We proposed a novel application of cobalt sulfide (CoS) in the configuration of transparent thin ... more We proposed a novel application of cobalt sulfide (CoS) in the configuration of transparent thin film as anode in p-type dyesensitized solar cell (p-DSC). The anodes here considered have been prepared using a water-based method that is suitable for the large scale production of large-area electrodes. The photoactive cathodes of the p-DSC were mesoporous nickel oxide (NiO) thin films deposited via rapid discharge sintering. The NiO electrodes were sensitized with the benchmark dye erythrosine B (ERY), while the couple I À /I 3 À was the redox mediator. The CoS anodes showed higher electrocatalytic efficiency in comparison with the commonly used platinized Fluorine-doped Tin Oxide (Pt-FTO). This was determined by means of electrochemical impedance spectroscopy of CoS based dummy cells, with CoS showing a lower charge-transfer resistance with respect to Pt-FTO. The overall conversion efficiency of the p-DSC employing ERY-sensitized NiO as photoactive cathode and CoS anode was 0.026 %, a value very close to that obtained with Pt-FTO anodes (0.030 %). The external quantum efficiency spectra of the p-DSCs with CoS anodes were similar to those obtained with Pt-FTO anodes under illumination with AM 1.5 solar simulator.
International Journal of Adhesion and Adhesives, 2014
Abstract This paper evaluates the use of an air based atmospheric pressure plasma jet for the act... more Abstract This paper evaluates the use of an air based atmospheric pressure plasma jet for the activation of carbon fibre epoxy composites. In addition the performance of the jet in the removal of a 5–8 nm thick layer of FreKote 710-NC from the composite surface prior to heat-cured epoxy adhesive bonding was also investigated. The plasma treated composites were examined using thermal infra-red imaging, WCA, profilometry and SEM. A reflectance-IR technique was used to monitor the removal of FreKote as well as changes in the functional chemistry of the plasma treated composite surface. The optimised plasma pre-treatment conditions were identified which help to maximise composite-to-composite adhesive bond strength based on LST. This involved a treatment at a 14 mm nozzle to composite distance and scan rate of 30 mm/s. The air plasma process was bench marked against a manual methanol solvent wipe process, for the removal of FreKote. Both the plasma and the solvent wipe treatments yielded similar composite to composite bond strengths, demonstrating that both were successful in the removal of the FreKote. A comparison was also made between the plasma treated composite and a composite which had been grit blasted using 50 μm aluminium oxide grit. A 7% enhancement in lap-shear strength was achieved with the plasma treatment compared to grit blasting.
Plasma diagnostics of atmospheric plasmas is a key tool in helping to understand processing perfo... more Plasma diagnostics of atmospheric plasmas is a key tool in helping to understand processing performance issues. This paper presents an electrical, optical and thermographic imaging study of the PlasmaStream atmospheric plasma jet system. The system was found to exhibit three operating modes; one constricted/localized plasma and two extended volume plasmas. At low power and helium flows the plasma is localized at the electrodes and has the electrical properties of a corona/filamentary discharge with electrical chaotic temporal structure. With increasing discharge power and helium flow the plasma expands into the volume of the tube, becoming regular and homogeneous in appearance. Emission spectra show evidence of atomic oxygen, nitric oxide and the hydroxyl radical production. Plasma activated gas temperature deduced from the rotational temperature of nitrogen molecules was found to be of order of 400 K: whereas thermographic imaging of the quartz tube yielded surface temperatures between 319 and 347 K.
There is considerable interest in processing technologies which can lead to more energy efficient... more There is considerable interest in processing technologies which can lead to more energy efficient sintering of metal powders. The use of microwave sintering in particular leads to reduced energy usage during sintering as the volumetric heating process is considerably more efficient compared with resistance heating. In this study the use of a novel plasma microwave processing technology for the sintering of nickel powder discs is evaluated. The sintering study was carried out on 20 mm diameter by 2 mm thick pressed discs of nickel powder, with mean particle size of 1 µm. The discs were fired in a 5 cm diameter microwave (2.45 GHz) plasma ball under a hydrogen atmosphere at a pressure of 2 kPa. The same discs were also sintered using both non plasma microwave and tube furnace firing. The microwave plasma sintering is very rapid with full disc strength of approx. 1000 N based on 3-point bend tests being achieved within 10 minutes. In contrast the sintering time in the tube furnace involved treatments of up to 6 hours. The non plasma microwave system involved intermediate treatment periods of 1 to 2 hours. Another advantage of the microwave plasma treatment is that the degree of sintering between the individual nickel powder particles can be precisely controlled by the duration of the treatment time in the plasma. There was a broadly linear increase in fired pellet breaking strength with plasma treatment duration. In addition to breaking load, the mechanical properties of the sintered nickel discs were compared based on Rockwell hardness tests and density measurements. The morphology of the sintered discs was compared using microscopy and SEM. This study demonstrated that the plasma microwave sintered discs produced similar or superior performance (depending on processing conditions) to discs fired using the non-plasma microwave and furnace firing conditions. Accurate control of the sample conditions and structure can easily be controlled with the plasma system compared with the conventional systems. The apparent volumetric heating in the microwave systems give a more uniform heating at lower temperatures and allows for greater control and homogeneity.
