The use of iron ore concentrate has become of interest as the demand for higher grade sinter feed is increasing. The fine nature of the concentrate however raises concerns as it can affect permeability during sintering which could have a drastic impact on sinter productivity. In this study the use of iron ore concentrate and micropellets as sinter feed was evaluated. Five mixtures containing different proportions of iron ore concentrate and micropellets, together with iron ore fines, were prepared. These mixtures were agglomerated through pilot scale sinter pot tests, followed by measuring the sinter strength, reduction disintegration and reducibility. Samples of the produced sinters were also subjected to XRF, XRD and SEM-EDS analyses for mineralogical analysis, and MF-XRT for sinter structure analysis. Granulation results revealed that the mixture containing 0% micropellets – 40% concentrate showed superiority in material transfer efficiency while the mixture containing 30% micropellets – 10% concentrate showed superiority in permeability. X-ray diffraction analysis (XRD) revealed an increase in hematite as micropellet content increased. The analysis also revealed high SFCA content for the 0% micropellet – 40% concentrate which decreased as micropellet content increased. This was mainly associated with a decrease in reactivity. Optical microscopy confirmed a large presence of SFCA and also revealed significant precipitation of magnetite and SFCA in the bonding phase structures of the high concentrate containing sinters. It also revealed a pronounced presence of acicular SFCA in the high micropellet containing mixtures. Scanning electron microscopy (SEM) and energy dispersion spectrometry (EDS) revealed a slight consistency in phase chemistry across the different sinter mixtures. This was mainly associated to similar chemical compositions of the starting mixtures. It also revealed that the micropellets maintained a hematite-silica core surrounded by a Fe-rich sintered matrix. It was further determined that the introduction of iron ore concentrate and micropellets did not significantly impact mechanical properties of sinter as similar tumble indices (TI) were observed. A slight variation in reduction disintegration index (RDI) was however seen with high micropellet sinters showing a larger degree of degradation compared to high concentrate containing sinters. Sinter reducibility (RI) also decreased slightly as micropellets increased and this was associated with the inability of micropellets to assimilate during sintering. It was therefore concluded that although micropellets and concentrate can be used as sinter feed, the optimum amount thereof was not yet determined. / Dissertation (MEng)--University of Pretoria, 2020. / Materials Science and Metallurgical Engineering / MEng / Unrestricted

The influence of various pressure-less heating schedules (CRH - Constant Rate of Heating, TSS - Two Step Sintering, RCS – Rate Controlled Sintering) on the final microstructure of cubic zirconia ceramics was studied in this master´s thesis. There were used nanopowders ZrO2 (stabilized with 8 mol.% Y2O3) with initial particle size 80 nm (TZ-8Y) and 140 nm (TZ-8YSB). Powders were cold isostatically pressed and pressure-less sintered in air by different heating regimes. It was found that for both studied materials the modification of conventional sintering (CRH) using lower sintering temperatures and longer sintering dwell times can result in samples with finer microstructure. For example, the sintering of TZ-8YSB ceramics at a relatively low temperature (1270 °C) but for very long time (60 h) led to ceramics with the same final density (99,25 % of theoretical density) and almost identical grains (1,31 m vs. 1,27 m) in compare with TSS (1440 °C/ 1290 °C/ 15 h). On the other hand, RCS method showed no positive effect on the microstructure of both materials in comparison with CRH method. The obtained results indicate that the microstructure of c-ZrO2 ceramics can be influenced rather in its third sintering stage (by CRH and TSS methods) than in the second sintering stage (by RCS method).

This diploma thesis deals with the study of microstructure development during the final stage of sintering of two zirconia based ceramic materials (tetragonal ZrO2 doped with 3 mol% of Y2O3 and cubic ZrO2 doped with 8 mol% of Y2O3). Conventional, microwave and SPS sintering methods were used. Achieved relative densities and mean grain sizes were evaluated for sintered materials. It was found that the non-conventional sintering methods are capable of rapid processing of ceramics with improved microstructure compared to conventional sintering.

Sintering is a one of the key step in a processing of bulk ceramic materials. New sintering methods were invented in the last years. These new sintering methods, according to their authors, can be used for obtaining finer final microstructure of ceramics only by modifying the heating schedule. This work is focused on an influence of the Two Step Sintering method on the final microstructure for oxide ceramics. Obtained experimental results have shown that the effectivity of the Two Step Sintering method is rising with crystallographic symmetry of used material. Thesis is also focused on a high-temperature dilatometry and concept of the Master Sintering Curve. This concept was used for calculation of the activation energy of sintering and finding different sintering mechanisms acting in the intermediate and final stage of sintering. Activation energy of sintering was compared with activation energy of grain growth with target to find the kinetic window. Kinetic window can allow a sintering without a grain growth. Master Sintering Curve method was also used in a pressure assisted unconventional sintering technique Spark Plasma Sintering. Master Sintering Curve applied to a Spark Plasma Sintering technique reveals and quantified different sintering mechanisms acting in Spark Plasma Sintering. These findings led to preparation of transparent tetragonal ZrO2.

