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Title:Amorphous nanocomposite of polycarbosilanes and aluminum oxide
Authors:ID Mavrič, Andraž (Copyright holder)
ID Mavrič, Andraž (Author)
ID Valant, Matjaž (Mentor) More about this mentor... New window
Files:.pdf Andraz_Mavric_thesis2018.pdf (5,07 MB)
MD5: D2FC1C200A3C0C09AD586688AD046D6F
 
Language:English
Work type:Doctoral dissertation
Typology:2.08 - Doctoral Dissertation
Organization:FPŠ - Graduate School
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.
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
Publication status:In print
Place of publishing:Nova Gorica
Year of publishing:2018
Number of pages:133
PID:20.500.12556/RUNG-3945-1420f80a-49df-90c0-55ec-a47e074507be New window
COBISS.SI-ID:5178107 New window
NUK URN:URN:SI:UNG:REP:YVHEPSKS
Publication date in RUNG:19.07.2018
Views:7345
Downloads:223
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Document is financed by a project

Funder:ARRS - Slovenian Research Agency
Project number:P2-0377

Secondary language

Language:Slovenian
Title:Amorfni nanokompozit polikarbosilanov z aluminijevim oksidom
Abstract:V doktorski disertaciji je predstavljen nov princip visokotemperaturne stabilizacije amorfnega aluminijevega oksida. Stabilizacijo dosežemo z disperzijo dendritičnih molekul polimetilsilana v gel aluminijevega hidroksida. Gel služi kot prekurzor, ki se pri termični obdelavi pretvori v amorfni aluminijev oksid, ki je stabilen do 900°C. Dispergirane makromolekule vnašajo v strukturo aluminijevega oksida napetosti, ki termodinamsko stabilizirajo amorfno fazo. V prvem delu je predstavljena sinteza, karakterizacija in topnost dendritičnega polimetilsilana. Polimerizacija je bila izvedena z elektrolizo trifunkcionalnega monomera, triklorometilsilana. Raziskan mehanizem polimerizacije predvideva le eno reakcijo na elektrodi, to je redukcija Si–Cl vezi na monomeru in nastanek silil aniona. Rast polimera poteka z adicijo aniona, na sosednji monomer, oligomer oziroma druge polimerne zvrsti. Za študij topnosti so bili uporabljeni trije kemijsko sorodni vendar različni polisilani: linearni polidimetilsilan, dendritični polimetilsilan in zamreženi polikarbosilan. Topnost in de-aglomeracijo smo zasledovali z dinamičnim sipanjem svetlobe in termično analizo. Pri sobni temperaturi so polimerne molekule aglomerirane. Velikost aglomeratov je v območju od 500 do 1300 nm. Pri temperaturah nad 40°C poteče de-aglomeracija. Nastanejo stabilne raztopine polimerov, v katerih je mogoče izmeriti velikost molekul. Hidrodinamski diameter posameznih molekul je v območju od 20 do 40 nm. Izmerjene velikosti dveh dendritičnih polimetilsilanov, sintetiziranih pod različnimi elektrolitskimi pogoji, so večje, kot lahko pričakujemo za strukturo idealnega dendrimera, ki bi se moral na vsakem monomeru vejati v tri smeri. Strukturna karakterizacija z 29Si NMR je pokazala, da je poleg atomov Si, ki se vejejo v tri smeri, prisoten tudi določen delež linearnih segmentov, ki predstavljajo defekte v vejanju. Rezultat prisotnosti teh defektov je rast polimera preko meje za idealen dendrimer. Dobljena struktura tako ni dendrimer ampak hiper-razvejani dendrit. Pokazali smo, da je dosežena velikost odvisna od deleža linearnih defektov. Večji kot je delež defektov, večji je lahko polimer. Posledično so polimeri z več defekti manj gosti. Sintetizirani polimetilsilan smo uspešno uporabili za pripravo nanokompozita. Kot prekurzor za nanokompozit smo uporabili s hidratacijo pripravljen gel aluminijevega hidroksida, ki je vseboval dispergirane molekule polimetilsilana. S termično obdelavo prekurzorja nastane amorfni aluminijev oksid, ki je stabilen do 900°C. Med termično obdelavo z aluminijevim oksidom se vzpostavitvijo kovalentne vezi. S tem homogeno preko celotnega nanokompozita ustvarjajo napetosti, ki ohranjajo matrico aluminijevega oksida amorfno. Amorfna struktura je bila potrjena z X-žarkovno in elektronsko difrakcijo, dodatno pa še z 27Al NMR, ki je pokazal prisotnostjo penta-koordiniranega aluminija in z UV-Vis refleksijsko spektroskopijo, ki je razkrila nizko vrednost prepovedanega pasu v primerjavi s kristaliničnimi polimorfi.
Keywords:amorfni aluminijev oksid, polimetilsilan, nanokompozit, elektropolimerizacija, topnost, aglomeracija, de-aglomeracija, dendrimer, hiper-razvejani dendrit, dinamično sipanje svetlobe, termična analiza, presevna elektronska mikroskopija, vrstična elektronska mikroskopija, X-žarkovna difrakcija, infrardeča spektroskopija, UV-Vis spektroskopija


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