Current Activated Processing of Milled Aluminum, Dissertation – Literature Example

Literature Review

Introduction

Powder metallurgy is a latest, fine powdered material blendingprocess which also involves heating the compressed material in a controlled atmosphere to shape the material. The powder metallurgy process primarily consists of four steps namely powder manufacture, blending and compacting and sintering (Subramanian 2008). The main purpose of literature review is to review the various literatures and studies conducted linked with preparation process of Carbon Nanotube reinforced Aluminum Composites prepared by powder metallurgical techniques.

Carbon Nanotubes composites have seen a rapid growth in last five years to an exciting and vibrant area of analysis and research. CNTs extraordinary properties have motivated various researchers and hence there are various studies conducted in the context of CNT reinforced Aluminum Composites prepared by powder metallurgical techniques. Also there are various researches conducted related to including nanotubes into metals, ceramics and various other forms of carbon. Most of the recent studies have solely been focusing one using the mechanical properties of these CNTs rather than its optical or electronic attributes. However, this review analyses the various experimental studies and findings conducted in the context of activated processing of milled aluminum and Single Matrix Aluminum CNT Composites.

Effect of CNT on Mechanical Properties

Increased concern has been shown in using Carbon Nanotubes (CNTs) as reinforcements for the metallic matrices in the last few years. The main focus was on identifying their contribution in the enhancement of mechanical performance of the ultimate composite. In traditional composites composite homogeneity, orientation of the CNT, adhesion of the Nanotube matrix, Nanotube volume fraction and Nanotube aspect ratio have been found to significantly influence the properties of Nano composite. It is quite a challenging task to control these factors in order to acquire an exceptional composite. In this context, Aluminum has greatly benefitted from extensive research work that will attribute CNTs with substantial improvements in its basic properties. The basic limitation in using carbon Nanotubes as super reinforcements for metal matrix composites is poor dispersion/ distribution and poor agglomeration within metallic matrix (Esawi, Morsi and Sayed 2010).

There are various research groups comprising of authors which investigated the application of ball milling as a technique of mechanical dispersion. Bakshi, Singh and Seal (2009) assert that distinct milling conditions like time and energy were examined. TEM and SEM images that showed extensively dispersed CNTs showed that it is quite a challenging task. Nevertheless, the concerns regarding amorphization and damage of CNT with exposure to harsh milling conditions have been elevated so that optimization becomes mandatory for milling. The contents of CNT up to 10wt% were examined. CNT additions showed enhancement of the mechanical properties. The maximum CNT content which produced maximum enhancements was varying depending upon the preparation and mixing technique used. Generally speaking, the enhancements were not as per expected (Esawi, Morsi and Sayed 2010).

The existence of non-melted and unreacted CNTs after exposing it to high temperatures in the plasma spraying technique was studied. For the researchers, processing the CNT-composites at very harsh temperatures was a matter of concern so as to prevent CNT damage and other chemical interfacial reactions within the matrix. There have been many discussions saying that the Al4C3 phase must not be considered detrimental as it can assist in enhancement of the Al-CNT bondage and lock the various Nanotubes in place contributing to the mechanical properties of the composite. A major problem identified earlier was the properties as well as fabrication of the dispersed Nanotube –Aluminum composite and CNT clustering while adding it to Aluminum powder (Bakshi, Singh and Seal 2009).

As per the analysis done by Bakshi, Singh and Seal (2009) the most hopeful technique for getting CNTs dispersed in the aluminum matrix is ball milling. In this process the tensile strength increased by 21% for 2wt% CNT that was reinforced aluminum that was processed by hot extrusion and cold compaction. The mechanical properties were enhanced only when the cold working effect was limited to the process of ball milling. Extrusion also promoted the alignment of the CNTs in the direction of the extrusion. Both TEM and XRD analysis revealed that there was very little growth in the mean size of the crystal after extrusion was done at 500? C and annealing was done at 500 ? C for 10 hours. The matrix Nanostructure was retained when the final product was prepared which had also contributed for the enhanced strength which was displayed equally by all the samples relative to un-milled aluminum (Bakshi, Singh and Seal 2009).

It has been seen that CNTs act as the nucleation sites for the formation of voids during the testing for tensile strength. In addition to this, both CNT inner tube slippages and CNT pullout was observed in all the fractured surfaces. The result found presently showed that the way powders are processed significantly affects the final behavior and properties of CNT-AL composite as this point did not receive enough attention till date. Present and future work in this area comprises of using electrically activated procedures for the promotion of simultaneous annealing and consolidation of the powders, study of the impact of aspect ratio on the CNTs, examining the impact of higher-weight fraction CNT on the strength of aluminum matrix and optimizing the ball-milling condition (Gupta and Sharon 2011)

Simultaneous Enhancement of ductility with magnesium reinforcement

The critical study conducted by Goha, et al.(2006) highlight that Metal Matrix Composites (MMCs) are fast becoming popular because of their enhanced mechanical and physical properties relative to monolithic metals. There are various types of MMCs, but Magnesium (Mg) matrix composites are the most crucial because of their unique application in the automotive and aerospace industries as lightweight structural materials. In comparison to fiber reinforced Mg composites, the particulate reinforced Mg composites are gaining popularity because of their superior production rate, easy fabrication process and minimized reinforcement costs (Goha, et al. 2006).

