Effect of Multiwalled Carbon Nanotubes on the Thermal

Effect of Multiwalled Carbon Nanotubes on the ThermalConductivity and Porosity Characteristics of BlastFurnace Carbon Refractories

YAWEI LI, XILAI CHEN, YUANBING LI, SHAOBAI SANG, and LEI ZHAO

Different amounts of carbon nanotubes (CNTs) (0–5 mass pct) containing carbon refractoryspecimens for a blast furnace were prepared and coked for 3 hours at 1473 K (1200 �C) and1673 K (1400 �C). The thermal conductivity and porosity characteristics of the coked specimenswere evaluated using the flash diffusivity technique and mercury porosimetry, respectively. Itwas found that CNTs acted as carbon source, and most of them were consumed during coking.With the increase of CNT content, the aggregation of CNTs became more severe, the amount ofSiC whiskers formed increased and their aspect ratio became larger, and the SiC whiskerstended to be distributed nonhomogeneously. The thermal conductivity of a 4 mass pct CNTcontaining a carbon specimen was highest because of the contributions of SiC and residualCNTs. The porosity characteristics of a 0.5 mass pct CNT containing a carbon specimen wasbest because of the uniform filling of SiC whiskers. The excessive addition of CNTs degradedthe porosity characteristics because of the severe aggregation of CNTs.

DOI: 10.1007/s11661-010-0300-9� The Minerals, Metals & Materials Society and ASM International 2010

I. INTRODUCTION

THE life of a blast furnace (BF) is determined mainlyby the rate of erosion and corrosion of hearth andbottom carbon refractories.[1,2] Currently, carbon refrac-tories for a BF are being developed with high thermalconductivity (TC) and a highly microporous structurefor two reasons: First, high TC can sustain the risk ofhigh thermal loads and large temperature gradients, andsecond, a highly microporous structure effectively canprevent the infiltration of molten iron and avoid theoccurrence of a brittle contact zone.[3–5] Commonly thereare two routes toward obtaining a highly microporousstructure. The first route is adding silicon, and the secondis decreasing the size of the filler grains.[6,7] In ourprevious work, it has been observed that the addition ofappropriate silicon content along with its particle size,[8–10]

activated alumina micropowder,[11] electric-calcinedanthracite aggregates,[12] and aluminum[13] can be favor-able for the formation of an excellent microporousstructure. Also, two routes are proposed toward increas-ing TC: adding a component with high thermal conduc-tivity and densifying the material.

Carbon nanotubes (CNTs) have been investigatedwidely for applications in the past decade because oftheir unique mechanical and physical properties. Someinvestigations show that the incorporation of CNTs inpolymer-[14] and ceramic-based composites can lead to

significant improvements in electrical and thermal prop-erties because of CNTs excellent electrical conductivity(106 S/m at 300 K (27 �C) for single-walled CNT[SWCNT] and >105 S/m for multiple-walled CNT[MWCNT])[15,16] and thermal conductivity (6600W/m 9 K for individual SWCNT and >3000 W/m 9 Kfor individual MWCNT).[17,18] In the case of ceramic-based composites, Jiang and Gao[19] reported that therewas a 97 pct enhancement in TC of a TiN-CNTcomposite at 703 K (430 �C) compared with that ofTiN because of the presence of 5 wt pct MWNTs. Zhanet al.[20] obtained an electrical conductivity of 3345 S/mfor a 15 vol. pct SWCNT–alumina nanocomposite,which represents an increase of 13 orders of magnitudeover that of pure alumina. However, until now, fewreports can be found in the literature about the effect ofCNTs addition on the TC and porosity characteristics ofcarbon refractories for a BF.This article will address the evolution of TC, as well as

the porosity characteristics and microstructures, of BFcarbon refractories with added CNTs during heattreatment in a CO-N2 atmosphere.

