Publications

Abstract not provided.

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.

Abstract not provided.

The microstructure and mechanical properties of niobium-modified lead zirconate titanate (PNZT) 95/5 ceramics, where 95/5 refers to the ratio of lead zirconate to lead titanate, were evaluated as a function of lead (Pb) stoichiometry. Chemically-prepared PNZT 95/5 is produced at Sandia National Laboratories by the Ceramics and Glass Processing Department (14154) for use as voltage elements in ferroelectric neutron generator power supplies. PNZT 95/5 was prepared according to the nominal formulation of Pb{sub 0.991+x}(Zr{sub 0.955}Ti{sub 0.045}){sub 0.982}Nb{sub 0.018}O{sub 3+x}, where x (-0.0274 {approx}< x {approx}< 0.0297) refers to the mole fraction of Pb and O that deviated from the stoichiometric value. The Pb concentrations were determined from calcined powders; no adjustments were made to Pb compositions due to weight loss during sintering. The microstructure (second phases, fracture mode and grain size) varied appreciably with Pb stoichiometry, whereas the mechanical properties (hardness, fracture toughness, strength and Weibull parameters) exhibited modest variation. Specimens deficient in Pb, 2.74% (x = -0.0274) and 2.15% (x = -0.02150), had a high area fraction of a zirconia (ZrO{sub 2}) second phase on the order of 0.02. As the Pb content in solid solution increased the ZrO{sub 2} content decreased; no ZrO{sub 2} was observed for the specimen containing 2.97% excess Pb (x = 0.0297). Over the range of Pb stoichiometry most specimens fractured predominately transgranularly; however, 2.97% Pb excess PNZT 95/5 fractured predominately intergranularly. No systematic changes in hardness or Weibull modulus were observed as a function of Pb content. Fracture toughness decreased slightly from 1.8 MPa{center_dot}m{sup 1/2} for Pb deficient specimens to 1.6 MPa{center_dot}m{sup 1/2} for specimens with excess Pb. Although there are microstructural differences with changes in Pb content, the mechanical properties did not vary substantially. However, the average failure stress and fracture toughness for PNZT 95/5 containing 2.97% excess Pb decreased slightly. It is expected that additional increases in Pb content would result in further mechanical property degradation. The decrease in mechanical properties for the 2.97% Pb excess ceramics could be the result of a weaker PbO-rich grain boundary phase present in the material. If better mechanical properties are desired, it is recommended that PNZT 95/5 ceramics are processed by a method whereby any excess Pb is depleted from the final sintered ceramic so that near-stoichiometric values of Pb concentration are reached. Otherwise, a PbO-rich grain boundary phase may exist in the ceramic which could potentially be detrimental to the mechanical properties of PNZT 95/5 ceramics.

The electrical properties of lead zirconate titanate ceramics near the 95/5 composition are extremely sensitive to the chemical composition and processing conditions. To precisely control the lead stoichiometry in a solid solution has been a challenge because of lead volatility during high temperature sintering. In this study, we investigated the effect of the amount of lead in the solid solution on crystal structure, dielectric behavior, and phase transformation characteristics for chemically prepared niobium modified PZT 95/5 ceramics. Implications are important for process control and assurance of material performance.

The influences of temperature and processing conditions (unpoled or poled-depoled) on strength, fracture toughness and the stress-strain behavior of tin-modified lead zirconate titanate (PSZT) were evaluated in four-point bending. PSZT exhibits temperature-dependent non-linear and non-symmetric stress-strain behavior. A consequence of temperature dependent non-linearity is an apparent reduction in the flexural strength of PSZT as temperature increases. At room temperature the average stress in the outer-fiber of bend bars was 84 MPa, whereas, for specimens tested at 120 C the average failure stress was only 64 MPa. The load-carrying capacity, however, does not change with temperature, but the degree of deformation tolerated by PSZT prior to failure increased with temperature.

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.