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排序方式: 共有291条查询结果,搜索用时 15 毫秒
71.
Yoshifuku S Chen S McMahon EM Yoshikawa A Sengupta PP Korinek J Belohlavek M 《Ultrasonics》2007,46(2):109-118
Objectives
We introduced a harmonic-to-fundamental ratio (HFR) of the radiofrequency (RF) signals that reduces confounding effects of attenuation. We studied whether HFR analysis of RF signals received from contrast microbubbles allows accurate measurement of the left ventricular (LV) cavity area under varying levels of attenuation.Background
Attenuation is a fundamental problem in ultrasound imaging and limits the use of clinical echocardiography.Methods
RF data from short axis systolic and diastolic scans were obtained from 14 open-chest dogs following left-atrial bolus of Optison. Attenuation was induced by interposed silicone pads calibrated to induce 7 dB or 14 dB reductions of the backscattered RF signal. RF images were reconstructed from the RF signals, HFR values calculated for each image pixel for 0 dB, 7 dB and 14 dB attenuation conditions, and LV area obtained by summation of “LV cavity pixels”. A reference LV cavity area was obtained from endocardial border tracings in enhanced scans by experts.Results
Correlation of the HFR-defined and reference areas at systole was R = 0.95, R = 0.94, and R = 0.91 for 0 dB, 7 dB and 14 dB levels of attenuation, respectively, and at diastole was R = 0.95 for 0 dB, 7 dB and 14 dB levels of attenuation. The mean difference from both systolic and diastolic values was <1.45 cm2 (i.e. negligible) in all attenuation settings.Conclusion
Our novel HFR method supports precise measurement of the LV cavity area in contrast images with simulated high attenuation of ultrasound signals. 相似文献72.
C. J. McMahon Jr. J. A. Pfaendtner R. C. Muthiah 《Czechoslovak Journal of Physics》1995,45(11):965-978
A mode of brittle fracture is described which is fundamentally different from the rapid transgranular cleavage or intergranular decohesion that is usually associated with that term. It involves stress-induced diffusion of surface-adsorbed embrittling elements along grain boundaries, and it occurs by slow, step-wise crack growth, the rate of which can, in principle, be calculated from the knowledge of the relevant intergranular diffusion coefficient, the stress profile at the crack tip and the dependence of the stress for grain-boundary decohesion on the concentration of the embrittling element. This mode of fracture is postulated to be possible in any high-strength alloy with a low-melting-point element adsorbed on the surface if the applied stress is high enough. Known examples include the brittle type of stress-relief cracking in steels, tin-induced cracking of Cu-Sn alloys, oxygen-induced cracking of iron-, copper-, and nickel-based alloys, and the group of phenomena known as liquid-metal embrittlement and solid-metal embrittlement.The paper is dedicated to Dr. Frantiek Kroupa in honour of his 70th birthday.This work is supported by National Science Foundation Grant CMS 95-03980. 相似文献
73.
Tuning Phenols with Intra‐Molecular Bond Shifted HYdrogens (IM‐SHY) as diaCEST MRI Contrast Agents 下载免费PDF全文
Xing Yang Nirbhay N. Yadav Xiaolei Song Prof. Sangeeta Ray Banerjee Hannah Edelman Il Minn Prof. Peter C. M. van Zijl Prof. Martin G. Pomper Prof. Michael T. McMahon 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(48):15824-15832
The optimal exchange properties for chemical exchange saturation transfer (CEST) contrast agents on 3 T clinical scanners were characterized using continuous wave saturation transfer, and it was demonstrated that the exchangeable protons in phenols can be tuned to reach these criteria through proper ring substitution. Systematic modification allows the chemical shift of the exchangeable protons to be positioned between 4.8 to 12 ppm from water and enables adjustment of the proton exchange rate to maximize CEST contrast at these shifts. In particular, 44 hydrogen‐bonded phenols are investigated for their potential as CEST MRI contrast agents and the stereoelectronic effects on their CEST properties are summarized. Furthermore, a pair of compounds, 2,5‐dihydroxyterephthalic acid and 4,6‐dihydroxyisophthalic acid, were identified which produce the highest sensitivity through incorporating two exchangeable protons per ring. 相似文献
74.
Swallen SF Mapes MK Kim YS McMahon RJ Ediger MD Satija S 《The Journal of chemical physics》2006,124(18):184501
The translational dynamics of the low molecular weight glass-former tris(naphthylbenzene) have been studied on the length scale of a few nanometers at the glass transition temperature Tg. Neutron reflectivity was used to measure isotopic interdiffusion of multilayer samples created by physical vapor deposition. Deposition with the substrate held at Tg-6 K allows observation of dynamics characterizing the equilibrium supercooled liquid. The diffusion coefficient measured at q = 0.03 A(-1) was determined to be 1x10(-17) cm2/s at 342 K (Tg). The self-part of the intermediate scattering function I(s)(q,t) decays exponentially. Samples deposited well below Tg show a substantial thermal history effect during subsequent translational motion at Tg. 相似文献
75.
