Search

Oxides of titanium have found applications in areas such as energy capture, cell attachment to biomedical devices and as catalytic surfaces. This study demonstrates that the use of a microwave plasma can significantly enhance both titanium oxide formation and growth rate, compared with those obtained using conventional furnace treatments. The ability of microwave plasma treatments to control the thickness and morphology of oxides grown on titanium substrates is demonstrated. For a fixed plasma treatment time of 10 min, depending on the input power and treatment pressure, the oxide-layer thickness of between 0.69 and 10.45 μm were obtained. The associated roughness (Ra) values were in the range of 0.11–0.67 μm. This study demonstrated that the morphology of the oxide-layer was particularly influenced by the input power to the plasma, while oxide thickness was depended on the treatment pressure and time. The mechanism of titanium oxidation was also investigated and based on the homogeneous oxidation observed across the titanium metal surface (under the oxide layer), it is also concluded that the mechanism of oxidation is ‘general corrosion’.

Plasma processing treatments are used widely in materials finishing. The plasmas used for these applications can be either ‘hot’ or ‘cold’. Examples of the former include those used in thermal spray and arc discharge machining. Cold plasmas, which are the focus of this review, are used for relatively low temperature surface cleaning and polymer activation. Cold plasmas can also be used for the deposition of coatings including plasma polymerised thin films. These nanometre thick polymer layers exhibit a tailored chemical functionality and can be used, for example, to control surface wetting properties ranging from superhydrophilic to superhydrophobic (SH). These cold plasmas can be operated under both low pressure and atmospheric conditions. Commercial materials finishing applications of ‘cold’ plasma treatments are highlighted.

Plasma polymerised PEO-like films were deposited from tetraethylene glycol dimethyl ether (TEGDME) and diethylene glycol vinyl ether (DEGVE) using an atmospheric plasma jet. Films formed from TEGDME with COC retention >50% showed anti-fouling properties, while DEGVE films with COC >60% did not. TEGDME films deposited at higher monomer flow rates had a lower density and more amorphous phases in the polymer network. Consequently, these films more readily facilitate penetration and binding of water to their surface in comparison to the denser DEGVE films. The difference in fouling properties identified through this comparative study has shown that % COC retention may not alone be an indication of non-fouling behaviour. Other factors such as polymer network structure may also play a crucial role in the prevention of surface fouling.

Interaction between the liquid precursor droplets and non-equilibrium atmospheric pressure plasma (APP) is identified as an important mechanism in two-phase flow. In this study, a two-dimensional integrated fluid-droplet model is developed to explore the behavior of this interaction under similar operating conditions of an experiment performed using PlasmaStream system. The evaporation of droplets is recognized as a primary mechanism in APP; however, the mutual interactions, such as grazing and coalescence between the droplets are dominant in the limit of higher precursor flow rates (>100 μL min−1). The spatio-temporal profiles of discharge plasma are contrasted by considering the effect of various liquid precursors, such as Hexamethyldisiloxane (HMDSO), nHexane, Tetraethyl orthosilicate and water in order to illustrate their significance during droplet-plasma interaction. In particular, we investigated the enhancement of evaporation of droplets by the increment of gas flow rates. Finally, the size distribution of HMDSO droplets measured with the experiment is compared with the results of the fluid-droplet model to provide the authenticity of numerical simulation outcomes.

A 3D coupled fluid–droplet model is developed specifically to characterize the significance of droplet-plasma interaction at atmospheric pressure. The liquid droplet introduces a perturbation in atmospheric pressure plasma (APP) and under many conditions, the behavior of this perturbation is not clear during transport in PlasmaStream system. In this study, we identify the importance of ionization mechanism in two-phase flow. The affect and spatial expansion of vaporization in discharge plasma depend on the flow rate of liquid precursors. Penning ionization is recognized as the leading process along the pulse of evaporating droplets as compared to other ionization processes that explain the relevance of small nitrogen impurities. The influence of different precursors, such as hexamethyldisiloxane, tetraethyl orthosilicate and water is described by contrasting the implication of the evaporation process. Finally, we validate the numerical simulation by comparison with the experimental observations of droplet size distributions using a laser diffraction particle size analysis technique as a part of APP jet deposition system.

In this study surfaces exhibiting a nano-structured morphology have been creating in a single step deposition process using an atmospheric pressure plasma jet. Plasma polymerized coatings were deposited from a HMDSO monomer which was introduced into a He/N2 plasma in the form of aerosolized droplets. By controlling the plasma discharge regime and monomer flow rate, the surface morphology was altered to create surfaces exhibiting an aligned fibrous morphology or a surface consisting of spherical agglomerates. Optical emission spectroscopy showed variation in photon emission energy depending on the He/N2 ratio. Both thermal imaging and numerical simulations indicate that vaporization of the monomer occurs, which facilitates monomer polymerization in the gas phase and subsequent particulate nucleation. XPS analysis identified a higher amount of carbon bound siloxy species at regions where greater particulate formation was observed on the substrate. Cross-sectional SEM and He-ion imaging indicate that the formation of fibrous structures is associated with localized surface charging, which gives rise to selective agglomeration of semi-amorphous particulates which become aligned on the substrate. Modeling of droplet behavior in the plasma indicates that under conditions of higher monomer flow rate, saturation within the plasma occurs which facilitates nucleation of particulates, while experimental and modeling results indicate surface driven growth mechanisms are more likely to occur at lower precursor flow rates.

This work directly compares vapour and liquid aerosol states for the deposition of perfluorocarbon coatings using an atmospheric pressure, non-thermal equilibrium plasma jet system. The objective of the study is to evaluate how the physical state of the precursor (gas or liquid), influences the fragmentation of the monomer molecules in the plasma and the subsequent coating properties. Specifically the effect of gas or liquid aerosol precursor feed on the ability to achieve a soft plasma polymerization (SPP) is assessed with a view to producing a coating that exhibits minimal fragmentation, while being well cross-linked. The precursor (perfluoro-1decene) was introduced into a helium plasma and coatings deposited at rates of up to