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DOI: 10.1016/S1872-5805(21)60050-1
Microstructure of high thermal conductivity mesophasepitch-based carbon fibers
YE Chong1,2,3, WU Huang1,2,3, ZHU Shi-peng4, FAN Zhen4, HUANG Dong1,2,3,*, HAN Fei1,3, LIU Jin-shui1,3,*, YANG Jian-xiao1,3, LIU Hong-bo1,3
(1. College of Materials Science and Engineering, Hunan University, Changsha 410082, China;
2. Hunan Province Engineering Research Center for High Performance Pitch Based Carbon Materials,
Hunan Toyi Carbon Material Technology Co., Ltd., Changsha 410000, China;
3. Hunan Province Key Laboratory for Advanced Carbon Materials and Applied Technology, Hunan University, Changsha 410082, China;
4. Key Laboratory of Advanced Functional Composite Materials, Aerospace Research Institute of Materials and
Processing Technology, Beijing 100076, China)
Abstract: The microstructural characteristics of high thermal conductivity mesophase pitch-based carbon fibers were investig-ated by XRD, Raman spectroscopy, SEM and TEM. The relationship between microstructural characteristics and thermal conductiv-ity is discussed. Results show that the radial structure is always accompanied by a splitting stucture. La has more significant impacton the thermal conductivity than Lc. The Raman spectroscopy ID/IG value of the cross section was used as an essential index to evalu-ate the thermal conductivity of the carbon fibers. The microstructural characteristics including large graphite crystallite size, high pre-ferred orientation along the fiber axis, and few defects contribute to the high thermal conductivity of the carbon fibers.Key words: Microstructure;Mesophase pitch;Carbon fiber;High thermal conductivity
1 Introduction
Mesophase pitch-based carbon fibers have high-er Young’s modulus and thermal conductivity com-pared with polyacrylonitrile-based carbon fibers, ow-ing to their well-developed graphite crystallites andhighly oriented crystalline along the fiber axis. Thesesuperior properties allow mesophase pitch-based car-bon fibers to be widely used in the fields of aerospacevehicles, electronic devices, and so on[1–3]. The highperformance mesophase pitch-based carbon fibers areusually fabricated by a series of processes includingpitch-synthesis, melt-spinning, pre-oxidation, carbon-ization, graphitization, surface treatment, and so on.Among them, melt-spinning plays a crucially import-ant role in controlling the microstructures and physic-al property of mesophase pitch-based carbon fibers.Until now, several various microstructural character-istics have been widely reported, such as the randomstructure, radial structure, onion-skin structure, flat-
layer structure and folded-radial structure, and thesemicrostructural characteristics are highly dependenton the melt-spinning conditions[4, 5].
Many studies have focused on the effects ofmelt-spinning technological parameters, spinneretdesign and chemical composition of pitch on the mi-crostructures and properties of the mesophase pitch-based carbon fibers[6–10]. Compared with chemicalcomposition of mesophase pitch, melt-spinning is adirect and effective approach to control the micro-structure and properties of carbon fibers. Many re-searchers investigated the relationship between thetypical microstructures of the mesophase pitch-basedcarbon fibers and the corresponding properties. Themicrostructure of mesophase pitch-based carbonfibers with ultra-high modulus was studied byMorinobu Endo[11]. The relationship between the mi-crostructure and the mechanical property of differentcarbon fibers (P25, P55, P75, P100 and P120, Cytec),with thermal conductivities in the range of 97−
Received date: 2019-08-21; Revised date: 2019-11-27Corresponding author: HUANG Dong, Ph. D. E-mail: [email protected];
LIU Jin-shui, Ph. D, Professor. E-mail: [email protected] introduction: YE Chong, Ph. D Candidate. E-mail: [email protected]
第 36 卷 第 5 期 新 型 炭 材 料 Vol. 36 No. 52021 年 10 月 NEW CARBON MATERIALS Oct. 2021
640 W·m−1·K−1, was investigated by YanlingHuang[12]. The microstructural difference of the meso-phase pitch-based fibers (E01, E35, E55, E75, E105,E120, E130, DU PONT) was also examined by G. M.Pennock[13]. Although much data have been reportedin the literature on the microstructures of mesophasepitch-based carbon fibers, there are few reports on themicrostructural features of the high thermal conduct-ive mesophase pitch-based carbon fibers, especiallyfor ultra-high thermal conductive (≥600 W·m−1·K−1)carbon fibers. To obtain a high thermal conductivity,it is necessary to systematically explore the micro-structures of mesophase pitch-based carbon fibers interms of the degree of graphitization, crystalline size,and preferred orientation.
