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Reactions and Mechanisms in Thermal Analysis of Advanced Materials
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Materials Degradation and Failure SeriesStudies and investigations on materials failure are critical aspects of science and engineering.
The failure analysis of existing materials and the development of new materials demands in-depth understanding of the concepts and principles involved in the deterioration of materials
The Material’s Degradation and Failure series encourages the publication of titles that are centered on understanding the failure in materials. Topics treating the kinetics and mechanism of degradation of materials is of particular interest. Similarly, characterization techniques that
record macroscopic (e.g., tensile testing), microscopic (e.g., in-situ observation) and nanoscopic (e.g., nanoindentation) damages in materials will be of interest. Modeling studies that cover failure
in materials will also be included in this series.
Series Editors: Atul Tiwari and Baldev RajDr. Atul Tiwari, CChem
Director, R&D, Pantheon Chemicals225 W. Deer Valley Road #4
Phoenix, AZ 85027 USAEmail: [email protected], [email protected]
Dr. Baldev Raj, FTWAS, FNAE, FNA, FASc, FNASc Director, National Institute of Advanced Studies
Indian Institute of Science Campus Bangalore 560 012, India
Email: [email protected], [email protected]
Publishers at ScrivenerMartin Scrivener([email protected])
Phillip Carmical ([email protected])
Reactions and Mechanisms in Thermal Analysis of
Advanced Materials
Edited by
Atul Tiwari and Baldev Raj
Copyright © 2015 by Scrivener Publishing LLC. All rights reserved.
Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Salem, Massachusetts.Published simultaneously in Canada.
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Cover design by Atul Tiwari and Russell Richardson
Library of Congr ess Cataloging-in-Publication Data:
ISBN 978-1-119-11757-5
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
v
Contents
Preface xv
Part 1: Degradation of Polymers
1 Thermal Stability of Organic Monolayers Covalently Grafted on Silicon Surfaces 3Florent Yang, Philippe Allongue, François Ozanam and Jean-Noël Chazalviel1.1 Introduction 3
1.1.1 Hydrogen-Terminated Si Surfaces 61.2 Alkyl-Grafted Surfaces 8
1.2.1 Preparation 81.2.2 Thermal Stability of Alkyl-Grafted Surfaces 91.2.3 Case of Substituted Alkyl Surfaces 14
1.3 Alkoxy-Grafted Surfaces 151.3.1 Preparation 151.3.2 Thermal Stability of Alkoxy-Grafted Surfaces 17
1.4 Surfaces Grafted with Aryl Groups 191.4.1 Preparation 191.4.2 Thermal Stability 20
1.5 Surfaces Grafted via Si–N Linkages 221.5.1 Preparation 221.5.2 Thermal Stability 23
1.5.2.1 The Thermal Treatment of the Si Surface with NH3 241.5.2.2 Thermal Stability of the Modified Surfaces 26
1.6 Summary 27References 30
2 Thermal Analysis to Discriminate the Stability of Biomedical Ultrahigh-Molecular-Weight Polyethylenes Formulations 39María José Martínez-Morlanes and Francisco Javier Medel2.1 Introduction 392.2 Suitability of TGA Analysis for the Study of Stability of
Medical Polyethylene 422.2.1 Introduction 422.2.2 Degradation Curves of UHMWPE Depending on the Reaction
Atmosphere 432.2.3 Decomposition Processes of UHMWPE in Air 45
2.2.3.1 Thermo-oxidation Process 452.2.3.2 Thermal Degradation Process of UHMWPE 47
2.2.4 Irradiation Effects on the Thermogravimetric Curves of UHMWPE 49
vi Contents
2.2.5 Stabilization of Polyethylene against Thermo-oxidative Degradation 50
2.3 Activation Energies of Degradation Processes in the Thermal Decomposition of UHMWPE 56
References 58
3 Materials Obtained by Solid-State Thermal Decomposition of Coordination Compounds and Metal–Organic Coordination Polymers 63Oana Carp3.1 Introduction 633.2 Coordination Compounds and Metal–Organic Coordination
Polymers as Precursors of Oxides 653.2.1 Coordination Compounds with Carboxylic Acid as Ligand 673.2.2 Coordination’s Compounds as Precursors in the Combustion
Synthesis of Oxides 693.2.3 Metal–Organic Coordination Polymers as Precursors of Oxides 71
3.3 Coordination Compounds and Metal–Organic Coordination Polymers as Precursors of Sulfides 72
3.4 Coordination Compounds as Precursors of Composites 743.5 Coordination Compounds and Metal–Organic Coordination
Polymers as Precursors of New Complexes 743.6 Coordination Compounds and Metal–Organic Coordination
Polymers as Precursor of Metals 753.7 Coordination Compounds as Precursor of Nitrides 763.8 Other Materials 773.9 Conclusions 77References 78
4 Methods for Limiting the Flammability of High-Density Polyethylene with Magnesium Hydroxide 85Joanna Lenża, Maria Sozańska and Henryk Rydarowski 4.1 Introduction 854.2 Experimental Part 88
4.2.1 Materials 884.2.2 Sample Preparation 894.2.3 Methods of Testing 89
4.3 Results and Discussion 914.3.1 Thermal Stability 914.3.2 Flammability 92
4.3.2.1 UL-94 Test 924.3.2.2 Limiting Oxygen Index (LOI) 944.3.2.3 Cone Calorimetry 95
4.3.3 Mechanical Properties 974.3.4 Microstructure of Fracture Surface of Composites 98
4.4 Conclusions 99References 100
Contents vii
5 Thermal Analysis in the Study of Polymer (Bio)-degradation 103Joanna Rydz, Marta Musioł and Henryk Janeczek 5.1 Introduction 1035.2 Differential Scanning Calorimetry 105
5.2.1 Melting Profile 1055.2.2 Glass Transition Study 1075.2.3 Cold Crystallization Analysis 1085.2.4 Crystallinity Degree 1095.2.5 Data Analysis of DSC Heat Effects for the Most
Representative (Bio)-degradable Polymers 1095.3 Dynamic Mechanical Analysis 112
5.3.1 Glass Transition and Melting Study 1125.3.2 Cold Crystallization Analysis 1135.3.3 Data Analysis of DMA Heat Effects for the Most
Representative (Bio)degradable Polymers 1145.4 Thermogravimetric Analysis 115
5.4.1 Thermal Stability 1155.4.2 Thermal Decomposition Kinetic Model 1165.4.3 Data Analysis of TGA Heat Effects for the Most
Representative (Bio)-degradable Polymers 1195.5 Conclusions 120Acknowledgments 121References 121
6 Thermal and Oxidative Degradation Behavior of Polymers and Nanocomposites 127Gauri Ramasubramanian and Samy Madbouly 6.1 Introduction 1276.2 Thermal Degradation 131
