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https://hdl.handle.net/1889/689
2024-01-16T14:46:05ZFabrication and characterization of UHTCMCs for tribological applications
https://hdl.handle.net/1889/5398
Title: Fabrication and characterization of UHTCMCs for tribological applications
Authors: Mor, Matteo
Abstract: The interest in the Ultra-High Temperature Ceramic Matrix Composites (UHTCMCs) increased over the last years due to their combination of toughness typical of CMCs with the high temperature stability of ultra-high temperature ceramics. The demand of UHTCMCs with better properties produced through more sustainable processes had led to the investigation of different approaches. This thesis deals with the fabrication and characterization of short pitch carbon fibres reinforced UHTCMCs for extreme applications, using a slurry infiltration process that avoids the problems related to the processes reported in literature. This work can be divided into four parts:
In the first part, a new technique for the manufacturing of short fibre reinforced ultra-refractory ceramics via slurry infiltration is presented. This technique - based on the use of natural thickeners and planetary mixer - allowed the fabrication of solvent-free, thin (100 µm), flexible and easy to handle sheets suitable for the fabrication of homogeneous structures by overlapping and Hot-Pressing sintering. A large range of compositions - in terms of matrix and fibre volumetric content - from 0-100% was possible with this method. The short carbon fibre amount incorporated in the sheets ranged from 20-50% vol and the fibre length ranged from 3 to 5 mm. The matrix composition investigated with this technique consisted of ZrB2 90 vol. % - SiC 10 vol. %. Microstructure and mechanical properties of different samples were investigated. The microstructure observed with SEM showed a homogeneous fibre distribution, low interaction between fibre and matrix and no damage in the fibre surface due to the mixing process. From mechanical tests, increasing the fibre amount from 35 to 50 vol.% led to an improvement of the mechanical properties. In fact, the 4-point flexural strength ranged from 107 to 140 MPa, depending on the amount of carbon fibres. The same behaviour was observed during the works of fracture tests (WOF). The WOF ranged from 108 to 253 J/m2.
The second part was focused on the processing and oxidation resistance study of graded materials composed with three different layers, with an amount of 0-20-50 vol. % of short carbon fibres respectively. Three different samples were fabricated: 1) ZrB2- 85 vol %, SiC- 10, vol % Y2O3- 5 vol %, 2) ZrB2- 85 vol % MoSi2- 15 vol %, 3) SiC- 95 vol %, Y2O3- 5 vol %. The analysis of the morphology showed that all samples presented a homogeneous fibre distribution and a good interaction between the layers without the presence of defects. The same samples were subjected to oxidation at 1650 °C in air and the behaviour of each layer composition was evaluated. SEM and EDS analysis were carried out to investigate the oxidation grade and the oxygen diffusion. The sample containing MoSi2 resulted the most interesting with a mass loss of – 0.56 mg/cm2 lower than the -4.51 mg/cm2 and -7.05 mg/cm2 of the others.
