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Abstract
In the context of optical fiber humidity sensing, the long-term stability of sensors in high humidity and dew environments such as bathrooms or marine climates remains a challenge, especially since many humidity sensitive materials are water soluble. In this study, we use methyldiethanolamine, pentaerythritol triacrylate and Eosin Y to form a liquid-solid structure humidity sensitive component, the outermost layer is coated with PDMS passivating layer to ensure the stability and durability of the humidity sensor under the conditions of dew and high humidity. The liquid microcavity of the sensor consists of methyldiethanolamine-pentaerythritol triacrylate composite solution, and the sensitivity is several times higher than that of the liquid-free cavity sensor. The sensitivity of the sensor to temperature is verified (0.43 nm/°C and 0.30 nm/°C, respectively) and temperature crosstalk is compensated using a matrix. The compact structure allows for ultra-fast response (602 ms) and recovery time (349 ms). Our work provides a promising platform for efficient and practical humidity and other gas monitoring systems.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the increased demand for precision detection and control of humidity in medical health, chemical synthesis, and intelligent manufacturing, humidity sensors have been rapidly developed [1,2]. Optical fiber relative humidity sensors have received considerable attention due to their many outstanding advantages, such as small size, high sensitivity, high flexibility, and good resistance to electromagnetic interference [3,4]. The most common and effective way to improve the performance of humidity sensors is to integrate moisture-sensitive materials on optical fibers to improve sensor sensitivity, hygroscopic properties and dehydrating properties [5–8]. Several moisture-sensitive materials have been reported, for example, Ruijie Tong et al. developed a compact non-crosstalk humidity sensor using graphene quantum dots and polyvinyl alcohol (GQDs-PVA) as moisture-sensitive materials based on surface plasmon resonance (SPR) and Mach Zehnder interference principles [7]. Yin Liu et al. developed a gelatin coated Michelson interferometric humidity sensor based on helical multi-core optical fibers, with a sensitivity of -0.185 nm/%RH [9]. In addition, there are also polyelectrolytes, polyimide (PI), agarose, biological materials, and so on [10]. However, high-performance is only half the story. The other, and often overlooked half is water-durability. Because many humidity-sensitive materials with good water absorption are water-soluble. The humidity sensing materials tend to swell, shrink, flake or peel off from the sensors under prolonged. This situation is exacerbated in high humidity or dew environment, thus making them unusable under harsh conditions such as high temperature and humidity or a dewy atmosphere. Therefore, there is a great need to develop all-fiber humidity sensors that are effective in high humidity or excessive dew conditions [11–13].
In the pursuit of better water-durable performance, copolymerized hydrophobic groups, grafted hydrophobic polymers, interpenetrating network structures and inorganic/organic composites with applied protective layers have been developed for practical applications of humidity sensors [14–16]. Among them, applying a protective layer on the outer side of the humidity sensing structure proved an efficient and convenient method to improve humidity sensors’ water-resistance and environmental stability. Polydimethylsiloxane (PDMS) is a stable hydrophobic polymer coated on solid surfaces to enhance water resistance [17]. PDMS is used in coating treatments to form a protective hydrophobic layer on surfaces to enhance their structural/chemical stability under wet conditions [18]. Inspired by these works, we expect that PDMS coatings can be used as hydrophobic layers to enhance the dew resistance of humidity sensors. The choice of PDMS offers many advantages over other materials. First, the PDMS layer not only confers a hydrophobic surface to the sensor but prevents the humidity-sensitive part from direct contact with water in a dewy environment, which can lead to structural damage. It also has good gas permeability, which does not affect water molecules from entering the humidity-sensitive structure and affecting the humidity response effect. Secondly, PDMS can resist degradation during long-term use and has low solubility and good stability. In addition, crosstalk effects caused by other factors (ambient temperature in most cases) will inevitably affect and degrade the measurement accuracy of relative humidity. Based on this, the best choice for practical applications is to develop a multiparameter humidity sensing system with temperature compensation. For example, Lingxin Kong et al. implemented a dual channel SPR sensor using PDMS and PVA coating to distinguish temperature and humidity changes [19]. Hailin Chen et al. utilized PDMS and PI to form two Fabry-Perot interference (FPI) cavities and achieved the Vernier effect for simultaneous temperature and humidity measurement [20]. In previous studies, many humidity sensors implemented temperature compensation [21]. However, it is a common problem to ignore the water resistance stability of humidity sensor.
