Journal of Materials Science

, Volume 31, Issue 11, pp 3021–3033 | Cite as

The influence of surface kinetics in modelling chemical vapour deposition processes in porous preforms

  • J. P. Dekker
  • R. Moene
  • J. Schoonman
Papers
Received:
Accepted:

Abstract

The isothermal chemical vapour infiltration (ICVI) process is a well known technique for the production of composites and the surface modification of porous preforms. Mathematical modelling of the process can provide a better understanding of the influence of individual process parameters on the deposition characteristics such as final porosity or deposition profiles in the pore network. The influence of different rate expressions for several binary compounds on the ICVI process is discussed. Experimental work is used to validate the importance of correct kinetic expressions in a continuous ICVI model for cylindrical pores. The predicted infiltration characteristics are compared with experimental results. The final densification and Thiele modulus, i.e. a number which is a measure for the diffusion limitations in a pore, are used for the evaluation of the presented model, and conditions are given for an optimal densification of a porous preform by the ICVI process for several binary compounds. The deposition profiles as predicted by the model calculations are in agreement with the experimentally determined deposition profiles of TiN and TiC in small tubes. Moreover, it can be concluded that the shape of the deposition profiles is determined by the heterogeneous reaction kinetics. There is only a qualitative agreement between the predicted densification and measured densification for the synthesis of TiN and TiB2 in sintered porous alumina. This mismatch can be explained in terms of a complexity of the pore network and differences in reaction kinetics. Model calculations reveal that there is a scattering for the predicted residual porosity as a function of the Thiele modulus for TiN. Moreover, this Thiele modulus can not fully account for the changes in densification at different temperatures. Given these uncertainties it is likely that a residual porosity of less than one percent can be obtained if the Thiele modulus is smaller than 1 × 10−4. However, a CVI process with such a small Thiele modulus will not be practical, because of the concomitant long process times. Therefore, more precise conditions for the individual process parameters, i.e. concentration, reactor pressure, and temperature are deduced from the model calculations.

Keywords

Pore Network Binary Compound Porous Alumina Chemical Vapour Deposition Process Residual Porosity 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Nomenclature

a, b, c

reaction order constants

Ci(x, t)

concentration of species i at axial position x and time t (mole m−3)

Cio

bulk concentration of species i (mole m−3)

Ci*(x, t)

dimensionless concentration of species i at axial position x and time t

De(x, t)

effective diffusion coefficient at axial position x and time t (m2s−1)

Dij(x, t)

binary diffusion coefficient (m2s−1)

DK(x, t)

Knudsen diffusion coefficient at position x and time t (m2s−1)

F

correction factor for effective diffusion coefficient

k

growth rate constant (ms−1(m3mole−1)a+b-1)

Ki

adsorption-desorption equilibrium constant (m3mole−1)

L

length of a pore (m)

Mi

molecular weight of species i (g mole−1)

Mij

harmonic mean of the molecular weights of species i andj (g mole−1)

Ms

molecular weight of deposit (g mole−1)

mt

measured mass increase (g)

ni

stoichiometric number

P

reactor pressure (Pa)

R(Ci)

growth rate (mole(m−2s−1))

r(x, t)

pore radius at position x and time t (m)

ro

initial pore radius (m)

r*

dimensionless pore radius

S

geometrical surface area (m2)

st

fraction of free titanium sites at the surface of TiN

sn

fraction of free nitrogen sites at the surface of TiN

T

temperature (K)

t

time (s)

tp

process time (s)

U

KHCl/(KH2CH2)1/2 (m3 mole−1)

V

volume of alumina substrate (m3)

W

KTiCl3(m3 mole−1)

X

volume of infiltrated deposit relative to initial pore volume

x

axial distance (m)

x*

dimensionless axial distance

z

number of time steps

α

dummy variable for integration

ɛ

porosity of sintered porous alumina substrate

λ

ratio of the volume over the surface area perpendicular to the flux (m)

ϱ

density deposit (kg m−3)

σij

a characteristic length (Å)

τ

tortuosity factor of substrate

ϕ

Thiele modulus

ΩD

collision integral

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Copyright information

© Chapman & Hall 1996

Authors and Affiliations

  • J. P. Dekker
    • 1
  • R. Moene
    • 1
    • 2
  • J. Schoonman
    • 1
  1. 1.Laboratory for Applied Inorganic ChemistryDelft University of TechnologyBL DelftThe Netherlands
  2. 2.Department of Chemical Process TechnologyDelft University of TechnologyBL DelftThe Netherlands

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