Abstract
Thermomagnetic analysis of magnetic susceptibility k(T) was carried out for a number of natural powder materials from soils, baked clay and anthropogenic dust samples using fast (11oC/min) and slow (6.5oC/min) heating rates available in the furnace of Kappabridge KLY2 (Agico). Based on the additional data for mineralogy, grain size and magnetic properties of the studied samples, behaviour of k(T) cycles and the observed differences in the curves for fast and slow heating rate are interpreted in terms of mineralogical transformations and Curie temperatures (Tc). The effect of different sample size is also explored, using large volume and small volume of powder material. It is found that soil samples show enhanced information on mineralogical transformations and appearance of new strongly magnetic phases when using fast heating rate and large sample size. This approach moves the transformation at higher temperature, but enhances the amplitude of the signal of newly created phase. Large sample size gives prevalence of the local micro- environment, created by evolving gases, released during transformations. The example from archeological brick reveals the effect of different sample sizes on the observed Curie temperatures on heating and cooling curves, when the magnetic carrier is substituted magnetite (Mn0.2Fe2.70O4). Large sample size leads to bigger differences in Tcs on heating and cooling, while small sample size results in similar Tcs for both heating rates.
Highlights
Analysis of the behavior of magnetic susceptibility during continuous heating up to 700◦C and the following cooling is a standard method for identification of magnetic mineralogy in natural samples through the observed Curie temperatures (Sanford, 1928; O’Reilly, 1984; Hrouda, 1994)
This implies that factors such as heating rate and sample size, which are critical for the performance and results of classical thermal analyses [thermogravimetry (TG), differential thermal analysis (DTA), effluent gas analysis (EGA), differential scanning calorimetry (DSC); Kissinger, 1956; Gallaghar, 1997; Brown, 1998; Fernandez et al, 2011; Kousksou et al, 2011], play a role in the recorded variations of magnetic susceptibility with increasing/decreasing temperature as well
The aim of the present study is to show through several experimental case studies of different natural samples how the two parameters—heating rate and sample size— influence the shape of the curves and behavior of low-field magnetic susceptibility with increasing temperature
Summary
Analysis of the behavior of magnetic susceptibility during continuous heating up to 700◦C and the following cooling is a standard method for identification of magnetic mineralogy in natural samples through the observed Curie temperatures (Sanford, 1928; O’Reilly, 1984; Hrouda, 1994). Frequently observed structural and mineralogical phase transformations during heating and cooling runs of the k-T analysis are linked to the kinetics and chemical thermodynamic of the processes involved. This implies that factors such as heating rate and sample size, which are critical for the performance and results of classical thermal analyses [thermogravimetry (TG), differential thermal analysis (DTA), effluent gas analysis (EGA), differential scanning calorimetry (DSC); Kissinger, 1956; Gallaghar, 1997; Brown, 1998; Fernandez et al, 2011; Kousksou et al, 2011], play a role in the recorded variations of magnetic susceptibility with increasing/decreasing temperature as well. The aim of the present study is to show through several experimental case studies of different natural samples how the two parameters—heating rate and sample size— influence the shape of the curves and behavior of low-field magnetic susceptibility with increasing temperature
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