Reduce synthesis temperature and improve dispersion of YAG nanopowders based on the co-crystallization method

Article abstract of DOI:10.1016/j.jallcom.2014.08.074

Pure yttrium aluminum garnet (YAG) nanopowders were synthesized at 950 °C from the co-crystallization precursor of Y(NO₃)₃·6H₂O and Al(NO₃)₃·9H₂O (nitrate process). When 17wt.% of Y(NO₃)₃·6H₂O was replaced by Y₂O₃ nanopowders, so as to make up a three-layer core-shell structure of the precursor, the synthesis temperature was reduced to 850 °C (Y₂O₃ process). Based on Y₂O₃ process, further reducing the synthesis temperature to 700 °C was realized by adding polyacrylic acid (PAA, 50% M), which was used to shorten the distance of the metal ions and provide combustion heat (PAA process). TEM characterizations indicated that the powders produced through nitrate and Y₂O₃ processes agglomerated, while the powders produced through PAA process were dispersed much better. The agglomerate size analysis results demonstrated that the powders produced through PAA process were with smaller agglomerate size and wider agglomerate size distribution than those through nitrate process or Y₂O₃ process. And they were more likely to be sintered to YAG transparent ceramics.

Full text of DOI:10.1016/j.jallcom.2014.08.074

Journal of Alloys and Compounds 618 (2015) 1–6
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Reduce synthesis temperature and improve dispersion of YAG
nanopowders based on the co-crystallization method
⇑
G.F. Fan, Y.Q. Tang, W.Z. Lu , X.R. Zhang, X. Xu
School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, PR China
Key Lab of Functional Materials for Electronic Information, MOE, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 May 2014
Received in revised form 8 July 2014
Accepted 7 August 2014
Available online 18 August 2014
Pure yttrium aluminum garnet (YAG) nanopowders were synthesized at 950 °C from the co-crystalliza-
tion precursor of Y(NO
was replaced by Y nanopowders, so as to make up a three-layer core–shell structure of the precursor,
the synthesis temperature was reduced to 850 °C (Y process). Based on Y process, further reducing
3
)
3
ꢀ6H
2
O and Al(NO
3
)
3
ꢀ9H
2
O (nitrate process). When 17 wt.% of Y(NO
3
)
3 2
ꢀ6H O
2 3
O
O
2 3
2 3
O
the synthesis temperature to 700 °C was realized by adding polyacrylic acid (PAA, 50% M), which was
used to shorten the distance of the metal ions and provide combustion heat (PAA process). TEM charac-
terizations indicated that the powders produced through nitrate and Y O processes agglomerated, while
2 3
the powders produced through PAA process were dispersed much better. The agglomerate size analysis
results demonstrated that the powders produced through PAA process were with smaller agglomerate
Keywords:
Rare earth alloys and compounds
Chemical synthesis
Phase transitions
Transmission electron microscopy, TEM
size and wider agglomerate size distribution than those through nitrate process or Y
were more likely to be sintered to YAG transparent ceramics.
2 3
O process. And they
Ó 2014 Elsevier B.V. All rights reserved.
1
. Introduction
ders, such as co-precipitation technique [7,8], sol–gel process
[
9,10], hydrothermal (or solvothermal) treatment [11,12], combi-
Yttrium aluminum garnet (YAG) is an excellent optical material
nation of the above methods [13]. YAG nanopowders produced
by the wet-chemical methods are more appropriate to produce
YAG transparent ceramics owing to higher sintering activity. How-
ever, these wet-chemical methods either are difficult to control
and time-consuming due to the needs of precise pH value [14]
and aging duration [15,16], or demand high requirement for the
equipment [17]. To get rid of the defects of these methods, a novel
easy method through just calcining the aqueous solution of the
due to its outstanding optical performance and high thermal stabil-
ity. One of its most important applications is as laser host material.
However, YAG single crystal with luminescence element is difficult
to achieve large size and heavy doping, resulting in inability to out-
put high power [1,2]. Since YAG transparent ceramic can overcome
the defects of YAG single crystal [3,4], as well as easier and cheaper
to produce, it has attracted much attention of researchers.
The sintering activity of YAG ceramic powders has great influ-
ence on the optical transmittance and the laser performance of
YAG transparent ceramics. There are two source powders to pro-
2 4
raw materials has been used to synthesize MgAl O transparent
ceramic nanopowders [18]. Key to this method is the close solubil-
ity of the salts in the solvent. Thus when the solvent is evaporated
off, the metal salts are co-crystallized, resulting in metal ions
mixed at an atomic level. Therefore, the synthesis temperature
can lower to the wet-chemical level while the process is easier
and yield is higher. So by analogy, if the solubility of an aluminum
salt is close to that of a yttrium salt, or solubility of them is very
high, YAG nanopowders can also be synthesized at a low tempera-
ture from the co-crystallization precursor, in which Y3+ and Al
have been mixed fully.
2 3 2 3
duce YAG transparent ceramics: (1) Y O and Al O as raw materi-
als for solid-state reactive method. (2) YAG nanopowders produced
by the wet-chemical methods. Conventional solid-state reactive
method is easy, but it requires extensive heat treatment to elimi-
nate the intermediate phases of YAM (Y
4 2 9 3
Al O ) and YAP (YAlO )
[
5,6]. Worse yet, many impurities are introduced during the
3+
repeated grinding and milling. To avoid these defects, wet-chemi-
cal methods have been used to produce phase-pure YAG nanopow-
In this research, pure YAG nanopowders were synthesized
through an easy method called co-crystallization method using
⇑
Corresponding author at: School of Optical and Electronic Information, Huaz-
hong University of Science and Technology, Wuhan 430074, PR China. Tel./fax: +86
2
the raw materials of Y(NO
3
)
3
ꢀ6H
2
O and Al(NO
3
)
3
2
ꢀ9H O, as the solu-
7 87543134.
bility of them is high [19], seen in Table 1. Y
2
O
3
nanopowders and
E-mail address: lwz@mail.hust.edu.cn (W.Z. Lu).
http://dx.doi.org/10.1016/j.jallcom.2014.08.074
925-8388/Ó 2014 Elsevier B.V. All rights reserved.
0

