Commercial α-Al2O3 Research Articles

Realizing the high loading amount of active Cu on Al2O3 to boost its CO catalytic oxidation

Catalytic oxidation of carbon monoxide (CO) by Cu/Al2O3 has garnered increasing interest in recent years due to its promising application prospects. Numerous investigations conducted on the Cu/Al2O3 system, but its catalytic performance for CO oxidation is still not as promising as that of precious metal catalysts. Increasing the loading amount of the active Cu on Al2O3 surface is a feasible method for improving its activity. However, with the increase of Cu loading, the agglomeration and enlargement of Cu particles is inevitable, which reduces the active Cu amount. Therefore, the utilization rate of Cu atoms is not high and the catalytic performance often can not further rise. Enhancing active Cu loading amount as high as possible is a prerequisite to further enlarge the activity of Cu/Al2O3 catalyst. Herein, self-synthesized Al2O3 nanofibers (Al2O3-nf) with high specific surface area and abundant penta-coordinated aluminum (AlV) are used as the support to maximize the Cu loading amount by chemical vapor deposition (CVD). And commercially available α-Al2O3 is used for comparative experiment. The high specific surface area could make Cu high dispersion on Al2O3, even at 20 wt% Cu loads, which is beneficial to high concentration load of active Cu. The catalytic activity of Cu/Al2O3-nf-CVD gradually increases with the increase of Cu loading from 2 wt% to 20 wt%, exhibiting a clear linear correlation with the surface content of Cu0 on the catalyst. Meanwhile, this result confirms that Cu0 plays a crucial role in CO oxidation of Cu/Al2O3. However, commercial α-Al2O3 reaches its highest activity when the Cu load is 5%, and then its activity begins to decrease due to the agglomeration of particles. Moreover, Cu/Al2O3-nf-CVD also exhibits remarkable thermal stability for CO oxidation. This work highlights a new strategy to synthesis of high Cu loading amount, high activity and thermostable Cu/Al2O3 catalyst for low-temperature oxidation of CO.

Influences of Raw Powder on Transparent Al2O3 Prepared by Two-Step Pulsed Electric Current Sintering

In order to fabricate Al2O3 ceramics with high transparency, six types of commercial α-Al2O3 powders were consolidated with a two-step pulsed electric current sintering (PECS) in this study. High density in Al2O3 usually has a positive effect on its transmittance, while as some α-Al2O3 powders show low transparency with almost full-densification. It suggested that abnormal grain growth may affect transparency of Al2O3 ceramics. Since pressure sintering in Al2O3 is influenced with dynamic grain phenomena, the differences in abnormal grain growth must be influenced with difference in properties of Al2O3 powder such as crystallinity.

Nanocomposites of poly(vinylidene fluoride) with oxide nanoparticles for barrier layers of flexible pipes

Poly(vinylidene fluoride) (PVDF) with commercial rutile-TiO2, SiO2, α-Al2O3, and Fe3O4 nanoparticles were prepared via extrusion using different contents, from 0.1 to 10 wt.%. The effect of the nanoparticles in the matrix on several material characteristics, such as the thermal and mechanical properties, crystallinity degree, and permeability to CO2, were examined. X-ray diffraction and infrared analyses revealed that the successful incorporation of increasing amounts of nanoparticles in the PVDF matrix reduced the total crystallinity but increased the β-phase content in almost all nanocomposites, suggesting an improved interaction between filler and polymer.The degree of crystallinity was between 30 and 38%, and good stability to thermal degradation of the obtained.The highlight of the present work is the observation that PVDF/rutile-TiO2 and PVDF/SiO2 nanocomposites showed an increased barrier to CO2 compared with PVDF. Such a conclusion tends to demonstrate that rutile-TiO2 and SiO2 nanoparticles associated with PVDF resulted in materials applicable in the barrier layer of flexible risers.

