Nanoparticle Coating in the Gas Phase

Transcription

1 Nanoparticle Coating in the Gas Phase Ruud van Ommen Product & Process Engineering Dept. of Chemical Engineering Faculty of Applied Sciences Delft University of Technology Challenge the future Particle Technology course 2010 Delft University of Technology Current Nanoparticle Synthesis Current practice q-dot synthesis: Transient fast batch reaction Huge gradients in reactor during synthesis Unsuited to investigate growth mechanism Poor Scalability Δ Precursors solvent ligands Monomers Brian Yen, MIT 2006

2 Reactor Eng. of Nanoparticle Synthesis gas-phase synthesis liquid-phase synthesis Chemseddine et.al., Eur.J.Inorg. Chem (Scalable, wide PSDs) (Narrow PSDs, not scalable) Murray et.al., JACS 1993 Beetstra et al, Chem. Vap. Dep Kreutzer et al, Anal. Chem Coating nanoparticles: approach? How to provide objects with an ultrathin film? Learn from semi-conductor industry Liquid phase coating Chemical Vapour Depisition (CVD) Atomic Layer Deposition (ALD) Gas phase coating: Less contamination, Less separation problems, Easier to scale-up 2 μm Pore et al., J. Am. Chem. Soc. 131 (2008) 3478

3 Atomic Layer Deposition A B A B A B A B A B etc. Number of cycles determines layer thickness

4 Atomic Layer Deposition Deposition of alumina layer using tri-methyl aluminum (TMA) and water: A Al-OH + Al(CH 3 ) 3 (g) Al-O-Al(CH 3 ) 2 + CH 4 (g) 2 Al-OH + Al(CH 3 ) 3 (g) ( Al-O) 2 -Al-CH 3 +2 CH 4 (g) B Al(CH 3 ) 2 + 2H 2 O(g) Al(OH) CH 4 (g) Al-CH 3 + H 2 O (g) Al-OH + CH 4 (g) A B A B A B A B etc. Periodic table of ALD Status August 2004 Puurunen, J. Appl. Phys. 97 (2005) 1

5 Applying ALD to particles Particles ~ 100 μm are industrially coated in a fluidized bed (pharma, food, etc.) This also be applied to nanoparticles using Atomic Layer Deposition (ALD) J.R. Wank, S.M. George, A.W. Weimer, J. Am. Ceram. Soc. 87 (2004) 762 How is it possible to fluidize nanoparticles? gas

6 Geldart s Fluidization Map (ρ p ρ g ) 10 3 [kg/m 3 ] Nanoparticles d ~ nm C A B Nanoagglomerates d ~ 200µm d p [μm] D Classification of Nanofluidization Agglomerate Particulate Fluidization (APF) Smooth fluidization, low U mf, negligible elutriation Large bed expansion (3 5 times), minimal bubbling even at high gas velocities Examples: Degussa Aerosil R974 (hydrophobic, d p = 12 nm) Degussa Aerosil A200 (hydrophilic, d p = 12 nm) Agglomerate Bubbling Fluidization (ABF) Exhibits slugs, ratholes, difficult to fluidize at low gas velocity Small bed expansion (< 2 times), high U mf, large bubbles, substantial elutriation Examples: Degussa Aeroxide TiO 2 P25 (hydrophilic, d p = 21 nm) Degussa Aerosil A90 (hydrophilic, d p = 20 nm). Wang, Y.; Gu, G.; Wei, F.; Wu, J. Powder Technology. 2002, 124,

7 Pressure Drop / Bed Weight Typical APF and ABF Fluidization Behaviour 1 APF Aerosil R974 (sieved to < 500µm) Umf=0.23cm/s ΔP u mf =0.23 cm/s Bed expansion ratio Pressure ( Increasing Ugas) Pressure (Decreasing Ugas) Bed Expansion (Increasing Ugas) Bed Expansion (Decreasing Ugas) Superficial Velocity (cm/s) Smooth liquid-like fluidization Large bed expansion Very low U mf Bed expands before U mf No bubbles observed at moderate U 0 Negligible elutriation Desirable Bed Expansion Ratio (H/Ho) Non-dimensional Bed [-] ABF Nanopowders (sieved) Gas Velocity, [m/s] Aerosil 90 Aeroxide TiO2 P25 Aeroxide Alu C Bubbling, Group B like fluidization Low bed expansion High U mf Expansion remains const. at U mb High elutriation Undesirable! Hierarchical Structure of Fluidized Fractal Agglomerates of Silica Nanoparticles Complex Agglomerate Simple agglomerate Large (d a ~ µm) complex agglomerates Simple agglomerates (d p ~ µm) Primary agglomerates (~ 1-4 µm) Net-like structures chains of primary 12 nm silica NPs Chains and nets of nanoparticles

