CERAMICSINTERNATIONAL

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nCorrespondinE-mail addre

(2015) 1271–1277

Ceramics International 41 www.elsevier.com/locate/ceramint

Synthesis and characterization of tungsten carbide fine powders

Jelena Lukovića,n, Biljana Babića, Dušan Bučevaca, Marija Prekajskia, Jelena Pantića,Zvezdana Baščarevićb, Branko Matovića

aVinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, 11001 Belgrade, SerbiabInstitute for Multidisciplinary Research, University of Belgrade, Kneza Višeslava 1, 11030 Belgrade, Serbia

Received 23 February 2014; received in revised form 20 July 2014; accepted 10 September 2014Available online 18 September 2014

Abstract

Fine tungsten carbide (WC) powder was prepared by solid state reaction between tungsten powder (W) and activated carbon cloth as a newcarbon (C) source. The effect of temperature and time of heat treatment as well as the effect of C/W ratio on WC phase formation was studied.The results obtained by X-ray powder diffraction (XRPD) show that obtained powder is single WC. Microstructure and morphology wasdeterminate by means of scanning electron microscopy (SEM). Brunauer–Emmett–Teller (BET) method was used for examining specific surfacearea and texture of obtained powders. It was found that WC powder was successfully synthesized in excess carbon after eight-hour heat treatmentat relatively low temperature (1000 1C).& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: WC powder; X-ray diffraction; Scanning electron microscopy; BET method

1. Introduction

Tungsten carbide belongs to the group of interstitial carbideswhich possess properties characteristic for both ceramics andmetals. These materials have high melting point, very highhardness, low friction coefficients, low reactivity, high oxidationresistance, and good thermal and electrical conductivity [1]. Thisunique combination of properties makes WC appropriate forvarious applications such as manufacturing of cutting tools andwear-resistant parts, often in form of WC-Co hard materials.Possibility to use WC in electrocatalysis has been intensivelystudied since 1970s when Levy and Boudart discovered that WCpossesses catalytic properties similar to those of platinum groupmetals [2]. WC has been used as the Pt electrocatalyst support forprocesses such as methanol oxidation [3–5], oxygen reduction[6,7] and nitrophenol oxidation [8,9]. Since high specific surfaceis essential for its effective use as catalyst support there is apermanent effort to synthesize high specific surface WC powderat temperature as low as possible in order to minimize production

10.1016/j.ceramint.2014.09.05714 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel./fax: þ381 11 340 8782.ss: jelenal@vinca.rs (J. Luković).

cost [6]. Although there is a number of different methods for WCpowder synthesis almost all of them are actually based on directcarburization of tungsten or carbothermal reduction of tungstenoxide to tungsten and subsequent carburization of tungsten.It is well known that that WC can be obtained by simple heatingof mixture of W powder and carbon black at temperature1400–1600 1C. However, there are studies which have shownthat the use of different sources of W and carbon can provideintimate mixing of W and C and allow formation of fine WCpowder at temperatures significantly lower than 1400 1C. Ele-mentary tungsten, tungsten trioxide (WO3) [6,8–10] and differenttungsten salts [11] have been used as tungsten source whereascarbon powders [3–8,10,12], ethylene (C2H4) [8], methane (CH4)[13] and glucose [10,11] have been used as a carbon source.During thermal treatment, the tungsten salts normally transforminto some form of tungsten oxide which undergoes carbothermalreduction to tungsten which finally transforms to WC bycarburization. Different methods for the synthesis of WC powdersuch as direct carburization of tungsten powder [15], carbother-mal reduction – carburization [10], mechanical milling andmechanochemical synthesis [3], gas–solid reaction [13], sol–gel procedure [7] and polymeric precursor routes using metal

J. Luković et al. / Ceramics International 41 (2015) 1271–12771272

alkoxides [6,10–12,14] have been developed. In general, themain goal of ongoing research is to synthesize fine and pure WCpowder at relatively low temperature using low cost procedure.The solid state reactions, which normally involve reactionbetween WO3 and carbon, are conducted at relatively hightemperature ranging from 1200 to 1300 1C. The synthesistemperature can be significantly lowered using techniques suchas hydrothermal method or gas–solid reaction in which tungstensalts were used as a source of tungsten. Although the synthesistemperature was lowered to 900 1C, the obtained WC powderswere fairly coarse, with particle size of about 1 μm [11,12].

