No description available.

No description available.

No description available.

No description available.

This thesis concerns the processing of thick lead zirconate titanate (PZT) films integrated with rigid substrates. The aim was to better understand the evolution of the microstructure, stress, and electrical properties of the films under constrained sintering conditions. This is an important process to understand because of the degrading effects that constrained sintering has on PZT films, which are employed vastly in commercial applications. It is hypothesised that the better understandings can lead to the development of PZT films that exhibit superior dielectric and piezoelectric properties than those in current production. The shrinkage of PZT films was examined in order to better understand the ways in which the rigidity of the substrate affects densification during sintering. This was done by processing isolated regions of PZT film on silicon substrates. These were sintered using a halogen bulb which exhibited a spot at a temperature of 725°C and with a ramp rate of less than 10 seconds. In this way the sintered regions could be ‘frozen’ mid sintering. The shrinkage of the films was determined at various sintering times. It was found that film shrinkage had finished within 2 minutes of sintering. The evolution of the constrained films during sintering was then examined as a function of the microstructure, stress and electrical property development. It was found that the grain sizes and electrical properties increased within 2 minutes of sintering. However, at longer sintering times there was a degradation of the films. Furthermore, tensile stresses developed during sintering which had degrading effects. This work was expanded upon by motioning the PZT films in a single line scan through the sintering spot to sinter larger areas of the film in one motion. This resulted in a high control over the sintering times, which was vital as the highest electrical properties were found at short sintering times. Next it was examined if the electrical properties could be further increased by applying a compressive stress. It was found that the dielectric properties increased as a result of increased domain wall vibrations. However, there was a decrease in domain reorientation during poling as a result of the effect of the compressive stress, thus the piezoelectric properties reduced. The evolution of PZT films under constrained sintering was better understood as a result of these studies, and led to the development of a sintering method in which the dielectric and piezoelectric properties were increased.

Over the last few decades, porous metals have received a large amount of interest in industry due to the rapid advancement in manufacturing techniques, design and possible applications. Their unique properties and multi-functionality allow them to be utilized in many different applications throughout different industrial sectors. This thesis manufactured porous steel using the Lost Carbonate Sintering (LCS) method and studied their mechanical and sound absorption properties. The dissolution, decomposition and re-sintering routes were studied and compared. The mechanical properties of the specimens manufactured by LCS via the dissolution and decomposition routes were measured by compression and three-point bending tests. The compression strength, elastic modulus and flexural strength of the porous steel specimens manufactured with both routes increased with increasing relative density, pore size and compaction pressure. Increasing the sintering temperature and time in the decomposition route served to increase the compression strength, elastic modulus and flexural strength. The advantages and disadvantages of the dissolution and decomposition routes were analyzed. The porous steel specimens manufactured by the decomposition route had better mechanical properties than those manufactured by the dissolution route. The acoustic absorption performance of porous steel manufactured by the LCS process via the dissolution route was assessed using the standing wave impedance tube method. The single layer specimens showed excellent sound absorption properties at high frequencies. Pore size of the porous steel had no significant effect on the sound absorption coefficient. Sound absorption at low frequencies can be improved by increasing the thickness of specimens, or by introducing an air gap behind the absorber. The sound absorption properties of the porous specimens of multi-layer assemblies with different porosities, pore sizes, thicknesses and air-gap depths were assessed. The porosity of the first layer of multi-layer-assembled specimens had a critical effect on the sound absorption coefficient and frequency of peak. Increasing this porosity increased the sound absorption coefficient at all frequencies after the peak. The effects of the porosities of the subsequent layers were smaller. When the first layer had a high porosity, increasing the porosity of the second layer increased the frequency of peak. When the first layer had a low porosity, increasing the porosity of the second layer enhanced the sound absorption coefficient of peak. The effects of pore size were not significant. Increasing the thickness of specimens and the depth of air-gap behind the specimens decreased the frequency and coefficient of the peak in the sound absorption curve.

No description available.

No description available.

