Molecular Dynamics Simulation of Gas Diffusion in B2O3 and SiC

J. Mater. Sci. Technol, Vol.19 Suppl.1, 200329Molecular Dynamics Simulation of Gas Diffusion in B2O3 and SiCYajing YE*, Litong ZHANG, Laifei CHENG and Yongdong XU 06 AState Key Laboratory of Solification Processing, Northwestern Polytechnical University, Xi an Shanxi 710072, China[ Manuscript received November 4, 2003, in revised form December 30, 2003]Molecular dynamics simulation using a universal force field has been employed to determine the diffusion cofficientsof O2 and Na2SO4 vapor into B2O3 and SiC from 700 K to 1273 K, respectively. Einstein diffusion was observedin a 250~300 ps simulation. The diffusion cofficient for the 02 range from about 9.279x10 9 cm2/s for B2O3to 2.275x10-10 cm2/s_ for SiC at a loading of 32 molecules per simulation box, that for the Na2SOs vapor rangefrom about 9.888x10- 7 cm2/s for B2O3 to 1.837x10-10 cm2/s for SiC at a loading of 8 molecules per simulationbox. Environment properties of C/SiC composite will be increased via the B2O3 preventing the diffusion of O2 andNa2SOs vapor into the pyrolytic interphase and carbon fibers.KEY WORDS: Molecular dynamics, Difusion cofficient, C/SiC composite, Environment properties1. Introductionsimulations, it is possible to estimate difusion coefficients inthe range from 10-10 to 10-5 cm2s-1 to within a factor ofA pyrolytic carbon (PyC) interphase is very necessary2to 58. The most frequently. used approach to obtain self-for improving the mechanical and environmental propertiesdifusion coefficients in molecular dynamics (MD) simulationof carbon fiber reinforced SiC composites (C/SiC). It canis by means of the Einstein relationshipl9l written in the formincrease strength of the composites by protecting the fibersfrom oxidation and corrosion in the chemical vapor infiltra-ion (CV1) processes or in the engine combustion environ-Da=;6N t-∞dt乙ll。2(r()- r()1)(1)ments. In general, PyC interphase can be oxidized freely inthe oxidation phenomenon above about 673 K{[] and erodedwhere N is the number of diffusing particles, r:(0) and r:(t)by acidic or alkaline gases (in general, simulated by Na2SO4),are the initial and final positions of the center of mass of par-and producing a lot of holes and channels for the diffusionticle i over the time interval t, and <{|r:(t) - r(0)|-)istheof gases into carbon fibers. Many methods have been devel-averaged mean-square displacement (MSD) of the ensemble.oped to protoct fbers from oxidizing and corroding, such as,The Einstein relationship assumes a random- walk motion forbut not limited to, chemical vapor deposition (CVD) multi-the difusing particlesllayer SiC coatings|2l , graded composite interphase, including(BN-SiC)n and (PyC-SiC)n fabricated by CVI processl3.4,2.2 Force fieldsputtered iridium coatingsI5l et al.A boron-containing matrix inhibitor including boron,The Universal 1.02 force field (UFF) of Cerius2 (versionboron carbide and borides forms a glassy boron-oxide layer,4.6, Accelrys, Inc.) was used to determine gas difusion coef-which can block the active sites on the carbon surface or act asficients. UFF is a purely diagonal, harmonic forcefeld. Bondstretching is described by a harmonic term, angle bending bya barrier to gases difusion, and exhibits the most promisinga three-term Fourier cosine expansion, and torsions and inver-inhibition efectl. A problem of surface modification of PyCsions by cosine Fourier expansion terms. The van der Waalsinterphase can be solved by using B ion implantation, whichinteractions are described by the Lennard-Jones potential.is the most important commercial tailored surface treatmentmethod.. In general, B ion implantation leads to amor-and a screened (distance-dependent) Coulombic term. UFFphous structure of PyC interphase due to the high implantinghas full coverage