Rheological properties, structural and thermal elucidation of coal-tar pitches used in the fabrication of multi-directional carbon-carbon composites

Journal Pre-proof Rheological properties, structural and thermal elucidation of coal-tar pitches used in the fabrication of multi-directional carbon-carbon composites

Nisar Ali, Hira Zaman, Wajed Zaman, Muhammad Bila PII:

S0254-0584(19)31374-4

DOI:

https://doi.org/10.1016/j.matchemphys.2019.122564

Reference:

MAC 122564

To appear in:

Materials Chemistry and Physics

Received Date:

26 September 2019

Accepted Date:

15 December 2019

Please cite this article as: Nisar Ali, Hira Zaman, Wajed Zaman, Muhammad Bila, Rheological properties, structural and thermal elucidation of coal-tar pitches used in the fabrication of multidirectional carbon-carbon composites, Materials Chemistry and Physics (2019), https://doi.org/10. 1016/j.matchemphys.2019.122564

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Rheological properties, structural and thermal elucidation of coal-tar pitches used in the fabrication of multi-directional carbon-carbon composites Nisar Ali*1,2, Hira Zaman3, Wajed Zaman4, and Muhammad Bilal*5 1Department

for Management of Science and Technology Development, Ton Duc Thang

University, Ho Chi Minh City, Vietnam, 2Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam; 3Institute of Chemical Sciences, University of Peshawar, Khyber Pakhtunkhwa, Pakistan; 4School of Material Science & Engineering, Northwestern Polytechnical University, Xi’an 710072, China; 5School of Life Science and Food

Engineering,

Huaiyin

Institute

of

Technology,

Huaian

223003,

China;

*Corresponding author E-mails: [email protected] (N. Ali); [email protected] (M. Bilal). Abstract Two types of coal-tar pitches varying by softening points and quinolone insoluble (QI) contents, used in the fabrication of carbon-carbon composites (C/C’s), were studied using various techniques regarding their physical and chemical transformations during pyrolysis and graphitization. These pitches were highly graphitizable and were found the best suited for fabricating high-density multi-directional C/C’s. Raman micro-spectroscopy (RMS) of the pitches showed a gradual decrease in the width and frequency of the G band as pyrolysis and graphitization proceeded, corresponding to the decrease of nonaromatic C–C bonds and increase in the ordered layered graphite network. The pitches showed non-Newtonian behavior because of the presence of QI particles, and their viscosity decreased with increasing shear rate and temperature. The decrease in viscosity with increasing shear rates was found constant at all measured temperatures. RMS and elemental analysis showed that structural transformation in the pitch during pyrolysis took place most noticeably beyond 800 oC, resulting in an abrupt increase in the carbon/hydrogen ratio, due to the vanishing of mesophase structure and extension of the distorted carbon network. Sigmoidal curve fitting using the Boltzmann equation was found the best fit for the weight loss of these pitches during pyrolysis. Mesophase formation was found to take place at 425 oC in both pitches and its contents increased with increasing 1

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the retention time. The mesophase contents in the pitches were directly related to their thermal stability, coke yield, and ease of graphitization. Taken together, these coal-tar pitches seem to be very promising carbon matrix precursor to produce high density and easily graphitized multidirectional C/C’s for aerospace applications. Keywords:

Carbon-carbon

composite;

Heat

treatment;

Graphitization;

Raman

spectroscopy; Rheological and thermal properties

1. Introduction Carbon-carbon composites (C/C’s) endure high thermal gradients and complex mechanical stresses and are well known for their excellent thermo-physical and thermomechanical properties in the field of aerospace and aeronautics [1-2]. The brake discs of almost all passenger and military aircraft are also made up of C/C’s. 4D C/C’s possess quasi-isotropic properties, better ablation resistance and endure extreme thermal shocks and are therefore regarded as a suitable material to make nozzles of rockets, especially solid rocket motor (SRM), reentry tips and leading edges of reentry vehicles and space shuttles [3-5]. The use of C/C’s in a wider range of other civil applications has been limited by their manufacturing cost. Efforts are being made to fabricate C/C’s in a cheaper way by finding the effective process and efficient matrix precursors to reduce the densification cycles. Isotropic pitches have considerable potential as inexpensive materials for the economical production of high-density multi-directional C/C’s [6-7]. Recently, coal-tar pitches have been extensively used as carbon matrix precursors for the fabrication of 1-dimensional to 3-dimensional C/C’s because coal-tar pitches as impregnation agents are an economic and simple procedure to effectively densify complex and large preforms [8-13]. Coal-tar pitches show promise as carbon matrix precursors because of their low price, high carbon yield and excellent graphitizability. The density of the resulting C/C’s more than 1.9 g cm-3 can be achieved using coal-tar pitches, which is required for better mechanical and ablation properties for aerospace applications [6]. The conversion of the isotropic pitch to anisotropic carbon matrix requires pyrolysis. In contrast to petroleum pitches, coal-tar pitches have unique characteristics of possessing small particles. These particles are not soluble even in a strong solvent like quinolone and are therefore termed as quinolone insoluble (QI). The carbonization behavior of the pitch is influenced by these particulate matters [14]. Elemental analyses 2

