Malaysian Journal of Analytical Sciences, Vol 23 No 4 (2019): 682 - 689
DOI: https://doi.org/10.17576/mjas-2019-2304-15

MALAYSIAN JOURNAL OF ANALYTICAL SCIENCES
Published by The Malaysian Analytical Sciences Society

ISSN
1394 - 2506

PRELIMINARY INVESTIGATION OF LOW CURRENT FLASH SINTERINGIN ZIRCONIA NANOPARTICLE COMPACT(Kajian Awal Persinteran Kilat Arus Rendah dalam Nanopartikal Zirkonia Kompak)
Michiyuki Yoshida1*, Mitsuki Hada1, Yutaka Shinoda2, Osamu Sakurada1, Fumihiro Wakai2
1

Department of Chemistry and Biomolecular Science,
Gifu University, 1-1 Yanagido, Gifu-city 501-1193, Japan
2
Laboratory for Materials and Structures,Tokyo Institute of Technology, 4259 Nagatsuta Midori-ku Yokohama-city,226-8503, Japan*Corresponding author: myoshida@gifu-u.ac.jp

Received: 31 March 2018; Accepted: 17 April 2019

Abstract
Flash sintering is a newly developed technique to quickly densify conductive ceramics in the presence of an electric field. At acritical combination of field and temperature, densification takes place in a few seconds. In this paper, the feasibility of thedensification in flash sintering with low current was investigated in the newly developed compact consisted of zirconia
nanoparticles. Homogeneous microstructure with densely packing of nano-particles was successfully obtained by gel-casting of
nano-suspension. The grain size and relative density of the sample flash sintered at 30 mA/mm 2 were 40.9 nm and 86.8%,
respectively. The sample densified with limited grain growth in low current flash sintering. Although the full densification was
not achieved in the present study, the effectiveness of the nanoparticle compact to reduce the current for the densification during
flash sintering was shown by comparing the results reported in flash sintering with the coarser microstructure.
Keywords: zirconia, flash sintering, gel-casting
AbstrakPensinteran kilat merupakan satu teknik yang baru dibangunkan untuk mempercepatkan pemampatan seramik konduktif dalamkehadiran medan elektrik. Pada kombinasi kritikal antara medan dan suhu, densifikasi berlaku dalam beberapa saat. Dalam
manuskrip ini, keberhasilan densifikasi dalam pensinteran kilat pada arus rendah telah dikaji dalam kompak yang baru
dibangunkan terdiri daripada zirkonia nanopartikel. Struktur mikro homogen dengan pengisian padat partikel nano telah berjaya
dihasilkan menggunakan acuan gel ampaian nano. Saiz butir dan ketumpatan relatif sampel bersinter pada 30 mA/mm 2 masingmasing adalah 40.9 nm dan 86.8%. Sampel telah dimampatkan dengan pertumbuhan butiran yang terhad di dalam arus
pensinteran kilat yang rendah. Walaupun pemampatan penuh tidak dapat dicapai di dalam kajian ini, namun keberkesanan
kompak nanopartikel untuk merendahkan aliran arus pemampatan ketika pensinteran kilat telah dapat dilihat dengan
membandingkan hasil laporan pensinteran kilat ketika struktur mikro yang lebih besar dijalankan.
Kata kunci: zirkonia, pensinteran kilat, acuan gel

Introduction
Sintering refers to the process of firing and consolidating a body shaped from powder particle, and is originatedfrom the fabrication of the pottery. During the sintering process, inter-particle pores in the green body are
eliminated by atomic diffusion driven by capillary forces. A variety of sintering techniques, such as normal
sintering, hot isostatic or uniaxial pressing has been applied in the production of ceramic materials. During the last682

