Doped Nanostructured Manganese Ferrites: Synthesis, Characterization, and Magnetic Properties

Nanocrystalline aluminum-doped manganese ferrite was synthesized by facile thermal treatment method. Nanostructure-doped ferrite with crystalline size that ranged between 3.71 and 6.35 nm was characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and vibrating-sample magnetometry (VSM). The Scherrer and Williamson-Hall hypothesis techniques were utilized to determine lattice constants and strain. Various types of structural properties including octahedral and tetrahedral site radius, bond lengths and angles, hopping parameter, oxygen positional parameters, site bonds, and edge lengths were determined from XRD spectrum analysis. Discrepancy in the hypothetically expected angle indicates improvement of A-B superexchange intercommunication. Furthermore, magnetic-hysteresis (M-H) and XPS analysis support the claim of enhancement. The presence of the ionic nature of iron and manganese in ferrite is FeII, FeIII, MnII, and MnIV as revealed by the results of XPS. Moreover, XPS assists in an excellent way to understand the properties such as configuration, chemical nature, and average inversion degree of doped ferrite samples. The spin noncollinearity and exquisite interaction amid the sublattice are responsible for the decrease in the saturation and remnant magnetization determined from the hysteresis loop at ambient temperature with maximum magnetic field of 1.8 T.

Processing and Thermal Conductivity of Bulk Nanocrystalline Aluminum Nitride

Producing bulk AlN with grain sizes in the nano regime and measuring its thermal conductivity is an important milestone in the development of materials for high energy optical applications. We present the synthesis and subsequent densification of nano-AlN powder to produce bulk nanocrystalline AlN. The nanopowder is synthesized by converting transition alumina (δ-Al2O3) with <40 nm grain size to AlN using a carbon free reduction/nitridation process. We consolidated the nano-AlN powder using current activated pressure assisted densification (CAPAD) and achieved a relative density of 98% at 1300 °C with average grain size, d¯~125 nm. By contrast, high quality commercially available AlN powder yields densities ~75% under the same CAPAD conditions. We used the 3-ω method to measure the thermal conductivity, κ of two nanocrystalline samples, 91% dense, d¯ = 110 nm and 99% dense, d¯ = 220 nm, respectively. The dense sample with 220 nm grains has a measured κ = 43 W/(m·K) at room temperature, which is relatively high for a nanocrystalline ceramic, but still low compared to single crystal and large grain sized polycrystalline AlN which can exceed 300 W/(m·K). The reduction in κ in both samples is understood as a combination of grain boundary scattering and porosity effects. We believe that these are finest d¯ reported in bulk dense AlN and is the first report of thermal conductivity for AlN with ≤220 nm grain size. The obtained κ values are higher than the vast majority of conventional optical materials, demonstrating the advantage of AlN for high-energy optical applications.

Size Effects in Internal Friction of Nanocrystalline Aluminum Films

Al thin film is extensively used in micro-electromechanical systems (MEMS) and electronic interconnections; however, most previous research has concentrated on their quasi-static properties and applied their designs on larger scales. The present study designed a paddle-like cantilever specimen with metal films deposited on the upper surface to investigate the quasi-static properties of Al thin film at room temperature under high vacuum conditions at microscopic scales. Energy loss was determined using a decay technique in the oscillation amplitude of a vibrating structure following resonant excitation. Grain size and film thickness size were strictly controlled considering the quasi-static properties of the films. This study found that the internal friction of ultra-thin and thin Al films was more dependent on the grain boundaries than film thickness.