Smart and Flexible Energy Devices

✍ Scribed by Ram K. Gupta (editor), Tuan Anh Nguyen (editor)

• Publisher: CRC Press
• Year: 2022
• Tongue: English
• Leaves: 621
• Edition: 1
• Category: Library

✦ Synopsis

The scientific community and industries have seen tremendous progress in efficient energy production and storage in the last few years. With the advancement in technology, new devices require high-performance, stretchable, bendable, twistable energy sources which can be integrated into next-generation wearable, compact, and portable electronics for medical, military, and civilian applications.

Smart and Flexible Energy Devices examines the materials, basic working principles, and state-of-the-art progress of flexible devices like fuel cells, solar cells, batteries, and supercapacitors. Covering synthesis approaches for advanced energy materials in flexible devices and fabrications and fundamental design concepts of flexible energy devices such as fuel cells, solar cells, batteries, and supercapacitors, top author teams explore how newer materials with advanced properties are used to fabricate energy devices to meet the future demand for flexible electronics.

Additional Features Include:

• Addresses the materials, technologies, and challenges of various flexible energy devices under one cover.

• Emphasizes future demand and challenges of the field.

• Considers all flexible energy types- fuel cells, solar cells, batteries, and supercapacitors.

• Suitable for undergraduate and postgraduate students of material science and energy programs.

This is a valuable resource for academics and industry professionals working in the field of energy materials, nanotechnology, and energy devices.

