Facile Preparation, Microstructure and Dielectric Properties of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ Perovskite High-Entropy Ceramics

Abstract

Preparation and properties of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics are investigated. La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics are prepared by a traditional two-step solid-state reaction method in air. La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ single-phase powders are synthesized by calcining the mixed oxides at 1000 °C for 20 h. The high-entropy ceramics are sintered at 1350–1650 °C in a muffle furnace for 4 h by using the above powders. The phase compositions of the high-entropy ceramics at different temperatures are characterized by X-ray diffraction (XRD) with Cu Kα radiation. A field-emission scanning electron microscope with energy-dispersive spectroscopy (EDS) is used to observe the microstructures and analyze the elemental distributions. The hardness and dielectric properties are measured and discussed.

1. Introduction

High-entropy materials, one of the most influential concepts in material science in the past ten years, refer to multicomponent solid solutions formed by five or more components [1]. The first high-entropy ceramic, (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O with regular Fm-3m rock-salt structure, was reported by Rost et al. in 2015 [2]. Since then, a series of high-entropy ceramics, including high-entropy oxide ceramics with fluorite [3,4,5,6], perovskite [7,8,9], spinel structures [10,11] and so on, as well as nonoxide high-entropy ceramics, such as borides [12,13,14], carbides [15,16,17,18], nitrides [19], and silicides [20,21], have been synthesized. Among them, perovskite-structure high-entropy oxides have excellent physical properties for applications in many different fields [7,8,9]. For example, they can be used as cathode materials for solid-oxide fuel cells (SOFCs) [22,23], dielectrics [24,25], and ferroelectric and multiferroic materials [26,27,28].

Lanthanum chromate (LaCrO₃) has a typical perovskite crystal structure, good chemical resistance, and chemical and physical stability at high temperature, but its sintering properties are poor [29,30]. In the current research reports, most of them improve the sintering and high-temperature properties of LaCrO₃ ceramics by doping the A and B sites and adding sintering aids [31,32,33,34]. For example, Ga et al. and Luo et al. [35,36] prepared highly stable composite negative-temperature coefficient (NTC) ceramics by introducing perovskite (LaCrO₃) into certain spinel oxides. Therefore, in this study when designing perovskite-type high-entropy oxides, the active transition metal elements Cr, Mn, Fe, Co, and Ni with close ionic radii were selected for the B site. Theoretically, the highest configurational entropy is obtained for some materials when all the elements in a sublattice have the same atomic fraction [21,37]. In 2021, Guo et al. [38] reported that La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ perovskite high-entropy powders prepared by the coprecipitation and combined with calcination method presented good rate properties for the application in supercapacitors. Vladimir et al. [39] reported that La(Cr_0.2Mn_0.2Fe_0.2 Co_0.2Ni_0.2)O₃ perovskite high-entropy ceramics sintered at 1300 °C for 10 h in air showed a single phase with a perovskite-like structure. It also exhibited good DC conductivities at room temperature for the application in SOFC. At high temperatures, the transition metal ions exhibit a variety of oxidation states, accompanied by a variety of charge disproportionation, such as Mn⁴⁺ + Ni²⁺ → Mn³⁺ + Ni³⁺ [40]. However, there are few reports on the microstructure and dielectric properties of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ perovskite high-entropy ceramics prepared at higher temperatures (>1300 °C).

In this study, La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ perovskite high-entropy ceramics were prepared at high temperatures of 1350–1650 °C. Next, phase composition and the microstructure of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ perovskite high-entropy ceramics sintered at different temperatures were investigated. Finally, the density, hardness, and dielectric properties of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics were analyzed.

