**2.1 Fabrication routes**

The fabrication of HEAs can be achieved through solid and liquid state routes. The processing of bulk HECs is preferred in solid state due to the high melting

points of ceramics. The solid-state processing includes high energy ball milling, conventional solid state sintering, self-propagating high temperature synthesis (SHS), and spark plasma sintering (SPS) under the combined effect of heating using electric current and pressure. The SPS is a rapid and facile method to produce nearly dense components and suitable for sintering of high-entropy ceramics. Typically, the precursor ceramic powders are mixed and filled into a graphite die and rapidly sintered under high vacuum, pulsed direct electrical current, and uniaxial pressure.

Different types of the ceramic precursors have been used to fabricate HECs. The most commonly used precursors are commercial ceramic powders. The ceramic powders are mixed in desired stoichiometric ratio and homogenized using ball milling followed by spark plasma sintering to a target temperature and uniaxial pressure. The synthesis of high-entropy ceramic powders has been investigated as well. The precursor ceramic powders can be pre-synthesized via thermal reduction (TR) where metal oxide powders are used as reactants (**Figure 1**). The advantages of using oxide powders as raw materials include reducing the cost of the starting materials and possibilities of producing high purity ceramic powders with controlled grain size. Feng et al. [1] used metallic oxides and carbon black powder as starting material to produce high-entropy (Hf, Zr, Ti, Ta, Nb)C powder. The carbothermal reduction of oxide ceramics was completed at 1873 K under vacuum in 1 hour. Subsequent solid solution formation was achieved at a higher temperature of 2273 K, demonstrating a single-phase rocksalt structure of the solid solution. Ye et al. [2] reported the synthesis of high-entropy (Zr0.25Ta0.25Nb0.25Ti0.25)C powders of a single-phase rocksalt crystal structure by carbothermal reduction of metal oxides and graphite.

Liu et al. [3] demonstrated the synthesis of high-entropy (Hf0.2Zr0.2Ta0.2 Nb0.2Ti0.2)B2 powders with single-phase hexagonal structure through borothermal reduction from metal oxides and amorphous boron powders at 1973 K under argon atmosphere. The synthesized high-entropy metal diboride powders showed a fine particle size of 310 nm. Instead of boron, Zhang et al. [4] used the combination of B4C, graphite, and metal oxides to synthesize (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2, (Hf0.2 Zr0.2Mo0.2Nb0.2Ti0.2)B2, and (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 powders by boro-/carbothermal reduction. The reaction is reported to be more effective than borothermal reduction, with a lower reduction temperature 1873 K.

Wei et al. synthesized high-entropy carbide (Ti0.2Zr0.2Nb0.2Ta0.2W0.2)C by employing different methods using commercial carbide powders and thermally reduced carbide powders and *in-situ* fabrication process from elemental powders [5]. All three compositions revealed single-phase face-centered cubic (FCC) structure according to X-ray diffraction (XRD) patterns. The comparison of the

**159**

*High-Entropy Ceramics*

powders and TR approach.

studied by Anand et al. [8].

insulation applications.

conventional diborides.

*2.2.2 High-entropy borides (HEBs)*

(LiPON) solid electrolyte (2 × 10<sup>−</sup><sup>6</sup>

>10<sup>−</sup><sup>3</sup>

Na<sup>+</sup>

*2.2.1 High-entropy oxides (HEOs)*

*DOI: http://dx.doi.org/10.5772/intechopen.89527*

**2.2 Overview of current researches on bulk HECs**

constant was discovered by Bérardan et al. [9]. With a Li<sup>+</sup>

three methods suggested the microstructure inhomogeneity in case of elemental

The first high-entropy ceramic was reported by Rost et al. [6], on the production of entropy stabilized metal oxides with rocksalt crystal structure, synthesized from equimolar CoO, CuO, MgO, NiO, and ZnO in a tube furnace at temperatures above 850°C. Homogenous distribution of the cations in the crystal structure was observed. The system showed a reversible transformation between the highentropy solid solution and multicomponent oxide mixture. Later on, detailed investigation using extended X-ray absorption fine structure (EXAFS) was performed to investigate the localize structure of the aforementioned high-entropy oxide [7], demonstrating that the anion lattice (O sublattice) has the most distortion to accommodate the size mismatch in the cation lattice (metal sublattice). The phase stability, as a result of configurational entropy, of the same HEO system was

