Thermally stable metallic glass strain sensors with extended sensing range and sensitivity

Abstract

Metallic glasses (MGs) are metal alloys with attractive properties for use as electrodes in strain sensors, such as high conductivity, mechanical strength, and strong corrosion resistance owing to their unique amorphous atomic arrangements, which are critical criteria for highly reliable sensors. However, MG-based strain sensors that may detect strains of >1% with high gauge factors have not yet been reported. In this study, an AlY-MG thin film in an amorphous phase is successfully deposited on a target substrate via sputter deposition. Whereas layer-by-layer-assembled graphene nanoplatelets (GNP) enable stable strain sensing over an extended range, the amorphous atomic arrangement of the MG film resulted in more distinct changes in electron conducting path under strains from 2% to 10%, which is induced by cracks initiated on the surface of the AlY film. Furthermore, the chemical stability of the sputtered AlY thin film is examined via electrical and optical analyses, which prove the high chemical stability at the interface between AlY and the GNP layer. As a result, a strain sensor based on the AlY film, which may detect various human motions when adhered to the skin, is developed.

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Introduction

Since the emergence of flexible devices and real-time monitoring technologies based on the internet of things, demand for high-performance wearable sensors, such as strain sensors, has continuously increased.^1–4 A strain sensor is a device that detects motion and deformation and converts the deformation into an electric signal. As commercially available strain sensors comprise rigid substrates that may not detect minute human signals and movements, wearable thin-film strain sensors that may be adhered to human skin are of interest to researchers. To accurately detect human motion, the strain sensor should detect a wide range of strains (∼30%) with high sensitivity and reliability.^5,6 Furthermore, to increase the range of applications, the component materials should be chemically stable to maintain the initial performance, even in the harsh environments of various outdoor sports.^2,7–11

Metallic glasses (MGs) are non-crystalline metals that exhibit short-range periodic atomic arrangements.^12,13 Unlike conventional metals, MGs lack grains and grain boundaries, which endows them with excellent chemical stability and mechanical strength.^12–14 Rapid oxidation and corrosion, which occur on crystalline metals, are often attributed to the grain boundaries on their surfaces because these are active sites where chemical reactions occur. Additionally, mechanical strains exerted on metallic electrodes usually cause degradation in the electrical conductivity, which is accompanied by the irreversible isolation of grains.^12,15 In contrast, the absence of grains and grain boundaries within MGs results in excellent mechanical strength and high oxidation resistance. Owing to these properties, numerous attempts to exploit their characteristics in various wearable and flexible devices have been reported.^16–19 Their high electrical conductivity and exceptional mechanical and chemical stabilities^20–22 convinced researchers that MGs are strong candidates for use as the key component materials for next-generation wearable devices.

In the case of strain sensors, Xian et al. deposited a Zr-based MG thin film on a polycarbonate film as a component of a strain sensor. The Zr-based MG thin film exhibited high thermal and chemical stability, which confirmed the efficiency of the MG in a wearable device.²³ In addition, MG thin film deposited on a flexible printed circuit board as a strain sensor²⁴ functioned with bending strains under 0.190%. The previous attempts^23,24 on applying MG as a strain sensing electrode have primarily focused on the MG's geometrical deformation without the formation of cracks, which usually occurs under low levels of strain.

Crack-based strain sensors^25–28 utilize changes in an electrically conductive path on the materials that are subjected to specific amounts of strain. To achieve a high gauge factor (GF), high conductivity materials such as metals are used as electrodes,^26–28 while elongation of the operation range of the sensors has been attempted by incorporating high flexibility materials such as CNTs.^29–31 In this respect, a stretchable strain sensor with an Al-based MG thin film and additional graphene layer stretched to 10% strain is fabricated. Due to the high stretchability and rearrangement property of graphene nanoplatelets (GNPs),³² the GNP layer combined with Al-based MG film would be capable of sensing extended ranges of strain. To form dense GNP on an elastomeric substrate, layer-by-layer assembly coating³¹ of aqueous polyvinyl alcohol (PVA) and GNP was conducted; solution-based coating of oppositely charged materials produced a flexible conductive path that withstands an extended range of strains. Furthermore, the short-range ordered atomic arrangement found in the amorphous materials resulted in different crack formation behavior to that of crystalline electrodes, which resulted in a higher gauge factor at larger strain levels. Furthermore, the Al-based MG electrode is found to be more chemically inert, which prevented degradation of device performance even under high temperatures.

