R. G. Pavelko

Sensors Based on Technology “Nano-on-Micro” for Wireless Instruments Preventing Ecological and Industrial Catastrophes

The problem of gas analyzers compatible with wireless networks can be solved by using sensors based on the “nano-on-micro” technology. The basis of this technology consists in nano-composite sensing metal oxide semiconductor or thermocatalytic materials deposited on a microhotplate fabricated using silicon or alumina microelectronic technology. As a result, the sensor combines the advantages of both technologies: on the one hand, high stability and sufficient selectivity of nano-composite materials, and, on the other hand, microprocessor compatibility, low-cost, mass-production possibilities, and low power consumption of microelectronic substrates. Two methods for the fabrication of microhotplates are the most promising: the silicon based technology of silicon oxide/silicon nitride membranes and the CeraMEMS technology of thin alumina films (TAF). The first technology enables the fabrication of microheaters with a power consumption around 20 mW for an operating temperature below 450°C. Advantages of CeraMEMS platforms are: (1) operation at temperature up to 600°C and, potentially, up to 800°C; (2) robustness compared with silicon chip with thin membrane; (3) perfect Pt and sensing layer adhesion without any adhesive layers; (4) low cost of middle scale production (104–107 chips per year) compared with the silicon technology. The CeraMEMS platform can be used for the fabrication of semiconductor and thermocatalytic gas sensors, as a source of IR radiation for optical gas sensors and as bolometers. The sensor withstands ∼7 × 106 on-off cycles. Heater resistance drift is below 3% per year at 550°C.

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.

Silicon carbide transport during carbothermic reduction of SiO2: Thermodynamic evaluation and experimental study

Mass-transfer processes during the high-temperature carbothermic reduction of silicon dioxide have been studied using thermodynamic modeling. The chemical vapor transport of silicon carbide has been investigated using SiO2 + xSiC mixtures—major reaction products in the SiO2-C system—as examples. Thermodynamic modeling results indicate that the vapor transport of silicon carbide is possible at temperatures from 1300 to 1500°C, and that the major gaseous species involved are Si and CO. Vapor transport processes have been studied experimentally. It is shown that the thermal reaction between carbon monoxide and silicon leads not only to direct conversion of silicon particles to silicon carbide but also to the growth of silicon oxycarbide fibers. The synthesized material has been characterized by x-ray diffraction and high-resolution optical microscopy.

Perchlorosilanes and perchlorocarbosilanes as precursors for SiC synthesis

Homologues with the general stoichiometry a(SiCl4): bSi: cC: d(SiC) are shown to be potential precursors for the low-temperature gas-phase synthesis of silicon carbide. Thermal decomposition of these precursors yields the chemically stable gaseous species SiCl4 and condensed Si, C, SiC, SiC + Si, or SiC + C. We use thermodynamic modeling of the thermal decomposition of octachlorotrisilane, Si3Cl8, to analyze the key features of the thermolysis of perchlorosilanes with the general stoichiometry a(SiCl4): bSi. The equilibrium compositions of reaction products in the Si3Cl8-CO system are determined. This reaction system enables low-temperature (400–1200 K) formation of silicon carbide.

Alumina MEMS Platform for Impulse Semiconductor and IR Optic Gas Sensors

In the presentation, we discuss the application of a novel MEMS technology based on a fabrication of thin alumina film (TAF). The membrane is fabricated by the electrolyte spark oxidation of aluminum. The membrane consists of nano-crystalline gamma-aluminum oxide and has a thickness of 10 - 30 microns. It was shown that this membrane chip can be used for the fabrication of gas sensors (semiconductor, thermocatalytic, and optic) operating in impulse regime. The thermal response time of the heater is of about 80 ms, the chip remains working after 7 millions on-off cycles at 450degC.

Scanning probe microscope-quartz crystal microbalances integrated system for in-situ study of sensor properties of microamounts of nanomaterials

Testing of scanning probe microscope (SPM)-quartz crystal microbalances (QCM) integrated system has been performed to measure the responses of tin oxide-based sensor nanomaterials on the effect of air mixtures containing hydrogen, methane, and ammonia.The change in the oscillation frequency of piezoquartz resonator with applied sensitive materials and change of the surface potential according to Kelvin probe microscopy (KPM) have been used as sensor signals.

Thermodynamic and experimental study of the interaction of silicon and carbon monoxide: Synthesis of silicon carbide nanofibers

The reaction of crystalline silicon with carbon monoxide to produce silicon carbide was studied. Thermodynamic simulation of the equilibrium phase composition of the nSi-mCO system was carried out in the range 300–2000 K (27–1727°C). Conditions required for silicon carbide was carried out applying various experimental modes (n, m, and T) and possible pathways of the reactions were determined. Interaction between crystalline silicon and carbon monoxide formation in a temperature range of 1000–1450°C. The order of the reaction in CO was found to be close to unity. Silicon carbide nanofibers with thicknesses of from 5 to 100 nm were synthesized and characterized by powder X-ray diffraction, mass-spectral elemental analysis, and scanning electron microscopy. A possibility of synthesizing high-purity silicon carbide fibers were experimentally evaluated.

Tin Oxide from Organo-Metallic Compounds: Material’S Properties and Sensor Characteristics

This chapter summarizes studies performed on tin dioxide materials, fabricated from different synthetic routes, for further use in semiconductor gas sensors. The synthesis of SnO2 was performed using a conventional precipitation technique starting from tin (IV) acetate. The material was compared with a commercial SnO2 nanopowder. On the basis of thermo XRD analysis followed by the calculation of average crystallite size, we estimated the thermal stability of the mentioned materials. FT-IR, Raman, SEM and laser spark element analyses were also used for material characterization. Thick film sensors, fabricated using synthesized and commercial SnO2 materials, were examined with regard to their responses to methane. Both undoped and doped synthesized SnO2 were found to exhibit the highest responses to methane compared with others materials and commercial tin oxide sensors.

Effect of ultrafine carbon precursors on the morphology of silicon carbide nanoparticles

A comparative analysis of carbon precursors used for the synthesis of silicon carbide nanoparticles has been carried out. Phenol-formaldehyde resin, colloidal graphite, poly(vinyl alcohol), carbon black obtained as a byproduct of fullerene synthesis, and multiwall carbon nanotubes have been used as carbon sources in the synthesis of the starting ultrafine composition nC + mSiO2. After carbothermic synthesis, the morphology of silicon carbide nanoparticles was studied by scanning and transmission electron microscopy. The morphologic features and particle-size distribution of the materials were comparatively studied.