Highly transparent alumina spark plasma sintered from common-grade commercial powder: The effect of powder treatment

Article abstract of DOI:10.1111/j.1551-2916.2009.03544.x

Transparent alumina were tentatively prepared by spark plasma sintering, using aggregated submicrometer-sized commercial Al₂O₃ powder. It is found that the pretreatment of the powder in hydrofluoric acid significantly reduces the agg

Full text of DOI:10.1111/j.1551-2916.2009.03544.x

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J. Am. Ceram. Soc., 93 [5] 1232–1236 (2010)
DOI: 10.1111/j.1551-2916.2009.03544.x
r 2010 The American Ceramic Society
ournal
J
Highly Transparent Alumina Spark Plasma Sintered from Common-Grade
Commercial Powder: The Effect of Powder Treatment
Xihai Jin, Lian Gao,w and Jing Sun
The State Key Laboratory on High Performance Ceramics and Superfine Microstructure,
Shanghai Institute of Ceramics, Shanghai, China 200050
Transparent alumina were tentatively prepared by spark plasma
sintering, using aggregated submicrometer-sized commercial
Al2O3 powder. It is found that the pretreatment of the powder
in hydrofluoric acid significantly reduces the aggregate size of
the powder, suppresses the grain growth, and improves the mi-
crostructure homogeneity of the sintered ceramics. The sample
sintered from the pretreated powder is highly transparent, while
the sample sintered from the untreated powder is translucent at
most. The fundamental reasons for this are investigated.
dispersed nanoparticles and excellent sinterability is needed.
The high price and limited sources of such a powder are not
beneficial for a large-scale production of the material.
In order to overcome the problem, transparent Al2O3 ce-
ramics were tentatively prepared by spark plasma sintering in
the present work, using common grade a-Al2O3 powder that
showed a primary particle size in the submicron range and was
aggregated as the raw material. We found that highly trans-
parent Al2O3 ceramics could be obtained after pretreatment of
the powder with hydrofluoric acid (HF), although the sample
from the untreated powder was translucent. This powder
treatment technique shows great promise for producing fine
grained transparent Al2O3 ceramics, using common-grade
Al2O3 powder.
I. Introduction
LUMINA (Al2O3) shows many interesting properties, such as
high strength, high hardness, and excellent corrosive re-
A
sistance. This makes transparent Al2O3 ceramics a promising
candidate for applications as electromagnetic windows, trans-
parent armor, and envelopes of high-pressure metal halide
lamps, etc. Traditional transparent Al2O3 ceramics are pre-
pared by sintering in hydrogen gas at temperature generally
above 17001C.1,2 The high sintering temperature causes exten-
sive grain growth and seriously affects the mechanical strength
and hardness of the material. What is more important, the
large grain size (410 mm) leads to significant light scattering
caused by the birefringence of coarse Al2O3 grains.3 As a re-
sult, although traditional transparent Al2O3 ceramics show a
high diffuse forward transmission, its in-line transmission is
typically below 10%, which makes the material appear trans-
lucent rather than transparent. The low strength and in-line
transmission pose an almost insurmountable obstacle for
application in fields where high transparence and good
mechanical properties are required.
II. Experimental Procedure
A common-grade a-Al2O3 powder (HFF5, Shanghai Wusong
Chemical Co. Ltd., Shanghai, China) with a purity higher than
99.99% was used to prepare the transparent Al2O3 ceramics.
The powder was dispersed in 10 wt% HF to form a slurry with a
solid load of 100 g/L; then, it was stirred for 6 h under contin-
uous ultrasonicating at room temperature. Afterwards, the pow-
der was collected through filtering and cleaned by successive
washing with distilled water for five times and ethanol alcohol
for two times. The powder was dried at 1001C for 8 h, calcinated
at 6001C in air for 1 h, and then spark plasma sintered (Dr.
Sinter 1020, Sumitomo Coal Mining Co. Ltd., Tokyo, Japan) in
a vacuum at a heating speed of 1001C/min. When the temper-
ature increased to 7001C, a pressure of 80 MPa was applied,
which remained constant during the later stage of sintering. The
temperature was measured by an optical pyrometer focused on
the half-through hole in the graphite die, and the variation of the
total height of the graphite mold assembly with the temperature
was recorded. After holding at the sintering temperature for
3 min, the applied pressure was removed and the power was
turned off immediately, allowing the sample to cool naturally in
the furnace. The final dimension of the sintered sample was
10 mm in diameter and 2.5 mm in thickness.
The surface layer of the sintered disk was removed (1 mm in
depth) by deep grinding and mirror polished on both sides to a
thickness of 0.5 mm, using 0.5 mm diamond paste. The micro-
structure was examined by scanning electron microscopy (SEM,
Instrument JSM 6700F JEOL Ltd., Tokyo, Japan) on the pol-
ished surface thermally etched at 11501C for 2 h. The in-line
transmission was measured using a double-beam spectropho-
tometer (Model U-2800, Hitachi, Tokyo, Japan); the total for-
ward transmission was measured using a PerkinElmer Lambda
950 (Perkin Elmer Inc., Waltham, MA) spectrophotometer
equipped with an integrating sphere. The impurity level of the
powder was analyzed using an inductively coupled plasma
atomic emission spectrometer (Vista, Palo Alto, CA); the par-
ticle size distribution was analyzed (Zetaplus, Brookheaven) af-
ter the powder was ultrasonically dispersed in water for 6 h.
Recently, fine-grained transparent Al2O3 ceramics has at-
tracted much attention,3–10 due to its superior mechanical and
optical properties. This material is prepared by hot isostatic
pressing or spark plasma sintering at a low temperature around
11501–13001C. The formation of fine-grained microstructure
(o1 mm) leads to a significant improvement in both the strength
and the transparency. It is reported that the strength of the fine-
grained transparent Al2O3 is up to 600–800 MPa together with a
high in-line transmission up to 60% for visible light.4,10 In com-
parison with traditional coarse-grained transparent Al2O3 ce-
ramics, the properties of the fine grained one are much more
superior. However, for the preparation of fine-grained transpar-
ent Al2O3 ceramics, high-grade a-Al2O3 powder with well-
G. Wei—contributing editor
Manuscript No. 26691. Received August 13, 2009; approved November 9, 2009.
This work was supported by the National Natural Science Foundation of China
(50672112), the Key Project for Fundamental Research of Shanghai (09JC1415400), and
the National Basic Research Program (2005CB623605).
wAuthor to whom correspondence should be addressed. e-mail: liangaoc@online.sh.cn
1232

