High specific surface area porous sic ceramics coated with reticulated amorphous sic nanowires

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

Physica E 40 (2008) 2540–2544

High specific surface area porous SiC ceramics coated

with reticulated amorphous SiC nanowires Limin Shi  , Hongsheng Zhao, Yinghui Yan, Ziqiang Li, Chunhe Tang

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China

Available online 18 October 2007

Abstract

High specific surface area porous SiC ceramics, coated completely with reticulated amorphous SiC nanowires, have been fabricated with commercially available phenolic resin and silicon powders using a novel method The results indicate that the specific surface area and porosity of the as-synthesized materials can be up to 112 m2g 1and 81%, respectively The coated amorphous SiC nanowires have uniform structure and smooth surfaces The electron emission turn-on field and threshold field are about 2.9 and 6.7 V mm 1, respectively This kind of materials may simultaneously possess the unique properties of porous materials, SiC, and nanowires And they may have promising applications in a wide variety of areas

r2007 Elsevier B.V All rights reserved

PACS: 61.46 w; 81.05.Je

Keywords: Porous; SiC; Nanowires; Ceramics

1 Introduction

Due to their excellent performances, such as low density,

high permeability, and high specific surface areas, porous

materials have found numerous applications, including

filters, catalyst supports, sensors, separations, and tissue

engineering [1,2] SiC is a typical transition metal carbide

with excellent physical, chemical, mechanical, and

electro-nic properties, which make it extensively used in many

fields such as metallurgy, refractory, and environment

protection [3,4] During the past two decades,

one-dimensional nanomaterials (nanotubes, nanowires,

nanor-ods, and nanofibers) have attracted considerable attention

because of their unique physico-chemical properties and

wide variety of potential applications [5,6] It has been

evidenced that the porous SiC ceramics combine the

properties of porous materials with those of SiC This is

the same with SiC nanowires In order to meet the stringent

requirements of some applications, various methods have

been developed for fabricating porous SiC ceramics [7,8] and SiC nanowires [9,10] Similarly, the preparation of high specific surface area porous SiC ceramics coated entirely by reticulated SiC nanowires may make it possible

to obtain a kind of functional and structural material, which perhaps can simultaneously possess the excellent properties of porous materials, SiC, and 1D nanomaterials Unfortunately, few reports on the fabrication of this unique material are available until now Here, we propose

a novel method to synthesize high specific surface area porous SiC ceramics, which are coated entirely with reticulated SiC nanowires Encouraged by the success in preparing SiC nanowires[11], Al was used as catalyst in the present experiments The specific surface area of the porous SiC ceramics produced in our process can be up to

112 m2g 1 The porosity is greater than 80% The electron emission turn-on field is 2.9 V mm 1 and the electron emission threshold field is as low as 6.7 V mm 1 The reticulated SiC nanowires, which are coated on the external surface of the as-synthesized porous SiC ceramics, are amorphous At the same time, the SiC nanowires have uniform structure and smooth surface

www.elsevier.com/locate/physe

1386-9477/$ - see front matter r 2007 Elsevier B.V All rights reserved.

doi: 10.1016/j.physe.2007.10.034

Corresponding author Tel.: +86 10 89796095; fax: +86 10 69771464.

E-mail address: slm02@mails.tsinghua.edu.cn (L Shi).

