Available online at ScienceDirect. Energy Procedia 69 (2015 )

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1 Available online at ScienceDirect Energy Procedia 69 (2015 ) International Conference on Concentrating Solar Power and Chemical Energy Systems, SolarPACES 2014 Enhancement of oxidation resistance of graphite foams by polymer derived-silicon carbide coating for concentrated solar power applications T. Kim a, D. Singh a, and M. Singh b a Energy Systems Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA b Ohio Aerospace Institute, Cedar Point Rd, Cleveland, OH 44142, USA Abstract Graphite foam with extremely high thermal conductivity has been investigated to enhance heat transfer of latent heat thermal energy storage (LHTES) systems. However, the use of graphite foam for elevated temperature applications (>600 due to poor oxidation resistance of graphite. In the present study, oxidation resistance of graphite foam coated with silicon carbide (SiC) was investigated. A pre-ceramic polymer derived coating (PDC) method was used to form a SiC coating on the graphite foams. Post coating deposition, the samples were analyzed by scanning electron microscopy and energy dispersive spectroscopy. The oxidation resistance of PDC-SiC coating was quantified by measuring the weight of the samples at several measuring points. The experiments were conducted under static argon atmosphere in a furnace. After the experiments, oxidation rates (%/hour) were calculated to predict the lifetime of the graphite foams. The experimental results showed that the PDC-SiC coating could prevent the oxidation of graphite foam under static argon atmosphere up to 900 C Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2015 The Authors. Published by Elsevier Ltd. ( Peer Peer review review by by the the scientific scientific conference conference committee committee of SolarPACES of SolarPACES 2014 under 2014 responsibility under responsibility of PSE AG of PSE AG. Keywords: graphite foam, latent heat, thermal energy storage, polymer derived coating, silicon carbide, oxidation 1. Introduction Thermal energy storage (TES) systems enable concentrated solar power (CSP) plants to operate when sun is not available. TES systems using sensible heat are incorporated in current CSP plants. For advanced CSP system whose operation temperature is higher th a phase change material (PCM), which is called a latent heat thermal energy storage (LHTES) system, has been considered to be utilized because it Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG doi: /j.egypro

