Abstract A high-precision fiber-optic gas sensing system integrating a graphite photoacoustic (PA) detector with 2f/1f self-calibration is presented to reduce the influence of laser power fluctuations while retaining noncontact detection. A graphite flake and a microcantilever are copackaged within a brass housing to form a solid-state PA detector. A modulated laser beam passes through the target gas in a Herriott multipass cell and strikes the graphite flake; the absorbed optical power is converted into a PA response, which is subsequently detected by a fiber-optic cantilever microphone. The multipass cell provides an effective optical absorption path length of 13.5 m, enhancing the PA response by approximately 1 order of magnitude. The fundamental optical-power modulation is converted into a 1f PA response that serves as an optical-power reference, whereas the 2f PA response contains the CH4 absorption information. The 2f/1f ratio is used to reduce the influence of incident optical-power variations on concentration retrieval. For CH4 detection at 1650.91 nm, the system achieved a minimum detection limit of 6.8 ppb with an integration time of 100 s, corresponding to a normalized noise-equivalent absorption coefficient of 1.17 × 10–9 cm–1·W·Hz–1/2. Even under conditions of significant optical power fluctuation, the normalized concentration deviation remained within approximately 3%. Ambient-air measurements yield a CH4 concentration of approximately 1.84 ppm, demonstrating the applicability of the proposed system to practical methane monitoring.

The evolution of surface morphology, mechanical properties, and microscopic characteristics in spontaneously combusted coal gangue (SCG) geopolymers activated with K2OnSiO(2)+KOH or Na2OnSiO(2)+NaOH was examined under varying silicate moduli (M = 1.0-2.0) and alkali contents (R2O = 6-14 %) over a two-year period. In particular, we conducted in depth analysis using FTIR Si-O-T deconvolution, TG-MS gel quantification, and SEM-EDS. The results indicate that the differences in ionic radius, hydration radius, and diffusion rate between K+ and Na+ significantly influence the reaction pathways, product phases, and hydration kinetics. Moderate parameters (M = 1.6, R2O = 10 %) optimize the depolymerization-polycondensation balance, whereas higher values (M = 2.0, R2O = 14 %) drive long-term secondary polymerization after 720 days, reorganizing low-polymerized aluminosilicates into highly crosslinked K/N-A-S-H phases, with blue shifts of Q(3) and Q(4). In sodium-based systems, NaFeSi2O6 initially formed and subsequently transformed into F-A-S-H, eventually yielding N-A-S-H, F-A-S-H, and C-(N)-A-S-H, accompanied by efflorescence, cracking, and strength degradation. In contrast, in potassium-based systems, F-A-S-H evolved into lamellar or blocky forms and synergized with K-A-S-H and C-(K)-A-S-H to form a three-dimensional cross-linked network that later developed semi-crystalline features. A high Al content yields polyhedral or layered morphologies, whereas high Si/Al ratios (>2) result in bulk/honeycomb structures. The gel phase composition typically maintains a Na/K:Al:Si = 1:1:3. This study systematically addresses the knowledge gap in the long-term evolution mechanisms of geopolymers. SEM first reveals the depolymerization-repolymerization process in K+/Na+ based geopolymer and induced crystal morphology evolution under varying Si/Al/K(Na) stoichiometries, enabling novel utilization of SCG.

