<< Calculus`VectorAnalysis`

SetCoordinates[Cartesian[x, y, z]];

Grad[T[x, y, z]]

Div[Grad[T[x, y, z]]] - Laplacian[T[x, y, z]]

rce = Div[Grad[T[x]]] == 0

res = DSolve[{rce, T[0] == T1, T[d] == T2}, T[x], 
          x][[1]]

teplota = T[x] /. res /. {d -> 0.1, T1 -> 20, 
            T2 -> -10};
Plot[teplota, {x, 0, 0.1}, 
      PlotStyle -> {Hue[0], Thickness[0.008]}]