Materialist

Introduction to Graphene and Graphene Nanoribbons-Part2

2011-06-04T21:02:00.000-07:00

Graphene Production

Graphene was first produced by mechanical exfoliation of graphite. This method provided a small amount of high quality samples for fundamental studies. Later on, several methods have been utilized to synthesize graphene sheets which might be categorized into bottom-up and top-down approaches with carbon containing molecules and graphite as initial materials, respectively. Cost, throughput, size of sheets, quality of sheets, chemical modification, and compatibility with commercial chip fabrication process are among the most notable considerations in selecting a method for synthesizing graphene. Some of the groundbreaking efforts in synthesizing graphene are Mechanical exfoliation, Supported growth (thermal decomposition of carbides or epitaxial growth by CVD and wet chemical routes)

· Mechanical exfoliation: (top- down approach)

With an inter-layer van der Waals interaction energy of about 2 eV/nm2, the order of magnitude of the force required to exfoliate graphite is about 300 nN/lm2 . This extremely weak force can be easily achieved with an adhesive tape as experienced each time one refreshes a graphite crystal substrate for AFM or STM imaging,

Micromechanical exfoliation remains the best method in terms of electrical and structural quality of the obtained graphene, primarily because it benefits from the high-quality of the starting single crystalline graphite source.

· Supported growth:

It has been known since the early 1970s that graphene could be grown directly on solid substrates and two different mechanisms can be exploited: the thermal decomposition of carbides or the epitaxial growth of graphene on metallic or metal carbide substrates by chemical vapor deposition of hydrocarbons.

The thermal treatment of silicon carbide at about 1300 C under vacuum results in the sublimation of the silicon atoms while the carbon-enriched surface undergoes reorganization and, for high enough temperatures, graphitization. The careful control of the sublimation has recently led to the formation of very thin graphene coatings over the entire surface of SiC wafers, with occasionally only one graphene layer being present.

Growth of few-layer graphene on Ni films, by CVD at atmospheric pressure is reported.

Some of the groundbreaking efforts in synthesizing graphene are summarized in Table 1

Graphene Nanoribbons (GNRs)

Graphene is a zero gap semiconductor, so that a field effect transistor (FET) will not have an ‘‘off’’ state unless a forbidden gap is created. Such a gap can be produced confining the electronic wave functions by etching narrow graphene nanoribbons (GNRs) typically of a few nanometers in width and with well defined crystallographic orientation.

So GNRs are 1D while Graphene layer is a 2D allotrop of carbon.

-Post process:

A suitable lithographic technique will allow the patterning of the ultimate, one atom thick nanoelectronics, that can operate initially most likely in combination with semiconductor- based circuitry, but with the potential that shortly graphene-only circuitry will become possible. To make possible the exploitation of the advantages arising from the possibility of lithographic patterning, two conditions have to be fulfilled: (i) the width of the GNRs has to be controlled down to a few nanometers in order to be able to achieve gap values that will allow room temperature operation; (ii) the crystallographic orientation of the GNRs has to be controlled equally precisely as a misorientation of only a few degrees can completely ruin the gap.

To be continued...

Introduction to Graphene and Graphene Nanoribbons-Part1

2011-05-25T09:16:00.000-07:00

Carbon is the fourth most abundant element in the universe after hydrogen, helium, and oxygen.

Soon after the discovery of the X-Ray diffraction in 1912, these two fundamental crystal structures i.e. cubic diamond and hexagonal graphite were identified

After the Second World War, in the middle of the last century, further tremendous progress in the science of carbon has lead to unexpected and fascinating discoveries.

Carbon Bonds:
Because the outermost shell of a carbon atom contains only four electrons, these atoms can gain stability by losing four electrons, adding four electrons, or sharing four additional electrons
from another atom. Carbon can form three different kinds of bonds that involve the sharing of electrons. These are called single, double, and triple bonds, depending on how many pairs of electrons are being shared with another atom. This can be another carbon atom, or it can be an atom of another element.
In a single bond, two atoms share one pair of electrons. In this electron pair, both electrons can come from one atom or each atom can donate an electron. In a double bond, two atoms share two pairs of electrons. In a triple bond, two atoms share three pairs of electrons.

