Origin of the biphase nature and surface roughness of biogenic calcite secreted by the giant barnacle Austromegabalanus psittacus

The calcite grains forming the wall plates of the giant barnacle Austramegabalanus psittacus have a distinctive surface roughness made of variously sized crystalline nanoprotrusions covered by extremely thin amorphous pellicles. This biphase (crystalline-amorphous) structure also penetrates through the crystal’s interiors, forming a web-like structure. Nanoprotrusions very frequently elongate following directions related to the crystallographic structure of calcite, in particular, the <− 441> directions, which are the strongest periodic bond chains (PBCs) in calcite. We propose that the formation of elongated nanoprotrusions happens during the crystallization of calcite from a precursor amorphous calcium carbonate (ACC). This is because biomolecules integrated within the ACC are expelled from such PBCs due to the force of crystallization, with the consequent formation of uninterrupted crystalline nanorods. Expelled biomolecules accumulate in adjacent regions, thereby stabilizing small pellicle-like volumes of ACC. With growth, such pellicles become occluded within the crystal. In summary, the surface roughness of the biomineral surface reflects the complex shape of the crystallization front, and the biphase structure provides evidence for crystallization from an amorphous precursor. The surface roughness is generally explained as resulting from the attachment of ACC particles to the crystal surface, which later crystallised in concordance with the crystal lattice. If this was the case, the nanoprotrusions do not reflect the size and shape of any precursor particle. Accordingly, the particle attachment model for biomineral formation should seek new evidence.

Study on MgO-Al2O3-TiO2 Inclusion in 409L Stainless Steel

The effects of deoxidization and wire feeding operation on the type of titanium, magnesium and aluminum spinel compound inclusion were investigated in 409L stainless steel production process. Further, the formation mechanism of the compound inclusion was also researched at the same time. The results show that the spherical compound inclusion formed in molten steel presents clearly hierarchy after Ti injection in VOD refining process, where the inclusion size is about 5μm to 10μm. The MgO-Al2O3-TiO2 phase of compound inclusion presents regular configuration. MgO and Al2O3 content in compound inclusion decreases gradually together with CaO and TiO2 content increasing from interior to exterior of inclusion. The biphase structure of MgO·Al2O3-TiO2 will be formed with TiO2 dissolving into MgO·Al2O3. On the condition that Ti content is controlled at 0.25 wt.% together with w(Al)<0.01 wt.%, TiO2 solubility is generally less than 20 wt.% in MgO·Al2O3-yTiO2, as y< 0.44.

Structure and dielectric properties of pyrochlore–fluorite biphase ceramics in the Bi2O3–ZnO–Nb2O5 system

The structure and dielectric properties of Bi2O3–ZnO–Nb2O5-based ceramics with pyrochlore–fluorite biphase structure were investigated. Mixed-sintered ceramics were prepared by two precalcined constituents in the system of x[Bi1.5Zn0.5(Zn0.5Nb1.5)O7]−(1 − x)Bi3/4Nb1/4O7/4 (0.05 ≤ x ≤ 0.35), where Bi1.5Zn0.5(Zn0.5Nb1.5)O7 is a cubic pyrochlore (α) and Bi3/4Nb1/4O7/4 is a defect cubic fluorite (F). The phase composition of the mixed-sintered ceramics were characterized as a biphasic structure with a distorted pyrochlore (β) and a fluorite (F) coexisting. The phase ratio of β pyrochlore and F fluorite is related to the chemical composition x. The dielectric properties of the ceramics gradually change with their structure.