On the Conversion of Bulk Polycrystalline Y2O3 into the Nanocrystalline State

Abstract

A reversible phase transformation (RPT) process is observed that converts fully dense polycrystalline Y2O3 directly into the nanocrystalline state. The process involves a forward transformation from cubic‐to‐monoclinic symmetry under a high pressure and a reverse transformation from monoclinic‐to‐cubic symmetry under a lower pressure. An example is given of a reduction in grain size of cubic‐Y2O3 from 300 to 0.1 μm in a single pressure‐induced RPT at 1000°C.

Surface Modification in Polycrystalline Y2O3 Under High‐Pressure/High‐Temperature Processing Conditions

“…Density measurements conducted on the processed samples showed that the forward phase transformation is accompanied by about a 6% decrease in volume, consistent with the calculated difference between cubic and monoclinic phases, and with XRD results. The measured densities of nanograined c‐Y 2 O 3 and m‐Y 2 O 3 phases are comparable with the theoretical densities of 5.03 and 5.41 g/cm 3 , respectively . In what follows, the discussion is focused on the first processing step (cubic→monclinic), where a surface modification is observed, Fig.…”

“…Hardness of a processed sample is measured using both Vickers and nanoindentation techniques. Vickers hardness of the transformed samples are measured at different loads (200, 500, and 1000 g) and compared with the unprocessed sample . A 20% increase in the hardness is observed in the bulk sample relative to the unprocessed sample.…”

“…However, the high cost of fabrication and its poor performance at high temperatures has initiated a search for suitable alternatives. Nanocrystalline Y 2 O 3 and Y 2 O 3 ‐base nanocomposites are attractive candidates, as they exhibit excellent transmittance in the mid‐to‐near infrared (2–8 μm) range . In addition, nano‐Y 2 O 3 has good mechanical properties over the temperature range of interest.…”

“…However, traditional ceramic processes require high sintering temperatures, which makes the fabrication of polycrystalline Y 2 O 3 with grain size <1 μm particularly challenging, even when starting with ultrafine powder. Three methods may be used to overcome this limitation: (i) start with a metastable nanopowder, produced by vapor condensation method, and control its decomposition kinetics during high‐pressure sintering to obtain a nanograin size. Typically, the pressure requirement is in the 3–8 GPa range, which places a practical limit on the part size that can be fabricated using today's hot‐pressing technologies; (ii) generate a biphasic nanocomposite, in which the two phases strongly impede grain growth, particularly when their volume fractions are comparable .…”

“…Typically, the pressure requirement is in the 3–8 GPa range, which places a practical limit on the part size that can be fabricated using today's hot‐pressing technologies; (ii) generate a biphasic nanocomposite, in which the two phases strongly impede grain growth, particularly when their volume fractions are comparable . A prerequisite for optical applications is that the refractive indices of the two phases should be similar, and the particle size should be < 1/20λ of the wavelength of light of interest; and (iii) utilize a pressure‐induced, reversible‐phase transformation, as discussed elsewhere . Recently, Yoshida et.…”