T-Axis Calibration

As with many other analytical techniques, the temperature axis used in differential thermal analysis (and DSC) must be calibrated with materials having known transition temperatures. The International Confederation of Thermal Analysis (ICTA) has been very active in developing a set of standard materials for this purpose (19) and has worked with the U.S. National Bureau of Standards to have these materials made commercially available (20). The U.S. National Bureau of Standards GM 754-GM 760 DTA temperature standards are listed in Table 6.2. They cover the temperature range from —83 to 925 C. The results of an ICTA round-robin study with 24 cooperating laboratories have been reported by Menis and Sterling (20). 

Freezing temperature standards are also available from the National 

Physical Laboratory, Teddington, England, U.K. These substances and their respective freezing points are given in Table 6.3. 

Judd and Pope (123) proposed the use of (SrBa)C03 mixtures as temperature standards in the temperature range 780-930°C. These double carbonates undergo an orthorhombic-hexagonal transition at a temperature that is dependent on the composition of the double carbonate. This transition temperature for the various double carbonates is shown in Table 6.4. In each case, a single narrow endothermic DTA peak is obtained with a peak 

Fairly high-resistance thermistors, 100,000Q. at ambient temperature, connected in a bridge circuit have been used to detect the differential temperature (22,23). This method does not normally require the use of a dc amplifier. Because their resistance decreases rapidly with increase in temperature, thermistors are generally only useful up to about 300°C (23). 

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