Phase evolution of YAG powders obtained by gel combustion combined with field-assisted rapid synthesis technique

Yttrium aluminum garnet (YAG) nanopowders were synthesized by a novel method combining gel combustion and field-assisted rapid synthesis technique. It is noted that hexagonal YAlO₃ (YAH) is formed first at 800–830 °C by crystallization from the amorphous, and completely transforms to YAG at about 925 °C. The grain size of YAG powders calcined at 925 °C for 3 min is about 60 nm. Moreover, the phase evolution due to different heat treatment methods was also investigated. The results indicate that the crystallization pathways are related to the atmosphere and heating rate. In air, YAG is directly crystallized from amorphous precursors without any intermediate phases. In an anoxic atmosphere, phase formation follows an amorphous-YAH-YAG route at a rapid heating rate, while the amorphous-YAH+YAG-YAG route is observed in the case of slow heating.     

Bioactivity modulation of Bioglass® powder by thermal treatment

A discussion of the effects of Bioglass^® powder crystallisation on the in vitro bioactivity in simulated body fluid (SBF) is presented.Starting from Bioglass^® powder, different glass–ceramics were obtained by thermal treatments between 580 °C and 800 °C, with variable crystallisation content (from 10 to 92 wt%). All samples (glass and glass–ceramics) showed apatite formation at their surface when immersed in SBF. In case of the glass and the samples with lowest crystallinity, the first step of apatite formation involved a homogenous dissolution followed by an amorphous calcium phosphate (CaP) layer precipitation. For the samples with a high crystallisation content, heterogeneous dissolution occurred. For the first time, the Stevels number of the amorphous phase is used to explain the possible dissolution of the crystalline phase present in materials with a similar chemical composition of the Bioglass^®. All samples presented at 21 days of immersion in SBF B-type hydroxycarbonate apatite crystals.     

Diphasic aluminosilicate gels with two stage mullitization in temperature range of 1200–1300 °C

The crystallization of mullite in amorphous diphasic gel aged for 6 months has been studied using non-isothermal differential scanning calorimetry (DSC) and powder X-ray diffraction with Rietveld structure refinement analysis. The diphasic premullite gels undergo structural changes by aging even when they are calcined at 700 °C. These changes imply segregation of the sample to Al₂O₃-rich and SiO₂-rich regions. From the Al₂O₃-rich region crystallizes poorly defined AlSi spinel at 977 °C followed by two-step mullite crystallization in the temperature interval of 1200–1300 °C. Two overlapped exothermic peaks on DSC scan of aged gel were observed; the first at 1233 °C and the second at 1261 °C. The former is attributed to mullite crystallization by transformation of AlSi spinel, by which excess alumina occurs, which in the second step of mullitization reacts with amorphous SiO₂-rich phase. The activation energy for mullite crystallization in the first step was E_a=935±14 kJ mol⁻¹ and the Avrami exponent n=2.5. The values E_a=1119±25 kJ mol⁻¹ and n=1.2 were obtained for mullite formation in the second step. If amorphous SiO₂-rich phase is extracted from the sample, the value E_a=805±26 kJ mol⁻¹ is obtained. Mullite crystallizing from AlSi spinel (when SiO₂-rich phase has been extracted) differentiates compositionally from that formed by both reactions. Smaller unit cell parameters and higher amount of oxygen vacancies are incorporated into tetrahedral positions of mullite structure, as was determined by Rietveld structure refinement method.     

Phase transformation and morphological evolution of electrospun zirconia nanofibers during thermal annealing

Pure zirconia nanofibers were fabricated by electrospinning zirconia-polymer precursor and subsequent annealing. Fiber properties such as polymer decomposition, crystallization formation, phase transformation, surface morphologies, etc., were investigated by various techniques, including thermogravimetric analysis (TGA) and differential thermal analysis (DTA), high temperature differential scanning calorimeter (HTDSC), powder X-ray diffractometer (XRD), field emission scanning electron microscopy (FESEM), etc. It was found that the crystallization of as-spun fibers started at 450 °C and the initial crystallized zirconia phase was tetragonal (t), which began transforming to monoclinic (m) phase at 650 °C as evidenced by XRD; HTDSC showed at different thermal circles, the m-to-t transformation temperatures remained virtually unchanged while the reverse t-to-m temperatures systematically shifted from 924.9 to 978.6 °C as the progress of thermal circles; FESEM examinations revealed that fibers calcined to 1000 °C went through thermal grooving due to surface diffusion during heat treatment; fibers heated to 1370 °C formed the so-called “bamboo wires”, where volume diffusion was the dominant driving force.     