Municipal Solid Waste Incineration fly ash (MSWI FA), as a typical high-risk solid waste, presents significant challenges in its safe disposal and resource utilization. In this study, MSWI FA and spontaneous combustion coal gangue (SCG) were synergistically used to synthesize composite geopolymers by leveraging their complementary high-calcium and aluminosilicate characteristics. The geopolymerization mechanism, phase evolution, chemical bonding states, and the speciation of Pb2+, Zn2+, and Cd2+ were systematically investigated over a one-year period, with their spatial association with the gel matrix examined by SEM-EDS. Under alkaline activation, reactive [SiO4]4-and [AlO4]5-species released from SCG interact with Ca2+ from MSWI FA, forming a highly polymerized C-(K)-A-S-H gel, and K+ further promotes structural densification. Optimal mechanical performance and heavy metal immobilization are achieved at 40% SCG content. Based on ionic potential theory, Pb2+ reacts with [SiO4] tetrahedra to form stable Pb-O-Si covalent bonds, whereas Zn2+ and Cd2+ are mainly immobilized through ion exchange and isomorphous substitution. Fractal analysis shows that the pore structure exhibits multi-fractal characteristics, and the pore volume fractal dimension (D) is negatively correlated with heavy metal leaching concentration. Higher D values indicate a more tortuous pore network, which effectively restricts longterm metal migration. This study provides a promising strategy for avoiding the high-energy calcination carbon footprint typically required for raw coal gangue activation, while offering important technical support for the long-term environmental risk assessment of hazardous waste stabilization.

A cantilever-enhanced fiber-optic photoacoustic sensor with pressure self-compensation is reported to address reduced detection accuracy in plateau regions for trace acetylene detection. Under low-pressure conditions, the alteration of thermophysical properties in gases increases the thermoacoustic dissipation, while the enhanced compressibility improves the cantilever acoustic sensitivity. Importantly, the pressure-induced modification of the spectral line profile of acetylene molecules results in non-monotonic dependence of the photoacoustic pressure wave on air pressure. The signal is detected by the second-harmonic wavelength modulation spectroscopy technique. In low-pressure environments, fluctuations in the acoustic sensitivity and the photoacoustic excitation are compensated via coordinated optimization of operating frequencies and laser modulation depth. The scheme eliminates the requirement for additional air pressure measurements to correct sensor sensitivity. The sensor with pressure self-compensation breaks the limitations of existing photoacoustic sensors whose detection accuracy is heavily dependent on environmental pressure. Within the pressure range of 60-100 kPa, two pressure-insensitive frequencies at 2110 Hz and 2300 Hz are observed under a laser modulation depth of 48 pm. The concentration measurement errors of the acetylene sensor remain within +/- 5 % and the detection limits are less than 0.1 ppm.

The fiber-optic gas sensor is an ideal choice for trace gas detection due to its high sensitivity, telemetry, and miniaturization. However, advanced technical methods based on a long optical path or functional coating to improve sensitivity limit the trace gas detection application of confined spaces and fast response. A fiber-tip Fabry-Perot (F-P) interferometric photothermal (PT) flexible gas sensor based on capillary microcavity is proposed for fast gas detection. An inner diameter of 130 μm capillary is used to align and fix the single-mode fiber and the short fiber coated with a gold reflective film, forming a high-contrast F-P cavity with a cavity length of about 800 μm. The volume of the gas chamber is only 10 nL. The capillary microcavity structure is optimized by simulation analysis and experiment. When the cavity length is 392-935 μm, the PT signal increases with the increase of the cavity length. The excitation light interference suppression and high-speed PT phase demodulation can be realized by a truncated spectral white-light interference demodulation method. The experimental results show that the sensor has a minimum detection limit of 0.8 ppm C2H2 and a response time of only 1 s.

Inhalable immunization is a multidimensional colloidal delivery process, involving aerosol particle transport in the airways and interaction with mucus at the mucosal interface. Primarily, the impact of airway anatomy, airflow, and particle's physicochemical properties on aerosol deposition and deep lung delivery are highly desired. In this study, a human airway geometry model was constructed, and the airflow field distribution and turbulence characteristics based on airway anatomy were visualized through in silico simulation. The dimensionless numbers of Stokes number (Stk) and Schmidt number (Sc) were introduced to mechanistically demonstrate the particle deposition under the impact of multiple parameters, including inlet airflow rate (Q), particle size (dp), and particle density (rho). With the increasement of Stk and Sc, the mechanism transitioned from Brownian diffusion- to inertial impact-dominated deposition. The response surface methodology (RSM) analysis indicated that the Q and interaction term between particle size and inlet airflow rate (dpQ) were the most critical parameters that dominated deep lung transportation fraction (DLF). The proposed regression equation provided quantitative design guidance for parameter of airflow and particle. This in silico methodology provides a rational predictive design strategy for inhaled aerosol formulations for both therapeutic and prophylactic applications.