6.2.1 Auto-oxidation 1326.2.2 Pyrolysis 134
6.3 Chemical and Oxidative Degradation 1376.4 Photo-oxidation 143
6.4.1 Organic Coatings 1456.4.2 Organic Photovoltaic Materials (OPVs) 148
6.5 Environmental and Biological Degradation 1486.6 Degradation of Polymer Nanocomposites 1546.7 Conclusions 162References 162
7 Thermal Degradation Effects on Polyurethanes and Their Nanocomposites 165Ivan Navarro-Baena, Marina P. Arrieta, Alicia Mujica-Garcia, Valentina Sessini, José M. Kenny and Laura Peponi7.1 Introduction 1657.2 Main Techniques Used for Studying the Thermal Degradation Process 1677.3 Degradation Mechanisms 169
viii Contents
7.4 Chemical Approaches Used to Improve the Thermal Stability of PU 1717.5 Thermal Degradation of PU Based on Natural Sources 172
7.5.1 Vegetable Oils-Based PU 1727.5.2 Bio-poly(ester-urethane)s 173
7.6 Nanocomposites 1747.6.1 Clays 1757.6.2 Polyhedral Oligomeric Silsesquioxanes 1767.6.3 Carbon Nanotubes 1777.6.4 Expandable Graphite and Graphene 1777.6.5 Cellulose, Starch, and Chitin Reinforcement 1787.6.6 Other Fillers 181
7.7 PU Electrospun Fibers 1817.8 Conclusions 184References 184
8 Controllable Thermal Degradation of Thermosetting Epoxy Resins 191Zhonggang Wang8.1 Introduction 1918.2 Ester-, Carbamate-, and Carbonate-Linked Reworkable Epoxy Resins 1938.3 Ether-Linked Reworkable Epoxy Resins 1958.4 Phosphate- and Phosphite-Linked Reworkable Epoxy Resins 196
8.4.1 Thermal Degradation of Phosphate and Phosphite-Linked Epoxy Resins Cured by Acid Anhydride 196
8.4.2 Comparison of Structure and Thermal Degradation of Phosphate-Linked Epoxy Resins between Anhydride and Cationic Curing Methods 199
8.4.3 Degradation Mechanism of Phosphate and Phosphite-Linked Epoxy Resins 201
8.5 Sulfite-Linked Reworkable Epoxy Resins 2048.5.1 Thermal Degradation Behavior of Epo-S and Its
Copolymers with ERL-4221 2048.5.2 Thermal Degradation Mechanism of the Cured Epo-S Network 206
References 207
9 Mechanism of Thermal Degradation of Vinylidene Chloride Barrier Polymers 209
Bob A. Howell9.1 Introduction 2099.2 Discussion 2109.3 Conclusions 218References 219
10 Role of Mass Spectrometry in the Elucidation of Thermal Degradation Mechanisms in Polymeric Materials 221Paola Rizzarelli and Sabrina Carroccio10.1 Introduction 22110.2 Thermogravimetry-Mass Spectrometry (TG-MS) 224
Contents ix
10.3 Gas Chromatography-Mass Spectrometry (GC-MS) and Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS) 228
10.4 Direct Pyrolysis Mass Spectrometry (DPMS) 23710.5 Matrix-Assisted Laser Desorption Ionisation Mass
Spectrometry (MALDI MS) 24210.6 Other Mass Spectrometric Techniques 24610.7 Conclusions 249References 251
11 The Mechanism of Poly(styrene) Degradation 259Bob A. Howell11.1 Introduction 25911.2 Discussion 26011.3 Conclusions 266References 266
12 The Use of Thermal Volatilization Analysis of Polylactic Acid and Its Blends with Starch 269Derval dos Santos Rosa, Claudio Roberto Passatore, and Jose Ricardo Nunes de Macedo12.1 Introduction 26912.2 Use of TVA 27112.3 TVA as an Analytic Technique 27212.4 TVA-PLA Investigation 27412.5 TVA – Thermoplastic Starch 27612.6 Analyses of TVA – PLA and Their Mixtures with Thermoplastic Starch 28012.7 Conclusions 282Acknowledgments 282References 282
Part 2: Degradation of Other Materials
13 Reaction Mechanisms in Thermal Analysis of Amazon Oilseeds 287Orquídea Vasconcelos dos Santos, Carlos Emmerson and Suzana Caetano da Silva Lannes 13.1 Introduction 287
13.1.1 Thermal Analysis of the Brazil Nut Oil in Nitrogen Atmosphere 29513.2 Oxidative Stability 297References 299
14 Thermal Degradation of Cellulose and Cellulosic Substrates 301Jenny Alongi and Giulio Malucelli 14.1 Introduction 30114.2 Thermal and Thermo-oxidative Degradation of Cellulose 302
14.2.1 Cellulose Pyrolysis: Reaction Mechanism and Degradation Products 304
x Contents
14.2.2 Cellulose Thermo-oxidation: Reaction Mechanism and Degradation Products 307
14.2.3 Volatile Species Evolved during Degradation 30814.2.4 Char Formation 313
14.3 Factors Affecting Cellulose Thermal Degradation: Charring/Volatilisation Competition 31814.3.1 Heating Rate 31814.3.2 Water 32014.3.3 Metals 32314.3.4 Phosphorus and Phosphorus/Nitrogen-Based Flame Retardants 32514.3.5 Hybrid Phosphorus- and Phosphorus/Nitrogen-Doped Flame
Retardant Silica Coatings 32914.4 Conclusions 329References 330
15 Thermal Decomposition Behavior of Sodium Alkoxides of Relevance to Fast Reactor Technology 333K. Chandran, M. Kamruddin, S. Anthonysamy and V. Ganesan 15.1 Introduction 33315.2 Preparation of Sodium Alkoxides 334
15.2.1 Literature Survey 33415.2.2 Chemicals and Their Purification Methods 33515.2.3 Experimental Setup for the Preparation of Alkoxides 33715.2.4 Preparation Procedure 337
15.3 Characterization of Sodium Alkoxides 33915.3.1 Estimation of Carbon and Hydrogen 33915.3.2 Estimation of Sodium 34115.3.3 Infrared Analysis 34215.3.4 X-Ray Diffraction Analysis 346
15.4 Thermal Decomposition of Sodium Alkoxides 34815.4.1 Characterization of Decomposition Residues 360
15.5 Kinetic Analysis 36415.5.1 Model-dependent Method for Non-isothermal Experiments 364
References 390
16 Thermal Degradation and Morphological Characteristics of Bone Products 393F. Miculescu, A. Maidaniuc, G.E. Stan, M. Miculescu, S.I. Voicu, L.T.Ciocan
16.1 Introduction and Objectives 39316.2 Short Overview on the Thermal Analysis Experimental Methods 396
16.2.1 Differential Thermal Analysis (DTA) 39616.2.2 Differential Scanning Calorimetry (DSC) 39616.2.3 Thermogravimetry Analysis (TGA) 39716.2.4 Dilatometry 39716.2.5 Thermal Analysis Experimental Parameters 398
16.2.5.1 Influence of Working Environment 398
Contents xi
16.2.5.2 Setting the Heating Rate 39916.2.5.3 Crucible Choice 399
16.2.6 Sample Preparation 39916.3 Morpho-structural Changes Induced by the Thermal Treatments
Applied to Hard Tissues. Bone Degradation Mechanism 40016.3.1 25–250 °C: Evaporation of adsorbed water 40116.3.2 250–550 °C: Degradation and Elimination of the
Organic Component 40116.3.3 600–800 °C: Elimination of Carbonate Groups.
