High-melting temperature oxides, carbides and nitrides are superior in hardness and strength to metals, especially in severe conditions. However, the extensive use of such ceramics in structural engineering applications often encountered critical problems due to their lack of damage tolerance and to the limited mechanical reliability. Several ceramic composites and, in particular, laminated structures have been developed in recent years to enhance strength, toughness and to improve flaw tolerance. Significant strength increase and improved mechanical reliability, in terms of Weibull modulus or minimum threshold failure stress, can be achieved by the engineering of the critical surface region in the ceramic component. Such effect can be realized by using a laminated composite structure with tailored sub-surface insertion of layers with different composition. Such laminate is able to develop, upon co-sintering, a spatial variation of residual stress with maximum compression at specific depth from the surface due to the differences in thermal expansion coefficient of the constituting layers. In the present work silicon carbide has been selected as second phase to graduate the thermal expansion coefficient of alumina due to its relatively low specific density that could allow the production of lighter components with improved mechanical performance, also for high temperature applications. Ceramic laminates with strong interfaces composed of Al2O3/SiC composite layers were produced by pressureless sintering or Spark Plasma Sintering (SPS) of green layers stacks prepared by tape casting water-based suspensions. Monolithic composites containing up to 30 vol% silicon carbide were fabricated and thoroughly characterized. Five engineered ceramic laminates with peculiar layers combination that is able to promote the stable growth of surface defects before final failure were also designed and produced. By changing the composition of the stacked laminae and the architecture of the laminate, tailored residual stress profile and T-curve were generated after co-sintering and successive cooling in each multilayer. The results of the mechanical characterization show that the engineered laminates are sensibly stronger than parent monolithic composite ceramic and exhibit surface damage insensitivity, according to the design. Such shielding effect is especially observed when macroscopic cracks are introduced by high load Vickers indentations. Some designed multilayers exhibit reduced strength scatter and higher Weibull modulus, which implies superior mechanical reliability. Fractographic observations on fracture surfaces of the engineered laminates show a graceful crack propagation within the surface layers in residual compressive stress which can be attributed to the stable growth of superficial cracks before final failure as it is predicted by the apparent fracture toughness curve. Such fracture behaviour is considered to be responsible for the peculiar surface damage insensitivity and the improved mechanical performance.

The objective of this work is the investigation of the industrial production process of the Color Masterbatches and of the parameters that influence the pigment dispersion into the polymer matrix. In particular, the project is focused on the production process optimization in order to increase the quality of the final product and to minimize their environmental impact.

Abstract Fuel cells are a very efficient way to transform the chemical energy of a fuel into electrical energy. Among the different kind of fuel cells, the solid oxide based ones (SOFC) are the most promising when cost is considered, since their high operative temperature allows the use of widely available non precious materials as catalysts. The worldwide research activity on SOFC is very active and many aspects are currently under investigation; nevertheless it is generally agreed that the development of suitable low-cost fabrication technologies is presently the key technical challenge which needs to be faced. In the present thesis the two most promising geometries have been developed, i.e. the planar configuration and the microtubular one. Innovative aspects have been considered in both directions. In both cases environmental impact and cost of the processing was minimized by using only water based ceramic powder production technologies. A first aim of this work was to develop a processing technique capable of producing planar anode supported SOFC with thin electrolyte by sequential tape casting method (thus avoiding the lamination step) and co-sintering. In order to do this, different aspects associated with the processing steps, from colloidal suspension stability, to defects related with green forming and sintering have been studied. One of the most critical factors in the co-sintering of ceramic multilayers is the stress generated because of different thermal expansion coefficients and sintering kinetics of the constituting layers. Such stresses, when not controlled, can lead to the formation of several types of defects, such as flaws, delaminations, retarded densification and warping. In order to understand and to solve the problems of defect formation and curvature development, a set up for monitoring in situ the sintering process has been built. The effect of different parameters (such as green composition, type and amount of doping elements, powder granulometry etc.) on the sintering kinetics of the layers constituting the cells has been studied. The different