Publications

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Fine powders of calcium zirconate (CaZrO{sub 3}, CZ) and calcium titanate (CaTiO{sub 3}, CT) were synthesized using a nonaqueous oxalate co-precipitation route from Ca(NO{sub 3}){sub 2}{center_dot}4 H{sub 2}O and group(IV) n-butoxides (Ti(OBu{sup n}){sub 4} or Zr(OBu{sup n}){sub 4}). Several reaction conditions and batch sizes (2-35 g) were explored to determine their influence on final particle size, morphology, and phase. Characterization of the as-prepared oxalate precursors, oven dried oxalate precursors (60-90 C), and calcined powders (635-900 C) were analyzed with TGA/DTA, XRD, TEM, and SEM. Densification and sintering studies on pressed CZ pellets at 1375 and 1400 C were also performed. Through the developed oxalate co-precipitation route, densification temperatures for CZ were lowered by 125 C from the 1500 C firing temperature required for conventional mixed oxide powders. Low field electrical tests of the CZ pellets indicated excellent dielectric properties with dielectric constants of {approx}30 and a dissipation factor of 0.0004 were measured at 1 kHz.

This report focuses on our recent advances in the fabrication and processing of barium strontium titanate (BST) thin films by chemical solution depositiion for next generation fuctional integrated capacitors. Projected trends for capacitors include increasing capacitance density, decreasing operating voltages, decreasing dielectric thickness and decreased process cost. Key to all these trends is the strong correlation of film phase evolution and resulting microstructure, it becomes possible to tailor the microstructure for specific applications. This interplay will be discussed in relation to the resulting temperature dependent dielectric response of the BST films.

The high permittivity of Pb(Zr,Ti)O3 and (Pb,La)(Zr,Ti)O 3 - PZT and PLZT, respectively - thin films and the flexibility of chemical solution deposition (CSD) make solution-derived P(L)ZT thin films extremely attractive for integrated capacitor applications. However, Pb-loss or cation segregation during processing results in degraded properties of the final film. Here, we have extended the use of multivariate statistical analysis (MSA) of energy-dispersive spectroscopy (EDS) spectrum images (SIs) in scanning transmission electron microscopy (STEM) to allow the two-dimensional (2D) quantitative analysis of cation segregation and depletion in P(L)ZT thin films. Quantified STEM-EDS SIs allow high-resolution (< ≈10 nm) quantification of these cation distributions. Surface Pb depletion is found after crystallization and is replenished by a unique post-crystallization PbO overcoat+anneal processes. Zr/Ti and La segregation are found to develop in a decidedly nonplanar fashion during crystallization, especially in PLZT 12/70/30 material, highlighting the need for 2D analysis. Quantitative 2D chemical information is essential for improved processing of homogeneous P(L)ZT films with optimal electrical properties. © 2008 Sandia Corporation. Journal compilation © 2008 The American Ceramic Society.

Lead loss during processing of solution-derived Pb(Zr,Ti)O3 (PZT)-based thin-films can result in the formation of a Pb-deficient, nonferroelectric fluorite phase that is detrimental to dielectric properties. It has recently been shown that this nonferroelectric fluorite phase can be converted to the desired perovskite phase by postcrystallization treatment. Here, quantitative standard-based energy-dispersive x-ray spectrometry (EDS) in a scanning transmission electron microscope (STEM) is used to study cation distribution before and after this fluorite-to-perovskite transformation. Single-phase perovskite PbZr0.53 Ti0.47O3 (PZT 53 /47) and Pb0.88 La0.12 Zr0.68 Ti0.29O3 (PLZT 12/70/30) specimens that underwent this treatment were found to be chemically indistinguishable from the perovskite present in the multiphase specimens prior to the fluorite-to-perovskite transformation. Significant Zr-Ti segregation is found in PLZT 12/70/30, but not in PZT 53/47. Slight La-segregation was seen in rapidly crystallized PLZT, but not in more slowly crystallized PLZT.

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This report focuses on our recent advances in the fabrication and processing of barium strontium titanate (BST) thin films by chemical solution deposition for next generation functional integrated capacitors. Projected trends for capacitors include increasing capacitance density, decreasing operating voltages, decreasing dielectric thickness and decreased process cost. Key to all these trends is the strong correlation of film phase evolution and resulting microstructure, it becomes possible to tailor the microstructure for specific applications. This interplay will be discussed in relation to the resulting temperature dependent dielectric response of the BST films.

