Size determination of superconducting MgB2 powder from magnetization curve, image analysis and surface area measurements

This paper reports on a method for estimating the average grain size of superconducting nanoparticles through their magnetic properties. The use of SQUID magnetometry to determine the average MgB2 particle size was investigated and the results were compared with those of several different techniques. In particular, the data obtained from zero field cooled magnetization measurements as a function of the temperature were compared with the results obtained by the scanning electron microscopy and Brunauer-Emmett-Teller techniques. The particle magnetization was measured by a commercial SQUID magnetometer in a magnetic field (10 G) and at temperatures ranging from 5 to 50 K by dispersing the powders in a grease medium. The grain size was obtained by fitting the data taking into account the Ginzburg-Landau temperature dependence of the London penetration depth. Variations in typical modeling parameters were explored in order to gain a better picture of the average grain size and the effectiveness of various measurement techniques. We find that it is possible to use the magnetization measurements to determine the average grain size, even though an SEM analysis coupled to the image analysis allows the extraction of more information about the grain-size distribution. Furthermore, a Matlab routine has been developed in order to automatically analyze several SEM images.