Neste trabalho foi investigada uma mudança na linha base observada em curvas de calorimetria exploratória diferencial em um trabalho onde microesferas de vidros aluminossilicatos contendo Ho foram estudados para a aplicação em radioterapia interna seletiva para o tratamento de carcinoma hepatocelular. Os vidros com composição nominal 53,7 SiO2 .10,5 Al2O3 . 35,8 MgO em %mol foram produzidos pelo método de fusão tradicional. As fritas obtidas foram moídas e peneiradas na faixa de 45 a 63 μm. O material foi utilizado para produzir microesferas pelo método de esferolização por queda gravitacional. O pó de vidro e as microesferas foram caracterizados por espectrometria de fluorescência de raios X, difração de laser, difração de raios X, calorimetria exploratória diferencial, análise térmica diferencial, termogravimetria, espectrometria de massa e microscopia eletrônica de varredura. Após as análises térmicas foram formadas pastilhas nos cadinhos que foram analisadas por microscopia eletrônica de varredura, difração de raios X e picnometria a gás He. A mudança na linha base foi associada ao processo de sinterização por fluxo viscoso e ocorre devido a diminuição do fluxo de calor detectado devido à retração da amostra. Outros processos como cristalização concomitante com a sinterização também foram estudados. / In this work a difference in the baseline in differential scanning calorimetry analyses, observed in a work where aluminosilicate glasses microspheres containing Ho were studied for application in selective internal radiotherapy as hepatocellular carcinoma treatment, was studied. The glasses with nominal composition 53,7 SiO2 .10,5 Al2O3 . 35,8 MgO in %mol were produced from traditional melting. The frits obtained were milled and sieved in the range of 45 a 63 μm. The material was used to produce glass microspheres by the gravitational fall method. The glass powder and the microspheres were characterized by X ray fluorescence spectrometry, laser diffraction, X ray diffraction, differential scanning calorimetry, differential thermal analysis, thermogravimetry, mass spectrometry, and scanning electron microscopy. After the thermal analyses, pellets were formed in the crucibles and were analyzed by scanning electron microscopy, X ray diffraction, and He picnometry. The difference in the baseline was associated to the viscous flow sintering process and happens because of the decrease in the detected heat flow due to the sample shrinkage. Other events as concurrent crystallization with the sintering process were also studied.

Ces travaux de thèse visent à améliorer la densification et la résistance à la rupture de céramiques silicatées élaborées principalement à partir de kaolin. Cette amélioration est effectuée en substituant une partie du kaolin par de l’halloysite. En effet, cette voie permet également de mettre en évidence l’influence de la morphologie et de l’organisation des particules sur les propriétés microstructurales. Premièrement, la dispersion des suspensions contenant soit du kaolin soit un mélange de kaolin et d’halloysite, a été optimisée en réduisant la taille des agglomérats et en augmentant la valeur absolue du potentiel zêta. Deuxièmement, les paramètres (broyage, rapport liant/plastifiant…) contrôlant le procédé de mise en forme de coulage en bande ont également été optimisés. Enfin, l’influence de deux types de traitement thermique à 1200°C (frittage conventionnel et par voie micro-ondes) sur les propriétés mécaniques (contrainte à la rupture en flexion biaxiale) des matériaux finis a été mise en évidence. Les résultats obtenus montrent que plusieurs compromis entre le taux de porosité, l’organisation de la microstructure et les propriétés mécaniques ont été obtenus i) soit avec un taux d’halloysite de 10 % en masse associée à un traitement thermique par voie micro-ondes à 1200°C ii) soit avec un taux de substitution d’halloysite égal à 50 % en masse associé à un frittage conventionnel à la même température. / This thesis is devoted to improve both densification and stress to rupture values of silicate ceramics mainly elaborated from kaolin. This improvement is achieved by replacing a proportion of kaolin by halloysite. Indeed, this way also allows to highlight the influence of the shape of particles and their organization on microstructural properties. First, the dispersion of suspensions containing only kaolin or a mixture of kaolin and halloysite was optimized by reducing the size of agglomerates and by increasing the absolute value of zeta potential. Second, parameters (milling, ratio binder/plasticizer...) controlling the tape casting shaping process were also optimized. Finally, the influence of two types of thermal treatment at 1200°C (conventional and micro-waves sintering) mechanical properties (stress to rupture determined by bixial bending tests) of final products was studied. Results obtained show that several compromises regarding porosity, the organization of microstructure and mechanical properties i) a proportion equals to 10 mass % of halloysite combined with a micro-waves thermal treatment at 1200°C ii) a substitution content equals to 50 mass % of halloysite combined with a conventional thermal treatment at the same temperature.