Recrystallization of Hydroxyapatite 40416.3.4 800–1100 °C: Transformation of Hydroxyapatite into
beta-Tricalcium Phosphate [β-TCP] 40516.3.5 >1100 °: Conversion of β-TCP to α-TCP 406
16.4 Conclusions 408References 408
17 Processes and Mechanisms in Hydrothermal Degradation of Waste Electric and Electronic Equipment 411Yu Luling, He Wenzhi and Li Guangming 17.1 Introduction 41117.2 Application of Hydrothermal Degradation in
Treatment of WEEE 41417.2.1 Hydrothermal Degradation of Brominated Epoxy Resin 41417.2.2 Hydrothermal Degradation of Liquid Crystal 41517.2.3 Hydrothermal Conversion of Waste Polarizing
Film to Acetic Acid 41617.2.4 Hydrothermal Renovation of LiCoO2 in Spent
Lithium-Ion Batteries (LIBs) 41717.3 Mechanism of Hydrothermal Degradation for Treatment of WEEE 418
17.3.1 Mechanism of Brominated Epoxy Resin Degradation 41817.3.2 Mechanism of Liquid Crystal Degradation 42217.3.3 Mechanism of Conversion of Waste Polarizing
Film to Acetic Acid 42617.3.4 Mechanism of Renovation of LiCoO2 in Cathode of Spent
Lithium-Ion Batteries 42817.4 Conclusion 431Acknowledgements 431References 431
18 Heat Transfer Mechanism and Thermomechanical Analysis of Masonry Structures (Mortars and Bricks) Subjected to High Temperatures 437M.E. Maciá Torregrosa and J. Camacho Diez 18.1 Introduction: State of the Art 43718.2 Heat Transfer Mechanisms through a Masonry
Element under Load 44218.3 Influence of High Temperatures on the Structural Behavior of a Masonry
Element 444
xii Contents
18.4 Factors Involved in the Behavior of the Masonry Subjected to High Temperatures 44418.4.1 Thermal and Mechanical Properties 44518.4.2 Thermal Properties 446
18.4.2.1 Density 44718.4.2.2 Specific Heat 44718.4.2.3 Thermal Conductivity 44718.4.2.4 Thermal Expansion Coefficient 448
18.4.3 Mechanical Properties 44818.4.3.1 Young’s Modulus 44818.4.3.2 Stress–Strain Diagram with Respect to Temperature 449
18.5 Properties of the Ceramic Pieces 44918.5.1 Density 44918.5.2 Specific Heat 44918.5.3 Thermal Conductivity 45018.5.4 Thermal Expansion Coefficient 45318.5.5 Young’s Modulus 45518.5.6 Stress–Strain Diagram with respect to Temperature 455
18.6 Properties of the Mortar 45618.6.1 Density 45618.6.2 Specific Heat 45618.6.3 Thermal Conductivity 45718.6.4 Thermal Expansion Coefficient 45718.6.5 Young’s Modulus 45818.6.6 Stress–Strain Diagram with respect to Temperature 460
References 463
19 Application of Vibrational Spectroscopy to Elucidate Protein Conformational Changes Promoted by Thermal Treatment in Muscle-Based Food 467A.M. Herrero, P. Carmona, F. Jiménez-Colmenero and C. Ruíz-Capillas19.1 Introduction 46719.2 Protein Structure 46819.3 Muscle-Based Food Proteins: Thermal treatment 46819.4 Vibrational Spectroscopic Methods and Protein Structure 469
19.4.1 Raman Spectroscopy 46919.4.1.1 Raman Spectrum and Protein Secondary Structure 47019.4.1.2 Raman Spectrum and Protein Tertiary Structure 471
19.4.2 Infrared Spectroscopy 47119.4.2.1 Infrared Spectra and Protein Structure 471
19.5 Vibrational Spectroscopy to Elucidate Structural Changes Induced by Thermal Treatment in Muscle Foods 47319.5.1 Raman Spectroscopy and Heat-Induced Protein
Structural Changes 47319.5.2 Infrared Spectroscopy and Heat-Induced Changes in
Protein Structure 47619.6 Conclusions 479
Contents xiii
Acknowledgements 479References 480
20 Thermal Activation of Layered Hydroxide-Based Catalysts 483Milica Hadnadjev-Kostic, Tatjana Vulic and Radmila Marinkovic-Neducin20.1 Introduction 48320.2 LDH General Properties 484
20.2.1 Synthesis of LDH-Based Catalysts 48820.3 Thermal Activation of LDH-Based Catalysts – Thermal Decomposition
Pathway from LDH to Mixed Oxides 49020.4 Properties of Thermally Activated LDHs 495
20.4.1 Structural Properties of Thermally Activated Mixed Oxides 49520.4.2 Textural Properties of Mixed Oxides 49720.4.3 Memory Effect 49920.4.4 Paracrystallinity 50020.4.5 Acid–Base and Redox Properties of Mixed Oxides 500
20.5 Application of LDH-Based Materials 50120.5.1 Application of LDH and Their Mixed Oxides in
Photocatalytic Reactions 50120.6 Synthesis Methods of Ti-Containing LDH-Based Materials 50220.7 Synthesis Methods for the Association of TiO2 and
LDH-Based Catalysts 50220.7.1 Novel Synthesis Methods and Their Application in
Photocatalytic Reactions 50320.8 Conclusions and Perspectives 509References 510
21 Thermal Decomposition of Natural Fibers: Kinetics and Degradation Mechanisms 515Matheus Poletto, Heitor L. Ornaghi Júnior and Ademir J. Zattera 21.1 Introduction 51521.2 Theoretical Background 516
21.2.1 X-Ray Diffraction 51621.2.2 Degradation Kinetics 517
21.2.2.1 Degradation Reaction Mechanisms – Criado Method 51821.3 Chemical Composition of the Natural Fibers 52221.4 XRD Analysis Applied to Natural Fibers 52421.5 Thermogravimetric Analysis of Natural Fibers 527
21.5.1 Thermal Degradation of the Main Components of the Lignocellulosic Fibers 527
21.6 Kinetic Degradation and Reaction Mechanisms in the Solid State of Natural Fibers 532
21.7 Conclusion 541References 541
xiv Contents
22 On the Kinetic Mechanism of Non-isothermal Degradation of Solids 547Lyubomir T. Vlaev, Velyana G. Georgieva, and Mariana P. Tavlieva22.1 Introduction 54722.2 Mathematical Background in the Thermogravimetry 549
22.2.1 Algebraic Expression of f(a) Functions and Their Corresponding Mechanism 549
22.2.2 Calculation Procedures Based on Different Integral Methods 55022.2.3 Calculation Procedures Based on Isoconversional
(Model-Free) Methods 55522.2.4 Calculation of the Activation Energy by Iterative Procedure 55622.2.5 Determination of the Most Probable Mechanism Function 55722.2.6 Calculation of the Pre-exponential Factor in the
Arrhenius Equation 55822.2.7 Calculation of the Transition-State Thermodynamic Functions 56022.2.8 Estimation of the Lifetime 561
22.3 Kinetic Mechanism of the Thermal Degradation of CaC2O4·H2O 56122.4 Kinetic Mechanism of the Thermal Degradation of Chitin 56722.5 Kinetic Mechanism of the Thermal Degradation of Rice Husks 57122.6 Conclusions 574Acknowledgments 575References 575
Index 579
xv
Preface
Knowledge and experience relating to the stability of man-made materials is of great impor-tance for realizing suitable and reliable technologies. Control over the physico-mechanical properties of a synthetic material can be achieved by selecting the proper chemical ingredi-ents and reacting them under appropriate optimized conditions. High chemical, environ-mental, mechanical and thermal stability is often desired from a high-performance material. Knowledge of the thermal stability of a material as a function of time and temperature could provide valuable insights related to performance during the service life span of the material and, indeed, for life management too. High thermal stability is desired in materials operating in extreme service conditions, while moderate to low for biological applications.
