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Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase
Journal of Rare Earths xxx (xxxx) xxx
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Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase* Xin Xiong, Zhoufu Wang*, Xitang Wang, Hao Liu, Yan Ma The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 January 2019 Received in revised form 20 March 2019 Accepted 20 March 2019 Available online xxx
Corundum porous materials of particle-packing type with different contents of in situ formed LaAl11O18 were prepared using tabular corundum, reactive alumina and La2O3 powder as raw materials. The effects of the introduction of LaAl11O18 on the microstructure, phase composition and properties of the porous materials were investigated. The specimens were characterized by scanning electron microscopy, X-ray diffraction and mercury porosimetry. Results show that platelet-like LaAl11O18 is formed in situ by the reactions of La2O3 and Al2O3. With a certain amount of La2O3 added, the cold crushing strength, cold modulus of rupture and hot modulus of rupture (1400 0.5 h) of the specimens are increased, and the air permeability is improved simultaneously. However, upon further increasing the amount of La2O3 added, the mechanical properties and air permeability of the porous materials then decrease gradually owing to the increased numbers of pores and cracks in the bonding phase. The enhanced mechanical properties of the specimens with La2O3 added are attributed to the strengthening effects of plate-like LaAl11O18 in the bonding phase and to the activated sintering of both Al2O3 powder and corundum coarse aggregate for the diffusion of La3þ in Al2O3 lattice. In addition, the improved air permeability of the specimens should be related to the decreased content of pores in the bonding phase and the reduced number of interfacial cracks. © 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Keywords: Porous materials of particle-packing type Lanthanum hexaluminate Mechanical strength Strengthening mechanism Air permeability Rare earths
1. Introduction Porous purging materials have been widely used in the fields of iron and steel smelting, aluminum smelting, and copper smelting to improve the quality of molten metal.1e5 The porous materials are mostly in direct contact with the molten metal, and as a result this porous medium should be subject to the huge hydrostatic pressure and flow scouring of molten metal. Besides, the flushing action of high pressure gas in service may also be destructive. From the foregoing, high mechanical strength of the porous medium is required. Pores in a porous medium can be achieved by particle packing,6 the pore-forming-agent method,7,8 direct foaming,9 the freeze-drying process,10e12 and gel casting.13 Fig. 1 shows the fabrication process and structural schematic of the porous materials of particle-packing type. As can be seen from the figure, pores in this type of porous material are mainly interconnected with each * Foundation items: Project supported by the National Natural Science Foundation of China (51672195, 51474166) and the Key Program of Natural Science Foundation of Hubei Province, China (2017CFA004). * Corresponding author. E-mail address: [email protected] (Z. Wang).
other, which lead to excellent air permeability of the porous medium. Nevertheless, poor mechanical strength may also occur. Obviously, increasing the bonding strength of coarse aggregate particles should be an ideal method of improving the mechanical strength of porous materials. Lanthanum hexaluminate (LaAl11O18) has thin plate-like morphology, high melting point,14 great heat and chemical stability, and can be compatible with Al2O3.15 These features make LaAl11O18 an ideal second phase for improving the mechanical properties of alumina.16,17 The results of Chen15 show that the fracture toughness of alumina composites with 30 vol% of in situ formed LaAl11O18 increased by over 40% when compared with pure Al2O3 materials; the main toughening mechanism was the crack-bridging action of the plate-like particles. Negahdari et al.18 studied the mechanical properties of alumina/lanthanum hexaluminate composite ceramics with different contents of lanthanum hexaluminate and indicated that the fracture toughness, hardness, and elastic modulus of the composite materials increased with the addition of a certain amount of in situ formed lanthanum hexaluminate. Upon further increasing the lanthanum hexaluminate content, the hardness and elastic modulus of the composites were degraded. Moreover, the
https://doi.org/10.1016/j.jre.2019.03.022 1002-0721/© 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Xiong X et al., Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.03.022
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Fig. 1. Fabrication process and structural schematic of porous materials of particle-packing type.
influence of lanthanum hexaluminate on mechanical properties was described by the microstructure, porosity and intrinsic characteristics of lanthanum hexaluminate. Wu et al.19 prepared Al2O3/ LaAl11O18 composites with 25 vol% of LaAl11O18, and the sintering and fracture of the resulting composites were investigated. They found that the decreased grain size of Al2O3 grains and the mechanism of crack deflection caused by the rod-like LaAl11O18 grains leads to the improvement in fracture toughness of the composites. Yamashitaw et al.20 prepared a translucent Al2O3 1 vol% LaAl11O18 composite with high fracture toughness, and indicated that crack deflection and crack bridging in LaAl11O18 should be responsible for the enhanced bending strength and fracture toughness. According to the above results, it is clear that the addition of a moderate amount of LaAl11O18 to the compositions will improve the mechanical properties of alumina materials. However, until now, the effects of LaAl11O18 on the structure and properties of corundum porous materials have not been reported. In the work described in this paper, LaAl11O18 was introduced to the bonding phase of corundum porous materials by in situ reaction of Al2O3 and La2O3, and the effects of in situ formed LaAl11O18 on the mechanical properties and air permeability of the resulting porous materials were investigated, and then the reinforcing and toughening mechanisms were discussed.