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have shown that the QI material has high carbon and oxygen contents and low hydrogen contents that support the presence of highly condensed aromatics and/or highly polar compounds with oxygen functionalities [15-17]. The pitch carbon yield is directly proportional to its primary QI contents. The presence of QI particles produce C/C’s with a high percentage of carbon matrix, fewer micro-cracks, and less closed porosity and hence increases the anti-ablation performance and mechanical properties of the composite [18-19]. The concentration of QI contents in a coal-tar pitch influence the development of a liquid crystalline polyaromatic structure called mesophase during pyrolysis, which solidifies the anisotropic solid material in the carbonization process. Mesophase formation is associated with the volatilization of low molecular mass constituents, polymerization, and condensation reactions of the hydrocarbons and the thermal cracking of aliphatic side groups of aromatic molecules to form free radicals. These aromatic free radicals react together to form a polyaromatic carbonaceous mesophase. Figueiras et al. [18] have demonstrated that mesophase has a significant effect on the structure and properties of unidirectional C/C’s. The impregnation efficiency or weight gain in each densification cycle of C/C not only depends on the experimental conditions but also on the type and properties of the impregnates like their chemical composition, QI content, softening point, fluidity, viscosity, etc. [20]. Aggarwall et al. [21] have reported the acceptable range of primary QI of the precursor pitch as 12%–15% for producing C/C’s with a density of 1.8 g cm-3. The rheological properties of the pitches have revealed that the viscosity of pitches depends on their chemical composition, size, shape, and distribution of the dispersed particles and temperature [22]. Many authors have studied the physicochemical properties of the pitches at different stages during their carbonization to better understand the fabrication of C/C’s. Herein, we studied two coal-tar pitches regarding their rheological properties, mesophase formation, coke yield and chemical changes as a function of temperature during pyrolysis to evaluate the optimum parameters for the efficient impregnation and densification of four-directional C/C’s using these pitches. The low melting points, high carbon yield and low viscosities of these pitches make them ideal matrix precursors for the fabrication of multidirectional C/C’s to the density of more than 1.9 g cm-3. The aim of the work described in this paper 3

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is to understand the initial changes in the rheological properties of coal-tar pitches with temperature and their structural transformation during pyrolysis and graphitization, using a variety of techniques. Since, to the best of our information, there is no direct study about the influence of mesophase contents of the pitches on the coke yield and degree of graphitization, in reference to multi-directional C/C composites. This paper reports the effect of retention time during the pyrolysis of coal-tar pitches at 425 oC on the mesophase formation and the effect of mesophase content on the carbonization rate, coke yield, and ease of graphitization of these coal-tar pitches. Moreover, most of the studies regarding the coal tar pitches have either been carried out to be used as a binder, for the purpose without their link to the c/c composite fabrication or their use in connection with unidirectional or 2D C/C composites. This report highlights, for the first time, the different aspects of coal-tar pitches that were successfully used in the fabrication of 4D C/C composites that are widely used in aerospace applications, especially in the rocket nozzle throat, nose tips and leading edges of missiles and space shuttles. The findings of this research work are very helpful to set the optimum process parameters for the efficient densification of 4D C/C’s. The four-directional C/C’s was fabricated with intermediate modulus carbon fibers and these two coal-tar pitches with a density of 1.92 gcm-3, using high-pressure impregnation carbonization (HIPIC). These coal-tar pitches seem to be very promising carbon matrix precursor to produce high density and easily graphitized multidirectional C/C’s. 2. Experimental Two coal tar pitches i.e. low density (LD) and high density (HD) pitch were supplied by the Institute of coal chemistry, Chinese Academy of Sciences, Xi’an, China. The softening point, density, and solubility of the pitches are shown in Table 1. The elemental compositions of the pitches were measured at different stages of pyrolysis using PerkinElmer 2400 Series II CHNS/O elemental analyzer system. 2.1 Rheological analyses Rheological properties of the pitches were studied using a rotational Brookfield DV-I digital viscometer and dilatometer (DIL-402C). The pitch samples were taken in a 500mL beaker and the viscosity of the pitch samples at different temperatures and different shear rates corresponding to each temperature was measured using spindle RV5, following 4

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ASTMD5018. The shear rate (  ) and viscosity (  ) are calculated from the angular velocity (  ) of the rotor and steady-state torque (  ), taking into account the ratio of the rotor-tocup radii ( k ) and rotor length ( L ), using the following equations (Equation 1 and 2);  

 1 k

(Eq. 1)

 k  2 R  k 2 L  k 3 R / 3) 2

(Eq. 2)