✦ Table of Contents

Cover
Half Title
Title Page
Copyright Page
Contents
Preface
Editors
Contributors
1. Smart and Flexible Energy Devices: Principles, Advances, and Opportunities
1.1 Introduction
1.2 Flexible supercapacitors
1.2.1 Flexible supercapacitors based on carbon
1.2.2 Flexible supercapacitors based on metal oxides and sulfides
1.2.3 Flexible supercapacitors based on nanocomposites
1.3 Flexible batteries
1.3.1 Flexible Li-ion and Li-sulfur batteries
1.3.2 Flexible metal-air batteries
1.4 Flexible proton exchange membrane fuel cells
1.5 Flexible solar cells
1.5.1 Dye-sensitized flexible solar cells
1.5.2 Perovskite-based flexible solar cells
1.6 Conclusion
References
2. Innovation in Materials and Design for Flexible Energy Devices
2.1 Introduction
2.2 Materials
2.2.1 Inorganic nanomaterials
2.2.1.1 1D materials
2.2.1.2 2D materials
2.2.2 Organic materials
2.2.2.1 Polymers
2.2.2.2 Other organic materials
2.3 Structural requirements
2.3.1 Flexible substrates and membranes
2.3.2 Thickness of compound/active layer
2.4 Wearability assessments
2.4.1 Softness
2.4.2 Stretchability: The residual strain
2.5 Self-healing mechanism
2.5.1 Intrinsic self-healing polymers with reversible bonds
2.5.2 Self-healing through exhaustion of healing agents
2.6 Design of flexible energy devices
2.7 Flexible energy storage and conversion devices
2.7.1 Energy conversion devices
2.7.1.1 Nanogenerator (NGs)
2.7.1.2 Photovoltaic
2.7.1.3 Other flexible generators
2.7.2 Energy storage devices (ESDs)
2.7.2.1 Flexible batteries (FBs)
2.7.2.1.1 Li-ion flexible batteries (LiBs)
2.7.2.1.2 Other flexible batteries
2.7.2.2 Supercapacitors (SCs)
2.8 Configuration designs for flexible ESDs
2.8.1 1D configuration of ESDs
2.8.1.1 Fiber-type
2.8.1.2 Spring types
2.8.1.3 Spine type
2.8.2 2D configuration of ESDs
2.8.2.1 Layered sandwich configuration
2.8.2.2 Planar interdigital configuration
2.8.2.3 Other 2D novel configurations
2.8.3 3D configuration ESDs
2.8.3.1 Origami/Kirigami/honeycomb-based structures
2.9 Summary
References
3. Basics and Architectural Aspects of Flexible Energy Devices
3.1 Introduction
3.2 Nanotechnology for flexible energy devices
3.3 Architectural concepts, structures, and materials for flexible solar cells
3.3.1 Flexible dye-sensitized solar cells FDSSCs
3.3.1.1 Structure design and basic concept
3.3.1.2 Flexible materials and fabrication process for FDSSCs
3.3.2 Quantum dot synthesized solar cell (QDSSCs)
3.3.2.1 Structure design and basic concept
3.3.2.2 Flexible materials and fabrication process for QDSSCs
3.3.3 Toward other flexible photovoltaic technologies
3.3.3.1 Inorganic materials based flexible solar cells
3.3.3.2 Organic materials based flexible solar cells
3.4 Architectural concepts, structures, and materials for flexible batteries
3.4.1 Lithium-ion batteries (LIBs)
3.4.1.1 Structure design and basic concept
3.4.1.2 Flexible materials for LIB structures
3.4.2 Zinc-ion batteries (ZIBs)
3.4.2.1 Structure design and basic concept
3.4.2.2 Flexible materials for ZIB structures
3.4.3 Flexible batteries advancement
3.5 Architectural concepts, structures, and materials for flexible supercapacitors
3.5.1 Structure design and basic concept
3.5.2 Flexible materials for SCs structures
3.6 Conclusion
References
4. Characterization Techniques of Flexible Energy Devices
4.1 Introduction
4.2 Characterization techniques for flexible energy devices
4.2.1 Scanning electron microscopy
4.2.2 Transmission electron microscopy
4.2.3 X-ray diffraction
4.2.4 Cyclic voltammetry
4.2.5 Galvanostatic charge-discharge test
4.2.6 Electrochemical impedance spectroscopy
4.2.7 Atomic force microscopy
4.2.8 Secondary ion mass spectroscopy
4.2.9 Inductively coupled plasma-mass spectroscopy
4.2.10 Fourier transform infrared spectroscopy
4.3 Summary
References
5. Micro- and Nanofibers-Based Flexible Energy Devices
5.1 Introduction of nanofibers and microfibers
5.1.1 Carbon fibers
5.1.2 Biopolymer fibers
5.1.3 Aramid fibers
