Sparking advanced ceramics
Fawad Inam, Haixue Yan, Wei Tu and Mike Reece of Nanoforce Technology Ltd, Queen Mary University of London, UK, describe developments using spark plasma sintering.
Ceramics are normally processed by consolidating powders using sintering at high temperatures. The time required for this process is usually measured in hours because of the slow rate of heating/cooling of the furnaces used. Spark plasma sintering (SPS), as it is commonly known, is a rapid sintering process, where the cycle can be completed in minutes. This generates the possibility of making ceramics with new microstructures and properties. Queen Mary University of London and its spin-out company, Nanoforce Technology Ltd, are working to exploit SPS technology in the UK. Spark plasma sintering has some similarities to hot-pressing, which involves applying pressure during sintering. The rapid heating rates in SPS are achieved by the direct Joule heating of the graphite dies in which the ceramics are sintered. This involves pulsed, direct current at low voltages (<10V) and high currents (typically >1,000A). The furnace at Nanoforce can achieve rates of up to 600°C/min. Because only the ceramic and die are heated, the cooling rates can be even faster because of their low thermal inertia and forced cooling.
Nanoforce is working with its academic and industrial collaborators to develop new structural and functional ceramics that cannot be prepared using other processing routes. The themes for this research are nanostructured materials, non-equilibrium composites, textured materials and difficult-to-densify systems.
There is a great interest in size effects in materials. There are many properties that change dramatically when the grain size or component dimensions are below 100nm. A wide range of nanoscale ceramic powders have been synthesised with dimensions in the nanometre range. However, during conventional sintering, the grain size can increase rapidly in the final stages of densification, leading to micrometre scale grains. The rapid heating/cooling afforded by SPS allows densification to be achieved with minimal grain growth.
The work at Nanoforce focuses on the effect of grain size on the mechanical properties of structural nanoceramics and the properties of ferroelectric and varistor nanoceramics.
Ferroelectrics are used for non-volatile memories, and actuators and sensors in microelectromechanical systems (MEMS). The increasing demand for size reduction in the microelectronics industry is approaching the nanometre scale, where experimental and theoretical work is showing that the properties of materials diverge. Nanoforce has demonstrated, for the first time, that Aurivillius phase ferroelectric ceramics with grain size <100nm maintain their ferroelectric properties, albeit reduced.
Spark plasma sintering opens up the possibility of producing new composites, with novel properties, that were previously not possible to fabricate using conventional processing routes, non-equilibrium composites. When two non-equilibrium phases are processed by conventional sintering they react to form equilibrium phases. This is used to good advantage in reactive sintering. However, this limits the composites that can be produced.
Using SPS, rapid densification can be achievedwith minimal reaction between components. Multiferroics materials have coupling of ferroelectric-ferromagnetic-ferroelastic properties, and are of potential interest for new sensors and memory devices. The materials that intrinsically demonstrate these properties are limited and the coupling tends to be weak. One solution is to rapidly co-sinter ferroelectric and ferromagnetic phases into a non-equilibrium composite.
The projects are using SPS to produce dense, well dispersed, carbon nanotube (CNT) nanocomposites (see image, right). For the exploitation of these novel nanocomposites, it is crucial not to damage the CNTs while sintering at high temperatures. Carbon nanotubes possess a heat-sensitive structure that degrades if processed using conventional routes such as hot-pressing. Spark plasma sintering can process these nanocomposites quickly without structural or chemical degradation of the CNTs, opening up the possibility of developing ceramics with new electro-mechanical properties and applications, such as, heating elements (see image, bottom, right), electro discharge machinability (EDM), and high electrical and thermal anisotropy.
Textured or grain-orientated ceramics can have anisotropic and improved properties compared to randomly grain-orientated ceramics. Nanoforce is working to develop a new commercial processing route for producing higher textured and denser ceramics. This involves a two-step process that relies on the rapid heating rates achieved in SPS.
In the first step, fine powders are densified with minimal grain growth, the second step, which is at a higher temperature, involves hot forging. Texturing is very important in some ferroelectric ceramics because of the crystallographic dependence of their properties. Using the two-step method Nanoforce has reported for the first time piezoelectric activity in the super high Curie point, Nd2Ti2O7 (1,482°C). These materials could produce a step change in the sensitivity and operating temperature for high temperature accelerometres and vibration sensors.
Carbide, nitride and boride ceramics are of interest for many applications because of their high melting temperatures, good mechanical properties, including high hardness, and low density. For example, the material with the highest known melting temperature is the carbide Ta4HfC5 at 4,215°C. While boron dicarbonitride (BC2N) is classed as the fourth hardest material known to man, with the commercially used boron nitride (B4C) sixth.
These materials require high temperatures and pressures to densify them. Even under these conditions they may not fully densify. Nanoforce is investigating the potential of SPS to densify these materials to close to theoretical density. The group is working with the UK’s Defence Science and Technology Laboratory and the Centre for Advanced Structural Ceramics at Imperial College London, UK, to develop the next generation of ultrahigh temperature ceramics based on the diborides of zirconium and hafnium (ZrB2, HfB2). These may find application in the leading edges of hypersonic vehicles, where they may need to operate at temperatures above 2,000°C while maintaining structural integrity.
The technique’s high throughput is being put to use in the production of ceramic sputtering targets for new compositions for structural coatings and functional thin films. However, the temperature gradients that are produced by SPS are a possible limitation. In ceramics that have relatively good electrical and thermal conductivities at high temperature, such as silicon carbide, this does not present a problem, even for large pieces. To guide optimisation of the processing computational modelling employing coupled thermal-electrical axisymmetric elements is used.
The research at Nanoforce is focused on the development of new materials using new processing routes. A long term objective of this work is to commercialise materials prepared by SPS through knowledge transfer and spin-outs. To achieve this, Nanoforce is working with industrial collaborators to demonstrate the feasibility of scaling-up.
Further information: Nanoforce