Sol–gel processing and phase characterization of Al2O3 and Al2O3/SiC nanocomposite powders

Monolithic Al₂O₃ and Al₂O₃/SiC nanocomposite powders were prepared by sol–gel processing. The process involved the precipitation of Al(NO₃)₃·9H₂O with NH₄OH in excess water to form boehmite (AlOOH). XRD indicates that the subsequent thermal reaction proceeds by the phase transformation sequence AlOOH, γ-, δ-, θ-, to α-Al₂O₃. The ²⁷Al NMR spectra indicate a gradual increase in the proportion of Al in the tetrahedral sites of the γ-, δ- and θ-Al₂O₃ formed at increasing calcination temperatures. Complete transformation to octahedral Al (α-Al₂O₃) is marked by the abrupt disappearance of tetrahedral Al. Al₂O₃/SiC nanocomposite powders were prepared by adding α-SiC powder to the boehmite precursor at the precipitation stage. Upon heating, the ²⁹Si NMR spectra of the Al₂O₃/SiC powders reveal α-SiC, Al₂O₃·xSiO₂ and SiO₂ phases. Stable α-Al₂O₃ and α-Al₂O₃/SiC nanocomposite powders are formed at 1200 and 1300 °C, respectively. It appears likely that the presence of SiC modifies the thermal behaviour of the Al₂O₃ in the nanocomposites by stabilising the Al₂O₃ phases with concomitant oxidation of SiO₂.     

Al2TiO5–Al2O3–TiO2 nanocomposite: Structure,mechanical property and bioactivity studies

Novel biomaterials are of prime importance in tissue engineering. Here, we developed novel nanostructured Al₂TiO₅–Al₂O₃–TiO₂ composite as a biomaterial for bone repair. Initially, nanocrystalline Al₂O₃–TiO₂ composite powder was synthesized by a sol–gel process. The powder was cold compacted and sintered at 1300–1500 °C to develop nanostructured Al₂TiO₅–Al₂O₃–TiO₂ composite. Nano features were retained in the sintered structures while the grains showed irregular morphology. The grain-growth and microcracking were prominent at higher sintering temperatures. X-ray diffraction peak intensity of β-Al₂TiO₅ increased with increasing temperature. β-Al₂TiO₅ content increased from 91.67% at 1300 °C to 98.83% at 1500 °C, according to Rietveld refinement. The density of β-Al₂TiO₅ sintered at 1300 °C, 1400 °C and 1500 °C were computed to be 3.668 g cm^?3, 3.685 g cm^?3 and 3.664 g cm^?3, respectively.Nanocrystalline grains enhanced the flexural strength. The highest flexural strength of 43.2 MPa was achieved. Bioactivity and biomechanical properties were assessed in simulated body fluid. Electron microscopy confirmed the formation of apatite crystals on the surface of the nanocomposite. Spectroscopic analysis established the presence of Ca and P ions in the crystals. Results throw light on biocompatibility and bioactivity of β-Al₂TiO₅ phase, which has not been reported previously.     

In situ growth of TiCxN1−x whiskers in Al2O3 matrix for ceramic cutting tools

For resolving the dispersing problem of whiskers and fabricating cutting tool materials with excellent properties, an in situ growth technology is used to directly synthesize TiC_xN_1?x whiskers in α-Al₂O₃ matrix by a carbothermal reduction process at a temperature range of 1250–1550 °C. The raw materials are consisted of TiO₂, carbon, nickel, and NaCl. Various molar ratios from 1:3, 1:4, 1:5 to 1:7 of TiO₂:C are experimentally used. For the molar ratio 1:3, a few whiskers can be found only at the synthesis temperature of 1550 °C. For the other molar ratios, large amount of whiskers can be observed at the whole synthesis temperature range. The highest yield of whisker is observed when the synthesis temperature is 1250 °C and the molar ratio of TiO₂:C is 1:4. The compound AlO appears at 1250 °C and AlN instead at 1550 °C. The majority of the synthesized whiskers display an ideal aspect ratio of 10–30 with a diameter of 1–3 μm. No obviously influence on the whiskers growth by the present of α-Al₂O₃ matrix powder can be noted.     

