Materials Science and Engineering of Carbon: Characterization

By Michio Inagaki and Feiyu Kang

Materials Science and Engineering of Carbon: Characterization discusses 12 characterization techniques, focusing on their application to carbon materials, including X-ray diffraction, X-ray small-angle scattering, transmission electron microscopy, Raman spectroscopy, scanning electron microscopy, image analysis, X-ray photoelectron spectroscopy, magnetoresistance, electrochemical performance, pore structure analysis, thermal analyses, and quantification of functional groups.

Each contributor in the book has worked on carbon materials for many years, and their background and experience will provide guidance on the development and research of carbon materials and their further applications.

• Focuses on characterization techniques for carbon materials
• Authored by experts who are considered specialists in their respective techniques
• Presents practical results on various carbon materials, including fault results, which will help readers understand the optimum conditions for the characterization of carbon materials

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 7, 2016
• ISBN: 9780128054680

Book preview

Materials Science and Engineering of Carbon

Characterization

Editor

Michio Inagaki

Professor Emeritus of Hokkaido University, 228-7399 Nakagawa, Hosoe-cho, Kita-ku, Hamamatsu 431-1304, Japan

Feiyu Kang

Graduate School at Shenzhen, Tsinghua University, University Town, Shenzhen, Guangdong Province 518055, China

Table of Contents

Cover image

Title page

Copyright

List of Contributors

Preface

Acknowledgments

Chapter 1. Introduction

1.1. Carbon Materials

1.2. Characterization of Carbon Materials

1.3. Structure of the Present Book

Chapter 2. X-ray Powder Diffraction

2.1. Introduction

2.2. X-ray Diffraction Pattern of Carbon Materials

2.3. Parameters Determined by X-ray Diffraction

2.4. Instrumentation

2.5. Specifications for Measurement

2.6. Degree of Graphitization

2.7. Key Issues for Measurement

2.8. Concluding Remarks

Chapter 3. Small-Angle X-ray Scattering

3.1. Introduction

3.2. Fundamentals

3.3. Key Issues for the Measurements

3.4. Applications for Carbon Materials

3.5. Concluding Remarks

Chapter 4. Transmission Electron Microscopy

4.1. Introduction

4.2. Modes of Transmission Electron Microscopy

4.3. Key Issues for Observation

4.5. Applications for Carbon Materials

4.6. Conclusions

Chapter 5. Scanning Electron Microscopy

5.1. Introduction

5.2. Instrumentation and Resolving Power

5.3. Specimen Preparation

5.4. Observation With the Out-Lens Objective Lens System

5.5. Observation With the Snorkel Objective Lens System

5.6. Observation With the In-Lens System

5.7. Electron Channeling Effect

5.8. Concluding Remarks

Chapter 6. Image Analysis

6.1. Introduction

6.2. Image Analysis Methods

6.3. Structure Analysis Through Transmission Electron Microscopy

6.4. Texture Analysis Through Scanning Electron Micrographs

6.5. Texture Analysis Through Optical Micrographs

6.6. Concluding Remarks

Chapter 7. Raman Spectroscopy

7.1. Introduction

7.2. Fundamentals

7.3. Key Issues for the Measurements

7.4. As a Measure for Structure Characterization

7.5. Concluding Remarks

Chapter 8. X-ray Photoelectron Spectroscopy

8.1. Introduction

8.2. Practical Side of Measurements

8.3. State Analysis

8.4. Semiquantitative Analysis

8.5. Concluding Remarks

Chapter 9. Magnetoresistance

9.1. Introduction

9.2. General Scheme of Δρ/ρ0 Change With Graphitization

9.3. Measurement of Magnetoresistance

9.4. Magnetoresistance Parameters for Coke

9.5. Magnetoresistance Parameters for Carbon Fibers and Extruded Coke

9.6. Magnetoresistance Parameters for Highly Crystallized Graphite Materials

9.7. Concluding Remarks

Supplement: Background of the Characterization of Carbon Materials With Δρ/ρ0

Chapter 10. Electrochemical Performance

10.1. Introduction

10.2. Fundamentals

10.3. Measurement Procedure

10.4. Concluding Remarks

Chapter 11. Gas Adsorption/Desorption Isotherm for Pore Structure Characterization