Diamon Crystal

FORMS OF CARBON (Carbon Alothrops)
Some elements have several different forms. These different forms of an element are called allotropes.

Carbon Allotropes:
Because of the flexibility of its bonding, carbon-based systems show an unlimited number of different structures with an equally large variety of
physical properties. These physical properties are, in great part, the result of the dimensionality of these structures. (diamond and graphite (3D) to graphene (2D), nanotubes (1D) or fullerenes (0D))
Fullerenes are molecules where carbon atoms are arranged spherically, and hence, from the physical point of view, are zero dimensional objects with discrete energy states.
Carbon nanotubes are obtained by rolling graphene along a given direction and reconnecting the carbon bonds. Hence carbon nanotubes have only hexagons and can be thought of as one-dimensional _1D_ objects.
Graphene A graphene crystal is an infinite two-dimensional layer consisting of sp2 hybridized carbon atoms , which belongs to one of the five 2D Bravais lattices called the hexagonal (triangular) lattice. It is noteworthy that by piling up graphene layers, in an ordered way, one can form 3D graphite. Graphene was initially considered as a theoretical building block used to describe the graphite crystal, and to study the formation of carbon nanotubes (rolled graphene sheets), and predict their fascinating electronic properties. This 2D atomic (one atom thick) crystal of carbon has as fingerprint a unique electronic structure with linear dispersion close to the Fermi level. Charge carriers in graphene are better described as massless Dirac fermions, which result in new phenomena.

Graphite, a three dimensional _3D_ allotrope of carbon, became widely known after the invention of the pencil in 1564, and its usefulness as an instrument for writing comes from the fact that graphite is made out of stacks of graphene layers that are weakly coupled by van der Waals forces. We can define graphite as an infinite three-dimensional crystal made of stacked layers consisting of sp2 hybridized carbon atoms ; each carbon atom is connected to other three making an angle of 120◦ with a bond length of 1.42 Å. Depending on the layers stacking, these crystals could be hexagonal (ABABAB. . .) or rhombohedral (ABCABC. . .). In both 3D crystals, the layers interact weakly through van der Waals forces. Graphite crystals can be found naturally, and can also be artificially synthesized by thermolytic processes; such as the production of highly oriented pyrolytic graphite (HOPG).

Carbon allotrops

Graphene:
Among systems with only carbon atoms, graphene, a two-dimensional (2D) allotrope of carbon, plays an important role since it is the basis for the understanding of the electronic properties in other allotropes. Graphene is made out of carbon atoms arranged on a honeycomb structure made out of hexagons , and can be thought of as composed of benzene rings stripped out from their hydrogen atoms
Although graphene is the mother for all these different allotropes and has been presumably produced every time someone writes with a pencil, it was only isolated 440 years after its invention
Having the title of the strongest material ever measured3, graphene is a two-dimensional (one-atom-thickness) allotrope of carbon with a planar honeycomb lattice . It is regarded as the basic building block of carbon nanotubes and large fullerenes. The properties of carbon nanotubes originate from graphene sheets.
With the exception of diamond, it is possible to think of fullerenes, nanotubes and graphite as different structures built from the same hexagonal array of sp2 carbon atoms, namely graphene. Indeed, fullerenes and nanotubes can be mentally visualized as a graphene sheet rolled into a spherical and cylindrical shape, respectively, and graphite can be described as a stack of alternately shifted graphene sheets.