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Chemical solution deposition has been used to fabricate continuous ultrathin lead lanthanum zirconate titanate (PLZT) films as thin as 20 nm. Further, multilayer capacitor structures with as many as 10 dielectric layers have been fabricated from these ultrathin PLZT films by alternating spin-coated dielectric layers with sputtered platinum electrodes. Integrating a photolithographically defined wet etch step to the fabrication process enabled the production of functional multilayer stacks with capacitance values exceeding 600 nF. Such ultrathin multilayer capacitors offer tremendous advantages for further miniaturization of integrated passive components.

A facile solution-based processing route using standard spin-coating deposition techniques has been developed for the production of reliable capacitors based on lead lanthanum zirconate titanate (PLZT) with active areas of ≥1 mm2 and dielectric layer thicknesses down to 50 nm. With careful control of the dielectric phase development through improved processing, ultrathin capacitors exhibited slim ferroelectric hysteresis loops and dielectric constants of >1000, similar to those of much thicker films. Thus, it has been demonstrated that chemical solution deposition is a viable route to the production of capacitor films which are as thin as 50 nm but are still macroscopically addressable with specific capacitance values >160 nF/mm2.

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The purpose of this SAND Report is to document efforts in the extraction and failure analyses of sleeve-style Lightning Arrestor Connectors (LACs). Several MC3080 and MC3079 LACs were recovered from the field and tested as part of the Enhanced Surveillance Campaign. A portion of these LACs failed retesting. Terry Ernest (01733), the LAC Component Engineer, provided eleven MC3080 LACs for evaluation where four of the LACs failed IR/DCW and one failed FRB requirements. The extraction of rutile sleeves from failed LACs was required to determine the source of failure. Rutile sleeves associated with connector function failures were examined for cracks, debris as well as any other anomalies which could have caused the LAC to not function properly. Sleeves that failed FRB or that experienced high FRB exhibited high symmetry, smooth surface, long-flow amicon, and slightly over-sized inside diameter. LACs that failed DCW or IR requirements had rutile sleeves that exhibited breakdown tracks.

Detailed studies of the properties of ceramic CaCu{sub 3}Ti{sub 4}O{sub 12} (CCTO) have clarified the physics of this interesting material and revealed several features not reported before. The dielectric relaxational properties of CCTO are explained in terms of a capacitive-layer model, as for an inhomogeneous semiconductor, consisting of semiconducting grains and insulating grain boundaries as also concluded by others. The kinetics of the main [low-temperature (T)] relaxation reveal that two different thermally activated processes in CCTO grains control the dynamics. A likely candidate defect responsible for the two processes is the oxygen vacancy which is a double donor. A higher-T relaxation is determined by grain boundary conduction. Both Nb and Fe doping lowered both the apparent dielectric constant {var_epsilon}{prime} and the dielectric loss, but increased Fe doping led to more dramatic effects. At 3 at.% Fe doping, the anomalous {var_epsilon}{prime}(T) response was removed, making the CCTO an intrinsic, very-low-loss dielectric. The intrinsic {var_epsilon}{prime}({approx}75) and its T dependence are measured and shown to be largely determined by a low-lying soft TO phonon. At low T, cubic CCTO transforms into an antiferromagnetic phase at T{sub N} = 25 K. T{sub N} is essentially independent of Nb doping (up to 4 at.%) and of hydrostatic pressure (up to {approx}7 kbar), but decreases significantly with Fe doping. Analysis of the high-T dependence of the magnetic susceptibility provided insight into the role of Fe as a dopant. Finally, an {var_epsilon}{prime}(T) anomaly associated with the onset of antiferromagnetic order has been discovered, providing evidence for coupling between the polarization and sublattice magnetization. The possible origin of this coupling is discussed.

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Chemically prepared zinc oxide powders are fabricated for the production of high aspect ratio varistor components. Colloidal processing in water was performed to reduce agglomerates to primary particles, form a high solids loading slurry, 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. Chemically prepared zinc oxide powders were processed for the production of high aspect ratio varistor components. Near net shape casting methods including slip casting and agarose gelcasting were evaluated for effectiveness in achieving a uniform green microstructure achieving density values near the theoretical maximum during sintering. The structure of the green parts was examined by mercury porisimetry. Agarose gelcasting produced green parts with low solids loading values and did not achieve high fired density. Isopressing the agarose cast parts after drying raised the fired density to greater than 95%, but the parts exhibited catastrophic shorting during electrical testing. Slip casting produced high green density parts, which exhibited high fired density values. The electrical characteristics of slip cast parts are comparable with dry pressed powder compacts. Alternative methods for near net shape forming of ceramic dispersions were investigated for use with the chemically prepared ZnO material. Recommendations for further investigation to achieve a viable production process are presented.