Seeding effect: impact and consequences on the characteristics of sol-gel derived α-Al2O3

Alumina (α-Al2O3) -an advanced ceramic with high mechanical strength, chemical stability and biocompatibility- can be effectively produced via the sol-gel technique. Seeding approach can be an alternative way to further improve the properties of sol-gel derived α-Al2O3 by promoting nucleation and growth. This work concentrates on the chemical/physical, microstructural, and mechanical characteristics of the sol-gel derived and seeded α-Al2O3 powders, and pellets. Seeding was applied by using commercial α-Al2O3 powders with wt.0%, 3%, 6%, and 10% of the precursor. As the seeding agent amount increased, bulk density values of the pellets also increased. However, the highest bulk density increase was observed for the samples heat treated at 1300 °C. Likewise, the compression strength enhanced but the best results were determined for the samples heat treated at 1300 °C. As the heat treatment temperature increased, heterogeneous grain growth occurred depending on the formation of large pores, and this situation reduced the compression strength.

Al2O3–Ce:YAG composite ceramics for high brightness lighting: Cerium doping effect

Efficient Al2O3–Ce:YAG composite ceramic phosphors were obtained via reactive vacuum sintering using commercial α-Al2O3, Y2O3, and CeO2 powders as starting materials, and SiO2, MgO as a complex sintering additive. The effect of cerium content (0.05–0.3 at%) on the structural-phase state, optical and photoluminescent properties of ceramic phosphors were investigated comprehensively by X-ray diffraction, high-resolution scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction, confocal laser scanning microscopy, ultraviolet-visible spectrophotometry, and photoluminescence examinations. It is shown that an increase in the cerium doping level of the composites initiates their partial recharging into the non-luminescent Ce4+ state and the parasitic introduction of magnesium ions into the aluminum oxide phase due to the stabilization of the crystal structure of Mg,Si,Ce:YAG solid solutions. The highest value of the photoluminescence intensity is observed for the 0.1 at% Ce3+ sample, which may be caused by the optimal ratio of its absorbance and radiative recombination rate compared to other samples. The obtained ceramic phosphors show excellent thermal quenching behavior – reduction in the photoluminescence emission intensities did not exceed 5% at heating temperatures to 150 °C, which can be regarded as almost unchanged. The correlated color temperature values of 1.0 and 0.4-mm-thick Al2O3–Ce:YAG samples with 0.05–0.3 at% Ce3+ doping varied between 4583 and 3890 K and 10,308–4146 K at color rendering index values of 61–35 and 74–55, respectively. It is concluded that 0.05–0.1 at% Ce3+-doped ceramic phosphors have an optimal balance between a high color rendering index, luminous efficiency, and appropriate correlated color temperature values for high-power and high-brightness natural white light-emitting diodes.

Wafer-scale epitaxial single-crystalline Ni(111) films on sapphires for graphene growth

The growth of graphene on Ni magnetic films is of great significance for graphene spintronics, whereas the existence of grain boundaries and twin crystal structures in Ni films is an obstacle for obtaining the large-scale and uniform graphene. In this paper, an epitaxial wafer-scale single-crystalline Ni(111) film with the flat and clean surface was successfully prepared on the commercial α-Al2O3(0001) substrate by a two-step method, which was demonstrated with several characterization methods. According to the abnormal grain growth mechanism, the clean and uniform sapphire surface plays a key role for the single crystallization of Ni films as it induces a weak interface energy difference between two atomic stacking structures (ABC and ACB), thus stimulating the evolution of Ni films from (111) out-of-plane textures to single crystals. Furthermore, an ultra-flat and wrinkle-free graphene monolayer was synthesized on the prepared Ni film, which further verified its high quality and effectiveness.

Spectroscopy and diode-pumped laser operation of transparent Tm:Lu3Al5O12 ceramics produced by solid-state sintering.

