Preparation of corundum-mullite refractories with lightweight, high strength and high thermal shock resistance

Journal Pre-proof

Preparation of corundum-mullite refractories with lightweight, high strength and high thermal shock resistance Yongqiang Chen , Guoqi Liu , Qiang Gu , Sai Li , Bingbing Fan , Rui Zhang , Hongxia Li PII: DOI: Reference:

S2589-1529(19)30313-8 https://doi.org/10.1016/j.mtla.2019.100517 MTLA 100517

To appear in:

Materialia

Received date: Accepted date:

7 July 2019 19 October 2019

Please cite this article as: Yongqiang Chen , Guoqi Liu , Qiang Gu , Sai Li , Bingbing Fan , Rui Zhang , Hongxia Li , Preparation of corundum-mullite refractories with lightweight, high strength and high thermal shock resistance, Materialia (2019), doi: https://doi.org/10.1016/j.mtla.2019.100517

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Preparation of corundum-mullite refractories with lightweight, high strength and high thermal shock resistance Yongqiang Chen*a,b, Guoqi Liub, Qiang Gua, Sai Lia, Bingbing Fana, Rui Zhanga, Hongxia Lia,b a. School of Materials Science and Engineering, Zhengzhou University, Henan 450001, PR China b. Sinosteel Luoyang Institute of Refractories Research Co., Ltd., Henan 471039, PR China

Graphical Abstract

Abstract: The influences of PMMA sphere, hollow SiO2 sphere, hollow corundum sphere and AlF3·3H2O on properties of corundum-mullite refractories have been investigated. The finite-element analysis was used to simulate thermal stress of the samples in the process of thermal shock. The PMMA, hollow SiO2 and hollow corundum were used to form different 

Corresponding Author. E-mail address: [email protected]. Tel: +86-0379-64205801.

E-mail address: [email protected] Tel: +86-0379-64206330. 1

pore structures. The AlF3·3H2O was used as a reaction medium to promote the mullite whiskers formation. F2 was formed by oxidation reaction between AlF3 and O2 which could react with edges of fused mullite aggregate besides corundum and silica. SEM results showed that much mullite whiskers formed in the contact area of matrix powder and fused mullite aggregate. Bridging mullite whiskers of in-situ formation between matrix and aggregates strongly improved strength performance and thermal shock resistance of corundum-mullite materials. The closed pores from hollow corundum sphere were beneficial to thermal shock resistance. The high open porosity after PMMA burning out is adverse to in-situ formation of mullite whisker. Hollow corundum sphere, AlF3·3H2O and suitable grain composition worked together providing corundum-mullite refractories of lightweight, high strength and high thermal shock resistance. Key words: Corundum-mullite; High thermal shock resistance; Finite-element analysis; characteristic; Whiskers in-situ formation.

2

Pore

1. Introduction Corundum-mullite refractories are widely applied to high temperature kiln furniture, such as push board, refractory slab, saggars, crucibles and rollers due to excellent high temperature strength performance [1, 2]. At present, the porosity and fracture strength of corundum-mullite refractories used as kiln furniture usually are below 15% and 15 MPa respectively [3-5]. Besides, the sintering temperature of corundum-mullite materials usually is around 1700 oC [6-8]. The development of modern high temperature industry has put forward higher requirements on long life and low energy consumption for refractories. So, it is very necessary to develop the corundum-mullite materials with lightweight, high strength and high thermal shock resistance by lower sintering temperature. However, the lightweight and low sintering temperature are adverse to high strength[2]. Although the corundum-mullite porous ceramics only consisting of fine powder have the higher fracture strength after low temperature about 1500oC, the fracture toughness is relatively poor due to dense combination of fine powder after sintering[9-12]. The adding or in-situ forming mullite-whiskers has been adopted to enhance the toughness of corundum-mullite materials[13-16]. Nevertheless, the effect of adding low-dimensional mullite-whiskers is not very good due to poor dispersion and joining in the matrix[16-19]. The in-situ forming of mullite whiskers have been explored by adding inducer such as V2O5, MnO2 and AlF3 [10, 19-21]. However, these methods only are reported on the basis of activity fine powders matrix. In addition, although the in-situ forming mullite-whiskers can improve the fracture toughness, the thermal shock resistance of corundum-mullite refractories is not still obviously enhanced [16, 20, 22, 23]. There are two reasons worth concern. The first, the content of forming mullite whisker is limited due to dense matrix after fine powder sintered. Meanwhile, 3

