This master’s thesis deals with the possibility of manufacturing gas analyzer by Rapid Prototyping Technology and it with the method Direct Metal Laser Sintering. The theoretical part describes the current production of component in the Frentech Aerospace LLC and innovation with the DMSL method in the company Innomia Furthermore JSC. Then an analysis of the principle of single methods Rapid Prototyping, especially the method of Direct Metal Laser Sintering, is implemented. The aim of the experimental part is to compare the mechanical properties and material structures, conventional metallurgy and powder metallurgy. The thesis also contains a technical-economic evaluation comparing the manufacture of mechanical part by conventional and advanced additive technology.

Les procédés de fabrication additive permettent, à partir d’un fichier de CAO, la fabrication de pièces complexes sans outillage, dans des délais de développement très courts et avec une grande flexibilité. Parmi les procédés de fabrication additive employés avec les polymères, le frittage laser de poudre est le plus utilisé. Ces travaux de thèse sont consacrés à l’étude et la compréhension des mécanismes fondamentaux impliqués lors du procédé de frittage laser de poudres de polyamide 12. Au cours du procédé de frittage laser de nombreux paramètres interviennent. Ainsi l’énergie fournie à la poudre par l’intermédiaire du rayon laser dépend de la puissance de celui-ci, de la vitesse de balayage et de l’espacement entre deux balayages successifs. De plus, le matériau subit un cycle thermique sévère : avant d’être frittée, la poudre est préchauffée. Puis, dans le bac de fabrication, la poudre non frittée ainsi que les pièces séjournent pendant toute la durée de la fabrication à des températures élevées. Cette histoire thermique entraîne un vieillissement et donc une modification des propriétés de la poudre ce qui complique sa réutilisation. L’influence de ces différents paramètres sur la microstructure et les propriétés mécaniques des pièces finales a été mise en évidence. De plus l’utilisation de différentes poudres de polyamide 12 a permis l’identification des paramètres-clés de la matière.Le frittage laser des polymères semi-cristallins est régi par plusieurs mécanismes fondamentaux : la fusion des particules de poudre, l’interdiffusion des chaînes macromoléculaires aux interfaces, la coalescence des particules fondues, la densification et enfin la cristallisation. L’étude et la modélisation de la cristallisation ont été effectuées pour l’un des polyamides 12 employés au cours de cette thèse. De ce traitement théorique ont pu être déduits les temps de maintien du polymère à l’état fondu au cours du procédé. Dans une seconde phase, des analyses rhéologiques menées dans le cadre de la viscoélasticité linéaires des polymères à l’état fondu ont permis de déduire les temps d’interdiffusion des chaînes macromoléculaires. Par ailleurs, le processus de coalescence des particules de poudres à l’état fondu a été suivi expérimentalement et modélisé pour différentes températures. Ces temps ont été confrontés à la durée de maintien du polymère à l’état fondu, confirmant ainsi la bonne consolidation obtenue lors du frittage du polyamide 12. En conclusion, ce travail contribue à la compréhension des différents mécanismes physico-chimiques intervenant au cours du frittage laser : il permet d’expliciter de façon assez approfondie les relations entre les propriétés des poudres, les paramètres du procédé et les propriétés finales des pièces. De nombreuses préconisations relatives à l’optimisation des propriétés des poudres pourront être déduites de ce travail et aideront au développement de nouveaux matériaux adaptés à ce procédé. / Additive processing technologies are aimed at manufacturing parts directly from a computer-aided design (CAD) file, without the need for tooling. Therefore flexibility of production increases and manufacturing of small to mid-size series of very complex or even customized parts becomes possible within reduced development time and expenses. Because of the good mechanical properties obtained in the parts, the most commonly used among additive technologies for polymers is laser sintering (LS). The objective of