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[Kimiyoshi Naito](https://orcid.org/0000-0002-3334-4876), Chiemi Nagai

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[Raman Scattering for Anisotropy of Polyacrylonitrile-Based and Pitch-Based Carbon Fibers](https://mdr.nims.go.jp/datasets/8da90383-2ec5-4073-ba2c-2d8534db3f82)

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Raman Scattering for Anisotropy of Polyacrylonitrile-Based and Pitch-Based Carbon FibersAcademic Editor: David P. HarperReceived: 31 May 2025Revised: 6 August 2025Accepted: 21 August 2025Published: 25 August 2025Citation: Naito, K.; Nagai, C. RamanScattering for Anisotropy ofPolyacrylonitrile-Based andPitch-Based Carbon Fibers. Fibers 2025,13, 114. https://doi.org/10.3390/fib13090114Copyright: © 2025 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license(https://creativecommons.org/licenses/by/4.0/).ArticleRaman Scattering for Anisotropy of Polyacrylonitrile-Based andPitch-Based Carbon FibersKimiyoshi Naito 1,2,* and Chiemi Nagai 11 Polymer Matrix Composites Group, Research Center for Structural Materials, National Institute for MaterialsScience (NIMS), Tsukuba 305-0047, Japan; nagai.chiemi@nims.go.jp2 Department of Aerospace Engineering, Tohoku University, Sendai 980-8579, Japan* Correspondence: naito.kimiyoshi@nims.go.jp; Tel.: +81-29-859-2803AbstractPolyacrylonitrile (PAN)-based and pitch-based carbon fibers exhibit significant anisotropiesin the radial and axial directions. Characterizing the anisotropy of the elastic propertiesof PAN-based and pitch-based carbon fibers is important for carbon fiber research com-munities. In this present study, the Raman scattering for anisotropy of PAN-based andpitch-based carbon fiber-reinforced plastic (CFRP) samples was investigated. The Ramanscattering parameters and ratios in the CFRPs with 0◦, 45◦, and 90◦ sections are related tothe tensile modulus. These linear trends for the PAN-based and pitch-based CFRPs with 0◦,45◦, and 90◦ sections intersect in the range of 400–700 GPa. The change in Raman scatteringparameters and ratios of PAN-based and pitch-based carbon fibers and CFRPs with a 0◦section are related to the tensile modulus. These linear trends also intersect in the rangeof 400–700 GPa. The intensity ratios increased with increase in the angle for each CFRPs.The intensity ratio in an arbitrary angle could be estimated using the rule of mixtures andcoordinate transformation equations. The Raman anisotropic nature of PAN-based andpitch-based fibers are identified experimentally and analytically.Keywords: carbon fiber; Raman; scattering parameter; scattering ratio; anisotropy1. IntroductionThe utilization of polyacrylonitrile (PAN)-based and pitch-based carbon fibers, whichexhibit elevated axial stiffness and strength, has been employed to reinforce polymer-matrixmaterials in advanced composites [1]. Carbon fiber-reinforced plastics (CFRPs) are widelyused in aerospace, automotive, energy, and marine industries due to their exceptionalstrength-to-weight ratio and corrosion resistance [2,3]. For instance, Hamzat et al. [2]provide a comprehensive review of CFRP applications in aircraft, helicopters, drones,wind turbines, and ships, emphasizing their performance under mechanical, thermal,and chemical stress conditions. Seo et al. [4] highlight the growing demand for CFRPcomponents and the need for precise manufacturing techniques to meet the stringentrequirements of these industries. To address environmental concerns, recent researchhas focused on sustainable production and recycling of CFRPs. Innovative closed-looprecycling methods have enabled the reintegration of carbon fibers into new compositematerials, promoting circular economy principles [3]. However, these fibers demonstratesubstantial anisotropies in radial and axial directions. Additionally, there is a dearth ofdata concerning the elastic properties of fibers in directions other than the longitudinaland transverse directions, as well as on the relationship between elastic properties andFibers 2025, 13, 114 https://doi.org/10.3390/fib13090114https://doi.org/10.3390/fib13090114https://doi.org/10.3390/fib13090114https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/fibershttps://www.mdpi.comhttps://orcid.org/0000-0002-3334-4876https://doi.org/10.3390/fib13090114https://www.mdpi.com/article/10.3390/fib13090114?type=check_update&version=1Fibers 