M. Tech. (Department of Metallurgical Engineering, Faculty of Engineering Technology), Vaal University of Technology. / Solid titanium (Ti), Ti-6Al-4V (wt.%), and Ti-24Nb-4Zr-8Sn (wt.%) materials were fabricated from powders using spark plasma sintering (SPS). The starting materials comprised of elemental powders of ASTM Grade 4 titanium (Ti), aluminium (Al), vanadium (V), niobium (Nb), zirconium (Zr), and tin (Sn). The powders were initially characterised and milled prior to sintering. The micronpowders were milled in an attempt to produce materials with nanostructured grains and as a result improved hardness and wear resistance. The produced solid Ti-24Nb-4Zr-8Sn alloy was compared to solid titanium (Ti) and Ti-6Al-4V (Ti64) on the basis of density, microstructure, hardness, corrosion, and biocompatibility. Relative densities above 99.0% were achieved for all three systems. CP-Ti and Ti64 had both 100% relative density, and Ti2448 showed a slightly lower density of 99.8%. Corrosion results showed that all three materials exhibited good corrosion resistance due to the formation of a protective passive film. In 0.9% NaCl Ti2448 had the highest current density (9.05 nA/cm2), implying that its corrosion resistance is relatively poor in comparison to Ti (6.41 nA/cm2) and Ti64 (5.43 nA/cm2), respectively. The same behavior was observed in Hank's solution. In cell culture medium, Ti2448 showed better corrosion resistance with the lowest current density of 2.96 nA/cm2 compared to 4.86 nA/cm2 and 5.62 nA/cm2 of Ti and Ti64 respectively. However, the current densities observed are quite low and insignificant that they lie within acceptable ranges for Ti2448 to be qualified as a biomaterial. Cell proliferation test was performed using murine osteoblastic cells, MC3T3-E1 at two cell densities; 400 and 4000 cells/mL for 7 days incubation. Pure titanium showed better cell attachment and proliferation under both conditions suggesting that the presence of other oxide layers influence cell proliferation. No significant difference in cell proliferation was observed between Ti64 and Ti2448.

No description available.

Microfabrication of ceramic features has become a critical issue in realizing the miniaturization of devices. Micromolding and sintering play critical roles in fabricating micron- and submicron-sized ceramic features using nanoparticles. Developed from soft lithography, replica molding has been proven a good method to prepare micron- and submicron-sized features. However, the fidelity of the features can be compromised by incomplete feature cavity filling and feature shrinkage during the forming process. In this study, centrifuge-aided micromolding is developed to prepare micron- and submicron-sized ZnO features. By introducing a centrifugal force, the shear-thinning behavior of the suspensions is utilized, and the cavity filling process and the diffusion of trapped air out of the features are accelerated. The drying shrinkage is decreased by increasing the density of the wet nanoparticle packing from the centrifugal process. The centrifugal force improves the fidelity of all the designed features. ZnO ridges from 0.4 μm to 2 μm size and rods of 1.6 μm size are prepared successfully. The wide applicability of this strategy has been demonstrated by preparing ZrO2 features via the same method. Sintering process has a significant influence on the morphology and microstructural evolution of micron-sized ceramic features. When ceramic features decrease to much smaller sizes, such as in the micron range, the dominating sintering mechanism(s) can be different from those of the bulk at large scales. However, limited effort has been devoted to understanding the sintering behaviors. In this study, the as-prepared micron-sized ZnO ridges and rods were sintered at 950oC for different time in air atmosphere. The sintering process destructs the ZnO features via abnormal grain growth and surface roughening. Destruction prediction of features using sintering time is established based on grain growth. Feature surface roughening is further analyzed with respect to thermodynamic fundamentals. Because of the evaporation tendency during zinc oxide sintering, sintering atmosphere has a significant influence on the sintering behavior and feature fidelity. In this study, micron-sized ZnO ridge features were sintered under air and argon atmospheres. Ridge size, line edge roughness, and grain size were characterized. Quantitative calculation of sintering behaviors was performed in order to obtain fundamental understating of the micron-sized ZnO feature sintering. It is found that oxygen partial pressure is the deciding factor for the ridge feature evolution. ZnO evaporation and defects diffusion are responsible for the ZnO bulk and ridge sintering behavior differences. / Master of Science / In order to produce portable devices with small sizes, novel techniques are required to make small components, which is called microfabrication. Since ceramic materials are widely used in various electronic devices, microfabrication of small ceramic features has become an important issue. When ceramic nanoparticles are used as the raw material, the fabrication of ceramic features mainly consists of two processes: micromolding and sintering, which are the problems that this thesis focuses on. In the micromolding process, the loose nanoparticles are packed to form features with specific shapes. In the sintering process, the nanoparticles in as-prepared features are bonded into a coherent and dense feature. For the micromolding process, a suspension made from the nanoparticles is poured into a mold with as-designed feature shape, and the dry feature is obtained after a drying process. In this study, the factors that will affect the shape of the features are studied. It is found that the major factors include completeness of the filling process and shrinkage during the drying process. By completing the micromolding process in a centrifugal machine, the micromolding process is accelerated, and the shrinkage during the drying process is decreased. Both the two aspects will benefit the feature quality. By using this technique, zinc oxide ridges from 0.4 μm to 2 μm size and rods of 1.6 μm size are fabricated successfully. It is also demonstrated that this technique can be applied to other ceramic materials. Sintering process can convert packed nanoparticles into a coherent object, which can help us to obtain dense ceramic features. However, the sintering process will cause the change in feature shape. For large size ceramic bulks, the sintering theory has been well established to explain these changes. When the size of ceramic materials decreases to very small scale, such as micron size, new sintering theory is needed to explain the change of ceramic features in the sintering process. In this study, micron-sized zinc oxide ridges and rods were sintered at 950oC for different time. It was found that the sintering process will distort the shape of the zinc oxide features. Based on thermodynamic views, the corresponding new theory was established. Because zinc oxide is relatively easy to evaporate during sintering, sintering atmosphere will also affect the shape of the features. In this study, micron-sized zinc oxide ridge features were sintered under air and argon atmospheres. It was found that oxygen content was the major factor that will affect the shape change. The corresponding theory was established to explain the effect of the sintering atmosphere based on thermodynamic views.