SiC particles of micrometer size are generally used for reinforcement in Magnesium as they are easily available and have low cost. Some of the mechanical properties associated with Mg like modulus and hardness can be greatly improved by SiCp reinforcement. The major problem faced by micrometer sized SiCp reinforcement with Mg is that it has low ductility and tensile strength resulting from matrix/particle interfacial failure and particle fracture. Thus, to combat these obstacles and further improve the mechanical properties, significant studies on Nano size reinforcements are being done. Nano size reinforcements have been expected to contribute excellent properties to Mg matrix by using limited reinforcement material (Goha, et al. 2006).

In the study of Goha, et al.(2006) the DMD process followed by the hot extrusion process was successful in fabricating the Mg-CNT Nano composites. Macro pores and blowholes were absent showing good solidification and constant flow of argon while melting and depositing did not result in entrapping of the gases. The results obtained from density measurement indicated that lighter Nano composites can be obtained by adding CNTs. Prior studies indicate that addi9ng ceramic materials like Al2O3 and SiCp as reinforcement materials, the density of the Mg composites can be enhanced. However, this is not desired as Mg composites have light-weight applications (Goha, et al. 2006).

Goha, et al. conducted a comparison which was made between the densities of Mg-1.3wt% CNT and Mg composites having SiC particles. The comparison showed that the density of Mg-SiC composites is 11% more than the density of Mg-1.3wt% CNT Nano composites. Nevertheless, the tensile strength achieved by Mg-SiC composites was found to be lower than latter.

Incorporating CNT into the matrix of Mg has negligible effects on the Nano composites macro hardness maintaining the 1.3wt% CNT threshold. Beyond this threshold, it is found that the macro hardness begins to decrease as a result of increase in the Mg matrix porosity affecting the integrity of this material. The increased porosity is the result of increased addition of the CNT as smaller clusters of CNTs result in enhanced Mg porosity. Maximum ductility, tensile strength and yield were observed in Mg-1.3wt% CNT Nano composite. This tensile strength begins to decline beyond the threshold of 1.3wt% CNT. Enhanced yield strength comes from geometrically essential dislocations generated in the Mg matrix around CNT resulting from coefficient of Thermal Expansion (CTE) and an elastic modulus mismatching between CNT and Mg. The degree of dislocation found as a result of CTE mismatch was proportional to the CNTs volume fraction and was inversely proportional to CNTs diameter. Greater dislocation density can be seen with lesser CNT diameter and larger CNT fraction which will give higher yield strengths. This process of enhanced yield strength along with greater CNT volume fraction can only be applied until the threshold of 1.3wt% CNT beyond which the yield strength normally degenerates as a result of larger amount of Mg matrix porosity. Sometimes the tensile strength gets enhanced to an additional 1.3wt% CNT which is the result of restricted CNT dislocation movement. Rod shape reinforcements have been introduced to inhibit the dislocation motion and promote effective strengthening of the matrix relative to spherical reinforcements as a result of minimized inter-reinforcement spacing (Goha, et al. 2006).

Goha, et al study clearly described the fact that increased ductility was seen in MG that was reinforced with 1.6wt% of CNTs. The maximum ductility improvement was 69% in Mg 1.3wt% CNT Nano composite. Mg that is having a hexagonal close packed structure (HCP) has 3 independent easy-slip systems which bring about limited ductility. It was observed previously in AZ31B alloy that a non-basal slip can get activated at the room temperature. When extensive non-basal cross slips get activated, it ensures 5 independent slip systems in the Mg matrix which promotes higher ductility. It has been studies earlier that existence of reinforcements can create a slip-mode transition in context with matrix/reinforcement interaction.Cross slip in the non-basal planes can get activated when CNTs are present which results in enhanced ductility. This deduction can be confirmed with the help of Transmission Electron Microscope (TEM) (Goha, et al. 2006).

Carbon Nanotube Aluminum Composite Coating via Cold Spraying

A recent study conducted by Bakshi, Singh and Seal (2009) demonstrates that cold spraying is comparatively a newer coating process where the powder particles are augmented to supersonic velocities with the help of a carrier gas flowing under great pressure differences via a de Laval patterned nozzle and made to create an impact on the substrate. It has various advantages like creating minimized impacts on material which is sprayed like phase changes, grain coarsening and oxidation that generates dense coatings and the substrate is not affected by the coating process. Some of the disadvantages are huge loss of carrier gas until it is recycled and the materials that can be plastically deformed can only be deposited. The particles do not melt and the bonding is the result of adiabatic shear instabilities that arise from the thermal softening at the interface of particle/particle and substrate/particle which is modeled with the help of a finite element technique. The plastic flow maintain constitutive relations used in modeling the binding and deformation manage the flow stress dependence on the pressure, temperature, strain rate and strain. The parameters that affect the entire process and the efficiency of spraying are particle density, particle size, density of gas, temperature of gas and the spraying angle. Different models have been suggested for identifying the effect of different parameters (Bakshi, Singh and Seal 2009).