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III. Results and Discussion
This chemical etching is especially severe at the jointing point
between the primary particles, due to its more disordered struc-
ture. Under the influence of an ultrasonic shock force and the
chemical etching by HF, the joint points tend to become loose,
leading to the disassociation of the aggregate and thereby a re-
duced aggregate size and narrowed particle size distribution in
the pretreated powder. The reduction in aggregate size leads to a
slight increase in the specific surface area of the powder from 4.7
to 5.2 m2/g.
Figure 2 shows the sintering behavior of the pretreated and
untreated Al2O3 powders, where the normalized displacement of
the total height of the graphite mold assembly is used to ap-
proximate the real-time shrinkage during the densification pro-
cess. It can be seen that both samples begin to shrink at about
9001C and the shrinkages increase abruptly above 9501C, reach-
ing their maxima at about 12501 and 13001C for the samples
from the pretreated and untreated powder, respectively. A com-
parison between the two displacement curves finds that the pre-
treatment in HF has substantially enhanced the sinterability of
the powder, which is mainly caused by the decrease in the ag-
gregate size and probably the activation of the particle surface
during the powder treatment process. The treatment of the pow-
der in HF not only led to a decrease in the aggregate size
as observed above, but also made the particle surface more
activated due to the strong chemical etching effect of HF on
the Al2O3 particle surface. Both of the effects are helpful for
sintering.
Chemical analysis indicates that the Al2O3 powder is highly pure
even before the treatment in HF; the only detectable impurities
in it are Si (9 ppm), Fe (9 ppm), K (7 ppm), Na (6 ppm), and Ca
(2 ppm). After the treatment in HF, the Si, K, and Na impurity
contents decrease to 7, 1, and 4 ppm, respectively; while the
content of Fe and Ca remain constant. It seems that the treat-
ment of the powder in HF has not brought about any obvious
change in its impurity level, probably because of the high purity
of the original powder.
Figures 1(a) and (b) show the particle size distribution of the
powders. The powder before treatment shows a bimodal particle
size distribution, with two peaks centered at 360 and 1150 nm,
respectively. After the treatment, the peak centered at 1150 nm is
replaced with a small peak centered at 70 nm, while the peak
centered at 360 nm was kept nearly unchanged. Figures 1(c) and
(d) shows the SEM image of the untreated and pretreated Al2O3
powders. In agreement with the result of the particle size distri-
bution measurement, a clear reduction in aggregate size is ob-
served after the powder treatment. For the untreated powder,
large aggregates up to 1–1.5 mm are frequently observed, while
almost all the aggregates in the pretreated powder remain below
0.8 mm. We believe that it is the disassociation of the aggregate
that causes the reduction in aggregate size and the narrowed
particle size distribution in the pretreated powder, as explained
below.
Density measurement indicates that the samples spark plasma
sintered at 12001–14001C are all fully densified, independent of
the powder treatment. Figure 3(a) shows the in-line transmission
of the spark plasma sintered samples from the pretreated Al2O3
powder. On the whole, the in-line transmission decreases with
an increase in the sintering temperature due to the increase in
light scattering caused by grain growth.3 Taking the in-line
transmission at the wavelength of 640 nm as an example,
When Al2O3 powder is dispersed in aqueous HF solution, the
Al2O3 particles can be etched by HF according to the following
reaction, which shows a negative Gibbs free energy change
(DG 5 ꢀ17 kJ/mol)11,12
:
A2O3ðsÞ þ 12HF
¼ 2AlF6H3ðaqÞ þ 3H2O
ðaqÞ
ðaqÞ
(b)
(a)
50
100
1000
50
100
1000
5000
5000
Particle size/ nm
Particle size/ nm
(d)
(c)
Fig. 1. Particle size distribution and scanning electron microscopy images of (a, c) untreated and (b, d) pretreated alumina powder.