This paper describes a method for synthesis of pure, fineWC powder at temperature as low as 1000 1C. The methodconsists of simple thermal treatment of a mixture of tungstenpowder and viscose rayon cloth. According to the author'sknowledge, viscose rayon cloth was used as a carbon sourcefor the first time. The effects of the carbon/tungsten ratio,temperature and duration of heat treatment on properties of theobtained powders were studied.

2. Experimental

2.1. Preparation of tungsten carbide nanopowders

For this study, viscose rayon cloth (Viskoza factory, Loznica,Serbia) was used as a carbon precursor. The cloth was impreg-nated with a mixture of NH4Cl and ZnCl2 aqueous solutions.It was found that the addition of NH4Cl and ZnCl2 can increasethe yield of reaction of carbonization [16] which was conductedat 1000 1C in a nitrogen flow. The carbonization process wasfollowed by activation in a CO2 flow at 850 1C for 1 h. Surfacecharacteristics of activated carbon cloth (ACC) was reported inwork by Sekulić et al. [16]. Activated carbon cloth was milled invibrating mill for 15 min.

Commercial tungsten powder (Koch-Light Laboratories, LTD,purity 99.9%) of average grain size of 1 mm according tomanufacturer specification and milled ACC as a carbon precursorwere mixed in vibrating mill for 15 min. The carbon/tungsten(C/W) molar ratio was varied in the range of 1–4. The preparedmixtures with different C/W molar ratio were thermally treated attemperature ranging from 700 to 1000 1C with an increment of100 1C. The heating rate was 1 1C/min. The mixtures were placed

Fig. 1. FE SEM images of ACC (A

in a middle of tube in order to provide uniform heating. The heattreatment was conducted in argon flow for 2, 4 and 8 h. After thetreatment, the furnace was cooled to the room temperature.The carbonization and activation of viscose rayon cloth, as

well as the W/C powders mixtures heat treatment were donein horizontal tube furnace (Protherm Furnaces, model PTF16/38/250, Turkey) under a controlled nitrogen (carboniza-tion), carbon dioxide (activation) or argon (powder mixturestreatment) flow. The employed gaseous contained less than5 ppm O2 and H2O. In all experiments, a gas flow of 0.5 1 per minwas used. The gaseous flow was maintained during cooling tillroom temperature.

2.2. Powder characterization

Adsorption and desorption of N2 were measured on milledACC, as well on the obtained powders, at �196 1C using thegravimetric McBain method. Specific surface area, SBET, pore sizedistribution, mesopore including external surface area, Smeso, andmicropore volume, Vmic, for the samples were calculated from theisotherms. Pore size distribution was estimated by applying BJHmethod [17] to the desorption branch of isotherms and mesoporesurface and micropore volume were estimated using the highresolution αs plot method [18–20]. Micropore surface, Smic, wascalculated by subtracting Smeso from SBET.SEM imaging of cloth before milling was done by Philips

XL-30 DX4i scaninng electron microsope. The morphology ofthe milled ACC was studied by field emission scanning electronmicroscopy (FESEM) TESCAN Mira3 XMU at 20 kV. Themorphology and microstructure of the obtained powders wereexamined using Tescan VEGA TS 5130 MM microscope.Powders obtained by thermal treatment were characterized by

X-ray diffraction (XRD) using Siemens D500 X-ray diffract-ometer with Cu Kα radiation and Ni filter. The scanning ofsamples was done at a speed 11/s in a range of diffraction angle2θ 5–1201, with the angular resolution of 0.021 for all XRDtests. Williamson–Hall analysis was used to evaluate thecrystalline sizes and lattice strain. This analysis is a simplifiedintegral breadth method where both size-induced and strain-induced broadening are deconvoluted by considering the peakwidth as a function of 2θ [21].

