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921.
Donley JP 《The European physical journal. E, Soft matter》2005,16(3):273-282
Density functional techniques are used to derive a charging expression for the non-uniform density of a molecular liquid. In the atomic limit the equation reduces to an exact form due to Fixman. The theory is simplified greatly via a physical approximation that accounts for three-body correlations beyond those included in the hypernetted chain (HNC) closure of the Ornstein-Zernike (OZ) equation. The radial distribution function is obtained as a special case. The theory is tested by examining the phase behavior of two fundamental complex fluids: the homopolymer blend and diblock copolymer melts. For the former it is found, contrary to HNC theory and its molecular generalizations, that a critical temperature Tc is predicted from the structure route. This Tc scales linearly with degree of polymerization N in agreement with Flory theory. The simplest form of the theory can be considered as a way to incorporate attractive interactions within a formalism that is very similar to that of the OZ or reference interaction site model (RISM). The relevance of the theory to charged liquids is also discussed. 相似文献
922.
The unzipping transition under the influence of external force of a dsDNA molecule has been studied using the Peyrard-Bishop Hamiltonian. The critical force Fc(T) for unzipping calculated in the constant force ensemble is found to depend on the potential parameter k which measures the stiffness associated with a single strand of DNA and on D, the well depth of the on-site potential representing the strength of hydrogen bonds in a base pair. The dependence on temperature of Fc(T) is found to be (TD - T)1/2 (TD being the thermal denaturation temperature) with Fc(TD) = 0 and Fc(0) =
. We used the constant extension ensemble to calculate the average force F(y) required to stretch a base pair a y distance apart. The value of F(y) needed to stretch a base pair located far away from the ends of a dsDNA molecule is found twice the value of the force needed to stretch a base pair located at one of the ends to the same distance for y 1.0 . The force F(y) in both cases is found to have a very large value for y 0.2 compared to the critical force found from the constant force ensemble to which F(y) approaches for large values of y. It is shown that the value of F(y) at the peak depends on the value of k which measures the energy barrier associated with the reduction in DNA strand rigidity as one passes from dsDNA to ssDNA and on the value of the depth of the on-site potential. The effect of defects on the position and height of the peak in the F(y) curve is investigated by replacing some of the base pairs including the one being stretched by defect base pairs. The formation and behaviour of a loop of Y shape when one of the ends base pair is stretched and a bubble of ssDNA with the shape of an eye when a base pair far from ends is stretched are investigated. 相似文献
923.
Vollmer A Jurchescu OD Arfaoui I Salzmann I Palstra TT Rudolf P Niemax J Pflaum J Rabe JP Koch N 《The European physical journal. E, Soft matter》2005,17(3):339-343
We use ultraviolet photoelectron spectroscopy to investigate the effect of oxygen and air exposure on the electronic structure of pentacene single crystals and thin films. It is found that O2 and water do not react noticeably with pentacene, whereas singlet oxygen/ozone readily oxidize the organic compound. Also, we obtain no evidence for considerable p-type doping of pentacene by O2 at low pressure. However, oxygen exposure lowers the hole injection barrier at the interface between Au and pentacene by 0.25 eV, presumably due to a modification of the Au surface properties. 相似文献
924.
Lagerwall JP Heppke G Giesselmann F 《The European physical journal. E, Soft matter》2005,18(1):113-121
We study the effects of mixing ferroelectric and antiferroelectric liquid-crystal compounds (FLCs and AFLCs) when the former
are strictly synclinic and the latter strictly anticlinic, i.e. one mixture component exhibits only SmC* and the other only SmC a* as tilted phase. Three different paths between syn- and anticlinicity were detected: transition directly between SmC* and
SmC a*, transition via the SmCβ* and SmCγ* subphases, or by “escaping” the clinicity frustration by reducing the tilt to zero, i.e. the SmA* phase is extended downwards in temperature, separating SmC* from SmC a* in the phase diagram. The most common path is the one via the subphases, demonstrating that these phases appear as a result
of frustration between syn- and anticlinic and, consequently, between syn- and antipolar order. For assessing the role of
chirality, we also replaced the FLC with non-chiral synclinics. With one of the AFLCs, the route via supbhases was detected
even in this case, suggesting that chirality --although necessary-- does not have quite the importance that has previously
been attributed to the appearance of the subphases. The path chosen in the mixture study seemed to be determined mainly by
the synclinic component, the subphase induction occurring only when the SmA*-SmC* transition was second order. 相似文献
925.
The full Landau potential was determined for a
ferroelectric liquid crystal doped with varying concentrations of the chiral dopant R1011 and its enantiomer S1011. A multi-curve
fitting procedure using temperature and electric field dependent tilt angle and polarization data was employed to the generalized
Landau model of ferroelectric liquid
crystals. From this analysis the three Landau coefficients as well as the polarization-tilt coupling parameters were obtained
as a function of dopant concentration and configuration. It is shown that the two most varied
parameters are those of the first Landau coefficient α and the
(chiral) linear polarization-tilt coupling constant C. The effect on the
first Landau term is equivalent for the two dopants of opposite handedness
indicating its achiral nature, while the effect on the (chiral) bilinear
coupling term differs for the R1011 and S1011 dopant, increasing and
decreasing the coupling between tilt and polarization respectively. This
difference in the bilinear coupling term quantifiably evidences that the
R1011 dopant increases and S1011 dopant reduces the inherent chirality in
this system. 相似文献
926.
