Alumina-silica composite coatings on graphite by CVD at 550[degrees]C.Alumina-silica composite coatings were prepared on the surface of graphite paper by chemical vapor deposition Vapor deposition Production of a film of material often on a heated surface and in a vacuum. Vapor deposition technology is used in a large variety of applications. using Al[Cl.sub.3]/Si[Cl.sub.4]/[H.sub.2]/C[O.sub.2] as precursor in the temperature range of 300 to 550[degrees]C. X-ray diffraction (XRD XRD X-Ray Diffraction XRD Crossroad XRD X-Ray Diode ) and scanning electron microscopy electron microscopy Technique that allows examination of samples too small to be seen with a light microscope. Electron beams have much smaller wavelengths than visible light and hence higher resolving power. (SEM) were used to examine the phase composition and the microstructure mi·cro·struc·ture n. The structure of an organism or object as revealed through microscopic examination. microstructure Noun a structure on a microscopic scale, such as that of a metal or a cell of the coating, respectively. The results indicated that a dense, uniform, and adherent adherent /ad·her·ent/ (-ent) sticking or holding fast, or having such qualities. alumina-silica composite coating can be prepared on graphite paper substrate by chemical vapor deposition at 550[degrees]C. Alumina-silica composite coating is composed of particles or nodules Nodules A small mass of tissue in the form of a protuberance or a knot that is solid and can be detected by touch. Mentioned in: Leprosy of varying size. Each particle is often composed of a number of finer particles. The phases of the 550[degrees]C composite coating include [gamma]-alumina and amorphous silica. The elemental chlorine content in the composite coating decreases with increasing deposition temperature. The surfaces of the alumina-silica composite coatings are affected by deposition temperature. There are some obvious micro-cracks in the 300[degrees]C composite coating, which are attributed to a mismatch of the coefficient of thermal expansion coefficient of thermal expansion, n See expansion, thermal coefficient. between composite coating and graphite paper. The 550[degrees]C alumina-silica composite coating can be completely turned into mullite after heat-treatment at 1350[degrees]C for 0.5 hr in argon argon (är`gŏn) [Gr.,=inert], gaseous chemical element; symbol Ar; at. no. 18; at. wt. 39.948; m.p. −189.2°C;; b.p. −185.7°C;; density 1.784 grams per liter at STP; valence 0. atmosphere. Keywords: Alumina-silica, composite coatings, graphite paper, chemical vapor deposition, mullitization ********** The tendency for carbon to oxidize oxidize /ox·i·dize/ (ok´si-diz) to cause to combine with oxygen or to remove hydrogen. ox·i·dize v. 1. To combine with oxygen; change into an oxide. 2. and become a vapor is a problem that has limited the use of carbon-based materials at high temperatures. Structural carbon materials, including carbon fibers, carbon-carbon composites, and graphite, are extensively used in aerospace and various industrial applications. Much attention had been given to the development of methods of oxidation protection of carbon materials. (1-2) Mullite (3[Al.sub.2][O.sub.3]x2Si[O.sub.2]), due to its thermal stability, superior corrosion resistance at high temperature, and low oxygen diffusion coefficient, has been targeted as a prime candidate coating material coating material, n a biologically acceptable, usually porous nonmetal applied over the surface of a metallic implant with the expectation that tissue ingrowth will occur in the pores. Often a carbon polymer or ceramic substance. . (3) Since mullite coating by chemical vapor deposition (CVD CVD Cardiovascular disease, see there ) using Si[Cl.sub.4]/Al[Cl.sub.3]/C[O.sub.2]/[H.sub.2] was proposed by Vinod Sarin sarin (zärēn`), volatile liquid used as a nerve gas. It boils at 147°C; but evaporates quickly at room temperature; its vapor is colorless and odorless. et al., (4) a comprehensive investigation of the CVD of Si[O.sub.2], [Al.sub.2][O.sub.3], and aluminosilicate Aluminosilicate minerals are minerals composed of aluminum, silicon, and oxygen. Andalusite, kyanite, and sillimanite are naturally occuring aluminosilicate minerals that have the composition Al2SiO5. coatings from mixtures of Si[Cl.sub.4] or