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Preparation of microporous mullite ceramics by foaming for high temperature thermal isolation
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Preparation of microporous mullite ceramics by foaming for high temperature thermal isolation Huishi Guo a, Wenfeng Li b,n, Fangbao Ye a,n a b
High Temperature Ceramics Institute, Henan Key Laboratory of High Temperature Functional Ceramics, Zhengzhou University, Zhengzhou, 450052 China College of Materials Science and Engineering, Henan University of Technology, Zhengzhou, 450007 China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 July 2016 Received in revised form 2 August 2016 Accepted 5 August 2016
To satisfy the need of thermal isolation in high temperature industry, microporous mullite ceramics with nest-like structures have been fabricated by foaming with kyanite and Al(OH)3 as raw materials, cement as the foam stabilizer. The effects of solid loading on viscosities of slurries, properties and microstructure of samples have been systematically investigated. It is found that foams can be stabilized by the hydration of cement in aqueous ceramic slurries, and then, they convert into pores in samples through curing and heating. With increasing solid loading, the number and size of pores decrease, but the mechanical strength and thermal conductivity increase. Meanwhile, a great amount of needle-like mullites are grown in-suit on pore walls, they interwoven into interlocked network structures, making the effective pore size and thermal conductivity smaller, while the mechanical strength stronger. In addition, to efficiently investigate the thermal insulation properties of samples at high temperature, a new thermal conductivity model has been built up, which combines the complexity of pore structure and thermal radiation, showing that the thermal radiation has an effect on the effective thermal conductivity at high temperature, and the experiment datas fit well with the model when di ¼3, kʹ ¼0.3 and a¼ 5 10 11 W/ mK4. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Microporous mullite Kyanite Foaming method Pore size distribution Thermal conductivity
1. Introduction In recent years, high temperature industry has become the major energy-consuming trade in the industrial production, but the heat loss is large owing to the poor thermal isolation of various kilns and heat transport pipelines. Therefore, it is important to develop effective thermal isolation technologies, which is beneficial to the energy saving and healthy development of society and environment. Porous ceramics have advantages of low thermal conductivity, low bulk density, high refractoriness and strength [1– 4], which make them can be used as high temperature thermal insulating materials in the metallurgy, refractory and ceramic industries [5]. Most applications of porous ceramics are mainly determined by factors of pore morphology, porosity and pore size distribution, which depend greatly on the producing processes [6,7]. Thus far, a variety of manufacturing methods have been developed, such as replication of a sacrificial foam template, direct foaming of a liquid slurry, or burn-out of fugitive pore former [8,9]. Among them, the direct foaming method is recognized to be an optimum method for n
Corresponding authors. Tel.: þ 86 371 67766196; fax: þ86 371 67763822. E-mail addresses: [email protected] (W. Li), [email protected] (F. Ye).
fabricating porous ceramics with high porosity and smaller-sized pores [4]. By using this method, porous ceramics can be produced through forming a certain amount of air bubbles directly in the aqueous ceramic powder suspension, which is subsequently consolidated in order to keep the pore structure created by air bubbles [10]. However, several transformations in the bubble structures may occur within the interval between foam generation and foam solidication, the liquid foam may collapse or even vanish due to the drainage, coalescence or Ostwald ripening effects, which have impacts on the final microstructure and properties of porous ceramics [11,12]. In order to avoid the collapsing of liquid foams and retain the pore morphology, researchers mainly take methods from two aspects. On the one hand, choosing proper solidication technologies, such as gel-casting or starch consolidation method, foams can be stabilized through the in situ polymerization of organic monomers or by the consolidation of starch subsequently [11,13]. On the other hand, selecting appropriate forming agents, such as ovalbumin, fish collagen or bovine serum albumin, which can be used not only as forming agents, but as binders [14,15]. However, their extensive applications have been restricted because the processes are difficult to control, and the starting materials are expensive, as well as some of the organic monomers have toxicity [16,17]. Recently, we have explored a cost-effective and eco-friendly
http://dx.doi.org/10.1016/j.ceramint.2016.08.029 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: H. Guo, et al., Preparation of microporous mullite ceramics by foaming for high temperature thermal isolation, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.029i
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method for the preparation of porous ceramics, which involves the foaming of suspensions and uses cement as the foam stabilizer. When fresh air-water interface is produced by mechanical frothing, surfactant molecules tend to absorb onto the air-water interface with a hydrophobic part being expelled from the solvent and a hydrophilic part remaining in contact with the liquid [11]. Through this mechanism, the surface tension of air-water interface can be reduced, and new bubbles are created. Meanwhile, the cement and other particles deposit onto the surface of bubble films, then the hydration reaction of gelatinization materials occurs, and new hydration products fill into blanks among water molecules, which can fix other particles and stabilize the bubbles. Finally, these bubbles convert into pores in samples through curing, drying and heating processes. In order to control pore structures and properties of porous ceramics, researchers usually adjust the kind or amount of foaming agent [18], but effects of solid loading on the final structure and properties of porous ceramics are rarely reported. In addition, the thermal conductivity plays an important role in performances of high temperature thermal isolations, but most thermal conductivity experiments are tested at room temperature, and the currently available structure models are constructed according to the porosity and pore distribution, the radiation component is neglected [13]. However, at temperatures higher than 1000 °C, the heat transfer by radiation becomes greater than the conduction and/or the convection. On the other hand, a few high temperature models are either empirical or they assume that ceramics have geometrically regular microstructures, but actual pore structures have random distributions, shapes and sizes, which results in the lack of agreement between experiments and theoretical predictions of these models [19–21]. In this paper, microporous mullite ceramics for high temperature thermal isolation are fabricated by foaming with kyanite and aluminum hydroxide (Al(OH)3) as starting materials, cement as the foam stabilizer. Effects of solid loading on the viscosities of slurries, properties (including bulk density, open porosity, mechanical strength, pore size distribution, high temperature thermal conductivity) and microstructure of samples are investigated. Meanwhile, a new high temperature thermal conductivity model is set up based on the complexity of pore structures and the thermal radiation.
