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WITHDRAWN: Microstructure and reactivity evolution of colloidal silica binder in different systems at elevated temperatures
Journal Pre-proof Microstructure and reactivity evolution of colloidal silica binder in different systems at elevated temperatures Jiancheng An, Yuping Wang, Quanli Jia, Fei Zhao, Xinhong Liu PII:
S0272-8842(20)30546-0
DOI:
https://doi.org/10.1016/j.ceramint.2020.02.220
Reference:
CERI 24443
To appear in:
Ceramics International
Received Date: 14 January 2020 Accepted Date: 22 February 2020
Please cite this article as: J. An, Y. Wang, Q. Jia, F. Zhao, X. Liu, Microstructure and reactivity evolution of colloidal silica binder in different systems at elevated temperatures, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2020.02.220. 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. © 2020 Published by Elsevier Ltd.
Microstructure and reactivity evolution of colloidal silica binder in different systems at elevated temperatures Jiancheng An1,2#, Yuping Wang1#, Quanli Jia1, Fei Zhao1*, Xinhong Liu1* 1
Henan Key Laboratory of High Temperature Functional Ceramics, Zhengzhou University, 75 Daxue Road, Zhengzhou 450052, China
2
Tongda Refractory Technologies, Co. LTD, 1Anningzhuang East Road, Haidian District, Beijing
100085, China *Corresponding authors. Tel: +86-371-67761908, Fax: +86-371-67763822 # The authors contributed equally to this work. E-mail address: [email protected] (F. Zhao), [email protected] (X. Liu) Abstract: To find a suitable application system for colloidal silica, phase and microstructure evolution of the dried silica gel with and without carbon inclusion were investigated after heated at different temperature, and reactivity of silica gel and alumina fines was also investigated. The results showed that atmosphere and carbon inclusions only slightly affected the phase composition of the silica gel at elevated temperatures, and the crystalline phases were composed of major α-cristobalite and minor α-tridymite. However, the morphology and particle size of the silica gel were greatly affected by atmosphere and carbon inclusions during heating. The spherical nano-silica particles with sizes of 40-50 nm grew rapidly to macroscale rod-like particles with temperature rise from 800-1000°C to above 1200°C in air, and the sintering of silica particles was observed. The reactivity decreased significantly, and no mullite was formed when the silica gel and alumina fines were heated at 1400°C, and only some mullite formed at 1500°C in air. However, the size and morphology of the spherical nano-silica particles remained almost unchanged on heating the silica gel and carbon in a reducing atmosphere; further, many developing well mullite crystals and some SiC whiskers were formed at 1500°C. Carbon inclusions retarded the growth of nano-silica particles and promoted the formation of SiC whiskers at high temperature. Thus, colloidal silica is suitable for application in carbon-containing refractory castables. Key words：Colloidal silica, reactivity, microstructure, sintering, particle size
1.
Introduction Refractory castables are universally used in high-temperature industries because of their unique
advantages, such as energy saving, environment friendliness, good performance, easy installation, and cost effectiveness [1,2]. Binding systems are very important for refractory castables, because the binders are closely related to achieving high-quality refractory castables with a long service life. Cement is a conventional binder and has been used in refractory castables for a long time, as it provides castables with good installation property and cold strength. However, the low melting point phases (e.g., anorthite and gehlenite) can form because of CaO in cement reacting with other components (such as microsilica) in 1
castables, leading to the low melting point phases (e.g., anorthite and gehlenite) formation, which degrades the high temperature performance of the castables. CaO in cement also easily reacts with Al2O3 in castables to form calcium dialuminate (CA2) and calcium hexaluminate (CA6) that cause volumetric expansion and generate many cracks at high temperature [3-7]. Therefore, the use of cement as a binder is usually formed detrimental components in refractory castables at high temperature, it is unsuitable for application in some high temperature critical areas. In recent years, hydratable alumina, colloidal silica, and colloidal alumina as new cement-free binders have been adopted by refractory castables [8-13]. Among these binders, colloidal silica has attracted considerable attentions from researchers, manufacturers, and consumers. Because colloidal silica has a high solid content (15–50 wt% nano-silica) and the nano-sized SiO2 can fill packing gaps, increasing the densification of castables [14,15]. Moreover, incorporating colloidal silica leads to forming mullite phase at low temperatures for alumina-rich castables [16]. However, the current research on colloidal silica is focused on its effects on the mixing behavior [17], rheological behavior [18], setting mechanism [19], green mechanical properties [20,21], and high-temperature properties of castables [9,22-24]. Less research has been focused on phase composition and microstructure evolution, and reactivity of silica gel particles came from colloidal silica in different systems at elevated temperatures. Different phases of silica may confer different properties to refractory castables, and the particle size of silica greatly affects the reactivity of silica and properties of the castables. Previous researches have showed that nano oxide particles easily undergo