Contribution of transmission electron microscopy to the elaboration of transparent (glass)ceramics
Authors: Genevois Cécile , Boyer Marina , Al‐Saghir Kholoud , Chenu Sébastien , Véron Emmanuel , Fayon Franck , Matzen Guy , Allix Mathieu 
Published: December 2016

Transparent polycrystalline (glass)ceramics present significant economical and functional advantages over single crystal materials for optical, communication, and laser technologies. To date, transparency in (glass)ceramics is ensured either by an optical isotropy (i.e. cubic symmetry) or a nanometric crystallite size. Polycrystalline ceramics offer several advantages, particularly in the fabrication of complex shapes and large‐scale industrial production, and enable greater and more homogenous doping of optically active ions than is possible in single crystals. Our recent work shows the possibility to obtain transparent polycrystalline ceramics by full and congruent crystallization from glass. Transparency in these materials is observed despite micrometer scale crystals and a non cubic symmetry (no structural isotropy) of the crystalline phase. Transmission electron microscopy used in TEM – STEM – High Resolution modes and coupled with EDX map or line scan, enables to complete the nanostructure characterization which explains the transparence properties and to highlight the crystallization processes occurring during the heat treatments. Three cases where transmission electron microscopy has played an important role to develop transparent (glass‐)ceramics will be presented.

Highly transparent ZnGa
2
O
4
glass‐ceramic materials are elaborated via a simple heat treatment of a 55SiO
2
–5Na
2
O–17ZnO–23Ga
2
O
3
parent glass composition, which presents nanoscale spinodal phase separation. This optimized glass‐ceramic exhibits 50 wt% of ZnGa
2
O
4
nanocrystals showing a homogeneous and tuneable size. To describe the crystallization process, the glass and glass‐ceramic nanostructures are studied by high resolution scanning transmission electron microscopy analysis coupled with in situ high temperature X‐ray diffraction and optical measurements. From these results, an original mechanism is proposed to explain the crystallization process occurring in a spinodal phase separated glass. Remarkably, red long‐lasting luminescence arising from the entire sample volume is observed in the Cr
3+
doped transparent glass‐ceramics, opening the route to a wider range of performing applications for this famous zinc gallate persistent phosphor.
[1]

A series of biphasic (100 − z)BaAl
4
O
7
−zBaAl
2
O
4
(0 < z ≤ 45) transparent polycrystalline ceramics have been synthesized by full crystallization from glass process. Despite being composed of two birefringent crystalline phases, these new materials exhibit improved transparency compared to the pure BaAl
4
O
7
ceramic recently reported to show remarkable scintillation properties. Multiscale structural characterizations demonstrate that this transparency enhancement can be explained by the presence of nanometer scale BaAl
2
O
4
crystals which crystallize coherently with the BaAl
4
O
7
matrix. We show that the BaAl
2
O
4
nanostructuration limits the BaAl
4
O
7
growth via an original Zener pinning effect, such decreasing light scattering due to the material birefringence. Interestingly, the BaAl
4
O
7
scintillation properties can be retained in these two‐phase transparent ceramics.
[2]

The full and congruent crystallization of Sr
1+x/2
Al
2+x
Si
2‐x
O
8
(0 < x ≤ 0.4) leads to new transparent polycrystalline ceramics with hexagonal symmetry. We use a controlled degree of chemical disorder in the structure to obtain optical isotropy at the micrometer length scale. The structure and the chemical disorder were characterized by X ray Diffraction, solid state NMR and STEM‐HAADF imaging. These materials reach the theoretical limit in transmittance that is to say 90%.
[3]