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Five types of ferrite-pearlite structure carbon steels with different carbon contents (IF steel, S12C, S20C, S35C, S50C) were friction stir welded under various welding conditions, and the mechanical properties and microstructures of the FSW carbon steel joints were evaluated. Compared with IF steel, the microstructures and mechanical properties of the carbon steel joints are significantly affected by the welding conditions. When the carbon content is less than or equal to 0.12 mass%, the welding produces ferrite-pearlite structures, and the strength slightly increases compared to the base metal due to the refined microstructure; when the carbon content is above 0.2 mass%, the welding produces ferrite-pearlite plus harder phases like the martensite and bainite microstructures, resulting in a significantly increased strength of the joints. These are dependent on each of the thermal-mechanical cycles.

The role of porosity on impedancemetric NO x sensing was investigated for sensors composed of a porous Y-stabilized ZrO 2 (YSZ) electrolyte and Au wire electrodes. NO x sensors were fabricated at firing temperatures of 950 • C, 1000 • C, and 1050 • C to establish different porous microstructures. Porosity calculations were determined from scanning electron microscopy images of the porous electrolytes using a three-dimensional statistical method. The mean porosity of sensors fired at 950 • C was 50.2%, and the mean porosity value decreased to 46.7% for sensors fired at 1050 • C. Impedance spectroscopy was used to measure the electrical response of the sensors while operating at 650 • C, in gas atmospheres of 0-100 ppm NO x and O 2 concentrations ranging from 1-18% in a balance of N 2 . The impedance of the sensors demonstrated a strong dependence on porosity as a decrease in porosity of about 4% resulted in nearly a 50% decrease in the impedance. Analysis of the impedance data indicted NO x sensitivity increased as the YSZ electrolyte porosity decreased. The response to NO x concentrations ≤ 10 ppm were distinguishable at operating frequencies as high as 40 Hz enabling rapid sensing.

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Using impedance spectroscopy the electrical response of sensors with various porous Y-stabilized ZrO 2 (YSZ) microstructures was measured for gas concentrations containing 0-100 ppm NO with 10.5%O 2 at temperatures ranging from 600-700 °C. The impedance response increased substantially as the sensor porosity increased from 46%-50%. Activation energies calculated based on data from the impedance measurements increased in magnitude (97.4-104.9 kJ/mol for 100 ppm NO) with respect to increasing YSZ porosity. Analysis of the oxygen partial pressure dependence of the sensors suggested that dissociative adsorption was the dominant rate limiting. The PWC/DNP theory level was used to investigate the gas-phase energy barrier of the 2NO+O 2 →2NO 2 reaction on a 56-atom YSZ/Au model cluster using Density Functional Theory and Linear Synchronous Transit/Quadratic Synchronous Transit calculations. The reaction path shows oxygen surface reactions that begin with NO association with adsorbed O 2 on a Zr surface site, followed by O 2 dissociative adsorption, atomic oxygen diffusion, and further NO 2 formation. The free energy barrier was calculated to be 181.7 kJ/mol at PWC/DNP. A qualitative comparison with the extrapolated data at 62% ± 2% porosity representing the YSZ model cluster indicates that the calculated barriers are in reasonable agreement with experiments, especially when the RPBE functional is used.