2. Experimental procedures 2.1. Materials
slurries at 0.3 wt%, followed by a vigorously whisked for 5 min to acquire uniform foamed slurries. Then the as-generated foamed slurries were poured into molds immediately. After 24–48 h of free curing, the samples gained sufficient mechanical properties for manipulation. After demoulding, the green bodies were dried and then sintered at 1500 °C for 3 h with the heating rate of 3 °C/min in a programmable electric furnace. 2.3. Characterization Viscosity behavior of foamed slurries was measured by a rotational viscometer (NDJ-1, Chengou Industry, China) within a plastic container (Ф30 60 mm) using 2# rotor. Bulk density (ρ) and open porosity (ε) of sintered samples were determined by Archimedes method in distilled water. True density (ρt) was examined according to ISO5018-1983 standard. Relative density was determined through the ratio of ρ and ρt. Total porosity (p) was evaluated using Eq. (1).
⎛ ρ⎞ p = ⎜⎜ 1− ⎟⎟ × 100% ρt ⎠ ⎝
(1)
Flexural strength was determined by the three-point bending method in a mechanical testing machine (HT-8391, Hongta Co, China) using a span of 50 mm and a cross-head speed of 10 mm/ min, three to five samples were used to determine the average flexural strength. Pore size distribution was determined with mercury porosimetry (PM-60GT, Quantachrome instruments, America). Thermal conductivity at 1100 °C was measured by a flat plate thermo-conductivity tester (PBDR-02P, Luoyang Precondar Instruments for Testing Refractoriness Co, China). Polished sections of samples were prepared for microstructure and composition analysis by scanning electron microscopy (SEM, JEOL JSM5160LV, Japan) equipped with an energy dispersive spectrometry (EDS, INCA2000, Oxford, UK).
3. Results and discussions 3.1. Viscosity of slurries Fig. 1 shows the viscosities of foamed slurries with different solid loading. It can be seen that with increasing test time, viscosities of slurries have no obvious changes in a short stage, which can reserve sufficient time for the stirring and casting of slurries. Meanwhile, viscosities of slurries increase with solid loading as a
Commercially available kyanite powder with d50 of 75 mm and Al2O3 content 50 71% (China Kyanite Company) was used as the main raw material. Al(OH)3 (Gibbsite, China aluminum Co., Ltd., China) with d50 of 84 mm was chosen as the auxiliary material. Sodium lauryl sulfate (SLS, Chemical purity, Tianjin Chemical Reagent Co., China) was selected as the foaming agent. Aluminate cement (Secar 71, Lafarge group, France) was used as the stabilizer of foams. 2.2. Sample preparation Mixtures were firstly prepared by putting ceramic powders (composed by kyanite, Al(OH)3 and aluminate cement at the weight ratio of 60:40:5) into a mixer (JJ-1, Jieruier Co., China) and dry mixed together for 2 h, then distilled water was added into the mixer with solid contents of 35 vol%, 40 vol%, 45 vol% and 50 vol%, respectively. The density of kyanite and Al(OH)3 were taken as 3.5 g/cm3 and 2.4 g/cm3 respectively in the solid loading calculations. After 3 min of wet mixing, the foaming agent was added into
Fig. 1. Viscosities of slurries with different solid loading.
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Fig. 2. SEM micrographs of microporous mullite samples with different solid loading: (a) 35 vol%, (b) 40 vol%, (c) 45 vol% and (d) 50 vol%.
result of the reduction of the free water among particles. In addition, the distance among particles reduces with increasing solid loading, which enhances the friction among particles and results in an increment in the viscosities of slurries [22].
3.2. Pore structure SEM micrographs of microporous mullite samples with different solid loading are shown in Fig. 2. As shown in Fig. 2a, when the solid loading is at 35 vol%, there are a large number of big pores in
Fig. 3. SEM photos and EDS analysis spectrum of needle-like mullite crystals in pores.
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the sample, some of them connect with each other, and the pore walls are thinner compared with other samples. With increasing solid loading at range of 40–50 vol% (Fig. 2b–d), the number and size of pores decrease, but the wall thickness becomes thicker, along with more closed-pores. Analysis of these is due to the fact that, with increasing solid loading, the amount of forming liquid in slurries reduces relative, and the foaming capacity of slurries decreases, which results in a reduction in bubble numbers. Meanwhile, viscosities of slurries increase with the increment of solid loading, which prevents the nucleation and development of air bubbles, and these bubbles are more easily destroyed due to the enhancement of friction among particles. In addition, as shown in Fig. 2, the pores show regular round in shape in polished sections, therefore, it can be inferred that the pore structure is nearly spherical in samples. Fig. 3 shows SEM micrographs of the pore morphology. It can be seen that the pores in samples look like birds' nests (Fig. 3a), and lots of needle-like crystals are grown in situ on the pore walls (Fig. 3b), most of them are 10–15 mm in length and 0.5–1 mm in diameter. These needle-like crystals have smooth and clean surfaces, they interwoven into interlocked network structures, in which there are a lot of micropores (Fig. 3c). The EDS (Fig. 3d) analysis exhibits that these needle-like crystals are mullite, they come from the mullitization of kyanite. These needle-like mullites have a preferred orientation along the direction of elongation of crystals, this may be attributed to the fact that the (001) crystal face has higher interfacial energies than that in other crystal faces [23,24], and the crystal structure of these mullites belongs to orthorhombic system, the symmetry of this structure makes it easily form needle-like in shape [25]. In addition, there are minor impurities in kyanite, they form low viscosity glassy phase at high temperature and promote the development of needle-like mullites [26].
