Phase transformation aluminas

Each form of crystalline alumina is only stable in a limited temperature range. Fig. 3.19 shows synthesis conditions and phase transformations of the most important aluminas. 

Topsee and coworkers—in situ XRD synchrotron studies indicate well-dispersed metallic Cu particles upon activation ZnO observed to strain Cu particles by EELS. Topsoe and coworkers,264 utilizing in situ XRD with synchrotron radiation, demonstrated that the Cu phase transforms primarily to a crystalline metallic Cu phase from CuO precursor during activation. Smaller particles were detected when the ternary A1203 component was present (9.5 nm versus 14 nm for the binary Cu/Zn catalyst), indicating that alumina acts primarily as a structural stabilizer, a spacer for well-dispersed Cu particles, which assists in minimizing sintering. 

Mitrovic and Knezic (1979) also prepared ultrafiltration and reverse osmosis membranes by this technique. Their membranes were etched in 5% oxalic acid. The membranes had pores of the order of 100 nm, but only about 1.5 nm in the residual barrier layer (layer AB in Figure 2.15). The pores in the barrier layer were unstable in water and the permeability decreased during the experiments. Complete dehydration of alumina or phase transformation to a-alumina was necessary to stabilize the pore structure. The resulting membranes were found unsuitable for reverse osmosis but suitable for ultrafiltration after removing the barrier layer. Beside reverse osmosis and ultrafiltration measurements, some gas permeability data have also been reported on this type of membranes (Itaya et al. 1984). The water flux through a 50/im thick membrane is about 0.2mL/cm -h with a N2 flow about 6cmVcm -min-bar. The gas transport through the membrane was due to Knudsen diffusion mechanism, which is inversely proportional to the square root of molecular mass. 

Fischer-Tropsch (FT) process is used for the production of hydrocarbon fuels. The process uses synthesis gases CO and H2O. It is shown that cobalt/alumina-based catalysts are highly active for the synthesis. The process is also used to convert coal to substitute or synthetic natural gas (SNG). The use of Fe-based catalysts is also believed to be attractive due to their high FT activity. HRTEM has played a major role in the study of phase transformations in Fe Fischer-Tropsch during temperature programmed reduction (TPR) using both CO and H2 (Jin et al 2000, Shroff et al 1995). TiClj/MgC -based (Ziegler-Natta) catalysts are used for polymerization of alkenes (Kim et al 2000) and EM is used to study the polymerization (Oleshko et al 2002). 

As the electrolytes, alkali metal sulfates(M=Li, Na, and K)(l-ll), 3-Alumina(12), and NASIC0N(13, 14) have been examined. Alkali metal sulfates are cation conductors at elevated temperature(>700 C). However, they have several disadvantages. One is the phase transformation of the sulfates(15-18). By this transformation, cracks occur in the electrolyte body and result in the permeation of the ambient gases. The other disadvantage is their low electrical conductivity. Mono, di, or tri-valent cations(19-24) have been doped so as to enhance their conductivity. Furthermore, they become ductile at a tem-... 

Spherical alumina can also be formed from commercial, low cost aluminum-oxides or even from aluminum-hydroxides. In the latter case energy of the plasma should provide not only the enthalpy of melting but that of dehydration and subsequent phase transformations of alumina as well. Under the aforementioned conditions particles below 45 pm have a good chance to be spherodized. Presumably the wide particle size distribution of starting gibbsite powder accounts for the less spheroidization rate of 70%. 

Several papers report [4] that liquid alumina solidifies not in the thermodynamically most stable phase of (X-AI2O3, but rather in the form of Y-AI2O3. This is attributed to the fact that the solidified phase structure is basically determined by the relative critical free enthalpies of nucleation of alternative crystal structures. Consequently, not surprising, that considerable part of spheroidized particles composed of y-AbOs and other metastable phases (such as 8, 0) of alumina (Fig. 7). The latter were formed from the y phase according to the usual route of phase transformation on cal-... 

Sanchez-Herencia, A.J., James, L., Lange, F., Bifurcation in alumina plates produced by a phase transformation in central, alumina/zirconia thin layers, J. Eur. Ceram. Soc., 20, 1297-1300, 2000. 

Because the currently used y-alumina is not stable in all acid and basic environments used in industry [2], the development of mesoporous layers other than y-alumina deserves attention as well. Most common materials that can be used for the mesoporous layer are zirconia and ti-tania [3,4], but recently also the preparation of mesoporous hafnia is described [5], Hafnia seems to be a very interesting membrane material, because it can, unlike zirconia and titania, be fired up to 1850°C without a phase transformation of its monoclinic form. Hafnia also has a high chemical resistance toward acid and basic media. Another interesting material, currently under investigation by the group of Brinker is mesoporous silica [6,7], This material is especially interesting because a tailor made morphology and pore-size is possible. 

