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My Ph.D. focused on Atmospheric Pressure Chemical Vapour Deposition (APCVD) of main group metal phosphides and oxides. I have experience of using a variety of other types of CVD methodologies including low pressure CVD and aerosol assisted CVD. I am also well acquainted with other coating methodologies such as screen printing, sol gel techniques (spin, dip etc.) and physical vapour deposition (PVD). One of my main interests is in developing hybrid CVD methodologies such as combined aerosol and atmospheric pressure systems. This particular system is interesting as it provides the benefits of aerosol assisted CVD such as a wide and exotic precursor choice with the benefits of APCVD such as the production of a hard and durable films well as new methodologies I am interested in energy efficient coatings for use in limiting the energy demand of buildings. Please click on this link to see a video of me talking about the hybrid CVD methodology applied to energy efficient window coatings. The information on this page is a very quick summary of some of my work for further information please contact me and/or look at my publications. For a more detailed description of each figure click on the figure or the link in the figure caption. |
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The use of air-conditioning equipment in order to maintain comfortable conditions inside buildings during the summer months is ever increasing and consumes vast amounts of electricity. As direct consequence carbon dioxide emissions increase, as well as other atmospheric pollutants. The effect is a self propagating cycle: the global temperature increases necessitating the further use of air conditioning in the summer months leading to a further growth in carbon dioxide emissions. One possible approach to break this is cycle is through the use of "smart windows". Solar control coatings are a technology applicable to all types of glazing, commercial or residential, to play an active role in improving the energy efficiency of the building. Current solar control coatings consist of an all-out approach that is applicable to constant climates (Figure 1). If an environment is consistently hot, tinted glass or thin metallic coatings can be used to reflect solar heat, preventing it from entering the building and thus limiting the need for internal cooling. Conversely, in a consistently cold environment heat may be retained in a building by the use of a wavelength selective coating which is transparent in the visible part of the spectrum but highly reflective in the infrared. Thus sunlight can enter the building but internally generated heat is prevented from escaping; reducing heating requirements.
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Figure 1. Static approaches to energy efficient glazing - explanation of this figure SMART glazing works by having a dynamic coating that varies its optical properties i.e. how much solar heat is let in and out of the building, depending on the temperature (Figure 2). This is known as thermochromism. In cold weather our coating will allow solar heat energy into the building but in hot weather this energy is reflected away. In this manner the heating and air conditioning costs of the building are reduced and a comfortable working environment is maintained. |
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Figure 2. The idea behind dynamic, thermochromic, SMART glazing - explanation of this figure
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Vanadium dioxide is one example of a material which is thermochromic. It is ideal for application in SMART glazing as; it is easily made using CVD methods, the change in optical properties is in the infra red and not the visible region of the electromagnetic spectrum meaning the view does not change, the transition temperature is tunable by varying the growth conditions and/or dopant.
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Figure 3. Example scanning electron
microscope images of vanadium dioxide films - explanation
of this figure
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The optical properties of the films are similar to those shown in figure 4. There is a large change in the infra red in both transmission (red and orange lines) and reflectance (blue lines). Indicating ideal behaviour for our SMART coating. Figure 4 also shows the thermochromic behaviour of our films; the transition temperature can be decreased by incorporating tungsten into the films which causes stress in the crystal structure and lowers the energy required for the thermochromic transition.
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Figure 4. The optical properties of
some vanadium dioxide films - explanation
of this figure
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One of the problems of regular APCVD is that we are limited in which precursors we can use. We are limited because there must be sufficient volatility in order for mass transport to the reactor to occur. The use of aerosol assisted CVD overcomes this hurdle as the precursors we use are not limited by volatility, rather solubility in whichever solvent we choose to use. In fact solubility is not the whole story; as long as we can make a dispersion and generate an aerosol we can transport our precursor to the reactor and make a film. Traditional aerosol assisted CVD grows films with poor adherence, using the hybrid methodology this is no longer a problem. Deposited films have mechanical properties similar to films grown by APCVD.
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Figure 5. Schematic and photograph
of hybrid CVD rig - explanation of
this figure
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The first system I looked at was vanadium dioxide grown by APCVD and gold nanoparticles from aerosol assisted CVD. Figure 6 shows some example properties of these films. The transmission and reflectance spectra still show a big change on transition, although less in the near infra red than had been seen previously. The hysteresis behaviour was also similar. One striking difference is the colour of the films (Figure 7). This is due to the presence of a surface plasmon resonance band which arises from the presence of gold nanoparticles in the film. Surface plasmon resonance is essentially a strong absorbence which is a quantum confinement effect of the very small size of the gold particles.
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Figure 6. Optical Properties of gold
doped vanadium dioxide films prepared using the hybrid method - explanation
of this figure
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By changing the amount of gold in the film, we shift the position of the surface plasmon resonance band. The position of this band is dependent on the size and distribution of gold nanoparticles, by using more gold we create bigger nanoparticles which are also closer together and hence we change the position of the surface plasmon resonance band; hence changing the colour of our film. Figure 7 shows a variety of different colours that have been produced.
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Figure 7. Changing the amount of gold
in the film dramatically effects the colour - explanation
of this figure
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