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111.
112.
Andrew P. Mendham John Spencer Babur Z. Chowdhry Trevor J. Dines Muhammad Mujahid Rex A. Palmer Graham J. Tizzard Simon J. Coles 《Journal of chemical crystallography》2011,41(9):1323-1327
Abstract
The cyclic di-amino acid peptide N,N′-diacetyl-cyclo(Gly-Gly), C8H10N2O4, crystallizes in the triclinic space group P[`1] P\bar{1} with unit cell parameters a = 9.4855(4) ?, b = 10.0250(3) ?, c = 10.0763(4) ?, α = 73.682(2)°, β = 82.816(2)°, γ = 81.733(2)°, V = 906.40(6) ?3, Z = 4 (2 molecules, A and B, per asymmetric unit), Dc = 1.452 g cm−3 and linear absorption coefficient 0.118 mm−1. The crystal structure determination was carried out with MoKα X-ray data measured at 120(2) K. In the final refinement cycle the data/restraints/parameter ratios were 4124/0/258 and goodness-of-fit on F2 = 1.0008. Final R indices for [I > 2σ(I)] were R1 = 0.0501, wR2 = 0.1007 and R indices (all data) R1 = 0.0864, wR2 = 0.11180. The largest electron density difference peak and hole were 0.241 and −0.232 e ?−3, respectively. The DKP rings in both molecules A and B have boat conformations with pseudo mm2 (C2v) symmetry if the N atoms and CH2 groups are considered identical. In each case, the prow and stern of the boat are the α-carbons C(3) and C(6). The overall molecular symmetry of molecules A and B is approximately C2 with the twofold symmetry axis of the DKP boat being maintained through the centre of the DKP ring. Details of the molecular geometry are compared with that of the parent compound cyclo(Gly-Gly) in which the DKP ring is planar with exact symmetry [`1] \bar{1} (Ci). 相似文献113.
114.
Precision capacitance dilatometry provides a sensitive measure of the thermal strain developed in a sample undergoing a structural distortion with its varying temperature. The A15 structure compounds, V3Si and Nb3Sn, are well known to undergo distortion from their cubic structures at room temperature to tetragonal structures (c/a > 1 for V3Si and c/a < 1 for Nb3Sn) at low temperatures. In the past, highly anomalous thermal expansion behaviour recorded for these materials has been attributed to a strongly anharmonic lattice potential manifesting itself in unusually high, and strongly temperature-dependent, Grüneisen parameters. Further studies on polycrystalline material revealed this anomalous expansion to be highly anisotropic at temperatures for which, according to conventional diffraction data, the materials are cubic. This behaviour was linked to control of sample morphology by a residual stress field resulting from sample preparation. More recent experiments, in which the transformation morphology has been controlled by the application of external stresses to single crystal V3Si and polycrystalline samples of Nb3Sn and Nb3(Sn1-x Sb x ), have confirmed the occurrence of significant anisotropy in the thermal strain in the cubic phase, well above the structural transformation. We link this departure from cubic symmetry with the well-known soft-mode character of these materials and the associated “central peak” scattering which is also observed well above the transformation temperature. We are led to propose that the “central peak” is the precursor to a Bragg reflection for the transformation structure. This coincidence between “central peak” scattering and the reciprocal lattice for the transformed phase in Ti-Ni has been termed a “ghost lattice”. 相似文献
115.
S. Friedrich C. R. Bates M. T. Burks O. B. Drury D. P. DiPrete 《Journal of Radioanalytical and Nuclear Chemistry》2013,296(2):927-930
The difficulty to directly detect plutonium in spent nuclear fuel due to the high Compton background of the fission products motivates the design of a gamma detector with improved sensitivity at low energies. We have built such a detector by operating a thin high-purity Ge detector with a large scintillator Compton veto directly behind it. The Ge detector is thin to absorb just the low-energy Pu radiation of interest while minimizing Compton scattering of high-energy radiation from the fission products. The subsequent scintillator is large so that forward-scattered photons from the Ge detector interact in it at least once to provide an anti-coincidence veto for the Ge detector. For highest sensitivity, additional material in the line of sight is minimized, the radioactive sample is kept thin, and its radiation is collimated. We will discuss the instrument design, and demonstrate the feasibility of the approach with a prototype that employs two large CsI scintillator vetoes. Initial spectra of a thin Cs-137 calibration source show a background suppression of a factor of ~2.5 at ~100 keV, limited by an unexpectedly thick 4 mm dead layer in the Ge detector. 相似文献
116.
