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951.
Ulrich Gesenhues 《Journal of Physics and Chemistry of Solids》2007,68(2):224-235
The extensive polygonization of 200 nm rutile crystals in high-energy dry milling allowed to study the spectral properties of grain boundaries and adjacent microstrained crystalline matter. Changes in UV, VIS, NIR, IR and FIR spectra during milling were followed. For the UV absorption edge the value of unstrained rutile was retained while residual traces of anatase, intergrown with the rutile phase, continued to act as traps for photoinduced charges. The evolving broad absorption in VIS and NIR could be attributed to electrons weakly bound to defects in the packing of oxygen anions at the grain boundaries, which may relax to face-sharing Ti3+-O octahedra. Among the IR-active lattice vibrations, the narrow Eu(2) band showed a shift to higher frequencies by 15 cm−1 which is definitively not due to phonon confinement or Fröhlich surface modes but probably to coupling of the bulk phonon to a plasmon at the grain boundary. At the external surface of the polygonized primary particle, the regular atomic order is destroyed by milling so that hydroxylation is replaced by physisorption of H2O, as shown by IR and TG. 相似文献
952.
The lithium ion conducting solid polymer electrolytes (SPE) based on PVAc-LiClO4 of various compositions were prepared by solution casting technique. Structure and surface morphology characterization were studied by X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) measurements, respectively. Thermal and conductivity behavior of polymer-salt complexes were studied by employing differential scanning calorimetry (DSC) and ac impedance measurements, respectively. XRD and SEM analyses indicate the amorphous nature of the polymer-salt complexes. DSC measurements show decrease in Tg with the increase in LiClO4 concentrations. The bulk conductivity of the PVAc:LiClO4 polymer electrolytes was found to vary between 7.6×10−7 and 6.2×10−5 S cm−1 at 303 K with the increase in salt concentration. The temperature dependence of the polymer electrolyte complexes appear to obey Arrhenius law. 相似文献
953.
The Maxwell-Wagner-Sillars (MWS) relaxation is studied for semi-crystalline polymers poly (ether ether ketone) (PEEK), in the range 20 Hz-1 MHz and temperature varying from 80 to 330 °C. The parameter is the crystallization condition in the case of PEEK, which is a semi-crystalline polymer considered as a particulate composite. The relaxation found in the semi-crystalline polymers above the α relaxation of the PEEK is ascribed to the trapping of conductive carriers at the interface between crystalline lamellae and the amorphous matrix. The study of PEEK microstructure is based on differential calorimetry and X-rays diffraction. Two lamellae populations have been detected, that depends on the crystallization temperature and its duration. The crystallinity rate is increasing with crystallization temperature and duration. In dielectric studies, the use of the electric modulus instead of permittivity allows us to minimize the ionic conduction and then leads to the appearance of the interfacial relaxation. According to our measurements, the crystallinity rate is not the main factor of the interfacial relaxation intensity, which also depends on the nature and degree of perfection of the lamellae. 相似文献
954.
According to stationary X-ray-excited luminescence spectra and thermally stimulated luminescence spectra of CaF2:Eu nanophosphors, it was found that Eu3+?→?Eu2+ conversion can occur during thermal annealing of fine-grained (d?=?25?nm) nanoparticles in the 200–800°C range, which is accompanied by an increase in their size within 40–189?nm. An important role of the exciton mechanism of Eu2+ luminescence excitation was revealed according to the temperature dependence of X-ray-excited luminescence spectra of CaF2:Eu nanoparticles of 114?nm size. The maximum of the X-ray-excited luminescence light output of CaF2:Eu nanophosphors in the Eu2+ ions’ emission band was traced out at 400–500°C annealing temperature and at the size of nanoparticles of 114–180?nm. The subsequent growth of the annealing temperatures, particularly in the 800–1000°C range, causes the reduction of X-ray-excited luminescence light output because of the increment of lattice defects’ concentration due to a sharp increase in the size of nanoparticles and their agglomeration. 相似文献
955.
采用溶胶凝胶法制备了Y_4Zr_3O_(12)∶Eu~(3+)纳米荧光粉,分别采用XRD、TEM和荧光光谱仪对样品的结构、形貌和发光性能进行了表征,探讨了烧结温度和Eu~(3+)掺杂浓度对荧光粉发光性能的影响。结果表明,样品可以被394 nm和467 nm的激发光有效激发。样品的最佳烧结温度和Eu~(3+)离子的最佳掺杂摩尔分数分别为1 400℃和18%。浓度猝灭主要归因于电偶极-电偶极相互作用。 相似文献
956.
