Orientador: Yoshitaka Gushikem / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-13T19:15:08Z (GMT). No. of bitstreams: 1 Marafon_Eduardo_D.pdf: 4220393 bytes, checksum: 1adee837c254c6481f606e3e1cf1dac1 (MD5) Previous issue date: 2008 / Resumo: O material carbono cerâmico condutor SiO2/ZrO2/C-grafite foi preparado pelo método sol-gel em duas composições: (A) SZC30 (SiO2 = 50 %, ZrO2 = 20 %, C = 30 %) e (B) SZC20 (SiO2 = 60 %, ZrO2 = 20 %, C = 20 %) em % m/m, caracterizado e utilizado para preparar eletrodos. O material carbono cerâmico foi aquecido nas temperaturas de 400, 600, 800 e 1000 °C, em atmosfera de nitrogênio, e foi verificado que o mesmo apresenta alta estabilidade térmica. Verificando-se a cristalização de ZrO2 em 1000 °C, na fase tetragonal, enquanto que SiO2 permanece amorfo. A área superficial específica dos materiais carbono cerâmico SZC20 e SZC30 como preparados, foram respectivamente 45 e 12 mg, enquanto que em temperaturas acima de 600 °C, devido ao colapso dos poros, a área superficial específica dos materiais diminuem para 3 e 4 mcm. As imagens de microscopia eletrônica de transmissão de alta resolução mostraram que as nanopartículas de grafite estão bem dispersas no interior da matriz de SiO2/ZrO2, sendo responsáveis pela elevada condutividade elétrica dos materiais carbono cerâmico SZC 20 e SZC30 prensados na forma de discos, 4 e 18 S cm, respectivamente. Com o material SZC30 foram preparados eletrodos e os mesmos foram modificados com hexacianoferrato(II) de cobre (CuHCF) e flavina adenina dinucleotídeo (FAD). A espessura do filme de CuHCF formado sobre a superfície do eletrodo SZC30 foi estimada como sendo 101 nm, e ele apresentou boa estabilidade química e baixa resistência de transferência de carga. O eletrodo SZC30 modificado com FAD mostrou ser eficiente nas reações de redução eletrocatalítica de íons bromato e iodato, em solução aquosa. A eletroredução de ambos os íons ocorreu num potencial de -0,41 V vs. Eletrodo de calomelano saturado. Os experimentos de cronoamperometria mostraram que o eletrodo carbono cerâmico modificado com FAD apresenta potencial para ser empregado como sensor amperométrico para determinação de bromato e iodato, apresentando limites de detecção de 2,3 e 1,5 mmol L, respectivamente / Abstract: SiO2/ZrO2/C-graphite was prepared by the sol-gel method in two compositions: (A) SZC30 (SiO2 = 50 %, ZrO2 = 20 %, C = 30 %) e (B) SZC20 (SiO2 = 60 %, ZrO2 = 20 %, C = 20 %) in % m/m, characterized and used to prepare electrodes. The carbon ceramic material was thermally treated at 400, 600, 800 and 1000 °C in nitrogen atmosphere, and it was observed that it presents high thermal stability and the crystallization of ZrO2 occurred at 1000 °C in tetragonal phase, while SiO2 remained amorphous. The specific surface area of SZC20 and SZC30 the carbon ceramic materials was, respectively, 45 and 12 mg, as prepared. For materials treated at temperatures higher than 600 °C, due to the collapse of the porous, specific surface area of the materials decrease to 3 and 4 mg. High resolution transmission electron microscopy micrographs showed that the graphite nanoparticles are well dispersed inside SiO2/ZrO2 matrix, and these nanoparticles are responsible for the high electrical conductivity of pressed disks of carbon ceramic materials, 4 and 18 S cm, respectively. The SZC30 material was used to prepare electrodes and they were modified with copper hexacyanoferrate(II) (CuHCF) and flavin adenine dinucleotide (FAD). The thickness of the CuHCF film was estimated as 101 nm, presenting good chemical stability and low charge transfer resistance. FAD modified SZC30 carbon ceramic electrode was efficient for electrocatalytic reduction of bromate and iodate ions, in aqueous solution. The electroreduction of both ions occurred at a peak potential of - 0,41 V vs. saturated calomel electrode. The cronoamperometry experiments showed that FAD modified SZC30 carbon ceramic electrode can be used as an amperometric sensor for bromate and iodate, presenting detection limit of 2,3 and 1,5 mmol L, respectively / Doutorado / Quimica Inorganica / Doutor em Ciencias Quimicas

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.

