Ag‐Loaded WO3 Ceramic Nanomaterials: Characterization and Moisture Sensing Studies

Pellet samples of 0–4% weight Ag‐loaded WO₃ prepared through soft chemical route were sintered at 700°C and exposed to humidity. Resistance of the pellets decreased with increase in relative humidity. Sensitivity increased with increase in the % loading of Ag. Four percent Ag‐loaded WO₃ showed maximum sensitivity of 2.38 MΩ/% RH in 20–90% relative humidity range. This sensing element manifests highest crystallinity as well as maximum void concentration. Hysteresis and repeatability for this sensing element after 6 months are within ±2%. A polynomial fit of the humidification data revealed a strong correlation between resistance and relative humidity.     

Photoluminescence properties of Eu-doped WO3-Eu2(WO4)3 composites and single-phase Eu2(WO4)3 powders

The development of highly stable and efficient oxide-based red phosphors is urgently required for next-generation lighting devices. Herein, we report the micro/crystal structures and luminescent properties of single-phase Eu₂(WO₄)₃ and Eu³⁺-doped WO₃-Eu₂(WO₄)₃ composite phosphors prepared by a one-step conventional solid-state reaction method in air atmosphere. As increasing Eu contents in the mixtures of WO₃ and Eu₂O₃, the intensities of the X-ray diffraction peaks of Eu₂(WO₄)₃ increased while that of WO₃ decreased. The photoluminescence intensity of the synthesized phosphors increased with increase in the Eu content when calcined at 900 °C, while it degraded at a higher temperature. Red-emitting single-phase Eu₂(WO₄)₃ powders were successfully obtained when the WO₃ and Eu₂O₃ powders were calcined in the ratio of 3:1. The intensity of the red emission spectra of the Eu₂(WO₄)₃ phosphor was higher than those of the 6, 12, and 24 at.% Eu-added WO₃ composites at excitation wavelengths of 394 and 465 nm. On the other hand, the intensity of emission from the single-phase phosphor was lower than that of the Eu-doped WO₃-Eu₂(WO₄)₃ composites under excitation of UV light at 254 nm. Thus, we propose two prospective phosphors for application as red phosphors at various wavelengths.     

Low temperature sintering of PZT powders coated with Pb5Ge3O11 by sol–Gel method

Low temperature sintering of PZT powders was investigated using Pb₅Ge₃O₁₁(PGO) as a sintering aid. PZT powders with 150 nm particle size were coated with PGO which was prepared from precursor solutions of Ge(OiPr)₄ and Pb(NO₃)₂ by sol–gel method. 1 wt% PGO-added PZT powders were densified at 750°C for 2 h to sintered bodies with the relative density of approximately 95%. An addition of PGO improved the sinterability of PZT powders with a reduction of sintering temperature by about 300°C. Dielectric and piezoelectric properties of PGO-added PZT ceramics sintered at ≦950°C were superior to those without PGO additives. However, a higher sintering temperature above 1000°C deteriorated the dielectric and piezoelectric properties of PGO-added PZT ceramics. This may be attributed to the change of microstructure involving the formation of solid solution between PZT and PGO. The 1 wt% PGO-added PZT bodies sintered at 750°C exhibited an electromechanical coupling factor, Kp, of about 56%.     

Phase Formation,Microstructure, and Densification of Yttrium‐Doped Barium Zirconate Prepared by the Sonochemical Method

Spherical monodispersed yttrium‐doped barium zirconate (BaY_0.1Zr_0.9O₃; BYZ) particles were successfully prepared in single step without the requirement of calcination process by the sonochemical method in highly basic aqueous solution. The stoichiometric solution of BaCl₂.2H₂O, ZrOCl₂.8H₂O, and YCl₄.6H₂O was precipitated in a 20 M NaOH under high‐intensity ultrasonic irradiation (20 kHz, 150 W/cm²) for 15, 30, and 60 min. As‐prepared powders were identified by XRD, FT‐IR, and Raman spectroscopy as cubic perovskite BYZ. The microstructure examined by SEM and TEM showed spherical‐shaped BYZ particles formed by aggregation of primary nanocrystals, and this unique morphology was induced by the effects of the ultrasonication and the strong alkaline environment. BYZ powders prepared under ultrasonication for 60 min had narrow‐sized distribution with the average particle size of 267 ± 26 nm and the specific surface area of 40.2 m²/g. BYZ ceramics sintered in the air at 1550°C for 20 h showed good densification (95%) and consisted of large grain size (7.67 ± 2.79 μm).     