sintering kinetics were related to the developed defects and curvature. Such analysis led to the fabrication of defect free cells which were successfully electrochemically characterised. In order to being able to quantify the stresses which are developed and to predict the curvature behaviour of a bi-layer upon sintering, the knowledge of the very high temperature mechanical parameters of the materials is needed. A technique for measuring the uniaxial viscosity of thin tape cast layers upon sintering was therefore developed and is described in a section of this thesis. Whereas planar SOFC seem to be the preferred choice for future stationary application, micro tubular ones may be much more attractive for portable applications. The first advantage of the micro tubular SOFC design, is the potential very high volumetric power density, being it inversely proportional to the electrolyte diameter. The other advantages are resulting from low thermal mass: (i) high thermal shock resistance, and (ii) rapid turn on/off capability. However, miniaturization issues are arising when the diameter is decreased to less than 1 mm, mainly due to difficulties associated with the application of an internal current collector. The novel approach, which is described in the present thesis, consists in fabricating the cell in the form of a fiber around a metallic wire. The support for the cell fabrication consists of a thin nickel wire, on to which the porous anode layer, the electrode and the cathode, are deposited in succession. This nickel support acts as the current collector as well, and this opens totally new possibilities for downscaling tubular cells. Abstract Le celle a combustibile rappresentano una via efficiente per la conversione diretta dell’energia chimica di un combustibile in energia elettrica. Tra le diverse tipologie di celle, quelle agli ossidi solidi (SOFC), sono le più promettenti in termini di costo, perché l’elevata temperatura operativa consente l’utilizzo di materiali largamente disponibili e non preziosi come catalizzatori. L’attività di ricerca sulle SOFC è fiorente, e diversi aspetti sono investigati attualmente; ciononostante, è generalmente riconosciuto che lo sviluppo di tecnologie di fabbricazione economiche per la produzione di celle rappresenta la sfida tecnica che deve essere affrontata più urgentemente. In questa tesi sono state sviluppate le due geometrie più promettenti: quella planare e quella micro-tubolare. Sono stati considerati aspetti innovativi per entrambe. L’impatto ambientale ed il costo dei processi sono stati minimizzati grazie all’utilizzo di tecnologie delle polveri ceramiche a base acquea. Un primo scopo di questo lavoro è stato quello di sviluppare una tecnologia di processo in grado di produrre celle planari supportate da anodo con elettrolita sottile, tramite colaggio su nastro sequenziale (evitando quindi la fase della laminazione) e co-sinterizzazione. A tal fine sono stati studiati i diversi aspetti delle fasi del processo, dalla stabilità delle sospensioni colloidali, ai difetti associati alla formatura del verde ed alla sinterizzazione. Uno dei fattori più critici nella co-sinterizzazione di multistrati ceramici sono gli sforzi generati dai diversi coefficienti di espansione termica e cinetiche di sinterizzazione dei singoli strati. Tali sforzi, se non controllati, possono portare alla formazione di diversi tipologie di difettosità, come cricche, delaminazioni, densificazione ritardata e curvature. Al fine di comprendere e risolvere il problema della formazione dei difetti e dello sviluppo della curvatura, è stato costruito un apparato atto al controllo visivo del processo di sinterizzazione. È stato quindi studiato l’effetto di diversi parametri (come composizione del verde, tipo e concentrazione di elementi dopanti, granulometria delle polveri, ecc.) sulla cinetica di sinterizzazione degli strati che costituiscono la cella. Le diverse cinetiche di sinterizzazione sono state messe in relazione ai difetti ed alla curvatura riscontrata sul prodotto finale. Al fine di quantificare gli sforzi sviluppati e predire la velocità di curvatura di un bi-strato durante la sinterizzazione, è necessario conoscere i parametri meccanici del materiale alle altissime temperature. A tal fine è stata sviluppata una tecnica di misura per la viscosità uniassiale di film sottili ottenuti per colaggio su nastro. Mentre le celle planari appaiono la scelta preferenziale per la generazione stazionaria, quelle micro-tubolari sono di interesse per applicazioni portatili. Il primo vantaggio della geometria micro-tubolare, è l’altissima densità di potenza volumetrica teorica, che è inversamente proporzionale al diametro dell’elettrolita. Gli altri vantaggi, che derivano dalla bassa massa termica, sono l’elevata resistenza allo shock termico e la possibilità di sopportare cicli rapidi di accensione/spegnimento. Ciononostante, la riduzione del diametro delle celle a meno di 1 millimetro è problematica, principalmente per le difficoltà associate all’applicazione di un collettore di corrente interno. L’approccio innovativo descritto in questa tesi consiste nella fabbricazione della cella in forma di fibra attorno ad un filo metallico. Il supporto per la fabbricazione della cella consiste in un filo sottile di nichel, sul quale vengono depositati in successione l’anodo poroso, l’elettrolita ed il catodo. Il supporto di nichel espleta anche la funzione di collettore di corrente, aprendo nuove possibilità per la miniaturizzazione di celle tubulari.

