Materials in security science

Materials World magazine
1 Nov 2017

Bill Lee, Max Swinscow-Hall, Deeph Chana and Chris Hankin* assess the role of materials in security science and technology.

Research in the field of security science and technology covers what is done to protect the civil population from a range of threats. Geo-political uncertainties, climate change and new cyber and physical attack methods are changing the security landscape. To combat these threats requires interdisciplinary research teams combining different skillsets, such as materials scientists, physicists and psychologists.

Those involved, in what the USA calls Homeland Security, therefore monitor arising threats and capitalise on technological advancements to rapidly develop solutions and bridge capability gaps. Recent terrorist attacks in Manchester and London, UK, and Barcelona, Spain, as well as the impact of the malware cyberattack on the UK’s National Health Service, highlight the need to be ahead of the game.

To achieve this, researchers put themselves in the mindset of the good and the bad guys. For example, while the former considers how to detect explosives being smuggled on to an aeroplane, the latter wants to avoid explosives being detected.

The ISST at Imperial College

Clearly, security is not just a technical issue, but demands cross-sectorial working with policy makers and industry. As scientists and engineers, it’s our responsibility to ensure that technical knowledge is available to underpin policy and develop the most cutting-edge technological solutions. It’s also important for another reason – government and industry tend to be driven by pressures that prioritise relatively short-term goals, such as election cycles and shareholder returns. However, security 20 years from now will be defined by the policies, systems and technologies already being developed. 

At the Institute of Security Science and Technology (ISST) at Imperial College London, UK, we actively pursue working with the UK Government and industry, acting as a security science, technology and innovation interface for these important stakeholders. Our work is broadly split into six security themes that reflect the current and evolving landscape – cyber, cyber physical, physical, healthcare, climate and environmental, and financial systems. 

The importance of materials

Materials sit at the heart of most technological innovations, and all of the above mentioned systems. From developing sensors for detecting physical threats to understanding what building materials protect against them. 

Materials are particularly important in the physical security space for the mitigation of threats and improving survivability and environmental impact. This includes detection and physical protection against CBRNE (chemical, biological, radiological, nuclear and explosive) materials.  

Since monitoring, detection and communication are vital, developments in sensors and detectors are a key area. We are all familiar with going through X-ray scanners at airports and having our hand luggage swabbed and monitored to check for explosive materials. Many airports now have scanners for analysis of liquids through plastic or glass bottles. Perhaps less obvious is that all containers entering the UK by rail, ship or air are scanned with a variety of detectors looking for CBRNE materials. 

Professor Norbert Klein in the Department of Materials at Imperial College has developed practical sensors for security applications, based on earlier work on microwave filters and oscillators for mobile communication. This is a good example of dual-use capability. Sometimes researchers are unaware that some of their work can have another so-called dual-use application from the one they are working on. Care must be taken with the term, however, because it also distinguishes civil and military uses.  

In 2007, Klein founded a company called EMISENS, which successfully commercialises a microwave-based bottle scanner and detector, called EMILI3, based on dual mode radio frequency/microwave and infrared sensor technology. 

EMILI 3 is a liquid explosive detection system (LEDS) and is classified by the European Civil Aviation Conference as a Type B Standard 3 LEDS, which is the highest currently available.

The EMILI 3 system can also be used to detect liquid drugs such as dissolved heroine, alcohol content, flammable liquids and strong acids. The properties of these substances are measured through the container wall within 1-3 seconds, ensuring that the can remains sealed. It uses three frequency bands of the electromagnetic spectrum - microwave and RF frequencies, the evanescent field of a patented dual-mode open dielectric loaded microwave cavity is used to measure the dielectric permittivity – the intermolecular relaxation and the ionic conductivity of the liquid under test, and infrared spectroscopy for identification of certain intramolecular bonds.

In 2016, EMILI 3 was identified as the fastest LEDS on the market by a series of extensive airport trials ordered by the European Commission. EMISENS is currently participating in a tendering process with European airports and the certification with the US Transportation Security Administration is in progress. 

Miniaturisation and nanotechnology

Materials not only help to make testing faster, miniaturisation and nanotechnology developments, in sensing or communications devices usually involve smart or functional materials as well. In addition, developments in memory materials, for energy harvesting and storage (when main power is unavailable), or communication devices that operate when the network is lost, are of interest, as are plastic electronics to defeat scanners, which can be flexible and wearable and may communicate via the internet.  