A cantilever-enhanced fiber-optic photoacoustic (PA) spectrophone is reported for trace gas detection at a low-pressure environment. A cantilever-based fiber-optic Fabry-Perot (F-P) interferometer (FPI) is utilized for simultaneous measurement of air pressure and PA pressure. Since the cantilever resonance frequency follows air pressure linearly, the fundamental frequency intensity modulation (1f-IM) technique is applied to scan the frequency response of the solid PA signal from tube wall absorption for tracking the cantilever resonance frequency in real time. The second-harmonic wavelength modulation spectroscopy (2f-WMS) technique is used to measure the gas PA pressure wave at the cantilever resonance. According to the inverse restriction relationship of air pressure on the PA excitation and cantilever detection, the measured gas PA signal at the low-pressure environment is enhanced. The target gas concentration is corrected by the measured air pressure, which makes the spectrophone generally applicable under any pressure. The experimental results indicate that the normalized noise equivalent absorption (NNEA) coefficients of the spectrophone in the standard atmospheric pressure and the low-pressure environment of 60 kPa are 2.2 x 10-9 and 2.0 x 10-9 cm-1WHz-1/2, respectively. 0.1 ppm acetylene (C2H2) can be detected at any air pressure. The detected maximum relative error of 10 ppm C2H2 gas under different pressures is less than +/- 9% and the error is reduced to less than +/- 2% when the concentration rises to 70 ppm. In the pressure range of 60-100 kPa, the cantilever-enhanced fiber-optic PA spectrophone has extremely high accuracy and pressure stability, covering the pressure range of most ground gas detection scenes.

An in-situ detection system of dissolved C2H2/CH4 with a frequency division-multiplexed fiber-optic photoacoustic (PA) sensor (FOPAS) is designed for diagnosing failures of large power transformers. The system relies on a fluorinated ethylene-propylene (FEP) membrane to extract gases and an all-optical gas sensing element, which has the advantages of non-consumption of oil, anti-electromagnetic interference and passive operation. The oil-gas separation unit and the PA excitation-detection unit are closely integrated into an independent system, communicating with a dual-frequency demodulator through two optical fibers. Two lasers emitting at 1532.83 nm and 1650.91 nm operate simultaneously, exciting PA signals of C2H2 and CH4, respectively. The modulation frequencies of the two lasers are 1486 Hz and 1490 Hz, and twice the frequencies fall within the resonant frequency band of the fiber-optic microphone. A custom-designed dual-channel digital lock-in amplifier is embedded in the demodulator to avoid crosstalk between frequencies, which achieves dual-component synchronous detection. The experimental results show that the temperature increase can promote oil-gas separation. The system can reach equilibrium within 2 h at 60 degrees C. The minimum detectable concentrations of dissolved C2H2 and CH4 are both about 0.1 mu L center dot L- 1, which meet the detection requirements of dissolved gases in transformer oil. The system has the potential for real-time monitoring of dissolved C2H2/CH4. The excellent detection performance provides technical support for the more accurate real-time condition monitoring and early fault warning of large power transformers.

A highly sensitive trace gas sensing system based on carbon black absorption enhanced photoacoustic (PA) spectroscopy (PAS) is reported. A carbon black sheet and a fiber-optic cantilever microphone (FOCM) are integrated to form a fiber-optic cantilever spectrophone (FOCS). The gas concentration is obtained by measuring the acoustic wave amplitude generated by the carbon black sheet, which absorbs the laser passing through the interest gas. Due to the higher laser absorption rate of carbon black than that of flake graphite, the excited solid-state PA pressure wave is enhanced. The ability of the FOCS to detect weak sound waves is related to the laser power and the absorption length. Therefore, an Erbium-doped optical fiber amplifier and a multipass cell are also used to increase laser absorption by the tested gas, which combines with the FOCM to achieve multimechanism enhancement of the system performance. Different from traditional PAS gas detection systems, this system is a noncontact measurement solution, which not only effectively avoids gas flow noise but also makes the sensing element immune to the damage of corrosive gases. The experimental results show that the sensitivity of the system is about four times higher than that of the system using flake graphite as the light-absorbing element. When the average time is 100 s, the minimum detection limit of acetylene is 0.31 ppb. The normalized noise equivalent absorption coefficient of the designed PA system is achieved to be 7.1 × 10-10 cm-1·W·Hz-1/2.

