Effect of Fe₂O₃ on Compressive Strength and Microstructure of Porous Acicular Mullite

Abstract

Porous acicular mullite was fabricated at 1300 °C starting from Al₂O₃ and mixture of SiO₂ and MoO₃ obtained by previous oxidation of waste MoSi₂. It was found that the presence of MoO₃ favors formation of acicular (prism-like) mullite grains with sharp edges. The effect of addition of Fe₂O₃ (4–12 wt.%) on phase composition, compressive strength, thermal conductivity and microstructure was studied. The addition of Fe₂O₃ improved the compressive strength from approximately 25 MPa in pure mullite to about 76 MPa in samples containing 12 wt.% Fe₂O₃, while the open porosity decreased from 55.4% to 51.8%. The presence of Fe₂O₃ caused a decrease in mullite formation temperature owing to the formation of liquid phase and accelerated diffusion. The solubility of iron oxide in mullite lattice was between 8 and 12 wt.% Fe₂O₃. The incorporated iron ions also promoted the rounding of sharp edges in prismatic mullite grains, leading to a reduced specific surface area of 0.55 m²/g in the sample with 12 wt.% Fe₂O₃. The thermal conductivity of mullite increased with addition of 12 wt.% Fe₂O₃ reaching value of 1.17 W/m·K.

1. Introduction

Porous acicular mullite (3Al₂O₃·2SiO₂) is a promising material for hot gases filtration owing to its excellent combination of thermal properties, such as good thermal shock resistance, low thermal expansion coefficient and good stability at elevated temperature [1,2]. Although filter materials are usually not exposed to considerable mechanical load, a sufficiently good rigidity is essential to allow easy handling and machining. Acicular (needle-like) mullite consisting of elongated, interlocked grains typically possesses a much better rigidity/porosity ratio than material consisting of equiaxed grains [3,4]. In addition, acicular microstructure creates unusually low pressure drop across the filter which is especially important for filtration of diesel engine exhaust [5]. Therefore, it has been essential to fabricate acicular mullite using an inexpensive method and cheap starting material such as kaolinite [6], which mainly consists of Al₂O₃ and SiO₂. Developing prism-like, elongate, mullite grains using conventional ceramic processing techniques has been found to be quite difficult. This unique grain morphology is normally developed using expensive techniques, which involve grain growth from either molten phase [7,8] or gaseous phase [9,10]. The Dow Chemical Company, Core R&D, Midland, USA developed acicular mullite in a reactor filled with SiF₄ [10]. Kaolinite clay as a starting material was first transformed into fluorotopaz as an intermediate product, which was subsequently transformed into acicular mullite. The process was conducted under careful monitoring of the SiF₄ gas flow. However, a similar microstructure was obtained in our previous study using a simple and inexpensive technique [11]. It was shown that waste MoSi₂ can be used as both a source of SiO₂ and a pore former. The oxidation (calcination) of crushed and pulverized waste MoSi₂ heating elements is given by the following Equation (1) [12]:

2MoSi₂ (s) + 7O₂ (g) → 2MoO₃ (s) + 4SiO₂ (s)

(1)

Thus, MoO₃ evaporates before the onset of the sintering process, creating additional porosity. The presence of a pore-forming agent is especially important for filter application, as required porosity is very high, typically between 50 and 80%. Such high porosity is normally accompanied by very low compressive strength, which sometimes can be too low to allow sample handling [13,14,15]. The required compressive strength varies with the application, but, in general, it should exceed 30 MPa. The compressive strength can be improved by simply increasing the sintering temperature and accelerating the sintering process. However, this is normally followed by an undesired decrease in porosity. Therefore, it would be of great importance to develop a method to improve strength without a considerable decrease in porosity. It is well known that the presence of metal cations in mullite precursors affects the temperature of mullite crystallization as well as the morphology of mullite grains [16,17]. Transition metal cations such as Fe³⁺, which preferably replace Al in mullite structure, show relatively high solubility, up to 12 wt.% [18,19]. Ye et al. [20] have shown that the addition of 9 wt.% of Fe₂O₃ can promote the formation of elongated mullite grains similar to those obtained in our study. Therefore, it would be interesting to study the effect of Fe₂O₃ addition on the properties of mullite having elongated grain morphology. This paper examines the effect of Fe₂O₃ addition on grain morphology, porosity, compressive strength, and thermal conductivity of acicular mullite. It will be shown that the addition of Fe₂O₃ promotes densification which leads to compressive strength improvement, followed by minimal porosity reduction.