The repair or replacement of damaged or diseased hard tissue is a biomedical field that has been the subject of more and more interest in many areas of research and especially in the development of new biomaterials. The rise in the average age of the world population, increasing osteoporosis treatments and the spread of cancer and genetic bone diseases, has brought about the need to find solutions for patient care. To achieve this target/objective, biomaterials must simulate the body environment as much as possible and favour tissue repair by integrating them into the host site. Calcium phosphates are used as medical implants because they have a chemical composition similar to the mineral of human bones, i.e. apatite. For this reason they are biocompatible and they can interact in a bioactive way with bone tissue. In the present work a specific form of bone graft, in the form of calcium phosphate granules, has been developed by using the droplet extrusion technique. The granules were characterized chemically and physically, with specific attention to in vivo and in vitro analyses. The proposed method has allowed us to obtain spherical granules in very narrow micrometric size distribution (300-1200 μm) without the use of solvents or oils thus avoiding time consuming washing processes. Granules were produced with several controlled mineralogical compositions including: pure Hydroxyapatite (HA) and β-Tricalcium Phosphate (βTCP), mixtures of HA/βTCP and Hydroxyapatite/Tetracalcium phosphate (HA/TTCP), and compositions doped with zinc (for antibacterial purposes) and strontium (for anti-osteoporosis purposes). Of several interesting features, the produced granules show high interconnected microporosity (0.1-10 μm) and surface roughness, properties necessary for osteoconductivity. The solubility behavior of granules was studied and demonstrated that the morphology and microporosity are more important in dissolution processes than chemical or mineralogical composition. Products were tested in simulated body fluid (SBF), and among the different compositions, HA/TTCP has been found to be bioactive during in vitro studies. In fact an intense precipitation of a carbonated layer of apatite was observed, associated with the high dissolution of a TTCP phase. All pure granules were demonstrated to not be cytotoxic. Bone implantations in different animal models (rabbits and primates) showed good performance of granules in the repairing of bone. The granules stimulated the bone growth without any inflammatory reactions. In particular, HA/TTCP granules exhibited excellent biomechanical properties by increasing the stability of neo-formed bone. These preliminary investigations were sufficient to show that the developed granules can be used for bone repair or replacement. However, more studies, especially for doped products, such as in vitro cells experiments, have to be performed to assure the biocompatibility and the effective stimulation of bone growth. This work was performed in collaboration with Eurocoating S.p.A. (Trento, Italy), a company expert in biomedical coatings for prostheses and implants, and it is a part of “CaP project†co-sponsored by Provincia Autonoma di Trento (Italy).