the relative dense ceramics are also adverse to thermal shock resistance. The second, although the high porosity corundum-mullite ceramics provide enough space for mullite-whiskers, the whiskers can not generate effective bridge between matrix. As we all know, the gas-gas reaction is the best approach to forming longer mullite-whiskers in corundum-mullite materials by the addition of AlF3·H2O[20, 23]. It should de noticed that high porosity reduces the reaction gas concentration which leads to limited generation of mullite-whisker. So, the synergy of light weight, high strength and high thermal shock resistance for corundum-mullite refractories still was a challenge by low temperature sintering. The corundum-mullite refractories usually are composed of aggregate and matrix[21, 22]. The aggregate is beneficial to thermal shock resistance. However, the exist of aggregate increases the sintering temperature due to low sintering activity. The bonding mode of aggregate and matrix is very important for mechanical properties which still lacks intensive research. The AlF3·H2O can promote mullite-whisker formation and generate network structure improving fracture properties and thermal shock resistance by the reaction (AlF3-Al2O3 and AlF3-SiO2)[24, 25]. The porosity structure can affect concentration and diffusion velocity of reaction gas (AlOF, SiF4 and O2). Meanwhile, the addition of AlF3·3H2O can influence the bonding characteristic of aggregate and matrix, thus improving the fracture behavior of corundum-mullite materials. Consequently, it is necessary to research porosity characteristic, binding mode of aggregate and matrix and forming process of mullite-whisker in order to optimize integrated performance of corundum-mullite refractories. In the present work, the influence of pore-forming materials, ratio of aggregate to matrix and aluminum fluoride on structure and properties of corundum-mullite refractories was 4

investigated. The aluminum fluoride was introduced to boundary area of aggregate and matrix by preprocess of aggregate. The thermal shock resistance mechanism was sufficiently researched by SEM analysis. According to related results, the optimistic scheme was gained to realize light weight, high strength and high thermal shock resistance for corundum-mullite refractories.

2. Materials and methods 2.1 Raw materials Commerial tabular alumina (Qingdao Almatis Co., Ltd.), fused mullite (Kaifeng Hecheng Special Refractory Materials Co., Ltd.), reactive alumina (Jinan Yongsheng Special Refractory Materials Co., Ltd.), and Elken 983 silica fumes were used as raw materials. The characterization of raw materials are shown in Table 1. AlF3·3H2O (Density=2.88g·cm-3, Sinopharm Chemical Reagent Co., Ltd.) was analytically pure. The PMMA (polymethyl methacrylate) sphere, hollow SiO2 sphere and hollow corundum sphere were used to form different pore structures. Thermoplastic phenolic resin were used as binders (Shengquan chemical industry Co. Ltd, China). The SEM of PAAM(a), hollow SiO2 sphere(b) and tabular corundum(c) can be seen in Fig. 1. Table 1 Chemical composition (wt%) and particle size of raw materials Al2O3

SiO2

Fe2O3

K2O

Na2O

Size

Tabular corundum (TC)

99.43

0.02

0.05

0.02

0.30

1-3mm, 0.5-1mm, 45μm

Fused mullite (FM)

75.64

23.29

≤0.1

-

≤0.26

1-3mm, 0.5-1mm, 45μm

α-Al2O3

99.7

0.08

0.03

-

0.07

D50=2μm

SiO2

0.20

98.0

0.05

-

0.30

<0.2μm

5

2.2 Preparation of mullite-corundum samples The AlF3·3H2O and aggregates were premixed with phenolic resin as binder in a high-speed mixer for30 min. Then, the mixed powders were dry at 110 oC for 24h. Subsequently, the pre-mixed powders and related matrix fine powders were mixed for 1h. The content of tabular corundum, AlF3·3H2O, pore-forming agents were adjusted according to related experiment design. The mixtures were compressed into rectangular samples in stainless-steel die of 150×25×25mm with the pressure of 80 MPa. The green samples were sintered with the heating rate of 2°C/min and dwelling at related temperature for 3h in an electric furnace. 2.3 Characterization of mullite-corundum samples The bulk density and apparent porosity of the samples were measured according to GB/T2997-2000. The flexural strength was measured according to GB/T3001-2007 at room temperature. The high temperature fracture strength was measured according to GB/T3002-2004 at 1400 oC for 30 min and loading rate of 0.15 MPa/s. The thermal shock resistance of the samples were evaluated by quenching samples in 0.1MPa compressed air keeping 30min (ΔΤ=1100°C, 3 cycles). The samples were put in 1100 oC electric furnace keeping 30min. Then, the samples were taken out and put under 0.1 MPa compressed air keeping 30min. The residual fracture strength of samples was measured to evaluate the thermal shock performance. The thermal expansion coefficient was measured according to YB/T 5205-93. The thermal conductivity was measured YB/T 059-94 with a plane table thermo-conductivity meter. The phase compositions of samples were characterized by X-ray diffractometry (XRD, X'Pert Pro, Philips, Netherlands) in a 2θ range from 10o to 80o with a scanning speed of 2°/minute, using Cu-Kα radiation (λ =1.542Å), operating at 40kV and 40mA. The microstructures of samples 6

