Narrowing size distribution widths of small α-Al2O3 nanoparticles by addition of large nanoparticles in coagulation separation

α-Al₂O₃ nanoparticles separated by fractionated coagulation still have broad size distributions which limit their wider applications. By adding 20-time mass of large α-Al₂O₃ (40.5 nm) into α-Al₂O₃ nanoparticles to be separated in coagulation separation, the average size of separated α-Al₂O₃ nanoparticles decrease from 6.6 nm without addition of large α-Al₂O₃ NPs to 4.4 nm, and the size distribution changes from 3–10 nm without addition of large α-Al₂O₃ NPs to 3–6 nm. With increasing amount of large α-Al₂O₃ NPs added, separated α-Al₂O₃ NPs exhibit smaller average sizes and narrower size distribution widths at the same separation concentrations. This approach may be applied to narrow size distribution widths in large-scale size-selective separations of other nanoparticles.     

Sintering kinetics of disperse ultrafine equiaxed α-Al2O3 nanoparticles

The sintering kinetics of ceramic nanoparticles is essential for preparing dense nanocrystalline ceramics with fine grains, but the sintering kinetics of disperse ultrafine α-Al₂O₃ nanoparticles has not been systematically explored so far. In this paper, the sintering kinetics of disperse ultrafine equiaxed α-Al₂O₃ nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm without any agglomeration was studied systematically. Finally, α-Al₂O₃ nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7% was sintered in air by two-step sintering (heated to 1100 °C without hold and then cooled down to 950 °C with a 40 h hold). The sintering temperature is the lowest for pressureless sintering of α-Al₂O₃ and almost fully dense α-Al₂O₃ nanocrystalline ceramic obtained also has the finest grain so far.     

Plastic deformation of dense nanocrystalline yttrium oxide at elevated temperatures

Nanocrystalline yttrium oxide, Y₂O₃ with 110 nm average grain size was plastically deformed between 800 °C and 1100 °C by compression at different strain rates and by creep at different stresses. The onset temperature for plasticity was at 1000 °C. Yield stress was strongly temperature dependent and the strain hardening disappeared at 1100 °C. The polyhedral and equiaxed grain morphology were preserved in the deformed specimens. The experimentally measured and theoretically calculated stress exponent n = 2 was consistent with the plastic deformation by grain boundary sliding. Decrease in the grain size was consistent with decrease in the brittle to ductile transition temperature.     

Direct dehydrogenation of n-butane over Pt/Sn/M/γ-Al2O3 catalysts: Effect of third metal (M) addition

A series of Pt/Sn/M/γ-Al₂O₃ catalysts with different third metal (M = Zn, In, Y, Bi, and Ga) were prepared by a sequential impregnation method for use in the direct dehydrogenation of n-butane to n-butene and 1,3-butadiene. In the direct dehydrogenation of n-butane, Pt/Sn/Zn/γ-Al₂O₃ catalyst showed the best catalytic performance. Catalytic performance decreased in the order of Pt/Sn/Zn/γ-Al₂O₃ > Pt/Sn/In/γ-Al₂O₃ > Pt/Sn/γ-Al₂O₃ > Pt/Sn/Y/γ-Al₂O₃ > Pt/Sn/Bi/γ-Al₂O₃ > Pt/Sn/Ga/γ-Al₂O₃. The catalytic performance increased with increasing metal–support interaction and Pt surface area of the catalyst.     

Novel nonaqueous precipitation synthesis of alumina powders

Alumina powders were prepared via a novel nonaqueous precipitation method with aluminum powders as aluminum source and anhydrous acetic acid as precipitant. The thermal decomposition and phase transformation of crystal precipitate and the influence of precipitate aging were investigated via TG-DTA-MS, XRD, TEM, BET, FE-SEM and performance tests of sintered bodies. The results show crystal precipitate C₄H₇AlO₇ transforms to amorphous Al₂O₃ at 300 °C, and then to γ-Al₂O₃ at 950 °C, and finally to α-Al₂O₃ at 1050 °C. The particle size of α-Al₂O₃ prepared at 1100 °C is 50–100 nm with BET surface area of 25.98 m²∙g⁻¹. FE-SEM morphology of sintered sample at 1400 °C shows excellent sinterability of the α-Al₂O₃ powders. Aging eliminates aggregation, and leads to highly homogenized and densified particles. It also affects the densification behaviour during sintering and further influences density, thermal expansion coefficient, flexural strength, volume resistivity and electric breakdown strength of sintered bodies     

