Temperature calibration Curie point method

For isothermal measurements, it is advisable to use a furnace of low thermal capacity unless suitable arrangements can be made to transport the sample into a preheated zone. The Curie point method [132] of temperature calibration is ideally suited for microbalance studies with a small furnace. A unijunction transistor relaxation oscillator, with a thermistor as the resistive part with completion of the circuit through the balance suspension, has been suggested for temperature measurements within the limited range 298—433 K [133]. 

Figure 11.4. Curie-point method of temperature calibration...

Figure 10.25 The Curie point method for temperature calibration. (Reproduced with kind permission of Springer Science and Business Media from M. Brown, Introduction to Thermal Analysis, Kluwer Academic Publishers, Dordrecht. 2001 Springer Science.)...

Figure 16.5 The TGA Curie point method records for each standard an apparent sharp weight change at a well-defined temperature, which corresponds to a known transformation in the standard s ferromagnetic properties at that temperature. The figure shows the relative temperature precision from three replicate calibration runs using alumel and nickel Curie point standards. (Courtesy of TA Instruments, New Castle, DE, www.tainst.com.)...

Curie point method The materials selected for temperature calibration are metals and alloys which are ferromagnetic at low temperature but which lose their ferromagnetism at well-defined Curie points. If the magnetic material is placed in the sample holder with a magnet placed below the sample, as shown in Figure 15.9c, the magnetic force on the sample will cause an apparent increase in mass. In a TG rim, the force will disappear fairly abruptly at the Curie point as shown in Figure 15.10. The mid-temperature... 

Temperature The sensor is located as near the sample as is practicable, and the reliability of measurements is increased by calibration, such as in the Curie point (magnetic) method. There may be delays in the transfer of heat to the sample due to conductivity and thermal inertia of both reactant and container, a temperature programmed change, the gas atmosphere flow, and reactant self-cooling or self-heating during the reaction. 

Norem et al. (14, 15) used the third method, as previously discussed, to calibrate the temperature of their type of furnace and/or sample container, A ferromagnetic material was placed in the sample container and suspended within a magnetic field. At the material s Curie point temperature, its equivalent magnetic mass diminishes to zero and the thermobalance indicates an apparent mass-loss. For calibration over the temperature range from ambient temperature to lOOOC, it is obvious that a number of ferromagnetic materials must be used. The criteria which were considered characteristic of an ideal standard were the following (15) ... 

The weight calibration of thermobalances is done using standard weights. The temperature calibration is more difficult. The method using the Curie point temperature, as... 

Temperature calibration caimot be carried out by the normal IPTS standards since these involve no mass change. A method has been devised using the Curie point (Tq) of metals. This is the temperature above which they have no ferromagnetism. For example, for nickel metal, the Curie px)int temperature is 353°C. By placing a magnet near a calibration sample of nickel, the mass is increased at temperatures below To, but not above that temperatiue. A step occurs in the TG trace that allows calibration. 

Calibration and Standards. Thermal analysis methods are not absolute and calibration is needed to record the correct abscissa value of temperature T (in Kelvin) and time t (in seconds or minutes). On the ordinate, calibration is necessary for the amplitude of deflection, AT, expressed as the difference in temperature (in Kelvin) for DTA or as heat flux, dQldT (in joules per second or watts) for DSC. Each instrument manufacturer provides methods and standard materials for these calibrations. In addition, ICTAC, in collaboration with the National Institute of Standards and Technology (NIST), has developed a series of materials as calibration standards for DSC/DTA. These reference materials can be used to calibrate both the temperature scale (K, abscissa) and heat flow (J/g, ordinate) on the basis of the integrated area under the curve. Figure 4 shows the heat flow-temperature relationship for various solid-solid and solid-liquid melting standards. Table 3 lists the solidi-to-solid2 transitions, melting points, and Curie temperatures of various pure metals, and also their transition enthalpies (J/g) (11). 

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