were observed by a scanning electron microscope (SEM, JSM-6610, ZESS, BRUKER Company, Germany) equipped with energy dispersive X-ray spectroscopy (EDS, QUANTAX200-3, BRUKER Company, Germany). 2.4 Finite-element analysis Finite element analysis is used for thermal stress numerical simulation that was combined with experimental results, in order to validate the related conclusion. The samples mainly was composed of corundum, mullite, pore and slight glass phase. The 3D solid size was set for 125×25×25mm. The mesh and nodes number of the overall model is 29,988 and 141,885 respectively. The thermal radiation and thermal convection were considered. The thermal stress of Y-axis have little effect on the samples. In order to simplify analysis, only the normal stress on the X-axis and Z-axis was output. According to YB/T376.2-1995, the temperature of furnace chamber was set at 1100oC holding 30 min, then the samples is cooled in room temperature.

3. Results and discussion 3.1 Influence of tabular corundum on mullite-corundum samples The tabular corundum is beneficial for the thermal shock resistance due to micro-pore structure. The influence of tabular corundum (1-3mm) on mullite-corundum materials was researched. Table 2 shows the chemical composition and particle size of different samples. Table 2 Chemical composition (wt%) and particle size of different samples. FM

FM

FM

TC

TC

TC

α- Al2O3

SiO2

S1

1-3mm 10

0.5-1mm 20

45μm 5

1-3mm 10

0.5-1mm 20

45μm 5

D50=2μm 20

<0.2μm 10

S2

5

20

5

15

20

5

20

10

S3

0

20

5

20

20

5

20

10

The samples (S1, S2 and S3) have the same ratio of aggregates and matrix. Firstly, the effect of sintering temperature was studied taking S1 as the object. Fig 1 shows the properties of 7

S1 samples after sintered at different temperature. As seen in Fig. 2 (a,b), the 1550 oC S1 has the highest bulk density (BD) and fracture strength (FS). This results indicates that the S1 samples have been sintered well. The BD and FS of 1650 oC S1 decrease due to the bulk expansion caused by excess mullite generation[26]. Taking 1550 oC and 1650 oC as sintering temperature, these samples ( S1, S2 and S3) with different tabular corundum content were researched. Fig. 3 shows the properties of S1, S2 and S3 samples. The BD and AP (apparent porosity) of S1, S2 and S3 samples are similar as seen in Fig. 3(a). Comparing 1550 oC samples with 1650 oC samples, the variation trend of the FS is same as shown in Fig. 3(b). The 1550 oC was considered as the optimum sintering temperature. The Fig. 3(c) shows the FS and R-FS ( residual-fracture strength) of 1550 oC samples. The S2 sample has the highest FS and R-FS. The suitable content tabular corundum benefited indeed the thermal shock resistance of corundum-mullite refractories. It is noteworthy that the excess tabular corundum is not beneficial to fracture strength due to lots of closed pores in its inner. So, the chemical composition of S2 was used as the basic formula in the following research. 3.2 Influence of AlF3·3H2O and pore-forming agents Corundum-mullite samples with different additives are shown in Table 3. Table 4 shows the properties of different corundum-materials after 1550 oC sintering. The FS, R-FS and RR-FS of N2 sample are 12.90MPa, 8.4MPa and 65.12% respectively as seen in Table 4. The addition of AlF3·3H2O obviously enhanced the fracture strength and thermal shock resistance. It can be explained that the AlF3·3H2O can promote the formation of mullite whisker by gas-solid and gas-gas reaction [27-29]. The related reaction processes are listed as following: AlF3∙3H2OAlF3+3H2O (1) AlF3+H2OAlOF+2HF (2) SiO2+4HFSiF4+2H2O (3) 8

6AlOF+2SiF4+7H2O3Al2O3∙2SiO2+14HF (4) 2AlF3+O22AlOF+2F2 (5) Al2O3+F22AlOF+0.5O2 (6) SiO2+2F2SiF4+O2 (7) 6AlOF+2SiF4+3.5O23Al2O3∙2SiO2+7F2 (8)

