Publications

The composition and phase fraction of the intergranular phase of 94ND10 ceramic is determined and fabricated ex situ. The fraction of each phase is 85.96 vol% Al₂O₃ bulk phase, 9.46 vol% Mg-rich intergranular phase, 4.36 vol% Ca/Si-rich intergranular phase, and 0.22 vol% voids. The Ca/Si-rich phase consists of 0.628 at% Mg, 12.59 at% Si, 10.24 at% Ca, 17.23 at% Al, and balance O. The Mgrich phase consists of 14.17 at% Mg, 0.066 at% Si, 0.047 at% Ca, 28.69 at% Al, and balance O. XRD of the ex situ intergranular material made by mixed oxides consisting of the above phase and element fractions yielded 92 vol% MgAl₂O₄ phase and 8 vol% CaAl₂Si₂O₈ phase. The formation of MgAl₂O₄ phase is consistent with prior XRD of 94ND10, while the CaAl₂Si₂O₈ phase may exist in 94ND10 but at a concentration not readily detected with XRD. The MgAl₂O₄ and CaAl₂Si₂O₈ phases determined from XRD are expected to have the elemental compositions for the Mg-rich and Ca/Si-rich phases above by cation substitutions (e.g., some Mg substituted for by Ca in the Mg-rich phase) and impurity phases not detectable with XRD.

Negative and zero coefficient of thermal expansion (CTE) materials are of interest for developing polymer composites in electronic circuits that match the expansion of Si and in zero CTE supports for optical components, e.g., mirrors. In this work, the processing challenges and stability of ZrW₂O₈, HfW₂O₈, HfMgW₃O₁₂, Al(HfMg)_0.5W₃O₁₂, and Al_0.5Sc_1.5W₃O₁₂ negative and zero thermal expansion coefficient ceramics are discussed. Al_0.5Sc_1.5W₃O₁₂ is demonstrated to be a relatively simple oxide to fabricate in large quantity and is shown to exhibit single phase up to 1300 °C in air and inert N₂ environments. The negative and zero CTE behavior was confirmed with dilatometry. Thermal conductivity and heat capacity were reported for the first time for HfMgW₃O₁₂ and Al_0.5Sc_1.5W₃O₁₂ and thermal conductivity was found to be very low (~0.5 W/mK). Grüneisen parameter is also estimated. Methods for integration of Al_0.5Sc_1.5W₃O₁₂ with other materials was examined and embedding 50 vol% of the ceramic powder in flexible epoxy was demonstrated with a commercial vendor.

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In this project, ceramic encapsulation materials were studied for high temperature (>~°500 C) applications where typical polymer encapsulants are unstable. A new low temperature (<~°200 C) method of processing ceramics, the cold sintering process was examined. Additionally, commercially available high temperature ceramic cements were investigated. In both cases, the mechanical strengths of available materials are less than desired (i.e., desired strengths similar to Si₃N₄), limiting applicability. Composite designs to increase mechanical strength are suggested. Additionally, non-uniformities in stresses and densification while embedding alumina sheets in encapsulants via cold sintering using uni-axial pressing led to fracture of sheets, and an alternative iso-static based approach is recommended for future studies.

Electrical conduction in silica-based capacitors under a combined effect of intermediate electric field and temperature (2.5 - 10 kV/mm, 50-300°C) is dominated by localized motion of high mobility ions such as sodium. Thermally stimulated polarization and depolarization current (TSPC/TSDC) characterization was carried out on poled fused silica and AF32 glass samples. Two relaxation mechanisms were found during the depolarization step and an anomalous response for the second TSDC peak was observed. Absorption current measurements were performed on the glass samples and a time-dependent response was observed when subjected to different electro-thermal conditions. It was found that at low temperature (T = 175 °C) and short times, the current follows a linear behavior (I α V) while at high temperature (T = 250 °C), the current follows V0.5. TSPC/TSDC and absorption current measurements results led to the conclusion that (1) Poole-Frenkel dominates conduction at high temperatures and at longer times and that (2) ionic blockage and/or H+/H3O+ injection are responsible for the observed anomalous current response.