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991.
992.
Nina Georgieva Kostova Elka Kraleva Alla A. Spojakina Erika Godocikova Peter Balaz 《Journal of Materials Science》2007,42(10):3321-3325
The Al-MCM-41 has been used as support to prepare Mo-containing catalysts. The 12- molybdophosphoric heteropoly acid (HPMo)
is used as initial compound. The catalysts are synthesized by two different methods: incipient impregnation with aqueous solution
of the acid and mechanochemical synthesis. The samples were tested in the reaction of the thiophene hydrodesulfurization after
activation with mixture H2 + H2S. The effect of the preparation method of the catalysts on their physicochemical and catalytic properties has been studied.
A partial destruction of the loaded compound is observed in mechanochemically treated sample whereas the aggregates are formed
from the particles of different size in the impregnated sample. The specific surface area of the sample prepared by mechanical–chemical
treatment decreases 2–3 times, while the total pore volume is about four times lower. The HDS activity is higher on the impregnated
sample than on the mechanochemically treated one. 相似文献
993.
Z. Ferhat-Hamida H. Phuong-Nguyen P. Bernazzani A. Haine G. Delmas 《Journal of Materials Science》2007,42(9):3138-3154
LLDPE samples synthesized with Ziegler–Natta (ZN) and Metallocene (MT) catalysts have been analyzed to investigate a potential
catalyst-dependent morphology and to find an explanation for the difficult processing of MT. Slow calorimetry at v = 0.02 K/min and IR at RT and in the melt are used. The differences between MT and ZN are assigned to their different composition,
MT not having the linear segments, which are present in ZN. Slow calorimetry is effectively a drawing process of the melt
with chain orientation followed by decay. The later event, characterized by an endotherm, ΔH
network, occurs at higher temperatures for MT, the presence of a regular distribution of methyl groups slowing down the process.
The rocking, gauche, bending and stretching regions of the IR spectra are analyzed. The nascent MT has more strained bands
in the rocking region. The wagging region reveals the more homogeneous environment of MT through the maximum absorbance at
1,368 cm−1. Decomposition of bands is made for the rocking and wagging regions. The orthorhombic crystallinity, αc (FTIR), measures the sum of long- and short-range orthorhombic order, the latter being obtained by αc (FTIR)-αc (X-rays). The values of αc (FTIR) for MT and ZN are very similar in conditions of equilibrium. The justifications for the molecular origin of ΔH
network are presented: (i) the slow relaxation of long chains strained and oriented in the melt measured by other techniques, (ii)
The correlation, for gels of a linear sample, made in different solvents, between the maximum drawability, λmax, and ΔH
network in a slow T-ramp. The range is 80–270 for λmax and 40–120 J/g for ΔH
network. (iii) The comparison of two traces of the same sample, between 140 °C and 270 °C, show that comparable events in the melt
appear in the integrated absorbance and in the slow calorimetry signal. Analysis on thin films of the little-studied CH2 stretching region reveals that their extinction coefficient, ε, and the shape of the bands are highly sensitive to the sample
history, ε diminishing by a large factor in slowly crystallized samples. Events in the slow T-ramp, followed by a fast crystallization, on the other hand, leads to materials with standard characteristics. Slow calorimetry
traces display more events (endothermic and exothermic) for MT than for ZN, a finding consistent with more flow irregularities
during processing. Equilibrium conditions and better processing could be reached for MT by extending time in the melt or using
higher temperatures. 相似文献
994.
Daniel X. Gouveia Odair P. Ferreira Antonio G. Souza Filho M. G. da Silva J. A. C. de Paiva Oswaldo L. Alves Josué Mendes Filho 《Journal of Materials Science》2007,42(2):534-538
The thermal decomposition properties of Mg–Fe hydrotalcites were studied through in situ 57Fe Mössbauer spectroscopy and in situ X-ray diffraction. Abrupt changes in the quadrupolar splitting measured in the Mössbauer spectra revealed a phase transition from the starting lamellar structure to a new crystalline arrangement. By analyzing the Mössbauer parameters we show that the material is highly disordered in the 300–400 °C temperature range. This hypothesis is confirmed by the X-ray results whose diffractograms indicated the collapse of the lamellar structure and the formation of a solid solution. 相似文献
995.
A consistent analytical model of nonlinear ion drift spectrometry for modern gas analyzers is developed. A procedure for determining the field dependence of the ion mobility using the experimental data is described. An ionogram is calculated for the case of a flat drift chamber and polynomial character of the field dependence of the ion mobility. 相似文献
996.
M. F. Laguna P. S. Cornaglia C. A. Balseiro 《Journal of Low Temperature Physics》2004,135(1-2):127-130
We model a vortex system in a sample with bulk pinning and superficial pinning generated by a magnetic decoration. We perform a sequence of finite temperature numerical experiments in which external forces are applied to obtain, a dynamically ordered vortex lattice. We analyze the final structures and the behavior of the total energy of the system. 相似文献
997.
Dmitri A. Parshin 《Journal of Low Temperature Physics》2004,137(3-4):233-250
Using a simple diagram technique we derive the electric-dipole echo amplitude from two-level systems with a quadrupole nuclear moment in glasses in an external magnetic field. We show, that due to the quadrupole moment interaction of a tunneling particle with a gradient of an internal electric field, the echo amplitude experiences oscillations in rather weak magnetic fields. With an increase of the magnetic field, when the Zeeman energy becomes larger than the quadrupole energy splitting, the average echo amplitude increases and saturates (for high magnetic fields) at some level which is above the average level of echo oscillations for small magnetic fields. PACS numbers: 77.22.Ch, 61.43.Fs, 76.60.Gv, 76.60.Lz 相似文献
998.
Neutron-irradiated quartz crystal is one of the interesting materials for the study of the glassy dynamics. The temperature and frequency dependence of the sound velocity in amorphous materials at temperatures above the low-temperature maximum is not well understood. In this paper we present the results of our measurements of the variation of the ultrasound velocity (270 – 650 MHz) in neutron-irradiated quartz crystals with different irradiation doses in temperature range 5 - 35 K. We have found that the temperature dependence of the ultrasound velocity is linear in the sample with a dose < 10 19 n/cm2 and is sublinear in samples with a dose > 10 19 n/cm2 . We discuss our results in the framework of the different theoretical approaches. PACS numbers: 61.43. j, 62.65.+k, 63. 相似文献
999.
In this paper we report the results of electrical resistivity (1.5≤ T ≤ 300K) and point contact spectroscopy (PCS) measurements on single crystals of metallic sodium tungsten bronze with varying sodium content. We have shown that the electron-phonon coupling function as measured through PCS can explain quantitatively the large temperature dependence of resistivity ρ seen in these materials over the entire temperature range. The electron-phonon coupling function shows predominately large peaks for phonon frequency range of 30 meV ≤ ω ≤ 100 meV which match well with the calculated optical phonons for WO6 octahedron. The integrated electron-phonon coupling constant λ from this data is ≈ 0.25-0.45, depending on the Na content. PACS numbers: 72.15.-v, 72.80.Ga 相似文献
1000.