Effect of calcination temperature on the textural properties of 3 mol% yttria-stabilized zirconia powders

Zirconia-based ceramics have been commonly used in many different technological fields. This kind of materials have consequently received a great deal of attention. The main goal of this study was to investigate the effect of the calcination temperature on the morphological and textural properties of 3 mol% yttria-stabilized zirconia powders (3YSZ). Scanning electronic microscopy (SEM), nitrogen adsorption isotherms, thermogravimetry-differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy have been used with such an aim. Once the 3YSZ powders were dried at 100 °C they were subjected to calcination at 300, 500 or 700 °C. The calcination temperature noticeably influences the surface and textural properties of the 3YSZ powders. Textural parameters appear to be closely related to the removal of the alcohol used in the sol-gel synthesis procedure. The largest values of specific surface area and pore volumes correspond to samples prepared at 100 and 500 °C (i.e., 161 and 62 m²/g). In the first case, this fact is attributable to the removal of residual water and alcohol occluded within the powder particles whereas in the second case it may be due to the elimination of gases produced during the calcination stage.     

Effect of Y2O3 addition on the phase transition and growth of YSZ nanocrystallites prepared by a sol-gel process

The effect of Y₂O₃ addition on the phase transition and growth of yttria-stabilized zirconia (YSZ) nanocrystallites prepared by a sol-gel process with various mixtures of ZrOCl₂ · 8H₂O and Y(NO₃)₃ · 6H₂O ethanol-water solutions at low temperatures has been studied. DTA/TGA, XRD, SEM, TEM and ED have been utilized to characterize the YSZ nanocrystallites. The crystallization temperature of 3YSZ, in which Y₂O₃/(Y₂O₃ + ZrO₂) = 0.03, gel powders estimated by DTA/TG is about 427 °C. When 3YSZ and 5YSZ gels are calcined at 500-700 °C, their crystal structures as composed of coexisting tetragonal and monoclinic ZrO₂, and tetragonal phase decreases with calcination temperature increasing from 500 to 700 °C. Pure cubic ZrO₂ is obtained when added Y₂O₃ is greater than 8 mol%. A nanocrystallite size distribution between 10 and 20 nm is obtained in TEM observations.     

Epitaxial films of metals from the platinum group (Ir,Rh, Pt and Ru) on YSZ-buffered Si(1 1 1)

In the present work we have grown twin-free single crystal metal films of iridium (Ir), rhodium (Rh), platinum (Pt) and ruthenium (Ru) on silicon (1 1 1) substrates via an yttria-stabilized zirconia (YSZ) buffer layer. A prerequisite for the realisation of heteroepitaxial metal films without additional texture components was the twin-free deposition of the YSZ films by pulsed laser deposition (PLD). For the metal films on top, a novel two-step growth process was applied with an extremely low deposition rate for the first 20 nm. For all metals, a drastic texture improvement by up to a factor of 9 could be observed compared to the oxide buffer layer. Minimum values were 0.18° (Ir) and 0.12° (Rh) for tilt and twist, respectively. For all four metals investigated, twin-free epitaxial films could be grown on YSZ/Si(1 1 1) whereas the twinning problem for platinum films was solved by decoupling the Pt-YSZ interface via an additional iridium interlayer. The grown metal/YSZ/Si(1 1 1) multilayer samples offer the possibility to integrate a variety of interesting nanostructures and functional materials on silicon. They are now available in 4 in wafer size.     

Synthesis of sodium silicate-based hydrophilic silica aerogel beads with superior properties: Effect of heat-treatment

This work demonstrates the synthesis of hydrophilic and hydrophobic high surface area silica aerogel beads with a large pore volume. Wet gel silica beads were modified and heat-treated under atmospheric pressure after modification of the surface by trimethychlorosilane (TMCS). The effects of heat treatment on the physical (hydrophobicity) and textural properties (specific surface area, pore volume, and pore size) of silica aerogel beads were investigated. The results indicated that hydrophobicity of the silica aerogel beads can be maintained up to 400 °C. The hydrophobicity of the silica aerogel beads decreased with increasing temperature in the range of 200-500 °C, and the beads became completely hydrophilic after heat treatment at 500 °C. The specific surface area, cumulative pore volume, and pore size of the silica aerogel beads increased with increasing temperature. Heating the TMCS modified bead gel at 400 °C for 1 h resulted in silica aerogel beads with high surface area (769 m²/g), and large cumulative pore volume (3.10 cm³/g). The effects of heat treatment on the physical and textural properties of silica aerogel beads were investigated by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), thermogravimetric and differential analysis (TG-DTA), Fourier-transform infrared spectroscopy (FT-IR), and Brunauer, Emmett and Teller (BET) and BJH nitrogen gas adsorption and desorption methods.     

