To make alumina bubble insulating bricks, different diameters of alumina bubbles are proportionally blended, then the proper ratio of binders, such as phosphate or aluminum phosphate are added into the raw material, after mixing, pre-sintered fine powders are added into the raw material, and mixed together, so the binders and fine powders can even be distributed on the surface of alumina bubbles. Through vibration molding and drying, the green alumina bubble bricks are sintered at a 1500℃ high temperature. Because the large diameter bubbles have lower mechanical strength, they are easy to crack during molding, the max. the diameter should not exceed 5mm. But small diameter bubbles and fine powders proportion should not too high, too, because the bulk density and thermal conductivity will significantly increase. Sintering temperature also has a great influence on alumina bubble bricks, with the increase of sintering temperature, the sintering degree is increased, and thermal conductivity increases accordingly.
By mineral composition and structure, alumina bubble bricks are divided into pure alumina bubble insulating bricks, mullite bonded alumina bubble bricks, Sialon bonded alumina bubble bricks, and so on. For mullite bonded alumina bubble insulating bricks, some ratio of SiO2 powder is added into powder material, during sintering, some quantity of mullite is formed in the matrix to replace the corundum phase, so the thermal shock resistance of alumina bubble insulating bricks is improved.
During the high-temperature nitridation sintering of Sialon bonded alumina bubble bricks, the matrix has a nitridation reaction and forms Sialoon bonded matrix phase. Sialon bonded alumina bubble bricks have high mechanical strength, good thermal shock resistance, strong corrosion-resistance, and so on.
| Item | LKZ-88 | LKZ-99 | Sialon bonded alumina bubble insulating brick | Zirconia bubble insulating brick |
| Al2O3 % | ≥88 | ≥99 | ≥70 | / |
| ZrO2 % | / | / | / | ≥98 |
| SiO2 % | / | ≤0.2 | / | ≤0.2 |
| Fe2O3 % | ≤0.3 | ≤0.15 | N≥5 | ≤0.2 |
| Bulk density g/cm3 | 1.30~1.45 | 1.45~1.65 | ≤1.5 | ≤3.0 |
| Cold crushing strength Mpa | 10 | 9 | 15 | 8 |
| Refractoriness under load (0.2Mpa, 0.6%) | 1650 | 1700 | 1700 | 1700 |
| Reheating linear change rate % | ±0.3 | ±0.3 | / | ±0.2 |
| Linear expansion coefficient /℃-1 | 8.0*10-6 | 8.6*10-6 | Thermal shock resistance ≥5（1100℃, water cooling） | / |
| Thermal conductivity /W(m·K)-1 | <0.9 | <1.0 | <1.1 | <0.5 |
| Max. working temperature ℃ | 1650 | 1800 | 1600 | 2000~2200 |
Alumina bubble insulating bricks applications
Compared with other lightweight insulating materials, alumina bubble insulating bricks have the advantages of high working temperature, high mechanical strength, low thermal conductivity. Their bulk density is 50%~60 lower than that of the same composition dense refractory bricks. Alumina bubble bricks can endure high-temperature flame impact and can be used as the inner lining of high-temperature kilns and furnaces, the main applications of alumina bubble bricks are Molybdenum silicide electric furnace, molybdenum wire furnace, tungsten furnace, high-temperature gas intermittent kiln, tunnel kiln, and so on, the energy consumption is saved by at least 20%.