Electron microdiffraction studies of zirconia particles

Abstract

A batch of zirconia was prepared at a pH of 2.95 using a sol-gel technique. The crystal structures formed during 500 ~ calcination was followed by X-ray diffraction. The tetragonal phase was the major component after the initial calcination period of 15.5 h; however, it gradually transformed to the monoclinic crystal form during 200 h of calcination at 500 ~ Electron microdiffraction was employed in the present investigation to determine the crystal structure of individual particles, and to identify whether these particles contained twin variants. A technique has been developed to get a dispersion of agglomerated particles by condensing and spreading the beam on the agglomerates at 200 kV. The data revealed that some of the individual zirconia particles are featureless and some of them appear to contain single or multiple twin variants.

Zirconia: A Review of a Super Ceramic

“…Following transformation of t-Zr0 2 to m-Zr0 2 many individual particles are observed to be twinned or multiply twinned. 58 The BET surface areas, following 4 hours of calcination at 600 0 e in air, fall within 40 + 10 m 2 /g and show no dependence upon the crystal phase of the calcined material. Thus, similar surface areas are obtained for t-Zr02 and m-Zr0 2 ; this provides an additional measure to show that similar particle sizes are obtained for both materials.…”

“…The particle indicated by an arrow in Figure 17 is seen to contain three or four variants in the particle with a diameter of 34 nm. 58 A still unresolved feature of the t -+ m transition in zirconia concerns twinning, which is commonly observed in monoclinic Zr0 2 . Twins are believed to occur in bulk materials to accommodate at least partially the shape change associated with the t -+ m transformation.…”

“…55 However, data in addition to that in Figure 17 have been obtained to provide strong evidence to show that ultrafine particles of zirconia do contain internal twins or hybrid crystal phases or poly domains. 58 The important question is how the changes in the initial particle size of the gel precipitated at different pH's occur. Zr0 2 from the material produced at a pH of 13.5 clearly shows that the critical size for t-Zr02' if it is stabilized by excess surface energy, must be greater than about 18 nm.…”

“…At the same time, it does not appear that the particles undergoing the t -+ m transformation increase significantly in physical size. 58 The "in-situ" X-ray diffraction technique was utilized to follow the phase transformation in zirconia. 62 It was found that irrespective of whether the rapid precipitation or slow preCipitation technique was utilized for the precipitation of zirconia, initially the amorphous zirconia transformed at about 450°C to a tetragonal form (Figure 18).…”

The Production and Characterisation of Trichotoxin Peptaibols, by Trichoderma asperellum

“…In the present study, MEA medium was shown to be advantageous for the confirmation of the putative peptaibol production. Addition of Aib to the growth medium has been shown to induce a higher level of peptaibol production [17]. It is also clear that MEA is a suitable medium for the production of peptaibols by T. asperellum, and this can be enhanced by the presence of a precursor of Aib (Fig.…”