Porous fibrous ZrO2-mullite ceramics prepared via tert-butyl alcohol-based gel-casting

Author’s Accepted Manuscript Porous fibrous ZrO2-mullite ceramics prepared via tert-butyl alcohol-based gel-casting Xinghui Hou, Zhenli Liu, Zongquan Liu, Lei Yuan, Jingkun Yu www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(18)31062-9 https://doi.org/10.1016/j.ceramint.2018.04.192 CERI18109

To appear in: Ceramics International Received date: 5 February 2018 Revised date: 13 April 2018 Accepted date: 21 April 2018 Cite this article as: Xinghui Hou, Zhenli Liu, Zongquan Liu, Lei Yuan and Jingkun Yu, Porous fibrous ZrO 2-mullite ceramics prepared via tert-butyl alcohol-based gel-casting, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.04.192 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 galley proof before it is published in its final citable 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.

Porous fibrous ZrO2-mullite ceramics prepared via tert-butyl alcohol-based gel-casting Xinghui Hou, Zhenli Liu, Zongquan Liu, Lei Yuan*, Jingkun Yu* School of Metallurgy, Northeastern University NO. 3-11, Wenhua Road, Heping District, Shenyang, 110819, PR China. * Corresponding author: [email protected] (L Yuan); [email protected] (J Yu)

Abstract Mullite fiber was used to fabricate ZrO2-mullite based porous ceramic via tert-butyl alcohol (TBA)-based gel-casting process using zirconite and bauxite as raw materials. Phase compositions, microstructure, pore size distribution, linear shrinkage, bulk density, apparent porosity, thermal conductivity, and compressive strength were analyzed to investigate influences of mullite fiber content and added Y2O3 on prepared porous ceramics. Results show that bird nest-like three-dimensional fibrous reticular skeleton structure was constructed with mullite fibers that evenly enwrapped rod-like mullite and ZrO2 grains. Prepared porous fibrous ZrO2-mullite ceramics had narrow pore size distribution that consisted of mullite and m-ZrO2. With an increase in mullite fiber content, linear shrinkage and bulk density decreased, apparent porosity increased, and relatively good thermal conductivity was obtained. In addition, added Y2O3 reacted with Al2O3 and SiO2 to form Y-Al-Si-O glass phase, which promoted sintering and densification of the ceramic, thus improving its compressive strength. Keywords: Mullite fiber; ZrO2; Porous ceramic; Gel-casting process; Y2O3 additive

1. Introduction Porous ceramics with high area ratio and good heat insulation have been widely used for filtration and catalysis in chemical industry, metallurgy, pharmaceuticals, aeronautics, and astronautics. Many methods, such as organic foam impregnation, foaming, pore-forming agents and freeze-drying, have been used to prepare porous ceramics with different pore parameters. For example, the pore size of porous

ceramics can be controlled using different polyurethane templates by the organic foam impregnation; nano-sized pores can be obtained by foaming method; the porosity of porous ceramics can be also obtained as high as 99% using foaming method; of course, the pore shape can be also changed by using different pore-forming agents [1-4]. In order to further explore the development of preparing porous ceramic, a new nearly net-shaped gel-casting technique has been proposed. Compared with the above other fabrication methods, the net-shaped gel-casting technique as an advanced and flexible preparation process has the following advantages: (1) it has a wide range of applications, such as preparing a single material or composite material. (2) The body has less shrinkage due to the slurry with low viscosity and high solid volume fraction, so the component with a near net size can be prepared. (3) The component with complex shapes can be prepared, because the flowing liquid slurry could fully fill in the mold. (4) The green body with high strength and good plasticity can machine into the more precise parts. (5) The requirements for the mold are not high. (6) The purity of the sintered components is high. It is feasible for using this technique to obtain the composite porous ceramics with higher porosity, homogeneity and reliability, and now has drawn increasing attention in recent years [5-8]. In the porous ceramics, the mullite porous ceramics fabricated via gel-casting are an aluminosilicate ceramic material and have the advantages of high-temperature mechanical properties, good chemical stability, low thermal conductivity, excellent creep resistance, and thermal shock resistance [9-11]. It can meet the specific requirements of pore sizes and structures for different applications such as hot gas and waste water filters, separation membranes, catalyst supports, and thermal insulation materials. However, in the process of application, its two most important evaluation parameters of porosity and mechanical strength are inversely proportional, and it is relatively difficult to balance the relationship between them [12-16]. Therefore, to address this issue, the ZrO2 considering the most promising high performance ceramic material can be introduced in porous mullite ceramics to produce ZrO2-mullite based multiphase porous ceramics, which can be used in high temperature environments. The second phase of ZrO2 in the matrix phase can hinder dislocation movement and