8 Hierarchical Structure of Fluidized Fractal Agglomerates of Silica Nanoparticles 2R max N=10,000 Simulated diffusion limited agglomerate SEM image of silica nano-agglomerate Behavior of nanoagglomerates is substantially different from free-flowing particulate systems. An estimate of mean size, voidage, and density of porous agglomerates composed of individual nanoparticles is desired. Assisted Nanofluidization NP agglomerates have been successfully fluidized using: Vertical vibration Stirring Sound waves Rotation (in a rotating fluidized bed) Collision with magnetic particles excited by an oscillating magnetic field Alternating electric field Secondary flow using a downward facing micro-jet

9 Intermezzo: Pulsed fluidization of wet micron-size particles gas velocity Akhavan et al., Ind. Eng. Chem. Res., 48, pp , Sound Assisted Nanofluidization Movie Fluidization Advantages: Will not contaminate the nanoparticles Smooth fluidization is obtained at very low Umf for APF type nanopowders Disadvantages: May not work well for ABF nanopowders Difficult to scale-up Large bubbles form at certain frequencies

10 Magnetically Assisted Nanofluidization Fluidization Behaviour of Hard Agglomerates Movies Without magnetic field With magnetic field U U 0 =13.15 cm/s cm/s U 0 =0.94 Cannot be fluidized Rotating Fluidized Bed (RFB) Fluidization air Adhesion Drag force Buoyancy Drag force Buoyancy Gravity Fluidization air Gravity Fluidization air Centrifugal force Conventional FB Rotating FB

11 Group A/C Classification in a Rotating Fluid Bed Density difference ( p- f) (g/cm 3 ) At higher g the transition curve between group A/C shifts to the left so that much smaller particles can be fluidized in a RFB 3 π d p 4.8 Adas ( ρp ρ f ) g ( ε 1) = z 8µm glass beads "g" 19 "g" C 100 g 7 "g" 1 "g" ε = 0.35 A 8 μ m glass beads 7 μm alumina particles 1 g P (Pa) Bed Mass: 0.2 kg (8 μm glass beads) 325 rpm (7 "g") 525 rpm (19 "g") 19 g 7 g Mean particle size d p (μm) U g (m/s) 8µm glass beads fluidize as Group A particles at 7 g and 19 g From: Qian,G-H., I. Bagyi, I. Burdick, R. Pfeffer, H. Shaw, J. Stevens, AIChE Journal, 47(5), , SEM-EDX: micromixing for rotating bed Scanning Electron Microscopy - Energy Dispersive X-ray spectroscopy Particles: Silica (12 nm) and Alumina (13 nm) G 0 = 40; U 0 /U mf = 1.5; mixing time = 6 min 100 μm 100 μm The images show that mixing occurs at the micron scale, and not at the nano scale for rotating bed fluidization. Nakamura and Watano, Powder Technology, 183 (2008)

12 Earlier findings: Micro-jets Micro-jets (nozzle diameter μm) are very effective in assisting nanofluidization Downward pointing jets are most effective Add alcohol to nitrogen flow to avoid static effects Pfeffer, R., Fluidization of nanopowders (PTF award lecture), presented at the AIChE Annual Meeting, Philadelphia PA, USA, November 16-21, paper 369a, 2008 Micro-jet assisted nanofluidization: Aerosil R974 (APF) Non-dimensional Bed Height, [H/H0] Gas Velocity, [m/s] Jet Assisted-9.5 g Jet Processing-9.5 g Jet Assisted-13 g Conventional-20 g Jet Processing-20 g Jet processed Conventional Conventional-9.5 g Conventional-13 g Jet Processing-13 g Jet Assisted-20 g Nozzle pressure: 8.3 bar Jet velocity: 320 m/s (sonic) 50x bed expansion with jet (5x bed expansion without) Bubbling hindered with jet From: J. A. Quevedo, A. Omosebi, R. Pfeffer, AIChE J. 56 (2010)

