Selective Corrosion Preparation and Sintering of Disperse α‐Al2O3 Nanoparticles

Disperse fine equiaxed α‐Al₂O₃ nanoparticles with a mean particle size of 9 nm and a narrow size distribution of 2–27 nm were synthesized using α‐Fe₂O₃ as seeds and isolation via homogeneous precipitation‐calcination‐selective corrosion processing. The presence of α‐Fe₂O₃ acting as seeds and isolation phase can reduce the formation temperature to 700°C and prevent agglomeration and growth of α‐Al₂O₃ nanoparticles, resulting in disperse fine equiaxed α‐Al₂O₃ nanoparticles. These α‐Al₂O₃ nanoparticles were pressed into green compacts at 500 MPa and sintered first by normal sintering to study their sintering behavior and finally by two‐step sintering (heated to 1175°C without hold and decreased to 1025°C with a 20 h hold in air) to obtain nanocrystalline α‐Al₂O₃ ceramics. The two‐step sintered bodies are nanocrystalline α‐Al₂O₃ with an average grain size of 55 nm and a relative density of 99.6%. The almost fully dense nanocrystalline α‐Al₂O₃ ceramic with finest grains achieved so far by pressureless sintering reveals that these α‐Al₂O₃ nanoparticles have an excellent sintering activity.     

Disperse Equiaxed α‐Al2O3 Nanoparticles Without Vermicular Microstructure

Nanocrystalline microstructure is regarded as a strategic approach to overcome the brittleness of alumina ceramics, and the preparation of disperse equiaxed α‐Al₂O₃ nanoparticles is an essential step for the preparation of nanocrystalline alumina ceramics. In this work, disperse equiaxed α‐Al₂O₃ nanoparticles were prepared using α‐Fe₂O₃ as seed and isolation phase. At first, the composite of α‐Al₂O₃ nanoparticles embedded in α‐Fe₂O₃ matrix was obtained by calcining the precursor powder containing γ‐AlOOH and Fe(OH)₃ (Fe³⁺/Al³⁺ mole ratio of 5) at 770°C for 2 h. Then disperse equiaxed α‐Al₂O₃ nanoparticles with a mean size of 12 nm and a size distribution from 2 to 40 nm without vermicular microstructure were obtained by removal of α‐Fe₂O₃ and other impurities in the composite through acid corrosion.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

High performance of la‐promoted Fe2O3/α‐Al2O3 oxygen carrier for chemical looping combustion

Iron oxide supported oxygen carrier (OC) is regarded to a promising candidate for chemical looping combustion (CLC). However, phase separation between Fe₂O₃ and supports often occurs resulted from the severe sintering of supports during calcination, which leads to the sintering and breakage of Fe₂O₃ thus the decrease of redox reactivity. In this article, La‐promoted Fe₂O₃/α‐Al₂O₃ were used as OCs for CLC of CH₄ and for the first time found that the OC with the addition of 18 wt % La exhibited outstanding reactivity and redox stability during 50 cycles of CLC of CH₄. Such a superior performance originated from the formation of LaAl₁₂O₁₉ hexaaluminate (La‐HA) phase with not only small particle size but also excellent thermal stability at CLC conditions, which worked as a binder to prevent the phase separation thereby the sintering and breakage of active species α‐Fe₂O₃ were avoided during reaction. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2827–2838, 2017     

Effects of Gd Substitution on Sintering and Optical Properties of Highly Transparent (Y0.95−xGdxEu0.05)2O3 Ceramics

