Carbonate-precipitation synthesis of Yb:Y2O3 nanopowders and its characteristics

Ytterbium-doped yttria (Yb³⁺:Y₂O₃) nanopowders for transparent ceramics were synthesized by using a carbonate-precipitation method. The characteristics of precursor and powders calcined at different temperatures were investigated. The pure yttria phase can form through calcining at 700 °C. The Yb³⁺:Y₂O₃ nanopowders calcined at 1100 °C were well dispersed with a spherical morphology, and had a narrow particle size distribution with a mean particle size of about 70 nm. By using 1100 °C-calcined powders, nearly full dense Yb³⁺:Y₂O₃ ceramics were fabricated at 1750 °C for 8 h without any additives under vacuum conditions. The fluorescence spectrum of the sintered ceramics illustrates that there are two emission peaks locating at 1028 and 1071 nm respectively, all corresponding to the ²F_5/2 → ²F_7/2 transitions of Yb³⁺ ion. Homogeneous Yb³⁺:Y₂O₃ nanopowders synthesized by carbonate-precipitation method are suitable for the fabrication of IR-transparent ceramics.     

Microwave-assisted solvothermal preparation and photoluminescence properties of Y2O3:Eu phosphors

Europium doped yttrium oxide phosphors were synthesized by a rapid microwave-assisted solvothermal method. The microwave processing time for synthesizing the precursors of Y₂O₃:Eu³⁺ powders was as short as 5 min. After calcination at 600 °C, a well-crystallized pure phase of Y₂O₃:Eu³⁺ was obtained. The morphology of the precipitated powders was spherical and composed of nano-sized grains. As the microwave irradiation time was increased, the average particle size of the spherical powders increased, and the crystallinity of heat-treated powders was also enhanced. The synthesized powders retained the spherical morphology after heating treatments. An intense red emission at 611 nm was assigned to the ⁵D₀-⁷F₂ transition of Eu³⁺.     

Agglomeration of α-Al2O3 powders prepared by gel combustion

Ultrafine α-Al₂O₃ powders were prepared by a gel combustion method and the agglomeration characteristic of the resultant powders was studied. A variety of fine crystallite α-Al₂O₃ powders with different agglomeration structures could be obtained by altering the citrate-to-nitrate ratio γ and calcining the precursors at 1050 °C for 2 h. All the powders were of nearly equivalent crystallite size (60–80 nm) except for the P1 powder (113 nm) from the gel with γ = 0.033. The primary crystallites of the obtained α-Al₂O₃ powders were formed into large secondary particles with different degree of agglomeration. Except for the powder P1, the mean particle sizes from specific surface area and particle size distribution measurement increase with increasing citrate-to-nitrate ratio in the fuel-lean condition and decrease in the fuel-rich condition. Densities of alumina ceramics from powders P4 and P5 sintered at different temperatures were relatively low due to the wide particle size distribution.     

Crystalline morphology and photoluminescent properties of YInGe2O7:Eu phosphors prepared from microwave and conventional sintering

This paper describes an investigation of the crystalline morphology and photoluminescent properties of YInGe₂O₇:Eu³⁺ powders using microwave assisted sintering. For comparison, the properties of YInGe₂O₇:Eu³⁺ powders sintered at 1200 °C in conventional furnace for 10 h were also investigated. X-ray powder diffraction analysis confirmed the formation of monoclinic YInGe₂O₇ without second phase or phases of starting materials as YInGe₂O₇:50 mol% Eu powders sintered at 1200 °C in microwave furnace for 1 h. Scanning electron microscopy showed smaller particle size and more uniform grain size distributions are obtained by microwave assisted sintering. In the PL studies, both microwave sintered and conventionally sintered powders emitted a maximum luminescence centered at 620 nm under excitation of 393 nm with similar luminescent intensity. The results show that microwave processing has the potential to reduce the time and required energy input for the production of YInGe₂O₇:Eu³⁺ phosphors without sacrificing the photoluminescence.     

Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.     

Synthesis of lanthanum beta-alumina powders by the polymeric precursor technique

La-β-Al₂O₃ (LaAl₁₁O₁₈) powders were synthesized by the polymeric precursor technique using lanthanum nitrate and aluminum nitrate. The transformations during thermal treatment of the precursor solution with ethylene glycol and citric acid were evaluated by thermal analysis. Fourier transform infrared spectroscopy analysis was performed after calcinations of the polymeric resin for determination of residual carbon. The specific surface area was evaluated by the BET method. Fine powders with ∼121 m²/g specific surface area and 20 nm average particle size were obtained and observed by scanning and transmission electron microscopy. Nearly single phase LaAl₁₁O₁₈ was obtained after pressing and sintering these powders at 1600 °C with small additions of MgO. The sintered pellets were characterized by X-ray diffraction and scanning electron microscopy. Impedance spectroscopy measurements carried out in the 1000–1200 °C range show the electrolytic behavior of the La-β-Al₂O₃ pellets, suggesting their application as solid electrolytes in high temperature potentiometric oxygen sensors.     

Synthesis of mono-dispersed spherical Nd:Y2O3 powder for transparent ceramics

Transparent Nd:Y₂O₃ ceramic was obtained by sintering mono-sized spherical powder. The powder was prepared by homogeneous precipitation method in aqueous media using urea to regulate the pH. The structure and morphology of the powder were investigated by TG-DTA, XRD, SEM and IR spectrum. The effect of aging temperature, time, and the concentration of urea, [Y³⁺], and [Nd³⁺] were investigated. Results showed that the obtained precursor was R₂(OH)CO₃·H₂O (R = Y, Nd), and the least size of mono-sized spherical yttria particles was 72 nm by a microwave oven method after calcinations at 850 °C for 4 h. After dry press and CIP, the particles accumulated closely, and no defects can be detected in the green body.     

Structural,morphological and up-converting luminescence characteristics of nanocrystalline Y2O3:Yb/Er powders obtained via spray pyrolysis

Sub-micronic, spherical Y₂O₃:Yb/Er particles comprising clustered nano-units (70 nm) were prepared via ultrasonic spray pyrolysis from pure nitrate precursor solutions with different Yb/Er dopant ratios. The particles were additionally thermally treated at 1100 °C for 12, 24 and 48 h. The structural and morphological characteristics of particles were studied by X-ray powder diffraction, Fourier transform infrared spectroscopy, field-emission scanning electron microscopy, energy dispersive X-ray analysis and specific surface area (BET) and were further correlated with their advanced optical properties. For the recorded up-conversion emissions, originating from the following Er³⁺ transitions: [²H_9/2→⁴I_15/2] in blue (407–420 nm); [²H_11/2, ⁴S_3/2→⁴I_15/2] green: 510–590 nm; and [⁴F_9/2→⁴I_15/2] in red (640–720 nm) spectral region, the corresponding lifetimes were acquired in the wide temperature range (10–300 K). The most intense green up-conversion emission with the long decay of 550 ms is recorded for Y_1.97Yb_0.02Er_0.01O₃ particles thermally treated at 1100 °C for 24 h.     

Effects of PVA content on the synthesis of LaFeO3 via sol-gel route

LaFeO₃ were synthesized via a sol-gel route based on polyvinyl alcohol (PVA). Differential scanning calorimetry (DSC), Thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Raman spectroscopy and field emission scanning electron microscopy (FESEM) techniques were used to characterize precursors and derived oxide powders. The effect of the ratios of positively charged valences to hydroxyl groups of PVA (Mⁿ⁺/-OH) on the formation of LaFeO₃ was investigated. XRD analysis showed that single-phase and well-crystallized LaFeO₃ was obtained from the Mⁿ⁺/-OH = 4:1 molar ratio precursor at 700 °C. For the precursor with Mⁿ⁺/-OH = 2:1, nanocrystalline LaFeO₃ with average particle size of ∼50 nm was formed directly in the charring procedure. With increase of PVA content to Mⁿ⁺/-OH = 1:1, phase pure LaFeO₃ was obtained at 500 °C.     