this work is to contribute to a better understanding of the different physical mechanisms involved during laser sintering of polyamide 12 powders. Many operating variables impact the laser sintering process. Especially, the energy supplied to the powder with the laser beam depends on its power, its displacement velocity and the scan spacing. Moreover, the polymer material undergoes a quite severe thermal treatment : before its sintering, the powder is preheated, then in the build tank the sintered parts and the un-sintered surrounding powder remain until the end of the job at elevated temperatures. This thermal history induces ageing, which modifies some powder features and hinders its future reuse. The influence of the parameters mentioned above on the part microstructure and mechanical properties was investigated. Moreover the use of different polyamide 12 powders enabled to identify the key material characteristics towards the physical processes involved in LS and towards the final properties of parts. The laser sintering of semi-crystalline polymers is governed by several fundamental mechanisms: melting of particles, interdiffusion of macromolecular chains at interfaces, coalescence of molten particles, then densification and finally crystallisation. The study and modelling of crystallisation were carried out with one of the PA12 powders used in the first part of this work. From this modelling, the time during which the polymer remains in the molten state during the process was estimated. Next, a rheological analysis made within the framework of linear viscoelasticity of polymer melts allowed to compute the interdiffusion time of the macromolecular chains. Moreover, the coalescence process of molten particles was observed at different temperatures and modeled. The characteristic times thus estimated for these physical processes were opposed to the time during which the polymer remains in the molten state and confirmed the good consolidation obtained by laser sintering of polyamide 12. In conclusion this work contributes to understand the different physico-chemical mechanisms involved during polymer laser sintering by specifying the relations between powder parameters, process variables and final properties of parts. Many recommendations for the optimisation of powder properties can be derived from this work for the purpose of developing new polymeric materials adapted to this process.

Additive manufacturing has increased in popularity in recent years partly due to the possibilities of producing complex geometries in a rapid manner. Selective laser sintering (SLS) is a type of additive manufacturing technique that utilizes polymer powder and a layer-by-layer technique to build up the desired geometry. The main drawbacks with this technique are related to the reproducibility, mechanical performance and the poor surface finish of printed parts. Surface roughness increase the risk of bacterial adhesion and biofilm formation, which is unbeneficial for parts to be used in the healthcare industry. This thesis investigated the possibility in reducing the surface roughness of SLS printed polyamide 12 with the fully automated post-processing technology PostPro3D. The postprocessing relies on chemical post-processing for smoothening of the parts’ surface. PostPro3D utilizes vaporized solvent which condenses on the printed parts causing the surface to reflow. By this roughness, in terms of unmolten particles, is dissolved and surface pores are sealed. The influence of alternating post-processing parameters; pressure, temperature, time and solvent volume was evaluated with a Design of Experiments (DoE). The roughness reduction was quantified with monitoring the arithmetic mean average roughness (Ra), the ten-point height roughness (Rz) and the average waviness (Wa) using a stylus profilometer and confocal laser scanning microscope (CLSM). The effect of post-processing on mechanical properties was evaluated with tensile testing and the effect on microstructure by scanning electron microscopy (SEM). A comparison was made between post-processed samples and a non-postprocessed reference, as well as between samples post-processed with different degree of aggressivity, with regards to the