An infinite composite wall slice is shown in the figure below. The wall material initially does

not contain Helium $(He).$ At time $t=0$ a constant flux, $G,$ of $He$ begins to pass through $x=0.$ The

concentration of $He$ is kept at zero at $x=x3.$ The material located between $0$ and $x1$ has a

diffusion coefficient D$1,$ between $x1$ and $x2$ a coefficient D$2$ and between $x2$ and $x3$ is D$3,$

where $D1>D2>$ D$3.$

a$.$ Plot the shape of the concentration profile across the wall under steady$-$state conditions.

Pay particular attention to end points, slopes, etc. Note important points and be quantitative

when possible.

b$.$ What is the flux at $x3$ in steady state?