Rocket science often seems to typify state-of-the-art technology. The extreme conditions of take-off, landing and space itself, the exacting specifications of the instrumentation required for rocket control and scientific data collection, and the costs per unit mass for launching the rocket would seem to leave no room for mediocrity. With their enhanced multifunctional properties nanomaterials offer a lot of bang per kilo for missions. Physics World Materials looks at what nanomaterials can offer to protect against the elements in space.
To merit incorporation in a billion-dollar space mission, new technologies need to deliver more than just a promise of enhanced functionality. The properties of new materials and the impact of their use on all other aspects of the space equipment need to be reliably defined. As Jamshid A Samareh and Emilie J Siochi from NASA’s Langley Research Center emphasize in a recent review, a key factor preventing greater uptake of nanomaterials in space missions has been a lack of a deep understanding of their behaviour within the complex and sophisticated systems of spacecraft. “The barrier is understanding the measurable benefits over materials that are currently being used – especially when you have to trade risk and cost with current paradigms,” says Emilie Siochi, senior materials scientist in the Advanced Materials and Processing Branch at NASA Langley Research Center in Virginia, USA. As for the electronics used, Meyya Meyyappan Chief Scientist for Exploration Technology at Ames Research Center points out that making sure the radiation tolerance and packaging meet requirements can preclude adoption of “the state-of-the-art”. Scaling up production from lab levels to the volumes needed for a rocket can also compromise the nanomaterial properties that recommended their use in the first place, deterring uptake.
Despite the overarching caution in space technology the lure remains for harnessing nanomaterials to take on the challenges of blasting free from Earth intact, facing the furnace of take-off and the chill of outer space, as well as the cosmic cocktail of radiation exposure. And there are a growing number of nanomaterials whose manufacture and device application has reached a level of maturity that allows for a valuable contribution in aerospace missions.
Know your strengths
Carbon nanotubes (CNT) were among the first nanostructures to capture the imagination of engineers in aerospace and other industries, and they form the focus of Samareh and Siochi’s review. A 2002 report of the potential applications of CNTs in space missions highlighted the mechanical strength enhancements CNTs bring to composites, with both polymer and aluminium matrix composites reinforced with single-wall CNTs showing orders of magnitude greater strength than the aluminium 2219-T87 typically used for high-temperature structural applications such as space boosters and fuel tanks.
Chiara Daraio and colleagues at Caltech, Rice University and Clemson University in the US combined carbon nanotubes with carbon fibres to harness the mechanical properties and energy dissipation characteristics of these structures. Their “bucky sponge” is capable of damping impact forces by as much as 50%, providing valuable protection given the high risks of potential collisions between spacecraft and other extraterrestrial debris.
In practice, Samareh and Siochi suggest that the numerous design considerations such as supporting structures and durability may have inhibited wider CNT use in aerospace composites so far. There has also been a frustrating compromise in the thermal conductivity, as well as mechanical and other properties of mass-produced CNT composites compared with those from a lab. As a result, commercial sectors with less exacting requirements on mass reduction, reliability and environmental durability have seen faster progress in exploiting carbon nanotube composites. Significant progress in manufacturing CNT composites over the past 15 years allows for a more optimistic prognosis of their future contribution in aerospace applications, but as Samareh and Siochi highlight, the ultimate outlook for this field will depend on how well the advantages identified in CNT technology match with the needs of aerospace to spur on investment in affordable mass production of materials that retain the properties observed in the lab.
Handling the heat
The thermal resilience of structural properties alone is not enough for a rocket to withstand the extremes of temperature in lift-off and re-entry. The spacecraft’s electronics and sensing devices also require protection. To meet these wide-ranging demands sophisticated thermal management systems draw on technologies ranging from effective heat conductors and radiators, to layers of sacrificial material on the rocket surface that absorb the heat generated as rockets slow down on entering an atmosphere. Nanostructures are promising materials for heat management systems, which can exploit their large surface area per unit volume for heat transfer.
The increasingly reduced size of electronic devices in itself causes heating. In the Nanotechnology’s Aerospace focus collection Michael T Barako, Vincent Gambin and Jesse Tice at NG Next, Northrop Grumman Corporation go over the heat challenges specifically associated with high power electronics in aerospace technologies, where the pressure to decrease device size can escalate heating effects that add to issues of heat exposure from the flight itself, culminating in what they describe as “the most aggressive thermal management challenges”. They describe some of the options nanomaterials can provide at different points from heat source to sink. These include the use of synthetic diamonds with high thermal conductivity to ease heat transport, nanoporous materials and vertically aligned copper nanowires as high performance thermal interface materials, and phase change materials (PCMs) that can absorb heat energy in changes in microstructure. While aerospace applications set a high bar on the ability to reproduce nanostructures exactly so that their performance can be reliably defined, these heat-transfer solutions are also increasingly relevant to nanoelectronics for other uses. Barako et al. suggest that in general, a bottle neck has arisen as thermal management technologies lag behind progress in device size reductions that exacerbate heating. As a result, solutions transferred from aerospace research may be welcome across a range of applications.
