Effect of Al2O3 addition on properties of non-sintered SiC–Si3N4 composite refractory materials

    Effect of Al2 O3 addition on properties of non-sintered SiC-Si 3 N4 composite refractory materials Jian Chen, Kai Chen, Yan-Gai Liu, Zhao-Hui Huang, Ming-Hao Fang, Jun-Tong Huang PII: DOI: Reference:

S0263-4368(14)00079-1 doi: 10.1016/j.ijrmhm.2014.05.001 RMHM 3796

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date: Revised date: Accepted date:

27 March 2014 26 April 2014 2 May 2014

Please cite this article as: Chen Jian, Chen Kai, Liu Yan-Gai, Huang Zhao-Hui, Fang Ming-Hao, Huang Jun-Tong, Effect of Al2 O3 addition on properties of non-sintered SiCSi3 N4 composite refractory materials, International Journal of Refractory Metals and Hard Materials (2014), doi: 10.1016/j.ijrmhm.2014.05.001

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ACCEPTED MANUSCRIPT Effect of Al2O3 addition on properties of non-sintered SiC-Si3N4 composite refractory materials

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Jian Chen, Kai Chen, Yan-Gai Liu, Zhao-Hui Huang, Ming-Hao Fang, Jun-Tong Huang School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, P.R. China

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Abstract

Preparation of SiC-Si3N4 composite refractory materials without sintering entails only low energy

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consumption and incurs little cost compared with traditional preparation methods. This paper investigated the effect of Al2O3 addition on bulk density, apparent porosity, linear shrinkage and oxidation resistance of as-fabricated non-sintered SiC-Si3N4 composite refractory materials. Meanwhile, the compressive and

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flexural strength both before and after heat treatment were analyzed. The mechanisms of oxidation

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resistance and cryolite resistance of the SiC-Si3N4 composite refractory materials are discussed. Increasing amounts of Al2O3 reduced linear shrinkage but oxidation resistance, and cryolite resistance.

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Moreover, compressive and flexural strength initially increased and then decreased, with maximum

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values achieved at an Al2O3 addition of 8% w/w. Keywords: Al2O3; non-sintered; SiC-Si3N4 composite; refractory; cryolite resistance

1. Introduction SiC-Si3N4 composite refractory materials are widely used in walls of blast furnaces, circulating fluidized bed boilers, and waste incinerators because of their inherent properties, such as high load softening point and resistance to creep, chemical attack, and thermal shock [1–2].



Corresponding author: Tel./Fax: +86-10-82322186

E-mail addresses: [email protected] (Y.G. Liu) 1

ACCEPTED MANUSCRIPT SiC-Si3N4 composite refractory materials are traditionally prepared by high-temperature nitridation reactions by using fine Si powders, Si3N4 powders, and SiC powders/particles as raw materials in a

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controlled nitrogen atmosphere furnace [3–5]. Such preparation is energy- and resource-intensive and potentially causes environmental pollution, thereby restricting its large-scale application. By contrast,

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non-sintered refractory materials are sintered in situ by surplus heat released from their industrial process applications, which avoid high-temperature nitridation reaction processing. Therefore, non-sintering

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methods are significant in refractory material development and energy conservation [6]. The effect of Al2O3 addition on the properties of composites has been widely reported [7–12]. Industrial-scale aluminum production is conducted through the Hall–Hèroult process, in which the

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aluminum oxide dissolved in a cryolitic-based electrolyte is reduced to metal at ~960 °C [13]. The

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chemical attack of cryolitic on the refractory lining influences cell efficiency and may terminate its life in extreme situations. The most important function of the refractory for aluminum cells is to act as a barrier

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against bath components and molten aluminum keep the insulation layer intact [14]. Therefore, the

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important properties of the refractory layer are chemical attack resistance, abrasive resistance, oxidation resistance, and compressive and flexural strength. Al2O3 is widely used as SiC-Al2O3 and Al2O3-Si3N4 matrix composites because of its high hardness, excellent chemical stability, oxidation resistance and abrasive resistance [15–18]. Al2O3 is dissolved by SiO2 glass oxidized from SiC and Si3N4, and forms a glass phase that shields against oxidation and corrosion during electrolysis process [19]. The silicate glass from surface oxidation changes from feldspar to mullite, which not only strengthens material bond strength but also decreases surplus liquid at high temperatures [20–21]. Allaire [22–23] stated that a high alumina/silica ratio (>0.9) refractory is preferred to minimize Na and NaF formation, which erode the composite. Therefore, Al2O3 is an ideal candidate for cryolite resistance. However, few studies have

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ACCEPTED MANUSCRIPT examined the effect of Al2O3 addition on the properties of SiC-Si3N4. Thus, the effect of Al2O3 addition on the properties of non-sintered SiC-Si3N4 composite refractory materials should be investigated.

