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1.
Aerogels are unique materials offering a combination of remarkable properties that make them useful in a wide range of applications. However, aerogel materials can be difficult to work with because they are fragile. The intent of the work presented here was to study the relationship between axial loading and pore structure in aerogel material. Silica aerogel samples with a bulk density of 0.1 g/mL were compressed by uni-axial force loads from 1 to 5 kN which resulted in stress levels up to 23 MPa. The resulting change in the pore distribution was observed using nitrogen desorption analysis and scanning electron microscopy. Uncompressed aerogel samples exhibit peak pore volume at diameters of about 20 nm. As the aerogels are subjected to increased loading, the location of the peak volume moves to smaller diameters with a reduced volume of pores occurring above this diameter. The peak diameter, the average pore diameter and pore volume all decrease and scale with increasing maximum stress while the surface area of the aerogel samples remains unaffected at about 520 m 2/g. When combined with data from the literature, the relation between maximum pore diameter and applied stress suggests a failure mechanism dominated by bending induced fracture. 相似文献
2.
The effect of physical aging on a silicate glass has been investigated by low-frequency Raman scattering. It was observed that the low-frequency side of the excess of Raman scattering, or boson peak, due to harmonic vibration modes decreases in intensity with the thermal annealing (aging) at a temperature lower than the glass transition temperature ( Tg), after quenching (rejuvenation) from a temperature higher than Tg. Moreover, it was found that the lowering of the very low-frequency scattering mainly due to anharmonic modes becomes more pronounced when the aging temperature is decreased. These observations are interpreted in the frame of the energy landscape, and by considering the model of the glass heterogeneous cohesion at the nanometric scale. 相似文献
3.
X-ray diffraction intensities were measured from antiferromagnetic NiO. The data were submitted to a nonparametric multipole analysis aimed at formulation of experimentally valid statements on the nature of charge distribution. Strong “bonding maxima” appear between the Ni〈100〉O nearest neighbors in the region of the oxygen atoms involving NiO coupling. An electronic cage structure is also observed. This is parallel to the observations on MnO and CoO but different in nature, being formed by separate cubic cages for the cations instead of the oxygen, and without a clear indication of an extended network buildup. 相似文献
4.
X-ray diffraction intensities from CoO were measured above and below the Néel temperature. The data were submitted to a non-parametric multipole analysis aiming at formulation of experimentally valid statements on the nature of the charge distribution. Strong “bonding maxima” are seen between the Co 〈110〉O nearest neighbors. In the paramagnetic state they are formed by the Co-ion alone, in the antiferromagnetic state they involve a strong CoO coupling. The outer electrons of oxygen give rise to slight maxima in the 〈110〉 directions, making the atom into an O 2? ion. In the paramagnetic state, their coupling with the bonding feature forms an octahedral electron cage around each oxygen atom. Broad density maxima between the atoms in 〈111〉 directions are present in both states. They connect the “bonding maxima” in the 〈110〉 directions, more strongly in the antiferromagnetic state, building up an interatomic three-dimensional network. Such electron network structures seem to be characteristic of metal oxides more generally. 相似文献
5.
It is shown that stacking disorder in layered semiconductors leads to a localization of the electronic wavefunctions over a finite number of layers. The electronic transport across the layers is described in terms of a hopping model. The experimentally observed strong anisotropy of the de-conductivity for low temperatures is explained as a consequence of the stacking disorder. 相似文献
6.
X-ray diffraction intensities from MnO and CoO were measured above and below their Néel temperatures and from NiO, below the Néel temperature. To detect possible characteristics of the paramagnetic and antiferromagnetic states of the crystals, the data were subjected to direct multipole analysis of the atomic-charge densities. For MnO, both spherical and nonspherical accumulation-of-charge densities indicate the exchange of the roles played by manganese and oxygen in the magnetic phase transition. Both spherical and nonspherical features characteristic of the ionic nature are inherent in both states. The electron counts of the density peaks correspond to Mn 2+ and O 1?, with the tenth electron of O2? being distributed in a wider region. In the paramagnetic state, there is an electronic Mn-Mn bond which seems to be formed due to coupling with the tenth electron of O 2? and builds up a three-dimensional net of the charge density with the “cages” surrounding oxygen atoms. In the antiferromagnetic state, some Mn-Mn bonds disappear, while the preserved nonspherical ionic features enhance the role of oxygen atoms in electronic coupling. 相似文献
8.
The possible resonance excitation of surface electromagnetic waves by the bulk waves at the interface between a positive transparent uniaxial crystal and an isotropic medium has been predicted. The existence of the surface waves is provided by anisotropy of one of the boundary media. The tensor relation between the vector amplitudes of an exciting bulk electromagnetic wave and the excited surface wave is established. The ratio of the moduli of the tangential components of these amplitudes is analyzed as a function of the angle of incidence and the polarization of the bulk wave. The numerical calculations are performed for the surface waves at the interface between paratellurite TeO 2 and a KRS-6 (TlBr 30%-TlCl 70%) crystal. 相似文献
10.
