carbon composites by lanthanum oxide

JOURNAL OF RARE EARTHS, Vol. 30, No. 2, Feb. 2012, P. 128

Catalytic graphitization of carbon/carbon composites by lanthanum oxide ZHANG Can (ᓴ♓)1, LU Guimin (䏃䌉⇥)2, SUN Ze (ᄭ⋑)1, YU Jianguo (Ѣᓎ೑)1, 2 (1. State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China; 2. National Engineering Research Center for Integrated Utilization of Salt Lake Resources, School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China) Received 15 July 2011; revised 1 August 2011

Abstract: Graphitized carbon/carbon composites were prepared by the process of catalytic graphitization with the rare-earth catalyst, lanthanum oxide (La2O3), in order to increase the degree of graphitization and reduce the electrical resistivity. The modified coal tar pitch and coal-based needle coke were used as carbon source, and a small amount of La2O3 was added to catalyze the graphitization of the disordered carbon materials. The effects of La2O3 catalyst on the graphitization degree and microstructure of the carbon/carbon composites were investigated by X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. The results showed that La2O3 promoted the formation of more perfect and larger crystallites, and improved the electrical/mechanical properties of carbon/carbon composites. Carbon/carbon composites with a lower electrical resistivity (7.0 P:·m) could be prepared when adding 5 wt.% La2O3 powder with heating treatment at 2800 ºC. The catalytic effect of La2O3 for the graphitization of carbon/carbon composites was analyzed. Keywords: lanthanum oxide; catalytic graphitization; C/C composites; rare earths

The graphitized carbon/carbon (C/C) composite has light weight, high thermal conductivity and high thermal stability, so it has been widely used in the space and aeronautic industries[1]. Graphitization is a transformation of the disordered carbon materials into three-dimensional graphite under high heat treatment temperature (HTT)[2,3]. Generally, when the extra energy is provided, for example, by the HTT at 3000 °C, the disordered carbon materials can be graphitized by atomic displacements. During the graphitization, the matrix microstructure has significant effect on the performance of C/C composites, particularly its mechanical properties. Therefore, many researches have been focused on the control of the matrix microstructure and the graphitization degree of carbon materials, in order to obtain the desired performances of carbon materials[4]. It has been found that some carbon materials were very difficult to graphitize even after being heated to temperatures above 3000 ºC. The methods of increasing pressure or adding catalysts can be used to accelerate the graphitization of a non-graphitizable carbon. These processes are respectively called stress graphitization and catalytic graphitization. The stress resulted from the anisotropic thermal expansion of carbon layers at high temperature, can accelerate the graphitization of the thermosetting matrix of C/C composites[5–8]. And, an extensive study has been made of the catalytic graphitization of carbon by various elements, such as Fe, Co, Ni, Ti, Zr, B, Be, Mn, Mo etc.[9–20] However, the catalytic effects of rare-earth elements on the graphitization of

non-graphitizing carbon have been seldom studied. Recently, Yi et al.[21,22] found that the rare-earth catalysts, yttrium nitrate and praseodymium nitrate, had good catalytic properties for graphitization of the disordered carbon materials. Compared to the other catalysts, with lower content of the rare-earth catalyst, a significant enhancement of graphitization of the non-graphitizing carbon could be observed. Lanthanum oxide (La2O3) has wide application due to special electronic structure and physicochemical properties. It has strong affinity with nitrogen and sulfur. It can be used as catalyst to reduce the sulfur dioxide and nitric oxide[23] and as “puffing” inhibitor during graphitization of carbon/carbon composites[24]. Also, La2O3 contains no nitrogen, a kind of “puffing” element, compared with yttrium nitrate and praseodymium nitrate. In addition, lanthanum in nature exists mainly as oxide compounds, easy to obtain. In the present work, we introduced lanthanum oxide to catalyze the graphitization of carbon/carbon composites, where the modified coal-tar pitch and coal-based needle coke were used as carbon source. The effects of the content of lanthanum oxide on the catalytic graphitization of C/C composites were investigated.

