important engineering materials -...

IMPORTANT ENGINEERING MATERIALS

Nanomaterials, Liquid Crystals

(c) Dr. Payal Joshi, NMIMS

Nanomaterials

© Dr. Payal Joshi, NMIMS

•Creation of nanostructures, functional materials, devices and components through control of matter on the nanometer length scale is nanotechnology. •Materials whose sizes of individual building blocks are less than 100 nm, at least in one dimension are called nanomaterials.


•How small is a nanometer? • 1 cm = 100th of a meter • 1 mm = 1000th of a meter • 1µm = 1 millionth of a meter • 1 nanometer = 10-9 meter---- 1 billionth of a meter • 10,000X smaller than the diameter of human hair.


Can we make miniature devices? • Electronic product designers can double the processor speed

every 18 months – Moore’s Law • Number of transistors squeezed onto a computer chip every 18

months


•Nanometer size of materials render them: •Large fraction of surface atoms •High surface energy •Spatial confinement •Reduced imperfections


Properties at nanoscale Applications Higher surface to volume ratio with enhanced reactivity

Catalysis, solar cells & batteries

Increased hardness with decreasing grain size

Hard coatings & thin protection layers

Narrower band gap with decreasing grain size

Electronics

Light in weight with great strength Sports goods


•Nanomaterials are described: a) Zero dimensional: metallic, semiconducting

and ceramic nanoparticles b) One dimensional: nanowires, nanotubes,

nanorods c) Two dimensional: Thin films (plates, and

networks) d) 3-D: Nanophases


Graphene: The Unrolled Carbon Nanotube

• Delicately cleaving a sample of graphite with sticky tape • Sheet of crystalline carbon just one atom thick- Honeycomb lattice


Carbon Nanotubes (CNTs)

z CNTs are rolled up crystalline sheets of graphene thousand times thinner than a human hair

z Single Walled (SWCNT) & Multiple Walled (MWCNT) z SWCNT consists of single

graphene cylinder z MWCNT consists of several

graphene cylinders arranged in a concentric form


Properties

z Atomic arrangement (geometry of the structure) determines the mechanical and electronic properties of CNTs

z Outstanding electrical properties surpassing standard conductors & semiconductors in many ways

z CNTs are extraordinarily flexible & do not break upon mechanical deformation


Applications z CNTs have structural & electronic defects which allow

SWCNTs to act as transistors

z They are so strong that it can act as satellite tethers & space elevators

z CNTs are used as electrodes in batteries and capacitors (energy storage)- MWCNTs are widely used in lithium ion batteries for notebook computers and mobile phones

z CNTs intrinsically have an enormously high surface area, making them active catalysts

z CNTs as components of medical and biosensor devices


Layered Nanomaterials

Graphite • Formed by overlapping flat

hexagonal layers of carbon atoms separated by 3.35 Å, C-C distance is 1.42 Å

• sp2 hybridized C atoms are arranged in a hexagonal lattice & every carbon is bonded to three others

• Graphite is soft, the in-plane C-C bond of individual layers is extremely strong


Fullerenes

z Named after architect Buckminster Fuller resembling geodesic domes, Also known as bucky ball

z Spherical cages of 60 C atoms arranged in interlocking 20 hexagons & 12 pentagons

z Extremely strong, able to resist great pressures-- they bounce back to original shape after subjected to extreme pressures (< 3000 Atm)

z Do not bond to each other chemically rather they stick together thro' weaker Vander-Waals forces

z Superconductivity & ferromagnetic-- Intercalation of alkali metal atoms leads to its metallic behaviour


Uses z Potential as catalysts e.g, Conversion of ethyl benzene

to styrene z Ultra-thin layers of fullerenes as data storage devices

in flexible organic solar cells, photodetectors for X-rays

z Radioactive material can be introduced inside, thus avoiding dangerous leaks


Applications of nanomaterials • Nanomaterials in Medicine: • Nanorobots (nanobots) are used as drug delivery devices

during heart surgery. • Quantum dots are nanomaterials that glow brightly when

illuminated by UV light. Quantum dots bind to proteins unique to cancer cells, literally bringing tumors to light and killing it.

• Ferromagnetic nanoparticles have been developed and optimized for targeted delivery of therapeutic drugs, genes or radionuclides.


•Nanomaterials in Catalysis •Nanomaterial-based catalysts are heterogeneous catalysts. •Nano-TiO2 in photocatalysis, the one containing more defects exhibits higher photocatalytic activity. •Fullerenes: Potential as catalysts e.g, Conversion of ethyl benzene to styrene


•Nanomaterials are thermodynamically metastable •Kinetics associated with nanomaterials is rapid. Nanoparticles are highly reactive, they inherently interact with impurities as well. • In order to retain nanosize of particles, they are encapsulated in a polymeric matrix.


LIQUID CRYSTALS


• Cholesteryl benzoate (C6H5COOC27H45) when heated undergoes two sharp phase transformations one after the other.

• It fuses sharply at 145qC to give a turbid liquid which on further heating changes suddenly in to clear liquid at 178qC. These changes reversed on cooling.

