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High temperature thermal barrier coatings from recycled fly ash cenospheres
Applied Thermal Engineering 48 (2012) 117e121
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
High temperature thermal barrier coatings from recycled fly ash cenospheres A. Arizmendi-Morquecho a, A. Chávez-Valdez b, J. Alvarez-Quintana a, * a b
Centro de Investigación en Materiales Avanzados S.C. Unidad Monterrey, Alianza Nte. 202, 66600 Apodaca, Nuevo León, Mexico Institute of Biomaterials, Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Cauerstr. 6, 91058 Erlangen, Germany
h i g h l i g h t s < New FAC thermal barriers with potentiality for high temperature applications (HTA). < A thermal conductivity value as low as 0.17 W/m K for FAC was found at 1200 K. < Low thermal expansion coefficient of 5.96 106 for FAC was found. This avoids microcracking at HTA. < HT values of a and e guarantee a good performance during unsteady state operation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 December 2011 Accepted 2 May 2012 Available online 11 May 2012
The high temperature behavior of electrophoretically deposited thermal barrier coatings based on recycled fly ash cenospheres is presented. Thermal properties such as thermal expansion coefficient, specific heat, thermal conductivity, thermal diffusivity and thermal effusivity of fly ash (FA) and fly-ash cenospheres (FAC) were measured in the temperature range of 373 Ke1173 K. Thermal conductivity values as low as 0.17 W/m K and 0.32 W/m K for FAC and FA respectively at 1200 K were found. These results confirm their potentiality as ultra-low thermal conductivity thermal insulators for high temperature applications. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: High temperature insulators Thermal barrier coatings Fly ash cenospheres
1. Introduction Coal power plants produce 41% of global electricity. Nevertheless, in addition to electricity these plants also produce a waste material because of the non-combustible mineral portion of coal. That material is compose of microscopic flakes and hollow spheres of mullite (3Al2O32SiO2) which are called fly ash cenospheres and fly ash cenospheres respectively. Mullite is a promising material as a coating because of its excellent corrosion resistance, high thermal stability, its permanence in severe chemical environments and its low thermal conductivity. Moreover, when compared to currently thermal protective coatings which are based on yttrium stabilized zircon (YSZ) it has a coefficient of thermal expansion much lower [1,2]. Globally, coal-fired power plants produce 500 million tons of fly ash each year [3]. The recoverability of fly ash depends significantly on the manner in which it is disposed, in addition there are regulations related with the areas can be used to place such wastes. Fly ash disposed of in a monofill or holding pond likely would be suitable for beneficial use because it has generally not been commingled with other materials [4]. Unfortunately, the American Coal
* Corresponding author. Tel.: þ52 8111560829; fax: þ52 8111560820. E-mail address: [email protected] (J. Alvarez-Quintana). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2012.05.004
Ash Association (ACAA) is unaware of data indicating how much of the 100e500 million tons of stockpiled fly ash is deposited in monofills or holding ponds [5]. In 2007, only the United States produced 131 million tons of coal combustion products. While 43 percent were used beneficially, nearly 75 million tons were disposed of [6]. It is clear that by using ashes instead of disposing of it in landfills are avoiding the environmental degradation and energy costs associated with mining virgin materials [4]. Unlike the earlier work where the thermal properties of the heat barriers were analyzed in the temperature range from 100 K to 500 K [7], here we deal with an extensive study of thermophysical properties of thermal barrier coatings (TBCs) based on recycled fly ash cenospheres and their thermal behavior beyond room temperature (from 300 K to 1173 K). In addition to the aim of this research, here we are extending the applications of recycled fly ash cenospheres in areas other than filler material or in the concrete industry, in order to reduce the environmental problems related with its disposal. The thermophysical properties governing the thermal energy propagation in solids are the thermal conductivity (k), thermal diffusivity (a) and thermal effusivity (e). However, while the role of k is well known, the role played by a and e is usually undervalued and misunderstood mainly because they describe the unsteady thermal state of the system. Thermal diffusivity is the quantity associated with the speed of propagation of thermal energy in
118
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a material, whereas the thermal effusivity is the capacity to exchange thermal energy with the surroundings [8]. The importance of a and e lies not only in the aforementioned characteristics, but also on doing a complete thermal characterization of the material in order to know a better understanding of its thermal behavior in a dynamic regime, especially in the evaluation of material performance as either heat insulator or as heat dissipator. High temperature thermal barrier applications require materials which can withstand increasing temperatures [9]. Applications such as, gas burners, molten metal filters, kitchen stoves and many others are subjected to thermal distributions which could induce microcracking by thermal expansion [10]. Therefore, it is important to know the high temperature behavior of thermophysical properties such as: specific heat, thermal conductivity, thermal diffusivity and thermal effusivity which are involved in these phenomena.