1 Experimental 1.1 Preparation of C/C composites with/without La2O3 additive

Foundation item: Project supported by the National High-Tech R&D Program (863 Program) of China (2009AA06Z102), the Fundamental Research Fund for the Central Universities and State Key Laboratory of Exploration Fund of China Corresponding authors: LU Guimin, SUN Ze (E-mail: [email protected], [email protected]; Tel.: +86-21-64252065) DOI: 10.1016/S1002-0721(12)60008-8

ZHANG Can et al., Catalytic graphitization of carbon/carbon composites by lanthanum oxide

The graphitized C/C composites were prepared with the modified coal-tar pitch and coal-based needle coke as carbon sources, and a small amount of La2O3 to catalyze the graphitization of carbon materials. The modified coal-tar pitch was produced by Shanghai Hongte Chemical Co., Inc., with size 76 Pm and softening point 107 ºC. The other carbon source, coal-based needle coke was produced by Shanghai Hongte Chemical Co., Inc, with the size distribution as follows: 1–2 mm 12.5 wt.%; 0.5–1 mm 22.5 wt.%; 0.15–0.5 mm 30 wt.%; 0.076–0.15 mm 25 wt.%; <0.076 mm 10 wt.%. The coal-based needle coke contains about 1.65 wt.% of sulfur (S) and 0.92 wt.% nitrogen (N). La2O3 was obtained from Shanghai Maikun Chemical Co., Ltd., with powder size <76 Pm. Modified coal-tar pitch (23 wt.%) was dispersed in coal-based needle coke, then a small amount of La2O3 was added in the mixture, such as 0%, 0.5%, 1.0%, 3.0%, 5.0% La2O3 by mass, respectively. The mixture was mixed at 155 ºC and hot-moulded under 23.6 MPa pressure at 135 ºC. The prepared samples are labeled with symbols CM-L0, CM-L0.5, CM-L1.0, CM-L3.0 and CM-L5.0. Sample CM-L0 represents C/C composite without La2O3 additive. These green composites were heated from room temperature to 600 ºC at the rate of 5 ºC/h, and from 600 to 1100 ºC at the rate of 10 ºC/h under nitrogen and carbon monoxide atmosphere. And two cycles of coal tar pitch impregnation/carbonization (PIC) were performed on the densification of C/C composites. Thereafter, all samples were graphitized at 2800 ºC with the heating rate of 600 ºC/h under an inert atmosphere (argon gas with high purity as the inert atmosphere). 1.2

Characterization of C/C composites with/without La2O3 additive

The degree of graphitization of the samples, the matrix microstructure and the properties of the composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. Samples for XRD analysis were mounted on a glass holder with 0.2 mm depth, and patterns were recorded on a Rigaku D/max2550VB+/PC system, using Cu KD radiation (O=0.154 nm, 40 kV, 100 mA) over the range of 10º–80º (2) at room temperature. SEM was taken on a JEOL (S4800) microscope operating at 15 kV. The Raman spectroscopy were collected

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using a Renishaw inVia Reflex system at an excitation wave length of 785 nm. The density of the prepared samples was determined by measuring the mass and geometry. Electrical resistivity at room temperature was measured by a Ke Mu Tan (GM-II) resistivity instrument.

2 Results and discussion 2.1 Effect of La2O3 on the extent of graphitization of C/C composites Using the mixture of modified coal-tar pitch and coal-based needle coke as carbon source to prepare C/C composites, the XRD data of samples show combined effects of two phases under normal circumstances, as shown in Fig. 1(a). XRD phase analysis of Fig. 1(b) demonstrates that La2O3 component in the graphitized composites exists mainly in the form of lanthanum (La). Also, lanthanum carbide (LaC2) probably coexists with La in the samples. Table 1 lists crystal parameters of the graphitized C/C composites with La2O3 catalyst and that of a composite without La2O3 additive (CM-L0), where all samples are prepared with the same process. As seen in Fig. 1, since the intensity of (002) diffraction peak of graphite is very high, crystallite size (La) is difficult to measure directly, and it can be calculated by the expression La=9.5/(d002–3.354)[15]. The degree of graphitization (g) is calculated by the expression g=(0.3440–d002)/(0.3440–0.3354). Compared with sample CM-L0, C/C composites containing La2O3 exhibit lower graphite interlayer distance, higher degree of graphitization and larger crystallite size in all case. The interlayer spacing of CM-L0 is 0.3381 nm, which is reduced to 0.3368 nm for CM-P3.0. With further increase in the concentration of La2O3, the interlayer spacing increases and the crystallite size decreases, for example, sample CM-L5.0 in Table 1. It is believed that high concentrations Table 1 Effect of La2O3 on graphitization of C/C composites Samples

