Introduction

Heavy and toxic haze pollution poses a serious risk to human health with detrimental effects on global mortality in recent years1,2,3. Particulate matter (PM) emission from cement factories, power plants, and metallurgical industry is a significant source contributing to acute haze formation4,5,6, producing particle sizes below 10 μm, and creating serious health concerns7,8,9,10,11. To tackle or mitigate the challenge, porous metals12,13 and porous ceramics14,15,16 are widely utilized at high temperatures, where PM is initially released to take full advantage of filtration efficiency. Although these porous materials are effective at capturing PM2.5, and PM1017, they, unfortunately, suffer in application owing to performance-related challenges such as poor corrosion and high-temperature oxidation resistivities, severe brittleness, and unworkability, and significant energy penalty associated with complicated material preparation procedures. As such, it is of considerable significance to develop functional porous materials for high-temperature PM capturing with a simple, highly efficient, and scalable approach.

TiAl porous alloy is expected to be the next-generation microfiltration membrane for potential application in high-temperature gas/solid or liquid/solid two-phase separation, owing to its characteristics of containing a mixture of metallic and covalent bonds that provide sound mechanical properties18,19, excellent high-temperature oxidation resistance, and corrosion resistance20,21 at elevated temperatures, particularly at above 600 °C. Unfortunately, the insufficient efficiency of microfiltration membrane severely limits their practical application in capturing Nano-scale PM, resulting in inadequate filtration performance22,23. The main cause of this problem for TiAl porous alloy prepared by powder metallurgy is the formation of micro-scaled pore structure rather than a dual-scale integrated micro/nanoporous network structure during high-temperature heat-treatment processing24,25,26.

To overcome these limitations, we offer an alternative approach for fabricating porous composite microfiltration membrane (PCMM) with hierarchical micro/mano-dual-scaled pore structure for efficient high-temperature PM capturing. Control of material nanostructure is an effective approach to modify material properties27,28,29. The introduction of Nano-ZrO2 in the high Nb-TiAl system provides a potential solution for high-temperature application because of its high melting point (2700 °C), low coefficient of thermal expansion, and good creep resistance30,31. More specifically, a micro-scale porous microstructure was constructed by controlling self-diffusion and mutual diffusion of Ti, Al, Nb powders below 900 °C, followed by building nano-scale pores on the inner surface of cell walls through chemical reaction between the porous support material and the Nano-ZrO2 particles at 900–1350 °C. Benefited from this structural feature, the designed PCMM demonstrates increased high-temperature PM removal efficiency and favorable high-temperature oxidation/sulfidation performance.

Results and discussion

Microstructure evaluation and phase transformation mechanism of PCMM

The crystal structure of Ti-48Al-6Nb porous alloy and the PCMM (with 4 wt.% Nano-ZrO2 addition) are presented in Fig. 1a. The strong signals of α2-Ti3Al and γ-TiAl in PCMM, as characteristic peaks of Ti-48Al-6Nb porous alloy, suggest the formation of high Nb-TiAl-based matrix. The diffraction peaks of α-Al2O3 occurring in the diffractograms indicate the formation of PCMM. Fig. 1b exhibits the X-ray diffraction (XRD) patterns of PCMM with 4 wt.% Nano-ZrO2 addition and annealed at different temperatures. It can be seen from Fig. 1b that the TiAl3 phase formed by reaction of Ti with Al at 600 °C and subsequently transformed into TiAl2, TiAl, and Ti3Al phases at a temperature range of 600–900 °C, whereas the NbAl3 phase formed by reaction of Nb with Al and NbAl3 transformed into Nb2Al phase at temperatures ranging from 900 to 1350 °C, forming a soluble solid in the matrix interior18,22. The Nano-ZrO2 react with Ti3Al and TiAl at the same time to form the Al2O3 phase, observable from the intensified diffraction peaks.

Fig. 1: XRD spectra of porous sample.
figure 1

a Ti-48Al-6Nb porous alloy (black) and PCMM (red); b PCMM annealed at different temperatures: 600 °C (black), 900 °C (red), and 1350 °C (blue).