2. Experimental procedure A wide explanation about samples preparation, chemical composition and structural analysis was previously reported [7]. Nevertheless, because of the geometrical requirements of the existing equipments for high temperature thermal characterization new samples have been synthesized. For thermal diffusivity measurements, cylindrical samples of 1 cm in diameter of FAC and FA specimens were prepared by uniaxial pressure at 250 MPa. Next, the specimens were sintered at 1000, 1100 and 1200 C during 3 h with heating rates of 5 K/min. It is important to note that uniaxial pressing does not affect the spherical structure of the cenospheres in FAC samples. The Laser Flash Analysis (LFA) method was used to do the thermal diffusivity measurements [11]. It is an ASTM standard method (E1461), which involves subjecting the entire front surface of a sample to a very short burst of energy from a CO2 laser. Specimen’s surfaces were gold coated to prevent the laser transmission and to ensure a good absorption of the IR energy. An IR detector monitors the temperature increment of the opposite side of the sample. Then, the thermal diffusivity is calculated from the temperature rise versus time profile in the temperature range from 298 K to 1173 K.
Differential Scanning Calorimetry (DSC) measurements to obtain specific heat were carried out for FAC and FA powders at a heating rate of 10 K/min in dry argon atmosphere using TA Q200 equipment (Thermal Analysis Co., USA). In this equipment the cell contains a unique TzeroÔ sensor that allows detection and compensation for resistance and capacitance imbalances that can negatively affect baseline flatness, sensitivity, and resolution. The resulting heat flow signal provided a more accurate representation of the actual heat flowing to and from the sample with temperature precision of 0.05 C, temperature accuracy of 0.1 C, baseline curvature of 10 mW, sensitivity of 0.2 mW and Indium height/width resolution of 30 mW/ C. The DSC module records time dynamics heat flow during sample heating allowing for determining specific heat capacity by comparing thermal response of the sample with that of a standard reference. Furthermore, sample density was determined from volumeemass measurements; here, the mass of the samples was measured in a Mettler Toledo XS205 dual range analytical balance with accuracy of 0.01 mg (0e81 g), readability of 0.01 mg/0.1 mg, repeatability of 0.02 mg/0.05 mg and linearity of 0.2 mg. The volume was calculated with the dimensions of the specimens, the diameter of 1 cm and thickness measured with a Vernier caliper instrument. Finally, the thermal conductivity and thermal effusivity were obtained directly from k ¼ aCpr and e ¼ a1/2Cpr, respectively by multiplying the experimental values of the thermal diffusivity, specific heat and density of the fly ash specimens. In order to get a complete thermal characterization, thermal expansion coefficients for FAC and FA samples were also measured by using a dilatometer (model Netzsch DIL 402 PC) in the temperature range from 300 to 1473 K and heating rate of 10 K/min. To perform this test, additional cylindrical samples with length of 40 mm and diameter of 1 cm were prepared by uniaxial pressing under 125 MPa.
3. Results and discussion The specific heat capacity of FAC and FA powders at different temperatures is presented in Fig. 1a. In general, this temperature dependence of the specific heat capacity is in line with the typical
Fig. 1. a) Specific heat capacity and b) thermal expansion coefficient of FAC and FA samples as a function of the temperature.
