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 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.
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.
Yttrium aluminate glass microspheres with the eutectic composition 76.8 mol. % Al2O3 and 23.2 mol. % Y2O3 were prepared by combining the sol-gel Pechini method with flame synthesis. The sol-gel ...method was applied to achieve the desired composition homogeneity of the prepared glass and hence, improve the microstructure homogeneity and mechanical properties of bulk polycrystalline materials. The latter were prepared by hot pressing, more specifically pressure assisted sintering, at 1050 °C, 1300 °C and 1600 °C using pressures of 30 MPa and 80 MPa and holding times between 0 and 30 min. This also led to the crystallization of the glass. A composite with the Vickers hardness 18.0 ± 0.7 GPa and an indentation fracture toughness 4.9 ± 0.3 MPa.m1/2 was obtained by sintering at 1600 °C, at the pressure of 80 MPa and with 30 min isothermal heating at the maximum temperature. Improved mechanical properties were observed when increasing the temperature of sintering and the holding time. This can be attributed to the formation of a unique microstructure consisting of α-Al2O3 grains in the μm-scale embedded in a YAG (yttrium-aluminium garnet) matrix in the hot-pressed samples.
The ytterbium aluminum garnet composition YbAG (62.5 mol.% Al
2
O
3
, 37.5 mol.% Yb
2
O
3
) was prepared in the form of glass microspheres by flame synthesis. Precursor powder for flame synthesis ...with high homogeneity was prepared by modified sol–gel Pechini method. XRD pattern of prepared glass microspheres indicated predominantly amorphous nature of the sample. Detailed study of morphology of the microspheres by scanning electron microscopy revealed the presence of a small fraction of partially or fully crystallized microspheres. The high-temperature X-ray powder diffraction analysis (HT XRD) was carried out in the temperature interval 750–1450 °C: The temperature dependence of phase composition was determined. Crystallization of Yb
3
Al
5
O
12
—ytterbium aluminum garnet phase—was observed in the temperature range 900–1200 °C. The DSC analysis with heating rates 2, 4, 6, 8, 10 °C min
−1
in temperature interval 25–1200 °C was performed in N
2
atmosphere to study thermal behavior and crystallization kinetics of prepared glass microspheres. The two exothermic effects at 918 and 939 °C were observed, which were attributed to Yb
3
Al
5
O
12
crystallization. The crystallization kinetics of prepared sample was examined with the use of JMAK model, and the kinetic triplet—frequency factor
A
= (1.8 ± 2.2) 10
+28
min
−1
(for the first peak),
A
= (1.2 ± 1.6) 10
+55
min
−1
(for the second peak), apparent activation energy
E
app
= (6.4 ± 0.1) 10
+02
kJ mol
−1
(for the first peak),
E
app
= (1.3 ± 0.1) 10
+03
kJ mol
−1
(for the second peak) and the Avrami coefficient
m
= 3 (for the first peak) and
m
= 2 (for the second peak)—was determined using RSS,
R
adj
2
, AIC and
W
AIC
criteria.
The Ni-doped Ca
2
Al
2
SiO
7
glass systems were prepared by flame synthesis. Solid-state reaction was used to prepare the powder precursors. The concentration of Ni was 0.5, 1 and 3 mol%. ...Polydisperse systems were prepared with diameters between 5 and 140 μm. Detailed examination of morphology of the glass microbeads by SEM revealed no features indicating the presence of crystalline phases. However, X-ray diffraction analysis showed that the samples GNi0.5 (0.5 mol% of Ni) and GNi1.0 (1.0 mol% of Ni) contained traces of crystalline gehlenite. HT-XRD was used to determine the temperature dependence of phase composition. For all prepared compositions, only one crystalline phase (Ca
2
Al
2
SiO
7
) was observed. DSC measurements in the temperature range 30–1200 °C at five different heating rates were carried out to study the thermal behavior. The DSC curves of all glasses contained one exothermic peak, which was attributed to crystallization of the gehlenite. The maximum of the peak decreased with increasing Ni content in the microspheres. The kinetic parameters (frequency factor
A
, apparent activation energy
E
app
and the Avrami coefficient
m
) of the crystallization were determined using the Johnson–Mehl–Avrami–Kolgomorov model. In case of GNi0.5 and GNi1.0 glasses, the nucleation’s rate had linear temperature dependence, the crystal growth interface is controlled by chemical boundary and the crystal growth is one-dimensional. The rate of nucleation is linear, the crystal interface growth is controlled by diffusion and one-dimensional crystal growth prevails in crystallization of the GNi3.0 (3.0 mol% of Ni).
Multiwall carbon nanotubes (MWCNT) were synthesized by catalytic chemical vapor deposition (CCVD) in a horizontal tube reactor using acetylene as the carbon source in a gas mixture with argon at ...700°C and 30minute reaction time. As a catalyst was used the sodium forms of natural platy nontronite particles Sampor or Washington and their iron modified forms. Additionally, hydrothermally synthesized hematite α-Fe2O3 and/or its heterocoagulates with nontronite particles were used to test the catalytic activity. Catalyst nanoparticles were used to modify the conditioned Si wafer surface used as the catalyst support. Before CNT growth the catalyst nanoparticles were activated by applying a hydrogen stream in the tube reactor at 700°C. The effects of catalyst type and the reaction conditions on MWCNT growth such as C2H2/Ar ratio, time and reaction temperature were investigated. The growth of MWCNT was affected by the density of catalyst particles covering the surface e.g. for hematite, the amount deposited on a silicon surface. Depending on the type of catalyst located on the Si substrate, the bamboo-like, curly shaped and straight individual MWCNT were formed. The quality of the synthesized MWCNT was investigated using Raman spectroscopy. According to the cation exchange to Fe-forms, the iron content in nontronites was increased by about 14.5wt.%. However, by the addition of hematite particles, the iron content was increased by about 13.0wt.% of the total iron present. Raman spectroscopy has proved that good quality ordered graphite structure of the carbon sheets in CNT was also achieved by using pure Na-forms of natural nontronites applicable as the low-cost catalyst nanoparticles. Therefore, no ion exchange modification using iron salt is necessary for this type of material.
