Preparation of ZrO2 fiber modified Al2O3 membrane supports with enhanced strength and permeability

Accepted Manuscript Title: Preparation of ZrO2 fiber modified Al2 O3 membrane supports with enhanced strength and permeability Authors: Weiya Zhu, Yang Liu, Kang Guan, Cheng Peng, Jianqing Wu PII: DOI: Reference:

S0955-2219(18)30727-1 https://doi.org/10.1016/j.jeurceramsoc.2018.12.009 JECS 12214

To appear in:

Journal of the European Ceramic Society

Received date: Revised date: Accepted date:

14 October 2018 27 November 2018 1 December 2018

Please cite this article as: Zhu W, Liu Y, Guan K, Peng C, Wu J, Preparation of ZrO2 fiber modified Al2 O3 membrane supports with enhanced strength and permeability, Journal of the European Ceramic Society (2018), https://doi.org/10.1016/j.jeurceramsoc.2018.12.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation of ZrO2 fiber modified Al2O3 membrane supports with enhanced strength and permeability Weiya Zhu, Yang Liu, Kang Guan, Cheng Peng, Jianqing Wu*

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Guangzhou 510640, People’s Republic of China

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School of Materials Science and Engineering, South China University of Technology,

Abstract:

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In this paper, Al2O3 membrane support with both enhanced permeability and

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strength was fabricated by introducing zirconia fiber (ZrO2(f)). Effects of the ZrO2(f)

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content and sintering temperature on the open porosity, shrinkage rate, microstructure, water permeance, bending strength and pore size of the support were investigated.

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Results reveal that apart from fiber reinforcement, ZrO2(f) was found to promote open

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porosity and permeability by changing pore morphology and reducing flow resistance. However, it damages the porous structure if the sintering temperature is excessive.

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Compared with the un-added sample, the bending strength and water permeance of the support with a ZrO2(f) addition of 4 wt% sintered at 1550 °C increased by 261%

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and 52%, respectively. The modified support meets the requirement for more demanding operation conditions and may achieve the objects of miniaturizing and lightening the final products.

Keyword: ZrO2 fiber; Al2O3; Ceramic supports; Strength; Permeability

1. Introduction Al2O3 porous ceramics is equipped with a unique set of characteristics: high

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strength, chemical corrosion resistance as well as temperature extremes [1, 2] and,

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therefore, it is applicable in a wide range of applications ,such as waste solution treatment [3], oil concentration [4] and gas separation process [5], etc. Ceramic

separation membrane support is regarded as one of the important applications of

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porous Al2O3 ceramics, which requires sufficient strength to provide mechanical

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support for the thin separation layer and high porosity to ensure excellent permeability

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and enough capillary force [6, 7]. It was widely acknowledged that membrane system with high surface area-to-volume ratio is ideal in promoting the separation efficiency,

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such as hollow fiber membranes, where elements need to be prepared in a smaller size

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in thickness, resulting in more rigorous demands for the supports strength [8]. Traditionally, the strength of support, formed by granular packing, is determined by

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the number of contact points and the adhesion between particles [9, 10]. In the process of sintering, the support is strengthened by the reduction of intergranular distance and

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growing sintering neck, thereby having fewer pores inside [11, 12]. In other words, the gradual increase in strength seems to inevitably cause the decrease of porosity and pore size, thus leading to low permeability [13]. Moreover, ceramic fibers (or whiskers) are widely employed as a reinforcement agent [14-16] due to the various toughening mechanisms, such as debonding, pull-out,

and crack-bridging [17]. Furthermore, they are used not only in dense materials but also in porous materials [18, 19]. It was also found that the interlocked fiber structure has the potential to achieve extremely high porosity with a well-connected pore structure [20, 21]. Zhu et al. [22] fabricated a support at 1200 °C exhibiting a high

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mechanical strength of 81.2 MPa with in-situ synthesized mullite whiskers, in which both the porosity and permeability were also significantly improved. Han et al. [23]

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used SiC whiskers as reinforcing agent and the experiments showed that the bending strength reached 28 MPa with 3.3 wt % SiC whiskers addition, nearly 3 times over

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that of blank samples, and the gas permeability was also improved. Therefore, it can

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be inferred that fibers may also perform well in Al2O3 supports, which has positive

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effects on both mechanical strength and permeability. Additionally, a proper length of

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fibers could maintain excellent performance of supports even in thinner conditions

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instead of limiting the support thickness.

