Effect of Residual Oxygen Concentration on the Lattice Parameters of Aluminum Nitride Powder Prepared via Carbothermal Reduction Nitridation Reaction.

Residual oxygen in wurtzite-type aluminum nitride (AlN) crystal, which significantly affects phonon transport and crystal growth, is crucial to thermal conductivity and the crystal quality of AlN ceramics. In this study, the effect of residual oxygen on the lattice of AlN was examined for as-synthesized and sintered samples. By controlling reaction time in the carbothermal reduction nitridation (CRN) procedure, AlN powder was successfully synthesized, and the amount of residual oxygen was systematically controlled. The evolution of lattice parameters of AlN with respect to oxygen conc. was carefully investigated via X-ray diffraction analysis. With increasing amounts of residual oxygen in the as-synthesized AlN, lattice expansion in the ab plane was induced without a significant change in the c-axis lattice parameter. The lattice expansion in the ab plane owing to the residual oxygen was also confirmed with high-resolution transmission electron microscopy, in contrast to the invariant lattice parameter of the sintered AlN phase. Micro-strain values from XRD peak broadening confirm that stress, induced by residual oxygen, expands the AlN lattice. In this work, the lattice expansion of AlN with increasing residual oxygen was elucidated via X-ray diffraction and HR-TEM, which is useful to estimate and control the lattice oxygen in AlN ceramics.

Direct Evidence on Effect of Oxygen Dissolution on Thermal and Electrical Conductivity of AlN Ceramics Using Al Solid-State NMR Analysis.

Aluminum nitride, with its high thermal conductivity and insulating properties, is a promising candidate as a thermal dissipation material in optoelectronics and high-power logic devices. In this work, we have shown that the thermal conductivity and electrical resistivity of AlN ceramics are primarily governed by ionic defects created by oxygen dissolved in AlN grains, which are directly probed using 27Al NMR spectroscopy. We find that a 4-coordinated AlN3O defect (ON) in the AlN lattice is changed to intermediate AlNO3, and further to 6-coordinated AlO6 with decreasing oxygen concentration. As the aluminum vacancy (VAl) defect, which is detrimental to thermal conductivity, is removed, the overall thermal conductivity is improved from 120 to 160 W/mK because of the relatively minor effect of the AlO6 defect on thermal conductivity. With the same total oxygen content, as the AlN3O defect concentration decreases, thermal conductivity increases. The electrical resistivity of our AlN ceramics also increases with the removal of oxygen because the major ionic carrier is VAl. Our results show that to enhance the thermal conductivity and electrical resistivity of AlN ceramics, the dissolved oxygen in AlN grains should be removed first. This understanding of the local structure of Al-related defects enables us to design new thermal dissipation materials.

Bulk Metamaterials Exhibiting Chemically Tunable Hyperbolic Responses.

Extraordinary properties of traditional hyperbolic metamaterials, not found in nature, arise from their man-made subwavelength structures causing unique light-matter interactions. However, their preparation requiring nanofabrication processes is highly challenging and merely provides nanoscale two-dimensional structures. Stabilizing their bulk forms via scalable procedures has been a sought-goal for broad applications of this technology. Herein, we report a new strategy of designing and realizing bulk metamaterials with finely tunable hyperbolic responses. We develop a facile two-step process: (1) self-assembly to obtain heterostructured nanohybrids of building blocks and (2) consolidation to convert nanohybrid powders to dense bulk pellets. Our samples have centimeter-scale dimensions typically, readily further scalable. Importantly, the thickness of building blocks and their relative concentration in bulk materials serve as a delicate means of controlling hyperbolic responses. The resulting new bulk heterostructured material system consists of the alternating h-BN and graphite/graphene nanolayers and exhibits significant modulation in both type-I and type-II hyperbolic resonance modes. It is the first example of real bulk hyperbolic metamaterials, consequently displaying the capability of tuning their responses along both in-plane and out-of-plane directions of the materials for the first time. It also distinctly interacts with unpolarized and polarized transverse magnetic and electronic beams to give unique hyperbolic responses. Our achievement can be a new platform to create various bulk metamaterials without complicated nanofabrication techniques. Our facile synthesis method using common laboratory techniques can open doors to broad-range researchers for active interdisciplinary studies for this otherwise hardly accessible technology.

