bluuebunny/crossref_metadata_2025_split huggingface dataset:
Estimation of Lattice Strain in 4H-SiC Commercial Wafer by Synchrotron Monochromatic X-ray Topographic Contour Mapping
Authors: Guo Jianqiu , Yang Yu , Raghothamachar Balaji , Dudley Michael 
Published: February 2020

Unique and excellent properties of 4H-SiC have rendered a series of successful applications in various energy-related fields such as Electric Vehicle (EV) and Smart Grid. While efforts have been made over decades towards the improvement of crystal quality, the densities of crystalline defects existing in the as-grown crystals are still significant. One of the major issues is the residual stress left by the inhomogeneous distribution of crystal defects, which will lead to lattice bending and lattice strain once the bulk as-grown crystals are sliced into wafers. Therefore, the level of lattice strain and bending equivalently characterizes the level of overall homogeneity of the sliced wafer.
X-ray topography, particularly synchrotron monochromatic X-ray topography (SMBXT), enables a highly strain-sensitive technique for imaging the near surface regions of single crystals. When a strained crystal is illuminated by synchrotron monochromatic radiation, the crystal does not diffract entirely. In fact, only a limited region fulfills the Bragg’s Law and diffracts. In order for other regions to diffract, the wafer has to be rocked through the perfect Bragg’s condition with tiny increments. The variation in diffraction condition can thus be mapped as a function of position, which is caused by local lattice bending and strain. By recording the same map with the opposite diffraction vector, i.e. rotating the wafer about the original diffraction vector by 180 degree, the strain component can be separated with the bending component so that two individual maps of lattice strain and lattice bending, respectively, can be obtained.
In this paper, we use this method to estimate the local lattice strain and lattice bending as a function of position in a 75mm 4H-SiC commercial substrate wafer. Results are compared with X-ray topographic images recorded simultaneously from the same wafer.

Figure 1
Studies on Doping Concentration Variations in 4H-SiC Substrates Using X-ray Contour Mapping
Authors: Yang Yu , Guo Jianqiu , Raghothamachar Balaji , Chan Xiaojun , Kim Taejin , Dudley Michael 
Published: February 2020

4H-SiC is one of the promising wide bandgap material that attracts considerable attention in power electronics industry. Several applications have benefited from the improved performances of devices made of 4H-SiC in terms of its higher breakdown fields, efficiency and reliability as well as excellent physical properties. However, the widespread commercialization of this material is still being hindered by the various defects introduced during the crystal growth which is found to degrade the device performance and lifetime. Stacking faults, for example, have been considered to be one of the most detrimental structural defects. It has been predicted theoretically that high nitrogen doping concentration level (above 2 x 1019cm-3) inside 4H-SiC crystal will increase propensity for formation of stacking faults. These can create quantum wells which can lower the free energy of the whole crystal once the barrier for partial dislocation motion is overcome by thermal energy (achieved by annealing above 1000o C) [1-2]. Moreover, from a perspective of high power electronic applications, other severe issues, such as inhomogeneous resistivity and a large {0001} surface roughness of substrate are characteristic of heavily nitrogen doped SiC crystal [3]. It is known that, the nitrogen incorporation kinetic is anisotropic in different crystallographic directions during PVT growth of SiC. Therefore, better understanding the nitrogen doping distribution inside the material is important. In our experiments, we conducted X-ray topographic contour mapping [4] using synchrotron monochromatic X-ray beam by rocking the heavily doped wafer surface successively with a fixed angular step size around the near-surface diffraction vectors 0008 and -0008 respectively. From these measurements, a strain map can be derived by deconvoluting the lattice parameter variations from the lattice tilt. The nitrogen doping concentration can be calculated from the isotropic lattice strain due to the incorporation of dopants according to the equation from H. Jacobson [5].
[1] Y. Yang, J. Guo, Ouloide Goue, B. Raghothamachar, M. Dudley, G. Chung, E. Sanchez, J. Quast, I. Manning, and D. Hansen, J. Crystal Growth, 452, 32 (2016).
[2] T.A. Kuhr, J. Liu, H. J. Chung et al., Journal of Applied Physics,92, 5863 (2002).
[3] N. Ohtani, M. Katsuno, M. Nakabayashi, T. Fujimoto et al., J. Crystal Growth, 311, 1475 (2009).
[4] S. R. Stock, Haydn Chen and H. K. Birnbaum, Philosophical Magzine A, 53(1), 73(1986).
[5] H. Jacobson, J. Birch, C. Hallin, A. Henry, R. Yakimova, T. Tuomi, E. Janzén, and U. Lindefelt, Applied Physics Letters 82 (21), 3689 (2003).