5.1.4 Ceramic fibers
5.2 Flexible energy devices based on nanofibers
5.2.1 Inorganic fibers for flexible energy devices
5.2.2 Metallic fibers for energy devices
5.2.3 Carbon-based fibers for energy devices
5.2.4 Biobased fibers for energy devices
5.2.4.1 Cellulose-based fibers
5.2.4.2 Keratin and chitin fiber composites
5.3 Electrospun fibers for flexible energy devices
5.4 Conclusions
References
6. 3D Printed Flexible Energy Devices
6.1 3D printing technologies
6.1.1 Direct ink writing
6.1.2 Fuse deposition modelling
6.1.3 Material jetting
6.1.4 Binder jetting
6.1.5 Directed energy deposition (DED)
6.2 Configuration of flexible energy device
6.2.1 Active materials
6.2.2 EES electrodes
6.2.3 Electrolyte and the solid-state devices
6.2.4 Configuration of EES devices
6.3 3D printed EES devices
6.3.1 3D printed electrodes
6.3.1.1 Carbon-based electrodes
6.3.1.2 Polymer-based electrodes
6.3.1.3 Others
6.3.2 3D printed electrolytes
6.3.3 3D printed device
6.4 Summary and outlook
6.4.1 Precision and resolution of 3D printing
6.4.2 New materials
6.4.3 Integration with multi-materials printing technology and the interface
6.4.4 4D printing
References
7. Environmental Impact of Flexible Energy Devices
7.1 Introduction
7.2 Technical description of flexible devices
7.2.1 Energy conversion devices
7.2.1.1 Flexible solar cells
7.2.2 Energy storage devices
7.2.2.1 Flexible supercapacitors
7.2.2.2 Modern designs of lithium-ion batteries
7.3 Flexible materials environmental effects
7.3.1 Cadmium
7.3.2 Amorphous silicon (a-Si)
7.3.3 Copper indium gallium diselenide (CIGS)
7.3.4 Lead halide
7.3.5 Carbon-based nanomaterials
7.3.6 Tellurium and indium
7.3.7 Toxic flexible substrate
7.4 Processing routes and design strategies for safe and sustainable manufacturing
7.4.1 Toxic materials replacement
7.4.1.1 Lead-free perovskite
7.4.1.2 Indium
7.4.2 Improving processing routes
7.4.3 Recycling
7.4.4 Encapsulation
7.5 Conclusion
References
8. Metal Oxide-Based Materials for Flexible and Portable Fuel Cells: Current Status and Future Prospects
8.1 Introduction
8.2 Current architecture and materials for flexible and portable fuel cells
8.3 Material challenges for flexible and portable fuel cells
8.4 Strategies to Tailor metal oxides for fuel cells
8.4.1 Morphological control
8.4.2 Phase structure engineering
8.4.3 Oxygen-vacancy control
8.4.4 Doping
8.4.5 Compositing with carbon/metal-based materials
8.5 Current status of metal oxide-based materials in flexible and portable fuel cells
8.5.1 Metal oxide-based catalysts
8.5.1.1 Simple metal oxides as catalysts
8.5.1.2 Perovskites as catalysts
8.5.1.3 Spinel oxides as catalysts
8.5.2 Metal oxide-based co-catalysts
8.5.3 Metal oxide-based catalyst supports
8.5.4 Metal oxide-based electrolytes/membranes
8.5.5 Metal oxide-based bipolar plates and substrates
8.5.6 Metal oxide-based current-collector
8.5.7 Metal oxide-based electrodes
8.6 Future avenues for metal oxide systems in empowering flexible and portable fuel cells
8.7 Acknowledgments
References
9. Flexible Fuel Cells Based on Microbes
9.1 Introduction
9.2 Basics of MFCs
9.2.1 Instrumental bases
9.2.2 Two-compartment MFCs
9.2.3 Single-compartment MFCs
9.3 Flexible MFCs
9.3.1 Electrodes
9.3.1.1 Carbonaceous material
9.3.1.2 Bacterial cellulose
9.3.1.3 Graphene sheet
9.3.1.4 Polypyrrole (PPy)
9.3.2 Membrane
9.3.3 Microorganism
9.3.4 Fabrication
9.3.5 Applications
9.3.5.1 Energy harvesting
9.3.5.2 Treatment of wastewater
9.3.5.3 Sensors and portable power machines
9.4 Future aspect
9.4.1 Large-scale uses
9.4.2 Anode manipulation
9.4.3 Membrane-free MFC
9.5 Conclusion
References
10. Flexible Silicon Photovoltaic Solar Cells
10.1 Introduction
10.2 Classification of flexible photovoltaic solar cells
10.2.1 Inorganic flexible photovoltaic solar cells
10.2.2 Organic flexible photovoltaic solar cells
10.2.3 Hybrid flexible photovoltaic solar cells
10.3 Flexible silicon (Si) photovoltaic solar cells