González-Mozuelos P Yeom MS Olvera de la Cruz M 《The European physical journal. E, Soft matter》2005,16(2):167-178
We study small rod-like molecular electrolytes solutions with
their corresponding atomic counterions. The asymptotic length
scales (decay length and wavelength) of the structural
correlations are analyzed using the formalism of the dressed
interaction site theory (DIST). The correlation functions are
determined using the reference interaction site model equation
complemented with a mixed approach in which the hypernetted-chain
closure is used for the repulsive interactions, and the mean
spherical approximation is used for the attractive interactions.
The results from this scheme are in good agreement with the Monte
Carlo computer simulations reported here. The asymptotic
properties of the correlation functions of this molecular system
are compared against those corresponding to two related simple
(atomic) electrolyte models. The main conclusion is that the
molecular structure of the ions lowers by two orders of magnitude
the concentration at which the transition from monotonic to
oscillatory decay occurs. 相似文献
927.
Cerisier P Porterie B Kaiss A Cordonnier J 《The European physical journal. E, Soft matter》2005,18(1):85-93
The motion of particles moving under gravity in the velocity field of a liquid in a Bénard hexagonal cell is studied experimentally
and numerically for Stokes flow conditions. We then explain the settlement of particles in the centers of cells to form a
regular quincunx. It is found that sedimentation also occurs preferentially along the lines connecting the centers of adjacent
cells to form a triangle deposition tessellation. Finally, it is explained why particles occupy the central part of each convective
cell while the peripheral part of the cell quickly becomes limpid. Numerical results are in agreement with the experimental
observations of Bénard and those of the present study. 相似文献
928.
Erukhimovich IY 《The European physical journal. E, Soft matter》2005,18(4):383-406
The statistical theory of microphase separation in the ternary ABC triblock
copolymers is presented and the corresponding phase diagrams are built both
for the linear and miktoarm copolymers. For this purpose the Leibler weak
segregation theory in molten diblock copolymers is generalized to multi-component monodisperse block copolymers with due regard
for the 2nd shell harmonics contributions defined in the paper. The Hildebrand approximation for the χ-parameters is used.
The physical meaning of
this and alternative choices for the χ-parameters is discussed. The symmetric
AfB1-2fCf copolymers with the middle block non-selective with respect to the side ones are shown to undergo the continuous ODT not
only into the lamellar phase but also, instead, into
various non-conventional cubic phases (depending on the middle block composition it could be the simple cubic, face-centered
cubic or non-centrosymmetric phase, which reveals the symmetry of I41 32 space group No. 214 first predicted to appear in molten block copolymers). For
asymmetric linear ABC copolymers a region of compositions is found where the weakly segregated gyroid (double gyroid) phase
exists between the planar hexagonal and lamellar or one of the non-conventional cubic phases up to the very critical point.
In contrast, the miktoarm (star) ABC block copolymers with one of its arm non-selective with respect to the two others are
shown
to reveal a pronounced tendency towards strong segregation, which is preceded by increase of stability of the conventional
BCC phase and a peculiar weakly segregated BCC phase (BCC3), where the dominant harmonics belong to the 3rd coordination sphere of the reciprocal lattice. The validity region of the
developed theory is discussed and outlined in the composition triangles both for linear and miktoarm copolymers. We present
also the list of the 2nd shell harmonics (SAXS
reflections) allowed and prohibited in some of the non-conventional morphologies due to the weak segregation considerations
and comparison of our results with the preceding SCFT treatment of the ABC copolymers by Matsen. 相似文献
929.
Zhao J Tian S Wang Q Liu X Jiang S Ji X An L Jiang B 《The European physical journal. E, Soft matter》2005,16(1):49-56
The surface topography of thin diblock copolymer films is studied by atomic
force microscopy (AFM). With AFM an island-to-ribbon transition is observed
for symmetric polystyrene-b-poly (4-vinylpyridine) (PS-b-P4VP) on mica with
increasing solution concentration. Our study also demonstrates how the
formation of the pattern strongly depends on the copolymer composition based
on the volume fraction. The substrate and solvent used both have great
effects on the morphology of the thin films. Only by using highly polar
substrate (mica), can we gain regular pattern. The reason why the regular
islands cannot be obtained with symmetric PS-b-P4VP on graphite is also
explained. On mica using nonselective and selective solvents, a rather
regular pattern can be obtained. The difference is only in the solution
concentration for forming regular patterns. 相似文献
930.
A model for polar filaments interacting via molecular motor complexes is investigated which exhibits bifurcations to spatial
patterns. It is shown that the homogeneous distribution of filaments, such as actin or microtubules, may become either unstable
with respect to an orientational instability of a finite wave number or with respect to modulations of the filament density,
where long-wavelength modes are amplified as well. Above threshold nonlinear interactions select either stripe patterns or
periodic asters. The existence and stability ranges of each pattern close to threshold are predicted in terms of a weakly
nonlinear perturbation analysis, which is confirmed by numerical simulations of the basic model equations. The two relevant
parameters determining the bifurcation scenario of the model can be related to the concentrations of the active molecular
motors and of the filaments, respectively, which both could be easily regulated by the cell. 相似文献