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Preparation of zirconium carbide foam by direct foaming method
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ScienceDirect Journal of the European Ceramic Society 34 (2014) 3513–3520
Preparation of zirconium carbide foam by direct foaming method Fei Li a,b , Zhuang Kang a , Xiao Huang a,∗ , Xin-Gang Wang a , Guo-Jun Zhang a,∗∗ a
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Shanghai 200050, China b University of Chinese Academy of Sciences, Beijing 100049, China Received 23 January 2014; received in revised form 21 April 2014; accepted 16 May 2014 Available online 11 June 2014
Abstract Ultra light, highly porous, closed-cell structured ZrC foam can be produced in two steps. First, pre-ceramic foam is prepared by direct foaming of zirconia sol and phenolic resin. In the next step, the foamed green body is converted into ZrC foam after carbothermal reduction at 1600 ◦ C under argon atmosphere. The obtained ZrC foam has porosity of 85% and possesses uniform cells with an average size of about 40 m. The foam also displays excellent thermal stability up to 2400 ◦ C. Its compressive strength and thermal conductivity at room temperature are 0.4 MPa and 0.94 W/(m K), respectively. © 2014 Elsevier Ltd. All rights reserved. Keywords: Ultra high temperature ceramic foams; Zirconium carbide; Direct foaming; Carbothermal reduction
1. Introduction Comparing to their organic partners, ceramic foams have excellent thermal stability and outstanding resistance to organic solvents and chemical corrosion, which can lead to various engineering applications, such as high temperature thermal insulation, catalysis, molten metal or hot gas filtration and more.1–9 Several approaches have been developed to produce ceramic foams and it appears that the microstructures/properties of ceramic foams are strongly dependent on the preparation protocols.1,4,5 Partial sintering is the most straightforward processing route for the preparation of porous ceramics.10,11 But this method usually results in porous ceramics with porosity lower than 60 vol%.1 Ceramic foams with high porosity are usually prepared by replica, sacrificial template and direct foaming methods, in which ceramic suspensions or preceramic polymers are usually involved.1,4,5 Many oxide and silicon-based non-oxide ceramic foams with porosity greater than 90 vol% have been prepared by the forementioned three methods and have found numerous ∗
Corresponding author. Tel.: +86 2152414318; fax: +86 2152413122. Corresponding author. E-mail addresses: [email protected] (X. Huang), [email protected] (G.-J. Zhang). ∗∗
http://dx.doi.org/10.1016/j.jeurceramsoc.2014.05.029 0955-2219/© 2014 Elsevier Ltd. All rights reserved.
applications.1,2,5 But mainly due to their relatively low melting points, few of these ceramic foams can be used at temperatures above 2000 ◦ C. Zirconium carbide (ZrC) is a typical member of the socalled ultra high temperature ceramic (UHTC) family. Due to its extremely high melting point (>3400 ◦ C), high hardness, excellent solid-phase stability and good thermomechanical properties, etc., ZrC has been considered as one of the most potential candidates for high temperature applications often associated with hypersonic aerospace vehicles and rocket propulsion systems.12–16 However, up to now, most researches on ZrC focus on the synthesis and sintering of the ceramic powder, the densification and the performance of the corresponding ceramic materials. Unlike silicon carbide, whose foams have been well studied and become commercially available,17 the researches on ZrC foams18 are very limited probably due to the harsh synthesis conditions and the absence of appropriate preceramic precursors. It has been reported that ZrC ultrafine powders can be prepared using various sol–gel precursors.12,13 More recently, Rambo et al.18 reported the preparation of porous biomorphic ZrC/C composite by impregnating the pine wood in zirconia sol and pyrolyzing the sample in inert atmosphere. In this work, we report the preparation of ZrC foams by employing a commercial foaming technique for phenolic resins. In the experiments,
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zirconia sol is used as the zirconia precursor, while commercial foamable phenolic resin is used as the carbon source and foaming aid. By thermal-setting the wet foam and pyrolyzing the foamed green body at 1600 ◦ C, highly porous ZrC foam can be obtained. 2. Experimental 2.1. Materials Zirconium oxychloride octahydrate (ZrOCl2 ·8H2 O, 99%) was purchased from Shanghai Diyang Chemical Co., Shanghai, China, used as received. Hydrogen peroxide 30% (AR grade) and ethanol (AR grade) were purchased from Sinopharm Chemical Reagent Co., Shanghai, China, and used without further purification. Deionized water (DI water) was prepared in-house by using a Thermo Scientific Barnstead Easypure II system. Phenolic resin, pentane (blowing agent), Tween 20 (emulsifier) and citric acid (curing agent) were obtained from Zhejiang Xinzun Energy Conservation Building Materials Co., Ltd, China. 2.2. Preparation of zirconia sol Zirconia sol was prepared according to the reported literature.19 Zirconium oxychloride (ZOC) was dissolved in a water/ethanol solution. Then hydrogen peroxide was slowly added into the ZOC solution. The overall molar ratio of water to ZOC was controlled at 10, while that of hydrogen peroxide to ZOC was 5. After hydrogen peroxide addition, the mixture was stirred at room temperature for 24 h before it was further concentrated to yield zirconia sol by rotary evaporation. The obtained sol has a solid content of 60%. 2.3. Foaming process A commercial foaming process for phenolic resin was applied here. Typically, phenolic resin, emulsifier (Tween 20), blowing agent (pentane), zirconia sol and curing agent (citric acid) were vigorously stirred to form a viscous foamable mixture. The mixture was stirred for 6 min, and then it was poured into a 100 mm × 100 mm × 20 mm rectangular Teflon mold and kept in an oven at 75 ◦ C for 4 h to let the foaming process complete and the foamed body ripen. The amount of phenolic resin used in the mixture was based on the carbon to zirconium molar ratio, which was controlled at 10. The amount of emulsifier, blowing agent and curing agent applied were 5%, 7% and 19% of the weight of phenolic resin, respectively. The flow chart of the ZrC foam preparation is illustrated in Scheme 1. 2.4. Heat treatment The foamed green body was pyrolyzed at 1400 ◦ C for 2 h or 1600 ◦ C for 1 h under flowing argon atmosphere in a tube furnace with a heating rate of 3 ◦ C/min. In order to investigate its high temperature stability, the obtained ZrC foam was cut into small blocks and each block was
Scheme 1. Flow chart of ZrC foam preparation.
