Chemically prepared zinc oxide powders are fabricated for the production of high aspect ratio varistor components. Colloidal processing was performed to reduce agglomerates to primary particles, form a high solids loadingslurry, and prevent dopant migration. The milled and dispersed powder exhibited a viscoelastic to elastic behavioral transition at a volume loading of 43-46%. The origin of this transition was studied using acoustic spectroscopy, zeta potential measurements, and oscillatory rheology. The phenomenon occurs due to a volume fraction solids dependent reduction in the zeta potential of the solid phase. It is postulated to result from divalent ion binding within the polyelectrolyte dispersant chain and was mitigated using a polyethylene glycol plasticizing additive. This allowed for increased solids loading in the slurry and a green body fabrication study to be presented in our companion paper.
Niobium doped PZT 95/5 (lead zirconate-lead titanate) is the material used in voltage bars for all ferroelectric neutron generator power supplies. In June of 1999, the transfer and scale-up of the Sandia Process from Department 1846 to Department 14192 was initiated. The laboratory-scale process of 1.6 kg has been successfully scaled to a production batch quantity of 10 kg. This report documents efforts to characterize and optimize the production-scale process utilizing Design of Experiments methodology. Of the 34 factors identified in the powder preparation sub-process, 11 were initially selected for the screening design. Additional experiments and safety analysis subsequently reduced the screening design to six factors. Three of the six factors (Milling Time, Media Size, and Pyrolysis Air Flow) were identified as statistically significant for one or more responses and were further investigated through a full factorial interaction design. Analysis of the interaction design resulted in developing models for Powder Bulk Density, Powder Tap Density, and +20 Mesh Fraction. Subsequent batches validated the models. The initial baseline powder preparation conditions were modified, resulting in improved powder yield by significantly reducing the +20 mesh waste fraction. Response variation analysis indicated additional investigation of the powder preparation sub-process steps was necessary to identify and reduce the sources of variation to further optimize the process.
The Materials Chemistry Department 1846 has developed a lab-scale chem-prep process for the synthesis of PNZT 95/5, referred to as the ''SP'' process (Sandia Process). This process (TSP) has been successfully transferred to and scaled-up by Department 14192 (Ceramics and Glass Department), producing the larger quantities of PZT powder required to meet the future supply needs of Sandia for neutron generator production. The particle size distributions of TSP powders routinely have been found to contain a large particle size fraction that was absent in development (SP) powders. This SAND report documents experimental studies focused on characterizing these particles and assessing their potential impact on material performance. To characterize these larger particles, fractionation of several TSP powders was performed. The ''large particle size fractions'' obtained were characterized by particle size analysis, SEM, and ICP analysis and incorporated into compacts and sintered. Large particles were found to be very similar in structure and composition as the bulk of the powder. Studies showed that the large-size fractions of the powders behave similarly to the non-fractionated powder with respect to the types of microstructural features once sintered. Powders were also compared that were prepared using different post-synthesis processing (i.e. differences in precipitate drying). Results showed that these powders contained different amounts and sizes of porous inclusions when sintered. How this affects the functional performance of the PZT 95/5 material is the subject of future investigations.
The particular lead zirconate/titanate composition PZT 95/5-2Nb was identified many years ago as a promising ferroelectric ceramic for use in shock-driven pulsed power supplies. The bulk density and the corresponding porous microstructure of this material can be varied by adding different types and quantities of organic pore formers prior to bisque firing and sintering. Early studies showed that the porous microstructure could have a significant effect on power supply performance, with only a relatively narrow range of densities providing acceptable shock wave response. However, relatively few studies were performed over the years to characterize the shock response of this material, yielding few insights on how microstructural features actually influence the constitutive mechanical, electrical, and phase-transition properties. The goal of the current work was to address these issues through comparative shock wave experiments on PZT 95/5-2Nb materials having different porous microstructures. A gas-gun facility was used to generate uniaxial-strain shock waves in test materials under carefully controlled impact conditions. Reverse-impact experiments were conducted to obtain basic Hugoniot data, and transmitted-wave experiments were conducted to examine both constitutive mechanical properties and shock-driven electrical currents. The present work benefited from a recent study in which a baseline material with a particular microstructure had been examined in detail. This study identified a complex mechanical behavior governed by anomalous compressibility and incomplete phase transformation at low shock amplitudes, and by a relatively slow yielding process at high shock amplitudes. Depoling currents are reduced at low shock stresses due to the incomplete transformation, and are reduced further in the presence of a strong electrical field. At high shock stresses, depoling currents are driven by a wave structure governed by the threshold for dynamic yielding. This wave structure is insensitive to the final wave amplitude, resulting in depoling currents that do not increase with shock amplitude for stresses above the yield threshold. In the present study, experiments were conducted under matched experimental conditions to directly compare with the behavior of the baseline material. Only subtle differences were observed in the mechanical and electrical shock responses of common-density materials having different porous microstructures, but large effects were observed when initial density was varied.
