Dynamic Measurement of Dielectric Properties of Materials at High Temperature During Microwave Heating in a Dual Mode Cylindrical Cavity

Abstract

Abstract-A microwave cavity and heating system for microwave processing and in situ dynamic measurements of the complex permittivity of dielectric materials at high temperatures ( 1000 C) has been developed. The method is based on a dual-mode cylindrical cavity where heating and testing are performed by two different swept frequency microwave sources. A cross-coupling filter isolates the signals coming from both sources. By adjusting the frequency bandwidth of the heating source and the level of coupling to the cavity, an automatic procedure allows for the establishment of a desirable level of heating rate to the dielectric sample to reach high temperatures in short cycles. Dielectric properties of materials as a function of temperature are calculated by an improved cavity perturbation method during heating. Accuracy of complex permittivity results has been evaluated and an error lower than 5% with respect to a rigorous analysis of the cavity has been achieved. The functionality of the microwave dielectric measurement system has been demonstrated by heating and measuring glass and ceramic samples up to 1000 C. The correlation of the complex permittivity with the heating rate, temperature, absorbed power, and other processing parameters can help to better understand the interactions that take place during microwave heating of materials at high temperatures compared to conventional heating.

In Situ Monitoring of Microwave Processing of Materials at High Temperatures through Dielectric Properties Measurement

“…A deviation of 5% in the sample height can lead to errors up to 40% in the dielectric properties determination [29]. Thus, a precise control of the sample height is essential if changes are expected during the process.…”

“…Since the pyrometer points to the surface of the quartz holder, a temperature calibration method has been previously performed to obtain the relationship between the bulk temperature of the sample and the pyrometer measurements [29]. …”

“…To overcome these drawbacks, [29] reports a new dual-mode technique which allows the dielectric characterization of the sample under microwave heating, without the need of mechanical adjustments of the measurement cell during the heating process. The solution is based on a microwave source emitting in a certain bandwidth.…”

“…In [29], the authors describe the design of the reactor and the main devices of the system, together with the electromagnetic algorithms (cavity perturbation method) which were modified to obtain improved accuracy in the dielectric characterization. However, this equipment presents additional innovative features which allow performing these dielectric measurements in a way that was not possible before.…”

Algoritmos de optimización en la estimación de propiedades termodinámicas en tiempo real durante el tratamiento térmico de materiales con microondas

“…Complementariamente, el calentamiento con microondas es un campo que desde su descubrimiento ha ido aumentando su popularidad [2], dentro de las ventajas potenciales de este calentamiento sobre los métodos convencionales se encuentran el reducido tiempo de procesado térmico del material y el calentamiento volumétrico, lo que permite que este sea uniforme dentro del sólido [3]. Ante estas ventajas, algunos investigadores han propuesto hacer uso de las microondas para la estimación de parámetros tales como las propiedades electromagnéticas del material [4], [5].…”