2. Materials and Methods

In this work, La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics were prepared by a typical two-step solid-state method using the analytical-grade metal oxides La₂O₃ (AR, ≥99.0%), Cr₂O₃ (AR, ≥99.0%), Co₂O₃ (AR, ≥99.0%), Fe₂O₃ (AR, ≥99.0%), MnO₂ (AR, ≥99.0%), and Ni₂O₃ (AR, ≥99.0%) as raw materials. Firstly, oxide powders were weighed according to the stoichiometric ratio of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ and then ball-milled with alcohol and agate balls in a planetary mill for 12 h. Then, the slurry was dried and passed through a 150 μm sieve. The above powder mixture was pressed into a disc and then calcined at 900–1200 °C in a muffle furnace (KSL-1700X, Kejing Company, Hefei, China) for 20 h in air. The synthesized disc was then crushed, pulverized, ball-milled, and passed through a 150 μm sieve again to obtain homogeneous powders. The ceramic discs of 10 mm in diameter and 2 mm in thickness were pressed at 25 MPa with 3 wt% polyvinyl acetate (PVA) as binder. The green bodies were sintered at 1350–1650 °C for 4 h with a heating rate of 5 °C·min⁻¹ and cooled naturally.

The phase compositions of the synthesized powders and sintered high-entropy ceramics were analyzed by a high-solution X-ray diffraction (XRD) (Bruker D8 Advance, CuKα target with λ = 0.15418 nm, (Bruker, Karlsruhe, Germany). The morphology and element distribution of the powders and morphology of the high-entropy ceramics were characterized by SEM and EDS (FEI Quanta 250 FEG microscope and EDS detector). The particle size and the distribution of the powders were calculated using Image-Pro software. The apparent porosity and bulk density of high-entropy ceramics were obtained according to the Archimedes method. The theoretical density of high-entropy ceramics was calculated based on the lattice parameters obtained from XRD patterns and the molecular weight of the stoichiometric ratio of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃. The relative density was calculated by dividing the measured sample bulk density by the theoretical density. The HV-2000 A micro-Vickers hardness tester (Shanghai Qinming, Shanghai, China) was employed to characterize the hardness of the high-entropy ceramics. The peak load was 19.61 N, and the pressure was maintained for 10 s. Temperature-dependent dielectric properties, at a frequency of 10 kHz of high-entropy ceramics with silver-pasted electrodes, were measured with an Agilent 4980A LCR meter when the samples were heated from 20 °C to 150 °C at a rate of 2 °C·min⁻¹.

3. Results

3.1. Phase Composition and Microstructural Analysis

Figure 1 shows XRD patterns of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy powders calcined at different temperatures. It can be seen that the prepared high-entropy powders are consistent with the orthorhombic perovskite structure with the space group Pnma (62), and no additional peaks are observed in any of the prepared samples. The diffraction peaks match well with JCPDS # 89–0478 of pure LaCrO₃ at room temperature [41]. It shows that the solid-state reaction at 1000 °C for 20 h provides sufficient time for atomic diffusion, thus forming a highly symmetrical perovskite phase-disordered solid solution [42]. According to the relationship between Gibbs free energy (ΔG_mix) and temperature, ΔG_mix decreases with increasing temperature, which stabilizes the high-entropy system. Therefore, 1000 °C is determined as an appropriate temperature for calcining powders in the first step. Additionally, the high sintering activity of the powders obtained at 1000 °C facilitates the formation of dense bulk ceramics at a higher sintering temperature.

For perovskite oxides, Goldschmidt [43] proposed in 1926 that the “tolerance factor” can be used to predict the single phase of perovskite, which can be calculated using Equation (1):

$$t=\frac{R_{\mathrm{A}}+R_{\mathrm{O}}}{\sqrt{2}\left(R_{\mathrm{B}}+R_{\mathrm{O}}\right)}$$

(1)

where R_A and R_B are the ionic radii of the cation at A site and B site, respectively, and R_O is the radius of the oxygen ion. In the case of multiple cations at a specific site, an average of the ionic radii is considered [7]. In an ideal case, t ≈ 1 is perhaps a necessary but not sufficient condition for the formation of a cubic structure; when t > 1.0, a tetragonal or hexagonal structure may be formed [44]; and when t < 1.0, an orthorhombic or rhombohedral structure may be formed, especially for systems with smaller A-site cations or bigger B-site cations [45]. The oxidation state, coordination number (CN), and corresponding cation radius (r_c) [45] of each element of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramic are shown in

Table 1. Oxidation state, coordination number (CN), and corresponding cationic radius (r_c) (data from Shannon).

