Highly infrared transparent nanometric tetragonal zirconia prepared by high-pressure spark plasma sintering

Article abstract of DOI:10.1111/j.1551-2916.2011.04680.x

Highly infrared transparent tetragonal ZrO₂ (3 mol% yttria-stabilized) was prepared by means of high-pressure spark plasma sintering (HP-SPS). The crystallite size of the resulting dense sample was about 80 nm. In-line transmittance for the 1.5 mm thick sample ranged between 63% and 67% in the wavelength of 3-5 μm, representing 81%-87% of the theoretic value. Meanwhile, the cutoff wavelength of tetragonal ZrO₂ was superior to the values of other high-strength transparent ceramics. This study highlighted the efficiency and simplicity of HP-SPS for obtaining transparent tetragonal ZrO₂ and the great potential of this ceramic applied for durable infrared windows.

Full text of DOI:10.1111/j.1551-2916.2011.04680.x

J. Am. Ceram. Soc., 94 [9] 2739–2741 (2011)
DOI: 10.1111/j.1551-2916.2011.04680.x
©
2011 The American Ceramic Society
ournal
J
Highly Infrared Transparent Nanometric Tetragonal Zirconia Prepared by
High-Pressure Spark Plasma Sintering
‡
Haibin Zhang, Zhipeng Li, Byung-Nam Kim, * Koji Morita, *
,†
§
‡,
‡,
‡
,
‡,
‡,
Hidehiro Yoshida, * Keijiro Hiraga, * and Yoshio Sakka *
‡
Advanced Materials Processing Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan
§
Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), National Institute for
Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan
5
Highly infrared transparent tetragonal ZrO
2
(3 mol% yttria-
recently, Krell et al. point out that nanometric microstruc-
ture is required for TZP to achieve good light transmittance
because of its high intrinsic birefringence (one order of mag-
nitude higher than that of alumina). The proposed scattering
stabilized) was prepared by means of high-pressure spark
plasma sintering (HP-SPS). The crystallite size of the resulting
dense sample was about 80 nm. In-line transmittance for the
5
1
.5 mm thick sample ranged between 63% and 67% in the
wavelength of 3–5 lm, representing 81%–87% of the theoretic
value. Meanwhile, the cutoff wavelength of tetragonal ZrO
model predicts that the in-line transmittance of 50% at the
visible wavelength range is expected at the grain size <40 nm
for a pore-free TZP with the thickness of 1 mm. This
explains the previous results of the low transmittance of TZP
2
was superior to the values of other high-strength transparent
ceramics. This study highlighted the efficiency and simplicity of
2
–5
in the visible wavelength,
since such fine grains are not
HP-SPS for obtaining transparent tetragonal ZrO
great potential of this ceramic applied for durable infrared
windows.
2
and the
attained. On the other hand, this model reveals the weak
influence of birefringent scattering in the infrared (IR) range
as grain sizes are lesser than 200 nm, which is verified by the
experimental result. The combination of pre-sintering and
HIP, together with the optimized powder processing, allows
5
I. Introduction
fabrication of nanometric (115 nm) and almost fully dense
tetragonal ZrO . The resulting specimen exhibits the maxi-
5
T
is well known that tetragonal zirconia polycrystals