improve deformation resistance, thus, reinforcing the multiphase porous ceramics [17-19]. The mullite fiber with high strength and lightweight can combine with the composites to synthesize fiber-reinforced composites that have high porosity and desirable strength, which make it a possible candidate for exploring preparation of porous mullite ceramics [20, 21]. Because of wide applications of rare earth oxides in ceramic materials, Y2O3, which has a high melting point, can be introduced to such ceramics for further promoting an increase in strength [22-24]. In the net-shaped gel-casting process, the tert-butyl alcohol (TBA) can be used as the solvent to form the premixed liquid with monomer, crosslinking agent and dispersant. TBA with low viscosity of 3.35 mPa·s can be conducive to preparing the uniform slurry with low viscosity and high solid volume fraction, which can easily fill every corner of the mold in the injection molding process. Then TBA becomes the crystal after dehydration, which reduce the shrinkage and deformation of the body. In the sintering stage, TBA and other organic compound exhausted from the body, and the porous ceramic was obtained. Therefore, in this paper, the TBA-based gel-casting technique was used to fabricate the porous fibrous ZrO2-mullite ceramics with mullite fiber as a matrix and with zirconite and bauxite as raw materials for binder. Effects of mullite fiber content and added Y2O3 on phase compositions, microstructure, pore size distribution, linear shrinkage, bulk density, apparent porosity, thermal conductivity and compressive strength were investigated.

2. Experimental 2.1 Raw materials Commercial polycrystalline mullite fibers (purity≥99.5%, Weiye Crystal Fiber Co., Ltd., Zhejiang, China) were used as starting materials. Submicro-sized zirconite (Zhengyang Foundry Material Plant, Zhengzhou, China) and bauxite (Chenyuan Powder Co., Ltd., Shandong, China) powders were used as high temperature binders; chemical compositions of bauxite and zirconite are shown in Table 1 and Table 2, respectively. Table 1. Chemical composition of bauxite (wt%).

Content

Al2O3

SiO2

TiO2

Fe2O3

CaO

Others

wt%

82.94

8.41

4.96

1.75

0.66

1.28

Table 2. Chemical composition of zirconite (wt%). Content

ZrO2

SiO2

TiO2

Fe2O3

CaO

Others

wt%

66.66

32.04

0.25

0.1

0.08

0.87

Tert-butyl alcohol (TBA) was used as the molding medium, and acrylamide (AM) was the organic monomer. N, N-Methylene-bisacrylamide (MBA) was a crosslinking agent. Citric acid was a dispersant, ammonium persulfate was an evocating agent, and N, N, N, N-tetramethyl ethylenediamine was a catalyst (National Drug Group Chemical Reagents Co., Ltd., Shenyang, China). 2.2 Preparation process (1) Pretreatment of mullite fiber: Mullite fibers were first calcined at 600 ºC for 2 h in a high temperature furnace. The fibers were then put into ethanol, stirred, and separated using an electric mixer and an ultrasonic oscillator. Last, they were dried at 60 ºC for 24 h. (2) Sample preparation: According to equations (1), (2), and (3), the stoichiometric ratio of bauxite and zirconite was about 100:75.1; bauxite and zirconite were fully reacted to form the ZrO2-mullite composite. To discuss effects of mullite fiber, ratios of mullite fiber-to-premix binder were 1:2 (S1), 1:1 (S2), and 2:1 (S3), (Table 3). Also, 2.5 wt% Y2O3 was added to the bauxite and zirconite mixture in S2 to investigate effects of additive on the porous ceramic; this sample was labeled as S4. ZrSiO4(s) = ZrO2(s) + SiO2(s)

(1)

2SiO2(s) + 3Al2O3(s) = Al6Si2O13(s)

(2)

