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Pyrolysis of polyurethane by microwave hybrid heating for the processing of NiCr foams
Journal of Materials Processing Technology 212 (2012) 1481–1487
Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Pyrolysis of polyurethane by microwave hybrid heating for the processing of NiCr foams Kangjian Wu ∗ , Ho-Seon Park, Monika Willert-Porada Chair of Materials Processing, University of Bayreuth, 95447 Bayreuth, Germany
a r t i c l e
i n f o
Article history: Received 26 September 2011 Received in revised form 17 January 2012 Accepted 17 February 2012 Available online 25 February 2012 Keywords: Metal foam Polyurethane Pyrolysis Microwave
a b s t r a c t Reticulated polyurethane foams are often used as templates for the processing of metal and ceramic foams, since polyurethane foams with a high homogeneity and uniformity of pores and a wide variety of porosities and pore sizes are commercially available. Current conventional methods to pyrolyse polyurethane have brought attention to issues such as long processing time, high costs and high contents of carbon residue. Microwave hybrid heating, as a new pyrolysis method, has been investigated in this research to overcome these issues. Two microwave hybrid and one conventional heating process as comparison were performed to pyrolyse binder and polyurethane for the processing of NiCr8020 foams, which were fabricated by dip-coating method in a powder metallurgical route. Compared to the conventional process, the processing time and residual carbon content of the pyrolysed and pre-sintered foams can be reduced significantly by utilizing microwave hybrid heating techniques, whereas the stability of microwave hybrid heating must be improved and a compromise must be made between the heating rate and the residual carbon content due to the formation of burst holes during the rapid decomposition of polyurethane. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Polyurethane foams, which have a high homogeneity and uniformity of pores, are often used as templates for the processing of metal and ceramic foams, e.g., Queheillalt et al. (2001) synthesized open-cell nickel foams by depositing nickel phase onto polyurethane foams followed by pyrolysis of polyurethane. Zhu et al. (2002) and Neukam (2011) fabricated reticulated SiC and NiCr8020 foams, respectively, by dip-coating the polyurethane foams with ceramic or metal slurries followed by pyrolysis of polyurethane and sintering of powders. Various conventional processes have been explored for the pyrolysis of polyurethane. In one process, powder-coated polyurethane templates were heated slowly at 0.5–1 ◦ C/min to a temperature between 250 and 600 ◦ C to avoid high stresses generated in struts and collapse of the foams during the heating period. Zhu et al. (2002) heated the samples at 1 ◦ C/min to 600 ◦ C for the decomposition of polyurethane. Neukam (2011) applied a lower heating rate of 0.5 ◦ C/min up to 350 and 500 ◦ C followed by 1 h dwell time, respectively. Such processes with very low heating rates need usually a long processing time. In another, polyurethane templates coated by a dense layer were rapidly exposed to 600–1000 ◦ C in less than 5 s (Cushnie
∗ Corresponding author. Tel.: +49 921 557211; fax: +49 921 557205. E-mail addresses: [email protected], [email protected] (K. Wu). 0924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2012.02.010
and Campbell, 1998). Cushnie and Campbell exposed nickel-coated polyurethane templates rapidly to 1000 ◦ C in a belt furnace and retained this temperature for 200 s under flowing nitrogen. In this patented process, small burst holes were generated on the struts due to the rapid decomposition of polyurethane. Additionally, numerous spots of carbon residual were visible surrounding the burst holes and the processed nickel foams had a residual carbon content of 0.195 wt%. Further reduction of the carbon content to 0.043 wt% was only possible with the help of a reactive atmosphere, containing 15% H2 , 30% H2 O and balance N2 . However, this kind of reactive gas treatment is not suitable for the processing of NiCr foams, because this alloy belongs to alloys with oxygen affine elements, such as Cr but also Al and Ti. Furthermore, conventional rapid heating process is difficult to be used to process metal foams fabricated in a powder metallurgical route, since rapid decomposition of polyurethane can probably lead to collapse of the foams. In powder metallurgy, microwave techniques have been used to sinter metal or ceramic green compacts as it offers specific advantages due to the following facts: reduced processing times (Leonelli et al., 2008), rapid heating rates (Clark et al., 2000), finer microstructures (Rödiger et al., 1998) and improved mechanical properties (Oh et al., 2001) as compared to the conventional processes. It was deemed that microwave processing of metallic materials is difficult since most metals generally have a microwave penetration depth of the micrometer order and the direct microwave heating tends to remain superficial, e.g., the penetration depth of pure nickel
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Fig. 1. Images of: (a) reticulated polyurethane foams (PPI 20) and (b) NiCr8020 powder (d90 < 45 m).