145qC 178qC

p-cholesteryl benzoate Ù p-cholesteryl benzoate Ù p-cholesteryl benzoate (solid) (liquid crystal) (liquid) Mesomorphic state


• This turbid liquid show anisotropy (direction dependent-tendency to point along a common axis-properties of material depends on direction in which they are measured)

• True liquid, on the contrary are isotropic • Since anisotropic properties are associated with crystalline state, the turbid liquids are called Liquid crystals


• Liquid crystals (LCs) are ‘orientationally ordered liquids’ or ‘positionally disordered crystals’ that combine the properties of both crystalline (optical and electrical anisotropy) and liquid (molecular mobility and fluidity) states


Mesogen: It is the fundamental unit of a liquid crystal that induces structural order in the crystals.

Solid Liquid Crystal Liquid • An intermediate state of a matter, in between the conventional

liquid and a solid crystal

• A liquid crystal may flow like a liquid, but have the molecules arranged and/or oriented in a crystal-like way.

• When viewed under a microscope using a polarized light source, different liquid crystal phases will appear to have a distinct texture


Substances Transition Point (qC)

Melting Point (qC)

Anisaldazine (CH3OC6H4CH:N:CHC6H4OCH3) 165 180

p-azoxy anisole (CH3OC6H4NONC6H4OCH3) 116 135

p-azoxy phenetole (C2H5OC6H4NONC6H4OC2H5) 137 167

Diethylbenzidine (C2H5NHC6H4C6H4NHC2H5) 115 120

p-methoxy cinnamic acid (CH3OC6H4CH=CHCOOH) 170 186


LIQUID CRYSTALS

i. NEMATIC LC

II. LYOTROPIC LC

ii. CHOLESTRIC LC iii. SMECTIC LC

I. THERMOTROPIC LC


Classification Thermotropic LCs :Thermotropic LCs exhibit a variety of phases (smectic or nematic) as temperature is changed.

• At high temperatures, thermal motion destroys ordering, pushing the material into a conventional liquid phase.

• At low temperatures, most LC materials will form a crystal. Eg, p-azoxy anisole


• Lyotropic Liquid Crystals • It consists of a flexible hydrophobic chain (the tail) and a polar, hydrophilic (ionic or non-ionic) head group– Amphiphilic molecules


Liquid crystals which are prepared by mixing two or more substances, of which one is a polar molecule, are known as lyotropic liquid crystals. Eg. Soap in water, biological and cell membranes

Hydrophobic end of the mesogen

Hydrophilic end of the mesogen

Exhibit phase transitions as a function of concentration of the mesogen in a solvent (typically water) as well as temperature.


Nematic or Thread like Liquid Crystals

• Thread-like, parallel arrangement of mesogen towards each other along the axis

• They are mobile in 3 directions & rotate in one direction

• Nematic liquid crystal have more fluidity than smectic types.

• An aligned nematic has the optical properties of a crystal and this makes them extremely useful in liquid crystal displays (LCD).


e.g. p-azoxy phenetole (137q – 167qC), dibenzal benzidine (234q – 260qC).


Smectic liquid crystals

• Smectic liquid crystals have a layered structure

• Inter layer attractions are weak as compared to lateral forces of attraction between molecules.

• When a stress is applied or allowed to flow, layers slide over one another like soap (e.g. Ethyl p-azoxy benzoate) but still retain their parallelism


• In the Smectic A phase, mesogen is oriented along the normal layer

• In the Smectic C phase they are tilted away from the layer.


Cholesteric liquid crystals

• This phase is called cholesteric because it was first observed for cholesterol derivatives

• These are optically active and similar to nematic kind in arrangement but show strong colour effect in polarised light.

• Optical activity of these crystal is many times higher than of its solid crystalline variety.


• Only those chiral molecules that lack inversion symmetry, can give rise to such a phase.

• Molecules are twisted about an axis. The twist may be right / left handed depending upon the conformation.


Liquid Crystalline behaviour and Chemical Structure • Molecular structures play an important role in determining the phase, transition temperatures, optical and electro-optical properties of liquid crystals.

• Many liquid crystals have chemical structure represented as

• R is the side chain group, Z is the linking group and X is terminal group.


• R : Alkyl, alkoxy or alkenyl groups. The length and flexibility of side chain affect the phase transition temperature and the type of liquid crystal phase.

• A and B : Aromatic ring A and B may be same or different.

• Substitution over the rings by – CN, – F, – Cl polar groups change the dielectric properties of liquid crystals.


• Z : Linking group makes contribution to phase transition temperature and other physical properties.

• Linking group can be like, Ester (– CO–O–) , Ethylene (–CH2–CH2–), azo (–N = N–),

–CH=CH– help for delocalization or resonance

• X : Terminal group X, contributes to dielectric anisotropy. Groups: –CN, –OR, –R, –Cl


Applications of Liquid Crystals • LCDs • Cholesteric liquid crystal are used for detecting tumors in human body. • Liquid crystals are used as solvents for spectroscopic studies. • Liquid crystal memory units with extensive capacity were used in Space Shuttle navigation equipment.


• Recording /Sensing temperature changes: • Thermotropic chiral LCs can be used as crude thermometers • Liquid crystal color transitions are used on many aquarium and pool thermometers. • Liquid crystal in fluid form is used to detect electrically generated hot spots for failure analysis in the semiconductor industry.


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