Facile fabrication of cordierite-based porous ceramics with magnetic properties

In this paper, cordierite-based porous ceramics with magnetic properties have been firstly in-situ synthesized by using MgO, Al2O3, and SiO2 powders as raw materials and Fe3O4 as a functional additive. Combining with the foam freeze casting method, near net size fabrication (total linear shrinkage < 2.86%) of the magnetic porous materials was realized by adjusting the amount of Fe3O4. The porosity, compressive strength, and saturation magnetization of the prepared materials were 83.9%–87.8%, 1.51–2.65 MPa, and 1.2–5.8 emu/g, respectively. The phase composition and microstructure evolutions during sintering were investigated briefly. The results showed that the synthesis temperature of cordierite was lowered about 100 °C due to the addition of Fe3O4. Except for the main phase-cordierite, Mg—Al—Fe spinel and α-Fe2O3 also existed in the final materials. The lattice parameters of the Mg—Al—Fe spinel and the amount of α-Fe2O3 changed obviously with the change in the sintering temperature and Fe3O4 amount, which mainly influenced the magnetic properties of the prepared materials. Thus, a facile fabrication method of the cordierite-based porous ceramics with the magnetic properties has been put forward in this paper.


Introduction 
Due to their excellent properties, porous cordierite ceramics have been widely applied in many fields, some results show that it is hard to obtain high-purity cordierite ceramics at low temperature through the solid-state reaction, and the most impurity phases usually have a negative performance impact [10][11][12][13].
To lower the synthesis or sintering temperature, additives have been widely used and proved to be effective [14][15][16][17][18]. For example, Li et al. [14] reported that the excess MgO increased the production of liquid and densification in the solid-state method. Guo et al. [15] studied the influence of La 2 O 3 on the preparation and performance of the porous cordierite from rice husk, and found that the cordierite formation temperature decreased greatly with the addition of La 2 O 3 . Senthil Kumar et al. [16] found that the addition of CeO 2 benefited the synthesis of cordierite and the mechanical properties of the prepared materials. Moreover, Li et al. [18] prepared cordierite ceramics using kaolin, silicon dioxide, and magnesium oxide as raw materials, and studied the effects of Fe 2 O 3 on the sintering behavior. They found that Fe 2 O 3 promoted the sintering process and the content of the cordierite phase also increased. However, all these sintering additives could not give the material functionality, such as magnetic properties, electric or dielectric properties. The magnetic porous materials show broad prospects applied in supported catalysts [19], microwave absorbing [20], oil adsorption capacity [21], water treatment [22], biotechnology [23], and other fields [24,25] because of their large specific surface area and stable magnetic properties. For example, porous Ni x Mg y Zn 1−x−y Fe 2 O 4 ferrites could be used in the area of magnetic carriers for the immobilized enzyme due to the strong adsorption properties, porous structure, and magnetic properties [25]. Introducing magnetism to the porous cordierite ceramics may improve their filtering or adsorption capacity to some certain extent due to the magnetic effect. Therefore, it is significant to find an additive that can not only reduce the synthesis temperature of cordierite, but also give the material magnetism.
To reduce the synthesis temperature of cordierite and give the material magnetism at the same time simply by introducing appropriate additives, Fe 3 O 4 has come into our sight. Firstly, Fe 3 O 4 has the same crystal structure as the intermediate phase spinel (MgAl 2 O 4 ) when MgO, Al 2 O 3 , and SiO 2 were using as raw materials [10]. Thus, it may react with MgO and Al 2 O 3 to form a Mg-Al-Fe-O solid solution with a spinel structure [26], which may be conducive to the synthesis of cordierite. Secondly, Fe 3 O 4 is a typical ferrite with an inverse spinel crystal structure, which was usually used as a magnetic component in the magnetic porous materials [27][28][29][30]. Finally, the formed Mg-Al-Fe-O solid solution is one kind of ferrite which also can give the material magnetism.
In this work, a series of cordierite-based porous ceramics were in-situ synthesized by the foam gel-casting/foam freeze casting method using magnesia, alumina, and silica as raw materials and Fe 3 O 4 as the functional additive. The effects of sintering temperature and Fe 3 O 4 amount on the phase composition, microstructure, and properties have been researched. The purpose of this work is to investigate the effects of Fe 3 O 4 on the phase composition evolution and magnetic properties of the cordieritebased porous ceramics, and finally to provide a strategy to endow and regulate the magnetic properties of the cordierite-based porous ceramics by introducing Fe 3 O 4 as the functional additive.

