A novel process to high-strength and controlled-shrinkage Al2O3 foams with open-cell by Al powder hollowing technology

Ceramic foams with open-cell structures have attracted extensive attention due to their unique structure and superior properties. But these materials often exhibit the weakness of high sintered shrinkage and low strength at high porosity levels. In this work, novel ceramic foams with open-cell structures have been obtained using Al powder by combining direct foaming and gelation freezing (DF–GF). The foams are assembled by hollow Al₂O₃ particles resulting from the Kirkendall effect, in which expanded particles overcome the shrinkage of sintering. The influence of sintering temperature on the microstructure and properties of foams are investigated. The Al₂O₃ foams show near-zero-shrinkage at 1773 K after undergoing the process of first expansion and then shrinkage. Compared to other conventional open-cell foam, this foam displays relatively high compressive strength of 0.35–2.19 MPa at high porosity levels of 89.45%–94.45%, attributed to hierarchical pore structure and reaction bonding between Al and O₂. This method from pore structure design provides a novel route for the preparation of controlled shrinkage and high-compressive strength alumina foam with open-cell toward potential application.     

Large-scale fine structural alumina matrix ceramic guideway materials improved by diopside and Fe2O3

Diopside and Fe₂O₃ were introduced in alumina matrix ceramic materials. Large-scale fine structural alumina matrix ceramic guideway materials were fabricated by the technology of pressureless sintering, during which liquid phase sintering took place and new phases such as 3Al₂O₃·2SiO₂, CaO·Al₂O₃·2SiO₂ and CaO·6Al₂O₃ were produced by the chemical reactions taking place among alumina and the additives. The hardness, the fracture toughness and the bending strength of the guideway products were tested. The influences of diopside and Fe₂O₃ additions were studied by microstructural observations and mechanical properties evaluations. Meanwhile, the expected improvement of mechanical properties compared with pure alumina was indeed observed. The fracture mechanism and porosity of large-scale fine structural alumina matrix ceramic guideway materials were analyzed.     

Influence of processing parameters on the surface quality of electrophoretically deposited alumina coatings on foam ceramics

Previous work showed that the electrophoretic deposition of coatings with aligned pore channels is technically feasible on Al₂O₃-C foams. In the next step, the amount and size of cracks in the sintered coatings should be reduced. The study revealed the drying conditions, alumina raw materials used, the chemistry of the foam skeleton and the sintering conditions as significant influences. The best drying procedure was freeze drying after sudden freezing in liquid nitrogen. Three alumina raw materials with different particle size distributions were tested with regard to linear shrinkage, number of cracks and number of channel-like pores. The CT 9 and CL 370 showed a low number of cracks, however CT 9 possessed almost no pores. The Al₂O₃-C foam skeletons electrophoretically coated with CL 370 and sintered at 1600 °C in air showed the best results with a low number of small cracks and high number of channel-like pores.     

Rheotechnological properties of mixed suspensions in the SiO2 - Al2O3 system and some properties of materials based on them. 1. Molten quartz – Alumina system

The rheotechnological properties of mixed suspensions in the SiO₂ - Al₂O₃ system obtained by the method of mixing of individual suspensions of molten quartz and alumina are described. Some properties of the materials after their heat treatment at 1000 – 1300°C are investigated. The ranges of compositions (30 – 40% Al₂O₃, 60 – 70% SiO₂) in which the parameters of thermal expansion of the materials are 2 – 3 times lower than those calculated under the condition of additivity are determined. The obtained materials possess an elevated mechanical strength. Translated from Ogneupory i Tekhnicheskaya Keramika, No.7, pp. 18 – 23, July, 2000.     

