Two and three dimensional electron backscattered diffraction analysis of solid oxide cells materials

There are two main technique were developed in this work: a technique to calculate grain boundary energy and pressure and a technique to measure lattice constant from EBSD. The techniques were applied to Nb-doped Strontium titanate (STN) and yttria stabilized zirconia (YSZ) which are commonly used in solid oxide fuel cell and electrolysis cell. Conductivity of STN is one of the important properties that researchers desire to improve. Grin boundary conductivity contributes to the overall conductivity of the STN. Grain boundary density controlled by mainly grain growth in material processing. Grain boundary migration in grain growth involves grain boundary mobility and net pressure on it. Thus grain boundary energy and pressure of STN were calculated in this work.
Secondary phase is undesired in STN and YSZ synthesis. The secondary phase in ceramics with the same compounds can have different lattice structure. In this case, lattice parameters analysis aid to differential the secondary phases. However lattice constant of secondary phase cannot measure by general tools such as x-ray diffraction due to its insufficiency. Point analysis in electron backscattered diffraction (EBSDX allows measuring the lattice constant. Both 2D and 3D EBSD were used in acquiring microstructure and crystallographic information of STN and YSZ.
Prior to EBSD data collection, effect of FIB milling on STN and YSZ was investigated to optimize EBSD data quality and acquisition time for 3D-EBSD experiments by FIB serial sectioning. Band contrast and band slope were used to describe the pattern quality. The FIB probe currents investigated ranged from 100 to 5000 pA and the accelerating voltage was either 30 or 5 kV. The results show that 30 kV FIB milling induced a significant reduction of the pattern quality of STN samples compared to a mechanically polished surface but yielded a high pattern quality on YSZ. The difference between STN and YSZ pattern quality is thought to be caused by difference in the degree of ion damage as their backscatter coefficients and ion penetration depths are virtually identical. Reducing the FIB probe current from 5000 to100pA improved the pattern quality by 20% for STN but only showed a marginal improvement for YSZ.
On STN, a conductive coating can help to improve the pattern quality and 5 kV polishing can lead to a 100% improvement of the pattern quality relatively to 30 kV FIB milling.
According to the study results a new technique to combine a high kV FIB milling and low kV polishing was developed for 3D-EBSD experiments of STN. A low kV ion beam was successfully implemented to automatically polish surfaces in 3D-EBSD of La and Nb-doped strontium titanate of volume 12.6x12.6x3.0 μm. The key to achieving this technique is the combination of a defocused low kV high current ion beam and line scan milling. The polishing performance in this investigation is discussed, and two potential methods for further improvement are presented.
La and Nb-doped strontium titanate (STLN) with different La contents (La = 0.000, 0.005, 0.01 and 0.02 mol%) are used in grain boundary energy and pressure calculation. 3D-EBSD of the four STLNs were collected. According to largeness of grain size in STLNs (La = 0.000 and 0.005 mol%), 3D-EBSD data of the sample contain in sufficient grains for calculation of gain boundary energy and pressure, thus only 3D-EBSD data of STLNs (La = 0.001 and 0.02 mol%) were used in the calculation. Relative grain boundary energy of STLN (La = 0.02 mol%) was successfully calculated. However in STLN (La = 0.01 mol%) the calculation was not success due to insufficiency of grain boundaries for the calculation.
In lattice constant measurement, lattice constants of cubic STN and cubic YSZ in STN-YSZ binary mixture samples were successfully measured from EBSPs collected at SEM 10 kV and EBSD detector distant 35.527 mm. The measurement error compare to the lattice constant measure from XRD peaks is in the order of 0.01 - 0.67%. Precision of lattice constant measurement by this method is limited mainly by censor resolution of EBSD detector. For a Nordlys S™ EBSD detector (Oxford Instruments, Hobro DK) used in this experiment the precision limit is in the order of 0.03-0.04 Å. The precision is not enough detect the lattice constant difference of STN and YSZ in each samples. Although both the techniques are partly success in applying to analyze STN and YSZ it will be an interesting task for future development.