer with any MWD. Retardation and inhibition are discussed in terms of (1) the leaving R group reactivity and (2) the intermediate radical termination model versus the slow fragmentation model. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3189–3204, 2005 相似文献
27.
The temperature dependences of the equilibrium constants of two chain reversible reactions in quinonediimine (quinonemonoimine)—2,5-dichlorohydroquinone
systems in chlorobenzene were studied. The enthalpy of equilibrium of the reversible reaction of quinonediimine with 4-hydroxydiphenylamine
was estimated from these data (ΔH = − 14.4±1.6 kJ mol−1) and a more accurate value of the N-H bond dissociation energy in the 4-anilinodiphenylaminyl radical was determined (D
NH = 278.6±3.0 kJ mol−1). A chain mechanism was proposed for the reaction between quinonediimine and 2,5-dichlorohydroquinone, and the chain length
was estimated (ν = 300 units) at room temperature. Processing of published data on the rate constant of the reaction of styrylperoxy
radicals with 2,5-dichlorohydroquinone in the framework of the intersecting parabolas method gave the O-H bond dissociation
energy in 2,5-dichlorohydroquinone: D
OH = 362.4±0.9 kJ mol−1. Taking into account these data, the O-H bond dissociation energy in the 2,5-dichlorosemiquinone radical was found: D
OH = 253.6±1.9 kJ mol−1.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1661–1666, October, 2006. 相似文献
28.
Summary Analytical peak-shape equations were derived for first-order reversible reactions occurring in a chromatographic reactor by
treating the reversible reactions as consecutive reactions with alternating products. The results of the analytical peak-shape
equations were compared with those from a numerical solution of the partial differential equation system modeling the chromatographic
reactor. For small to medium conversions the correspondence was found to be sufficiently close to enable substitution of the
numerical solution in fitting procedures for the determination of rate constants.
Presented at the 21st ISC held in Stuttgart, Germany, 15th–20th September, 1996 相似文献
29.
Purification and reversible immobilization of d-amino acid oxidase from Trigonopsis variabilis could be simultaneously accomplished by hydrophobic interaction on Phenyl Sepharose CL-4B in the presence of 50 mM pyrophosphate buffer (pH 8.5). The presence of a high salt concentration of 2M, which is generally required for the hydrophobic interactions, was not essential for the hydrophobic immobilization. The
enzyme in free as well as immobilized form was optimally active between pH 7.0 and 9.0. The immobilized preparation could
be reused in a batch process for the conversion of d-amino acids to α-keto acids. When the activity of the preparation dropped below practical limits, the gel could be regenerated
by water wash and recharged with fresh crude extract from yeast. 相似文献
30.
Different grades of linear low density polyethylenes (LLDPEs) have been quenched cooled step-wise and crystallised isothermally
at (a series of increasing) temperatures in a DSC (thermal fractionated samples). These samples have been investigated by
temperature modulated DSC (MDSC). The heat flow curves of the thermal fractionated materials were compared with those obtained
from samples crystallised at a relatively slow cooling rate of 2 K min-1(standard samples).
The melting enthalpy obtained from the total heat flow of the thermal fractionated samples was 0-10 J g-1higher than those of standard samples. The melting enthalpy obtained from the reversing heat flows was 13-31 J g-1lower in the thermal fractionated samples than in the standard samples. The ratio of the reversing melting enthalpy to the
total melting enthalpy increased with decreasing density of the PE. The melting temperature of the endotherms formed by the
step-wise cooling was 9 K higher than the crystallisation temperature.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献