A reutilização e a reciclagem de produtos em final de vida útil são cruciais quando se consideram os problemas ambientais causados pelos resíduos gerados. Vitrocerâmicas podem ser produzidas a partir da cristalização controlada do vidro. Nesta dissertação, vitrocerâmicas são obtidas a partir de pó de vidro oriundo de para-brisas descartados. Adicionalmente, junto a esse pó de vidro, foram utilizadas diferentes concentrações de pentóxido de nióbio para atuar como agente nucleante e facilitar a cristalização. Foram obtidas quatro composições: sem Nb2O5 e com 5%, 10% e 15% de Nb2O5. No processo de produção foram adotadas duas temperaturas de sinterização, 700°C e 800°C, e dois tempos de sinterização, 1 h e 3 h. Foi realizado um estudo das propriedades microestruturais, mecânicas e elétricas, além das propriedades de densidade, porosidade e absorção de água do material produzido. Adcionamelte, foram avaliadas as características do pó de vidro utilizado para sua produção. Para caracterização do pó de vidro, foram feitas a Análise Granulométrica via úmido, a Fluorescência de Raios X e as análises térmicas TG e DTA. Já para as vitrocerâmicas foram realizadas: DRX, Método de Arquimedes, MEV, Retração Linear do diâmetro, Resistência à Flexão Biaxial e Tração Diametral. Também foram determinadas a Constante Dielétrica e a Condutividade Elétrica das vitrocerâmicas produzidas. Constatou-se que o uso de partículas menores que 1,2 μm, consideradas muito pequenas, prejudica a densificação do material, enquanto que a distribuição de tamanhos larga favoreceu o empacotamento das partículas. A utilização de maiores temperaturas de sinterização e de maior tempo de sinterização aumentam a porosidade e reduzem a densidade. Em contrapartida, a temperatura de 800°C permite a cristalização sem o uso de agente nuclenate. A adição de Nb2O5 promoveu a cristalização do material já em 700 °C, fato que não ocorre sem sua adição a esta temperatura. Ele também permitiu a formação da Isoleucita, estrutura perosviskita que apresenta boas propriedades ferroelétricas. Assim, as vitrocerâmicas com as maiores concentrações desse pentóxido obtiveram as maiores constantes dielétricas e condutividades elétricas, sendo de 63,597 e 4,283 μS/m, respectivamente, ambas encontradas nas vitrocerâmicas produzidas com 15% de Nb2O5, sinterizadas em 700°C por 1 h. Por outro lado, houve perda de resistência mecânica tanto na flexão biaxial quanto na tração diametral, sendo observada uma queda de 34,83 MPa para 14,34 MPa na resistência à Flexão biaxial. Apesar disso, as vitrocerâmicas produzidas ainda apresentam resistências superiores a de concretos estruturais. Em relação ao desvio padrão das propriedades mecânicas, houve redução de forma expressiva com a adição do agente nucleante. Os resultados obtidos permitiram concluir que é viável a produção de vitrocerâmicas a partir do vidro de para-brisas, tendo sido obtidas propriedades mecânicas e elétricas satisfatórias.