In the third part, the preparation and properties of complex structures with changing matrix compositions and complex shapes starting from the new slurry infiltration process presented in this thesis were investigated. In detail, four different structures were analyzed. The first sample was obtained hot pressing the sheets with the pressure applied perpendicularly to the pile-up direction. The sheets with a vertical orientation sintered by Hot-Pressing and composed by a matrix of ZrB2-85 vol.%, SiC- 10 vol.%, reinforced with 40 vol.% of 3 mm long pitch carbon fibers (Z3-40-Z). It was characterized by a good fiber distribution and an interesting sheet bending that improved the mechanical properties with a value of flexural strength of 130 MPa. The second sample was a square plate composed by an external part of ZrB2-35 vol. %, SiC- 60 vol. %, Y2O3-5 vol. % reinforced with 40 vol. % of short pitch carbon fibres with a length of 3 mm (ZSY605) and an internal square part composed by a matrix of ZrB2-85 vol. %, SiC- 10 vol. %, Y2O3-5 vol. % reinforced with 40 vol. % of short pitch carbon fibres with a length of 3 mm (ZSY105). The two joined interfaces displayed good interaction, but defects originating from different stress distribution during sintering resulted in a low flexural strength of 45 MPa. To investigate the possibility to produce samples with a non-square shape, a cylindrical shape was fabricated (Z3-40-C). It was made by vertical sheets of UHTCMCs and sintered by Hot pressing, resulting in a final sample composed by a core of ZrB2-35 vol.%, SiC- 60 vol.%, Y2O3-5 vol.% reinforced with 40 vol. % of short pitch carbon fibres with a length of 3 mm and an external shell of ZrB2-85 vol.%, SiC- 10 vol. %, Y2O3-5 vol.% reinforced with 40 vol. % of short pitch carbon fibres with a length of 3 mm. In this case, the sample resulted with a good fibre distribution and good matrix interaction at the interface between the compositions. Finally, a cone was obtained by pressure-less sintering from the bending of a UHTCMC sheets composed by ZrB2-85 vol.%, SiC- 10 vol.%, Y2O3-5 vol.% reinforced with 40 vol. % of short pitch carbon fibres with a length of 3 mm, which demonstrated the feasibility to obtain complex shapes with different techniques.
In the fourth part, the tribological properties of Ultra-high-temperature ceramic matrix composites (UHTCMCs) have been examined to give a better understanding of this material class for friction applications. Two pads consisting of pitch carbon fiber (Cf) reinforced ZrB2-10%SiC were prepared with long fibers arranged in 0°/90° orientation (ZL) and randomly oriented chopped fibers (ZS). Microstructure, mechanical properties and tribological behavior on a self-designed dynamometer were tested, and the friction film was analyzed to examine relevant wear mechanisms. As disc, three different compositions were used: a carbon fiber reinforced carbon−silicon carbide disc (C/C-SiC), a Steel disc and a carbon fiber reinforced carbon disc (C/C). Tests conducted against a C/C-SiC disc showed a promising stable braking performance. However, wear and mechanical stress were high and could be tolerated only with long carbon fiber reinforcement. In the case of the test conducted against the Steel disc - most specifically for the ZS material, it was observed that abraded material from the disc formed a stable friction film by fusing together harder pad particles with abraded steel dust. This reduced wear and stabilized the braking performance. Finally, the interaction between the UHTCMCs materials and the C/C disc showed an interesting low coefficient of friction without pads damage and with an operating temperature of ≈ 550 °C.2023-01-01T00:00:00ZInnovative materials for miniaturized sample preparation techniques, environmental and food packaging applications
https://hdl.handle.net/1889/5397
Title: Innovative materials for miniaturized sample preparation techniques, environmental and food packaging applications
Authors: Fornari, Fabio
Abstract: In Europe approximately 2.5 billion tons/year of waste are produced, deriving from households, agriculture, livestock, and industrial activities. Moreover, it has been estimated that, in 2011, 20% of the manufactured food went to waste, for a total of nearly 130 thousand tons. The recent increase in worldwide population produced an acceleration of economic development, intensifying manufacturing activities to satisfy the worldwide demand, thus requiring a great effort to food system and environmental policies in keeping pace.
Economic processes still based on a linear approach constitute a serious threat to the environment, sustenance of food system, and biodiversity. This is not only for the amount of waste that is produced, but also for the emission into the environment of substances and biological entities that alter the equilibrium in ecosystems and pose risks to human health. Up till now only a reduced portion of contaminants are monitored by competent authorities, whereas a lot of emerging contaminants are not under control due to the lack of knowledge and absence of specific regulations.
The European Union has enacted a series of directives and regulations that aim at improving the waste management policies and regulating certain pollutants for a more sustainable development. Examples are represented by the Directive EU 2018/851 for waste management, the Regulation EU 2019/1021 for limiting the emission of persistent organic pollutants (POPs), and the NORMAN project (started in 2005) to gain knowledge towards emerging contaminants.
