Electrical resistivity cerium

Figure 1. Electrical resistance as a function of hydrogen content for cerium-hydrogen and praseodymium-hydrogen systems (6,7)...

Most of the high-pressure studies have involved the determination of the phase boundaries as a function of pressure and temperature using volumetric, electrical resistivity and X-ray techniques. As a result of these studies a few new allotropic phases have been discovered. Other studies include the superconducting behavior of these elements, some magnetic property measurements, and the valence fluctuation behavior of cerium. And as a result of these investigations several surprises were found, which are noted below. 

Fig. 11.2. Cerium solute contribution to the electrical resistivity of YCe alloys, normalized to its value at 0 K, vs temperature. The Ce contribution at 0 K is equal to 12.0 1.5 ftH cm/at.% Ce [after Sugawara and Yoshida (1968)].

Electrical resistance measurements have been reported on PANI/cerium oxide composites [48], polyester fibres and carbon containing epoxy composites [74]. 

Parvatikar and co-workers [48] measured the electrical resistance of PANI/cerium oxide composites and the change in this with relative humidity. The results showed that there was potential for these composites to be nsed as humidity sensors. 

Kondo-like behavior was observed in the lanthanide compoimds, typically in Ce and Yb compounds (Buschow et al. 1971, Parks 1977, Falicov et al. 1981). For example, fire electric resistivity in Cej Laj j Cu6 increases logarithmically with decreasing temperature for all the Af-values (Sumiyama et al. 1986), as shown in fig. 1. The Kondo effect occurs independently at each cerium site even in a dense system. Therefore, this phenomenon was called the dense Kondo effect. 

Altunbas and Harris (1980) studied the cerium-praseodymium alloy system using electrical resistivity. X-ray diffraction and differential thermal analysis (DTA) techniques. In most of the research they used standard commercial material but some relatively pure praseodymium (purified by solid state electrolysis) was used in the DTA measurements. Appropriate amounts of the component metals were arc-melted in purified argon, turned and remelted several times. This was followed by a seven day vacuum anneal at 600°C with slow cooling to room temperature. Their electrical resistivity curves for the praseodymium sample indicated only one solid phase transformation (dhep bcc) whereas the curves for cerium and the Ce-Pr alloys exhibited two transitions, dhep fee (below 61°C for pure cerium) and fee bcc. 

Chnard (1967) used electrical resistivity measurements to study the -phase forming tendencies of cerium alloys between 300 and 1.5 K. The addition of 2 at% samarium to cerium resulted in a hysteresis loop in the temperature vs. resistivity curve between approximately 40 and 170 K. This was attributed to the formation of some aCe during thermal cycling. 

Spedding and Daane (1961) have reported that a 50% alloy of ytterbium in cerium was observed to contain two immiscible liquids. An X-ray study of these phases indicated a solubility of 1 at% Ce in Yb and 3 at% Yb in Ce. King (1969) studied electrical resistance of ytterbium-rich alloys of the cerium-ytterbium phase diagram and observed that an addition of 10 at% cerium moved the resistance peak in pure ytterbium from 3.937 to 5.27 GPa at room temperature. 

Much less efforts have been put in the study of the cerium monochalcogenides CeY (Y = S, Se and Te), since they were considered as rare examples of cerium compounds with normal behaviour. However, the first resistivity measurements on CeS (Schoenes and Hulliger 1985) displayed a temperature dependence similar to that found in CeAl2- This prompted a more systematic investigation of the electrical resistivity of CeS, CeSe and CeTe on single crystals in a large temperature region (2 to 1000 K). 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

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