The Bonding Between Binder Coke and Filler Particles in Carbon and Graphite Electrodes

&A /IICICJ^ 5SN 1018-5593

Commission of the European Communities technical coal research

THE BONDING BETWEEN BINDER COKE AND FILLER PARTICLES IN CARBON AND GRAPHITE ELECTRODES

Report EUR 13993 EN

Blow-up from microfiche original

Commission of the European Communities technical coal research

THE BONDING BETWEEN BINDER COKE AND FILLER PARTICLES IN CARBON AND GRAPHITE ELECTRODES

J. W. PATRICK Department of Chemical Engineering Loughborough University of Technology Loughborough, Leicestershire LE11 3TU United Kingdom

Contract No 7220-EC/839

FINAL REPORT

Directorate-General . _,^. Energy PARL EUROP. Biblioth. 1992 N.CEUR 13993 EN cT Published by the COMMISSION OF THE EUROPEAN COMMUNITIES Directorate-General Telecommunications, Information Industries and Innovation L-2920 LUXEMBOURG

LEGAL NOTICE Neither the Commission of the European Communities nor any person acting on behalf of the Commission is responsible for the use which might be made of the following information

Catalogue number: CD-NA-13993-EN-C

CONTENTS

Page

Summary V

1 Introduction 1

2 Experimental Methods 3 2.1 Assessment of binder/filler interface quality 3 2.2 Measurement of the tensile strength of electrodes 7 2.3 Measurement of the electrical resistivity of electrodes 7 2.4 Measurement of pore structural parameters 8

3 Experimental Studies 10 3.1 Raw materials and interface quality 10 3.2 Processing conditions and interface quality 14 3.3 Development of interfaces during processing 17 3.4 Examination of commercial electrode carbons 18 k General Discussion 21

5 Conclusions 31

References 33 Tables 35 Figures 53

V SUMMARY

Coal-tar-pitch bonded carbons and graphites find wide application as electrodes. This study was concerned primarily with electrode carbons used in the aluminium industry. Since they are two-phase carbon composites, the quality of the interfaces between the filler and the mixture of small filler particles and pitch constituting the binder phase is expected to exert an important influence on their properties. With the object of promoting the use of coal-derived products for this application, the aim of the present study was to gain a better understanding of the factors, including pitch quality, influencing the quality of binder/filler properties in electrode carbons and the effect of interface quality on technically-important electrode properties.

Both industrial and pilot-scale electrodes were studied. Differing quality interfaces in electrode carbons could be identified by examination of etched electrode surfaces in a scanning electron microscope (SEM) and classified according to the degree of continuity between binder and the larger (>50 um) filler particles into five categories: voided, partially- and completely- fissured, pored and continuous. The proportions of the various interfaces present in an electrode were quantified by point counting and the data obtained was used to calculate a single value index, the IQI, indicative of the quality of interfaces present. Features formed by failure of weak interfaces during tensile testing of electrode carbons could be identified by SEM study of fracture surfaces. The mode of failure of electrodes was characterised by comparing the proportion of features formed by transgranular fracture of the larger (>50 pm) filler particles with those resulting from interfacial failure.

To investigate the development of interfaces during processing of an anode mix, small test pieces were cut from a green block and heated to temperatures in the range 200 - 1000°C. The interface quality of the green block and the heat-treated test pieces was assessed by SEM examination of etched surfaces. The green block contained a high proportion of partially fissured interfaces which were considered to result from the relaxation of stresses associated with the 3-5X increase in volume which occurred on release of the moulding pressure. A marked increase in the number of voided interfaces was observed VI near the temperature of maximum volatile release from the pitch while shrinkage associated with the resolidification stage of pitch carbonization resulted in the formation of fissured interfaces.

The influence of raw materials on interface quality was studied using four filler cokes, three petroleum cokes and an electro-calcined anthracite, and four coal-tar pitches varying in nature and quantity of quinoline-insoluble components. Test electrodes were prepared using a pitch content and processing conditions giving optimum electrode strength. For each filler used, the IQI values of the four electrodes produced using the four pitches were quite similar, but marked variations between the four fillers were observed. Thus it appears that the filler particles have a greater influence on the quality of the binder/filler interfaces in the electrodes than the variations in the nature of the pitches used.

The effect of pitch content and mixing time and temperature were studied using a single pitch/filler combination. For the range of time and temperature studied, pitch content appears to be the most critical parameter. Consistently high IQI values were achieved at the optimum pitch level while at other levels more variation in IQI was observed. Insufficient pitch resulted in increased observations of fissured interfaces while too much pitch induced voided interfaces.

Clear trends were observed between increasing IQI values and increasing tensile strength but decreasing electrical resistivity. Both electrode properties were also dependent on the bulk density of the electrode. Thus both interface quality and porosity have an important bearing on the quality of electrode carbons. However, attempts to obtain relationships between the tensile strength of laboratory electrode carbons and combinations of interface quality and pore structural parameters were not successful.

For the range of commercial electrode carbons studied, tensile strength was not dependent upon the incidence of interfacial failure features in their fracture surfaces. Their tensile strengths could be related to combinations of pore structural parameters determined by image analysis of incident-light microscope images of polished electrode surfaces. However, coefficients in the relationships varied depending on the nature of the filler particle VII present, electro-calcined anthracite fillers producing electrodes whose strength was less sensitive to variations in pore structural parameters than the strengths of electrodes made using petroleum cokes as fillers.

1 INTRODUCTION

Coal-tar-pitch bonded carbons are used directly as electrodes in aluminium and fluorine production or, after heating to 2700°C, as graphite electrodes in electric-arc steel-making1. In the European Community consumption of coal- tar pitch for these purposes amounts to approximately 2 x 105 t/annum2. The bulk of this pitch is used to make anodes for aluminium reduction cells; hence the consideration given to this use in this study.

Aluminium is produced by the electrolytic reduction of molten alumina in the Hall-Heroult cell3. The alumina is dissolved in cryolite at 970°C and electrolysed between a consumable carbon anode and a carbon cathode which constitutes the lining of the cell. Both anodes and cathodes must possess adequate strength to withstand the mechanical handling encountered during their production and installation in the cell and to resist thermal stresses during use*. Other important properties of anodes are resistivity, which influences the amount of waste heat generated, and oxidation resistance.

Two types of anodes are in general use, the Soderberg anode which is carbonized within the cell environment, and the pre-baked anode1. The latter are produced by the slow carbonization of a compacted mixture of petroleum coke filler particles and a binder-grade coal-tar pitch. Cathode carbons are obtained by a basically similar process except that a variety of fillers, for example electro- or gas-calcined anthracite, petroleum coke, scrap graphite or mixtures thereof, are used.

Intra- and inter-particular voids are present in the uncarbonized green blocks and devolatilisation pores will be generated during carbonization of the pitch phase. Both anodes and cathodes are therefore porous, two-phase carbon/carbon composites so that their strength will be dependent upon both their porous nature and the quality of the interfaces between binder and filler. These properties will also be important in considerations of oxidation resistance and electrical resistivity.

The principal objective of this study was to gain a fuller understanding of the factors influencing the quality of the binder/filler interfaces in electrode carbons with the aim of furthering the use of coal-based materials in electrode manufacture.

For this study two basic approaches were adopted. The first approach involved the laboratory production of test electrodes with the aim of identifying raw material and process variables which influence the nature of the binder/filler interface. In the second, a range of commercially-produced anode and cathode carbons was examined in order to establish the variability of the quality of binder/filler interfaces in such materials and to identify binders or fillers associated with inferior quality binder/filler interface bonding.

After a section describing the experimental techniques used to assess the quality of binder/filler interfaces in electrode carbons and to determine other electrode properties, the four phases of the work are described. Three of the phases are concerned with laboratory studies of the influence of raw material and processing conditions on interface quality. The final phase involved studies of commercial electrode carbons. For each phase, relevant experimental methods are described and the results obtained presented. The findings of all the experimental work are then considered in a final "General Discussion" section. 3 2 EXPERIMENTAL METHODS

2.1 THE ASSESSMENT OF BINDER/FILLER INTERFACE QUALITY

Previous studies had demonstrated that anode carbons were amenable to study by scanning electron microscopy (SEM) of either etched5 or fractured surfaces6. Both methods have been further developed during this study and applied to both anode and cathode carbons.

The Use of Etched Surfaces

Sample Preparation

Specimens for SEM examination were prepared by first polishing epoxy-resin mc-nted test pieces to a standard suitable for optical microscopy, etching in atomic oxygen, to induce a surface topography reflecting the structural order of the surface material, and gold-coating to produce a conducting surface.

SEM Examination

A low magnification electron micrograph of the etched surface of an anode carbon is shown in Figure 1. Etching attacks the resin in the pores preferentially leaving the carbon standing slightly proud. Preferential etching of parts of the carbon structure induces a surface topography characteristic of the carbon present. The lamelliform nature of the carbon in a large petroleum coke filler particle is shown at F while an area of binder phase is shown at B. Here the term binder phase is used to refer to the mixture of pitch coke and small filler particles filling the interstices between the large filler particles. The pitch coke in commercial electrode materials is distributed so thinly over the surface of the filler particles that it cannot normally be identified by either scanning or optical microscopy.

**Further details of the extensive numerical data generated during this study are available in a PhD thesis submitted to Loughborough University of Technology by Mr G N Ogden. Careful SEM examination of a range of anode carbons showed that poor quality interfaces were associated primarily with the larger filler particles. Interfaces between such particles (>50 um in size) and the binder could be classified, according to the degree of contact between binder and filler, into five categories; voided (V), partially (Fp) and completely (Fc) fissured, pored (P) and continuous (C). These are illustrated in Figures 2a - e and described in Table 1. Figure 2a shows a voided interface where the filler particle, F, is adjacent to a large pore, P. Fissured interfaces where the binder and filler are separated, completely or partially, by a fissure are shown in Figures 2b and c. A pored interface containing a number of small pores is shown in Figure 2d while Figure 2e shows an interface where binder and filler are in continuous contact.

Graphite particles in cathode blocks are similar in appearance to petroleum coke particles. Calcined anthracite consists mostly of a blocky material with no obvious structure. An example of a continuous interface involving such a particle is shown in Figure 3-

Ey the application of point counting techniques during the examination of a large number of randomly selected interfaces, 0.5 nun apart, quantitative data can be obtained. Errors involved vary with the number of interfaces examined. At the 50% level of observation they are calculated to fall from ±"]% for 200 counts to ±3% for 500 counts.

