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SP 2005 Lecture04 Preview

1. This lecture will cover the process of chemical vapor deposition. Key concepts that will be discussed include mean free path, flux, pressure, and gas kinetics. 2. The rate of a chemical reaction depends on the concentration of reactants present and can be first-order or second-order. Flux is proportional to the reaction rate constant. 3. Laminar flow and the thickness of the "dead layer" near surfaces where flow velocity decreases to zero will also be examined.

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75 views2 pages

SP 2005 Lecture04 Preview

1. This lecture will cover the process of chemical vapor deposition. Key concepts that will be discussed include mean free path, flux, pressure, and gas kinetics. 2. The rate of a chemical reaction depends on the concentration of reactants present and can be first-order or second-order. Flux is proportional to the reaction rate constant. 3. Laminar flow and the thickness of the "dead layer" near surfaces where flow velocity decreases to zero will also be examined.

Uploaded by

stansilaw
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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3.155J/6.

152J Spring Term, 2005

Preview and preparation for Lecture 4, Feb.14: CVD

In this lecture we will go over the process of Chemical Vapor deposition. It requires that
you understand mean free path, flux, pressure, and gas kinetics. In addition, we will do a
little laminar flow and dead layer analysis (velocity of airflow over your bike helmet
actually decreases to zero right at the helmet surface no matter how fast you ride).

Concepts you are expected to become familiar with in the context of CVD are listed
below. (Blue ones are carry-overs from Lec. 3.)

1. Flux, J, of particles: the number of particles flowing through a unit area per
unit time:
J = N /(area ¥ time) .

2. Concept of flux, J, of a species diffusing down the concentration gradient,


∂c/∂x (c = number /volume), in a solid. J is proportional to diffusion constant
D (cm2/s): †
∂c
J x = -D
∂x
Plummer sections 7.1 to 7.2.2 should get you up to speed.

3. Mean free path, l: the average distance a particle travels between collisions,
† its momentum.
i.e. events that change

4. Ideal gas law relating pressure, volume, temperature, and number of particles:
pV = NkB T (or pV = NRT )
or p = nkB T , i.e. n = N/V

Check your high school or freshman chemistry books


† †
† reaction depends on the concentration, c, of reactants
5. The rate of a chemical
present. They can be first-order reactions, proportional to ci, second-order
going as A c + Bc2 etc. We will assume all first-order reactions, and the flux
of species proceeding from that reaction is proportional to the reaction rate
constant, k which has units of velocity: J = k c

6. In addition, we will do a little laminar flow and dead layer analysis (velocity
of airflow over your bike helmet actually decreases to zero right at the helmet
surface no matter how fast you ride). We’ll calculate the thickness of this
layer. †
7. Because chemical reactions involve different species surmounting a potential
barrier before getting to a lower (more stable energy state) the reaction
Ê DG ˆ
constant has an Arrhenius-like dependence on temperature: k µ expÁ- ˜.
Ë kB T ¯
And this is perhaps new to you:
8. The concept we’ll see that is probably the most elusive for non-chemists is the
DG
P ⋅P -
“equilibrium constant” of a reaction: K ≡ A B =†K 0e kT . It indicates the
PAB
degree to which the concentration or partial pressure of the products of the
reaction, A and B, exceed the concentration of the parent compound, AB.
Large K indicates a bias toward the products.
†

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