Materials Research Express PAPER Carbon nanofibers obtained from electrospinning process To cite this article: Juliana Bovi de Oliveira et al 2018 Mater. Res. Express 5 025602   View the article online for updates and enhancements. Related content Development of structurally stable electrospun carbon nanofibers from polyvinyl alcohol Ashish Gupta and Sanjay R Dhakate - Electrical property of macroscopic graphene composite fibers prepared by chemical vapor deposition Haibin Sun, Can Fu, Yanli Gao et al. - Dielectric transition of polyacrylonitrile derived carbon nanofibers Jiangling Li, Shi Su, Lei Zhou et al. - This content was downloaded from IP address 186.217.236.60 on 20/05/2019 at 18:03 https://doi.org/10.1088/2053-1591/aaa467 http://iopscience.iop.org/article/10.1088/2053-1591/aa6a89 http://iopscience.iop.org/article/10.1088/2053-1591/aa6a89 http://iopscience.iop.org/article/10.1088/2053-1591/aa6a89 http://iopscience.iop.org/article/10.1088/1361-6528/aac260 http://iopscience.iop.org/article/10.1088/1361-6528/aac260 http://iopscience.iop.org/article/10.1088/1361-6528/aac260 http://iopscience.iop.org/article/10.1088/2053-1591/1/3/035604 http://iopscience.iop.org/article/10.1088/2053-1591/1/3/035604 https://oasc-eu1.247realmedia.com/5c/iopscience.iop.org/528685444/Middle/IOPP/IOPs-Mid-MREX-pdf/IOPs-Mid-MREX-pdf.jpg/1? Mater. Res. Express 5 (2018) 025602 https://doi.org/10.1088/2053-1591/aaa467 PAPER Carbon nanofibers obtained from electrospinning process Juliana Bovi deOliveira1,4 , LíliaMüllerGuerrini2, Silvia SizukaOishi3 , Luis Rogerio deOliveiraHein1, Luíza dos Santos Conejo1 ,Mirabel Cerqueira Rezende2 andEdsonCocchieri Botelho1 1 Universidade Estadual Paulista (UNESP), Faculty of Engineering,Materials andTechnologyDepartment, Guaratinguetá, Brazil 2 Universidade Federal de São Paulo (UNIFESP), São José dosCampos, Brazil 3 InstitutoNacional de Pesquisas Espaciais, São José dosCampos, Brazil 4 Author towhomany correspondence should be addressed E-mail: juliana_bovi@hotmail.com Keywords: polyacrylonitrile, electrospinning, carbon nanofibers, carbonization Abstract In recent years, reinforcements consisting of carbon nanostructures, such as carbon nanotubes, fullerenes, graphenes, and carbon nanofibers have received significant attention duemainly to their chemical inertness and goodmechanical, electrical and thermal properties. Since carbon nanofibers comprise a continuous reinforcingwith high specific surface area, associatedwith the fact that they can be obtained at a low cost and in a large amount, they have shown to be advantageous compared to traditional carbon nanotubes. Themain objective of this work is the processing of carbon nanofibers, using polyacrylonitrile (PAN) as a precursor, obtained by the electrospinning process via polymer solution, with subsequent use for airspace applications as reinforcement in polymer composites. In this work,firstly PANnanofibers were produced by electrospinningwith diameters in the range of (375±85)nm, using a dimethylformamide solution. Using a furnace, the PANnanofiber was converted into carbon nanofiber.Morphologies and structures of PANand carbon nanofibers were investigated by scanning electronmicroscopy, Raman Spectroscopy, thermogravimetric analyses and differential scanning calorimeter. The resulting residual weight after carbonizationwas approximately 38% inweight, with a diameters reduction of 50%, and the same showed a carbon yield of 25%. From the analysis of the crystalline structure of the carbonizedmaterial, it was found that thematerial presented a disordered structure. 1. Introduction Carbonmaterials present scientific and technological interests, due to their wide applicability in different areas of knowledge. Thesematerials aremostly applied in airspace, automotive,marine,medical and electrochemical fields aswell as in the nuclear industry [1, 2]. Carbonmaterials present advantageous properties, such as, chemical inertia, resistance to themost of common corrosive reagents and stability in a broad range temperatures under inert atmosphere (sublimating at 3900 Kunder atmospheric conditions andmelting temperature at 4800 K). Furthermore, the carbon has a low density in comparisonwithmetals and alloys,making it appropriate for compact and lightweight applications. Thus, among the carbonmaterials nowadays available can be cited carbon foam, carbon nanofibers, carbon nanotubes and carbonmatrix composites [3–5]. As result of their excellentmultifunctional properties and various technological applications, especially in airspace and biomedicalfields, the processing of