A. Arizmendi-Morquecho et al. / Applied Thermal Engineering 48 (2012) 117e121
behavior for solid materials. At low temperatures, it can be observed the tendency to the T3 law behavior. For high temperatures, the specific heat increases rapidly to the constant high temperature limit predicted for polyatomic solids [12]. Specific heat capacity values of FAC are higher than values for FA. The behavior of specific heat capacity in our materials agrees with the previously reported works [13e16] where the difference in this property is attributed to differences in their chemical composition. For instance; due to the presence of a high content of Fe2O3, a higher specific heat capacity values in FAC samples are presented than in FA samples. We have also measured the thermal expansion coefficient of the FAC and FA samples, Fig. 1b shows the experimental data obtained. The change in dimensions is result of an increase in the amplitude of vibration between atoms in the materials, both materials present similar temperature behaviors. The main difference between FAC and FA materials takes place in the range of 50e300 C, where a significant expansion is seen for FA because the aeb transition of the high temperature polymorphic form of silica (cristobalite). This happens due to the low density and the high specific volume of the high temperature phase, which corresponds to a discontinuous increase in volume as previously observed [17]. The thermal expansion coefficients of the materials observed in this temperature range were 10.2 106 and 6.3 106/ C for FA and FAC samples, respectively. In the temperature range of 570e1270 C, the correspondents thermal expansion coefficients of FA and FAC samples were estimated to be 5.96 106 and 6.13 106/ C; these values are very similar to those reported for ceramic materials [18]. Other changes in the materials occur at temperatures of 1390 and 1410 K for FA and FAC samples: respectively, where the samples begin to shrink very fast because of the sintering in the presence of liquid. Fig. 2 shows the thermal diffusivity behavior of FAC and FA samples. It can be observed that it depends on the densification degree of the specimens. For instance; thermal diffusivity values for FAC and FA denser specimens (sintered at 1200 C) turn out to be higher for the whole temperature range in comparison with diffusivity of the samples sintered at lower temperatures (1000 and
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1100 C). Besides, thermal diffusivity values for FAC and FA are almost temperature independent and it can be observed that these values are nearly constant in the whole temperature range varying from 2 107 to 7 107 m2/s and 2 107 to 5 107 m2/s for FAC and FA respectively. Nevertheless; contrary to the expected, although FAC samples have lower thermal conductivity than FA samples [7], FAC samples shows slightly higher thermal diffusivity values than FA samples. The possible reason is related to the air filled core; air shows thermal diffusivity values as high as that of metals [8], and this might help to enhance the diffusivity of FAC samples. It is clear that microstructural and morphological characteristics also influence significantly the thermal diffusivity behavior. In time-dependent heat transfer problems another important material’s thermal property is its thermal effusivity. In this situation the heat flow through the interface between the material and its surroundings is not proportional to the thermal conductivity of the material, as under steady conditions, but to its thermal effusivity [8,19]. Fig. 3 shows thermal effusivity values for FAC and FA samples. It can be observed that it depends on the specimen’s densification as well. Thermal effusivity values for FAC and FA are temperature dependent and these values change in the whole temperature range from 170 to 550 m2/s and from 400 to 900 m2/s for FAC and FA samples, respectively. Therefore, FA samples present higher effusivity values than FAC samples. As mentioned before, it could be due to the air filled cores in FAC samples; in this case, air has very low thermal effusivity values in comparison with other materials [8]. High thermal effusivity values in FA samples mean that under transient conditions they could extract more heat than FAC samples. Thermal conductivity behavior is widely explained in the temperature range from 100 to 500 K in reference [7]. Therefore, in this section we only highlight the most important points presented in Fig. 4. In this figure, solid symbols represent the low temperature thermal conductivity values previously reported [7], and open symbols represent the high temperature values obtained in this work. It is clear that high temperature data follow the tendency of the low temperature data. Solid lines represent the previous
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Fig. 2. Thermal diffusivity as a function of the temperature for FAC and FA samples sintered at different temperatures. Dotted lines are only a guide to the eye.
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Fig. 3. Thermal effusivity as a function of the temperature for FAC and FA samples sintered at different temperatures. Dotted lines are only guide to the eye.