Comparison of two low temperature deposition methods for HA Combining silica shot with a flow of ... more Comparison of two low temperature deposition methods for HA Combining silica shot with a flow of HA powder worked effectively Replacing the silica with alumina abrasive produced a more adherent coating Chemistry and crystallinity of the deposited HA coatings were comparable to the feedstock powder Results show favourable coating properties for CoBlast in comparison to the Shot
The thermal stability of siloxane films deposited on Vycor glass and silicon substrates using atm... more The thermal stability of siloxane films deposited on Vycor glass and silicon substrates using atmospheric pressure plasma was studied. Siloxane films were deposited from a liquid tetraethyl orthosilicate (TEOS) precursor which was nebulised into a helium/oxygen plasma. The thickness of the siloxane films was in the range 1-12 nm as measured using both ellipsometry and X-ray reflectometry. Film composition and chemical properties were examined using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analysis. The thermal stability of the films was evaluated under argon atmosphere at temperatures up to 700°C. The films were found to exhibit good adhesion without cracks or delamination after the thermal treatment. A decrease in coating thickness and a reduction in surface roughness was however observed indicating, a change in coating chemistry. This was confirmed by the reduction in carbon concentration observed by XPS. Particulates generated by excess gas phase reaction of TEOS were observed on the surface of the coating. A larger reduction in particulates size was observed after thermal treatment compared with the reduction in thickness of the bulk coating. This indicates that the particulates may have a different composition to that of the coating.
Titanium dioxide (TiO 2) thin films, doped with chromium (Cr) and codoped with chromium-carbon (C... more Titanium dioxide (TiO 2) thin films, doped with chromium (Cr) and codoped with chromium-carbon (Cr, C) and chromium-nitrogen (Cr, N) of various concentrations, were deposited using magnetron sputtering. Postdeposition thermal treatments were carried out at 450 • C for 5 h to change the as-deposited amorphous coatings to their crystalline form. The crystalline phase was found to be dependent on the amount and type of dopant present. Ultraviolet-visible (UV-Vis) absorption data and band gap energies calculated by spectroscopic ellipsometry showed that, on increasing Cr concentration, a shift of the absorption edge towards visible light and a reduction of the band gap occurred. This was further improved by codoping of Cr with either N or C, with the latter case exhibiting more photo-activity towards visible light. In addition, hybrid density functional theory (DFT) calculations were performed for Cr-, N-. and C-monodoping, together with Cr-C and Cr-N codoping, in both rutile and anatase phases of crystalline titania. Using this method, absorption coefficients and band gaps were determined to explore photo-activity. Very good, semiquantitative agreement was found between the DFT and experimental approaches for these quantities, underlining the key role of state-of-the-art quantum calculations in interpreting and guiding experimental studies of doping in metal oxides.
This paper presents the mechanical performance of the annealed NiTi Shape Memory Alloy (SMA) coat... more This paper presents the mechanical performance of the annealed NiTi Shape Memory Alloy (SMA) coating deposited onto 316L stainless steel substrate. The as-deposited SMA coating, Ni55.9 Ti44.1, showed an amorphous behaviour. The crystalline NiTi (SMA) coating was produced by annealing the as-deposited NiTi with a thickness about 2.0 µm, at above its crystallisation temperature in a vacuum ambient. The annealed NiTi coatings were characterised to determine the effect of the annealing parameters on their mechanical behaviour. The NiTi phases and structures were determined by x-ray diffraction (XRD) and scanning electron microscopy (SEM) whereas the mechanical properties were measured using the Rockwell C adhesion test. Three main phases; NiTi B2 parent phase, Ni3Ti and TiO2 were found in the annealed samples and the intensities of each phase were dependent on the annealing temperature and annealing time. Each phase significantly affected the mechanical behaviour of the coatings. Higher...
