Table 1: Design factors and their levels.

Design factorsUnitLevels
123

Concentration of source of nickel (nickel sulfate solution) (A)g/L20 2 5 a 30
Concentration of reducing agent (sodium hypophosphite solution) (B)g/ L14 1 7 a 20
Concentration of source of tungsten (sodium tungstate) (C)g/ L15 2 0 a 25
Annealing temperature (D) °C400 5 0 0 a 600

a Initial condition.