Trans. Nonferrous Met. Soc. China 22(2012) 129-133

Effects of synthesis temperature and raw materials composition on preparation of β-Sialon based composites from fly ash

MA Bei-yue, LI Ying, YAN Chen, DING Yu-shi

School of Materials and Metallurgy, Northeastern University, Shenyang 110819, China

Received 30 October 2010; accepted 27 May 2011

Abstract: β-Sialon based composites were successfully prepared from fly ash and carbon black under nitrogen atmosphere by carbothermal reduction-nitridation process. Effects of heating temperature and raw materials composition on synthesis process were investigated, and the formation process of the composites was also discussed. The phase composition and microstructure of the composites were characterized by X-ray diffraction and scanning electronic microscopy. The results show that increasing heating temperature or mass ratio of carbon black to fly ash can promote the formation of β-Sialon. The β-Sialon based composites can be synthesized at 1723 K for 6 h while heating the sample with mass ratio of carbon black to fly ash of 0.56. The as-received β-Sialon in the composites exists as granular with an average particle size of 2-3 μm. The preparation process of β-Sialon based composites includes the formation of O′-Sialon, X-Sialon and β-Sialon as well as the conversion processes of O′-Sialon and X-Sialon to β-Sialon.

Key words: Sialon; composites; carbothermal reduction-nitridation process; fly ash; synthesis temperature; raw materials composition

1 Introduction

Sialon includes β-Sialon (Si_6-zAl_zO_zN_8-z, 02-xAl_xO_1+xN_2-x, 0xSi_12-(m+n)Al_m+nO_nN_16-n, 0 pyrophyllite [8], bauxite [9], zeolite [10], coal gangue [11, 12] and rice husks [13].

Fly ash emerges as a by-product from the combustion of raw coal in thermal power plants. Until 2020, the present accumulating amount of fly ash in China is approximately three billion tons. The mass waste causes serious environmental problems. Thus, the effective and comprehensive utilization of waste fly ash can provide an important route to solve these problems. Meanwhile, the high value-added products with low cost such as mullite and mullite-zirconia-corundum composites can be produced using fly ash [14, 15].

In the present work, the β-Sialon based composites were synthesized by carbothermal reduction-nitridation process, with fly ash and carbon black as raw materials. The effects of heating temperature and raw materials composition on synthesis process were investigated, and the formation process of the composites was also discussed.

2 Experimental

2.1 Raw materials

Fly ash (mesh size≤74 μm) and carbon black (mesh size≤30 μm) were used as raw materials, and the chemical composition of fly ash (mass fraction) was Al₂O₃ 41.20%, SiO₂ 48.49%, Fe₂O₃ 3.37%, CaO 3.31%, TiO₂ 1.30% and MgO 0.20%. In Fig. 1 the crystalline phases of fly ash mainly include mullite as well as small amounts of corundum (Al₂O₃) and quartz (SiO₂). In addition, the mass fraction of C in carbon black and the volume fraction of nitrogen gas (N₂) were 98.0% and 99.99%, respectively.

Fig. 1 XRD pattern of fly ash

2.2 Preparation of samples

The overall chemical reaction equation for synthesizing β-Sialon from fly ash by carbothermal reduction-nitridation process can be expressed as

Al₆Si₂O₁₃(s)+4SiO₂(s)+15C(s)+5N₂(g)=

    2Si₃Al₃O₃N₅(s)+15CO(g)                   (1)

According to reaction (1), the mass ratio of carbon black to fly ash (m_c/m_f) was 0.25. Fly ash was mixed with carbon black in m_c/m_f of 0.28, 0.30, 0.42 and 0.56. This over-stoichiometry of carbon black was necessary to promote the carbothermal reduction-nitridation reaction. Mixing was performed in a ball mill with anhydrous ethanol for 12 h, and then the mixed powders were pressed to form the samples with 20 mm in diameter and 10 mm in thickness under a pressure of 20 MPa. Then the formed samples were dried fully at 393 K and put into a graphite crucible. The crucible was placed in an atmosphere-controlled tubular furnace and then heated up to 1673 K and 1723 K for 6 h, respectively. During the synthesis process, the N₂ flow remained at 1.0 L/min. After the predetermined temperature and time reached, the system was cooled to room temperature.

2.3 Characterization of samples

The samples with different m_c/m_f values synthesized at 1673 and 1723 K were oxidized in air at 973 K for 2 h to remove residual carbon. The phase composition and microstructure of the products were characterized by X-ray diffractometer (XRD, Cu K_α radiation, 30 kV and 30 mA), scanning electronic microscopy (SEM) and energy dispersive spectrum (EDS).

3 Results and discussion

3.1 Phase composition and microstructure

Figure 2 shows the XRD patterns of the samples with different m_c/m_f values synthesized at 1673 K for 6 h. With increasing m_c/m_f from 0.28 to 0.30 (Figs. 2(a) and (b)), the diffraction intensities of mullite and O′-Sialon phases weaken gradually, and the diffraction intensities of β-Sialon and corundum phase strengthen. This indicates that increasing carbon content in a sample can promote the carbothermal reduction-nitridation reaction. The samples with m_c/m_f of 0.28 and 0.30 all consist of mullite and O′-Sialon. In Fig. 2(c), β-Sialon can be detected in the sample with m_c/m_f of 0.42, and the sample is composed of mullite, O′-Sialon and β-Sialon. In   Fig. 2(d), corundum can be detected in the higher carbon content sample (m_c/m_f=0.56), and the sample mainly includes mullite, corundum, O′-Sialon and β-Sialon.

Fig. 2 XRD patterns of samples with different m_c/m_f values synthesized at 1673 K for 6 h: (a) 0.28; (b) 0.30; (c) 0.42;    (d) 0.56

Figure 3 shows the XRD patterns of the samples with different m_c/m_f values synthesized at 1723 K for 6 h. With increasing m_c/m_f from 0.28 to 0.56, the diffraction intensities of mullite and X-Sialon phases weaken gradually, and the diffraction intensities of β-Sialon and corundum phase strengthen. In Figs. 3(a) and (b) the samples with m_c/m_f of 0.28 and 0.30 all consist of mullite, corundum and X-Sialon. β-Sialon can be detected in the sample with m_c/m_f of 0.42 (Fig. 3(c)), and the mullite phase vanishes completely, showing that the sample is composed of X-Sialon, β-Sialon and corundum. In Fig. 3(d) the diffraction intensity of X-Sialon in the sample with m_c/m_f of 0.56 weakens remarkably, and the main crystalline phase is β-Sialon. Thus, increasing carbon content in a sample can promote the decomposition of mullite and the formation of β-Sialon.

Figures 4(a) and (b) show the SEM images of the samples with m_c/m_f of 0.56 synthesized at 1673 K and 1723 K for 6 h, respectively. The particles in the composites exist as granular. The average particle size synthesized at 1673 K and 1723 K reaches 1-2 μm (Fig. 4(a)) and 2-3 μm (Fig. 4(b)), respectively. EDS analysis (Fig. 4(c)) indicates that the particle (point 1) as shown in Fig. 4(b) is composed of Si, Al, O and N elements. By combing with XRD pattern (Fig. 3(d)), point 1 is β-Sialon.

Fig. 3 XRD patterns of samples with different m_c/m_f values synthesized at 1723 K for 6 h: (a) 0.28; (b) 0.30; (c) 0.42;    (d) 0.56

Fig. 4 SEM images of samples with m_c/m_f of 0.56 synthesized at 1673 K (a) and 1723 K (b), and EDS pattern of point 1 in  Fig. 4(b)

3.2 Analysis of formation process

The carbothermal reduction-nitridation reaction process for synthesizing β-Sialon based composites from fly ash is very complex and involves many chemical reactions. During the synthesis process, CO, SiO, Si₂N₂O, Si₃N₄ and AlN are regarded as important intermediate products. With changing heating temperature and raw materials composition, O′-Sialon, X-Sialon and β-Sialon may be formed.