reported modeling in the framework of the self-consistent field concept (SCF) and a modified Maxwell equation (MME) [[7] and references therein]. It can be observed that high temperature thermal conductivity data fits relatively well the proposed models. As expected, FAC coatings show lower thermal conductivity values than FA coatings due to the hollow nature of the cenospheres which limits the heat transfer. The minimum thermal conductivity values at 1200 K are approximately 0.17 W/m K and 0.32 W/m K for FAC and FA samples sintered at 1000 C respectively. These values are relatively lower than values reported for highly disordered oxides (amorphous or polycrystalline) such as fused Silica and Yttria Stabilized Zirconia (YSZ),
which are currently the main materials used as thermal barrier coatings [20,21]. For instance, the thermal conductivity of YSZ as a function of grain size, 10e100 nm, has been measured by Yang et al. [22] up to 500 K with values ranging from 1.2 W/m K to 2.3 W/m K. Moreover, when compared with porous and dense mullite (3Al2O3$2SiO2 or 2Al2O3$SiO2), it has been reported that porous mullite with porosity of 57% and pores sizes about 40 mm shows a thermal conductivity of around 1 W/m K and almost constant in the temperature range from 200 to 1300 K [23]. On the other hand, in the same temperature range, dense mullite present values of thermal conductivity between 5 W/m K and 3.5 W/m K respectively [23]. Therefore, FAC and FA materials here
Fig. 4. Thermal conductivity as a function of the temperature for FAC and FA samples sintered at different temperatures. Solid lines represent the modeling previously reported [7].
A. Arizmendi-Morquecho et al. / Applied Thermal Engineering 48 (2012) 117e121
reported present lower values of thermal conductivity at high temperatures. In general, it is clear that the thermal conductivity of porous materials is greatly influenced by the gaseous component which has significantly lower values of thermal conductivity in comparison with the solid component [21]. In the solid, increasing temperature increases the Umklapp collisions between the phonons which result in a decrease in thermal conductivity. In gases, heat transfer is controlled by direct collisions between molecules, and as would be expected, their thermal conductivity is low compared to most solids since they are dilute media. Therefore, in a two phase system with solid and gaseous compound, the thermal conductivity will depend significantly on individual thermal conductivity of the constitutive phases [21]. 4. Conclusions According to the high temperature behavior of thermophysical properties, thermal barrier coatings based on recycled FAC and FA sintered at 1000 C could be regarded as effective thermal insulators with thermal conductivity values as low as 0.17 W/m K and 0.32 W/m K respectively at 1200 K. Besides this, thermal diffusivity and thermal effusivity behavior guarantees an excellent performance during unsteady state operation. Samples with low thermal expansion coefficients of 5.96 106 and 6.13 106/ C for FA and FAC respectively were obtained. This result is important in order to reduce the microcracking present at high temperatures due to thermal expansion. In general, we can conclude from the high temperature measurements an apparent high thermal stability presented in recycled FAC and FA thermal barrier coatings which confirm their potentiality as thermal insulators for high temperature applications. Acknowledgements The authors express their gratitude to Dr. Bertrand Laine and Pierre Beauchene for thermal diffusivity measurements at ONERA.
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References [1] K.N. Lee, R.A. Miller, N.S. Jacobson, New generation of plasma-sprayed mullite coatings on silicon carbide, J. Am. Ceram. Soc. 78 (3) (1995) 705e710. [2] S.L. Shindé, J.S. Goela, High Thermal Conductivity Materials, in: High Thermal Conductivity SiC and Applications, Springer, NY, 2006 pp. 167e198 (Chapter 6). [3] R. Kikuchi, Application of coal ash to environmental improvement, transformation into zeolite, potassium fertilizer and FGD absorbent, Resour. Conserv. Recycl. 