This paper reports the preparation of cubically ordered, self-assembled nanostructural crystals on micropatterned surfaces. Large ordered arrays of octahedral crystals were formed on substrates with triangular-shaped micropatterns that match the shape of the {111} surface morphology of the crystals. Many crystals became self-aligned in X, Y, and Z orientations through the interaction with the matching micropatterns. However, line micropatterns did not produce such well-defined crystals and crystal alignment. This work is among the first examples to show 3D crystal alignment independent of the long rodlike micellar structure that is characteristic of the hexagonal phases. Significantly, it also suggests that the geometry of the underline patterns has a large effect on the crystal morphology and orientation. We hope that these results will be helpful in developing microdevices based on self-assembled nanoscale materials.
We have undertaken the synthesis of a thin film ''All Ceramic Battery'' (ACB) using solution route processes. Based on the literature and experimental results, we selected SnO{sub 2}, LiCoO{sub 2}, and LiLaTiO{sub 3} (LLT) as the anode, cathode, and electrolyte, respectively. Strain induced by lattice mismatch between the cathode and bottom electrode, as estimated by computational calculations, indicate that thin film orientations for batteries when thicknesses are as low as 500 {angstrom} are strongly controlled by surface energies. Therefore, we chose platinized silicon as the basal platform based on our previous experience with this material. The anode thin films were generated by standard spin-cast methods and processing using a solution of [Sn(ONep)]{sub 8} and HOAc which was found to form Sn{sub 6}(O){sub 4}(ONep){sub 4}. Electrochemical evaluation showed that the SnO{sub 2} was converted to Sn{sup o} during the first cycle. The cathode was also prepared by spin coating using the novel [Li(ONep)]{sub 8} and Co(OAc){sub 2}. The films could be electrochemically cycled (i.e., charged/discharged), with all of the associated structural changes being observable by XRD. Computational models indicated that the LLT electrolyte would be the best available ceramic material for use as the electrolyte. The LLT was synthesized from [Li(ONep)]{sub 8}, [Ti(ONep){sub 4}]{sub 2}, and La(DIP){sub 3}(py){sub 3} with RTP processing at 900 C being necessary to form the perovskite phase. Alternatively, a novel route to thin films of the block co-polymer ORMOLYTE was developed. The integration of these components was undertaken with each part of the assembly being identifiably by XRD analysis (this will allow us to follow the progress of the charge/discharge cycles of the battery during use). SEM investigations revealed the films were continuous with minimal mixing. All initial testing of the thin-film cathode/electrolyte/anode ACB devices revealed electrical shorting. Alternative approaches for preparing non-shorted devices (e.g. inverted and side-by-side) are under study.
The Materials Chemistry Department 1846 has developed a lab-scale chem-prep process for the synthesis of PNZT 95/5, a ferroelectric material that is used in neutron generator power supplies. This process (Sandia Process, or SP) has been successfully transferred to and scaled by Department 14192 (Ceramics and Glass Department), (Transferred Sandia Process, or TSP), to meet the future supply needs of Sandia for its neutron generator production responsibilities. In going from the development-size SP batch (1.6 kg/batch) to the production-scale TSP powder batch size (10 kg/batch), it was important that it be determined if the scaling process caused any ''performance-critical'' changes in the PNZT 95/5 being produced. One area where a difference was found was in the particle size distributions of the calcined PNZT powders. Documented in this SAND report are the results of an experimental study to determine the origin of the differences in the particle size distribution of the SP and TSP powders.