Yoshida et al: PRELIMINARY INVESTIGATION OF LOW CURRENT FLASH SINTERING IN ZIRCONIA
NANOPARTICLE COMPACT

10–15 years, field assisted sintering (FAST) and microwave sintering have grown as a consequence of advantages
including higher heating, cooling rates and lower sintering temperatures, compared with the traditional techniques.
Quite recently, “Flash Sintering”, as an extremely fast sintering method, has attracted much interest since it was firstreported by Cologna et al. in 2010 [1]. In their original report, a DC electric field is applied to a sample (3 mol%
Y2O3-stabilized ZrO2) while it is heated in a conventional furnace and at a certain furnace temperature thedensification takes place in a few seconds. Flash sintering has now been demonstrated with a wide range of different
ceramics with submicron or micrometer scale microstructure [2-9].
One of the confirmed obstacles to achieve the industrialization of flash sintering is the homogeneous heating of thesample. In flash sintering, the final specimen temperature achieved, which can be expected to determine the extent
of sintering, is controlled primarily by the limiting current in the power supply. However, too high a limiting current
gives more scope for temperature gradients and catastrophic localization of the current[10]. For practical applicationof flash sintering, therefore, more sophisticated powder technology, which allow low currents to obtain thehomogeneous heating of the sample. It is well known that the sinterability is expected to be improved for applying
nanoparticles as a starting material. The green body consisted with nano-particles is expected to be densified byflash sintering with the homogeneous heating in low current.
Recently, our group successfully fabricated the densely packed green body by gel-casting of zirconia nanosuspension [11]. The initial compact prepared by the gel-casting in our study had the densely packed structure witha narrow pore size distribution. Almost fully dense sample with 99.5% of theoretical density was successfully
obtained by the pressureless sintering at 1100 °C, which is 300 °C lower than that prepared by conventional process.
In this work, the feasibility of the densification in flash sintering with low current was investigated in the newly
developed zirconia nanoparticle compact.
Materials and Methods
In this study, the green body consisting of 3mol% yttria stabilized zirconia (3YSZ) nanoparticle was prepared bygel-casting. 3YSZ nano-suspension with the average particle size of 16 nm (MEL Chemicals Ltd., U.K.) was usedas the starting material. The solids loading of the starting suspension measured by gravimetric analysis was 7.0
vol%. The as-received suspension with weak acidic pH of 2.7 was modified by adding tetra methyl ammoniumhydroxide (Sigma-Aldrich, U.S.A) to pH 9.5 for the complete dissociation of citrate. Subsequently, 4.5 wt.% oftriammonium citrate (99% purity, Kanto Chemical Co. Inc., Japan) was added as the dispersant. 0.6 wt.% of agarose(LM, Kanto Chemical Co. Inc., Japan) was added to the prepared suspension as the gelling agent. The zirconia
suspension was heated before adding agarose and kept at a temperature of 80 °C in a closed vessel. The preparedsuspension was casted to a plastic mold. After one week drying, the green body was calcined at the temperature of
650 °C for decomposing organic additions. The calcined sample with cylindrical shape was machined to have adimension of φ4.5 mm×3.8mm for flash sintering.
The sample was sintered in a vertical split furnace fixed on a universal testing machine (AG-I, Shimadzu Co.,
Kyoto, Japan). The sample was initially heated in the furnace at a rate of 20 °C /min to 500 °C and then at 10 °C/min. A schematic illustration of the experimental setup for flash sintering is shown in Figure 1. Platinum electrodes
on top and bottom of the sample ensured the application of the electrical field. These electrodes were maintained
between alumina disks and specimen under a constant load of 10 N (~0.6 MPa). The constant electric field, E = 60
V cm−1 was applied by a commercial DC power source (ZX-400HN, Takasago Ltd, Kawasaki, Japan) from the startof heating. The current limit, at which the power supply switched from voltage control to current control, was set to
30 mA/mm2, which was 4 times lower than the reported value of 120mA/mm2 to reach the full densification in flash
sintering with submicron microstructure [12]. The voltage across the Platinum electrodes and the current passing
through the sample were measured using digital multimeters. After switching form voltage control to current
control, the electric field was loaded on the sample for 10sec, and then the furnace was switched off to cool it down.
Relative density of the flash sintered sample was calculated using Archimedes’ method by taking 6.10 g/cm 3 as thetheoretical density of tetragonal zirconia. Microstructure of the sample was observed from the fractured surfaces.
The samples were coated with osmium and observed by scanning electron microscope (Model S4800, Hitachi Co.,
Japan). The equivalent circle diameter measured by the image analysis of SEM photographs was defined as thegrain size.