C[H.sub.3]Si[Cl.sub.3] and Al[Cl.sub.3] in C[O.sub.2] and [H.sub.2] has been carried out. (5-7) However, because of the high deposition temperature (above 950[degrees]C) and resultant strong oxidation of carbon materials by [H.sub.2]O, a SiC barrier layer between the mullite coating and carbon material is necessary. (8-9) Even so, an Si[O.sub.2] interface layer resulting from the oxidation of SiC by [H.sub.2]O appears between the mullite and SiC coating. (3) Because the melting point melting point, temperature at which a substance changes its state from solid to liquid. Under standard atmospheric pressure different pure crystalline solids will each melt at a different specific temperature; thus melting point is a characteristic of a substance and of Si[O.sub.2] is lower than mullite, the erosion resistance of the SiC/mullite coating is weakened. Pulsed laser deposition, electrophoretic deposition Electrophoretic deposition (EPD), is a term for a broad range of industrial processes which includes electrocoating, electrophoretic coating, or electrophoretic painting. , dip-coating, sol-gel, and other new methods are being investigated to prepare oxidization-resistant coatings for carbon-based materials. (10-12) The present author prepared an alumina-silica composite coating on the surface of graphite paper by CVD at 550[degrees]C using Si[Cl.sub.4]/Al[Cl.sub.3]/C[O.sub.2]/[H.sub.2], and the graphite paper was not oxidized oxidized having been modified by the process of oxidation. oxidized cellulose see absorbable cellulose. by [H.sub.2]O. The mullite coating can be obtained by heat-treating the 550[degrees]C composite coating in argon atmosphere. Low temperature CVD and subsequent heat-treatment provide a method of fabricating mullite coating onto carbon-based materials, which can avoid the oxidization of the carbon-based materials. The main objective of this article is to investigate the microstructure, composition, and mullitization of the alumina-silica composite coating. EXPERIMENTAL PROCEDURE The alumina-silica composite coating examined in the present study was prepared by CVD at low pressure and various temperatures ranging from 300 to 550[degrees]C using an Al[Cl.sub.3]/Si[Cl.sub.4]/[H.sub.2]/C[O.sub.2] system on graphite paper. The Al[Cl.sub.3] evaporator was maintained at 130[degrees]C in an oil bath. The Si[Cl.sub.4] bubbler was maintained at 0[degrees]C in a refrigerated re·frig·er·ate tr.v. re·frig·er·at·ed, re·frig·er·at·ing, re·frig·er·ates 1. To cool or chill (a substance). 2. To preserve (food) by chilling. bath. The gas lines from the Al[Cl.sub.3], evaporator to the reactor were heated to 200[degrees]C in order to prevent Al[Cl.sub.3] from condensing con·dense v. con·densed, con·dens·ing, con·dens·es v.tr. 1. To reduce the volume or compass of. 2. To make more concise; abridge or shorten. 3. Physics a. . All reactant reactant /re·ac·tant/ (re-ak´tant) a substance entering into a chemical reaction. re·ac·tant n. gases were mixed just before being introduced into the reactor to prevent any premature reaction, as they react readily on contact. The stoichiometry stoichiometry Determination of the proportions (by weight or number of molecules) in which elements or compounds react with one another. The rules for determining stoichiometric relationships are based on the laws of conservation (see of the input Al[Cl.sub.3]/Si[Cl.sub.4] mol ratio and the total pressure in the reaction chamber were fixed at 3:1 and 75 torr torr (tōr), n a unit of pressure equivalent to 0.001316 atmosphere; named after the physicist Torricelli. Also called mm Hg. , respectively. The total coating deposition time was two hours. The 550[degrees]C composite coating was heat-treated in argon atmosphere at temperatures in the range of 1000~1350[degrees]C for 0.5 hr to obtain mullite coating. [FIGURE 1 OMITTED] [FIGURE 2 OMITTED] The composite coatings were examined by X-ray diffraction (XRD, Rigaku D/Max-B) for phase identification using Ni-filtered Cu[K.sub.a] radiation at a scanning rate of 0.5[degrees]/sec and scanned from 20[degrees] to 70[degrees] of 2[theta Theta A measure of the rate of decline in the value of an option due to the passage of time. Theta can also be referred to as the time decay on the value of an option. If everything is held constant, then the option will lose value as time moves closer to the maturity of the option. ]. The surface morphologies of the composite coatings were observed by scanning electronic microscopy (SEM, Model JEOL JEOL Japan Electron Optics Laboratory 840). The chemical compositions of the composite coatings were analyzed by energy dispersive dispersive /dis·per·sive/ (-per´siv) 1. tending to become dispersed. 