Calcination, precursor powders

Rocha, J., del Arco, M., Rives, V. and Ulibarri, M. A. (1999). Reconstruction of layered double hydroxides from calcined precursors a powder XRD and Al-27 MAS NMR study. J. Mater. Chem. 9, 2499. 

The segregation observed in these precursor powders is often lost by calcining the powders where solid state interdiffiision and reaction of the different metals takes place. If the segregation is large scale, then it will take a long time for this interdiffiision to take place. For this reason an understanding of the segregation processes in coprecipitation is important. 

Zeta-process and conventionally calcined powders can be made flowable for rapid mold filling in automated isostatic pressing of thin-walled cylindrical tubes by the technique of slurry spray drying (S D). Here the calcined and milled precursor powder is dispersed in an aqueous slurry (i.e., a stable, high solids content, low viscosity suspension) prior to spray-drying into spherical agglomerates. 

The foregoing powder synthesis techniques, along with others such as polymerization and decomposition of complex double alkoxides and the melt and spray decomposition of mixed nitrates, produce precursor powders with a high degree of chemical homogeneity. However upon subsequent calcination of the products of these reactions, the crystallization path invariably passes through the following sequence of reactions ... 

Fig. 4. The XRD patterns of samples, (a) Metal support oxidized at 1100 C (b) Sample 9 (c) Sample 3 (d) Sample 6 (e) Precursor powder calcined at 1000 °C...

Figure 3 shows the X-ray diffraction powder patterns of Rh-Sn/Si02 reduced at different temperatures. The catalyst which were reduced below 573 K showed no shift of Rh (111) diffraction peak as shown in Fig. 3-a. The reduction over 673 K brought about a shift toward lower diffraction angle, which indicates well-mixing between Rh and Sn [3, 4,11]. The Sn oxide in the calcined precursor... 

Preparation of precursor powders for polycrystalline processing 18, 17.3.7.1.1 Blending, calcination and milling procedures ... 

Figure 5. XRD patterns for the precursor powder obtained at pH=6 and its calcined products.

Flow chart of the mixed precipitation method is shown in Fig. 3.42. Briefly, alumina precursor powder was first synthesized by adding aluminum nitrate solution (0.15 M) dropwisely into the ammonium bicarbonate solution (1.5 M) under vigorous stirring. The as-prepared alumina precursor powder was then dispersed in yttrium nitrate solution to form uniform suspension. After that, ammonium bicarbonate solution (0.5 M) was added dropwisely into the suspension, so that yttrium precipitate was coated on the alumina precursor powder. The mixed precursor was aged, washed, dried, and then calcined to form yttrium aluminum garnet, with a phase formation temperature about 300 °C lower than that required by the traditional solid-state reaction process. Alternatively, it is also worth trying to coat yttria with alumina. 

In another study, it was found that the way to treat the precursor powder had significant effect on performance of YAG ceramics by using SPS [142]. When sintering YAG synthesised by using reverse-strike co-precipitation with SPS, the precursor powder was calcined at 1000 °C and dispersed by using ball milling with... 

The Nd YAG ceramics were prepared from precursor powder synthesized by using chemical coprecipitation method, with mixed aqueous solutions of aluminum, yttrium, and neodymium chlorides as sources of the cations and solution of ammonium hydrogen carbonate as precipitant. The precursor was calcinated at 1200 °C to obtain oxide raw powders. The powders were compacted and finally sintered at 1750 °C in vacuum for 5-20 h [90]. 

Dielectric Bai cSr cTi03 (BST) precursor was prepared by the co-precipitation of Ti0S04, Ba -", and Sr "". NaOH and NaCOj were used as precipitating agents to adjust the pH to 9. Inexpensive Ti0S04 was used as the Ti source. After reaction for 180 min, the precipitate was washed with alcohol once, and then dried for about 12 h in an electric oven at 100°C to obtain the dried precursor powder. The dried powder was calcined at 950°C for 4 h. The residue solids were washed with deionized water to remove the remaining salts and dried in air at 100°C for 12 h. Bao.sSro.sTiOs powder was obtained as follows ... 

An amorphous precursor powder was desired to facilitate iron dissolution and geopolymer formation. However, when using the PVA method, a minimum calcination of 600 C was necessary to ensure PVA polymer removal. Shown in Fig. 1(a), the low angle peak near 10 20 was due to the presence of PVA, but was removed after the powder was heated to 600 C (Fig. 1(b)). The broadness of the peaks in Fig. 1(b) indicates that the calcined powder was poorly crystalline. In addition, calcination of the precursor powder caused its color change from a yellow-red to a deep ted color. 

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