The new biaxial alignment technique based on a combination of magnetic field and mechanical force alignment (acting normal to each other) has been applied for the first time to an RE-123 superconductor ceramic, where RE - Eu .92 Yb .08. Biaxial (and hence triaxial) alignment was achieved, at least in the surface layer, as demonstrated by XRD and observation of the twin boundary pattern.

A recently developed method for producing triaxial alignment of single crystalline grains may be applicable to ceramics other than the high-Tc cuprate superconductors for which it was designed as a means of reducing the grain boundary weak links due to grain orientational misfit. This technique uses a suitable combination of a mechanical force and a magnetic field acting on the moment of a rare earth element incorporated into the ceramic; a "granular single crystal" is thus formed. A detailed step-by-step procedure is presented here to facilitate use of the new approach.

Two forming techniques, i.e., slip casting and dry compaction, have been employed to make SiC(platelet)-A1203 composites. The orientation of SiC platelets in the composites, before and after hot pressing, was studied using optical microscopy and a pole-figure X-ray device. It was found that the preferred orientation in the dry-pressed green bodies increased with increasing pressure, whereas the platelets in the slip cast bodies were close to a random orientation. The pressure during the hot pressing process did, however, align the platelets of slip-cast composites during densification. Optical micrographs confirmed the platelets tend to align with their faces perpendicular to the hot pressing direction. The effect of platelet size on the orientation was also investigated. X-ray analysis showed that small platelets (∼ 1.6 μm) do not give rise to preferred orientation both in green and hot pressed bodies; whereas larger platelets (∼24 μm) with larger aspect ratio do yield a stronger preferred orientation. The causes for the platelet alignment will be discussed.

Tape casting technology has been developed for the production of ceramic composites using fine powders of alumina and zirconia with an average particle size in the range 0.25-0.3 μm. These powders require careful control of the dispersant/solids ratio in order to minimize the slip viscosity, and to prevent the rejection of excess dispersant during solvent evaporation. In this regard phosphate ester has been found to be an effective dispersant. The choice of solvent is also important, with a non-polar solvent exhibiting more forgiving behaviour. The use of such fine powders lowers the solids loading permitted in the tape casting slip to about 22 vol.%, with a resulting green density of less than 54% of theoretical. However, high sintered densities can be achieved in both single tapes and laminates. Constraints imposed on the tape during solvent evaporation lead to a difference in green density within the tapes, and strategies need to be developed to counteract this.

Stable Si3N4/SiCw co-dispersions containing up to 20 wt.% of whiskers have been prepared. Homogeneous green bodies with relative green density above 55% and pore size in the range 40-50 nm have been obtained by pressure assisted colloidal filtration. By optimizing the temperature and/or the amount of sintering additives, densities above 90-95 % have been achieved by pressureless sintering of these consolidated compacts.

A colloidal processing technique based on the heterocoagulation of selected ceramic oxide systems, and analogous to reaction injection molding of polymers, is described. Commercial colloidal silica and boehmite sols, which have oppositely charged particles in the pH range 2.0-9.0, and mixtures of these suspensions with micron sized oxide powders, have been used to determine whether a liquid to solid transformation could be induced when the two materials are simultaneously injected at high velocities into a mold cavity. Rheological changes have been measured as a function of relative flow rates of the two suspensions, solids loading and oscillatory frequency. Viscosity is observed to increase by as much as six orders of magnitude under optimized conditions.

Solid Freeform Fabrication (SFF) is a new computer fabrication technique that does not require any part specific tooling. The starting material can be either solid, liquid or gaseous. The part can be made from metallic, ceramic, polymeric or a composite material. The concept is to use a solid modeling system to define the part of interest and to reduce the model to a set of toggle point data that totally define the geometry. In Selective Laser Sintering the sectioned component is then combined with a rastered laser system that impinges on the precursor powder materials in a layered reconstruction of the three dimensional CAD designed part. The part is then formed in this manner. This approach to producing the part involves a great deal of understanding of the laser materials interactions, the appropriate choice of materials specific to this processing and how the total process integrates. Application to ceramic powders will be described. An alternative approach to SFF is Selective Area Laser Deposition where the three dimensional part is made from the gas phase. The initial gas deposition studies involving deposition of carbon from hydrocarbons will be discussed. For both of the above SFF approaches the laser beam powder and gas phase interactions and the microstructure of the resulting three dimensional forms as a function of system parameters will be described.

