Simulation and Prediction of 3-D Microstructural Evolution of Ni Cermet Anode

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Phebe James
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1 Simulation and Prediction of 3-D Microstructural Evolution of Ni Cermet Anode Xin Wang and Alan Atkinson Department of Materials, Imperial College London

$3D Microstructure of Ni Cermet Anode Volume fractions, particle size and$

2 3D Microstructure of Ni Cermet Anode Volume fractions, particle size and porosity Triple phase boundary (TPB) and tortuosity Obtained by FIB slice and view YSZ Ni

3 Degradation of Microstructure YSZ Ni As-reduced specimen Specimen Niparticle size (μm) Total TPB (μm/ μm 3 ) as-reduced aged Ni YSZ Aged 3000 h at 800 C in 97% H 2 / 3% H 2 O Particle coarsening Reduction of TPB

4 Simulation of Microstructure Evolution To better understand microstructure degradation and develop anode materials with long-lasting electrochemical performance, it is important to have available a reliable way of modelling real microstructure evolution. We have developed an algorithm of calculating the structural imbalance of materials surface and use cellular automata approach to model the microstructure of this materials

5 What Is Structural Imbalance? A B C D E A>B>C>D>E The structural imbalance describes the degree of deviation of a surface element from being a bulk element. The environment of surface atoms differs from that of bulk atoms in that the surface atoms have unbalanced bonding. The imbalanced atomic interactions gives rise to the higher energy.

6 Structure Imbalance of a Surface Voxel Mesoscopic voxel interaction model

7 Structural imbalance=surface/interface energy 1 Gibbs-Thompson equation Free energy is proportional to local curvature A rough surface with two sharp peaks Curvature dependence of the calculated interface energy for different circle sizes A circle with a radius of 40 cells

8 Cellular Automata In CA, space is discretized and made up of cells. The system evolves in discrete time steps, like simple automata, according to a set of rules: At each time step, each voxel is allowed a chance of changing its state (evaporate or deposit). The probability of the change is proportional to the local energy (structural imbalance). C A B Evaporation: A>B>C>D>E D E Deposition: E>D>C>B>A

9 Wetting and Coarsening Young s equation: γ βg = γ αβ + γ αg cosθ

10 Grain Growth

11 2D particle coating Poor wettability Good wettability

12 Wettability contact angle of Ni on YSZ Tsoga et al, Acta mater , 1996, measured the minimum contact angle to be 117º Duh et all, J.Mater. Sci Lett., 8, , 1989 measured =112º.

13 Equivalent 2D Contact Angle N θ N θz θ P Z P Equivalent 2-D contact angle where X= Z, N or P

14 Correlation between different contact angles N ϒ NP ϒ NZ θ N ϒ NP θ θz θ P ϒ NZ ϒ ZP Z P ϒ ZP Equivalent 2-D contact angle Standard contact angle The standard contact angle is related to the equivalent 2-D contact angle by:

15 Contact angle changes after aging As-reduced specimen Contact angles θ Z θ N θ P θ As reduced Aged 147 ± ± ± ±24 69 ±22 81 ± Aged 3000 h at 800 C in 97% H 2 / 3% H 2 O 2D contact angles can also be computed based on interfacial normal vectors Sun et al., Materials Characterization 96 (2014)

16 Contact angle determined by CA As reduced, Cmis=0.89 eq. contact angle=130 Aged, Cmis=0.8 eq. contact angle=120

17 Simulation on real 3D microstructure V=6x6x12µm=432µm 3 Immobile YSZ framework

18 Mutating Ni Structure

19 TPB evolution

20 Coarsening & TPB reduction Deq (μm) TPB (μm/ μm 3 ) Experimental Aged at 800C for 3000h Simulation 1000 ts

21 Summary We have demonstrated that the 3-D real microstructure of Ni cermet can be simulated by cellular automata approach, using the newly developed algorithm for surface free energy. Wettability is a key parameter in controlling the microstructure changes. Two new methods for determining the contact angle from reconstructed 3-D microstructure are proposed. The contact angle of Ni on YSZ after aging at 800 C in low water vapour decreased from 129 (as-reduced) to 122. The simulation shows that generally Ni particle size increases and TPB decreases with time. The prediction of microstructure evolution is feasible using the newly developed simulation approach.

22 Thank you very much for your attention!

Fuel Cell Preparation. The anode supported SOFC studied was Ni-YSZ/YSZ/LSM-

Supplemental Information Fuel Cell Preparation. The anode supported SOFC studied was Ni-YSZ/YSZ/LSM- YSZ,LSM (YSZ = 8 mol% Y 2 O 3 -stabilized ZrO 2 and LSM = La 0.8 Sr 0.2 MnO 3 ). The anode substrates,