2. promoting dispersion. spectroscopy (EDS (Electronic Data Systems, Plano, TX, www.eds.com) Founded in 1962 by H. Ross Perot (independent candidate for the President of the U.S. in 1992), EDS is the largest outsourcing and data processing services organization in the country. , link-860). [FIGURE 3 OMITTED] RESULTS AND DISCUSSION Characterization of the Alumina-Silica Composite Coating Figures 1 and 2 show the XRD pattern and EDS spectrum of the surface of the alumina-silica composite coating deposited on the graphite paper at 550[degrees]C, respectively. Because the alumina-silica composite coating and graphite paper adhere to adhere to verb 1. follow, keep, maintain, respect, observe, be true, fulfil, obey, heed, keep to, abide by, be loyal, mind, be constant, be faithful 2. each other, it is not possible to pull them apart completely. Therefore, strong graphite peaks are observed in the XRD patterns. The strong adhesion between coating and graphite indicate that the graphite is not oxidized by C[O.sub.2] and [H.sub.2]O formed by the reaction of [H.sub.2] and C[O.sub.2] during CVD. Figures 1 and 2 indicate that the 550[degrees]C composite coating is composed of [gamma]-alumina and amorphous silica. According to the breadth of the diffraction peaks, the size of the y-alumina crystals can be calculated from the following Scherrer equation. (13) D = 0.89[lambda]/[beta] cos[theta] (1) where [lambda] is the wavelength of the characteristic X-rays, [theta] is the Bragg angle, and [beta] is the calibrated cal·i·brate tr.v. cal·i·brat·ed, cal·i·brat·ing, cal·i·brates 1. To check, adjust, or determine by comparison with a standard (the graduations of a quantitative measuring instrument): width of the half-height of diffraction peaks. The average size of the [gamma]-alumina crystals calculated by (1) was about 10~20 nm. Figure 3 shows the elemental chlorine content in alumina-silica composite coatings deposited at different temperatures. It indicates that the higher the deposition temperature, the less the Cl content in the deposit. For the deposition temperature of 550[degrees]C, Cl element is absent in the coating. Morphology of the Alumina-Silica Composite Coating Typical SEM micrographs of the surface of the alumina-silica composite coating deposited on graphite paper at different deposition temperatures are shown in Figure 4. There are some micro-cracks in the surface of the 300[degrees]C composite coating and a trace of micro-cracks in the surface of the 400[degrees]C composite coating, which could be attributed to the difference of the coefficient of thermal expansion between the alumina-silica composite coatings containing Cl element and the graphite paper. However, no micro-cracks were observed within the 450[degrees]C and 550[degrees]C coatings, which could be attributed to a good thermal match between the composite coatings and the graphite paper. The morphology of the alumina-silica composite coating is dependent upon the deposition temperature. In the 300[degrees]C, 400[degrees]C, and 450[degrees]C composite coatings, some egg-like particles with a circular boundary were observed on the surface of the host particles with a polygonal boundary. The egg-particles lie either at the junction of several host particles or on the surface of one host particle. The wetting angle of the egg-particles on the surface of a host particle is obviously less than 90[degrees]. In general, gaseous reactants undergo gas phase reactions, thus forming intermediate species during the CVD process. Then diffusion/convection of the intermediate species across the boundary layer occurs. The boundary layer, a thin layer close to the substrate surface, is defined as the distance where the velocity of gas increases from zero at the substrate surface to the bulk value. (14) It is the absorption of gaseous reactants onto the heated substrate, and the heterogeneous reaction that occurs at the gas-solid interface (i.e., heated substrate), which