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991.
Yuhui Qu Wanyong Ma Xiufang Bian Hongwei Tang Weixing Tian 《International journal of quantum chemistry》2006,106(4):960-967
The geometry, electronic configurations, harmonic vibrational frequencies, and stability of the structural isomers of boron phosphide clusters have been investigated using density functional theory (DFT). CCSD(T) calculations show that the lowest‐energy structures are cyclic (IIt, IVs) with Dnh symmetry for dimers and trimers. The caged structure for B4P4 lie higher in energy than the monocyclic structure with D2d symmetry (VIs). The B–P bond dominates the structures for many isomers, so that one preferred dissociation channel is loss of the BP monomer. The hybridization and chemical bonding in the different structures are also discussed. Comparisons with boron nitride clusters, the ground state structures of BnPn (n = 2, 3) clusters are analogous to those of their corresponding BnNn (n = 2, 3) counterparts. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 相似文献
992.
Mukund V. Deshpande 《Applied biochemistry and biotechnology》1992,36(3):227-234
Using cellulase/hemicellulase complex of Sclerotium rolfsii UV-8 mutant and Saccharomyces cerevisiae for fermentation, the coupled saccharification/fermentation (CSF) of 15% AT-rice straw was carried out at 40 degrees C, pH 4.5 for the first 24 h and further incubation was performed at 30 degrees C for 72 h. Increasing the amount of cellulase activity from 3-12 IU FPA/g of substrate resulted in increased yields of ethanol from 1.5-3.6% in 96 h. It has been observed that the coupled system was advantageous over the two stage (separate hydrolysis/fermentation) system as it produced higher amounts of ethanol from cellulose (3.6% as compared to 2.3% ethanol from rice straw). 相似文献
993.
María Beln Ruiz 《International journal of quantum chemistry》2005,101(3):246-260
A general expression for the nonrelativistic Hamiltonian for n‐electron atoms with the fixed nucleus approximation is derived in a straightforward manner using the chain rule. The kinetic energy part is transformed into the mutually independent distance coordinates ri, rij, and the polar angles θi, and φi. This form of the Hamiltonian is very appropriate for calculating integrals using Slater orbitals, not only of states of S symmetry, but also of states with higher angular momentum, as P states. As a first step in a study of the Hylleraas method for five‐electron systems, variational calculations on the 2P ground state of boron atom are performed without any interelectronic distance. The orbital exponents are optimized. The single‐term reference wave function leads to an energy of ?24.498369 atomic units (a.u.) with a virial factor of η = 2.0000000009, which coincides with the Hartree–Fock energy ?24.498369 a.u. A 150‐term wave function expansion leads to an energy of ?24.541246 a.u., with a factor of η = 1.9999999912, which represents 28% of the correlation energy. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
994.
We consider a quantum many-body system on a lattice which exhibits a spontaneous symmetry breaking in its infinite-volume ground states, but in which the corresponding order operator does not commute with the Hamiltonian. Typical examples are the Heisenberg antiferromagnet with a Néel order and the Hubbard model with a (superconducting) off-diagonal long-range order. In the corresponding finite system, the symmetry breaking is usually obscured by quantum fluctuation and one gets a symmetric ground state with a long-range order. In such a situation, Horsch and von der Linden proved that the finite system has a low-lying eigenstate whose excitation energy is not more than of orderN
–1, whereN denotes the number of sites in the lattice. Here we study the situation where the broken symmetry is a continuous one. For a particular set of states (which are orthogonal to the ground state and with each other), we prove bounds for their energy expectation values. The bounds establish that there exist ever-increasing numbers of low-lying eigenstates whose excitation energies are bounded by a constant timesN
–1. A crucial feature of the particular low-lying states we consider is that they can be regarded as finite-volume counterparts of the infinite-volume ground states. By forming linear combinations of these low-lying states and the (finite-volume) ground state and by taking infinite-volume limits, we construct infinite-volume ground states with explicit symmetry breaking. We conjecture that these infinite-volume ground states are ergodic, i.e., physically natural. Our general theorems not only shed light on the nature of symmetry breaking in quantum many-body systems, but also provide indispensable information for numerical approaches to these systems. We also discuss applications of our general results to a variety of interesting examples. The present paper is intended to be accessible to readers without background in mathematical approaches to quantum many-body systems. 相似文献
995.
Osipovs S 《Analytical and bioanalytical chemistry》2008,391(4):1409-1417
Biomass tar mainly consists of stable aromatic compounds such as benzene and polyaromatic hydrocarbons, benzene being the biggest tar component in real biomass gasification gas. For the analysis of individual tar compounds, the solid-phase adsorption method was chosen. According to this method, tar samples are collected on a column with an amino-phase sorbent. With a high benzene concentration in biomass tar, some of the benzene will not be collected on the amino-phase sorbent. To get over this situation, we have installed another column with activated charcoal which is intended for collection of volatile organic compounds, including benzene, after the column with the amino-phase sorbent. The study of maximal adsorption amounts of various compounds on both adsorbents while testing different sampling volumes led to the conclusion that benzene is a limiting compound. The research proved that the use of two sorbents (500 mg + 100 mg) connected in series allows for assessment of tar in synthesis gas with a tar concentration up to 30-40 g m(-3), which corresponds to the requirements of most gasifiers. 相似文献
996.
In this paper we consider the Lane–Emden problem adapted for the p-Laplacian
where Ω is a bounded domain in , n ≥ 2, λ > 0 and p < q < p* (with if p < n, and p* = ∞ otherwise). After some recalls about the existence of ground state and least energy nodal solutions, we prove that,
when q → p, accumulation points of ground state solutions or of least energy nodal solutions are, up to a “good” scaling, respectively
first or second eigenfunctions of −Δ
p
.
