Abstract
The crystallization of glass microspheres in the Y
2
O
3
–Al
2
O
3
-system produced from precursor powders of four different nominal compositions via flame synthesis is analyzed in detail by ...electron microscopy with a focus on electron backscatter diffraction (EBSD). Growth models are formulated for individual microspheres crystallized during flame synthesis as well as after an additional heat treatment step. 16 different types of crystallized bodies are cataloged for future reference. They are presented without regard for their relative occurrence; some are extremely rare but illustrate the possibilities of flame synthesis in the analyzed system. All three phases in the binary Y
2
O
3
–Al
2
O
3
-phase diagram (Y
3
Al
5
O
12
, YAlO
3
and Y
4
Al
2
O
9
) and α-alumina are located by EBSD. Energy dispersive X-ray spectrometry results obtained from these microspheres show that their chemical composition can deviate from the nominal composition of the precursor powder. The multitude of differing microsphere types showing polygon and dendritic crystal growth as well as phase separation indicate that flame synthesis can lead to a wide variety of parameters during microsphere production, e.g. via irregular flight paths through the flame, contaminants or irregular cooling rates.
Bi doped 2CaO-Al
2
O
3
-SiO
2
(gehlenite) glasses were prepared by conventional melting. The concentration of Bi
3+
was 0, 0.25, 0.50 and 1.50 mol. %. The prepared samples were X-ray amorphous except ...of the undoped sample and the sample with the highest content of Bi
3+
, for which XRD records revealed traces of crystalline gehlenite.
Differential thermal analysis (DTA) was used to study the thermal properties of prepared glasses, performed in the atmosphere of nitrogen at 5 different heating rates. The DTA records of all studied samples contained only one exothermic effect. Based on the results of XRD analysis, the exothermic effect was attributed to the crystallization of gehlenite. The XRD patterns of all samples measured after DTA analysis contained gehlenite as the main crystalline phase (01–074-164 COD). The DTA records measured at a heating rate of 10 °C.min
−1
revealed a significant decrease in the temperatures of maxima of the exothermic peaks (from 982 °C to 943 °C) with the increasing Bi doping. The model approach using the Johnson–Mehl–Avrami-Kolgomorov (JMAK) equations with six different values (1.5, 2, 2.5, 3, 3.5 and 4) of Avrami parameters (
m
) was used to study crystallization kinetics of gehlenite glasses. The model that best describes the experimental data was found, using the AIC (Akaike criterium) and wAIC (Akaike weight) criteria:
A
(frequency factor) and
E
app
(apparent activation energy) were determined. Based on kinetic parameters values, it is concluded that the nucleation rate is constant. Irrespective of the Bi content, the movement of the growth zone is controlled by diffusion and 3D growth is characteristic for the gehlenite crystals growth.
Five compositions in the system Al2O3–Y2O3 with high level of homogeneity were prepared in the form of glass microspheres by flame synthesis. The amorphous nature of prepared glasses with highly ...disordered structure was confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman and nuclear magnetic resonance (NMR) spectroscopy. In the NMR spectra, typical signals with chemical shifts of 75, 42 and 12 ppm were observed, which were attributed to the presence of AlO4, AlO5 and AlO6 motifs in the glass structure. The ratio of individual motifs in glass samples did not change significantly with the composition. The crystallization of yttrium-aluminium garnet (YAG) phase was observed as a major process in the glasses thermally treated up to 1450 °C, with slow crystallization of θ- and α-Al2O3 phases detected in the temperature interval 980–1450 °C. IR and Raman spectra of the microspheres crystallized at 998, 1300 and 1500 °C for 4 h contained typical bands, that were assigned to the vibrations of AlO4 and AlO6 groups in YAG and Al2O3 structures. The comparison of 27Al and 89Y magic angle spinning (MAS) NMR spectra showed the presence of only YAG and α-Al2O3 phase in the samples crystallized at 1500 °C and the presence of a trace amount of θ-Al2O3 in the sample crystallized at 998 and 1300 °C. The yttrium aluminium perovskite (YAP) and yttrium aluminium monoclinic (YAM) phases, expected in this system, were no detected.
