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Phase Transitions in NanoPhase Transitions in Nano--

Structured MaterialsStructured MaterialsDr. Salamat AliDr. Salamat Ali

Govt. College University, LahoreGovt. College University, Lahore

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Outline

Introduction Phase Transformation Phase Diagram of a Simple System

Classification of Phase Transitions Some Tools to detect Phase Transition Other forms of Phase Transitions

Melting Temperature Some Results from Literature/GCUL Conclusion References

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Introduction

Phase: It is a region of material that is chemically uniform, physically

distinct, and (often) mechanically separable.

In thermodynamics,phase transition orphase change is the transformation of a

thermodynamic system from one phase to another.

Crystallisation is a condensation process involving the creation of a crystalline

daughter phase from a parent phase.

The formation of any new phase from a bulk parent phase requires the creation

of an interface between the two phases.

The creation of this interface requires an amount of work, dependant upon the

surface/interfacial tension of the interface.

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Phase Transformation

The nomenclature for the different phase transitions.

The distinguishing characteristicof a phase transition is an

abrupt sudden change in one ormore physical properties, inparticular the heat capacity, witha small change in a

thermodynamic variable such asthe temperature.

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Phase Diagram of a Simple System

The crystal phase is stable at high P and low T.

The liquid phase is stable at high P and high T.

The vapour phase is stable at low P and low/high T.

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Other Forms of Phase Transitions

The transition between the ferromagnetic(Anti-ferromagnetic)andparamagneticphases ofmagneticmaterials at the Curie

point(Neels Temperature).

The emergence ofsuperconductivityin certain metals whencooled below a critical temperature.

Changes in the crystallographicstructure such as betweenferrite (bcc) andaustenite (fcc) ofiron.

Order-disorder transitions such as in alpha-titaniumaluminides.

Quantum condensation ofbosonic fluids, such as Bose-Einstein

condensation and the superfluid transition in liquid helium.

The martensitic transformation which occurs as one of the manyphase transformations in carbon steel and stands as a model for

displacive phase transformations.

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First Order and Second Order Phase Transitions

First-order phase transitions exhibit a discontinuity in thefirst derivative of the free energywith a thermodynamic

variable.i.e.

(dG/dX (where X is any thermodynamic variable)

Second-order phase transitions have a discontinuity in asecond derivative of the free energy.

Modern Classification is basd upon latent heat first order involes latent heat while 2nd order does not).

How to Classify Phase Transitions

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Some Tools to detect Phase Transitions

XRD (For Structure)

Resistivity ()

Susceptibility () Specific Heat (Cp)

Pressure

Volume

(Should be a function of Temperature)

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Phase Transition in Nanostructures

The same thermodynamic and kinetic principles that controlphase transitions in bulk materials can be applied to

nanostructuredmaterials. However, oftentimes common simplifications that are used in

these two fields are not appropriate for use with nanophase

materials. Therefore, there are a number of noteworthy phasetransition phenomena that occur in nanomaterials. SayMelting Point.

The melting point of a solid is size dependent. In the nanoregime, this dependence becomes significant.

Why?

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Melting Temperature Environmental effects

Pressure of the system Contaminants or Impurities

Structural effects Grain size

Phases present

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Unruh, Patterson, and Shah

Studied the effects of particle size onthe melting behavior of Bix(SiO2)100-xfilms

Amorphous SiO2 substrate created byvapor deposition

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X-ray Diffraction and DSC (Unruh et. al.)

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Thermodynamics Pressure in the solid does not equal the pressure in

the liquid: Ps = P l +2/r

d = -SdT + VdP

SmdT = 2V dr/r2 Bulk melting temperature: Tm=Hm/Sm Simplifying and combining all the above Eqs. Thefinal

result is:

T = (2VTm) / (rHm)

(which means that change in M.P. is inverselyproportional to the radius of the sphere).

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Calculated Melting Point of Gold(Ichimose et al.)

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Observed Melting point of Bi (Unruh et. al)

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Observed Melting Temperature of CuO

Nano-Particles

0 5 10 15 20 25 30

870

875

880

885

890

895

900

905

MeltingTemperat

ure(0C)

Particle size (nm)

Melting Temperature of Bulk CuO ~ 12000C

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The Crystallization of Metallic

GlassesMainly two factors are important:

Nucleation

Critical radius requirement causes anucleation energy barrier G*

Crystal (Grain) Growth

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Grain Growth

G =H -TS

Initially reduced the free energy of thesystem due to the reduction in surface tovolume ratio

Ultimately slows due to entropyrequirements

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Grain Growth Kinetics Driving force for grain growth:

v 2M/D dD / dt

Where: v = the driving force for grain growth

M = Mobility

= surface energy

D = Mean grain diameter t = time of reaction

Mean grain diameter: D = Ktn

(K is proportionality constant that increases withtemperature, n is the fractional constant)

Composition and temperature dependent

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DSC and X-ray Diffraction (Baricco et.al.)

