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841.
A new structural model with fluctuant Si-trimers and missing Si-adatom is proposed for Si-terminated 6H-SiC(0 0 0 1)(3 × 3) reconstruction. The atomic and electronic structures of the model are studied using first principles pseudopotential density-functional approach. The calculated surface electronic density of states coincides quantitatively with the experimental results of photoemission and electron energy loss spectroscopy. Based on the calculations, the Patterson map and scanning tunneling microscopic (STM) images simulated for the new model agree more satisfactorily with the experimental X-ray diffraction and STM observations than that for previously proposed models. The calculations of formation energies suggest that the new structure would be formed under the environment of dilute Si vapor around the surface in the preparation process. 相似文献
842.
It has recently been shown that the first passage time problem for a certain class of one-dimensional processes that includes shot noise can be formulated in terms of a set of integral equations. These are found by exact enumeration of all possible trajectories. We show that the equations can be found by more direct means for processes described by the evolution equation
, wheren(t) is time-localized shot noise. 相似文献
843.
烟草中Fe,Co一阶导数分光光度法同时测定的研究 总被引:4,自引:0,他引:4
本文研究了在pH4.0时,meso-四(4-磺酸基苯基)卟啉与铁、钴同时络合显色的反应条件以及一阶导数光谱行为。此体系一阶导数的灵敏度比零阶导数灵敏度高。Fe ̄(3+)~0.18μg/mL、Co ̄(2+)0~0.24μg/ml,范围内符合比耳定律;检测限为:Fe ̄(3+)=0.48ng/mL,Co ̄(2+)=0.2ng/mL。回收率为:Fe98.5%~100.8%,Co99.2%~101.3%。此方法用于烟草中痕量Fe、Co测定,与AAS值相比较,结果令人满意。 相似文献
844.
Tokutaro Ogata 《Tetrahedron》2004,60(18):3941-3948
The first total and biomimetic synthesis of violet-quinone (1), which has a dinaphthofuranquinone (DNFQ) framework, is described. This synthesis features the oxidative dimerization of 1-naphthol 4 and the construction of the DNFQ framework by photochemical ring closure of 2,2′-binaphthoquinone 7 as a key intermediate. Compound 7 was prepared by the novel oxidative dimerization of 4 with a semiconductor (such as ZrO2) in the presence of dioxygen, followed by oxidation of the resulting 2,2′-binaphthyl-1,1′-quinone 6 with HNO3. 相似文献
845.
Simultaneous spectrophotometric determination of three food dyes by using the first derivative of ratio spectra 总被引:1,自引:0,他引:1
Ternary mixtures of colorants E-123 (Amaranth), E-124 (Ponceau 4R) and E-120 (Carminic acid) are resolved by using the first derivative ratio spectrum-zero crossing method without the need for any separation step. Calibration graphs were linear up to 64 mg l−1 of carminic acid, 32 mg l−1 of Amaranth and 32 mg l−1 of Ponceau 4R. The method was applied to different commercial food products and results concordant with high performance liquid chromatography were obtained. 相似文献
846.
The constitutive equations for liquid crystalline polymers recently proposed by one of us [1] are applied here to interpret the behaviour of the shear viscosity η and the first normal stress difference N1() measured for liquid crystalline (LC) solutions of hydroxypropylcellulose in acetic acid. N1( ) is observed to change from positive to negative and again to positive, as the shear rate increases, at lower concentrations, in the LC phase. The -values at which N1 changes sign depend on the molecular mass (degree of polymerization) and on the concentration. η shows a small Newtonian plateau at low shear rates and a strong shear-thinning at higher values of . The rate of decrease of η in this region shows an “hesitation” similar to one previously observed in LC solutions of poly-γ-benzyl-L-glutamate PBLG. All these observations can be rationalized within the frame-work of Martins' theory. The expressions for N1() and η derived from this theory fit very well (quantitatively) to the experimental data and some fundamental viscoelastic parameters of the system under study are thereby obtained for the first time. 相似文献
847.
Karakaş A Elmali A Unver H Svoboda I 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(13-14):2979-2987
N-(4-nitrobenzylidene)-o-fluoroamine (1) and N-(3-nitrobenzylidene)-p-fluoroamine (2) have been synthesized. The crystal structures of both compounds have been defined by X-ray diffraction analysis, and characterized by FT-IR and UV-visible instrumental methods. The recorded spectrum by UV-visible spectroscopy for the investigated compounds show good transparency in the visible region, and have solvatochromic behavior in the UV region, implying nonzero microscopic first hyperpolarizability. We also report ab initio quantum chemical calculations of the electric dipole moments (mu) and the first hyperpolarizabilities (beta) of the studied compounds. Our results suggest that the investigated ligands might have microscopic nonlinear optical (NLO) behavior with nonzero values. 相似文献
848.
Enhancement spectra of the collision-induced absorption in the first overtone region 5500-6750 cm−1 of D2 in the D2-Ar, D2-Kr, and D2-Xe binary mixtures were studied at 298 K for base densities of D2 in the range 55-251 amagat and for partial densities of Ar, Kr, and Xe in the range 46-384 amagat. The observed spectra consist of the following quadrupolar transitions: O2(3), O2(2), Q2 (J), J = 1-5 and S2 (J), J = 0-5 of D2. Binary and ternary absorption coefficients were determined from the integrated absorption coefficients of the band. Profile analyses of the spectra were carried out using the Birnbaum-Cohen (BC) lineshape function and characteristic lineshape parameters were determined from the analyses. 相似文献
849.
In this piece of work, we introduce a new idea and obtain stability interval for explicit difference schemes of O(k2+h2) for one, two and three space dimensional second-order hyperbolic equations utt=a(x,t)uxx+α(x,t)ux-2η2(x,t)u,utt=a(x,y,t)uxx+b(x,y,t)uyy+α(x,y,t)ux+β(x,y,t)uy-2η2(x,y,t)u, and utt=a(x,y,z,t)uxx+b(x,y,z,t)uyy+c(x,y,z,t)uzz+α(x,y,z,t)ux+β(x,y,z,t)uy+γ(x,y,z,t)uz-2η2(x,y,z,t)u,0<x,y,z<1,t>0 subject to appropriate initial and Dirichlet boundary conditions, where h>0 and k>0 are grid sizes in space and time coordinates, respectively. A new idea is also introduced to obtain explicit difference schemes of O(k2) in order to obtain numerical solution of u at first time step in a different manner. 相似文献
850.
We performed total energy electronic-structure calculations based on DFT that clarify the intrinsic magnetism of undoped GaN. The magnetism is due to Ga, instead of N, vacancies. The origin of magnetism arises from the unpaired 2p electrons of N surrounding Ga vacancy. At a vacancy concentration of 5.6%, the ferromagnetic state is 181 meV lower than the antiferromagnetic state. Our findings are helpful to gain a more novel understanding of structural and spin properties of Ga vacancy in wurtzite GaN and also provide a possible way to generate magnetic GaN by introducing Ga vacancies instead of doping with transition-metal atoms. 相似文献