As well as keeping the electronics for the instrumentation cool manned space missions need to be equipped to protect humans from the extremes of temperatures as well, and here again PCMs can come in handy. NASA has already invested in the potential of PCM technology for thermal regulation of space suits. While a lot of PCMs are based on hydrated salts, the textiles for space suits use microcapsules of a combination of paraffins with different crystallization and melting points. Global Markets Insights Inc have forecast a compound annual growth rate of 15.8% in the PCM market worldwide, exceeding US$4 billion by 2024. The growth is largely driven by their environmental benefits as alternatives to fossil-fuel-powered heaters and coolers, with textiles contributing significantly to that growth.
Once outside the Earth’s atmosphere radiation from the Sun becomes a significant thermal management issue. Traditional quartz tiles are used as solar reflectors but while these provide excellent thermal properties for radiating heat from the spacecraft and reflecting heat from the Sun, they are heavy and fragile. Researchers at the University of Southampton have now developed a coating for satellites and rocket exteriors that reduces the mass burden and is easier to apply to curved or even flexible surfaces. They use an aluminium-doped ZnO metamaterial that combines infrared plasmonics and the high visible transparency of transparent conducting oxides for a broadband spectral response. They conclude that their material provides “a new ultrathin approach to thermal radiators that could eventually replace conventional technologies such as metallized quartz tiles for use in space.”
Oxford nanoSystems have also developed a coating for thermal regulation. Theirs is a paint that dries with a nanoscale dendritic structure, which encourages bubble formation in fluids for enhanced cooling. The initial market for the innovation was boiler systems but the company has since begun investigating the fit of their technology with a range of other markets. As a high-cost low-volume application, where the profit on delivery may justify the substantial investments to optimize the product, aerospace has cropped up as a tempting potential application.
Electronics aside, knowledge of how to deal with temperatures exceeding several thousand degrees Kelvin turns out to be quite a transferrable skill. When Kevin Jordan, chief engineer of BNNT LLC wanted to produce boron nitride nanotubes (BNNTs) without catalysts to improve the quality and purity, he needed to work at temperatures of around 4000 °C. “I have a friend who is a rocket scientist, so we were able to work at these temperatures,” he says. The BNNT LLC approach allowed production of higher quality BNNTs at costs that were actually lower than catalytically produced counterparts. While the company does not work directly with space missions, the wide-ranging applications of BNNTs – as additives for improving properties such as thermal stability, piezoelectric properties and radiation shielding – suggests a plausible role in future space exploration.
New and old connections
In their new vision for Human and Robotics Space Exploration, NASA identified two key technology requirements for aerospace nanoelectronics. The first is highly integrated electronics with over a 1 billion devices per chip to minimize mass, a goal that would seem to favour next-generation nanoelectronics technologies over traditional electronics, where the constant reduction in feature sizes seen over the past 50 years is now struggling to keep pace with Moore’s law. However, as Meyya Meyyappan, Jessica E Koehne and Jin-Woo Han at Ames Research Center point out in a recent review – in reality the electronic devices onboard missions into space are more likely to be “a few generations behind the state-of-the-art” allowing time to check for the necessary packaging and radiation tolerance. Radiation tolerance is the second of the two key technology requirements for aerospace nanoelectronics.
In fact guarding against radiation may give older technologies cause for re-invention. Charge transport in vacuum is immune to radiation damage, so while silicon has mostly superseded vacuum-based electronics for terrestrial technologies, advantages remain for space. “The shortcomings of vacuum devices include high operating voltages, fragility, and large sizes relative to solid-state devices, and as a result, they have disappeared except for niche applications, for example, in satellites,” explain Meyyappan, Koehne and Han. However, although most vacuum electronics is on the microscale, the clunky vacuum equipment of the 1960s has been significantly upgraded since, and Meyyappan, Koehne and Han add, “Efforts to combine the best of vacuum transport and silicon technology have yielded nanoscale vacuum devices with electrode gaps of 50–150 nm.” An absence of scatterers also makes transport in vacuum faster.
Since their discovery in 1990 the field emission properties of carbon nanotubes have been an enduring topic of research, particularly with the ability to produce vertically aligned CNT arrays. As Valerie J Scott and colleagues from the Jet Propulsion Centre and Chevron Energy Technology Company in the US point out in their report in the Nanotechnology focus collection on aerospace, “The high aspect ratio and electrical properties of carbon nanotubes (CNTs) make these materials ideal candidates for use as field emitters in miniature vacuum electronics.” The drawback has been the absence of a robust adherence between CNT array and substrate, which introduces both mechanical and electrical weaknesses. Scott et al. look into how they can make these field emitters less fragile. By growing CNTs directly onto titanium they show they can anchor them more robustly to the substrate instead of relying on the weak van der Waals forces attaching them to silicon substrates. The result is low threshold voltages, high field enhancement factors, and long operating lifetimes, with current densities of 25 mA cm−2 held for over 24 h. They also show a transfer process of CNTs from polished silicon to titanium with copper, offering further performance improvements.
