Synthesis of Cerium‐Doped Gd3(Al,Ga)5O12 Powder for Ceramic Scintillators with Ultrasonic‐Assisted Chemical Coprecipitation Method

Cerium‐doped Gd₃(Al,Ga)₅O₁₂ powders have been synthesized with ultrasonic‐assisted chemical coprecipitation method (UACC), and the traditional chemical coprecipitation method (CC) was also employed for comparison. The structure and morphology of powders were investigated by XRD, BET, and TEM. The powders were used for preparing ceramics at different temperatures. The specific surface areas of UACC and CC powders calcined at 800°C were 66 and 29 m²/g, respectively. Ceramics derived from UACC and CC powders were sintered at 1600°C, and the densities are 6.67 and 6.48 g/cm³, respectively. UACC is an attractive method for synthesizing GAGG powder for preparing ceramic scintillators.     

Preparation and Electrochemical Properties of Sr‐doped K2NiF4‐type Cathode Material Pr1.7Sr0.3CuO4 for IT‐SOFCs

A novel K₂NiF₄‐type oxide Pr_1.7Sr_0.3CuO₄ (PSCu) is studied to obtain its electrochemical properties as the cathode for intermediate‐temperature solid oxide fuel cells (IT‐SOFCs). The PSCu cathode powder and Ce_0.8Sm_0.2O_1.9 (SDC) electrolyte powder were synthesized by sol‐gel method and glycine‐nitrate method, respectively. The crystal structure of PSCu powder and PSCu‐SDC composite powder were identified with X‐ray diffraction (XRD). It is shown that PSCu belongs to tetragonal K₂NiF₄‐type and has good chemical compatibility with SDC. The thermal expansion coefficient (TEC) of PSCu is close to that of SDC. The conductivity of PSCu tested with four‐probe method exhibits a semiconductor‐pseudometal transformation at 400–450 °C, where the maximum conductivity of 103.6 S cm⁻¹ is obtained. The polarization test indicates the area specific resistance (ASR) of PSCu decreases with increasing temperature, reaching 0.11 Ω cm² at 800 °C. The activation energy of oxygen reduction reaction during 600–800 °C is 1.19 eV. The single fuel cell performance test reveals the open circuit voltage (OCV) and resistivity of PSCu reduce with increasing temperature, but the power density ascends with increasing temperature. The maximal power density is 243 mW cm⁻² at 800 °C, and the corresponding current density and OCV are 633 mA cm⁻² and 0.77 V, respectively.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

Understanding the Role of Triblock Copolymers for the Synthesis of Mesoporous Alumina,and Its Adsorption Efficiency for Congo Red

Mesoporous alumina (MA)was synthesized by sol–gel based evaporation‐induced self‐assembly process using aluminum isopropoxide as alumina source in the presence of three different types of triblock copolymers (TBCs), F68, F127, and L64. The role of different TBCs as surfactants on thermal, crystallization, textural, and microstructural properties of the alumina powders was studied. To understand the effects of different copolymers, the adsorption efficiency of the samples for Congo red (CR) was studied. For all the surfactants, the XRD results showed the stability of γ‐Al₂O₃ phase up to 1000°C for 1 h dwell time. A maximum value (431.8 m²/g) of Brunauer–Emmet–Teller surface area was obtained for the 400°C‐treated powder prepared from F68 surfactant. The transmission electron microscopy micrograph exhibited worm‐like mesoporous structures of the 400°C‐treated powders prepared from F68 and F127 surfactants_. The adsorption performance for CR of the 400°C‐treated powders for different surfactants was in the order of F68 > F127 > L64. A tentative mechanism was illustrated to understand the roles of different block copolymers on the properties of the prepared MA.     

