A steel with a grain size of 25 mm has a yield stress of 200 MPa and with a grain size of 9 mm a yield stress of 300 MPa. A dispersion of non-deformable particles is required to raise the strength to 500 MPa in a steel with grain size 100 mm. What would be the required dispersion spacing? (Assume the shear modulus μ = 80 GPa and the Burgers vector b = 0.2 nm.)