2. Material and methods

2.1 Fabrication of the metallic glass sputtering targets

MG sputtering targets with Al-based atomic composition (AlY) were produced via powder metallurgy processing using metallic glass powder. First, a master alloy with the designed atomic composition of Al₉₀Y₁₀ was prepared in a vacuum induction furnace. Then, MG alloy powders were produced using an Ar gas atomizer. The powders were then sintered via hot isostatic pressing at a respective temperature and pressure of 813 K and 60 kg mm⁻² to produce consolidated billets. Subsequently, the Al-based MG was sintered at 723 K. Finally, disc-type targets with thicknesses of 2 in. were fabricated via cutting and grinding.

2.2 Characterization of the Al and AlY thin film electrodes

The amorphous property of the AlY thin film was evaluated via X-ray diffraction (XRD) of films with thicknesses of 100 nm on clean glass substrates using a D8 Advance X-ray diffractometer (Bruker, Billerica, MA, USA). Reflectance spectra of Al and AlY film were obtained with a UV-Vis spectrophotometer (Lambda 365, PerkinElmer, United States). Strain sensing performances of strain sensor devices with Al and AlY were evaluated with a Keithley 2400 sourcemeter.

2.3 Strain sensor fabrication

A stretchable polydimethylsiloxane (PDMS) substrate was prepared by mixing the base and curing agent (20 : 1, w/w) and then pouring the mixture onto a clean Si wafer. After 2 h of thermal curing at 80 °C in a convection oven, the PDMS substrate was carefully peeled from the Si wafer and cut to the desired size. Subsequently, a dense layer of GNPs was coated on the PDMS substrate via LbL assembly, as previously reported.^32,33 A PVA solution for use in LbL coating was prepared by dissolving 0.2 wt% PVA in deionized (DI) water and stirring the solution at 80 °C. The GNPs were dispersed in DI water with polystyrene sulfonate (PSS, 0.2 wt%) via sonication for 3 h. To form a GNP layer on the hydrophobic PDMS surface, the PDMS substrate was immersed in the PVA solution for 5 min, rinsed with DI-water to remove the excess PVA, and then dried thoroughly by heating on a hotplate (80 °C, 10 min). The PDMS coated with hydrophilic PVA was then immersed in the GNP solution for 5 min, followed by rinsing and heating (Fig. S1, ESI†). The PVA and GNP coating was repeated 10 times to form an LbL-assembled GNP film on the PDMS. Formation of GNP layers on the PDMS via LbL assembly was confirmed through SEM images and transmittance shown in Fig. S2 (ESI†). Subsequently, the metallic electrodes, such as Al and AlY, were deposited on the GNP films via direct current (DC) sputtering. For deposition of both Al and AlY films, sputter deposition was conducted at a base pressure of 2 × 10⁻⁶ Torr under an Ar gas flow of 60 sccm with a working pressure of 7 mTorr. The Al and AlY electrodes were deposited at a sputter power of 100 W, and the thicknesses of the films were measured using alpha-step surface profilometry (DektakXT, Bruker) and confirmed to be 100 nm. After sputtering, the substrates were carefully handled such that no external stress was applied before strain sensor evaluation. Electrical contacts on the sputtered metallic electrodes were prepared using Ag paste and Cu tape, and then the GNP- and metal-coated PDMS substrates were encapsulated with PDMS for stable strain sensor operation.

