Carbon Fiber onboards SpaceX

Carbon fiber industry has seen a boom in the past decade as once, in 2005, three companies Toray, Toho Tenax and Mitsubishi Rayon Co. Ltd held 70% of the share in carbon fiber supplies, now are down to 45% of the holdings as more and more investors join in the profitable sales of carbon fiber. The prominent user of this material has been the aerospace industry which looks upon materials that could provide a higher strength to weight ratio, so that it could cut off the fuel expenditure. A Boeing 787 plane constitutes of 50% composite, 20% aluminium, 15% titanium and the remaining is steel. It has been suggested that the need for more composite structures in aeroplane components is needed. Such suggestion would worry the metal producers but we have to see how much of the advice would be acted upon.

Recently, the real life Tony Stark, Elon Musk, presented his vision of mars plane with the concept of SpaceX. During the presentation Musk presented that plan of the craft to be made of carbon fiber composite. The reason for the composite introduction is because using solely carbon fiber is difficult so there needs the combination of resin with it. The introduction of this material on a larger scale in SpaceX aircrafts is the core reason for Musk plan to have an efficient journey on the part of quality as well as quantity. Because his vision already involves the population to be of a ‘million’. So it would be a continuous plan and rather seems pragmatic one as well, because as per Colin Sirret, Head Research for Airbus in UK, one kilogram of weight removed, saves around a $1million in costs over the lifetime of an aircraft. So it certainly shows the outcome of getting an efficient product in the end.

But true mystery is how the CFRP is going to perform in the conditions during its journey because a lot of variations in pressure and temperatures would be observed which the SpaceX organisers suggest are sustainable for the material. But ‘how?’ is the real question, since CFRP are considered as materials that are prone to cracking and delaminating when exposed to extreme conditions and such conditions would be observed as the CFRP tank would hold liquid methane and oxygen. Both are gases at room temperature. So to endure the temperature and allow an impermeable layer to hold on any gas and liquid if somehow either goes a change in its state. That is something to look out for. Plus the size of the components for the spacecraft is going to be large and such manufacturing would be a great task for the SpaceX engineers.

SpaceX programme is certainly the ‘Giant leap’ Neil Armstrong was talking about, it’s a breakthrough for mankind and as well as for aspiring material scientist. Like any other good research for any man to understand this concept other than Elon Musk, I would say for myself and everyone ‘further study is needed’.

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No Match for Gold!

Ask yourself the question if you were to choose an element for currency which one would it be? There are 118 elements in the periodic table why in the world people came up with valuing gold over the other as the preferred material. To answer this question we need to categorize each of the element with the character it holds and the one that is being demanded. This has to be kept in mind the element can be such which is valuable so that not it can not be acquired through wrong means and unreactive so that it can be preserved and saved for ages.

Let us cut short the research and cross out all the gaseous elements, so off with Hydrogen, Oxygens and the Nobles. Kicking out the liquid bromine and mercury as well. We happen to still have Marvel's favourite group 'Radioactive elements', Stay away from them, Seriously!. That leaves us with alkaline, rare earth and the transition elements. Alkaline are too reactive they might not even wait for the trade to finish before reacting. Then comes rare earth, well they are hard to distinguish from each other so skipping those as well. Left we are with the transition group. The 49 element large group holds all variety, we can not make use of the elements as titanium and zirconium, for they are pretty tough.but hard to smelt. Iron easily rusts, aluminum is hard to extract and is flimsy, copper is corrosion resistant but still it has better competitors to challenge. Palladium, platinum, iridium, rhodium, osmium, silver and gold. These elements can resist corrosion to high degree but they all too carry a drawback except for silver and gold the rest are rarer then the rare earth metals and also hard to smelt.

Gold and silver held the higher ground when it came to making money. However, it has been found that gold is the better of the two, as Silver tarnishes when it reacts with sulphur from the environment. The additional advantage of gold is its malleability. It is the most malleable and ductile element of them all. No wonder you see a new design, if you are known to the south Asian culture. Speaking of which comes a story of choosing the element for forming currency coins. Muhammad Taghluq in the early 14th century forged copper coins for his kingdom's trade measures. What happened was the whole people forged their own coins with the availability of the given material, soon every one was a millionare, The Sultan had to revert his decision which cost him more as he asked for the return of all copper coins in return of which the people will get the gold and silver coins back.

