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Fabrication of porcelain foam substrates coated with SiC, Ni, and Cr using the dip-coating technique
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Fabrication of porcelain foam Substrates coated with SiC, Ni, and Cr using the dip-coating technique Abdul Rashid Jamaludin, Shah Rizal Kasim, Ahmad Kamal Ismail, Mohd Zukifly Abdullah, Zainal Arifin Ahmad
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S0272-8842(14)01672-1 http://dx.doi.org/10.1016/j.ceramint.2014.10.123 CERI9382
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Ceramics International
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Cite this article as: Abdul Rashid Jamaludin, Shah Rizal Kasim, Ahmad Kamal Ismail, Mohd Zukifly Abdullah, Zainal Arifin Ahmad, Fabrication of porcelain foam Substrates coated with SiC, Ni, and Cr using the dip-coating technique, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2014.10.123 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Fabrication of Porcelain Foam Substrates Coated with SiC, Ni, and Cr Using the DipCoating Technique
Abdul Rashid Jamaludin1, Shah Rizal Kasim1, Ahmad Kamal Ismail2, Mohd Zukifly Abdullah2, Zainal Arifin Ahmad1,* 1
Structural Materials Niche Area, School of Materials and Mineral Resources, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia 2
Porous Media Combustion Laboratory, School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia *Corresponding author. Tel.:+60 45996127; fax: +60 45941011 E-mail address: [email protected]
Abstract This report presents a study on the properties of porcelain foam substrate fabricated via the sponge replication technique. The sintered porcelain foam substrates were dip-coated with SiC-, Cr-, and Ni-based coating slurries, and sintered again at 900-1250 ºC. The XRD results of coated samples show existence of secondary phases (Cr2O3, Al3Cr7, NiO2, and NiAl2O4) due to sintering, mostly at higher temperatures. The porosity of the plain substrate was reduced significantly after dip-coating and proportionally decreased as the sintering temperature increased, therefore influencing the compressive strength value. Furthermore, the thermal conductivity of coated substrate sintered at 1250 ºC was obtained in the range of 0.0921-0.1358 W/mK, when compared to the plain substrate at 0.0461 W/mK. In general, a reduction in the number of pores, cracks and open cells was observed, with significant improvement in the compressive strength of the coated substrate.
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Keywords: Sintering (A); Porosity (B); Thermal conductivity (C); Fracture (C); Porcelain (D)
1.
Introduction
Coating of ceramic, polymer, or metal, safeguards certain properties of the material from external degradation. Ceramic coating materials have received considerable attention due its ability to withstand high temperature, resistance to corrosion, wear and tear, extension of material service life, economic consideration, and improvement of certain other material properties such as strength and conductivity (thermal and electrical). This kind of coated ceramic substrates have been utilized in a wide range of applications such as the filtration process [1], membrane layer for micro and ultra-filtration [2], separator [3], catalyst [4], and porous media burner applications [5]. Recently, Mueller et al. [5] used porous alumina substrate dip-coated with SiC in a porous medium burner (PMB). The results obtained showed that SiC-coated substrate facilitated good combustion for stoichiometric fuel-to-air ratio. In another study, Chung [6] proposed SiC, as suitable filler materials to fabricate ceramic and metal-matrix composites for applications requiring high thermal conductivity. Besides that, oxide (Al2O3) and base metal catalyst (Ni/SiC and Ni/Al2O3) were previously used in partial oxidation of methane [7,8].
Nowadays, there are many processing methods or techniques used in the fabrication of coating layers. The more traditional methods of coating a porous substrate involve the use of dip-coating method or also known as chemical solution deposition (CSD) technique [9,10]. Besides, the dip-spin coating [11], and spray-coating [12] are also commonly used. Futhermore, some advanced processes such as the atomic layer deposition [13], chemical
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vapour deposition (CVD) [14], physical vapour deposition (PVD) [15], electrodeposition[16], and magnetron sputtering [17] techniques can be found in the literature. However, these techniques have their drawbacks namely; high cost, involvement of corrosive materials or aggressive chemicals, high thermal mismatch between substrate and coating layer, cracking of coating layer, inhomogeneity in terms of thickness of deposited coating and restriction on the shape of the substrate [18].
The principle of dip-coating involves the wetting of the substrate with a solution. The capillary suction pressure of the porous substrate creates a wet cake from the accumulation of particles (that cannot enter the pores) [19]. This process can produce a coating layer/thickness in the range of 100 nm to 100 µm [9]. Over the years, the dip-coating technique has been popularly used on account of the fact that it is simple, low cost, less time consuming, cheaper base material and precursors can be used, good control on layer thickness, and less wastage during fabrication of both porous and dense coatings [9,10,20]. Moreover, dip-coating is a very straightforward process and the most suitable technique used to coat porous substrates. This is because, the porous substrates have reticulated structure which can be difficult to coat using other coating methods [5]. The immersion of substrate in the coating slurry or suspension ensures that most of the material surface is completely coated. Layers are easily obtained by evaporation of solutions containing metal oxide precursors, organic matters, monomers, polymers or various kinds of nanoparticles. Solvents are used, usually alcohol, because of its’ low surface tension and fairly high volatility [9]. However, any aqueous solution can be easily deposited when proper processing conditions are applied.
The polymeric sponge method is widely used in the fabrication of reticulated ceramics. Although, it creates more open cell structures in the range of few millimeters, but is
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still prone to struts cracks [21,22]. Occurrence of cracks on the porcelain foam struts can be attributed to the pyrolysis of the sponge template during the sintering process. This can be overcome by forming a thick ceramic coating or introducing a secondary coat over the virgin template [20,23,24].
Although, the thermodynamic properties of SiC, Ni and Cr are well known, the use of SiC-, Ni- and Cr-based materials as coating additives for porcelain foam substrate has not been used. This is the first such attempt to develop porcelain foam substrate coated with SiC, Ni and Cr based materials. It is expected that, coating of the ceramic substrate will eliminate defects and cracks. Besides, it is expected to significantly improve the thermal efficiency of the coated substrate with the addition of conductive base coating materials to the slurry.
2.
