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Vacuum 82 (2008) 336–339

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Direct determination of carbothermal reduction temperature forpreparing silicon carbide from the vacuum furnace thermobarogram

Yong Zhenga, Ying Zhenga,�, Rong Wangb, Kemei Weib

aCollege of Chemistry and Materials Science, Fujian Normal University, Fuzhou, Fujian 350007, ChinabNational Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou 350002, China

Received 28 January 2007; received in revised form 25 April 2007; accepted 29 April 2007

Abstract

In this study, we combined the traditional vacuum furnace with a sur-high accuracy vacuum gauge for measuring the change of gas

pressure in the furnace as a function of temperature. The modified vacuum furnace was used for preparing silicon carbide (SiC), and the

recorded data named thermobarogram were applied to detect the reaction temperature for synthesis of SiC, which, in particular, changes

as catalysts are used. The results showed that the carbothermal reduction temperature for synthesizing SiC, under dynamic vacuum

condition, would decrease to 850 1C when the mole ratio of Fe/Si ¼ 0.06 was employed in the SiC precursor.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Carbothermal reduction; Silicon carbide; Vacuum furnace; Synthesis

1. Introduction

For the past several decades, silicon carbide (SiC) hasbeen known for its many outstanding properties [1–3], suchas high thermal stability, high mechanical strength, andhigh thermal conductivity. Therefore, it has not only beenwidely used in the ceramic [4] and abrasive industry [5], butalso being studied for catalyst [6] and sorbent support [7].In addition, its wide band gap and high electronic mobilitymake SiC a promising semiconductor material for thefabrication of electronic devices [8,9]. The carbothermalreduction of sand by coke, known as the Acheson process[8], is the main method of SiC production. However, thismethod is an energy-intensive process and involves manysteps for synthesizing a phase-pure SiC. In the last decade,considerable attention has been focused on developingvarious methods for preparing SiC. The conventionalprocessing techniques, such as solid–solid (carbothermalreduction) reaction [10], gas–solid (chemical vapor deposition)reaction [11], hot-pressing and sintering [12], are oftenemployed to prepare SiC for ceramic or abrasive materials.

ee front matter r 2007 Elsevier Ltd. All rights reserved.

cuum.2007.04.037

ing author. Tel./fax: +86 591 83465225.

ess: zhn63201@163.com (Y. Zheng).

The improved methods, such as sol–gel process [13,14],nanocasting process [15], shape-memory-synthesis (SMS)method [16], microwave heating technique [17], andtemplating procedure [18], are used to obtain SiC materialswith a well-defined morphology. Among these, Rameshand co-workers [17] have synthesized SiC powder from Siand C using a kitchen microwave oven at 980 1C, which is arelatively low temperature. However, with different meth-ods or reactants, the reaction temperature for preparingSiC would vary from 1000 to 1800 1C. Therefore, thesuitable reaction temperature for preparing SiC must betested one by one when the method or reactant changes.These massive repetition tests would be time consumingand energy intensive.In our experiments, carbothermal reduction was carried

out in an improved vacuum furnace combined with a sur-highaccuracy vacuum gauge. The change of gas pressure in thevacuum furnace with temperature was monitored duringthe process of carbothermal reduction. This method iscalled the modified thermobaric analysis, and the recordedcurve is called thermobarogram. From the thermobaro-gram, the lowest carbothermal reduction temperature forpreparing SiC could be determined directly. It has beenproved to be a very useful tool in studying the effect of Fe

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Fig. 1. The thermobarogram of the control sample.

Fig. 2. The XRD patterns of control sample obtained at 1000, 1100, and

1300 1C.

Y. Zheng et al. / Vacuum 82 (2008) 336–339 337

catalysis in the SiC precursors on carbothermal reductiontemperature for preparing SiC.

2. Experimental

First, 25 g saccharose (AR grade) and a small amount offerric nitrate were dissolved in the mixed solution of 20mlabsolute ethanol (AR grade) and 25ml deionized water bymagnetic stirring. Next, 50ml tetraethoxysilane (TEOS,AR grade) and 1ml concentrated nitric acid solution(AR grade) were added to the solution slowly, and then thesolution was kept stirring at 60 1C to enhance thehydrolysis of TEOS until it formed a gelatin. Xerogel wasobtained after the gelatin was aged and dried at 100 1C inan oven for 24 h to remove the excess water and othersolvents. A control experiment without ferric nitrate wasalso performed.