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Malaysian Journal of Analytical Sciences, Vol 23 No 4 (2019): 682 - 689
DOI: https://doi.org/10.17576/mjas-2019-2304-15

Power supply
SiC RodSpecimen

Inslation(Alumina)

HeaterVoltage

Pt plate

Pt wireCurrent
Digital Multimeter

Figure 1. Schematic illustration of the experimental set up for flash sintering

Results and Discussion
Initial compact
The photograph of the gel-casted sample after drying is shown in Figure 2. After complete drying, no micro
cracking and uniform shrinkage in a radial direction were observed. Figure 3 shows the FE-SEM micrograph for thefractured surface of the sample calcined at 650 °C. The grain size and relative density for the calcined sample were
28 nm and 52%, respectively. The calcined compact had a very narrow pore size distribution, and the average poresize was 15 nm. Homogeneous microstructure with densely packing of nano-particles was successfully obtained byslip-casting of nano-suspension.

Figure 2. Photograph of the gel-casted sample

Figure 3. Fractured surface for the gel-casted
sample calcined at 650 °C

General features of flash sintering
Current, voltage, resistance and power versus furnace temperature graphs during flash sintering are shown in
Figure 4. The current rises slowly at first and then more rapidly until it reaches a pre-programmed limit (Figure4(a)). The rapid increase in current at constant voltage shows that the specimen resistance was decreasing
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Yoshida et al: PRELIMINARY INVESTIGATION OF LOW CURRENT FLASH SINTERING IN ZIRCONIA
NANOPARTICLE COMPACT

throughout the process, as shown in Figure 4(c) so that the switch to current control results in a power spike (Figure4(d)). These features demonstrate that the original results reported in the literature were successfully reproduced inthis study. On the other hand, the observed width of the power spike in Figure 4(d) was about 150 sec, which wasmuch longer than that reported in the literature (several seconds).
0.5

50

(b)

(a)
40

Voltage / V

Current / A

0.4

0.3

0.2

0.1

30

20

10

0

0
500

600

700

800

900

1000

500

600

Furnance Temperature /℃
400

800

900

1000

20

(d)

(c)
300

15

Power / W

Resistance / kΩ

700

Furnance Temperature /℃

200

100

10

5

0

0
500

600

700

800

900

Furnance Temperature /℃

1000

500

600

700

800

900

1000

Furnance Temperature /℃

Figure 4. Current, voltage, resistance and power versus furnace temperature graphs during flash sintering
In the previous works, the connection between the electrodes and the specimen was made by inserting the electrodes
through the holes in the specimens. This resulted in the improvement of the contact between the Pt and the specimen
due to local melting/sintering around the contacts [13]. On the other hand, in this study, the contact between theelectrodes and the sample was maintained by holding with SiC rods under a constant load of 10 N as shown in
Figure 1. In this case, the rapid densification during flash sintering resulted in the poor contact between theelectrode and the sample. This might cause an inhibiting effect on the runaway Joule heating and widen the width ofthe power spike in this study.
Specimen temperature and densification
The specimen temperature can rise considerably above the furnace temperature from Joule heating. We obtained ananalytical estimate on the assumption of a uniform sample temperature, TS, from the black body radiation model
based on the Stefan-Boltzmann law [14]. According to this model, the sample temperature at steady state can be
expressed as follows:
𝑇𝑠 = (𝑇04 +
685

𝑊
𝐴𝜀𝜎𝑠

1/4

)

(1)

Malaysian Journal of Analytical Sciences, Vol 23 No 4 (2019): 682 - 689
DOI: https://doi.org/10.17576/mjas-2019-2304-15

where T0 is the furnace temperature, W is the electrical power dissipated, A is the instantaneous surface area, ε is theemissivity and σs is Stefan–Boltzmann constant. The height of the specimens deduced from the displacement wasused for calculating A in this case; it was assumed that the cross section of the specimen was constant during thetest. The choice of emissivity is particularly problematic as it can be expected to vary with density during sintering
as well as with surface roughness, wavelength and temperature. In this study, a value of 0.7 was chosen, based on
the measurements of Tanaka et al. [15] on sintered zirconia. The relationship between the furnace temperature andthe specimen temperatures calculated from eq. (1) are given in Figure 5(a). The dotted line in Figure 5(a) indicatesthe specimen temperature for no Joule heating, in which the specimen temperature is exactly same as the furnace
temperature. The specimen temperature started to increase with Joule heating at around the furnace temperature of
600 °C, and then rapidly increased to reach 1270 °C at the furnace temperature of 880 °C. The displacement versus
furnace temperature is shown in Figure 5(b), where the crosshead displacement was corrected with subtracting thethermal expansion of the SiC rods. The shrinkage of the specimen started to increase at the furnace temperature of
700 °C. The rapid densification was observed at the furnace temperature of 850 °C and was coincided with the rapid
Joule heating of the sample apparently from Figure 5(a).

