Carbon Nanofoam Structure and Functions

Abstract

Carbon nanofoam is the fifth allotrope of Carbon after graphite, diamond, fullerene (e.g., C-60 molecules), and Carbon nanotubes. It was discovered in 1997 by Andrei V. Rode and his team at the Australian National University in Canberra, in collaboration with Ioffe Physico-Technical Institute in St Petersburg. The molecular structure of Carbon nanofoam consists of Carbon tendrils bonded together in a low-density, mist like arrangement.

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This paper talks about the physical structure, chemical properties, preparation methods and applications of Carbon nanofoam. The most unusual property of Carbon nanofoam is its ferromagnetism; it gets attracted to magnets, like iron. At a temperature as low as -183 ÌŠC, Carbon nanofoam behaves like a magnet. Also, the foam is a semiconductor, making it attractive for device applications. The reason for the foam’s magnetic property has been explained in the paper.

Carbon nanofoam is hence the first pure-Carbon magnet and also one of the lightest known solid substances (with a density of ~2 mg/cm3), when used along with aerogel. The Carbon nanofoam is believed to remove “magnetic prejudice” among the known elements, the idea than an element should be stereotyped as either magnetic or nonmagnetic.

1. INTRODUCTION

Carbon nanofoam was discovered by 1″Andrei V. Rode and co-workers, in collaboration with Ioffe Physico-Technical Institute in St Petersburg at the Australian National University in Canberra in the year 1997. It is the fifth allotrope of Carbon after graphite, diamond, fullerene and Carbon nanotubes.

The molecular structure of Carbon nanofoam consists of Carbon tendrils, bonded together to form a cluster- like assembly of low density in a loose three- dimensional web pattern. The width of each cluster is about 6 nanometers, consisting of about 4000 Carbon atoms. These Carbon atoms are linked in the form of graphite- like sheets but consist of heptagonal structures included among the hexagonal patterns, giving it a negative curvature, (Figure 1(a)) unlike the Buckminster fullerenes [1] in which the inclusion of pentagonal structures gives the Carbon sheet a positive curvature. The density of Carbon nanofoam is approximately 2 mg/cm3, which makes it one of the lightest known solid substances, the other being aerogels whose density is about 100 times more than that of Carbon nanofoam [1].

According to Rode and his colleagues [1], nanofoam contains a number of unpaired electrons due to the Carbon atoms with only three bonds, found at topological and bonding defects. This gives rise to the most unusual feature of Carbon nanofoam, which is that it is attracted to magnets. Moreover, below ˆ’183°C Carbon nanofoam acts as a magnet itself. Another property of Carbon nanofoam is that unlike aerogels, Carbon nanofoam is a poor conductor of electricity.

The clause for the magnetic property of Carbon nanofoam is that only freshly produced Carbon nanofoam is ferromagnetic; Carbon nanofoam is strongly attracted to a permanent magnet at room temperature, initially. This room temperature ferro- magnetic behavior disappears after a few hours of preparation of the Carbon nanofoam, when the temperature eventually fluctuates to go above the room temperature. However, the ferro-magnetic property persists at lower temperatures.

Depending on the pressure of the ambient Argon gas inside the chamber where high- pulse, high- energy laser ablation [3] [4] [5] and deposition of Carbon vapors is performed, different Carbonaceous structures are formed. At a pressure of 0.1 Torr*, diamond- like Carbon films are formed. As the pressure is increased to greater than 0.1 Torr, diamond like Carbon- nanofoam is produced. The density of the Carbon nanofoam depends on the density and the polymerization chemistry used during the sol-gel process [3] [4]. The particle diameter of low-density foams is the largest, which is up to 100 nanometers, with a pore size of at least 500 nanometers. The high- density Carbon foams have pores of size less than 1000-Angstrom Units and the particles are ultra-fine, the density being approximately 0.8 grams/cubic centimeter. Electrically conductive Carbon nanofoams are also under production, which has many properties of the traditional aerogel material. Prepared by sol- gel methods, these materials are available in the form of monoliths, granules, powders and papers. The foams prepared by these methods are typically of low density, continuous porosity and high capacitance.

The most intriguing property of Carbon nanofoam is its Ferro magnetism (Figure 1(b)). The reason for the existence of this unusual property attributed to an allotrope of Carbon, which is conventionally believed to be a non- magnetic element, is due to the complex microstructure of the nanofoam. Few researchers claimed that the ferromagnetism is due to the presence of traces of iron and nickel impurities in their foam. Later they calculated that the small amounts of these magnetic materials could only account for 20% of the strength of the ferromagnetic fields in the foam and concluded that the ferromagnetism is an intrinsic property of this allotrope of Carbon. The unpaired electron that does not form a chemical bond in the 7- corner, 7- edged polygons present in the structure of Carbon nanofoam has a magnetic moment, which is suspected to be the reason of its magnetism.