In this article, we propose an optical fiber humidity sensor based on methyldiethanolamine (MDEA)-photopolymer-PDMS structure. It focuses on addressing a humidity sensor challenge named the balance of high durability and performance in harsh environments. To be specific, we develop a PDMS layer to overcome the failure of the sensor in high humidity and dew environment. Furthermore, considering addressing temperature and humidity crosstalk, we calibrate the temperature effect to achieve simultaneous temperature and humidity measurements. The humidity-sensitive part of most optical fiber sensors is made of solid materials. However, many liquid materials exhibit higher humidity responses than solid materials, such as, glycerol, ionic liquids, and other materials [22–25]. The MDEA-photopolymer component has significant benefits in terms of sensitive improvement, which is up to 0.9596 nm/%RH and several times to tens of times compared with a single polymer humidity sensor [26,27]. MDEA has a good absorption effect on acidic gases and is often used as a detection agent for carbon dioxide and sulfur dioxide [28]. MDEA also has an excellent ability to dissolve water [22]. However, to the authors’ best knowledge, there are no application examples where MDEA has been applied to humidity sensors, and the humidity sensing potential of MDEA has yet to be exploited [29]. In addition, liquid materials have been applied to measure parameters such as temperature and magnetic field [30,31], but liquid-core optical fiber humidity sensors have yet to be reported. This is because, unlike other parameters, humidity measurement requires sensitive components to contact water molecules, which is a challenge for packaging liquid materials. Here, we use the MDEA-photopolymer-PDMS sandwich structure to ensure the interaction between water molecules and humidity-sensitive components. Finally, the compact structure, high hydrophilicity, and rapid adsorption and desorption of sensor ensure the advantages of fast response and low sensor hysteresis. In the humidity range of 53%RH to 69.9%RH, the response and recovery time of the sensor are as low as 602 ms and 349 ms, respectively. We achieve an optical fiber humidity sensor that balances the advantages of simple process, high sensitivity, fast response time and water-resistance, which has a more excellent prospect of application in biomedical, chemical manufacturing, and environmental science fields.
2. Fabrication and principle
2.1 Materials
Eosin Y, MDEA, and pentaerythritol triacrylate (PETA) were purchased from Aladdin, China. PDMS (Sylgard 184-A, Sylgard 184-B) was purchased from DOWSIL, U.S.A. All the above chemicals were purchased commercially and were not further purified. The hollow core fiber (HCF) inner diameter is 50 µm, and the outer diameter is 125 µm. The diameters of the single mode fiber (SMF) core and cladding are 8.2 µm and 125 µm, respectively.
2.2 Sensing mechanism
The schematic diagram of the sensor is shown in Fig. 1. The PDMS is located at the outermost layer of the sensor and serves as a waterproof layer to improve the stability of the humidity sensor in dew environments. Here, the external PDMS does not interfere with humidity monitoring. PDMS coating can effectively waterproof, and allowing water vapor pass through it [17,32]. The humidity sensitive component composed of liquid and photopolymer are located inside the HCF. The sensor is composed of multiple FPI cavities cascaded together, and different microcavities have different responses to external relative humidity and temperature. The incident light enters the optical fiber and is reflected by the fiber/MDEA-photopolymer interface, MDEA-photopolymer/PDMS interface, and PDMS/air interface, respectively. Three beams of reflected light will interfere. The total intensity of the superimposed beam can be expressed as [33]:
Here, symbols, ${A_1}$, ${A_2}$ and ${A_3}$ are the amplitudes of incident light reflected by above three interfaces, respectively. The symbols ${\varphi _1} = \frac{{4\pi {n_1}{l_1}}}{\lambda }$ and ${\varphi _2} = \frac{{4\pi {n_2}{l_2}}}{\lambda }$ represent the phase differences of reflected light, respectively. ${n_1}$ and ${n_2}$ are the refractive index of the humidity sensitive part and PDMS, respectively, while ${l_1}$ and ${l_2}$ are the cavity lengths of the humidity sensitive part and PDMS, respectively. $\lambda$ is the light wavelength.