Experimental procedures
2.1
Preparation of porcelain foam substrate
The porcelain foam substrate was prepared using the sponge replication technique. Ready-made porcelain raw powder supplied by Ipoh Ceramic Sdn Bhd was used for slurry preparation. The solid to liquid content ratio was fixed at 3:2. Sodium silicate (0.5wt% Na2SiO3) was added as a dispersing agent for the mixing process. The slurry was prepared by mixing the mixture for 24 h in a polyethylene bottle. A commercial polyurethane (PU) sponge with interconnected open cells (average cell of 20 pores per inch) purchased from CCT Automation Sdn. Bhd. was chosen as the template and further modified into smaller dimensions (10 cm × 10cm × 2.5 cm). Initially, the sponge template was submerged and compressed in the porcelain slurry. This ensured the template was completely filled with slurry and removed all the trapped air. Next, the impregnated sponge was taken out, squeezed
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manually by hand and pressed between two preset rollers. This process was repeated three times to facilitate adequate filling of slurry and to remove excess slurry, as well as to achieve a well-distributed coating over the template.The impregnated sponge was dried at room temperature for 48 h before being put in an oven for 24 h at 80 ºC. Sintering was done using an electrical furnace (Unitek Muffle Furnace 1600) with a starting heating rate of 5oC/min up to 500 ºC (soak for 1 h); and then further heating up to 1250 ºC (soaked for 2 h) before cooling down to room temperature.
2.2
Process of coating
The coating slurries were prepared by wet-mixing the raw materials as shown in Table 1. The ratio of solid to liquid content was fixed at 1:4. Polyvinyl alcohol (PVA), which acts as a binder was diluted in distilled water prior to the mixing process. The batches were mixed for 24 h in a polyurethane bottle. The process of infiltrating the coating slurry into the sintered porcelain foam substrate was accomplished by dipping the substrate for a period of 15 s. This impregnated substrate was dried for 20 min before being dipped again in the same coating slurry. The impregnated substrate was rinsed to remove excess solution after each dipping process. Next, the coated substrate was dried for 24 h at room temperature and later subjected to further drying at 80 ºC for another 24 h in an oven. The sintering process was carried out in normal furnace atmosphere of 900 ºC, 1150 ºC, and 1250 ºC,with a 5 ºC/min heating rate and 2 h of soaking time .
Table 1 Batch composition used in the experiment Sample name
*
Material composition (wt%)
Sintering
SiC Cr Ni Bentonite Alumina Kaolin PVA
condition (ºC)
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P.B20.SiC62.900 P.B20.SiC62.1150
900 62
20
1
10
7
1150
P.B20.SiC62.1250
1250
P.B20.Cr62.900
900
P.B20.Cr62.1150
62
20
1
10
7
1150
P.B20.Cr62.1250
1250
P.B20.Ni62.900
900
P.B20.Ni62.1150
62
20
1
10
7
P.B20.Ni62.1250
1150 1250
*Symbols (P.B20.x62.T) of the sample notation are: “P” is porcelain, “B” is bentonite, “x” are SiC, Ni, and Cr, respectively. The number “20” and “62” are amount of subjected material in weight percent, while “T” represents the sintering temperature.
The particle size distributions for the raw coating powders were determined using the Mastersizer S, Malvern Instrument Ltd., UK. The thermal analysis of the sponge was performed under athmospheric air, at a heating rate of 10 °C/min using thermal gravimetric analysis (TGA/DSC Linsesis, Germany). The microstructural morphology of the sintered porcelain foam substrate and coating layer were established from the scanning electron microscopy (SEM) method using table top TM3000 Hitachi, Japan and Zeiss Supra 35VP FE-SEM, Germany. The phase analysis of the sintered samples was measured via the X-ray diffraction method (XRD Bruker D8 Advance, Germany) with CuKĮ monochromatic radiation (Ȝ= 1.5406 Å). In addition, the Archimedes displacement technique was used to measure the bulk density and total porosity of the sintered samples.
The compressive strengths of porcelain foam substrate before and after coating were measured using a universal testing machine (Instron 5982, USA) with a crosshead speed of 1 mm/min and load cell of 5 kN. Square-shaped samples (15 mm × 15 mm × 15 mm) were 6
used for this measurement. The value of compressive strength associated with the load applied on the sample until it fractured, was calculated by dividing the maximum load at fracture over the cross-sectional area. Finally, the thermal conductivity test for the porcelain foam and coated substrate (10 mm × 10 mm × 5 mm) was conducted using the Hot-Disk Thermal Constants Analyzer, UK.
3.
Results and discussion
Fig. 1 shows the SEM images of the base coating materials used in this study. The particles of SiC (9.92µm to 92.6µm) and Cr (35.46µm to 103.2µm) observed are in the form of flakes. In contrast, Ni particles (8.18ȝm to 191.57ȝm) are spherical in shape and found to be agglomerated.
Fig. 2 presents SEM images of raw PU sponge and porcelain foam substrate (post sintering process). The sponge used was similar to the one used in our previous study [25]. The sponge template in Fig. 2(a) represents the sponge characteristics before the replication and sintering process. On the other hand, Fig. 2(b) describes the porcelain foam after the replication and sintering process. It is evident from Fig. 2 that the porcelain foam has open cell structure. The sintering process resulted in the formation of cracks as can be seen from Fig. 2(b). This is possibly due to the pyrolysis of the sponge template usually associated with the sintering process. According to Brown and Green [23], the occurance of cracks was because of strut expansion (volumetric increase) when the ester-type polyurethane was heated beyond its melting point. Several studies have suggested the use of thick coating or secondary coating as solution to overcome this problem [15, 23, 24].
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No significant weight change was observed during the initial test for the sponge as presented in [25]. However, it started to decompose in a two-step manner with an obvious steep decrease at approximately 250-330 ºC due to the decomposition of urethane bonds; the second decrease happened between 350 ºC and 420 ºC, attributed to loss of ester groups [26]. Evidently, no further weight loss was observed until the end of the test. Therefore, the 500 ºC soaking during sintering process was enough to eliminate the sponge template.
The sintered porcelain foam is composed of crystalline and glassy phases as is evident from Fig. 3. The crystalline structures is comprised of quartz (ICDD: 00-046-1045) and mullite (ICDD: 01-088-0890). The presence of glassy phase is obvious as shown by a bulge distortion at 2-theta range, between 15-35 °. According to Han et al. [27], the existence of the glassy phase contributes to the strength of the structure.