2. Materials and Methods

2.1. Sample Preparation

The used MoSi₂ heating element (Super Kanthal 1700 °C, made by Bulten–Kanthal AB, Hallstahammar, Sweden) was crushed and pulverized in a vibratory mill for 1 h. The obtained powder was subsequently calcined at 500 °C for 24 h in the air. The powder denoted as PHE500 (Pulverized Heating Element) consisted of MoO₃, residual MoSi₂, and amorphous SiO₂, and was therefore used as a source of SiO₂ for the mullite fabrication. X-ray diffraction (XRD) analysis conducted on Siemens D500 diffractometer (Siemens, Karlsruhe, Germany) showed that the crystalline fraction of PHE500 powder consisted of 13 wt.% of residual MoSi₂ and 87 wt.% of MoO₃ [11]. Bearing in mind that XRD cannot detect the amorphous SiO₂ present in PHE500, it can be concluded that the fractions of MoO₃ and residual MoSi₂ in PHE500 are lower than those determined by XRD. Assuming that the oxidation of MoSi₂ follows Equation (1) it was calculated that PHE500 powder should approximately contain 7.5 wt.% of the residual MoSi₂, 50.4 wt.% of MoO₃, and 42.1 wt.% of amorphous SiO₂. The residual MoSi₂ is expected to completely oxidize during sintering (>1000 °C) and yield an additional amount of SiO₂ prior to the mullite formation. Simple calculation based on Equation (1) shows that 100 g of PHE500 will yield ~48 g of SiO₂. Knowing that the Al₂O₃/SiO₂ weight ratio in mullite (3Al₂O₃·2SiO₂) is 72/28, it was estimated that the Al₂O₃/ PHE500 weight ratio in the starting mixture must be 55/45 in order to fabricate mullite. The amount of Fe₂O₃ was adjusted to 4, 8 and 12 wt.%. The calculated amounts of PHE500 and Al₂O₃ powder (Alcoa A-16 SG, Alcoa Corporation, Pittsburgh, PA, USA) and Fe₂O₃ (Sigma Aldrich Chemie GmbH, Schnelldorf, Germany) were homogenized by ball milling in a plastic jar using Al₂O₃ balls as milling media and methanol as a liquid vehicle. After milling for 12 h, the mix was dried at 90 °C followed by sieving. After sieving, the mixture was uniaxially cold pressed into cylindrical compacts (Ø = 8 mm, height ~10 mm) under a pressure of 60 MPa. The samples were sintered in a tube furnace (Elektron, Banja Koviljača, Serbia) at a temperature of 1300 °C for 4 h in air with a heating rate of 5 °C /min. A four-hour hold at 750 °C was applied to allow the majority of MoO₃ to evaporate slowly. Continuous heating can often lead to sample disintegration due to the rapid evaporation of MoO₃ near 800 °C. Although MoO₃ is toxic if inhaled or in contact with skin, it should be noted that the described procedure poses no significant environmental risk. The MoO₃ vapor condenses immediately upon leaving the heating zone, forming a network of long, interlocked fibers with diameters similar to those of human hair. Tube furnaces are particularly suitable for this sintering process, as the fibers collect along the tube walls near the outlet, making them easy to gather and safely store.

The sintered samples were machined in order to make the upper and lower planes perfectly parallel.

2.2. Sample Characterization

The phase composition of starting powders as well as the sintered samples was determined by X-ray diffraction (XRD) analysis on a Siemens D500 diffractometer (Siemens, Karlsruhe, Germany) using Cu Ka radiation with a Ni filter. Open porosity was determined by Archimedes’ method using xylene (Merck, Darmstadt, Germany). Compressive strength measurement was carried out on Instron M 1185 testing machine (Instron, Norwood, MA, USA). Six samples of each composition were tested. Microstructure and elemental composition were examined by Scanning electron microscopy (SEM) using a JEOL-JSM-5800LV (JEOL, Tokyo, Japan) microscope equipped with an Energy Dispersive Spectroscopy (EDS) detector. Thermal conductivity at 300 °C was measured on 5 mm × 5 mm × 3 mm machined specimens, using a Thermal Transport Option (TTO) of Physical Properties Measurement System (PPMS, Model 6000, Quantum Design, San Diego, CA, USA). Specific surface area and BJH (Barrett-Joyner-Halenda) pore size were measured by the Bruner-Emmett-Teller (BET) method on a high-speed gas sorption analyzer (NOVA4000, Quantachrome Instruments, Boynton Beach, FL, USA).