Gold malleability and corrosion resistant does make it valuable but it does not end here as the shine and glare gives it a unique character, which is the reason its symbol is 'Au'. The symbol comes from the Greek work aurum which means sunshine and glow. Corrosion resistance helps gold preserve its character and as it may be added if you get a gold statue as a gift, for 1000 years it will be preserved for your great grand-ever so infinite-children. So its all due to the ever lasting and unmatched beauty of the material that has caused it to be the talk of the town.

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CONDUCTIVE POLYMERS

It has been a convention and a natural selection for every science student to define and distinguish materials on the basis of their properties, one such property being electrical conductivity. Through the ages we see metals as our optimum choice for conducting electric current. An addition to the Power Set of conductors was Semiconductor, that allow electric charge to pass under induced doping mechanism. Above all this Polymer has been unanimously accepted and known to be an insulator or in some cases dielectric, and so has been used to coat metal wires for a safety purpose for the property of insulation.

The name conducting polymer is kind of an oxymoron. But this was proved wrong, after it was predicted back in 1962 by Walmslay. Walmslay along with a colleague presented the idea of in poly-acetylene. According to that solitons are form of mobile defects that can be charged to give conductive properties. Similarly in 1963, Weiss and his colleagues worked on conducting polymers as they reported high conductivity in polypyrrole oxided with iodine. The journey continued when in 1973 a key discovery was made by Walatka and his co-workers as they found an inorganic polymer that is a metal, the polymer was polysulphur nitride. They predicted that the characteristic of metal, found in this polymer, is intrinsic. It was due to an unpaired electron present between each S-N unit, which showed that the valance band is half occupied. Hence forbidden gap is absent, which allows for the movement of these unpaired electrons whenever an electric field is applied.

So in 1977, the hard-work over the years was paid off, after Heeger, Mcdiarmad and Shirakawa discovered the property of conductance in polyacetylene (PA) by the oxidation of halogens vapours as chlorine, bromine and iodine. These vapours made the conducting property of PA films to increase by a billion times. Such form of halogen treatment is called doping just as so the term used in semi-conductor study. The doped form of PA showed the conductance of tenth of a million S/m compared to other polymers, it’s the highest, as for example Teflon ten quadrillionth S/mand if compared to metals silver and copper show 100 million S/m polymer is that along the backbone, conjugated double bonds are present. By conjugation it is meant that between the carbon atoms there are single and double bonds in alternating positions. Sigma bond that gives a strong chemical bond is present in every bond. In addition to it localised weak pi bond is also present in double bonds. To make the material conductive, charge carriers in the form of extra electron or holes are injected that allows for the current to pass.

Conductivity:

Conductivity of a material can be defined from ohms law that states that “current through a conductor between two points is directly proportional to the potential difference across the two points” Where R is the resistance, the reciprocal of resistance is called conductance and is measure in Siemens and of Conductivity is Siemens/meter. Conductivity of a material depends on the number of charge carriers and the mobility of the charge, as in how fast they can move. The conductivity of a material also depends on temperature as in it shows a decreasing character, when the material is metal and increasing when it’s a semi conductor or insulator.

What makes materials conductive?

In most of the materials be it polymer or any crystalline structure metal, the property of conductance is directional, called anistropic property. For example, carbon allotropic forms diamond and graphite show different characteristics adding to that we compare polyacetylene character as well. Diamond, graphite and polyacetylene has three, two and one dimensional structured carbon atoms respectively. Unlike graphite and diamond, Polyacetylene has hydrogen atoms between the carbon atoms. Due to the presence of strong sigma bond and symmetrical structure diamond is isotropic and insulator while in graphite and polyacetylene the presence of mobile pi electrons gives space for the material to conduct electrical charge in certain directions.

Synthesis & Processing:

With temperature change Shirakawa was able to form copper coloured Cis and silvery trans polyacetylene. With trans showing modest conductivity compared to Cis. Also the trans form is more stable thermodynamically. Shirakawa, during experimentation, found that the transmission was reduced when the film was exposed for a few minutes to bromine and chlorine, but when exposed for higher rates the film gave high IR transmission. The doping was studied by Mcdiarmid and Heeger. The halogen doping that induces conductivity in polymers is oxidation (p type). The doped polymer forms a salt. And the charge on the polymer is what constitute the charge carrier and by applying electric field in the direction perpendicular to the film, the counter ions can be made to diffuse both from and into the structure. Thus, giving the character of turning conductivity on and off. The later years synthesis include one as formation of a more denser film of polyacetylene, left after the evaporation of bis-triflouromethylebenzene. An advancement in electrical properties was achieved in the year 1987 when the conductivity of PA was claimed to be equal to copper’s. The experiment was same as Shirakawa.

