Degradation of structural ceramics by erosion
Materials wastage by solid-particle erosion can be severe and limit lifetimes. This paper will review the theoretical description of solid-particle erosion in brittle materials, which is well-developed for monolithic ceramics. The models can usually account for effects from the principal projectile properties: size, impact velocity, and impact angle. Materials parameters such as fracture toughness and hardness can be included. Steady-state erosion measurements on a wide variety of ceramics, ranging from Si single crystals to SiC-whisker-reinforced A12O3 are reviewed and compared to the models. It is believed thatR-curve behavior and/or particle fragmentation is responsible for discrepancies between the theory and experimental results for composite ceramics. Additionally, the theories make no attempt to describe threshold or incubation effects.
Erosion of a silicon carbide whisker reinforced silicon nitride
The steady-state solid particle erosion behavior of hot-pressed Si/sub 3/N/sub 4/ reinforced with 0, 10, and 20 weight percent SiC whiskers has been investigated at room temperature using angular alumina particles (63 to 270..mu..m diameter) as the erodent. The impact angle was varied from 30 to 90/sup 0/ and particle velocities were varied from 80 to 140 m/s. These materials were found to be very erosion resistant. However, there was little effect of the SiC whisker reinforcement on either the absolute erosion rate or on the velocity or particle-size exponents for the rates. The lack of a fiber reinforcement effect occurs even though the fiber additions provide an increase in the fracture toughness at high fiber contents. Results on differently processed Si/sub 3/N/sub 4/ base materials showed that microstructural variations due to processing history have a very significant influence on the erosion resistance of the base (matrix) material. The results substantiate the idea that microstructure plays an important, but not fully understood, role in the erosion processes for brittle materials.
A bifunctional Cr/Yb/Tm:Ca3Ga2Ge3O12 phosphor with near-infrared long-lasting phosphorescence and upconversion luminescence.
Currently, upconversion nanocrystals and long-lasting phosphorescent particles have attracted extensive research interest for their possible applications as bioimaging probes. However, there are few reports concerning the achievement of both upconversion luminescence of lanthanide ions and long-lasting phosphorescence of transition metal ions in a sole host so far. Herein, we demonstrate a novel calcium gallium germanium garnet (Ca3Ga2Ge3O12) host where lanthanide ions such as Tm(3+)/Yb(3+) and transition metal ions such as Cr(3+) can be easily incorporated through substituting the Ca(2+) and Ga(3+) respectively. This Cr/Yb/Tm:Ca3Ga2Ge3O12 phosphor exhibits both broadband near-infrared long-lasting phosphorescence of Cr(3+) with an afterglow time of more than 7000 s and near-infrared to near-infrared upconversion luminescence of Tm(3+). Impressively, it is evidenced that the addition of Yb(3+)/Tm(3+) into Cr:Ca3Ga2Ge3O12 not only results in Tm(3+) upconversion luminescence but also greatly increases Cr(3+) afterglow time. Based on excitation/emission, three-dimensional thermoluminescence, and time-resolved luminescence spectra, the related long-lasting phosphorescence and upconversion luminescent mechanisms are systematically discussed as well.