The formation of the nanocrystalline state under the action of a concentrated load applied to crystals is studied. It is established that the relaxation of the enormous stresses (on the order of the shear modulus) arising under the indenter proceeds through twinning, which results in the fragmentation of the structure into nanoblocks. The properties of the nanocrystalline material thus obtained are explained based on the consideration of grain boundaries as twin boundaries that represent another phase of the material. 相似文献
11.
The reflectometry curves of an Nb(4 nm)/YFe 2(40 nm, 〈110〉)/Fe(1.5 nm)/Nb(50 nm) sample measured in the synchrotron radiation of right- and left-handed circular polarization
for a set of wavelengths near the Y L
2,3 absorption edges have been used to determine the energy dependences of the component magnitudes for the X-ray susceptibility
tensor of YFe 2 near the yttrium absorption edges, including off-diagonal magnetic additives. Our result is in good agreement with the normalized
experimental absorption and circular dichroism spectra and their Kramers-Kronig transformations. 相似文献
12.
The factors responsible for the formation of different configurations of boundaries between adjacent crystallites during their growth from melt by Bridgman and Czochralski methods have been considered by an of example Fe–20 wt % Ga alloy and Ni bicrystals. It is found that the configuration of intercrystallite boundary is related to the features of crystallite growth, caused by the strained state of intercrystallite and interphase (crystal–melt) boundaries, the difference in the linear thermal expansion coefficients of the crystallite boundaries and bulk, and the shape (geometry) of the bicrystal cross section. It is suggested that the strained state of boundaries and the formation of substructure in crystallites during directional crystallization from metal melt are significantly affected by their deformation under the melt weight. 相似文献
13.
The state of manganese impurity in implanted silicon at implantation doses of up to 5 × 10 16 cm ?2 has been investigated by X-ray diffraction and transmission electron microscopy. It is established that, after short-term vacuum annealing at 850°C, most of the implanted manganese impurities are in microinclusions up to 20 nm in size formed by a tetragonal silicide phase of the Mn 15Si 26 type. 相似文献
14.
The structure, dc and ac magnetic properties of sonochemically prepared nano-particles of La 0.9Ca 0.1MnO 3 compound were studied as compared to those of the bulk sample of the same composition. It is shown that transition to the nanometer sized scale results in a change of the magnetic order from a mixed antiferro + ferromagnetic to a pure ferromagnetic one due to the improvement of crystallinity and suppression of chemical/magnetic disorder in nano-particles. The surface contribution to its magnetism leads to a super-paramagnetic-like ordering supposedly due to the interfacial exchange coupling via surface tunneling of carriers above the Curie point ( TC). Below TC ∼ 90 K, a notable contribution of surface spin disorder reduces the spontaneous magnetization of the nano-sample as compared to the bulk-one. 相似文献
15.
A kinematical approximation has been derived from the general formulas for calculating the reflectivity from a multilayer anisotropic structure. An explicit form of reflection matrices at the boundary of layers and the refraction tensors has been obtained for a particular case where the layer magnetization is oriented in the surface plane and for an arbitrary form of X-ray susceptibility tensors. Calculations of the reflectivity from a Nb(4 nm)/[Dy(5 nm)/Lu(3 nm)] 420 model structure with helicoidal ordering of Dy magnetic layers show that kinematical approximation can be applied for grazing angles that are larger than the critical total-reflection angle and that the “magnetic” refraction must be taken into account. 相似文献
16.
Crystallography Reports - The specific features of magnetron sputtering deposition of thin SmS films for thermoelectric generators that do not require any forced formation of temperature gradient... 相似文献
17.
A computer simulation and measurements of the light transmittance of germanium and paratellurite crystals of different thickness were used to show that, at scattering probabilities of photons comparable to their absorption probabilities, the standard methods for calculating light extinction coefficients on the basis of the Bouguer law lead to rough errors in estimation of the optical quality of a material. 相似文献
18.
The characteristic A 1 peak at 199 cm −1 in the Raman spectrum of amorphous GeSe 2 were compared with the peaks at 211 and 216 cm −1 in the spectrum of crystalline GeSe 2. It was proved that the crystalline 216 cm −1 peak is an intrinsic mode which is enhanced by the bulk exciton transition. From a model calculation using a valence force field and bond polarizability, the 211 cm −1 peak was assigned to in-phase breathing vibrations extended along the GeSe 4 tetrahedral chain structure, while the 216 cm −1 peak was attributed to in-phase breathing vibrations quasi-localized at the GeSe 4 edge-sharing tetrahedra. The phonon density of states in the crystal has a doublet peak similar to the amorphous Raman spectrum. A correspondence between the amorphous and the crystalline Raman spectra was proposed. 相似文献
20.
The preparation of pure crystalline cholsteryl oleyl carbonate is described. The melting point, the mesophase transition temperatures, and the enthalpy of the melting process are recorded, and a method of maintaining cholesteryl oleyl carbonate in a pure state over extended periods of time is suggested. 相似文献
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