Figure 1
Effects of GDL Compression on SGL Binder
Authors: Lamibrac Adrien , Büchi Felix N , Eller Jens 
Published: February 2020

The gas diffusion layer (GDL) is the key component for efficient mass and heat transport between the flow field structures and the catalyst layer of polymer electrolyte fuel cells (PEFC). The characterization of its transport properties under different compression levels is important to understand the implications of the inhomogeneous GDL compression on the channel-rib scale. While experimental and numerical studies show good agreement for GDLs with solid binder [1], the contributions of the porous binder in SGL type GDLs remain unclear [2, 3].
This work reports on the compression dependent characterization of the GDL morphology, mass and energy transport by means of X-ray tomographic microscopy (XTM) and numerical transport simulations of SGL 24 BA material. X-ray tomographic microscopy images were recorded with a 2.2 um voxel edge length. Though the pores in the binder are not resolved with such voxel dimensions, a specific segmentation procedure of the reconstructed images makes possible to distinguish between fibres, binder and void (see Figure 1, left). The porous binder domains are identified as stiff GDL component, as the volume of the binder domain remains constant for compression levels between 0 – 25 % (see Figure 1, right). In this compression range only the volume of the identified void domain reduces, the fibers deform but the binder domains are not affected. For compression levels above 25 % the binder starts to be compressed and the volume of the binder domains reduces similar as the overall GDL volume. Comparing numerical transport simulations with experimental GDL effective diffusivity and conductivity data, the binder effective transport parameters are determined for different compression levels.
References
[1] J. Becker, R. Flückiger, M. Reum, F. N. Büchi, F. Marone, M. Stampanoni, J. Electrochem. Soc., 156, B1175, 2009.
[2] R. Flückiger, S. A. Freunberger, D. Kramer , A. Wokaun, G. G. Scherer, F. N. Büchi, Electrochim. Acta 54, 551–559, 2008.
[3] A. Rashapov, J. T. Gostick, Transport in Porous Media, 115, 411-433, 2016.
Figure 1: Left) 3D rendering of a ternary segmented SGL 24BA gas diffusion layer at 0 % compression ; green: fibres, red: binder, white: void; Right) relative volume fractions of the components of the ternary segmentation as function of compression.

Figure 1
Introducing Microporous Layer (MPL) Attached Catalyst Coated Membrane for PEFCs
Authors: Tanuma Toshihiro 
Published: February 2020

A brand-new membrane electrode assembly (MEA) fabrication method was conceived. The performance of the MEA by using an MPL attached catalyst coated membrane (CCM) was equivalent to that by using the gas diffusion electrode (GDE) method, indicating that the coated interface between the catalyst layer and the MPL plays a significant role in achieving better MEA performance.
For the last few years, we have been doing research on hydrophilic MPLs. We have already reported (1, 2) that the MEA using a hydrophilic MPL showed much better performance in a wide range of pressure and humidity conditions than that using a conventional hydrophobic MPL. We also found that the MEA using the GDE method on the hydrophilic MPL showed the highest cell voltage compared to the MEAs using the decal method and the membrane coating method. In the case of the MEA using the GDE method on the hydrophilic MPL, the catalyst layer (CL) is integrated into the MPL to form a firm interface between the CL and the hydrophilic MPL. We suggested that this interface allows product water and oxygen gas to transport more smoothly (3).
In order to examine the significance of the interface between the CL and the MPL in this work, we used an MPL attached CCM as illustrated in Fig.1, which was fabricated by hot-pressing a cathode CL coated hydrophilic MPL to the membrane with an anode CL. The hydrophilic MPL consists of carbon fiber and ionomer. The MEA employing the MPL attached CCM showed equivalent performance to that by using the gas diffusion electrode (GDE) method as shown in Fig. 2. This result clearly indicates that the coated interface between the CL and the MPL is more favorable than the hot-pressed interface. The MPL attached CCM is easy to handle because it shows little dimensional change despite employing a non-reinforced membrane. As any GDL substrate is applicable to this MPL attached CCM, this is a unique and promising method for MEA fabrication.
References
1. T. Tanuma and S. Kinoshita, Energy Procedia, 28, 12 (2012).
2. T. Tanuma, M. Kawamoto and S. Kinoshita, Journal of The Electrochemical Society, 164, F499 (2017).
3. T. Tanuma and S. Kinoshita, Journal of The Electrochemical Society, 161, F94 (2014).

