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171.
Rajagopalan R Uetrecht P Bugaj JE Achilefu SA Dorshow RB 《Photochemistry and photobiology》2000,71(3):347-350
Indocyanine green is a medically useful dye that absorbs and fluoresces in the near infrared and has been sporadically employed clinically as an optical tracer agent for liver function evaluation and cardiac output measurements. The poor stability of this dye in aqueous solution, especially at the high concentrations needed for bolus injection, has been a hindrance in clinical application. However, by using carefully chosen macromolecular additives, the stability of these aqueous dye solutions may be enhanced significantly. Such noncovalent binding between dye and carrier molecules was found to preserve substantially the dye in aqueous solutions for several weeks with no apparent changes in the measured in vivo biological properties. 相似文献
172.
We have used ion-mobility mass spectrometry to examine the conformations of the protonated complex formed between AcA(7)KA(6)KK and AcEA(7)EA(7), helical alanine-based peptides that incorporate glutamic acid (E) and lysine (K). Designed interactions between the acidic E and basic K residues help to stabilize the complex, which is generated by electrospray and studied in the gas phase. There are two main conformations: (1) a coaxial linear arrangement where the helices are tethered together by an EKK interaction between the pair of lysines at the C-terminus of the AcA(7)KA(6)KK peptide and a glutamic acid at the N-terminus of the AcEA(7)EA(7) peptide and (2) a coiled-coil arrangement with side-by-side antiparallel helices where there is an additional EK interaction between the E and K residues in the middle of the helices. The coiled-coil opens up to the coaxial linear structure as the temperature is raised. Entropy and enthalpy changes for the opening of the coiled-coil were derived from the measurements. The enthalpy change indicates that the interaction between the E and K residues in the middle of the helices is a weak neutral hydrogen bond. The EKK interaction is significantly stronger. 相似文献
173.
ESR study on the stability of W/O gel-emulsions 总被引:1,自引:0,他引:1
W/O gel-emulsions (high-internal-phase-volume-ratio emulsions) form in water (or brine) /tetraethyleneglycol dodecyl ether/heptane system above the HLB (hydrophile-lipophile balance) temperature of the system. A salt, which largely decreases cloud temperature in a water-nonionic surfactant system, makes the surfactant film rigid and the gel-emulsions hence become very stable. The effect of aded salt on the apparent order parameter S, and the isotropic hyperfine splitting constant a
N in gel-emulsions was determined by the ESR spin probe method using 5-doxyl stearic acid as the spin probe. The apparent order parameter S, and the isotropic hyperfine splitting constant a
N increase with increasing salinity in Na2SO4, CaCl2, and NaCl systems. It is considered that the surfactant molecules are tightly packed in these systems and this tendency is highly related to the stability of gel-emulsions. The salt dehydrates the hydrophilic moiety of surfactant and hence the lateral interactions of surfactant molecular layer at the water-oil interface increases.The observed difference in the apparent order parameter between the ordinary emulsions and the gel-emulsions suggests that most of the surfactant molecules are adsorbed at the oil-water interface (the surface of the water droplet) in gel-emulsions. 相似文献
174.
Astashkin AV Johnson-Winters K Klein EL Feng C Wilson HL Rajagopalan KV Raitsimring AM Enemark JH 《Journal of the American Chemical Society》2008,130(26):8471-8480
Electron paramagnetic resonance (EPR) investigation of the Mo(V) center of the pathogenic R160Q mutant of human sulfite oxidase (hSO) confirms the presence of three distinct species whose relative abundances depend upon pH. Species 1 is exclusively present at pH < or = 6, and remains in significant amounts even at pH 8. Variable-frequency electron spin echo envelope modulation (ESEEM) studies of this species prepared with (33)S-labeled sulfite clearly show the presence of coordinated sulfate, as has previously been found for the "blocked" form of Arabidopsis thaliana at low pH (Astashkin, A. V.; Johnson-Winters, K.; Klein, E. L.; Byrne, R. S.; Hille, R.; Raitsimring, A. M.; Enemark, J. H. J. Am. Chem. Soc. 2007, 129, 14800). The ESEEM spectra of Species 1 prepared in (17)O-enriched water show both strongly and weakly magnetically coupled (17)O atoms that can be assigned to an equatorial sulfate ligand and the axial oxo ligand, respectively. The nuclear quadrupole interaction (nqi) of the axial oxo ligand is substantially stronger than those found for other oxo-Mo(V) centers studied previously. Additionally, pulsed electron-nuclear double resonance (ENDOR) measurements reveal a nearby weakly coupled exchangeable proton. The structure for Species 1 proposed from the pulsed EPR results using isotopic labeling is a six-coordinate Mo(V) center with an equatorial sulfate ligand that is hydrogen bonded to an exchangeable proton. Six-coordination is supported by the (17)O nqi parameters for the axial oxo group of the model compound, (dttd)Mo(17)O((17)Otms), where H2dttd = 2,3:8,9-dibenzo-1,4,7,10-tetrathiadecane; tms = trimethylsilyl. Reduction of R160Q to Mo(V) with Ti(III) gives primarily Species 2, another low pH form, whereas reduction with sulfite at higher pH values gives a mixture of Species 1 and 2, as well as the "primary" high pH form of wild-type SO. The occurrence of significant amounts of the "sulfate-blocked" form of R160Q (Species 1) at physiological pH suggests that this species may be a contributing factor to the lethality of this mutation. 相似文献
175.