at a microwave frequency of 2.45 GHz under room temperature is 2.7 m (Gupta et al., 2007). With increasing temperature, however, the penetration depth increases slowly and can be raised by a factor of two to ten times when the temperature is approaching the melting point of the metal (Huey and Morrow, 2004). Moreover, metals can absorb microwaves and be heated directly when they are present in the form of powders or small particles (Gupta et al., 2007). Luo et al. (2004) suggested that the heating of powdered metals in a single- or multimode microwave system can be attributed to the result of eddy currents or magnetic induction heating. Veltl et al. (2004) reported the presence of another microwave effect that the high frequency electric field acting between the metal particles generates plasma which promotes mass transport into the contact areas between particles. These conclusions also constitute the theoretical basis of microwave-assisted pyrolysis with metallic materials. However, processing of metallic materials which have sharp edges such as metal foams by direct microwave heating is problematic since microwave field tends to concentrate itself at the sharp edges of materials (Chan and Reader, 2000). If the field concentration at sharp edges exceeds the dielectric strength of gas medium, plasma can be generated and leads to overheating or melting of the materials (Park, 2005). Microwave hybrid heating can be a solution to this problem. Microwave hybrid heating combines the direct microwave heating with a microwave-coupled external heating source by using microwave susceptors such as SiC, which has a high dielectric loss already at low temperatures (Baeraky, 2002). Park (2005) sintered green compacts of steel gears under inert atmosphere by microwave heating. Without susceptor gas plasma was always ignited at the shape edges of gears during the heating period, whereas the risk of plasma formation was reduced significantly by using SiC plates as susceptor, because SiC absorbed microwave effectively and weakened the field concentration at the sharp edges of the gears. The aim of this study is to investigate the possibility of applying microwave hybrid heating techniques in the powder metallurgical processing process of polyurethane-template based NiCr foams, which are attractive candidates for the applications such as high-temperature catalyst substrate, filter or heat exchanger due to their superior oxidation resistance and mechanical properties even under elevated temperature (Choe and Dunand, 2004). Two microwave hybrid heating and one conventional heating process as comparison were investigated experimentally in order to identify the benefits and drawbacks of both heating methods. The pyrolysed and pre-sintered NiCr foams were characterized and compared based on their macro-, microstructures and residual carbon contents, since the carbon content has to be as low as possible to avoid the formation of (Ni)–(Crx Cy )-eutectics upon sintering (Velikanova et al., 1999). Neukam (2011) found that liquid phase formed already
at 1232 ◦ C during sintering for the NiCr8020 foams with 2.71 wt% residual carbon content, while the solid phase maintained stable up to 1300 ◦ C for the foams with 0.14 wt% carbon content. 2. Experimental procedure 2.1. Materials and dip-coating Fig. 1 illustrates the reticulated polyurethane foams (PPI 20, EMW Filtertechnik, Germany) and water-atomized NiCr8020 powders (d90 < 45 m, H.C. Starck, Germany) used in this research. Slurry with 80 wt% solid content was made by mixing NiCr8020 powders with 5 wt% PVA aqueous solvent homogenously and it was stirred continuously in order to avoid the sedimentation of metal phase. Polyurethane foams with a size of 30 mm × 30 mm × 10 mm were immersed into the slurry and compressed while submerged in order to fill all of the pores. Powders were able to be coated onto the templates by means of the adhesive force of PVA. The impregnated templates passed then through rollers with a preset separation to remove the excess slurry, which is shown in Fig. 2. Subsequently, the coated polyurethane templates were dried at 80 ◦ C in air. 2.2. Pyrolysis and pre-sintering After drying the binder and the polyurethane templates were pyrolysed followed by pre-sintering of NiCr8020 foams at 1000–1050 ◦ C under Ar/H2 (5% H2 ) at ambient pressure. Pyrolysis by conventional heating was performed in a quartz tube furnace (HST 12/400, Carbolite, Germany). The experimental setup of pyrolysis by microwave hybrid heating is shown in Fig. 3. The microwave furnace is an electrical furnace equipped with resistively heated Mo-heating elements and with a multi-mode microwave cavity integrated into it. The outer wall of the furnace is made of steel and the inner wall of the microwave cavity is made of molybdenum. The applied microwave frequency is 2.45 GHz and the maximal output powder is 6 kW. A porous Al2 O3 casket with a dimension of 135 mm × 135 mm × 75 mm was used as thermal
Fig. 2. Process of removing the excess slurry by a preset roller.
K. Wu et al. / Journal of Materials Processing Technology 212 (2012) 1481–1487
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Fig. 3. Experimental set-up of pyrolysis by microwave hybrid heating.
insulation for the samples. The samples were put on an Al2 O3 sheet with a thickness of 1 mm to avoid direct contact of the samples with the SiC plate, which was used as susceptor and was placed inside the casket. The temperature was measured by a high-temperature pyrometer (for T > 200 ◦ C) through a quartz glass window at the top of the furnace. The pyrometer focus was adjusted through a hole on the casket to the upper side of the samples. Fig. 4 shows the temperature profiles applied during each of the three pyrolysis processes. The processes are described as 3-step conventional, 3-step and 1-step microwave hybrid heating processes. Neukam (2011) determined the decomposition behaviors of PVA and polyurethane under Ar/H2 by DSC measurements. Peaks were observed at 326 and 442 ◦ C, 323 and 424 ◦ C for the decomposition of PVA and polyurethane, respectively. For each organic substance, the two peaks correspond to the maximal decomposition rates of its side groups and main chains, respectively. In
the 3-step conventional heating process, the samples were thus heated with 0.5 ◦ C/min to 350 and 500 ◦ C followed by 1 h dwell time, respectively. After pyrolysis the samples were heated with 5 ◦ C/min to 1000 ◦ C and held at this temperature for 1 h. Such a process takes totally about 20 h until the samples were cooled down to 200 ◦ C. In the 3-step microwave hybrid heating process, the samples were heated with 3–5 ◦ C/min to 350 and 500 ◦ C followed by 0.5 h dwell time, respectively. Subsequently, the samples were heated to 1050 ◦ C with approximately 10 ◦ C/min followed by 0.5 h dwell time. Gas plasma was ignited after the two pyrolysis steps, which can be indicated by the peaks in the temperature profile. The plasma was able to be extinguished by reducing the output power and increasing it again. In the 1-step microwave hybrid heating process, the samples were heated with a heating rate of approximately 85 ◦ C/min to 1050 ◦ C followed by 1 h dwell time. No plasma was observed during the whole process. The processing time of 3-step and 1-step microwave hybrid heating correspond to a reduction of processing time by about 80% and 90%, respectively, as compared to the conventional process.