Densification and biocompatibility of sintering 3.0 mol% yttria-tetragonal ZrO2 polycrystal ceramics with x wt% Fe2O3 and 5.0 wt% mica powders additive

The densification and biocompatibility of sintered 3.0 mol% yttria-tetragonal zirconia polycrystal (3Y-TZP) ceramics, with X wt% Fe₂O₃ and 5.0 wt% mica powders (denoted by 3Y-TZP: X-5.0 wt% mica) have been studied. When the pellets of 3Y-TZP: X-5.0 wt% mica were sintered at 1300 °C for 1 h, the relative shrinkage increases from 19.20–19.43% with the X increased from 0.3 to 1.0. The relative shrinkage of pellets containing 1.0 wt% Fe₂O₃ (X=1.0) increased from 19.43–19.59% when sintering temperatures were raised from 1300 °C to 1450 °C. X-ray diffraction results show that the pellets of 3Y-TZP: X-5.0 wt% mica sintered at 1400 °C for 1 h only contained single phase of tetragonal ZrO₂ (t-ZrO₂). When the sintering temperature was higher than 1400 °C, the Vickers microhardness was greatest in the pellets with X=0.5. Within pellets with the same Fe₂O₃ content, the dominant wavelength (λ_d) was only slightly different for pellets sintered at 1300 °C and those sintered at 1450 °C. The results of the materials were evaluated in vitro cytotoxicity tests reveals that the powders and sintered pellets are safe materials. The oral mucosa irritation tests did not find erythema or histopathological change including normal epithelium, and was free from leucocyte infiltration, vascular congestion and oedema.     

Effect of Ga2O3 on Sintering and Phase Stability of Sc2O3‐Doped Tetragonal Zirconia Prepared via Aqueous Solution Route

The effects of the presence of Ga₂O₃ on low‐temperature sintering and the phase stability of 4, 5, and 6 mol% Sc₂O₃‐doped tetragonal zirconia ceramics (4ScSZ, 5ScSZ, and 6ScSZ, respectively) were investigated. A series of zirconia sintered bodies with compositions (ZrO₂)_0.99?x(Sc₂O₃)_x(Ga₂O₃)_0.01, x = 0.04, 0.05, and 0.06 was fabricated by sintering at 1000°C to 1500°C for 1 h using fine powders that were prepared via the combination of homogeneous precipitation method and hydrolysis technique using monoclinic zirconia sols synthesized through the forced hydrolysis of an aqueous solution of zirconium oxychloride at 100°C for 168 h. The presence of 1 mol% Ga₂O₃ was effective in reducing sintering temperature necessary to fabricate dense bodies and enabled to obtain dense sintered bodies via sintering at 1100°C for 1 h. The phase stability, that is, low‐temperature degradation behavior of the resultant zirconia ceramics was determined under hydrothermal condition. The zirconia ceramics codoped with 1 mol% Ga₂O₃ and 6 mol% Sc₂O₃ (1Ga6ScZ) fabricated via sintering at 1300°C for 1 h showed high phase stability without the appearance of monoclinic zirconia phase, that is the tetragonal‐to‐monoclinic phase transformation was not observed in the 1Ga6ScZ after treatment under hydrothermal condition at 150°C for 30 h.     