13 U jet = 0 m/s U jet 300 m/s Exp. work: Sam Johnson Particle Diameter [μm] (10 nm) Gas-phase coating of particles ALD ALD (Atomic Layer or MLD Deposition) CVD CVD (Chemical Vapour Deposition) (1μm) (10μm) Film Thickness [nm] King, Liang, and Weimer, ALD review paper, Journal of the Electrochemical Society, 2010 Number of cycles determines layer thickness Residence time determines layer thickness

14 Examples of ALD-coated NPs ZnO TiO 2 TiO 2 TiO 2 TiO 2 5 nm TiO 2 20 nm SiO 2 TiO 2 TiO 2 ZnO From Weimer & co-workers Si 5 nm 3 N 4 Setup Fluidized bed: diameter 26 mm 120 g powder 160 C & 1 bar

15 Two examples Coating cathode materials for Li-ion batteries Coating catalyst particles for attrition resistance

16 Delft University of Technology Challenge the future Amsterdam Brussels 12 h Lyon 5 min maps.google.com

17 Improving Li ion batteries current equipment electrolyte membrane Smaller Coating Nanoparticles: Faster Reduce charging aging Particles used in this study Material: LiMn 2 O 4 Primary particles with a diameter of nm Static aggregates with a size of µm 20 μm 2 μm Beetstra et al., Chem. Vap. Dep. 15 (2009)

18 TEM pictures of results 20 nm 20 nm 5 nm Uncoated Coated (5 ALD cycles) Obtained at atmospheric pressure! Beetstra et al., Chem. Vap. Dep. 15 (2009) Energy Dispersive X-ray Spectrocopy A B A B 5 nm Beetstra et al., Chem. Vap. Dep. 15 (2009)

19 Results: BET analysis # ALD Cycles Surface area [m 2 /g] D equiv [nm] Initial material: Primary particles of nm Static aggregates of µm Beetstra et al., Chem. Vap. Dep. 15 (2009) Two examples Coating cathode materials for Li-ion batteries Coating catalyst particles for attrition resistance

20 Coating of particles for catalysis Strengthen the carrier particles Attrition during fluidization reduced HOWEVER Reduction of reactant diffusion Activity reduced Very thin layer needed Experimental approach 2 H 2 3 H 2 Pd/BaSO 4 Pd/BaSO 4 naphthalene tetralin decalin Catalyst: Pd 5wt% (~10 nm) on BaSO 4 BET surface area: 3.9 m 2 /g volume fraction 12% 8% 4% Particle size distribution Apply 5 ALD cycles to coat with Al 2 O 3 0% diameter (µm) van Ommen et al., AIChE annual meeting 2009

21 Results coating Atomic layer deposition: TEM pictures 5 nm 5 nm Non-coated 5 ALD cycles coated Results coating Atomic layer deposition: TEM pictures Conformal amorphous layer Thickness ~ 2 nm Growth per cycle: 0.4 nm/cycle 20 nm

22 EDX EDX analysis shows the presence of aluminum 20 nm

23 Results Atomic Force Microscopy nm 70 uncoated nm 8 coated nm nm van Ommen et al., AIChE annual meeting 2009 Exp. approach activity test 2 H 2 3 H 2 Pd/BaSO 4 Pd/BaSO 4 naphthalene tetralin decalin Reaction conditions: Feed: 3wt.% naphthalene solution in n-decane Catalyst: 1g of Pd/BaSO 4 non-coated or coated with alumina Temperature: 150 C Pressure: 50bar of H 2 Reaction time: 5 hours 18 GC samples taken

24 Results activity test Naphtalene conc. (wt.%) 2,5% 2,0% 1,5% 1,0% 0,5% 0,0% coated uncoated Time (min) van Ommen et al., AIChE annual meeting 2009 Approach attrition test Same fluidized bed columns as used for coating 20 g sample (4 cm settled bed height) 60 min of fluidization at 9.4 cm/s (higher than during coating)

25 Results attrition test Particle size distributions uncoated coated Volume fraction Before attrition test After Volume fraction [-] Before After diameter (µm) diameter (µm) The coating protects the particles from attrition van Ommen et al., AIChE annual meeting 2009 Conclusions Nanoparticles can be fluidized as dynamic agglomerates. Fluidization quality can be much improved with the aid of an external force field such as vibration, sound waves, magnetic agitation, a centrifugal field, or a microjet. Successful coating of nanoparticles with ALD. Aging of cathode material for Li-ion batteries can be reduced.