New Methods for Preparing Submicrometer Powders of The Tungstate‐Ion Conductor Sc2(WO4)3 and its Al and In Analogs

Two new methods for preparing submicrometer powders of M₂(WO₄)₃, M = Sc, In, and Al via combustion synthesis are reported. Stoichiometric combinations of trivalent metal nitrates, ammonium metatungstate, and either urea or carbohydrazide as the fuel were reacted at 550°C, producing amorphous or poorly crystallized powders with an average particle size ranging from 164 to 350 nm. Calcining the powders at 800°C for 1 h produced well‐crystallized, phase‐pure powders with an average particle size ranging from 210 to 711 nm. Powders sintered at 1000°C for 14 h resulted in pellets that were 87%–95% of the theoretical density, which is notably higher than typically obtained from powders prepared by solid‐state reaction. Whereas there was little difference in the microstructure of Al₂(WO₄)₃ pellets prepared with the two different powders, the carbohydrazide‐derived powders resulted in In₂(WO₄)₃ and Sc₂(WO₄)₃ pellets with a larger grain size than those prepared with urea‐derived powders. The electrical conductivity of the sintered pellets, while comparable to that reported for polycrystalline M₂(WO₄)₃ prepared by solid‐state reaction, was strongly influenced by grain‐boundary effects.     

Low‐Cost Silicon Nitride from β‐Silicon Nitride Powder and by Low‐Temperature Sintering

Aiming to manufacture low‐cost silicon nitride components, a low‐cost β powder was chosen as a raw powder and low‐temperature sintering at 1550–1600°C under atmospheric pressure nitrogen was carried out. The silicon nitride from β powder with 5 wt% Y₂O₃ and 5 wt% MgAl₂O₄ additives and sintered at 1600°C for 8 h was successfully densified, and it exhibited moderate strength and toughness of 553 MPa ± 22 and 3.5 MPa m^1/2, respectively. The results indicate that the low‐temperature sintering of the low‐cost β powder has a potential to reduce cost of components.     

Creating a Protective Shell for Reactive MoSi2 Particles in High‐Temperature Ceramics

Alumina encapsulated molybdenum silicide (MoSi₂) intermetallic particles were synthesized using a simple precipitation method followed by calcining at temperatures of 800°C–1000°C, to prevent the premature oxidation of MoSi₂ at high temperatures. The shell composition and the influence of the calcining temperature on microcapsule integrity were investigated by means of X‐ray photoelectron spectroscopy, X‐ray diffraction, scanning electron microscopy, and thermogravimetric analysis. The results demonstrate that the composition and the mechanical stability of the alumina shell can be tuned by the annealing temperature. After calcining at 800°C and 850°C the alumina shell remains intact. Calcining at higher temperature promotes the formation of mullite, which leads to cracking of the shell. However, when annealed at 1000°C for 24 h these cracks were filled with mullite and preserved the molybdenum silicide particles. Furthermore, the mechanical stability of the shell was improved by applying an intermediate calcining treatment at 450°C prior to the annealing process at 1000°C.     

The dispersants effect on physical properties of long‐lasting luminescent polypropylene

Two series of blends, O‐PP₁₅ and O‐PP₃₅, were prepared by mixing polypropylene (PP), luminescent powders (SrAl₂O₄: Eu²⁺, Dy³⁺) of 15 and 35 μm average particle diameter, and hydrophobic dispersant at about 190°C in the Brabender mixer. The effect of amounts and diameter of luminescent powders on the physical properties of PP material were discussed herein. The luminescence and afterglow time tests indicated that the initial luminescence of all blends increased with the luminescent powders amounts. O‐PP₃₅ blends showed lower afterglow luminance than O‐PP₁₅ blends at low luminescent powder amounts. The melting and crystallization temperatures of the blends appeared at 152–168°C and 87–103°C, respectively. The blends displayed peaks attributable to a α crystal structure at 2θ = 18°–19°. The β crystal structure was only evident from its characteristic 2θ peak at 15°–16° in the WAXD pattern of the O‐PP₃₅ blends with high luminescent powder amounts. All of the blends had lower tensile strengths. However, the improvement in the luminescent powder distribution was evident from the SEM images after adding hydrophobic dispersant. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Processing of alumina‐coated glass‐bonded silicon carbide membranes for oily wastewater treatment

