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

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22 CHAPTER 3. THEORY OF THE CLARK SENSOR And from this the conditions for membrane controlled diffusion becomes: dm Pm ≫ dL PL (3.20) In order to achieve this condition, a relatively thick membrane with a low oxygen permeability have to be used. When this condition is achieved, the oxygen sensor output depends only on membrane properties as given by Eq. 3.10 and the sensor calibrated in one liquid can be used in other liquids without recalibration. However there is always a liquid film (however thin it may be) and this causes variations in calibration in different liquids. 3.2 The Potentiostat Most of us can probably recall from the early chemistry lessons in school , that if you take a piece of iron and dips it into a sulphuric acid, it will start to dissolve, or rather corrode. Though in the 17th century Sir Humphrey Davy, following Galvani’s experiments [29] discovered that if a piece of non corrosive metal, like Platinum, was put in the acid, and then connected to a positive pole. While at the same time connecting the piece of iron to the negative pole of the same current source. The corrosion of the iron would slow down, or even come to a complete halt if a high enough amount of voltage was applied. Vice versa the corrosion would increase with the voltage, if the iron was connected to the positive pole and the platinum to the negative pole. Though above a specific current(the exact one varying with the temperature, composition, and size of the metals) the current will suddenly drop to low values and the iron would cease to corrode (A phenomenon discovered first by Michael Faraday.[30]). The understanding and explanation for this was later brought on by the invention of the potentiostat. As mentioned at the beginning of this chapter, a potentiostat is used in order to measure the current, so what does this device do, and what is the principle behind it. Basically the potentiostat, is an amplifier that sets and maintains a voltage between two electrodes at a specific value, a Working Electrode (also sometimes referred to as the Indicator Electrode[31]) and a Reference electrode.
3.2. THE POTENTIOSTAT 23 Reference Electrode Constant E required Working Electrode Figure 3.5: A basic electrochemical cell with a cathode (Working Electrode) and a anode (Reference Electrode). However in order for this to be accomplished some conditions have to be fulfilled[32]. The Reference electrode maintains a constant voltage referred to as the hydrogen electrode potential, an often used metal for this (as is also the case in this project) is Ag covered with a layer of AgCl. The moment a current passes through this electrode it will become polarized, eg its potential will vary accordingly to the current. Therefore in order for a steady potentential can be maintained, no current must be allowed to pass through it. This is accomplished by having a constant potential difference between the two electrodes. However in order to achieve this a third electrode have to be added to the cell, a Counter Electrode (also known as a Auxiliary Electrode). Hence a current is forced between the Counter Electrode and the Working Electrode, this current is high enough and in the proper polarity to keep the potential of the Working Electrode at a set value in comparison to the Reference Electrode. There will be two focuses of the Potentiostat[33]: 1. Measuring the potential difference between the Reference and Working Electrode, in a way that leaves the Reference Electrode unpolarized, while comparing this potential difference to an already preset voltage. 2. Force a current between the Counter Electrode and the Working Electrode to counterbalance the difference between the Working Electrode potential and the preset voltage. This is realized electronically by using a operational amplifier also known as an OPAMP, the operational amplifier will has two inputs, an inverting and
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 and 26: 3.1. CHEMISTRY OF THE CLARK SENSOR
Page 31: 3.1. CHEMISTRY OF THE CLARK SENSOR
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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