Basic Knowledge About Graphite Intercalation

Published Date: 02 Nov 2017

1.0 CHAPTER 1

1.1 INTRODUCTION

The diverse properties of graphite intercalation compounds suggest a number of applications of these materials to mechanical, electrical and chemical engineering processes. Though graphite intercalation compounds are not yet widely used in industrial applications, the parent compound graphite is an extensively used industrial material. Intercalation offers the interesting possibility of both major and minor modifications to the properties of graphite.

An attractive feature of graphite intercalation compounds is that the host material graphite can be prepared on a commercial scale from readily available raw materials. The preparation of intercalation compounds could also be carried out with reasonably modest equipment on a commercial basis. For these reasons, industrial use for graphite intercalation compounds should find ready acceptance provided suitable applications can be identified.

Some of the possible applications of graphite intercalation compounds that have been made or proposed are battery and electrode materials, chemical catalysis and catalytic applications, conductivity applications, carbon fibres and other applications but in this paper we will focusing in conductivity application that include the electrical properties.

The high room temperature conductivities achieved with such intercalation compounds offer promise for practical applications certain acceptor compounds as electrical conductors.

Considerable interest in the composite wire approach was stimulated by the report by Vogel (1977) of the fabrication of a composite wire consisting of a graphite-SbF5 core, embedded in a copper sheath, and exhibiting room temperature conductivity greater than that of copper. The copper sheath was used to provide encapsulation of a high conductivity intercalation compound, since compounds exhibiting the highest electrical conductivities tend to be highly reactive in air and to desorbs readily. The copper sheath is also useful for several steps in the fabrication process, which started with the insertion of graphite powder and SbF5 into a copper tube which was then sealed. The loaded copper tube was then swaged or drawn into a wire, which also served to compact the core material. Heat treatment of the composite wire produced an intercalation compound in the core material, with a high degree of preferential orientation of the c-axis in the radial direction of the wire.

For a wire, consisting of 2/3 (by volume) of Cu with the remaining 1/3 as the intercalated graphite-SbF5 core, Vogel (1977) reported a composite conductivity of 5:95 x 105 (Ωcm)-1. Because this conductivity was comparable to that of copper (slightly greater in fact), considerable interest in these composite wires was stimulated. However, later work on a similar type of composite wire by Singhal (1980) failed to confirm these high composite conductivity values. Although Singhal's graphit-SbF5 core material showed the preferential orientation of the c-axis, the maximum composite conductivity that Singhal obtained was 4:5 x 105 (Ωcm)-1,from which he concluded that the intercalate core did not contribute significantly to the composite conductivity. Although Vogel's composite wire approach offers considerable potential, it remains to be demonstrated that intercalated graphite cores enhance the composite wire conductivity or that such composites could be produced on a commercial basis with properties superior to conventional wires. Concurrently, considerable work is in progress on intercalated graphite fibres in an effort to assess the potential of intercalated fibre bundles for application as electrical conductors.

Other promising applications areas for intercalated graphite involve exploitation of the high electrical anisotropy of these materials. In this class of applications, high conductivity in one direction could be realized with poor conductivity perpendicular to this direction

A major simplification is made in treating the electronic structure of graphite intercalation compounds by recognizing the strong similarity of the structural and electronic properties of graphite intercalation compounds to those of their parent constitutive materials, the graphite host and the intercalate. The physical basis for this identification arises from the strong interlayer bonding in both graphitic and intercalate layers, and the relatively weak interlayer bonding between graphite intercalate and graphite-graphite layers in the intercalation compounds.

A close relationship between the graphite intercalation compounds and their parent materials implies that in the dilute intercalate concentration limit, the electronic structure is closely related to that of pristine graphite. The discussion of the electronic structure for the graphite intercalation compounds therefore begins with a review of the graphite electronic structure and its application in dilute limit models for the intercalation compounds.

1.2 PROBLEM STATEMENT

Graphite Intercalation compound has advent of practicable in development of many application. To investigate the possibility of this compound to increase in the electrical conductivity generally we must did a simulation. We should have enough knowledge to do the simulation to vary the result and compare it with experimental result.

1.3 OBJECTIVE

(i) To study the band gap of graphite compound C6Yb and C6Ca

(ii) To study the superconduction of the graphite compound C6Yb and C6Ca

(iii) To calculate the critical temperature of C6Yb and C6Ca using DFT calculation and compare it with experimental result.