aged under argon atmosphere at 1800, 2000, 2200 ◦ C for 1 h or 2400 ◦ C for 10 min (graphite furnace, 10 ◦ C/min), respectively. 2.5. Characterization The morphology of the foam was observed using a JEOL JSM-6700F scanning electron microscope. The average foam cell sizes were determined by image analyses on SEM images using Image-Pro Plus 5.0. More than 100 cells were measured to give out average cell diameter for each sample. Transmission electron micrographs (TEM) were recorded on a JEOL JEM-2100F transmission electron microscope at 200 kV. Photographs were taken by a HTC One S camera. X-ray diffraction (XRD) patterns were collected by using a Rigaku D/Max-2200 PC X-ray diffractometer with Cu target (40 kV, 40 mA). The pore structure of the foams was characterized by mercury intrusion porosimetry (MIP, AutoPore 9500). Compressive strength was measured by an Instron 5592 universal testing machine with a crosshead speed of 0.5 mm/min on samples with dimensions of 10 mm × 10 mm × 10 mm. Thermogravimetric analysis was performed on a Netzsch STA 449F3 in oxygen at a heating rate of 10 ◦ C/min to calculate the weight percentage of ZrC in the pyrolyzed foam.
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Fig. 1. A photograph of a cut foamed green body.
The thermal conductivity, λ, of the foam was determined by laser flash diffusivity technique. The thermal diffusivity, α, was measured on a Netzsch LFA 427 in the temperature range 25 ◦ C to 300 ◦ C in air (sample: Φ 10 mm by 1.5 mm thickness). The specific heat capacity, Cp , was measured on a Netzsch STA 449F3 at temperature range 25–300 ◦ C in flowing air with a heating rate of 10 ◦ C/min. The bulk density, ρ, Cp and α were then used to calculate the thermal conductivity based on Eq. (1): λ = a × r × Cp
(1)
3. Results and discussion 3.1. Preparation of the ZrC foams Fig. 1 shows a photograph of the foamed green body which was cut into a block of 10 cm × 5 cm × 2 cm. The red outlook is coming from the phenolic resin. In the picture, the foam shows fine pore structure by naked eyes. Both the surface and the cross section of the foam seem homogeneous and no obvious voids are observed. It has a bulk density of 0.17 g/cm3 and a porosity of 85% (based on MIP, vide infra). Apparently, the mixture of our zirconia sol and phenolic resin at C/Zr of 10 has excellent foamability. Foaming process is quite complicated. Many technical factors related to the foamability and foam stability as well as environmental and cost effective issues need to be taken into account during processing. To simplify the situation, a mature and widely used commercial phenolic resin foaming process is applied. The key in our protocol is to prepare a zirconia sol which has good compatibility with foamable phenolic resin. In our experiments, the foamability worsens as the Zr content increases because the inorganic zirconia sol affects the foamability of the organic phenolic resin. The research work to make zirconium-containing precursor of better compatibility with foamable polymers is still under progress. The foamed green bodies were pyrolyzed at 1400 ◦ C for 2 h or 1600 ◦ C for 1 h respectively. The XRD patterns of the pyrolyzed foams are shown in Fig. 2. It appears that after pyrolysis at 1400 ◦ C for 2 h, the existence of m-ZrO2 and t-ZrO2 is still obvious. Since the carbon from phenolic resin is excessive, it indicates that the carbothermal reduction has not completed and higher reaction temperature is needed. After pyrolysis at 1600 ◦ C for 1 h, the carbothermal reduction has completed as
Fig. 2. XRD patterns of the foamed green bodies after being pyrolyzed at 1400 for 2 h and 1600 ◦ C for 1 h in Ar.
only ZrC phase can be indexed in the XRD pattern. Based upon weight loss of the obtained ZrC foam in oxygen atmosphere and assuming ZrO2 as the only product,20 the ZrC content in the aspyrolyzed foam is calculated to be 53 wt%, and the other 47 wt% is amorphous carbon. The pictures of ZrC foams obtained after pyrolysis at 1400 and 1600 ◦ C are shown in Fig. 3. After pyrolysis, the foams maintain their original shapes with ∼65% volume shrinkage and no structural damages are observed. The pictures of 1600-pyrolyzed ZrC foams after high temperature aging are also shown in Fig. 3. Similarly, no geometric changes and structural damages are noticed in the ZrC foam after aging, indicating its excellent dimensional stability at high temperatures. The densities of the foams, which are calculated by mass over volume, are all around 0.17 g/cm3 . The pre-ceramic and ceramic foams having similar density is probably because the weight loss during pyrolysis is compensated by the volume shrinkage.
Fig. 3. Photographs of the as-pyrolyzed foams at 1400 and 1600 ◦ C in Ar, and the 1600-pyrolyzed foams after high temperature aging in Ar.
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Fig. 4. SEMs of the (a) pre-ceramic foam, (b) ZrC foam after 1600 ◦ C pyrolysis in Ar and ZrC foams after further aging at (c) 1800 ◦ C, (d) 2000 ◦ C, (e) 2200 ◦ C for 1 h in Ar and (f) 2400 ◦ C for 10 min in Ar. The scale bars are 30 m.