Microwave Dielectric Properties of LiKSm2(MoO4)4 Ceramics with Ultralow Sintering Temperatures

In this work, a novel low‐temperature firing microwave dielectric ceramic LiKSm₂(MoO₄)₄ was prepared via solid‐state reaction method. Ceramic samples with relative densities about 94.6% were obtained at sintering temperature 640°C–680°C. The best microwave dielectric properties was obtained in ceramic sample sintered at 620°C with a permittivity about 11.5, a Q × f value about 39 000 GHz and a temperature coefficient of frequency about ?15.9 ppm/°C. According to XRD patterns and backscattered electron micrograph, combined with Energy Dispersive Spectra analysis, of cofired samples with 30 wt% aluminum sintered at 620°C/4 h, the LiKSm₂(MoO₄)₄ ceramic was found to be chemically compatible with Al but react seriously with Ag, forming AgSmMo₂O₈ phase, at its sintering temperature.     

Effect of calcination temperature on morphology of mesoporous YSZ

A nano-structured mesoporous yttria-stabilized zirconia (YSZ) powders were prepared for the first time using cetyltrimethylammonium bromide (CTAB) as the surfactant and urea as the hydrolyzing agent and using ZrO(NO₃)·6H₂O and Y(NO₃)₃·6H₂O as inorganic precursors. The Brunauer–Emmett–Teller (BET) surface area, Barrett–Joyner–Halender (BJH) pore size distribution and crystallite/particle size of mesoporous YSZ varied with calcine temperatures were studied. Characterizations revealed that the mesoporous YSZ powder calcined at 600 °C was weakly agglomerated and had a high surface area of 137 m²/g with an average grain size of ∼5.8 nm. It was demonstrated that the mesoporous structure remained up to 900 °C. The low-densified YSZ sample with porosity as high as 33% was prepared from mesoporous YSZ powder sintered at 1500 °C for 6 h.     

Pb(Zr0.95Ti0.05)O3 powders and porous ceramics prepared by one-step pyrolysis process using non-aqueous Pechini method

Using non-aqueous Pechini method, Pb(Zr_0.95Ti_0.05)O₃ powders were prepared at low temperature by one-step pyrolysis process. The polymeric gels and powders were characterized using a range of techniques, such as DTG, XRD, SEM, Raman spectroscopy, and laser particle size distribution. The perovskite phase was formed at about 350–400 °C and some oxocarbonate impurities can be detected in all samples after calcining at 400–850 °C by one-step pyrolysis process. Phase pure and porous Pb(Zr_0.95Ti_0.05)O₃ ceramics were obtained without pore formers from the powders by one-step pyrolysis process at 500 °C for 4 h. The relative densities were 87%, 91% and 94% for the ceramics sintered at 1100, 1150 and 1200 °C for 2 h, respectively. The porous ceramics sintered at 1200 °C for 2 h have homogeneously dispersed pores and fine-grain structures with an individual grain size of 0.7–2 μm.     

Mechanical properties of 1 mol% CeO2‐10 mol% Sc2O3 co‐doped and stabilized ZrO2 synthesized through hydro/solve‐thermal method

Ultra‐fine 1 mol% CeO₂‐10 mol% Sc₂O₃ co‐doped and stabilized ZrO₂ (1Ce10ScSZ) powders with average grain size less than 10 nm in diameter were prepared by hydro/solve‐thermal method using either deionized water, ethanol, or methanol as solvent. As‐synthesized powders were characterized in terms of phase structure, particle morphology, and chemical composition by X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), high‐resolution transmission electron microscopy (HRTEM), and inductively coupled plasma‐optical emission spectroscopy (ICP‐OES), respectively. Sintering studying was conducted on pellets of 15 mm in diameter and 3 mm in thickness under uniaxial compaction using 25 MPa at either 600, 800, 1000, 1100, 1200, 1400, or 1500°C for 1 hour. Phase transitions and grain morphologies of those sintered samples were characterized by XRD and field emission scanning electron microscopy (FESEM). Mechanical properties were characterized on dense pellets sintered at 1500°C by nanoindentation. Experimental results showed that ethanol was more effective to synthesize agglomerate‐free 1Ce10ScSZ powders as compared with deionized water and methanol. Choice of solvent affected the environment of hydro/solve‐thermal solution, which led to variation of chemical compositions of powders and porosities of sintered pellets, and therefore, influenced their mechanical performance. Our study showed that solvent was important to make dense, thin, and mechanically robust 1Ce10ScSZ electrolyte for potential applications in electrochemical devices. Absolute values of hardness (H) and Young's modulus (E) measured from our samples are much higher and more consistence than those results obtained from commercial vendors reported in literatures.     

