In this work, crystallization, thermal expansion and wetting behavior of ZnO–Al2O3–SiO2 (ZAS) glass were first investigated. The results showed that ZnAl2O4 was precipitated from ZAS glass after crystallization treatment. Crystallization increased the coefficient of thermal expansion (CTE) of ZAS glass ceramic due to the high CTE of ZnAl2O4. In addition, ZAS glass exhibited good wettability on the surface of MgAl2O4 substrate. On this basis, ZAS glass was used to join MgAl2O4 ceramic, and the microstructure and mechanical properties of joints obtained with different cooling methods were investigated. The flexural strength of joints was related to the content of ZnAl2O4 crystals in the brazing seams. Additional nucleation and crystallization treatment during cooling process improved the crystallinity of brazing seam, resulting in better matching of the CTE of brazing seam with that of MgAl2O4 ceramic. The maximum flexural strength of joints reached 201 MPa, which was equivalent to the strength of MgAl2O4 ceramic. 

Joining MgAl2O4 ceramics using ZnO–Al2O3–SiO2 glass ceramic with precipitated ZnAl2O4

Diffusion bonding is a viable technique for fabricating larger parts or composite transparent ceramics. Spark plasma sintering (SPS) and hot isostatic pressing (HIP) allow for simultaneous application of high temperature and pressure, making them suitable for bonding various materials, including transparent ceramics. In this study, we demonstrated the combined use of SPS followed by HIP for successful diffusion bonding of transparent MgAl2O4 (spinel) and Y3Al5O12 (YAG) ceramics. The YAG samples had superior surface quality and were adequately bonded using SPS alone, while rougher spinel samples showed inadequate bonding initially due to insufficient surface contact. However, subsequent HIP treatment effectively closed the voids at the interface and promoted grain growth across the joint, resulting in a strong seamless bond. The combination of SPS and HIP, as a two-step process, holds promise for diffusion bonding of ceramics and offers a potential solution for compensating surface quality issues in the bonded parts. 

Diffusion bonding of transparent ceramics by spark plasma sintering (SPS) complemented by hot isostatic pressing (HIP)

Influence of Gd2O3 on the microstructure and properties of transparent MgAl2O4: Cr3+ ceramic

Highly transparent 4.5 at.% Sm:YAG ceramics were fabricated by solid-state reactive sintering using tetraethyl orthosilicate (TEOS) and CaO as sintering additives. The influences of TEOS content on phase formation, microstructure evolution, and optical properties were investigated. The 4.5 at.% Sm:YAG ceramics with 0.3–0.6 wt% TEOS all have a single yttrium aluminum garnet phase after pre-sintering at 1725 °C for 30 h. With TEOS increasing from 0.3 to 0.6 wt%, residual pores were gradually discharged, the Sm:YAG ceramics with 0.5 and 0.6 wt% TEOS showed fully dense microstructure after vacuum sintering at 1725 °C for 30 h. After hot isostatic pressing (HIP) at 1750 °C for 3 h, the average grain size of the final Sm:YAG ceramics increased from 9.5 to 15.8 μm with TEOS increasing from 0.3 to 0.6 wt%, and all the ceramic samples showed uniform microstructures. The 4.5 at.% Sm:YAG ceramics with 0.6 wt% TEOS achieved high in-line transmittance of 83.6% at 808 nm and absorption coefficient of 3.32 cm−1 at 1064 nm, which means it can effectively suppress amplified spontaneous emission and parasitic oscillation. 

