The present thesis work is focused on the low-temperature transformation of food industry wastes like mussel shells into nanocrystalline ions-substituted hydroxyapatite powder, having similarities with natural bone apatite, on the consolidation of such powder by cold sintering, and on the physicochemical characterization of the raw materials, synthesised powders and sintered pellets. Nonetheless the evaluation of the mechanical and biological properties was carried out to address cold sintered bodies to possible scaffolds for bone tissue engineering applications. Mussel shells, like other biogenic source of calcium carbonate/phosphate, have the attractive of being a “zero”-cost raw material because they are a waste, but also of having trace elements (Mg, Na, Sr, etc.) which, if found in a bioceramic, have a positive effect on the biological properties. Therefore, mussel shell-derived hydroxyapatite could resemble the mineralized bone tissue, being natural apatite nanometric, ion substituted and with low crystalline tenor. In the first part of the manuscript, two production methods were explored: mechanochemistry and dissolution-precipitation synthesis. Mechanochemistry was carried out at room temperature by directly mixing crushed mussel shells with phosphoric acid in a ball mill. Nanocrystalline multi-ions substituted hydroxyapatite was produced after 4 h of milling and drying at 150°C. Conversely, dissolution-precipitation synthesis was carried out in two steps: the dissolution of crushed mussel shells by adding phosphoric and chloric acid occurred at room temperature, whereas the precipitation of calcium phosphates induced by soda solution, occurred at 45°C. Dissolution-precipitation was further implemented to produce a homogeneous composite material in a single-step by introducing chitosan (in a 2/5/10 wt%) during the dissolution step. The idea was to produce a composite material able to mimic the natural bone tissue composition. In the second part of the manuscript, cold sintering was investigated for the consolidation of the synthesised hydroxyapatite and hydroxyapatite-based composites at a maximum temperature of 200 °C to avoid phase transformation, limit grain growth and preserve the osteoconduction of the bioceramic materials. The effect of the main process parameters such as solvent amount, pressure, temperature and holding time was discussed. Pressure-solution creep and plastic deformation were pointed out as the fundamental consolidation mechanisms in cold sintering, the pressure playing the major role. With a synergistic combination of pressure (600 MPa), temperature (200°C) and liquid phase (20 wt%) it was possible to consolidate hydroxyapatite above 80% relative density in only 15 min. Furthermore, pressure and temperature act a complementary agent during cold sintering. In fact, it was possible to consolidate nanometric HAp and HAp/chitosan composites above 90% relative density by increasing the applied pressure up to 1.5 GPa at room temperature. The mechanical properties of cold sintered pellets were investigated, and resulted in a flexural bending strength and Vickers microhardness, respectively, of 45 MPa and 1.1 GPa for pure hydroxyapatite and of 55 MPa and 0.8 GPa for HAp/chitosan composite. In the frame of bone tissue engineering applications, cold sintered bodies were also preliminarily tested in vitro to establish their bioactivity, their cellular viability through cytotoxicity assessment, and the ability to sustain cells adhesion, osteogenic differentiation. And extracellular matrix mineralization.

Cold sintering process (CSP) je nová metoda pro slinování keramik a skel. Tato metoda vede ke snížení teploty (

[pt] A busca pela diminuição da dependência de combustíveis fósseis faz com que a ciência avance, diariamente, na utilização de combustíveis ecológicos, como o H2. Uma das formas de sua obtenção é através da fotocatálise. Esse processo consiste em uma reação catalítica com o uso de energia, na forma de luz. A fotólise da água é amplamente utilizada, principalmente com luz solar como fonte luminosa, que é abundante e reduz os custos de sua produção. O desempenho desta reação depende da posição das bandas de condução (BC) e valência (BV) do fotocatalisador. O TiO2 é utilizado como fotocatalisador em diversas reações, inclusive para a produção de H2. Os fotocatalisadores utilizados na forma de pós nanométricos apresentam dificuldade de separação após a reação. A transformação do pó em um material compacto é uma alternativa para retirá-lo do meio reacional evitando perdas e custos com separação. Desse modo, a compactação do pó é uma alternativa para facilitar sua reciclagem. O principal método de sua produção é pelo processo de sinterização, que envolve temperaturas elevadas (geralmente, 75 por cento do ponto de fusão do material) e longo tempo, podendo durar até dias. Para diminuir os gastos energéticos, o processo de sinterização a frio é uma opção, que consiste na densificação do material com uso de pressão e um solvente (aquoso ou não) e, temperaturas de sinterização de até 500 Graus C. O objetivo do estudo consistiu na produção de pastilhas de TiO2, comercial, e P25, através de uma variação do método de sinterização a frio, onde aplicou-se a pressão no pó, junto com o solvente, antes de seu tratamento térmico. As pastilhas produzidas foram caracterizadas pelas técnicas de TGA/DSC, XRD, MEV, CV, e DRS. / [en] The quest to reduce dependence on fossil fuels makes science advance, daily, in the use of ecological fuels, such as H2. One of the ways to obtain it is through photocatalysis. This process consists on a catalytic reaction using energy, in the form of light. Water photolysis is widely used, mainly with sunlight as a light source, which is abundant and reduces production costs. The performance of this reaction depends on the position of the conduction (CB) and valence (VB) bands of the photocatalyst. TiO2 is used as a photocatalyst in several reactions, including the production of H2. Photocatalysts used in the form of nanometric powders have difficulty in separating after the reaction. The transformation of the powder into a compact material is an alternative to remove it from the reaction medium, avoiding losses and costs with separation. Thus, the compaction of the powder is an alternative to facilitate its recycling. The main method of its production is through the sintering process, which involves high temperatures (generally 75 percent of the material s melting point) and a long time, which can last up to days. To reduce energy costs, the cold sintering process is an option, which consists of densifying the material using pressure and a solvent (aqueous or not) and sintering temperatures of up to 500 C degrees. The aim of the study was the production of commercial TiO2 and P25 pellets, through a variant of the cold sintering method, where pressure was applied to the powder, with the proper solvent, before the heat treatment. The pellets produced were characterized by the techniques of TGA/DSC, XRD, SEM, CV, and DRS.