Monolithic Alumina Research Articles

Microstructure and mechanical properties of alumina ceramics reinforced by boron nitride nanotubes

Boron nitride nanotubes (BNNTs)/alumina composites were fabricated by hot pressing. The mechanical properties of the composites are greatly dependent upon the content of BNNTs. In comparison with monolithic alumina, the incorporation of BNNTs results in the improvement of bending strength and fracture toughness owing to the effective inhibition of grain growth. A routine toughening mechanism, especially the bridging of BNNTs at grain boundaries and the sufficient physical bonding between BNNTs and alumina matrix, is dominantly responsible for the increase in mechanical properties.

Thermal Shock Resistance Behavior of Alumina Ceramics Incorporated with Boron Nitride Nanotubes

Alumina-matrix composite containing 2.0 wt% boron nitride nanotubes and monolithic alumina were fabricated by hot pressing, and their thermal and mechanical properties were investigated. Thermal shock resistance was evaluated by water quenching and subsequently by measuring the residual flexural strength by three-point bending test. Though the residual flexural strength of the composite is higher than that of the monolith at temperature differences lower than 280°C, the thermal shock of the composite is more sensitive to temperature change, exhibiting no significant improvement in thermal shock resistance. The variation in thermal shock resistance for the composite is resulted from the introduction of BNNTs.

Isolation of titania nanoparticles in monolithic ultraporous alumina: Effect of nanoparticle aggregation on anatase phase stability and photocatalytic activity

We report on the preparation of a new photocatalyst by impregnation of size-selected TiO 2 nanoparticles into monolithic ultraporous alumina of θ and α polymorphs. The alumina matrix is prepared by selective surface oxidation of liquid Hg-alloys in a humid atmosphere at room temperature with subsequent annealing at 1100–1300 °C. An additional treatment with trimethylethoxysilane was used to mechanically reinforce the ultraporous structure. Monodispersed titanium-oxo-alkoxy nanoparticles were generated in a sol–gel reactor with rapid micromixing, then deposited by liquid colloid impregnation and thermally treated above 350 °C to form anatase TiO 2. High-resolution TEM images show evidence of non-aggregated 5-nm nanoparticles in θ-alumina matrix at a mass loading of 26 wt.%, which conserve their anatase crystalline structure after the thermal treatment at temperatures as high as 1000 °C. The rutile phase appears only at T ≥ 1100 °C when particle size increases to ∼17 nm due to the aggregation. The photocatalytic activity (ethylene decomposition) of the non-aggregated nanoparticles is conserved over a wide temperature range, and it decreases as soon as the aggregation takes place.

One-pot synthesis of hierarchically macro/mesoporous Al 2O 3 monoliths from a facile sol–gel process

Based on the preparation of ordered mesoporous Al 2O 3 powder using P123/isopropoxide aluminum/HNO 3 reaction system, hierarchically macro/mesoporous alumina monoliths were successfully fabricated by introducing PEG8000 as the phase-segregation initiator to generate macropores. The effects of PEG8000 and P123 contents on the monolithic and hierarchical morphology were investigated systematically. With appropriate selection of the starting composition, monolithic skeletons with bimodal macropores interconnected by mesostructured walls can be synthesized in both the dried gels and calcined samples. The well-defined macropores are in close-celled structure dispersed in large-sized mesoporous matrix. The mesopore size can be regulated by adjusting the PEG8000 content while its shape is governed by P123 content. The well-developed alumina monolith, MMA-P20, exhibits high surface area of 287 m 2/g and narrowly distributed mesopores with median size of 13 nm after calcinaton.

