X-Ray Diffraction

INTRODUCTION

CHAPTER 3 CHAPTER 3

3.2 X-Ray Diffraction

tannic acid mass fraction and tends to zero for mass fractions higher than 18%.

These materials, however, undergo plastic deformation and pore crushing as indi- cated by a large ireversible intrusion that occurs abruptly when the pressure reaches approximately 2100 atm.

3.1.5 Elemental analysis

Carbon and hydrogen contents and carbon-to-hydrogen ratios for the chars are given in Table IV. The remaining mass is mostly oxygen which, however, was not measured directly in this study. The amount of carbon in the copolymer formed from tannic acid decreases with increasing amount of tannic acid, since pure tannic acid chars are shown to have a much lower carbon content than the plain polymer.

The oxygen content of the heat-treated tannic acid appears to be very large ( ;::::;20%) as might be anticipated for an oxygen rich starting material (C76H52046 ). Upon partial combustion, of the materials most of the oxygen and hydrogen leave, and the carbon concentrations assume very high values. The nitrogen content in the particles was very low, below 0.25%. Sulfur was introduced in the particles by the polymerization catalyst (p-toluene sulfonic acid), but Leco analysis indicated that sulfur levels were quite low (0.08%).

kinetics of glassy carbons have been shown by Fischbach[20] to be rather slow, and a true graphitic structure is never reached even at heat treating temperatures in excess of 3000 K. The interlayer spacing

d

decreases from 4.0

A

at 1000 K to a limiting value of 3.43

A

at 3500 K. These spacings are significantly larger than the 3.354

A

spacing of the three-dimensionally ordered graphite. If graphitization involves joining small layer planes together to form larger ones and then rotating and translating these larger planes to appropriate stacking registry (alternate layers are laterally displaced) with neighboring planes and, at the same time, eliminating defects in the planes[20], then the long molecules and the high porosity of glassy carbons will limit the extend of crystallization since extensive reorganization will be required. TEM studies show crystallization of the materials at 3000 K in the form of randomly distorted turbostratic layers[l9].

The x-ray diffraction spectra of glassy and other disordered carbons typically contain {002} peaks that result from stacks of parallel layer planes, and (hk} peaks that result from the two-dimensional structure within the individual layer plane segments[20]. Three-dimensional peaks are rarely present, indicating minimal or nonexistent stacking order of parallel layers. Even after heat treatment for 2 s at temperatures as high as 1600 K the diffraction spectra for the materials produced in the present study show no signs of graphitization, as expected. At small angles there is a very strong scattering region, which is attributed to x-ray scattering by the extensive pore network[l8]. At larger angles there are three broad peaks corresponding to

d

002 ,

d

100 and

d

110 spacings. These peaks become slightly sharper and shift to higher angles with heat treatment temperature, as shown in Fig. 9.

Table V shows the average interlayer spacings and crystallite sizes, derived assuming disorganized graphitic structures[21]: basal plane dimension La, and height Le, normal to the {002} planes calculated using the Scherrer-type modified Warren equation[22] for line broadening at 3/ 4 of the peak height. The observed line breadth

was corrected for instrumental line broadening[21,23], although, for the very broad peaks in these materials, this correction is almost negligible.

The interlayer spacings decrease and the crystallite dimensions increase with heat treatment. The interlayer spacings (

d

002 ) are rather large when compared to to the average interlayer spacing of 3.364 Atypically observed for "turbostratic"

carbons, i.e, carbons with graphite layers randomly stacked. The apparent crystal- lite dimensions are also rather small in comparison to those of graphitizing carbons.

These low values, however, are to be expected, given the moderate heat treatment temperatures and short time durations of the present experiments. Moreover, the significance of La and Le is ambigous. In particular, the basal plane dimension La should be best interpreted as an indication of relative layer flatness and per- fection, not as real crystallite layer diameters[20]. Hence, layers that are bent or perforated, will give small La values even though the extent of the structure may be large. The crystallite dimensions of the present chars are consistent with with values reported for glassy carbons produced for a phenolic resin[19], indicating sim- ilar physical properties for glassy carbons made from different starting materials as postulated by Fitzer et al.[l]. Partial combustion in air at 1600 K for 2 s, promoted structural ordering as evidenced by the appearance of the {100} peak, shown in Fig. 9. Heat treatment at higher oxygen levels (30% 02 ) enhanced crystal forma- tion indicated by the presence of taller and narrower peaks. The particles did not ignite at a reactor temperature of 1600 K and oxygen levels below 30%, but rather burned at particle temperatures close to that of the furnace. The small tempera- ture rise was confirmed by optical pyrometry. The calculated temperature rise due to measured surface reaction rates is small (

<

15 K) for oxygen levels below 21 %.

Hence, the enhanced graphitization cannot be explained by temperature differences alone. Catalysis of graphitization by oxidizing gases has been previously reported by Noda and Inagaki[24]. At higher oxygen levels (21 - 30%), the temperature rise

may become more significant; it is estimated to be 40 K at 30% 02 . It was not posi- ble to measure such a small temperature rise accurately in the present experiments.