Application of the Method

In an attempt to assess the significance of the method, it was applied to individual specimens of near mean tensile strength from a series of four experimental anode carbons. Results based on 200 counts per specimen are given in Table 2.

No information is available regarding any possible variation in the strength of continuous interfaces. Thus it is assumed that the strength of an interface is dependent only on the degree of contact between the binder phase and filler particle. On this basis it is considered that the strength of interfaces falls in the order continuous > pored > partially fissured, with voided and completely fissured interfaces having no strength. To obtain an index indicative of the overall quality of interfacial bonding in the electrodes, multiplication factors were ascribed to the three strongest interface types. Values of the multiplication factors were calculated by an iterative method so that the IQI values obtained using them, ranked the four electrodes in order of their measured tensile strength. Interface quality indices (IQI) calculated using multiplication factors of 0.4, 1.1 and 2.2 for the partially fissured, pored and continuous interfaces respectively, are compared in Table 2 with the tensile strengths of the specimens measured by the method given below. It is apparent that the IQI calculated in this way provides a reasonable ranking of the anodes in terms of their tensile strength.

As will be discussed later, it is recognised that quality of binder/filler interfaces is only one of the many factors on which the strength of carbons depend. Nevertheless the derived interface quality index appears to be a useful single-value indicator of the quality of interfaces and is so used in this report when comparing the effects of raw material and processing parameters.

The Use of Fractured Surfaces

A representation of a fracture crack passing through an anode carbon is given in Figure 4. From this it is evident that the fracture crack will create features in all anode fracture surfaces resulting from transgranular fracture of filler coke particles, fracture of binder phase and interfacial failure at weak binder/filler interfaces. The fracture crack will also encounter various types of voids, interparticular and devolatilisation pores, large or small fissures and, in some anodes, highly porous areas of binder or filler.

Sample Preparation for SEM Examination

Preparation of fracture surfaces merely involved cleaning the surface of debris using an ultrasonic cleaning bath, drying and gold coating. SEM Examination

In Figures 5a and b are shown low magnification views of a fracture surface of a cathode carbon. In both figures transgranular fracture of a blocky form of electro-calcined anthracite (ECA) has given the relatively smooth surfaces at C. Such fracture of petroleum coke particles in anode carbons reveals clearly the lamellar structure of the material.

Also shown in Figures 5a and b are features resulting from the failure of a weak binder/filler interface. The filler side is shown in Figure 5a at B, small pieces of binder phase being visible adhering to the surface of the ECA particle. At A in Figure 5b the large flattened area is the binder side of an interfacial feature.

AT. area of fractured binder phase in an anode carbon is shown in Figure 6. The lamellar nature of the small petroleum coke particles is apparent at L. In contrast, as Figure 7 shows, the material in the surfaces of pores is covered by a thin film of pitch coke so that surface detail is obscured.

As for etched surfaces, quantitative data can be obtained from the examination of fracture surfaces by the application of a point counting technique. However, results are liable to higher error than those from the examination of etched surfaces due to the greater difficulty in differentiating accurately between some types of feature.

Application of the Method

The results obtained from the examination of 500 positions on the fracture surface of an anode carbon broken in diametral compression are shown in Figure 8. This shows that the majority of the observations fell on pores, fractured filler and binder and features indicative of interfacial failure. In this case the number of observations of the filler and binder side of interfacial failures were almost equal. However, it is often difficult to identify the bir.der side.

The results obtained when fractographic analysis was applied to individual specimens of the four anode carbons considered previously are given in Figure 9. For each carbon, 250 positions on each side of the fracture were examined, the feature present being allocated to one of the categories above. However they are grouped into broader categories for the purposes of Figure 9. Tensile strengths of the four specimens ranged from 3.9 to 7.1 MPa, the strength increasing from left to right in each histogram in Figure 9. It is clear that for these anodes, strength increases as the prevalence of interfacial failure decreases and consequently binder or filler fracture becomes more important.

Thus it is evident that direct evidence of poor quality binder/filler bonding in anode carbons can be obtained from a study of fracture surfaces. Fractographic analysis however, is more difficult to carry out than the examination of etched carbon surfaces, some features, for example fractured binder and binder in pore walls, being difficult to separate. When used in this work therefore, the analysis has been simplified so that only the larger filler particles, ie, greater than 50 um are considered and interfacial quality is then expressed in terms of the ratio of transgranular to interfacial failure.

2.2 Measurement of the Tensile Strength of Electrodes

The tensile strength of electrodes was measured using the diametral ccapression method in which a cylindrical test piece is loaded across a diameter but fails along that diameter as a result of induced tensile forces. Test pieces 10 mm long by 15 mm diameter were generally used in this study. They were loaded using an Instron universal testing machine operating with a cross head speed of 0.5 mm/minute. Tensile strengths quoted are mean values obtained from the testing of 25 - 50 individual test pieces.

2. 3 Measurement of the Electrical Resistivity of Electrodes

The method developed in this study to measure the resistivity of laboratory electrodes involves the direct measurement of resistance using a Cropico D04 digital ohm-meter. Reproducibly good contact with the 70 mm long by 15 mm diameter test specimens was achieved using screw clips tightened to constant torque. To obtain the contact resistance correction total resistance measurements were obtained for four separations of the clips. The contact 8 resistance is then equal to the intercept of the resistance/separation line, this being obtained using a least squares method. For each test specimen four values of resistivity were calculated. To obtain the mean resistivity of the standard sized laboratory electrode block (90 mm long by 70 mm diameter) this procedure was repeated for 10 test specimens. The upper and lower sets were discarded and the mean resistivity calculated from the remaining 32 individual test results.

2.4 Measurement of Pore Structural Parameters

Relevant pore structural parameters were measured using a Joyce-Loebl mini- Magiscan image analysis system. The TV image for analysis was obtained, using a microscope fitted with a x4 objective, from a resin-impregnated polished carbon block viewed under incident light. Each block contained 14 rectangular carbon surfaces each approximately 15 mm by 10 mm, two from each of seven tensile test pieces. These comprised the specimen of mean strength and those with strengths ±1/3, ±2/3 and ±1 standard deviation from the mean value. Each carbon surface was cut as closely as possible to the fracture surface and was polished to a scratch and relief-free condition before examination.

The mini-Magiscan is capable of measuring numerous parameters from each field of view. Of relevance to this study are the following:

Total field area TA Total pore area PA Number of pores N Mean pore height H Mean pore width W Mean pore length Fmax Mean pore breadth Fmin

The pore length is the maximum pore dimension while the pore breadth is the dimension at right angles to the direction of maximum dimension. These two dimensions are often referred to as the maximum and minimum Ferets diameters, hence the abbreviations used. Heights and breadths are the maximum dimensions of the pores parallel to the vertical and horizontal edges of the field of view. These basic measurements are used to calculate the following derived values:

Porosity P = PA/TA Mean pore size P = PA/(N x [H + W]/2) Mean wall size W = (TA - PA)/(N x [H + W]/2)

The ratio of the mean pore size derived by this method to the maximum Ferets diameter depends on pore shape but for regularly-shaped features, since it represents the average length of the horizontal and vertical scan lines which traverse the pores in a field of view, the mean pore size should be less than the maximum Ferets diameter. It is used in this work since a corresponding value for the wall size, ie, the interpore spacing can be obtained.

Pore structural measurements quoted in this report are either mean values for each individual tensile test pieces or mean values for the electrode. Values for individual test pieces were obtained from the examination of 50 fields of view spread over the two polished surfaces. Mean values for an electrode were obtained by averaging the values for the seven individual test pieces. 10

3 EXPERIMENTAL STUDIES

3.1 RAW MATERIALS AND INTERFACE QUALITY

The objective of this part of the study was to seek to identify the extent to which raw materials can influence the binder/filler interface quality in fully-processed laboratory electrodes. Accordingly, selected examples of filler cokes and binder pitches, each covering a range of quality, were used in the production of test electrodes and their interface quality index assessed by examination of etched surfaces.

3.1.1 Experimental Procedures

Raw Materials

To produce the 16 test electrodes examined in this part of the study, four fillers were used in conjunction with four coal-tar pitches. The four fillers and their identification letters were:

Identification Filler type

A a regular grade petroleum coke B a normal grade premium petroleum coke C a base grade premium petroleum coke D an electro-calcined anthracite

The four pitches and their identification numbers were:

Identification Pitch type

1 a carbon black modified pitch

2 a pitch with low anthracene insoluble content 3 a normal binder grade pitch 4 a high mesophase content pitch

Analytical data for the pitches are given in Table 3. 11

Test Electrode Production

To produce test electrodes, 3 kg of filler, sized as follows:

Size Proportion (wt%) coarse (>1200 pm) 43.5 medium (300 - 1200 pm) 34.1 fine (<300 µm) 22.4 were electrically heated to 165°C in a 4.5 1 capacity double Z-blade mixer. The hot filler grist was then mixed for 10 minutes with 600 g of coal-tar pitch before the paste was moulded at a pressure of 20 MPa for one minute to form a block 90 mm diameter by 70 mm long. The pitch content used, 20 wt% when expressed as a percentage of the filler weight (equivalent to 16.7 wt% of the green block), was chosen after preliminary experiments which showed that this level gave an electrode with good strength while avoiding any tendency for deformation (slumping) or coke pick up during carbonization. The cooled green blocks were packed in fine petroleum coke and carbonized in flowing nitrogen at 15 K/hour to 1030°C. After soaking for 12 hours they were then cooled at 20 K/hour to room temperature.

Test Electrode Examination

The green apparent densities were determined from the weight and size of the blocks before carbonization while the baked apparent densities quoted are mean values obtained from the weights and dimensions of the tensile strength test pieces. Cylindrical cores 15 mm in diameter were cut from the baked electrodes using a diamond tipped core drill. These were used for electrical resistivity measurements. The cores were cut into 10 mm tall cylinders for the determination of tensile strength by the diametral compression method, the quoted mean strengths being obtained from the results of 40 - 50 individual test pieces. Specimens of mean strength and with strengths ±1/3, 2 ± /3 and ±1.0 standard deviation from the mean were used for measurement of pore structural parameters and, after etching, for interface quality assessment. 12

3.1.2 Consideration of Results

Density, tensile strength and resistivity data for the test electrodes produced are listed in Table 4. Each electrode is identified by a letter and number which refer respectively to the filler and pitch used in its production. Data obtained from the SEM assessment of the quality of the interfaces present are given in Table 5. for each electrode the percentage observations of the various types of interface being recorded. The interface quality index (IQI) and tensile strength test results are also included. Mean pore structural parameters for the electrodes produced are given in Table 6. These data are referred to as Phase 1 data in some parts of this report.