The reactions (1-4) mainly happen at the low temperature stage. The reactions (5-8) mainly happen at the high temperature stage. The bridging of mullite whiskers could reinforce the bonding strength of matrix and aggregates. Meanwhile, the whiskers with outstanding mechanical strength and resilience could effectively consume thermal stress, thus, enhancing thermal shock resistance of corundum-mullite samples. In addition, the related gases would discharge from the sample in the process of solids translated into gases. So, the apparent porosity of N2 sample is higher than that of N1 sample as shown in Table 4. Comparing N3 with N1, the H-A reduces the FS. However, the RR-FS reachs 95.67%. In addition, the BD of N3 sample are obviously lower than that of N1(decreasing for 7.89%) while the AP of N1 and N3 has small difference (2.45%). There are two reasons. The first, the added H-A equivalently increased the aggregates content accordingly weakening the bonding strength of matrix. The second, the hollow structure increased the closed porosity content that might benefit thermal shock resistance of corundum-mullite materials, meanwhile, the hollow-Al2O3 with closed pore caused BD obviously decreasing and AP slight increasing. The H-A and AlF3·3H2O worked together providing corundum-mullite refractories with high apparent porosity and low bulk density. It is speculated that the hollow Al2O3 is corroded by F2 as the reaction (5). So, the closed pore became to open pore. The FS and R-FS of N4 are higher than that of N3 due to the addition of AlF3·3H2O. The RR-FS of N4 is higher than that of N2, but lower than that of N3. This results indicate that, to a certain extent, increasing porosity is beneficial to improving thermal shock resistance, and real pores are more beneficial to thermal shock resistance. The hollow SiO2 was used to increase the porosity of corundum-mullite materials. Meanwhile, the 9

α-Al2O3 active powder also was added for decreasing the relative content of SiO2. The FS and AP of N5 samples are higher than that of N3 samples. However, the R-FS and RR-FS of N5 samples are lower. The results reveals that the addition of SiO2 is adverse to improve thermal shock resistance. The FS, R-FS and RR-FS of N6 samples are higher than that of N5 samples due to the addition of AlF3·3H2O. The effect of PMMA was related to N7 and N8 samples. The PMMA was burnt out leading to highest AP. So, the FS of N7 samples was lowest. Although the highest content AlF3·3H2O was added, the FS of N8 samples was still very low. The N8 samples with 47.43% AP have the 75% RR-FS which is lower than 80.73% RR-FS of N4 samples. This phenomenon further indicates that the lots of open pores are adverse to thermal shock resistance of corundum-mullite materials. The high temperature fracture strength of all samples has the same variation trend as room fracture strength. The H-FS of N5 sample has obvious reduction compared with FS. This results indicate that SiO2 can damage H-FS. Table 3 Chemical composition (wt%) of corundum-mullite samples N1

N2

N3

N4

N5

N6

N7

N8

S2

100

100

100

100

100

100

100

100

AlF3·3H2O H-A H-S PMMA α-Al2O3

0 0 0 0 0

+5 0 0 0 0

0 +20 0 0 0

+5 +20 0 0 0

0 0 +5

+5 0 +5

+10

+10

+10 0 0 +10 0

+15 0 0 +10 0

H-A: Hollow Al2O3 sphere（0.1-1mm); H-S: Hollow SiO2 sphere ( D50=50μm); PMMA: Polymethyl methacrylate sphere. Table 4 Properties of corundum-mullite samples after sintering at 1550oC

FS/MPa H-FS/MPa R-FS/MPa RR-FS/% BD/kg.cm-3 AP/%

N1 11.77 8.47 2.64 22.43 2.79 17.41

N2 12.90 10.06 8.4 65.12 2.62 19.77

N3 7.16 5.79 6.85 95.67 2.57 19.86

N4 8.72 7.15 7.04 80.73 2.37 27.24

N5 9.21 5.34 2.44 26.49 2.39 26.27

N6 9.54 5.82 3.28 34.38 2.31 29.43

N7 2.74 2.33 1.95 71.17 1.91 43.04

N8 3.08 2.67 2.31 75 1.76 47.43

FS:Fracture strength; H-FS: High temperature fracture strength; R-FS: Residual fracture strength; RR-FS: Residual ratio of fracture strength; BD: Bulk density; AP: Apparent porosity. 10