The XRD patterns of porcelain foam coated with SiC-, Cr-, and Ni-based coating slurry at different sintering temperatures are presented in Fig. 4. The XRD pattern in Fig. 4(a) confirms that SiC phase was deposited on or into the porcelain substrate with moissanite-4H (ICDD: 00-029-1127) and moissanite-6H (ICDD: 01-072-0018) as the main constituents. The intensity of moissanite-6H lowered as the sintering temperature increased. The diffraction pattern for Cr-based coating on porcelain foam substrate is shown in Fig. 4(b). Residual Cr can still be seen after sintering at 900 ºC. Furthermore, an intense formation of Cr2O3 was observed at 1150 ºC. As stated by Caplan et al. [28], the formation of Cr2O3 can be associated to nodule growth and ballooning formations on the surface of the sample, as seen in Fig. 5(c)(ii). Sintering at higher temperatures for Cr-based coating caused the formation of other constituent compounds such as the aluminum chromite (Al3Cr7, ICCD: 03-065-6108) and chromium spinel phase. These phases were believed to be the by-products of the reaction
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between the coating layer and porcelain melt which created a consolidated surface. The formation of nickel oxide (NiO2, ICDD: 01-089-5881) is apparent after the porcelain foam coated with Ni-base coating slurry was sintered at 900 ºC and 1150 ºC as seen in the XRD pattern in Fig. 4(c). Further sintering up to 1250 ºC produced NiAl2O4 phase which can be considered as a secondary constituent formed due to the reaction of additional coating material composition in the slurry.
The other visible phases in the XRD patterns are mullite, quartz, and cristobalite. The intensities of mullite and quartz phases appeared to be reduced at higher sintering temperatures, whereas the cristobalite phase increased. The observation of mullite, quartz, and cristobalite peaks in the XRD patterns dictate that they were originated from the porcelain substrate body and components of coating slurry (alumina, kaolin, and bentonite).
Fig. 5 shows the surface morphologies of SiC-, Ni-, and Cr-base coating layers, dipcoated and sintered at 1250 ºC. The inserts correspondingly signify an image enlargement of the coated substrate’s surface at 900 ºC, 1150 ºC, and 1250 ºC. In the dip-coating process, porcelain foam substrate is submerged into the respective coating slurry and then withdrawn at a constant rate. During the immersion and withdrawal process, the coating particles in the slurry gets deposited on the surface of the porcelain foam. The capillary forces associated with the pores and cracks, acts as a driving force for coating, in addition to the adhesion of particles by the binder. Another observation is that, the particles accumulated on the substrate body obstructs the open cell pathways. This creates a cluttered formation of accumulated particles, resulting in a closed cell morphology. Ahmad et al. [20] states that, a structure with an open cell morphology (foam) induces a lower capillary suction force for coating, relative to that prompted by micro-pores on the struts of a substrate. Moreover, the inserts shows that
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the coating particles are almost uniformly deposited. The particles are extended to the entire surface except for a few uncoated regions which are hard for the coating particles to cling on; such as the curve of the triangular struts.
Fig. 5 also exhibits the effects of sintering temperature (increases from 900 ºC to 1250 ºC) on the coating quality, as indicated by the XRD analyses (Fig. 4). At 900 ºC, all coating materials showed that the particles are distributed randomly and adhered loosely but still maintain their own shapes. However, for sintering temperatures of 1150 ºC and 1250 ºC, each coating material behaves differently due to the formation of new compounds. For SiC-based coating material, only the sample sintered at 1250 ºC shows a more intact layer; the particles clinging on the surface appears to be locked on a densified surface indicating the occurance of liquid phase sintering. This can be attributed to the dissolution of additional coating materials (bentonite and kaolin) and pre-melting of the porcelain foam substrate at elevated temperatures, causing the SiC particles to compact tightly over the surface of the substrate. A similar observation can be made for the Ni-based coating material. At 1250 ºC, the grain size of the coating layer appears to grow larger. The surface of the coating layer also appears to be relatively more consolidated compared to the sample sintered at lower temperatures due to the melting of the additional coating additives, thus creating a denser coating surface. For the Ni-based coating, the formation of new compounds (NiO2 and NiAl2O4) above 900 ºC, produced different surface morphologies. For Cr-based coating, the surface shows nodule growth and ballooning, caused by the formation of Cr2O3 and Al3Cr7. The adhesion between the coating particles and substrate for all coating materials is very strong at high temperatures and thus prevents the formation of an interlayer without causing any delamination.
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The porosity of porcelain foam and coated substrates after sintering are shown in Fig. 6. The results indicate that the porosity of coated porcelain foams (<76 %) are lesser compared to the original porcelain foam substrate (82 %). Furthermore, the porosity value also decreases as the sintering temperature increased throughout the coating. This fact is associated to the condition after dip-coating and sintering, in which the amount of open cells in the porcelain foam substrate are reduced significantly, as seen in Fig. 5. However, according to Yao et al. [29], macro and micro-cracks prone to occur in the skeleton of a reticulated porous structure especially in the sponge replication technique. The existence of pores and cracks in the porcelain foam substrate are believed to contribute to the increase in porosity value, apart from the high counts of open cells. However, the high percentage of porosity is compensated by the coating of the porcelain foam substrate. This eventually reduces the abundance of pores, cracks and open cells,and porosity of the plain substrate.
The compressive strength of the porcelain foam and coated substrates with respect to the sintering temperature of the coating are shown in Fig. 7. The compressive strength values of the porcelain foam and coated substrates are varied between 0.33-0.67 MPa, while the higher values of 0.38-0.67 MPa are recorded in the coated samples against the lower value (0.33 MPa) obtained in the plain substrate. The sintering temperature of coating at 1150 ºC indeed produces an intermediate compressive strength. Moreover, the samples sintered at the highest temperature indicate high compressive strength values. Besides, there is not much difference in terms of compressive strength value between SiC-, Cr-, and Ni-based coatings after sintering at the respective temperatures. According to Peng et al. [21], the compressive strength of ceramic foam increases with the rise in solid volume fraction and the strength of the strut. The solid volume fraction is associated with the increment of closed cell morphologies after dip-coating process. The latter can be related to the development of
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thicker struts due to the dissolution of additional coating materials, as well as the pre-melting of the surface of the porcelain foam substrate caused by liquid phase sintering at 1250 ºC.