2025, 13, 114 2 of 15fiber structures. Pitch-based carbon fibers, especially, offer a high modulus and thermalstability, making them suitable for applications requiring dimensional precision and highstiffness, such as aerospace structures, robotics, and precision instruments [5]. Their uniquegraphitic microstructure also contributes to pronounced anisotropic properties, which arecritical for directional mechanical performance and thermal conductivity [5]. A number ofstudies have been conducted to examine the structure of carbon fibers employing a varietyof techniques, including X-ray diffraction (XRD) [6,7] and transmission electron microscopy(TEM) [8,9]. The anisotropy of the elastic properties of carbon fibers has been evaluated bynanoindentation [10,11], ultrasonic spectroscopy (RUS) [12], and radial compression testsof the carbon fibers [13,14]. Additionally, Naito et al. [15] also characterized the anisotropyof the elastic properties of various PAN-based and pitch-based carbon fibers using thenanoindentation test.Raman spectroscopy is a non-destructive, contactless technique capable of providinglocalized structural information with high spatial resolution and sensitivity to microstruc-tural variations [16]. It is particularly sensitive to crystallinity, defect density, and molecularorientation, which are critical parameters in assessing the anisotropic nature. Unlike tech-niques such as XRD or TEM, which often require extensive sample preparation or providelimited surface information, Raman spectroscopy enables rapid, in situ characterizationof material surfaces without altering the sample preparation [16]. Raman spectroscopyhas been extensively applied in a variety of studies on carbon materials, and its efficacyin characterizing these structures has been proven. Preliminary research on carbon ma-terials indicated that a Raman band at approximately 1600 cm−1 can be associated withthe graphite mode (G-band). Another band in polycrystalline line graphite, observed atapproximately 1300 cm−1, is attributable to the defect mode (D-band) [17]. The D- andG-bands in Raman spectra are sensitive to the degree of graphitization, defect density,and orientation of sp2 carbon domains. These spectral features are strongly correlatedwith mechanical properties such as tensile strength and modulus [18]. Brubaker et al. [18]demonstrated that the position and shape of the peak in Raman spectra exhibit robust corre-lations with mechanical performance across a wide range of PAN-based carbon fibers [18].The ratio of the intensities of the two bands, ID/IG, is one of the effective ways to evaluatethe difference in modulus and anisotropy of PAN-based and pitch-based carbon fibers.Naito et al. [19] also characterized the parameters and ratios related to Raman scatteringand stress measurement for the G- and D-bands of various PAN-based and pitch-basedcarbon fibers. These Raman scattering parameters and ratios include peak values of Ramanshifts (RG, RD), full width at half maximum (FWHMG, FWHMD), peak value slopes (|AG|,|AD|), peak value intercepts (BG, BD), intensity ratio (ID/IG), peak value ratio (RD/RG),full width at half maximum ratio (FWHMD/FWHMG), slope ratio (AD/AG), and interceptratio (BD/BG). These correlations were verified by evaluating PAN-based and pitch-basedcarbon fibers separately, and these parameters and ratios were correlated to the tensile mod-ulus (E) and the X-ray diffraction structure parameters of interlayer spacing (d002), latticespacing (d10), and crystalline size (Lc and La). These make Raman spectroscopy particularlysuitable for evaluating anisotropy, which is inherently linked to the alignment and orderingof carbon structures. Additionally, Raman spectroscopy allows polarization-dependentmeasurements [18], enabling the assessment of anisotropy by comparing spectral responsesalong different fiber orientations. This capability is crucial for understanding the directionaldependence of structural properties.The evaluation of the anisotropy of various PAN-based and pitch-based carbon fibersusing Raman spectroscopy and the understanding of the relationship between the Ramanscattering parameters and the mechanical/structural properties are important for thedesign and development of carbon