abnormal growth and partial sintering at high temperature in oxidizing atmosphere, and that the particle size remains in the nanoscale at high temperatures in the presence of carbon in a reducing atmosphere [25-27], but some nano particles easily decompose into other phases in a reducing atmosphere [27]. If nano-silica particles grow into micro- or macroscale particles at high temperature, their high reactivity decreases, which may be detrimental to the filling effect, sinterablity and mullite formation at lower temperature, and the cost-effectiveness of the nano particles will be disappeared. If nano silica particles transform into other substances, the properties of the castables will be greatly affected. However, there are few reports on changes in silica gel in different systems at elevated temperatures. In addition, there are diversified research results on mullite formation temperature in refractory castables with colloidal silica as binder. Chen [28] prepared mullite precursor mixture powder by mixing alumina powder and silica sol, and the initial mullite formation temperature was 1250 . J.Q. Xiong et al 2
[29] addressed that the introduced silica sol could react with alumina to form mullite at about 1100°C, leading to the suitable densification of materials. However, some studies show that silica sol combined with alumina cannot produce mullite at low temperature. The research results of Zhigang Li et al [30] showed that mullite formed at 1547°C by heating mixture of silica gel and Al2O3 micropowders in air, i.e. mullite could not form at low temperature. Therefore, in order to tailor the microstructure and properties of castables using silica sol, the reactivity of silica gel with alumina at high temperature should be studied. Nowadays, carbon-free and carbon-containing castables are widely used in practice. Therefore, in this study, the existing commercial colloidal silica sol was adopted, and the phase composition and microstructure evolution as well as reactivity of the silica gel in systems with and without carbon and in the presence of Al2O3 fines during heating were investigated. The possibility of mullite formation at low temperature by heating a mixture of silica gel and Al2O3 fines and the stability of the silica gel in a reducing atmosphere were discussed. The suitable system for silica sol application in refractory castables was also proposed.
2.
Experimental procedure Commercial colloidal silica (Shuanglian Chemical Co. Ltd, China; solid content (SiO2): 30%)
generally used in castables was used as the main material, and carbon black (N220, Tianjin Ebory Chemical Co. Ltd, China) and tabular alumina fines (Almatis Co. Ltd, Al2O3>99%, 40µm) were used as accessory materials. Silica gel was prepard by dried the colloidal silica at 110°C for 24 h. Morphology of the dried gel was shown in Fig. 1, and the gel particles with diameter of 35-40 nm.
Fig. 1 Morphology of silica gel. There were three groups of samples: (1) Dried silica gel powders were fired at 800-1500 °C for 3 h in oxidizing atmosphere, this group was designated as S0. (2) Dried gel and carbon black powders were 3
fired at 800-1500°C for 3 h in reducing atmosphere, this group was designated as S1. Carbon was excess in the large sagger and thus created a reducing atmosphere (including CO and N2). (3) Silica gel powder was mixed with tabular alumina fines (m(Al2O3):m(SiO2) = 3:2) and heated at 1400-1500°C in different atmosphere (in air and in reducing atmosphere); this group was designated as AS. Differential thermal and thermogravimetric analysis (DTA-TG) of silica gel was tested using a thermal analysis apparatus (Netzsch STA-449C). The crystalline phases of the samples were determined by X-ray diffraction (XRD; Philips X′Pert Pro). The microstructure of the samples was characterized by scanning electron microscopy (SEM, JEOL JSM-6500F) equipped with energy-dispersive spectroscopy (EDS).
3.
Results and discussion
3.1. TG-DTA of silica gel powder The TG - DTA curves of silica gel powder is shown in Fig.2. The endothermic peak at 55°C with weight loss of 5.8% is the weight loss of free water. The exothermic peak at 980°C with no further weight loss is due to α-tridymite and α-cristobalite crystallization [31]. Based on this result, the heating temperature is selected as 800°C to 1500°C for the following experiments to investigate the silica gel changes in different system at elevated temperature.
Fig. 2 DTA-TG curves of silica gel powder. 3.2. Phase evolution of silica gel powder in different atmosphere XRD results of the samples after fired at 800-1500°C are shown in Fig. 3. Fig.3a shows that the main phase of the S0 series samples is α-cristobalite and the peaks corresponding to α-tridymite are very weak after heated at 800°C in air. On increasing temperature from 1000°C to 1500°C, the intensity of the α-tridymite peaks increases notably, and major α-cristobalite phase and minor α-tridymite phase co-exist at high temperatures. 4
(a) Samples of group S0 heated in air
(b) Samples of group S1 heated in reducing atmosphere. Fig.3 XRD patterns of silica gel heated in different atmosphere Fig.3b shows that the main phase is α-cristobalite and the minor phase is α-tridymite after heated at 800°C in a reducing atmosphere. On increasing temperature from 800°C to 1200°C, the intensity of the α-tridymite peaks increases significantly and then decreases slightly after heated at 1500°C. The phase composition of sample S1 after heated at 1500°C in a reducing atmosphere is a little different from that of sample S0 heated in air, illustrating that carbon and atmosphere have little effect phase composition of silica gel at elevated temperature.
