Alumina Nanohoneycombs with Controlled Spatial Ordering of Pore Channels Fabrication, Theory and Properties K.Y. Ng, C. Cheng and A.H.W.

Transcription

1 Alumina Nanohoneycombs with Controlled Spatial Ordering of Pore Channels Fabrication, Theory and Properties K.Y. Ng, C. Cheng and A.H.W. Ngan Department of Mechanical Engineering, University of Hong Kong

2 Anodized Aluminium Oxide (AAO) or Porous Aluminium Oxide (PAA) - a nano-pore-channel material: Grain 1 Grain 2

3 Examples of AAO Applications: As template for nanotube/nanowire arrays: CNT arrays [Hu et al. APL, 2001] Bi 2 Te 3 nanowire arrays [Sander et al. Adv. Mater., 2002] Electroosmotic pump [Vajandar et al. Nanotech. 2007]

4 Ammonia gas sensor [Varghese et al. Sensors & Actuators B, 2003] Physisorption of ammonia onto AAO changes impedance and phase angle:

5 Sea-water desalination [Chang et al., Int. Conf. on Miniaturized Systems for Chemistry and Life Sciences, Seattle, 2011] Overlap of electric-double-layers blocks one ion-species Charging the membrane switches on the EDL and lowers the conductivity of water flowing through:

6 Industrial-scale applications of AAO require controllable (i.e. selectively good or bad) pore-channel ordering over wide areas of AAO film.

7 Some examples of AAO produced in the literature: Poorer pore-channel ordering Better pore-channel ordering Variations in quality exist! Crouse et al. APL, (2000) Jessensky et al. APL, (1998) Jessensky et al. APL, (1998) Su and Zhou, Adv. Mater. (2008)

8 Known factors of AAO growth: Electrolyte and concentration Temperature Voltage Time Surface pre-texture Surprisingly, Al substrate orientation has not been studied!

9 This talk will cover: (1) A method to produce large-area homogeneous AAO films with selective pore-channel ordering. (2) Mechanical properties of AAO films.

10 Our Experiments: Al substrates of grain size from mm to cm were produced by recrystallization of high-purity Al. These were used as substrates for anodization.

11 Different ordering of AAO pore channels was observed on Al grains with different orientations:

12 Cross-sectional TEM: Near top of AAO film: Near bottom of film:

13 Quantifying the pore ordering - RDF Pore i

14 Over 100 grains studied. Ordering of pore channels found to be in the sequence: [001] > [111] > [101] of Al substrate orientation

15 Substrate Orientation vs Other Factors Electrolyte used: oxalic acid

16 Conclusions (1): By carefully conditioning the crystallographic orientation of the Al substrate, mm to cm (or even larger) AAO with homogeneous yet selective ordering of pore-channels can be produced. Ordering in the sequence [001] > [111] > [101] is not explainable by mechanical stresses or crystal symmetry (wrong sequence).

17 Theoretical Modeling of Pore Growth: o/e interface j O, o/ e j Al, o/ e j O, ox j Al, ox

18 Simulation Results Order and disorder domains of the model parameters exist: Nonporous Initial configuration: After 300s: Porous structures after anodization for 300s starting from the same initial configuration under conditions of: (a) B Al = 0.12 A/m 2 ; (b) B Al = 0.36 A/m 2 ; (c) B Al = 0.54 A/m 2 ; (d) B Al = 0.78 A/m 2. (40 V; B O = A/m 2 ; k O /k Al = 1.5; k O = 3.8 nm/v; β = 3/7)

19 Self-ordered Growth Simulation conditions: 40 V B O = A/m 2 B Al = 0.72 A/m 2 k O /k Al = 1.5, k O = 3.8 nm/v β = 3/7 All 3 simulations are at same conditions, except the spacing of initial pores: Periodic boundary conditions used on left and right

20 The Substrate Influence Effect of b : o/e interface Ordered porous j O, ox j Al, ox Disordered porous (oxidation) m/o interface (ionization) On increasing b, porous structure can transit from unstable (disordered) to stable (ordered). R4 is needed to relieve volume expansion on oxidation due to R3. b = the ratio of rates of R4 to R3 b should be affected by the substrate orientation. b s effect on pore growth stability can explain the observed dependence of ordering on Al substrate orientation.

