Influence of MgO-Al₂O₃ hollow sphere content on the microstructure and mechanical properties of calcium hexaluminate porous ceramics

ABSTRACT

Calcium hexaluminate (CA₆) porous ceramics were prepared by foaming and pore-forming agent methods, with Al₂O₃ and calcium aluminate cement (CAC) as raw materials and magnesium aluminum spinel hollow spheres (MASHSs) as pore-forming agent. The effects of the amount of magnesium aluminum spinel hollow sphere on the phase composition, bulk density, apparent porosity, flexural strength, microstructure, and thermal conductivity of CA₆-based porous ceramics were systematically studied. The pores of such porous ceramics are mainly formed by the enhanced amount of MASHSs and the generation of CA₆ phase. The results demonstrate that with the elevation of MASHSs content, the microstructure of the samples transforms from a uniform distribution of closed pores to an interconnected micro-pore arrangement. Notably, the fracture paths transition from cohesive to interfacial interactions between MASHSs. This intricate interplay culminates in a remarkable outcome – our ceramics exhibit thermal conductivities ranging from 0.210 to 0.451 W/(m/k), an augmented gas permeability of 25.96 × 10⁻¹¹m², and exceptional compressive strengths reaching up to 22.3 MPa. Overall, this study provides insights into the influence of MASHSs content on the microstructure and thermal-mechanical properties of porous materials, which may contribute to the development of advanced ceramic materials with tailored properties.

KEYWORDS:

• MA hollow spheres
• calcium hexaluminate
• porous ceramics
• thermal conductivity
• mechanical property

1. Introduction

Calcium hexaluminate-based porous ceramic comprise a significant class of high temperature materials that are composed of calcium hexaluminate (CA₆) phase of magnetoplumbite structure (P6₃/mmc) that are cross-linked, chemically or physically, with the capacity to absorb alkali vapor corrosion, low thermal conductivity, and good volume stability in complex extreme environment [1–7]. These porous ceramics have recently emerged as promising materials for applications such as gas filtration [8–12], refractory support [13–17], and thermal insulation [18,19]. However, CA₆-based porous ceramic generally exhibit relatively low insulation performance, and as porous materials, their porosity responses to external forces convey unstable mechanical property due to unexpected open pores and microstructure [20–23]. Therefore, the development of porous ceramics with high porosity, good mechanical properties and thermal conductivity has garnered significant attention in the past few years.

To date, various porous ceramics have been developed by the different material selection and preparation technology. These materials commonly are generated by the polymer sponges [24–27], using pore-forming agent method or by direct foaming [28–34]. Furthermore, using hollow sphere (HSs) ceramics is another attracting alternative way to produce porous ceramics [35–41]. Among them, magnesium aluminum spinel hollow sphere has shown particular promise and attracted significant interest in porous materials on account of its high-temperature phase (MgAl₂O₄) with a melting point of 2135°C, and the remarkable resistance to alkali vapor corrosion [42,43]. More importantly, in our previous study, a novel approach to produces highly open pore structures by foaming method with magnesium aluminum (MA) spinel hollow spheres as pore-forming agent and with CAC as binder was introduced, which are ideal for porous material applications. These results are obtained because high temperature accelerates the formation of the denser structure between matrixes and MA hollow sphere, which is conducive to changing the direction of crack. Moreover, the foaming method with calcium aluminate cement (CAC), as a binder, would help enhance the apparent porosity. As such, the links between MASHSs and matrixes provides a denser structure to change the directions of crack from the surface to the interior MASHSs, and therefore facilitating the mechanical performance of the porous materials [42].

Given that optimizing the processing parameters is significantly essential for porous ceramic, numerous studies have focus on the fabrication of porous ceramic with hollow spheres (HSs) [36,41,42]. For example, glass hollow spheres can be joined together by carefully heating them above their melting temperature to form porous glass [10,44]. According to the Green et al, they produced hollow sphere glasses by sintering glass HSs at various temperatures and obtain samples with mechanical strength equivalent to that of other lightweight glasses with higher densities [45,46]. Similarly, metallic hollow spheres can be sintered together to form hollow sphere structures. Sanders and coworkers reported that hollow spheres structures could potentially have improved mechanical properties compared to porous ceramics prepared from other manufacturing processes using theoretical studies [47,48].