Fabrication of asymmetric alumina membranes: I. Effect of SrO addition on thermal stabilization of transition aluminas

The effect of SrO addition on the thermal stabilization of transition aluminas with the aim of producing membrane layers (supported and unsupported) has been investigated. Al₂O₃–x wt.% SrO composite powders (x = 1, 3, 5, 8) were synthesized by co-precipitation of the hydroxides from solutions of AlCl₃ and Sr(NO₃)₂ salts using NH₄OH as a precipitating agent. Optimum SrO dopant concentration regarding the transition aluminas stabilization effect was determined to be 5 wt.% based on XRD analysis. STA analysis showed a 30 °C shift versus higher temperatures in the transformation of final transitional alumina (θ-Al₂O₃) to stable alpha phase due to addition of 5 wt.% SrO. The mechanism of transition aluminas thermal stabilization as a result of SrO addition is thoroughly discussed. Unsupported alumina membranes were prepared by drying boehmite sols at 600, 800, 1000 and 1100 °C. The effect of calcination temperature on surface area, pore size distribution of unsupported membranes containing 5 wt.% SrO has been investigated. The microstructure of unsupported and supported membranes revealed quite different. Smaller grains in the supported layers were attributed to the interaction between support and membrane.     

Dispersion of aqueous nano-sized alumina suspensions using cationic polyelectrolyte

In this paper, we investigate and report the effects of a cationic polyelectrolyte, polydiallydimethylammonium chloride (PDADMAC), on the stability of nano-sized alumina (α-Al₂O₃) suspension. Due to the static electrical repulsion interactions, PDADMAC show significant adsorption on α-Al₂O₃ only at alkaline pH. Considerable amount of non-adsorbed free PDADMAC exist in α-Al₂O₃ suspensions dispersed by PDADMAC at acidic and neutral pH. At 1.0 wt.% PDADMAC addition, α-Al₂O₃ became highly positively charged (>50 mV) in the entire pH range contrast with that the stability of α-Al₂O₃ suspension is enhanced only over the basic pH range in the presence of anionic polyelectrolyte. Free PDADMAC electrolyte in suspension results in increasing viscosity of α-Al₂O₃ suspensions dispersed with PDADMAC at alkaline and neutral pH. However, non-adsorbed free PDADMAC in suspension do not contribute to viscosity increase in acidic α-Al₂O₃ suspensions and α-Al₂O₃ suspensions dispersed with PDADMAC show the best flow behavior at acidic pH.     

Synthesis of γ-Al2O3 nanopowders by freeze-drying

This work studies the synthesis of γ-Al₂O₃ nanopowders by a freeze-drying method. Aqueous solutions of Al₂(SO₄)₃·18H₂O were used as precursors to Al concentrations of 0.76, 1.00 and 1.40 M. Homogeneous spherical granules with diameters ranging from 1 to 100 μm have been obtained. These porous granules are constituted by soft agglomerates of nanoparticles with primary particle size lower than 20 nm. The microstructure of the agglomerates largely depends on the freezing kinetics. After drying amorphous aluminium sulphate powder is obtained that decomposes at 825 °C leading to the formation of γ-Al₂O₃. Physicochemical study of the freeze-dried powders is performed through particle size distribution and zeta potential measurements. The characterisation of the powders is evaluated considering the influence of processing parameters such as the salt concentration, the freezing rate and the thermal treatment for the synthesis and the dispersing conditions of the obtained powders. By adjusting the dispersing conditions a minimum particle size <30 nm is measured, thus confirming that granules can be easily dispersed into nanoparticles.     

Ageing response of a Al–Cu–Li 2198 alloy

The effects of different ageing treatments on microstructure evolution, properties and fracture are investigated in the present study. 2198 alloy exhibits strong ageing response during ageing. It is found that tensile properties, hardness and conductivity of 2198 alloy are very sensitive to ageing temperatures, which corresponds to different microstructures. In the naturally-aged condition (T3), only δ′ (Al₃Li) was detected. After artificial ageing (T8), large amounts of precipitates emerged and major precipitates that were detected turned to be δ′, θ′ (Al₂Cu) and T₁ (Al₂CuLi) phase. Exposure to higher temperature caused greater amounts of the precipitation. The constitution and morphology of precipitates varies with different ageing temperature; the major precipitates are δ′, θ′ when ageing below 160 °C, while above 160 °C, T₁ phase comes out in large numbers, becoming dominate strengthening phases gradually. Fracture transforms from a typical dimple type to a dimple-intergranular mixed type with the rise of ageing temperature.     