Jeffrey M. McMahon Anne-Isabelle Henry Kristin L. Wustholz Michael J. Natan R. Griffith Freeman Richard P. Van Duyne George C. Schatz 《Analytical and bioanalytical chemistry》2009,394(7):1819-1825
Finite element method calculations were carried out to determine extinction spectra and the electromagnetic (EM) contributions
to surface-enhanced Raman spectroscopy (SERS) for 90-nm Au nanoparticle dimers modeled after experimental nanotags. The calculations
revealed that the EM properties depend significantly on the junction region, specifically the distance between the nanoparticles
for spacings of less than 1 nm. For extinction spectra, spacings below 1 nm lead to maxima that are strongly red-shifted from
the 600-nm plasmon maximum associated with an isolated nanoparticle. This result agrees qualitatively well with experimental
transmission electron microscopy images and localized surface plasmon resonance spectra that are also presented. The calculations
further revealed that spacings below 0.5 nm, and especially a slight fusing of the nanoparticles to give tiny crevices, leads
to EM enhancements of 1010 or greater. Assuming a uniform coating of SERS molecules around both nanoparticles, we determined that regardless of the
separation, the highest EM fields always dominate the SERS signal. In addition, we determined that for small separations less
than 3% of the molecules always contribute to greater than 90% of the signal.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
76.
McMahon SA Leonard GA Buchanan LV Giraud MF Naismith JH 《Acta crystallographica. Section D, Biological crystallography》1999,55(5):1108
In the paper by McMahon, Leonard, Buchanan, Giraud & Naismith [Acta Cryst. (1999). D55, 399-402] an author's error has resulted in the fifth sentence of the Abstract being incorrect. The sentence should read 'They are monoclinic, space group P21, with unit-cell dimensions a = 71.12, b = 58.42, c = 96.38 A, beta = 96.38 degrees. 92% (native) and 94% (selenomethionine) complete data sets have been recorded to 2.9 A (Rmerge = 5.0%) and 3.0 A (Rmerge = 6.9%), respectively.' 相似文献
77.
Fourier transform ion cyclotron resonance (FT-ICR) ligand exchange equilibrium experiments have been used to establish a relative scale of sodium binding free energies of about fifty organic molecules. Ab initio calculations yield accurate enthalpies and entropies of complexation for a new set of 30 molecules. These calculations establish an absolute basis for the relative experimental free energy scale. In addition, they provide structural information for the complexes which permits considerable insight into the nature of sodium ion binding. We found that when the binding site is a first row atom, the sodium ion aligns with the molecular dipole axis in order to maximize charge-dipole electrostatic interactions. Strong deviations from this behavior occur when the ion is attached to a heavier atom such as sulfur, chlorine or bromine. For flexible molecules such as the isomers of butyl chloride, there are several isomers of low energy, and differences exist between the enthalpy and free energy orders of stability. Finally, sodium ion affinities have been obtained for several aromatic molecules which lend support to the importance of charge-quadrupole interactions in such cation-pi complexes. 相似文献
78.
Stracener LL Halter RJ McMahon RJ Castro C Karney WL 《The Journal of organic chemistry》2000,65(1):199-204
Ab initio (MP2, CCSD(T)) and density functional theory (BLYP, B3LYP) calculations provide insight concerning novel aspects of structure and bonding in cyclobutylidene (1). Singlet cyclobutylidene ((1)1) adopts a bicyclobutane-like structure (C(s) symmetry) that includes a weak, transannular bonding interaction between the carbene carbon and the opposing CH(2) group. Conformational ring inversion in (1)1 occurs through a transition state of C(2)(v)() symmetry (TS(1)1) with an enthalpy barrier of approximately 3 kcal/mol. Stabilization afforded the singlet state by the transannular interaction appears to be largely offset by a loss of hyperconjugative stabilization from the adjacent C-H bonds. Triplet cyclobutylidene ((3)1) exhibits a C(2)(v)() structure and conventional bonding. The triplet state lies 5.9 kcal/mol above the singlet ground state at the CCSD(T)/TZP//CCSD(T)/DZP level of theory. The singlet-triplet energy gap of cyclobutylidene (-5.9 kcal/mol) lies between that of an acyclic analogue, dimethylcarbene (-1.6 kcal/mol), and a highly strained analogue, cyclopropylidene (-13.8 kcal/mol). The magnitude of the energy gap suggests that triplet cyclobutylidene ((3)1) will be thermally accessible under a variety of experimental conditions. 相似文献
79.
Herman Gershon Donald D. Clarke John J. McMahon Muriel Gershon 《Monatshefte für Chemie / Chemical Monthly》2001,132(7):833-838
Summary. 2,5-, 3,5-, 3,6-, 3,7-, 4,5-, 5,6-, 5,7-, and 6,7-dibromo-8-quinolinols were prepared and tested against six fungi (Aspergillus niger, Aspergillus oryzae, Myrothecium verrucaria, Trichoderma viride, Mucor cirinelloides, and Trichophyton mentagrophytes) in Sabouraud dextrose broth. Most of the compounds were strongly antifungal, inhibiting five of the fungi below 1μg/cm3, but were somewhat less fungitoxic than the corresponding chloro analogues. M. cirinelloides was not inhibited by any of the compounds except 4,5-dibromo-8-quinolinol. This high level of fungitoxicity is attributed
to intramolecular synergism.
Received December 28, 2000. Accepted (revised) February 3, 2001 相似文献
80.
I.R. Gabitov A.O. Korotkevich A.I. Maimistov J.B. McMahon 《Applied Physics A: Materials Science & Processing》2007,89(2):277-281
Light propagation in a Bragg periodic structure containing thin films with metallic nanoparticles is studied. The plasmonic
resonance frequency, Bragg frequency, and the light carrier frequency are assumed to be close. Exact solutions describing
solitary gap-waves are found, and a light arrest phenomenon due to the nonlinearity of plasmonic oscillations is studied.
PACS 42.55.Tv; 42.65.-k; 42.70.Qs; 81.07.Bc 相似文献