Figure 1
Investigating the Effect of Non-Uniform Microporous Layer Intrusion on Oxygen Transport in Dry and Partially Saturated Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers
Authors: Wong Andrew , Bazylak Aimy 
Published: February 2020

State-of-the-art gas diffusion layers (GDL) used in polymer electrolyte membrane (PEM) fuel cells have a bi-layered structure comprising of a carbon fiber substrate and a microporous layer (MPL). Addition of the MPL has been shown to improve the fuel cell performance [1-2]. Earlier studies have also shown that the configuration of the MPL relative to the GDL has an effect on gas transport through the combined diffusion medium [3-4]. This study investigates the effect of the MPL intrusion and morphology on the oxygen transport in PEM fuel cell GDLs. In particular, we investigate the effect of varying MPL layer characteristics, both interior and exterior to the carbon fiber substrate in dry and saturated conditions, on the oxygen diffusivity and liquid water saturation at breakthrough conditions. Stochastic generation methods were utilized to create GDLs with varying MPL intrusion depths and morphologies. The generated morphologies of the non-uniform MPL layers were validated using reconstructed microscale X-ray-computed tomography.
Pore network modelling was applied to simulate the oxygen transport behavior within these stochastically generated materials. The liquid water saturation was calculated using an invasion percolation algorithm. Three distinct scenarios were studied: varied MPL intrusion depth, varied MPL outer layer thickness, and constant MPL thickness with varied intrusion depth. In controlling the MPL intrusion, it was found that ~50-60 µm is the critical MPL thickness beyond which the saturated oxygen effective diffusion coefficient rapidly increases and the substrate saturation rapidly decreases (Figure 1). This study suggests that when the GDL is invaded by liquid water through cracks in the MPL, preferential MPL configurations exist for improving oxygen transport and minimizing substrate saturation. The results of this study can be used to guide the continued improvement of MPL fabrication.

References

[1] J. Lee, R. Yip, P. Antonacci, N. Ge, T. Kotaka, Y. Tabuchi, and A. Bazylak, “Synchrotron Investigation of Microporous Layer Thickness on Liquid Water Distribution in a PEM Fuel Cell,” J. Electrochem. Soc., vol. 162, no. 7, pp. F669–F676, Apr. 2015.
[2] Z. Qi and A. Kaufman, “Improvement of water management by a microporous sublayer for PEM fuel cells,” J. Power Sources, vol. 109, no. 1, pp. 38–46, 2002.
[3] C. Chan, N. Zamel, X. Li, and J. Shen, “Experimental measurement of effective diffusion coefficient of gas diffusion layer/microporous layer in PEM fuel cells,” Electrochim. Acta, vol. 65, pp. 13–21, 2012.
[4] N. Zamel, J. Becker, and A. Wiegmann, “Estimating the thermal conductivity and diffusion coefficient of the microporous layer of polymer electrolyte membrane fuel cells,” J. Power Sources, vol. 207, pp. 70–80, 2012.

Figure 1
Nano-Scale Simulation of Platinum Particle Dissolution, Re-Deposition, and Transport
Authors: Ogawa Shohei , Yarlagadda Venkata , Kongkanand Anusorn , Litster Shawn 
Published: February 2020

Dissolution and redeposition of platinum (Pt) in the catalyst layer of polymer electrolyte fuel cells (PEFCs) results in a loss of electrochemically active surface area and increases in the overpotential for the oxygen reduction reaction, reducing efficiency.1 Typically, carbon supported Pt (Pt/C) catalyst exhibits Pt particles both on the external surface of the carbon support as well as within the internal micro-pores and smaller mesopores of the support that are known to be inaccessible to ionomer. Experimental evidence suggests that there may be preferential exchange of Pt between external and internal Pt.2,3Here, we apply a computational model of the ionomer coated catalyst surface in combination with a Pt particle embedded within a water-filled micro-pore. To evaluate the transfer of Pt between internal and external Pt, we have developed a three-dimensional computational model to simulate proton and Pt ion transport using a Poisson-Nernst-Planck model in combination with Pt dissolution and redeposition as well as Pt oxide coverage models (Figure 1a). Our results suggest that the external Pt particle in contact with the incomer increases in a diameter, whereas the Pt particle in the pore reduces its diameter. The Figure 1 c) shows the change of the diameter of the external and internal particles with the electrode potential cycling.
This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