A transparent Tm:Lu3Al5O12 ceramic is fabricated by solid-state reactive sintering at 1830 °C for 30 h using commercial α-Al2O3 and Lu2O3/Tm2O3 powders and sintering aids - MgO and TEOS. The ceramic belongs to the cubic system and exhibits a close-packed structure (mean grain size: 21 µm). The in-line transmission at ∼1 µm is 82.6%, close to the theoretical limit. The spectroscopic properties of the ceramic are studied in detail. The maximum stimulated-emission cross-section is 2.37×10-21 cm2 at 1749nm and the radiative lifetime of the 3F4 state is about 10 ms. The modified Judd-Ofelt theory accounting for configuration interaction is applied to determine the transition probabilities of Tm3+, yielding the intensity parameters Ω2 = 2.507, Ω4 = 1.236, Ω6 = 1.340 [10-20 cm2] and α = 0.196×10-4 cm. The effect of excited configurations on lower-lying interconnected states with the same J quantum number is discussed. First laser operation is achieved under diode-pumping at 792 nm. A 4 at.% Tm:Lu3Al5O12 ceramic laser generated 3.12 W at 2022-2035nm with a slope efficiency of 60.2%. The ceramic is promising for multi-watt lasers at >2 µm.

Highly active Pd-Fe/α-Al2O3 catalyst with the bayberry tannin as chelating promoter for CO oxidative coupling to diethyl oxalate

A novel Pd-Fe/α-Al2O3 catalyst was synthesized by incipient-wetness impregnation method with bayberry tannin as chelating promoter and commercial hollow column Raschig ring α-Al2O3 as support for the synthesis of diethyl oxalate from CO and ethyl nitrite. A variety of characterization techniques including N2 physical adsorption, optical microscopy, scanning electron microscopy and energy dispersive system (SEM-EDS), inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), were employed to explore the relationship between the physicochemical properties and activity of catalysts. It indicated that a large number of phenolic hydroxyl groups in bayberry tannin can efficiently anchor the active component Pd, reduce the particle size and make the active Pd as a multi-ring distribution on the commercial α-Al2O3 support, which were beneficial to improve the catalytic activity for the production of diethyl oxalate from CO and ethyl nitrite. 0.3 wt% Pd-Fe/α-Al2O3 showed excellent catalytic activity and selectivity in a continuous flow, fixed-bed reactor with the loading amount of 10 mL catalysts. Under the mild reaction conditions, the space-time yield of diethyl oxalate was 978gL−1 h−1 and CO conversion was 44% with the selectivity to diethyl oxalate of 95.5%.

One-step synthesis of few-layered-graphene/alumina powders for strong and tough composites with high electrical conductivity

The chemical vapor deposition of carbon onto a commercial α-Al2O3 powder bed produces a pristine film of few-layered-graphene (FLG) uniformly covering the α-Al2O3 grains. This obviates both the manipulation of nanocarbons, lengthy mixing steps and the risk of damaging any pre-existing graphene platelets. The powders are consolidated to 99 % by SPS, producing samples where a FLG film is located along the grain boundaries of the submicron α-Al2O3. Compared to the pure α-Al2O3, the composites are moderately stronger and similarly tough and hard due to crack-deflection and crack-bridging and they are electrically conducting with a percolation threshold below 0.74 vol.% of carbon. The high conductivity values reflect the high quality of the thin FLG film and its continuous nature over very long distances. The samples are characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy.

Influence of co-doped alumina on the microstructure and radioluminescence of SrHfO3:Ce ceramics

SrHfO3:Ce and SrHfO3:Ce,Al powders were prepared by the solid-state synthesis using the commercial HfO2, SrCO3, CeO2 and α-Al2O3 as the starting materials. All the SrHfO3:Ce and SrHfO3:Ce,Al powders calcined at different temperatures within 1100 °C–1300 °C for 8 h in air exhibited a pure SrHfO3 phase. Meanwhile, the effects of different calcination temperatures and co-doped alumina on the microstructure of SrHfO3:Ce,Al powders were discussed. SrHfO3:Ce and SrHfO3:Ce,Al ceramics were successfully fabricated via the vacuum sintering at 1800 °C for 20 h. The alumina acted as the sintering aid, which promoted the densification and eliminated the intragranular pores of SrHfO3:Ce,Al ceramics. The charge disbalance was effectively compensated by co-doped alumina. SrHfO3:Ce,Al ceramics exhibited a higher light yield of 5700 ph/MeV compared with 4600 ph/MeV obtained in SrHfO3:Ce ceramics. The charge traps in ceramics and their effects on scintillation properties were also investigated by thermoluminescence measurement.

