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3.6.2 Short description

Nano-ceramics describe a relatively large family of various materials with different properties and application potential. To achieve a concise structure, it makes sense to separate the ceramics material class roughly into oxide and non-oxide ceramics.

In the following, a number of relevant nano-ceramic materials will be shortly described:

Oxide nano-ceramics:
The main part of nano-ceramic materials is consisting of oxide ceramics. Various nano-scale metal oxides are currently investigated for different applications and in more or less advanced research state. However, only a minority has already found their way into widespread commercial use.

Indium-Tin oxide (ITO) is one of the most prominent nano materials is. ITO is a semi-conducting, mainly transparent material consisting of Indium(III)-oxide (In2O3) to around 90 % and Tin(IV)-oxide (SnO2) to around 10 %. It is used as thin-film material for the production of transparent electrodes in liquid crystal displays, touch screens, organic LEDs, thin-film solar cells, semiconducting sensors etc. Due to its IR-radiation reflectivity it is often used as thermal insulation coating on window glass. Its anti-static properties make it additionally suitable e. g. for packing and storage of sensitive electronic components. However, as ITO-prices have drastically increased due to a global indium production shortage within the last years, research for alternatives has got intensified. Promising candidates are Fluor Tin Oxides (FTO), Aluminium Zinc Oxides (AZO) and Antimony Tin Oxides (ATO).

Aluminium oxide (Al2O3) nanoparticles and nanoparticulate aluminium oxide are in widespread commercial use. Particles have a size of about 20 nm and quite high sur-face-volume ratio. They find application in heat and wear resistant coatings e. g. for cutting and grinding tools and as transparent coatings. nanoparticulate bulk aluminium oxide is a bulk material with a nano-grain internal structure similar to nanocrystalline metals.

Silicon dioxide (SiO2) is a material of widespread appearance. It is a main fraction e. g. of sands. SiO2 is chemically robust and is the main part of quartz glasses. Even nanopar-ticulate SiO2 is already in widespread use. As commercially availabe ingredient it appears as additive in rubber products for vehicle tires as well as in photo resists in lithography processes to optimize the desired material properties. In a nanoporous, foam-like form it is the main base material even for the new material type of aerogels.

Fig. 6: Aerogel protecting a hand from a flame (Source: Lawrence Berkeley National Lab).

Aerogels are high porosity solids with up to 95 % of the volume consisting of nanoscale pores. The base underlying manufacturing process is a solgel-synthesis. Different types of aerogels have been created depending on the base material. Hence, there are carbon, metal oxide, polymer aerogels or even aerogels from other substances. However, the most common aerogels are based on silicates. Note that Silica (silicon dioxide, SiO2) although being a special case with no negative charge and no need for counter-ions will be explicitely considered as silicates and thus base material for aerogels.

Due to their high porosity aerogels have an extremely high surface area of up to 1000 m2/g and a high low thermal conductivity. Thus, they are suitable e. g. as insulation and as filter materials. A further predominat property is their low specific weight, making them a research subject for lightweight construction.

Besides, aerogels are of interst due to their optical properties. They exhibit high optical transparencies with refractions indices close to unity. Silica aerogels are appearing slightly blue.

The base-particle size of silica aerogels is ~10 nm, pore sizes are arranged in ~10 to ~100 nm distance. They are resistant and chemically inert to liquid metals, heat resistant up to 1200 °C and non-toxic.

Fig. 7: TiO2-nanoparticles (Source: BASF).

Titanium oxide nanoaprticles (TiO2) have got manifold application potentials. With crystallit sizes of 3-5 nm they are about 100-fold smaller compared to conventional TiO2-pigments and show clearly modified physical and chemical properties. They are mainly suitable for transparent coatings, as light scattering is reduced due to their small particle size. Thus, nanoscale TiO2 acts as an invisible physical barrier for UV-radiation, making it a base ingredient e. g. for new generation sun screen cosmetics. But also other coatings e. g. for glasses, metals, wood products etc benefit from TiO2 nanoparticle additives. TiO2 nanoparticles have a large surface-volume ratio, which enhances their catalytic activity. This property in increasingly used for chemical catalysis applications such as e. g. the photo-catalytic purification of water and air leading to a destruction of organic pollutants. Another application for TiO2 nanoparticles has developed in novel organic solar cells, making use of the particle properties in light induced electrical charge separation.

