United Arab Emirates University Faculty of Engineering College Requirements Devision

 

ENGINEERING MATERIAL - (MECH 390)

 NANO-STRUCTURED MATERIAL

Section No.

Prepared By

Student Name                                      ID

Student Name                                      ID

Student Name                                      ID

Supervising Instructor: Dr. Ra’a Said

Dec. 20^th, 2000

1. Introduction

Nano-structured materials have miniature crystal structure that enables them to posses special properties. Nano-structured materials are exceptionally strong, hard, ductile at high temperatures, wear-resistant, erosion-resistant, corrosion-resistant, and chemically very active. This allows the use of nanostructured materials in applications that are of great importance to the advancement of human life and technology. Although are currently utilized in many application, many aspects of nanostructured materials are still under research and development .

2. What Are Nano-Structured Materials

Nano-structured materials are those that have internal structural elements (basic building blocks or crystals) with a maximum dimension of few hundred nano-meters. They are not different from metals, ceramics, or polymers from classification point of view, to the contrary, nano-structured materials could be of any type but with a crystal structure in the nano-meter range. Since atoms average sizes are within few Angstroms (1 Å = 10^-10 m, a tenth of a billion of a meter), one nano-meter (1nm = 10^-9 m) would be composed of less that 10 atoms. Thus the grain size of nanostructured materials are composed of very few atom. This minute size of the builiding block of nanostructured materials provides them with many superior properties when compared to conventional materials.

3. Examples of Nano-structured Materials

Nanostructured materials are encountered in many structures and applications. For example paints, cosmetics, powders, and pharmaceutical products are all composed of nanostructured materials.

4. Properties

The properties of nanostructured materials are dependent on several features, such as the grain size and composition.  Generally the properties of the nanostructrued materials show outstanding improvement or deviation from the properties exhibited by the conventional materials.

4.1 Mechanical properties:

Module of elasticity:

Figure (1) shows a comparison between the stress-strain behavior of pure epoxy and nanotube-epoxy composite, it obvious that the module of elasticity (E) for tension and compression of the nanostructued material is higher.

Hardness:

Hardness increases with the decreasing grain size, for nanocrystalline pure metals (~ 10 nm grain size) the hardness is 2 to 7 times higher than those of larger grained (>1μ m) metals.

Another example is the nanophase copper and palladium assembled from clusters with diameters in the range 5-7 nm can have hardness and yield strength values up to 500% greater than in the conventionally produced metal.

Fig. 1:  The creep rate behavior for 1

Ductility:

Ductility is strongly affected by the change in the grain size.  Ceramics which are usually exhibit brittle behavior, are capable to undergo appreciable plastic deformation before fracture (ductility), in contrast to the normal ceramics, which are normally difficult to deform and hence very brittle [3].

Creep:

Nanostructured materials show different creep behaviors than that of materials with those of conventional grain size at different stress levels [2].  At high stresses nanostructured materials creep rate is slower, but at lower stresses and higher temperature, the creep behavior is faster than the conventional material, as shown in the figure (2) below.

4.2 Electrical properties:

Nanostructured materials have lower electrical resistance than conventional material that have ordinary grain size.

4.3 Magnetic properties:

Magnetic nanoparticles show a variety of unusual magnetic behaviors when compared to the bulk materials.

Remanence:

Remanence is magnitude of reflux density that remains when a magnetic field is removed.  Magnetic nanoparticle shows a reduction in the remanence behavior.

Coercivity:

It is the applied magnetic field necessary to reduce to zero the magnetic flux density of magnetized material.  Nanostructured magnetic-materials posses coercivity 1-2 orders of magnitude higher than conventional materials.

GMR (Giant MagnitoResistance):

It is a unique phenomena possessed by magnetic nanostructured materials, which means reduction of the electrical resistance of a material when exposed to magnetic field, [4].

