Reaction sintering of colloidal processed mixtures of sub-micrometric alumina and nano-titania

The fabrication of composites formed by alumina grains (95 vol%) in the micrometer size range and aluminium titanate nanoparticles (5 vol%) by reaction sintering of alumina (Al₂O₃) and titania (TiO₂) is investigated. The green bodies were constituted by mixtures of sub-micrometric alumina and nano-titania obtained from freeze-drying homogeneous water based suspensions, and pressing the powders. The optimization of the colloidal processing variables was performed using the viscosity of the suspensions as control parameter. Different one step and two step sintering schedules using as maximum dwell temperatures 1300 and 1400 °C were established from dynamic sintering experiments. Specimens cooled at 5 °C/min as well as quenched specimens were prepared and characterized in terms of crystalline phases, by X-ray diffraction, and microstructure by scanning electron microscopy of fracture surfaces.Even though homogeneous final materials were obtained in all cases, full reaction was obtained only in materials treated at 1400 °C. The microstructure of the composites obtained by quenching was formed by an alumina matrix with bimodal grain size distribution and submicrometric aluminium titanate grains located inside the largest alumina grains and at triple points. However a cooling rate of 5 °C/min led to significant decomposition of aluminium titanate. This fact is attributed to the small size of the particles and the effect of the alumina surrounding matrix.     

The effect of polyvinyl alcohol as a binder and stearic acid as an internal lubricant in the formation, and subsequent sintering of spray-dried alumina

The production of ceramic components using fewer processing steps on a shorter timescale is very important when considering the industrial and economic aspects of the manufacture of these materials in bulk. Spray-dried granules are expected to give compacts with fewer defects due to their low shear strength compared to conventional powders. Several extent studies show results for product of high relative densities (∼50% at 10 MPa), however, this study arrives at a process for making ceramic components with comparable density (48-49%) at 10 MPa), using less processing time and fewer processing steps which becomes extremely important when one considers the industrial aspects such as bulk production and manufacturing cost. In the present investigation, 35 vol% alumina slurries with 0.5% weight dispersant (ammonium polyacrylate) have been synthesized with different contents of binder (PVA) and lubricant (stearic acid). It is found that variations in the amounts of these additives plays a significant role in the formation of spray-dried granules, as well as the subsequent consolidation and densification of the compacts made using the granule particles. There is support for adopting the concept of a ‘compact process’.     

Sintering behavior of nano alumina powder shaped by pressure filtration

In the present study, the sintering behavior of a commercial nano alumina powder with an initial particle size of 100 nm was investigated. The shrinkage response of the powder formed by pressure filtration (PF) during non-isothermal sintering was measured in a laser assisted dilatometer at three different heating rates of 2, 10 and 25 °C min⁻¹ up to 1400 °C. In order to calculate the activation energy of sintering, constant rate of heating (CRH) was employed and the activation energy was found to be 608 ± 20 kJ mol⁻¹ for iso-density method. The heating rate was demonstrated to have a vital role on densification behavior and final grain size. The mean grain size of the full dense specimens decreased from 875 to 443 nm when the heating rate increased from 2 to 25 °C min⁻¹.     

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.     

Effect of freeze-drying treatment on the optical properties of SPS-sintered alumina

This study looks at the influence of alumina powder processing on the preparation of transparent alumina by Spark Plasma Sintering (SPS). Zeta potential measurements were carried out on alumina suspensions in order to determine the best dispersion conditions. Stable slurries were submitted to a spray freeze drying process and their sintering behavior was compared with the corresponding non spray freeze dried powders. Transparent alumina samples were successfully prepared from alumina powders by Spark Plasma Sintering. An optical model considering pore and grain size distributions has been developed to obtain information about porosity in dense materials. It was found that the final density and, accordingly, the optical properties were improved when spray freeze dried starting powder was used.     

A new direct coagulation casting process for alumina slurries prepared using poly(acrylate) dispersant

Direct coagulation casting (DCC) of concentrated aqueous alumina slurries prepared using ammonium poly(acrylate) dispersant has been studied using MgO as coagulating agent. Addition of small amounts of MgO increased the viscosity of the concentrated alumina slurries with time and finally transformed it in to a stiff gel. Sufficient working time for degassing and casting could be achieved by cooling the slurries to a temperature of ∼5 °C after proper homogenization after the addition of MgO. The DCC slip with alumina loading in the range of 50–55 vol% showed relatively low viscosity (0.12–0.36 Pa s at shear rate of 93 s⁻¹) and yield stress (1.96–10.56 Pa) values. The wet coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% had enough compressive strength (45–211 kPa) for handling during mould removal and further drying. The coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% showed linear shrinkage in the range of 4.8–2.3 during drying and 17.1–16.2 during sintering respectively. Near-net-shape alumina components with density >98% TD could be prepared by the DCC process.     

Creep behaviour of alumina/YAG composites prepared by different sintering routes

Al₂O₃-5 vol.% Y₃Al₅O₁₂ (YAG) composite powders have been prepared by surface doping of α-alumina powders by an yttrium chloride aqueous solution. Two commercial, one submicron-sized, the other ultra-fine, alumina powders were compared as matrix materials. YAG phase was yielded by an in situ reaction promoted by the subsequent thermal treatment of the doped powders. In particular, a flash soaking into a tubular furnace kept at a fixed temperature in the range 1050-1150 °C was employed, for inducing the crystallization of yttrium-aluminates on the alumina particles surface, but avoiding a relevant crystallites growth. After that, aqueous suspensions of the calcined powders were dispersed by ball-milling and cast into porous moulds or simply dried in a oven. Slip cast green bodies were densified by pressure-less sintering, while powdered samples were consolidated by hot pressing or spark plasma sintering. The low- and high-temperature mechanical performances of the sintered materials were investigated and related to monolithic aluminas behaviour as well as to the composites microstructures. It is shown that the hot-pressed and spark plasma sintered composites present a significantly lower creep rate as compared to reference, monolithic alumina samples.     