Mechanochemical synthesis of Al2O3/Co nanocomposite by aluminothermic reaction

In this work, Al₂O₃/Co nanocomposite was successfully prepared by mechanochemical reaction between Co₃O₄ and Al powders in a planetary high energy ball mill. The mechanism of the reaction was dealt using X-ray diffraction (XRD), differential thermal analysis (DTA), and thermodynamics calculations. It was found that Co₃O₄ reacts with Al through a self-sustaining combustion reaction after an incubation period of 50 min and the reaction between Co₃O₄ and Al involves two steps. First, Co₃O₄ reacts with Al to form CoO and Al₂O₃ at the temperature around melting point of Al, and at higher temperature, CoO reacts with remaining Al to form Co and Al₂O₃. Mechanical activation process decreases the reaction temperature from 1041 °C for as-received Co₃O₄ and Al powder mixture to 869 °C for 45 min milled powders. After annealing of powder milled for 12 h, no phase transformation has been detected. The crystallite sizes of both α-Al₂O₃ and Co remained in nanometeric scale after annealing at 1000 °C for 1 h.     

Fracture behaviour of α-Al2O3 ceramics reinforced with a mixture of single-wall and multi-wall carbon nanotubes

The extraordinary mechanical properties of single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs) have generated interest in incorporating them as toughening agents in ceramics. This work describes the fracture behaviour of an alumina (Al₂O₃) ceramic reinforced with a mixture of 0.05 wt% MWCNTs + 0.05 wt% SWCNTs. The CNT/Al₂O₃ nanocomposite was pressureless sintered in air using graphite powder as bed powder at 1520 °C for 1 h. The hardnesses and fracture toughnesses were lower than for pure Al₂O₃ and Al₂O₃ + 0.1 wt% SWCNTs and Al₂O₃ + 0.1 wt% MWCNTs. A predominantly transgranular fracture mode with a decrease in crack deflection and no pull-out was observed in the SWCNT + MWCNT–Al₂O₃ nanocomposite. MWCNTs had to the best reinforcing effect in Al₂O₃ nanocomposite.     

A study on in situ growth of TaC whiskers in Al2O3 matrix powder for ceramic cutting tools

In situ growth of tantalum carbide (TaC) whiskers was synthesized in an α-Al₂O₃ matrix powder via a carbothermal reduction technique within a temperature range of 1350–1500 °C in an argon atmosphere. The starting materials consisted of Ta₂O₅, C, Ni and NaCl powders. Different mixing methods and various reaction temperatures were employed. Most of the prepared whiskers were 0.2–0.5 μm in diameter and 5–15 μm in length. The reaction temperature of 1400–1450 °C was suitable for the growth of TaC whiskers and a wet mixing method was beneficial to increase the whisker yield. Some of the whiskers exhibited the needle shape while others exhibited the screw shape. The growth mechanism of the whiskers was a complex mechanism involving a helical screw dislocation mechanism and a vapor–liquid–solid process. No obvious influences of the Al₂O₃ matrix powder on the growth of TaC whiskers were found and the major impurities in the obtained powder were TaC particles, nickel and unreacted carbon.     

Phase change in calcination process for BaTi2O5 and proposal of a new temperature profile

The mixture of BaCO₃ and rutile-type TiO₂ powders mixed with wet ball-milling was calcined in air and the phase change in calcination process was investigated in detail. The slow heating in the temperature range from 800 to 850 °C was effective to change BaTiO₃ to BaTi₂O₅ in the following heating process. The influence of two factors in calcination for the synthesis of BaTi₂O₅, which are temperature profile with slow heating and packing state of the mixture, was investigated. The slow heating in two temperature ranges, 800–850 °C and 1100–1160 °C, was effective for the synthesis of BaTi₂O₅. The production amount of BaTi₂O₅ was increased by using powder compacts rather than powder form. By calcining powder compacts at 1160 °C for 2 h, the powder of BaTi₂O₅ having an α_s value of 0.93, which is semi-quantitatively corresponding to BaTi₂O₅ production ratio, was obtained.     