10.3.1 Flexible crystalline silicon solar cells
10.3.1.1 Recent progress in flexible crystalline silicon solar cells
10.3.2 Flexible thin-film amorphous silicon solar cells
10.3.2.1 Recent progress in flexible amorphous silicon solar cells
10.3.3 Silicon nanostructures for flexible solar cells
10.3.3.1 Silicon nanowire flexible solar cells
10.3.3.2 Silicon nanopyramid solar cells
10.3.3.3 Silicon nanoparticles for solar cells
10.3.3.4 Silicon ink-based solar cells
10.4 Outlook and conclusions
Acknowledgement
References
11. Flexible Solar Cells Based on Metal Oxides
11.1 Introduction
11.2 Substrate materials in flexible solar cells
11.3 Flexible dye-sensitized solar cells based on metal oxides
11.4 Flexible organic solar cells based on metal oxides
11.5 Flexible perovskite solar cells based on metal oxides
11.6 Other flexible solar cells based on metal oxides
11.7 Conclusion
References
12. Inorganic Materials for Flexible Solar Cells
12.1 Introduction
12.2 Inorganic photoactive devices
12.3 Cu(In,Ga)Se2 (CIGS) solar cells
12.4 Cu2ZnSn(S,Se)4 solar cells
12.5 CdTe solar cells
12.6 Sb2Se3 solar cells
12.7 CsPb(I1-xBrx)3 solar cells
12.8 Environmental and economic concerns
12.9 Conclusion
References
13. Efficient Metal Oxide-Based Flexible Perovskite Solar Cells
13.1 Introduction
13.2 Requirement for alternate energy resources
13.3 Metal oxide nanostructures
13.4 Metal oxide based flexible solar cells
13.5 Metal oxides based flexible perovskite solar cells
13.6 Recent advancement in metal oxide-based flexible perovskite solar cells
13.7 Summary and outlook
Acknowledgments
References
14. Flexible Solar Cells Based on Chalcogenides
14.1 Introduction
14.2 Merits of flexible solar cells
14.3 Progress and development on different substrates
14.3.1 CIGS
14.3.1.1 Polyimide
14.3.1.2 Metal foils
14.3.1.3 Ceramic and other materials
14.3.2 CdTe
14.3.2.1 Metal Foils
14.3.2.2 Polymer
14.3.2.3 Ceramics
14.3.3 CZTS/CZTS(Se)
14.3.3.1 Metal foils
14.3.3.2 UTG
14.3.3.3 Polymer and other materials
14.3.4 Sb2Se3
14.4 Fabrication issues and challenges with flexible solar cells
14.4.1 Crack initiation
14.4.2 Performance degradation under bending
14.4.3 Substrate choice
14.4.4 Electrodes issues
14.4.5 Stability and scalability issues
14.5 Future prospects and strategies for further advancements
14.5.1 Absorber optimization
14.5.2 New chalcogenide materials
14.5.3 Optimizing every layer of solar module
14.5.4 Material database and machine learning algorithms
14.5.5 Rigorous testing
14.5.6 Development of transparent/semitransparent solar cells
14.5.7 Integration with existing technologies
14.6 Conclusion
References
15. Perovskite-Based Flexible Solar Cells
15.1 Introduction
15.2 Device structure and development of FPSCs
15.2.1 Device structure of FPSC
15.2.2 Development of FPSCs
15.3 FPSC fabrication methods
15.3.1 Laboratory scale fabrication methods
15.3.1.1 Spin coating
Advantages
Disadvantages
15.3.1.2 Thermal evaporation
Advantages
Disadvantages
15.3.2 Large scale fabrication methods
15.3.2.1 Inkjet printing
Advantages
Disadvantages
15.3.2.2 Blade coating
Advantages
Disadvantages
15.3.2.3 Spray coating
Advantages
Disadvantages
15.3.2.4 Slot-die coating
Advantages
Disadvantages
15.4 Materials for FPSCs
15.4.1 Perovskite absorber layer
15.4.2 Charge transport layers
15.4.2.1 Electron transport layer
15.4.2.2 Hole transport layer
15.4.3 Flexible substrates
15.4.3.1 Polymer (or plastic) substrates
15.4.3.2 Metal substrates
15.4.3.3 Fiber shaped PSCs
15.4.3.4 Other flexible substrates
15.4.4 Transparent conducting layer
15.4.5 Encapsulation
15.5 Recycling of FPSCs
15.6 Challenges and future perspectives
15.6.1 Environmental stability
15.6.2 Mechanical stability
15.6.3 High manufacturing cost
15.6.4 Large-area fabrication
15.6.5 Toxicity
15.7 Applications of FPSCs
15.8 Conclusion
References
16. Quantum Dots Based Flexible Solar Cells
16.1 Introduction
16.2 Theoretical background of QDs
16.2.1 Quantum size effect
16.2.2 Multiple exciton generation