3. Results and Discussion

Phase Composition

The XRD pattern of the starting mixture containing 12 wt.% Fe₂O₃ is presented in Figure 1. The figure confirms the presence of MoO₃, Al₂O₃ and Fe₂O₃ in the form of hematite. As expected, the amorphous SiO₂ was not detected.

The XRD patterns of samples sintered at 1300 °C are given in Figure 2. It was shown that mullite is the main crystalline phase in all samples. It has an orthorhombic structure, consisting of octahedral chains of AlO₆ interconnected by tetrahedral chains of aluminum-oxygen and/or silicon-oxygen (AlO₄/SiO₄). It is important to note the presence of small amounts of SiO₂ (cristobalite) and Al₂O₃ (corundum) in the sample without Fe₂O_3, which indicates that amorphous SiO₂ first crystallizes as quartz, then undergoes phase transformation into cristobalite, which subsequently reacts with Al₂O₃ to form mullite. The absence of SiO₂ in samples containing Fe₂O₃ reveals that the reaction between SiO₂ and Al₂O₃ and therefore mullite formation, might be accelerated by the Fe₂O₃ presence. The analysis of the XRD pattern of the sample containing 12 wt.% Fe₂O₃ shows the presence of hematite (Fe₂O₃), indicating that the solubility of Fe₂O₃ in mullite is between 8 wt.% and 12 wt.%.

The comprehensive study on iron solubility in mullite solid solution conducted by Ye et al. confirmed that most iron ions in the mullite are in the form of Fe³⁺ due to a smaller atomic radius than that of Fe²⁺ [21]. The effective ionic radius of Fe²⁺ (six-fold coordination) is 0.061 nm, notably larger than that of Al³⁺ (six-fold coordination, 0.0535 nm), which introduces significant internal strain [22]. In contrast, the ionic radius of Fe³⁺ (six-fold coordination) is 0.055 nm, closely matching that of Al³⁺. This similarity enables Fe³⁺ ions to partially substitute for Al³⁺ ions within the mullite [AlO₆] octahedra. The formation of a solid solution of Fe₂O₃ within the mullite structure, involving the substitution of Al³⁺ by Fe³⁺, leads to an expansion of the mullite lattice. This is evidenced by a shift in the characteristic mullite XRD peaks toward lower angles, corresponding to increased interplanar spacings (larger d-values). The shifting is especially pronounced at larger 2θ values. The addition of 4 and 8 wt.% Fe₂O₃ caused a continuous increase in the shifting of characteristic mullite peaks to the left. However, the shifting caused by the increase of Fe₂O₃ amount from 8 wt.% to 12 wt.% was much smaller and could be detected only at larger values of 2θ. This finding indicates that the maximum solubility of Fe₂O₃ is slightly higher than 8 wt.%. This is consistent with the results obtained in a study conducted by Ye et al., reporting that iron solubility is between 6 and 9 wt.% [21]. Zhan et al. [23] have also found that critical Fe₂O₃ doping threshold is 10 wt.%. The unit-cell parameters of the orthorhombic structure of mullite containing different amounts of Fe₂O₃ are listed in

Table 1. Unit-cell parameters of mullite with varying Fe₂O₃ content, determined from XRD patterns using Rietveld refinement in the FullProf (Version 5.60) software package. The estimated standard deviations are given in round brackets.

Fe2O3 Content	a (Å)	b (Å)	c (Å)
0	7.5534 (8)	7.6895 (8)	2.8851 (3)
4	7.5721 (9)	7.7132 (9)	2.8960 (4)
8	7.5864 (12)	7.7331 (12)	2.9042 (5)
12	7.5880 (11)	7.7340 (11)	2.9051 (4)

. The unit-cell parameters increase with increasing Fe₂O₃ content as the larger Fe³⁺ ions substitute for Al³⁺ ions.

The parameters given in Table 1 were plotted against Fe₂O₃ content to assess their linear relationship. As shown in Figure 3, the relationship between Fe₂O₃ content and unit-cell parameters is nearly linear for samples containing 0–8 wt.% Fe₂O₃. However, the unit-cell parameter of the sample with 12 wt.% Fe₂O₃ deviates from linearity, indicating that the maximum Fe ion solubility has been reached in this sample, leaving no scope for further increase in the unit-cell dimensions.