Effect of citric acid on the microstructure and optical properties of transparent YAG ceramics by reactive-SPS

Commercial α-Al2O3 and Y2O3 powders with 0.5 wt% TEOS were used to fabricate transparent polycrystalline YAG ceramics by reactive spark plasma sintering. The powder mixtures were dispersed with dispersants of PAA and citric acid in aqueous slurry. YAG specimens sintered at 1400 °C for 3 min were used to investigate the effect of processing parameters on the microstructure and the optical properties. It is found that a pretreatment of the powders mixture with 0.6 wt% citric acid can significantly improve the microstructure homogeneity of the as-sintered YAG ceramics. After post-annealing in wet air, the specimen sintered from the treated powders mixture was transparent, while the specimen from the untreated powders mixture was hazy. The effects of citric acid for this were investigated in this paper.

Hydrothermally stable mesoporous ZrO2 membranes prepared by a facile nanoparticle deposition process

Stabilized ZrO2 membranes (e.g. 8YSZ) have great potential due to their excellent thermal stability. In this paper, we present a new synthetic methodology to produce 8YSZ membranes on commercial macroporous α-Al2O3 supports. The resulting membranes have an average pore size of ∼5 nm and ∼3 nm, respectively, and a thickness approaching 2 μm. In various harsh stability tests, these membranes were tolerant to hydrothermal conditions (300 °C, 15 bar), high temperature (700 °C), and a high pH (NaOH, pH 14, 80 °C). They also demonstrated a suitable MWCO (15000 Da PEG; <3000 Da PEG) and permeance (145 kg h−1 m−2 bar−1; 40–50 kg h−1 m−2 bar−1) for commercial ultrafiltration processes. Importantly, they can also be considered for the production of novel graded macro-/meso-/microporous membrane systems, replacing the widely used but unstable mesoporous γ-Al2O3 membranes. At the end of this paper, we demonstrate that the synthetic methodology presented is also useful for producing membranes of other promising materials, including stabilized CeO2.

Densification and Mechanical Properties of YAG Ceramics Fabricated by Air Pressureless Sintering

Yttrium aluminum garnet (Y3Al5O12, YAG) ceramics with a relative density of about 99.5% were fabricated by solid-state reaction and pressureless sintering using commercial α-Al2O3 and Y2O3 powders as raw materials in air. The effect of different sintering additives, e.g. tetraethyl orthosilicate (TEOS), MgO and TEOS + MgO, on the relative density and mechanical properties of the YAG ceramic samples were investigated. When the dopant content of TEOS exceeded 0.5 wt.%, a sudden grain growth appeared, as well as a residual second phase located in triangle grain boundary. MgO was more effective as a sintering additive than silica in promoting the densification of the YAG ceramics. The relative density of the ceramics gradually decreased with the increase of MgO doping content and fixed TEOS content. The mechanical properties of the YAG ceramics fabricated by pressureless sintering in air at 1600–1710°C were tested. The fracture surface morphology of the MgO-doped ceramic samples was mainly transgranular, whereas the TEOS-doped samples were mainly intergranular.

