Platinum Ceramic Composites as new Electrode Materials: Fabrication, Sintering, Microstructures and Properties

Abstract

In a variety of applications metal matrix composites (MMC) show very beneficial properties. A well-known example are oxide dispersion strengthened (ODS) materials. Their high temperature mechanical properties can be noticeably improved by adding ceramic particles to a metal matrix. A similar approach was used for the development of wearresistant materials, e.g. silver-based contact materials for switches [1] or iridium-based electrode materials. [2] Both electrical contacts and electrodes are damaged by an arc discharge. The crater formation can be explained by the particle-ejection-model [3] which describes the loss of material as melting and splashing of electrode material.The wear resistance of contact materials is increased by the addition of particles like SnO 2 , CdO, In 2 O 3 to a silver matrix. In presence of such particles, electrical discharges cause smaller and flatter craters. [4,5] As a result of the increased viscosity of the molten material, less particles are ejected from the crater. [4] This effect can only operate if the melting point of the particles is higher than that of the matrix material.Furthermore, the wear resistance can be raised by the addition of particles with a sublimation temperature close to the melting point of the matrix material. The sublimation of the particles, e.g CdO consumes energy which cools the melt. This reduces the amount of molten material, thus leading to less erosion. [6] In order to improve the erosion resistance of electrode materials Al 2 O 3 , MgO, and rare earth metal oxides, especially Y 2 O 3 and La 2 O 3 have been added to iridium and ruthenium, respectively. [7,8] Two theories are discussed to explain the increase in wear resistance. On the one hand, these oxides impede oxidation of the electrode surface. [9,10] On the other hand, the large atomic weight of the oxides, especially of rare earth metals, leads to a reduced sputter rate during the glow discharge. [11] In a previous paper [12] we showed that oxidation plays a dominant role in the erosion of electrode materials. In the case of platinum, erosion stems from the plasma-assisted formation of volatile oxides in the cathode spot.In this paper, the fabrication, microstructural design and characterization of platinum composites with 14 vol.% ceramic dispersoids are presented. Metal oxides of different atomic weight and high melting points were chosen: MgO (M Mg = 24.3 g/mol), ZrO 2 (M Zr = 91.2 g/mol) and CeO 2 COMMUNICATIONS ADVANCED ENGINEERING MATERIALS 2006, 8, No. 1-2 a) b) Fig. 1. Metallographic cross sections of Pt/MgO samples after the second sintering at 1600°C: (a) mechanically alloyed and (b) chemically alloyed samples. The microstructure of the chemically alloyed sample is much more homogeneous.