Thermophysical properties of nominally phase pure boride ceramics

This research focusses on the thermophysical properties of nominally phase pure boride ceramics. As interest in ultra high temperature ceramics increases due to a renewed interest in hypersonic flight vehicles and with the expanding materials design space accompanying interest in high entropy materials, it is imperative to understand the intrinsic properties of boride ceramics. By reducing Hf content in ZrB2 from the natural abundance, ~1.75 at% in this case, to ~100 ppm, thermal conductivity increased from 88 W/m·K to 141 W/m·K. Removal of Hf allowed the thermal conductivity of ZrB2 with small transition metal solute additions to be measured without being masked by Hf impurity effects. Additions of Ti and Y reduced thermal conductivity by 20% and 30% respectively. The melting temperatures of two different types of ZrB2 were also studied. A commercially available grade of ZrB2 (~ 1.75 at% Hf) had a melting temperature of 3280°C while a low Hf (100 ppm) ZrB2 had a melting temperature of 3273°C. The kinetics of the final stage of densification was also studied for nominally phase pure ZrB2. Dislocation motion with an activation energy of 162 kJ/mol was determined to be the dominant mechanism in the absence of competing mechanisms such as grain pinning or solute drag caused by secondary phases and impurity soute atoms. The effect of configurational entropy on the solubility of yttrium in high entropy borides was investigated. No significant difference in yttrium solubility was found between nominally pure ZrB2 and a four component high entropy boride (Ti,Zr,Nb,Hf)B2. Mitigation of impurity atoms and secondary phases minimized extrinsic effect and elucidated intrinsic properties of boride ceramics --Abstract, page iv

Effects of transition metals on thermal properties of ZrB2

Nominally phase pure zirconium diboride ceramics were synthesized to study their intrinsic thermal properties. Ceramics for this study were synthesized by reaction hot pressing of reactor grade ZrH2 and B to minimize impurities commonly found in commercial powders such as the natural abundance (1-4 wt%) of Hf. Starting powders contained \u3c200 ppm Hf. Previous results showed that Hf impurities present in quantities comparable to commercial powders masked the effect of other transition metal additions. For example, additions of 3 at% Ti and Y had no apparent effect on thermal conductivity of ceramics produced from commercial ZrB2. Lowering the Hf content to 0.4 at% increased thermal conductivity from ~90 W/m•K for ZrB2 ceramics prepared from commercial powders to ~100 W/m•K for low-Hf content ZrB2 at 25 °C. Lowering the Hf content also increased the thermal conductivity at 2000°C from ~70 W/m•K to ~80 W/m•K. For the low Hf ZrB2, adding 3 at% TiB2 decreased thermal conductivity ~15 W/m•K at 25°C while adding 3 at% MoB2 decreased thermal conductivity ~45 W/m•K at 25°C. For the present study, transition metals such as Hf, Ti, Y, Ta, and W were added individually to nominally phase pure ZrB2 to study the effects on thermal diffusivity, thermal conductivity and heat capacity at temperatures from 25°C to 2000°C. These properties will be compared to values obtained for ceramics prepared from commercial ZrB2 powders, which contained the natural abundance of Hf. Most previous reports have relied on heat capacity values from the NIST-JANAF thermodynamic tables to calculate thermal conductivity of ZrB2 ceramics. However, the heat capacity of ZrB2 with low Hf content was approximately 10% greater than widely accepted values. Due to this difference, heat capacity will be measured for each composition, and these values will be used to calculate thermal conductivity. The intrinsic thermal properties of ZrB2will be discussed as well as the effect of transition metal additions on the thermal properties of ZrB2 with low and naturally abundant quantities of Hf

Measurement of the Melting Temperature of ZrB₂ as Determined by Laser Heating and Spectrometric Analysis

The melting temperatures of two different ZrB2 ceramics were studied using laser induced melting. ZrB2 having a low Hf content, produced by reaction hot pressing, had a melting temperature of 3546 K and a commercial grade ZrB2 had a melting temperature of 3553 K. Uncertainty of the temperature measurements was 1% of the absolute temperature, or ~35 K for both materials based upon 2-sigma and a 95% confidence interval. While these values were consistent with the previously reported ZrB2 melting temperature of 3518 K, this study was able to measure Tm with less uncertainty than previous studies (± 45 K). Furthermore, this study assessed the effect of Hf content on melting temperature, finding that melting temperature did not change significantly for hafnium contents of 1.75 to 0.01 at%. This study also measured a normal spectral emissivity of 0.34 for ZrB2 at 3000 K. The emissivity decreased to 0.28 at the melting temperature, then, stabilized at 0.30 in a liquid phase

Densification Behavior of ZrB₂-MoSi₂ Ceramics: The Formation and Evolution of Core-Shell Solid Solution Structures