Preparation and characterization of monolithic alumina aerogels

Alumina aerogels were prepared by a sol–gel method combined with the ethanol supercritical drying technique using aluminum tri-sec butoxide and nitric acid as the precursor and catalyzer respectively. This method affords high-surface-area alumina aerogel monoliths without the use of complexing agents. The structure and morphology of the aerogels were investigated by TEM, XRD, FTIR and BET techniques. The results confirmed that the as-prepared alumina aerogel possessed a network microstructure made up of pseudoboehmite fibers and a surface area of 690m2/g. It was transformed to γ-Al2O3 after heat treatment at 800°C without a significant loss in surface area. DMA analysis and hotdisk thermal analysis were performed to characterize the mechanical and thermal properties of the samples. The results indicated that the alumina aerogel was robust and exhibited excellent thermal insulating properties. The elastic modulus was up to 11.4MPa after drying, which is the one of the highest modulus of alumina aerogels ever reported. The thermal conductivities at 30°C and 400°C were 0.028W/mK and 0.065W/mK respectively.

Two steps bulk-surface functionalization of nanoporous alumina by methyl and vinyl-silane adsorption. Evidence for oxide surface highly reactive sites creation

Functionalization of a novel nanoporous monolithic alumina synthesized from amalgam is investigated. The structure is studied by X-ray diffraction, BET, MEB and IR spectroscopy, before and after chemical functionalization by trimethylethoxy silane adsorption and annealing at high temperature. These treatments retain both monolith microstructure and nanostructure while strongly improving material mechanical properties. Allyldimethoxysilane and alcohol adsorption on the annealed samples, proves that highly reactive sites are available for further polymer grafting, as demonstrated by a significant shift of allyldimethoxysilane ν(SiH) to 2,215 cm(-1) and adsorbed acetate formation. Simple quantum computations on model systems support this conclusion. Chemical processes reported in this paper, allow a nanostructured alumina monoliths functionalization to optimize ceramics-polymer bonds, and to tune new hybrid biomaterial properties.

Abrasive wear resistance of SiO2-doped polycrystalline alumina

A range of silica-doped alumina ceramics with addition of 0, 0.5, 1, 5 and 10 wt.% of SiO2 was prepared by hot pressing at 1450 °C. Controlled grain growth was achieved by high temperature annealing of hot pressed specimens at 1600 °C. Resistance against wet abrasive wear was tested using custom-built sample holder on a standard grinder/polisher machine. The wear rates increased with increasing rotation speed and loading force, and with size of alumina grains, and decreased with increasing content of SiO2 addition. The measured wear rates were comparable to those of alumina–SiC nanocomposites with similar alumina matrix grain size, but inferior to pure alumina ceramics with submicrometre microstructure. The wear rates of silica doped specimens increased, similarly to monolithic alumina, with the time of high temperature isothermal treatment, which primarily resulted in the growth of alumina grains. The change of the phase composition (crystallisation of low thermal expansion phase mullite) and resulting change of the stress state of the annealed silica doped specimens had negative impact on both the hardness and wear resistance.

In situ sol–gel preparation of porous alumina monoliths for chromatographic separations of adenosine phosphates

A method enabling the in situ preparation of porous alumina monoliths within 100 μm i.d. fused silica capillaries has been developed. These monoliths were prepared using the sol–gel process from a mixture consisting of an inorganic aluminum salt, a porogen, an epoxide, and a solvent. We investigated the effects of varying the preparation conditions on the physical characteristics of the monoliths with respect to their potential application in chromatographic separations. The best columns were obtained from a mixture of aluminum chloride hexahydrate, N,N-dimethylformamide, water, ethanol and propylene oxide. Adenosine phosphates were then separated in the optimized column with retention increasing according to number of phosphate functionalities.