The data in Table 5 are organised to facilitate the comparison of the effect of the various fillers on the IQI of the electrodes. Excluding the one anomalous result for electrode A3, for each filler coke used the IQI values are reasonably constant but marked variations between fillers are evident. Thus it appears that the filler particle has a greater influence on the quality of binder/filler interfaces in the electrodes than the variations in the nature of the pitch used.

Comparing the effect of different filler cokes, the order of increasing mean ICI is A, C, B, D. This implies that, under the conditions used in this study to produce test electrodes, the electro-calcined anthracite is capable of forming higher quality binder-filler interfaces than the petroleum cokes. No systematic variation in the IQI values with the pitches used is evident from the data in Table 5« Thus differences in the nature of the pitch do not appear to play any significant role in determining the nature of the interfaces, at least as assessed by this technique.

Regarding the influence of interface quality on electrode strength, no general correlation is apparent between the interface quality index and the tensile strength of the test electrodes. Thus, amongst the electrodes made using petroleum coke as filler particles, the group made using filler A had both the highest strength and the lowest IQI. In contrast, although the electro- calcined anthracite containing materials may not be strictly comparable with those containing petroleum coke, as a group they were the strongest and had the highest IQI. 13

To compare the influence of binder pitches and filler particles on test electrode quality, selected items from the data presented in Tables 4 to 6 are plotted in Figures 10 to 13. The variation in properties of the four electrodes made using one filler coke and the four pitches are given in Figures 10 and 11 while corresponding data for electrodes made using individual pitches and four filler cokes are given in Figures 12 and 13. The two series of graphs allow ready comparison of the effect of pitch type and filler type respectively.

Regarding the effect of pitch type. Figures 10 and 11 indicate that there is no significant difference between the behaviour of pitches 1 to 3 but that the use of pitch 4, the high-mesophase pitch, resulted in significantly lower baked densities and tensile strengths and, except when combined with filler C, higher electrical resistivities. The general impression gained from comparing the shape of the histograms in the figures is that increases in tensile strength are accompanied by decreases in electrical resistivity. Thus there is the possibility that similar factors influence electrode strength and electron flow. It is also evident from Figures 10 and 11 that electrode tensile strength appears to vary with baked apparent density. Only in the case of the four electrodes made using filler D, the electro-calcined anthracite, does the tensile strength and resistivity appear to vary systematically with individual pore structural parameters.

Comparing the effect of filler type on electrode properties, the Figures 10 and 12 show that filler D, the electro-calcined anthracite, produced a group of electrodes with similar tensile strengths to those produced from regular petroleum coke, filler A. The groups of electrodes produced from the two premium grade petroleum cokes, fillers B and C, had similar mean strengths, these being lower than those obtained using filler coke A. Again,from the shape of the bar charts, it is generally noticeable that the tensile strength and electrical resistivity of the electrodes varies with the baked apparent density but not with individual pore structural parameters.

The interdependence of various electrode properties are shown in Figures 14 to 17- For the electrodes as a whole there is a general trend, associated with a low correlation coefficient, of increasing tensile strength with 14 increasing interface quality index (Figure 14), a trend which appears to be marginally improved if sets of electrodes made using the individual filler cokes, A to D, are considered. When the electrodes containing the individual filler cokes are considered, good correlations are obtained between the baked apparent densities and their tensile strengths (Figure 15) and electrical resistivities (Figure 16). The correlations are generally positive for tensile strength and negative for electrical resistivity. The exception is the electrical resistivity variation of the electrodes made using filler C but in this instance the spread of values was very small. The relationship between tensile strength and electrical resistivity of the test electrodes is shown in Figure 17. Two good correlations are evident, one for the electrodes containing petroleum coke as filler, the other for ECA-containing electrodes.

3.2 PROCESSING CONDITIONS AND INTERFACE QUALITY

This part of the research programme aimed to investigate the influence of some processing conditions on the binder/filler interface quality in fully- processed laboratory test electrodes. A single binder-pitch/filler coke combination was used, the processing conditions studied being pitch content, and mixing time and temperature. These were the parameters originally thought to have an important bearing on interface quality.

3-2.1 Experimental Procedures

Processing Parameters Studied

For this study the effect of processing parameters on the quality of interfaces in test electrodes made using premium filler coke C and the high mesophase binder pitch 4 was investigated. In the study of the effect of raw materials this combination of pitch and filler coke did not produce high quality electrode carbons. It was chosen for this study in order to see what improvement could be achieved by varying the processing conditions. Three concentrations of pitch were used 18, 20 and 22 wt% pitch when expressed as a percentage of the filler coke weight. These corresponded to 15-3. 16.7 and 18.0 wt% in the green mix. Pitch/filler mixtures were mixed at three temperatures I65, 180 and 200°C for 10, 20 and 30 minutes before moulding for 15

one minute at a moulding pressure of 20 MPa. They were then carbonized as described previously.

Test Electrode Examination

Using the methods applied when assessing the effect of raw materials on interface quality, measurements of interface quality, green and baked apparent density, tensile strength, electrical resistivity and pore structural parameters were obtained for the test electrodes prepared.

3.2.2 Consideration of Results

Table 7 lists the physical property data, ie, density, tensile strength and electrical resistivity, of the test electrodes produced. The results of the interface quality assessment are given in Table 8 while the pore structure parameters measured by image analysis are listed in Table 9- These data are referred to as Phase 2 data in some parts of this report.

The variation with processing parameters of the interface quality index, IQI, tensile strength, electrical resistivity and baked apparent density of the test electrodes are illustrated in the histograms in Figures 18 while the variation of pore structural parameters is shown in Figure 19• For each property, for each mixing temperature and pitch content, the three property values plotted from left to right refer to the mixing times of 10, 20 and 30 minutes.

As regards the effect on the interfacial quality it is apparent from Table 8 and Figure 18 that generally lower interface quality indices were observed at the lowest pitch level used, the effect of the two higher pitch concentrations being quite similar. Usually small improvements in interface quality index were observed at the intermediate mixing time of 20 minutes. The effect of mixing temperature was dependent on the pitch content of the green mix. Thus for the three pitch levels of 18, 20 and 22 \nt% respectively the highest interface quality indices were 64.4 at l65°C, 79.2 at 200°C and 77.7 at l80°C respectively. 16

From examination of Figure 18, it is evident that relative to the influence of pitch content and mixing temperature, mixing time played a subordinate role ir. determining the other physical properties of the test electrodes. In general however, for the quantities of materials mixed in this study, there appears to be a slight advantage, in terms of strength and electrical resistivity, to be gained from using a mixing time of 20 minutes rather than the 10 minutes used in other parts of this study.

Regarding the influence of pitch content, for the combination of binder pitch and filler coke used. Figure 18 shows that a pitch content of 20 vt% gives electrode carbons with an optimum combination of high density and strength and low electrical resistivity. The mixing temperature giving the optimum combination of electrode properties appears to be complexly dependent on the pitch content of the green mix. Thus for pitch contents of 18, 20 and 22 wt#, optimum quality electrodes were obtained by mixing at l80°C, 200°C and 180 - 200°C respectively.

It is difficult to distinguish any pattern in the variation of pore structural data with mixing conditions. The fractional volume porosities of the test electrodes all lay within the range of 0.3 * 0.05 and showed no obvious variation with the pitch content of the green mix. However there does appear from Figure 19 to be a trend for the pore and wall sizes both to increase with pitch content. This is accompanied by a tendency for the number of pores to fall as the pitch content increases.

Regarding the interdependence of electrode properties, Figures 20 and 21 show broad correlations between interface quality and tensile strength, and tensile strength and baked apparent density. However, for this series of experiments, the dependence of electrical resistivity on baked apparent density was poor (Figure 22) with the result that no correlation between tensile strength and electrical resistivity was obtained (Figure 23). Again no obvious trends were apparent between interface quality and physical properties and any individual pore structural parameter. 17

3.3 DEVELOPMENT IN INTERFACES DURING PROCESSING

The object of this phase of the work was to investigate the development of the binder/filler interfaces as carbonization proceeds. Accordingly, small test pieces were cut from a green block obtained using filler A and binder pitch 3. Heat-treatment of these small test electrodes was interrupted at temperatures in the range 200 - 1000°C and the interfaces present were studied by examining etched surfaces in an SEM.

3.3.1 Experimental Procedures

The green block was prepared by mixing the regular grade petroleum coke A with 22 vt% of coal-tar pitch 3 (to give a concentration of 18 wt% in the green block) at 165°C. The coke/pitch mix was moulded for one minute at 150°C at a pressure of 20 MPa to produce a green block 90 mm diameter by 90 mm tall. Small test pieces 40 x 20 x 10 mm were cut from the large block and these were heated under nitrogen at 5 K/minute to various temperatures in the range 200 - 1000°C.

Apparent densities of the green and heat-treated test pieces were obtained from their weight and dimensions. Usually the heat-treated specimens were distorted and were re-cut to regular shape before measurement. Thus dimensional changes resulting from heat-treatment could not be obtained.

3.3.2 Consideration of Results

For the range of test electrodes examined. Figure 24a - d shows the variation in the proportion of the different interfaces with the heat-treatment temperature. No graph is given for pored interfaces since their frequency of observation was less than 5% and showed little variation. The variation of the apparent density of the test electrodes with heat-treatment temperature is illustrated in Figure 25.

Figure 24 shows that the interfaces in the moulded mix are largely of the continuous, voided or partially-fissured types. The changes which occur on heat-treatment are conveniently considered in three temperature ranges 200 - 350°C, 350 - 600°C and 600 - 1000°C. In the first of these, only relatively 18 small changes in the proportions of the various interfaces were observed despite, as Figure 25 shows, the apparent density decreasing considerably.

