Influence of texture on electrical properties of ZnO ceramics prepared by extrusion and spark plasma sintering

FAST/SPS sintering of nanocrystalline zinc oxide—Part II: Abnormal grain growth, texture and grain anisotropy

“…Díaz-Chao et al [23] concluded that the crystallographic preferred orientation is a direct consequence of the needle-like morphology, which reoriented under the uniaxial pressure. Similar investigations about the sintering of needle-like ZnO particles also resulted into crystalline texture [20,43,57,68]. These particles may rotate under mechanical pressure into a direction perpendicular to it and finally align, which is an important mechanism for this formation of texture [69].…”

“…On the other hand, the particles may already be aligned during the casting process of the green body prior to sintering. This procedure is often referred to template growth [43] and may be realized via slip casting [68] or tape casting of platelet [22] and rod-like [20,43] crystallite. However, in the present work only isometric particles are initially present, which represents no good explanation for the occurrence of crystalline texture.…”

“…Interestingly, the presence of adsorbed water on nanocrystalline ZnO strongly promotes diffusion resulting into significant decrease of sintering temperatures necessary for full densification, which offers a reduction of grain boundary mobility [19]. Furthermore, the growth of polycrystalline microstructure may result in the formation of texture, which affects electrical [20][21][22][23] and mechanical [24] properties. In general, texture is a preferred alignment of grains or crystallographic axis [22], but crystalline texture and morphologic texture (anisotropy of grain shape) do not necessarily need to coincide [23].…”

“…Here, for pure ZnO no study exists that reports the development of grain anisotropy from isometric powder particles. From the application point of view a texture will affect the electrical [20] and optical [44] properties, whereas anisotropy of electrical properties seems promising for ceramic in electric industry [20,23].…”

Shaping ceramics by plastic processing

“…In another example, rod-like ZnO particles have been extruded to create textured microstuctures with reduced electrical resistivity compared to uniformly textured samples. 23 This alignment is most apparent when extruding fibre-loaded composite formulations 93 as shown in Fig. 4.…”