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801.
High pressure experimental studies show that in certain mesogenic materials, the nematic-smectic A (N-Sm A) transition temperature
TAN exhibits nonlinear pressure dependence. As a consequence, the material
shows reentrant phenomena that is a phase sequence nematic — smectic A — reentrant nematic appears. The
characteristic features of this phenomenon have been addressed here within
the framework of Landau-de-Gennes theory, where the coupling between nematic
and smectic A order parameters (γ, λeff) plays an important role. The cubic coupling γ is chosen to be negative in order to form Sm A phase whereas the biquadratic
coupling λeff is made large and positive to obtain reentrant behaviour. In the present work, we incorporate the pressure dependence in
the theory through γ and λeff which justifies the experimental pressure dependence in the reentrant transition temperature
. The pressure dependence of γ and λeff are employed in the calculation of excess specific heat capacity near the reentrant transition. The computed heat capacity
shows strong pressure dependence near the reentrant transition which
can be confirmed from high pressure measurement. 相似文献
802.
The formal [2sigma + 2sigma + 2pi] cycloaddition of quadricyclane, 1, with dimethyl azodicarboxylate, 2, in water has been studied using DFT methods at the B3LYP/6-31G** and MPWB1K/6-31G** levels. In the gas phase, the reaction of 1 with 2 has a two-stage mechanism with a large polar character and an activation barrier of 23.2 kcal/mol. Inclusion of water through a combined discrete-continuum model changes the mechanism to a two-step model where the first nucleophilic attack of 1 to 2 is the rate-limiting step with an activation barrier of 14.7 kcal/mol. Analysis of the electronic structure of the transition state structures points out the large zwitterionic character of these species. A DFT analysis of the global electrophilicity and nucleophilicity of the reagents provides a sound explanation about the participation of 1 as a nucleophile in these cycloadditions. This behavior is reinforced by a further study of the reaction of 1 with 1,1-dicyanoethylene. 相似文献
803.
A method for speciation, preconcentration and separation of Fe(II) and Fe(III) in different matrices was developed using solvent extraction and flame atomic absorption spectrometry. 4-Acetyl-5-methyl-1-phenyl-1H-pyrazole-3-carboxylic acid (AMPC) was used as a new complexing reagent for Fe(III). The Fe(III)-AMPC complex was extracted into methyl isobutyl ketone (MIBK) phase in the pH range 1.0-2.5, and Fe(II) ion remained in aqueous phase at all pH. The chemical composition of the Fe(III)-AMPC complex was determined by the Job's method. The optimum conditions for quantitative recovery of Fe(III) were determined as pH 1.5, shaking time of 2 min, 1.64 × 10−4 mol L−1 AMPC reagent and 10 mL of MIBK. Furthermore, the influences of diverse metal ions were investigated. The level of Fe(II) was calculated by difference of total iron and Fe(III) concentrations. The detection limit based on the 3σ criterion was found to be 0.24 μg L−1 for Fe(III). The recoveries were higher than 95% and relative standard deviation was less than 2.1% (N = 8). The validation of the procedure was performed by the analysis of two certified standard reference materials. The presented method was applied to the determination of Fe(II) and Fe(III) in tap water, lake water, river water, sea water, fruit juice, cola, and molasses samples with satisfactory results. 相似文献
804.
Driss Mountassif Pierre Andreoletti Zakaria El Kebbaj Adnane Moutaouakkil Mustapha Cherkaoui-Malki Norbert Latruffe M'hammed Saïd El Kebbaj 《BMC biochemistry》2008,9(1):26
Background
The interconversion of two important energy metabolites, 3-hydroxybutyrate and acetoacetate (the major ketone bodies), is catalyzed by D-3-hydroxybutyrate dehydrogenase (BDH1: EC 1.1.1.30), a NAD+-dependent enzyme. The eukaryotic enzyme is bound to the mitochondrial inner membrane and harbors a unique lecithin-dependent activity. Here, we report an advanced purification method of the mammalian BDH applied to the liver enzyme from jerboa (Jaculus orientalis), a hibernating rodent adapted to extreme diet and environmental conditions. 相似文献805.
René B Masliah G Antri SE Fermandjian S Mauffret O 《The journal of physical chemistry. B》2007,111(16):4235-4243
The dynamics of the DNA phosphodiester backbone conformations have been studied for a strong topoisomerase II cleavage site (site 22) using molecular dynamics simulations in explicit water and in the presence of sodium ions. We investigated the backbone motions and more particularly the BI/BII transitions involving the epsilon and zeta angles. The consensus cleavage site is adjacent to the phosphate which shows the most important phosphodiester backbone flexibility in the sequence. We infer that these latter properties could be responsible for the preferential cleavage at this site possibly through the perturbation of the cleavage/ligation activities of the topoisomerase II. More generally, the steps pur-pur and pyr-pur are those presenting the highest BII contents. Relations are observed between the backbone phosphodiester BI/BII transitions and the flexibility of the deoxyribose sugar and the helical parameters such as roll. The roll is sequence dependent when the related phosphate is in the BI form, whereas this appears not to be true when it is in the BII form. The BI/BII transitions are associated with water migration, and new relations are observed with counterions. Indeed, it is observed that a strong coupling exists between the BII form and the presence of sodium ions near the adjacent sugar deoxyribose. The presence of sodium ions in the O4' surroundings or their binding could assist the BI to BII transition by furnishing energy. The implications of these new findings and, namely, their importance in the context of the sequence-dependent behavior of BI/BII transitions will be investigated in future studies. 相似文献
806.