Apart from polyacetylene other polymers have also been developed that are more stable to air and oxygen effects and also are more easily process able, compared to PA, but have lower conductivities. These include polyparaphenylene, polyparaphenylenevinylene, polypyrrole, polythiophene and polyaniline and their derivatives.

Mechanism:

The lower binding energy of electrons in metals gives them highly dense electronic state as well as provides the free electrons to move easily, under the application of electric field, from atom to atom. The structure also has a defining role In determining the electrical properties of materials. As in metals, we see there is an overlap of orbitals across each atom giving a molecular orbital. Hence, greater atomic orbitals intersection more will be molecular orbitals formation. These molecular orbitals combine to form an energy gap or band. In metals the valance band is unfilled. So we can say that molecular orbital above some energy will also be unfilled or will be empty. The gap or the energy spacing between two bands, highest energy and other lowest energy is called band gap. The conduction band is one that has the lowest energy and is the unoccupied band while the opposite is valance band. The electric field response is shown by a material when the energy gap is zero or the conduction band is partially filled. For chained structure the same model goes but it can be added through using quantum mechanical model and so making use of Pauli Exclusion Principle we find that increasing the polymer length decreases the band gap.

Another model is Ab-inito model that considers ladders of filled and empty orbitals. The combined wave function of the orbitals is called state. Cases are, one when there are even number of electrons in the lower part of the energy ladder and occupying same orbital, these electrons will have opposite spins and so the spin angular momentum be zero. Another case is when the electron is excited to move to a higher energy state. The energy difference is called band gap. If the spin of these different state electrons is same than such is called triplet and acts as a paramagnetic and non magnetic if the spin is opposite.

We mentioned that the band gap will be zero if the chain lengthens but such is not predicted and its experimentally found that it depends on the wavelength of the first absorption band. it has also been seen beyond a certain limit no change is expected in further conjugation. So the result shows that with energy levels and band gap of significant size the PA acts as a semi conductor.

Question arises, as if why does PA behave as metal when doped? Some models suggest an answer to that as the conjugation of the double bond. That is different from ordinary in a way that the next bond is known to other and another reason presented is the delocalised pi electrons between these double bonds that are evenly spaced but it has been found that they are not. Such idea was presented back in 1930s when a student of Heisenberg put forward the hypothetical presentation that the chain of sodium if distorted as in making the equal chains into alternating long and short ones we see that the metal becomes an insulator or semiconductor and the conductivity decreases with temperature.

Dopant role is found to either add electron or remove it. For example if iodine atom undergoes oxidation it is predicted to produce a hole that is delocalised but it does not do that completely. As the electron is removed a radical cation is formed the radical is called polaron and is localised because of the coulomb attraction to its counterion. The counterion is found to have a lower mobility and is made to surround the polaron so to allow the charge carrying to be made. Another reason for localisation of polaron is the equilibrium change of the radical cation.

If another electron is removed from the already oxidised section the electron then moves as a pair. A defect in conducting polymer called solitons formed after thermal isomerisation of trans PA. The stable free radical formed is neutral although it does not itself carry as it propagates but can help in transfer from chain to chain.

Another mechanism of charge transfer intersolitons hopping. In this the solitons move all around and exchange electrons with adjacent localised soliton that are charged.

Special Features:

• Molecular Disorder:

Unlike inorganic semiconductors, polymers exhibit molecular character with lacking of long range order. The motion involving electrons is one dimensional in individual molecules. The dimensional reduction gives polymer the nature to generate electronic properties by certain states called Fermi surface instabilities.

• Nature of Doping:

Doping occurring in inorganic semiconductor involves the dopant to occupy a position within the lattice of the material being doped. Thus, resulting in electron- rich or deficient sites, there is no charge transfer between the two sites. In polymers we see the doping of a material involves charge transfer reaction, the polymer undergoes oxidation and reduction.