3.2. Microstructure analyses MIP is a very powerful analytical tool for porous materials with pore size between 0.0035 and 500 m.21 MIP results of the pre-ceramic and ceramic foams are summarized in Table 1. Table 1 Summary of MIP data. Sample
Bulk density at 0.53 psia (g/mL)
Porosity (%)
Median pore size (m)
Foamed green body Pyrolyzed at 1600 ◦ C Thermal aging Aged at 1800 ◦ C Aged at 2000 ◦ C Aged at 2200 ◦ C Aged at 2400 ◦ C
0.17 0.16
86 85
3.4 8.0
0.17 0.19 0.19 0.16
84 84 83 90
11.3 11.6 11.8 14.7
The results show that the bulk densities and porosities of all the foams are close. And the bulk density data from MIP are consistent with the densities calculated by mass over volume. SEMs of the pre-ceramic and ceramic foams after 1600 ◦ C pyrolysis are shown in Fig. 4a and b. Fig. 4a clearly shows the closed cells structure of the foam with the existence of several broken cells possibly from mechanical cutting, which resembles the foamed phenolic resin (without zirconia sol). The commercial foaming process applied here is known to produce foam with closed cell structure.22,23 The cells in Fig. 4a look slightly elongated to one direction. Actually, it elongated to the rise direction, which is commonly observed in open-mold foaming process.24 After high temperature pyrolysis, the ceramic foam (Fig. 4b) maintains the similar morphology as the pre-ceramic foam, except that the cell elongation became less obvious probably because the cells relaxed during heat treatment. The
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Fig. 5. SEMs of the struts in (a) pre-ceramic foam and ZrC foams aged at (b) 2000 ◦ C, (c) 2200 ◦ C for 1 h in Ar and (d) at 2400 ◦ C for 10 min in Ar. The scale bars are 2 m.
precursor-to-ceramic conversion in our cases at least involves the decomposition of the phenolic resin and zirconia sol and carbothermal reduction of zirconia with carbon. Thus it is reasonable to observe the reduction in cell size, which decreases from about 60 m in the pre-ceramic foam to about 40 m in the foam after pyrolysis at 1600 ◦ C for 1 h. Fig. 4c–f shows that there are no obvious morphological changes after the pyrolyzed foams were further aged at 1800–2400 ◦ C, which is also an indication of the good high temperature stability of the ZrC foam. Fig. 5 shows some typical struts of the pre-ceramic foam and
high temperature aged ZrC foams. The thickness of the struts is all about 3–4 m. Again, high temperature aging did not cause obvious changes to the struts structure. TEM images of one typical strut and cell wall of ZrC foam after pyrolysis at 1600 ◦ C are shown in Fig. 6. In TEM images, the dark phases are ZrC, and light phases are amorphous carbon. It is obvious that the phase distributions in the strut and cell wall are different. In the cell wall, it appears that carbon is the continuous phase with ZrC particles embedding in the carbon matrix, while in the struts ZrC forms the dominant phase. This
Fig. 6. TEM images of one typical (a) strut and (b) cell wall of ZrC foam after pyrolysis at 1600 ◦ C.
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might be due to the drainage and mass migrated to the strut during foaming process.4 SEM results show that there are no obvious changes in the cell size after high temperature pyrolysis and thermal aging. However, very interestingly, the average pore sizes of the foams and pore size distribution from MIP (Table 1 and Fig. 7) show some significant changes. The average pore size increases from 3.4 m in pre-ceramic foam to 8.0 m in ZrC foam after pyrolysis. When the ZrC foams were further aged at high temperatures, their average pore sizes based on MIP seem to increase further as aging temperature. In principle, MIP gives out the size of the largest entrance toward a pore, not the actual inner size of a pore.21,25 According to Washburn equation (a modified Young–Laplace equation, Eq. (2)),21 in MIP experiment, the pore size is calculated based on the pressure applied to force mercury intruding the pores. The larger the pressure is, the smaller the pore size is. P = 2 × γ × cos θ/rpore
(2)
where P is the pressure across the interface, γ is the surface tension of mercury, θ is the contact angle between the solid and mercury, and rpore is the calculated radius. In our cases, the closed cell walls can be broken when the applied pressure reaches a certain value. And then mercury intrudes into the cells to give out information of the pore structure. Because extra pressure is needed to break the cell walls, the calculated pore sizes are much smaller than cell sizes observed from SEMs.21 The increase of pore sizes after high temperature pyrolysis and aging indicates that less pressure is required to force mercury to intrude into the cells, which implies that the cell wall strength decreases after pyrolysis and further decreases upon aging. 3.3. Compressive strength of the ZrC foams A preliminary study on the mechanical properties of the ZrC foam obtained from 1600 ◦ C pyrolysis was carried out. Fig. 8a shows a typical compressive strain-stress curve of the ZrC foam, which has a compressive strength of 0.4 MPa. Generally, the
Fig. 7. Pore size distribution of the pre-ceramic and ZrC foams by MIP.
dependence of the strain-stress of ceramic foams can be divided into two different stages: linear elastic and a plateau region.26,27 Upon compression, the foam undergoes a progressive collapse of the cells, with the lower part of the foam remains completely undamaged.28 In the plateau region, the stress is assumed to be independent of the strain as part of the structure collapses, while other parts of the structure remain elastic.28 Beyond the plateau, densification takes place and the stress rises sharply as complete densification begins. These characteristics are also typical of other ceramic foams, and have already been discussed by Ashby and Gibson8,9 and other researchers.26–29 In Fig. 8b, the compressive strength decreases with pyrolysis and aging temperatures initially, and then shows a slightly increase after aging at 2400 ◦ C. The compressive response of a closed ceramic foam is largely depended on the strength of the cell walls and the struts.28,30 The decrease of the compressive strength of the ZrC foams in Fig. 8b is probably due to the decrease of the strength of the cell walls, which is consistent with the MIP observations. After 2400 ◦ C aging, the strength of the struts could be enhanced by the neck formation between ZrC particles,31 which leads to the exceptional increase of the compressive strength.