prepared specimens were fired at 1750 C at a heating rate of 5 C/min for 3 h and then furnace-cooled. 2.2. Testing and characterization methods Apparent porosity (AP), bulk density (BD), cold crushing strength (CCS) and cold modulus of rupture (CMOR) of the specimens were measured according to ISO Standards 5017:2013 and 5014:1997. The hot modulus of rupture (HMOR) was measured according to ISO Standard 5013:1985, which was performed at 1400 C. The microstructure and composition of the specimens were characterized by using scanning electron microscopy (SEM, JSM-6610, JEOL, Tokyo, Japan) with energy-dispersive spectroscopy (EDS, QUANTAX200-30, Bruker Corp., USA). The phase composition of the specimens was investigated by X-ray diffraction (XRD, X'Pert Pro, Philips Corp., USA). The pore-size distribution was examined by mercury porosimetry (Quantachrome PM60GT-18, Quantachrome Instruments Ltd., USA). The air permeability was measured with a permeability tester (TQD-02, Sinosteel, China) at room temperature, and N2 was selected as the air source. 3. Results and discussion 3.1. Phase composition
2. Experimental 2.1. Specimen preparation Tabular alumina (0.5e1 mm, 0.045 mm, T60/64, Almatis, Tsingtao, China), reactive alumina powder (d50 ¼ 0.5 mm, CT 3000 SG, Almatis, Tsingtao, China), and La2O3 (99.99% purity, Zibo Weijie Rare Earth Co., Ltd., China) powder were used as raw materials. Aqueous solution of maltose (25 wt% concentration) was used as binding agent. The weight ratio of coarse aggregate, powder, and binder was 83:17:3.5. Different proportions of La2O3 were added into the compositions by equal replacement of the tabular alumina powder (0.045 mm); the detailed compositions are listed in Table 1. The powder mixtures were ball-mixed for 3 h at a speed of 200 r/min to ensure that the La2O3 powder was dispersed homogeneously in the compositions. The raw materials were mixed in an intensive mixer, and then bar-shaped (25 mm 25 mm 125 mm) and cylinder-shaped (F50 mm 50 mm) specimens were uniaxially pressed at 75 MPa. After drying at 110 C for 24 h, the as-
Fig. 2(a) shows the XRD patterns of the specimens after heat treatment at 1750 C for 3 h, and Fig. 2(b) shows the magnified local XRD peaks at approximately 35.2 in Fig. 2(a) that belong to the (110) peak of a-Al2O3. As shown in Fig. 2(a), the main crystalline phase of all the specimens is a-Al2O3 and the diffraction peaks of LaAl11O18 appeared in specimen L0.5. Upon further increasing the amount of La2O3 added, the diffraction intensity of the LaAl11O18 peaks increased simultaneously. More interestingly, as shown in Fig. 2(b), the diffraction peak of a-Al2O3 shifted to small angles for specimens L0.1, L0.5, L1 and L2, and the decreasing tendency of the diffraction angle was more obvious for the specimens with increasing amount of La2O3 added. During heat treatment, a few La3þ ions diffused into the a-Al2O3 lattice and led to lattice expansion for the larger radii of the La3þ (0.1032 nm) to Al3þ (0.0535 nm) ions.21 Nevertheless, the solubility of lanthanum in Al2O3 would not be high for the huge radius difference between La3þ and Al3þ ions, and the exact value is still unknown for the detection limit of most experimental
Table 1 Composition of the specimens. Raw materials Tabular alumina Reactive alumina La2O3 powder
0.5e1 mm 0.045 mm d50 ¼ 0.5 mm 2e3 mm
L0
L0.1
L0.5
L1
L2
83 10 7 e
83 9.9 7 0.1
83 9.5 7 0.5
83 9 7 1
83 8 7 2
Please cite this article as: Xiong X et al., Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.03.022
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Fig. 2. XRD patterns of specimens with and without La2O3 addition (a) and magnified local XRD peaks around 35.2 (b).
methods.22,23 Thompson et al.22 reported a possible bulk solubility value of about 80 ppm, and the real value could be much lower. Therefore, most of lanthanum was more likely to react with Al2O3 and form LaAl11O18 in the specimens. The formation of LaAl11O18 by the solid-phase reaction of La2O3 and Al2O3 contains two main stages19,24e26: La2O3þAl2O3/2LaAlO3,
(1)
LaAlO3þ5Al2O3/LaAl11O18.
(2)
First, La2O3 reacted with Al2O3 to form LaAlO3 at relatively low temperature (800e1000 C) and completed rapidly. Second, the generated LaAlO3 further reacted with Al2O3 to form LaAl11O18 with a slow reaction rate, which was the limiting factor of the formation reaction of LaAl11O18. Ropp and Carroll26 studied the solid reaction kinetics of La2O3 and Al2O3 and presented two possible diffusion mechanisms of Eq. (2) in air. If La3þ diffuses into Al2O3 and O2 moves through the gas phase, the diffusion equations are expressed as follows:
Fig. 3. Precipitation of LaAl11O18 from Al2O3 grain in specimen L0.5.
2LaAlO3/2La3þþ3O2þ12eþAl2O3,
(3)
3.2. Specimen microstructure
2La3þþ11Al2O3þ3O2þ12e/2LaAl11O18.
(4)
Fig. 4 shows the polished cross-sections of resin-infiltrated specimens, in which the black region corresponds to cured epoxy resin and the gray region to the coarse aggregate particles and the binder phase between them. In addition, in specimens with La2O3 additions, some white particles can be seen, which were identified as the in situ formed LaAl11O18 by EDS. As shown in Fig. 4(a), some interfacial cracks between coarse aggregate and bonding phase were observed in specimen L0, which should be due to the different shrinking percentages of coarse aggregate and powder in the sintering process. However, these interfacial cracks nearly disappeared in specimens L0.1 and L0.5, as presented in Fig. 4(b, c), and in which the coarse aggregate particles were bonded tightly with a few LaAl11O18 particles in the bonding phase. Upon further increasing the amount of La2O3 added, some cracks and pores can be seen in the bonding phase, which tend to be more significant in the specimens with more LaAl11O18 formation, as shown in Fig. 4(d, f). This should be attributed to the great volume expansion in forming excessive LaAl11O18 and the low sinterability of its plate-like grains27; in addition, large amounts of plate-like LaAl11O18 in the bonding phase may simultaneously inhibit the sintering of Al2O3. Furthermore, when the additions of La2O3 were above 1 wt%, several corundum coarse aggregate particles were also involved in the formation of LaAl11O18, as depicted in Fig. 4(e, f). More interestingly, some plate-like LaAl11O18 grains were located inside the coarse aggregate particles in specimen L1, as
If Al3þ diffuses into La2O3 and O2 moves through the gas phase, the diffusion equations are 10Al2O3/20Al3þþ15O2þ60e,
(5)
2LaAlO3þ20Al3þþ15O2þ60e/2LaAl11O18.
(6)
Ropp and Carroll also demonstrated that Eq. (2) involves firstorder reaction kinetics, and the controlling factor is the conversion of O2 to O2 at the boundary of LaAl11O18. However, they did not point out the primary diffusion mechanisms for the absence of data concerning the diffusion of La3þ and Al3þ in LaAlO3. As shown in Fig. 3, some plate-like LaAl11O18 grains were precipitated from an Al2O3 grain in specimen L0.5, which can also be seen in all the specimens containing in situ formed LaAl11O18. This should be attributed to the reaction of Al2O3 and the dissolved La3þ ions in this Al2O3 grain, as demonstrated in the XRD results shown in Fig. 2(b). Thus, it is clear that Eqs. (3) and (4) should be the primary diffusion mechanisms of the formation of LaAl11O18 in this work. Furthermore, the porous structure in the specimens may also be beneficial for the diffusion of O2 in the reaction system forming LaAl11O18, and then accelerate this reaction.
