Continuous synthesis of Zn sub(2)SiO sub(4):Mn super(2+) fine particles in supercritical water at temperatures of 400-500 degree C and pressures of 30-35 MPa

Sub-micron sized Zn sub(2)SiO sub(4):Mn super(2+) phosphors particles were continuously synthesized in supercritical water with a flow reactor. Colloidal silica or sodium silicate was used as the Si source. Zn and Mn sources were chosen from their nitrates, sulfates, and acetates. The syntheses were carried out at temperatures from 400 to 500 degree C, at pressures from 30 to 35 MPa, at NaOH concentrations from 0.014 to 0.025 M, and for residence times from 0.025 to 0.18 s. Sodium silicate formed alpha - and beta -Zn sub(2)SiO sub(4):Mn super(2+) phases regardless of the Zn and Mn sources, while colloidal silica formed phases dependent on the type of Zn and Mn sources used in addition to the use of alkali. As the reaction temperature increased, the crystallinity of alpha -Zn sub(2)SiO sub(4):Mn super(2+) phase increased and the Mn substitution into the Zn sites of the alpha -Zn sub(2)SiO sub(4) phase decreased. Of the conditions studied, the most highly crystalline alpha -Zn sub(2)SiO sub(4):Mn super(2+) was produced at a temperature of 400 degree C, a pressure of 30 MPa, a NaOH concentration of 0.14 M, and a residence time of 0.13 s with Zn and Mn sulfates and colloidal silica as starting materials. The alpha -Zn sub(2)SiO sub(4):Mn super(2+) fine particles synthesized were round in shape, had an average diameter of 268 nm, and exhibited a green-emission with a peak wavelength of 524 nm.