II. EXPERIMENTAL

A. Raw Materials and Refractories Fabrication

Electric-calcined anthracite granules and fine powder(3–5 mm, 1–3 mm, 0–1 mm, and <0.088 mm; FangdaCarbon Co., Ltd., Beijing, China), flaky graphite(<0.075 mm; Qingdao Hengsheng Graphite Co., Ltd.,Qingdao, China), brown corundum (<0.075 mm;Zhengzhou Zhenjin Refractory Co., Ltd., Zhengzhou,China), metallic silicon (<0.045 mm; ZhejiangKaiyuantong Silicon Co., Ltd., Kaiyuan, China),

YAWEI LI, Professor, XILAI CHEN, Ph.D. Student, YUANBINGLI, Professor, SHAOBAI SANG, Teacher, and LEI ZHAO, Professor,are with the Hubei Province Key Laboratory of Ceramics andRefractories, Wuhan University of Science & Technology, Wuhan430081, P.R. China. Contact e-mail: [email protected]

Manuscript submitted March 9, 2010.Article published online May 15, 2010

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 41A, SEPTEMBER 2010—2383

and CNTs (Alpha Nano Tech. Inc., Chengdu, China;diameter: >50 nm, length: ~20 lm, purity: >95 masspct, ash:<1.5 mass pct, electrical conductivity:>104 S/m)were used as the raw materials. Thermosetting phenolicresin (50 mass pct of carbon yield; Yongli RefractoryCo., Ltd., Zibo, China) was used as a binder. Thechemical composition of all the raw materials is shownin Table I. The scanning electron microscopy (SEM)(Figure1(a)) and transmission electron microscopy(TEM) (Figure 1(b)) images show that the CNTs hadan aspect ratio of about 2 9 102 and they were multi-walled. The batch containing 66 mass pct electric-calcined anthracite, 20 mass pct graphite, 6 mass pctbrown corundum, and 8 mass pct Si was used as thebasis. To study the effect of CNTs, different amounts ofCNTs were added in the following formula: 0.5, 1, 2, 3,4, and 5 mass pct, respectively. Eleven mass pct resinand 5 mass pct absolute alcohol were added for allformulations. For the homogeneous dispersion of CNTsin the mixture, CNTs were first mixed with resin for30 minutes with the aid of absolute alcohol and thenadded together. The raw materials were mixed for30 minutes in a mixer with the rotating speed of 80–100revolutions per minute. After kneading, cylindricalspecimens 50 mm in diameter and 50 mm in heightwere pressed under the pressure of 100 MPa. Afterpressing, the specimens were cured at 383 K (110 �C)and 473 K (200 �C) for 10 hours, respectively, in amuffle furnace. The coking of as-prepared specimenswas carried out at 1473 K (1200 �C) and 1673 K(1400 �C) for 3 hours, respectively, in a crucible filledwith industrial applied carbon grit, in which the silicaand alumina are the main impurities.

B. Tests and Characterization Methods

The bulk density and apparent porosity of the firedspecimens were measured according to the water-immersion method. The cold crushing strength wasevaluated in terms of GB/T 2997–85. The micropore sizedistribution was examined from specimens of approxi-mately 6 9 6 9 6 mm by a mercury porosimeter(Autopore IV9500; Micromeritics Instrument Corp.,Norcross, GA). The TC was calculated from theproduct of thermal diffusivity measured using the flashdiffusivity technique (Flashline 5000; Anter Corp.,Pittsburgh, PA); bulk density and heat capacity werealso measured. The heat capacity was determined from ameasured standard graphite sample. The specimens were12.5 mm in diameter and 2.5 mm thick. The diffusivitywas measured in a direction parallel to the formingdirection of the initial carbon refractories. The samplesfor cross-sectional analysis were prepared from thefractured specimens and were observed by a field-emission SEM (Nova400 Nano FESEM; FEI Co.,Philips’, Hillsboro, OR).

III. RESULTS AND DISCUSSION

A. Microstructure Analysis

Figure 2 shows SEM images of fractured specimenswith different CNT contents after coking at 1473 K(1200 �C). For the specimen without CNTs added,visual inspection revealed that a gray-green phase wasdistributed homogeneously within the matrix; microob-servation by SEM suggested that large amounts of

Table I. Chemical Compositions of the Raw Materials (Mass Percent)

C Al2O3 SiO2 Si Al Ash Volatile Na2O Fe TiO2

Electric-calcined anthracite 93 3.87 3.96 7.34 1.35Flaky graphite ‡97 1.4Brown corundum ‡96 £0.9 £0.2 £0.15 2.5Metallic silicon ‡98 0.5

Fig. 1—SEM image (a) and TEM image (b) of CNTs.