If magnesium aluminum spinel hollow spheres (MASHSs) are able to play the role of pore-forming agent of the porous ceramics, it would be expected that the porous ceramic would have good porosity and thermal conductivity properties with addition of MASHSs even if the compressive strength of ceramics is somewhat decreased. It is not clear if sufficient mechanical properties and thermal conductivity properties of the porous ceramics could be maintained when the amount of MASHSs is increased. Furthermore, in the porous ceramic field, very little attention has been focused on magnesium aluminum spinel hollow spheres (MASHSs), if this factors well considered in our joint pore former methods, in accordance with the desired application, porous materials can also ensure enhanced performance without noticeably decreasing the thermo-mechanical properties and its service life.

In this work, we investigated the porosity, volume stability and thermo-mechanical properties of porous materials with varied amounts of MASHSs additions. We also discussed the changes in properties of ceramics with respect to the phase development and microstructure evolution in this material.

2. Material and methods

2.1. Materials preparation

Table 1 presents the compositions of five distinct formulations of porous ceramics, which vary in the content of magnesium aluminum spinel hollow spheres (MASHSs) (Zibo Wanbang nonmetallic materials Co., Ltd. China). The starting materials utilized in this study were alumina powder (CL370, Al₂O₃ ≥99.56 wt%, d₅₀ = 2.14 μm; Almatis Aluminum (Qingdao) Co., Ltd., China). To achieve the desired properties of the samples, CAC (Secar 71, d₅₀ = 13.6 μm; Imerys, Tianjin, China) was used as a high-temperature binder, while sodium lauryl sulfate (Tianjin Guangfu Fine Chemical Research Institute, Chemical pure) was selected as the foaming agent. The samples with varying concentrations of MASHSs (60, 65, 70, 75, and 80 wt%) were coded as HS0, HS5, HS10, HS15, and HS20, respectively.

Table 1. Formulations of porous ceramic composed of varied MA hollow spheres content (wt%).

Raw materials	Specification	HS0	HS5	HS10	HS15	HS20
MA Hollow spheres	0.9 ~ 0.6 mm	40	45	50	55	60
MA Hollow spheres	0.6 ~ 0.5 mm	20	20	20	20	20
Alumin micro-powder	d50 = 2.14 μm	33	28.87	24.75	20.62	16.5
Secar71	d50 = 13.6 μm	7	6.13	5.25	4.38	3.5
Foaming agent		0.043	0.043	0.043	0.043	0.043
Foam stabilizer		2.86	2.86	2.86	2.86	2.86
PAM		23.43	23.43	23.43	23.43	23.43

2.2. Preparation of porous ceramic

The experimental procedure for producing porous ceramics containing MASHSs is detailed in Figure 1. Initially, polyacrylamide (PAM) and lauryl alcohol (C₁₂H₂₆O, CHO) were dissolved in a mixer at a mass ratio of 4:6 to prepare a foam stabilizer solution with a concentration of 1.5 wt%. Then, 33% carboxymethylcellulose sodium (CMC) was added successively into the foam stabilizer solution to obtain the appropriate premix solution, which was stirred sufficiently. Next, CAC, MASHSs, sodium dodecyl sulfate (SDS), and Al₂O₃ were added to the premix solution to form a uniform slurry. The slurry was allowed to polymerize to a gelled body at 25°C for 24 h, after which green bodies were produced from 40 mm × 40 mm × 160 mm molds with vibration and dried at 110°C for 24 h. The samples were then heated under an air atmosphere from 300 to 1100°C at a heating rate of 3°C/min, followed by an acceleration of the heating rate to 5°C/min from 1100 to 1700°C. Finally, porous ceramics containing MASHSs were obtained after a dwell time of 3 h at 1700°C. The entire process is illustrated in Figure 1.

Figure 1. Flow chart of the experimental procedure.

2.3. Sample characterization

The bulk density (BD), apparent porosity (AP), and linear shrinkage of the samples were determined according to Chinese standards GB/T 2997–2000 and GB/T 5988–2007, respectively. The compressive strength (CSS) of the materials was measured using a universal testing machine with a crosshead speed of 0.5 mm/min. The thermal conductivity of the samples was measured using a thermal analyzer (YB/T 430–2005) at different temperatures (400°C, 800°C, and 1200°C). The linear shrinkage and the permanent linear change (PLC) of sintered specimens was calculated as the percentage difference between the final and initial lengths, following the GB/T 1548–1992 and GB/T 5988–2007 standard, respectively. Each measurement was replicated three times, and the reported results represent the mean of these three values. Additionally, the phase compositions of each sample after firing were determined using an X-ray diffractometer (XRD; D4 Endeavor, Bruker, Germany). The microstructure of the specimens was characterized via field emission scanning electron microscopy (SEM, SIGMA HD, Zeiss, Germany), which took into account the effect of added MASHSs on the related performance of the samples.