2.3. Materials characterization
Fig. 4. Temperature profiles of the three heating processes.
The macro- and microstructure of pyrolysed and pre-sintered NiCr8020 foams were characterized by stereomicroscope (SZX12, Olympus, Germany) and scanning electron microscope (JSM-840A, Jeol, Japan). The SEM samples were prepared by gluing foam struts directly onto an aluminum stub with silver paste in order to avoid any other carbon contaminations. The interfacial chemistry and element distributions were studied by electron dispersive X-ray analysis (Oxford Instruments, UK). The porosity of pre-sintered metal struts was determined by linear analysis by using the software “Lince”. The residual carbon content of the pre-sintered foams was determined by CHN elementary analysis (EA 3000, HEKAtech, Germany). Metal struts were cut from the foams and ground to fine powders. For each elementary analysis, 10 mg powders were used and for each heating process, powders from three samples were analyzed.
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3. Results 3.1. Macro- and microstructures Fig. 5 shows the powder-coated polyurethane template after drying. Some struts at the top and bottom portion of the template were not able to be coated completely and the uncoated parts can be used as burst openings for the decomposition of polyurethane during the pyrolysis. The macro- and microstructure of the pre-sintered NiCr8020 foams are shown in Fig. 6. No burst holes or cracks were observed on the struts of the conventionally pyrolysed foams (Fig. 6(a)). Nevertheless, numerous small spots were formed on the particle surfaces, as shown in Fig. 6(b). Table 1 shows the EDX results of spectrum 1 and 2, which were detected direct on the small spots and away from the spots, respectively. In spectrum 1 carbon has a high content of 10.84 wt% and the chromium content of 25.90 wt% is higher compared to the nominal alloy content. The element
Fig. 5. NiCr8020 powder-coated polyurethane template after drying.
Fig. 6. Macro- and microstructures of the pre-sintered NiCr8020 foams by: (a and b) conventional heating; (c and d) 3-step microwave hybrid heating; (e and f) 1-step microwave hybrid heating.
K. Wu et al. / Journal of Materials Processing Technology 212 (2012) 1481–1487 Table 1 EDX results detected on the NiCr8020 particle processed by the conventional heating.
Spectrum 1 Spectrum 2
Element (wt%) C
Cr
Ni
10.84 2.47
25.90 18.53
63.26 78.99
distributions in Fig. 7 indicate that almost no nickel phase was detected on the small spots whereas chromium and carbon tend to concentrate themselves at the sites where the small spots are located. It is thus suggested that the small spots on the NiCr8020 particles are composed of chromium carbide. Foams obtained by 3-step microwave hybrid heating were exposed to a comparable temperature profile as those obtained by conventional process. However, due to the ignition of plasma during the heating period one corner of the foams was molten. One of the molten struts is shown in Fig. 6(c). In addition, several small burst holes on the metal struts were observed. On the particle surfaces the number of chromium carbide spots decreased significantly as compared to the foams obtained by conventional process (Fig. 6(d)). For the 1-step microwave hybrid heating process, burst holes on the struts and cracks of several fine struts were generated (Fig. 6(e)), whereas no chromium carbide spots on the particle surfaces can be observed (Fig. 6(f)). The strut porosity of the NiCr8020 foams processed by the three different heating processes is listed in Table 2. 3.2. Elementary analysis of carbon A comparison of the residual carbon contents of the pyrolysed and pre-sintered NiCr foams obtained by the three different
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Table 2 Metal strut porosity after the three heating processes. Process
Strut porosity (%)
3-Step conventional heating 3-Step microwave hybrid heating 1-Step microwave hybrid heating
38.78 ± 2.41 34.17 ± 3.49 40.57 ± 4.07
heating processes is present in Fig. 8. The conventionally pyrolysed NiCr8020 foams had the highest average residual carbon content of 0.14 wt% while the foams fabricated by 3-step microwave hybrid heating had a carbon content of only 0.045 wt%. The lowest carbon content of 0.016 wt% was achieved by 1-step microwave hybrid heating. The average residual carbon contents of 3-step and 1-step microwave hybrid heating correspond to a reduction of carbon content by about 70% and 90%, respectively, as compared to the conventional process. 4. Discussion 4.1. Processing time By utilizing microwave hybrid heating, a reduction of processing time by about 80–90% as compared to the conventional process was realized, since in powder metallurgy conventional pyrolysis of polyurethane for the processing of metal foams is usually performed with a very low heating rate of 0.5–1 ◦ C/min, otherwise the decomposition rate of polyurethane will be too high and probably lead to collapse of the foams. By utilizing microwave hybrid heating, however, it was possible to heat the powder-coated polyurethane foams with a much higher heating rate of even about 85 ◦ C/min without collapse and the processing time can be reduced
Fig. 7. SEM image and EDX element maps of a NiCr8020 particle surface after the processing by conventional heating. (a) SEM image; (b) nickel (white); (c) chromium (green); (d) carbon (red). All elements were recorded from their K␣ line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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plays an important role on the stability of microwave hybrid heating for the processing of metal foams and the stability of microwave hybrid heating can be firstly improved by increasing the heating rate, reducing the sintering temperature and processing time. Secondly, application of pulsed microwave radiation could be another solution to reduce the risk of plasma ignition and improve the stability of these processes. 4.3. Residual carbon content
Fig. 8. Residual carbon contents of processed NiCr8020 foams for the three heating processes.