Microstructure refinement of alumina: Optimisation by gas pressure sintering process

This paper investigates densification and grain growth evolutions during gas pressure sintering of alumina. Isothermal sintering runs are performed under different nitrogen pressures: atmospheric pressure and 2 MPa. Experimental data are fitted thanks to constitutive laws in order to understand nitrogen pressure effect on densification and grain growth mechanisms of a fine-grained alumina. An optimal run of densification is proposed as an application of these results.     

Effects of the nature of the doping salt and of the thermal pre-treatment and sintering temperature on Spark Plasma Sintering of transparent alumina

A slurry of α-Al₂O₃ was doped with Mg, Zr and La nitrates or chlorides, in various amounts in the range 150-500 wt ppm and then freeze-dried to produce nanosized doped powder (∼150 nm). The powder was sintered by SPS to yield transparent polycrystalline alpha alumina. The influence of the nature of the doping element and the starting salt, the thermal treatment before sintering and the sintering temperature on the transparency of the ceramics were investigated. The transparency of the ceramics of nanosized Al₂O₃ was shown to depend mainly on the way the powder was prepared, the nature of the doping salt also had an effect. Finally, a high real inline transmittance, reaching 48.1% was achieved after optimization.     

Sliding wear behaviour of alumina/nickel nanocomposites processed by a conventional sintering route

The wear resistance of Al₂O₃/2.5 vol.% Ni nanocomposites sintered by a conventional route was studied under ball-on-disk dry sliding conditions and compared with the same nanocomposites but consolidated by spark plasma sintering, together with alumina obtained by the same technique and by hot pressing. The results showed an improvement of about 0.5, 1 and 2 orders of magnitude, respectively. Thus, alumina/Ni nanocomposites processed by conventional route can compete, in cost and wear performance, with nanomaterials obtained by more sophisticated techniques.     

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.     

Influence of green state processes on the sintering behaviour and the subsequent optical properties of spark plasma sintered alumina

The present investigation gives a quantitative correlation between different green microstructures, and their sintering behaviour during spark plasma sintering. The green microstructures were elaborated via various green shaping processes such as direct casting and direct coagulation casting compared to uniaxial compaction of the as-received sub-micron grained corundum powder. Narrowing pore size distribution and reducing pore size (≈40 nm) in the green compact could favour cold densification during initial uniaxial pressing by grain sliding and rearrangement. This is attributed to the soft homogeneous touching network in direct-cast green samples. Consequently, grain growth was impeded and the onset of shrinkage was delayed. Moreover, the small pores and the narrow pore size distribution in the homogeneous green bodies led to higher final densities, with better optical properties compared to the less homogeneous green samples.     

Joining of alumina with a porous alumina interlayer

In the joining of structural ceramics, a porous interlayer is generally believed to deteriorate the mechanical properties of the joint. This paper, however, shows that a porous interlayer can sustain high adhesion strength when cavities or interfacial cracks are eliminated. The characteristic of the new slurry approach, described in this work, is that a pure alumina slurry interlayer is dried between two adjoining dense alumina plates and sintered with a negligible external pressure to form the porous interlayer. The effect of slurry concentration was studied to optimize the microstructure of interlayer. By controlling the interlayer microstructure and nature of the flaws, it was possible to fabricate high-strength bonds. The new slurry approach opens up the possibility of pure diffusion bonding which requires neither high pressure during heat treatments nor flat surfaces.     

Effect of Y2O3 additive on conventional and microwave sintering of mullite

Mullite has become a strong candidate material for advanced structural and functional ceramics. Much interest has recently focused on sintering aids for mullite. The aim of this study was to evaluate the effect of Y₂O₃ as a sintering aid in the conventional and microwave sintering of mullite. To accomplish this study, a highly pure industrial mullite was used. Mullite with and without Y₂O₃ was pressed under a cold isostatic pressure of 200 MPa. Samples were sintered conventionally at 1400, 1450, 1500, 1550 and 1600 °C for 2 h and microwave-sintered for up to 40 min using a large range of power. The microstructure and physical properties of the microwave-sintered samples were compared to those of the conventionally sintered samples. The results showed that Y₂O₃ improved the densification of mullite bodies in the conventional and microwave sintering processes, but high densifications were achieved in just a few minutes when Y₂O₃ was used with microwave processing.     

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.     

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.     

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.     

Fabrication of alumina parts by electrophoretic deposition from ethanol and aqueous suspensions

This paper discusses the effect of the properties of alumina suspensions in ethanol and in water, on green and sintered ceramic parts formed by electrophoretic deposition. The results of the study demonstrate that a small amount of water present in ethanol suspensions as a hidden additive due to the hygroscopicity of alumina powder and ethanol can detrimentally affect the behaviour of the suspension, thus lowering the reliability of the process. Electrophoretic deposition from aqueous suspensions appears to be advantageous over ethanol, from a reliability standpoint, and due to higher achievable green and sintered densities of the deposits and higher deposition rates. Dolapix CE64 appears to be superior surfactant in water as it results in deposits with the lowest green and sintered porosities.     

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.