Cold sprays are used for depositing different materials like composite materials, alloys and pure metals. In all the above mentioned spraying composite coatings, the 2^nd phase was found to be uniformly distributed within the matrix. Presently great interest has developed in cold spraying composites that contain Nano fillers as the reinforcement material by the technique of cold spraying.

As per the analysis of Bakshi, Singh and Seal (2009) Aluminum composites that were reinforced with CNTs were found to be successfully synthesized by the process of cold spraying of blended powders. This mixture was made of pure Aluminum powder and spray-dried agglomerates of the Al-Si eutectic powder which contained either 20wt% or 10wt% CNTs. Coating that were 500 ?m thick were sprayed easily on the aluminum alloy AA6061 substrate which had nominal CNTs of 0.5 and 1wt%. CNTs were found to be easily retained as well as located between the interfaces of the splat and also seen to be embedded within the matrix. Nevertheless, CNTs were shorter in length as a result of fracture as an impact of shearing in between the aluminum matrix and Al-Si eutectic particles. The composite was Nano indented which yielded various values of the elastic modulus in between 40 and 120 GPA. The lower values denoted the porous regions of the deposit and the higher values denoted the regions that were Si rich. Some of the regions had very elastic modulus which was as high as 299 GPA for the Al 0.5wt% CNT and 191 GPA for the Al 1wt% CNT coating (Bakshi, Singh and Seal 2009).

Pressure-less infiltration technique in CNTs reinforced Al composites fabrication and Tri-biological properties

Carbon Nanotubes (CNTs) are fast becoming popular in the technological and scientific field because of their unique physical and chemical properties. Sufficient work has been done on accelerating their physical properties like Young’s modulus and bending stiffness with the help of electronic transition microscopy or atomic force microscopy (Zhou and Zhang 2006).

Zhou and Zhang(2006) examinedYoung’s modulus for isolated nanotubes by analyzing its intrinsic thermal vibrations and amplitude in the TEM in order to get the average value for Young’s modulus which was found to be 1.8TPa. The atomic force microscopy was employed to realize the mechanical properties of these nanotubes and it was seen that their tensile strength reached 150GPa. CNTs have their best application as super powerful nano-tubular reinforcements to construct nano-composites that possess unusually high strength. Such nano-reinforcements are used to enhance its tensile strength. The properties as well as processing of the CNTs reinforced Al-based composite were prepared by the hot-pressing technique followed by the method of hot-extrusion. It was observed that none of the CNTs got damaged while preparing the composite and no reaction product was found at the interface of the nanotube and Al after annealing at 983K for 24 hours. When annealing is done at 873K the strength of the composite undergoes changes (Zhou and Zhang 2006).

A recent investigation of the CNTs reinforced Cu-matrix composites showed that the composites wear resistance improved and also the friction coefficient decreased as a result of the CNTs effect (Jiangab and Gao 2004). The main idea of Zhou and Zhang (2006) was to report the production of Al composites that were CNT reinforced by the pressure less infiltration method and other tribological properties that have never been reported before. It was also seen that the maximum volume content of CNT was 10-15% in CNTs/Al and CNTs/Cu composites and the CNTs/Al composite showed lower wear rates and friction coefficient (Zhou and Zhang 2006).

In another study, the technique of pressure-less infiltration was employed for spontaneous infiltration of some molten Al alloy into the CNTs-Mg-Al performs and not of ceramic performs and the CNTs/Al composites were also fabricated. The wear and friction element of the composite was measured with the help of a pin-on-desk wear tester specifically under non-lubricated conditions. The CNTs reinforced aluminum composites were made with the pressure less infiltration technique. In order to successfully infiltrate the Al alloys into the CNTs performs by the pressure less method requires N2 atmosphere and Mg to be present either in perform or the alloy (Koch and Whittenberge 1995).

Zhou and Zhang (2006) highlighted that the CNTs are very well-dispersed and uniformly embedded within the Al matrix composites. As the volume fraction of the CNTs increased, the composite hardness also increased but also decreased when the volume fraction increased further. The composite with CNTs having 15 vol% was the hardest. The friction coefficient of this composite minimized as the CNT volume friction increased because of its unique topological structure and self-lubrication properties. When measured within the volume fraction range of the CNTs from 0-20%, it was seen that the composite’s wear rate decreased steadily with increase in the CNTs volume fraction within the composite. CNTs bring about a favorable impact on the wear resistance and this was because of its excellent mechanical properties, uniform dispersion within the composite and the efficiency of CNTs reinforcement (Zhou and Zhang 2006).

