Nano-structured vanadium oxide thin films for gas sensors

Vanadium thin films are prepared by DC magnetron sputtering using the GLancing Angle Deposition (GLAD) method. This technique will allow the growth of porous nano-structures with inclined columns, as well as zig-zags and spirals. Also pulses of oxygen combined with GLAD will be used during sputtering to study the properties of the films. All films are oxidized after being heated for several temperature cycles, taking the forms of V2O5 and VOX compositions. The aim of this work is to characterize coatings containing primarily the VO2 phase; given this phase has a reversible semi-conductor to metal transition close to room temperature (Approximately 68º). Due to this behaviour, VO2 becomes useful in sensor applications, especially with gases. The best operating conditions to favour the formation of VO2 and its most significant metal-to-semiconductor transition will be examined. The column angle β is measured by scanning electron microscopy (SEM) and the electrical conductivity of the films exhibiting inclined columns, zig-zags and spirals nano-structures as well as the films sputtered with oxygen pulses is investigated with DC-resistivity measurements. It is found that the column angle β must reach a threshold value to significantly reduce the electrical conductivity of the vanadium thin films close to one order of magnitude. The most suitable nano-structures for gas sensor applications found were: Inclined columns of incidence angle α=85º annealed to 550ºC and inclined columns of incidence angle α=85º with oxygen pulses during deposition with a period of 16 seconds and duty cycles dc=0,124 and dc=0,375, with annealing treatments to 550ºC and 500ºC respectively. The film with dc=0,375 annealed to 550ºC was tested for ozone sensing. The optimum working temperature found was of 370ºC degrees, with a maximum variation of resistivity in front of time of 0,05534 Ω/s.Outgoin

Silicon Rich Nanoparticles and their Effect on Creep Deformation in Ti-6Al-2Sn-4Zr-2Mo

Validerat; 20160704 (global_studentproject_submitter

Nano-structured vanadium oxide thin films for gas sensors

Vanadium thin films are prepared by DC magnetron sputtering using the GLancing Angle Deposition (GLAD) method. This technique will allow the growth of porous nano-structures with inclined columns, as well as zig-zags and spirals. Also pulses of oxygen combined with GLAD will be used during sputtering to study the properties of the films. All films are oxidized after being heated for several temperature cycles, taking the forms of V2O5 and VOX compositions. The aim of this work is to characterize coatings containing primarily the VO2 phase; given this phase has a reversible semi-conductor to metal transition close to room temperature (Approximately 68º). Due to this behaviour, VO2 becomes useful in sensor applications, especially with gases. The best operating conditions to favour the formation of VO2 and its most significant metal-to-semiconductor transition will be examined. The column angle β is measured by scanning electron microscopy (SEM) and the electrical conductivity of the films exhibiting inclined columns, zig-zags and spirals nano-structures as well as the films sputtered with oxygen pulses is investigated with DC-resistivity measurements. It is found that the column angle β must reach a threshold value to significantly reduce the electrical conductivity of the vanadium thin films close to one order of magnitude. The most suitable nano-structures for gas sensor applications found were: Inclined columns of incidence angle α=85º annealed to 550ºC and inclined columns of incidence angle α=85º with oxygen pulses during deposition with a period of 16 seconds and duty cycles dc=0,124 and dc=0,375, with annealing treatments to 550ºC and 500ºC respectively. The film with dc=0,375 annealed to 550ºC was tested for ozone sensing. The optimum working temperature found was of 370ºC degrees, with a maximum variation of resistivity in front of time of 0,05534 Ω/s.Outgoin

Hot deformation behaviour of sintered cobalt

Hot deformation of sintered cobalt during hot compression testing was investigated in the temperature range of 700–1000 °C and at strain rates ranging from 0.0005 to 0.1 s−1. Cobalt underwent considerable dynamic recrystallization (DRX) during hot deformation, with stress-strain flow curves exhibiting one or several peaks followed by significant flow softening and leading to a steady-state stress. Constitutive equations were used to derive the flow stress behaviour. A physically based model to describe the strain rate as a function of stress was suggested for temperatures ranging between 775 and 1000 °C. In this case, a creep exponent (n) of 5 indicated that the deformation mechanism was controlled by the glide and climb of dislocations. The activation energy coinciding with the one for self-diffusion of ferromagnetic cubic cobalt implied a diffusion-controlled mechanism and the presence of face-centered cubic (FCC) cobalt during deformation. Interestingly, the results at 700 °C could not be perfectly fitted to this model and exhibited a higher resistance to deformation. This revealed that the glide and climb deformation mode was close to the transition where glide mode was dominant, and thus mostly glide occurred at 700 °C, especially for the largest strain rates

Contact fatigue behavior of a-Al2O3-Ti(C,N) CVD coated WC-Co under dry and wet conditions

The response to cycling contact fatigue load of a WC-6%Co carbide coated with a Ti(C,N)/a-Al2O3 CVD multilayer was investigated in dry and wet conditions. Imprints in dry conditions were characterized by small thin cracks forming a circumference at the maximum radii of the imprint. The damaged coating was totally present in the final imprint of the dry test. Wet indentations showcase an area in the imprint where the a-Al2O3 layer has been removed throughout a ring but was kept at the center of the indentation, suggesting that the coating damage under cycling contact load in wet conditions is dominated by a-Al2O3 degradation, associated with a fretting effect or tangential loads accelerating the fatigue-corrosion of the alumina layer.Peer ReviewedPostprint (author's final draft

Contact fatigue behaviour of CVD coated cemented carbides in dry and wet conditions

Cycling contact fatigue indentation tests in dry and wet conditions were conducted in cemented carbides CVD-coated with Ti(C,N)/a-Al2O3 and Zr(C,N)/a-Al2O3 systems. In dry contact experiments, all imprints presented a deformed area where the coating was still present after the test. Formation of cracks was visible at the edge of the contact point of the imprint. On the other hand, under wet conditions, almost all imprints presented a ring where the a-Al2O3 had been removed, driven by a corrosion-fatigue process. Exceptionally, for Cr-containing cemented carbides combined with Zr(C,N)/a-Al2O3 coating, no delamination of a-Al2O3 was observed after the test under wet conditions. This result suggests that the combination of fine-grained cemented carbides and coating systems with low coefficient of thermal expansion may be of advantage to resist degradation by cyclic contact fatigue. Furthermore, the response to fatigue by contact loads can be enhanced by tailoring/combining cemented carbides and CVD coating systems which present higher chemical stability, and both higher intrinsic plasticity and resistance to plastic deformation.Peer ReviewedPostprint (author's final draft