Marked changes in both density and the proportions of the different types of interfaces took place in the temperature range 350 - 600°C. Voided interfaces increased in number especially between 350 and 400°C. The number of continuous interfaces fell throughout the 350 - 600°C range, initially gently then more sharply. The gradual initial decrease in the number of partially- fissured interfaces is reversed above 450°C while the number of completely- fissured interfaces rises, initially slowly, but more sharply around 600°C. In this temperature range, the apparent density of test electrodes falls to a minimum value near 450°C before rising sharply.

Above 600°C, the changes are again relatively small. The density stays relatively constant but there is some shift in the numbers of interface types from continuous to fissured.

3.4 EXAMINATION OF COMMERCIAL ELECTRODE CARBONS

In this phase of the research study commercially-produced electrode carbons were examined with the object of demonstrating the variability of binder/filler interface quality in such materials, to identify, if possible, those raw materials associated with poor interface quality and to investigate the effect of poor interface quality on electrode properties.

3.4.1 Experimental Procedures

Electrode Carbons Examined

The electrodes examined are listed in Table 10. They were all produced for use in the aluminium industry as either cathodes (1, 2 and 10) or anodes. The cathodes, contained as filler cokes, a mixture of gas-calcined anthracite and graphite, electro-calcined anthracite and graphite respectively. All the ar.ode carbons were made using petroleum coke as the filler. Table 10 also contains data from the diametral tensile strength testing of the carbons. The cathodes were generally weaker than the anode carbons. 19 Electrode Examination

Attempts were made to assess the quality of binder/filler interfaces by the indirect method of examining fracture surfaces in the SEM. For the present study, SEM fractography was carried out using both halves of seven broken tensile test pieces. To permit realistic comparison between electrodes, the seven test pieces chosen were those of highest and lowest tensile strength and those whose tensile strengths lay close to the mean value and to ±0.75 and si.5 times the standard deviation away from the mean value.

To obtain quantitative information, 50 randomly chosen positions on the surface of each specimen were examined, the feature present being allocated to one of four classes, transgranular failure of the filler, interfacial failure, binder failure or pore surface. Transgranular fracture failure features were further subdivided according to whether the fracture crack had traversed a blocky material present in electro-calcined anthracite or a lamellar form of carbon present in petroleum coke and as a porous form in calcined anthracite. Interfacial failures were also subdivided in order to distinguish between binder and filler side, the type of filler present being noted for filler side features.

Fore structural data were obtained from polished surfaces of four of the electrode carbons listed in Table 10 using the procedures described earlier.

3.4.2 Consideration of Results

Mean tensile strength data for the ten commercial electrode carbons examined are included in Table 10. Data obtained from the SEM examination of broken tensile test pieces are given in Table 11, the data given for each electrode being the mean data from the five individual specimens examined. In view of the difficulty sometimes pxprirneed in identifying features formed by interfacial failure, the correspondence between the total filler side and binder side features is considered to be quite good. Table 12 summarises the data on transgranular and interfacial failure summing the data given in subdivided form in Table 11. 20 It is evident from the number of features present in fracture surfaces which were formed as a result of interfacial failure during tensile testing (Table 11), that all the commercial electrode carbons contained poor quality binder/filler interfaces. In this respect the cathode carbons appeared to have a higher proportion of low quality binder/filler interfaces than anode carbons. Thus the frequency of interfacial failure values ranged from 5-0 to c.1% for the cathodes but from 2.6 to 5.3% for the anodes.

For these materials no simple relationship is apparent between the extent of interfacial failure and the strength of the carbon. Table 12 contains values for the ratio of observations of transgranular failure of the larger filler particles to the total observations of interfacial failure. Neither this value nor the observations of interfacial failure vary systematically with the tensile strength of the carbons.

Pere structural parameters measured by image analysis of polished surfaces are given in Tables 13 to 16, each table giving the pore structural data and the tensile strength for the seven specimens examined from one electrode. No simple systematic variation of the tensile strength with any individual pore structural parameter is apparent. The use of this data in seeking more complex explanation of the role of pore structure and interface quality on the strength of carbons is reserved for the General Discussion section. 21 i» GENERAL DISCUSSION

It can be inferred from consideration of the method of production of electrode carbons by the carbonization of coal-tar pitch-bonded filler particles that the product is a composite composed of two solids, albeit both carbon, and hence that their properties should be dependent upon the quality of the bond between binder and filler. However, no previous attempt has been made to quantify the quality of binder/filler interfaces in electrode carbons so that no direct information is available on the influence of variations in interface quality on electrode properties and performance.

In confirmation of previous findings6, in electrodes examined in the present work, made using a mixture of filler coke sizes including very fine particles, carbon residues from the binder pitch could not be identified except as thin films on pore walls. Instead scanning electron microscopy shows that the larger filler particles in electrode carbons are bound together by a binder phase consisting of a mixture of pitch coke and small filler particles. In the present work it has been demonstrated that variations in the quality of the interfaces between large filler particles and this binder phase can be detected by the examination of etched electrode surfaces in a scanning electron microscope and quantified by applying a point counting technique. Fcor quality interfaces influence the mode of passage of a crack propagating through a carbon under stress and induce features in the fracture surface indicative of interfacial failure. These features can be detected when fractured surfaces are examined in a scanning electron microscope. Quantitative data indicative of the quality of the interfaces present in a carbon can be obtained by point counting but the results obtained are considered less accurate than those obtained using etched surfaces because of the difficulty of identifying some features with certainty.

Notwithstanding the inability to detect coke residues from the pitch in electrode carbons, it is recognised that the pitch is the only component present in the green block capable of acting as an adhesive for the filler cske particles. To achieve a sound binder/filler bond in the electrode the pitch must be given the opportunity during mixing and moulding to contact and to wet all the surfaces of the filler coke particles. Weak bonds, for example cipole/dipole interactions, may be formed at this stage. Contact must then 22 be maintained until the adhesive sets, ie, the pitch is carbonized, and strong bonding is achieved. The nature of the bonding at a strong binder/filler interface in a carbon is not known. However the bonding at a good continuous interface is sufficiently strong that the interface is not preferentially attacked by the atomic oxygen etchant used in this study.

The various types of inferior quality interfaces identified in this work result from the inability to maintain the pitch/filler contact during the various processes involved in electrode fabrication. Consideration of the data obtained from the study of the variation of interface types during processing offers some insight into the factors responsible for poor quality interfaces.

The aim of the moulding stage, apart from obtaining a suitable size and shape of electrode, is to eject air trapped in the mix so that both maximum green block density and good adhesive/filler contact are achieved. During moulding the pitch-coated filler particles initially form a load-resisting network which undergoes deformation as the mixture of pitch and small filler particles constituting the binder phase yields7. The consequent flow into the spaces between the larger filler particles improves binder/ filler contact. The porosity of the mix decreases as the air in the interstices is ejected. Ideally, for a homogeneous mixture, with the optimum binder/filler ratio and filler size distribution, compression of the green mix should be halted as soon as all the air has been ejected and good contact between binder phase and filler particles achieved. Such conditions are difficult to identify so that, in practice, further pressure is exerted and this results in thinning of the binder phase between filler particles until eventually point contacts between these particles are established. These then become load-bearing. The stressed filler/filler contacts will endure various degrees of strain. Distortion, including bending of appropriately stressed planar particles will occur, possibly to the point of fragmentation.

Expansion of the compact occurs on release of the load, primarily due to relaxation of the elastic stresses and shape distortions associated with the filler particles. The extent of this expansion for the pitch 3/fiHer A mix used in the study of interfaces during processing was measured to be ^.k vol%. This expansion can result in some filler/filler contact points being broken 23 as two filler particles pull apart. The effect on the binder bridges will depend on the viscosity of the material present. Easier movement of the pitch constituent of the binder phase on compression during moulding may allow sufficient preferential movement that the binder phase becomes inhomogeneous. Two extremes of behaviour can then be envisaged as the filler particles draw apart. Unless the extent of relaxation is high, low viscosity bridges will merely stretch and retract. However, this mechanism of stress relief is not readily available to areas of binder with high content of fine filler particles which have high viscosity. Instead, at the extreme, stress on the binder/filler interfaces induced by the pulling apart of large filler particles will exceed the interfacial strength so that completely or partially fissured interfaces are formed. This explains the presence of many fissured interfaces in the unheated test pieces in the present work. Stress relaxation will gradually cease as the moulded block cools find the pitch hardens.

Because no air-curing of the binder takes place in anode production, further changes in the relative positions of the binder and filler may occur on heat- treatment in the temperature range below that at which active decomposition of the pitch takes place.

The dilatometric behaviour of anode mixes on heating depends, in a complex way, upon the mixing and moulding conditions, the granulometry of the filler, the pitch type and content, and the heating rate so that it is impossible to anticipate the form of the dilatometric curve. In general two temperature ranges of rapid dilatation have been observed, namely near 200°C and near the temperature of active pitch decomposition. However, under some conditions the two zones merge into one8. Clearly this has occurred in the present study with the result that the density of the heat-treated test electrodes fell progressively on heating between 200°C and 'JCWC (Figure 25).

At the lower end of this temperature range the fall reflects a continued relaxation of stresses induced during moulding. Little overall change in the number of fissured interfaces was observed, presumably because the low viscosity of the pitch at these temperatures permitted stress relaxation by binder bridge retraction rather than interface fissuring. Thermogravimetric analysis showed (Figure 26) that active decomposition of pitch 3 commences near 350°C. Pore formation induced by the volatile matter evolution then 24 becomes the important factor influencing the density and is responsible for the marked formation of voided interfaces from continuous ones observed near this temperature.

Eventually, as the pitch carbonization continues, the ordering associated with, firstly mesophase coalescence and subsequently resolidification, induces shrinkage of the pitch phase. This becomes the dominant influence on the test electrode density which in the present work increased between 450°C and 550°C (Figure 25). In this temperature range, the number of continuous interfaces fell while fissured interfaces became more numerous (Figure 24). Apparently the shrinkage imposes stresses which can most readily be accommodated by failure of the weakest continuous binder/filler interfaces.