Electric current effects on wear and friction properties of the CNT-Ag-G composites:

As per study conducted by Feng, Zhang and Xu (2005), in the electrical brush-rotor system, electric currents are made to pass in between the rotor and the brush which slide against each other. The way an electrical brush performs is largely affected by the total effects of the sliding motion and the current flow. Excessive heat generated as a result of intense frictional and electrical heating can raise the temperature at the interface of the electrical contact. This can deteriorate he brushes performance, change the friction coefficient and cause severe wear. When the electrical current is absent wear results from abrasive wear forms and fatigue. When the current was intensified, the wear was the result of intensified abrasive properties of the body surface of the metal counter. Research studies also indicated that wear also gets affected by the high temperature resulting from high speed sliding with the current oxidation of the carbon materials. Thus, it is important for the brush to possess little thermal and electrical resistance and enhanced resistance to material transfer and wear (Feng, Zhang and Xu 2005).

The silver-graphite composite is a classic electrical contact material and comprise of two distinct components like graphite and silver. Graphite is beneficial because of its exclusive anti-friction properties but causes problems due to brittleness, reduced current carrying capacity and low strength. With increase in the graphite content, the brushes hardness and electric conductivity decreases. It is obvious for new components to enhance the thermal and electrical conductivity along with the hardness of the brush materials (Feng, Zhang and Xu 2005).

Feng, Zhang and Xu (2005) also indicate that CNTs have always been a matter of interest for considerable research. Various investigators have highlighted the unique mechanical and physical properties of this new type of carbon. CNTs have exclusive thermal conductivity and electronic properties better than diamond and superior mechanical properties like strength, reliance and stiffness better than any current materials. They offer great opportunities for developing a fundamentally novel material system. Adding 1% CNT by weight, the elastic stiffness increased from 36 to 42% and the tensile strength increased by 25% of the CNT-polystyrene composites. When the Ma fabricated CNTs-nano-Sic ceramic composites were produced by the hot-press technique, the fracture toughness and strength increased by 10% relative to the monolithic ceramics. It was also seen that the fracture toughness and the bending strength of 5vol% CNT-SiO2 composite as compared to monolithic SiO2 increased by 65% and 100% respectively (Feng, Zhang and Xu 2005).

Most of the research studies on CNT composites focused on ceramic or polymer matrix materials along with their structural applications and minimum stress was laid on metal-matrix composites particularly for the poor wear performance and basic friction of the electrical contact materials. The main idea was to study the impact of electrical currents on wear performance and friction of the CNT-graphite-silver composite and examine the worn surface also.

Overall Feng, Zhang and Xu (2005) highlighted certain facts like excessive heat generated by release of the current resulted in roughening of the surface of the counter, higher wear rate for the brush and mechanical weakening of the brushes near surface layer. The friction coefficient reduced in absence of the current and this was the result of the lubricating film, which altered the severities of nature of contact from metal-metal to metal-lubricating film-metal specifically on the contact interface of the ring and the brush. The greater wear volume of the positive brush relative to the negative brush can also be the result of oxidation reactions between the negative oxygen ions and the positive metal ions. The contact voltage for the composites initially dropped low during wear but the rose to a specific level resulting from the lubricating film particularly of the sliding interface and then became constant. When compared with the Ag-G composites, the CNT-Ag-G composites undergo lesser wear volume and similar friction coefficient value (Feng, Zhang and Xu 2005).

CNTs-metal nitride composites:

A new interest has been developed in the property characterization and fabrication of the CNT-combined composites since multi-walled CNTs (MWNTs) have been bulk produced. CNTs have superior properties like excellent electrical conductivity, superior mechanical strength and non-linear optical properties. These properties are of great use in composites as it is predicted to deliver these CNT properties in a synergistic and process-able host. Specifically speaking, these unique properties characteristic to CNTs have made them quite popular for combining CNTs with the composites in order to fabricate Nano-electronic elements. Experimental and theoretical results exhibit excellent electrical properties of the CNTs as it has electric-current-carrying capacity which is 1000 times more than the copper wires. Thus, the use of CNTs as additives to better electrical properties of the Nano-composite materials has been predicted (Jiangz and Gao 2005).

Jiangz and Gao (2005) has analyzed CNTs use as conductive fillers in metal oxide, metal and polymer matrices to enhance their nano-electric device applications. A specific electrochemical route was employed for the fabrication of CNTs/ polypyrrole (PPy) that can have potential applications for CNT-based nano-electronic structures. Another study on CNTs/ polyaniline (PANI) indicated an increased order of magnitude in the electrical conductivity relative to neat PANI. In addition to this, CNT/metal oxide or CNT/metal composites have been produced for better electrical conductivity. The CNT/Al composites resulted in an increase in the electrical conductivity along with increase in the volume fraction of the aluminum nanotubes. For bulk alignment of the CNTs, the high-temperature extrusion technique was employed (Jiangz and Gao 2005).