2
G.F. Fan et al. / Journal of Alloys and Compounds 618 (2015) 1–6
Table 1
Solubility of Y(NO
Ref. [20]. The low decomposition temperatures of Y(NO
3 3
) and
3
)
3
and Al(NO
3 3
) in water.
Al(NO is favor of a low synthesis temperature of YAG, as the
3 3
)
Material
Solubility (g/100 ml)
2 3 2 3
active precursors of Y O and Al O are readily to react with each
other.
0
°C
20 °C
40 °C
60 °C
Fig. 2 shows the XRD patterns of the powders calcined from
50 °C to 1100 °C for 3 h through nitrate process. The powders
Y(NO
Al(NO
3
)
3
93.1
60.0
123
73.9
163
88.7
200
106
8
3
)
3
are amorphous when calcined at 850 °C. As the temperature
increases to 900 °C, YAG is the dominant phase with small amount
of secondary phase of YAM. Pure YAG is synthesized until 950 °C.
From 950 °C to 1100 °C none change occurs except for sharpening
of the diffraction peaks. The result indicates that the crystallinity of
the powders is improved gradually.
polyacrylic acid (PAA) were selected to lower the synthesis tem-
perature and improve dispersion. Effects of them were discussed
in detail.
Fig. 3 shows TEM micrographs of the powders calcined at
500 °C, 850 °C, 950 °C, 1000 °C for 3 h through nitrate process
respectively. It can be seen that the particle sizes are about
2
. Experimental procedures
YAG nanopowders were synthesized based on the co-crystallization method.
1
0 nm, 20–30 nm, 30–40 nm when calcined at 500 °C, 850 °C,
The raw materials were Y(NO
3
)
3
ꢀ6H
2
O (99.9%), Al(NO
3
)
3
ꢀ9H
2
O (99%), Y
2
O
3
(40 nm,
ꢀ6H O to
9
9.99%) and PAA. Y nanopowders were used to partly replace Y(NO
2
O
3
3
)
3
2
950 °C respectively, indicating the particle size increases as the cal-
cination temperature increases. However, the powders are dis-
persed badly. It is hard to find a single particle when the
calcination temperature is higher than 850 °C. When calcined at
lower YAG synthesis temperature, while PAA was used to overcome the agglomer-
ation and lower the synthesis temperature further. (I) During nitrate process,
Y(NO
3 3
)
ꢀ6H
2
O and Al(NO
3
)
3
ꢀ9H
2
O were weighted according to the stoichiometric
3+
ratio of Y
3
Al
5
O
12 and dissolved in distilled water with the concentration of Al
5
2 3 2 3
00 °C, the particles of Y O and Al O precursors cluster together,
being 1.65 M. The solution was stirred at a proper rotating speed by a magnetic stir-
rer at 100 °C until it became too sticky for the magnetic stirrer to rotate. Then the
sticky mixture (YAG precursor) was put into a crucible and calcined at different
leaving little space between them, seen in Fig. 3(a). Therefore,
slight agglomeration has been formed below 500 °C already. The
agglomeration is caused by the big surface tension, capillary force
and hydrogen bond of the crystal water contained in the nitrates
[21], as the crystal water cannot be removed at 200 °C [22,23].
And the agglomeration will be aggravated as the calcination tem-
perature increases. From 500 °C to 1000 °C, the necklike connec-
tions between the particles become more evident, demonstrating
the surface diffusion and grain boundary diffusion are easy to be
achieved among the agglomerated particles.
temperatures for 3 h. (II) During Y
replaced by Y nanopowders. Later process was the same as above. (III) During
PAA process, PAA which was 50 wt.% of Y(NO O, Al(NO O and Y
process. Later process was still
2
O
3
process, 17 wt.% of Y(NO
3
)
3
ꢀ6H
2
O was
2 3
O
3 3
) ꢀ6H
2
)
3 3
ꢀ9H
2
2 3
O
was added into the solution based on
unchanged.
2 3
Y O
Crystal structure of the powders was examined by X-ray diffraction(XRD, Riga-
ku Dmax-rC, Japan) using Cu K radiation in the range of 2h = 10–65° with a scan-
a
ning speed of 10°/min, while the powders used for particle size calculating by
Scherrer formula were scanned at the speed of 2°/min again. Morphology was char-
acterized by transmission electron microscope (TEM, Tecnai G2 20, Netherlands)
with accelerating voltage of 200 kV. The agglomerate size was measured by laser
particle size analyzer (Mastersizer 3000, England). Thermal gravimetric analysis
on the nitrates was carried out by thermogravimetric analyzer and differential
scanning calorimeter (TGA/DSC, Mettler-Toledo 1, Switzerland) with the heating
rate of 10°C/min.
2 3 2 3
3.2. The effect of Y O nanopowders through Y O process
As the synthesis temperature is a little high through nitrate pro-
cess, Y nanopowders were added as the raw materials to lower
the synthesis temperature. The XRD patterns of the powders cal-
cined at 800 °C, 850 °C, 950 °C for 3 h through Y process are
3
. Results and discussions
2 3
O
3.1. The effect of synthesis temperatures through nitrate process
2 3
O
shown in Fig. 4(a–c). It can be seen that the powders are amor-
phous at 800 °C, and pure YAG is synthesized at 850 °C. The syn-
thesis temperature has been reduced by 100 °C compared with
nitrate process. Difference between the diffraction peak intensity
of the powders calcined at 950 °C through nitrate and Y O process
2 3
is also shown in Fig. 4(c and d). The stronger diffraction peak
Fig. 1 shows the TG curves of Y(NO
of temperature. It can be seen that the mass losses of the two salts
are very limited above 500 °C, indicating Y(NO
almost have decomposed to the precursors of Y
3 3 3 3
) and Al(NO ) as a function
3
)
3
and Al(NO
3
)
3
3
2
O
3
and Al O
2
respectively. The result is in agreement with the study made by
Fig. 2. XRD patterns of the powders calcined at different temperatures for 3 h
Fig. 1. TG curves of Y(NO
3
)
3
and Al(NO
3
3
) .
through nitrate process.