) before and (B) after milling.

Figofsym

TaPo

Sa

Mi

0 2 4 6 8 10 120

5

10

15

20

ΔVp/Δ

r p (c

m3 /g

nm

)

rp / nm

Fig. 3. Pore size distributions of ACC sample.

100

150

200

250

300

V /

cm3 g

-1

J. Luković et al. / Ceramics International 41 (2015) 1271–1277 1273

3. Results and discussion

3.1. Activated carbon cloth

Scanning electron micrograph of a cloth before milling(Fig. 1A) shows the large chain-like woven fibers. Aftermilling (Fig. 1B) fibers are broken into smaller pieces.

Since the powder particles become finer with milling time,their surface area increases, so rate of chemical reaction can beraised by exposing more particles to other reactant. This resultin an increased chance of collision between reactant particles,so there are more collisions in any given time [22].

Nitrogen adsorption isotherms for milled ACC, as the amountof N2 adsorbed at �196 1C, are shown in Fig. 2. Specific surfacearea calculated by BET equation, SBET, is given in Table 1.According to the IUPAC classification [23] isotherms are oftype-I, which is associated with microporous materials. There is apronounced step related to the micropore filling and, after therelative pressure of about 0.1 is reached, further adsorption isrelatively low which indicates low mesopore volume. Such asmall amount of mesopores leads to long equilibration times,and presence of hysteresis loop at low pressure. The retainedadsorbate persists after prolonged outgassing and can be removedonly by pumping at elevated temperature.

Pore size distributions (PSD) of ACC sample is shown inFig. 3. PSD shows that ACC sample is microporous, mesopor-ous surface is almost negligible.

The αs plot, obtained on the basis of standard nitrogenadsorption isotherm is shown in Fig. 4. The straight line gives amesoporous surface area including the contribution of external

0.0 0.2 0.4 0.6 0.8 1.00

2

4

6

8

10

12

14

n (m

mol

g-1

)

P / P0

. 2. Nitrogen adsorption isotherms – the amount of N2 adsorbed as a functionrelative pressure for milled ACC sample. Solid symbols – adsorption, openbols – desorption.

ble 1rous properties of milled ACC.

mple SBET (m2/g) Smeso (m2/g) Smic (m2/g) Vmic (cm

3/g)

lled ACC 942 12 930 0.451

0.0 0.5 1.0 1.5 2.0 2.5 3.00

50

αs

Fig. 4. αS – plot for nitrogen adsorption isotherm of ACC sample.

surface, Smeso, determined by its slope, and micropore volume,Vmic, is given by the intercept. Calculated porosity parameters(Smeso, Smic, Vmic) are given in Table 1.

3.2. XRD analysis

XRD analysis was employed to analyze the effect of heattreatment time, temperature and C/W ratio on phase evolution.The results are in good agreement with literature data whichsuggest that formation of WC proceeds through the following,consecutive reactions [24,25]:

2WþC ¼ W2C or ð1Þ

WþC ¼ WC ð2Þ

W2CþC ¼ 2WC ð3ÞFig. 5 shows the effect of heat treatment time on phase

composition of powders with C/W¼3 heat treated at 1000 1C.It can be seen that W2C and W are the major phases whereasWC is minor phase in powder heat treated for 2 h. However,

J. Luković et al. / Ceramics International 41 (2015) 1271–12771274

after prolonged heat treatment, W2C which can be consideredas an intermediate product, reacts with C to form WC. AsFig. 5 indicates, W2C is fully converted to WC after 8 h longheat treatment.

For this reason, samples with C/W¼3 heat treated for 8 h atdifferent temperature were selected to present the effect oftemperature on phase composition of the obtained powders. AsFig. 6 evidences, W2C, which is dominant phase in the samplesheat treated at 700 and 800 1C completely converts into WC whentemperature is increased to 1000 1C.

