JEMEA Journal Vol.1 (2017.12)

学術論文
TiO2-VO2二成分系スピノーダル分解におけるマイクロ波照射効果
Microwave irradiation effects on the spinodal decomposition of TiO2-VO2 system
青柳 宗一郎、福島 潤 *、林 大和、滝澤 博胤
東北大学大学院 工学研究科
〒980-8579 宮城県仙台市青葉区荒巻字青葉 6-6-07
Department of Applied Chemistry, School of Engineering, Tohoku University
6-6-07, Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan
corresponding author*, e-mail address: fukushima@aim.che.tohoku.ac.jp
Keywords: microwave heating, spinodal decomposition, selective heating, metal−insulator transition, VO2, TiO2AbstractIn this study, TiO2–VO2 solid solutions were heated by microwaves and by an electric-furnace to investigate the spinodal
decomposition behavior of a TiO2–VO2 system. Single-mode and multi-mode type microwave irradiation devices were used toinvestigate the spinodal decomposition of TiO2–VO2. In the conventional process, spinodal decomposition required 12 h ofannealing. However, in the microwave process, spinodal decomposition proceeded at a low temperature for 30 min. Additionally,
the metal–insulator transition temperature (TMI) of the prepared sample was lower than that of pristine VO2. This indicated theexistence of anisotropic pressure on the boundary between the Ti-rich phase and the V-rich phase. Furthermore, themicrowave-irradiated samples exhibited broadened double endothermic peaks in the DSC profiles. The broad peaks indicated
that the samples exhibit several values of TMI, implying variations in the composition of the V-rich phase. The phase variation
resulted from the selective heating of the V-rich layer that induced one-directional material diffusion from the V-rich phase to theTi-rich phase.

1. Introduction
The metal−insulator transition temperature (TMI) oftransition is on the sub picosecond scale2), and there are

vanadium dioxides (VO2) is approximately 69 °C. Hence,

significant changes in the optical absorption and resistivity

VO2 behaves like a metal at temperatures exceeding 69 °C

across the transition3). Therefore, VO2 may be applied toand an insulator at temperatures below the same.

ultrafast optical switching devices and thermochromic

Metal−insulator transition is due to the transformation of thematerials4-8).

crystal structure from tetragonal (P42/mnm) rutile (R) in theApplications involving thermochromic materials require

high-temperature region to monoclinic (P21/c) at TMI at a

effective control of TMI. Previous studies indicate that the TMI1)

low temperature . The time taken for complete phase

value of VO2 increases with isotropic pressure (0.6 K/GPa)

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JEMEA Journal Vol.1 (2017.12)

and decreases with anisotropic pressure along the c-axis (-12K/GPa)9). Another study reported that TMI alternatively
decreases

along

thec-axis

through

thespinodal

decomposition of TiO2-VO2 system. In the spinodal
decomposition, an epitaxial layer is selectively formed along
the c-axis because the difference in their c-axis lattice
parameters exceeds that along the a-axis 10). Another method
to decrease TMI involves stacking a VO2 nanolayer on asubstrate to induce anisotropic pressure between VO2 andthe substrate. The VO2 thin film is prepared by various
methods, including pulse laser decomposition (PLD),
physical vapor decomposition (PVD), and magnetron
spattering11-15). Among these, spinodal decomposition is thesimplest and easiest method to decrease TMI since solid state
reactions, which involve mixing powders, pressing them into
pellets, and heating in a furnace, are employed.
Intrinsically,

solid-state

reactions

involve

Fig. 1: Schematic view of the microwave
prolong

irradiation device of single-mode type (a),

high-temperature treatment. Hence, they are time- andmulti-mode type (b).

energy-consuming processes. Specifically, the spinodal
the conventionally processed system.

decomposition of VO2 and TiO2 requires several hours10). In
order to shorten the time and decrease the process
temperature, the present study focuses on microwave

2. Experimental procedure

processing. A previous study used microwave processing

Two types of microwave irradiation devices were used toand reported a decrease in solid-state reaction time. For

investigate the spinodal decomposition of TiO2–VO2. Figure

example, the study reported on the increase in material

1 shows the schematic views of each of the devices. One oftransfer between SrTiO3 and CaTiO316) and rapid synthesis

the devices was a single-mode type microwave irradiationof Fe2O3(ZnO)m17). Another report demonstrated that thedevice. The device was composed of a magnetron

spinodal decomposition of a TiO2–SnO2 system was(IMP-15ENA IDX Co., Ltd., Tochigi, Japan), an isolator,

accelerated with microwave irradiation18). Furthermore, thethree stub tuners, a plunger, and a TE102 cavity, whichauthors suggested that only SnO2 was associated with high

specially divided the maximum point of E-field and H-field

temperature in a TiO2–SnO2 system and that theseintensity. The sample was inserted into a test tube composed

anisothermal conditions led to spinodal decompositionof quartz and sealed in a vacuum. The sample temperature

within a short period. In addition to the TiO2–SnO2 system,

was measured by a radiation thermometer through a hole at

it is considered that the anisothermal conditions occurred in

the side of the cavity. The other device was a multi-mode

the TiO2–VO2 system because VO2 was selectively heated.

