Synthesis of tetragonal and orthorhombic polymorphs of Hf3N4 by high-pressure annealing of a prestructured nanocrystalline precursor.

Hf3N4 in nanocrystalline form is produced by solution phase reaction of Hf(NEtMe)4 with ammonia followed by low-temperature pyrolysis in ammonia. Understanding of phase behavior in these systems is important because early transition-metal nitrides with the metal in maximum oxidation state are potential visible light photocatalysts. A combination of synchrotron powder X-ray diffraction and pair distribution function studies has been used to show this phase to have a tetragonally distorted fluorite structure with 1/3 vacancies on the anion sites. Laser heating nanocrystalline Hf3N4 at 12 GPa and 1500 K in a diamond anvil cell results in its crystallization with the same structure type, an interesting example of prestructuring of the phase during preparation of the precursor compound. This metastable pathway could provide a route to other new polymorphs of metal nitrides and to nitrogen-rich phases where they do not currently exist. Importantly it leads to bulk formation of the material rather than surface conversion as often occurs in elemental combination reactions at high pressure. Laser heating at 2000 K at a higher pressure of 19 GPa results in a further new polymorph of Hf3N4 that adopts an anion deficient cottunite-type (orthorhombic) structure. The orthorhombic Hf3N4 phase is recoverable to ambient pressure and the tetragonal phase is at least partially recoverable.

Templated non-oxide sol-gel preparation of well-ordered macroporous (inverse opal) Ta3N5 films.

Reactions of Ta(NMe2)5 and n-propylamine are shown to be an effective system for sol-gel processing of Ta3N5. Ordered macroporous films of Ta3N5 on silica substrates have been prepared by infiltration of such a sol into close-packed sacrificial templates of cross-linked 500 nm polystyrene spheres followed by pyrolysis under ammonia to remove the template and crystallize the Ta3N5. Templates with long-range order were produced by controlled humidity evaporation. Pyrolysis of a sol-infiltrated template at 600 °C removes the polystyrene but does not crystallize Ta3N5, and X-ray diffraction shows nanocrystalline TaN plus amorphous material. Heating at 700 °C crystallizes Ta3N5 while retaining a high degree of pore ordering, whereas at 800 °C porous films with a complete loss of order are obtained.

The effect of image enhancements and dual observers on proximal caries detection.

OBJECTIVES: The aim of this study was to determine if the use of certain image enhancements and dual observers had an effect on the detection of caries, dentin extension, and cavitation. STUDY DESIGN: Seven observers viewed unenhanced and enhanced images taken on photostimulable phosphor plates (PSP) and Schick 33 sensors and were asked to determine whether proximal caries lesions, dentin extension, and cavitation were present. Pairs of observers also evaluated the unenhanced PSP images and recorded their confidence. Micro-computed tomography was used as the gold standard. RESULTS: For caries lesion detection, PSP outperformed Schick sensors, although the differences are most likely not clinically significant. Observers (single and dual) and filters had no effect on any of the diagnostic tasks. Schick sensors and unfiltered images were more specific for dentin extension. CONCLUSIONS: Caries detection was statistically greater with the PSP plate, but both detectors allowed for high accuracy. Expensive software or time-consuming consultations did not improve outcomes.