The measurement of air quality, particularly the concentration of carbon dioxide (CO2), has gained significant interest due to increased public health awareness. Elevated concentrations of CO2 in the air have been linked to negative effects on cognitive functions and directly correlate with the quality of indoor air. This work presents the prototype of a miniaturized sensor module designed for sensing CO2 concentrations in the air. The sensor module utilizes the two-chamber photoacoustic concept and features a compact design combining an absorption cell with a wafer-bonded detector based on silicon MEMS (micro-electro-mechanical systems) microphone technology. The CO2-filled detector chip uses the photoacoustic effect and is able to detect pressure oscillations within an enclosed volume generated by the absorption of modulated infrared (IR) light by CO2 molecules. The complete sensor module measures only 9×13×7.8 mm3 and the used detector chip is the smallest detector for two-chamber photoacoustic sensors reported to date. Experimental characterization showed that the prototype achieves a detection limit of 81 ppm CO2 and exhibits a response time τ63 of 53 s. The compact size and performance characteristics make the proposed sensor module suitable for applications in indoor air quality monitoring.

The ever-increasing demand for low-cost air quality sensors paved the way for miniaturized photoacoustic spectroscopy (PAS) gas sensors. We present a sophisticated system model to predict the optical output of a critical component of a PAS sensor—the infrared (IR) emitter, suited to optimize the overall system stability further. It consists of validated sub-models of the microheater, the optical filter, and the optical path in between. It is verified by measurements of the emitter output for a wide range of gas concentrations below 10.000 ppm inside the emitter.

We present a sophisticated method for improving the sensitivity of CO 2 -detecting non-resonant MEMS-based photoacoustic gas spectroscopy systems by more than 50 % compared to state-of-the-art approaches. The method based on signal demodulation can also be used to linearize the measured system’s response in order to reduce the calibration effort for future sensors. Tuning the filter transmission spectra accordingly is utilized to further increase the linearity of the system response and enhancing the sensitivity by more than 200 %.

This study proposes a model-based method to extract relative humidity (RH) values from non-resonant photoacoustic gas spectroscopy (PAS) sensors without using an additional physical humidity sensor. PAS sensors detect gases by injecting optical power of a specific wavelength into a closed pressure chamber, where the target gas absorbs the energy and releases thermal energy through collision processes, leading to a pressure signal that is detected by a microphone. However, PAS sensors are prone to cross sensitivity, especially from humidity, which can alter relaxation paths and interfere with the measured sound signal. The proposed method combines an analytical model that calculates the thermal conductivity of the atmosphere inside the pressure chamber with measured values of the energy content of the transient microphone signal to determine the RH values. The study demonstrates that this method can be applied to complement the system with the capability to detect humidity as well, i.e., to establish a humidity sensor without adding additional components or sensors, but only by the smart evaluation of the PAS signal. The so-established RH sensor in PAS system exhibits an absolute mean error of smaller than 5 % RH and makes the PAS systems more reliable and robust.

We present a concept for a wafer-level manufactured photoacoustic transducer, suitable to be used in consumer-grade gas sensors. The transducer consists of an anodically bonded two-layer stack of a blank silicon wafer and an 11 µm membrane, which was wet-etched from a borosilicate wafer. The membrane separates two cavities; one of which was hermetically sealed and filled with CO2 during the anodic bonding and acts as an infrared absorber. The second cavity was designed to be connected to a standard MEMS microphone on PCB-level forming an infrared-sensitive photoacoustic detector. CO2 sensors consisting of the detector and a MEMS infrared emitter were built up and characterized towards their sensitivity and noise levels at six different component distance ranging from 3.0 mm to 15.5 mm. The signal response for the sample with the longest absorption path ranged from a decrease of 8.3% at a CO2 concentration of 9400 ppm to a decrease of 0.8% at a concentration of 560 ppm. A standard deviation of the measured values of 18 ppm was determined when the sensor was exposed to 1000 ppm CO2.

We demonstrate a novel approach for the simulation of miniaturized non-resonant photoacoustic gas spectroscopy (PAS) sensors by applying a one-dimensional signal flow model. Using the described method, these non-resonant PAS sensors systems can be optimized with regard to their applicability to different target gases. The model is validated with lab measurements and reveals potentials and limitations with a view to optimization endeavours.

We present a novel modeling approach that integrates the sub-models of the different physical domains of a PAS sensor. In this way, a connected system-level-model is created considering the optical power of the infrared source, the transmissivity of the optical filter, the absorption behavior of a target gas, and the sensitivity of the microphone used. Based on this model, the influence of the individual components on the overall system is determined in order to identify possible candidates as target gases for miniaturized PAS systems.

Nature is often more resourceful than traditional technical approaches when it comes to finding efficient solutions to engineering problems. For example, the wavelike manner in which fish move—the so-called undulation—reveals the most energy-efficient way to transfer momentum from a solid to a fluid. This bionic principle can be made technically applicable, for example as an integrated cooling at chip-level, as a propulsion for microswimmers, or for efficient gas transport in lab-on-chip or gas sensing applications, to name just a few. The technical implementation of this concept is a thin and laterally extended monomorphic piezoelectric structure with segmented electrodes on top, which geometrically mimics a fish fin. The key of this concept is the phase-shifted excitation of the flapper by the segmented electrodes. To this end, various electrode designs and different fin geometries are evaluated with respect to their capability to generate undulation, which gives directly an estimate of the achievable mass flow. This is done by means of Laser Doppler Vibrometry. Additionally, these measurements serve as validation of coupled finite element models of momentum transfer between solid and fluid, which enable to evaluate the resulting flow field and to predict the achievable mass flow rates. This way, recommendations for the design of future MEMS flappers are derived and the most promising design variants are identified.