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XPS analysis was also carried out to investigate the composition of PCMM (Figs 2a–d). Ti 2p spectra in the PCMM with 4 wt.% Nano-ZrO2 addition (Fig. 2a) can be divided into two edge splits, Ti 2p3/2 and Ti 2p1/2, and Ti 2p signal suggests that Al and O groups bind to Ti by four coordination modes: The Ti 2p3/2 and Ti 2p1/2 peaks are located 454.06 and 460.16 eV for Ti-Al (TiAl and Ti3Al), 455.50 and 461.10 eV for Ti-O (TiO), 457.3 and 462.5 eV for Ti-O (Ti2O3), 458.82 and 464.52 eV for Ti-O (TiO2)32,33. The high-resolution spectra Al 2p in the PCMM (Fig. 2b) revealed the presence of Al-Ti/Al-Nb (TiAl and Nb2Al) bonds at 72.00 eV and Al-O (Al2O3) bond at 74.30 eV, suggesting that formation of TiAl, Ti3Al, Nb2Al and Al2O3 phases34. The high-resolution spectra Nb 3d in the PCMM (Fig. 2c) can be divided into two edge splits: Nb 3d5/2 and Nb 3d3/2, with Nb 3d5/2 and Nb 3d3/2 peaks located at 202.7 and 205.48 eV for Nb-Al (Nb2Al), 205.00 and 207.72 eV for Nb-O (NbO2), and 207.20 and 209.92 eV for Nb-O (Nb2O5), correspondingly35. The high-resolution spectra Zr 3d in the PCMM (Fig. 2d) revealed the presence of Zr metal at 178.54 and 180.97 eV, and Zr-O (ZrO) bond at 180.68 and 183.11 eV, Zr-O (ZrO2) bond at 182.39 and 184.8236. Therefore, the PCMM (with 4 wt.% Nano-ZrO2 addition) obtained at 1350 °C are mainly composed of TiAl, Ti3Al, Nb2Al, TiO, Ti2O3, TiO2, Al2O3, NbO2, Nb2O5, Zr, ZrO, ZrO2. As such, the phase formation and transformation for the mixture of the Ti-48Al-6Nb matrix and the Nano-ZrO2 powder heated at different temperatures could proceed as following:

Fig. 2: XPS patterns of PCMM (with 4 wt.% Nano-ZrO2 addition).
figure 2

a Ti 2p; b Al 2p; c Nb 3d; d Zr 3d.

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at 600 °C22:

$${\mathrm{Ti}} + 3{\mathrm{Al}} \to {\mathrm{TiAl}}_3$$
(1)

at 600~900 °C:

$${\mathrm{Nb}} + {\mathrm{TiAl}}_{\mathrm{3}} \to {\mathrm{NbAl}}_3 + {\mathrm{Ti}}$$
(2)
$$2{\mathrm{TiAl}}_3 + {\mathrm{Ti}} \to 3{\mathrm{TiAl}}_2$$
(3)
$${\mathrm{TiAl}}_3 + 8{\mathrm{Ti}} \to 3{\mathrm{Ti}}_3{\mathrm{Al}}$$
(4)
$${\mathrm{TiAl}}_3 + 2{\mathrm{Ti}} \to 3{\mathrm{TiAl}}$$
(5)
$${\mathrm{TiAl}}_2 + 5{\mathrm{Ti}} \to 2{\mathrm{Ti}}_3{\mathrm{Al}}$$
(6)
$${\mathrm{TiAl}}_2 + {\mathrm{Ti}} \to 2{\mathrm{TiAl}}$$
(7)