Fig. 4 shows the XRD patterns of different corundum-mullite samples (N1-N8). The all samples were composed of Al2O3 and mullite that could be confirmed by XRD patterns from Fig. 4. The Al2O3 content of N3 and N4 samples is higher than that of N1 and N2 which is correlated to the addition of hollow Al2O3 spheres. The diffraction peak at (40o-45o) obviously reveals the relative content of Al2O3 and mullite. It is should be noticed that the N8 sample with 15% AlF3·3H2O has the highest relative mullite content. This result further indicates that the AlF3·3H2O is more beneficial to the formation of mullite instead of forming Al2O3. So, the Al2O3 content of N6 sample is lower than that of N5 sample due to the addition of AlF3·3H2O according to the diffraction peak at (25o-30o). In general, the in-situ formation of mullite is through the solid-solid reaction of alumina and silica. Therefore, the alumina and silica which are away from contact area don’t react with each other. However, the AlF3 presence changed the reaction process. According to the reactions (5-8), the gas phase intermediates broke the reaction limit of directly contact for alumina and silica. So, the mullite content increased obviously. The SEM and EDS of fracture surface for N1 (a) and N2 (b) sample are shown in Fig. 5. There are clear differences in the contact form of aggregates and matrix for N1 and N2 samples. Fig. 5(a) shows that there are clear boundaries between aggregate and matrix as labeled by red circle. It is different from Fig. 5(a) that the Fig. 5(b) has a transition region between aggregate and matrix. In addition, the network structures formed by whiskers are found at the transition region from Fig. 5(b). The EDS shows that the aggregate is mullite. The mullite whiskers formed effective bridging between mullite aggregate and matrix. Because of the gas phase reaction, the mullite whisker nucleated on mullite aggregate surface, then grew to the matrix. The matrix fine powders could be sintered effectively at lower temperature. So, the in-situ formation mullite whiskers through gas-gas reaction enhanced the bonding strength of aggregate and matrix.

Meanwhile, the sintering temperature also can be decreased. The synergetic 11

enhancement mechanism of FS and RR-FS of corundum-mullite materials further was confirmed by SEM results. In order to further explore the bonding mode of aggregate and matrix, the different magnification morphologies were observed by SEM. Fig. 6 shows the SEM images for N3 (a, c, e) and N4 (b, d, f) sample. The number 1, 2, and 3 refer to mullite aggregate, matrix and tabular corundum respectively as shown in Fig. 6(c). The three morphologies are very different and can be easily distinguished. The tabular corundum contains a lot of holes which is beneficial to thermal shock resistance. Comparing Fig. 6(a,c) with Fig 5(b,d), the much micro cracks are observed in Fig. 6(a, c). Besides, the obvious boundaries can be found between aggregate and matrix as seen Fig. 6(c) labeled by red circle and Fig. 6(e). There are no obvious cracks and boundaries observed in Fig. 6 (b,d and f). The cracks were formed due to the sintering shrinking of matrix fine powders. Of course, if the aggregate was surrounded by a mass of fine powders, the bonding of aggregate and matrix will become more compact after sintering. This situation is correlated to weak thermal shock resistance (RR-FS, 22.43%) of N1 samples. The N4 sample has better bonding between aggregate and matrix due to the addition of AlF3·3H2O. The gas phase reactants (AlOF, SiF4 and O2) broke the distance limit of solid phase transmission. Meanwhile, the nucleation at the surface of aggregate became the basis for solid phase reaction. In fact, the micro cracks are useful for improving thermal shock resistance by dispersing thermal stress. However, the micro cracks also can reduce the strength. So, the N3 samples had the higher RR-FS and lower FS compared with N4 samples. It should be noticed that the R-FS of N4 samples was still higher than that of N3 samples. This results reveal that the AlF 3 can simultaneously enhance the strength and the thermal shock resistance of corundum-mullite refractories. Fig. 7 shows the morphologies of N3(a) and N4(b) sample after thermal shock. The aggregates and matrix were partly destroyed with a large number of cracks as seen in Fig 6(a). 12

Compared with Fig. 6(a), the cracks content of Fig. 7(a) is more which is correlated to FS reduction of N3 sample after thermal shock. Comparing the change of Fig. 6(a) to Fig. 7(a) with Fig. 6(b) to Fig. 7(b), the cracks increase extent of N4 sample is obviously higher than that of N3 sample. So, the RR-FS of N3 sample is higher than that of N4 sample as shown in Table 4. The pore content of Fig. 7(b) is higher than that of Fig. 7(a). Meanwhile, the cracks content of Fig. 7(b) is lower than that of Fig. 7(a). The reason was that the AlF3·3H2O caused gas phase generation which partly escaped from the materials leaving pores. These pores were beneficial to thermal shock resistance. Fig. 8 shows the SEM of fracture surface and etched polished section about the N5 sample. The short columnar particles were found in Fig. 8(a). This can be explained that the addition of SiO2 increased the low temperature glass phase, then, the glass phase became liquid which promoted the columnar grain forming and growing. The aggregates, matrix and glass phase tended to form a whole as seen from Fig. 8(a). So, the R-FS of N5 sample sharply decreased due to broken glass phase after thermal shock. Fig. 8(b) reveals that the glass phase is corroded leaving many pores in the matrix. So, the hollow SiO2 is not suitable for increasing porosity of corundum-mullite refractories. Fig. 9 shows the SEM images of N7(a, c, e) and N8(b, d, f) samples after 1550 oC sintering. Comparing Fig. 9(a) with Fig. 9(b), the mullite whiskers of N7 sample are shorter and less than that of N8 sample. This phenomenon was related to the reaction gases (AlOF, SiF4) concentration. The PMMA agents were burn out leaving lots of open pores. Then, the reaction gases ran away through these open pores that lowed the gases concentration. So, the formation of mullite whiskers was limited. The increasing of AlF3 content could increase the reaction gases concentration, thus, promote the generation and growth of mullite whisker as seen in Fig. 9 (b). 13