C. Madhukar Reddy G.R. Dillip B. Deva Prasad Raju 《Journal of Physics and Chemistry of Solids》2011,72(12):1436-1441
Lead containing calcium zinc sodium fluoroborate (LCZSFB) glasses doped with different concentrations of trivalent dysprosium ions were prepared and investigated by the XRD, FTIR, optical absorption, photoluminescence and decay curve analysis. The experimentally determined oscillator strengths have been determined by measuring the areas under the absorption peaks and the Judd–Ofelt (J–O) intensity parameters were calculated using the least squares fit method. From the evaluated J–O parameters the radiative transition probability rates, radiative lifetimes and branching ratios were calculated for 4F9/2 excited level. Room temperature photoluminescence spectra for different concentrations of Dy3+-doped LCZSFB glasses were obtained by exciting the glass samples at 386 nm. The intensity of Dy3+ emission spectra increases with increasing concentration of 0.1, 0.25, 0.5 and 1.0 mol% and beyond 1.0 mol% the concentration quenching is observed. The measuring branching ratios are reasonably high for transitions 4F9/2→6H15/2 and 6H13/2, suggesting that the emission at 484 and 576 nm, respectively, can give rise to lasing action in the visible region. From the visible emission spectra, yellow–blue (Y/B) intensity ratios and chromaticity color coordinates were also estimated. The lifetimes of 4F9/2 metastable state for the samples with different concentrations were also measured and discussed. 相似文献
957.
958.
959.
Living polymers are formed by reversible association of primary units (unimers). Generally the chain statistical weight involves
a factor σ < 1 suppressing short chains in comparison with free unimers. Living polymerization is a sharp thermodynamic transition
for σ ≪ 1 which is typically the case. We show that this sharpness has an important effect on the kinetics of living polymerization
(one-dimensional association). The kinetic model involves i) the unimer activation step (a transition to an assembly-competent
state); ii) the scission/recombination processes providing growth of polymer chains and relaxation of their length distribution.
Analyzing the polymerization with no chains but unimers at t = 0 , with initial concentration of unimers M ≳ M
* (M* is the critical polymerization concentration), we determine the time evolution of the chain length distribution and find
that: 1) for M
* ≪ M ≪ M
*/σ the kinetics is characterized by 5 distinct time stages demarcated by 4 characteristic times t1, t2, t3 and t*; 2) there are transient regimes (t
1 ≲ t ≲ t
3) when the molecular-weight distribution is strongly non-exponential; 3) the chain scissions are negligible at times shorter
than t2. The chain growth is auto-accelerated for t
1 ≲ t ≲ t
2 : the cut-off chain length (= polymerization degree 〈n〉w
N
1 ∝ t
2 in this regime. 4) For t
2 < t < t
3 the length distribution is characterized by essentially 2 non-linear modes; the shorter cut-off length N1 is decreasing with time in this regime, while the length scale N2 of the second mode is increasing. (5) The terminal relaxation time of the polymer length distribution, t*, shows a sharp maximum in the vicinity of M*; the effective exponent
is as high as ∼ σ-1/3 just above M*. 相似文献
960.
We present the infrared and Raman study of the optical phonon modes of the defective compounds ZnGa2Se4 and ZnGa2S4. Most of the compounds have been found to crystallize in the thiogallate structure (defect chalcopyrite) with space group where all cations and vacancies are ordered. For some Zinc compounds a partially disordered cationic sublattice with various degrees of cation and vacancy statistical distribution, which lead to the higher symmetry (defect stannite), has been reported. For ZnGa2Se4 we have found three modes of A symmetry, showing Raman activity only. In addition, we have observed each five modes of B and E symmetry, showing infrared as well as Raman activity. The number of modes and their symmetry assignment, based on polarized measurements, clearly indicate space group for the investigated crystals of ZnGa2Se4.Regarding ZnGa2S4 we have found three modes exclusively showing Raman activity (2A⊕1B1), and only eight modes showing infrared as well as Raman activity (3B2⊕5E). The assignment of the modes has been derived by analyzing the spectral positions of the vibrational modes in comparison to a number of compounds. From the number and symmetry assignment of the optical phonon modes we confirm that ZnGa2S4 most likely crystallizes in space group . 相似文献