In the present work, the relationship between mi-crostructural characteristics and thermal conductivityof mesophase pitch-based carbon fibers was investig-ated. The morphology, crystalline size, preferred ori-entation, and other microstructural characteristics ofthe carbon fibers were systematically analyzed byXRD, Raman spectroscopy, SEM and TEM. Thiswork will provide meaningful guidance for the fabric-ation of the high thermal conductive mesophase pitch-based carbon fibers.
2 Experimental
2.1 MaterialsFive kinds of high thermal conductive meso-
phase pitch-based carbon fibers were employed in thiswork. The fibers were XN-90 (Nippon Graphite FiberCorp), K13C2U (Mitsubishi Chemical FunctionalProducts Inc.), K13D2U (Mitsubishi Chemical Func-tional Products Inc.), K1100 (Cytec) and HNU-3000
(homemade) respectively.
2.2 Microstructural characterizationCrystallite structural parameters were tested by
X-ray diffraction (XRD) on a D/Max-2550 PC dif-fractometer apparatus with Cu Kα radiation (λ=0.154 184 nm) with Si as an internal standard. Inter-layer spacing d002, crystalline sizes (La, Lc) and de-gree of graphitization (g) were calculated based on theXRD data[14]. Misorientation angles (Z) of graphenesheets with reference to the fiber axis were measuredby full width at half maxima (FWHM) of the diffrac-tion profile from the azimuthal scan.
Morphology and microstructures of the fiberswere examined by scanning electron microscopy(SEM, NOVA 400 NANO), transmission electron mi-croscopy (TEM, Titan G260-300) and Raman spectro-scopy (Horiba JY, XploRA, λ=633 nm), respectively.
2.3 Thermal conductivity testThermal conductivity of the carbon fibers was
obtained by an indirect method. The axial electricalresistivity of carbon fibers was measured with a four-probe method, and the thermal conductivity valueswere calculated by different empirical formulas. Thecalculated values were compared with the manufactur-ers’ data[15–17].
The empirical formula λ1=1261/ρ was establishedby Zhang X[18]. λ2=1272.4/ρ−49.4 was established byYamamoto[19], and λ3=440000/(100ρ+258)−295 wascreated by Lavin[20]. The results show that the thermalconductivity values calculated from the formula λ1=1261/ρ are consistent with the reported values whenthe thermal conductivity exceeds 600 W·m−1·K−1.When the thermal conductivity is less than 600W·m−1·K−1, the formula λ=440000/(100ρ +258)−295is more accurate. The properties of the 5 mesophasepitch-based carbon fibers are listed in Table 1.
Table 1 Properties of the five types of mesophase pitch-based carbon fibers.
