No description available.

No description available.

No description available.

No description available.

No description available.

A study has been made of the factors which affect anatase retention during papermaking. By using electrophoretic, streaming potential and adsorption techniques a study of the effect of pH, rosin size, aluminium sulphate and various physical parameters on the electrokinetics of the system has been made. Using the Derjaguin–Landau–Verwey and Overbeek theory of colloid interaction it is found that maximum retention of anatase occurs when flocculation of the system is predicted by the above theory. This maximum retention occurs below a pH of 4.0 when the paper 'is made from cellulose fibres and anatase only. On the addition of aluminium sulphate however a high retention is obtained up to a pH of 7 to 9, the limit being dependant on the amount of aluminium sulphate. This increase over the pH range 4 to 7 is attributed to the adsorption of aluminium species on the anatase over this range which causes a large shift in the isoelectric point. A parallel study of the retention of rosin size has been made, and from the results obtained it is proposed that not only are the retention mechanisms of size similar to that of the anatase, but in fact it is the adsorption of one which causes the retention of the other.

The work presented in this thesis is focused on the development of a novel low processing temperature PZT (lead zirconate titanate) ink for direct writing of functional microsystems. The work examines both the synthesis of PZT powder for use in the ink as well as the formulation of the ink. Two different routes were investigated for the powder synthesis: electro hydrodynamic atomisation (EHDA) and molten salt synthesis (MSS). EHDA is a technique that leads to the formation of small and spherical droplets that, after drying, result in solid particles. Several process parameters were investigated in order to determine their influence on particle size. PZT sols with concentrations up to 0.6 M were electrosprayed under different conditions: the flow rate was varied from 0.2 to 0.6 ml h1 and the distance between the needle and the bottom electrode was increased from 20 to 40 mm. The solvent was dried by the use of a focused lamp with temperature between 200 and 680oC. It was determined that, in order to reduce the size of the PZT particles, low concentration and low flow rate were needed. The needle-electrode distance was found not to have a strong effect on size. However an increase in the focus temperature to a threshold of 520oC led to a reduction of the particle diameter. A further increase of the degree of heating led to the formation of big and irregular particles. Small PZT particles with diameter of 260 nm were obtained under the following conditions: flow rate of 0.2 ml h1, sol concentration of 0.2 M, needle-electrode distance of 30 mm and drying temperature of 520oC. The synthesised powder was spherical in shape, that made it suitable for IJP, but the yield was very low. In order to overcome the problem associated with the low yield of EHDA, MSS was investigated. Also in this case process parameters were studied with the purpose to reduce particle size. The optimum synthesis conditions were found to be 1 hour at 850oC, with a ramp rate of 3.3oC min1. Under these circumstances, PZT particles with a mean diameter of 340 nm were synthesised. From the investigation it came to light that long times and higher temperatures led to an increase in particle size due to coarsening process. Short times and low temperatures led instead to an incomplete reaction between the starting oxides. A reaction mechanism for the formation of PZT is also proposed: fully dissolved Pb reacts with the insoluble TiO2 to form PbTiO3. Then ZrO2 reacts with PT and the remaining Pb to form PZT. Composite inks were formulated from the powders synthesised by both the routes. In the case of EHDA however the formulation was hampered due to a low amount of powder available. This resulted in a quick powder sedimentation that led to nozzle clogging during printing. Inks composed of MSS powder were formulated with different solid loadings to identify under which conditions the nozzles were clogging. Two different patterns were printed on the substrate: a 20x20 drops for the identification of the ink behaviour on the wafer, and a line array pattern to determine in which conditions printing quality was enhanced.