Highly transparent (Y_0.95?xGd_xEu_0.05)₂O₃ (x = 0.15–0.55) ceramics have been fabricated by vacuum sintering at the relatively low temperature of 1700°C for 4 h with the in‐line transmittances of 73.6%–79.5% at the Eu³⁺ emission wavelength of 613 nm (~91.9%–99.3% of the theoretical transmittance of Y_1.34Gd_0.6Eu_0.06O₃ single crystal), whereas the x = 0.65 ceramic undergoes a phase transformation at 1650°C and has a transparency of 53.4% at the lower sintering temperature of 1625°C. The effects of Gd³⁺ substitution for Y³⁺ on the particle characteristics, sintering kinetics, and optical performances of the materials were systematically studied. The results show that (1) calcining the layered rare‐earth hydroxide precursors of the ternary Y–Gd–Eu system yielded rounded oxide particles with greatly reduced hard agglomeration and the particle/crystallite size slightly decreases along with increasing Gd³⁺ incorporation; (2) in the temperature range 1100°C–1480°C, the sintering kinetics of (Y_0.95?xGd_xEu_0.05)₂O₃ is mainly controlled by grain‐boundary diffusion with similar activation energies of ~230 kJ/mol; (3) Gd³⁺ addition promotes grain growth and densification in the temperature range 1100°C–1400°C; (4) the bandgap energies of the (Y_0.95?xGd_xEu_0.05)₂O₃ ceramics generally decrease with increasing x; however, they are much lower than those of the oxide powders; (5) both the oxide powders and the transparent ceramics exhibit the typical red emission of Eu³⁺ at ~613 nm (the ⁵D₀→⁷F₂ transition) under charge transfer (CT) excitation. Gd³⁺ incorporation enhances the photoluminescence and shortens the fluorescence lifetime of Eu³⁺.     

Fabrication,Microstructure, and Luminescent Properties of Ce3+ ‐Doped Lu3Al5O12 (Ce:LuAG) Transparent Ceramics by Low‐Temperature Vacuum Sintering

The fabrication of 0.5 mol% Ce:LuAG transparent ceramics starting from synthetic nanosized Ce:LuAG powders was investigated by low temperature vacuum sintering. It was found that high quality optical Ce:LuAG ceramics could be densified successfully by vacuum sintering (<10^–3 pa) at 1750°C for 10 h. The in‐line optical transmittance of as‐sintered Ce:LuAG ceramics with thickness of 0.7 mm could reach 73.48% at the wavelength of 550 nm. The microstructure observations revealed that transparent Ce:LuAG ceramics were composed of uniform LuAG grains with average size of 9 μm and HRTEM morphology indicated that no impurity segregation existed at grain boundaries or within Ce:LuAG grains. It was also demonstrated that the annealing treatment (at 1450°C for 20 h in air) could greatly enhance the luminescent intensity of as‐sintered Ce:LuAG ceramics under excitation of X‐ray radiation (75 kV, 25 mA), which makes it a potential candidate to be applied in radiation detector.     

Perfect High‐Temperature Plasticity Realized in Multiwalled Carbon Nanotube‐Concentrated α‐Al2O3 Hybrid

We investigate the high‐temperature compressive deformation behavior of a novel, fully dense and structurally uniform, 20 vol% multiwalled carbon nanotube (MWCNT)–α‐Al₂O₃ matrix hybrid, which has a strong room‐temperature interfacial shear resistance (ISR) and a unique MWCNT‐concentrated grain‐boundary (GB) structure. We realized a perfect plastic deformation at 1400°C and a rather high initial strain rate of 10^?4 s^?1 by a low ~30 MPa flow stress, which is contrary to the strain hardening response of fine‐grain monolithic Al₂O₃. This unique performance in CNT–ceramic system in compression is explained as follows: the concentrated network of individual MWCNTs perfectly withstands the high‐temperature and shear/compressive forces, and strongly preserves the nanostructure of Al₂O₃ matrix by preventing the dynamic grain growth, even during a large ~44% deformation. Furthermore, the presence of large amount of radially soft/elastic, highly energy‐absorbing MWCNTs in the GB and specially multiple junction areas, and a potentially weak 1400°C‐ISR, could greatly facilitate the GB sliding process (despite the hybrid's strong room‐temperature ISR), as evidenced by the formation of some submicrometer‐scale MWCNT aggregates in GB area, the equiaxed grains and dislocation‐free nanostructure of the deformed hybrid. The results presented here could be attractive for the ceramic forming industry and could be regarded as a reference for oxide systems in which, the GB areas are occupied with soft/elastic, highly energy‐absorbing nanostructures.     