Creep mechanisms of laminated alumina/zirconia-toughened alumina composites

High-temperature plastic deformation of laminar composites containing alternate layers of Al₂O₃ and a mixture of 60 vol.% Al₂O₃ + 40 vol.% 3 mol% Y₂O₃-stabilized tetragonal ZrO₂ (ZTA) produced by tape casting is investigated in isostrain compression testing at temperatures between 1400 and 1500 °C. The stress exponent n and the creep activation energy Q are close to 1 and 700 kJ/mol, respectively. Microstructual observations reveal the lack of differential features in the ZTA layers and a general creep damage of the Al₂O₃ layers, with little microcracking by cavity coalescence even up to strains of 30%. The layer interfaces maintain their initial structural integrity after testing. An isostrain composite creep model predicts correctly the overall mechanical behavior of the laminates, which is dictated by the alumina phase via diffusional creep controlled by oxygen grain boundary diffusion.     

Preparation of Al2O3–ZrO2 nanocomposite films by laser chemical vapour deposition

Alumina (Al₂O₃)–zirconia (ZrO₂) nanocomposite films were prepared by laser chemical vapour deposition. α-Al₂O₃–ZrO₂ and γ-Al₂O₃–t-ZrO₂ nanocomposite films were prepared at 1207 and 1000 K, respectively. In the nanocomposite films, 10-nm-wide t-ZrO₂ nanodendrites grew inside the α- or γ-Al₂O₃ columnar grains. The γ-Al₂O₃–t-ZrO₂ nanocomposite films exhibited high nanoindentation hardness (28.0 GPa) and heat insulation efficiency (4788 J s^−1/2 m⁻² K⁻¹).     

Creep behavior of a zirconium diboride–silicon carbide composite

Flexural creep studies of ZrB₂–20 vol% SiC ultra-high temperature ceramic were conducted over the range of 1400–1820 °C in an argon shielded testing apparatus. A two decade increase in creep rate, between 1500 and 1600 °C, suggests a clear transition between two distinct creep mechanisms. Low temperature deformation (1400–1500 °C) is dominated by ZrB₂ grain or ZrB₂–SiC interphase boundary and ZrB₂ lattice diffusion having an activation energy of 364 ± 93 kJ/mol and a stress exponent of unity. At high temperatures (>1600 °C) the rate-controlling processes include ZrB₂–ZrB₂ and/or ZrB₂–SiC boundary sliding with an activation energy of 639 ± 1 kJ/mol and stress exponents of 1.7 < n < 2.2. In addition, cavitation is found in all specimens above 1600 °C where strain-rate contributions agree with a stress exponent of n = 2.2. Microstructure observations show cavitation may partially accommodate grain boundary sliding, but of most significance, we find evidence of approximately 5% contribution to the accumulated creep strain.     

Oxidation behavior characteristics of an aluminized Ni-based single crystal superalloy CM186LC between 900 °C and 1100 °C in air

Oxidation behavior of an aluminized Ni-based single crystal superalloy CM186LC was performed between 900 °C and 1100 °C in air. The oxidation kinetics approximately followed a parabolic oxidation law at 900 °C and 1100 °C. The mass gains were significantly increased owing to the formation of θ-Al₂O₃ during initial oxidation stage. After 100 h oxidation, the mass gain rates were then decreased due to the transformation from θ-Al₂O₃ to α-Al₂O₃. The microstructures after 500 h oxidation at all temperatures generally consisted of scale, coating layer, interdiffusion zone (IDZ), substrate diffusion zone (SDZ) accompanied with the topologically close-packed (TCP) and substrate.     