The hydrostatically induced ferroelectric(FE)-to-antiferroelectric(AFE) phase transformation for chemically prepared niobium modified PZT 95/5 ceramics was studied as a function of density and pore former type (Lucite or Avicel). Special attention was placed on the effect of different pore formers on the charge release behavior associated with the FE-to-AFE phase transformation. Within the same density range (7.26 g/cm3 to 7.44 g/cm3), results showed that ceramics prepared with Lucite pore former exhibit a higher bulk modulus and a sharper polarization release behavior than those prepared with Avicel. In addition, the average transformation pressure was 10.7% greater and the amount of polarization released was 2.1% higher for ceramics with Lucite pore former. The increased transformation pressure was attributed to the increase of bulk modulus associated with Lucite pore former. Data indicated that a minimum volumetric transformational strain of -0.42% was required to trigger the hydrostatically induced FE-to-AFE phase transformation. This work has important implications for increasing the high temperature charge output for neutron generator power supply units.

Chemically prepared zinc oxide powders were processed for the production of high aspect ratio varistor components (length/diameter >5). Near-net-shape casting methods including slip casting and agarose gelcasting were evaluated for effectiveness in achieving a uniform green microstructure that densifies to near theoretical values during sintering. The structure of the green parts was examined by mercury porisimetry. Agarose gelcasting produced green parts having low solids loading values and did not achieve high fired density. Isopressing the agarose cast parts after drying raised the fired density to greater than 95%, but the parts exhibited catastrophic shorting during electrical testing. Slip casting produced high green density parts, which exhibit high fired density values. The electrical characteristics of slip-cast parts are comparable with dry-pressed powder compacts.

Combined XRD/neutron Rietveld refinements were performed on PbZr{sub 0.30}Ti{sub 0.70}O{sub 3} powder samples doped with nominally 4% Ln (where Ln = Ce, Nd, Tb, Y, or Yb). Resulting refined structural parameters indicated that the lattice parameters and volume changes in the tetragonal perovskite unit cell were consistent with A and/or B-site doping of the structure. Ce doping is inconsistent with respect to its rather large atomic radius, but is understood in terms of its oxidation to the Ce{sup +4} oxidation state in the structure. Results of the B-site displacement values for the Ti/Zr site indicate that amphoteric doping of Ln cations in the structure results in superior properties for PLnZT materials.

Development of next generation electronics for pulse discharge systems requires miniaturization and integration of high voltage, high value resistors (greater than 100 megohms) with novel substrate materials. These material advances are needed for improved reliability, robustness and performance. In this study, high sheet resistance inks of 1 megohm per square were evaluated to reduce overall electrical system volume. We investigated a deposition process that permits co-sintering of high-sheet-resistance inks with a variety of different material substrates. Our approach combines the direct write process of aerosol jetting with laser sintering and conventional thermal sintering processes. One advantage of aerosol jetting is that high quality, fine line depositions can be achieved on a wide variety of substrates. When combined with laser sintering, the aerosol jetting approach has the capability to deposit resistors at any location on a substrate and to additively trim the resistors to specific values. We have demonstrated a 400 times reduction in overall resistor volume compared to commercial chip resistors using the above process techniques. Resistors that exhibited this volumetric efficiency were fabricated by 850 C thermal processing on alumina substrates and by 0.1W laser sintering on Kapton substrates.

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.

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.

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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.

A lattice-Monte Carlo approach was developed to simulate ferroelectric domain behavior. The model utilizes a Hamiltonian for the total energy that includes electrostatic terms (involving dipole-dipole interactions, local polarization gradients, and applied electric field), and elastic strain energy. The contributions of these energy components to the domain structure and to the overall applied field response of the system were examined. In general, the model exhibited domain structure characteristics consistent with those observed in a tetragonally distorted ferroelectric. Good qualitative agreement between the appearance of simulated electrical hysteresis loops and those characteristic of real ferroelectric materials was found.

The authors used micro-Raman spectroscopy to monitor the ferroelectric (FE) to antiferroelectric (AFE) phase transition in PZT ceramic bars during the application of uniaxial stress. They designed and constructed a simple loading device, which can apply sufficient uniaxial force to transform reasonably large ceramic bars while being small enough to fit on the mechanical stage of the microscope used for Raman analysis. Raman spectra of individual grains in ceramic PZT bars were obtained as the stress on the bar was increased in increments. At the same time gauges attached to the PZT bar recorded axial and lateral strains induced by the applied stress. The Raman spectra were used to calculate an FE coordinate, which is related to the fraction of FE phase present. The authors present data showing changes in the FE coordinates of individual PZT grains and correlate these changes to stress-strain data, which plot the macroscopic evolution of the FE-to-AFE transformation. Their data indicates that the FE-to-AFE transformation does not occur simultaneously for all PZT grains but that grains react individually to local conditions.

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.