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reportEquipment and Infrastructure Protection
9.11.3 Short description

The technology analysis is further been done based on protection by reinforcement of structures, protection against fire and electromagnetic shielding of information and communication infrastructure. A short description of the developments is available from the subsequent sub-sections.

 

9.11.3.1 Reinforcement of structures

 

Protection of critical infrastructure against explosions and natural disasters are of primary
important in order to retain organisational functions of civil society. Enhanced safety and security of constructions through incorporation of nanomaterials can be achieved. Products such as safety glasses, plating, concrete reinforcement, fire protection and self healing are additional security features. Metal foams have been mentioned in the literature for protection against ballistic projectiles. Aluminium foams sandwiched between steel plates have shown to have high tolerance to shock waves [266]. Nanometre sized precipitates are reported to increase the
strength of steel alloys making them suitable for ballistic impact applications. The material demonstrates very high strength, hardness, ductility, toughness and good corrosion resistance [267]. Ceramic composites backed by metal layers have been used in ballistic armour applications. Function grade material with nanoscale coating was used to enhance the weakness at the interface of composite and metal. It was demonstrated to have enhanced ballistic resistance [268].

 

Vertically aligned carbon nanotubes have been shown to have super compressible foam like behaviour. These nanotubes fabricated through the chemical vapour deposition process show very high compressive strength, recovery rate, sag factor and excellent breath ability. These have foams have demonstrated greater compressibility and pressure resistance than polymer foams, while offering chemical resistance similar to metal foams. The experimentally demonstrated prototype is expected to be used in energy absorbing surfaces for earthquake and explosion protection [269].

 

Inorganic fullerenes such as tungsten and molybdenum sulphide have been demonstrated to have excellent antishock behaviour with the ability to handle upto 25GPa. The close cage structure of inorganic fullerenes provides it very high mechanical strength and shock resistance behaviour. The fullerenes when combined in a metal, alloys and polymers can be used in applications for protection against explosive ballistic projectiles [270]. 

 

9.11.3.2 Protection against fire

 

Fire resistant coatings have been mentioned in the literature. Nanometer scale layered double
hydroxide (LDH) and nanoscaled titanium dioxide have been mentioned in the literature for fire resistance coating. Experimental studies performed have investigated the effects of nano-LDH and nano-titanium dioxide in improving fire resistance and anti aging properties of coatings. The coatings were shown to have great improvements in properties though by addition of nano scale
additives [271]. The improvement of acrylic nanocomposites with nanoscale silicon dioxide has also been studied in fire resistant properties of flame retardant coatings. The nanoscale silicon dioxide enhances the anti-oxidation, char accumulation and char structure. The fire protection properties of acrylic nanocomposites have been reported to be better than conventional acrylic resin [272]. Acrylic nanocomposite coating containing nanoclays have been developed for fire
protective coatings. The influence of nanoclays on properties was studied using scanning
electron microscopy and fire tests. Nanocomposite coatings with 1.5% nanoclay have demonstrated good fire resistance and aging [273].

 

A comparative study of fire resistance of polymer nanocomposites filled with organoclays, polyhedral silsesquioxanes and carbon nanotubes has been mentioned in the literature. The experimental study using TEM showed that good dispersion of fillers improve flame retarding ability but failed in flammability tests. It was demonstrated that best results were obtained using a combination of flame retardants and Nanofillers [274]. Polymer and layered silicate nanocomposites have been experimentally studied, and shown that indefinite protection against fire could not be achieved. A number of approaches were suggested for improving fire retardancy
like improving coupling of silicate layers in the char, incorporating additional additives as second layers of defence, to improve the effectiveness [275].

 

Buckyball nanocomposites have been investigated for their flame retardant properties. Dispersion of buckyballs in a polypropylene matrix was studies using TEM. The presence of C60 demonstrated to have a marked increase in the flame retardant properties of the nanocomposites. The flame retardancy was found to increase with the increase in increasing loading of the C60 in the polymer matrix. The mechanism of the buckyballs for trapping the free radical has been proposed [276].