Figure 1
Understanding Binary Interactions and Aging Effects in Catalyst Layer Inks for Controlled Manufacturing
Authors: Dixit Marm , Hatzell Kelsey B 
Published: February 2020

Controlled manufacturing and processing of multi-material electrodes and catalyst layers for energy storage and conversion applications is important for achieving low-cost and durable systems. Typically electrodes are coated using solution-based or wet- methods to achieve high aspect ratio thin films. The processing conditions as well as the ink formulation affect the ultimate material arrangment and electrode properties. Currently, ink formulation primarily focuses on macro-scale process-specific optimization (i.e. viscosity and surface/interfacial tension) rather than understanding local and nano-scale interactions between in constituents in an ink. Herein, we investigate local interactions between carbon and polymer components in a carbon based ink. We specifically investigate the role of active material surface chemistry plays on aggregation, stability, and sedimentation dynamics in multi-component inks1,2. The material arrangement and organization in a cast thin film is discerned from both shear and elongational flow processing methods. Finally, we introduce novel direct and indirect (rheological ) techniques to probe shear induced transformationd in inks or viscoelastic materials3,4.
[1] Holdcroft, S. (2013). Chemistry of Materials, 26(1), 381-393.
[2] Takahashi, S., Shimanuki, J., Mashio, T., Ohma, A., Tohma, H., Ishihara, A.,& Miyazawa, A. (2017). Electrochimica Acta, 224, 178-185.
[3] Hatzell, K. B., Eller, J., Morelly, S., Tang, M., Alvarez, N. J., & Gogotsi, Y. (2017). Faraday Discussions

[4] Hatzell, K. B., Boota, M., & Gogotsi, Y. (2015). Chemical Society Reviews, 44(23), 8664-8687.
Understanding Binary Interactions in Fuel-Cell Catalyst-Layer Inks
Authors: Berlinger Sarah A , McCloskey Bryan D , Weber Adam Z. 
Published: February 2020

The catalyst layer (CL) in a fuel cell comprises the majority of the stack cost. Therefore, for mass commercialization of fuel cells, it is critical to understand the formation process of the CL, thereby informing the design and optimization of next-generation CLs.
CLs are made from precursor inks that are colloidal dispersions of a catalyst (typically platinum on carbon), an ion-conducting polymer (traditionally perfluorinated sulfonic acid or PFSA, the most widely used of which is Nafion), and a solvent traditionally composed of different low-weight alcohols and water. Ink properties directly impact the resultant morphology, transport, and electrochemical properties of the CL.[1] While there has been much work on ink composition, the research to date has been almost entirely empirical, resulting in limited understanding of fundamental species interactions in the ink and their effect on the resultant CL structure. Furthermore, though the structure of nafion in solution has been extensively studied,[2-4] there has been little work on applying this information to decribe interactions with other ink components.
In this study, we create a series of inks composed of commerical PFSA dispersions of different equivalent weights and side chains, a variety of solvents, and model compound particles. Effects of solvent composition, ionomer loading, and processing conditions are investigated on both the solution phase and the resulting cast solid layers including ionomer thin films and micrometer layers with and without solid particles. Using microscopy, rheology, and dynamic light scattering, we explore the aggregation behavior and fundamental forces governing the behavior of these ionomer/particle, particle/solvent, and solvent/ionomer interactions. Using these insights, design metrics and governing design rules are elucidated, which can help in fabrication of next-generation CLs.

Acknowledgements

This work was mainly funded under the Fuel Cell Performance and Durability Consortium (FC PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231. S.B also acknowledges support from National Science Foundation Graduate Research Fellowship under Grant No. DGE 1106400.