11.1. Introduction

11.2. Fundamentals

11.3. Key Issues for the Measurements and Analyses

11.4. Application to Carbon Materials

11.5. Concluding Remarks

Chapter 12. Thermal Analysis

12.1. Introduction

12.2. Fundamentals in Thermal Analyses

12.3. Key Issues for the Measurements

12.4. Applications of TG and DTG for Carbon Materials

12.5. Applications of DTA and DSC for Carbon Materials

12.6. Concluding Remarks

Chapter 13. Titration Method for the Identification of Surface Functional Groups

13.1. Introduction

13.2. Basic Concept of Titration Method

13.3. Instrumentation

13.4. Specification for the Methodology

13.5. Analysis of the Titration Results

13.6. Key Points for the Titration Measurements

13.7. Concluding Remarks

Chapter 14. Temperature Programmed Desorption

14.1. Introduction

14.2. TPD Experimental Conditions and Apparatus

14.3. Assignment of TPD Peaks to Surface Functional Groups

14.4. Secondary Reactions During a TPD Run

14.5. Effect of Air Exposure on TPD Patterns

14.6. Effect of Inorganic Matter in Carbons

14.7. Concluding Remarks

Index

Copyright

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List of Contributors

Sylvie Bonnamy,     Director of Research, ICMN, CNRS, University of Orléans, 1b rue de la Férollerie, 45071 Orléans Cedex 2, France

Ming-ming Chen,     Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

Katsuya Fukuyama,     Center for Liberal Arts, Meiji Gakuin University, Kamikurata, Totsuka, Yokohama 244-8539, Japan

Yoshikiyo Hatakeyama,     College of Humanities and Sciences, Nihon University, Sakurajosui, Setagaya, Tokyo 156-8550, Japan

Yoshihiro Hishiyama,     Professor Emeritus of Tokyo City University, 1-28-1 Tamazutsumi, Setagaya-ku, Tokyo 158-8557, Japan

Michio Inagaki,     Professor Emeritus of Hokkaido University, 228-7399 Nakagawa, Hosoe-cho, Kita-ku, Hamamatsu 431-1304, Japan

Takafumi Ishii,     Institute for Multidisciplinary Research for Advance Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

Norio Iwashita,     National Institute of Advanced Industrial Science and Technology (AIST), Onogawa, Tsukuba, Ibaraki 305-8569, Japan

Yutaka Kaburagi,     Professor Emeritus of Tokyo City University, 1-24-12-204 Kagahara, Tsuzuki-ku, Yokohama, Kanagawa 224-0055, Japan

Feiyu Kang,     Graduate School at Shenzhen, Tsinghua University, University Town, Shenzhen, Guangdong Province 518055, China

Yern Seung Kim,     Carbon Nanomaterials Design Laboratory, Global Research Laboratory, Research Institute of Advanced Materials, and Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea

Hidetaka Konno,     Professor Emeritus of Hokkaido University, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan

Takashi Kyotani,     Institute for Multidisciplinary Research for Advance Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

Yoko Nishi,     Toyo Tanso Co. Ltd., Takuma Division, 2791 Matsusaki, Takuma-cho, Mitoyo, Kagawa 769-1102, Japan

Agnès Oberlin,     Director of Research, CNRS, Saint Martin de Londres, France

Kyoichi Oshida,     National Institute of Technology (NIT), Nagano College, 716 Tokuma, Nagano 381-8550, Japan

Chong Rae Park,     Carbon Nanomaterials Design Laboratory, Global Research Laboratory, Research Institute of Advanced Materials, and Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea

Soshi Shiraishi,     Graduate School of Science and Technology, Gunma University, Kiryu, Gunma 376-8515, Japan

Akira Yoshida,     Professor Emeritus of Tokyo City University, 1-19-32, Aoba-ku, Yokohama, Kanagawa 225-0002, Japan