roughness values, mechanical properties and the microstructure. Results indicated that solvent volume and time had the largest effect in reducing the roughness parameters Ra and Rz, while time had the largest influence in increasing the elongation at break, tensile strength at break and toughness. The post-processing’s effect on waviness and Young’s modulus was less evident. SEM established that complete dissolution of powder particles was not achieved for the tested parameter ranges, but a clear improvement of the surface was observed for all different post-processing conditions, as compared to a non-post-processed specimen. The reduction in roughness by increased solvent volume and time was thought to be due to increased condensation of solvent droplets on the SLS-parts. The increase in mechanical properties was likely related to elimination of crack initiation points at the surface. In general, the mechanical properties experienced a wide spread in the results, this was concluded to be related to differences in intrinsic properties of the printed parts, and highlighted the problems with reproducibility related to the SLS. An optimal roughness of Ra less than 1 µm was not obtained for the tested post-processing conditions, and further parameter optimization is required. / Möjligheten att tillverka komplexa geometrier på ett snabbt sätt, har fått additiv tillverkning att öka i popularitet. Selective laser sintering (SLS) är en typ av additiv tillverkning där polymer pulver sintras samman succesivt lager för lager. Dessa lager bygger tillsammans upp den önskade geometrin. De största nackdelarna med SLS är att de tillverkade delarna har bristande mekaniska egenskaper, har brister i reproducerbarheten samt att ytan har en dålig kvalitet, den är ojämn. Ytojämnheten ökar risken för att bakterier fastnar och ett en biofilm bildas. Då produkten ska användas inom sjukvården, är det viktigt att biofilm bildning undviks. Den här uppsatsen har undersökt möjligheterna att reducera ytojämnheten av SLS-printad polyamid 12 med hjälp av kemisk efterbehandling i PostPro3D. Denna maskin är helt automatisk och åstadkommer ytbehandling genom att förånga lösningsmedel som sedan kondenserar på det SLS-printade materialet. Ytan på materialet löses upp vilket minskar ytojämnheter i form av pulver partiklar samt sluter porer på ytan. Genom att ändra på parametrarna för efterbehandlingen kan graden av aggressivitet påverkas, detta gäller tryck, temperatur, tid och lösningsmedels volym. De optimala parametrarna för att åstadkomma en jämn yta utvärderades med en Design of Experiments (DoE). Reducering av ytojämnhet mättes med hjälp av aritmetisk genomsnittlig ojämnhet (Ra), tio-punkts höjd ojämnhet (Rz) och medel-vågighet (Wa), med nålprofilometer och konfokal mikroskop. Efterbehandlingens påverkan på de mekaniska egenskaperna utvärderades i ett dragprov, medan mikrostrukturen undersöktes med svepelektronmikroskop (SEM). Ytjämnheten, de mekaniska egenskaperna och mikrostrukturen jämfördes mellan icke behandlade prover och ytbehandlade prover, med varierad grad av aggressivitet. Resultaten indikerade att tid och volym hade störst effekt på Ra och Rz, medan tid hade störst positiv inverkan på töjning, styrka och seghet. Effekten på styvheten (E-modulen) och vågigheten (Wa) var mindre uppenbar, och någon tydlig påverkan kunde inte observeras. SEM-analys visade att fullständig upplösning av partiklar på ytan inte sker för de testade behandlingarna, men en tydlig förbättring kunde ses vid jämförelse av ett obehandlat prov och ett behandlat prov. Den ökade ytjämnheten för längre tid och högre volym tros bero på en ökad kondensering av lösningsmedel på ytan under efterbehandlingen. Ökningen i mekaniska egenskaperna är troligtvis relaterade till eliminering av kritiska defekter på ytan. Generellt visade de mekaniska egenskaper en stor spridning i resultaten, detta tros bero på inneboende egenskaper i provstavarna. Denna slutsats understryker den bristande reproducerbarheten för SLS-printning. En optimal ytjämnhet antas vara ett Ra värde under 1 µm, denna ytjämnhet har inte uppnåtts med de testade efterbehandlingsparameter värdena, därför krävs ytterligare parameter optimering för att nå optimal efterbehandling.

