China's Refractories

Mullite has a high melting point, high hardness, good thermal shock resistance, corrosion resistance, and high-temperature mechanical properties. Mullite raw materials are mostly synthesized by the sintering method and electric fused method. The research and application progress of pure mullite and its composite refractories in fields such as ceramics, metallurgy, aerospace, military industry, cement and glass were reviewed. The future development trend of mullite in the application of refractories was prospected.

In order to utilize coal gangue and steel slag with high added value, foamed ceramics were prepared by using coal gangue as the main raw material, and steel slag as the auxiliary raw material, adding appropriate amount of flux (talc and potassium feldspar) and foaming agent silicon carbide (SiC). The effects of the steel slag addition (equivalent replacement of coal gangue, 5%, 10%, 15%, 20%, 25%, 30%, and 35%, by mass) and flux addition (fixing the total addition of talc and potassium feldspar at 20 mass%, replacing potassium feldspar with talc in equal amount of 0, 5%, 10%, 15%, and 20%, by mass) on the physical properties, microstructure and phase composition of the foamed ceramics were studied. The results show that: (1) with the increase of steel slag additions, the addition of SiO₂ in the skeleton structure decreases, the addition of CaO, MgO and other oxides increases, and the viscosity decreases; excessive steel slag addition is not conducive to the formation of moderate-size and uniform distributed pores due to the low addition of SiO₂; the steel slag addition shall not exceed 30%; (2) the influence of talc and potassium feldspar flux on the foamed ceramics is mainly to change the contents of alkali metal oxides, as well as Al₂O₃ and SiO₂ which constitute the ceramic skeleton; (3) the optimum foamed ceramic formulation is m(coal gangue) : m(steel slag) : m(potassium feldspar) : m(talc)=50 : 30 : 10 : 10, extra-adding 0.1 mass% SiC.

In order to improve the densification of Si₃N₄ bonded SiC refractories and reduce the nitriding temperature of Si powder, Si₃N₄ bonded SiC refractories were produced by reaction sintering at 1 350 °C for 5 h under a carbon embedded atmosphere, using SiC particles and fine powder, and Si powder as the main raw materials, and introducing Ti-Si-Fe alloy extracted from high-titanium blast furnace slag to partially replace the Si powder. The effects of the Ti-Si-Fe alloy addition (0, 1.8%, 3.6%, 5.4%, and 7.2%, by mass) on the nitriding behavior of Si powder, as well as on the mechanical properties and microstructure of the material were investigated, and the nitriding reaction sintering mechanism was also explored. The results show that: (1) with the increase of the Ti-Si-Fe alloy addition, the cold mechanical properties and the hot modulus of rupture of the refractories are obviously improved, and the refractoriness under load exceeds 1 700 °C; the property enhancement slows down with Ti-Si-Fe alloys addition above 3.6%; (2) Ti-Si-Fe alloy promotes the complete nitridation of Si powder and the reaction sintering of the material at a lower temperature; the volume growth during the nitridation process of the Ti-Si-Fe alloys and Si powder can effectively fill pores, nitriding products improve the bonding state between aggregates and matrix, and that inside matrix, thereby increasing the densification and improving the mechanical properties of the material; (3) after the introduction of Ti-Si-Fe alloys, the liquid phase rich in Ti, Si, N, and Fe components is formed in the reaction system; besides the traditional VS and VLS mechanisms, the dissolution-precipitation mechanism plays a leading role in the formation of short columnar β-Si₃N₄ and granular TiN; and the cross-linked α-Si₃N₄ whisker, short columnar β-Si₃N₄ and granular TiN enhance the mechanical properties of the material.