In this work, La2Ce2O7/40 wt% YSZ (LC40Z) mixed powders were used to fabricate composite bulk ceramics with their potential use as a material for high-performance thermal barrier coatings (TBCs). For ...this purpose, the effect of sintering temperature on the chemical reactivity of LC and YSZ was investigated via hot-pressing at the temperatures of 1300, 1400 and 1500 °C. X-ray diffraction analysis (XRD), Raman spectroscopy, and scanning electron microscopy (SEM) were utilized to characterize the phase and microstructure evolution in both LC40Z powder mixtures and composite bulk ceramics. Results showed that solid-solution reactions occurred between the YSZ and LC during hot-pressing of the LC40Z powder mixtures, indicated by a new phase of La2Ce0·2Zr1·8O7 (LCZ) observed in the XRD patterns. The detailed analysis of Raman spectra confirmed the gradual transition from fluorite LC to pyrochlore LCZ structure, which was demonstrated by the appearance of characteristic pyrochlore bands. All composites densified by hot-pressing exhibited a high relative density above 95%. The average grain size of the LC40Z composites increased significantly with increasing sintering temperature, while gradual pore-healing was observed. The associated mechanical properties of LC40Z ceramics were also reported. The Vickers hardness values increased with increasing sintering temperature, which is consistent with the microstructure evolution and relative density variations. The highest hardness, with a value of 10.99 ± 0.23 GPa, was achieved for the composite hot-pressed at 1500 °C. The fracture toughness results showed the same dependence on sintering temperature. The fracture toughness increased from 1.97 ± 0.15 to 2.4 ± 0.14 MPa m1/2, indicating that the mechanical properties of the LC40Z composites can be tailored by changing the sintering temperature during hot-pressing.
Yttrium aluminate glasses (76.8 mol% of Al
2
O
3
, 23.2 mol% of Y
2
O
3
) doped with Er
3+
and Nd
3+
ions at different concentration levels (0.25 mol%, 0.5 mol% and 0.75 mol% Er
2
O
3
/Nd
2
O
3
) ...were prepared by flame synthesis in the form of glass microspheres. The prepared samples were XRD amorphous, without presence of any crystalline phases in measured patterns. The two exothermic effects (~ 940, ~ 1010 °C), which can be assigned to the two steps of YAG crystallization, were observed in the DSC records of all prepared samples. The high temperature XRD measurements showed YAG (900–1200 °C) and α-Al
2
O
3
(1300–1450 °C) phase crystallization. The emission spectra were measured in the VIS and NIR regions for Er-doped samples and in the NIR region for Nd-doped samples. All measured emission spectra contain of characteristic bands due to the typical 4
f
–4
f
transitions within the Er
3+
and Nd
3+
ions. Comparison of the measured intensities of Er-doped samples made it evident that the highest intensities were obtained for the 0.5 mol% Er
2
O
3
-doped sample (in both the NIR and VIS spectral regions). The maximum intensity for Nd-doped samples was found when the sample was doped with 0.75 mol% of Nd
2
O
3
. The slowly increasing of emission intensities in samples after 20 min annealing at 1000 °C and Stark splitting of emission bands in samples after 40 and 60 min annealing at 1000 °C and after 20, 40 and 60 min annealing at 1500 °C was observed.
The ytterbium-aluminate microspheres with yterbium-aluminium garnet composition doped by 0.25, 0.5, 0.75, 1.0, 3.0 and 5.0 mol.% of Er2O3 were prepared by combination of sol-gel method and flame ...synthesis. The prepared systems were mostly X-ray amorphous with the presence of traces of ytterbium-aluminium garnet phase (YbAG), and ytterbium oxide. The thermal analysis of prepared systems shows crystallization of the prepared systems in the temperature interval 914–984 °C in one or two steps, depending on the Er content. Ytterbium aluminate garnet was found to crystallize as the prevailing phase in all heat-treated samples. An almost pure red up-conversion photoluminescence emission with the intensity influenced by the Er2O3 content was observed in prepared systems.