In the restoration field the synthetic resins are commonly used and they are selected in order to replace natural products and possibly overcome their drawbacks. Nevertheless, these resins are not completely able to cover the specific mechanical properties required in each restoration work. The main aim of this research is the development of innovative micro/nanocomposite materials with enhanced features for artwork conservation operations. The introduction of appropriate amounts of micro- and nanofillers within commercially available art preserving polymers may allow the improvement of their mechanical deficiencies, without impairing their physical and chemical properties. Cellulose microcrystals (CMC) and nanocrystals (CNC) were selected as natural reinforcing fillers for a commercial acrylic copolymer (Paraloid B72) widely applied as a consolidant of wooden objects and two molecular weights of a thermoplastic water soluble adhesive used in the restoration of oil paintings (Aquazol 200 and 500). In particular, melt-compounded Paraloid and Aquazol based microcomposites with various amounts (5÷30 wt%) of CMC and thin films of Aquazol 500 micro- and nanocomposites produced by the solution mixing method with a CMC and CNC content of 5-10-30 wt% were investigated. Several characterization techniques were used in order to assess the effect of micro- and nanocellulose on the physical and thermo-mechanical behavior of these three thermoplastic polymers. In the first part of the work, the characterization of melt-compounded and compression molded microcomposites under dry and conditioned state (T= 23°C, RH= 55%) was performed. All dried and wet formulations showed a similar stabilizing effect of CMC flakes with an increase of elastic modulus and a decrease of thermal expansion coefficient and creep compliance, regardless of the moisture content. Interestingly, conditioned composites exhibited the enhancement of the tensile properties at break, in contrast to dried microcomposites that reported a drop in these properties. On the other hand, the highest amount of CMC led to a chromatic change of the three matrices towards yellow-brown tones. In the second part of the work, the characterization of solution mixed micro and nanocomposites based on Aquazol 500 highlighted the systematic increment of the dimensional stability of the neat resin due to the presence of both CMC and CNC particles. Remarkably, CNC proved to be more effective than CMC in increasing of the stiffness and the elongation at break of Aquazol as the filler loading increased, without impairing the good optical properties of this material. In the third part of the work, the application of melt-compounded Paraloid/CMC composites as consolidants for damaged wood was investigated. CMC introduction did not change the good viscosity of the neat matrix and especially its good water repellency. Wood samples treated with microfilled Paraloid exhibited an increment of the stiffness and the flexure strength under quasi-static and impact conditions and, additionally, a systematic enhancement of the radial and tangential surface hardness almost up to the intact wood values was observed. In the last part of this thesis, the practical application of CMC and CNC Aquazol based adhesive films made by melt-compounding and solution mixing was investigated for the lining of oil paintings. Single-lap shear tests confirmed the stabilization action of both CMC and CNC particles on all experimental formulations with a progressive reduction of the compliance proportionally to the filler loading. Even in this case the increment of the Aquazol dimensional stability was mainly imparted by the presence of CNC. Only for solution mixed CMC composites a dramatic drop in the adhesive strength as the filler content increased was detected.