3.3. Microstructure evolution of silica gel in different atmosphere Fig. 4 shows the microstructure of the samples heated at 800-1500°C in air. Figs.4a and 4b indicate that the size of the spherical nano-silica particles is less than 60 nm after heated at 800-1000°C. With temperature increasing up to 1200°C, the silica particle size significantly increases, forming microparticles, and sintering of the particles was observed (Fig.4c). With the temperature further 5
increasing to 1500°C, the silica particles grew on a micro- or macroscale, and particles were sintered and closely bound to each other (as shown in Fig. 4d). This suggested that the growth of the nanoscale silica particles has closely related to the heated temperature in oxidizing atmosphere. The high reactivity and excellent properties of the nanoscale silica particles are lost when they are abnormally grown on a microor macroscale at high temperatures.
Fig. 4 Microstructure of S0 sample after fired in air: (a) 800°C, (b) 1000°C, (c) 1200°C, and (d) 1500°C. SEM images of the S1 series of samples after fired at 800-1500°C are shown in Fig. 5. The size of the nano-silica particles is 40–50 nm and remains almost unchanged with the temperature increasing up to 1500°C. Further, the spherical morphology of the nano-silica particles showed no change with temperature rise from 800°C to 1500°C, indicating that the carbon can inhibit the nano-silica particles growing, thus retaining their fine size and spherical shape at high temperatures. The spherical shape of the nanoscale silica particles will lead to the good filling effect and high reactivity, which contribute to the formation of mullite at lower temperatures [32] and improvement in high temperature properties of refractory castables.
6
Fig. 5 Microstructure of S1 sample after fired in reducing atmosphere: (a) 800°C, (b)1000°C, (c) 1200°C, and (d) 1500°C. The above results illustrate that carbon black has a key effect on retarding the growth of nanoscale silica particles at high temperatures. In order to illustrate the role and distribution of carbon black in the samples, EDS analysis is selectively checked on sample S1 after heated at 1000°C (Fig. 6). The EDS maps reveal that the sample contains Si, O and C. Carbon is well distributed along the particle boundaries of the nano-silica particles, which restricts the growth and sintering of these nanoparticles.
Fig. 6 EDS maps of Si, O, and C elements of sample S1 after heated at 1000°C. In air, silica particles directly contact with each other, resulting in agglomeration at low temperature, 7
and on increasing the temperature, the particles tend to grow and sinter. In a reducing atmosphere, carbon black is around the nanoscale silica particles boundaries, creating a steric hindrance effect, leading to separating each other of different particles, and a schematic of the effect of carbon is shown in Fig. 7.
Fig. 7 Schematic of steric hindrance effect of carbon. 3.3. Reactivity of silica gel powder with alumina fines There are diverse opinions about mullite formation in the presence of colloidal silica at low temperature in alumina rich castables. Therefore, to investigate if the mullite phase can form at relatively low temperatures in the presence of silica sol, a mixture of the dried silica gel powder and tabular alumina fines (n(Al2O3): n(SiO2) = 3:2) was fired at 1400-1500 °C for 3 h both in air and in reducing atmosphere. Fig. 8a shows the XRD results of samples after heated at 1400°C, and the crystalline phases indicate the presence of corundum and cristobalite; further, no mullite crystal phase is observed in samples fired in air and in reducing atmosphere. Fig.8b shows that mullite can form in samples fired in both atmospheres at 1500°C, the intensity of mullite peaks in sample fired in reducing atmosphere is a little stronger than those fired in air, and the intensity of cristobalite peaks in sample heated in reducing atmosphere is a little weaker than those heated in air. Indicating more silica gel has reacted with alumina to form more mullite in reducing atmosphere. Fig. 4 indicated that nanoscale silica particles grew into micro- or macroscale particles after heating at 1200°C and their reactivity degraded. Generally, mullite formation temperature is above 1200 °C, so mullite can not form by heating silica gel and alumina fines at 1400°C in air which is due to the abnormal growth of nanoscale silica particles. Though nano silica particles are in nanoscale at 1400°C in reducing atmosphere (Fig.5), and mullite diffraction peaks are not observed after fired at 1400°C(Fig.8a), which may be due to the carbon inclusion retard the reaction between silica and alumina. Therefore, silica gel 8
can not react with alumina to form mullite at low temperature (≤1400°C) in high purity system.
Fig. 8 XRD patterns of AS sample after fired at 1400-1500°C. Fig. 9 indicates that the SEM image and EDS results of sample AS after heated at 1400°C in air.
Fig. 9 SEM image (a) and EDS results (b, c) of AS sample after heated at 1400°C. Fig. 9 illustrates that only silica and alumina particles are present in the AS sample, further confirming the absence of mullite at 1400°C, and the result was in accordance with XRD result in Fig.8a. The SEM image and EDS result of AS sample heated at 1500°C in air are shown in Figs.10(a) and (b).
9
Fig. 10 Microstructure (a) and EDS result (b) of AS sample fired at 1500°C in air. Figs.10a and b indicate that some mullite crystals with needle-like are formed in sample after fired at 1500°C in air, and the mullite crystals are fine, develop not good enough. The sintering phenomenon of sample is observed from Fig.10a, and it is difficult to find nanosilica particles. Figs.11(a)-(b) show the SEM image and EDS result of sample AS fired at 1500°C in reducing atmosphere.
