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rspa.royalsocietypublishing.org
Printing low-melting-point
alloy ink to directly make a
solidified circuit or functional
device with a heating pen
Lei Wang1 and Jing Liu1,2
Research
Cite this article: Wang L, Liu J. 2014 Printing
low-melting-point alloy ink to directly make a
solidified circuit or functional device with a
heating pen. Proc. R. Soc. A 470: 20140609.
http://dx.doi.org/10.1098/rspa.2014.0609
Received: 8 August 2014
Accepted: 25 September 2014
Subject Areas:
mechanical engineering, materials science,
electrical engineering
Keywords:
printed electronics, liquid metal,
low-melting-point metal ink, heating pen,
direct writing
Author for correspondence:
Jing Liu
e-mail: jliu@mail.ipc.ac.cn
1 Beijing Key Lab of CryoBiomedical Engineering and Key Lab of
Cryogenics, Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, Beijing 100190, People’s
Republic of China
2 Department of Biomedical Engineering, School of Medicine,
Tsinghua University, Beijing 100084, People’s Republic of China
A new method to directly print out a solidified
electronic circuit through low-melting-point metal
ink is proposed. A functional pen with heating
capability was fabricated. Several typical thermal
properties of the alloy ink Bi35 In48.6 Sn16 Zn0.4 were
measured and evaluated. Owing to the specifically selected melting point of the ink, which is
slightly higher than room temperature, various
electronic devices, graphics or circuits can be
manufactured in a short period of time and then
rapidly solidified by cooling in the surrounding
air. The liquid–solid phase change mechanism of
the written lines was experimentally characterized
using a scanning electron microscope. In order
to determine the matching substrate, wettability
between the metal ink Bi35 In48.6 Sn16 Zn0.4 and
several materials, including mica plate and silicone
rubber, was investigated. The resistance–temperature
curve of a printed resistor indicated its potential
as a temperature control switch. Furthermore, the
measured reflection coefficient of a printed doublediamond antenna accords well with the simulated
result. With unique merits such as no pollution, no
requirement for encapsulation and easy recycling,
the present printing approach is an important
supplement to current printed electronics and has
enormous practical value in the future.
2014 The Author(s) Published by the Royal Society. All rights reserved.
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1. Introduction
(a) Preparation of Bi35 In48.6 Sn16 Zn0.4 metal ink
The four metals (bismuth, indium, tin and zinc) with purity above 99.99% (4N) were purchased
from Shenzhen Zhongjin Lingnan Non-ferrous Metal Co., Ltd, China, and Wuxi Bokai Analytical
Instrument Manufacture Co., Ltd, China, for making the alloy ink. They were weighed according
to the ratio of 35 : 48.6 : 16 : 0.4. Then, these pure metals were put in a beaker for 5 h at 245◦ C in
an electric vacuum drying oven. The mixture was stirred in the beaker, which was then put in
a water bath at 85–90◦ C for 30 min. Finally, the beaker was placed in the electric vacuum drying
oven for 2 h to further ensure a well-mixed alloy solution. This preparation process, which took
only half of a day, is much simpler than the preparation of silver nanoparticles [32], especially
when mass production is required.
(b) The structure of the heating pen and thermal properties of the metal ink
The heating pen loaded with Bi35 In48.6 Sn16 Zn0.4 needs to be heated since the melting point
of Bi35 In48.6 Sn16 Zn0.4 (approx. 58.3◦ C) is slightly higher than room temperature. The external
appearance of the heating pen is shown in figure 1a. The pen container loaded with liquid metal
...................................................
2. Material and methods
rspa.royalsocietypublishing.org Proc. R. Soc. A 470: 20140609
Printed electronics is of growing scientific interest for applications in a number of areas, including
transistors [1,2], antennae [3], printed circuit boards (PCBs) [4], light-emitting diodes [5–7],
radio frequency identification [8], photovoltaics [9], sensors [10,11], memories [12] and so on.