The polished fracture surfaces of N7 sample and N8 sample are Fig. 9 (c, e) and Fig. 9( d, f) respectively. From Fig. 9(b), the interlaced whiskers can be faintly observed that is correlated with higher fracture strength of N8 samples. In addition, there are more small particles in N8 samples as seen in Fig. 9(f). The result further indicates that the AlF3 can react with big aggregates which is related to Fig. 5. In order to further illustrate the effect of gas concentration, the mullite whiskers characteristics of N8 and N2 samples are presented in Fig. 10. The mullite whiskers of N2 sample grow well than that of N8 samples. It should be noticed that the added AlF3·3H2O content（+5%）of N2 sample was obviously less than that of N8 sample (+15%). However, the higher apparent porosity of N8 sample reduced the reaction gases concentration limited the generation of mullite whiskers. 3.3 Integrative optimization of mullite-corundum composites. According to the above analysis, the hollow Al2O3 sphere was the best pore forming agent to ensure strength and thermal shock resistance. So, the effect of hollow Al2O3 sphere on thermal physics properties of mullite-corundum materials was investigated. Meanwhile, the finite element analysis was used to simulate the temperature change and thermal stress in the process of thermal shock resistance. Based on S2 from Table 2, the different content hollow were extra added in S2 basic formula. The related samples were signed as T1 (+5% hollow Al2O3 spheres) and T2 ( +60% hollow Al2O3 spheres). Fig. 11 shows the thermal physics properties of T1 and T2 samples. The thermal expansion coefficient of T2 sample is higher than that of T1 sample due to increased Al2O3 content. The high thermal expansion coefficient is adverse to thermal shock resistance. The thermal conductivity of T2 sample is lower that of T1.

14

In general, the low thermal conductivity is also adverse to thermal shock resistance. The heat transfer ways of ceramics materials mainly are crystal lattice vibration and thermal radiation. So, the thermal conductivity of T1 and T2 samples increased with the increasing of temperature. It should be noticed that the specific heat capacity of T2 sample is higher than that of T1 sample. This result is attributed to high porosity of T2 sample. The specific heat capacity of atmosphere is higher than that of solids. The thermal diffusivity of T2 sample has little change with temperature increasing. It could be explained that the atmosphere in closed porosity absorbed heat increasing internal energy due to specific heat capacity increasing of atmosphere in the process of temperature rise. In addition, the thermal transfer between gas and solid is weaker that of solid. So, the low thermal conductivity caused by high porosity have little effect on the thermal shock resistance of ceramics materials. Therefore, the thermal expansion coefficient and strength of corundum-mullite materials should be controlled by the reasonable addition of hollow-Al2O3 sphere. Fig. 12 shows the temperature changes of T1 and T2 samples in the process of thermal shock by finite element analysis. When the samples were set in 1100 oC electric furnace, the maximum temperature and minimum temperature were simulated in the samples. The Max and Min temperatures of T1 and T2 samples reach the same at 1350 s and 1650 s respectively. The difference value of the Max temperature and Min temperature of T1 sample was lower than that of T2 samples when the samples were rapidly removed to air from electric 1100 oC furnace. At 3000 s, the T2 sample still had temperature difference. This results indicated that the T2 sample had high thermal stress in the process of thermal shock resistance. Fig. 13 shows the simulated thermal stress of the samples in thermal shock process. The stress caused by quick heating is lower than the stress caused by quick cooling for T1 and T2 15

samples. For T1 sample, the maximum tensile stress in the direction of X and Z are16.42 MPa and 11.28 MPa respectively in the process of rapid heating, and the maximum tensile stress in the direction of X and Z are 19.89 MPa and 19.66 MPa respectively in the process of cooling. The stress of T1 sample is lower than that of T2 sample at each stage. The simulation results indicate that the excess hollow-Al2O3 sphere is adverse to thermal shock resistance of corundum-mullite materials. According to above analysis, the different content hollow Al2O3 sphere and 10wt% AlF3·3H2O were extra added into S2 basic formula. Fig 13 shows the thermal shock resistance characteristic of different samples after 1550 oC sintering. The sample with 5wt% hollow Al2O3 has the highest residual fracture strength and residual ratio. In addition, the residual FS ratio decreases with the increasing of hollow Al2O3 sphere. This experiment results are consistent with simulation results. The residual FS decreases and then increases with hollow Al2O3 content increasing. It is equivalent to increasing the particle material in the sample and reducing the bonding strength between particles and matrix after hollow Al2O3 addition. However, the hollow Al2O3 sphere has higher sintering activity than aggregates. So, the extra hollow Al 2O3 can increase bonding strength, thus increase residual fracture strength.