Municipal solid waste incineration (MSWI) fly ash containing heavy metals is hazardous solid waste that requires safe disposal. Alkali-activated treatment can effectively improve the environmental safety of MSWI fly ash. Due to the low alumina content of MSWI fly ash, the addition of nanoalumina (NA) solves the defect to enhance the ability of solidification/stabilization (S/S) and maximize the utilization of MSWI fly ash. NA/MSWI fly ash solidified body was prepared by alkali-activated technology in this work. In this study, NA was added (0-3 wt.%) to explore its effects on mechanical properties and heavy metal immobilization efficiency of NA/MSWI fly ash solidified body. Research results revealed that when the addition of NA was 2 wt.%, the compressive strength of solidified bodies can significantly increase by 38.0% and 42.8% at 7 and 28 days, respectively. Meanwhile, the leaching concentration of heavy metals in NA/MSWI fly ash solidified body can noticeably reduce with the immobilization efficiency of above 99.5%. Adding NA promoted the ratio of stable state on Pb and Zn in solidified bodies. A series of microscopic characterization analyses indicated that NA stimulated gels and Friedel's salt formation, and N-A-S-H was also found in the NA/MSWI fly ash solidified body, enhancing S/S heavy metals by physical adsorption, physical encapsulation, and chemical bonding. Therefore, this study paves a potential new way for the application of nanomaterials in S/S MSWI fly ash through alkali-activated technology, and nanoalumina is a more promising candidate.

Aiming at the need of rapid and sensitive detection of acetylene dissolved in oil, a small-volume T-type photoacoustic (PA) cell was proposed. By optimizing the design parameters, the volume of the T-type cell was reduced to 30 mL, which resulted in a significant reduction in the sampling amount of the transformer oil. A laser PA spectroscopy system for dissolved gas analysis was built combined with the headspace degassing method. A near-infrared tunable laser with wavelength modulation spectroscopy was employed to detect the escaped acetylene gas from oil. Test results show that the detection limit of dissolved acetylene reaches 0.2 mu L/L when the sampled oil volume is 50 mL. In addition, the degassing balance time can be shortened to 270 s. This scheme has the superiority of small oil extraction volume, high accuracy, and short response time, providing a new method for analyzing the dissolved characteristic gases in oil.

A fiber-optic trace gas sensing system based on photoacoustic (PA) effect of solids is reported. The modulated laser light absorbed by the gas is incident on a graphite sheet. The graphite sheet and cantilever are enclosed together to form an optical power detector based on PA effect of solids. This scheme is different from the traditional PA trace gas detection, achieving the separation of the gas chamber and the PA detector, thereby isolating the interference of airflow and environmental noise. In addition, due to the large size of the graphite sheet, the laser incident on the graphite sheet does not need to be converged through a lens, which makes it much easier to adjust the optical path. The PA pressure wave generated by the graphite sheet leads to the vibration of the optical interferometric cantilever. A fiber-optic white-light interferometer is used for high-resolution cantilever readout to detect the PA signal generated by solid excitations. The trace acetylene gas is used to verify the system performance. The experimental results indicate that when the averaging time is 100 s, the minimum detection limit (MDL) is 24 ppb. The normalized noise equivalent absorption (NNEA) coefficient of the designed PA system is 5.4x10(-9) cm(-1)center dot W center dot Hz(-1/2). This gas sensing system has the characteristics of intrinsic safety and high sensitivity, which can realize remote measurement of corrosive, flammable and explosive gases.