Crack‐free γ‐Al₂O₃‐coated glass‐bonded SiC membranes were successfully prepared using a simple heat‐treatment and dip‐coating process at a temperature as low as 850°C in air. The changes in the porosity, flexural strength, flux, and oil rejection rate of the membranes were investigated while changing the initial SiC particle size. Larger SiC particles led to bigger pores, resulting in higher flux in the oily water and a lower oil rejection rate. The SiC membranes with a support prepared from 10 μm SiC powder showed an exceptionally high oil rejection rate (99.9%) with a feed oil concentration of 600 mg/L at an applied pressure of 101 kPa. The typical porosity, flexural strength, steady state flux, and oil rejection rate of the alumina‐coated SiC membrane were ~45%, ~81 MPa, 1.78×10^?5 m³ m^?2s^?1, and 99.9%, respectively.     

Effect of Pluronic F-127 debinding temperature on the sintering and densification of alumina by direct ink writing

Hydrogel-based alumina (Al₂O₃) inks were prepared using Pluronic F-127 with 65 wt% of solid loading (Al₂O₃). The Al₂O₃ inks were deposited, and the freestanding samples were studied using TGA/DTA. Significant weight loss was observed between 180 and 360°C. A two-stage hydrogel debinding process of Al₂O₃ samples was carried out at 180 and 360°C with holding times of 30, 60, 90, 120, and 150 min. The Al₂O₃ samples were then sintered at 1600°C. X-ray diffraction was used for the phase analysis of the alumina inks, and a scanning electron microscope was used microstructural analysis. Based on the TGA/DTA analysis, a two-stage debinding process was adopted. Significant effect of hydrogel debinding temperature was observed on the sintering and densification behavior of alumina. It was observed that the porosities in the alumina samples were increasing when the debinding time was increased from 30 to 150 min, with the debinding temperature at 180 and 360°C. Moreover, the nature of the porosities was changing from closed porosities to interconnected porosities.     

Performance Evaluation of Solid Oxide Fuel Cells with Thin Film Electrolyte Fabricated by Binder‐Assisted Slurry Casting

A gas‐tight yttria‐stabilized zirconia (YSZ) electrolyte film was fabricated on porous NiO–YSZ anode substrates by a binder‐assisted slurry casting technique. The scanning electron microscope (SEM) results showed that the YSZ film was relatively dense with a thickness of 10 μm. La_0.8Sr_0.2MnO₃ (LSM)–YSZ was applied to cathode using a screen‐print technique and the single fuel cells were tested in a temperature range from 600 to 800 °C. An open circuit voltage (OCV) of over 1.0 V was observed. The maximum power densities at 600, 700, and 800 °C were 0.13, 0.44, and 1.1 W cm^–2, respectively.     

Sinterability of Forsterite Prepared via Solid‐State Reaction

In this work, the sinterability of forsterite powder synthesized via solid‐state reaction was investigated. X‐ray diffraction (XRD) analyses indicate that the synthesized powder possessed peaks that correspond to stoichiometric forsterite. Scanning electron micrographs revealed that the powders were formed agglomerates, which were made up of loosely packed fine particles. Subsequently, the forsterite powders were cold isostatically pressed into a disk shape under 200 MPa and sintered in air at temperature ranging from 1200°C to 1500°C (interval of 50°C) with ramp rate of 10°C/min and dwelling time of 2 h. The sinterability of each sintered samples was examined in terms of phase stability, relative density, Vickers hardness, fracture toughness, and microstructural examination. XRD examination on all the sintered samples exhibited pure forsterite, in which the generated peaks were found to be in a good agreement with JCPDS card no. 34‐0189. The densification of forsterite progressed to reach a maximum relative density of ~91% at 1500°C. This study also revealed that high‐strength forsterite ceramic can be synthesized via solid‐state reaction as forsterite attained favorable mechanical properties, having fracture toughness of 4.88 MPam^1/2 and hardness of 7.11 GPa at 1400°C.     

Low‐Temperature Synthesis of Nanocrystalline NbB2 Powders by Borothermal Reduction in Molten Salt

Nanocrystalline NbB₂ powders were successfully prepared by borothermal reduction in molten salt at 800°C–1000°C. Due to the more homogeneous mixing and more rapid diffusion of species in the liquid state than in the solid state, the synthesis temperature of pure NbB₂ phase was greatly decreased by the presence of molten NaCl/KCl salt. The NbB₂ powders synthesized at 1000°C had the largest specific surface area of 27.09 m²/g and the lowest equivalent average particle size of 32 nm, respectively.     