Effect of TEOS content on microstructure evolution and optical properties of Sm:YAG transparent ceramics

For improving the mechanical properties of MgAl2O4 ceramic and maintaining its wide band transparency, the sapphire/MgAl2O4/sapphire laminated transparent composites with thin sapphire layers were fabricated by brazing with BaO-B2O3-SiO2 (BBS) glass filler. The BBS glass filler had an appropriate CTE (coefficient of thermal expansion) value and good wettability on the surface of sapphire and MgAl2O4 ceramics. Joining experiments were first carried out to optimize the preparation process parameters of laminated composites. The results indicated that high bonding temperature promoted mutual dissolution and diffusion of BBS glass and MgAl2O4 ceramic, but led to the intergranular infiltration of glass filler and the formation of pores due to volatilization of B2O3. When the bonding temperature was 1000 °C, the sapphire/MgAl2O4 joints with the best transparency and acceptable shear strength could be obtained. On this basis, the laminated composites with 0.15 mm thick sapphire layers were fabricated by brazing using BBS glass at 1000 °C. The flexural strength of the laminated composite reached 320 ± 10 MPa, which was higher than that of MgAl2O4 (195 ± 15 MPa). The in-line transmittance of laminated composite was 80.9% at 1000 nm, which was slightly lower than that of MgAl2O4 (82.2%). 

Fabrication of sapphire/MgAl2O4/sapphire laminated transparent composite by brazing with BaO-B2O3-SiO2 glass filler

Ceramic joining by glass is a promising method of the preparation of large transparent ceramics from small blocks. The chemical composition of glass was optimised to match the coefficient of thermal expansion and refractive index of transparent magnesium aluminate (MgAl2O4) ceramics. A two-step joining method was developed to join MgAl2O4 ceramics with a reduced number of bubbles in the joint, and the thermal properties of the optimised glass were evaluated to determine the joining temperature. Two transparent MgAl2O4 blocks were joined by a glass layer that was approximately 20 µm thick. The joint area could not be distinguished with a naked eye. The transmittance of the joined body vertical to or parallel through the glass layer was approximately the same as that of the ceramics. The average three-point bending strength of the joint reached 202 ± 33 MPa, which was 64% that of the ceramic body. 

Joining transparent spinel ceramics using refractive index–matched glass

In this paper, YOF was used as the sintering additive for obtaining MgAl2O4 transparent ceramics with excellent optical properties. It is found that YOF enhances the densification of MgAl2O4 ceramics. In addition, YOF can effectively suppress grain growth in the final stage because of the reduction of grain boundary energies and the pinning effect at the grain boundary region during the hot isostatic pressing (HIPing) process. Nevertheless, it is important to carefully control the content of YOF. Although the formation of a low amount of Y3Al5O12 (YAG) is advantageous because its grains are doing the pinning effect at the grain boundary, an excessive amount is not desired as it is likely to cause scattering. After optimizing the YOF content and sintering temperature, the final sample (4 mm thick after polishing) with 0.01 wt% YOF HIPed at 1550 °C is obtained, showing the high in-line transmittance of 82.6% at 400 nm and high flexural strength of 271.6 MPa. 

Influence of YOF on the preparation and optical quality of MgAl2O4 transparent ceramics

To achieve optimal light transparency in MgAl2O4 transparent ceramics, a higher sintering temperature is required to eliminate pores. However, inevitable rapid grain growth occurs, leading to lower mechanical properties. To address this problem, calcium fluoride (CaF2) is utilized as a sintering additive to enhance the transparency of MgAl2O4 ceramics. The influence of CaF2 on the densification process and microstructure evolution is investigated. CaF2 demonstrates notable abilities in facilitating densification and lowering the sintering temperature. Moreover, it inhibits grain growth during the final sintering stage. However, the sintering temperature must be controlled because the secondary phases (calcium aluminate) precipitate at a higher temperature, thereby affecting the homogeneity of the ceramic microstructure. After optimizing the sintering process, ceramics (with a thickness of 5 mm) containing 0.05 wt% CaF2 exhibited the highest in-line transmittance of 81 % and 87 % in the visible and near-infrared wavelengths, respectively, after pre-sintering at 1505 °C for 3 h and subsequent hot isostatic pressing (HIPing) at 1550 °C for 3 h. 

Effect of CaF2 on the preparation and optical property of MgAl2O4 transparent ceramics

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Joining transparent spinel ceramics using refractive index–matched glass

Influence of YOF on the preparation and optical quality of MgAl2O4 transparent ceramics

Effect of CaF2 on the preparation and optical property of MgAl2O4 transparent ceramics