Synthesis of alumina nanofibers by a mercury-mediated method

Alumina nanofibers were successfully synthesized in mercury media at room temperature. Structure and morphology of the nanofibers were characterized by TEM, EDX, FESEM, XRD, TG, DTA and N₂ adsorption–desorption. The results show that the as-grown alumina nanofibers are amorphous, and have diameters of 5–15 nm and lengths up to several micrometers. After calcinated at 850 °C for 2 h, the amorphous alumina nanofibers convert to γ-Al₂O₃ nanofibers. The mechanism for the growth of alumina nanofibers was discussed and a model representing the growth process was presented. During the process, mercury will be produced by metathesis reaction of HgCl₂ and Al, Al atoms continuously dissolve into mercury and diffuse to amalgam/air interface, and then Al atoms react with oxygen and water in air, finally alumina nanofibers can be formed.     

Hydration of CAC cement in a castable refractory matrix containing processing additives

A growing demand for refractory castables with a particular behavior has been inducing a continuous technological evolution, where one of the most important aspects, is an in-depth knowledge of hydraulic binders. These materials greatly influence the rheological properties and mechanical strength evolution of castables, defining their workability range and demolding time, respectively. The hydration process of hydraulic binders is influenced by the presence of matrix and additives (dispersants and accelerators), which affect the setting and demolding time of shaped bodies. In this work, the influence of these variables on the hydration process of calcium aluminate cement was studied by means of temperature measurements, oscillatory rheometry and normal force measurement. These techniques were able to evaluate the setting behavior of different binders, either in plain water or in matrix-representative suspensions. In both cases, the dispersants presented a retarding effect on the hydration process, which was more significant for citric acid and diammonium citrate. The combination of these additives with an accelerator (Li₂CO₃) was shown to be an efficient tool to control the setting time of castables.     

High temperature mechanical characterisation of an alumina refractory concrete for Blast Furnace main trough: Part I. General context

Blast Furnace main trough is an industrial structure submitted to severe high temperature cyclic loading applied on its inner lining made of refractory concrete. The attempt to increase the lifetime of such a structure by numerical simulation requires a proper experimental characterisation of all materials involved and particularly of the refractory concrete. The present paper exposes an analysis of the conditions required for an experimental setup in accountancy with the material working conditions. Then, the development of a performing high temperature mechanical testing device aimed at characterising the castable behaviour in its service conditions is introduced. In particular an original extensometer allowing high temperature direct measurement of the specimen height variation has been developed. Lastly, results of an uniaxiale compression test carried out at intermediate temperature are presented and discussed.     

Preparation of coatings on alumina ceramic for wettability

To improve the wetting ability between ceramic and metal, titanium and copper coatings on alumina ceramics were prepared by infiltration of molten salt and electroless plating, respectively. A Ti/Cu bi-layer was also obtained by combining the two methods. The preparation process was optimized. The phase composition of the coatings was analyzed by X-ray diffraction (XRD). And the wettability of the titanium coating was investigated. The results showed that the copper droplet was easily spread on the surface of titanium coated alumina. The titanium coated alumina was well composited with high chromium white cast iron and the interface between ceramic and metal was well combined.     

Workability and setting parameters evaluation of colloidal silica bonded refractory suspensions

The efficiency of colloidal silica as a binder agent for castable matrix suspension in the presence of different setting agents and curing temperatures was evaluated. The tests were carried out trough rheometric techniques according to a systemic approach specifically developed for ceramic systems (oscillatory and normal force tests). Colloidal silica performed well as a binder agent for refractory suspensions when a suitable additive was selected. Among the additives analyzed, magnesium oxide was the most suitable for the evaluated systems. MgO addition in the range of 0.3–0.6 wt% and curing temperature of 25 °C were the suggested parameters for alumina and microsilica systems.     

