Structure Of Al2O3 Research Articles

The network structure and self-diffusion in Al2O3 and Al2O3·2SiO2 system: insight from data mining of molecular dynamics data

The network structure and self-diffusion in Al2O3 and Al2O3·2SiO2 system: insight from data mining of molecular dynamics data

Thermal conductivity enhancement and shape stabilization of phase change thermal storage material reinforced by combustion synthesized porous Al2O3

The utilization of paraffin in latent heat storage system is limited by its leakage and low thermal conductivity. To address such challenges, we prepare composite phase change materials (PCMs) with enhanced thermal conductivity and shape stability. Paraffin is supported by porous Al2O3 which is synthesized by solution combustion synthesis (SCS). The porous structure of Al2O3 is quite influenced by the fuel species in SCS reactions. The glycine-induced Al2O3 shows highly hierarchical porous structure, which can improve the anti-leakage performance of the composites. The thermal conductivity of Al2O3/PW reaches 1.27 W/m•K with an Al2O3 mass ratio of 50 wt%, which is 5 times of pure paraffin. The composite also presents acceptable latent heat capacity of higher than 100 J/g and very good thermal cycling stability. These characteristics assure the promising application of Al2O3/PW composites for thermal energy storage.

Influence of µCT scanning resolution and volume on FEM-simulation of periodic 3D-printed porous ceramics

Complex cellular ceramic structures are commonly fabricated via replica technique, gel-casting, or freeze-drying. These structures exhibit a manufacturing-related heterogeneity in their microstructure associated with a large scattering range of their physical properties. This further complicates the determination of the representative volume-of-interest (VOI) for the corresponding characterization method. Periodic cellular ceramics can be systematically investigated by µCT analysis to determine the influence of the scan resolution on the results of the mechanical FEM simulation under the same boundary conditions (load, displacement). In this work, a complex 3D printed Al2O3 structure with 3×3×3 arranged cubic unit cells is scanned with three different µCT resolutions (6.87 µm, 13.56 µm and 27.11 µm). From the scans, FE models and sub-volumes are created with a gradation by a factor of 2 down to 1/8 of the initial volume to determine the representative VOI. It was demonstrated that a good agreement with the experimental data can only be achieved with the highest scanning resolution, but here a reduction of the VOI by a factor of four is possible to obtain identical results despite slight production-related deviations.

Ordered Mesoporous Alumina with Tunable Morphologies and Pore Sizes for CO2 Capture and Dye Separation.

We describe a versatile and scalable strategy toward long-range and periodically ordered mesoporous alumina (Al2O3) structures by evaporation-induced self-assembly of a structure-directing ABA triblock copolymer (F127) mixed with aluminum tri-sec-butoxide-derived sol additive. We found that the separate preparation of the alkoxide sol-gel reaction before mixing with the block copolymer enabled access to a relatively unexplored parameter space of copolymer-to-additive composition, acid-to-metal molar ratio, and solvent, yielding ordered mesophases of two-dimensional (2D) lamellar, hexagonal cylinder, and 3D cage-like cubic lattices, as well as multiscale hierarchical ordered structures from spinodal decomposition-induced macro- and mesophase separation. Thermal annealing in air at 900 °C yielded well-ordered mesoporous crystalline γ-Al2O3 structures and hierarchically porous γ-Al2O3 with 3D interconnected macroscale and ordered mesoscale pore networks. The ordered Al2O3 structures exhibited tunable pore sizes in three different length scales, <2 nm (micropore), 2-11 nm (mesopore), and 1-5 μm (macropore), as well as high surface areas and pore volumes of up to 305 m2/g and 0.33 cm3/g, respectively. Moreover, the resultant mesoporous Al2O3 demonstrated enhanced adsorption capacities of carbon dioxide and Congo red dye. Such hierarchically ordered mesoporous Al2O3 are well-suited for green environmental solutions and urban sustainability applications, for example, high-temperature solid adsorbents and catalyst supports for carbon dioxide sequestration, fuel cells, and wastewater separation treatments.