Fig. 11 Microstructure (a) and EDS result (b) of AS sample fired at 1500°C in reducing atmosphere. Figs.11 (a) and (b) demonstrate that many columnar mullite crystals develop well in sample fired in reducing atmosphere at 1500°C, and the mullite crystals are easily find by SEM observation. And many nanoscale silica particles are observed, and the particle size of nano silica is about 50nm, the silica particles have small size and high reactivity, which are benefit for the further application. Moreover, many whiskers were observed on surface of the sample S1 (silica gel) fired at 1500°C in the reducing atmosphere. The microstructure and EDS result of the whiskers are shown in Fig. 12(a)-(c).
10
Fig. 12 Microstructure and EDS result of S1 sample after fired at 1500°C in reducing atmosphere. The EDS result in Fig. 12 indicates that these whiskers are SiC whiskers, and that SiC whisker had a chain-bead structure, with O element present in the bead. The obtained SiC whiskers are several tens of microns in length and several hundreds nanometer in diameters. The in situ formed SiC whiskers exhibited good strengthening and toughening effects, thus improving the mechanical properties of refractory materials. Nanoscale silica particles react with C to form SiO (g) and CO (g), and SiO (g) further reacts with C or CO (g) to form SiC whiskers and SiO2. From the above results, we can conclude that nano silica particles are abnormal growth in oxidizing atmosphere, and their small size loses and reactivity decreases. However, the size of the spherical nanoscale silica particles does not change. Silica gel can not react with alumina at heating temperature ≤1400°C both in oxidizing atmosphere and in reducing atmosphere, but mullite can form at 1500°C in both atmospheres. Compared with mullite formed in oxidizing atmosphere, there have more mullite crystals formation in reducing atmosphere, and mullite crystals develop well. In addition, with carbon inclusions, many nano silica particles with high reactivity still exist at 1500°C. In conclusion, the small size of nano silica particles and the formed SiC whiskers would improve high temperature properties of castables, making the colloidal silica suitable for application to carbon-containing castables.
4.
Conclusions The phase composition and microstructure evolutions, and reactivity of silica gel powders in
different systems at elevated temperature was investigated. The results showed that carbon inclusions and atmosphere slightly affected the phase composition of the silica gel powder during heating. The crystalline phases included the main α-cristobalite phase and minor phase α-tridymite both in air and in reducing atmosphere. Carbon inclusions and atmosphere greatly affected the morphology and particle size of the silica gel during heating. The spherical nano-silica particles grew rapidly to form macroscale rod-like particles with temperature increasing from 800-1000°C to above 1200°C in air, and the silica particles sintered with each other. However, the particle size of nano-silica with high reactivity was 11
almost unchanged on heating a mixture of silica gel and carbon in a reducing atmosphere, and some SiC whiskers were formed on sample surface at 1500°C. No mullite phase formed on heating the silica gel powder with alumina fines at 1400°C, and mullite formed at 1500°C, more mullite formed in reducing atmosphere than that in air. Carbon inclusions retarded the growth of nanoscale silica particles and promoted SiC whisker formation, thus making colloidal silica suitable for application in carbon-containing refractory castables.
ACKNOWLEDGEMENTS The work was financially supported by the National Natural Science Foundation of China (51672253, 51872266).
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Chakraborty, Easy-to-use mullite and spinel sols as bonding agents in a high-alumina based ultra low cement castable, Ceram. Int, 28 (2002), p.719. [14] H. Peng, and B. Myhre, Microsilica-gel bonded refractory castables with improved set-behaviour and mechanical properties, Refract. Worldforum, 7 (3) (2015), p.69. [15] J. Q. Xiong, Y. T. Peng, D. Y. Xie, and X. S. Mao, The characteristics of silica-sol combining refractories, Adv. Mater. Res, 396–398 (2011), p.288. [16] A. K. Singh, and R. S, Nano mullite bonded refractory castable composition for high temperature applications, Ceram. Int, 42 (2016), p.12937. [17] R.G. Pileggi, A.R. Studart, and V.C. Pandolfelli, How mixing affects the rheology of refractory castables, Am. Ceram. Soc. Bull, 80 (6) (2001), p.27. [18] S.H. Badiee, and O.Sasan, Non-cement refractory castables containing nano-silica: performance, microstructure, properties, Ceram.Silik, 53 (4) (2009), p.297. [19] R.K. Iler, The Chemisty of Silica: Solubility, Polymerization, Cocolloid and Surface Properties and Biochemistr, John Wiley, New York, 