2ZrSiO4(s) + 3Al2O3(s) = 2ZrO2(s) + Al6Si2O13(s)

(3)

Table 3. Composition of raw materials of samples (wt%). Samples

Mullite fibers

Premix binder

Bauxite

Zirconite

Y2O3

S1

33.33

38.10

28.57

-

S2

50.00

28.57

21.43

-

S3

66.67

19.05

14.28

-

S4

50.00

27.14

20.36

2.50

(3) Sample treatment: First, monomer (AM) and crosslinking agent (MBA) (in a ratio of 29:1) were added to TBA to prepare a premixed liquid with a concentration of 20%. Citric acid was added as a dispersant, and prepared binders were also added. The mixture was wet-milled at a rate of 500 r/min for 5 h to obtain a stable uniform ceramic slurry. Second, treated fiber was added (in the required proportion) into a slurry containing the initiator and the catalyst. After mechanical stirring and ultrasonic shaking, a mold containing the slurry was put in a drying oven of 50 ºC to crosslink AM and MBA to prepare a uniform fiber ceramic body. Finally, the dried sample (with exhausted organic matter) was removed from the mold and sintered at 1600 ºC for 4 h with a heating rate of 4 ºC/min. 2.3 Characterization Field emission scanning electron microscopy (FE-SEM, Model Ultra Plus, ZEISS, Germany) with incidental X-ray energy dispersive spectroscopy (EDS, Oxford, UK) was used to detect the microstructure of the porous ceramic samples. Phase compositions were identified using X-ray diffraction (XRD, Model D500, Siemens) with Cu 𝐾𝛼 radiation, λ of 0.154056 nm, tube voltage of 40 kV, tube current of 40 mA, scanning speed of 8°/min, and scanning range of 5~90°. Using Archimedes’ principle, apparent porosity and bulk density were determined using water as the medium. The linear shrinkage ratio was calculated using the following equation: shrinkage = [(l1-l2)/l1]*100%, where l1 and l2 are the diameters of dried bodies and sintered

samples,

respectively.

Mercury

Porosimetry

(Autopore

IV9500,

Micromeritics Instrument Corp., USA) was used to examine the pore size distribution of samples, thermal-conductivity instrument (LFA-1000, Linseis/Laser Flash Series,

Germany) used to measure the thermal conductivity of samples with 25.4 mm diameter and 3 mm thickness, and hydraulic pressing (5015 type, China) used according to GB/T 5072-2008 to examine its compressive strength.

3. Results and discussion The XRD pattern of the porous ceramic prepared using the TBA-based gel-casting method was shown in Fig. 1, it can be seen that the phase composition of the sintered porous ceramic was mullite and m-ZrO2. This indicates that there was no ZrSiO4 phase in the porous ceramic because of complete decomposition of zirconite at a sintering temperature of 1600 ºC. The decomposition product was fully reacted and combined with bauxite to form the ZrO2-mullite composite. The values of Gibbs free energy for the corresponding reactions are shown in Table 4. When reactants and products are pure solids, their activity can be chosen as 1. According to principles of thermodynamics, smaller negative values of ∆G indicate that a reaction occurs more easily, and this is especially true for equation (3). In addition to composite, introducing a second phase of ZrO2 in the mullite ceramic can further enhance properties of porous ceramics.

Mullite





Intensity ( a.u. )



m-ZrO2

 



  



20

  



                           

40

60

80

2 (  ) Fig. 1. XRD pattern of the prepared porous ceramic.

Table 4. Gibbs free energy of equations (2) and (3).

∆G [kJ/mol]

Reactions T=1400K

T=1600K

T=1800K

T=2000K

Eq (2)

-20.01

-25.62

-31.4

-36.65

Eq (3)

-6191.87

-6348.2

-6516.26

-6709.25

Fig. 2 shows fracture micro-morphologies of samples S1-S3 at magnifications of both 500 times and 1000 times.

Fig. 2. SEM images of fractures for samples S1-S3: (a) S1×500, (b) S1×1000, (c) S2×500, (d) S2×1000, (e) S3×500, and (f) S3×1000.

