microstructure, elastic and inelastic properties of...

ISSN 1063�7834, Physics of the Solid State, 2015, Vol. 57, No. 3, pp. 586–591. © Pleiades Publishing, Ltd., 2015.Original Russian Text © T.S. Orlova, B.K. Kardashev, B.I. Smirnov, A. Gutierrez�Pardo, J. Ramirez�Rico, J. Martinez�Fernandez, 2015, published in Fizika Tverdogo Tela, 2015,Vol. 57, No. 3, pp. 571–577.

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1. INTRODUCTION

Biomorphic carbon matrices produced by carbon�ization of natural wood or wood fiber�boards arepromising materials for various practical applications.Being characterized by natural open porosity (to75%), they can be used as matrices for developing newcomposites such as bioC–metal, bioC–polymer, andfor producing light porous biomorphic ceramics SiCand composites SiC/Si on their basis [1–3].

Different porous carbons are conventionally usedas materials for electrodes, which is caused by theirchemical resistance to different electrolytes, wideoperating temperature ranges, and simple fabricationof electrodes. The use of biocarbon materials as elec�trodes for energy accumulator systems (electrochemi�cal capacitors) seems promising, since they have largesurface areas, high conductivity, interconnectedporous structure, controllable pore sizes appropriatefor electrolyte ions, and electrochemically stable sur�faces [4, 5]. They do not require binding materials forproducing electrodes; in this case, the entire carbonskeleton is involved in electrode operation.

In recent years, particular interest is attracted bygraphitized biocarbon matrices produced also by nat�ural wood carbonization only in the presence of tran�sition metal catalysts [6–8]. Due to the retention ofthe natural porosity inherent to natural wood and highheat and electrical conductivity of graphite, the fabri�cation of graphitized biocarbon matrices is of greatpractical interest. Conventional pyrolysis (carboniza�tion) of wood does not allow production of graphitizedcarbon even as the treatment temperature is increasedto 3000°C [1, 9]. In this case, the so�called turbostaticcarbon is formed, which consists of misoriented car�bon layers spaced by distances exceeding the inter�plane distance (3.354 Å) of perfect graphite [10]. Toimprove ordering in the structure of non�graphitizedcarbons, transition metal catalysts based on iron,cobalt, nickel, and copper are used, whose use resultsin the formation of graphitized carbon even at Tcarb >500°C [11–14]. The graphitization of carbons usingtransition metal catalysts well proved in the case of thinfilms, aerogels, and cellulose powders. Only few innumber studies are devoted to graphitization of bulk

MECHANICAL PROPERTIES, PHYSICS OF STRENGTH, AND PLASTICITY

Microstructure, Elastic and Inelastic Properties of Partially Graphitized Biomorphic Carbons

T. S. Orlovaa, b, *, B. K. Kardasheva, B. I. Smirnova, **, A. Gutierrez�Pardoc, J. Ramirez�Ricoc, and J. Martinez�Fernandezc

a Ioffe Physical�Technical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia * e�mail: [email protected]** e�mail: [email protected]

b National Research University of Information Technologies, Mechanics and Optics, Kronverkskii pr. 49, St. Petersburg, 197101 Russia

c Departamento de Fisica de la Materia Condensada—Instituto de Ciencia de Materiales de Sevilla (ICMSE), Universidad de Sevilla, Apartado de Correos 1065, Sevilla, ES41080 Spain