•Sodium forms of nontronites were used as catalyst for synthesis of carbon nanotubes.•Catalytic performance of nontronites vs. synthetic hematite nanoparticles was slightly better.•The best catalyst was the heterocoagulate based on nontronite and synthetic hematite nanoparticles.•Natural nontronites can be used as efficient low-cost catalyst for CVD synthesis of carbon nanotubes.
The ALaE glass with eutectic composition (76.2 mol% Al2O3, 23.8 mol% La2O3) and the ALaP glass with LaAlO3 (lanthanum-aluminium perovskite) composition (50 mol% Al2O3, 50 mol% La2O3) were prepared by ...combination of sol-gel Pechini method and flame synthesis in the form of glass microspheres (diameter ≈ 10 μm). The prepared glasses were characterized by OM, SEM, XRD and DSC. The prepared samples contained only completely re-melted spherical particles. However, more detailed study by SEM revealed small fraction of crystalline microspheres. Traces of crystalline LaAlO3 were detected in X-ray powder diffraction patterns of both glasses. DSC analysis in the temperature range 35–1200 °C with heating rates 2, 4, 6, 8 and 10 °C/min was performed in nitrogen atmosphere to study the thermal behaviour and crystallization kinetics of prepared glasses. In the DSC curve of the eutectic ALaE glass recorded at 10 °C/min two exothermic effects were observed at 920 and 936°. Only one exothermic effect at 868 °C was present in the DSC curve of the ALaP sample with perovskite composition. The high-temperature X-ray powder diffraction analysis was carried out in the temperature interval 25–1200 °C. For both compositions formation of LaAlO3 as the main crystalline phase was observed in the whole measured temperature range. In order to study the crystallization kinetics, the DSC curves were transformed into dependence of fractional extent of crystallization, α on temperature. The JMAK model was found suitable for description of crystallization kinetics of both studied systems. The kinetic parameters (A, Eapp and m) were calculated for this model and crystallization mechanisms have been proposed.
Yttrium aluminate glasses with eutectic AY-E and near-eutectic composition AY-NE were prepared in the form of glass microspheres. Their basic characterization was carried out by XRD, optical ...microscopy and SEM. In DSC records of both samples, two exothermic peaks in temperature interval 940–1027 °C were observed. In both samples, YAG phase crystallized in two steps, as determined by HT XRD. DSC experiments conducted in the temperature interval 35–1200 °C at heating rates 2, 4, 6, 8 and 10 °C min
−1
were performed, and the kinetic parameters of crystallization were determined with the use of the JMAK model. Crystallization in both samples was controlled by diffusion flow with linear nucleation rate time dependence. One-dimensional growth and formation of needle-like (dendritic) YAG crystals was observed in AY-E glass crystallized at 932 °C corresponding to the first exothermic maximum at the DSC curve. Two-dimensional growth and the presence of plate-like YAG crystals were observed in AY-NE glass crystallized at 996 °C. For the second exothermic effect, plate-like crystals crystallized at higher temperatures (996 and 1020 °C) in both compositions. The results of SEM analysis are in agreement with the results of kinetic calculations in the prepared systems.
DSC, SEM–EDS, XRD and high-temperature XRD analysis was used to study thermal and crystallization behaviour of yttrium aluminate glasses prepared in the form of microspheres. The glasses YA-E ...(eutectic composition from the pseudo-binary system Al
2
O
3
–Y
3
Al
5
O
12
) and YA-G (a composition identical to the stoichiometric Y
3
Al
5
O
12
(YAG) phase) were prepared by combination of the Pechini method with flame synthesis. The resulting microspheres were largely amorphous, but contained traces of yttrium–aluminium garnet as the main crystalline phase embedded in the yttrium aluminate glass matrix. Crystallization of the YAG phase was observed as the dominant exothermic process on DSC curves. From the DSC records, the basic thermal characteristics of the matrix glass, i.e.
T
g
(glass transition temperature),
T
x
(onset of crystallization peak temperature),
T
f
(temperature of the inflection point of the crystallization peak) and
T
p
(maximum of crystallization peak temperature), were determined. HT XRD experiments in the temperature interval 750–1200 °C and isothermal HT XRD experiments at 932, 998 and 1200 °C with 6-h holding time were also performed. Crystallization experiments at lower temperatures 932 °C (YA-E) and 915 °C (YA-G) were conducted to study phase development in a low-temperature region. Crystallization experiments at higher temperatures (1000, 1300 and 1500 °C) with maximum holding time of 6 h were performed to study crystallization of α-Al
2
O
3
in the eutectic system. The SEM and SEM–EDS examination of polished cross sections of crystallized microspheres revealed slow volume crystallization of the YAG phase in the AY-E glass. Eventually, polycrystalline microspheres with fine-grained microstructure were prepared after 6-h treatment at 1500 °C.