3.3. Bulk density and open porosity Bulk density and open porosity of samples with different solid loading are shown in Fig. 4. As showing, with increasing solid loading, the bulk density increases gradually from 0.49 g/cm3 to 1.02 g/cm3, while the open porosity decreases from 83.02% to 65.32%. This may be ascribed as this, with the increment of solid loading, the total pore volume in samples reduces due to the reduction in the number and size of pores, which leads to the rise of bulk density and the decline of open porosity.
Fig. 4. Dependence of the bulk density and open porosity of samples on the solid loading.
Fig. 5. Double log plot of relationship between relative density and flexural strength.
3.4. Mechanical properties Reports point out that the mechanical properties of porous ceramics are complicated, they may depend mainly on the strut strength, relative density, pore size and distribution [18]. According to Ashby's theory, when materials and pore size distribution have no obvious changes, the relationship between strength and relative density of porous materials can be expressed by the following equation [27].
⎛ ρ ⎞m σ = K ⎜⎜ ⎟⎟ σt ⎝ ρt ⎠
(2)
where st and ρt are the strength and true density of pore wall materials, s and ρ represent the strength and density of porous materials, respectively; K is a dimensionless constant and the exponent m is 1.5 or 2, depending on whether the pore morphology is open- or closed-pore, respectively. In this paper, the relationship between the flexural strength and relative density of samples is shown in Fig. 5, showing that the flexural strength is in direct proportion to the relative density, and their relationship in the present case has a slope of m ¼2.1 in the double log plot. Therefore, the mechanical strength significantly increases accompanied with the increase of relative density. The influence of solid loading on the flexural strength of samples is shown in Fig. 6. It is found that the values of flexural strength are 2.9 MPa, 5.25 MPa, 5.91 MPa and 10.55 MPa for the solid loadings of 35 vol%, 40 vol%, 45 vol% and 50 vol%. This can be attributed to the fact that the strength of porous ceramics depends mainly on the area within the structure that acts as load-bearing struts [28]. In comparison with other samples, for the sample with 35 vol% solid loading, there are a large quantity of big pores, which results in a low relative density of sintered samples and more defects on the walls, the area of load-bearing struts is less, thus the flexural strength is smaller. With increasing solid loading, the number and size of pores decrease, which leads to an increase in the relative density. A denser packing of ceramic particles around these areas helps to increase the effective load bearing area, thereby the strength increases. In addition, samples in this study show high mechanical strength, this is possible because that the mullite crystals present needle-like in shape, which plays an important role in reinforcing and toughening the samples due to their high aspect ratio and high strength [29].
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Fig. 6. Dependence of the flexural strength of samples on the solid loading.
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Fig. 8. Dependence of thermal conductivity of samples on the solid loading.
3.5. Pore size distribution
3.6. Thermal conductivity
Pore size distribution of microporous mullite samples is shown in Fig. 7. It is found that the pore size distribution is closely related to the solid loading of slurries, the mean pore sizes of samples prepared at solid loadings of 35 vol%, 40 vol%, 45 vol% and 50 vol% are 17.8 mm, 13.4 mm, 8.8 mm and 3.9 mm, respectively, the values decrease with increasing solid loading, similar reason applies to this trend as the case for the microstructures discussed before that the high viscosity of slurries prevents the nucleation and development of bubbles. Meanwhile, it can be seen from Fig. 7 that the pore size distribution of samples shows multimodal characteristic, indicating that pores in samples have multiple sizes. According to the pore's generating sources, pore size distribution and microstructures of samples, the pores in this study can be classified into four kinds: (і) bubbles in foamed slurries convert into pores with the size of 30–100 mm through caking and sintering processes; (іі) windows on the pore walls in size of 10–30 mm; (ііі) needle-like mullite crystals interwoven into network structures, in which there are a large amount of micropores in size of 1–10 mm; (iv) the mullitization of kyanite generates volume expansion, a lot of micropores with the size of below 1 mm are formed between mullite crystals.
Thermal conductivity of samples with different solid loading is shown in Fig. 8, showing that, at 1100 °C, with increasing solid loading from 35 vol% to 50 vol%, the thermal conductivity increases gradually from 0.28 W/mK to 0.42 W/mK. This is probably due to that the samples can be seen composed by mullite skeleton and air, because the thermal conductivity of air is much lower than that of mullite, the air trapped in pores plays a role of better thermal insulator, the higher of the porosity, the lower of the thermal conductivity will be [30], thus the porosity decreases with increasing solid loading, which results in an increase in the thermal conductivity. Reports show that the thermal conductivity of porous ceramics is related with not only the porosity, but also the microstructure [13]. There are five basic thermal conductivity models (including the Series, Parallel, Maxwell-Eucken 1, 2 and EMT models) for twocomponent materials according to the microstructure, they are listed in Table 1, many effective thermal conductivity models in reports are based on one or more of these five basic structural models. Among them, the Series and Parallel models represent a laminate structure of the components aligned either perpendicular or parallel to the heat flow. Two Maxwell-Eucken models arise depending on which component forms the continuous phase. Maxwell-Eucken 1 model is set up in the case that the thermal conductivity of the continuous phase is larger than that of the dispersed phase, while Maxwell-Eucken 2 model is set up for the opposite. And the EMT model supposes a completely random distribution of all the components. Each of them only represents one kind of structure and can not be suitable for all types of structures, therefore it is difficult to choose a proper model to accurately predict the porosity dependence of thermal conductivity [13]. Recently, Wang [31] has developed a unifying equation and found that these five basic models may be derived from Eq. (3) by suitable choice of parameters di and kʹ. m
ku =
Fig. 7. Pore size distribution of samples with different solid loading.