The submerged entry nozzle (SEN) has been used to transport the molten steel from tundish to the mould. The main purpose of the SEN usage is both to prevent oxygen and nitrogen pick-up by molten steel and to achieve the desired flow condition in the mould. Therefore, the SEN can be considered as a vital factor for a stable casting process and the steel quality. Furthermore, the steelmaking processes occur at high temperatures around 1873 K so the interaction between the refractory materials of the SEN and molten steel is unavoidable. Therefore, the knowledge of the SEN behaviors during pre-heating and casting is necessary for the design of the steelmaking processes. The internal surfaces of modern SENs are coated with a glass/silicon powder layer to prevent the SEN graphite oxidation during pre-heating. The effects of the interaction between the coating layer and the SEN base refractory materials on clogging were studied in supplement 1. The results of the study indicated the penetration of the formed alkaline-rich glaze into the Alumina/graphite base refractory during pre-heating. More specifically, the alkaline-rich glaze reacts with graphite to form carbon monoxide gas. Thereafter, dissociation of CO at the SEN/molten metal interface takes place. This leads to reoxidation of dissolved REM (Rare Earth Metal), which form the “In Situ” REM oxides at the interface between the SEN and the REM alloyed molten steel. Also, the interaction of the penetrated glaze with alumina in the SEN base refractory materials leads to a formation of a high-viscous alumina-rich glaze during the SEN pre-heating process. This in turn, creates a very uneven surface at the SEN internal surface. The “In Situ” formation of the REM oxides together with the uneven internal surface of the SEN may facilitate the accumulation of the primary inclusions. Supplement 1 revealed the disadvantages of the glass/silicon powder layer. On the other hand the carbon oxidation is a main industrial problem for un-coated Alumina/Graphite Submerged Entry Nozzles (SEN) during pre-heating. This led to the proposal of a new refractory material for the SEN. In supplement 2, the effect of ZrSi2 antioxidant and the coexistence of antioxidant additive and (4B2O3 ·BaO) glass powder on carbon oxidation were investigated at simulated non-isothermal heating conditions in a  controlled atmosphere. Also, the effect of ZrSi2 antioxidant on carbon oxidation was investigated at isothermal temperatures at 1473 K and 1773 K. The specimens’ weight losses and temperatures were plotted versus time and compared to each others. The thickness of the oxide areas were measured and also examined using XRD, FEG-SEM and EDS. The coexistence of 8 wt% ZrSi2 and 15 wt% (4B2O3 ·BaO) glass powder of the total alumina/Graphite base refractory materials, presented the most effective resistance to carbon oxidation. The 121% volume expansion due to the Zircon formation during heating and filling up the open pores by (4B2O3 ·BaO) glaze during green body sintering led to an excellent carbon oxidation resistance. In supplement 3, decarburization behaviors of Al2O3-C, ZrO2-C and MgO-C refractory materials constituting a commercial Submerged Entry Nozzle (SEN), have been investigated in different gas atmosphere consisting of CO2, O2 and Ar. The (CO2/O2) ratio values were kept the same as it is in propane combustion flue gas at Air Fuel Ratio (AFR) values equal to 1.5 and 1 for both Air-fuel and Oxygen-fuel combustions. Laboratory experiments were carried out non-isothermally in the temperature range 873 K to 1473 K at 15 K/min followed by isothermal heating at 1473 K for 60 min. The decarburization ratio (α) values of the three refractory types were determined by measuring the weight losses of the samples. The results showed that the decarburization ratio (α) values of the MgO-C refractory became 3.1 times higher for oxygen-fuel combustion compared to air-fuel combustion both at AFR equal to 1.5 in the temperature range 873 K to 1473 K. The decarburization ratio (α) values for Al2O3-C samples were the same as for the isothermal heating at 1473 K and non-isothermal heating in the temperature range 473  to 1773 K with a 15 K/min heating rate. It substantiates the SEN preheating advantage at higher temperatures for shorter holding times instead of heating at lower temperatures for longer holding times. Jander’s diffusion model was proposed for estimating the decarburization rate of Al2O3-C refractory in the SEN. The activation energy for Al2O3-C samples heated at AFR equal to 1.5, for air-fuel and oxygen-fuel combustions were found to be 84.5 KJ/mol and 95.5 KJ/mol respectively during non-isothermal heating in the temperature range 873 K to 1473 K. / QC 20101008