polymericmaterials onmicrometric and nanometric dimensions come lately, attractingmore attention to specific purposes. Examples in this area are the carbon micro and nanofibers, which have beenwidely studied, because they present excellent thermal, electrical and mechanical properties. Thesefibers have goodmechanical strength, good electrical and thermal conductivities, high temperature resistance, great chemical stability andwide surface area, and yet can bemanufactured easily and in large quantities. The carbonmicro and nanofibers also have the advantage of having low cost compared RECEIVED 3 September 2017 REVISED 19December 2017 ACCEPTED FOR PUBLICATION 29December 2017 PUBLISHED 15 February 2018 © 2018 IOPPublishing Ltd https://doi.org/10.1088/2053-1591/aaa467 https://orcid.org/0000-0003-2460-2930 https://orcid.org/0000-0003-2460-2930 https://orcid.org/0000-0001-5540-3382 https://orcid.org/0000-0001-5540-3382 https://orcid.org/0000-0001-5533-1374 https://orcid.org/0000-0001-5533-1374 mailto:juliana_bovi@hotmail.com http://crossmark.crossref.org/dialog/?doi=10.1088/2053-1591/aaa467&domain=pdf&date_stamp=2018-02-15 http://crossmark.crossref.org/dialog/?doi=10.1088/2053-1591/aaa467&domain=pdf&date_stamp=2018-02-15 with carbon nanotubes. Thus, thesematerialsmay be applied in several areas, including reinforcingmaterials, catalyst supports, high temperaturefilter, in orthopedic implants and others [6–9]. Still in thefield of nanotechnology, which has shown an accelerated development in experimental area, we highlight contributions involving nanocomposites, cosmetics, flame resistantmaterials,materials with electro- optical and antibacterial properties, among others. Basically, the nanoscale is a transition zone between the macro andmolecular levels. Nanocompositematerials appear as suitable alternatives to overcome the limitations ofmicrocomposites. It is found that the interactions at the interfaces of composites are improved when there is the use of nanomaterials in thematrix of composites, with the consequent improving of their properties [10–13]. Inside in this context, the electrospinning via polymer solution has been considered an effective alternative to produce uniform and continuousmicrometric and nanometric fibers. This technique highlights due to its easily of using and experimental condition control. In this process the polymer solution is in a capillary tube which is connected to an electrode and the same is coupled to a source of high voltage that can be positive or negative. Atfirst, the solution displays a format of a drop at the end of the capillary. Then, the drop surface extends to produce a cone (called Taylor cone) as result of the increased voltage. Subsequently, a jet of solution is ejected. In this step, the solvent evaporates and the polymer solidifies, producing afibermat that is deposited on ametallic collector under grounding. During this process, some factorsmay influence themorphology and structure ofmicro and nanofibers produced. Among these themain processing parameter are: the applied voltage, the solutionflow in the capillary, the concentration of polymer solution, and theworking distance. The working distance is defined as the distance between the exit of solution and the collector [14–17]. Among the polymeric precursors used tomanufacture carbon nanofibers through the electrospinning processmay be considered the polyacrylonitrile (PAN), because it present a high carbon yield in the carbonization heat treatment and simple preparation process [6, 18]. This work has as objective the production of carbon nanofibers based on carbonization of PANmat precursor, obtained by electrospinning process of polymer solution. In order to obtain the PANnanofibermat, a study concerning themain variables related to the electrospinning process to obtainmicrometers and nanometers with homogeneous distribution and free of defects was done. The characterization of PANmats considered the thermal behavior (by thermogravimetric and differential scanning calorimetry (DSC) analyses), morphological aspects (by scanning electronmicroscopy, SEM) and crystalline structure (byRaman spectroscopy). 2.Materials andmethods 2.1.Materials The PANused in this workwas supplied by Radici company in powder form. The dimethylformamide solvent (DMF), with concentration�99%,was supplied byMallinckrodt Chemicals company. 2.2. The solution preparation A solution containing 4%, 5% and 6% (wt/wt) of PAN inDMFwas prepared. The homogenization of this solutionwas guaranteed by constant agitation for 3 h, at room temperature. 