Electrical conductivity anisotropy was well exhibited by the ceramic-matrix nano-composites and their resulting materials. Previous works focused on a simple solvo-thermal technique for manufacturing CNT-magnetic nano-composite in situ accompanied by strengthened electrical properties. According to these studies, CNTs create an obvious impact on enhancing the electrical properties in various NT-combined composites. However, studies concerning the electrical and fabrication properties of CNT-nitride composites are lacking till date.

Metal nitrides are quite a gifted material as they have excellent attributes which can be applied to wide-ranging applications which require electrical properties. They have also found advanced application in developed microelectronic devices. For instance, Titanium Nitride (TiN) has gained popularity as an electrode in various electrochemical capacitors and as electrical conductors in electronic devices. Iron Nitride (FexN) is a newly invented ceramic material used in the electrical industry. It is believed that CNTs have huge potential for developing the electrical properties of a new type of CNT-nitride materials. As per this study, CNT-Fe2N and CNT-TiN nano-composites were successfully characterized and fabricated. Also, the electrochemical and electrical properties and the microstructures of CNT-nitride nano-composites were also studied. This study will prove to be a foundation for developing a new type of CNT-nitride composites which will display improved electrical properties in the electronic devices that are CNT-based (Jiangz and Gao 2005).

Jiangz and Gao study also demonstrated that CNT-Fe2N and CNT-TiN nano composites were produced with superior electrical properties. When having 12 vol% CNTs, the CNT-TiN composites showed 45% enhancement in electrical conductivity relative to TiN material. Adding CNTs does not alter the original electrochemical stability of the TiN making the composite the most suitable candidate for being used as electrodes in various electrochemical capacitors. The creation of tiny TiN nanoparticles that are attached to individual CNTs results in good dispersion of CNTs in the composite and also promotes interfacial adhesion of the nitride matrix and CNTs. In absence of this nanoparticle-attached microstructure, the CNT-Fe2N composites showed an improvement by 12% in their electrical properties by addition of 12 vol% CNTs. The potential mechanisms that improve the electrical conductivity are bridge-connected functions of the CNTs between the various conducting domains and CNT introduced conducting paths. These CNT-nitride composites are a new type of material having better electrical properties that can have applications in the CNT-based nano-electronic industries, production of electrochemical capacitors and other crucial fields (Jiangz and Gao 2005).

CNT Tensile behavior and microstructures

CNTs have been identified as ideal reinforcements for improving the mechanical performance of the mechanical performance as a result of higher elastic modulus, aspect ratio and strength. In the recent past, there have been studies showing that adding CNTs can boost the toughness, strength and conductivity of ceramics and polymers. Adding CNTs as reinforcement in CNT/polymer nano-composite enhances the tensile strength of this polymer matrix manifold. The fracture toughness and bending strength of the CNT/silica nano-composites increases more than three times when fabricated by the sol-gel process relative to monolithic silica. CNT or aluminum nano-composites fabricated by molecular level mixing exhibit toughening and strengthening on addition of CNTs (Kima and Cha 2006).

In the study conducted by Kima and Cha (2006) Nano-sized Cu powders were manufactured by the spray drying technique followed by reduction and burn-up processes. A water solution made of [Cu (NO3)2].3H2O was simply spray dried on the hot wall with 15,000rpm. This dried powder was then burned-up at around 300? C to create oxide powder. This oxide powder was then reduced to a Cu powder at about 200? C in a H2 atmosphere. The size of the Cu powder was from 200 to 300 nm.Multi-walled CNTs that were fabricated by the chemical vapor deposition (CVD) technique having an average diameter of 40 nm and length of few meters was supplied. The CNTs used in this particular study has a density of 1.8 g/cm3 as derived from the TEM observation and the density of Cu was found to be 8.9g/cm3. In this specific study, the wt% of CNT is 1.0 for every 5 vol% of CNT/Cu nano-composite and around 2.2 for every 10 vol% of CNT/Cu nano-composites. The nano-sized CNTs and Cu powder were well-mixed into a composite powder with the help of high-energy ball milling method by use of planetary miller with 150 rpm for 24 hours. The volume fraction of the CNT ranges from 0 to 10%. The CNT/Cu composite powders were initially pre-compacted in some graphite mold under 10MPa pressure (Kima and Cha 2006).

This pre-compacted powder was sintered by the spark plasma sintering system at a temperature of 700? C for about 1 min in a vacuum atmosphere of 10-3 torr under 50MPa pressure. Heating was constant at 100? C/min. the CNT/Cu nano-composites that were spark plasma sintered were subjected to cold rolls up to a 50% reduction and then to full annealing for 3 hours at 650? C. the CNT/Cu composite powders and the sintered nano-composites microstructures were measured by the help of high-resolution scanning electron microscopy (HRSEM) examined by optical microscopy. The CNT volume fraction in context with CNT/Cu nano-composites was also identified by measuring the carbon content by C/S analyzer and Elemental Analyzer. The tensile strength was tested by INSTRON 5583 under the 0.2mm/min speed. Sub-size specimens particularly dog-bone shaped with a gage length of about 9mm and having a width of 2.5mm was based on ASTME8M for testing tensile strength. The nano-scale hardness of the CNT/Cu nano-composite was analyzed by the nano-indentation test (Kima and Cha 2006).