4. Conclusion The corundum-mullite refractories with excellent comprehensive performance were successful prepared by controlling the in-situ formation of mullite whiskers and pore characteristic. The closed pores are more conducive to realize synergistic improvement of light weight, high strength and high thermal shock resistance. The finite element analysis and experiment results confirmed that the suitable content hollow Al2O3 was beneficial to thermal shock resistance and fracture strength of corundum-mullite materials. Meanwhile, the simulation results indicated that the cold-shock produced higher thermal stress compared with heat-shock. The AlF3·3H2O could change the bonding mode of matrix and aggregates 16

improving thermal shock resistance of corundum-mullite refractories. Acknowledgements This work was sponsored by the National Natural Science Foundation of China (NSFC) (51932008 and 51772277). National Key R＆D Program of China (2017YFB0304000). The authors would say thanks for the support. The authors declare that they have no conflict of interest.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References [1] C. Sadik, I.-E. El Amrani, A. Albizane, Recent advances in silica-alumina refractory: A review, Journal of Asian Ceramic Societies, 2 (2014) 83-96. [2] A.V. Maldhure, H.S. Tripathi, A. Ghosh, Mechanical Properties of Mullite–Corundum Composites Prepared from Bauxite, International Journal of Applied Ceramic Technology, 12 (2015). [3] X. Kong, Y. Tian, Y. Chai, P. Zhao, K. Wang, Z. Li, Effects of pyrolusite additive on the microstructure and mechanical strength of corundum–mullite ceramics, Ceramics International, 41 (2015) 4294-4300. [4] M.I. Osendi, C. Baudín, Mechanical properties of mullite materials, Journal of the European Ceramic Society, 16 (1996). [5] G. Chen, J. Chen, X. Zhi, C. Srinivasakannan, J. Peng, Synthesis and Microwave Absorption Characteristics of Corundum-Mullite Refractories, Refractories And Industrial Ceramics, 55 (2014) 231-235. [6] C. Aksel, The effect of mullite on the mechanical properties and thermal shock behaviour of alumina–mullite refractory materials, Ceramics International, 29 (2003) 183-188. [7] T. Takei, Y. Kameshima, A. Yasumori, K. Okada, Crystallization kinetics of mullite from Al2O3–SiO2 glasses under non-isothermal conditions, Journal of the European Ceramic Society, 21 (2001) 2487-2493. [8] J.M. Amigó, F.J. Serrano, M.A. Kojdecki, J. Bastida, V. Esteve, M.M. Reventós, F. Martí, X-ray diffraction microstructure analysis of mullite, quartz and corundum in porcelain insulators, Journal of the European Ceramic Society, 25 (2005) 1479-1486. [9] I. Zake-Tiluga, R. Svinka, V. Svinka, Highly porous corundum–mullite ceramics – Structure and properties, Ceramics International, 40 (2014) 3071-3077. [10] G. Xu, Y. Ma, G. Ruan, H. Cui, Z. Zhang, B. Bai, Preparation of porous Al2TiO5 ceramics reinforced by in situ formation of mullite whiskers, Materials & Design, 47 (2013) 57-60. 17