Among the various printing techniques, such as micro-contact printing [13], nanoimprinting [14],
screen printing [15], stencil printing [16,17], drop-on-demand inkjet printing [18] and roll-toroll printing [19], direct additive printing has many advantages such as low cost and easy
operation compared with the conventional vacuum deposition and photolithographic patterning
methods [20]. In recent years, Russo et al. [21] demonstrated a facile pen-on-paper method to
directly create flexible printed electronics using conductive silver ink. Although conductive tracks
can be directly written upon paper via this method, there still exist several limitations that need to
be overcome. The preparation of conductive silver ink is a complicated process and the ink needs
to be stored in properly sealed containers. To achieve better properties of the printed silver ink, a
certain annealing temperature and time are necessary. For example, only after annealing to higher
temperatures (greater than or equal to 170◦ C) can one expect a comparatively ideal electrical
resistivity (4.34 × 10−8 Ω m) of the silver ink (50 wt% silver). Undoubtedly, write and play cannot
be realized easily via this type of conductive silver ink. Also, recycling the printed silver ink is
another difficult task that produces material waste when printing with this ink. Dickey et al. [22]
investigated a type of liquid metal GaIn24.5 (melting point approx. 15.7◦ C), which is in the liquid
state at room temperature, and evaluated its rheological behaviour and oxidation properties.
Zheng et al. [23] and Boley et al. [24] used such an ink in direct writing technology. However,
the patterns printed with this ink need to be packaged or they can be easily damaged when
exposed to air because of the flowability of the ink. Similar to the conductive silver ink, GaIn24.5
ink is not easily recycled after printing. In order to provide a new candidate ink for pen-on-paper
writing technology, a type of alternative Bi-based alloy, Bi35 In48.6 Sn16 Zn0.4 [25], is introduced as
the printing ink in this article. The measured melting point of Bi35 In48.6 Sn16 Zn0.4 is 58.3◦ C, which
is slightly lower than the Bi–In–Sn alloys (approx. 60◦ C) [26]. Bi35 In48.6 Sn16 Zn0.4 is a non-toxic
material, and the four constituent metals (Bi, In, Sn and Zn) are in widespread use [27–30], e.g.
Bi compounds are widely used in cosmetics and pharmaceuticals [31]. This metal ink has unique
merits such as it is easy to prepare, it has high electrical conductivity, it is non-polluting and it
is easy to recycle. Direct writing with such an ink will have important practical significance and
application value to supplement the current PCB manufacturing technology.
2
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(a)
(b)
3
liquid metal ink
aluminium heating muff
thermocouple
constantan heating wire
pen nib
Figure 1. (a) The external appearance of the heating pen loaded with Bi35 In48.6 Sn16 Zn0.4 liquid metal ink. Inset: The macrograph
of the pen nib. Scale bar is 5 mm in length. (b) The sectional structure of the heating pen. (Online version in colour.)
(b)
12
k
L
k (106 . S m–1)
g (mPa s)
4.0
3.5
3.0
2.5
0
–0.2
9
L (DL/L, %)
(a) 4.5
–0.4
2.0
50
100
150 200
T (°C)
250
300
6
–150
–100
–50
T (°C)
0
50
Figure 2. (a) The dynamic viscosity (γ ) of the liquid metal ink Bi35 In48.6 Sn16 Zn0.4 measured as a function of temperature (T).
(b) The electrical conductivity (κ)–temperature (T) and the thermal expansion rate (Λ)–temperature (T) measurement curves
of the liquid metal ink Bi35 In48.6 Sn16 Zn0.4 in the solid state. The data are from [33]. (Online version in colour.)
ink is installed in the aluminium heating muff. A plug is located at the tail of the pen holder to
stopper the pen container. The aluminium heating muff is wound with a constantan wire which is
powered by a DC (direct current) regulated power supply. The sectional structure of the heating
pen is shown in figure 1b. The pen holder is wrapped with a yellow high-temperature adhesive
tape to reduce heat dissipation and to fix the constantan heating wire. Joule energy is generated
when current is passed through the constantan wire and the aluminium heating muff and the
liquid metal ink will thus be heated. The temperature of the heating pen is monitored via a kthermocouple which is located in the middle of the pen holder. The k-thermocouple is connected
to a data acquisition instrument from which the temperature change of the pen holder can be
observed.