3. Results and discussion

In Figure 2(a), the linear shrinkage of samples with varying MASHS content after firing at 1700°C is presented. The results showed that the linear shrinkage increased from 5.09% to 6.03% as the MASHS content increased from 40 wt% to 60 wt%. This phenomenon can be attributed to the decreased matrix content due to the enhanced HSs, resulting in the generation of a large number of pores between the enhanced MASHSs, and producing linear shrinkage after sintering.

Figure 2. Linear shrinkage and permanent linear changes (PLC) of porous ceramic with different MASHSs addition.

To investigate the effect of different MASHS content on the volume stability of the samples, the permanent linear change (PLC) of the samples fired at 1700°C was measured, as shown in Figure 2(b). The results indicate that the porous ceramic exhibited excellent volume stability during elevated temperature, with a change rate of PLC highly close to zero. Meanwhile, the maximum change rate was 0.11%, while the minimum was 0.02%. Further detailed analysis on the variation with respect to the volume expansion of the samples will be discussed below.

The X-ray diffraction (XRD) analysis of samples fired at 1700°C (shown in Figure 3) confirms the formation of CA₆ phase in all samples due to the addition of CAC in the matrix composition (as indicated in Table 1). The intensity of the CA₆ peaks is similar for all samples, implying that the amount of CAC added is adequate to produce the same amount of CA₆ phase. However, the different amounts of added MASHSs lead to the generation of varying amounts of CA₆ phase, which is due to the presence of SiO₂ and Al₂O₃ in MASHSs that can react with CaO or Al₂O₃ in the CAC during firing. This exothermic reaction causes volumetric expansion of the samples [35,42].

Figure 3. X-ray diffraction (XRD) patterns of the porous ceramic with different MASHSs addition after firing at 1700°C for 3 h.

In addition to the CA₆ phase, peaks attributed to MgAl₂O₄ spinel and MgO are also detected in the XRD patterns of samples [42]. The produced MgO could react with Al₂O₃ in the matrix to form MgAl₂O₄ spinel, which also leads to volumetric expansion of the samples. Interestingly, the intensity of the MgO peak decreases, while the intensity of the MgAl₂O₄ peak slightly enhances with the addition of MASHSs. This finding suggests that the in-situ formation of MgAl₂O₄ spinel is enhanced with the enhanced MASHSs content.

Moreover, the permanent linear change (PLC) results of samples fired at 1700°C (as shown in Figure 2(b)) demonstrate that the addition of MASHSs can effectively suppress the volumetric expansion of the samples caused by the formation of CA₆ phase and MgAl₂O₄ spinel [49–51]. The PLC values of samples decrease gradually as the MASHSs content increases, implying that the volume stability of the samples is improved. For instance, the HS20 sample exhibits a positive PLC value of 0.08%, indicating that the sample experienced linear expansion after firing. Overall, these results suggest that the addition of MASHSs can effectively adjust the phase composition of the matrix and improve the volume stability of the porous ceramic.

The microstructural analysis presented in Figure 5 provides insight into the underlying mechanism behind the enhanced physical properties of samples with varying MASHSs content. It is noteworthy that the presence of CA₆ plates is observed in all samples, and the thickness of these plates varies depending on the amount of MASHSs added. Thinner CA₆ plates are observed in samples with higher MASHSs content (HS0- HS10), resulting in less local volume expansion in the sample matrix. In contrast, samples with lower MASHSs content (HS20) exhibit thicker CA₆ grains, leading to higher local volume expansion in the sample matrix. Furthermore, the observed sintering neck between two MASHS particles decreases as the added amount of MASHSs is reduced, which is insufficient to form strong bonding among the particles. As a result, the porosity of the samples is enhanced, as shown in Figure 4. This is consistent with previous research indicating that the presence of porosity in ceramics can improve their mechanical properties [42]. The increased porosity also leads to a higher degree of pore connectivity, improving the permeability and thermal insulation properties of the samples. Overall, the microstructural analysis suggests that the addition of MASHSs can lead to the formation of thinner CA₆ plates, reduced sintering necks, and increased porosity and pore connectivity, all of which contribute to the improved physical properties of the samples.

Figure 4. Changes of bulk density and apparent porosity of the porous ceramic with different MASHSs addition.

