A review of two-step sintering for ceramics

Effect of sintering temperature on microstructural and optical properties of transparent yttria ceramics fabricated by spark plasma sintering

Yttrium oxide (yttria) with monoclinic structure exhibits unique optical properties; however, the monoclinic phase is thermodynamically stable only at pressures higher than ∼16 GPa. In this study, the effect of grain size and plastic strain on the stability of monoclinic phase is investigated by a high-pressure torsion (HPT) method. A cubic-to-monoclinic phase transition occurs at 6 GPa, which is ∼10 GPa below the theoretical transition pressure. Microstructure analysis shows that monoclinic phase forms in nanograins smaller than ∼22 nm and its fraction increases with plastic strain, while larger grains have a cubic structure. The band gap decreases and the photoluminescence features change from electric dipole to mainly magnetic dipole without significant decrease in the photoluminescence intensity after formation of the monoclinic phase. It is also suggested that monoclinic phase formation is due to the enhancement of effective internal pressure in nanograins.

Optical Properties of Nanocrystalline Monoclinic Y2O3 Stabilized by Grain Size and Plastic Strain Effects via High-Pressure Torsion.

Sintering is a critical phase in the production of ceramic bodies. By controlling the density and microstructure formation, sintering now emerged as a processing technology of ceramic materials. Tailoring the structural, mechanical, electrical, magnetic and optical properties is widening the application of ceramics in various fields. Recently, many advanced sintering methods have reported to fabricate ceramic materials with controlled properties. Two‐stage sintering (TSS) is one of the simple and cost‐effective methods to obtain near‐theoretical density materials with controlled grain growth without adding any dopants. Many recent works have reported the use of TSS as a processing method to fabricate nanoceramics for various applications. With this background, this chapter reviews the advantages of TSS in ceramic preparation based on properties and materials and explores the future directions.

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Two-Step Sintering of Ceramics

Transparent ceramic scintillators with the composition Gd0.3Lu1.6Eu0.1O3 (GLO:Eu) have been prepared by different sintering profiles: a traditional profile consisting of a slow ramp followed by a dwell, and a two-step profile consisting of a fast ramp and short dwell followed by a long dwell at a lower temperature. A subsequent Hot Isostatic Press (HIP) step was used to achieve full density and transparency. Two-step sintering allowed full transparency to be achieved after HIPing at 1525°C, while traditionally sintered samples required 1850°C in the HIP to achieve high transparency indicating that two-step sintering is successful in maintaining a small grain size and therefore allowing densification to be decoupled from grain growth during the low temperature HIP step. HIPing at elevated temperatures between 1525 and 1850°C resulted in rapid grain growth from sub-micron to ~300 µm grains. Radioluminescence spectra show negligible difference between samples with sub-micron grain size and those with 300 µm grains.

Two-step sintering of Gd_03Lu_16Eu_01O_3 transparent ceramic scintillator

Undoped Y2O3 and Y2O3 doped with either Si4+, Er3+, Yb3+, Al3+ or Ti4+ were fabricated by a two-step sintering method. The concentration of each dopant cation (Si4+, Er3+, Yb3+, Al3+, and Ti4+) was adjusted so that the dopant:yttria molar ratio was 0.01 and the mechanical properties and the microstructures of each were characterized. The Vickers hardness and the bending strength of cation-doped Y2O3 increased in the order Si4+, Er3+, Yb3+, Al3+, and Ti4+ paralleling increases in the relative density. Ti-doped-Y2O3 had the highest density and the highest strength. Transmission electron microscopy analysis revealed that Ti segregated to the grain boundaries of Y2O3. The two-step sintering method was optimized to enhance the mechanical properties of Y2O3. As a consequence, Ti-doped Y2O3 with the Ti4+:yttria molar ratio of 0.01 sintered at the optimized heating schedule was found to have a Vickers hardness and three-point bending strength as high as 874 Hv and 307.0 MPa, respectively.

Effect of cation doping on mechanical properties of yttria prepared by an optimized two-step sintering process

A systematic series of experiments was performed to determine the optimum firing schedule of Y2O3. A two-step sintering with subsequent profile succeeded to fabricate Y2O3 ceramic with the relative density of 97.6% and the average grain size of 0.46 μm: first step and second step temperature of T1 = 1490 °C and T2 = 1350 °C, respectively and second step holding time of t2 = 20 h. After the firing schedule of pure Y2O3 was determined, two types of TiO2/Y2O3 and ZrO2/Y2O3 composites were fabricated for the further improvement of mechanical properties. According to results, ZrO2/Y2O3 composite showed a dramatic increase in strength and toughness as 417 MPa and 4.4 MPa.m1/2, respectively.

Hadi Razavi Khosroshahi

Papers

Effect of cation doping on mechanical properties of yttria prepared by an optimized two-step sintering process

Effect of Nanosized Particles on the Mechanical Properties of Y2O3