Received September 15, 2014

Abstract—The microstructural characteristics and amplitude dependences of the Young’s modulus E andinternal friction (logarithmic decrement δ) of biocarbon matrices prepared by beech wood carbonization attemperatures Tcarb = 850–1600°C in the presence of a nickel�containing catalyst have been studied. UsingX�ray diffraction and electron microscopy, it has been shown that the use of a nickel catalyst during carbon�ization results in a partial graphitization of biocarbons at Tcarb ≥ 1000°C: the graphite phase is formed as 50�to 100�nm globules at Tcarb = 1000°C and as 0.5� to 3.0�μm globules at Tcarb = 1600°C. It has been found thatthe measured dependences E(Tcarb) and δ(Tcarb) contain three characteristic ranges of variations in theYoung’s modulus and logarithmic decrement with a change in the carbonization temperature: E increases andδ decreases in the ranges Tcarb < 1000°C and Tcarb > 1300°C; in the range 1000 < Tcarb < 1300°C, E sharplydecreases and δ increases. The observed behavior of E(Tcarb) and δ(Tcarb) for biocarbons carbonized in thepresence of nickel correlates with the evolution of their microstructure. The largest values of E are obtainedfor samples with Tcarb = 1000 and 1600°C. However, the samples with Tcarb = 1600°C exhibit a higher suscep�tibility to microplasticity due to the presence of a globular graphite phase that is significantly larger in size andtotal volume.

DOI: 10.1134/S106378341503018X

PHYSICS OF THE SOLID STATE Vol. 57 No. 3 2015

MICROSTRUCTURE, ELASTIC AND INELASTIC PROPERTIES 587

carbons carbonized from natural wood [6–8]. In [6],using liquid catalysts based on nickel in wood carbon�ization, porous carbons with a graphitized phase con�tent typical of carbons produced from pitch�basedgraphite were produced retaining the biocarbon porousstructure. These biomorphic graphitized carbonsshowed a significantly higher thermal conductivity incomparison with ordinary quasi�amorphous biocar�bons [7]. However, elastic, microplastic, and strengthproperties of graphitized biocarbons were not studied.

In the present work, using a similar carbonizationmethod in the presence of catalyst with varied carbon�ization temperature Tcarb, beech wood biocarbonswere produced, and the dependences of the Young’smodulus and internal friction on Tcarb in comparisonwith their microstructural features were studied for thefirst time. These data are compared with similardependences we previously obtained in [15] for bio�carbons carbonized under the same conditions, but inthe absence of catalyst.

2. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE

The samples of biocarbon produced by beech woodcarbonization at different temperatures Tcarb = 600–1600°C were studied in the absence and in the pres�ence of Ni�containing catalyst. As a precursor, woodpieces 75 × 15 × 15 mm in size were used, which werepreliminarily dried in a furnace to eliminate residualwater from pores to prevent cracking during subse�quent pyrolysis. A saturated solution of nickel nitrate(Ni(NO3)2 · 6H2O) in isopropyl alcohol was used as acatalyst, into which samples were placed for fillingtheir pores. To provide complete filling of the entirepore space by solution, air from pores was preliminar�ily pumped out by placing the sample to a vacuumchamber. The samples were kept in the solution for acertain time, and then were dried in air at room tem�perature for 24 h followed by additional drying at 80°Cuntil complete drying which was controlled by achange in the precursor weight. The pyrolysis processwas performed in an inert gas (99.999% nitrogen) flow.The heating rate was 1°C/min to 500°C, then5°C/min to Tcarb (600, 850, 1000, 1150, 1300, 1400,1500, and 1600°C) at which the sample was kept for30 min and then was cooled with a rate of 5°C/min.Then Ni catalyst particles were removed by washingthe samples in concentrated HCl acid (37%, Panreac)for 2 h followed by washing in deionized water withneutral pH and drying in a furnace. This method forproducing graphitized biocarbons is described in detailin [8]. In what follows, we present the results forgraphitized samples after removing residual nickelfrom them, except for specially mentioned cases. Forcomparison, some samples were prepared at the sameheat treatment parameters, but in the absence of cata�lyst. In what follows, we denote beech biocarbon sam�

ples carbonized at certain Tcarb after treatment by a Ni�containing catalyst by BE�C�Tcarb(Ni); i.e., BE�C�850(Ni) is biocarbon produced by beech wood car�bonization at Tcarb = 850°C in the presence of Ni cat�alyst. Accordingly, samples carbonized without nickelare denoted by BE�C�600, BE�C�850, and so on.

