China's Refractories

With the increase in global warming, building energy saving becomes a principal policy for most countries. About 70% of the energy consumption loss of buildings is through the external walls of the buildings. Ultra-lightweight foam concretes with dry density of 150-300 kg·m ^-3as the thermal insulation materials have been commercially prepared and used in building insulation engineering in China. This paper reports the raw materials and procedures for preparation of ultra-lightweight foam concretes by chemical foaming (UFC-C) and physical foaming (UFC-P). The characteristics of the air-voids structure, the mechanical properties, the water absorption, the thermal conductivity and the fire resistance of UFC-C and UFC-P were summarized in this paper. Based on existing research progress, the future research works were presented.

Highly porous silica ceramics with different pore sizes were fabricated by adjusting the mixed surfactants addition. The effect of the pore size on the cold compressive strength and the thermal conductivity of the ceramics was researched. The results show that the smaller pore size can improve the compressive strength and the thermal conductivity. With the mixed surfactants addition increasing from 0.1 mass% to 0.4 mass%, the porosity is close, in the range of 88.10%-88.31%, and the average pore size decreases from 190 μm to 97 μm; the compressive strength is enhanced from 2.97 MPa to 3.55 MPa; and the thermal conductivity decreases from 0.104 W·m^-1·K^-1 to 0.089 W·m^-1·K^-1.

Porous alumina ceramics are widely used in various fields because of their unique characteristics in structure and properties. This article mainly provided an overview of preparation techniques of porous alumina ceramics; besides, the principles and the characteristics of pore-forming agents processes, gel-casting processes, freeze-casting processes, anodizing processes and other common processes in obtaining the required properties of porous alumina ceramics were introduced. Meanwhile, the relevant applications and the performance of porous alumina ceramics were described. The research prospects of porous alumina ceramics were put forward based on the research status at the end of this paper.

Corundum-mullite porous ceramics (CMPCs) were fabricated using fly ash and aluminum ash as the raw materials, adding different amounts of starch (0, 10%, 20%, and 30%, by mass) as the pore-forming agent, molding and firing at 1 200 °C for 1 h. The apparent porosity, the bulk density, the cold compressive strength, the thermal shock resistance and the thermal conductivity of the CMPCs were tested. The phase composition and the microstructure of the CMPCs were characterized by XRD and SEM. The effect of the starch addition on the properties of the CMPCs was studied. The results show that: (1) the CMPCs with high porosity can be successfully prepared by adding starch as the pore-forming agent and firing at 1 200 °C for 1 h; (2) with the starch addition increasing, the porosity increases; the cold compressive strength and the number of the quenching cycles of the CMPCs decrease, but they remain at high levels; meanwhile, the thermal conductivity decreases; (3) generally, when adding 20 mass% starch, the CMPC has the apparent porosity of 48.6%, the cold compressive strength of 52.1 MPa, the quenching cycles of 5 and the thermal conductivity of 1.63 W·m^-1·K^-1 and 1.52 W·m^-1·K^-1, respectively, at 25 °C and 500 °C, showing good performance.

Ultra-high temperature ceramics (UHTCs) are a family of borides, carbides and nitrides of transition elements such as hafnium, zirconium, tantalum and niobium. They exhibit the highest known melting points, good mechanical strength, good chemical and thermal stability under certain conditions. In last decade, researchers dedicated to characterize porous UHTCs aiming to develop novel thermal insulating materials that could withstand temperatures over 2 000 °C. In this article, the preparation and characteristics of porous UHTCs were reviewed. Dry processing, colloidal processing and solution processing routes have been used to prepare porous UHTCs with porosities ranging from 5% to 97% and pore sizes ranging from hundreds of nanometers to hundreds of micrometers. The obtained porous UHTCs are chemically and dimensionally stable at temperatures up to 2 000 °C during static state high-temperature thermal aging.

Magchrome bricks, as the inner lining of RH snorkels, have played a vital role in the operation of RH degassers for a long term. In chrome-free campaigns, resin-bonded, Al-containing magnesia bricks have been an alternative of magchrome bricks with a comparable performance in the last decade. It is important to have found whisker formation in the matrix of Al-containing magnesia bricks above 1 100 °C and in the correlation to their high performance of RH snorkels. In this paper, the bonding mechanisms of both refractories are investigated to differentiate from other refractories. In magchrome bricks, the bonding modes of fused magchrome grains are characterized by the reactions between magnesia and chrome ore at different burning temperatures. At 1 500 °C, liquid forms around chromite grains. It is sucked into surrounding magnesia and a gap forms around chromite grains at 1 600 °C. Plenty of Fe₂O₃, Cr₂O₃ and Al₂O₃ have diffused from chrome ore into magnesia at 1 670 °C. A complete dissolution of the chrome ore takes place at 1 750 °C, with chromite precipitating entirely. In unburnt, Al-containing magnesia bricks, a dense network of whiskers is formed during heating, which is a prevailing bonding feature, instead of traditional particle growing and merging. It is believed that the whiskers are formed by vapour-solid mechanism since there is no liquid droplet observed at the tip of whiskers. In most stringent working conditions of RH snorkels, the bonding mechanisms are emphasized for their application, instead of chromia component.

In order to improve the service performance and explore the damage mechanism of silicon carbide-mullite bricks for the transition zone of cement rotary kilns, the phase composition and the microstructure of a used brick in the transition zone of a cement rotary kiln were analyzed by XRD, SEM and EDS. The results show that the liquid and alkali vapor phases generated by the reaction between cement materials and the silicon carbide-mullite brick mostly enter the silicon carbide-mullite brick through the pores; meanwhile, Ca⁺ and K⁺ in the cement penetrate through the liquid maintaining a high chemical potential energy to dissolve Al₂O₃ and SiO₂ at the top of the liquid phase thus enhancing the phase penetration; with the decreasing temperature, crystals such as gehlenite, potassium feldspar and potassium chloride are precipitated, which destroy the original structure and increase the difference of thermal expansion coefficient between the high temperature dense end and the metamorphic layer thus resulting in cracks, spalling, and rupture.

In this paper, Si₃N₄ fiber materials were fabricated by nitridation of porous Si green bodies, which were prepared by the foaming method combined with gel-casting. The effect of the nitriding process (the heating rate and the nitrogen flow rate) on the phase and the microstructure was studied. The results show that decreasing the heating rate and the nitrogen flow rate is both beneficial to the growth of Si₃N₄ nanofibers and promoting the disintegration of the pore wall structure of the porous green bodies. The optimized nitrogen flow rate and the heating rate for the growth of silicon nitride fibers are 150 mL/min and 0.5 °C/min, respectively.

Corundum spinel castable was prepared using tabular corundum as aggregates, white fused corundum powder, spinel powder and alumina powder as the matrix, pure calcium aluminate cement as a binder, and extra adding calcium chloride (0, 1%, 2%, and 3%, by mass). The effects of the CaCl₂ addition on the cold physical properties, the hot strength, the thermal shock resistance and the microstructure of the castable were studied. The results show that, for the corundum spinel castable fired at 1 550 ℃, with the increase of the CaCl₂ addition from 0 to 3%, the cold strengthes first increase, then decrease, the apparent porosity increases, the volume density decreases, and the linear change rate first decreases and then increases, while the overall change is not significant; however, the hot modulus of rupture and the thermal shock resistance are obviously improved. This is mainly due to that, CaCl₂ is evenly distributed in the castable in the form of solution, and reacts with Al₂O₃ to form small flake CA₆ crystals, which evenly distributed in the sample matrix strengthening and toughening the material.