Microporous and mesoporous 8YSZ materials derived from polymeric acetylacetone-modified precursors for inorganic membrane applications

Both microporous and mesoporous 8% mol-yttria-stabilized zirconia (8YSZ) material can be prepared from the same 8YSZ sols. Stable 8YSZ sols with particle sizes smaller than 10 nm were prepared by using acetylacetone as a precursor modifier in combination with carboxylic acids. Nonanic and caproic acids are preferred due to their higher stability in combinations of highly concentrated sols. Cubic 8YSZ microporous material can be prepared by using acetylacetone as a precursor modifier with caproic acid as the catalyst. These materials can be transformed into mesoporous material composed of crystalline primary cubic YSZ nanoparticles by simply mixing with structure-directing agents. Use of the block copolymer F127 resulted in the largest specific surface area of 100 m²/g. Thin 8YSZ layers of these sols can be prepared on glass slips and on graded porous alumina supports with thicknesses between 20 and 100 nm.     

Ho~(3+)/Yb~(3+)∶8YSZ纳米粉体制备及发光性能研究

通过化学共沉淀法制备Ho~(3+)/Yb~(3+)共掺杂的氧化钇稳定的氧化锆(8YSZ)纳米粉体。研究了煅烧温度对Ho~(3+)/Yb~(3+)共掺杂8YSZ纳米粉体荧光光谱和上转换发光性能的影响。结果表明:在448 nm波长激发光激发下,Ho~(3+)/Yb~(3+)∶8YSZ纳米粉体有一个549 nm的绿光发射峰,随着煅烧温度的升高,Ho~(3+)/Yb~(3+)∶8YSZ纳米粉体的荧光强度先升高后降低。980 nm波长激光激发下,Ho~(3+)/Yb~(3+)∶8YSZ纳米粉体在450~750 nm范围内有一个中心波长在539 nm的绿光发射峰和一个中心波长在650 nm的红光发射峰,随着煅烧温度的升高,纳米粉体的颗粒尺寸增大的同时,其上转换发光强度也逐渐增大。     

Microstructural and physical properties of the ambient pressure dried hydrophobic silica aerogels with various solvent mixtures

The experimental results on the microstructural and physical properties of the ambient pressure dried hydrophobic silica aerogels with various solvent mixtures have been reported. The aerogels were prepared with sodium silicate precursor, ammonium hydroxide catalyst, trimethylchlorosilane (TMCS) silylating agent, solvent mixture of methanol-isopropanol (MeOH/IPA) and various aprotic solvent mixtures namely, hexane and benzene (HB), hexane and toluene (HT), hexane and xylene (HX), heptane and benzene (HpB), heptane and toluene (HpT), heptane and xylene (HpX). The physical properties of the aerogels such as % of volume shrinkage, density, % of optical transmission, surface area, % of porosity, pore volume, thermal conductivity and heat capacities of the aerogels were studied. The hydrophobicity of the aerogels was studied by contact angle measurements. The HX and HpX aerogels have been found to be more hydrophobic (contact angle, θ > 155°) than the other aerogels. It has been observed that the % of weight increase is highest (1%) for the HT aerogels and lowest (0.25%) for HpX aerogels by keeping them at 70% humidity for 350 h. Further, the aerogels have been characterized by pore size distribution (PSD), Fourier transform infra red spectroscopy (FTIR) and thermogravimetric and differential thermal (TG-DGA) analysis and transmission electron microscopy (TEM) techniques. The results have been discussed by taking into account the surface tension, vapor pressure, molecular weight and chain length of the solvents. Low density (0.051 g/cc), hydrophobic (165°), transparent (85%), low thermal conductive (0.059 W/m K), low heat capacity (180 kJ/m³ K) and highly porous (97.38%) silica aerogels were obtained with HpX solvent mixture.     