 

Buckypaper is another name for carbon nanotubes membrane, which is made up of tangled carbon nanotubes ropes. The bucky paper was incorporated on the surface of polyhedral oligomeric silsesquioxane and glass fibre composite to experimentally study the fire resistance. The buckypaper was shown to have effectively reduced flammability for covering glass fibre composites. The advantages offered by the buckypaper are its thermal stability and ability to act as a barrier in reducing the degradation of products [277].

 

9.11.3.3 Electromagnetic shielding of information and communication equipment

 

Electromagnetic interference (EMI) is a well known problem for security electronic equipment. In a highly integrated information and telecommunications networked society, electromagnetic pulses or fields could be present a threat to the security of vital networks. Electromagnetic pulses may have their origin in thermonuclear explosions or an electromagnetic bomb. A number of different materials are under consideration and are being researched to protect devices and networks from EMI.

 

Electromagnetic radiation is also a by product of the vast increase of 1-5 GHz consumer
electronic applications that interfere with large and critical systems. The need for protection from EMI has thus become an essential functional need of critical electronic and communications equipment. Literature has reported the most cost effective commercially available to be magnetic fillers and dielectrics [ 278, 279]. Intrinsically conductive polymers have been considered as protection against EMI due to their high conductivity, environmental stability and simple synthesis
methods [280]. Electromagnetic shielding properties of polyaniline and polyurethane
nanocomposites were optimised by modelling using a genetic algorithm and then experimentally tested. EMI shielding obtained attenuation higher than 40 or 80dB based on application in the microwave band [281]. Intrinsically conductive polymers are applied in form of thermoplastic polymer thin films such as polyvinyl chloride and polystyrene [282]. The synthesis and characterisation of nanocrystalline silver coated fly ash cenosphere particles are used in producing conductive polymer composites for EMI shielding has been mentioned in the literature
[283].

 

In order to improve the performance and properties of intrinsically conductive polymers, lamellar nanocomposites based on conductive polymers like polyaniline have been reported in the literature. An investigation into the thermal, mechanical, electrical and microwave radiation absorbing properties of conductive polymers have been studies. Nanocomposites of polyaniline and organoclays doped with dodecylbenzenesulphonate have been experimentally studied. The nanocomposites showed to have high conductivities and good mechanical properties for conducting composites. These nanocomposites were shown to act effectively against radiation between 8 -12 GHz. These could be potentially used in antistatic packaging layers as well [284].

 

EMI shielding has been experimentally demonstrated using nanocomposite comprising of carbon nanotubes and carbon nanofibres in a polymer matrix. Carbon nanofibres of diameter 100-200nm in diameter and 30-100 µm length, with carbon nanotubes with 10-20 nm diameter and 5 -20 µm were used in the experimental research in a polystyrene matrix. A relative comparison of carbon nanotubes composite, nanofiber based composite and the combination of the two materials was done in experimental research. Electrical conductivity of the filler material, shape, size, and distribution in the matrix determine the electromagnetic shielding of the device. EMI shielding effectiveness for commercial applications has been mentioned in the literature as 20 dB. This was successfully demonstrated by nanocomposites containing 10 wt% of carbon nanofibres and a minimum of 1wt% carbon nanotubes. Other advantages offered by these nanocomposites are the light weight, low cost and excellent mechanical properties [285]. Vapour grown carbon nanofibres (VGCNF) in a polymer matrix have been investigated for their effectiveness in shielding and the relationship with processing variables. The effect of frequency (8 -12.5 GHz) was related to electrical characteristic like impedance. The research concluded that all VGCNF reinforced composites can find applications in electromagnetic shielding applications [286].