Please cite this article as: Xiong X et al., Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.03.022
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Fig. 4. SEM images of specimens L0 (a), L0.1 (b), L0.5 (c), L1 (d, e) and L2 (f).
Fig. 4(e) shows, as well as in the other specimens containing in situ formed LaAl11O18. This further validates the diffusion of La3þ ions into Al2O3 during the heating process. Obviously, the formed solid solution of La2O3 and Al2O3 in corundum coarse aggregate should be conducive to increasing the sintering activity of these particles. 3.3. Physical properties of specimens The physical properties of the fired specimens are shown in Figs. 5 and 6, respectively. As can be seen from the figures, with increasing amount of La2O3 added, the bulk density, cold crushing
strength, cold modulus of rupture, and hot modulus of rupture of the specimens first increased and then decreased, while the apparent porosity showed a reverse trend. Specimen L0.1 has the minimum percentage of linear change, the lowest apparent porosity, and the maximum bulk density. This means that the sinter-ability of corundum porous materials could be improved by addition of 0.1 wt% La2O3. Specimen L0.5 exhibits the maximum cold crushing strength, cold modulus of rupture, and hot modulus of rupture, which increased by 29%, 15.2% and 23.8%, respectively, when compared with the specimen with no La2O3 added. During the process of heating, a portion of the La3þ ions diffused into the a-Al2O3 lattice, resulting in lattice distortion of Al2O3,
Fig. 5. Percentage of linear change (a), apparent porosity (b) and bulk density (c) of specimens with different La2O3 contents.
Please cite this article as: Xiong X et al., Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.03.022
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Fig. 6. The mechanical properties of specimens with different La2O3 contents. (a) CCS; (b)CMOR; (c) HMOR.
which was favorable for improving the sintering activity of both Al2O3 powders and corundum coarse aggregate. Furthermore, some in situ formed LaAl11O18 at the grain boundary of Al2O3 could help limit the movement of the grain boundary and accelerate the exclusion of pores. Thus, the sintering properties of corundum porous materials can be improved with a small amount of La2O3 added, which should be responsible for the change of physical properties of specimen L0.1. The enhanced mechanical properties of specimens L0.1 and L0.5 should be attributed to the decreased apparent porosity, tighter connection of coarse aggregate particles, and the reinforcing and toughening effects of the in situ formed LaAl11O18. For specimens L1 and L2, the decreased bulk density and mechanical properties should be due to the large amount of pores and cracks in the bonding phase, as shown in Fig. 4(def). Fig. 7 shows the fracture morphology of specimens (a) L0 and (b) L0.5 at low magnification. A rough fracture surface of specimen L0 can be seen in Fig. 7(a), from which the morphology of the coarse aggregate particles at that section mostly remain intact. This means that the fracture of specimen L0 mainly occurred in the bonding
phase between coarse aggregate particles. The fracture surface of specimen L0.5 was smoother, and the fractured-through part of the coarse aggregate particles can be observed in Fig. 7(b). This indicates that a suitable amount of in situ formed LaAl11O18 should be conducive to optimizing the bonded interfaces between coarse aggregate particles and the bonding phase in the specimens, which is also verified in Fig. 4(a)e(c). Fig. 8 shows the fracture morphology of the bonding phase in specimens L0 and L0.5. Specimen L0 possesses a typical intergranular fracture at the bonding phase, as presented in Fig. 8(a), in which Al2O3 grains exhibit irregular shapes and inhomogeneous sizes. In specimen L0.5, plate-like LaAl11O18 grains can be observed mainly located at the grain boundary of Al2O3, as shown in Fig. 8(b), and the grain sizes of Al2O3 in the bonding phase were more uniform and smaller. Fig. 8(c) is a magnified image of micro-zone 1 in Fig. 8(b), from which the trans-granular fracture through part of the Al2O3 and LaAl11O18 grains can be observed. In addition, the crackpropagation paths in Fig. 8(c) also reveal the mechanism of transgranular fracture in specimen L0.5. From the foregoing, the more
Fig. 7. Fracture morphology of specimens L0 (a) and L0.5 (b) at low magnification.
Please cite this article as: Xiong X et al., Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.03.022
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Fig. 8. Fracture morphology of matrix phase in specimens L0 (a) and L0.5 (b), magnified micrograph of micro-zone 1 in (b) and (c).
uniform and smaller grain sizes of Al2O3 in the bonding phase and the reinforcement effect of LaAl11O18 should be responsible for the enhanced mechanical strength of the bonding phase in specimens with a certain amount of La2O3 added. In addition, in specimens L1 and L2, part of the corundum coarse aggregate also participated in the formation reaction of LaAl11O18, forming a transition layer at the surface of these particles. This should be beneficial for optimizing the interfaces between coarse aggregate and the bonding phase. However, the numerous cracks and pores in the bonding phase will significantly weaken the bonding strength of the coarse aggregate particles, which should be the critical factor for the decreased mechanical properties of specimens L1 and L2. 3.4. Pore-size distribution The pore-size distributions of specimens L0, L0.1 and L2 are presented in Fig. 9. The specimen without La2O3 added shows a typical bimodal distribution and the peaks are observed to be approximately 113.04 and 1.74 mm. The larger pores originated
from the stacking of coarse aggregate particles and the smaller pores were mainly located in coarse aggregate and the bonding phase. For the specimen with 0.1 wt% La2O3 added, the peak of the larger pores increased to 161.12 mm, while the peak intensity of the smaller pores was reduced obviously. This implies that the introduction of a certain amount of La2O3 can help increase the size of larger pores that were derived from the stacking of coarse aggregate particles, which should be attributed to the decrease of interfacial cracks between coarse aggregate and the bonding phase. In addition, the activated sintering of Al2O3 powders caused by the diffusion of La3þ ions in the a-Al2O3 lattice also accelerates the exclusion of pores in the bonding phase during the sintering process and gives rise to the decreased peak intensity of pores smaller than 10 mm. However, with an excess amount of La2O3 added, e.g., in specimen L2, a third peak at approximately 31.06 mm emerged, which mainly belongs to the generated pores during the formation of LaAl11O18, as shown in Fig. 4(f). Moreover, both the value and the intensity of the peak at approximately 113.04 mm were decreased, while, regarding the peak of pores smaller than 10 mm, the intensity increased and the value
Fig. 9. Pore-size distribution of specimens with different La2O3 contents. (a) Discrete frequency distribution; (b) Cumulative distribution.