Preface

There are few books that offer fundamental knowledge on carbon materials and those that are available must be handy and easy to buy. Therefore, we (M. I. and F. K.) published the book from Tsinghua University Press, China, entitled Carbon Materials Science and Engineering: From Fundamentals to Applications in 2006. The book aimed to give comprehensive information firstly on the fundamental science of preparation and characterization of various carbon materials, and secondly on engineering and applications of various carbon materials. Many copies have been sold, mostly in China. However, the progress in carbon materials science research and engineering was rapid not only on nanocarbons, represented by carbon nanotubes and graphenes, but also on classic carbons, such as porous carbons and their application to electric double-layer capacitors, so that we believed it was necessary to revise our 2006 book, even though we added some descriptions on newly developed carbons in 2011. We strongly felt that the revision of the previous book was not enough, mainly because it has to contain the fundamentals of carbon materials. Therefore, we decided to publish the advances in carbon materials science as a new book, with the collaboration of two more authors, H. Konno and M. Toyoda. It was published in September 2013 as Advanced Science and Engineering of Carbon from two publishers, Tsinghua University Press and Elsevier. At the same time, we revised and updated the previous book and in 2014 published the second edition of the book entitled Materials Science and Engineering of Carbon: Fundamentals from Tsinghua University Press and Elsevier.

During the preparation of these two books, Fundamentals and Advanced, we found that we did not have enough space for the descriptions on the characterization techniques for carbon materials, and also that we could not cover all of the techniques by ourselves. In addition, we found out that special considerations on the preparation of specimens and the conditions applied were required for the characterization of carbon materials, particularly for fundamental techniques of characterization. We found that some of papers that were published in well-respected journals did not employ the proper procedures for specimen preparation and the optimum conditions for the measurements. For our new book, we selected 13 techniques that were commonly used in the research work on carbon materials: X-ray powder diffraction, small-angle X-ray scattering, transmission electron microscopy, scanning electron microscopy, image analysis technique, Raman spectroscopy, X-ray photoelectron spectroscopy, magnetoresistance measurement, electrochemical performance determination, gas adsorption/desorption for pore structure analysis, thermal analysis, titration method for identification of surface functional groups and temperature programmed desorption. We asked the experts who had been working on carbon materials by using these respective techniques to focus on writing about the practical applications of the techniques on carbon materials.

Michio Inagaki

Feiyu Kang

Editors

Acknowledgments

All of the authors would like to express their sincere thanks to the people and the organizations that kindly provided the data and figures for this book. We are also extremely grateful to all of the people who worked on this book at both Tsinghua University Press and Elsevier.

Chapter 1

Introduction

Michio Inagaki     Professor Emeritus of Hokkaido University, 228-7399 Nakagawa, Hosoe-cho, Kita-ku, Hamamatsu 431-1304, Japan

Feiyu Kang     Graduate School at Shenzhen, Tsinghua University, University Town, Shenzhen, Guangdong Province 518055, China

Abstract

Carbon materials have wide varieties in structure, nanotexture, microtexture and properties due to different carbon–carbon bonding natures, sp³, flat sp²  +  π, curved sp²  +  π and sp  +  2π. Importance of the characterization of carbon materials is emphasized, and 13 techniques are selected by explaining key issues for the application of these techniques, particularly to carbon materials in the graphite family.

Keywords

Carbon family; Carbon materials; Characterization; Microtexture; Nanotexture

Chapter Outline

1.1 Carbon Materials 1

1.2 Characterization of Carbon Materials 3

1.3 Structure of the Present Book 5

References 6

1.1. Carbon Materials

A variety of carbon materials have been developed and have provided distinguished contributions to the development of science and engineering, particularly of modern science and engineering [1,2]. In Fig. 1.1 carbon materials are classified on the basis of carbon–carbon bonds of the hybrid orbitals of sp³, planar sp²  +  π, curved sp²  +  π and sp  +  2π.

