J.G.P. Binner

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‐8 wt% 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 focussed ion beam and subsequently observed using transmission electron microscopy. XRD scan analysis shows that Mg3N2 formed in the foam at the molten alloy–ceramic infiltration front, whereas transmission electron microscopy 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 Al, Mg and the N2 atmosphere that improve the wettability between molten Al and Al2O3 and induce spontaneous infiltration.

Microwave Sintering of Ceramics: What Does it Offer?

Microwave heating is fundamentally different from conventional sintering in that the energy is deposited volumetrically rather than relying on thermal conduction from the surface. Properly manipulated this may lead to a number of benefits, including greater microstructural control, improved product properties and reductions in manufacturing costs due to energy savings and shorter processing times. This paper seeks to review some of the opportunities and challenges that exist when microwaves are applied to the process of ceramic sintering.

Tape casting aqueous alumina suspensions containing a latex binder

The tape casting of ceramic membrane substrates of ∼1 mm thickness has been studied using two grades of ammonium polymethylacrylate (PMA) as a dispersing agent, dibutylphthalate as a plasticizer and polyvinylacetate (PVAc) latex as a binder. The colloidal stability of the alumina particles in the precursor slip was characterized using zeta potential measurements and the rheological behaviour of the slips. After tape casting, the density, strength and flexibility of the green tapes were studied in terms of changes in the plasticizer concentration and pH of the slip. The results indicated that alumina particles could be stabilized at pH 6–10 at a PMA15 concentration of 10 mg g−1. The slips prepared with the polyelectroyte also displayed a strong shear thinning effect, which is important for tape casting. With the proper addition of plasticizer and adjustment of pH, it was found that a PVAc latex-based system can yield green alumina tapes with high green strength and flexibility.

Enhanced casting rate by dynamic heating during slip casting

Abstract The slip casting process is widely used to consolidate ceramic particles from aqueous suspensions to form green compacts, particularly those with complicated shapes. Raising the slip temperature during slip casting is known to increase the casting rate, this is believed to be via a decrease in water viscosity. However, differences have been observed when using convection and microwave heating to raise the temperature. In the present work, it has been found that the use of short-pulses of microwave energy to heat the casting system dynamically causes a greater degree of acceleration than when using conventional radiant heating. The increased uptake of water from the slip by the porous mould is believed to be indicative of a vaporisation–condensation cycle mechanism. A negative pressure would be created during the condensation stage of the cycle, acting as an additional suction force to the capillary action and hence accelerating the casting process.

Oxyacetylene torch testing and microstructural characterization of tantalum carbide

Tantalum carbide samples have been subjected to high‐temperature testing at ∼2300°C using an oxyacetylene torch to evaluate their potential for ultra‐high temperature applications. While large samples cracked during the rapid heating, indicating their inability to withstand thermal shock, small samples survived the severe test conditions. The oxidation products formed were characterized and found to comprise different phases of Ta2O5. The ultra‐high temperature experienced by the samples resulted in the formation of many interesting microstructures, including the formation of submicron sized grains, which has not been reported previously in the literature, as well as the expected evidence of melting.

Green Forming with Nanoparticles

The potential for green forming with nanosized yttria-doped, partially stabilised zirconia (YPSZ) powder using both uniaxial compaction and colloidal suspensions has been investigated. Green compacts were fabricated from a Y-PSZ nanopowder, a submicron powder of similar composition and combinations of the two by die pressing at varying axial stress, with the effect of a micronising milling process on the green density also being examined. Results indicated that the incorporation of the nanopowder into the submicron powder increased green density by up to 17% whilst micronising helped to achieve green densities as high as 60% of theoretical, even with the pure nanopowder. The zeta potential, viscosity and mean particle size of two different Y-PSZ nanosuspensions were measured for two different anionic polyelectrolytes, polyacrylic acid (PAA) and polymethacrylic acid (PMAA). The results showed that both led to flocculation under acidic conditions and hence increasing viscosity with increasing polyelectrolyte concentration.

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

Abstract Abstract 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 contents. 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 submicrometre 3 mol.% YSZ. The amount of transformation was found to be much lower for nanozirconia compared to the submicrometre zirconia with similar yttria content.

Densification of nanostructured YSZ green compacts

Conventional and microwave sintering experiments have been performed on nanocrystalline yttria partially stabilised zirconia (Y-PSZ) green compacts produced by die pressing and slurry casting. Whilst one of the nanopowders used developed spherical inclusions measuring 1 – 4 μm in diameter during sintering, the combination of microwave sintering and the second nanopowder yielded sintered bodies of >97% of theoretical density and with an avConventional and microwave sintering experiments have been performed on nanocrystalline yttria partially stabilised zirconia (Y-PSZ) green compacts produced by die pressing and slurry casting. Whilst one of the nanopowders used developed spherical inclusions measuring 1 – 4 μm in diameter during sintering, the combination of microwave sintering and the second nanopowder yielded sintered bodies of >97% of theoretical density and with an average grain size of 140 nm.

Gel Cast Foam Diesel Particulate Filters

A new manufacturing route for foam ceramics based on gel casting has been developed and is being commercialised. Gel casting employs an organic monomer that is polymerised to cause the insitu gelation of a foamed aqueous ceramic slurry. The primary advantage is the inherent flexibility of the process; the foams can be near net shape manufactured in a variety of shapes and sizes and after production are simply dried and fired. In addition, the porosity and pore size distribution can be varied to suit the application and a wide range of ceramics can be foamed with densities ranging from 5-40% of theoretical. Applications are diverse and include the potential to be used as diesel particulate filters (DPF). The present work examines this and concludes that filtration efficiencies of ≥90% are achievable without generating a significant backpressure for the engine.

When should microwaves be used to process technical ceramics

This paper attempts to shed light on why the stand alone microwave processing of technical ceramics, despite being one of the most popular field with respect to volume of research performed, is still struggling to achieve priority status with respect to commercialisation. To obtain some answers to this enigma and determine when microwaves should be used to process technical ceramics, three case studies are explored. The conclusion is that microwaves should be used to process technical ceramics when specific advantage can be taken of the intrinsic nature of microwave energy and not simply as an alternative energy source. In addition, it is concluded that from a commercialisation view point hybrid processing is often a better approach than the use of pure microwaves.