Effect of pressure and temperature on densification,microstructure and mechanical properties of spark plasma sintered silicon carbide processed with β-silicon carbide nanopowder and sintering additives

The effects of applied pressure and temperature during spark plasma sintering (SPS) of additive-containing nanocrystalline silicon carbide on its densification, microstructure, and mechanical properties have been investigated. Both relative density and grain size are found to increase with temperature. Furthermore, with increase in pressure at constant temperature, the relative density improves significantly, whereas the grain size decreases. Reasonably high relative density (~96%) is achieved on carrying out SPS at 1300 °C under applied pressure of 75 MPa for 5 min, with a maximum of ~97.7% at 1500 °C under 50 MPa for 5 min. TEM studies have shown the presence of an amorphous phase at grain boundaries and triple points, which confirms the formation of liquid phase during sintering and its significant contribution to densification of SiC at relatively lower temperatures (≤1400 °C). The relative density decreases on raising the SPS temperature beyond 1500 °C, probably due to pores caused by vaporization of the liquid phase. Whereas β-SiC is observed in the microstructures for SPS carried out at temperatures ≤1500 °C, α-SiC evolves and its volume fraction increases with further increase in SPS temperatures. Both hardness and Young׳s modulus increase with increase in relative density, whereas indentation fracture toughness appears to be higher in case of two-phase microstructure containing α and β-SiC.     

Formation and micro-Raman spectroscopic study of Aurivilius and fluorite-type SrBi2Nb2O9 nanocrystallites obtained using an ‘amorphous citrate’ route

The crystallization of a SrBi₂Nb₂O₉ gel-glass obtained using the amorphous citrate method was studied by micro-Raman scattering, X-ray diffraction, and electron microscopy techniques. A citric acid–ethanolamine gel with the stoichiometric proportion of the metallic cations was prepared as polymeric precursor and calcined to obtain the amorphous complex. Nanocrystallites with a metastable fluorite-type structure nucleate from the amorphous complex below 500 °C, as shown by X-ray scattering and confirmed by electron microscopy. The morphological study by scanning electron microscopy revealed the nucleation of nanocrystals in the glass-like amorphous powder after thermal treatment at 500 °C. Raman features characteristic of the stable Aurivillius nanocrystals can be detected after thermal treatment at 550 °C, while using X-ray diffraction the crystallization of the Bi-layered perovskite phase is observed only after treatment at 625 °C or higher temperatures. Both X-ray and Raman scattering detected single phase nanocrystallites with Aurivillius structure above 650 °C. The distinctive Raman features of the different SrBi₂Nb₂O₉ nanocrystallites and its evolution with thermal treatment is presented.     

Luminescence effect in amorphous PLT

Amorphous and crystalline powder of PLT phase was synthesized by using the Pechini method. Infrared (FTIR) analysis of the polymeric resin shows intense bands of organic materials from 250 to 1620 cm⁻¹. X-ray diffraction (XRD) and Raman spectra of calcined powder at different temperatures show amorphous phase at 450 °C/3 h, semi-crystalline phase at 550 °C/3 h and a crystalline phase at 800 °C/3 h. Luminescence effect was observed in amorphous powder calcined from 300 to 350 °/3 h with broad absorption peaks in 579 nm at 300 °C/3 h and 603 nm at 350 °C/3 h, respectively. The photoluminescence effect is attributed to emissions of Ti→O directly from the oxygen 2p orbital (valence band) to the titanate 3d orbital (conduction bands).     

Microstructural characterization of spark plasma sintered boron carbide ceramics

Fully dense boron carbide specimens were fabricated by the spark plasma sintering (SPS) technology in the absence of any sintering additives. Densification starts at 1500 °C and the highest densification rate is reached at about 1900 °C. The microstructure of the ceramic sintered at 2200 °C, with heating rates in the 50–400 °C/min range, displays abnormal grain growth, while for a 600 °C/min heating rate a homogeneous distribution of finely equiaxed grains with 4.05 ± 1.62 μm average size was obtained. TEM analysis revealed the presence of W-based amorphous and of crystalline boron-rich B₅₀N₂ secondary phases at triple-junctions. No grain-boundary films were detected by HRTEM. The formation of a transient liquid alumino-silicate phase stands apparently behind the early stage of densification.     

