Microwaves Solve Some Nanostructured Materials Processing Problems
Nanostructured materials including ceramics and metal-ceramic composites have received much attention in recent years. The appeal is their ability to display unusual physical and mechanical properties such as superplasticity in ceramics and transparency for usually opaque materials among many other unique properties. For example, the use of nanocomponents could pave the way to merge and shrink components such as a computer, TV, telephone and GPS system into a device as small as a wrist watch. While nanosize powders offering these properties have currently been produced successfully-sometimes in relatively large quantities-a number of challenges still need to be surmounted if engineering parts are to be manufactured. Most likely, "bottom-up" approaches will probably be the long-term solution. However, these may not be available for several years, and will require industry to completely retool. Therefore there is considerable mileage to be gained by examining what can be achieved practically now using a "top-down" approach based on existing manufacturing facilities. This brief discussion outlines work underway at Loughborough University in the field of advanced ceramics.
Challenges that are currently being investigated at Loughborough include:
- The ability to control the inherent agglomeration of nanopowders, something that arises from the massive surface areas that these powders have
- The ability of nanopowders to flow when dry
- The difficulty in producing high solids loading suspensions that have low viscosities
Developments in each of these areas have been achieved and patents are being filed. However, the greatest opportunity for using microwave processing could be the need to convert the unfired compacts made from nanopowders into dense, sintered forms while retaining the nanostructure.
Obtaining densification without suffering from significant grain growth is extremely difficult because the driving forces for densification and grain coarsening are comparable in magnitude, both being proportional to the reciprocal grain size. Thus, success is dependent on close control of the competition between them.
The greatest achievements to date have all involved the use of pressure; for example, via hot pressing, hot isostatic pressing, sinter forging and the misnamed "spark plasma sintering" routes. While these routes have shown good potential for achieving the dual goals of near-theoretical densities and <100 nm grain sizes, they have associated disadvantages, not least their cost and often a very limited shape capability.
A second strategy has been to add solutes or second-phase particles that respectively reduce grain boundary mobilities or pin the grain boundaries. This approach has been successful, but it can only be pursued in a few systems due to the fact that the addition of different phases can unacceptably degrade the final properties of the product for many applications.
A third approach, and the one adopted at Loughborough, is to use two-stage sintering. Previous work by Chen and co-workers (Chen et.al., Nature, 404, p 168-171, 2000) has shown that the majority of the grain growth that occurs takes place during the initial heating step to temperature T1 (Fig. 1). Research at Loughborough is exploiting the very fast and uniform heating profiles generated by a microwave/conventional hybrid furnace. This approach yields fully dense samples having average grain sizes of <65 nm (Fig. 2) using precursor YSZ and ZnO powders approximately 20 nm in diameter. Restricting grain growth to a factor of just ~3.5 while achieving full density via pressureless sintering is believed to be a world first, and patenting is in progress. In addition, sintering temperatures were 300°C (540°F) lower, and only 20% of the time was required compared with the sintering of equivalent conventional powders.
While substantial characterisation work is still required, preliminary results suggest a grain size-dependent shift in the phase boundary composition for the nano YSZ ceramics. This will not only have significant implications for achieving the peak mechanical and electrical performance, but may also lead to a redrawing of the phase diagrams for other nanostructured advanced ceramics.
As indicated above, one of the features of working with nanopowders is that densification takes place at temperatures a few hundred degrees below those of equivalent, larger grained powders. Besides the potential for retaining a nanostructure discussed above, these low sintering temperatures also open up the possibility of new applications for which conventional powders are not suitable. One example would be in composites in which a high sintering temperature renders certain material combinations incompatible. An example that is being researched at Loughborough is the sintering of metal-ceramic composites. Judicious selection of the right materials can result in the ability to co-fire these materials without melting the metal structure. IH