Sintering techniques for microstructure control in ceramics

Andrew T Rosenberger, Purdue University

Abstract

Sintering techniques can be manipulated to enhance densification in difficult to sinter materials and to produce property enhancing microstructures. However, the interplay between materials, sintering techniques, and end properties is not fully understood in many material systems, and some fundamental aspects of sintering such as the nature of the effects of electric fields remains unknown. The processing property relationships were examined in two classes of materials; zirconium diboride ultra high temperature ceramic composites, and all solid lithium-ion battery phosphate materials. Investigation of zirconium diboride ceramics focused on the effects of zirconium carbide as a secondary or tertiary phase in ZrB2 and ZrB2 – SiC. Addition of zirconium carbide was observed to increase flexural strength of composites up to 590MPa at 50wt% ZrC, significantly higher than the flexural strength of 380MPa observed in similarly prepared ZrB2 – SiC. This difference was attributed to the absence of CTE mismatch induced residual stresses in the ZrB2 – ZrC composites. A high temperature reaction between ZrB2 and TiC producing Zr1-xTixB2 – ZrC composites was discovered and found to enhance densification while reducing the average grain size to as small as 1.4μm, lower than the starting powder size of 1.8μm. While a high flexural strength of 670MPa was observed, a strength dependence on the ZrC grain size indicative of CTE mismatch residual stresses was also seen. Finally, the oxidation and ablation resistance of ZrB2 – ZrC – SiC composites as a function of ZrC fraction and ZrC:SiC ratio was investigated. Above 5vol% ZrC, the oxidation and ablation resistance of the composites was significantly reduced due to ZrC oxidation, regardless of SiC content. While ZrC can significantly enhance the mechanical properties of the composite, the volume fraction must be kept low to avoid an undesirable reduction in the oxidation resistance. The influence of applied electrical fields during sintering on microstructure and electronic properties of lithium aluminum titanium phosphate (LATP) electrolyte material was investigated by sintering LATP pellets under DC voltages of 0V, 2V, 10V, and 20V. Application of a DC voltage increased relative density from 86% to a maximum of 95.5%. However, unlike reports on other material systems such as zirconia, a high DC voltage induced, rather than restrained, abnormal grain growth. Conductivity decreased with applied voltage from 4.8*10 -4 S/cm at 0V to 1.3*10-4 S/cm at 20V, which was attributed to the high faceting and poor grain-to-grain contact of the grains sintered under 10V and 20V. This indicates that field-assisted sintering techniques may actually be detrimental to solid state battery materials, and that the field effects are significantly different from those observed in other systems in the literature.

Degree

Ph.D.

Advisors

Stanciu, Purdue University.

Subject Area

Materials science

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