3. Results and discussion

Fig. 1 shows the schematics of the strain sensor structure and methods of GNP layer coating and metal deposition. LbL assembly is an effective method for forming thin films on various substrates. The technique is used for coating various substances, such as CNTs and graphene,^32–36 to yield a desired density and thickness. PVA is effectively attached to the PDMS surface via noncovalent bonding interactions, such as hydrogen bonding and van der Waals and hydrophobic attraction. The aqueous GNP solution is prepared by adding PSS,^32,37–41 which facilitates the dispersion of the GNPs in the polar solvent. Furthermore, the sulfonic groups of PSS also contribute to the formation of hydrogen bonds with PVA.³² The GNPs are strongly attached to PVA by a combination of van der Waals forces, charge transfer, and hydrophobic interactions. Subsequently, Al or AlY film with a thickness of 100 nm was deposited on the GNP layer via sputter deposition. A unique property of the MG thin film is the absence of grains and grain boundaries; for the thin film to exhibit this property, sputter deposition is widely employed, as sputtering ensures the rapid cooling of the metallic atoms upon deposition on the target substrate under controlled conditions.⁴² To ensure that the deposited AlY film was in the amorphous phase, the sputtered Al and AlY thin films were subjected to XRD.

Fig. 1 Schematics of the device structure and fabrication of the AlY thin film-based strain sensor. (a) Schematic of sputter deposition on the layer-by-layer-assembled GNP layer. (b) Schematic of GNP/AlY under strain. PDMS, polydimethylsiloxane.

The XRD patterns of the sputter-deposited Al and AlY thin films are shown in Fig. 2. As commonly observed in the XRD patterns of conventional metals, sharp Bragg peaks including the peak that corresponds to the (111) plane at 2θ = 40° indicate that the sputtered Al film exhibits highly crystalline properties with long-range-ordered atomic arrangements.^43,44 However, the XRD pattern of the AlY film does not indicate such characteristics, but exhibits only broad maxima, as observed in previous studies regarding Al-based MGs.^44–48 Therefore, the atomic arrangement in the sputtered AlY thin film did not induce Bragg diffraction, which suggests that an amorphous atomic arrangement is dominant throughout the sputtered thin film. The XRD pattern shown in Fig. S3 (ESI†) reveals that the AlY thin film with a thickness of 100 nm deposited on the GNP layer also maintained its stable amorphous phase.

Fig. 2 X-ray diffraction patterns of the Al and AlY thin films sputtered on glass substrates. The inset shows the sputtered thin film on the glass substrate, and the scale bar represents 2 mm.

The difference revealed by the XRD patterns directly affects the crack formation behaviors of the crystalline and MG thin films, as shown in the optical microscopy images of the Al (Fig. 3a–c) and AlY films (Fig. 3d–f) deposited on LbL-coated GNPs. Compared to the Al film, the AlY thin film exhibited stronger resistance to crack formation under a lower stretching strain. As a result, for a 5% to 10% strain, a noticeable difference in the amount of cracking on the MG surface was observed. At a 10% strain, several newly formed short cracks were observed that may not be observed at lower strains. On the surface of the Al film on the GNP layer, transverse cracks were formed at lower stretching strain, and at the higher levels of exerted strain there are more rearrangements in the grains rather than the formation of new cracks.

Fig. 3 Optical microscopy images of GNP/Al and GNP/AlY under (a) and (d) 0%, (b) and (e) 5%, and (c) and (f) 10% strain, and a schematic of the crack formation behaviors of the (g) Al and (h) AlY thin films.

Strain sensors with crystalline metal thin films often suffer from short detection ranges in terms of strain because the crystalline metals are often abruptly isolated between the metallic domains along the grain boundaries at low strains (< 2%).^31,49–51 As shown in Fig. 3g, ductile deformation often occurs with the formation of intergranular fractures along slip planes in the crystalline structure, such as grain boundaries and dislocations. These slip planes act as resistance against external stress, which minimizes the electrical resistance change in Al thin films deposited on GNPs. Thus, since these local strains of Al thin films are not high enough to cause plastic deformation of the metal network due to torsion and out-of-plane deflection, when the metal film on the elastomer is stretched, the Al thin films reveal relatively insufficient resistance change to be applied as a strain sensor. Even at higher tensile strains (10%), the grains are isolated and slip instead of exhibiting new transgranular cracks,^12,52 causing a minor change in the electrically conductive pathway of the Al thin film from that of the electrode at a lower applied tensile strain, as shown in Fig. 4a. Conversely, suppressed crack formation at a lower strain and a gradual increase in the number of cracks at a higher strain can be observed in the AlY thin film, which may affect the intensity of the stable signal under low external strain. When tensile strain is applied to the AlY thin film on GNPs, the initial shear band expands in a direction perpendicular to the stretching direction. Accordingly, the conductive paths in the AlY thin film become narrower and longer due to obstruction by increased shear bands. As a result, the resistance of the AlY thin film increases during the stretching process due to the formation of the shear fracture, which contributes to the large difference in resistance and high gauge factor, as shown in Fig. 4b. After the strain is released, the AlY thin film returns to the initial state, recovering its original electrically conductive path.

