Highly Ordered and Dense Thermally Conductive Graphitic Films from a Graphene Oxide/Reduced Graphene Oxide Mixture

Abstract

•G-O films are prepared from nematic-phase precursors using the doctor blade method•Films are graphitized at 3,000°C, pressed, and heated again at 3,000°C•Incorporating chemically reduced G-O improves film density and crystallinity•Mechanical press and second heat treatment further improve density and crystallinity Highly oriented graphitic films are used as thermal interface materials in portable electronics. Graphitic films derived from graphene oxide (G-O) could provide advantages, but graphitization of these films results in significant expansion due to heat-induced decomposition of functional groups on G-O. We report methodology that significantly improves the quality of graphitic films derived from G-O. By incorporating a small fraction of “very small”-diameter reduced G-O platelets prior to film casting, we observed less expansion and higher densities after graphitization. Mechanical pressing of these G-O-derived graphitic films can increase density but introduced defects. A subsequent heat treatment improved the crystallinity beyond that prior to mechanical pressing. Combining both approaches yielded a dense, high-quality graphitic film. The addition of reduced G-O could be used in other heat-treated G-O materials where increased density is desirable, e,g., G-O-derived carbon fibers. We report a new approach to making highly dense, oriented, and crystalline graphite films from heat-treated and pressed graphene oxide (G-O). By introducing small-diameter reduced graphene oxide (rG-O) flakes into the graphene oxide starting material, we found that after heat treatment at 3,000°C, the sample density and atomic order substantially improved over a film composed, at the outset, only of pure G-O flakes. A subsequent mechanical press increased the density but reduced the atomic order. A second 3,000°C heat treatment restored the graphitic structure with graphitization metrics exceeding even those of the first heat treatment. The optimized graphitic film with an original concentration of 15 wt % reduced G-O in G-O gave well-oriented graphitic films with a density of 2.1 g cm−3, cross-plane thermal conductivity of 5.65 W m−1 K−1, and in-plane thermal conductivity of 2,025 ± 25 W m−1 K−1. We report a new approach to making highly dense, oriented, and crystalline graphite films from heat-treated and pressed graphene oxide (G-O). By introducing small-diameter reduced graphene oxide (rG-O) flakes into the graphene oxide starting material, we found that after heat treatment at 3,000°C, the sample density and atomic order substantially improved over a film composed, at the outset, only of pure G-O flakes. A subsequent mechanical press increased the density but reduced the atomic order. A second 3,000°C heat treatment restored the graphitic structure with graphitization metrics exceeding even those of the first heat treatment. The optimized graphitic film with an original concentration of 15 wt % reduced G-O in G-O gave well-oriented graphitic films with a density of 2.1 g cm−3, cross-plane thermal conductivity of 5.65 W m−1 K−1, and in-plane thermal conductivity of 2,025 ± 25 W m−1 K−1. Space constraints in miniature devices do not allow for large heatsinks, thus necessitating thin, highly conductive, and flexible heat spreaders for managing the thermal dissipation effectively. A single layer of suspended graphene has one of the highest reported thermal conductivities exceeding 5,000 W m−1 K−11Balandin A.A. Ghosh S. Bao W. Calizo I. Teweldebrhan D. Miao F. Lau C.N. Superior thermal conductivity of single-layer graphene.Nano Lett. 2008; 8: 902-907Crossref PubMed Scopus (9927) Google Scholar,2Ghosh d. Calizo I. Teweldebrhan D. Pokatilov E.P. Nika D.L. Balandin A.A. Bao W. Miao F. Lau C.N. Extremely high thermal conductivity of graphene: prospects for thermal management applications in nanoelectronic circuits.Appl. Phys. Lett. 2008; 92: 151911Crossref Scopus (1571) Google Scholar but is not practical to use as a heatsink material. Current industrial processes for the production of flexible heat