As seen in Fig. 2, samples S1-S3 of the formed porous ceramics had almost the same microstructure, which was like a bird nest and was constructed from a large

number of interlaced mullite fibers. Intercrossed connections between fibers caused the prepared ceramic to have high porosity, and porosity increased with an increased amount of fibers because more adjacent fibers were attached to one another and produced more pores. Energy spectrum analysis results of point components in Fig. 2b are shown in Table 5. It was determined from calculation and analysis that compositions of Points 1 and 2 were mainly mullite and ZrO2, respectively. This indicates that the binder components were mainly gray mullite and white ZrO2, and these held the mullite fibers together to increase strength. Table 5. EDS microanalysis of points in Fig. 2. Position

Zr

Al

Si

O

Y

Fe

Ti

Point 1

0.00

38.13

13.29

48.58

0.00

0.00

0.00

Point 2

65.65

0.00

0.00

31.02

0.00

1.06

2.27

Hence, the porous fibrous ZrO2-mullite ceramic was successfully prepared, and its three-dimensional reticular skeleton structure was constructed from fibers that were connected and uniformly wrapped by rod mullite and ZrO2 grains, thus ensuring that the generated porous ceramic had high porosity and certain strength. These factors had effects on sintering behaviors and relative properties. Fig. 3 displays pore size distributions of the porous fibrous ZrO2-mullite ceramic samples with different amounts of mullite fiber. As seen in Fig. 3, pore size distributions of the porous ceramics all have a narrow single peak. Each peak was approximately between 3 μm and 5 μm, and the average pore sizes of the samples were all about 4.9 μm. This means that the pore size distribution of each sample did not show a significant numerical difference with an increase in porosity (Fig. 2), and differences in pore size distribution trends were very small. In general, different amounts of mullite fiber had little effect on pore size distribution. Thus, it is concluded that TBA-based gel-casting is a near net-shape forming method, and prepared samples did not easily shrink with the stable pore size after sintering.

Log differential pore volume ( ml / g )

S1 with 33.33% mullite fiber S2 with 50% mullite fiber S3 with 66.67% mullite fiber

12 10 8 6 4 2 0 1

10

100

Pore size ( m ) Fig. 3. Pore size distributions of porous fibrous ZrO2-mullite ceramic samples with different amounts of mullite fiber.

Fig. 4 shows changes in linear shrinkage ratio for samples S1-S3 sintered using the same heating system with different amounts of mullite fiber.

Linear shrinkage ratio ( % )

12.5 12.0

11.5 11.0 10.5 30

40

50

60

70

Amount of mullite fiber ( wt % ) Fig. 4. Changes in linear shrinkage ratio of porous fibrous ZrO2-mullite ceramic samples with different amounts of mullite fiber.

As shown in Fig. 4, it is clear that the linear shrinkage ratios of samples S1-S3 ranged from 10.52% to 12.30% and that the ratio decreased with an increase in the

amount of mullite fiber. In our preparation process, rigid three-dimensional reticular skeleton structure was formed with a large amount of mullite fibers. Also, fibers were combined via a binder of mullite and ZrO2, which effectively prevented shrinkage of samples during sintering. In addition, introduced ZrO2 with low modulus of elasticity had small deformation, resulting in low linear shrinkage ratio. Therefore, when more amounts of mullite fibers were added, the interlaced arrangement of fibers was more complex, formed structure was stronger, and linear shrinkage ratio was lower. Fig. 5 shows changes in apparent porosity and bulk density of porous fibrous ZrO2-mullite ceramic samples with different amounts of mullite fiber.

Apparent porosity Bulk density

1.2 1.0

76

0.8

72

0.6

68

0.4

3

80

Bulk density ( g / cm )

Apparent porosity ( % )

84

30

40

50

60

70

Amount of mullite fiber ( % ) Fig. 5. Changes in apparent porosity and bulk density of porous fibrous ZrO2-mullite ceramic samples with different amounts of mullite fiber.

