PREPARATION, PROPERTIES PREPARATION OF METALLIC PARTICLES AND APPLICATIONS • Phase ‘break down’ - Milling/grinding - Atomization OF • Phase ‘transformation’ - Thermolysis/Pyrolysis HIGHLY DISPERSED METALLIC - Reduction PARTICLES • Phase ‘build-up’ - Condensation in gas phase (Me 0 ) g - Condensation in liquid phase (Me 0 ) l Dan Goia Clarkson University PHASE ‘BREAK DOWN’ / ATOMIZATION PHASE ‘BREAK DOWN’ / MILLING Spraying/pulverization of molten metals Size reduction of coarse/agglomerated metallic powders • Large particles, broad size distributions - Mechanical energy (shear, collision) - Monodispersed particles - Dispersion media (liquid or gas) - Sub-micrometer size - Dispersing agents - Controlled atmosphere and temperature frequently required • Capable to produce a large variety of alloy powders • Suitable for some applications (mechanical alloying) • Low manufacturing costs • Rarely yields highly monodispersed, spherical particles • Inert carrier gases may be required 1
PHASE TRANSFORMATION PHASE TRANSFORMATION AEROSOL AEROSOL REDUCTION THERMOLYSYS MeX 2 MeX 2 Metallic T, n e - particle T +2 e - - 2X +n e - - Difficult to control the size distribution of precursor droplets - n X - Agglomeration of droplets/particles due to collisions Wide particle size distribution Metallic compounds Metallic particles PHASE ‘BUILD UP’ / CONDENSATION SPRAY PYROLYSIS/AEROSOL THERMOLYSIS Decomposition of liquid precursors in gas phase Condensation from Condensation from liquids gas phase (Chemical Precipitation) >850 0 C >120 0 C >1,000 0 C CVD Plasma (Me n+ ) l (Me 0 ) s (MeX) g + ne - Pd(NO 3 ) 2 droplet Pd(NO 3 ) 2 crystal Polycrystalline Highly crystalline Pd particle Pd particle T T - X Size, uniformity, and degree of agglomeration of Me particles depends on: (Me 0 ) l (Me 0 ) g a) Size and size distribution of droplets → pneumatic/spraying - Droplet generation → ultrasonic Nucleation - Size control → pressure and → transducers’ frequency, size ~ ν Growth → various approaches (momentum, gravitation force) - Size distribution b) Stability of the aerosols (droplets, intermediates, and final particles) - Laminar flow during the process (Me 0 n ) gas (Me 0 n ) liquid - Working below the critical concentration) 2
CHEMICAL PRECIPITATION 2.0 - + 4H - + 3 e - → MnO 2 + 2H 2 O MnO 4 Au 3+ + 3 e - → Au 0 Metal atoms generated ‘via’ redox reactions: Pt 2+ + 2 e - → Pt 0 Pd 2+ + 2 e - → Pd 0 1.0 Me n+ + Red → Me 0 + Ox Ag + + e - → Ag 0 Cu + + 2 e - → Cu 0 H + + e - → ½ H 2 Driving force: 0.0 Co 2+ + 2 e - → Co 0 ∆ E 0 = E 0 1 - E 0 2 Fe 2+ + 2 e - → Fe 0 C 6 H 8 O 6 → C 6 H 8 O 6 + 2 e - + 2H + Zn 2+ + 2 e - → Zn 0 ln K e = nF ⋅ ∆ E 0 /RT R-CH 2 OH → R-COH + 2 e - + 2H + -1.0 N 2 H 4 + 4OH - → N 2 + 4 e - + H 2 O V 2+ + 2 e - → V 0 ∆ E 0 → critical supersaturation Ti 2+ + 2 e - → Ti 0 → nucleation rate Al 3+ + 3 e - → Al 0 -2.0 TAILORING ∆ E 0 TAILORING ∆ E 0 Ag + + 1e - → Ag 0 E 0 = +0.799V • Effect of the pH Ag + + Cl - → AgCl • Precipitation → Whenever H + or OH - species are involved in the reaction K sp = 1.82 x 10 -10 → Ag 0 + 1e - + Cl - Examples AgCl E 0 AgCl = E 0 Ag+ - 0.059/1 log[Cl - ]/K sp = 0.799 - 0.059(log[Cl - ] – logK sp ) = 0.222V C 6 H 6 O 6 + 2e - + 2H + → C 6 H 8 O 6 a) E 0 = -0.244V E 0 = -0.152V AgI K sp = 3.0 x 10 -17 E 0 = -0.710V K sp = 6.3 x 10 -50 Ag 2 S E 0 = E 0 - 0.059/2 log[C 6 H 8 O 6 ]/[H + ] 2 [C 6 H 6 O 6 ] = -0.244 - 0.059 (pH) • Complexation Ag + + 2NH 3 → Ag[NH 3 ] 2 + pK f = 10 -7.4 [H + ] ↑ , pH ↓ ⇒ C 6 H 8 O 6 less strong reductant → Ag 0 + 2NH 3 Ag[NH 3 ] 2 + + 1e - b) N 2 + 4e - + 4H 2 O → N 2 H 4 + 4OH - E 0 = -1.160V Ag+ - 0.059/1 log[Ag + ][NH3] 2 /[Ag(NH3)2] + = 0.799 - 0.059(pK f ) = 0.373V E 0 Ag[NH3]2 = E 0 3- + 1e - → Ag + + 2SO 3 E 0 = 0.430V Ag(SO 3 ) 2 2- pK f = 8.68 E 0 = E 0 - 0.059/4 log1/[OH - ] 4 = -1.160 + 0.059 (14 - pH) 3- + 1e - → Ag + + 2S 2 O 3 E 0 = 0.010V 2- Ag(S 2 O 3 ) 2 pK f = 13.46 → Ag + + 2CN - E 0 = -0.290V Ag(CN) 2 - + 1e - pK f = 19.85 [H + ] ↑ , pH ↓ ⇒ Hydrazine becomes a less strong reductant • Concentration E = E 0 - 0.059 log [Ag 0 ]/[Ag + ] = 0.799 + 0.059 log[Ag + ] E 0 = 0.777V [Ag + ] = 10 3 M 3
REDOX DIAGRAMS CONDENSATION FROM LIQUID PHASE METAL IONS/COMPLEXES E (V) Pd 2+ + 2e - → Pd 0 Reduction + METAL ATOMS (~3Å) Ag + + e - → Ag 0 CLUSTERS + + 1e - → Ag 0 + 2NH 3 Ag[NH 3 ] 2 NUCLEI (~8-10Å) pH 0 2+ + 2e - → Pd 0 + 42NH 3 Pd[NH 3 ] 4 Diffusional growth NANOSIZE C 6 H 6 O 6 + 2e - + 2H + → C 6 H 8 O 6 PRIMARY PARTICLES (1-30 nm) Diffusional growth/ Effective Stabilization Aggregation Coagulation N 2 + 4e - + 4H 2 O → N 2 H 4 + 4OH - - TRUE AGGREGATED LARGE PARTICLES NANOSYSTEMS NANOSIZE SYSTEMS (Crystalline / Polycrystalline) EXPERIMENTAL CRITICAL PROPERTIES ‘DIRECT’ ADDITION ‘REVERSED’ ADDITION ‘DOUBLE-JET’ ADDITION Me n+ /MeX m Red Me n+ /MeX m Red n n • Particle size and size distribution • Internal structure MeX m Red n + Me n+ Disp. Disp. Disp. • Particle morphology • Internal composition C C C Red • Surface properties Me n+ Me n+ Red Me n+ Red T n T f Time T n T f Time T n T f Time 4
Platinum Particles (~ 2.0 nm) PREPARATION OF NANOSIZE METALLIC PARTICLES a) Generate a large number of nuclei b) Involve a large fraction ( f) of atoms in the nucleation step R p = r n × ( 100 / f ) 1/3 Final size in the nanosize range → Provide high supersaturation (large ∆ E ) → Use suitable dispersion media → Work in dilute systems → Use surfactants c) Prevent the aggregation of primary particles → Maximize electrostatic repulsive forces (dilute systems) → Minimize/screen attractive forces (dispersing agents) PREPARATION OF LARGE PARTICLES A. CRYSTALLINE → diffusion growth - Slow nucleation (small ∆ E, strong metallic complexes) - Slow addition of precursors in the system - Use of seeds - Very effective stabilization B. POLYCRYSTALLINE PARTICLES → aggregation - Control the attractive/repulsive forces by adjusting: - Ionic strength - pH - Activity of the dispersant/protective colloid - More versatile in controlling the size of the particles Nanosize Silver Particles (~90 nm ) 5
