The Improvement in Energy Efficiency Based on Nano-structure Materials

Transcription

1 International Workshop on 1iGO Science and Technology 2010 The Improvement in Energy Efficiency Based on Nanostructure Materials Chien Chon Chen Department of Energy and Resources, National United University, Taiwan

2 Abstract Increases of energy demands and concerns about global warming compel scientists to keep looking for potential renewable energy sources unremittingly. For alternative energy sources, efficiently harvesting solar energy to generate electric power using various photovoltaic technologies have undergone rapid development over the past few years because the sun is the most important inexhaustible and clean energy source. Nanomaterials could be a key materials for energy saving and improvement of solar energy conversion efficiency.

3 AAO template Anodic alumina has called by various names; such as, anodic aluminum oxide (AAO), anodic alumina nanoholds (AAN), anodic alumina membrane (AAM), or porous anodic alumina(paa). When anodized in an acidic electrolyte and controlled in the suitable conditions, aluminum forms a porous oxide with very uniform and parallel cell pores. Its structure is described as a closepacked array of columnar cells. The pore diameters and pore densities of anodic alumina film can be finely controlled by appropriately carrying out the electrochemical process parameters.

4 Fabrication of AAO General purity (99.7%) commercial aluminum is used instead of the highpurity aluminum required in conventional anodization. The disordered arrangements of pores that initially form on the surface of aluminum rearranged by selfdiffusion inside the anodic aluminum oxide (AAO) layer via longterm heat treatment. E ( V ) [ Al +3 ]=10 6 [ AlO 2 ]=10 6 (Corrosion) (Passivation) (Corrosion) Al +3 Al 2 O 3 Al +3 AlO Al 2 O 2 3 Al Al +3 Al Al 2 O 3 (Immunity) Al Al 2 O 3 AlO 2 Al AlO Electrolyte Time (hr) Voltage (V) Temp. ( o C) ph 3 wt.% H 2 C 2 O vol.% H 2 SO 4 3 wt.% H 2 C2O 4 10 vol.% H 2 SO 4 15~20 vol. % H 2 SO 4 3~5%(H 2 C 2 O 4 )+4%(H 2 SO 4 ) 4~8 vol.% H 3 PO 4 15 vol.% H 2 SO 4 3 wt.% H 2 C 2 O 4 0.5M (H 2 SO 4 ) 0.3M (H 2 C 2 O 4 )+0.3M (H 2 SO 4 ) ~25 15~120 40mA/cm ~10 0~10 0~ High anodizing voltage, low electrolyte temperatures and low acid concentrations favor film growth. However, low applied voltage, high acid concentrations and high anodizing temperature favor film dissolution. When the rate of film growth has fallen to the rate of dissolution of the film in the electrolyte, the thickness of the film remains constant.

5 Fabrication of AAO General purity (99.7%) commercial aluminum is used instead of the highpurity aluminum required in conventional anodization. The disordered arrangements of pores that initially form on the surface of aluminum rearranged by selfdiffusion inside the anodic aluminum oxide (AAO) layer via longterm heat treatment. E ( V ) [ Al +3 ]=10 6 [ AlO 2 ]=10 6 (Corrosion) (Passivation) (Corrosion) Al +3 Al 2 O 3 Al +3 AlO Al 2 O 2 3 Al Al +3 Al Al 2 O 3 (Immunity) Al Al 2 O 3 AlO 2 Al AlO Electrolyte Time (hr) Voltage (V) Temp. ( o C) ph 3 wt.% H 2 C 2 O vol.% H 2 SO 4 3 wt.% H 2 C2O 4 10 vol.% H 2 SO 4 15~20 vol. % H 2 SO 4 3~5%(H 2 C 2 O 4 )+4%(H 2 SO 4 ) 4~8 vol.% H 3 PO 4 15 vol.% H 2 SO 4 3 wt.% H 2 C 2 O 4 0.5M (H 2 SO 4 ) 0.3M (H 2 C 2 O 4 )+0.3M (H 2 SO 4 ) ~25 15~120 40mA/cm ~10 0~10 0~ High anodizing voltage, low electrolyte temperatures and low acid concentrations favor film growth. However, low applied voltage, high acid concentrations and high anodizing temperature favor film dissolution. When the rate of film growth has fallen to the rate of dissolution of the film in the electrolyte, the thickness of the film remains constant.