fiber-reinforced composites and structures [20,21].Fibers 2025, 13, 114 3 of 15However, the anisotropy of PAN-based and pitch-based carbon fibers using the Ramanspectroscopy and the relationship between the Raman scattering parameters and themechanical properties are not yet understood.In this study, Raman scattering was employed to assess the anisotropy of a series ofcommercially available carbon fibers, including high-strength PAN-based, high-modulusPAN-based, high-modulus pitch-based, and high-ductility pitch-based carbon fibers, alongwith carbon fiber-reinforced plastic (CFRP) samples. It was found that the X-ray diffractionstructure parameters were correlated to the tensile modulus in the previous investiga-tion. Therefore, the relationship between the Raman scattering parameters and the tensilemodulus as mechanical properties was evaluated.2. Materials and MethodsMaterialsCFRP prepreg sheets were cured by an autoclave (ACA Series, Ashida Mfg. Co., Ltd.,Osaka, Japan) in a laboratory setting to prepare the CFRP samples. The prepreg sheetswere trimmed to the requisite dimensions and fiber orientation prior to being placed inthe autoclave chamber. CFRP laminates were fabricated using a hand lay-up and vacuumbagging technique (without a bleeder). The curing conditions, layer sequences, and carbonfiber volume fractions of the CFRPs are summarized in Table 1.Table 1. Physical and Raman properties of the PAN-based and pitch-based CFRPs.HS-PAN HM-PAN HM-Pitch HD-Pitch HS-PAN HM-PitchT700SC M60JB K13D XN05 T300 T800SC IMS60 K13CCuring conditions 180 ◦C 4 h 120 ◦C 4 h135 ◦C 2 h 180 ◦C 4 h 180 ◦C 4 h 180 ◦C 4 h 180 ◦C 4 h 180 ◦C 4 h 120 ◦C 4 h135 ◦C 2 hLayer sequence [0/+45/90/−45]14[0/+45/90/−45]26[0/+45/90/−45]26[0/+45/90/−45]34[0/+45/90/−45]20[0/+45/90/−45]s[0/+45/90/−45]24[0/+45/90/−45]28Fiber volume fraction, Vf 0.539 0.574 0.545 0.565 0.492 0.523 0.567 0.542Tensile modulus offiber, Ef (GPa) *1 230 588 935 54 230 294 285 900Peak Ramanshift(G-band)RG (cm−1)Fiber *20◦45◦90◦1595.7 (0.9)1602.7 (0.9)1603.6 (0.7)1604.2 (0.9)1582.2 (0.6)1583.7 (0.3)1585.0 (0.3)1585.4 (0.3)1581.5 (0.5)1582.2 (0.3)1582.9 (0.3)1583.2 (0.4)1597.5 (0.2)1596.0 (1.5)1595.9 (1.2)1595.6 (1.3)1594.8 (0.9)1601.5 (1.2)1602.2 (1.0)1602.8 (0.8)1598.6 (1.6)1603.1 (0.6)1603.9 (0.5)1604.7 (0.7)1597.6 (1.6)1603.0 (0.7)1603.7 (0.6)1604.2 (0.9)1583.2 (0.4)1582.8 (0.3)1583.3 (0.3)1584.1 (0.3)Peak Ramanshift(D-band)RD (cm−1)Fiber *20◦45◦90◦1360.9 (1.3)1356.6 (0.8)1356.2 (0.9)1355.9 (0.9)1350.0 (0.8)1351.1 (0.2)1351.7 (0.3)1352.0 (0.3)1349.1 (0.8)1351.5 (0.5)1352.8 (0.5)1354.0 (0.7)1348.2 (0.3)1350.4 (1.0)1350.1 (0.4)1349.9 (0.6)1357.5 (1.4)1357.8 (1.4)1356.4 (1.2)1357.1 (1.0)1354.6 (1.1)1355.9 (1.0)1355.1 (0.6)1354.7 (0.6)1357.3 (1.6)1355.9 (0.7)1354.8 (0.7)1355.8 (1.0)1351.2 (0.5)1351.2 (0.4)1351.9 (0.4)1352.5 (0.4)G-band fullwidth at halfmaximumFWHMG(cm−1)Fiber *20◦45◦90◦88.1 (3.4)71.5 (2.4)67.7 (2.4)65.3 (3.1)25.2 (0.5)30.2 (0.5)32.7 (0.8)33.5 (1.1)18.0 (0.6)19.8 (0.9)21.1 (0.9)20.8 (1.0)66.4 (0.5)53.6 (4.3)46.5 (4.4)44.4 (4.6)85.3 (3.8)79.4 (4.4)74.7 (3.1)75.3 (2.6)75.5 (4.0)71.4 (3.2)69.0 (1.4)66.7 (2.3)81.3 (1.2)72.6 (1.6)69.0 (1.7)72.1 (4.1)19.0 (0.8)21.9 (0.5)23.3 (0.5)25.8 (1.3)D-band fullwidth at halfmaximumFWHMD(cm−1)Fiber *20◦45◦90◦188.7 (3.8)166.8 (5.7)163.2 (6.0)159.2 (7.7)34.4 (0.6)34.0 (0.5)36.4 (0.8)37.4 (1.0)45.9 (4.2)36.8 (1.1)39.0 (1.0)40.3 (1.1)68.1 (0.5)72.5 (2.9)68.2 (1.0)71.4 (2.7)196.0 (4.3)190.9 (10.6)185.5 (5.6)195.0 (7.5)138.9 (4.8)158.9 (7.1)161.2 (4.3)161.1 (4.0)128.8 (1.3)159.5 (3.8)155.9 (5.2)164.0 (6.4)37.9 (1.0)38.9 (0.8)41.5 (0.8)44.1 (1.4)Peak Ramanshift ratioRD/RGFiber *20◦45◦90◦0.8530.8460.8460.8450.8530.8530.8530.8530.8530.8540.8550.8550.8440.8460.8460.8460.8510.8480.8470.8470.8470.8460.8450.8440.8500.8460.8450.8450.8530.8540.8540.854Full width athalfmaximumratioFWHMD/FWHMGFiber *20◦45◦90◦2.1422.3332.4112.4391.3671.1271.1131.1162.5451.8591.8491.9361.0271.3541.4661.6102.2972.4032.4852.5901.8382.2242.3352.4141.5852.1972.2592.2741.9991.7771.7811.707Fibers 2025, 13, 114 4 of 15Table 1. Cont.HS-PAN HM-PAN HM-Pitch HD-Pitch HS-PAN HM-PitchT700SC M60JB K13D XN05 T300 T800SC IMS60 K13CIntensityratioID/IGFiber *20◦45◦90◦0.959 (0.032)1.339 (0.108)1.394 (0.077)1.427 (0.076)0.390 (0.018)0.836 (0.056)1.235 (0.037)1.338 (0.040)0.106 (0.029)0.715 (0.043)0.842 (0.024)0.861 (0.037)1.467 (0.109)1.731 (0.242)2.191 (0.146)2.418 (0.233)0.934 (0.023)1.272 (0.100)1.325 (0.078)1.405 (0.049)0.888 (0.046)1.244 (0.055)1.311 (0.042)1.422 (0.071)0.905 (0.016)1.246 (0.033)1.292 (0.051)1.367 (0.081)0.187 (0.016)0.728 (0.034)0.820 (0.015)0.829 (0.014)VolumefractionVOS (%)VOS (fiber) (%)0.5374.376.3624.11.5555.845.761.71.054.741.275.451.295.191.4028.3*1 Producer’s data sheet T700SC, M60JB, T300, and T800SC: catalog for TORAYCA, Toray Industries, Inc. (Toray),high performance carbon fiber Torayca in Japanese. 