The softening point of the pitches was measured using the dilatometer (DIL-402C) by scanning the pitch samples of dimensions 10 × 4 × 4 mm3 at 1 °C/min. The softening point was determined by the intersection of the tangents to the curves obtained by plotting the change in length of the material as a function of temperature. 2.2 Pyrolysis and mesophase formation Pyrolysis of the pitches (around 5 g) at different temperatures and at the different retention time was carried out in a digitally controlled horizontal tube furnace at 5 °C/min under flowing argon (Fig. 1) to follow the similar procedure adopted during the production of C/C composites at industrial scale. For mesophase study in the pitches, the samples were heat-treated from ambient to 425 °C and held at 425 °C for 1, 6 and 10 h. These heattreated pitches with different retention times were further characterized using polarized light optical microscopy (PLM), thermogravimetric analysis (TGA), Raman microspectroscopy (RMS), X-rays diffraction (XRD) and elemental analysis. To study the mesophase formation and final morphologies of the pitches as a matrix for C/C’s, the heat-treated pitch samples were embedded in the epoxy resin and were ground and polished using various grades of silica and diamond paste following the procedure in reference [6] and were observed using PLM at a different resolution. 2.3 Thermal analyses Thermogravimetric and differential thermal analyses (TGA-DTA) of the pitch samples and heat-treated pitches were carried out on an STA409 Netzsch analyzer under nonisothermal conditions at a heating rate of 5 °C/min from 50-900 °C, using about 12 mg of the sample under 250 mL/min flow of argon. 2.4 Structural analyses

5

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Raman microspectroscopy is a non-destructive technique that has been widely applied to the study of pitches and other carbon materials to get information about the chemical bonding, phase composition, and crystalline state [23-26]. This technique is based on the analysis of the spectral characteristics of vibrational modes associated with the Raman peaks. RMS analyses were carried out on both pitches before and after heat treatment and pyrolysis. All the spectra were recorded in the absorption range of 400-2000 cm-1 using a 514 nm incident laser beam of 5 mW power with a 200 magnification. The same polished samples prepared for PLM study were used for RMS analysis.The XRD measurements were performed with an X’Pert PRO PANalytical diffractometer (λ(CuKα1)) to measure the graphitizability and the influence of earlier heat treatment (mesophase formation) during pyrolysis of the pitches on the degree of graphitization in 1600 and 2100 °C. All the spectra were recorded by making pellets from the powdered samples of the original and pyrolyzed pitches and scanned in the 2θ range of 20° to 90°. 2.5 Fabrication of multi-directional C/C composites Four directional (4D) preform was prepared from T800 carbon fiber, where three directions were on the plane (i.e. 0, +60, -60) and the fourth one perpendicular to the plane in the z-direction. The 4D-preforms with dimensions 150 x 300 were densified using these two coal-tar pitches using the HIPIC process at 75 MPa. After densification, the C/C’s was subjected to high-temperature treatment at 2300 °C for graphitization. The detailed process of fabrication has been given in earlier reports [6, 27]. MRS, XRD and microscopic examination of the C/C’s matrix were carried out after graphitization to study morphology and structural changes in these pitches. 3. Results and discussion It is observed from Table 1 that because of high QI and toluene insoluble characteristics of pitch-HD, its density, melting point, and softening point are higher than the pitch-LD, which corresponds to the presence of a higher concentration of aromatic compounds in it. Because the QI is associated with the low H and high C and O contents (i.e. high aromatic characteristics and high polar compound with oxygen functionalities), the extent of aromatic characteristics and polarity in QI is directly related to its influence on the overall physical properties of the coal-tar pitch. The RMS analyses show the presence of 6

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the G band at around 1595 cm-1 and a broad complex band at around 1350 cm-1, which represents a wavenumber for the D band, as shown in Fig. 2 (a and b). These bands are associated with the different vibration modes of C–C bonds in non-aromatic, aryl–aryl and polyaromatic molecules and are the characteristic features of mesophase pitches. The bands at nearly 1350 cm-1 can be associated with the Ag vibrations of the benzene skeleton of the molecules present in the coal tar pitch. The D band, as shown in Fig. 2, consists of many components that correspond to the different vibrations of the distorted carbon network. Both spectra corresponding to pitch-HD and LD have an intense fluorescence background that designates the high concentration of hydrogen contents in these pitches [28]. Thermal analysis (TGA-DSC) of these pitches show that the thermal stability of pitch-HD at any temperature during pyrolysis is higher than the pitch-LD (Fig. 3), and the coke yield of pitch-LD and pitch-HD at 900 °C is about 32% and 62%, respectively. These results reveal that the coke yield of pitch HD is 30% higher than the pitch-LD, which is probably because of the higher QI contents and aromatic composition of pitch-HD. For both pitches, most of the gaseous products of pyrolysis release in a narrow temperature range (i.e., around ∆ 250 °Cat 5 °C min-1), that is, for pitch-LD, this range is from 250 to 510 °C and it is about 280 to 520 °C for pitch HD. The derivative of the weight loss with respect to temperature (DTG) reveals that the initial weight loss of pitch-LD and pitch-HD starts at 160 and 250 °C and their maximum weight loss takes place at 270 and 370 °C, respectively. Sigmoidal curve fitting using the following Boltzmann equation (equation 3) was found the best fit for the pyrolysis of both pitch-LD and pitch-HD at 5 °C/min, as shown with the dash lines in Fig. 3. y

( A1  A2)  A2 [1  exp( x  xo ) / dx)]

(Eq. 3)

Where, A1 and A2 is the initial and final y value (left and right horizontal asymptote) respectively, xo is the center (point of inflection), dx is the width (the change in x corresponding to the most significant change in y values) The y value at xo is the halfway between the two limiting values A1 and A2 , that is, y ( xo ) 