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In this paper, SiC-Si3N4 composite refractory materials were prepared by using quartz, coke, and SiC as starting materials, powdered Al2O3 as additive, and phenolic resin and urotropin as complex adhesives.

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This paper studied the effects of Al2O3 dose on apparent porosity, bulk density, linear shrinkage, oxidation resistance, compressive strength, flexural strength, cryolite resistance, and microstructure. The

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mechanisms of oxidation resistance and cryolite resistance of the SiC-Si3N4 composite refractory materials are discussed.

2. Experimental

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2.1. Starting materials

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The primary raw materials used were α-SiC particles (SiC ≥ 98% w/w, grain size: 0.50 mm), α-SiC powders (SiC ≥ 97% w/w, grain size: 45 μm to 61 μm, Luoyang Refractory Research and Industry Trade

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Co., Ltd., China), α-Al2O3 powder (Al2O3 ≥ 99.9% w/w, grain size < 0.5 μm, Aluminum Corporation of

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China, Ltd.), and Si3N4 powders (grain size < 74 μm), which were synthesized by carbothermal reduction nitridation (CRN) of quartz and carbon coke at 1600 °C. A complex adhesive was formed from mixed phenolic resin and urotropin at a weight ratio of phenolic resin to urotropin of 8:1. The composition of the specimen is shown in Table 1.

2.2. Preparation of specimens In our previous studies [24–25], quartz and coke were used as raw materials to prepare high-purity Si3N4 powder by using CRN. SiC particles, which were the aggregate of the green body, were then mixed in the proportions mentioned in Table 1 and formed a frame of compound. The complex adhesive was

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ACCEPTED MANUSCRIPT slowly added. After a uniform mixture was produced, the mixtures, SiC, Si 3N4, and Al2O3 powders were inter-mixed and ball milled for 6 h. The second mixing caused the powders to uniformly disperse in the

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aggregate to increase stacking density. The powder mixtures were uniaxially compacted into cylindrical pellet specimens, which were further compacted by cold isostatic pressing at 200 MPa. These green

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specimens then were oven-dried at 100 °C for 24 h. The specimen height and diameter were both 20 mm

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for compressive strength tests and 3 mm × 4 mm × 40 mm bar samples for flexural strength tests.

2.3. Characterization

The apparent porosity and bulk density of specimens were determined by Archimedes’ method. The

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permanent linear change was tested via means of linear shrinkage after heat treatment at 1000 °C for 2 h.

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To assess the inoxidizability of the specimens in use, their oxidation loss rate was computed by comparing their masses before and after heat treatment at 1000 °C for 2 h. The compressive strength and

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flexure strength were measured by a microcomputer controlled hydraulic universal testing machine

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(WEW-50B, Longhua Instrument, China). To investigate the cryolite resistance of the as-prepared non-sintered SiC-Si3N4 refractory materials in industrial applications, we conducted static cryolite resistance tests on crucible specimens and analyzed their SEM micrographs and EDS spectra. The size of the crucible was Φ 50 mm × 50 mm with the Ф 20 mm × 25 mm of the inner bore, and the cryolite and Al2O3 as erosive media had masses of 6.0 and 1.2 g, respectively. The crystalline phases of synthesized products were examined by X-ray diffraction (XRD; D8 Advance diffractometer, Germany), using Cu-Kα1 radiation (λ = 1.5406 Å) with a step of 0.02° (2θ) and a scanning rate of 4 °·min-1. The microstructure and micro-area chemical analysis of the products were analyzed by scanning electron

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ACCEPTED MANUSCRIPT microscopy (SEM; JSM-6460LV microscope, JEOL, Japan) equipped with an energy dispersive spectrum

Results and Discussion

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(EDS; INCAx-sight, Oxford Instrument, UK).

