High efficiency hierarchical porous composite microfiltration membrane for high-temperature particulate matter capturing

Porous intermetallic membrane with extensive interconnected pores are potential candidates as functional materials for high-temperature particulate matter (PM) capturing. However, fabrication of intermetallic membrane with a combined performance of high filtration efficiency and high-temperature oxidation resistance remains a challenge. To tackle this issue, a hierarchical micro-/nano-dual-scale sized pores was constructed on the inner cell walls of a porous support through mutual diffusion and chemical reaction. Benefited from its hierarchical micro/nano-dual-scaled pore structural features, the high Nb containing TiAl-based porous composite microfiltration membrane demonstrates ultrahigh PM>2.5 removal efficiency (99.58%) and favorable oxidation/sulfidation performance at high temperature. These features, combined with our experimental design strategy, provide insight into designing high-temperature PM filtration membrane materials with enhanced performance and durability.


INTRODUCTION
Heavy and toxic haze pollution poses a serious risk to human health with detrimental effects on global mortality in recent years [1][2][3] . Particulate matter (PM) emission from cement factories, power plants, and metallurgical industry is a significant source contributing to acute haze formation 4-6 , producing particle sizes below 10 μm, and creating serious health concerns [7][8][9][10][11] . To tackle or mitigate the challenge, porous metals 12,13 and porous ceramics [14][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 PM 2.5 , and PM 10 17 , they, unfortunately, suffer in application owing to performance-related challenges such as poor corrosion and hightemperature 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 hightemperature 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 hightemperature 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 properties 18,19 , excellent high-temperature oxidation resistance, and corrosion resistance 20,21 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 performance 22,23 . 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 processing [24][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 properties [27][28][29] . The introduction of Nano-ZrO 2 in the high Nb-TiAl system provides a potential solution for hightemperature application because of its high melting point (2700°C), low coefficient of thermal expansion, and good creep resistance 30,31 . 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-ZrO 2 particles at 900-1350°C. Benefited from this structural feature, the designed PCMM demonstrates increased high-temperature PM removal efficiency and favorable hightemperature 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-ZrO 2 addition) are presented in Fig. 1a. The strong signals of α 2 -Ti 3 Al 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-Al 2 O 3 occurring in the diffractograms indicate the formation of PCMM. Fig. 1b exhibits the X-ray diffraction (XRD) patterns of PCMM with 4 wt.% Nano-ZrO 2 addition and annealed at different temperatures. It can be seen from Fig. 1b that the TiAl 3 phase formed by reaction of Ti with Al at 600°C and subsequently transformed into TiAl 2 , TiAl, and Ti 3 Al phases at a temperature range of 600-900°C, whereas the NbAl 3 phase formed by reaction of Nb with Al and NbAl 3 transformed into Nb 2 Al phase at temperatures ranging from 900 to 1350°C, forming a soluble solid in the matrix interior 18,22 . The Nano-ZrO 2 react with Ti 3 Al and TiAl at the same time to form the Al 2 O 3 phase, observable from the intensified diffraction peaks.
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-ZrO 2 addition (Fig. 2a) can be divided into two edge splits, Ti 2p 3/2 and Ti 2p 1/2 , and Ti 2p signal suggests that Al and O groups bind to Ti by four coordination modes: The Ti 2p 3/2 and Ti 2p 1/2 peaks are    36 . Therefore, the PCMM (with 4 wt.% Nano-ZrO 2 addition) obtained at 1350°C are mainly composed of TiAl, Ti 3 Al, Nb 2 Al, TiO, Ti 2 O 3 , TiO 2 , Al 2 O 3 , NbO 2 , Nb 2 O 5 , Zr, ZrO, ZrO 2 . As such, the phase formation and transformation for the mixture of the Ti-48Al-6Nb matrix and the Nano-ZrO 2 powder heated at different temperatures could proceed as following: at 600~900°C: at 900~1350°C: 22,24,26 2TiAl 2 þ Ti 3 Al ! 5TiAl (8) 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-ZrO 2 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-ZrO 2 addition, the minimum average pore diameter (7.25 μm) was achieved when adding 8 wt.