2 Experimental

Commercially available phenolic resin (barium phenolic

resin; Beijing Fiberglass-Reinforced Plastics Research

and Design Institute, Beijing, China) and silicon powder

(average particle size of 9.4 mm; Beijing Da Di Zelin-Silicon

Limited Company, Beijing, China) were used as carbon

source and silicon source, respectively Analytical ethanol

was used as solvent and de-ionized water as precipitator

Aluminum powders were used as catalyst The fabrication

of porous SiC ceramics constructed by SiC nanowires

consisted of the following steps: (1) Preparation of

precursor powders based on coat-mix process[12–14]: After

being properly weighed, 10 g phenolic resin was first

dissolved into 20 g ethanol to form the binder solution,

which was stirred vigorously at 45 1C for 1 h Ten grams of

silicon powders were subsequently introduced into the

binder solution After being homogeneously mixed at 45 1C

for another 1 h, the obtained slurry was injected into

de-ionized water Then, the aqueous solution was stirred at

45 1C for about 2 h This step was followed by decanting,

filtering and drying At last, the precursor powders,

core–shell silicon-phenolic resin powders, could be

ob-tained (2) Molding: The obtained precursor powders were

pressed into the cylindrical green compacts with a diameter

of 10 mm using a stainless steel mould The temperature,

pressure and soaking time were maintained at 80 1C,

0.1 MPa and 1 h, respectively (3) Carbonizing: This step

was performed by heating the green bodies in a

program-mable quartz tube furnace in a flowing Ar atmosphere For

the sake of not destroying the samples, they were heated

from room ambient temperature to 800 1C with a heating

rate of 0.5 1C min 1 The holding time is 2 h at 800 1C (4)

Sintering: The resulting carbonized compacts and Al

powders, which were held in a graphite crucible with a

graphite lid, were sintered in a graphite furnace in Ar

atmosphere Here, the graphite crucible covered by a

graphite lid was used to provide a relatively closed

environment for the synthesis of porous SiC ceramics

coated entirely by SiC nanowires The heat treatment

started at ambient temperature and stopped at 1500 1C

with a heating rate of 5 1C min 1, then cooled in the

furnace to room ambient temperature The pressure of Ar

and soaking time were 0.15 MPa and 2 h, respectively

X-ray diffraction (XRD) pattern of the prepared porous

SiC ceramics was recorded at room temperature on a Japan

D/max-IIIA X-ray diffractometer using standard Cu Ka

radiation Renishaw RM1000 Raman microscope was used

to further investigate the structure of the as-synthesized

products A Hitachi S-3000N scanning electron microscope

(SEM), operated at an accelerating voltage of 10 kV, was

employed to characterize the morphology of the porous

ceramics Further structure characterization and fast

four-ier transform (FFT) of the SiC nanowires, which coated

with the external surface off fabricated porous SiC

ceramics, were investigated on a Tecnai TF20

high-resolution transmission electron microscope (HRTEM)

operated at 200 kV Energy dispersive X-ray spectroscopy (EDX; attached to the Technai TF20) was employed for identifying the elemental composition of the SiC nano-wires The specific surface area, median pore diameter (volume), median pore diameter (area), average pore diameter (4 V A 1), bulk density, apparent density, and porosity of the fabricated porous SiC ceramics were determined by mercury intrusion porosimetry (AutoPore

IV 9510, Micromeritics, USA) The field emission proper-ties were measured in a vacuum chamber at room temperature

3 Results and discussion

The color of the carbonized compacts is black However,

it can be seen directly that after heating at 1500 1C for 2 h with Ar pressure of 0.15 MPa, the color of the samples has turned into light green This implies that a complete conversion of silicon and phenolic resin-derived carbon into SiC has been achieved at such an experimental condition The light green color of the resulted products also indicates that the as-synthesized porous SiC ceramics possess a relatively higher purity

Fig 1(a) and (b)shows the XRD patterns of the external and internal surfaces of the porous ceramics fabricated at

1500 1C for 2 h with Ar pressure of 0.15 MPa using our process, respectively It can be found fromFig 1(a)that no obvious diffraction peaks of SiC crystals existed in the XRD pattern of the external surface of the as-synthesized materials This indicates that the external surface of the resulting ceramics is amorphous state As shown in Fig 1(b), the five sharp characteristic peaks correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of b-SiC

Fig 1 XRD patterns ((a) external and (b) internal) of high specific surface area porous SiC ceramics coated with reticulated SiC nanowires fabricated at 1500 1C for 2 h with Ar pressure of 0.15 MPa using our proposed process The amorphous nature of the external of the as-synthesized SiC materials is revealed by the XRD patterns.

phase according to the standard JCPDS cards (29-1129).