ZrB2–SiC Nano‐Powder Mixture Prepared Using ZrSi2 and Modified Spark Plasma Sintering

ZrB₂–SiC nano‐powder mixture was synthesized using ZrSi₂ source material and a modified spark plasma sintering apparatus. The particle size of ZrB₂ and SiC was about 80 and 20 nm, respectively. The molecular‐level homogeneity of Zr/Si source and fast heating/cooling rate by SPS caused the formation of homogeneously intermixed nano‐powders. A strong exothermal reaction occurred at around 860°C, which caused strong agglomeration and growth of the synthesized powder mixture. The rapid reaction could be controlled by adding 20 wt% of NaCl, which acted as an inert filler.     

Preparation and Characterization of Silver‐Doped Cu(In,Ga)Se2 Films via Nonvacuum Solution Process

Cu(In,Ga)Se₂ films doped with different contents of silver ions (Ag⁺) were successfully prepared using nonvacuum spin coating followed by selenization at elevated temperatures. Increasing the Ag⁺ ion content increased the lattice parameters of the chalcopyrite structure, and shifted the A1 mode in the Raman signals to low frequencies. The band gaps of the prepared (Ag,Cu)(In,Ga)Se₂ (ACIGS) films were considerably increased, thereby increasing the open‐circuit voltage (V_oc) of the solar cells. As Ag⁺ ion content increased, the microstructures of ACIGS films became densified because the formed (Cu,Ag)₂In alloy phase with a low melting point facilitated liquid‐phase sintering. The evaporation of selenium species was correspondingly suppressed in the films during selenization, thereby reducing the selenium vacancies. The improvement in the microstructures and the defects of ACIGS films increased short‐circuit current (J_sc) and fill factor of the solar cells. The spectral response of the solar cells was also enhanced remarkably. This study demonstrated that incorporation of Ag⁺ ions into Cu(In,Ga)Se₂ films substantially improved the efficiency of the solar cells.     

Photocatalytic activity of titanium dioxide coatings: Influence of the firing temperature of the chemical gel

Heterogeneous photocatalysis can be exploited for the decomposition of micro-organisms which have developed on the surfaces of building materials. In this work, the efficiency of titanium dioxide coatings on fired clay products is examined. The sol–gel method is used to synthesize a fine TiO₂ powder with a specific surface area of 180 m² g^?1. Thermal treatment of the chemical gel at 340 °C leads to crystallisation in the anatase phase and with further temperature increase, crystallite growth. For thermal treatments in the range 580–800 °C, there is a progressive transition from anatase to rutile. However, despite a decrease in specific surface area of the powder attributed to aggregation/agglomeration, the coherent domain size deduced from X-ray diffraction measurements remains almost constant at 23 nm. Once the transition is completed, increase of thermal treatment temperature above 800 °C leads to further crystallite growth in the rutile phase. The thermally treated titania powders were then sprayed onto fired clay substrates and the photocatalytic activity was assessed by the aptitude of the coating to degrade methylene blue when exposed to ultraviolet light. These tests revealed that the crystallite size is the important controlling factor for photocatalytic activity rather than the powder specific surface area or the anatase/rutile polymorph ratio.     

New Method of Synthesizing Aluminum Oxynitride Spinel Powders

Aluminum oxynitride spinel (AlON) powders were synthesized by aluminothermic reaction in a reducing N₂‐CO atmosphere. Low cost and easily available aluminum and γ‐Al₂O₃ alumina micrometer‐sized powders were employed as starting materials. Mixed powders consisting of 75 wt% Al and 25 wt% Al₂O₃ were milled together and pressed into billets with diameter of 20 mm and height of 15 mm. Green‐body billets were then calcined in charcoal‐protected condition (namely in a N₂‐CO atmosphere) at 1600°C. Phase composition and microstructure of final sintered products were analyzed. The results showed that AlON phase with AlN as a minor phase was formed at 1600°C for 3 h. At the same time, grains of AlON were tabular in shape and whiskers can be found in samples after being sintered at 1600°C.     