Four silicon nitride powder blends vary in starting powder characteristics, glass chemistry, and phase composition. This work focuses on how these properties influence densification behavior, microstructural development, and the resulting mechanical performance of dense ceramics. Previous work completed on alpha-rich, low oxide containing (8 wt%), and fine silicon nitride powder (GS-44) showed high hardness equiaxed with grained ceramic. GS-44 served as an excellent precursor for the matrix phase material in graphene reinforced composites, which resulted in 235% increase in toughness and high hardness retention [1] with the addition of 1.5 vol% graphene. As the GS-44 powder is no longer in production, investigative work into other commercial powders and customization of powder blends was initiated. Commercial blends were selected based on availability, high alpha content, fine particle size, and additive chemistry (Al2O3, MgO, and Y2O3). The objective was to understand which powder characteristics led to a ceramic design that contained high hardness, strength, and toughness properties in order to increase the use of silicon nitride in extreme temperature environments. One such example is aerospace and structural applications that require a high-performance material that is lightweight and good thermal stability. Strong covalent bonding in silicon nitride make densification of powders extremely difficult; thereby, sintering additives are necessary to promote liquid phase sintering processes. Compaction of ceramic powders was carried out using a spark plasma sintering (SPS) furnace by utilizing a pulsed direct current through a conductive graphite die that encapsulates the sample powder. SPS was preferred over other conventional sintering methods owing to its high heating rate and short dwell times at the sintering target temperature. Thus, SPS provides superior control for tailoring the final silicon nitride properties by producing a hard alpha-phase and tough beta-phase microstructures. The custom blend developed had an appreciable amount of media wear included during the milling process that increased the additive content. Development of the custom blend was used to understand the effect of a larger additive content. Commercial GS-44 blend was used as the control to track the effect of adjusting specific surface area and oxide content in silicon nitride powder systems (HCS-M, C-R3, and UA-SN). The mechanical results for the four matrix systems, showed that toughness increased with grain coarsening and minimization of alumina content in beta silicon nitride. Based on these findings it is important to determine tradeoffs (i.e. balance of high hardness, toughness, and strength) to engineer an optimal ceramic that can be used for structural and aerospace applications.

Abstract The recent trend in low temperature co-fired ceramic (LTCC) technology is to integrate more elements into multilayer modules. This thesis describes work specifically aimed at developing ferroelectric barium strontium titanate (BST) for integration into such modules. In particular, an objective was the development of a novel, electric field controlled, tunable component to be used at microwave frequencies (2–26 GHz). For the application envisaged, relative permittivity is required to be low (100–1000) and adjustable by a suitable applied electric field, the dissipation factor at room temperature must be low (~0.001) at 2–26 GHz, and most importantly, the sintering temperature must be suited to the LTCC technology (~900 °C) Initial work was focused on sol-gel derived Ba0.7Sr0.3TiO3 powders with boron oxide addition, which were sintered at 900 °C, the dissipation factor was 0.006. The dissipation factor was not low enough for the desired microwave application, and attention turned to powders prepared by the mixed-oxide route. The Ba0.7Sr0.3TiO3 powders, fluxed with the optimum amounts of boron oxide and lithium carbonate, could be sintered at 890 °C to the same density as is achieved with un-fluxed Ba0.7Sr0.3TiO3 sintered at 1360 °C. The dissipation factor for this fluxed powder was acceptably low, although permittivity was too high for the particular objective. Subsequently, research was on BST modified by magnesia, 0.4Ba0.55Sr0.45TiO3-0.6MgO (BSTM). With the optimum fluxing additives, the sintering temperature necessary to achieve a dense BSTM-based ceramic was reduced to 950 °C. The developed microstructure was good, and the relative permittivity and dissipation factor values (221, 0.0012 at 1 kHz) at room temperature indicated good microwave properties. Studies were also undertaken with organic-based tape-casting slurries, laminating procedures and burn-out and sintering schedules. Several kinds of tapes were fabricated and characterized. A test structure for the measurement of dielectric properties at 26 GHz of the optimized BSTM-based ceramic was constructed. The specimen was 50 μm thick layer of BST on an alumina substrate. The relative permittivity and tunability were 130 and >15 % at 4 V μm-1 at room temperature. A tunable phase-shifter was fabricated from the same BSTM-based tape using a novel gravure printing technique, and measurements at 26 GHz showed phase shift from 10 to 35° when the electric field was increased from 1 V μm-1 to 2.5 V μm-1. Some exploratory experiments are described to assess the compatibility of the developed BST-based LTCC with commercial LTCC and some electroceramics.