In this study the use of microwave plasma sintering of nickel oxide (NiO x) particles for use as ... more In this study the use of microwave plasma sintering of nickel oxide (NiO x) particles for use as ptype photoelectrode coatings in dye-sensitized solar cells (DSSCs) is investigated. NiO x was chosen as the photocathode for this application due to its stability, wide band gap and p-type 2 nature. For high light conversion efficiency DSSCs require a mesoporous structure exhibiting a high surface area. This can be achieved by sintering particles of NiO x onto a conductive substrate. In this study the use of both 2.45 GHz microwave plasma and conventional furnace sintering were compared for the sintering of the NiO x particles. Coatings 1 to 2.5 µm thick were obtained from the sintered particles (mean particle size of 50 nm) on 3 mm thick fluorine-doped tin oxide (FTO) coated glass substrates. Both the furnace and microwave plasma sintering treatments were carried out at ~450 ºC over a 5 minute period. Dye sensitization was carried out using Erythrosin B and the UV-vis absorption spectra of the NiO x coatings were compared. A 44% increase in the level of dye adsorption was obtained for the microwave plasma sintered samples as compared to that obtained through furnace treatments. While the photovoltaic performance of the DSSC fabricated using the microwave plasma treated NiO x coatings exhibited a tenfold increase in the conversion efficiency in comparison to the furnace treated samples. This enhanced performance was associated with the difference in the mesoporous structure of the sintered NiO x coatings.
Crystalline titanium dioxide (TiO 2) coatings have been widely used in photoelectrochemical solar... more Crystalline titanium dioxide (TiO 2) coatings have been widely used in photoelectrochemical solar cell applications. In this study, TiO 2 and carbon-doped TiO 2 coatings were deposited onto unheated titanium and silicon wafer substrates using a DC closed-field magnetron 2 sputtering system. The resultant coatings had an amorphous structure and a post-deposition heat treatment is required to convert this amorphous structure into the photoactive crystalline phase(s) of TiO 2. This study investigates the use of a microwave plasma heat treatment as a means of achieving this crystalline conversion. The treatment involved placing the sputtered coatings into a 2.45 GHz microwave-induced nitrogen plasma where they were heated to approximately 550°C. It was observed that for treatment times as short as 1 minute, the 0.25-µm thick coatings were converted into the anatase crystalline phase of TiO 2. The coatings were further transformed into the rutile crystalline phase after treatments at higher temperatures. The doping of TiO 2 with carbon was found to result in a reduction in this phase transformation temperature, with higher level of doping (up to 5.8% in this study) leading to lower anatase-to-rutile transition temperature. The photoactivity performance of both doped and un-doped coatings heat-treated using both furnace and microwave plasma was compared. The carbon-doped TiO 2 exhibited a 29% increase in photocurrent density compared to that observed for the un-doped coating. Comparing carbon-doped coatings heat-treated using the furnace and microwave plasma, it was observed that the latter yielded a 19% increase in photocurrent density. This enhanced performance may be correlated to the differences in the coatings' surface morphology and band gap energy, both of which influence the coatings' photoabsorption efficiency.
ABSTRACT Siloxane coatings were deposited using two different atmospheric plasma systems namely a... more ABSTRACT Siloxane coatings were deposited using two different atmospheric plasma systems namely a reel-to-reel atmospheric plasma liquid deposition system called Labline™ and an atmospheric plasma jet system called PlasmaStream™. Both systems combine an atmospheric plasma, with the use of liquid precursors. The influence of the plasma source and processing conditions on the deposited coating properties were studied. The coatings were deposited onto Vycor glass and silicon wafer substrates from a liquid tetraethyl orthosilicate (TEOS) precursor which was nebulized into both He and He/O2 plasmas. Higher film growth rates were obtained using the plasma jet system; however, the reel-to-reel system facilitated the larger area coating of webs. The thickness (spectroscopic ellipsometry measurements) and water contact angle profile of the films deposited using the jet system on silicon wafer substrates in static mode were investigated. Amongst the other coatings characterization techniques used to evaluate the deposited coatings were optical profilometry, XPS, SEM, and AFM. The formation of particulates due to excess gas-phase reactions during the atmospheric plasma deposition of coatings has been widely reported. In this study, larger number of particulates were observed under the conditions of higher plasma power, with the addition of O2 into the He plasma and also at low TEOS flow rates. The introduction of N2 into the He/O2 plasma, during the deposition of siloxane coatings led to a significant reduction in the number of particulates generated for both plasma sources. Nitrogen gas flow rate was varied systematically and a correlation was obtained on the influence of the flow rate of this gas on surface roughness and particulate formation.
