Synthesis of high performance ceramic fibers by chemical vapor deposition for advanced metallics reinforcing

The chemical vapor deposition (CVD) synthesis of fibers capable of effectively reinforcing intermetallic matrices at elevated temperatures which can be used for potential applications in high temperature composite materials is described. This process was used due to its advantage over other fiber synthesis processes. It is extremely important to produce these fibers with good reproducible and controlled growth rates. However, the complex interplay of mass and energy transfer, blended with the fluid dynamics makes this a formidable task. The design and development of CVD reactor assembly and system to synthesize TiB2, CrB, B4C, and TiC fibers was performed. Residual thermal analysis for estimating stresses arising form thermal expansion mismatch were determined. Various techniques to improve the mechanical properties were also performed. Various techniques for improving the fiber properties were elaborated. The crystal structure and its orientation for TiB2 fiber is discussed. An overall view of the CVD process to develop CrB2, TiB2, and other high performance ceramic fibers is presented

Temperature Oscillations of Alternating-Current-Heated Thin Filaments in Ceramic Fiber Production

Resistively heated filaments are used to manufacture ceramic fibers. When an ac power source is used for heating, the temperature of the filament oscillates. These oscillations influence the deposition rate on the surface of the filament. An analysis of this problem is presented, and it is also shown that these oscillatory effects diminish as the filament gauge increases. The analysis can help to decide whether an ac or a dc power source should be used

Modeling of a-Si : H deposition in a DC glow discharge reactor

PECVD reactors are increasingly used for the manufacturing of electronic components. This paper presents a reactor model for the deposition of amorphous hydrogenated silicon in a dc glow discharge of Ar-SiH4 The parallel-plate configuration is used in this study. Electron and positive ion densities have been calculated in a self-consistent way. A macroscopic description that is based on the Boltzmann equation with forwardscattering is used to calculate the ionization rate. The dissociation rate constant of SiH4 requires knowledge about the electron energy distribution function. Maxwell and Druyvesteyn distributions are compared and the numerical results show that the deposition rate is lower for the Druyvesteyn distribution. The plasma chemistry model includes silane, silyl, silylene, disilane, hydrogen, and atomic hydrogen. The sensitivity of the deposition rate toward the branching ratios SiH3 and SiH2 as well as H2 and H during silyl dissociation is examined. Further parameters that are considered in the sensitivity analysis include anode/cathode temperatures, pressure, applied voltage, gap distance, gap length, molar fraction of SiH4, and flow speed. This work offers insight into the effects of all design and control variables

Convective Regimes in reactive Fluid media due to the interaction with Catalytic Surfaces

Reactive fluid media enclosed in a cavity with a catalytic surface are analyzed. Nonisothermal chemical reactions on this surface can lead to convective instabilities. A simplified model is developed by using a low-order truncation of a Fourier-type expansion and employing the Galerkin method. A linear stability analysis is presented and it is shown that, under certain conditions, the marginal curve for the onset of oscillatory instabilities can lie below that for monotonic ones. The stability of the convective modes is studied by nonlinear stability analysis and it is shown how they can evolve into periodic and nonperiodic motion patterns. Numerical results are provided to support and confirm analytical predictions

Simulation of the growth of CVD films

Chemical vapor deposition (CVD) is the preferred method of manufacture for solid films used in many industrially important thin and thick film applications. Requirements for the physical, mechanical and electrical properties of these films are becoming increasingly difficult to achieve, and deposition morphology plays an important role in this regard. Recently, we proposed a continuum model describing the evolution of a gassolid interface during atmospheric pressure CVD (Viljoen er al., 1994). A linear stability analysis (LSA) was used to determine the effect of reactor conditions on planar growth stability. The present paper discusses numerical solution of this model, and uses simulation examples to illustrate interface evolution under typical deposition conditions and from arbitrary initial interface shapes

Numeriacal solution of nonlinier boudary value problems with applications

x, 323 p.; 23 cm