( ( dik′)/( di − 1)k′ + ki) ( ( dik′)/( di − 1)k′ + ki)
∑i = 1 kivi m ∑i = 1 vi
(3)
where v and k are volume fraction and thermal conductivity. Subscripts of u, 1, and m represent the porous material, component 1 and component m, respectively. The di parameter is related to the sphericity of the dispersed phase, and the most approach is to take di ¼ 3. The kʹ parameter is associated with pore size and pore distribution [32]. In this study, component 1 and component 2 are
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Table 1 Five basic effective thermal conductivity structure models for two-component materials.
Model
Structure schematic
Effective thermal conductivity equation
K
Parallel model
v1k1 v2 k 2 1
K
Series model
v1 / k1 v2 / k2
3k1 2k1 k2 K 3k1 v1 v2 2k1 k2 3k2 v2 k2 v1k1 2k2 k1 K 3k2 v2 v1 2k2 k1 v1k1 v2 k2
Maxwell-Eucken 1 (k1 = continuous phase, k2 = dispersed phase) Maxwell-Eucken 2 (k1 = dispersed phase, k2 = continuous phase)
EMT model
dense mullite and air, the values of their thermal conductivity are chosen to be 3.8 W/mK and 0.06 W/mK at 1100 °C, respectively [19,33], the kʹ parameter is chosen to be 0.3. Fig. 9 shows the influence of porosity on the thermal conductivity, together with the predicted values from five basic structural models and the universal model. It can be seen that the experimental thermal conductivities aren't fit with the values computed from five basic thermal conductivity equations, but the experimental values are between the Maxwell-Eucken 1 and EMT models, which means that the distribution of some pores in samples is uniform, but that of some others is nonuniform. It also can be seen that the trend of experimental values is fit with the universal model, but the experimental values are a little higher
v1
k1 K k1 2 K
v2
k2 K k2 2 K
0
than the predicted values, this is probably due to the fact that the thermal radiation has an effect on the effective thermal conductivities at high temperature. In order to include such a radiation term, Wagh gives the following expression for the thermal conductivity of porous ceramics by radiation [21]:
k r = 8εσT 3r0p
(4)
where ε is the emissivity of pore surfaces and has a value anywhere between 0 and 1, s is the Stefan-Boltzmann constant equal to 5.735 10 8 J/m2K4s, T is the average temperature of samples, r0 is the maximum pore size, and p is the porosity. In this study, ε ¼0.85 [34], r0 ¼17.8 mm. Therefore, the effective high temperature (HT) thermal conductivity of porous materials can be written: m
k ef = k u + ξ⋅k r =
( ( dik′)/( di − 1)k′ + ki) + ξ⋅8εσT3r p 0 ( ( dik′)/( di − 1)k′ + ki)
∑i = 1 kivi m ∑i = 1 vi
(5)
where ξ is an empirical coefficient, thus the HT mode can be represented in the form: m
k ef =
( ( dik′)/( di − 1)k′ + ki) + aT3p ( ( dik′)/( di − 1)k′ + ki)
∑i = 1 kivi m
∑i = 1 vi
(6)
and
a = 8ξεσr0
Fig. 9. Comparison of experimental thermal conductivity datas and calculated values from different effective thermal conductivity models.
(7)
where α is an empirical coefficient which can be used to fix the contribution of radiation at a given temperature. In this research, a parameter is chosen to be 5 10 11 W/mK4. Fig. 10 shows the influence of porosity on the thermal
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References
Fig. 10. Comparison of experimental thermal conductivity datas and calculated values from HT and universal models.
conductivity, together with the predicted values from HT and universal models. It is found that the experiment datas can be matched by the HT model. The good agreement between experiment datas and HT model calculated values demonstrates that the microporous mullite ceramics have complex structures, they may contain two or three of the five basic structures. Meanwhile, the datas derived from the HT model are higher than that from the universal model at the same porosity, indicating that the thermal radiation has an effect on the thermal conductivity at high temperature. In addition, samples in this study show a low thermal conductivity, this may be attributed to the high porosity and uniformly distributed micropores. Moreover, the needle-like mullite crystals form interlocked network structures, which have longer heat transfer paths, and the thermal resistance on the joints is very large, which enhances photon scatting of the lattice [2]. All these factors are beneficial to decrease the thermal conductivity.
4. Conclusions In this study, needle-like microporous mullite ceramics with low bulk density (0.49–1.02 g/cm3), high open porosity (65.32– 83.02%), high flexural strength (2.9–10.55 MPa), small mean pore size (3.3–17.07 mm) and low thermal conductivity (0.28–0.42 W/ mK at 1100 °C) have been successfully prepared by foaming with kyanite and Al(OH)3 as raw materials, cement as the foam stabilizer. Results show that, the solid loading has significant effects on properties and microstructure of samples, the increase of solid loading leads to a decrease in the number and size of pores but an increase in the pore wall thickness, which is beneficial to reduce the open porosity and improve the bulk density, mechanical strength and thermal conductivity. The mullite crystals generated from the mullitization of kyanite show needle-like in shape, they interwoven into interlocked network structures, and the pore structure looks like a bird's nest, this microstructure of samples makes them have both high mechanical strength and low thermal conductivity. In addition, a new HT thermal conductivity model has been set up, which combines the complexity of pore structure and the thermal radiation, the experimental datas are in good agreement with the theoretical values derived from the HT model when di ¼3, kʹ ¼0.3 and a ¼5 10 11 W/mK4.
