Permanent linear change (PLC), also known as residual linear change or high-temperature volume stability, refers to the irreversible change in the dimensions (length) of refractory products after being heated to high temperatures. It is expressed as a percentage, where a positive value indicates expansion (referred to as permanent linear expansion) and a negative value indicates shrinkage (referred to as permanent linear shrinkage). This property can be measured through experiments, where a refractory sample is heated to a specified temperature, held for a certain period, and then cooled to room temperature to determine the residual expansion or contraction in length. The temperature and duration used in the test should comply with the technical standards for each type of product.
During the use of refractory products, further sintering and phase changes may occur, leading to additional volume changes and resulting in residual shrinkage or expansion. This is an important aspect of high-temperature volume stability and a key quality indicator for refractory materials. For example, excessive permanent linear shrinkage in roof products can lead to brick detachment or even roof collapse. This indicator is particularly critical for unburned products and monolithic refractories.
The permanent linear change rate is a vital measure of the high-temperature volume stability of refractory materials, also known as residual linear change. For insulating products, this rate is typically evaluated based on the percentage of permanent linear change under no-load conditions. The calculation formula is as follows:
Where:
-ΔL represents the permanent linear change rate.
– L0 and L1 represent the lengths before and after reheating, respectively.
A positive value indicates expansion, while a negative value indicates shrinkage.
Due to the 5-8% volume expansion associated with the solid-phase synthesis of magnesium-aluminum spinel, permanent linear expansion (PLC) primarily occurs when the spinel formation reaction is incomplete during the firing process. On the other hand, permanent linear shrinkage is mainly caused by insufficient firing temperature or inadequate holding time, leading to incomplete sintering. Typically, materials with lower bulk density exhibit a higher permanent linear change rate. This is because the larger interparticle distances in low-density materials make it difficult to form sintering necks during firing, resulting in incomplete sintering.
According to the national standard GB/T5988-1986, the permanent linear change of spinel-based insulating refractory materials was measured. After firing at 1550°C for 3 hours, the permanent linear change rate was ±0.5% for samples with a bulk density of 1.0 g/cm³ and ±0.2% for samples with a bulk density of 0.5 g/cm³. For insulating refractory products, spinel-based insulating materials demonstrate a relatively low permanent linear change rate.
When the density of the reaction product is lower than that of the reactants, expansion occurs. Examples include the mullitization of andalusite, sillimanite, and kyanite, as well as the reaction between magnesia (MgO) and alumina (Al₂O₃) to form spinel. Conversely, when the reaction product has a higher density than the reactants, shrinkage occurs.
Sintering is a critical process during reheating and a major cause of permanent linear shrinkage. The porosity, liquid phase content, liquid phase composition, and grain size of refractory materials significantly impact the sintering process. Higher liquid phase content, smaller grain size, and greater porosity facilitate sintering, leading to increased permanent shrinkage.
Increasing the firing temperature and extending the holding time can bring the refractory material closer to its thermodynamic equilibrium state, thereby reducing the rate of permanent linear change.
Permanent linear change (PLC) refers to the residual shrinkage or expansion, expressed as a percentage of the original length, after a fired refractory product is reheated to a specified temperature, held for a certain duration, and cooled to room temperature. It is a crucial technical indicator of product quality, reflecting the high-temperature dimensional stability of the material. A negative sign (“-“) indicates shrinkage, while a positive sign (“+”) denotes expansion, representing the residual change in dimension.