Please cite this article as: Xiong X et al., Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.03.022
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decreased. Obviously, numerous cracks and smaller pores in specimen L2 should be the reasons for the above phenomenon. Fig. 9(b) shows the cumulative volume percentage of specimens L0, L0.1, and L2. As can be seen in the figure, the median diameter of pores in the specimen with 0.1 wt% La2O3 added increased from 59.87 to 83.43 mm, which decreased to 37.48 mm for the specimen with 2 wt% La2O3 added.
3.5. Air permeability of specimens Fig. 10 shows the pressure-drop curves obtained at room temperature for the specimens with different amounts of La2O3 added. The pressure drop shifted to the lowest values for the specimen with 0.1 wt% La2O3 added. Upon further increasing the amount of La2O3 added, the pressure drops of the specimens increased to higher values, which were much higher than that of specimen L0 with 2 wt% La2O3 added. More remarkably, the relationships between pressure drop and air-flow rate for all the specimens are nonlinear. Obviously, Darcy's law is not suited to study the air permeability behavior of this type of porous material, which can be described using Forchheimer's equation applicable for the compressible flow of gases,28,29 as follows:
p21 p22 mvs rv2s ¼ þ ; 2p2 L k1 k2
(7)
where p1 and p2 are the absolute air pressures at the entrance and exit of the specimen (Pa), respectively, L is the thickness of the specimen (m), m the gas viscosity (Pa$s), Vs the gas velocity (m/s), r the gas density (kg/m3), and k1 and k2 are, the Darcian (m2) and non-Darcian (m) permeability coefficients, respectively. The parameters k1 and k2 are introduced to describe the viscous and inertial effects on the flow systems. The values of k1 and k2 were obtained by fitting the curves to Forchheimer's equation as displayed in Fig. 8, and the results are listed in Table 2. The correlation coefficients R2 of all the specimens were higher than 0.999, which further verified the effectiveness of Forchheimer's equation in evaluating the permeability of this type of porous material. The values of k1 show little change when the addition of La2O3 is lower than 1 wt%, and which increased to a certain extent for the specimen with 2 wt% La2O3 added. With increasing amount of La2O3 added, the permeability coefficient k2 increased to a maximum value for the specimen with 0.1 wt% La2O3 added, and then decreased gradually. It is worth noting that
Fig. 10. Pressure-drop curves obtained at room temperature for specimens L0eL2.
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Table 2 Results of polynomial fitting using Forchheimer's equation. Specimen
k1 (1010 m2)
k2 (106 m)
R2
L0 L0.1 L0.5 L1 L2
2.24 2.19 2.32 2.29 2.54
1.49 1.94 1.65 1.45 1.00
0.9997 0.9996 0.9998 0.9998 0.9997
specimen L2 exhibits a significantly decreased k2 despite its maximum apparent porosity. The values of k1 of this type of porous medium can be predicted with the Kozeny-Carman equation30:
k1 ¼
ε3 kk s20 ð1 εÞ2
;
(8)
where kk is the Kozeny parameter and ε and s0 are the porosity and the specific surface of porous medium, respectively. kk defines the shape factor and tortuosity of the flowing channel, and is equal to 5 in most cases. From Eq. (8), the changes in apparent porosity should be responsible for the variation of k1 for the specimens with different amounts of La2O3 added, as shown in Table 2. The variation of k2 should be related to the structural changes of the flow channel, i.e., the connected pores in the specimens. For the specimens with a certain amount of La2O3 added, the introduction of La2O3 contributes to the activated sintering of both Al2O3 powder and coarse aggregate, and accelerates the exclusion of pores in the bonding phase during the heat-treatment process. The decreased content of smaller pores in the bonding phase and the reduced number of interfacial cracks can help reduce the air-flow resistance. This should account for the increased k2 value of the specimen with 0.1 wt% La2O3 added. However, with an excessive amount of La2O3 added, numerous smaller pores and cracks can be observed in the specimens. This will increase the resistance to air flow obviously, and lead to a minimum value of k2 in specimens with 2 wt% La2O3 added, despite its maximum apparent porosity.
4. Conclusions (1) The mechanical properties of corundum porous materials can be improved with addition of a certain amount of in situ formed LaAl11O18, and the air permeability can also be increased. (2) The strengthening mechanisms of LaAl11O18 for the corundum porous materials are activated sintering of both Al2O3 powder and corundum coarse aggregate in specimens for the diffusion of La3þ ions to the Al2O3 lattice, forming a solid solution, which enhanced the bonding strength between the coarse aggregate particles; reinforcement effects of the in situ formation of plate-like LaAl11O18 grains in the bonding phase. In this work, the cold crushing strength, cold modulus of rupture, and hot modulus of rupture (1400 C 0.5 h) of the specimen with 0.5 wt% La2O3 added were increased by approximately 29%, 15.2% and 23.8%, respectively, compared with the specimen without any La2O3 added. (3) Forchheimer's equation is well suited to describe the permeability behavior in this type of porous material. With a certain amount of La2O3 addition that does not exceed 1 wt%, the k2 values of the specimens can be increased, while the k1 values show little change. However, with excessive amount of in situ formed LaAl11O18 added to the specimens, the k2
Please cite this article as: Xiong X et al., Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.03.022
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values will be significantly decreased despite the increased apparent porosity and k1 value.
