Microwave-assisted sintering of ceramic materials has proven to be a very favorable processing technique that can promote lower grain growth and densification with shorter dwell times. Hematite is considered a good microwave absorber due to its high loss tangent value, calculated in the range of 0.001 to 0.2 between room temperature and 750 ºC for 2.45 GHz1, thus showing good interaction with this electromagnetic radiation. However, there are few studies on grain growth kinetics of hematite during microwave sintering, as well as the relationship with grain growth parameters. Therefore, the aim of this work was to evaluate the effect of the heating rate and dwell time on grain growth kinetics, during microwave-assisted sintering (2.45 GHz) of hematite nanopowders. In an initial characterization, dilatometry tests were performed by conventional heating and microwave-assisted heating using the same heating rate (20 °C/min). From these results, the temperature ranges of the initial and intermediate stages of sintering and the onset of linear shrinkage were determined. Considering these results, the samples were sintered in a conventional oven from 750 ºC to 1200 ºC with increments of 50 ºC, varying the dwell time in 6, 12 and 36 minutes. Thus, the diffusional mechanism (N) could be calculated, a value used for the approximate calculation of microwave sintering kinetics. Additionally, sintering was performed in a microwave oven using three heating rates (20, 30 and 50 °C/min) to evaluate the effect of the heating rate on grain growth. The estimated activation energies for the grain growth process during microwave sintering were approximately 237.5 to 272.3kJ/mol, which were higher compared to conventional sintering, in the range of 206.0 to 242.0 kJ /mol. It was found that with increasing microwave heating rate, the activation energy for grain growth tends to be higher.

En este trabajo se discute cómo influye la temperatura en la obtención de polvos cerámicos monofásicos y de tamaño nanométrico con composición K0,5Na0,5NbO3 (KNN), sintetizados por el método de combustión. Fueron usados como fuentes de cationes: K2CO3, Na2CO3, NH4[NbO(C2O4)2(H2O)] . H2O y una mezcla de úrea/glicina en relación 50/50 como combustible. La solución que contiene precursores y combustible fue colocada en un horno precalentado, donde se realizó la combustión; en este proceso fue analizado el efecto de cuatro temperaturas diferentes (500, 600, 700 y 800ºC). Las fases presentes en los polvos cerámicos, su microestructura y morfología, fueron determinadas por difracción de rayos-X (DRX), espectroscopia infrarroja con transformada de Fourier (FTIR) y microscopia electrónica de barrido (MEB). Los resultados experimentales mostraron que la fase perovskita pura fue formada a temperaturas tan bajas como 500ºC. El polvo obtenido posee simetría monoclínica y tamaño medio de partícula de ~ 27 nm. Se concluye que la homogeneidad de la solución precursora y una baja temperatura de procesamiento, juegan un papel clave en la formación de nanopartículas de KNN.

This paper discusses how temperature influences the obtention of single-phase K0.5Na0.5NbO3 (KNN) nanometric ceramic powders, synthesized by the combustion method. K2CO3, Na2CO3, and NH4[NbO(C2O4)2(H2O)] . H2O were used as cation sources and a mixture of urea / glycine in relation 50/50 was used as fuel. The solution containing precursors and fuel was placed in a preheated muffle furnace, where the combustion took place. In this process, the effect of four different temperatures (500, 600, 700 and 800°C) was analyzed. The phases present in the ceramic powders, the microstructure and the morphology were determined by X-ray diffraction (XRD), Fourier transform infrared (FTIR) Spectroscopy, and scanning electron microscopy (SEM). Experimental results show that pure perovskite phase was formed at temperatures as low as 500°C. Furthermore, the obtained powder has monoclinic symmetry and average particle size of ~ 27 nm. We conclude that the homogeneity of the precursor solution and a low processing temperature play a key role in the formation of nano-particles of KNN.