Proceedings of the 41st International Conference on Advanced Ceramics and Composites

By Narottam P. Bansal

This proceedings contains a collection of 24 papers from The American Ceramic Society’s 41st  International Conference on Advanced Ceramics and Composites, held in Daytona Beach, Florida, January 22-27, 2017. This issue includes papers presented in the following symposia:

•             Symposium 3     14th International Symposium on Solid Oxide Fuel Cells (SOFC)

•             Symposium 8     11th International Symposium on Advanced Processing & Manufacturing Technologies for Structural & Multifunctional Materials and Systems

•             Symposium 11   Advanced Materials and Innovative Processing ideas for the Production Root Technology

•             Symposium 12   Materials for Extreme Environments: Ultrahigh Temperature Ceramics (UHTCs) and Nano-laminated Ternary Carbides and Nitrides (MAX Phases)

•             Symposium 13   Advanced Materials for Sustainable Nuclear Fission and Fusion Energy

•             Symposium 14   Crystalline Materials for Electrical, Optical and Medical Applications

•             Symposium 15   Additive Manufacturing and 3D Printing Technologies

•             Focused Session 1  Geopolymers, Chemically Bonded Ceramics, Eco-friendly and Sustainable Materials

• Language: English
• Publisher: Wiley
• Release date: Mar 5, 2018
• ISBN: 9781119474739

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Introduction

This Ceramic Engineering and Science Proceedings (CESP) issue consists of 24 papers that were submitted and approved from select symposia held during the 41st International Conference on Advanced Ceramics and Composites (ICACC), held January 22-27, 2017 in Daytona Beach, Florida. ICACC is the most prominent international meeting in the area of advanced structural, functional, and nanoscopic ceramics, composites, and other emerging ceramic materials and technologies. This prestigious conference has been organized by the Engineering Ceramics Division (ECD) of The American Ceramic Society (ACerS) since 1977.

The 41st ICACC hosted more than 1,000 attendees from 41 countries that gave over 850 presentations. The topics ranged from ceramic nanomaterials to structural reliability of ceramic components, which demonstrated the linkage between materials science developments at the atomic level and macro level structural applications. Papers addressed material, model, and component development and investigated the interrelations between the processing, properties, and microstruc-ture of ceramic materials.

The 2017 conference was organized into the following 15 symposia and 3 Focused Sessions and two Special Sessions:

The proceedings papers from this meeting are published in the below two issues of the 2017 Ceramic Engineering and Science Proceedings (CESP):

CESP Volume 38, Issue 2 (includes 23 papers from Symposia 1, 2, 4, 5, and GYIF)

CESP Volume 38, Issue 3 (includes 24 papers from Symposia 3, 8, 11, 12, 13, 14, 15 and FS1)

The organization of the Daytona Beach meeting and the publication of these proceedings were possible thanks to the professional staff of ACerS and the tireless dedication of many ECD members. We would especially like to express our sincere thanks to the symposia organizers, session chairs, presenters and conference attendees, for their efforts and enthusiastic participation in the vibrant and cutting-edge conference.

ACerS and the ECD invite you to attend the 42nd International Conference on Advanced Ceramics and Composites (http://www.ceramics.org/icacc2018) January 21-26, 2018 in Daytona Beach, Florida.

To purchase additional CESP issues as well as other ceramic publications, visit the ACerS-Wiley Publications home page at www.wiley.com/go/ceramics.

Surojit Gupta, University of North Dakota, USA

Jingyang Wang, Institute of Metal Research, Chinese Academy of Sciences, China

Volume Editors

August 2017 ICACC

Solid Oxide Fuel Cells

MITIGATION OF COMPRESSOR STALL/SURGE IN A HYBRID SOLID OXIDE FUEL CELL-GAS TURBINE SYSTEM

M. Azizia and J. Brouwerb

National Fuel Cell Research Center, University of California, Irvine, CA 92697, USA