The role of solid solution shells in the densification behavior of ZrB2-MoSi2 ceramics was analyzed for varying ZrB2 starting particle sizes and MoSi2 contents. The formation of core-shell structures in hot-pressed ceramics in the ZrB2-MoSi2 system was confirmed by SEM, TEM and X-ray diffraction analysis. Microstructure analysis established that each ZrB2 core and its (Zr,Mo)B2 solid solution shell were isostructural with no detectable crystallographic misfit. Two single phase (Zr1-XMoX)B2 solid solutions were synthesized by reactive hot pressing to confirm that ZrB2-MoB2 solid solutions obey a quasi-linear Vegard\u27s law. Additional (Si-free) ZrB2-Mo compositions, ZrB2/Mo/ZrB2 sandwich structures, or ZrB2-MoSi2 ceramics (quenched to room temperature) were hot pressed to study a transient liquid phase and its interdependency on ZrB2 powder purity, connectivity of pathways for mass transfer, wettability, or solubility as a function of applied pressure. The solid solution shells were the result of mass transfer by competing solid state mechanisms of surface and grain boundary diffusion during sintering that may have been boosted by a fugitive Si-based transient liquid phase. Mo was incorporated in diffusion-deposited diboride material at particle-particle necks. The volume fraction of solid solution shells and their Mo contents were affected by processing temperatures. In addition, plastic deformation of MoSi2 filled some closed porosity to aid the ceramics in achieving near full density

Final‐stage densification kinetics of direct current–sintered ZrB 2

Final-stage sintering was analyzed for nominally phase pure zirconium diboride synthesized by borothermal reduction of high-purity ZrO2. Analysis was conducted on ZrB2 ceramics with relative densities greater than 90% using the Nabarro–Herring stress–directed vacancy diffusion model. Temperatures of 1900°C or above and an applied uniaxial pressure of 50 MPa were required to fully densify ZrB2 ceramics by direct current sintering. Ram travel data were collected and used to determine the relative density of the specimens during sintering. Specimens sintered between 1900 and 2100°C achieved relative densities greater than 97%, whereas specimens sintered below 1900°C failed to reach the final stage of sintering. The average grain size ranged from 1.0 to 14.7 μm. The activation energy was calculated from the slope of an Arrhenius plot that used the Kalish equation. The activation energy was 162 ± 34 kJ/mol, which is consistent with the activation energy for dislocation movement in ZrB2. The diffusion coefficients for dislocation motion that controls densification were 5.1 × 10−6 cm2/s at 1900°C and 5.1 × 10−5 cm2/s at 2100°C, as calculated from activation energy and average grain sizes. This study provides evidence that the dominant mechanism for final-stage sintering of ZrB2 ceramics is dislocation motion

Galanin transgenic mice display cognitive and neurochemical deficits characteristic of Alzheimer's disease

Galanin is a neuropeptide with multiple inhibitory actions on neurotransmission and memory. In Alzheimer's disease (AD), increased galanin-containing fibers hyperinnervate cholinergic neurons within the basal forebrain in association with a decline in cognition. We generated transgenic mice (GAL-tg) that overexpress galanin under the control of the dopamine β-hydroxylase promoter to study the neurochemical and behavioral sequelae of a mouse model of galanin overexpression in AD. Overexpression of galanin was associated with a reduction in the number of identifiable neurons producing acetylcholine in the horizontal limb of the diagonal band. Behavioral phenotyping indicated that GAL-tgs displayed normal general health and sensory and motor abilities; however, GAL-tg mice showed selective performance deficits on the Morris spatial navigational task and the social transmission of food preference olfactory memory test. These results suggest that elevated expression of galanin contributes to the neurochemical and cognitive impairments characteristic of AD

Engineered immunogens to elicit antibodies against conserved coronavirus epitopes

Abstract Immune responses to SARS-CoV-2 primarily target the receptor binding domain of the spike protein, which continually mutates to escape acquired immunity. Other regions in the spike S2 subunit, such as the stem helix and the segment encompassing residues 815-823 adjacent to the fusion peptide, are highly conserved across sarbecoviruses and are recognized by broadly reactive antibodies, providing hope that vaccines targeting these epitopes could offer protection against both current and emergent viruses. Here we employ computational modeling to design scaffolded immunogens that display the spike 815-823 peptide and the stem helix epitopes without the distracting and immunodominant receptor binding domain. These engineered proteins bind with high affinity and specificity to the mature and germline versions of previously identified broadly protective human antibodies. Epitope scaffolds interact with both sera and isolated monoclonal antibodies with broadly reactivity from individuals with pre-existing SARS-CoV-2 immunity. When used as immunogens, epitope scaffolds elicit sera with broad betacoronavirus reactivity and protect as “boosts” against live virus challenge in mice, illustrating their potential as components of a future pancoronavirus vaccine