Figure 5. SEM of the porous ceramic with different MASHSs addition after sintering at 1700°C. [(a, f) HS0 sample; (b, g) HS5 sample; (c, h) HS10 sample; (d, i) HS15 sample; (e, j) HS20 sample].

Note that in Figure 5, four types of porosity are generated in these samples, including pores originating from the MASHSs (labeled with number 1 in Figure 5), pores produced between the hollow spheres (labeled with number 2), small pores generated by the foaming method (labeled with number 3), and micrometer pores (about 10 μm) between the CA₆ grains (labeled with number 4). As the content of HSs increases, the content of the matrix decreases, and thus decreasing the pores produced by the foaming agent, while the connected pores between hollow spheres augment from HS0 to HS20. This is due to the MASHSs-crosslinked network displays a continuous transition from compact contact to point contact, generating higher porosity (shown in Figure 4).

These findings also confirm that the hindered CA₆ growth and increased porosity level by enhanced MASHSs content are conducive to inhibit volume expansions accompanied by in situ CA₆ and MgAl₂O₄ spinel formation, thereby supporting the volume stability of materials as shown in Figure 2. Overall, these results suggest that the addition of MASHSs can be an effective strategy for improving the physical performance of materials by controlling the microstructure and porosity of the materials, which can in turn mitigate issues such as volume expansion during the firing process.

Figure 6 presents the impact of MASHSs addition on the gas permeability (GP) and cold crushing strength (CCS) of porous ceramic fired at 1700°C. The results show that the CCS continuously decreases, while the GP increases with an increase in the MASHSs content. For instance, the CCS decreases from 22.3 MPa to 21.2 MPa when the added MASHSs is varied from 60 to 65 wt%. On the other hand, the GP of the MASHSs-containing ceramic is higher, which is consistent with the porosity results. Specifically, the GP of samples gradually increases from 2.67 × 10⁻¹¹m² to 25.96 × 10⁻¹¹m² with the increased MASHSs content.

Figure 6. Changes of gas permeability (GP) and cold crushing strength (CCS) of the porous ceramic with different MASHSs addition.

The increased GP of materials caused by the added MASHSs is likely the reason for the decrease in CCS values, as the porous structure is found to decline during firing (as seen in Figure 5). Nevertheless, the interlocked structure between CA₆ is beneficial for the overall improvement of strength. As observed in Figures 4 and 5, formed different types of pores at elevated temperatures promote the porosity connectivity degree of the sample, and thus enhancing the GP performance of such materials. These findings suggest that the addition of MASHSs can be a potential approach to tune the microstructure and porosity of materials, leading to adjusted GP and CCS properties, which are critical for applications such as gas separation and filtration.

The effect of MASHSs addition on the thermal insulation properties of porous materials was investigated by evaluating the thermal conductivity at different temperatures (400°C, 800°C, and 1200°C) after firing, as shown in Figure 7. The results show that the thermal conductivity of the materials increases with increasing MASHSs content. Among the samples, the HS0 sample with 60% MASHSs content exhibited thermal conductivity values of 0.210 W/mK, 0.342 W/mK, and 0.451 W/mK at 400°C, 800°C, and 1200°C, respectively. Compared to previous studies, our MASHSs-containing ceramic demonstrates improved thermal insulation properties up to 0.210 W/mK, while maintaining considerable strength and porosity [35,36,42]. This is attributed to the increased pore content within the HS0 sample containing 60% MASHSs, which act as pore-forming additives to form well-interconnected pore structures (Figure 5), thus reducing heat conduction and favoring the thermal insulation properties of the materials. The intricate interplay between the material’s structural attributes and its thermal conductivity will be thoroughly scrutinized and elaborated upon in subsequent sections of this study.

Figure 7. Thermal conductivity of the porous ceramic with different MASHSs addition.