Synthesis and photoluminescence study of La1.8Eu0.2O3 coating on nanometric α-Al2O3

In this work the La_1.8Eu_0.2O₃ coating on nanometric alpha-alumina, α-Al₂O₃@La_1.8Eu_0.2O₃, was prepared for the first time by a soft chemical method. The powder was heat-treated at 100, 400, 800 and 1200 °C for 2 h. X-ray powder diffraction patterns (XRD), transmission electronic microscopy (TEM), emission and excitation spectra, as well as Eu³⁺ lifetime were used to characterize the material and to follow the changes in structure as the heating temperature increases. The Eu³⁺ luminescence data revealed the characteristic transitions ⁵D₀ → ⁷F_J (J = 0, 1 and 3) of Eu³⁺ at around 580, 591 and 613 nm, respectively, when the powders were excited by 393 nm. The red color of the samples changed to yellow when the powder was annealed at 1200 °C. The decrease in the (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) ratio from around 5.0 for samples heated at lower temperatures to 3.1 for samples annealed at 1200 °C is consistent with a higher symmetry of the Eu³⁺ at higher temperature. The excitation spectra of the samples also confirms this change by the presence of a more intense and broad band at around 317 nm, instead of the presence of the characteristic peak at 393 nm, which corresponds to the ⁷F₀ → ⁵L₆ transition of the Eu³⁺. The lifetimes of the ⁵D₀ → ⁷F₂ transition of Eu³⁺ for the samples heat-treated at 100, 400, 800 and 1200 °C was evaluated as 0.57, 0.72, 0.43 and 0.31 ms, respectively.     

Heteroepitaxial growth of δ-Bi2O3 thin films on CaF2(1 1 1) by chemical vapour deposition under atmospheric pressure

We have succeeded in achieving the heteroepitaxial growth of a δ-Bi₂O₃ thin film on a CaF₂(1 1 1) substrate by means of chemical vapour deposition under atmospheric pressure. The film grew with a strong (1 1 1) orientation. From X-ray pole figures, it was observed that the δ-Bi₂O₃ film grew on the CaF₂(1 1 1) substrate with 60° rotational in-plane domains. The growth mode was of a 3D island type, and the grain size decreased with increasing oxygen pressure during the δ-Bi₂O₃ film growth, improving the overall surface smoothness.     

Facile route to obtain a highly bioactive SiO2–CaO–Na2O–P2O5 crystalline powder

This work describes a facile method to obtain highly bioactive crystalline powders of the SiO₂–CaO–Na₂O–P₂O₅ system using a simple route: solid state reaction. Success in obtaining the highly bioactive crystal phase of interest (sodium calcium silicate Na₂Ca₂Si₃O₉ containing phosphorus) involves heating the starting reactant powder mixture under an oxidizing atmosphere for 480 min in the temperature range 950–1000 °C. Despite a significant loss of phosphorus at heat treatment temperatures above 950 °C, the resulting Na₂Ca₂Si₃O₉ crystal phase is thermally stable up to 1100 °C. Longer treatment times favor the formation of a secondary phase (sodium calcium phosphate NaCaPO₄), which, according to recent studies, further increases the bioactivity of a similar material. Finally, in vitro bioactivity tests in acellular simulated body fluid (SBF) of a powder containing only the Na₂Ca₂Si₃O₉ phase has shown behavior similar to that of Biosilicate® — an ~ 99.5% crystalline glass–ceramic whose outstanding characteristics of interaction with living tissue have already been reported in the literature.     

Effects of multicomponent catalyzer on preparation of ultrafine α-Al2O3 at low sintering temperature

α-Al₂O₃ powder, with a purity of 99.95% and an average particle size of 80 nm, was prepared via the thermal decomposition of AACH precursor. During the AACH preparation, OP-10 (alkylphenol ethoxylates) was used as a dispersant and a deflocculating agent, and a self-confected multicomponent catalyzer (MC) was used to prevent powder agglomeration. Our experiments illustrated that: apart from the necking and agglomeration-preventing effect during the sintering process, MC had potential promotive effects on the reduction of α-Al₂O₃ phase transformation temperature by maintaining the powder particles in a comparatively dynamic-sintering state, thus significantly improving the dispersibility of the sintered powder. BET and ICP analyses indicated that MC was propitious for increasing the specific surface area of the formed α-Al₂O₃ powder and induced very tiny impurities in the sintered products.     

Effect of aluminum source on the synthesis of AlN powders from combustion synthesis precursors

In the present work, the carbothermal reduction method was employed to fabricate the AlN powders by utilizing the combustion synthesized precursor derived from the mixed solution comprised of an aluminum source (Al(NO₃)₃ or Al₂(SO₄)₃ or AlCl₃), glucose, nitric acid, and urea. Effects of aluminum source on the particle size and morphology of precursors as well as synthesized AlN powders were studied in detail. The size and morphology of precursors, derived from various aluminum sources, had exhibited significant differences. The precursor from Al(NO₃)₃ source had completed the nitridation reaction at 1500 °C in 2 h. However, the nitridation reactions of the precursors from Al₂(SO₄)₃ or AlCl₃ source furnished at increased temperature of 1550 °C in 2 h. Moreover, the AlN powders from various aluminum sources have been synthesized directly from γ-Al₂O₃ without γ-Al₂O₃ to α-Al₂O₃ phase transition. The AlN powders from Al(NO₃)₃, calcined at 1550 °C for 2 h, were comprised of well-distributed spherical particles with an average size of 80 nm. While the AlN powders from AlCl₃ or Al₂(SO₄)₃ consisted of heterogeneously distributed spherical particles ranging from 100 to 200 nm or from 80 to 150 nm, respectively.     