2384—VOLUME 41A, SEPTEMBER 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A

cristobalite particles and SiC whiskers are observed inthe matrix with pores (Figure 2(a)), and the samephenomenon was also found in previous work.[10] Thegray-green phase was proved to be SiC whiskers by ahigh-temperature microscope observation of the reac-tion of Si at increased temperatures in carbon blocks.[21]

Cristobalite and SiC were formed by the in situ reactionsof Si with O and C to form the corresponding oxide andcarbide. Interestingly, it was found that the gray-greenphase tends to be distributed nonhomogeneously in thematrix of specimens with increasing CNTs amount,which indicates that SiC whiskers were locally formed.The amount of cristobalite decreased and that of SiCincreased with the addition of CNT (Figures 2(b) and(c)), which was attributed to the oxygen partial pressurereduction by adding CNTs. Another notable phenom-enon is that the aspect ratio of SiC whiskers becamelarger with the increased amount of CNTs. Fromprevious work[22] it was deduced that the porousstructures of specimens became worse. Temperatureplayed an important role in the evolutions of cristobaliteand SiC whiskers. For the specimens prepared by cokingat 1673 K (1400 �C), although the similar visual inspec-tion was also found, the gray-green color became darkercompared with that at 1473 K (1200 �C). As comparedwith Figure 2 in microscale, the amount of cristobalite

decreased and that of SiC increased (Figure 3), whichreveals that the elevated temperature can be favorablefor the formation of SiC.It was difficult to detect CNTs in the coked specimens.

High-magnification SEM indicates that some CNTswere aggregated in the resin locally and were sur-rounded by graphite flake (Figure 4). Agglomerationobserved in coked body resulted from poor dispersion ofCNTs during mixing process, which hardly can beavoided because of the high specific surface area ofCNTs. It is difficult to achieve a tight interface bonding

between the CNTs and the surrounding matrix,[23–25]

and thus, some CNTs were surrounded by the presenceof oxygen. Combined with the previous observations, itcan be deduced that the CNTs act as carbon sourcebased on their high reactivity[26] and were consumed bythe following reaction during heating process:

3C sð Þ þO 2 gð Þ þ Si sð Þ � SiC sð Þ þ 2CO gð Þ ½1�

It seems that the surrounding atmosphere can pro-duce a significant effect on the survival of CNTs. Underthe N2

[27] or vacuum[28] atmosphere, CNTs remainedintact after the heating treatment. In this work, althoughthe main atmosphere consists of CO and N2

[29] intheory, in fact, locally the oxygen partical pressure is

Fig. 2—SEM images of fractured specimens with different CNT contents after coking at 1473 K (1200 �C): (a) 0, (b) 2 mass pct, and (c) 5 mass pct.

Fig. 3—SEM images of fractured specimens with different CNT contents after coking at 1673 K (1400 �C): (a) 0, (b) 2 mass pct, and (c) 5 mass pct.


high, especially in the pores and interfaces, plus the highactivation of CNTs, and thus CNTs can be consumedeasily in increased temperature.

B. Thermal Conductivity (TC)

The variations of TC (298 K (25 �C)) are shown inFigure 5. The TC of the specimens coked at twodifferent temperatures was increased with the increasedamount of CNTs up to 4 mass pct. For example, at1673 K (1400 �C), the TC was enhanced from 16.5W/m 9 K for the no CNT specimen to 20.8 W/m 9 Kfor the 4 mass pct CNT specimen, but it decreasedslightly again by adding more CNTs. For each indicatedcarbon specimen, the TC of specimens coked at 1673 K(1400 �C) was higher than that at 1473 K (1200 �C).The enhancement of TC with the addition of CNTs wasexplained by the increased formation of SiC and thepresence of more residual CNTs. The degradation of TC