*1 Torr is approximately equal to 1 mmHg; 1 Torr = 133.322368 Pascal

Due to the magnetic properties of Carbon nanofoam, it can be used in a number of applications namely, medicine, optics, fuel cells and other electronic devices. They are also being used as lightweight, high temperature insulation materials, absorbents and coating agents and as electrodes for water deionization cells. In biomedicine, Carbon nanofoams are used as tiny ferromagnetic clusters, which could be injected in blood vessels, in order to increase the quality of magnetic resonance imaging. Another application of Carbon nanofoams is in spintronic devices, whose operations are based on the material’s magnetic properties.

The researchers also have preliminary indications that the novel magnetic behavior also occurs in another nano-compound made of boron and nitrogen, two other elements that are ordinarily non-magnetic.

The following parts of this paper discuss in detail, the

• Molecular Structure
• Synthesis methods
• Properties, and
• Applications of Carbon nanofoam.

2. STRUCTURE OF CARBON NANOFOAM

Carbon nanofoam consists of Carbon atoms bonded by both sp2 and sp3 hybridizations, unlike the other allotropes of Carbon such as graphite and diamond which have only sp3 hybridization and C60 and Carbon nanotubes that have only sp2 hybridization [7]. Around 4000 such Carbon atoms are bonded together in the form of a cluster-like assembly of low density. In other words, these Carbon atoms are bonded in the form of graphite-like sheets but consist of heptagonal structures included among the hexagonal patterns, giving it (Carbon nanofoam) a hyperbolic pattern, as proposed for schwarzite[6]

The percentage distribution of the sp2 and sp3 hybridizations can be controlled by during the synthesis of the nanofoam. High pulse-rate Laser Ablation method for the synthesis of Carbon nanofoam by A. V. Rode et al [1] demonstrates that there are two types of particles in the foam and that here is a small amount of particles with a high sp2 fraction (~0.9) of graphite-like bonds, due to crystalline graphite used in the experiment. Particles with a fraction, generally lower than 0.8 sp2 are inferred to consist of amorphous Carbon with a mixture of sp2 and sp3 bonding. Particles with lower sp2 content and a higher Plasmon energy are more “diamond-like”, as they have higher density and a higher fraction of sp3 bonds. Upon measurement, it has been observed that these is a high sp3 content at the edges of the foam and at the edges of the cluster, which is a clear indication that the sp3 bonding atoms are located at the surface of the clusters and that the connections between the clusters are due to the sp3 bonding.

3. SYNTHESIS OF CARBON NANOFOAM

The synthesis of Carbon nanofoam is done on a laboratory scale and is not produced industrially, in bulk. Two methods are adapted for the preparation of Carbon nanofoams, depending on different types of requirements such as particle size, density, resistivity, etc. The two methods are listed and explained below.

3.1. Laser Ablation

Laser ablation is the process of removing material from a solid (or occasionally liquid) surface by exposing it to radiation with a laser beam. Depending on the flux density of the laser, the effect of laser ablation varies. For a more clear description; at low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to plasma. Usually, laser ablation refers to removing material with a pulsed laser, but it is possible to ablate material with a continuous wave laser beam if the laser intensity is high enough.

High-repetition-rate laser ablation and deposition of Carbon vapors results in the formation of quite different Carbonaceous structures depending on the pressure of the ambient Ar gas in the chamber. Diamond-like Carbon films form at a pressure below 0.1 Torr whereas a diamond-like Carbon nano-foam is created above 0.1Torr. The creation of particular molecular structures involves “atom-to-atom” attachment in appropriate physical conditions at an appropriate rate.

3.1.1. Experimental Setup

The experimental setup of the experiment conducted by E.G. Gamaly and piers is as follows: a 42-W, 120-ns pulse-width Q-switched Nd: YAG laser (λ = 1.064 mm) with variable repetition rate of 2-25 kHz was used. Laser of intensity approximately 109 Watts/cm2, averaged over the pulse duration was created on the glossy Carbon target, keeping the repetition rate fixed at 10 kHz and focal spot scanned over a 2X2 cm area of the target surface.

3.1.2. Formation of Carbon Nanofoam in Ar ambient temperature

The diamond-like Carbon (DLC) films is deposited in vacuum of approximately 106 Torr.

Transformation to a different form of Carbon material occurs in an Ar-filled chamber at a pressure around 0:1Torr. At this pressure, the mean free path for collisions of the evaporated Carbon atoms is in the order of 1 cm. Thus, Carbon-Carbon and Carbon-argon collisions in the chamber start to play a dominant role in the formation of Carbonaceous structures in Ar-filled chamber.

The high-repetition-rate laser evaporation of a Carbon target in a 1-100 Torr Ar atmosphere produces a higher evaporation rate of Carbon atoms and ions than conventional laser ablation techniques.