Fig. 8. (a) Typical compressive stress-strain response of ZrC foam from 1600 ◦ C pyrolysis; (b) Compressive strength of the pre-ceramic foam, the ZrC foam from 1600 ◦ C pyrolysis, and ZrC foams being aged at 1800, 2000, 2400 ◦ C respectively (data are average of 5 individual measurements).
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general approach to prepare other UHTC foams and SiC foam as well. Acknowledgements Financial support from the Chinese Academy of Sciences under the Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program), Science and Technology Commission of Shanghai Municipality (# 11ZR1442200) and the National Natural Science Foundation of China (Nos. 11205229, 51002168) are gratefully acknowledged. References
Fig. 9. Temperature dependence of thermal conductivity of the ZrC foams obtained from pyrolysis at 1600 ◦ C for 1 h.
3.4. Thermal conductivity of the ZrC foam The thermal conductivity of the ZrC foam is determined by laser flash technique, which is 0.96 W/(m K) at 50 ◦ C and 1.36 W/(m K) at 300 ◦ C (limited by our current apparatus). As shown in Fig. 9, the thermal conductivity increases as temperature, more rapidly in the range 50–200 ◦ C, and more slowly in the range 200–300 ◦ C. The heat conduction in a cellular solid is a quite complicated.30 To simplify, the overall thermal transfer in the ZrC foam can be the sum of the heat conduction through the cell walls or struts and radiative heat transfer.30,32 Although the heat conduction through the solid decreases with increasing temperature for ceramic materials, radiative heat transfer through the cells increases as the cube of the temperature,27,30 which is the major reason for the increase of thermal conductivity versus temperature. 4. Conclusions Low density, highly macroporous, closed cell structured ZrC foam can be prepared by a commercial foaming technique for polymeric foams and a following high temperature ceramization. Pre-ceramic foam is first prepared by co-blowing of a zirconia sol and phenolic resin. The foamed green body is converted to ceramic foam after 1600 ◦ C pyrolysis under argon thereafter. The ZrC foam obtained has a large porosity of 85% and cells with an average size of about 40 m. Thermal aging experiments illustrate that the obtained ZrC foam has excellent thermal stability up to 2400 ◦ C, which is very promising for potential ultra high temperature applications. There are no obvious macro- and micro-structure changes after thermal aging by direct and SEM observations. MIP results reveal that the thermal aging may weaken the strength of the cell walls, thus lead to the decrease of the compressive strength of the ZrC foam. Our work demonstrates that zirconium-based sol-gel precursor can be an efficient precursor for making ZrC foams. We also believe that this easy, low-cost foaming method could be a
1. Ohji T, Fukushima M. Macro-porous ceramics: processing and properties. Int Mater Rev 2012;57:115–31. 2. Colombo P. Engineering porosity in polymer-derived ceramics. J Eur Ceram Soc 2008;28:1389–95. 3. Colombo P. In praise of pores. Science 2008;322:381–3. 4. Studart AR, Gonzenbach UT, Tervoort E, Gauckler LJ. Processing routes to macroporous ceramics: a review. J Am Ceram Soc 2006;89: 1771–89. 5. Colombo P. Conventional and novel processing methods for cellular ceramics. Philos Trans R Soc A 2006;364:109–24. 6. Colombo P, Bernardo E. Macro- and micro-cellular porous ceramics from preceramic polymers. Compos Sci Technol 2003;63:2353–9. 7. Fujiu T, Messing GL, Huebner W. Processing and properties of cellular silica synthesized by foaming sol-gels. J Am Ceram Soc 1990;73:85–90. 8. Ashby MF, Sammis CG. The damage mechanics of brittle solids in compression. Pure Appl Geophys 1990;133:489–521. 9. Ashby MF, Medalist RFM. The mechanical properties of cellular solids. Metall Trans A 1983;14:1755–69. 10. Zhang G-J, Yang J-F, Ohji T. Fabrication of porous ceramics with unidirectionally aligned continuous pores. J Am Ceram Soc 2001;84:1395–7. 11. Yang J-F, Zhang G-J, Ohji T. Porosity and microstructure control of porous ceramics by partial hot pressing. J Mater Res 2001;16:1916–8. 12. Katoh Y, Vasudevamurthy G, Nozawa T, Snead LL. Properties of zirconium carbide for nuclear fuel applications. J Nucl Mater 2013;441:718–42. 13. Sacks MD, Wang CA, Yang ZH, Jain A. Carbothermal reduction synthesis of nanocrystalline zirconium carbide and hafnium carbide powders using solution-derived precursors. J Mater Sci 2004;39:6057–66. 14. Wang X-G, Liu J-X, Kan Y-M, Zhang G-J. Effect of solid solution formation on densification of hot-pressed ZrC ceramics with MC (M = V, Nb, and Ta) additions. J Eur Ceram Soc 2012;32:1795–802. 15. Wang X-G, Zhang G-J, Xue J-X, Tang Y, Huang X, Xu C-M, et al. Reactive hot pressing of ZrC–SiC ceramics at low temperature. J Am Ceram Soc 2013;96:32–6. 16. Wang X-G, Guo W-M, Kan Y-M, Zhang G-J, Wang P-L. Densification behavior and properties of hot-pressed ZrC ceramics with Zr and graphite additives. J Eur Ceram Soc 2011;31:1103–11. 17. Eom J-H, Kim Y-W, Raju S. Processing and properties of macroporous silicon carbide ceramics: a review. J Asian Ceram Soc 2013;1: 220–42. 18. Rambo CR, Cao J, Rusina O, Sieber H. Manufacturing of biomorphic (Si, Ti, Zr)-carbide ceramics by sol–gel processing. Carbon 2005;43: 1174–83. 19. Zhou C-H, He C, Hu X-F, Luo H-J. Preparation and characterization of stable zirconia sol. Glass Enamel 2001;29:41–4. 20. Garcia-Marquez A, Portehault D, Giordano C. Chromium nitride and carbide nanofibers: from composites to mesostructures. J Mater Chem 2011;21:2136–43. 21. Giesche H. Mercury porosimetry: a general (practical) overview. Part Part Syst Char 2006;23:9–19. 22. Lunt J, MacPherson EJ, Meunier PJ. Closed cell phenolic foam. US Patent 4,545,119; 1986.