<p dir="ltr">Current industrial processes are energy and carbon emission intensive. Amidst the growing demand for decarbonization, it is critical to utilize alternate sources of energy and innovative technologies that could improve efficiency and reduce power consumption. In this context, twin-screw compressors are used extensively in commercial and industrial applications. Profile optimization and capacity modulation solutions (e.g., slide valves, variable-speed, etc.) are continuously investigated to improve the performance and operation of the compressors. This study focuses on an exploratory investigation of an additively manufactured twin-screw compressor with internal cooling channels to achieve a near isothermal compression process by evaluating both the potential compressor performance improvement and the structural integrity by means of rotordynamics and fatigue analyses.</p><p dir="ltr">To predict the compressor performance, complex coupling between compression process and heat transfer during the operation of the compressor must be investigated. The interactions between solid (i.e., rotors) and fluid phases (i.e., air and coolant) were modeled using a transient 3D CFD model with conjugated heat transfer (CHT). The CFD model predicted compressor performance parameters such as isentropic efficiency, heat transfer rate, work input and compression forces on the rotors. The performance of the twin-screw compressor with internal cooling channels has been compared with a conventional twin-screw compressor for which experimental data was available. Further investigations have been conducted at different operating conditions, including various pressure ratios, rotational speeds, and mass flow rates to improve the compressor efficiency. The results of the CFD model were used to quantify compression loads, assess the characteristics of the heat transfer processes, and optimize the internal flow through the cooling channels. As the rotors can be affected by stress accumulation and deformations due to their hollowness and reduced wall thickness over time, this study also established a detailed rotordynamic simulation model and a fatigue model using the actual compression forces obtained from previous CFD studies. Both hollow and solid rotors have been analyzed and compared. The bearing loads have been verified against Campbell diagrams whereas the fatigue results have been compared with experimental testing. With the validated model, the hollow rotor compressor durability was analyzed and compared with the conventional rotors. Lastly, a general mechanistic model to better understand bearing loads and frictional losses in a twin-screw compressor is also established and studied.</p><p dir="ltr">The CHT study concluded that the hollow rotor with single-phase internal cooling yielded to an increase in isentropic efficiency of 1% for the higher pressure ratio and 2% for lower pressure ratio at 19,000 RPM. More importantly, the hollow rotors also showed a decrease of 40 K and 20 K in discharge temperatures for the two operating conditions respectively, thereby arriving closer to isothermal conditions and reducing the thermal stresses on the rotors. The rotordynamic study revealed that the male rotor would endure highest amount of von Misses stress reaching up to 338 MPa for the pressure ratio of 3.29 bar and 19,000 RPM. Because of this, a maximum fatigue factor of safety of 5 occurs on the male rotor. From the analyses, the rotors were deemed to be safe and optimized for the designed operating conditions and proof of concept rotors were additively manufacturers with an Inconel alloy through Direct Metal Laser Sintering.</p>

<div>Powder Bed Fusion Additive Manufacturing enables the fabrication of metal parts with complex geometry and elaborates internal features, the simplication of the assembly process, and the reduction of development time. However, the lack of consis-tent quality hinders its tremendous potential for widespread application in industry. This limits its ability as a viable manufacturing process particularly in the aerospace and medical industries where high quality and repeatability are critical. A variety of defects, which may be initiated during the powder-bed fusion additive manufacturing process, compromise the repeatability, precision, and resulting mechanical properties of the final part. The literature review shows that a non-uniform temperature distribution throughout fabricated layers is a signicant source of the majority of thermal defects. Therefore, the work introduces an online thermography methodology to study temperature distribution, thermal evolution, and thermal specications of the fabricated layers in powder-bed fusion process or any other thermal inherent AM process. This methodology utilizes infrared technique and segmentation image processing to extract the required data about temperature distribution and HAZs of the layer under fabrication. We conducted some primary experiments in the FDM process to leverage the thermography technique and achieve a certain insight to be able to propose a technique to generate a more uniform temperature distribution. These experiments lead to proposing an innovative chessboard scanning