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91.
The permeabilities and solubilities of five gases are reported for bisphenol-A polycarbonate (PC), tetramethyl polycarbonate (TMPC), and tetramethyl hexafluoro polycarbonate (TMHFPC) at temperatures up to 200°C. The temperature dependence of permselectivity is discussed in terms of solubility and diffusivity selectivity changes with temperature for CO2/CH4 and He/N2 gas separations. The activation energies for permeation and diffusion and the heats of sorption are also reported for each gas in the three polycarbonates. Analysis of these values provides a better fundamental understanding of the effect of polymer-penetrant interactions and polymer backbone structure on the temperature dependence of the transport and sorption properties of gases in membrane separation processes. Important factors affecting the solubility and diffusivity selectivity losses or gains with increased temperature are also identified through correlation of these data with physical properties of the gases and polymers. These conclusions provide a framework for choosing the most promising membrane materials for particular gas separations at elevated temperatures. © 1994 John Wiley & Sons, Inc. 相似文献
92.
The dissolution of an iron-containing substance in orthophosphoric acid either alone or mixed with nitric acid or nitric acid-sulphuric acid mixture at 270-280 degrees has been examined, to determine the composition of the solutions which are used for analytical determinations. The possibility of condensation of H(3)PO(4) and formation of polyphosphoric acids during the dissolution has been proved. Solutions which may contain pyrophosphoric acid, tripolyphosphoric acid and tetrapolyphosphoric acid have been obtained. The exact composition of these solutions depends on the initial H(3)PO(4) concentration, the heating time, and, in some cases, the presence of nitric acid and sulphuric acid. It is assumed that the nitric and sulphuric acid affect the loss of water and therefore the thermal dehydration of the orthophosphoric acid during the dissolution. No qualitative changes in the phosphoric acid composition have been observed in the presence of Fe(III). Formation of Fe(III) polyphosphates is assumed. 相似文献
93.
Degradation induced by ionizing radiations on polyphosphazenes, dissolved in chlorinated methanes, is the subject of this investigation. Both steady gamma and pulse radiolysis techniques have been employed.The labile sites for radical attack in poly[bis(p-tolylamino)] phosphazene were identified as the aniline N-H and benzyl C-H bonds of the pendant groups. All the primary radicals, by themselves or after peroxidation, can give rise to macroradicals by hydrogen abstraction from the polymer molecules. Appreciable crosslinking appeared only in the presence of quite a high concentration of oxygen, whereas, in its absence or after it had been consumed, practically no changes were detected. Optical spectra of intermediates formed after pulse radiolysis are reported and attributed to radical-polymer charge transfer complexes, anilinium ions and toluidine radicals. p-Toluidine is formed as an end-product by hydrogen abstraction from polymer molecules. In solvents where hydrogen chloride can be formed, the amino functional group is affected. 相似文献
94.
The gamma induced polymerizations of iso-butyl- (i-BMA) and dodecyl-(DMMA) methacrylates under non-stationary conditions have been studied, utilizing a calorimetric method already used for other methacrylates.For both monomers, the polymerization rate was found to vary linearly with at the dose rates and temperatures examined. The rate constants for propagation and termination are compared with those of methyl-, ethyl-, n-butyl- and cyclohexyl methacrylate; the results are discussed. 相似文献
95.
Georgieva DN Genov N Nikolov P Aleksiev B Rajashankar KR Voelter W Betzel C 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2003,59(3):617-627
The neurotoxic complex Vipoxin is the lethal component of the venom of Vipera ammodytes meridionalis, the most toxic snake in Europe. It is a complex between a toxic phospholipase A2 (PLA2) and a non-toxic and catalytically inactive protein, stabilizing the enzyme and reducing the activity and toxicity. Structure-function relationships in this complex were studied by spectroscopic methods. A good correlation between the ionization behaviour and accessible surface area (ASA) of the tyrosyl residues was observed. In the toxic PLA2 subunit phenolic groups participate in H-bonding network that stabilizes the catalytically and pharmacologically active conformation. The tryptophan fluorescence decay of Vipoxin is well fitted by two exponentials with lifetimes of 0.1 (54%) and 2.5 (46%) ns. W20P, W31P and W31I are located in the interface between the two subunits and participate in hydrophobic interactions stabilizing the complex. Dissociation of the complex leads to a transition of the tryptophans from hydrophobic to hydrophilic environment, which influences mainly tau2. The longer lifetime is more sensitive to the polarity of the environment. Circular dichroism measurements demonstrate that the two components of the neurotoxin preserve their secondary structure after dissociation of the complex. The results of the spectroscopic studies are in accordance with a mechanism of blockade of transmission across the neuromuscular junctions of the breathing muscles by interaction of a dissociated toxic PLA2 with a membrane. The loss of toxicity is connected with slight changes in the secondary structure of PLA2. CD studies also show a substantial contribution of disulfide bonds to the stability of the neurotoxic complex and its components. 相似文献
96.