Innovation in ceramic foam filters manufacturing process

Ceramic filters have singular properties such as high permeability and specific surface area associated to low density. Some examples of their use can be found in liquid metal deep filtration, water treatment, air purification, and others. These particular properties are dependent on the filters' manufacturing process. One of the most used techniques to produce filters is the replication method, which provides a tridimensional network of struts and interconnected pores. However, a common issue of all ceramic filters produced by this technique is their low mechanical strength due to the hollow struts and microcracking generated during the thermal decomposition of the sponge. Intending to address this limitation and produce filters with higher mechanical strength, this paper analyzed a modified process route for manufacturing filters. Samples of preheated Al₂O₃ filters were vacuum infiltrated with ceramic suspensions (Al₂O₃ and SiO₂) of different solids concentrations (15-40 wt%), particle size distribution (nano to micrometric size) at distinct processing times (1-5 minutes). The data analysis indicated that the best balance among mechanical strength, mass, and strut thickness was achieved by infiltration with SiO₂ colloidal suspension (40 wt%) per 1 minute. SEM analysis confirmed that the struts were filled and microcracks were sealed with colloidal particles.     

Interaction of slip‐ and flame‐spray coated carbon‐bonded alumina filters with steel melts

Ceramic foam filters play an essential role in the quest for cleanliness of cast steel parts as they facilitate turbulence reduction during mold filling as well as removal of nonmetallic inclusions. A coating on these filters is able to increase their strength and filtration efficiency by improving the adhesion of inclusions to the filter strands. In this study, Al₂O₃‐C filters were coated with an alumina slip via slip and flame spraying. The phase composition and the microstructure of the coatings were investigated before and after immersion into molten steel contained in a metal casting simulator. After contact with molten steel, Al₂O₃‐C reference filter shows intense decarburization which often influence the quality of cast steel parts due to formation of gas bubbles. Slip‐sprayed alumina coatings on such a filter promote the deposition of inclusions due to formation of a vitreous alumina layer but will also cause gas bubble formation as they exhibit a high porosity. Flame‐spray coatings have low porosity and hence, prevent formation of gas bubbles. Furthermore, they showed the highest reactivity toward the steel melt and hence, are recommended for filtration of cast products with a high demand on cleanliness.     

Reactivity of supported platinum nanoclusters studied by in situ GISAXS: clusters stability under hydrogen

This paper is an initial study of the reactivity and thermal stability of atomic platinum clusters supported on Al₂O₃/SiO₂/Si(100) as a function of the thickness of the alumina film and presence of hydrogen. Extremely high thermal stability of Pt_7–10 clusters in vacuo as well as in the presence of hydrogen is observed on SiO₂/Si(100) coated with six cycles of Al₂O₃ film prepared by an atomic layer deposition technique.     

Effect of different contents of zirconia and yttrium oxide on microstructure and properties of high alumina ceramics

The effects of zirconia and yttrium oxide addition on microstructure, bulk density, microhardness, flexural strength, and wear resistance of high alumina ceramics (>97 wt% Al₂O₃, MSA ceramics) composed of MgO–SiO₂–Al₂O₃ system have been investigated. The results show that the addition of zirconia makes the mechanical properties and wear properties of ceramics composed of MgO–SiO₂–Al₂O₃–ZrO₂ (MSAZ ceramics) system have been greatly improved compared with MSA ceramics. In addition, the ceramics composed of MgO–SiO₂–Al₂O₃–ZrO₂–Y₂O₃ (MSAZY ceramics) system have better mechanical properties and wear properties than MSAZ ceramics. With the contents of zirconia and yttrium oxide increase, the bulk density, microhardness, and flexural strength of MSAZ and MSAZY ceramics increased at first and then decreased. However, the wear rate shows the opposite. When 0.4 wt% ZrO₂ and 0.6 wt% Y₂O₃ were added to the matrix, the wear rate of MSAZY ceramics reached a minimum of 0.042%, and the wear resistance was improved by about 73.8% compared with MSA ceramics with a wear rate of 0.16%. In addition, the optimum additions of zirconia and yttria are 0.4% and 0.6%, respectively.     