A comparative study of dry reforming of methane over nickel catalysts supported on perovskite-type LaAlO3 and commercial α-Al2O3

A systematic and comparative study was made to determine the influence of perovskite-type LaAlO3 and commercial α-Al2O3 on the performance of nickel-based catalysts in dry reforming of methane (DRM). The perovskite-type LaAlO3 was selected due to its characteristics of solid state semiconductor with oxygen vacancies and high structural stability. The catalysts were characterized by X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), N2 adsorption-desorption, temperature programmed reduction (TPR-H2), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The catalyst performance was evaluated based on activity tests (600–800 °C) and short- and long-term stability (10 and 20 h) at 700 °C at a GHSV (Gas Hourly Space Velocity) of 18 and 72 L g−1 h−1. The TPR-H2 profiles indicate that the oxygen vacancies on the perovskite surface exerted a strong effect on the reduction temperature and reducibility of the NiO nanoparticles, resulting in weak Ni0/support interaction. The results of the tests after 10 h under GHSV of 18 L g−1 h−1 indicate that the Ni/LaAlO3 catalyst is 7.8 and 11.5% more stable than Ni/α-Al2O3 in the conversions of CH4 and CO2, respectively. The higher stability and activity of Ni/LaAlO3 is directly ascribed to the presence of NiO (3.38 wt%) after activation, which promoted the formation of carbon nanotubes (CNT) and increased the dispersion of the metallic phase. Even under severe conditions of activation and reaction (high GHSV), as in the long-term test, the Ni/LaAlO3 catalyst showed a 37.2% higher H2 yield than the Ni/α-Al2O3. Analyses by TEM indicate that the Ni/α-Al2O3 catalyst exhibited deactivation problems associated with sintering effects. Thus, the presence of structural defects and surfaces rich in oxygen vacancies makes LaAlO3 perovskite a potential support for application in methane catalytic reforming processes.

Fabrication and microstructural characterization of the novel optical ceramic consisting of α-Al2O3@amorphous alumina nanocomposite core/shell structure

In this study, α-Al2O3@amorphous alumina nanocomposite core-shell structure was synthesized from AlCl3 and the commercial α-Al2O3 nanoparticles as the starting materials via a wet chemical route. The results indicated that the shell material mainly comprised of ammonium chloride and boehmite phases. Boehmite was transformed to the amorphous and γ-Al2O3 phases after the calcination process and the shell material was completely converted to γ-Al2O3 at 1000 °C. However, for the α-Al2O3@amorphous alumina core-shell nanoparticles were completely converted to α-Al2O3 at 1000 °C. It can be concluded that α-Al2O3 core particles, as the seed crystalline, help to transforming of γ-Al2O3 phase as the shell material directly without forming transitional phases to α-Al2O3. The optical polycrystalline alumina was fabricated using spark plasma sintering of α-Al2O3@amorphous alumina core-shell nanocomposite. The body sintered has a final density of ∼99.8% and the in-line transmittance value is ∼80% within the IR range.

Effects of operating parameters and ionic liquid properties on fabrication of supported ionic liquid membranes based on mesoporous γ-Al2O3 supports

Supported ionic liquid membranes (SILMs) prepared with inorganic supports are more stable under high temperatures and differential pressures for gas separation than those based on organic membranes. In this study, a hot coating method was used to modify commercial α-Al2O3 tubular substrates to prepare high-quality membrane supports. Following impregnation of an ionic liquid (IL; [BMIM][BF4] or [BMIM][Ac]) into the supports over a short time, the as-prepared SILMs were used for separation of N2 and CO2. The effects of preparation parameters, including impregnation time (0–1800s) and impregnation temperature (18–59°C) on SILM performance and IL loading were systematically investigated to gain a better understanding of the SILM preparation process. The optimum impregnation time was determined by considering both the permeance and the selectivity. Further, a high temperature was found to be beneficial for the preparation process because the viscosity of ILs is usually low at higher temperatures. However, when the viscosities of the two ILs ([BMIM][BF4] and [BMIM][Ac]) were maintained at the same value by controlling the impregnation temperature, the qualities of the as-prepared SILMs were different, which may be due to the different surface tensions of these ILs.