Spinal cord injury (SCI) is an extremely serious condition which leads to a drastic decrease on patient mobility. Neural tissue engineering (NTE) has been trying to promote solutions by combining different materials such as polymers, cells, and specific architectures to assist regeneration of the injured tissue. This work objective is the conceptualization and manufacture of a multilayer multifunctional nanocomposite and study its cytocompatibility with neural stem cells (NSCs) envisioning NTE application. For that, vertically aligned carbon nanotubes (VACNTs) were grown by thermal chemical vapor deposition onto a silica substrate (SiO₂). Then, as-grown VACNTs were transferred onto polydimethylsiloxane (PDMS) creating a VACNT-PDMS layer with a stable interface, maintaining CNT alignment, confirmed by scanning electron microscopy. By transferring VACNTs onto this PDMS layer, flexibility and handling of the nanocomposite was obtained eliminating some problems presented by native VACNTs when coupled with their native substrate which is rigid. However, the nanocomposite VACNT/PDMS is inert and requires a component that mimics the neural microenvironment. So, a third polymeric layer was needed. To integrate this layer. VACNT and PDMS underwent a series of surface treatments (i.e., ultraviolet coupled with ozone (UV/O3) and oxygen (O₂) plasma) leading to changes on wettability and surface energy assessed indirectly by water contact angle (WCA) and attenuated-total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used to identify chemical changes post treatments. O₂ plasma treatment led to most significative change in wettability with the introduction of silanol (Si-OH) groups on PDMS and carboxylic (COOH) groups on CNTs. For the third layer two strategies were employed. The first (S1) relied on the creation of a hydrogel composed of gelatin and alginate methacrylamide (GelMA and AlgMA) capable of photo- and -ionic crosslink. The hydrogel showed to support NSC proliferation and viability, however, when transferred to the VACNT-PDMS layer the hydrogel would delaminate. The alternative solution (S2) consisted in the crosslink of gelatin and alginate induced by 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide coupled with N-hydroxysuccinimide on top of a previously silanized VACNT-PDMS layer. This was a successful approach creating a nanocomposite capable of sustaining adhesion and proliferation of NSCs. Furthermore, with immunocytochemical staining it was possible to observe neuronal differentiation. The results obtained demonstrated that the conceptualized multilayer multifunctional nanocomposite favors proliferation and differentiation of NSCs, making it an electro-responsive platform and a candidate to be electrically stimulated for neural tissue engineering.

Gadolinium aluminate (GdAlO3) and its calcium-substituted derivates (Gd1-xCaxAlO3-δ; x = 0.05, 0.10 and 0.15) are prepared via one-step mechanochemical processing of simple oxide precursors at ambient temperature. The mechanochemical reaction is accompanied by Gd2O3 phase transformation. The as-prepared and sintered materials are characterized by X-ray diffraction and scanning electron microscopy. Sintering at 1450°C resulted in relatively porous ceramics (with density <90% of theoretical value) except Gd0.85Ca0.15AlO3-δ. The electrical conductivity of the sintered samples was investigated by impedance spectroscopy at 350-1000°C in air. Undoped GdAlO3 ceramics exhibit low conductivity, ~10-6 S/cm at 800°C. Acceptor-type substitution of 5 at.% of gadolinium by calcium results in ~3 orders of magnitude increase in both total and bulk conductivity associated with a substantial enhancement in oxygen-ionic transport. Further doping has a limited effect on the electrical transport properties, and electrical conductivity remains nearly composition-independent in the range x = 0.05-0.15. Grain boundaries are demonstrated to have a significant contribution to the total resistivity of prepared calcium-substituted Gd1-xCaxAlO3-δ ceramics with grain sizes in the range of 1.1-1.5 μm.

Manganites present fascinating physical properties which can be tuned according with the method and processing conditions. In this work, Ca1-xPrxMnO3 (x = 0.03, 0.06, 0.1) materials were prepared by the Laser Floating Zone technique (LFZ), under different pulling rates (25 to 100 mm/h) in air and argon atmospheres, obtaining fibres with 2.5 mm diameter. Such processing conditions promote the formation of secondary phases in (Ca,Pr)MnO3 fibres. The formation of the perovskite phase at slow pulling rates was observed. The effects of the LFZ conditions on the phase composition, microstructural features, electric and magnetic properties of as-grown and thermally post-processed fibres are analysed and discussed. The results indicate that the LFZ processing conditions have a great impact on the electrical and magnetic properties of the prepared manganites. Secondary phases increase with pulling rate and the effects imposed by their formation can be minimized by additional heat treatment at high temperatures, improving the electric conductivity and magnetization of the heattreated fibres.

The present investigation was carried out to develop dense ceramics of silicon nitride-titanium nitride (Si3N4–TiN) with low electrical resistivity and excellent mechanical properties. The objective was to employ these ceramics as substrates of conductive diamond electrodes produced by chemical vapor deposition (CVD) for electrochemical applications. TiN powder was added to a Si3N4 matrix powder composition at varying volume fractions (21–30%). Disc-shaped samples were fabricated by mixing and pressing the powders, followed by pressureless sintering. The crystalline phase composition and microstructure were analyzed using X-ray diffraction and scanning electron microscopy, while the electrical resistivity was measured with a four-point probe configuration. The composites transitioned from insulating to conductive behavior between 23 and 27%vol TiN. The developed compositions displayed superior hardness, fracture toughness, elastic modulus, and thermal conductivity compared to the Si3N4 matrix. Notably, the composition containing 30%vol TiN displayed noteworthy properties, including a hardness value of 16.1 GPa, fracture toughness of 7.0 MPa m1/2, and electrical resistivity of 8.9 × 10−1 Ω cm. Finally, the proof-of-concept experiment demonstrated the potential of Si3N4–TiN composites as robust and electroconductive substrates for depositing conductive diamond electrodes. This was achieved by successfully depositing conductive diamond films on Si3N4–TiN substrates using the Hot Filament Chemical Vapor Deposition (HFCVD) technique.