 

The mechanical, electrical, thermal, and electromagnetic shielding properties of multiwalled carbon nanotubes embedded in rubber sheets have been investigated. The amount and alignment of nanotubes determined the shielding effectiveness. These nanotubes demonstrated effective shielding for the range between several hundred MHz upto 1GHz. Due to shielding effectiveness of above 60dB these are expected to be used in a number of industrial electrical equipments [287]. Multiwalled carbon nanotubes have been demonstrated for their electromagnetic shielding in liquid crystal polymers and melaine polymer matrix. The highest shielding effectiveness at 60 dB experimentally demonstrated is suitable for industrial scale applications. The higher aspect ratio carbon nanotubes demonstrated higher shielding effectiveness and are considered suitable for use in conductive filler in plastic package [288]. Single walled carbon nanotubes in polymer matrix have been reported in the literature for EMI shielding applications. The highest shielding effectiveness demonstrated was 49dB at 10MHz for 15 wt% SWCNT. A shielding effectiveness of between 15-20 dB was observed for 500MHz -1.5 GHz frequencies [289].

 

Sol-gel based coatings have been mentioned in the literature for electromagnetic wave shielding coatings. These are composed of ultra fine metal particles in a silicon dioxide matrix. Silver colloid particles 10 nm in diameter were used with TiOxNy-ATO particles to obtain a protective film. The electromagnetic shielding effect depends on the surface conductivity which in turn is affected by the electric conductivity of the material and structure of the particles. The experimental research demonstrated that the surface conductivity of the film can be controlled by controlling the shape of the nanoparticles [290]. Nanoparticles of nickel and iron alloys were mentioned in the literature to be dispersed in expanded graphite for electromagnetic shielding applications. The alloy nanoparticles showed high shielding effectiveness for low frequency. Expanded graphite is electrically conductive, and shows good shielding effectiveness at high frequencies. The material thus demonstrated high shielding effectiveness for a wide range of frequencies [291].

 

Electromagnetic wave absorption has been mentioned in the literature using nano tetrapleg zinc oxide as absorbent and epoxy resin as binder in a coating. The nano tetrapleg zinc oxide coatings were observed to have excellent absorption for wave band between 15 -18 GHz [292]. Carbon nano onions (average particle size of 4 -7 nm) and detonation nano diamond are considered to be a candidate material for applications of electromagnetic shielding. Nanocomposites of carbon nano onions have been experimentally shown to attenuate electromagnetic shielding radiation for 12- 230 THz. It has also been mentioned that through mixed use of carbon nanomaterials, further enhanced shielding can be achieve for a spectral range of ultraviolet to terahertz and microwave [293].

 

Electromagnetic shielding in the low frequency range (30 kHz - 1.5GHz) has been experimentally demonstrated using carbon matrix composites with self assembled interconnected carbon nanoribbon network. These were fabricated using low cost natural materials such as rice husks as source for carbonaceous sources and transition metals as catalysts. The composites with carbon nanoribbons were show to have higher electromagnetic shielding value and higher electrical conductivities, than composites without carbon nanoribbons. Low frequency shielding has important applications for portable electronic devices that are used by civilian security agencies [294].

 

Carbon nanotubes and shape memory alloy composites have been developed for electromagnetic shielding applications. The shielding effectiveness of the composite was shown to have a dependency on the content of the nanotubes and the thickness of the composites. Three frequency bands 8-26.5 GHz (K band), 33-50 GHz (Q band) and 50-75 GHz (V band) were used for the experimental studies, with the highest frequency demonstrating the highest shielding efficiency [295]. 

 

 

 

 

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Visits: 1189, Published on: May, 18th 2009, 04:30 PM, Last edit: May, 26th 2009, 03:41 PM Size: 13 KByte

Tags: metallic foams, ceramic composites, CNT for compressible foams, inorganic fullerenes, nanoscale LDH, titanium dioxide, silicon dioxide, nanoclays, buckyballs, Carbon Nanotubes, carbon nanofibres, magnetic fillers, Nanocomposites, metal nanoparticles, carbon nanoribbon

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