16.2.3 Ultrafast charge transfer
16.3 Synthesis and characterization of QDs
16.3.1 Colloidal synthesis
16.3.2 Surface engineering
16.4 QDs based flexible heterojunction solar cell
16.5 QD based flexible sensitized solar cells
16.6 QDs based flexible perovskite solar cells
16.7 Flexible QD-silicon hybrid solar cells
16.7 Conclusion
References
17. A Method of Strategic Evaluation for Perovskite-Based Flexible Solar Cells
17.1 Introduction
17.2 Perovskite-based solar cells' working mechanism
17.2.1 The future of perovskite-based solar cells
17.3 Methodology
17.3.1 AHP analysis
17.4 Conclusions
References
18. Flexible Batteries Based on Li-Ion
18.1 Introduction
18.2 Flexible electrodes
18.2.1 Flexible anodes
18.2.1.1 Carbon materials
18.2.1.2 Mxenes
18.2.2 Flexible cathodes
18.3 Flexible electrolytes
18.4 Battery structures
18.5 Fabrication of FLIBs
18.6 Conclusion
References
19. Flexible Na-Ion Batteries
19.1 Introduction
19.2 Flexible Na-ion batteries
19.2.1 Configurations
19.2.2 Electrolytes
19.2.3 Electrode materials
19.2.4 Separators
19.3 Conclusion
References
20. Flexible Batteries Based on K-ion
20.1 Introduction
20.2 Working principle
20.3 Influence of electrolytes and solid electrolyte interphase in K-ion based batteries
20.3.1 Thermodynamic understanding of electrolyte reduction
20.3.2 Comparison of K-ion SEI and Li-/Na-ion SEI
20.3.3 Effects of electrolyte selection on SEI
20.3.4 Mechanical stability of SEI
20.4 Anode materials for K-ion based flexible batteries
20.4.1 Carbon materials
20.4.2 Phosphorus compounds
20.4.3 Titanium-based compounds
20.4.4 Alloying-type compounds
20.4.5 Organic compounds
20.5 Cathode materials for K-ion based flexible batteries
20.5.1 Hexacyanometallate
20.5.2 Layered metal oxides
20.5.3 Polyanionic compounds
20.5.4 Organic materials
20.6 Summary and future outlooks
References
21. Flexible Batteries Based on Zn-Ion
21.1 Introduction
21.2 Zinc-ion batteries and mechanisms
21.3 Flexible zinc-ion batteries
21.3.1 Polymer electrolytes
21.3.1.1 PEO and derivatives
21.3.1.2 PVA and derivatives
21.3.1.3 PAM and derivatives
21.3.2 Functionalities
21.3.3 Flexible device constructions (electrodes)
21.4 Current challenges and perspectives
21.4.1 Voltage issue
21.4.2 Structural enhancement
21.4.3 Multifunctionalities
21.5 Conclusions
Reference
22. Fabrication Techniques for Wearable Batteries
22.1 Introduction
22.2 Wearable batteries
22.3 Electrode fabrication approaches
22.3.1 Substrate-enabled techniques
22.3.1.1 Chemical vapor deposition
22.3.1.2 Hydrothermal deposition
22.3.1.3 Electrochemical deposition
22.3.1.4 Electrospinning and electrospraying
22.3.1.5 Solution dip coating
22.3.1.6 Spray painting
22.3.1.7 Biscrolling
22.3.2 Substrateless techniques
22.3.2.1 Electrospinning
22.3.2.2 Wet spinning
22.3.2.3 Melt spinning
22.3.2.4 3D extrusion printing
22.4 Structures and approaches for unification of electrodes
22.4.1 Winding
22.4.2 Twisting
22.4.3 Coaxial assembly
22.5 Integration of fiber electrodes/batteries into textiles for wearable applications
22.5.1 Weaving
22.5.2 Knitting
22.6 Conclusion
References
23. Carbon-Based Advanced Flexible Supercapacitors
23.1 Introduction
23.2 Carbon-based materials for FSCs
23.2.1 Graphene
23.2.2 Carbon nanotubes
23.2.3 Bio-based carbon
23.3 Synthesis of carbon-based materials
23.4 Mechanisms of energy storage in supercapacitors
23.4.1 Electrochemical double-layer capacitors
23.4.2 Pseudo-capacitors
23.4.3 Hybrid capacitors
23.5 Carbon-based flexible supercapacitors
23.5.1 Graphene-based flexible supercapacitors
23.5.2 CNT-based flexible supercapacitors
23.5.3 Bio-based FSCs
23.6 Conclusion
References
24. 2D Materials for Flexible Supercapacitors
24.1 Introduction
24.2 Classification, methods, and merits
24.2.1 Graphene and its carbonaceous analogs
24.2.1 TMCs and TMDs
24.2.3 MXenes
24.2.4 2D MOFs
24.2.5 Other 2D/layered materials
24.2.5 Hybrid 2D nanostructures
24.2.6 Configurations of FSCs
24.3 Challenges and prospects