Sample R(Ω) S(μm2) ρ(μΩ·m) λ1(W·m−1·K−1) λ2(W·m−1·K−1) λ3(W·m−1·K−1) λ*(W·m−1·K−1)XN-90 1023.17 73.27 2.95 428.89 383.76 501.27 500K13C2U 789.00 65.94 2.07 609.87 566.39 651.64 620K13D2U 423.33 95.42 1.58 799.08 757.30 762.98 800K1100 362.33 80.47 1.15 1099.42 1058.14 884.86 1100
HNU-3000 192.57 165.78 1.12 1127.00 1088.19 893.27 −
Note: λ* manufacturers’ data[24, 25]
第 5 期 YE Chong et al: Microstructure of high thermal conductivity mesophase pitch-based carbon fibers · 981 ·
3 Results and discussion
3.1 SEMSEM micrographs and schematic diagrams of
these carbon fibers are shown in Fig. 1. It can be seenthat all the fibers present obvious graphite layer-struc-ture. XN-90, K13C2U, and K13D2U are all round-shape, while K1100 and homemade HNU-3000 exhib-it a round-split structure. Some irregular hole-defectsare observed on the cross section of XN-90 . The out-er region of XN-90 exhibits a folded-radial structure,while the central region shows onion-skin structure(Fig. 1(a)). Both K13C2U and K13D2U present vis-ible folded-like structure. It should be noted that thegraphite layers in the outer region of K13D2U are lar-ger in size and more ordered along the axial directionin comparison with K13C2U (Fig. 1(b) and
Fig. 1(c)). As a result, K13D2U possess higherthermal conductivity than K13C2U. Both K1100 andHNU-3000 present well-developed graphite layers, asshown in Fig. 1(d) and Fig. 1(e). Notably, K1100show a perfect radial-split structure. However, HNU-3000 seems to be folded-radial structure with thethicker graphite layers. Among these 5 types of fibers,K1100 and HNU-3000 have the largest graphite layer,the highest orientation and the highest thermal con-ductivity. Based on the above results, it can be foundthat the orientation and size of the graphite layers arehighly correlated with the thermal conductivity of thecarbon fibers, and the carbon fibers with a radialstructure unavoidably exhibit wedge-splitting features.
3.2 XRDThe XRD patterns of the carbon fibers are shown
in Fig. 2. As seen, all of the 5 types of carbon fibersexhibit narrow and sharp peak of (002) and distinctpeak of (004) in the equatorial scan patterns (Fig. 2(a)),indicating the large crystallite size and small interlay-er spacing. The results demonstrate that the graphitelayers are compactly stacked and orderly alignedalong the fiber axis direction.
Fig. 2(b) displays the meridional scan patterns ofthe 5 kinds of carbon fiber. The diffraction peaks of(100) and (101) planes are clearly displayed in thesepatterns, indicating the three-dimensional orderedstructure of the graphite crystallites in all the 5 kindsof carbon fiber.
The misorientation angle Z is determined byFWHM of the diffraction profile from the azimuthalscan (Fig. 2(c)), as listed in Table 2. Among them, theZ value of K1100 carbon fiber is the smallest, indicat-ing the highest orientation degree of the graphite lay-ers in K1100. In XN-90, K13C2U, K13D2U andK1100, the Z value gradually decreases and thethermal conductivity increases in turn. However, it isinteresting to find that HNU-3000 have a larger Zvalue than K13C2U, K13D2U and K1100, which doesnot agree with the above mentioned results. This res-ult is attributed to the larger graphite layers in HNU-3000, which inhibits the regular arrangement ofgraphite crystallites along the fiber axis direction dur-
(a)
2 μm 1 μm(b)
2 μm 1 μm
(c)
2 μm 1 μm
(d)
2 μm 1 μm
(e)
2 μm 1 μm
Fig. 1 SEM micrographs and sketches of mesophase pitch-based carbonfibers: (a) XN-90, (b) K13C2U, (c) K13D2U, (d) K1100 and (e) HNU-3000.
· 982 · 新 型 炭 材 料 第 36 卷
ing graphitization.As shown in Fig. 2(d), the (002) peak is narrow
and sharp, and the (004) peak can also be clearly seenin all the fibers. The interlayer spacing of d002,graphene-layer stacking height of Lc, graphite crystal-lite width of La and graphitization degree of g are cal-culated according to the XRD powder diffraction pat-terns, as listed in Table 2. From Table 1 and 2, it canbe found that the La, Lc and g values all have positivecorrelations with thermal conductivity of the fibers,while the d002 value has a negative correlation with it.Considering the larger La, smaller Lc and higher
thermal conductivity in HNU-3000 than K1100, itcan be concluded that the La has more significant im-pact on the thermal conductivity than the Lc. Smallerd002, larger La, Lc and g mean more perfect crystalwith fewer defects, contributing to the increase of themean free path and the decrease of the scattering ofphonons, further leading to higher thermal conductiv-ity[21, 22].