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Crystallization of Ceramic Glasses

Spassov and Meinhardt studied Co33Zr67 Unable to experimentally conclude whether nanophase

was due to polymorphic transformation or primarycrystallization

Kinetic analysis showed it to be a time dependentprimary crystallization

Hi h P Ph

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High Pressure Phase

Stabilization

Unexpected metastable phases arepresent in some nano-structured ceramics

Trend seems to be that these phases aretypically seen under high-pressureconditions

Some are observed in doped materials ofthe same chemical composition

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Solid-state Diffusion

Solid-state diffusion does not control nano-

materials in the same manner as it does inbulk materials

Mathematically the same, but occurs

relatively quickly due to the finite size ofthe particle

Temperature, but not pressure, dependent

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SnO2 Used in oxygen sensors

Normal annealing results in a tetragonalphase

Under high pressure conditions,

orthorhombic crystal structure can form Not stable under ambient conditions

(cannot generally be forced throughdoping)

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Nanophase SnO2

Shows evidence of a post-anneal

orthorhombic phase when heat treated inO2 deficient environment

Evidence suggests the material is not

stoichiometric SnO2 Overall stress of the system is reduced by

the formation of orthorhombic structure

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ZrO2

Used in oxygen sensors, and the

fabrication of fuel cells Microcrystalline material is tetragonal

High temperature or high pressures cancause a reversible martensitictransformation to monoclinic

Dopants can be used to stabilizemonoclinic at ambient conditions

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Nanocrystalline ZrO2

Monoclinic phase is stable at ambient

conditions Produced by sol-gel method Appears to be the result of preparation pH

Transformation is neither martensitic or reversible

OH- ions coupled with Zr vacanciesprovide the required stress on the system

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Grain size Stability

Grain size stability in nanocrystalline materials

has been found to be closely related to the: structural characteristics of the material, such as

grain size and its distribution,

grain morphologies,

triple junctions,

porosity in the sample, and so on.

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Conclusions

Thermodynamics and kinetics both playan important role in phasetransformations.

The phase transformations andstabilizations are particle size dependent.

This dependence is critical in the nanoregime.

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References

Baricco, M. et al. Formation of Nanophases by Crystalization of

Amorphous AlloysMaterial Science Forum vol. 195 Trans TechPublications, 1995.

Bokhimi, X. et al. Tetragonal Nanophase Stabilization in NondopedSol-Gel Zirconia Prepared with Different Hydrolysis CatalystsJournalof Solid State Chemistry, vol 135 p. 28-35, 1998.

Gaskell, David R. Introduction to the Thermodynamics of Materials, 3rded. Taylor and Francis, Washington, DC. 1995.

Hampel, G. et al. Structure Investigations on Annealed Fe(CuNb)SiBAlloys with Different Si-B ContentsPhysica Status Solidi A, vol. 149

no. 1 p. 515-33, J une 16, 1995. Ichimose, N. et al. Superfine Particle TechnologySpringer-VerlagLondon, 1992.

Porter, D. A. and Easterling K. E. Phase Transformations in Metals andAlloys 2nd ed. Chapman and Hall, London, 1992.

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References (cont.)

Potter, Mark C., ed. Fundamentals of Engineering, 5th ed. Great LakesPress, Okemos, MI 1996.

Ragone, David V. Thermaodynamics of Materials Vol. IIJ ohn Wiley &Sons. Inc. New York, 1995.

Shek, C. H. et al. Nanomicrostructure, Chemical Stability, andAbnormal Transformation in Ultrafine Particles of Oxidized TinJournal

of Physics and Chemistry of Solids, vol. 58 no. 1 p.13-17, J an 1997.

Sheng, P and M. Zhou Melting and Thermal Characteristics of SmallGranular ParticlesMaterials Research Society SymposiumProceedings, vol. 195. Materials Research Society, Apr 16-20 1990.

Spassov, T. et al. Nanocrystalization of Co33Zr67 GlassesJournal ofMaterials Science, vol 32 p. 1483-6, Mar 15, 1997.

Unruh, K. M. et al. Melting Behavior in Granular Metal Thin FilmsMaterials Research Society Symposium Proceedings, vol. 195.

Materials Research Society, Apr 16-20 1990.

Our Group

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Our GroupMaterial Science/Nano-Technology Research Laboratory

Department of PhysicsGCU, LahoreThe Group stepped into the field of Nano-science in 2005

Running ProjectsColossal Magneto-resis tant (CMR) Nanomaterials

Metal Oxide Nanomaterials

Doped Metal Oxide NanomaterialsDoped Sulfide Nanomaterials

Transparent Conducting Oxides for Solar cells

Carbon Nano-tubes

Nanotechno logy for Cancer Treatment

Facilities Available:Fabrication of Nano-Materials:

Include Two State of the Art Furnaces

1). RT to 14000C (Muffle)

2). Tube Furnace (RT to 18000C) with controlled environment

3). PVD Set-up (under construct ion)

Characterization Techniques

Structural and Phase Analysis using XRDThermal Properties by DSC/TGA (RT to 15000C) and a DSC for low temperature down to 90 K

Magnetic Properties by AC Susceptibi lity Measurement (77-300K)

Electrical Properties using Resist ivity Measurement (77- 300K)

Open to Collaborate