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141.
The complex conductivity in polycrystalline C60 and C70 has been investigated for frequencies 20 Hz≤ν≤106 Hz and temperatures 10 K≤T≤750 K. The high-frequency dielectric constants εα= 2.6±0.1(C60) and ε∞= 4.6±0.1 (C70) were deduced from these experiments. The observed low temperature relaxation process in C60 fits well into the relaxation dynamics of the C60 molecules as determined by many other experimental techniques operating on very different time scales. In addition to the study of the dipolar relaxation process, the dc and ac conductivities were determined. From the temperature dependence of the dc conductivities energy barriers of EG=1.75±0.1 eV (C60) and EG=1.7±0.1 eV (C70) were estimated. In C70 we found indications for small polaron tunneling. 相似文献
142.
An attempt is made to investigate theoretically the effective electron mass in ternary chalcopyrite semiconductors at low temperatures on the basis of a newly derived dispersion relation of the conduction electrons under cross fields for the more generalized case which occurs from the consideration of the various types of anisotropies in the energy spectrum. It is found, taking degeneraten-CdGeAs2 as an example, that the effective electron mass at the Fermi level along the direction of magnetic quantization depends on both the Fermi energy and the magnetic quantum number due to the combined influence of the crystal field splitting parameter and the anisotropic spin-orbit splitting parameter respectively, resulting in different effective masses at the Fermi level corresponding to different magnetic sub-bands. It is also observed that the same mass at the Fermi level in the direction normal to both magnetic and electric fields also varies both with Fermi energy and magnetic sub-band index, and the characteristic feature of cross-fields is to introduce the index-dependent oscillatory mass anisotropy. The theoretical results are in good agreement with the experimental observations as reported elsewhere. 相似文献
143.
Bhaskar Sengupta 《European Journal of Operational Research》1981,7(3):257-264
This paper considers a class of Markov Decision problems subjects to partial observation. We develop a methodology for early detection of failure of systems that do not display symptoms of failure. We assume that the inspections used for detection are error prone and there is a fixed probability of not detecting the failure. The problem is studied as a non-stationary, finite horizon Markovian Decision Process with two states. We demonstrate that the optimal policy has a two region structure that intuition suggests. This characterization can be used to reduce the computational burden of finding the optimum inspection schedule. We also show that the form of the optimum policy remains unchanged for the unbounded horizon problem. Finally, we discuss a potential application of the model to medical diagnosis. 相似文献
144.
145.
Courtney?L.?Sherman Jennifer?S.?BrodbeltEmail author Alan?P.?Marchand Bhaskar?Poola 《Journal of the American Society for Mass Spectrometry》2005,16(7):1162-1171
The self-assembly of ligand-metal-ligand sandwich complexes involving a novel quinoxaline-containing crown ether, 1, was studied by electrospray ionization mass spectrometry (ESI-MS). Donor-acceptor π-stacking interactions between the electron-poor
quinoxaline group of 1 and electron-rich benzene groups from benzo- or dibenzo-18-crown-6 were found to significantly enhance the formation of mixed-ligand
sandwich complexes with a free energy gain of up to 17 kJ/mol relative to sandwich formation involving macrocycles without
such aromatic functionalities. The relative intensities of the sandwich complexes were greatest with the alkali metals Na+, K+, and Rb+ as well as with the ammonium ion in equimolar concentrations with the macrocycles. The preferential formation of the mixed-ligand
sandwich complexes demonstrated that donor-acceptor π-stacking interactions contribute to the assembly of molecular structures
and can be monitored by ESI-MS. 相似文献
146.
