Processing of cellular ceramics by foaming and in situ polymerisation of organic monomers

This paper describes studies on a new processing route for fabricating highly porous ceramics. The method is based on the generation of a foam from an aqueous suspension of ceramic powder and the subsequent stabilisation of the structure by in situ polymerisation of organic monomers. The influence of the slip viscosity on the foam volume and stability was determined using concentrated alumina suspensions containing dispersing agents and two commercial foaming agents. The in situ polymerisation of organic monomers led to fast solidification, resulting in strong, porous bodies which could withstand machining. The resulting ceramic foams consisted of a highly interconnected network of spherical cells with densities as low as 6% of theoretical. The distribution of cell size was dependent both on the density of the specimen produced and on the time for polymerisation onset. The size ranged from approximately 30 to 600 mm. Enlargement of cell size to achieve materials of higher permeability was possible through expansion of the foam via pressure reduction before polymerisation. The creation of highly densified struts between the cells led to flexural strengths as high as 26 MPa

Effect of triblock copolymer non-ionic surfactants on the rheology of 3 mol% yttria stabilised zirconia nanosuspensions

The effect of three different molecular weights of a triblock copolymer nonionic surfactant composed of poly(ethylene oxide) and poly(propylene oxide) have been investigated on the zeta potential, stability and rheology of a commercial nanosuspension of 3 mol% yttria stabilised zirconia (3YSZ). Whilst the surfactants showed some evidence of being adsorbed onto the nanoparticle surfaces, it was in insufficient quantities to achieve complete coverage and measurement of the total organic carbon content suggested that the bulk of the surfactants remained in the liquid medium. As a result, there was only a small effect on the zeta potential. Nevertheless, the stability of the suspension was not affected up to solids contents as high as 54 wt%. Whilst the viscosity of the nanosuspension increased slightly with increasing surfactant concentration due to the presence of the polymer molecules, for the lowest molecular weight surfactant the effect was relatively small. Finally, it was observed that if the ionic strength of the suspension was reduced via the removal of free electrolytes in the suspension by dialysis, the viscosity decreased significantly. The reverse behaviour was also observed when extra NH4Cl electrolyte was added to the nanosuspension

Effect of dispersants on the rheology of aqueous silicon carbide suspensions

The effect of cationic and anionic dispersants on aqueous suspensions of as-received and surface-modified silicon carbide particles has been studied via observation of the rheological behaviour. Only the cationic dispersants were effective for the as-received SiC, with polyethyleneimine being superior to Hyamine 2389 probably as a result of a greater electrosteric interaction. SiC particles modified using Al(NO3)3 behaved like alumina and so could be dispersed using the anionic dispersants ammonium polyacrylate and polymethacrylate. Such dispersions displayed no heteroaggregation when alumina was added, although the order of mixing could significantly affect the rheological behavior of the suspension. Nevertheless, the suspensions appeared robust to slight fluctuations in pH

Processing of bulk nanostructured ceramics

Conventional ceramic forming routes have been adapted for the processing of ~16 nm, 3 mol% yttria stabilized zirconia nanopowders leading to the production of ~99% dense nanostructured ceramics that display average grain sizes as fine as ~65 nm. The precursor material is in the form of ~5 vol% solids content nanosuspensions produced commercially; these can now be concentrated up to ~37 vol% whilst retaining the viscosity at ~0.05 Pa s. A patent application has been submitted related to the process. The concentrated suspensions have then been used to produce granulated powders suitable for dry forming via spray-freeze drying. Whilst powders have been produced that will yield green bodies with densities of ~50% of theoretical, currently the powders suffer from either poor flow and low fill densities or granules that are too strong to crush during pressing, even at pressures up to 500 MPa. The same suspensions have also been slip cast into extremely homogeneous green bodies with densities of ~54% of theoretical after drying using a humidity drier. Higher densities are currently blocked by cracking of the samples during drying and/or burnout of the organics if the solids content of the suspensions exceeds ~20 vol%. Radiant and hybrid pressureless sintering experiments have been performed on the dry and wet processed green bodies using both conventional single step and two-step sintering cycles. Whilst densities >98% of theoretical were achievable by all combinations, a nanostructure could only be retained using the two stage sintering approach. With hybrid heating the average grain sizes for die pressed samples were in the range 70 – 80 nm whilst for the more homogeneous slip cast samples a final average grain size of just 64 nm was achieved for a body with a final density of ~99.5%. It is believed that the primary advantage offered by hybrid heating is the ability to use a much faster initial heating rate, 20 versus just 7oC min-1, without risking damage to the samples. Whilst detailed characterisation of the properties of these nanostructured ceramics has begun, preliminary results have suggested that the toughness is lower and hardness roughly equivalent to submicron grain-sized 3-YSZ, although the resistance to wear and hydrothermal ageing may have been improved. As a result of detailed crystallographic characterisation this is believed to be due to a grain size dependent shift in the phase boundary composition for nano YSZ ceramics leading to ‘over stabilisation’ at any given yttria content. Current work is focused on investigating the effect of both yttria content and average grain size on the properties of these new materials

