Microwave and flash processing of functional materials: Are there (m)any similarities?

Electroceramic devices such as varistors and capacitors are used in most of the modern day electronic appliances and constitute a multi-billion$ market. Conventional fabrication methods of these devices involve high sintering temperatures and long processing time. Since sintering controls the electrical properties, it is necessary to develop simpler and less demanding processing methods. Microwave sintering (MS) was demonstrated to be a viable alternative for rapid processing electroceramics as they respond to the E-field component of the electromagnetic radiation. In a recent report on ‘flash sintering’ (FS) it was demonstrated that full sintering of dog-bone shaped zirconia ceramics can be achieved at 850oC in just 5 seconds (at moderate E-fields) rather than normally used 1450oC for few hours. This opens up the possibility of achieving significant energy savings during manufacture and the ability to produce fine grained ceramics. However the exact mechanisms by which this phenomena occur is not fully clear yet and the methodology is untested for the sintering of other complex functional materials. At Loughborough we investigated the feasibility of sintering nanocrystalline ZnO-varistors, BaTiO3-capacitors and CCTO dielectrics using both MS and FS methods along with simultaneous measurements of shrinkage, online thermal distribution mapping and atmospheric control. This allowed the fabrication of disc-shaped functional ceramic devices using both these techniques and the properties of the devices are compared with conventionally sintered components. This talk will review these new developments on FS along with the operative mechanisms in comparison with microwave processing

Field assisted processing of 3D printed ceramics

Advanced ceramic products for highly demanding applications in electronics, energy, healthcare and defence sectors require densification/sintering, a high temperature process (~1000–2000oC) that in industry can take days. The amount of energy needed, and CO2 emitted, is therefore very significant. Conventional processing of these functional devices/components are often plagued by interfacial issues, unwanted grain growth and limitations of co-firing dissimilar materials. Thus, rapid and efficient sintering methods such as SPS, Microwave Assisted Sintering (MAS) and Flash Sintering (FS) are continuously being developed. These approaches referred as Field Assisted Sintering Techniques (FAST) use an external field that was demonstrated to have a positive effect on densification. For example, the FS method, for reasons that are far from fully understood, has yielded full densification in very short periods (5 s) at very low furnace temperatures (850oC) for zirconia, and at a surprisingly low temperature of 325oC for Co2MnO4 spinel ceramics. The associated time and energy advantage is estimated to be staggering, as well as the ability to tailor the microstructure. In this talk, we will have a closer look at MAS and FS methods– one a well-established and the other a newly emerging densification method. The MAS method can be suitable for the processing of various simple and complex shaped engineering components, the early use of FS method was restricted to dog-bone shaped ceramic specimens – that are both difficult to make and do not have much industrial applicability. However, the recent developments have demonstrated that FS can also be used to sinter different sample shapes. We investigated the feasibility of sintering of 3D printed ultra-low loss 5G microwave dielectrics, YSZ/ZTA biomedical components using MS and FS methods along with measurements of shrinkage and thermal mapping. This talk will review these developments on FS along with the operative mechanisms in comparison with MAS

Evaluation of the high temperature performance of HfB2 UHTC particulate filled Cf/C composites

This is the peer reviewed version of the following article: PAUL, A. ... et al, 2017. Evaluation of the high temperature performance of HfB2 UHTC particulate filled Cf/C composites. International Journal of Applied Ceramic Technology, 14 (3), pp. 344–353, which has been published in final form at https://doi.org/10.1111/ijac.12659. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.Room and high temperature flexural strength and coefficient of thermal expansion (CTE) of HfB2 ultra-high temperature ceramic (UHTC) particulate filled Cf/C composites are determined along with UHT oxidation behavior. Both room and high temperature strength of the composites were found to be broadly comparable to those of other thermal protection system materials currently being investigated. The CTE of the composites was measured both along and perpendicular to the fiber direction up to 1700°C and the values were found to depend on fiber orientation by approximately a factor of 3. Arc-jet testing of the UHTC composites highlighted the excellent ultra-high temperature oxidation performance of these materials

Compositional effects in nanostructured yttria partially stabilised zirconia

There is a considerable current interest in learning how to process genuinely nanostructured ceramics as they offer the potential for significantly enhanced properties; however, it is often difficult to make large enough components to allow more than the most basic of property measurements. In this work, densified components measuring up to 50 mm in size have been produced and a number of very interesting properties have been measured and demonstrated

A comparative study of the synthesis of nanocrystalline Yttrium Aluminium Garnet using sol-gel and co-precipitation methods

This paper is in closed access.Nanocrystalline yttrium aluminium garnet (nYAG) powder has been synthesized via sol-gel and co-precipitation methods using nitrate precursors. Thermal evolution and crystallisation kinetics of both the methods were investigated. The optimised calcination condition for the formation of nYAG was also examined. It was found that a complete transformation to nYAG was observed at 925 C/2 h and 1000 C/1 h for the coprecipitation and sol-gel samples respectively. An intermediate YAlO3 phase was formed at 900 C in all powders regardless of the synthesis methods. The powder morphologies obtained from TEM revealed very similar particle sizes for the two routes (20-30 nm); whilst the extent of agglomeration was higher for the sol-gel method. It was also observed that by controlling the pH in a narrow range, maintaining the precipitate processing temperature and dehydrating excess OH- ions in the precipitates using n-butanol treatment, the extent of agglomeration was further reduced in the co-precipitated nYAG powder. © 2013 Elsevier Ltd and Techna Group S.r.l

Improved transparency and hardness in α-alumina ceramics fabricated by high-pressure SPS of nanopowders

Nanocrystalline alumina powder with an average crystallite size of ≤50 nm has been consolidated by spark plasma sintering (SPS) and hot pressing (HP) with a view to achieving dense, fine grained alumina bodies that display transparency. When as-synthesised powder was densified directly, excessive grain growth resulted from both the SPS and HP techniques and hence a large final grain size was observed. Attempts to improve the uniformity of the green microstructure prior to densification were unsuccessful when spray freeze dried granules were used, whether pre-pressed into a compact or not. The use of 53% dense slip cast green compacts, however, enabled final density of ~99.96% and a mean grain size of ~0.32 μm to be achieved when SPS conditions of 1200˚C and 500 MPa were applied for 20 minutes. These samples offered in-line transmittance values of up to ~80% and microhardness values of 22 GPa