Search

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 (MDTA) 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 permittivity) alter (generally smoothly) 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. [ABSTRACT FROM AUTHOR]

This article describes the design and operation of a new thermal analysis instrument which uses microwaves to simultaneously heat and detect thermally induced transformations in samples with masses in the range of 50 mg to 0.5 g. The data acquisition and control software developed for the instrument support a range of experimental techniques including constant power, linearly ramped power, linearly ramped temperature, and various modulated methods. Microwave thermal analysis utilizes the fact that physical or chemical alterations in a material, caused by processes such as melting, decomposition, or solid-solid phase changes, cause variations in its dielectric properties. These can be revealed by a variety of means including changes in the sample temperature, the differential temperature, or the shape of the power profile during linear heating experiments. The scope of the instrument is demonstrated with the decomposition of basic copper carbonate. The large temperature increase (∼100 °C) observed on the formation of the strongly coupling oxide indicates the potential sensitivity of using the thermal effects of dielectric changes as a means of detection. Further fine detail can be revealed by the use of derivative plots of either the applied power or the temperature. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Focuses on the application of sample controlled thermal analysis (SCTA) to the study of the reactivity of acid-base catalysts. Use of SCTA to overcome limitations of conventional thermal methods; Sensitivity of SCTA to catalytic reaction; Implications of the study for the reactivity of catalyst.

Sample controlled thermal analysis techniques such as constant rate transformation analysis or stepwise isothermal analysis, where the transformation rate of the sample itself is used to control the experiment, are becoming increasingly important [1]. The measurements are normally carried out using changes in the sample mass, sample dimensions or in the evolved gas, as the property used to control the experiment, and enable reactions to be studied in greater detail than is possible using linear heating techniques. A new approach is described here where a thermomicroscopy system has been developed to enable the intensity of the light reflected or transmitted by the sample to be used as the controlling signal [2]. [ABSTRACT FROM AUTHOR]

Focuses on the development of a preparative scale sample controlled thermogravimetric (SCTG) system. Importance of the SCTG system for larger scale sample controlled thermobalance; Objective of the SCTG concept; Performance of the system in studies on organic compounds.