Thermocatalytic Sensors Research Articles

Microscale chemiresistive hydrogen sensors: Current status and recent developments

Hydrogen is known for its efficient combustion, abundant natural availability, and environmentally friendly characteristics. It is recognized as a promising energy source for the future and is already utilized in various industries, including petrochemicals, electronics, food processing, aerospace, and new energy vehicles. However, challenges arise in the storage and use of hydrogen owing to its tendency to leak, its potential for explosion within a specific concentration range of 4%–75%, and itslow ignition energy requirements. Consequently, there is a demand for hydrogen sensors capable of quickly and accurately detecting low levels of hydrogen leaks. Microelectromechanical systems-based chemiresistive hydrogen sensors offer advantages such as low cost, compact size, low energy consumption, and superior sensing performance, making them a major focus of recent research. This article provides a comprehensive overview and comparison of the sensing principles of various hydrogen sensors, including chemiresistive sensors, electrochemical sensors, thermocatalytic sensors, acoustic sensors, and mechanical sensors. Micro-chemiresistive hydrogen sensors exhibit high sensitivity, low cost, and ease of integration, making them highly promising for practical applications. In response to the challenges encountered in practical applications of chemiresistive hydrogen sensors, such as high operating temperatures and high power consumption, this review explores emerging trends in chemiresistive hydrogen sensor technology from the perspectives of novel materials and activation methods. Finally, it discusses the applications and potential further developments of chemiresistive hydrogen sensors.

ИДЕНТИФИКАЦИЯ ГОРЮЧИХ ГАЗОВ ТЕРМОКАТАЛИТИЧЕСКИМИ СЕНСОРАМИ

Thermocatalytic gas sensors are among the most widely used for measuring concentrations of combustible gases. However, in spite of the variety of advantages of this type of sensors, there are also shortcomings, one of which is low selectivity to target combustible gases. This study presents the results of the application of machine learning methods for the identification of flammable gases in the air using thermocatalytic sensors. The sensor signal measurements were performed using the multistage measuring pulse method, which was described in our previous papers. To enrich the sensor signal with information about ambient gas composition, the number of stages of the method was drastically increased. Methane and vapors of acetone, ethanol and gasoline were used as the target gases. The multivariate data was processed with the support vector machine method. The results show that the proposed method allows for gas identification using a single catalytic gas sensor. The method can be used in the development of highly selective gas detectors responding only to target gases.

МЕТОД ГЛАВНЫХ КОМПОНЕНТ КАК АЛЬТЕРНАТИВНЫЙ АЛГОРИТМ ОБРАБОТКИ ДАННЫХ ТЕРМОКАТАЛИТИЧЕСКИХ СЕНСОРОВ

In this article the study of the possibility and evaluation of the use of the principal component analisys as an alternative algorithm for processing data of thermocatalytic sensors is carried out. Based on the obtained results it was found that the effect of the sensor's operating temperature on the response is minimal, which makes it possible to combine them into a single system. At the same time the different concentration of the supplied gas is uniquely determined. The research methodology is defined and the results are presented, showing that the principal component analysis makes it possible to distinguish sensor responses at different concentrations without using additional mathematical operations to prepare the obtained data.

Improving the Efficiency of Measurements of Combustible Gases Concentration by a Thermocatalytic Sensor

Thermocatalytic sensors are widely used in gas analysis systems and have high reliability and low cost. However, errors in measuring the concentration of combustible gases related to the non-linearity of the conversion characteristic and the influence of ambient temperature fluctuations significantly limit the scope of their application.Purpose of reseach. Development of a method for measuring gas concentration by thermocatalytic sensors, which allows reducing measurement errors by tuning out due to ambient temperature influence and linearization of the conversion characteristic.Problems. They are as follows: to develop a method for temperature stabilization of a thermocatalytic sensor; to make a structural and functional scheme for the sensor activation; to obtain a mathematical description of the method and substantitation for tuning out as a result of temperature influence; to experimentally confirm the possibility of linearization of the sensor conversion function in the thermal stabilization mode.Methods. The mathematical description of the method applies the theory of heat transfer and the theory of electrical circuits with discrete signals. When analyzing existing solutions and synthesizing the device, methods for calculating circuits with nonlinear elements and the theory of measurement systems have been used. The real conversion function has been obtained through an experimental method.Results. A method for measuring gas concentration by a thermocatalytic sensor with the use of a microcontroller and PWM has been developed. It allows reducing errors due to tuning out as a result of ambient temperature influence. A mathematical description of the method has been given. An experiment has been performed. It demonstrates the effectiveness of using temperature stabilization to linearize the conversion characteristic.Conclusion. The paper proposes a method for temperature stabilization of thermocatalytic gas sensors. The method makes it possible to increase the accuracy of measurements by tuning out due to the influence of temperature fluctuations and linearization of the conversion function. The possibility of linearization of the sensor function has been experimentally confirmed. It characterizes the dependence of the output signal on the concentration of combustible gas. Using this method allows you to reduce the cost of the sensor, improve the quality factors of the sensor, such as the reliability and stability of parameters.