117.
Dawud H. Tan Chunfeng Zhou Christopher J. Ellison Satish Kumar Christopher W. Macosko Frank S. Bates 《ournal of non Newtonian Fluid Mechanics》2010,165(15-16):892-900
Both melt viscosity (ηo) and elasticity (correlated here with the longest melt relaxation time λ1) were found to control the diameter distribution of meltblown fibers. Fibers were formed by melt blowing binary polystyrene (PS) blends containing widely differing component molecular weights using a custom-built laboratory apparatus. Varying the concentration and molecular weight of a high molecular weight PS provided independent control over ηo and λ1. These rheological parameters influence the average diameter (dav) and the distribution of diameters (coefficient of variation, CV) of meltblown fibers in different ways. Increasing ηo leads to an increase in dav but has little impact on CV. On the other hand, increasing λ1 beyond a threshold value reduces CV while simultaneously increasing dav. A one-dimensional slender-jet theoretical model with both upper convected Maxwell and Phan–Thien and Tanner constitutive equations was developed to investigate the influence of viscoelasticity and processing parameters on the properties of meltblown fibers. This model predicts a strong dependence of fiber diameter on the air shear stress and variations in fiber diameter with viscoelasticity that are in qualitative agreement with the experimental results. We believe these results suggest that carefully controlling the viscoelastic profile of polymers used in melt blowing is a viable approach for producing nanofibers with narrow fiber diameter distributions using current commercial equipment. 相似文献
118.
EricJ. May AnneBuyle Padias RobertB. Bates HenryK. Hall 《Helvetica chimica acta》2005,88(6):1397-1404
Bifunctional vinyl ethers react with electron‐poor alkenes to cyclobutanes in good yields. The second C?C bond reacted with neither the cyclobutane nor its zwitterion intermediate, even on heating. Cyclobutanes formed from ‘tetracyanoethylene’ ( 8 ) were transformed into tetrahydropyridines by reaction of the corresponding zwitterion with MeCN as the solvent. In contrast, cyclobutanes formed from dimethyl (dicyanomethylidene)propanedioate ( 9 ) did not react with MeCN, which is ascribed to diminished stabilization of the zwitterion intermediate, and increased steric effects. These results extend the classical studies of Huisgen and his co‐workers. 相似文献
119.
Nataliya Sakhnenko Alexander Nerukh Trevor Benson Phillip Sewell 《Optical and Quantum Electronics》2007,39(10-11):825-836
The resolvent method approach is proposed for analytically solving the time domain Volterra integral equation (TDVIE) which describes the electromagnetic fields in 2-D cylindrical structures with time changes in the permittivity. It is shown that the approach allows investigation of the electromagnetic field transformation due to an abrupt time change in dielectric permittivity inside a circular cylinder as well as the construction of the Green’s function for an initial-boundary value problem. Key mechanisms of electromagnetic field transformation are investigated and a qualitative distinction of the processes is shown that depends on an initial field configuration. 相似文献
120.
Karen A. Switek Kwanho Chang Frank S. Bates Marc A. Hillmyer 《Journal of polymer science. Part A, Polymer chemistry》2007,45(3):361-373
Block copolymers containing polystyrene and polycyclooctene were synthesized with a ring‐opening metathesis polymerization/chain‐transfer approach. Polystyrene, containing appropriately placed olefins, was prepared by anionic polymerization and served as a macromolecular chain‐transfer agent for the ring‐opening metathesis polymerization of cyclooctene. These unsaturated polymers were subsequently converted to the corresponding saturated triblock copolymers with a simple heterogeneous catalytic hydrogenation step. The molecular and morphological characterization of the block copolymers was consistent with the absence of significant branching in the central polycyclooctene and polyethylene blocks [high melting temperatures (114–127 °C) and levels of crystallinity (17–42%)]. A dramatic improvement in both the long‐range order and the mechanical properties of a microphase‐separated, symmetric polystyrene–polycyclooctene–polystyrene block copolymer sample was observed after fractionation. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 361–373, 2007 相似文献