The research presented in this thesis was part of a larger collaborated project (LastForm Programme) to research engineering solutions for the rapid manufacture of large scale (0.5m – 5.0m in length) low, medium and high temperature tooling (from room temperature to 1000C) for use in the automotive and aerospace industry. All research was conducted using small scale investigations but with a final discussion including implications of the work in future large scale planning. The aim of the work presented in this thesis was to develop current understanding about the sintering and melting behaviour of metal powders by Selective Laser Sintering (SLS). The powder used in the research was an argon atomised austenitic stainless steel of type 314s HC. The powder was supplied in four batches, each differentiated by particle size distribution; -300+150m, -150+75m, -75+38m and -38m. The characteristics of each powder, in particular flow properties, differed considerably allowing powder handling and powder flow during melting to also be explored in this work. Three different environmental conditions were also investigated to asses the role of atmospheric and residual (powder) oxygen: (1) air atmosphere (control), (2) argon atmosphere and (3) argon atmosphere with argon percolation through the powder layer. In this, the design of an environmental control chamber and its integration into a research SLS machine was central to the work. Experimental studies of the selective laser sintering/melting process on room temperature stainless steel 314s powder beds has been successfully carried out. The methodology was progressive; from tracks to layers to multiple layers. Single tracks were produced by melting the powder by varying laser power and scan speed. Results from experiments have been used to construct a series of process maps. Each map successfully charts the heating and melting behaviour of the irradiated powder. Behaviours can now be predicted with reasonable accuracy over a dense power and speed range, including laser powers up to 200W and scan speeds up to 50mm/s. The experiments also allowed melt pool geometries to be investigated. Three types of melt cross-section were categorised; flattened, rounded and bell shape. Flat tracks generally occurred at low speed (0.5mm/s) but also occurred up to 4mm/s at lower power (77W). Rounded tracks occurred between 1mm/s and 4mm/s and had a much larger area than expected. In the rounded track regime tracks sink well into the powder bed. Powder to either side of a track collapses into it, leaving a trench surrounding the track. The admission of extra powder is thought to be one cause of increased mass. However, a remaining question that still needs answering is what causes the change from a flattened to a rounded track. Values of laser absorptivity were also estimated from track mass per unit length data and from melting boundaries displayed within the process maps. The results showed that absorptivity changed considerably depending on the powder, process conditions and atmospheric conditions. Within an argon atmosphere an „effective‟ absorptivity from mass data was estimated to range from 0.1 to 0.65, the lower value at low speed scanning (0.5mm/s) and the higher value from high speed scanning (>4mm/s). These values were much higher than expected for a CO2 laser. Melt pool balling was found to be a big problem, limiting the process speed at which continuous tracks could be successfully constructed (<12mm/s). Comparisons between a mathematical model developed in this work and experimental results suggested that balling within an air environment occurred when the ratio of melt pool length to width reached a critical value close to . Balling within an argon atmosphere was more difficult to model due to higher viscous melts caused by the take up of surrounding powder. Melted single layers were produced by varying laser power, scan speed, scan length and scan spacing or melt track overlap. Scan length proved to be a significant factor affecting layer warping and surface cracking. Provided the scan length remained below 15mm, layer warping could be largely avoided. Multiple layer blocks were produced by melting layers, one on top of the other. They were constructed over a range of conditions by varying laser power, scan speed, scan spacing and layer thickness. Layer thickness was a crucial parameter in controlling the interfacial bond between layers, but the spreading mechanism proved to be the overriding factor affecting layer thickness and therefore the quality and density of the blocks.

Sintering of silicon carbide (SiC) requires high temperature and pressure due to the covalent nature of its bonding. Therefore, sintering additives are used to lower the sintering temperature and to control the microstructure. In this work, role of aluminium nitride (AlN) and carbon in pressure assisted densification is studied as the literature was not clear on whether AlN always induces liquid phase sintering (LPS). It is shown that mixing suffices to produce green bodies in which AlN is present as individual particles. When heated above 1700⁰C the AlN redistributes to grain boundaries and triple junctions through vapour transport and grain boundary diffusion, which causes the onset of densification. Addition of AlN and carbon together leads to microstructure more consistent with solid state sintering (SSS) than with LPS which was induced with the addition of yttria and AlN. Nano-indentation was used to measure hardness as a function of strain rate and temperature. It was found that hardness increases 0.8 GPa per decade strain rate and decreases with increasing temperature. Since in the absence of cracking hardness is a function of stiffness and yield stress, nano-indentation was used to calculate the Peierls stress (12 ± 1 GPa), activation energy (1.1 ± 0.3 eV) and the activation volume (1.44 x 10⁻²⁹ m³) of dislocation glide in SiC. Grain size was found to have a minimal effect on plasticity of the material when indents are small. Consistent with widely reported trends, the hardness was found to decrease when higher loads are used. It is argued that this decrease in hardness is due to an increase in crack length relative to the indent size. An empirical model, based on dimensional analysis, describes the observed decrease in hardness rather well. Observations of a new damage mechanism after unloading of large load indents are presented and a mechanism is proposed.