type microwave irradiation device, μReactor Ex (Shikoku

In the present study, a solid solution of TiO2–VO2 was

Keisoku Co., Ltd., Takamatsu, Japan), as shown in Fig. 1 (b).

subjected to microwave irradiation to investigate theIn this device, microwaves were uniformly irradiated to aspinodal decomposition behavior of the TiO2–VO2 system.

sample. The sample was also sealed in a test tube, and theThe composition and phase transition temperature of thetest tube was surrounded by SiC, which acted as a susceptor.

microwave-irradiated system were compared with those ofWe measured the susceptor temperature, as it is close to the10

JEMEA Journal Vol.1 (2017.12)

and a solid solution was obtained by rapid quenching. The
solid solution pellet was sealed into quartz test tube, andheated by each of the microwave irradiation devices for 30
min at 300, 400 and 500 °C. For comparison with
microwave processing, a conventionally heated sample wasprepared by using an electric furnace at 900 °C for 1 h and
then heated at 400 °C for 12 h.
All products were examined by powder X-ray diffraction
(XRD RINT-2000PC Rigaku Co., Tokyo, Japan) using
Cu-Kα radiation to determine the lattice parameters. The
microstructure of the sample was observed by using
scanning

transmission

electron

microscopy

(STEM,HD-2700 Hitachi High-Technologies Co., Tokyo, Japan) andFig. 2: Microwave absorption properties of TiO2

an energy-dispersive X-ray (EDX) analyzer. Differential

and VO2

scanning calorimetry (DSC Q2000, TA Instruments Japan

sample temperature given the susceptor absorbs most of theInc., Tokyo, Japan)) was used to determine TMI.

microwave energy.
The solid solution Ti0.4V0.6O2 was prepared from a mixture

3. Results and discussionof TiO2 (99.9%, 2 μm, Kojundo Chemical Laboratory Co.,

3-1. Spinodal decomposition by microwave irradiation

Ltd., Saitama, Japan), V2O5 (99.0% Wako Pure Chemical

In this system, the microwave energy absorbed by VO2Industries, Ltd. Osaka, Japan), and V2O3 (99.9%, 180 μm,exceeds that absorbed by TiO2. Figure 2 shows theKojundo Chemical Laboratory Co., Ltd., Saitama, Japan).

microwave absorption properties of TiO2 and VO2. In order

The powders were mixed with ethanol with an agate mortar

to investigate the microwave absorption properties, eachand a pestle and dried. The dried powder was pressed into a

material was subjected to microwave irradiation from apellet (diameter: ⌀ 6 mm, height: 2.5 mm) and sealed into a

single-mode cavity and the temperature was measured. Eachquartz test tube. The sample was heated at 600 °C for 1 d,

powder was pelletized and set in the quartz test tube, and theFig. 3: XRD patterns of samples prepared by the single-mode microwave furnace (a) and multi-mode
furnace (b).

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JEMEA Journal Vol.1 (2017.12)

Fig. 4: STEM images of the sample prepared approximately at 300, 400 and 500 °C by (a) the single-mode
microwave furnace and (b) the multi-mode microwave furnace.
test tube was set at the point of H-field intensity maxima.

the Ti-rich and V-rich phases, respectively. The width of theThe temperature of TiO2 did not increase, although that ofsplit peaks increased with a decrease in the holding

thetemperature. This suggests that the difference in lattice

microwave irradiation. The resistivity of TiO2 is 107 Ω cm

parameters along the c-axis increased between the Ti-rich

although that of VO2 is 10 Ω cm. During H-field heating,and V-rich phases. The results corresponded to the phase

conductive materials are likely to absorb microwave energy.

diagram obtained in a previous study10). Peaks of l ≠ 0 atThus, VO2 is selectively heated in the system during

300 °C in the multi-mode did not clearly show the split.

microwave irradiation.