On the role of oxygen vacancies, aliovalent ions and lattice strain in the in vivo wear behavior of alumina hip joints

We have visualized at the nanometer scale the topological, chemical and mechanical characteristics of long-term in vivo exposed bearing surfaces of femoral heads made of monolithic alumina. Four self-mated alumina retrievals were studied, which were exposed in the human body for relatively long periods of time ranging between 7.7 and 10.7 yrs. Besides conventional morphological features, monitored by atomic force microscopy, the topographic distributions of point defects and lattice strain on the surface of the heads were systematically probed by collecting high spatially and spectrally resolved cathodoluminescence spectra from zones of different wear severity. Three types of optically active point-defect site could be detected: (i) oxygen vacancies; (ii) substitutional (aliovalent) cations; and, (iii) interstitial aluminum cations. These luminescent sites represent the main defects progressively developed in the alumina lattice during exposure in human hip joints. A clear evolution toward (environmentally driven) off-stoichiometry was found with progressing wear. Moreover, the shallow electro-stimulated optical probe also detailed the presence of lattice strain fields (of both elastic and plastic nature) stored in the very neighborhood of the bearing surface. The present spectroscopic characterizations enable substantiating important tribochemical interactions between bearing surfaces and in vivo environment as pivotal parts of progressive events of wear degradation.

Electroconductive Alumina–TiC–Ni nanocomposites obtained by Spark Plasma Sintering

In the present work, the processing and characterization of electroconductive Alumina–TiC–Ni nanocomposites obtained by Spark Plasma Sintering (SPS) are described. These nanocomposites are singular due to the excellent mechanical properties they present (particular regarding Vickers hardness, 25.6 ± 0.7 GPa), as well as their extremely good wear behaviour, studied under “ball-on-disk” dry sliding conditions. The wear rate obtained was 25 times (almost 1.5 orders of magnitude) smaller than the value obtained for a monolithic alumina sintered under the same conditions. Flexural strength had been improved up to 75% with respect to the monolithic alumina processed under the same conditions. As these nanocomposites can be machined by electroerosion (EDM), they can adopt any shape for devices requiring a good mechanical performance and low wear rates.

Thermal and hydrothermal stability of a metal monolithic anodic alumina support for steam reforming of methane

In this work, a plate-type Al/Fe–Cr alloy/Al clad material was prepared into a metal-monolithic anodic alumina support (Al 2O 3/Fe–Cr alloy/Al 2O 3) to investigate its thermal and hydrothermal stability and potential use in the steam reforming of methane (SRM). re-heating of the base material at 500 °C for 3 h or longer significantly enhanced the thermal resistance temperature of the support, which was associated with the formation of the diffusion layer between the alumina layer and the alloy layer. Pore widening treatment (PWT) in 4.0 wt.% oxalic acid solution at 20 °C for 4–8 h provided an excellent anodic support that was strong enough to endure a 1000 °C calcination without the alumina layer shelling off. The appearance of an increased number of smaller fragments and cracks on the support surface was considered to be one of the main reasons for the promotion effect of PWT. During a thermal shock test of the heating–cooling cycles from ambient temperature to 800 °C for 5000 times, no alumina layer was found to shell off from the alloy interlayer, which highlighted the good adhesion between the alumina layer and the alloy layer in the anodic alumina support. Although an obvious decrease of surface area was observed when calcining the support in air or steam at 700 °C, the anodic alumina-supported Ni catalyst showed favorable catalytic durability and hydrothermal stability in the stationary SRM test for 3000 h and the DSS-like (daily start-up and shut-down) test for 500 times in a typical hydrothermal environment (700 °C, Steam/Carbon = 3, 157,000 mL/(h g)). Especially, under an electrical-heating pattern, the SRM reaction system could reach stability within ca. 10 min, offering a strong possibility for shortening the start-up time of conventional SRM reformers from 1 to 2 h to a few minutes.

Colloidal processing and CO2-capture performance of hierarchically porous Al2O3-zeolite 13X composites

Hierarchically porous composites for CO2-capture have been produced by coating the inner walls of foam-like macroporous alumina monoliths, produced by templated synthesis, with microporous zeolite ...