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23. Cornick M. Foam. In: Pilato L, editor. Phenolic resins: a century of progress. New York: Springer; 2010. p. 189–208. 24. Gibson LJ, Ashby MF. Cellular solids: structure and properties. 2nd ed. Cambridge, UK: Cambridge University Press; 1997. 25. León, León CA. New perspectives in mercury porosimetry. Adv Colloid Interfac 1998;76-77:341–72. 26. Tondi G, Zhao W, Pizzi A, Du G, Fierro V, Celzard A. Tannin-based rigid foams: a survey of chemical and physical properties. Bioresour Technol 2009;100:5162–9. 27. Bourret J, Michot A, Tessier-Doyen N, Naït-Ali B, Pennec F, Alzina A, et al. Thermal conductivity of very porous kaolin-based ceramics. J Am Ceram Soc 2014;97:938–44.
28. Brezny R, Green DJ. Mechanical behavior of cellular ceramics. In: Mater sci tech-lond. Weinheim: Wiley-VCH; 2006. p. 467–516. 29. Colombo P, Modesti M. Silicon oxycarbide ceramic foams from a preceramic polymer. J Am Ceram Soc 1999;82:573–8. 30. Scheffler M, Colombo P. Cellular ceramics: structure, manufacturing, properties and applications. Weinheim, Germany: Wiley-VCH; 2005. 31. Landi E, Sciti D, Melandri C, Medri V. Ice templating of ZrB2 porous architectures. J Eur Ceram Soc 2013;33:1599–607. 32. Smith DS, Alzina A, Bourret J, Nait-Ali B, Pennec F, Tessier-Doyen N, et al. Thermal conductivity of porous materials. J Mater Res 2013;28: 2260–72.
ScienceDirect Journal of the European Ceramic Society 34 (2014) 3513–3520
Preparation of zirconium carbide foam by direct foaming method Fei Li a,b , Zhuang Kang a , Xiao Huang a,∗ , Xin-Gang Wang a , Guo-Jun Zhang a,∗∗ a
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Shanghai 200050, China b University of Chinese Academy of Sciences, Beijing 100049, China Received 23 January 2014; received in revised form 21 April 2014; accepted 16 May 2014 Available online 11 June 2014
Abstract Ultra light, highly porous, closed-cell structured ZrC foam can be produced in two steps. First, pre-ceramic foam is prepared by direct foaming of zirconia sol and phenolic resin. In the next step, the foamed green body is converted into ZrC foam after carbothermal reduction at 1600 ◦ C under argon atmosphere. The obtained ZrC foam has porosity of 85% and possesses uniform cells with an average size of about 40 m. The foam also displays excellent thermal stability up to 2400 ◦ C. Its compressive strength and thermal conductivity at room temperature are 0.4 MPa and 0.94 W/(m K), respectively. © 2014 Elsevier Ltd. All rights reserved. Keywords: Ultra high temperature ceramic foams; Zirconium carbide; Direct foaming; Carbothermal reduction
1. Introduction Comparing to their organic partners, ceramic foams have excellent thermal stability and outstanding resistance to organic solvents and chemical corrosion, which can lead to various engineering applications, such as high temperature thermal insulation, catalysis, molten metal or hot gas filtration and more.1–9 Several approaches have been developed to produce ceramic foams and it appears that the microstructures/properties of ceramic foams are strongly dependent on the preparation protocols.1,4,5 Partial sintering is the most straightforward processing route for the preparation of porous ceramics.10,11 But this method usually results in porous ceramics with porosity lower than 60 vol%.1 Ceramic foams with high porosity are usually prepared by replica, sacrificial template and direct foaming methods, in which ceramic suspensions or preceramic polymers are usually involved.1,4,5 Many oxide and silicon-based non-oxide ceramic foams with porosity greater than 90 vol% have been prepared by the forementioned three methods and have found numerous ∗
Corresponding author. Tel.: +86 2152414318; fax: +86 2152413122. Corresponding author. E-mail addresses: [email protected] (X. Huang), [email protected] (G.-J. Zhang). ∗∗
http://dx.doi.org/10.1016/j.jeurceramsoc.2014.05.029 0955-2219/© 2014 Elsevier Ltd. All rights reserved.