Extensive amounts of published literature are available on the thermal analysis of mate-rials. However, the content of most of the published papers is limited to monitoring of thermal degradation steps. It is extremely difficult to find articles on the mechanisms of decomposition of materials, partly because such studies are difficult to establish and justify. Several books are available on fundamental aspects of the techniques; however, applica-tion of these techniques to new materials has not yet been properly documented. We have noticed that researchers find it difficult to understand and explain the reasons behind the multiple-step decomposition patterns in their materials. The poor availability of under-stood investigations with good insights into the mechanistic routes and associated reac-tions has been the primary reason for such a lacuna. In this book, we have invited authors who have expertise in dealing with thermal analysis of materials and suggested that they contribute chapters focusing on the reactions occurring during thermal decomposition and mechanisms of reactions during such processes.
This edited volume consists of twenty-two chapters relating to the reactions and associ-ated mechanisms observed in advanced materials as a function of time and temperature. It is divided into two parts; the first part containing information about degradation of poly-meric materials and the second part devoted to materials other than polymers. The first chapter discusses thermal stability of organic layers grafted onto silicon and the reaction mechanisms of thermal degradation occurring in such hybrid systems. The use of thermo-gravimetric analysis to characterize biomedical ultrahigh molecular weight polyethylenes is discussed in a separate chapter, followed by a study on how thermal analysis controls the phase composition, size, and porosity of materials. Similarly, the reactions occurring in composites of high-density polyethylene containing magnesium hydroxide as flame retardant are presented in a committed chapter. Another chapter is devoted to the study of changes in thermal properties, crystallinity and decomposition kinetics during the deg-radation of aliphatic polyesters. A detailed investigation of polymer degradation modes and degradation mechanisms under thermal, chemical, biological, and radiation effects has also been provided as a separate chapter. Additionally, this chapter provides a fundamental
xvi Preface
overview of widely-accepted mechanisms and methods of applying these mechanisms, either individually or in combination with the existing high-end polymers. Thermal deg-radation of polyurethanes is covered in another chapter, and several techniques to study the degradation processes, degradation mechanisms, and ways for improving their thermal stability are highlighted in a chapter related to polyurethanes. Likewise, the design, degra-dation behavior, associated mechanisms, and structure-property relationship of thermally reworkable epoxy resins have been added in a separate chapter.
The degradation pattern and mechanism of thermal decomposition of vinylidene chlo-ride polymers have been systematically described in a separate chapter, followed by an over-view chapter on the use of mass spectrometry as an analytical tool to investigate the thermal degradation mechanisms in macromolecules. Another important study on the degradation behavior of general purpose poly(styrene) has been carefully described in a separate chap-ter. The reaction mechanisms involving the degradation of lipid-based compounds such as Brazil nut oil have been explored in an individual chapter, followed by a study on the reac-tion mechanism during thermal degradation of cellulose and cellulosic substrates such as cotton and paper.
An interesting study reporting on sodium alkoxides under isothermal and non-isother-mal conditions using the thermogravimetric method has its own chapter, followed by a chapter on the influences of temperature on the products obtained through thermal pro-cessing of hard tissues. A unique chapter on the application of the hydrothermal method for the treatment of waste electric and electronic equipment has also been included. Another chapter describes the theoretical criteria for carrying out the evaluation of a masonry struc-ture affected by high temperature firing. It is evident that the heat processing step produces molecular changes in muscle proteins that affect the functional and textural properties, resulting in the alteration of the quality of the final product. A chapter highlights the use of spectroscopic techniques in determining the protein structural changes in muscle food products and blood plasma during heating. The effect of temperature on the structural and textural properties of layered double hydroxide-based catalyst has also been postulated on, followed by a study to evaluate the degradation process of ten different natural fibers by X-ray diffraction and thermogravimetry techniques. The kinetic mechanism for non-iso-thermal degradation of solids has been described in the final chapter.
We are confident that after reading these chapters, students and researchers will develop a deeper understanding of the degradation patterns in advanced next-generation materi-als and could devise the mechanistic routes of decomposition in their materials based on robust insights. Such contributions by the readers could further push the frontiers of the science and technology of degradations, a vital pursuit. This is a useful book for readers from diverse backgrounds in chemistry, physics, biology, materials science and engineer-ing, including chemical engineering. This book can be used as a reference for students and research scholars and as a guide for technologists working in the industry.
Atul Tiwari, PhDBaldev Raj, PhD
June 1, 2015
Part 1 DEGRADATION OF POLYMERS
3
Atul Tiwari and Baldev Raj, Reactions and Mechanisms in Thermal Analysis of Advanced Materials, (3–38) © 2015 Scrivener Publishing LLC
*Corresponding author: [email protected]
1
Thermal Stability of Organic Monolayers Covalently Grafted on Silicon Surfaces
Florent Yang1,*, Philippe Allongue2, François Ozanam2, and Jean-Noël Chazalviel2
1Department of Materials and Earth Sciences, Surface Science Division, Darmstadt University of Technology, Darmstadt, Germany
2Physique de la Matière Condensée, Ecole Polytechnique, CNRS, Palaiseau, France
AbstractOrganic modification of silicon surfaces is a topic of high interest in fundamental surface chemistry research as well as for the development of technological applications ranging from microelectronics to photovoltaics and biotechnology. Over the past decades, many approaches to anchor covalently organic monolayers to hydrogen-terminated silicon surfaces have been investigated. These organic monolayers may bear specific terminal groups depending on the application aimed at. Also, they may be used as buffer layers to protect the silicon surface against oxidation in contact with atmospheric environment or aqueous media. In the context of further modification of the silicon surfaces (e.g., in microelectronics), thermal processes at tempera-tures higher than ambient are necessary; thus, the understanding of the reaction mechanisms of the thermal decomposition of such organic layers may become an important issue. In this chap-ter, the thermal stability of organic layers grafted onto silicon surfaces is reviewed, and the reac-tion mechanisms of the thermal degradation occurring on these hybrid systems are discussed.
