UV Raman spectroscopic study on the phase transformation of ZrO₂, Y₂O₃–ZrO₂ and SO₄²⁻/ZrO₂

Abstract

The phase evolution of zirconia (ZrO 2 ), sulfated zirconia (SO 4 2− /ZrO 2 ) and yttrium oxide incorporated zirconia (Y 2 O 3 -ZrO 2 ) from the tetrahedral phase to the monoclinic phase was studied using UV Raman spectroscopy, visible Raman spectroscopy and x-ray diffraction (XRD). It is clearly observed that there are discrepancies between the results from the UV Raman spectra, visible Raman spectra and XRD patterns. The phase change from tetragonal to monoclinic is always earlier or at lower calcination temperatures as observed by UV Raman spectroscopy than by visible Raman spectroscopy and XRD. UV Raman spectroscopy is found to be more sensitive at the surface region while visible Raman spectroscopy and XRD supply the information mainly from the bulk. The inconsistency in the results from the three techniques suggests that the phase transformation of zirconia starts from its surface region and then gradually develops into its bulk. For SO 4 2− /ZrO 2 and Y 2 O 3 -ZrO 2 , the transformation from the tetragonal to the monoclinic phase is significantly retarded owing to the presence of the sulfated groups and the yttrium oxide. Particularly, the tetragonal phase of Y 2 O 3 -ZrO 2 can be maintained up to 800 • C although its phase at the surface region changed into monoclinic at 500 • C.

Synthesis by coprecipitation of india-stabilized zirconia and codoping with MoO3, WO3, TaO2.5, or NbO2.5 for application as thermal barrier coatings

“…1, the distinction between tetragonal and cubic phase (c-) is not apparent, since the distinction between the peaks (002) t /(110) t at 2θ ≈ 35º cannot be observed unambiguously because of peak-broadening effects 32,33 . For this reason, visible Raman spectra were used to observe the existence of the single t-phase 2,34 . Raman spectra for InTaSZ compositions are shown in Fig.…”

Design of new nanomaterials for their use in catalysis and photochemistry

“…This fact can be explained due to the presence of WO x which inhibits the surface mobility of zirconia, avoiding in this way the sintering process and keeping high surface area values 9 of the final material, or to the stabilizing effect of W which substitute OH and the defects in the material. 19 Regarding the molar W/Zr ratio determined by ICP-AES, the nominal value determined by this technique is quite near to the theoretical value introduced in the synthesis, supporting the idea that all tungsten and zirconium is precipitated during 35 the microwave assisted synthesis. In addition, this molar ratio is constant upon heating indicating that tungsten is not lost during the calcination step.…”

“…Several synthetic pathways have been proposed to synthetize W/ZrO 2 materials, such as deposition of tungsten salts over hydrous zirconia, 11,18 anion exchange, 19,20 coprecipitation, 21,22 hydrothermal synthesis 23 , sol-gel, 24 etc. In the three last methods mentioned, treatments at relatively high temperatures are required in order to activate the materials, probably because the tungsten is located inside the zirconia and it has to migrate to the surface in order to be catalytically active.…”

“…Periodic density functional theory (DFT) calculations were performed using the Perdew−Wang (PW91) exchange-correlation functional within the generalized gradient approach (GGA) 19,20 as implemented in the VASP code. 21 The valence density was expanded in a plane wave basis set with a kinetic energy cutoff of 450 eV, and the effect of the core electrons in the valence density was taken into account by means of the projected augmented wave (PAW) formalism.…”

“…[12][13][14][15][16] Dichas fases presentan diferente acidez y basicidad en función de su estructura cristalina, lo que les confiere diferente actividad y selectividad en diversas reacciones catalíticas. [17][18][19][20][21] Las propiedades ácidas del ZrO 2 se pueden incrementar con la introducción de molibdeno, tunsgeno o iones sulfato. De entre ellos el tungsteno es el preferido ya que los materiales con molibdeno no presentan tanta acidez y los que contienen sulfatos se desactivan por la pérdida de éstos en los procesos catalíticos.…”