spreaders rely on flexible pyrolytic graphite or graphitized polyimide having in-plane thermal conductivity of ca. 1,950 W m−1 K−1 for the best samples.3Panasonic CorporationPGS Graphite Sheets.https://industrial.panasonic.com/cdbs/www-data/pdf/AYA0000/AYA0000C27.pdfDate: 2017Google Scholar Graphite oxide is a form of graphite in which the layers of graphite are highly oxidized.4Dreyer D.R. Park S. Bielawski C.W. Ruoff R.S. The chemistry of graphene oxide.Chem. Soc. Rev. 2010; 39: 228-240Crossref PubMed Scopus (8490) Google Scholar The oxygen-containing functional groups on the basal plane and edges allow stable and high-concentration dispersions of graphene oxide (G-O) in water and many other polar solvents.5Cote L.J. Kim J. Tung V.C. Luo J. Kim F. Huang J. Graphene oxide as surfactant sheets.Pure Appl. Chem. 2010; 83: 95-110Crossref Scopus (328) Google Scholar, 6Paredes J. Villar-Rodil S. Martínez-Alonso A. Tascon J. Graphene oxide dispersions in organic solvents.Langmuir. 2008; 24: 10560-10564Crossref PubMed Scopus (2168) Google Scholar, 7Akbari A. Sheath P. Martin S.T. Shinde D.B. Shaibani M. Banerjee P.C. Tkacz R. Bhattacharyya D. Majumder M. Large-area graphene-based nanofiltration membranes by shear alignment of discotic nematic liquid crystals of graphene oxide.Nat. Commun. 2016; 7: 10891Crossref PubMed Scopus (346) Google Scholar This liquid dispersibility allows for thin films (“G-O paper”) to be made by various techniques including vacuum filtration,8Rozada R. Paredes J.I. Villar-Rodil S. Martínez-Alonso A. Tascón J.M. Towards full repair of defects in reduced graphene oxide films by two-step graphitization.Nano Res. 2013; 6: 216-233Crossref Scopus (149) Google Scholar,9Dikin D.A. Stankovich S. Zimney E.J. Piner R.D. Dommett G.H. Evmenenko G. Nguyen S.T. Ruoff R.S. Preparation and characterization of graphene oxide paper.Nature. 2007; 448: 457Crossref PubMed Scopus (4477) Google Scholar spray deposition,10Pham V.H. Cuong T.V. Hur S.H. Shin E.W. Kim J.S. Chung J.S. Kim E.J. Fast and simple fabrication of a large transparent chemically-converted graphene film by spray-coating.Carbon. 2010; 48: 1945-1951Crossref Scopus (257) Google Scholar,11Xin G. Sun H. Hu T. Fard H.R. Sun X. Koratkar N. Borca-Tasciuc T. Lian J. 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Tkacz R. Bhattacharyya D. Majumder M. Large-area graphene-based nanofiltration membranes by shear alignment of discotic nematic liquid crystals of graphene oxide.Nat. Commun. 2016; 7: 10891Crossref PubMed Scopus (346) Google Scholar The oxidation of graphite disrupts the extended network of sp2 hybridized carbon and results in very poor thermal and electrical conductivities.15Mu X. Wu X. Zhang T. Go D.B. Luo T. Thermal transport in graphene oxide–from ballistic extreme to amorphous limit.Sci. Rep. 2014; 4: 3909Crossref PubMed Scopus (152) Google Scholar Reduction of G-O can remove these functional groups, but high temperatures are necessary to restore the graphitic sp2 structure.8Rozada R. Paredes J.I. Villar-Rodil S. Martínez-Alonso A. Tascón J.M. Towards full repair of defects in reduced graphene oxide films by two-step graphitization.Nano Res. 2013; 6: 216-233Crossref Scopus (149) Google Scholar When stacked/overlapped G-O (including G-O paper) is thermally reduced, interstitial water, as well as oxygen-containing functional groups in the G-O material itself, are released in gaseous form, resulting in significant expansion of the film due to the voids created in the film.16McAllister M.J. Li J.-L. Adamson D.H. Schniepp H.C. Abdala A.A. Liu J. Herrera-Alonso M. Milius D.L. Car R. Prud'homme R.K. Aksay I.A. Single sheet functionalized graphene by oxidation and thermal expansion of graphite.Chem. Mater. 2007; 19: 4396-4404Crossref Scopus (2927) Google Scholar This can be desirable for some applications, such as electrochemical energy storage, but not for thermal management.17Wei X.H. Liu L. Zhang J.X. Shi J.L. Guo Q.G. Mechanical, electrical, thermal performances and structure characteristics of flexible graphite sheets.J. Mater. Sci. 2010; 45: 2449-2455Crossref Scopus (46) Google Scholar The density of graphitic films can be increased by heating under pressure18Chen X. Deng X. Kim N.Y. Wang Y. Huang Y. Peng L. Huang M. Zhang X. Chen X. Luo D. Graphitization of graphene oxide films under pressure.Carbon. 