2.0 CHAPTER 2

2.1 LITERATURE REVIEW

2.1.1 BASIC KNOWLEDGE ABOUT GRAPHITE INTERCALATION

COMPOUND

Graphite is in form of carbon which the atom is arranged hexagonally in the two-dimensional sheet. Pure graphite showed a very interesting compound that displaying unusual properties such as the in the form of zero band gap the two-dimensional sheets form a semi-conductor. However, in bulk graphite these two dimensional carbon layers are held together via weak van der Waals forces. In this feature it possible to introduce metal atoms between the layers of carbon. This phenomena is known as the intercalation compounds in which an ordered structure is formed. These processes result in a marked modification of both the physical and electronic properties. In particular, unlike pure graphite, some graphite intercalation compounds are found to superconductor.

2.1.2 BAND STRUCTURE OF GRAPHITE INTERCALATION COMPOUND

The band dispersion of various elements on highly oriented pyrolytic graphite (HOPG) has been studied by angle-resolved photoemission spectroscopy (ARPES). The ARPES around the element such ytterbium and calcium point shows shifts of the á´« and á´«* bands toward higher binding energies upon the adsorption of that element in the submonolayer regime, showing charge transfer from the adsorbed of that alkali material to the graphite á´«* band and an ionic bond between the element and graphite. There are many type of graphite intercalation compound that behave as the superconductor material including the ytterbium-graphite compound C6Yb and calcium-graphite compound C6Ca. So, the band gap of these materials always zero or approaching to zero because of the superconductor behaviour.

2.1.3 SUPERCONDUCTION BEHAVIOUR OF GRAPHITE INTERCALATION

COMPOUND

Two of the principle signatures of superconductivity are the absence of electrical resistivity and the development of a diamagnetic moment below the ordering transition. These graphite intercalation compounds provide an excellent laboratory in which to study low dimensional electronic systems in a controlled fashion. In particular, the introduction of these metal atoms is thought both to donate electrons to carbon layers and to change the spacing of these layers. These processes result in a marked modification of both the physical and electronic properties. In particular, unlike pure graphite, some graphite intercalation compounds are found to be superconductor. The first of these to be reported was C8K, which has a superconducting transition temperature of 0.15K. Interestingly, while the metastable high pressure phase C2Li exhibits a superconducting transition at 1.9K, the compounds C6Li and C3Li are found to not superconduct down to the lowest measured temperatures. In all these compounds the transfer of charge from the metal to the graphite is thought to play an important role in the superconductivity. However, we see that there must be additional factors at work as both potassium (K) and lithium (Li) would be expected to donate one electron each to the graphite and C8K superconducts while C6Li does not. Interestingly, superconducting upper critical field studies and resistivity measurements suggest that these compounds are significantly more isotropic than pure graphite. This is unexpected as the effect of introducing the intercalant is to move the graphite layer further apart. In this paper we will discuss about the superconducting behaviour of C6Yb and C6Ca.

2.1.3.1 PHENOMENON OF CRITICAL TEMPERATURE Tc IN

SUPERCONDUCTION BEHAVIOUR OF GRAPHITE INTERCALATION

COMPOUND

The first measurement of superconductivity in a graphite intercalation compound (GIC) was made by Hannay et al. in 1965. on the first stage KC8 compound which exhibits a very low critical temperature: 0.14 K. A larger intercalated metal concentration allows to increase the critical temperature up to 3 K for KC3, 5 K for NaC2 and 5.5 K for KC4. These compounds were obtained by means of high pressure reactions but they become meta stable under room conditions. However, several ternary graphite intercalation compounds promote superconductivity, for example, at 1.4 K for KHgC8 or 2.7 K for KTl1.5C4. More recently, two binary and one ternary GICs were discovered to be superconducting at higher temperatures: YbC6 exhibits a critical temperature of 6.5 K , meanwhile for CaC6 and Li3Ca2C6, Tc reaches 11.5 K and 11.15 K , respectively. In this paper we will discuss about the critical temperature of C6Yb and C6Ca.

3.0 CHAPTER 3

3.1 METHODOLOGY

3.1.1 METHOD

3.1.1.1 SIMULATION USING DFT (DENSITY FUNCTIONAL THEORY)

The density-functional theory is used to obtain the solution of the Schrodinger equation. DFT theory is widely used to study atomic and electronic structure of molecules and condensed matters. The electron density is used to differentiate the Schrodinger equation so that Kohn-Sham equations are obtained. These equations are solved in the local-density approximation (LDA). It is also possible to obtain the result in the generalized gradient approximation (GGA) but these results are very close to that of LDA. Hence, it is not necessary to obtain the result in both the approximations.

