The effects of using different sintering techniques (conventional, flash and spark plasma sintering) on grain boundary segregation were investigated in a 3-phase polycrystalline ceramic containing cubic 8 mol% Y2O3 stabilized ZrO2 (YSZ), α-Al2O3 and MgAl2O4. Six types of interfaces for each sintered sample were analyzed for grain boundary chemistry. Using aberration-corrected STEM and EDS, we show Al segregation at YSZ-YSZ boundaries, and Y/Zr segregation at Al2O3-Al2O3, MgAl2O4-MgAl2O4 and MgAl2O4-Al2O3 boundaries. YSZ-MgAl2O4 and YSZ-Al2O3 heterointerfaces, in contrast, do not show elemental segregation. Our results show that the type of segregation at different boundaries does not change with different sintering processes, indicating that elemental segregation mainly depends on initial sample composition, although the amount of segregation can vary. Quantitative analyses reveal that spark plasma sintering (950°C for 5 min, 100 MPa) results in relatively higher average segregation at grain boundaries and heterointerfaces compared to conventional sintering (1550°C for 10 h), suggesting that low temperatures result in higher grain boundary segregation. The average segregation concentrations at the grain boundaries of our flash sintered sample (estimated to reach 1700°C for 6 s) were found to be consistently similar to long-annealed spark-plasma sintered sample (1350°C for 20 h), suggesting that very high temperatures reached during flash process can achieve similar segregation concentrations compared to a SPS sample annealed for several hours at lower temperature. The presence of other phases in multi-phase systems can modify the grain boundary chemistry and segregation compared to segregation in single-phase ceramics.

Radiation damage tolerance for a variety of ceramics at high temperatures depends on the material's resistance to nucleation and growth of extended defects. Such processes are prevalent in ceramics employed for space, nuclear fission/fusion and nuclear waste environments. This report shows that random heterointerfaces in materials with sub-micron grains can act as highly efficient sinks for point defects compared to grain boundaries in single-phase materials. The concentration of dislocation loops in a radiation damage-prone phase (Al₂O₃) is significantly reduced when Al₂O₃ is a component of a composite system as opposed to a single-phase system. These results present a novel method for designing exceptionally radiation damage tolerant ceramics at high temperatures with a stable grain size, without requiring extensive interfacial engineering or production of nanocrystalline materials.

We report on the electrical properties of 2.0 eV bandgap (EG) CuInGaS2 (CIGS) solar absorbers integrated on SnO2:F (FTO) substrates and interfaced with CdS buffer layers for multijunction solar cells and photoelectrochemical water splitting devices. The averaged short-circuit photocurrent density measured on nine ITO/ZnO/CdS/CIGS/FTO cells was 10.0 mA cm-2, a value corresponding to 70% of the optical limit for a 2.0 eV-bandgap absorber. However, the averaged power conversion efficiency was low (avg.: 2.4%) and for the most part limited by modest open circuit voltage values (avg.: 587 mV). Solid-state analyses performed at low temperatures revealed poor energetic alignment at the CdS/CIGS top interface. An activation energy (1.1 eV) for the dominant recombination mechanism significantly lower than the CIGS bandgap was measured, implying that recombination takes place near the CdS/CIGS interface. This finding is supported by the large difference found between the quasi Fermi level splitting of the bare absorber (1.17 eV) and the open circuit voltage of the device. Complementary theoretical calculations identified Fermi level pinning as a possible cause for the poor interface energetics through Cd incorporation on both the Cu and the group-III sites, resulting in a large conduction band offset (∼0.9 eV) at the CdS/CIGS interface. This work underlines the need for new EG-tunable buffers coupled with optimized CIGS surface energetics (e.g. ordered vacancy compounds) for future chalcopyrite-based multijunction solar cells and photoelectrochemical water splitting devices. This journal is

Amorphous silicates in meteoritic samples are significant as potential surviving interstellar dust from the formation of the Solar System. Amorphous silicate-rich grains called GEMS, glass with embedded metal and sulfides, are abundant in anhydrous interplanetary dust particles and some ultracarbonaceous Antarctic micrometeorites that have high presolar grain abundances. Some GEMS within these objects have been confirmed to be presolar. GEMS-like material, consisting of amorphous silicate with opaque inclusions, has been identified in the matrices of some primitive meteorites. We use specialized thin specimen preparation and transmission electron microscopy methods to compare the GEMS-like material in Paris (CM) and Acfer 094 (ungrouped) chondrites with GEMS in anhydrous, chondritic interplanetary dust particles. Specifically, we compared the amorphous silicate morphology, degree of partial ordering, elemental composition and dominant iron oxidation state. We find that the amorphous silicates in the GEMS-like materials in Paris and Acfer 094 show incipient ordering and are Fe-rich and highly oxidized, while those in GEMS are fully amorphous, Fe-poor, and anhydrous. From examination of various formation routes for the amorphous silicates, we conclude that the GEMS-like material in Paris and Acfer 094 is not GEMS and that it is unlikely that GEMS-like material accreted as primary amorphous material that retained its original amorphous structure since accretion. We also conclude that the presence of GEMS-like amorphous silicates in chondrite matrix is not a reliable indicator for high degrees of primitivity or pristinity since even mild, complex alteration processes may significantly alter silicate structure. Finally, we propose searches for presolar amorphous silicates in chondrite matrix samples that have been precharacterized by transmission electron microscopy methods to enable direct comparisons of presolar amorphous silicate abundances.