7

( A1  A2) . 2

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It shows that the carbonization of coal-tar pitch is a single step degradation process, which initially started with low rates and then increased with the temperature, following “S-curve” with plateau top and bottom of the specified temperature range. The DSC curves of the pitches in Fig.4 display that the pyrolysis of coal tar pitches is an endothermic process and the initial process of carbonization starts at around 320°C in both pitches. The elaboration of the DSC curve during pitch pyrolysis is very complicated because of the involvement of many kinds of reactions, for example, the escape of volatiles, dehydrogenation, alkylation, condensation, thermal cracking, polymerization, etc., taking place at the same time. The main difference in the DSC curves of pitch-A and pitch-B lies at a temperature higher than 600 °C. After 600 °C, the rate of net endothermic reactions of pitch-HD is lower than the pitch-LD, depicting the earlier completion of the coke formation in pitch-HD. The reason might be the presence of more primary QI and aromatic contents in pitch-HD than pitch-LD which assist coke formation. 3.1 Study of the rheological properties of the pitches The viscosity of both pitches was measured at three different temperatures above their melting points as a function of shear rate corresponding to each temperature. The results presented in Fig. 5A and B reflect that the viscosity of pitch-B at any given temperature is higher than the pitch-A. The reason may be the presence of a higher concentration of molecular species (and polarized molecules) especially QI and TI particles in pitch-B than pitch-A. The viscosity of both pitches decreases with increasing temperature and shear rates. The Arrhenius equation (equation 4) was used to study the exponential decrease in the fluidity of these pitches with respect to temperature.   A1 exp

A2 T

(Eq. 4)

Where  is the viscosity of the pitch at any temperature T, A1 and A2 are the coefficients. The exponential decrease in viscosity of these pitches has been plotted for comparison in Fig. 6 at three different shear rates, which follows Arrhenius straight line equation. The coefficients of Arrhenius equation i.e., A1 and A2 corresponding to pitch-LD and pitch-HD at the different shear rates were calculated respectively from the slope and intercept of the straight lines, as summarized in Table 2. The decrease in viscosity in pitch-LD and pitch-HD at all measured temperatures (i.e., at 120, 140 and 160 °C in case of pitch-LD 8

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and at 160, 180 and 200 °C in case of pitch-HD) with increasing shear rates from 5 rpm to 20 rpm is almost constant at 40.2±3.6% and 45.0±4.7%, respectively. This big change in viscosity with a shear rate in both pitches can be attributed to the presence of 7%–12% QI particles in these pitches. The effect of shear rate on the viscosity of pitch-HD is slightly higher than pitch-LD that corresponds to the presence of a high concentration of QI particles like any other kinds of particles, dispersed in pitch-HD than pitch-LD. Similar behavior has been reported by Nutz et al. [29] by increasing the concentration of graphite particles in pitches. The decrease in viscosity of pitch-LD and pitch-HD with increasing shear rates from 10 to 20 rpm at any measured temperature is almost constant at 14.6±1.9% and 16.3±3.9%, respectively. Initially, at a lower value of shear rates i.e. from 5 to 10 rpm, the decrease in viscosity with increasing shear rate is much higher than at higher shear rates i.e., from 10 to 20 rpm, at all measured temperatures. Fig.7A and B represent the change in length of the coal-tar pitch-LD and HD, respectively, as a function of temperature. The softening point of the pitch was calculated from the tangent of the curve at the point where the negative thermal expansion in the material starts. The softening points of pitch-LD and HD were found as 39.1 and 91.9 °C, respectively. This considerable difference in their softening points indicates the higher aromatic character in pitch-HD than pitch-LD. 3.2 Influence of mesophase contents in coal-tar pitches In isotropic coal-tar pitches, upon heating, an anisotropic liquid crystalline phase called mesophase is formed upon heating between 300-500 °C, which can be observed as bright regions in PLM micrographs. Fig 8a-10f depicts the influence of holding time (retention time) of pitch-A from 1 to 10 h at 425 °C, on mesophase formation. By pyrolyzing the pitch-LD at 425 °C for 1 h, small anisotropic crystals appeared. The size and concentration of anisotropic spherules increase with increasing the retention time at 425 °C. After 10 h heat treatment, the PLM micrograph of pitch-LD shows the existence of the high concentration of anisotropic domains uniformly distributed into the isotropic phase. Similar behavior is portrayed with pitch-HD under the same heat treatment as pitch-HD. The only difference is the presence of a greater concentration of anisotropic texture in pitch-HD than pitch LD below 10 h of retention time. The reason might be the presence of higher viscosity due to the higher amount of primary aromatic compounds, 9

