In the thesis work an experimental study of different materials with a great applicative interest has been developed. In particular working on: 1) ancient and modern blue and white dyes, 2) pure and doped Zinc ferrites (ZnFe2O4), the results obtained from local investigations performed by the Electron Paramagnetic Resonance spectroscopy, have been correlated to structural elements investigated by micro-Raman spectroscopy. 1) An essential task for preservation and restauration of coloured pieces of artworks is the deep knowledge about the used dyes and the materials used as the binding medium on the massive substrate. With regard to the dyes, it is crucial to assess the defects ensamble in terms of intrinsic or extrinsic defects and impurities; from them, together with the bulk optical properties of the matrix, the colour rendering and its temporal evolution depends, which is an essential element in the art works. It is also important to characterize the crystalline structure of the matrix hosting the active centers, this because all the physical phenomena related to the optical properties of a material such as for example luminescence, trapping and transport charge mechanisms, are affected by the environment of the active center. EPR measurements both at room and low (170K) temperature have been performed on: i) historical and synthesized blue pigments coming from collections of the Egyptian Museum in Turin and ii) a set of synthesized ZnO-based white pigments. On the same sets of samples, micro-Raman characterizations have been performed obtaining a mapping of the host phases. Based also on comparison to calculated spectra, EPR data allowed to identify specific paramagnetic centers and correlate them with the observed fluorescence lifetime obtained on the same samples by the group leaded by Prof. Comelli at POLIMI. 2)The chance to synthesized spinel ferrite nanoparticles, MeFe2O4 (Me = Ni2+, Co2+, Zn2+, Mg2+, etc.), has opened new application fronts, for example biotechnology, to this class of materials which have been mostly used in electronics, till now. Typically, the properties of the ferrites depend on the composition, the cation distribution, the synthesis methods and on the nanoparticles dimensions. Among all ferrites, zinc ferrite exhibits unusual behavior in nanoregime. Bulk ZnFe2O4 has a normal spinel structure, in which all Zn2+ ions occupy tetrahedral positions (A-sites) and all Fe3+ ions octahedral positions (B-sites). At room temperature, zinc ferrite is paramagnetic, and the transition to a magnetically ordered state occurs only at temperatures of about 10 K. The magnetic properties change significantly when the particle size decreases to a few tens of nanometers: at this scale, a partial cation inversion occurs. Thus, there is a transformation of structure from normal to mixed spinel, which can lead to a superparamagnetic behaviour at room temperature. A part of the thesis work has been thus dedicated to the analyses of EPR and micro-Raman data collected from a series of pure and substitued Zinc ferrites obtained from classical solid state procedure and from solution after microwave or coprecipitation methods. The samples have been prepared at the Dipartimento di Chimica of the University of Pavia. The temperature beahviour of Fe3+ EPR signals from different samples has been correlated to the structural information derived from micro-Raman experiments. The results seems to validate an intrinsic nature of the observed superparamagnetic behavior excluding the presence of nanoclusters of maghemite as extrinsic phase.
Nel lavoro di tesi è stato sviluppato uno studio sperimentale di alcuni materiali ad elevato interesse applicativo. In particolare lavorando su: 1) pigmenti blu e bianchi sia antichi che moderni, 2) ferriti di zinco (ZnFe2O4) pure e drogate, i risultati ottenuti da indagini locali eseguite tramite la spettroscopia di Risonanza Paramagnetica Elettronica (EPR) sono stati correlati ad elementi strutturali investigati tramite la spettroscopia micro-Raman. 1) Un aspetto essenziale nella conservazione e nel restauro di opere d'arte, è la dettagliata conoscenza dei coloranti utilizzati e leganti impiegati nella stesura dei pigmenti stessi sui supporti massivi. Per quanto riguarda i pigmenti, è cruciale caratterizzare la popolazione di difetti, intrinseci ed estrinseci, e le impurezze; da essi, unitamente alle proprietà ottiche di bulk della matrice, viene a dipendere la risposta cromatica e la sua evoluzione temporale, elemento fondamentale nelle opere d'arte. È inoltre importante caratterizzare la struttura cristallina della matrice che ospita i centri attivi, questo perché tutti i fenomeni fisici correlati alle proprietà ottiche di un materiale come ad esempio la fotoluminescenza, meccanismi di trasporto di carica e trapping, sono condizionati dall'ambiente che circonda il centro attivo. Misure EPR sia a temperatura ambiente sia a bassa temperatura (170K) sono state eseguite su i) pigmenti blu storici e sintetizzati provenienti dalle collezioni del Museo Egizio di Torino e ii) un set di pigmenti bianchi sintetizzati a base di ZnO. Sugli stessi campioni sono state condotte analisi micro-Raman che hanno fornito una mappatura delle fasi ospiti. Le analisi EPR hanno permesso di identificare specifici centri paramagnetici e di correlarne la presenza con i risultati da analisi di fotoluminescenza condotte sugli stessi campioni dal gruppo di ricerca della Prof.Comelli al POLIMI; la determinazione dei centri EPR presenti è stato possibile anche grazie alla simulazione di alcuni segnali. 