Since Fe₂O₃ was found to accelerate the mullite formation, a set of samples was sintered at 1200 °C in order to further examine the effect of Fe₂O₃ on mullite formation and grain morphology. The temperature lower than the sintering temperature was convenient due to a lower degree of reaction between SiO₂ and Al₂O₃. These samples are not considered significant from the standpoint of mechanical properties, as they do not comprise pure mullite. The XRD patterns of samples containing a different amount of Fe₂O₃ sintered at 1200 °C are given in Figure 4. As observed, the sample without Fe₂O₃ contains a large amount of SiO₂ and Al₂O₃. The amount continuously decreases with the increase of Fe₂O₃ content, whereas the amount of mullite increases. This result clearly demonstrates the capability of Fe₂O₃ to promote mullite formation.

The phase composition of samples sintered at 1200 °C was determined using the RIR (Reference Intensity Ratio) method within PDXL2 (version 2.0.3.0) software. Only three crystalline phases; SiO₂, Al₂O₃ and mullite were selected in order to study the change in their content during mullite formation in sample containing different amount of Fe₂O₃. The results are given in Figure 5, showing the continuous decrease of SiO₂ and Al₂O₃ content and increase in mullite content with the amount of Fe₂O_3, which confirms the ability of Fe₂O₃ to accelerate the mullite formation. The phase composition of samples sintered at 1200 °C with a particular emphasis on the presence of Al₂(MoO₄)₃ will be discussed in detail following SEM analysis, which will be focused on the effect of Fe₂O₃ on microstructural development at 1200 °C.

The effect of Fe₂O₃ on the microstructure of samples sintered at 1200 °C is presented in Figure 6. Figure 6a shows that the formation of prismatic mullite grains is preceded by the formation of mullite whiskers from the equiaxed particles of starting powder. As the figure evidences, a bunch of mullite whiskers start to grow at one point, and after that, they spread out. The above made conclusion that the presence of Fe₂O₃ promotes mullite formation is also confirmed by Figure 6b,c. Figure 6b shows the microstructure of a sample having 4 wt.% of Fe₂O₃. The comparison of the microstructure of the sample without Fe₂O₃ (Figure 6a) and the microstructure of the sample containing 4 wt.% Fe₂O₃ shows that the elongated mullite grains are much larger in the sample with 4 wt.% Fe₂O₃. To be more precise, the tine mullite whiskers grew into prismatic, rectangular grains. At the same time the amount of starting, equiaxed particles decreased due to the reaction between SiO₂ and Al₂O₃. This effect is even more pronounced in samples containing 8 wt.% Fe₂O₃ (Figure 6c) and 12 wt.% Fe₂O₃ (Figure 6d). As Figure 6d shows, the microstructure of sample containing 12 wt.% Fe₂O₃ predominantly consists of mullite grains with small amount of unreacted starting powder located between them. It would be of great importance to point out the interlocking of acicular mullite grains that is denoted in Figure 6d. The strong bonding between interlocked grains provides a skeleton for a rigid structure that possesses high compressive strength and high porosity at the same time. This kind of microstructure normally causes a relatively small pressure drop when it comes to gas filtration, as the gas can flow along the elongated grains.

It is evident from Figure 6 that the presence of Fe₂O₃ accelerates mullite formation and promotes the grain growth in an elongated fashion, which is related to its crystal structure. Mullite has an orthorhombic structure, consisting of octahedral chains of AlO₆ located at the vertices and in the center of the unit cell, with edges aligned in parallel along the “c” crystallographic axis. The AlO₆ chains are interconnected by double tetrahedral chains of aluminum–oxygen and/or silicon–oxygen (AlO₄/SiO₄), also aligned in parallel with the c axis [24,25,26]. Ye et al. [20] made a calculation which showed that the presence of Fe₂O₃ causes a decrease in surface energy of the (001) mullite plane, which subsequently causes an increase in the growth tendency on the (001) plane and therefore preferential growth along the c-axis. A similar effect was observed in boron-dopped mullite [27]. The addition of ~6 wt.% of B₂O₃ caused a considerable increase in the aspect ratio of mullite grains as well as the reduction in mullite formation temperature. It was found that the presence of boron reduces the viscosity of the SiO₂ containing amorphous phase and enhances atomic diffusion facilitating mullite nucleation and anisotropic grain growth. Similarly to boron, it was documented that iron also participates in the formation of the liquid phase, which promotes mullite formation. To be more precise, the presence of iron triggers the formation of an iron–silicate liquid phase which subsequently reacts with Al₂O₃ to form mullite. As previously mentioned, the presence of a liquid phase enhances diffusion, thereby facilitating nucleation of mullite grains, which is essential for reducing the mullite formation temperature and promoting the growth of elongated grains.