27 (1999) 333e346. [4] Advanced Notice of Proposed Rulemaking e Identification of Nonhazardous Secondary Materials that Are Solid Waste Coal Combustion Residuals e Coal Fly Ash, Bottom Ash, and Boiler Slag. February 3, 2011 (EPA-HQ-RCRA-20080329-1815 Fly ash: http://www.epa.gov/epawaste/nonhaz/define/index.htm). [5] Coal Ash, Fact Sheet, American Coal Council & American Coal Ash Association. [6] http://www.coalashfacts.org/; http://www.acaa-usa.org/. [7] A. Chávez-Valdez, A. Arizmendi-Morquecho, G. Vargas, J.M. Almanza, J. Alvarez-Quintana, Ultra-low thermal conductivity thermal barrier coatings from recycled fly ash cenospheres, Acta Mater. 59 (2011) 2556. [8] A. Salazar, On thermal diffusivity, Eur. J. Phys. 24 (2003) 351. [9] J. Saggio-Woyansky, C. Scott, W.P. Minnear, Processing of porous ceramics, Am. Ceram. Soc. Bull. 7 (1992) 1674. [10] V. Tvergaard, W.J. Hutchinson, Microcracking in ceramics induced by thermal expansion or elastic anisotropy, J. Am. Ceram. Soc. 71 (1988) 157. [11] T.M. Tritt, Thermal Conductivity, Theory, Properties, and Applications, first ed. Springer, New York, 2004. [12] Z.M. Zhang, Nano/Microscale Heat Transfer, first ed. McGraw-Hill, New York, 2007. [13] Y. Xu, D.D.L. Chung, Cement Concr. Res. 29 (7) (1999) 1117. [14] Y. Xu, D.D.L. Chung, Cement Concr. Res. 30 (1) (2000) 59. [15] Y. Xu, D.D.L. Chung, Cement Concr. Res. 30 (7) (2000) 1175. [16] S. Krishnaiah, D.N. Singh, Constr. Build. Mater. 20 (2006) 193. [17] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, second ed. Wiley & Sons, New York, 1976. [18] D. Krause, H. Bach, Low Thermal Expansion Glass Ceramics, second ed. Springer, Berlin, 2005. [19] H.S. Carlslaw, J.C. Jaeger, Conduction of Heat in Solids, second ed. Oxford University Press, New York, 1959. [20] P.W. Scott, C.M. Bristow, Industrial Minerals and Extractive Industry Geology, Geological Society Pub House, London, 2002. [21] A. Zerroung, K. Zehar, L. Refoufi, Thermal conductivity models of porous materials, J. Eng. Appl. Sci. 2 (2007) 722. [22] H.S. Yang, G.R. Bai, L.J. Thomson, J.A. Eastman, Interfacial thermal resistance in nanocrystalline yttria-stabilized zirconia, Acta Mater. 50 (9) (2002) 2309. [23] R. Barea, M.I. Osendi, J.M. Ferreira, P. Miranzo, Thermal conductivity of highly porous mullite material, Acta Mater. 53 (11) (2005) 3313.
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
High temperature thermal barrier coatings from recycled fly ash cenospheres A. Arizmendi-Morquecho a, A. Chávez-Valdez b, J. Alvarez-Quintana a, * a b
Centro de Investigación en Materiales Avanzados S.C. Unidad Monterrey, Alianza Nte. 202, 66600 Apodaca, Nuevo León, Mexico Institute of Biomaterials, Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Cauerstr. 6, 91058 Erlangen, Germany
h i g h l i g h t s < New FAC thermal barriers with potentiality for high temperature applications (HTA). < A thermal conductivity value as low as 0.17 W/m K for FAC was found at 1200 K. < Low thermal expansion coefficient of 5.96 106 for FAC was found. This avoids microcracking at HTA. < HT values of a and e guarantee a good performance during unsteady state operation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 December 2011 Accepted 2 May 2012 Available online 11 May 2012
The high temperature behavior of electrophoretically deposited thermal barrier coatings based on recycled fly ash cenospheres is presented. Thermal properties such as thermal expansion coefficient, specific heat, thermal conductivity, thermal diffusivity and thermal effusivity of fly ash (FA) and fly-ash cenospheres (FAC) were measured in the temperature range of 373 Ke1173 K. Thermal conductivity values as low as 0.17 W/m K and 0.32 W/m K for FAC and FA respectively at 1200 K were found. These results confirm their potentiality as ultra-low thermal conductivity thermal insulators for high temperature applications. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: High temperature insulators Thermal barrier coatings Fly ash cenospheres
1. Introduction Coal power plants produce 41% of global electricity. Nevertheless, in addition to electricity these plants also produce a waste material because of the non-combustible mineral portion of coal. That material is compose of microscopic flakes and hollow spheres of mullite (3Al2O32SiO2) which are called fly ash cenospheres and fly ash cenospheres respectively. Mullite is a promising material as a coating because of its excellent corrosion resistance, high thermal stability, its permanence in severe chemical environments and its low thermal conductivity. Moreover, when compared to currently thermal protective coatings which are based on yttrium stabilized zircon (YSZ) it has a coefficient of thermal expansion much lower [1,2]. Globally, coal-fired power plants produce 500 million tons of fly ash each year [3]. The recoverability of fly ash depends significantly on the manner in which it is disposed, in addition there are regulations related with the areas can be used to place such wastes. Fly ash disposed of in a monofill or holding pond likely would be suitable for beneficial use because it has generally not been commingled with other materials [4]. Unfortunately, the American Coal