Multiple FPIs form multiple beam interference, the spectrum is composed of high-frequency signals contributed by the MDEA-photopolymer cavity and low-frequency signals contributed by the PDMS cavity [26]. Fast Fourier transform is used to distinguish between low-frequency and high-frequency signals in the spectrum. Due to differences in cavity length and working medium, the response of the two cavities to humidity and temperature is not the same. By tracking the dip wavelength shift of different cavity contribution spectra, temperature and humidity can be simultaneously detected through the sensitivity matrix. This has practical application value; many relative humidity sensors have temperature crosstalk. For relative humidity measurements with significant temperature changes (such as humidity study in thermal power plants [34]), it is necessary to measure relative humidity and ambient temperature to correct for deviations caused by temperature changes in relative humidity measurements. Therefore, we have to take measures to compensate for the impact of temperature. As the humidity and temperature of the surrounding environment change, the wavelength shift of low-frequency and high-frequency signals can be expressed as:
Here, $\Delta {\lambda _L}$ and $\Delta {\lambda _H}$ are the wavelength shift of the low-frequency and high-frequency signals in interference spectra, respectively. $K_{RH}^L$ and $K_{RH}^H$ are the humidity sensitivity coefficients of two frequency signals interference spectra dips, $K_T^L$ and $K_T^H$ are the temperature sensitivity coefficients of two interference spectra dips, while the humidity and temperature variations are $\Delta RH$ and $\Delta T$, respectively.
According to Eq. (2), the change of temperature and humidity can be expressed as:
The humidity sensitive component of the sensor includes the liquid cavity and the photopolymer part. The liquid cavity composed of MDEA and PETA. To the best of our knowledge, there is preceding work of liquid materials used in humidity-sensitive applications have only used glycerin as an optical fiber liquid whispering-gallery mode resonator sensor [24]. This design significantly improves the sensitivity but is needed to consider the stability of the liquid resonator structure. A more stable structure design can achieve the unity of robustness and sensitivity. The photopolymer solution formed by Eosin Y, MDEA, and PETA, which is irradiated by green light and then solidified to form a solid cavity. The ability of photopolymer to sense humidity is related to the alignment and copolymerization of monomers in light radiation to produce crosslinked polymers. The polymer network's bulk pores with appropriate dimensions exhibit better water absorption ability [26]. The water molecules can be bound to the surface or diffuse into the matrix of the polymers while relative humidity increases. For the humidity sensitivity of the sensor, it can be inferred from the Eq. (1) that the humidity sensitivity can be detected as [26]:
In particular, for the MDEA liquid material which plays a major role in sensor enhancement, the influence of water absorption on the interference cavity length can be expressed as follows [35]:
Here, ${\left( {\frac{{{V_{final}}}}{{{V_{initial}}}}} \right)_T}$ is the final expansion ratio of droplet volume in T temperature, ${\left( {\frac{{{x_{initial}}}}{{{x_{final}}}}} \right)_T}$ and ${\left( {\frac{{{\rho_{initial}}}}{{{\rho_{final}}}}} \right)_T}$ are the ratio of droplet concentration and density before and after absorbing water, respectively.
2.3 Fabrication of MDEA-photopolymer-PDMS humidity sensor
First, use a fiber cutter to cut commercial SMF. Then, an HCF is fused to it using an optical fiber fusion splicer (88S, Fujikura, Japan). To ensure no collapse fusion, the fusion splicer uses manual mode and sets the fusion parameters as follows: prefuse time of 180 ms, the discharge time of 700 ms, discharge strength of 70 units, and end face spacing of 15 µm between SMF and HCF. The spliced SMF-HCF structure is cut on a precision cutting table to obtain the required HCF length. Subsequently, we mix 8 wt. -% MDEA and 92 wt. -% PETA evenly and then fed into the HCF. Controlling the length of the liquid portion contributes to a compact structure and rapid response. Thirdly, a photopolymer solution composed of 0.5 wt. -% Eosin Y, 8 wt. -% MDEA, and 91.5 wt. -% PETA was prepared. The homogeneously mixed photopolymer is fed into the HCF, and the length of moisture sensitive component is 27.0 µm. Then rapidly irradiating it with a 532 nm laser to solidify it. Due to the absence of photosensitizers in the MDEA and PETA portions on the inner side of the cavity, curing will not occur, while the Eosin Y contained on the outer side will undergo a photopolymerization reaction with the MDEA and PETA under green light irradiation, forming a porous polymer solid network. Cured photopolymer encapsulates the liquid in HCF. Finally, fill the port of the HCF with PDMS and the thickness is 4.3 µm, which is conducive to the diffusion of water molecules. Subsequently, curing is achieved by baking at 80°C for 3 hours.