Erosion behaviors and the control of fiber structure in Al2O3,f/TiAl composites

Continuous Al2O3 fiber reinforced TiAl composite (Al2O3,f/TiAl) is expected to be promising to satisfy the tougher conditions in space industry. However, the thermostability of Al2O3 fibers in TiAl alloy has not been well studied. In this work, continuous Al2O3 fibers (containing 28 mol.% SiO2) have been introduced in Ti-48Al-4Nb-2Cr alloy by powder metallurgy technology at different hot pressing temperatures. The erosion reactions of Al2O3 fibers in TiAl matrix have been investigated by thermodynamic calculation. A evolution model of the interface regions has been established. It has been determined in this study that micropores can be eliminated completely at hot pressing temperature of 1050 °C while ensuring the integrity of Al2O3 fibers. Thermal exposure tests revealed the erosion reaction of fibers was mainly dominated by element diffusions of Ti and Si, and the erosion product was determined to be Ti5Si3. Based on the regulation of erosion reactions, a novel multilayer fiber structure with higher modulus has been achieved which could be matched with TiAl alloy and expected to improve strength/toughness of the Al2O3,f/TiAl composite. This structure contains a core of original fiber and coated by three composite ceramic layers, which can be obtained at hot pressing condition of 1050 °C/50 MPa/2 h and thermal exposure of 1050 °C/60 min

Pt-Sn clusters anchored at Al3+penta sites as a sinter-resistant and regenerable catalyst for propane dehydrogenation

Pt-based catalysts are widely used in propane dehydrogenation reaction for the production of propylene. Suppressing irreversible deactivation caused by the sintering of Pt particles under harsh conditions and regeneration process is a significant challenge in this catalyst. Herein, a series of highly ordered mesoporous Al2O3 supports with different levels of Al3+penta sites, are fabricated and used as the support to disperse Pt-Sn2 clusters. Characterizations of Pt-Sn2/meso-Al2O3 with XRD, NMR, CO-IR, STEM, TG, and Raman techniques along with propane dehydrogenation-regeneration cycles test reveal the structure-stability-regenerability relationship. The coordinatively unsaturated pentacoordinate Al3+ (Al3+penta) can strongly anchor Pt atoms via a formation of Al-O-Pt bond, and thus stabilize the Pt-based particles at the surface of Al2O3. The stability and regenerability of Pt-Sn2/meso-Al2O3 are strongly dependent on the content of Al3+penta sites in the Al2O3 structure, and a high level of Al3+penta sites can effectively prevent the agglomeration of Pt-Sn2 clusters into large Pt nanoparticles in the consecutive dehydrogenation-regeneration cycles. The Pt-Sn2/meso-Al2O3-600 with the highest level of Al3+penta (50.8%) delivers the best performance in propane dehydrogenation, which exhibits propane conversion of 40% and propylene selectivity above 98% at 570 °C with 10 vol% C3H8 and 10 vol% H2 feed. A slow deactivation in this catalyst is ascribed to the formation of coke, and the catalytic performance can be fully restored in the consecutive dehydrogenation-regeneration cycles via a simple calcination treatment.

Efficient peroxymonosulfate nonradical activity of Zn-Mn-Al2O3@g-C3N4 via synergism of Zn, Mn doping and g-C3N4 composite

Electronic structural engineering is critical to high-performance catalysis. However, it is difficult to achieve the best state with a single method to control the electronic structure. In this, Zn, Mn orbital hybridization, and polarization built-in electric field (heterostructure of g-C3N4 composite ZnMn-Al2O3) was used to control the electronic structure of Al2O3 to catalyze peroxymonosulfate to deal with environmental pollution. The obtained Zn-Mn-Al2O3@g-C3N4 (ZnMnAlO-CN) exhibits an intrinsic activity 71.1 times higher than that of Al2O3. Experiments and theoretical calculations have determined that the hybridization of Zn and Mn orbitals generates new orbitals that cross the Fermi level, and the polarization built-in electric field (polarization electric field) promotes the charge polarity on ZnMnAlO-CN. Orbital hybridization regulation and polarized electric field can coordinately adjust the electronic structure, thereby synergizing the non-radical reaction of surface reactive complexes and 1O2. The optimized catalytic mechanism shows good adaptability to pH, anions, and actual water bodies (tap water, surface water, and groundwater). It can stably remove environmental pollutants in continuous flow reaction. The coordinated use of multiple control methods is expected to provide ideas for releasing the inherent activity of the catalyst.