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Vol. 93, No. 5
mission of 0.33 and 0.64 at a sample thickness of 1.0 and
0.8 mm, and a wavelength of 640 nm. If their sample thickness
are reduced to 0.5 mm as in the present work, the in-line trans-
mission of 0.53 and 0.71 are expected according to the equation
T2 5 (1ꢀR)[T1/(1ꢀR)]d2/d1, where T1 and T2 represent the in-line
transmission at a sample thickness of d1 and d2, and R 5 0.14, is
the reflection loss at the sample surfaces.4 We can see that, the
in-line transmission of the samples sintered at 13001–13501C
from the pretreated Al2O3 powder in the present work are ob-
viously lower than that of the samples prepared by Krell, be-
cause of their larger grain size (1.8–2.2 mm), as will be revealed
later; However, they are still nearly equivalent to that of the
sample prepared by Kim, despite the much smaller grain size of
the Kim’s sample. The reason for the latter case is not clear, and
most probably related with the difference in the sintering
processes. Because of the slow heating rate (21C/min) and
prolonged holding time (20 min) at the sintering temperature,
carbon contamination was relatively heavy in the sample
prepared by Kim, which affected its transparency. While in
comparison, carbon contamination should be much reduced in
the present work, because of the fast heating rate (1001C/min)
and short holding time (3 min) in sintering, as well as the rel-
atively coarse powder used. Coarse powder with a small specific
surface area can effectively depress the adsorption of the carbon
species that is released from the graphite mold assembly, and
thus reduces carbon contamination in the sintered sample.
XPS measurement found no carbon in the sintered samples,
probably due to the very low carbon content, which was below
the detecting limit of the instrument (100 ppm). The low
carbon contamination makes the samples in the present work
show in-line transmission that are comparable with that of the
sample prepared by Kim, although their grain sizes are much
larger than the latter.
pretreated
untreated
1.0
0.8
0.6
0.4
0.2
0.0
800
900
1000
1100
1200
1300
Sintering temperature/ °C
Fig. 2. Spark plasma sintering behaviors of the untreated and the
pretreated alumina powders.
the samples sintered at 13001, 13501, and 14001C show an in-line
transmission of 0.53, 0.51, and 0.44, respectively. These in-line
transmissions are much higher than that of the conventional
transparent Al2O3 ceramics, and are even comparable with the
in-line transmission of some of the fine-grained transparent
Al2O3 reported before.
Using a highly sinterable Al2O3 powder (TM-DAR, purity
499.99%; Taimei Chemical Company Ltd., Nagano, Japan) as
the raw material, fine-grained transparent Al2O3 ceramics with
grain sizes about 0.3 and 0.6 mm were prepared by Kim and
Krell through spark plasma sintering and hot isostatic pressing,
respectively.4,8 These ceramics showed respective in-line trans-
Figure 3(b) show the total forward transmission of the sam-
ples sintered at 13001 and 13501C, which have the best trans-
parency. In comparison with the in-line transmission, the total
forward transmissions are much higher, especially at a low wave
length range. This means that light scattering by grain bound-
aries and residual pores is still relatively significant in the sam-
ples. In fact, the gradual increase in both the total forward and
in-line transmission with the wave length distinctively indicates
the existence of light scattering within the samples. Especially
the light scattering by residual pores, it has a strong influence on
not only the in-line transmission but also the total forward
transmission as well.8
Figure 4 shows the optical images of the spark plasma sin-
tered samples from the untreated and pretreated Al2O3 powders.
Unevenness in the sample color, which is usually caused by
heavy carbon contamination during spark plasma sintering,8 is
not observed for all these samples. The samples from the pre-
treated powder are highly transparent, and the underlying im-
ages immediately or 2.5 mm below the sample can be clearly
seen, which is in good agreement with their high in-line trans-
mission. As for the sample from the untreated powder, it is
translucent at most. It is interesting that a pretreatment of the
Al2O3 powder in HF can lead to such a great improvement in
the transparency of the sintered material.
As well known, light scattering caused by the birefringent
Al2O3 grains and residual pores are the two major factors af-
fecting the transparency of high-purity Al2O3 ceramics. Except
for some special cases,2 large grain size and high porosity usually
lead to a strong light scattering and a serious deterioration of the
material transparency. Figure 5(a) shows the SEM image of the
sample that was spark plasma sintered at 13501C from the un-
treated Al2O3 powder. This sample possesses a coarse-grained
microstructure with an average grain size of about 5.2 mm. Fur-
thermore, owing to an exaggerated grain growth induced by the
large aggregates in the starting powder and the entrapment of
pores in the rapidly growing grains, large grains over 10 mm and
intragranular pores larger than 100 nm are frequently observed
in it. The resulting strong light scattering significantly reduces
the transparency of the material, making it appear translucent.
Fig. 3. (a) In-line transmission and (b) total forward transmission of
the alumina ceramics that were spark plasma sintered at a different tem-
perature from the pretreated powder. The shoulder at B350 nm on the
in-line transmission curve is a noise signal from the instrument.