Figure 2a illustrates the change in dynamic viscosity of the liquid metal ink Bi35 In48.6 Sn16 Zn0.4
at different temperatures. The measurements were performed with a high-temperature molten
metal viscometer fabricated at Shandong University based on an oscillating-cup method [34].
The dynamic viscosity of the ink is measured at five temperature points (74◦ C, 95◦ C, 150◦ C, 203◦ C
and 295◦ C) and measurement at each temperature point is repeated three times. It can be seen that
the dynamic viscosity of the ink decreases with increasing temperature, and the descent velocity
...................................................
pen container
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plug
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4
100
80
1
10
RMS roughness (nm)
100
Figure 3. Contact angles of a Bi35 In48.6 Sn16 Zn0.4 droplet on five substrates with different RMS roughness values. Five scatter
points represent (from left to right) mica plate, PVC plastic film, silicone rubber, stainless steel and office paper. Inset presents
an illustration of the contact angle between the droplet and the substrate. Error bars represent the standard deviation of the
mean. (Online version in colour.)
changes from fast to slow. When the temperature of the heating pen becomes too low, the metal
ink cannot smoothly flow when writing because of its high viscosity. On the other hand, when the
temperature of the heating pen is too high, the metal ink will flow uncontrolledly even when not
writing because of its low viscosity. In our experience, the temperature of the heating pen should
be regulated within the range of approximately 70–80◦ C (the corresponding dynamic viscosity
range is from approx. 4.1 to 3.9 mPa s) to write/print smoothly.
The electrical conductivity (κ) and the thermal expansion rate (Λ) of Bi35 In48.6 Sn16 Zn0.4 in
the solid state are measured by a four-probe electrometer method using a physical property
measurement system (PPMS, Quantum Design) at a heating rate of 5◦ C min−1 . It can be observed
that, with the increase of T, κ approximates to a linear decrease while Λ approximates to a
linear increase. The electrical conductivity of Bi35 In48.6 Sn16 Zn0.4 is 7.31 × 106 S m−1 at room
temperature. This is approximately twice that of GaIn24.5 (3.36 × 106 S m−1 ) [35], which was used
as the printing ink by Zheng et al. [23], and approximately 14 times that of conductive silver ink
(50 wt% silver, 5.03 × 105 S m−1 ), which was used by Russo et al. [21] before heat treatment. The
comparatively higher electrical conductivity of Bi35 In48.6 Sn16 Zn0.4 makes it suitable for writing
circuit lines. It can be seen from the Λ–T measurement curve that Bi35 In48.6 Sn16 Zn0.4 expands
when heated and contracts when it is cold. The measurement curve shows excellent linearity, and
the thermal expansion coefficient is approximately 30.5 × 10−6 per ◦ C at room temperature.
3. Experimental results
(a) Wettability between liquid metal ink and several substrates
To determine the appropriate substrates on which to perform direct-write technology with the
heating pen, wettability between Bi35 In48.6 Sn16 Zn0.4 and different substrates was investigated.
The measured RMS (root mean square) roughness of the five substrates—mica plate, PVC
(polyvinyl chloride) plastic film, silicone rubber, stainless steel and office paper—is 0.432, 3.81,
5.86, 7.38 and 202 nm, respectively. In addition, the contact angles of a Bi35 In48.6 Sn16 Zn0.4 droplet
on these five substrates, measured by means of a five-point fitting method with a contact angle
meter (JC2000D3, Shanghai), are shown in figure 3. Each contact angle is the arithmetic average of
five measured values. It can be seen that each contact angle is greatly influenced by the roughness
of the substrate, which is in agreement with the results of Kramer et al. [36], whose research focus
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120
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contact angle (°)
140
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5
(a)
conductive track
57.70 mm
PVC film
Figure 4. (a) The conductive tracks written by a Bi35 In48.6 Sn16 Zn0.4 filled heating pen on the PVC plastic film substrate. Panels
(b) and (c) are, respectively, ESEM images of the width and thickness of the tracks shown in (a). (Online version in colour.)