No change in density was observed on heating these particular test electrodes above 600°C (Figure 25). A small further shrinkage can be expected to occur near 700°C and this is considered responsible for the minor increases in the numbers of the two types of fissured interfaces observed above 600°C (Figure

It is recognised that no single set of experimental conditions can lead to a general description of the development of binder/ filler interfaces during anode carbon production. Also the effects noted in the present work were no doubt emphasised by the high heating rate used. However, poor quality interfaces were only slightly more prevalent in these materials than in commercially-produced anode carbons. It therefore appears justifiable to conclude that, in electrode carbons, voided interfaces are formed by volatiles released during active decomposition of the pitch while fissured interfaces result from relaxation of stresses induced during the moulding stage and the shrinkage of the pitch phase as it resolidifies.

The present study has shown that poor quality interfaces, especially fissured ones, are associated primarily with the larger filler particles. It is unlikely that this is caused by any variation in the basic nature of the surface of filler cokes with particle size. More probably the effect is associated with the stresses imposed on the interfaces by pitch coke shrinkage and relaxation of moulding stresses being concentrated around larger filler particles. Provided the pitch content is adequate, and the discussion below 25 indicates that the pitch concentration used in the present study is near the optimum level, good mixing and moulding ensures that the pitch is ideally positioned to act as an adhesive for the filler particles.

During carbonization, provided the interface remains unfissured, the bonds formed at the interfaces on wetting of the filler by the pitch become transformed into stronger bonds. The nature of neither type of bond is established. However, examination of etched surfaces of petroleum coke based electrodes in an SEM reveals the position of the constituent carbon lamellae. It was quite apparent when assessing interface quality that the fissured interfaces occurred predominantly when the carbon lamellae lay parallel to the interface (cf. Figure 2b). Conversely, continuously-bonded interfaces were usually observed when the lamellae intersected the interface at an angle (cf. Figure 2e) . Identifying the lamellae with graphite-like structures having few valency electrons parallel to the basal planes but many at the prismatic edges, then these findings imply that valency electrons in the carbon become involved in bonding at the binder/filler interface. It is considered that such electrons play a role in both the relatively weak pitch/filler bonding and in the stronger bonds formed on carbonization. The strength of the pitch/filler bond, which probably involves oxygenated-functional groups on the filler surface, governs its resistance to fissuring on carbonization. Electro-calcined anthracite does not soften on carbonization so that large lamellae are not formed. If the smaller graphite-like layers present are not well ordered then valency electrons would be present in any surface. This probably accounts for the good interface quality in the test electrodes based on ECA. In this work, no evidence was found to suggest that physical interlocking associated with the penetration of pitch into fissures in petroleum coke particles played any role in determining interface quality.

According to aluminium industry criteria9, the pitches used in this study varied in quality. Hence the lack of any marked influence of the pitches on interface quality suggests that any variation in the affinity of the pitches to bond to filler coke surfaces is too small to be identified by present methods. It has been claimed that the nature and concentration of insolubles in the pitch influences binder/filler bonding10. Thus, the mechanical action of mixing is considered to spread any mesophase spheres present in the pitch along the filler coke surface aligning the constituent lamellar aromatic 26 molecules parallel to the surface thereby minimising the availability of bonding functional groups in the interface. This effect does appear to occur in electrode-like carbons11. However, clear evidence showing that mesophase spheres can wet and bond to surfaces of carbons possessing less order than petroleum coke suggests that mesophase spheres can bond firmly to ends of lamellae in anode carbon fillers.

It has been suggested that primary quinoline insolubles and carbon black particles interfere with the mesophase growth and coalescence on carbonization12 so resulting in a less well-ordered pitch coke which is more capable of bonding to filler particles than the better ordered structures otherwise formed. Such views tend to be based on the behaviour of pitches when carbonized alone. When subjected to spatial constraint in commercial anode mixes, it is unlikely that growth of mesophase to a size capable of producing well-ordered structures can occur. Thus it can be argued convincingly that, for acceptable quality binder pitches, if mixing and moulding operations are carried out near to the optimum conditions no marked influence of the pitches on interface quality in electrode carbons should be observed.

The primary objective of the mixing operation in electrode manufacture should be to achieve a homogeneous mixture in which all the filler surfaces are evenly coated with pitch. The optimum proportion of pitch to filler will depend on the granulometry of the filler. Too low a pitch content will lead to inadequate coverage of the filler surface with pitch and to areas of unbonded filler in a weak electrode. Too high a pitch content, apart from inducing slumping, may result in weak, highly-porous regions in the electrode. Eoth features may occur in the same electrode if mixing is inadequate. It can be envisaged that the mixing time and temperature will both effect the efficiency of mixing, the optimum time being dependent on the quantity being mixed and the optimum temperature on pitch properties.

The results obtained in this study are in general accord with these views. The range of mixing times employed in this work apparently was too narrow to have a large effect on the interface quality index. The optimum pitch level in the green block appeared to be 16.7 wt%, this level resulting in high interface quality for all mixing times and temperatures. All electrodes made 27 with lower pitch concentrations had lower interface quality. At higher concentrations the IQI of the electrodes varied markedly depending on the mixing conditions. Good interface quality was only observed for electrodes mixed at l80°C. In accord with the views above, low values were associated primarily with increases in the number of voided interfaces present. Thus although the results are in general agreement with expected behaviour, the interface quality in electrodes apparently depends, in a manner difficult to predict, on the three processing conditions, pitch content and moulding time and temperature.

Regarding the influence of binder/filler interface quality on technical important properties of electrode carbons, in the experimental sections of this report, trends between increasing tensile strength and decreasing electrical resistivity with increasing interface quality index have been identified. For tensile strength and interface quality index all the data available from this study are summarised in Figure 27. The data fall into three groups, ie, the data for the four commercial materials and the two sets of data from Phases 1 and 2 of this work. For each individual group the previously noted trend between tensile strength and interface quality index is still evident but it is also apparent that the data do not fit a single general pattern.

Although the Phase 2 data and those for the commercial carbons listed in Table 2 appear to follow the same trend, the Phase 1 data from those carbons prepared to study the influence of raw materials on interface quality appear to form a group where high interface quality index has not resulted in high strength.

The reason for this is not clear. It is recognised that the interface quality index is an empirical one calculated to provide a single-value index of the interfaces observed in electrode carbons. The coefficients used in calculating the index were chosen to reflect the variation in strength of the four commercial anode carbons, hence the points for these materials fall on a straight line in Figure 27. It is possible that the use of other coefficients might reflect more accurately the variation of strength of the test electrode carbons with the proportion of the various interface types 28 present, but it is considered unlikely that any single set of coefficients would have general applicability to all the carbons here considered.

In addition to the quality of binder/filler interfaces the tensile strength cf carbons is considered to be dependent on any variation in strength of the continuous interfaces present, the fractional volume porosity which governs the area under stress and the size and shape of the pores which influence the extent to which they act as stress concentrators13. Thus to explain the variation of tensile strength amongst electrode carbons with precision, all these factors must be taken into account. Only by doing so can a generally applicable relationship between tensile strength and carbon structure be expected. Thus it is perhaps not surprising that the empirical approach used here, involving only one of the factors considered important, does not result in a general explanation of the variation of strength of the electrode carbons examined.

Previous studies had shown that the tensile strength, S, of electrode carbons could be related to their pore structural parameters, determined by image analysis, using an equation of the form:

S x N = K x W/P2 + C where N is the number of pores per unit area, P and W are the pore and wall sizes respectively and K and C are constants 14.

A further equation found useful to describe the tensile strength of metallurgical cokes in terms of pore structural data is:

S = KFmax"1/2 x exp(-2[Fmax/Fmin]1/2 x p) where Fmax and Fmin are the maximum and minimum Feret diameters, p is the fractional volume porosity and K is a constant13. Since the constants, K, in both equations govern the manner of the variation of strength with pore structural parameters, there is reason to believe that they are material properties indicative of the inherent pore free strength of the material considered. 29 Both the equations were used in an attempt to relate the tensile strength and pore structural parameters of individual specimens from electrodes produced in the studies described in Sections 3-1 and 3-2, the intention being to try to relate the constants in the relationships to the mean interface quality indices of the various electrode carbons. Unfortunately it was found that, possibly due to the range of strength values being low, the data obtained did not fit equations of the form shown above with sufficient accuracy to justify continuing with this approach.

In Figures 28 and 29 mean tensile strength and mean pore structural data for all the test electrodes produced during this study are plotted in the form of the above equations. It is clear from these figures that neither equation cffers a general explanation for the variation in strength of these materials.

However, the situation was different for the four commercial carbons studied in Section 3.4 of this report. For these materials the variation in tensile strengths of individual specimens from each material could be explained well in terms of pore and wall sizes and the number of pores per unit area (Figure 30). The coefficients in the equations for the four electrode carbons examined are given in Table 17. It is evident that the values of the constant K in the equations, which express the slopes of the lines in Figures 1-4, vary considerably from 1101 for electrode 2 to 7^48 for electrode 3.

If K is a materials property indicative of the pore-free strength, it might be expected to be influenced by various factors including the average binder- filler interfacial strength. High binder/filler interface strength would be expected to result in features originating from transgranular fracture of filler particles being observed in fracture surfaces more often than those indicative of interfacial failure. Thus a high T/I ratio would be expected. Unfortunately comparison of the T/I ratios in Table 12 with the K values in Table 17 reveals no simple relation between them.

Comparison of the equations in Table 17 suggest that the strength of electrode r.'jmber 2, made using electro-calcined anthracite, is less sensitive to variations in porous structure than the other three electrodes. Figure 30 shows this clearly. Accordingly electrodes 1, 3 and 10 were regarded as a single population and a general equation relating their strengths and porous 30 structures sought. The equation obtained is given in Table 17 and this describes the data with a high coefficient of determination of 0.92. This would appear to justify considering these electrodes as part of a single papulation and is indicative of a common feature in their structures. All three carbons contained petroleum coke either in the original form or as scrap graphite and it appears to be this feature which distinguishes between these electrodes and electrode 2 made using electro-calcined anthracite. It would appear therefore that, as well as the nature of the porous structure and the quality of the binder-filler interface, the nature of the filler particle must also be considered as a factor influencing the quality of carbon electrodes.

As regards the effect of interface quality on electrical conductivity, mean data for all the test electrodes are plotted against the mean interface quality index in Figure 31- A broad general trend covering all the test electrodes is evident. Again other factors, eg, porosity, degree of carbon crder in the filler coke, would be expected to have a bearing on the electrical conductivity of electrode carbons, but nevertheless it is again clear that binder/filler interface quality has an important bearing.