* Corresponding author. Tel.: þ52 8111560829; fax: þ52 8111560820. E-mail address: [email protected] (J. Alvarez-Quintana). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2012.05.004
Ash Association (ACAA) is unaware of data indicating how much of the 100e500 million tons of stockpiled fly ash is deposited in monofills or holding ponds [5]. In 2007, only the United States produced 131 million tons of coal combustion products. While 43 percent were used beneficially, nearly 75 million tons were disposed of [6]. It is clear that by using ashes instead of disposing of it in landfills are avoiding the environmental degradation and energy costs associated with mining virgin materials [4]. Unlike the earlier work where the thermal properties of the heat barriers were analyzed in the temperature range from 100 K to 500 K [7], here we deal with an extensive study of thermophysical properties of thermal barrier coatings (TBCs) based on recycled fly ash cenospheres and their thermal behavior beyond room temperature (from 300 K to 1173 K). In addition to the aim of this research, here we are extending the applications of recycled fly ash cenospheres in areas other than filler material or in the concrete industry, in order to reduce the environmental problems related with its disposal. The thermophysical properties governing the thermal energy propagation in solids are the thermal conductivity (k), thermal diffusivity (a) and thermal effusivity (e). However, while the role of k is well known, the role played by a and e is usually undervalued and misunderstood mainly because they describe the unsteady thermal state of the system. Thermal diffusivity is the quantity associated with the speed of propagation of thermal energy in
118
A. Arizmendi-Morquecho et al. / Applied Thermal Engineering 48 (2012) 117e121
a material, whereas the thermal effusivity is the capacity to exchange thermal energy with the surroundings [8]. The importance of a and e lies not only in the aforementioned characteristics, but also on doing a complete thermal characterization of the material in order to know a better understanding of its thermal behavior in a dynamic regime, especially in the evaluation of material performance as either heat insulator or as heat dissipator. High temperature thermal barrier applications require materials which can withstand increasing temperatures [9]. Applications such as, gas burners, molten metal filters, kitchen stoves and many others are subjected to thermal distributions which could induce microcracking by thermal expansion [10]. Therefore, it is important to know the high temperature behavior of thermophysical properties such as: specific heat, thermal conductivity, thermal diffusivity and thermal effusivity which are involved in these phenomena.
2. Experimental procedure A wide explanation about samples preparation, chemical composition and structural analysis was previously reported [7]. Nevertheless, because of the geometrical requirements of the existing equipments for high temperature thermal characterization new samples have been synthesized. For thermal diffusivity measurements, cylindrical samples of 1 cm in diameter of FAC and FA specimens were prepared by uniaxial pressure at 250 MPa. Next, the specimens were sintered at 1000, 1100 and 1200 C during 3 h with heating rates of 5 K/min. It is important to note that uniaxial pressing does not affect the spherical structure of the cenospheres in FAC samples. The Laser Flash Analysis (LFA) method was used to do the thermal diffusivity measurements [11]. It is an ASTM standard method (E1461), which involves subjecting the entire front surface of a sample to a very short burst of energy from a CO2 laser. Specimen’s surfaces were gold coated to prevent the laser transmission and to ensure a good absorption of the IR energy. An IR detector monitors the temperature increment of the opposite side of the sample. Then, the thermal diffusivity is calculated from the temperature rise versus time profile in the temperature range from 298 K to 1173 K.