The infinite plane of a perfect graphene shows a zero electronic band gap with electrons having zero effective mass where valance and conduction bands meet4. This makes graphene an anomalous material, which does not behave as either a metal or a semiconductor.
Graphene exhibits a number of exotic physical properties, previously not observed at the nanoscale. The observation of room-temperature quantum Hall Effect, ultrahigh electron mobility and ballistic transport, long electron mean free paths, superior thermal conductivity, great mechanical strength, and remarkable flexibility are among the striking properties of graphene. Hence, its desirability in electronics.
Graphene has first attracted the curiosity of mesoscopic physicists owing to its peculiar electronic behavior under magnetic field and at low temperature. The investigation and tailoring of its transport properties from macroscopic to molecular scales captures a large share of the current research effort. Technologists and materials scientists have rapidly grabbed some of the assets of graphene and are already exploring the ways of incorporating graphene into applied devices and materials. Eventually, chemists and surface physicists remembered their ancient recipes which could provide solutions to mass produce the elusive carbon monolayer either in bulk suspensions or in substrate-supported forms.
Now, graphene is a new and hot research topic with a strong vow to address the global demand for a new revolutionary material.

To be continued...

Ref:

1. West, K., Carbon Chemistry. 2008: Infobase Publishing.

2. Loiseau, A., Understanding carbon nanotubes : [ressource âelectronique] from basics to

applications. Lecture notes in physics; ; 677. 2006, Berlin ; New York: Springer. xvi, 552 p.

Strengthening by particles or Dispersion strengthening

2010-04-06T11:48:00.000-07:00

Introduction to Biomaterials

2010-02-12T19:06:00.000-08:00

A BIOMATERIAL is used to make devices to replace a part or a function of the body in a safe, reliable, economic, and physiologically acceptable manner [Hench and Erthridge, 1982]. A variety of devices and materials presently used in the treatment of disease or injury include such commonplace items as sutures, needles, catheters, plates, tooth ﬁllings, etc. Over the years, various deﬁnitions of the term biomaterials have been proposed. For example, a biomaterial can be simply deﬁned as a synthetic material used to replace part of a living system or to function in intimate contact with living tissue. The Clemson University Advisory Board for Biomaterials has formally deﬁned a biomaterial to be “a systemically and pharmacologically inert substance designed for implantation within or incorporation with living systems.” Black deﬁned biomaterials as “a nonviable material used in a medical device, intended to interact with biological systems” [Black, 1992].

Other deﬁnitions have included “materials of synthetic as well as of natural origin in contact with tissue, blood, and biological ﬂuids, and intended for use for prosthetic, diagnostic, therapeutic, and storage applications without adversely affecting the living organism and its components” [Bruck, 1980] and “any substance (other than drugs) or combination of substances, synthetic or natural in origin, which can be used for any period of time, as a whole or as a part of a system which treats, augments, or replaces any tissue, organ, or function of the body” [Williams, 1987]. By contrast, a biological material is a material such as skin or artery, produced by a biological system. Artiﬁcial materials that simply are in contact with the skin, such as hearing aids and wearable artiﬁcial limbs, are not included in our deﬁnition of biomaterials since the skin acts as a barrier with the external world.

According to these deﬁnitions one must possess knowledge in a number of different disciplines or collaborate with individuals from a wide variety of different specialties in order to properly develop and use biomaterials in medicine and dentistry (see Table 1). Table 2 provides some examples of the uses of biomaterials, which include replacement of a body part that has lost function due to disease or trauma, to assist in healing, to improve performance, and to correct abnormalities. The role of biomaterials has been inﬂuenced considerably by advances in many areas of biotechnology and science. For example, with the advent of antibiotics, infectious disease is less of a threat than in former times, so that degenerative diseases assume a greater importance. Moreover, advances in surgical technique and instruments have permitted materials to be used in ways that were not possible previously.

What is composite materials?

2009-02-08T05:26:00.000-08:00

As the term indicates, composite material reveals a material that is different fromcommon heterogeneous materials. Currently composite materials refers to materials having strong ﬁbers—continuous or noncontinuous—surrounded by a weaker matrix material. The matrix serves to distribute the ﬁbers and also to transmit the load to the ﬁbers.

The bonding between ﬁbers and matrix is created during the manufacturing phase of the composite material. This has fundamental inﬂuence on the mechanical properties of the composite material.

Fibers consist of thousands of ﬁlaments, each ﬁlament having a diameter of between 5 and 15 micrometers, allowing them to be producible using textile machines, for example, in the case of glass ﬁber, one can obtain two semi-products as shown in Figure 1. These ﬁbers are sold in the following forms:

a) Short ﬁbers, with lengths of a few centimeters or fractions of millimeters are felts, mats, and short ﬁbers used in injection molding.

b)Long ﬁbers, which are cut during time of fabrication of the composite material, are used as is or woven.