Low temperature sintering has become a very important research area in ceramics processing and sintering as a promising process to obtain grain size below 100nm. For electronic ceramics, low temperature sintering is particularly difficult, because not only the required microstructure but also the desired electronic properties should be obtained. In this dissertation, the effect of liquid sintering aids and particle size (micrometer and nanometer) on sintering temperature and Positive Temperature Coefficient Resistivity (PTCR) property are investigated for Ba1-xSrxTiO3 (BST) doped with 0.2-0.3mol% Sb3+ (x = 0.1,0.2,0.3,0.4 and 0.5). Different sintering aids with low melting point are used as sintering aids to decrease the sintering temperature for micrometer size BST particles. Micrometer size and nanometer size Ba1-xSrxTiO3 (BST) particles are used to demonstrate the particle size effect on the sintering temperature for semiconducting BST. To reduce the sintering temperature, three processes are developed, i.e. 1 using sol-gel nanometer size Sb3+ doped powders with a sintering aid; 2 using micrometer size powders plus a sintering aid; and 3 using nanometer size Sb3+ doped powders with sintering aids. Grain size effect on PTCR characteristics is investigated through comparison between micrometer size powder sintered pellets and nanometer size powder sintered pellets. The former has lower resistivity at temperatures below the Curie temperature (Tc) and high resistivity at temperatures above the Curie temperature (Tc) along with higher ñmax/ñmin ratio (ñmax is the highest resistivity at temperatures above Tc, ñmin is the lowest resistivity at temperatures below Tc), whereas the latter has both higher ñmax and ñmin. Also, ñmax/ñmin is smaller than that of pellets with larger grain size. The reason is that the solid with small grain size has more grain boundaries than the solid with large grain size. The contribution z at room temperature and high temperature and a lower ñmax/ñmin ratio value.

The effect of replacing fine ore by coarse ore on sintering reactions was investigated using an infrared furnace on laboratory scale and sinter pots on pilot plant scale. Five sinter mixes were prepared by changing the percentage coarse ore from 0% to 100% in 25% increments. Coarse ore fraction, sintering temperature, holding time and oxygen partial pressure were selected as sintering parameters, and two-level factorial design was used for identification of parameters that significantly influence the formation of sinter phases. Experimental results showed that the coarse ore fraction has a higher effect on the sintering process compared to those of other parameters. The experiment design also enabled to set these parameters to their optimum values. The porosity of compacted pellets was measured using a helium pycnometer. The replacement of fine ore by coarse ore resulted in a decrease in porosity (increase in packing density) of compacted pellets. The particles are closer to each other in pellets consisting of more coarse particles than fine particles. Laboratory experiments were performed at 1300°C in air, using a high heating rate (15°C/s). The holding time was set to 2.5 minutes. X-ray diffraction (XRD), reflected light microscopy (RLM), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were used to characterize sintering reactions and sinter phases. XRD analysis revealed that sintered pellets consisted of hematite, SFCA, SFCA-I and calcium silicate. The proportions of SFCA slightly increased when the fraction of coarse ore varied from 0% to 25%, but decreased with a further increase in percentage coarse ore. At 25% coarse ore fraction, the porosity of the compacted pellets decreased, resulting in an increase in packing density and sintering rate. More hematite reacted, resulting in the formation of high amounts of SFCA. Above 25% coarse ore fraction, the amount of hematite increased, and the concentrations of columnar SFCA decreased despite a further decrease in porosity. This was attributed to the decrease in reaction surface area for coarse ore, and the short reaction time, which limited the extent of reaction of the coarse particles. The variation of SFCA-I and calcium silicate was not significant under laboratory conditions. Reflected light microscopy and SEM analysis easily identified two major sinter phases: hematite and SFCA. A clear distinction between the different types of SFCA could not be made using EDS analysis. Sinter pot tests were carried out in order to examine the effect of coarse ore fraction on physical and metallurgical properties of sinters. The tumbler and reduction disintegration indexes increased with increasing coarse ore fraction in the sinter bed. This was presumably due to the increase in amounts of hematite and decrease in surface area for reaction. Consequently, the reducibility of sinter decreased as the percentage coarse ore increased. This study has concluded that the presence of 25% coarse ore in the sinter mix led to enhance sintering reactions. The amounts of SFCA increased, and sinter quality was improved. It is recommended that in future work, sintering reactions should further be investigated by also measuring the permeability of the sinter bed and the reaction surface area of solid particles. Copyright / Dissertation (MSc)--University of Pretoria, 2012. / Materials Science and Metallurgical Engineering / unrestricted