(TZP) have excellent strength, toughness, wear resistance,
2
I
mum theoretical value of in-line transmittance (77%) in the
IR region of 4–5 lm, implying the great potential of tetrago-
nal ZrO applied for durable IR windows.
2
In this communication, we present the idea that, compared
to the method of pre-sintering plus HIP, high-pressure spark
plasma sintering (HP-SPS) is an extremely simple and effective
chemical resistance, and biocompatibility. The combination
of these salient properties makes TZP an excellent candidate
for various structural and biomedical applications, such as
femoral ball heads, dental restoratives, extrusion dies, seal
1
faces and in milling media.
Transparent tetragonal ZrO
2
will be desirable for optical
2
approach to obtain highly IR transparent tetragonal ZrO .
applications owing to the outstanding mechanical properties.
Nevertheless, as a hard-to-sinter ceramic, polycrystalline TZP
samples usually are opaque even after high-temperature
sintering because the existence of residual pores considerably
scatters incident light and deteriorates the transparency.
Hence polycrystalline TZP attracts little attention in optical
applications.
II. Experimental Procedure
(1) Sample Preparation
The starting materials were commercial 3 mol% Y O -doped
ZrO powders (TZ-3Y; Tosoh Corporation, Tokyo, Japan).
2
The commercial powders were consolidated directly, without
any treatment or additives, using a spark plasma sintering
machine (SPS-1050; SPS Syntex Inc., Kawasaki, Japan) in
2
3
If porosity is controlled to an extremely low level, it is pos-
sible to obtain transparent polycrystalline TZP, for example,
2
using hot pressing, spark plasma sintering (SPS)
3–4
À3
6–7
and hot
isotropic pressing (HIP). However, all these results show the
low transmittance of TZP in the visible wavelength region.
vacuum (10 torr). The applied pressure was 400 MPa
5
and the heating rate was 50°C/min. The optimum sintering
temperature and duration time was 1050°C and 10 min,
respectively. The as-sintered sample disk had a diameter of
10 mm and a thickness of about 1.6 mm. The annealing
treatment was conducted at 900°C for 2 h in air.
2–5
For noncubic ceramics, the optical anisotropy causes dif-
fuse scattering due to birefringence at grain boundaries.
Thus, the transparency of pore-free noncubic ceramics
depends on the degree of birefringence and grain size. More
(2) X-Ray Diffraction
The as-synthesized sample was analyzed using X-ray diffrac-
tion in a D/max-rA diffractometer (Rigaku, Tokyo, Japan)
with CuKa radiation.
J. Blendell—contributing editor
Manuscript No. 29432. Received March 10, 2011; approved May 11, 2011.
*
Member, American Ceramic Society
(
3) Measurement of In-Line Transmittance
This work was supported by World Premier International Research Center Initia-
tive, MEXT, Japan. This work was partly supported by the Grant-in Aid for Scientific
Research (C-22560675) from the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
The in-line transmission of the polished specimen disk with a
thickness of 1.5 mm was measured using a spectrophotometer
(SolidSpec-3700DUV; Shimazu Corporation, Kyoto, Japan)
†
Author to whom correspondence should be addressed. e-mail: zhang.haibin@nims.
go.jp
2739