Two step hot pressing sintering of dense fine grained WC–Al2O3 composites

WC–40 vol% Al₂O₃ composites were prepared by high energy ball milling and the following two step hot pressing sintering (TSS). The tungsten carbide (WC) and commercial alumina (Al₂O₃) powders composed of amorphous Al₂O₃, boehmite (AlOOH) and χ-Al₂O₃ were used as the starting materials. The feasibility of two step sintering (TSS) method to WC–40 vol% Al₂O₃ composites was demonstrated, optimum TSS regime was discussed and phase transformation during TSS process was investigated. The results showed that both the pre-sintering at a proper first step temperature (T₁) to obtain a critical initial density and isothermal hot pressing at an appropriate second step temperature (T₂) to inhibit grain boundary migration (GBM) were of significant importance. When the as milled WC–40 vol% Al₂O₃ powders were hot pressed under TSS₄ regime, a relative density of 99% and a grain size of 2.38 μm were obtained, an excellent Vickers hardness of 19.71 GPa was achieved, combining a fracture toughness of 12 MPa m^1/2 with a flexural strength of 1285 MPa. Compared with the near full dense samples consolidated under CS₁ regime (1540 °C for 90 min), the grain size decreased, the Vickers hardness, fracture toughness and flexural strength were all improved due to the refined microstructure and the transgranular fracture mode. The amorphous Al₂O₃, AlOOH and χ-Al₂O₃ were transformed to α-Al₂O₃ completely during the sintering process.     

Microstructure and high temperature 4-point bending creep of sol–gel derived mullite ceramics

Four-point bending creep behavior of mullite ceramics with monomodal and bimodal distribution of grain sizes was studied in the temperature range of 1320–1400 °C under the stresses between 40 and 160 MPa. Mullite ceramic with bimodal grain size distribution was prepared using aluminum nitrate nonahydrate as alumina precursor. When γ-Al₂O₃ or boehmite were used as alumina precursors, mullite grains are equiaxial with mean particle size of 0.6 μm for the former and 1.3 μm for the latter alumina precursor. The highest creep rate exhibited the sample with monomodal morphology and grains in size of 0.6 μm, which is about one order of magnitude greater than that for the monomodal morphology but with grains in size of 1.3 μm. The highest activation energy for creep (Q = 742 ± 33 kJ/mol) exhibits mullite with equiaxial grains of 1.3 μm, whereas for sample with smaller equiaxial grains the activation energy is much smaller and similar to mullite ceramics with bimodal grain morphology. Intergranular fracture is predominant near the tension surface, while transgranular more planar fracture is predominant near the compression surface zone.     

Low firing and microwave dielectric properties of BaTi4O9 with B2O3–ZnO–La2O3 glass addition

A high-Q low firing ceramics material was fabricated from a composite of 80 wt% of BaTi₄O₉ powder and 20 wt% of B₂O₃–ZnO–La₂O₃ glass flux. The sample sintered at 900 °C for 3 h had the maximum bulk density. B₂O₃–ZnO–La₂O₃ glass was found to act as the sintering aid. The material sintered at 900 °C for 3 h consists of BaTi₄O₉, LaBO₃, an unidentified crystal phase and residual glass phase, and possesses excellent microwave dielectric properties: permittivity k ≈ 27, quality factor Q × f ≈ 20,000 GHz, temperature coefficient of resonant frequency τ_f ≈ 6.5 ppm/°C.     

Mechanochemical-molten salt synthesis of α-Al2O3 platelets

The polyaluminium chloride (PACl) precursor was used for a simple and scaled-up mechanochemical-molten salt synthesis of α-Al₂O₃ platelets. PACl, as a low temperature α-Al₂O₃ precursor, was firstly mechanically activated by high-energy ball milling for 5 min, followed by a next 5 min ball milling in the presence of a NaCl–KCl salt mixture. The starting formation temperature of the α-Al₂O₃ phase was 600 °C. In the subsequent annealing in the temperature range of 660–1000 °C, the α-Al₂O₃ phase with a well developed plate-like morphology was obtained. The products were characterized by X-ray powder diffractometry, scanning electron microscopy (SEM), and thermal analysis (DTA, TG) and solution ²⁷Al NMR spectroscopy.     

