研究目的
To investigate the effect of different inorganics (activated carbon and lithium chloride) on the performance of polyimide-based fiber Bragg grating humidity sensors, aiming to improve sensitivity, stability, and response time.
研究成果
The experimental results demonstrate that adding activated carbon (AC) or lithium chloride (LiCl) to polyimide coatings improves the humidity sensitivity of FBG sensors, with AC providing better stability and LiCl enhancing sensitivity but reducing repeatability. The combined LPC structure (LiCl-doped PI covered with AC) offers the highest sensitivity increase (3.9 times) while maintaining good stability and short response time. The sensors show linear responses to RH and temperature, making them suitable for high-precision humidity measurements. Future work could focus on optimizing inorganic content and exploring other materials for further enhancements.
研究不足
The main limitation noted is the time required for the THTC to form stable RH environments, which can delay testing. Saturated salt solutions were used to mitigate this for response time measurements. Other potential limitations include the sensitivity to temperature variations and the complexity of coating processes, which might affect reproducibility.
1：Experimental Design and Method Selection:
The study used a dip coating process to deposit organic-inorganic hybrid coatings on fiber Bragg gratings (FBGs) to form humidity sensors. The rationale was to combine the advantages of polyimide (PI) with inorganics like lithium chloride (LiCl) and activated carbon (AC) to enhance sensor performance. Theoretical models included the Bragg wavelength shift principle for FBGs (Eq. 1-3 in the paper).
2：Sample Selection and Data Sources:
Bare FBGs were used as the base sensors. Coatings were applied using polyamic acid (PAA) with additions of LiCl (1% by weight) and AC. Saturated salt solutions and a temperature & humidity test chamber (THTC) provided controlled RH environments for testing.
3：List of Experimental Equipment and Materials:
Equipment included a stepping motor for coating deposition, an oven for heating, an optical performance monitor (OPM) module with light source and demodulation, a THTC for environmental control, and saturated salt solutions. Materials included FBGs, Piranha solution (H2SO4 and H2O2), silane coupling agent (KH550), polyamic acid, lithium chloride, activated carbon, deionized water, and ethyl alcohol.
4：Experimental Procedures and Operational Workflow:
FBGs were cleaned with Piranha solution, treated with silane coupling agent, and coated via dip coating with PAA-based solutions (pure PAA, PAA+AC, LiCl-doped PAA, LiCl-doped PAA+AC). Coating was repeated four times, followed by heating for imidization and stress relief. Sensors were tested in THTC and with saturated salt solutions for RH sensitivity, temperature sensitivity, repeatability, stability, and response time, with data recorded by a computer.
5：Data Analysis Methods:
Data analysis involved calculating wavelength shifts, sensitivities (pm/%RH and pm/°C), response times, and deviations in stability tests. Statistical measures like maximum deviation and standard deviation were used for repeatability assessment.
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