Influence of processing conditions on the ionic conductivity of holmium zirconate (Ho2Zr2O7)

Nanopowders of holmium zirconate (Ho₂Zr₂O₇) synthesised through carbon neutral sol-gel method were pressed into pellets and individually sintered for 2 h in a single step sintering (SSS) process from 1100 °C to 1500 °C at 100 °C interval and in a two step sintering (TSS) process at (I) −1500 °C for 5 min followed by (II) - 1300 °C for 96 h. Relative density of each of the sintered pellet was determined using the Archimedes’ technique and the theoretical density was calculated from crystal structure data. Grain size was obtained from SEM micrographs using ImageJ. Pellets processed by TSS have been found to be denser (98 %) with less grain growth (1.29 μm) as compared to the pellets processed using SSS process. Ionic conductivity of Ho₂Zr₂O₇ pellets sintered by two different processes was measured using ac impedance spectroscopy technique over the temperature range of 350 °C–750 °C in the frequency range of 100 mHz–100 MHz for both heating and cooling cycles. The temperature dependence of bulk (2.67⨯10⁻³ Scm⁻¹) and grain boundary (2.50⨯10⁻³ Scm⁻¹) conductivities of Ho₂Zr₂O₇ prepared by TSS process are greater than those processed by SSS process suggesting the strong influence of processing conditions and grain size. Results of this study, indicates that the TSS is the preferable route for processing the holmium zirconate as it can be sintered to exceptionally high densities at lower temperature, exhibits less grain growth and enhanced ionic conductivity compared with the samples processed by SSS process. Hence, holmium zirconate can be considered as a promising new oxide ion conducting solid electrolyte for intermediate temperature SOFC applications between 350 °C and 750 °C temperature range.     

Negative thermal expansion coefficient of Sc-doped indium tungstate ceramics synthesized by co-precipitation

Co-precipitation was successfully applied to synthesize the Sc³⁺ doped In_2-xSc_x (WO₄)₃ (x = 0, 0.3, 0.6, 0.9 and 1.2) compounds. The composition- and temperature-induced structural phase transition and thermal expansion behaviors of Sc³⁺ doped In₂(WO₄)₃ were investigated. Results indicate that In_2-xSc_x (WO₄)₃ crystalizes in a monoclinic structure at 300 °C for x ≤ 0.3 and changes into hexagonal structure for x ≥ 0.6. Hexagonal In_1.1Sc_0.9(WO₄)₃ displays negative thermal expansion (NTE) with an average linear coefficient of thermal expansion (CTE) of −1.85 × 10⁻⁶ °C ⁻¹. After sintering at 700 °C and above, a phase transition from hexagonal to orthorhombic phase was observed in In_2-xSc_x (WO₄)₃ (x ≥ 0.6). Sc³⁺ doping successfully reduce the temperature-induced phase transition temperature of In_2-xSc_x (WO₄)₃ ceramics from 250 °C (x = 0) to room temperature (x = 0.9). When x = 0.9 and 1.2, the average linear CTEs of In_2-xSc_x (WO₄)₃ ceramics are −5.45 × 10⁻⁶ °C⁻¹ and -4.43 × 10⁻⁶ °C⁻¹ in a wider temperature range of 25–700 °C, respectively.     