The percentage of thermal dehydration and decomposition of ordinary and deuterated hydrates of barium acetate (tri- and monohydrates) were studied with a derivatograph and by DSC method. The observed phase transitions were identified and their corresponding enthalpy changes determined. The latter were compared with those estimated on thermodynamic data. The conclusions made on the dehydration process in correlation with IR spectra were used as information about the structure of the dehydrated water in the compounds investigated. The end product was also identified as barium carbonate. 相似文献
97.
Artuso F D'Archivio AA Lora S Jerabek K Králik M Corain B 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(21):5292-5296
The microporous (gel-type) functional resin co-poly-N,N-dimethylacrylamide (DMAA) (88 % mol)/methacrylic acid (MAA) (8 % mol)/N,N'-methylenebisacrylamide (MBAA) (4 % mol) (MPIF(H)) is employed as the hosting framework for the production of resin-supported Pd(0) nanoclusters. The obtained composite MPIF(-)Na(+)/Pd(0) is prepared upon reducing, in ethanol, MPIF(-)Pd(2+) (0.5), obtained upon previous homogeneous dispersion of "Pd(2+)" inside the resin particles (XRMA control) through ion-exchange. Metal nanoclusters appear to be size-controlled (2.0+/-0.2 nm) and are seen to reasonably fit the predominant resin "nanopores" diameter, determined in ethanol (3.2 nm) by means of inverse steric exclusion chromatography (ISEC). 相似文献
98.
L. Minkova E. Lefterova Ts. Koleva E. Nedkov M. Nikolova 《Colloid and polymer science》1988,266(10):898-905
Isotactic polypropylene films, Buplen Type, 40m thick, irradiated by a60Co source to doses 0.37–37 Mrad, are investigated by means of optical microscopy, WAXS, thermogravimetry, DSC and DTA. The original film exhibits a paracrystal structure. Irradiation does not change the films' structure. The kinetic parameters of the non-isothermal destruction and the thermodynamic parameters of melting are obtained. The samples irradiated to small doses (up to 3 Mrad) are thermally more stable; the activating energy of the destruction is higher than that of the original film. The temperature of melting slightly increases, while the enthalpy of melting decreases. For the range of doses of 3.7–37 Mrad, the films show low thermal stability and the destruction proceeds with low activating energy. From the results of the data obtained, the following assumptions are made: the-irradiation causes simultaneous crosslinking and chain scission at random sites along the chains. Fragments of partially crosslinked molecules and fractions of low molecular linear segments are formed. The destruction caused by radiation prevails above 3 Mrad. 相似文献
99.
M. Carenza S. Lora P. Caliceti O. Schiavon F. M. Veronese 《Radiation Physics and Chemistry》1993,42(4-6):897-901
The controlled release of peptides and proteins from hydrogels obtained by radiation-induced polymerization of 2-hydroxyethyl methacrylate at a low temperature was studied. It was found that the extent of release progressively decreased as protein molecular weight increased until no further release occurred above a critical value of the latter. However, an increasing rate of protein release was found if the polymerization was carried out in the presence of poly(ethylene glycol), PEG. Moreover, only with high molecular weight PEGs were large proteins released. The release data as a function of swellability and porosity of polymer matrices were discussed. 相似文献
100.
Galabov B Nikolova V Wilke JJ Schaefer HF Allen WD 《Journal of the American Chemical Society》2008,130(30):9887-9896
The S N2 identity exchange reactions of the fluoride ion with benzyl fluoride and 10 para-substituted derivatives (RC6H 4CH 2F, R = CH3, OH, OCH 3, NH2, F, Cl, CCH, CN, COF, and NO2) have been investigated by both rigorous ab initio methods and carefully calibrated density functional theory. Groundbreaking focal-point computations were executed for the C6H5CH 2F + F (-) and C 6H 5CH2Cl + Cl (-) SN2 reactions at the highest possible levels of electronic structure theory, employing complete basis set (CBS) extrapolations of aug-cc-pV XZ (X = 2-5) Hartree-Fock and MP2 energies, and including higher-order electron correlation via CCSD/aug-cc-pVQZ and CCSD(T)/aug-cc-pVTZ coupled cluster wave functions. Strong linear dependences are found between the computed electrostatic potential at the reaction-center carbon atom and the effective SN2 activation energies within the series of para-substituted benzyl fluorides. An activation strain energy decomposition indicates that the SN2 reactivity of these benzylic compounds is governed by the intrinsic electrostatic interaction between the reacting fragments. The delocalization of nucleophilic charge into the aromatic ring in the SN2 transition states is quite limited and should not be considered the origin of benzylic acceleration of SN2 reactions. Our rigorous focal-point computations validate the benzylic effect by establishing SN2 barriers for (F (-), Cl (-)) identity exchange in (C6H5CH2F, C6H 5CH2Cl) that are lower than those of (CH3F, CH3Cl) by (3.8, 1.6) kcal mol (-1), in order. 相似文献