Synthesis of nano-sized Yb3Al5O12 powders by the urea co-precipitation method

Nano-sized polycrystalline ytterbium aluminum garnet (YbAG, Yb₃Al₅O₁₂) powders were successfully synthesized by a simple urea co-precipitation method. The thermal behavior of the YbAG precursor was investigated. The calcined amorphous YbAG precursor was directly converted to 20–30 nm monophase YbAG at as low as temperature 900 °C, without any other intermediate phases. The nano-sized YbAG powders calcined at 900 °C distributed evenly with a slight aggregation, and the specific surface area of the powders reached 29.7 m² g⁻¹. The absorption spectrum of the YbAG powders was measured, and there were two strong absorption peaks centered at 931 and 965 nm. The results did not correspond to the absorption peaks of Yb:YAG crystal exactly, as the lattice parameters were different between YbAG and YAG.     

Fabrication and laser behaviors of Nd:YAG ceramic microchips

Transparent Nd:YAG ceramic microchips were fabricated through the slip casting shaping directly from the slurry formed by the commercial Al₂O₃/Y₂O₃/Nd₂O₃ powders, and followed by the vacuum sintering procedure. Viscosity of the slurries, the phase evolution and the densification behavior were investigated. For the Al₂O₃/Y₂O₃/Nd₂O₃ compound slurries system, the optimal condition is 0.5 wt.% NH₄PAA dispersant and 30 wt.% solid loading at pH ≥ 8. The YAG phase started to form at 1250° C and pure YAG phase could be obtained at 1400° C. The optical in-line transmittance of the Nd:YAG ceramics with thickness of 2 mm was about 83.8% at 1064 nm and 82.5% at 400 nm, which hit the upper limit of the theoretically calculated value. For the 1.0 at.% Nd:YAG ceramic microchip, the slope efficiency was 43% for 1.0 at.% Nd:YAG ceramic pumped by 920 mW cw Ti:sapphire tunable laser, and the maximum laser output power 246 W was obtained for 2.0 at.% Nd:YAG ceramics pumped by 925 W LD.     

An approach to Y2O3:Eu optical nanostructured ceramics

Y₂O₃:Eu³⁺ (1 at.%) translucent nanostructured ceramics with total forward transmission achieving ∼70% of the theoretical limit has been obtained by the transformation-assisted consolidation of custom-made cubic Y₂O₃:Eu³⁺ nanopowders under high pressure (HP). Sintering under the pressure of 7.7 GPa and temperatures in the 100-500 °C range leads to the partial cubic-to-monoclinic phase transition that results in two-phase Y₂O₃:Eu³⁺ nanoceramics. The average grain size of ceramics d ≤ 50 nm for both Y₂O₃:Eu³⁺ polymorph is comparable with crystallite size of initial nanopowders (d ∼ 40 nm), indicating that the grain growth factor is near unity. The phase compositions, morphology, densities, preliminary optical and luminescent properties of synthesized nanostructured ceramics have been studied.     

Structural and optical properties of Tm:Y2O3 transparent ceramic with La2O3, ZrO2 as composite sintering aid

The paper reports the use of La₂O₃ and ZrO₂ co-doping as a composite sintering aid for the fabrication of Tm:Y₂O₃ transparent ceramics. Two groups of experiments were conducted for investigating the influences of composite sintering aids on the microstructures and the optical properties of Tm:Y₂O₃ transparent ceramics in contrast to single La³⁺ and single Zr⁴⁺ doped Tm:Y₂O₃. Samples with composite sintering aids could realize fine microstructures and good optical properties at relatively low sintering temperatures. Grain sizes around 10 μm and transmittances close to theoretical value at wavelength of 2 μm were achieved for the 9 at.% La³⁺, 3 at.% Zr⁴⁺ co-doped samples sintered at 1500-1600 °C. The influences of the composite sintering aids on the emission intensities and the phonon energies of Tm:Y₂O₃ ceramics were also investigated.     

