Influence of the conformation method on flash sintering of ZnO ceramics

Flash Sintering has been shown to be a promising alternative in obtaining high-density ceramics with grain growth control. However, some conditions are still under development. Regarding the dimension of the ceramic, there is a limitation related to obtaining a homogeneous material in large specimens, in order to avoid the formation of preferential current flow paths. Depending on the temperature required for the flash event to occur, some electrode materials have a high cost, or controlled atmosphere operations are required. Typically, the electrodes used in flash sintering consist of platinum, however, in some cases, other materials may be a cheaper suitable alternative, such as stainless steel[1] or nickel-chromium alloys. Also, the use of different compositions in the electrode influences the conductivity of the material, which affects the onset of thermal runaway[2]. In this regard, different conformation techniques were studied in order to improve the homogeneity of the sintered ZnO ceramic body. Additionally, the feasibility of the use of Inconel (nickel-chromium) electrodes in the replacement of platinum electrodes was evaluated. Then, ZnO specimens were conformed (cylindrical shape - 6 mm diameter and 5 mm thickness) by uniaxial pressing under 140MPa and 300 MPa, isostatic pressing under 200 MPa, and slip casting. All experiments were conducted isothermally at 800 °C in an adapted tube furnace[3], with an applied field of 60 V/cm and 200mA/mm2 as maximum current density, using either platinum or Inconel electrode. Figure 1 shows the variating in time incubation with respect to the conformation method used. The distinct incubation times are justified by the difference in the pore distribution, which affects the electrical resistivity of the samples. Also, according to Figure 1, it can be seen that in addition to presenting a higher electrical resistivity, which increased the incubation time, the samples conformed by slip casting are also characterized by the high formation of hotspots, which can be observed by the appearance of several spikes during the occurrence of the flash event. Figure 2 shows, for the samples conformed by uniaxial pressing (140 MPa), the difference between incubation times in relation to the material used for the electrode. It can be seen that, in this case, the use of Inconel increased the incubation time. Microstructural and further analyzes are being conducted. Please click Additional Files below to see the full abstract

Influence of the flash sintering parameters on the densification and microstructure of ZnO

Técnicas alternativas à sinterização convencional vêm sendo estudadas visando o controle da microestrutura e crescimento de grãos, além de um menor consumo energético. Dentre tais técnicas tem-se a sinterização flash, a qual apresenta rápida retração e densificação da peça na ordem de segundos e pode ser aplicada a diversos materiais, como o ZnO, um semicondutor do tipo n utilizado em diversos dispositivos eletrônicos e optoeletrônicos. Apesar dos inúmeros avanços, a sinterização flash ainda apresenta alguns desafios científicos a serem superados para sua viabilidade em aplicações industriais. Para contribuir nos avanços deste campo científico, o presente trabalho objetivou avaliar os parâmetros da sinterização flash na homogeneidade microestrutural do ZnO e possibilidades de aplicação. Durante a etapa de parametrização, foi possível avaliar as condições ideais de processo, como parâmetros elétricos, térmicos e preparo do material. Posteriormente, foi avaliada a influência dos métodos de conformação e os consequentes gradientes de densidades formados em corpos de prova de ZnO na densificação e microestrutura, quando sinterizados via sinterização flash, pela primeira vez na literatura. Os resultados indicaram que a distribuição da densidade a verde gerada durante a conformação deve ser considerada um fator decisivo na avaliação dos corpos de prova submetidos à sinterização flash. Avaliou-se também a sinterização flash quando aplicada juntamente ao isolamento térmico com manta de alumina (TIFS). Os resultados mostraram que a TIFS forneceu uma microestrutura mais homogênea, reduzindo a diferença de tamanho de grão entre o núcleo e a superfície do corpo de prova. O TIFS, além de proporcionar uma diminuição de 450 ºC na temperatura de início do evento flash, também conseguiu atingir uma temperatura estimada durante o estágio III apenas 100ºC inferior à alcançada para sinterização flash sob as mesmas condições, evidenciando economia de energia de até 78% com o processo de isolamento, em comparação com os experimentos de sinterização flash sem isolamento. O presente trabalho visou ainda avaliar o desempenho do ZnO processado usando sinterização flash quando aplicado como fotocatalisador, uma vez que a sinterização flash pode apresentar grande potencial de aplicação para a obtenção de corpos com maior controle no crescimento de grão e melhorada reatividade de superfície. Foi possível a obtenção de corpos com elevada porosidade e integridade suficiente para serem submetidos à sinterização flash. Os resultados dos ensaios fotocatalíticos dos corpos submetidos à sinterização flash mostraram-se promissores frente aos materiais submetidos à sinterização convencional, demonstrando o potencial de aplicação desta rota de sinterização no processamento de fotocatalisadores.Alternative techniques to conventional sintering have been studied for controlling microstructure and grain growth, besides lower energy consumption. Among such techniques flash sintering stands out, providing fast shrinkage and densification in seconds. It applies to several materials, such as ZnO, an n-type semiconductor used in many electronic and optoelectronic devices. Despite numerous advances, flash sintering still faces some scientific challenges to overcome for its viability in industrial applications. To contribute to the advances in this scientific field, the present work aimed to evaluate the flash sintering parameters on ZnO microstructural homogeneity and application possibilities. During the parameterization step, it was possible to analyze the ideal process conditions, such as electrical and thermal parameters and material preparation. Subsequently, the influence of forming methods and the consequent density gradients formed in ZnO specimens on densification and microstructure when sintered via flash sintering was evaluated for the first time in the literature. The results indicated that the green density distribution generated during forming should be considered a decisive factor when evaluating specimens submitted to flash sintering. Also, flash sintering was investigated when coupled with thermal insulation with an alumina blanket (TIFS). The results showed that TIFS provided a more homogeneous microstructure, reducing the difference in grain size between the core and the surface of the specimen. TIFS, besides providing a 450°C decrease in the flash event onset temperature, was also able to achieve an estimated temperature during stage III only 100°C lower than that achieved for flash sintering under the same conditions, evidencing energy savings of up to 78% with the insulation process, compared to the uninsulated flash sintering experiments. The present work also aimed to evaluate the performance of flash-sintered ZnO when applied as a photocatalyst, since it may present great potential for application in the production of samples with refined microstructure, improving the surface reactivity. It was possible to prepare specimens with high porosity and sufficient integrity to be submitted to flash sintering. The results of the photocatalytic tests of the flash-sintered specimens were promising compared to the conventional-sintered ones, demonstrating the potential application of this alternative sintering route in the processing of photocatalysts

Effect of thermal insulation on microstructural homogeneity and onset temperature of flash sintered materials

The present work aimed to reduce the microstructure heterogeneity inherent to flash sintering by using alumina blankets as a thermal insulator around ZnO cylindrical samples during the sintering process, under different electric field conditions. Thermal insulation significantly reduced the flash onset temperature and the grain size heterogeneity. For higher electric fields, a temperature reduction as high as 480 °C was observed, which also led to lower densification. These findings were discussed in terms of changes in the heat loss dynamics coupled with the adsorbed water retention, both promoted by the applied thermal insulation. A model to estimate the temperature at stage III of flash sintering was proposed. The final temperature reached with thermal insulation did not differ significantly from the ones without it. Thus, thermal insulation could represent an alternative route to flash sinter materials with lower furnace temperatures with energy savings up to 78 % and a more homogeneous microstructure