The photocatalytic splitting of water into hydrogen and oxygen using a photoelectrochemical (PEC)... more The photocatalytic splitting of water into hydrogen and oxygen using a photoelectrochemical (PEC) cell containing titanium dioxide (TiO2) photoanode is a potentially renewable source of chemical fuels. However, the size of the band gap (-3.2 eV) of the TiO2 photocatalyst leads to its relatively low photoactivity toward visible light in a PEC cell. The development of materials with smaller band gaps of approximately 2.4 eV is therefore necessary to operate PEC cells efficiently. This study investigates the effect of dopant (C or N) and co-dopant (C+N) on the physical, structural and photoactivity of TiO2 nano thick coating. TiO2 nano-thick coatings were deposited using a closed field DC reactive magnetron sputtering technique, from titanium target in argon plasma with trace addition of oxygen. In order to study the influence of doping such as C, N and C+N inclusions in the TiO2 coatings, trace levels of CO2 or N2 or CO2+N2 gas were introduced into the deposition chamber respectively. The properties of the deposited nano-coatings were determined using Spectroscopic Ellipsometry, SEM, AFM, Optical profilometry, XPS, Raman, X-ray diffraction UV-Vis spectroscopy and tri-electrode potentiostat measurements. Coating growth rate, structure, surface morphology and roughness were found to be significantly influenced by the types and amount of doping. Substitutional type of doping in all doped sample were confirmed by XPS. UV-vis measurement confirmed that doping (especially for C doped sample) facilitate photoactivity of sputtered deposited titania coating toward visible light by reducing bandgap. The photocurrent density (indirect indication of water splitting performance) of the C-doped photoanode was approximately 26% higher in comparison with un-doped photoanode. However, coating doped with nitrogen (N or N+C) does not exhibit good performance in the photoelectrochemical cell due to their higher charge recombination properties.
The objective of this study is to investigate the influence of alcohol addition on the incorporat... more The objective of this study is to investigate the influence of alcohol addition on the incorporation of metal oxide nanoparticles into nm thick siloxane coatings. Titanium oxide (TiO2) nanoparticles with diameters of 30-80 nm were incorporated into an atmospheric plasma deposited tetramethylorthosilicate (TMOS) siloxane coating. The TMOS/TiO2 coating was deposited using the atmospheric plasma jet system known as PlasmaStream. In this system the liquid precursor/nanoparticle mixture is nebulised into the plasma. It was observed that prior to being nebulised the TiO2 particles agglomerated and settled over time in the TMOS/TiO2 mixture. In order to obtain a more stable nanoparticle/TMOS suspension the addition of the alcohols methanol, octanol and pentanol to this mixture was investigated. The addition of each of these alcohols was found to stabilise the nanoparticle suspension. The effect of the alcohol was therefore assessed with respect to the properties of the deposited coatings. It was observed that coatings deposited from TMOS/TiO2, with and without the addition of methanol were broadly similar. In contrast the coatings deposited with octanol and pentanol addition to the TMOS/TiO2 mixture were significantly thicker, for a given set of deposition parameters and were also more homogeneous. This would indicate that the alcohol precursor was incorporated into the plasma polymerised siloxane. The incorporation of the organic functionality from the alcohols was confirmed from FTIR spectra of the coatings. The difference in behaviour with alcohol type is likely to be due to the lower boiling point of methanol (65 degrees C), which is lower than the maximum plasma temperature measured at the jet orifice (77 degrees C). This temperature is significantly lower than the 196 degrees C and 136 degrees C boiling points of octanol and pentanol respectively. The friction of the coatings was determined using the Pin-on-disc technique. The more organic coatings deposited with octanol and pentanol exhibited friction values of 0.2, compared with approx. 0.8 for the coatings deposited from TMOS/TiO2 mixture (with and without methanol). Wear performance comparison between the two types of coating again indicated a significant organic component in the coatings deposited from the higher boiling point alcohols.
There is considerable interest in processing technologies which can lead to more energy efficient... more There is considerable interest in processing technologies which can lead to more energy efficient sintering of abrasive metal matrix composites. In this study the use of a novel microwave plasma processing technique called Rapid Discharge Sintering (RDS) for sintering nickel-diamond metal matrix composites (MMCs) is evaluated. Nickel-diamond powder composites (80-20 % by weight respectively) were uniaxially pressed into 20 mm discs at compaction pressures of 100, 200 and 300 MPa. The discs were sintered using a microwave plasma formed with hydrogen and hydrogen/nitrogen as the discharge gases. For comparison, discs were also sintered using a tube furnace in a gas flow of hydrogen and nitrogen (3:1). Discs pressed to 300 MPa were treated at both 850 and 1000 o C. The properties of the sintered nickel-diamond composites were characterised using density, flexural stress, hardness, wear resistance, SEM and XRD. The RDS samples sintered at 1000 o C achieved the maximum disc strength of approximately 470 MPa within a 20 minute chamber processing time, compared with 6 hours for furnace sintered samples. RDS samples exhibited increased hardness values and a finer nickel matrix over furnace sintered samples. RDS has shown the ability to process nickel-diamond MMCs without oxidation or graphitisation at higher temperatures. As a result, minimal diamond destruction was observed during abrasive wear testing for RDS samples.