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especially BI contents in pitch-HD than pitch-LD, which facilitates mesophase formation. The anisotropic character (mesophase concentration) of both pitches after 10 h of retention time is almost the same as evident in Fig. 8c and 8f. However, the anisotropic regions in the LD-pitch consist of comparatively bigger spheres than HD-pitch. The reason behind is the difference between these two pitches in their primary QI contents. Because primary QI contents have been reported to attach to the surface of mesophase spheres and restrict their coalescence [18]. The coal-tar pitch with less amount of QI content, therefore, results in coalesced mesophase, as shown in the optical micrograph of LD-pitch (Fig. 8c). On the basis of these results, during the production of multidirectional aerospace-grade C/C composite, the green composite after impregnation by these coal tar pitches was treated at 425 °C for 10 h in the carbonization furnace during the process of carbonization for maximum carbonization efficiency. The process of fabrication of multi-directional C/C composite has been addressed in more detail in our earlier reports [6,27]. The RMS analyses of the heat-treated pitches are the same as untreated pitches showing D and G bands. Fig. 2 depicts that the Raman spectra are apparently not related to the anisotropic character of the pitches and there is no marked difference in the Raman spectra of both pitches after heat treatment for different retention time. It means that the RMS analysis is not able to address the mesophase formation in coal tar pitches. The XRD data of the pitches show a sharp peak (at around 2θ=26) with their intensity depending upon the temperature. Table 3 illustrates the effect of mesophase contents (retention time at 425 °C) on the degree of graphitization of pitch-LD and pitch-HD at 1600 °C and 2100 °C. The XRD data shows that the mesophase contents increase the ease of graphitization in both pitches and the degree of graphitization increases with increasing mesophase contents. At high temperature, i.e., 2100 °C, the influence of mesophase in the degree of graphitization is however not very significant. Fig. 9A and B show the thermogravimetric analysis results of pitch-LD and pitch-HD pyrolyzed at 425 °C for different holding time. Since the holding time at 425 °C is directly proportional to the concentration of mesophase in these pitches. Therefore, we can figure out the influence of mesophase concentration on the rate of weight loss and coke yield of pitch-LD and pitch-HD, using these thermograms. Results showed that the temperature 10

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of initial weight loss of these untreated pitches and semi-coke is independent of the retention time. In both pitches, the coke yield increases with increases the retention time. There is however no marked influence on coke yield by increasing the retention time of these pitches from 0.1 to 1 h. This can be attributed to the small difference in concentration of mesophase in pitches between these two retention times, as supported by PLM micrographs in Fig.10. The coke yield at 900 °C of the heat-treated pitch-LD is higher by 38.7% to 50% with the increase in retention time to 6 h and 10 h, respectively as compared to untreated pitch-LD. Similar behavior is shown by pitch-HD where the increase in coke yield at 900 °C is 12.87% and 21.5% for holding the sample at 6 and 10 h, respectively, as compared to an untreated pitch-HD. Fig.13 elucidates the percentage amount of coke yield at 900 °C as a function of holding time of pitch-LD and pitch-HD at 425 °C, as derived from thermogravimetric analysis results in Fig. 9A and B. The coke yield of untreated pitch-HD is higher than the pitch-LD by 30%. It can be attributed to the increase in the concentration of high molecular weight compounds and high QI and BI contents in pitch-HD than pitch-LD. After heat treatment, the coke yield of pitch-LD increases more significantly than pitch-HD. The difference in the coke yield between heattreated pitch-LD and pitch-HD with 6 h and 10 h holds/relaxation time is only around 4% and 1.5% respectively, as shown in Fig. 10. Under PLM images, it is clear that there is no big difference in the anisotropic contents, that is, mesophase contents, in both pitches after 6 and 10 h of relaxation time at 425 oC. There is no noticeable change in the Raman spectra of these two pitches having a variable concentration of mesophase contents, as shown in Fig.11, which designates that the RMS analysis is not able to address the presence or change in mesophase contents in coal tar pitches. 3.3 Carbonization and graphitization of pitches as a matrix of C/C’s RMS analyses of the C/C composites fabricated with pitch-LD and pitch-HD were analyzed at different stages of fabrication. It is found that there is no marked change in the characteristic RMS spectra of pitches pyrolyzed at different temperatures from 400800 °C (Fig.12). The G-band has two components, that is, at 1582 cm-1 and 1602 cm-1. All the spectra from 400-800 °C have an intense fluorescence background, which designates the presence of a large amount of hydrogen in the specimens. The fluorescence background of the Raman spectra of pyrolyzed pitch becomes less intense 11

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beyond 800 °C, indicate the extension of graphene layers and aromatization that results in the decrease of hydrogen contents in the resulting cake. There is a significant change in the Raman spectra after pyrolyzing Pitch-HD at 1600 °C, which shows three prominent Raman peaks at 790, 1362 and 1595 cm-1, which represent the formation of a distorted graphite network (Fig. 13). The 2D band, which is sometimes also referred to as G’-band and represents the graphene layer thickness in graphitic material, appears at around 2710 cm-1 at 2100 °C as shown in Fig 13. 2D band is sharp and narrow and appears at lower frequency (around 2650 cm-1) for a single graphene layer, but in our case, the 2D peak is wide and appears at a relatively higher frequency, which represents the formation of multi-layer graphitic material or polycrystalline graphite. The relative intensity of the two bands i.e. (ID/IG) varies from 0.42 to 1.58 at different stages of coke formation and graphitization, as shown in Fig.14. The (ID/IG) first increases with increasing temperature up to around 700 °C, then decreases sharply beyond 800 °C and then increases during the post carbonization process and reaches to 1.58 at 1600 °C. This is because of the structural transformation that takes place at around and beyond 800 °C, associated with the elimination of a large amount of hydrogen and the disappearance of mesophase structure during early carbonization and the formation of more distorted carbon network in the post-carbonization process. During the graphitization process at 2100 °C, the ID/IGis found to decrease due to the formation of microcrystalline graphite, which is associated with a sharp decrease in the width of the G band. The post carbonization and graphitization steps are also associated with the shifting of the G band towards lower frequency, which represents the gradual increase in the ordered layer structure of graphite network. The XRD data of the pitches show that both coal tar pitches are graphitizable and the degree of graphitization increases with increasing the heat treatment temperature. Fig.15A and B show that in both cases, the intensity of the 2θ peak increases and their width decreases with increasing the heat treatment temperature. The intensity of the peaks that appears between 40o and 85o was weak, therefore it has been amplified separately in Fig.15. The carbonization of both pitches at 1000 oC shows a broad (002) reflection at around 25.6o and broad much weaker reflection of (100) and (004). At 1600 oC,