The principal object of this thesis is the investigation of silicon oxycarbide (SiOC) ceramics as anode material for Li-ion batteries. The investigated materials are prepared by cross linking commercial polymer siloxanes via hydrosylilation reactions or hybrid alkoxide precursors via sol-gel. The cross linked polymer networks are then converted in to ceramic materials by a pyrolysis process in controlled argon atmosphere at 800-1300 °C. In details the influence of carbon content on lithium storage properties is addressed for SiOC with the same O/Si atomic ratio of about 1. Detailed structural characterization studies are performed using complementary techniques which aim correlating the electrochemical behavior with the microstructure of the SiOC anodes. Results suggest that SiOC anodes behave as a composite material consisting of a disordered silicon oxycarbide phase having a very high first insertion capacity of ca 1300 mAh g-1 and a free C phase. However, the charge irreversibly trapped into the amorphous silicon oxycarbide network is also high. In consequence the maximum reversible lithium storage capacity of 650 mAh g-1 is measured on high-C content SiOCs with the ratio between amorphous silicon oxycarbide and the free C phase of ï ¾ 1:1. The high carbon content SiOC shows also an excellent cycling stability and performance at high charging/discharging rate with the stable capacity at 2C rate being around 200 mAh g-1. Increasing the pyrolysis temperature has an opposite effect on the low-C and high-C materials: for the latter one the reversible capacity decreases following a known trend while the former shows an increase of xi the reversible capacity which has never been observed before for similar materials. The influence of pyrolysis atmosphere on lithium storage capacity is investigated as well. It is found that pyrolysis in Ar/H2 mixtures, compared to the treatment under pure Ar, results into a decrease of the concentration of C dangling bonds as revealed by electron spin resonance (ESR) measurements. The sample prepared under Ar/H2 mixture shows an excellent cycling stability with an increase in the specific capacity of about 150 mAh g-1 compared to its analogues pyrolysed in pure argon atmosphere. In order to study the role of porosity towards the lithium storage properties, a comparison of dense and porous materials obtained using same starting precursors is made. Porous SiOC ceramics are prepared by HF etching of the SiOC ceramics. HF etching removes a part of the amorphous silica phase from SiOC nanostructure leaving a porous structure. Porous ceramics with surface areas up to 640 m2 g-1 is obtained. The electrochemical charging/discharging results indicate that the porosity can help to increase the lithium storage capacity and it also leads to an enhanced cycling stability. This work demonstrates clearly that silicon oxycarbide (SiOC) ceramics present excellent electrochemical properties to be applied as a promising anode material for lithium storage applications.

An innovative current collection architecture for micro-tubular solid oxide fuel cells (SOFC) has been developed. A nickel wire is coiled around a thin carbon composite rod in order to fabricate cell supports. Different carbon composites such as pencil leads and carbon fibres were investigated. The cell support was then coated with ceramic slurries based NiO/YSZ and YSZ for anode and electrolyte, respectively, by successive dip coatings. Effect of thermal behaviour, porosity, amount of binder and dip coating parameters were conjunctly analysed to produce anode and electrolyte crack-free layers with the thickness desired. Pyrolisable materials were then eliminated under air atmosphere at 800ºC followed by co-sintering of half-cells at 1380ºC for 2 h in argon to avoid the oxidation of the nickel wire. In order to complete the cells, sintered half-cells were dipped into cathode inks consisted of LSM-YSZ composite for a functional layer and LSM pure to increase the electrical conductivity of the cathode. The cathode was also sintered at 1150ºC for 2 h under argon atmosphere. Complete cells with an outer diameter below 1.2 mm and length of 30 mm with an effective cathode length of 20 mm and whose active cathode area is 0.75 cm2 were produced. The efficiency of the current collector method developed is evaluated by comparison with the performance of a micro-tubular cell produced and tested under similar conditions, but with a common current collection method. The results of I-V curves shown that the innovative current collection method enhances the performance of a typical micro-tubular cell in the order of 3-4 times. The improvement in performance is attributed to the reduction of current paths of the micro-tubular cells. Suggestions for the production and characterization of current collector-supported micro-tubular cells are also given.