Acknowledgments The authors greatly appreciate National Natural Science Foundation of China for financial support (NSFC-No. 51032007).
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Preparation of microporous mullite ceramics by foaming for high temperature thermal isolation Huishi Guo a, Wenfeng Li b,n, Fangbao Ye a,n a b
High Temperature Ceramics Institute, Henan Key Laboratory of High Temperature Functional Ceramics, Zhengzhou University, Zhengzhou, 450052 China College of Materials Science and Engineering, Henan University of Technology, Zhengzhou, 450007 China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 July 2016 Received in revised form 2 August 2016 Accepted 5 August 2016
To satisfy the need of thermal isolation in high temperature industry, microporous mullite ceramics with nest-like structures have been fabricated by foaming with kyanite and Al(OH)3 as raw materials, cement as the foam stabilizer. The effects of solid loading on viscosities of slurries, properties and microstructure of samples have been systematically investigated. It is found that foams can be stabilized by the hydration of cement in aqueous ceramic slurries, and then, they convert into pores in samples through curing and heating. With increasing solid loading, the number and size of pores decrease, but the mechanical strength and thermal conductivity increase. Meanwhile, a great amount of needle-like mullites are grown in-suit on pore walls, they interwoven into interlocked network structures, making the effective pore size and thermal conductivity smaller, while the mechanical strength stronger. In addition, to efficiently investigate the thermal insulation properties of samples at high temperature, a new thermal conductivity model has been built up, which combines the complexity of pore structure and thermal radiation, showing that the thermal radiation has an effect on the effective thermal conductivity at high temperature, and the experiment datas fit well with the model when di ¼3, kʹ ¼0.3 and a¼ 5 10 11 W/ mK4. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Microporous mullite Kyanite Foaming method Pore size distribution Thermal conductivity
1. Introduction In recent years, high temperature industry has become the major energy-consuming trade in the industrial production, but the heat loss is large owing to the poor thermal isolation of various kilns and heat transport pipelines. Therefore, it is important to develop effective thermal isolation technologies, which is beneficial to the energy saving and healthy development of society and environment. Porous ceramics have advantages of low thermal conductivity, low bulk density, high refractoriness and strength [1– 4], which make them can be used as high temperature thermal insulating materials in the metallurgy, refractory and ceramic industries [5]. Most applications of porous ceramics are mainly determined by factors of pore morphology, porosity and pore size distribution, which depend greatly on the producing processes [6,7]. Thus far, a variety of manufacturing methods have been developed, such as replication of a sacrificial foam template, direct foaming of a liquid slurry, or burn-out of fugitive pore former [8,9]. Among them, the direct foaming method is recognized to be an optimum method for n
Corresponding authors. Tel.: þ 86 371 67766196; fax: þ86 371 67763822. E-mail addresses: [email protected] (W. Li), [email protected] (F. Ye).
fabricating porous ceramics with high porosity and smaller-sized pores [4]. By using this method, porous ceramics can be produced through forming a certain amount of air bubbles directly in the aqueous ceramic powder suspension, which is subsequently consolidated in order to keep the pore structure created by air bubbles [10]. However, several transformations in the bubble structures may occur within the interval between foam generation and foam solidication, the liquid foam may collapse or even vanish due to the drainage, coalescence or Ostwald ripening effects, which have impacts on the final microstructure and properties of porous ceramics [11,12]. In order to avoid the collapsing of liquid foams and retain the pore morphology, researchers mainly take methods from two aspects. On the one hand, choosing proper solidication technologies, such as gel-casting or starch consolidation method, foams can be stabilized through the in situ polymerization of organic monomers or by the consolidation of starch subsequently [11,13]. On the other hand, selecting appropriate forming agents, such as ovalbumin, fish collagen or bovine serum albumin, which can be used not only as forming agents, but as binders [14,15]. However, their extensive applications have been restricted because the processes are difficult to control, and the starting materials are expensive, as well as some of the organic monomers have toxicity [16,17]. Recently, we have explored a cost-effective and eco-friendly
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method for the preparation of porous ceramics, which involves the foaming of suspensions and uses cement as the foam stabilizer. When fresh air-water interface is produced by mechanical frothing, surfactant molecules tend to absorb onto the air-water interface with a hydrophobic part being expelled from the solvent and a hydrophilic part remaining in contact with the liquid [11]. Through this mechanism, the surface tension of air-water interface can be reduced, and new bubbles are created. Meanwhile, the cement and other particles deposit onto the surface of bubble films, then the hydration reaction of gelatinization materials occurs, and new hydration products fill into blanks among water molecules, which can fix other particles and stabilize the bubbles. Finally, these bubbles convert into pores in samples through curing, drying and heating processes. In order to control pore structures and properties of porous ceramics, researchers usually adjust the kind or amount of foaming agent [18], but effects of solid loading on the final structure and properties of porous ceramics are rarely reported. In addition, the thermal conductivity plays an important role in performances of high temperature thermal isolations, but most thermal conductivity experiments are tested at room temperature, and the currently available structure models are constructed according to the porosity and pore distribution, the radiation component is neglected [13]. However, at temperatures higher than 1000 °C, the heat transfer by radiation becomes greater than the conduction and/or the convection. On the other hand, a few high temperature models are either empirical or they assume that ceramics have geometrically regular microstructures, but actual pore structures have random distributions, shapes and sizes, which results in the lack of agreement between experiments and theoretical predictions of these models [19–21]. In this paper, microporous mullite ceramics for high temperature thermal isolation are fabricated by foaming with kyanite and aluminum hydroxide (Al(OH)3) as starting materials, cement as the foam stabilizer. Effects of solid loading on the viscosities of slurries, properties (including bulk density, open porosity, mechanical strength, pore size distribution, high temperature thermal conductivity) and microstructure of samples are investigated. Meanwhile, a new high temperature thermal conductivity model is set up based on the complexity of pore structures and the thermal radiation.