CM-L0

CM-L0.5

CM-L1.0

CM-L3.0

CM-L5.0

d002 /nm

0.3381

0.3375

0.3373

0.3368

0.3371

g/%

68.60

75.58

77.91

83.72

80.23

La/nm

35.19

45.24

50.00

67.86

55.88

Fig.1 XRD patterns of sample CM-L0 (a) and sample CM-L5.0 (b) at 2800 ºC HTT for 2 h

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(more than 3 wt.%) of catalyst result in localized graphitization of the matrix, causing a decrease in the crystallite size and an increase in the concentration of crystallite boundaries. This demonstrates that only a small quantity of La2O3 (3 wt.%) in the composite matrix is the most effective as catalyst to enhance the degree of graphitization. The Raman spectrum of carbon contains valuable information about the atomic level microstructure[25]. Carbons show mainly two Raman bands, 1580 and 1360 cm–1. The former, which is referred as the G-mode, corresponds to the E2g mode of graphite, and is assigned to the “in-plane” displacement of the carbons strongly coupled in the hexagonal sheets. The latter, which is absent in the single crystal graphite, corresponds to the defect-induced Raman band called the D-mode[26]. The structural changes can be monitored by measuring the ratio of the intensities of the 1360 cm–1 (D) to the 1580 cm–1(G) lines (ID/IG). The Raman spectra measured for the graphitized C/C composites are shown in Fig. 2. It is shown that D band of sample CM-L3.0 is of the lowest intensity. The specimens of CM-L0 and CM-L5.0 have much higher intensity of D band. It shows that the microstructure of the composites transits to perfect graphite with 3 wt.% La2O3 concentration. In Table 2, the value ID/IG of sample CM-L0 is 0.68, and it is 0.41 for sample CM-L3.0. The smaller ratio of intensities (ID/IG) means better extent of graphitization[25]. On the basis of the test results, the degree of graphitization is improved with the addition of La2O3. In contrast, the values ID/IG increase with further increase in the concentration of La2O3, for example, sample CM-L5.0 in Table 2. The results of Raman conform to those of the XRD. The fracture morphologies of the graphitized C/C composites are presented in Fig. 3. Compared sample CM-L0 with sample CM-L3.0, graphite laminar structure becomes better due to the addition of lanthanum oxide. It demonstrates that the addition of lanthanum oxide can promote the growth of graphite crystal. 2.2 Effect of La2O3 on electrical resistivity and bulk density of C/C composites The resistivity value varies greatly for different graphite,

as a function of bulk density and the degree of graphitization. As far as the conductivity of carbon materials is concerned, conductivity is positively related to the degree of graphitization, the size of graphite crystallites (La) and bulk densities[27]. The bulk density behaviors and the resistivity behaviors of the composites with La2O3 addition are shown in Fig. 4. It is shown that the bulk densities of specimens increase dramatically with the content of La2O3 addition and the highest bulk density is obtained at La2O3 content of 5%. Also, the lowest electrical resistivity of 7.0 P:·m appears at La2O3 content of 5%. During the graphitization, the elements S and N are released mainly at 1400–2200 ºC[24]. When the stress produced by the released gas exceeds the stress of the material, a “puffing” effect may be produced. The lanthanum oxide can react with S and N and is decomposed at a higher temperature than that at which puffing occurs[24]. This increases the temperature range of the S and N release and can inhibit the “puffing” effect, as seen in Fig. 5. The expansivity decreases with the content of La2O3. This is the reason

Fig. 2 Raman spectra for the samples after graphitization at 2800 ºC HTT for 2 h (1) CM-L0; (2) CM-L0.5; (3) CM-L1.0; (4) CM-L3.0; (5) CM-L5.0 Table 2 Raman analysis results for specimens after graphitization at 2800 ºC HTT for 2 h Samples

CM-L0

CM-L0.5

CM-L1.0

CM-L3.0

CM-L5.0

ID/IG

0.68

0.60

0.49

0.41

0.55

Fig. 3 SEM pictures of samples with different contents of La2O3 after graphitization at 2800 ºC HTT for 2 h (a) CM-L0; (b) CM-L3.0