The study on various wet chemistry techniques on YBa2Cu3O7−δ superconducting oxides powder preparation

This paper reports on the results of four different wet chemical techniques used for the synthesis of superconducting oxides, YBa₂Cu₃O₇ (YBCO). YBCO samples were prepared by complexion citric gel route (CT), acetate-tartrate sol-gel (ASG), oxalate/ethanol coprecipitation (COP), and acetate-tartrate sol-gel solid-state reaction (ASG-SSR) techniques. Each of the YBCO powders was annealed for 4 h at 880 °C, 900 °C, 920 °C, and 950 °C to find the optimum temperature. The optimum sintering temperature for the formation of Y-123 phase is very much dependent on the type of wet chemical technique used to prepare the powders.     

Structural evolutions of hydrothermally prepared mesostructured MCM-48 silica using differently manufactured amorphous silica powders

The hydrothermal synthesis of Si-MCM-48 mesoporous molecular sieves was carried out using a ternary SiO₂:CTAOH:H₂O system wherein differently manufactured amorphous silica powders such as fumed silica (FMDS), spray dried precipitated silica (SDPS) and flash dried precipitated silica (FDPS) were used as silica source materials. The changes in structural/textural properties were evaluated using powder XRD, N₂ adsorption-desorption and scanning electron microscopy techniques. Studies on the progressive development of MCM-48 mesophases revealed that, the reactivity of the silica source follow the trend: FMDS > SDPS > FDPS. MCM-48 synthesized using low cost FDPS has exhibited thicker pore walls but poorer orderness, while MCM-48 prepared from relatively expensive FMDS has thinner pore walls and more ordered structure. Moreover, the extent of contraction caused by calcinations, agglomerate size and structural stability were found to depend on the reactivity of the silica source used.     

Structure and growth morphology of Gd2O3-doped CeO2 thin films

Gd₂O₃-doped CeO₂ (Gd_0.1Ce_0.9O_1.95, GDC) thin films were synthesized on (1 0 0) Si single crystal substrates by a reactive radio frequency magnetron sputtering technique. Structures and surface morphologies were characterized by X-ray diffraction (XRD), Atomic Force Microscopy (AFM) and one-dimensional power spectral density (1DPSD) analysis. The XRD patterns indicated that, in the temperature range of 200–700 °C, f.c.c. structured GDC thin films were formed with growth orientations varying with temperature—random growth at 200 °C, (2 2 0) textures at 300–600 °C and (1 1 1) texture at 700 °C. GDC film synthesized at 200 °C had the smoothest surface with roughness of R_rms=0.973 nm. Its 1DPSD plot was characterized with a constant part at the low frequencies and a part at the high frequencies that could be fitted by the f^−2.4 power law decay. Such surface feature and scaling behavior were probably caused by the high deposition rate and random growth in the GDC film at this temperature. At higher temperatures (300–700 °C), however, an intermediate frequency slope (−γ₂≈−2) appeared in the 1DPSD plots between the low frequency constant part and the high frequency part fitted by f⁻⁴ power law decay, which indicated a roughing mechanism dominated by crystallographic orientation growth that caused much rougher surfaces in GDC films (R_rms>4 nm).     

Synthesis optimization of organic xerogels produced from convective air-drying of resorcinol-formaldehyde gels

Resorcinol-formaldehyde gels were produced at 50, 70 and 90 °C and with three different R/C ratios (500, 1000 and 2000). The effect of these variables combined with that of aging time was studied in order to optimize the synthesis conditions. The convective air-drying process was used, and the drying duration was studied with regard to the synthesis conditions. The aging time has no effect on the pore texture after 24 h at 90 °C or 48 h at 70 °C, whatever the R/C value. The synthesis-aging step can be shortened by increasing the temperature. Nevertheless, the pore size tends then to decrease, especially when R/C is high, but this can be counterbalanced by increasing R/C. Moreover, bubbles often appear in the gel at high synthesis temperature, which limits the temperature to about 70 °C in the case of monolithic parts. At 70 °C and with an air velocity of 2 m/s, the elimination of 90% of the solvent requires 1 h drying when the pore size reaches 400-600 nm, 2.5 h for 50 nm wide pores and 3 h when the pore size decreases to 15-20 nm. The drying duration does not exceed 8 h in all cases and could be shortened by increasing the temperature at the end of the process.     