1 Materials
High-purity ceramic powders of MgO (0.05 μm), Al 2 O 3 (0.3 μm), and SiO 2 (10 μm) were used to synthesize the cordierite, and the Fe 3 O 4 (0.5 μm) powders were used as the functional additive. The details of the raw materials and other reagents used in the experiment are shown in Table 1.

2 Preparation
To study the effects of the sintering temperature, the porous samples were fabricated by the foam gelcasting method. Firstly, an initial solution containing AM, MBAM, ammonium polyacrylate, and deionized water was prepared. Then, the initial solution and the ceramic powders were ball-milled for 15 h to obtain a uniform slurry. The volume ratio of the ceramic powders to the deionized water was 1 : 3 in the slurry. After the addition of SDS and lauryl alcohol, the slurry was stirred (15 min) by a mechanical agitator rapidly. Then, TEMED and APS were added into the foamed slurry and stirred for another 5 min. Next, the obtained foamed slurry was poured into disposable paper molds with a diameter of 36 mm and gelled in 30 min to get a green body in the air. Lastly, the samples were dried by microwave equipment (BDMW-M-M-80, Henan Boda the mass ratio of the deionized water, AM, and MBAM was 100 : 10 : 1; the mass of APS was 15 wt% of AM; the mass of TEMED was 5 wt% of AM; the amount of ammonium polyacrylate was 6 wt% of ceramic powders; and the concentration of SDS was 2 g/L. In order to adjust the magnetic properties on a large scale, the amount of Fe 3 O 4 was designed from 8 to 25 wt%. Besides, the foam freeze-casting method [32] was selected to achieve near net size preparation of cordierite-based magnetic porous ceramics. The volume ratio of the ceramic powders to the deionized water was 1 : 4 in the slurry, the dosage of gelatin was 0.05 g/mL, and the concentration of SDS was 2 g/L. All the samples were sintered at 1340 ℃.

3 Characterization
The dried green body and sintered specimens were ground to powders with a mortar. Then, the phase composition was investigated by X-ray diffraction (XRD, D8 ADVANCE A25, Bruker, Germany) with Cu Kα (λ = 1.5418 Å) as the radiation, and the scanning speed was 6 (°)/min. Standard chromium powders were added to the sample powders in order to correct the instrument error, and then the XRD data were refined by Jade software. After that, the lattice parameters of the cordierite and Mg-Al-Fe spinel were calculated by Bragg equation (n·λ = 2d·sinθ). The microstructure, morphology, and elemental distribution of typical samples were examined by a scanning electron microscope (EVO 18, Carl Zeiss, Germany) equipped an energy dispersive spectroscope (EDS) operating at 15 kV. The heat and weight changes of the dried green body were evaluated by using a differential scanning calorimeter (DSC, 409 PC, Netzsch, Germany) in the air at a heating rate of 10 /min up to 1400 . ℃ ℃ The prepared materials were ground into powders, and the magnetic properties of the related powders were measured by a vibration sample magnetometer (VSM, 8604, Lakeshore, USA) at room temperature. Open porosity and bulk density were tested by Archimedes method in distilled water. The compressive strength (the size of samples was ϕ20 mm × 20 mm) was tested by a universal testing machine (WDW-100E, Jinan Times Assay Test Instrument Co. Ltd., China), and the loading rate was 0.5 mm/min. The drying linear shrinkage, sintering linear shrinkage, and total linear shrinkage were calculated by the inner diameter of the mold, dried green body, and sintered samples. A permeability tester (DSY Porous Ceramic Permeability Tester, Xiangtan Instrument and Instrument Co., Ltd., China) was used to measure the permeability of the distilled water, and the sample size was ϕ20 mm × 10 mm. All these data were obtained from at least three samples.