Now, samples with different C/W ratio heat treated at1000 1C for 8 h will be used to study the effect of C/W ratioon phase composition which is presented in Fig. 7. As thefigure shows an excess of carbon is necessary to obtain pureWC. In samples with C/W¼1 the major phase is W2C whichconverts to WC with an increase in C/W ratio. It is evident thatfrom Fig. 7 that C/W¼3 is sufficient to obtain pure WC.Based on the results presented in Figs. 5, 6, 7 it can be

20 40 60 80 100 120

**

-W-WC*-W2C

*

********

*

Inte

nsity

(a. u

.)

2θ

2 h

4 h

8 h

*

Fig. 5. Effect of the retention time on the synthesis at constant temperature(1000 1C) and constant C/W molar ratio (3).

20 40 60 80 100 120

*

**

*

*

*

1000 °C

900 °C

800 °C

Inte

nsity

(a. u

.)

2θ

700 °C

-W-WC* -W2C

Fig. 6. Effect of synthesis temperature at constant C/W molar ratio (3) andconstant retention time (8 h).

concluded that pure WC powder can be fabricated by 8 h longheat treatment of mixture with C/W=3 at 1000 1C.The internal strain of sample obtained at 1000 1C for 8 h

was estimated from the Williamson–Hall plots (Fig. 8) whichwere drawn using following equation [21]:

βtotalcosθ¼ 0:9λ=Dþ4Δd=dsinθ

where βtotal is the full width half maximum of the XRD peak,λ is the incident x-ray wave length, θ is the diffraction angle,D is crystallite size and Δd is the difference of the d spacingcorresponding to a typical peak. By plotting βtotal Ucosθ versussinθ it is possible to obtain D from the intercept and Δd=dfrom the slope.The Williamson–Hall plot for the obtained powder is shown in

Fig. 8. The presence of slopes on the βUcosθ axis indicates theinternal strain of nanocrystals. An increasing value of the slopesexhibits a clear contribution of strain effect. The strain is presentalong crystalline boundaries due to lattice mismatch. From thelinear fit to the data, the crystalline size was estimated from the

20 40 60 80 100 120

*******

*

**

Inte

nsity

(a. u

.)

2θ

1

2

3

4

-WC * -W2C -W

Fig. 7. Effect of C/W molar ratio at constant temperature (1000 1C) andconstant retention time (8 h).

Fig. 8. Williamson–Hall plot of the WC phase obtained at 1000 1C for 8 hwith C/W molar ratio 3.

TT

SC

34

J. Luković et al. / Ceramics International 41 (2015) 1271–1277 1275

y-intercept, and the strain d = d, from the slope of the fit. Unit cellparameters, crystallite size and lattice strain of WC are given inTable 2.

3.3. BET measurements

In Fig. 9 one can see the adsorption and desorption isothermsof N2 on WC powders with C/W ratio 3 and 4, obtain after 8 hheat treatment at 1000 1C given as amount of adsorbed N2 as afunction of relative pressure. Solid symbols are for adsorption,open symbols – desorption. These sorption isotherms exhibithysteresis type H4, according to IUPAC [23] classification andindicate the presence of mesopores. The relatively small amountof adsorption of nitrogen at low relative pressure indicates thatmicropores are not present in the samples.

The values of the overall specific surface of WC samples arelisted in Table 3. It can also be seen that the overall specific

Table 2Unit cell parameters, crystallite size and lattice strain of WC.

Latticeparameter,a0 ½A ̊�

Latticeparameter,b0 ½A ̊�

Latticeparameter,c0 ½A ̊�

Crystallitesize,D ½nm�

Latticestrain,ε

2.9015 2.9015 2.8344 99.2 0.005231

able 3he values of the overall specific surface of WC samples.

ample SBET Smeso Smicro Vmicro

/W ratio (m2 g�1) (m2 g�1) (m2 g�1) (cm3 g�1)

:1 14 14 – –

:1 30 30 – –

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

n / m

ol g

-1

P / P0

34

Fig. 9. Nitrogen adsorption isotherms, as the amount of N2 adsorbed asfunction of relative pressure for powder obtained at 1000 1C for 8 h and withC/W molar ratio 3 and 4. Solid symbols – adsorption, open symbols –

desorption.