2. Experimental procedures 2.1. Materials
slurries at 0.3 wt%, followed by a vigorously whisked for 5 min to acquire uniform foamed slurries. Then the as-generated foamed slurries were poured into molds immediately. After 24–48 h of free curing, the samples gained sufficient mechanical properties for manipulation. After demoulding, the green bodies were dried and then sintered at 1500 °C for 3 h with the heating rate of 3 °C/min in a programmable electric furnace. 2.3. Characterization Viscosity behavior of foamed slurries was measured by a rotational viscometer (NDJ-1, Chengou Industry, China) within a plastic container (Ф30 60 mm) using 2# rotor. Bulk density (ρ) and open porosity (ε) of sintered samples were determined by Archimedes method in distilled water. True density (ρt) was examined according to ISO5018-1983 standard. Relative density was determined through the ratio of ρ and ρt. Total porosity (p) was evaluated using Eq. (1).
⎛ ρ⎞ p = ⎜⎜ 1− ⎟⎟ × 100% ρt ⎠ ⎝
(1)
Flexural strength was determined by the three-point bending method in a mechanical testing machine (HT-8391, Hongta Co, China) using a span of 50 mm and a cross-head speed of 10 mm/ min, three to five samples were used to determine the average flexural strength. Pore size distribution was determined with mercury porosimetry (PM-60GT, Quantachrome instruments, America). Thermal conductivity at 1100 °C was measured by a flat plate thermo-conductivity tester (PBDR-02P, Luoyang Precondar Instruments for Testing Refractoriness Co, China). Polished sections of samples were prepared for microstructure and composition analysis by scanning electron microscopy (SEM, JEOL JSM5160LV, Japan) equipped with an energy dispersive spectrometry (EDS, INCA2000, Oxford, UK).
3. Results and discussions 3.1. Viscosity of slurries Fig. 1 shows the viscosities of foamed slurries with different solid loading. It can be seen that with increasing test time, viscosities of slurries have no obvious changes in a short stage, which can reserve sufficient time for the stirring and casting of slurries. Meanwhile, viscosities of slurries increase with solid loading as a
Commercially available kyanite powder with d50 of 75 mm and Al2O3 content 50 71% (China Kyanite Company) was used as the main raw material. Al(OH)3 (Gibbsite, China aluminum Co., Ltd., China) with d50 of 84 mm was chosen as the auxiliary material. Sodium lauryl sulfate (SLS, Chemical purity, Tianjin Chemical Reagent Co., China) was selected as the foaming agent. Aluminate cement (Secar 71, Lafarge group, France) was used as the stabilizer of foams. 2.2. Sample preparation Mixtures were firstly prepared by putting ceramic powders (composed by kyanite, Al(OH)3 and aluminate cement at the weight ratio of 60:40:5) into a mixer (JJ-1, Jieruier Co., China) and dry mixed together for 2 h, then distilled water was added into the mixer with solid contents of 35 vol%, 40 vol%, 45 vol% and 50 vol%, respectively. The density of kyanite and Al(OH)3 were taken as 3.5 g/cm3 and 2.4 g/cm3 respectively in the solid loading calculations. After 3 min of wet mixing, the foaming agent was added into
Fig. 1. Viscosities of slurries with different solid loading.
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Fig. 2. SEM micrographs of microporous mullite samples with different solid loading: (a) 35 vol%, (b) 40 vol%, (c) 45 vol% and (d) 50 vol%.
result of the reduction of the free water among particles. In addition, the distance among particles reduces with increasing solid loading, which enhances the friction among particles and results in an increment in the viscosities of slurries [22].
3.2. Pore structure SEM micrographs of microporous mullite samples with different solid loading are shown in Fig. 2. As shown in Fig. 2a, when the solid loading is at 35 vol%, there are a large number of big pores in
Fig. 3. SEM photos and EDS analysis spectrum of needle-like mullite crystals in pores.
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the sample, some of them connect with each other, and the pore walls are thinner compared with other samples. With increasing solid loading at range of 40–50 vol% (Fig. 2b–d), the number and size of pores decrease, but the wall thickness becomes thicker, along with more closed-pores. Analysis of these is due to the fact that, with increasing solid loading, the amount of forming liquid in slurries reduces relative, and the foaming capacity of slurries decreases, which results in a reduction in bubble numbers. Meanwhile, viscosities of slurries increase with the increment of solid loading, which prevents the nucleation and development of air bubbles, and these bubbles are more easily destroyed due to the enhancement of friction among particles. In addition, as shown in Fig. 2, the pores show regular round in shape in polished sections, therefore, it can be inferred that the pore structure is nearly spherical in samples. Fig. 3 shows SEM micrographs of the pore morphology. It can be seen that the pores in samples look like birds' nests (Fig. 3a), and lots of needle-like crystals are grown in situ on the pore walls (Fig. 3b), most of them are 10–15 mm in length and 0.5–1 mm in diameter. These needle-like crystals have smooth and clean surfaces, they interwoven into interlocked network structures, in which there are a lot of micropores (Fig. 3c). The EDS (Fig. 3d) analysis exhibits that these needle-like crystals are mullite, they come from the mullitization of kyanite. These needle-like mullites have a preferred orientation along the direction of elongation of crystals, this may be attributed to the fact that the (001) crystal face has higher interfacial energies than that in other crystal faces [23,24], and the crystal structure of these mullites belongs to orthorhombic system, the symmetry of this structure makes it easily form needle-like in shape [25]. In addition, there are minor impurities in kyanite, they form low viscosity glassy phase at high temperature and promote the development of needle-like mullites [26].