(a)

(b)

Figure 5. Sample temperature and displacement vs furnace temperature graphs during flash sintering
Microstructure
The micrograph for the fractured surface of the sample after flash sintering is shown in Figure 6. The grain size andrelative density for the flash sintered sample were 40.9nm and 86.8%, respectively. In the case of the sample withsubmicron microstructure, the almost same density as in this study was obtained under the current of 60 mA/mm 2,
which was twice as high as in this study [12]. This indicates that the nanoparticle compact reduces the current forthe densification during flash sintering.
Average grain size is plotted as a function of density in Figure 7. For comparison, the results of normal sintering ofgel casted sample [11], in which the samples were heated at the sintering temperatures for 2 hours, were also plottedin Figure 7. In normal sintering, the grain size increased with increasing the density. The hindered grain growth up
to around 90% was shown in Figure 7. Subsequently, the exaggerated grain growth was observed at a density higher
than 90% in which the relationship between grain size and relative density was parabolic. The transition frominsignificant grain growth to rapid grain growth at around 90% density is believed to change from open to closed
porosity, and this is the typical sintering behavior reported in the literature [16]. As shown in Figure 7, the result of
flash sintered sample is in the region under a curve corresponding to the results of the normal sintering. This
indicates that zirconia nanoparticle compact densified with limited grain growth in low current flash sintering.

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Yoshida et al: PRELIMINARY INVESTIGATION OF LOW CURRENT FLASH SINTERING IN ZIRCONIA
NANOPARTICLE COMPACT

Figure 6. Fractured surface for the flash sintered sample at 30mA/mm2

140
Normal Sintering
Flash sintering

Grain Size / nm

120
100
80
60
40
20
0
50

60

70

80

90

100

Relative Density / %Figure 7.

The relationship between relative density and grain size of the flash sintered sample. Results fornormal sintered samples are plotted for comparison

The mechanisms responsible for flash sintering remain open to question [17, 18]. Recently, some researchers haveshown that the power surge can be explained satisfactorily as thermal runaway resulting from the negative
temperature coefficient of resistivity of most ceramics [10, 19, 20] whilst comparison with the sintering of powder
compacts rapidly heated without an electric field suggests that the high densification rate may be at least in part a
consequence of the fast heating in flash sintering [21]. Maximum heating rate observed in this study was 360◦C/min, which is 36 times higher than set up heating rate for the furnace, and comparable to that reported in theliterature [21].
Although the full densification was not achieved, zirconia nanoparticle compact, obtained from gel-casting of nanosuspension, densified with the limited grain growth in low current flash sintering, and reduced the current for thedensification during flash sintering. The optimization of the condition for the electrical loading, such as voltage andcurrent set up in the power supply, is necessary to achieve the full densification of the nanoparticle compact.
Conclusion
In this study, we prepared the zirconia nanoparticle compact by gel-casting and demonstrated flash sintering. The
conclusions obtained are summarized as follows: (1) Homogeneous microstructure with densely packing of nanoparticles was successfully obtained by slip-casting of nano-suspension. The grain size and relative density of the687

Malaysian Journal of Analytical Sciences, Vol 23 No 4 (2019): 682 - 689
DOI: https://doi.org/10.17576/mjas-2019-2304-15

compact were 28nm and 52%, respectively. The sample had a very narrow pore size distribution with average poresize of 15nm, (2) While the original results reported in the literature were successfully reproduced in this study. theobserved width of the power spike in this study was about 150 sec, which was longer than that reported in theliterature (several seconds), (3) The grain size and relative density of the sample flash sintered at 30 mA/mm2 were
40.9nm and 86.8%, respectively. The sample densified with limited grain growth in low current flash sintering.
Although the full densification was not achieved in the present study, the effectiveness of the nanoparticle compacts
to reduce the current for the densification during flash sintering was shown by comparing the result reported in flash
sintering with the coarser microstructure.
Acknowledgement
This work was supported (in part) by the Collaborative Research Project of Materials and Structures Laboratory,
Tokyo Institute of Technology.
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NANOPARTICLE COMPACT

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