Mechanochemical synthesis and structural characterization of nano-sized amorphous tricalcium phosphate

Nano-sized amorphous tricalcium phosphate powders were synthesized through different mechanochemical reactions. The influence of milling parameters and chemical composition of reagents on the formation of amorphous tricalcium phosphate was investigated. In all the experiments, the mole ratio of calcium to phosphorous oxide was 3:1, i.e. the stoichiometric Ca/P content in the composition of amorphous tricalcium phosphate (Ca/P=1.5). Results revealed that the phase purity, structural features, and morphological characteristics of products were significantly influenced by the chemical composition of raw materials and milling parameters. For all the reactions, amorphous tricalcium phosphate was formed as the main product of mechanical activation after 10 h. After annealing at 1100 °C, crystallization of amorphous phase occurred, and consequently high crystalline β-tricalcium phosphate was generated. According to FT-IR findings, the synthesized powders had high chemical purity. After 10 h of milling, the obtained nanopowders through four distinct reactions exhibited crystallite sizes about 20, 69, 58 and 55 nm. The results from scanning electron micrographs showed that the mean size of agglomerate was in the range of 1–5 μm. Detailed study of morphological features by using transmission electron microscopy confirmed the formation of nano-sized amorphous tricalcium phosphate with spheroidal and ellipse-like morphologies.     

Characterization of lanthanum zirconate prepared by a nitrate-modified alkoxide synthesis route: From sol to crystalline powder

We studied the synthesis of lanthanum zirconate from lanthanum nitrate and zirconium butoxide in the transition from liquid to amorphous and crystalline phase. Thermal behaviour of lanthanum nitrate dissolved in 2-methoxyethanol is different from that of the salt, which we ascribe to the potential coordination of the solvent to lanthanum atoms. IR analysis shows that lanthanum nitrate remains strongly associated in the solvent. The lanthanum environment in the lanthanum nitrate and lanthanum zirconate sols, determined by EXAFS, is similar to that of the lanthanum nitrate hydrate. The environment is completely changed in the dried lanthanum zirconate precursor powder and powder heated at 500 °C due to the decomposition of nitrates. Zirconium species form polynuclear oxo-alkoxide complexes during thermolysis and they are stable from room temperature to 500 °C. The powder heated up to 700 °C for 1 h is amorphous, while after heating at 800 °C for 1 h pure pyrochlore phase crystallizes. No link between La and Zr species is established either in the sol or in the dried precursor powder or amorphous powder heated at 500 °C. The reaction between the metallic species proceeds as a solid-state reaction. The synthesis route ensures a very good mixing of the species at the nanometre level.     

Lanthanum cobaltite nanoparticles using the polymeric precursor method

The present study reports the evolution of reactive lanthanum cobaltite nanoparticles obtained by a polymeric precursor route, using citric acid as chelating agent. The crystallization from amorphous precursor, particle growth and the formation of nanoparticle agglomerates at different calcination temperatures was carried out by conventional and high-resolution electron microscopy, electron diffraction and energy-dispersive X-ray analysis and Raman spectroscopy. Microstructure measurements were compared with X-ray diffraction and chemical analysis results. Electron diffraction, combined with TEM, was used to determine the proportion of amorphous phase. The presence of amorphous carbon during the decomposition of the amorphous precursor was analyzed by Raman spectroscopy. The coherent crystalline domain size and the particle size have been monitored by XRD and electron microscopy in order to determine the evolution of both crystal size and the temperature onset for the formation of polycrystalline aggregates.The results demonstrate that at 550 °C we obtain pure single-phase equiaxed nanopowders of LaCoO₃ with crystal size of 20 nm, free of amorphous carbon and without the presence of polycrystalline aggregates.     

The impact of heat treatment on the microstructure of a clay ceramic and its thermal and mechanical properties

This paper presents the results of an experimental study on the microstructure, the thermal and the mechanical properties of a clay-based ceramic used in building applications. The X-ray tomography analysis showed a layered microstructure of clay with 200 µm sheets of porosity after the extrusion process. The gas release from the dehydration, dehydroxylation and decarbonation induced a 7 vol% formation of porosity during the heat treatment of the clay-based ceramic up to 850 °C. The porosity increase and the development of metakaolin led to a 38% decrease in the thermal conductivity. On the other hand, the Young's modulus of the clay-based ceramic was conserved due to the formation of smaller pores than the 200 µm sheets of porosity. The densification and the crystallization of amorphous phases also led to a 110% increase of the Young's modulus from 850 °C to 1050 °C. The Young's modulus of the clay-based ceramic was only decreased by the β→α quartz inversion of the cooling due to sand addition. Hence, this study provided a useful insight into how the microstructure of fired clay bricks can be specifically transformed by the porosity during the heat treatment to control the thermal and mechanical properties.     