• Solitons, bipolarons and polarons

Conductivity increase with doping was thought to be due to formation of unfilled band of electron. In PA and PPP it has been assumed that the conductivity was dependant on spinless charge carriers. Increase in conductivity is due to formation of polaron,bipolaron and soliton. These particles form due to electron phonon interaction and these are the charge carrying species. In degenerate polymers as trans, solitons are the charge carriers whereas in non generate the polarons and when combined bipolarons are the charge carriers.

Applications:

• Polyaniline is used as inhibitor (reducing rate of corrosion) and as in electromagnetic shielding as a conductor.
• Poly(ethylenedioxythiophene) is used in coating to protect a material from electrical discharge and also used as an injector of holes in light emitting diodes.
• The derivatives of polythiopene are used in field effective transistors.
• Polypyrrole can be used in stealth devices as it has been found that it can absorb microwaves and so can be used in sensing devices.
• Poly(phenylene vinylidene) can be used in protecting electroluminescent displays.

References:

Suarez Harrera, Marco F. ,Conducting Polymers, Electrochemistry Norden Bengt, Krutmeijer Eva, 2000, Conductive Polymers, The Nobel Prize in Chemsitry

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Carbon Fiber Composite for Prosthesis

Prostheses have been in use over a very long period of time. Prosthesis is defined as an artificial attachment that replaces a missing portion of body. Prosthetic limbs are used to replace arms or legs; its need arising due to various circumstances including disease and accidents. The type of artificial limb used depends on the degree of loss of the limb.
Recent advances in the field of prosthesis have led to the widespread use of advanced plastics and carbon fiber composites. These materials lead to the production of a lighter, stronger and a more suitable prosthetic limb compared to the rudimentary artificial attachments in the less advanced times.
The incorporation electronics in prosthesis provides more flexibility and functionality of the limb. It is much more realistic and has the feature of adapting their function during under certain conditions, such as gripping or walking.
Although newer materials and technology have revolutionized prosthesis, the basic components remain the same.

Components of a prosthesis

Pylon

It is the framework upon which the prosthetic limb is built. It is used to provide structural support, like the skeleton in human body, and conventionally metal rods were used as pylons. In the modern age, lighter carbon-fiber is used to form pylons. Pylons are sometimes covered by foam like material which is colored to match the skin of the recipient giving the prosthetic a more life-like appearance.

Socket

This is the interface between the stump (remaining part of the human limb) and the prosthetic. The socket must be fitted accurately as it forms the part of region from where mechanical forces are transferred to the patient’s body. This careful fitting ensures that no damage is done to the skin or the tissues. One or more layers of socks are worn to facilitate better fitting.

The suspension system

This is responsible for keeping the prosthetic attached to the body. There can be several kinds of suspension systems. There’s the harness system, where straps or belts or sleeves are used to attach the prosthetic. In some cases prosthetic remains attached to the body by just fitting around the shape of the limb. A common mechanism is suction whereby attachment is maintained by snugly fitting the prosthetic to the residual limb and using an airtight seal to keep it together.
Although most prosthesis retain these components in some form, each device is unique. Amputation above or below a joint makes a huge difference in the type of prosthetic required, for example, amputation above knee will require a device having artificial knee while amputation below knee means the patient could use his or her own knee.

Design and fabrication

This process consists of various steps starting with the precise measurements which include detailed measurements of the patient’s body such as height, weight etc.
Following this, amputation surgery and time for the wound to heal is given, after which a plaster mold of the residual limb is taken. A duplicate is constructed and is used to test the fit of prosthetic limb.
The interface between the residual limb and the prosthetic socket needs to be meticulously monitored as the residual limb will shrink due to muscle atrophy i.e. shrinkage due to lack of use. Children amputees have to visit the doctor regularly because their prosthesis need to be resized and reshaped to cope with their natural growth.

Thermosets vs. thermoplastics as the polymer matrix

A prosthetic nowadays is made using plastic polymer laminates. Thermosets such as acrylic, polyester or usually epoxy is used. The advantage these thermosets offer is that they allow greater degree of control over controlling variables such as strength, stiffness and thickness of the final product. This control allows the prosthetic to be strong and stiff in certain areas and thin and light in others.
For instance, during above knee prosthesis, it is essential that the weight bearing area must be stiff so as to not bend under the weight of the body. To achieve this, extra reinforcement is applied in this area. In the rest of the socket, these requirements can be sacrificed so the laminates are thin and lighter prosthesis can be made.Similarly, rigidity or flexibility can be achieved by adjusting the resin and fiber content.
However, once fabrication is complete the laminate cannot be remolded and it is difficult to make adjustments in case of thermosetting resins compared to thermoplastics on the account of the cross-linking in them.