The submerged entry nozzle (SEN) is used to transport the molten steel from a tundish to a mould. The main purpose of its usage is to prevent oxygen and nitrogen pick-up by molten steel from the gas. Furthermore, to achieve the desired flow conditions in the mould. Therefore, the SEN can be considered as a vital factor for a stable casting process and the steel quality. In addition, the steelmaking processes occur at high temperatures around 1873 K, so the interaction between the refractory materials of the SEN and molten steel is unavoidable. Therefore, the knowledge of the SEN behaviors during preheating and casting processes is necessary for the design of the steelmaking processes  The internal surfaces of modern SENs are coated with a glass/silicon powder layer to prevent the SEN graphite oxidation during preheating. The effects of the interaction between the coating layer and the SEN base refractory materials on clogging were studied. A large number of accretion samples formed inside alumina-graphite clogged SENs were examined using FEG-SEM-EDS and Feature analysis. The internal coated SENs were used for continuous casting of stainless steel grades alloyed with Rare Earth Metals (REM). The post-mortem study results clearly revealed the formation of a multi-layer accretion. A harmful effect of the SENs decarburization on the accretion thickness was also indicated. In addition, the results indicated a penetration of the formed alkaline-rich glaze into the alumina-graphite base refractory. More specifically, the alkaline-rich glaze reacts with graphite to form a carbon monoxide gas. Thereafter, dissociation of CO at the interface between SEN and molten metal takes place. This leads to reoxidation of dissolved alloying elements such as REM (Rare Earth Metal). This reoxidation forms the “In Situ” REM oxides at the interface between the SEN and the REM alloyed molten steel. Also, the interaction of the penetrated glaze with alumina in the SEN base refractory materials leads to the formation of a high-viscous alumina-rich glaze during the SEN preheating process. This, in turn, creates a very uneven surface at the SEN internal surface. Furthermore, these uneven areas react with dissolved REM in molten steel to form REM aluminates, REM silicates and REM alumina-silicates. The formation of the large “in-situ” REM oxides and the reaction of the REM alloying elements with the previously mentioned SEN´s uneven areas may provide a large REM-rich surface in contact with the primary inclusions in molten steel. This may facilitate the attraction and agglomeration of the primary REM oxide inclusions on the SEN internal surface and thereafter the clogging. The study revealed the disadvantages of the glass/silicon powder coating applications and the SEN decarburization. The decarburization behaviors of Al2O3-C, ZrO2-C and MgO-C refractory materials from a commercial Submerged Entry Nozzle (SEN), were also investigated for different gas atmospheres consisting of CO2, O2 and Ar. The gas ratio values were kept the same as it is in a propane combustion flue gas at different Air-Fuel-Ratio (AFR) values for both Air-Fuel and Oxygen-Fuel combustion systems. Laboratory experiments were carried out under nonisothermal conditions followed by isothermal heating. The decarburization ratio (α) values of all three refractory types were determined by measuring the real time weight losses of the samples. The results showed the higher decarburization ratio (α) values increasing for MgO-C refractory when changing the Air-Fuel combustion to Oxygen-Fuel combustion at the same AFR value. It substantiates the SEN preheating advantage at higher temperatures for shorter holding times compared to heating at lower temperatures during longer holding times for Al2O3-C samples. Diffusion models were proposed for estimation of the decarburization rate of an Al2O3-C refractory in the SEN. Two different methods were studied to prevent the SEN decarburization during preheating: The effect of an ZrSi2 antioxidant and the coexistence of an antioxidant additive and a (4B2O3 ·BaO) glass powder on carbon oxidation for non-isothermal and isothermal heating conditions in a controlled atmosphere. The coexistence of 8 wt% ZrSi2 and 15 wt% (4B2O3 ·BaO) glass powder of the total alumina-graphite refractory base materials, presented the most effective resistance to carbon oxidation. The 121% volume expansion due to the Zircon formation during heating and filling up the open pores by a (4B2O3 ·BaO) glaze during the green body sintering led to an excellent carbon oxidation resistance. The effects of the plasma spray-PVD coating of the Yttria Stabilized Zirconia (YSZ) powder on the carbon oxidation of the Al2O3-C coated samples were investigated. Trials were performed at non-isothermal heating conditions in a controlled atmosphere. Also, the applied temperature profile for the laboratory trials were defined based on the industrial preheating trials. The controlled atmospheres consisted of CO2, O2 and Ar. The thicknesses of the decarburized layers were measured and examined using light optic microscopy, FEG-SEM and EDS. A 250-290 μm YSZ coating is suggested to be an appropriate coating, as it provides both an even surface as well as prevention of the decarburization even during heating in air. In addition, the interactions between the YSZ coated alumina-graphite refractory base materials in contact with a cerium alloyed molten stainless steel were surveyed. The YSZ coating provided a total prevention of the alumina reduction by cerium. Therefore, the prevention of the first clogging product formed on the surface of the SEN refractory base materials. Therefore, the YSZ plasma-PVD coating can be recommended for coating of the hot surface of the commercial SENs. / <p>QC 20111014</p>