An ultrahigh-speed phase demodulation system was designed for the Fabry–Perot (F–P) interferometric sensor based on fiber array parallel spectral detection. A high-power amplified spontaneous emission (ASE) source served as the broadband detection light. The spectrum generated by the dispersion of the F–P interference light through an arrayed waveguide grating (AWG) was incident into the fiber array and was detected in parallel by 48 photodiodes. The 48-channel signals were acquired synchronously and processed in real time to achieve a phase demodulation for the F–P cavity at 200 kHz. As a result, a low-resolution spectral detection and demodulation system was constructed with high speed. The length demodulation range of the F–P cavity was 60–700 µm, and the demodulation resolution was as high as 0.22 nm. The designed high-sensitivity demodulator is expected to be used for ultrasonic and high-frequency vibration detection.

A high-sensitivity fiber-optic photoacoustic (PA) gas microsensor is demonstrated with dual enhancement based on acoustics and detection. Due to the characteristic of small size, a Helmholtz resonator is integrated into a miniature PA sensor. The acoustically amplified PA signal is detected by a high-sensitivity fiber Fabry-Perot (F-P) interferometric cantilever. The first-order resonant frequencies of the interferometric cantilever and Helmholtz resonator are matched by subtle adjustments. The weak PA signal is significantly enhanced in a volume of only 0.35 mL, which breaks the volume limitation of the resonance modes in traditional PA sensing systems. To improve the resolution of the microsensor, a white light interferometry (WLI)-based spectral demodulation algorithm is utilized. The experimental results indicate that the minimum detection limit of acetylene (C2H2) drops to about 15 ppb with an averaging time of 100 s, corresponding to the normalized noise equivalent absorption (NNEA) coefficient of 2.7 x 10(-9) Wcm(-1)Hz(-1/2). The dual resonance enhanced fiber-optic PA gas microsensor has the merits of high sensitivity, intrinsic safety, and compact structure.

SF6 leaked from gas insulation equipment will interfere with the detection of dissolved characteristic gases in transformer oil. In order to determine the impact of SF6 on the detection of dissolved C2H4 gas in oil, a photoacoustic (PA) C2H4 detection system based on the combination of mid-infrared thermal radiation source and near-infrared laser was constructed. The excitation light from two bands was incident on a non-resonant PA cell. The thermal radiation light source and the laser light source were switched to evaluate the interference level of SF6 on the detection of C2H4. The experimental results show that SF6 has significant interference to C2H4 when using a mid-infrared thermal radiation light source. The interference coefficient is 2014%. The near-infrared laser based on PA detection method can effectively suppress the interference of SF6 on C2H4. The minimum detection limit of C2H4 reaches 0.6 ppm when the average time is 60 s.

The rapid development of hydrogen refueling station (HRS) has induced potentially serious risk exposure to public safety. Consequently, the safety issues associated with HRS have become a research hotspot. A comprehensive methodology for risk analysis of hydrogen facilities was proposed. A failure tree analysis (FTA) was used for hazard analysis while a bow-tie diagram and Bayesian network were applied to model the worst-case accident scenario and to analyse the risks of root event. In addition, the fuzzy set theory combined with FTA was employed to calculate the failure probability of the hydrogen facilities. The risk analysis of a HRS was implemented with the proposed method. The results showed that the vapour cloud explosion (VCE) of compressors in the gas HRS was the most risky scenario with the probability of 8.12E-04 before taking safety measures. The death, severely injured, and minor injured radius of the VCE from compressors was 35.5, 20.0, and 10.3 m, respectively. Results also revealed that after taking several safety barriers, the risk level converted from an intolerable risk level to a tolerable risk with control level.