3. Experiments and results
3.1 Experiment setup
The experimental setup of humidity and temperature response experiment is shown in Fig. 2. The light from super continuous light sources (Wavelength range 470-2400 nm, YSL Photonics, China) is coupled to the sensor through a circulator. The reflected light is recorded by a spectrometer (Resolution: 0.02 nm, AD6370D, Yokogawa, Japan). The sensor is placed in a temperature and humidity chamber (Temperature error: ± 0.5 °C, Humidity error: ± 3%RH, ST-50LA, Yishite Instrument Co., Ltd, China) for temperature and humidity control.
3.2 Performance of the MDEA-photopolymer-PDMS sensor
To study the humidity sensing performance, the sensor is placed in a temperature and humidity chamber with a humidity control range of 25%RH to 90%RH, and the temperature is maintained at 20 °C. The reflection spectrum is recorded through a spectrometer. When the relative humidity rises, it leads to an increase in the effective length of the interference cavity, which causes a redshift in the reflection spectrum. At the same time, the increase of water molecules may cause the refractive index of the sensor to move steadily toward the refractive index value of water, resulting in a blue shift. In this experiment, it is shown that the increase of the effective length is the main influencing factor of the spectral variation [26]. Figure 3(a) and (c) show the spectra of the optical fiber sensor’s low-frequency and high-frequency signals recorded under different humidity. Figure 3(b) and (d) show the curve fitting of the characteristic wavelength shift. We find that the humidity sensitivity of low-frequency signal in the interference spectrum of the sensor is 0.94 nm/% RH. The relationship between high-frequency signals in interference spectrum is 0.96 nm/%RH.
Fig. 3. The performance of the optical fiber humidity sensor based on MDEA-photopolymer-PDMS under varied relative humidity levels. (a) is the spectra of low-frequency signal under different relative humidity; (b) is the relationship between low-frequency signal’s wavelength and relative humidity. (c) is the spectra of high-frequency signal under different relative humidity; (d) is the relationship between high-frequency’s wavelength and relative humidity.
3.3 Temperature compensation of the MDEA-photopolymer-PDMS sensor
To measure the sensor's response to temperature, set the ambient test humidity to 55%RH, and increase the temperature from 20 °C to 60 °C. The test results are recorded as follows. The reflection spectrums are shown in Fig. 4(a) and (c). We can see that in a wide temperature range between 20 °C and 60 °C, the relationship between temperature and wavelength shift of the two frequency signals for temperature changes are 0.43 nm/°C and 0.30 nm/°C, respectively, with good linearity. The results are shown in Fig. 4(b) and (d).
Fig. 4. Temperature responses from sensors (a) and (c) are the low-frequency signals and the high-frequency signals in the interference spectrum of the sensor at different temperatures, respectively. (b) and (d) are the relationship between two kinds of frequency signals and temperature.
To sum up, the variation in humidity and temperature can be expressed as:
4. Discussion
For the sensor to be used in practice, stable and repeatable measurements must be present. To illustrate the usefulness of the optical fiber sensor, we verify its accuracy, durability, stability, repeatability and response speed.
4.1 Humidity and temperature cross testing
We perform a series of measurements under different temperature and humidity conditions. The sensor is placed in a temperature and humidity chamber with known temperature and humidity conditions, and five sets of spectral data are random collected for different temperature and humidity values. Here, the temperature and humidity measurements are obtained directly from the wavelength brought into the equation, and the experimental results are shown in Table 1. The results show that the measured temperature and humidity values are in good agreement with the actual values, with an average error of 0.80% and 0.59%, respectively. The errors are mainly caused by the instability of the measurement process and the deviation of the response sensitivity under different temperature and humidity conditions. Nevertheless, the optical fiber sensor proposed in this paper for simultaneous temperature and humidity measurement has achieved accurate measurements.
Table 1. Comparison between experimental actual values and calculated results
4.2 Durability of MDEA-photopolymer-PDMS humidity sensor
Humidity sensors should have high stability in various environments when directly exposed to the atmosphere. Dew is often exposed during the measurement of the sensor under high humidity conditions. For many humidity sensing materials, these materials are easily dissolved in water. Stability to water is very important for humidity sensors. In order to assess the stability of the humidity sensors in water, they were placed in water immersed for a longer period of time and then dried. The response of the humidity sensor under different humidity was tested, and the results are shown in Fig. 5(a). It can be clearly seen that the humidity response characteristics of the sensor did not undergo large changes after 200 hours of immersion in water at 20 °C. The results show that the MDEA-photopolymer-PDMS structure has good stability and high water-resistance, which can prevent sensor failure. For the fitting line of the signal and its corresponding humidity in the two frequency ranges, the maximum change in slope is 3.2% and the maximum change in intercept is 2.36 nm. Compared to the relationship between the wavelength position and humidity of a MDEA-photopolymer structure humidity sensor without PDMS layer protection before and after immersion for 100 hours is shown in Fig. 5(b). The response curve of the sensor shifts upward and the sensitivity decreases, which can be explained by the large amount of water absorption and expansion of the sensor, the deformation of the polymer network, and the inability to recover the initial state after leaving the water. After soaking for 200 hours, the sensing structure fall off.