The densification mechanisms of polycrystalline α-alumina by spark plasma sintering are highly contradictory, with different research groups suggesting diffusion to dislocation controlled mechanisms to be rate controlling. The specific objective of this work was to investigate densification mechanisms of α-alumina during the intermediate and final stages of sintering by SPS, analyze the microstructural development and establish sintering trajectories. In addition, zirconia and yttria were added in different weight percentages to study the effect of solute concentration on the densification kinetics of spark plasma sintered alumina. The present work adopts a different approach from the classical method adopted previously to analyze the sintering kinetics and densification mechanisms of alumina in SPS, although existing models for hot pressing were adopted for the basic analysis. The densification behavior was investigated in the temperature range 1223-1573 K under applied stresses of 25, 50 and 100 MPa and grain sizes between 100 and 250 nm. The SEM micrographs reveal equiaxed grains with no abnormal grain growth in the dense samples. The ‘master sintering curve’ shows grain size to be primarily dependent on density, irrespective of the applied stresses or temperature. The stress exponent of 1 along with an inverse grain size exponent of 3 and activation energy of 320-550 kJ mol-1 suggests Al3+ grain boundary diffusion as the rate controlling densification mechanism in alumina. The densification rates are marginally slower in compositions with 0.1% Y2O3 and ZrO2 content possibly due to the smaller grain sizes used in this study which leads to faster rates compared to earlier reports. However, higher Y2O3 and ZrO2 content led to decrease in densification rate by more than an order of magnitude possibly due to presence of a second phase which increases the effective path length for diffusion, thereby reducing the densification rates. Presence of Y2O3 and ZrO2 in the compositions with 0.1% Y2O3 and ZrO2 were confirmed by TEM studies. The Y3Al5O12 (YAG) phase developed between 1223 and 1273 K and suppressed densification and grain growth in alumina. In spite of higher temperatures required for alumina-YAG and alumina-zirconia composites to attain density ~99%, the alumina grain size in the composites was smaller than that in pure alumina due to the Zener drag effect. The stress exponents obtained for Y2O3 and ZrO2 composites at both the concentrations yield a value of n~ 2, which indicates a change in densification mechanism from pure alumina. The higher stress dependence of these composites could be due to presence of solute and second phase formation, both of which retard densification rates. The inverse grain size exponents obtained are between 1 and 2; both stress exponent and grain size exponent values suggest an interface reaction controlled diffusion mechanism occurring in these composites, independent of the Y2O3 and ZrO2 content. Higher activation energies are obtained with the Y2O3 and ZrO2 composites of higher content, respectively, due to presence of second phase particles at grain boundaries. The presence of solutes at grain boundaries hinders grain boundary diffusion of alumina, leading to interface reaction controlled process; this is confirmed by superimposing standard aluminum grain boundary and lattice diffusion data on to stress-densification rate data obtained in this work. A comparison of stress exponents using current experimental data adopting the present and the classical approaches show a wide difference in their values indicating a change in the rate controlling diffusion path, necessitating a review of the assumptions made on the basic equations used in previous SPS studies.

