Unlike many conventional materials, ceramic fiber modules do not follow the typical principle of thermal expansion. Instead, they tend to undergo shrinkage when exposed to high temperatures. Let's explore the physics behind this unique behavior.
Shrinkage under heat is a critical characteristic of ceramic fibers, directly related to crystallization and grain growth. At temperatures below the crystallization point, refractory fibers show almost no shrinkage. However, as the temperature rises to the crystallization threshold, noticeable shrinkage begins to occur.
Crystallization marks the onset of shrinkage and initiates a process often referred to as "fiber curling." The curling or deformation of individual fibers is the fundamental mechanism leading to the overall shrinkage of the fiber assembly.
As crystalline phases begin to form, an ultra-fine crystalline structure emerges. Over time and with increasing temperature, these crystal grains continue to grow. Once the grain size approaches the diameter of a single fiber, the fiber's strength decreases. Continued exposure to high temperatures results in progressive shrinkage, reducing the effective fiber length and leading to overall dimensional changes after long-term use.
It is worth noting that at 1200°C, high-grade ceramic fibers (such as those classified as 1400-type with higher alumina content) exhibit greater shrinkage compared to 1260-type fibers. However, at 1400°C, the 1400-type fibers show significantly lower shrinkage rates than the 1260-type.
Therefore, in practical applications, both the maximum service temperature and the shrinkage rate at the actual operating temperature should be considered when selecting the appropriate material. Simply choosing a higher temperature grade is not always optimal. For example, at 1000°C, 1260 high-purity fibers often deliver better performance and durability than 1400 high-alumina fibers. In contrast, for environments around 1200°C, high-alumina or low-zirconia types generally offer more balanced overall performance.
