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Nano-Particulate Technology: Synthesis
Feynman’s Vision in 1959
“There is plenty of room at the bottom”
Microtechnology is a frontier to be pushed back, like HP, HV, LT
Ordinary machines could build small machines, which could build smaller machines,…. to atomic level
22 years later, first journal publication of article on molecular nanotechnology (Drexler, 1981)
Excerpts from “The Hindu” interview with Prof. Pradeep, Dept of Chemistry, IIT Madras; March 28, 2007
Excerpts from “The Hindu” interview with Prof. Pradeep, Dept of Chemistry, IIT Madras; March 28, 2007
Why is it necessary to know about nano technology?
Well, look at nature. Leaves make complex molecules called carbohydrates starting from a single carbon molecule, carbon-dioxide, present in air. These molecules make life possible for all of us. Every molecular assembly in nature is by this atom-by-atom approach. From amoeba to elephant is made this way. These synthetic routes are the most energy efficient, green and sustainable. The motion of a muscle fibre, or a flagellum is the result of nano technologies. Therefore, ultimately an understanding of these will help us to do things better, with improved efficiency — in much more eco-friendly, sustainable manner. Of course when you look at properties at this length scale, one may find new things. That drives the other side of scientific enquiry — curiosity.
Nano-Engineered Products
Semiconductor nano-crystallites for use in microelectronics
Ceramics for use in demanding environments
Polymers with enhanced functional properties
Transparent coatings with UV/ IR absorption properties, abrasion resistance
Static dissipative/ conductive films
Enhanced heat-transfer fluids
Catalysis
Topical personal care (e.g., sunscreen) & pharmaceutical applications
Ultrafine polishing of e.g., rigid mememory disks, optical lenses, etc.
Functional Polymer Fillers
To improve viscoplastic properties
By addition of inorganic fillers
Glass fiber, talcum, kaolin
20-60% dosage
Disadvantage: incresed density of the composite materials
Late ’80s: Toyota developed nano-clays (“bentonite”) for automotive applications
Functional polymers are very versatile, even tiny amounts can have dramatic impact
Other Applications
Nanowire & Nanotube arrays for EMI Shielding
Superior thermal, electrical, mechanical properties
SWNT, MWNT
Metallic or semiconducting
Carbon nanotubes provide special advantage in shielding
Chemical Gas Sensing
Low-power sensor arrays with high sensitivity, selectivity
e.g., humidity sensors, solid-state resistive sensors, combustible gas sensors, etc.
Ceramic MEMS
2D & 3D microcomponents & microelectromechanical devices for harsh environments
Energy Conversion:
Photo-voltaics, radiation detection, electroluminescent devices, etc.
Electronics & Related Fields:
Scanning probe, scanning microscopy standards
Storage & memory media
Flat panel displays, etc.
Other Applications, cont’d
Marine Anti-Fouling:
Nanoparticles held in coating lattice, not leached out by marine environment
Slowly release ions to provide long-term protection
Assure longevity of antimicrobial activity
Textile Fibers:
Nanoparticles in nylon, PP for antimicrobial character in extreme environments, after extensive thermal cycling
Nanosized ZnO and CuO in synthetic fibers with minimal effects on color & clarity
Permanent Coatings:
For long-term antimicrobial protection in many coating formulations
Healthcare, insdustrial, food processing, general paints & coatings
Catalysts:
Allows thinner active layers, less usage of precious metals
High, stable solids dispersions
Key application: automotive catalytic converters
Other Applications, cont’d
Fuel Cells:
Rare-earth metal oxides , nanoceria
As components in electrodes
As low-temperature electrolytes in solid xide fuel cells (SOFC)
Sunscreen:
To protect human screen from harmful UV rays
Nanomaterials are effective sun blockers
Semiconductor Polishing:
CMP slurries with fumed silica, collidal silica
Ceria, alumina dispersions in nano-sizes
High planarity, efficient removal, unique surface chemistry
Nano-Particles
Fundamental building blocks of nano-technology
Starting point for “bottom-up” approaches for preparing nano-structured materials & devices
Their synthesis is an important research component
Nano-Particle Synthesis Methods
Colloidal processes
Bognolo, 2003
Liquid-phase synthesis
Grieve et al., 2000
Gas-phase synthesis
Kruis et al., 1998
Vapor-phase synthesis
Swihart, 2003
Sono-fragmentation
Gopi, 2007! (Ph.D. thesis)
Colloidal Process
Nanoparticles produced directly to required specifications, assembled to perform a specific task
Involves use of surface-active agents
e.g., CdS 50 nm particles by mixing two solutions containing inverted micelles of sodium bis(2-ethyl hexyl) sulfosuccinate in heptane
e.g., antiferromagnetic nanoparticles of Fe2O3 by decomposition of Fe(CO)5 in a mixture of decaline and oleyl sarcosine
Coordinating ligands used to produce nanoclusters
Surfactants play a major role