A multiplexing scheme of fiber-optic photoacoustic (PA) gas sensors is demonstrated for simultaneous detection of multi-point gases. By sharing the PA demodulation device, the average cost of single point measurement can be significantly reduced. The passive fiber-optic PA gas sensing probe is integrated by a PA tube and a FabryPerot (F-P) interferometric (FPI) cantilever. The laser excited the PA signals is simultaneously incident into two sensing probes. The superimposed F-P interference spectrum containing multi-point gas concentration information is received by a high-speed spectrometer. For synchronous measurement of multi-point gases, a whitelight interferometry (WLI) based frequency division multiplexing (FDM) spectral demodulation method is exploited. A multi-point acetylene gas remote monitoring system is established by two PA gas sensing probes with 3 km fiber cable. The experimental results show that the sensing probes have achieved the detection limit of sub-ppm. The crosstalk between the two sensors is characterized by the gas concentrations at the two points, indicating a crosstalk of about - 35 dB. This multiplexing scheme of fiber-optic PA sensing probes has the merits of remote monitoring, low crosstalk, high sensitivity, intrinsic safety and low cost. It can be applied for coal spontaneous combustion monitoring, dissolved gas analysis and gas micro-leakage monitoring.

A fiber-optic Fabry-Perot (F-P) vibration/acoustic sensing system based on high-speed phase demodulation was developed. The demodulation part is mainly composed of a super luminescent diode (SLD), a miniature high-speed spectral module, and a field programmable gate array (FPGA) based spectral signal acquisition and pro-cessing circuit. The spectral acquisition rate is up to 20 kHz. Real-time spectral acquisition, fast Fourier transform (FFT), frequency estimation, phase calculation, and mode hopping suppression are all performed by FPGA to obtain absolute cavity length of a F-P interferometric sensor. As a result, the integrated design significantly improves the speed and stability of the phase demodulation. The performance of the system was verified by a fiber-optic F-P cantilever microphone. Experimental results show that the sensing system can realize highly sensitive detection of acoustic signals with the frequency between 20 Hz and 8 kHz, which basically covers the main range of human voice frequencies. The noise equivalent minimum detection limit of sound pressure reaches 2.8 mu Pa/Hz1/2 at 1 kHz. The designed fiber-optic acoustic sensing system has the features of resistance to electromagnetic interference, intrinsic safety, remote detection and small size. It can be used for vibration monitoring of transformers and large structures, voice communication in coal mines and nuclear magnetic resonance rooms.

A high-sensitivity fiber-optic photoacoustic sensor with pressure compensation is proposed to analyze the decomposition component SO2 in high-pressure gas insulation equipment. The multiple influence mechanism of pressure on photoacoustic excitation and cantilever detection has been theoretically analyzed and verified. In the high-pressure environment, the excited photoacoustic signal is enhanced, which compensates for the loss of sensitivity of the cantilever. A fiber-optic F-P cantilever is utilized to simultaneously measure static pressure and dynamic photoacoustic wave, and a spectral demodulation method based on white light interference is applied to calculate the optical path difference of the F-P interferometer (FPI). The real-time pressure is judged through the linear relationship between the average optical path difference of FPI and the pressure, which gives the proposed fiber-optic photoacoustic sensor the inherent advantages of being uncharged and resistant to electromagnetic interference. The average optical path difference of FPI is positively related to pressure, with a responsivity of 0.6 mu m/atm, which is based on changes in the refractive index of gas. In the range of 1-4 atm, the SO2 sensor has a higher detection sensitivity at high-pressure, which benefits from the pressure compensation effect. With the pressure environment of gas insulation equipment at 4 atm as the application background, the SO2 gas is tested. The detection limit is 20 ppb with an averaging time of 400 s.

The water vapor in the ambient air affects the accuracy of the photoacoustic (PA) dissolved gas analysis system for transformer health monitoring. A laser PA system was evaluated by dry and humidified standard gases to study the influence of water vapor concentration on PA gas detection. Theoretical analysis was conducted on the effect of gas molecule relaxation on PA signal detection. A high-frequency resonant PA cell and a low-frequency nonresonant PA cell were used to detect acetylene (C2H2) and carbon monoxide (CO), respectively. The experimental results show that the PA signal of humidified CO is about 12 times higher than PA signal of dry gas for the resonant PA detection system, respectively. In addition, as the frequency is increased from 30 to 980 Hz, the PA signals of humidified and dry CO attenuate by 1.5 and 6.9 times, respectively.