Development of a novel instrument for microwave dielectric thermal analysis

Abstract

The advantages of microwave heating in industrial processing are becoming more widely appreciated and the technique is of increasing commercial significance. However, knowledge of a material's dielectric properties as a function of temperature is of considerable importance as they determine the efficiency with which the material converts microwave energy into heat. This article describes the development of a novel instrument for a new form of thermal analysis, microwave dielectric thermal analysis ͑MDTA͒ in which the sample is heated not conventionally but by microwaves, while its dielectric properties at microwave frequencies are determined quasisimultaneously using a network analyzer. A plot of the dielectric properties against temperature then gives the MDTA curve. The fundamental principle underlying the use of MDTA is that not only do a material's dielectric properties ͑both the real and imaginary parts of the complex permittivity͒ alter ͑generally smoothly͒ as a function of temperature but dramatic differences can be found when chemical or physical changes occur. MDTA measures the sample's dielectric properties as a function of temperature before, during, and after the chemical and/or physical changes occurring as it is heated in the microwave field and thus provides a unique way of studying them. Furthermore, due to the quasisimultaneous nature of the measurements, MDTA can also make a valuable contribution to the investigation of the so-called "microwave effect" anomalies that occur when certain materials are heated in a microwave field. We show that the technique can be used to study a wide range of thermal transformations, including solid-solid phase changes, melting, and chemical reactions, e.g., dehydration, via the complex permittivity-temperature profiles generated in MDTA.

Evaluation of Microwave Vacuum Drying as an Alternative to Freeze-Drying of Biologics and Vaccines: the Power of Simple Modeling to Identify a Mechanism for Faster Drying Times Achieved with Microwave

“…As per Eq. 2 , the power dissipated ( P d ) from microwave absorption is in terms of watts/m 3 and is proportional to a constant K , the dielectric constant ɛ , the loss factor tan( δ ), the microwave frequency f , and the electric field E ( 23 , 24 ).…”

Thermal Analysis

“…With the use of tablets of paracetamol and hyproxypropyl methylcellulose, it is demonstrated that the technique is capable of differentiating between the components in scanning mode, whereas three-dimensional imaging can be accomplished by allowing the probe to melt through the paracetamol down to the underlying polymer. Nair et al (34) have developed a novel instrument for a new type of thermal analysis, microwave dielectric thermal analysis, in which dielectric properties of a sample are determined while it is heated by microwaves. A plot of the dielectric properties against temperature allows one to observe the effect of the temperature as well as of chemical and/or physical changes on material's dielectric properties (both the real and imaginary parts of the complex permittivity).…”