G.F. Fan et al. / Journal of Alloys and Compounds 618 (2015) 1–6
3
Fig. 3. TEM micrographs of the powders calcined at different temperatures for 3 h through nitrate process (a) 500 °C, (b) 850 °C, (c) 950 °C and (d) 1000 °C.
to crystallize on, then the later crystallized Y(NO
Al(NO again. In this manner, the three materials make up a
three-layer core–shell structure, shown in Fig. 5. And the distance
3 3
) adheres on
3 3
)
3
+
3+
between Y and Al is shortened by the three-layer core–shell
structure. As a consequence, mass transfer is promoted when cal-
cined at high temperature and the synthesis temperature is
reduced.
Fig. 6 shows TEM micrographs of YAG nanopowders calcined at
8
2 3
50 °C, 950 °C through Y O process. The particle sizes are 20–
4
0 nm, 30–50 nm when calcined at 850 °C, 950 °C respectively,
bigger than those through nitrate process. And the particles
agglomerate again by the same mechanism as nitrate process.
However, the agglomeration is weakened when calcined at the
same temperature. This is because the crystallinity of the powders
calcined at 850 °C is improved through Y O process, the activity or
2 3
the surface energy of the particles is lowered, resulting in less
potential of clustering together and weaker agglomeration.
Fig. 4. XRD patterns of the powders calcined at different temperatures for 3 h
through Y process and nitrate process (a) 800 °C through Y process, (b)
50 °C through Y process, (c) 950 °C through Y process and (d) 950 °C
through nitrate process.
2
O
3
2 3
O
8
2
O
3
2 3
O
intensity indicates the better crystallinity of the powders produced
through Y process at 950 °C.
The reason of lowering the synthesis temperature via adding
is as follow. During the evaporation process, Al(NO crystal-
lizes earlier, for solubility of Al(NO is smaller than that of
Y(NO . So segregation between Al(NO and Y(NO occurs,
2 3
O
Y
O
2 3
3 3
)
3 3
)
3
)
3
3
)
3
3 3
)
3
+
3+
resulting in difficulties in mass transfer between Y and Al
during the high temperature, so as to raise the synthesis tempera-
ture. However, when Y
2 3
O nanopowders are dispersed in the
solution uniformly, they provide heterogeneous nuclei for Al(NO
3
)
3
2 3 3 3 3 3
Fig. 5. Model of three-layer core–shell structure formed by Y O , Al(NO ) ,Y(NO ) .

