Electrical Conductivity and Structural Properties of Cesium Iron Phosphate Glasses: A Potential Host for Vitrifying Nucl
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Electrical Conductivity and Structural Properties of Cesium Iron Phosphate Glasses: A Potential Host for Vitrifying Nuclear Waste Mogus-Milankovic1, K. Furic1 and D. E. Day2 1 Ruder Boskovic Institute, Department of Physics, 10000 Zagreb, Croatia 2 University of Missouri-Rolla, Graduate Center for Materials Research, Rolla, MO 65409-1170, USA ABSTRACT The thermally stimulated current (TSC) and dc conductivity for iron phosphate glasses containing up to 28 mol% Cs2O have been measured in a temperature range from 120 to 400 K. The dc conductivity and activation energy were constant and independent of Cs2O content. With increasing cesium concentration in cesium iron phosphate glasses the slowly moving cesium ions are more tightly bound to the non-bridging oxygen ions and make no measurable contribution to dc conductivity. The dc conduction in these glasses is totally electronic, controlled by electron hopping between iron ions. The ionic conduction is immeasurably small because of the low mobility of the cesium ions. This agreement is reinforced by the excellent chemical durability of the glasses, where the dissolution rate at 90oC changes little with increasing Cs2O content. Raman spectroscopy indicated that the structure of these glasses was composed of predominantly pyrophosphate (P2O7) groups, but the metaphosphate chains (PO3) also existed. INTRODUCTION The most common glass materials used and currently approved, for vitrifying high level nuclear waste (HLW) are borosilicate glasses with several different compositions. However, some of the HLWs at Hanford, WA [1] contain components such as phosphates, halides and heavy metals, which make them poorly suited for disposal in borosilicate glasses. Also, much of the radioactivity (~25%) at Hanford is in the form of radioactive 137Cs sludges [2]. If the radioactive Cs is not removed from HLW sludges, the large amount of thermal energy produced during radioactive decay would limit the concentration of waste that could be immobilized in a glassy form due to extensive heating [3]. Such additional pre-treatment process increases the cost of cleaning up the HLW sludges. Iron phosphate glasses containing from 25 to 40 mol% Fe2O3 appear to be highly effective for vitrifying those HLWs. An advantage of using the iron phosphate glass for vitrifying nuclear waste is related to the fact that the Cs is known to be volatile at higher temperature [4]. Iron phosphate glasses can be melted at temperature as low as 1225 K, which is at least 200 K lower than melting temperature of borosilicate glasses (~1450 K). In the present study the glass formation, structural and electrical properties for the iron phosphate glasses containing up to 28 mol% Cs2O have been investigated. Such a basic knowledge for these glasses is essential for a comprehensive scientific assessment of their use in nuclear waste disposal.
EXPERIMENTAL The glasses were melted in an alumina crucible in air at ∼1300 K for approximately 2 h using a homogeneous mixture of reagent grade of iron oxide, Fe2O3, phosphorus pent
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