EE C245 ME C218 Introduction to MEMS Design Fall 2007

Transcription

1 EE C245 ME C218 Introduction to MEMS Design Fall 2007 Prof. Clark T.-C. Nguyen Dept. of Electrical Engineering & Computer Sciences University of California at Berkeley Berkeley, CA Lecture 20: Lossless Transducers EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 1 Announcements Hand back graded midterm today Midterm Statistics: Max. Possible Score Top Score Average Standard Deviation Median Come to my office if you would like to see the details of your Z-score EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 2 1

2 Lecture utline Reading: Senturia Chpts. 10, 6 Lecture Topics: Project Description Energy Conserving Transducers Charge Control Voltage Control Linearizing Capacitive Actuators EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 3 Project Description EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 4 2

3 Go Through the Project Handout EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 5 Micro-Scale Power Generation EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 6 3

4 Micro-Scale Power Generation Goal: generate power at the micron scale with superior energy density compared to batteries Motivation: enable standalone micro sensors and micro actuators with wireless communication pursuant to realizing large wireless sensor networks Actuators Sensors Fuel storage Heat engine/ Fuel reformer Thermal/Exhaust processor TE Converter/ Fuel cell ASIC/CPU RF/ptical Comm 1 mm Approach: harness fuels with energy density Li Battery Methanol Ethanol Diesel Gasoline Methane Propane Energy Density (kw-hr/kg) EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 7 Approach: Fuel Cell Common elements among fuel cells: fuel storage/delivery anode and cathode electrodes catalyst to dissociate fuel (e.g., into and e - ) at anode and combine products at cathode ion exchange medium (i.e., electrolyte) Chemical Energy Fuel Storage Chemical Energy Fuel Delivery Porous Anode Electrode e - Catalyst C e - (e.g., platinum) 2 2 Load Electrolyte 20 50% eff. Porous Cathode Electrode Electrical Energy EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/ V out - Electrical Energy 4

5 Metal-Hydride Micro Fuel Cell bjective: provide 3.2V output continuous power with 30mA 100ms pulses for comms for 3 month operation Specific energy: 0.95 W-hr/g Water and metal hydride powder fuel kept separate until power is needed shelf life > 10 years Tiny Tiny Fuel Fuel Cell Cell Regulating Check Check Valve Valve LiAlH LiAlH 4 Fuel 4 Fuel [Honeywell] For 3 month operation: need 0.9g of LiAlH 4 (1.4cc) Need 1.6g of (1.6cc) Polymer Block Block EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 9 Hydrogen Generator/Regulator LiAlH 4 + reaction rate regulated by a pneumatic valve a completely mechanical feedback system requiring no electrical power here 2 here consumed Pressure rises rises by by fuel fuel cell cell Membrane deflects Pressure drops drops Water Valve Valve opens opens again Water Valve Valve closes closes again evaporates Diaphram e - 2 e - + V out Water Chamber Seal Valve Disk generated 2 when when 2 reaches LiAlH LiAlH 4 powder 4 LiAlH 4 Powder Chamber 2 generated EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/

6 Metal-Hydride MFC Performance Right: actual AMPGen hydride fuel cell, including fuel storage Performance: (as advertised) steady hydrogen regulator steady 3.2V output voltage under a 0.5mA load Series Series Connection of of 5 Micro Micro Fuel Fuel Cells Cells Vfc (Volts), H2 Pressure (psi) over-pressure utput Volts (Load current 70uA) Time (days) Air Humidity Air Temperature EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/ [Honeywell] Air RH (%), Air T ( C), Valve Position (um) Total Mass: Total Volume: Total Energy: Energy Density: Specific Energy: 3.36 g 3.55 cc W- W-hrs hr/cc 0.95 W-hr/g 5X 5X Better!!! Compare: CR2430 Li Li Battery 4.6g, 4.6g, 1.3cc, 1.3cc, W-hrs W-hrs W-hr/cc, W-hr/g Metal-Hydride Micro Fuel Cell bjective: provide 3.2V output continuous power with 30mA 100ms pulses for comms for 3 month operation Specific energy: 0.95 W-hr/g Water and metal hydride powder fuel kept separate until power is needed shelf life > 10 years Tiny Tiny Fuel Fuel Cell Cell Regulating Check Check Valve Valve LiAlH LiAlH 4 Fuel 4 Fuel [Honeywell] For 3 month operation: need 0.9g of LiAlH 4 (1.4cc) Need 1.6g of (1.6cc) Polymer Block Block EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/