3.3. Bulk density and open porosity Bulk density and open porosity of samples with different solid loading are shown in Fig. 4. As showing, with increasing solid loading, the bulk density increases gradually from 0.49 g/cm3 to 1.02 g/cm3, while the open porosity decreases from 83.02% to 65.32%. This may be ascribed as this, with the increment of solid loading, the total pore volume in samples reduces due to the reduction in the number and size of pores, which leads to the rise of bulk density and the decline of open porosity.
Fig. 4. Dependence of the bulk density and open porosity of samples on the solid loading.
Fig. 5. Double log plot of relationship between relative density and flexural strength.
3.4. Mechanical properties Reports point out that the mechanical properties of porous ceramics are complicated, they may depend mainly on the strut strength, relative density, pore size and distribution [18]. According to Ashby's theory, when materials and pore size distribution have no obvious changes, the relationship between strength and relative density of porous materials can be expressed by the following equation [27].
⎛ ρ ⎞m σ = K ⎜⎜ ⎟⎟ σt ⎝ ρt ⎠
(2)
where st and ρt are the strength and true density of pore wall materials, s and ρ represent the strength and density of porous materials, respectively; K is a dimensionless constant and the exponent m is 1.5 or 2, depending on whether the pore morphology is open- or closed-pore, respectively. In this paper, the relationship between the flexural strength and relative density of samples is shown in Fig. 5, showing that the flexural strength is in direct proportion to the relative density, and their relationship in the present case has a slope of m ¼2.1 in the double log plot. Therefore, the mechanical strength significantly increases accompanied with the increase of relative density. The influence of solid loading on the flexural strength of samples is shown in Fig. 6. It is found that the values of flexural strength are 2.9 MPa, 5.25 MPa, 5.91 MPa and 10.55 MPa for the solid loadings of 35 vol%, 40 vol%, 45 vol% and 50 vol%. This can be attributed to the fact that the strength of porous ceramics depends mainly on the area within the structure that acts as load-bearing struts [28]. In comparison with other samples, for the sample with 35 vol% solid loading, there are a large quantity of big pores, which results in a low relative density of sintered samples and more defects on the walls, the area of load-bearing struts is less, thus the flexural strength is smaller. With increasing solid loading, the number and size of pores decrease, which leads to an increase in the relative density. A denser packing of ceramic particles around these areas helps to increase the effective load bearing area, thereby the strength increases. In addition, samples in this study show high mechanical strength, this is possible because that the mullite crystals present needle-like in shape, which plays an important role in reinforcing and toughening the samples due to their high aspect ratio and high strength [29].
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Fig. 6. Dependence of the flexural strength of samples on the solid loading.
5
Fig. 8. Dependence of thermal conductivity of samples on the solid loading.
3.5. Pore size distribution
3.6. Thermal conductivity
Pore size distribution of microporous mullite samples is shown in Fig. 7. It is found that the pore size distribution is closely related to the solid loading of slurries, the mean pore sizes of samples prepared at solid loadings of 35 vol%, 40 vol%, 45 vol% and 50 vol% are 17.8 mm, 13.4 mm, 8.8 mm and 3.9 mm, respectively, the values decrease with increasing solid loading, similar reason applies to this trend as the case for the microstructures discussed before that the high viscosity of slurries prevents the nucleation and development of bubbles. Meanwhile, it can be seen from Fig. 7 that the pore size distribution of samples shows multimodal characteristic, indicating that pores in samples have multiple sizes. According to the pore's generating sources, pore size distribution and microstructures of samples, the pores in this study can be classified into four kinds: (і) bubbles in foamed slurries convert into pores with the size of 30–100 mm through caking and sintering processes; (іі) windows on the pore walls in size of 10–30 mm; (ііі) needle-like mullite crystals interwoven into network structures, in which there are a large amount of micropores in size of 1–10 mm; (iv) the mullitization of kyanite generates volume expansion, a lot of micropores with the size of below 1 mm are formed between mullite crystals.
Thermal conductivity of samples with different solid loading is shown in Fig. 8, showing that, at 1100 °C, with increasing solid loading from 35 vol% to 50 vol%, the thermal conductivity increases gradually from 0.28 W/mK to 0.42 W/mK. This is probably due to that the samples can be seen composed by mullite skeleton and air, because the thermal conductivity of air is much lower than that of mullite, the air trapped in pores plays a role of better thermal insulator, the higher of the porosity, the lower of the thermal conductivity will be [30], thus the porosity decreases with increasing solid loading, which results in an increase in the thermal conductivity. Reports show that the thermal conductivity of porous ceramics is related with not only the porosity, but also the microstructure [13]. There are five basic thermal conductivity models (including the Series, Parallel, Maxwell-Eucken 1, 2 and EMT models) for twocomponent materials according to the microstructure, they are listed in Table 1, many effective thermal conductivity models in reports are based on one or more of these five basic structural models. Among them, the Series and Parallel models represent a laminate structure of the components aligned either perpendicular or parallel to the heat flow. Two Maxwell-Eucken models arise depending on which component forms the continuous phase. Maxwell-Eucken 1 model is set up in the case that the thermal conductivity of the continuous phase is larger than that of the dispersed phase, while Maxwell-Eucken 2 model is set up for the opposite. And the EMT model supposes a completely random distribution of all the components. Each of them only represents one kind of structure and can not be suitable for all types of structures, therefore it is difficult to choose a proper model to accurately predict the porosity dependence of thermal conductivity [13]. Recently, Wang [31] has developed a unifying equation and found that these five basic models may be derived from Eq. (3) by suitable choice of parameters di and kʹ. m
ku =
Fig. 7. Pore size distribution of samples with different solid loading.