produces the deposit and by-product by·prod·uct or by-prod·uct n. 1. Something produced in the making of something else. 2. A secondary result; a side effect. by-product Noun 1. species. The deposits will diffuse along the heated substrate surface forming the crystallization Crystallization The formation of a solid from a solution, melt, vapor, or a different solid phase. Crystallization from solution is an important industrial operation because of the large number of materials marketed as crystalline particles. center and growth of the film. At increasingly high deposition temperatures and reactant concentrations, homogeneous gas phase reactions will occur in addition to the heterogeneous reaction. This will lead to particle formation in the gas phase and interrupt the growth of coatings, and hence the microstructure purity and adhesion. (15) If the CVD process below 450[degrees]C was a completely heterogeneous reaction, the egg-particles could not be present. According to the above SEM micrographs and discussion, the CVD processes below 450[degrees]C contain both heterogeneous reaction and homogeneous gas phase reactions. The main chemical precursors used in the CVD of films and coatings and the associated chemical reactions can be categorized into the following types of reactions: thermal decomposition (pyrolysis py·rol·y·sis n. Decomposition or transformation of a chemical compound caused by heat. pyrolysis (pīrol´isis), n ), reduction, oxidation, hydrolysis hydrolysis (hīdrŏl`ĭsĭs), chemical reaction of a compound with water, usually resulting in the formation of one or more new compounds. , nitridation, disproportionation Disproportionation or dismutation is used to describe two particular types of chemical reaction:[1]
[FIGURE 4 OMITTED] It had been widely proposed that the CVD of alumina and silica from an Al[Cl.sub.3]/Si[Cl.sub.4]/[H.sub.2]/C[O.sub.2] mixture follows the following equations: Gas reaction [H.sub.2] (g) + C[O.sub.2] (g) [left and right arrow] [H.sub.2]O (g) + CO (g) (2) Surface reaction 2Al[Cl.sub.3] + 3[H.sub.2]O [left and right arrow] [Al.sub.2][O.sub.3] + 6HCl (3) Si[Cl.sub.4] + 2[H.sub.2]O [left and right arrow] Si[O.sub.2] + 4HCl (4) Needless to say, the actual chemistries of the deposition processes are more complex than what the above equations suggest. In addition to the homogeneous mechanism, a detailed surface chemistry model for the co-deposition of the two oxides was proposed in reference 17. The model assumed that the products of the steps that led to solid formation were Si[O.sub.2] and [Al.sub.2][O.sub.3]. These solid species could subsequently combine, in solid-solid reaction steps, to yield the aluminosilicate phase that was thermodynamically ther·mo·dy·nam·ic adj. 1. Characteristic of or resulting from the conversion of heat into other forms of energy. 2. Of or relating to thermodynamics. favored to be present at the conditions of deposition. The following reaction sequence, first proposed by Bradford, (18) is the most widely accepted simplified mechanism for the generation of water from [H.sub.2] and C[O.sub.2] (water gas-shift reaction, WSGR WSGR Wilson, Sonsini, Goodrich and Rosati (Palo Alto, CA law firm) WSGR Water Gas Shift Reaction ). [H.sub.2] + M [right arrow] 2H + M (5) C[O.sub.2] + H [right arrow] CO + OH (6) [H.sub.2] + OH [right arrow] O + [H.sub.2]O (7) M stands for all species present in the gas phase. Equations (6) and (7), which are the main routes for the generation of CO and [H.sub.2]O, respectively, proceed at the same rate, which is higher than the rate of any other reaction. Despite the fact that this reaction proceeds at relatively high rates, hydroxyl radicals are consumed by other reactions, and thus, the concentration of [H.sub.2]O is larger by several orders of magnitude than the concentration of OH. Therefore, even though OH is expected to be much more reactive toward the surface of the substrate than [H.sub.2]O, it may not play a very important role as an O donor for Si[O.sub.2] and [Al.sub.2][O.sub.3] formation on the surface. (17) [FIGURE 5 OMITTED] Even though at room temperature, the chloride free-water acutely reacts with [H.sub.2]O to form compound colloids. Because the Cl element is present in the composite coatings below 450[degrees]C, basic chloride colloids like [Al(OH)[.sub.n] [Cl.sub.6-n]][.sub.m] must