at 900~1350 °C:22,24,26

$$2{\mathrm{TiAl}}_2 + {\mathrm{Ti}}_3{\mathrm{Al}} \to 5{\mathrm{TiAl}}$$
(8)
$${\mathrm{NbAl}}_3 + 5{\mathrm{Nb}} \to 3{\mathrm{Nb}}_2{\mathrm{Al}}$$
(9)
$$4{\mathrm{TiAl}} + 7{\mathrm{ZrO}}_2 \to 5{\mathrm{Zr}} + 2{\mathrm{ZrO}} + {\mathrm{TiO}} + {\mathrm{Ti}}_2{\mathrm{O}}_3 + {\mathrm{TiO}}_{\mathrm{2}} + 2{\mathrm{Al}}_2{\mathrm{O}}_3$$
(10)
$$2{\mathrm{Ti}}_3{\mathrm{Al}} + 6{\mathrm{ZrO}}_2 \to 5{\mathrm{Zr}} + {\mathrm{ZrO}} + 3{\mathrm{TiO}} + {\mathrm{Ti}}_2{\mathrm{O}}_3 + {\mathrm{TiO}}_2 + {\mathrm{Al}}_2{\mathrm{O}}_3$$
(11)
$$4{\mathrm{NbAl}}_3 + 14{\mathrm{ZrO}}_2 \to 13{\mathrm{Zr}} + {\mathrm{ZrO}} + 2{\mathrm{NbO}}_2 + {\mathrm{Nb}}_2{\mathrm{O}}_5 + 6{\mathrm{Al}}_2{\mathrm{O}}_3$$
(12)
$$3{\mathrm{NbAl}}_2 + 9{\mathrm{ZrO}}_2 \to 7{\mathrm{Zr}} + 2{\mathrm{ZrO}} + {\mathrm{NbO}}_2 + {\mathrm{Nb}}_2{\mathrm{O}}_5 + 3{\mathrm{Al}}_2{\mathrm{O}}_3$$
(13)

The pore parameters are crucially important to the performance of high-temperature PM capturing, especially the pore diameter distribution. The pore diameter distribution of Ti-48Al-6Nb porous alloy and PCMM with 0.5–8 wt.% Nano-ZrO2 addition analyzed by mercury intrusion porosimeter are exhibited in Fig. 3a and b. Compared with the average pore diameter of Ti-48Al-6Nb porous alloy (9.53 μm), the average pore diameter of PCMM clearly showed a substantial decrease. Regarding the cases at 0.5–8 wt.% Nano-ZrO2 addition, the minimum average pore diameter (7.25 μm) was achieved when adding 8 wt.% Nano-ZrO2, and the cumulative pore volume for the smaller pores of 0 ∼ 5 μm also increased. This phenomenon could be attributed to the fact that the Nano-sized pores were fabricated on the inner cell walls of high Nb-TiAl support through chemical reaction between TiAl/Ti3Al and Nano-ZrO2 at high temperature (>900 °C). In addition, Table 1 quantitatively presents the effect of Nano-ZrO2 mass on the total pore volume and area, porosity, and the density of PCMM. Compared with the Ti-48Al-6Nb porous alloy, the total pore area extended from 0.05 m2/g to 0.096 m2/g, the total pore volume reached from 0.131 cm3/g to 0.196 cm3/g, whereas the porosity increased from 28.79% to 44.78% for the PCMM with 4 wt.% Nano-ZrO2 addition. It is interesting to note that the addition of Nano-ZrO2 can change the minimum pore diameter of Ti-48Al-6Nb porous alloy while improving other pore parameters.

Fig. 3: Pore diameter distribution curve of Ti-48Al-6Nb porous alloy and PCMM.
figure 3

a Ti-48Al-6Nb porous alloy (black) and PCMM with 0.5 wt.% (red), 1 wt.% (green), 2 wt.% (blue), 4 wt.% (cyan) and 8 wt.% (magenta) Nano-ZrO2 additions; b locally magnified curves of Fig. 3a.

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Table 1 Pore parameter for Ti-48Al-6Nb porous alloy and PCMM with different amounts of Nano-ZrO2 addition (0.5–8 wt.%).
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The pore formation and distribution are presented in Fig. 4 and Fig. 5, and the detailed pore parameters of the PCMM heat-treatment at different temperatures (600 °C, 900 °C, 1000 °C, 1100 °C, 1200 °C, 1350 °C) are listed in Table 2. Fig. 4 depicts typical surface field emission scanning electron microscopy (FESEM) images of Ti-48Al-6Nb porous alloy and PCMM with 4 wt.% Nano-ZrO2 addition heated at 600 °C, 900 °C, and 1350 °C, showing the formation of a skeleton structure consisting of particles and possibly Kirkendall voids at 600 °C (Fig. 4a), 900 °C (Fig. 4c). and 1350 °C (Fig. 4e)22,24. Similar Kirkendall voids formation is also observable from the samples of PCMM with 4 wt.% Nano-ZrO2 addition (Fig. 4b and Fig. 4d). Fig. 4f shows the micro/nano-pore formation on the inner cell walls of high Nb-TiAl porous supporting, featuring a hierarchical micro/nano-dual-scaled porous structure, possibly through chemical reaction of Nb-TiAl with Nano-ZrO2 at 900–1350 °C.

