A microcontroller-based differential thermal analysis instrument

Abstract

Differential thermal analysis is employed to characterize new materials. The structure of a prototype of a differential thermal analysis instrument, based on a microcontroller is described. The design considerations of the temperature controller to vary the temperature of the heating unit at a constant rate are discussed. Experimental and simulation results are presented. I. INTRODUCTION Thermal analysis has been traditionally used in the characterization of clays and their natural and artificial mixtures. This technique in different forms, is also being increasingly applied in the characterization of polymers and plastics. The differential thermal analysis (DTA) is conducted to detect amongst other things, the endothermic or exothermic reactions that occur in the material under test, when the same is subjected to a specified constant heating rate. The test, in its essence, consists in monitoring the changes in the temperature of the sample under test with respect to the temperature of inert rcfcrence material when both are subjected to the same constant heating rate. The other thermal analysis tests which are more frequently used, in the field of polymers and plastics are differential scanning calorimetry (DSC), the therniomechanical analysis (TMA), the dynamic mechanical analysis (DMA) and the thermal gravimetric analysis (TGA) (I). This last is also useful in the field of clays and ceramics to determine quantitative relations of different phases in a typical thermal reaction which involves loss of some mass in the sample under test. A conventional typical DTA instrument (fig. 1) consist of a heating unit (HU), a programable temperature controller (TC), a sample holder (SH) temperature sensing thermocouples, aiid associated low drift d.c. amplifiers as well as a strip chart recorder (2,3). There is usually a provision for heating the sample to upto 1200 C at an adjustable constant heating rate varying from 2 C/min to 20 C/min. Typically the duration of the experiment may vary between 1 hour and 10 hours. In this paper, a compact prototype, based on a 16-bit microeontroller, for a DTA instrument, is proposed. The programable temperature control of the heating unit and acquisition of the differential temperature of sample under test w.r.t. the temperature of the reference material is done in the microcontroller. The design of the temperature controller for the Ku assembled in the laboratory, based on the experimentally determined model of the HU, is discussed. The simulation aiid experimental results for the P and PLD controllers are presented. The experimental results of a typical Merentia1 thermal analysis test conducted with the proposed prototype under the control of a P.C. compatible microcomputer are presented.