Visualizing particle and pore arrangements in dry-pressed spray-dried alumina (Al₂O₃) as a function of processing parameters

Variability in green bulk density in ceramic green bodies has been a continuing issue in the ceramic processing industry. Dry-pressing of ceramic powders is a widely used process due to its low cost, high production rates, and shape forming abilities. Spray-drying is the most widely used granulation technique for large production of dry-pressed ceramics. One mechanism that can cause variation in green bulk density is inadequate cohesion of the spray-dried granules during compaction. Organic binders are needed to create temporary adhesion and strength prior to the firing process. The mechanical properties of the binder and the morphology of the spray-dried granules govern the compaction behavior of the granules and the resultant green bulk density of the compacted body. Determining the roles and processing parameters that may lead to microstructural uniformity in processing dry-pressed green compacts will help narrow the problem of green bulk variation during processing. These parameters may include the type of organic binders, aqueous slurry characteristics prior to spray-drying, and pressing conditions. Visualizing this issue during the processing stage of green dry-pressed alumina is the focus of this dissertation and proves a need in the ceramic industry. Alumina (Al2O3) is an important material in the technical ceramics industry due to its favorable thermal, mechanical, and electrically insulating properties. As previously stated, spray-drying is the most widely used granulation technique for large production of dry-pressing ceramics. This candidate system and granulation process were solely investigated for this dissertation. The effects that processing parameters have during the spray-drying process on the morphology of spray-dried granules promoted the understanding of how granule characteristics effect the compaction behavior of the granules and the rearrangement of the particles that comprise them. This provided an understanding of what granule characteristics promote or inhibit microstructural uniformity. The processing roles that were varied included the viscosity, the type and percentage of organic binder, and the specific gravity of the slurry. Alumina granules were compacted at various uniaxial pressures and characterized using a field emission scanning electron microscope (FESEM). The solids loading of the alumina slurries affected the generated density of the spray-dried granules. The binder amount, binder type, and viscosity of the alumina slurries affected the droplet formation of the atomized slurry, and therefore, the generated morphology of the spray-dried granule. The acrylic emulsion binder resulted in spray-dried granules with a higher density and uniform internal structure when compared to the polyvinyl alcohol binder system whose internal structure consisted of an abundance of hollow coring, or microstructural voids, remaining after the spray-drying process. In comparison, the higher percentage of polyvinyl alcohol binder resulted in the lowest spray-dried granule densities and resulted in a higher degree of hollow coring occurring. This difference was noted in the resultant compacted microstructures of the compacted green bodies as well, with a higher degree of microstructural uniformity formed from the green compacts processed with the acrylic emulsion binders. The generated voids within the polyvinyl alcohol binders were difficult to remove during compaction, leaving behind closed porosity and a higher degree of microstructural defects within the visualized compacted microstructure. Moisture content was added to these granules in hope to remove these defects. Since moisture is a plasticizer for polyvinyl alcohol, the moisture content softened the granule and resulted in the removal of these defects. A higher degree of microstructural uniformity was visualized as a function of compaction moisture. However, adding moisture content could affect the overall density distribution within the alumina compact, therefore, microstructural analysis in three dimensions was necessary. In terms of compaction behavior, the density of the granules affected the initial fill density but the morphology and binder type affected the overall knitting behavior and compaction rate of the generated granules. A three-dimensional layering method was developed to stack two-dimensional FESEM micrographs in three-dimensions to visualize the microstructural characteristics of varying processing parameters. This method resulted in an accurate micron sized measurement to calculate the material removal based on the depths of micro-hardness indentations with a confocal laser scanning microscope throughout the varying removed layers. However, this method was time sensitive, generating small volumes in a matter of weeks. This method was compared to micro X-ray computed tomography (CT) data analyses. The FESEM method resulted in higher resolution three-dimensional microstructures of a finite volume whereas the micro X-ray CT analysis was sufficient to visualize the particle and pore arrangements throughout the full compact at a resolution twelve times lower. This lower resolution resulted in depleting the overall microstructural information gained. Even though the micro X-ray CT data was sufficient to visualize the particle and pore arrangements as well as the density distribution throughout the whole green alumina compact, the three-dimensional FESEM microstructures gained the closed porosity and sub-micron information needed to be able to better characterize these green alumina compacts. Similar visualization results were obtained for the three-dimensional microstructures in comparison to the two-dimensional microstructural characterization analysis. The higher degree of polyvinyl alcohol binder resulted in a higher degree of microstructural defects and density distributions throughout the compact. The highest degree of uniformity was visualized from the compacts processed with the acrylic emulsion binder and the added moisture content to the polyvinyl alcohol polyvinyl alcohol spray-dried granules. Image processing techniques were implemented on both data analyses to threshold the micrographs into a binary image that distinguished the micrographs into material and porosity based on pixel intensity on a greyscale. The porosity segmented analysis for the three-dimensional FESEM method returned similar results to the geometric porosity calculated from the geometric density measurements. Due to the lower resolution capabilities, the segmented porosity calculations obtained from the micro X-ray CT data were an underestimation in comparison to the geometric porosity measurements. Mercury porosimetry proved difficult to conduct as the green alumina compacts would break during mercury intrusion. This technique was insufficient in characterizing the porosity of soft, green alumina compacts even at slow intrusion rates. That is why the image processing and characterization techniques used for this dissertation are valuable in characterizing green ceramic microstructures. The sub-micron and density distribution information gained throughout both visualization techniques can be used as a tool to understand what processing parameters as a function of what ceramic forming method will promote microstructural uniformity in the unfired state and more reliable ceramic parts. The work completed in this dissertation involved analyzing the compaction behavior of granulated alumina as a function of processing parameters and visualizing the microstructural variations that comprised the final green microstructure. This dissertation developed and determined methods to understand particle and pore arrangements in dry pressed alumina green compacts. To understand this phenomenon, visualizing the internal structure of the green compacts in both two and three dimensions promoted the understanding of distinguishing what process parameters produce fewer green bulk microstructural variations. The improved microstructural characterization techniques accomplished in this work provided an improved method of characterizing green microstructures to help guide improved outcomes in ceramic processing.Ph.D.Includes bibliographical reference