Chemical gelation of cerium (III)-doped yttrium aluminium
oxide spherical particles
L.T. Su, A.I.Y. Tok,
a)
and F.Y.C. Boey
School of Materials Science and Engineering, Nanyang Technology University, Singapore 639798
J.L. Woodhead
Advanced Material Resources (Europe) Ltd., Abingdon, United Kingdom, OX14 3YS
(Received 3 March 2006; accepted 3 May 2006)
A novel low-temperature (900 °C) chemical gelation method was developed to
synthesize spherical and nonagglomerated Ce
3+
-doped yttrium aluminum oxide
particles (YAG:Ce
3+
). This represents a process with a much lower processing
temperature than current solid-state reaction processes (1400 °C). Characterization of
the particles via x-ray diffraction and thermoanalytical methods showed that calcination
at 900 °C for 2 h allowed direct crystallization from the amorphous phase, inferring
that this process allows homogeneous mixing and increased precursor reactivity.
Electron microscopy results showed that the spherical particles (∼100 to ∼3 m) were
the flocks of crystallites. The crystallite sizes (Rietveld refinement) grew linearly from
27 nm (900 °C) to 114 nm (1300 °C). The surface area decreased from 40 m
2
/g
(900 °C) to 5 m
2
/g (1300 °C) because of the coagulating and growing of crystallites to
bigger grains at 1300 °C. Single-crystal nanoparticles (around 100 nm) were obtained
with this process and their atomic structures were revealed via high-resolution
transmission electron microscopy.
I. INTRODUCTION
The phosphor that is commercially used for white
light emitting diode (LED) lighting applications is a
cerium (III)-doped yttrium aluminum oxide (garnet crys-
tal; YAG:Ce
3+
). The garnet crystal has a general formula
[RE
3
c
][Al
2
a
][Al
3
d
][O
12
], where RE are rare-earth ions
and the superscripts c, a, and d denote the Wyckoff sym-
bols for the ions’ occupancy site in the lattice.
1–4
. All of
the metal cations are trivalents. The garnet’s crystal
structure (space group number 230; Ia 3
¯
d) is cubic with
160 atoms and eight formula units per unit cell. Each
formula unit consists of an ion arranged on a body cen-
tered cubic (BCC) lattice with c and d ions lying on the
faces. Each a and d ion occupies an octahedral site (i.e.,
AlO
6
) and tetrahedral sites (i.e., AlO
4
), respectively.
These polyhedrals are not regular because the oxygen
lattice is distorted. Each c ion is surrounded by eight
oxygen ions, forming a severely distorted cube.
It has been found that YAG:Ce
3+
coated onto a blue
LED is excited by the blue radiation to give white
light.
5,6
Critical property requirements of the phosphor
include its’ being small with spherical particles to
achieve a thinner layer and a more homogeneous coating
on the LED with less material loading.
Chemical synthesis such as homogeneous precipita-
tion,
7
coprecipitation,
8,9
and combustion synthesis,
10
and
physical methods such as spray pyrolysis
11
and solid-
state reaction
12
are common methods in synthesizing
YAG particles. However, it is difficult to crystallize pure
garnet crystal phase at low temperatures (<1000 °C) be-
cause yttrium and aluminium ions have different solu-
bilities and precipitate at different pHs. Several phases
[perovskite YAlO
3
(YAP), hexagonal YAlO
3
(YAH),
and monoclinic Y
4
Al
2
O
9
(YAM)] other than garnet are
present after calcination, even when reacting a stoichio-
metric (garnet YAG;Y:Al 3:5) mixture of precursors.
Therefore, it is necessary to control the precursors’ com-
positions during the preparation. One of the methods is
using emulsions as reported by Hardy et al.
13
They syn-
thesized the undoped yttrium aluminium oxide by gelling
the alkoxide sols in an organic solvent. However, the
precursors (alkoxide sols) must be prepared at an el-
evated temperature.
This article reports on the chemical gelation method
(an emulsion-based technique), which uses mixed salt-
solutions as the precursors; no heat is required for the
preparation. Chemical gelation was developed to synthe-
size spherical and nonagglomerated YAG:Ce
3+
particles
that crystallize with pure cubic garnet phase at low
a)
Address all correspondence to this author.
e-mail: miytok@ntu.edu.sg
DOI: 10.1557/JMR.2006.0312
J. Mater. Res., Vol. 21, No. 10, Oct 2006 © 2006 Materials Research Society 2510