It can be concluded that the literature data ascribes the elongation of mullite grains and lowering of mullite formation temperature to the presence of dopants, i.e., iron. While this study confirms that iron can lower the temperature required for mullite formation, it does not verify iron’s critical role in the development of elongated mullite grains. Figure 7. shows samples with different amounts of Fe₂O₃ sintered at 1300 °C. The images were taken at two different magnifications in order to provide a comprehensive insight into the grain size and morphology.

As illustrated in Figure 7a,a’, the sample without Fe₂O₃ contains elongated prism-like mullite grains, which means that the growth of acicular mullite grains was initiated without the presence of iron. The morphology of mullite grains in the sample with 12 wt.% Fe₂O₃ sintered at 1200 °C (Figure 6d) was quite similar to that of the sample without Fe₂O₃ sintered at 1300 °C. It would be important to point out the sharp edges of rectangular mullite prisms. As shown in Figure 7b,b’, these edges stay sharp in the sample containing 4 wt.% Fe₂O₃ sintered at 1300 °C. However, the edges become rounded in samples containing 8 wt.% (Figure 7c,c’) and 12 wt.% Fe₂O₃ (Figure 7d,d’) indicating that dissolved iron accelerates diffusion and therefore material transport. The FeO–SiO₂–Al₂O₃ phase diagram [28] shows that the reaction between Fe ion and SiO₂ can produce a liquid phase at temperatures as low as 1185 °C. The liquid phase then reacts with Al₂O₃ to form an iron-containing solid solution, which aligns well with the earlier conclusion that the presence of iron promotes mullite formation at 1200 °C (Figure 6). As shown in Figure 2, iron ions dissolve into the mullite structure, suggesting that their concentration in the liquid phase decreases during sintering as well as mullite grain growth. Consequently, it can be inferred that the edge rounding observed in samples with 8 and 12 wt.% Fe₂O₃ is primarily the result of rapid solid-state diffusion occurring at 1300 °C, facilitated by the significant amount of iron ions dissolved in mullite grains. A similar conclusion was reached by Tu et al., [29] who observed that substituting Al³⁺ with Gd³⁺ in MgAl₂O₄ markedly enhanced mass transport, resulting in an almost fully densified material.

As a consequence, the grain edges become rounded as a result of material’s tendency to reduce its specific surface. This is confirmed by specific surface analysis results presented in Figure 8 showing that the specific surface continuously decreases from 0.95 m²/g in the sample without Fe₂O₃ to 0.55 m²/g in the sample containing 12 wt.% Fe₂O₃. As revealed in Figure 7d,d’, the grains in the sample containing 12 wt.% Fe₂O₃ predominantly exhibit rounded edges, making it difficult to identify grains with a rectangular cross-section. In addition, the Fe₂O₃ phase present in the samples containing 12 wt.% Fe₂O₃ was in the form of equiaxed grains embedded between elongated mullite grains, as marked in Figure 7d. Now the question arises: what causes the acicular grain growth? According to our previous research on mullite without an additive [11], the key component for acicular grain growth is MoO₃ which reacts with Al₂O₃ above 500 °C to form Al₂(MoO₄)₃ according to the following reaction [30]:

$${\mathrm{Al}}_2{\mathrm{O}}_3+3{\mathrm{MoO}}_3\stackrel{500°\mathrm{C}}{\to }{\mathrm{Al}}_2{\left({\mathrm{MoO}}_4\right)}_3$$

(2)