The submerged entry nozzle (SEN) is used to transport the molten steel from a tundish to a mould. The main purpose of its usage is to prevent oxygen and nitrogen pick-up by molten steel from the gas. Furthermore, to achieve the desired flow conditions in the mould. Therefore, the SEN can be considered as a vital factor for a stable casting process and the steel quality. In addition, the steelmaking processes occur at high temperatures around 1873 K, so the interaction between the refractory materials of the SEN and molten steel is unavoidable. Therefore, the knowledge of the SEN behaviors during preheating and casting processes is necessary for the design of the steelmaking processes  The internal surfaces of modern SENs are coated with a glass/silicon powder layer to prevent the SEN graphite oxidation during preheating. The effects of the interaction between the coating layer and the SEN base refractory materials on clogging were studied. A large number of accretion samples formed inside alumina-graphite clogged SENs were examined using FEG-SEM-EDS and Feature analysis. The internal coated SENs were used for continuous casting of stainless steel grades alloyed with Rare Earth Metals (REM). The post-mortem study results clearly revealed the formation of a multi-layer accretion. A harmful effect of the SENs decarburization on the accretion thickness was also indicated. In addition, the results indicated a penetration of the formed alkaline-rich glaze into the alumina-graphite base refractory. More specifically, the alkaline-rich glaze reacts with graphite to form a carbon monoxide gas. Thereafter, dissociation of CO at the interface between SEN and molten metal takes place. This leads to reoxidation of dissolved alloying elements such as REM (Rare Earth Metal). This reoxidation forms the “In Situ” REM oxides at the interface between the SEN and the REM alloyed molten steel. Also, the interaction of the penetrated glaze with alumina in the SEN base refractory materials leads to the formation of a high-viscous alumina-rich glaze during the SEN preheating process. This, in turn, creates a very uneven surface at the SEN internal surface. Furthermore, these uneven areas react with dissolved REM in molten steel to form REM aluminates, REM silicates and REM alumina-silicates. The formation of the large “in-situ” REM oxides and the reaction of the REM alloying elements with the previously mentioned SEN´s uneven areas may provide a large REM-rich surface in contact with the primary inclusions in molten steel. This may facilitate the attraction and agglomeration of the primary REM oxide inclusions on the SEN internal surface and thereafter the clogging. The study revealed the disadvantages of the glass/silicon powder coating applications and the SEN decarburization. The decarburization behaviors of Al2O3-C, ZrO2-C and MgO-C refractory materials from a commercial Submerged Entry Nozzle (SEN), were also investigated for different gas atmospheres consisting of CO2, O2 and Ar. The gas ratio values were kept the same as it is in a propane combustion flue gas at different Air-Fuel-Ratio (AFR) values for both Air-Fuel and Oxygen-Fuel combustion systems. Laboratory experiments were carried out under nonisothermal conditions followed by isothermal heating. The decarburization ratio (α) values of all three refractory types were determined by measuring the real time weight losses of the samples. The results showed the higher decarburization ratio (α) values increasing for MgO-C refractory when changing the Air-Fuel combustion to Oxygen-Fuel combustion at the same AFR value. It substantiates the SEN preheating advantage at higher temperatures for shorter holding times compared to heating at lower temperatures during longer holding times for Al2O3-C samples. Diffusion models were proposed for estimation of the decarburization rate of an Al2O3-C refractory in the SEN. Two different methods were studied to prevent the SEN decarburization during preheating: The effect of an ZrSi2 antioxidant and the coexistence of an antioxidant additive and a (4B2O3 ·BaO) glass powder on carbon oxidation for non-isothermal and isothermal heating conditions in a controlled atmosphere. The coexistence of 8 wt% ZrSi2 and 15 wt% (4B2O3 ·BaO) glass powder of the total alumina-graphite refractory base materials, presented the most effective resistance to carbon oxidation. The 121% volume expansion due to the Zircon formation during heating and filling up the open pores by a (4B2O3 ·BaO) glaze during the green body sintering led to an excellent carbon oxidation resistance. The effects of the plasma spray-PVD coating of the Yttria Stabilized Zirconia (YSZ) powder on the carbon oxidation of the Al2O3-C coated samples were investigated. Trials were performed at non-isothermal heating conditions in a controlled atmosphere. Also, the applied temperature profile for the laboratory trials were defined based on the industrial preheating trials. The controlled atmospheres consisted of CO2, O2 and Ar. The thicknesses of the decarburized layers were measured and examined using light optic microscopy, FEG-SEM and EDS. A 250-290 μm YSZ coating is suggested to be an appropriate coating, as it provides both an even surface as well as prevention of the decarburization even during heating in air. In addition, the interactions between the YSZ coated alumina-graphite refractory base materials in contact with a cerium alloyed molten stainless steel were surveyed. The YSZ coating provided a total prevention of the alumina reduction by cerium. Therefore, the prevention of the first clogging product formed on the surface of the SEN refractory base materials. Therefore, the YSZ plasma-PVD coating can be recommended for coating of the hot surface of the commercial SENs.