2.3. Electrospinning The electrospinning systemused in this work (figure 1) consists of a high voltage source (0–30 kV) the Faísca brand, a cylinder underground (collector) that is rotated by a gearmotor the Tekno brand,MRT910model (0–300 rpm) and aTekno brandCVET2002model controller, a glass syringewith 20 ml that has a stainless steel fitting and a stainless needleHamilton type. To control the humidity during this process was used a dehumidifier Thermomatic brand,Desidrat Plusmodel and thermohygrometer watchMinipa brandMT-241model. To quantify the cylinder speedwas used a tachometerMinipa brand andMDT2238Amodel. The parameters used in the processing of PANblankets by electrospinningwere: a stainless steel needle of 10 mm−1 long and 1.5 mmdiameter, a cylinder rotation around 24 rpm, applied voltage range of 18.8–27.1 kV, working distance of 10 cm, humidity range of 35%–48%, room temperature range of 29.4–37.5 °Cand collection time of 1, 2, 6 and 12 h. These parameters were stablished in a previous work of the research group [19]. The equipment used in the treatment of themats does not have a control of solutionflow. The exit of the solution of the needle tip is controlled by gravity (9.8 m s−2). 2 Mater. Res. Express 5 (2018) 025602 J B deOliveira et al 2.3.1. PAN characterization The thermogravimetric analyses (TGA)were conducted in a SEIKOSIINanotechnology equipment, EXSTAR 6000 (TG/DTG6200)model, using about 8 mg of PANmat in a platinumpan, at a heating rate of 15 °Cmin−1, under a nitrogen flowof 100 ml min−1, in the temperature range of 30–1000 °C. TheDSC analyses were performed in a TA Instruments equipment, Q20 series, with controlled nitrogen flowof 40 ml min−1, using approximately 5.7 mg of PANmat, and the heating rate of 10 °Cmin−1, in the temperature range of 25–400 °C. Themorphological analyses of nanofibers were performed in a scanning electronmicroscope of ZEISS, EVO|LS15model, equippedwithOXFORD link formicroanalysis and tungsten filament. The analysis conditionswere electron beamwith a resolution of 20 μm, 10 kV and vacuumof 10−3 Pa. Themats were cut, pasted into a double-sided tape of carbon andmetallizedwith gold. The Image J programwas used for the image analyses. 2.4. Carbonization and characterization of carbonfibers The PANfibers obtained from the electrospinning process were carbonized in an electric furnacewith cylindrical chamber of quartz, under constant flowof nitrogen. During this step, it was used the heating rate of 1 °Cmin−1, starting from ambient temperature up to 1000 °C, remaining 1 h at themaximum temperature. A stainless steel screen 304 of 100mesh, diameter of wire of 0.10 and 0.154 mmopeningwas used towrap and protect the nanofibers to avoid that they do notflewduring the carbonization process. 2.4.1. Characterizationmethod of the carbonizedmaterial Morphological analyses of carbon nanofibers was held in a scanning electronmicroscope of ZEISS,model EVO| LS15with tungsten filament. Just the samewas used in the PANmats characterization. The carbonmats were glued on a double-sided tape of carbon andmetallizedwith gold. The image analyses were also performed using the Image J program. In this researchwork, analyses of the carbonmats by Raman spectroscopywas also performed on an optical microscope Renishaw 2000, with laser at 514.5 nm.Calibrationwas previously performedwith diamond. 3. Results and discussion 3.1. PANnanofibers produced by eletrospinning In this work 3 processing parameters of the electrospinning process were evaluated for production of the nanofibers: solution concentration, voltage and processing time. 3.1.1. Solution concentration The solution concentration used to produce nanofibers were 4%, 5% and 6% (wt/wt). Figure 2 shows themats based on PANfibers produced by electrospinning process, using PANconcentrations of 4%, 5% and 6% (wt/ wt), processed under the conditions described in item 2.3. Concerning about the use of 4%wt/wt PAN/DMF contents was necessary to raise the solution voltage to 27.1 kV for the production of PAN fiber jet in the needle. This problemoccurs due to the low concentration of polymer in solution and consequently its low viscosity. Figure 1. Schematic diagramof the electrospinning systemused. 