Manufacturing of nano mullite-silicon carbide filters by in situ reaction bonding

In the present study, the effect of silica nanoparticles on the formation of nano-mullite phase for use in the manufacture of silicon carbide based ceramic foam filters has been investigated. Polyurethane foam filters were impregnated with nanosilica particles by slip casting. In this method, the effect of different percentages of nanosilica particles in the slurry on compressive strength, density and porosity of ceramic foam filters was investigated. The effect of silica nanoparticles on viscosity of slurry was studied using rheometric test. So, sample S₁₅ was selected to proceed. For thermal treatment of ceramic foams, different sintering temperatures were investigated and the best temperature was reported at 1250 °C. Compressive strength results showed that with increasing nano-silica content, CCS increased. XRD results from the samples showed that the nano-mullite phase was formed at 1250 °C along with silicon carbide and alumina phases. Scanning electron microscope images (SEM) showed that the mullite phase was formed in nano-dimensions in ceramic foam bodies. The formation of mullite phase in the microstructure of the filters is one of the factors of strengthening and increased refractory characteristics. EDS analysis by the scanning electron microscopy of the filter which passed ductile iron melt showed that cast iron inclusions and impurities were mostly consisted from FeO, MnO, SiO₂, Al₂O₃, MgO and CO, which were trapped inside the ceramic filter.     

Nano-functionalization of carbon-bonded alumina using graphene oxide and MWCNTs

This paper focused on the nano-functionalization of carbon-bonded alumina using graphene oxide (GO) and multi-walled carbon nanotubes (MWCNTs). GO was prepared according to the modified Tour's method. The rheology of suspensions containing GO, CNTs and both substances were analyzed and discussed. Xanthan proved to be a suitable stabilizer for the three systems. The spraying process of the suspensions was investigated with the aid of a high-speed camera. Al₂O₃-C filters and flat samples were spray coated, for investigations in contact with steel. The ceramic foam filters as well as the starting suspensions were analyzed by several analytic techniques to investigate the microstructure and other properties. Moreover, the hot stage microscope was used to study the steel/coating wetting behavior during operation at high temperatures. The results showed that the formulation containing both GO and CNTs delivered the best performance in contact with the melt. Similar coatings based on these nano-sized materials may offer an innovative route to improve purification of steel melts by filtration.     

Effect of glass additives on the strength and toughness of polycrystalline alumina

The effect of stress at grain boundaries on the mechanical properties of alumina ceramics was investigated. Residual stresses at grain-boundaries resulted from a mismatch in thermal expansion coefficient (TEC) between the alumina matrix and the glass-phase segregated at grain-boundaries. The BaO–Al₂O₃–SiO₂ (BAS) system and the Li₂O–Al₂O₃–SiO₂ (LAS) system glasses were chosen to have a higher and a lower TEC than that of alumina, respectively, resulting in microscopic tensile and compressive stresses at grain-boundaries for Al₂O₃/BAS and Al₂O₃/LAS composites, respectively. The experimental results showed that the Al₂O₃/BAS composite fractured intergranularly with a fracture toughness higher than that of monolithic alumina. On the other hand, the Al₂O₃/LAS composite experienced transgranular fracture and high bending strength despite its low toughness. Both composites could be sintered to full density at 1500°C for 2 h due to the presence of a liquid phase. It was concluded that strengthening and toughening of alumina ceramics could be tailored by designing their grain-boundary microstresses.     

Combustion Prevention of Iron Powders by a Novel Coating Method

The effectiveness of a novel coating method in providing fine iron powder particles with a protective barrier against rapid oxidation was systematically studied. Particles were individually coated with an alumina‐based (Al₂O₃) ultra‐thin film using the Atomic Layer Deposition method. The oxidation resistance of the coating layer was found to be greatly dependent on the film thickness. Furthermore, for each film thickness there was a corresponding temperature above which the film drastically lost its protective effectiveness, primarily due to cracking caused by the thermal expansion mismatch between the particle and the alumina film. This problem was largely overcome when, instead of Al₂O₃, the protective film was generated from alternating layers of Al₂O₃/ZnS to ensure that the thermal expansion properties of the resultant film matched that of iron. The technique employed in this study is quite robust and can be adapted for combustion prevention in other types of metal powders.     