Biodegradable and bio-based polymers have raised great attention since sustainable development policies tend to become more and more important with the growing concern for the environment and the decreasing reserve of fossil fuel [1]. The increasing demand for environmentally friendly materials attracted the attention on biopolymers reinforced with cellulose, that is a virtually inexhaustible source of raw material [2] and on new manufacturing ways such as additive manufacturing (AM) [3]. The most diffused AM technology for polymers is Fused Deposition Modelling (FDM), a technique where a filament of thermoplastic polymer is extruded through a nozzle and deposited layer by layer to form the final object with the support of computer aided design. The aim of this work is the development of different kind of thermoplastic biodegradable composites based on commercially available polymers reinforced with cellulose and to study their applicability in fused deposition modeling (FDM). The final goal is the production of plastic filaments suitable to feed a commercially available FDM 3D-printing machine. Starting from microcrystalline cellulose (MCC), two different types of nanocellulose: crystalline nanocellulose (CNC) and nanofibrillated cellulose (NFC) were produced and studied to be applied as natural reinforcing fillers for selected types of biopolymers. Cellulose nanocrystals in water solution were prepared from micro-cellulose through a sulfuric acid hydrolysis while the fibrillated nanocellulose was obtained with high energy ultrasonication. The commercial grade polymer matrices selected in this research were: i. polyvinyl alcohol (PVA), a water-soluble biodegradable material; ii. poly(lactic acid) (PLA), a biodegradable polymer that comes from the fermentation of agricultural waste; iii. poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) that belongs to the family of polyhydroxyalkanoates (PHA) and it is entirely synthesized by microorganism as an intracellular storage product under particular growth conditions. Composite materials containing various amounts of cellulose fillers produced by solution or melt mixing were grinded and extruded through a single screw extruder to obtain filaments. With the aid of a desktop 3D printer, dumbbell specimens were fabricated, and their mechanical properties determined. Several characterization techniques were used in order to assess the effect of micro- and nanocellulose on the physical and thermo-mechanical behavior of these thermoplastic composites. According to SEM analysis, CNC particles appear homogeneously dispersed in PVA without noticeable aggregates. Thermal degradation of PVA was shifted towards higher temperatures with the increase of filler content, enhancing the thermal stability of the composites as compared with neat PVA. An enhancement in the storage modulus with the amount of CNC was observed in both filament and 3D printed specimens. In particular, an increase of about three times in the storage modulus at room temperature was reached in 3D samples with a CNC concentration of 10wt%. An improvement of the dimensional stability was observed with a reduction of the creep compliance with the filler content. Quasi-static tensile tests evidenced an increase of the stiffness and the strength of PVA due to the CNC introduction. A comparison between the reinforcing effect of nanocellulose and microcellulose in 3D printed samples highlighted the higher efficiency of CNC over MCC in reducing the rubber-like behavior of polyvinyl alcohol. Maleic anhydride (MAH) was employed to improve the interaction between hydrophilic microcrystalline cellulose and the PLA matrix. Infrared spectroscopy confirmed the grafting of maleic anhydride on the PLA backbone during melt mixing and SEM analysis revealed that microcellulose was well dispersed in PLA and maleic anhydride was able to enhance the interface between the two components. Thermal degradation of PLA was not affected by the presence of MAH. On the other hand, glass transition temperature, crystallization temperature and melting temperature were lowered by the increasing amount of MAH. Glass transition temperature at 10wt% of MAH decreased from 70°C to 48°C. Tensile tests highlighted that microcellulose in low concentration was able to improve the stiffness and the stress at break of 3D printed specimens. The maximum in term of stiffness and strength is reached for composite at 1wt% of MCC and at 5 wt% with the presence of MAH. NFC was dispersed in PLA by solution mixing and nanocomposites were printed and characterized. The creep compliance curves of the 3D printed samples were well fitted by a power law model and resulted that NFC was able to reduce the time-dependent linear response under constant load conditions, improving the geometrical stability. Static tensile test on plates obtained by solution casting displayed an increase in stiffness of the filament samples with increasing amount of nanocellulose. The same effect was not observed on 3D printed samples where a poor adhesion between subsequent layers was evidenced from SEM analysis upon the introduction of NCF. Lauryl functionalized nanocellulose was incorporated in PLA with solution mixing technique but the limited quantity of materials did not permit to go further with the production of filaments. Scanning electron microscopy indicated that up to a filler content of 6.5 wt. %, LNC was well dispersed. Nanocomposites with 3 and 5 wt. % of LNC showed the highest strain at break and a large amount of plastic deformation due to a strong interfacial adhesion between the PLA and filler particles while for higher LNC fractions the presence of aggregates weakened the nanocomposite. A decrease in stiffness was measured upon the introduction of LNC related to the low stiffness of the short aliphatic chains attached to the surface of the cellulose and so the formation of a soft phase between filler and the matrix as highlighted also by gas permeability tests. Finally, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) was successfully extruded and 3D printed. PHBH and NCF were mixed in solution and extruded in form of filaments used to feed a 3D printing machine. The reinforcing effect of the nanocellulose in terms of stress at break and of elongation at break showed a maximum at a content of 0.5 wt%. An increase in stiffness for filament with increasing amount of nanocellulose was measured but also in this case it was not observed in 3D printed samples. Anyway, the presence of NCF did not affect the thermal behavior of the materials.