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Rapid Communications of the American Ceramic Society
Vol. 94, No. 9
3
–7
for the visible range and a FTIR spectrometer (Nicolet 6700;
Thermo Electron Corporation, Madison, WI, USA) for the
infrared range. A small aperture of 0.31° used in the Shimazu
oxygen back. Therefore, the discoloration obviously weak-
ens and the appearance alters to lighter white upon annealing
(not shown here). But some color centers still remain, and
the annealing extending even for 100 h at 900°C cannot fur-
ther reduce the color centers (not shown here). The retention
of color centers in tetragonal zirconia after annealing treat-
spectrophotometer allows the real in-line transmittance
(
8
RIT) to be measured. Calibrating using the RIT in the over-
lapping 1.25–1.6 lm wavelength of two equipments, the mea-
sured transmittance of the FTIR spectrometer was 5% higher.
Thus, the data of the FTIR spectrometer was multiplied
5
ment is also observed by Krell et al.
Figure 2 exhibits the typical in-line transmittance spectra.
The in-line transmittance is considerably increased by anneal-
ing due to the reduction of color centers. In addition, anneal-
ing in air induces the presence of an absorption peak at
4.2 lm, which very likely originates from the incorporation
9
5.2% to eliminate the influence of scattering light.
(
4) Microstructure Observation
For transmission electron microscope (TEM) observation, a
thin sheet with a thickness of 500 lm was cut by a low speed
diamond cutter, mechanically polished to a thickness of
2
of CO . Krell et al. also observed a similar absorption peak
5
in their annealed TZP samples. For the sample with 1.5 mm
thickness, the in-line transmittance in the 3–5 lm IR region
ranges between 63%–67%. Due to the reflective losses of the
end surfaces, the maximum theoretical transmittance for TZP
is about 77% in the IR region for the refractive index of
1
00 lm, followed by further thinning with an Ar ion-milling
machine. The TEM observations were performed using
a JEOL-2010 F (JEOL Ltd., Akishima, Tokyo, Japan)
operated at 200 kV.
1
2
2.10. Thus, the reported in-line transmittance in this work
corresponds to 81%–87% of the theoretical value. Obviously,
this value is increased with the decrease of sample thickness
because of birefringence. Taking account of the equation of
The grain size was measured using the TEM by obtaining
the average cross-section area per grain and by assuming
spherical grains. The measured grain size was an apparent
one, so it was multiplied by 1.225 to determine the true grain
8
thickness effect, the transmittance measured in this work is
comparable to the extremely high transmittance (77%) using
6
,7
size. The mean grain size was the average value of 5 TEM
images.
5
a 0.5 mm-thick sample. Another important factor for IR
applications is the long wave cutoff. The longer the cutoff
wavelength, the more useful is the window at elevated temper-
III. Results and Discussions
12
2
atures. The cutoff wavelength of tetragonal ZrO is superior
1
3
Figure 1 shows the XRD profile of the annealed zirconia
bulk sample. The resultant zirconia is identified as the tetrag-
onal modification. The XRD profile is enlarged in the 2h
range of 72–76° because this region can visually distinguish
the reflection peaks of cubic from tetragonal phases. It can
be seen that (004) and (220) reflection peaks are assigned to
the tetragonal phase, which excludes the existence of cubic
to the values of other high-strength transparent ceramics,
such as alumina, MgAl O spinel, and AlON. These results
highlight the great potential of TZP applied for IR windows.
2
4
9
ZrO
(
2
. The as-sintered sample displays a dark appearance
not shown here), which is typically related to the existence
3–7
of color centers (oxygen vacancies with trapped electrons).
It is well known that thermal or electro-reduction can easily
cause the formation of color centers in zirconia.
1
0–11
Both
the reduction conditions are readily met in the SPS process:
high-temperature vacuum and graphite dies yield thermal
reduction, and the applied current produces electro-reduc-
tion. As a result, color centers are formed in the as-sintered
TZP. The negative influence of color centers is related to the
3–7,10–11
absorption of incident light. Annealing in oxidizing
atmosphere can reduce these defects through diffusing
Fig. 2. Typical in-line transmittance spectra of the as-sintered and
annealed tetragonal ZrO . The sample thickness was 1.5 mm.
2
Fig. 1. XRD profiles of zirconia bulk sample prepared by high-
pressure spark plasma sintering at 1050°C for 10 min under the
pressure of 400 MPa and annealed at 900°C for 2 h in air.
Fig. 3. Conventional bright-field transmission electron microscopic
images of the annealed tetragonal ZrO
2
.

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8
3%–67% in the wavelength of 3–5 lm, corresponding to
1%–87% of the maximum allowable value. Meanwhile, the
intrinsic cutoff wavelength of tetragonal ZrO
to the values of other high-strength transparent ceramics,
such as Al , MgAl spinel, and AlON. The dark colora-
2
was superior
2
O
3
2 4
O
tion of the HP-SPSed zirconia was attributed to the presence
of color centers, which could be reduced by annealing treat-
ment at 900°C for 2 h. This work revealed the efficiency and
simplicity of HP-SPS for obtaining transparent tetragonal
zirconia, and the great potential of this ceramic for durable
infrared windows.
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8
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h