Combined with the above XRD analyses, it can be

concluded that the as-synthesized materials are SiC

ceramics coated with amorphous layers

The SEM images shown in Fig 2(a) and (b) are

the typical morphologies of the external surface and

fracture surface of the resulted porous SiC ceramics

coated with reticulated SiC nanowires, respectively

Fig 2(a) clearly reveals that the external surface of the

fabricated ceramics are constructed completely by

reticu-lated SiC nanowires with uniform diameter distribution

in the range of 30–80 nm, and lengths over several tens

of micrometers It can also be found from Fig 2(b) that

the fabricated ceramics have large amounts of irregular

pores

The further morphology characterization of SiC nano-wires, which coated on the surface of the as-synthesized porous SiC ceramics, can be seen clearly by HRTEM imaging shown in Fig 3(a) It indicates that the SiC nanowires have uniform structure and smooth surface The chemical composition of the corresponding SiC nanowire presented in Fig 3(a) is characterized by EDX spectro-scopy, as shown isFig 3(b) The EDX peaks at 0.26, 0.52, 1.51, 1.73, 0.94, 8.02, and 8.92 keV correspond to the Ka lines of carbon, oxygen, silicon, aluminum, and the La, Ka, and Kb lines of copper, respectively The peaks of copper are originated from the copper grid, which is used to prepare the TEM sample The oxygen is probably formed during the TEM sample preparation This reveals that the nanowires are SiC nanowires with a small amount of

Fig 2 The representative SEM images ((a) external surface and (b) fracture surface) of the as-synthesized porous SiC ceramics It can be found that the fabricated SiC ceramics with large amounts of pores are coated completely with reticulated nanowires with uniform structure.

Fig 3 (a) Representative TEM images, (b) EDX spectra of SiC nanowires, and (c) HRTEM lattice image of the reticulated SiC nanowires which are coated on the external surfaces of the porous SiC ceramics prepared using our process It can be evidenced that the SiC nanowire with a uniform structure

is amorphous, which is further confirmed by the FFT shown in (d).

aluminum The corresponding HRTEM lattice image and

FFT shown in Fig 3(c) and (d) indicate that the SiC

nanowires are amorphous This is in accordance with the

XRD analysis mentioned above

Table 1 lists the data for the obtained high specific

surface area porous SiC ceramics coated completely by

reticulated SiC nanowires measured by mercury intrusion

porosimetry It can be found that the median pore diameter

(volume) and average pore size are 1.6 mm and 25 nm,

respectively The bulk density is 0.61 g cm 3 and the

apparent (skeletal) density is 3.19 g cm 3 The open

porosity can be up to 81% Moreover, it is surprising to

find that the specific surface area of the porous materials

prepared by our process is greater than 100 m2g 1 These

unique properties may make this novel material an

attractive candidate for many potential applications such

as catalysis supports

Fig 4shows the typical plot of emission current density versus electric field (J–E) for the obtained high specific surface area porous SiC ceramics coated entirely by reticulated SiC nanowires The electron emission is observed at an electric field of about 2.8 V mm 1 The electron emission turn-on field (Eto) and threshold field (Ethr), defined as the macroscopic fields required to produce a current density of 10 mA cm 2and 10 mA cm 2, are about 2.9 and 6.7 V mm 1, respectively The low Ethr

value indicates that this material produced by our method may have promising applications in flat panel displays By plotting ln(J
E 2) versus E 1, a Fowler–Nordheim (F–N) curve can easily be obtained (inset inFig 4) The linearity

of the curve shows that the obtained materials possess a conventional field emission mechanism