Thermal analysis
The heat and weight changes of the dried green body during the sintering are shown in Fig. 1. From the differential thermal analysis (DTA) curve, two endothermic peaks (at 105 and 930 ) and three exothermic peaks ℃ (at 380, 1270, and 1310 ) were observed. Compared ℃ with the DTA curve without Fe 3 O 4 as a sintering additive [10], the exothermic peak at 1369 relating ℃ to the synthesis of cordierite decreased obviously after the addition of Fe 3 O 4 , which indicated that the synthesis temperature of cordierite was lowered.
From the thermogravimetry (TG) curve in Fig. 1, four stages of the weight change were observed, which were different from the results without the addition of Fe 3 O 4 in Ref. [10]. In that study, only three weight changes occurred below 1000 [10]. This result ℃ indicated that the addition of Fe 3 O 4 also influenced the weight change during the sintering. Combined with the TG-DTA curves in Fig. 1 and our previous study [10,33], the initial weight loss (about 5 wt%) occurred from 50 to 200 , which was caused by the removal ℃ of residual water and intramolecular dehydration of organics, corresponding to the above endothermic peak at 105 ; the second part of the weight loss (about 1 ℃ 4.7 wt%) occurred between 200 and 600 was mainly ℃ due to the oxidative decomposition of organic residues, corresponding to the above exothermic peak at 380 ; ℃ and the third part of the weight loss (about 1.1 wt%) occurred at 800-1000 , which was due to the ℃ elimination of the sulfur coming from APS and SDS. Above 1000 , the fourth part of the weight loss occurred, ℃ which might be related to the addition of Fe 3 O 4 .