have existed in the boundary layer. The basic chloride colloids are absorbed onto the surface of the coating to form the egg-particles. With the deposition and surface diffusion of the egg-particles toward the host particles, it gradually is reduced to a spherical-cap-like particle, subsequently turning into a part of the host particle. As a result, the host particles increase in size. According to the Gibbs-Thomson relation, (19) the correspondence between the crystal size (B) (or the new generated particle) and supersaturation supersaturation, n the addition to or presence of an ingredient in a solution in greater quantity than the solvent can permanently take up. ([DELTA][mu]) of the reactant species can be expressed by the following equation: B [proportional] 1/ln ([DELTA][mu]) (8) Because the increase of deposition temperature results in a faster chemical reaction rate, the rate of [H.sub.2]O production at 550[degrees]C is higher than that below 450[degrees]C. According to equation (8), the higher [DELTA][mu] of [H.sub.2]O leads to a dramatic decrease in the size of the compound particles generated in either boundary layer or solid surface. These fine-particles absorb and diffuse along the heated substrate surface thus forming the crystallization center and growth of the film. Figure 5 shows a magnified SEM micrograph micrograph /mi·cro·graph/ (-graf) 1. an instrument used to record very minute movements by making a greatly magnified photograph of the minute motions of a diaphragm. 2. of Figure 4d. It indicates that the composite coating is composed of a large number of particles of unequal size, and that each particle is composed of a number of finer particles, so that egg-particles are not present in the 550[degrees]C composite coatings. [FIGURE 6 OMITTED] Figure 6 shows the SEM micrograph of the cross-section of the alumina-silica composite coating deposited at 550[degrees]C. It indicates the coating is dense and uniform. [FIGURE 7 OMITTED] Mullitization of 550[degrees]C Composite Coating Figure 7 shows the XRD patterns for the heat-treated 550[degrees]C alumina-silica composite coatings in argon atmosphere at temperatures in the range of 1000~1350[degrees]C for 0.5 hr. The 1050[degrees]C sample is composed of [delta]-[Al.sub.2][O.sub.3], 3[Al.sub.2][O.sub.3]x2Si[O.sub.2], amorphous mixture, and a trace amount of cristobalite cristobalite (kristō´b  līt),n . The 1260[degrees]C sample is composed of mostly well-crystallized orthorhombic or·tho·rhom·bic adj. Of or relating to a crystalline structure of three mutually perpendicular axes of different length. orthorhombic mullite, [alpha]-[Al.sub.2][O.sub.3], and a small amount of [theta]-[Al.sub.2][O.sub.3] and cristobalite. The 1350[degrees]C sample is composed of mostly well-crystallized orthorhombic mullite and [alpha]-[Al.sub.2][O.sub.3]. The residual [alpha]-[Al.sub.2][O.sub.3] is necessary to eliminate any residual trace amount of cristobalite and enhance the erosion resistance of the mullite coating. Figure 7 also indicates that the 550[degrees]C alumina-silica composite coating can be completely turned into mullite after heat-treatment at 1350[degrees]C for 0.5 hr. CONCLUSIONS A dense, uniform, and adherent alumina-silica composite coating can be prepared on the surface of a graphite paper substrate by chemical vapor deposition at 550[degrees]C using Si[Cl.sub.4]/Al[Cl.sub.3]/C[O.sub.2]/[H.sub.2]. The 550[degrees]C alumina-silica composite coating is composed of [gamma]-alumina and amorphous silica. The 550[degrees]C alumina-silica composite coating can be completely turned into mullite after heat-treatment at 1350[degrees]C for 0.5 hr in argon atmosphere. The alumina-silica composite coating is composed of a number of unequal particles, and each particle often is composed of a number of finer particles. The surfaces of the alumina-silica composite coatings are affected by deposition temperature. The chemical vapor deposition process below 450[degrees]C contains both heterogeneous reaction and homogeneous gas phase reactions. References (1) Dzyadikevich, Y.V. and Olejnik, V.E., "The Ways of Protection of the Graphite Materials Against Oxidation," (review), Poroshkovaya Metallurgiya, 3/4, 41-47 (1996). (2) Lu, W. and Chung, D.D.L., "Oxidation Protection of Carbon Materials by Acid Phosphate Impregnation impregnation /im·preg·na·tion/ (im?preg-na´shun) 1. fertilization. 