3 Mater. Res. Express 5 (2018) 025602 J B deOliveira et al After performing the electrospinning process, it wasmeasured the thickness ofmats, using amicrometer device. Themats based on 4%, 5% and 6% (wt/wt) solutions presented, respectively, the following average thickness values: (0.069±0.004)mm; (0.050±0.002)mmand (0.066±0.004)mm. It was observed that the thickness ofmats producedwith the 5% (wt/wt)PAN/DMF solution presented lower valueswhen compared with the others, but all values are similar. However, it was verified that the process that used 4% (wt/wt) solution consumed twicemore than themat obtained from electrospinning process using 6% (wt/wt)PAN solution, keeping the same final thickness. Thus, in order to promote the PANmat productionwith the solution containing 4% (wt/wt) of PAN, high voltages andmore solutionwere required, being a disadvantage. Considering these arguments, in this work themats based on PAN fibers were producedwith 6% (wt/wt)PAN/ DMF solution. 3.1.2. Voltage variation in the electrospinning process In order to produce PAN fibers with smaller diameters, in the electrospinning process were tested the voltages of 18.8, 20.4 and 21.8 kV. Applying the voltages previously cited and keeping constant the other parameters (concentration of solution in 6% (wt/wt); drum rotation of 24 rpm,work distance of 10 cm and time collection in 2 h, humidity of 35%; and temperature of 37.5 °C it was possible to obtain PANnanofibers as depicted in figures 3–5. Thesefigures show themorphologies offibers and histograms of distribution of frequency as a function offiber diameter. Figure 2.PANmats obtainedwith PAN/DMF solutions of: (a) 4%, (b) 5%and (c) 6% (wt/wt). Figure 3. (a) SEMof PANnanofibers obtained from electrospinning with voltage of 18.8 kV and (b) frequency distribution of the fibers. 4 Mater. Res. Express 5 (2018) 025602 J B deOliveira et al Figures 3(b), 4(b) and 5(b) show the highest frequency of diameters in the range of 300–400 nm for the three different voltages used. According tofigures 3(a), 4(a) and 5(a), themorphologies of the fibers are similar, without defects. The average diametersmeasuredwere (359±49)nm, (359±73)nmand (373±82)nm for fibers obtained at 18.8 kV, 20.4 kV and 21.8 kV, respectively. Comparing the diameters of thefibers obtained with different voltages no significant variation is observed. Since the voltage of 21.8 kV remainedmore stable for a longer period of processing time, this value was used in this work. 3.1.3. Processing time The variation of processing time is extremely important because this parameter determines the amount offibers obtained for the subsequent carbonization heat treatment. Thus, with this purpose in this workwas first used PAN/DMF solutionwith concentration of (6%wt/wt) for 1 h, and the parameters used in the electrospinning process were: stainless steel needle of 10 mm length and 1.5 mmdiameter; drum rotation of 24.1 rpm; applied voltage of 21.8 kV and theworking distance of 10 cm; humidity 32% and temperature 33.5 °C.However, when the PANmatwith thickness of (0.041±0.002)mmwas carbonized it was verified no carbon yield in the end of the heat treatment. The same behavior happenedwith themat electrospinned for 2 h, using the same processing parameters cited. Despite greater thickness ((0.066±0.004)mm)when compared to themat electrospinned for 1 h, this thicker sample was not proper for the carbonization yet due probably to the insufficientmass of PAN mat exposed to the heat treatment. Due to this difficulty, it was carried out the electrospinning of PAN/DMF solution of 6% (wt/wt) for 6 h. In this case the thickness reached the value of (0.082±0.003)mm, using the following parameters: applied voltage of 22.2 kV, humidity of 48%and temperature of 31.6 °C. The PANnanofibermat obtained in 6 h of electrospinning, after carbonization, presented a good carbon yield. In order to increase the PANmat production it was used 12 h of electrospinning. In this case, using the applied voltage of 21.8 kV; humidity of 43% and 29.4 °C. In this condition, the thickness of themat produced increased for (0.103±0.003)mm.The increased time of electrospinning process also resulted in an increase in Figure 4. (a) SEMof PANnanofibers obtained from electrospinning with voltage of 20.4 kV and (b) frequency distribution of the fibers. Figure 5. (a) SEMof PANnanofibers obtained from electrospinning with voltage of 21.8 kV (b) and histogramof frequency distribution. 