For example, Raman microscopy was applied to investigate the sequestration of C02 at 150 °C and at a partial pressure of C02 of 15 MPa in magnesite (MgC03) (Wolf et al., 2004). A cell with a Raman microprobe was used to monitor phase transformations of C0M0O4 and NiMo04 upon heating in air and to follow the sulfidation of 7-alumina-supported NiMo04 catalysts at 320 °C in mixtures of H2 and H2S (Payen et al., 1980). 

Alumina membranes. It has been established that several phases of alumina exist and a particular phase of alumina is determined not only by the temperature it has experienced but also by the chemical path it has taken. For commercial membrane applications, the alpha- and gamma-phases of alumina are the most common. Alpha-alumina membranes are well known for their thermal and hydrothermal stabilities beyond 1,000 C. In fact, other transitional forms of alumina will undergo transformation towards the thermodynamically stable alpha-alumina at elevated tcmjxratures beyond 900 C. On the contrary, commercial gamma-alumina membranes are typically calcined at 400-600 C during production and are, therefore, subject to potential structural changes beyond 600°C. Moreover, alumina chemistry reveals that phase transition also occurs beyond that temperature [Wefers and Misra, 1987]. 

Han et al (61) reported that the Al2(Mo04)3 phase on alumina is easily hydrated by moisture in air and transforms into amorphous M0O3, whereas AIPO4 only shghtly reacts with water. Further addition of phosphorus decreases the formation of Al2(Mo04)3 since competitive adsorption of phosphorus and molybdenum oxo-species occurs on the alumina surface. Phosphorus inhibits the formation of Al2(Mo04)3 in the presence of nickel (62). The number of deposited polymeric phosphorus-oxo compounds decreases in the presence of molybdenum, probably through the formation of dispersed Mo—P heteropoly compounds (63). 

Wood E. J. and Rubie D. C. (1996) The effect of alumina on phase transformations at the 660-kilometer discontinuity from Fe-Mg partitioning experiments. Science 273, 1522-1524. 

The importance of phase transformations Is easily seen by reference to alumina. The most stable phase is o-alumina, with a surface area of ca 2 7 alumina is widely used as a support, with a surface area of ca 150-It is obvious that phase transformation of 7-AI 0 to a-Al 0 will... 

The sequence of phase transformations shown in Figure 2 is an approximationp largely because process variables such as time, atmosphere and properties of precursor hydroxides are not included. Thus, for example, bayerite and glbbslte may be converted to boehmite and thence to y-alumina during calcination if the particle size Is large and the precipitate Is moist [18]. 

The presence of small amounts of impurities can either accelerate or decelerate phase transformations In alumina. The presence of platinum at concentrations from 0.1 to 3 v% has been reported to accelerate sintering, with the production of the S phase, in particular, being favoured (Figure 3) [27]. On the other hand, 3% Mi was found to lead to the Initial stabilisation of alumina surface area, followed by rapid collapse of the structure (Figure... 

It is clear from the above discussion that despite the importance of alumina as a catalyst support, the detailed mechanism of sintering is far from clear. There is good understanding of the effect of different factors on sintering, but the detailed ntechanlsra of phase transformations is, in many cases, open to considerable uncertainty. 

Chang PL, Yen FS, Cheng KC, Wen HL (2001) Examirratiorrs on the critical and primary crystallite sizes during 0-to a-phase transformation of rrltrafine alumina powders. Nano Letters 1 253-261 Charlet L, Manceau A (1992) X-ray absorption spectroscopic study of the sorption of Cr(IIl) at the oxide/water interface. 11 Adsorptiorr, coprecipitation and surface precipitation on ferric hydrous oxides. J Colloid Interface Sci 148 425-442... 

The maximum stress is obtained after each cycle and so no stress relaxation occurs. Note that the deflection measurements start with a dried membrane which already shows a certain deflection which is equivalent to a tensile stress level of 30 0 MPa. It is not clear at the moment whether it is allowed to sum up these two contributions or that the drying stress relaxes during heating and is replaced by stresses originating in the phase transformation/thermal mismatch processes. In any case when summing up is allowed the final stress in the y-alumina after cooling down is not greater than 30 MPa in the other case it is zero. 

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