Screen-printing is one of the oldest printing processes, yet its market share remains very limited due to its slower printing speeds compared to the other available processes. This is mainly because of the reciprocating motion of the squeegee upon the printing screen. In order for screen-printing to become more competitive, the concept of a high-speed continuous belt screen-printing press was developed. However, this will produce an increase in squeegee wear and friction of the squeegee upon the screen. For this reason, this work investigated the use of a roller squeegee that could rotate across the screen. It has been proven that screen-printing with a roller squeegee can be successfully achieved. Additionally, in terms of density and tone gain, these images were comparable to those produced with traditional blade squeegees. A numerical model has been developed to simulate the characteristics that will be encountered within the ink film when printing with a roller squeegee. Numerical simulations were run where the settings corresponded to the parameters utilised in experimental trials. Here, it was discovered that an increase in squeegee diameter will increase the ink film on the squeegee and will also increase the contact width of the screen upon the substrate. This will have the effect of increasing the pumping capacity of the squeegee, which will therefore increase the ink deposit. This was confirmed in the experimental trials. It was also shown that the locking of the squeegee increased the shear mechanism within the ink film, resulting in a reduction in the ink viscosity within the nip contact region. This had the effect of reducing the ink film thickness on the squeegee, which reduces the pumping capacity of the squeegee, thus producing a reduced ink deposit. Additionally, this work is the first method that has been able to estimate the height of the ink deposit for a range of halftone open areas where the results correspond almost identically to the actual printed heights of the prints obtained in experimental studies. This work has improved the fundamental understanding of the mechanics and the process physics within the ink transfer mechanism in the screen-printing process. Use of experimental and numerical models has resulted in new theories being developed that will further the knowledge of the process. This has led to the design and manufacture of a high-speed rotary screen-printing press that will enable high-speed, continuous screen-printing.

No description available.

There is an expanding interest in rapid prototyping capabilities of direct write technologies. Due to this, a number of candidate deposition systems have been used for fabricating microscale structures for electronics and biomedical applications such as fabricating conducting tracks for next generation electronic devices, bone replacement parts, flat screen displays and polymeric light emitting diodes, etc. In response to this growing interest in direct write technologies, an experimental investigation was carried out in order to find out whether electrohydrodynamic atomisation can be used as a competing direct write technology. In this research, initially, gold and silver alcosols were subjected to electrohydrodynamic atomisation. Different modes of atomisation and their parameters such as flow rate and voltage were recorded and a mode selection map was constructed for further studies. The main objective was to investigate the feasibility of electrohydrodynamic atomisation to fabricate films of thickness ranging between 100 nm - 1 urn to be used as substrates for e.g. in surface enhanced Raman spectroscopy, cell biology and bio-engineering. The gold alcosol was subjected to atomisation and the droplets resulting from jet break-up were collected on a substrate in order to produce dense gold films. Different flow rates and voltages were used and the optimum conditions such as flow rate, applied voltage and distance between the substrate and the capillary exit, for film fabrication were obtained. The deposition time was varied from 60 - 900 s and the variation of film thickness and surface morphology was observed with increasing deposition time. Dense film thicknesses ranging from -0.5 - -2 um were fabricated by electrohydrodynamic deposition technique. In order to pattern microscale structures on a substrate, gold and silver alcosols were subjected electrohydrodynamic atomisation printing. The main objective was to investigate whether using a low concentrated metal particle solution in conjunction with electrohydrodynamic atomisation printing technique can be utilised to produce conducting tracks in the range of 20 - 200 urns in diameter to be used in next generation electronic devices. Tracks containing metal particles were deposited at various flow rates. With increasing flow rate, the track width increased and the finest track width of -110 urn was achieved. Templates were used to fabricate finer tracks in conjunction with electrospraying and in this case gold tracks of -20 um were patterned on silicon wafer. Due to low metal particle concentration conductive tracks were not possible to produce by these methods. Layer-by-layer deposition method was used with electrohydrodynamic printing to fabricate conducting tracks. The electrical resistivity of the printed gold track in this study was measured to be 1.8 x 10" Qm. The gold hydrosol containing spherical shaped nanoparticles and a silver suspension containing micro size particles were subjected to co-axial electrohydrodynamic atomisation to study the encapsulation of these metals with polyethylene glycol (PEG) : polyethylene oxide (PEO) fibres. The main objective was to investigate feasibility of encapsulating metal particles in polymeric fibres. PEG and PEO was used as a model system to encapsulate silver and gold particles and this technique can be further developed to produce composite fibres to be used in optical, electrical and sensing devices. The relationships between the jet speed and particle arrangement in the encapsulation sheaths were also characterised using advance analytical techniques.