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631.
J. Carter A. A. Cowley H. Diesener R. W. Fearick S. V. Frtsch M. N. Harakeh J. J. Lawrie S. J. Mills P. von Neumann-Cosel R. T. Newman J. V. Pilcher A. Richter K. Schweda F. D. Smit G. F. Steyn S. Strauch D. M. Whittal 《Nuclear Physics A》1998,630(3-4)
The 40Ca(p,p′ α) reaction has been studied at an incident proton energy Ep = 99.5 MeV for proton laboratory scattering angles Θplab = 17°, 23° and 27°. Emission of α particles coincident with the scattered proton has been measured for an angular range Θα 0° − 180° relative to the recoil axis. A multipole decomposition for the α0-decay channel to the 36Ar ground state has been performed from the angular-correlation functions. The energy distribution of the dominating E2 strength deduced in the excitation energy range Ex = 11–21 MeV agrees reasonably well with the results from electron and α-induced α0-decay investigations. The exhaustion of the E2 energy-weighted sum rule in this channel up to an energy of 17 MeV is 16.1(4.0)%, in accord with the study of the (α, α′ α0) reaction. However, this value is twice what is found in the (e,e′ α0) experiment in the same energy region. Thus, the puzzling discrepancy in the E2 strengths derived from electromagnetic and hadronic probes remains unsolved. 相似文献
632.
Crystallization of compression-molded isotactic polypropylene and polyethylene is invariably spherulitic; generally, nucleation occurs randomly throughout the sample. In a special case where nucleation predominates at the surface, spherulitic growth centers become crowded and are forced to propagate unidirectionally into the bulk (transcrystallinity). Conditions for the formation of transcrystallinity have been investigated by optical and scanning electron microscopy. The occurrence of transcrystallinity is attributed to heterogeneous nucleation induced at the mold surface. To be effective, the mold surface must have a nucleating efficiency equal to or greater than that of adventitious nuclei present in the polymer. As the crystallization temperature approaches the melting point, the activity of mold surfaces is found to increase leading invariably to transcrystalline formation. The degree of activity of various mold surfaces correlates with the known activity of specific dispersed nucleating agents having similar chemical structures. Contrary to claims in the literature, the surface energy of the mold surface and temperature gradients across the melt surface do not play a primary role in transcrystalline formation of polypropylene. 相似文献
633.
J. W. Lee Y. Takase B. A. Newman J. I. Scheinbeim 《Journal of Polymer Science.Polymer Physics》1991,29(3):279-286
We have discovered recently that melt-quenched and cold-drawn Nylon - 11 films exhibit very clear ferroelectric hysteresis behavior. In the present study, a remanent polarization as high as 86 mC/m2 has been found in Nylon - 7 samples; this is significantly higher than that usually observed in poly (vinylidene fluoride) films. The effect of annealing on the electric displacement versus electric field characteristics of both Nylon - 11 and Nylon - 7 films have been studied and show an increased coercive field (62 to 115 MV/m for Nylon - 11 and 79 to 97 MV/m for Nylon - 7) and a decreased remanent polarization (51 to 17.3 mC/m2 for Nylon - 11 and 86 to 70.5 mC/m2 for Nylon - 7) with increasing annealing temperature from 25 to 145°C. 相似文献
634.
Y. Takase J. I. Scheinbeim B. A. Newman 《Journal of Polymer Science.Polymer Physics》1990,28(9):1599-1609
To obtain information about microscopic processes involved in the polarization switching in uniaxially oriented poly (vinylidene fluoride) film, a least-squares estimation of nonlinear parameters was developed to yield parameters of an equation which describes the nucleation and domain growth process. Time domain measurements of polarization reversal revealed that switching times decreased as the annealing temperature Ta increased (67.0, 52.4, and 41.3 μs at –20°C under a 200 MV/m pulse field for the as-stretched samples, the samples being annealed at 120 and 160°C, respectively). The analysis showed that the value of the domain growth speed increased as Ta increased. This is consistent with x-ray diffraction data which indicated that the annealing process brought about better chain packing and increased crystallite perfection. The analysis also showed that the nucleation probability significantly increased as Ta increased. This result was interpreted in terms of a morphological transformation, which was indicated by the decrease in elastic modulus with increasing Ta with no corresponding loss of orientation. It is suggested that the annealing process brought about an increase in the number of nucleation sites as a result of a transformation from a fibrous structure to a crystal-amorphous series structure which has increased boundary zone area. 相似文献
635.
636.
Time-related luminescence spectroscopic investigation of electron trapping and recombination in infrared stimulated luminescence 总被引:1,自引:0,他引:1
Infrared stimulated luminescence (ISL) occurred in CaS:Eu,Sm due to formation of luminescent centres Eu2+ and electron trapping centres Sm3+. The electron trapping centres Sm3+ became occupied (forming Sm2+ by trapping excited electrons) in photoluminescence (PL) excitation (PLX) process causing simultaneous ionization of luminescent centres Eu2+ (leaving Eu3+ by losing an electron or capturing a hole). In this paper, the electron trapping in PLX and the recombination in ISL were examined by the time-related PL and ISL spectra of CaS:Eu,Sm. The spectroscopic evidence confirmed that the ISL in CaS:Eu,Sm was produced due to recombination of de-trapped electrons and previously ionized luminescent centres (Eu3+). It was believed that the electron trapping occurred concurrently as occurrence of the PL of Eu2+ in PLX process. However, the recombination of de-trapped electrons and previously ionized luminescent centres took about 10 s or even more to occur after infrared irradiation. PACS 78.55.-m; 78.45.+h 相似文献
637.
Cui XY Medvedeva JE Delley B Freeman AJ Newman N Stampfl C 《Physical review letters》2005,95(25):256404
Results of extensive density-functional studies provide direct evidence that Cr atoms in Cr:GaN have a strong tendency to form embedded clusters, occupying Ga sites. Significantly, for larger than 2-Cr-atom clusters, states containing antiferromagnetic coupling with net spin in the range 0.06-1.47 muB/Cr are favored. We propose a picture where various configurations coexist and the statistical distribution and associated magnetism will depend sensitively on the growth details. Such a view may elucidate many puzzling observations related to the structural and magnetic properties of III-N and other dilute semiconductors. 相似文献
638.
We present a general theorem restricting properties of interfaces between thermodynamic states and apply it to the spin glass excitations observed numerically by Krzakala and Martin and separately by Palassini and Young in spatial dimensions d = 3,4. We show that such excitations, with interface dimension d(s) < d, cannot yield regionally congruent thermodynamic states. More generally, zero density interfaces of translation-covariant excitations cannot be pinned (by the disorder) in any d but rather must deflect to infinity in the thermodynamic limit. Additional consequences concerning regional congruence in spin glasses and other systems are discussed. 相似文献
639.
640.