References

1. Holdcroft, S., Fuel Cell Catalyst Layers: A Polymer Science Perspective. Chemistry of Materials, 2014. 26(1): p. 381-393.
2. Lee, S.-J., et al., Solution properties of nafion in methanol/water mixture solvent. Polymer, 2004. 45(8): p. 2853-2862.
3. Welch, C., et al., Nafion in Dilute Solvent Systems: Dispersion or Solution? ACS Macro Letters, 2012. 1(12): p. 1403-1407.
4. Kim, Y.S., et al., Origin of Toughness in Dispersion-Cast Nafion Membranes. Macromolecules, 2015. 48(7): p. 2161-2172.
Three-Dimensional Water Distribution and Interfacial Effects through Simultaneous Neutron and X-Ray Tomography
Authors: LaManna Jacob M , Yue Yang , Trabold Thomas A , Fairweather Joseph D. , Hussey Daniel S , Baltic Elias , Jacobson David L 
Published: February 2020

It is often difficult to fully resolve non-destructively both liquid water and solid structure within an operating fuel cell. Both neutrons and X-rays offer non-destructive probes to understand internal structure of objects of interest. By combining these two probes it is possible to obtain three-dimensional information on liquid water distribution and its interaction with the material structure and interfaces. The NIST neutron imaging facility offers the ability to perform simultaneous neutron and X-ray tomography by orienting a micro-focus X-ray tube orthogonal to the neutron beam. A small, 0.36 cm2, fuel cell was developed specifically to facilitate simultaneous neutron and X-ray imaging and was operated at high flow rates to avoid water slug formation in the gas channels. The presentation will describe initial results and ongoing efforts in simultaneous tomography of fuel cells to further understanding of interfacial effects and overall water distribution. Shown in the figure are three-dimensional volume renderings of the composite neutron and X-ray tomography reconstructions giving water distribution with respect to fuel cell hardware and material interfaces. This work was partially supported by GM through an Academic Partnership Single Project Agreement with Rochester Institute of Technology.

Figure 1
Low Pt Content Proton Exchange Membrane Fuel Cell Stack
Authors: Matolin Vladimir , Fiala Roman , Dubau Martin , Ostroverkh Anna , Lavkova Jaroslava , Kus Peter , Vaclavu Michal , Matolinova Iva 
Published: February 2020

Pt doped cerium oxide thin films with a nano-porous structure was prepared by reactive rf magnetron deposition on a fuel cell gas diffusion layer [1] coated with nitrogenated amorphous carbon films (CNx) as an interlayer. We show that the structure and morphology of the cerium oxide-CNx composites depend on the oxygen and nitrogen concentration in working gas used for catalyst deposition. Considerable plasma erosion of the carbonaceous interlayer accompanied by the formation of highly porous carbon/cerium oxide bilayer systems is reported. Oxygen plasma etching of the carbon interlayer occurring simultaneously with cerium oxide film growth is considered to be the driving force for the formation of cerium oxide film nanoporosity with large active surface.
Thin film techniques permits to grow the catalyst film characterized by highly dispersed platinum [2] showing high hydrogen oxidation activity and stability. These new materials may help to substantially reduce the demand for expensive noble-metals in proton exchange membrane fuel cell (PEMFC) catalytic applications.
We prepared 50 cm2 MEAs with 6 μg Pt/cm2 anode and 300 μg Pt/cm2 cathode catalysts. The MEAs activity were measured in 10 cell hydrogen/air water cooled PEMFC stack, developed in our laboratory. Efficiency measurements performed at different pressures, temperatures, and hydrogen flow conditions were compared with performance of standard reference MEAs. We showed that MEAs made by assembling thin film Pt-CeOx/CNx anodes, containing 6 μg Pt per 1 cm2 only, and reference cathodes can be used for fabrication of state of the art low Pt content PEMFC stacks delivering high specific power. It is shown that the almost Pt free anode technology represents promising solution for future fabrication of PEMFC stacks using Pt free cathodes under development.
Reference
[1] Fiala, R; Figueroba, A; Bruix, A; Vaclavu, M; Rednyk, A; Khalakhan, I; Vorokhta, M; Lavkova, J; Illas, F; Potin, V; Matolinova, I; Neyman, KM; Matolin, V, High efficiency of Pt2+- CeO2 novel thin film catalyst as anode for proton exchange membrane fuel cells, Appl. Catal. B-Environ., 197 : 262–270, 2016.
[2] Dvorak, F; Camellone, MF; Tovt, A; Tran, N-D; Negreiros, FR; Vorokhta, M; Skala, T; Matolinova, I; Myslivecek, J; Matolin, V; Fabris, S, Creating single-atom Pt-ceria catalysts by surface step decoration, Nat. Commun., 7 (Feb): Art. No. 10801, 2016.