Figure 8 presents a comparative analysis of porosity and mechanical strength, encompassing data obtained in this study and findings from prior literature sources [34,52–56]. The literature data encapsulate the performance metrics of diverse ceramics fashioned through various preparation methodologies. As illustrated in the figure, the porosity values, both from literature data and our experimental dataset, tend to cluster around the 70% mark. However, what sets our approach apart is the considerably higher mechanical strength achieved in comparison to alternative methods. Notably, our method demonstrates an impressive range of mechanical strength, spanning three orders of magnitude, with values ranging from 17.1 to 22.3 MPa, depending on the chosen processing technique. It is particularly noteworthy that the maximum mechanical strength attained in our MASHSs ceramic foams reaches an impressive 22.3 MPa in this work, surpassing the mechanical performance of ceramic foams produced via direct foaming and gel-casting methods. This achievement can be attributed to the unique presence of windows on the MASHSs. Consequently, the incorporation of MASHSs has a discernible impact on the theoretical apparent porosity (AP) and geometric porosity (GP), as exemplified in Figures 4 and 6. For instance, the porosity range attained through our approach (ranging from 66.5% to 73.9%) falls within the spectrum between that of HA ceramics crafted through direct forming and Al₂O₃ porous ceramics generated via pore-forming agent methodologies [54]. Figure 8 presents a comparative analysis of porosity and mechanical strength, encompassing data obtained in this study and findings from prior literature sources. In essence, our approach demonstrates not only competitive porosity levels but also an impressive mechanical strength profile, showcasing the potential for advanced ceramic materials with tailored properties.

Figure 8. Comparative assessment of porosity and mechanical strength between the as-prepared porous ceramic and diverse porous ceramics fabricated utilizing varied preparation techniques as documented in the literature.

The primary objective of this study was to elucidate the influence of MA Hollow Sphere content on the performance of CA₆-based porous ceramics, employing MASHSs as a pore-forming agent. To assess the effectiveness of these porous MA ceramics, striking a balance between mechanical strength and insulation performance for structural applications, we conducted a comparative analysis against existing literature sources [57–69]. As depicted in Figure 9, a substantial portion of the investigated porous ceramics exhibited compressive strengths ranging from 1.5 MPa to 17.1 MPa, which generally fell below the impressive compressive strength achieved in our study (22.3 MPa). Notably, certain literature sources reported thermal conductivities within a comparable range, spanning from 0.02 W/(m·K) to 0.09 W/(m·K). For instance, the thermal conductivity value of 0.02 W/(m·K) observed in mullite fiber composites aligns with the thermal conductivity of other specimens (ranging from 0.03 to 0.24 W/(m·K)). However, it is crucial to note that the corresponding compressive strength of the mullite fiber composite stands at 1.8 MPa, which is one order of magnitude lower than the achievement in our study. This substantial difference ultimately compromises the insulation performance of the material. On the other hand, the highest compressive strengths recorded in Si₃N₄-silica aerogel composites (14.7 MPa) and attapulgite fiber-reinforced silica aerogel composites (15.3 MPa) are on par with our previous study [42] but they still are lower than this work. Nevertheless, it is worth highlighting that these composites involve the use of costly fiber materials and often entail protracted or high-temperature processing procedures, rendering them economically impractical for widespread use. Therefore, our study represents a significant advancement by presenting lightweight porous MA ceramics that concurrently exhibit both high compressive strength and low thermal conductivity. This unique combination renders them a promising choice for structural applications.

Figure 9. Comparative analysis of maximum compressive strengths and corresponding minimum thermal conductivities of the porous ceramics in comparison with previously reported porous ceramics.

Of note, the innovative methods introduced in this study, alongside the acquired results, shed light on a compelling dynamic: as the proportion of MA hollow spheres content increased, there was a notable and significant rise in thermal conductivity, while the compressive strength of the samples decreased. To delve deeper into the intricacies of this observed enhancement in thermal insulation, it is paramount to recognize the multifaceted interplay between the solid and gas phases within porous materials. A pivotal aspect to consider is that heat conduction within these materials is profoundly influenced by both these phases. In particular, the gas-filled voids or air pockets nestled within the porous structure exhibit heightened sensitivity to temperature fluctuations. The remarkable augmentation in thermal insulation performance, vividly depicted in Figures 5 and 7, can be attributed to the strategic incorporation of a higher content of MA hollow spheres. This integration effectively fosters a more abundant presence of air or gas-filled voids within the porous architecture [69]. In the realm of heat conduction, gases emerge as exemplary thermal insulators when compared to their solid counterparts. Their inherent property of lower thermal conductivity serves as a pivotal asset, leading to an overall reduction in the thermal conductivity of sample [56]. Furthermore, the infusion of a higher content of MAHSHs initiates an intriguing interplay between HSs and the irregular CA₆ solid phase, as ingeniously illustrated in Figure 5. This dynamic arrangement orchestrates an intricate choreography for heat flow. With multiple reflections and scattering events punctuating its trajectory, direct heat transmission is significantly curtailed. This intricate ballet of redirection and scattering contributes substantially to the insulation effect [62]. Moreover, this novel approach ushers in the creation of a composite structure, inherently capable of further mitigating solid thermal conductivity through an extended solid heat transfer path [55]. This adds yet another layer to the material’s prowess in curtailing heat transmission. In this intricate interplay of effects, the amalgamation of these factors notably diminishes the thermal conductivity of the final products at room temperature. These collective attributes firmly establish porous ceramics as exemplary candidates for high-temperature thermal insulation materials, as illustrated in Figure 7. It is precisely this intricate interconnection of structural modifications, gas-phase inclusions, and composite behaviors that invigorates the landscape of advanced thermal insulation materials with promising potential.