Mechanical properties of in-situ toughened Al2O3/Fe3Al

Mechanical properties of in-situ toughened Al₂O₃/Fe₃Al nano-/micro-composites were measured. Effects of Fe₃Al content, sintering temperature and holding time on properties and microstructure of the composites were investigated. The addition of Fe₃Al nano-particles decreased the aspect ratio and grain size of Al₂O₃, and changed the fracture mode of composites. The maximum bending strength and fracture toughness were 832 MPa and 7.96 MPa m^1/2, which were obtained in Al₂O₃/5 wt.% Fe₃Al sintered at 1530 °C and Al₂O₃/10 wt.% Fe₃Al sintered at 1600 °C, respectively. Compared to monolithic alumina, the strength increased by 132% and the toughness increased by 73%. The improvement in the mechanical properties of the composites was attributed to the change in fracture mode from intergranular fracture to transgranular fracture, the “in-situ reinforced effect” arising from the platelet grains of Al₂O₃ matrix, refined microstructure by dispersoids, as well as crack deflection and bridging of intergranular and intragranular Fe₃Al.     

Effect of B2O3 and CuO additives on the sintering temperature and microwave dielectric properties of Ba(Mg1/3Nb2/3)O3 ceramics

B₂O₃ added Ba(Mg_1/3Nb_2/3)O₃ (BBMN) ceramics cannot be sintered below 930 °C. However, when CuO was added to them, they were sintered even at 850 °C. The amount of the Ba₂B₂O₅ second phase, which was formed in the BBMN ceramics decreased with the addition of CuO. Therefore, the CuO additive is considered to react with the B₂O₃ inhibiting the reaction between B₂O₃ and BaO. A dense microstructure without pores developed with the addition of a small amount of CuO. The bulk density, dielectric constant (ɛ_r) and Q-value increased with the addition of CuO, but decreased when a large amount of CuO was added. Excellent microwave dielectric properties were obtained for the Ba(Mg_1/3Nb_2/3)O₃ + 2.0 mol% B₂O₃ + 10.0 mol% CuO ceramic sintered at 875 °C for 2 h, with values Q_xf = 21 500 GHz, ɛ_r = 31 and temperature coefficient of resonance frequency (τ_f) = 21.3 ppm/°C.     

Plasticized alkaline solid polymer electrolyte system

Alkaline solid polymer electrolytes were prepared by utilizing poly(vinyl alcohol) (PVA), potassium hydroxide (KOH), α-Al₂O₃ and different amounts of propylene carbonate (PC). The addition of PC to the PVA:KOH:α-Al₂O₃:H₂O increased its conductivity by three orders of magnitude to the reading of ∼ 10^− 4 S cm^− 1. The plot for log σ − 1 / T showed a transformation from liquid-like conductivity to Arrhenius type. The dielectric constant (ε_r) of the samples increases with increasing PC concentrations and temperatures. Scanning electron microscopy also shows the effect of PC on the polymer electrolytes surface. Thermogravimetric studies show that the thermal stability of the polymer electrolytes decreases with the addition of PC.     

Preparation and luminescent properties of Cr3+:Al2O3 nano-powders by low-temperature combustion synthesis

Cr³⁺:Al₂O₃ nano-powders were prepared through low-temperature combustion synthesis (LCS) method by using glucose as a dispersion agent for the first time. The Cr³⁺:Al₂O₃ nano-powders samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and luminescence spectrometer. XRD results showed that pure α-Al₂O₃ phase was obtained for the sample fired at 1100 °C for 0.5 h. TEM results indicated that nano-powders were well dispersed. Luminescence spectrum analysis results indicated that the excitation spectrum of Cr³⁺:Al₂O₃ nano-powders consisted of two bands peaking at 462 nm and 579 nm, respectively, and the emission spectrum consisted of two bands peaking at 692 nm and 668 nm, respectively.