The CNT/Cu nano-composites have also been produced by spark plasma sintering method of the high-energy ball milling nano-sized Cu powders having multi-walled CNTs were also subjected to cold-rolling technique. The CNT/Cu nano-composite microstructure was classified into two-region structures comprising of the CNT free matrix region and fibrous CNT/Cu composite region. The distribution of the CNTs within the Cu matrix in a characteristic inhomogeneous manner made the CNT/Cu nano-composites exhibit mechanical properties that showed a yielding behavior made of two steps displayed in a stress-strain curve. The primary as well as secondary yield strength as measured of the CNT/Cu nano-composites agreed with the estimated values that are based on the model of generalized shear-lag. This depicts that the strengthening mechanism results from substantial load transfer of the elastically stiff CNT within the plastic matrix. The CNTs were homogenously distributed within the fibrous CNT/Cu composite area showing significant improvement in the yield strength. These results showed that having the CNTs homogenously distributed within the metal matrix is a critical issue for enhancing the mechanical properties of the metal/CNT nano-composites (Kima and Cha 2006).

Coating multi-walled carbon nanotubes with metal sulfides

According to Wei and Song (2005) to maximize carbon nanotubes uses in different applications, it is essential to connect in the surface other useful groups or other nanostructures. Combining carbon nanotubes with other Nano crystals are highly functional for applications in sensors, catalysts, Nano electronic devices, field emission displays, data storage/processing devices, as well as ceramic or polymer reinforcement. A number of semiconductor nanoparticles like SnO2, CdS [3], CdSe,CdTe,Eu2O3 etc have been attached to CNTs’ surfaces. Multi-walled carbon nanotubes externally measured averagely with diameter between 20 and 50 nm with a length measurement of about a dozen micrometers were arranged by the thermal catalytic decomposition of hydrocarbon and over 90 percent of purity (Wei and Song 2005).

Wei and Song’s process implementation in the preparation of the metal sulfide composites went as follows. In a usual mixture, MWNTs were initially detached in a 1 wt.% sodium dodecyl sulfate (SDS) aqueous solution by ultra-sonication for 3 h, to be able to created SDS adsorb on the MWNTs surface. After repeatedly rinsing and drying, 100 mg SDS adsorbed MWNTs was sonicated in a solution of 20 ml 0.1 mol/l metal salt (such as CdCl2, or AgNO3, or Hg (NO3)2) for 5 min. After that, 20 ml 0.1 mol/l Na2S solution was slowly added into the mixed solutions and then vigorously stirred. Following the 30 h reaction, the finish products was then rinsed more with water and then dried up at 80 ?C for 10 h. With the use of Hitachi Model H-800 transmission electron microscope, TEM images were taken, with 200 kV of accelerating voltage. X-ray powder diffraction (XRD) was done on a Shimadzu (Japan) XRD-6000 X-ray diffract meter with Cu K_ radiation (? = 0.15406 nm, U=40 kV, I=30mA) at 0.02?/s scanning rate in the 2? range from 10? to 80?. The Hitachi F- 4500 fluorescence spectrometer were used to record the photoluminescence spectra (Wei and Song 2005).

The study demonstrated that sulfides CdS, Ag2S and HgS nanoparticles with less than 30 nm of size were successfully coated on MWNTs easily and effectively situ synthetic method with no severe effects on the MWNTs energy states. This method is very versatile and it can be used to other kinds of metal compound nanostructures transition. It should be noted too that this innovative hybrid carbon nanotubes type with sidewall coated metal sulfides nanoparticles may contain more remarkable possible applications in nanometer-scale optoelectronic devices or field emitters (Wei and Song 2005).

Mechanical milling of intermetallics

Intermetallic compounds contain the class of metallic materials that recently have the pleasure of concentrated study by materials engineers and scientists. These materials are indispensable already in a lot of applications and provide the prospect of presenting supplementary breakthroughs in performance in high temperature magnetic materials, hydrogen storage materials and structural materials. The large amount of information regarding intermetallics was recently gathered in books and journals, written in recent years.

In this assessment the major categories for the application of MA/MM to the synthesis of intermetallic compounds was also addressed. These categories are; the synthesis of intermetallics which are hard to conventionally prepare, MA/MM for microstructural modification for properties improvement, and synthesis of non-equilibrium structures/microstructures combination at intermetallic compositions. The combined study on the intermetallic compounds and MA/MM already required extreme effort therefore it is not possible to attempt a complete compilation in this research. Only important topics, selected specifically, will be covered and the emphasis will be the most recent results (Koch and Whittenberge 1995).