4
G.F. Fan et al. / Journal of Alloys and Compounds 618 (2015) 1–6
2 3
Fig. 6. TEM micrographs of YAG powders calcined at different temperatures for 3 h through Y O process (a) 850 °C and (b) 950 °C.
3
.3. The effect of PAA through PAA process
Y
2
O
3
process. And this is the lowest temperature to synthesize YAG
without the use of autoclave. The so low synthesis temperature can
be explained in two aspects: (1) The effect of Y nanopowders.
To improve the dispersion and lower the synthesized tempera-
2 3
O
ture further, PAA is added in as the raw material based on Y
2
O
3
As electronegativity of Al is higher than that of Y, the formation
speed of –COOAl1/3 is faster than that of –COOY1/3. Therefore, seg-
regation between Y and Al takes place in polymeric chelate.
process. In this process, it plays as a chelating agent, as it can react
with most high valence metal ions at a certain temperature [24]. So
3
+
3+
3+
3+
by stirring at 100 °C, PAA can react with Y /Al as Eq. (1):
Similar to Y
2
O
3
process, a three-layer core–shell structure is again
ð1Þ
M stands for Y/Al. YAG precursor has been evolved to polymeric
chelate of Y /Al during PAA process.
formed by Y
2
O
3
nanopowders, aluminum chelate and yttrium che-
3
+
3+
late. (2) The properties of PAA. The close distance of –COO– in PAA
results in closer distance of –COOAl1/3 and –COOY1/3, so mass
transfer between Al and Y at high temperature is promoted
again. Moreover, PAA provides combustion heat required to syn-
thesize YAG [25]. As a consequence, the synthesis temperature is
reduced again.
Fig. 7 shows the XRD patterns of the powders calcined at 650 °C,
3
+
3+
7
00 °C, 800 °C for 3 h through PAA process. As can be seen from it,
pure YAG is synthesized at 700 °C, 150 °C lower than that through
Fig. 8 shows TEM micrographs of YAG nanopowders calcined at
8
5
50 °C, 950 °C through PAA process. The particle sizes are 30–
0 nm, 40–70 nm when calcined at 850 °C, 950 °C respectively.
The sizes are larger than those through the other two methods.
And the particles show much better dispersion, reflecting that
PAA has significant effect on dispersion. The good dispersion is
3
+
3+
due to insolubility of polymeric chelate of Y /Al . So water can
be removed out easily at a low temperature, then hard agglomer-
ation is greatly weakened.
Fig. 9 shows the photographs of powders calcined at 850 °C for
3
2 3
h through PAA process and Y O process. The powders form small
granules through PAA process, while they form flakes through Y
2 3
O
process. The macro-figure also indicates PAA can improve
dispersion.
3.4. Comparison of the powders produced through the three processes
Table 2 shows the particle sizes and agglomerate sizes of the
powders produced through nitrate process at 950 °C (P1), Y
2 3
O
Fig. 7. XRD patterns of the powders calcined at different temperatures for 3 h
through PAA process.
process at 850 °C (P2) and PAA process at 700 °C (P3). The mean