Element	Oxidation	CN	rc (Å)
La	+3	XII	1.36
Cr	+3	VI	0.615
Co	+3	VI	0.545
Fe	+3	VI	0.55
Mn	+4	VI	0.53
Ni	+3	VI	0.56
O	−2	VI	1.40

. It can be seen from Table 1 that t ≈ 0.966, and a single-phase perovskite high-entropy ceramic material can be formed. This is consistent with the conclusion reported by Jiang et al. [7], that stable, single-phase perovskite high-entropy ceramics are formed when t ≈ 1. The slightly smaller t value may be due to lattice distortion caused by the combination of various elements in the high-entropy material.

Figure 2 shows SEM images of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ powders calcined at different temperatures. It can be seen that the grain size increases obviously with the increase in calcination temperature. The powders calcined at 900 °C have poor dispersibility and form small grain agglomerates due to the small grain size. The grain size of the powders calcined at 1000 °C increases slightly, and there is no obvious agglomeration. Compared with Figure 2a, the grain-size distribution is relatively more uniform, and the grain size is in the range of 200–400 nm. When calcination temperature increases to 1100 °C and 1200 °C, the grain size of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ powders increases obviously, showing a large agglomeration state. In addition, a wider particle-size distribution can be found.

The XRD results of the crystal structure of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics are shown in Figure 3. It can be seen that the diffraction peaks of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics sintered at 1350 °C and 1400 °C correspond well to the standard card of LaCrO₃ (JCPDS # 89-0478). When sintering temperatures increase to 1450 °C and 1500 °C, a small amount of the second-phase LaMnO₃ appears. When the sintering temperature is 1550 °C, in addition to the presence of the second-phase LaMnO₃, the chromium-containing oxides start to volatilize and the peaks contain some mixed peaks of chromium oxide and cobalt oxides, such as CrO₂ and Co₃O₄. When the sintering temperature further increases to 1600 °C, the LaMnO₃ peak disappears and new peaks of CrO₃ appear. When the sintering temperature reaches 1650 °C, the diffraction peaks match well with the standard card, while the peaks shift to the right slightly. This may be due to the formation of the transient liquid phase by doping Co at a high temperature (1650 °C), which promotes sintering [46].

Figure 4 shows SEM images of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics sintered at different temperatures. The results show that a relatively dense microstructure is obtained with a grain size of about 1.6 μm when La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramic is sintered at 1350 °C. To further verify the uniformity of element distribution, La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics sintered at 1350 °C are analyzed with EDS. As shown in Figure 5a, all six elements are uniformly distributed without any element segregation or aggregation. The atomic ratios obtained by semiquantitative elemental analysis, which are almost equal to the designed perovskite composition, indicate that there is no obvious composition change during the sintering process. Based on the analysis results, the atomic fractions of each element are shown in Figure 5b. It can be seen from the inset that the ratio of atom and mass of the five elements is about 5%, and the variation is ±0.5%. The atomic and mass fractions of the five elements are extremely close, providing further evidence for the equiatomic composition of Cr³⁺, Mn⁴⁺, Fe³⁺, Co³⁺, and Ni³⁺ at the B site (ABO₃). When the sintering temperature is 1350 °C, it can be seen that the grain size of the high-entropy ceramics is relatively small. When the sintering temperature increases to 1400–1450 °C, some pores appear and the density decreases in the microstructure of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics. The grain size increases from 1.6 μm to about 2.6 μm. When the sintering temperatures are higher than 1500 °C (1500–1600 °C), the second phase (small white particles) first precipitates at the grain boundary and then precipitates along the surface, which is consistent with the XRD results. Moreover, there are no obvious pores observed in SEM images and the grain size further increases from 8.6 μm to 17.4 μm. The grain size of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics grows significantly with a clear grain boundary and the grains are closely adjacent to each other.

When the sintering temperature reaches 1650 °C (Figure 6), pores and liquid phases can be found in La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics through the local magnification of the SEM images. When the temperature reaches 1650 °C, the volatilization of Cr in the ceramic matrix leads to the formation of pores, and the doping of Co forms a transient liquid phase to promote sintering [47].