Fig. 4 Mechanism of changes in conductive paths of (a) GNP/Al and (b) GNP/AlY. Crystalline characteristic of Al results in a minor change in conductive path, while the amorphous property of AlY results in distinct crack formation on the electrode, which results in higher G.F. at higher strain.

The differences in the crack formation behaviors of Al and AlY on the GNPs in Fig. 3 are also related to the results of the strain sensors based on their respective structures. Fig. 5 compares the performance and parameters of the strain sensors of the Al and AlY thin films deposited on the GNPs. Fig. S4 (ESI†) shows that the conductive path on the GNP layer is preserved under 10% strain and the resistance changes reversely after each stretching cycle. This confirms that it can be used in organic-metal bilayered sensors and exhibits stable strain-sensing capacity. Overall, the additional metallic film on the LbL-coated GNP layer significantly increases the GF because the electrically conductive metal layer provides a higher contrast upon crack formation due to external stress. The relative change in resistance before and after the stretching (ΔR/R_o) of the GNP/Al layer (Fig. 5c) increased with the amount of strain for a strain of <6%. However, the intensity of the resistive response at a strain of <10% was not discernible from that at a strain of <8%, and thus, the sensor may not detect a strain of >8%. GF of a strain sensor is calculated with the relative change in resistance and the amount of strain applied:

where R_o is the original resistance before the application of strain and ε is the amount of strain. The respective GFs of the GNP/Al device at 2% and 10% strains were 133.1 and 110.2. The relative change in resistance and GF of GNP/AlY increased noticeably as the strain increased from 2% to 10%. Whereas the GF was comparable to that of GNP/Al at 2% strain, the GF at 10% strain was 501.6, which is 4.5-fold larger than that of GNP/Al. Furthermore, the stability of the device with GNP/AlY was evaluated under cyclic strain (Fig. 5e), which confirmed that GNP/AlY is applicable for use in sensing various repeated human motions.

Fig. 5 Resistive responses of (a) GNP/Al and (b) GNP/AlY under various stretching strains (0–10%). (c) Relative changes in resistance of the strain sensors and (d) gauge factors as functions of the strain. (e) Relative change in the resistance of GNP/AlY over 100 cycles of stretching. ΔR/R_o, the relative change in resistance before and after stretching.

To evaluate whether GNP/AlY could accurately detect human motions, the strain sensor was attached to an index finger and fixed with a bandage for stable measurement while bending. The resistive responses to the bending of the index finger at angles of 30°, 90°, and 110° (Fig. 6a) are shown in Fig. 6b. From 30° to 90°, the relative change in resistance increased from 0.6 to 1.0, which indicates that the sensor can discern the different strains at low and high bending angles. At 110°, the intensity of the resistive signal considerably exceeded the intensities observed at 30° and 90° bending, as the sensor was subjected to not only a larger bending strain but also a larger lateral stretching strain than at lower-angle movements. Additionally, to examine whether the GNP/AlY device could also detect various types of strains, such as external pressure, its resistive responses were recorded while its center was pushed at different forces (Fig. 6c and d). The strain sensor exhibited a resistive response with increasing intensity as the device was pressed with a stronger force. Thus, GNP/AlY provides a suitable structure for fabricating strain sensors that can precisely detect various motions.

Fig. 6 (a) Bending behavior of the strain sensor worn on a finger joint. (b) Resistive responses caused by the bending motions of various angles. (c) Exerting pressure on the strain sensor. (d) Relative changes in resistance during the touch sensing studies under different pressures. ΔR/R_o, relative change in resistance before and after stretching.

Application in the detection of actual human motions requires stability under various temperatures: motion sensors original performance should be preserved under changing environments of various outdoor activities. However, crystalline metals that have been used for strain sensors often suffer oxidation, and the degradation accelerates when the metallic parts are in contact with moisture or organic substances. MGs are renowned for their high levels of chemical inertness due to their randomly oriented atomic arrangements. Fig. 7 compares the strain sensing performances of devices with Al (a) and AlY (b) before and after aging at 100 °C for 60 hours. The gauge factor measured before and after aging is summarized in Fig. S5(a) (ESI†). After 60 hours of aging, the sensitivity of the sensor with the Al film decreased sharply from 133.1 to 46.0 under 2% strain and 110.2 to 84.0 under 10% strain. Conversely, the GF of the sensor with the AlY film remained unchanged before and after heating for 60 h: the GFs were 108.6 and 129.8 (at 2% strain) and 501.6 and 517.4 (at 10% strain) before and after aging. In addition, Fig. 7a shows that the original resistance (R_o) value did not fully recover after each bending cycle. To examine the interfaces between the metal films and LbL-deposited GNP layers, two samples were analyzed using reflectance spectroscopy (Fig. S5b, ESI†). After aging, a sharp decrease in the reflectance of the Al surface was observed, which suggests that the Al thin film underwent oxidation reactions with the GNPs, PVA, and PSS.^53,54 Conversely, the reflectance of AlY was not affected by the aging process, which should be due to the chemical inertness of the AlY thin film. The formation of insulating oxide layers at the interface between the metallic electrode and the graphene layer hinders the recovery of the conductive path as Fig. 7b exhibits a slower decrease of ΔR/R_o for each bending cycle. The undesirable oxidation often causes competition between construction and destruction of conductive paths in the device⁵⁵ upon release of the tensile strain. This should delay the recovery of the conductive paths of the strain sensor with Al. However, Fig. 7c and d exhibit excellent stability of the strain sensor device with AlY, with well-recovered GF and R_o for each bending cycle. The stability results suggest that the MG thin film is a favorable candidate for use in a strain sensor electrode in several practical strain sensing applications that require electrode materials with high thermal and corrosion resistance.

Fig. 7 Strain sensing performances of (a) and (b) GNP/Al and (c) and (d) GNP/AlY under 2% strain, before and after storage at 100 °C for 60 hours. The device with Al exhibited a significant reduction in GF and incomplete recovery of R_o after each strain cycle, compared to highly stable results produced from the device with AlY.

4. Conclusions

An Al-based MG (AlY) thin film was sputter-deposited on an LbL-assembled GNP layer to form a stretchable strain sensor. Unlike the conventional Al thin film, the AlY film showed crack propagation over a wider range of strains because of its amorphous nature. However, compared to the Al thin film, the AlY film did not display rapid crack propagation under a low strain, which resulted in a gradual increase in the relative change in resistance and an increase in the GF of the strain sensor at strains of 2–10%. Furthermore, the amorphous property of the AlY film on the LbL-assembled GNP layer rendered it chemically stable at a high temperature (100 °C). Therefore, the performance of the strain sensor with AlY maintained its original sensitivity, even under such harsh conditions. Thus, the resistive responses and stability analyses confirm that AlY is a promising material for use in wearable strain sensors that may be utilized in different environments with high reliability.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Basic Science Research Program via the National Research Foundation of Korea, funded by the Ministry of Science and ICT of the Republic of Korea (No. 2023R1A2C2008021 and RS-2023-00217270). This work was also supported by the Technology Innovation Program (no. 20017439, ‘Development of manufacturing process technique on high-speed signal transmission line for 6G device,’ and no. 20021915, ‘Development on Nanocomposite Material of Optical Film (GPa) for Foldable Devices’) funded by the Ministry of Trade, Industry, and Energy (Republic of Korea).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc00748d

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