In December 2019, the European Commission announced the so-called European Green Deal, comprising a series of policies aimed at meeting the climate objectives of the next future. The main figures of merit include the reduction of European greenhouse gasses emission by 55% before 2030 (with respect to 1990 levels) and the achievement of the climate-neutrality by 2050 to create a toxic-free environment. Along with the regulation of certain pollutants, as among which the POP regulation previously mentioned, it is fundamental to re-design economic processes to make them more circular.
Within this frame of reference, the advancements in the field of Materials Science might help in reaching the climate requirements set by the European Green Deal. This Thesis, in the framework of the PhD Program in Materials Science and Technology (Department of Chemistry, Life Sciences and Environmental Sustainability; University of Parma) aims at exploring the potential of novel materials for applications in the field of Analytical Chemistry, and on materials devoted to packaging, and environmental applications.
The development of analytical methodologies able to keep pace with very low contamination thresholds and fast-changing regulations is of vital importance, not only to comply to the regulating authorities, but to enable people in making decision with confidence. Additionally, the advent of Green Analytical Chemistry has posed new challenges in the development of analytical methods focusing on their impact on the environment. In this context, novel sorbent materials and devices for miniaturized sample treatment offer a plethora of possibilities for the development of methods with enhanced selectivity, sensitivity, and lower detection limits, reducing both the amount of sample required for the analyses the use of organic solvents. The chapters dedicated to this topics explores the capabilities of carbon nanotubes and magnetic composites for the miniaturized extraction of emerging contaminants and priority pollutants from water samples, paying attention to the Green Analytical Chemistry principles.
Along with the introduction of technologies with a reduced climate footprint, it is fundamental to implement strategies able to remediate contaminated ecosystems to effectively depollute the environment. In this context, bioremediation provides a sustainable and cost-effective approach for the removal of pollutants from contaminated ecosystems. Along with the metabolic activity of living organisms, the use of carbon-based materials could simultaneously aid the removal of certain pollutants, promoting the survival of microbial communities, to increase their bioremediating effect. The chapter dedicated to this topic explores the potential of carbon-based materials and aquatic plants in reducing the concentration of a class of organic micropollutants in sediments, for the development of a green bioremediation technology.
Finally, the reduction of waste or its reuse as raw material for novel applications are fundamental to decrease the overall climate footprint. Within this frame of reference, the manufacturing of active food packaging could help in preventing food waste. The impact of active food packaging materials could increase even more dramatically if the active ingredients exploiting antibacterial and antioxidant activity can be derived from natural and renewable sources, like in the case of essential oils and their active components, substituting conventional food preservatives. However, efforts have to be made in making such substances more appealing for technological applications. About this topic, one chapter is dedicated to the use of chemometrics for predicting the formation of cocrystals based on the active components of essential oils, to extend their applicability in fields ranging from agriculture to food packaging. On a closing note, the fabrication of biocomposites to be used as building materials or secondary packaging materials could offer a valuable solution for the valorization of waste in the context of circular economy. This process could become even more interesting when the feedstock is represented by toxic wastes, integrating bioremediation and the production of safe biocomposite materials in a single step. In this context, a chapter is dedicated to the development of a biocomposite material starting from cosmetic waste.2023-01-01T00:00:00ZSpectroscopic study of phase transitions and polymorphism in molecular crystals
https://hdl.handle.net/1889/5396
Title: Spectroscopic study of phase transitions and polymorphism in molecular crystals
Authors: Ferrari, Elena
Abstract: This spectroscopic study is dedicated to molecular crystals with polymorphism and phase transitions induced by temperature or pressure. Charge transfer cocrystals, single component organic semiconductors and liquid crystals were investigated by IR and Raman spectroscopies, to achieve both relevant microscopic parameters and information on the crystal packing.2023-01-01T00:00:00ZResponsive materials via vinylogous urethane chemistry and host-guest interactions.
https://hdl.handle.net/1889/5341
Title: Responsive materials via vinylogous urethane chemistry and host-guest interactions.
