Tougher carbon fibre using CNTs

Originally appeared here: http://www.materialstoday.com/composites/news/tougher-carbon-fibre-using-cnts/?sf4650592=1

Engineers from McGill University have definitively demonstrated that multi-wall carbon nanotubes (MWCNTs) can improve the mechanical toughness of carbon fibre laminates.

Carbon fibre composites have been in widespread use for decades – in Formula1, such materials form the chassis of every car, and up to 50% of an aircraft’s structure is now composite-based. It is all about their mechanical properties – when compared to metals, composites offer a superior strength-to-weight ratio, so in mass-critical applications, carbon fibre composites are the material of choice.

But the performance of these materials is not defined by the individual fibres – when it comes to determining damage initiation and growth in the composite, it is the properties of the polymer matrix that dominate. The most widely used polymeric resins tend to provide high stiffness but low fracture toughness, which can result in delamination in the final composite. Now, a team from Quebec’s McGill University have a demonstrated that the inclusion of multi-wall carbon nanotubes (MWCNTs) in the matrix significantly improves its fracture toughness, leading to a new generation of tougher carbon fibre composites.

Published in Carbon 79 (2014) 413-423 [DOI: 10.1016/j.carbon.2014.07.084], this work focused on modifying the brittle thermoset resin used in most carbon-based composites. Two different formulations were used – in the first, functionalised MWCNTs were mixed with the resin. The second formulation combined functionalised MWCNTs with a more traditional acrylate-based toughening agent. A technique called Resin Film Infusion (RFI) was then used to flow the MWCNT-filled resin through layers of carbon fibre mats, to produce the laminated composites. RFI is used in the aerospace industry to produce composites impregnated with rubber particles, but McGill researcher Pascal Hubert used it to ensure an even dispersion of aligned carbon nanotubes throughout the resin.

Fracture toughness tests were carried out on the MWCNT-filled resins and on the final laminates. The mechanical properties of the raw polymer resins were only marginally improved by the addition of MWCNTs. But, the final laminated composites exhibited significant improvement in their delamination properties (up to 143% in the case of Mode II fracture toughness). Hubert and his team believe that when the resin flows through the carbon fibre fabric, the fibres act as a sieve, ensuring a more even dispersion of MWCNTs, and improved mechanical properties. The team believe that this work can lead to a new generation of nano-enhanced carbon fibre composites, but further work on scaling up their system is still needed.

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No Lab Coat Needed logo

What do you think guys? It took me all of 3 mins to do! I am hoping to get a proper designer to create something a bit “sexier” soon, but I think this will do for the moment :)

LOGO

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Vibration filtering in nature: how a spider hears

This article appeared here on 29th August

A collaboration of US and EU researchers has found that the viscoelectric properties of a spider’s leg helps it to detect vibrations

Biological sensory organs help us to receive, interpret and respond to environmental stimuli. In the world of invertebrates, these sensors are remarkably complex – spiders ‘hear’ – or more accurately, sense vibrations – through strain-sensitive grooves, called lyriform organs, distributed along their legs. One species of nocturnal spider found in Central America -Cupiennius salei – optimizes its ‘hearing’ by sitting on mechanically stiff plants, ensuring that vibrations from nearby prey, predators or sexual partners can be easily sensed.

The lyriform organ is extremely sensitive to substrate vibrations – at high frequencies (> 40 Hz) deflections as small as 10-9 – 10-8 elicit a response in the leg. As well as being highly sensitive, the system can also filter out low-frequency background noise – a challenge facing those designing bio-inspired sensing systems. An international team of researchers believe that they have discovered how this ‘filter’ works, and say that their results will establish a basis for bio-inspired sensor design.

spider

Led by the Georgia Institute of Technology [Acta Biomaterialia (2014) DOI:10.1016/j.actbio.2014.07.023], this work focused on the mechanical properties of a skin-pad close to the sensory organ. The pad is found between the metatarsus (second-last segment) and tarsus of each leg, adjacent to the lyriform organ. Earlier research suggested that this pad contributed to the filtering mechanism, but details were unclear. By using surface force spectroscopy (SFS), the team directly measured the mechanical response of the pad’s viscoelastic surface. By mapping the pad’s surface at a range of temperatures (between 15–40 °C) and frequencies (from 0.05 to 40 Hz), it was possible to define the thermomechanical behavior of the material under typical environmental conditions experienced by the spider.

The group found that the viscoelastic properties of the pad surface were highly temperature-sensitive. At around 20 °C, it became highly viscous, meaning that the spider is particularly sensitive to substrate vibrations at this temperature. This matches closely with the environment Cupiennius – the mountainous region it inhabits has an average night-time temperature of 19 °C. The viscoelastic properties of the pad also define the filtering effect at low frequencies – the mechanical contact between the pad and the tarsus displays a higher effective modulus at high frequencies than at low frequencies. This suggests that mechanical energy is more efficiently transmitted to the sensory grooves at high frequencies.