Keywords: Thermal stability, silicon, organic modification, grafting, organic monolayers, alkyl/alkoxy chains, aryl groups, Si–N linkages, heterostructures, silicon hybrid systems, reaction mechanisms
1.1 Introduction
Organic modification of semiconductor materials [1–5], especially silicon (Si) [1, 2, 5], has attracted increasing interest because of the importance of understanding, control-ling, and tailoring the properties of these organic/inorganic interfaces. Nowadays, the importance of surface chemistry in fundamental research and technological applica-tions has grown up together with the ability to selectively modify oxide-free Si surfaces. Functionalization of Si surfaces by grafting organic molecules onto the hydrogen-ter-minated Si surface, which leads to the formation of covalent Si–C, Si–O–C, or Si–N bonding, opens the way to various applications ranging from molecular electronics
4 Reactions and Mechanisms in Thermal Analysis of Advanced Materials
[6–8], microelectronics [9–13], and photovoltaics [14–18] to photoelectrochemical devices [19, 20]. Two different pathways can be used to tether organic species on Si surfaces. One is to start from clean and reconstructed Si surfaces in ultrahigh vacuum (UHV) conditions (e.g., by heating the sample up to 1000 °C), which can subsequently react with organic compounds in UHV [3, 5, 21, 22]. Another pathway, which will be our main focus, consists in reacting an H- or Cl-terminated Si surface with organic functions at ambient pressure using suitable precursor molecules. The processes are often conducted by dissolving the precursor in an organic solvent or with the neat liq-uid. Figure 1.1 summarizes the main methods to covalently anchor organic molecules to oxide-free Si surfaces by wet chemical means. The formation of well-ordered organic monolayers covalently bonded to Si using the latter process has been the subject of many studies. Among those are studies on alkyl [23–33], alkenyl [34–38], alkynyl [38–43], and alkoxy chains [44–54], aryl groups [55–67], and chains bearing a functional termination [68–73] to name a few. Moreover, the presence of functional groups, such as –OH, –COOH, or –N3, on top of the grafted organic monolayer, may be a start-ing point to extend the library of available chemical terminal groups, in particular to develop Si-based chemical and biochemical sensors [74–78], e.g., using click chem-istry [79–83]. The organic/Si hybrid system can be achieved via a wide variety of wet chemical [84], photochemical [85], and electrochemical [86] methods. Among the wet chemical methods, the hydrosilylation route [87, 88], which is a well-known reaction in organic chemistry, is widely used to make alkenes, alkynes, or other unsaturated hydro-carbon compounds react with a H-terminated Si surface. Electrochemical grafting can be made from organolithium or Grignard reagents [26, 40, 89] in anodic conditions, or from diazonium compounds in cathodic conditions to obtain aryl-grafted surfaces [55, 90–92]. Among these methods, those which have been used in connection with thermal stability studies will be briefly described in the corresponding subchapters. However, since a thorough description of these organic grafting methods is beyond the scope of this chapter, we suggest the interested reader to refer to the several excel-lent reviews on this topic already published over the past decades [1, 2, 84, 93–95]. The different approaches to form oxide-free organic/Si-based hybrid interfaces presented in this chapter are summarized in Figure 1.1, in the case of H-terminated Si(111) sur-faces, but can also be extended to other types of Si surfaces. Organically modified Si surfaces exhibit excellent electronic quality (low concentration of surface states in the mid-gap) and improved chemical resistance against the formation of an oxide layer under exposure to ambient air [27, 90, 96–99] or in aqueous media [100–104].
Thermal stability of the grafted organic monolayers may also become an issue in the context of microelectronics because there is an increasing need to replace the silicon dioxide (SiO2) gate insulator in the elementary metal-oxide-semiconductor field-effect transistors (MOSFETs) by a material with a higher dielectric constant (so-called “high-κ oxide”, e.g., HfO2, whose dielectric constant is 4–6 times as high as that of SiO2) [105–107]. The replacement of the SiO2 gate oxide layer is necessary to keep the switching performances of individual MOSFETs and to reduce the leakage current appearing upon decreasing the size of the elementary MOSFETs [108–110]. However, the deposition of high-κ oxides on silicon substrates is accompanied with the formation of an SiO2 interlayer and/or silicate between the silicon and the high-κ oxide, leading to
Thermal Stability of Organic Monolayers Covalently Grafted 5
Figu
re 1
.1 S
chem
atic
illu
stra
tion
of th
e di
ffere
nt ch
emic
al m
odifi
catio
n pa
thw
ays t
o ob
tain
org
anic
ally
mod
ified
Si s
urfa
ces f
rom
atom
ical
ly fl
at H
-ter
min
ated
Si
(111
) sur
face
s as s
tart
ing
subs
trat
es.
Alk
oxy-
graf
ted
Si(1
11) s
urfa
ces
Ary
l-gra
fted
Si(1
11) s
urfa
ces
Gra
ftin
g th
roug
h Si
–N li
nkag
es
Chem
ical
fu
nctio
nalit
ies
avai
labl
e
Shor
t cha
ins
avai
labl
e
CH3
CH3
CH3
HH
H
HH
OO
OSi
SiSi
SiSi
SiSi
Si
HH
HH
SiSi
SiSi
O
e.g.
R or
HO
HSi
SiSi
Si
Si(1
11)
Si(1
11)
Si(1
11)
Si(1
11)
Si(1
11)
Si(1
11)
Si(1
11)
Si(1
11)
Si(1
11)
Si(1
11)
Star
ting
subs
trat
e
Anhydro
us MeO
HR-M
gXCH
3
CH3
CH3
CH3
OO
HH
SiSi Si
(111
)
Chlo
rinat
ion/
Alco
hol R
oute
R-O
H
Cl SiSi
SiSi
SiSi
SiSi
SiSi
SiSi
HH
SiSi
SiSi
Spec
ial c
ase
for m
ethy
l – 10
0% c
over
age
PCI5
inbe
nzoy
l per
oxid
eR-
MgX
or R
-Li
PCI5
inbe
nzoy
l per
oxid
e
ClCl
ClCl
ClCl
Cl
CH3
CH3
CH3 Si(1
11)
SiSi
SiSi
CH3
CH3
CH3
CH3
CH3
or
COO
H
Hyd
rosi
lyla
tion
Anod
ic G
raft
ing
Cath
odic
Gra
ftin
gCh
lorin
atio
n/Am
ine
Rout
e
Aryl
diaz
oniu
m sa
ltsH
HH
H
SiSi
SiSi
Si(1
11)
SiClCl
ClCl
ClCl
NR
SiSi
Si
Si(1
11)
SiR-
NH
2Si
SiSi
SiSi
Si
X
X
H
N2+
H
X Si
Anod
ic G
raft
ing
Chlo
rinat
ion/
Grig
nard
Rou
te
Prec
urso
rs
OH
-RO
=CH
-RCH
2=CH
-RPr
ecur
sors
e.g.