strategy called tessellation algorithm, which can generate more uniform temperature distribution and diminish the layer warpage consequently especially throughout the layers with either geometry that is more complex or poses relatively longer dimensions. In the next step, this work develops a new technique in ABAQUS to verify the proposed scanning strategy. This technique simulates temperature distribution throughout a layer printed by chessboard printing patterns in powder-bed fusion process in a fraction of the time taken by current methods in the literature. This technique compares the temperature distribution throughout a designed layer printed by three presented chessboard-scanning patterns, namely, rastering pattern, helical pattern, and tessellation pattern. The results conrm that the tessellation pattern generates more uniform temperature distribution compared with the other two patterns. Further research is in progress to leverage the thermography methodology to verify the simulation technique. It is also pursuing a hybrid closed-loop online monitoring (OM) and control methodology, which bases on the introduced tessellation algorithm and online thermography in this work and Articial Neural Networking (ANN) to generate the most possible uniform temperature distribution within a safe temperature range layer-by-layer.</div>

Purdue School of Engineering and Technology, Indianapolis / Powder Bed Fusion Additive Manufacturing enables the fabrication of metal parts with complex geometry and elaborates internal features, the simplification of the assembly process, and the reduction of development time. However, the lack of consistent quality hinders its tremendous potential for widespread application in industry. This limits its ability as a viable manufacturing process particularly in the aerospace and medical industries where high quality and repeatability are critical. A variety of defects, which may be initiated during the powder-bed fusion additive manufacturing process, compromise the repeatability, precision, and resulting mechanical properties of the final part. The literature review shows that a non-uniform temperature distribution throughout fabricated layers is a significant source of the majority of thermal defects. Therefore, the work introduces an online thermography methodology to study temperature distribution, thermal evolution, and thermal specifications of the fabricated layers in powder-bed fusion process or any other thermal inherent AM process. This methodology utilizes infrared technique and segmentation image processing to extract the required data about temperature distribution and HAZs of the layer under fabrication. We conducted some primary experiments in the FDM process to leverage the thermography technique and achieve a certain insight to be able to propose a technique to generate a more uniform temperature distribution. These experiments lead to proposing an innovative chessboard scanning strategy called tessellation algorithm, which can generate more uniform temperature distribution and diminish the layer warpage consequently especially throughout the layers with either geometry that is more complex or poses relatively longer dimensions. In the next step, this work develops a new technique in ABAQUS to verify the proposed scanning strategy. This technique simulates temperature distribution throughout a layer printed by chessboard printing patterns in powder-bed fusion process in a fraction of the time taken by current methods in the literature. This technique compares the temperature distribution throughout a designed layer printed by three presented chessboard-scanning patterns, namely, rastering pattern, helical pattern, and tessellation pattern. The results confirm that the tessellation pattern generates more uniform temperature distribution compared with the other two patterns. Further research is in progress to leverage the thermography methodology to verify the simulation technique. It is also pursuing a hybrid closed-loop online monitoring and control methodology, which bases on the introduced tessellation algorithm and online thermography in this work and Artificial Neural Networking (ANN) to generate the most possible uniform temperature distribution within a safe temperature range layer-by-layer.

No description available.

Pour les applications aéronautiques, les fibres en céramique piézoélectrique à âme métallique permettent d'imaginer des solutions pour avoir des dispositifs actifs et des capteurs complétement intégrés dans des structures, telles que les composites renforcés par des fibres.La démarche de cette étude est d'élaborer et de caractériser de telles fibres qui présentent de nombreux avantages : l'activation en mode radial permet d'utiliser de faibles tensions de commandes, l'utilisation d'un cœur et d'une matrice conducteurs permet de s'affranchir du dépôt d'électrodes et de garantir leur continuité, la présence d'un coeur métallique améliore la résistance mécanique de la fibre, l'utilisation sous forme de fibres fines et longues permet de l'intégrer à des profils de formes complexes sur de grandes longueurs. Dans un premier temps, le procédé d'enduction est utilisé pour la réalisation de ces fibres