In forming ﬁber reinforcement, the assembly of ﬁbers to make ﬁber forms for the fabrication of composite material can take the following forms:

Unidimensional: unidirectional tows, yarns, or tapes

Bidimensional: woven or nonwoven fabrics (felts or mats)

Tridimensional: fabrics (sometimes called multidimensional fabrics) with ﬁbers oriented along many directions (>2)

Before the formation of the reinforcements, the ﬁbers are subjected to a surface treatment to

Decrease the abrasion action of ﬁbers when passing through the forming machines.

Improve the adhesion with the matrix material.Other types of reinforcements, full or empty spheres (microspheres) or powders, are also used.

The matrix materials include the following:

Polymeric matrix: thermoplastic resins (polypropylene, polyphenylenesulfone, polyamide, polyetheretherketone, etc.) and thermoset resins (poly-esters, phenolics, melamines, silicones, polyurethanes, epoxies). Their principal physical properties are indicated in the Table 1.

Mineral matrix: silicon carbide, carbon. They can be used at high tem-peratures.

Metallic matrix: aluminum alloys, titanium alloys, oriented eutectics.

Applications of composites:

The range of applications is very large. A few examples are shown below. Electrical, Electronics Insulation for electrical construction Supports for circuit breakers Supports for printed circuits Armors, boxes, covers Antennas, radomes Tops of television towers Cable tracks Windmills Buildings and Public Works Housing cells Chimneys Concrete molds Various covers (domes, windows, etc.) Swimming pools Facade panels Proﬁles Partitions, doors, furniture, bathrooms

Road Transports Body components Complete body Wheels, shields, radiator grills, Transmission shafts Suspension springs Bottles for compressed petroleum gas Chassis Suspension arms Casings Cabins, seats Highway tankers, isothermal trucks Trailers

Rail transports: Fronts of power units Wagons Doors, seats, interior panels Ventilation housings Marine Transports: Hovercrafts Rescue crafts Patrol boats Trawlers Landing gears Anti-mine ships Racing boats Pleasure boats Canoes Cable transports: Telepherique cabins Telecabins Air transports All composite passenger aircrafts All composite gliders Many aircraft components: radomes, leading edges, ailerons, verticalstabilizers Helicopter blades, propellers Transmission shafts Aircraft brake discs

Space Transports Rocket boosters Reservoirs Nozzles Shields for atmosphere reentrance General mechanical applications Gears Bearings Housings, casings Jack body Robot arms Fly wheels Weaving machine rods Pipes Components of drawing table Compressed gas bottles Tubes for offshore platforms Pneumatics for radial frames Sports and Recreation Tennis and squash rackets Fishing poles Skis Poles used in jumping Sails Surf boards, Roller skates Bows and arrows Javelins Protection helmets Bicycle frames Golf clubs Oars

Ref: Composite MaterialsDesign and aplications by Daniel GaySuong ,V. HoaStephen ,W. Tsai,CRC press