ABSTRACT

One of the main purposes of an SOFC-GT hybrid system is for distributed power generation applications. This study investigates the possible use of an SOFC-GT hybrid system to power multi-MW dynamic loads. Based upon the integration of commercially available gas turbine technology, control strategies for the SOFC-GT hybrid system are investigated for different stationary power applications. Risk analysis of compressor stall/surge in the hybrid SOFC-GT power system as it is dynamically dispatched to meet demand is assessed in transient pre-load and post-load modes. Optimal control algorithm is proposed and applied to mitigate stall/surge in compressor as a response to sudden power demand change. This study aims to study compressor stall/surge mitigation assuming a connecting pipe that reduces the back pressure on the compressor in order to maintain the compressor mass flow rate at a specific setpoint.

INTRODUCTION

A better understanding of turbulent unsteady flows in compressor and gas turbine systems is a necessary step toward a breakthrough in compressor applications for hybrid fuel cell-gas turbine (FC-GT) systems transient operation. Hybrid fuel cell-gas turbines are among the many low emission power generation systems. In the previous studies at National Fuel Cell Research Center (NFCRC) at University of California, Irvine, compressor stall/surge analysis for a 4 MW locomotive hybrid solid oxide fuel cell-gas turbine (SOFC-GT) engine has been performed based on the 1.7 MW multi-stage air compressor similar to available commercial compressors¹. Controls methods have been previously developed for these types of systems in order to avoid stall/surge in the compressor². Computational fluid dynamics (CFD) tools can provide a better understanding of flow distribution and instabilities near the stall/surge line. In this study a mechanism is presented in order to mitigate stall/surge in the compressor assuming a connecting pipe between the compressor inlet and outlet that maintains constant air mass flow rate at the design condition of the compressor. This mechanism will avoid secondary stall/surge occurance in the compressor while the hybrid system is exposed to a sudden increase in power demand change from 3 MW to 3.5 MW.

TURBOMACHINERY MODELING

Shear stress transport (SST k-co) fluid model is used for faster convergence in the turbomachinery problem. The pressure dynamics of the compressor outlet has been solved in the MATLAB/Simulink platform that was previously developed at NFCRC. Computational fluid dynamics analysis of the compressor is accomplieshed using ANSYS CFX software³. Power demand variation in the hybrid SOFC-GT system causes pressure change at the compressor outlet. The pressure variation is set as a boundary condition for the turbomachinery analysis. The results show that by using a pipe guiding the exit air flow rate to the compressor inlet, the compressor mass flow rate could be maintained at the design condition of 7 kg/s. 1.7 MW compressor is an appropriate choice among the industrial compressors to be used in a 4 MW hybrid locomotive SOFC-GT system with topping cycle design due to the enhanced ability to maintain air flow rate through the compressor during the sudden transient step-load change.

RESULTS AND DISCUSSION

Figure 1 shows the pressure variation contour on the compressor front and rear impellers post stall/surge. The pressure on the compressor outlet is reduced while the compressor mass flow converges to the design condition at a constant value.

Figure 1: Pressure reduction and stall/surge mitigation of 1.7 MW multi-stage compressor based on controlled stall/surge assuming a connecting pipe between the compressor outlet and inlet that keeps the compressor mass flow rate at a constant rate.

Figure 2. shows the air mass flow rate increase post stall/surge on the rear impeller blades due to the controlled air flow rate to meet the mass flow rate at set point.

Figure 2: Air mass flow rae incease at the front impeller inlet due to the reduced back pressure on the compressor.

Figure 3. shows that it takes 10 rotor revolutions post stall/surge so that the air mass flow rate can be reached to the steady normal operating condition of the hybrid SOFC-GT system. Control algorithms are topics of future research in order to reduce the delay time between the stall/surge and the normal operating condition.

Figure 3: Compressor air mass flow rate increase post stall/surge to the normal design value of 7 kg/s.

CONCLUSION

In this study, analysis of post stall/surge of a 1.7 MW multi-stage compressor is investigated assuming a connecting pipe between the inlet and outlet of the compressor maintaining the compressor air mass flow rate at the 7 kg/s design condition. The reduced back pressure significantly reduces the risk of flow reversal in the 4 MW hybrid SOFC-GT system. As a result, a second stall/surge is less likely to occur after a sudden increase in the power demand is applied to the hybrid system. The response time of the air mass flow rate variation could help in future design of control systems for faster mitigation of compressor stall/surge.