Synthesis and Characterization of Alumina-Ceria Core-Shell Nanoparticles Using a Wet Chemical Method

In this study, α-Al2O3-CeO2 core-shell nanoparticles were synthesized from the cerium acetate and the commercial α-Al2O3 nanoparticles as the starting materials via a wet chemical method. Poly (acrylic acid) (PAA) as an additive compound was used for the surface modification of alumina nanoparticles. Also, the effects of PAA content, pH value and calcination temperature on the synthesis behavior of α-Al2O3-CeO2 nanoparticles were investigated. The formation of core-shell structure was investigated using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) equipped with energy dispersive X-ray spectroscopy (EDS). The results indicated that at the PAA=1.5 wt. %, pH=6 and calcination temperature=1150°C (as optimal conditions), the core-shell nanoparticles with alumina core and ceria shell and homogeneous size distribution were synthesized successfully.

Fabrication, microstructure and magneto-optical properties of Tb3Al5O12 transparent ceramics

Terbium aluminum garnet (Tb3Al5O12, TAG) transparent ceramics were fabricated by the solid-state reactive sintering of a mixture of commercial Tb4O7 and α-Al2O3 powders with tetraethoxysilane (TEOS) and MgO as sintering aids. Samples sintered at 1700 °C and 1725 °C for 20 h were utilized to examine the phase compositions, optical quality, microstructure and magneto-optical property. X-ray diffraction (XRD) results show that the sample sintered at 1700 °C has a pure garnet crystal structure, while the characteristic diffraction peaks of TAG disappear when sintered at 1725 °C. The sample sintered at 1725 °C shows high transparency and the optical transmittance is beyond 80% in the region of 600–1500 nm. It is found that the sample sintered at 1700 °C exhibits homogeneous grains with the average size of about 5.97 μm, however, no TAG grains are observed in the sample sintered at 1725 °C. The Verdet constants of the samples sintered at 1700 °C and 1725 °C are −179.4, −179.3 rad T−1 m−1 at 633 nm, respectively. The thermal depolarization of the sample sintered at 1725 °C increases up to 2 × 10−4 at maximum laser power of 91 W, which corresponds to the isolation ratio of 37 dB.

Effect of Nd3+ ions on phase transformations and microstructure of 0–4 at.% Nd3+:Y3Al5O12 transparent ceramics

Neodymium doped yttrium aluminum garnet (1–4 at.%) Nd3+:Y3Al5O12 laser ceramics were obtained by the reactive sintering method using commercial α-Al2O3, Y2O3, and Nd2O3 powders as starting materials, and SiO2, ZrO2 as a complex sintering aid. Phase composition and microstructure of Nd3+:Y3Al5O12 substitutional solid solutions were studied by XRD, SEM, XPS and UV-vis spectrophotometry methods. The regularities for entering of iso- (Nd3+) and heterovalent (Si4+, Zr4+) impurities into the garnet structure were revealed based on an assumption of formation of substitutional solid solutions. The influence of impurity point defects on optical properties of Nd3+:Y3Al5O12 laser ceramics was studied.

Fabrication and properties of transparent Tb:YAG fluorescent ceramics with different doping concentrations

Terbium doped yttrium aluminum garnet (Tb:YAG) transparent ceramics with different doping concentrations were fabricated by the solid-state reaction method using commercial Y2O3, α-Al2O3 and Tb4O7 powders as raw materials. Samples sintered at 1750°C for 20h were utilized to observe the optical transmittance, microstructure and fluorescence characteristics. It is found that all the Tb: YAG ceramics with different doping concentrations exhibit homogeneous structures with grain size distributions around 22–29µm. For the 5at% Tb:YAG transparent ceramics, the grain boundaries are clean with no secondary phases. The photoluminescence spectra show that Tb:YAG ceramics emit predominantly at 544nm originated from the energy levels transition of 5D4→7F5 of Tb3+ ions, and the intensity of the emission peak reaches a maximum value when the Tb3+ concentration is 5at%. The in-line transmittance of the 5at% Tb:YAG ceramics is 73.4% at the wavelength of 544nm, which needs to be further enhanced by optimizing the fabrication process. We think that Tb:YAG transparent ceramics may have potential applications in the high-power white LEDs.