Material of the month: Aggregated diamond nanorods - carbon strength examined
Unlike our recent focuses on aluminium foil and stainless steel, you won’t find this month’s material in your kitchen. Aggregated diamond nanorods form a special material that has only been around since 2005, reports Maria Felice.
Carbon is often used in GCSE Chemistry as an example of an element that exhibits allotropy, which means it can exist in more than one multi-atomic structure. The list started off with amorphous carbon (a constituent of charcoal), graphite and diamond, and now also includes buckminsterfullerene, carbon nanotubes, carbon nanofoam and many more. So revealing how many allotropes you learnt about at school can give away your age.
In its graphite form, carbon consists of carbon atoms bonded triagonally to three others by strong covalent C–C bonds, forming a two-dimensional network. Layers of these atoms are held together by weak van der Waals forces. In buckminsterfullerene (C60), the carbon atoms are arranged in hexagons and pentagons to form a football-shaped molecule, with each carbon atom covalently bonded to three others. There are 60 atoms in each molecule, hence the formula C60. In the solid form, these molecules are joined together by weak van der Waals forces.
In the diamond form, each carbon atom is bonded to four others covalently and tetrahedrally, resulting in a strong three-dimensional network. Most natural diamonds are single-crystal, so it is highly anisotropic and can cleave in some directions. Despite being an excellent abrasive, its use as a superabrasive is limited as it can be susceptible to graphitisation and softening. Polycrystalline diamond would be preferable but is rarely found naturally, so synthetic types are required. Aggregated diamond nanorods (ADNRs) are one example of this and were first created by Natalia Dubrovinskaia and her team in 2005 at the University of Bayreuth, in Germany.
The team subjected various forms of carbon to different temperatures and pressures in an attempt to produce nanorods, which, in theory, had very attractive properties. The required parameters were found to be a starting material of C60, temperature of 2,000°C and pressure of 20GPa. The output was solid cylinders, 3mm in height and 1.8mm in diameter, made of a material that was compact and translucent. Its X-ray diffraction patterns indicated that the material had a diamond-structure and its Raman and IR spectra were similar to those obtained for nanodiamond. High resolution transmission electron microscopy showed that the bulk material consisted of elongated crystals of aggregated diamond nanorods. The crystals were approximately 1µm in length and 20nm in diameter.
The density of ADNRs was found to be 3.532g/cm3 – slightly higher than diamond. One explanation for this is that contraction of the outerlayer of the rods causes the C-C bonds to bulk modulus is a measure of a material’s resistance to a change in volume when pressure is applied at a constant temperature. The larger the value, the more resistant the material is to compression. The bulk modulus of ADNRs was found to be 491GPa – the highest ever recorded experimentally. For comparison, the bulk modulus of diamond is 442GPa and that of steel is 160GPa. The nanorods were found to have a wear resistance of two to three times that of single-crystal diamond and three times that of one of the best PCDs available, making them an excellent abrasive.
Wear resistance is dependent on fracture toughness, Young’s modulus and hardness. Nanorods were found to have a fracture toughness (resistance to fracture once a crack is present) of two to three times that of diamond. This is probably due the suppression of the preferred cleavage plane thanks to the tiny nanorods, their random orientation and the compact mutual intergrowth of the material. In polycrystalline materials, the presence of grain boundaries can reduce performance, but the Young’s modulus of ADNRs is equal to that of a single-crystal diamond. This shows that the intergrain spacing is extremely small and grain boundaries are dense and immobile.
The orientation of the nanorods is not uniform and this random intergrowth might be the cause of the extreme hardness ADNRs exhibit. The microhardness of a material can be determined by the Vickers test, which involves pressing a diamond pyramid into the surface of the material and measuring the indentation. As expected, the Vickers test on ADNRs did not work and the conclusion was therefore that ADNRs are at least as hard as conventional diamond.
ADNRs could be a valuable material for machining ferrous alloys and ceramics, thanks to their mechanical properties and resistance to graphitisation. They may also have applications in precise machining because of their nanocrystalline structure.
Dubrovinskaia has carried out some other very interesting work, including the synthesis of a boron nitride nanocomposite. This is the first non-carbon based bulk material with a hardness approaching that of any form of diamond. She has also worked on a novel method of achieving high pressure for studying materials under extreme conditions. The diamond anvil cell technique has been used since the 1950s to do this, and the maximum static pressure achievable at room temperature is in the region of 400GPa. In 2012, Dubrovinskaia and her team raised this to 600GPa by implementing micro-ball nanodiamond anvils as second stage anvils in conventional diamond anvil cells. This method is an improvement on the usual process of achieving super-high pressure through the use of shock waves, which grants only nanoseconds of observation time and generates a lot of heat in addition to pressure.
Materials science is constantly evolving and facts such as ‘diamond is the hardest material’ come with an expiry date. I don’t think nanomaterials will ever replace single-crystal diamond as a girl’s best friend, but they are certainly capable of scratching the gemstone.
All of Maria Felice’s Material of the month and Materials through the ages columns are available online at www.iom3.org/materialsworld If you have any ideas for materials you would like to see covered, get in touch and let us know, firstname.lastname@example.org