Full length articleReduction in sintering temperature for flash-sintering of yttria by nickel cation-doping
Graphical abstract
Introduction
Electric field-assisted sintering techniques (FAST), which are new sintering methods using electrical fields, have shown that ceramics can be quickly sintered at low temperatures [1], [2], [3], [4], [5], [6]. As an example, direct current (DC) electrical fields of approximately 20 V/cm lowered the sintering temperature of the 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) from about 1400 °C to 1300 °C [5], [6]. Furthermore, it has been demonstrated that 3Y-TZP can be fully-densified at 850 °C within 5 s under an electric field of 120 V/cm [7]. This phenomenon is called flash-sintering. The flash-sintering is characterized by almost immediate densification (typically occurs in just a few seconds) and a non-linear increase in electric conductivity under a threshold condition of temperature and applied field [7], [8]. The nature of the flash-sintering is fundamentally different from FAST, in which fields lead to a gradual enhancement in the sintering without any change in the specimen conductivity. Abrupt densification at low temperature by flash-sintering has been demonstrated in various ceramics [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21].
In our previous study, we have demonstrated that DC fields greater than 300 V/cm can trigger the flash-sintering in undoped Y2O3 [17]. Y2O3 ceramics have special chemical and physical properties, such as high resistance to halogen-plasma corrosion [22] and thermal stability [23], [24], [25], but they are difficult to sinter. Conventional sintering requires very high temperatures, typically >1400 °C, and a vacuum or hydrogen atmosphere [26], [27], [28], [29], [30], [31], [32], [33]. However, by applying an electric field of 1000 V/cm, for instance, a dense Y2O3 polycrystal was obtained at 985 °C in less than 10 s [17].
On the other hand, it has been reported that the sintering temperature of Y2O3 for the conventional sintering process can be reduced by the doping of divalent alkaline metal cations, such as Mg2+ and Ca2+ [34]. According to the literature, grain growth in Y2O3 is rate-controlled by the Y3+ diffusion via a cation interstitial mechanism, and the grain boundary mobility is higher in a reducing atmosphere than in air [34], [35], [36]. The defect reaction in divalent oxide-doped Y2O3 can be written as follows [35], [36], [37]:where MO is the oxide of the divalent cation of M, and is a divalent cation on a yttrium site in Y2O3 with an excess −1 charge, is a vacant oxygen anion with a double positive effective charge relative to the perfect Y2O3 lattice, and is an oxygen anion on an oxygen anion site in Y2O3. The formation of an oxygen anion vacancy due to divalent cation-doping is supposed to accelerate the Y3+ diffusion in the Y2O3 [36]. More recently, we have found that the doping of 1 mol% Ni2+ significantly enhanced the densification rate and reduced the sintering temperature of Y2O3 by about 400 °C under pressureless sintering in air [38], [39]. An interdiffusion experiment of the diffusion couples of polycrystalline Er2O3 and Y2O3 revealed that the grain boundary and lattice diffusion coefficients of Y3+ in Y2O3 are accelerated by the Ni2+-doping [40]. The accelerated diffusivity of Y2O3 is responsible for the improved sinterability of the Ni-doped Y2O3.
In the present study, the doping effect of 1 mol% Ni2+ on the field-assisted sintering and flash-sintering in Y2O3 was investigated under a DC electric field up to 1000 V/cm. It was demonstrated that the Ni-doping significantly facilitated the field-assisted sintering and flash-sintering of Y2O3.
Section snippets
Material
The materials used in this study were undoped Y2O3 and 1 mol% Ni2+-doped Y2O3. The starting raw materials were commercially available, high-purity Y2O3 powder (BB-type; 99.9% purity, Shin-Etsu Chemical, Japan) and nickel acetate (99.99% purity, Rare Metallic, Japan). The manufacturer-specified average particle size and BET multi-point specific surface area of the Y2O3 powder were 20 nm and 37.0 m2/g, respectively. The Y2O3 powder and nickel acetate were ball-milled with zirconia balls in
Field-assisted sintering and flash-sintering
The sintering densification curves for the 1 mol% Ni2+-doped Y2O3 materials measured for different applied DC field values as a function of the furnace temperature are shown in Fig. 2 . The data for conventional sintering, without an electric field, are shown as 0 V/cm. For comparison, the data for the undoped Y2O3 under the fields of 0 V/cm and 500 V/cm are also plotted in Fig. 2. The undoped Y2O3 at 0 V/cm (conventional sintering) exhibited the relative density of 79% at 1400 °C, and showed
Discussion
The present EELS analysis revealed that the concentration of the extrinsic oxygen anion vacancies is increased by the flash-sintering in the undoped and Ni-doped Y2O3. The chemical shift in the O-K edge ELNES has also been observed in Y2O3 produced by spark plasma sintering (SPS) [53]. The Y2O3 was fully-densified at 1050 °C in a carbon die and punches in which a pulsed DC current and compressive stress were applied. The O-K edge ELNES taken from the SPSed Y2O3 exhibited a relatively high
Summary
The doping effect of 1 mol% Ni2+ on the field-assisted sintering and flash-sintering in Y2O3 was investigated under a DC electric field up to 1000 V/cm. The EELS analysis indicated that an oxygen anion vacancy is an important factor in the occurrence of the flash-sintering in the present materials. The results are summarized as follows:
- (1)
In the 1 mol% Ni2+-doped Y2O3, the field of 300 V/cm enhances the densification rate by field-assisted sintering. At the fields of 500 V/cm–1000 V/cm,
Acknowledgments
This work was financially supported by a Grant-in-Aid for Scientific Research (KAKENHI) on Innovative Areas 2505-25106001, 25106004 and 25106006 from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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