The main objective of this work was to assess the possibility of the enhancement of the electrocatalytic activity of Sr(Ti,V)O3-δ fuel electrode components for high-temperature solid electrolyte cells by introducing Ni into the B-sublattice of the perovskite structure with in-situ nanostructuring under operation conditions by exsolution. The work was motivated by the drawbacks of commonly used cermet Ni-YSZ cermet anodes such as long-term microstructural degradation and intolerance to redox changes, sulfur poisoning and carbon deposition. Strontium titanate-vanadates were considered as suitable ceramic components stable under fuel electrode operation conditions and with prospects for sulfur and carbon deposition tolerance, while nano-dispersed Ni was expected to enhance the electrocatalytic activity while avoiding the disadvantages of Ni-YSZ cermets. High-energy mechanochemical route in combination with thermal treatments under controlled reducing atmosphere were employed for the preparation of selected materials with a nominal composition Sr1-xTi1-y-zVyNizO3 (x = 0-0.04, y = 0.2-0.4, z = 0.02-0.12). Detailed analysis of the phase formation in this system revealed that comparatively high thermal treatment temperature (1200°C), required to eliminate the undesired insulating Sr3(VO4)2 intermediate phase in the course of synthesis of Sr(Ti,V)O3 perovskites, results in a segregation of Ni in the form of poorly dispersed submicron metallic particles. Prepared Sr1-x(Ti,V)O3-δ-Ni ceramic materials exhibited a moderate thermal expansion coefficients compatible with that of YSZ solid electrolyte. The electrical conductivity was found to increase with increasing vanadium content in the perovskite phase, while the nominal A-site deficiency had an opposite effect. The electrochemical impedance spectroscopy studies revealed a rather poor activity of Sr1-x(Ti,V)O3-δ-Ni porous electrodes for hydrogen oxidation reaction. This was ascribed to insufficient intrinsic electrocatalytic activity of Sr1-x(Ti,V)O3 perovskites, low expected ionic conduction in these phases, and segregation of Ni particles in the course of synthesis. It was demonstrated that the electrochemical performance of these electrodes can be substantially improved by the infiltrations of gadolinia-doped ceria as oxygen-ion conducting component and small extra amounts of well-dispersed Ni as an electrocatalyst

Bulk samples of the ternary TiO2–V2O5–P2O5 system have been synthesized by the melt-quenching technique. X-ray diffraction and scanning electron microscopy with energy-dispersive spectroscopy were applied for structure and composition identification, whereby the presence of NASICON and Rutile type structures were identified. Dielectric properties were measured using impedance spectroscopy, at frequencies from 10 mHz to 1 MHz, and temperatures between 40 °C and 190 °C. Different relaxation models were applied to fit the experimental data. The electrical conductivity and activation energy of the glass matrix and crystalline particles were calculated and correlated to the structure of the material. The impedance spectra were analyzed with the complex dielectric modulus and the Distribution Function of Relaxation Times method to corroborate and complement the results of equivalent circuit models. The grain conductivity was shown to reach 10-1 S/m at 190 °C for the NASICON sample.