It was also presented that MA is useful in helping in the combination of a lot of new high energy product magnets. These energy products got their magnetism properties through intermetallic compounds. The Nd-Fe-B system was initially evaluated by refining the pure component powders collectively in a planetary ball mill. MA formed a superior encrusted structure of Nd/Fe lamellae trapped B particles at the Nd/Fe interfaces. Compound formation transpires on succeeding annealing to outline the Nd, Fe, 4B phase(Koch and Whittenberge 1995).

Optimum magnetic coercivities are acquired by annealing at 700°C while the Nd2Fe14B phase contains about 50 nm grain sizes. Annealing of the as-milled powders produces magnetically isotropic powder, such as random crystallite alignment per powder particle. Magnetically anisotropic samples can be arranged from compacted isotropic magnets through texturing with the use of hot deformation. The remains of the anisotropic samples are 1.25 T and its energy product, (BH), is 295 kJ/m” which is equivalent to the values taken in commercial magnets through powder metallurgical or via quick solidification methods.

Additionally, it was reported that usage of MA to organize hard magnetic materials was based on Sm (Fe, TM), compounds with the ThMn, crystal structure. TM here means ‘transition metal’ which can either be MO, V, or Ti. In the case of NddFe-B, MA only offers an intimate homogeneous combination of the elemental components as well as the magnetic compounds which were improved by annealing at temperatures of 600-900°C. These experiments proved that MA is a superior technique to effortlessly acquire initial results on new compounds’ hard-magnetic properties of new compounds (Koch and Whittenberge 1995).

Dispersing multi-walled CNT with water:

CNTs or carbon nanotubes have caught a lot of attention due to their stimulating potential applications in Nano composites, sensors, molecular devices or advanced materials with electronic and optical properties. Though, CNTs dispersion in solvents and polymer matrixes is a hindrance for supplementary applications because of they are not easily processed. In this manner, a great deal of hard work has been conducted on the CNTs surface modification mostly to improve their ability and solubility (Wang, Liu and Zhu 2007)

Wang, Liu and Zhu(2007) implemented a methodology through modifications with macromolecules utilizing defect chemistry of oxidized CNTs by amidation and esterification. With the use of this methodology, different organic molecules that include small molecules, copolymers, dendritic, hyper branched polymers and linear polymers were grafted successfully onto the CNTs  tips and sidewall through a series of reaction strategies like atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT), ring-opening polymerization (ROP), and more.  Furthermore, straight covalent modifications of the static sidewall through a number of strong reactions like fluorination, osmylation, azomethine ylides and ozonolysis,were applied too. Even though direct modification is concise and the multi-step reaction is broken out, it is restricted in some detailed alteration reactions and entails challenging experimental conditions. Usually, the covalent modifications are frequently monotonous and awkward for additional applications and constantly end up in the loss of CNTs intrinsic electronic structures (Wang, Liu and Zhu 2007)

Modifications on non-covalent surface were also developed even though the comparatively frail relations between CNTs and the modifiers. Numerous approaches have been accounted through non-covalent adsorption like small molecular surfactants, p–p interaction utilizing pyrene-containing molecules, non-specific or specific adsorption of protein and DNA. Amphiphilic block copolymers, forming solvent micelles, have been used too in dissolving the immaculate SWCNTs with the use of hydrophobic interaction for selective solvents. However, this method frequently fails to break up the modified products from the selective solvents because of the physical adsorption nature; therefore, the assessment of the applications of the solid properties and modified products was suppressed (Wang, Liu and Zhu 2007).

There was also a report of a non-covalent procedure through zwitterionic functionalization of SWCNTs with small molecules containing monoamine functional group. Also addressed was SWCNTs functionality with diamine- terminated oligomeric poly (ethylene glycol) through zwitterionic interaction. Despite the functionalization strategies’ success, long reaction time and high temperature are always necessary. Momentarily, because of single anchoring site between modifiers and SWCNTs, the finished products were not established enough when compared with the covalent strategies and from time to time unsuccessful in exfoliation of the CNTs bundles to individual tubes.

In spite of a huge improvement in alteration of the static CNTs surface, which has been achieved recently, simple approaches are still very much required, specifically when it comes for the modified CNTs applications in the polymer based Nano composites, where it requires high scalable and efficient requirement. Zwitterionic interaction is the non-covalent interaction’s simplest form. Zwitterionic functionalization of CNTs with a block copolymer that contain multiple reactive sites can successfully boost the relations between the modifiers and the CNTs. Non-binding segment would improve the tubes’ solubility and execute some handy functions. Hence, block copolymer modification may provide considerable advantages to the final CNTs like in preparation of CNTs-based Nano composites, service as a catalyst support, direction of nanoparticle assembly or use as biosensor. It is understood that there is no account regarding the modification of oxidized CNTs based on multiple zwitterionic interactions with definite double-hydrophilic block copolymer.