Pressureless sintered Al2O3–SiC nanocomposites

Al₂O₃ + 5 vol% SiC composite ceramics were prepared via a conventional powder processing route followed by pressureless sintering. Commercially available Al₂O₃ and SiC powders were milled together in an aqueous suspension. The slurry was freeze granulated, and green bodies were obtained by cold isostatic pressing of the granules. Pressureless sintering was carried out in a nitrogen atmosphere at 1750 and 1780 °C. Near full density (>99%) was achieved at 1780 °C. Densification at the lower sintering temperature was promoted by smaller additions of MgO. Vickers hardness and indentation fracture toughness varied around 18 GPa and 2.3 MPa m^1/2 after sintering at 1780 °C. Transmission electron microscopy revealed that the SiC particles were located predominantly to the interior of the matrix grains and well distributed throughout the composite microstructures. The intragranular particles had sizes in the range 50–200 nm while the intergranular particles were larger, typically 200–500 nm in diameter.     

Synthesis and tunable thermal expansion properties of Sc2−xYxW3O12 solid solutions

A new series of rare earth solid solutions Sc_2−xY_xW₃O₁₂ was successfully synthesized by the conventional solid-state method. Effects of doping ion yttrium on the crystal structure, morphology and thermal expansion property of as-prepared Sc_2−xY_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TG), field emission scanning electron microscope (FE-SEM) and thermal mechanical analyzer (TMA). Results indicate that the obtained Sc_2−xY_xW₃O₁₂ samples with Y doping of 0≤x≤0.5 are in the form of orthorhombic Sc₂W₃O₁₂-structure and show negative thermal expansion (NTE) from room temperature to 600 °C; while as-synthesized materials with Y doping of 1.5≤x≤2 take hygroscopic Y₂W₃O₁₂·nH₂O-structure at room temperature and exhibit NTE only after losing water molecules. It is suggested that the obvious difference in crystal structure leads to different thermal expansion behaviors in Sc_2−xY_xW₃O₁₂. Thus it is proposed that thermal expansion properties of Sc_2−xY_xW₃O₁₂ can be adjusted by the employment of Y dopant; the obtained Sc_1.5Y_0.5W₃O₁₂ ceramic shows almost zero thermal expansion and its average linear thermal expansion coefficient is −0.00683×10⁻⁶ °C⁻¹ in the 25–250 °C range.     

Transparent yttria produced by spark plasma sintering at moderate temperature and pressure profiles

Transparent yttria (Y₂O₃) bodies were fabricated by spark plasma sintering, and the effects of the sintering temperature on relative density, microstructure, and the optical and mechanical properties of Y₂O₃ bodies were investigated. Fully dense Y₂O₃ bodies were obtained at sintering temperatures 1473-1873 K. The average grain size was 0.24-0.32 μm at 1473-1573 K, and steadily increased to 1.97 μm with an increase in temperature to 1823 K. The highest transmittance was obtained in the Y₂O₃ body sintered at 1573 K and annealed at 1323 K, showing 81.7% (99% of the theoretical value) at a wavelength of 2000 nm.     

Compressive creep behavior of spark plasma sintered La2O3–YSZ composite

The present paper describes compressive creep behavior of cubic 8 mol% yttria stabilized zirconia+10 mol% La₂O₃ (fabricated by Spark Plasma Sintering) in the temperature range of 1300–1330 °C at a stress level of 45–78 MPa in vacuum. The pre- and post-creep microstructures, relative magnitudes of the stress exponent (n=1.7–2.1) and the activation energy (540–580 kJ/mol) suggest that grain boundary sliding aided by inter-diffusion of La and Zr leading to the formation of pyrochlore La_1.6Y_0.4Zr₂O₇ phase at the grain boundaries during creep is the active creep mechanism in this composite.     