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16. Negahdari Z, Willert-Porada M. Tailoring the microstructure of reactionsintered alumina/lanthanum hexaaluminate particulate composites. J Eur Ceram Soc. 2010;30(6):1381. 17. Yasuoka M, Hirao K, Brito ME, Kanzaki S. High-Strength and high-fracturetoughness ceramics in the Al2O3/LaAl11O18 Systems. J Am Ceram Soc. 1995;78(7):1853. 18. Negahdari Z, Willert-Porada M, Pfeiffer C. Mechanical properties of dense to porous alumina/lanthanum hexaaluminate composite ceramics. Mater Sci Eng A. 2010;527(12):3005. 19. Wu YQ, Zhang YF, Huang XX, Li BS, Guo JK. Preparation, sintering and fracture behavior of Al2O3/LaAl11O18 ceramic composites. J Mater Sci. 2001;36(17):4195. 20. Yamashita I, Tsukuma K, Kusunose T. Translucent Al2O3/LaAl11O18 composite. J Am Ceram Soc. 2010;92(9):2136. 21. Qian FL, Xie ZP, Sun JL, Wang F. Effect of La2O3 addition on microstructure and microwave dielectric properties of Al2O3 ceramics. J Chin Ceram Soc. 2012;40(12):1708. 22. Thompson AM, Soni KK, Chan HM, Harmer MP, Williams DB, Chabala JM, et al. Dopant distributions in rare-earth-doped Alumina. J Am Ceram Soc. 1997;80(2):373. 23. Ren T, Nforbi LNN, Kanakala R, Graeve OA. Phase stability and mechanisms of transformation of La-doped g-Alumina. Inorg Chem. 2018;57:3035. 24. Sung GY, Kang KY, Park SC. Synthesis and preparation of lanthanum aluminate target for radio-frequency magnetron sputtering. J Am Ceram Soc. 1978;74(2):437. 25. Nair J, Nair P, Mizukami F, Ommen JGV, Doesburg GBM, Ross JRH, et al. Pore structure evolution of lanthana-alumina systems prepared through coprecipitation. J Am Ceram Soc. 2000;83(8):1942. 26. Ropp RC, Carroll B. Solid-state kinetics of LaAl11O18. J Am Ceram Soc. 1980;63(7e8):416. 27. Cao XQ, Zhang YF, Zhang JF, Zhong XH, Wang Y, Ma HM, et al. Failure of the plasma-sprayed coating of lanthanum hexaluminate. J Eur Ceram Soc. 2008;28(10):1979. 28. Innocentini MDM, Pardo ARF, Pandolfelli VC. Influence of Air compressibility on the permeability evaluation of refractory castables. J Am Ceram Soc. 2000;83(6):1536. 29. Innocentini MDM, Pardo ARF, Pandolfelli VC, Menegazzo BA. Permeability of high-alumina refractory castables based on various hydraulic binders. J Am Ceram Soc. 2002;85(6):1517. 30. Scheffler M, Colombo P. Cellular ceramics: structure, manufacturing, properties and applications. Weinheim: WILEY-VCH Verlag GmbH & Co. KGaA; 2005.
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Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase* Xin Xiong, Zhoufu Wang*, Xitang Wang, Hao Liu, Yan Ma The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 January 2019 Received in revised form 20 March 2019 Accepted 20 March 2019 Available online xxx
Corundum porous materials of particle-packing type with different contents of in situ formed LaAl11O18 were prepared using tabular corundum, reactive alumina and La2O3 powder as raw materials. The effects of the introduction of LaAl11O18 on the microstructure, phase composition and properties of the porous materials were investigated. The specimens were characterized by scanning electron microscopy, X-ray diffraction and mercury porosimetry. Results show that platelet-like LaAl11O18 is formed in situ by the reactions of La2O3 and Al2O3. With a certain amount of La2O3 added, the cold crushing strength, cold modulus of rupture and hot modulus of rupture (1400 0.5 h) of the specimens are increased, and the air permeability is improved simultaneously. However, upon further increasing the amount of La2O3 added, the mechanical properties and air permeability of the porous materials then decrease gradually owing to the increased numbers of pores and cracks in the bonding phase. The enhanced mechanical properties of the specimens with La2O3 added are attributed to the strengthening effects of plate-like LaAl11O18 in the bonding phase and to the activated sintering of both Al2O3 powder and corundum coarse aggregate for the diffusion of La3þ in Al2O3 lattice. In addition, the improved air permeability of the specimens should be related to the decreased content of pores in the bonding phase and the reduced number of interfacial cracks. © 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Keywords: Porous materials of particle-packing type Lanthanum hexaluminate Mechanical strength Strengthening mechanism Air permeability Rare earths
1. Introduction Porous purging materials have been widely used in the fields of iron and steel smelting, aluminum smelting, and copper smelting to improve the quality of molten metal.1e5 The porous materials are mostly in direct contact with the molten metal, and as a result this porous medium should be subject to the huge hydrostatic pressure and flow scouring of molten metal. Besides, the flushing action of high pressure gas in service may also be destructive. From the foregoing, high mechanical strength of the porous medium is required. Pores in a porous medium can be achieved by particle packing,6 the pore-forming-agent method,7,8 direct foaming,9 the freeze-drying process,10e12 and gel casting.13 Fig. 1 shows the fabrication process and structural schematic of the porous materials of particle-packing type. As can be seen from the figure, pores in this type of porous material are mainly interconnected with each * Foundation items: Project supported by the National Natural Science Foundation of China (51672195, 51474166) and the Key Program of Natural Science Foundation of Hubei Province, China (2017CFA004). * Corresponding author. E-mail address: [email protected] (Z. Wang).
other, which lead to excellent air permeability of the porous medium. Nevertheless, poor mechanical strength may also occur. Obviously, increasing the bonding strength of coarse aggregate particles should be an ideal method of improving the mechanical strength of porous materials. Lanthanum hexaluminate (LaAl11O18) has thin plate-like morphology, high melting point,14 great heat and chemical stability, and can be compatible with Al2O3.15 These features make LaAl11O18 an ideal second phase for improving the mechanical properties of alumina.16,17 The results of Chen15 show that the fracture toughness of alumina composites with 30 vol% of in situ formed LaAl11O18 increased by over 40% when compared with pure Al2O3 materials; the main toughening mechanism was the crack-bridging action of the plate-like particles. Negahdari et al.18 studied the mechanical properties of alumina/lanthanum hexaluminate composite ceramics with different contents of lanthanum hexaluminate and indicated that the fracture toughness, hardness, and elastic modulus of the composite materials increased with the addition of a certain amount of in situ formed lanthanum hexaluminate. Upon further increasing the lanthanum hexaluminate content, the hardness and elastic modulus of the composites were degraded. Moreover, the
https://doi.org/10.1016/j.jre.2019.03.022 1002-0721/© 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Xiong X et al., Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.03.022
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Fig. 1. Fabrication process and structural schematic of porous materials of particle-packing type.
influence of lanthanum hexaluminate on mechanical properties was described by the microstructure, porosity and intrinsic characteristics of lanthanum hexaluminate. Wu et al.19 prepared Al2O3/ LaAl11O18 composites with 25 vol% of LaAl11O18, and the sintering and fracture of the resulting composites were investigated. They found that the decreased grain size of Al2O3 grains and the mechanism of crack deflection caused by the rod-like LaAl11O18 grains leads to the improvement in fracture toughness of the composites. Yamashitaw et al.20 prepared a translucent Al2O3 1 vol% LaAl11O18 composite with high fracture toughness, and indicated that crack deflection and crack bridging in LaAl11O18 should be responsible for the enhanced bending strength and fracture toughness. According to the above results, it is clear that the addition of a moderate amount of LaAl11O18 to the compositions will improve the mechanical properties of alumina materials. However, until now, the effects of LaAl11O18 on the structure and properties of corundum porous materials have not been reported. In the work described in this paper, LaAl11O18 was introduced to the bonding phase of corundum porous materials by in situ reaction of Al2O3 and La2O3, and the effects of in situ formed LaAl11O18 on the mechanical properties and air permeability of the resulting porous materials were investigated, and then the reinforcing and toughening mechanisms were discussed.
