Eﬀect of Glass Transition Temperature of Stabilizing Polymer
of Air-Stable Gelatin-Stabilized Copper Fine Particles during
Redox Two-Step Low-Temperature Sintering Process
Masaki Matsubara,³ Tetsu Yonezawa,* and Hiroki Tsukamoto
Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University,
Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628
E-mail: tetsu@eng.hokudai.ac.jp
Received: September 3, 2015; Accepted: September 10, 2015; Web Released: September 17, 2015

Tetsu Yonezawa
Professor Tetsu Yonezawa was born in Kobe in 1965. He graduated from Graduate School of Engineering,
The University of Tokyo and received a Ph.D. degree in 1994. After spending 2 years as a post-doc in Tokyoand France, he was appointed as an assistant professor in the Kunitake Lab., Faculty of Engineering,
Kyushu University. After staying at Nagoya University and The University of Tokyo as an associate professor,
he has now been working at Division of Materials Science and Engineering, Faculty of Engineering,
Hokkaido University, since 2008. He received the SPSJ Hitachi Chemical Award 2011. He has publishedmore than 170 original papers and 20 books.

Abstract
It has been found in this study that the glass transition
temperatures can be considered as the lowest temperature forsintering metal ﬁne particles to obtain electro-conductive
metallic layers from the diﬀerential scanning calorimetry
measurement of the ﬁne particles. Gelatin-stabilized copper
ﬁne particles could be sintered to form an electrically conductive copper layer with a resistivity of 1.9 © 10¹5 ³ cm at as
low as 150 °C using oxidation­reduction two-step sintering.
Many eﬀorts have been made to develop low-temperatureinterconnection and nanostructured formation1­3 processes forprinted electronics because of their high potential for use in
ﬂexible electronic devices,4 especially organic devices, compared to existing general electronic circuit technology, suchas photolithography,5 vapor deposition,6 and sputtering.7 In
particular, conductive pastes and inks that consist of metal
nanoparticles and ﬁne particles are key materials in this ﬁeld
because they can provide a cost-saving wet process application. Metallic silver inks,8­10 possessing high stability underambient conditions and a low resistivity, are one of the mostrepresentative materials for the production of metallic conductive ﬁlms. However, their high cost and the electromigrationof silver still limit their use in large-scale printed electronic
applications. In contrast, metallic copper pastes and inks are
preferable alternatives because of their low cost and high
³ Present address: Department of Materials and Environmental
Engineering, National Institute of Technology, Sendai College,
48 Nodayama, Medeshima-Shiote, Natori, Miyagi 981-1239
Bull. Chem. Soc. Jpn. 2015, 88, 1755–1759 | doi:10.1246/bcsj.20150305

ability to resist electromigration. So far, however, their poor
oxidation resistance has prevented the practical use of copper
nanoparticles and ﬁne particles as electro-conductive materials.
Polymers are among the preferable candidates for preventing
oxidation of copper surfaces because of their gas barrier properties. In order to overcome this issue, studies on antioxidation11­14 and sintering techniques15,16 of copper nanoparticles
and ﬁne particles have been performed extensively.
In our previous studies, we have proposed a two-step sintering process for copper ﬁne particles stabilized by gelatin17 or
alkylamines.18 Gelatin, well known as a most popular biopolymer, is a good stabilizer of copper ﬁne particles and acts as
an antioxidation reagent for particles synthesized by chemical
reduction of copper(II) oxide (CuO)17,19­21 and a copper salt.22
Once gelatin protects the surface of copper, gelatin-stabilized
copper nanoparticles and ﬁne particles show a high antioxidative ability due to the glass state of gelatin-layers. Alkylamine can also prevent oxidation under ambient conditions,18
but gelatin-stabilized copper ﬁne particles are easier to handle.
Gelatin-stabilized copper ﬁne particles could be sintered at
as low as 200 °C using a two-step sintering process. In the ﬁrst
oxidative annealing of the two-step sintering process, nanosized copper(I) oxide (Cu2O) projections with a size of approximately 10 nm were predominantly observed on the surface of
metallic copper ﬁne particles. Copper ﬁne particles connected
via these Cu2O nanoprojections, which penetrated the gelatin
layers. Then, they were reduced at 200 °C under a ﬂow of
reductive gas composed of 3% H2 in N2 gas for 4 h. Because of
the small size (ca. 10 nm) of Cu2O nanoprojections, the temperature for reductive sintering is very low even though bulk Cu2O
is usually reduced at higher than 400 °C.23 Even though the© 2015 The Chemical Society of Japan | 1755