In order to prolong the service life of ZrO₂ thermal barrier coatings, HS188 alloy was used as the substrate, and NiCrAlY powder (base layer, d₅₀=106 μm) and yttria stabilized zirconia (YSZ, d₅₀=50 μm) were employed as the sprayed feeder to prepare NiCrALY+YSZ thermal barrier coatings using high-energy plasma spraying equipment with a progressive exploration method. The microstructure of the coatings was controlled by adjusting the stand-off distance (85, 95, 105 and 115 mm) and air-cooling manner (rear air cooling, rear air cooling + gun cooling, front air cooling, and front air cooling + gun cooling). The bonding strength of the coatings was evaluated by the drawing method. The results indicate that under the conditions of long stand-off distance and low cooling rate, it is difficult for the coatings to form a vertical crack structure due to the low instantaneous heating temperature and insufficient quenching on the sample surface. However, when the instantaneous heating temperature reaches the critical value and the air cooling rate is excessive, un-melted spray particles exist. When the stand-off distance is 85 mm and the cooling method is front air cooling, a thermal barrier ceramic coating with vertical crack morphology was obtained on HS188 alloy. Simultaneously, it has a moderate density of vertical cracks, the highest bonding strength and the best toughness.

：To optimize the preparation of alumina aerogels, the sol-gel method was combined with supercritical drying, using aluminum isopropoxide (AIP) as the precursor, ethanol (EtOH) as the solvent, and acetic acid (HAc) as the catalyst. The effects of the ethanol addition [n(AIP) : n(EtOH) : n(H₂O) : n(HAc)=1 : 8 : 0.8 : 2, 1 : 12 : 0.8 : 2, 1 : 16 : 0.8 : 2, and 1 : 20 : 0.8 : 2] and acetic acid addition [n(AIP) : n(EtOH) : n(H₂O) : n(HAc)=1 : 16 : 0.8 : 1, 1 : 16 : 0.8 : 2, 1 : 16 : 0.8 : 3, and 1 : 16 : 0.8 : 4] on the pore structure and specific surface area of Al₂O₃ aerogels were studied. The results show that ethanol affects the pore structure of Al₂O₃ aerogels by affecting the condensation probability of Al—OH troups, and acetic acid affects the pore structure of Al₂O₃ aerogels by affecting the reaction rate; when n(AIP) : n(EtOH) : n(H₂O) : n(HAc)= 1 : 16 : 0.8 : 2, the Al₂O₃ aerogel has a porous microstructure, and its internal pore diameters is generally smaller than the average free path of air molecules, showing excellent thermal insulation characteristics, low density of 0.20 g·cm^-3, and the specific surface areas of 458, 102, and 51 m²·g^-1 at 25, 1 000, and 1 200 °C, respectively.

The lining materials for blast furnaces have evolved from brick masonry to monolithic refractory materials. Monolithic castables are widely used in various industrial furnaces. The rapid development of silica sol materials, combined with the commonly used construction methods of casting and spraying, offers greater flexibility, efficiency, environmental friendliness, and a longer lifespan. Typical monolithic refractory technologies, such as the integral casting technique for the furnace hearth, the pre-applied slag coating technique for the furnace belly and waist, and the inner lining gunning technique, are grounded in scientific theory and practice, thereby advancing the development of refractory materials and enhancing the operational quality of blast furnaces.

In order to solve the problems of traditional long sintering time and relatively low density of synthesized Al₄SiC₄, Al₄SiC₄ was prepared via hot pressing sintering using aluminum powder, silicon carbide powder, flake graphite, and pretreated synthetic Al₄C₃ as raw materials. The phase composition of the Al₄C₃ specimen, which was prepared by pretreating aluminum powder and flake graphite at different temperatures, was investigated. The effects of the heat treatment temperature and duration on the phase composition, microstructure, physical properties, and oxidation resistance of the Al₄SiC₄ specimen were also explored. The results show that under the condition of firing at 1 650 °C for 3 h, the Al₄SiC₄ specimen exhibits an apparent porosity of 13.1%, a bulk density of 2.88 g·cm^-3, a cold compressive strength as high as 63.23 MPa, and a mass loss rate of 1.2%. The preparation of relatively-high-density Al₄SiC₄ ceramics by hot pressing sintering has potential industrial application prospects.