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981.
The authors have previously shown how to achieve a 40-fold reduction in system background for above-ground germanium detectors. Such systems are generally sufficient for examining small hydrogenous samples or samples of non-hydrogenous materials such as soil, glass, metals, etc. However, samples with a large hydrogen content yield a seriously degraded background due to a number of neutron-induced interactions, such as Compton scatter following hydrogen capture of thermalized neutrons. A study was performed to better understand the sources of thermal neutron flux in above-ground detector systems. The effects of different quantities of hydrogen in the sample were also examined. Methods are presented for additional lowering of system background for both hydrogenous and non-hydrogenous samples. 相似文献
982.
Rate constants for the gas-phase reactions of NO3 radicals with a series of cycloalkenes have been determined at 298 ± 2 K, using a relative rate technique. Using an equilibrium constant for the NO2 + NO3 ? N2O5 reactions of 3.4 × 10?11 cm3 molecule?1, the following rate constants (in units of 10?13 cm3 molecule?1 s?1) were obtained: cyclopentene, 4.52 ± 0.52; cycloheptene, 4.71 ± 0.56; bicyclo[2.2.1]-2-heptene, 2.41 ± 0.28; bicyclo[2.2.2]-2-octene, 1.41 ± 0.17; bicyclo[2.2.1]-2,5-heptadiene, 9.92 ± 1.13; and 1,3,5-cycloheptatriene, 12.6 ± 2.9. When combined with previous literature rate constants for cyclohexene and 1,4-cyclohexadiene, these data show that the rate constants for the nonconjugated cycloalkenes studied depend to a first approximation on the number of double bonds and the degree and configuration of substitution per double bond. No obvious effects of ring strain energy on these NO3 radical addition rate constants were observed. Our previous a priori predictive techniques for the alkenes and cycloalkenes can now be extended to strained cycloalkenes. 相似文献
983.
The kinetics of the gas-phase reactions of the OH radical with (C2H5O)3PO and (CH3O)2P(S)Cl and of the reactions of NO3 radicals and O3 with (CH3O)2P(S)Cl have been studied at room temperature. Using a relative rate technique, the rate constants determined for the reactions of the OH radical with (C2H5O)3PO and (CH3O)2P(S)Cl at 296 ± 2 K and 740 torr total pressure of air were (5.53 ± 0.35) × 10?11 and (5.96 ± 0.38) × 10?11 cm3 molecule?1 s?1, respectively. Upper limits to the rate constants for the NO3 radical and O3 reactions with (CH3O)2P(S)Cl of <3 × 10?14 cm3 molecule?1 s?1 and <2 × 10?19 cm3 molecule?1 s?1, respectively, were obtained. These data are compared and discussed with previous literature data for organophosphorus compounds. 相似文献
984.
985.
Peter A. Chetcuti Andr Ligard Grety Rihs Günther Rist Arthur Schweiger 《Helvetica chimica acta》1991,74(7):1591-1599
Bimetallic inclusion complexes have been synthesized by a secondary coordination interaction between the guest complex [Fe(η5-C5H5)(CO)2(NH3)][PF6] and copper(II) complex 1a or nickel(II) complex lb containing crown-ether hosts. The X-ray crystal-structure analysis established that the Cu,Fe inclusion complex 2 crystallize as a centrosymmetric dimer with a Cu? Cu separation of 3.73 Å and a novel out-of-plane Cu? N interaction. The magnetic parameters for 2 were obtained by ESR and ENDOR spectroscopy. ESR susceptibility measurements down to 6 K exclude the presence of any antiferromagnetic coupling interaction between the CuIIcenters of the dimer. 相似文献
986.
987.
Reduction of [(η5-CH3C6H6)Fe(CO)3]+PF6? by NaBH4, LiBH4, LiEt3BH and KBu53BH becomes less regioselective at lower temperatures. Regioselectivity of reduction by 9-BBN, however, improves as the temperature is lowered. Factors affecting nucleophilic addition are discussed in terms of likely transition states for the competing processes. 相似文献
988.
Hans G Kaper Gary K Leaf Arthur J Lindeman 《Journal of Mathematical Analysis and Applications》1975,50(1):42-65
The one-group neutron transport equation is commonly given as an integrodifferential equation for the neutron density ψ(x, ω) over a domain G × S in the five-dimensional phase space . In this paper we show how, by decomposing the domain of the transport operator into a complementary pair of manifolds by means of a projection operator, any transport problem can be formulated, on either manifold, in terms of a symmetric positive definite operator. We use Friedrichs' method to extend the operator to a selfadjoint operator and look for a generalized solution by minimizing a certain functional over the appropriate Hilbert space. A Ritz-Galerkin type approximation procedure is formulated, and an estimate for the difference between the exact and approximate solution is given. The procedure is illustrated for a special choice of finite dimensional subspace. 相似文献
989.
Reversible intermolecular energy transfer between saturated amines and benzene in non-polar solution
Excitation of a mixture of dimethylethylamine (DEMA) and benzene in n-hexane at 222 nm primarily produces excited amine, while at 261 nm excited benzene predominantly results. The fluorescence spectra appreciably overlap. With 222 nm excitation, DEMA fluorescence is quenched by benzene at the diffusion-controlled rate; this quenching results with nearly unit efficiency in sensitized benzene fluorescence. With 261 nm excitation, some sensitized DEMA fluorescence is observed: the rate constant for tins process is ≈ 2.6 × 109 M?1 s?1. 相似文献
990.
Arthur C. Brett Samuel Levine Robert N. OBrien 《Journal of Electroanalytical Chemistry》1978,89(1):83-86
A theory developed in a previous paper of the streaming of the electrolyte solution at the polarographic maximum of the first kind with the dropping mercury electrode, is applied to the negative maximum. Stream function contours and fluid velocities are calculated and comparison is made with the experimental results of O'Brien and Dieken. Calculated separations in flow below the neck of the mercury drop are in agreement with predictions of other authors. 相似文献