Received: 29 April 2008 相似文献
997.
P.Y.M. Yew D.D. Zhu Q.Y. Lin L. Jiang P.L. Chee H.S. Leong Z. Dong X.D. Guo D. Kai X.J. Loh 《Materials Today Chemistry》2021
There are various concerns with current sunscreen actives in the sun care industry which pushes for a natural-based active that is sustainable as well as effective. Lignin is a natural polymer, capable of ultraviolet (UV) filtration and maintaining photostability. In this study, a variety of tuneable lignin-poly(ethylene glycol) (LP) nanodiamond (LP–ND) complexes were developed with varying ratios of LP to NDs. There are two fractions of LP–NDs, which use detonation NDs (LP–rNDs) and furnace NDs (LP–mNDs). Both complexes demonstrated favorable UV filtration, enhanced photostability, and uniform dispersion in water and cream. The LP–mND had maintained its particle size at <200 nm with an increasing LP ratio from 0.5 to 20 times as opposed to the increased particle size of the LP–rND. The LP–mND complex excelled in its photostability during solar radiation from 1 h to 6 h, demonstrating its effectiveness in absorbing 70% in the UV B region. Results had indicated enhanced synergistic effect of LP–rNDs and LP–mNDs with commercial sunscreen. The LP–rND complex had enhanced the sunscreen from 29 to 62, whereas the LP–mND had further boosted the sun protection factor to 89. In a nutshell, the lignin nanodiamond complexes offer a sustainable alternative to current actives in the industry. 相似文献
998.
T. Ritschel Ch. Zuhrt L. Zülicke P. J. Kuntz 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,41(1):127-141
ArnHCl+ van-der-Waals clusters for n = 1–13 are investigated
with the “minimal diatomics-in-molecules (DIM) model”
using ab-initio input data obtained from multi-reference
configuration-interaction calculations plus subsequent projection onto
valence-bond wavefunctions.
The results for the complexes with n = 1–3 are checked against
ab-initio calculations at the
coupled-cluster (CCSD) level with the same one-electron atomic basis set as
for the input data generation (aug-cc-pVTZ from Dunning).
In addition to the electronic ground state,
the first excited
state for the triatomic
complex (n = 1) is also studied.
The results
from the DIM model are shown to be in fair agreement with those from
advanced conventional ab-initio calculations, although there are
differences in detail. The comparison
justifies the extension of the DIM approach to n > 3.
Systematic analysis of the local minima of the multi-dimensional
potential-energy surfaces (PESs), carried out with the combined
method
described in part I (Monte-Carlo sampling plus subsequent steepest-descent
optimization), reveals simple building-up regularities for the most
stable structures (i.e. those corresponding to the global PES minimum)
at each n: apart from always having a nearly linear
(Ar–H–Cl)+ fragment as core, the aggregates show little
or no symmetry. Secondary local minima are also determined and their
structures interpreted.
The PESs for the low-lying excited states reveal a much more complicated
topography compared to the ArnH+ clusters allowing a variety
of photo-processes.
The energy level sequence of the first five excited electronic states
and the stability of the clusters in these states is studied
as a function of the cluster size n. 相似文献
999.
K. von Haeften K. Fink 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,43(1-3):121-124
We have investigated the electronic energies of the ground and excited
states of an octahedral helium
cluster by quantum chemical ab initio calculations. The excited levels
were calculated for the central atom for a set of different
inter atomic separations. Our approach yields potential energy curves which
are suited to describe a density dependence of the spectral features
as previously observed in photo excitation experiments. The potential
energy curves of the 2s and 2p states show a hump at
? caused by the strong perturbation of neighbouring atoms. The
existence of this hump explains the
experimentally observed blue shift and its dependence on the cluster
size or density, respectively.
The potential curves of the higher levels show almost constant
energies. Perturbations of these levels are small, because the overlap
between the Rydberg orbital and the orbitals of the surrounding atoms is
small. This is the case for both small R values
where the Rydberg orbital is well outside the cluster as well as
for large R where the density drastically decreases.
These findings coincide with the
un-shifted features of small clusters observed in experiments. 相似文献
1000.
采用共沉淀法制备了不同Fe/Ni摩尔比(0、0.125、0.25、0.5和0.75)的Al_(2)O_(3)负载型NiFe单、双金属催化剂,并考察了它们在生物糠醛衍生2-羟基四氢吡喃(2-HTHP)还原胺化合成高附加值5-氨基戊醇反应中的催化性能.研究发现当Fe/Ni摩尔比为0.25时表现出最高的5-氨基戊醇收率(在80℃和2 MPa优化条件下达90%).利用N_(2)吸脱附、X射线衍射(XRD)、氢气程序升温还原(H_(2)-TPR)、透射电镜(TEM)等技术对催化剂进行了表征,结果表明适量Fe的掺入可以提高催化剂的分散性和还原性,同时在还原活化催化剂中形成Ni-Fe合金相,继而使相应催化剂表现出更优异的催化活性.通过固定床对比考察了催化剂的稳定性,NiFe(0.25)-Al-O双金属催化剂表现出明显优于NiFe(0)-Al-O单金属催化剂的稳定性,运行120 h前者只有微弱失活而后者显著失活.对比反应后催化剂表征发现,金属Fe的修饰可以有效抑制金属活性颗粒的烧结和流失,从而显著提高其反应稳定性. 相似文献