As seen in Fig. 5, apparent porosity and bulk density of the porous ceramic ranged from 68.1-84.3% and from 0.49-1.09 g/cm3, respectively. Also, with an increasing amount of fiber, apparent porosity significantly increased and bulk density decreased, and this was consistent with the trend in linear shrinkage ratio. In this preparation process, added polycrystalline mullite fiber was composed of high melting point metal oxide microcrystals, which had stable crystal structure and high specific surface area. After sintering, a crisscrossed complex fibrous reticular skeleton structure was formed, and a greater amount of added mullite fiber led to

higher specific surface area for the porous ceramic. As a result, higher apparent porosity and lower bulk density were obtained, and this had great influence on performance in adsorption and catalysis applications. The thermal conductivity and compressive strength of porous fibrous ZrO2-mullite ceramic samples with different amounts of mullite fiber were shown in Fig. 6, the thermal conductivity and compressive strength of the samples decreased with an increasing amount of mullite fiber. Thermal conductivity ranged from 0.244-0.566

Thermal conductivity Compressive strength

0.64

6.0

0.56 4.5 0.48 3.0 0.40 1.5 0.32 0.0 0.24 30

40

50

60

Compressive strength ( MPa )

Thermal conductivity ( W / m )

W/m·K, and compressive strength ranged from 0.54-6.32 MPa.

70

Amount of mullite fiber ( % ) Fig. 6. Changes in thermal conductivity and compressive strength for porous fibrous ZrO2-mullite ceramic samples with different amounts of mullite fiber.

Generally, thermal conductivity of a material increases with its density, and thermal conductivity of a solid is always greater than that of a gas. In this preparation process, thermal conductivity of all samples was low, and this was because gas heat conduction in pores of the porous ceramic mainly determined heat conduction of the porous ceramic. The porous fibrous ZrO2-mullite ceramics have high porosity in the bird nest-like structure. Therefore, as the amount of mullite fiber was increased, higher porosity led to more gas in the pores used for heat conduction, and thus, lower thermal conductivity was obtained, which is beneficial for applications in high temperature thermal insulation.

When the ratio of mullite fiber-to-ceramic powder was 1:2, the binder that formed at 1600 ºC combined the mullite fibers, and there was a large contact area between fibers, resulting in relatively strong bonds. With an increase in the proportion of mullite fiber (Fig. 2), the amount of binder on fibers and the contact area between them were both reduced, and then the interfacial bonding between fibers became weak. This indicates that a decrease in the load area of the porous ceramic led to a decrease in mechanical strength. Moreover, pores in the ceramic could be considered as the source of cracking; this caused stress concentration and separation of the matrix material, which further affected mechanical properties.

Fig. 7. SEM images of (a) original mullite fibers and (b) mullite fibers in the fabricated porous ceramic after sintering at 1600 ºC.

However, compared to the smooth cylindrical original mullite fibers without sintering and bonding (Fig. 7a), the sintered mullite fibers shown in Fig. 7b had a rougher surface and larger contact area. Thus, the obtained porous fibrous ZrO2-mullite ceramic had relatively high compressive strength for applications, and this was attributed to the binder of the ZrO2-mullite composite formed from bauxite and zirconite, which played an important role in combining fibers. Generally, the system tends toward the direction of reduced free energy. In reaction, contact between ceramic powder and mullite fiber reduced the surface area of the generated mullite and mullite fiber, and reducing this surface area as much as possible decreased the free surface energy. The more that the free energy of the system was reduced, the more the contact there was between ceramic powder and mullite fiber, the larger that

the load area between fibers was, and the higher that the strength was [25-27]. Therefore, ceramic binder grew with mullite fiber as a crystallizing particle and then adhered to the fiber surface. Compared with direct contact between fibers, binder that formed on fiber could combine two fibers and increase contact area between fibers, and this improved compressive strength of the formed porous ceramic. On this basis, to further improve use performance of the porous ceramic prepared via TBA-based gel-casting, 2.5 wt% Y2O3 was added to mullite fiber and premixed binder, which were in a 1:1 ratio, and this sample was marked as S4 (Table 3). Y2O3 was used because of its extensive application in ceramic materials. The fabricated porous ceramics without (S2) or with (S4) added Y2O3 were compared, and the differences in properties are shown in Table 6. Table 6. Changes in properties of S2 and S4. Sample

Shrinkage

Porosity

Density

Thermal conductivity

Strength

(%)

(%)

(g/cm3)

(W/m·K)

(MPa)