prominent broad bands appear at around 43.0o and 53.7 o corresponding to (101) and 12

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(004). There is also a very weak broad reflection of (110) appears at 1600 oC. But as the temperature becomes high to 2100 oC, the pitch becomes graphitized and not only the (002) peak become sharper and strong but also the (100), (101), (004) and (110) peaks become prominent and sharper, which confirms that a 3D-graphitic structure has formed. The d-spacing in the pitches decreases with the increase in temperature and earlier retention time at 425 oC. The d-spacing, which is related to the alignment of graphene layers and degree of graphitization (g), by equation 5, decreases with increasing the temperature as summarized in Table 3.

g=

3.440[Å ]-d 002 [Å ] 3.440[Å ]-3.354[Å ]

(Eq. 5)

It is interesting to see that the values of d-spacing at 1600 and 2100oC decreases for both pitches as the retention time during the earlier heat treatment at 425 oC increases. Since the retention time is linearly related to the mesophase concentration, therefore the results show the mesophase contents during the early stages of coal-tar pitches have a positive effect on their degree or ease of graphitization during high-temperature treatments. The elemental analysis of the pyrolyzed pitches at a different temperature, as shown in Fig.16, reveals that carbon to hydrogen ratio (C/H) increases with increases temperature which corresponds to the conversion of aliphatic molecules to polycyclic and aromatic compounds. The C/H ratio is often regarded as the condensed state of the pitch molecules. The graph shows a very small change in the C/H ratio of up to 800 oC. However, there is a significant increase in the C/H ratio after 800 oC, which implies the extension of the graphene layer, aromatization and the formation of a distorted carbon network of the coke. The same result is supported by comparing the Raman spectra of the pyrolyzed pitches from 400-1000 oC during RMS analysis, as discussed earlier. The PLM micrographs of pitch-LD and pitch-HD as a matrix in C/C composites after heat treatment at 2300 oC are shown in Fig. 17. The matrix of both pitches shows optical activity. The pitch-LD matrix exhibits optically active large anisotropic domains while the pitch-HD shows flow type morphology because of the difference in their QI and BI contents. The mechanical and thermo-mechanical properties of the as-prepared 4D C/C composite are shown in Table 4. The ablation performance, and thermo-oxidative 13

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behavior and thermo-mechanical properties of the 4D C/C composites, which are widely used in aerospace applications, densified with these two pitches have been reported in our previous published manuscripts [6,27,30] and the static and dynamic mechanical behavior of the as-prepared 4D C/C composites will be addressed in our next report. 4. Conclusions Raman spectra of coal tar pitches show two bands i.e. D and G at 1350 cm-1 and 1590 cm-1, respectively, corresponding to the C-C vibrations in alkyl-alkyl and aryl-aryl molecules. The Raman spectra do not address the anisotropy or mesophase formation in these pitches, which is formed at around 425 oC, however, the relative intensity (ID/IG) significantly increases due to the formation of the distorted carbon network during the post carbonization process and then decreases due to the formation of microcrystalline graphite during graphitization processes. There is a sharp decrease in the width and frequency of the G band as the graphitization process proceeds, which represent the gradual increase in the ordered layered structure of graphite network. Anisotropic characteristics or liquid crystalline mesophase contents of the pitches increase with increasing the retention time during heat treatment at 425 oC. The coal tar pitches exhibit non-Newtonian behavior and their viscosity decreases with increasing temperature and shear rate due to the presence of QI particles in these pitches. The viscosity of the pitch with a higher concentration of BI and QI is higher at all measured temperatures. Thermal stability and coke yield of the heat-treated pitches depends on the retention time at 425 oC,

which corresponds to the concentration of mesophase in these pitches. Raman

spectra showed a decrease in the intense fluorescence background and the G band shift towards the lower wavenumber with pyrolysis most obviously starts shifting at around 800oC, and also the elemental analyses show a significant increase in the C/H ratio beyond 800 oC. These Raman and elemental analyses imply to the major structural changes corresponding to the elimination in the hydrogen contents, the disappearance of mesophase structure, an extension of the distorted carbon network and formation of nano-crystalline graphite. The XRD studies show that the mesophase contents increase the ease or degree of graphitization of these pitches. These coal tar pitches are found suitable to fabricate multidirectional C/C composite of density as high as 1.92 g cm-3 for aerospace applications. The retention time of 6-10 h at 425 oC during the carbonization 14

