Significant hidden temperature gradients in thermogravimetric tests

In thermal analysis, correct measurement of temperature is usually ensured by means of a calibration procedure. However, in addition to make sure that the right value of temperature is measured, estimation of temperature gradients into the sample is important. That is especially important in thermogravimetric (TG) analysis, where combinations of some of the common sample sizes heating rates could lead to important differences of temperature into the sample. If there is a significant gradient between different parts of the sample, then the temperature of the thermocouple, although correct, does not actually represent the temperature of the whole sample. While the correctness of the temperature is always important, errors in temperature measurement are critical in kinetic studies. Thus, estimations of the temperature gradients that appear into the sample as a result of a given treatment are of highest interest to choose the right operational conditions that minimize that gradient. That is particularly important for kinetic studies. In this work, thermal gradients originated into a sample during a typical TG test are estimated through a simulation study performed on the Comsol ™ software. A typical vertical TG furnace, sample size of about 125 mg, and several heating rates were used. Additionally, samples of different void contents were considered. The results of the simulation show that significant gradients of temperature can be achieved into the sample with experimental conditions like those that are often used. It is also observed that the difference of temperature between the sample and the furnace wall not only depends on the heating rate, which can be easily corrected by calibration at the corresponding heating rate, but also varies with temperature, which makes highly recommended to calibrate in more than one temperature point when broad ranges of temperature are considered.