-Organic-Inorganic Conversion Process for Material Creation –Formation and Function of Characteristic Nanostructures–

In the case that an organometallic compound or an organic metal complex is used as the raw material in an “organic–inorganic conversion” process, the desired shape and characteristic nanostructure can be formed. This nanostructure can provide unique functionality and lead to the creation of original materials. From a ﬁbrous form of polycarbosilane, which is an organosilicon polymer, made by melt spinning in an inert gas, continuous inorganic SiC ﬁbers can be obtained. These ﬁbers are not easily oxidized and their tensile strength does not decrease even in a high temperature in air. They can therefore be used in ﬁber-reinforced composite materials. Polycarbosilane is also an excellent binder for ceramic powder that is diﬃcult to sinter. In addition, it is an excellent impregnation agent for ceramic compacts. Thus, preparation of ceramic compacts having high mechanical strength and oxidation resistance is possible using polycarbosilane. On the other hand, when bis(acetylacetonato)zinc; (Zn(acac) 2 ), which is an organic metal complex, is made into a ﬁbrous form by sublimation and pyrolysis with superheated steam, inorganic ZnO ﬁbers can be obtained. Such ﬁbers exhibit visible-light photocatalytic ability and eﬀectively decompose volatile organic gases.


I. INTRODUCTION
"Organic-inorganic conversion" is an excellent method for synthesizing materials because the desired shape and fine structure of the material can be produced. Recently, advance in high temperature engineering, nuclear engineering and rocket propulsion have stimulated interest in SiC as a refractory because this material is expected to have high resistance to oxidation, corrosion and thermal shock [1]. However, SiC cannot be sintered easily by the ordinary methods [2][3][4][5][6][7][8][9]. Because the interatomic force of SiC is highly directional and its coordination number is low, the vacancy diffusion required for sintering is strongly inhibited. Yajima and his group have developed SiC fibers with high tensile strength and elasticity on the basis of the concept of "organic-inorganic conversion"; polycarbosilane was used as a precursor matter [10][11][12]. The success of this research served the basis for a broad-range investigation of the application of polycarbosilane [13][14][15][16]. It was found that polycarbosilane could be converted to fine crystalline SiC particles through heat treatment. Polycarbosilane acted as an excellent binder for SiC powders and it has enabled to produce shaped SiC bodies [14][15][16].

A. Silicon carbide fibers
Continuous fibers of inorganic SiC are prepared from the organometallicpolymer of polycarbosilane [10- 12]. Polycarbosilane is prepared from dechlorinated dimethyldichlorosilane by polymerization in an autoclave at 450 • C for 15 h (Fig. 1). Obtained polycarbosilane is formed as a precursor fiber by melt-spinning. Then, precursor fiber is heated to produce the inorganic SiC fiber (Fig. 1, process (A)). Thus, the organic precursor is prepared in a subtle form of fiber which is extremely difficult to produce from an inorganic precursor [10]. On heating the organic precursor a ceramic skeleton remains. This inorganic residue is consisted of Si and C and it possesses many excellent characteristics not previously reported. The SiC fiber with 10 µm diameter ( high tensile strength (350 kg·mm −2 ) (Fig. 3) and high Young's modulus (20 tonnes mm). The tensile strength and the Young's modulus of the SiC fibers are almost independent of temperature (between room temperature and 1400 • C) in vacuo [11]. Figure 4 shows the X-ray (Ni-filtered CuKα) diffraction patterns of the precursor fiber heated at (a) 1000, (b) 1100, (c) 1200, (d) 1300 and (e) 1500 • C. The X-ray diffraction pattern on heating at 1500 • C indicates a larger fraction of crystallized β-SiC in comparison with the patterns of fibers heated at 1100-1300 • C. Figure 5 shows a high-resolution electron micrograph of the pulverized SiC fiber prepared by heating a precursor fiber at 1300 • C in a vacuum [12]. The lattice fringe corresponding to a 0.25 nm interlayer spacing of β-SiC (111) planes is observed. The β-SiC phase has granular morphology and its grain size is estimated as about 2-4 nm.
Obtained SiC fibers are very flexible and heat-resisting. The Youngs modulus of these fibers are also very high. These fibers are well suited for the reinforcement of metals, ceramics and plastics [31][32][33].

B. Silicon carbide compacts
Polycarbosilane acted as an excellent binder for ceramic powder that is difficult to sinter such as SiC. A method was developed to produce shaped SiC bodies using polycarbosilane as a binder (Fig. 1, process (B)) [14][15][16]. The polycarbosilane was added to the α-SiC powder (filler) and the mixture was sintered at several temperatures in N 2 stream. Hot pressing was not required. The SiC compacts obtained have a low density but a high bend strength as shown in Fig. 6 [14]. Polycarbosilane is also an excellent impregnation reagent [15]. Figure 7 shows that the number of impregnation affects the density and In the case of the as-sintered specimen ( Fig. 8(a)) the sharp edge of the α-SiC particles is clearly observed. The micrograph of the impregnated sample ( Fig. 8(b)) shows that all α-SiC particles are fully covered with β-SiC from the pyrolyzed polycarbosilane. The polycarbosilane decomposes to β-SiC and that the decomposition products are fine particles (≈10 nm); these fine particles bind strongly with the α-SiC particles and can sinter rapidly. Since polycarbosilane is used as a binder, impurities are avoided; therefore, a high purity SiC compacts are also ensured. Polycarbosilane is useful to prepare another ceramic shaped bodies such as Si 3 N 4 , BN and B 4 C [16]. Composite materials of ceramic dispersed metals and alloys are also prepared using polycarbosilane [34][35][36][37].