2) La possibilità di realizzare nanoparticelle di ferriti a struttura a spinelli, MeFe2O4 (Me = Ni2+, Co2+, Zn2+, Mg2+, etc.), ha aperto nuovi fronti applicativi, ad es. la biotecnologia, a questa classe di materiali molto impiegati fin qui soprattutto nell'elettronica. Tipicamente le proprietà funzionali delle ferriti dipendono dalla composizione, dall'ordine cationico, dalle condizioni di sintesi e dalle dimensioni delle nanoparticelle. Tra tutte le ferriti, la ferrite di zinco esibisce un comportamento insolito quando si passa alla scala nanometrica. In forma massiva ZnFe2O4 ha una struttura cosiddetta a spinello normale in cui tutti gli ioni Zn2+ occupano i siti a coordinazione tetraedrica (siti A) e tutti gli ioni Fe3+ i siti a coordinazione ottaedrica (siti B). Alla temperatura ambiente, la ferrite di zinco è paramagnetica e la transizione ad un ordine magnetico avviene solo a temperature dell'ordine di 10K. Le proprietà magnetiche cambiano significativamente quando le dimensioni delle particelle raggiungono valori dell'ordine di una decina di nanometri: a queste dimensioni si verifica una parziale inversione cationica. Conseguentemente, si assiste ad una trasformazione strutturale, da spinello normale a spinello misto, che può portare ad un comportamento superparamagnetico a temperatura ambiente. I campioni studiati, sintetizzati presso il Dipartimenti di Chimica Fisica dell'Università di Pavia, sono ferriti di zinco pure e drogate sintetizzate con diversi metodi (sintesi a stato solido e sol gel). L'andamento in temperatura del segnale EPR dello ione Fe3+ nei campioni è stato correlato a informazioni strutturali ottenute da esperimenti condotti con la spettroscopia micro-Raman. I risultati sembrano avvalorare l'ipotesi di una natura intrinseca del comportamento superparamagnetico osservato nei diversi campioni di ferrite di zinco, escludendo la presenza di un'eventuale fase maghemite nei campioni stessi.
Correlazione tra centri paramagnetici e strutture di fase in diversi materiali applicativi: coloranti artistici e ferriti.
ALBINI, BENEDETTA
2015/2016
Abstract
In the thesis work an experimental study of different materials with a great applicative interest has been developed. In particular working on: 1) ancient and modern blue and white dyes, 2) pure and doped Zinc ferrites (ZnFe2O4), the results obtained from local investigations performed by the Electron Paramagnetic Resonance spectroscopy, have been correlated to structural elements investigated by micro-Raman spectroscopy. 1) An essential task for preservation and restauration of coloured pieces of artworks is the deep knowledge about the used dyes and the materials used as the binding medium on the massive substrate. With regard to the dyes, it is crucial to assess the defects ensamble in terms of intrinsic or extrinsic defects and impurities; from them, together with the bulk optical properties of the matrix, the colour rendering and its temporal evolution depends, which is an essential element in the art works. It is also important to characterize the crystalline structure of the matrix hosting the active centers, this because all the physical phenomena related to the optical properties of a material such as for example luminescence, trapping and transport charge mechanisms, are affected by the environment of the active center. EPR measurements both at room and low (170K) temperature have been performed on: i) historical and synthesized blue pigments coming from collections of the Egyptian Museum in Turin and ii) a set of synthesized ZnO-based white pigments. On the same sets of samples, micro-Raman characterizations have been performed obtaining a mapping of the host phases. Based also on comparison to calculated spectra, EPR data allowed to identify specific paramagnetic centers and correlate them with the observed fluorescence lifetime obtained on the same samples by the group leaded by Prof. Comelli at POLIMI. 2)The chance to synthesized spinel ferrite nanoparticles, MeFe2O4 (Me = Ni2+, Co2+, Zn2+, Mg2+, etc.), has opened new application fronts, for example biotechnology, to this class of materials which have been mostly used in electronics, till now. Typically, the properties of the ferrites depend on the composition, the cation distribution, the synthesis methods and on the nanoparticles dimensions. Among all ferrites, zinc ferrite exhibits unusual behavior in nanoregime. Bulk ZnFe2O4 has a normal spinel structure, in which all Zn2+ ions occupy tetrahedral positions (A-sites) and all Fe3+ ions octahedral positions (B-sites). At room temperature, zinc ferrite is paramagnetic, and the transition to a magnetically ordered state occurs only at temperatures of about 10 K. The magnetic properties change significantly when the particle size decreases to a few tens of nanometers: at this scale, a partial cation inversion occurs. Thus, there is a transformation of structure from normal to mixed spinel, which can lead to a superparamagnetic behaviour at room temperature. A part of the thesis work has been thus dedicated to the analyses of EPR and micro-Raman data collected from a series of pure and substitued Zinc ferrites obtained from classical solid state procedure and from solution after microwave or coprecipitation methods. The samples have been prepared at the Dipartimento di Chimica of the University of Pavia. The temperature beahviour of Fe3+ EPR signals from different samples has been correlated to the structural information derived from micro-Raman experiments. The results seems to validate an intrinsic nature of the observed superparamagnetic behavior excluding the presence of nanoclusters of maghemite as extrinsic phase.È consentito all'utente scaricare e condividere i documenti disponibili a testo pieno in UNITESI UNIPV nel rispetto della licenza Creative Commons del tipo CC BY NC ND.
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https://hdl.handle.net/20.500.14239/18509