This PhD thesis focuses on the synthesis and characterization of multifunctional polymer-derived SiOCN ceramics. The main objective is to synthesize N-doped silicon oxycarbides (SiOCs), to characterize their structure and their properties (mainly electrical conductivity) and correlate them with the presence of N in the structure. We also aim to understand how the architecture of the starting polymer can influence the retention of N into the SiOC structure. First, N-doped SiOC polymer precursors were synthesized via hydrosilylation reaction between Si-H groups present in a commercial polysiloxane (PHMS) and –CH=CH2 groups of three different commercial N-containing compounds. The structural characterization of as-synthesized preceramic polymer precursor was investigated by FT-IR and NMR. Thermal degradation was studied by TGA. The results show that the architecture of the polymer precursors plays an important role on the pyrolythic transformation. Then, SiOCN ceramics were obtained by pyrolysis of the as-synthesized polymer precursors in nitrogen atmosphere at various temperatures for 1h using a tubular furnace. Subsequently, high temperature structural evolution was studied using combined techniques such as XRD, FT-IR, NMR, Elemental analysis, and XPS. The obtained results show that the type of N-containing compounds impacts on the crystallization behavior of the final ceramics. Elemental analysis clearly indicates that N is present in the SiOC matrix and the degree of N retention after pyrolysis is related to the type of N-containing starting compounds. XPS data indicate that N-C bonds are present in the SiOC ceramic samples even if only N-Si bonds exist in the starting N-containing precursor. However, a larger fraction of N-C bonds is present in the final SiOCN ceramic when N atoms form bonds with sp2 carbon atoms in the pre-ceramic polymer. We have also studied electrical and optical properties of the SiOCNs. Electrical conductivity of the powdered ceramic samples was determined using powder-solution-composite technique. The results show an increase in room temperature AC conductivity of three orders of magnitude, from ≈10-5 (S/cm) to ≈10-2 (S/cm), with increasing pyrolysis temperature from 1000 to 1400 °C. Furthermore, the electrical conductivity of the SiOCN ceramic derived from N-C bond bearing precursor is three to five times higher than that of the sample derived from N-Si containing precursor at each pyrolysis temperature. The combined structural study by Raman spectroscopy and chemical analysis suggests that the increase of electrical conductivity with the pyrolysis temperature is due to the sp3-to-sp2 transition of the amorphous carbon phase. The higher conductivity of the amine-derived SiOCN is also discussed considering features like the volume % of the free-carbon phase and its possible N-doping. Fluorescence of the SiOCN samples treated at low temperatures, 400 and 600 °C, has been studied. The spectra show that the heated precursors fluoresce in the visible range with a dominant blue emission. Since the non-heated polymer precursors do not fluoresce, emitting centers must be formed during the polymer-to-ceramic transformation and associated with the structural changes. The origin of the luminescence could be originated from defects related to C, O and/or Si. Finally, we investigated the gas sensing behavior of the SiOCNs pyrolyzed at low and high temperatures. Regarding the electrical gas sensing of the SiOCN ceramics pyrolyzed at 1400 °C, the response to two target gases NO2 and H2 was tested by in situ DC conductance measurements at operating temperatures from 200 to 550 °C. The SiOCN ceramics are sensitive to NO2 at temperatures below 400 °C and to H2 at temperatures above 400 °C. In addition, the response observed for the studied SiOCN ceramics is higher than that reported in the previous studies for SiOC ceramic aerogels. With regard to the optical gas sensing of the SiOCNs obtained from the heat treatment of the polymer precursors at 400 and 600 °C, fluorescence spectra in the presence of organic vapors such as acetone and hexane were recorded. The results show that these tested vapors quench the fluorescence of the studied SiOCN. In conclusion, the SiOCN ceramics can be promising materials for the gas sensing application.