significantly. This advantage can be attributed to the higher efficiency of microwave hybrid sintering of metal green bodies as compared to the conventional process, since in the microwave hybrid heating process metal green bodies can be heated not only by the conventional infrared radiation heating from the microwave heated SiC susceptor, but also by the dissipation of microwave radiation in the volume penetrated by the radiation and possibly by electric arcs among the particles. Therefore, microwave hybrid heating makes the rapid processing of the metal foams possible, which are fabricated in a powder metallurgical route. However, due to the higher heating rates burst holes or cracks of several struts were generated during the decomposition of polyurethane, similar to the results obtained by the conventional rapid heating process (Cushnie and Campbell, 1998). 4.2. Stability of microwave hybrid heating In the 3-step microwave hybrid heating process, gas plasma was ignited after the two pyrolysis steps and one corner of the sample was molten. The plasma was able to be extinguished by reducing the output power and increasing it again. For 1-step microwave hybrid heating, however, no plasma was ignited during the whole process. The probable reason is, 3-step microwave hybrid heating process has a relative longer heating period of about 3 h as compared to the 1-step heating process with only about 1 h heating period (Fig. 4). This difference resulted in the formation of thicker sintering necks (Fig. 6(d and f)) and a lower strut porosity (Table 2) for the former, which further resulted in a higher electric conductivity. For conductors, the relationship between electric conductivity and penetration depth of microwave radiation, which is defined as the depth into the conductor from the surface at which the current density is 1/e of its value at the surface, can be expressed by following equation (Meredith, 1998):
ı=
1 f0 r
(1)
In which ı is the penetration depth of electromagnetic field, is the electric conductivity, f is the frequency, 0 is the magnetic constant and r is the relative permeability. According to the equation, the penetration depth of microwave radiation decreases with increasing electric conductivity of metal foams and the increasing unabsorbed microwave radiation tends to concentration itself at sharp edges. In excess of the dielectric strength of gas medium, plasma can be ignited. It is thus suggested that electric conductivity
The residual carbon content of the foams processed by microwave hybrid heating was reduced by about 70–90% compared to the conventional process, because within a microwave hybrid heating process multiple paths to heat organic substances can be utilized simultaneously as compared to the conventional processes with only one heating source. In our research, organic substances were heated by three heating sources during the microwave hybrid heating processes: (1) direct heating by dissipation of microwave radiation into heat in the volume penetrated by the radiation; (2) indirect heating by metal particles which were heated inductively upon exposure to the electromagnetic field and possibly by electric arcs among the particles; (3) indirect infrared radiation heating form the microwave heated SiC susceptor. Therefore, microwave hybrid heating is more effective to remove organic substances in metal foams compared to the conventional processes. In addition, heating rates have also an influence on the pyrolysis effect. For the decomposition of polyurethane under inert gases, the kinetic constant increases with increasing heating rate (Bilbao et al., 1996). By using rapid heating methods the organic substances can be thus removed before the formation of closed pores. However, due to the formation of burst holes and cracks, a compromise must be made between the heating rate and the residual carbon content. 5. Conclusions Pyrolysis of binder and polyurethane for the processing of NiCr8020 foams was conducted successfully by one conventional and two microwave hybrid heating processes. By utilizing microwave hybrid heating, a reduction of processing time and residual carbon content by about 80–90% and 70–90% as compared to the conventional process, respectively, was realized. The stability of microwave hybrid heating processes for the processing of metal foams can be improved by increasing the heating rate, reducing the sintering temperature and processing time. High heating rates are also beneficial for the removal of organic substances, but due to the formation of burst holes and cracks, a compromise must be made between the heating rate and the residual carbon content. Acknowledgments The authors would like to acknowledge the graduate school 1229 of German Science Foundation for the financial support and Ms. Birgit Brunner from Department of Chemical Engineering for the elementary analysis. References Baeraky, T.A., 2002. Microwave measurements of the dielectric properties of silicon carbide at high temperature. Egypt. J. Sol. 25, 263–273. Bilbao, R., Mastral, J.F., Ceamanos, J., Aldea, M.E., 1996. Kinetics of the thermal decomposition of polyurethane foams in nitrogen and air atmospheres. J. Anal. Appl. Pyrolysis 37, 69–82. Chan, T.V.C.T., Reader, H.C., 2000. Understanding Microwave Heating Cavities. Artech House Inc., Norwood, MA, USA. Choe, H., Dunand, D.C., 2004. Mechanical properties of oxidation-resistant Ni–Cr foams. Mater. Sci. Eng. A 384, 184–193. Clark, D.E., Folz, D.C., West, J.K., 2000. Processing materials with microwave energy. Mater. Sci. Eng. A 287, 153–158.
K. Wu et al. / Journal of Materials Processing Technology 212 (2012) 1481–1487 Cushnie, K.K., Campbell, S.T., 1998. Process for removal of polymer foams from nickel-coated substrates, U.S. Patent 5,735,977. Gupta, M., Leong, W.W., Eugene, 2007. Microwaves and Metals. John Wiley & Sons (Asia) Pte Ltd., Singapore. Huey, H.E., Morrow, M.S., 2004. Microwave interactions in the melting of metals. In: Schulz, R.L., Folz, D.C. (Eds.), Microwave and Radio Frequency Applications. Proceedings of the Fourth World Congress on Microwave and radio Frequency Applications. The Microwave Working Group Ltd., Arnold, MD, USA. Leonelli, C., Veronesi, P., Denti, L., Gatto, A., Iuliano, L., 2008. Microwave assisted sintering of green metal parts. J. Mater. Process. Technol. 205, 489–496. Luo, J., Hunyar, C., Feher, L., Link, G., Thumm, M., Pozzo, P., 2004. Theory and experiments of electromagnetic loss mechanism for microwave heating of powdered metals. Appl. Phys. Lett. 84, 5076–5078. Meredith, R.J., 1998. Engineers’ Handbook of Industrial Microwave Heating. The Institution of Electrical Engineers, London, UK. Neukam, M., 2011. Process development for metal–ceramic composites on the basis of cellular nickel-base alloys (in German). Shaker Verlag, Aachen, Germany.