2004. K13D and K13C: catalog for Carbon Fiber Tow(Continuous Fiber), Mitsubishi Chemical Corp., DIALEAD. 2022. XN-05: catalog for GRANOC Yarn, NipponGraphite Fiber Corp. (NGF), technical data XN and XNL. *2 Single fiber data from previous investigation [19]. ( )indicate standard deviations.The high-strength PAN-based (HS-PAN) (T700SC), high-modulus PAN-based (HM-PAN) (M60JB), high-modulus pitch-based (HM-pitch) (K13D), and high-ductility pitch-based (HD-pitch) (XN05) CFRPs with nominal thicknesses of approximately 14 mm wereused. The HS-PAN (T300 and IMS60) and HM-pitch (K13C) CFRPs with nominal thick-nesses of approximately 14 mm and the HS-PAN (T800SC) CFRP with a nominal thicknessof approximately 1 mm were also tested for comparison purposes. T700SC, M60JB, T300,and T800SC PAN-based carbon fibers were supplied by Toray Industries, Inc., Tokyo, Japan.IMS60 PAN-based carbon fiber was supplied by Teijin Ltd., Tokyo, Japan. K13D and K13Cpitch-based carbon fibers were supplied by Mitsubishi Chemical Corp., Tokyo, Japan. XN05pitch-based carbon fiber was supplied by Nippon Graphite Fiber Corp., Hyogo, Japan. Thetensile modulus of PAN-based and pitch-based carbon fibers is summarized in Table 1.T700SC, M60JB, T300, T800SC, and K13C prepregs were supplied by Toray Industries, Inc.,Tokyo, Japan. IMS60 prepreg was supplied by Teijin Ltd., Tokyo, Japan. K13D prepregwas supplied by Mitsubishi Chemical Corp., Tokyo, Japan. XN05 prepreg was suppliedby Nippon Graphite Fiber Corp., Hyogo, Japan. The fabricated CFRPs were cut into10 × 10 × 10 (10 × 10 × 1 for T800SC) mm3 pieces using a rotary cutting machine (Refinecutter RCA-234, Refine Tec Ltd., Kanagawa, Japan) at 2500 rpm with an abrasive cuttingwheel (GC150NB, Heiwa Technica Co., Ltd., Tokyo, Japan). The CFRP samples were em-bedded in the epoxy resin and subsequently polished by an automatic polishing machine(Automet 2000, Buhler Ltd., Yokohama, Japan) with TexmetP and polycrystalline diamondsuspensions of 9 and 3 µm, followed by a MasterTex and MasterPrep (Al2O3) suspensionof 0.05 µm. This process was undertaken to produce cross sections of the carbon fibers foranisotropy Raman spectra measurement testing. The specimens’ surface was polished toclearly define the carbon fibers and resin phases [15]. Two specimens were prepared for allCFRP samples. Figure 1a–e shows the anisotropy Raman spectra measurement samples.Raman spectra measurement tests were carried out on axial (0◦), 45 degree (45◦), andcross (90◦) sections of each carbon fiber to investigate the anisotropy of the Raman spectraof the carbon fibers in the chamber of a laser Raman spectrometer (NRS-7100. JASCOCorp., Tokyo, Japan). The experimental apparatus utilized a laser excitation wavelength of532 nm, a diffraction grating of 1800 L/mm, a CCD detector, a long working distance objectlens (100×) (spot size of 1 µm), a redactor with an OD of 1, a laser power of less than 3 mW,an exposure time of 60 s, and a neon lamp calibration. Raman spectra were measured overthe 1200–1700 cm−1 range. At least twenty specimens were tested at different locations ofall carbon fibers.Fibers 2025, 13, 114 5 of 15  (a) (b)   (c) (d)  (e) Figure 1. Anisotropy Raman spectra measurement samples: (a) T700SC; (b) M60JB; (c) K13D;(d) XN05; and (e) schematic view.3. Results and Discussion3.1. Raman SpectrumThe Raman spectra were obtained from a neon lamp calibration (1712.71 cm−1) andbaseline correction. Figure 2a–d shows the Raman spectra of PAN-based and pitch-basedcarbon fibers and CFRPs with axial (0◦), 45 degree (45◦), and cross (90◦) sections. TheG-band peak at 1600 cm−1 and D-band peak at 1300 cm−1 are clearly visible in the PAN-based and pitch-based carbon fibers and CFRPs with 0◦, 45◦, and 90◦ sections. In order tocorrectly describe the parameters, Raman spectra were analyzed using a combination