The problem that I am trying to solve is the temperature distribution in two media (with different thermal conductivities) joined at a common surface . I have had success with a 1 D model but can not seem to be able to extend the analysis to two dimensions.
Here is my 1-D model

1 D

In[]:=
k1=2;(*thermalconductivitiyinregion1*)​​k2=4;(*thermalconductivitiyinregion2*)
In[]:=
eqs=(*equations*)​​​​k1
2
∇
{x}
T1[x]0,(*region1*)​​k2
2
∇
{x}
T2[x]0(*region2*)​​;
In[]:=
bcs=(*boundaryconditions*)​​{​​T1[0]20,​​T2[1]10,​​T1[1/2]T2[1/2],​​k1T1'[1/2]k2T2'[1/2]​​};
In[]:=
eqsbcs=Join[eqs,bcs](*equationsandboundaryconditions*)
Out[]=
2
′′
T1
[x]0,4
′′
T2
[x]0,T1[0]20,T2[1]10,T1
1
2
T2
1
2
,2
′
T1

1
2
4
′
T2

1
2

In[]:=
{sol1,sol2}=NDSolveValue[eqsbcs,​​{T1,T2},{x,0,1},WorkingPrecision30]
Out[]=
InterpolatingFunction
Domain: {{0,1.00000000000000000000000000000}}
Output: scalar
,InterpolatingFunction
Domain: {{0,1.00000000000000000000000000000}}
Output: scalar

In[]:=
T1plot=Plot[sol1[x],{x,0,0.5}];
In[]:=
T2plot=Plot[sol2[x],{x,0.5,1.0}];
In[]:=
Show[T1plot,T2plot,PlotRangeAll]
Out[]=
0.2
0.4
0.6
0.8
1.0
10
12
14
16
18
20
Here is my 2D model

2 D

In[]:=
k1=2;​​k2=4;
In[]:=
eqs=​​​​k1
2
∇
{x,y}
T1[x,y]0,​​k2
2
∇
{x,y}
T2[x,y]0​​;
In[]:=
bcs=​​{​​T1[0,y]20,​​T2[1,y]10,​​T1[1/2,y]T2[1/2,y],​​k1(D[T1[x,y],x]/.x1/2)k2(D[T2[x,y],x]/.x1/2)​​};
In[]:=
eqsbcs=Join[eqs,bcs]
Out[]=
2
(0,2)
T1
[x,y]+
(2,0)
T1
[x,y]0,4
(0,2)
T2
[x,y]+
(2,0)
T2
[x,y]0,T1[0,y]20,T2[1,y]10,T1
1
2
,yT2
1
2
,y,2
(1,0)
T1

1
2
,y4
(1,0)
T2

1
2
,y
In[]:=
NDSolveValue[eqsbcs,​​{T1[x,y],T2[x,y]},{x,0,1},{y,0,1},WorkingPrecision30]
NDSolveValue
:Boundary condition T1
1
2
,yT2
1
2
,y is not specified on a single edge of the boundary of the computational domain.
Out[]=
NDSolveValue2
(0,2)
T1
[x,y]+
(2,0)
T1
[x,y]0,4
(0,2)
T2
[x,y]+
(2,0)
T2
[x,y]0,T1[0,y]20,T2[1,y]10,T1
1
2
,yT2
1
2
,y,2
(1,0)
T1

1
2
,y4
(1,0)
T2

1
2
,y,{T1[x,y],T2[x,y]},{x,0,1},{y,0,1},WorkingPrecision30