The obtained xerogels were placed into the graphitevacuum furnace. A lower than 10�2 Pa vacuum wasgenerated inside the furnace. Then the furnace was keptbeing evacuated (under dynamic vacuum condition) whileit was heated at a rate of 20 1C/min. At the same time, thechange of gas pressure in the furnace was recorded as afunction of temperature. We named the recorded data as athermobarogram. From this thermobarogram, the lowesttemperature of carbothermal reduction for preparing SiCcan be directly determined. When the lowest temperaturewas determined, the vacuum furnace was held at thistemperature for carbothermal reduction process until thegas pressure decreased to below 10�2 Pa.

Structural analysis of the obtained samples was carriedout on a Bruker D8 Advance X-ray diffractometer with CuKa radiation (40 kV, 40mA) at a scanning rate of 11/min.

3. Results and discussion

The corresponding overall carbothermal reduction thattakes place in the vacuum furnace may be written asfollows [19]:

SiO2ðsÞ þ 3CðsÞ ! SiCðsÞ þ 2COðgÞ:

During this solid–solid reaction, gaseous products aregenerated when the carbothermal reduction reaction takesplace. Obviously, once the SiC is formed, the gas pressureof vacuum furnace should be increased. In other words,whether the SiC has formed or not can be judged bymonitoring the change of gas pressure in the vacuumfurnace.

Fig. 1 shows the thermobarogram of the control sample.During the carbothermal reduction process, the gaspressure increased dramatically at about 150–200 and350–600 1C, which may be due to the pyrolysis of xerogel atthese two temperature regions. When the temperaturewas raised above 800 1C, the pyrolysis of xerogel wouldbe completed [20], and mixture of C and SiO2 would beformed. At the last temperature region (1100–1400 1C), thegas pressure increased dramatically again; this may be due

to the generation of the CO product. It may suggest theoccurrence of carbothermal reduction reaction of C andSiO2. We stopped the reaction process immediately whenthe temperature was achieved at 1000, 1100, and 1300 1C,respectively. Then the cooled samples were characterizedby X-ray diffraction (XRD). The results are shown inFig. 2.At 1000 1C only amorphous silica appeared in the XRD

profile, which indicated that SiC did not form at thistemperature. The XRD profile at 1100 1C displays foursmall peaks at 2y ¼ 35.61, 41.41, 59.91, and 721, whichcorrespond to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes ofthe b-SiC according to the Ramsdell notation [21],respectively. It indicated the formation of b-SiC. Theintensity of characteristic diffraction peaks of SiC en-hanced when the carbothermal reduction temperature wasincreased to 1300 1C. Obviously, the rate of carbothermalreduction reaction would be increased as the temperaturerises. In other words, the generation rate of CO gas wasincreased with rising temperature, which resulted in adramatical increase of the gas pressure in the vacuum

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Fig. 3. The XRD patterns of control sample maintained at 1000, 1100,

and 1300 1C, until the gas pressure in the vacuum furnace decreased to

below 10�2 Pa.

Fig. 4. The thermobarograms of samples containing Fe/Si mole ratios of

0.02, 0.04, and 0.06.

Y. Zheng et al. / Vacuum 82 (2008) 336–339338

furnace. We can conclude that the dramatical change of gaspressure at the last temperature region from the thermo-barogram is due to the reaction of C and SiO to form SiC.To discuss whether 1100 1C is the optimal temperature forproducing large quantities of SiC, the temperature wasmaintained at 1000, 1100, and 1300 1C, respectively, untilthe gas pressure in the vacuum furnace decreased to below10�2 Pa. For different samples, this process usually takes2–3 h, and it is much shorter than the Acheson process [8].Then the obtained samples were analyzed by XRD. Theresults are shown in Fig. 3.

At 1000 1C, only amorphous silica appeared in the XRDprofile, indicating that the SiC was not yet formed at thistemperature for the control sample in these experimentalconditions. However, both samples heat treated at 1100and 1300 1C display the strong intensity of characteristicdiffraction peaks of b-SiC. As the carbothermal reductiontemperature at 1100 1C is enough for preparing largequantities of SiC, it is not necessary to increase thetemperature. As is known, the partial pressure of CO gasproducts will affect the rate of solid–solid reactionaccording to the principle of chemical kinetics. Obviously,when the temperature achieved a suitable degree, the lowerpartial pressure of CO will be beneficial to the rate ofcarbothermal reduction reaction. Conversely, if CO main-tains in the system, it would decrease the rate of thecarbothermal reduction reaction. So it is necessary toenhance the reaction temperature for preparing SiC. In theexperimental conditions, the gaseous product was con-tinuosly evacuated from the reaction system. It wouldmaintain the relatively lower pressure of CO in the systemand make the rate of carbothermal reduction steady.Therefore, the temperature at 1100 1C is enough forpreparing large-scale SiC.

