Recent development on low–carbon/ultra-low carbon refractories with superior properties for the rapid development of green low–carbon economy and high-quality clean steel is a research hotspot. The structure and properties of low–carbon refractories in a variety of ways are optimized to solve the problem of deterioration of high-temperature properties caused by simple reduction of graphite content. The introduction of carbon nanomaterials can significantly reduce the overall carbon content of the material. Also, the mechanical strength and thermal shock resistance of carbon-containing refractories are improved simultaneously because the nanostructures absorb and buffer the thermal stress generated by a large number of thermal shocks. In addition, reinforcement phases such as carbides, borides and nitrides are also applied to improve the organizational structure and properties of low–carbon refractories to solve the long-standing problem of carbon source oxidation. However, the poor dispersion of nanomaterials in the matrix is a significant technical problem, and the synthesis process of composite powders is complex and challenging. High production costs and the difficulty of achieving large-scale application restrict the industrial development of these materials. The structural degradation at high temperatures leads to a decline in the strength of refractories and even deterioration of comprehensive performance, which cannot guarantee the stability of long-term service of refractories

The MAX phase materials were expected to achieve the balance between low carbonization and service performance of carbon-containing refractories due to their graphite-like layered structure, high thermal conductivity, high damage tolerance and good oxidation resistance and thermal shock resistance. In this work, in-situ Ti₃Si(Al)C₂ reaction bonded low carbon Al₂O₃–C refractory was prepared by a reactive melt infiltration method based on the preparation process of ceramic matrix composite materials. The effects of the amount of TiC/graphite in the prefabricated material and the type of Si–Al alloy as the infiltration medium on the phase composition, microstructure, physical properties, mechanical properties and thermal shock resistance of the refractories were investigated.

The phase composition and performance (i.e., the apparent porosity, bulk density and flexural strength) are different at different compositions of the preforms. Corundum, Ti₃Si(Al)C₂ and TiC phases appear in all the samples, and trace amounts of SiC, TiAl_xSi_y and residual Al also occur. When the preform does not contain graphite, the content of Ti₃Si(Al)C₂ in the sample is higher, and the apparent porosity is much lower. The flexural strength of the sample increases from 9.0 MPa and 23.4 MPa to 79.4 MPa and 82.2 MPa, respectively, when the graphite removes from the raw material. Also, the bulk density of refractories is higher. This is due to the poor wettability of graphite and alloy, which prevents the infiltration of alloy melt into the preform. Graphite reacts with Si to form SiC phase and consumes Si element, which affects the growth of subsequent Ti₃Si(Al)C₂ phase. The content of titanium carbide in the raw material also affects the generation of Ti₃Si(Al)C₂, increasing the content of titanium carbide is beneficial to generating more Ti₃Si(Al)C₂, which causes a lower porosity and a higher bulk density of Al₂O₃–Ti₃Si(Al)C₂–C efractories. The overall apparent porosity of samples Al₅₀Si₅₀ and Al₉₅Si₅ is lower than that of sample Al₅Si₉₅, and the bulk density is relatively high. This is related to a fact that the presence of Al promotes the infiltration of alloy melt into the precast and is conducive to the formation of Ti₃Si(Al)C₂. The flexural strength of the sample T25@Al₅₀Si₅₀ using Al₅₀Si₅₀ alloy as a penetration medium increases from 75.5 MPa to 110.5 MPa after thermal shock. The rich toughening mechanism of Ti₃Si(Al)C₂ helps to improve the thermal shock resistance of the refractory, and the oxidation products generated after the oxidation of Ti₃Si(Al)C₂ can make the structure of the refractory more dense. The presence of whiskers in the refractory is also beneficial for better thermal shock resistance.

Low carbon Al₂O₃–Ti₃Si(Al)C₂–C refractory could be prepared after the preform was infiltrated with Al₅₀Si₅₀ alloy. The presence of graphite in the preform obstructed the penetration of Si–Al alloy and ultimately affected the mechanical properties of the sample. The flexural strength of the preform without graphite was up to 82.2 MPa after the infiltration reaction at 1550 ℃. The content of Al in the infiltration medium had an effect on the production of Ti₃Si(Al)C₂ in the refractory. The presence of Al was conducive to the reduction of twinning grain boundary energy between TiC grains, effectively promoting the formation of TiCx twins. Ti₃Si(Al)C₂ phase was also easy to nucleate from the TiCx twin and grow in the Ti–Si–Al liquid phase. The Ti₃Si(Al)C₂ phase consumed energy through various deformation mechanisms, thus increasing the flexural strength of the refractory material at room temperature. Also, Ti₃Si(Al)C₂ could form a continuous and dense oxide layer during the thermal shock process, effectively preventing the Ti₃Si(Al)C₂ inside the sample from being further oxidized. The formation of the dense layer could restore or even strengthen the mechanical properties of the refractory after thermal shock. The flexural strength of sample T25@Al₅₀Si₅₀ increased from 75.5 MPa to 110.5 MPa after thermal shock.