Phase composition of samples sintered at different temperatures
To study the evolution of the phase composition during sintering, the dried green body was sintered at different temperatures. The typical images and XRD patterns of the dried green body and samples sintered at different temperatures are shown in Fig. 2. As shown in Fig. 2(a) (the diameter of the cylinders is about 30 mm), the sample color changed significantly from black to yellow, pale brown, brown, dark brown, and dark gray with increasing the sintering temperature. The color change of the samples sintered at different temperatures indicated that the phase composition changed obviously during the sintering, which was proved by the XRD patterns in Fig. 2(b). From Fig. 2(b), the dried green body was composed of periclase, corundum, quartz, and magnetite which was basically consistent with the raw materials. Besides, the sintering temperature affected the phase composition greatly before 1320 . The major phases ℃ of the samples sintered at 1240 were ℃ quartz, cristobalite, Mg-Al-Fe spinel (the crystal structure was consistent with that in JCPDS 01-089-8734), and sapphirine (JCPDS 00-044-1430), which was different from that of the dried green body. Especially, dark Fe 3 O 4 changed into the Mg-Al-Fe spinel, leading the sample color change to yellow. When the sintering www.springer.com/journal/40145 temperature increased to 1260 , cristobalite became ℃ the main phase and the content of quartz decreased significantly. Meanwhile, the diffraction peaks of cordierite appeared. This phenomenon indicated that the cordierite was synthesized at 1260 , which was ℃ much lower than that of the samples without Fe 3 O 4 as the additive [10]. This result showed that the addition of Fe 3 O 4 promoted the synthesis of cordierite effectively. According to the study of Wu et al. [34], Fe 3+ can substitute Mg 2+ or Al 3+ to form the solid solution in the MgO-Al 2 O 3 -SiO 2 system, which is beneficial for the synthesis of cordierite. In this study, the formation of the Mg-Al-Fe spinel had the same role. As sintering temperature increased further, the content of cordierite increased, and it became the first main phase in the samples sintered at 1300 . Above ℃ 1300 , the diffraction peak intensity of cordierite ℃ changed a little, showing that the cordierite had been mostly synthesized at this temperature. The synthesis temperature of cordierite was about 100 lower than ℃ that of the samples without Fe 3 O 4 [10]. As sintering temperature increased to 1320 , α ℃ -Fe 2 O 3 with a rhombohedral crystal system (JCPDS 01-079-1741) appeared, and the diffraction peaks of the Mg-Al-Fe spinel shifted to the left. Over 1320 , the diffracti ℃ on peaks of the Mg-Al-Fe spinel and iron oxide were almost unchanged, while the diffraction peaks of cristobalite decreased gradually and finally disappeared at 1360 . ℃ According to the study on cordierite glass ceramics, irons could enter the cordierite lattice to form iron cordierite [31]. However, herein the diffraction peaks of the iron cordierite were not detected; the diffraction peaks of the cordierite sintered at different temperatures did not shift obviously and the lattice parameters of cordierite calculated by the Bragg equation were almost the same as those (a = 16.9750 Å, b = 9.7300 Å, c = 9.3560 Å) of cordierite in Ref. [35]. In contrast, the positions of diffraction peaks of Mg-Al-Fe spinel varied obviously with the increase of sintering temperature. When sintering temperature increased from 1280 to 1320 , the diffraction peaks of the ℃ Mg-Al-Fe spinel shifted to left gradually, which indicated the lattice parameters of the Mg-Al-Fe spinel increased. The average ionic radius of Fe ions was bigger than those of Mg 2+ and Al 3+ . The large lattice parameter indicated that the proportion of Fe ion in the Mg-Al-Fe spinel was heightened at a high temperature. The high proportion of Fe ion in the Mg-Al-Fe spinel ferrite is favorable for its magnetism.

Typical microstructure of samples sintered at different temperatures
Typical SEM images in Fig. 3 show the pore structure, skeletal microstructure of dried green body, and samples sintered at different temperatures. As shown in Fig. 3(a), micron spherical pores were obtained by this method, and the pore size was about 200 μm. After being sintered, many small window pores appeared on the large spherical pore wall. In addition, the number and size of the window pores expanded obviously as sintering temperature increased, which may be due to the uneven sintering shrinkage and the densification of the ceramic skeleton. It should be noted that the size of the large spherical pore is almost unchanged before and after the sintering. The SEM images at high magnification showed that the Before the sintering, ceramic particles were connected by organics (polyacrylamide). As sintering temperature increased to 1240 , sintering necks formed between ℃ the ceramic particles, and the skeleton became more densified. In addition, the liquid phase appeared at 1300 and its amount increased as the sintering ℃ temperature raised. The increase in the liquid phase was correlated to the change of cristobalite.
To determine the morphology of different phases, the element distribution of the typical samples was investigated by the EDS analysis, as shown in Fig. 4. According to the elemental mappings in Fig. 4(a), Si enrich particles were observed inside yellow circles. The size and shape of the Si enrich particles were consistent with the raw material of SiO 2 . In addition, the distribution of Mg, Al, and Fe was basically the same, indicating the existence of the Mg-Al-Fe solid solution. From Fig. 2(b), cordierite also appeared at 1260 , but the cordierite grains could not be ℃ distinguished from Fig. 4(a). This was mainly because the diffraction peak intensity of cordierite at 1260 in ℃ Fig. 2(b) was weak. As shown in Fig. 4(b), iron element is enriched in the small particles circled by the red line, and the distribution of Mg, Al, and Si was uniform in the matrix phase. Combining with Fig. 2(b), the small particles were the Mg-Al-Fe spinel or Fe 2 O 3 , and the matrix was mainly cordierite. Figure  4(b) also shows that the magnetic particles of the Mg-Al-Si spinel and Fe 2 O 3 bonded tightly with the cordierite matrix.