2018; 132: 294-303Crossref Scopus (35) Google Scholar or the application of pressure after heating,11Xin G. Sun H. Hu T. Fard H.R. Sun X. Koratkar N. Borca-Tasciuc T. Lian J. Large-area freestanding graphene paper for superior thermal management.Adv. Mater. 2014; 26: 4521-4526Crossref PubMed Scopus (282) Google Scholar,19Huang Y. Gong Q. Zhang Q. Shao Y. Wang J. Jiang Y. Zhao M. Zhuang D. Liang J. Fabrication and molecular dynamics analyses of highly thermal conductive reduced graphene oxide films at ultra-high temperatures.Nanoscale. 2017; 9: 2340-2347Crossref PubMed Google Scholar,20Peng L. Xu Z. Liu Z. Guo Y. Li P. Gao C. Ultrahigh thermal conductive yet superflexible graphene films.Adv. Mater. 2017; 29: 1700589Crossref Scopus (238) Google Scholar but reported densities of films made in this way are limited to about 2.0 g cm−3, some 10% lower than the single-crystal density of graphite (2.266 g cm−3). The reported thermal conductivities of the pressed graphitic films made from G-O precursors vary widely, from below 1,000 W m−1 K−1 to 1,940 W m−1 K−1 (an extensive literature survey of the properties of graphitic films derived from G-O is presented in Table S5). Here, we report a method to fabricate highly dense, oriented, and thermally conductive graphitic films from G-O by high-temperature treatment of a shear-aligned nematic mixture of G-O flakes and ultrasmall-diameter reduced G-O flakes. Graphitizable precursors including polymers21Li H. Dai S. Miao J. Wu X. Chandrasekharan N. Qiu H. Yang J. Enhanced thermal conductivity of graphene/polyimide hybrid film via a novel “molecular welding” strategy.Carbon. 2018; 126: 319-327Crossref Scopus (54) Google Scholar,22Wang K. Li M. Zhang J. Lu H. Polyacrylonitrile coupled graphite oxide film with improved heat dissipation ability.Carbon. 2019; 144: 249-258Crossref Scopus (11) Google Scholar and G-O23Xin G. Yao T. Sun H. Scott S.M. Shao D. Wang G. Lian J. Highly thermally conductive and mechanically strong graphene fibers.Science. 2015; 349: 1083-1087Crossref PubMed Scopus (419) Google Scholar have been used for densification of G-O-derived films and fibers but our addition of small-diameter reduced G-O prevents the G-O film from large expansion during the heat-treatment processes, increasing the density and improving the crystallographic texture of the resulting graphitic film. The details of our approach for the preparation of graphitic films are depicted in Figure 1 (experimental information can be found in Supplemental Information). Two precursors were prepared for film fabrication: (1) a highly viscous nematic-phase gel of G-O and (2) a dispersion of small-diameter reduced G-O flakes (hereafter referred to as rG-O) that was prepared by ultrasonication of commercial rG-O flake material. The ultrasonicated rG-O was dispersed in the nematic-phase G-O gel by mechanical grinding at concentrations from 0 to 25 wt %. Films were prepared using shear alignment by a doctor blade7Akbari A. Sheath P. Martin S.T. Shinde D.B. Shaibani M. Banerjee P.C. Tkacz R. Bhattacharyya D. Majumder M. Large-area graphene-based nanofiltration membranes by shear alignment of discotic nematic liquid crystals of graphene oxide.Nat. Commun. 2016; 7: 10891Crossref PubMed Scopus (346) Google Scholar to create films with G-O flakes aligned highly parallel to the film.7Akbari A. Sheath P. Martin S.T. Shinde D.B. Shaibani M. Banerjee P.C. Tkacz R. Bhattacharyya D. Majumder M. Large-area graphene-based nanofiltration membranes by shear alignment of discotic nematic liquid crystals of graphene oxide.Nat. Commun. 2016; 7: 10891Crossref PubMed Scopus (346) Google Scholar,24Shaibani M. Akbari A. Sheath P. Easton C.D. Banerjee P.C. Konstas K. Fakhfouri A. Barghamadi M. Musameh M.M. Best A.S. Suppressed polysulfide crossover in Li–S batteries through a high-flux graphene oxide membrane supported on a sulfur cathode.ACS Nano. 2016; 10: 7768-7779Crossref PubMed Scopus (117) Google Scholar,25Akbari A. Meragawi S.E. Martin S.T. Corry B. Shamsaei E. Easton C.D. Bhattacharyya D. Majumder M. Solvent transport behavior of shear aligned graphene oxide membranes and implications in organic solvent nanofiltration.ACS Appl. Mater. Interfaces. 