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Oh, S.T., Tajima, K., Ando, M., Ohji, T., 2001. Fabrication of porous Al2 O3 by microwave sintering and its properties. Mater. Lett. 48, 215–218. Park, H.S., 2005. Microwave Sintering of Metallic Materials (in German). VDI Verlag, Düsseldorf, Germany. Queheillalt, D.T., Hass, D.D., Sypeck, D.J., Wadley, H.N.G., 2001. Synthesis of opencell metal foams by templated directed vapor deposition. J. Mater. Res. 16, 1028–1036. Rödiger, K., Dreyer, K., Gerdes, T., Willert-Porada, M., 1998. Microwave sintering of hardmetals. Int. J. Refract. Met. Hard Mater. 16, 409–416. Velikanova, T.Ya., Bondar, A.A., Grytsiv, A.V., 1999. The chromium–nickel–carbon (Cr–Ni–C) phase diagram. J. Phase Equilib. 20, 125–147. Veltl, G., Petzoldt, F., Pueschner, P.A., 2004. Effects of microwaves on sintering processes. In: Danninger, H., Ratzi, R. (Eds.), Proceedings of Euro PM2004 Conference. Vienna, Austria. Zhu, X., Jiang, D., Tan, S., 2002. Preparation of silicon carbide reticulated porous ceramics. Mater. Sci. Eng. A 323, 232–238.
Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Pyrolysis of polyurethane by microwave hybrid heating for the processing of NiCr foams Kangjian Wu ∗ , Ho-Seon Park, Monika Willert-Porada Chair of Materials Processing, University of Bayreuth, 95447 Bayreuth, Germany
a r t i c l e
i n f o
Article history: Received 26 September 2011 Received in revised form 17 January 2012 Accepted 17 February 2012 Available online 25 February 2012 Keywords: Metal foam Polyurethane Pyrolysis Microwave
a b s t r a c t Reticulated polyurethane foams are often used as templates for the processing of metal and ceramic foams, since polyurethane foams with a high homogeneity and uniformity of pores and a wide variety of porosities and pore sizes are commercially available. Current conventional methods to pyrolyse polyurethane have brought attention to issues such as long processing time, high costs and high contents of carbon residue. Microwave hybrid heating, as a new pyrolysis method, has been investigated in this research to overcome these issues. Two microwave hybrid and one conventional heating process as comparison were performed to pyrolyse binder and polyurethane for the processing of NiCr8020 foams, which were fabricated by dip-coating method in a powder metallurgical route. Compared to the conventional process, the processing time and residual carbon content of the pyrolysed and pre-sintered foams can be reduced significantly by utilizing microwave hybrid heating techniques, whereas the stability of microwave hybrid heating must be improved and a compromise must be made between the heating rate and the residual carbon content due to the formation of burst holes during the rapid decomposition of polyurethane. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Polyurethane foams, which have a high homogeneity and uniformity of pores, are often used as templates for the processing of metal and ceramic foams, e.g., Queheillalt et al. (2001) synthesized open-cell nickel foams by depositing nickel phase onto polyurethane foams followed by pyrolysis of polyurethane. Zhu et al. (2002) and Neukam (2011) fabricated reticulated SiC and NiCr8020 foams, respectively, by dip-coating the polyurethane foams with ceramic or metal slurries followed by pyrolysis of polyurethane and sintering of powders. Various conventional processes have been explored for the pyrolysis of polyurethane. In one process, powder-coated polyurethane templates were heated slowly at 0.5–1 ◦ C/min to a temperature between 250 and 600 ◦ C to avoid high stresses generated in struts and collapse of the foams during the heating period. Zhu et al. (2002) heated the samples at 1 ◦ C/min to 600 ◦ C for the decomposition of polyurethane. Neukam (2011) applied a lower heating rate of 0.5 ◦ C/min up to 350 and 500 ◦ C followed by 1 h dwell time, respectively. Such processes with very low heating rates need usually a long processing time. In another, polyurethane templates coated by a dense layer were rapidly exposed to 600–1000 ◦ C in less than 5 s (Cushnie
∗ Corresponding author. Tel.: +49 921 557211; fax: +49 921 557205. E-mail addresses: [email protected], [email protected] (K. Wu). 0924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2012.02.010
and Campbell, 1998). Cushnie and Campbell exposed nickel-coated polyurethane templates rapidly to 1000 ◦ C in a belt furnace and retained this temperature for 200 s under flowing nitrogen. In this patented process, small burst holes were generated on the struts due to the rapid decomposition of polyurethane. Additionally, numerous spots of carbon residual were visible surrounding the burst holes and the processed nickel foams had a residual carbon content of 0.195 wt%. Further reduction of the carbon content to 0.043 wt% was only possible with the help of a reactive atmosphere, containing 15% H2 , 30% H2 O and balance N2 . However, this kind of reactive gas treatment is not suitable for the processing of NiCr foams, because this alloy belongs to alloys with oxygen affine elements, such as Cr but also Al and Ti. Furthermore, conventional rapid heating process is difficult to be used to process metal foams fabricated in a powder metallurgical route, since rapid decomposition of polyurethane can probably lead to collapse of the foams. In powder metallurgy, microwave techniques have been used to sinter metal or ceramic green compacts as it offers specific advantages due to the following facts: reduced processing times (Leonelli et al., 2008), rapid heating rates (Clark et al., 2000), finer microstructures (Rödiger et al., 1998) and improved mechanical properties (Oh et al., 2001) as compared to the conventional processes. It was deemed that microwave processing of metallic materials is difficult since most metals generally have a microwave penetration depth of the micrometer order and the direct microwave heating tends to remain superficial, e.g., the penetration depth of pure nickel
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Fig. 1. Images of: (a) reticulated polyurethane foams (PPI 20) and (b) NiCr8020 powder (d90 < 45 m).
