Strengthening of Oxide Ceramics by Transformation-Induced Stress

Alumina-zirconia composites were fabricated by isostatic pressing and sintering of powder mixtures in such a way that bar-shaped specimens consisted of three layers. The outer layers contained A1₂O₃ and unstabilized ZrO₂ while the central layer contained A1₂O₃ and partially stabilized ZrO₂ (with 2 mol% Y₂O₃). When cooled from the sintering temperature, some of the zirconia in the outer layers transformed to the monoclinic form while zirconia in the central layer was retained in the tetragonal form. The transformation of zirconia in the outer layers led to the establishment of surface compressive stresses and balancing tensile stresses in the bulk. The existence of surface compressive stresses was verified by a strain gauge technique and bending strength measurements on samples with varying thickness of the outer layers. The layered composites exhibited greater strength in comparison with monolithic Al₂O₃-ZrO₂ specimens. Further, variation of strength in bending with outer layer thickness (for a fixed total thickness) indicated that failure occurred from internal flaws. Scanning electron microscopy of fracture surfaces revealed that strength-limiting flaws were voids located in the central layer near the interface separating the central and the outer layers.     

Spinodal Decomposition in Ionic Compounds

Spinodal decomposition in ionic solid solutions of the form AX-BX where A and B are cations and X is the anion is examined along similar lines as proposed by Cahn. Assuming anions to be immobile, electrostatic energy arising due to the differences in cationic mobilities is determined. This electrostatic energy is shown to retard the kinetics of spinodal decomposition. The role of aliovalent dopants on the kinetics of decomposition is also examined. Subsequently, the probable role of aluminum on spinodal decomposition in the system TiO₂-SnO₂ is assessed.     

Fracture Properties of Polycrystalline Lithia-Stabilized β"- Alumina

The critical stress-intensity factor, K_1C , and the fracture strength, σ _f , have been investigated on both hot-pressed and sintered lithia-stabilized β "-alumina. The hot-pressed material possessed a strong preferred orientation with many of the basal planes aligned perpendicular to the direction of hot-pressing. Both K_1C and σ _f were found to be orientation-dependent. Two regimes of fracture were identified. In fine-grained material (<120 μm), the strength was slightly dependent on grain size.
For larger grain sizes, the strength decreased rapidly with increasing grain size and the fracture mode was almost entirely transgranular. The K_1C values for sintered β "-alumina were in the same range as those obtained on the hot-pressed material.     

Effect of geological medium on seismic signals from underground nuclear explosion events – A case study for Baneberry site

Seismic signals due to any underground nuclear explosion events are known to be influenced by the local geology of the test site and the yield level. In this paper, transient three-dimensional finite element code SHOCK-3D developed for the simulation of underground nuclear explosion events has been used to obtain synthetic acceleration signals for Baneberry site (Nevada) single and composite rock media. At this site an underground nuclear test of 10 kT conducted on 18th December 1970 at source depth of 278 m resulted into venting as reported by Terhune et al with 2D simulation results and later by us through 3D simulation in Ranjan et al. First, the reasons of the venting for this event are summarized. After the successful validation of the 3D numerical model for Baneberry site rock media, parametric studies are carried out for 1 and 8 kT yields at 100 m depth (Scaled Depths of Burst SDOB ～ 100 and 50 m/kT^1/3, respectively) for homogeneous and composite Paleozoic and Tuff media of Baneberry site. It is demonstrated that the near source local geological formations and associated nonlinear effects significantly influence the seismic signals. With this study the seismic decoupling of the source by an order of magnitude has been illustrated. Finally, it is concluded that the seismic signals alone, in the absence of in-depth information of the local geology of the specific test site, are not appropriate measures of the source strength.     

Dental pulp stem cells and banking of teeth as a lifesaving therapeutic vista

Exfoliated deciduous or an extracted healthy adult tooth can be used to harvest, process, and cryogenically preserve dental pulp stem cells. Future stem cell-based regenerative medicine methods could benefit significantly from these mesenchymal stem cells. Teeth serve as a substantial source of mesenchymal stem cells, otherwise disposed of as medical waste. Care should be taken to store this treasure trove of stem cells. Collective responsibility of patients, dentists, and physicians is necessary to ensure that this valuable resource is not wasted and that every possible dental pulp stem cell is available for use in the future. The dental pulp stem cells (DPSC) inside teeth represent a significant future source of stem cells for regenerative medicine procedures. This review describes the ontogeny, the laboratory processing and collection, and isolation methods of DPSC. This review also discusses currently available stem cell banking facilities and their potential use in regenerative medicine procedures in dental and general medical applications in the future.