Fast grain growth phenomenon in high-entropy ceramics:A case study in rare-earth hexaaluminates

It is generally reported that the grain growth in high-entropy ceramics at high temperatures is relatively slower than that in the corresponding single-component ceramics owing to the so-called sluggish diffusion effect.In this study,we report a fast grain growth phenomenon in the high-entropy ceramics(La_(0.2)Nd_(0.2)Sm_(0.2)Eu_(0.2)Gd_(0.2))MgAl_(11)O_(19)(HEMA)prepared by a conventional solid-state reaction method.The results demonstrate that the grain sizes of the as-sintered HEMA ceramics are larger than those of the corresponding five single-component ceramics prepared by the same pressureless sintering process,and the grain growth rate of HEMA ceramics is obviously higher than those of the five single-component ceramics during the subsequent heat treatment.Such fast grain growth phenomenon indicates that the sluggish diffusion effect cannot dominate the grain growth behavior of the current high-entropy ceramics.The X-ray photoelectron spectroscopy(XPS)analysis reveals that there are more oxygen vacancies(OV)in the high-entropy ceramics than those in the single-component ceramics owing to the variable valance states of Eu ion.The high-temperature electrical conductivities of the HEMA ceramics support this analysis.It is considered that the high concentration of OV and its high mobility in HEMA ceramics contribute to the accelerated migration and diffusion of cations and consequently increase the grain growth rate.Based on this study,it is believed that multiple intrinsic factors for the high-entropy ceramic system will simultaneously determine the grain growth behavior at high temperatures.

Synergistic effects of high-entropy engineering and particulate toughening on the properties of rare-earth aluminate-based ceramic composites

Rare-earth aluminates(REAIOs)are potential thermal barrier coating(TBC)materials,but the relatively high thermal conductivity(ko,~13.6 W·m^(-1)·K^(-1))and low fracture toughness(K_(1c),-1.9 MPa·m^(1/2))limit their application.This work proposed a strategy to improve their properties through the synergistic effects of high-entropy engineering and particulate toughening.High-entropy(La_(0.2)Nd_(0.2)Sm_(0.2)Eu_(0.2)Gd_(0.2)AlO_(3))(HEAO)-based particulate composites with different contents of high-entropy(La_(0.2)Nd_(0.2)Sm_(0.2)Eu_(0.2)Gd_(0.2))_(2)Zr_(2)O_(7)(HEZO)were designed and successfully prepared by solid-state sintering.The high-entropy feature of both the matrix and secondary phases causes the strong phonon scattering and the incorporation of the HEZO secondary phase,remarkedly inhibiting the grain growth of the HEAO phase.As a result,HEAO-xHEZO(x=0,5%,10%,25%,and 50%in volume)ceramic composites show low thermal conductivity and high fracture toughness.Compared to the most commonly applied TBC material-yttria stabilized-zirconia(YSZ),the HEAO-25%HEZO particulate composite has a lower thermal conductivity of 0.96-1.17 W·m^(-1)·K^(-1)(298-1273 K),enhanced fracture toughness of 3.94±0.35 MPa-m,and comparable linear coefficient of thermal expansion(CTE)of 10.5×10^(-6)K^(-1).It is believed that the proposed strategy should be revelatory for the design of new coating materials including TBCs and environmental barrier coatings(EBCs).