Three Dimensional Printing is a process for rapid prototyping of functional components. Thin layers of powder are bound in selected areas by ink-jet printing of a liquid binder. Layers are added sequentially until a three dimensional part is completed. The unbound powder is then removed. The raster scan pattern and layering used in forming has an observable effect on the surface finish and microstructure of the part, as does the chemistry of the particular materials system used. Important phenomena include ballistic interaction of the jet with the powder bed and wicking of the binder. Process parameters such as layer height, flow rate, and powder density were studied. Microstructural examination of the printed components is also reported.

The low-temperature, Zr/Ii ordered, form of zirconium titanate has been investigated using high-resolution transmission electron microscopy in order to characterize the incommensurate structure of phases with compositions ZrTiO4 to near Zr5Ti7O24. Electron diffraction reveals that compositions with Zr:Ti between 5:7 and 1:1 have incommensurate superstructures, and phases close to 1:1 are commensurate with an a-axis repeat 2× that of the disordered structure. High-resolution images show that the a-doubling in ZrTiO4 corresponds to a new structure, one that consists of two Zr-rich, distorted octahedral layers alternated with two Ti-rich octahedral layers. The incommensurate compositions are composed of blocks of the 1:1 structure intercalated with blocks of the commensurate 5:7 structure, the latter having a tripled a-repeat and a ZTTZTT sequence of cation layers. The intercalation can be described as an “interface-modulated” structure resulting from the quasi-periodic insertion of (100) faults with displacement vector ℝ= -l/3aord in the ordered 5:7 phase. Although their spacing is variable, the faults are uniformly distributed in such a way as to produce incommensurate satellite reflections.

Binder design is an important issue in ceramic injection molding technology. The binder decomposition mechanism, which involves thermodynamics, kinetics, as well as heat and mass transfer, controls the binder removal process. This process, in turn, is governed by the thermal and physical characteristics of the organic waxes used, and is the most critical step in injection molding ceramics. In this paper, we present the binder design philosophy and the method of binder selection. A systematic binder removal study focusing on heating rate, setter powder, and sublimable materials was carried out with the selected compositions. As a result of this study, we concluded that the fluid wicking controls the binder removal at the molten temperature of the binder, and the diffusion and permeation-controlled mechanism dominate at the decomposition temperature range of the binder. With the right binder selection, it is feasible to produce internal and external crack-free large cross-section injection-molded ceramic parts.

The binder burn-out behavior in PMMA/glass powder composites was studied as a function of glass composition in the PbO-SiO2-B203 system. The PbO content was varied to obtain glasses having optical basicities in the range of 0.62-0.81. TGA measurements on PMMA/glass composites show that the main decomposition end temperatures of PMMA increase with decreasing PbO content, and therefore, decreasing optical basicity. This indicates that PMMA interacts strongly with more acidic surfaces. The effect of the milling fluid was also studied. Milling in toluene does not significantly alter the behavior. Milling in water of a PbO-rich glass leads to significant leaching of lead. During drying, the leached lead deposits on the glass particle surfaces and drastically alters the thermal behaviour of the glass powder and PMMA/glass composites.

Both the binder viscosity and the % saturation will change during the removal of multicomponent binder systems. The effects of these changes were evaluated for the polyvinyl butyral (PVB) - dibutyl phthlate (DBP) system as the plasticizing constituent was selectively removed at isothermal conditions (Tiso < 170°C). Experiments were performed to determine the viscosities of the PVB-DBP system at various isotherms and at different concentrations of DBP. A scaling model was developed to determine the relative importance of capillary forces on the distribution of binder within ceramic compacts during thermolysis. This model was modified to account for the influence of fluid saturation on both the capillary driving force and the permeability of the wetting phase (i.e., binder). Two important results are derived from this analysis: (1) the change in binder viscosity affects the length scale (h) over which capillary forces act to redistribute the PVB-DBP to a greater degree than the change in saturation, and (2) the value of h can be significantly lower than the representative macroscopic dimension of the green compact and still permit a capillary-controlled distribution process for multicomponent binders.

A series of alumina particles coated with inorganic polymers, that have utility as preceramic dispersants, have been pyrolyzed and the products of the pyrolysis studied to help determine the possible decomposition routes. The inorganic polymers, bis(amino)polyphosphazenes and siloxanes, appear to undergo loss of the organic sidechains via a nucleophilic process which would incorporate inorganic backbone into the alumina matrix. The bis(alkoxy)polyphosphazenes decompose by nucleophilic attachment but also exhibit polymer loss by formation of cyclic oligomers.

