The first example of metathesis(solvent free or solid state)
reaction was performed in the presence of MW irradiation producing spherical nHA from calcium chloride and sodium phosphate in solid phases and was reported by Parhi et al, 2004 where the particles were obtained in an irregular diameter distribution [19]. MW heating as a processing method to synthesize HA by one-step solid-state reaction at room
temperature was further in troduced to obtain Nano-rods HA [43]. In addition, results showed that continuous MW heating led to large sized HA Nano-rods when compared with the interval heating method. Spherical nHA particles (CDHA, 50 nm and Ca/P¼1.5) were also prepared through a mechanically-activated solid state reaction in adomestic MW for only 4 min, starting with stoichiometric molar feed ratio of calcium and phosphorous precursors [