Evaluation of Color and Structure of α-Fe2O3 Nanocapsules by Tuning of the SiO2 Morphology.

In this study, we synthesized a core/yolk-shell α-Fe2O3@SiO2 structure by treatment of SiO2 shell with mild basic solution. Spindle shaped α-Fe2O3 nanoparticles were initially prepared as core materials and subsequently coated with silica via sol-gel method. The yolk shell of α-Fe2O3 was produced by dissolution of silica shell from core-shell type hematite in NH4OH solution. In addition, the correlation between the density and morphology of the silica coating layer and the chromaticity according to the NH4OH concentration and the etching time was confirmed. We found that, after the conversion, the spindle shapes of the composite colloids were retained for both the hematite core and the silica shell, but the physical properties had a median value.

Color evolution and phase transformation behaviors of core-shell yellow iron(III) oxy-hydroxide pigments.

This manuscript reports on color evolution and phase transformation of alpha-FeOOH and silica-coated alpha-FeOOH pigment. Goethite alpha-FeOOH, a yellow pigment, is the principle coloring agent in yellow, but it is not stable in the high temperature. To obtain stable and bright color of alpha-FeOOH, alpha-FeOOH nanorods were coated with silica. Core-shell pigments were calcined at high temperatures (300 and 1000 degrees C) and characterized by scanning electron microscopy (SEM), CIE L*a*b* color parameter measurements, transmission electron microscopy (TEM) and UV-vis spectroscopy. The yellow alpha-FeOOH was transformed to alpha-Fe2O3 with red, brown at 300, 1000 degrees C, respectively. In contrast, the silica-coated a-FeOOH retained a red coloration with high a* value at 300 and 1000 degrees C.

Thermal behavior and coloration study of silica-coated alpha-Fe2O3 and beta-FeOOH nanocapsules.

This manuscript reports characterization of the colorations and thermal behaviors of the silica-coated alpha-Fe2O3 and beta-FeOOH nanocapsules. Prepared beta-FeOOH and alpha-Fe2O3 nanoparticles were coated with silica using tetraethylorthosilicate (TEOS) and cetyltrimethyl-ammonium bromide (CTAB) as a surface modifier for the comparison of physical properties of both samples. XRD patterns of the silica-coated beta-FeOOH and alpha-Fe2O3 nanoparticles were heated to 1000 degrees C, show a hematite (alpha-Fe2O3) structure. The silica-coated beta-FeOOH nanoparticles became almost entirely hollow at 1000 degrees C due to their volume reduction. In addition, the coloration values of the transformation nano capsule alpha-Fe2O3 are lower than those of the silica-coated alpha-Fe2O3 nanostructures. On the other hand, the silica-coated alpha-Fe2O3 nanoparticles retained their colorations and shapes after being heated to 1000 degrees C. The morphologies, crystal structures and colorations of the as prepared samples were analyzed by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction and CIE colorimeter.

One-step silica coating of cetyltrimethyl ammonium bromide-stabilized alpha-Fe2O3.

The alpha-Fe2O3 nanoparticles were coated with silica using intermediated surfactant such as cetyltrimethyl ammonium bromide (CTAB). After the formation of a thin intermediate layer in basic solution without any purification, TEOS was added directly to the solution for further silica growing. Silica thickness can be controlled through the various synthetic conditions (5 to 50 nm). The synthesized powder was characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and UV-visible spectroscopy. XRD and UV-visible spectroscopy of silica coated alpha-Fe2O3 is preserved constant after coating silica, suggesting that the SiO2 layers can be regarded as protecting layers without destroying the properties of the alpha-Fe2O3 nanoparticles.