applications.1,2,5 But mainly due to their relatively low melting points, few of these ceramic foams can be used at temperatures above 2000 ◦ C. Zirconium carbide (ZrC) is a typical member of the socalled ultra high temperature ceramic (UHTC) family. Due to its extremely high melting point (>3400 ◦ C), high hardness, excellent solid-phase stability and good thermomechanical properties, etc., ZrC has been considered as one of the most potential candidates for high temperature applications often associated with hypersonic aerospace vehicles and rocket propulsion systems.12–16 However, up to now, most researches on ZrC focus on the synthesis and sintering of the ceramic powder, the densification and the performance of the corresponding ceramic materials. Unlike silicon carbide, whose foams have been well studied and become commercially available,17 the researches on ZrC foams18 are very limited probably due to the harsh synthesis conditions and the absence of appropriate preceramic precursors. It has been reported that ZrC ultrafine powders can be prepared using various sol–gel precursors.12,13 More recently, Rambo et al.18 reported the preparation of porous biomorphic ZrC/C composite by impregnating the pine wood in zirconia sol and pyrolyzing the sample in inert atmosphere. In this work, we report the preparation of ZrC foams by employing a commercial foaming technique for phenolic resins. In the experiments,
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zirconia sol is used as the zirconia precursor, while commercial foamable phenolic resin is used as the carbon source and foaming aid. By thermal-setting the wet foam and pyrolyzing the foamed green body at 1600 ◦ C, highly porous ZrC foam can be obtained. 2. Experimental 2.1. Materials Zirconium oxychloride octahydrate (ZrOCl2 ·8H2 O, 99%) was purchased from Shanghai Diyang Chemical Co., Shanghai, China, used as received. Hydrogen peroxide 30% (AR grade) and ethanol (AR grade) were purchased from Sinopharm Chemical Reagent Co., Shanghai, China, and used without further purification. Deionized water (DI water) was prepared in-house by using a Thermo Scientific Barnstead Easypure II system. Phenolic resin, pentane (blowing agent), Tween 20 (emulsifier) and citric acid (curing agent) were obtained from Zhejiang Xinzun Energy Conservation Building Materials Co., Ltd, China. 2.2. Preparation of zirconia sol Zirconia sol was prepared according to the reported literature.19 Zirconium oxychloride (ZOC) was dissolved in a water/ethanol solution. Then hydrogen peroxide was slowly added into the ZOC solution. The overall molar ratio of water to ZOC was controlled at 10, while that of hydrogen peroxide to ZOC was 5. After hydrogen peroxide addition, the mixture was stirred at room temperature for 24 h before it was further concentrated to yield zirconia sol by rotary evaporation. The obtained sol has a solid content of 60%. 2.3. Foaming process A commercial foaming process for phenolic resin was applied here. Typically, phenolic resin, emulsifier (Tween 20), blowing agent (pentane), zirconia sol and curing agent (citric acid) were vigorously stirred to form a viscous foamable mixture. The mixture was stirred for 6 min, and then it was poured into a 100 mm × 100 mm × 20 mm rectangular Teflon mold and kept in an oven at 75 ◦ C for 4 h to let the foaming process complete and the foamed body ripen. The amount of phenolic resin used in the mixture was based on the carbon to zirconium molar ratio, which was controlled at 10. The amount of emulsifier, blowing agent and curing agent applied were 5%, 7% and 19% of the weight of phenolic resin, respectively. The flow chart of the ZrC foam preparation is illustrated in Scheme 1. 2.4. Heat treatment The foamed green body was pyrolyzed at 1400 ◦ C for 2 h or 1600 ◦ C for 1 h under flowing argon atmosphere in a tube furnace with a heating rate of 3 ◦ C/min. In order to investigate its high temperature stability, the obtained ZrC foam was cut into small blocks and each block was
Scheme 1. Flow chart of ZrC foam preparation.
aged under argon atmosphere at 1800, 2000, 2200 ◦ C for 1 h or 2400 ◦ C for 10 min (graphite furnace, 10 ◦ C/min), respectively. 2.5. Characterization The morphology of the foam was observed using a JEOL JSM-6700F scanning electron microscope. The average foam cell sizes were determined by image analyses on SEM images using Image-Pro Plus 5.0. More than 100 cells were measured to give out average cell diameter for each sample. Transmission electron micrographs (TEM) were recorded on a JEOL JEM-2100F transmission electron microscope at 200 kV. Photographs were taken by a HTC One S camera. X-ray diffraction (XRD) patterns were collected by using a Rigaku D/Max-2200 PC X-ray diffractometer with Cu target (40 kV, 40 mA). The pore structure of the foams was characterized by mercury intrusion porosimetry (MIP, AutoPore 9500). Compressive strength was measured by an Instron 5592 universal testing machine with a crosshead speed of 0.5 mm/min on samples with dimensions of 10 mm × 10 mm × 10 mm. Thermogravimetric analysis was performed on a Netzsch STA 449F3 in oxygen at a heating rate of 10 ◦ C/min to calculate the weight percentage of ZrC in the pyrolyzed foam.
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Fig. 1. A photograph of a cut foamed green body.