Carbon materials composed principally of one of these hybrid orbitals are represented by diamond, graphite, fullerenes and carbynes, respectively, and called carbon families [1]. Each family has its own variety, as written in Fig. 1.1. In the diamond family, there are highly crystalline diamonds with cubic and hexagonal crystal symmetries, but also there are noncrystalline (amorphous) materials that have almost the same properties as crystalline diamond, which are called diamond-like carbons (DLCs). Carbon materials belonging to the carbyne family are composed of either double or triple carbon–carbon bonds, cumulene or polyyne type, using sp  +  2π hybrid orbitals. Carbon materials in the fullerene family contain pentagons together with hexagons of carbon atoms using electronic orbitals of curved sp²  +  π, which leads to closed cages, the smallest cage being C60. Diversity observed in the fullerene family occurs in the size of the cage, C60, C70,…, and also in the thickness of the cage wall, single- and multi-walled.

Figure 1.1  Classification of carbon materials based on carbon–carbon bond nature and varieties in structures and textures.

Carbon materials belonging to the graphite family consist of carbon hexagons based on the flat sp²  +  π hybrid orbitals, of which the fundamental structural unit is anisotropic crystallite of flat hexagonal carbon layers stacked in parallel. The graphite family has wide variation in structure, nanotexture and microtexture, which produces a variety of carbon materials. Some of them are listed in Fig. 1.1, a number of them having been produced in industries and utilized in various fields. The structure of carbon materials is evaluated by stacking regularity (degree of graphitization P1) and interlayer spacing d002 from highly crystalline graphite to amorphous carbon (glass-like carbon). The nanotexture in carbon materials is classified four ways on the basis of orientation scheme of anisotropic hexagonal carbon layers; firstly, into two, oriented and random (nonoriented), and then the former into three schemes of planar, axial and point orientation, each nanotexture giving characteristic carbon materials, glass-like carbon, kish graphite (single crystal graphite), pyrolytic carbons, different cokes, carbon fibers, activated carbons and carbon blacks. Many of these carbon materials consist of anisotropic particles, of which the orientation creates another variety, microtexture, in bulk materials. Microtexture is not classified systematically, but an example can be found in the composites of carbon fiber with plastics (carbon fiber–reinforced plastics, CFRPs). In polycrystalline graphite blocks, anisotropic particles of needle-like coke are orientated during formation into large-sized rods with binder pitch to produce graphite electrodes for metal processing. On the other hand, anisotropic coke particles are formed with binder pitch under isostatic pressure to produce isotropic high-density graphite blocks, in which anisotropic coke particles are randomly oriented. In porous carbons, including activated carbon, the control in pore structure, not only their sizes but also in their morphologies, is now important for their applications.

In two books published previously [1,2], the preparation processes were discussed mainly in relation to the properties and functionalities, which were characterized by different techniques, for each carbon material, but the details of characterization techniques for the structures and textures of carbon materials could not be given, mainly because of space limitations in the books.

1.2. Characterization of Carbon Materials

Carbon materials, particularly those classified into the graphite family, have a variety of structures and textures, nanotexture and microtexture, as shown in Fig. 1.1. Inevitably they have to be characterized by different techniques to understand their structure and textures for the control of various properties and further development of carbon materials, not only for improving their properties but also for creating new properties (new functions). In Fig. 1.2, to understand the characterization of carbon materials there are five steps: bonding nature, structure, nanotexture, microtexture and properties (functions). Numerous techniques, from classic to modern, have been applied for the characterization of carbon materials on the basis of different purposes, to develop new carbon materials, to understand the processes for the fabrication of various carbon materials, to improve the functions of carbon materials, etc.

Figure 1.2  Five steps to characterize the carbon materials and 13 techniques applicable for characterizing carbon materials.

In this book, 13 techniques have been selected as the fundamental techniques for the characterization of carbon materials among numerous techniques, which have frequently been applied to carbon materials and are expected to be continued to be used in the future; they are related to the five steps in Fig. 1.2, aimed at understanding characteristics of carbon materials hierarchically and correctly.

