Barry A. Bender

Effect of thermochemical treatments on the strength and microstructure of SiC fibres

The room-temperature strength of commercially available polymer-derived SiC fibres degrades during the typical high-temperature thermal cycle used in ceramic matrix composite fabrication. Substantial improvements in retained room-temperature strength for two different commercially available fibres were observed after annealing in carbon powder at temperatures up to 1600 °C. Further improvements in strength were observed for both fibres when heat treated in CO atmospheres. X-ray diffraction, TEM, SEM, auger electron spectroscopy, and optical microscopy were used to characterize the microstructure and chemistry of these heattreated fibres in order to understand better the degradation mechanisms of the fibres as well as their improved strength retention.

Prototype capacitor produced by freeze tape-casting

In previous research freeze casting was used to construct ceramic-polymer composites in which the two phases are arranged in an electrically parallel configuration. By doing so, the composites exhibit dielectric constant (K) up to two orders of magnitude higher than that of composites with ceramic particles randomly dispersed in a polymer matrix. This technique has been successful with both an aqueous and camphene based ceramic slurry that is frozen uni-directionally to form a template such that ceramic aggregates are aligned in the temperature gradient direction. This technique has also been modified to process the slurry at room temperature using a tape caster. This alleviates the need for liquid nitrogen and a freeze-dryer. Transitioning the directional freezing technique to use a tape caster was successful and 20 vol.% BaTiO3 X7R ceramic plates were produced from camphene based tapes fired at 1300 °C for 2hrs. A 10 layer prototype capacitor was produced with a capacitance of 1.4 nF (K ∼ 490) and low loss. This work shows that it is viable to scale the freezing technology up for future high K composite capacitor production.

The Effect of Interfacial Polarization on the Energy Density of Ferroelectric Glass-Ceramics

In the last two years, the US Naval Research Laboratory has been able to synthesize barium strontium titanate (BST)-based glass-ceramics with dielectric breakdown strength as high as 800 kV/cm and dielectric constant up to 1200. Unfortunately, the energy density of the candidate glass-ceramics was only ~1 joule/cc when measured using a discharge measurement circuit. Polarization-electric field measurements revealed wide open hysteresis loops, indicating that most of the electrical energy was not released during discharge. Subsequent experiments showed that the buildup of interfacial polarization was the likely cause in this composite dielectric system. Using the Maxwell-Wagner capacitor model, we were able to quantify the dielectric response of composites based on the permittivities and conductivities of the constituent phases. The response was used to plot polarization-electric field hysteresis for energy density predictions. The results indicated that the aluminosilicate glass phase is the major contributor to the interfacial polarization in this glass-ceramic system.

Casting high-T c superconducting BiSCCO

Bi-Sr-Ca-Cu-O (BiSCCO) was melted and cast into molds in order to render the superconductor into fully dense and useful shapes. Special conditions for casting and subsequent heat treatments were required to ensure structural integrity and a high transition temperature,Tc.The history of the castings during heat treatments was studied by thermal analysis, X-ray diffraction, and microscopy.

Slow crack growth in Si3N4 at room temperature

Tests of Si3N4 hot pressed with various types and levels of oxide additives show evidence of room temperature slow crack growth in delayed failure tests (using natural flaws), but not in fracture mechanics (e.g. DCB or DT) tests consistent with more limited literature data for these two types of tests. Neither type of test showed slow crack growth in either CVD Si3N4 or RSSN. Further the fracture mode in the latter two materials was essentially all transgranular, while it was predominantly (e.g. 80%) intergranular in the hot-pressed materials. It was thus postulated that (1) the oxide grain boundary phase is responsible for slow crack growth and; (2) varying distribution of the oxide boundary phase and grain boundary character result in sufficient boundaries not susceptible to slow crack growth to pin cracks with macroscopic crack lengths (i.e. as in DCB and DT tests). Both the much smaller crack front lengths and the large number of small (natural, e.g. machining) flaws allows some of these small flaws to grow to critical size, thus leading to delayed failure in the hot-pressed materials.