ofmulti peak fitting functions. Specifically, two or three Lorentzian functions were selectedwith the intention to fit the G- and D-bands [21,22]. This package of fitting functions isdesigned to be applicable to the Raman spectra of carbon fibers. These fitting lines are alsoshown in Figure 2a–d.Fibers 2025, 13, 114 6 of 15  (a) (b)   (c) (d) 120013001400150016001700Intensity(a.u.)Raman shift (cm−1)0500100015002000Fiber0°45°90°120013001400150016001700Intensity(a.u.)Raman shift (cm−1)02000400060008000Fiber0°45°90°120013001400150016001700Intensity(a.u.)Raman shift (cm−1)02000400060008000Fiber0°45°90°120013001400150016001700Intensity(a.u.)Raman shift (cm−1)0500100015002000Fiber0°45°90°Figure 2. Raman spectra of PAN-based and pitch-based carbon fibers [19] and CFRPs with 0◦, 45◦,and 90◦ sections. Black lines show single fiber data from previous investigation [19]: (a) T700SC;(b) M60JB; (c) K13D; and (d) XN05.A discernible discrepancy was evident in the peak values of the Raman shifts for the G-(RG) and D-bands (RD). The HM-PAN M60JB and HM-pitch K13D carbon fibers and CFRPsexhibited sharp G- and D-band behaviors, characterized by full widths at half maximumof G (FWHMG) and D-bands (FWHMD), whereas HS-PAN T700SC and HD-pitch XN05carbon fibers and CFRPs exhibited broader behaviors, characterized by larger FWHMG andFWHMD values. The RG, RD, FWHMG, and FWHMD values are summarized in Table 1.The results for the HS-PAN (T300, T800SC, and IMS60) and HM-pitch (K13C) carbon fibersare also summarized in Table 1. In our previous investigation, linear relationships wereobserved between the RG, RD, FWHMG, and FWHMD values and E. Similar relations wereobserved in the PAN-based and pitch-based CFRPs with 0◦, 45◦, and 90◦ sections, as shownin Figure 3a–d, and the values of RG, RD, FWHMG, and FWHMD in the CFRPs with 0◦,45◦, and 90◦ sections were also related to the structural parameters from our previousinvestigation [19].Fibers 2025, 13, 114 7 of 15  (a) (b)   (c) (d) Peak Raman shift (G-band), RG(cm−1)Tensile modulus, E (GPa)1560157015801590160016100 200 400 600 800 1000PAN-basedpitch-basedT700SCM60JBK13DXN-05HS-PANHM-pitchFiber0°45°90° Peak Raman shift (D-band), RD(cm−1)Tensile modulus, E (GPa)0 200 400 600 800 1000133013401350136013701380PAN-basedpitch-basedT700SCM60JBK13DXN-05HS-PANHM-pitchFiber0°45°90°Full width at half maximumFWHMG(G-band) (cm−1)Tensile modulus, E (GPa)0 200 400 600 800 1000020406080100120PAN-basedpitch-basedT700SCM60JBK13DXN-05HS-PANHM-pitchFiber0°45°90°Full width at half maximumFWHMD(D-band) (cm−1)Tensile modulus, E (GPa)0 200 400 600 800 1000050100150200250PAN-basedpitch-basedT700SCM60JBK13DXN-05HS-PANHM-pitchFiber0°45°90°Figure 3. Raman scattering parameters as a function of the tensile modulus of PAN-based andpitch-based carbon fibers [19] and CFRPs with 0◦, 45◦, and 90◦ sections. Black and gray lines andopen symbols show single fiber data from previous investigation [19]: (a) RG vs. E; (b) RD vs. E;(c) FWHMG vs. E; and (d) FWHMD vs. E.The Raman intensity ratio of the G- and D-bands, ID/IG, is a useful parameter forcharacterizing carbon fibers [21–24]. Similarly, the peak Raman shift ratio of the G- andD-bands, RD/RG, and the full width at half maximum ratio for the G- and D-bands,FWHMD/FWHMG, are effective parameters, as indicated in our previous investigation. Thevalues of ID/IG, RD/RG, and FWHMD/FWHMG are summarized in Table 1. The error valueswere found to be affected not only by the material itself but also by the angle. Similar resultswere observed in [15]. Furthermore, the error values were affected by the fitting process.Similar results were found in [19]. However, in this study, the coefficients of variation(standard deviations/means) of RD/RG and FWHMD/FWHMG for all samples were lessthan 0.15% and around 5%, respectively, indicating that the error was quite small. Thecoefficients of variation (standard deviations/means) of ID/IG were around 10% at most forall samples, indicating that the error was considered small. Figure 4a–c shows the Ramanscattering ratios (ID/IG, RD/RG, and FWHMD/FWHMG values) as functions of the tensilemodulus, E. The ID/IG values for PAN-based and pitch-based carbon fibers demonstrateda decrease with increasing E. In contrast, the RD/RG and FWHMD/FWHMG values forpitch-based carbon fibers exhibited an increase with increasing E, while those for PAN-Fibers 2025, 13, 114 8 of 15based carbon fibers increased (RD/RG) or decreased (FWHMD/FWHMG) with increasingE. Linear relationships were observed between the ID/IG, RD/RG, and FWHMD/FWHMGvalues and E. Furthermore, the linear trends for the PAN-based and pitch-based carbonfibers intersected within the range of 400–700 GPa. Such ratio evaluation techniques havebeen employed to assess the efficacy of physical properties and anisotropy [10–15,19,25,26].  (a) (b)   (c)  Tensile modulus, E (GPa)0 200 400 600 800 1000Intensity ratio, ID/I GPAN-basedpitch-based03.52.520.511.53T700SCM60JBK13DXN-05HS-PANHM-pitchFiber0°45°90°Peak Raman shift ratio, RD/RGTensile modulus, E (GPa)0 200 400 600 800 10000.840.8450.850.8550.86PAN-basedpitch-basedT700SCM60JBK13DXN-05HS-PANHM-pitchFiber0°45°90°Full width at half maximum ratioFWHMD/FWHMGTensile modulus, E (GPa)0 200 400 600 800 1000012343.52.51.50.5PAN-basedpitch-basedT700SCM60JBK13DXN-05HS-PANHM-pitchFiber0°45°90°Figure 4. Raman scattering ratio as a function of tensile modulus of PAN-based and pitch-basedcarbon fibers [19] and CFRPs with 0◦, 45◦, and 90◦ sections. Black and gray lines and open symbolsshow single fiber data from previous investigation [19]: (a) ID/IG vs. E; (b) RD/RG vs. E; and(c) FWHMD/FWHMG vs. E.3.2. Raman Scattering Parameter and Ratio Differences Between Carbon Fibers and CFRPs inAxial (0◦) SectionA comparative analysis of the fiber and 0◦ Raman scattering parameter and ratio canelucidate the axial Raman characteristics, which consider the internal structure. Figure 5a–dshows the scanning probe microscope (SPM) images of the fiber and 0◦ surfaces. The SPMimages of the surface of fibers show that the surface of the HS-PAN (T700SC) and HD-pitch(XN05) fibers are smooth, while the HM-PAN (M60JB) fiber has groove-like features thatare parallel to the fiber axis. The HM-pitch (K13D) fiber exhibits rough texture, whichis mainly because the fiber possesses a sheet-like structure. The HS-PAN (T700SC) andHD-pitch (XN05) CFRPs with a 0◦ section have comparatively smoother surfaces, whileHM-PAN (M60JB) CFRP has a slightly sheet-like or fibrillar structure that is parallel toFibers 2025, 13, 114 9 of 15the fiber axis. The HM-pitch (K13D) CFRP has a sheet-like structure that is parallel to thefiber axis.  (a) (b)   (c) (d) Figure 5. Scanning probe microscope images of PAN-based and pitch-based carbon fibers and CFRPswith longitudinal section: (a) T700SC; (b) M60JB; (c) K13D; and (d) XN05.Figures 6a,b and 7a–c show the change in the Raman scattering parameters (RG, RD,FWHMG, and FWHMD) and ratios (ID/IG, RD/RG, and FWHMD/FWHMG) of PAN-basedand pitch-based carbon fibers and CFRPs with a 0◦ section.Fibers 2025, 13, 114 10 of 15  (a) (b) Figure 6. Change in Raman scattering parameters as a function of tensile modulus of PAN-basedand pitch-based carbon fibers and CFRPs with 0◦, 45◦, and 90◦ sections: (a) RG, RD vs. E; and(b) FWHMG, FWHMD vs. E.  (a) (b)  (c)  Figure 7. Change in Raman scattering ratio as a function of tensile modulus of PAN-based andpitch-based carbon fibers and CFRPs with a 0◦ section: (a) ID/IG vs. E; (b) RD/RG vs. E; and(c) FWHMD/FWHMG vs. E.The changes in RG and RD were less than 1% and were almost the same value. Thechange in FWHMG exhibited an increase with an increase in E, whereas that in FWHMDdemonstrated a decrease. The change in RD/RG was also less than 1% and were almost theFibers 2025, 13, 114 11 of 15same value. The change in ID/IG exhibited a rapid increase with an increase in E, whereas thatin FWHMD/FWHMG demonstrated a decrease. The Raman crystalline ordered or disorderedstructures were more visible in the 0◦ section sample. In addition, these linear trends for thePAN-based and pitch-based carbon fibers also intersected in the range of 400–700 GPa.3.3. Raman Scattering for Anisotropy of Carbon FibersFigure 8a–e shows the intensity ratios ((ID/IG)0, (ID/IG)45, and (ID/IG)90) of PAN-basedand pitch-based CFRPs as a function of θ. The error might appear relatively large due tobeing highlighted. However, in this study, the coefficient of variation in the intensity ratios((ID/IG)0, (ID/IG)45, and (ID/IG)90) were around 10% at most for all samples, indicatingthat the error was considered small.  (a) (b) Intensity ratio(I D/I G) 0, (ID/I G) 45, (I D/I G) 90Angle, θ (deg)0 15 30 45 60 75 9011.71.11.51.21.31.41.6Intensity ratio(I D/I G) 0, (ID/I G) 45, (I D/I G) 90Angle, θ (deg)0 15 30 45 60 75 900.61.60.81.411.2  (c) (d)  (e) Intensity ratio(I D/I G) 0, (ID/I G) 45, (I D/I G) 90Angle, θ (deg)0 15 30 45 60 75 900.510.60.90.70.8Intensity ratio(I D/I G) 0, (ID/I G) 45, (I D/I G) 90Angle, θ (deg)0 15 30 45 60 75 9013.51.5322.5Intensity ratio(I D/I G) 0, (ID/I G) 45, (I D/I G) 90Angle, θ (deg)0 15 30 45 60 75 90023.50.52.511.5T700SCM60JBK13DXN-05Figure 8. The intensity ratios ((ID/IG)0, (ID/IG)45, and (ID/IG)90) of PAN-based and pitch-basedCFRPs: (a) T700SC; (b) M60JB; (c) K13D; (d) XN05; and (e) summary display.Fibers 2025, 13, 114 12 of 15The intensity ratios increased in the order of (ID/IG)0 < (ID/IG)45 < (ID/IG)90 for eachcarbon fiber, and a linear relationship was found between (ID/IG)0, (ID/IG)45, and (ID/IG)90and E, as shown in Figure 4a. These intensity ratio values were higher than the (ID/IG)fiber,as also shown in Figure 4a. The T700SC, M60JB, K13D, and XN05 fibers exhibited signif-icant variation coefficients in the 0◦ direction. Additionally, T300 and K13C fibers alsodemonstrated the most significant disparities between samples in the 0◦ direction. TheT800SC and IMS60 fibers indicated significant disparities in the 90◦ direction. Carbon fiberis a material that exhibits high orientation in the longitudinal direction (0◦ direction) and issensitive to structural changes. It has been hypothesized that local structural variationswere discerned through the application of Raman spectroscopy. In M60JB, K13D, andK13C fibers, which exhibit high orientations and high elastic moduli, the coefficient ofvariation for individual fibers [19] and in the 0◦ direction is substantial. Many defects inthe carbon fiber are created during the manufacturing process of the precursor materialand the subsequent heat treatment. These include fibrillar misalignment, ultra-micro pores,etc. [27,28]. The presence of these defects in the carbon fibers results in higher (ID/IG)0,(ID/IG)45, and (ID/IG)90. Similar results of angle-dependence were observed in the charac-terizing anisotropy of the elastic properties of carbon fibers, although ID/IG exhibited theopposite trend to the angle [10–15].The intensity ratio, (ID/IG)θ , in an arbitrary angle, θ, was calculated using the followingequation (coordinate transformation):(IDIG)θ=(IDIG)0cos4θ +(IDIG)90sin4θ +(IDIG)ACsin2θcos2θ, (1)in which θ is the fiber angle from the measurement plane, as shown in Figure 1e. (ID/IG)ACis the intensity ratio in the (axial cross-section) shear direction and is given by.(IDIG)AC= 4(IDIG)45−(IDIG)0−(IDIG)90, (2)The estimated lines obtained from experimental results were also shown in Figure 8a–e.The experimental results were found to agree with the predictions. Raman intensity ratio(ID/IG)θ of PAN-based and pitch-based CFRPs could be estimated from these equations.The intensity ratios (ID/IG)0 and (ID/IG)90 were calculated using a simple rule ofmixtures.1(IDIG)0=VOS(IDIG)OS+(1 − VOS)(IDIG)DS, (3)(IDIG)90=(IDIG)OSVOS +(IDIG)DS(1 − VOS), (4)in which VOS is the volume fraction of Raman ordered structure. (ID/IG)OS and (ID/IG)DSare the intensity ratios in the Raman ordered and disordered structures. The above correla-tions were verified by evaluating PAN-based and pitch-based fibers separately. Therefore,(ID/IG)OS and (ID/IG)DS were estimated with PAN-based and pitch-based fibers separately(the structure of XN05 HD-pitch and K13D (also K13C) HM-pitch fibers were quite different;(ID/IG)OS and (ID/IG)DS were estimated with HD-pitch and HM-pitch fibers separately).The estimated (ID/IG)OS and (ID/IG)DS of the PAN-based fiber were 0.119 and 1.419, respec-tively. (ID/IG)OS and (ID/IG)DS of the HM-pitch fiber were 0.063 and 0.856, and (ID/IG)OSand (ID/IG)DS of the HD-pitch fiber were 1.076 and 3.547. (ID/IG)OS of the HM-pitch fiberwas the lowest among other fibers, and (ID/IG)DS of the HD-pitch was the highest amongother fibers. The VOS of the fibers are shown in Table 1. The VOS of the HS-PAN andHM-PAN fibers were ≈1% and 6%, respectively. The VOS of the HM-pitch and HD-pitchFibers 2025, 13, 114 13 of 15fibers were ≈1% and 46%. These values strongly depended on the precursor materials offiber and Raman disordered structure. The Raman intensity ratio (ID/IG)θ of PAN-basedand pitch-based CFRPs was also calculated from