Crystallization of Eutectic Microstructures from Oxide and Carbide Melts

The theoretical aspect of the directional solidification :is briefly described. Two different experimental techniques were used for the production of directionally solidified high refractory oxide and carbide eutectics. Microstructure, crystallography, mechanical properties and mechanism of failure were investigated. Some of the inves·tLgated eutectics retain considerable strength at elevated temperature, whereas the majority of alloys and sintered oxide bodies show rapid loss in strength. Eutectic in the system B4C-SiC shows excepti.onally high wear r~sisfance

Crystallization of Eutectic Microstructures from Oxide and Carbide Melts

The theoretical aspect of the directional solidification :is briefly described. Two different experimental techniques were used for the production of directionally solidified high refractory oxide and carbide eutectics. Microstructure, crystallography, mechanical properties and mechanism of failure were investigated. Some of the inves·tLgated eutectics retain considerable strength at elevated temperature, whereas the majority of alloys and sintered oxide bodies show rapid loss in strength. Eutectic in the system B4C-SiC shows excepti.onally high wear r~sisfance

Relative energetics and structural properties of zirconia using a self-consistent tight-binding model

We describe an empirical, self-consistent, orthogonal tight-binding model for zirconia, which allows for the polarizability of the anions at dipole and quadrupole levels and for crystal field splitting of the cation d orbitals. This is achieved by mixing the orbitals of different symmetry on a site with coupling coefficients driven by the Coulomb potentials up to octapole level. The additional forces on atoms due to the self-consistency and polarizabilities are exactly obtained by straightforward electrostatics, by analogy with the Hellmann-Feynman theorem as applied in first-principles calculations. The model correctly orders the zero temperature energies of all zirconia polymorphs. The Zr-O matrix elements of the Hamiltonian, which measure covalency, make a greater contribution than the polarizability to the energy differences between phases. Results for elastic constants of the cubic and tetragonal phases and phonon frequencies of the cubic phase are also presented and compared with some experimental data and first-principles calculations. We suggest that the model will be useful for studying finite temperature effects by means of molecular dynamics.Comment: to be published in Physical Review B (1 march 2000

Boron and Lithium in Calcium Sulfate Veins: Tracking Precipitation of Diagenetic Materials in Vera Rubin Ridge, Gale Crater

The NASA Curiosity rover’s ChemCam instrument suite has detected boron in calcium‐sulfate‐filled fractures throughout the sedimentary strata of Gale crater including Vera Rubin ridge (VRR). The presence of elevated B concentration provides insights into Martian subsurface aqueous processes. In this study we extend the dataset of B in Ca‐sulfate veins across Gale crater, comparing the detection frequency and relative abundances with Li. We report 33 new detections of B within veins analyzed between sols 1548 and 2311 where detections increase in Pettegrove Point and Jura members, which form VRR. The presence of B and Li in the Ca‐sulfate veins is possibly due to dissolution of pre‐existing B in clays of the bedrock by acids or neutral water and redistribution of the elements into the veins. Elevated frequency of B detection in veins of Gale crater correlate with presence of dehydration features such as desiccation cracks, altered clay minerals and detections of evaporites such as Mg‐sulfates, chloride salts in the host rocks. The increased observations of B also coincide with decreased Li concentration in the veins (average Li concentration of veins drops by ~15 ppm). Boron and Li have varying solubilities and Li does not form salts as readily upon dehydration as B, causing it to remain in the solution. So, the weak negative correlation between B and Li may reflect the crystallization sequence during dehydration on Vera Rubin ridge

Effect of Dopants on Zirconia Stabilization—An X-ray Absorption Study: III, Charge-Compensating Dopants

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66100/1/j.1151-2916.1994.tb05404.x.pd

Contribution à l'étude de la structure des halloysites et essais de synthèse

On a pu obtenir des particules d'halloysite par précipitation à partir d'un mélange d'aluminate et silicate de soude, en proportions convenables. La formation du précipité est favorisée par l'introduction d'une faible quantité d'halloysite naturelle. Le corps obtenu est étudié par l'analyse thermique différentielle et contrôlé au microscope électronique. Les résultats sont discutés.Karsulin M., Stubican V. Contribution à l'étude de la structure des halloysites et essais de synthèse. In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 76, 10-12, 1953. pp. 434-437