ACKNOWLEDGEMENTS

The authors would like to thank Federal Railroad Administration (FRA) for its support during this research.

REFERENCES

Azizi, Mohammad Ali, and Jacob Brouwer. Transient Analysis of 220 kW Solid Oxide Fuel Cell-Gas Turbine Hybrid System Using Computational Fluid Dynamics Results. ECS Transactions 71.1 (2016): 289-301.

McLarty, Dustin Fogle. Thermodynamic Modeling and Dispatch of Distributed Energy Technologies including Fuel Cell-Gas Turbine Hybrids. 2013.

ANSYS® Academic Research, Release 16.0

CHARACTERISTICS OF PROTECTIVE SPINEL MANGANESE COBALTITE COATINGS PRODUCED BY APS FOR Cr-CONTAINED SOFC INTERCONNECTS

Chun-Liang Chang*, Chang-sing Hwang, Chun-Huang Tsai, Sheng-Fu Yang, Te-Jung Huang, Ming-Hsiu Wu and Cheng-Yun Fu

Physics Division, Institute of Nuclear Energy Research, Taiwan, ROC

ABSTRACT

The chromium-contained ferritic stainless steels are widely employed as metallic interconnects in intermediated temperature solid oxide fuel cells. However, the chromium content of these steels would cause obvious degradation phenomena such as the growth of Cr2O3, chromium poisoning and delamination of oxide scales. Due to the high electrical conductivity of about 60 S/cm at 800oC in air, the manganese cobaltite with spinel structures are employed as protective coatings on Cr-contained steel interconnects. In this study, Ce-doped manganese cobaltite (CMC) protective coatings are produced by the promising atmospheric plasma spraying technique on the pre-oxidized Crofer22H, Crofer22APU and SS441 substrates. The obtained CMC layers reveal relatively dense microstructure due to the employed process parameters. The ASR values of the coated interconnects were measured by a four-probe dc technique at 800°C in air. After about 4,343 hours ageing at 800oC in air, the initial and final ASR values of the coated Crofer22APU sample with pre-oxidation treatment are 1.50 and 1.86 mQcm², respectively. The smallest increasing rate of ASR in this study is only about 0.083

Qcm²/hr. X-ray diffraction analysis was adopted to identify the crystal structures of the obtained spinel coatings. The morphology and cross-sectional observations of the coatings on interconnects were characterized by scanning electron microscopy equipped with energy dispersive spectrometer.