en déposant des particules céramiques à base de titano-zirconate de plomb (PZT) sur des fils de platine. Le développement et l'optimisation d'un procédé multicouche permet de réaliser des fibres avec des épaisseurs parfaitement contrôlées pour obtenir les capacités de déformations optimales en alternant des cycles dépôt/traitement thermique avant une opération de frittage finale. La caractérisation d'échantillons massifs traités dans les mêmes conditions permet de s'assurer des propriétés piézoélectriques atteintes pendant les différents traitements thermiques. Les caractérisations électromécaniques réalisées sur les fibres permettent de vérifier le comportement en tant qu'actionneur et que capteur, bien qu'il s'avère difficile de remonter aux caractéristiques intrinsèques des fibres.Dans un second temps, une réflexion est menée sur les moyens à mettre en oeuvre pour envisager un développement à grande échelle de ce type de fibre. Dans ce sens, des expérimentations sur la mise en place du procédé continu de coextrusion avec un polymère chargé sont menées, de même que sur la réduction de la température de frittage à l'aide d'additifs pour substituer les fils en platine, mais aussi sur la réduction du temps de frittage à l'aide de techniques non conventionnelles comme le frittage laser et le frittage micro-ondes. Ces investigations ouvrent des pistes sérieuses pour imaginer une production continue de fibres piézoélectriques à âme métallique.Enfin, des travaux de modélisation par éléments finis du comportement de ces fibres, intégrées ou non dans une structure, permettent de mettre en évidence l'influence du dimensionnement des fibres sur les déformations résultantes, en fonction notamment de l'épaisseur du matériau actif déposée et des propriétés élastiques des différents éléments. Différentes configurations sont imaginées pour utiliser ces fibres dans des structures en tant qu'actionneur et capteur. / Metal core piezoelectric fibres are suitable for active devices and sensors fully embedded in structuresas fibres reinforced polymers for aeronautic applications.The aim of this study is to develop and characterize such fibres that have many advantages: radial mode activation allows the use of low voltage control, the use of a core and a conductive matrix eliminates the deposition of electrodes and the necessity to maintain their continuity, the presence of a metal core improves the mechanical strength of the fibre, the use of thin and long fibres permits their integration in profiles with complex shapes over long distances.In a first step, a dip-coating process is used to realize such fibres by depositing ceramic particles based on lead zirconium titanate (PZT) on platinum wire. The development and optimization of a multilayer process, by alternating deposition cycles / heat treatment prior to the final sintering step, lead to the production of fibres with perfectly controlled thickness in order to obtain optimal strain capability. Characterization of bulk samples under the same thermal conditions allows to measure equivalent piezoelectric properties as fibres submitted to the same heat treatments. Electromechanical characterizations performed on the fibres confirm their behaviour as actuator and sensor, although it is still difficult to determine the effective piezoelectric properties of the fibres.In a second step, the possibility to develop a large-scale production of this type of fibre is investigated.In this regard, experiments are carried on coextrusion process with a PZT loaded polymer, as well as the reduction of the sintering temperature by using additives to replace the platinum core. In addition, reducing the sintering time using unconventional techniques such as laser sintering and microwave sintering are investigated. It is then open serious leads to imagine a continuous production of metal core piezoelectric fibres.Finally, a finite element modelling approach of the behaviour of these fibres, integrated or not in a structure, allows to highlight the influence of fibre sizing on the resulting strains, in particular according to the thickness of the active deposited material and elastic properties of the individual elements (metal core, matrix). Different configurations are analysed to use these fibres in structures as actuator and sensor.

Materiais da família ACu3Ti4O12 (ACTO) são potenciais candidatos para aplicação como dielétricos em capacitores cerâmicos devido aos seus altíssimos valores de constante dielétrica (&epsilon;\'), podendo chegar a 105 à temperatura ambiente. Entretanto, a origem deste fenômeno, denominado constante dielétrica gigante (CDG), é ainda uma questão altamente discutida na literatura. Deste modo, para compreender melhor os mecanismos por trás da manifestação desta CDG, neste trabalho os compostos ACu3Ti4O12 (com A = Ca, Bi2/3) foram sintetizados por uma rota baseada no método dos precursores poliméricos, sendo as reações envolvidas durante a síntese investigadas por análise térmica diferencial (ATD) e termogravimentria (TG). O subsequente processamento cerâmico foi realizado via sinterização tanto convencional quanto não convencional, utilizando, neste último caso, sinterização a laser e assistida por campo elétrico. As