Peach MJ Tellgren EI Sałek P Helgaker T Tozer DJ 《The journal of physical chemistry. A》2007,111(46):11930-11935
The bond length alternation (BLA), the highest-occupied-lowest-unoccupied (HO-LU) orbital energy gap, and the corresponding excitation energy are determined for trans-polyacetylene (PA) and polyyne (PY) using density functional theory. Results from the Coulomb-attenuated CAM-B3LYP functional are compared with those from the conventional BHHLYP and B3LYP hybrid functionals. BLA values and HO-LU gaps are determined using both finite oligomer and infinite chain calculations, subject to periodic boundary conditions. TDDFT excitation energies are determined for the oligomers. The oligomer excitation energies and HO-LU gaps are then used, in conjunction with the infinite chain HO-LU gap, to estimate the infinite chain excitation energy. Overall, BHHLYP and CAM-B3LYP give BLA values and excitation energies that are larger and more accurate than those obtained using B3LYP. The results highlight the degree to which excitation energies can be approximated using the HO-LU gaps-at the infinite limit, this approximation works well for B3LYP, but not for the other functionals, where the HO-LU gap is significantly larger. The study provides further evidence for the high-quality theoretical predictions that can be obtained from the CAM-B3LYP functional. 相似文献
807.
Mohammadine El Haddad Rajâa Boulahjar Rachid Slimani Marie-Claude Viaud-Massuard Saïd El Antri Saïd Lazar 《Journal of Analytical Chemistry》2011,66(6):586-590
The retention of alkylphosphonic acids (MPA, EPA, PPA, and BPA) and inorganic anions (Cl−, NO3−, and SO42−) were studied on two anion exchangers, e.g., Dionex IonPac AS4A-SC (250 ± 4 mm I.D.) and Satisfaction P 4000-SAX (50 ± 4.6
mm I.D.) under isocratic elution conditions with evaporative light scattering detector. The plots of retention factors, k, of organic anions vs. the reciprocal of eluent ion concentration show good linearity. The major retention mechanisms are
interpreted as ion exchange and some others interactions. The C effect of organic modifier added to the aqueous buffered mobile
phase is also investigated. 相似文献
808.
Akbar Ghazavizadeh Gregory C. Rutledge Ali A. Atai Saïd Ahzi Yves Rémond Nasser Soltani 《Journal of Polymer Science.Polymer Physics》2013,51(16):1228-1243
The interphase layer in semi‐crystalline polyethylene is the least known constituent, compared to the amorphous and crystalline phases, in terms of mechanical properties. In this study, the Monte Carlo molecular simulation results for the interlamellar domain (i.e. amorphous+ interphases), reported in (Macromolecules 2006, 39, 439–447) are employed. The amorphous elastic properties are adopted from the literature and then two distinct micromechanical homogenization approaches are utilized to dissociate the interphase stiffness from that of the interlamellar region. The results of the two micromechanical approaches match perfectly. Interestingly, the dissociated interphase stiffness lacks the common feature of positive definiteness, which is attributed to its nature as a transitional domain between two coexisting phases. The sensitivity analyses reveal that this property is insensitive to the non‐orthotropic components of the interlamellar stiffness and the uncertainties existing in the interlamellar and amorphous stiffnesses. Finally, using the dissociated interphase stiffness, its effective Young's modulus is calculated, which compares well with the effective interlamellar Young's modulus for highly crystalline polyethylene, reported in an experimental study. This satisfactory agreement along with the identical results produced by the two micromechanical approaches confirms the validity of the new information about the interphase elastic properties in addition to making the proposed dissociation methodology quite reliable when applied to similar problems. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1228–1243 相似文献
809.
810.
Antonija Lesar Sa?a Kova?i? Milan Hodo??ek Margret G. Stadler Max Mühlh?user Sigrid D. Peyerimhoff 《Molecular physics》2013,111(17):2375-2380
A computational study of the low-lying singlet and triplet states of bromine nitrite, cis-BrONO, is presented. Calculations of excitation energies and oscillator strengths are reported using multi-reference configuration interaction, MRD-CI, methods in conjunction with the cc-pVDZ?+?sp and cc-pVTZ?+?sp basis sets. In agreement with recent experimental work the calculations find two important transitions, namely to the states 21 A′ at 3.99?eV and 41 A′ at 5.27?eV. The corresponding measurements obtained 3.87 and 5.44?eV, respectively. Both states show multi-reference character, representing a linear combination of 3a″?→?4a″ and 9a′?→?10a′ transitions. The potential energy curves for the Br+ONO dissociation are repulsive for both states indicating possible photodissociation. BrO+NO products are not favoured because the corresponding potential energy curves show barriers on the order of 0.5?eV. 相似文献