CRYSTALLINE GOLD POWDER INTERNAL PARTICLE STRUCTURE Diffusional Growth Effective colloid stabilization Small ∆ E, supersaturation Crystalline 0.15 µ m 0.30 µ m Particles Nanosize Primary Particles Coagulation/Aggregation Poor colloid stabilization Large ∆ E, supersaturation Polycrystalline 0.5 µ m 1.0 µ m Particles AgPd Spherical Alloy Particles 6
INTERNAL PARTICLE STRUCTURE METAL PARTICLES FORMATION IMPORTANCE OF PARTICLE STRUCTURE Polycrystalline Crystalline Monodispersed Gold Monodispersed Gold A. Electronics/Thick film Due to the absence of internal grain boundaries, highly crystalline particles of PM yield dense, continuous, thinner, and more conductive ‘fired’ films. B. Electronics/Oxidation of base metals Highly crystalline base metals (Cu, Ni) are more resistant against oxidation when used as precursors for thick film conductors. C. Medicine/Biology Highly crystalline, dense gold particles are more effective as carriers of drugs/vaccines through biological tissues. 2 µ m 2 µ m PARTICLE MORPHOLOGY PARTICLE MORPHOLOGY Crystalline Pd Particles Hexagonal Gold platelets 7
SURFACE PROPERTIES INTERNAL COMPOSITION IMPACT Bimetallic particles → electronics (wide range of properties attainable) • Dispersibility in liquids → catalysis (enhanced catalytic activity) • Self assembly properties • Sintering characteristics • Core/Shell structure • Catalytic activity • Adhesion properties 1 ≠ E 0 → the most electropositive element will form the core E 0 2 • Corrosion → the most electronegative element will form the shell ⇒ Precipitation order can be tailored by appropriate complex formation TAILORING SURFACE BEHAVIOR • ‘Solid solutions’/Alloys • Selection of precipitation environment (reductant, dispersant, solvent) • Subsequent surface treatment (performed on either wet or dry powders) 1 ≅ E 0 → similar reduction rates E 0 2 ∆ E 0 1 , ∆ E 0 2 >> → fast reactions - Coating with organic compounds - Coating with inorganic compounds ⇒ Uniformly mixed crystalline lattices - Coating with metals ELECTROLESS PLATING ELECTRODISPLACEMENT Ag + Ag + Nucleation Growth Cu 2+ e - Ag 0 Ag 0 Ag 0 Ag 0 e - Homogeneous Heterogeneous Ag 0 Cu 0 Ox Ox Ox Ox e - e - Ag + Ag + Red Red Red Red Ag + Ag + e - e - Ag + Ag + Cu 2+ e - Copper e - e - e - e - e - e - e - Ag + Ag n 0 e - Ag n 0 Ag + Ag + Ag + Ag + Ag n 0 Ag + Ag + e - Cu 2+ Metal e - Ag 0 Ag 0 Ag 0 Ag 0 Ag 0 Ag 0 cluster Cu 0 Red: C 6 H 8 O 6 , N 2 H 4 Substrate 8
THICK FILM TECHNOLOGY APPLICATIONS OF MONODISPERSED METALLIC PARTICLES Conductive layers a) ‘Fired’/Sintered films • Electronics • Catalysis ‘Green’ layer Metallic layer Metal paste • Biology and medicine Drying Sintering • Pigments substrate • Obscurant smokes b) ‘Non-Fired’ films • Nonlinear optics Metal-filled polymer film • Transparent conductive coatings Metal paste • Ferromagnetic fluids Drying/Curing • High density magnetic storage substrate SECTION THROUGH A MLCC ULTRATHIN METALLIC LAYERS (Up to 800 alternative layers) dielectric layers (~ 6 µ m) metallic layers (~ 1 µ m) Human hair (~ 60 µ m) Dielectric Tape Electrode film 1.0 µ m 9
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