Theoretical study on residual infrared absorption of Ti:sapphire laser crystals

Residual infrared absorption is a key problem affecting the laser emission efficiency of Ti:sapphire crystal. In this paper, the origin of residual infrared absorption of Ti:sapphire crystal is systematically studied by using the first principles method. According to the contact conditions of O octahedron in the crystal structure of Al 2 O 3 , four Ti 3 + − Ti 3 + ion pair models and three Ti 4 + − Ti 3 + ion pair models were defined and constructed. For what we believe is the first time, the near-infrared absorption spectra consistent with the experimental results were obtained in specific theoretical models. The electronic structures, absorption spectra, and charge distributions calculated show that the line-contact Ti 3 + − Ti 3 + ion pair with antiferromagnetic coupling and the face-contact Ti 4 + − Ti 3 + ion pair are two main contributors to the residual infrared absorption of Ti:sapphire, while some other ion pair models provide a basis to explain more complex residual infrared absorption.

Experimental investigations of Al2O3- and ZrO2-based coatings deposited by detonation spraying

This paper presents the results of experimental investigations of the physical and mechanical properties, structure and surface morphology of Al2O3- and ZrO2-based coatings deposited by detonation spraying. The coatings were sprayed onto titanium-alloyed steel 12Ch18N10T (GOST 4986-79). The thickness of the coatings ranged from 250 to 1100 μm. After the deposition of the coatings the mechanical properties of the steel’s surface layer significantly improved. Its microhardness increased threefold, reaching the maximum value of over 11 GPa. Also the tribological properties of the material detonation sprayed onto the steel were tested. A significant decrease in the kinetic friction coefficient was observed. For the steel with the ZrO2 coating the friction coefficient decreased by nearly half, reaching the value of 0.330. An increase in abrasive wear resistance was noted. Moreover, the results of electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction examinations evaluating the structural properties and surface morphology of the coatings are presented.

Effect of particle size and thermal activation on the coal gangue based geopolymer

To better improve the preparation processing of coal gangue based geopolymer and understand the influence of particle size and thermal activation on the coal gangue based geopolymer, this paper introduces two kinds of geopolymers prepared by different methods using coal gangue and blast furnace slag as raw materials. The influence of particle size and activation temperature on the coal gangue was analyzed by XRD, FTIR, SEM, and TG-DTA. Then the mineral composition, microstructure and pore structure on the properties of the coal gangue based geopolymer were studied through XRD, MIP, and SEM. The results show that the optimum particle size is 200 mesh and the optimum activation temperature is 700 °C; Both geopolymers have obvious mechanical characterization; The main change of coal gangue under mechanical grinding is that the original structure is destroyed, the active reaction site and the potential gelling activity are both increased; While the main change of coal gangue under thermal activation is the crystal structure of Al2O3·2SiO2·2H2O in the coal gangue is destroyed and a phase transformation occurs; The analysis of XRD and SEM-EDS manifested that both the final products are composed of N-C-A-S-H gel and some unreacted crystalline phases are still present.