Dilatometric study of anisotropic sintering of alumina/zirconia laminates with controlled fracture behaviour

Al₂O₃ and ZrO₂ monoliths as well as layered Al₂O₃/ZrO₂ composites with a varying layer thickness ratio were prepared by electrophoretic deposition. The sintering shrinkage of these materials in the transversal (perpendicular to the layers, i.e. in the direction of deposition) as well as in the longitudinal (parallel with layers interfaces) direction were monitored using high-temperature dilatometry. The sintering of layered composites exhibited anisotropic behaviour. The detailed study revealed that sintering shrinkage in the longitudinal direction was governed by alumina (material with a higher sintering temperature), whilst in the transversal direction it was accelerated by the directional sintering of zirconia layers. For interpretation of such anisotropic sintering kinetics, the Master Shrinkage Curve model was developed and applied. Crack propagation through laminates with a different alumina/zirconia thickness ratio was described with the help of scanning electron microscopy and confocal laser microscopy.     

A nitride whisker template for growth of mullite in SiC reticulated porous ceramics

Silicon carbide reticulated porous ceramics (SiC RPCs) were fabricated by polymer replica technique. The effects of nitride whisker template on the growth of mullite, the strut structure and mechanical properties of SiC RPCs were investigated. Prepolyurethane (PU) open-cell sponge was first coated by SiC slurry consisting of SiC, reactive Al₂O₃, microsilica and Si powder, then it was nitridized at 1400 °C in a flowing N₂ atmosphere to prepare SiC preforms. Subsequently, these preforms were treated by vacuum infiltration of alumina slurry and fired at 1450 °C in air. The results showed that Si₂N₂O whiskers grew on the surface and in the matrix of SiC preforms after nitridation. The diameter of struts in SiC RPCs increased after vacuum infiltration process because alumina slurry was easily adhered by the surface nitride whiskers. In addition, such whiskers inside the strut of SiC preforms acted as the template to promote the growth of column-liked mullite in SiC RPCs. The mechanical properties and thermal shock resistance of SiC RPCs were greatly improved due to the special interfacial characteristics of multi-layered struts as well as better interlocked column-liked mullite in SiC skeleton.     

Mixture Design and Response Surface Analysis of Densification of Silicon Carbide Ceramics with (SiO2–Dy2O3–Al2O3) Additives

Statistical mixture designs are used to systematically study the densification properties of silicon carbide (SiC) ceramics sintered with SiO₂, Dy₂O₃, and Al₂O₃. Mixture models for percentage theoretical density and SiC weight loss as a function of the SiO₂, Dy₂O₃, and Al₂O₃ oxide proportions have been determined and validated by analysis of variance. The results indicate a region confined by about 0–20 mol% silica, 50–65 mol% dysprosia, and 40–65 mol% alumina, with all samples containing 10% by volume of additives, and simultaneously maximization of density values and minimization of weight loss during SiC-based ceramic sintering.     

Wetting properties and chemical interactions of copper with industrial filter materials

Copper foundries have started using ceramic foam filters to increase copper melt purity and improve cast quality. For the selection of filter materials, the interaction between the copper melt and the filter materials is a significant criterion. This study aims to determine interactions and wetting behavior between commercially available filter materials and pure copper melt using sessile drop technique. SEM/EDX analysis revealed no interactions between Cu and the filter materials, ZrO₂, Al₂O₃–ZrO₂, Al₂O₃–P, and Al₂O₃–Si. Although Cu and SiC showed no visible reaction, a kind of Cu infiltration into the SiC substrate was observed. The wetting behavior was different for the different substrate materials characterized by the increasing contact angle from Cu/ZrO₂, Cu/SiC, Cu/Al₂O₃–ZrO₂, Cu/Al₂O₃–P to Cu/Al₂O₃–Si.     

Phase composition of alumina–mullite–zirconia refractory materials

Refractories in the Al₂O₃–SiO₂–ZrO₂ system are widely used in many applications, for ceramic rollers in particular, and are characterized by high mechanical strength, excellent thermal shock resistance, resistance to corrosion by alkaline compounds and low creep at high temperature. Their performances greatly depend on the amount and chemical composition of crystalline and glassy phases, which were investigated by quantitative XRPD (RIR–Rietveld) and XRF in order to assess the effect of various Al₂O₃/SiO₂ ratios of starting batches and different alumina particle size distributions. Refractories consist of mullite, corundum, zirconia polymorphs and a vitreous phase in largely variable amounts. The mullite percentage, unit cell parameters and composition vary with sintering temperature, being mostly influenced by the Al₂O₃/SiO₂ ratio of the batch. Its orthorhombic unit cell increased its volume from 1400 to 1500 °C, while its stoichiometry became more aluminous. The corundum stability during firing is strongly affected by the Al₂O₃/SiO₂ ratio, but not by the particle size distribution of alumina raw materials. Zirconia raw materials are involved in the high temperature reactions and about one-third of the available ZrO₂ is dissolved in the glassy phase, ensuring excellent resistance to alkali corrosion, mainly depending on the fraction of coarse alumina. The phase composition of the vitreous phase increased with sintering temperature, being over 20% when the fractions of coarse alumina in the starting batch are between 0.2 and 0.5.     