surface of the WC samples depends on C/W ratio i.e., samplewith higher excess of carbon has higher specific surface due tohigh specific surface area of C precursor.The pore size distributions of the WC samples with C/W

ratio 3 and 4, obtained after 8 h heat treatment at 1000 1C areshown in Fig. 10. WC samples are completely mesoporous andthe pore radius is above 2 nm.The αs plots, obtained using standard nitrogen adsorption

isotherm, are shown in Fig. 11. The straight line in the high αsregion gives the mesoporous surface areas, which include thecontribution of the external surface, Smeso, determined by itsslope, while the micropore volume, Vmic, is given by theintercept. The calculated porosity parameters (Smeso, Smic, Vmic)are shown in Table 3. Analysis of the experimental data confirmsthat CW samples are mesoporous. The presence of mesopores isessential for some applications, therefore the preparation of CWsamples according to this method is an effective way to expandthe scope of potential application of WC.

0.0 0.5 1.0 1.5 2.0 2.5 3.00

5

10

15

20

25

30

35

34

V /

cm3 g

-1

αs

Fig. 11. αs-plots for nitrogen adsorption isotherm of obtained WC powder at1000 1C for 8 h and with C/W molar ratio 3 and 4.

0 2 4 6 8 10 120

2

4

6

8

10

34

ΔVp/

Δrp

/ cm

3 g-1

nm

-1

rp / nm

Fig. 10. Pore size distribution (PSD) for obtained WC powder at 1000 1C for8 h and with C/W molar ratio 3 and 4.

Fig. 12. SEM micrographs of WC powder obtained at 1000 1C for 8 h and with C/W molar ratio (A), (B) 3 and (C), (D) 4 at different magnifications.

J. Luković et al. / Ceramics International 41 (2015) 1271–12771276

3.4. SEM analysis

The SEM micrographs of obtained WC powders are shownin Fig. 12. To show comparison between the WC powdersobtained at same heating temperature (1000 1C) and sameheating time (8 h) but with different molar ratios (3 and 4)micrographs of these powders are given. For each powdersame magnifications were used. One can easily see that obtainpowders have almost same microstructure.

The morphology of powders shows granular structure whereindividual particles are cemented in the form of agglomeratedlumps. An individual particle possesses nearly equiaxed shapewith narrow size distribution. However, irregular crystal shapecan be ascribed to no sufficient high annealing temperaturewhich provides fast diffusion necessary for crystallite growth.It can be roughly estimated that the WC grains lie in the rangeof 0.3–0.5 mm. Since the measured crystalline size estimatedby XRD shows that average size is about 0.1 mm, it can beassumed that agglomerates consist of approximately 5 crystal-lites, which growth together during thermal treatment.

Powder obtained from C/W molar ratio 3 has better connectedgrains. Because of smaller grains of powder obtained from C/W

molar ratio 4 there are more space between them and porosity isbigger. This is in accordance with the results of BET analysisand specific surface area.

4. Conclusion

It was found that pure fine WC powder could be obtainedusing tungsten powder and viscose rayon cloth after milling asa carbon precursor. It was found that optimal experimentalcondition for WC synthesis is C/W molar ratio 3 and reactiontemperature at 1000 1C, under an argon atmosphere, and 8 h ofdwell time. The obtained powders are agglomerated withirregular shape morphology of individual particles.Sorption isotherms indicate the presence of mesopores and

the pore radius is above 2 nm. The relatively small amount ofadsorption of nitrogen at low relative pressure indicates thatmicropores are not present in the samples. The specific surfacearea of the obtained powder, SBET, is 14 m2 g�1 The resultsshow that a carbon cloth has a potential as a new carbonprecursor for WC production.

J. Luković et al. / Ceramics International 41 (2015) 1271–1277 1277

Acknowledgment

This project was financially supported by the Ministry ofEducation and Science of the Republic of Serbia (Project no.45012)

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