Hyd
rosi
lyla
tion
SiSi
Alk
yl-g
raft
ed S
i(111
) sur
face
s
Chem
ical
func
tiona
litie
sav
aila
ble
Shor
t cha
ins
avai
labl
e
∆, h
v, or
cata
lyst
∆, h
v, or
cata
lyst
6 Reactions and Mechanisms in Thermal Analysis of Advanced Materials
an unwanted series capacitance [111]. In this context, the passivation of the silicon sur-face with an ultrathin organic layer such as a short alkyl or alkoxy layer [13], or by the formation of Si–N linkages [112] was proposed to reduce SiO2 formation. For instance, oxygen diffusion was retarded by depositing HfO2 on a nitride-passivated Si surface, which minimized the formation of interfacial SiO2 compared to the case of HfO2 grown on an H-terminated Si surface [113]. In many instances, the deposition of the high-κ oxide is performed at temperatures well above 300 K, and post-annealing treatments may be necessary to remove organic contaminations and/or cure out defects in the as-deposited oxide. This raises the question of the thermal stability of the organic layers, which is the key issue. The reaction mechanisms of the thermal decomposition of these organic layers will be described in this chapter.
For good electronic properties of the grafted Si surface, an obvious prerequisite is that the starting surface (H-terminated Si surface) has to be as clean as possible, pre-senting a high degree of passivation without defects.
1.1.1 Hydrogen-Terminated Si Surfaces
Obtaining an H-terminated Si surface is easily achieved at ambient by selective etching of the native oxide in aqueous fluoride solutions or even in alkaline solutions where Si is etched at a high rate [114–117]. This surface chemistry, also theoretically explained by ab initio calculations [118], confers this surface unique properties. It is quite hydro-phobic (contact angle ~90 °), stable in air, and it takes several hours to grow a native oxide layer. From an electronic viewpoint, H-terminated Si surfaces are known to pres-ent the slowest recombination velocity ever reported due to the absence of any surface states in the band gap [119]. From a structural viewpoint, Si etching becomes more and more anisotropic with increasing pH. In alkaline solutions [120, 121] and in 40% NH4F [122], the (111) planes are etching stop planes for steric reasons while the (100) and (110) planes undergo faster dissolution. At low pH, the overall etching is quasi isotropic.
Obtaining an H-terminated Si(111) model surface with 100 nm wide terraces (mis-cut 0.2 °) free of etch pits and separated by rectilinear atomic steps, such as the one in Figure 1.2a, is feasible because this plane is an etch-stop plane. This requires, however, careful operating conditions. The NH4F solution must be oxygen-free [123], by bub-bling it with N2 or adding sulfite ions as an oxygen scavenger [124], the sample must present a rough part to act as sacrificial anode [117, 125] and the miscut must be pre-cisely aligned toward the [11 2–] direction [117, 125]. On the atomic scale the terraces present a (1 × 1) structure (see inset of Figure 1.2a) with 7.8 1014 vertical monohydride (≡Si–H) sites per cm2 in agreement with the narrow IR band at 2083 cm-1 observed in p-polarization only (see Figure 1.2b). If one of these conditions is not fulfilled, a larger density of structural defects, associated with dihydride (=SiH2) sites, is observed (these are located at triangular etch pits [126] and at irregular steps [117]). Obtaining an H-terminated Si(100) surface with wide atomically smooth terraces is much more difficult since this plane does not correspond to an etch-stop plane. A flat but atomi-cally rough surface is obtained at low pH and elevating the pH of the solution leads to facets or even pyramids in strongly alkaline solutions. However, recently, Hines and
Thermal Stability of Organic Monolayers Covalently Grafted 7
coworkers revealed that NH4F etching of Si(100) surfaces may actually result in near atomically flat H-terminated Si(100) surfaces, consisting of well-defined bilayer struc-tures, as shown in Figure 1.2c [127–129].
In some applications, porous silicon (PSi) is, however, desired to increase the spe-cific surface area (up to 1000 m2/cm3). Several books and reviews have been already published about PSi formation mechanisms, morphologies and optical properties (cf. [130–133]). Briefly, PSi is usually prepared from (100) Si wafers by constant-current anodization in an ethanolic solution (mixture of HF with ethanol). The characteris-tics of the pores are determined by the doping of the substrate, the HF concentration and the current density used during the anodization process [134–139]. PSi surfaces present ≡Si–H, =SiH2, and trihydride (–SiH3) sites. Potential steric hindrances could appear most likely when micropores formation is obtained. It should be mentioned that PSi received increasing interest in the 1990s because of its luminescence proper-ties in the visible range.
Long-term stability of H-terminated Si surfaces in air (or aqueous media) is an important issue. Upon exposure to air, H-terminated single-crystal Si surfaces tend to become slowly covered with a thin, native silicon oxide layer, which makes the surface improper for further chemical modification. Moreover, this oxidation in ambient is associated with a degradation of the electronic properties of the surface (high charge carrier surface recombination velocities, reflecting a significant density of active trap sites on the surface) [96, 119, 140–142]. Therefore, after the etching step, freshly pre-pared H-terminated Si surfaces should be used for organic modification. Care should also be given to the chemical solution used for grafting, which should be as much as possible free of water and oxygen to avoid silicon oxidation.
Figure 1.2 Atomic force microscopy (AFM) topography image (a) and polarized infrared (IR) absorption spectra (b) of an atomically flat H-terminated (111) Si surface prepared by etching in oxygen-free 40% NH4F solution (pH = 7.8). After Ref. [117]. (c) Scanning tunneling microscopy (STM) image of an H-terminated (100) Si surface etched in 40% NH4F solution presenting well-defined bilayer structures. After Ref. [128]. Inset in (a) displays a surface with a (1 × 1) structure on the atomic scale. (c) is adapted and reprinted with permission from Refs. [128,129]. Copyright 2009 and 2012 by American Chemical Society.
(a)
Abs
orba
nce
per r
e�ec
tion
Si(111)–H Si(100)–H2083 cm-1
H
Si
p-pol.s-pol.
2000 2100Wave number (cm-1)
200 nm
(b) (c)
5 nm
0.76 nm
0.38 nm
8 Reactions and Mechanisms in Thermal Analysis of Advanced Materials
1.2 Alkyl-Grafted Surfaces
1.2.1 Preparation
The hydrogenated silicon surface can be grafted with alkyl groups by using various reaction routes. The most popular one by far has been the hydrosilylation of an alkene precursor:
≡Si–H + H2C=CH–R ∆
→, ,h orn
catalyst ≡Si–CH2–CH2–R. (1.1)
This reaction, originally promoted by radical initiation using acyl peroxides [88], can actually be promoted by various other means: thermal activation (typically 200 °C overnight [88, 143, 144]), photochemical activation (typically 3 hours under 5 mW/cm2 irradiation at 312 nm [25, 85], or under white light [34, 37]), or use of a catalyst (typi-cally 100 °C overnight in the presence of the Lewis-acid catalyst EtAlCl2 [25, 35, 145]). In all of the cases, the grafting reaction is in competition with the oxidation of the sili-con surface, which may be prevented by working under strictly anhydrous conditions [146] (note that such conditions are automatically fulfilled in the presence of EtAlCl2). It has been proposed that the grafting reaction proceeds by formation and propagation of a surface silyl radical (dangling bond) [88]. This proposal has received support from scanning tunneling microscopy (STM) investigations under UHV [147, 148], though it may not apply to all of the aforementioned experimental protocols [2, 149–151].