1979, p.866. [20] M. Nouri-Khezrabad, A.P. Luz, V.R. Salvini, F. Golestani-Fard, H.R. Rezaie, and V.C. Pandolfelli, Developing nano-bonded refractory castables with enhanced green mechanical properties, Ceram. Int, 41 (2015), p.3051. [21] L.Y. Wang, Y.H. Liang, Y.C. Yin, L. Zhao, M. F. Cai, and J. H. Nie, Enhancing the green mechanical strength of colloidal silica-bonded alumina castables using a silane coupling agent, Ceram. Int, 42 (9) (2016), p.11496. [22] E. Prestes, J. Medeiros, D.T. Gomes, J.L.B.C. Veiga , and V.C.Pandolfelli , Hot-erosion of nano-bonded refractory castables for petrochemical industries, Ceram. Int, 39 (3) (2013), p.2611. [23] M.V. M. Magliano, E. Prestes, J. Medeiros, J.L.B.C .Veiga , and V.C. Pandolfelli , Colloidal silica selection for nano bonded refractory castables, Refract. Appl. News, 15(3) (2010), p.14. [24] M.A.L. Braulio, G.G. Morbioli, J. Medeiros, J.B.Gallo , and V. C. Pandolfelli, Nano-bonded wide temperature range designed refractory castables, J. Am. Ceram. Soc, 95(3) (2012), p.1100. [25] Y.B. Wen, X.H. Liu, X.Y. Chen, Q.L. Jia, R.H. Yu, and T. Ma, Effect of heat treatment conditions on the growth of MgAl2O4 nano particles obtained by sol-gel method, Ceram. Int, 43 (2017), p.15246. [26] R.T. Wang, X.P. Liang, Y. Peng, X.W. Fan, and J.X. Li, Effect of the reaction temperature on nano crystallites MgAl2O4 spinel ceramic precursor, J. Ceram. Process. Res,10 (2009), p.780. [27] Y.B. Wen, X.H. Liu, X. Y. Chen, L. Yang, and T. Ma, Effect of heat treatment on size of ZnAl2O4 Nano-particles, J. Chin. Ceram. Soc, 45 (2017), p.880. [28] Chen Z, Zhang L, Cheng L, et al. Novel method of adding seeds for preparation of mullite[J]. Journal of Materials Processing Technology, 2005, 166(2):183-187. [29] J. Q. Xiong, Y. T. Peng, D. Y. Xie, X.S.Mao, The characteristics of silica-sol combining refractories, Adv. Mater. Res. 396–398 (2011)288–291. [30] Z.G. Li, Z.Y. Zhang, G.W. Ren, Properties of silica sol bonded corundum and corundum-mullite castables, Refract (Chinese). 46(2) (2012)90-95. 13
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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. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0272-8842(20)30546-0
DOI:
https://doi.org/10.1016/j.ceramint.2020.02.220
Reference:
CERI 24443
To appear in:
Ceramics International
Received Date: 14 January 2020 Accepted Date: 22 February 2020
Please cite this article as: J. An, Y. Wang, Q. Jia, F. Zhao, X. Liu, Microstructure and reactivity evolution of colloidal silica binder in different systems at elevated temperatures, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2020.02.220. 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. © 2020 Published by Elsevier Ltd.
Microstructure and reactivity evolution of colloidal silica binder in different systems at elevated temperatures Jiancheng An1,2#, Yuping Wang1#, Quanli Jia1, Fei Zhao1*, Xinhong Liu1* 1
Henan Key Laboratory of High Temperature Functional Ceramics, Zhengzhou University, 75 Daxue Road, Zhengzhou 450052, China
2
Tongda Refractory Technologies, Co. LTD, 1Anningzhuang East Road, Haidian District, Beijing
100085, China *Corresponding authors. Tel: +86-371-67761908, Fax: +86-371-67763822 # The authors contributed equally to this work. E-mail address: [email protected] (F. Zhao), [email protected] (X. Liu) Abstract: To find a suitable application system for colloidal silica, phase and microstructure evolution of the dried silica gel with and without carbon inclusion were investigated after heated at different temperature, and reactivity of silica gel and alumina fines was also investigated. The results showed that atmosphere and carbon inclusions only slightly affected the phase composition of the silica gel at elevated temperatures, and the crystalline phases were composed of major α-cristobalite and minor α-tridymite. However, the morphology and particle size of the silica gel were greatly affected by atmosphere and carbon inclusions during heating. The spherical nano-silica particles with sizes of 40-50 nm grew rapidly to macroscale rod-like particles with temperature rise from 800-1000°C to above 1200°C in air, and the sintering of silica particles was observed. The reactivity decreased significantly, and no mullite was formed when the silica gel and alumina fines were heated at 1400°C, and only some mullite formed at 1500°C in air. However, the size and morphology of the spherical nano-silica particles remained almost unchanged on heating the silica gel and carbon in a reducing atmosphere; further, many developing well mullite crystals and some SiC whiskers were formed at 1500°C. Carbon inclusions retarded the growth of nano-silica particles and promoted the formation of SiC whiskers at high temperature. Thus, colloidal silica is suitable for application in carbon-containing refractory castables. Key words：Colloidal silica, reactivity, microstructure, sintering, particle size
1.