Fig. 4: Surface FESEM images of porous sample.
figure 4

Ti-48Al-6Nb porous alloy at various heat-treatment temperatures: a 600 °C, c 900 °C, e 1350 °C; PCMM at various heat-treatment temperatures: b 600 °C, d 900 °C, f 1350 °C.

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Fig. 5: Pore diameter distribution curves of PCMM at various heat-treatment temperatures.
figure 5

600 °C (black), 900 °C (red), 1000 °C (green), 1100 °C (blue), 1200 °C (cyan), 1350 °C (magenta).

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Table 2 Pore parameter of PCMM at various heat-treatment temperatures.
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The pore diameter distribution of the PCMM with 4 wt.% Nano-ZrO2 addition at various heat-treatment temperatures (600 °C, 900 °C, 1000 °C, 1100 °C, 1200 °C, 1350 °C) are presented in Fig. 5, further illustrating the relationship between microstructure and heat-treatment temperatures. By increasing the heat-treatment temperature from 600 °C to 1350 °C, the average pore diameter of PCMM continues to increase. More specifically, the average pore diameter increases from 3.22 μm to 3.95 μm when temperature was elevated from 600 to 900 °C due possibly to the mutual diffusion between Ti and Al, and between Nb and Al, along with the transformation of TiAl3 into TiAl and Ti3Al phases18,22. Upon further increasing the heat-treatment temperature from 900 to 1200 °C, ZrO2 began to react with Ti3Al and TiAl, creating spherical TiO, Ti2O3, TiO2, Al2O3, NbO2, Nb2O5, Zr, ZrO, and ZrO2. The occurrence of further phase transformation could result in further increase of average pore diameter from 3.95 μm to 7.24 μm. The skeletal structure was further homogenized with concurring dimensional shrinkages to form a hierarchical micro/nano-dual-scaled porous structure at 1350 °C with a final average pore diameter of 7.25 μm. In addition, detailed pore parameters of the PCMM with the addition of 4 wt.% ZrO2 are given in Table 2.

Fig. 6 provides typical FESEM images of Ti-48Al-6Nb porous alloy and PCMM with 4 wt.% Nano-ZrO2 addition. As shown in Fig. 6a and b, the Ti-48Al-6Nb porous alloy mainly exhibits irregular spherical interconnected particles and the formation of a microporous skeleton. Unlike the Ti-48Al-6Nb porous alloy, however, smaller pores were observed in the PCMM (with 4 wt.% Nano-ZrO2 addition), forming nano-sized pores on the inner cell walls of the Ti-48Al-6Nb microporous skeleton (Fig. 6c, d). It is of interest to note that the hierarchical micro/nano-dual-scaled pore structure of porous ceramics was reported to be beneficial to the capturing of high-temperature PM37,38,39,40 because of the effectively increased contact area between PM and porous structures. However, the reported hierarchical micro/nano-dual-scaled porous structures were fabricated by combining chemical grafting of pore-forming agents and polyurethane, which could be easily ablated under high temperature, a potential detriment to practical application39,40.

Fig. 6: FESEM image of porous sample.
figure 6

Ti-48Al-6Nb porous alloy: a surface; b cross-sectional; PCMM: c surface; d cross-sectional.