Chemically prepared Pb(Zr0.95Ti0.05)O3 (PZT 95/5) ceramics were fabricated with a range of different porosity levels, while grain size was held constant, by systematic additions of added organic pore former (Avicel). Use of Avicel in amounts ranging from 0 to 4.0 wt% resulted in fired ceramic densities that ranged from 97.3% to 82.3%. Hydrostatic-pressure-induced ferroelectric (FE) to antiferroelectric (AFE) phase transformations were substantially more diffuse and occurred at lower hydrostatic pressures with increasing porosity. An ∼12 MPa decrease in hydrostatic transformation pressure per volume percent added porosity was observed. The decrease in transformation pressure with decreasing density was quantitatively consistent with the calculated macroscopic stress required to achieve a specific volumetric macrostrain (0.40%). This strain was equivalent to experimentally measured macrostrain for FE-to-AFE transformation. The macroscopic stress levels were calculated using measured bulk modulus values that decreased from 84 to 46 GPa as density decreased from 97.3% to 82.3%. Good agreement between calculated and measured values of FE-to-AFE transformation stress was obtained for ceramics fired at 1275° and 1345°C.
Palmierite (K{sub 2}Pb(SO{sub 4}){sub 2}) has been prepared via a chemical synthesis method. Intensity differences were observed when X-ray powder data from the newly synthesized compound were compared to the published powder diffraction card (PDF) 29-1015 for Palmierite. Investigation of these differences indicated the possibility of preferred orientation and/or chemical inhomogeneity affecting intensities, particularly those of the basal (00{ell}) reflections. Annealing of the Palmierite was found to reduce the effects of preferred orientation. Electron microprobe analysis confirmed K:Pb:S as 2:1:2 for the annealed Palmierite powder. Subsequent least-squares refinement and Rietveld analysis of the annealed powder showed peak intensities very close to that of a calculated Palmierite pattern (based on single crystal data), yet substantially higher than many of the PDF 29-1015 published intensities. Further investigation of peak intensity variation via calculated patterns suggested that the intensity discrepancies between the annealed sample and those found in PDF 29-1015 were potentially due to chemical variation in the K{sub 2}Pb(SO{sub 4}){sub 2} composition. X-ray powder diffraction and crystal data for Palmierite are reported for the annealed sample. Palmierite is Trigonal/Hexagonal with unit cell parameters a = 5.497(1){angstrom}, c = 20.864(2) {angstrom}, space group R-3m (166), and Z = 3.
Robocasting, a computer controlled slurry deposition technique, was used to fabricate ceramic monoliths and composites of chemically prepared Pb(Zr{sub 0.95}Ti{sub 0.05})O{sub 3} (PZT 95/5) ceramics. Densities and electrical properties of the robocast samples were equivalent to those obtained for cold isostatically pressed (CIP) parts formed at 200 MPa. Robocast composites consisting of alternate layers of the following sintered densities: (93.9%--96.1%--93.9%), were fabricated using different levels of organic pore former additions. Modification from a single to a multiple material deposition robocaster was essential to the fabrication of composites that could withstand repeated cycles of saturated polarization switching under 30 kV/cm fields. Further, these composites withstood 500 MPa hydrostatic pressure induced poled ferroelectric (FE) to antiferroelectric (AFE) phase transformation during which strain differences on the order of 0.8% occurred between composite elements.
Self-assembled monolayers (SAMS) are commonly produced by immersing substrates in organic solutions containing trichlorosilane coupling agents. Unfortunately, such deposition solutions can also form alternate structures including inverse micelles and lamellar phases. The formation of alternate phases is one reason for the sensitivity of SAM depositions to factors such as the water content of the deposition solvent. If such phases are present, the performance of thin films used for applications such as minimization of friction and stiction in micromachines can be seriously compromised. Inverse micelle formation has been studied in detail for depositions involve 1H-, 1H-, 2H-, 2H-perfluorodecyltrichlorosilane (FDTS) in isooctane. Nuclear magnetic resonance experiments have been used to monitor the kinetics of hydrolysis and condensation reactions between water and FDTS. Light scattering experiments show that when hydrolyzed FDTS concentrations reach a critical concentration, there is a burst of nucleation to form high concentrations of spherical agglomerates. Atomic force microscopy results show that the agglomerates then deposit on substrate surfaces. Deposition conditions leading to monolayer formation involve using deposition times that are short relative to the induction time for agglomeration. After deposition, inverse micelles can be converted into lamellar or monolayer structures with appropriate heat treatments if surface concentrations are relatively low.
