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Page 2 of 17 Liu et al. J Mater Inf 2023;3:17 https://dx.doi.org/10.20517/jmi.2023.19

INTRODUCTION
Being one kind of ultra-high temperature ceramics [1-10] , transition metal carbide ceramics of the IVB, VB,
and VIB groups have found wide applications in various fields, such as hard cutting tools, high-temperature
abrasives, aerospace industry, submarine equipment, and nuclear energy [1,2,10-13] . They also present several
attractive properties, including excellent thermal and electrical conductivity, chemical corrosion resistance,
high melting point and hardness, abrasion resistance, oxidation and radiation resistance, and high
temperature stability [14-16] . In recent years, a new approach to alloy design has emerged with the development
of polymetallic solid solution compounds. These alloys, known as high entropy materials (HEMs) [17-21] , are
formed by combining five or more principal elements, each with a concentration between 35 and 5 at.% and
exhibiting a mixing entropy S > 1.5R. Owing to the contributions of high entropy, HEMs exhibit unique
characteristics different from those of metal simple substances, which can be summarized as four major
effects, including high entropy effects, sluggish diffusion effects, severe lattice distortion effects, and cocktail
effects.

High entropy carbide ceramics (HECs) are single-phase solid solutions composed of four or more transition
metal (TM) atoms. Theoretical studies have helped scientists to prepare and study HECs more efficiently
and economically through different simulation methods [22,23] . For instance, high-throughput computing and
machine learning (ML) have been used to study HEMs [22,24-29] . Castle et al. experimentally fabricated the high
entropy carbides (HECs) and inherited the four effects of HEMs [30,31] . Medium entropy carbides (MECs)
were then derived from the concept of HECs, with a mixing entropy between 1R and 1.5R. MECs, such as
HECs, have excellent mechanical and functional properties. The study of the properties of MECs in
multicomponent solid solutions has been widely reported [32-36] . Compared to traditional binary carbides,
HECs and MECs exhibit superior physical and chemical performances, such as high thermal stability, good
corrosion resistance, high fracture toughness, high Young’s modulus, and high hardness [24,30,37-42] .

In the field of nuclear energy, the Nuclear Thermal Propulsion (NTP) engine systems are attractive options
for planetary exploration applications due to their high performance, such as Mars exploration [43-46] . In order
to achieve better performance of NTP systems, the NTP reactors will be utilized at temperatures above
2,500 K and maintaining pressures exceeding 3 MPa. Historically, uranium fuel (UC) has been the primary
focus for NTP reactors. The design and study of UC as the initial fuel element have received significant
attention [47-50] . However, relying solely on UC and UC would impose severe limitations on the core
2
[51]
operating temperature, resulting in a reduction in NTP specific impulse .
In recent decades, there has been a progression in the development of carbide fuels, transitioning from
single-component compositions [52-57] to multicomponent compositions, such as (ZrU)C and (ZrNbU)C .
[35]
Notably, Pelaccio et al. have designed solid-solution (ZrU)C ceramic fuels, which currently serve as the
primary source of nuclear fuel for NTP reactors, offering improved reliability . Composite ceramic fuels,
[58]
including UC or (ZrU)C, exhibit advantageous properties such as high melting point temperatures and high
thermal conductivity. Unfortunately, the compatibility of composite ceramics, such as UC or (ZrU)C, with
fuse-resistant alloys is limited at higher temperatures. Further exploration of incorporating a third metal
atom into the (ZrU)C system holds the potential to unveil the future prospects of carbide fuels, specifically
MEC ceramic fuels, for advanced nuclear power systems. Multicomponent (ZrNbU)C fuels are regarded as
the preferred choice for realizing high thrust nuclear-powered spacecraft in the future, and in the study, a
powder metallurgical process combining carbothermal reduction and liquid phase sintering is used to
prepare poly (ZrNbU)C fuels. The mechanisms and laws of the process parameters on the reaction kinetics,