at a microwave frequency of 2.45 GHz under room temperature is 2.7 m (Gupta et al., 2007). With increasing temperature, however, the penetration depth increases slowly and can be raised by a factor of two to ten times when the temperature is approaching the melting point of the metal (Huey and Morrow, 2004). Moreover, metals can absorb microwaves and be heated directly when they are present in the form of powders or small particles (Gupta et al., 2007). Luo et al. (2004) suggested that the heating of powdered metals in a single- or multimode microwave system can be attributed to the result of eddy currents or magnetic induction heating. Veltl et al. (2004) reported the presence of another microwave effect that the high frequency electric field acting between the metal particles generates plasma which promotes mass transport into the contact areas between particles. These conclusions also constitute the theoretical basis of microwave-assisted pyrolysis with metallic materials. However, processing of metallic materials which have sharp edges such as metal foams by direct microwave heating is problematic since microwave field tends to concentrate itself at the sharp edges of materials (Chan and Reader, 2000). If the field concentration at sharp edges exceeds the dielectric strength of gas medium, plasma can be generated and leads to overheating or melting of the materials (Park, 2005). Microwave hybrid heating can be a solution to this problem. Microwave hybrid heating combines the direct microwave heating with a microwave-coupled external heating source by using microwave susceptors such as SiC, which has a high dielectric loss already at low temperatures (Baeraky, 2002). Park (2005) sintered green compacts of steel gears under inert atmosphere by microwave heating. Without susceptor gas plasma was always ignited at the shape edges of gears during the heating period, whereas the risk of plasma formation was reduced significantly by using SiC plates as susceptor, because SiC absorbed microwave effectively and weakened the field concentration at the sharp edges of the gears. The aim of this study is to investigate the possibility of applying microwave hybrid heating techniques in the powder metallurgical processing process of polyurethane-template based NiCr foams, which are attractive candidates for the applications such as high-temperature catalyst substrate, filter or heat exchanger due to their superior oxidation resistance and mechanical properties even under elevated temperature (Choe and Dunand, 2004). Two microwave hybrid heating and one conventional heating process as comparison were investigated experimentally in order to identify the benefits and drawbacks of both heating methods. The pyrolysed and pre-sintered NiCr foams were characterized and compared based on their macro-, microstructures and residual carbon contents, since the carbon content has to be as low as possible to avoid the formation of (Ni)–(Crx Cy )-eutectics upon sintering (Velikanova et al., 1999). Neukam (2011) found that liquid phase formed already
at 1232 ◦ C during sintering for the NiCr8020 foams with 2.71 wt% residual carbon content, while the solid phase maintained stable up to 1300 ◦ C for the foams with 0.14 wt% carbon content. 2. Experimental procedure 2.1. Materials and dip-coating Fig. 1 illustrates the reticulated polyurethane foams (PPI 20, EMW Filtertechnik, Germany) and water-atomized NiCr8020 powders (d90 < 45 m, H.C. Starck, Germany) used in this research. Slurry with 80 wt% solid content was made by mixing NiCr8020 powders with 5 wt% PVA aqueous solvent homogenously and it was stirred continuously in order to avoid the sedimentation of metal phase. Polyurethane foams with a size of 30 mm × 30 mm × 10 mm were immersed into the slurry and compressed while submerged in order to fill all of the pores. Powders were able to be coated onto the templates by means of the adhesive force of PVA. The impregnated templates passed then through rollers with a preset separation to remove the excess slurry, which is shown in Fig. 2. Subsequently, the coated polyurethane templates were dried at 80 ◦ C in air. 2.2. Pyrolysis and pre-sintering After drying the binder and the polyurethane templates were pyrolysed followed by pre-sintering of NiCr8020 foams at 1000–1050 ◦ C under Ar/H2 (5% H2 ) at ambient pressure. Pyrolysis by conventional heating was performed in a quartz tube furnace (HST 12/400, Carbolite, Germany). The experimental setup of pyrolysis by microwave hybrid heating is shown in Fig. 3. The microwave furnace is an electrical furnace equipped with resistively heated Mo-heating elements and with a multi-mode microwave cavity integrated into it. The outer wall of the furnace is made of steel and the inner wall of the microwave cavity is made of molybdenum. The applied microwave frequency is 2.45 GHz and the maximal output powder is 6 kW. A porous Al2 O3 casket with a dimension of 135 mm × 135 mm × 75 mm was used as thermal
Fig. 2. Process of removing the excess slurry by a preset roller.
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Fig. 3. Experimental set-up of pyrolysis by microwave hybrid heating.
