On the Development of Thermally Etched Morphology of α-Zn2SiO4 Polycrystals

• Main Authors: chang chih chiang, 張志強
• Other Authors: Professor Pouyan Shen
• Format: Others
• Language: en_US
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/31141265324156701197

碩士 === 國立中山大學 === 材料科學研究所 === 87 === Nearly equi-axed a-Zn2SiO4 (willemite) polycrystals, with or without Mn-dopant and prepared via a sintering route (1350oC for 4 h to 100h), were thermally etched at 1250oC for 1 min to 12 h and studied by scanning electron microscopy regarding the effects of Mn- dopant and lattice imperfections, viz. grain boundary, dislocation arrays (dislocation line vetor parallel to <2110> in addition to [0001]), subgrain boundaries, faults and predominately {2110} faceted intragranular pores as characterized by transmission electron microscopy (TEM), on the etched morphology. In the nascent etching stage of gel-derived a-Zn2SiO4 polycrystals, a ratter rapid and sharp grooving at grain boundaries with accompanied anisotropic etching of the nearby lattice was found to cause hill-and-valley structure with ridges. The grooves and ridges were subsequently obscured presumably due to the increase of deposition rate and the decrease of sublimation rate at area of increasing negative curvature. In the final stage, intragranular etching at dislocation outcrops and subgrain boundary became prominent on a rather smooth surface. As for Mn-doped a-Zn2SiO4 polycrystals, grooving at grain boundaries and less at subgrain boundaries was evident in the incipient etching stage. In the final stage, surface diffusion and anisotropic lattice plane etching (0001) > {1010} and {1120} caused the formation of terraces and ledges for Mn-doped a-Zn2SiO4 but not for gel-derived a-Zn2SiO4 in the whole etching process. This can be attributed to ubiquitous dislocation networks, loops and submicron-sized intragranular pores and more dislocations of the latter which effectively obscured the anisotropic lattice etching. Alternatively, etch hillocks could be due to Mn-dopant acting as an effective inhibitor for etching at dislocation outcrops and subgrain boundaries. The faceted pores were dragged by dislocation networks with traces parallel to <1120> and <0001>, and hence could hardly migrate and coalesce during thermal etching. And the dislocation outcrops, appeared to be dragged by surface ledges. The lower temperature limit for appreciable thermal etching was estimated to be about 1050oC as indicated by negligible grain boundary grooving for Mn-doped a-Zn2SiO4 at this temperature. On the other hand, partial melting above 1350oC caused significant spiral growth for (0001) exposed willemite grains more or less facilitated by VLS mechanism.