Authors: Soavi, Giuseppe
Abstract: Responsive materials exhibit remarkable macroscopic responses upon specific external stimuli, providing solutions to today’s progress challenges. The growing demand for more sustainable technological development is becoming a necessity in many fields, prompting the scientific community to push the boundaries of the progress even further. The goal is to provide ever more performing materials with a well-defined end-of-life management strategy. For this purpose, polymers incorporating dynamic bonds or weak interactions represent the best tradeoff between a well-consolidated platform of synthetic polymers and a new generation of materials, more durable or, alternatively, able to easily be reintroduced in a circular economy.
The envisaged dynamicity in these new polymeric materials can be imparted by the incorporation of weak interactions, such as hydrogen bonding, host-guest interactions, metal-ligand coordination, or more robust reversible covalent bonds, giving arise to covalent adaptable networks.
This PhD thesis reports design, synthesis, and properties of responsive materials for application in a wide variety of fields: from promoter for interlayer adhesion in composites until the tailoring of new recyclable insulators, as well as the generation of new self-reporting polymers.
The main investigated covalent adaptable networks (CANs) were those based on the vinylogous urethane chemistry. In Chapter 2, a systematic study of the thermal, rheological, and mechanical properties of phenoxy-based vitrimers was presented. They relied on the transamination of vinylogous urethane. The aforementioned vitrimers were obtained by a two-steps synthesis from a commercial phenoxy resin via partial conversion of hydroxyl groups to acetoacetates (AcAc), followed by network formation by reaction with a 20% molar excess of m-xylylendiamine (XYDIA) as crosslinker. Three different vitrimers with variable crosslinking density were obtained by tuning the density of AcAc moieties along the phenoxy resin scaffold (5%, 10% and 15% conversion of hydroxyl groups). The conversion of linear polymers to dynamic crosslinked networks was confirmed by DMTA and rheology measurements, followed by stress relaxation tests to investigate the kinetics of bond exchanges. The calculation of activation energies for the relaxation process showed the negligible existence of additional relaxation modes, compared to the relaxation due to vinylogous bond exchanges, especially for higher vinylogous urethane crosslinking %. Tensile tests as a function of reprocessing cycles revealed an increase of the maximum elongation and stress at break and proved the good recyclability of the vitrimers. Enhanced adhesive properties compared to pristine phenoxy resins were demonstrated, including the possibility to thermally re-join the assembly after its mechanical failure. Finally, the solvent-free preparation of vitrimer was exploited for 5% crosslinked vitrimer via melt reactive blending, providing a valuable alternative to the less environmentally sustainable synthesis in solution.
As adhesion promoters for carbon fiber-reinforced epoxy matrices, phenoxy resins and their functionalized counterparts with acetoacetate units were studied in Chapter 3. Multilayer objects are known to be susceptible to failure in a variety of modes in the thickness direction since their main weakness is the fiber/matrix interface. This phenomenon is called interlaminar delamination. Delamination is detrimental for composites because it is capable to worsen drastically their mechanical properties, therefore the enhancement of the interlaminar adhesion becomes a crucial factor. Our purpose was to address this issue playing on different factors, including the type of bonds introduced to the polymeric component of the composite, the topology of connection between the epoxy resin of the composite, the polymeric chains of the adhesion promoter and the ability to dissipate mechanical stresses.
We proposed the application of two commercial phenoxy resins with different average molecular weights (PKHB and PKHP). They are amorphous thermoplastic mixable with epoxy resin, ensuring mutual diffusion of the relevant polymeric chains, hence able to give arise to a physical entanglement, which is one of the enhancing factors of interlaminar adhesion. A further step was the introduction of a properly functionalized phenoxy resin (PKHP-AcAc 20% and PKHB-AcAc 20%) It was thought that the presence of acetoacetate units could be crucial for the formation in situ of a vitrimeric network, once crosslinked with the amines of the crosslinker present in the epoxy matrix of the composite. The interlaminar adhesion would also be enhanced due to the development of a stitch-stitch topology entanglement, which entails the diffusion of acetoacetylated phenoxy chains into the epoxy networks of the two adherends, crosslinking into a third covalent adaptable-based polymer, in topological entanglement with both pre-existing polymer networks. The third polymer network acts like a molecular suture. To separate, at least one of the three polymer networks must break. These polymeric adhesion promoters were applied to a commercial prepreg (DT120), as thin films or dissolved in a suitable solvent. Mechanical characterization of bonded joint was evaluated in single lap shear configuration and fracture toughness configuration.
