Fabrication of highly porous Y2O3:Ho,Yb ceramic and its thermometric applications

Article abstract of DOI:10.1016/j.jallcom.2018.01.229

Highly porous Y₂O₃:Ho,Yb ceramic with relative high compressive strength was successfully fabricated by using NaF as pore-forming agent for the first time. The porosity, pore size, compressive strength and temperature-dependent upcon

Full text of DOI:10.1016/j.jallcom.2018.01.229

Journal of Alloys and Compounds 741 (2018) 1158e1162
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Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Fabrication of highly porous Y2O3:Ho,Yb ceramic and its thermometric
applications
,
, *
Yanyan Guo a b, Dianyuan Wang c, Yong He a
a Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, 430074, China
b College of Mechanical and Materials Engineering, Jiujiang University, Jiujiang, 332005, China
c College of Science, Jiujiang University, Jiujiang, 332005, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly porous Y2O3:Ho,Yb ceramic with relative high compressive strength was successfully fabricated
by using NaF as pore-forming agent for the first time. The porosity, pore size, compressive strength and
temperature-dependent upconversion luminescence of the as-fabricated porous Y2O3:Ho,Yb ceramic
were investigated in detail. It was found that the open porosity and compressive strength of the as-
fabricated porous Y2O3:Ho,Yb ceramic were 43.12% and 28.53 MPa, respectively, and the pore size was
Received 21 November 2017
Received in revised form
15 January 2018
Accepted 16 January 2018
4e8 mm. Under 980 nm excitation, the upconversion luminescence intensity ratio (I550/I665) of the
550 nm and 665 nm emissions varied from 8.10 to 4.63 with the temperature increasing from 293 K to
873 K. Porous Y2O3:Ho,Yb ceramic with such properties could be applied to ultra-high temperature
ceramic structures for space application, luminescent probes and temperature sensors.
© 2018 Elsevier B.V. All rights reserved.
Keywords:
Porous ceramics
Y2O3:Ho,Yb
Microstructure
Compressive strength
Upconversion luminescence
1. Introduction
ions doped materials using a fluorescence intensity ratio (FIR)
technique, have drawn much attention due to their potential ap-
It is well known that Y2O3 crystalline has a high melting point
(~2430 ꢀC), high thermal conductivity (13.6 W/m,k), good chemical
durability, thermal stability, wide spectral transparency range
plications, such as temperature monitoring in highly corrosive
media, electrical power stations, oil refineries, coal mines and
building fire detection [14e20]. In this paper, we successfully
fabricated highly porous Y2O3:Ho,Yb ceramic with relative high
compressive strength by using NaF as pore-forming agent. The
porosity, pore size, compressive strength and temperature-
dependent upconversion luminescence of the as-fabricated
porous Y2O3:Ho,Yb ceramic were investigated in detail. Further-
more, the thermometry properties based on the upconversion
emissions of Ho3þ ions were investigated. The results of this study
can be beneficial to the promotion of porous Y2O3:Ho,Yb ceramics
as candidates for ultra-high temperature ceramic structures for
space application, luminescent probes and temperature sensors.
(0.23e8 m
m), and relatively low phonon energy (~377 cmꢁ1) [1,2]. A
number of published works have focused on the fabrication and
properties of Y2O3 transparent ceramics due to their potential ap-
plications, including phosphor matrices, laser hosts, and trans-
parent windows [3,4].
Porous ceramics have been of considerable interest and tech-
nical importance due to their extensive applications, including
supports for catalysts, artificial bones, filters, chemical sensors and
light-weight thermal insulation parts [5e9]. The advantages of
using porous ceramics are usually high porosity, large surface area,
low density and low thermal conductivity which can be tailored by
controlling the microstructure of pores according to specific ap-
plications [10e13]. However, we found that the studies on porous
Y2O3 ceramics are very scarce.
2. Experimental
2.1. Preparation of Y2O3:Ho,Yb nanopowders and porous
Y2O3:Ho,Yb ceramic
Moreover, in recent years the optical temperature sensors,
which are based on upconversion (UC) emissions of rare earth (RE)
Yttrium oxide (Y2O3, 99.99%), erbium oxide (Ho2O3, 99.95%), and
ytterbium oxide (Yb2O3, 99.95%) were purchased from Grirem
Advanced Materials Co., Ltd (Beijing, China). Nitric acid (HNO3,
* Corresponding author.
E-mail address: yonghe246@126.com (Y. He).
https://doi.org/10.1016/j.jallcom.2018.01.229
0925-8388/© 2018 Elsevier B.V. All rights reserved.