2 T. Kim et al. / Energy Procedia 69 ( 2015 ) has high specific energy capacity and narrow operating temperature range. However, charging and discharging of LHTES system is limited by PCM s low thermal conductivity. Numerous approaches to improve thermal performance of PCMs have been investigated [1-8]. Recently, graphite foam with extremely high thermal conductivity [9, 10] has been considered to improve thermal performance of LHTES system. However, the use of foam [11]. Oxidation protection for graphite materials has been extensively studied and ceramic coatings are commonly employed to improve the oxidation resistance of graphite materials [12 17]. Fuji et al. [12] found that the compositionally gradient SiC/C layers protected the graphite from oxidation at high temperatures in air. Chunhe and Jie [13] investigated oxidation resistance of SiC coating on graphite. They found that weight of graphite decreased by 68%, whereas SiC coated graphite showed only 1.7% reduction in weight after 8 hours exposure in air at 1000 C. Fergus and Worrell [14] studied oxidation protection of graphite using siliconcarbide/boron-containing coatings. Their results showed that the coating consisting of a BNC-B 4 C and SiC can protect the graphite from oxidation under 1 atm oxygen at 1500 C for 9 days. Zhu et al. [15] studied oxidation resistance of a dense functional gradient SiC coating on graphite. They showed that the gradient SiC coatings improved oxidation resistance and thermal shock resistance. They attributed high thermal shock resistance to the gradient SiC/C layer which could mitigate the thermal stress between SiC coating and graphite. Zhao et al. [16] used SiC/Si MoSi2 coating on graphite matrix. Their results showed that the porosity and pore radius of graphite materials had marked effect on the oxidation behavior. Yang et al. [17] investigated the oxidation resistance enhancement of carbon/carbon(c/c) composites and graphite with SiC coating. Their oxidation test showed that SiC coated graphite had a better oxidation resistance than SiC coated C/C composites. Oxidation behavior of SiC coated C/C composites were linear, while those of SiC coated graphite followed a quasi-parabolic manner. Based on the results, it was inferred that oxidation mechanism of C/C composite was controlled by chemical reaction while for graphite was controlled by oxygen diffusion. As described above, SiC is considered to be among the best coating material due to its good mechanical properties, low density, good physical chemical compatibility with carbon and excellent oxidation resistance below [19-21]. Klett et al. [19] studied oxidation behavior of graphite foam with SiC coating. They used chemical vapor infiltration (CVI) process to apply SiC coatings on the graphite. Their result showed that coating the foam with SiC enhanced the oxidation resistance of the graphite foam. Duston et al. [20] used preceramic polymers based systems to coat SiC on the graphite foams. They found that the compressive strength of the graphite foams increased by 2.5 times through the SiC coating on the graphite foams with negligible influence on thermal properties. Kim et al. [21] investigated the oxidation resistance of uncoated graphite foams and chemical vapor reaction (CVR) SiC coated graphite foams. They found that CVR-SiC coating could prevent the oxidation of graphite foam under static argon atmosphere. In the present study, pre-ceramic polymer derived coating (PDC) method was used to form a SiC coating on the graphite foams. After the coating process, samples were analyzed by scanning electron microscope (SEM), energy dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD) to verify formation of SiC coating on the graphite foam. The oxidation resistance of PDC-SiC coating was quantified by measuring the weight of the samples at various time intervals. After the experiments, oxidation rates (%/hour) were calculated and used to predict the lifetime of the graphite foams. 2. Experiments 2.1. Deposition of SiC coatings on graphite foam & characterization Dip coating solutions were prepared by dissolving different amounts of poly-carbosilane polymer in highpressure liquid chromatography (HPLC) grade Toluene. Two types of graphite foam samples 0.23 g/cm 3 (medium density) and 0.44 g/cm 3 (high density) were baked in oven in air at 100 C to remove any moisture. After the drying treatment, dimensions and weights of each sample were measured. Medium and high-density samples (5 each) were dip coated and cured in oven at 100 C for 1 hour and then 150 C for 1 hour. After curing, all the samples were

3 902 T. Kim et al. / Energy Procedia 69 ( 2015 ) pyrolyzed up to 1000 C in argon (60 C/hr to 1000 C and 30 minute hold at 1000 C). After pyrolysis, weights were again recorded. Single and three dip coatings were prepared with medium and high-density foams. In 3-dips process, samples were dipped in the precursor solution and dried three times before pyrolysis, whereas, in 1-dip this was done only once. The typical weight gains in medium and high-density samples were around 10-12% and 7-8%, respectively. For SEM, EDS, and XRD analysis, medium density foam with 3-dips coating was used. It is important to point out that traditional mounting and polishing of these specimens is extremely complicated due to soft graphite ligaments. In addition, there are issues with contamination with polishing materials as well as mounting epoxy. In addition, breaking of specimens can always cause secondary damage to coatings. Fig. 1 shows the SEM micrograph and EDS analysis of medium density 3-dips coated samples. The EDS analysis shows the existence of silicon elements on the surface of the graphite foam. Fig. 2 shows the XRD analysis of powder of medium density 3-dips coated samples. The XRD analysis confirms the formation of silicon carbide phase on the surface of the graphite foam as evidenced by the presence of SiC diffraction peaks. (a) (b) Fig. 1. (a) SEM Micrographs and (b) EDS analysis of medium density foam with 3-dips coating at the location shown in 1(a). Fig. 2. XRD analysis of powder of medium density foam with 3-dips coating.