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High-temperature oxidation and sulfidation performance of composite microfiltration membrane

Given that corrosion and oxidation are key challenges for porous materials application under rigorous conditions, two typical intermetallic porous materials (Ti-48Al and Ti48Al6Nb porous alloys) were also included in our study for comparison with the PCMM. Shown in Fig. 7 are the isothermal oxidation kinetics curves of Ti-48Al, Ti-48Al-6Nb porous alloys and PCMM (with 4 wt.% Nano-ZrO2 addition) at 900 °C. The obtained mass gain of the Ti-48Al and the Ti-48Al-6Nb porous alloys were 35.024 g/m2 and 10.231 g/m2, respectively, after 900 °C/100 h isothermal oxidation treatment, whereas the PCMM might characterize a relatively more sluggish high-temperature oxidation kinetics with a mass gain of only 3.235 g/m2. The isothermal oxidation results obtained at 900 °C confirmed the beneficial effect of Nano-ZrO2 introduction on the practical application potential of TiAl-based porous alloys for gas/solid or liquid/solid separation with a broadened operation window. This could be the result of a large amount of various ceramic foams and the higher porosity, which leads to a lower thermal conductivity based on Effective Medium Theory, consistent with the previous work by Smith and Mohanta41,42.

Fig. 7: Mass gain of porous sample at 900°C.
figure 7

Ti-48Al porous alloy (black), Ti-48Al-6Nb porous alloy (red), and PCMM with 4 wt.% Nano-ZrO2 addition (green).

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The pore structure stability, i.e., oxidation/sulfidation resistivity of the PCMM was studied at 900 °C and compared with the Ti-48Al and the Ti-48Al-6Nb porous alloys, the most popular intermetallic porous materials for high-temperature applications43,44. Fig. 8 shows that the open pores of Ti-48Al and Ti-48Al-6Nb porous alloys are completely blocked by oxidation/sulfidation products within a 100 h and a 10 h test, respectively, possibly attributable to the large-sized TiO2 grains formed on the entire surfaces of the Ti-48Al and Ti-48Al-6Nb porous alloy (Fig. 8a and c), consistent with our previous work45,46. The PCMM, however, maintained the structural integrity with no obvious pore size variation under the same testing condition (Fig. 8e), possibly attributable to the favorable presence of Al2O3, TiO, Ti2O3, TiO2, NbO2, Nb2O5, Zr, ZrO, ZrO2 particles that hinders the mutual diffusion of oxygen and titanium during high-temperature oxidation. This finding is consistent with the energy dispersion spectrometry (EDS) composition of surface scans of Ti-48Al, Ti-48Al-6Nb porous alloys, and PCMM results shown in Table 3.

Fig. 8: FESEM images of porous materials treated by isothermal oxidation and isothermal sulfidation.
figure 8

a, b Ti-48Al porous alloy; c, d Ti-48Al-6Nb porous alloy; e, f PCMM with 4 wt.% Nano-ZrO2.

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Table 3 EDS composition analysis of porous sample after isothermal oxidation treatment.
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The high-temperature sulfidation behavior of the Ti-48Al, Ti-48Al-6Nb porous alloys, and PCMM with the addition of 4 wt.% Nano-ZrO2 were also investigated with the corresponding results shown in Fig. 8b, d, f. Fig. 8b shows that the Ti-48Al porous alloy was severely corroded with all the pores covered by the small-sized TiO2 grains after undertaking a 10 h isothermal sulfidation at 900 °C. As for the Ti-48Al-6Nb porous alloy, the multiple corroded areas are detectable with some pores also blocked by the formation of the TiO2 and Al2O3 mixture, though predominantly of TiO2 (Fig. 8d). With respect to the PCMM, however, most of the pores are retained even though corrosion still appears to have occurred (Fig. 8f). Furthermore, surface composition of samples subjected to sulfidation are analyzed by EDS with the corresponding results listed in Table 4, showing the presence of only Ti and O. For the Ti-48Al-6Nb porous alloy, however, Al and Nb were also identifiable. As for the PCMM, the amount of Al has significantly increased possibly due to its high sulfidation resistance.