is the wettability between gallium–indium alloy droplets and thin metal films. Among the five
contact angle values, the contact angle between Bi35 In48.6 Sn16 Zn0.4 and the office paper is the
greatest, which indicates the poor wettability of Bi35 In48.6 Sn16 Zn0.4 on office paper. By contrast,
the contact angle between Bi35 In48.6 Sn16 Zn0.4 and the mica plate substrate is the smallest, which
is due to the smooth surface of the mica plate. However, there will be scratches on the mica
plate when printing on it, because mica plate is thin and brittle. The contact angles between
Bi35 In48.6 Sn16 Zn0.4 and silicone rubber, PVC plastic film and the stainless steel are between the
two extremes and are similar to each other. These three substrates can all have traces written
on them. However, as the heating pen is fixed on a liquid metal printer [35] to provide accurate
printing, the PVC plastic film performed better as the substrate than stainless steel and silicone
rubber. The reason for this is that the stainless-steel plate cannot be bent and moved freely, and
the thin (0.1–0.5 mm) silicone rubber is not manufactured as A4 paper size. PVC plastic films,
purchased from Shanghai Yuanhao Office Equipment Co., Ltd, China, are used as the printing
substrate in this article to perform the fabrication of patterns.
(b) Fabrication of conductive patterns and circuits
Figure 4a shows the printing tracks directly written with the liquid metal ink Bi35 In48.6 Sn16
Zn0.4 . The environmental scanning electron microscope (ESEM) images of the width and
thickness of the track are shown in figure 4b and 4c, respectively. The scrimps on the surface
...................................................
109.3 mm
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(c)
(b)
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(a)
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(b)
Figure 5. The printed patterns on PVC plastic film substrate. (a) A printed sailing boat shape and (b) a PCB. Scale bars is 2 cm.
(Online version in colour.)
of the printed tracks are caused by the cold contraction of the liquid metal ink when it was
cooled to the solid state from liquid. It can be seen that the measured width and thickness of the
track are, respectively, 109.3 and 57.7 μm, which are much smaller than previous studies [21,23].
Undoubtedly, a higher resolution can be realized if a finer pen nib is used.
When the heating pen is held on a liquid metal printer, the location of the pen can be accurately
controlled and complex patterns can be directly printed out. A sailing ship shape conductive
pattern thus made is presented in figure 5a, which indicates the feasibility of drawing with liquid
metal ink. In addition, the printed circuit on PVC plastic film substrate as shown in figure 5b
can also be realized. Rapid manufacturing of circuits via this direct-write method may bring a
revolutionary change to the current PCB manufacturing. A prominent problem that needs to be
solved is the effective connection between the component pins and the liquid metal ink.
(c) Characterization of the printed resistor and the double-diamond antenna
To illustrate the practical value of the present method, a printed resistor is shown in figure 6a.
The total length of the printed line between the two endpoints is 0.495 m, and the value of
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(a)
(b) 24
7
resistance (W)
18
16
14
20
30
40
50
temperature (°C)
60
Figure 6. (a) A printed resistor on the PVC plastic film substrate (scale bar is 1 cm in length) and (b) the resistance–temperature
curve of the printed resistor. (Online version in colour.)
this resistor is 14.43 Ω as measured by an LCR digital electric bridge at room temperature. The
resistance-temperature curve of this printed resistor is shown in figure 6b. Generally speaking, the
resistance value increases with increasing temperature. As the temperature becomes lower than
approximately 47◦ C, the curve changes slightly. However, there is a steep change in the resistance
value near the melting point of the liquid metal Bi35 In48.6 Sn16 Zn0.4 ink. This is due to the phase
change of the printed liquid metal line in this temperature range. As the temperature rises to
become higher than the melting point of the metal ink, the resistance–temperature curve changes
slightly again. Such a device, which should be called a phase change resistor, could be used in
temperature control systems because of its evident thermal sensitivity characteristic.