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process increases the carbon yield and thus the C/C’s production efficiency as a whole. These pitches exhibit highly anisotropic characteristics under PLM, however, because of the different QI and BI insoluble contents; pitch-LD matrix exhibits optically active large anisotropic domains while the pitch-HD has flow type morphology. Acknowledgment All listed authors are grateful to their representative departments and universities for the financial support and analytical services used in this study. Conflict of interest The authors declare that they have no conflict of interest. References [1]

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List of Figures Fig. 1. Schematic of the horizontal tube furnace used for the heat treatment of pitches under N2 flow. Fig. 2. Raman spectra of as-received coal tar pitch-LD and pitch-HD showing D and G bands. Fig. 3. Thermal analysis showing the weight loss during the pyrolysis of pitch-LD and pitch-HD at 5 oC/min, with Sigmoidal Boltzmann curve fitting. Fig. 4. Comparison of the DSC curves during the pyrolysis of pitch-LD and HD at 5 °C /min Fig. 5. Viscosity of (A) pitch-LD and (B) pitch-HD as a function of temperature and shear rate. Fig. 6.Linear Arrhenius plots of pitch-LD and pitch-HD at different shear rates. Fig. 7. Change in length of (A) Pitch-LD and (B) Pitch-HD as a function of temperature, to find its softening point. Fig. 8. The PLM images showing the influence of retention time on the mesophase formation at 420 oC in (a-c) pitch-LD and (d-f) pitch-HD Fig. 9. Thermogravimetric analysis of the heat-treated A) HD-type and B) LD-type pitches for different retention time. Fig. 10. The influence of retention time on the coke yield of heat-treated pitches at 425 oC.

Fig. 11. Raman spectra of coal tar pitch after heat treatment at 425 oC for different retention times. Fig. 12.Influence of temperature on the Raman spectra of pyrolyzed pitch-HD as a matrix in C/C’s. Fig. 13. Influence of high temperature on the Raman spectra of LD and HD types pitches used as a matrix of C/C’s. Fig. 14. Influence of carbonization and graphitization on the Raman spectra of pitch-HD used as a matrix in C/C’s. Fig. 15. Comparison of X-ray diffractograms of carbonized and graphitized pitch-HD and LD at different temperatures. 19

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Fig. 16. Effect of heat treatment temperature on the carbon to hydrogen ratio of pitch-LD and pitch-HD used as a matrix for C/C’s. Fig. 17. PLM micrographs of pitch matrix used in the fabrication of 4-directional C/C composites after graphitization temperature treatment at 2300 oC.

20

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Sample chamber

Furnace chamber

N2 Inlet

N2 out let

Sample

Controller

Fig. 1.

21

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2500

G band D band

2000

Counts

Pitch HD

1500 1000

Pitch LD

500 600

800

1000

1200

1400

1600

Wavenumber (cm-1)

Fig. 2.

22

1800

2000

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Pitch LD Pitch HD Sigmodial Boltzmann curve fit

120

100 90

0.0 DTG

TGA

-0.1

80 70

-0.2

60 50

-0.3

40 30 100

200

300

400

500

600 o

Temperature ( C) Fig. 3.

23

700

800

900

-0.4

DTG (% oC-1)

Residual weight (%)

110

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-------- (LD-Pitch) ----- (HD-Pitch)

Fig.4.

24

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(LD-Pitch)

(HD-Pitch)

Fig. 5.

25

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9.6

-1

Pitch HD (5 min ) -1 Pitch HD (10 min ) -1 Pitch HD (20 min ) -1 Pitch LD (5 min ) -1 Pitch LD (10 min ) -1 Pitch LD (20 min )

9.3

Ln h cp)

9.0 8.7 8.4 8.1 7.8 7.5

2.1

2.2

2.3 2.4 -3 -1 T (10 x K ) -1

Fig. 6.

26

2.5

2.6

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(LD-Pitch)

(HD-Pitch)

Fig. 7.

27

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(d) 1 h

20 µm (b) 6 h

20 µm (e) 6 h

20 µm (c) 10 h

20 µm (f) 10 h

20 µm

20 µm

Fig. 8.

28

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110

Pitch LD-untreated Pitch LD-1 h Pitch LD-6 h Pitch LD-10 h

(Pitch-LD)

100 Residual weight (%)

90 80 70 60 50 40 30 100

200

300

400

500

600

700

800

900

Temperature (oC)

Residual weight (%)

Pitch HD-untreated Pitch HD-1 h Pitch HD-6 h Pitch HD-10 h

(Pitch-HD)

100 90 80 70 60 100

200

300

400

500

600

Temperature (oC)

Fig. 9.

29

700

800

900

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90

Pitch B Pitch A

Coke yield (%)

80 70 60 50 40 30 0

2

4 6 8 Holding time (h)

Fig. 10.

30

10

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Pitch LD, 10 h

Pitch LD, 6 h

Pitch LD,1 h

400

600

800

1000 1200 1400 1600 1800 2000 Wavenumber (cm-1)

Fig. 11.