Y. Guo et al. / Journal of Alloys and Compounds 741 (2018) 1158e1162
1159
65e68%, GR), glycine (AR, 99.5e100.5%), sodium fluoride (NaF, GR,
ꢂ99.0%) and ethanol (AR, ꢂ99.7%) were purchased from Sinopharm
Chemical Reagent Co, Ltd (Shanghai, China). All the chemicals were
used as received without further purification.
Y2O3:0.5%Ho,2%Yb nanopowders were prepared by low tem-
perature combustion method. RE2O3 (RE ¼ Y, Ho, Yb) were dis-
solved in dilute HNO3 under heating to prepare the RE(NO3)3 stock
solution. In a typical synthesis, 39 mL of 0.3 mol/L Y(NO3)3, 3 mL of
0.02 mol/L Ho(NO3)3, 12 mL of 0.02 mol/L Yb(NO3)3 aqueous solu-
tions and 0.9 g glycine were mixed and underwent boiling to
dehydration. The mixture then ignited and the combustion process
lasted only a few seconds. The voluminous and foamy product was
calcined at 700 ꢀC for 1 h in air, and then Y2O3:Ho,Yb nanopowders
were obtained.
The as-prepared Y2O3:Ho,Yb nanopowders and the pore-
forming agent NaF with a mass ratio 9:1 were mixed via ball
milling at room temperature. The obtained slurry was hot-press
sintered at 1000 ꢀC for 2 h at a pressure of 80 MPa under a vac-
uum of about 10ꢁ2 Pa. After the hot pressing, the Y2O3:Ho,Yb
sample was annealed at 1300 ꢀC for 5 h in air. Therefore, the porous
Y2O3:Ho,Yb ceramic sample was obtained.
Fig. 1. XRD patterns of the Y2O3:Ho,Yb nanopowders and the porous Y2O3:Ho,Yb
ceramic.
2.2. Characterization
The as-prepared Y2O3:Ho,Yb nanopowders and the sintered
Y2O3:Ho,Yb ceramic sample were characterized by an X-ray
diffractometer (SHIMADZU XD-D1, Rigaku, Japan) with Cu Ka-ra-
diation (
l
¼ 0.15418 nm) in the range of 2
q
¼ 20e90ꢀ. The powders
morphology, microstructure and EDS spectrum of the ceramic were
observed by scanning electron microscopy (VEGA II LSU, Tescan,
Czech Republic). The shape and particle size of the Y2O3:Ho,Yb
nanopowders were determined by high-resolution transmission
electron microscopy (HRTEM, FEI Tecnai G2 F20, FEI). The bulk
density of the ceramic sample was calculated from the mass and
dimension of samples. Open porosity of the sintered ceramic
sample was determined by the water-immersion technique using
the Archimedes method. The compressive strength was measured
by a universal testing machine (XWW, Beijing Shengxin detecting
instrument, China) with acrosshead speed of 0.5 mm/min. The
ceramic sample was cylindrical, 12 mm in diameter and 10 mm in
height. The upconversion luminescence spectra were measured
and analyzed by a fluorescence spectrometer (F-4500, Hitachi,
Japan) under 980 nm laser diode (LD) (Newport, USA) excitation.
Fig. 2. EDS spectrum of the porous Y2O3:Ho,Yb ceramic.
diameter of the particles is about 30 nm, and the shape is approx-
imately spherical. The porosity and compressive strength of the
fabricated sample were characterized to further study the micro-
structure and mechanical property. The bulk density and open
porosity of the as-fabricated porous Y2O3:Ho,Yb ceramic are
determined to be 2.85 g/cm3 and 43.12%, respectively. Moreover,
the total porosity calculated from the relative density was almost
the same as that measured using the Archimedes method, indi-
cating that most of the pores in the sample were open. Fig. 3(c)
shows the typical microstructure of the porous Y2O3:Ho,Yb ceramic
sample. It can be found that the pores in the ceramic sample are
3. Results and discussion
The XRD patterns of the as-prepared Y2O3:Ho,Yb nanopowders
and the porous Y2O3:Ho,Yb ceramic are presented in Fig. 1. It can be
found that the patterns are consistent with the cubic Y2O3 phase
(JCPDS 25-1200), and no NaF phase was detected in the ceramic
sample. These results indicate that NaF does not participate in the
reaction, and most of the pore-forming agent NaF has volatilized
during the sintering process in air atmosphere.
Fig. 2 shows the EDS spectrum of the porous Y2O3:Ho,Yb
ceramic. The automated qualitative elemental analysis of the EDS
spectrum reveals that elements present in the porous Y2O3:Ho,Yb
ceramic sample are Y, O, Ho, Yb, Na and F only, the atomic% of Y, O,
Ho, Yb, Na and F elements were detected as 32.36%, 64.65%, 0.54%,
2.01%, 0.06% and 0.38%, respectively. This result confirms the
presence of Ho and Yb elements in the porous Y2O3 ceramic sample.
Furthermore, only trace Na and F elements were detected by EDS,
which is consistent with the result in XRD pattern of the porous
ceramic sample mentioned above.
abundant and the pore size is 4e8 mm, which indicates that NaF is
good pore-forming agent. Highly porous ceramic fabricated by
conventional methods often contain defects, such as cracking and
surface flaws [21]. These defects in the cell structure would
significantly reduce the strength of porous ceramic. However, there
were no noticeable macroscopic defects, such as cracking or
distortion in the as-fabricated porous Y2O3:Ho,Yb ceramic sample,
which indicates that the sintering process in air atmosphere did not
cause any destruction of the pore structure. And the compressive
strength of the as-fabricated porous ceramic sample is relatively
high to be 28.53 MPa. It is because that the vacuum hot-pressing
sintering method was employed in the fabrication of the porous
Y2O3:Ho,Yb ceramic.
Fig. 4 shows the temperature-dependent upconversion lumi-
nescence spectra of the porous Y2O3:Ho,Yb ceramic under 980 nm
Fig. 3(a), (b) show the SEM and TEM morphology of the as-
prepared Y2O3:Ho,Yb nanopowders. It can be clearly seen that the