Analysis of reaction mechanism
To explore the influence of Fe 3 O 4 on the in-situ synthesis of cordierite, the green body was calcined at 700, 800, 850, 900, and 930 . The phase compositions of the ℃ samples were analyzed by the XRD and the results are shown in Fig. 5 Fig. 4(a) were basically the same, which proved that these particles were the Mg-Al-Fe spinel. After the appearance of cristobalite at 1240 , the cordierite ℃ appeared at 1260 . The possible reaction is presented ℃  2+ and Al 3+ from the Mg-Al-Fe spinel reacted with the cristobalite to form the cordierite. Thus, a large amount of cordierite was generated, and the diffraction intensity of cordierite became the highest at 1300 . ℃ Because part of Mg 2+ and Al 3+ from the Mg-Al-Fe spinel reacted to form the cordierite, the proportion of Fe ions in the Mg-Al-Fe spinel increased. The average ionic radius of Fe ions was bigger than those of Mg 2+ and Al 3+ , so the diffraction peaks of the Mg-Al-Fe spinel shifted to the left. At the same time, the sapphirine reacted with the cristobalite to form the cordierite [37]. The possible reaction is presented in Eqs. (8) and (9) (9) As sintering temperature increased to 1320 , ℃ the diffraction intensity of the Mg-Al-Fe spinel dropped slightly, and the diffraction peak of α-Fe 2 O 3 appeared again (Fig. 2). From Fig. 4(b), the distribution position of Mg, Al, and Si tended to be uniform at 1320 , ℃ which further proved that these particles are cordierite; and the proportion of Mg and Al was quite low in the Mg-Al-Fe spinel. The possible reaction is shown in Eq. (10) ℃ In addition, it can be seen from the microstructure at low magnification in Fig. 3(f) that the small pores on the spherical pore wall continued to grow with the increase of temperature. The growth of the small pores on the pore wall may be mainly related to the appearance of the liquid phase in the later stage of the sintering. Besides, the microstructure at high magnification also showed the existence of the liquid phases at 1360 . ℃

Porosity and compressive strength
The sintering linear shrinkage, open porosity, and compressive strength of the samples sintered at different temperatures are shown in Fig. 6. The large sintering shrinkage (over 14%) at 1360 caused ℃ cracking and failure of the samples. Thus, the properties of those samples were not tested.
As sintering temperature increased from 1240 to 1320 , the sintering shrinkage firstly remained ℃ unchanged, then decreased, and finally increased. However, the variation range of the sintering shrinkage was narrow, only changing from 2.15% to 3.29%. In addition, the sintering shrinkage of these samples was generally small, because the volume expansion derived from the phase evolution could partly compensate for the sintering shrinkage [38]. The cordierite has a relatively lower theoretical density than the main raw materials, which can lead to volume expansion during the sintering. Combined with Fig. 2(b), a large amount of cordierite was synthesized at 1300 . Thus, the ℃ sintering shrinkage did not increase but decreased. The increase of the sintering shrinkage at 1340 was ℃ mainly due to the appearance of silicon-rich liquid phase.
As shown in Fig. 6, the open porosity was 83.9%-88.3%, and the compressive strength was 1.46-2.65 MPa. Meanwhile, the trend of compressive strength was opposite to the change of open porosity. Compared Sintering shrinkage, open porosity, and compressive strength of samples sintered at different temperatures.
with the materials prepared by other methods, the results (Fig. 6) showed that the materials prepared here presented both higher porosity and compressive strength. The porous cordierite prepared by the in-situ solid-state reactions [39] showed a porosity of 33.16% and a compressive strength of 72.64 MPa. The porous cordierite with the porosity of 86.9%-89.5% and the compressive strength of 0.22-0.44 MPa was obtained by thermo-foaming [40]. Herein, the high porosity of the samples was due to the high foam volume during the preparation and the small shrinkage during the sintering. The high strength was caused by the high equability of the spherical pores and the small sintering deformation [41]. The compressive strength of the samples with an open porosity of ~83% in this paper was slightly smaller than that of the samples without additive [10], which was mainly because there were a large number of window pores on the spherical pore wall (Fig. 3).