Introduction Refractory castables are universally used in high-temperature industries because of their unique
advantages, such as energy saving, environment friendliness, good performance, easy installation, and cost effectiveness [1,2]. Binding systems are very important for refractory castables, because the binders are closely related to achieving high-quality refractory castables with a long service life. Cement is a conventional binder and has been used in refractory castables for a long time, as it provides castables with good installation property and cold strength. However, the low melting point phases (e.g., anorthite and gehlenite) can form because of CaO in cement reacting with other components (such as microsilica) in 1
castables, leading to the low melting point phases (e.g., anorthite and gehlenite) formation, which degrades the high temperature performance of the castables. CaO in cement also easily reacts with Al2O3 in castables to form calcium dialuminate (CA2) and calcium hexaluminate (CA6) that cause volumetric expansion and generate many cracks at high temperature [3-7]. Therefore, the use of cement as a binder is usually formed detrimental components in refractory castables at high temperature, it is unsuitable for application in some high temperature critical areas. In recent years, hydratable alumina, colloidal silica, and colloidal alumina as new cement-free binders have been adopted by refractory castables [8-13]. Among these binders, colloidal silica has attracted considerable attentions from researchers, manufacturers, and consumers. Because colloidal silica has a high solid content (15–50 wt% nano-silica) and the nano-sized SiO2 can fill packing gaps, increasing the densification of castables [14,15]. Moreover, incorporating colloidal silica leads to forming mullite phase at low temperatures for alumina-rich castables [16]. However, the current research on colloidal silica is focused on its effects on the mixing behavior [17], rheological behavior [18], setting mechanism [19], green mechanical properties [20,21], and high-temperature properties of castables [9,22-24]. Less research has been focused on phase composition and microstructure evolution, and reactivity of silica gel particles came from colloidal silica in different systems at elevated temperatures. Different phases of silica may confer different properties to refractory castables, and the particle size of silica greatly affects the reactivity of silica and properties of the castables. Previous researches have showed that nano oxide particles easily undergo abnormal growth and partial sintering at high temperature in oxidizing atmosphere, and that the particle size remains in the nanoscale at high temperatures in the presence of carbon in a reducing atmosphere [25-27], but some nano particles easily decompose into other phases in a reducing atmosphere [27]. If nano-silica particles grow into micro- or macroscale particles at high temperature, their high reactivity decreases, which may be detrimental to the filling effect, sinterablity and mullite formation at lower temperature, and the cost-effectiveness of the nano particles will be disappeared. If nano silica particles transform into other substances, the properties of the castables will be greatly affected. However, there are few reports on changes in silica gel in different systems at elevated temperatures. In addition, there are diversified research results on mullite formation temperature in refractory castables with colloidal silica as binder. Chen [28] prepared mullite precursor mixture powder by mixing alumina powder and silica sol, and the initial mullite formation temperature was 1250 . J.Q. Xiong et al 2
[29] addressed that the introduced silica sol could react with alumina to form mullite at about 1100°C, leading to the suitable densification of materials. However, some studies show that silica sol combined with alumina cannot produce mullite at low temperature. The research results of Zhigang Li et al [30] showed that mullite formed at 1547°C by heating mixture of silica gel and Al2O3 micropowders in air, i.e. mullite could not form at low temperature. Therefore, in order to tailor the microstructure and properties of castables using silica sol, the reactivity of silica gel with alumina at high temperature should be studied. Nowadays, carbon-free and carbon-containing castables are widely used in practice. Therefore, in this study, the existing commercial colloidal silica sol was adopted, and the phase composition and microstructure evolution as well as reactivity of the silica gel in systems with and without carbon and in the presence of Al2O3 fines during heating were investigated. The possibility of mullite formation at low temperature by heating a mixture of silica gel and Al2O3 fines and the stability of the silica gel in a reducing atmosphere were discussed. The suitable system for silica sol application in refractory castables was also proposed.
2.
Experimental procedure Commercial colloidal silica (Shuanglian Chemical Co. Ltd, China; solid content (SiO2): 30%)
generally used in castables was used as the main material, and carbon black (N220, Tianjin Ebory Chemical Co. Ltd, China) and tabular alumina fines (Almatis Co. Ltd, Al2O3>99%, 40µm) were used as accessory materials. Silica gel was prepard by dried the colloidal silica at 110°C for 24 h. Morphology of the dried gel was shown in Fig. 1, and the gel particles with diameter of 35-40 nm.
Fig. 1 Morphology of silica gel. There were three groups of samples: (1) Dried silica gel powders were fired at 800-1500 °C for 3 h in oxidizing atmosphere, this group was designated as S0. (2) Dried gel and carbon black powders were 3
fired at 800-1500°C for 3 h in reducing atmosphere, this group was designated as S1. Carbon was excess in the large sagger and thus created a reducing atmosphere (including CO and N2). (3) Silica gel powder was mixed with tabular alumina fines (m(Al2O3):m(SiO2) = 3:2) and heated at 1400-1500°C in different atmosphere (in air and in reducing atmosphere); this group was designated as AS. Differential thermal and thermogravimetric analysis (DTA-TG) of silica gel was tested using a thermal analysis apparatus (Netzsch STA-449C). The crystalline phases of the samples were determined by X-ray diffraction (XRD; Philips X′Pert Pro). The microstructure of the samples was characterized by scanning electron microscopy (SEM, JEOL JSM-6500F) equipped with energy-dispersive spectroscopy (EDS).
3.