6 Equilibrium for AAO making Teflon has considerable chemical stability in most electrolytes and is strong enough not to have chemical reactions with most of materials When a positive voltage is applied to an aluminum substrate in a suitable electrolyte, pores form on the surface at almost random positions. However, under specific conditions, almost perfect hexagonally ordered pores in anodic alumina can be obtained

7 Selfrepairing of AAO by heat treatment The process makes the random nanochannels to rearrange to an ordered array on AAO even when using a commercial Al as substrate. (a) The holes in AAO were random when Al with a purity of 99.7% was used after anodization; (b) the pores decrease in size towards the center of pore after heat treatment; (c) the pores were expanded by upon pore widening; (d) after longterm pore widening, uniformsized closepacked ordered nanopore on AAO;

8 Various Pore Size of AAO (a) (b) (c) (d) (e) (f) AAO pore size of (a)10 nm and (b)20 nm were fabricated by 10 vol.% H 2 SO 4 electrolyte; (c) 30 nm, (d) 40 nm, (e) 50nm, and (f) 70 nm were fabricated by 3 wt.% C 2 H 2 O 4 electrolyte; (g) 250 nm, (h) 350 nm, and (i) 500 nm were fabricated by 1 vol.% H 3 PO 4 electrolyte.

9 Evaluation of pore density with various AAO pore size Given eight types of AAO with pore diameters of 15, 60, 85, 100, 200, 300, 400, and 500 nm, the pore density can be computed as , , , and , , , , , and pore/cm 2, and Pore area fraction can be evaluated as 15.8, 36.7, 56.7, 59.6, 65.6, 70.1, 75.4, and 92.2% when the pores are closet packing on the AAO.

10 Evaluation of pore density and pore area fraction with various AAO pore size Theoretical results Experimental results Pore diameter (nm) Pore density (pore.cm 2 ) Pore area fraction (%) Pore diameter (nm) Pore density (pore.cm 2 ) Pore area fraction (%) Pore area fraction= (pore density pore area) / unit area Al 2 O 3 area fraction= 1 pore area fraction

11 The schematic diagram of metal/ceramic composite of the experimental sample High density, ordering, and uniform nanowires stands on the AAO would be useful for the device that require high current density or the high surface area application. 1000μm Metal 10μm AAO/ wires Furthermore, keeps Sn metal as the substrate, but remove Al foil, barrier layer, and partial of AAO, so the ordered and short Sn nanowires can expose on the AAO. 0.04μm 1000μm Barrier layer Al

12 Nanowires forming in the AAO, and the partial of nanowires expose on the AAO

13 Nanospheres forming on the AAO Sealed glass tube melt AAO melt oxide oxide AAO melt AAO oxide Schematic diagram nanoparticles melt oxide AAO AAO melt nanoparticles

14 Ag nanowires forming by anodization and electroless deposition The thermoconductivity film was fabricated by the process that deposited silver into AAO template. AAO fabrication process was started from aluminum foil. Aluminum foil (99.7%) was put inside the electrochemical holder forming AAO through electrolyte polishing and anodization in an electrochemical bath. Electropolish aluminum substrate. Anodization. Selfdiffussion of AAO Remove Al and barrier Widen pore diameter of AAO template. A conductive electrode forming by sputtering. Deposited Ag into AAO forming Ag nanowires. Crystallization of Ag nanowires.

15 Theoretical evaluation of AAO surface areas and Ag nanowires/aao thermal resistance Surface area /m nm 60nm 85nm 100nm 200nm 300nm 400nm 500nm Thermal resistance /K.k 1 W nm 60nm 85nm 100nm 200nm 300nm 400nm 500nm Thickness /μm Thickness /μm Silver inside AAO template by electrodeposition method. AAO with aligned silver wires inside can be a low resistance TIM. Thermal conductivity is the quantity of heat transmitted, due to unit temperature gradient, in unit time under steady conditions in a normal direction to a surface of unit area.

16 Thermal resistance R = L KA R: thermal resistance K/W L: material thickness (m) K: Thermal conductivity (W/(m.K)) A : heat transfer area (m 2 ) K silver = 430 W/(m.K) L AAO = 30 μm Ag nanowires Heat sink AAO R AAO, φ15nm = K/W R AAO, φ60nm = K/W R AAO, φ85nm = K/W R AAO, φ100nm = K/W R AAO, φ200nm = K/W Chip R AAO, φ300nm = R AAO, φ400nm = K/W K/W R AAO, φ500nm = K/W The thermal resistance values were very lower than the normal thermal resistance required of 50~ K/W

17 AAO is a suitable template for TIM fabrication. AAO structure of pore size, porosity, pore density, thickness, and surface area (with 30μm thickness) can be controlled in 10~500 nm, 16~92%, 4.7~ pore.cm 2, 0.1~100 μm, and 0.02~0.25 m 2 ranges, respectively. TIM structure with silver inside AAO has theoretical thermal resistance from 2.19~0.77 mk.w 1 range under 15~500 nm AAO/wires. the values were very lower than the normal thermal resistance required of 50~100 mk.w 1 This method of manufacturing TIM with silver nanowires combines the traditional anodization process and electrodeposition with nanotechnology, which can potentially be useful in keeping the cost of thermal conductor materials fabrication down.

