Contribuição ao estudo de sinterização sem pressão assistida por campo elétrico de zircônia tetragonal estabilizada com ítria

Abstract

Experiments on sintering ZrO2: 3 mol% Y2O3 polycrystalline ceramics (Y-TZP, hereafter 3YSZ) were carried out by three methods: heating following the room temperature-1400 o C-room temperature profile (conventional sintering), heating from room temperature to 1000-1100 o C under an applied AC electric field (dynamic electric field-assisted sintering), and heating to 1000-1100 o C for application of an AC electric field (isothermal electric field-assisted sintering). The last method was performed under different specimen conditions (green pellets, green pellets isostatically pressed with different loads, pellets pre-sintered at 1400 o C) and different experimental conditions (different frequencies of the AC electric field, DC electric fields, different limitation of the electric current densities, applying loads simultaneously to application of the electric field). All 3YSZ sintered samples, besides having their apparent densities determined, had their surfaces observed in a scanning electron microscope to evaluate average grain size and distribution of grain sizes (some, along the surface from the center to the border). Moreover, impedance spectroscopy analyses were carried out to evaluate the intergranular (mainly grain boundary) and intragranular (bulk) contributions to the electrical resistivity. The primary idea was to collect data on electric field-assisted sintering looking for understanding the mechanisms behind that sintering method, known to produce dense ceramic pieces at temperatures lower than those used in conventional sintering, in short times and inhibiting grain growth. The main results show that 1) the shrinkage level depends on the AC frequency, 2) the larger the porosity the higher the electric field effect, 3) higher current densities promotes higher densification up to a limit that could damage the sample, 4) the electric current pulse follows preferentially the intergranular instead of the bulk pathway, and 5) electric field-assisted sintered specimens show enhanced grain boundary conductivity. A mechanism for the electric field-assisted sintering is proposed based on that 1) Joule heating is the primary event, 2) the electric current, as a result of the electric field, follows the intergranular pathway, 3) Joule heating diffuses chemical species depleted at the interfaces back to the bulk, increasing the defect concentration, leading to the enhancement of the bulk conductivity, and 4) that same Joule heating is responsible for the decrease of the potential barrier at the space charge region, inhibiting the blocking of oxide ions at the grain boundaries.

Comportamento de compósitos ZrO>sub<2>/sub< / Al>sub<2>/sub<O>sub<3>/sub< usando os processos de >i<flash sintering>/i< e sinterização convencional

“…A energia de ativação do contorno de grão, em sua grande parte, foi ligeiramente maior que do grão devido às características resistivas da região(M´PEKO;FRANCIS;LIU et al, 2016a). Todos os valores de energia de ativação de todas as sinterizações estão de acordo com os reportados na literatura para a ZrO2 e compósitos a base de ZrO2, em torno de 1,0 eV (BARSOUKOV;MACDONALD, 2018;M´PEKO;FRANCIS;CARVALHO, 2018;BICHAUD et al, 2015;LIU et al, 2016a).Para Liu et al (2016a), o fato das energias de ativação serem praticamente as mesmas em todas as condições sugere que as diferentes condutividades na FS pode ser associada a densidade de defeitos.Embora Du et al (2016) não encontraram influência na resposta elétrica depois do FS, e Biesuz e Sglavo (2019) sugeriram que a característica de mudança da condutividade seria referente ao tipo de corrente elétrica, há trabalhos que utilizaram AC e encontraram o aumento de condutividade da FS em relação a 1T (MUCCILLO; CARVALHO; MUCCILLO, 2018; CARVALHO; MUCCILLO; MUCCILLO, 2018). Essas diferenças nos valores de resistência ou condutividade são devido às variações da forma/dimensões da amostra bem como dos parâmetros envolvidos na FS (MUCCILLO; MUCCILLO, 2013).…”