Zirconia-toughened alumina (ZTA) ceramic is regarded as one of the well-known

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structural materials [24, 25]. For the purpose of pursuing higher strength and density,

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zirconia is usually added in the form of nanoparticles and it is expected to appear as submicron tetragonal zirconia grains which are capable of increasing the crack propagation path and fracture energy. Compared to pure Al2O3, the integration of 5-30

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wt% ZrO2 in Al2O3 leads to considerably enhanced fracture toughness and flexural strength [26]. In addition, it has also been reported that the zirconia fiber possesses great reinforcement effects. Hua et al [27] proposed that the mechanical properties of NiFe2O4 ceramic matrix was effectively enhanced by adding 3 wt% ZrO2(f), where the

bending strength reached 88.92 MPa, and the weak interface bonding between ZrO2(f) and NiFe2O4 played an important role in fiber enhancement. However, it is known that there is a lack of research in reinforcing Al2O3 membrane supports with ZrO2(f). Therefore, it is considered to be employed to improve the properties of ceramics and

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obtain supports with both high mechanical strength and good permeability. In this study, Al2O3 membrane supports were prepared with ZrO2(f). The effects of the

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amount of ZrO2(f) on the shrinkage rate, porosity, bending strength, mean pore size and permeability of the supports were investigated respectively. Moreover,

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characterizations were also made to understand the mechanism according to which

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ZrO2(f) acts.

2. Experimental procedure 2.1. Chemicals High-purity alumina powder (α-Al2O3, 99.5%, D50=10 μm), which was commercially available (AS-250, Showa Denko K.K., Japan), was selected as matrix

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material. Zirconia fiber (diameter 3~20 μm, length 30~200 μm; Shandong Zhongbote

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Advanced Ceramic, China) was employed as the reinforcement, and its microstructure

morphology is shown in Fig. 1. TiO2 (D50=0.3 μm, Tianjin Fuchen chemical, China) was selected as the sintering aid to lower the sintering temperature, and it proved that

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the material performed well in promoting sintering in Al2O3 ceramic system [28, 29].

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Polyvinyl alcohol (PVA-AH26, Sinopharm Chemical Reagent, China) was utilized as an

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organic binder. Corn starch (Biochemical Reagent, Shanghai Yuanju Biological Technology, China) was selected as a pore forming agent. Deionized water (18 MΩ)

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was used in the whole process during preparation.

Fig. 1. Morphology of the ZrO2(f). 2.2. Preparation of the supports

100 g and 0.8 g of Al2O3 and TiO2 powders were mixed into a homogenized suspension with 12 g of polyvinyl alcohol solution (10 wt%) and 58 g of deionized water, and then they were mixed in a ball mill for 1 h. Then ZrO2(f) was added into the slurry and rigorously stirred for another 30 min. In addition, all the samples were named

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from Z0 to Z6 according to the fiber content (Z=zirconia, e.g. Z2: formulation with 2 g ZrO2(f) , Z0 is the control sample). The mixtures were dehydrated in a plaster mold and

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further dried at 60 ℃ in a drying oven till the water content of 3-6 wt%. Afterwards, the

mixtures with the particles between 40 mesh and 80 mesh sieves were obtained by

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crushing and sieving. Finally, the mixture powder were pressed into green disk bodies

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with the sizes of 27 mm in diameter and 3 mm in thickness under 35 MPa. The green

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disk bodies were sintered in air from room temperature to the highest temperature at a

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rate of 10 °C/min and then soaked for 2 h. After that, the samples were cooled to room

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temperature naturally in the furnace. 2.3 Measurement methods

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The diameter of support was measured and the shrinkage rate was calculated as

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(L1-L2) /L1 where L1 and L2 refer to the length before and after firing, respectively. The open porosity of the sample was measured by employing the Archimedes method. Moreover, the measurements of three-point bending strength were performed on the

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3mm×4 mm×25mm rectangular bars using a mechanical testing machine (Instron5567, Instron, USA) with a 20 mm span and a crosshead speed of 0.5 mm/min. The bending strength was taken from the average value of six individual samples. The morphology of support was observed by SEM (EVO 18, ZEISS, Germany). Mercury porosimetry

(Auto Pore IV 9500, Micromeritics Instrument, America) was adopted to determine the average pore size and pore size distribution of the Al2O3 supports. Permeance of the membrane was tested on a fully automated fluid and gas handling system OSMO Inspector 2.0 (Poseidon, Convergence, Netherlands), which was determined by

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collecting the permeation shown in a mass flow meter of the OSMO Inspector and timing the collection period. To avoid non-stationary transient effects, the membrane

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was saturated with deionized water (18 MΩ) before the pressure was applied. The

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effective filtration area of each sample was measured and calculated as 2.64×10-4 m2.