TGA and FTIR were employed to investigate the effects of dibutyl phthalate and butyl benzyl phthalate on the thermal degradation behavior of PVB, a binder used commonly for tape casting. Butyl benzyl phthalate was found to enhance the crosslinking of the PVB binder when heated in a nitrogen atmosphere during binder pyrolysis, which retarded its thermal degradation.

The interaction between AIN powder and PVB binder was also studied by using TGA, XPS and a special furnace for collecting residual carbon. Intrinsic char and gas phase mediated char were studied separately.

Polycarbosilane and polysilazane, precursors of SiC and Si3N4, have been incorporated into the binder system for injection-molded Si3N4-based parts. Property requirements for these precursors as the injection-molding binder are briefly described. The binder removal mechanism is proposed and discussed based on the experimental results. Instead, the gas diffusion effect, rather than fluid wicking, is a key factor. Controlling the thermal decomposition rate of the binder is the key for achieving crack-free dense sintered parts.

Preceramic polysilazane was added to high purity Si powders as a reactive binder during synthesis of reaction bonded Si3N4 parts to improve the defect structures in both the unfired and nitrided parts. This study examined how the presence of Si powder affected the polymer-to-ceramic transformation of the polysilazane. Pyrolysis of both polysilazane and polysilazane+Si samples were characterized by TGA. Also, the gaseous volatiles evolved during pyrolysis were characterized by infrared and mass spectrometry. Results show that between 100- 550°C, silicon powder inhibits the loss of silazane oligomers from the polysilazane. However, for temperatures above 550°C, the weight loss differences between the pure polysilazane and the polysilazane+Si samples were less pronounced. Specific solvents used in the preparation of the polysilazane+Si mixtures affected the quantity and type of species in the volatiles during pyrolysis.

Surface oxidation kinetics of an a-Si3 N4 submicron size and an amorphous nano-size powder have been studied using x-ray photoelectron spectroscopy (XPS) and Bremsstrahlung-excited Auger electron spectroscopy (AES). The samples were oxidized by heating in air at temperatures between 850°C and 1000°C. The oxide thickness for each heating time and temperature was determined both from the relative 0 Is and N Is XPS peak intensities and from the Si02 and Si3 N4 Si KLL peak intensities. In each case, there was a good agreement between the oxide thickness value calculated from the XPS data and that obtained from the AES data. At these temperatures, oxidation followed a linear rate law. Activation energies of 15±1 kcal/mole and 27±1 kcal/mole were measured for the a-powder and the amorphous powder, respectively.

In the combined stage sintering model previously introduced[l], a single equation was derived that quantifies sintering as a continuous phenomenon from beginning to end. The microstructure is characterized by two separate parameters representing the geometry (Γ) and the scale (G). Calculations of Γ from experimental data show the effect of surface diffusion and microstructural geometry on Γ. The model is reviewed and discussed with respect to the sintering of alumina and spherical nickel powder.

A cellular automaton algorithm is described that simulates the evolution of a surface driven by the reduction of chemical potential differences on the surface. When the surface tension is isotropic, the chemical potential is proportional to the curvature at the surface. This process is important in the development of microstructure during the sintering of powders. The algorithm is implemented in two and three dimensions in a digital image mode, using discrete pixels to represent continuum objects. The heart of the algorithm is a pixel-counting-based method for computing the potential at a pixel located in a digital surface. This method gives an approximate measure of the curvature at the given surface pixel. The continuum version of this method is analytically shown to give the true curvature at a point on a continuum surface. The digital version of the curvature computation method is shown to obey the scaling laws derived for the continuum version. The evolution of the surface of a three dimensional loosely packed powder, along with the percolation characteristics of its pore space, are computed as an example of the algorithm.

Effect of glass infiltration distance, controlled by particle sizes of glass and ceramic filler, and green density, on densification kinetics of glass-filled ceramics has been studied using borosilicate glass-cordierite as the model system. Within the particle size range investigated, the densification is found to be significantly enhanced by increasing cordierite size, reducing glass size and increasing green density. The above results are attributed to both increased driving force of densification by reducing glass particle size, and decreased glass redistribution distance by either increasing green density of compacts or increasing the particle size ratio between cordierite and glass powders.