4 Conclusions

In summary, we have demonstrated a process for the fabrication of high specific surface area porous SiC ceramics, which are coated completely with reticulated amorphous SiC nanowires Commercially available phe-nolic resin and silicon powders are employed as carbon source and silicon source, respectively The reticulated SiC nanowires coated on the external surface of the as-synthesized materials are amorphous phase Meanwhile, these SiC nanowires have uniform structure and smooth surface The as-synthesized porous SiC ceramics, coated completely with reticulated amorphous SiC nanowires, possess high specific surface areas and porosity The specific surface area and porosity can be up to 112 m2g 1 and 81%, respectively The field emission properties of the obtained high specific surface area porous SiC ceramics coated by reticulated SiC nanowires are measured The Eto

and Ethr are about 2.9 and 6.7 V mm 1, respectively Thus,

we can envisage that the prepared materials may find interesting applications in filters, catalyst supports, and electronic devices Given the simplicity of the procedures and the unique properties of the as-synthesized materials, the method described here should attract a great deal of attention Other high specific surface area porous carbide ceramics coated with carbide nanowires may also be fabricated by this method

Acknowledgment

This work is supported by Key Faculty Support Program of Tsinghua University

References

[1] R.A Caruso, J.H Schattka, Adv Mater 12 (2005) 1921.

[2] Y.J Wang, F Caruso, Adv Funct Mater 14 (2004) 1012 [3] T Ishikawa, Y Kohtoku, K Kumagawa, T Yamamura,

T Nagasawa, Nature 391 (1998) 773.

[4] D Nakamura, I Gunjishima, S Yamaguchi, T Ito, A Okamoto,

H Kondo, S Onda, K Takatori, Nature 430 (2004) 1009.

Table 1

The major properties of the porous SiC ceramics coated entirely by

reticulated SiC nanowires fabricated at 1500 1C for 2 h with Ar pressure of

0.15 MPa using our proposed process

Specific surface area (m 2 g 1 ) 112

Median pore diameter (volume) (mm) 1.6

Median pore diameter (area) (nm) 6

Average pore diameter (4 V A 1 ) (nm) 25

Apparent (skeletal) density (g cm 3 ) 3.19

It is surprising to find that the specific surface area of the resulting porous

SiC ceramics is greater than 100 m2g 1, which may help this novel

material find many applications.

Fig 4 The typical emission J–E plot from the obtained high specific

surface area porous SiC ceramics coated with reticulated SiC nanowires.

The E to and E thr are about 2.9 and 6.7 V mm 1, respectively Inset: F–N

plot The linearity of the curve implies that the emission of the high

specific surface area porous SiC ceramics coated with reticulated SiC

nanowires agrees with the properties expected for field emission.

[5] M Law, L.E Greene, J.C Johnson, R Saykally, P.D Yang, Nat.

Mater 4 (2005) 455.

[6] S Iijima, Nature 354 (1991) 56.

[7] K Sonnenburg, P Adelhelm, M Antonietti, B Smarsly, R No¨ske,

P Strauch, Phys Chem Chem Phys 8 (2006) 3561.

[8] Y Shin, C.M Wang, G.J Exarchos, Adv Mater 17 (2005) 73.

[9] Z.W Pan, H.L Lai, F.C.K Au, X.F Duan, W.Y Zhou, W.S Shi,

N Wang, C.S Lee, N.B Wong, S.T Lee, S.S Xie, Adv Mater 12

(2000) 1186.

[10] H Ye, N Titchenal, Y Gogotsi, F Ko, Adv Mater 17 (2005) 1531.

[11] S.Z Deng, Z.S Wu, J Zhou, N.S Xu, J Chen, J Chen, Chem Phys Lett 356 (2002) 511.

[12] D Simwonis, H Thu¨len, F.J Dias, A Naoumidis, D Sto¨ver,

J Mater Process Technol 92 (1999) 107.

[13] H Luhleich, J Dias, H Nickel, Carbon 35 (1997) 95.

[14] L Shi, H Zhao, Y Yan, Z Li, C Tang, Powder Technol 169 (2006) 71.