In the last decade the increase in energy demand, the awareness of limited availability of fossil fuels and the need to reduce green house gases emission impelled governments and research institutions to focus on the study of renewable energy sources such as solar, wind and biomass derived energy and on the increase in the energy production devices efficiency. Within such scenario a relevant contribution is given by fuel cells technologies as advanced power generation system. Fuel cells are high efficiency devices and comply with the request of environmental friendly source of energy. They convert directly fuel energy into power and heat by electrochemical reactions without the need for combustion as intermediate step. The possibility to use Hydrogen makes fuel cells virtually zero-emission devices being water the only reaction product; nevertheless, also the use of hydrocarbons as fuel reduces considerable CO2 emissions. Among the different systems, solid oxide fuel cells (SOFCs) operate at high temperatures (650-1000°C) and allow to achieve the highest electrical efficiency, from 45 to 60% for common fuels, values not attainable by traditional electrical power generation methods, and up to 70% in combination with a gas turbine for Hybrid Power System generation, with an overall electrical and thermal efficiency higher than 90%. Moreover, such technology presents many advantages such as the possibility to be fed with different fuels, the absence of moving parts, modularity and limited emissions. These characteristics make SOFC suitable for application in the distributed generation market. Despite all the mentioned advantages, SOFCs show problems that make these devices not suitable for the production on industrial scale yet. In particular they present low reliability and are not competitive with traditional powers sources. SOFCs are constituted by single cells (consisting of an anode and a cathode separated by a solid electrolyte) that are collected together in a stack by interconnects in order to obtain the required power. This means that a stack is a multilayer assembly of materials with different thermal, mechanical and chemical properties that need to fulfil many prerequisites for their own function. Moreover, some of these properties must match for other connected components; for instance they have to show similar thermal expansion coefficients, to be stable at high temperatures and during thermal transients. Due to the high working temperature, stack components are necessarily subjected to degradation phenomena, which reduce their long term reliability. Among them, poisoning of cathode by Chromium evaporation from metallic interconnects, chemical interactions between glass–ceramic sealants and ferritic steel interconnects, anode poisoning caused by carbon or sulphur deposition, reduction of electrical conductivity are worthy of mention. Furthermore, various cycling conditions such as thermal cycle, redox cycle, and load cycle affect stability of SOFCs. All these degradation phenomena must be minimized in order to increase SOFC reliability. All these issues are object of intense research. The research work of the present thesis has been focused on the increase of redox stability of anode supported cells, which is considered one of the key point to improve stack reliability. The state of the art materials for the anode is Ni/YSZ cermet due to its high performance. Nevertheless, this cermet is prone to severe degradation upon redox cycling. Due to the high operating temperatures, Nickel particles tend to coalesce and coarsen. Fuel supply interruptions, over-potentials and leakages can cause the re-oxidation of Ni to NiO with a consequent volumetric expansion that can generate internal stresses and lead to cracks formation within the YSZ network and the electrolyte resulting in cell failure. Different approaches can be taken in account in order to minimize redox instability. In order to study redox phenomena and produce redox stable cells many aspects related to the modification of anodic microstructure were analyzed. Among these, one of the most promising method is to modify the anode microstructure by increasing its porosity. The present thesis is divided in two parts. In the first section the theoretical background of fuel cells, specifically SOFCs, is reported. A particular attention is dedicated to describe redox phenomenon and the state of the art of the research in this field. The second experimental part concerns with the production of anodes with improved microstructure. The modification of microstructure was realized by using different powders and by adding different pore formers and doping elements. A detailed study of the effects on redox stability of the microstructure modifications induced by the addition of each of the aforesaid substances is described.