[11] Z. Hou, B. Zhang, R. Zhang, L. Liu, Fabrication of highly porous mullite microspheres via oil-drop molding accompanied by freeze casting, Ceramics International, 43 (2017) 8809-8812. [12] N. Li, X.-Y. Zhang, Y.-N. Qu, J. Xu, N. Ma, K. Gan, W.-L. Huo, J.-L. Yang, A simple and efficient way to prepare porous mullite matrix ceramics via directly sintering SiO2-Al2O3 microspheres, Journal of the European Ceramic Society, 36 (2016) 2807-2812. [13] B. Meng, J. Peng, Effects of in situ synthesized mullite whiskers on fracture strength and fracture toughness of corundum-mullite refractory materials, Ceramics International, 39 (2013) 1525-1531. [14] X. Wang, A. Guo, J. Liu, Y. Wang, J. Liu, H. Du, F. Hou, Effects of in situ synthesized mullite whiskers on compressive strength of mullite fiber brick, Ceramics International, 42 (2016) 13161-13167. [15] K.K. Chawla, Interface engineering in mullite fiber/mullite matrix composites, Journal of the European Ceramic Society, 28 (2008) 447-453. [16] B. Meng, J. Hui Peng, Optimization on Influencing Factors of Fracture Toughness of Corundum-Mullite Toughened by In Situ Synthesized Mullite Whiskers by Response Surface Methodology, 2012. [17] D.-J. Zeng, Y.-M. Zhang, J.-F. Yang, B. Wang, J. Matsushita, Fabrication and Property of Mullite Whiskers Frameworks with an Ultrahigh Porosity by Expandable Mesocarbon Microbeads, Journal Of the American Ceramic Society, 99 (2016) 2226-2228. [18] T. Takei, Y. Kameshima, A. Yasumori, K. Okada, Crystallization kinetics of mullite in alumina-silica glass fibers, Journal Of the American Ceramic Society, 82 (1999) 2876-2880. [19] H.J. Choi, J.G. Lee, Synthesis of mullite whiskers, Journal Of the American Ceramic Society, 85 (2002) 481-483. [20] X. Chen, T. Li, Q. Ren, X. Wu, H. Li, A. Dang, T. Zhao, Y. Shang, Y. Zhang, Mullite whisker network reinforced ceramic with high strength and lightweight, Journal of Alloys and Compounds, 700 (2017) 37-42. [21] C. Aksel, The role of fine alumina and mullite particles on the thermomechanical behaviour of alumina–mullite refractory materials, Materials Letters, 57 (2002) 708-714. [22] I. Zake-Tiluga, V. Svinka, R. Svinka, L. Grase, Thermal shock resistance of porous Al2O3-mullite ceramics, Ceramics International, 41 (2015) 11504-11509. [23] L. Xu, X. Xi, W. Zhu, A. Shui, W. Dai, Investigation on the influence factors for preparing mullite-whisker-structured porous ceramic, Journal of Alloys and Compounds, 649 (2015) 739-745. [24] Z. Zhu, Z. Wei, J. Shen, L. Zhu, L. Xu, Y. Zhang, S. Wang, T. Liu, Fabrication and catalytic growth mechanism of mullite ceramic whiskers using molybdenum oxide as catalyst, Ceramics International, 43 (2017) 2871-2875. [25] T.V. Vakalova, V.M. Pogrebenkov, I.B. Revva, P.G. Rusinov, D.I. Balamygin, Effect of fluorine-containing additive on the synthesis and sintering of compositions from natural raw materials in the “cordierite –mullite” system, Ceramics International, 45 (2019) 9695-9703. [26] Z. Zhu, Z. Wei, J. Shen, Z. Li, X. Lei, Y. Zhang, W. Shuai, L. Tong, Fabrication and catalytic growth mechanism of mullite ceramic whiskers using molybdenum oxide as catalyst, Ceramics International, 43 (2016) S0272884216320247. [27] K. Okada, N. Ōtsuka, Synthesis of mullite whiskers by vapour-phase reaction, Journal of Materials Science Letters, 8 (1989) 1052-1054. [28] R.R. Monteiro, A.C.S. Sabioni, Preparation of mullite whiskers derived from topaz doped with rare earth oxides for applications in composite materials, Ceramics International, 42 (2016) 49-55. 18

[29] M. Rashad, M. Balasubramanian, Characteristics of porous mullite developed from clay and AlF 3 ·3H 2 O, Journal of the European Ceramic Society, (2018) S0955221918301274.

Fig. 1 SEM of PMMA, SiO2 hollow sphere and Tabular corundum.

19

Fig. 2.

Properties of S1 samples after sintered ( 1350 oC, 1450oC, 1550oC and 1650oC).

20

Fig. 3. Properties of S1, S2 and S3 samples after sintered ( 1550 oC and 1650 oC).

Fig. 4 XRD patterns of different corundum-mullite samples after sintering at 1550oC. 21

Fig. 5 Fracture morphology and EDS of N1 sample (a) and N2 sample (b).

Fig. 6 SEM images of N3(a,c,e) and N4(b,d,f) samples.

22

Fig. 7 SEM images of N3(a) and N4(b) samples after thermal shock.

Fig. 8 SEM images of N3 sample (a-fracture surface) and (b-polished surface).

23

Fig. 9 SEM images of N7 (a, c, e) and N8 (b, d, f) samples.

Fig. 10 Mullite whiskers SEM Images of N8 (a) and N2 (b) samples.

24

Fig. 11 Thermal physics properties of T1 and T2 samples.

Fig. 12 Temperature changes simulation of T1 and T2 samples in the process of thermal shock.

25

Fig. 13 Simulated thermal stress of T1 and T2 samples in thermal shock process.

Fig. 14 Thermal shock resistance characteristic of corundum-mullite samples with different hollow Al2O3 content.

26