Information on structure is obtained from X-ray diffraction (XRD) analysis of carbon materials, via the determination of interlayer spacing d002, crystallite sizes of Lc and La, and degree of graphitization P1; information on nanotexture, orientation scheme and degree is also obtained quantitatively. Nanotexture on the surface can be observed by scanning electron microscopy (SEM), but image analysis techniques are necessary for quantitative evaluation. Structure of carbon materials can be identified also by transmission electron microscopy (TEM), Raman spectroscopy and magnetoresistance measurements. Magnetoresistance is one of the parameters in electromagnetic properties, but it works as a measure of structural perfection and also of nanotextures [3]. For SEM and TEM, processing using image analysis is a powerful tool used to analyze micrographs quantitatively. Small-angle X-ray scattering (SAXS) and gas adsorption/desorption isotherm measurement provide information on pore structure in carbon materials; the former detects both closed and open pores, but the latter can detect only the pores open for adsorbent gas, mostly N2. Recently, identification and control of the surface functional groups on carbon materials have attracted attention, reflecting biomedical and electrochemical applications, and so titration method and temperature programmed desorption (TPD) have been applied for their characterization. For the same purpose as above, X-ray photoelectron spectroscopy (XPS) is a useful tool, to identify the hybrid orbitals employed by carbon atoms and also the bonding states of carbon atoms with heteroatoms, such as H, N, O, etc. XPS has been frequently employed to identify the state of foreign atoms in carbon materials, either substituted for carbon atoms or attached as functional groups.

In addition, thermal analysis and electrochemical performance are included in the present book for the following reasons. Processes of pyrolysis and carbonization of precursors are also important to control the structure, texture and properties of resultant carbons, which have been discussed for various carbon materials in the previous books [1,2]. To understand these processes, thermal analysis techniques have frequently been employed and very helpful. Now, carbon materials have become important component materials in various energy-storage devices, such as electrochemical capacitors, lithium-ion rechargeable batteries and fuel cells, in which electrochemical performance of carbon materials has to be correctly and accurately evaluated in the context of electrochemistry.

Characterization of carbon materials has to be carefully done because the optimum condition of each technique for carbon materials is quite different from that which has usually been employed for metals and ceramics, as mentioned below.

X-ray powder diffraction (XRD) has been the most frequently applied technique for determining the fundamental characterization parameter. Carbon has a very low absorption coefficient for X-rays and so X-rays penetrate deeply into carbon, which makes the diffraction profile shift to a lower angle side and broaden markedly. Therefore, it is strongly recommended to measure the powder patterns by using a thin specimen and internal standard of silicon, which is proposed in reference [4] as the specified standard procedure.

The scanning electron microscope is also frequently applied for various carbon materials. Observation by SEM is recommended in many cases to be done at low acceleration voltage of electron beam at 2–5  kV, much lower than 25  kV for metals and ceramics, because electron beams can penetrate deeply due to very low absorption coefficient of carbon; therefore it is essential to understand the structure of the surface of carbon materials.

Raman intensity ratio ID/IG had been used as a measure of graphitization and even to estimate La. However, the relation between ID/IG and La is not confirmed experimentally and theoretically. The dependence of ID/IG on heat treatment temperature, in other words, on graphitization degree, is different with respect to each carbon material, although Raman spectroscopy itself is a useful technique for characterization of the carbon material.

The application of carbon materials as electrodes for energy storage devices, lithium-ion rechargeable batteries and electric double-layer capacitors is now an important field of research. The specific parameters obtained from the analyses of energy storage capability, such as capacity for the charge/discharge of lithium ions and capacitance by the formation of electric double layers on the surface of carbon electrodes, and also dependence of energy storage capability on the charge/discharge rate and cycle, are often used as characterization parameters for carbon materials. Regrettably incorrect applications of the formulae used to calculate these parameters are often found in the literature. These are the reasons why the present book is being published in addition to the books previously published [1,2].