Magnetic properties
The samples sintered at or above 1300 could be ℃ easily attracted by magnets and dragged off the ground, which are shown in Figs. 7(a) and 7(b). Considering the magnetic properties and phase composition, the hysteresis loops of the two typical samples sintered at 1300 and 1340 were measured by vibrating the ℃ sample magnetometer, as shown in Fig. 7(c). When the sintering temperatures were 1300 and 1340 , the ℃ values of M s were calculated to be 3.7 and 4.8 emu/g, the values of M r were 0.2 and 0.3 emu/g, and the H c were 20 and 30 Oe, respectively. The results indicated that the composite ceramics were easy to be magnetized by an external magnetic field and easy to demagnetize due to their low coercive force. The hysteresis loops showed that the samples were soft magnetic materials with low magnetizing power and hysteresis loss. The variation in the M s value of the samples sintered at 1300 or 1340 was mainly due to the phase evolution. ℃ Firstly, the diffraction peaks of the Mg-Al-Fe spinel became sharper at 1340 in Fig.  ℃ 2(b), suggesting the improvement of crystallinity. Secondly, the lattice constant of the Mg-Al-Fe spinel rose from 8.29 to 8.33 Å when the temperature increased from 1300 to 1340 , indicating that the proportion of iron ions in ℃ the Mg-Al-Fe spinel increased at 1340 . Finally, ℃ α-Fe 2 O 3 occurred in the samples sintered at 1340 , ℃ which also had magnetism [42].

Phase composition
The XRD patterns of the samples with different

Microstructure
The SEM images of the cordierite-based porous ceramics with different Fe 3 O 4 amounts are shown in Fig. 9. From the microstructure at low magnification, the pore size of the spherical pores in different samples changed slightly. However, the number of window pores on the spherical pore wall increased with the rise of Fe 3 O 4 amount. As seen from the microstructure at high magnification in Fig. 9, the grains grew obviously with the increase of Fe 3 O 4 content. From Fig. 9(j), microcracks appeared in the samples with 25 wt% Fe 3 O 4 , which would affect the strength of the materials.