SOI Based Low Power Thermocatalytic Sensor with Nanostructured Gas Sensitive Material

Introduction Catalytic gas sensors are essential devices for detection of combustive gases near lower explosion limit (LEL). As the minimum power dissipation of Pt coil based sensors (pellistors) are 120 - 150 mW [1] intensive research is devoted to reduce it to be better compatible with portable devices while preserving sensitivity and stability. The reported microheater structures can operate up to 600 oC at a cost of 20-50 mW power consumption [2]. Nowadays the research activity is focused on development of stablenanostructured catalyst layer effective at low temperature, whereas compatible with MEMS thick film technology. Gas Sensitive Catalytic Materials The approach of fabrication gas sensitive material for coil and silicon membrane sensors is different. In first case the catalyst is bulky and forms bead or cylinder with diameter 400-500µm. In MEMS structures the catalyst is deposited on a microhotplate with a characteristic diameter of 100µm, de facto forming 2D surface. In the present work nanodespersed Al2O3 and ZrO2 ceramic carriers were prepared as presented on figs.5 and 6, respectively. Each material was divided into two equal parts - an active catalytic layer from one part and a comparative element from the second part were made exhibiting equal surface area. .In order to impregnate the catalyst support with the catalyst metal, salts of palladium chloride (PdCl2) and platinum acid (H2PtCl6) were used. Having annealed at high temperature metal clusters were formed in the catalyst support. Finally the active and reference materials were mixed with an organic binder to make the paste suitable for drop-coating deposition to MEMS silicone microheater. SOI Based MEMS Microheater Uniform and reproducible crystalline Si filaments were formed from SOI (silicon on insulator) wafers, because the buried oxide provides uniform thickness of the device layer and guarantees identical geometry. Cantilevers are suspended on stress compensated SiO2-Si3N4 membrane to increase their mechanical stability and eliminate their bending out of the original plane (fig. 1-4). Thereby the reduced stress provides longer lifetime. The higher resistivity of device silicon ensures higher filament resistance at the same temperature compared to its thin film metal reference, therefore the cross section of the current routes should be increased to achieve the sufficient resistance. A plausible advantage of the single crystalline filament material and the design is the minimized degradation effect of electromigration, thereby the lifetime of the heater is expected to achieve 6000-8000 hours. Moreover, the heated area of filament can be completely covered with catalyst or passive material, similarly to the coil-type filament devices.The opened side chip design facilitates catalyst deposition. Results and Conclusions Significant issues arise when the design of thermocatalytic sensors are transferred from the volumetric to the microplanar approach. First of all, the catalytic gas-sensitive layer must provide chemical activities:3∙10-6÷10-5mW/μm3. The solution of the problem is to choose classical materials alreadybeen used for many years in coil types pellistors - catalysts of platinum group metals on Al2O3 or ZrO2 ceramic carriers. The stability and behavior of these materials at high working temperatures has been already tested over tens of years in real working conditions (mains, gas line pipes, leakage alarm systems and etc.). The decrease in the quantity of the catalytic material deposited on the microheater leads to insufficient catalytic activity of the sensor as a whole. An increase in the operating temperature can correct the situation, but it is limited by the long-term stability of the microheater and the transformation of the crystallographic phase of the ceramic catalyst carrier. The critical temperature is around 550 °C. The Pt-Pd mixed-catalysts can be applied in the microplanar structure if uniform hotplate temperature is provided and the active and the reference sensing layers are deposited such as to minimize imbalance between the two elements. Acknowledgement This research was sponsored by the Sponsored by the National Research, Development and Innovation Office Foundation, Hungary, funding No. 2017-2.3.4-TeT-RU-2017-00006, and the Ministry of Science and Higher Education of the Russian Federation founding with unique identifier RFMEFI58718X0053.