Synthesis of tungsten carbide nanopowders by direct carbonization of tungsten oxide and carbon: Effects of tungsten oxide source on phase structure and morphology evolution

In the paper, WC nanopowders are successfully prepared by carbothermal reduction method, and the effect of tungsten oxide source on the phase structure evolution and products properties of the as-synthesized WC nanopowders has been investigated. Four tungsten oxide powders are chosen as tungsten oxide sources, e.g., rods-like WO₃ , WO₃ nanopartiles, WO₃ micro-particles and WO_2.9 micro-particles. Compared with other three tungsten oxide sources, the WO₃ micro-particles possesses small particle size, less agglomerates and good dispersity and the uniform tungsten oxide-carbon mixture after ball milling can be easily obtained. The appropriate tungsten oxide source can result in lower processing temperature (≤1200 °C) and shorter holding time (≤3 h). Single-phase WC powders with average particle size of 100 nm and uniform particle distribution can be achieved by micro-particle-like WO₃ at 1100 °C for 3 h. The as-prepared WC products by other three tungsten oxide sources exhibit problems of more aggregates, non-uniform particle size and large particle size (250 nm), respectively. In addition, the method can provide a facile, low-cost, efficient, and industrially feasible pathway for large scale preparation of WC nanopowders.     

Dense nanocrystalline UO2+x fuel pellets synthesized by high pressure spark plasma sintering

Nanocrystalline UO_2+x powders are prepared by high‐energy ball milling and subsequently consolidated into dense fuel pellets (>95% of theoretical density) under high pressure (750 MPa) by spark plasma sintering at low sintering temperatures (600°C‐700°C). The grain size achieved in the dense nano‐ceramic pellets varies within 60‐160 nm as controlled by sintering temperature and duration. The sintered fuel pellets are single phase UO_2+x with hyper‐stoichiometric compositions as derived by X‐ray diffraction, and micro‐Raman measurements indicate that random oxygen interstitials and Willis clusters dominate the single phase nano‐sized oxide pellets of UO_2.03 and UO_2.11, respectively. The thermal conductivities of the densified nano‐sized oxide fuel pellets are measured by laser flash, and the fuel stoichiometry displays a dominant effect in controlling thermal transport properties. A reduction in thermal conductivity is also observed for the dense nano‐sized pellets as compared with micron‐sized counterparts reported in the literature. The correlation among the SPS sintering parameters—microstructure control—properties is established, and the nano‐sized UO_2+x pellets with controlled microstructure can serve as the model systems for fundamental understandings of fuel behaviors and obtaining critical experimental data for multi‐physics MARMOT model validation.     

A novel route to synthesize metal titanium phosphate ceramic pigments: Co,Ni, and Cu-incorporated α-Ti(HPO4)2⋅H2O from ilmenite

Solid-state reaction of α-Ti(HPO₄)₂·H₂O (α-TiP) and acetates of Co, Ni, or Cu at 500 °C and 800 °C produce a number of different metal titanium phosphate (MTiP) inorganic pigments with different colors and shades. At a given metal to α-TiP ratio and calcination temperature, the reaction produces several types of pigments such as phosphates M_0.5TiO(PO₄), M_0.5Ti₂(PO₄)₃, M₂P₂O₇, and M₃(PO₄)₃ (M = Co, Ni, and Cu) with a unique color. α-TiP, an iron free base material was prepared by digestion of high-grade natural ilmenite beach sand (FeTiO₃) with 85 wt% H₃PO₄ at 120 °C. TGA, XRD, SEM/EDX, Diffuse reflectance UV–vis, and Raman spectroscopy techniques were used for characterization. Synthesized MTiP pigments after enameling at a single fire glazing at 1100 °C for 45 min exhibit vivid colors, ranging from purple, yellow, and green with different shades.     

Effect of MgO and PVA on the Synthesis and Properties of Negative Thermal Expansion Ceramics of Zr2(WO4)(PO4)2

A green synthesis of Zr₂(WO₄)(PO₄)₂ ceramics from ZrO₂, WO₃ and P₂O₅ is presented. It is shown that the ceramics can be synthesized by one‐step sintering within 60 min. The relative density of the ceramics can be enhanced from about 75% without sintering additives to 99.8% of the theoretical value with 1.0 wt% MgO and 2.0 wt% polyvinyl alcohol. The grain sizes of the ceramics are smaller and more uniform with MgO added in the raw materials than with MgO added in the Zr₂(WO₄)(PO₄)₂ powder. The coefficients of thermal expansion are about ?2.325 × 10^?6, ?1.406 × 10^?6, ?1.509 × 10^?6 and ?1.384 × 10^?6°C^?1 for the samples without MgO, with MgO added in the raw materials, with MgO added in the Zr₂(WO₄)(PO₄)₂ powder and with MgO and PVA added in the raw materials, respectively.     

