AnnualReport2020

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: Jena University - DE
In residence at: Extreme Conditions
and Materials: High Temperature and
Irradiation (CEMHTI) - Orléans
Nationality: German
Dates: September 2019 to August 2020
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, a critical view on results obtained during the search for a phase
of the composition ZnY2O4 in solid state ceramics was published in early
2020 [1]. Further fundamental work was addressed towards the significant
information depth of XRD and showed it to range from at least 30 µm up to
more than 100 µm in the analyzed material instead of the 10 µm sometimes
assumed in the literature [3].
Wolfgang Wisniewski worked in Jena, Germany
from 2010-2018 where he focused on applying the
method of electron backscatter diffraction (EBSD)
to glass-ceramics 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 67 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
The EBSD-analyses of beads produced using levitation melting showed
that various crystal growth mechanisms can occur during cooling. Figure
1 illustrates three possible types of crystal growth via the image quality
maps of EBSD-scans. Figure 1 a) shows the entire cross section through
a fully crystallized bead where many nucleation events occurred but it can
be shown that each grain shows its own, independent crystal orientation.
Figure 1 b) illustrates the microstructure in a different bead with a higher
magnification to illustrate the tree-like (central trunk with branches to each
side) morphology characteristic for dendritic growth. It can be shown that all
the crystallites within the white frames show only one orientation per frame,
confirming that they are connected outside of the current cross section
and hence their formation via dendritic growth. Figure 1 c) presents the
microstructure in a segment of a bead that fractured during cooling: apart
from some polishing artifacts and two pores, it contains no grain boundaries
and can be shown to have only a single crystal orientation. While it is only
part of a bead and the scan only covers a cross section, the scanned area
covers more than 7.2 mm2, proving that macroscopically homogeneous
crystals can be produced by levitation melting. The ability to produce
spherical single crystals e.g. 5 mm in diameter would allow their use in as
optical components that the production via levitation melting is much faster
than the classical methods used for single crystal production [4].
Materials & Energy Sciences 2020
28
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 glassceramics
exhibiting controlled nanostructuration.
He is author or co-author of more than 130 scientific
publications (H-index = 30), he is also co-inventor
of 5 recent international patents on transparent
alkaline earth aluminate glass and nanostructured
glass and glass-ceramics. He was awarded the
CNRS bronze medal in 2013.
Figure 1: IQ-maps of microstructure cross sections through beads produced
via levitation melting: a) polycrystalline b) dendritic and c) single crystalline.
While the Corona Pandemic of 2020 caused severe restrictions on the ability to
access laboratory equipment, the extensive home office hours provided enough
time to finalize a manuscript titled: “Oriented Surface Nucleation in Inorganic
Glasses - A Review” [2] (summarizing 10 years of work in this field) and publish
it in the high-ranking journal Progress in Materials Science (impact factor: 31).
Materials & Energy Sciences 2020
29