The thermal conductivity, λ, of the foam was determined by laser flash diffusivity technique. The thermal diffusivity, α, was measured on a Netzsch LFA 427 in the temperature range 25 ◦ C to 300 ◦ C in air (sample: Φ 10 mm by 1.5 mm thickness). The specific heat capacity, Cp , was measured on a Netzsch STA 449F3 at temperature range 25–300 ◦ C in flowing air with a heating rate of 10 ◦ C/min. The bulk density, ρ, Cp and α were then used to calculate the thermal conductivity based on Eq. (1): λ = a × r × Cp
(1)
3. Results and discussion 3.1. Preparation of the ZrC foams Fig. 1 shows a photograph of the foamed green body which was cut into a block of 10 cm × 5 cm × 2 cm. The red outlook is coming from the phenolic resin. In the picture, the foam shows fine pore structure by naked eyes. Both the surface and the cross section of the foam seem homogeneous and no obvious voids are observed. It has a bulk density of 0.17 g/cm3 and a porosity of 85% (based on MIP, vide infra). Apparently, the mixture of our zirconia sol and phenolic resin at C/Zr of 10 has excellent foamability. Foaming process is quite complicated. Many technical factors related to the foamability and foam stability as well as environmental and cost effective issues need to be taken into account during processing. To simplify the situation, a mature and widely used commercial phenolic resin foaming process is applied. The key in our protocol is to prepare a zirconia sol which has good compatibility with foamable phenolic resin. In our experiments, the foamability worsens as the Zr content increases because the inorganic zirconia sol affects the foamability of the organic phenolic resin. The research work to make zirconium-containing precursor of better compatibility with foamable polymers is still under progress. The foamed green bodies were pyrolyzed at 1400 ◦ C for 2 h or 1600 ◦ C for 1 h respectively. The XRD patterns of the pyrolyzed foams are shown in Fig. 2. It appears that after pyrolysis at 1400 ◦ C for 2 h, the existence of m-ZrO2 and t-ZrO2 is still obvious. Since the carbon from phenolic resin is excessive, it indicates that the carbothermal reduction has not completed and higher reaction temperature is needed. After pyrolysis at 1600 ◦ C for 1 h, the carbothermal reduction has completed as
Fig. 2. XRD patterns of the foamed green bodies after being pyrolyzed at 1400 for 2 h and 1600 ◦ C for 1 h in Ar.
only ZrC phase can be indexed in the XRD pattern. Based upon weight loss of the obtained ZrC foam in oxygen atmosphere and assuming ZrO2 as the only product,20 the ZrC content in the aspyrolyzed foam is calculated to be 53 wt%, and the other 47 wt% is amorphous carbon. The pictures of ZrC foams obtained after pyrolysis at 1400 and 1600 ◦ C are shown in Fig. 3. After pyrolysis, the foams maintain their original shapes with ∼65% volume shrinkage and no structural damages are observed. The pictures of 1600-pyrolyzed ZrC foams after high temperature aging are also shown in Fig. 3. Similarly, no geometric changes and structural damages are noticed in the ZrC foam after aging, indicating its excellent dimensional stability at high temperatures. The densities of the foams, which are calculated by mass over volume, are all around 0.17 g/cm3 . The pre-ceramic and ceramic foams having similar density is probably because the weight loss during pyrolysis is compensated by the volume shrinkage.
Fig. 3. Photographs of the as-pyrolyzed foams at 1400 and 1600 ◦ C in Ar, and the 1600-pyrolyzed foams after high temperature aging in Ar.
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Fig. 4. SEMs of the (a) pre-ceramic foam, (b) ZrC foam after 1600 ◦ C pyrolysis in Ar and ZrC foams after further aging at (c) 1800 ◦ C, (d) 2000 ◦ C, (e) 2200 ◦ C for 1 h in Ar and (f) 2400 ◦ C for 10 min in Ar. The scale bars are 30 m.
3.2. Microstructure analyses MIP is a very powerful analytical tool for porous materials with pore size between 0.0035 and 500 m.21 MIP results of the pre-ceramic and ceramic foams are summarized in Table 1. Table 1 Summary of MIP data. Sample
Bulk density at 0.53 psia (g/mL)
Porosity (%)
Median pore size (m)
Foamed green body Pyrolyzed at 1600 ◦ C Thermal aging Aged at 1800 ◦ C Aged at 2000 ◦ C Aged at 2200 ◦ C Aged at 2400 ◦ C
0.17 0.16
86 85
3.4 8.0
0.17 0.19 0.19 0.16
84 84 83 90
11.3 11.6 11.8 14.7
The results show that the bulk densities and porosities of all the foams are close. And the bulk density data from MIP are consistent with the densities calculated by mass over volume. SEMs of the pre-ceramic and ceramic foams after 1600 ◦ C pyrolysis are shown in Fig. 4a and b. Fig. 4a clearly shows the closed cells structure of the foam with the existence of several broken cells possibly from mechanical cutting, which resembles the foamed phenolic resin (without zirconia sol). The commercial foaming process applied here is known to produce foam with closed cell structure.22,23 The cells in Fig. 4a look slightly elongated to one direction. Actually, it elongated to the rise direction, which is commonly observed in open-mold foaming process.24 After high temperature pyrolysis, the ceramic foam (Fig. 4b) maintains the similar morphology as the pre-ceramic foam, except that the cell elongation became less obvious probably because the cells relaxed during heat treatment. The
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Fig. 5. SEMs of the struts in (a) pre-ceramic foam and ZrC foams aged at (b) 2000 ◦ C, (c) 2200 ◦ C for 1 h in Ar and (d) at 2400 ◦ C for 10 min in Ar. The scale bars are 2 m.
precursor-to-ceramic conversion in our cases at least involves the decomposition of the phenolic resin and zirconia sol and carbothermal reduction of zirconia with carbon. Thus it is reasonable to observe the reduction in cell size, which decreases from about 60 m in the pre-ceramic foam to about 40 m in the foam after pyrolysis at 1600 ◦ C for 1 h. Fig. 4c–f shows that there are no obvious morphological changes after the pyrolyzed foams were further aged at 1800–2400 ◦ C, which is also an indication of the good high temperature stability of the ZrC foam. Fig. 5 shows some typical struts of the pre-ceramic foam and
high temperature aged ZrC foams. The thickness of the struts is all about 3–4 m. Again, high temperature aging did not cause obvious changes to the struts structure. TEM images of one typical strut and cell wall of ZrC foam after pyrolysis at 1600 ◦ C are shown in Fig. 6. In TEM images, the dark phases are ZrC, and light phases are amorphous carbon. It is obvious that the phase distributions in the strut and cell wall are different. In the cell wall, it appears that carbon is the continuous phase with ZrC particles embedding in the carbon matrix, while in the struts ZrC forms the dominant phase. This
Fig. 6. TEM images of one typical (a) strut and (b) cell wall of ZrC foam after pyrolysis at 1600 ◦ C.