Results and discussion
3.1. TG-DTA of silica gel powder The TG - DTA curves of silica gel powder is shown in Fig.2. The endothermic peak at 55°C with weight loss of 5.8% is the weight loss of free water. The exothermic peak at 980°C with no further weight loss is due to α-tridymite and α-cristobalite crystallization [31]. Based on this result, the heating temperature is selected as 800°C to 1500°C for the following experiments to investigate the silica gel changes in different system at elevated temperature.
Fig. 2 DTA-TG curves of silica gel powder. 3.2. Phase evolution of silica gel powder in different atmosphere XRD results of the samples after fired at 800-1500°C are shown in Fig. 3. Fig.3a shows that the main phase of the S0 series samples is α-cristobalite and the peaks corresponding to α-tridymite are very weak after heated at 800°C in air. On increasing temperature from 1000°C to 1500°C, the intensity of the α-tridymite peaks increases notably, and major α-cristobalite phase and minor α-tridymite phase co-exist at high temperatures. 4
(a) Samples of group S0 heated in air
(b) Samples of group S1 heated in reducing atmosphere. Fig.3 XRD patterns of silica gel heated in different atmosphere Fig.3b shows that the main phase is α-cristobalite and the minor phase is α-tridymite after heated at 800°C in a reducing atmosphere. On increasing temperature from 800°C to 1200°C, the intensity of the α-tridymite peaks increases significantly and then decreases slightly after heated at 1500°C. The phase composition of sample S1 after heated at 1500°C in a reducing atmosphere is a little different from that of sample S0 heated in air, illustrating that carbon and atmosphere have little effect phase composition of silica gel at elevated temperature.
3.3. Microstructure evolution of silica gel in different atmosphere Fig. 4 shows the microstructure of the samples heated at 800-1500°C in air. Figs.4a and 4b indicate that the size of the spherical nano-silica particles is less than 60 nm after heated at 800-1000°C. With temperature increasing up to 1200°C, the silica particle size significantly increases, forming microparticles, and sintering of the particles was observed (Fig.4c). With the temperature further 5
increasing to 1500°C, the silica particles grew on a micro- or macroscale, and particles were sintered and closely bound to each other (as shown in Fig. 4d). This suggested that the growth of the nanoscale silica particles has closely related to the heated temperature in oxidizing atmosphere. The high reactivity and excellent properties of the nanoscale silica particles are lost when they are abnormally grown on a microor macroscale at high temperatures.
Fig. 4 Microstructure of S0 sample after fired in air: (a) 800°C, (b) 1000°C, (c) 1200°C, and (d) 1500°C. SEM images of the S1 series of samples after fired at 800-1500°C are shown in Fig. 5. The size of the nano-silica particles is 40–50 nm and remains almost unchanged with the temperature increasing up to 1500°C. Further, the spherical morphology of the nano-silica particles showed no change with temperature rise from 800°C to 1500°C, indicating that the carbon can inhibit the nano-silica particles growing, thus retaining their fine size and spherical shape at high temperatures. The spherical shape of the nanoscale silica particles will lead to the good filling effect and high reactivity, which contribute to the formation of mullite at lower temperatures [32] and improvement in high temperature properties of refractory castables.
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Fig. 5 Microstructure of S1 sample after fired in reducing atmosphere: (a) 800°C, (b)1000°C, (c) 1200°C, and (d) 1500°C. The above results illustrate that carbon black has a key effect on retarding the growth of nanoscale silica particles at high temperatures. In order to illustrate the role and distribution of carbon black in the samples, EDS analysis is selectively checked on sample S1 after heated at 1000°C (Fig. 6). The EDS maps reveal that the sample contains Si, O and C. Carbon is well distributed along the particle boundaries of the nano-silica particles, which restricts the growth and sintering of these nanoparticles.
Fig. 6 EDS maps of Si, O, and C elements of sample S1 after heated at 1000°C. In air, silica particles directly contact with each other, resulting in agglomeration at low temperature, 7
and on increasing the temperature, the particles tend to grow and sinter. In a reducing atmosphere, carbon black is around the nanoscale silica particles boundaries, creating a steric hindrance effect, leading to separating each other of different particles, and a schematic of the effect of carbon is shown in Fig. 7.
Fig. 7 Schematic of steric hindrance effect of carbon. 3.3. Reactivity of silica gel powder with alumina fines There are diverse opinions about mullite formation in the presence of colloidal silica at low temperature in alumina rich castables. Therefore, to investigate if the mullite phase can form at relatively low temperatures in the presence of silica sol, a mixture of the dried silica gel powder and tabular alumina fines (n(Al2O3): n(SiO2) = 3:2) was fired at 1400-1500 °C for 3 h both in air and in reducing atmosphere. Fig. 8a shows the XRD results of samples after heated at 1400°C, and the crystalline phases indicate the presence of corundum and cristobalite; further, no mullite crystal phase is observed in samples fired in air and in reducing atmosphere. Fig.8b shows that mullite can form in samples fired in both atmospheres at 1500°C, the intensity of mullite peaks in sample fired in reducing atmosphere is a little stronger than those fired in air, and the intensity of cristobalite peaks in sample heated in reducing atmosphere is a little weaker than those heated in air. Indicating more silica gel has reacted with alumina to form more mullite in reducing atmosphere. Fig. 4 indicated that nanoscale silica particles grew into micro- or macroscale particles after heating at 1200°C and their reactivity degraded. Generally, mullite formation temperature is above 1200 °C, so mullite can not form by heating silica gel and alumina fines at 1400°C in air which is due to the abnormal growth of nanoscale silica particles. Though nano silica particles are in nanoscale at 1400°C in reducing atmosphere (Fig.5), and mullite diffraction peaks are not observed after fired at 1400°C(Fig.8a), which may be due to the carbon inclusion retard the reaction between silica and alumina. Therefore, silica gel 8
can not react with alumina to form mullite at low temperature (≤1400°C) in high purity system.