SiO₂(s)+C(s)=SiO(g)+CO(g)                   (2)

2SiO(g)+C(s)+N₂(g)=Si₂N₂O(s)+CO(g)          (3)

2SiO₂(s)+3C(s)+N₂(g)=Si₂N₂O(s)+3CO(g)        (4)

3SiO(g)+3C(s)+2N₂(g)=Si₃N₄(s)+3CO(g)         (5)

3SiO₂(s)+6C(s)+2N₂(g)=Si₃N₄(s)+6CO(g)        (6)

3Si₂N₂O(s)+3C(s)+N₂(g)=2Si₃N₄(s)+3CO(g)      (7)

Al₂O₃(s)+3C(s)+N₂(g)=2AlN(s)+3CO(g)         (8)

1.83Si₂N₂O(s)+0.17Al₂O₃(s)=2Si_1.83Al_0.17O_1.17N_1.83(s)(9)

Al₂O₃(s)+Si₃N₄(s)+AlN(s)=Si₃Al₃O₃N₅(s)        (10)

1/2Al₆Si₂O₁₃(s)+2SiO₂(s)+15/2C(s)+5/2N₂(g)=

Si₃Al₃O₃N₅(s)+15/2CO(g)                 (11)

0.17/6Al₆Si₂O₁₃(s)+5.32/3SiO₂(s)+5.49/2C(s)+

1.83/2N₂(g)=Si_1.83Al_0.17O_1.17N_1.83(s)+5.49/2CO(g)(12)

Al₆Si₂O₁₃(s)+SiO₂(s)+3C(s)+N₂(g)=

Si₃Al₆O₁₂N₂(s)+3CO(g)                   (13)

The Gibbs free energies of formation of Al₆Si₂O₁₃, SiO₂, Al₂O₃, CO, O′-Sialon and β-Sialon are listed in Table 1 [16, 17]. When the partial pressure of N₂ is equal to p^Q(0.1 MPa), the Gibbs free energies for reactions (11) and (12) as well as the relationship between partial pressure of CO gas (p_CO)and temperature can be further obtained. It is noted that the Gibbs formation free energy of O′-Sialon (x=0.17) is lack, so it is approximately equal to that of O′-Sialon (x=0.2) during the process of calculating relational thermodynamics data.

,

    lg(p_CO/p^Q)=5.88-10637.65/T               (14)

,

lg(p_CO/p^Q)=5.59-6446.46/T                (15)

Table 1 Gibbs free energies of formation of some compounds in Al₂O₃-SiO₂-C-N₂ system

Figure 5 shows the predomination region diagram for Al₂O₃-SiO₂-C-N₂ system plotted by the thermodynamic data as shown in Eqs. (14) and (15). It indicates that when the partial pressure of CO gas (p_CO) in the reacting furnace remains constant, with increasing heating temperature, C in a sample can react with SiO₂, Al₂O₃ and N₂ to form O′-Sialon (reactions (2)-(4), (9), (12)) and further produce β-Sialon (reactions (2), (5)-(8), (10), (11)). The predomination regions change from Al₂O₃(s) + SiO₂(s) + C(s) to Al₂O₃(s) + O′-Sialon(s) + C(s) and Al₂O₃(s) + β-Sialon(s) + C(s), respectively. When the temperature remains constant, with decreasing p_CO, the predomination regions change from Al₂O₃(s) + SiO₂(s) + C(s) to Al₂O₃(s) + O′-Sialon(s) + C(s) and Al₂O₃(s) + β-Sialon(s) + C(s), respectively. Thus, proper partial pressure of CO gas and temperature are key factors for the synthesis of β-Sialon based composites.