The resulting increased average temperature and density of the C-Ar mixture in the experimental chamber increases the probability of the formation of higher energy Carbon-Carbon bonds.

The resulting increased collision frequency from the above deposition conditions encourages diffusion-limited aggregation of Carbon atoms into fractal structures, and the formation of low density Carbon foam.

Figures 3.1.2. (a) and 3.1.2. (b) show the scanning and transmission electron microscope images respectively, showing the free-standing Carbon foam. These images are scaled to 1 mm and 100 nm respectively.

The analysis of these images reveal that the foam represents a fractal-like structure which consists of Carbon clusters with the average diameter of 6 nm randomly interconnected into web-like foam.

The foam looks like a capricious mixture of “strings of pearls”. [3]

Initially, the flow of atomic Carbons is created by the laser ablation near the target surface. After the chamber is filled with an inert ambient gas, it results in the collision of Carbon atoms with the ambient gas atoms, as the Carbon plume expands. Hence, the Carbon atoms collide, diffuse through the gas, exchanging their energy, and finally cool down to the average Carbon-gas temperature.

3.2. Sol Gel Process

The sol-gel process, also known as chemical solution deposition, is a wet-chemical technique widely used in the fields of materials science and ceramic engineering. Such methods are used primarily for the fabrication of materials (typically a metal oxide) starting from a chemical solution (or sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers. Typical precursors are metal alkoxides and metal chlorides, which undergo various forms of hydrolysis and polycondensation reactions.

Carbon nanofoam is also prepared from the pyrolysis of organic precursors, such as organic aerogels produced through sol-gel processes (such as resorcinol formaldehyde sol-gels) (Figure 3.2.). The sol-gel solution is cast into the desired shape and after the formation of a highly cross-linked gel the solvent is removed from the pores of the gel. The remaining rigid monolithic shape consists of covalently bonded, nanometer-sized particles that are arranged in a 3-dimensional network. Precursor RF gels can be applied to a fine Carbon felt which is Carbonized to form Carbon nanofoam electrodes [9].

The Carbon nanofoam thus prepared usually has low density and very high specific surface areas (up to ˆ¼1200m2 g-1), and they can be produced in different forms, such as monoliths, fine particles or films. The final shape and properties depend strongly on the sample history, as is the case with all amorphous Carbons.

4. PROPERTIES OF CARBON NANOFOAM

Many of the properties of Carbon nanofoams match with those of the traditional aerogel materials. Carbon nanofoams are available in the form of monoliths, granules, powders and papers. They are electrically conductive, synthetic and lightweight foams in which the solid matrix and pore spaces have nanometer-scale dimensions.

Prepared by sol-gel methods, nanofoams typically have low density, continuous porosity, high surface area, and fine cell/pore sizes. The foams are also electrically conductive and have a high capacitance. Standard densities of Carbon nanofoams range from 0.25 to 1.00 g/cm3. Carbon nanofoams precursors can be infiltrated into a Carbon fiber mat that, when Carbonized, will result in paper-like electrode material 0.007 to 0.050 inches thick.

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Morphology examination by scanning electron microscope shows an open cell structure and continuous porosity. The particle size and pore spacing is a function of density and the polymerization chemistry used during the sol-gel process. Low density Carbon nanofoams (~0.25 g/cm3) have the largest cell/pore size with particle diameters of up to 100 nm and pores at least 500 nm. High density Carbon foams (abt. 0.8 g/cm3) have ultra-fine particles and pores of less than 1000Å.

The nanofoam contains numerous unpaired electrons, which Rode and colleagues propose is due to Carbon atoms with only three bonds that are found at topological and bonding defects. This gives rise to what is perhaps Carbon nanofoam’s most unusual feature: it is attracted to magnets, and below ˆ’183 °C can itself be made magnetic.

4.1. Ferro magnetism of Carbon nanofoam

It is a well-known fact that Carbon and its allotropes are among those materials which do not get attracted to magnets. Although, it has been discovered that Carbon nanofoam is attracted to magnets, and below ˆ’183 °C can itself be made magnetic. This behavior of Carbon nanofoam is unusual as against the magnetic property generally attributed to Carbon. However, at room temperature, the nanofoam’s magnetization disappears a few hours after the material is produced.

The reason for the magnetic behavior of Carbon nanofoam is discovered to be its molecular structure; it consists of a number of unpaired electrons due to the Carbon atoms with only three bonds that are found at topological and bonding defects. The unpaired electrons contribute towards the existence of magnetic moment in the nanofoam, which is believed to be the reason for its ferro magnetic character.