1.3. Structure of the Present Book

Thirteen characterization techniques, including electrochemical performance and thermal analysis, are explained in the book, focusing on their applications to carbon materials, mostly those belonging to the graphite family. Most of them have been commonly used for the characterization of various materials, and their fundamental theories and instrumentations are published in specialized reviews and books. This book, therefore, emphasizes key issues for the practical application of these techniques to carbon materials, by showing some experimental results. The chapter authors are the specialists who have been working on these various carbon materials.

References

[1] Inagaki M, Kang F. Materials science and engineering of carbon: fundamentals. 2nd ed. Elsevier/Tsinghua University Press; 2014.

[2] Inagaki M, Kang F, Toyoda M, Konno H. Advanced materials science and engineering of carbon. Elsevier/Tsinghua University Press; 2014.

[3] Hishiyama Y, Kaburagi Y, Inagaki M. Characterization of structure and microtexture of carbon materials by magnetoresistance. In: Chemistry and physics of carbon. vol. 23. Marcel Dekkar Inc; 1991:1–68.

[4] Iwashita N, Park C.R, Fujimoto H, Shiraishi M, Inagaki M. Specification for the procedure of x-ray diffraction measurements on carbon materials. Carbon. 2004;42:701–714.

Chapter 2

X-ray Powder Diffraction

Norio Iwashita     National Institute of Advanced Industrial Science and Technology (AIST), Onogawa, Tsukuba, Ibaraki 305-8569, Japan

Abstract

X-ray diffraction (XRD) techniques have been widely applied for the characterization of crystal structure of carbon materials and have given useful information, lattice constants, crystallite size and degree of the graphitization. Graphitization is a characteristic phenomenon for carbon materials and can be analyzed by different techniques, including XRD. For the analysis of XRD patterns, there are key points that the researchers have to pay attention in order to measure the diffraction patterns and to analyze them correctly. This chapter introduces the standard methodology to obtain reliable values of the lattice constants and the crystallite sizes of carbon materials.

Keywords

Crystal structure characterization; Crystallite size; Degree of graphitization; Graphitization; Interlayer spacing; Lattice constant; X-ray diffraction (XRD)

Chapter Outline

2.1 Introduction 7

2.2 X-ray Diffraction Pattern of Carbon Materials 8

2.3 Parameters Determined by X-ray Diffraction 10

2.4 Instrumentation 11

2.5 Specifications for Measurement 14

2.5.1 Preparation of Sample for X-ray Measurements 14

2.5.2 Measurement and Intensity Correction of Diffraction Profiles 14

2.5.3 Correction of Diffraction Angle With Internal Standard 16

2.5.4 Determination of Full Width at Half Maximum Intensity 17

2.5.5 Accuracy of the Values Determined 18

2.6 Degree of Graphitization 18

2.7 Key Issues for Measurement 21

2.7.1 Diffraction Pattern 21

2.7.2 Use of Internal Standard 21

2.7.3 Use of Thin Sample Holder 22

2.7.4 Indexing the Diffraction Line 23

2.7.5 Separation into Component Profiles 23

2.8 Concluding Remarks 24

References 24

2.1. Introduction

All carbon materials in the graphite family, which include highly graphitized (highly ordered) carbons, eg, natural graphite and highly oriented pyrolytic graphite (HOPG), to disordered carbons, eg, glass-like carbon and nanosized carbon blacks, consist of the same elemental domains of parallel stacking of carbon hexagonal net layers. These elemental domains are the basic structural unit in carbon materials and have been called crystallites, even though the basal planes are stacked in parallel without regularity. In carbon materials derived from organic materials heated to about 1000–1500°C, crystallites have been shown to be parallel stacks of two or three layers having the size 1–1.5  nm without regularity (turbostratic structure) under transmission electron microscopy [1]. A wide range of structures from turbostratic stacking with small-sized layers in a small number of layers to regular graphitic stacking, either hexagonal (ABABAB) or rhombohedral (ABCABC) regularity, with large-sized and a large number of layers occur in various carbon materials. The structure is known to depend strongly on the precursors, their carbonization conditions and high temperature treatment conditions. X-ray diffraction (XRD) is one of the powerful tools used to characterize these structures, particularly powder diffraction techniques that are commonly employed in various fields of research, production and application of carbon materials.