Furthermore, the brittle foam or cellular material crushing strength theory by Ashby and Medalist [30], has demonstrated a relationship between mechanical behavior and relative density (
ρ ), as shown in Equation 3.1. The strength ( σ ) is associated to the relative density ρo
via the expression of:
§ȡ · ı =K¨ ¸ ıs © ȡo ¹
m
(3.1)
where, σ s is the strength of the strut material, whilst ρ and ρo are the bulk and theoretical density of the strut material, respectively. Meanwhile, K is the cellular geometry constant and exponent m equals to 3/2 or 2 depending on the cell morphology i.e. whether it is open or closed cells. In this case, the morphology of the porcelain foam substrate is similar to the open cells structure containing interconnected pores with struts as the wall.
The thermal conductivity measurements of porcelain foam and coated substrate sintered at 1250 ºC (measured at room temperature until 350 ºC) are shown in Fig. 8. The porcelain foam substrate, with 82 % porosity, has a kT value of 0.0461 W/mK at room temperature, while those of P.B20.SiC62.1250(55.9% porosity), P.B20.Cr62.1250(57.8 % porosity), and P.B20.Ni62.1250 (58.5 % porosity), have kT values in the order of 0.1326W/mK, 0.1358W/mK and 0.0921 W/mK respectively. The high porosity, larger cell size, and high counts of open cells are responsible for the low conductivity of plain substrate when compared to the coated samples, which showed better kT values. The other possible cause of kT difference is the states of cell walls (Fig. 5), which are either dense or porous. Dense cell walls show good continuity between the neighboring cells, consequently improving thermal conduction property. However, the porosity of ceramic substrate is the most dominant factor.
The distinct results of the porosity dependence of thermal conductivity [ kT (T ) ] can be partially explained by the rule-of-mixtures in Equation 3.2 [31]:
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kT (T ) = φ k g (T ) + (1 − φ ) ks (T )
(3.2)
where φ is the porosity, k g (T ) is the gaseous thermal conductivity of air at room temperature, and ks (T ) is the intrinsic thermal conductivity of the ceramic material. The value of k g (T ) is relatively small compared to the thermal conductivity of ceramic material (porcelain), and can be ignored especially at room temperature.
The thermal conductivity and insulation mechanism of a porous ceramic is controlled by the transports of phonon (transfer of energy associated by vibrations of crystal lattice) at room temperature and by photon (a form of electromagnetic radiation) at the radiation temperature [31]. Therefore, the overall conductivity measured at room temperature is only dominated by the phonon transporting through the solid phase. Thus, any factor that influences the reduction of solid phase or decreases the connectivity of the substrate’s skeleton, generally obstructs the transport of phonons in ceramic substrate. As a consequence, materials with high porosity and large cell size impede phonon transport, and is therefore responsible for the low thermal conductivity value.
At higher temperatures, the influence of photon at or near the radiation or glowing temperature of the porcelain foam and coated substrate is predominant. A two to four fold increase in the value of thermal conductivity was observed at 350 ºC when compared to the results taken at room temperature. This can be directly attributed to the influence of the radiation effect.
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The influence of an almost identical porosity for both porcelain foam and coated substrate to their respective thermal conductivity is presented in Fig. 9. The result shows that the thermal conductivity for coated porcelain foam is much higher than the uncoated substrate. About 150% increase in thermal conductivity can be obtained when the porcelain foam is coated. This has many applications related to porous media combustion. Higher thermal conductivity materials are expected to improve combustion and reduce NOx emissions [32], besides can be used as a stable and active catalyst material. This increase in thermal conductivity is despite 5% decrease in porosity for coated ceramics. This shows that, coating substances influence the properties of the thermal conductivity of the porcelain foam.
4.
Conclusion
This work has succesfully demonstrated the preparation of porcelain foams using the sponge replication technique, as well its coating with SiC-, Ni- and Cr-based materials using the dip-coating method. The XRD results of coated samples show existence of secondary phases (Cr2O3, Al3Cr7, NiO2, and NiAl2O4) due to sintering, mostly at higher temperatures. The porosity of coated porcelain foams (< 76 %) was less than the porosity of the original porcelain foam substrate (82 %). In general, a reduction in the number of pores, cracks and open cells was observed, with significant improvement in the compressive strength of the coated substrate. The thermal conductivity of coated substrate sintered at 1250 ºC was obtained in the range of 0.0921-0.1358 W/mK, when compared to the plain substrate at 0.0461 W/mK.
Acknowledgments
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The authors would like to acknowledge the technical staff of School of Materials and Mineral Resources Engineering (SMMRE) for their support and Universiti Sains Malaysia (USM) for funding provided under the RU-PRGS grant (1001/PBAHAN/8046032).
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Y. S. Han, J. B. A. O. Li, B. O. Chi, Z. H. E. Wen, The effect of sintering temperature on porous silica composite strength, J. Porous Mater. 10 (2003) 41-45.
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D. Caplan, A. Harvey, M. Cohen, Oxidation of chromium at 890-1200 ºC, Corros. Sci. 3 (1963) 161-175.
[29]
X. Yao, S. Tan, Z. Huang, D. Jiang, Effect of recoating slurry viscosity on the properties of reticulated porous silicon carbide ceramics, Ceram. Int. 32 (2006) 137142.
[30]
R. E. M. Medalist, The mechanical properties of cellular solids, Metal. Trans. A 14 (1983) 1755-1769.
[31]
Y.W. Lo, W.C.J. Wei, C.H. Hsueh, Low thermal conductivity of porous Al2O3 foams for SOFC insulation, Mater. Chem. Phys. 129 (2011) 326-330.
[32]
A.K. Ismail, M.Z. Abdullah, M. Zubair, Z.A. Ahmad, A.R. Jamaludin, K.F. Mustafa, A. R. Jamaludin, K.F. Mustafa, M. Z. Abdullah, Application of porous medium burner with micro cogeneration system, Energy. 50 (2013) 131–142.
List of table Table 1 Batch composition used in the experiment.
18
List of figures
Fig. 1 SEM micrographs of the base coating particles: (a) SiC, (b) Cr, and (c) Ni powder.
Fig. 2 SEM images of (a) sponge template and (b) porcelain foam substrate after sintered.
Fig. 3 XRD pattern of sintered porcelain foam substrate.
Fig. 4 XRD patterns of porcelain foam coated with (a) SiC-, (b) Cr-, and (c) Ni-based coating slurry and sintered at 900 ºC, 1150 ºC, and 1250 ºC, respectively.
Fig. 5 SEM micrographs of (a) SiC-, (b) Ni-, and (c) Cr-based coating layers sintered at 1250 ºC. The inserts (roman numerical) show magnified images of the coated surface at different sintering temperature (i) 900 ºC, (ii) 1150 ºC, and (iii) 1250 ºC, respectively.