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Y. Guo et al. / Journal of Alloys and Compounds 741 (2018) 1158e1162
Fig. 3. SEM image (a) and (b) TEM image of the Y2O3:Ho,Yb nanopowders, (c) SEM image of the fracture plane of the porous Y2O3:Ho,Yb ceramic.
Fig. 5. Pump power dependence of the upconversion emission intensity of the porous
Y2O3:Ho,Yb ceramic (under the 980 nm excitation).
Fig. 4. Temperature-dependent upconversion luminescence spectra of the porous
Y2O3:Ho,Yb ceramic under 980 nm excitation.
excitation. Very intense green emission with peak at 550 nm, and
considerably intense red emission at 665 nm were detected. The
transitions of Ho3þ corresponding to the emission peaks were also
presented in Fig. 4 [22]. In upconversion mechanism, the emission
intensity Iup is proportional to the nth power of the pump power as
I
upfPn, where n is the number of NIR photons required to absorb
for emitting one visible photon. A plot of logarithm Iup versus
logarithm P yields a straight line with slope n [23], as shown in
Fig. 5. The values of n for green 550 nm and red 665 nm emissions
are near 2, indicating that two-photon process was involved in
green and red upconversion emission mechanisms. Combined the
slope n and energy diagrams of Ho3þ and Yb3þ ions, the proposed
mechanism for upconversion emission are demonstrated in Fig. 6.
About the upconversion of green and red emission, it was explained
as follows [24,25]: the energy transfer (ET) from Yb3þ could excite
Ho3þ from the ground state 5I8 to 5I6, and finally to 5F4, 5S2 and 5F5
levels. The transitions from 5F4, 5S2 to 5I8 state result in 550 nm
emission. The electrons of Ho3þ in the 5F4, 5S2 levels may also relax
nonradiatively to 5F5 state. The transition from 5F5 to 5I8 state re-
sults in 665 nm emission.
Fig. 6. The proposed upconversion mechanism in porous Y2O3:Ho,Yb ceramic under
980 nm excitation.
range of 293e873 K. The pumping power of 980 nm diode laser was
set at 670 mW. As shown in Fig. 4, the emission intensities of green
and red light are all sensitive to temperature in this system. It can
be observed that the luminescence intensity decreased gradually
Furthermore, to investigate the influence of temperature on
photoluminescence properties, the upconversion luminescence
spectra of the porous Y2O3:Ho,Yb ceramic were measured in the
spectral range of 520e680 nm as a function of temperature in the

