The aim of this PhD thesis is to develop a non-destructive procedure for the micro-scale study of materials where an agent diffuses in a solid, turbid (non-transparent) matrix. The procedure was developed using micro-spatially offset Raman spectroscopy (micro-SORS), a combination of SORS and microscopy, that permits the non-destructive retrieval of subsurface composition of a material at the micro-scale directly from the surface, without the need of cross-sectional analysis. The research is focused on materials encountered in the Cultural Heritage field, including plasters and stuccos treated with ammonium oxalate (AmOX) or Paraloid B72, and absorbing surfaces cleaned with an innovative, eco-friendly deep eutectic solvent. Moreover, dye sensitized solar cells (DSSC) photo-electrode were investigated, demonstrating the applicability of the procedure to a wider range of situations also outside heritage science field. The impact of this study relies on the evaluation of the efficacy of conservation treatments, the monitoring of the solvents diffusion and the quality control of photo-electrodes without damaging the materials. A systematic investigation of micro-SORS potentials and limitations were carried out using mock-up samples with controlled characteristics, namely penetration depth of the agent, its concentration on the surface and its concentration profile inside the matrix. The results of the experiments show a decrease of the agent/matrix Raman intensity ratio with the increase of the defocusing distance or the spatial offset. The trend of this intensity decrease was found to be related to the penetration depth of the agent and to its diffusion profile. The data interpretation was supported by a physical model, that was designed to simulate the scattering of light and the Raman photons collection in micro-SORS measurements, thus providing analytic expressions which describe the relationship between micro-SORS agent/matrix intensity ratio and the concentration profile of an agent dispersed in the turbid medium. A case study was included in the experimentation: plasters treated with AmOX collected from an historical building, Palazzo Besta in Teglio (SO), were analysed obtaining relevant information about newly formed oxalates distribution inside the plaster matrix, paving way for the use of micro-SORS as a routine method for non-destructively evaluating conservation products distribution and treatments efficacy. Additionally, a preliminary application of PCA to experimental data provided promising results, that encourages further exploration on the use of statistical multivariate analysis on micro-SORS spectra.
Lo scopo della tesi consiste nello sviluppare una procedura non-distruttiva per lo studio a scala micrometrica di materiali in cui un’agente diffonde in una matrice altamente diffusiva. La procedura è stata sviluppata utilizzando il micro-spatially offset Raman spectroscopy (micro-SORS), una tecnica che combina SORS e microscopia e permette di determinare non-distruttivamente la composizione sub-superficiale di un materiale non trasparente direttamente dalla superficie dello stesso, senza ausilio di una sezione trasversale, e con una risoluzione nell’ordine di grandezza dei micron. La ricerca è stata principalmente focalizzata su materiali che appartengono all’ambito dei Beni Culturali, tra i quali intonaci e stucchi trattati con ossalato di ammonio (AmOX) e Paraloid B72, e superfici porose soggette a pulitura con un innovativo ed ecocompatibile deep eutectic solvent (DES). Inoltre, per dimostrare l’applicabilità del metodo anche al di fuori dell’ambito della scienza della conservazione, indagini micro-SORS sono state condotte sul foto-elettrodo di celle solari dye sensitized (DSSC). L’impatto della ricerca riguarda la valutazione dell’efficacia di trattamenti conservativi, il monitoraggio della diffusione di solventi durante le operazioni di pulitura e il controllo qualità di dispositivi DSSC, senza danneggiare i materiali indagati. Uno studio sistematico di potenzialità e limiti del micro-SORS è stato condotto su provini di laboratorio preparati con specifiche concentrazioni dell’agente in superficie, specifici profili di concentrazione e profondità di penetrazione dell’agente all’interno della matrice. All’aumentare della distanza di offset o di quella di defocusing è stata osservata una progressiva diminuzione dell’intensità delle bande Raman dell’agente rispetto a quelle della matrice. L’andamento della diminuzione di intensità Raman relativa è risultato essere correlato alla profondità di penetrazione dell’agente e al suo profilo di diffusione. L’interpretazione dei dati è stata supportata da un modello fisico, sviluppato con l’intento di simulare lo scattering della luce e la raccolta dei fotoni Raman durante un esperimento micro-SORS. Il modello consente di esprimere in termini analitici la relazione tra il profilo di concentrazione dell’agente e il rapporto di intensità delle bande Raman di agente e matrice. La ricerca ha incluso un caso studio riguardante l’analisi micro-SORS di intonaci trattati con AmOX, collezionati da una delle facciate dipinte di Palazzo Besta (Teglio, SO). Le indagini hanno consentito di ottenere informazioni sulla distribuzione delle fasi di ossalato di neoformazione all’interno della matrice dell’intonaco, aprendo la strada all’utilizzo del micro-SORS come metodo di routine per valutare non-distruttivamente l’efficacia di trattamenti applicati a fini conservativi. Inoltre, l’analisi delle componenti principali (PCA) condotta su spettri micro-SORS ha fornito risultati che incoraggiano l’ulteriore esplorazione dell’uso di metodi di analisi multivariata su dataset micro-SORS.
Micrometric-scale, non-destructive investigation of diffusion process in turbid materials
BOTTEON, ALESSANDRA
2021/2022
Abstract
The aim of this PhD thesis is to develop a non-destructive procedure for the micro-scale study of materials where an agent diffuses in a solid, turbid (non-transparent) matrix. The procedure was developed using micro-spatially offset Raman spectroscopy (micro-SORS), a combination of SORS and microscopy, that permits the non-destructive retrieval of subsurface composition of a material at the micro-scale directly from the surface, without the need of cross-sectional analysis. The research is focused on materials encountered in the Cultural Heritage field, including plasters and stuccos treated with ammonium oxalate (AmOX) or Paraloid B72, and absorbing surfaces cleaned with an innovative, eco-friendly deep eutectic solvent. Moreover, dye sensitized solar cells (DSSC) photo-electrode were investigated, demonstrating the applicability of the procedure to a wider range of situations also outside heritage science field. The impact of this study relies on the evaluation of the efficacy of conservation treatments, the monitoring of the solvents diffusion and the quality control of photo-electrodes without damaging the materials. A systematic investigation of micro-SORS potentials and limitations were carried out using mock-up samples with controlled characteristics, namely penetration depth of the agent, its concentration on the surface and its concentration profile inside the matrix. The results of the experiments show a decrease of the agent/matrix Raman intensity ratio with the increase of the defocusing distance or the spatial offset. The trend of this intensity decrease was found to be related to the penetration depth of the agent and to its diffusion profile. The data interpretation was supported by a physical model, that was designed to simulate the scattering of light and the Raman photons collection in micro-SORS measurements, thus providing analytic expressions which describe the relationship between micro-SORS agent/matrix intensity ratio and the concentration profile of an agent dispersed in the turbid medium. A case study was included in the experimentation: plasters treated with AmOX collected from an historical building, Palazzo Besta in Teglio (SO), were analysed obtaining relevant information about newly formed oxalates distribution inside the plaster matrix, paving way for the use of micro-SORS as a routine method for non-destructively evaluating conservation products distribution and treatments efficacy. Additionally, a preliminary application of PCA to experimental data provided promising results, that encourages further exploration on the use of statistical multivariate analysis on micro-SORS spectra.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/187913