The specimens’ relative bulk density and apparent porosity are shown in Fig. 1. Increased Al2O3

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addition reduced the bulk density and increased apparent porosity of the specimens. When 16% w/w

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Al2O3 was added, the bulk density reached its lowest value of 2.28 g·cm-3 and the apparent porosity peaked at 22.57 %, which can be explained from two aspects. On one hand, increasing Al2O3 addition reduced the interface contact area between SiC and Si3N4 but increased that between Al2O3 and SiC and

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Si3N4. Apparent porosity also increased because of weak activity between Al 2O3 and SiC-Si3N4 at low temperatures [26]. On the other hand, increasing Al2O3 addition significantly changed the

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aggregate/particle size ratio in the composite. On account of the SiC particles, the aggregate of the green

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body occupied a reduced percentage in the composite, and the stacking density of the composite rapidly reduced, especially with the addition of 4% w/w Al2O3 to 8% w/w. This result indicated that an increased

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Al2O3 adversely affects the product’s compactness. To further investigate the specimens’ performance, their microstructures were observed. Fig. 2 shows SEM micrographs of the surface of the specimens that contain different Al2O3 doses. As Fig. 2 shows, the extent to which particles combined in the specimens gradually reduced with increased Al2O3 dose. Figure 3 shows the variation in linear shrinkage and oxidation loss rate, with increased Al2O3 dose. In general, variations in linear shrinkage diminished, albeit inconspicuously, the minimum linear shrinkage was 4.2% in specimen A1 (4% w/w Al2O3). This result indicates that the linear shrinkage of the non-sintering SiC-Si3N4 refractory materials was low and compliant with typical industrial specifications.

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ACCEPTED MANUSCRIPT The oxidation loss rate reduced with increased Al2O3 dose, which implies that Al2O3 addition enhances oxidation resistance. SiC-Si3N4 refractory materials react with oxygen at high temperature,

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according to Eqs. (1)–(3). Pores in the matrix cause oxidation to occur mainly on the specimens’ surfaces and only to a lesser extent within the specimens’ interiors. In the first oxidation stage, the matrix was

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surface oxidized; subsequently, the interior was oxidized through open pores. The oxidation rate was then reduced as this oxide layer formed on the surface of the matrix, while the matrix was still oxidized

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through the diffusion of the oxidant. As Fig. 4 shows, a glass phase developed and blocked the direct route transmission of the oxidizing medium. The glass phase was a SiO2-based silicate glass, which formed as Al2O3 was dissolved by SiO2 glass according to Eqs. (4) and (5) [19]. Therefore, the oxidizing

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media only diffused through the glass phase. However, the oxygen diffusion coefficient was infinitesimal

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in SiO2-based silicate glass, and thus greatly retarded the oxidation rate. In addition, a sharp decrease in oxidation was observed from 12% to 16% w/w Al2O3 addition, which indicates that when the glass phase

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is sufficient, increasing with the amount of Al2O3 addition, the glass phase was changed to mullite in

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quantity [27]. The oxygen diffusion coefficient of mullite (1.5×10-11 cm2/s-1) was less than that of SiO2-based silicate glass (1.5×10-9 cm2/s-1) [26]. 2SiC + 3O2 → 2SiO2 + 2CO

(1)

SiC + O2 → SiO2 + CO2

(2)

Si3N4 + 3O2 → 3SiO2 + 2N2

(3)

2SiO2 + Al2O3 → 2SiO2·Al2O3

(4)

xSiO2 + yAl2O3 + z (2SiO2·Al2O3) → Silicate Glass

(5)

The compressive, and flexural, strengths of the specimen are shown in Fig. 5. The compressive strength increased with increased Al2O3 dose and this effect diminished beyond an Al2O3 dose of 8 % w/w.

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ACCEPTED MANUSCRIPT Meanwhile, the flexural strength increased initially with increasing Al2O3 dose, because Al2O3 has high hardness and is anisotropic, which enhanced its strength. Besides, as shown in Fig.2, when a small

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amount of Al2O3 dose was introduced, a compact structure was observed in specimen A1 and A2 [see Figs 2(a) and (b)], and the holes were relatively regular in shape in specimens A1 and A2, which indicates

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that the insetting and pulling out of large particles strengthened the specimen. However, the flexural strength reached its maximum value (9.10 MPa) at 8 % w/w Al2O3, and then decreased. It is due to the

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interface contact area between SiC and Si3N4 and the percentage of the aggregate in the green body were reduced when excessive Al2O3 doses were introduced, thereby reducing strength. Additionally, as the Figs 2(c) and (d) shows, excessive Al2O3 loosens the structure and decreases the insetting and pulling out

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of large particles. Therefore, aforementioned results indicate that introducing a small amount of Al2O3

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could enhance the strength and 8% w/w is the optimum Al2O3 dose. To evaluate the compressive and flexural strength after industrial application, the specimens were

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buried in carbon insulation at 1000 °C for 2 h, and their compressive and flexural strength was tested. As