Differential Scanning Calorimetry (DSC) measurements to obtain specific heat were carried out for FAC and FA powders at a heating rate of 10 K/min in dry argon atmosphere using TA Q200 equipment (Thermal Analysis Co., USA). In this equipment the cell contains a unique TzeroÔ sensor that allows detection and compensation for resistance and capacitance imbalances that can negatively affect baseline flatness, sensitivity, and resolution. The resulting heat flow signal provided a more accurate representation of the actual heat flowing to and from the sample with temperature precision of 0.05 C, temperature accuracy of 0.1 C, baseline curvature of 10 mW, sensitivity of 0.2 mW and Indium height/width resolution of 30 mW/ C. The DSC module records time dynamics heat flow during sample heating allowing for determining specific heat capacity by comparing thermal response of the sample with that of a standard reference. Furthermore, sample density was determined from volumeemass measurements; here, the mass of the samples was measured in a Mettler Toledo XS205 dual range analytical balance with accuracy of 0.01 mg (0e81 g), readability of 0.01 mg/0.1 mg, repeatability of 0.02 mg/0.05 mg and linearity of 0.2 mg. The volume was calculated with the dimensions of the specimens, the diameter of 1 cm and thickness measured with a Vernier caliper instrument. Finally, the thermal conductivity and thermal effusivity were obtained directly from k ¼ aCpr and e ¼ a1/2Cpr, respectively by multiplying the experimental values of the thermal diffusivity, specific heat and density of the fly ash specimens. In order to get a complete thermal characterization, thermal expansion coefficients for FAC and FA samples were also measured by using a dilatometer (model Netzsch DIL 402 PC) in the temperature range from 300 to 1473 K and heating rate of 10 K/min. To perform this test, additional cylindrical samples with length of 40 mm and diameter of 1 cm were prepared by uniaxial pressing under 125 MPa.
3. Results and discussion The specific heat capacity of FAC and FA powders at different temperatures is presented in Fig. 1a. In general, this temperature dependence of the specific heat capacity is in line with the typical
Fig. 1. a) Specific heat capacity and b) thermal expansion coefficient of FAC and FA samples as a function of the temperature.
A. Arizmendi-Morquecho et al. / Applied Thermal Engineering 48 (2012) 117e121
behavior for solid materials. At low temperatures, it can be observed the tendency to the T3 law behavior. For high temperatures, the specific heat increases rapidly to the constant high temperature limit predicted for polyatomic solids [12]. Specific heat capacity values of FAC are higher than values for FA. The behavior of specific heat capacity in our materials agrees with the previously reported works [13e16] where the difference in this property is attributed to differences in their chemical composition. For instance; due to the presence of a high content of Fe2O3, a higher specific heat capacity values in FAC samples are presented than in FA samples. We have also measured the thermal expansion coefficient of the FAC and FA samples, Fig. 1b shows the experimental data obtained. The change in dimensions is result of an increase in the amplitude of vibration between atoms in the materials, both materials present similar temperature behaviors. The main difference between FAC and FA materials takes place in the range of 50e300 C, where a significant expansion is seen for FA because the aeb transition of the high temperature polymorphic form of silica (cristobalite). This happens due to the low density and the high specific volume of the high temperature phase, which corresponds to a discontinuous increase in volume as previously observed [17]. The thermal expansion coefficients of the materials observed in this temperature range were 10.2 106 and 6.3 106/ C for FA and FAC samples, respectively. In the temperature range of 570e1270 C, the correspondents thermal expansion coefficients of FA and FAC samples were estimated to be 5.96 106 and 6.13 106/ C; these values are very similar to those reported for ceramic materials [18]. Other changes in the materials occur at temperatures of 1390 and 1410 K for FA and FAC samples: respectively, where the samples begin to shrink very fast because of the sintering in the presence of liquid. Fig. 2 shows the thermal diffusivity behavior of FAC and FA samples. It can be observed that it depends on the densification degree of the specimens. For instance; thermal diffusivity values for FAC and FA denser specimens (sintered at 1200 C) turn out to be higher for the whole temperature range in comparison with diffusivity of the samples sintered at lower temperatures (1000 and
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1100 C). Besides, thermal diffusivity values for FAC and FA are almost temperature independent and it can be observed that these values are nearly constant in the whole temperature range varying from 2 107 to 7 107 m2/s and 2 107 to 5 107 m2/s for FAC and FA respectively. Nevertheless; contrary to the expected, although FAC samples have lower thermal conductivity than FA samples [7], FAC samples shows slightly higher thermal diffusivity values than FA samples. The possible reason is related to the air filled core; air shows thermal diffusivity values as high as that of metals [8], and this might help to enhance the diffusivity of FAC samples. It is clear that microstructural and morphological characteristics also influence significantly the thermal diffusivity behavior. In time-dependent heat transfer problems another important material’s thermal property is its thermal effusivity. In this situation the heat flow through the interface between the material and its surroundings is not proportional to the thermal conductivity of the material, as under steady conditions, but to its thermal effusivity [8,19]. Fig. 3 shows thermal effusivity values for FAC and FA samples. It can be observed that it depends on the specimen’s densification as well. Thermal effusivity values for FAC and FA are temperature dependent and these values change in the whole temperature range from 170 to 550 m2/s and from 400 to 900 m2/s for FAC and FA samples, respectively. Therefore, FA samples present higher effusivity values than FAC samples. As mentioned before, it could be due to the air filled cores in FAC samples; in this case, air has very low thermal effusivity values in comparison with other materials [8]. High thermal effusivity values in FA samples mean that under transient conditions they could extract more heat than FAC samples. Thermal conductivity behavior is widely explained in the temperature range from 100 to 500 K in reference [7]. Therefore, in this section we only highlight the most important points presented in Fig. 4. In this figure, solid symbols represent the low temperature thermal conductivity values previously reported [7], and open symbols represent the high temperature values obtained in this work. It is clear that high temperature data follow the tendency of the low temperature data. Solid lines represent the previous
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Fig. 3. Thermal effusivity as a function of the temperature for FAC and FA samples sintered at different temperatures. Dotted lines are only guide to the eye.
