Cristobalite peak

It is known SiC starts to oxidize at -750 °C. The initial produet is amorphous silica, which begins to crystallize into cristobalite at - 1100 °C. Fig. 1 shows the XRD patterns of porous SiC ceramics sintered at different temperatures for 2 h. At 1200 °C, obvious cristobalite peaks were found. In addition, a substantial spinel forms as the result of the diffusive reaction between AI2O3 and MgO in solid state, a-cordierite peaks appear at 1300 °C and their intensity is found to increase notably as the sintering temperature is further increased to 1350 °C. On the other hand, the peak intensity... 

XRD patterns before and after the oxidation for 100 h are shown in Figure 4.11. For Zl, see Figure 4.11a, there are no visible changes for oxidation at 1000 "C compared with the as-sintered product. However, after 1200 "C oxidation, a visible silica (cristobalite) peak was detected and a very few muUite peaks were also detected. When the temperature reached to 1400 "C, the intensity of mullite peaks became stronger and more peaks were visible, and the peak of cristobalite was still... 

All the solid phases were identified and characterized for crystallinity by X-ray powder diffraction (Philips PW 1730/10 diffractometer, Cu Kq radiation equipped with a PW 1030/70 vertical goniometer and connected to a P.C. computer for quantitative analyses). Crystallinities for Nu-10 and cristobalite were computed by comparing the intensity of the most characteristic diffraction peaks of each sample to that of the corresponding pure 100% crystalline phases used as standards. In some cases calibration curves derived from Nu-10/cristobalite mechanical mixtures were used. Si, Al, and alkali contents were determined either on precursors or calcined samples (900 C, air flow, 4h) by atomic absorption, using a Perkin-Elmer 380 AA instrument after digestion and dissolution of the samples in H,S04/HF solutions and further elimination of HF by gentle heating at 60 C for 12 n. 

Refinement of the framework coordinates was difficult because of the acentricity and pseudosymmetry. A stable least-squares solution was obtained for the framework atoms in which the T-0 distances indicate alternation of A1 and P atoms. However, the 0 atoms showed large displacements from the centroid, particularly 0(2) for which a difference-Fourier map indicated three spearate peaks (Figure 3). Because there is no optical or X-ray evidence for symmetry lower than hexagonal, it is assumed that there are microdomains with tilted tetrahedra, as proposed for high-cristobalite(6) and high-tridymite( 7). For convenience, the displacements of the oxygen atoms are approximated by ellipsoids. 

Figure 4 X-ray diffraction patterns contrasting various crystallinities of silica (a) radiolarian silica, Porcelanite (opal-CT) and a-Cristobalite (made by heating silica gel at 1,350 °C for 4 h) from Calvert (1983) (b) diatom assemblage from Antarctic plankton tow, deep-sea siliceous ooze (Holocene in age) from beneath the Antarctic Polar Front, and two chert deposits from state of New York. The sharpness of the silica peak(s) between 20° and 26° two theta increases as silica undergoes diagenetic transformation from a fresh-diatom assemblage to buried sediment for...

Low cristobalite is metastable with respect to quartz but persists up to is taken as the temperature at the peak... 

Figure 10. Neutron total scattering S(Q) functions from powdered samples of the high and low temperature phases of cristobalite and quartz, shown over two ranges of Q. The scattering functions show well-defined Bragg peaks at lower values of Q superimposed above structured diffuse scattering, and at high Q the scattering shows well-defined oscillations.

Figure 11. T(r) functions for the high and low temperatures phases of cristobalite obtained from neutron total scattering measurements. The two peaks at low-r correspond to the Si-0 and 0-0 bonds. The dashed lines trace features in T(r) for p-cristobalite that are not seen in the T(r) for a-cristobalite.

Figure 7. MAS-NMR spectra for spectra for cristobalite as a function of temperature, across the a-p transition. The chemical shift decreases with increasing temperature in the a-phase and across the a-p transition. A distribution of transition temperatures causes the presence of peaks for both phases to be present at 230°. [Used by permission of the editor of Physics and Chemistry of Minerals, from Spearing and Stebbins (1992), Fig. 7, p. 313, Springer-Verlag 1992]...

The authors described the reaction mechanism as occurring by impregnation of the sample, bond reaction, and bond depletion. Immediately the alkali consumes the cristobalite and attacks the glass of the matrix bond. Next the fine crystalline mullite bridges associated with the bonding matrix react with the soda. The mullite x-ray peak intensity is quickly reduced. The extent of attack on the coarser crystalline mullite is intensified at higher temperatures or as the reaction proceeds. When equilibrium... 

Figure 11. X-ray patterns for silica gel heated at 700°C over 100 hrs (curve a), CPG (curve b) and CPG heated at 700°C during 105 hrs (curve c). (A) peaks correspond to o-cristobalite, (B) peaks correspond to Q-quartz.

TG and DTA profiles of AP-25HDO-X materials were similar to that AP-24PDO-X ones although the temperature of the exothermic peak (1238 K) did not show any change with 25HDO/A1 molar ratio. The exothermic peak was due to ciystallization into a-cristobalite AIPO4. 

The data for the NaP03-Si02 system are given in Table IX. Crystalline phases of all the samples are composed of a-cristobalite and trisodium trimetaphosphate. Some of them contain small amounts of or-tridymite. Ortho-, di-, and triphosphate were found by paper chromatography in the aqueous solutions of the samples. These ortho- and chain phosphates may be derived from amorphous substances, because the x-ray diffraction patterns of these samples do not exhibit any peaks attributable to phosphates other than trisodium trimetaphosphate. However, the structures of these amorphous substances cannot be determined from the data presented here. It has been shown that the crystals of neither of the systems contain P-O-Si linkages. 

She e/ alP have determined, using XRD, the major phases present in porous SiC ceramics sintered at different temperatures for 4 h. At 1400°C, porous SiC ceramics consist mainly of SiC, cristobalite and a- ALO3, but slight mullite peaks can be found. When the temperature increases to 1450°C, the mullite peaks are obvious. At 1500 and 1550°C, the amount of a- AljOj decreases abruptly and more extensive mullitization occurs. 

---