( ( dik′)/( di − 1)k′ + ki) ( ( dik′)/( di − 1)k′ + ki)
∑i = 1 kivi m ∑i = 1 vi
(3)
where v and k are volume fraction and thermal conductivity. Subscripts of u, 1, and m represent the porous material, component 1 and component m, respectively. The di parameter is related to the sphericity of the dispersed phase, and the most approach is to take di ¼ 3. The kʹ parameter is associated with pore size and pore distribution [32]. In this study, component 1 and component 2 are
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Table 1 Five basic effective thermal conductivity structure models for two-component materials.
Model
Structure schematic
Effective thermal conductivity equation
K
Parallel model
v1k1 v2 k 2 1
K
Series model
v1 / k1 v2 / k2
3k1 2k1 k2 K 3k1 v1 v2 2k1 k2 3k2 v2 k2 v1k1 2k2 k1 K 3k2 v2 v1 2k2 k1 v1k1 v2 k2
Maxwell-Eucken 1 (k1 = continuous phase, k2 = dispersed phase) Maxwell-Eucken 2 (k1 = dispersed phase, k2 = continuous phase)
EMT model
dense mullite and air, the values of their thermal conductivity are chosen to be 3.8 W/mK and 0.06 W/mK at 1100 °C, respectively [19,33], the kʹ parameter is chosen to be 0.3. Fig. 9 shows the influence of porosity on the thermal conductivity, together with the predicted values from five basic structural models and the universal model. It can be seen that the experimental thermal conductivities aren't fit with the values computed from five basic thermal conductivity equations, but the experimental values are between the Maxwell-Eucken 1 and EMT models, which means that the distribution of some pores in samples is uniform, but that of some others is nonuniform. It also can be seen that the trend of experimental values is fit with the universal model, but the experimental values are a little higher
v1
k1 K k1 2 K
v2
k2 K k2 2 K
0
than the predicted values, this is probably due to the fact that the thermal radiation has an effect on the effective thermal conductivities at high temperature. In order to include such a radiation term, Wagh gives the following expression for the thermal conductivity of porous ceramics by radiation [21]:
k r = 8εσT 3r0p
(4)
where ε is the emissivity of pore surfaces and has a value anywhere between 0 and 1, s is the Stefan-Boltzmann constant equal to 5.735 10 8 J/m2K4s, T is the average temperature of samples, r0 is the maximum pore size, and p is the porosity. In this study, ε ¼0.85 [34], r0 ¼17.8 mm. Therefore, the effective high temperature (HT) thermal conductivity of porous materials can be written: m
k ef = k u + ξ⋅k r =
( ( dik′)/( di − 1)k′ + ki) + ξ⋅8εσT3r p 0 ( ( dik′)/( di − 1)k′ + ki)
∑i = 1 kivi m ∑i = 1 vi
(5)
where ξ is an empirical coefficient, thus the HT mode can be represented in the form: m
k ef =
( ( dik′)/( di − 1)k′ + ki) + aT3p ( ( dik′)/( di − 1)k′ + ki)
∑i = 1 kivi m
∑i = 1 vi
(6)
and
a = 8ξεσr0
Fig. 9. Comparison of experimental thermal conductivity datas and calculated values from different effective thermal conductivity models.
(7)
where α is an empirical coefficient which can be used to fix the contribution of radiation at a given temperature. In this research, a parameter is chosen to be 5 10 11 W/mK4. Fig. 10 shows the influence of porosity on the thermal
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References
Fig. 10. Comparison of experimental thermal conductivity datas and calculated values from HT and universal models.
conductivity, together with the predicted values from HT and universal models. It is found that the experiment datas can be matched by the HT model. The good agreement between experiment datas and HT model calculated values demonstrates that the microporous mullite ceramics have complex structures, they may contain two or three of the five basic structures. Meanwhile, the datas derived from the HT model are higher than that from the universal model at the same porosity, indicating that the thermal radiation has an effect on the thermal conductivity at high temperature. In addition, samples in this study show a low thermal conductivity, this may be attributed to the high porosity and uniformly distributed micropores. Moreover, the needle-like mullite crystals form interlocked network structures, which have longer heat transfer paths, and the thermal resistance on the joints is very large, which enhances photon scatting of the lattice [2]. All these factors are beneficial to decrease the thermal conductivity.
4. Conclusions In this study, needle-like microporous mullite ceramics with low bulk density (0.49–1.02 g/cm3), high open porosity (65.32– 83.02%), high flexural strength (2.9–10.55 MPa), small mean pore size (3.3–17.07 mm) and low thermal conductivity (0.28–0.42 W/ mK at 1100 °C) have been successfully prepared by foaming with kyanite and Al(OH)3 as raw materials, cement as the foam stabilizer. Results show that, the solid loading has significant effects on properties and microstructure of samples, the increase of solid loading leads to a decrease in the number and size of pores but an increase in the pore wall thickness, which is beneficial to reduce the open porosity and improve the bulk density, mechanical strength and thermal conductivity. The mullite crystals generated from the mullitization of kyanite show needle-like in shape, they interwoven into interlocked network structures, and the pore structure looks like a bird's nest, this microstructure of samples makes them have both high mechanical strength and low thermal conductivity. In addition, a new HT thermal conductivity model has been set up, which combines the complexity of pore structure and the thermal radiation, the experimental datas are in good agreement with the theoretical values derived from the HT model when di ¼3, kʹ ¼0.3 and a ¼5 10 11 W/mK4.
Acknowledgments The authors greatly appreciate National Natural Science Foundation of China for financial support (NSFC-No. 51032007).
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Please cite this article as: H. Guo, et al., Preparation of microporous mullite ceramics by foaming for high temperature thermal isolation, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.029i