5. Synthesis of Nano-structured Materials

There are five widely known methods to produce nanomaterials, and they are as follows:

All these processes synthesize nanomaterials to varying degrees of commercially viable quantities. To date, of all the above process, only sol-gel synthesis can produce materials (both metals and ceramics) at ultra-low temperature, large quantities relatively cheaply, synthesize almost any material, co-synthesize two or more materials simultaneously, coat one or more materials onto other materials (metal or ceramic particulates, and three-dimensional objects), produce extremely homogeneous alloys and composites, synthesize ultra-high purity (99.9999%) materials, tailor the composition very accurately even in the early stages of the process, because the synthesis is actually performed on an atomic level, precisely control the microstructure of the final products, and precisely control the physical, mechanical, and chemical properties of the final products.

6. Typical applications

There are typical different applications for the nanostructure materials in different fields; this is according to the enhancement it does to certain properties as mentioned above. Here is some of these applications:

6.1 Catalytic application:

Catalysts are substances that increase the rate of chemical reaction without being consumed in the process.  Catalysts are used in many petroleum and petrochemical processes, chemical separations, air separation applications, and environmental clean up.  In catalysis the key goal is to promote reactions that have high selectivity with high yield. It is anticipated that this goal will be more closely approached through tailoring a catalyst particle via nanoparticle synthesis, that it performs only specific chemical conversions, performs these at high yield, and does so with greater energy efficiency. In addition to selectivity, the surface area is also an important characteristic of catalyst, high surface area can be attained by creating materials where surface area is high compared to the amount of the bulk support material.  

Nanostructured materials, such as zeolites, with their potentially high surface areas compared to conventional  materials, make them suitable for a variety of catalytic applications. Beside that, the control of surface structure at the nanostructure level makes it possible to modify separation or catalytic process selectivity by several orders of magnitude [7].

6.2 Fuel cell:

Fuel cells are devices convert the chemical energy directly to electrical energy.  It operates like battery, however it do not run down or require recharging.   Figure (3:A) below shows one form of these cells in schematic forms.  A simple fuel cell consists of two electrodes sandwiched around an electrolyte.  The anode receives the hydrogen steam and the cathode receives the oxygen, generating electricity, water and heat.

The hydrogen atom splits into proton (which passes through the electrolyte), and electrons enters the electrical circuit. 

One obstruction in the usage of fuel cell, is the storage of H₂ , since it require a large storage tank,  nanoscale materials or structure with exceedingly high storage capacity per unit volume and weight for gases such as H₂ , is a solution for this problem. Carbon nanotube which are cylindrical sheet of graphite whose diameter is in nanoscale (2-20 nm), is ideal storage media for hydrogen due to their porous nature, and assumed capillary forces which would serve to draw hydrogen into the spaces between the carbon atoms [7].  See Fig. 3b and 3c.

7. Advanced applications

7.1 Nanostructured solar cell

Solar cells are devices that convert the energy of sunlight directly to electricity, they made of semicanductors such as silicon. A classic solar cell, see figure (4), consists of two layers of semiconductor, one p-type and the other n-type, sandwiched together to form a “pn junction”.  The pn junction induces an electricity field a cross the device. When particle of light “photons” are absorbed by the semiconductor, they transfer their energy to some of the semiconductor’s electron , which are able to move about through the material.  For each charged electron, a corresponding positive charge is created known as a “hole”.  The movement of the electrons and holes across the pn juncion  induce  a voltage across the device. After a short time these electrons and holes recombine [5].

In nanostructured solar cell, a nanostructured semiconductor with a high surface area is used, rather than the flat semiconductor in the classic solar cell, that result in greater absorption of light and higher efficient energy conversion.  Furthermore, electron-hole recombination in the semiconductor which seriously affects the efficiency of classic solar cell, dose not occur in this case [6]. In figure (5), a schematic view of the nanostuctured solar cell and its component.