While more research is needed, the authors believe that this work will help in the design and development of efficient bio-inspired sensors.

To download the article related to this news story, please click here.

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Nickel alloys and the challenge of thermal analysis

This was written as part of Materials Today’s new section on Thermal Analysis and can be read in its original location here. Many thanks to engineer / metallurgist Lindsay Chapman for speaking to me – couldn’t have written this without her input. She’s a total legend.

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“….Laurie Winkless looks at the many challenges surrounding the characterization of nickel-based superalloys.”

Superalloys, also called high-performance alloys, have certainly earned their name – this group of materials have found numerous applications due to their excellent mechanical properties at elevated temperatures. Their resistance to mechanical creep, corrosion and oxidation makes them indispensable to the aeronautical industry, where nickel alloys (Ni-alloys) have been used in turbine engines since World War II. Designing nickel alloys is not without its challenges – in a single alloy, there could be 13 different alloying elements designed in, not to mention any impurities that are present. So they are very difficult to model thermodynamically. And even if their composition is relatively well-defined, often it is difficult to establish what compounds form within the alloy, or where they can be found in the structure. But even with all these challenges, Ni-alloys are everywhere.

The mechanical strength of most metals decreases as the temperature goes up, but nickel based superalloys buck that trend – and it’s all down to their microstructure. Ni-alloys have a base matrix very similar to that of stainless steel – a face centred cubic (fcc) structure, referred to as the gamma (γ) phase. But, what makes them particularly interesting is that they also have a strengthening phase called gamma prime (γ’), consisting of ordered intermetallic particles such as Ni3Al and Ni3Ti, that stop the alloys from softening, even at high temperatures. According to Lindsay Chapman, Senior Scientist at the National Physical Laboratory (NPL), it’s this phase that provides “…the magic of nickel alloys – they can operate at a greater percentage of their melting point than any other alloys – you would struggle to get a steel to be effective so close to its melting point”.

This remarkable property of Ni-alloys also provides a big measurement challenge. Most thermal analysis techniques involve heating and cooling a sample at a defined rate. But in Ni-alloys, there is a strong relationship between heating rate and the material’s microstructure. This is not a surprise – alloy producers use heat treatments to alter the microstructure and ‘tune’ alloys for a particular application (e.g. where creep resistance is key). However, characterisation techniques such as differential scanning calorimetry (DSC) can do the same thing – just in the process of measuring an alloy, it can effectively “heat-treat” it. And as the microstructure can effect both the thermal and mechanical properties of these materials, there’s no guarantee that the sample that comes out is the same one that went in. In short, the process of measuring Ni-alloys can actually alter their properties!

Another measurement challenge is the fact that each technique offers a different thermal environment – systems like laser flash measure thermal diffusivity in a quasi-equilibrium environment, but a standard DSC system ramps up a sample’s temperature by several °C per minute. And neither of those environments reflect that under which Ni-alloys are likely to perform, when used in a power plant turbine, for example. As with all materials research, there is the question of scalability – do the measured properties of a 3 mm sample reflect those of the alloy in a metre long turbine blade? And is it possible to ensure that in a given alloy, each sample machined from a single block all have the same microstructure? It seems that, despite their widespread use, Ni-alloys still offer many challenges to the material producers, kit manufacturers and end users.

NPL sits right at the interface of these stakeholders, working alongside other measurement labs, universities and industrial partners to establish the measurement standards needed to meet these challenges. Chapman is an expert in high temperature thermal analysis and regularly works with customers from the aeronautical, automotive and power plant industries, to help them understand these alloys and their limitations. Historically, very few customers have asked specific questions of the alloy microstructure. But Chapman believes that this is changing. She says, “One of the main problems facing alloy producers is the scarcity of rare earth metals. As they start to look at substituting in other elements, they will need to fully understand the microstructure in order to move forward”.

For many materials-based industries, research from metrology labs such as NPL will help to lead the way and define what is possible at elevated temperatures. Identifying and reducing measurement uncertainties is the cornerstone of this effort, alongside establishing reliable high-temperature thermal analysis techniques for complex materials. For Ni-alloys, measurement is the key.

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Interview with Lindsay Chapman (audio)

Hi all,

Something a bit different today. As part of a new section on thermal analysis on the Materials Today website, I interviewed  NPL Senior Research Scientist Lindsay Chapman. We talked about some of the measurement challenges in thermal analysis, and on the role of the National Physical Lab (yes, my “alma mater”) in establishing good practise in this area. Lindsay is a true expert in her field, so its really worth a listen.

http://www.materialstoday.com/characterization/podcasts/thermal-analysis/

Enjoy :)

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Aerogels for insulation: it’s all about particle size

This news story originally appears here: http://www.materialstoday.com/energy/news/aerogels-for-insulation-particle-size/

A team of Norwegian researchers have shown that the thermal and optical properties of aerogels depend on their particle size – useful in the design of insulating windows.