5 Mater. Res. Express 5 (2018) 025602 J B deOliveira et al the scattering of PANnanofibers on the aluminum foil, used as a collector, hindering a significant increase in the finalmaterial thickness. Nevertheless, the fibers produced by this process, after carbonization, resulted in carbonmats. Figure 6 depict, respectively, themorphologies obtained by SEMand the histograms of the distribution frequency of thefibers obtained in the electrospinning of 12 h. As can be observed infigure 6(a), thefiber structure presents few defects such as junctions (indicated by arrows). These defectsmay occurwhen the nanofibers are deposited on the collector, and these one get in contact with each other under the influence of solvent still present, join or coalesce each other. However, it was verified that this fiber generatesmats almost no defects like drops. According to the results observed infigure 6(b), it is seen that the highest frequency of diameters is in the range of 300–400 nm (with average diameters equal to (375±85)nm). According to the literature [20, 21], the distribution of diameters of PANnanofibers, produced by electrospinning, is in the range of 0–2000 nm,with an average diameter between 100 and 280 nm. 3.1.4. Thermal behavior of PANblankets produced by electrospinning process Figure 7 presents the thermal decomposition obtained byTGA for PAN fibers processed from the temperature range of 30–1000 °C, at a heating rate of 15 °Cmin−1, under a nitrogen flowof 100 ml min−1. According to the TGA, the thermal degradation of PAN fiber occurs in at least 4 different stages: 1st stage— presence ofmoisture; 2nd stage—presence of solvent; 3rd stage—degradation of the PAN fiber and 4th stage— gases that are volatilized, as identified in accordancewith theDTG results. Initially, there is aweight loss of 1.2% in the TGA curve, as shown in the derivative curve. This variation occurs at 90 °C, possibly, due to the presence ofmoisture or lowmolecular weight fractions in themat. Then, it is observed a secondweight loss of around 0.9%, that occurs between the temperatures of 150 and 230 °C,whichmay be related to the presence ofDMF (solvent used in the electrospinning process). According to the literature, the boiling temperature ofDMF is Figure 6. (a) SEMof PANnanofibers obtained after 12 h in electrospinning process and (b) histogramof diameter distribution. Figure 7.TGA/DTGof PANfiber. 6 Mater. Res. Express 5 (2018) 025602 J B deOliveira et al equal to 153 °C [22]. In the temperature of 278 °C, begins the degradation of PAN fibers; theDTG curve shows that this stepfinalizes at 312 °C.Theweight loss of PAN starts at 278 °C, and goes until about 1000 °C,where is consumed 59.9%of the PANweight. These losses are attributed to different gases that are volatilized during the decomposition of PAN, such asH2O, CO2, CO, CH4,NH3 andHCN [23, 24]. According toXue et al [25], the formation ofNH3 andHCN is originated from the terminal amino groups of the cyclized structure of the PAN copolymer. In accordancewith the literature, the onset of PANdegradation occurs around 160 °C [26]. Other authors, such as Brito Jr et al [27], reported that was not observed byTGA technique a significant weight loss below 280 °C. TheDSC curves of PAN fibers produced by electrospinning are presented infigure 8. Figure 8(a) shows the thermal behavior of PAN fiber obtained from solution of 6% (wt/wt)processed for 6 h. Figure 8(b) presents the results of PAN fiberwith concentration of 6% (wt/wt) electrospinned for 12 h. According to these curves, the PANpresents a glass transition temperature value (Tg)well defined. The Tg of PAN fiber, obtained from electrospinning process of 6 h, is 103 °C.This value is very close to the Tg value obtained for the PAN fibers from electrospinning process of 12 h, which is 102 °C.This Tg values are found slightly lower than that presented in the literature, i.e., around 125 °C [28]. Both curves observed (figure 8) present a peak of degradation (exotherm)with amaximum temperature of 292 °C (starting in 238 °Cand ending in 329 °C) to themat after electrospinning for 6 h and a peakwith a maximum temperature of 290 °C (with onset in 222 °Cand endset in 323 °C) for themat after electrospinning for 12 h. The degradation values presented in both graphs are close. These values are in accordance with the literature for PAN [29], which reports value of 293 °C and also the value of PANdegradation temperature found infigure 7, which is 297 °C. The enthalpy of degradation reaction for PANobtained from the area contained under the exothermic peak is 827.4 J g−1 (figures 8(a)), and 629.1 J g−1 (figure 8(b)), considering PAN fibers obtained after 6 h and 12 h, respectively, being found in thework of Santos [29] a value lower but closed than verified in this study, equal to 515 J g−1. Based onfigure 8 it was not possible to detect a peak related to themelting of PAN, only the peaks relating to degradation. This is possibly as a consequence of the final degradation reaction occurs at temperatures near to themelting temperature (320–326 °C), asmentioned in the literature [30]. 