Why use Epoxy?

Acrylics and epoxies are popular thermosetting resin systems employed in production of laminates for prosthesis purposes providing different advantages. Epoxy results in a strong, stiff structure with very good adhesion to carbon fibers. It has a longer setting time so it is preferred in situations where bigger devices are to be produced.
Acrylics produce a colorless transparent structure which is skin friendly and has antibacterial properties. It has a shorter setting time but its adherence to carbon fiber is not as good as epoxy resin.

Epoxy, polyesters or vinyl esters?

Epoxies are the most expensive of these three resins but they justify their cost. They are almost three times stronger than the next strongest resin. They adhere to the fibers and older epoxies much better than the other two and form a virtually leak proof barrier.
Vinyl esters are not very strong (about one-third the strength of epoxy) and are usually only used for aesthetic purposes. Due to their decreased strength and adherence, they are not effective materials for prosthetics.
Polyesters are the cheapest and have poor bonding capabilities especially with carbon fibers so they cannot be used in load bearing circumstances.

Why use carbon fibers as reinforcements?

Reinforcement fabrics for prosthesis include fiberglass, carbon and Kevlar. Each comes with its own pros and cons. Fiberglass for instance is the most economical and easily saturates with resin and can be found in different forms. Fiberglass is durable and flexible because glass fibers are twice as strong under compression as in tension. But, fiberglass is heavier than the other two and is not as strong or stiff as the carbon fiber.
Kevlar is the lightest material and provides excellent fracture toughness under impact loading. It can also absorb high levels of torque and stress. Unfortunately, Kevlar is not very good at maintaining form or structure under load and it is very difficult to saturate it with resin.
Carbon fiber is the most suitable alternative. It is almost as light as Kevlar and is able to hold its shape under load due to excellent stiffness and strength in both compression and tension. In addition, carbon fiber has excellent fatigue resistance and is bio-compatible. Carbon fibers are very versatile and innovative designs can enhance the effect of their mechanical properties. Laminates can be made to be either stiff or strong. Fiber orientation can improve properties in more than one direction.
Structural compromises can overcome the undesirable properties of carbon fiber composites such as brittleness. All three fibers can be used in unison to obtain a hybrid which can exhibit the properties of all three fibers.

Manufacturing

As mentioned earlier, the process of prosthetic production starts with taking careful measurements of the residual limb and other key factors such as the weight of the patient, length of the other whole limb etc. Once the wound is healed, a plaster mold of the residual is created. Using this mold a duplicate residual limb is produced. This duplicate is used for testing with the prosthetic limb.
The prosthetic limbs are created by using prepreg carbon fiber. This is basically carbon fiber that has been pre-impregnated with epoxy resin and the epoxy is allowed to be partially cured. These prepregs come in sheets or rolls with optimum balance between the fiber and the resin.
A plaster model of the prosthetic is dried in an over to remove moisture from the mold. Prepregs are meanwhile stored in a freezer to prevent further curing. Patterns are traced on a plastic sheet such as PVC foils and then these patterns are cut out of a single sheet of prepreg. Prepregs come in different orientations (bi-directional, unidirectional) and depending on the area of the prosthetic and the amount of load that particular area is going to bear, several layers of prepregs are applied with varying orientations to accommodate torsion and flexion.
This assembly is sealed in a vacuum bag and it goes into an oven for 4-5 hours of curing. High vacuum environment is maintained to remove excess resin. Finally, buffing is done to remove sharp edges.

Other applications

Carbon fiber composite’s properties have made it indispensable to various applications. It is now quickly displacing metal alloys and other materials.

Aerospace

Carbon fiber has been drafted rapidly in this industry. The high strength-to-weight makes it a suitable structural material, but mainly the weight savings associated with it is what makes it a necessity. Each pound lost in the final structure leads to huge amounts of savings in fuel consumption.

Sporting goods

Sports are another market that utilizes carbon fiber because it offers higher performance due to decreased weight without compromising strength.

Automotive

For enhanced performance as required in formula 1, NASCAR and in other high end cars, carbon fibers are being used. It is not just because of the mechanical properties, but also about the aesthetics. The distinctive carbon fiber weave has become a fashion statement for high end cars.

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