The structure of carbonized samples was studied byX�ray diffraction and scanning electron microscopy(SEM) methods.

Samples for acoustic studies were prepared as rods~20 mm long with approximately square cross section~16 mm2 in area. Samples oriented by the long sidealong growth fibers of primary wood we studied. TheYoung’s modulus E and ultrasonic vibration decre�ment δ were measured using the composite vibratormethod. Longitudinal resonant vibrations wereexcited in samples at a frequency of ~100 kHz. TheYoung’s modulus E and ultrasonic vibration decre�ment δ were measured as functions of the acousticvibration amplitude. The range of vibrational strainamplitudes ε was from ~10–7 to 2 × 10–4. The methodis described in detail in [16]. To measure the amplitudedependences E(ε) and δ(ε), the acoustic system (sam�ple and quartz converter) was placed into a vacuumchamber, from which air was pumped out to~10⎯3 mmHg. Then the amplitude dependences of theYoung’s modulus E and decrement δ were measured atroom temperature.

3. EXPERIMENTAL RESULTS AND DISCUSSION

3.1. Structural Characterization of Samples

First of all, the effect of catalytic pyrolysis (carbon�ization) on changes in the crystal structure and thedegree of residual disordering was measured by X�dif�fraction analysis methods. Figure 1 shows the exampleof X�ray diffraction patterns of biocarbon samples car�bonized at different temperatures using a nickel�con�taining catalyst. We can see that the use of catalyst atTcarb ≥ 1000°C results in the formation of a graphitizedphase, as suggested by the appearance of the strong(002) peak at 2θ = 26.6° in the X�ray pattern (Fig. 1).At lower carbonization temperatures, the use of cata�lyst was inefficient for forming an appreciable graphitephase fraction.

The degree of ordering of the biocarbon structurewas studied by Raman spectroscopy in [8]. These dataalso show a substantial increase in the degree of graph�itization in the samples treated by Ni�catalysts, incomparison with untreated samples with increasedTcarb ≥ 1000°C [8].

The use of catalyst had no effect on themacroporous structure of biocarbon samples: at allcarbonization temperatures, it remained similar andwas characterized by the presence of pores of small(3–7 μm) and large (22–38 μm) diameters, oriented

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along the wood growth. The total volume of suchmicroscopic pores in the samples was 65 ± 5% [6].Measurements of the density of the carbon skeletonsthemselves using a helium picnometer [8] indicate theformation of additional nanoporosity in biocarbonswith Tcarb > 1000°C comparable to that of BE�C sam�ples [15].

The biocarbon microstructure was studied by scan�ning electron microscopy. Electron microscopy imagesof the BE�C(Ni) sample structure with Tcarb = 1000and 1600°C are shown in Figs. 2a and 2b, respectively.At Tcarb = 1000°C, a graphite phase is formed near Niparticles (Fig. 2a). Lower carbonization temperatureswere inefficient for graphitization. The fine structure ofthe nucleating graphite phase was studied in detail in[8] by transmission electron microscopy. It was shownthat the nucleating phase represents graphene layersgrowing around Ni particles and forming hollow nano�spheres 50–100 nm in size after nickel removal. InbioC�1600(Ni) samples, large graphite globules(grains) 0.5–3.0 μm in size are formed (Fig. 2b). Thefraction of the formed graphite phase in samples withTcarb = 1000°C is small (Fig. 2a), whereas it occupies asignificant part of the sample volume in samples withTcarb = 1600°C (Fig. 2b). It should be noted that graph�ite phase globules larger than 1 μm were formed only atcarbonization temperatures exceeding the nickel melt�ing temperature of ~1450°C.