prepared specimens were fired at 1750 C at a heating rate of 5 C/min for 3 h and then furnace-cooled. 2.2. Testing and characterization methods Apparent porosity (AP), bulk density (BD), cold crushing strength (CCS) and cold modulus of rupture (CMOR) of the specimens were measured according to ISO Standards 5017:2013 and 5014:1997. The hot modulus of rupture (HMOR) was measured according to ISO Standard 5013:1985, which was performed at 1400 C. The microstructure and composition of the specimens were characterized by using scanning electron microscopy (SEM, JSM-6610, JEOL, Tokyo, Japan) with energy-dispersive spectroscopy (EDS, QUANTAX200-30, Bruker Corp., USA). The phase composition of the specimens was investigated by X-ray diffraction (XRD, X'Pert Pro, Philips Corp., USA). The pore-size distribution was examined by mercury porosimetry (Quantachrome PM60GT-18, Quantachrome Instruments Ltd., USA). The air permeability was measured with a permeability tester (TQD-02, Sinosteel, China) at room temperature, and N2 was selected as the air source. 3. Results and discussion 3.1. Phase composition
2. Experimental 2.1. Specimen preparation Tabular alumina (0.5e1 mm, 0.045 mm, T60/64, Almatis, Tsingtao, China), reactive alumina powder (d50 ¼ 0.5 mm, CT 3000 SG, Almatis, Tsingtao, China), and La2O3 (99.99% purity, Zibo Weijie Rare Earth Co., Ltd., China) powder were used as raw materials. Aqueous solution of maltose (25 wt% concentration) was used as binding agent. The weight ratio of coarse aggregate, powder, and binder was 83:17:3.5. Different proportions of La2O3 were added into the compositions by equal replacement of the tabular alumina powder (0.045 mm); the detailed compositions are listed in Table 1. The powder mixtures were ball-mixed for 3 h at a speed of 200 r/min to ensure that the La2O3 powder was dispersed homogeneously in the compositions. The raw materials were mixed in an intensive mixer, and then bar-shaped (25 mm 25 mm 125 mm) and cylinder-shaped (F50 mm 50 mm) specimens were uniaxially pressed at 75 MPa. After drying at 110 C for 24 h, the as-
Fig. 2(a) shows the XRD patterns of the specimens after heat treatment at 1750 C for 3 h, and Fig. 2(b) shows the magnified local XRD peaks at approximately 35.2 in Fig. 2(a) that belong to the (110) peak of a-Al2O3. As shown in Fig. 2(a), the main crystalline phase of all the specimens is a-Al2O3 and the diffraction peaks of LaAl11O18 appeared in specimen L0.5. Upon further increasing the amount of La2O3 added, the diffraction intensity of the LaAl11O18 peaks increased simultaneously. More interestingly, as shown in Fig. 2(b), the diffraction peak of a-Al2O3 shifted to small angles for specimens L0.1, L0.5, L1 and L2, and the decreasing tendency of the diffraction angle was more obvious for the specimens with increasing amount of La2O3 added. During heat treatment, a few La3þ ions diffused into the a-Al2O3 lattice and led to lattice expansion for the larger radii of the La3þ (0.1032 nm) to Al3þ (0.0535 nm) ions.21 Nevertheless, the solubility of lanthanum in Al2O3 would not be high for the huge radius difference between La3þ and Al3þ ions, and the exact value is still unknown for the detection limit of most experimental
Table 1 Composition of the specimens. Raw materials Tabular alumina Reactive alumina La2O3 powder
0.5e1 mm 0.045 mm d50 ¼ 0.5 mm 2e3 mm
L0
L0.1
L0.5
L1
L2
83 10 7 e
83 9.9 7 0.1
83 9.5 7 0.5
83 9 7 1
83 8 7 2
Please cite this article as: Xiong X et al., Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.03.022
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Fig. 2. XRD patterns of specimens with and without La2O3 addition (a) and magnified local XRD peaks around 35.2 (b).
methods.22,23 Thompson et al.22 reported a possible bulk solubility value of about 80 ppm, and the real value could be much lower. Therefore, most of lanthanum was more likely to react with Al2O3 and form LaAl11O18 in the specimens. The formation of LaAl11O18 by the solid-phase reaction of La2O3 and Al2O3 contains two main stages19,24e26: La2O3þAl2O3/2LaAlO3,
(1)
LaAlO3þ5Al2O3/LaAl11O18.
(2)
First, La2O3 reacted with Al2O3 to form LaAlO3 at relatively low temperature (800e1000 C) and completed rapidly. Second, the generated LaAlO3 further reacted with Al2O3 to form LaAl11O18 with a slow reaction rate, which was the limiting factor of the formation reaction of LaAl11O18. Ropp and Carroll26 studied the solid reaction kinetics of La2O3 and Al2O3 and presented two possible diffusion mechanisms of Eq. (2) in air. If La3þ diffuses into Al2O3 and O2 moves through the gas phase, the diffusion equations are expressed as follows:
Fig. 3. Precipitation of LaAl11O18 from Al2O3 grain in specimen L0.5.
2LaAlO3/2La3þþ3O2þ12eþAl2O3,
(3)
3.2. Specimen microstructure
2La3þþ11Al2O3þ3O2þ12e/2LaAl11O18.
(4)
Fig. 4 shows the polished cross-sections of resin-infiltrated specimens, in which the black region corresponds to cured epoxy resin and the gray region to the coarse aggregate particles and the binder phase between them. In addition, in specimens with La2O3 additions, some white particles can be seen, which were identified as the in situ formed LaAl11O18 by EDS. As shown in Fig. 4(a), some interfacial cracks between coarse aggregate and bonding phase were observed in specimen L0, which should be due to the different shrinking percentages of coarse aggregate and powder in the sintering process. However, these interfacial cracks nearly disappeared in specimens L0.1 and L0.5, as presented in Fig. 4(b, c), and in which the coarse aggregate particles were bonded tightly with a few LaAl11O18 particles in the bonding phase. Upon further increasing the amount of La2O3 added, some cracks and pores can be seen in the bonding phase, which tend to be more significant in the specimens with more LaAl11O18 formation, as shown in Fig. 4(d, f). This should be attributed to the great volume expansion in forming excessive LaAl11O18 and the low sinterability of its plate-like grains27; in addition, large amounts of plate-like LaAl11O18 in the bonding phase may simultaneously inhibit the sintering of Al2O3. Furthermore, when the additions of La2O3 were above 1 wt%, several corundum coarse aggregate particles were also involved in the formation of LaAl11O18, as depicted in Fig. 4(e, f). More interestingly, some plate-like LaAl11O18 grains were located inside the coarse aggregate particles in specimen L1, as
If Al3þ diffuses into La2O3 and O2 moves through the gas phase, the diffusion equations are 10Al2O3/20Al3þþ15O2þ60e,
(5)
2LaAlO3þ20Al3þþ15O2þ60e/2LaAl11O18.
