Thermal-etching development of α-Zn2SiO4 polycrystals:: effects of lattice imperfections, Mn-dopant and capillary force

Willemite (alpha-Zn2SiO4) polycrystals, prepared by sintering at 1350 degrees C with or without Mn-dopant, were thermally etched at 1250 degrees C for 1 min to 12 h and studied by scanning electron microscopy with regard to the effects of lattice imperfections, Mn-dopant and capillary force on etching development. For alpha-Zn2SiO4 without dopant, a rather rapid grooving at grinding scratches, grain boundaries and {11 (2) over bar 0} faceted pores caused a hill-and-valley structure in the nascent etching stage. The grooves were subsequently obscured due to increasing deposition rate and decreasing sublimation rate with increasing negative curvature and hence, a prominent capillary effect. In the later stage, etching at {11 (2) over bar 0} faults, dislocations (parallel to [10 (1) over bar 0] and [0001]) and subgrain boundaries became prominent. Thermal grooving at these lattice imperfections was significantly suppressed by Mn-dopant. Under such circumstances, surface diffusion and anisotropic lattice plane etching, the etching rate being faster for (0001) than {10 (1) over bar 0} and {11 (2) over bar 0}, caused the formation of surface hillocks with terraces and ledges. (C) 2000 Elsevier Science S.A. All rights reserved.