3.2. Elastic and Inelastic Properties

Figure 3 shows the dependences of the modulus Eand decrement δ on the carbonization temperature,measured in vacuum after pumping air for bioC(Ni)biocarbon matrices produced by beech carbonizationin the presence of Ni�containing catalyst (curves 1). In

this case, we note that the measured (effective) valuesof E and δ were determined proceeding from the totalsample cross section, rather than that of the compos�ing biocarbon material. For comparison, there are alsoshown the dependences E(Tcarb) and δ(Tcarb) we previ�ously obtained in [15] for beech biocarbon matricescarbonized under the same conditions, but withoutcatalyst, which can be conditionally divided into twolinear ranges: as Tcarb is increased to ~1000°C, E rap�idly increases, while δ decreases; at Tcarb > 1000°C,these variations become significantly slower (Figs. 3aand 3b, curves 2). The rapid variations of E and δ withtemperature in the range Tcarb < 1000°C are associatedwith the increase in the nanocrystalline phase fractionin the amorphous matrix; at Tcarb > 1000°C, elasticproperties are controlled by the almost formed nanoc�rystalline phase [15]. In the same temperature rangeTcarb = 900–1000°C, the insulator—strongly disor�dered metal transition known from [17, 18] isobserved; furthermore, the behavior of thermal prop�erties also changes [19–21]. For the bioC(Ni) sampleseries, the dependences E(Tcarb) and δ(Tcarb) can be

1.0

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0.4

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0

Inte

nsi

ty,

arb.

un

its

10 20 30 40 50 60 702θ, deg

1000°C

800°C

600°C

1300°C

Tcarb = 1600°C

Fig. 1. X�ray diffraction patterns of biocarbon samples car�bonized at different temperatures in the presence of Nicatalyst. Asterisks indicate the peaks corresponding toresidual Ni after sample etching in HCl.

Ni

5 μm

Ni

G�phase

G�phase

(a)

(b)3 μm

NiNi

Fig. 2. Typical SEM microstructure images of the samplescarbonized at Tcarb = (a) 1000 and (b) 1600°C in the pres�ence of Ni catalyst. G is the graphite phase, Ni are nickelcatalyst particles. Arrows indicate the Ni�phase aroundwhich graphite phase nucleates at Tcarb = 1000°C.



conditionally divided into three regions: E increasesand δ decreases in the ranges Tcarb < 1000°C and Tcarb> 1300°C; in the range 1000 < Tcarb < 1300°C, Eabruptly decreases while δ increases. In fact, in therange 850 < Tcarb < 1000°C, the dependences E(Tcarb)and δ(Tcarb) for BE�C(Ni) and BE�C samples are sim�ilar. To all appearance, the graphite phase (G�phase) isstill formed very slowly in this range, which is sug�gested by the data of X�ray and microstructural studies(see Subsection 3.1); the increase in E and decrease inδ with Tcarb are controlled for BE�C(Ni) samples, asfor BE�C, by the growing nanocrystalline phase andamorphous matrix. According to the structural studies,the G�phase rapidly grows in the region Tcarb > 1300°C,which does control the strong increase in E and thedecrease in δ with Tcarb. It seems interesting that,despite the G�phase formation, an abrupt decrease in Efrom 8.8 to 2.8 GPa and an increase in the decrementfrom 4 × 10–3 to 8 × 10–3 is observed in the range 1000 <Tcarb < 1300°C. Such low E and high δ can be explainedassuming that the G�phase formation prevents thenanocrystalline phase formation, which seems reason�able, since the crystallization processes of nanocrystal�line and graphite phases seem to be competing. Thenthe formed graphite globules “float” in the amorphousmatrix at Tcarb ~ 1300°C, their fraction is small, whichdoes cause the low value of E mainly controlled by theamorphous phase. Then in the region Tcarb > 1400°C, Eincreases due to a rapid increase in the G�phase frac�tion in the amorphous matrix. The increase in the elas�tic modulus with the carbonization temperature in theregion Tcarb > 1300°C is certainly related to the degreeof biocarbon crystallization. It follows from themicroscopy data (Fig. 2b) that, although a large num�ber of graphite globules are formed at Tcarb = 1600°C,the amorphous (or quasi�amorphous) phase fractionstill occupies a significant sample volume. It is possiblethat the fact that the Young’s modulus for bioC�1600(Ni) samples does not reach the value of E at Tcarb= 1000°C for both systems of BE�C and BE�C(Ni)samples indirectly argues in favor of our assumptionthat the nucleating graphite phase suppresses thenanocrystalline phase formation.