We proposed a novel application of cobalt sulfide (CoS) in the configuration of transparent thin ... more We proposed a novel application of cobalt sulfide (CoS) in the configuration of transparent thin film as anode in p-type dyesensitized solar cell (p-DSC). The anodes here considered have been prepared using a water-based method that is suitable for the large scale production of large-area electrodes. The photoactive cathodes of the p-DSC were mesoporous nickel oxide (NiO) thin films deposited via rapid discharge sintering. The NiO electrodes were sensitized with the benchmark dye erythrosine B (ERY), while the couple I À /I 3 À was the redox mediator. The CoS anodes showed higher electrocatalytic efficiency in comparison with the commonly used platinized Fluorine-doped Tin Oxide (Pt-FTO). This was determined by means of electrochemical impedance spectroscopy of CoS based dummy cells, with CoS showing a lower charge-transfer resistance with respect to Pt-FTO. The overall conversion efficiency of the p-DSC employing ERY-sensitized NiO as photoactive cathode and CoS anode was 0.026 %, a value very close to that obtained with Pt-FTO anodes (0.030 %). The external quantum efficiency spectra of the p-DSCs with CoS anodes were similar to those obtained with Pt-FTO anodes under illumination with AM 1.5 solar simulator.
International Journal of Adhesion and Adhesives, 2014
Abstract This paper evaluates the use of an air based atmospheric pressure plasma jet for the act... more Abstract This paper evaluates the use of an air based atmospheric pressure plasma jet for the activation of carbon fibre epoxy composites. In addition the performance of the jet in the removal of a 5–8 nm thick layer of FreKote 710-NC from the composite surface prior to heat-cured epoxy adhesive bonding was also investigated. The plasma treated composites were examined using thermal infra-red imaging, WCA, profilometry and SEM. A reflectance-IR technique was used to monitor the removal of FreKote as well as changes in the functional chemistry of the plasma treated composite surface. The optimised plasma pre-treatment conditions were identified which help to maximise composite-to-composite adhesive bond strength based on LST. This involved a treatment at a 14 mm nozzle to composite distance and scan rate of 30 mm/s. The air plasma process was bench marked against a manual methanol solvent wipe process, for the removal of FreKote. Both the plasma and the solvent wipe treatments yielded similar composite to composite bond strengths, demonstrating that both were successful in the removal of the FreKote. A comparison was also made between the plasma treated composite and a composite which had been grit blasted using 50 μm aluminium oxide grit. A 7% enhancement in lap-shear strength was achieved with the plasma treatment compared to grit blasting.
Plasma diagnostics of atmospheric plasmas is a key tool in helping to understand processing perfo... more Plasma diagnostics of atmospheric plasmas is a key tool in helping to understand processing performance issues. This paper presents an electrical, optical and thermographic imaging study of the PlasmaStream atmospheric plasma jet system. The system was found to exhibit three operating modes; one constricted/localized plasma and two extended volume plasmas. At low power and helium flows the plasma is localized at the electrodes and has the electrical properties of a corona/filamentary discharge with electrical chaotic temporal structure. With increasing discharge power and helium flow the plasma expands into the volume of the tube, becoming regular and homogeneous in appearance. Emission spectra show evidence of atomic oxygen, nitric oxide and the hydroxyl radical production. Plasma activated gas temperature deduced from the rotational temperature of nitrogen molecules was found to be of order of 400 K: whereas thermographic imaging of the quartz tube yielded surface temperatures between 319 and 347 K.
There is considerable interest in processing technologies which can lead to more energy efficient... more There is considerable interest in processing technologies which can lead to more energy efficient sintering of metal powders. The use of microwave sintering in particular leads to reduced energy usage during sintering as the volumetric heating process is considerably more efficient compared with resistance heating. In this study the use of a novel plasma microwave processing technology for the sintering of nickel powder discs is evaluated. The sintering study was carried out on 20 mm diameter by 2 mm thick pressed discs of nickel powder, with mean particle size of 1 µm. The discs were fired in a 5 cm diameter microwave (2.45 GHz) plasma ball under a hydrogen atmosphere at a pressure of 2 kPa. The same discs were also sintered using both non plasma microwave and tube furnace firing. The microwave plasma sintering is very rapid with full disc strength of approx. 1000 N based on 3-point bend tests being achieved within 10 minutes. In contrast the sintering time in the tube furnace involved treatments of up to 6 hours. The non plasma microwave system involved intermediate treatment periods of 1 to 2 hours. Another advantage of the microwave plasma treatment is that the degree of sintering between the individual nickel powder particles can be precisely controlled by the duration of the treatment time in the plasma. There was a broadly linear increase in fired pellet breaking strength with plasma treatment duration. In addition to breaking load, the mechanical properties of the sintered nickel discs were compared based on Rockwell hardness tests and density measurements. The morphology of the sintered discs was compared using microscopy and SEM. This study demonstrated that the plasma microwave sintered discs produced similar or superior performance (depending on processing conditions) to discs fired using the non-plasma microwave and furnace firing conditions. Accurate control of the sample conditions and structure can easily be controlled with the plasma system compared with the conventional systems. The apparent volumetric heating in the microwave systems give a more uniform heating at lower temperatures and allows for greater control and homogeneity.