Our observations have unveiled a noteworthy trend in the cold crushing strength (CCS) of the materials, which consistently decreases with an increase in MASHSs content, as depicted in Figure 6. Our hypothesis posits that this trend primarily arises from the interstitial spaces introduced by the presence of MASHSs [42]. This hypothesis finds robust support in the porosity results, which indicate that the interconnected micro-pore structure bears a striking resemblance to a configuration akin to randomly packed spheres. Consequently, the observed enhancement in air permeability (AP) and gas permeability (GP) with augmented MASHSs content predominantly stems from the reduction in surface contact points between the MASHSs (as seen in Figure 6). For instance, the CCS of sample HS0, characterized by a higher content of hollow spheres, registers an impressive 22.3 MPa, surpassing the values recorded for other samples. As elucidated in previous research, the reinforcement of the bond between CA₆ and MASHSs during the firing process at elevated temperatures plays a pivotal role in augmenting the interfacial strength between MASHSs [42]. This, in turn, is conducive to lower AP and GP values within the sample. Consequently, cracks tend to propagate along shorter pathways, extending through the MASHSs [Figure 11(a)], and eventually fracturing them, as illustrated in Figs. 10(a-c). As a result, under the influence of this compact structure, the compressive strength of the HS0 sample soars to an impressive 22.3 MPa.

Figure 10. Fracture morphology of the porous ceramic with different MASHSs addition after sintering at 1700°C. [(a) HS0 sample; (b) HS5 sample; (c) HS10 sample; (d) HS15 sample; (e) HS20 sample].

Figure 11. Schematic representation of two fracture paths of MASHSs when the HSs are (a) strongly or (b) weakly bonded together. The insets in each picture show the cross section of the MASHSs.

However, the increased content of MASHSs leads to a reduction in the volume of the matrix, as exemplified by the HS20 sample. Consequently, HS20 exhibits a higher gas permeability (GP) while concurrently displaying a lower cold crushing strength (CCS). In this scenario, the fracture path shifts from a through-HSs fracture to one that follows along the HSs, as confirmed by the SEM image presented in Figure 10(e). As a consequence, the MASHSs within the structure are considered to be “loosely connected,” a factor that is believed to weaken the mechanical strength of the ceramic, particularly under the condition of overfiring, as illustrated in Figure 11(b). Nonetheless, this adjustment serves to further optimize the thermal insulation properties of the porous ceramic. Remarkably, this enhancement extends to composite structures involving ceramics, thereby broadening the scope of applications in this field, as demonstrated in Figures 8 and 9.

4. Conclusion

In conclusion, our study explored the influence of varying magnesium aluminum spinel hollow sphere (MASHSs) content on CA₆-based porous ceramics. We observed that as MASHSs content increased, thermal conductivity rose significantly, while compressive strength decreased. The incorporation of MASHSs fostered an interconnected micro-pore structure and enhanced air and gas permeability. This intricate interplay between structural pores and gas-phase inclusions culminated in a notable reduction in thermal conductivity ranging from 0.210 to 0.451 W/(m/k). Our ceramics exhibited exceptional performance, with compressive strengths reaching up to 22.3 MPa, surpassing those in existing literature. Furthermore, the unique balance between mechanical strength and thermal insulation established our ceramics as promising materials for structural applications. Notably, certain compositions with “loosely connected” MASHSs displayed even greater thermal insulation properties, albeit with reduced mechanical strength. This study advances the field of advanced thermal insulation materials by leveraging the synergistic effects of structural modification and gas-phase inclusions. Our findings open new avenues for the development of high-performance ceramics with tailored properties, showcasing their potential in diverse structural applications.

Acknowledgments

The work was financially supported by the National Natural Science Foundation of China (No. 12002226).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the National Natural Science Foundation of China [No. 12002226].