INTRODUCTION

A fuel cell is an electrochemical device which can directly convert energy of applied fuel to electricity. The solid oxide fuel cells (SOFCs) have some unique advantages over other types of fuel cell or traditional power generation technologies, including inherently high efficiency, low gas pollution and high fuel flexibility.1,2 SOFCs with reduced operation temperatures (500-700°C) or intermediate temperature SOFCs (ITSOFCs) provide numerous advantages, such as the application of low-cost component materials, the improvement of sealing capability, the reduction of the interfacial reaction during cell operation. Due to low material cost, good mechanical properties, high electrical conductivity, high thermal conductivity and easy manufacturing process to large area of metallic materials, traditional ceramic interconnectors such as LaCrO3 are replaced by high temperature alloys in SOFC stack. Many high temperature alloys such as Cr-based, Ni-based and Fe-based superalloys have been studied as metallic interconnector. Although Cr-based superalloy has good matching in thermal expansion with other SOFC components, the large amount of chromium trioxide formation and evaporation and further the degradation resulted from Cr-poison still need to be overcome. The Ni-based superalloys generally have a thermal expansion coefficient mismatch and an expansive material cost. The Cr-contained stainless steels with the chromium content about 20 wt.% (Fe-based superalloy) are commonly considered as interconnectors among these promising candidate materials due to their good electrical conducting oxide scales, low material cost, high thermal conductivity and compatible thermal expansion coefficients (TECs) with other cell components, etc.3,4 However, the Cr-poison effect and growing of oxide scale still needing to be reduced for the Cr-contained interconnector. During the operating period of a SOFC stack, the oxide scale, Cr2O3, easily reacts with O2 and/or H2O and transforms to chromium trioxide (CrO3) or chromium hydroxide (CrO2OH) vapors. These gaseous species may transform back to solid chromium oxides at the triple phase boundaries (TPBs) of cathodes and cause dramatic performance degradations of SOFCs.5,6 Furthermore, the oxide scales of Cr-contained steels are getting thicker with exposure time during operation, eventually result in large interfacial resistances. In general, the acceptable interfacial contact resistance of interconnector should be lower than 100 mficm² and the life-time of interconnector should be over 40,000 hours. Therefore, it is necessary to improve the performances of metallic interconnectors by applying effective protective coatings on surfaces of Cr-contained stainless steels to suppress the chromium poisoning and oxide scale growth. A protective coating acts as a mass barrier to chromium cation, oxygen anion and Cr-contained molecule to transport through it.7,8 In addition, a protective coating should reveal a higher electrical conductivity than that of Cr2O3 to minimize interfacial contact resistances at the interfaces between electrodes and interconnectors. An excellent protective coating must be an excellent electron conductor with negligible oxygen ion conductivity and dense microstructure. In recent years, several studies suggest that the spinel coating is a promising protective coating due to the high electrical conductivity, excellent chromium retention and good thermal expansion coefficient (TEC) match with Cr-contained ferritic stainless steel.⁹ According to the results published by of Petric and Ling et al.,0010 the electrical conductivity of Mn-Co spinel is 60 S/cm at 800°C which is almost highest value among the vast variety of binary spinels composed of Al, Mg, Cr, Fe, Co, Ni Cu and Zn. The TEC value of Mn-Co spinel is around 9.7x10−6 °C−1 which is closed to 11x10−6 °C−1 of Cr-contained steel. Yang's study shows that the Ce doped Mn-Co spinel coatings retained the advantages of the undoped Mn-Co spinel coatings, acting effectively as a Cr-outward and O-inward diffusion barrier and improving the electrical performance of the ferritic stainless steel. In addition, the RE addition to the spinel appeared to alter the scale growth behavior beneath the coating, leading to a more adherent scale/metal interface.¹¹ Atmospheric plasma spraying (APS) is a promising technique to produce ceramic coatings with thick films on various substrates materials. Therefore, APS process was employed to produce protective ceramic coatings of Cr-contained steel due to its flexibility, high deposition rate and low cost. In this study, Ce0.05Mn1.475Co1.475O4-8 (CMC) powders were synthesized by sol gel method and employed as feedstock powders for APS process to produce protective coatings on the substrates of Crofer22H, Crofer22APU and SS441 Cr-contained steels with pre-oxidation treatment. The properties of the coated samples were examined and discussed by means of X-ray diffractometer (XRD), scanning electron microscope (SEM) and DC four-point measurement.

EXPERIMENTAL

The CMC powders were synthesized using following starting materials: Ce(NO3)^6H2O, Mn(NO3)2-4H2O, Fe(NO3)3-9H2O and citric acid monohydrate (C6HsO^H2O). Stoichiometric amounts of Ce-nitrate, Mn-nitrate and Co-nitrate were firstly dissolved into deionized water to obtain a cation solution. The citric acid monohydrate was further added into the cation solution to form complex solution and then the solution was heated to 80°C and dwelled for 1 hour to improve the chelating reaction. The molar ratio of citric acid to total metal ions and the concentration of the total metal ions in the deionized water were kept at 2 and 1.0 M, respectively. The solution was then evaporated until a gel was formed. This gel was further heated to 110 and 300°C in sequence until a brown spongy-like solid foam was formed. The solid foam was ground and heated to 600°C for 2 hours to remove the residual organic reagents.