Comparison of two low temperature deposition methods for HA Combining silica shot with a flow of ... more Comparison of two low temperature deposition methods for HA Combining silica shot with a flow of HA powder worked effectively Replacing the silica with alumina abrasive produced a more adherent coating Chemistry and crystallinity of the deposited HA coatings were comparable to the feedstock powder Results show favourable coating properties for CoBlast in comparison to the Shot
The thermal stability of siloxane films deposited on Vycor glass and silicon substrates using atm... more The thermal stability of siloxane films deposited on Vycor glass and silicon substrates using atmospheric pressure plasma was studied. Siloxane films were deposited from a liquid tetraethyl orthosilicate (TEOS) precursor which was nebulised into a helium/oxygen plasma. The thickness of the siloxane films was in the range 1-12 nm as measured using both ellipsometry and X-ray reflectometry. Film composition and chemical properties were examined using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analysis. The thermal stability of the films was evaluated under argon atmosphere at temperatures up to 700°C. The films were found to exhibit good adhesion without cracks or delamination after the thermal treatment. A decrease in coating thickness and a reduction in surface roughness was however observed indicating, a change in coating chemistry. This was confirmed by the reduction in carbon concentration observed by XPS. Particulates generated by excess gas phase reaction of TEOS were observed on the surface of the coating. A larger reduction in particulates size was observed after thermal treatment compared with the reduction in thickness of the bulk coating. This indicates that the particulates may have a different composition to that of the coating.
Titanium dioxide (TiO 2) thin films, doped with chromium (Cr) and codoped with chromium-carbon (C... more Titanium dioxide (TiO 2) thin films, doped with chromium (Cr) and codoped with chromium-carbon (Cr, C) and chromium-nitrogen (Cr, N) of various concentrations, were deposited using magnetron sputtering. Postdeposition thermal treatments were carried out at 450 • C for 5 h to change the as-deposited amorphous coatings to their crystalline form. The crystalline phase was found to be dependent on the amount and type of dopant present. Ultraviolet-visible (UV-Vis) absorption data and band gap energies calculated by spectroscopic ellipsometry showed that, on increasing Cr concentration, a shift of the absorption edge towards visible light and a reduction of the band gap occurred. This was further improved by codoping of Cr with either N or C, with the latter case exhibiting more photo-activity towards visible light. In addition, hybrid density functional theory (DFT) calculations were performed for Cr-, N-. and C-monodoping, together with Cr-C and Cr-N codoping, in both rutile and anatase phases of crystalline titania. Using this method, absorption coefficients and band gaps were determined to explore photo-activity. Very good, semiquantitative agreement was found between the DFT and experimental approaches for these quantities, underlining the key role of state-of-the-art quantum calculations in interpreting and guiding experimental studies of doping in metal oxides.
This paper presents the mechanical performance of the annealed NiTi Shape Memory Alloy (SMA) coat... more This paper presents the mechanical performance of the annealed NiTi Shape Memory Alloy (SMA) coating deposited onto 316L stainless steel substrate. The as-deposited SMA coating, Ni55.9 Ti44.1, showed an amorphous behaviour. The crystalline NiTi (SMA) coating was produced by annealing the as-deposited NiTi with a thickness about 2.0 µm, at above its crystallisation temperature in a vacuum ambient. The annealed NiTi coatings were characterised to determine the effect of the annealing parameters on their mechanical behaviour. The NiTi phases and structures were determined by x-ray diffraction (XRD) and scanning electron microscopy (SEM) whereas the mechanical properties were measured using the Rockwell C adhesion test. Three main phases; NiTi B2 parent phase, Ni3Ti and TiO2 were found in the annealed samples and the intensities of each phase were dependent on the annealing temperature and annealing time. Each phase significantly affected the mechanical behaviour of the coatings. Higher...