Processing high-density, fine-grained Ba2YCu3O7 superconductors

The techniques for processing Ba2YCu3O7 fine-grained, high-density superconductors are described. Densification and microstructure are found to be strongly dependent on the choice of the barium precursor. The resulting microstructures are characterized using transmission electron microscopy.

Crystallization Kinetics of Barium Strontium Titanate Glass-Ceramics

Barium strontium titanate has been targeted as one potential ferroelectric glass-ceramic for high energy density dielectric materials. Previous testing has shown that the dielectric constant of these materials was as high as 1000 and dielectric breakdown strength up to 800 kV/cm. This did not, however, result in exceptional energy density (~ 0.90 J/cm3). In order to increase overall energy density refining agents can be added to the melt, but the nucleation and growth of the ceramic particles can also play a role. Therefore, in this study the crystallization kinetics were observed to more fully understand how the barium strontium titanate (BST) phase forms so that the optimal energy density could be achieved. It was found that the activation energy was 400 -430 kJ/mol, while the average Avrami parameter was 2.2 -2.5 for BST 70/30 with various additives. The activation energy is close to the disassociation of the Si-O bonds, while crystallization most likely occurs in the bulk with the mechanism of growth being interface controlled.

HP001: Piezoelectric ceramics for explosively-driven high voltage RF pulse generation

Na0.5Bi0.5TiO3 doped with varying levels of BaTiO3 (BT) was synthesized. Remnant polarization, coercive field, d33, permittivity and grain size were measured. Output voltage generated under fast impact was measured. The piezoelectric doped with 4 mol.% BT generated the highest output voltage and was selected for explosive impact testing. Results showed that it generated an output voltage twice that of the standard of PZT-5A..

The Effect of Doping and Composition on the Dielectric Properties and Sintering of a Lead Magnesium Niobate-Lead Titanate-Based Ceramic Dielectric Composite

A series of lead magnesium niobate-lead titanate ((1-x)Pb(Mg 1/3Nb 2/3)O3-xPbTiO3) relaxor ferroelectrics (PMN-PT) are being used in the composite design of a stable high temperature high permittivity ceramic dielectric. However, pure PMN has a dielectric maximum no lower than -10degC. To extend this range to lower temperatures niobium was replaced with tantalum. The tantalum-based component (PMTa-PT) is more refractory and does not sinter as easily as the PMN-PT components of the dielectric composite. The effect of lead oxide and lithium oxide as sintering aids was explored. Both dopants enhanced the sinterability of the tantalum-based component, while the lead oxide reduced its permittivity. At compositions above 0.70PMN-0.30PT, PMN-PT becomes less relaxor-like limiting the design of the stable dielectric to 140degC. To extend this range to higher temperatures a composite approach was taken. A series of four compositions between 0.63 to 0.65 PMN-PT was fabricated. Trilayer composites were made from these compositions with 0.70PMN-0.30PT. By varying the thickness of the composite layers an optimal configuration for the high-temperature component of the composite dielectric was determined which extended the dielectric stable temperature range of the composite dielectric to 185degC.

Dielectric properties of lead barium zirconate titanate-based relaxors for power electronics applications

In this study, we examined the effects of niobium oxide and lanthanum oxide dopants on the dielectric properties of a lead barium zirconate titanate (PBZT) relaxor material. Specifically, our goal was to maximize PBZTs dielectric constant (>6000) while maintaining its stability under DC electric field for filter capacitor applications. Our results showed that the Nb addition did not have a significant effect on the dielectric behavior of PBZT, except the 1% Nb addition which increased the PBZTs dielectric constant by 17%. The cause of the increase is speculated in this article. On the other hand, the La addition caused a monotonic decrease of dielectric constant and a significant shift of the dielectric peaks toward low temperature.