Transparent ceramics are a kind of ceramic materials integrating structural and functional properties. A variety of ceramic materials such as oxides, nitrides, oxynitrides and fluorides are developed. Among them, spinel transparent ceramics are suitable for transparent armor, mid-wave infrared windows and fairings due to their low density, high hardness, high strength, high temperature stability, especially the wide transmittance range and high transmittance from near ultraviolet to mid-infrared. The development of transparent ceramics becomes a hot research spot. The design and performance optimization of new transparent ceramics are mainly realized through composition design because transparent ceramics have some strict requirements for microstructure and phase purity.

In recent years, the novel properties of the high-entropy effect in materials provide an approach for the development of materials. In this paper, a spinel-type (Li,Mg,Zn,Al,Ga)₃O₄ (LMAZGO) compound was designed with MgAl₂O₄, MgGa₂O₄, ZnAl₂O₄, ZnGa₂O₄, LiGa₅O₈ and LiAl₅O₈ as components. The sintering process of LMAZGO high-entropy transparent ceramic materials was investigated, and the microstructure of transparent ceramics was optimized. In addition, the optical transmittance and mechanical properties of LMAZGO high-entropy transparent ceramic materials were also analyzed.

Commercial powders of Li₂CO₃, MgO, α-Al₂O₃, ZnO, and Ga₂O₃ (purity of 99.99%) were used as raw materials, and the proportion of each raw material was calculated according to the elemental composition of LMAZGO. After mixing evenly, the high-entropy ceramic was synthesized in air by a high-temperature solid-state reaction method, and then ground to the target powder. LMAZGO blanks with a diameter of 20 mm were prepared via axial compression combined with cold isostatic pressing. Subsequently, the transparent ceramic pre-sintered body was prepared via pressureless pre-sintering in a muffle furnace, and the residual pores were further removed by hot isostatic post-treatment. The prepared LMAZGO transparent ceramics were ground and polished on the both sides for further characterization.

The phases of LMAZGO powders and bulk were characterized by a model X'Pert PRO X-ray diffractometer under Cu target at 40 kV and 40 mA. The density of ceramics was measured based on the Archimedes drainage principle. The thermal behavior of the blank was determined by a model DIL402SE thermodilatometer at different temperatures from room temperature to 1 500 ℃ at a heating rate of 10 ℃/min. The microstructure of powders and ceramics was characterized by a model S-3 400 scanning electron microscope, and the average grain size of the grains was determined by a linear intercept method. The linear transmission spectra of the samples were measured by a model Lambda 750S UV-NIR spectrophotometer and a model 6 700 Fourier transform infrared absorption spectrometer. The Vickers hardness was measured by a model 430SVD hardness tester. The flexural strength of high-entropy transparent ceramics was determined based on a three-point flexure test by a model MTS810 universal materials testing machine. The elastic properties of transparent ceramics were determined by a model MK7 elastic modulus meter.

The spinel LMAZGO compounds were synthesized at 1 100 ℃ for 2 h. Based on the XRD and SEM results, the synthesized powder has a spinel phase, uniform and fine grains and little agglomeration, and the average grain size is 290 nm. The configuration entropy of LMAZGO is 3.18R, calculated by the sublattice model, which has a high crystal structure stability.

The results of thermal expansion show that the shrinkage densification rate of LMAZGO ceramic blank is further accelerated from 1 200 ℃, and the maximum densification rate of LMAZGO ceramic blank occurs at 1 380 ℃ in the middle stage of sintering. According to the thermal expansion analysis of LMAZGO, 1 300, 1 350, 1 400 and 1 450 ℃ were selected as the pressureless pre-firing temperatures, and the pre-firing process of the green body was optimized. The optical transmittance of the transparent ceramic samples obtained by subsequent HIP sintering firstly increases and then decreases with the increase of pre-firing temperature. Based on the results of the sintering curve, the pressureless sintering process of LMAZGO transparent ceramics can be optimized at a sintering temperature of 1 400 ℃ for holding time of 6 h. The average grain size of the prepared LMAZGO transparent ceramic pre-burned body is 1.81 μm, and the density is 98.7%. The superior optical transparency is obtained after hot isostatic sintering (sintering conditions: argon atmosphere, incubation at 1 550 ℃ for 5 h), and there is no abnormal growth of grain size.

The optical transmittance range of the obtained high-entropy spinel LMAZGO transparent ceramics is 0.23-7.35 μm, the maximum linear transmittance is 85.2%, the Vickers hardness (H_V1) is 12.7 GPa, the Young modulus is 265 GPa, and the flexural strength is (332±30) MPa.

A spinel LMAZGO high-entropy compound with a configuration entropy of 3.18R and a high crystal structure stability was synthesized. The preparation process of LMAZGO high-entropy transparent ceramics was systematically optimized via pressureless sintering combined with hot isostatic post-treatment, and the prepared LMAZGO transparent ceramics had an excellent optical transparency, with an optical transmittance range of 0.23-7.35 μm, the maximum linear transmittance of 85.2%, the Vickers hardness (H_V1) of 12.7 GPa, the Young modulus of 265 GPa, and the flexural strength of (332±30) MPa. The results showed that the high-entropy spinel LMAZGO transparent ceramic material exhibited broad spectral transparency and excellent mechanical properties, having a promising application in the field of mid-infrared transparent window. This study provided some ideas for the development of new transparent ceramics.