Mondal JA Ghosh HN Ghanty TK Mukherjee T Palit DK 《The journal of physical chemistry. A》2006,110(10):3432-3446
Ultrafast relaxation dynamics of the excited singlet (S(1)) state of Michler's ketone (MK) has been investigated in different kinds of solvents using a time-resolved absorption spectroscopic technique with 120 fs time resolution. This technique reveals that conversion of the locally excited (LE) state to the twisted intramolecular charge transfer (TICT) state because of twisting of the N,N-dimethylanilino groups with respect to the central carbonyl group is the major relaxation process responsible for the multi-exponential and probe-wavelength-dependent transient absorption dynamics of the S1 state of MK, but solvation dynamics does not have a significant role in this process. Theoretical optimization of the ground-state geometry of MK shows that the dimethylanilino groups attached to the central carbonyl group are at a dihedral angle of about 51 degrees with respect to each other because of steric interaction between the phenyl rings. Following photoexcitation of MK to its S1 state, two kinds of twisting motions have been resolved. Immediately after photoexcitation, an ultrafast "anti-twisting" motion of the dimethylanilino groups brings back the pretwisted molecule to a near-planar geometry with high mesomeric interaction and intramolecular charge transfer (ICT) character. This motion is observed in all kinds of solvents. Additionally, in solvents of large polarity, the dimethylamino groups undergo further twisting to about 90 degrees with respect to the phenyl ring, to which it is attached, leading to the conversion of the ICT state to the TICT state. Similar characteristics of the absorption spectra of the TICT state and the anion radical of MK establish the nearly pure electron transfer (ET) character of the TICT state. In aprotic solvents, because of the steep slope of the potential energy surface near the Franck-Condon (FC) or LE state region, the LE state is nearly nonemissive at room temperature and fluorescence emission is observed from only the ICT and TICT states. Alternatively, in protic solvents, because of an intermolecular hydrogen-bonding interaction between MK and the solvent, the LE region is more flat and stimulated emission from this state is also observed. However, a stronger hydrogen-bonding interaction between the TICT state and the solvent as well as the closeness between the two potential energy surfaces due to the TICT and the ground states cause the nonradiative coupling between these states to be very effective and, hence, cause the TICT state to be weakly emissive. The multi-exponentiality and strong wavelength-dependence of the kinetics of the relaxation process taking place in the S1 state of MK have arisen for several reasons, such as strong overlapping of transient absorption and stimulated emission spectra of the LE, ICT, and TICT states, which are formed consecutively following photoexcitation of the molecule, as well as the fact that different probe wavelengths monitor different regions of the potential energy surface representing the twisting motion of the excited molecule. 相似文献
147.
Superheated emulsion detector is known to detect neutrons, γ-rays and other charged particles. The present work includes the study of nucleation efficiency of superheated drops of one
of the CFC-free liquids, R134a (C2H2F4), to fast neutrons, its response to γ-rays from 241Am and 137Cs and compare its nucleation efficiency with that of R12. The observation indicates that because of the presence of hydrogen,
the nucleation efficiency is less in R134a than in R12 in the present neutron energy range of consideration. R134a is one
of the most environment-friendly, commercially available liquid that is suitable for superheated drop detector, specially
in neutron dosimetry and one needs to investigate it in detail. 相似文献
148.
S. Dechoudhury Vaishali Naik Manas Mondal Hemendra Kumar Pandey Avik Chatterjee Dirtha Sanyal Debasis Bhowmick Alok Chakrabarti 《Pramana》2010,75(3):485-499
The design of a medium energy beam transport (MEBT) line comprising of a re-buncher and four quadrupoles, two upstream and
the other two downstream of the re-buncher, has been presented. The design was done to ensure almost 100% transport of heavy-ion
beams of about 99 keV/u energy from RFQ having a q/A not less than 1/14 through the re-buncher and then through IH Linac of about 0.6 m length in which beam would be accelerated
to about 185 keV/u. The re-buncher has been designed to operate at 37.8 MHz, the resonating frequency of both the RFQ and
the IH Linac. The entire beam line has been installed and recently O5+ beam from RFQ has been transported through the re-buncher and subsequently accelerated in the IH Linac successfully. 相似文献
149.
We used variationally improved perturbation theory (VIPT) in calculating the slope and curvature of Isgur-Wise (I-W) function
with the Cornell potential −$
\frac{{4\alpha _s }}
{{3r}}
$
\frac{{4\alpha _s }}
{{3r}}
+br + c instead of the usual stationary state perturbation theory as done earlier. We used −(4α
s
/3r), i.e. the Coulombic potential, as the parent and the linear one, i.e. br+c as the perturbed potential in the theory and calculated the slope and curvature of Isgur-Wise function including three states
in the summation involved in the first-order correction to wave function in the method. 相似文献
150.
The generalized Stokes parameters, which are two-point extensions of conventional Stokes parameters, are determined for a pair of points in the cross-section of a partially coherent broadband light beam. For this purpose, using a two-mirror and two-beam-splitter assembly, unpolarized, linearly polarized and partially polarized electromagnetic beams are generated. This simple experimental method for determining generalized Stokes parameters establishes an analogy with the experimental scheme of determining conventional Stokes parameters, which in succession contributes to appreciable reduction in uncertainty in the measurements. 相似文献