One-step hydrothermal synthesis of yttria-stabilized tetragonal zirconia polycrystalline nanopowders for blue-colored zirconia-cobalt aluminate spinel composite ceramics

A novel approach for the preparation of blue-color giving zirconia nanopowders by doping of 3?mol% Y₂O₃ through simple one-step hydrothermal process is proposed. A blue-color giving yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) powders were prepared by urea-based solution, zirconium acetate, CoCl₂·6H₂O and AlCl₃ precursors in hydrothermal vessel at 24?h, 150?°C and 20 bars. Based on the results, the synthesized blue-color giving Y-TZP nanopowders have entirely tetragonal structure as mono phase with 3.8?±?0.2?nm average grain size and (Y, Co, Al)_xZrO₂; x?≤?0.03?at. with chemical composition. Thermal treatment was also applied to synthesized Y-TZP powders at 1200?°C and 1450?°C to observe the color evolution. Only sharp blue was obtained in Y-TZP powders resulting the development of zirconia-cobalt aluminate spinel (ZrO₂-CoAl₂O₄) composite ceramic structure for both temperatures after heat-treatment. Herein, not only formation of CoAl₂O₄ but also incorporation of cobalt (Co) and aluminum (Al) into the Y-TZP grains plays a critical role on evolving of blue color. This synthesized Y-TZP nanopowders can be a good candidate for one-step production of blue-color sintered ZrO₂-CoAl₂O₄ spinel composite ceramics in numerous ceramic applications due to their superior structural and functional properties.     

Effect of in-situ formed whiskers diameter from tourmaline on microstructure and mechanical properties of 3Y-TZP ceramics

In this paper, in-situ whiskers reinforced 3 mol% Y₂O₃ stabilized tetragonal ZrO₂ (3Y-TZP) ceramics with different diameters were prepared using pressureless sintering by introducing tourmaline with different particle sizes into 3Y-TZP powders. The purpose of this research was to investigate the influence of in-situ formed whisker diameters on the densification, microstructure and mechanical properties of 3Y-TZP ceramics. The prepared ceramics were characterized by X-ray diffraction, scanning electron microscope and transmission electron microscope. Findings indicated that in-situ mullite whiskers formed by phase transformation of tourmaline particles can promote the densification of 3Y-TZP ceramics, and further improve the dispersion of mullite whiskers in the 3Y-TZP ceramics. More importantly, the average diameter of mullite whiskers can be controlled by altering the tourmaline particle size. When the average particle size of tourmaline is 500 nm, 3Y-TZP composites have a near-fully dense microstructure of 99.09%, with the ZrO₂ grain size of about 335 nm, the average diameter of mullite whiskers is 330 nm. Both the bending strength and fracture toughness reached optimal values of 836 ± 24 MPa and 10.6 ± 0.5 MPa m^0.5, respectively. This paper provides a new way to design of the microstructure and strength-toughness of zirconia composite ceramics.     

Sintering kinetics of ZrO2 nanopowders modified by group IV elements

The sintering behavior of tetragonal zirconia nanopowders modified by the group IV elements at the initial sintering stage was investigated. It was found that different additives SiO₂, SnO₂, and GeO₂ have a significant influence on the densification kinetics of 3Y-TZP nanopowders obtained by coprecipitation during sintering as it depends on the amount of additives (0-5 wt%). The shrinkage of zirconia-based specimens during the nonisothermal sintering was analyzed using the dilatometric data. The constant rate of heating technique was applied in order to determine the dominant mass transfer mechanism at the initial stage of sintering in modified zirconia nanopowders. It was found that there was a change in the mass transfer mechanism and diffusion activation energy in 3Y-TZP as a result of the additives. The dominant sintering mechanism in 3Y-TZP changed from the volume diffusion to the grain boundary diffusion due to the addition of SiO₂ and SnO₂ and the sintering activation energy increased in these cases. However, GeO₂ additive activated the viscous flow mechanism in sintering process of 3Y-TZP nanopowders which led to acceleration of the densification due to the decrease in the diffusion activation energy.     

Synthesis of vanadium carbide nanopowders by thermal processing and their characterization

Vanadium carbide (V₈C₇) nanopowders were successfully prepared by thermal processing the precursor which originated from the mixture of ammonium vanadate (NH₄VO₃) and nanometer carbon black. The products were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) methods. The single phase V₈C₇ powders can be prepared at ∼ 1100 °C for 1 h with the average crystallite size of 32.6 nm and the powders show good dispersion and are mainly composed of uniformly-sized spherical particles with a mean diameter of ∼ 100 nm.     

Dual role of activated carbon as fuel and template for solution combustion synthesis of porous zinc oxide powders

In this work, nanosized zinc oxide (ZnO) powders were fabricated by urea–nitrate solution combustion synthesis using activated carbon as a structure-directing template and secondary fuel at different fuel–oxidant ratios. The as-synthesized powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption–desorption measurements, UV–Vis diffuse reflectance spectroscopy, and photoluminescence. The effect of fuel amount on photocatalytic activity of ZnO powders was evaluated by the degradation of an azo dye Orange G. It was observed that combustion synthesis with activated carbon as a secondary fuel had a profound effect on reducing crystallite size and enhancement of specific surface area. The crystallite size of the as-synthesized powders varied from 46 to 26 nm. The ZnO powder prepared at a fuel–oxidant ratio of 1.8 possessed the small crystallite size and high specific surface area of 69 m²/g. It correspondingly resulted in the highest dye removal percentage of 99% with a rate constant of 0.027 min⁻¹. The improvement in dye degradation can be due to the synergistic interaction and interplay of enhanced surface area and catalytic ability of the photocatalyst. This study provides a simple single-step synthesis methodology to produce metal oxide nanopowders with tunable surface properties for high potential applications in catalysis, optoelectronics, and gas sensors.     