Fig. 6 The percentage of porosity for the porcelain foam and coated substrate with respect to the sintering temperature of the coating.
Fig. 7 The compressive strength of the porcelain foam and coated substrate with respect to the sintering temperature of coating.
Fig. 8 Thermal conductivity of porcelain foam and coated substrate as a function of different testing temperatures.
19
Fig. 9 Dependence of thermal conductivity (bar – left ordinate) and porosity (line – right ordinate) of porcelain foam and coated substrate (sintered at 900 ºC and having an almost similar porosity).
20
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Fabrication of porcelain foam Substrates coated with SiC, Ni, and Cr using the dip-coating technique Abdul Rashid Jamaludin, Shah Rizal Kasim, Ahmad Kamal Ismail, Mohd Zukifly Abdullah, Zainal Arifin Ahmad
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(14)01672-1 http://dx.doi.org/10.1016/j.ceramint.2014.10.123 CERI9382
To appear in:
Ceramics International
Received date: Revised date: Accepted date:
29 August 2014 21 October 2014 21 October 2014
Cite this article as: Abdul Rashid Jamaludin, Shah Rizal Kasim, Ahmad Kamal Ismail, Mohd Zukifly Abdullah, Zainal Arifin Ahmad, Fabrication of porcelain foam Substrates coated with SiC, Ni, and Cr using the dip-coating technique, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2014.10.123 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Fabrication of Porcelain Foam Substrates Coated with SiC, Ni, and Cr Using the DipCoating Technique
Abdul Rashid Jamaludin1, Shah Rizal Kasim1, Ahmad Kamal Ismail2, Mohd Zukifly Abdullah2, Zainal Arifin Ahmad1,* 1
Structural Materials Niche Area, School of Materials and Mineral Resources, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia 2
Porous Media Combustion Laboratory, School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia *Corresponding author. Tel.:+60 45996127; fax: +60 45941011 E-mail address: [email protected]
Abstract This report presents a study on the properties of porcelain foam substrate fabricated via the sponge replication technique. The sintered porcelain foam substrates were dip-coated with SiC-, Cr-, and Ni-based coating slurries, and sintered again at 900-1250 ºC. The XRD results of coated samples show existence of secondary phases (Cr2O3, Al3Cr7, NiO2, and NiAl2O4) due to sintering, mostly at higher temperatures. The porosity of the plain substrate was reduced significantly after dip-coating and proportionally decreased as the sintering temperature increased, therefore influencing the compressive strength value. Furthermore, the thermal conductivity of coated substrate sintered at 1250 ºC was obtained in the range of 0.0921-0.1358 W/mK, when compared to the plain substrate at 0.0461 W/mK. In general, a reduction in the number of pores, cracks and open cells was observed, with significant improvement in the compressive strength of the coated substrate.
1
Keywords: Sintering (A); Porosity (B); Thermal conductivity (C); Fracture (C); Porcelain (D)
1.
Introduction
Coating of ceramic, polymer, or metal, safeguards certain properties of the material from external degradation. Ceramic coating materials have received considerable attention due its ability to withstand high temperature, resistance to corrosion, wear and tear, extension of material service life, economic consideration, and improvement of certain other material properties such as strength and conductivity (thermal and electrical). This kind of coated ceramic substrates have been utilized in a wide range of applications such as the filtration process [1], membrane layer for micro and ultra-filtration [2], separator [3], catalyst [4], and porous media burner applications [5]. Recently, Mueller et al. [5] used porous alumina substrate dip-coated with SiC in a porous medium burner (PMB). The results obtained showed that SiC-coated substrate facilitated good combustion for stoichiometric fuel-to-air ratio. In another study, Chung [6] proposed SiC, as suitable filler materials to fabricate ceramic and metal-matrix composites for applications requiring high thermal conductivity. Besides that, oxide (Al2O3) and base metal catalyst (Ni/SiC and Ni/Al2O3) were previously used in partial oxidation of methane [7,8].
Nowadays, there are many processing methods or techniques used in the fabrication of coating layers. The more traditional methods of coating a porous substrate involve the use of dip-coating method or also known as chemical solution deposition (CSD) technique [9,10]. Besides, the dip-spin coating [11], and spray-coating [12] are also commonly used. Futhermore, some advanced processes such as the atomic layer deposition [13], chemical
2
vapour deposition (CVD) [14], physical vapour deposition (PVD) [15], electrodeposition[16], and magnetron sputtering [17] techniques can be found in the literature. However, these techniques have their drawbacks namely; high cost, involvement of corrosive materials or aggressive chemicals, high thermal mismatch between substrate and coating layer, cracking of coating layer, inhomogeneity in terms of thickness of deposited coating and restriction on the shape of the substrate [18].
The principle of dip-coating involves the wetting of the substrate with a solution. The capillary suction pressure of the porous substrate creates a wet cake from the accumulation of particles (that cannot enter the pores) [19]. This process can produce a coating layer/thickness in the range of 100 nm to 100 µm [9]. Over the years, the dip-coating technique has been popularly used on account of the fact that it is simple, low cost, less time consuming, cheaper base material and precursors can be used, good control on layer thickness, and less wastage during fabrication of both porous and dense coatings [9,10,20]. Moreover, dip-coating is a very straightforward process and the most suitable technique used to coat porous substrates. This is because, the porous substrates have reticulated structure which can be difficult to coat using other coating methods [5]. The immersion of substrate in the coating slurry or suspension ensures that most of the material surface is completely coated. Layers are easily obtained by evaporation of solutions containing metal oxide precursors, organic matters, monomers, polymers or various kinds of nanoparticles. Solvents are used, usually alcohol, because of its’ low surface tension and fairly high volatility [9]. However, any aqueous solution can be easily deposited when proper processing conditions are applied.
The polymeric sponge method is widely used in the fabrication of reticulated ceramics. Although, it creates more open cell structures in the range of few millimeters, but is
3
still prone to struts cracks [21,22]. Occurrence of cracks on the porcelain foam struts can be attributed to the pyrolysis of the sponge template during the sintering process. This can be overcome by forming a thick ceramic coating or introducing a secondary coat over the virgin template [20,23,24].