organic components included in the Cu ﬁne particle paste did
not fully decompose at this temperature, a low resistivity of
8.2 © 10¹6 ³ cm was obtained. Alkylamine-stabilized particles
could also be sintered at 250 °C using a similar two-step sintering process. In that case, Cu2O nanoprominences and CuOnanowires were generated during oxidative sintering, which
could also be reduced to metallic copper under reductive
sintering, connecting the particles.17,18,20
In this study, we tried to expand our novel strategy of low
temperature sintering and have explored the minimum temperature for the sintering of gelatin-stabilized Cu ﬁne particles.
We focus here on the eﬀect of glass transition of polymers
used to stabilize copper ﬁne particles. It was found that theglass transition temperatures of the polymers are very importantfactor to design the sintering process. We have successfully
performed the two-step sintering at a low temperature of 150 °C
and the resultant conductive copper ﬁlms show a resistivity of
as low as 1.9 © 10¹5 ³ cm. Diﬀerential scanning calorimetry(DSC) measurements of gelatin and the binder polymer
revealed that their glass transition temperatures (Tg) are key
parameters for this sintering process. The eﬀect of polymer
content on the reduction time was also investigated.
Experimental
The Cu ﬁne particles were prepared following a method
described in a previous report.17 First, 32 g of gelatin was
dissolved in warm water (950 mL, 60 °C), and then, 80 g ofCuO microparticles (Nisshin Chemco) were dispersed in thissolution. Then, 28 wt % aqueous ammonia was added and the
pH was adjusted to ca. 11. The reduction of CuO was performed by the addition of 120 mL of hydrazine monohydrate(Wako) at 80 °C in air with stirring for 2 h. The brown products
were then collected by decantation after introducing a saturated
aqueous citric acid solution to reduce the dispersion pH toca. 8.5, and washed with water twice and ethanol twice again,
followed by drying the collected Cu ﬁne particle powder at
60 °C under N2 atmosphere. The resulting Cu particles werepulverized and dispersed into 50 wt % of commerciallyavailable paste vehicle with 5 wt % polyamine-type dispersing
agent. The copper content of the obtained paste was 48 wt %.
The vehicle used here contained 10 wt % ethyl cellulose. Thus,
the paste consisted of 45 wt % of a liquid component and
7 wt % of polymers such as gelatin, the dispersing agent, andethyl cellulose, and 48 wt % of copper. The obtained Cu paste
was deposited on a glass or an aluminum oxide (Al2O3)
substrates using the doctor blade method to obtain a ﬁlm of
40 ¯m thickness and 20 mm width. After drying in N2 at 60 °C
for 1 h, the substrates were cut into 20 mm © 20 mm pieces andannealed at 150 °C in air for 4 h, and subsequently reduced in
3% H2 in N2 gas. The reductive annealing times were varied
from 4 to 8 h to determine the reduced state of copper. The gas
ﬂow rates were adjusted to 2 L min¹1 for both air and 3% H2 in
N2 gas during annealing. We also prepared a Cu paste without
the vehicle by mixing 50 wt % of Cu particle powder and
50 wt % of α-terpineol to decrease the polymer content of thepaste. Scanning electron microscopy (SEM) of the particles
was performed using a JEOL JSM-6701F ﬁeld emission SEM
with an acceleration voltage of 15 kV. X-ray diﬀraction (XRD)
patterns were obtained using a Rigaku Mini Flex II (Cu Kα).
1756 | Bull. Chem. Soc. Jpn. 2015, 88, 1755–1759 | doi:10.1246/bcsj.20150305

DSC measurements were performed using a Rigaku DSC 8270.
Transmission electron microscopic (TEM) images were obtained with a Hitachi H-9500 with an acceleration voltage of
300 kV. To prepare TEM samples, the annealed copper layer
was detached from the Al2O3 substrate by scratching, then dispersed in ethanol and applied to a carbon-coated copper grid.
Results and Discussion
Gelatin-stabilized Cu ﬁne particles with uniform size could
be prepared by the reduction of CuO in the presence of gelatin.
As shown in Figure 1a, the shape of the particles was nearly
spherical and the average particle size was calculated asca. 130 nm. A thin layer of gelatin covered the surface of theparticles, forming a smooth surface. Cu paste, which includes
7 wt % of polymers, was printed on a glass substrate (for XRD)
or an Al2O3 substrate using the doctor blade method and driedat 60 °C under N2 ﬂow to avoid surface oxidation. After
printing, the particles became covered with polymer layers
show in Figure 1b, so that the Cu particle core did not contact
neighbors directly. As discussed in a previous paper,17 these Cu
particles can be oxidized above ca. 120 °C under air. Moreover,nanosized particle-like Cu2O is formed on the surface of
metallic Cu particles when the Cu particles were oxidatively