A comparison of the effects of multi-wall and single-wall carbon nanotube additions on the properties of zirconia toughened alumina composites

The use of multi-wall carbon nanotubes (MWCNTs) or single-wall carbon nanotubes (SWCNTs) as filler in ceramic matrices could create composites with exceptional mechanical properties. We have prepared dense monolithic alumina (Al 2O 3) and zirconia-toughened alumina (ZTA) composites with additions of 0.01 wt% of MWCNTs or 0.01 wt% of SWCNTs by conventional sintering and have demonstrated that the mechanical properties depend on (a) the distribution of CNTs in the matrix and (b) the interaction between the ceramic phases and CNTs. The fracture toughness of Al 2O 3 ceramics reinforced with SWCNTs was significantly better than those reinforced with MWCNTs. However, fracture toughness in MWCNT-reinforced ZTA increased 41% over ZTA free of the toughening agent and 44% over ZTA reinforced with SWCNTs. A well dispersed and small amount of MWCNTs was enough to produce an increase of fracture toughness in ZTA composites.

Improved wear behaviour of alumina–aluminium titanate laminates with low residual stresses and large grained interfaces

The wear behaviour of a monolithic alumina and an alumina–aluminium titanate laminated structure was studied. The laminate, containing surface fine grained alumina layers and internal composite layers with 10 vol.% of aluminium titanate, showed relatively low (≅20 MPa) compressive residual stresses at the surface. Interfaces between layers were constituted by large alumina grains (up to ≅50 μm) that promoted toughening due to crack deflection and branching. Wear tests were performed on square specimens (30 mm × 30 mm × 6 mm) using the pin-on-disc method. The laminates showed higher wear resistance than the monolithic alumina. The analysis of the results together with SEM-EDX observations was performed to identify possible wear mechanisms. The wear resistance improvements are discussed in terms of the residual stresses in the laminate and the properties provided by the special microstructure of the interfaces.

Relationship between microstructure and abrasive wear resistance of Al 2O 3–FeAl 2O 4 nanocomposites produced via solid-state precipitation

The work investigates the correlation between the microstructure and wear behaviour of novel Al 2O 3–FeAl 2O 4 nanocomposites, developed by precipitation of FeAl 2O 4 particles through reduction aging of Al 2O 3–10 wt.% Fe 2O 3 solid solutions in N 2/4%H 2. Reduction aging at 1450 °C for 10 and 20 h resulted in considerable improvements in abrasive wear resistance. The nanocomposites developed from solid solutions doped additionally with ∼250 ppm of Y 2O 3 contained finer intergranular second phase particles (by a factor of ∼2) and showed further improvements in the wear resistance. Doped nanocomposites reduction aged for 20 h at 1450 °C exhibited the minimum wear rate (reduced by a factor of ∼2.5 with respect to monolithic Al 2O 3). The suppression of fracture-induced surface pullout in the presence of intragranular nanosized second phase particles was the major factor responsible for the improved wear resistance of the nanocomposites with respect to monolithic alumina; microstructures without these intragranular nanoparticles showed no improvement. Higher aging temperature led to the presence of coarse (>2 μm) intergranular FeAl 2O 4 particles which had a detrimental effect on the wear resistance.

Ballistic and physical properties of highly fractured alumina

It is known that ceramic materials in armour are often shattered by a penetrating projectile and the resistance to penetration is therefore dependent upon the properties of this failed material. A small explosive charge was used to shatter a confined 95% alumina tile by passing the stress wave from an explosive detonation through a steel cover plate and into the ceramic. This fractured material was shown to have an elastic modulus of 130 GPa compared to 330 GPa for the monolithic alumina. Impact indentation tests and ballistic depth of penetration tests were performed on the intact ceramic, shattered ceramic and a pressed ceramic powder compact. It was found that the fractured material had a ballistic efficiency approximately 70% of that of the monolithic alumina and was also able to cause significant erosion to the projectile.