Microwave dielectric characteristics of Li2(Mg0.94M0.06)Ti3O8 (M=Zn,Co, and Mn) ceramics

Li₂(Mg_0.94M_0.06)Ti₃O₈ (M=Zn, Co, and Mn) ceramics were synthesized by the conventional solid-state reaction route. The effect of M (Zn, Co, and Mn) substitution on the structure, microstructure and microwave dielectric properties of Li₂(Mg_0.94M_0.06)Ti₃O₈ has been investigated. The XRD patterns of sintered samples revealed the single-phase formation with spinel structure. With the increase in ionic radius of M, the Qf value decrease is attributed to the decrease of packing fraction and grain size. The Li₂(Mg_0.94Zn_0.06)Ti₃O₈ ceramic sintered at 1075 °C for 4 h showed the best microwave dielectric properties with a dielectric constant of 27.1, a Qf value of 44 800 GHz, and a temperature coefficient of resonant frequency of (+)1.9 ppm/°C.     

Bulk Iodoapatite Ceramic Densified by Spark Plasma Sintering with Exceptional Thermal Stability

Iodoapatite powder prepared by high‐energy ball milling is densified by Spark Plasma Sintering to ~96% theoretical density. X‐ray Diffraction and First‐principle Calculation indicates the sintered phase is iodine‐deficient apatite with chemical composition of Pb_9.85(VO₄)₆I_1.7 and iodine confinement over 8 wt%. Thermogravimetric analysis shows the bulk iodoapatite displays exceptionally stability without iodine release until 670°C. The greatly improved iodine confinement can be attributed to the dense matrix upon rapid consolidation from highly activated powders by mechanical attrition.     

Preparation of Co3O4/ZrO2(Y2O3) Catalyst and its Enhanced Catalytic Oxidation of CO

Yttria-stabilized zirconia powders were prepared by the sol–gel method coupled with supercritical CO₂ fluid-drying technology, using ZrOCl₂·8H₂O as the precursor, urea as the precipitant, and yttria as the stabilizer. The particles were characterized by X-ray diffraction, TEM and BET. The Co₃O₄/ZrO₂(Y₂O₃) catalysts were prepared by the impregnation method. The content of cobalt was varied from 5 to 12 wt%. The prepared catalysts were calcined at 200–500 °C and the pretreating temperature was varied from 200–400 °C. The performance of CO catalytic oxidation was tested and the catalyst with 8% Co loading, calcined at 200 °C, and with a pretreating temperature of 300 °C, showed the highest catalytic activity. The temperature for 95% CO conversion was as low as 113 °C; and, the catalyst showed both good cycling stability and excellent long-term stability.     

Microstructure and mechanical characterization of Cu–Ni/Al2O3 nanocomposites fabricated using a novel in situ reactive synthesis

Chemical-based synthesis of co-formed oxide (CuO–NiO–Al₂O₃) nanoparticles, followed by selective hydrogen reduction of the Cu and Ni oxides and ultimately consolidation into pellets, produced various compositions of Cu–Ni/Al₂O₃ nanocomposites. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy analyses were used to characterize the powders. The generated powders ranged in size from 20 to 70 nm, with a considerable presence of agglomerates. According to SEM examination, the powders were homogeneous in shape and particle size. Cold pressed nanocomposite powders were sintered for 2 h at 950 °C. SEM with energy dispersive spectroscopy (EDS) was also used to study the microstructure of the sintered specimens. In addition, the physical and mechanical properties of sintered specimens were studied. When the Al₂O₃ content increased, a more uniform distribution of nanosized Al₂O₃ particles in the Cu–Ni matrix was attained, resulting in a reduction in particle size. The results also demonstrated that as the Al₂O₃ concentration was raised, the microhardness and compressive strength of the nanocomposites rose by 74% and 67% compared to pure alloy, with the exception of fracture strain, which decreased dramatically.