Tato doktorská práce se zabývala mikrostrukturálním vývojem vybraných oxidových keramických materiálů během konvenčního slinování (CS), rychlého slinování (RRS), flash slinování (FS) a slinování pomocí plazmatu (SPS). S ohledem na keramiku pro strukturální aplikace byly pomocí RRS připraveny relativně velké (1 cm3), bez defektní a téměř hutné pelety oxidu hlinitého a yttriem stabilizovaného oxidu zirkoničitého (YSZ) s homogenní mikrostrukturou. RRS bylo také shledáno jako optimální metoda pro přípravu vysoce hutné bezolovnaté piezoelektrické keramiky s podobnými vlastnostmi, jako byly získány po časově a energeticky náročnějším CS. Metoda SPS dále zlepšila vlastnosti bezolovnaté piezoelektrické keramiky a produkovala plně hutné vzorky, což je dobrým předpokladem pro translucenci a z níž vyplývajícím optoelektrickým vlastnostem. Nejoptimálnějších výsledků – plné hustoty a vysokých piezoelektrických vlastností – bylo dosaženo kombinací SPS a RRS. Analýzy provedené v této studii také poukázaly na důležitost eliminace těkavých nečistot před rychlým ohřevem. Jinak totiž dochází k zachycení těchto látek ve slinuté keramice, což ve výsledku limituje její konečnou hustotu. Ukázalo se, že nízké konečné hustoty RRS YSZ jsou spojeny se zachycením zbytkového chloru pocházejícího ze syntézy prášku. Pokud byl zbytkový chlor odstraněn vysokoteplotním žíháním keramických kompaktů před zahájením RRS, byly touto metodou získány téměř plně hutné YZS vzorky. Negativní vliv zbytkového chloru na zhutnění byl viditelný také u flash slinovaných YSZ vzorků. Navíc FS YSZ často vede ke zrychlení růstu zrn v jádře vzorku, v důsledku vyšší teploty a elektrochemické redukce. Ve spektru procesních parametrů použitých v rámci této práce dokonce došlo k abnormálnímu růstu zrna (AGG). Silně bimodální distribuce velikosti zrn ukázaná v této práci nebyla dříve nalezena u flash slinutého YSZ. AGG byl vysvětlen dvěma přispívajícími faktory – relativně velkou velikostí vzorku, která vedla k lokalizaci elektrického proudu a vzniku horkých míst (z angl. hot-spots), a celkově akcelerovanou kinetikou růstu zrn v jádře vzorku způsobenou elektrochemickou redukcí.

Sintering is an important processing step for obtaining the necessary mechanical stability and rigidity of ceramic bulk materials. Both mass and heat transfer are essential in the sintering process. The importance of radiation heat transfer is significantly enhanced at high temperatures according to the well-known Stefan-Boltzmann’s law. In this thesis, we modified the pressure-less spark plasma sintering set-up to generate intense thermal radiation, aiming at rapid consolidation of ceramic bulk materials. This approach was named as “Sintering by Intense Thermal Radiation (SITR)” as only thermal radiation contributed. Firstly, the heat and mass transfer mechanisms during the SITR process were studied by choosing zirconia ceramics as references. The results revealed that the multiple scattering and absorption of radiation by the materials contributed to the heat diffusion. The observed enhanced densification and grain growth can be explained by a multiple ordered coalescence of zirconia nanocrystals using high heating rates. Secondly, the temperature distribution during the SITR process was investigated by both numerical simulation and experimental verifications. It showed that the radiator geometry, sample geometry and radiating area were influencing factors. Besides, the change of material and geometry of the radiators resulted in an asymmetric temperature distribution that favored the formation of SiC foams. The foams had gradient structures with different open porosity levels and pore sizes and size distributions. Finally, ceramic bulk materials were successfully fabricated by the SITR method within minutes. These materials included dense and strong ZrO2 ceramics, Si3N4 foams decorated with one-dimensional nanostructures, and nasal cavity-like SiC-Si3N4 foams with hierarchical heterogeneities. Sufficient densification or formed strong necks were used for tailoring these unique microstructures. The SITR approach is well applicable for fast manufacture of ceramic bulk materials because it is clean and requires low energy consumption and properties can be controlled and tuned by selective heating, heating speed or temperature distribution. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Manuscript.</p><p> </p>