2018; 10: 2067-2074Crossref PubMed Scopus (36) Google Scholar Because the oxygen present in G-O releases significant gas during heat treatment,8Rozada R. Paredes J.I. Villar-Rodil S. Martínez-Alonso A. Tascón J.M. Towards full repair of defects in reduced graphene oxide films by two-step graphitization.Nano Res. 2013; 6: 216-233Crossref Scopus (149) Google Scholar,26Cunning B.V. Wang B. Shin T.J. Ruoff R.S. Structure-directing effect of single crystal graphene film on polymer carbonization and graphitization.Mater. Horiz. 2019; 6https://doi.org/10.1039/C8MH01507DCrossref Google Scholar the films were first “reduced” (i.e., deoxygenated) by HI (aq) treatment prior to the first heating step; this reduces the gas-induced expansion during heat treatment. The films were then heated to 3,000°C in stages. Because the density of high-temperature-treated G-O films is significantly lower than that of graphite, compression of the films after heat treatment11Xin G. Sun H. Hu T. Fard H.R. Sun X. Koratkar N. Borca-Tasciuc T. Lian J. Large-area freestanding graphene paper for superior thermal management.Adv. Mater. 2014; 26: 4521-4526Crossref PubMed Scopus (282) Google Scholar,19Huang Y. Gong Q. Zhang Q. Shao Y. Wang J. Jiang Y. Zhao M. Zhuang D. Liang J. Fabrication and molecular dynamics analyses of highly thermal conductive reduced graphene oxide films at ultra-high temperatures.Nanoscale. 2017; 9: 2340-2347Crossref PubMed Google Scholar,20Peng L. Xu Z. Liu Z. Guo Y. Li P. Gao C. Ultrahigh thermal conductive yet superflexible graphene films.Adv. Mater. 2017; 29: 1700589Crossref Scopus (238) Google Scholar or during heat treatment18Chen X. Deng X. Kim N.Y. Wang Y. Huang Y. Peng L. Huang M. Zhang X. Chen X. Luo D. Graphitization of graphene oxide films under pressure.Carbon. 2018; 132: 294-303Crossref Scopus (35) Google Scholar is an effective way of increasing their density, although to the best of our knowledge all studies on G-O samples have used static pressing. Static pressing is not ideal for large-scale production, so we have used a rolling press that is more suitable for incorporation as part of a production line. Because the mechanical treatment of graphite is known to introduce defects in the lattice,27Huang J. HRTEM and EELS studies of defects structure and amorphous-like graphite induced by ball-milling.Acta Mater. 1999; 47: 1801-1808Crossref Scopus (105) Google Scholar we subjected the samples to a second high-temperature treatment at 3,000°C in an attempt to restore crystallographic order. We use the following nomenclature for describing the samples in this article. In rG-O_X-YC, X indicates the wt % of small-diameter rG-O flakes added to the nematic-phase G-O during film preparation, and YC describes the temperature of the heat treatment. “As prep.” and “HI” are used to indicate samples after doctor blading and HI reduction, respectively. As an example, rG-O_20-2000C indicates the sample containing 20 wt % of rG-O and that the final treatment stage was heating at 2,000°C. Furthermore, we introduce YC_P (P for “pressed”) to describe the samples after rolling-mill treatment (performed after the first 3,000°C heat treatment), and YC_PH to describe samples after the second 3,000°C heat treatment. The evolution of G-O films in response to reduction and heat treatment has been extensively reported,11Xin G. Sun H. Hu T. Fard H.R. Sun X. Koratkar N. Borca-Tasciuc T. Lian J. Large-area freestanding graphene paper for superior thermal management.Adv. Mater. 2014; 26: 4521-4526Crossref PubMed Scopus (282) Google Scholar,18Chen X. Deng X. Kim N.Y. Wang Y. Huang Y. Peng L. Huang M. Zhang X. Chen X. Luo D. Graphitization of graphene oxide films under pressure.Carbon. 2018; 132: 294-303Crossref Scopus (35) Google Scholar, 19Huang Y. Gong Q. Zhang Q. Shao Y. Wang J. Jiang Y. Zhao M. Zhuang D. Liang J. Fabrication and molecular dynamics analyses of highly thermal conductive reduced graphene oxide films at ultra-high temperatures.Nanoscale. 