Introduction
In recent years, the room-temperature liquid metal (LM), such as eutectic Galinstan composed of gallium, indium and tin, has been attracting great interest due to its unique attributes such as excellent fluidity, extremely low evaporation, high thermal conductivity and electrical conductivity, high surface tension, and generally chemical stability and nontoxicity. Recently intensive research efforts made in liquid metal have led to the development of emerging applications such as stretchable and soft electronics [
1,
2], microfluidics [
3–
5], energy management and storage [
6–
8], biomedical technology [
9], regulation of chemical reaction, actuators and soft robotics [
10–
12], and functional materials [
13–
16].
Cu-based coil is an indispensable electronic element of the electronic system, which has been widely applied for electromagnetic signal generation and reception, and electromagnetic actuator. However, Cu-based coil is hard to be stretched for dynamical transformation of complicated shapes. Fortunately, the room-temperature liquid metal makes flexible coil possible. Guo et al. [
17] have designed an electromagnetic actuator based on LMC (liquid metal coil) with a complete flexibility. Lazarus et al. [
18] have developed a multilayer LMC for stretchable inductors. Jin et al. [
19] have applied LMC to fabricate a stretchable acoustic device. Fassler and Majidi [
20] have prepared hyperelastic strain sensing and stretchable electronics based on LMC. Zhao et al. [
21] have designed a flexible electronic system based on LMC for eye movement tracking. Guo et al. [
22] have developed a wearable healthcare device with a wireless power transfer based on LMC. However, few studies can be found on LMC to electromagnetic collection for energy harvesting and electromagnetic generation for wireless charging. In fact, electromagnetic energy harvesting from wire carrying alternating current, such as medium-voltage power lines, is an emerging technology to power supply for sensors and electronic measurement systems [
23,
24].
In this paper, the flexible LMC was prepared instead of the traditional rigid coils for electromagnetic energy harvesting and wireless charging. The liquid metal of Galinstan was used to prepare an LMC, which could sustain stretching, twisting and bending with large deformation, and had a good electrical contact performance with the external circuit. The LMC based magnetic energy harvester was then designed and demonstrated to collect magnetic field energy induced by wire carrying alternating currents. In addition, the flexible spiral-shaped LMC was also designed and demonstrated to power cellular telephone through wireless charging.
Materials and methods
The room-temperature liquid metal of Galinstanalloy was adopted here, which is composed of 68.5 wt% of gallium, 21.5 wt% of indium, and 10 wt% of tin. These primitive elements which were supplied by Zhuzhou Yilong Hung Industrial Co., Ltd. were mixed together by the given proportion at 200°C in a vacuum drying oven for 2 h to prepare the eutectic Galinstan alloy. The strip-shaped nanocrystalline iron alloy film with a thickness of 0.1 mm and a width of 14 mm was obtained from the Advanced Technology and Materials Co., Ltd., and silicone of Ecoflex 00–30 from Smooth-On Inc.
Figure 1 shows the preparation process of LM wire. First, the LM was injected to the elastic tube through a syringe pump driven by a motor. To expel air bubbles inside, the elastic tube perfused with LM was kept tight tension, which was important for avoiding the LM disconnection under large deformation as illustrated in Fig. 1(a). Secondly, Cu wire was used to insert elastic tube to get in touch with the LM for the external circuit, which was then tightly fastened using the thin thread as shown in Fig. 1(b). It is noteworthy that the oxide layer of Cu wire would severely weaken the electrical conduction between Cu and LM. Although the oxide layer of Cu was removed during the preparation process, its interface would quickly generate a new layer of oxide from the LM. To resolve this problem, the Cu wire surface was deposited with a layer of conductive CuGa
2, which had a good wetting performance with the LM. Thus, it was used to prevent the oxidation generation and maintain a good electrical contact. As previously described in Ref. [
25], it is very convenient to plate CuGa
2 only by inserting the Cu wire into the Galinstan immersed in 0.5 M NaOH solution and applying a direct current of 0.1 A, which was demonstrated in Fig. 1(c). The as-prepared LM wire is shown in Fig. 1(d) and could be stretched to 200%.
Results and discussion
Deformation performances of LM wire and coil
The major advantage of the LM wire is that it can sustain a larger tensile deformation compared with Cu wire. Figure 2 shows the electric resistances of the LM wires at stretching, twisting, bending and folding, respectively, where the lengths of all the LM wires are fixed as 100 mm. It is observed from Fig. 2(a) that the electric resistance of the LM wire with an inner diameter of 1 mm reaches up to its 200% at strain 150%, while increasing 145% for an inner diameter of 3 mm. The electric resistance increases due to its diameter decrease after stretching. Figure 2(b) presents the impact of twisting on the electric resistance of LM wire. The results indicate that the torsion with 720 degrees has less effect on the electric resistance. The bending test results shown in Fig. 2(c) have similar results with the case for twisting. However, the folding has severely weakened the electric resistance of the LM wire as exhibited in Fig. 2(d). The above results suggest that the LM wire has an ability of large deformation without leakage due to the flexible silicone tube encapsulation.
To prepare the spiral LMC consisted of a flexible LM wire with an inner diameter of 1mm, the silicone of Ecoflex 00–30 was applied to fix the spiral LMC into the flexible disc shape as depicted in Fig. 3(a). The disc-shaped LMC has an outer diameter of 4 cm and an inner diameter of 1.5 cm, where the total length of the LM wire is about 130 cm. Figure 3 shows the impact of LMC deformation on its electric resistance. The results indicate that the spiral LMC can sustain a variety of large-scale deformation, which may be very useful for self-adaptation to the system shape with dynamical deformation.