Handling the rays
Meyyappan, Koehne and Han’s review outlines three different types of radiation damage that affect electronics in space: single event effects (SEE) from collisions with a single energetic particle; long-term effects defined by a total ionizing dose (TID) and displacement damage from lattice atoms knocked out of place (DD). Should the technology reach a sufficient stage of maturity, CNT field effect transistors may fare particularly well as they have 45% greater resistance to SEEs that propagate through the circuit (single event transients) than conventional electronics. They remain susceptible to TID.
Memory technology that does not rely on charge transport is also immune to radiation damage, and here the review points out that PCMs offer a potential solution. Setting the phase of chalcogenide materials – compounds of group 16 anions, particularly sulphides, selenides, and tellurides such as Ge2Sb2Te5 – as either amorphous or crystalline is a way of defining 0 and 1 logic bits. So far the current needed to provide the thermal energy for the phase change has been a limitation of phase change random access memory but using nanowires such as GeTe instead of thin films greatly reduces the energy needed to write bits with this technology, alleviating current restrictions.
Protecting the vulnerable
There will inevitably be times where radiation-resistant device options fall short, or humans are manning the flight, and here materials that effectively protect against radiation are crucial. As John Yeow and colleagues at the University of Waterloo and the Canadian Space Agency point out in their Nanotechnology focus collection on aerospace report, although materials from elements with a high charge to atomic mass ratio are better at absorbing radiation, larger atoms are more prone to breaking into smaller atoms, which lead to further secondary radiation damage. Hydrogen, which has the highest charge to mass ratio is too difficult to store even as water, but polymers that have a low charge to mass ratio but a high hydrogen content could make good radiation-resistant materials. However radiation-resistant materials also need to meet the stringent mechanical and thermal resistance requirements of a space rocket, and in polymers are generally weak in terms of mechanical and thermal properties.
“Numerous studies have investigated different types of polymer-based composites in order to find lightweight alternatives to aluminium alloys with acceptable strength and good performance against radiation,” explain Yeow and colleagues in their report. While adding nanomaterials to polymer composites has been widely explored to improve thermal and mechanical properties, there has been little progress in identifying additives that improve the radiation resistance as well. Yeow and colleagues form a composite from multiwalled CNTs and polymethyl methacrylate (PMMA) and report a weight reduction of 18–19% against aluminium alloys both with and without the CNTs. In addition, they note that adding the CNTs improved both the mechanical strength and thermal resilience of the material while also reducing the generation of secondary neutron radiation by up to 5%.
The effects of radiation damage include changes to the threshold voltages, drive and leakage currents and ultimately lead to failure of the device completely. The damage is the result of trapped charges and increases with their concentration, which nanoscale device sizes can exacerbate
In 2016 Jin-Woo Han and Meyya Meyyappan at NASA Ames Research Center, and Mo Kebaili at KEBAILI Corp. in the US developed a thermal treatment system for repairing various device aging mechanisms including TID. “If the degradation can be healed during the life of post-processed silicon akin to a human immune system, the circuit reliability and lifetime can be improved,” they suggest in a report of the work. They explain the benefit of matching the treatment temperature to the damage, showing that annealing at 200 °C for 3 hours can recover drain current versus gate voltage characteristics for their device. They fabricate the microheater and the system on chip it is designed to repair separately, and then combine them in a stack, making the approach applicable to “any arbitrary commercial-off-the-shelf device”. “I am hoping to fly something like this to the Moon perhaps by 2020,” Meyya Meyyappan tells Physics World Materials.
In the Nanotechnology focus collection on aerospace Sedki Amor at the Université Catholique de Louvain and colleagues in Belgium and Tunisia also look at thermal repair for radiation damage. They point out that while the damage is receptive to repair when subject to annealing at 300-400 °C for an hour, or even just room temperatures after a significantly more protracted period of over seven months, for a speedier recovery they propose higher temperatures with a built-in MEMS device. With their silicon-on-insulator micro-hotplates for low-power and in situ annealing mitigation, they show they can restore normal device operation in just 8 minutes.
Mature enough to reach for the stars
While the technology of the tiny may seem an unlikely ally for taking on the vast unknown of space, nanostructures clearly have a lot to offer when dealing with harsh environments. While it remains the case that the level of investment required for each mission discourages gambling with new technologies, comparatively mature fields of CNT and PCM technologies are increasingly staking their claim to play a role in space exploration. The push-pull factor identified by Samareh and Siochi holds relevance for all potential aerospace nanotechnologies, and what is ultimately incorporated in space missions will depend on whether the apparent advantages match with the needs of those in aerospace tasked with the optimization for space missions. The sheer volume of promising alternative nanotechnologies with potential in this sphere however, suggests that when it comes to big space projects, the future is nano.