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21.
Experimental and computational studies were performed to understand the electronic structure of ternary perovskites (ASnO(3), A = Ca, Sr, Ba, Cd), pyrochlores (RE(2)Sn(2)O(7), RE = Y, La, Lu; Cd(2)Sb(2)O(7)), and defect pyrochlore oxides (Ag(2)Sb(2)O(6)) containing the main group ions Sn(4+) and Sb(5+). In all compounds, the lowest energy states in the conduction band arise primarily from the antibonding Sn/Sb 5s-O 2p interaction. In the alkaline-earth stannate perovskites (BaSnO(3), SrSnO(3), and CaSnO(3)) the conduction bandwidth decreases strongly in response to the octahedral tilting distortion triggered by the decreasing size of the alkaline-earth cation. This in turn leads to a corresponding increase in the band gap from 3.1 eV in BaSnO(3) to 4.4 eV in CaSnO(3). The band gap of CdSnO(3) is relatively small (3.0 eV) considering the large octahedral tilting distortion. The origin of this apparent anomaly is the mixing between the empty Cd 5s orbitals and the antibonding Sn 5s-O 2p states. This mixing leads to a widening of the conduction band and a corresponding decrease in the band gap. The participation of the normally inert A-site cation in the electronic structure near the Fermi level can be considered an inductive effect, as it utilizes substitution on the A-site to directly modify the electronic structure of the SnO(3)(2)(-) framework. While the pyrochlore structure is more complicated, the energy level and width of the lowest energy conduction band can be analyzed in a manner similar to that utilized on the perovskite structure. The Sn-O-Sn and Sb-O-Sb bonds are highly distorted from linear geometry in pyrochlore, leading to a relatively narrow conduction band and a wide band gap. In Cd(2)Sb(2)O(7) and Ag(2)Sb(2)O(6) the Cd(2+) and Ag(+) ions exhibit a strong inductive effect that widens the conduction band and lowers the band gap significantly, very similar to the effect observed in the perovskite form of CdSnO(3). 相似文献
22.
A crude preparation of Aspergillus niger β-glucosidase (27.5 cello-biase U/mg protein at 40°C, pH 5.0) was immobilized on
concanavalin A-Sepharose (CAS). The cellobiase activity of the immobilized enzyme was 1334 U/mg dried CAS or 108 U/mL CAS
gel. The β-glucosidase-CAS complex was entrapped within crosslinked propylene glycol alginate/bone-geletin gel spheres that
possessed between 0.67 and 2.35 cellobiase U/mL spheres, depending on their size. The effect of cellobiose concentration (10–300
mM) on the activity of native, immobilized, and gel-entrapped enzyme was determined. It was shown that concentrations of cellobiose
between 10 and 180 mM were not inhibitory to the entrapped enzyme, although inhibition was found to occur with the native
and immobilized enzyme. Exogenous ion addition was not necessary to maintain the structural integrity of the spheres, which
were stable for 4 d at 40°C. 相似文献
23.
Schumm BA Koetke DS Adolphsen CE Alexander JP Averill D Barish BC Barklow T Barnett BA Blockus D Boyarski A Brabson B Breakstone A Bulos F Burchat PR Burke DL Cence RJ Chapman J Chmeissani M Cords D Coupal DP Dauncey P DeStaebler HC Dorfan JM Drell PS Drewer DC Durrett D Elia R Feldman GJ Field RC Ford WT Fordham C Frey R Fujino D Gan KK Gero E Gidal G Glanzman T Goldhaber G Gomez Cadenas JJ Gratta G Hanson G Harr R Harral B Harris FA Hayes K Hearty C Heusch CA Hildreth MD Himel T Hinshaw DA 《Physical review D: Particles and fields》1992,46(1):453-456
24.
Chen JP Meziani ZE Beck D Boyd G Chinitz LM Day DB Dennis LC Dodge G Filippone BW Giovanetti KL Jourdan J Kemper KW Koh T Lorenzon W McCarthy JS McKeown RD Milner RG Minehart RC Morgenstern J Mougey J Potterveld DH Rondon-Aramayo OA Sealock RM Smith LC Thornton ST Walker RC Woodward C 《Physical review letters》1991,66(10):1283-1286
25.