Fabrication of Transparent Pb(Mg1/3Nb2/3)O3–PbTiO3 Based Ceramics by Conventional Sintering

Transparent 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃ (PMN‐PT) based ceramics were prepared by a conventional solid‐state synthesis without using a hot‐press method. The ceramics became transparent when they were sintered in an O₂ atmosphere. The optical transmission increased with decreasing diameter of the calcined powder, which was controlled by the size of zirconia ball‐milling media. Substitution of 3 mol% La for Pb in PMN‐PT further increased the optical transmission to 68% at the wavelength of 2000 nm, which was comparable to that of hot‐pressed Pb(Mg_1/3Nb_2/3)O₃‐PbTiO₃ based transparent ceramics.     

Fabrication of Aluminum Oxynitride (γ‐AlON) Transparent Ceramics with Modified Gelcasting

Transparent aluminum oxynitride (AlON) ceramics have been prepared through aqueous gelcasting forming technique starting from the raw materials of single phase AlON powders. The powder was specially treated for anti‐hydrolysis in ethanol before the shaping technique. The surface‐treated AlON powders could then be dispersed in an aqueous‐organic solution to prepare stable slurries containing 35 vol% solids loading. The obtained stable slurries were subsequently casted, calcined, and pressureless sintered at 1950°C for 8 h in nitrogen atmosphere. High transparent AlON ceramics with an average grain size of 112 μm and the in‐line transmittance of 81% at wavelength 1100 nm have been obtained.     

Thermal Reaction of Cristobalite in Nano‐SiO2/α‐Al2O3 Powder Systems for Mullite Synthesis

Nanoscaled cristobalite and α‐Al₂O₃ powders were used as the starting materials for synthesizing mullite by solid‐state reaction. The thermal reaction of the cristobalite with α‐Al₂O₃ during the thermal treatment was examined. Cristobalite powder with a D₅₀ value of 430 nm was adopted to mix with α‐Al₂O₃ powders with a D₅₀ values of 230, 310, and 400 nm in a stoichiometric composition of 3Al₂O₃?2SiO₂ (71.8 wt% α‐Al₂O₃ and 28.2 wt% SiO₂). Samples for thermal reaction were prepared using uniaxial pressed from the three mixtures that showed various particle number ratios of SiO₂/Al₂O₃ due to the different particle sizes of α‐Al₂O_3. Examinations were performed by differential thermal analysis, X‐ray diffraction, scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, and transmission electron microscopy techniques. The results showed that cristobalite particles amorphized during the thermal treatment, and then reacted with the α‐Al₂O₃ particle to form mullite via nucleation and growth. The amorphization temperature can be reduced by using finer‐sized α‐Al₂O₃ powders, thus leading to a lower temperature for mullite formation. Mullite crystals with a multidomain structure were observed in the α‐Al₂O₃ particle matrixes. The crystal orientation of the mullite was controlled by the α‐Al₂O₃ matrix, that is, [001] α‐Al₂O₃ → [001] mullite. These results indicate that the amorphization of cristobalite may trigger the reaction of SiO₂ with α‐Al₂O₃, initiating the nucleation of mullite. The α‐Al₂O₃ particles act as the hosts for mullite formation and determine the size of the mullite particles.     

Synthesis of γ‐Fluoro‐α, β‐unsaturated Carboxylic Esters from Saturated α‐Fluoro Aldehydes

γ‐Fluoro‐α, β‐unsaturated carboxylic esters 7a, 7b and 7d and 4‐fluoro‐4‐phenylbut‐3‐enoic ester ( 8 ) are obtained by two alternative pathways from 2‐fluoro aldehydes 5a—d , either by Horner—Wadsworth—Emmons reaction or by Wittig reaction. The aldehydes 5a—d are prepared by Swern oxidation of the corresponding fluorohydrins 4a—d . These are available from α‐olefins by bromofluorination, bromineby‐acetate replacement and subsequent hydrolysis.     