Structural and thermodynamic aspects of the evolution of supersmooth surface of oxide crystals

Evolution of the α-Al2O3, La3Ga5SiO14, and LaMgAl11O19 single crystals supersmooth surfaces at annealing in air was investigated using thermodynamic calculations. In particular the characteristic temperatures and annealing ranges were calculated when the surface melting phenomena took place or atomically smooth steps were formed and moved due to the surface diffusion processes. Later conclusion was confirmed in the surface probe microscopy experiments with regard to the crystal structure data of α-Al2O3, La3Ga5SiO14, and LaMgAl11O19, specifically the surface diffusion processes dominated at comparably low annealing temperatures (less than surface melting temperature Tsm). In this temperature range the crystal structure near the surface was rather stable, that is why one could assume the electrically neutral clusters of Al2O3, LaMgAl11O19 и La3Ga5SiO14 involved in the surface diffusion processes instead of single atoms. A further increase in annealing temperature to Tsm resulted in the La3Ga5SiO14 crystal surface thermodestruction due to probably incongruent melting of La3Ga5SiO14 compound and related friability of its structure if to compare it with Al2O3 and LaMgAl11O19 structures with the layers of close-packed oxygen atoms.

Simultaneous Improvement in the Corrosion Resistance and Visible Transparency of ZnO/Cu/ZnO Transparent Heaters with Reactively Sputtered Al2O3 Layers

In this study, we report a methodology that simultaneously improved the corrosion resistance and visible transparency of ZnO/Cu/ZnO transparent heaters with the addition of reactively sputtered Al2O3 layers. To assess corrosion resistance, ZnO and Al2O3 layers with thicknesses in the range of 0-20 nm were deposited onto 20 nm-thick Cu layer, and the corrosion behavior of the underlying Cu layers was investigated by evaluating the sheet resistance change in an 85 °C/85% humidity test. While the 20 nm-thick ZnO layer was not an effective moisture barrier, showing a sheet resistance increase of more than 50% after 10,000 m, the sheet resistance of the Cu layer below the 5 nm-thick Al2O3 layer did not show an observable increase for equal duration. Optical transmittance was also investigated by constructing glass/ZnO/Cu/ZnO/Al2O3 structures with varying thicknesses for the top ZnO and Al2O3 layers, in the range of 10-30 and 10-60 nm, respectively The thicknesses of the bottom ZnO and Cu layers were fixed at 30 nm and 10 nm. The results revealed that the average visible light transmittance of the ZnO/Cu/ZnO/Al2O3 structure increased by 2.5% over the optimized ZnO/Cu/ZnO structure.

Processing and thermal diffusivity measurements of compositionally graded Al-12Si to Al2O3 structures

The directed energy deposition (DED)-based additive manufacturing (AM) was used to create compositionally graded pure Al-12Si to pure Al2O3 structures varying the powder feed rates during deposition. Thermal diffusivity of Al-12Si + Al2O3 structures was reduced by >60% compared to pure Al-12Si. With a pure Al2O3 ceramic layer on Al-12Si + Al2O3, our results confirm the feasibility of designing and manufacturing metal-ceramic composites via AM with tailored thermal properties.

Enhanced ionic conductivity and lithium dendrite suppression of polymer solid electrolytes by alumina nanorods and interfacial graphite modification

Poor room-temperature ionic conductivity and lithium dendrite formation are the main issues of solid electrolytes. In this work, rod-shaped alumina incorporation and graphite coating were simultaneously applied to poly (propylene carbonate) (PPC)-based polymer solid electrolytes (Wang et al., 2018). The obtained alumina modified solid electrolyte membrane (Al-SE) achieves a high ionic conductivity of 3.48 × 10−4 S/cm at room temperature with a wide electrochemical window of 4.6 V. The assembled NCM622/Al-SE/Li solid-state battery exhibits initial discharge capacities of 198.2 mAh/g and 177.5 mAh/g at the current density of 0.1 C and 0.5 C, with the remaining capacities of 165.8 mAh/g and 161.3 mAh/g after 100 cycles respectively. The rod-shaped structure of Al2O3 provides fast transport channels for lithium ions and its Lewis acidity promotes the dissociation of lithium salts and release of free lithium ions. The lithiophilic Al2O3 and Graphite form intimate contact with metallic Li and create fast Li+ conductive layers of Li-Al-O layer and LiC6 layer, thus facilitating the uniform deposition of Li and inhibiting Li dendrite formation during long-term cycling. This kind of composite Al-SE is expected to provide a promising alternative for practical application in solid electrolytes.