Ball milling assisted hydrothermal synthesis of ZrO2 nanopowders

The formation of ZrO₂ nanopowders under various hydrothermal conditions such as temperature, time, autoclave rotation speed, heating rate and particularly assistance of ball milling during reaction was investigated. Full ZrO₂ formation (with monoclinic phase) from zirconium solution was completed at shorter times with increasing temperature such as after 4 h at 150 °C, 2 h at 175 °C and less than 2 h at 200 °C. Crystallite size increased from 2.9 to 4 nm with increasing reaction temperature from 125 °C to 200 °C, respectively. Ball milling assisted hydrothermal runs were performed to understand the effect of mechanical force on phase formation, crystallinity and particle size distribution. Monoclinic ZrO₂ was formed in both milled and non-milled runs when zirconium solution was used. Mean particle size for the 2 M solution was measured to be 94 nm for the milled and 117 nm for the non-milled powders. However, when amorphous aqueous zirconia gels (precipitated at pH 5.8) were used, tetragonal phase was also formed in addition to monoclinic phase. Mean particle size was measured to be 0.7 μm (d₉₀≅1.3 μm) for the milled and 7.9 μm (d₉₀≅13 μm) for the non-milled powders. Ball milling during hydrothermal reactions of both zirconium solution and aqueous zirconium gel resulted in smaller crystallite size and mean particle size and, at the same time, effectively controlled particle size distribution (or agglomeration) of nanopowders.     

Characterization of low-temperature coal ash behaviors at high temperatures under reducing atmosphere

The coal ash obtained at 815 °C under oxidizing atmosphere was further treated at 1300 °C and 1400 °C under reducing atmosphere. The resultant ashes were examined by XRD, SEM/EDX and FTIR. The results show that the residence time of coal ash at high temperatures has considerable influences on the compositions of coal ash and little effect on the amounts of unburned carbon. The amorphous phase of mineral matters increases with the increasing temperature. The FTIR peaks due to presence of different functional groups of minerals support the findings of XRD, and supply additional information of amorphous phase which cannot be detected in XRD. The ash samples generated from a fixed bed reactor during char gasification were also studied with FTIR. The temperatures of char preparation are responsible for the different transformation of minerals during high temperature gasification.     

Sintered material from alkaline basaltic tuffs

The possibility to obtain sintered material from alkaline basaltic tuffs is demonstrated. The parent rock was milled for 10–15 min, the resulting powder was pressed at 100 MPa and the obtained samples were heat-treated in the range of 1000–1140 °C. The sintering behaviour and the phase formation were studied by pycnometry, dilatometry, DTA, XRD and SEM.The final material was obtained by sintering at 1100 °C and is characterized by zero water absorption, 8–9 vol.% closed porosity and a structure similar to a glass-ceramic. Due to high crystallization trend of used composition, phase formation takes place during the sintering and cooling steps; this leads to a crystallinity of ～60% and formation of different crystal phases (pyroxene, anorthite, spinel and hematite).Despite the low-cost production cycle the obtained material is characterized by high mechanical properties: bending strength of 100 MPa and Young modulus of 90 GPa.     

Structural,optical, and electrical properties of conducting p-type transparent Cu–Cr–O thin films

Cu–Cr–O films were prepared by DC magnetron co-sputtering using Cu and Cr targets on quartz substrates. The films were then annealed at temperatures ranging from 400 °C to 900 °C for 2 h under a controlled Ar atmosphere. The as-deposited and 400 °C-annealed films were amorphous, semi-transparent, and insulated. After annealing at 500 °C, the Cu–Cr–O films contained a mixture of monoclinic CuO and spinel CuCr₂O₄ phases. Annealing at 600 °C led to the formation of delafossite CuCrO₂ phases. When the annealing was further increased to temperatures above 700 °C, the films exhibited a pure delafossite CuCrO₂ phase. The crystallinity and grain size also increased with the annealing temperature. The formation of the delafossite CuCrO₂ phase during post-annealing processing was in good agreement with thermodynamics. The optimum conductivity and transparency were achieved for the film annealed at approximately 700 °C with a figure of merit of 1.51×10⁻⁸ Ω⁻¹ (i.e., electrical resistivity of up to 5.13 Ω-cm and visible light transmittance of up to 58.3%). The lower formation temperature and superior properties of CuCrO₂ found in this study indicated the higher potential of this material for practical applications compared to CuAlO₂.     

Effect of heat treatment time on local structure of lanthanum-aluminum–gallium-borate xerogels

The aim of this study was to investigate the effect of 850 °C heat treatment time on LaAlGaB₅O₂₄ amorphous xerogel. Structural changes were evidenced by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. If xerogel remains non-crystalline after 5 min treatment, nanocrystallites of LaAl_2.03(B₄O₁₀)O_0.54 type are developed after 15 min treatment and this is the sole crystalline phase preserved by increasing the treatment time up to 24 h. Boron, aluminum and gallium units from amorphous xerogel are drastically changed by crystallization, but they are only slightly affected by increasing the 850 °C treatment from 15 min to 24 h.     

