Physico-Chemistry of Solid-Gas Interfaces Concepts and Methodology for Gas Sensor Development

Fundamental elementary facts and theoretical tools for the interpretation and model development of solid-gas interactions are first presented in this work. Chemical, physical and electrochemical aspects are presented from a phenomenological, thermodynamic and kinetic point of view. The theoretical aspects of electrical properties on the surface of a solid are also covered to provide greater accessibility for those with a physico-chemical background. The second part is devoted to the development of devices for gas detection in a system approach. Methods for experimental investigations concerning solid-gas interactions are first described. Results are then presented in order to support the contribution made by large metallic elements to the electronic processes associated with solid-gas interactions.

Preface xiii

Chapter 1. Adsorption Phenomena 1

1.1. The surface of solids: general points 1

1.2. Illustration of adsorption 2

1.2.1. The volumetric method or manometry 3

1.2.2. The gravimetric method or thermogravimetry 4

1.3. Acting forces between a gas molecule and the surface of a solid 4

1.3.1. Van der Waals forces 4

1.3.2. Expression of the potential between a molecule and a solid 6

1.3.3. Chemical forces between a gas species and the surface of a solid 7

1.3.4. Distinction between physical and chemical adsorption 8

1.4. Thermodynamic study of physical adsorption 8

1.4.1. The different models of adsorption 8

1.4.2. The Hill model 9

1.4.3. The Hill-Everett model 10

1.4.4. Thermodynamics of the adsorption equilibrium in Hill’s model 10

1.4.4.1. Formulating the equilibrium 10

1.4.4.2. Isotherm equation 11

1.4.5. Thermodynamics of adsorption equilibrium in the Hill-Everett model 12

1.5. Physical adsorption isotherms 13

1.5.1. General points 13

1.5.2. Adsorption isotherms of mobile monolayers 15

1.5.3. Adsorption isotherms of localized monolayers 15

1.5.3.1. Thermodynamic method 16

1.5.3.2. The kinetic model 17

1.5.4. Multilayer adsorption isotherms 18

1.5.4.1. Isotherm equation 18

1.6. Chemical adsorption isotherms 23

1.7. Bibliography 27

Chapter 2. Structure of Solids: Physico-chemical Aspects 29

2.1. The concept of phases 29

2.2. Solid solutions 31

2.3. Point defects in solids 33

2.4. Denotation of structural members of a crystal lattice 34

2.5. Formation of structural point defects 36

2.5.1. Formation of defects in a solid matrix 36

2.5.2. Formation of defects involving surface elements 37

2.5.3. Concept of elementary hopping step 38

2.6. Bibliography 38

Chapter 3. Gas-Solid Interactions: Electronic Aspects 39

3.1. Introduction 39

3.2. Electronic properties of gases 39

3.3. Electronic properties of solids 40

3.3.1. Introduction 40

3.3.2. Energy spectrum of a crystal lattice electron 41

3.3.2.1. Reminder about quantum mechanics principles. 41

3.3.2.2. Band diagrams of solids 45

3.3.2.3. Effective mass of an electron 52

3.4. Electrical conductivity in solids 55

3.4.1. Full bands 55

3.4.2. Partially occupied bands 56

3.5. Influence of temperature on the electric behavior of solids 57

3.5.1. Band diagram and Fermi level of conductors 57

3.5.2. Case of intrinsic semiconductors 61

3.5.3. Case of extrinsic semiconductors 62

3.5.4. Case of materials with point defects 64

3.5.4.1. Metal oxides with anion defects, denoted by MO1x 65

3.5.4.2. Metal oxides with cation vacancies, denoted by M1xO 66

3.5.4.3. Metal oxides with interstitial cations, denoted by M1+xO 67

3.5.4.4. Metal oxides with interstitial anions, denoted by MO1+x 67

3.6. Bibliography 68

Chapter 4. Interfacial Thermodynamic Equilibrium Studies 69

4.1. Introduction 69

4.2. Interfacial phenomena 70

4.3. Solid-gas equilibriums involving electron transfers or electron holes 71

4.3.1. Concept of surface states 72

4.3.2. Space-charge region (SCR) 73

4.3.3. Electronic work function 77

4.3.3.1. Case of a semiconductor in the absence of surface states 77

4.3.3.2. Case of a semiconductor in the presence of surface states 78

4.3.3.3. Physicists’ and electrochemists’ denotation systems 79

4.3.4. Influence of adsorption on the electron work functions 80

4.3.4.1. Influence of adsorption on the surface barrier VS 80

4.3.4.2. Influence of adsorption on the dipole component VD. 90

4.4. Solid-gas equilibriums involving mass and charge transfers 91