Fig. 8 XRD patterns of AS sample after fired at 1400-1500°C. Fig. 9 indicates that the SEM image and EDS results of sample AS after heated at 1400°C in air.
Fig. 9 SEM image (a) and EDS results (b, c) of AS sample after heated at 1400°C. Fig. 9 illustrates that only silica and alumina particles are present in the AS sample, further confirming the absence of mullite at 1400°C, and the result was in accordance with XRD result in Fig.8a. The SEM image and EDS result of AS sample heated at 1500°C in air are shown in Figs.10(a) and (b).
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Fig. 10 Microstructure (a) and EDS result (b) of AS sample fired at 1500°C in air. Figs.10a and b indicate that some mullite crystals with needle-like are formed in sample after fired at 1500°C in air, and the mullite crystals are fine, develop not good enough. The sintering phenomenon of sample is observed from Fig.10a, and it is difficult to find nanosilica particles. Figs.11(a)-(b) show the SEM image and EDS result of sample AS fired at 1500°C in reducing atmosphere.
Fig. 11 Microstructure (a) and EDS result (b) of AS sample fired at 1500°C in reducing atmosphere. Figs.11 (a) and (b) demonstrate that many columnar mullite crystals develop well in sample fired in reducing atmosphere at 1500°C, and the mullite crystals are easily find by SEM observation. And many nanoscale silica particles are observed, and the particle size of nano silica is about 50nm, the silica particles have small size and high reactivity, which are benefit for the further application. Moreover, many whiskers were observed on surface of the sample S1 (silica gel) fired at 1500°C in the reducing atmosphere. The microstructure and EDS result of the whiskers are shown in Fig. 12(a)-(c).
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Fig. 12 Microstructure and EDS result of S1 sample after fired at 1500°C in reducing atmosphere. The EDS result in Fig. 12 indicates that these whiskers are SiC whiskers, and that SiC whisker had a chain-bead structure, with O element present in the bead. The obtained SiC whiskers are several tens of microns in length and several hundreds nanometer in diameters. The in situ formed SiC whiskers exhibited good strengthening and toughening effects, thus improving the mechanical properties of refractory materials. Nanoscale silica particles react with C to form SiO (g) and CO (g), and SiO (g) further reacts with C or CO (g) to form SiC whiskers and SiO2. From the above results, we can conclude that nano silica particles are abnormal growth in oxidizing atmosphere, and their small size loses and reactivity decreases. However, the size of the spherical nanoscale silica particles does not change. Silica gel can not react with alumina at heating temperature ≤1400°C both in oxidizing atmosphere and in reducing atmosphere, but mullite can form at 1500°C in both atmospheres. Compared with mullite formed in oxidizing atmosphere, there have more mullite crystals formation in reducing atmosphere, and mullite crystals develop well. In addition, with carbon inclusions, many nano silica particles with high reactivity still exist at 1500°C. In conclusion, the small size of nano silica particles and the formed SiC whiskers would improve high temperature properties of castables, making the colloidal silica suitable for application to carbon-containing castables.
4.
Conclusions The phase composition and microstructure evolutions, and reactivity of silica gel powders in
different systems at elevated temperature was investigated. The results showed that carbon inclusions and atmosphere slightly affected the phase composition of the silica gel powder during heating. The crystalline phases included the main α-cristobalite phase and minor phase α-tridymite both in air and in reducing atmosphere. Carbon inclusions and atmosphere greatly affected the morphology and particle size of the silica gel during heating. The spherical nano-silica particles grew rapidly to form macroscale rod-like particles with temperature increasing from 800-1000°C to above 1200°C in air, and the silica particles sintered with each other. However, the particle size of nano-silica with high reactivity was 11
almost unchanged on heating a mixture of silica gel and carbon in a reducing atmosphere, and some SiC whiskers were formed on sample surface at 1500°C. No mullite phase formed on heating the silica gel powder with alumina fines at 1400°C, and mullite formed at 1500°C, more mullite formed in reducing atmosphere than that in air. Carbon inclusions retarded the growth of nanoscale silica particles and promoted SiC whisker formation, thus making colloidal silica suitable for application in carbon-containing refractory castables.
ACKNOWLEDGEMENTS The work was financially supported by the National Natural Science Foundation of China (51672253, 51872266).
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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. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: