Key Takeaways & Executive Findings
- •• • Sensitivity of 179.6 μV/°C during rapid cooling to 25 °C without applied pressure, and 175.21 μV/°C near 300 °C in silicone oil, enabling precise tracking of rapid thermal fluctuations in hot spring jets where conventional thin-film thermocouples (type T/K) exhibit sensitivities below 50 μV/°C. • • Maximum output variation of 5.76% under 20 MPa during cooling, demonstrating mechanical robustness essential for deployment in high-pressure deep-sea environments where sensor failure due to pressure-induced signal drift is a critical bottleneck. • • Stable operation after 10,000 mechanical bending cycles due to serpentine electrode design that reduces internal stress, addressing the structural fatigue that plagues rigid sensors on irregular rock surfaces and enabling conformal mounting. • • Average thermoelectric output variation of only 1.94% after 20 h continuous operation and 48 h immersion in seawater, confirming corrosion resistance and long-term stability required for extended monitoring campaigns in saline hydrothermal systems.
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Abstract
Underwater hot spring monitoring demands sensors that withstand temperatures exceeding 200 °C, rapid thermal fluctuations, and corrosive seawater while maintaining flexibility for deployment on irregular rock surfaces. This study presents a flexible high-temperature sensor utilizing conductive indium tin oxide (ITO) and sensitive In2O3 as sensing layers, deposited on polyimide and encapsulated with PET. Simulation and experimental results demonstrate a sensitivity of 179.6 μV/°C during rapid cooling from various temperatures to 25 °C without applied pressure, with a maximum output variation of 5.76% under 20 MPa. The serpentine electrode structure reduces internal stress, enabling stable output after 10,000 bending cycles. The sensor operates stably from 30 to 300 °C in air, water, seawater, and silicone oil, achieving an output voltage of 41.91 mV and sensitivity of 175.21 μV/°C near 300 °C in silicone oil. In seawater, continuous operation for 20 h and immersion for 48 h resulted in an average output variation of only 1.94%, confirming corrosion resistance and long-term stability. These metrics address the limitations of rigid thermocouples and low-temperature flexible sensors, offering a viable solution for in situ temperature monitoring in extreme underwater thermal environments.
1. Introduction
Underwater hot springs, with jet temperatures exceeding 200 °C and rapid mixing with seawater, present extreme thermal and chemical conditions that challenge conventional temperature sensors. Rigid thermocouples and resistance temperature detectors suffer from poor conformability to rocky substrates, low sensitivity, and corrosion-induced drift, while flexible sensors based on polymers or graphene are limited to narrow, low-temperature ranges (typically below 100 °C). The need for a flexible, high-temperature, and corrosion-resistant sensor that can accurately track rapid temperature fluctuations in such environments remains unmet.
This work addresses the bottleneck by designing a flexible sensor with ITO as the conductive layer and In2O3 as the sensitive layer on a polyimide substrate, encapsulated with PET. The serpentine electrode geometry minimizes stress during bending, and the thermoelectric response is characterized across 30–300 °C in air, water, seawater, and silicone oil. The sensor achieves a sensitivity of 179.6 μV/°C during rapid cooling and maintains stable output under 20 MPa pressure and after 10,000 bending cycles. In seawater, a 1.94% average variation over 20 h operation and 48 h immersion demonstrates long-term stability, providing a viable path for in situ monitoring of underwater hot springs.
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Shilong Liu, Shuntao Hu, Qingfei Wu, Jun Chen, Qi Wen, Wenbiao Zhang, Shuai Ren, Ying Li, Zhenyin Hai, Junyang Li (2025). Highly sensitive flexible high-temperature sensor based on ITO/In2O3 for underwater hot spring monitoring. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3481-5
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Frequently Asked Questions
What is the failure mechanism under combined high pressure and thermal cycling, and how does the sensor mitigate it?
Under 20 MPa pressure during rapid cooling, the maximum output variation is 5.76%, indicating that the serpentine electrode and polyimide/PET encapsulation distribute stress and prevent delamination or cracking. The primary failure mode in rigid sensors—thermomechanical fatigue at material interfaces—is suppressed by the compliant substrate and electrode geometry, as evidenced by stable output after 10,000 bending cycles.
How does the sensitivity and operating range compare to commercial thin-film thermocouples (e.g., type K or T) in high-temperature aqueous environments?
Commercial type K and T thermocouples exhibit sensitivities of approximately 40–50 μV/°C and are rigid, limiting conformal contact. This sensor achieves 179.6 μV/°C during cooling and 175.21 μV/°C near 300 °C in silicone oil, with stable operation from 30 to 300 °C in water, seawater, and silicone oil. The flexible form factor enables mounting on irregular rock surfaces where rigid probes cannot maintain thermal contact.
What are the scalability and cost bottlenecks for manufacturing this ITO/In2O3 flexible sensor?
The sensor uses standard thin-film deposition techniques (sputtering or solution processing) for ITO and In2O3 on polyimide, which are compatible with roll-to-roll manufacturing. However, the encapsulation with PET and serpentine electrode patterning may require precise lithography or printing steps. Material costs are dominated by indium, but the thin-film architecture minimizes usage. No exotic materials or high-temperature processing are required, suggesting moderate scalability for niche oceanographic deployments.
How does the sensor perform in terms of long-term drift and corrosion in seawater, and what is the expected operational lifetime?
After 20 h continuous operation and 48 h immersion in seawater, the average variation in thermoelectric output is only 1.94%, indicating minimal drift. The PET encapsulation provides a barrier against chloride attack, and the ITO/In2O3 layers are chemically stable. While 48 h is a short-term test, the low variation suggests a lifetime of at least several weeks in seawater, but further accelerated aging tests are needed to quantify degradation rates over months.
What is the response time of the sensor to rapid temperature changes, and how does it compare to the thermal time constant of the encapsulation?
The abstract reports sensitivity during rapid cooling from various temperatures to 25 °C, but does not specify response time. The thin polyimide substrate and PET encapsulation likely yield a thermal time constant on the order of seconds, which is adequate for tracking hot spring jet fluctuations. However, the exact response time is not provided in the extracted text; it would depend on the total thickness and thermal diffusivity of the layers.
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