Development of a Oxygen Sensor for Marine ... - DTU Nanotech

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16 CHAPTER 3. THEORY OF THE CLARK SENSOR Anodic reaction: Overall reaction: O2 + 2H2O + 2e − → H2O2 + 2OH − H2O2 + 2e − → 2OH − O2 + H2O + 4e − → 4OH − Ag + Cl − → AgCl + e − 4Ag + O2 + 2H2O + 4Cl − → 4AgCl + 4OH − Two pathways are possible for the reduction of oxygen at the noble metal surface. One is a 4-electron pathway where the oxygen in the bulk diffuses to the surface of the cathode and is converted to H2O via H2O2. The other is a 2-electron pathway where the H2O2 diffuses directly from the cathode surface into the bulk liquid. Also as can be seen above from the reaction, hydroxyl ions are constantly being substituted for chloride ions once the reaction starts, therefore KCl or NaCl have to be used as an electrolyte. However this electrolyte will slowly be depleted of Cl − , so something has to be done to keep this depletion from occurring to fast, or to replenish it somehow. Hence this is one of the problems that have to be faced when building a Clark sensor, as it can be one of the main limits for the lifetime of the sensor. 3.1.1 One layer electrode model Now three assumptions will be used to look at a one dimensional diffusion equation[26]: 1. The cathode is well polished and the membrane is tightly fit over the cathode surface such that the thickness of the electrolyte layer between the membrane and the cathode is negligible. Therefore the membrane will define the oxygen flux to the cathode surface. 2. The liquid around the sensor is well agitated so the partial pressure of oxygen at the membrane surface is the same as that of the bulk liquid. 3. Oxygen diffusion occurs only in one direction, perpendicular to the cathode surface.
3.1. CHEMISTRY OF THE CLARK SENSOR 17 PSfrag replacements y Cathode Membrane Liquid x = 0 x = dm Figure 3.3: One layer electrode model to illustrate the three assumptions, as well as to help derive the equations based on them. Using Fick’s second law and the coordinate system from Figure 3.3, the unsteady diffusion in the membrane will be described by: ∂p ∂t ∂ = Dm 2p ∂x2 p0 x (3.1) Where Dm is the oxygen diffusivity in the membrane, p the partial pressure of oxygen in the membrane, and x is the distance from the cathode surface. Under the assumption that diffusion have yet to take place, the initial boundary conditions are: p = 0 at t = 0 (3.2) p = 0 at x = 0 (3.3) p = p0 at x = dm (3.4) where dm is the membrane thickness and po is the partial pressure of oxygen in the bulk liquid.
Page 1 and 2: Development of a Oxygen Sensor for
Page 3: Abstract This thesis described the
Page 7 and 8: Contents 1 Introduction 1 1.1 Disso
Page 9 and 10: CONTENTS vii B Fabrication Process
Page 11 and 12: Chapter 1 Introduction Roughly 70 %
Page 13 and 14: 1.1. DISSOLVED OXYGEN LEVEL 3 K = a
Page 15 and 16: 1.2. THESIS OUTLINE 5 In Chapter 8
Page 17 and 18: Chapter 2 Optodes and ISFET There a
Page 19 and 20: 2.2. ISFET (ION SELECTIVE FIELD EFF
Page 21 and 22: 2.3. SUMMARY 11 of oxygen molecules
Page 23 and 24: Chapter 3 Theory of the Clark Senso
Page 25: 3.1. CHEMISTRY OF THE CLARK SENSOR
Page 29 and 30: 3.1. CHEMISTRY OF THE CLARK SENSOR
Page 31 and 32: 3.1. CHEMISTRY OF THE CLARK SENSOR
Page 33 and 34: 3.2. THE POTENTIOSTAT 23 Reference
Page 35 and 36: 3.2. THE POTENTIOSTAT 25 Working El
Page 37 and 38: Chapter 4 System Design The sensor
Page 39 and 40: 4.2. ACTUAL DESIGN 29 seen on figur
Page 41 and 42: 4.2. ACTUAL DESIGN 31 The third des
Page 43 and 44: 4.3. TEMPERATURE SENSOR 33 Figure 4
Page 45 and 46: 4.3. TEMPERATURE SENSOR 35 a Pt300,
Page 47 and 48: 4.4. PACKAGING 37 Figure 4.11: Clos
Page 49 and 50: 4.4. PACKAGING 39 90 mm 15 mm 6 mm
Page 51 and 52: Chapter 5 Fabrication The fabricati
Page 53 and 54: 5.1. PROCESSING 43 Figure 5.4: Alig
Page 55 and 56: 5.2. BACK-END PROCESSING 45 under e
Page 57 and 58: 5.2. BACK-END PROCESSING 47 5.2.2 A
Page 59 and 60: Chapter 6 Problems and Solutions In
Page 61 and 62: 6.1. WIREBONDING 51 make a line bet
Page 63 and 64: 6.2. THE FLEX PRINT 53 The reason b
Page 65 and 66: 6.3. THE ’O’-RING 55 Figure 6.8
Page 67 and 68: Chapter 7 Evaluation of Results and
Page 69 and 70: 7.2. TEMPERATURE SENSOR 59 Figure 7
Page 71 and 72: 7.4. SUMMARY 61 Figure 7.5: Polarog
Page 73 and 74: Chapter 8 Conclusion The primary go
Page 75 and 76: Bibliography [1] http://earthobserv
Page 77 and 78:
BIBLIOGRAPHY 67 [26] W.E. Morf. The
Page 79 and 80:
Appendix A Silicide Recipe The foll
Page 81 and 82:
Appendix B Fabrication Process 71
Page 84 and 85:
74 APPENDIX B. FABRICATION PROCESS
Page 86:
76 APPENDIX C. PROCESS SEQUENCE
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