Photo-quenched of the luminiscence signal in Co(II)-doped alumina prepared by the sol–gel method

Co(II)-doped alumina monoliths prepared by the sol–gel method were laser irradiated producing fluorescence. The intensity of this defect-induced fluorescence was exponentially reduced with the irradiation time to practically disappear. The rate the fluorescence intensity decays was modeled as a double exponential function of the irradiation time; the characteristic times associated with the decays are in the range of seconds. The suppression of the luminescence was associated with the local heating produced by the highly focused laser beam in a small area (≈ 2 μm in diameter) on the sample. This heating process reduces physical (grain boundaries and surface states) and chemical (oxygen vacancies) defects present in the sol–gel samples. Some residual fluorescence still remains after long periods of illumination. The characteristic times for alumina samples are compared with those obtained for other metal oxides prepared also by the sol–gel method.

Shrinking Core Model for Knudsen Diffusion-Limited Atomic Layer Deposition on a Nanoporous Monolith with an Ultrahigh Aspect Ratio

ZnO was grown on nanoporous alumina monoliths by atomic layer deposition (ALD) from diethylzinc (DEZ) and water. The alumina monolith has a cylindrical shape with a length of 6.2 ± 2.0 mm and a diameter of 1.93 ± 0.78 mm, of which the average pore diameter and specific surface area are 8.9 nm and 256 m2/g, respectively. The ALD process in the nanoporous monolith, of which the aspect ratio is higher than 105, was limited by the Knudsen diffusion of DEZ within the pores. The minimum exposure time (τcy) of DEZ for the complete coverage of ZnO on the cylindrical monolith was predicted to be τcy = ρOHR2/4DeCo by using the shrinking core model (SCM) where ρOH is a molar volumetric density of OH groups, R is a radius of the monolithic cylinder, De is an effective diffusion coefficient of DEZ in porous alumina, and Co is a concentration of DEZ on the exterior surface. Using a value of De (1.4 × 10−2 cm2/s) obtained from the SCM, the minimum exposure time was predicted to be ∼66 min. Indeed, the complete coverage of ZnO was experimentally accomplished in an exposure time of 70 min. In addition, it is also demonstrated that the minimum exposure time is largely dependent in the shape (e.g., planar, cylindrical, or spherical) of the monolith.

Facile synthesis of ordered mesoporous γ-alumina monoliths via polymerization-based gel-casting

Porous γ-alumina monoliths with hexagonal mesoporous structures were prepared starting from well-ordered mesoporous alumina powders which were firstly synthesized by the self-assembly of aluminum isopropoxide and a triblock copolymer in the presence of nitric acid. The alumina powders were fused together to form alumina monoliths by the integration of polymer gel-casting together with the calcination at different temperatures in air. The powdery and monolithic products were investigated using transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), small-angle X-ray scattering (SAXS), powder wide-angle X-ray diffraction (WA-XRD) and the nitrogen adsorption–desorption isotherm measurements. As results, the as-prepared powdery alumina possesses ordered p6 mm hexagonal mesoporosity, high BET surface area and narrow pore-size distribution. After high-temperature calcination, the obtained alumina monoliths still exhibit ordered mesoporosity with hexagonal symmetry, which is very similar to that of the original powdery sample except for the reduced pore volume and specific surface area. The method can be extended to prepare other porous metal oxide monoliths when appropriate powdery metal oxides with a connected pore system are applied.

Facile Synthesis of Hierarchical Alumina Monoliths with Tubular Macropores and Ordered Mesoporous Walls

Abstract A facile approach of evaporation-induced self-assembly plus coating was developed using nonwoven fabrics (NF) as a sacrificial scaffold, leading to the acquisition of hierarchically macro/mesoporous alumina monolith. The macropores templated by NF show a tubular structure of about 10 µm in channel diameter. The monolith is composed of ordered mesostructured wall with large-sized mesopores and high thermal stability. The regularity of the mesopores can be maintained up to 900 °C, even after the amorphous Al2O3 transforms to crystalline structure (γ-Al2O3). This facile route can be extended to the preparation of other hierarchically macro/mesoporous monoliths such as TiO2, ZrO2, ZnO, et al.