Influence of hydrogen on structural and optical properties of low temperature polycrystalline Ge films deposited by RF magnetron sputtering

To improve the properties of polycrystalline Ge thin films, which are a candidate material for the bottom cells of low cost monolithic tandem solar cells, ∼300 nm in situ hydrogenated Ge (Ge:H) thin films were deposited on silicon nitride coated glass by radio-frequency magnetron sputtering. The films were sputtered in a mixture of 15 sccm argon and 10 sccm hydrogen at a variety of low substrate temperatures (T_s)≤450 °C. Structural and optical properties of the Ge:H thin films were measured and compared to those of non-hydrogenated Ge thin films deduced in our previous work. Raman and X-ray diffraction spectra revealed a structural evolution from amorphous to crystalline phase with increase in T_s. It is found that the introduction of hydrogen gas benefits the structural properties of the polycrystalline Ge film, sputtered at 450 °C, although the onset crystallization temperature is ∼90 °C higher than in those sputtered without hydrogen. Compared with non-hydrogenated Ge thin films, hydrogen incorporated in the films leads to broadened band gaps of the films sputtered at different T_s.     

Synthesis and characterization of ilmenite NiTiO3 and CoTiO3 prepared by a modified Pechini method

Powders of ilmenite structure NiTiO₃ and CoTiO₃ were prepared by a simple method based on the modified Pechini process. The raw compounds and citric acid (CA) were mixed in ethanol (EA) with the molar ratio Ni(Co)/Ti/CA/EA = 1/1/1/7.5. The DTA curve shows exothermic peaks only around 300-350 °C and 600 °C, which correspond to the decomposition of the organic compound and direct crystallization of the ilmenite phase. X-ray diffraction patterns indicated that the ilmenite phase was successfully synthesized as the Ni(Co)-Ti precursor calcined above 600-900 °C for 3 h, and the activation energies of NiTiO₃ and CoTiO₃ were calculated to be about 8.84 and 13.23 kJ/mol. TEM bright field images showed that the grain sizes of powders of NiTiO₃ and CoTiO₃ at 600-900 °C were estimated to be about 10-250 and 20-200 nm depending on the nature of the aggregate. The samples of NiTiO₃ calcined at 600-800 °C have the larger specific surface area of 31.51, 18.78, and 6.01 m²/g, respectively. The UV-Vis diffuse reflectance spectra show the optical band gaps of NiTiO₃ and CoTiO₃ as 3.02 and 2.43 eV.     

Structural evolution up to 1100 °C of xerogels prepared from TEOS sonohydrolysis and liquid phase exchanged by acetone

Silica xerogels were prepared from sonohydrolysis of tetraethoxysilane and exchange of the liquid phase of the wet gel by acetone. Monolithic xerogels were obtained by slow evaporation of acetone. The structural characteristics of the xerogels were studied as a function of temperature up to 1100 °C by means of bulk and skeletal density measurements, linear shrinkage measurements and thermal analyses (DTA, TG and DL). The results were correlated with the evolution in the UV-Vis absorption. Particularly, the initial pore structure of the dried acetone-exchanged xerogel was studied by small-angle X-ray scattering and nitrogen adsorption. The acetone-exchanged xerogels exhibit greater porosity in the mesopore region presenting greater mean pore size (∼4 nm) when compared to non-exchanged xerogels. The porosity of the xerogels is practically stable in the temperature range between 200 °C and 800 °C. Evolution in the structure of the solid particles (silica network) is the predominant process upon heating up to about 400 °C and pore elimination is the predominant process above 900 °C. At 1000 °C the xerogels are still monolithic and retain about 5 vol.% pores. The xerogels exhibited foaming phenomenon after hold for 10 h at 1100 °C. This temperature is even higher than that found for foaming of non-exchanged xerogels.     

Influence of annealing on the physical properties of filtered vacuum arc deposited tin oxide thin films

Tin oxide (SnO₂) thin films were deposited on UV fused silica (UVFS) substrates using filtered vacuum arc deposition (FVAD). During deposition, the substrates were at room temperature (RT). As-deposited films were annealed at 400 and 600 °C in Ar for 30 min. The film structure, composition, and surface morphology were determined as function of the annealing temperature using X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The XRD patterns of the SnO₂ thin films deposited on substrates at RT indicated that the films were amorphous, however, after the annealing the film structure became polycrystalline. The grain size of the annealed films, obtained from the XRD analysis, increased with the annealing temperature, and it was in the range 8-34 nm. The AFM analysis of the surface revealed an increase in the film surface average grain size from 15 nm to 46 nm, and the surface roughness from 0.2 to 1.8 nm, as function of the annealing temperature. The average optical transmission of the films in the visible spectrum was >80%, and increased by the annealing ∼10%. The films’ optical constants in the 250-989 nm wavelength range were determined by variable angle spectroscopic ellipsometry (VASE). The refractive indexes of as-deposited and annealed films were in the range 1.83-2.23 and 1.85-2.3, respectively. The extinction coefficients, k(λ), of as-deposited and annealed films were in the range same range ∼0-0.5. The optical energy band gap (E_g), as determined by the dependence of the absorption coefficient on the photon energy at short wavelengths, increased with the annealing temperature from 3.90 to 4.35 eV. The lowest electrical resistivity of the as-deposited tin oxide films was 7.8 × 10⁻³ Ω cm, however, film annealing resulted in highly resistive films.     