A modified model for alumina membranes formed by gel-casting followed by dip-coating

α-Al₂O₃ membrane coating (top layer) was formed by dip-coating and investigated based on full factorial design of experiments. Statistical analysis showed that the effects of two variables; dipping time and withdrawal speed of the substrate, were highly significant in determining the thickness of the top layers (α<0.001). A modified model was developed to interpret quantitatively the formation process of the top layers onto the porous substrate during dip-coating. In this model, the effects of all variables affecting the top layer thickness were considered simultaneously. The experimental data corresponding to α-Al₂O₃ membranes obtained from suspensions with different volume fractions of ceramic powder showed good agreement with the model. The root mean square deviation was less than 0.02.     

Effects of different production techniques on glass–alumina functionally graded materials

Glass–alumina functionally graded materials were obtained using two different methods: percolation, which was representative of natural transport based processes, and plasma spraying, which was representative of constructive processes. The specimens produced in this way were investigated to evaluate the effect of production techniques on the final microstructure and gradient, which, in turn, govern the properties and performances of the graded systems. Moreover, post-production heat treatments were performed in order to improve the reliability of the materials examined.     

Injection molding of surface modified powders with high solid loadings: A case for fabrication of translucent alumina ceramics

Solid loading is a critical key to the fabrication of ceramic compacts with high densities via ceramic injection molding. As reported in most previous work, solid loading of ultra-fine alumina feedstock system could be achieved only up to ∼58 vol% with stearic acid (SA) as the surface modification agent. In present work, different from the traditional work in which SA has been introduced just in the powder blending process, we have successfully prepared the feedstock with a much higher solid loading up to ∼64 vol% by a prior ball milling treatment of ceramic powders with a small amount of SA before the traditional blending process. It can be attributed to that SA can be coated homogeneously around the powder surfaces by a chemical reaction induced by ball milling treatment. Highly translucent Al₂O₃ ceramics have been fabricated, which suggests an alternative route for fabrication of translucent ceramics with high quality.     

Calcination and associated structural modifications in boehmite and their influence on high temperature densification of alumina

A systematic study is reported on the calcination of boehmite and its associated structural changes, and their effect on densification features. Boehmite precursor gels have been calcined in the temperature range 250-1200 °C. The associated structural changes are identified by FTIR and XRD. The specific surface area measurements indicated a relatively high value of 169 m²/g for boehmite calcined at 400 °C; this value reduced to 4 m²/g on calcination at 1200 °C. In the temperature range 400-1000 °C, the coordination of aluminium changes from a quasioctahedral to a tetrahedral nature, which reverts to octahedral at 1200 °C. The precursor containing γ-alumina gives a 92.1% theoretical density, on sintering at 1500 °C due to the highly unstable quasioctahedral coordination. Boehmite precursors calcined at 400 °C and 1000 °C produced a density of 88.2% and 96.9%, respectively, in the sintered compact at 1500 °C. Boehmite calcined to α-alumina (1200 °C) possesses an octahedral structure having a density of 97.6% at 1500 °C.     

Micro powder injection moulding of alumina micro-channel part

A feedstock consisting of submicron alumina powder and a formulated binder, was developed to fabricate alumina micro-channel part by micro powder injection moulding. During small scale-mixing, the mixing torques of feedstocks with four different powder loadings were used to establish a suitable powder loading. The thermal and rheological properties of the selected feedstock were examined and used to establish conditions for large scale mixing, debinding and injection moulding. The micro-channel parts were pressureless sintered at different temperatures. The results showed that the moulded, debound and sintered micro-channel parts had good shape retention. The dimensions of the micro-channel part changed with the different processing steps. High densification of the micro-channel parts was achieved at sintering temperatures of 1350 °C and above. Above 1350 °C, the grain grew significantly with increasing the sintering temperatures and thus it led to a decrease in the microhardness.     

