PHASE TRANSFORMATION METHODS

Fine particles of metallic compounds converted into metallic particles

MeXn : oxides, carbonates, chlorides, nitrates, etc.

Types of transformations:

MeXnMe

Advantages: - The size and shape of the precursors can be preserved - Preparation of highly crystalline metallic particles - Preparation of composite alloy metallic particles (MexMeyMez)

THERMOLYSYS

MeXn

T

MeXn

+n e-

- nX

Metallicparticle

REDUCTION

- nX

PHASE TRANSFORMATION

T, ne-

+n e-

- n X

Metallic particlesMetallic compounds

1 m

Haematite particles Iron particles

Precursors particles thermodynamically unstable at elevated temperatures

THERMAL DECOMPOSITION

Liquid: Chlorides, nitratesSolid: Oxides, carbonates, organic and organometallic compounds

AgNO3

PdO

> 6000CAg0

Pd0

Ag2O 2Ag + ½ O2

PdO Pd + ½ O2

Continuous phase Gas - Reducing- Reacting- Inert

Liquid - High temperature liquids (polyols, mineral oil)- Salt-melts (300 – 5000C)- Low temperature transformations (Ag2O , SnO2 , Bi2O3*)

Solid - Inorganic compounds (ceramics, salts)

> 8200C

MeXn Me0 + nX

*Preparation of colloidal bismuth particles in polyols, J. Mater. Res., Vol. 20, No. 6, Jun 2005

- Precursors particles in direct contact with a reducing agent- Electron transfer from a reducing agent to the precursors

Reducing agent

Gas - Reducing carrier gas- Reducing gases (H2) mixed with an inert carrier gas (N2)

Mechanisms: - Diffusion of gas molecules into the precursor lattice - ‘Electron hopping’

Liquid - The molecules of the liquid in which the precursor particles are dispersed (polyol)

- Reducing agent molecules dissolved in the dispersion medium

Mechanisms: - Dissolution/Precipitation- ‘Electron hopping’ (reduction of copper oxide)*

Transformation temperature Depends on - The ‘strength’ or concentration (H2) of the reducing agent

- The stability of the precursors

CHEMICAL TRANSFORMATION / REDUCTION

Men+ + ne- Me0

*"Formation of Uniform Colloidal Copper Particles in Polyols by Aggregation of Nanosize Precursors", Goia, D.V.*, Crnjak-Orel, Z., Matijevic, E., J. of Mater. Res., Vol 18, 4, 2003.

METALLIC PRECURSORS

• State of aggregation - Liquid Solution droplets in either a gas or liquid medium- Solid Particles - in gas phase (aerosols)

- in liquid phase (suspensions) - in solid phase (mixed salts, blends)

• Degree of dispersion - Dispersed particles - agglomerated - non-agglomerated

- Bulk powders

• Size Nanosize to micron size

• Shape - Spheres, rods, platelets, etc.- Various aspect ratios

• Chemical nature - Inorganic salts (oxides, carbonates, nitrates, chlorides, etc.)- Organometallic compounds*

Processes Transformations in gas phase/Spray pyrolysis Transformations in high temperature liquids Transformations in refractory solid phases

*Formation of uniform colloidal ceria in polyol", D. Andreescu, E. Matijević, D.V. Goia*, Colloids and Surfaces A: Physiochemical and Engineering Aspects, 291 (2006) 93-100.

SPRAY PYROLYSIS/AEROSOL THERMOLYSIS

Pd(NO3)2 droplet Pd(NO3)2 crystal Polycrystalline Highly crystalline Pd particle Pd particle

Size, uniformity, and degree of agglomeration of Me particles depends on:

a) Size and size distribution of droplets- Droplet generation pneumatic/spraying

ultrasonic- Size control pressure

transducers’ frequency, size ~ - Size distribution various approaches (momentum, gravitation force)

b) Stability of the aerosols (droplets, intermediates, and final particles)- Laminar flow during the process- Working below the critical concentration)

Decomposition of liquid precursors in gas phase

>1200C >8500C >1,0000C

'Method of making metallic powders by aerosol thermolysis, Ranade, M.B., Goia, D.V., Varga, G.J., Gamson, B.W., Bara, J., US 5,928,405, July 27,1999.

SPRAY PYROLYSIS/AEROSOL THERMOLYSIS

Carrier gas - air thermal decomposition (non-oxidizing metals)- inert thermal decomposition (oxidizing metals)- reducing chemical reduction

Heat sources - Plasma (plasma pyrolysis)- Flame (flame pyrolysis)- Hot wall/furnace

Particle properties- Highly crystalline (severe internal sintering, even melting) excellent sintering properties

‘inter-particles’ shrinkage only- Size: - from nanosize to micron size particles- Size distribution: - wide (due to droplet and particle collisions)

- classification required to obtain narrow size distributions (methods)- Shape: - spherical- Cost: - high capital investment

- increases with the decrease in size

Examples a) Preparation of noble metals - simple (Au, Ag, Pd) and composite (Ag/Pd)

- decomposition of salts in air or inert gasesb) Preparation of non-noble metals (Cu, Co, Ni, etc.)

- decomposition of salts in reducing atmosphere

Applications - Electronics (Shoei/Japan)- Thousand of tons of powders (Ag/Pd, Ni, Cu)

PHASE TRANSFORMATIONS IN LIQUID PHASE

Precursors Finely divided solids well-dispersed in liquids with high boiling temperatures Metallic compounds unstable at temperatures below Tb

Example

Bi2(CO3)3 Bi2(Poly)3 Bi particles(various shapes possible)

Liquids Organic - Mineral oil- Polyols

Inorganic - Melted salts

Advantage - May preserve the size and shape of the precursor particles - The liquid phase more effective in preventing inter-particle collisions

Challenge - Prevent the aggregation of the resulting particles (easier with polyols)

Solution - Particle encapsulation in temperature resistant coatings- Use of effective dispersants- Deagglomeration via ‘media milling’

Polyol, ~1800C Polyol, >2100C

PHASE TRANSFORMATIONS IN SOLID PHASE

• Precursors Finely divided solids well-dispersed in a solid continuous phase Metallic compounds unstable at temperatures below Tm

• Transformations: Both chemical and thermal

• Examples

NiCl2 Ni particles PdO Pd particles

• Continuous phase - Inorganic salts

• Precursor dispersion: - Evaporation of dispersions in concentrated salt solutions- dispersion technique, choice of solvent is very important- solvent evaporation (under mixing, spray drying, etc.)

- Milling of precursor particles with inorganic salts

• Advantages - Very low cost - May preserve the size and shape of the precursor particles - The solid phase the most effective in preventing inter-particle collisions

• Challenges - Prevent the aggregation of the resulting particles (dispersion of precursor particles) - Find a salt which is not reacting with the precursors or the final particles

- Deagglomeration via ‘media milling’

H2 (4500C) Air >850

BULK POWDER TRANSFORMATIONS

• Concept

• Limitation Only nonagglomerated metallic precursors

• Type of transformation Both chemical reduction and thermal decomposition eventually possible

• Advantage low cost

• Challenge prevent particle aggregation due to sintering

Solution encapsulate the precursor particles with thin films of refractory ceramics (0.1 – 1%)

• Example

• Steps dispersion coating drying transformation

PdO/ZrO2 Pd/ZrO2

- PSD can be very effectively preserved during the process- Post-transformation milling can be eventually used to break any agglomerates formed

T, Red