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Fig. 6 illustrates, at an Al2O3 dose of 8 % w/w, compressive strength and flexural strength reached their peak values of 44.39 and 4.29 MPa respectively. In order to investigate the relationship between the Al2O3 dose and strength after heat treatment, the SEM micrographs of fracture morphology of specimens with different Al2O3 content after heat treatment at 1000 °C for 2 h is shown in Figure 6. It is obviously shows that Gum-like polymers in specimens A1, A2, and A3 connected the particles of different sizes and filled the interstices. The polymer was a reticulated carbide layer formed in the composite refractory materials after phenolic resin was carbonized during heat treatment; this polymer could improve the specimens’ stability. Although the polymer could also be found in specimens A1 and A3, it was sparsely distributed, whereas specimen A2 exhibited broader distribution and more dense carbon layers, and no

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ACCEPTED MANUSCRIPT polymer was found in specimen A4. Hence, with further increased Al2O3 dose, the strengths decreased to different extents.

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Fig. 7 shows the SEM micrographs and EDS spectra of the specimens after static cryolite resistance tests at 1000 °C for 2 h with 4%, 8%, 12%, and 16% w/w Al2O3 dose. The specimens’ EDS spectra were

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scanned from the surface into the interior of the crucible. The erosion of fluorine concentration in specimen A1 was 6.3% and that of sodium was 13.0%, whereas with increased Al2O3 content, erosion

sodium was small and only partial.

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resistance increased. At an Al2O3 dose of 16% w/w [see Fig. 7(d)], the degree of erosion of fluorine and

The XRD spectra of the specimen with 8% w/w Al2O3 addition are shown in Fig. 8, in which mullite

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could be found. Theory suggests that the silicate glass from surface oxidation was phase changed from

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feldspar to mullite [21,28], according to Eqs. (6) and (7). (6)

3(2Al2O3·3SiO2) → 2(3Al2O3·2SiO2) + 5SiO2

(7)

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2(Al2O3·2SiO2) → 2Al2O3·3SiO2 + SiO2

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The traditional erosion process of cryolite can be expressed by Eqs. (8) to (12). SiC + 3CO2 → SiO2 + 4CO

(8)

Si3N4 + 6CO2 → 3SiO2+ 2N2+6CO

(9)

2SiO2 + NaAlF4 → SiF4 + NaAlSiO4

(10)

2Si3N4 + 3NaAlF4 + 6CO2 → 3SiF4 + 3NaAlSiO4 + 6C + 4N2

(11)

2SiC + NaAlF4 + 2O2 → SiF4 + NaAlSiO4 +2C

(12)

This process mainly occurred from the surface to the interior between the specimen’s matrix and the eroding medium. The corrosion process and mechanism of SiC-Si3N4 refractory materials can be explained as follows:

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ACCEPTED MANUSCRIPT (1) The glass phase of the matrix was corroded by the erosion medium and reacted with a small quantity of Al2O3 from the medium, which would produce the feldspar glass phase with low viscosity and melting

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point [29]. Therefore, the influx of the erosive medium was difficult to resist, thereby reducing the SiC particles.

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(2) SiC and Si3N4 continually reacted with carbon dioxide produced from oxidation according to Eqs. (8)–(9) to produce SiO2 which was, in turn, corroded by the erosive medium. This corrosion led to further

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oxidation.

(3) SiO2 reacted with HF which escaped from the erosive medium to produce gaseous SiF 4 and led to further damage.

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Given its high melting point, mullite not only strengthened the material bond strength but also

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decreased surplus liquid at high temperatures. Moreover, Al2O3 reacted with SiO2 and NaF and produced

(14).

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nepheline (NaAlSiO4) when the weight ratio of Al2O3/SiO2 reached 0.9 [22], as shown in Eqs. (13) and

Na3AlF6 + 3Na → 6NaF + Al

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(13)

4NaF + 2Al2O3 + 5SiO2 → 4NaAlSiO4 + SiF4

(14)

The as-form nepheline obstructed the open pores and reduced the erosion of Na and NaF. Fig. 9 indicates that the melting temperature of the ternary system of Na2O-Al2O3-SiO2 overstepped to 732 °C when the weight ratio of Al2O3/SiO2 exceeded 0.85 [23]. Therefore, the infiltrate curdled on the surface and stopped continually permeating. Al2O3 is also oxidation- and cryolite-resistant. Thus, it effectively reduces oxidation and erosion rates. Therefore, the addition of Al2O3 enhanced cryolite resistance. We compare the properties of carbon refractory, SiC-Si3N4 composite refractory, and non-sintered SiC-Si3N4 composite refractory, as shown in Table 2. The properties of non-sintered SiC-Si3N4 composite

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ACCEPTED MANUSCRIPT refractory are not as good as those of SiC-Si3N4 composite refractory [30], whereas surpass carbon refractory which is widely used in aluminum oxide reduction cells [31–32]. Therefore, the composite

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refractory in this work has potential application in the industrial aluminum electrolytic cells.