Effect of varied powder processing routes on the stabilizing performance and coordination type of polyacrylate in alumina suspensions

The influence of initial pH and energy input during suspension homogenization on the stabilizing performance and coordination type of commercial available polyacrylate dispersant were studied. Additionally to widely used rheology and electroacoustic measurement techniques the alumina suspensions were analysed with centrifugal separation and in situ ATR-FTIR to study the impact of varied powder processing in detail. In contrast to zeta potential analysis and viscosity measurements only the determination of sedimentation properties by centrifugal separation shows the effect of macroscopic changes in powder processing. A combination of positively charged alumina surface and a high shear homogenization leads to the most stable suspension. Accordingly ATR-FTIR results show a correlation between improved suspension stability and inner-sphere coordination of polyacrylate. Moreover it was possible to determine an optimal pH range for inner-sphere adsorption. It can be shown that macroscopic changes in powder processing influence the coordination of dispersant and thus the suspension stability.     

In situ vibration enhanced pressure slip casting of submicrometer alumina powders

This paper presents a novel method for improvement of particle packing in consolidation of submicrometer alumina powders by pressure slip casting. In this method, filtration cell is subjected to a mechanical vibration field with constant frequency of 50 Hz and vibration amplitudes ranging from 0 (no vibration) to 2 mm. Filtration rate, thickness and green density of the fabricated samples were measured to investigate the influence of vibration on filtration characteristics. It was revealed that employment of vibration can significantly increase filtration rate. Furthermore, there is an optimum vibration amplitude which results in the structure with the highest packing density. This value is shifted to higher vibration amplitudes as more concentrated alumina slurries is used. As the available formulation based on Darcy's law could not predict the results of the present investigation, a “Correction Factor” was utilized in order to increase the accuracy of the prediction in the presence of a vibration field.     

Growth mechanism of single-crystal α-Al2O3 nanofibers fabricated by electrospinning techniques

Crystal-growth-related microstructures and the length-to-diameter ratio of a single-crystal-type α-Al₂O₃ nanofiber were examined using HR-TEM techniques. The fibers exhibited diameters ranging from 50 to 100 nm and lengths of several tens of micrometers. During thermal treatments, the alumina fiber went through phase transformations similar to boehmite. Therefore, the phase evolution, especially the final θ- to α-Al₂O₃ stage of the phase transformation, may be the determining factor in the microstructural evolution of the nanofibers. HR-TEM techniques were utilized to demonstrate that the single crystals were formed by the coalescence of well-elongated α-Al₂O₃ colonies. The fibers grew in the [1 1 0] or [1 1 2] direction instead of [0 0 1]. A thermodynamic analysis revealed that if the α-Al₂O₃ nanofiber that transformed from θ-Al₂O₃ behaved in a stable manner, there could be a size ratio limit for the length and diameter of each α-Al₂O₃ colony. The smallest potential diameter was calculated to be around 17 nm.     

Production of highly aligned porous alumina ceramics by extruding frozen alumina/camphene body

We herein propose a new technique for producing highly aligned porous ceramics by extruding a frozen ceramic/camphene body. To accomplish this, an alumina/camphene slurry with an initial alumina content of 10 vol% was first frozen unidirectionally in a 20 mm × 20 mm mold and extruded through a reduction die with a cross-section of 5 mm × 5 mm at room-temperature. This simple process enabled the formation of porous alumina ceramics with highly aligned pores as a replica of the camphene dendrites with a preferential orientation parallel to the extrusion direction. The sample showed much higher compressive strength of 280 ± 80 kPa with a porosity of 83 vol% when tested parallel to the direction of pore alignment. In addition, these materials could be used as a valuable framework for the production of ceramic/epoxy composites, particularly with a lamellar structure, which would result in a remarkable increase in mechanical properties.     