reported modeling in the framework of the self-consistent field concept (SCF) and a modified Maxwell equation (MME) [[7] and references therein]. It can be observed that high temperature thermal conductivity data fits relatively well the proposed models. As expected, FAC coatings show lower thermal conductivity values than FA coatings due to the hollow nature of the cenospheres which limits the heat transfer. The minimum thermal conductivity values at 1200 K are approximately 0.17 W/m K and 0.32 W/m K for FAC and FA samples sintered at 1000 C respectively. These values are relatively lower than values reported for highly disordered oxides (amorphous or polycrystalline) such as fused Silica and Yttria Stabilized Zirconia (YSZ),
which are currently the main materials used as thermal barrier coatings [20,21]. For instance, the thermal conductivity of YSZ as a function of grain size, 10e100 nm, has been measured by Yang et al. [22] up to 500 K with values ranging from 1.2 W/m K to 2.3 W/m K. Moreover, when compared with porous and dense mullite (3Al2O3$2SiO2 or 2Al2O3$SiO2), it has been reported that porous mullite with porosity of 57% and pores sizes about 40 mm shows a thermal conductivity of around 1 W/m K and almost constant in the temperature range from 200 to 1300 K [23]. On the other hand, in the same temperature range, dense mullite present values of thermal conductivity between 5 W/m K and 3.5 W/m K respectively [23]. Therefore, FAC and FA materials here
Fig. 4. Thermal conductivity as a function of the temperature for FAC and FA samples sintered at different temperatures. Solid lines represent the modeling previously reported [7].
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reported present lower values of thermal conductivity at high temperatures. In general, it is clear that the thermal conductivity of porous materials is greatly influenced by the gaseous component which has significantly lower values of thermal conductivity in comparison with the solid component [21]. In the solid, increasing temperature increases the Umklapp collisions between the phonons which result in a decrease in thermal conductivity. In gases, heat transfer is controlled by direct collisions between molecules, and as would be expected, their thermal conductivity is low compared to most solids since they are dilute media. Therefore, in a two phase system with solid and gaseous compound, the thermal conductivity will depend significantly on individual thermal conductivity of the constitutive phases [21]. 4. Conclusions According to the high temperature behavior of thermophysical properties, thermal barrier coatings based on recycled FAC and FA sintered at 1000 C could be regarded as effective thermal insulators with thermal conductivity values as low as 0.17 W/m K and 0.32 W/m K respectively at 1200 K. Besides this, thermal diffusivity and thermal effusivity behavior guarantees an excellent performance during unsteady state operation. Samples with low thermal expansion coefficients of 5.96 106 and 6.13 106/ C for FA and FAC respectively were obtained. This result is important in order to reduce the microcracking present at high temperatures due to thermal expansion. In general, we can conclude from the high temperature measurements an apparent high thermal stability presented in recycled FAC and FA thermal barrier coatings which confirm their potentiality as thermal insulators for high temperature applications. Acknowledgements The authors express their gratitude to Dr. Bertrand Laine and Pierre Beauchene for thermal diffusivity measurements at ONERA.
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