insulation for the samples. The samples were put on an Al2 O3 sheet with a thickness of 1 mm to avoid direct contact of the samples with the SiC plate, which was used as susceptor and was placed inside the casket. The temperature was measured by a high-temperature pyrometer (for T > 200 ◦ C) through a quartz glass window at the top of the furnace. The pyrometer focus was adjusted through a hole on the casket to the upper side of the samples. Fig. 4 shows the temperature profiles applied during each of the three pyrolysis processes. The processes are described as 3-step conventional, 3-step and 1-step microwave hybrid heating processes. Neukam (2011) determined the decomposition behaviors of PVA and polyurethane under Ar/H2 by DSC measurements. Peaks were observed at 326 and 442 ◦ C, 323 and 424 ◦ C for the decomposition of PVA and polyurethane, respectively. For each organic substance, the two peaks correspond to the maximal decomposition rates of its side groups and main chains, respectively. In
the 3-step conventional heating process, the samples were thus heated with 0.5 ◦ C/min to 350 and 500 ◦ C followed by 1 h dwell time, respectively. After pyrolysis the samples were heated with 5 ◦ C/min to 1000 ◦ C and held at this temperature for 1 h. Such a process takes totally about 20 h until the samples were cooled down to 200 ◦ C. In the 3-step microwave hybrid heating process, the samples were heated with 3–5 ◦ C/min to 350 and 500 ◦ C followed by 0.5 h dwell time, respectively. Subsequently, the samples were heated to 1050 ◦ C with approximately 10 ◦ C/min followed by 0.5 h dwell time. Gas plasma was ignited after the two pyrolysis steps, which can be indicated by the peaks in the temperature profile. The plasma was able to be extinguished by reducing the output power and increasing it again. In the 1-step microwave hybrid heating process, the samples were heated with a heating rate of approximately 85 ◦ C/min to 1050 ◦ C followed by 1 h dwell time. No plasma was observed during the whole process. The processing time of 3-step and 1-step microwave hybrid heating correspond to a reduction of processing time by about 80% and 90%, respectively, as compared to the conventional process.
2.3. Materials characterization
Fig. 4. Temperature profiles of the three heating processes.
The macro- and microstructure of pyrolysed and pre-sintered NiCr8020 foams were characterized by stereomicroscope (SZX12, Olympus, Germany) and scanning electron microscope (JSM-840A, Jeol, Japan). The SEM samples were prepared by gluing foam struts directly onto an aluminum stub with silver paste in order to avoid any other carbon contaminations. The interfacial chemistry and element distributions were studied by electron dispersive X-ray analysis (Oxford Instruments, UK). The porosity of pre-sintered metal struts was determined by linear analysis by using the software “Lince”. The residual carbon content of the pre-sintered foams was determined by CHN elementary analysis (EA 3000, HEKAtech, Germany). Metal struts were cut from the foams and ground to fine powders. For each elementary analysis, 10 mg powders were used and for each heating process, powders from three samples were analyzed.
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3. Results 3.1. Macro- and microstructures Fig. 5 shows the powder-coated polyurethane template after drying. Some struts at the top and bottom portion of the template were not able to be coated completely and the uncoated parts can be used as burst openings for the decomposition of polyurethane during the pyrolysis. The macro- and microstructure of the pre-sintered NiCr8020 foams are shown in Fig. 6. No burst holes or cracks were observed on the struts of the conventionally pyrolysed foams (Fig. 6(a)). Nevertheless, numerous small spots were formed on the particle surfaces, as shown in Fig. 6(b). Table 1 shows the EDX results of spectrum 1 and 2, which were detected direct on the small spots and away from the spots, respectively. In spectrum 1 carbon has a high content of 10.84 wt% and the chromium content of 25.90 wt% is higher compared to the nominal alloy content. The element
Fig. 5. NiCr8020 powder-coated polyurethane template after drying.
Fig. 6. Macro- and microstructures of the pre-sintered NiCr8020 foams by: (a and b) conventional heating; (c and d) 3-step microwave hybrid heating; (e and f) 1-step microwave hybrid heating.
K. Wu et al. / Journal of Materials Processing Technology 212 (2012) 1481–1487 Table 1 EDX results detected on the NiCr8020 particle processed by the conventional heating.
Spectrum 1 Spectrum 2
Element (wt%) C
Cr
Ni
10.84 2.47
25.90 18.53
63.26 78.99
distributions in Fig. 7 indicate that almost no nickel phase was detected on the small spots whereas chromium and carbon tend to concentrate themselves at the sites where the small spots are located. It is thus suggested that the small spots on the NiCr8020 particles are composed of chromium carbide. Foams obtained by 3-step microwave hybrid heating were exposed to a comparable temperature profile as those obtained by conventional process. However, due to the ignition of plasma during the heating period one corner of the foams was molten. One of the molten struts is shown in Fig. 6(c). In addition, several small burst holes on the metal struts were observed. On the particle surfaces the number of chromium carbide spots decreased significantly as compared to the foams obtained by conventional process (Fig. 6(d)). For the 1-step microwave hybrid heating process, burst holes on the struts and cracks of several fine struts were generated (Fig. 6(e)), whereas no chromium carbide spots on the particle surfaces can be observed (Fig. 6(f)). The strut porosity of the NiCr8020 foams processed by the three different heating processes is listed in Table 2. 3.2. Elementary analysis of carbon A comparison of the residual carbon contents of the pyrolysed and pre-sintered NiCr foams obtained by the three different
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Table 2 Metal strut porosity after the three heating processes. Process
Strut porosity (%)
3-Step conventional heating 3-Step microwave hybrid heating 1-Step microwave hybrid heating
38.78 ± 2.41 34.17 ± 3.49 40.57 ± 4.07
heating processes is present in Fig. 8. The conventionally pyrolysed NiCr8020 foams had the highest average residual carbon content of 0.14 wt% while the foams fabricated by 3-step microwave hybrid heating had a carbon content of only 0.045 wt%. The lowest carbon content of 0.016 wt% was achieved by 1-step microwave hybrid heating. The average residual carbon contents of 3-step and 1-step microwave hybrid heating correspond to a reduction of carbon content by about 70% and 90%, respectively, as compared to the conventional process. 4. Discussion 4.1. Processing time By utilizing microwave hybrid heating, a reduction of processing time by about 80–90% as compared to the conventional process was realized, since in powder metallurgy conventional pyrolysis of polyurethane for the processing of metal foams is usually performed with a very low heating rate of 0.5–1 ◦ C/min, otherwise the decomposition rate of polyurethane will be too high and probably lead to collapse of the foams. By utilizing microwave hybrid heating, however, it was possible to heat the powder-coated polyurethane foams with a much higher heating rate of even about 85 ◦ C/min without collapse and the processing time can be reduced
Fig. 7. SEM image and EDX element maps of a NiCr8020 particle surface after the processing by conventional heating. (a) SEM image; (b) nickel (white); (c) chromium (green); (d) carbon (red). All elements were recorded from their K␣ line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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plays an important role on the stability of microwave hybrid heating for the processing of metal foams and the stability of microwave hybrid heating can be firstly improved by increasing the heating rate, reducing the sintering temperature and processing time. Secondly, application of pulsed microwave radiation could be another solution to reduce the risk of plasma ignition and improve the stability of these processes. 4.3. Residual carbon content
Fig. 8. Residual carbon contents of processed NiCr8020 foams for the three heating processes.