Effect of Al2O3 particle size on preparation and properties of ZTA ceramics formed by gelcasting

Zirconia-toughened alumina (ZTA) ceramics were prepared using three different kinds of Al₂O₃ powders (marked PW-A average particle size: 7.53 μm, marked PW-B average particle size: 1.76 μm, marked PW-C average particle size: 0.61 μm) by gelcasting. Effect of Al₂O₃ particle size on zeta potential, dispersant dosage and solid volume fractions of ZTA suspensions as well as the mechanical properties of ZTA green bodies and ceramics were investigated. The optimum dosages of dispersant for ZTA suspensions prepared by PW-A, PW-B and PW-C are 0.4 wt%, 0.5 wt% and 0.7 wt%, respectively. The highest solid volume fractions of ZTA suspensions can reach 62 vol% (SP-A), 60 vol% (SP-B) and 52 vol% (SP-C), respectively. The green bodies show a bending strength as high as 20 MPa, which can meet the requirement of machining. The Al₂O₃ powder with fine particle size is beneficial to the improvement of mechanical properties. The ZTA ceramics prepared by PW-B Al₂O₃ powder show the highest bending strength (680 MPa) and toughness (7.49 MPa m^1/2).     

Lithium-deficient LiYMn2O4 spinels (0.9 ≤ Y < 1): Lithium content, synthesis temperature, thermal behaviour and electrochemical properties

Lithium-deficient Li_YMn₂O₄ spinels (LD-Li_YMn₂O₄) with nominal composition (0.9 ≤ Y < 1) have been synthesized by melt impregnation from Mn₂O₃ and LiNO₃ at temperatures ranging from 700 °C to 850 °C. X-ray diffraction data show that LD-Li_YMn₂O₄ spinels are obtained as single phases in the range Y = 0.975-1 at 700 °C and 750 °C. Morphological characterization by transmission electron microscopy shows that the particle size of LD-Li_YMn₂O₄ spinels increases on decreasing the Li-content. The influence of the Li-content and the synthesis temperature on the thermal and electrochemical behaviours has been systematically studied. Thermal analysis studies indicate that the temperature of the first thermal effect in the differential thermal analysis (DTA)/thermogravimetric (TG) curves, T_C1, linearly increases on decreasing the Li-content. The electrochemical properties of LD-Li_YMn₂O₄ spinels, determined by galvanostatic cycling, notably change with the synthesis conditions. So, the first discharge capacity, Q_disch., at C rate increases on rising the Li-content and the synthesis temperature. The sample Li_0.975Mn₂O₄ synthesized at 700 °C has a Q_disch. = 123 mAh g⁻¹ and a capacity retention of 99.77% per cycle. This LD-Li_YMn₂O₄ sample had the best electrochemical characteristics of the series.     

Preparation, characterization and dielectric properties of CaCu3Ti4O12 ceramics

CaCu₃Ti₄O₁₂ nano-sized powders were successfully prepared by sol-gel technique and calcination at 600-900 °C. The thermal decomposition process, phase structures and morphology of synthesized powders were characterized by IR, DSC-TG, XRD, TEM, respectively. It was found that the main weight-loss and decomposition of precursors occurred below 450 °C and the complex perovskite phase appeared when the calcination temperature was higher than 700 °C. Using above synthesized powders as starting materials, CCTO-based ceramics with excellent dielectric properties (?₂₅ = 5.9 × 10⁴, tan δ = 0.06 at 1.0 kHz) were prepared by sintering at 1125 °C. According to the results, a conduction mechanism was proposed to explain the origin of giant dielectric constant in CCTO system.