Although, the thermodynamic properties of SiC, Ni and Cr are well known, the use of SiC-, Ni- and Cr-based materials as coating additives for porcelain foam substrate has not been used. This is the first such attempt to develop porcelain foam substrate coated with SiC, Ni and Cr based materials. It is expected that, coating of the ceramic substrate will eliminate defects and cracks. Besides, it is expected to significantly improve the thermal efficiency of the coated substrate with the addition of conductive base coating materials to the slurry.
2.
Experimental procedures
2.1
Preparation of porcelain foam substrate
The porcelain foam substrate was prepared using the sponge replication technique. Ready-made porcelain raw powder supplied by Ipoh Ceramic Sdn Bhd was used for slurry preparation. The solid to liquid content ratio was fixed at 3:2. Sodium silicate (0.5wt% Na2SiO3) was added as a dispersing agent for the mixing process. The slurry was prepared by mixing the mixture for 24 h in a polyethylene bottle. A commercial polyurethane (PU) sponge with interconnected open cells (average cell of 20 pores per inch) purchased from CCT Automation Sdn. Bhd. was chosen as the template and further modified into smaller dimensions (10 cm × 10cm × 2.5 cm). Initially, the sponge template was submerged and compressed in the porcelain slurry. This ensured the template was completely filled with slurry and removed all the trapped air. Next, the impregnated sponge was taken out, squeezed
4
manually by hand and pressed between two preset rollers. This process was repeated three times to facilitate adequate filling of slurry and to remove excess slurry, as well as to achieve a well-distributed coating over the template.The impregnated sponge was dried at room temperature for 48 h before being put in an oven for 24 h at 80 ºC. Sintering was done using an electrical furnace (Unitek Muffle Furnace 1600) with a starting heating rate of 5oC/min up to 500 ºC (soak for 1 h); and then further heating up to 1250 ºC (soaked for 2 h) before cooling down to room temperature.
2.2
Process of coating
The coating slurries were prepared by wet-mixing the raw materials as shown in Table 1. The ratio of solid to liquid content was fixed at 1:4. Polyvinyl alcohol (PVA), which acts as a binder was diluted in distilled water prior to the mixing process. The batches were mixed for 24 h in a polyurethane bottle. The process of infiltrating the coating slurry into the sintered porcelain foam substrate was accomplished by dipping the substrate for a period of 15 s. This impregnated substrate was dried for 20 min before being dipped again in the same coating slurry. The impregnated substrate was rinsed to remove excess solution after each dipping process. Next, the coated substrate was dried for 24 h at room temperature and later subjected to further drying at 80 ºC for another 24 h in an oven. The sintering process was carried out in normal furnace atmosphere of 900 ºC, 1150 ºC, and 1250 ºC,with a 5 ºC/min heating rate and 2 h of soaking time .
Table 1 Batch composition used in the experiment Sample name
*
Material composition (wt%)
Sintering
SiC Cr Ni Bentonite Alumina Kaolin PVA
condition (ºC)
5
P.B20.SiC62.900 P.B20.SiC62.1150
900 62
20
1
10
7
1150
P.B20.SiC62.1250
1250
P.B20.Cr62.900
900
P.B20.Cr62.1150
62
20
1
10
7
1150
P.B20.Cr62.1250
1250
P.B20.Ni62.900
900
P.B20.Ni62.1150
62
20
1
10
7
P.B20.Ni62.1250
1150 1250
*Symbols (P.B20.x62.T) of the sample notation are: “P” is porcelain, “B” is bentonite, “x” are SiC, Ni, and Cr, respectively. The number “20” and “62” are amount of subjected material in weight percent, while “T” represents the sintering temperature.
The particle size distributions for the raw coating powders were determined using the Mastersizer S, Malvern Instrument Ltd., UK. The thermal analysis of the sponge was performed under athmospheric air, at a heating rate of 10 °C/min using thermal gravimetric analysis (TGA/DSC Linsesis, Germany). The microstructural morphology of the sintered porcelain foam substrate and coating layer were established from the scanning electron microscopy (SEM) method using table top TM3000 Hitachi, Japan and Zeiss Supra 35VP FE-SEM, Germany. The phase analysis of the sintered samples was measured via the X-ray diffraction method (XRD Bruker D8 Advance, Germany) with CuKĮ monochromatic radiation (Ȝ= 1.5406 Å). In addition, the Archimedes displacement technique was used to measure the bulk density and total porosity of the sintered samples.
The compressive strengths of porcelain foam substrate before and after coating were measured using a universal testing machine (Instron 5982, USA) with a crosshead speed of 1 mm/min and load cell of 5 kN. Square-shaped samples (15 mm × 15 mm × 15 mm) were 6
used for this measurement. The value of compressive strength associated with the load applied on the sample until it fractured, was calculated by dividing the maximum load at fracture over the cross-sectional area. Finally, the thermal conductivity test for the porcelain foam and coated substrate (10 mm × 10 mm × 5 mm) was conducted using the Hot-Disk Thermal Constants Analyzer, UK.
3.
Results and discussion
Fig. 1 shows the SEM images of the base coating materials used in this study. The particles of SiC (9.92µm to 92.6µm) and Cr (35.46µm to 103.2µm) observed are in the form of flakes. In contrast, Ni particles (8.18ȝm to 191.57ȝm) are spherical in shape and found to be agglomerated.
Fig. 2 presents SEM images of raw PU sponge and porcelain foam substrate (post sintering process). The sponge used was similar to the one used in our previous study [25]. The sponge template in Fig. 2(a) represents the sponge characteristics before the replication and sintering process. On the other hand, Fig. 2(b) describes the porcelain foam after the replication and sintering process. It is evident from Fig. 2 that the porcelain foam has open cell structure. The sintering process resulted in the formation of cracks as can be seen from Fig. 2(b). This is possibly due to the pyrolysis of the sponge template usually associated with the sintering process. According to Brown and Green [23], the occurance of cracks was because of strut expansion (volumetric increase) when the ester-type polyurethane was heated beyond its melting point. Several studies have suggested the use of thick coating or secondary coating as solution to overcome this problem [15, 23, 24].
7
No significant weight change was observed during the initial test for the sponge as presented in [25]. However, it started to decompose in a two-step manner with an obvious steep decrease at approximately 250-330 ºC due to the decomposition of urethane bonds; the second decrease happened between 350 ºC and 420 ºC, attributed to loss of ester groups [26]. Evidently, no further weight loss was observed until the end of the test. Therefore, the 500 ºC soaking during sintering process was enough to eliminate the sponge template.