Diploma thesis occupy with preparation of porous material from magnesium powder with a HAp admixture by cold pressing followed by spark plasma sintering (SPS). This thesis contain both preparation of bulk material, diffusion plot and charakterization of materials based on the compaction process conditions. On the basis of physical mechanical characteristics, the impact of the pressing process on the subsequent sintering and the resulting material properties are evaluated. Bulk material is characterized considering to structure and physical–mechanical properties. Properties of final metarial will serve to optimize conditions for process of bulk material preparation.

This diploma thesis deals with research and preparation of bulk materials based on the Mg–Al–Ti system. The theoretical part summarizes the basic knowledge about magnesium alloys, focusing mainly on Mg–Al and Mg–Ti systems. Furthermore, basic information on powder metallurgy methods was included here, from the production of powder materials, through their compaction, to heat treatment and spark plasma sintering (SPS). The theoretical part ends with literature review on the current research of the Mg–Al–Ti system. In the experimental part, bulk materials based on the Mg–Al–Ti system was prepared using traditional methods of powder metallurgy, as well as using the SPS method. The microstructure of the material, elemental and phase composition was examined in this thesis. Subsequently, Vickers hardness and flexural strength were measured, and fractographic observation of the fracture surface was performed. It was found that the aluminum was completely dissolved during the heat treatment, but the titanium particles remained almost intact in the material and worked as a particulate reinforcement. Materials prepared by methods of conventional powder metallurgy showed increased porosity compared to materials prepared by the SPS, resulting in lower hardness and flexural strength. The hardness increased with increasing the amount of aluminum and titanium and with the amount of magnesium phase . Fractographic observation of the fracture surface suggests that a diffuse connection between the reinforcement and the matrix may have occurred after the sintering process.