(6)
Ropp and Carroll also demonstrated that Eq. (2) involves firstorder reaction kinetics, and the controlling factor is the conversion of O2 to O2 at the boundary of LaAl11O18. However, they did not point out the primary diffusion mechanisms for the absence of data concerning the diffusion of La3þ and Al3þ in LaAlO3. As shown in Fig. 3, some plate-like LaAl11O18 grains were precipitated from an Al2O3 grain in specimen L0.5, which can also be seen in all the specimens containing in situ formed LaAl11O18. This should be attributed to the reaction of Al2O3 and the dissolved La3þ ions in this Al2O3 grain, as demonstrated in the XRD results shown in Fig. 2(b). Thus, it is clear that Eqs. (3) and (4) should be the primary diffusion mechanisms of the formation of LaAl11O18 in this work. Furthermore, the porous structure in the specimens may also be beneficial for the diffusion of O2 in the reaction system forming LaAl11O18, and then accelerate this reaction.
Please cite this article as: Xiong X et al., Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.03.022
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Fig. 4. SEM images of specimens L0 (a), L0.1 (b), L0.5 (c), L1 (d, e) and L2 (f).
Fig. 4(e) shows, as well as in the other specimens containing in situ formed LaAl11O18. This further validates the diffusion of La3þ ions into Al2O3 during the heating process. Obviously, the formed solid solution of La2O3 and Al2O3 in corundum coarse aggregate should be conducive to increasing the sintering activity of these particles. 3.3. Physical properties of specimens The physical properties of the fired specimens are shown in Figs. 5 and 6, respectively. As can be seen from the figures, with increasing amount of La2O3 added, the bulk density, cold crushing
strength, cold modulus of rupture, and hot modulus of rupture of the specimens first increased and then decreased, while the apparent porosity showed a reverse trend. Specimen L0.1 has the minimum percentage of linear change, the lowest apparent porosity, and the maximum bulk density. This means that the sinter-ability of corundum porous materials could be improved by addition of 0.1 wt% La2O3. Specimen L0.5 exhibits the maximum cold crushing strength, cold modulus of rupture, and hot modulus of rupture, which increased by 29%, 15.2% and 23.8%, respectively, when compared with the specimen with no La2O3 added. During the process of heating, a portion of the La3þ ions diffused into the a-Al2O3 lattice, resulting in lattice distortion of Al2O3,
Fig. 5. Percentage of linear change (a), apparent porosity (b) and bulk density (c) of specimens with different La2O3 contents.
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Fig. 6. The mechanical properties of specimens with different La2O3 contents. (a) CCS; (b)CMOR; (c) HMOR.
which was favorable for improving the sintering activity of both Al2O3 powders and corundum coarse aggregate. Furthermore, some in situ formed LaAl11O18 at the grain boundary of Al2O3 could help limit the movement of the grain boundary and accelerate the exclusion of pores. Thus, the sintering properties of corundum porous materials can be improved with a small amount of La2O3 added, which should be responsible for the change of physical properties of specimen L0.1. The enhanced mechanical properties of specimens L0.1 and L0.5 should be attributed to the decreased apparent porosity, tighter connection of coarse aggregate particles, and the reinforcing and toughening effects of the in situ formed LaAl11O18. For specimens L1 and L2, the decreased bulk density and mechanical properties should be due to the large amount of pores and cracks in the bonding phase, as shown in Fig. 4(def). Fig. 7 shows the fracture morphology of specimens (a) L0 and (b) L0.5 at low magnification. A rough fracture surface of specimen L0 can be seen in Fig. 7(a), from which the morphology of the coarse aggregate particles at that section mostly remain intact. This means that the fracture of specimen L0 mainly occurred in the bonding
phase between coarse aggregate particles. The fracture surface of specimen L0.5 was smoother, and the fractured-through part of the coarse aggregate particles can be observed in Fig. 7(b). This indicates that a suitable amount of in situ formed LaAl11O18 should be conducive to optimizing the bonded interfaces between coarse aggregate particles and the bonding phase in the specimens, which is also verified in Fig. 4(a)e(c). Fig. 8 shows the fracture morphology of the bonding phase in specimens L0 and L0.5. Specimen L0 possesses a typical intergranular fracture at the bonding phase, as presented in Fig. 8(a), in which Al2O3 grains exhibit irregular shapes and inhomogeneous sizes. In specimen L0.5, plate-like LaAl11O18 grains can be observed mainly located at the grain boundary of Al2O3, as shown in Fig. 8(b), and the grain sizes of Al2O3 in the bonding phase were more uniform and smaller. Fig. 8(c) is a magnified image of micro-zone 1 in Fig. 8(b), from which the trans-granular fracture through part of the Al2O3 and LaAl11O18 grains can be observed. In addition, the crackpropagation paths in Fig. 8(c) also reveal the mechanism of transgranular fracture in specimen L0.5. From the foregoing, the more
Fig. 7. Fracture morphology of specimens L0 (a) and L0.5 (b) at low magnification.
Please cite this article as: Xiong X et al., Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.03.022
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Fig. 8. Fracture morphology of matrix phase in specimens L0 (a) and L0.5 (b), magnified micrograph of micro-zone 1 in (b) and (c).
