Synthesis of monolithic mesoporous silicon carbide from
resorcinol–formaldehyde/silica composites
Yong Kong,Ya Zhong,Xiaodong Shen n,Longhua Gu,Sheng Cui,Meng Yang
College of Material Science and Engineering,Nanjing University of Technology,Nanjing210009,PR China
a r t i c l e i n f o
Article history:
Received7October2012
Accepted9February2013
Available online17February2013
Keywords:
Silicon carbide
Aerogel
Carbothermal reduction
Sol–gel process
Porous materials
a b s t r a c t
Resorcinol–formaldehyde/silica composite(RF/SiO2)gels were synthesized in one pot using a facile
process.RF/SiO2aerogels were obtained after supercritical carbon dioxidefluid drying.Monolithic
mesoporous silicon carbide(SiC aerogels)was prepared from RF/SiO2aerogels after carbothermal
reduction and calcination.The as-prepared SiC products exhibited monolithic mesoporous morphology
and possd a BET specific surface area of251m2/g and a pore volume of0.965cm3/g.X-ray
diffraction(XRD)and transmission electron microscopy(TEM)demonstrated that the resulting SiC
aerogels were compod of a-SiC nanocrystals.The bulk density and skeleton density of SiC products is
0.288g/cm3and3.12g/cm3,respectively.The porosity of SiC products is90.8%.The SiC aerogels were
stable up to temperatures near6501C.
&2013Elvier B.V.All rights rerved.
1.Introduction
路易十四的情人Silicon carbide(SiC)shows high hardness,good thermal shock
resistance,high thermal conductivity and stability,low thermal
expansion coefficient,superior chemical inertness and large band
gap,and therefore is considered as a promising material in many
fields of catalysis,high-power and high-frequency electronics,
茯苓有什么功效photoelectric,anti-radiation,and wave-absorbing devices[1–4].
Typically,porous SiC is prepared by carbothermal reduction of
silica and carbon at a high temperature,and binary carbonaceous
silica composites are commonly ud as precursors of porous SiC
[5–7].However,the products yielded by the techniques are
usuallyfibers and whiskers,and the process does not adequately
克什米尔公主号事件prerve the starting network structure of the precursors.Never-
theless,monolithic SiC aerogels have been successfully synthe-
sized from polymer cross-linked silica aerogels recently[8].The
resulting SiC prented monolithic and porous structure,and had
a BET specific surface area of$20m2/g.Alternatively,monolithic
SiC aerogels with a BET specific surface area of232m2/g have
been prepared by carbothermal reduction from resorcinol–
formaldehyde/silica composite(RF/SiO2)aerogels[9].However,
the methods employed to form carbon–silica hybrid gels are
complicated and time-consuming.In tho techniques,tetraethy-
lorthosilicate(TEOS)and tetramethylorthosilicate(TMOS)are
usually ud as silicon sources,and acids and alkalis are involved
as catalysts.Moreover,there is a very different gelation time
between RF gels and silica gels.Therefore,to form carbon–silica
hybrid gels,the silica sol and carbonaceous sol had to be prepared
parately,and the processing went through multiple-step
sol–gel process.
In the past,(3-aminopropyl)triethoxysilane(APTES)and(3-
aminopropyl)trimethoxysilane(APTMS)were commonly ud as a
amino-functionalized modifier in the synthesis of porous silica
[10–16],but have never been ud solely as a silica source.There-
fore,we propo a facile synthesis of RF/SiO2aerogels.RF/SiO2gels
were synthesized in one pot by simply mixing the monomers,and
no catalysts were required.SiC aerogels were formed from RF/SiO2
aerogels after thermal treatment.
2.Experimental
Sample preparation:APTES,resorcinol(R),formaldehyde
(F,37%w/w aqueous solution)and anhydrous alcohol(EtOH)
自制酸豆角were ud as raw materials.All of the reagents and solvents were
analytical grade and ud as received without further purification.
果断的反义词
R,F,APTES,EtOH and deionized water(W)were mixed in a pot at
room temperature,with R:F:APTES:EtOH:W prepared at a molar
ratio of1:2:2:25.7:2.Subquently,the compound was trans-
ferred into plastic molds(40mm in inner diameter).After gela-
tion,the wet gels were aged at701C for24h and simultaneously
washed with ethanol every8h.The alcohol gels were dried in an
autoclave(HELIX1.1system,Applied Separations,Inc.,Allentown,
PA)with supercriticalfluid CO2to form RF/SiO2aerogels.The
thermal treatment of RF/SiO2aerogels was performed in a tube
furnace(72mm inner diameters of tube).The temperature was
first raid to15001C with a rate of21C/min underflowing argon
小学数学学情分析
(100ml/min),and maintained at that level for5h.Subquently,
Contents lists available at SciVer ScienceDirect
journal homepage:/locate/matlet
Materials Letters
0167-577X/$-e front matter&2013Elvier B.V.All rights rerved.
dx.doi/10.1016/j.matlet.2013.02.047
n Corresponding author.Tel.:þ862583587235;fax:þ862583221690.
E-mail address:xdshen@njut.edu,
(X.Shen).
Materials Letters99(2013)108–110
the temperature was lowered to 6001C,flowing argon was changed to flowing air,and excess of carbon was burned off by maintaining the temperature at that level for 2h.