The sintered porcelain foam is composed of crystalline and glassy phases as is evident from Fig. 3. The crystalline structures is comprised of quartz (ICDD: 00-046-1045) and mullite (ICDD: 01-088-0890). The presence of glassy phase is obvious as shown by a bulge distortion at 2-theta range, between 15-35 °. According to Han et al. [27], the existence of the glassy phase contributes to the strength of the structure.
The XRD patterns of porcelain foam coated with SiC-, Cr-, and Ni-based coating slurry at different sintering temperatures are presented in Fig. 4. The XRD pattern in Fig. 4(a) confirms that SiC phase was deposited on or into the porcelain substrate with moissanite-4H (ICDD: 00-029-1127) and moissanite-6H (ICDD: 01-072-0018) as the main constituents. The intensity of moissanite-6H lowered as the sintering temperature increased. The diffraction pattern for Cr-based coating on porcelain foam substrate is shown in Fig. 4(b). Residual Cr can still be seen after sintering at 900 ºC. Furthermore, an intense formation of Cr2O3 was observed at 1150 ºC. As stated by Caplan et al. [28], the formation of Cr2O3 can be associated to nodule growth and ballooning formations on the surface of the sample, as seen in Fig. 5(c)(ii). Sintering at higher temperatures for Cr-based coating caused the formation of other constituent compounds such as the aluminum chromite (Al3Cr7, ICCD: 03-065-6108) and chromium spinel phase. These phases were believed to be the by-products of the reaction
8
between the coating layer and porcelain melt which created a consolidated surface. The formation of nickel oxide (NiO2, ICDD: 01-089-5881) is apparent after the porcelain foam coated with Ni-base coating slurry was sintered at 900 ºC and 1150 ºC as seen in the XRD pattern in Fig. 4(c). Further sintering up to 1250 ºC produced NiAl2O4 phase which can be considered as a secondary constituent formed due to the reaction of additional coating material composition in the slurry.
The other visible phases in the XRD patterns are mullite, quartz, and cristobalite. The intensities of mullite and quartz phases appeared to be reduced at higher sintering temperatures, whereas the cristobalite phase increased. The observation of mullite, quartz, and cristobalite peaks in the XRD patterns dictate that they were originated from the porcelain substrate body and components of coating slurry (alumina, kaolin, and bentonite).
Fig. 5 shows the surface morphologies of SiC-, Ni-, and Cr-base coating layers, dipcoated and sintered at 1250 ºC. The inserts correspondingly signify an image enlargement of the coated substrate’s surface at 900 ºC, 1150 ºC, and 1250 ºC. In the dip-coating process, porcelain foam substrate is submerged into the respective coating slurry and then withdrawn at a constant rate. During the immersion and withdrawal process, the coating particles in the slurry gets deposited on the surface of the porcelain foam. The capillary forces associated with the pores and cracks, acts as a driving force for coating, in addition to the adhesion of particles by the binder. Another observation is that, the particles accumulated on the substrate body obstructs the open cell pathways. This creates a cluttered formation of accumulated particles, resulting in a closed cell morphology. Ahmad et al. [20] states that, a structure with an open cell morphology (foam) induces a lower capillary suction force for coating, relative to that prompted by micro-pores on the struts of a substrate. Moreover, the inserts shows that
9
the coating particles are almost uniformly deposited. The particles are extended to the entire surface except for a few uncoated regions which are hard for the coating particles to cling on; such as the curve of the triangular struts.
Fig. 5 also exhibits the effects of sintering temperature (increases from 900 ºC to 1250 ºC) on the coating quality, as indicated by the XRD analyses (Fig. 4). At 900 ºC, all coating materials showed that the particles are distributed randomly and adhered loosely but still maintain their own shapes. However, for sintering temperatures of 1150 ºC and 1250 ºC, each coating material behaves differently due to the formation of new compounds. For SiC-based coating material, only the sample sintered at 1250 ºC shows a more intact layer; the particles clinging on the surface appears to be locked on a densified surface indicating the occurance of liquid phase sintering. This can be attributed to the dissolution of additional coating materials (bentonite and kaolin) and pre-melting of the porcelain foam substrate at elevated temperatures, causing the SiC particles to compact tightly over the surface of the substrate. A similar observation can be made for the Ni-based coating material. At 1250 ºC, the grain size of the coating layer appears to grow larger. The surface of the coating layer also appears to be relatively more consolidated compared to the sample sintered at lower temperatures due to the melting of the additional coating additives, thus creating a denser coating surface. For the Ni-based coating, the formation of new compounds (NiO2 and NiAl2O4) above 900 ºC, produced different surface morphologies. For Cr-based coating, the surface shows nodule growth and ballooning, caused by the formation of Cr2O3 and Al3Cr7. The adhesion between the coating particles and substrate for all coating materials is very strong at high temperatures and thus prevents the formation of an interlayer without causing any delamination.
10
The porosity of porcelain foam and coated substrates after sintering are shown in Fig. 6. The results indicate that the porosity of coated porcelain foams (<76 %) are lesser compared to the original porcelain foam substrate (82 %). Furthermore, the porosity value also decreases as the sintering temperature increased throughout the coating. This fact is associated to the condition after dip-coating and sintering, in which the amount of open cells in the porcelain foam substrate are reduced significantly, as seen in Fig. 5. However, according to Yao et al. [29], macro and micro-cracks prone to occur in the skeleton of a reticulated porous structure especially in the sponge replication technique. The existence of pores and cracks in the porcelain foam substrate are believed to contribute to the increase in porosity value, apart from the high counts of open cells. However, the high percentage of porosity is compensated by the coating of the porcelain foam substrate. This eventually reduces the abundance of pores, cracks and open cells,and porosity of the plain substrate.
The compressive strength of the porcelain foam and coated substrates with respect to the sintering temperature of the coating are shown in Fig. 7. The compressive strength values of the porcelain foam and coated substrates are varied between 0.33-0.67 MPa, while the higher values of 0.38-0.67 MPa are recorded in the coated samples against the lower value (0.33 MPa) obtained in the plain substrate. The sintering temperature of coating at 1150 ºC indeed produces an intermediate compressive strength. Moreover, the samples sintered at the highest temperature indicate high compressive strength values. Besides, there is not much difference in terms of compressive strength value between SiC-, Cr-, and Ni-based coatings after sintering at the respective temperatures. According to Peng et al. [21], the compressive strength of ceramic foam increases with the rise in solid volume fraction and the strength of the strut. The solid volume fraction is associated with the increment of closed cell morphologies after dip-coating process. The latter can be related to the development of
11
thicker struts due to the dissolution of additional coating materials, as well as the pre-melting of the surface of the porcelain foam substrate caused by liquid phase sintering at 1250 ºC.
