Effect of Binder Size and Chemistry on Pure Alumina Vacuum-Formed Ceramic Fiber Boards

Researchers Balazs Borbas et al. from the Department of Physical Chemistry and Materials Science, Faculty of Chemical Technology and Biotechnology, Budapest University of Technology and Economics, published a paper in ScienceDirect titled “Effect of binder’s size and chemistry on pure aluminium-oxide vacuum formed ceramic fibre boards.”

In this study, the authors synthesized a new generation of alumina fibers using the sol–gel method, and successfully fabricated pure alumina vacuum-formed fiber boards. The research investigated the influence of binder chemistry and particle size on the properties of the vacuum-formed boards.

Original Article: https://www.sciencedirect.com/science/article/pii/S2666539524000178

Research Content
The vacuum forming technique enables the rapid production of ceramic fiber materials. In this process, the required slurry mainly consists of ceramic fibers, suitable binders, and water. During vacuum forming, pre-cut ceramic fibers are suspended in a slurry containing either inorganic binders (such as boehmite) or organic binders (such as starch), and then compacted under the influence of vacuum pressure.

The vacuum serves to remove moisture and fix the solid ceramic fibers within the mold. After demolding and drying, a ceramic fiber preform is obtained. By applying vacuum, the driving force for liquid removal through the porous mold can be enhanced by up to five times compared with non-vacuum conditions.

To prevent fiber entanglement during the forming process and ensure their uniform distribution in the final product, flocculants are commonly employed. In order to enhance the thermal resistance of the material, the addition of non-alumina-based binders should be avoided as much as possible. The present study demonstrates that boehmite, nano-sized aluminum hydroxide, and aluminum chloride hydrate can all serve as effective binders in our slurry system. Among them, aluminum chloride hydrate exhibits strong chemical bonding capability, enabling the formation of robust chemical bonds between the castable components. Furthermore, the gel network formed by the binder helps prevent material migration during the forming and drying processes.

As shown in Figure 1, pure alumina fibers were synthesized via the sol–gel method, and the sol-derived fibers were converted into ceramics by heat treatment at 500 °C. At this temperature, crystallization does not occur, and the resulting fibers remain amorphous.

Figure 1: Scanning electron microscopy (SEM) image and X-ray diffraction (XRD) pattern of the alumina fibers used.

Figure 2: Schematic diagram of the sample preparation process.


Table 1: Data of density, shrinkage, and weight loss rate.

As shown in Table 1, when the shrinkage was reduced by 15%, no significant differences in isotropic shrinkage were observed. Although a notable weight loss was detected, the sample density increased substantially due to high-temperature heat treatment.

Figure 3: Two types of stress–strain curves: (a) with and (b) without local maxima.

Figure 4: (a) Modulus and (b) compressive strength as a function of density.
 

As shown in Figure 3, when the compressive strength is identified as a local maximum, the inflection point can be determined numerically to approximate the compressive strength value. Both the modulus and compressive strength are dependent on density (Figure 4). Samples with higher density exhibit greater strength and higher modulus. Both values increase further as a result of heat treatment.

Figure 5: (a) Modulus and (b) compressive strength as a function of binder particle size. Hollow symbols represent boehmite-containing samples.

The results show that the mechanical properties are strongly dependent on the binder system employed, as illustrated in Figure 5. Among samples with different chemical compositions, those containing boehmite exhibited superior performance, with the highest values of modulus and compressive strength. A consistent trend was also observed when comparing the three aluminum hydroxide–based systems.
 

Figure 6: FESEM micrographs of non-calcined and calcined samples.

The micro-scale morphology was examined using FESEM. As shown in Figure 6, the cross-sectional scans of each sample revealed heterogeneities. On the side near the filtration surface, more binder was observed between the fibers compared to regions farther from the fiber surfaces.

Figure 7: Detailed FESEM micrographs of non-calcined and calcined samples.

Figure 8: (a) Cross-section and crystal size distribution of crystallized fibers measured from FESEM images.

As shown in Figure 7, heat treatment at 1500 °C for 24 hours promoted crystal growth: the binder particles disappeared, and both the fibers and binder system exhibited well-developed crystals. Samples with smaller binder particle sizes formed correspondingly smaller crystallized grains. Compared with samples using micron-sized binders, those with nano-sized powder binders exhibited smaller crystal sizes (Figure 7). Relative to OH90, the crystal size was approximately half that of the average crystal (see Figure 8).

Figure 9: X-ray diffraction (XRD) patterns of vacuum-formed samples.

The crystalline phases of the samples were analyzed using powder X-ray diffraction (XRD). In the as-prepared, non-heat-treated samples, no distinct peaks were observed due to the presence of amorphous components. In contrast, the calcined samples exhibited sharp diffraction peaks, indicating the formation of crystalline phases. Further analysis identified these crystalline phases as α-alumina, the most thermodynamically stable form of alumina at high temperatures.

Conclusions

Vacuum-formed pure alumina fiberboard samples were successfully prepared using different binder systems. Aluminum hydroxide with particle sizes of 10 nm, 6.5 μm, and 90 μm, as well as a 100 μm reference, was employed as the binder, while polyaluminum chloride was used as a flocculant. The samples were subjected to high-temperature treatment at 1500 °C for 24 hours, during which isotropic shrinkage was observed.

The micro-scale morphology was investigated using field-emission scanning electron microscopy (FESEM). Compared with other samples, only a small amount of binder particles was observed in the boehmite-containing samples. Calcined samples exhibited well-developed crystals, with both the fibers and binder transforming into crystalline phases. Smaller binder particle sizes resulted in correspondingly smaller crystal sizes. In all cases, the crystalline phase was identified as α-alumina.