características (micro)estruturais foram avaliadas por meio de difratometria de raios X (DRX), microscopia eletrônica de varredura (MEV) e espectroscopia de energia dispersiva de raios X (EDX). Já as propriedades (di)elétricas foram estudadas, em nível microestrutural, utilizando espectroscopia de impedância (EI). Destas caracterizações, verificou-se que tanto as características (micro)estruturais quanto as propriedades (di)elétricas são fortemente influenciadas pelas condições de processamento. Neste sentido, mostramos que estes materiais podem apresentar baixos valores de permissividade à temperatura ambiente (&epsilon;\' ~ 102), típicos da resposta do volume, quando possuem grãos resistivos. Em contrapartida, quando as cerâmicas apresentam grãos semicondutores, valores de constante dielétrica gigante (&epsilon;\' >103) são verificados à temperatura ambiente devido à manifestação de efeitos de polarização interfacial. O caráter semicondutor dos grãos surge de maneira termicamente assistida. Isto ocorre porque, em maiores temperaturas, há uma migração de Cu para as regiões intergranulares das cerâmicas e também uma reação de redução do Cu2+ em Cu+. Durante o resfriamento o Cu+ reoxida, dando origem a semicondutividade dos grãos (deficientes em Cu). Como as condições empregadas na sinterização influenciaram as propriedades finais das cerâmicas, incluindo tamanho médio de grãos, decidimos inovar no processamento cerâmico ao aplicar um campo elétrico durante o tratamento térmico partindo de um pó ainda amorfo. Isto levou à observação de dois cenários: i) em altos campos, o pó sai de seu estado amorfo, passa pela cristalização de fases intermediárias, seguida de síntese ultrarrápida (flash synthesis), sem densificação; ii) em baixos campos, o pó transita do estado amorfo à fase final (passando pela cristalização das fases intermediárias), acompanhada de sinterização ultrarrápida (flash sintering), com alta densificação, tudo isso em um único experimento (FAST O3S). Finalmente, mostramos assim que utilizar um campo elétrico durante o tratamento térmico pode acelerar significativamente as taxas tanto de síntese quanto de sinterização, o que abre um novo paradigma no processamento de materiais cerâmicos. / Materials of the ACu3Ti4O12 (ACTO) family are potential candidates for application as dielectric in ceramic capacitors due to their extremely large dielectric constant (&epsilon;\'), which can reach 105 at room temperature. However, the origin of such large &epsilon;\' values, known as giant dielectric constant (GDC), is still an open debate in the literature. In order to better understand the mechanisms behind the manifestation of the GDG phenomenon, in this work, the compounds ACu3Ti4O12 (with A = Ca, Bi2/3) were synthesized by applying a modified polymeric precursor method. The reactions taking place during the powders synthesis were investigated through differential thermal analysis (DTA) and thermogravimetry (TG). The ceramic processing was then performed via conventional as well as non-conventional sintering, using, in the latter case, both laser and electric field-assisted sintering. The (micro)structural characteristics were evaluated by X-ray diffraction (DRX), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). Regarding the (di)electric properties, they were examined (at the microstructural level) using impedance spectroscopy (IS). We have shown from these characterizations that both (micro)structural and (di)electric features are strongly dependent on processing conditions. In this sense, we have demonstrated that these materials might present a low permittivity at room temperature (&epsilon;\' ~ 102), typical from the bulk response, when the ceramic grains are resistive. On the other hand, when the grains are semiconducting, giant dielectric constant values (&epsilon;\' >103) are verified at room temperature due to the manifestation of interfacial polarization effects. The semiconducting nature of the grains is promoted by a thermally-assisted mechanism, i.e., at higher temperatures there is Cu migration towards the ceramic intergranular areas and also a reduction of Cu2+ to Cu+. During cooling, the Cu+ re-oxidizes yielding the grain (Cu-deficient) semiconductivity. Since the conditions employed during the sintering have influenced the final ceramic properties, including the average grain size, we propose a novel approach to the ceramic processing by applying an electric field during the heat treatment starting from an amorphous powder, which led to the following scenarios: i) powder crystallization into the intermediate phases and then instantaneously into the final phase (flash synthesis), displaying no densification, at high fields; ii) powder transition from amorphous to the final phase (through crystallization into intermediate phases), followed by sintering with high densification, at low fields: this is the fast one-step synthesis plus sintering (FAST O3S) of materials, which ends with the so-called flash sintering. Finally, we have demonstrated that both synthesis and sintering rates can be enhanced by applying an electric field during the heat treatment, opening a new paradigm for ceramic processing.

No description available.