Vapor-Phase Synthesis
Vapor phase mixture rendered thermodynamically unstable relative to formation of desired solid material
“supersaturated vapor”
“chemical supersaturation”
particles nucleate homogeneously if
Degree of supersaturation is sufficient
Reaction/ condensation kinetics permit
Once nucleation occurs, remaining supersaturation relieved by
Condensation, or
Reaction of vapor-phase molecules on resulting particles
This initiates particle growth phase
Rapid quenching after nucleation prevents particle growth
By removing source of supersaturation, or
By slowing the kinetics
Coagulation rate proportional to square of number concentration
Weak dependence on particle size
At high temperatures, particles coalesce (sinter) rather than coagulate
Spherical particles produced
At low temperatures, loose agglomerates with open structures formed
At intermediate temperatures, partially-sintered, non-spherical particles form
Control of coagulation & coalescence critical
Nanoparticles in gas phase always agglomerate
Loosely agglomerated particles can be re-dispersed with reasonable effort
Hard (partially sintered) agglomerates cannot be fully redispersed
Liquid-Phase Synthesis
Used widely for preparation of “quantum dots” (semiconductor nanoparticles)
“Sol-Gel” method used to synthesize glass, ceramic, and glasss-ceramic nanoparticles
Dispersion can be stabilized indefinitely by capping particles with appropriate ligands
Sol-Gel Method
Aqueous or alcohol-based
Involves use of molecular precursors, mainly alkoxides
Alternatively, metal formates
Mixture stirred until gel forms
Gel is dried @ 100 C for 24 hours over a water bath, then ground to a powder
Powder heated gradually (5 C/min), calcined in air @ 500 – 1200 C for 2 hours
Allows mixing of precursors at molecular level
better control
High purity
Low sintering temperature
High degree of homogeneity
Particularly suited to production of nano-sized multi-component ceramic powders
Gas-Phase Synthesis
Supersaturation achieved by vaporizing material into a background gas, then cooling the gas
Methods using solid precursors
Inert Gas Condensation
Pulsed Laser Ablation
Spark Discharge Generation
Ion Sputtering
Methods using liquid or vapor precursors
Chemical Vapor Synthesis
Spray Pyrolysis
Laser Pyrolysis/ Photochemical Synthesis
Thermal Plasma Synthesis
Flame Synthesis
Flame Spray Pyrolysis
Low-Temperature Reactive Synthesis
Inert Gas Condensation
Suited for production of metal (e.g., Bi) nanoparticles
Reasonable evaporation rates at attainable temperatures
Procedure:
Heat solid to evaporate it into a BG gas
Mix vapor with a cold inert gas to reduce temperature
Include reactive gas (e.g., O2) in cold gas stream to prepare compounds (e.g., oxides)
Cntrolled sintering after particle formation used to prepare composite nanoparticles (e.g., PbS/ Ag, Si/In, Ge/In, Al/In, Al/Pb)
Pulsed Laser Ablation
Use pulsed laser to vaporize a plume of material
Tightly confined, spatially & temporally
Can generally only produce small amounts of nanoparticles
But can vaporize materials that cannot be easily evaporated
e.g., synthesis of Si, MgO, titania, hydrogenated-silicon nanoparticles
Strong dependence of particle formation dynamics on BG gas
Spark Discharge Generation
Charge electrodes made of metal to be vaporized in presence of inert BG gas until breakdown voltage is reached
Arc formed across electrodes vaporizes small amount of metal
e.g., Ni
Produces very small amounts of nanoparticles
but in a reproducible manner
Reactive BG gas (e.g., O2) can be used to make compounds (e.g., oxide)
BG gas can be pulsed between electrodes as arc is initiated
Pulsed arc molecular beam deposition system
Ion Sputtering
Sputter solid with beam of inert gas ions
e.g., magnetron sputtering of metal targets
Low pressure (appr 1 mTorr) required
Further processing of nanoparticles in aerosol form difficult
Chemical Vapor Synthesis
Vapor phase precursors brought into a hot-wall reactor under nucleating condition
Vapor phase nucleation of particles favored over film deposition on surfaces
CVC reactor (Chemical Vapor Condensation) versus CVD
Very flexible, can produce wide range of materials
Can take advantage of huge database of precursor chemistries developed for CVD processes
Precursors can be S, L or G under ambient conditions
but delivered to reactor as vapor (using bubbler, sublimator, etc)
Examples:
Oxide-coated Si nanoparticles for high-density nonvolatile memory devices
W nanoparticles by decomposition of tungsten hexacarbonyl
Cu and CuxOy nanoparticles from copper lacetonate
Allows formation of doped or multi-component nanoparticles by use of multiple precursors
nanocrystalline europium doped yttria from organometallic yttrium & europium precursors
erbium in Si nanoparticles
zirconia doped with alumina
one material encapsulated within another (e.g., metal in metal halide)
Can prevent agglomeration
Spray Pyrolysis
Use of a nebulizer to inject very small droplets of precursor solution
Also known as aerosol decomposition synthesis, droplet-to-particle conversion