It seems interesting to compare the amplitudedependences of the Young’s modulus and decrementfor partially graphitized BE�C(Ni) samples with Tcarbfrom different temperature ranges: 1000 < Tcarb <1300°C and Tcarb > 1300°C. Figures 4 and 5 show thedependences E(ε) and δ(ε) for BE�C�1150(Ni) andBE�C�1400(Ni) samples, respectively. In the depen�dences E(ε) and δ(ε) for BE�C�1400(Ni) (Fig. 5), asfor BE�C�1500(Ni) and BE�C�1600(Ni), the follow�ing characteristic features may be noted. The modulusgradually decreases with increasing vibration ampli�tude. As for the decrement, in BE�C(Ni) samples withTcarb > 1300°C, it remains almost unchanged withamplitude, but increases as large amplitudes are

reached, ε > 10–4 (Fig. 5b). For BE�C(Ni) sampleswith Tcarb < 1300°C, the decrement does not increaseeven upon reaching maximum amplitudes used in thisstudy (Fig. 4b). This indicates a decreased plasticity ofsamples with Tcarb < 1300°C. It should be noted thatthe plasticity effect (a decrease in E and an increase inδ with ε) manifests itself in BE�C(Ni) beginning withTcarb > 1300°C when an already significant graphitephase volume is probably formed, graphite globulesreach micrometer sizes, and the G�phase controls thebehavior of elastic and inelastic properties of a mate�rial. It seems that dislocation glide providing plasticflow can be easier implemented in the crystal structure(even if it represents a set of graphite globules withsizes from fractions of micrometer to severalmicrometers), than in the amorphous structure. In thestudy of elastic and inelastic properties of pine biocar�bon with Tcarb = 1000 and 2400°C, it was shown thatthe crystallite size approximately twofold increaseswith increasing Tcarb, which leads to a decrease in the

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(a)

600 800 1000 1200 1400 16000

20

40

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δ,

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Tcarb, °C

(b)

Fig. 3. Dependences of the (a) Young’s modulus E and(b) decrement δ of biomorphic carbon samples on theircarbonization temperature Tcarb. (1) BE�C(Ni) sampleseries (present study) and (2) BE�C sample series [15].

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conditional yield stress, i.e., microplastic deformationfacilitation [22]. In beech biocarbon carbonized with�out Ni catalyst, the plasticity effect (a decrease in Eand an increase in δ with ε) appeared even at Tcarb ≥1000°C, which was associated with nanocrystallinephase formation [15]. The absence of an increase inthe microplasticity in BE�C(Ni) samples with 1000 <Tcarb < 1300°C even at large acoustic vibration ampli�tudes (in contrast to BE�C samples), as noted above,indirectly argues in favor of our assumption about thesuppression of nanocrystalline phase formation duringactive nucleation of the graphite phase.

Thus, the biocarbon BE�C matrices with Tcarb ≥1300°C, studied in the present work, exhibit micro�plastic properties. Figure 6 shows the stress–micro�plastic strain diagrams obtained by the results ofacoustic measurements in vacuum for the BE�C(Ni)system samples. The diagrams were constructed usingthe dependences E(ε) similar to the curves in Figs. 4and 5, obtained with increasing amplitudes. The con�struction procedure for such diagrams is given in [23].