Furthermore, the brittle foam or cellular material crushing strength theory by Ashby and Medalist [30], has demonstrated a relationship between mechanical behavior and relative density (
ρ ), as shown in Equation 3.1. The strength ( σ ) is associated to the relative density ρo
via the expression of:
§ȡ · ı =K¨ ¸ ıs © ȡo ¹
m
(3.1)
where, σ s is the strength of the strut material, whilst ρ and ρo are the bulk and theoretical density of the strut material, respectively. Meanwhile, K is the cellular geometry constant and exponent m equals to 3/2 or 2 depending on the cell morphology i.e. whether it is open or closed cells. In this case, the morphology of the porcelain foam substrate is similar to the open cells structure containing interconnected pores with struts as the wall.
The thermal conductivity measurements of porcelain foam and coated substrate sintered at 1250 ºC (measured at room temperature until 350 ºC) are shown in Fig. 8. The porcelain foam substrate, with 82 % porosity, has a kT value of 0.0461 W/mK at room temperature, while those of P.B20.SiC62.1250(55.9% porosity), P.B20.Cr62.1250(57.8 % porosity), and P.B20.Ni62.1250 (58.5 % porosity), have kT values in the order of 0.1326W/mK, 0.1358W/mK and 0.0921 W/mK respectively. The high porosity, larger cell size, and high counts of open cells are responsible for the low conductivity of plain substrate when compared to the coated samples, which showed better kT values. The other possible cause of kT difference is the states of cell walls (Fig. 5), which are either dense or porous. Dense cell walls show good continuity between the neighboring cells, consequently improving thermal conduction property. However, the porosity of ceramic substrate is the most dominant factor.
The distinct results of the porosity dependence of thermal conductivity [ kT (T ) ] can be partially explained by the rule-of-mixtures in Equation 3.2 [31]:
12
kT (T ) = φ k g (T ) + (1 − φ ) ks (T )
(3.2)
where φ is the porosity, k g (T ) is the gaseous thermal conductivity of air at room temperature, and ks (T ) is the intrinsic thermal conductivity of the ceramic material. The value of k g (T ) is relatively small compared to the thermal conductivity of ceramic material (porcelain), and can be ignored especially at room temperature.
The thermal conductivity and insulation mechanism of a porous ceramic is controlled by the transports of phonon (transfer of energy associated by vibrations of crystal lattice) at room temperature and by photon (a form of electromagnetic radiation) at the radiation temperature [31]. Therefore, the overall conductivity measured at room temperature is only dominated by the phonon transporting through the solid phase. Thus, any factor that influences the reduction of solid phase or decreases the connectivity of the substrate’s skeleton, generally obstructs the transport of phonons in ceramic substrate. As a consequence, materials with high porosity and large cell size impede phonon transport, and is therefore responsible for the low thermal conductivity value.
At higher temperatures, the influence of photon at or near the radiation or glowing temperature of the porcelain foam and coated substrate is predominant. A two to four fold increase in the value of thermal conductivity was observed at 350 ºC when compared to the results taken at room temperature. This can be directly attributed to the influence of the radiation effect.
13
The influence of an almost identical porosity for both porcelain foam and coated substrate to their respective thermal conductivity is presented in Fig. 9. The result shows that the thermal conductivity for coated porcelain foam is much higher than the uncoated substrate. About 150% increase in thermal conductivity can be obtained when the porcelain foam is coated. This has many applications related to porous media combustion. Higher thermal conductivity materials are expected to improve combustion and reduce NOx emissions [32], besides can be used as a stable and active catalyst material. This increase in thermal conductivity is despite 5% decrease in porosity for coated ceramics. This shows that, coating substances influence the properties of the thermal conductivity of the porcelain foam.
4.
Conclusion
This work has succesfully demonstrated the preparation of porcelain foams using the sponge replication technique, as well its coating with SiC-, Ni- and Cr-based materials using the dip-coating method. The XRD results of coated samples show existence of secondary phases (Cr2O3, Al3Cr7, NiO2, and NiAl2O4) due to sintering, mostly at higher temperatures. The porosity of coated porcelain foams (< 76 %) was less than the porosity of the original porcelain foam substrate (82 %). In general, a reduction in the number of pores, cracks and open cells was observed, with significant improvement in the compressive strength of the coated substrate. The thermal conductivity of coated substrate sintered at 1250 ºC was obtained in the range of 0.0921-0.1358 W/mK, when compared to the plain substrate at 0.0461 W/mK.
Acknowledgments
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The authors would like to acknowledge the technical staff of School of Materials and Mineral Resources Engineering (SMMRE) for their support and Universiti Sains Malaysia (USM) for funding provided under the RU-PRGS grant (1001/PBAHAN/8046032).
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List of table Table 1 Batch composition used in the experiment.
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List of figures
Fig. 1 SEM micrographs of the base coating particles: (a) SiC, (b) Cr, and (c) Ni powder.
Fig. 2 SEM images of (a) sponge template and (b) porcelain foam substrate after sintered.
Fig. 3 XRD pattern of sintered porcelain foam substrate.
Fig. 4 XRD patterns of porcelain foam coated with (a) SiC-, (b) Cr-, and (c) Ni-based coating slurry and sintered at 900 ºC, 1150 ºC, and 1250 ºC, respectively.
Fig. 5 SEM micrographs of (a) SiC-, (b) Ni-, and (c) Cr-based coating layers sintered at 1250 ºC. The inserts (roman numerical) show magnified images of the coated surface at different sintering temperature (i) 900 ºC, (ii) 1150 ºC, and (iii) 1250 ºC, respectively.
Fig. 6 The percentage of porosity for the porcelain foam and coated substrate with respect to the sintering temperature of the coating.
Fig. 7 The compressive strength of the porcelain foam and coated substrate with respect to the sintering temperature of coating.
Fig. 8 Thermal conductivity of porcelain foam and coated substrate as a function of different testing temperatures.
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Fig. 9 Dependence of thermal conductivity (bar – left ordinate) and porosity (line – right ordinate) of porcelain foam and coated substrate (sintered at 900 ºC and having an almost similar porosity).
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Figure 1
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Figure 9