4.4.1. Solids with anion vacancies 92

4.4.2. Solids with interstitial cations 94

4.4.3. Solids with interstitial anions 94

4.4.4. Solids with cation vacancies 96

4.5. Homogenous semiconductor interfaces 97

4.5.1. The electrostatic potential is associated with the intrinsic energy level 103

4.5.2. Electrochemical aspect 104

4.5.3. Polarization of the junction. 107

4.6. Heterogenous junction of semiconductor metals 107

4.7. Bibliography 108

Chapter 5. Model Development for Interfacial Phenomena 109

5.1. General points on process kinetics 109

5.1.1. Linear chain 111

5.1.1.1. Pure kinetic case hypothesis 114

5.1.1.2. Bodenstein’s stationary state hypothesis 118

5.1.1.3. Evolution of the rate according to time and gas pressure 119

5.1.1.4. Diffusion in a homogenous solid phase 121

5.1.2. Branched processes 125

5.2. Electrochemical aspect of kinetic processes 126

5.3. Expression of mixed potential 133

5.4. Bibliography 136

Chapter 6. Apparatus for Experimental Studies: Examples of Applications 137

6.1. Introduction 137

6.2. Calorimetry 138

6.2.1. General points 138

6.2.1.1. Theoretical aspect of Tian-Calvet calorimeters 139

6.2.1.2. Seebeck effect 139

6.2.1.3. Peltier effect 140

6.2.1.4. Tian equation 140

6.2.1.5. Description of a Tian-Calvet device 142

6.2.1.6. Thermogram profile 144

6.2.1.7. Examples of applications 146

6.3. Thermodesorption 156

6.3.1. Introduction 156

6.3.2. Theoretical aspect 157

6.3.3. Display of results 161

6.3.3.1. Tin dioxide 161

6.3.3.2. Nickel oxide 163

6.4. Vibrating capacitor methods 172

6.4.1. Contact potential difference 172

6.4.2. Working principle of the vibrating capacitor method 176

6.4.2.1. Introduction 176

6.4.2.2. Theoretical study of the vibrating capacitor method 176

6.4.3. Advantages of using the vibrating capacitor technique 179

6.4.3.1. The materials studied 179

6.4.3.2. Temperature conditions 179

6.4.3.3. Pressure conditions 181

6.4.4. The constraints 181

6.4.4.1. The reference electrode 181

6.4.4.2. Capacitance modulation 182

6.4.5. Display of experimental results 182

6.4.5.1. Study of interactions between oxygen and tin dioxide 184

6.4.5.2. Study of interactions between oxygen and beta-alumina 185

6.5. Electrical interface characterization 187

6.5.1. General points 187

6.5.2. Direct-current measurement 189

6.5.3. Alternating-current measurement 191

6.5.3.1. General points 191

6.5.3.2. Principle of the impedance spectroscopy technique 191

6.5.4. Application of impedance spectroscopy – experimental results 196

6.5.4.1. Protocol 196

6.5.4.2. Experimental results: characteristics specific to each material 197

6.5.5. Evolution of electrical parameters according to temperature 202

6.5.6. Evolution of electrical parameters according to pressure 208

6.6. Bibliography 212

Chapter 7. Material Elaboration 215

7.1. Introduction 215

7.2. Tin dioxide 216

7.2.1. The compression of powders 216

7.2.1.1. Elaboration process and structural properties 216

7.2.1.2. Influence of the morphological parameters on the electric properties 217

7.2.2. Reactive evaporation 219

7.2.2.1. Experimental device 219

7.2.2.2. Measure of the source temperature 222

7.2.2.3. Thickness measure 222

7.2.2.4. Experimental process 224

7.2.2.5. Structure and properties of the films 224

7.2.3. Chemical vapor deposition: deposit contained between 50 and 300 Å 236

7.2.3.1. General points 236

7.2.3.2. Device description 238

7.2.3.3. Structural characterization of the material 242

7.2.3.4. Influence of the experimental parameters on the physico-chemical properties of the films 245

7.2.3.5. Influence of the structure parameters on the electric properties of the films 250

7.2.4. Elaboration of thick films using serigraphy 252

7.2.4.1. Method description 252

7.2.4.2. Ink elaboration 253

7.2.4.3. Structural characterization of thick films made with tin dioxide 254

7.3. Beta-alumina 255

7.3.1. General properties 255

7.3.2. Material elaboration 257

7.3.3. Material shaping 261

7.3.3.1. Mono-axial compression 261

7.3.3.2. Serigraphic process 262

7.3.4. Characterization of materials 263

7.3.4.1. Physico-chemical characterization of the sintered materials 263

7.3.4.2. Physico-chemical treatment of the thick films 266

7.3.5. Electric characterization 273

7.4. Bibliography 275

Chapter 8. Influence of the Metallic Components on the Electrical Response of the Sensors 277