A Thin-Film Platform for Chemical Gas Sensors

In the study, a technique for formation of planar microheaters that make it possible to heat an active zone to a temperature higher than 500°С is proposed and successfully implemented. The developed heating elements are distinguished by low power consumption, short response time, and extremely high resistance to impact loads. The synthetic approaches used in the work (anodic oxidation, photolithography, and magnetron sputtering) feature manufacturability and scaling simplicity. This makes planar heating elements a promising platform based on which semiconductor and thermocatalytic sensors of toxic and explosive gases may be created.

Expanding Analytical Potential of Hybrid Sensor Arrays

Three types of array from resistive sensors based on polymer composites with carbon filler, tin oxide and thermo-catalytic sensors were compared. Special attention was given to ways of expanding gas analysis systems by using hybrid sensor arrays and kinetic parameter of sensory response.

Fabrication of Microhotplate by Selective Laser Sintering of Micropowder for Thermal Conductivity Measuring Sensors

The layout of experimental samples of microhotplate based on nanocomposite material consisting of ruthenium dioxide and borosilicate glass binder were formed by selective laser sintering method. The resulting structures were used as microhotplate to measure changes in thermal conductivity of gases. The comparison of power consumption of standard platinum wire coil element widely used in thermocatalytic (calorimetric) sensors and microhotplate produced by the new technology showed strong advantage of the latter one.

Control of the Diffusion Flow in Thermocatalytic Fuel Gas Sensors

Brief descriptions of different thermocatalytic sensors are given. Problems that arise due to instability of the sensors during operation are considered. The use of limited access of the mixture being analyzed to the reaction chamber, while satisfying certain conditions, is proposed to increase the stability. The diffusion processes are analyzed and recommendations are made for increasing the measurement stability severalfold.

Diffusion processes in surface thermal catalysis and improvement of the stability of combustible gas sensors

We briefly describe the development of thermocatalytic sensors. Problems associated with instability of sensors during their operation are outlined. We propose that for improving stability, the access of the mixture being analyzed to the reaction chamber must be limited so that the condition in which the sensor output is by many times larger than the diffusion flux of the combustible gas through the calibrated orifice is observed. Diffusion processes are analyzed to work out recommendations for improving the measurement stability by several times.

Studies of thermal stability of nanocrystalline SnO2, ZrO2, and SiC for semiconductor and thermocatalytic gas sensors

The thermal stability of synthesized and commercial SnO2, ZrO2, and SiC nanopowders is compared. The crystallite growth rate during the isothermal annealing of the materials at 700°C for 30 h is evaluated. The crystallites’ average size was determined by X-ray phase analysis (using the Scherrer method). The effect of impurity content on the kinetics of crystallite growth is studied for the synthesized SnO2 and ZrO2. Semiconductor and thermocatalytic sensors, based on the synthesized and commercial materials, are manufactured. The long-term stability of the sensors’ signal is compared with the thermal stability of the nanopowders.

Thermocatalytic sensors with Pd-Pt-Al 2O 3 catalyst

The pecularities of the temperature dependence of the output signals of thermocatalytic sensors (TCS), having metal-oxide catalysts of various composition (Pd-Al 2O 3, Pt-Al 2O 3, Pd-Pt-Al 2O 3), annealed in air and in methane-air mixture, in the process of methane detection are investigated. The obtained results were analysed with regard to the data of catalyst phase composition change in situ and of the Pd and Pt valency state. The conclusion was made that one of the possible reasons for the higher output signal stability in thermocatalytic sensors with the Pd-Pt-Al 2O 3 catalyst is the stability increase of Pt in the presence of Pd + and Pd 2+ palladium ion forms, which are most active in the reaction of deep methane oxidation.