Synthesis of mesoporous alumina with CO2 expanded carbonation and its catalytic oxidation of cyclohexanone

A CO₂ expanded carbonation technique is proposed for direct synthesis of alumina powders that does not require structure directing substances or templates. Mesoporous amorphous flower-like alumina was synthesized at relatively low volume expansions (lower ethanol to water volume ratio), whereas mesoporous crystalline honey-comb-like alumina was synthesized at high volume expansions. The alumina powders exhibited high surface area and pore size with small crystallite sizes. The alumina structures were stable from 400 to 800 °C. Experimental tests showed that the alumina powders could catalytically convert cyclohexanone to ɛ-caprolactone efficiently. The use of the calcined catalysts (at 400 and 800 °C; flower-like alumina) at equal ethanol to water volume ratio avoids the usual and inevitable hydrolysis of ɛ-caprolactone to ɛ-hydroxyhexanoic acid. The catalyst was recyclable and stable for up to five reaction cycles.     

High hardness cubic boron nitride with nanograin microstructure produced by high‐energy milling

High‐energy shaker milling of hexagonal boron nitride (hBN) powders was used to produce powders rich in sp³ bonding. The powders contained up to 68% sp³ bonding and were found to nucleate nanosize cBN grains during consolidation at 5.5 GPa and 1400°C. The effect of hBN starting particle size, milling time, and powder‐to‐milling ball ratio were studied. The amount of sp³ bonding for milled hBN powders was determined, using ¹¹B solid‐state NMR. The milled material was also analyzed by XRD, Raman spectroscopy, and HRTEM. The results indicate that the material has a nanosized microstructure comprised of a disordered hBN matrix and cBN nuclei in the form of sp³‐rich domains. Eight different milled powders were produced and consolidated at pressures of either 5.5 or 6.5 GPa and temperatures of either 1400°C or 1450°C into 12 mm diameter and 5 mm thick pellets. Consolidated pellets formed from milled hBN with 68% sp³ bonding had Vickers hardness of 42 ± 1 GPa and fracture toughness 3.8 ± 0.1 MPa.m^1/2. Vickers hardness of 49 ± 1 GPa and fracture toughness of 4.6 ± 0.1 MPa.m^1/2 was achieved with a precursor that contained milled hBN and 50 vol. % of 0.5 μm diameter cBN crystals.     

Molten Salt Synthesis of Bismuth Ferrite Nano‐ and Microcrystals and their Structural Characterization

Bismuth ferrite nano‐ and microcrystals were prepared by a facile molten salt technique in two kinds of molten‐salt‐based systems (NaCl–KCl and NaCl–Na₂SO₄). In the NaCl–KCl salt system, a systematic study indicating the effects of process parameters (e.g., calcination temperature, holding time as well as the molten salt ratios) on the bismuth ferrite formation mechanism and structural characteristics is reported. The results show that almost pure phase BiFeO₃ powders with minimum impurity phase of Bi₂Fe₄O₉ were synthesized at temperatures of 700°C–800°C, whereas high calcination temperature (e.g., 900°C) resulted in the formation of almost pure phase Bi₂Fe₄O₉ powders. The prolonged holding time increased the particle size via the Ostwald ripening mechanism; however, there was little effect on the particle morphology. Similar phenomenon occurred as increasing the molten salt ratios. In the NaCl–Na₂SO₄ salt systems, it is found that low NP‐9 (nonylphenyl ether, NP‐9) surfactant content (0–5 mL) led to the formation of almost pure phase BiFeO₃ powders, whereas high NP‐9 surfactant content (e.g., 20 mL) resulted in pure phase Bi₂Fe₄O₉ powders. The average particle size of the BiFeO₃ powders was decreased as increasing the NP‐9 surfactant content, whereas their morphologies did not change significantly. Because of the simplicity and versatility of the approach used, it is expected that this methodology can be generalized to the large‐scale preparation of other important transitional metal oxides with controllable sizes and shapes.