Catalytic pellistor-type sensors are widely used for the analytical detection of flammable/explosive gases. Further improvement of the characteristics of the sensors can be achieved by the development of miniature sensing elements based on planar microhotplates, which allows one to decrease the power consumption substantially. Here we demonstrate the microhotplate catalytic sensors based on anodic aluminium oxide supports for methane detection. The sensor response approaches the value of 15 mV/vol. % CH4 at the optimal supply voltage. A wide hysteresis in sensor response is observed during the cycling of the supply voltage. This phenomenon is studied under operando conditions using X-ray diffraction and thermogravimetry. The obtained results have a significant impact on the development of optimal working regimes, and the production of planar catalytic sensors on porous supports.

Microhotplate catalytic sensors based on porous anodic alumina: Operando study of methane response hysteresis

Explosion alarm in atmospheric condition requires detection of hydrocarbons at the level of volume percent concentration range. This is best achieved by the combustion type detection, contrary to the available sensitive conductive type sensors, which would be driven to saturation by concentration close to LEL. Therefore, for catalytic gas sensing a micro-heater system was prepared with an integrated thin film porous Anodic Aluminium Oxide (porous AAO) on the heated area which offers simultaneously large surface support material and better thermal control. Uniformly distributed Pt catalyst was selectively deposited in the porous AAO by atomic layer deposition (ALD). The device was tested and characterized by detection of methane and propane concentrations between 20 and 100% of their Lower Explosion Limit (LEL), and provided readings of >30 mV for 20% LEL (>5 vol%) of both methane and propane. The combination of porous alumina with ALD deposited Pt exhibited catalytic activity for both gases. As the temperature of the micro-heater is limited to ca. 700 °C, during methane detection the sensor can be operated in the reaction kinetic controlled regime, whereas for propane measurement the temperature range of the ideal mass transport control can be set. The sensitivity loss was analysed in terms of possible ways for optimization.

ALD nano-catalyst for micro-calorimetric detection of hydrocarbons

When a reasonable image is captured using the method of computational ghost imaging (CGI), it usually requires a number of illuminations. For example, to fully sample an <inline-formula> <tex-math notation="LaTeX">$N \times N$ </tex-math></inline-formula> pixel object, traditional CGI needs <inline-formula> <tex-math notation="LaTeX">$N^{2}$ </tex-math></inline-formula> illuminations. When the target is dynamic and its lateral motion distance exceeds the range of a single pixel, the correlation between the detected signal and the illumination pattern is lost, which degrades the imaging quality. To overcome the imaging problem of moving objects, we propose an image reconstruction method based on the artificial bee colony (ABC) algorithm, which is used to estimate the motion velocity, and the image is reconstructed by actively shifting patterns according to the estimated velocity. Numerical simulations and experiments demonstrate the effectiveness of the proposed method. The results show that the velocity of the target can be retrieved using the ABC algorithm and that the image can be reasonably reconstructed.

Moving Target Imaging via Computational Ghost Imaging Combined With Artificial Bee Colony Optimization

Temporally and spatially variant-resolution illumination patterns in computational ghost imaging

Micromachining Based on Mask-Free Direct Writing: An Advanced Approach to Innovative MEMS Gas Sensors

A low power consumption micro electro-mechanical system catalytic combustible low explosion limit (LEL) sensor was fabricated. The microheater was characterized by a suspending microhotplate over the silicon substrate. The alumina slurry and Pd-Pt catalyst solution were precisely and repeatedly coated on the microhotplate by a novel digital microfluid technique, respectively. Furthermore, the alumina layer and the alumina/catalyst layer on the microheater showed the collinear resistance versus voltage thermal characteristic curves during the solidification, which indicated that a good match could be directly made between them. During aging, the alumina/catalyst element demonstrated a high initial signal and then fell rapidly before coming to a stable value. Through pairing the alumina reference and alumina/catalyst sensitive elements in the wheatstone bridge, the output voltage could be up to 36 mV at the 50% LEL level of CH4 at the working temperature of 400 °C and the operation voltage was 2.6 V. The power consumption and the signal sensitivity could be also down to 75 mW and up to 0.702 mV/LEL%, respectively.

Zhimin Zhou

Papers

Demonstration of Reduced Power Consumption MEMS LEL Sensor Prepared by a Novel Digital Microfluid Technique