of the periodic table. UFF is moderatelyimpantngrate while B4C crystal will be fourafter annealing at 1773 Kaccurate for predicting geometries and conformational energyof B ion implanted PyC interphasel6l. Once B4C formed ondifferences of organic molecules, main-group inorganics, andthe surface of PyC interphase, it would react with O2 andmetal complexes. It is recommended for organometallic sys-Na2SO4 vapor, and form vitriform B2O3, which is viscid andtems and other systems for which other forcefields do not havecan flow into cracks on the interphase and seal them and pre-parameters.vent gases from difusing into interphase, consequently serveas a barrier to oxidation and corrosion.In this paper, an atomistic force field (Universal 1.02)2.3 Building simulation boxeshas been used to determine difusion coefficients for O2 andmodule, and B SiC simulation box was moved from ceramicNazSO4 vapor in B2O3 and SiC at 700 K~1000 K，andmodels library provided by the Cerius2. The two simula-1157 K~1273 KK, rspetively. Difusion cofficients have beention boxes were built of the elementary crystal units repeatedobtained form 250~300 ps MD simulation using the Einsteinin each direction 5 to 6 times, respectively, as ilustrated inrelationship (Eq(1)). By comparing difusion cofficients ofFig.1. Structures of O2 and Na2SO4 were built using the 3D-the two gases in B2O3 and SiC, blocking function of B2O3 offSketcher of Cerius2 . And then they were copied and insertedthe two gases can be analyzed. Difusion activation energies ofinto B2O3 and 8-SiC simulation boxes, respectively.O2 and Na2SO4 were computed at last. All simulations wereperformed on a 2-processor (R12000 A, 400 MHz) worksta-2.4 Simulation of X-ray difractiontion (Silicon graphics).The difraction crystal module of Cerius2 was used to cal-culate the powder X-ray diffraction pattern at the Cu Ka2. Computational Detailswave2.1 Molecular dynamics simulations of difusion coefficientFig.2_ntal difraction patternslt the current state of development of molecularrepoiYHay parameters of B2O3in litC N M H Glation esut lustata↑Ph.D. Candidate, to whom correspondence should be addesedin Fig.2(b) which indicate the appropriateness of the β-SiCE-mail: yyjcathy@yahoo.com.cnand B2O3 simulation boxes used in these simulations.30J. Mater. Sci. Techol, Vol.19 Suppl.1, 2003t (a)Si stom手120 stomC stom10[曼0。LFig.1 Snapshots of (a) B2O3 and (b) SiC crystal structures(b)25i 0.0r。(a)虽“.10foos100Time/ DsFig.3 Plots of MSD (nm) us t for the difusion O2 in(a)B2O3, (b) SiC over 300 ps at 700 Kb)tation PyC interphase, O2 which passes through the SiC ma-trix cracks to the B4C layer reacts firstly with B4C otherx0↑than PyC, thereby protects PyC interphase and inside car-bon fibers.. Reaction (3) leads to the formation of B2O3 andwas accompanied by a 250% volume increase. The formedB2O3 glass then probably seals the transverse; cracksIn theactual gas turbine engine environment, there isacidic and alkaline gas which can be replaced by Na2O4.201 deg.Above 1157 K (the melting point of Na2O4), Na2O4 vaporand liquid are coexistent. Therefore, PyC interphase reacts .Fig.2 Simulated X-ray difraction pattern for (a) SiC andwith Na2O4 vapor coming from crack tunnels in the SiC ma-trix above 1157 K,(b) B2O32C(2) + Na2SO4()→2CO2↑+Na2S(a)(4)2.5 Molecular dynamicsThe simulation boxes were minimized using the High-which leading to weightlessness of interphase and gaps in theConvergence 1000 steps (total of 1 ps) to 4.186x10-2 RMS .interphase which are channels for Na2O4 vapor difusing intoforce (kJ/mol.nm). .Dynamics were then performed using acarbon fibers. △rG =(kJ-mol-") of reactants and productsNVT ensemble from 700 K to 1273 K for the two simulationat 1200 K are 0, -893.538, - 369.098 and - 275.86413), sep-boxes, respectively. Simulations were performed for 250~300arately. So, O.Gm= -120.522(kJ-.mol-1)<0. B4C can reactp8 using an 1 fs time step. Cut-offs of two simulation boxeswith Na2SO4of 0.95 nm or 1.05 nm were used for calculation of nonbondedpotentials. Difusion coficients were calculated by use ofB4C(a) + 2Na2SO4()→CO2(g)↑+2Na2S(e) + 2B2O30) (5)Eq.