However, peaks of l ≠ 0 at 400 °C and 500 °C were split, andFigures 3(a) and (b) show the XRD patterns of the samples

the lattice parameter along the c-axis between the Ti-rich andprepared by using the single-mode and multi-mode

V-rich phase increased when the holding temperature wasmicrowave irradiation devices, respectively. All thelow. The absence of spinodal decomposition at 300 °C in

irradiated samples exhibited peaks with hk0 reflections that

multi-mode irradiation could be attributed to susceptor

were the same as those of a starting solid-solution sample:

heating. Although the susceptor contributed to thethe (020) and (220) peaks were not split, and this implied

stabilization of the holding temperature, it prevented

that a-axis lengths between the two phases are equal.

spinodal decomposition in a short period because the thermal

However, peaks with l ≠ 0, such as (101), (111), and (121),

effect was dominant.

were split into two sets of peaks, thereby indicating

Figure 4 (a) shows STEM images of the sample prepared

differences in c-axis length of two phases. The l ≠ 0 peaks of

by the single-mode microwave furnace at 300, 400 and

the decomposed samples at low and high 2θ values

500 °C for 30 min, and Figure 4 (b) shows STEM images of

corresponded to the Ti-rich and V-rich phases, respectively.

the sample prepared by the multi-mode microwave furnace

The a-axis length of TiO2 exceeded that of VO2 to maintain

in the same conditions. With the exception of the sample

the cell volume, and thus, a misfit strain between these two

prepared at 300 °C by the multi-mode microwave furnace,

phases forced the c-axis length to increase and decrease in

STEM images of all samples showed a layer (with a few

VO2

immediatelyincreased

aftercommencing

-2

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JEMEA Journal Vol.1 (2017.12)

Table1: Volume of unit cell and Ti content calculated from lattice parameters obtained from the XRD
results

nanometer thickness) of bright and dark contrast stacked in aeach sample prepared at 400 °C for 30 min by using asingle direction. It is considered that the stacking direction ismulti-mode type microwave furnace, a single-mode

along the c-axis of each phase given the XRD peaks split in l

microwave furnace, and an electric furnace and that of the≠ 0. The thickness of each layer was approximately 15–40

sample prepared at 400 °C for 12 h after heating in an10, 15)

.

electric furnace at 900 °C for 1 h. The conditions for theThe absence of a stacking structure was shown in the sample

sample prepared at 400 °C for 12 h after heating at 900 °C

prepared at 300 °C by the multi-mode microwave furnace.

for 1 h by using electric furnace were similar to those for theThis result indicated that spinodal decomposition did notsample prepared in a previous study. Thus, in this sample,proceed, and this was in agreement with the XRD results.

the spinodal decomposition proceeded to completion andnm, and this corresponds to that in a previous study

peaks with l ≠ 0 were split. Typical split peaks with l ≠ 03-2. Behavior of spinodal decomposition in thewere also detected for the microwave-irradiated samples.

microwave irradiation method

However, the sample prepared via the electric furnace at

In order to verify the effect of microwave irradiation on the400 °C for 30 min did not exhibit the fore-mentioned clear

spinodal decomposition behavior, the samples prepared bysplit

microwave heating were compared with those prepared by

decomposition preceded microwave irradiation in a short

conventional heating. Figure 5 shows the XRD patterns of

time period when compared to conventional heating.

peaks.

Theseresults

suggest

that

spinodal

Table 1 shows that the volume of unit cell and Ti contentcalculated from lattice parameters obtained from the XRD
results and Vegard’s law10). The Ti content of the sample
prepared by the multi-mode furnace was similar to that for
the sample prepared by the electric furnace at 400 °C for 12
h after heating at 900 °C for 1 h. In contrast, the Ti content of
the sample prepared by single-mode exceeded that of thesample prepared via multi-mode furnace. It is considered
that the thermal effect for spinodal decomposition wasdominant due to the susceptor used in the multi-mode
furnace. Thus, the composition of each phase after spinodal
Fig. 5: XRD patterns of samples prepared by using

decomposition varied with the method of heating.

each method

Figure 6 shows the STEM images and line analyses of13

JEMEA Journal Vol.1 (2017.12)

Fig. 6: STEM images and line analysis of EDX in each sample along the vertical direction of stacking
layers

EDX in each sample along the vertical direction of stacking

modulated layers.

layers. The STEM images and EDX results indicate that

Figure 7 shows the DSC results of each sample. Latent heat

layers of bright and dark contrast correspond to the Ti-rich

requires the occurrence of the metal–insulator transition ofphase and V-rich phase, respectively. Both samples prepared

VO2, and this implies that TMI is associated with theby microwave processing and conventional processing

endothermic peak of the DSC curve. Thus, TMI is determined

exhibited similar microstructures that consist of nanoscale-

from each endothermic peaks (Table 2). The TMI of each
sample decreased when compared to that of the pristine VO2.
Previous studies reported that the anisotropic pressure toVO2 relative to the c-axis results in a decrease in TMI by -12
K/GPa9), and the decrease in TMI indicates the existence of
anisotropic pressure on the boundary between the Ti-rich
phase and V-rich phase.
The

microwave-irradiatedsamples

exhibited

twoTable 2: TMI of each sample, as determined from DSC
results

Fig. 7: DSC results of each sample

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JEMEA Journal Vol.1 (2017.12)