uniform and smaller grain sizes of Al2O3 in the bonding phase and the reinforcement effect of LaAl11O18 should be responsible for the enhanced mechanical strength of the bonding phase in specimens with a certain amount of La2O3 added. In addition, in specimens L1 and L2, part of the corundum coarse aggregate also participated in the formation reaction of LaAl11O18, forming a transition layer at the surface of these particles. This should be beneficial for optimizing the interfaces between coarse aggregate and the bonding phase. However, the numerous cracks and pores in the bonding phase will significantly weaken the bonding strength of the coarse aggregate particles, which should be the critical factor for the decreased mechanical properties of specimens L1 and L2. 3.4. Pore-size distribution The pore-size distributions of specimens L0, L0.1 and L2 are presented in Fig. 9. The specimen without La2O3 added shows a typical bimodal distribution and the peaks are observed to be approximately 113.04 and 1.74 mm. The larger pores originated
from the stacking of coarse aggregate particles and the smaller pores were mainly located in coarse aggregate and the bonding phase. For the specimen with 0.1 wt% La2O3 added, the peak of the larger pores increased to 161.12 mm, while the peak intensity of the smaller pores was reduced obviously. This implies that the introduction of a certain amount of La2O3 can help increase the size of larger pores that were derived from the stacking of coarse aggregate particles, which should be attributed to the decrease of interfacial cracks between coarse aggregate and the bonding phase. In addition, the activated sintering of Al2O3 powders caused by the diffusion of La3þ ions in the a-Al2O3 lattice also accelerates the exclusion of pores in the bonding phase during the sintering process and gives rise to the decreased peak intensity of pores smaller than 10 mm. However, with an excess amount of La2O3 added, e.g., in specimen L2, a third peak at approximately 31.06 mm emerged, which mainly belongs to the generated pores during the formation of LaAl11O18, as shown in Fig. 4(f). Moreover, both the value and the intensity of the peak at approximately 113.04 mm were decreased, while, regarding the peak of pores smaller than 10 mm, the intensity increased and the value
Fig. 9. Pore-size distribution of specimens with different La2O3 contents. (a) Discrete frequency distribution; (b) Cumulative distribution.
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decreased. Obviously, numerous cracks and smaller pores in specimen L2 should be the reasons for the above phenomenon. Fig. 9(b) shows the cumulative volume percentage of specimens L0, L0.1, and L2. As can be seen in the figure, the median diameter of pores in the specimen with 0.1 wt% La2O3 added increased from 59.87 to 83.43 mm, which decreased to 37.48 mm for the specimen with 2 wt% La2O3 added.
3.5. Air permeability of specimens Fig. 10 shows the pressure-drop curves obtained at room temperature for the specimens with different amounts of La2O3 added. The pressure drop shifted to the lowest values for the specimen with 0.1 wt% La2O3 added. Upon further increasing the amount of La2O3 added, the pressure drops of the specimens increased to higher values, which were much higher than that of specimen L0 with 2 wt% La2O3 added. More remarkably, the relationships between pressure drop and air-flow rate for all the specimens are nonlinear. Obviously, Darcy's law is not suited to study the air permeability behavior of this type of porous material, which can be described using Forchheimer's equation applicable for the compressible flow of gases,28,29 as follows:
p21 p22 mvs rv2s ¼ þ ; 2p2 L k1 k2
(7)
where p1 and p2 are the absolute air pressures at the entrance and exit of the specimen (Pa), respectively, L is the thickness of the specimen (m), m the gas viscosity (Pa$s), Vs the gas velocity (m/s), r the gas density (kg/m3), and k1 and k2 are, the Darcian (m2) and non-Darcian (m) permeability coefficients, respectively. The parameters k1 and k2 are introduced to describe the viscous and inertial effects on the flow systems. The values of k1 and k2 were obtained by fitting the curves to Forchheimer's equation as displayed in Fig. 8, and the results are listed in Table 2. The correlation coefficients R2 of all the specimens were higher than 0.999, which further verified the effectiveness of Forchheimer's equation in evaluating the permeability of this type of porous material. The values of k1 show little change when the addition of La2O3 is lower than 1 wt%, and which increased to a certain extent for the specimen with 2 wt% La2O3 added. With increasing amount of La2O3 added, the permeability coefficient k2 increased to a maximum value for the specimen with 0.1 wt% La2O3 added, and then decreased gradually. It is worth noting that
Fig. 10. Pressure-drop curves obtained at room temperature for specimens L0eL2.
7
Table 2 Results of polynomial fitting using Forchheimer's equation. Specimen
k1 (1010 m2)
k2 (106 m)
R2
L0 L0.1 L0.5 L1 L2
2.24 2.19 2.32 2.29 2.54
1.49 1.94 1.65 1.45 1.00
0.9997 0.9996 0.9998 0.9998 0.9997
specimen L2 exhibits a significantly decreased k2 despite its maximum apparent porosity. The values of k1 of this type of porous medium can be predicted with the Kozeny-Carman equation30:
k1 ¼
ε3 kk s20 ð1 εÞ2
;
(8)
where kk is the Kozeny parameter and ε and s0 are the porosity and the specific surface of porous medium, respectively. kk defines the shape factor and tortuosity of the flowing channel, and is equal to 5 in most cases. From Eq. (8), the changes in apparent porosity should be responsible for the variation of k1 for the specimens with different amounts of La2O3 added, as shown in Table 2. The variation of k2 should be related to the structural changes of the flow channel, i.e., the connected pores in the specimens. For the specimens with a certain amount of La2O3 added, the introduction of La2O3 contributes to the activated sintering of both Al2O3 powder and coarse aggregate, and accelerates the exclusion of pores in the bonding phase during the heat-treatment process. The decreased content of smaller pores in the bonding phase and the reduced number of interfacial cracks can help reduce the air-flow resistance. This should account for the increased k2 value of the specimen with 0.1 wt% La2O3 added. However, with an excessive amount of La2O3 added, numerous smaller pores and cracks can be observed in the specimens. This will increase the resistance to air flow obviously, and lead to a minimum value of k2 in specimens with 2 wt% La2O3 added, despite its maximum apparent porosity.
4. Conclusions (1) The mechanical properties of corundum porous materials can be improved with addition of a certain amount of in situ formed LaAl11O18, and the air permeability can also be increased. (2) The strengthening mechanisms of LaAl11O18 for the corundum porous materials are activated sintering of both Al2O3 powder and corundum coarse aggregate in specimens for the diffusion of La3þ ions to the Al2O3 lattice, forming a solid solution, which enhanced the bonding strength between the coarse aggregate particles; reinforcement effects of the in situ formation of plate-like LaAl11O18 grains in the bonding phase. In this work, the cold crushing strength, cold modulus of rupture, and hot modulus of rupture (1400 C 0.5 h) of the specimen with 0.5 wt% La2O3 added were increased by approximately 29%, 15.2% and 23.8%, respectively, compared with the specimen without any La2O3 added. (3) Forchheimer's equation is well suited to describe the permeability behavior in this type of porous material. With a certain amount of La2O3 addition that does not exceed 1 wt%, the k2 values of the specimens can be increased, while the k1 values show little change. However, with excessive amount of in situ formed LaAl11O18 added to the specimens, the k2
Please cite this article as: Xiong X et al., Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.03.022
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values will be significantly decreased despite the increased apparent porosity and k1 value.
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Please cite this article as: Xiong X et al., Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.03.022