Conclusions

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This paper revealed the effect of added Al2O3 dose on the properties of non-sintered SiC-Si3N4 composite

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refractory materials. With increased Al2O3 dose, linear shrinkage of the SiC-Si3N4 composite refractory materials decreased, whereas their oxidation resistance increased. In addition, flexural strength initially increased and the specimens reached their peak (9.1 MPa) at 8% w/w Al2O3 dose, and then decreased.

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Compressive strength increased with increased Al2O3 content. When the increase was reduced beyond an Al2O3 dose of 8 % w/w, the maximum value reached 68.22 MPa. After heat-treatment, the compressive,

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and flexural, strengths peaked at 44.39 and 4.29 MPa respectively at 8 % w/w Al2O3 dose. Cryolite

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resistance exhibited an opposite tendency with increasing Al2O3 dose. Cryolite erosion was barely noticeable until 16% w/w Al2O3 addition. Considering these results, Al2O3 addition enhanced the strength,

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oxidation resistance, and cryolite resistance of the as-fabricated non-sintered SiC-Si3N4 composite refractory materials, and the optimum Al2O3 dose was 8% w/w. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51032007), the Program for New Century Excellent Talents in University of Ministry of Education of China (Grant No. NCET-12-0951) and the Fundamental Research Funds for the Central Universities (Grant No. 2652013040).

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Tables

SiC Si3N4 0.061mm

32

20

A2

32

20

A3

32

20

A4

32

20

28

20

adhesive

4

10

28

20

8

10

28

20

12

10

28

20

16

10

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A1

0.045mm

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0.5mm

Complex

Al2O3

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Specimens

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Table 1 Proportions of the specimens (% w/w).

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Table.2 Physical properties of carbon, SiC-Si3N4 and non-sintered SiC-Si3N4 composite refractory Compressive

Flexural

Linear

porosity(%)

(g/cm3)

strength(MPa)

strength(MPa)

shrinkage(%)

≤24

≥1.52

≥32

≥5

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Bulk density

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Carbon *1

Apparent

SiC-Si3N4

18~22

2.6~2.75

150

40

0.47

non-sintered

22.52~24

2.18~2.28

35~68.2

4.5~9.1

0.41~1.12

SiC-Si3N4 *1 Measured by Chinese metallurgy Industrial Standard (YB/T 2805-2006)

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ACCEPTED MANUSCRIPT Figure captions

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Figure 1 Effect of Al2O3 addition on bulk density and apparent porosity of specimens.

Figure 2 SEM micrographs of specimens with different amounts of Al 2O3: (a) A1, (b) A2, (c) A3, (d) A4.

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Figure 3 Effect of Al2O3 addition on linear shrinkage and oxidation loss rate of specimens after heat-treatment at 1000oC for 2h.

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Figure 4 SEM of the surface of the specimen with 8 % w/w Al2O3 addition after oxidation. Figure 5 Effect of Al2O3 addition on compressive strength and flexure strength of specimens. Figure 6 SEM micrographs of specimens with different amounts of Al 2O3 after heat-treatment: (a) A1, (b) A2, (c) A3,

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(d) A4.

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Figure 7 SEM micrographs and EDS spectra of the specimens after static cryolite resistance experiment with

different amounts of Al2O3: (a) A1, (b) A2, (c) A3, (d) A4.

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Figure 8 XRD pattern of the specimen with 8 % w/w Al2O3 addition.

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Figure 9 Partial phase diagram of Na2O-Al2O3-SiO2 ternary system.

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Figure 1

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Figure 3

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ACCEPTED MANUSCRIPT Graphical Abstract Manuscript Title: Effect of Al2O3 addition on properties of non-sintered SiC-Si3N4

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composite refractory materials

Authors: Jian Chen, Kai Chen, Yan-gai Liu, Zhaohui Huang, Minghao Fang, Juntong

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ACCEPTED MANUSCRIPT Research Highlights 1. The non-sintering SiC-Si3N4 refractories with Al2O3 addition were prepared.

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2. The Al2O3 additive benefited to enhance compressive and flexure strength. 3. The oxidation and cryolite resistance of refracteries were improved by adding Al2O3.

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4. The oxidation and cryolite resistance of matrix was analyzed comprehensively.

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