G.F. Fan et al. / Journal of Alloys and Compounds 618 (2015) 1–6
5
Fig. 8. TEM micrographs of YAG powders calcined at different temperatures for 3 h through PAA process (a) 850 °C and (b) 950 °C.
2 3
Fig. 9. Photographs of powders calcined at 850 °C through different processes for 3 h (a) PAA process and (b) Y O process.
Table 2
Particle sizes and agglomerate sizes of YAG powders produced through nitrate
process at 950 °C (P1),Y process at 850 °C (P2) and PAA process at 700 °C (P3).
2 3
O
Powder
D
XRD
(
l
m) m) m)
D
TEM
(
l
50
D ( l
P1
P2
P3
0.0404
0.0356
0.0174
0.030–0.040
0.020–0.040
–
50.0
33.4
8.19
primary size particle calculated from the XRD peak profile is
defined as DXRD. The mean crystallite size estimated from the
TEM photograph is denoted as DTEM. And the mean agglomerate
size D50 is the median diameter determined from the agglomerate
size distribution measured by the laser particle size analyzer. DTEM
of the powders through PAA process at 700 °C is not given here, as
they have not been tested by TEM. The DTEM’s of P1and P2 almost
agree with the respective values of DXRD within error, while the
agglomerate sizes D50 are much bigger than the particle sizes DXRD
.
The values of D50/DXRD of P1, P2 and P3 are 1237,938 and 470
respectively, indicating the agglomeration is weakened by these
processes gradually, consistent with the TEM results. According
to WH Rhodes, agglomerate limits attainable green density and
final ceramic density [26]. The big size agglomerates lead to big
pores in the compact. So the green density is limited. And the
dense agglomerates can readily become single grains at the initial
stage of sintering, leaving cracks between the surrounding grains,
which will evolve to isolated pores at the final stage of sintering.
As a result, the final density is limited. What’s more, the mecha-
nism increases with the increasing agglomerate size. So P3 may
be most beneficial for sintering while P1 worst.
Fig. 10. Agglomerate size distributions of the powders through nitrate process at
950 °C (P1),Y O3 process at 850 °C (P2) and PAA process at 700 °C (P3).
2
Fig. 10 shows the agglomerate size distributions of the three
powders. The agglomerate sizes of P1and P2 range from 0.48
to 115 m, while that of P3 ranges from 0.35 m to 105 m. The
double peaks in profile of P3 indicate the agglomerate sizes are
mainly distributed in two ranges: 1 m and 20 m. The more
lm
l
l
l
l
l
smooth profile of P3 indicates the agglomerate size distribution
is wider than that of P1 and P2, as the agglomerate sizes of P1,
P2 are mainly distributed in the narrow larger-size area. Since
the fine particles or small agglomerates can fill the gaps in the

6
G.F. Fan et al. / Journal of Alloys and Compounds 618 (2015) 1–6
compacter, the green density of powders with wider agglomerate
size distribution will be increased, leading to enhanced densifica-
tion of the compacts at the initial stage of sintering [27].
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3 5
Al O
[
7
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(
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based on the three-layer core–shell structure. The smaller agglom-
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duced through this process showed more possibility to be
sintered to transparent YAG ceramics.
[
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Acknowledgements
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This work was supported by the National Natural Science Foun-
dation of China through Grant Nos. 61172004 and 61201051. It
was also supported by the Fundamental Research Funds for the
Central Universities, HUST: 2014TS046 and the Education Depart-
ment Funds of Hubei Province, No. D20141503. Electron micros-
copy analysis was conducted at the analytical and testing center
of Huazhong University of Science and Technology.
2
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)
3 3
2
3
using poly(acrylic acid),
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