Lightweight refractory materials can be broadly classified as a combination of solid and gaseous phases. The solid phase primarily consists of inorganic non-metallic oxides (both crystalline and glassy phases). While these oxides generally have a high thermal conductivity, their refractory nature offers minimal insulation. The gaseous phase, present in the pores of the solid phase, significantly contributes to the material’s insulation properties due to the low thermal conductivity of gases.
For instance, the thermal conductivity of air at 0°C is 0.023 W/m·°C, and it remains below 0.03 W/m·°C at room temperature and 0.06 W/m·°C at 500°C. Consequently, lightweight refractory materials with high porosity, formed by solid and gaseous phases, exhibit excellent insulation properties. Additionally, bulk or fibrous materials filled with gas also provide good thermal insulation.
Lightweight refractory materials come in various types and can be classified based on different criteria. Firebird’s lightweight refractory products include FJM Insulating Firebrick, Silica Insulating Firebrick, GMK23 Anorthite Insulating Firebrick, Calcium Silicate Board, lightweight high-alumina, and alumina hollow ball products. Based on their mineral composition, these materials can be categorized as quartz-based, mullite-based, cordierite-based, corundum-based, and more. Typically, the bulk density of lightweight refractory products ranges from 0.6 to 1.2 g/cm³, and their operating temperatures are between 900 and 1350°C.
According to their service temperature, they can be divided into three categories: low-temperature insulation materials (≤600°C), medium-temperature insulation materials (600°C–1200°C), and high-temperature insulation materials (≥1200°C). For example, alumina hollow ball lightweight refractory products can withstand prolonged use at temperatures below 1800°C. Based on their structural characteristics, lightweight refractories can be classified into three types: continuous gaseous phase structure, continuous solid phase structure, and a mixed type where both phases are continuous.
Lightweight refractory materials are characterized by high-temperature mechanical properties and good volume stability. They have high porosity, low bulk density, and low thermal conductivity, providing excellent insulation performance. Due to the large number of pores within these materials, heat transfer occurs through both the solid and gaseous phases. Solid-phase heat transfer primarily involves conduction, while gas-phase heat transfer is more complex. When heat is transferred from a high-temperature area to the interior of the insulation material, the process initially occurs within the solid phase. Upon encountering pores, the heat transfer process follows two paths: one continues through the solid phase, while the other occurs through the gaseous phase.
In the solid-phase heat transfer path, the presence of pores disrupts the heat conduction direction, increasing the heat resistance. In the gaseous phase path, heat transfer includes conduction, convection, and radiation. The heat transfer process through insulating refractory materials, therefore, involves two channels—through the solid phase and the gaseous phase. The insulation effect is achieved because of the large number of pores, with the gas inside the pores playing a crucial insulating role. Heat conduction through the gaseous phase involves three processes: conduction, convective heat transfer, and thermal radiation.
Since gases have very low thermal conductivity, as shown in the table, heat conduction through gases is minimal. Additionally, most insulating refractory materials have small pore sizes, which limit gas movement and reduce the impact of convective heat transfer. In radiative heat transfer, certain gases, such as H₂, O₂, and N₂, exhibit minimal absorption and emission of radiation due to their symmetrical diatomic molecular structure. Since air primarily consists of oxygen and nitrogen, their contribution to radiative heat transfer is negligible.
In conclusion, the presence of pores in insulating refractory materials provides the material with effective insulation properties. Generally, smaller pore sizes are more favorable for improving the insulation performance of the material.Firebird offers high-quality lightweight refractory materials. If you are in need of them, feel free to contact me anytime.