Figure 7a shows a printed double-diamond antenna on a PVC plastic film substrate. Each
side of the diamond is 30 mm and the line width is approximately 0.4 mm. The frequency
response of the reflection coefficient of this antenna is measured with a network analyser (Agilent
N5230A), compared with the simulated result using Ansoft HFSS, as shown in figure 7b. The
measured resonant frequency is 2.47 GHz, while the simulated value is 2.5 GHz. The good
agreement between the measured and simulated results indicates that the printed antenna works
as predicted.
4. Discussion
Printing with the heating pen loaded with the metal ink Bi35 In48.6 Sn16 Zn0.4 introduced in this
article is an important supplement to the current direct-write technology. Various patterns and
circuits can be directly printed in a short time. The electrical conductivity of the metal ink
is important in printing devices or circuits. Adding small amounts or nanoparticles of highconductivity metals is an effective way of improving the electrical conductivity. Wettability
between the substrate and the metal ink plays a decisive role in successful printing. More
substrates which can ‘match’ with the metal ink should be developed. The articles printed with
the ink Bi35 In48.6 Sn16 Zn0.4 do not have good flexibility since they remain in the solid state at
room temperature. Therefore, this metal ink is more suitable to be written onto solid substrates to
manufacture instant PCBs, which will be the focus of our future investigations. When cracks arise
in the wires of the printed circuit after long-term use, one just needs to heat the circuit to above
the melting point of the metal ink to bridge the cracks together. In addition, the articles fabricated
with this direct-write technology are easy to recycle and cause almost no pollution. A comparison
between recycling GaIn24.5 and Bi35 In48.6 Sn16 Zn0.4 is shown in figure 8. It can be seen that the
solid-state Bi35 In48.6 Sn16 Zn0.4 ink can be easily peeled off, whereas the liquid-state GaIn24.5 ink
cannot. One matter that has yet to be resolved when using GaIn24.5 is how to handle any unused
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(a)
reflection coefficient (dB)
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(b)
8
0
–5
–10
–15
measured
simulated
–20
1.0
1.5
2.0
2.5
frequency (GHz)
3.0
3.5
Figure 7. (a) A direct-write double-diamond antenna on a PVC plastic film substrate. Scale bar is 30 mm. (b) Comparison
between the simulated and the measured reflection coefficients of the printed antenna. (Online version in colour.)
(a)
(b)
Figure 8. Comparison between recycling (a) Ga24.5 In and (b) Bi35 In48.6 Sn16 Zn0.4 . (Online version in colour.)
liquid metal on the substrate. This liquid metal sticks easily to one’s skin and may potentially
cause health problems because the surface of Ga is easily oxidized, which improves the adhesion
between Ga and other surfaces. However, with Bi35 In48.6 Sn16 Zn0.4 , one can conveniently peel the
metal traces off the substrate, collect them together and then put them in a heating container to
melt. This recycling measure causes no pollution, and saves the metal ink.
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5. Conclusion
heating pen and to Mr Jun Yang for providing the double-diamond antenna pattern.
Funding statement. This work was partially supported by the National Natural Science Foundation of China
(grant no. 51301186) and the Research Fund of the Chinese Academy of Sciences (grant no. KGZD-EW-T04-4).
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rspa.royalsocietypublishing.org Proc. R. Soc. A 470: 20140609
In summary, we have presented a method of directly printing solidified electronic circuits or
devices with the low-melting-point alloy Bi35 In48.6 Sn16 Zn0.4 as the ink and suggesting the best
material as the printing substrate. For future reference, the viscosity–temperature, electrical
conductivity–temperature and thermal expansion–temperature curves of the metal ink were
measured. At this stage, the achieved size of the printing tracks is already much smaller than
previous efforts. Overall, the solid-state patterns and circuits could be fabricated via this directwrite method in a short period of time. For practical illustration, a liquid metal resistor was
printed with its resistance–temperature curve measured, which indicated that such a resistor
has the potential to be used in a temperature control system. Furthermore, a double-diamond
antenna was also manufactured and there was good agreement between the measured reflection
coefficient and the simulated result. With merits such as no pollution, no requirement for
encapsulation and easy recycling, the present method is expected to be very useful in a wide
variety of emerging printed electronics areas.
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