3.3 Raman spectraFig. 3 shows Raman spectra of the 5 kinds of car-
bon fibers on their surface (Fig. 3(a)) and cross sec-tion (Fig. 3(b)). D peak at 1 360 cm−1 represents thenumber of the disordered layer structures, which iscaused by lattice defects, small sizes of crystallitesand low degrees of orientation. The G peak at1 580 cm−1 is attributed to the in-plane bond-stretch-ing vibration of sp2-hybridized C atoms. The intensityratio of D peak to G peak, denoted as ID/IG, is definedas the degree of disorder R[23]. There is a negative cor-relation between the thermal conductivity and R onthe cross section of the 5 kinds of fiber. A smaller R
(a)
10 20 30 40 50 60
Inte
nsity (
a.u
.)
2θ (°)
XN-90
K13C2U
K13D2U
HNU-3000
K1100
(004)
(002)
XN-90
K13C2U
K13D2U
HNU-3000
K1100
(b)
(100) (101)
60 70 80
Si(311)Si(111) C(004)
C(002)
XN-90
K13C2U
K13D2U
K1100
HNU-3000
(d)
−90 −60 −30 0 30 60 90
(c)
K13C2U
XN-90
K1100
HNU-3000
K13D2U
10 20 30 40 50 60
Inte
nsity (
a.u
.)
2θ (°)
Inte
nsity (
a.u
.)
β (°)
10 20 30 40 50 90
Inte
nsity (
a.u
.)
2θ (°)
Fig. 2 XRD patterns of the five kinds of carbon fiber with high thermal conductivities: (a) Equatorial scan, (b) Meridional scan,(c) Azimuthal scan on (002) crystal face and (d) powder diffraction.
Table 2 Crystalline parameters and degree of graphitiza-tion of carbon fibers.
Sample 2θ002(°) d002(nm) Lc(002)(nm) La(100)(nm) g(%)a Z(°)XN-90 26.38 0.3379 23.07 38.70 71.46 9.33K13C2U 26.41 0.3375 26.05 46.00 75.99 9.20K13D2U 26.43 0.3373 28.95 50.61 78.17 9.04K1100 26.47 0.3368 34.22 70.30 84.29 8.06
HNU-3000 26.48 0.3366 34.67 78.26 85.88 9.23Note: aDegree of graphitization (g) was calculated by the equation g=(0.3440−d002)/(0.3440−0.3354)
第 5 期 YE Chong et al: Microstructure of high thermal conductivity mesophase pitch-based carbon fibers · 983 ·
value in carbon fiber represents a higher thermal con-ductivity. The difference of R values on the surfaceand cross section is mainly ascribed to the skin-corestructure in the carbon fibers. It is worth noting thatK1100 and HNU-3000 have larger R values on thesurface but smaller R values on the cross section com-pared with K13D2U. It is believed that R value on thecross section can be used as an important index toevaluate the thermal conductivity of carbon fibers.
3.4 HRTEMThe HRTEM micrographs of the 5 kinds of car-
bon fibers are shown in Fig. 4. As seen, all the carbonfibers present an ordered lattice structure along the
fiber axis direction. There is a positive correlationbetween the thermal conductivity and crystal size. Theresult well agrees with the Lc values from XRD ana-lysis in Table 2. In addition, many crystalline defectssuch as distortion, disorientation, dislocation and dis-continuity of crystallites can also be found in HRTEMimages. In particular, the crystalline defects in K1100and HNU-3000 are rarely found. Considering thethermal conductivity, it is evident that fewer crystal-line defects will bring higher thermal conductivity.
Based on the above analysis, the axial structuralcharacteristics of the 5 high thermal conductive car-bon fibers are depicted in Fig. 5. For the 5 kinds of
1000 1200 1400 1600 1800 2000
G
R=0.087
R=0.076
R=0.072
R=0.120
K1100
HNU-3000
K13D2U
K13C2U
XN-90
(a)
Wave number (cm−1) Wave number (cm−1)
R=0.144D
1000 1200 1400 1600 1800 2000
GD
XN-90
K13C2U
K13D2U
HNU-3000
K1100
R=1.18
R=1.13
R=1.00
R=0.81
R=0.75
(b)
Inte
nsi
ty (
a.u
.)
Inte
nsi
ty (
a.u
.)
Fig. 3 Raman spectra of the 5 kinds of carbon fiber: (a) surface and (b) cross section.