3.2. Carbonnanofiber The PANmats produced by electrospinning for 6 and 12 hwere carbonized in an electric furnace. After carbonization process, these blankets exhibit black color, and fragile behavior (brittle) andweight loss of approximately 75%. The highweight loss exhibited by carbon nanofiber blanket is justified probably by the non- execution of the thermal stabilization step of PAN. Thermal stabilization typically occurs between the temperature range of 200–300 °C,with themain purpose tomake the PANprecursor fibers stable for the subsequent heat treatment process, preventing themelting of PANduring the carbonization process, as reported in the literature [31]. Figure 9(a) shows themorphology of PANfiber after electrospinning for 12 h and figure 9(b) presents the samemat after carbonization process originating carbon nanofibers, both assessed by SEM.As can be seen from figure 9(a), PANmat has a constitution of well defined and spaced fibers, overlapped in a randomarrangement, forming different planes.However, figure 9(b) depicts amore homogeneous structure generated after the carbonization of the PANfibers into carbon fibers. In other words, in his case it is observed amore compact Figure 8.DSC curve for PAN fiber obtained after electrospinning for: (a) 6 h and (b) 12 h. 7 Mater. Res. Express 5 (2018) 025602 J B deOliveira et al material, not being possible to distinguish onefiber from the other. Figure 10(a)presents the same carbon nanofiber, analyzed in highermagnification. This figure shows the presence offibers separated andwell defined, superposed in several planes. Itmay also be seen that occurred the breakage of some fibers after carbonization and that they presented, visibly, a reduction in their diameters. This break probably happened due to the release of volatilematerials of lowmolarmass. This reduction in diameter is expected due to volumetric shrinkage that the polymericmaterialsmay suffer when they are converted in carbonaceousmaterials, often this shrinkage is of approximately 30%by volume [32]. This change can also be observed in the histogramof diameter distribution of the fibers, according tofigure 10(b). According tofigure 10(b), it was observed that the carbon nanofibers obtained fromPAN fibers exhibitmore homogeneous dimensions, with 65%of thefibers presenting diameters values in the range of 200–300 nm. The average diameter of thesefibers is equal to (186±45)nm. This figure shows also that the carbonized fibers present a decrease of approximately 50% in their diameters. According to the literature this reductionmay be attributed to the heat treatment which provides theweight loss with the release of volatiles such as ammonia, hydrocyanic acid andwater, reducing, thus, the diameter offibers [21]. According toAdabi [21], it is possible to use nanofibers that were processed by electrospinning and subsequently carbonized, such as nanoelectrodes, but to function as a nanoelectrode, thefiber diameters have to be between 75 and 80 nm, because belowor above these values there is a decrease in the conductivity of the carbon fiber, not serving the function in question. t has been the application for composite reinforcement area requires that the blanket hasmade a suitable size so it can be, for example, used in a press with a composite. The carbonized blankets had small dimensions, since the tubular furnace used for the procedure had a diameter of approximately 60 mm. 3.2.1. Raman spectroscopy analysis of carbon nanofiber The carbon element can present different crystalline andmorphological structures with different characteristics andRaman spectroscopy is usually used for qualitative evaluation of the crystallographic ordering concerning about carbonaceousmaterials or to evaluate the effect of heat treatment on cokes or organic precursors. TheD band at∼1360 cm−1 is related to defects such as edge effects, impurities and finite size, corresponding to disordered structure of carbon and reflecting the sp2 vibration of the ring. TheGband at∼1590 cm−1, reflects Figure 9. SEMof (a)PAN fibers obtained after electrospinning; (b)PANfibers after carbonization (carbon nanofibers). Figure 10. (a) SEMof carbon nanofibers and histogramof diameter distribution of carbon nanofibers. 