Reaction takes place in solution in the droplets, followed by solvent evaporation
e.g.: preparation of TiO2 and Cu nanoparticles
Laser Pyrolysis/ Photothermal Synthesis
Precursors heated by absorption of laser energy
Allows highly localized heating & rapid cooling
Infrared (CO2) laser used
Energy absorbed by precursors, or by inert photosensitizer (SF6)
e.g.: Si from silane, MOS2, SiC
Pulsed laser shortens reaction time, allows preparation of even smaller particles
Thermal Plasma Synthesis
Inject precursors into a thermal plasma
Precursors generally decomposed fully into atoms…
Which then react or condense to form particles
When cooled by mixing with cool gas, or expansion through a nozzle
Used for production of SiC and TiC for nanophase hard coatings
Flame Synthesis
Particle synthesis within a flame
Heat produced in-situ by combustion reactions
Most commercially successful approach
Millions of metric tons per year of carbon black and metal oxides produced
Complex process, difficult to control
Primarily useful for making oxides
Recent advances:
 g-Fe2O3 nanoparticles
 Titania, silica sintered agglomerates
Application of DC electric field to flame can influence particle size
Flame Spray Pyrolysis
Directly spray liquid precursor into flame
Allows use of low-vapor-pressure precursors
Applied to synthesis of silica particles from hexamethyldisiloxane
Low-Temperature Reactive Synthesis
React vapor phase precursors directly w/o external addition of heat
and w/o significant production of heat
e.g.: ZnSe nanoparticles from dimethylzinc-trimethylamine and hydrogen selenide
by mixing in a counter-flow jet reactor at RT
heat of reaction sufficient to allow particle crystallization
Advances in Instrumentation for Nano-Particle Synthesis
Need to analyze processes with short time-scales, in small regions of a reactor, in complex mixtures
FTIR spectroscopy (in emission & transmission modes) to simultaneously characterize
gas temperature,
gas concentrations,
particle temperature, and
particle concentration during synthesis
Localized thermophoretic sampling and in-situ light scattering measurements of
particle concentration,
size, and
morphology
Particle mass spectrometry and TEM imaging of extracted samples
Advances in Modeling for Nano-Particle Synthesis
Compute particle nucleation rates based on detailed chemical reaction kinetics
in cases where nucleation does not occur by simple condensation of a supersaturated vapor
Model multi-dimensional particle size distributions
where both particle volume and surface area are explicitly treated
Model simultaneous coagulation and phase segregation in multi-component particles containing mutually immiscible phases
Sonochemical Nano-Synthesis
Sonochemistry: molecules undergo a chemical reaction due to application of powerful ultrasound (20 kHz – 10 MHz)
Acoustic cavitation can break chemical bonds
“Hot Spot” theory: As bubble implodes, very high temperatures ( 5,000 – 25,000 K) are realized for a few nanoseconds; this is followed by very rapid cooling (1011 K/s)
High cooling rate hinders product crystallization, hence amorphous nanoparticles are formed
Superior process for:
Preparation of amorphous products (“cold quenching”)
Insertion of nano-materials into mesoporous materials
By “acoustic streaming”
Deposition of nanoparticles on ceramic and polymeric surfaces
Formation of proteinacious micro- and nano-spheres
Sonochemical spherization
Very small particles
Sonochemical Nano-Synthesis: Examples
S-2, Se-2, Te-2
used in non-linear optic detectors, photorefractive devices, photovoltaic solar cells, optical storage media
Gold, Co, Fe, Pg, Ni, Au/Pd, Fe/Co
Nanophased oxides (titania, silica, ZnO, ZrO2, MnOx
More uniform dispersion, higher surface area, better thermal stability, phase purity of nanocrystalline titania reported
MgO coating on LiMn2O4
Magnetic Fe2O3 particles embedded in MgB2 bulk
Nanotubes of C, hydrocarbon, TiO2, MeTe2
Nanorods of Bi2S3, Sb2S3, Eu2O3, WS2, WO2, CdS, ZnS, PbS, Fe3O4
Nanowires of Se
Sono-Processing of Nanocomposites
Power ultrasound can assist in synthesis, blending, dispersion & erosion-testing of nano-composites
dispersed phase having at least one dimensin < 100 nm
High-intensity ultrasound used with melt processing for polymer-clay nano-composites
e.g., PP/PS-clay & PMMA/clay nano-composites prepared by ultrasonic-assisted melt mixing
clay aggregates more finely dispersed
Superior overall homogeneity of composite, improved performance
Sono- Fragmentation
(Size Reduction)
Sono- Fragmentation
(Size Reduction)
Sono- Fragmentation
(Size Reduction)
Feed Sample
Sonication
Sono-Processed Sample
Sono-Processed Sample
(stratified Mix)
Sono-Blending
(Particle Size De-stratification)
Sono-Blended Particles For Composite Formulation
Polymer Precursor Preparation
Polymer Precursor Preparation
Polymer Matrix
Particle Reinforced Polymer Matrix
Caviation Erosion On the ceramic Particle Reinforced Polymer Matrix
Superior Cavitation Erosion Resistance on Nano-Composites