18 Characterization of porous WO 3 electrochromic device cell When voltages between 100 to 160V were applied to tungsten film for 1 hr under 0.4 wt. % NaF electrolyte, porous WO 3 was formed. The film, which had a large surface area, was used as electrochromic film for EC glass. The film, which had a large surface area, was used as electrochromic film for EC glass. The average transmittance in the visible region of the spectrum for a 144 cm 2 EC device was above 75% in the bleached state and below 40% in the bleached state. The average transmittance in the colored and bleached states was 15.7% and 60.2%, respectively.

19 Electrochromic devices with a configuration consisting of glass/ito/wo 3 /1M LiClO 4 PC/Pt NiO/ITO/glass and a size of 144 cm 2 were obtained by assembling two pieces of coated glass as follows. The two electrodes were assembled into a sandwich type cell and sealed with a hotmelt film (SX1170, Solaronix, thickness 0.1 mm). Electrolyte was injected into the space between the two electrodes with a syringe to fabricate an EC glass device. The device was then sealed with vacuum glue (Torr Seal).

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22 Morphologic Characterization of Anodic Titania Nanotube Arrays for DyeSensitized Solar Cells A dyesensitized solar cell (DSSC) comprises an anode, an electrolytic solution and a cathode, The electron transfer path in DSSC can be explained as: electron is generated from dye and then transfers to TiO 2, TCO, external circuit, TCO/Pt, and electrolyte. The transfer characteristics of ion and electron in DSSC can be simulated using an equivalent circuit which included charge transfer resistances, chemical capacitance, contact resistances, and double layer capacitances, and Warburg impedance. The properties of DSSC equivalent circuit was detected and analyzed by EIS technique.

23 DSSC Structure

24 IVcurve measurements Computer GPIB Source meter IV curve AM1.5 solar simulator Sample

25 Photovoltaic performance of the NT DSSCs as a function of the tube length (L) under AM1.5 illumination % L /μm % 0.25 % 0.15 % Absorbance μm (a) 26μm μm 28μm μm 14μm 0.6 9μm 6μm t /h Wavelength /nm t /h L /μm J SC /ma cm V OC /V FF η (%) Current /ma (b) 30 μm 26 μm 28 μm 23 μm 14 μm 18 μm 9 μm 6 μm Voltage /V

26 The Analysis of Interface Impedances of DyeSensitized Solar Cell Ti R a Ti0 2 Electrolyte ITOPt C 1 Ti0 2 NT R 2 C 2 C 3 C 4 R 0 R 1 R 3 R 4 R b Ti0 2 Load L 0 C 2 C 4 C 3 R a TiO 2 TiO 2 / Electrolyte Electrolyte (Pt/ITO)/ Electrolyte R b ITO C 1 R 3 R 2 R 4 L 0 Wire R 0 Ti Ti/ TiO 2 R 1

27 TiO 2 NT Surface Area

28 Current DensityVoltage Curves of 12 TiO 2 NT DSSC Current Density/ ma cm % sun 88% sun 64% sun 25% sun 11% sun 3% sun 0% sun Voltage/ V

29 Bode Plots of TiO 2 NT DSSC 5 0% sun Log impedance/ Ohm % sun 11% sun 25% sun 64% sun 88% sun 100% sun Log frequency/ Hz

30 0 DSSC without illumination (dark) Z / Ω 0.5 L 0 R 0 R b C C C 3 R C R Measurement Fitting R R

31 0 DSSC under AM1.5 illumination 1.E05 C L 0 R 0 R b 9.E06 6.E C 2 R 1 3.E R E E E E C C Measurement Fitting R 3 R

32 EIS experimental data fitting Elements L 0 (μh) R 0 (ohm) R a (ohm) R b (ohm) C 1 (μf) R 1 (ohm) C 2 (μf) R 2 (ohm) C 3 (μf) R 3 (ohm) C 4 (μf) R 4 (ohm) Dark % AM % AM % AM Ti R a Ti0 2 Electrolyte ITOPt C 1 Ti0 2 NT R 2 C 2 C 3 C 4 R 0 R 1 R 3 R 4 R b Ti0 2 Load L 0

33 Conclusions The principal methods of fabricating nanochannel templates Al 2 O 3, WO 3, and TiO 2 include bottomup and selfassembly processes. The template synthesis method can produce nanomaterials of uniform size easily. Metal casting and anodizing are wellestablished metal processing method and has been used in metallurgy for several decades. The experiences gained from this traditional fabrication processes combined with emerging nanotechnology methodologies can potentially be useful in keeping the cost of nanomaterials fabrication low, improve the fabrication efficiency and reduce the overall complexity of the fabrication process.

34 Thank you