From the analysis above, the lowest carbothermalreduction temperature for preparing SiC would be directlydetermined from the beginning of the last temperature

region (1100–1300 1C) of the thermobarogram, which wasrecorded during the reaction process. Keller et al. [22] andLi et al. [23] have studied the carbothermal reduction forsynthesizing SiC by the vacuum furnace. They all carriedout the carbothermal reduction reaction at high tempe-ratures (1300–1600 1C) directly, but did not notice thechange of gas pressure in the vacuum furnace. And theydid not used the relationship between the change of gaspressure and the reaction temperature to study thepreparation temperature of SiC. In this work, the thermo-barogram was proposed for determining the lowestcarbothermal reduction temperature for preparing SiCfor the first time. From this kind of thermobarogram, westudied the effect of Fe catalysis in the SiC precursor oncarbothermal reduction temperature for preparing SiC.The results of corresponding thermobarogram for differentmole ratios of Fe/Si are shown in Fig. 4.From these thermobarograms, we can see that with the

increase of the mole ratio of Fe/Si in the precursor, thecorresponding last temperature region of the gas pressurechange shifts toward the lower temperature region. Itindicated that the lowest temperature for generating COgas decreased with the increased mole ratio of Fe/Si. Thelowest temperatures for the mole ratio of Fe/Si ¼ 0.02,0.04, and 0.06 are 1150, 1000, and 850 1C, respectively,which indicated that the corresponding carbothermalreduction temperatures for preparing SiC would be 1150,1000, and 850 1C. Then the reaction temperatures werekept at 1150, 1000, and 850 1C, respectively, until the gaspressure in the vacuum furnace decreased to below 10�2 Pa.The obtained samples were analyzed by XRD. The resultsare shown in Fig. 5.All three profiles show the intense characteristic diffrac-

tion peaks of b-SiC and no trace of silica or silicon could beobserved. It is indexed that SiC is a major constituent of allsamples, which are obtained from the correspondingcarbothermal reduction temperatures at 1150, 1000 and850 1C for the mole ratios of Fe/Si ¼ 0.02, 0.04, and 0.06,

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Fig. 5. The XRD patterns of samples containing Fe/Si mole ratios of 0.02,

0.04, 0.06, and maintained at 1150, 1000, and 850 1C, until the gas pressure

in the vacuum furnace decreased to below 10�2 Pa.

Y. Zheng et al. / Vacuum 82 (2008) 336–339 339

respectively. In other words, from the correspondingthermobarograms, we can directly determine the lowestcarbothermal reduction temperature for preparing SiCfrom the SiC precursors, which contained the Fe catalyst.In these studies, we have prepared SiC under relatively lowtemperature at 850 1C. To our knowledge, this is the lowesttemperature for preparing SiC by the method of carbother-mal reduction. The decrease in reaction temperature forpreparing SiC may be due to the formation of Fe–Sieutectoid composition [24] during the carbothermal reduc-tion process. In addition, the addition of Fe in the SiCprecursor may also affect the pyrolysis of xerogel. There-fore, the changed process of gas pressure in the vacuumunder 800 1C of the samples containing Fe is very differentfrom the control sample. The temperature of completepyrolysis of xerogel was decreased with the increased of themole ratio of Fe/Si in the precursor. When the mole ratioof Fe/Si is 0.06, the temperature would drop to 550 1C.

4. Conclusions

In conclusion, we have proposed a new method for thedirect determination of the lowest carbothermal reductiontemperature for preparing SiC from the thermobarograms.This method has been successfully applied to study theeffect of Fe catalyst in the SiC precursor on carbothermalreduction temperature for preparing SiC, and the lowestcarbothermal reduction temperature for preparing SiC hasbeen determined from the corresponding thermobarogram.The results showed that the carbothermal reductiontemperature for preparing SiC, under dynamic vacuum

condition, would drop to 850 1C when the mole ratio ofFe/Si ¼ 0.06 was employed in the SiC precursor. Thismethod could be hopefully applied to all high-temperaturesolid-state reactions, which produce gaseous products, forthe research of the reaction mechanism.

Acknowledgments

The authors are grateful for the financial supportfrom National Natural Science Foundation of China(20576021), Science and Technology Priority Project ofFujian Province (2005HZ01-2) and Science Foundationof Fujian Province Education Commission of China(2005K015).

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