Therefore,decomposition in a TiO2–VO2 system during microwave

microwave-irradiated samples exhibited several TMI. The

processing. Specifically, the TiO2–VO2 solid solution was

two peaks corresponded to metal–insulator transition derived

used as the starting material, and spinodal decomposition

from a starting solid solution and a phase formed by theproceeded within a short period (30 min). It is suggested that

microwave irradiation. Moreover, broad endothermic peaks

the development of concentration miscibility results from theindicated variation in the composition of the V-rich phase.

nonuniform sample temperature due to the nonuniformity of

This is consistent with the XRD results as the XRD peaks

thewere also broad. In the sample prepared by the single-mode

Additionally, the XRD and DSC results indicate that

furnace, an endothermic peak was approximately close to themicrowave selective heating induces one-directional material

peak of the completely spinodal-decomposed sample, anddiffusion from the V-rich phase to Ti-rich phase, and this

another peak was observed on the low-temperature side of

results in variations in the composition of the V-rich phase.

broadened

endothermic

peaks.

microwave

electromagnetic

fielddistribution.

the pristine VO2 peak. Conversely, in the sample prepared bymulti-mode microwave furnace, an endothermic peak was5. Acknowledgement

observed at the low-temperature side of the completely

This study was supported by a JSPS Grant-in-Aid for

spinodal-decomposed sample’s peak, and another peak was

Scientific Research (S) Number JP17H06156.

observed approximately near the peak of the starting sample.
We discuss the effect of microwave irradiation on theReferences

behavior of spinodal decomposition and material diffusion.

1) F. Morin, Phys. Rev. Lett., 3, (1959) 34.

In the conventional process of spinodal decomposition, a2) M. Becker, A. Buckman, R. Walser et al., Appl. Phys. Lett.,

sample temperature increases above the miscibility gap

65, (1994) 1507-1509.

curve in the first step of heating (900 °C), and then the3) M. Gurvitch, S. Luryi, A. Polyakov et al., J. Appl. Phys.,

sample is annealed at a low temperature (400 °C) to ensure

102, (2007) 033504.

concentration miscibility. Finally, a nanoscale stacked4) W. Roach, I. Balberg, Solid State Commun., 9, (1971)

structure is achieved by autonomously developing the551-555.

miscibility. In contrast, sample temperature is not uniform

5) H. Choi, J. Ahn, J. Jung et al., Phys.l Rev. B, 54, (1996)

during microwave irradiation due to the nonuniformity of4621-4628.

microwave

electromagnetic

6) A. Cavalleri, C. Tóth, C. Siders, et al., Phys. Rev. Lett., 87,

temperature

gradient

fieldensures

thedistribution.
developmentThis(2001) 237401.

ofconcentration miscibility, and the nanoscale stacked structure

7) J. Yu, S. Nam, J. Lee et al., Materials, 9, (2016) 556.

is formed in a short period. Additionally, it is considered that8) S. Kumar, F. Maury, N. Bahlawane, Scientific Reports, 6,

selective heating between Ti-rich layer and V-rich layer

(2016) 37699.

occurs after spinodal decomposition. The selective heating

9) L. A. Ladd, W. Paul, Solid State Commun. 7, (1969) 425.

induces one-directional material diffusion from the V-rich

10) Z. Hiroi, H. Hayamizu, T. Yoshida, et al., Chem. Mater,

phase to Ti-rich phase and especially during single-mode

25, (2013) 2202-2210.

microwave irradiation. One-directional material diffusion

11) Y. Muraoka, Z. Hiroi, Appl. Phys. Lett., 80, (2002)

leads to various compositions of the V-rich phase when

583-585.

compared to those of conventional heating samples.

12) Z. Yang, C. Ko, S. Ramanathan, J. Appl. Phys., 108,(2010) 073708.

4. Conclusion

13) K. Shibuya, J. Tsutsumi, T. Hasegawa, et al., Appl. Phys.

This study investigated the behavior of spinodal

Lett., 103, (2013) 2.

15

JEMEA Journal Vol.1 (2017.12)

14) H. Zhon, J. Li, Y. Xin et al., Ceram. Int., 42, (2016)
7655-7663.
15) G. Sun, H. Zhou, X. Cao et al., Appl. Mater. Interfaces, 8,(2016) 7054-7059.
16) A. Kishimoto, Y. Kamakura, T. Teranishi et al., Mater.
Chem. Phys., 139, (2013) 825-829.
17) D. Nagao, J. Fukushima, Y. Hayashi, H. Takizawa,Ceram. Int., 41, (2015) 14021-14028.
18) H. Takizawa, A. Hagiya, et al., Chem. Lett., 37, (2008)
714-715.
▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁Manuscript received: September 29, 2017
Revised: November 27, 2017
Accepted: December 4, 2017

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