Zinc oxide (ZnO) has some very similar properties. ZnO is a II-VI-semiconductor and applicable e. g. in transparent conductive thin-films in blue laser diodes, solar cells and liquid crystal displays. Since some time ZnO-nanopowder is also utilized as broadband UV-filter in commercial sunscreens. ZnO-particles act as inorganic UV-filter by reflecting the incident light. Conventional microscale ZnO-particles are white causing an unwanted "white skin effect". To avoid this, transparent ZnO-nanoparticles are increasingly utilized. Besides, the larger surface makes their protection functionality much more effective. The same holds for UV-protection of e. g. lacquer films and polymer surfaces. Compared to TiO2 the photo catalytic effect of ZnO is much weaker. ZnO shows a strong tendency towards self organized growth in nanostructures. By a variation of the conditions, crystal growth in various shapes is possible. In research labs, nanoscale wires, rods, rings etc have been grown. Of particular interest are ZnO nanocolumns. With these, laser emission at room temperature has been observed. ZnO nanocolumns thereby act both as optical amplification medium and as laser resonator.

Fig. 8: ZnO-nanocolumns on saphire substrate (Source: University of Ulm).

Comparable to ZnO rare-earth oxides are also UV-reflective and transparent. They are thus effective in protecting surfaces from degeneration by exposure to UV-radiation. A series of nanoscale particles such as Dy-, Er-, Eu-, Gd-, Nd-, Sm-, Tb-,Yb- and Y-oxides are available on small scale.

Zirconium and cerium oxides are even available as nanoparticles on small scale. Their average particle sizes are below 100 nm. They are investigated to improve catalysic reactions within catalytic converters on vehicles as well as in solid-oxide fuel-cells. To achieve better performance they are frequently doped with rare-earths such as yttrium.

Iron oxide nanoparticles are particles with up to 100 nm in size that are composed of different ferric oxides. The are consisting oxides of either oxidation level 2 or 3 (Fe(II)-oxide; Fe(III)-oxide) or as a mixture of both. The nanoparticles are processed by either chemical vapour deposition, sol-gel-processing or flame pyrolysis. Conventional iron oxide particles are currently in large scale use in pigments for lacquers and paints. However, nanoparticles are utilized in medical research e. g. as contrast agents in MRI-tomography or in cancer therapy. The main part of the medical applications is based on the magnetic properties of the particles.

Non-oxide nano-ceramics:
Within the group of non-oxide ceramics and ceramic powders there are carbides and nitrides dominating.

One of the most prominent carbide ceramic nanoparticles is tungsten carbide (W2C), a chemical compound of tungsten and carbon. Tungsten carbide has a high melting point and is extremely hard. Besides it shows a high electrical conductivity comparable to metals. Tungsten carbide is mainly used for hardening surfaces of cutting machinery. The aim is to improve wear and temperature resistance. It is even contained in "cement carbide" (tungsten carbide cobalt) which is a metal matrix composite, with tungsten carbide nanoparticles being aggregated fillers embedded into a cobalt matrix. Tungsten carbide nanoparticles are already in use and at the barrier to large scale production. Other carbide nano-ceramics are in stages of base and applied research. This hold e. g. for boron nitride (BN) which, embedded in nano-composites exhibits an optimized heat, strain and fracture resistance. Nanofilms of carbide ceramics based on e. g. Si, N, B, Ti are mainly investigated as thermal and mechanical reinforcing or friction lowering coatings.

In a slightly more advanced research state is nanoparticulate silicon nitride (Si3N4) which is is a hard, solid substance and shows a higher shock resistance tha other ceramics. Due to its shock and heat-resistant characteristics it is frequently used e. g. in ball bearings.