Cue to the wide variety of raw materials used in electrode manufacture, it is recognised that the findings of the present work may not be applicable in every industrial situation. That was not the intention. The aim of the work was to draw attention to one aspect of the structure of anode carbons, the quality of the binder/filler interfaces present and its effects on electrode quality which had not hitherto been given much consideration. It is now clear that interface quality in electrodes can be quantified, the data so obtained being useful in understanding both the effect of raw material and processing variables on the properties of fully-processed carbons and the factors during processing which influence interface quality. The bearing of interface quality on strength and electrical resistivity has also been demonstrated. Thus the work has made a contribution to the understanding of the structure ar.d properties of electrode carbons and to the use of coal-tar pitch as binders in their production. 31 CONCLUSIONS

Interfaces between the binder phase and filler particles in electrode carbons can be classified during SEM examination of etched surfaces according to the degree of continuity of structure at the interface.

The interface quality of an electrode can be assessed by point counting and the data obtained used to give a single-value interface quality index, IQI.

Interface quality has a bearing on both the strength and electrical resistivity of electrode carbons.

The relaxation of moulding stresses and shrinkage associated with pitch carbonization both result in fissured interfaces while voided interfaces are formed on volatile release from the pitch. These are the two principal types of poor quality interfaces.

Mixing time and temperature, and pitch content of the pitch/ filler mix all influence interface quality, pitch content appearing to be the most critical parameter.

At optimum pitch levels, the variation in quality of coal-tar pitches used as binders has less effect on IQI than variations amongst fillers.

SEM examination of fractured surfaces also reveals evidence of poor quality binder/filler interfaces. Interface quality can be assessed by- point counting but the data obtained could not be related to the strength of commercial electrode materials.

The strength of commercial electrode carbons is related to combinations of pore structural parameters but the values of coefficients in the relationships vary with the nature of the filler used.

33 REFERENCES

1 Mantell, C L, 'Carbon and Graphite Handbook', Interscience, New York, 1968.

2 'European Marketing Data and Statistics 1990' , 25th Edition, Euromonitor Publications Limited, London, 1990.

3 Gilchrist, J D, 'Extractive Metallurgy', 2nd Edition, Pergammon, Oxford, 1980.

Belitkus, D, Metallurgical Trans, 1978, 9B, 705.

5 Hays, D, Patrick, J W and Walker, A, Fuel 1983, 62, 946.

: Bennet, D, Gibson, A, Patrick, J W and Walker, A, Carbon 1988, 26, 653.

Taylor, J W in 'Carbon '88, Proc Int Conf on Carbon and Graphite', IOP Publishing Limited, Bristol, 1988, 461.

8 Martirena, H, Light Metals 1983, AIME, 1983, p749.

9 Jones, S S in 'Petroleum Derived Carbons', (Eds J D Bacha, J W Newnan and J L White), Acs Symposium Series 303. American Chemical Society, Washington DC, 1986, p234.

10 Jones, S S and Hildebrandt. R D, Light Metals 1975. AIME, 1975, p291.

11 Hays, D. Patrick. J W and Walker, A, Fuel 1983, 62, 1145.

12 Brooks, J D and Taylor, G H in 'Chemistry and Physics of Carbon', (Ed P L Walker jnr) Edward Arnold, London, 1968, Vol 4, p243.

13 Patrick, J W and Stacey. A E. ISS Transactions 1983, 3, 1.

14 Patrick, J W. Sorlie, M and Walker, A, Carbon 1989, 27, 469, 461.

Table 1. Description of interface types.

Interface type Designation Figure number Description

Voided V 2a Filler particle adjacent to a pore or void.

Fissured: 00 completely 2b Fissure runs whole length of interface. en partially 2c Fissure interrupted by areas of binder/filler contact.

Pored P 2d Small pores evident along interface.

Continuous C 2e Good binder/filler contact along length of interface. Table 2. Interface types in industrial anode carbons.

Observation of interface types, % Tensile

Electrode strength V Fc FP P C IQI (MPa) 1 6.8 22 14 11 10 43 110.0 2 6.1 22 13 6 23 36 106.9 3 5.3 11 19 18 31 22 89.7 4 4.7 29 16 7 24 25 84.4 Table 3. Characterisation data for electrode binder pitches.

Pitch number Nature of insoluble Softening point Anthracene oil Fixed carbon matter (KS),°C insolubles, wt% (SERS), wt%

1 Carbon black 91.0 12.4 55.0 00 ^1 2 Primary, low 94.4 7.2 53.3 concentration 3 Primary, optimum 90.5 14.5 56.5 concentration

4 Mesophase 94.0 12.6 56.2 Table 4. Physical data for Phase 1 electrodes.

Green Baked Electrode apparent apparent Tensile Electrical density density strength resistivity (kg/m3) (kg/m3) (MPa) (|i£2m) Al 1537 1430 3.85 65.6 A2 1554 1411 3.23 70.5 A3 1549 1441 3.70 62.1 A4 1551 1391 2.42 74.2 00 Bl 1629 1495 3.02 73.0 B2 1655 1488 3.06 74.1 B3 1642 1492 3.04 67.6 B4 1623 1447 2.41 78.2 CI 1592 1476 3.08 74.9 C2 1619 1462 2.96 74.6 C3 1614 1461 2.71 74.6 C4 1624 1426 2.39 71.8 Dl 1581 1440 3.53 54.8 D2 1606 1476 4.19 46.2 D3 1605 1466 3.99 52.2 D4 1568 1348 2.36 61.8 Table 5. Interface data for Phase 1 electrodes.

Observation of interface types, % Tensile

Electrode strength V Fc FP P C IQI (MPa) Al 3.85 27.0 21.5 18.0 3.6 29.8 76.7 A2 3.23 25.9 21.3 20.8 2.8 29.2 75.6 A3 3.70 21.9 14.6 12.6 4.2 46.7 112.4 A4 2.42 21.1 23.5 18.6 1.4 35.3 86.6 Bl 3.02 23.1 19.3 13.6 9.0 35.0 92.3 B2 3.06 20.7 14.6 13.8 9.3 41.6 107.3 B3 3.04 24.0 16.6 11.6 9.4 38.4 99.5 B4 2.41 31.6 15.8 12.0 6.9 33.7 86.5 CI 3.08 30.4 16.3 10.3 6.1 36.9 92.0 C2 2.96 28.9 16.4 13.6 7.3 33.7 87.6 C3 2.71 26.0 17.7 12.5 8.0 35.8 92.6 C4 2.39 33.3 16.0 12.0 4.9 33.8 84.6 Dl 3.53 25.8 10.8 10.0 12.7 40.8 107.7 D2 4.19 24.2 10.3 9.2 21.9 34.4 103.5 D3 3.99 26.2 9.8 8.7 19.4 35.9 103.8 D4 2.36 29.7 14.2 10.0 7.2 38.9 97.5 Table 6. Pore structural data for Phase 1 electrodes.

Mean pore structural parameters Feret diameters Electrode Tensile Fractional Pore size Wall size F F Pores/mm2 SxN W/P2 i 1 max ■* nun 1 h strength volume (um) (um) (um) (urn) (MPa) (urn ) *■ mix (MPa) porosity xlO* F . y *■ mm / Al 3.85 0.312 81.8 181 63.6 33.3 77.4 298 271 1.38 A2 3.23 0.287 89.8 224 69.0 36.6 59.0 191 278 1.37 A3 3.70 0.288 77.3 192 61.6 30.9 78.4 290 321 1.41 A4 2.42 0.288 91.1 225 68.1 36.8 59.4 144 271 1.36

Bl 3.02 0.260 60.3 172 52.4 26.1 105.1 317 473 1.42 o B2 3.06 0.288 73.7 183 59.7 29.2 85.2 261 337 1.43 B3 3.04 0.253 67.2 199 64.7 31.6 75.7 230 441 1.43 B4 2.41 0.265 70.7 196 66.0 30.6 76.4 184 392 1.47 CI 3.08 0.261 71.9 204 64.3 32.5 73.3 227 395 1.41 C2 2.96 0.263 75.8 213 60.8 31.8 72.6 215 371 1.38 C3 2.71 0.245 71.2 220 65.9 33.1 67.2 182 434 1.41 C4 2.39 0.275 73.9 195 61.7 31.6 76.8 184 357 1.40 Dl 3.53 0.219 59.0 211 51.1 27.1 91.3 322 606 1.37 D2 4.19 0.166 45.7 229 48.4 25.8 94.4 396 1096 1.37 DD43 3.99 0.176 49.2 230 50.3 26.6 89.8 358 950 1.38 1 2.36 0.249 73.4 222 56.6 30.4 75.3 178 412 1.36 Table 7. Physical data for Phase 2 electrodes. Mixing conditions Green Baked Pitch content Temp. Time apparent apparent Tensile Electrical (wt%) (°C) (min) density density strength resistivity (kg/m*> (kg/m3) (MPa) (HΩm) 10 1510 1384 2.53 99.8 18 165 20 1510 1389 2.65 86.4 30 1514 1383 2.55 100.4 10 1504 1401 2.69 92.9 18 180 20 1531 1429 3.20 75.7 30 1543 1426 3.03 79.6 10 1515 1416 2.75 86.6 18 200 20 1519 1435 2.86 97.2 30 1503 1433 2.86 102.0 10 1579 1335 1.88 81.5 20 165 20 1581 1383 2.67 66.3 30 1588 1380 2.82 72.1 10 1569 1423 3.39 76.5 20 180 20 1581 1432 3.58 69.0 30 1604 1411 3.81 66.5 10 1595 1447 3.71 72.2 20 200 20 1605 1449 4.12 71.3 30 1594 1431 3.96 67.1 10 1569 1394 2.77 77.6 22 165 20 1606 1365 2.63 63.5 30 1601 1338 2.78 79.7 10 1605 1395 3.27 81.2 22 180 20 1610 1374 2.96 79.5 30 1618 1376 3.21 75.6 10 1622 1402 3.09 73.5 22 200 20 1625 1410 3.34 76.8 30 1621 1400 3.58 76.7 Table 8. Interface data for Phase 2 electrodes.