Photovoltaic Materials

2007-10-21T02:59:00.000-07:00

Photovoltaic Materials(pv)
Photovoltaics (PV) comprises the technology to convert sunlight directly into electricity. The term "photo" means light and "voltaic," electricity. A photovoltaic (PV) cell, also known as "solar cell," is a semiconductor device that generates electricity when light falls on it . Although photovoltaic effect was observed in 1839 by the French scientist Edmund Becquerel, it was not fully comprehensible until the development of quantum theory of light and solid state physics in early to middle 1900s. Since its first commercial use in powering orbital satellites of the US space programs in the 1950s, PV has made significant progress with total U.S. photovoltaic module and cell shipments reaching $131 million dollars in 1996.
While most PV cells in use today are silicon-based, cells made of other semiconductor materials are expected to surpass silicon PV cells in performance and cost and become viable competitors in the PV marketplace.
Photovoltaics and Photovoltaic Cells
When sunlight strikes a PV cell, the photons of the absorbed sunlight dislodge the electrons from the atoms of the cell. The free electrons then move through the cell, creating and filling in holes in the cell. It is this movement of electrons and holes that generates electricity. The physical process in which a PV cell converts sunlight into electricity is known as the photovoltaic effect.
One single P V cell produces up to 2 watts of power, too small even for powering pocket calculators or wristwatches. To increase power output, many PV cells are connected together to form modules, which are further assembled into larger units called arrays. This modular nature of PV enables designers to build PV systems with various power output for different types of applications.
A complete PV system consists not only of PV modules, but also the "balance of system" or BOS -- the support structures, wiring, storage, conversion devices, etc. i.e. everything else in a PV system except the PV modules. Two major types of PV systems are available in the marketplace today: flat plate and concentrators.
As the most prevalent type of PV systems, flat plate systems build the PV modules on a rigid and flat surface to capture sunlight. Concentrator systems use lenses to concentrate sunlight on the PV cells and increase the cell power output. Comparing the two systems, flat plate systems are typically less complicated but employ a larger number of cells while the concentrator systems use smaller areas of cells but require more sophisticated and expensive tracking systems. Unable to focus diffuse sunlight, concentrator systems do not work under cloudy conditions.
Types of PV cell materials
PV cells are made of semiconductor materials. The major types of materials are crystalline and thin films, which vary from each other in terms of light absorption efficiency, energy conversion efficiency, manufacturing technology and cost of production.

Do You know what is Material science?

2007-09-11T10:31:00.000-07:00

First of all I think it's better to know what is materials science and what do the materialist(Materials scientists or metallurgist or Material Engineers)do;

What is Materials Science and Engineering?
Materials science and engineering involves the characterization of the physical and chemical properties of solid materials—metals and alloys, ceramics, magnetic materials, polymers, optical materials, semiconductors, superconductors, and composites—for the purpose of using, changing, or enhancing inherent properties to create or improve end products.
Materials science and engineering involves examining how the microstructure (crystalline or amorphous) [some microstructures are shown in the image] of a material can be changed to influence the strength, electrical conductivity, optical, or magnetic properties of a material. This field is inherently multidisciplinary, encompassing mechanical, chemical, biomedical, civil, electrical, and aerospace engineering; physics; and chemistry.

Materials science comprises the study of materials from the macro to the atomic scale—from highway building materials to carbon nanotubes—but, independent of scale, the study of materials is concerned fundamentally with the effect of structure and chemistry on the properties of materials.
Materials have historically been so important that different eras of civilization were named according to the materials from which tools were fabricated; for example, the Stone Age, the Bronze Age, and the Iron Age. The development of the semiconductor spawned the modern era of information technology often called the Silicon Age. Advances in materials science might make this new millennium the Biomaterials/Nanomaterials/Optical Materials Age.

What do Materials Scientists and Engineers do?
In industry, materials scientists and engineers work with natural or synthetic materials and, most often, with combinations of materials, to improve existing products or to develop novel products.
Other materials scientists are on the forefront of the revolution in biotechnology, developing materials for the components of artificial joints, heart valves, and other replacement body parts. Smart materials show a tremendous potential in medical and dental applications, such as compressible stents that reform to their intended shape upon contact with body heat once inserted into an artery, ceramic cement for bone repair, or shape-memory alloys to correct misplaced teeth or spine curvature. (Smart materials have one or more properties that can be dramatically altered, such as multiviscosity oil, with a viscosity that varies with temperature.)Related research involves developing smaller and more reliable components, such as ferromagnetic activators acting as tiny machines in military and other applications.
In aerospace engineering, materials scientists are developing airframe and fuselage materials with high strength-to-weight ratios, as well as developing smart materials into integrated sensors and actuators for reconfigurable wings and other adaptive structures.(from www.imse.ir)

welcome

2007-07-28T12:41:00.000-07:00

Hi everybody
welcome to my weblog
you will see news and views about material science and engineering or other intresting news in few days on this weblog
you can visit www.imse.ir if you are intresting in material science