4 T. Kim et al. / Energy Procedia 69 ( 2015 ) Oxidation studies Oxidation resistance of PDC-SiC coating on the graphite foams was quantified by measuring weight of the coated samples with increasing exposure times at 900 C using a Mettler Toledo s scale with resolution of ±0.005 mg. Each specimen was weighed 3 times at the various test interruption points. The standard deviation of 3 measurements for each sample was too small to be seen in the plots. The oxidation tests were conducted under static argon atmosphere in a vacuum furnace. The test furnace was evacuated to 20 mtorr using a rough pump and backfilled with argon gas prior to heating. After the experiments, weight loss rates (%/hour) were calculated to predict the lifetime of the graphite foams. All tested samples are listed in Table 1. Table 1. Test matrix. Sample Atmosphere Temperature Exposure Time 0.23 g/cm 3 Uncoated Graphite Foam 0.23 g/cm 3 PDC-SiC Coated Graphite Foam 3 Dips 0.44 g/cm 3 PDC-SiC Coated Graphite Foam 1 Dip 0.44 g/cm 3 PDC-SiC Coated Graphite Foam 3 Dips Static Argon Up to 500 hours 3. Results and discussion Uncoated graphite foams and PDC-SiC coated graphite foams were exposed to static argon atmosphere at 900 C for up to 500 hours. It should be noted that the LHTES system being designed is not expected to operate over 800 C. Based on the measurements of the weight changes of the samples, the oxidation rate, R(t), between each m( t) m( t t) 1 R(t) *100 * (1) m(0) t where, t is the exposure time (in hours), m(t) is the sample weight at time t (in grams), and m(0) is the initial weight of the sample. Fig. 3 shows the weight % measurements for the uncoated graphite foams and PDC-SiC graphite foams as a function of exposure time. The standard deviation of 3 measurements for each sample was too small to be seen in the plots. Kim et al. [21] reported that 0.23 g/cm 3 CVR-SiC coated graphite foams did not show any appreciable oxidation under static argon atmosphere with both high vacuum condition and low vacuum condition. Similarly, there were almost no weight losses under static argon atmosphere for 500 hours of exposure time for all PDC-SiC coated samples, while uncoated samples were susceptible to oxidation under same condition. In the case of 0.23 g/cm 3 3-dips PDC-SiC coated graphite foams, the weight of samples slightly increased with increasing exposure time. The initial weight change is relatively rapid and then rate of weight change decreased. The average oxidation rates of 0.23 g/cm 3 3-dips coated graphite foams at 144 hours and 500 hours of exposure time were -2.78*10-5 %/h and -0.71*10-5 %/h, respectively. It increased with increasing exposure time and approached to zero (Fig. 4. (a)). Based on the oxidation rate, 0.23 g/cm 3 3-dips coated graphite foams can be durable under static argon atmosphere.

5 904 T. Kim et al. / Energy Procedia 69 ( 2015 ) (a) (b) Weight (%) g/cm 3 Uncoated - # g/cm 3 Uncoated - # Weight (%) g/cm 3 PDC SIC Coated - 1 Dip g/cm 3 PDC SIC Coated - 3 Dips 0.23 g/cm 3 PDC SIC Coated - 3 Dips 0.23 g/cm 3 PDC SIC Coated - 3 Dips Fig. 3. Weight (%) vs. time for (a) uncoated foams, and (b) PDC-SiC coated foams. (a) (b) g/cc PDC SiC Coated - 3 Dips 0.23 g/cc PDC SiC Coated - 3 Dips g/cc PDC SIC Coated - 1 Dip 0.44 g/cc PDC SiC Coated - 3 Dips Oxidation Rate (%/h)* Oxidation Rate (%/h)* Fig. 4. Oxidation rate (%/h) of PDC- 3, (b) 0.44 g/cm 3. Similar to the 0.23 g/cm 3 PDC-SiC coated graphite foams, 0.44 g/cm 3 PDC-SiC coated graphite foams did not show any considerable weight gain or weight loss for 500-hours of exposure time. It was interesting to see that the 1 dip coated sample showed a weight loss, whereas, the 3dip coated sample showed a weight increase (Fig. 3. (b)). This was probably indicative of sufficient foam coverage for the 3 dip approach and the weight increase might be from some un-reacted coating precursor. The oxidation rate of 0.44 g/cm 3 coated graphite foams leveled off with increasing exposure time and approached to zero (Fig. 4. (b)).