2017; 9: 2340-2347Crossref PubMed Google Scholar, 20Peng L. Xu Z. Liu Z. Guo Y. Li P. Gao C. Ultrahigh thermal conductive yet superflexible graphene films.Adv. Mater. 2017; 29: 1700589Crossref Scopus (238) Google Scholar and the characterization of our films after reduction and the heat treatments is documented extensively in Supplemental Information. In this paper we focus on the effect of the incorporation of rG-O in the film and our mechanical and subsequent heat treatment on the morphology and graphitization of the film. To measure the effect of our treatment on the atomic order of our films, we used X-ray diffraction (XRD) to measure the mean domain size in the (002) direction as a metric for through-plane order (see Supplemental Information for details). Because of the strong texture of our films and the Bragg-Brentano geometry of our X-ray diffractometer, we could not probe the in-plane order of our films with XRD. Instead, we used the full width at half-maximum (FWHM) of the Raman G band as a metric for in-plane order.28Kudin K.N. Ozbas B. Schniepp H.C. Prud'Homme R.K. Aksay I.A. Car R. Raman spectra of graphite oxide and functionalized graphene sheets.Nano Lett. 2008; 8: 36-41Crossref PubMed Scopus (3470) Google Scholar For XRD analysis, a higher value for the mean domain size indicates higher order, while for Raman analysis, a smaller FWHM indicates higher in-plane order. Mean domain sizes in the (002) direction and Raman G FWHM for samples 3000C, 3000C_P, and 3000C_PH are detailed in Figures 2A and 2B . From measurements of both in-plane and through-plane graphitization metrics, we observed a clear trend in the improvement of graphitization metrics by the addition of rG-O for samples subjected to 3,000°C heat treatment but only up to the addition of 15 wt %. Additions larger than 15 wt % resulted in metrics that were worse than the samples without the rG-O addition. The Raman and XRD results also demonstrated that the rolling press treatment (3000C_P) reduced both in-plane and cross-plane atomic order. However, the second high-temperature heat treatment after rolling improved the atomic order beyond that seen before rolling. We also measured all of the 3000C_PH samples under tensile loading (Figure 2C) and found the tensile strength also improved with increasing rG-O content, consistent with the trends from XRD and Raman analysis. To investigate the possible cause for the lower graphitization metrics in the samples that contained rG-O additions higher that 15 wt %, we compared the As prep. films using a polarized light microscope (Figure S4). We observed for rG-O_0 and rG-O_15 that the G-O sheets are aligned parallel to the film surface; however, at higher concentrations they are randomly oriented. Because graphite contains parallel planes of carbon atoms, we postulate that having G-O sheets initially oriented almost parallel to the surface rather than at random angles allows for improved graphitization. From the results presented in Figures 2A–2C, it was clear that 15 wt % (rG-O_15) is the optimum addition of rG-O for high-quality graphitic films. To emphasize the comparison between the optimum rG-O addition and pure G-O films, we focused our comparisons on the rG-O_0 and rG-O_15 samples. In Figures 2D and 2E, we compare the tensile strengths and sheet resistances of rG-O_0 and rG-O_15 at all stages of film production. For rG-O_0 films, strong interlayer bonding in pure G-O results in high tensile strengths, including in chemically reduced (HI) and low-heat-treatment rG-O (1000C). At a heat-treatment temperature at 2,000°C, graphitizable carbons undergo significant in-plane atomic reorganization forming planar (turbostatic) sp2 hybridized carbon and removing non-carbon atoms8Rozada R. Paredes J.I. Villar-Rodil S. Martínez-Alonso A. Tascón J.M. Towards full repair of defects in reduced graphene oxide films by two-step graphitization.Nano Res. 2013; 6: 216-233Crossref Scopus (149) Google Scholar,11Xin G. Sun H. Hu T. Fard H.R. Sun X. Koratkar N. Borca-Tasciuc T. Lian J. Large-area freestanding graphene paper for superior thermal management.Adv. Mater. 