Microstructural features of nanocomposite of alumina@carbon nanotubes/alumina nanoparticles synthesized by a solvothermal method

The present study focuses on the synthesis of nanocomposite gamma alumina (γ-Al₂O₃), boehmite and multi- walled carbon nanotubes (MWCNTs) via a solvothermal procedure. The method is based on the ex situ filling of opened CNTs by liquid reactants. The microstructure and morphology of the synthesized nanocomposite Al₂O₃@CNTs/Al₂O₃ was characterized by high resolution transmission electron microscopy (HRTEM), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) and N₂ adsorption–desorption analysis.Based on the experimental results, it was determined that the volume ratio of γ-Al₂O₃/MWCNTs and the surface tension of the solvent both greatly influence the morphology of the nanocomposite. The resultant MWCNTs were coated and filled by homogeneous and uniform boehmite and γ-Al₂O₃ layers and nanoparticles with thicknesses of 1–3 nm and diameters of 20–40 nm, when the volume ratio of γ-Al₂O₃/MWCNT_S is 1 and the surface tension of the solvent is approximately 26 mN m^?1 at 20 °C, far below the maximum value (100–200 mN m^?1) for MWCNT filling.     

Catalytic conversion of CO,NO and SO2 on the supported sulfide catalyst: I. Catalytic reduction of SO2 by CO

Catalytic reduction of SO₂ to elemental sulfur by CO has been systematically investigated over γ-Al₂O₃-supported sulfide catalysts of transition metals including Co, Mo, Fe, CoMo and FeMo with different loadings of the metals. The sulfided CoMo/Al₂O₃ exhibited outstanding activity: a complete conversion of SO₂ was achieved at a temperature of 300°C. The reaction proceeds catalytically and consistently over time and most efficiently at a molar feed ratio CO/SO₂ = 2. A precursor CoMo/Al₂O₃ oxide which experienced sulfurization through the CO–SO₂ reaction yielded a working sulfide catalyst having a yet lower activity than the CoMo catalyst sulfided before reaction (pre-sulfiding). The catalytic activity of various metal sulfides decreased in order of 4% Co 16% Mo > 4% Fe15% Mo > 16% Mo ≥ 25% Mo > 14% Co ≥ 4% Co > 4% Fe. A DRIFT study showed that CO adsorbs exclusively on CoMo phase and that SO₂ predominantly on γ-Al₂O₃. It is suggested that the Co–Mo–S structure is more adequate than the other metal-sulfur structures for the formation of a carbonyl sulfide (COS) intermediate because of the proper strength of metal–sulfur bond, and catalytically works with γ-Al₂O₃ for the COS–SO₂ reaction.     

Fracture toughness of Al2O3 fibers with an artificial notch introduced by a focused-ion-beam

The present work was carried out to estimate the fracture toughness of two types of Al₂O₃ fibers (85Al₂O₃–15SiO₂, Altex^® (Sumitomo Chemical Co., Ltd) and α-Al₂O_3, Almax^® (Mitsui Mining Co., Ltd)) and to elucidate the transition from the intrinsic defects-induced fracture to introduced notch-induced one. With an application of the focused-ion (Ga⁺)-beam micromachining method, a mode I type straight-fronted edge notch with a notch-tip radius around 25 nm was introduced in fiber specimen. The fracture toughness K_Ic was estimated for each fiber specimen based on the fracture mechanical approach in which the measured values of notch depth, fiber diameter, fracture strength and calculated correction factor were substituted. The fracture toughness values of the 85Al₂O₃–15SiO₂ and α-Al₂O₃ fibers were estimated to be 1.86 ± 0.24 and 2.05 ± 0.13 MPa m^1/2, respectively. The fracture toughness value was almost independent of the fiber diameter and notch depth in both fibers tested. From the obtained fracture toughness value and the measured fracture strength of the original fiber, the notch depth at the transition from intrinsic defects-induced fracture to notch-induced one, corresponding to the equivalent size of the intrinsic defects that determines the strength of the original fiber, were estimated to be 0.3 and 0.8 μm for 85Al₂O₃–15SiO₂ and α-Al₂O₃ fibers, respectively.     