6 T. Kim et al. / Energy Procedia 69 ( 2015 ) The oxidation rates of 0.44 g/cm 3 3-dips coated graphite foam at 106 hours and 500 hours of exposure time were -2.91*10-5 %/h and -0.55*10-5 %/h, respectively. Based on the oxidation rate, 0.44 g/cm 3 3-dips coated graphite foams can be utilized under static argon atmosphere for more than 20 years. The criterion used to determine useful lifetime of the foam was 10% allowable weight reduction. The oxidation rates of 0.44 g/cm 3 1-dip coated graphite foam at 106 hours and 500 hours of exposure time were 0.88*10-5 %/h and 0.33*10-5 %/h, respectively. Based on the oxidation rate at 500 hours of exposure time, the lifetime (time to reach 90% of the initial weight) of 0.44 g/cm 3 1-dip coated graphite foam would be more than 20 years. It should be noted that these lifetime estimates are based on the assumption that the oxidation rates will be same long-term as those observed in 500 h tests. To further ascertain this, much longer-term experiments will have to be conducted. However, from the oxidation studies with PDC-SiC coated graphite foams presented here, we could conclude that the PDC-SiC coating could prevent the oxidation of graphite foam under static argon atmosphere up to 900 C. Finally, solution based pre-ceramic polymer derived coating processes are not expected to have major size limitations and can prove to be quite cost effective and easy to scale-up without major capital investments in real application for CSP plants. However, some of the coating process parameters and size effect in this system have to be optimized and evaluated. 4. Conclusions A method to enhance oxidation resistance of graphite foams was investigated in this study. PDC method was used to coat graphite foams with two densities with SiC coating. Post coating, samples were analyzed by SEM, EDS, XRD to verify formation of SiC coating on the graphite foam. The oxidation resistance of PDC-SiC coating on the graphite foams was investigated by measuring weight of the samples with increasing exposure time at elevated temperatures under static argon atmosphere. There were almost no weight changes for the PDC-SiC coated samples, while uncoated samples were susceptible to oxidation under identical test conditions. The oxidation rate of PDC-SiC coated graphite foams leveled off with increasing exposure time and approached to zero. These oxidation experimental results indicated that the PDC-SiC coating could prevent the oxidation of graphite foam under static argon atmosphere up to 900 C. Acknowledgements This work was supported by the US Department of Energy s EERE Solar Energy Technology Program (Sunshot Initiative) at Argonne National Laboratory, a US Department of Energy s Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC. Authors would like to acknowledge fruitful discussions with Dr. Levi Irwin of DOE s Sunshot Initiative. References [1] Sparrow EM, Larson ED, Ramsey JW. Freezing on a finned tube for either conduction-controlled or natural-convection-controlled heat transfer. Int J [2] Smith RN, Koch JD. Numerical solution for freezing adjacent to a finned surface. In Proceeding of the Seventh International Heat Transfer Confer [3] Lacroix M. Study of the heat transfer behavior of a latent heat thermal energy storage unit with a finned tube. Int J Heat Mass Trans 1993; [4] Velraj R, Seeniraj RV, Hafner B, Faber C, Schwarzer K. Experimental analysis and numerical modelling of inward solidification on a finned vertical tube for a latent heat storage unit. Sol Energy [5] Stritih U. An experimental study of enhanced heat transfer in rectangular PCM thermal s [6] Agyenim F, Eames P, Smyth M. A comparison of heat transfer enhancement in a medium temperature thermal energy storage heat exchanger using fins. Sol Energy [7] Siegel R. Solidification of low conductivity material containing dispersed high conductivity particles. Int J Heat Mass Trans [8] Seeniraj RV, Velraj R, Narasimhan NL. Heat Transfer Enhancement Study of a LHTS Unit Containing Dispersed High Conductivity Particles. J Sol Energy Eng

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