3. Results and discussions As shown in Fig. 2a, the open porosity of all samples (Z0 to Z6) decreased with the temperature varying from 1450 °C to 1600 °C, while the shrinkage rate showed the opposite trend (Fig. 2b). Obviously, an increase in the sintering temperature would enable the material

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structure to be denser, resulting in decreased pore volume and intergranular space, which can

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be observed in Fig.3f to 3i. Besides, the specific open porosity of Z6 sintered at 1500 °C and 1550 °C were 33.45% and 27.88% respectively, which is superior to that of Z0 (29.34% and

25.57%, respectively). These results suggest the ZrO2(f) is beneficial in improving the porosity.

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It is probably because pores distributed around the fibers greatly increases the probability of

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interconnecting due to the smooth surface and straightness of ZrO2(f), thus reducing the

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amount of dead-end pores which almost has no contribution to the open porosity. However, as the temperature rose higher to 1600 °C, Z4 and Z6 indicated a noticeable

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reducing trend, whose open porosity were 23.3% and 20.3%, respectively. The shrinkage rate

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also showed an obvious growing tendency with the increase of ZrO2(f) content at 1600 °C, the shrinkage rate of Z6 was 8.95%, about 45% higher than that of Z0. Dramatic changes of these

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two properties was likely due to the positive effect of ZrO2(f) on sintering. ZrO2 particles was employed to lower the sintering temperature of porous alumina [28, 29], and it exhibits higher

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sintering activity in the Al2O3-ZrO2-TiO2 system because the diffusion of Zr cations is significantly promoted by Ti cations via weakened ionic bonds [28, 31]. It can be observed in Fig. 3i that ZrO2(f) gradually lost its fibrous structure and partially moved to the grain boundary after sintering at 1600 °C. In this case, it no longer produces connected pores but leads to a denser part in the structure. Moreover, Zou et al. [32] added mullite fiber to reduce

the shrinkage rate of the support made of fly ash. The difference in the effects of shrinkage rate may be caused by the different fiber amount. In this system, the amount of ZrO2(f) is far from being able to form a connected network (as shown in Fig. 3a to 3e); therefore, the promotion of regional shrinkage can reflect to the whole shrinkage rate without being

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restricted by a rigid fiber net.

Fig. 2. Performance changes of supports with different ZrO2(f) content at different sintering

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temperatures: (a) open porosity; (b) shrinkage rate; (c) water permeance; (d) bending strength.

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Fig. 3 Morphology of Al2O3 supports added with ZrO2(f): (a) Z0, (b) Z1, (c) Z2, (d) Z4, (e) Z6;

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(the brighter one is ZrO2(f)).

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and fracture surface of Z4 sintered at: (f) 1450 °C, (g) 1500 °C, (h) 1550 °C and (i) 1600 °C

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The water permeance of Al2O3 supports added with ZrO2(f)was shown in the Fig. 2c. As the fiber content, increases, water permeance increases obviously accordingly, especially at a

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relatively low sintering temperature. When the sintering temperature was 1450 °C, the water

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permeance of Z6 and Z0 was 9.13 and 4.81 m3m-2h-1bar-1, respectively. This may, on the one hand, result from the increase in open porosity mentioned previously. On the other hand, it may also attribute to the smooth ZrO2(f) surface which can lower the flow resistance and the

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straight fiber morphology that reduces the tortuosity factor of pores. This means the addition of ZrO2(f) introduces fast flow paths into the structure. Moreover, as the ZrO2(f) content increased, fibers interlaced more frequently (Fig. 3d and 3e), so the connectivity of pores was further strengthened (Fig. 3g and 3h). A significant increase in permeability was therefore

observed when the fiber content increased from 2 wt% to 4 wt%. However, as the densification process underwent acceleration at 1600 °C, the water permeance of all samples indicated serious decline, and the flux of Z0 and Z6 finally decreased to 1.88 and 2.31 m3m-2h-1bar-1.

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Fig. 2d reveals the variation of bending strength. Owing to the excellent mechanical properties of ZrO2(f), support strength increased evidently even at 1450 °C. Fibers led to crack

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deflection and prevented crack propagation in this situation. It was discovered that the

sintering neck between ZrO2(f) and Al2O3 particles continued to be strengthen as the

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temperature increases. At 1550 °C, the bending strength of Z6 is nearly 3 times that of Z0.