3.2. Density, Apparent Porosity, and Hardness

Figure 7 shows the relative density, apparent porosity, and hardness of La(Cr_0.2Mn_0.2 Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics sintered at different temperatures. It can be seen that when sintering temperature is 1350 °C, the relative density is 93.0%, apparent porosity is 2.2%, and the hardness is 14.7 GPa. When the sintering temperature is 1400 °C, relative density decreases to the minimum (88.4%), apparent porosity increases to the maximum (5.6%), and the hardness is 14.4 GPa. When the sintering temperature increases to 1450 °C, the hardness reaches the minimum of 13.2 GPa. When the sintering temperature increases from 1450 °C to 1500 °C, the relative density increases from 88.7% to 96.8%, an increase of 8.1%. When the sintering temperature is 1500–1600 °C, the relative densities of high-entropy ceramics increase continuously. When the sintering temperature is 1600 °C, the relative density reaches the maximum of 99.5% and the porosity reaches the minimum of 1.0%. When the sintering temperature continues to increase from 1600 °C to 1650 °C, the relative density decreases to 92.1%. The change law of apparent porosity is just the opposite. The hardness values first decrease and then increase. When the sintering temperature is 1650 °C, the hardness value reaches the maximum of 18.7 GPa.

3.3. Dielectric Properties

The temperature dependence of dielectric constant and dielectric loss of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics sintered at different temperatures are shown in Figure 8, respectively. It can be seen that La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramics have a giant dielectric constant, which is significantly larger than that of pure LaCrO₃ ceramics (the dielectric constant is about 1500 when the test temperature is 40 °C [40]). The higher dielectric constant may be obtained by the lattice distortion, which is often caused by multielement doping [37,48]. In the low-frequency region, the dipole orientation polarization is the main reason for the higher dielectric constant, while in the high-frequency region, the dipole cannot be alternating with the electric field. With a further increase in frequency, the dipole oscillation induces polarization to stop and the partial spin polarization induced by the elemental solid solution produces the higher dielectric constant [49,50,51]. The dielectric constant increases with the increase in the test temperature, which is consistent with the results reported by Boudad et al. [52] and Coşkun et al. [53] on the dielectric properties of doped LaCrO₃. The increased dielectric constant should be attributed to strong effects of the hopping of small polarons and carriers at the grain boundary, especially for the doping system [52,53]. The dielectric constant of the sample sintered at 1350 °C increases rapidly with the increase in the test temperature. In addition, when the test temperature is 20–120 °C, the dielectric loss increases continuously, and when the test temperature is higher than 120 °C, the dielectric loss shows a decreasing trend. For the samples sintered at 1400–1550 °C, the dielectric constant increases with the increase in sintering temperature, which is consistent with the increasing trend of relative density in Figure 7. The dielectric loss shows a different increase trend with the increase in test temperature. This is due to the presence of many point defects (such as vacancies, substituted atoms, and interstitial atoms) in multielement, high-entropy solid solutions that can act as polarization centers and as local stresses caused by lattice disorder that can suppress dipole reorientation under alternating electric fields, thereby increasing the dielectric loss in the material [54,55,56,57]. When the sintering temperature is further increased to 1600 °C, the dielectric constant of the samples is significantly lower than those of high-entropy ceramics sintered at 1500 °C and 1550 °C at the same test temperature, which may be related to the jumping of oxygen vacancies due to the volatilization of elements, such as Cr and Co in the second phase of the sample. It affects the consistency of the ceramic matrix, which is also one of the factors affecting the dielectric constant. This is consistent with the results of XRD and SEM testing.

4. Conclusions

The phase composition, microstructure, hardness, and dielectric properties of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ perovskite high-entropy ceramics prepared through a two-step solid-state reaction technique were investigated. When the sintering temperatures were 1350 °C and 1400 °C, the phase of La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramic presented a pure, single-perovskite structure. The relative density reached 93.0% and 88.4%, and the hardness reached 14.7 GPa and 14.4 GPa, respectively. When the sintering temperature increased to 1450–1600 °C, the pure perovskite structure became unstable and the second-phase precipitates appeared in the XRD and SEM results. When the sintering temperature increased to the range of 1450–1600 °C, the relative density and hardness of the high-entropy ceramics increased with the increase in sintering temperature. Furthermore, the dielectric constant increased with the increase in test temperature, showing excellent mechanical and dielectric properties. These results indicate that La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ high-entropy ceramic is a potential dielectric material that can be utilized at high temperatures.