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991.
I. V. Avilov I. V. Filatov É. I. Zen'kevich A. M. Shul'ga 《Journal of Applied Spectroscopy》2001,68(1):14-25
We have made a theoretical analysis of the factors influencing the electronic absorption spectra of monomers of porphyrins and chlorins as well as of their chemically bound dimers in which the monomeric subunits are bound through the CH2–CH2 group. On the basis of quantum–chemical calculations by the semiempirical method CNDO/S it is shown that the addition of extra–ligands causes a change in the conformation of the dimers. 相似文献
992.
A.M. Serra 《Talanta》2009,78(3):790-794
A new methodology for the in-line preconcentration, clean-up and speciation of mercury by use of an anion-exchange membrane is proposed. The speciation of mercury is based on retention of its tetrachloro complex onto the membrane while organic mercury flows freely through it. A multisyringe is used as a liquid driver and a cold vapour atomic fluorescence detector is employed to ensure a high sensitivity. Organic mercury is decomposed into to inorganic mercury by using a UV lamp. The carrier and reductant streams consist of 1.5% (m/v) hydrochloric acid and 2% (m/v) tin chloride, respectively. Certified reference material DORM-2 was digested with 37% hydrochloric acid and analysed directly without the need for extraction. The proposed system is more environmental friendly than the classical liquid-liquid extraction procedure. Mercury recoveries from spiked samples and the reference material were all close to 100%. An LOD of 14 and 16 ng/L was obtained for total and organic mercury, respectively, both with an RSD less than 1.3%. 相似文献
993.
Hongmin Chen Mei‐Ling Cheng Y. C. Jean L. James Lee Jintao Yang 《Journal of Polymer Science.Polymer Physics》2008,46(4):388-405
Free‐volume properties, size and distribution, in amorphous polystyrene exposed to CO2 gases have been measured as a function of pressure to 800 psi (5.5 MPa), of time, and of temperature using positron annihilation lifetime spectroscopy. The free volume increases significantly and its distribution broadens as a function of pressure. The free volume relaxes as a function of time with a characteristic time of 15 h, and 5.7 h for 400, and 800 psi, respectively, after depressurizing under vacuum. A portion of free volume created by CO2 exposure remains permanently in the polymer after CO2 exposure. The glass transition temperature decreases significantly as a function of CO2 pressure from the free‐volume data and is compared with the differential scanning calorimeter results. The observed free‐volume variations as a function of pressure, time, and temperature are discussed in terms of hole expansion, creation, free‐volume relaxation, plasticization, and hole filling in amorphous polymers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 388–405, 2008 相似文献
994.
Sture Nordholm 《Molecules (Basel, Switzerland)》2021,26(12)
Pauling introduced the concept of electronegativity of an atom which has played an important role in understanding the polarity and ionic character of bonds between atoms. We set out to define a related concept of atomic reactivity in such a way that it can be quantified and used to predict the stability of covalent bonds in molecules. Guided by the early definition of electronegativity by Mulliken in terms of first ionization energies and Pauling in terms of bond energies, we propose corresponding definitions of atomic reactivity. The main goal of clearly distinguishing the inert gas atoms as nonreactive is fulfilled by three different proposed measures of atomic reactivity. The measure likely to be found most useful is based on the bond energies in atomic hydrides, which are related to atomic reactivities by a geometric average. The origin of the atomic reactivity is found in the symmetry of the atomic environment and related conservation laws which are also the origin of the shell structure of atoms and the periodic table. The reactive atoms are characterized by degenerate or nearly degenerate (several states of the same or nearly the same energy) ground states, while the inert atoms have nondegenerate ground states and no near-degeneracies. We show how to extend the use of the Aufbau model of atomic structure to qualitatively describe atomic reactivity in terms of ground state degeneracy. The symmetry and related conservation laws of atomic electron structures produce a strain (energy increase) in the structure, which we estimate by use of the Thomas-Fermi form of DFT implemented approximately with and without the symmetry and conservation constraints. This simplified and approximate analysis indicates that the total strain energy of an atom correlates strongly with the corresponding atomic reactivity measures but antibonding mechanisms prevent full conversion of strain relaxation to bonding. 相似文献
995.
R. Valenzuela G. Alvarez H. Montiel M.P. Gutirrez M.E. Mata-Zamora F. Barrn A.Y. Snchez I. Betancourt R. Zamorano 《Journal of magnetism and magnetic materials》2008,320(14):1961-1965
A low-field non-resonant microwave absorption has recently been observed in a variety of magnetically ordered materials at low DC fields (−1000 Oe HDC+1000 Oe), which is known as low-field microwave absorption (LFA). It has been shown that LFA is essentially similar to giant magnetoimpedance (GMI), and clearly different from ferromagnetic resonance (FMR). LFA strongly depends on the anisotropy field of the sample. In contrast with FMR (which can be described as the homogeneous precession of spins in the saturated state), LFA can be thought as a spin rotation process occurring during the magnetic saturation. In this work, we present a detailed study of the basic features of LFA in several types of materials: ferrites and amorphous microwires and ribbons; in particular the effects sample shape, temperature up to the Curie transition, the influence of easy axis and the effects of annealings. These examples show that once LFA is fully understood, it can become a powerful characterization tool. 相似文献
996.
Richard Nader Michel Kazan Elie Moussaed Thomas Stauden Merten Niebelschütz Pierre Masri Jörg Pezoldt 《Surface and interface analysis : SIA》2008,40(9):1310-1317
The influence of Ge deposition prior to carbon interaction with 3° off‐axis Si(111) substrates on the structural and morphological properties of the formed silicon carbide (SiC) layer is studied. In situ reflection high‐energy electron diffraction (RHEED) and X‐ray diffraction (XRD) revealed the formation of the cubic silicon carbide (3C‐SiC) modification. In situ spectroscopic ellipsometry measurements revealed a decreasing 3C‐SiC thickness with increasing Ge predeposition. Atomic force microscopy (AFM) studies revealed that the surface overlayer morphology is mainly formed by periodic step arrangements whose relevant geometric parameters, i.e. lateral separation, height and terrace width, depend on the Ge content. Besides the changes of the step morphology, the surface roughness and the grain size and the strain of the formed 3C‐SiC decreases with increasing germanium precoverage. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
997.
998.
999.
SONG Wei QIN Tao 《理论物理通讯》2008,49(6):1515-1518
We present a feasible scheme to implement the 1→2 optimal cloning of arbitrary single particle atomic state into two photonic states, which is important for applications in long distance quantum communication. Our scheme also realizes the tele-NOT gate of one atom to the distant atom trapped in another cavity. The scheme is based on the adiabatic passage and the polarization measurement. It is robust against a number of practical noises such as the violation of the Lamb Dicke condition, spontaneous emission, and detection inefficiency. 相似文献
1000.
James N. OShea J. Ben Taylor Louise C. Mayor Janine C. Swarbrick Joachim Schnadt 《Surface science》2008,602(9):1693-1698
Here we present the characteristic signatures in X-ray absorption and photoemission spectroscopy for molecular damage in adsorbed monolayers of bi-isonicotinic acid on a rutile TiO2(1 1 0) surface. Bi-isonicotinic acid is the anchor ligand through which many important inorganic complexes are bound to the surface of TiO2 in dye-sensitized solar cells. The nature of the damage caused by excessive heating of the adsorbed monolayer is consistent with splitting the molecule into two adsorbed isonicotinic acid molecular fragments. The effect on the lowest unoccupied molecular orbitals (involved in electron transfer in the molecule) can be understood in terms of the adsorption geometry of the reaction products and their nearest neighbor interactions. 相似文献