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101.
Filinchuk YE Birkedal H Cerný R Hostettler M Yanson TI Bodak OI Yvon K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(12):2972-2976
An X-ray study of single crystals extracted from an arc-melted Yb-Fe-Ga alloy showed that the diffraction pattern can be modeled by an intergrown crystal that has three sorts of domains: one hexagonal (1, LuFe(9.5) type) and two rhombohedral (2 a and 2 b, PrFe(7) type), the last two twinned by reticular merohedry. Crystals 1 and 2 are essentially polytypes with maximum degree of order (MDO polytypes), built up of nearly identical slabs that are stacked along [001] in ABAB em leader (1). and ABCABC em leader (2). sequences. Structure refinement was performed by a newly developed program that allowed us to refine several structures on a single data set. We found that the hexagonal and rhombohedral domains differ in chemical composition: while 1 shows a higher rate of Yb substitution by Fe(2) dumbbells, 2 shows partial substitution of Fe by Ga. Our observation of the nanoscale phase segregation is supported by latest finding of nonrandom distribution of stacking faults in a similar 2:17 alloy. An unequal distribution of chemical substitutions in 1 and 2 apparently compensates the inherent mismatch of basal plane dimensions of the individual MDO polytypes and thus constrains their cell parameters within the syntaxy. According to our knowledge this is the first example of two chemically distinct polytypes constituting a single crystal, refined on a single set of diffraction data. 相似文献
102.
Sans résuméAvec 4 figures et 1 table 相似文献
103.
K. Yvon 《Solid State Communications》1978,25(5):327-331
X-Ray diffraction measurements on single crystals of rhombohedral MxMo6S8 compounds (M = Cu, Ag, In, Sn, Gd, La), yield evidence for a systematic delocalisation of the cation M from the inversion centre. The delocalisation is proportional to the rhombohedral angle, but is not a simple function of the ionic radii as tabulated for M. It is suggested that this effect is temperature dependent and responsible for the occurence of lattice instabilities in the Chevrel phases. A possible correlation between delocalisation and superconductivity is discussed. 相似文献
104.
105.
Interactions between the ionophore nigericin AH and alkali metal cations M+ were studied using both potentiometry and calorimetry. Two types of complexes are formed: AHM+ and AM. Formation constants as well as ΔG, ΔH, and ΔS of formation are reported. These results are discussed in terms of the ‘open’ and ‘closed’ structure of various forms of nigericin (AH, A?, AM, AHM+). 相似文献
106.
Crozet Yvon Wen James J. Loo Rachel O. Andrews Philip C. Spatola Arno F. 《Molecular diversity》1997,3(4):261-276
We describe the first examples of a series of cyclic pseudopeptide libraries that have been prepared in a systematic approach in order to facilitate both synthesis and subsequent deconvolution attempts. Our synthetic strategy involved the attachment of a trifunctional amino acid (Asp, Asn or Glu) to a polystyrene resin via its side chain, and stepwise chain elongation using either protected amino acids or a pseudodipeptide building block. Head to tail cyclic peptides were formed by removal of the temporary N- and C-terminal protecting groups followed by ring closure by amide formation. Cyclization of the hexa, hepta, and octapseudopeptides on the resin avoided dimer formation, as monitored by mass spectrometry. We utilized a psi-scan approach in which a second fixed position was serially addressed by stepping a dipeptide surrogate, Pro[CH2S]Gly around the rings to generate a group of cyclic pseudopeptide sub-libraries. Oxidation of [CH2S] to [CH2SO] helped validate the synthesis and also provides a strategy for forming a new set of pseudopeptide libraries (previously described as libraries from libraries). Our results suggest that libraries of cyclic pseudopeptides are an efficient method of preparing and assaying these synthetically more challenging entities as potential drug leads. 相似文献
107.
Summary In order to approximate the Navier-Stokes equations with periodic boundary conditions in two directions and a no-slip boundary condition in the third direction by spectral methods, we justify by theoretical arguments an appropriate choice of discrete spaces for the velocity and the pressure. The compatibility between these two spaces is checked via an infsup condition. We analyze a spectral and a collocation pseudo-spectral method for the Stokes problem and a collocation pseudo-spectral method for the Navier-Stokes equations. We derive error bounds of spectral type, i.e. which behave likeM
– whereM depends on the number of degrees of freedom of the method and represents the regularity of the data. 相似文献
108.
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110.
Nguetnkam JP Kamga R Villiéras F Ekodeck GE Razafitianamaharavo A Yvon J 《Journal of colloid and interface science》2005,289(1):104-115
Two clays of the areas of Kaélé and Kousseri (extreme North Cameroon) containing mainly smectites and minor amounts of kaolinite were activated with sulfuric acid (1 to 8 N). Crystal-chemical properties were studied using X-ray diffraction, Fourier transform infrared spectroscopy, and chemical analysis, while textural properties were analyzed by step-by-step nitrogen adsorption at 77 K and low-pressure quasi-equilibrium argon adsorption at 77 K. As is generally observed, smectite is more sensitive to acid leaching than kaolinite. As a result of smectite decomposition, amorphous Al-containing silica forms, leading to an increase in the specific surface area of the leached materials. The content of the clay minerals and amorphous silica can be estimated on the basis of changes in the chemical composition of the samples upon acid leaching. As far as adsorption energy distributions derived from low-pressure argon derivative adsorption isotherms are concerned, the main modifications occur when 1 N sulfuric acid is used, due to the replacement of calcium and sodium compensating cations by protons. When higher acid concentrations are used, variations in adsorption energy distribution can be assigned to the presence of amorphous silica. It was possible to model experimental adsorption energy distributions as weighted sums of argon adsorption energy distributions obtained on (i) 1 N samples representing protonated clays and (ii) a silica gel used as a reference aluminous silica. Using such an approach, increasing acid concentration results in an increase in the surface area of silica, whereas the surface area of the remaining clay minerals remains roughly constant. 相似文献