OPTICAL CHARACTERISTICS OF Cr2O3/Al2O3 COMPOSITE STRUCTURE

The absorption, luminescence excitation, and photoluminescence (PL) spectra were studied for the nanocomposite chromium oxide (Cr2O3)/aluminum oxide (Al2O3) structure in the crystalline state produced by annealing pre-impregnated anodic alumina chromium anhydride at high temperatures. A PL band with maximum intensity at 615 nm, which is not characteristic of bulk materials, was found in a Cr2O3/γ-Al2O3, structure annealed at Тa = 900°С. This band was attributed to hexavalent chromium (Cr6+) ions formed as a result of trivalent chromium (Cr3+)-Cr6+ redox processes on the surface of Cr2O3 crystallites at annealing temperatures of 400-900°С. The emergence of an additional excitation band of Cr6+ ions at 520 nm was explained, with good agreement with the experimental results. It is shown that the photoluminescent properties of the nanocomposite Cr2O3/γ-Al2O3 structure are different for different ambient temperatures: at liquid nitrogen temperature (Т = 77.4 K), two broad bands were observed at 500 and 670 nm, while there was no PL at room temperature. For the nanocomposite Cr2O3/α-Al2O3 structure annealed at 1300°C, the photoluminescent properties were caused by radiative transitions in the structure of the energy levels of the Al2O3 defects of the anion-vacancy type and by impurity centers. In addition, the R-line half-width of Cr3+ for the nanocomposite Cr2O3/ Al2O3 structure was ~ 250 cm-1, which was larger than ~ 100 cm-1 for the original anodic alumina. This may be due to the greater defectiveness of the crystal lattices. Also, for the Cr2O3/Al2O3 structures, a significant increase in absorption in the ultraviolet range was found compared to the original Al2O3.

Fabrication of Mn2+ doped Al2O3 bulk crystal by flame fusion technique and their properties

Abstract In this work, Mn2+ doped Al2O3 bulk crystal has been growth by the flame fusion technique (FFT) and investigated their properties. The crystal structure of growth sapphire was studied by X-Rays Diffractometer (XRD) found to be Al2O3 structure according to the standard XRD pattern and no extra diffraction peaks were detected. The optical absorption spectra of sample were measured in the wavelength range of 350–1000 nm and shown only one broad band centered at 480 nm. The photoluminescence property was carried out using excitation wavelengths of 325 nm. The unusual redshift of Mn d–d emission was observed at 650 nm from 4T1g(D) → 6A1g(S) transition

Material extrusion additively manufactured alumina monolithic structures to improve the efficiency of plasma-catalytic oxidation of toluene

Abstract Non-thermal plasma (NTP) technology is one of the strategies with the most potential for the degradation of volatile organic compounds. Exploring the combination of plasma and catalysts in one system is of considerable significance to improve energy efficiency and decrease harmful by-products. The current geometrical design of monolithic catalysts is not optimal due to the limitations of traditional manufacturing processes. Here, the use of material extrusion additive manufacturing, followed by debinding and sintering to construct alumina monolithic catalysts with two-dimensional channels is suggested. A new feedstock material was adapted to the additive manufacturing process, and Al2O3 structures with unique axial and radial channels coated with Mn-Co nanospheres were successfully prepared for the plasma-catalytic degradation of toluene. The monoliths with radial and axial channels considerably increased energy efficiency with a minor increase in by-products. This work demonstrates the advantages of using additive manufacturing and provides a general strategy for the design and fabrication of monolithic catalysts with two-dimensional channel structures for heterogeneous catalysis.