Les conditions nécessaires pour obtenir des céramiques transparentes (absence de porosité, absence de seconde phase) requièrent une totale maitrise de chacune des étapes (synthèse, mise en forme, frittage) intervenant dans le processus, ce qui rend encore difficile l'industrialisation d'un procédé d'élaboration reproductible. Dans ce contexte, ce travail de thèse s’est focalisé à développer et évaluer la capacité du procédé de frittage micro-ondes à améliorer la robustesse du processus d'élaboration de spinelle transparent. La mise en place d'un dispositif de dilatométrie optique et une nouvelle technique de calibration originale, basée sur la fusion d'oxyde, ont permis de caractériser le frittage micro-ondes du spinelle pur avec une plus grande confiance. Même si aucun effet lié au procédé micro-onde n’a pu être mis en évidence pour le frittage du spinelle pur (trajectoire de frittage, mécanisme de densification et évolution de la porosité identiques), ces travaux ont démontré que l’impact de dopants pouvait être amplifié en présence du rayonnement micro-ondes. Ainsi, un décalage des courbes de retrait vers les basses températures a été observé lors du frittage micro-ondes du spinelle dopé avec TiO2 et MgO. L'existence d'un couplage particulier entre les défauts ponctuels chargés (lacunes, cations interstitiels) et le champ électrique pourrait être à l'origine de ce phénomène. Ces travaux de thèse ont également démontré la faisabilité de fritter des pièces de spinelle carrées de grandes dimensions (< 65 mm) compatibles avec un post-traitement HIP, en vue d'obtenir des pièces transparentes. Pour cela, un four micro-ondes monomode 915 MHz a été automatisé et une cellule de frittage adaptée a été développée. A l'issue du traitement HIP, les pièces ont présenté une transparence et des propriétés mécaniques (dureté; ténacité) comparables à celles des pièces pré frittées par voie conventionnelle. / The elaboration of transparent ceramics (no porosity, no second phase) requires a strict control of all steps of the process (synthesis, shaping, sintering). As a result, the industrialization of a reproducible process is still difficult to achieve. In this context, this thesis has focused on developing and evaluating the ability of the microwave sintering process to improve the robustness of the transparent spinel elaboration process.The set-up of an optical dilatometer and a new original calibration method, based of melting of different oxides, allowed to characterize the microwave sintering with a greater confidence. Even if no microwave effect has been observed on the sintering of pure spinel (identical sintering trajectory, densification mechanism and porosity evolution), this work has shown an amplification of the impact of dopants during microwave sintering. A shift of the shrinkage curves towards the low temperatures was observed during the microwave sintering of the spinel doped with TiO2 and MgO. The existence of a particular interaction between charged point defects (vacancies, interstitial cations) and the electric field could explain this phenomenon.This work thesis has shown also the feasibility of sintering of large square spinel pieces (< 65 mm) compatible with post treatment HIP in order to produce transparent ceramics. For this purpose, the microwave system working at 915 MHz was automatized and a new sintering configuration was developed. After a post-treatment, the transparency and mechanical properties (hardness; tenacity) of microwave pre-sintered samples were comparable with those of samples sintered by conventional heating.

Ce travail de thèse a pour objectif l’obtention de nanocomposites denses ZnO/SiO2 afin d’améliorer les propriétés thermoélectriques de l’oxyde de zinc. Ce manuscrit décrit différents aspects de l’élaboration tant en terme de synthèse que de densification de nanocomposites ZnO/SiO2 ainsi que leur caractérisation. Afin d’obtenir des nanoparticules en grande quantité, de bonne cristallinité et de taille inférieure à 10 nm, l’optimisation d’une synthèse par voie polyol en jouant sur différents paramètres (pH, température, taux d’hydrolyse, solvant, surfactant) a été réalisée. Nous avons pu mettre en évidence l’intérêt d’utiliser l’acide benzoïque comme surfactant pour éviter l’agglomération de ces nanoparticules. La modification de surface des nanoparticules par de la silice a ensuite été explorée. Cette modification a été réalisée par une méthode classique, le procédé Stöber, ainsi que par une technique moins conventionnelle, l’ALD. Une étude approfondie de la densification par SPS à la fois de l’oxyde de zinc et des nanoparticules recouvertes a été décrite. L’influence de la couche amorphe déposée sur la croissance cristalline des nanoparticules a été démontrée. Nous avons par ailleurs pu mettre en exergue une pollution importante par du carbone lors de la densification des composés entrainant des modifications importantes des propriétés de transport. Le résultat majeur de cette thèse est la mise en évidence de clusters d’oxyde de zinc fortement dopés dans ces composés qui remet en question les mécanismes de transport dans le ZnO. / This study is focusing on the synthesis of nanocomposites of Al doped ZnO/SiO2 with high density in order to increase the thermoelectric properties of ZnO. This work describes the optimization of the synthesis by investigating the effect of different experimental parameters (temperature, type of surfactant, degree of hydrolysis, nature of the solvent, pH) to obtain large amount of nanoparticles with size below 10 nm and good crystallinity. We have identified that using benzoic acid as surfactant could avoid the formation of particle aggregates. The modification of nanoparticles surfaces with SiO2 was investigated by using two methods the Stöber process and ALD. The possibility of ZnO and nanocomposite powder densification by spark plasma sintering was also tackled as well as the role played by the main parameters of the method (applied pressure and the best moment of its application, heating rate). The influence of the amorphous shell on the limiting grain growth during the sintering was demonstrated. Furthermore, a carbon accumulation which modifies the thermoelectric properties in the densified pellet was demonstrated. The source of it was assigned in part to the densification process. The most significant result of this study was the finding of the presence of ZnO clusters strongly doped wich could have fundamental implications as it may reopen the discussion on the transport mechanism in ZnO.