2. saturation (1). impregnation 1. the act of fertilizing or rendering pregnant. 2. saturation. ," Carbon, 40 (8), 1249-1254 (2002). (3) Hou, P., Basu, S.N., and Sarin, V.K., "Nucleation nu·cle·a·tion n. 1. The beginning of chemical or physical changes at discrete points in a system, such as the formation of crystals in a liquid. 2. The formation of cell nuclei. Mechanisms in Chemically Vapor Deposited Mullite Coatings on SiC," J. Mater. Res., 14 (7), 2952-2958 (1999). (4) Sarin, V.K. and Mulpuri, R.P., "Chemical Vapor Deposition of Mullite Coatings," U.S. Patent 5,763,008, 1998. (5) Nitodas, S.F and Sotirchos, S.V., "Codeposition of Silica, Alumina, and Aluminosilicates from Mixtures of C[H.sub.3]Si[C.sub.13], Al[Cl.sub.3], C[O.sub.2], and [H.sub.2]. Thermodynamic ther·mo·dy·nam·ic adj. 1. Characteristic of or resulting from the conversion of heat into other forms of energy. 2. Of or relating to thermodynamics. Analysis and Experimental Kinetic Investigation," Chemical Vapor Deposition, 5, 219 (1999). (6) Nitodas, S.F and Sotirchos, S.V., "Chemical Vapor Deposition of Aluminosilicates from Mixtures of Si[Cl.sub.4], Al[Cl.sub.3], C[O.sub.2], and [H.sub.2]," J. Electrochem. Soc., 147 (3), 1050-1058 (2000). (7) Nitodas, S.F and Sotirchos, S.V., "Deposition of Compositionally Graded Mullite/Alumina Coatings from Mixtures of Si[Cl.sub.4], Al[Cl.sub.3], C[O.sub.2], and [H.sub.2]," Adv. Mater., 15 (6), 99-104 (2003). (8) Yamamoto, O., Sasamoto, T., and Inagaki, M., "Effect of Mullite Coating on Oxidation Resistance of Carbon Materials with SiCgradient," J. Mater. Sci. Lett., 19 (12), 1053-1055 (2000). (9) Huang, J., Zeng, X., and Li, H. et al., "Mullite-[Al.sub.2][O.sub.3]-SiC Oxidation Protective Coating for Carbon/Carbon Composites," Carbon, 41 (14), 2825-2829 (2003). (10) Damjanovic, T., Jojic-Nedeljkovic, J., Fritze, H., and Borchardt, G. et al., "Mullite Diffusion Barriers on Carbon Based Composites for High Temperature Applications," Annales de Chimie: Science des Materiaux, 28 (Suppl.1), S71-S78 (2003). (11) Fritze, H., Schnittker, A., and Witke, T. et al., "Mullite Diffusion Barriers for SiC-C/C Composites Produced by Pulsed Laser Deposition," Mater. Res. Soc. Symp. Proc., 555, 79-84 (1999). (12) Damjanovic, T., Leipner, H., and Argirusis, C. et al., "Sol-Gel Route for Electrophoretic Deposition of Mullite Diffusion Barriers on C/C-SiC Composites," Mater. Sci. Forum, 453-454, 343-348 (2004). (13) Klug, H.P and Alexander, L.E., X-ray Diffraction Procedures for Polycrystalline Adj. 1. polycrystalline - composed of aggregates of crystals; "polycrystalline metals" crystalline - consisting of or containing or of the nature of crystals; "granite is crystalline" and Amorphous Materials, Wiley, New York New York, state, United States New York, Middle Atlantic state of the United States. It is bordered by Vermont, Massachusetts, Connecticut, and the Atlantic Ocean (E), New Jersey and Pennsylvania (S), Lakes Erie and Ontario and the Canadian province of , 687-690, 1974. (14) Schlichting, H., Boundary Layer Theory, McGraw-Hill, New York, 1960. (15) Choy, K.L., "Chemical Vapour Deposition of Coatings," Prog. Mater. Sci., 48, 57-170 (2003). (16) Lili, V.C, in Handbook of Thin Film Process Technology, Institute of Physics, Bristol (UK), p. B1.0:12, 1995. (17) Nitodas, S.F and Sotirchos, S.V., "Homogeneous and Heterogeneous Chemistry Models of the Codeposition of Silica, Alumina, and Aluminosilicates Development and Experimental Validation," J. Electrochem. Soc., 149 (11), C555-C566 (2002). (18) Bradford, B.W., J. Chem. Soc., 1557 (1933). (19) Givargiaov, E.J., Current Topics in Materials Science, North-Holland, New York, p. 91, 1978. Zhaofeng Chen**--Nanjing University of Aeronautics & Astronautics astronautics: see space science. Astronautics Flash Gordon space-traveling hero. [Am. Comics and Cin.: Halliwell] From the Earth to the Moon * Min Li and Yikai Shi--Northwestern Polytechnical University ([dagger]) * College of Material Science and Technology, Nanjing 210016, P.R. China. ([dagger]) College of Mechanical and Electrical, Xi'an 710072, P.R. China. ** Author to whom correspondence should be addressed. Email: Zhaofeng_chen@163.com; fax: +86-25-84236300. |
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