Finally, the powders were heated at 600 and 800°C for 4 hrs to complete the powder synthesis process.

As shown in Figure 1(a), the APS system consisted of a plasma torch (TriplexPro™-200, Sulzer Metco), a powder feeder system, a cooling system, a furnace, an IR (Infrared) detector and a Fanuc Robot ARC Mate 120iC system to scan plasma torch. The plasma torch was operated at medium current around 420 A and high voltage around 118 V. The mixed gas composed of argon, helium and nitrogen was used as plasma forming gas. Argon, helium and nitrogen flow rates were controlled by using mass flow controllers. Details of experimental apparatus and typical plasma spraying parameters were given in another published paper.12,13 A re-granulation process of the CMC powders was conducted to form sphere-like agglomerates via the spray drying equipment. The morphology of re-granulated powders is shown in Figure 1(b). The agglomerated CMC powders sieved with the particle sizes between 20 to 45 |xm were employed as feedstock. After re-granulation, feedstock powders keep its spinel crystalline structure and Ce0.05Mn1.475Co1.475O4 chemical composition respectively.

Figure 1: (a) Schematic diagram of the APS system. (b) Re-granulated CMC powders.

The Cr-contained steels, Crofer 22 H, Crofer 22 APU and SS441, were firstly cut into 1x1 cm² substrates and then their surfaces were blasted by abrasive Al2O3 powders via a sandblasting machine to increase the surface roughness. These substrates were heated to 800°C and held for 12 hrs to complete the pre-oxidization treatment.

The phase purity of coatings was determined by a XRD (Bruker D8) at room temperature. The scanning rate was 1 min/degree and the X-ray raddiation was Cu lk > . Surface morphology and cross-section observation were conducted by SEM (Hitachi S4800 and Jeol JSM-5310) equipped with an energy dispersive X-ray spectroscope (EDS). A DC four-point method was employed to measure the area specific resistance (ASR) values of the coated samples at 800°C.

RESULTS AND DISCUSSION

The ex-situ XRD patterns of the annealed CMC powders and CMC coatings are shown in Figure 2. Form Figure 2(a), it can be seen that cubic and tetragonal spinel crystalline structures are both exist in the annealed CMC powders at room temperature. These cubic and tetragonal spinel phases refer to cubic MnCo2O4 (JCPDF card No.23-1237) and Mn2CoO4 (JCPDF card No.77-0471), respectively. This observation is consistent with earlier work by Brylewski et al. and Naka et al showing the co-existence of the cubic- and the tetragonal-spinel for Mm + sCo2-8O4 with 0.3 < 8 < 0.9.14,15 According to the research results reported by Brylewski et al., the dual phase spinel has structural transformation with temperature. The tetragonal phase disappears at 400oC and only the cubic phase is then observed. As shown in Figure 2(b), it can be seen that the as-sprayed CMC coating still reveals dual phase as those of CMC powders. Even though a heat treatment of 800°C for 4343 hours was applied to the CMC coating, there is still no undesired phase shown in Figure 2(b). No obvious phase change is observed in the CMC coated samples. In addition, no CrO2 phase signals are detected in these samples.

Figure 2: The XRD results of (a) annealed CMC powders with different heating temperature; (b) CMC coated samples after the heat treatment of 800oC for 4,343 hrs.

The microstructure of the CMC coating on a Crofer 22 H substrate is shown in Figure 3. Because the same APS spraying parameters are applied for preparing all the specimens, the same microstructures of these specimens can be assumed and a typical microstructure is shown here only. The observations show that the as-sprayed CMC coating reveals a relatively dense microstructure with few closed pores and without cracks. From cross-sectional observation, it is shown that the thickness of APS-LSM coating applied in this study is around 17 |im. The EDS results listed in Figure 3(b) show that the stoichimetric ratio of the obtained CMC coating is Ce0.049Mm.488Co1.463MnO4-6, which is very close to Ce0.05Mm.475Co1.475O4-s of synthesised CMC powders. These EDS results imply that the APS process does not cause a serious evaporation for a particular element. From Figure 4, it can be found that the CMC coating still remains a dense microstructure without induced cracks and pores after a heat treatment of 800°C for 12 hours.