LMC for electromagnetic energy harvesting
The principle of an electromagnetic energy harvester is based on inductive coupling, which is like a current transformer. That is, a primary wire carrying alternating current produced varying magnetic flux in annular magnetic core, which, in turn, generates the alternating current in the LMC around the annular magnetic core as illustrated in Fig. 4(a). The induced alternating current in the LMC can be transformed to direct current by the AC-DC chip. First, AC-output is filtrated and smoothed to be considered as the input of DC-DC transformer of MAX5035D. This chip is designed to allow the DC input with a voltage range of 7.6–76 V, and DC output with a constant voltage of 5 V. The test platform of the electromagnetic energy harvester is illustrated in Fig. 4(a), and the corresponding experimental device diagram presented in Fig. 4(c). The annular magnetic core is composed of the nanocrystalline alloy with an initial relative permeability of about 5000, which has an outer diameter of 100 mm, an inner diameter of 50 mm, and a height of 25 mm as shown in Fig. 4(b), respectively. The LMC turn number of 100 is adopted, where the LM wire has an inner diameter of 1 mm, a length of 1300 mm, and an electric resistance of 4 W. The oscilloscope Tektronix TDS2024C was used to measure the AC voltage from the LMC, and an electronic load ITECH IT8511A was applied to connect the DC output from the chip of MAX5035D. In addition, the AC power was applied to generate the AC current from 0 to 200 A with a frequency of 50 Hz.
Figure 5 shows the output voltage of the LMC for different electronic loads and the currents carrying in the primary wire. It is found that the effective voltages of the LMC output increase with the primary current increasing, while the large primary current makes the magnetic core quickly approach to the saturation region, which induces the voltage wave pattern deformity. When there is no electronic loading, the voltage peak can increase from 12.5 V to 21.3 V when the primary current changes from 2 A to 14 A. In fact, the magnetic core keeps the saturation region at a small primary current of 2 A. It is noteworthy that increasing the electronic load could prevent the magnetic core saturation from occurring at the large primary current, which could be observed from Figs. 5(a)–5(e). When the electronic load is about 20 W, the magnetic core cannot approach to the saturation region for a primary current smaller than 60 A.
Figure 6 shows the impact of electronic load on the output power of AC-DC chip. It is observed that the output power reaches up to the saturation sate for a large primary current. An output power of 0.26 W could be obtained for the electronical loading of 50 W and the primary current of 10 A, which could be applied as the power supply for many sensors with low power consumption.
The above magnetic core is rigid which limits the flexibility of LMC. Thus, the flexible magnetic core was designed and prepared here. The nanocrystalline iron alloy strips with a thickness of 0.1 mm and a width of 20 mm were considered as magnetic materials, which were shaped in rings with a turn number of 25. The annular strips were then encapsulated by silicone composites, which were composed of Ecoflex 00–30 with 80 wt% of nanocrystalline iron microparticles. The as-prepared flexible magnetic core with an outer diameter of 45 mm, an inner diameter of 20 mm, and a height of 15 mm was presented in Fig. 7(a). It is observed from Fig. 7(a) that the nanocrystalline iron microparticles with a size of 5–10 mm are uniformly distributed in an elastic silicones matrix. The flexible magnetic core can be stretched to 150% and compressed to 50% as shown in Figs. 7(b) and 7(c), respectively. In addition, it can sustain twist with a large deformation.
The LMC turn number of 100 was adopted for flexible magnetic core, where the LM wire had an inner diameter of 0.5 mm, a length of 800 mm, and an electric resistance of 6.5 W as shown in Fig. 8.
Figures 9(a)–9(e) show the output voltage of LMC based flexible magnetic core for different electronic loads and the currents of primary wire. It is observed from Fig. 9 that the flexible magnetic core more easily approaches to saturation compared with the rigid core due to its smaller permeability. Thus, it would generate the smaller output voltage. However, it may be very useful for many applications where the shape needs dynamical deformation.
LMC for cellular telephone wireless charging
Another interesting application presented here is to charge cellular telephones wirelessly using LMC as electromagnetic generation. The spiral LMC presented in Fig. 8 and the magnetic conductor were prepared as the high frequency magnetic field generator through the DC-AC chip, which would generate the induction current in the receiving coil built in the mobile phone. The flexible magnetic conductor was prepared through silicone composites imbedded with 80 wt% of nanocrystalline iron microparticles. The induction current is further more transformed to DC for power supply for cellular telephones. The principle of wireless charging to cellular telephones is illustrated in Fig. 10(a). Figure 10(b) shows the image of the wireless charging system for the cellular telephone which is located on top of the spiral LMC.
The high frequency alternating current input to spiral LMC is presented in Fig. 11(a). It is observed from Fig. 11(b) that the wireless charging system based on the spiral LMC charges the SM-G9250 cellular telephone with a built-in battery of 2600 mAh from 20% to 100% in about 300 min.
Conclusions
In summary, a flexible LMC was successfully prepared. The applications based on LMC for electromagnetic energy harvester and wireless charging for cellular telephone were designed and demonstrated. The as-prepared LMC can sustain stretching, twisting, and bending with a large deformation. Besides, it has a good electrical contact stability with external circuit. The present study opens the way for further development of elastic LMC applications in electromagnetic energy harvesting and charging.
Higher Education Press and Springer-Verlag GmbH Germany, part of Springer Nature
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