Electrostatic Self-Assembled Composite Abrasives for Chemical Mechanical Polishing of A-Plane Sapphire

Sapphire substrates with different orientations have wide applications due to their excellent physical, chemical and optical properties. However, the chemical mechanical polishing of sapphire is challenging due to its chemical inertness, extreme hardness and brittleness. Herein, chemical mechanical polishing of A- and C-plane sapphire was systematically studied using α-Al2O3 and silica abrasives and polishing mechanism was analyzed by X-ray photoemission spectroscopy (XPS) and nanoindentation meter. The high MRR selectivity for C-plane sapphire in α-Al2O3 slurry is the synergy of selective hydration of C-plane and stronger crystal structure of A-plane. The low MRR selectivity for C-plane sapphire in silica slurry can be attributed to the formation of Al2SiO5 on both planes which reduced the impact of strong mechanical effect of α-Al2O3 abrasives. To improve the MRR of A-plane sapphire, a new nanocomposite particle with alumina as the core and silica as the soft shell was prepared by an electrostatic self-assembly method. The new composite abrasives combined the mechanical effect of α-Al2O3 abrasives and chemical effect of silica abrasives and demonstrated substantially higher MRR for A-plane sapphire than pure alumina abrasives, pure silica abrasives and physical mixture of alumina+silica abrasives.

Metal-Ceramic Beads Based on Niobium and Alumina Produced by Alginate Gelation

Full metal-ceramic composite beads containing different amounts of niobium and alumina, particularly 100 vol% alumina, 100 vol% niobium, and 95/5 vol% niobium/alumina, were produced by the alginate gelation process. The suspension for bead fabrication contained sodium alginate as gelling agent and was added dropwise into a calcium chloride solution to trigger the consolidation process. After debinding in air, sintering of the composite beads was performed under inert atmosphere. Samples in green and sintered state were analyzed by digital light microscopy and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy. Investigations by mercury intrusion porosimetry revealed that pure alumina beads featured smaller pores compared to composite beads, although the open porosities were comparable. The fracture strength was evaluated on single beads. Contrary to the pure alumina, the composite beads showed a clear plastic deformation. Pure niobium beads showed a ductile behavior with very large deformations. XRD analyses revealed the presence of calcium hexaluminate and beta-alumina as minor phases in the alumina beads, while the composite ones contained about 25 wt% of impurities. The impurities comprised NbO arising from the oxidation, and β-Nb2C, from the reaction with the residual sodium alginate.

Process characterization and analysis of ceramic powder bed fusion

Powder bed fusion (PBF) of ceramics is often limited because of the low absorptance of ceramic powders and lack of process understanding. These challenges have been addressed through a co-development of customized ceramic powders and laser process capabilities. The starting powder is made of a mix of pure alumina powder and alumina granules, to which a metal oxide dopant is added to increase absorptance. The performance of different granules and process parameters depends on a large number of influencing factors. In this study, two methods for characterizing and analyzing the PBF process are presented and used to assess which dopant is the most suitable for the process. The first method allows one to analyze the absorptance of the laser during the melting of a single track using an integrating sphere. The second one relies on in-situ video imaging using a high-speed camera and an external laser illumination. The absorption behavior of the laser power during the melting of both single tracks and full layers is proven to be a non-linear and extremely dynamic process. While for a single track, the manganese oxide doped powder delivers higher and more stable absorptance. When a full layer is analyzed, iron oxide-doped powder is leading to higher absorptance and a larger melt pool. Both dopants allow the generation of a stable melt-pool, which would be impossible with granules made of pure alumina. In addition, the present study sheds light on several phenomena related to powder and melt-pool dynamics, such as the change of melt-pool shape and dimension over time and powder denudation effects.

Na-Influenced Bulk and Surface Properties of the So-Called Iota(ι)-Alumina: Spectroscopy and Microscopy Studies

The test alumina (the so-called ι-Al2O3) was thermally recovered at 1,100°C from chitosan-AlOx hybrid films and found to contain Na and Ca impurity ions inherited from the parent chitosan. Two different modifications of pure alumina, namely, γ- and α-Al2O3, were adopted as control samples. The test and control aluminas were examined for 1) the bulk elemental constitution by atomic absorption spectroscopy (AAS), 2) the surface chemical composition by X-ray photoelectron spectroscopy (XPS), 3) the bulk phase composition by X-ray powder diffractometry (XRD), ex-situ Fourier-transform infrared spectroscopy (IR), and Laser Raman (LRa) spectroscopy, 4) the surface area, topography, and morphology by N2 sorptiometry, and atomic force (AFM) and scanning electron microscopy (SEM), 5) the surface adsorptive interactions with pyridine and 2-propanol gas-phase molecules by in-situ IR spectroscopy of the adsorbed species, and 6) the surface catalytic interactions with 2-propanol gas-phase molecules by in-situ IR spectroscopy of the gas phase. Results obtained could clearly show that the test alumina (ι-Al2O3) is only hypothetically pure alumina since in reality its bulk structure is majored by mullite-type Na-aluminate (Na0.67Al6O9.33/NaAlO2) and minored by Na-β-alumina (Na1.71Al11O17) and β-alumina (NaAl11O17). Consistently, observed Na-influenced modifications of the surface chemistry, topology, and morphology, as well as adsorptive and catalytic interactions with pyridine and 2-propanol gas-phase molecules, showed significant deviations from those exhibited by the control pure aluminas (γ- and α-Al2O3).