In this study the use of microwave plasma sintering of nickel oxide (NiO x) particles for use as ... more In this study the use of microwave plasma sintering of nickel oxide (NiO x) particles for use as ptype photoelectrode coatings in dye-sensitized solar cells (DSSCs) is investigated. NiO x was chosen as the photocathode for this application due to its stability, wide band gap and p-type 2 nature. For high light conversion efficiency DSSCs require a mesoporous structure exhibiting a high surface area. This can be achieved by sintering particles of NiO x onto a conductive substrate. In this study the use of both 2.45 GHz microwave plasma and conventional furnace sintering were compared for the sintering of the NiO x particles. Coatings 1 to 2.5 µm thick were obtained from the sintered particles (mean particle size of 50 nm) on 3 mm thick fluorine-doped tin oxide (FTO) coated glass substrates. Both the furnace and microwave plasma sintering treatments were carried out at ~450 ºC over a 5 minute period. Dye sensitization was carried out using Erythrosin B and the UV-vis absorption spectra of the NiO x coatings were compared. A 44% increase in the level of dye adsorption was obtained for the microwave plasma sintered samples as compared to that obtained through furnace treatments. While the photovoltaic performance of the DSSC fabricated using the microwave plasma treated NiO x coatings exhibited a tenfold increase in the conversion efficiency in comparison to the furnace treated samples. This enhanced performance was associated with the difference in the mesoporous structure of the sintered NiO x coatings.
Crystalline titanium dioxide (TiO 2) coatings have been widely used in photoelectrochemical solar... more Crystalline titanium dioxide (TiO 2) coatings have been widely used in photoelectrochemical solar cell applications. In this study, TiO 2 and carbon-doped TiO 2 coatings were deposited onto unheated titanium and silicon wafer substrates using a DC closed-field magnetron 2 sputtering system. The resultant coatings had an amorphous structure and a post-deposition heat treatment is required to convert this amorphous structure into the photoactive crystalline phase(s) of TiO 2. This study investigates the use of a microwave plasma heat treatment as a means of achieving this crystalline conversion. The treatment involved placing the sputtered coatings into a 2.45 GHz microwave-induced nitrogen plasma where they were heated to approximately 550°C. It was observed that for treatment times as short as 1 minute, the 0.25-µm thick coatings were converted into the anatase crystalline phase of TiO 2. The coatings were further transformed into the rutile crystalline phase after treatments at higher temperatures. The doping of TiO 2 with carbon was found to result in a reduction in this phase transformation temperature, with higher level of doping (up to 5.8% in this study) leading to lower anatase-to-rutile transition temperature. The photoactivity performance of both doped and un-doped coatings heat-treated using both furnace and microwave plasma was compared. The carbon-doped TiO 2 exhibited a 29% increase in photocurrent density compared to that observed for the un-doped coating. Comparing carbon-doped coatings heat-treated using the furnace and microwave plasma, it was observed that the latter yielded a 19% increase in photocurrent density. This enhanced performance may be correlated to the differences in the coatings' surface morphology and band gap energy, both of which influence the coatings' photoabsorption efficiency.
ABSTRACT Siloxane coatings were deposited using two different atmospheric plasma systems namely a... more ABSTRACT Siloxane coatings were deposited using two different atmospheric plasma systems namely a reel-to-reel atmospheric plasma liquid deposition system called Labline™ and an atmospheric plasma jet system called PlasmaStream™. Both systems combine an atmospheric plasma, with the use of liquid precursors. The influence of the plasma source and processing conditions on the deposited coating properties were studied. The coatings were deposited onto Vycor glass and silicon wafer substrates from a liquid tetraethyl orthosilicate (TEOS) precursor which was nebulized into both He and He/O2 plasmas. Higher film growth rates were obtained using the plasma jet system; however, the reel-to-reel system facilitated the larger area coating of webs. The thickness (spectroscopic ellipsometry measurements) and water contact angle profile of the films deposited using the jet system on silicon wafer substrates in static mode were investigated. Amongst the other coatings characterization techniques used to evaluate the deposited coatings were optical profilometry, XPS, SEM, and AFM. The formation of particulates due to excess gas-phase reactions during the atmospheric plasma deposition of coatings has been widely reported. In this study, larger number of particulates were observed under the conditions of higher plasma power, with the addition of O2 into the He plasma and also at low TEOS flow rates. The introduction of N2 into the He/O2 plasma, during the deposition of siloxane coatings led to a significant reduction in the number of particulates generated for both plasma sources. Nitrogen gas flow rate was varied systematically and a correlation was obtained on the influence of the flow rate of this gas on surface roughness and particulate formation.