Experimental investigation on Laser Directed Energy Deposition based additive manufacturing of Al2O3 bulk structures

In the present study, the effect of various processing parameters is investigated for building Al2O3 ceramic structures by laser additive manufacturing using the direct energy deposition technique (LAM-DED). The study shows that uniform single tracks and three dimensional (3D) structures of Al2O3 can be fabricated on alumina and Ti–6Al–4V substrates at the laser power of ~225 and ~300 W, respectively, for a fixed scanning strategy (θ = 0°), feed rate (8 g min−1) and scanning speed (0.6 m min−1). Optical microscopy, synchrotron-based X-ray micro-computed tomography (SR-μCT) examination and density measurement reveals that the bulk Al2O3 structures built on Ti–6Al–4V substrate have lower defects (porosity and cracks) and higher relative density (RD) = 85% than that fabricated on the alumina substrate (RD = 70%). This is primarily attributed to uniform melt pool formation on Ti–6Al–4V substrate at higher laser power. In addition to laser power, it is also found that the thermo-physical properties of the substrate material play a vital role on melt pool formation, which in turn decides the defect structure in built bulk Al2O3 components. Further, the change in scanning strategy to 45° and 67° increases the density of bulk Al2O3 structures built on Ti–6Al–4V substrate to 90% and 95%, respectively. The SR-μCT analysis of bulk structures built with different scanning strategies on the Ti–6Al–4V substrate confirmed that scanning strategy plays a vital role in the shape, size, and distribution of pores that formed during LAM-DED process. The study paves a way for LAM-DED of Al2O3 bulk structures by deriving the advantage of scan strategy and optimized processing parameters.

The color center of beryllium-treated yellow sapphires

Yellow color in gem corundum is commonly caused by Fe3+ impurity replacing Al3+ in the Al2O3 structure. For two decades beryllium-assisted heat treatment has been introduced to produce yellow sapphires from colorless, green, or light blue sapphires. The roles of beryllium atoms in corundum structure have been proposed in different ways either triggering structural defects or being as catalysts. Thus, the research experiments were conducted to evaluate these contradictions by applying the UV–vis excitation spectra and Fe K-edge x-ray absorption near edge structure spectra (XANES) of the samples, in combinations with Tauc plots of the UV–vis absorption spectra. The trace element content of the samples was analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). As a result, iron impurity in the samples has been confirmed as Fe3+ by XANES spectra, and hence, revealed as the cause of yellow color in natural yellow sapphire samples. Besides, the absorptions at 423 nm, 457 nm, 487 nm and 553 nm and emissions at 609 nm and 841 nm of Fe3+-Be2+ mixed donor states were detected by UV–vis excitation, which is the novel finding on the origin of yellow coloration in beryllium-treated sapphires. Therefore, an energy band model for mixed Fe3+-Be2+ donor states is proposed to be responsible for the yellow color center in beryllium-treated yellow sapphires, comparable to Fe3+ and Fe3+/Fe3+ states for the natural yellow sapphires commonly contain high iron.

Al2O3 + Graphene Low-Friction Composite Coatings Prepared By Sol–Gel Method

In this work, Al2O3 + graphene coatings were prepared using the sol–gel method. The aim of the study was preliminary determination of the influence of size and amount of graphene nanoplatelets on morphology, chemical structure, and basic tribological properties of Al2O3 + graphene composite coatings. Two types of reduced graphene oxide (rGO) nanoplatelets with different lateral size and thickness were used to prepare the coatings. To characterize them, scanning electron microscope (SEM), glow discharged optical emission spectrometer (GDOES), Fourier-transform infrared (FTIR), reflectance spectrometer, and ball-on-disk tribological tests were used. It was found that the presence of graphene in the Al2O3 + graphene coatings did not fundamentally change the chemical transformation of ceramic Al2O3 matrix. Morphology examinations of coatings containing larger graphene nanoplatelets revealed a tendency to their parallel arrangement in relation to the coated surface. The tribological properties of Al2O3 + graphene coatings turned out to be strongly dependent on the size of graphene nanoplatelets as well as on the heat treatment temperature. The friction coefficient as low as 0.11 and good durability were obtained for the Al2O3 + graphene coating with larger nanoplatelets and heat-treated at 500 °C. The results of conducted research indicate the potential use of Al2O3 + graphene composite coatings prepared by the sol–gel method as low-friction ceramic coatings.