Other topical areas include developments in:

  • Manufacturing technology, including materials produced from commercially available chemicals and in additive layer manufacturing, which can be acquired and used by anyone.
  • Lightweight materials and miniaturised components, which can be used in drones.
  • Autonomous vehicles and robots (and disabling them).
  • Biomimicry and disguise of devices as biological entities.
  • Resilient devices that will survive extreme environments, for example blast waves or radiation.
  • Protection of buildings, people and vehicles from blast and fire.
  • Physical protection of computers with tamper-proof hardware. 
  • Protection of people in crowded spaces including modelling of spread of contamination and link to evacuations. 
  • Use of materials for experimental evidence gathering in blast characterisation as used in the Centre for Blast Injury Studies and the Institute of Shock Physics at Imperial College. This is particularly important for subsequent modelling work.

Graphene-containing composites

Since the discovery of graphene, much research has been aimed at its production and application. The C-atoms in graphene are densely packed in a regular atomic-scale chicken wire (hexagonal) pattern. Effectively, graphene is a two-dimensional lattice of C-atoms one atom thick. Each atom has four bonds, one π-bond with each of its three neighbours and one π-bond that is oriented out of plane. Graphene's stability is due to its tightly packed carbon atoms and a sp2 orbital hybridisation – a combination of orbitals s, px and py that constitute the π-bond. The final pz electron makes up the π-bond. 

The π-bonds hybridise together to form the π-band and π*-bands. These bands are responsible for most of graphene's notable electronic properties, via the half-filled band that permits free-moving electrons. Graphene has generated much excitement because of an outstanding combination of properties, including being 200 times stronger than steel and having the highest electrical conductivity of any material measured to date. 

Many potential uses for graphene are being researched, including in composite, coating, membrane, electronic, sensor and biomedical applications. 

It is also of interest in the security arena. Many smart polymers are weak and cannot be used for structural applications. Dr Eleonora D’Elia and Professor Eduardo Saiz in the Materials Department at Imperial College are designing smart materials that perform self healing, shape changing and sensing functions without compromising the strength and toughness of the final product. These materials combine the strength of ceramic platelets with the functionality of smart polymers. The polymers can be chosen within a wide palette of polyurethanes and epoxies capable of adapting and responding to their environment. A fine conductive network of graphene provides the composites with sensing capabilities, allowing them to detect internal damage and external stresses such as pressure, tension and bending. Possible applications range from medicine – devising stitches capable of self-tightening around a wound – to robotics, where self-healing, skin-like sensing materials could close the gap between humans and artificial machines. There are also defence applications, where aerospace engineering and personal defence could take advantage of respectively morphing wings and sensing/self-healing armour capable of detecting damage.

In the field of security, smart graphene/polymer composites could be used as sensors and could be, for example, embedded in identity credentials, such as credit cards, to obstruct counterfeiting. Their highly tuneable properties will allow for localised sensing and actuation. Furthermore, carefully designed internal networks will provide a full palette of combined properties, allowing them to act as sensors for damage or differential stimuli and morph to provide an output signal for the correct identification of the input received.

Blast-resistant materials

Research in Dr Luke Louca’s group in Civil Engineering at Imperial College is examining the development of blast-resistant materials for building and personnel protection. His work includes experimental and numerical studies into the influence of material properties on the response of plates subjected to air-blast loading. The failure of mild steel, armour steel, aluminium alloy and fibre-reinforced polymer composite plates has been investigated by detonating disks of plastic explosives at small standoff distances. Permanent mid-point displacement increased linearly with increasing impulse for each material type, up to rupture. 

At higher charge masses, mild steel plates exhibited ductile tensile rupture, while armour steel plates, which ruptured at the same impulse, exhibited a more brittle type of failure. Aluminium alloy plates exhibited signs of melting and spraying radially outwards, resulting in material loss in the plate centre followed by rupture at higher impulses. Fibre-reinforced polymer composites showed evidence of fibre fracture at lower impulses than the other equivalent mass materials. 

In addition, the non-dimensional rupture impulse did not correlate with tensile strength or material ductility, but escalated with increasing specific energy to tensile fracture. These results indicate that the energy absorption capacity of the materials obtained from these simple tensile tests provides an approximate indication of their blast performance, making them a factor to consider when designing buildings.

Materials scientists and engineers, therefore, can make important contributions to protecting civilian populations worldwide.

*Professors Bill Lee and Chris Hankin  are Co-Directors of the Institute of Security Science and Technology (ISST).

Dr Deeph Chana is Deputy Director at the ISST.

Max Swinscow-Hall is Communications Manager of the ISST.