% Nano-ZrO 2 , 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/Ti 3 Al and Nano-ZrO 2 at high temperature (>900°C). In addition, Table 1 quantitatively presents the effect of Nano-ZrO 2 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 m 2 /g to 0.096 m 2 /g, the total pore volume reached from 0.131 cm 3 /g to 0.196 cm 3 /g, whereas the porosity increased from 28.79% to 44.78% for the PCMM with 4 wt.% Nano-ZrO 2 addition. It is interesting to note that the addition of Nano-ZrO 2 can change the minimum pore diameter of Ti-48Al-6Nb porous alloy while improving other pore parameters. The pore formation and distribution are presented in Fig. 4 and Fig. 5, and the detailed pore parameters of the PCMM heattreatment 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-ZrO 2 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-ZrO 2 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-dualscaled porous structure, possibly through chemical reaction of Nb-TiAl with Nano-ZrO 2 at 900-1350°C.
The pore diameter distribution of the PCMM with 4 wt.% Nano-ZrO 2 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 TiAl 3 into TiAl and Ti 3 Al phases 18,22 . Upon further increasing the heat-treatment temperature from 900 to 1200°C, ZrO 2 began to react with Ti 3 Al and TiAl, creating spherical TiO, Ti 2 O 3 , TiO 2 , Al 2 O 3 , NbO 2 , Nb 2 O 5 , Zr, ZrO, and ZrO 2 . 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.% ZrO 2 are given in Table 2. Fig. 6 provides typical FESEM images of Ti-48Al-6Nb porous alloy and PCMM with 4 wt.% Nano-ZrO 2 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-ZrO 2 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 PM 37-40 because of the effectively increased contact area between PM and porous structures. However, the reported hierarchical micro/nano-dualscaled 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 application 39,40 .
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-ZrO 2 addition) at 900°C. The obtained mass gain of the Ti-48Al and the Ti-48Al-6Nb porous alloys were 35.024 g/m 2 and 10.231 g/m 2 , 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/m 2 . The isothermal oxidation results obtained at 900°C confirmed the beneficial effect of Nano-ZrO 2 introduction on the practical application potential of TiAlbased 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 Mohanta 41,42 .
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 applications 43,44 . 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 TiO 2 grains formed on the entire surfaces of the Ti-48Al and Ti-48Al-6Nb porous alloy (Fig. 8a and c), consistent with our previous work 45,46 . 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 Al 2 O 3 , TiO, Ti 2 O 3 , TiO 2 , NbO 2 , Nb 2 O 5 , Zr, ZrO, ZrO 2 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.
The high-temperature sulfidation behavior of the Ti-48Al, Ti-48Al-6Nb porous alloys, and PCMM with the addition of 4 wt.% Nano-ZrO 2 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 smallsized TiO 2 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 TiO 2 and Al 2 O 3 mixture, though predominantly of TiO 2 (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.
Filtration performance of PCMM Ti-48Al-6Nb porous alloy and PCMM with the addition of 4 wt.% Nano-ZrO 2 were tested (750°C, 4000 Pa), with the results shown in Fig. 9a. The removal efficiency was calculated using Eq. (1): where ξ1 and ξ2 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 mm 2 (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 PM 2.5-5 and PM 5-10 , 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 PM 1-2.5 , PM 0.5-1 , PM 0.3-0.5 , PM <0.3 , respectively, as shown in Fig. 9e. 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-ZrO 2 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 studies [38][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.
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 PM 2.5 by conventional porous materials, is much more challenging because of the ultra-low mass and small particle size of PM 2.5 49 . 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-ZrO 2 and Ti 3 Al/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.

Materials
Ti, Al, and Nb powders (99.9%) with an average particle size of <50 μm and ZrO 2 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-ZrO 2 (average particle sizes of <50 nm) by ball milling at 130 rpm in a ball crusher for 10 h (ball-to-powder weight ratio   900°C for 10 h. All the samples were tested at 900°C for high-temperature sulfidation with continuous ventilation of SO 2 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 SO 2 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 incense 50 , 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 mm 2 (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-ZrO 2 addition were tested to ensure filtration measurement accuracy. Fig. 11 Schematic illustration of PM removal mechanism for as-prepared porous sample. a Ti-48Al-6Nb porous alloy; b PCMM.