Properties
The linear shrinkage of the samples with different Fe 3 O 4 amounts is shown in Fig. 10. With the increase of Fe 3 O 4 amount, the drying shrinkage changed slightly only fluctuating between 1.20% and 1.45%, but the sintering shrinkage increased obviously from −0.94% to 6.99%. The results indicated that the amount of Fe 3 O 4 had little effect on the drying shrinkage, but influenced the sintering linear shrinkage prominently. It was found that the samples expanded during the sintering when the amount of Fe 3 O 4 was 8 wt%. In addition, when the amount of Fe 3 O 4 rose from 8 to 16 wt%, the total shrinkage was between 0.50% and 2.86%, lower than 3% [43], which indicated that the near net shape fabrication of the cordierite-based porous ceramics with magnetism was realized.  The open porosity, bulk density, compressive strength, and permeability coefficient of the samples with different Fe 3 O 4 amounts were tested, and the results are shown in Fig. 11. With the increase of Fe 3 O 4 amount, the open porosity of the samples decreased from 87.8% to 86.0%, and the bulk density increased from 0.31 to 0.40 g/cm 3 . The variation of porosity is mainly derived from the change of sintering shrinkage. As shown in Fig. 11, when the Fe 3 O 4 amount increased from 8 to 25 wt%, the compressive strength increased first from 1.51 to 1.92 MPa, and then decreased to 1.36 MPa. As shown in Fig. 9(j), the microcracks generally existed in the samples when the Fe 3 O 4 content was 25 wt%, resulting in the lowest compressive strength (1.36 MPa) of the samples. Even so, its compressive strength was higher than that (1.22 MPa) of the samples with the same porosity [10]. As the Fe 3 O 4 amount increased from 8 to 25 wt%, the permeability coefficient changed from 2.2×10 −11 to 7.4×10 −11 m 2 roughly, showing an increasing trend. The changing trend of permeability was opposite to that of porosity. Combined with the pore structure in Fig. 9, both the number and the size of the window pores became larger, which help fluid pass through the pores. Thus, the permeability enlarged obviously under the condition of little change in the porosity. Moreover, the permeability of the materials with 25 wt% Fe 3 O 4 is twice that (3.65×10 −11 m 2 ) of the samples without Fe 3 O 4 as the additive [44].
The hysteresis loops of the samples with different Fe 3 O 4 contents are presented in Fig. 12, and the values of M s , M r , and H c were calculated and summarized in Table 2. As the Fe 3 O 4 content increased from 8 to 25 wt%, the M s increased from 1.2 to 5.8 emu/g, the H c changed from 80 to 145 Oe, and the M r was 0.3-1.4 emu/g. The change of M s was mainly caused by the Fig. 11 Open porosity, bulk density, compressive strength, and permeability of samples with different Fe 3 O 4 amounts.   The increase of iron proportion in the Mg-Al-Fe spinel will also increase the M s of the materials. Furthermore, the H c and M r of the samples were low, which indicated that the prepared porous ceramic was soft magnetic material. On the whole, the addition of Fe 3 O 4 not only promoted the synthesis of cordierite, but also lowered the sintering temperature of the prepared materials. Especially, it endowed magnetic properties to the prepared cordierite-based porous ceramics. Thus, through this method, the cordierite-based porous ceramics with the magnetic properties can be easily prepared, and the magnetic properties can be manipulated by adjusting the sintering temperature and Fe 3 O 4 amount. Meanwhile, the prepared samples possess high porosity, high strength, and high permeability.

Conclusions
In this paper, cordierite-based porous ceramics with magnetic properties, high porosity, and high strength have been easily prepared by combining the in-situ synthesis with the foam gel-casting method, wherein MgO, Al 2 O 3 , and SiO 2 are used as raw materials and Fe 3 O 4 as the functional additive.
As the sintering temperature increased from 1240 to 1340 , the main phases changed vastly, and the final ℃ crystalline phases were the cordierite, Mg-Al-Fe spinel, and α-Fe 2 O 3 . The open porosity of the samples sintered at different temperatures was between 83.9% and 88.3%, and the compressive strength was 1.46-2.68 MPa. When the sintering temperature was 1300 and 1340 , the values o ℃ f M s were 3.7 and 4.8 emu/g, respectively. The Mg-Al-Fe spinel and iron oxide were the main origin of magnetism.
The magnetic properties of the cordierite-based porous ceramics could also be manipulated by adjusting the Fe 3 O 4 amount. When the weight ratio of Fe 3 O 4 was 8-25 wt%, the M s was 1.2-5.8 emu/g, the open porosity was 86.0%-87.8%, the bulk density was 0.31-0.40 g/cm 3 , the compressive strength was 1.36-1.92 MPa, and the permeability coefficient was 2.2×10 −11 -7.4×10 −11 m 2 . When the weight ratio of Fe 3 O 4 was 8-16 wt%, the near net shape fabrication of the cordierite-based magnetic porous ceramics was realized with a total sintering shrinkage of 0.50%-2.86%.
This work provides a strategy to endow and regulate the magnetic properties of the cordierite-based porous ceramics through the incorporation of Fe 3 O 4 as the functional additive.