Efficient Near Infrared to Visible and Near‐Infrared Upconversion Emissions in Transparent (Tm3+, Er3+)‐α‐Sialon Ceramics

Intense visible and near‐infrared frequency upconversion and a frequency downconversion photoluminescence have been reported for the first time in Tm³⁺ and Er³⁺ co‐doped transparent α‐Sialon ceramics under 980 nm excitation. The α‐Sialon ceramics were prepared by hot‐press sintering technique. Intense upconversion bands at 554, 678, 803 nm, and a downconversion band at 1530 nm were observed as a result of the efficient energy transfer between Er³⁺ and Tm³⁺ ions. The quadratic dependence of upconversion intensities on the excitation power indicates that the upconversion process is governed by two‐photon absorption process. The sintered samples of thickness 0.20 mm have transparency above 80% in the range of 2000 to 4200 nm and it reaches as high as 82% at 3350 nm. Moderately low phonon energy was found with the highest frequency band at 828 cm^?1. These novel properties manifest the potential applications of transparent α‐Sialon ceramics as a multifunctional material.     

High performance ZSM‐5 membranes on coarse macroporous α‐Al2O3 supports for dehydration of alcohols

High‐performance ZSM‐5 membranes with a low Si/Al ratio of 10.3 were prepared on cheap coarse macroporous α‐Al₂O₃ tubes by fluoride route without organic template. The effects of crystallization time and aluminum source on the growth, morphology and pervaporation (PV) performances of the as‐synthesized membranes were investigated. The feasibility of preparing ZSM‐5 membranes with different Si/Al ratio which was implemented by using different Al₂(SO₄)₃·18H₂O content in synthesis gel were discussed. It was found that the aluminum source had significant effect on the synthesis of membranes. The ZSM‐5 membranes prepared by using Al₂(SO₄)₃·18H₂O as an aluminum source from synthetic gel with composition of 1SiO₂/0.05Al₂O₃/0.17Na₂O/0.9NaF/45H₂O showed high reproducibility and high PV performance with flux of 3.85 kg/(m²·h) and separation factor of higher than 10,000 in dehydration of 90 wt % i‐PrOH/H₂O at 348 K. Moreover, the ZSM‐5 membranes exhibited high water perm‐selectivity performance for dehydration of 90 wt % n‐PrOH/H₂O, n‐BtOH/H₂O, and i‐BtOH/H₂O mixtures, respectively. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2813–2824, 2016     

Low‐Temperature Sintering Microwave Characteristics of B2O3‐doped CaLa4Ti4O15 Dielectric Ceramics

Preparation and microwave dielectric properties of B₂O₃‐doped CaLa₄Ti₄O₁₅ ceramics have been investigated. X‐ray diffraction data show that CaLa₄Ti₄O₁₅ ceramic has a trigonal structure coupled with a second phase of CaLa₄Ti₅O₁₇. The CaLa₄Ti₄O₁₅ ceramic with addition of 0.5 wt% B₂O₃, sintered at 1220°C for 4 h, exhibits microwave dielectric properties with a dielectric constant of 45.8, Q × f value of 24,000 GHz, and temperature coefficient of resonant frequency (τ_f) of ?19 ppm/^°C. B₂O₃‐doped CaLa₄Ti₄O₁₅ ceramics, which have better sintering behavior (decrease in sintering temperature ~ 330°C) and dielectric properties than pure CaLa₄Ti₄O₁₅ ceramics, are candidates for applications in microwave devices.     