Poly (ethylene oxide)-b-poly [3-(N, N-dimethylaminoethyl) methacrylate] (PEO-b-PDMA) is a multi-functional water soluble block copolymer. Consequently, PDMA includes pendent tertiary amines that could intermingle with metal ions and acids. PEO, on the other hand, is a prototypic biocompatible polymer that is vital for the application in biotechnologies and pharmaceutics because it is resistant of protein adsorption and cell adhesion. In this study, a simple and effective method was reported to modify multi-walled carbon nanotubes (MWCNTs) with the use of a well-defined PEO-b-PDMA block copolymer through multiple zwitterionic interactions. In comparison with other reported CNTs strategies for modification, numerous advantages could be established with the use of this process, like in a mild research conditions, benign environmental solvent involved, ambient temperature, limited reaction time and sonication free. The finished MWCNTs could be separately divided in water and in some organic solvents too. Meanwhile, the modified MWCNTs are useable as metal nanoparticle assembly for one-dimensional template with the use of amino groups of PDMA block free from zwitterionic interaction. Additionally, non-covalent modified MWCNTs that are coated with PEO corona may be used to serve as potential non-biofouling materials applicable for applications in biotechnology and biosensor, where either non-specific or specific adhesions of cells or proteins must be evaded (Wang, Liu and Zhu 2007).

The MWCNTs employed and processed in this research are acquired from two different sources: the corroded MWCNTs, MWCNT–COOH (90 wt% pure CNTs and 5 wt% carboxylic acid groups; 8–15 nm in diameter and hundreds of nanometer in length, Times Nano, Inc., Chengdu), were produced through chemical vapor deposition and utilized as acquired, and branded as MWCNTs(CD). In order to effectively compare, oxidation of the pristine MWCNTs (purity 90%, 10–15 nm in diameter and several micrometers in length, Shenzhen Nanotech Port Co., Lt.) were done accordingly with the use of the method reported, and branded as MWCNT (SZ). FT-IR spectroscopy confirmed the carboxylic acid groups of the oxidized MWCNTs (Wang, Liu and Zhu 2007).

Overall, Wang, Liu and Zhu (2007) research presented a simplistic and competent approach to graft a distinct double-hydrophilic block copolymer to the oxidized MWCNTs surface through multiple zwitterionic interactions. The PDMA blocks that contain amino groups joined with the COOH groups of MWCNTs and secured into the surface of the MWCNT as the PEO segment structured a corona to improve the solubility. This relationship between the MWACNTs and the polymers ought to be extremely strong because of the multiple anchoring sites all along a single polymer chain. The modified MWCNTs can be separately liquefied in different solvents and shape up homogenous solutions. Additionally, the block copolymer has the capacity perform a different function, like it has the ability to capture metal nanoparticles. For some examples, Au and Pt nanoparticles have been triumphantly connected into the modified MACNTs surface.  Aside from these properties, the stabilized by the block copolymer modifier could discover possible application in biotechnology since the PEO segment has the ability to effectively defy the specific or non-specific adsorption of proteins on the MWCNTs’ surface. Consequently, CNTs modification with existing block copolymer has exclusive advantages when compared with minute molecular amine (Wang, Liu and Zhu 2007).

Conclusion

The above conducted literature review highlight the fact that there are various advanced powder metallurgical techniques of preparing Carbon Nanotube reinforced Aluminum Composites. The possible applications of carbon nanotubes composites range widely, from electrically conductive paints to optical devices. Only a relatively small number of these new materials have been used commercially, however, and there are still many problems to overcome before the full potential of nanotube containing composites can be realized. Not least of these problems is the high cost of good quality carbon nanotubes.

Great interest has been generated in Carbon Nanotubes (CNTs) since its discovery in 1991 and its application as reinforcements in metal matric composites, ceramic and polymer. The major reason is its superior mechanical properties like stiffness and strength up to 63 GPA and ~1 TPA respectively and its thermal conductivity up to 3000 W/m K. Using CNTs in the form of reinforcements contributes stiffness and strength of polymers and metals and improves the thermal conductivity and fracture toughness of ceramics. There are various techniques implemented for fabrication of MMC that is CNT reinforced and they are electroplating, conventional powder metallurgy methods and electro less plating via CNT containing electrolytic baths, mechanical alloying, thermal spraying and spark plasma sintering. It is quite a challenging task to uniformly disperse and align the nanotubes within a metal matrix composite. In the last few years, success has been achieved in aligning and dispersing the nanowires within blown bubble films of the epoxies that contain dispersed CNTs (Agarwal and Bakshi 2010).

The development of new techniques for the low cost synthesis of high quality nanotubes is clearly essential. This might be achieved through improved by catalytic processes, and there is currently a large amount of work being carried out at this end. Alternatively, it may be possible to develop new non-catalytic techniques for the synthesis of fullerene related nanotubes. In this connection it is interesting to note that nanotubes can be produced by the high temperature heat treatment of conventional carbons non-graphitising carbon.

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