8 Mater. Res. Express 5 (2018) 025602 J B deOliveira et al the degree of graphitization of the studiedmaterial (crystalline and atomic arrangements) [33–35]. Themost important parameter calculatedwith this technique is the ID/IG intensity ratio which is useful to estimate the degree of ordering in carbonaceousmaterial [36]. The second order Raman is related to the stacking disorder along the crystallographic c-axis [37]. Figure 11 shows thefirst and second order Raman spectrumof the carbonizedmaterial which displays two broadD andGbands characteristic of disordered carbon. In addition, the second order region does not show any band development which indicates the heat treatment temperature used (1000 °C) is not enough to improve crystallographic ordering of carbon nanofiber. Table 1 contains the positions of D andGbands and ID/IG intensity ratio obtained from the Raman spectra of the samplemeasured in three different places. According to the data presented in table 1, D andGbands positions correspond to a graphitic disordered system [38] due to the bands around 1360 cm−1 and 1590 cm−1, respectively. The ID/IG intensity ratio is 0.921±0.009, slightly smaller than for carbon nanofibers produced by catalytic thermal chemical vapor deposition (CVD) (ID/IG=1.35) [36]. Since ID/IG ratio decreases as sample disorder decreases, then, the produced nanofibers aremore ordered than the as-produced nanofibers byCVDmethod. According toRen et al [39], themicrostructure of carbon fibers changes along the axial direction indicating the surface heterogeneity. Moreover, the amount of amorphous carbon varied significantly in different regions. In this work, the low standard deviation shows the nanofibers are quite homogeneous, probably related to the reduced diameter of thefibers. 4. Final conclusions The polyacrylonitrilematwas produced by electrospinning generating nanofiberwith an average diameter of (375±85)nm. Themore appropriate processing conditionswere: applied voltage of 21.8 kV, distance working of 10 cm and rotation of the cylinder of 24.7 rpm, PAN/DMFconcentration solution of 6%m/mand time of material collection of 6 and 12 h. The thermal degradation of the PANnanofibers obtained from electrospinning process occurred in four steps and the residualmass that remained after this degradationwas 38%,when analyzed in an inert atmosphere. Figure 11. First and second order Raman spectrumof carbonizedmaterial. Table 1.DandGbands positions and ID/IG intensity ratio of the carbonized nanofiber (measured in three different places). Measure D band (cm−1) Gband (cm−1) ID/IG 1 1361.1 1587.2 0.918 2 1359.5 1590.3 0.930 3 1358.1 1590.3 0.914 Average 1359.6±1.5 1589.3±1.8 0.921±0.009 9 Mater. Res. Express 5 (2018) 025602 J B deOliveira et al InDSC analysis were found the glass transition temperature de values of 103 and 102 °C respectively for the PAN nanofibers obtained by electrospinning considering 6 and 12 h. The carbon nanofibermatwith a diameter of (186±45)nmwas successfully obtained from the carbonization of the PANnanofiber produced by electrospinning process. After carbonization, there occurred a reduction of 50% in the carbon nanofiber diameter in relation to the diameters of themat before carbonization. The aspect of themat observedwas brittle andwith black color, and the same presented amass yield of 25% when comparedwith the no carbonizedmat. Themore appropriate conditions obtained in the carbonization process were: heating rate of 1 °Cmin−1, starting from room temperature up to 1000 °C, remaining 1 h at the final temperature selected under a constant flowof nitrogen; Raman spectroscopy analysis showed thematerial can be classified as disordered carbon and is quite homogeneous due to the low standard deviation for ID/IG ratiowhich indicated the good structural quality of the carbon nanofibers. Acknowledgments The authors acknowledgefinancial support received fromFAPESP, CAPES/PVNS andCNPq. ORCID iDs Juliana Bovi deOliveira https://orcid.org/0000-0003-2460-2930 Silvia SizukaOishi https://orcid.org/0000-0001-5540-3382 Luíza dos SantosConejo https://orcid.org/0000-0001-5533-1374 References [1] StrelkoVV, Stavitskaya S S andGorlov Y I 2014 Proton catalysis with active carbons and partially pyrolyzed carbonaceousmaterials Chin. J. 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Introduction 2. Materials and methods 2.1. Materials 2.2. The solution preparation 2.3. Electrospinning 2.3.1. PAN characterization 2.4. Carbonization and characterization of carbon fibers 2.4.1. Characterization method of the carbonized material 3. Results and discussion 3.1. PAN nanofibers produced by eletrospinning 3.1.1. Solution concentration 3.1.2. Voltage variation in the electrospinning process 3.1.3. Processing time 3.1.4. Thermal behavior of PAN blankets produced by electrospinning process 3.2. Carbon nanofiber 3.2.1. Raman spectroscopy analysis of carbon nanofiber 4. Final conclusions Acknowledgments References