The Freezer/Mill®, a cryogenic mill is beneficial in areas where other grinding processes fail. Flexible materials such as biological samples and synthetics are embrittled at Liquid Nitrogen temperatures. The sample is hermetically sealed; therefore cross contamination is not possible. Since the Freezer/Mill is powered magnetically, there is no wear and tear on the movable components.
Nafion® , a polymer similar to Teflon (perfluorosulfonate polymer), has been used for more than three decades as a proton conducting membrane in fuel cells, and currently serves as an electrolyte. Owing to its properties it proved very appropriate for this application. The DLR has currently developed a technique to apply the Nafion in powder form. For the manufacture of this Nafion powder, the Freezer/Mill, acryogenic mill is employed. As a polymer, Nafion is highly flexible and soft which evidently makes milling hard. The Nafion granules (5 mm particle size) are minimized to approximately 10 µm during grinding. The resulting powder is blended with a catalyst and sprayed dry onto to the 25 - 175 µm thick membrane.
The fuel cell produces electrical output via electro-chemical oxidization of hydrogen into a proton and an electron as well as oxygen reduction as counter reaction. So as to reduce the required activation energy for this reaction, a catalyst has to be used. In the Polymer Electrolyte-Membrane-Fuel Cell (PEM-MC) platinum is used as a catalyst. The direct methanol fuel cell, in addition, needs ruthenium as a catalyst. These polymer membranes are employed in a range of fuel cells.
In spite of the special properties of Nafion, which as a polymer is very flexible and soft, it was possible to achieve a very fine powder after a combined grinding time of 20 minutes using the freezer/mill. No particle size determination was performed, although because of the thickness of the applied layer the maximum particle size can be stated as less than 25 µm.