Effect of metal‐support interface on hydrogen permeation through palladium membranes

Thin palladium membranes of different thicknesses were prepared on sol‐gel derived mesoporous γ‐alumina/α‐alumina and yttria‐stabilized zirconia/α‐alumina supports by a method combining sputter deposition and electroless plating. The effect of metal‐support interface on hydrogen transport permeation properties was investigated by comparing hydrogen permeation data for these membranes measured under different conditions. Hydrogen permeation fluxes for the Pd/γ‐Al₂O₃/α‐Al₂O₃ membranes are significantly smaller than those for the Pd/YSZ/α‐Al₂O₃ membranes under similar conditions. As the palladium membrane thickness increases, the difference in permeation fluxes between these two groups of membranes decreases and the pressure exponent for permeation flux approaches 0.5 from 1. Analysis of the permeation data with a permeation model shows that both groups of membranes have similar hydrogen permeability for bulk diffusion, but the Pd/γ‐Al₂O₃/α‐Al₂O₃ membranes exhibit a much lower surface reaction rate constant with higher activation energy, due possibly to the formation of Pd‐Al alloy, than the Pd/YSZ/α‐Al₂O₃ membranes. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Additive manufactured polyamide foams with periodic grid as templates for the production of functional coated carbon-bonded alumina foam filters

For the industrial production of metal melt filters, the replica technique is established since several decades. The polyurethane foams used as templates show a rather random structure with several defects which are transferred into the final filter structure after the replication. In order to generate filters with periodic structure and low amount of defects, a periodic foam model was used and open cell foams were produced by selective laser sintering of aged polyamide 12 (PA12). The PA12 foams were then used as sacrificial templates for the replica technique in the production route of Al₂O₃-C filters with functional coatings based on a cold sprayed Al₂O₃-C coating or a flame sprayed coating based on Al₂O₃. The differences in geometry between the computer-generated model foam, the sacrificial PA12 foam, the foam after carbonization, and the additional functionalized filters with the cold and hot coating were analyzed by computed tomography. Based on CT-data isosurfaces of the foams were generated to virtualize and distinguish the differences. Preliminary mechanical tests showed a higher cold crushing strength for the filters coated via flame-spray technique than the cold coating.     

Grain boundary diffusion effect on mineral transition of calcium sulpho-aluminate with sodium dopant

The mineral formation-transition mechanism, microstructure morphology evolution, pulverization property and chemical reactivity of calcium sulpho-aluminate with sodium dopant are systematically studied with the molar ratio of CaO to Al₂O₃ is 0.8, the molar ratio of CaO to SiO₂ is 1.5, the mass ratio of Al₂O₃ to SiO₂ is 2.0, and the SO₃ accounts for 4%. The results show that sodium dopant could promote 3CaO·3Al₂O₃·CaSO₄, CaO·2Al₂O₃ and 2CaO·Al₂O₃·SiO₂ transform into 2CaO·SiO₂, CaO·Al₂O₃, 12CaO·7Al₂O₃ and 2Na₂O·3CaO·5Al₂O₃, and it also can decrease the decomposition temperature of reactive materials and the initial phases formation temperature significantly. Sodium diffusion can destroy the clear grain boundary of calcium aluminate and calcium silicate phases, change the macro-microstructure morphology, improve the crystalline degree of the samples, and then improve the pulverization and alumina leaching property. However, excessive sodium dopant can decrease the melting temperature, inhibit the transformation process of 2CaO·SiO₂ from β to γ, and then deteriorate the pulverization property significantly again.