Annual-Report-2019

MECHANISMS OF GLASS CRYSTALLIZATION
ANALYSED BY ELECTRON BACKSCATTER
DIFFRACTION (EBSD)
Dr Wolfgang Wisniewski
LE STUDIUM / Marie Skłodowska-Curie
Research Fellow
Smart Loire Valley General Programme
From: Friedrich Schiller University Jena - DE
In residence at: Extreme Conditions and
Materials: High Temperature and Irradiation
(CEMHTI) - Orléans
Nationality: German
Dates: September 2019 to August 2020
Wolfgang Wisniewski worked in Jena, Germany from
2010-2018 where he focused on applying the method
of electron backscatter diffraction (EBSD) to glassceramics
for which he received his Ph.D. in 2011. In
early 2019 he became a Visiting Scientist in Trencin,
Slovakia and is currently a Le Studium Research
Fellow at the CEMHTI in Orléans, France. While his
primary work has remained the EBSD-analysis of
crystallized glasses, he has contributed to more
than 65 articles published in peer reviewed journals
concerning glass-ceramics, the information depth
of EBSD, ceramics, dewetted metal nano particles,
super conductors and solar cell materials.
In 2015 he received a Best Presentation Award at
the 11th International Symposium on Crystallization
in Glasses and Liquids in Nagaoka 2015). He also
contributed to work concerning solar cell materials
which became an ESRF Scientific Highlight in 2018
(Nano Energy, 2017, Vol. 42, 307–313).
Dr Mathieu Allix
Host scientist
Mathieu Allix, completed his PhD at the
University of Caen in 2004. After three years at
the University of Liverpool (U.K.), he joined the
CNRS in Orléans at the CEMHTI laboratory. His
research covers synthesis and characterization
of inorganic materials with a special interest on
new transparent ceramics. He has patented and
published (i) the first transparent polycrystalline
ceramics obtained by full crystallization from glass
(http://www.cnrs.fr/inc/communication/direct_
labos/allix.htm) and (ii) new highly transparent
glasses and glass-ceramics exhibiting controlled
nanostructuration. He is author or co-author of
more than 120 scientific publications (H-index =
29), he is also co-inventor of 5 recent international
patents on transparent alkaline earth aluminate
glass and nanostructured glass and glassceramics.
He was awarded the CNRS bronze
medal in 2013.
This project is aimed towards studying crystallization during or after the process
of levitation melting. This includes the crystallization of melts during cooling
but also the more controlled crystallization of glasses in a subsequent thermal
treatment. In order to determine the occurring crystallization mechanism, it is
necessary to analyze the crystallographic orientation relationships amongst
the various components of a microstructure. While phase identification and
characterization can be achieved using X-ray diffraction (XRD), determining
local orientation relationships in the microstructure necessitates the use of
electron backscatter diffraction (EBSD) which is performed in a scanning
electron microscope (SEM). EBSD can also be used to locate and identify
phases which occur in a quantity below the detection threshold of XRD.
It is essential to know and understand the possibilities and limits of the
applied methods when performing high-level analysis. With this aspect in
mind, critically viewing the results obtained during the search for a phase of
the composition ZnY 2
O 4
in solid state ceramics led to an observation affecting
the application of EBSD: misindexing due to EBSD-pattern superposition. The
Figure below illustrates this artifact: during the performed EBSD-scan, most
data points are correctly attributed to Y 2
O 3
, an example is featured as EBSDpattern
1. The patterns 2 and 3 originate from ZnO and were acquired at the
locations 2 and 3. EBSD-pattern 4, however, is indexed as ZnY 2
O 4
, although
it is in fact a superposition of two Y 2
O 3
patterns, individual components are
highlighted in pattern 4a. EBSD-pattern 5 is also indexed as ZnY 2
O 4
although
it is a superposition of the Y 2
O 3
patterns 5a and 5b acquired from neighboring
grains. This observation illustrates, that is essential to confirm certain indexing
results of the software by a detailed analysis of the obtained EBSD-patterns.
Keeping such possible artifacts in mind is essential when performing
measurements. In the context of performing EBSD-analysis of a phase
with the composition Ga 3
La 2
O 7.5
synthesized using levitation melting, this
critical view enabled the results presented below. The SEM-micrograph (left)
illustrates components of the Ga 3
La 2
O 7.5
microstructure after dendritic growth:
the primary compact crystals (bright) with a secondary crystallization (fine
lamellae) of Ga 3
La 2
O 7.5
in the interdendritic spaces and residual glass (dark).
The EBSD-pattern 1 represents the high pattern quality obtainable from the
primary dendritic crystals. Pattern 2 was obtained from the interdendritic
crystallization and is of lower quality due to residual glass within the information
volume of this pattern. The identical positions of the bands show that the
interdendritic crystals do not originated from independent nucleation, but are
a continuation of the primary crystal lattice in a second crystallization step.
Pattern 3 was acquired from the circled area: the somewhat broader bands
and deviating dominant zone axis (white arrow) imply that it may originate
from a different, perhaps unknown crystal phase. Further research is needed
to confirm this result.
Materials & Energy Sciences 2019
27