Amorphous nanocomposite of polycarbosilanes and aluminum oxideAndraž Mavrič
, 2018, doctoral dissertation
Abstract: This work presents a paradigm for high temperature stabilization of bulk amorphous aluminium oxide. The thermodynamic stabilization is achieved by preparing a nanocomposite, where polymethylsilane dendritic molecules are dispersed in an aluminium hydroxide gel. Upon heat-treatment the gel transforms to the amorphous aluminium oxide that is stable up to 900°C. The dispersion of the macromolecules and their covalent bonding to the alumina matrix induce homogeneously distributed strain fields that keep the alumina amorphous.
The first part of the thesis focuses on the synthesis, characterization and solubility properties of the dendritic polymethylsilane. The polymethylsilane is synthetized by electrochemical polymerization from trichloromethylsilane monomer. The polymerization mechanism, involving a single polymerization pathway, is identified. The polymer growth proceeds through reduction of the monomers to the silyl anions and their addition to the growing polymer.
The solubility of three chemically related but topologically different polysilanes (linear, dendritic and network) were studied by dynamic light scattering. At room temperature the agglomerates in a range from 500 to 1300 nm are present. They undergo de-agglomeration at slightly elevated temperatures of around 40°C. The de-agglomeration results in formation of stable solutions, where a hydrodynamic diameter of the individual polymer molecules was measured to be in a range from 20 to 40 nm.
The obtained diameters of two dendritic polymethylsilane macromolecules, synthesized under different electrolysis conditions, are much larger than the theoretical size estimated for an ideal dendrimer. We determined by 29Si NMR that the reason for this is in a large number of branching irregularities (defects) contained in the molecular structure. Combining the experimental values obtained by DLS and density measurements with a structural model that considers the branching irregularities, it is shown that the inclusion of the defects allows the dendritic polymer to exceed the sterical limitations and form the hyperbranched dendritic structure. The final size depends on a relative amount of the branching defects.
In the second part, the synthetized polymethylsilane molecules were successfully used for the nanocomposite formation. The aluminium hydroxide gel with the dispersed polymethylsilane molecules was prepared as a precursor. Upon heat-treatment it gives the amorphous aluminium oxide stable up to 900°C. The dispersed macromolecules induce homogeneously distributed strain fields that keep the aluminium oxide amorphous during the thermal treatment the dispersed macromolecules covalently bind to the matrix, inducing the interface strain. The amorphous state was confirmed by the presence of penta-coordinated aluminium detected by 27Al NMR and a low bandgap measured by UV-vis absorption spectroscopy.
Found in: ključnih besedah
Summary of found: ...for high temperature stabilization of bulk amorphous aluminium oxide. The thermodynamic stabilization is achieved by...
Keywords: amorphous aluminium oxide, polymethylsilane, nanocomposite, electropolymerization, solubility, agglomeration, de-agglomeration, dendrimer, hyperbranched dendritic structure, dynamic light scattering, thermal analysis, transmission electron microscopy, scanning electron microscopy, X-ray diffraction, infrared spectroscopy, UV-Vis spectroscopy
Published: 19.07.2018; Views: 4940; Downloads: 204
Fulltext (5,07 MB)
High-temperature stabilization of bulk amorphous Al2O3Andraž Mavrič
, Matjaž Valant
, Gregor Mali
, Mattia Fanetti
, 2018, original scientific article
Abstract: We present a method for high temperature stabilization of bulk amorphous aluminium oxide. The stabilization is achieved by dispersing polysilane dendritic molecules in aluminium hydroxide gel, which upon thermal treatment gives amorphous aluminium oxide stable up to 900 °C. The dispersed macromolecules covalently bind to the alumina matrix and induce homogeneously distributed strain fields that keep the alumina amorphous. The thermal conversion of the precursor system was followed by thermogravimetry with an evolved gas analysis, infrared spectroscopy and 29Si NMR. The amorphous structure of aluminium oxide was confirmed with an X-ray and electron diffraction. Additionally, the amorphous state was supported by presence of penta-coordinated aluminium detected by 27Al NMR and a low bandgap measured by a UV–visible absorption spectroscopy.
Found in: ključnih besedah
Summary of found: ...for high temperature stabilization of bulk amorphous aluminium oxide. The stabilization is achieved by dispersing...
Keywords: Amorphous aluminium oxide, Polysilane, Nanocomposite, Stabilization
Published: 30.07.2018; Views: 3575; Downloads: 11
Fulltext (2,67 MB)
Infrared spectra in amorphous alumina: A combined ab initio and experimental studyLuigi Giacomazzi
, Nikita S. Shcheblanov
, Mikhail E. Povarnitsyn
, Yanbo Li
, Andraž Mavrič
, Barbara Zupančič
, Jože Grdadolnik
, Alfredo Pasquarello
, 2023, original scientific article
Abstract: We present a combined study based on the experimental measurements of an infrared (IR) dielectric function and first-principles calculations of IR spectra and the vibrational density of states (VDOS) of amorphous alumina (am−Al2O3). In particular, we show that the main features of the imaginary part of the dielectric function ε2(ω) at ∼380 and 630 cm−1 are related to the motions of threefold-coordinated oxygen atoms, which are the vast majority of oxygen atoms in am-Al2O3. Our analysis provides an alternative point of view with respect to an earlier suggested assignment of the vibrational modes, which relates them to the stretching and bending vibrational modes of AlOn (n=4, 5, and 6) polyhedra. Our assignment is based on the additive decomposition of the VDOS and ε2(ω) spectra, which shows that (i) the band at ∼380cm−1 features oxygen motions occurring in a direction normal to the plane defined by the three nearest-neighbor aluminum atoms, i.e., out-of-plane motions of oxygen atoms; (ii) Al-O stretching vibrations (i.e., in-plane motions of oxygen atoms) appear at frequencies above ∼500cm−1, which characterize the vibrational modes underlying the band at ∼630cm−1. Aluminum and fourfold-coordinated oxygen atoms contribute uniformly to the VDOS and ε2(ω) spectra in the frequency region ∼350–650 cm−1 without causing specific features. Our numerical results are in good agreement with the previous and presently obtained experimental data on the IR dielectric function of am−Al2O3 films. Finally, we show that the IR spectrum can be modeled successfully by assuming isotropic Born charges for aluminum atoms and fourfold-coordinated oxygen atoms, while requiring the use of three parameters, defined in a local reference frame, for the anisotropic Born charges of threefold-coordinated oxygen atoms.
Found in: ključnih besedah
Summary of found: ...Amorphous materials, Density functional calculations, Infrared techniques, aluminium oxide...
Keywords: Dielectric properties, Microstructure, Amorphous materials, Density functional calculations, Infrared techniques, aluminium oxide
Published: 10.05.2023; Views: 117; Downloads: 2
Fulltext (1,11 MB)
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