Mixing conditions Observation of interface types, % Pitch Tensile F P C content Temp. Time strength V Fc P IQI (wt%) (°C) (min) (MPa) 10 2.53 41.2 19.0 13.0 2.3 24.4 61.4 18 165 20 2.65 44.4 17.5 16.4 4.2 24.2 64.4 30 2.55 41.1 18.1 12.6 5.6 22.4 60.5 10 2.69 43.1 18.1 9.2 6.6 22.8 61.1 18 180 20 3.20 39.6 19.7 11.3 6.3 22.9 61.8 30 3.03 39.7 20.4 10.9 6.8 22.0 60.2 10 2.75 38.5 21.7 11.3 6.3 22.1 60.1 18 200 20 2.86 38.9 22.2 12.1 5.5 21.1 57.3 30 2.86 38.6 21.1 13.6 5.3 21.2 57.9 10 1.88 40.5 16.1 10.9 4.7 27.5 70.0 20 165 20 2.67 38.5 15.5 11.2 4.9 29.7 75.2 30 2.82 40.5 17.2 9.1 6.0 26.9 69.4 10 3.39 37.6 16.0 14.7 4.6 26.9 70.1 20 180 20 3.58 34.6 19.5 13.6 4.9 27.2 70.7 30 •3.81 39.1 17.1 12.7 4.2 26.7 68.4 10 3.71 33.2 21.6 11.4 0.5 32.8 77.3 20 200 20 4.12 33.0 21.6 11.1 0.0 34.0 79.2 30 3.96 36.3 21.0 11.3 1.1 30.1 72.0 10 2.77 35.7 19.8 15.4 1.1 27.9 68.8 22 165 20 2.63 37.6 21.2 14.3 0.5 26.3 64.1 30 2.78 42.1 20.5 14.0 0.7 22.6 56.1 10 3.27 39.3 19.0 11.2 0.0 30.4 71.4 22 180 20 2.96 34.0 20.7 12.1 0.0 33.1 77.7 30 3.21 40.6 18.6 14.0 0.4 26.2 63.7 10 3.09 34.9 21.5 12.7 0.0 30.7 72.6 22 200 20 3.34 33.5 20.3 14.2 0.0 31.9 75.9 30 3.58 36.9 21.4 14.6 0.7 26.2 64.3 Table 9. Pore structural data for Phase 2 electrodes. Mean pore structural parameters Mixing conditions Feret diameters Pitch Tensile Fractional Pore size Wall size F F Pores/mm2 SxN W/P2 content Temp. Time strength volume (um) (Um) F max * min (MPa) (urn1) (um) (urn) 4 (wt%) CO (min) (MPa) porosity xlO \ k nun , 10 2.53 0.270 65.3 177 62.4 31.0 85.2 216 415 1.42 18 165 20 2.65 0.279 65.5 169 53.4 28.7 100.6 267 394 1.36 30 2.55 0.326 82.4 170 63.4 32.2 80.1 204 250 1.40 10 2.69 0.337 75.1 148 57.7 30.2 98.7 266 262 1.38 18 180 20 3.20 0.318 78.9 169 51.6 27.6 97.9 313 271 1.37 30 3.03 0.338 79.9 157 56.5 27.9 96.1 291 246 1.42 10 2.75 0.327 64.7 133 53.0 28.2 120.2 331 318 1.37 18 200 20 2.86 0.266 63.1 174 58.9 30.7 91.0 260 437 1.39 30 2.86 0.278 60.2 156 50.6 25.6 116.1 332 430 1.41 10 1.88 0.295 79.5 190 58.8 30.6 80.0 150 299 1.39 20 165 20 2.67 0.275 74.8 197 59.6 30.2 78.8 210 352 1.40 3O 30 2.82 0.304 73.0 167 59.3 30.1 89.4 252 313 1.40 10 3.39 0.312 72.9 161 59.4 28.8 92.5 314 303 1.44 20 180 20 3.58 0.320 70.0 149 52.4 26.7 110.6 396 304 1.40 30 3.81 0.286 76.0 190 56.7 29.2 84.2 321 329 1.39 10 3.71 0.306 74.5 169 59.5 29.7 88.5 328 304 1.42 20 200 20 4.12 0.257 69.8 202 55.2 27.9 84.7 349 415 1.41 30 3.96 0.273 70.1 187 53.9 26.5 92.1 365 381 1.43 10 2.77 0.306 71.1 161 50.8 25.2 108.2 300 318 1.42 22 165 20 2.63 0.304 90.8 208 60.7 31.0 70.7 186 252 1.40 30 2.78 0.298 83.2 196 57.2 28.2 80.4 224 283 1.42 10 3.27 0.300 92.2 215 56.3 28.1 73.8 241 253 1.42 22 180 20 2.96 0.286 85.3 212 54.6 27.6 78.0 231 291 1.41 30 3.21 0.316 92.1 202 55.6 27.3 78.4 252 238 1.43 10 3.09 0.293 83.6 202 59.7 29.9 74.9 231 289 1.41 • 22 200 20 3.34 0.319 86.8 185 55.5 27.0 85.1 284 246 1.43 30 3.58 0.336 90.4 178 58.0 29.5 81.8 293 218 1.40 Table 10. Tensile strengths of commercial electrodes used in the aluminium industry.

Tensile strength, MPa

Electrode Filler type Mean Standard deviation 1 GCA + graphite 5.01 0.78 2 ECA 3.29 0.52 3 PC 5.88 0.64 4 PC 6.62 1.01 5 PC 6.03 0.92 6 PC 5.82 0.88 7 PC 6.29 1.04 8 PC 5.91 1.00 9 PC 6.57 0.74

10 Graphite 3.35 0.71 Table 11. Data from SEM fractography of broken tensile test pieces.

Proportion of features, % Transgranular Interfacial Electrode Blocky Lamellar Blocky Lamellar Binder Binder failure Pore surface 1 22.9 3.1 2.1 0.4 2.5 17.9 51.1 2 13.4 11.1 4.0 0.8 3.2 15.8 51.7 3 0.5 18.3 0 3.3 4.8 22.6 50.5 45 4 0 20.1 0 2.0 3.3 23.2 51.4 5 0 19.0 0 1.5 2.0 32.7 44.8 6 0.2 19.7 0 2.1 2.3 27.6 48.1 7 0 13.8 0 1.8 2.7 30.0 51.7 8 0 20.4 0 1.1 2.3 25.2 51.0 9 0 16.3 0 0.8 1.8 27.6 53.5 10 0 17.5 0 3.1 3.1 24.2 52.7 Table 12. Transgranular and interfadal failure data for commercial electrodes.

Proportion of features, % Electrode Transgranular fracture Interfadal failure T/I Tensile strength, ratio MPa 1 26.0 5.0 5.0 5.01 2 24.5 8.0 3.1 3.29 3 18.8 8.1 2.3 5.88 45. 4 20.1 5.3 3.8 6.62 5 19.0 3.5 5.4 6.03 6 19.9 4.4 4.5 5.82 7 13.8 4.5 3.1 6.29 8 20.4 3.4 7.8 5.91 9 16.3 2.6 6.3 6.57 10 17.5 5.6 3.1 3.35 Table 13. Pore structrural data for electrode 1.

Mean pore structural parameters Feret diameters 2 Electrode Tensile Fractional Pore size Wall size F Fmin Pores/mm * max strength volume (Hm) (urn) (um) (um) (MPa) porosity 4=» 1 6.31 0.153 42.8 238 45.1 37.1 97 ^1 2 6.27 0.171 39.5 191 41.3 33.3 129 3 5.59 0.184 51.2 274 49.4 40.6 90 4 5.01 0.161 40.5 211 40.1 32.7 122 5 4.38 0.204 43.4 169 47.3 39.9 124 6 3.78 0.202 57.7 202 51.4 42.5 94 7 3.21 0.219 50.1 179 48.9 ' 38.3 111 Table 14. Pore structrural data for electrode 2.

Mean pore structural parameters Feret diameters 2 Electrode Tensile Fractional Pore size Wall size F F . Pores/mm F max * mm strength volume (^im) (*im) (|im) (urn) (MPa) porosity

1 4.18 0.104 51.8 444 48.8 39.8 53 00 2 3.91 0.136 47.9 303 39.9 33.4 90 3 3.71 0.166 47.2 238 41.9 32.9 105 4 3.29 0.128 41.1 279 38.4 31.8 102 5 2.91 0.145 52.4 310 40.9 33.7 84 6 2.52 0.197 70.7 289 52.2 43.1 68 7 2.21 0.185 60.4 265 51.3 41.1 76 Table 15. Pore structural data for electrode 3.

Mean pore structural parameters Feret diameters Electrode Tensile Fractional Pore size Wall size F F . Pores/mm2 Fmax Fmin strength volume (Hm) (|im) (^m) (nm) (MPa) porosity 4* 1 6.95 0.152 32.4 180 39.3 32.1 149 2 6.73 0.151 29.1 162 38.8 30.9 169 3 6.35 0.159 28.9 153 38.1 30.1 181 4 5.88 0.170 29.1 142 40.3 32.3 182 5 5.41 0.173 30.2 145 39.6 31.8 181 6 4.92 0.161 28.8 150 38.1 30.8 184 7 3.88 0.179 30.7 140 39.3 33.8 187 Table 16. Pore structrural data for electrode 10.

Mean pore structural parameters Feret diameters Electrode Tensile Fractional Pore size Wall size F F Pores/mm2 x strength volume (urn) (^m) (um max) (um* nun) (MPa) porosity en 1 4.84 0.158 29.2 156 32.9 26.9 198 O 2 4.25 0.162 28.9 150 33.2 26.5 206 3 3.78 0.186 32.3 142 34.2 28.5 204 4 3.35 0.180 30.1 137 32.9 26.7 222 5 2.92 0.193 32.5 136 33.6 28.0 214 6 2.52 0.185 31.3 138 32.2 26.6 225 7 2.31 0.194 31.9 132 32.8 27.0 224 Table 17. Electrode strength /pore structure equations.

Electrode Strength x pores/mm2 Coefficient of k T/I ratio (SxN) = determination

1 55.4 + 4804 W/P2 0.71 4804 5.0

2 143 +1082 W/P2 0.25 1082 3.1 3 -293+ 7530 W/P2 0.56 7530 2.3

10 -210 + 6221 W/P2 0.76 6221 3.1

1+3 + 10 -19.2 + 5515 W/P2 0.92 n/a n/a

JF*

■am- *

C200{1

Fig. 1. Low magnification view of an etched electrode surface. 54

IB P

20^1

-JET

*%' !J^ "■"•cy*^* «, B

20fl

F • r '. ■R

20|1

Fig. 2a-e. Interface types in petroleum-coke-based electrode carbon. 55

B ^\

20U.

