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Introduction Hydrogen can be released during the thermal decomposition of organic materials; therefore, monitoring its level in the working industrial high-voltage transformer oil allows you to identify the development of degenerative processes in advance, because these processes can lead to an accident in the future. In experiments has shown that highly sensitive and small-sized field effect gas sensor based on the metal-insulator-semiconductor (MIS) structure can be used for measuring of H2 in oil with direct contact of its structure with transformer oil. Hydrogen Diffusion in Transformer Oil Numerical estimates show that with the diffusion coefficient D = 4*10-9 m2/s [1] (at room temperature), the time required for H2 saturation in a transformer oil with 10 cm depth to a level 0,9C0 is more than 1 year. In reality, there are convective flows occurred due to the temperature gradient which are combined with diffusion. Nevertheless, such a large diffusion time must be taken into account when designing the experimental setup and the obtained results interpreting. Field-Effect Gas Sensor A schematic representation of the field-effect gas sensor is shown on Figure 1 and described in work [2]. The measuring H2 concentration with the field-effect gas sensor is as follows. When hydrogen molecules from the external environment interact with the metal electrode, H2 molecules decompose into atoms and diffuse to the metal-insulator interface. It is known that on the surface of palladium hydrogen atoms have a dipole moment. The electric field of dipoles leads to a change in the electric field in the dielectric and in the surface layer of the semiconductor. As a result, the capacitance of the MIS-structure changes, the value of which is fixed by the electronic unit while maintaining a constant bias voltage on the MIS-structure. Results and Conclusions The setup used in experiment (Fig. 2) provides similar to a transformer conditions of H2 diffusion. Both field-effect sensors were preliminarily calibrated for H2 in air (in the concentration range 5-100 ppm) at the sensing element temperatures of 23 and 100°С. The optimal temperature of the sensor structure is 100-150°C. However, in experiments, the main sensor could not be maintained at this temperature due to intense heat exchange with oil. For this reason, its heating element was turned off and equal to the temperature of the oil and was 23°C. The reference sensor was maintained at a temperature of 100°C. Relative humidity above oil was 25%. The experiment was carried out as follows. Through drainage tubes, fixed H2 concentrations in the range from 5 ppm to 100 ppm were fed into the container. After the mixture was supplied, the tubes were blocked and, subsequently, the H2 concentration in the vessel was monitored by sensor (Fig. 2 (3)). To accelerate the process of oil saturation with H2, a mixing device was used, the rotation frequency of which was selected to exclude the formation of air bubbles in the oil. Figure 3 shows the dependence of the response of the sensors (main and reference) on the concentration of H2. Table 1 shows the measurement results. The experiment showed that the temperature of the sensor has a significant effect on the detection time of H2. Immersion of the sensor in oil leads to an increase in the noise level of the signal and a decrease in the sensitivity of the sensor in the low concentrations range. Also, the presence of transformer oil does not change the functional dependence of the sensor capacity on the H2 concentration. The response time to H2 is significantly less than the time of its diffusion in oil, which makes it possible to use the sensor as a sensitive element in the control systems for the level of dissolved H2 in the oil of a working transformer. Acknowledgment This work was funded by the Russian Science Foundation under Grant no 18-79-10230 from 08.08.2018. References [1] Bychkov A.L., Korobeynikov S.M., Ryzhkina A.Y., Determination of the hydrogen diffusion coefficient in transformer oil, Technical Physics. The Russian Journal of Applied Physics. 81 (2011) 106-107. [2] Nikolaev, I.N., Kalinina, L.N., Litvinov, A.V. MIS sensors for hydrogen content measurements in 10 -4 -10 2 volume percent range (2011) Automation and Remote Control, 72 (2), pp. 442-448. Figure 1

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. References [1] Karpova, E., Mironov, S., Suchkov, A., Karelin, A., Karpov, E.E., Karpov, E.F. Increase of catalytic sensors stability (2014) Sensors and Actuators, B: Chemical, 197, pp. 358-363. [2] Bíró, F., Dücső, C., Radnóczi, G.Z., Baji, Z., Takács, M., Bársony, I. ALD nano-catalyst for micro-calorimetric detection of hydrocarbons (2017) Sensors and Actuators, B: Chemical, 247, pp. 617-625. Figure 1