Table 4 EDS composition analysis of porous sample after isothermal sulfidation treatments.
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Filtration performance of PCMM

Ti-48Al-6Nb porous alloy and PCMM with the addition of 4 wt.% Nano-ZrO2 were tested (750 °C, 4000 Pa), with the results shown in Fig. 9a. The removal efficiency was calculated using Eq. (1):

$$\eta = 1 - \xi 1/\xi 2 \times 100\%$$
(1)

where \(\xi 1\) and \(\xi 2\) represent the concentrations of incense PM in the downstream and upstream of the filter, respectively. During the experiment, the PM-containing air flows at a constant velocity of 2 L/min through the samples with an effective area of ~1256 mm2 (sample specifications: Φ40 × 2.4 mm), as shown in Fig. 9b and c. The PM concentration after filtration using Ti-48Al-6Nb porous alloy or PCMM filtration is much lower than the concentration before filtration in both cases (Fig. 9d). For PM with sizes >2.5 μm, the removal efficiency does not vary significantly for Ti-48Al-6Nb porous alloy or PCMM, although the separation efficiencies can be enhanced to 99.58% and 99.98% from 97.38% and 97.89% for PM2.5–5 and PM5–10, respectively. However, for PM with sizes <2.5 μm, the removal efficiency of PCMM is found to be greatly increased to 99.23%, 98.51%, 91.36% and 79.66% from 85.03%, 43.54%, 23.63% and 5.13% corresponding to PM1–2.5, PM0.5–1, PM0.3–0.5, PM<0.3, respectively, as shown in Fig. 9e.

Fig. 9: High-temperature filtration performance test of porous sample.
figure 9

a Schematic of high-temperature PM filtration setup. b, c The pictures of PCMM sample; d PM concentration of dust-laden smoke (original) and after filtration under test condition; e removal efficiency.

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The high-temperature PM (including PM>2.5 μm, PM<2.5 μm) was filtered and compared through Ti-48Al-6Nb porous alloy and PCMM with 4 wt.% Nano-ZrO2 addition. The test was conducted for 60 min at an airflow rate of 2 L/min, with the results shown in Fig. 10 confirming that both high-temperature PM<2.5 μm and PM>2.5 μm can be filtered through Ti-48Al-6Nb membrane with a separation efficiency of 33.32% (SD: ± 0.28%) and 97.46% (SD: ± 0.41%), respectively. In contrast, the high-temperature PM<2.5 and PM>2.5 separation efficiencies can be enhanced to 91.25% (SD: ± 0.21%) and 99.58% (SD: ± 0.22%) when using PCMM. Furthermore, comparison between our PCMM and various porous materials in previous studies38,39,40,47,48 shows that our PCMM with a hierarchical micro/nano-dual-scaled pore structural feature exhibits a relatively higher PM>2.5 μm removal efficiency at a much higher pressure (refer to Table 5). It is of note that the upper limit of service temperature of the as-prepared PCMM could be much higher than the testing temperature since PCMM could survive a temperature of up to 900 °C. These results indicate that our PCMM with hierarchical micro/nano-dual-scaled porous structure can achieve flow-through filtration with high removal efficiency, showing great commercialization prospects for high-temperature PM filtration.

Fig. 10: Removal efficiency of porous sample.
figure 10

Ti-48Al-6Nb porous alloy (black) and PCMM (red) after 60 min filtration test.

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Table 5 Comparison of PM>2.5 μm removal performance of PCMM with reported results.
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The PM filter model of as-prepared Ti-48Al-6Nb porous alloy and PCMM are proposed and schematically illustrated in Fig. 11. The capturing of PM2.5 by conventional porous materials, is much more challenging because of the ultra-low mass and small particle size of PM2.549. Conventional filters characteristic of regular pores could obstruct to the PM-transport, and effectively intercept larger PM. For the as-prepared Ti-48Al-6Nb porous alloy, the minimum filterable particle size is determined by “the size of the pore-throat” in the pore tunnel, and exhibits inferior filtration performance for PM<2.5, because PM with a diameter <2.5 μm could easily pass through its large pore-throat without being captured (Fig. 11 a). Compared with the Ti-48Al-6Nb porous alloy, the filtering efficiency of PCMM was greatly improved benefited from the smaller pores fabricated on the inner cell walls through our approach of chemical reaction between Nano-ZrO2 and Ti3Al/TiAl at 900–1350 °C, effectively forming a hierarchical micro/nano-dual-scaled porous structure (Fig. 11 b). The PCMM with hierarchical micro/nano-dual-scaled porous structure by combining diffusion forming and chemical reaction forming technologies demonstrates potential for applications in environment related fields where highly effective and robust high-temperature filtration is required.