Microwave dielectric properties of novel temperature stable high Q Li2Mg1−xZnxTi3O8 and Li2A1−xCaxTi3O8 (A = Mg,Zn) ceramics

The Li₂Mg_1?xZn_xTi₃O₈ (x = 0–1) and Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) ceramics are synthesized by solid-state ceramic route and the microwave dielectric properties are investigated. The Li₂MgTi₃O₈ ceramic shows ?_r = 27.2, Q_u × f = 42,000 GHz, and τ_f = (+)3.2 ppm/°C and Li₂ZnTi₃O₈ has ?_r = 25.6, Q_u × f = 72,000 GHz, and τ_f = (?)11.2 ppm/°C respectively when sintered at 1075 °C/4 h. The Li₂Mg_0.9Zn_0.1Ti₃O₈ dielectric ceramic composition shows the best dielectric properties with ?_r = 27, Q_u × f = 62,000 GHz, and τ_f = (+)1.1 ppm/°C. The effect of Ca substitution on the structure, microstructure and microwave dielectric properties of Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) has also been investigated. The materials reported in this paper are excellent in terms of dielectric properties and cost of production compared to commercially available high Q dielectric resonators.     

Thermolysis of acidic aluminum chloride solution and its products

A saturated acidic aluminum chloride solution with a total composition of AlCl₃·HCl·12H₂O was obtained, and its behavior under thermal treatments was studied using thermogravimetry, differential scanning calorimetry and mass spectrometry techniques. The thermolysis solid products were characterized with XRD and SEM. Four stages of the thermolysis could be distinguished. Initially, the solution lost free water molecules, and an amorphous precipitate with an approximate composition AlCl₃·HCl·12 H₂O was obtained as a product. The precipitate released eight water molecules in the temperature range 390–425 K. Then, all chlorine atoms in the form of HCl and two water molecules were outgassed at 425–485 K. The product completely lost water up to 650 K. The crystallization of the solid begins with appearance of the phase γ-Al₂O₃ at 1073 K, and the final product, α-Al₂O₃, is observed at 1323 K. The application of the saturated trichloride solutions as a binder and a promoter for activated sintering of composite ceramics on the base of alumina was examined.     

Creep behaviour of membrane and substrate materials for oxygen separation units

Advanced oxygen separation units are based on thin mixed ionic–electronic conducting ceramic layers that are mechanically supported by a porous substrate. One of the most important aspects for the long-term stability at elevated temperatures is the creep behaviour, which was assessed in the current study for various potential membrane and substrate materials. Systematic creep tests were carried out in air in the temperature range 700–1200 °C, under compressive loads of 20–120 MPa. The steady-state strain rates were described by a standard creep equation. LSCF38, MgO and all investigated cerium oxides (CeO_2?δ and its three Gd-/Pr-doped variations: Ce_0.8Gd_0.2?xPr_xO_{2?0.5(0.2?x)?δ}, with x = 0, 0.1 and 0.2) satisfied at around 800 °C a creep limit criterion, suggested to be necessary to warrant reliable long-term stability for membrane systems, but only ceria materials and MgO fulfilled this requirement also at higher temperatures.     

Methylcyclopentane transformation on Ge–Rh bimetallic catalysts prepared by organometallic grafting

Ge–Rh/Al₂O₃ catalysts were prepared by organometallic grafting, with nominal Ge loadings of 1/8, 1/4, 1/2 and 1 monolayer and actual loading of 300, 600, 2000 and 2100 ppm. The dispersion decreased proportionally to the Ge loading up to 600 ppm Ge. Methylcyclopentane was used as catalytic test reaction. The selectivity of ring opening was 70–90%. Addition of 300 and 600 ppm Ge divided the contiguous Rh sites to smaller ensembles. Here mostly 2-methylpentane gave C₁–C₅ fragments, its surface intermediate undergoing fragmentation without desorption, like on Rh with D < 60–70%. More Ge (2000 ppm) divided further Rh and also may have penetrated into subsurface layers. Here all primary ring opening products gave fragments, similarly to small Rh particles.