**1. Approach to sintering a yttria monolith to full density by surface and interfacial engineering**

#### **1.1 Introduction**

Yttria, Y2O3, has great potential as a host material for solid-state lasers such as yttrium aluminum garnet-doped neodymium, Nd:YAG, and yttrium oxide doped with ytterbium, Yb:Y2O3 [1, 2]. Other commonly known oxides, such as Al2O3, MgO, and ZrO2 ceramics, have attracted more interest than yttria [3, 4]. Therefore, the application of yttria ceramics in modern industry has been limited. Yttria ceramics are strongly dependent on not only their intrinsic properties but also their crystal structure. Yttria, which has a C-type rare earth sesquioxide structure, needs

to be derived from the cubic fluorite-type structure by removing one quarter of the oxygen atoms [5]. Yttria has 32 yttrium and 48 oxygen ion sites per unit cell. This structure has large interstitial sites with the same size as an oxygen ion in the anion sublattice. Yttria has a cubic or alpha-type crystal structure until the temperature reaches 2325°C. The phase transition to a tetragonal crystal structure also occurs at 2325°C. A stable phase, known as hexagonal or beta-type, is maintained until it reaches the melting temperature, 2340°C [6].

Recently, yttria, which is used widely in partially stabilized zirconia and sintering aids, was the center of attention in the semiconductor industry because yttria is superior to quartz, Al2O3, ZrO2, BN, and SiC in terms of its radical or cationic resistance activated by plasma [7] and sintering-limiting property [6].

Nevertheless, when the synthesis of nanoparticles using coprecipitation was carried out and the influence of precursors or additives was examined, it was reported that yttria, sintered under vacuum or hydrogen conditions, was close to the theoretical density [8–10]. The additives affected the grain boundary migration. This compensated for the charge, cation diffusion, solute transport, and additivedefect interaction [11, 12]. According to Huang et al. [13], the theoretical density of yttria ceramic was obtained by the two-step sintering and vacuum sintering of a lanthanum-doped yttria ceramic combination. Previous studies [14–16] attempted to improve the yttria transmittance and electrical conduction of the body sintered using special techniques, such as spark plasma sintering, microwave-flash combustion synthesis, hot isostatic pressing, and the addition of tri- or tetravalent additives. Therefore, it is necessary to examine the intrinsic sintering characteristics of yttria itself. More recently, Choi et al. [17] reported not only the behavior, color, and density of yttria ceramics but also the weight change due to oxygen vacancies and oxygen diffusion in a sintered body as a function of the sintering temperature. They suggested that the changes in color, density, weight, and microstructure of grains according to the sintering temperature were related to the volatilization of yttrium ions at oxygen vacancies in the lattice site at high temperatures [8, 10, 11]. On the other hand, the sintering property of the starting material, calcined yttria, from which the hydration reaction had been eliminated, as expressed in Eq. (1), but oxygen diffusion in oxygen vacancies and yttria powder, which removes gas spouting of the precursor, occurred continuously up to 1250°C, is unknown [13, 15]:

$$\text{RE} \left\{ \text{H}\_{2}\text{O} \right\}\_{\text{n}}^{3+} + \text{H}\_{2}\text{O} \left[ \text{RE} \text{(OH)} \left( \text{H}\_{2}\text{O} \right)\_{\text{n}}^{-1} \right]^{2+} + \text{H}\_{3}\text{O}^{+}\tag{1}$$

where *RE* and *n* represent the coordination number of rare earth and cation, respectively.

To determine the effects of oxygen vacancies and hydration on the sintering properties of yttria, this study studied the sintering characteristics of yttria calcined without a gas or hydration reaction containing a hydration reagent of precursors. In addition, when oxygen diffusion occurred in the oxygen vacancies, the yttria powder was heated repeatedly to adopt the result of density and increasing weight. The plasma resistivity of the yttria ceramic was compared with that of the control group.