Figure 3: (a) Surface morphology of as sprayed CMC coatings on Crofer 22H and (b) EDS results obtained from the red square area in (a).

Figure 4: Micrographs in (a) low and (b) high magnifications of the sprayed CMC coating on a Crofer 22 H substrate and after the heat treatment at 800oC for 12 hours.

Figure 5 shows the surface morphologies of CMC coated Crofer22H specimen. The CMC coatings still remain crack-free microstructure after a long-term ASR measurement applied at 800°C for 4,343 hours. Comparing to Figure 3 and 2.4, it is found that the crystalline grain of annealed CMC coatings reveal unique pyramidal shape. According to the EDS results, it is indicated that the pyramidal CMC coatings were composed of Mn, Co and Cr. It implies that the Cr element diffuses from substrate to CMC coating and forms (Mn, Co, Cr)3O4 crystalline structure. Figure 6 shows the corresponding cross-sectional microstructures of CMC coatings on the aforementioned specimens. It is clearly found that the oxide scales was composed of CrO2 layers between CMC spinel coating and metal substrates. Among these specimens the oxide scale of CMC-coated Crofer22APU specimen has the smallest thickness only about 2∼3 |im after a heat treatment for 4,343 hours. Except CMC-coated SS441 specimen, CMC coatings were adhered well on the surfaces of Crofer22H and Crofer22APU substrates as shown in Figure 6(a) and 6(b). Figure 6(c) shows that many cracks paralleled to the interface between CMC coating and SS441 substrate are found in the SS441 specimen. These cracks are not induced by the APS process because the as-sprayed CMC coating on SS441 substrate reveals a crack-free morphology. The occurence of cracks might be due to the lack of rare earth element in SS441 steel so as to result in a poor oxide scale adhesion on SS441 substrate after 4,343 hours ASR measurement.

Figure 5: Surface morphology and its EDS analysis results of CMC/22H specimen after long-term durability test at 800oC for 4,343 hours in air.

Figure 6: Cross-sectional micrographs and its element distribution images obtained from EDS analysis of (a) CMC/22H; (b) CMC/APU and (c) CMC/SS441 specimens after long-term durability test at 800oC for 4,343 hours in air.

From the ASR measurement, Arrhenius plots of the CMC coated specimens are given in Figure 7. The range of measurement temperature is from 600 to 800°C. The slopes of the CMC-coated Crofer22H and CMC-coated Crofer22APU specimens are similar. It implies that these specimens have the same conduction mechanism. It is attributed to the fact that similar oxide scales were formed at these specimens. The slope of CMC-coated SS441 specimen is significantly from the slopes of other specimens, this implies that the oxide scale or interface of this specimen has a different structure and conduction mechanism. The long-term ASR measurement results of the CMC-coated and LSM-coated specimens tested at 800°C in air for 4,343 hours are shown in Figure 8 and Table 2.1. These LSM coatings were prepared by APS process on the same substrates as that of CMC-coated specimens. It is clear to see that almost the CMC-coated specimens have lower initial ASR values than those of LSM-caoted specimens. It is attributed to the higher conductivity of CMC than those of LSM. The ASR increasing rates of CMC-coated specimens are all much lower than those of LSM-coated specimens. Among these specimens, the CMC-coated Crofer22APU specimen reveals the lowest ASR increasing rate of 0.083 £2cm²/hr, its ASR values varies from 1.50 to 1.86 mQcm² after a long-term ASR measurement at 800°C for 4,343 hours in air. The final ASR values of CMC-coated Crofer22H, Crofer 22APU and SS441 specimens are only 3.75, 1.86 and 7.11 mQcm², respectively, which are all much lower than 100 inHcrm². Their ASR increasing rates are all lower than the threshold value of 2.5 ficm²/hr as well.The ASR results indicate that dense and crack-free CMC coatings with acurrate spinel phase and chmical composition can be produced by APS process and reveal outstanding electrical performances.