(1) using the analysis module of Cerius2. Only the linearportions of the MSD versus time plots were used in analysisEach OrGm (kJ-mol-1) at 1200 K of reactants and productsof the trajectories and all gas molecules in the system werein reaction (5) is 4.164, - 892.663, - 396.098, - -271.189 andincluded in the determination of the MSD of the ensemble.-975.368131, respectively. Then, ArGi=- 1081.05, far lessOrG2(= - 120.522) of reaction (4). So NazSO4 will react with3. Computation Results and DiscussionB4C firstly rather than PyC at 1200 K thereby protecting thePyC interphase and inside carbon fiber from corroding. One3.1 Thermodynamics analysisof the products of reaction (3) and (5), namely, vitreous B2O3In oxidization atmosphere, PyC interphase reacts with O2can come into being protecting layers to seal cracks scatter-ing on the PyC interphase and prevent O2 and Na2SVanorfronm difusing further int BsClayers Wnd Na2SO4 vaporCa) +02(g)→CO2()↑.(2)ich has not reactedyet. In the following part of this paper, the authors will ana-O;GE(kJ-mol-1) of reactants and products at 700 Klyze the blocking diffusion function of the B2O3.are.0， 0 and- 395.398， respectively.T hen,OrG=-395.398(kJ-mol- "). At high temperature, boron3.2 Difusion cofficients calculationscarbide undergoes the following reactionAs an ilustration of the simulation results for O2 diffu-BsCo) + 4O2(g)→2B20O3q) + CO2(g)↑3)sionox' 中国煤化工° n 700 K (giminn in Fig.3. The difu-O;G(kJ.mo1-1) of reactants and products at 700 K are ._s 9.279x10- 9cm2/s. In-61.494，0, - 1085.668 and - -395.398, respectively. Then,orderYHC N M H G difusio coficient forO,G∞=- 2372.434 (kJ-mol~') is far less the value of reactionO2 in SiC Was computed also to compare with that of O2 in(2). That is to say, for C/SiC composites with B ion implan-B2O3. The result is 2.003x10 10 cm2 /s. The former is biggerJ. Mater. Sei. Technol, Vol.19 Suppl.1, 2003Table 1 Difusion activation energies of O2and Na2SO4 in B2O3 and SiC(kJ/mol)(8282O2NagSO4128.7143.2B2O3111.3127.3difusion resistance with SiC to O2 and Na2SO4.noL -4. Conclusiono3(bThermodynamics calculation of several chemical reactionsand difusion coefficients of O2 and NazSO4 vapor in B2O3can be calculated to analyze the heal properties of B4C in theC/SiC composites system in oxidation or corrosion environ-Difusion coficients of the two gases in siC were cal-ere cal-culated also for comparing with difusion resistance of B2O3.2oL6一10150-2020self-healing properties because of small difusion coeficientsTIme/ psof O2 and Na2SO4 vapor into B2O3. Oxidation and corrosionFig.4 Plots of MSD (nm) us t for the difusion Na2SO4 va-resistance of C/SiC composites could be improved via B ionimplanting into PyC interphase.por in (a)B2O3, (b) SiC over 250 ps at 1200 Kthan the latter of just about one order of magnitude. As weAcknowledgementknow, SiC is widely used as a barrier coating for its abilitiesThe authors would like to thank Prof. K.H.Su for providingof hindering gas from difusing into itl4. ScB2O3 can holdthermnodynanis data. Mr Y.S.Pam for diecussingx-ray difracup O2 difusing into interphase at 700 K while O2 can pervadeJ.X.Shen and Dr. X.Zhang for their luseful discussions and opirinto interphase and react with B4C.discussions and opin-ions.Diffusion of Na2SO4 was simulated at 1200 K using dy-namics module. Plots of MSD U8 t at 1200 K are given inFig.4. The diffusion coficients gained from simulation areREFERENCES9.888x10-7 cm2/s and 1.873x10 10 cm2 /s, respectively. 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