James F. Woodward 《Foundations of Physics》1982,12(5):467-478
The Faradayan hypothesis of inductive coupling of the electromagnetic and gravitational fields is briefly discussed. An experiment designed to test the hypothesis wherein samples are spun to see if any electrogravitational charge is induced is described. Results of the experiment are reported. They imply the induction of a charge density * for spinning samples that behaves as *=ma, where m is the mass density of an element of matter experiencing an acceleration a, and is the coupling coefficient for the hypothetical electrogravitational induction effect. In this experiment, is found to have the value(9.6±3.3)×10
–13
statcoulombs/dyne. Tests that seem to rule out explanations of the observed charges in terms of conventional charging mechanisms are considered. 相似文献
26.
YBaFe(2)O(5) has been synthesized by heating a nanoscale citrate precursor in a carefully controlled reducing environment. Successful synthesis of a single-phase sample can only be achieved in a narrow window of oxygen partial pressures and temperatures. YBaFe(2)O(5) adopts an oxygen-deficient perovskite-type structure, which contains double layers of corner sharing FeO(5) square pyramids separated by Y(3+) ions. At T(N) congruent with 430 K, tetragonal (P4/mmm) and paramagnetic YBaFe(2)O(5) orders antiferromagnetically (AFM) experiencing a slight orthorhombic distortion (Pmmm). Around this temperature, it can be characterized as a class-III mixed valence (MV) compound, where all iron atoms exist as equivalent MV Fe(2.5+) ions. The magnetic structure is characterized by AFM Fe-O-Fe superexchange coupling within the double layers and a ferromagnetic Fe-Fe direct-exchange coupling between neighboring double layers. Upon cooling below approximately 335 K, a premonitory charge ordering (2Fe(2.5+) --> Fe(2.5+delta) + Fe(2.5)(-delta)) into a class-II MV phase takes place. This transition is detected by differential scanning calorimetry, but powder diffraction techniques fail to detect any volume change or a long-range structural order. At approximately 308 K, a complete charge ordering (2Fe(2.5+) --> Fe(2+) + Fe(3+)) into a class-I MV compound takes place. This charge localization triggers a number of changes in the crystal, magnetic, and electronic structure of YBaFe(2)O(5). The magnetic structure rearranges to a G-type AFM structure, where both the Fe-O-Fe superexchange and the Fe-Fe direct-exchange couplings are antiferromagnetic. The crystal structure rearranges (Pmma) to accommodate alternating chains of Fe(2+) and Fe(3+) running along b and an unexpectedly large cooperative Jahn-Teller distortion about the high-spin Fe(2+) ions. This order of charges does not fulfill the Anderson condition, and it rather corresponds to an ordering of doubly occupied Fe(2+) d(xz) orbitals. Comparisons with YBaMn(2)O(5) and YBaCo(2)O(5) are made to highlight the impact of changing the d-electron count. 相似文献
27.
Jonathan D. Woodward 《Journal of solid state chemistry》2005,178(9):2922-2926
The reaction of UO3 and TeO3 with a KCl flux at 800 °C for 3 days yields single crystals of K4[(UO2)5(TeO3)2O5]. The structure of the title compound consists of layered, two-dimensional sheets arranged in a stair-like topology separated by potassium cations. Contained within these sheets are one-dimensional uranium oxide ribbons consisting of UO7 pentagonal bipyramids and UO6 tetragonal bipyramids. The ribbons are in turn linked by corner-sharing with trigonal pyramidal TeO3 units to form sheets. The lone-pair of electrons from the TeO3 groups are oriented in opposite directions with respect to one another on each side of the sheets rendering each individual sheet nonpolar. The potassium cations form contacts with nearby tellurite units and axial uranyl oxygen atoms. Crystallographic data (193 K, MoKα, ): triclinic, space group , , , , α=99.642(1)°, β=93.591(1)°, γ=100.506(1)°, , Z=1,R(F)=4.19% for 149 parameters and 2583 reflections with I>2σ(I). 相似文献
28.
‘7-aminocephalocillanic acid’ 1 , an intermediate for the preparation of new β-lactam antibiotics was prepared by transforming the β-lactam carbinol 2a to the phosphorane 9a through several steps. Oxydation of 9a with DMSO/Ac2O led directly to the cyclized ester 11a , which has been converted into 1 by known methods. 相似文献
29.
This article describes Bezout and Prüfer f-rings in terms of their localizations. All f-rings here are corrmutative, semi prime and possess an identity; they also have the bounded inversion property: a >1 implies that a is a multiplicative unit. The two main theorems are as follows: (1) A is a Bezout f-ring if and only if each localization at a maximal ideal is a (totally ordered) valuation ring; (2) Each Prufer f-ring is quasi-Bezout, and if each localization of A is a Prufer f-ring then so is A. We give a counter-example to show that the converse of the last assertion is false. 相似文献
30.