31

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o

Pitch HD, 1000 C

o

Pitch HD, 800 C

o

Pitch HD, 600 C

o

Pitch HD, 400 C

400

600

800

1000 1200 1400 1600 1800 2000 Wavenumber (cm-1)

Fig. 12.

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Pitch HD (2100 oC)

Pitch HD (1600 oC)

Pitch LD (2100 oC)

Pitch LD (1600 oC)

1000

1500

2000

Wavenumber (cm-1) Fig. 13.

33

2500

3000

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1610

1.6

ID/IG

G

1.4

1605 1600

1590

0.8 1585

0.6 1580

0.4 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Temperature (oC)

Fig. 14.

34

1575

-1

G (cm

ID/IG

1595

1.0

)

1.2

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o

(002)

1000 C o 1600 C (100) o (101) 2100 C

(Pitch-LD)

(004)

o

2100 C (110)

o

1600 C

o

1000 C

40

20

25 (002)

30

35

40

50

45

60

50

2

o

1000 C o 1600 C (100) o 2100 C (101)

55

70

60

65

80

70

75

80

85

(Pitch-HD)

(004)

o

2100 C

(110)

o

1600 C

o

1000 C

40

50

60

70

80

20 25 30 35 40 45 50 55 60 65 70 75 80 85

2

Fig. 15.

35

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20 Pitch LD Pitch HD

18

C/H (atomic ratio)

16 14 12 10 8 6 4 2 0

0

300 600 900 1200 1500 o Heat treatment temperature ( C)

Fig. 16.

36

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Pitch LD

Pitch HD

20µm

20µm

Fig. 17.

37

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Table 1. Characterization of pitches used in the densification of four-directional C/C composite. Property

Pitch-LD

Pitch-HD

Density (gcm-3)

1.21

1.35

69±3

140±5

Softening temperature (°C)

39.1

91.9

QI (wt.%)

<7

<12

TI (wt.%)

24-28

~31

Volatiles (wt.%)

59-63

53-57

Carbon (wt.%)

>90

>92

Hydrogen (wt.%)

5.2

4.5

Nitrogen (wt.%)

2.1

0.85

Sulphur (wt.%)

0.7

0.79

O & the rest (wt.%)

<1.2

<1.2

Ash contents (wt.%)

0.2

0.2

Melting point (°C)

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Table 2. Coefficients of Arrhenius equations at different shear rates corresponding to pitch-LD and pitch-HD. Material pitch LD

pitch HD

Arrhenius exponential equation A1 (cP) A2 (K-1) 9.189 2710.85 1.671 3296.98 0.036 4720.69 5.707 3242.15 0.420 4286.18 0.018 5533.09

Shear rate (min-1) 5 10 20 5 10 20

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Table 3. Influence of retention time at 425 °C on the d-spacing in the pitches at 1600 °C and 2100°C. d002 spacing (Å) at 1600 and 2100 °C Pitch LD

Retention time at 425 °C (h)

Pitch HD

1600 (°C)

2100 (°C)

1600 (°C)

2100 (°C)

0

3.440122564

3.39454945

3.431256751

3.388615298

6

3.42857074

3.390375566

3.424802045

3.388615298

10

3.411286926

3.384481633

3.418990884

3.384368009

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Table 4. Properties of the 4D C/C composite fabricated with intermediate modulus carbon fiber using both LD and HD coal tar pitches. Property

Value

Density (g/cm3)

1.92±0.01

Compressive strength (MPa)

83-89

Flexural strength (MPa)

129-139

Tensile strength (MPa)

75-81

CTE (µm/mK) @ RT-1000 °C

˂1.9 ×10-6

Ablation rate (mm/s) @ 3000 °C

~0.003

Thermal Conductivity (W/m K)

62-70

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Conflicts of Interest Statement Subject: Original Manuscript Submission Manuscript entitled Rheological properties, structural and thermal elucidation of coal tar pitches used in the fabrication of multi-directional carbon-carbon composites

All the authors certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript. I look forward to hearing from you with a positive response in the near future Sincerely, .................................... Muhammad Bilal, PhD Assistant Professor School of Life Science and Food Engineering, Huaiyin Institute of Technology, Huaian 223003, China. ...………....................... Part-time Researcher Ton Duc Thang University, Ho Chi Minh City, Vietnam, ................................... Emails: [email protected]; [email protected] WEB: ORCiD Profile: https://orcid.org/0000-0001-5388-3183 Research Gate Profile: https://www.researchgate.net/profile/Muhammad_Bilal123 Google Scholar Profile: https://scholar.google.com/citations?hl=en Editorships: Science of the Total Environment - Guest Editor Mathematical Bioscience and engineering - Guest Editor

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Research Highlights  Two types of coal-tar pitches have been selected for efficient densification of 4D C/C composites.  Carbon-carbon composites were studied regarding their physical and chemical transformations.  Initial heat treatment at 425 oC for 6-10 h is essential for high carbon yield and easily graphitizable pitch matrix.  Coal-tar pitches are suitable for efficient production of high density 4D C/C’s used in aerospace applications.