Processing of ceramic-metal interpenetrating composites

Interpenetrating composites consist of 3-dimensionally interpenetrating matrices of two different phases; in the present work they were alumina and aluminium-magnesium alloys and were produced by infiltrating ceramic foams with molten alloys. The foams were made by mechanically agitating ceramic suspensions to entrain gases and then setting the structure via the in-situ polymerisation of organic monomers, a process known as gel casting. This resulted in the foams having a very open and interconnected structure that could be easily infiltrated by the molten metals. Previous composites have been produced at Loughborough University using squeeze casting; however, whilst infiltration was usually accomplished in a matter of seconds, the resulting size and shape of the composite was limited. Hence the present work has focused on investigating the potential for using gravity-fed infiltration. Whilst this was much slower, often taking several minutes, when optimised it is believed it will offer the potential for the production of large and complex-shaped pieces. The composites were produced at atmospheric pressure by infiltrating 2-10 wt.% magnesium content Al-Mg alloys into 20% dense Al2O3 foams with highly interconnected porosity. The processing parameters of temperature, ≥ 900°C, and atmosphere, flowing N2- Ar, were investigated to determine the processing window and infiltration kinetics. In-situ observation of the process shows that infiltration is faster at higher temperatures, Mg contents and N2 partial pressures. Both optical and scanning electron microscopy (SEM) have been used to characterize the composites

Interface study by dual-beam FIB-TEM in a pressureless infiltrated Al(Mg)–Al2O3 interpenetrating composite

This paper considers the microstructures of an Al(Mg)/Al2O3 interpenetrating composite produced by a pressureless infiltration technique. It is well known that the governing principle in pressureless infiltration in Al/Al2O3 system is the wettability between the molten metal and the ceramic phase; however, the infiltration mechanism is still not well understood. The objective of this research was to observe the metal / ceramic interface to understand the infiltration mechanism better. The composite was produced using an Al-8wt.% Mg alloy and 15% dense alumina foams at 915°C in a flowing N2 atmosphere. After infiltration, the composite was characterized by a series of techniques. Thin film samples, specifically produced across the Al(Mg)-Al2O3 interface, were prepared using a Dual Beam Focused Ion Beam (FIB) and subsequently observed using Transmission Electron Microscopy (TEM). XRD scan analysis shows that Mg3N2 formed in the foam at the molten alloy-ceramic infiltration front whilst TEM analysis revealed that fine AlN grains formed at the metal / ceramic interface and MgAl2O4 and MgSi2 grains formed at specific points. It is concluded that it is the reactions between the Al, Mg and N2 atmosphere that improve the wettability between molten Al and Al2O3 and induce spontaneous infiltration

Preparation of high solids content nanozirconia suspensions

A new colloidal route leading to the production of ~99% dense 3 mol% yttria stabilized zirconia nanostructured ceramics, whilst retaining a final average grain size of ~75 nm, has been developed. The process was based on the production of stable, homogeneous nanosuspensions with solids contents of up to 28 vol% (70 wt%) but viscosities less than 0.05 Pa s at any shear rate in the range of study were obtained. The suspensions were formed by the concentration and optimization of precursor, dilute (5.0 vol%) commercial nanosuspensions, the approach requiring a change of pH, from the 2.4 of the as-received suspension to 11.5, and the use of an appropriate anionic dispersant. Exposure of the nanosuspensions to ultrasound also helped to reduce the viscosity further, though it only worked when the dispersant was optimized. The nanosuspension was slip cast to form homogenous green bodies with densities of ~55% of theoretical without agglomeration in the nanostructure; these were subsequently densified using a two-step sintering techniqu

Micro-Raman spectroscopy of indentation induced phase transformation in nanozirconia ceramics

Micro-Raman spectroscopy has been employed as an effective technique to determine the phase transformations in nanostructured yttria stabilised zirconia (YSZ) ceramics with different yttria content. Samples have been prepared with varying mean grain sizes by a slip casting route followed by a microwave assisted two-step sintering cycle starting with aqueous nanozirconia suspensions. Indents were generated using a Vickers pyramidal indenter at different loads and the resulting phase transformations were mapped using micro-Raman spectroscopy. The results were compared to those of a commercial submicron 3YSZ. The amount of transformation was found to be much lower for nanozirconia compared to the submicron zirconia with similar yttria content

Analysis and optimization of gel-cast ceramic foam diesel particulate filter performance

Gel-cast ceramic foams potentially offer a more robust configurable alternative filtration medium to monolithic wall flow filters (WFFs) for the reduction in particulate matter (PM) emissions from diesel internal combustion engines. The fundamental back pressure and filtration efficiency characteristics of gel-cast ceramic foam diesel particulate filters (DPFs) have been investigated. Methodology is developed for the first time that allows the calculation of the effect of local PM loading on the pressure drop characteristics from experimental data without problems caused by the non-uniform PM loading in the filter that can be applied to all depth bed filtration media. The back pressure and filtration efficiency relationships were used to develop graphical design spaces to aid development of application-specific DPFs. Effects of PM distribution on the pressure drop of the filter are presented. Filters with a non-even distribution of PM were found to have lower pressure drops than filters with an evenly distributed PM for the same average specific PM loadings. The predictions showed that gel-cast ceramic foams can achieve comparable back pressure, filtration volume, and PM holding capacity with WFFs with a lower filtration efficiency of about 80 per cent. The model demonstrated that greater than 90 per cent filtration efficiency can be achieved with filter volumes of about 0.6 times the volume of a WFF with a lower PM holding capacity

Measuring pore diameter distribution of gelcast ceramic foams from two-dimensional cross sections

Increasing applications for gelcast ceramic foams is making the effective, accurate and cost effective measurement of pore diameter and distribution of significant value to a wide range of research fields. Current methods either do not directly measure pore diameter or they require high equipment and time costs. Measuring pore diameter directly from sample cross sections is both rapid and cost effective but, due to the random nature of the pore location during sectioning of the sample, it under predicts the pore diameter. The proposed method identified that the mean measured pore diameter was 79% (2 s.f.) of the actual pore diameter. Numerical methods for correcting the pore distribution as well as the average pore diameter are presented