New Advances in Forming Functional Ceramics for Micro Devices

Micro electromechanical systems (MEMS) are finding uses in an increasing number of diverse applications. Currently the fabrication techniques used to produce such MEMS devices are primarily based on 2-D processing of thin films. The challenges faced by producing more complex structures (e.g. high aspect ratio, spans, and multi-material structures) require the development of new processing techniques. Potential solutions to these challenges based on low temperature processing of functional ceramics, selective chemical patterning, and micro-moulding are presented to show that it is possible to create complex functional ceramic structures which incorporate non-ceramic conducting and support structures. The capabilities of both techniques are compared and the relative advantages of each explored

Molten hydroxide synthesis as an alternative to molten salt sythesis for producing K0.5Na0.5NbO3 lead free ceramics

Lead-free piezoelectric materials have grown in importance through increased environmental concern and subsequent EU and worldwide legislation, with the aspiration to reduce the use of Pb-based materials in all sectors. Integration of the next generation of lead-free piezoelectric materials with substrates to form functional micro devices has received less attention. Low temperature synthesis methods for K0.5Na0.5NbO3 powder were developed to overcome the issue of poor purity of the final product during high temperature sintering. Molten hydroxide synthesis (MHS), derived from molten salt synthesis (MSS), has been developed to overcome a Na ion preference in the molten salt synthesis reaction that leads to NaNbO3 production instead of K0.5Na0.5NbO3 when stoichiometric amounts of precursors are used. MHS makes use of a KOH molten reaction aid in place of the NaCl/KCl molten salt mix of the MSS. In a two stage reaction K rich intermediate niobates are produced and subsequent reactions with Na species produce KNN

Investigation of the mechanism for current induced network failure for spray deposited silver nanowires

Silver nanowires are one of the prominent candidates for the replacement of the incumbent indium tin oxide in thin and flexible electronics applications. Their main drawback is their inferior electrical robustness. Here, the mechanism of the short duration direct current induced failure in large networks is investigated by current stress tests and by examining the morphology of failures. It is found that the failures are due to the heating of the film and they initiate at the nanowire junctions, indicating that the main failure mechanism is based on the Joule heating of the junctions. This failure mechanism is different than what has been seen in literature for single nanowires and sparse networks. In addition, finite element heating simulations are performed to support the findings. Finally, we suggest ways of improving these films, in order to make them more suitable for device applications

Synthesis and development of lead zirconate titanate inks for direct writing

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.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Stress analysis, dielectric, piezoelectric, and ferroelectric properties of PZT thick films. Fabrication of a 50MHz Tm-pMUT annular array

PZT films up to 35 μm thick were fabricated, using a composite sol gel route combining a PZT powder and a PZT sol. The maximum temperature for the process was 710°C. A demonstration of single layer and multilayer structures was given to show the flexibility of this technology. With Stoney’s Equation, studies of the in-situ film stress development as a function of the film thickness and density was effectuated. It helped to understand that the internal forces increase considerably with the film thickness and density. This study yields to set up experimental conditions in which a crack free surface finish of a 28μm thick film revealed the adaptability of the spin coating technique to fabricate thick films. The wet etching technology revealed the possibility of a great adaptability to pattern and shape innovative devices such as bars 10 μm wide of 21μm PZT thick film. The results open the way to a wide range of new industrial application requiring small features and/or multilayer PZT thick film with embedded electrodes. The single element and annular array devices have been shown to resonate at approximately 60MHz in air and 50 MHz in water. Three types of the composite thick film – 2C+4S, 2C+5S and 2C+6S – were used to fabricate the Tm-pMUT devices. In each case the most effective poling was obtained by maintaining the poling field of 8.4V/μm during cooling from the poling temperature (200ºC) to ‘freeze’ poled domains in place. This ‘freezing’ was required to prevent the tensile stresses within the film from reorienting the domains at high temperatures when the poling field is removed. Increasing values of thickness mode coupling coefficient (kt) were observed with increasing levels of sol infiltration (decreasing density). Such behaviour is thought to be due to non linear effects on the piezoelectric coefficient (e33) at high levels of porosity. For very dense thick film material a kt of 0.47 was observed which is comparable to that observed for the bulk material.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Design and development of micro-electromechanical acoustic emission sensors

Non-Destructive Testing (NDT) is a vital technique in modern engi- neering, enabling the monitoring of the structural health of a com- ponent and therefore enabling the prediction of component failure. Once the structural health of a component is known, timely main- tenance can be carried out to prevent component failure which may have resulted in costly downtime or injury. One NDT technique which has been of increasing interest over recent years is Acoustic Emission (AE) monitoring. AE monitoring technology has been successfully combined with preventative maintenance, saving millions of pounds worldwide.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Innovative method to produce large-area freestanding functional ceramic foils

Using thick and thin films instead of bulk functional materials presents tremendous advantages in the field of flexible electronics and component miniaturization. Here, a low-cost method to grow and release large-area, microscale thickness, freestanding, functional, ceramic foils is reported. It uses evaporation of sodium chloride to silicon wafer substrates as sacrificial layers, upon which functional lead titanate zirconate ceramic films are grown at 710 °C maximum temperature to validate the method. The freestanding, functional foils are then released by dissolution of the sacrificial sodium chloride in water and have the potential to be integrated into low-thermal stability printed circuits and flexible substrates. The optimization of the sodium chloride layer surface quality and bonding strength with the underlying wafer is achieved thanks to pre-annealing treatment

Investigation of ZnO nanrod solar cells with layer-by-layer deposited CdTe quantum dot absorbers

Innovation in solar cell design is required to reduce cost and compete with traditional power generation. Current innovative solar technologies include nanostructured dye-sensitised solar cells and polymer solar cells, which both contain organic materials with limited lifetime. This project aims to combine the advantages of ZnO nanorods and quantum dot (QD) absorbers in an all-inorganic solar cell, using the layer-by-layer (LbL) process to increase light absorption in the cell. Cont/d.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Non-holomorphic Corrections from Threebranes in F Theory

We construct solutions of type IIB supergravity dual to N=2 super Yang-Mills theories. By considering a probe moving in a background with constant coupling and an AdS_{5} component in its geometry, we are able to reproduce the exact low energy effective action for the theory with gauge group SU(2) and N_{f}=4 massless flavors. After turning on a mass for the flavors we find corrections to the AdS_{5} geometry. In addition, the coupling shows a power law dependence on the energy scale of the theory. The origin of the power law behaviour of the coupling is traced back to instanton corrections. Instanton corrections to the four derivative terms in the low energy effective action are correctly obtained from a probe analysis. By considering a Wilson loop in this geometry we are also able to compute the instanton effects on the quark-antiquark potential. Finally we consider a solution corresponding to an asymptotically free field theory. Again, the leading form of the four derivative terms in the low energy effective action are in complete agreement with field theory expectations.Comment: 23 pages, uses harvmac, References added, typos corrected and minor improvements to discussion of N dependence, to appear in Phys. Rev.