Industrial operations lead to energy consumption and environmental pollution with the economy development. In developed countries, the thermal efficiency of ceramic kilns in ceramic industry reaches over 50%, while in China it is only about 28%. Combined with the pollution reduction and carbon reduction in recent years, the gradual development of industrial kilns towards green and intelligent direction becomes an inevitable trend in the context of dual carbon. As is well known, microwave energy can reduce energy consumption and greenhouse gas emissions in drying processes. Microwave sintering technology is also regarded as a ‘new generation sintering technology of the 21st century‘. This technology is an efficient and environmentally friendly method used in metallurgy, powder and ceramics preparation due to its advantages such as volumetric heating, selective heating, time-saving and high efficiency. Compared with conventional sintering methods, a coupling effect between microwaves and materials is utilized in microwave sintering process, thereby producing dielectric loss, converting microwave energy into thermal energy. The microwave sintering furnace mainly involves microwave generator, waveguide tube and sintering chamber. In the operation, the microwave generated by the microwave generator is transmitted through a wave guide, thus entering the sintering chamber after passing through multiple layers of insulation, interacting with the material in the crucible, generating dielectric loss inside the material and converting microwave energy into thermal energy. For this purpose, some imperative requirements put forward for the selection of furnace lining materials. In addition to conventional fire resistance, the lining material used for microwave sintering furnaces also has good wave transmission properties. However, the existing research on transparent materials mostly focus on aircraft radome, and there is still a lack of systematic introduction on wave-transparent materials used in microwave sintering furnaces.

This review briefly introduces the working principle of microwave sintering technology and the wave transmission mechanism of materials, and summarizes several common types of high-temperature wave transmitting materials like ceramic firebrick, ceramic fiberboard, ceramic aerogel. Among them, ceramic aerogels have typically nano-pores of up to 90%, which can reduce the dielectric constant of the material significantly. Ceramic fiberboard has a low density and thermal conductivity. However, the application of these two materials is limited due to their lower operating temperatures. Ceramic firebrick with its superior mechanical properties and high-temperature stability shows a broad application prospect in fused silica, alumina, silicate and phosphate ceramics, and these materials with porous structures are more widely used because of the lower dielectric constant of air. Some influencing factors on the dielectric properties of materials are described. Combination with the mechanical properties and dielectric properties of materials, the performance of each ceramic is analyzed, and the advantages and disadvantages of different high-temperature wave transmitting materials are given. Among them, aluminum silicate ceramic shows a promising application prospect in the lining materials for microwave furnace. However, the comprehensive properties of the material in the industrial microwave frequency range still need to be further investigated.

Although the favourable dielectric property of ceramic aerogel and fiberboard is proved in the selection of lining materials for microwave sintering, some problems of operating temperature still remain in the practical application. For ceramic firebrick, compared with nitride ceramics (i.e., Si₃N₄、SiAlON and Si₂N₂O), some porous oxides ceramics (i.e., fused quartz, alumina, silicate and hosphate ceramics) become a research hotspot because of the lower cost and better antioxidant properties. All the materials show some advantages in mechanical properties and dielectric properties. For instance, fused quartz with relatively stable dielectric properties has a lower operating temperature. Alumina ceramics have a higher operating temperature, but their thermal shock resistance needs to be improved and its dielectric properties have a temperature dependence. Mulite ceramics have excellent characteristics such as high melting point (i.e., 1830 ℃ ), low thermal expansion coefficient (i.e., 4.5×10^–6 K^–1), and low thermal conductivity, which can be used in air at 1750 ℃ without high temperature oxidation. In addition, mulite ceramics also have the superior dielectric properties (i.e., ε is about 3–6, tanδ is about 10^–3). Some refractory materials with mullite as a main component are commonly used as lining materials in conventional high-temperature furnace. Furthermore, according to the existing studies, the increase of porosity can effectively reduce the dielectric constant of materials, and has a negative impact on the mechanical properties of materials. In this case, mullite ceramics are easy to form grains with acicular morphology during sintering, which is beneficial to maintaining high mechanical properties of the material at a high porosity. However, some challenges still remain in the scientific researches and practical application, such as some related studies on the dielectric properties at high temperatures are limited at <1200 ℃ , but it is far from enough for the lining materials of microwave kiln, in which the working temperature can often reach 1700 ℃ . The change of the transmittance of materials at higher temperatures is still of great research value. Also, it is necessary to optimize the dielectric constant test system at high temperatures. Finally, some novel material systems of oxides ceramic in a largescale need to be further explored for the microwave sintering application. Meanwhile, exploring the relevance between microstructure and properties and further accelerating the upgrading and application of ceramic based wave-transmitted materials in microwave sintering technology become some research hotspots, which can reduce the pollution and carbon emissions caused by the use of industrial kilns and lay a foundation for the realization of ’carbon neutrality’.