Sample characterization :Bulk densities (r b )were calculated from the weight and the physical dimensions of the samples.Skeletal densities (r s )were determined by helium pycnometry using a Micromeritics AcuuPyc II 1340instrument.Porosity was determined from the r b and r s values,porosity ¼1Àr b /r s .The microstructure was surveyed by LEO-1530VP scanning electron microscopy (SEM).The pha composition was evaluated by ARL ARLX’TRA X-ray diffraction (XRD)using a Cu-K a radiation.Trans-mission electron microscopy (TEM)was conducted using a JEOL JEM-2010electron microscope.Surface areas,pore volume and pore size distribution were measured by nitrogen adsorption/desorption isotherms using a Micromeritics ASAP2020surface
area analyzer.The specific surface area (s )was calculated using Brunaur–Emmett–Teller (BET)methods.The pore size distribu-tion was derived from the desorption branch of isotherms by using the non-local density functional theory (NLDFT)model.The pore volume was estimated from the adsorbed amount at a relative pressure p/p 0of 0.986.Thermogravimetric analysis (TGA)was performed by NETZSCH STA449C Thermogravimetric Analyzer to determine the thermal stability under a constant air flow of 30ml/min at a heating rate of 201C/min.
3.Results and discussion
Fig.1shows the XRD patterns of SiC aerogels.The peaks with 2y values of 341,35.71,38.21,41.41,601,65.61,71.81,73.61,and 75.51correspond to the crystal planes of 101,102,103,104,110,109,202,203and 204,respectively,for (6H of a -)SiC (PDF#29-1128).No other crystalline phas of silica,carbon or other impurities were detected.Analysis of the peaks using the Scherrer equation indicates that the average crystallite size of SiC is approximately 6.1nm.
Fig.2shows the photograph,SEM and TEM images of SiC aerogels.Although the as-prepared SiC aerogels exhibited significant weight loss and shrinkage relative to RF/SiO 2aerogels,they still prerved monolithic morphology.For SiC aerogels,the mass loss approaches 70%and the linear shrinkage is about 40%relative to RF/SiO 2aerogels.The bulk densities of RF/SiO 2and SiC aerogels are 0.206cm 3/g and 0.288cm 3/g,respectively.The skeleton density of SiC aerogels is 3.121g/cm 3,therefore,the porosity of SiC aerogels is 90.8%.As shown in the SEM and TEM images (Fig.2(b)and (c)),SiC aerogels prent porous structures of a typical colloidal gel,which is consistent of nanopores and SiC nanocrystalline framework.As obrved from the high-resolution transmission electron image (HRTEM,Fig.2(d)),the lattice fringe,with spacing of approximately 0.262nm,corresponds to the 101crystal plane of 6H–SiC (PDF#29-1128).Selected area electron diffraction (SAED)patterns are shown as an int in Fig.2(c),and the crystal planes are marked.In combination with the XRD patterns,the SAED rings and the
HRTEM
Fig.1.XRD patterns of SiC
aerogels.
Fig.2.(a)Photograph,(b)SEM image,(c)TEM image,(d)HRTEM image of SiC aerogels.Ints in (c):SAED patterns.
Y.Kong et al./Materials Letters 99(2013)108–110109
image reveal that the resulting product is (6H of a -)SiC.However,the porous SiC synthesized from C/SiO 2composites by carbothermal reduction are generally b pha [8,9,17–20].b -SiC reportedly starts forming at 12501C from C/SiO 2aerogels under dynamic Ar-flow [19].Similarly,small amount of 6H–SiC reportedly starts forming from 16001C [8].In our work,SiC can not be detected in samples prepared below 15001C,and there is no evidence of existence of b -SiC in the SiC products.
Fig.3shows the nitrogen adsorption/desorption isotherms of SiC aerogels.It is Type IV curves with type H1hysteresis loop in the IUPAC classification,characteristic of mesoporous structure with cylindrical pores [21].The BET specific surface area of SiC aerogels is 251m 2/g,the pore volume is 0.965cm 3/g.The pore-size distribution (shown as int in Fig.3)in the range of 1–27nm indicates the prence of well-defined nanopores.However,it can be worked out that a sample with a porosity of 90.8%and a bulk density of 0.288g/cm 3should have a total porosity of 3.15cm 3/g.Comparing the S
EM and TEM images with the N 2adsorption/desorption analysis,it is found that there is a significant level of porosity prent which the N 2analysis will not measure.The int of Fig.3shows the largest pores to be approximately 30nm while Fig.2(c)shows a mass of macropores.Therefore,it is worth noting that the N 2adsorption analysis can only measure the nanopores,and the macropores can not be measured by the N 2adsorption analysis.
TG curve (Fig.4)of the SiC aerogels shows that almost no loss of weight was obrved below 6501C,which suggests that the residual carbon was completely removed.The products gained weight above 6501C,denoting the oxidation of SiC nanocrystals in air.
4.Conclusions
In conclusion,a simple method for the synthesis of RF/SiO 2aerogels was prented.RF/SiO 2aerogels were converted to monolithic SiC aerogels after carbothermal reduction and calcina-tion.XRD and TEM analys indicated that nanocrystalline a -SiC was formed after carbothermal reaction.The as-synthesized SiC aerogels showed mesoporous structure with cylindrical pores.
The SiC monoliths exhibited good anti-oxidation property below 6501C in air.This new class of materials can be potentially ud in catalytic,electronic,thermal and photoelectric applications.
Acknowledgment
This work was supported by the support from the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD)and the Program for Changjiang Scholars and Innovative Rearch Team in University (PCSIRT).
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Fig. 3.Nitrogen adsorption/desorption isotherms of SiC aerogels.Ints:the corresponding pore-size
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Fig.4.TG curve of SiC aerogels in air.
Y.Kong et al./Materials Letters 99(2013)108–110
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