21 Ordered porous Disordered porous b = ratio of Al ionization rate to oxidation rate at m/o interface High b means more Al ionized and migrate away, than oxidized by incoming oxygen ions, and so the resultant oxide should contain a higher O:Al ratio. Experimental EDAX measurements indeed show that Oxygen content in ordered AAO is typically ~3% higher than disordered case: Theoretical analysis shows that a rise of Db ~ 0.1 would cause the Oxygen content to increase by ~ 4%, so results are agreeing.

22 Conclusions (2): A kinetics model based on ion migration across the Cabrera-Mott barriers at o/e interface, and a fixed ratio b of Al and O ion currents, is proposed and numerically implemented. The model predicts domains of system parameters in which pore-channel growth is self-ordered or disordered. This is the first ever successful attempt to simulate deep pore growth of AAO. Dependence of b value on Al substrate orientation could be a factor leading to the observed dependence of pore ordering on substrate orientation.

23 This talk will cover: (1) A method to produce large-area homogeneous AAO films with selective pore-channel ordering. (2) Mechanical properties of AAO films.

24 Nanoindentation on AAO with various spatial ordering Highly Ordered AAO Intermediate Ordered AAO Poorly Ordered AAO 20mm

25 Nanoindentation on AAO with various spatial ordering Increasing pore ordering

26 Deformation Microstructure of AAO In general, two systems of cracks observed in an indent: bilinear and median. Cross-section Plan-view Very sharp elastoplastic boundary: deformation appears to be all-or-none.

27 All-or-none Deformation Mildly deformed region: columns tilted Heavily deformed region: columns collapsed Very clear-cut elastoplastic boundary No deformation at all Also seen in micro-pillar compression: 500nm 500nm

28 All-or-none Deformation Energy Consideration (Ng, Lin, Ngan, 2011) d (i) Tilting Energy: z r elastic energy in unaltered zone plastic energy in tilted zone Suppose tilting is mainly resisted by a critical wall friction t c, so Equilibrium reached when i.e. Residual thickness of tilted zone A column approximation q in reasonable agreement with experiment. z r also seen to thicken with d.

29 All-or-none Deformation Energy Consideration d (ii) Full Compaction unaltered zone compaction energy tilting energy z r Suppose compaction of tilted thickness z c takes place against a constant stress s c : Equilibrium when Compacted zone thickness: From experiment, z r indeed seen to decrease with increasing d. Transition from tilting to compaction predicted to occur at

30 Summary of Predictions: Tilted zone Crack Compacted zone d q z r d z r Elastoplastic boundary Tilted zone thickness: Compacted zone thickness: increasing with d. Angle of tilting: decreasing with d.

31 Anomalously High Friction at Small Loads (Wang, Ngan, Ng, 2012) Different scaling of tangential and normal force w.r.t. D due to the discontinuous structure: μ = F T F N D D 2 1 F N

32 Related Publications: 1) K.Y. Ng, Y. Lin and A.H.W. Ngan, (2009), Deformation of anodic aluminum oxide nano-honeycombs during nanoindentation, Acta Materialia, 57, ) K.Y. Ng and A.H.W. Ngan, (2011), Precise control of nano-honeycomb ordering over anodic aluminum oxide of centimeters-sq. to achieve consistent sensor performance, Chem. Mater. 23, ) C. Cheng and A.H.W. Ngan, (2011), Modeling and simulation of anodic porous alumina formation, Electrochimica Acta 56, ) C. Cheng and A.H.W. Ngan, (2011), Quantitative characterization of acid concentration and temperature dependent self-ordering conditions of anodic porous alumina, AIP Advances, 1, ) S. Wang, A.H.W. Ngan, K.Y. Ng, (2012), Anomalous load-dependence in single-asperity tribological behaviour of anodic aluminium oxide nanohoneycombs, Scripta Mater. 67,

33 ~ End ~