MRS Fall Meeting: A selective review
Polymer-ceramics and metal-ceramic nanocomposites E P Giannelis
(Cornell University, USA)
New techniques are being exploited for making molecularsized sandwich structures of composite materials that could find an immediate use in various industries. The techniques provide unprecedented control in making layered structures of composites, where the layers alternate between plastic-like polymeric material and hard ceramics. So precise is the process that a layer of polymer no more than one molecule thick can be sandwiched between ultra-thin layers of ceramic measuring between 5 and 15 nm (Fig. 8). The result is a nanocomposite that marries the properties of both substances to produce a unique material. Among the applications being investigated by Emmanuel Giannelis are packaging materials used in the electronics industry. Packaging includes the protective enclosure of electronic devices and the substances that contain printed circuit boards and other electronic assemblies. It represents more than 50% of the engineering and manufacturing costs for most electronic products now in production, according to statistics from the Semiconductor Research Corp. Nanocomposites have electrical, thermal and mechanical properties that are equal to or superior to the polymers or ceramics now used in the manufacture of electronic parts. In addition, they should be cheaper to manufacture. The only commercial use of a nanocomposite at present is in a timing belt cover for cars that is manufactured by Toyota Corporation and possesses superior mechanical properties. Development of nanocomposites has lagged in the United States, in his opinion. Among the significant properties of nanocomposites is that they do not melt or decompose at temperatures where polymers by themselves would do so. There is little deterioration in properties with rising temperature. For example, one nanocomposite (a polyaniline-silicate) showed no glass transition up to 250°C, which was the limit of the apparatus, even though the polymer itself melts at 110°C. Conventional techniques for forming layered structures involve successively depositing individual layers. The new technique, in contrast, is capable of assembling hundreds of thousands of layers in a single step - a process known as self-assembly. Using systems with self-assembling characteristics to build complex structures is not a new idea. Natural systems such as abalone shells and some biological membranes have used the same design and processing approaches quite successfully to build materials with unique properties, ranging from strong and tough composites to highly selective sensing devices. In a variation of this approach, Giannelis and his graduate students have fabricated nanocomposites with several layers of polymer betweeen silicate-based ceramics. These substances, first explored by Japanese researchers, have a much higher proportion of polymer to silicate. Because so little silicate is used, the nanocomposite is lightweight. At the same time, it can be processed much like a plastic and is easily moulded. According to Giannelis the process of making nanocomposites resembles pouring flavoured syrup into the crystals of a thin Italian ice: gaps formed between the layers of certain ceramic oxides are infused with either polymers or
the chemicals that form polymers. Researchers are able to control carefully the amount of polymeric material infused. The result is a structure with layers so thin that they are essentially two-dimensional. Studies have shown that the properties of a polymer at or near the surface of a ceramic are different from those of a polymer surrounded by other polymers. This difference in chemistry at the surface as well as the shape and thickness of the ceramic filler, account for the improved properties of nanocomposites compared with conventional composites, where polymers and ceramics are mixed in a more random, bulk fashion. Giannelis holds patents on two nanocomposites and is pursuing commercial applications of his work with interested companies. This work is funded by Corning IBM, the Air Force Office of Scientific Research, the Materials Science Center at Cornell and the industry-funded Alliance for Electronic Packaging at Cornell. 1o I
Schematic structures of polymer-ceramic nanocomposites obtained using layered ceramics. Rectangular bars represent the ceramic layers and they are molecularly dispersed in the polymer matrix. The new composites can be designed to exhibit the overriding strength of a ceramic, crack deflecting properties of polymers, coefficient of thermal expansion closely matching the ceramic, and higher thermal stabili~ Furthermore, because of the molecular dispersion the desired properties can be achieved with very low ceramic Ioadings.
MATERIALS& DESIGNVol. 13 No. 2 APRIL1992