Investigation of Fluorescence Degradation Mechanism of
Hydrated BaMgAl
10
O
17
:Eu
2+
Phosphor
K. C. Mishra,
a,
*
,z
M. Raukas,
a,
*
G. Marking,
b
P. Chen,
c
and P. Boolchand
c
a
Osram Sylvania, Central Research, Beverly, Massachusetts 01863, USA
b
Osram Sylvania, Precision Materials and Components, Towanda, Pennsylvania 18848, USA
c
Department of Electrical Engineering, Computer Engineering and Computer Science,
University of Cincinnati, Cincinnati, Ohio, USA
We have investigated the degradation of BaMgAl
10
O
17
:Eu
2+
phosphor BAM using
151
Eu Mössbauer and fluorescence spectro-
scopic methods. The
151
Eu Mössbauer measurements show that upon hydration, a significant fraction of divalent europium ions is
oxidized. The degradation process induced by the intercalated water molecules into the host lattice appears to be a unique process.
It has many features that are both similar to and distinct from those induced by oxidation in air and those caused by discharge
during the lamp life. A plausible mechanism of degradation of BAM caused by the intercalated water molecules is discussed.
© 2005 The Electrochemical Society. DOI: 10.1149/1.2041927 All rights reserved.
Manuscript submitted March 31, 2005; revised manuscript received May 12, 2005. Available electronically September 14, 2005.
The barium magnesium aluminate phosphor activated by diva-
lent europium ions BAM continues to be a subject of great interest
in the fluorescence community because of its importance as an effi-
cient blue-emitting phosphor. However, its relative instability in a
variety of lamp-related processing conditions and also during the
lamp life continues to be a major concern.
1-7
A better alternative to
this blue-emitting phosphor has yet to be developed. Therefore, a
great deal of attention is focused worldwide on understanding the
underlying mechanisms of various degradation processes. It is
hoped that such an understanding could help in improving the main-
tenance of this phosphor. The present paper reports results from a
study involving degradation of BAM induced by intercalated water
molecules.
Intercalation of water into the host lattice of BAM
BaMgAl
10
O
17
has been reported to play a critical role in the deg-
radation process of this blue-emitting phosphor.
6,7
The baking steps
during the fabrication of fluorescent lamps and plasma display pan-
els produce conditions favorable for intercalating water molecules
into the -alumina lattice of BAM. Intercalation of water molecules
at the baking temperature 400–500°C has been observed, caus-
ing oxidation of Eu
2+
ions and color shift of the phosphor emission.
6
It has also been proposed that the water molecules contribute to
degradation of the phosphor during the lamp life.
6
The degradation process induced by water molecules during
lamp baking has many features that are both similar to and distinct
from those resulting from the oxidative degradation of BAM from
firing in air or degradation during the lamp life. One observes a blue
shift of the main emission peak from BAM near 450 nm caused by
oxidation of the activator ions by firing in the air. In contrast, the
emission spectrum of hydrated BAM shifts toward the green region
of the visible spectrum after baking accompanied by a long tail
extending to the deep red. This color shift is usually observed during
aging of this phosphor in mercury discharge lamps.
Poor maintenance and loss of brightness of hydrated BAM are
attributed mainly to oxidation of europium ions during the baking
process
6
in addition to absorption of ultraviolet UV radiation from
discharge by the water molecules and vibronic coupling of divalent
europium ions with adjacent water molecules.
It has been observed that water molecules can be easily interca-
lated into the intermediate plane of aluminates in the -alumina
structure.
8
Using X-ray and neutron diffraction methods and infrared
spectroscopy, Bates et al. investigated the location and geometry of
water molecules in lithium -aluminate.
8,9
The relative openness of
the intermediate plane in the -alumina lattice is considered to be
responsible for the observed absorption of water. The diffraction
measurements locate the water molecules in this plane.
8
That the
water molecules are inside the bulk material is also confirmed by the
observation of the infrared stretching frequency near 2881.2 cm
-1
which is distinct from the corresponding vibrational frequencies of
water molecules adsorbed on the surface.
6
Additionally, a significant
fraction of the adsorbed water molecules is also believed to be dis-
sociating to OH
-
and H
2
O
x
H
+
groups. These molecular groups
are located within the -alumina lattice.
6,8
A proper understanding of the degradation process induced by
the intercalated water molecules begs a complete picture of the
structure, location, and geometry of water molecules in the BAM
lattice, direct evidence of the generation of trivalent ions, and a
thorough characterization of the fluorescence spectra of fresh and
degraded phosphor samples. With the availability of this informa-
tion, a coherent picture of the degradation process can be developed,
and the underlying chemical processes can be explored to prevent
the degradation process. In a related theoretical study,
10
the struc-
tural questions regarding the intercalated water molecules were stud-
ied using atomistic simulation methods and possible mechanisms of
the oxidation processes were explored. In the present paper, we have
probed the fresh and hydrated samples by
151
Eu Mössbauer spec-
troscopy and solid-state fluorescence techniques to establish the
growth of trivalent europium ions during the hydration process and
spectroscopic characterization of the degradation process.
Sample Preparation
The BAM phosphor used in this work was obtained from the
BAM phosphor lot SSX-1 prepared for plasma display panel
PDP application, and is referred to as PDP BAM. The formulation
is almost stoichiometric. The material has a blue emission color. The
starting mixture was composed of 10.5 mol % Al OH
3
,
1.05 mol % MgCO
3
, 0.83 mol % BaCO
3
, 0.09 mol % BaF
2
, and
0.04 mol % Eu
2
O
3
. In order to produce material with the small par-
ticle size necessary for use in PDPs, we use small particle size
Hydral 710 aluminum hydroxide. The MgCO
3
weight is corrected
for a 97% assay; all other materials had an assumed assay of 100%.
The materials are weighed into a drum and drum blended for
30 min. The material was fired in a standard square Pell tray on a C.
I. Hayes type MY-6013130 continuous furnace at 1650°C under
reducing conditions. The phosphor was wet-milled in 3.5 kg batches
for 55 min on a small Sweco M-18 mill using low-density alumina
media and then wet-sifted by 378 mesh. The material was then fil-
tered, dried, and sifted through 64 mesh to break up the dried cakes.
The particle size distribution exhibits a 50% size of 2.74 m with
99.93% 9 m.
The humidity treatment was performed as follows. Approxi-
mately 11.2 g of phosphor were placed in two small silica boats
0.5 0.5 4.0 in., which were loaded into a 3-in. diam quartz
tube inside a tube furnace. The ends of the tube were partially sealed
* Electrochemical Soceity Active Member.
z
E-mail: kailash.mishra@sylvania.com
Journal of The Electrochemical Society, 152 11 H183-H190 2005
0013-4651/2005/15211/H183/8/$7.00 © The Electrochemical Society, Inc.
H183