8.1. Introduction 277

8.2. General points 278

8.2.1. Methods to deposit the metallic parts on the sensitive element 278

8.2.2. Role of the metallic elements on the sensors’ response 279

8.2.3. Role of the metal: catalytic aspects 282

8.2.3.1. Spill-over mechanism 283

8.2.3.2. Reverse spill-over mechanism 284

8.2.3.3. Electronic effect mechanism 284

8.2.3.4. Influence of the metal nature on the involved mechanism 286

8.3. Case study: tin dioxide 288

8.3.1. Choice of the samples 288

8.3.2. Description of the reactor 289

8.3.3. Experimental results 291

8.3.3.1. Influence of the oxygen pressure on the electric conductivity 291

8.3.3.2. Influence of the reducing gas on the electric conductions 295

8.4. Case study: beta-alumina 296

8.4.1. Device and experimental process 297

8.4.2. Influence of the nature of the electrodes on the measured voltage 298

8.4.2.1. Study of the different couples of metallic electrodes 299

8.4.2.2. Electric response to polluting gases 301

8.4.3. Influence of the electrode size 303

8.4.3.1. Description of the studied devices 303

8.4.3.2. Study of the electric response according to the experimental conditions 304

8.5. Conclusion 306

8.6. Bibliography 307

Chapter 9. Development and Use of Different Gas Sensors 309

9.1. General points on development and use 309

9.2. Examples of gas sensor development 310

9.2.1. Sensors elaborated using sintered materials 310

9.2.2. Sensors produced with serigraphed sensitive materials 312

9.3. Device designed for the laboratory assessment of sensitive elements and/or sensors to gas action 316

9.3.1. Measure cell for sensitive materials 317

9.3.2. Test bench for complete sensors 319

9.3.3. Measure of the signal 319

9.3.3.1. Measure of the electric conductance 319

9.3.3.2. Measure of the potential 322

9.4. Assessment of performance in the laboratory 322

9.4.1. Assessment of the performances of tin dioxide in the presence of gases 322

9.4.2. Assessment of beta-alumina in the presence of oxygen 327

9.4.2.1. Device and experimental process 327

9.4.2.2. Electric response to the action of oxygen 327

9.4.3. Assessment of the performances of beta-alumina in the presence of carbon monoxide 329

9.4.3.1. Measurement device 329

9.4.3.2. Electric results 329

9.5. Assessment of the sensor working for an industrial application 332

9.5.1. Detection of hydrogen leaks on a cryogenic engine 333

9.5.1.1. Context of the study 333

9.5.1.2. Study of performances in the presence of hydrogen 333

9.5.1.3. Test carried out in an industrial environment 337

9.5.2. Application of the resistant sensor to atmospheric pollutants in an urban environment 341

9.5.2.1. Measurement campaign conducted at Lyon in 1988 342

9.5.2.2. Measurement campaign conducted at Saint Etienne in 1998 345

9.5.3. Application of the potentiometric sensor to the control of car exhaust gas 347

9.5.3.1. Strategy implemented to control the emission of nitrogen oxides 347

9.5.3.2. Strategy implemented to control nitrogen oxide traps 349

9.5.3.3. Results relative to the nitrogen oxides traps 350

9.6. Amelioration of the selectivity properties 352

9.6.1. Amelioration of the selective detection properties of SnO2 sensors using metallic filters 352

9.6.1.1. Development of a sensor using a rhodium filter 352

9.6.1.2. Development of a sensor using a platinum filter 354

9.6.2. Development of mechanical filters 356

9.6.2.1. Development of a sensor detecting hydrogen 356

9.6.2.2. Development of a protective film for potentiometric sensors 356

9.7. Bibliography 359

Chapter 10. Models and Interpretation of Experimental Results 361

10.1. Introduction 361

10.2. Nickel oxide 362

10.2.1. Kinetic model 365

10.2.2. Simulation of a kinetic model using analog electric circuits 370

10.2.2.1. Simulation of the curves displaying a maximum 370

10.2.2.2. Simulation of the curves displaying a plateau 377

10.2.3. Physical significance of the measured electric conductivity 380

10.3. Beta-alumina 380

10.3.1. Physico-chemical and physical aspects of a phenomenon taking place at the electrodes 380

10.3.1.1. Oxygen species present at the surface of the device 380

10.3.1.2. Origin of the electric potential 384

10.3.2. Expression of the model 385

10.3.2.1. The electrode potential 385

10.3.2.2. Expression of the coverage degree 389

10.3.2.3. Expression of the theoretical potential difference at the poles of the device 394

10.3.3. Simulation of the results obtained with oxygen 395

10.3.3.1. Behavior as a function of temperature and pressure 395

10.3.3.2. Behavior as a function of electrode size 397

10.3.3.3. Evolution of the surface potential 399

10.3.4. Simulation of the phenomenon in the presence of CO 401

10.3.4.1. Description of the mechanisms considered 401

10.3.4.2. Oxidation mechanisms of carbon monoxide 402

10.3.4.3. Results of the simulation 405

10.4. Tin dioxide 409

10.4.1. Introduction 409

10.4.2. Proposition for a physico-chemical model 410

10.4.3. Phenomenon at the electrodes and role of the thickness of the sensitive film 415

10.4.3.1. Calculation of the conductance G as a function of the thickness of the film 416

10.4.3.2. Mathematical simulation 423

10.5. Bibliography 428

Index 431