We’ve all seen images of the ghostly-looking material aerogel. Famously, in 2006, panels of it were used on NASA’s Stardust mission to capture tiny samples of interstellar dust. But here on Earth, its low density and thermal conductivity have attracted the interest of a much more ‘urban’ research effort – in the development of insulating windows.

Photograph of Aerogel-AB (a). Scale bar: 10 mm.

Photograph of Aerogel-AB (a). Scale bar: 10 mm.

Windows have a huge impact on a building’s energy efficiency, with some figures suggesting that ~50% of the total energy loss from a standard office building happens through its windows. As global efforts to produce ‘green’ buildings become ever more ambitious, we’re seeing a growth in research programmes on windows. So far, there have been several window innovations which have shown potential to meet the requirement of energy efficient buildings – multi-layered, vacuum, and silica aerogel windows.

Arild Gustavsen and his team at the Norwegian University of Science and Technology are focused on the use of silica aerogel granules as the “filler” in double-glazed windows [Applied Energy 128 (2014) 27-34 DOI: 10.1016/j.apenergy.2014.04.037]. Because aerogel is mechanically very weak, much of the current research on aerogel glazing units (AGUs) focuses on the synthesis of the aerogel. But Gustavsen and his team specifically looked at the effect that aerogel granule size and layer thickness have on the thermal and optical properties of standard double-glazings.

Both AGUs show improved thermal insulation performance when compared to double glazings – AGUs containing ‘large’ aerogel granules (diameter 3–5 mm) showed a 58% reduction in heat loss. Smaller particles (<0.5 mm) had an even larger effect on the thermal conductivity of the window unit – there, the team saw a 63% reduction in heat losses. However, the introduction of these granules did have an effect on the optical transmittance of the windows – Gustavsen showed that the smaller the particle, the more diffuse the transmitted light. The team believe that this property may be useful in situations where glare and/or privacy need to be considered.

Highly insulating glazing units are defined as those with U-values of about 0.5–0.7 W/(m2K) – so far, results on these AGUs fall short. But this work has opened the debate on how to optimise not only the aerogel, but the design of the final glazing units for a range of building applications.

To download the article related to this news story, please click here.

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Roll-to-roll synthesis of CNT supercapacitor electrodes

Another news story for Materials Today – originally appeared here on 7th July

US researchers have developed a scalable process to produce continuous ribbons of aligned carbon nanotubes (CNTs), for the next generation of double-layer capacitors.

In the last decade, there has been a considerable growth in the wide-spread use of carbon nanomaterials across a range of industries. But the most common bottleneck to any further development is the scalability of their production. Although CNTs can be synthesised in large quantities, present processes for the growth of vertically-aligned CNTS – particularly of interest to the electronics market – are limited to a small range of substrate materials.

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Image credit: A.J. Raghavendra, Clemson Nanomaterials Center

But a group of researchers from Clemson University in the US have developed a relatively low-cost roll-to-roll method – their system can grow vertically-aligned CNTs (VACNTs) directly onto aluminium foil ribbons that are continuously draw through a reactor. Their process produces high density, high capacity (~50 F/g) forests of aligned CNTs that outperform commercial CNTs. The team also used these ribbons of aligned CNTs as the electrodes in a range of high-performance supercapacitor cells.

Today’s supercapacitors tend to use carbon materials in their electrodes, with their performance related to the electrode’s surface area. So, considerable research effort has focussed on using CNTs as supercapacitor electrodes. But issues of substrate preparation and high operating temperatures have rendered the system complex and inefficient.  What the Clemson team have done is develop a system that negates these issues – by adapting a standard Chemical Vapour Deposition (CVD) system, they have managed to decrease the growth temperature to 600 °C, which is below the melting temperature of aluminium. This means that it can be used to directly synthesise VACNTs onto a current collector substrate – in this case, aluminium foil ribbons.

The work, recently published in Nano Energy (2014) 9-16 [DOI: 10.1016/j.nanoen.2014.05.004], also reports on the direct assembly of these VACNT ribbons into supercapacitors. When compared with capacitors made with buckypaper and CNT forests from a stationary CVD set-up, the roll-to-roll electrodes performed well, with a charge capacity of 24.8 mAh/g. But their discharge time (630 ms), energy density (11.5 Wh/kg) and power density (1270 W/kg) all vastly outperformed the other electrodes. The roll-to-roll devices also showed excellent cycle stability, with no loss of performance over more than a thousand cycles.

These results demonstrate the real potential for this technique, and the team believe that it offers a viable process for the production of supercapacitor electrodes.

To download the article related to this news story, please click here.

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