The separated shear layer in the near wake of a circular cylinder was investigated using a single hot wire probe, with special attention given to the shear layer instability characteristics. Without end plates to force parallel vortex shedding, the critical Reynolds number for the onset of the instability was 740. The present data, together with all previously published data, show that the ratio of the instability frequency fsl to the vortex shedding frequency fv varies as Re0.65, which is in agreement with the Re0.67 dependence obtained by Prasad and Williamson [1997, J Fluid Mech 333:375–402]. However, the distribution of fsl/fv and the spectra of the longitudinal velocity fluctuation (u) suggest that, on either side of Re=5,000, the shear layer exhibits lower and upper subcritical regimes, in support of the observations by Norberg [1987, publication no. 87/2, Chalmers University of Technology, Sweden] and Prasad and Williamson [1997, J Fluid Mech 343:235–265]. The spectra of u provide strong evidence for the occurrence of vortex pairing in wake shear layers, suggesting that the near wake develops in a similar manner to a mixing layer. 相似文献
176.
Summary The paper deals with the study of the nature of secondary flow of aRivlin-Ericksen fluid, contained between two concentric spheres, which perform oscillations about a fixed diameter. The steady part of the secondary flow is discussed in detail in the following three cases (i) the outer sphere at rest, the inner oscillating, (ii) the two spheres oscillating with the same angular velocity in the same sense and (iii) the spheres oscillating with the same angular velocity in opposite sense. In a previous paper, a similar problem was discussed for theOldroyd fluids. We find that the secondary flow is strongly dependent on the common frequency of oscillation of the two spheres and on the rotational nature of the motion for the present investigation also. Certain contrasting features of interest between the secondary flow field of the two fluids are also noted. 相似文献
177.
Padmavathy Rajagopalan Andreas T. Tsatsas William M. Risen 《Journal of Polymer Science.Polymer Physics》1996,34(1):151-161
Fully exchanged, anhydrous ionomers of ethylene-co-acrylic acid (EAA) copolymers and ethylene-co-methacrylic acid (EMAA) copolymers containing Dy+3, Er+3, Sm+3, Tb+3, Tm+3, and Yb+3, and mixtures of them, were synthesized and studied in the near infrared region by reflection and Fourier Transform Laser Raman spectroscopies. The EAA copolymers ranged from 1.4 to 8.7 mol % acid and the EMAA copolymers were 7.3 and 16.2 mol % acid. The ionomers were shown to be essentially free of carboxylic acid groups, water, or other forms containing O (SINGLE BOND) H groups and were characterized by infrared and other methods. They are light and heat stable, and become thermoplastic and moldable at ca. 220°C under pressure. When excited at 1.064 μ with a Nd: YAG laser, these ionomers exhibit novel, lanthanide-dependent near infrared luminescence and strong Raman scattering in the near infrared region. The strongest luminescence is observed with Sm+3 ionomers. The Dy+3 ionomer Raman-shifts this source to emit light most strongly in the 1.53–1.55 μ range where the ionomer also transmits light well. © 1996 John Wiley & Sons, Inc. 相似文献
178.
S. Kumar K. Subramanian R. Srinivasan K. Rajagopalan A. M. M. Schreurs J. Kroon T. Steiner 《Journal of Molecular Structure》2000,520(1-3):131-139
The crystal structure of N-(p-methoxyphenyl)-N-prop-2-ynyl-urea contains five symmetry-independent molecules (Z′=5), which is a high and rarely occurring value. The five molecules have similar but not identical conformations, and form similar but not identical intermolecular interactions. The terminal alkyne groups of the five molecules are oriented differently, and form different hydrogen bond interactions (C–HO and C–Hπ). This is an example of how simple molecules can form a highly complicated solid state structure. 相似文献
179.
R. Ehrenborg D. Johnston R. Rajagopalan M. Readdy 《Discrete and Computational Geometry》2000,23(2):261-271
We show how the flag f -vector of a polytope changes when cutting off any face, generalizing work of Lee for simple polytopes. The result is in
terms of explicit linear operators on cd-polynomials. Also, we obtain the change in the flag f -vector when contracting any face of the polytope.
Received July 13, 1998, and in revised form April 14, 1999. 相似文献
180.
Dilip Rajagopalan 《Rheologica Acta》2000,39(2):138-151
Recent computational analysis of entrance flows (Mitsoulis et al. 1998) suggests that the entrance pressure drop is insensitive
to large changes in steady extensional viscosity-a result that directly contradicts a large body of experimental work in this
area. A re-examination of entrance flows using numerical simulations is presented in this work which shows that entrance pressure
drops do depend on the steady extensional viscosity, provided the extension rate in the entrance flow is large enough. Numerical
simulations are presented using both the strain thinning and thickening versions of the Phan-Thien–Tanner (PTT) constitutive
model. Several techniques for extracting extensional viscosity from entrance pressure are applied to the results of these
simulations. The resulting predictions of extensional viscosity are compared to the steady extensional viscosity curves predicted
by the PTT constitutive model used to generate the simulated pressure drop curves. The analytical techniques examined here
are shown to provide reasonably accurate estimates of the steady extensional viscosity. This work also clearly demonstrates
the advantage of using variable power-law coefficients for the rheological properties, used as inputs to the analyses, to
capture the extensional behavior at deformation rates below the power law region more accurately.
Received: 23 July 1999/Accepted: 24 November 1999 相似文献