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

Field assisted sintering of nanostructured zirconia-alumina ceramics for demanding applications

Flash and Field Assisted Sintering Techniques (FAST) have been demonstrated in many singular ceramic systems. However, little has been done to thoroughly investigate this phenomenon in binary systems. Zirconia-alumina composites are of interest because of their widespread use in demanding situations for health care, petro-chemical and energy applications. The prospect of minimising grain growth associated with FAST whilst achieving maximum densification is vital for the above applications to improve performance. Flash sintering behaviour of several zirconia-alumina formulations was investigated under a range of DC electric fields up to 700 V/cm. At low alumina contents (75 wt%) the effectiveness of field assisted sintering drops off. These pointers allowed to gain further insight into the mechanisms of flash sintering and will help to develop pathways for the adaptation of the technique to process complex ceramic systems

Erosion and mechanical properties of hydrothermally-resistant nanostructured zirconia components

Large scale 50 × 50 mm sintered nanostructured zirconia ceramics were fabricated via industrially viable dry pressing routes. The green bodies were sintered by a two-stage process and the optimised sintering conditions are reported. The suitability of nanostructured zirconia for demanding applications in petrochemical valve components was investigated by slurry impingement erosion experiments. Zirconia showed a 60-fold improvement compared to commonly used stellite-coated commercial stainless steel specimens under test conditions while no tetragonal to monoclinic phase transformation was observed after erosion. The enhanced performance was also valid when compared with reported erosion resistant properties of alumina and zirconia components by a factor of 36 and 3, respectively. This suggests nanostructured zirconia as a potential robust alternative material for construction of internal trim components of petrochemical valves

Electromagnetic simulation studies of microwave assisted heating for the processing of nanostructured iron oxide for solar driven water splitting

Microwave assisted preparation has been shown to improve the performance of hematite photoelectrodes for solar driven water splitting. To understand the microwave heating process further, the distribution of the electromagnetic (EM) fields within the material is analysed using finite-difference time-domain (FDTD) EM software. The rate of the increase in temperature is calculated from the simulated EM field distributions. In order to validate the simulation results, the calculated temperatures were compared with the experimental temperatures obtained using a thermal imaging camera

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

Comparison of spray freeze dried nanozirconia granules using ultrasonication and twin-fluid atomisation

Granulation of nanostructured 3. mol% yttria stabilised zirconia using spray freeze drying was investigated to achieve flowable and crushable granules for subsequent die pressing. Commercial nanosuspension consisting of ~16. nm particles was concentrated to ~55. wt% solids content via a patented technique, followed by spraying into liquid nitrogen using either a vibrating ultrasonic probe or a twin-fluid atomizer and freeze dried to yield spherical granules. Control of the granule size fractions was investigated by changing the amplitude and the feeding rate of the nanosuspension during ultrasonication, whilst the flow rate of compressed air used for spraying was varied during twin-fluid atomisation. Granules retaining good crushability for pressing were in a size range of 125-250. μm, which were achieved with ~60. wt% yields using the atomisation route, whilst a maximum of 35. wt% of granules in this size range were produced in previous research using ultrasonication. © 2013 Elsevier Ltd

Microwave-enhanced densification of sol-gel alumina films

Alumina films prepared by the sol-gel method were sintered at 1160 °C and 1200 °C using a 2.45 GHz microwave / conventional hybrid furnace in order to study the influence of microwave power on the sintering process and resultant samples. Experiments were designed to ensure that each series of samples underwent an identical thermal history in terms of temperature / time profiles. Sintering was carried out using three different heating approaches: pure conventional heating and hybrid heating with 600 W and 1000 W of microwave radiation, respectively. The results obtained showed that, compared with pure conventional heating, the presence of the microwave field led to higher sintered densities and crystallinity in the samples, indicating that the microwave field enhanced the sintering of the sol-gel alumina films and supporting the existence of the microwave effect

Investigation of long-term hydrothermal stability of NANO 3YSZ at operation temperatures of SOFCS

The study of hydrothermal degradation of 3 mol% yttria partially stabilized zirconia was carried out on various grain sizes with the target to prove the long term stability of nano sized 3YSZ applied in solid oxide fuel cells (SOFC5). Samples with grain sizes lower than 100 nm (nano) and higher than 100 nm (submicron) were produced by uniaxial pressing and slip casting. Tests of their mechanical performance such as hardness, however, showed that the way of processing does not have significant impact to the sintered specimen. Therefore are the samples of either process valuable for hydrothermal ageing experiments, which proved so far a complete resistance against water exposure in an autoclave up to 21 days

Microwave assisted large scale sintering of multilayer electroceramic devices

The feasibility of employing the microwave methodology for the processing of integrated passive devices (IPDs), nanocrystalline ZnO radials and nano multilayer varistor (MLVs) devices was explored. Methodical microwave sintering experiments were carried out using a multimode, 2.45 GHz microwave applicator. Effect of various experimental parameters such as heating rate, cooling rate, soaking time, sintering temperature etc. on the processing of these device components was investigated in detail. The resultant products were characterized for microstructure, composition and electrical performance. The various stages involved in taking the laboratory research to industrial scale-up production were also examined. The use of microwaves for the processing of MLVs was found to genuinely improve the electrical properties in both small scale (~200 devices/ batch) and large scale (~12000 devices/batch) sintering situations. For a stand alone microwave heating process a back-toback cascading /conveyer belt arrangement is recommended for continuous large scale production. However hybrid heating methodology was found to provide the capability of stacking operations and could be helpful in avoiding the use of ‘casketing’, besides providing the possibility of achieving uniform temperature across a large volume. The technique seems to be attractive in terms of its simplicity, rapidity, economic viability and the superior product performance achieved in all the cases augers well for its general applicability

Quantitative analysis of the residual stress and dislocation density distributions around indentations in alumina and zirconia toughened alumina (ZTA) ceramics

Alumina, 10% and 20% ZTA with 1.5mol% yttria stabiliser were subjected to Vickers indentation testing with loads from 1 to 20kg. Cr fluorescence and Raman spectroscopy were applied to the indent centre and around the indentation in order to investigate the origin of the signal, the effect of indentation loads and zirconia phase transformation on the residual stress and plastic deformation in the plastic zone. The results suggested that with very strong laser scattering, the depth resolution of ZTA materials was very poor, which lead to a very significant amount of the signal being collected from the subsurface regions below the plastic zone. It was also found that zirconia phase transformation reduced the compressive residual stress in the alumina matrix within the plastic zone, except at the indentation centre, due to the tensile residual microstress generated by the zirconia phase transformation. In addition, the dislocation density on the indent surface of the ZTA samples was significantly reduced due to the restriction of crack propagation and energy absorption during the phase transformation process. At the indent centre, the zirconia phase transformation was suppressed by the high compressive stress, therefore, no significant difference between alumina and ZTA in terms of their residual stress and dislocation density were observed. Using TEM observation, it was found that the plastic zone microstructure of pure alumina is different from that of ZTA, which is consistent with the Cr fluorescence results