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might be due to the drainage and mass migrated to the strut during foaming process.4 SEM results show that there are no obvious changes in the cell size after high temperature pyrolysis and thermal aging. However, very interestingly, the average pore sizes of the foams and pore size distribution from MIP (Table 1 and Fig. 7) show some significant changes. The average pore size increases from 3.4 m in pre-ceramic foam to 8.0 m in ZrC foam after pyrolysis. When the ZrC foams were further aged at high temperatures, their average pore sizes based on MIP seem to increase further as aging temperature. In principle, MIP gives out the size of the largest entrance toward a pore, not the actual inner size of a pore.21,25 According to Washburn equation (a modified Young–Laplace equation, Eq. (2)),21 in MIP experiment, the pore size is calculated based on the pressure applied to force mercury intruding the pores. The larger the pressure is, the smaller the pore size is. P = 2 × γ × cos θ/rpore
(2)
where P is the pressure across the interface, γ is the surface tension of mercury, θ is the contact angle between the solid and mercury, and rpore is the calculated radius. In our cases, the closed cell walls can be broken when the applied pressure reaches a certain value. And then mercury intrudes into the cells to give out information of the pore structure. Because extra pressure is needed to break the cell walls, the calculated pore sizes are much smaller than cell sizes observed from SEMs.21 The increase of pore sizes after high temperature pyrolysis and aging indicates that less pressure is required to force mercury to intrude into the cells, which implies that the cell wall strength decreases after pyrolysis and further decreases upon aging. 3.3. Compressive strength of the ZrC foams A preliminary study on the mechanical properties of the ZrC foam obtained from 1600 ◦ C pyrolysis was carried out. Fig. 8a shows a typical compressive strain-stress curve of the ZrC foam, which has a compressive strength of 0.4 MPa. Generally, the
Fig. 7. Pore size distribution of the pre-ceramic and ZrC foams by MIP.
dependence of the strain-stress of ceramic foams can be divided into two different stages: linear elastic and a plateau region.26,27 Upon compression, the foam undergoes a progressive collapse of the cells, with the lower part of the foam remains completely undamaged.28 In the plateau region, the stress is assumed to be independent of the strain as part of the structure collapses, while other parts of the structure remain elastic.28 Beyond the plateau, densification takes place and the stress rises sharply as complete densification begins. These characteristics are also typical of other ceramic foams, and have already been discussed by Ashby and Gibson8,9 and other researchers.26–29 In Fig. 8b, the compressive strength decreases with pyrolysis and aging temperatures initially, and then shows a slightly increase after aging at 2400 ◦ C. The compressive response of a closed ceramic foam is largely depended on the strength of the cell walls and the struts.28,30 The decrease of the compressive strength of the ZrC foams in Fig. 8b is probably due to the decrease of the strength of the cell walls, which is consistent with the MIP observations. After 2400 ◦ C aging, the strength of the struts could be enhanced by the neck formation between ZrC particles,31 which leads to the exceptional increase of the compressive strength.
Fig. 8. (a) Typical compressive stress-strain response of ZrC foam from 1600 ◦ C pyrolysis; (b) Compressive strength of the pre-ceramic foam, the ZrC foam from 1600 ◦ C pyrolysis, and ZrC foams being aged at 1800, 2000, 2400 ◦ C respectively (data are average of 5 individual measurements).
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general approach to prepare other UHTC foams and SiC foam as well. Acknowledgements Financial support from the Chinese Academy of Sciences under the Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program), Science and Technology Commission of Shanghai Municipality (# 11ZR1442200) and the National Natural Science Foundation of China (Nos. 11205229, 51002168) are gratefully acknowledged. References
Fig. 9. Temperature dependence of thermal conductivity of the ZrC foams obtained from pyrolysis at 1600 ◦ C for 1 h.
3.4. Thermal conductivity of the ZrC foam The thermal conductivity of the ZrC foam is determined by laser flash technique, which is 0.96 W/(m K) at 50 ◦ C and 1.36 W/(m K) at 300 ◦ C (limited by our current apparatus). As shown in Fig. 9, the thermal conductivity increases as temperature, more rapidly in the range 50–200 ◦ C, and more slowly in the range 200–300 ◦ C. The heat conduction in a cellular solid is a quite complicated.30 To simplify, the overall thermal transfer in the ZrC foam can be the sum of the heat conduction through the cell walls or struts and radiative heat transfer.30,32 Although the heat conduction through the solid decreases with increasing temperature for ceramic materials, radiative heat transfer through the cells increases as the cube of the temperature,27,30 which is the major reason for the increase of thermal conductivity versus temperature. 4. Conclusions Low density, highly macroporous, closed cell structured ZrC foam can be prepared by a commercial foaming technique for polymeric foams and a following high temperature ceramization. Pre-ceramic foam is first prepared by co-blowing of a zirconia sol and phenolic resin. The foamed green body is converted to ceramic foam after 1600 ◦ C pyrolysis under argon thereafter. The ZrC foam obtained has a large porosity of 85% and cells with an average size of about 40 m. Thermal aging experiments illustrate that the obtained ZrC foam has excellent thermal stability up to 2400 ◦ C, which is very promising for potential ultra high temperature applications. There are no obvious macro- and micro-structure changes after thermal aging by direct and SEM observations. MIP results reveal that the thermal aging may weaken the strength of the cell walls, thus lead to the decrease of the compressive strength of the ZrC foam. Our work demonstrates that zirconium-based sol-gel precursor can be an efficient precursor for making ZrC foams. We also believe that this easy, low-cost foaming method could be a
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