Microwave-absorption properties of SiOC ceramics derived from novel hyperbranched ferrocene-containing polysiloxane

In this contribution, we design a novel strategy to synthesize SiOC ceramics by pyrolysis of hyperbranched ferrocene-containing polysiloxane (HBPSO-VF) which are synthesized by the reaction of polysiloxane (PSO) with 1,1′-Bis(dimethylvinylsilyl)ferrocene (VF). This SiOC ceramics show much lower crystallization temperature because of the capability of HBPSO-VF to incorporate metallic iron into the backbone of PSO. The usage of HBPSO-VF offers enhanced ceramic yield of 83 wt% at 1200 °C due to the deep cross-linking of hydrosilylation. Nano-sized SiC and turbostratic carbons are separated from amorphous SiOC phase when it is annealed at 1100 °C, while crystallization temperature is 1400 °C when PSO is used as polymer precursors. The minimum reflection coefficient (RC_min) of this nanocrystal-containing ceramic reaches −46 dB, exhibiting a promising prospect as a kind of electromagnetic wave (EMW) absorbing materials. This method also can be further extended to develop other functional Si-based polymer derived ceramic (PDC) systems for EMW absorption and shielding applications.     

Decomposition of nonionic surfactant on a nitrogen-doped photocatalyst under visible-light irradiation

A visible-light-active N-containing TiO₂ photocatalysts were prepared from crude amorphous titanium dioxide by heating amorphous TiO₂ in gaseous NH₃ atmosphere. The calcination temperatures ranged from 200 to 1000 °C, respectively. UV–vis/DR spectra indicated that the N-doped catalysts prepared at temperatures <400 °C absorbed only UV light (E_g = 3.3 eV), whereas samples prepared at temperatures ≥400 °C absorbed both, UV (E_g = 3.10–3.31 eV) and vis (E_g = 2.54–2.66 eV) light. The chemical structure of the modified photocatalysts was investigated using FT-IR/DRS spectroscopy. All the spectra exhibited bands indicating nitrogen presence in the catalysts structure. The photocatalytic activity of the investigated catalysts was determined on a basis of a decomposition rate of nonionic surfactant (polyoxyethylenenonylphenol ether, Rokafenol N9). The most photoactive catalysts were those calcinated at 300, 500 and 600 °C. For the catalysts heated at temperatures of 500 and 600 °C Rokafenol N9 removal was equal to 61 and 60%, whereas TOC removal amounted to 40 and 35%, respectively. In case of the catalyst calcinated at 300 °C surfactant was degraded by 54% and TOC was removed by 35%. The phase composition of the most active photocatalysts was as follows: (a) catalyst calcinated at 300 °C—49.1% of amorphous TiO₂, 47.4% of anatase and 3.5% of rutile; (b) catalyst calcinated at 500 °C—7.1% of amorphous TiO₂, 89.4% of anatase and 3.5% of rutile; (c) catalyst calcinated at 600 °C—94.2% of anatase and 5.8% of rutile.     

Time-resolved powder neutron diffraction study of the phase transformation sequence of kaolinite to mullite

Mullite formation from kaolinite was studied by means of high-temperature in situ powder neutron diffraction by heating from room temperature up to 1370 °C. Neutron diffractometry under this non-isothermal conditions is suitable for studying high-temperature reaction kinetics and to identify short-lived species which otherwise might escape detection. Data collected from dynamic techniques (neutron diffraction, DTA, TGA and constant-heating rate sintering) were consistent with data gathered in static mode (conventional X-ray diffraction and TEM). The full process occurs in successive stages: (a) kaolinite dehydroxylation yielding metakaolinite in the ∼400–650 °C temperature range, (b) nucleation of mullite in the temperature range ∼980–992 to ∼1121 °C (primary mullite) side by side with a crystalline cubic phase (Si-Al spinel) detected in the ∼983–1030 °C temperature interval; (c) growth of mullite crystals from ∼1136 °C, (d) high (or β) cristobalite crystallization at T > ∼1200 °C and (e) secondary mullite crystallization at T > ∼1300 °C. The calculated activation energy for the kaolinite dehydration was 115 kJ/mol; for the mullite nucleation was 278 kJ/mol and for the growth of mullite process was 87 kJ/mol; finally for cristobalite nucleation the calculated apparent activation energy was 481 kJ/mol.