Fig. 3. Continuous interface in electro-calcined-anthracite- based electrode carbon.

FRACTURE FEATURE CRACK IDENTIFIER THROUGH

Filler Particle FF

Binder BF

Interface jp 4. Between Binder and Filler IB "*

Highly Porous HPF Filler' Highly Porous HPB Binder

Pore PS

Interparticular IPF Fissure Devolatilisation DPF Pore Interlamellar ILF Fissure

Fig. 4. Representation of the formation of fracture surface features when a crack passes through an electrode.

]'^3}'%'^ 400 M-

W --vrc *"7?

* > i . *.-*

400 PL

Fig. 5. Complementary fracture surface of an ECA based electrode carbon showing interfacial failure at A and B, and transgranular fracture at C. 58

Fig. 6. Fractured binder phase in an anode carbon, small lamellar petroleum coke panicles being evident at L.

Mm ***•

1 1MB 200M-

Fig. 7. Binder phase in pore surface of anode carbon covered by thin film of pitch coke. 59

Pores

Devolatilisation pores

Interlamellar fissures

Gross fissures

Filler fracture

Binder fracture

Interfacial failure : binder

: filler

Highly porous areas

10 20 30 Percentage observations

Fig. 8. Typical point-counting results. 60

0 3-9 MPa 7 S 5-1 MPa D 5-3 MPa 50- / E3 7-1 MPa / \ 7 / • 40 \ c o / / \ * \ > / / \ 2o 30 \ o / / \ « \ / / (0 c \ a> \ / / S 20 \ \ / / \ \ / / \ \ 10 / / \ \ / / \ \ / / A nmn CO. Pores Binder and Interfacial Highly filler failure porous fracture areas

Fig. 9. Point-counting results for anode carbons. 61

Filler coke

A B C D

120 3

u ^ 100 -

u 80 - - |

BO 4 - c 3 - -

c u 2 - - H

70 - -

a a 60 - u "E 50 ou 3 1 c « 1500 - - e J 1400 - °» >; -* C - d 4J 1300 - - CQ "^

12 3 4 12 3 4 12 3 4 12 3 4 Binder pitch

Fig. 10. Variation of physical properties of Phase 1 electrodes with pitch type. 62

Filler coke

A B C D

0.30 o Q. "si 0.25 - c _g u 2 0.20 - 1 1

100 - e 80 - '3 B 60 - o 40 -

230

220 E 3. 210

- at 200

190 -

180 - -

100 - - - s E 80 - - - - o - - - - 60 -

1 2 3 < 12 3'* 12 3 4 12 3 4 Binder pitch

Fig. 11. Variation of pore structural parameters of Phase 1 electrodes with pitch type. 63

Binder pitch

1 2 3 4

120 — m tS S 0* | 100 - u u 80 - J_ "l

4 c 1 3 - £ 2

> 70 ■a E 2 . a 60 u a. •fi 50 o 5 1 c "E 1500 D. "3b Q. CO >; 1400 o CO CQ 1 1300 -

ABCD ABCD ABCD ABCD Filler coke

Fig. 12. Variation of physical properties of Phase 1 electrodes with filler type. 64

Binder pitch

1 2 3 4

0.30 ■ o o. 0.25 - - c o o 0.20 2 | 1

100 E if 80 -

60 o a. 40

230

220 E 210 -

200 -

190

180

100 "

80 - a.o 60 —

A I3C D ABCD AlBC D A EJ C D Filler coke

Fig. 13. Variation of pore structural parameters of Phase 1 electrodes with filler type. 5 i-

A =B v=C ♦ = D

c ♦ u

IA 1 H

J_ X 70 80 90 100 110 120 Interface quality index

Fig. 14. Variation of tensile strength with interface quality index for Phase 1 electrodes. 5 i-

4 - ft*

■w eOJB

"35 en s H 3 -

1340 1360 1380 1400 1420 1440 1460 1480 1500 Baked apparent density, kg/m3

Fig. 15. Variation of tensile strength with baked apparent density for Phase 1 electrodes. 80 r-

70 -

E t

1 60 u "3

50 -

40 1340 1360 1380 1400 1420 1440 1460 1480 1500

Baked apparent density, kg/m3 Fig. 16. Variation of electrical resistivity with baked apparent density for Phase 1 electrodes. 80 i-

70 - a

\* en "g 60 u "5u 00 8 5

40 2 3 4 Tensile strength, MPa Fig. 17.Relationship between electrical resistivity and tensile strength for Phase 1 electrodes. 69

Pitch content, wt%

18 20 22

2 80 - - 5* i 70 - i I - t: = l l u 60 1 i i 1 I ■ i 1 i i i i i 1 1 II

10 4 r - i i 3 i .■S I i 1 i 1 1 i 2 1 l £ 1 i 1 i 1 1 l 1

100

'5 c i a 80 1 i - a 1 3 =»■ I i i i i •c 60 1 i i i 1 i i 1 i • i 1 i i 1 i I 1 c 1500 - H G acj. i i o. 1400 i «j 1 1 1 i •a 1 1 i i I i u ■a 1300 - - C3 1 i i i l l C9 1 1 1 1 1 i ! 65 180 200 165 180 200 165 180 200 Mixing temperature, °C

Fig. 18. Variation of physical properties of Phase 2 electrodes with processing parameters. 70

Pitch content, wt%

18 20 22

0.35 o a "3 0.30 - - 1 e 1 _o i l 1 l i 3 i l 1 0.25 i . i i £2 i i I 1 i i 1 1 i I i i 1 I I

90 1 1 1 1 80 1 1 i 1 I 1 1 70 - 1 o 1 1 1 cu 1 i 1 60 i i 1 1 i i 1 1 1 1 | i 1 I I

220 1 e 200 l 3. 1 1 180 - 1 - l i 1 1 160 1 i 1 i i 1 I 1 i i 140 i l i i 1 i 1 I! 1 1 I i

120

5 100 - s GO 1 a 80 1 - o 1 l i cu 1 1 1 60 1 I l 1 1 1 1 165 180 200 165 180 200 165 180 200

Mixing temperature, °C

Fig. 19. Variation of pore structural parameters of Phase 2 electrodes with processing parameters. 5 i-

□ = Phase 2

4 - □

□

VB 3 w "55 E*3 ■ c H

-I I 1 1 1 1 1 L _L -I I i_ _L J u -J 1 L. 55 60 65 70 75 80 Interface quality index

Fig. 20. Variation of tensile strength with interface quality index for Phase 2 electrodes. □ = Phase 2

a a. □ □ 6*

(A □□ '35 e ro

2 -

1300 1350 1400 1450

Baked apparent density, kg/m3 Fig. 21.Variation of tensile strength with baked apparent density for Phase 2 electrodes 110 i-

o □ « Phase 2 100 %

E □ i 90 I« u GJ *£ 80 TT -nr J3- w □ a □ a □a 70

□

60 _L J 1300 1350 1400 1450

Baked apparent density, kg/m3 Fig. 22.Variation of electrical resistivity with baked apparent density for Phase 2 electrodes. 110 i-

Q = Phase 2 □ 100 t? □

£ 90 > a a "8

80 a a 1 -p. t3a □

a □ □ 70 - a a Q □ a

60 J 5 Tensile strength, MPa

Fig. 23. Plot of electrical resistivity against tensile strength for Phase 2 electrodes. 75

50 \. 40

i . i J 1 i i i ' i i i

at V) 20 V) a "w o> w 10 u 01 ■ ■ i ■ i ■ i ■ i ■ ■ ■

Fe 10 > u

J ' ' 200 300 400 500 600 700 800 900 1000

Heat treatment temperature, °C.

Fig. 24. Development of interface types with heat treatment temperature. 1600

1500 -

£ ~ofc £ 1400 *55 c

1100 ± ± J_ J 100 200 300 400 500 600 700 800 900 1000 Heat treatment temperature, °C Fig. 25. Variation of baked apparent density with heat treatment temperature. 77

in O

0 100 200 300 400 500 600 Temperature, °C

Fig. 26. Thermogravimetric analysis data for pitch 4. □ = Industrial A = Phase 2 v= A ♦ = B + = C ea x= D □ S x e X («

2i X e 09 H A A A ^ A % ♦ A A A^ A A& + * X

iL _L 50 60 70 80 90 100 110 120

Interface quality index Fig. 27.Variation of tensile strength with interface quality index for all electrodes studied. 400 D 380

360 Q 340 E E 320 □ a. A A a S 300 □ A A - A a a. A P-o 280 - u .2 260 - a E - A 3 240 _ A «3 e a = Phase 1 M) A

180 D %D

160 A 140 x x X _L X 02 .03 .04 -05 .06 .07 .08 09 10 11 Wall size / pore size1, urn"1 Fig. 28. Variation of tensile strength with pore structural parameters (pore and wall size and pores/mm2) for Phase 1 and Phase 2 electrodes. □

a A a A 0- a

A *P A c fc % u A a a en A A A o» AA 'S5 A A s A 00 a a a a = Phase 1 o H □ A = Phase 2

05 .06 07 .08 .09 . 10

-1/2 -II expl-2(Fm„/Fmil,rPUm Fig. 29.Variation of tensile strength with pore structural parameters (maximum and minimum Ferets diameters and volume porosity) for Phase 1 and Phase 2 electrodes. 81

Q Electrodes 1. 3. 10 1200-

• Electrode 2

2 800 - c/T o Ui tf O O. <*• o 600 - B u t> a x> £ 3 C 400 - X a or ■S CD C t> u 200 - C/5 -*■/♦ ♦

0.00 0.10 0.20 Wall size / pore size2, [lm'1

Fig. 30. Variation of tensile strength with pore structural parameters (pore and wall size and pores/mm2) for industrial electrodes. 110 r-

□ = A 100 A = B v= C ♦ = D 90 + = Phase 2 + + a + + 6 80 + + A

8 A L. "u3 70 □ 00 □ □ 60

40 _L 50 60 70 80 90 100 110 120 Interface quality index

Fig. 31.Variation of electrical resistivity with interface quality index for Phase 1 and Phase 2 electrodes. 83

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