The entropy stabilized oxide ((Mg, Ni, Co, Cu, Zn)O with colossal dielectric

S/cm, which is much higher compared to lithium phosphorous oxynitride

. The potential of high-entropy oxide in the lithium battery has been reported by Sarkar et al. [11] with improved storage capacity retention and cycling stability. Jiang et al. [12] synthesized high-entropy perovskite oxides from multiple ABO3 perovskite oxides. Djenadic et al. [13] utilized nebulized spray pyrolysis (NSP) to synthesize single-phase rare earth oxide powders from seven equiatomic rare earth oxides. In the REO system, the importance of selecting the starting component in the reported multicomponent oxide system was highlighted, with cerium (Ce4+) addition being confirmed to improve the formation of single-phase solid solution. Gild et al. [14] fabricated high-entropy fluorite oxides from five fluorite oxides via high-energy ball milling and SPS, followed by various annealing treatments. Most of the fabricated HEOs revealed as nearly fully dense pellets with single-phase Fm-3 m crystal structure. Phonon scattering effect in the HEO system resulted in low thermal conductivity, making the synthesized HEOs desirable for thermal

The high-entropy borides are designed and fabricated from transition metal diborides, HfB2, ZrB2, TaB2, etc., aiming at developing a new class of ultrahigh temperature metal diboride materials with superior mechanical properties over

Gild et al. [15] sintered HEBs from five-component equimolar metal diborides via SPS. The fabricated HEBs from compositions like (Hf0.2Zr0.2Ta0.2Mo0.2Ti0.2) B2 and (Mo0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2 possess single-phase hexagonal AlB2 structure (**Figure 2**) as the parent metal diborides, with alternating hexagonal metal cations net and rigid boron net in the high-entropy structure. The authors reported that in case of using W2B5 in the starting precursors, single-phase solid solution was not successfully formed. Notably, W2B5 has a different crystal structure with other utilized metal borides, as well as limited solubility in other borides, which could

was suggested to be an excellent substituent as superionic conductors for Li<sup>+</sup>

ionic conductivity of

and

S/cm) [10], the produced high-entropy oxide

**Figure 1.** *The fabrication routes of dense high-entropy ceramics.*

*Engineering Steels and High Entropy-Alloys*

uniaxial pressure.

oxides and graphite.

points of ceramics. The solid-state processing includes high energy ball milling, conventional solid state sintering, self-propagating high temperature synthesis (SHS), and spark plasma sintering (SPS) under the combined effect of heating using electric current and pressure. The SPS is a rapid and facile method to produce nearly dense components and suitable for sintering of high-entropy ceramics. Typically, the precursor ceramic powders are mixed and filled into a graphite die and rapidly sintered under high vacuum, pulsed direct electrical current, and

Different types of the ceramic precursors have been used to fabricate HECs. The most commonly used precursors are commercial ceramic powders. The ceramic powders are mixed in desired stoichiometric ratio and homogenized using ball milling followed by spark plasma sintering to a target temperature and uniaxial pressure. The synthesis of high-entropy ceramic powders has been investigated as well. The precursor ceramic powders can be pre-synthesized via thermal reduction (TR) where metal oxide powders are used as reactants (**Figure 1**). The advantages of using oxide powders as raw materials include reducing the cost of the starting materials and possibilities of producing high purity ceramic powders with controlled grain size. Feng et al. [1] used metallic oxides and carbon black powder as starting material to produce high-entropy (Hf, Zr, Ti, Ta, Nb)C powder. The carbothermal reduction of oxide ceramics was completed at 1873 K under vacuum in 1 hour. Subsequent solid solution formation was achieved at a higher temperature of 2273 K, demonstrating a single-phase rocksalt structure of the solid solution. Ye et al. [2] reported the synthesis of high-entropy (Zr0.25Ta0.25Nb0.25Ti0.25)C powders of a single-phase rocksalt crystal structure by carbothermal reduction of metal

Liu et al. [3] demonstrated the synthesis of high-entropy (Hf0.2Zr0.2Ta0.2 Nb0.2Ti0.2)B2 powders with single-phase hexagonal structure through borothermal reduction from metal oxides and amorphous boron powders at 1973 K under argon atmosphere. The synthesized high-entropy metal diboride powders showed a fine particle size of 310 nm. Instead of boron, Zhang et al. [4] used the combination of B4C, graphite, and metal oxides to synthesize (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)B2, (Hf0.2 Zr0.2Mo0.2Nb0.2Ti0.2)B2, and (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2 powders by boro-/carbothermal reduction. The reaction is reported to be more effective than borothermal

Wei et al. synthesized high-entropy carbide (Ti0.2Zr0.2Nb0.2Ta0.2W0.2)C by employing different methods using commercial carbide powders and thermally reduced carbide powders and *in-situ* fabrication process from elemental powders [5]. All three compositions revealed single-phase face-centered cubic (FCC) structure according to X-ray diffraction (XRD) patterns. The comparison of the

reduction, with a lower reduction temperature 1873 K.

**158**

**Figure 1.**

*The fabrication routes of dense high-entropy ceramics.*

three methods suggested the microstructure inhomogeneity in case of elemental powders and TR approach.