Fig. 5. Comparison of humidity response before and after soaking. (a) The humidity sensor with PDMS protective layer, the upper line is the fitting line of the high-frequency signal in the spectrum, and the lower line is the fitting line of the low-frequency signal. (b) Humidity sensor without protective layer.
Based on these results, the MDEA-photopolymer-PDMS optical fiber humidity sensor exhibits considerable durability in dew atmospheres and high humidity environments. This is mainly because the PDMS layer effectively prevents dew from entering the interior of the humidity sensor, and the PDMS and photopolymer materials exhibit good high-temperature stability. On the other hand, traditional polymer sensors have poor durability in dew and high temperature and humidity environments due to polymer swelling or sensor structure detachment.
4.3 Response and recovery time
Since ambient humidity can change rapidly quickly, whether a sensor can respond accurately in real-time is an essential criterion for evaluation. According to the relative humidity table of saturated salt solution in a closed environment, potassium iodide saturated salt solution was selected to be sealed in a conical flask to generate an environment with 69.9%RH [36]. The sensor head was inserted into each sealed conical bottle through small holes in the stopper and kept the saturated salt solution in the air. A 1550 nm laser, photodetector, and oscilloscope were used to record the sensor response continuously. The data is transmitted to the computer. By moving the sensor head into or out of the conical flask containing the saturated salt solution, a fast conversion of the relative humidity environment around the sensor between 53%RH (the relative humidity value of the room) and 69.9%RH (the relative humidity value of the saturated salt solution) can be achieved, respectively. The experimental results are shown in Fig. 6. When the ambient relative humidity of the sensor changes rapidly between 53%RH and 69.9%RH, the voltage changes by about 0.08 V, then the voltage can return to the initial value. Due to the MDEA-photopolymer structure as an effective sensing element, the breathability of PDMS provides a channel for the penetration of water molecules, ensuring rapid interaction between water molecules and the humidity sensitive component. The response time of the sensor is as short as 602 ms, and the recovery time is as short as 349 ms. Compared to other with or without waterproofing humidity sensors, the response time and recovery time are increased by 1-5 orders of magnitude of optical fiber humidity sensor based on liquid-solid PDMS due to its compact structure and material advantages [20]. In particular, the response time is similar to recovery time mean that compared to other sensor configurations [14,37–39], the liquid-solid structure combines an excellent ability to absorb and release water molecules, ensuring the real-time response accuracy of the sensor. Therefore, the sensor is particularly suitable for monitoring humidity in environments where the humidity changes rapidly. At the same time, we know that only when the relative humidity of the environment reaches 55-65%RH can it promote the treatment and recovery of severe respiratory diseases such as bronchitis, pneumonia, and tuberculosis [40,41]. Therefore, accurate monitoring of the relative humidity of the treatment environment for patients with respiratory diseases is essential. The excellent performance of our developed sensors in the relative humidity range of 53%RH-69.9%RH is expected to be used in medical applications, such as precision and rapid medicine for respiratory diseases.
In this experiment, when the conical bottle is opened and closed, the relative humidity around the sensor does not change instantaneously, and there will be a slight difference compared to the set humidity value. At the same time, during operation, sensor bumps can also affect the voltage value to a certain extent, manifested as instability after the voltage value transitions in the figure. However, the MDEA-photopolymer-PDMS humidity sensor still exhibits an extremely high response speed.