S2

11.06

79.3

0.68

0.449

1.22

S4

22.41

69.9

1.02

0.489

5.12

From Table 6, it can be concluded that when 2.5 wt% Y2O3 was added to the raw materials, apparent porosity of the porous ceramic S4 was a little lower than that of S2 (without Y2O3). However, shrinkage ratio, bulk density, and thermal conductivity were higher than those of S2, and notably, compressive strength was greatly improved from 1.22 MPa (without Y2O3) to 5.12 MPa with Y2O3. This may be attributed to effects of added Y2O3 on phase composition and microstructure of the prepared porous ceramic. The XRD pattern of the porous fibrous ZrO2-mullite ceramic sample with added Y2O3 was shown in Fig. 8. The phase composition of the sample with added Y2O3 included the main crystal phase of mullite and m-ZrO2, and a small phase of t-ZrO2. Compared to the sample without Y2O3 (Fig. 1), for the sample with Y2O3, the diffraction peak corresponding to m-ZrO2 decreased and the diffraction peak

corresponding to t-ZrO2 was observed. It can be explained that doping with Y2O3 induced a martensitic transformation of zirconia from m-ZrO2 to t-ZrO2 at high temperature and that adding Y2O3 made the t-ZrO2 phase (which is stable at high temperature) also stable at normal temperature. Zirconia partially stabilized with Y2O3 could play an important role in strengthening compressive strength of porous fibrous ZrO2-mullite ceramic.

Mullite  m-ZrO2



Intensity ( a.u. )

 t-ZrO2  







 

 

20

 

 

 

            

  

40

60

80

2 ( ) Fig. 8. XRD pattern of porous fibrous ZrO2-mullite ceramic sample with added Y2O3.

Fig. 9 shows an SEM image and EDS element mappings of porous fibrous ZrO2-mullite ceramic with added Y2O3. Compared with the SEM image shown Fig. 2d, the SEM image in Fig. 9 for the sample with added Y2O3 shows that the bonding phase between mullite fibers was obviously denser and the porosity was smaller, which make the bonding more reliable and this were also consistent with the results given in Table 6. From EDS element mappings shown in Fig. 9, it is clear that ZrO2 and a yttrium-containing compound were wrapped around mullite fibers and were uniformly distributed in the porous ceramic.

Fig. 9. SEM image and EDS element mappings of porous fibrous ZrO2-mullite ceramic sample with added Y2O3.

EDS analysis of Point 3 from Fig 9 is shown in Fig. 10. Compared with Point 1 from Fig. 2, the main component of gray binder for Point 3 from Fig 9 was mullite, and there was also a small amount of yttrium-containing compound. At high temperature, a low melting point eutectic Y-Al-Si-O glass phase was formed to promote sintering of binder and to promote nucleation and precipitation of mullite, which reduced micro-pores in the fibrous skeleton structure and made bonding more dense [28].

Al

12000

Point 3

Intensity ( a.u.)

10000

Element

wt%

Atomic percentage

O

47.18

60.93

Al

37.32

28.58

Si

13.69

10.07

Y

1.81

0.42

O

8000 6000 4000

Si

Total amount: 100.00

100.00

2000 Y 0 0

2

4

6

8

Energy ( keV ) Fig. 10. EDS analysis of point 3 from Fig. 9 with added Y2O3.

4. Conclusions (1) Porous fibrous ZrO2-mullite ceramics were successfully prepared via TBA-based gel-casting with mullite fibers, zirconite, and bauxite as raw materials. A three-dimensional fibrous reticular skeleton structure that resembled a bird nest was constructed from combined mullite fibers and binder. (2) After sintering, the fabricated porous ceramic had a narrow pore size distribution and consisted of mullite and m-ZrO2. Values of linear shrinkage ratio and thermal conductivity decreased and apparent porosity increased with an increase in the amount of mullite fibers. Also, compressive strength was relatively great. (3) With the addition of 2.5 wt% Y2O3, a new phase of t-ZrO2 was formed, and compressive strength of the porous ceramic was obviously improved from 1.22 MPa to 5.12 MPa. This improvement was because of formation of a Y-Al-Si-O glass phase, which promoted sintering.

Acknowledgments The authors would like to express their gratitude for the financial support from the National Natural Science Foundation of China (Grant No. 51404056) and the Fundamental Research Funds for the Central Universities of China (Grant Nos.

N130402015).

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