Fig. 11: Schematic illustration of PM removal mechanism for as-prepared porous sample.
figure 11

a Ti-48Al-6Nb porous alloy; b PCMM.

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Methods

Materials

Ti, Al, and Nb powders (99.9%) with an average particle size of <50 μm and ZrO2 nanoparticles (99.9%) with an average particle size of less than 50 nm were used, purchased from Beijing DK nano technology Co., LTD.

Characterization analysis

XRD (Multipurpose X-ray Diffractometer TTR III) and XPS (Thermo Kalpha) were used to analyze the phase formation and crystal structure of Ti-48Al-6Nb porous alloy and the PCMM. The morphological features of Ti-48Al-6Nb porous alloy and PCMM were investigated using FESEM (ZEISS SUPRA55), whereas the surface composition analysis of the Ti-48Al-6Nb porous alloy and PCMM were performed by applying EDS. The pore parameters of the Ti-48Al-6Nb porous alloy and the PCMM were measured using mercury intrusion porosimetry (MIP, Quantachrome AUTOSCAN-33). The weights of Ti-48Al-6Nb porous alloy and PCMM were measured using a METTLER TOLEDO XSE electronic analytical balance with an accuracy of ±0.01 mg. Laser PM sensor (A4-CG; YEETC Co., Ltd, Beijing, China) was used to measure particle size and number before and after filtration.

Preparation of PCMM

PCMM was prepared using a modified powder metallurgy method as described below. Commercial Ti, Al, and Nb powders (average particle sizes of <50 μm) with the atomic ratio of 46:48:6 were mixed with 0.5–8 wt.% Nano-ZrO2 (average particle sizes of <50 nm) by ball milling at 130 rpm in a ball crusher for 10 h (ball-to-powder weight ratio of 5:1), then pressed into green pellets with a diameter of 20 mm under the pressure of 200 MPa. Subsequently, a four-step heat-treatment under vacuum was carried out to fabricate PCMM (120 °C/1 h, 600 °C/3 h, 900 °C/3 h, 1350 °C/3 h). The Ti-48Al and Ti-48Al-6Nb porous alloys obtained from the same processing procedures without any addition of Nano-ZrO2 was also prepared for comparison.

Isothermal oxidation and isothermal sulfidation tests

Isothermal oxidation tests were conducted in a quartz tube furnace. The isothermal oxidation behaviors of the Ti-48Al, Ti-48Al-6Nb porous alloys and PCMM (with 4 wt.% Nano-ZrO2 addition) were observed at 900 °C for 100 h. All the samples were tested at 900 °C for high-temperature oxidation, removed from the furnace at various oxidation intervals (2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 h), cooled in air for 1 h for weighing, and then, where applicable, placed back into the furnace for continued oxidation until 100 h. The isothermal sulfidation behavior of Ti-48Al porous alloy, Ti-48Al-6Nb porous alloy, and PCMM were observed at 900 °C for 10 h. All the samples were tested at 900 °C for high-temperature sulfidation with continuous ventilation of SO2 gas of 99% in purity, removed from the furnace with a total sulfidation duration of 10 h, cooled in air for 2 h before further characterizations, and the residual SO2 gas was removed using saturated sodium hydroxide aqueous solution.

High-temperature filtration performance tests

An in-house air filtration apparatus was applied to evaluate the filtration performances. The PM in the current experiment was generated by burning incense50, whereas the size and concentration of PM before and after filtration were measured by two laser PM sensors. During the experiment, the PM-containing air flows at a 2 L/min constant velocity through the samples with an effective area of about 1256 mm2 (sample specifications: Φ40 × 2.4 mm). Nine samples divided into three groups from each Ti-48Al-6Nb porous alloy and PCMM with 4 wt.% Nano-ZrO2 addition were tested to ensure filtration measurement accuracy.