Y. Guo et al. / Journal of Alloys and Compounds 741 (2018) 1158e1162
1161
with temperature due to the increased non-radiative relaxation of
electrons between rare earth energy levels. In addition, Fig. 7(a)
shows the plot of FIR of the 550 nm and 665 nm UC emissions
against the absolute temperature. The ratio I550/I665 decreased from
8.10 to 4.63 with the temperature increasing from 293 K to 873 K.
This is due to the fact that possibilities of non-radiative transition of
5F4,5S2/5F5 becomes stronger as the temperature increases by the
assistance of phonons of the matrix. Fig. 7(b) shows the plot of
ln(I550/I665) as a function of inverse absolute temperature, which
presents a straight line with a slope 251.77 K. It means that the
Boltzmann's law dominates the temperature dependence of the
luminescence.
Moreover, the relative sensitivity S is a very important param-
eter to evaluate the performance of the sensor, which can be
described by the following formulum:
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
1 dR
R dT
S ¼
;
(1)
Fig. 8. Relative sensitivity of the porous Y2O3:Ho,Yb ceramic at various temperature
calculated by Equation (1).
where R is the luminescence intensity ratio. This equation means
the relative change of emission intensity with respect to temper-
ature variation. The calculated result is described in Fig. 8. As
shown, the relative sensitivity reached the maximum value of
0.0038/K at 293 K. This temperature sensitivity can be comparable
that of Yb3þeEr3þ doped Gd2O3 nanophosphor (0.0039/K) and Er3þ
doped Sr2Bi4Ti5O18 (0.0042/K) [26,27]. Hence, the porous
Y2O3:Ho,Yb ceramic possesses good temperature sensitivity, and
can be used as a potential temperature sensor material based on its
upconversion luminescence.
4. Conclusion
In summary, highly porous Y2O3:Ho,Yb ceramic with relative
high compressive strength was successfully fabricated by using NaF
as pore-forming agent. The open porosity and compressive strength
of the as-fabricated porous Y2O3:Ho,Yb ceramic were 43.12% and
28.53 MPa, respectively, and the pore size was 4e8 mm. Under
980 nm excitation, the upconversion luminescence intensity ratio
(I550/I665) of the 550 nm and 665 nm emissions varied from 8.10 to
4.63 with the temperature increasing from 293 K to 873 K. It can be
concluded that the as-fabricated porous Y2O3:Ho,Yb ceramic had
high porosity and compressive strength, good temperature sensi-
tivity, indicating its applications in the ultra-high temperature
ceramic structures for space application, luminescent probes and
temperature sensors.
Acknowledgements
The work was supported by the NSFC (Grant No. 51062008,
21463014), Key Project of Chinese Ministry of Education (Grant No.
212095), and Foundation of Jiangxi Education Commission (Grant
No. GJJ11618).
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