The application of vitrimeric networks as recyclable alternative to the wide-spread thermosets-based insulators was analyzed in Chapter 4. The replacement of crosslinked polyethylene is becoming a prominent issue to relieve the environmental imprint due to the end-life high voltage cables disposal. Thereby, the design of a new vitrimer which intrinsically presenting high free volume was identified as the best solution to address this problem. As building blocks, triptycene-based molecules were chosen thanks to the ability to create interstitial space around them.
We were interested to develop a vinylogous transamination-based CAN, attainable by the reaction between aminic crosslinkers and acetoacetylated amorphous thermoplastics. Several routes were explored to generate triptycene-based phenoxy resins, in which the triptycene unit ideally replaces the bisphenol A. The synthetic attempts concerned the reactions between the diglycidylated triptycene (9,10-benzenoanthracene-1,4-diol diglycidyl ether) with the hydroquinone homologous (9,10-benzenoanthracene-1,4-diol) or primary amines. The polymerization experiments showed only the formation of oligomers even varying the conditions in terms of temperature, base/catalyst, and time.
To overcome the limited growth of the polymeric chain, a more straightforward approach was also studied, thus avoiding the challenging synthesis of the linear polymer and its functionalization. The 9,10-benzenoanthracene-1,4-diol was properly acetoacetylated, becoming a suitable raw material for the reaction with tris(2-aminoethyl) amine (TREN), to generate a vinylogous urethane-based vitrimer.
In Chapter 5 cavitand-based host-guest chemistry has been tested to produce self-diagnostic polyurethanes. The purpose was to introduce a supramolecular unit in a polyurethane matrix, designed for the non-destructive and non-invasive detection of micro–damages. A nonemissive supramolecular complex was prepared and embedded into a rigid PU for the assessment of the mechanical-induced stress in the polymeric matrix. Indeed, internal stress detection in polymers used for high performance applications is critical in preventing structural failures when the integrity is crucial. The investigated system is based on the complex between a fluorescent N-methyl pyridinium salt (PyPyr-OH) and a tetraphosphonate cavitand, CavPOPh (mono -OH). Both the host and the guest have a peripheral OH group to be embedded in the polyurethane matrix. In the complex the radiation emitted by the fluorophore is immediately quenched upon the complexation and theoretically restored by the mechanical stress-induced dissociation of the supramolecular complex. The photophysical behavior of the host-guest probe was investigated. Absorption and fluorescence emission titrations were performed, proving the emission quenching of PyPyr-OH in the complex. The spectroscopic characterization in solution was preparatory to perform investigation when the system was chemically linked to a polyurethane matrix. A polyol, in which a solution of the probe was dispersed, was polymerized with an isocyanate. The non-fluorescent behavior of the complex was retained in the polymer matrix. Tensile tests were carried out on dog-bone specimens to evaluate how the fluorescent response of the probe changed as stress level increased.
The fluorescence of PyPyr-OH remained quenched in the specimens after a mechanical stress application.
Finally, the effect of the host-guest interaction between cucurbit[8]uril (CB[8]) and a model trimethine indocyanine (Cy3) on dye spectral properties and aggregation in water was investigated in Chapter 6. Inclusion of polymethine cyanine dyes in the cavity of macrocyclic receptors is an effective strategy to alter their absorption and emission behavior in aqueous solution. Solution studies, performed by a combination of spectroscopic and calorimetric techniques, indicate that the addition of CB[8] disrupted Cy3 aggregates, leading to the formation of a 1:1 host-guest complex with an association constant of 1.5×106 M-1. At concentrations suitable for NMR experiments, the slow formation of a supramolecular polymer was observed, followed by precipitation. Single crystals X-ray structure elucidation confirmed the formation of a polymeric assembly with 1:1 stoichiometry in the solid state.2023-01-24T00:00:00Z