7 Empty Water Chamber peration LiAlH 4 + reaction rate regulated by a pneumatic valve a completely mechanical feedback system requiring no electrical power get get very very dry dry Pressure rises rises condition on on left left Membrane deflects water water diffuses Valve Valve closes closes towards left left Diaphram e - e V out Water Removed Seal Valve Disk generated 2 when when 2 reaches LiAlH LiAlH 4 powder 4 EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 13 LiAlH 4 Powder Chamber 2 generated - Power (uwatts) Specific Energy [W-hr/g] CR2430 Li Battery Water-Less peration Improves AMPGen Performance Water-less peration of AMPGen prototype with LiAlH4 fuel Time (days) 12x 5x LiAlH4 w/ Water 14x LiBH4 w/ Water LiAlH4 No Water Power Source Type 35x Water-less operation for for days, days, stepping up up power power level level from from 0.05, 0.05, 0.16, 0.16, and and to to mw mw Extracting water water from from exit exit fuel fuel air air can can still still yield yield >0.2mW Volumetric Energy density 2.4x 2.4x LiAlH4 LiAlH4 AMPGen W-hr/ccW Specific energy energy 2.7x 2.7x LiAlH4 LiAlH4 AMPGen W-hr/gW LiBH4 No Water This This is is 14x 14x than than the the CR2430 Li Li Battery! EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/

8 Radioisotope Power Sources? EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 15 Why Radioisotope Fuels? For micro-scale systems, energy and power density are paramount Micro-Scale Power Generation (MPG) Li Battery Methanol Ethanol Diesel Gasoline Methane Propane ~5-10 ~5-10 W-hr/g W-hr/g Specific Energy Density [W-hr/g] Methanol Diesel Gasoline Propane Pu-238 (80% Pu) Pm-147 Tl-204 Sr-90 U-235 Deuterium Radio Isotope Micro-Scale Power Sources (RIMS) Logarithmic Scale ~500-10,000 W-hr/g W-hr/g Radioisotopes can can store store orders orders of of magnitude more more energy energy in in a given given volume! ,000 10, ,000 1,000,000 10,000,000 Specific Energy Density [W-hr/g] EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/ ,000,000 8

9 Radioisotopes: Tiny Suns The Sun Equivalent to to a tiny tiny sun sun Half-life up up to to yrs! yrs! Radioisotope Photon Load i p h+ hν ~1.1eV ~1.1eV h+ h+ ~ kev kev beta h+ h+ h+ Fixed Charge E-Field alpha ~5 ~5 MeV MeV Problem: High α energy can can damage the the semiconductor e- e- n e- e- e- e- Fixed greatly reduces Charge converter lifetime EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 17 Previous Radioisotope Power Sources Problem: past radioisotope power sources had been plagued by numerous deficiencies (e.g., low efficiency, low damage threshold) that limited them to low output power Example: betavoltaic (attaining efficiency ~ 1.7%) i Radioisotope β h+ e- Betas Betas going going upward upward unused unused p n β Betas Betas absorbed absorbed within within radioisotope radioisotope Fixed Charge E-Field Silicon Silicon pn pn junction junction Fixed Charge Energy Energy lost lost Energy Energy lost lost to to heat heat Significant Significant leakage leakage current current High High energy energy particle particle can can damage damage the the semiconductor semiconductor Increase Increase in in recombination recombination Poor Poor Efficiency Efficiency Low Low utput utput Power Power EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/

10 Benefits of Miniaturization Conventional Betavoltaic Cell: Radioisotope β Fixed h+ p Charge Miniaturize & Retain E-Field Safe Betas i MEMS-Based Betavoltaic Cell: p Radioisotope e- n Fixed Charge n Miniaturize & Use Higher Charging Gas Cell: Energy Alphas Alpha Alpha Particles Particles Allowed Deep Allowed Deep RIE RIE Trenches Trenches E-Field 5 5 MeV MeV particle particle energy energy high high surface-to-volume surface-to-volume Radioisotope High Work Function Metal gas gas absorbs absorbs energy energy power power density density power power density density III-V III-V Semiconductor Semiconductor + Tiny Tiny Chamber Chamber Volume Volume efficiency efficiency ion allows allows high high pressure pressure better better damage damage resilience resilience α i better better ionization ionization eff. eff. e- Micro-Scale Micro-Scale Plate Plate Spacing Spacing Result: Result: output output allows allows high high E-field E-field power power using using smaller smaller reduces reduces ion-electron ion-electron radioisotope radioisotope volume High Press. Gas recombination volume Low Work Function Metal recombination EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 19 i Go Through the Project Handout EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/

11 Energy Conserving Transducers EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 21 Basic Physics of Electrostatic Actuation Goal: Determine gap spacing g as a function of input variables First, need to determine the energy of the system Two ways to change the energy: Change the charge q Change the separation g ΔW(q,g) = VΔq + F e Δg dw = Vdq + F e dg Note: We assume that the plates are supported elastically, so they don t collapse EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/

12 Here, the stored energy is the work done in increasing the gap after charging capacitor at zero gap Charge-Control Case Find force and voltage: Need stored energy Can find by recognizing that the energy in the final state is just the energy stored in capacitor charged to q EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/07 23 Having found stored energy, we can now find the force acting on the plates and the voltage across them: Charge-Control Case + - V EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/

13 Voltage-Control Case Practical situation: We control V Charge control on the typical sub-pf MEMS actuation capacitor is difficult Need to find F e as a partial derivative of the stored energy W = W(V,g) with respect to g with V held constant? But can t do this with present W(q,g) formula Solution: Apply Legendre transformation and define the co-energy W (V,g) EE C245: Introduction to MEMS Design Lecture 20 C. Nguyen 11/6/