by adding CNTs of more than 4 mass pct may beattributed to the matrix structure deterioration of cokedspecimens, which can be shown from the variationsin apparent porosity (Figure 6) and bulk density(Figure 7). The apparent porosity increased and thebulk density decreased with increasing CNT content.The formation of more interfaces and difficulty in CNTdispersion can be responsible for such variations. Indraet al.[23] reported that the interface has a negative effecton TC of the composites. Jiang and Gao[19] andTsuyohiko et al.[24] found that the homogeneous dis-persion of CNTs produced a gradual increase in the TC.In our previous work,[10] we found that the high TC SiCphase content increased with coking temperature, whichresulted in the TC increase.Theoretically, CNTs can improve the TC of coked

specimens considerably because of their high TC.However, most CNTs were consumed and thus unused,which was contrary to the original expectation. So, howto retain the CNTs intact in the coked specimens is an

Fig. 4—Aggregated CNTs in the matrix.

Fig. 5—The variations of TC (298 K (25 �C)) as a function of CNTcontent.

Fig. 6—The variations of apparent porosity as a function of CNTcontent.

Fig. 7—The variations of bulk density as a function of CNT content.


important challenge. Morisada et al.[30] reported thatCNTs can be coated with a SiC layer using SiO vapor invacuum, thereby enhancing the oxidation resistance ofCNTs. Liang et al.[31] found that uniform and compactSiC coatings on the surfaces of CNTs can be synthesizedin the initial polycarbonsilane/xylene concentrationsection of 10–15 pct. Incorporating or in situ formingsuch CNTs coated with SiC in carbon refractories mayresult in the retention of CNTs.

C. Porosity Characteristics

Figure 8 shows the variations of porosity character-istics. The mean pore diameter for the coked specimenswas decreased by the addition of 0.5 mass pct CNTs,but it increased gradually again by subsequent additionof CNTs, e.g., at 1673 K (1400 �C), from 0.18 lm for0.5 mass pct CNTs to 0.44 lm for 5 mass pct CNTs.Correspondingly, the trend of <1 lm pore volume wasopposite to that of mean pore diameter, e.g., at 1673 K(1400 �C), from 80.8 pct for 0.5 mass pct CNTs to66.7 pct for 5 mass pct CNTs. The variations of poros-ity characteristics were related to the filling of SiC. As0.5 mass pct CNTs were added, a lesser amount ofCNTs can be dispersed in the matrix easily as a filler andcan result in an increased particle packing density.Thereafter, according to reaction [1], SiC is formedhomogeneously and improves porosity characteristics.The degradation of porosity characteristics by addingCNTs of more than 0.5 mass pct can be explained bytwo aspects: first is the localized formation of SiCbecause the diffusion path of SiO (g) was shortened anddecreased the pore filling; second is the increasedagglomeration of CNTs. The degraded porosity char-acteristics can produce directly a negative influence onstrength (Figure 9). From Figure 9, it can be observedthat the cold crushing strength declined drastically byadding CNTs of more than 1 mass pct.

IV. CONCLUSIONS

The following conclusions are made on the basis ofthe study of effect of CNTs on the thermal conductivityand porosity characteristics of blast furnace carbonrefractory specimens after coking for 3 hours at 1473 K(1200 �C) and 1673 K (1400 �C).

1. With the increase of CNT content, the aggregationof CNTs became more severe; the formationamount of SiC whiskers increased and their aspectratio became larger, and the SiC whiskers tended tobe distributed nonhomogeneously.

2. CNTs acted as carbon source, and most of themwere consumed by reaction with Si and O2.

Fig. 8—The variations of porosity characteristics as a function of CNT content. (a) Mean pore diameter and (b)<1-lm pore volume.

Fig. 9—The variations of cold crushing strength as a function ofCNT content.


3. The TC of 4 mass pct CNTs containing the carbonspecimen was the highest, which was attributed tothe contributions of SiC and residual CNTs.

4. The porosity characteristics of 0.5 mass pct CNTscontaining the carbon specimen was best because ofthe uniform filling of SiC whiskers. The excessive addi-tion of CNTs degraded the porosity characteristics.

ACKNOWLEDGMENT

We thank the Department of Education of Chinafor the financial support for this research under theprogram of the New Century Excellent Talents inUniversity (Grant No. NCET-06-0676).

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Effect of Multiwalled Carbon Nanotubes on the Thermal