C. Zinc oxide fibers
As shown in Figs. 9 and 10, zinc oxide (ZnO) fiber was prepared from the precursor Zn(acac) 2 [22][23][24]. Fine polycrystalline Zn(acac)2 fiber was obtained by the sublimation of Zn(acac) 2 powder. When the Zn(acac) 2 fiber was pyrolyzed with superheated steam (Fig. 11), it was converted to ZnO fiber, while maintaining the original shape of the Zn(acac) 2 fiber. At this stage, polycrystalline ZnO, aggregated ZnO single crystal grains, which had diameters of a few nanometers, were present along with relatively large amounts of carbon (C) and hydrogen (H). The ZnO fiber sample produced after crystal grain growth by heat treatment at 800 • C contained almost no C and H, and nearly complete mineralization was achieved [25][26][27][28]. XRD patterns of compounds involved in the organicinorganic conversion are shown in Fig. 12. Final product of inorganic ZnO fiber consisted of ZnO microrods, and these microrods consisted of ZnO nanorods (Fig. 13). The surface of the ZnO nanorods was covered with scale-like nanosized-single-crystal grains (Fig. 14), which had high crystallinity, clean surfaces, and diameters of a few tens of nanometers [29,30]. This fiber efficiently decomposed a volatile organic compound (VOC), xylene as shown in Fig. 15. This decomposition of xylene was achieved by oxidation on the fiber surface under the illumination of white fluorescent light, which had a color temperature of 3500 K where visible light is dominant. This means that this fiber has a photocatalytic ability in response to visible light [22][23][24]. The visible-light photocatalytic ability of the ZnO fiber may be strongly related to the nanosizedsingle-crystal grains [29,30].
ZnO and TiO 2 have energy gaps of 3.2-3.4 eV, show the photocatalytic activity under near UV irradiation. The ultraviolet light irradiated into the photocatalyst forms the electron-hole pair in these oxide materials, and generates the active oxygen and the hydroxyl radical. The activators remove the toxic substance such as xylenes by the oxidative decomposition reaction. Therefore, ZnO and TiO 2 do not show the photocatalytic activity by the irradiation energy less than direct band-gap of 3.2 eV.
The PL spectrum of the ZnO fibers were obtained using a fluorescence spectrophotometer (Shimazu RF-5300PC) with the violaceous LED (400 nm) as the excitation source. Figure 16 is a result of comparing the PL property of the commercially available ZnO powder and the ZnO fiber. PL spectrum is hardly seen from the ZnO powder though luminescence around 440 nm can be confirmed from the ZnO fiber. The intraband level of the zinc oxide material is analyzed by many research groups, and the PL spectrum originates in the intraband level. However, these results cannot prove the photocatalytic activity by the visible-light irradiation. The photocatalytic activity appears by the electron that exists in the conduction band. We paid attention to the bonding interface of nanosized-single-crystal grains, and proposed the band bending model like Fig. 17. In this model, the electron excited to the localized level under the conduction band moves to the conduction band via the bent position of the band.
It can be concluded that the excitation electron gen- erated by the visible light irradiation exhibites the photocatalytic activity. In addition, when Zn(acac) 2 fiber is used as a raw material in chemical vapor deposition, extremely flat ZnO thin films can be formed which are promising as substrate materials [38][39][40].

III. CONCLUSIONS
Material creation based on the "organic-inorganic conversion" has been described. This method enabled to form the desired shape and characteristic nanostructure. This high Young's modulus (20 tonnes mm). The tensile strength and the Young's modulus of the SiC fiber are almost independent of temperature (between room temperature and 1400 • C) in vacuo.
2) Polycarbosilane acted as an excellent binder for ceramic powder that is difficult to sinter such as SiC.
The SiC compact using polycarbosilane as a binder has high bend strength despite its low density as compared with SiC compacts prepared by other methods and can be produced at relatively low temperatures without hot pressing.
3) ZnO fiber was synthesized from the precursor, Zn(acac) 2 , which is a zinc organic complex. The ZnO fiber consisted of ZnO microrods, and each microrod further consisted of ZnO nanorods. The surface of the ZnO nanorods were covered with nanosized-single-crystal grains, which had high crystallinity, clean surfaces, and diameters of a few tens of nanometers. This fiber has visiblelight photocatalytic ability.
S. Tozawa and S. Ito of IMR in Tohoku Univ. for their technical contributions. Chemical analyses by staff of AR-CAM and Dr. T. Ashino of IMR in Tohoku Univ. are also appreciated.