Figure 1. (a­d) SEM images of the surface of Cu ﬁneparticle layer. (a) As-prepared Cu particles (after drying),(b) as-printed Cu layer, (c) layer after oxidative annealing,and (d) layer after reductive annealing for 8 h. (e) A TEM
image of Cu particles after reductive annealing for 8 h.
(f ) Image with a larger magniﬁcation of the boxed area in(e). Necking structure of particles can be clearly observed.

© 2015 The Chemical Society of Japan

Heat Flow/mW
2θ /degree

Figure 2. XRD patterns of the Cu layer on a glass substrate
before and after annealing. (a) The as-printed Cu layer,(b) the layer after oxidative annealing, and the layer after
reductive annealing for (c) 4 h, (d) 6 h, and (e) 8 h. The
peaks of metallic copper and Cu2O are marked with closed
circles ( ) and open circles ( ), respectively.

annealed. Figure 1c shows that small particles were also
formed to make the surface rough in the case of annealing at
150 °C under airﬂow. As indicated in the SEM image, the sizeof the small particles on the metallic copper ﬁne particles is
ca. 12.8 nm. The particles can be connected to each other via
these small particles. The polymer layer surrounding the Cu
particles was still present because the polymers, gelatin, andthe binder, could not be decomposed at 150 °C. After 8 h of
reductive annealing at 150 °C in 3% H2 in N2 gas, the surface
became comparatively smooth, and necking was observed(Figure 1d). Figures 1e and 1f show the TEM images of theCu particles annealed under the reductive gas for 8 h, also
displaying the necking structure of particles. The resistivity of
the reductive annealed ﬁlm was 4.1 © 10¹5 ³ cm.
Figure 2 shows the XRD patterns of Cu thin ﬁlms on glass
substrates. After oxidative annealing, peaks corresponding to
Cu2O appear (Figure 2b). They indicate that the small particles
seen in Figure 1c were Cu2O, as observed after sintering at
200 °C in air.17 These Cu2O peaks gradually decreased withincrease in the reductive annealing time. A very small change
in the intensity of the Cu2O peaks can be observed up to 4 h of
reductive annealing, as shown in Figure 2c. After an additional
2 h of reductive annealing (total 6 h of annealing), the peaks
corresponding to Cu2O slightly reduced (Figure 2d). As shownin Figure 1e, the XRD spectrum indicates that Cu2O wascompletely reduced by annealing for 8 h at 150 °C.
Cu ﬁne particles and nanoparticles without any protecting
layer such as gelatins are oxidized instantly from the particle
surface under ambient conditions. On the contrary, our Cu ﬁneparticles are stable at temperatures below 120 °C.17 In order tounderstand this oxidation behavior, DSC measurements of bothgelatin and ethyl cellulose were performed (Figure 3). DSC
proﬁles were collected in the ﬁrst cooling cycle (2.5 °C min¹1)
over a temperature range of 180 to 60 °C after heating up to
200 °C. Gelatin and ethyl cellulose form an amorphous solid
below Tg, because of their high molecular weight. From the
DSC result, it is observed that the gelatin used in this study
Bull. Chem. Soc. Jpn. 2015, 88, 1755–1759 | doi:10.1246/bcsj.20150305

Temperature/°C

Figure 3. DSC proﬁles of the gelatin and ethyl cellulose(1st cooling, 2.5 °C min¹1).