A substantial decrease in hydrostatic ferroelectric (FE) to antiferroelectric (AFE) transformation pressure was measured for Pb(Zr{sub 0.949}Ti{sub 0.051}){sub 0.989}Nb{sub 0.0182}O{sub 3} ceramics with decreasing grain size. The 150 MPa decrease in hydrostatic FE to AFE transformation pressure over the grain size range of 8.5 {micro}m to 0.7{micro}m was shown to be consistent with enhanced internal stress with decreasing grain size. Further, the Curie Point decreased and the dielectric constant measured at 25 C increased with decreasing grain size. All three properties: dielectric constant magnitude, Curie point shift and FE to AFE phase transformation pressure were shown to be semi-quantitatively consistent with internal stress differences on the order of 100 MPa. Calculations of Curie point shifts from the Clausius-Clapeyron equation, using internal stress levels derived from the hydrostatic depoling characteristics, were consistent with measured values.
Chemically prepared Pb(Zr{sub 0.951}Ti{sub 0.949}){sub 0.982}Nb{sub 0.018}O{sub 3} ceramics were fabricated that were greater than 95% dense for sintering temperatures as low as 925 C. Achieving high density at low firing temperatures permitted isolation of the effects of grain size, from those due to porosity, on both dielectric and pressure induced transformation properties. Specifically, two samples of similar high density, but with grain sizes of 0.7 {micro}m and 8.5 {micro}m, respectively, were characterized. The hydrostatic ferroelectric (FE) to antiferroelectric (AFE) transformation pressure was substantially less (150 MPa) for the lower grain size material than for the larger grain size material. In addition, the dielectric constant increased and the Curie temperature decreased for the sample with lower grain size. All three properties: dielectric constant magnitude, Curie point shift, and FE to AFE phase transformation pressure were shown to be semi-quantitatively consistent with internal stress levels on the order of 100 MPa.
To characterize particle size/magnetic property relationships, 9 to 50 nm in diameter barium hexaferrite, BaFe{sub 12}O{sub 19} (BHF), particles were prepared using a new synthesis route. By replacing the conventional 50 to 100 nm particles of goethite with nanocrystalline goethite produced via the microwave anneal method of Knight and Sylva, nanocrystalline BHF was synthesized using the hydrothermal method. Evolution of particle size and morphology with respect to concentration and heat treatment time is reported. Hysteresis properties, including coercivity (0.2--1.0 kOe), magnetization saturation (0.1--33.4 emu/g), and magnetization remanence (0.004--22.5 emu/g) are discussed as a function of particle size. The magnetization saturation and remanence of the 7 nm particles is nearly zero, suggesting the superparamagnetic threshold size for BHF is around this size. In addition, the equilibrium morphology of BHF crystals was calculated to be truncated hexagonal prisms which was verified by experiment, and the isoelectric point, pH of 4.1, was measured for 18 nm BHF particles.
A nonaqueous coprecipitation process has been developed to prepare controlled stoichiometry lithium manganese oxalate precipitates. The process involved mixing a methanolic Li-Mn nitrate solution with a methanolic solution containing tetramethylammonium oxalate as the precipitating agent. The resulting oxalates were readily converted to a variety of phase pure lithium manganese oxides at moderate temperatures ({le}600{degrees}C), where the phase formed was determined by the initial Li/Mn ratio in the starting solution. Metal cation dopants have been incorporated into the oxalate precipitate by dissolving the appropriate metal nitrate in the Li-Mn precursor solution The various starting solutions, oxalate precipitates, and calcined oxides have been extensively characterized using a variety of techniques, including {sup 7}Li NMR, TGA/DTA, SEM, and XRD. Results indicate that a strong interaction occurs between Li and Mn in the nitrate solution which carries over into the oxalate phase during precipitation. The morphology and the crystallite size of the oxide powders were shown to be controlled by the morphology of the oxalate precursor and the oxalate calcination temperature, respectively. The results of initial cathode performance tests with respect to dopant type (Al, Ni, Co) and concentration for LiMn{sub 2}O{sub 4} are also reported.
The authors have characterized the pyrochlore-to-perovskite crystallization process in solution-derived Pb(Zr{sub 0.20}Ti{sub 0.80})O{sub 3} thin films on (100) MgO single crystal substrates. It has been determined that the perovskite phase nucleated preferentially at the film/MgO interface out of a nanocrystalline ({approx}100{angstrom} grains) pyrochlore matrix. During the early stages of the pyrochlore-to-perovskite conversion process, perovskite growth proceeded nearly isotropically from the substrate to form hemispherically shaped grains. Deviations from isotropic growth were shown to result from a growth dependence based on the crystallographic orientation of a growing perovskite grain relative to the orientations of pyrochlore grains being transformed. The volume change that occurs during the pyrochlore-to-perovskite transformation along with two-dimensional grain growth has been used to develop a mechanism for formation of porosity that commonly is concentrated in grain boundary regions.