UC San Diego

Since their inception in 2004, high entropy alloys have become a new area of intense

research in the metallurgical community. Over five hundred distinct compositions have been

examined and they have proven to have unique and promising characteristics in the mechanical,

thermal, and chemical fields. The field of high entropy ceramics, however, only began in 2016

and is still a fledgling field of research. Here, the bulk synthesis and fabrication of high entropy

diborides, carbides, fluorite oxides, and silicides is examined. In the first part of the dissertation,

the fabrication of high entropy diborides from commercial powder via high energy ball milling

and subsequent spark plasma sintering is investigated. The materials are found to have superior

hardness and oxidation resistance to the majority of binary diborides. In the second section, flash

spark plasma sintering is utilized to fabricate high entropy diboride and high entropy carbide

composites with graphite in 90 seconds. The presence of graphite was found to be necessary for

the homogenization of the diborides even at very high temperatures. In the third section, the

direct synthesis of high entropy diborides via borocarbothermal reduction of metal oxides and

spark plasma sintering; this successfully created fully dense, homogenous materials. In the fourth

part, the production of high entropy fluorite oxides based off the yttria-stabilized zirconia (YSZ)

structure is performed. These materials were found to have significantly reduced thermal and

ionic conductivities compared to 8YSZ, a commonly used fluorite oxide material. In the final

part, a high entropy silicide is fabricated via milling and spark plasma sintering. It was found to

have a thermal conductivity of approximately one third that of similar disilcides. The production

of a wide swath of materials allowed for similar characteristics to be found amongst the

materials. In particular, significant reductions in thermal conductivity and improvements in

hardness (both Vickers and nanoindentation) appear to be uniform properties among all the high

entropy ceramics, similar to what is observed in the metallurgical field. The discovery of these

materials opens up a wide new range of compositional space for refractory and rare earth

ceramics.