significantly. This advantage can be attributed to the higher efficiency of microwave hybrid sintering of metal green bodies as compared to the conventional process, since in the microwave hybrid heating process metal green bodies can be heated not only by the conventional infrared radiation heating from the microwave heated SiC susceptor, but also by the dissipation of microwave radiation in the volume penetrated by the radiation and possibly by electric arcs among the particles. Therefore, microwave hybrid heating makes the rapid processing of the metal foams possible, which are fabricated in a powder metallurgical route. However, due to the higher heating rates burst holes or cracks of several struts were generated during the decomposition of polyurethane, similar to the results obtained by the conventional rapid heating process (Cushnie and Campbell, 1998). 4.2. Stability of microwave hybrid heating In the 3-step microwave hybrid heating process, gas plasma was ignited after the two pyrolysis steps and one corner of the sample was molten. The plasma was able to be extinguished by reducing the output power and increasing it again. For 1-step microwave hybrid heating, however, no plasma was ignited during the whole process. The probable reason is, 3-step microwave hybrid heating process has a relative longer heating period of about 3 h as compared to the 1-step heating process with only about 1 h heating period (Fig. 4). This difference resulted in the formation of thicker sintering necks (Fig. 6(d and f)) and a lower strut porosity (Table 2) for the former, which further resulted in a higher electric conductivity. For conductors, the relationship between electric conductivity and penetration depth of microwave radiation, which is defined as the depth into the conductor from the surface at which the current density is 1/e of its value at the surface, can be expressed by following equation (Meredith, 1998):
ı=
1 f0 r
(1)
In which ı is the penetration depth of electromagnetic field, is the electric conductivity, f is the frequency, 0 is the magnetic constant and r is the relative permeability. According to the equation, the penetration depth of microwave radiation decreases with increasing electric conductivity of metal foams and the increasing unabsorbed microwave radiation tends to concentration itself at sharp edges. In excess of the dielectric strength of gas medium, plasma can be ignited. It is thus suggested that electric conductivity
The residual carbon content of the foams processed by microwave hybrid heating was reduced by about 70–90% compared to the conventional process, because within a microwave hybrid heating process multiple paths to heat organic substances can be utilized simultaneously as compared to the conventional processes with only one heating source. In our research, organic substances were heated by three heating sources during the microwave hybrid heating processes: (1) direct heating by dissipation of microwave radiation into heat in the volume penetrated by the radiation; (2) indirect heating by metal particles which were heated inductively upon exposure to the electromagnetic field and possibly by electric arcs among the particles; (3) indirect infrared radiation heating form the microwave heated SiC susceptor. Therefore, microwave hybrid heating is more effective to remove organic substances in metal foams compared to the conventional processes. In addition, heating rates have also an influence on the pyrolysis effect. For the decomposition of polyurethane under inert gases, the kinetic constant increases with increasing heating rate (Bilbao et al., 1996). By using rapid heating methods the organic substances can be thus removed before the formation of closed pores. However, due to the formation of burst holes and cracks, a compromise must be made between the heating rate and the residual carbon content. 5. Conclusions Pyrolysis of binder and polyurethane for the processing of NiCr8020 foams was conducted successfully by one conventional and two microwave hybrid heating processes. By utilizing microwave hybrid heating, a reduction of processing time and residual carbon content by about 80–90% and 70–90% as compared to the conventional process, respectively, was realized. The stability of microwave hybrid heating processes for the processing of metal foams can be improved by increasing the heating rate, reducing the sintering temperature and processing time. High heating rates are also beneficial for the removal of organic substances, but due to the formation of burst holes and cracks, a compromise must be made between the heating rate and the residual carbon content. Acknowledgments The authors would like to acknowledge the graduate school 1229 of German Science Foundation for the financial support and Ms. Birgit Brunner from Department of Chemical Engineering for the elementary analysis. References Baeraky, T.A., 2002. Microwave measurements of the dielectric properties of silicon carbide at high temperature. Egypt. J. Sol. 25, 263–273. Bilbao, R., Mastral, J.F., Ceamanos, J., Aldea, M.E., 1996. Kinetics of the thermal decomposition of polyurethane foams in nitrogen and air atmospheres. J. Anal. Appl. Pyrolysis 37, 69–82. Chan, T.V.C.T., Reader, H.C., 2000. Understanding Microwave Heating Cavities. Artech House Inc., Norwood, MA, USA. Choe, H., Dunand, D.C., 2004. Mechanical properties of oxidation-resistant Ni–Cr foams. Mater. Sci. Eng. A 384, 184–193. Clark, D.E., Folz, D.C., West, J.K., 2000. Processing materials with microwave energy. Mater. Sci. Eng. A 287, 153–158.
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