shows a structural change at 140 °C and transitions into a glass
structure at 90 °C upon cooling. Thus, it forms a solid glass
state below 90 °C and a soft gel state above 140 °C. Gelatin isa polypeptide, and the conformation of polypeptides changes
below 100 °C. The high transition temperature of gelatinmolecules observed in this DSC curve can be attributed to thecoordinative interaction between gelatin molecules and thecopper surface, because gelatin molecules have amino and thiolgroups in their branches. On the other hand, ethyl cellulose
may change into a soft state at ca. 130 °C during cooling,although the Tg could not be clearly observed in the measurement range. Therefore, softening of both gelatin and ethyl
cellulose occurs at around 130­140 °C. Below Tg, the polymer
layers surrounding particles form a rigid glass state and separate the particle surface from air. Above Tg, however, polymers
transition into a soft gel state, which allows gas molecules todiﬀuse into the polymer layers. Thus, we can easily control
the surface state of copper by controlling temperatures andgas atmospheres. These hardening and softening behaviors ofpolymers control the diﬀusion of gas molecules surrounding
the particle surface during annealing. Below Tg, polymers forma glassy state, and oxygen molecules cannot diﬀuse into thepolymer layers. When the temperature is higher than Tg, indicating that the softening of both gelatin and ethyl cellulose
occurs, oxygen molecules can diﬀuse easily into polymer
layers and oxidize the surface of Cu ﬁne particles during theoxidative annealing.24 The oxidative mechanism of gelatinstabilized Cu particles is summarized in Figure 4.
In the reductive annealing conditions, hydrogen molecules
should also penetrate the polymer layers above the Tg of thepolymers. For the complete reduction of Cu2O nano-objects on
the Cu ﬁne particle surface at 150 °C, 8 h of reduction annealing is required. It takes a much longer time to sinter the ﬁlms at
150 °C because of the lower sintering temperature. However,
we also demonstrated that we can reduce the sintering time bydecreasing the amount of polymer content in the Cu paste eventhough the ﬁlms are sintered at 150 °C. Cu paste consisting ofgelatin-stabilized Cu particles and α-terpineol without ethyl
cellulose was prepared to decrease the polymer content of theCu paste as shown in Figure 5a. In the XRD spectrum, a small
amount of Cu2O was formed after printing on the substrates© 2015 The Chemical Society of Japan | 1757

Conclusion

Figure 4. Schematic illustration of gas diﬀusion in thesoftened polymer layer in oxidative annealing. Above Tg,
the polymer layer becomes a soft layer, which can allow
penetration of oxygen molecules.

Figure 5. SEM images of the Cu particle layer without
ethyl cellulose. (a) The as-printed Cu layer, (b) the layer
after oxidative annealing, and (c) the layer after reductive
annealing for 4 h.

We have demonstrated an oxidative­reductive two-stepannealing of gelatin-stabilized Cu ﬁne particles at a temperature of as low as 150 °C to obtain a highly conductive Cu
layer. In our system, the polymer components included in theCu paste did not decompose. However, as 150 °C is higher than
the glass transition temperatures of the polymers, polymer
molecules can move on the particle surfaces and gas molecules
can diﬀuse into the soft polymeric layers. During the oxidative
annealing, Cu2O nanoprojections, which play a key role in
the two-step sintering process, were also generated at 150 °C,
resulting in the formation of necking structure after reductive
annealing. The thermal diﬀusion rate of hydrogen moleculesin the polymeric layers is also important. The resistivity of the
conductive ﬁlm obtained by Cu paste without ethyl cellulose
was as low as 1.9 © 10¹5 ³ cm. Our results strongly suggestthat the glass transition temperatures of the polymer components in the conductive pastes are the key temperatures to beconsidered for low-temperature sintering.
This work is partially supported by a Grant from Hokkaido
University, Shinsei Foundation, Murata Foundation, and Grantin-Aid for Scientiﬁc Research (A) (No. 24241041) from JSPS,
Japan. The authors thank Dr. Y. Ishida (Hokkaido Univ.) for
fruitful discussions.
References2θ /degree

Figure 6. XRD patterns of the Cu layer without ethyl
cellulose on a glass substrate before and after annealing.
(a) The as-printed Cu layer, (b) the layer after oxidative
annealing, and (c) the layer after reductive annealing for
4 h. The peaks of metallic copper and Cu2O are marked
with closed circles ( ) and open circles ( ), respectively.

(Figure 6a). The surface of Cu ﬁne particles was partially
oxidized during drying in the N2 atmosphere because of thepresence of thin polymer layers. Cu2O nanoprojections are also
observed after annealing at 150 °C in air (Figures 5b and 6b).
This ﬁlm can be reduced completely in 4 h, which is half as
long as the time required for the ethyl cellulose-containing
paste, by annealing at 150 °C in 3% H2 in N2 (Figures 5c
and 6c). The resistivity of this ﬁlm was 1.9 © 10¹5 ³ cm, which
was only about 10 times higher than that of bulk copper (1.7 ©
10¹6 ³ cm) and rated on par with that of Cu ﬁlms annealed at
higher temperature of 200 °C (8.2 © 10¹6 ³ cm).17
1758 | Bull. Chem. Soc. Jpn. 2015, 88, 1755–1759 | doi:10.1246/bcsj.20150305

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