The glass of gehlenite composition was prepared by flame synthesis in the form of microspheres. The powder precursor was synthesised by standard solid-state reaction method using SiO
2
, Al
2
O
3
and ...CaCO
3
. The prepared glasses were characterized from the point of view of surface morphology, phase composition and thermal properties by optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. The prepared samples contained only completely re-melted spherical particles. SEM did not reveal any features indicating the presence of crystalline phases. However, traces of crystalline gehlenite were detected by XRD. The high-temperature XRD measurements (HT XRD) were carried out to identify the phase evolution during glass crystallization. In the studied temperature range, gehlenite phase was identified as the main crystalline phase. Non-isothermal DSC analysis of prepared glass microspheres was carried out from room temperature up to 1200 °C at five different heating rates: 2, 4, 6, 8 and 10 °C/min to determine the thermal properties of microspheres. In order to study the crystallization kinetics, the DSC curves were transformed into dependence of fractional extent of crystallization (
α
) on temperature. The Johnson–Mehl–Avrami–Kolmogorov model was found to be suitable for description of crystallization kinetics. Frequency factor
A
= 5.56 × 10
29
± 1.73 × 10
29
min
−1
, apparent activation energy
E
app
= 722 ± 3 kJ mol
−1
and the Avrami coefficient
m
= 2 were determined. In the studied system, the linear temperature dependence of nucleation rate, diffusion controlled crystal growth interface and a 2D crystal growth were confirmed.
Crystallization of TiO2 xerogel Pagáčová Jana; Alfonz, Plško; Papučová Iveta ...
Journal of thermal analysis and calorimetry,
12/2020, Letnik:
142, Številka:
5
Journal Article
Recenzirano
For utilization of TiO
2
in advanced applications, TiO
2
after thermal treatment has to be in crystalline phase of anatase, because it gives the desired (photocatalytic) properties to material. The ...work deals with the study of anatase crystallization in TiO
2
xerogel (dried at 80 °C) which was prepared from sol in “titanium isopropoxide–isopropyl alcohol–nitric acid–distilled water” system. DTA/TG analysis at heating rates of 10 and 2 °C min
−1
was used for study of processes which take place during the thermal treatment of xerogel. Besides assumed anatase crystallization, the processes connected with mass loss were observed, and therefore, the process of thermal treatment of xerogel was observed using high-temperature Raman spectroscopy and high-temperature X-ray diffraction. Based on the comparison of results of all analyses, it can be concluded that the anatase is formed from xerogel by sequent processes in the temperature range of 280–400 °C—the first process represents the decomposition of xerogel and subsequent formation of amorphous TiO
2
and the second process represents the formation of crystalline anatase, respectively.
Bi-doped CaO–Al
–SiO
glass microspheres with Ca
Al
SiO
(gehlenite) composition were prepared by combination of solid-state reaction and flame synthesis. The concentration of Bi was 0.0, 0.5, 1 and ...3 mol%. The chemical composition of prepared glass microspheres was determined by X-ray fluorescence (XRF). The structural and magnetic properties of prepared glass microspheres and their polycrystalline analogues were studied by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Raman spectroscopy and SQUID magnetometry. The closer inspection of glass microspheres surface by SEM confirmed smooth surface and revealed no features indicating presence of crystalline phases. All Bi-doped microspheres are X-ray amorphous, however in case of undoped microspheres XRD detected traces of crystalline gehlenite. XRD analysis of samples crystallized at 1273 K for 10 h revealed the presence of gehlenite as the main crystalline phase. The presence of gehlenite in crystallized samples were also confirmed by Raman spectroscopy. All samples (glass microspheres and their crystalline analogues) showed diamagnetic or weak ferromagnetic behavior at room temperature, whereas paramagnetic or weak ferromagnetic behavior was observed at 2 K.
Glass microspheres with yttria-alumina eutectic composition (76.8 mol% Al
and 23.2 mol% Y
) were prepared by sol-gel Pechini method and flame synthesis with or without subsequent milling. Prepared ...amorphous powders were studied by X-ray powder diffraction (XRD), particle size analysis (PSA), scanning electron microscopy (SEM) and differential thermal analysis (DTA). Hot pressing (HP), rapid hot pressing (RHP) and spark plasma sintering (SPS) were used to sinter amorphous precursor powders at 1600 °C without holding time (0 min). The preparation process including milling step resulted in amorphous powders with narrower particle size distribution and smaller particle size. All applied pressure assisted sintering techniques resulted in dense bulk samples with fine grained microstructure consisting of irregular α-Al
and Y
Al
(YAG) grains. Milling was beneficial in terms of final microstructure refinement and mechanical properties of sintered materials. A material with the Vickers hardness of HV = (17.1 ± 0.3) GPa and indentation fracture resistance of (4.2 ± 0.2) MPa.m
was prepared from the powder milled for 12 h.
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