4.4 Repeatability and stability of MDEA-photopolymer-PDMS humidity sensor
An experiment with temperature and humidity variations is conducted to evaluate the repeatability of the sensor. We place the sensor in a temperature and humidity chamber to vary the test conditions. First, the temperature repeatability of the sensor is verified by switching the temperature from 20 °C to 60 °C at a constant 55%RH for ten rounds. The wavelength values of low-frequency and high-frequency signals are shown in Fig. 7(a) and (b), respectively, with the maximum wavelength fluctuation of the spectra is 0.92 nm. Subsequently, the humidity response repeatability of the sensor is verified by repeatedly changing the humidity to 25%RH and 90%RH ten rounds at a fixed 20 °C. The results are shown in Fig. 7(c) and (d), and the maximum wavelength fluctuation of the spectra is 1.68 nm. Considering the instability of the temperature and humidity chamber, we can determine that the sensor has high repeatability.
Fig. 7. The repeatability test of the sensor. (a) and (b) are the relationship between the sensor switching between 20 °C and 60 °C, its low-frequency and high-frequency signal wavelength position and temperature. (c) and (d) are the relationship between the sensor switching between 25%RH and 90%RH, its low-frequency and high-frequency signal wavelength position and temperature.
The stability test of the sensor is conducted by setting the humidity to 90%RH at 20 °C and recording the spectral dips position every 10 minutes for 6 consecutive times. On the other hand, place the sensor at 55%RH and 60 °C to record the spectrum every ten minutes, recording 6 times. The experimental results are shown in Fig. 8, and the maximum wavelength shift in the experiment is 0.98 nm and 0.74 nm, respectively.
A detailed comparison has been made with other reported studies. The MDEA-photopolymer-PDMS humidity sensor proposed here demonstrates its unique advantages, as shown in Table 2. In addition to good water-resistance, the relative humidity sensitivity and time response are higher and lower than most works, respectively. Some sensors also overlook the impact of temperature crosstalk. The response time of these research reports is too long, which clearly cannot be applied to real-time and rapid humidity monitoring. And the sensor response time is short [26], but its relative humidity sensitivity is low, making it difficult to accurately monitor environmental humidity. Compared to optical fiber humidity sensors made only of photopolymers or other types of polymers in the past, it is found that the sensitivity of the solid-liquid structured sensor is obviously enhanced by about several times or even tens of times. In summary, the MDEA-photopolymer-PDMS structure fully considers the trade-off between relative humidity sensor performance, especially relative humidity sensitivity and responsiveness, as well as practicality.
Table 2. Comparisons of the proposed sensor with other typical structures
5. Conclusion and outlook
In summary, an optical fiber humidity sensor combining liquid and solid humidity-sensitive portions is shown, with a PDMS hydrophobic layer coated on the surface to improve the sensor's survivability. Under the protection of the PDMS hydrophobic layer, the sensor exhibits good hydrothermal, and high-humidity stability, robustness, and repeatability. Thanks to MDEA in liquid components, humidity sensitivity has been dramatically improved, and temperature compensation measures have been taken to measure temperature and humidity simultaneously. The compact structure and excellent absorption and release ability of photopolymer materials allow the sensor to have a response and recovery time of only 602 ms and 349 ms in the relative humidity range of 53%RH-69.9%RH, respectively. The sensor displays its usability in long-term high humidity environments such as bathroom, marine climate, and unsaturated soil. In previous designs, photopolymer materials have demonstrated the ability to monitor various volatile organic compounds in addition to water molecules. We believe that PDMS protective layers and temperature compensation methods provide a simple and powerful strategy for designing various gas sensors with good durability and long-term stability under high humidity, and dew atmospheres.
Funding
National Natural Science Foundation of China (61975039, 62175046, 62205086, 62305080); Taishan Scholars Program; China Postdoctoral Science Foundation (2022M720940); Qingdao Natural Science Foundation (23-2-1-214-zyyd-jch); Natural Science Foundation of Heilongjiang Province (YQ2020F011); Postdoctoral Foundation of Heilongjiang Province of China (LBH-Z21128); Postdoctoral Innovation Project of Shandong Province (SDCX-ZG-202203017); Postdoctoral Applied Research Project of Qingdao; Fundamental Research Funds of Harbin Engineering University.
Acknowledgments
This research is supported by the National Natural Science Foundation of China (62205086, 62175046, 61975039); China Postdoctoral Science Foundation (2022M720940); Heilongjiang Provincial Natural Science Foundation of China (YQ2020F011); Postdoctoral Foundation of Heilongjiang Province of China (LBH-Z21128); Postdoctoral Innovation Project of Shandong Province (SDCX-ZG-202203017); Postdoctoral Applied Research Project of Qingdao; 111 Project (B13015) and Fundamental Research Funds of Harbin Engineering University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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