Microstructural development of interface layers between co-sintered alumina and spinel compacts

Tests were performed to investigate the microstructure of the interface between alumina and spinel materials after high temperature thermal treatment (1500 °C). The first test involved co-sintering of co-pressed alumina and spinel compacts. Microstructures were investigated by SEM, EDS, WDS and EBSD. A microstructurally distinct layer with columnar grains of up to 40 μm length and 5 μm width was observed after 16 h at 1500 °C. Growth rate of the columnar spinel grains from parent spinel towards alumina follows parabolic kinetics, controlled by a mixed process of O²⁻ ion diffusion and interface reaction. Diffusion couples of spinel and alumina were investigated. Same columnar spinel grains were observed at the interface which grew into alumina during thermal treatment with the same kinetics as in co-sintering experiments. The shape of the phase boundaries between spinel and alumina can be a further indication of the direction of their growth.     

The effect of alumina contamination from the ball-milling of fused silica on the high temperature properties of injection moulded silica ceramic components

The amount of alumina contamination present in ball-milled silica powders has been shown to increase with increased mill time for materials manufactured during the same time period. This alumina contamination level has also been observed to vary depending on the date, and possibly the state of repair, of the ball mill itself. The associated alumina level has been shown to significantly influence the high temperature properties (at 1475 °C) of the materials, with high contamination levels not only resulting in increased flexural strength and creep resistance, but also increasing the thermal contraction of the materials when dilatometer measurements were performed to 1600 °C.     

A study in the mechanical milling of alumina powder

The mechanical milling of alumina in order to reduce grain sizes to ≤100 nm has been proposed as a means of reducing sintering temperatures and improving pressureless sintered density, particularly as a means of allowing co-firing with metallic components for biomedical implants. There is a persistent problem with contamination from the milling media, usually hardened steel which can be only partially alleviated by acid leaching. We have explored the use of alternative milling media with a view to reducing the levels of contamination. Alumina powders were milled with hardened steel, tungsten carbide, alumina and zirconia milling media under identical conditions of ball mass:powder mass ratio 10:1 and target milling times of 32 h. All of the milling media were found to cause unacceptable levels of contamination. Zirconia media gave the lowest contamination (3–4%) and in some circumstances, the addition of a small amount of zirconia may lead to increased toughness without loss of bio-compatibility.     

Field assisted and flash sintering of alumina and its relationship to conductivity and MgO-doping

We show that flash-sintering in MgO-doped alumina is accompanied by a sharp increase in electrical conductivity. Experiments that measure conductivity in fully dense specimens, prepared by conventional sintering, prove that this is not a cause-and-effect relationship, but instead that the concomitant increase in the sintering rate and the conductivity share a common mechanism. The underlying mechanism, however, is mystifying since electrical conductivity is controlled by the transport of the fastest moving charged species, while sintering, which requires molecular transport or chemical diffusion, is limited by the slow moving charged species. Joule heating of the specimen during flash sintering cannot account for the anomalously high sintering rates. The sintering behavior of MgO-doped alumina is compared to that of nominally pure-alumina: the differences provide insight into the underlying mechanism for flash-sintering. We show that the pre-exponential in the Arrhenius equation for conductivity is enhanced in the non-linear regime, while the activation energy remains unchanged. The nucleation of Frenkel pairs is proposed as a mechanism to explain the coupling between flash-sintering and the non-linear increase in the conductivity.     

Microstructure and phase evolution of alumina-spinel self-flowing refractory castables containing nano-alumina particles

The microstructure and phase composition of alumina-spinel self-flowing refractory castables added with nano-alumina particles at different temperatures are investigated. The physical and mechanical properties of these refractory castables are studied. The results show that the addition of nano-alumina has a great effect on the physical and mechanical properties of these refractory castables. With the increase of nano-alumina content in the castable composition, the mechanical strength is considerably increased at various temperatures. It is shown that nano-alumina particles can affect formed phases after firing. The platy crystals of CA₆ are detected inside the grain boundaries of tabular alumina and spinel grains in samples fired at 1500 °C. CA₆ phase can be formed at lower temperatures (1300 °C) with the addition of nano-alumina particles. As a result of using nanometer-sized alumina particles with high surface area, the solid phase sintering of the nano-sized particles and CA₆ formation can occur at lower temperatures.