However, the hydrosilylation method is unpractical for the short alkyl chains (below C6), for which the alkene precursors are gaseous, and it does not allow at all for the grafting of methyl groups. In these cases, alternate methods are based on the use of a Grignard precursor (RMgX). Direct reaction of the Grignard precursor with the silicon surface may be carried out by thermal activation [25, 152, 153] or by electrochemical treatment through anodic decomposition of the Grignard and formation of reactive R• radicals [26, 154]. The latter method is very fast and easy to implement because the Grignards in ether solution are largely dissociated and behave as good electrolytes.
RMgX → R• + MgX+ + e–, (1.2)
≡Si–H + R• → ≡Si• + H–R, (1.3)
≡Si• + RMgX → ≡Si–R + MgX+ + e–. (1.4)Note that the mechanism of the thermal reaction has actually been reported to con-
sist of a zero-current electrochemical mechanism [153]. Alternately, the hydrogenated Si surface may be activated by halogenation (chlorination with PCl5 [23, 28, 155] or bromination with bromochloroform or N-bromosuccinimide [56]). The halogenated surface subsequently reacts with a Grignard [23, 28, 155] or an organolithium com-pound [56]. We write here the reactions for the chlorination/Grignard route:
≡Si–H + PCl5 → ≡Si–Cl + HCl + PCl3, (1.5)
≡Si–Cl + RMgX → ≡Si–R + MgXCl. (1.6)Note that the Grignard and organolithium compounds are extremely reactive toward
water, which ensures that anhydrous conditions are automatically warranted in their
Thermal Stability of Organic Monolayers Covalently Grafted 9
presence. Alkyl radicals may also be obtained and grafted on Si by cathodic reduction of haloalkanes, but rigorous exclusion of water is then mandatory [156].
All of these methods lead to essentially dense layers (though the thermal hydro-silylation method may be somewhat superior in this respect [157]) because the alkyl coverage is limited by steric hindrance among the alkyl chains. At a (111) Si surface, the alkyl chains are tilted by an angle of about 30 ° with respect to the surface normal, and the coverage (fraction of substituted SiHs) is at most of ca. 50% [150, 157]. However, the case of methyl is special: due to the small size of the methyl group, a 100% coverage at an atomically flat (111) Si surface may be reached [26, 158].
In a similar manner to the grafting of alkyl chains to an Si surface, alkenyl layers can be obtained by performing a hydrosilylation reaction with an alkyne precursor instead of an alkene:
≡Si–H + HC≡C–R ∆
→, ,h orn
catalyst ≡Si–CH=CH–R. (1.7)
Alkynes appear to be more reactive than alkenes, and the obtained alkenyl layers have been reported to be more dense and resistant toward chemical oxidation than their alkyl counterparts [159–165].
1.2.2 Thermal Stability of Alkyl-Grafted Surfaces
The stability of alkyl-grafted surfaces has been investigated by high-resolution elec-tron energy loss spectroscopy (HREELS), synchrotron X-ray photoelectron spectros-copy (SXPS), and IR spectroscopy. In a pioneering work, Sung et al. annealed octadecyl monolayers at increasing temperatures in UHV and monitored the HREELS spectrum at each annealing step [166]. They found that the characteristic vibrational modes of the grafted layer (νCH at 2920 cm-1, δCHx at 1280–1450 cm-1) stay unchanged up to about 600 K, then they decrease and SiH vibrational modes appear instead (Figure 1.3a). These observations led them to conclude that the layers desorb as a whole, according to a b-elimination mechanism, which is just the reverse of the hydrosilylation reaction:
≡Si–CH2–CH2–R ∆>600K → ≡Si–H + H2C=CH–R. (1.8)
Also, they observed that the wetting angle decreases upon annealing, a variation that correlates with the changes in chemical composition. In a later study of long-alkyl monolayers using IR spectroscopy, Yamada et al. observed a loss in the νCH intensity (2800–3000 cm-1 region) after annealing above 490 K (half of the intensity being lost at 540 K, the maximum temperature reached in their work), together with a small high-energy shift of the νCH bands, apparent above 440 K and attributed to an irreversible loss in the organization of the layer [167].
Faucheux et al. investigated the chain-length dependence by the annealing of alkyl monolayers (from C2 to C18) using IR spectroscopy in either a primary vacuum or a controlled atmosphere (oxidizing, reducing, or neutral) at low pressure [29]. Typical IR spectra are shown in Figure 1.3b. These results essentially agree with those of Yamada et al. and extend them to higher temperatures. A difference, however, concerns the Si–H vibration modes which do not reappear. Instead, Si–O (broad band at ca. 1100 cm–1)
10 Reactions and Mechanisms in Thermal Analysis of Advanced Materials
and Si–H with oxidized Si backbonds (bands at 2200 and 2250 cm–1, see Figure 1.3b) were observed. This difference is plausibly due to an imperfect control of the atmo-sphere (e.g., a higher amount of moisture may favor oxidation of the SiHs). The major difference between this work and that of Sung et al. is that the loss of organic material was found to start at around 250 °C (523 K) instead of 600 K and to be about com-plete at 350 °C. The major part of this difference can be accounted for by the different experimental protocol (15 min spent at each annealing temperature, comparable to the duration in Yamada’s work, but much longer than the duration of 1 min in Sung’s work). Based on the observation of a weak δSCH3 mode at 1255 cm–1, characteristic of Si–CH3 groups, Faucheux et al. considered an alternate desorption pathway for chains longer than C2 [29]:
≡Si–CH2–CH2–CH2–R ∆> 523K
→ ≡Si–CH3 + H2C=CH–R. (1.9)
However, the magnitude of the IR signals led them to conclude that this desorp-tion mechanism is secondary [29, 168]. The characteristic desorption temperature was found to be essentially independent of the alkyl chain length, except for the shortest ones (about 25 °C higher for ethyl). The experimental data were analyzed in the frame-work of a thermally activated first-order desorption model, i.e., desorption rate dnS/dt = nSw0 exp(-Ea/kBT). Assuming a plausible value of w0 = 109 s-1, the experimental results were quantitatively accounted for with an activation enthalpy Ea = 1.34 eV, consistent with the DH ~ 1 eV enthalpy change of the b-elimination reaction (see Figure 1.4; note that the activation enthalpy is expected to be larger than the reaction enthalpy) [29]. The somewhat higher desorption temperature found for ethyl is consistent with the ~0.1 eV higher value of DH as compared to longer chains. Finally, it was reported that the methylated Si surface (≡Si–CH3) is by far more stable, exhibiting no significant change up to 400 °C [29]. This is consistent with the fact that the b-elimination mecha-nism can operate only for chain lengths containing at least two carbon atoms; hence,
780
x 2
x 2
x 2
2100 2920
(a) (b)Wave number (cm-1)
Inte
nsit
y (a
rb. u
nits
)
0 1000 2000 3000 4000
785 K
5
4
3
2
1
0
-1
-2
νSiO
νSiH
x 8
400ºC νCH
350ºC
300ºC
250ºC
200ºC150ºC100ºC
750 K
720 K
680 K
650 K
615 K
unannealed
1000 1500 2000 2500 3000Wave number (cm-1)
103 ×
Cha
nge
in IR
abs
orba
nce
In (I
0/I)
1060
Figure 1.3 (a) Changes in the HREELS spectra of an octadecyl-grafted (100) Si surface after successive annealing by steps of ca. 35 °C, 1 min duration. (b) Changes in IR absorbance of a decyl-grafted atomically flat (111) Si surface after successive annealing by steps of 50 °C, 15 min duration. Note that the IR spectra represent the difference spectra from the freshly grafted surface to the grafted surface after annealing. Adapted and reprinted with permission from Refs. [166] (a), and [29] (b), respectively. Copyright 1997 (a) by American Chemical Society; Copyright 2006 (b) by AIP Publishing Group.