(a)
10 nm
20 nm 20 nm
10 nm 10 nm
Lc=10 nm
Lc=
17 n
m
Lc=
64 n
m
Lc=
38 n
m
Lc=
26 n
m
Fib
er a
xis
Fiber axis
Fiber axis
Fiber axis
Fiber axis
(b) (c)
(e)(d)
Fig. 4 HRTEM images of the five kinds of carbon fiber: (a) XN-90, (b) K13C2U, (c) K13D2U, (d) K1100 and (e) HNU-3000.
· 984 · 新 型 炭 材 料 第 36 卷
mesophase pitch-based carbon fiber, the large graph-ite crystalline size, high preferred orientation degreealong the axis direction, and few crystalline defects incarbon fibers lead to the high thermal conductivity.
4 Conclusion
Based on the comparison of the five kinds of dif-ferent high thermal conductive mesophase pitch-basedcarbon fiber in this work, it is found that thehomemade HUN-3000 carbon fiber possessesthe highest thermal conductivity of 1 127 W·m−1·K−1.The result shows that the radial structure is always ac-companied by a split structure and high thermal con-ductivity. La has more significant impact on thethermal conductivity than the Lc. Smaller d002, largerLa, Lc and g mean more perfect crystal structures withfewer defects, thus contributing to the increase ofmean free path and the decrease of the scattering ofphonons, leading to higher thermal conductivity. Rvalue on the cross section can be used as the import-ant index to evaluate the thermal conductivity of car-bon fibers. The microstructure of the carbon fiberswith a large graphite crystallite size, high preferredorientation degree along the axis direction, and fewcrystalline defects is conductive to the high thermalconductivity.
AcknowledgementsThe Innovation and Entrepreneurship Invest-
ment Project of Hunan Provincial Science and Tech-nology Department (2018GK5065); Special Fund for
Innovative Construction Province of Hunan(2019GK2021, 2019RS2058).
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Fig. 5 Sketches of the crystallite structure in the longitudinal section forthe high thermal conductive carbon fibers.
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高导热中间相沥青基炭纤维的微观结构研究
叶 崇1,2,3, 吴 晃1,2,3, 朱世鹏4, 樊 桢4, 黄 东1,2,3,*, 韩 飞1,3, 刘金水1,3,*, 杨建校1,3, 刘洪波1,3
(1. 湖南大学 材料科学与工程学院,湖南 长沙 410082;
2. 湖南东映碳材料科技有限公司 沥青基高性能碳材料湖南省工程研究中心,湖南 长沙 410000;
3. 湖南大学 先进炭材料及应用技术湖南省重点实验室,湖南 长沙 410082;
4. 航天材料及工艺研究所 先进功能复合材料技术重点实验室,北京 100076)
摘 要: 本文研究了高导热(500~1 127 W·m−1·K−1)中间相沥青基炭纤维的微观结构特征,初步建立了微观结构特征与
导热性能之间的影响关系。通过 XRD、拉曼光谱、SEM和 TEM对纤维的显微结构进行了系统表征,结果表明,辐射状的
纤维结构热导率较高,并伴随着劈裂状结构特征。La对热导率的影响比 Lc更加显著,纤维截面上的 R 值可作为评估热导
率的重要参照指标。在本研究所涉及的纤维中,石墨微晶尺寸越大,微晶缺陷越少,石墨微晶片层沿纤维轴向取向度越
好,则炭纤维的热导率越高。
关键词: 微观结构特征;中间相沥青;炭纤维;热导率
文章编号: 1007-8827(2021)05-0980-07 中图分类号: TQ536.2 文献标识码: A
基金项目:湖南省科学技术厅创新创业技术投资项目(2018KG5065);湖南创新型省份建设专项经费资助(2019GK2021,2019RS2058).
通讯作者:黄 东,博士. E-mail:[email protected];
刘金水,博士,教授. E-mail:[email protected]作者简介:叶 崇,博士研究生. E-mail:[email protected]本文的电子版全文由 Elsevier 出版社在 ScienceDirect 上出版(https://www.sciencedirect.com/journal/new-carbon-materials/)
· 986 · 新 型 炭 材 料 第 36 卷