The path to decarbonization requires a substantial reduction of global greenhouse gas emissions. This challenge encourages the research of alternative energy sources. Fuel cell devices are one of the most promising hydrogen-based technological solutions for energy conversion. Nowadays, proton-conducting polymer electrolyte membrane fuel cells are typically used for temperatures below 100 °C, while solid oxide fuel cells are used between 800 and 1000 °C. Intermediate temperature fuel cells to bridge this temperature gap remain under development since new capable electrolytes operating between 200 and 500 °C are still needed. Nonetheless, this class of intermediate fuel cells can potentially overcome some of the challenges in low and high-temperature fuel cells nowadays, such as avoiding non-metal catalysts, decreased costs, and higher longevities. Inorganic solid acids such as KH₂PO₄, CsHSO₄, CsH₂PO₄, and their derivations, have drawn considerable attention due to their high protonic conductivity under temperatures ranging from 150 °C to 300 °C. The electrical conductivity of CsH₂PO₄ shows a steep increase around 235 °C due to a phase transition from monoclinic to a superprotonic phase with cubic symmetry. Above this temperature, the conductivity exceeds 10⁻²S/cm, making it a promising material for electrolytes in fuel cells. However, the material is dehydrated beyond this temperature when the atmosphere is not sufficiently humidified. In this respect, two different approaches can be made to extend the narrow operating temperature range of this material: 1) improve the thermal stability of the material to restrict dehydration; 2) increase the conductivity of the material below the superprotonic phase transition. In the present study, we studied the CsH₂PO₄ properties as a function of grain size and processing route. The particle size was decreased by wet ball-milling, while the grain growth was achieved by the cold sintering process. The electrical conductivity of the superprotonic phase was found to be independent of the grain size, which suggests that transport through grain interiors is the dominant process. As for the electrical conductivity of the monoclinic phase, the brick layer analysis revealed that, for the as-grown and ball-milled samples, the conductivity is governed by the conduction pathways along the parallel grain boundaries. Additionally, we prepared several CsH₂PO₄ – CsH₅(PO₄)₂ composite systems to enhance the electrical conductivity of the electrolyte below the superprotonic phase transition. The addition of polycrystalline CsH₅(PO₄)₂, even in small amounts (4 wt %), has a significant impact on the electrical conductivity of the electrolyte. After the first reverse transition from the superprotonic state to the monoclinic phase, the proton conductivity was found to increase by over one order of magnitude in the low-temperature phase (below 235 °C). Conversely, at higher temperatures, the superprotonic nature of the sample remained unchanged, and the conductivity was found to be very close to the pure CsH₂PO₄ material. On adjusting the amount of CsH₅(PO₄)₂, we found that 26 wt % was the optimal quantity to provide superprotonic conductivity over a wide temperature range (160 – 270 °C). The enhanced conductivity at lower temperatures could be explained by forming a liquid CsH5(PO₄)₂ layer along with the interfaces and grain boundaries of the material. Above 151 °C the melting of this highly conductive CsH₅(PO₄)₂ phase was found to assist the proton conductivity of the composite.

This work seeks possibilities for advancing the thermoelectric oxide technology by exploring a module design involving materials produced by laser processing. A tubular thermoelectric generator of modular construction with functional elements located parallel to a pipe-shaped heat source was designed and manufactured. The p- and n-type counterparts based on Ca3Co4O9 and Ca0.95Pr0.05MnO3 were grown by the laser floating zone (LFZ) technique, ensuring highly dense microstructures and giving the unique possibility for fast and crucible-free processing of the legs with desirable geometry. The detailed structural and microstructural characterization indicated that the LFZ processing should be accompanied with an additional thermal annealing step to equilibrate the phase composition. Although the measured electrical performance was found close to or slightly higher compared to that of similar materials produced by other routes, it was still notably suppressed by the remaining phase impurities affecting the module's output. The maximum observed power output of the module containing 12 Ca3Co4O9/Ca0.95Pr0.05MnO3 thermoelectric couples reached up to 20 mW at a temperature gradient of 389 °C and a hot side temperature of 525 °C. The contribution of various factors to the overall performance was analyzed. The results suggest that the power output could be significantly enhanced by decreasing the contact resistance at the cold side and proper optimization of the LFZ processing conditions.

The synthesis of crystalline one-dimensional polymers provides a fundamental understanding about the structure− property relationship in polymeric materials and allows the preparation of materials with enhanced thermal, mechanical, and conducting properties. However, the synthesis of crystalline one-dimensional polymers remains a challenge because polymers tend to adopt amorphous or semicrystalline phases. Herein, we report the synthesis of a crystalline one-dimensional polymer in solution by dynamic covalent chemistry. The structure of the polymer has been unambiguously confirmed by microcrystal electron diffraction that together with charge transport studies and theoretical calculations show how the π-stacked chains of the polymer generate optimal channels for charge transport.