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The mechanism of fiber reinforcement can be found in the micrograph, such as fiber bridging,

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breaking, and interface debonding. As the diffusion sintering became very strong at 1600 °C,

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Al2O3 grains grew intensely and ZrO2(f) formed tight bond with the matrix. Eventually, the

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bending strength of Z0 reached 101.8±8.4 MPa and Z6 increased to the maximum value of 221.9±19.3 MPa at the same temperature.

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Despite the obvious improvement in strength, the sintering temperature was not expected

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to be too high, which lied in that the densification process at 1600 °C causes serious damage to porosity and water permeance. More importantly, the performance of the ZrO2(f) itself is weakened due to the intense diffusion. The optimized sintering temperature hence was

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1550 °C in this system. Results also showed that when the fiber content increased from 4 wt% to 6 wt% (Z4 and Z6) at this temperature, the water permeance of the support did not increase significantly, and the porosity even has a reducing trend. Therefore, Z4 is more favorable in terms of giving full play to ZrO2(f). The bending strength and water permeance of Z4 sintered

at 1550 °C increased by 261% and 52%, respectively, compared with the un-added sample

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(Z0).

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Fig. 4. Pore size distribution of supports with different ZrO2(f) addition sintered at 1550 °C.

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For ceramic supports, the pore size is an important parameter, which has significant

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effects on both mechanical performance and permeability. Hence the effects of pore size

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distribution of supports with different fiber addition sintered at 1550 °C were also studied. Fig. 4 shows that as the fiber content increased, the trend of change is increased first and then

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decreased, and the pore size distribution varied slightly compared to other properties. The

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mean pore size of Z0 was 1.92 μm, while Z2 had the largest mean pore size, 2.19 μm, among all samples. This indicates that the ZrO2(f) has little influence on the pore size, and it mainly improve the performance of the support by changing the morphology of pores.

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Table 1 illustrates a comparison of performance between Al2O3 supports in this paper

(Z4) and those in literatures. As can be seen, the support prepared by us has a higher bending strength compared to that of others in the case of similar pore size or open porosity. Thus, it can be predicted that the Al2O3 support prepared with ZrO2(f) addition can be employed in

more severe conditions and achieve the objects of miniaturizing and lightening the final products. Table 1 Comparison of properties and performances of Al2O3 supports in this work and in literatures

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Water permeance Ref. 3 -2 -1 -1 (m m h bar ) 5.67 This work [6] 45 [33] [34] 17.9 [35] 3.7 [36] 0.7 [37]

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Bending Pore size strength (μm) (MPa) 2.21 127 8 55.4 6.8 32.7 5.1 35.5 4.6 61 2.1 68.7 1.5 27

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Open Additives porosity (%) ZrO2 fiber 27.4 TiO2 nanoparticles 38.2 TiO2 particles 41.4 Boehmite sol 53.7 Boehmite sol 31 Boehmite sol 47.8 Clay 49

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Sintering temperature (℃) 1550 1650 1400 1500 1550 1450

4. Conclusions By introducing the ZrO2(f), both mechanical strength and permeability of Al2O3 membrane supports were improved. Straight shape and smooth surfaces of ZrO2(f) were found to reduce flow resistance and increase the connectivity of the pores, thus enabling porosity

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and water permeance increase with the addition content. Moreover, obvious evidences of fiber reinforcing mechanism were observed on the fracture surface, indicating the effect of ZrO2(f)

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in improving support strength. ZrO2(f) also exhibits high activity when sintering temperature exceeded 1550 °C and promotes densification of the system, but it finally had a serious

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negative effect on open porosity. The performance of Al2O3 supports was clearly improved

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when a small quantity of ZrO2(f) (4 wt%) was incorporated. The prepared support exhibited an

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open porosity of 27.4%, a mean pore size of 2.21 μm, a bending strength of 127±6.3 MPa and

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a water permeance of 5.67 m3m-2h-1bar-1. The high-performance support may reduce the use

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high productivity.

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of raw materials and it can also be applied to harsh conditions, hence gaining low cost and

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Acknowledge

This work was financially supported by the National Natural Science Foundation of

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China (Grant No. 51472092 and Grant No. 51702100) and the Major Science and Technology Project of Foshan City, Guangdong, China (Grant No. 2016AG101315)

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