In vivo performance of Al2O3-Ti bone implants in the rat femur

Background Alumina-titanium (Al2O3-Ti) biocomposites have been recently developed with improved mechanical properties for use in heavily loaded orthopedic sites. Their biological performance, however, has not been investigated yet. Methods The aim of the present study was to evaluate the in vivo biological interaction of Al2O3-Ti. Spark plasma sintering (SPS) was used to fabricate Al2O3-Ti composites with 25 vol.%, 50 vol.%, and 75 vol.% Ti content. Pure alumina and titanium were also fabricated by the same procedure for comparison. The fabricated composite disks were cut into small bars and implanted into medullary canals of rat femurs. The histological analysis and scanning electron microscopy (SEM) observation were carried out to determine the bone formation ability of these materials and to evaluate the bone-implant interfaces. Results The histological observation showed the formation of osteoblast, osteocytes with lacuna, bone with lamellar structures, and blood vessels indicating that the healing and remodeling of the bone, and vasculature reconstruction occurred after 4 and 8 weeks of implantation. However, superior bone formation and maturation were obtained after 8 weeks. SEM images also showed stronger interfaces at week 8. There were differences between the composites in percentages of bone area (TB%) and the number of osteocytes. The 50Ti composite showed higher TB% at week 4, while 25Ti and 75Ti represented higher TB% at week 8. All the composites showed a higher number of osteocytes compared to 100Ti, particularly 75Ti. Conclusions The fabricated composites have the potential to be used in load-bearing orthopedic applications.

High Temperature Metamaterial Enhanced Electromagnetic Absorbing Coating Prepared With Alumina Ceramic

At present, a series of challenges are impeding the large-scale application of the high temperature radar absorbing coatings (RACs), such as the complex preparation of raw material and the complicated technology of processing. In this paper, a new, thin and high-temperature metamaterial RAC (MRAC) with strong absorption was designed and experimentally demonstrated, composing of a radar absorbing coating and a layer of metamaterial. To avoid the need for the complex preparation of raw materials, pure alumina was selected as the radar absorbing coating. Meanwhile, plasma spraying and screen printing were employed to simplify the manufacture technology to produce the absorbing coating and the metamaterial layer, ensuring its feasibility and practicality. The metamaterial layer was prepared with a high temperature conductive paste and it improved the impedance matching of the coating and regulated the electromagnetic (EM) resonance. With the ability to consume more incident EM waves, excellent absorption performance at high temperature was achieved with relatively small thickness. In the 8 ~ 18 GHz band, the MRAC bandwidth for reflection loss (RL) below -5dB almost covered the frequency of 10 ~18 GHz with a thickness of only 1.5 mm at 800 ℃. This new metamaterial has broad application prospects by the virtue of its simplicity and ease of production.

Methanol oxidation over copper supported catalysts.

Oxidation of methanol has been studied over a range of temperature and contact times with a synthesised, 1wt % copper catalysts supported over Al2O3 and SiO2. Characterisation of these catalysts was performed by nitrogen adsorption and porosity measurements (BET), X-Ray Diffraction (XRD), and IR spectroscopy of adsorbed CO. BET measurement revealed surface areas reducing from 105 to 104 m2g-1 for pure alumina to copper supported catalyst while the silica ones reduced from 247 to 240 m2g-1 for pure silica to the copper supported respectively. Pore sizes also reduced from 32 – 23 nm and 23 – 20.3 nm for the alumina and silica catalysts respectively. No crystalline phases from the diffraction patterns of the loaded metals were found to be present on the XRD. CO adsorption studies showed the presence of small cluster metal atoms adsorbed on the surface of the catalysts with Temperature Programmed Reduction (TPR) experiments revealing the presence of partially oxidised and well dispersed Cu atoms. The alumina supported catalyst were more active than the silica ones while for selectivity and yield for formaldehyde, the reverse was the case. The alumina supported significantly showed high yields of Dimethyl ether (DME) while the silica showed high yield for methyl formate (MF) with COx and CH4 detected in very small quantities.

Effect of Silver Addition on the Adsorption Properties of Y Zeolite

The present work is aim to study the adsorption/desorption properties of the Ag-modified Y zeolite towards toluene as well as its high temperature behavior under the prompt thermal aging conditions. The Ag/Y samples were obtained by an ion exchange technique and mixed with pure alumina used as a binder. The reference samples were prepared by an incipient wetness impregnation of alumina with a solution of silver nitrate and mixed with pure HY zeolite. The initial and aged samples were characterized by TEM and XRD methods. It was found that the Ag-modified Y zeolite strongly adsorbs toluene. Irreversible sorption of toluene over the most active silver sites was shown to exclude them from the participation in oxidation processes, thus diminishing the overall efficiency of the adsorption-catalytic system.

Heteroleptic Derivatives of Aluminium(III) as Precursors for Nano-Al2O3: A Study on the Influence of Ligands on Morphology of Alumina

Three metalloorganic derivatives of aluminium(III) [(CH3COCHCOOCH3)Al(OC6H4CH=NC6H4O)(EtOH)] (1), [(CH3COCHCOCH3)2Al(o- MeC6H4O)]2 (2) and Al(OiPr)3 (3) were exploited as precursors of alumina through well-established sol-gel hydrolytic pathway. The resultant white alumina powders (a), (b) and (c) were characterized by FTIR, powder XRD, TEM and EDX analyses. Formation of pure alumina was confirmed through EDX and FTIR techniques. The powder XRD patterns of alumina samples established their existence in γ-phase. The morphological examination of all oxidic ceramics through TEM illustrated that the alumina powders (a) and (b) were fine and discrete nanorods whereas alumina powder (c) was highly agglomerated with irregular morphology.