The newly created Al₂(MoO₄)₃ is unstable above 800 °C and therefore it decomposed into Al₂O₃ and MoO₃ during heating. The positive outcome of this transformation is grain refinement, which yields very fine, highly reactive MoO₃ and Al₂O₃ particles. The melting point of MoO₃ is 795 °C; however, it exhibits volatile behavior near the melting point, which was confirmed by thermogravimetric analysis conducted in our previous study [11]. A considerable weight loss due to MoO₃ evaporation was measured in the temperature range from 800 to 900 °C. It was estimated that about 65 wt.% of the total amount of MoO₃ evaporated during heating to 900 °C. The residual liquid MoO₃ kept evaporating in the temperature interval from 900 °C to 1350 °C which was the highest temperature of thermogravimetric analysis. It is important to note that the evaporation rate significantly decreased at temperatures above 900 °C, suggesting that molten MoO₃ forms a liquid mixture in which its partial pressure is reduced. The XRD analysis results, shown in Figure 4, confirm the presence of Al₂(MoO₄)₃ in the samples sintered at 1200 °C. The characteristic peaks are located between 22 and 24°. Further increase in temperature to 1300 °C led to complete decomposition of Al₂(MoO₄)₃. As Figure 2 evidences characteristic peaks of Al₂(MoO₄)₃ phase were not detected in samples sintered at 1300 °C. It is believed that the presence of MoO₃ favors the formation of silica-rich glassy phase, which promotes dissolution of Al₂O₃ and accelerates mullitization reaction [31]. EDS analysis of mullite grains in samples sintered at 1300 °C did not detect the presence of Mo, suggesting that Mo ions are not incorporated into the mullite lattice. The substitution of Al³⁺ by Mo⁶⁺ is unlikely due to differences in their charges and ionic radii [22]. The ionic radius of Mo⁶⁺ in six-fold coordination is 0.059 nm, which is larger than that of both Al³⁺ (0.0535 nm) and Fe³⁺ (0.055 nm). For this reason, the sintering process in samples without Fe₂O₃ is not as fast as that in samples containing Fe ions dissolved into the mullite lattice. Consequently, the sharp edges of elongated prismatic grains in samples without Fe₂O₃ are preserved even at temperatures as high as 1300 °C.

The total amount of MoO₃ in the starting mixture was 23 wt.%. After intensive evaporation at temperatures between 800 and 900 °C, the residual amount of MoO₃ was estimated at 8 wt.%. This is very close to the 7 wt.% identified by Chen et al. [30] as the optimal concentration for the longitudinal growth of mullite whiskers. The presence of MoO₃ lowered the melting temperature of silica-rich glassy liquid and decreased the high-temperature viscosity of the liquid system which resulted in a decrease in activation energy for mullite whisker growth. To be more precise, the activation energy of longitudinal grain growth was reduced, suggesting a stronger catalytic effect on longitudinal than transverse grain growth.

As mentioned, the source of SiO₂ in this study was pulverized and calcined MoSi_2, which consisted of amorphous SiO₂ and MoO₃. The starting mixture contained 23 wt.% of MoO₃ in order to ensure the correct stoichiometric ratio of components for mullite formation. This is a relatively large amount when compared to other studies investigating the effect of MoO₃ presence [30]. The advantage of a large amount of MoO₃ is the empty space in samples that is formed in the temperature range 800–900 °C due to MoO₃ evaporation. Since evaporation takes place prior to mullite formation, the resulting empty space allows for the unconstrained growth of elongated mullite grains. Figure 9 illustrates the formation of a mullite whisker, which is expected to develop further into a rectangular prism-like grain. It is evident that the length of the whisker is limited by the space available for its growth. The growth of the whisker is ultimately obstructed by the neighboring grains, suggesting that large pores in the green samples contribute to the high aspect ratio of mullite grains.

Figure 10 presents the effect of Fe₂O₃ amount on open porosity and compressive strength. The decrease in porosity is expected, knowing that iron dissolved in the mullite lattice enhances mass transport and therefore accelerates the sintering/densification process. It is important to note that the reduction in porosity is relatively minor from a filtration performance perspective. The addition of 12 wt.% of Fe₂O₃ caused a decrease in open porosity of only 3.6%, from 55.4% in pure mullite to 51.8% in the sample containing 12 wt.% Fe₂O₃. Despite the large amount of additives, the porosity of the sample containing 12 wt.% of Fe₂O₃, reaching a value of ~52%, is considered sufficiently high for filtering of diesel engine exhaust gases.