The thermal behavior of 8 mol% yttria-stabilized zirconia nanocrystallites prepared by a sol–gel process

Eight mole percent yttria-stabilized zirconia (8YSZ) nanocrystallites were synthesized at a relatively low temperature using ZrOCl₂ · 8H₂O and Y(NO₃)₃ · 6H₂O as starting materials in an ethanol–water solution by a sol–gel process. The thermal behavior of the 8YSZ nanoparticles was investigated by differential thermal analysis, Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy and electron diffraction. The crystallization temperature of the 8YSZ gel powders was estimated to be about 729 K by DTA analysis. When calcined from 773 to 1273 K, the crystallization of the cubic phase was observed by XRD. The crystallite size of the 8YSZ increased from 7.14 to 20.10 nm with calcining temperature increasing from 773 to 1273 K. The activation energy for growth of 8YSZ nanoparticles was found to be 7.26 kJ/mol.     

X-ray scattering studies of polymeric zirconium species in aqueous xerogels

X-ray scattering in the small angle range was used to investigate the structure and size of the polymeric species present in zirconium acetate (ZrAc)-based gel powders (GP) and thin films (TF). The formation of amorphous zirconia aggregates/linear polymers was studied by measuring their gyration radii and their correlation and hydrodynamic lengths via Guinier, `longrods', Zimm and Porod plots. Scattering data obtained in this investigation suggest that both amorphous ZrAc-based GP and TF contain `tetrameric' units, arranged in a cylindrical fashion, that polymerize upon thermal treatment. Housdorff fractal dimensionality, D, of amorphous zirconia GP (D∼2.25) and TF (D∼2.5) indicates a diffusion-limited aggregation mechanism. Tricyclic cluster aggregates (Porod D∼1.85) with a M_w∼12 000 were found within amorphous ZrAc-based GP. This structure is preserved over the entire curing range. In contrast, a linear polymeric chain (Porod D∼1.52), with a M_w∼20 000 below 165 °C and ∼13 000 at higher temperatures, was suggested for amorphous ZrAc-based TF.     

Effect of frit size on sintering, crystallization and electrical properties of wollastonite glass-ceramics

Wollastonite glass-ceramics were prepared through pressureless sintering. The sinterability of the prepared samples of the glassy powder in the system (SiO₂-CaO-Na₂O-Fe₂O₃-WO₃) was investigated in the temperature range 720-900 °C and soaking time of 180 min. The influence of the increase in the glass powder particle size on the sinterability and dielectric properties of the glass-ceramic samples was studied.The sintered specimens were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods. XRD analysis revealed that wollastonite was the main crystalline phase in the sintered glass-ceramics. Dielectric properties such as dielectric constant and dielectric loss were measured via a network analyzer at 10 GHz.It was observed that the increase of the glassy powder particle size improved the sinterability and dielectric properties of the glass-ceramic specimens. Wollastonite glass-ceramics with 16 μm particle size had maximum constant and minimum loss (ε_r = 10.10 and tan δ = 0.005) compared with the other glass-ceramics.     

Polyvinyl alcohol cross-linked macroporous polymeric hydrogels: Structure formation and regularity investigation

A series of novel polyvinyl alcohol (PVA) hydrogels were synthesized by cross-linking of acrylate-modified PVA in aqueous solutions. Hydrogels were prepared at a temperature range −7.5 to −25 °С, macromer concentration 4-12 wt.%, and initiator concentration 0.4 to 1.6 mg/ml. The swelling behavior of polymeric hydrogels in aqueous media with different pH and ionic strength values was investigated. It was shown that they possess a high level of water absorption. The influence of different factors (porosity, pore size, and pore size distribution) and reaction conditions on the hydrogel structure was studied. The interior morphology of the hydrogel networks exhibits a complicated structure filled by fibrillar, lamellar and dendritic formations consisting of cross-linked polymer. The dispersed pores which are randomly distributed can be observed inside these formations and between them.