ZHANG Can et al., Catalytic graphitization of carbon/carbon composites by lanthanum oxide

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curs through the carbide. Many researchers considered that titanium reacts readily with carbon to form carbide following the third mechanism[15]. The exact mechanism involved in the catalysis is obscure. Perhaps the following explanation is relatively easy to comprehend in this study. After being introduced into carbon substrate, La2O3 reacts with disorganized carbon to form LaC2 when the temperature is higher than 1600 ºC[32], which will decompose to form graphite and the metal gas. The transformations are shown as Eq. (1–2). t1600 q C

Fig. 4 Electrical resistivity and bulk density of samples with different contents of La2O3 after graphitization at 2800 ºC HTT for 2h

o LaC2 C(disordered–carbon)+La6O11  (1) LaC2C(graphite)+La(g) (2) There is equilibrium between the solid LaC2 and gas La. In the process, the conversion from disordered carbon to graphite occurs, and the LaC2 is the catalyst. La2O3 can produce the catalytic effect after the LaC2 appears. The melting point of LaC2 is 2360 ºC[33]. When the temperature reaches the melting point, the liquid LaC2 can dissolve carbon and precipitate graphite through Eq. (3). LaC ( l ), t 2360 q C

Fig. 5 Expansivity of samples with different contents of La2O3 during graphitization at 2800 ºC HTT for 2 h

for the increasing bulk density of specimens produced by the addition of La2O3. As concluded above, the highest degree of graphitization occurs at 3 wt.% of La2O3 addition. However, resistivity decreases as the content of La2O3 increases. It is mainly because the resistivity is the function of bulk density and degree of graphitization. The higher the bulk density and the better the graphitization degree, the lower the resistivity[28,29]. Compared with sample CM-L3.0, sample CM-L5.0 makes graphitization degree slightly lower and bulk density higher. In the experiments, bulk density is predominant in the two factors that affect the resistivity, so the resistivity of sample CM-L5.0 is the lowest. 2.3 Catalytic mechanism of lanthanum oxide At present, three mechanisms are proposed to explain catalytic graphitization of catalytic element to carbon substrates[30]. The first is disordered carbon dissolution into the catalyst and subsequent precipitation as graphite. The group VIII metals such as ferrum, cobalt, nickel and so on, catalyze the graphitization according to this mechanism. The second mechanism is carbide formation through the reaction of carbon with metal catalyst and subsequent decomposition to produce graphite. Silicon catalyst exhibits this mechanism[31]. The third is the dissolution-precipitation sequence that oc-

2 o C(graphite) (3) C(disordered–carbon)  The driving force for the process, which occurs isothermally, is the excess free energy of disordered carbon structure. Evidently, the carbide melt wets and flows into the capillary-like network of very fine pores in the carbon, ingesting the disordered carbon on the leading surface of the particles and precipitating it as nonreactive small graphite crystallites, without significant change in the composition of the carbide. With further heat-treatment, the precipitated graphite crystallites slowly grow in size by which structural defects are removed and distortions are relaxed. From the analysis above, the mechanism of La2O3 catalysis can be described by carbide formation through the reaction of carbon with metal catalyst and subsequent decomposition to produce graphite before the melting point and the dissolution-precipitation sequence occurring through the carbide above the melting point.

3 Conclusions By comparison of the samples prepared with and without La2O3 additive, it was found that lanthanum oxide could catalyze significantly the graphitization of C/C composites. The interlayer spacing (d002) of C/C composites was increased dramatically when the content of La2O3 was more than 3%. Scanning electron microscope micrographs of C/C composites showed that an excellent crystallite arrangement appeared for 3% La2O3 addition. Only a small quantity of La2O3 (less than 3%) in the composite matrix was effective as catalyst to increase the degree of graphitization. There existed an optimal content for La2O3 additive, 5 wt.%, with which the prepared C/C composite had the lowest electrical resistivity of 7.0 P:·m. The catalytic mechanism of La2O3 involved the carbide formation through the reaction of carbon with metal catalyst and subsequent decomposition to produce graphite before the melting point and the dissolu-

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tion-precipitation sequence occurring through the carbide when the temperature was above the melting point.

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