When comparing the curves σ(εd) shown in Fig. 6,we can see that BE�C(Ni) samples with Tcarb ≥ 1300°Chave lower values of the conditional microyield stressσy (the stress at εd = 1.0 × 10

–8), which is obviouslyassociated with a significant increase in the volumefraction of the graphite phase and the formation ofmicrometer graphite globules. It should be noted thatthe BE�C samples prepared by beech carbonization atsimilar carbonization temperatures, but in the absence

7.7460

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7.7464E

, G

Pa

7.7458

7.7456

7.7540500

δ,

10−

5

400

300100 1000

ε, 10−7

(b)

(a)

Fig. 4. Amplitude dependences of the (a) Young’s modulusE and (b) decrement δ for BE�C(Ni) samples produced atTcarb = 1150°C in the presence of Ni catalyst. Measure�ments were performed in vacuum on samples subjected tohigh amplitudes at room temperature. Arrows indicate theε variation direction.

500

450100 1000

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600

650

δ,

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3.182

3.184

3.186

E,

GP

a

3.188(a)

(b)

Fig. 5. Amplitude dependences of the (a) Young’s modulusE and (b) decrement δ for BE�C(Ni) samples produced atTcarb = 1400°C in the presence of Ni catalyst. Measure�ments were performed in vacuum on samples subjected tohigh amplitudes at room temperature.

0 20 40 60εd, 10

−8

0.001

0.01

0.1

1

10

σ,

MP

a

3 2

16

5

4

Fig. 6. Stress–microplastic strain curves obtained by thedata of acoustic measurements in vacuum on BE�C(Ni)samples carbonized in the presence of Ni catalyst. Tcarb =(1) 850, (2) 1000, (3) 1150, (4) 1300, (5) 1400, and (6)1600°C.



of Ni�containing catalyst, did not exhibit similarmicroplastic flow [15]. Thus, porous BE�C(Ni) bio�carbon matrices with Tcarb = 1000 and 1600°C showthe highest Young’s moduli; however, the samples withTcarb = 1600°C exhibit a significantly higher suscepti�bility to plastic flow.

4. CONCLUSIONS

The amplitude dependences of the Young’s modu�lus E and ultrasonic vibration decrement δ were deter�mined for the first time for partially graphitized porousbiocarbon matrices produced by beech wood carbon�ization at different temperatures Tcarb in the presenceof nickel�containing catalyst.

The X�ray diffraction and electron microscopyinvestigations showed that the presence of a Ni catalystduring the carbonization results in a partial graphitiza�tion of biocarbons at Tcarb ≥ 1000°C. In this case, thegraphite phase is formed as globules whose sizeincreases from 50–100 nm at Tcarb = 1000°C to 0.5–3.0 μm at Tcarb = 1600°C. The total volume fraction ofthe graphite phase also increases with an increase inTcarb. The measured dependences E(Tcarb) and δ(Tcarb)contain three characteristic ranges of variations in theYoung’s modulus and logarithmic decrement with achange in the carbonization temperature. Theobserved behavior of the dependences E(Tcarb) andδ(Tcarb) for biocarbons carbonized in the presence ofnickel correlates with the evolution of their micro�structure: E increases while δ decreases with anincrease in Tcarb in the ranges Tcarb < 1000°C and Tcarb >1300°C, which is associated with the formation ofnanocrystalline and graphite phases in the amorphousmatrix in the former and latter cases, respectively. Inthe range 1000 < Tcarb < 1300°C, E abruptly decreasesand δ increases, which most probably indicates thesuppression of the formation of a nanocrystalline phaseby the formed graphite phase. In the range Tcarb ≥1300°C, it was found that the microplastic deformationis facilitated (the conditional yield stress σy decreases).The largest values of E were obtained for samples withTcarb = 1000 and 1600°C. However, the samples withTcarb = 1600°C showed a higher susceptibility to micro�plastic deformation, which is caused by a significantlylarger volume fraction of the graphite phase.

ACKNOWLEDGMENTS

This study was supported in part by the RussianFoundation for Basic Research (project no. 14�03�00496), the Presidium of the Russian Academy of Sci�ences (program no. P�20), and the Junta de Andalu�cia, Spain (grant no. P09�TEP�5152).

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Translated by A. Kazantsev

microstructure, elastic and inelastic properties of...

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