• Detailed explanation

Speaking in terms of magnetic susceptibility, in general, all known Carbon allotropes exhibit diamagnetic susceptibility in the range of χ =ˆ’(10ˆ’5-10ˆ’7) emu/g Oe with the exception of:

Polymerized C60 prepared in a two-dimensional rhombohedral phase of χ= +(0.25ˆ’1.3)*10ˆ’3 emu/g Oe (depending on the orientation of the magnetic field relative to the polymerized planes) which shows ferromagnetism

The disordered glass-like magnetism observed in activated Carbon fibers due to nonbonding π-electrons located at edge states, and

The unusual magnetic behavior observed in single wall Carbon nanohorns ascribed to the Van Vleck paramagnetic contribution.

Although ferromagnetism in polymerized C60 is noteworthy, the exceptionally large magnetic signal in Carbon nanostructures such as Carbon nanofoam remains a case of special interest.

In order to study the ferro magnetism of Carbon nanofoam, an experiment was conducted by Rode and his colleagues. They prepared Carbon nanofoam by high-pulse-rate laser ablation of an ultrapure glassy carbon target in a vacuum chamber made of stainless steel, filled with high-purity (99.995%) Argon gas, inside a 2 inch cylinder made of fused silica (SiO2). This setup results in the formation of carbon nanofoam, with a combination of sp2 and sp3 hybridization. The reason for the magnetic character of Carbon nanofoam was then discovered to be the ferromagnetic interaction of the spins of the unpaired electrons, separated by sp3 centers. [6]

A possible mechanism for magnetic moment generation would be a simple indirect exchange interaction through conduction electrons located on the hexagons. Low temperature magnetization curves indicate a saturation magnetization of approximately 0.35emu/g at 2 K. [10]

5. APPLICATIONS OF CARBON NANOFOAM

Carbon nanofoam is one of the lightest known solid substances till date. Hence, it finds its application in a number of fields. Although there are no immediate applications of Carbon nanofoams, a few of the possible areas where there can be applied are as follows:

They could be used in spintronic devices, which are based on a material’s magnetic properties.

In biomedicine: the Nano metric scale ferromagnetic clusters could be injected into blood vessels to enhance magnetic resonance imaging. It could also be implanted in tumors, where it could turn radio waves into a source of heat that would destroy the tumor but leave surrounding tissue unharmed.

Carbon nanofoam can replace the nanofoams of other metals because of its low density, high conductivity, light weight and also its ferro magnetic property.

As coatings or absorbents in specialty optics

As flexible electrodes for deionization and fuel cells

Carbon nanofoam paper

Making of High-Sensitivity Ultrasonic Transducer in Air

High-performance metal-air batteries

Spintronics, meaning spin transport electronics is also known as magnetoelectronics. It is an emerging technology which, in addition to its fundamental electronic charge, exploits both the intrinsic spin of the electron and its associated magnetic moment, in solid-state devices. Spintronic devices find their application in perhaps the most important computer subsystems: random access memories and high density non-volatile storage media. Hence in order to develop large memories on a small chip, making the chip as light weighted as possible is also very important. This is where the use of carbon nanofoam gives the desired result.

Carbon nanofoam paper is another interesting application of carbon nanofoams. Due to its composition, carbon nanofoam paper has proven very difficult to cut using traditional methods such as metal blades. It was found that using 100 W of power at a speed of 250 inches per minute (IPM) the 0.0075-inch thick carbon nanofoam paper was cleanly cut. [11]

For a high-sensitivity ultrasonic transducer in air, nanofoam can be considered to be applied to its acoustic matching layer. Since nanofoam has extremely low acoustic impedance, it is effective for the acoustic matching layer of an ultrasonic transducer in air. The sensitivity of the developed ultrasonic transducer can be made up to about twenty times higher than that of a conventional ultrasonic transducer in air. [12]

The desirable structural characteristics of carbon nanofoams can be exploited to design and produce electrocatalytic structures for O2 reduction that will enable high-performance metal-air batteries. While the native carbon nanofoam structure exhibits modest activity for O2 reduction, further functionalization of the nanofoam is necessary to achieve technologically relevant performance. [13]

6. CONCLUSION

In conclusion, this term paper throws light on a recently discovered allotrope of carbon called as Carbon Nanofoam, whose molecular structure and properties are different from the other allotropes of carbon such as graphite, diamond, C60, amorphous carbon, carbon nanotubes and fullerene. Carbon nanofoam is found to be one among the lightest known solid substances, which gives it an advantage over other substances in a number of varied applications.

The most intriguing feature of carbon nanofoam is its magnetic property. This novel magnetic behavior found in carbon nanofoam has made many renowned scientists and researchers rethink about what makes a material magnetic, since ferro-magnetism is not one of the attributed properties of carbon in any of its forms. Furthermore, this ferro-magnetic characteristic of carbon nanofoam, along with its other characteristics such as extremely low acoustic impedance, low density, continuous porosity, high surface area, fine cell/pore sizes, electrical conductivity and high capacitance is believed to have wide applications in the developing current technology whose motto is “The smaller the better”!