During the carbothermal reduction-nitridation reaction process, X-Sialon can be formed (reaction (13)). With increasing synthesis temperature and carbon content, X-Sialon and O′-Sialon can be nitridized to produce β-Sialon (reactions (16) and (17)). This can be confirmed by the XRD patterns as shown in Figs. 2   and 3.

Si₃Al₆O₁₂N₂(s)+3SiO₂(s)+12C(s)+4N₂(g)=

2Si₃Al₃O₃N₅(s)+12CO(g)                  (16)

Si_1.83Al_0.17O_1.17N_1.83(s)+0.83Al₂O₃(s)+1.83C(s)+ 0.61N₂(g)

=0.61Si₃Al₃O₃N₅(s)+1.83CO(g)           (17)

Additionally, Al₂O₃ and SiO₂ in fly ash can also react with C and N₂ to form O′-Sialon, X-Sialon and β-Sialon (reactions (18)-(20)):

0.17Al₂O₃(s)+3.66SiO₂(s)+5.49C(s)+1.83N₂(g)=

2Si_1.83Al_0.17O_1.17N_1.83(s)+5.49CO(g)          (18)

3Al₂O₃(s)+3SiO₂(s)+3C(s)+N₂(g)=

Si₃Al₆O₁₂N₂(s)+3CO(g)                   (19)

3Al₂O₃(s)+6SiO₂(s)+15C(s)+5N₂(g)=

2Si₃Al₃O₃N₅(s)+15CO(g)                  (20)

Fig. 5 Predomination region diagram for Al₂O₃-SiO₂-C-N₂ system (=0.1 MPa)

The formation process of β-Sialon based composites can be summarized as follows.

1) In a sample with lower carbon content (m_c/m_f= 0.28-0.42), O′-Sialon and X-Sialon can be fabricated at 1673 K and 1723 K, respectively. And they can be further converted into β-Sialon by increasing synthesis temperature and carbon content.

2) In a sample with higher carbon content (m_c/m_f= 0.56), β-Sialon can be directly produced at 1673 K and 1723 K.

4 Conclusions

1) The β-Sialon based composites can be successfully prepared from fly ash by carbothermal reduction-nitridation process, and the proper technological parameters are m_c/m_f of 0.56 and heating temperature of 1723 K.

2) The as-received β-Sialon exists as granular, and the average particle size of β-Sialon is 2-3 μm.

3) The preparation processes of β-Sialon based composites includes the formation of O′-Sialon, X-Sialon and β-Sialon as well as the conversion process of O′-Sialon and X-Sialon to β-Sialon.

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合成温度和配料组成对粉煤灰制备β-Sialon基复合材料的影响

马北越，厉 英，闫 晨，丁玉石

东北大学 材料与冶金学院，沈阳 110819

摘  要：以粉煤灰和炭黑为原料，采用碳热还原氮化法成功制备出β-Sialon基复合材料。研究了加热温度和配料组成对合成过程的影响，分析了材料的生成过程。采用XRD和SEM手段表征了合成材料的相组成和显微结构。结果表明：升高加热温度，增大炭黑与粉煤灰的质量比均可以促进β-Sialon的生成；将炭黑与粉煤灰质量比为0.56的试样加热至1723 K并保温6 h，可以合成β-Sialon基复合材料；合成材料中β-Sialon多以粒状形式存在，平均粒径为2~3 μm；β-Sialon基复合材料的生成过程包括O′-Sialon、X-Sialon和β-Sialon的生成及O′-Sialon和X-Sialon向β-Sialon的转化过程。

关键词：赛隆；复合材料；碳热还原氮化法；粉煤灰；合成温度；配料组成

(Edited by LI Xiang-qun)

Foundation item: Project (51074038) supported by the National Natural Science Foundation of China; Project (N100302002) supported by the Fundamental Research Funds for the Central Universities, China

Corresponding author: LI Ying; Tel: +86-24-83688995; E-mail: liying@mail.neu.edu.cn; beiyue_ma@yahoo.com.cn

DOI: 10.1016/S1003-6326(11)61151-5