Thermal Stability of Organic Monolayers Covalently Grafted 11
the desorption of methyl calls for another mechanism. The desorption experiments in N2 atmosphere of Li led to essentially similar conclusions [169].
Finally, a detailed analysis of the annealing of ≡Si–C2H5 and ≡Si–CH3 surfaces using surface-sensitive SXPS was carried out by Hunger et al. [170] and Jaeckel et al. (Figure 1.5a-d) [171]. Their results indicate that for ethyl, two desorption mechanisms are present: the b-desorption mechanism, which leads to the desorption of ethylene, and a second mechanism, which leaves a methyl group on the surface:
≡Si–CH2–CH3 ∆> 713K
→ Si–CH3 + (CH2)g, (1.10a)
2 × (CH2)g → (C2H4)g. (1.10b)
Two desorbed methylene groups are thought to couple, forming an ethylene mol-ecule [171]. Such a mechanism is similar to the one considered by Faucheux et al. (1.9) and regarded as secondary. The apparently larger contribution of this reaction pathway in the experiments of Jaeckel et al. can plausibly be attributed to the different sequence of the annealing temperatures in the two experiments [171].
The case of the methylated surface is especially interesting. Jaeckel et al. found that it is stable in UHV up to at least 440 °C, and annealing steps up to that temperature were found to be useful for removing adventitious hydrocarbon contamination [32, 172] (Figure 1.5c: see the decrease of the C 1s contribution at around 285.5 eV in the 300–440 °C range, while the main C–Si contribution at 284.5 eV remains quasi unchanged). At 530 °C, the methyl layer was found to disappear and XPS lines characteristic of silicon carbide were observed instead (Figure 1.5d). However, a closer examination of the methylated Si surfaces in the 390–450 °C range was also investigated by Yang et al. (Figure 1.6) [32], and more particularly in the case of CH3- and CD3-terminated Si(111) surfaces. They found that a small contribution attributed to silicon carbide (see
Temperature (ºC)(a) (b)
Temperature (ºC)
3
2
1
0
3
2
1
0
100 200 300 400 100 200 300 400500
Surf
ace
conc
entr
atio
n (1
014 c
m-2
)
Surf
ace
conc
entr
atio
n (1
014 c
m-2
)
Figure 1.4 Change in the surface concentration of alkyl chains upon annealing. (a) Experimental, corresponding to Figure. 1.3a (1 min annealing steps, after Ref. [166], circles) and 1.3b (15 min annealing steps, after Ref. [29], squares). (b) simulated annealing steps (1-min steps for the circles, 15-min steps for the squares) assuming a first-order rate constant w0 exp(-Ea/kBT), with w0 = 109 s-1 and Ea = 1.34 eV (same for the two simulations). Note that the different duration of the annealing steps largely accounts for the apparent discrepancy between the two data sets in (a) (the gap between the two simulations would be increased further by choosing lower values for w0 and Ea). The curves (dashed curve for the 1-min steps, solid curve for the 15-min steps) are only guides for the eye. Adapted and reprinted with permission from Ref. [29]. Copyright 2006 (b) by AIP Publishing Group.
12 Reactions and Mechanisms in Thermal Analysis of Advanced Materials
Si-C2H
5 Si-C2H
5
CCH2
CSi Si carbide
530 ºC
530 ºC440 ºC
440 ºC300 ºC
300 ºC
freshfresh
Si-CH3 Si-CH
3
Si-H
102 101 100 99
530 ºC
440 ºC
300 ºC
fresh
Si-H
288 286 284 282Binding energy (eV)
530 ºC
550 ºC
500 ºC
440 ºC
300 ºC
0.2
0.1
0
0.05
0
2800 2900 3000 3100700 800
νSiC + ρCH3
Wave number (cm-1)
fresh
Inte
nsity
(arb
. uni
ts)
Inte
nsity
(arb
. uni
ts)
Inte
nsity
(arb
. uni
ts)
Binding energy (eV)
Inte
nsity
(arb
. uni
ts)
IR a
bsor
ptio
n In
(I0/I)
IR a
bsor
ptio
n In
(I0/I)
450 ºC
400 ºC
350 ºC300 ºC
200 ºC
23 ºC
550 ºC
500 ºC
450 ºC
400 ºC
350 ºC
300 ºC
200 ºC
23 ºC
νSCH
3ν
ASCH
3
Wave number (cm-1)
(a) (b)
(c)
(e) (f)
(d)
C 1s Si 2p
Figure 1.5 Annealing of methyl- and ethyl-grafted silicon surfaces. (a–d) Changes in the C 1s (a,c) and Si 2p (b,d) photoemission spectra of an ethyl-grafted (111) Si surface (a,b) and a methyl-grafted (111)Si surface (c,d) upon successive annealing at 300 °C, 440 °C and 530 °C. The spectra were recorded at a photon energy of 330 eV. After Ref. [171]. (e–f) Change in the IR transmission spectrum of a methyl-grafted porous silicon layer upon successive annealing by increments of 50 °C from 50 °C to 550 °C (there is no measurable change up to ca. 400 °C; this is why not all of the spectra are shown). After Ref. [173]. (a–d) Adapted and reprinted with permission from Ref. [171]. Copyright 2007 by American Chemical Society.
C4 in Figure 1.6a) already starts to grow at 390 °C in the case of CH3, but is not observ-able in the case of CD3-terminated Si surfaces, indicating the superior performance in the case of CD3. After a subsequent annealing to 450 °C, the silicon carbide contribu-tion increased by a factor 3 of its original value, simultaneously with a strong decrease of the contribution from the remnant adventitious aliphatic hydrocarbons, as shown in Figure 1.6a. These experiments demonstrate that an annealing treatment can be beneficial for methyl-terminated Si surfaces to get rid of organic contaminants with-out damaging the Si/organic interface. The presence of silicon carbide upon annealing may strongly depend on the grafting method, the quality of the solution containing the organic compound, as well as the annealing dwelling time. Finally, in an oxygen-contaminated atmosphere and using methyl-grafted porous silicon, Faucheux found