2014; 26: 4521-4526Crossref PubMed Scopus (282) Google Scholar,18Chen X. Deng X. Kim N.Y. Wang Y. Huang Y. Peng L. Huang M. Zhang X. Chen X. Luo D. Graphitization of graphene oxide films under pressure.Carbon. 2018; 132: 294-303Crossref Scopus (35) Google Scholar (see elemental analysis in Table S3). As a result, the tensile strength of pure G-O films reduces significantly when heated to 2,000°C.11Xin G. Sun H. Hu T. Fard H.R. Sun X. Koratkar N. Borca-Tasciuc T. Lian J. Large-area freestanding graphene paper for superior thermal management.Adv. Mater. 2014; 26: 4521-4526Crossref PubMed Scopus (282) Google Scholar,23Xin G. Yao T. Sun H. Scott S.M. Shao D. Wang G. Lian J. Highly thermally conductive and mechanically strong graphene fibers.Science. 2015; 349: 1083-1087Crossref PubMed Scopus (419) Google Scholar In the case of rG-O_15 films, the incorporation of rG-O disrupted the interlayer bonding and reduced film strength. As a result, we observed a significant decrease in the strengths for As prep., HI, and 1000C but the improvement in sample quality due to the improved graphitization metrics results in higher tensile strengths than pure G-O films for treatment temperatures higher than 2,000°C. The addition of rG-O causes the sheet resistance to decrease for As prep. samples. We observed lower sheet resistance values for the rG-O_15 samples for all treatment conditions except at 3,000°C. To examine the effect of our treatments and rG-O incorporation on film morphology, we performed gas pycnometry density measurements and cross-section imaging using both scanning and transmission electron microscopy (SEM and TEM). We found that cross-section preparation by “cryogenic cracking” could not accurately reproduce the true structure of the film interior (see Supplemental Information for cross-sections prepared by cryogenic cracking and related discussion), and therefore prepared the cross-section samples by a focused ion beam (FIB). The densities of As prep., 3000C, and 3000C_PH are shown in Figure 3A. For As prep. films, we observed that the addition of rG-O slightly decreased the density with increasing rG-O content. After the sample had been heat treated to 3,000°C the sample density decreased significantly, and was much lower than the theoretical density of graphite (≈2.26 g cm−3). The density ranged from 0.3 g cm−3 for rG-O_0 to 1.5 g cm−3 for rG-O_15. The change of density with rG-O content matched the trends seen in the graphitization metrics of Figure 2. This significant difference in density is highlighted in the cross-sectional images of rG-O_0-3000C and rG-O_15-3000C in Figures 3B and 3C. Despite all As prep. films having almost the same thickness, we observed that the cross-section of rG-O_0-3000C is clearly larger than that of rG-O_15-3000C (Figures 3B and 3C) due to its significant porosity. The incorporation of the rG-O clearly has a positive effect on sample density prior to mechanical pressing, which we attribute to two reasons. First, because the rG-O is already reduced it does not form gaseous decomposition products when the film is heated; there is therefore less gas generated when compared with the pure G-O film. Second, the presence of rG-O causes a reduction in the density of the As prep. samples (Figure 3A), and the lower density may provide channels for gas escape from the film interior during heat treatment. Compression of the films followed by a second heat treatment resulted in high-density graphitic films, up to 2.1 g cm−3 for rG-O_15. The SEM cross-sections of rG-O_0- and rG-O_15-3000C_PH are shown in Figures 3D and 3E (other cross-sections can be seen in Figures S14–S20). Small voids were still visible in the rG-O_0 sample but none were visible in the rG-O_15 sample. We also imaged these cross-sections under TEM, detailed in Figure 4. Low-resolution imaging shows the crystallographic texture of the (002) planes from contrast differences that arise from regions of differing electron transparency (due to sample porosity). For the rG-O_0 sample, the regions through the sample thickness that are more electron transparent are not parallel with the film surface, whereas in the rG-O_15 sample they lie almost