Equations (1) and (2). The estimated lineswere almost same as the above lines obtained from experimental results.In addition, the intensity ratio (ID/IG)fiber was calculated using the rule of mixtures.1(IDIG)f iber=VOS( f iber)(IDIG)OS+(1 − VOS( f iber))(IDIG)DS(5)in which VOS(fiber) is the volume fraction of Raman ordered structure in fibers. The VOS(fiber)of fibers were also shown in Table 1. The VOS(fiber) of HS-PAN and HM-PAN fibers were≈5% and 24%, respectively. The VOS(fiber) of HM-pitch and HD-pitch fibers were 56%and 62%, respectively. The analytical results were found to agree with the experimentalresults. The Raman anisotropic nature of PAN-based and pitch-based fibers and CFRPswas identified experimentally and analytically.The anisotropic nature of carbon fibers, arising from their highly oriented graphiticmicrostructure, plays a critical role in determining mechanical and thermal properties.Understanding and quantifying this anisotropy is essential for optimizing composite per-formance and design, particularly in load-bearing and directional applications [3]. Ramanspectroscopy, among other techniques, has proven effective in evaluating orientation-dependent structural features, which directly influence mechanical behavior. The insightsgained from this study can be extended to other fiber precursors such as cellulose, lignin,and polyethylene-based fibers, which are being explored for low-cost and sustainable alter-natives to PAN- and pitch-based carbon fibers [5]. Furthermore, the anisotropy analysisframework developed here may be applied to hybrid composites, enabling broader materialoptimization across diverse engineering applications.4. ConclusionsRaman scattering for anisotropy was conducted on commercially available high-strength PAN-based, high-modulus PAN-based, high-modulus pitch-based, and high-ductility pitch-based carbon fiber-reinforced plastic (CFRP) samples. The results are brieflysummarized as follows:1. The Raman scattering parameters and ratios in the CFRPs are related to the tensilemodulus. These linear trends for the PAN-based and pitch-based CFRPs intersect inthe range of 400–700 GPa.2. The change in Raman scattering parameters and ratios of PAN-based and pitch-basedCFRPs are related to the tensile modulus. These linear trends also intersect in therange of 400–700 GPa.3. The intensity ratio in an arbitrary angle could be estimated using the rule of mixturesand coordinate transformation equations.Author Contributions: K.N.: Conceptualization, Methodology, Software, Validation, Formal analysis,Investigation, Resources, Data curation, Writing—original draft, Writing—review and editing, Vi-sualization, Supervision. C.N.: Software, Validation, Formal analysis, Investigation, Data curation,Writing—review and editing, Visualization. All authors have read and agreed to the publishedversion of the manuscript.Funding: This paper is based on results obtained from a future pioneering project commissionedby the New Energy and Industrial Technology Development Organization (NEDO) and innovativescience and technology initiative for security projects (JPJ004596) commissioned by the Acquisition,Technology & Logistics Agency (ATLA).Fibers 2025, 13, 114 14 of 15Data Availability Statement: The datasets supporting the conclusions of this article are includedwithin the article.Conflicts of Interest: The authors declare no potential conflicts of interest with respect to the research,authorship, and/or publication of this article.References1. Chand, S. Review-Carbon fibers for composites. J. Mater. Sci. 2000, 35, 1303–1313. [CrossRef]2. Hamzat, A.K.; Murad, M.S.; Adediran, I.A.; Asmatulu, E. 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[CrossRef]Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individualauthor(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury topeople or property resulting from any ideas, methods, instructions or products referred to in the content.https://doi.org/10.1007/s42452-022-05183-whttps://doi.org/10.1016/j.jmrt.2022.02.134https://doi.org/10.1016/S0008-6223(01)00139-7 Introduction  Materials and Methods  Results and Discussion  Raman Spectrum  Raman Scattering Parameter and Ratio Differences Between Carbon Fibers and CFRPs in Axial (0) Section  Raman Scattering for Anisotropy of Carbon Fibers  Conclusions  References