The photocatalytic splitting of water into hydrogen and oxygen using a photoelectrochemical (PEC)... more The photocatalytic splitting of water into hydrogen and oxygen using a photoelectrochemical (PEC) cell containing titanium dioxide (TiO2) photoanode is a potentially renewable source of chemical fuels. However, the size of the band gap (-3.2 eV) of the TiO2 photocatalyst leads to its relatively low photoactivity toward visible light in a PEC cell. The development of materials with smaller band gaps of approximately 2.4 eV is therefore necessary to operate PEC cells efficiently. This study investigates the effect of dopant (C or N) and co-dopant (C+N) on the physical, structural and photoactivity of TiO2 nano thick coating. TiO2 nano-thick coatings were deposited using a closed field DC reactive magnetron sputtering technique, from titanium target in argon plasma with trace addition of oxygen. In order to study the influence of doping such as C, N and C+N inclusions in the TiO2 coatings, trace levels of CO2 or N2 or CO2+N2 gas were introduced into the deposition chamber respectively. The properties of the deposited nano-coatings were determined using Spectroscopic Ellipsometry, SEM, AFM, Optical profilometry, XPS, Raman, X-ray diffraction UV-Vis spectroscopy and tri-electrode potentiostat measurements. Coating growth rate, structure, surface morphology and roughness were found to be significantly influenced by the types and amount of doping. Substitutional type of doping in all doped sample were confirmed by XPS. UV-vis measurement confirmed that doping (especially for C doped sample) facilitate photoactivity of sputtered deposited titania coating toward visible light by reducing bandgap. The photocurrent density (indirect indication of water splitting performance) of the C-doped photoanode was approximately 26% higher in comparison with un-doped photoanode. However, coating doped with nitrogen (N or N+C) does not exhibit good performance in the photoelectrochemical cell due to their higher charge recombination properties.
The objective of this study is to investigate the influence of alcohol addition on the incorporat... more The objective of this study is to investigate the influence of alcohol addition on the incorporation of metal oxide nanoparticles into nm thick siloxane coatings. Titanium oxide (TiO2) nanoparticles with diameters of 30-80 nm were incorporated into an atmospheric plasma deposited tetramethylorthosilicate (TMOS) siloxane coating. The TMOS/TiO2 coating was deposited using the atmospheric plasma jet system known as PlasmaStream. In this system the liquid precursor/nanoparticle mixture is nebulised into the plasma. It was observed that prior to being nebulised the TiO2 particles agglomerated and settled over time in the TMOS/TiO2 mixture. In order to obtain a more stable nanoparticle/TMOS suspension the addition of the alcohols methanol, octanol and pentanol to this mixture was investigated. The addition of each of these alcohols was found to stabilise the nanoparticle suspension. The effect of the alcohol was therefore assessed with respect to the properties of the deposited coatings. It was observed that coatings deposited from TMOS/TiO2, with and without the addition of methanol were broadly similar. In contrast the coatings deposited with octanol and pentanol addition to the TMOS/TiO2 mixture were significantly thicker, for a given set of deposition parameters and were also more homogeneous. This would indicate that the alcohol precursor was incorporated into the plasma polymerised siloxane. The incorporation of the organic functionality from the alcohols was confirmed from FTIR spectra of the coatings. The difference in behaviour with alcohol type is likely to be due to the lower boiling point of methanol (65 degrees C), which is lower than the maximum plasma temperature measured at the jet orifice (77 degrees C). This temperature is significantly lower than the 196 degrees C and 136 degrees C boiling points of octanol and pentanol respectively. The friction of the coatings was determined using the Pin-on-disc technique. The more organic coatings deposited with octanol and pentanol exhibited friction values of 0.2, compared with approx. 0.8 for the coatings deposited from TMOS/TiO2 mixture (with and without methanol). Wear performance comparison between the two types of coating again indicated a significant organic component in the coatings deposited from the higher boiling point alcohols.
There is considerable interest in processing technologies which can lead to more energy efficient... more There is considerable interest in processing technologies which can lead to more energy efficient sintering of abrasive metal matrix composites. In this study the use of a novel microwave plasma processing technique called Rapid Discharge Sintering (RDS) for sintering nickel-diamond metal matrix composites (MMCs) is evaluated. Nickel-diamond powder composites (80-20 % by weight respectively) were uniaxially pressed into 20 mm discs at compaction pressures of 100, 200 and 300 MPa. The discs were sintered using a microwave plasma formed with hydrogen and hydrogen/nitrogen as the discharge gases. For comparison, discs were also sintered using a tube furnace in a gas flow of hydrogen and nitrogen (3:1). Discs pressed to 300 MPa were treated at both 850 and 1000 o C. The properties of the sintered nickel-diamond composites were characterised using density, flexural stress, hardness, wear resistance, SEM and XRD. The RDS samples sintered at 1000 o C achieved the maximum disc strength of approximately 470 MPa within a 20 minute chamber processing time, compared with 6 hours for furnace sintered samples. RDS samples exhibited increased hardness values and a finer nickel matrix over furnace sintered samples. RDS has shown the ability to process nickel-diamond MMCs without oxidation or graphitisation at higher temperatures. As a result, minimal diamond destruction was observed during abrasive wear testing for RDS samples.
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Papers by Denis Dowting