K0.5Na0.5NbO3‐Based Lead‐Free Transparent Electro‐Optic Ceramics Prepared by Pressureless Sintering

Transparent lead‐free electro‐optic (EO) ceramics (K_0.5Na_0.5) _0.9Li_0.1Nb_0.9Bi_0.1O₃ have been fabricated by pressureless sintering. The ceramics have a fine‐grained structure and cubic‐like symmetry. The comodification with Li and Bi induces a diffuse phase transition, causing the ceramics become relaxor‐like and contain polar nanoregions. Our results reveal that excess Bi₂O₃ can further enhance the changes in the crystal structure and dielectric behavior of the ceramics, causing them become more cubic‐like and more relaxor‐like, respectively. These can reduce the light scattering arisen from birefringence and domain walls, and thus improving the optical properties. For the ceramics added with 4 and 6 mol% excess Bi₂O₃, the optical transmittance reaches a high value of 60%–70% in the near‐infrared region. The ceramics also exhibits a good linear EO response, giving an effective EO coefficient of 30–40 pm/V.     

Production of Al2O3‐Stabilized Tetragonal ZrO2 Nanoparticles for Thermal Barrier Coating

Al₂O₃‐stabilized tetragonal ZrO₂ nanoparticles were obtained through hot‐air spray pyrolysis and characterized after postsynthesized treatments. The produced nanoparticles were 26 nm in size with surface area of 59 m²/g. A multilayer thermal barrier coating of nanostructured Al₂O₃‐ZrO₂‐embedded silicate was applied to the mild steel (EN3) specimen using spin‐coating technique and characterized comprehensively employing X‐ray diffraction and scanning electron microscope. The Al₂O₃‐stabilized ZrO₂ with silicate matrix facilitates the formation of zirconium silicate nanostructured surface‐protective coating on EN3 specimen. The Al₂O₃‐ZrO₂/SiO₂ matrix‐based hybrid inorganic coating shows effective thermal barrier for EN3 after firing at a high temperature of 600°C.     

低温烧结SiC-Al2O3-Y2O3复相陶瓷的研究

以SiC微粉为主要原料,添加适量氧化物复合烧结助剂在相对低的温度下烧结制成性能优良的精细SiC复相陶瓷材料,能满足耐磨陶瓷部件、机械密封件的使用要求,生产成本低,具有较强的实用价值。     

Nano‐engineered nickel catalysts supported on 4‐channel α‐Al2O3 hollow fibers for dry reforming of methane

A nickel (Ni) nanoparticle catalyst, supported on 4‐channel α‐Al₂O₃ hollow fibers, was synthesized by atomic layer deposition (ALD). Highly dispersed Ni nanoparticles were successfully deposited on the outside surfaces and the inside porous structures of hollow fibers. The catalyst was employed to catalyze the dry reforming of methane (DRM) reaction and showed a methane reforming rate of 2040 Lh^?1gNi^?1 at 800°C. NiAl₂O₄ spinel was formed when Ni nanoparticles were deposited on alpha‐alumina substrates by ALD, which enhanced the Ni‐support interaction. Different cycles (two, five, and ten) of Al₂O₃ ALD films were applied on the Ni/hollow fiber catalysts to further improve the interaction between the Ni nanoparticles and the hollow fiber support. Both the catalyst activity and stability were improved with the deposition of Al₂O₃ ALD films. Among the Al₂O₃ ALD coated catalysts, the catalyst with five cycles of Al₂O₃ ALD showed the best performance. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2625–2631, 2018     

Dielectric Properties of Ultra‐Low Sintering Temperature Al2O3–BBSZ Glass Composite

Ceramic composites of B₂O₃–Bi₂O₃–SiO₂–ZnO (BBSZ) glass mixed with Al₂O₃ (10–50 vol%) were sintered at 450°C, and their microstructural and dielectric properties investigated. Dense structures were obtained when the Al₂O₃ content was lower than 30 vol%. Raman, XRD, and FESEM showed the existence of a secondary phase, Bi₂₄Si₂O₄₀, in all samples. The dielectric properties of the composite with 30 vol% addition of Al₂O₃ showed good dielectric properties with ε_r of 14.8 and 20.8 and 32.5 at 100 kHz and 100 MHz and 1 GHz, respectively. The tanδ values at the same frequencies were 0.004 and 0.006 and 0.016. The results show that BBSZ glass with different amounts of Al₂O₃ exhibit widely applicable relative permittivity values and affordable loss and are thus promising candidates for ultra‐low sintering temperature applications.