The present article presents a review of recent DFT applications to spectroscopic problems based on a specific computer code, CASTEP. CASTEP uses the plane-wave pseudopotential method to solve one-electron Kohn–Sham equations. The wavefunctions are expanded in a plane-wave basis set defined by the use of periodic boundary conditions and Bloch’s theorem. The electron–ion potential is described by means of ab initio pseudopotentials within either norm-conserving or ultrasoft formulations. Direct energy minimization schemes are used to obtain, self-consistently, the Kohn–Sham wavefunctions and corresponding charge density.

C:\Users\zul\Desktop\assigntment\subject 2013\thesis\1-s2.0-S0022286011008167-gr12.jpg C:\Users\zul\Desktop\assigntment\subject 2013\thesis\1-s2.0-S0022459609002199-gr16.jpg

Figure 3.1: Illustration of the software

3.1.1.2 PROPERTIES OF THE MATERIAL

For the material we will choose two type of intercalation compound that are C6Yb and C6Ca. We will focusing on the electrical properties of this compound since it related with the band gap and superconducting material. For the properties of Ytterbium-graphite compound C6Yb it just same as the calcium-graphite compound C6Ca that the crystallises in a rhombohedral unit cell belonging to the R-3m space group and with the following parameters: a=517 pm; α=49.55o as shown in Fig. 3. The use of samples synthesised from HOPG makes the crystallographic study easier since it is possible to study separately 00l, hk0 and hkl reflections. In the case of CaC6, the structure was entirely specified. The 00l reflections lead to a repeat distance of 452.4 pm for this first stage compound. The study of hk0 and hkl reflections shows that the c parameter corresponds to three times the interlayer distance: c=1357.2 pm=452.4 x 3, corresponding to an AαAβAγAαAβ .c-axis stacking (A: graphene sheets; α, β, γ: sites occupied by calcium atoms). For the lattice constant of this compound, there are three plane for the compound which been mark by a, b and c.

3.1.1.3 DEVELOPE ON THE STRUCTURE OF MATERIAL

Ytterbium-graphite compound C6Yb structure is just as same as the calcium-graphite compound C6Ca which is a hexagonal layered structure (P63/mmc) in which the intercalant atoms form a triangular array between every graphite layer (stage 1 intercalation). The alternate carbon and metal layers have an AαAβ registration where the A represents the carbon layers and α and β the intercalant layers. This is also shown in Figure 3 for the unit cell of C6Ca.

Figure 3 : Rhombohedral unit cell of C6Ca and set of joined unit cells.

3.1.1.4 OPTIMIZE TO THE GROUND STATE ENERGY

After the development process on the structure of the material, we will optimize that compound at the ground state energy which from a hexagonal layered structure that form a triangular array between every graphite layer (stage 1 intercalation) of the graphite intercalation compound. All of the angle, parameter, lattice constant, space group will be reset to the actual value to make sure that the result that we get is precise.

3.1.1.5 CALCULATE THE BAND STRUCTURE AND THE CRITICAL

TEMPERATURE Tc

For the band structure we will calculate band gap of the C6Yb and C6Ca compound that mostly they are zero band gap or approaching zero. Besides, compound we will calculate the critical temperature of that compound using DFT calculation to measure the superconductor of the C6Yb and C6Ca.

3.1.1.6 COMPARATIVE RESULTS BETWEEN SIMULATION AND THE

EXPERIMENT

After designing the structure and get the results or measurement from the simulation then we compare the simulation with the exact experiment. The

There will be a comparison of results between simulation and the experiment. Our calculation of band gap and critical temperature for C6Yb and C6Ca will be compared with the experimental result done by Weller et al. (2007). So, we must make sure that all the parameter and the structure of the C6Yb and C6Ca are correct to get the best result that similar with the experimental result.

MATERIAL

DEVELOPE ON STRUCTURE OF MATERIAL

OPTIMIZE TO THE GROUND STATE ENERGY

NO

CALCULATE THE BAND STRUCTURE AND THE CRITICAL TEMPERATURE

DATA COLLECTION

COMPARE WITH THE EXPERIMENTAL RESULT

YES

STOP

3.2 EXPECTED RESULT

Based on the simulation, we can detect the band gap and critical temperature of the two graphite intercalation compounds. We will compare the result of critical temperature of superconducting condition with the previous experiment. The critical temperature of the previous experiment for Ytterbium-graphite compound C6Yb is 6.5K and calcium-graphite compound C6Ca is 11.5K.

3.3 CONCLUSION

The main objectives of this research proposal are to study the band gap and superconducting of graphite compound C6Yb and C6Ca using the DFT calculation. Using the CASTEP software we can find the band gap of that compound and we also can find the critical temperature to detect their superconducting behaviour. This research proposal is for development or analytical the structure of graphite intercalation compound by doing the simulation process.

3.4 GANTT CHART