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Fig. 4. Optical images of the alumina (Al2O3) ceramics that were spark
plasma sintered at (a) 12501C, (b) 13001C, and (c–e) 13501C, using (a–c)
the pretreated and (d) the untreated Al2O3 powder. All the samples are
immediately on the text except sample (e), which is 2.5 mm above the
text.
However, with respect to the samples from the pretreated pow-
der, the results are quite different.
The pretreatment of Al2O3 powder in HF not only reduces
the aggregate size but also makes the remaining aggregates more
liable to be disrupted under the mechanical and electric shock
pressure during spark plasma sintering, because of the decreased
aggregate strength caused by the powder treatment process. This
significantly suppresses the abnormal grain growth induced by
the large aggregates in the powder during sintering, leading to a
remarkable improvement in the microstructure of the sintered
materials. Figures 5(b) and (c) show the SEM images of the
samples that were spark plasma sintered at 13001 and 13501C
from the pretreated Al2O3 powder. These samples show rela-
tively fine-grained microstructures with average grain sizes of 1.8
and 2.2 mm, respectively. In comparison with the sample from
the untreated powder shown in Fig. 5(a), the average grain
sizes of these samples are reduced by more than one half, and
the grain size distribution also becomes more uniform. What’s
more important, exaggerated grain growth and residual pores
that are found in the sample from the untreated powder are
scarcely observed here, although the gradual variation of light
transmission, especially the total forward transmission with
wave length implies that some residual pores may still present
there. Such microstructure improvements effectively suppress
light scattering, and cause a remarkable increase in the material
transparency.
Fig. 5. Scanning electron microscopy images of the alumina (Al2O3)
ceramics that were spark plasma sintered at (a, c) 13501C and (b)
13001C, using (a) the untreated and (b, c) the pretreated Al2O3 powder.
The bright spots on image (a) and (c) are contaminant particles that fell
on the sample surface during the thermal etching process.
the microstructure homogeneity and the optical property of the
spark plasma sintered sample. In comparison with the sample
sintered from the untreated powder, the samples sintered from
the pretreated powder showed much more homogeneous micro-
structures with significantly reduced grain size and residual po-
rosity. As a result, the transparency of the material was greatly
improved, due to the suppression of light scattering. Highly
transparent ceramics with an in-line transmission up to 0.53 at
the visible light range were obtained, using the pretreated Al2O3
powder.
IV. Summary and Conclusions
In summary, the pretreatment of the Al2O3 powder in HF led
to a decrease in the aggregate size, and substantially improved