7.2 Neural prostheses

Neural prostheses is a surgical implants for patient with neural damage (neuron is the brain cell responsible for conducting the electrical stimulus between cell).  The damage of these cells, result in inactivation of a portion of the brain needed for normal daily activity. Through this surgical implants the damaged portion of the brain is electrically stimulate, patient could then recover the lost function. 

Compared to conventional large grain-size materials, nanophase materials exhibit improved current capacitance, that is, the ability for a material to conduct a current overtime.  Currently, the used materials have shorter lifetime, and thus they require a frequent replacement. Neural prostheses composed of nanostructured materials, which has the ability to electrically stimulate other neurons longer, and that eliminates the repeated need for surgical replacement.

8. Current Status

Currently nanostructured materials are under extensive development and investigation.  Due to their processing and remarkable range of application, there is a wide range of disciplines contributing to the developments in nanostructure science and technology worldwide. Each year sees an ever-increasing number of researchers from diverse disciplines enter the field and an increasing breadth of novel ideas and exciting new opportunities explode on the international nanostructure scene.

The rapidly mounting level of interdisciplinary activity in nanostructuring is truly exciting. The intersections between the various disciplines are where much of the novel activity resides, and this activity is growing in importance.  Current reserches are focused on developing new synthesis routes of nanostructured metals, nanostructured ceramic, nanostructured polymers and their composite.

The properties of this materials (i.e. electrical, magnetic, mechanical, optical ….. etc) are comprehensively under study.  New applications for this material is also under progress, example are nanoparticle fillers in metal,ceramic, or polymer matrices, which can yield a very wide range of nanocomposites with unique properties.

Nanoscale devices are also under development. One of the major researches and development effort worldwide in nanoscale devices is focused on the magnetic devices using Giant MagnetoResistance (GRM), which is the decrease of electrical resistance of materials when exposed to a magnetic field.

9. Future  Issues

Nanotechnolgy is the creation and utilization of material, devices, and systems through the control of matter on the nanometer length scale, that is, at the level of atoms and molecule.  This new founded field is a natural result of the nanostructured materials and there use in different applications.   All natural materials and systems establish their foundation at the nano-scale.  The control of matter at molecular levels means tailoring and manipulating the fundamental properties, and processes exactly at the nano-scale, where the basic properties are determined.

Nanotechnology will be a strategic branch of science and engineering for the current century, thus there would be unbounding restructure for the technologies currently used for manufacturing, medicine, defense, energy production, environmental management, transportation, communication, computation and education, thus the nano-science and engineering will most likely produce the breakthrough of tomorrow.

One of the outstanding application of nanotechnology (molecular manufacturing), is the building of central processing unit (CPU) for a computer that occupies a volume less than 1 cubic micrometer, consumes roughly 100 nanowatts of power, and executes about 1 billion instructions per second (1 GHz).

In the medical field, the application of molecular manufacturing will lead to development of nanoscale medical devices of greater complexity and capability than modern drug molecules, and of far greater precision than modern surgical instruments.  More information about this topic, is available in references [9] and [10].

References

[1] http://www.rci.rutgers.edu/~majohnm/nanomat.html.

[2] http://www.rpi.edu/~crawfp/nano/.

[3] http://www.itri.loyola.edu/nano/06-01.htm.

[4] http://www.itri.loyola.edu/nano/06-03.htm.

[5] http://www.pv.unsw.deu.au/info/solarcel.html.

[6] http://www.epfl.ch/icp/ICP-2/solarcell-E.html.

[7] H. Gleiter, “Nanostructured materials: Basic Concepts and Microstructure”, Acta Materialia 48, pages 1-29, 2000.

[8] Mc GRAW-HILL, Encyclopedia of Science and Technology. 8^th edition, pages 624- 627.

[9] K.Eric and Chris, Unbounding the Future: The Nanotechnology Revelution. William Morrow and Company Inc., New York, 1991.

[10] B.C. Crandall, Nanotechnology. Massachusetts Institute of  Technology, London, 1996.