References
25. Flexible Supercapacitors Based on Metal Oxides
25.1 Introduction
25.2 Key characteristics of flexible supercapacitors
25.3 Equations of super-capacitance measurement
25.4 Metal oxide based-flexible supercapacitors
25.4.1 Drawbacks of MOs for supercapacitors
25.4.2 How to overcome the drawbacks?
25.4.3 Various electrolytes in supercapacitors
25.4.4 Various metal oxides in supercapacitors
25.4.4.1 Ruthenium oxide-based supercapacitors
25.4.4.2 Manganese-oxide-based supercapacitors
25.4.4.3 Other metal oxide-based supercapacitors
25.4.4 Transparent supercapacitors
25.5 Conclusions
Acknowledgments
References
26. Recent Advances in Transition Metal Chalcogenides for Flexible Supercapacitors
26.1 Introduction
26.2 Substrates for flexible supercapacitors
26.3 Metal chalcogenides and their categories
26.3.1 Unary-metal chalcogenides
26.3.2 Binary or ternary metal chalcogenide
26.4 How does the electrode system work?
26.5 Recent developments in the metal chalcogenide-based Flexible Supercapacitors
26.6 Conclusion
Acknowledgment
References
27. MOFs-Derived Metal Oxides-Based Compounds for Flexible Supercapacitors
27.1 Introduction
27.2 MOFs-derived metal oxides for flexible supercapacitors
27.2.1 MOFs-derived binary metal oxides
27.2.1.1 Co-oxides materials
27.2.1.2 Fe-oxides materials
27.2.1.3 Ce-oxides materials
27.2.2 MOFs-derived ternary or TMMOs
27.2.2.1 Porous ZnCo2O4 material
27.2.2.2 MnCo2O4 materials
27.2.2.3 NixCo3-xO4 material
27.2.3 MOFs-derived metal oxide/composite
27.3 Conclusions
References
28. Textile-Based Flexible Supercapacitors
28.1 Introduction
28.2 Classification of supercapacitor based on storage mechanism
28.2.1 Electrical double layer capacitors
28.2.2 Pseudocapacitors
28.2.3 Hybrid supercapacitors
28.3 Components of supercapacitors
28.3.1 Electrode
28.3.2 Electrolyte
28.3.3 Separator
28.3.4 Current collector
28.4 Textile-based supercapacitors
28.4.1 Fiber-based supercapacitors
28.4.1.1 Parallel fiber structure
28.4.1.2 Twisted fiber structure
28.4.1.3 Coaxial fiber structure
28.4.2 Fabric supercapacitors
28.5 Fiber-based electrodes
28.5.1 Conventional fibers based fiber electrodes
28.5.2 Metal yarns/wires/threads-based fiber electrodes
28.5.3 Graphene yarn-based electrodes
28.5.4 CNT yarn-based fiber electrodes
28.5.5 Hybrid fiber-based supercapacitor electrodes
28.6 Fabric-based electrodes for supercapacitors
28.6.1 Metal mesh-based electrodes
28.6.2 Carbon fabric-based electrodes
28.6.3 Conventional fabric-based electrodes
28.7 Applications
28.8 Conclusion and future perspective
Reference
29. Current Development and Challenges in Textile-Based Flexible Supercapacitors
29.1 Introduction
29.2 Classification and properties of textile fabrics
29.2.1 Classification of textile fabrics
29.2.1.1 Man-made fibers
29.2.1.2 Natural fibers
29.2.2 Properties of textile fibers for supercapacitor applications
29.3 Textile based flexible supercapacitors (TFSCs)
29.4 Design strategies for TFSCs application
29.5 Conclusion and future perspective
References
30. Flexible Supercapacitors Based on Nanocomposites
30.1 Introduction
30.2 Carbon nanomaterial-incorporated nanocomposites for FSCs
30.2.1 Carbon nanomaterials
30.2.2 Carbon-metal oxide nanocomposites
30.2.3 Carbon-conducting polymer nanocomposites
30.2.4 Carbon-mxene nanocomposites
30.3 Device configurations of nanocomposite-based FSCs
30.3.1 One-dimensional fiber-shaped FSCs
30.3.2 Two-dimensional film-shaped FSCs
30.3.3 Three-dimensional structural FSCs
30.4 Practical applications of nanocomposite-based FSCs
30.4.1 FSCs for wearable electronic devices
30.4.2 FSCs for flexible electronic devices
30.5 Summary and perspectives
Acknowledgments
References
31. Textile-Based Flexible Nanogenerators
31.1 Introduction
31.2 Piezoelectric nanogenerators
31.3 Textile-based piezoelectric nanogenerators
31.4 Pyroelectric and hybrid nanogenerators
31.5 Triboelectric nanogenerators (TENG)
31.6 Conclusion
References
Index