parallel to the film surface. High-resolution imaging of the (002) lattice planes shows similar behavior. We observed numerous regions in the rG-O_0-3000C_PH sample where (002) planes were found to intersect and then diverge. In contrast, the rG-O_15-3000C_PH sample had (002) planes parallel in most regions of the sample. We attribute this observation to the large difference in thermally induced film expansion and void creation experienced by the samples (Figures 3B and 3C). From our results, the rG-O_15 samples result in denser and well-aligned graphitic films. We attempted to characterize the thermal properties of the rG-O_0- and rG-O_15-3000C_PH samples. Our samples were too thin to obtain a direct experimental measurement of the in-plane thermal conductivity, so we used the optothermal Raman technique in conjunction with finite-element analysis of the laser-induced heat flow (using COMSOL Multiphysics software; see Supplemental Information for detailed analysis) to determine the sample in-plane thermal conductivity.29Malekpour H. Balandin A.A. Raman-based technique for measuring thermal conductivity of graphene and related materials.J. Raman Spectrosc. 2018; 49: 106-120Crossref Scopus (60) Google Scholar Because the thermal conductivity for oriented graphite samples is anisotropic, we required a cross-plane thermal conductivity value to accurately model the heat flow. Using the 3ω method,30Cahill D.G. Thermal conductivity measurement from 30 to 750 K: the 3ω method.Rev. Sci. Instr. 1990; 61: 802-808Crossref Scopus (1480) Google Scholar we extracted a value of 5.65 W m−1 K−1 for rG-O_15-3000C_PH. In the case of the rG-O_0 sample, the surface roughness was too high for us to be able to accurately measure cross-plane thermal conductivity, and we analyzed only the thermal conductivity of rG-O15-3000C_PH. Finite-element analysis gave an in-plane thermal conductivity of 2025 ± 25 W m−1 K−1 (Figure 5A; a range is presented due to uncertainties in the laser spot size). We also compared the heat-flow characteristics of these samples by bonding thin strips (5 × 40 mm) to a heating element held at 95°C and imaging with an infrared camera (Figure 5B). The lateral heat transfer is higher for the rG-O_15-3000C_PH film. Here, we have described a new approach for creating highly oriented graphitic films by combining G-O with small-diameter rG-O flakes. Films were cast using the doctor blade method followed by heat treatment, roller pressing, and a second heat treatment. By comparing different percentages of rG-O additions, we have determined that the optimal amount of rG-O was 15 wt %. Reduction and heat treatment significantly expanded the film thickness due to the release of gas from the G-O, but the addition of rG-O significantly reduced the film expansion during heating. Room-temperature rolling of these films after high-temperature treatment increased the film density but also increased atomic disorder. A final high-temperature treatment was crucial in achieving highly oriented graphitic films with large in-plane and cross-plane grain sizes. The thermal conductivity for our optimized sample exceeded 2000 W m−1 K−1. Furthermore, our experimental approach is adaptable to large-scale preparation techniques: doctor blading, roll pressing, and heat treatment are well-established industrial processing techniques. For full details, please refer to Supplemental Information. The authors would like to thank Dr. Fariborz Kargar for helpful discussions. This work was supported by the Institute for Basic Science ( IBS-R019-D1 ). A.A., B.V.C., and R.S.R. conceived the project. A.A. and B.V.C. performed and analyzed a majority of the experiments. B.V.C. and R.S.R. supervised the project. Thermal characterization experiments were performed by S.R.J., B.V.C., and G.-H.K. S.C., V.M., C.C., C.W.B., and P.B. assisted in data acquisition and analysis. R.S.R. secured funding for the project. A.A., B.V.C., and R.S.R. wrote the manuscript with input from all authors. The authors declare no competing interests. Download .pdf (3.23 MB) Help with pdf files Document S1. Supplemental Experimental Procedures, Figures S1–S21, and Tables S1–S5