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YB22C2N is one of a series of rare earth borocarbonitrides and is potentially the long awaited n-type counterpart to boron carbide. We conducted studies on YB22C2N spark plasma sintered with additions of YB4 and YB25C, including the investigations of the densification process and the thermoelectric properties of the material. We discovered that a small amount of dopants can lower the starting temperature of densification during spark plasma sintering (SPS). Variations of pressure and temperature during the sintering process are also found to have an effect. Electrical conductivity of the dense samples has increased due to insertion of metal borides and also because of the improvement of the relative density. At the same time, only a slight reduction was observed for the Seebeck coefficient leading to an important improvement of power factor. The highest density of more than 95% was achieved with 5 wt% of YB25(C) dopant. I. INTRODUCTION

Because of their interesting physical properties, higher boride compounds such as REB44Si2 (RE = rare earth) and REB22C2N are under investigation, since they are high-temperature materials and exhibit intrinsic low thermal conductivity, making them potentially good high temperature thermoelectric materials.1–3 The rare earth borocarbonitrides REB17CN, REB22C2N, and REB28.5C4 are particularly promising as n-type counterparts for p-type boron carbide.3,4 This series of homologous compounds has a structure related to boron carbide as is shown by the structures of YB22C2N and “B4C” in Fig. 1. “B4C” is in quotation marks since it has been the traditionally used nomenclature for boron carbide, however, in actuality such a carbon-rich composition is difficult (or impossible) to attain. The quotation marks are omitted for clarity in the rest of the paper. Similar to B4C, these compounds are difficult to densify and due to their low density, their figures of merit have appeared to be low. Attempts to reach a higher relative density for YB22C2N has been limited to 75% by spark plasma sintering (SPS).4 Various studies have already been carried out on the sintering of boron carbide with the help of transition elements, transitional element oxides, or nonoxide materials such as Al, Fe, TiO2, SiC, and TiC.5–7 All of these sintering aids have been effective for sintering. However in such cases, the dopants change physical properties because of the presence of large amounts of secondary phases. For example, the addition of TiB2 to B4C affected the thermoelectric properties by a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0100 J. Mater. Res., Vol. 25, No. 4, Apr 2010

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lowering the value of ZT compared to pure B4C.7 Additionally, a sintering temperature in the range 1900– 2300
C is needed to obtain relative density above 90%. In consideration of previous work on B4C, we have focused on the densification behavior of YB22C2N when doped with Y-B-(C) compounds. In this way, we avoid inserting fo