In 1963, the 117th Committee (carbon materials) of the Japan Society for the Promotion of Sciences (JSPS) a specified standard procedure for the determination of interlayer spacing and crystallite sizes of carbon materials by powder diffraction technique, especially graphitized materials, the so-called Gakushin (JSPS) method [2]. In 2002, the committee had discussed the revision of the specification in order to accept some modern computing processes, such as automatic step-scanning measurements of diffraction intensities, profile fitting for diffraction lines, etc. The English version of the revised specification was published in 2004 [3]. The methodology of the specification was standardized in Japanese Industrial Standard (JIS) R7651:2007 [4].

In this chapter, the characterization of crystal structure of carbon materials by X-ray powder diffraction technique is interpreted. The standard procedures for the determination of average interlayer spacing and crystallite sizes of carbon materials are introduced. Some key points are pointed out for the measurement and interpretation of the powder pattern observed.

2.2. X-ray Diffraction Pattern of Carbon Materials

X-ray powder patterns of natural graphite and coke heat-treated at 1200°C are shown in Fig. 2.1. For X-ray powder pattern of the coke having turbostratic structure, it is characteristically observed that 10 and 11 diffraction profiles are asymmetric and broad, as compared with symmetrical and sharp 100 and 110 diffraction profiles of natural graphite, as well as broadening of 00l diffraction profiles. In the turbostratic structure, each hexagonal net layer forms two-dimensional lattice independently, in other words, the c-axis cannot be defined, and correspondingly these diffraction peaks have to be indexed hk, not three-dimensional hkl.

The diffraction lines of graphite (Fig. 2.1A) are classified into three groups, the lines with 00l, hk0 and hkl indices, mainly because of the strong anisotropy in structure. The lines with 00l indices are due to the reflection from crystallographic basal planes (hexagonal carbon layers of graphite), where only the even l indices are allowed because of the extinction rule due to the parallel stacking of the layers. The lines with hk0 indices are due to the reflection from the crystallographic planes perpendicular to the basal planes, and the lines with hkl indices come from the planes declining to the basal planes, where the three-dimensional structure with regular stacking of layers has to be established. Therefore, the hkl lines are called three-dimensional lines and their appearance in the powder pattern is the experimental evidence for the formation of three-dimensional stacking (graphite structure).

Figure 2.1  X-ray powder patterns of carbon materials, (A) natural graphite and (B) coke heat-treated at 1200°C.

On the other hand, the powder pattern that is given by low-temperature-treated carbons is quite different from well-crystallized graphite because it consists of random stacking of small layers, as shown on a petroleum coke heat-treated up to 1200°C (Fig. 2.1B). The diffraction pattern is characterized by very broad 00l lines due to the small number of stacked layers, and by unsymmetrical hk lines and no hkl lines, as explained above.

By heat treatment of carbon materials at temperatures above 2000°C, carbon hexagonal net layers are extended and the number of layers stacked in parallel increases in crystallites. Moreover, three-dimensional stacking order occurs in crystallites. As a consequence of growth of carbon hexagonal net layers and appearance of three-dimensional stacking order, average interlayer spacing between layers, d002, approaches 0.3354  nm, as the value of graphite. To characterize the carbon materials prepared from different precursors and produced by different processing, the size, shape and crystalline perfection of carbon hexagonal net layers in the crystallites have to be exactly understood. By XRD technique, average interlayer spacing d002, crystallite size along the a- and c-axes La and Lc, respectively, and the probability for adjacent hexagonal net layers in parallel to have graphitic regular stacking P1 have been determined as fundamental parameters for the crystal structure and discussed on the correspondence to various properties of carbon materials.

2.3. Parameters Determined by X-ray Diffraction

The crystal structure of graphite is illustrated in Fig. 2.2A. In the hexagonal net layers of carbon atoms, each carbon atom