Unlike the relatively small change in porosity, the change in compressive strength was substantial. As shown in Figure 10, increasing the Fe₂O₃ content corresponded with a rise in strength, reaching 76 MPa in the sample containing 12 wt.% Fe₂O₃. This value was three times as high as that of mullite without Fe₂O₃. The improved compressive strength of samples containing Fe₂O₃ could be ascribed to the accelerated mass transport, which promotes sintering and therefore reinforces the contact between elongated mullite grains. As revealed in Figure 11, the two mullite grains in the sample containing 12 wt.% Fe₂O₃ are strongly bonded at the contact point, which reinforces the mullite skeleton and enhances its resistance to deformation under compression. However, this type of reinforcement does not significantly impact porosity because of the initially high porosity of the samples and the substantial gaps between grains formed during MoO₃ evaporation before sintering and densification.

Another important characteristic of material for diesel engine exhaust filtration is thermal conductivity [32,33]. Since the captured soot will eventually clog the filter, it is essential to periodically burn off the accumulated soot. The soot burnout generates local overheating which commonly causes filter cracking due to thermal shock [34]. Therefore, it is important to design a porous filter material with adequate thermal conductivity (>0.5 W/m·K) to ensure more uniform heat distribution. As presented in Figure 11, the addition of 4 wt.% Fe₂O₃ does not change the thermal conductivity. It is the same as the conductivity of the sample without Fe₂O₃. The porosity (Figure 10), phase composition (Figure 2) and microstructure (Figure 7) of the samples with 0 wt.% and 4 wt.% Fe₂O₃ are very similar, as is the thermal conductivity. The increase in thermal conductivity was recorded for samples containing 8 and 12 wt.% Fe₂O₃. The coalescence of mullite grains in these samples provides paths for heat transfer, causing an increase in conductivity [35]. In addition, the thermal conductivity of Fe₂O₃ at 300 °C is about 6 W/m·K, which is higher than that of mullite. Therefore, the presence of Fe₂O₃ secondary phase in samples containing 12 wt.% Fe₂O₃ is likely responsible for the high thermal conductivity of these samples. The thermal conductivity is inversely proportional to the pore size. As shown in Figure 12, the average pore sizes in samples containing 0 wt.% and 4 wt.% Fe₂O₃ are nearly identical, which aligns well with the observation that their thermal conductivities are also similar. The increase in Fe₂O₃ content led to a reduction in pore size, which, as expected, resulted in an increase in thermal conductivity of samples containing 8 and 12 wt.% Fe₂O₃. Considering that diesel particulate sizes range from 10 to 200 nm, it is likely that the significant reduction in pore size observed in samples containing 12 wt.% Fe₂O₃ could increase the risk of particulate clogging [36].

4. Conclusions

Porous acicular mullite was fabricated at 1300 °C starting from Al₂O₃ and a mixture of SiO₂ and MoO₃ obtained by previous oxidation of waste MoSi₂. The addition of Fe₂O₃ was an effective way to improve the compressive strength of acicular mullite with minimal decrease in porosity, primarily through densification and improved microstructural characteristics. The presence of 12 wt.% Fe₂O₃ improved compressive strength from ~25 MPa (pure mullite) to ~76 MPa, whereas the porosity decreased from 55.4% (pure mullite) to 51.8%. It was found that the presence of Fe₂O₃ reduces the mullite formation temperature, which was ascribed to the formation of a liquid phase and accelerated diffusion. The solubility limit of iron oxide in mullite was between 8 and 12 wt.%. This study demonstrated that the presence of Fe₂O₃ was not a prerequisite for the formation of mullite acicular (prismatic) grains. It was shown that the presence of MoO₃ also initiates the formation of elongated mullite grains. Although mullite formation with MoO₃ alone (without Fe₂O₃) occurs at a slightly higher temperature, the grains maintain their rectangular shape even at temperatures as high as 1300 °C, resulting in a higher specific surface area compared to samples containing Fe₂O₃. The presence of iron ions, along with enhanced mass transport, promoted the rounding of sharp edges in prismatic mullite grains. This led to a reduction in specific surface area to 0.55 m²/g in the sample with 12 wt.% Fe₂O₃, which is significantly lower than the 0.94 m²/g observed in pure mullite. Enhanced diffusion also contributed to the increased thermal conductivity, reaching a value of 1.17 W/m·K in the sample with 12 wt.% Fe₂O₃. The measured values of the three key properties; compressive strength, porosity, and thermal conductivity, are within the target ranges necessary for materials intended for diesel engine exhaust filtration.