Hence, the roles of temperature and oxygen level on catalyzing graphitization can- not be separated in the higher oxygen level experiments. The powdery nature of the chars also helped promote graphitization[25] and allowed 0₂ to penetrate the interior of the particles effectively. The TEM micrographs of Fig. 6 support the argument of 02 induced graphitization. The partially combusted sample (6b) pos- sesses regions where crystallites are oriented parallel to each other (and parallel to the electron beam); meanwhile, the carbonized (in N₂ at the same temperature) sample (6a) appears completely disordered.

The effect of 0₂ on graphitization of the copolymer chars was variable. Some of the copolymers underwent substantial graphitization upon partial combustion in air at 1600 K for 2 s. For these chars, a sharp graphitic and/or turbostratic component appears superimposed on the broad

{002}

band of the disordered car- bon matrix as shown in Fig. 9. The onset of this limited (two or three phase) graphitization appears suddenly over a fairly narrow range of treatment tempera- tures. The narrow diffraction peaks indicate the appearance of large crystallites.

The particles formed from 17% tannic acid and 83% polyfurfuryl alcohol (PFA) possess a turbostratic

{002}

component (d-3.49A). The particles formed from 50%

tannic acid and PFA exhibited a graphitic

{002}

component (d=3.356A) and the particles formed from 18% PEG and PFA showed a turbostratic together with a faint graphitic component. For the last two materials, {hk} peaks appeared, indica- tive of two-dimensional ordering. Appearance of the

{103}

peak for the 18% PEG material signifies the onset of three-dimensional ordering.

The origin and relative amounts of graphitic phases in the copolymer chars is uncertain at present. The graphitic phases may originate from the pore former materials themselves as they decompose upon oxidative heating. It has been pro-

posed[26] that extensive graphitization of carbons occurs only if the carbonizing mass passes through a plastic state during which anisotropic droplets that contain highly oriented polynuclear molecules separarated from the fluid mass, known as

"mesophase," are formed. SEM observations suggest that some of the copolymer materials and, in particular, those containing tannic acid have undergone partial melting and resolidification during the course of the 1600 K oxidation. Tannie acid melts at temperatures around 800 K. Hence if the mesophase arises from the molten components, graphitic crystals might result even at 1600 K aided by 02 catalysis. A polished section of a 50% tannic acid particle is shown in Fig. lOa. The dark ridges might be due to a separated, decomposed phase of tannic acid. Figure lOb shows a 50% tannic acid particle after partial burning. Many submicron spherical parti- cles are embedded on the surface of the latter particle. Previous work on sucrose and rayon carbons suggests that such spheres might be the graphitizing crystal- lites[27]. Franklin[28] suggests that the formation of the minor graphitic phase is due to temperature-dependent internal stresses induced by differential thermal ex- pansion of the small anisotropic crystallites, which are strongly bound together by extensive cross-link bonding. At high temperatures these crystallites rupture their cross-linked bonds and rearrange into highly oriented graphitic aggregates under the compressive stresses imposed by the surrounding carbon matrix.

Particles formed from 25% carbon black spheres as filler in the PFA resin also develop of a second crystalline phase upon partial combustion, as indicated by faint {002} peak and a few "two-dimensional" peaks. Since the resin alone, carbonized under the same conditions, did not exhibit any crystalline peaks, it appears that the graphitization of the binder was induced by the filler. Previous investigators have reported that furfuryl acohol and other resins graphitized significantly when mixed with natural and artificial graphite and carbon black fillers[29,30]. Moreover, the graphitization of carbon fiber/ glassy carbon composites starts at the boundary

between the matrix and the fiber and proceeds into the matrix[31]. This was at- tributed to stress accumulation that was caused by a large volume shrinkage ( 4 7%) observed during carbonization.

The transitional pore network existing in this material could also have con- tributed to the formation of the crystalline phase by differential expansion of the isotropic matrix and the carbon around the pores. Kawamura and Tsuzuku[32] ob- served that high-porosity glassy carbons graphitize more readily than low-porosity ones. Kamiya and Suzuki[33] reported a preferential alignment of carbon layers around macropores in phenol formaldehyde resin char that graphitized upon treat- ment to high temperatures (2700 K).

3.2.1 X-Ray Density

The density of the materials is estimated from x-ray diffraction data[34]:

7.627

d(oo2)

g/cm³,

(4)

where Z=4 denotes the number of carbon atoms in a unit cell, A=l2 is the atomic weight of carbon, mH = 1.66 x 10-²⁴ is the mass of a hydrogen atom, ac=2.456 is the lattice constant of graphite. X-ray densities for partially burned materials are tabulated in Table V. Since some of the copolymer chars exhibit multiphase graphitization, two density values have been reported, one corresponding to the disordered carbon matrix and one corresponding to the crystalline phase. If more than one crystalline phase is present, the density corresponding to the strongest peak is tabulated. The density of the unoxidized plain polymer sample is 1. 7 g/ cm³ and the density of the disordered component of the partially oxidized samples varies between 1.91to1.96 g/cm³. The density of the crystalline component varies between 2.1 to to 2.27 g/cm³, the last value being the density of graphite. Since the relative amounts of the different phases present are not known, an average value cannot be

deduced. However, it is expected that the average value will be very close to the density of the dominant disordered phase.