Investigadores de la Universidad Tecnológica de Hamburgo y la Universidad de Kiel encuentran el material más ligero del planeta: la aerografita.
Aerographites are ultralow density solids that resemble classical aerogels in many regards (in fact, by some definitions, aerographites are aerogels, simply produced by an unconventional process). At their essence they are frameworks with hollow struts made of graphite deposited through chemical vapor deposition onto a template that disappears as the material is made.
- Zinc metal powder with a grain size of 1–5 μm
- Poly(vinyl butyral)
- High-temperature air furnace with minimum upper temperature of 1200°C
- High-temperature ceramic rings to serve as molds
- Multizone (two zones or greater) split-hinge (“clamshell”) tube furnace
- Quartz processing tube (this recipe used a tube with length = 1300 mm diameter = 110 mm, dependent on tube furnace size)
- A liquid feedstock injector for CVD furnaces (e.g., syringe pump attached to a injector nozzle)
First, a template made from zinc oxide (ZnO) is prepared. This will serve as the “evaporating template” upon which the aerographite will be formed.
- Mix zinc (metal) powder with a grain size 1–5 μm is mixed with poly(vinyl butyral) powder in a mass ratio of 1 to 2.
- Heat the zinc powder/poly(vinyl butyral) mixture in a furnace (in air) to a temperature of 900 °C at a rate of 60 °C min−1 and hold for 30 min. This should produce a loose powder of ZnO tetrapods. Further synthesis methods and established recipes for the fabrication of ZnO structures can be found in Ozgur, et al. “A Comprehensive Review of ZnO Materials and Devices”, Journal of Applied Physics, 2005, 98, 041301, http://dx.doi.org/10.1063/1.1992666.
- Compress the ZnO tetrapod powder to a density ranging from 0.15 g cm−3 to 0.8 g m−3.
- Reheat the ZnO pellet for 3 to 4 h at 1200 °C in a ceramic rings (e.g., a ring with height = 10 mm and diameter = 15 mm) to fuse the tetrapods in the powder together.
Chemical Vapor Deposition of Aerographite Form
- Place the ZnO template into the second zone (the “processing zone”) of the multizone clamshell tube furnace.
- Heat the first zone of the furnace to 200 °C–this is the “injection zone”.
- Heat the second the zone of the furnace to 760 °C.
- Introduce a flow of 20 sccm (0.02 L min−1) of argon at atmospheric pressure.
- Begin injection of toluene at a rate of 5.5 mL h−1.
- Increase the flow of argon to 200 sccm and introduce a flow of 20 sccm hydrogen.
- Discontinue injection of the toluene.
- Turn off the flow of argon and increase the hydrogen to 600 sccm.
- Wait 45 min.
- Reintroduce a flow of 200 sccm argon and decrease the hydrogen to 20 sccm.
- Continue injection of the toluene.
- Wait 120 min.
- Discontinue injection of the toluene, turn off the flow of argon, and increase the flow of hydrogen to 600 sccm.
- Wait 20 min.
- Introduce a flow of 600 sccm argon and turn the hydrogen off. Turn the furnace off and wait for it to cool below 200 °C before removing samples.
Variables You Can Play With
For ultralow density hollow framework aerographite, use:
- A toluene injection rate of 2 mL h−1 with 200 sccm Ar and 60 sccm H2 at 760°C
- A 1-h post-treatment with no toluene injection under 90 sccm H2 at 800 °C
Obtenido en aerographite