Preparation and characterization of tetragonal-ZrO2 nanopowders by a molten hydroxides method

Pure tetragonal-ZrO₂ nanopowders are prepared by a molten hydroxides method, using hydrated zirconium nitrate as the starting material at 200 °C. X-ray diffraction analysis and transmission electron microscopy observation reveal that the nanopowders exhibit poor crystalline quality. After heat treated at 400 °C for 10 h in air, the nanopowders are crystalline with size range of ∼10–12 nm and most of them are agglomerated. The formation mechanism of the ZrO₂ nanopowders has been proposed. The heat treated nanopowders have a BET surface area of ∼27.3 m²/g due to agglomeration. The photoluminescence of the heat treated nanopowders has been investigated at room temperature.     

Up-scalable preparation of nano zirconium carbide powder in liquid polymeric precursor and its pyrolysis mechanism

Nano size ZrC powder was prepared by liquid polymeric precursor method. Zirconium n-butoxide (Zr(OⁿBu)₄) and benzoylacetone (BA) were mixed directly with different molar ratios to synthesize transparent liquid zirconium carbide single-source precursors. The carbon content in the precursor could be changed by adding different amount of BA. X-ray pure ZrC was obtained when the molar ratio of BA/Zr(OⁿBu)₄ was 4.6:1. The viscosity of the precursor was very low (<8 mPa s) without the addition of solvents. Zirconium carbide powders were fabricated by the pyrolysis at 800 °C in argon and subsequent heating at various temperatures in vacuum for carbothermal reduction reaction. The pyrolysis behavior, phase composition and transformation, and microstructure of the as-fabricated ZrC powders were analyzed. The gases of CH₄, CO and CO₂ released due to decomposition and evaporation of the organic component and transformation from ZrO₂ to ZrC during pyrolysis resulted in total 60–70% mass loss. The average grain size of the synthesized X-ray pure ZrC powders was less than 30 nm. Meanwhile, the pyrolysis mechanism of nano zirconium carbide powder was deduced.     

Dielectric,mechanical and thermal properties of ZrO2–TiO2 materials obtained by microwave sintering at low temperature

The sinterability of 3Y-TZP/TiO₂ materials using micrometre-sized ZrO₂ and nanometre-sized TiO₂ (16 wt%) by one-step fast microwave sintering at low temperature (1200–1300 °C) was investigated. Firstly, in situ detailed analysis of the dielectric properties of the material with temperature was carried out in order to measure the capacity of the material to transform microwave energy into heat. Another related parameter associated to microwave sintering is the penetration depth of the microwave radiation into the material, which showed great homogeneity from 400 °C. Secondly, the effect of sintering conditions on microstructure, density, hardness and coefficient of thermal expansion was evaluated. The X-ray diffraction study and microstructural characterization demonstrate that it is possible to obtain fully dense pieces (>99%) by microwave sintering, a condition yielding to a coarse-grained (~1–2 μm), quite hard (~13.7 GPa) 3Y-TZP/TiO₂ material. However, the most important feature is the significant reduction of the thermal expansion coefficient (8·10⁻⁶ K⁻¹) as compared to that of 3Y-TZP. In addition, the results from conventional sintering at 1400–1500 °C with 2 and 6 h of dwell time are examined and compared. The materials obtained at 1500 °C showed high density with grain size and hardness similar to those obtained by microwave but with a dramatic difference in the power consumption of the sintering cycle, since the materials obtained by microwave used a maximum absorbed power of 120 W and a heating cycle of only 40 min.     

Effect of activation on the carbon fibers from phenol–formaldehyde resins for electrochemical supercapacitors

Activated carbon fibers (ACF) are prepared from phenol–formaldehyde resin fibers through chemical activation and physical activation methods. The chemical activation process consisted of KOH, whereas the physical activation was performed by activation in CO₂. The characteristics of the electrochemical supercapacitors with carbon fibers without activation (CF), carbon fibers activated by CO₂ (ACF-CO₂), and carbon fibers activated by KOH (ACF-KOH) have been compared. The activated carbon fibers from phenol–formaldehyde resins present a broader potential range in aqueous electrolytes than activated carbon and other carbon fibers. Activation does not produce any important change in the shape of starting fibers. However, activation leads to surface roughness and larger surface areas as well as an adapted pore size distribution. The higher surface areas of fibers treated by KOH exhibited higher specific capacitances (214 and 116 F g⁻¹ in aqueous and organic electrolytes, respectively) and good rate capability. Results of this study suggest that the activated carbon fiber prepared by chemical activation is a suitable electrode material for high performance electrochemical supercapacitors.     

Effects of zirconium source and content on zirconia crystal form,microstructure and mechanical properties of ZTM ceramics

ZTM ceramics were successfully derived from coal gangue. The effect of zirconium source (ZrO₂ and ZrOCl₂?8H₂O) and content on properties of the ZTM ceramics has been studied. The phase composition, density, and microscopic morphology were characterized with X-ray diffraction (XRD), Archimedes method and scanning electron microscopy (SEM). The influence of zirconium source, sintering temperature, zirconia content on flexural properties and fracture toughness was studied. The sample, with ZrOCl₂?8H₂O added 12% zirconia (Z2TM12) sintered at 1475 °C for 3 h has the highest density of 2.83 g/cm³. Partially stable t-ZrO₂ was present in samples with zirconium oxychloride (ZrOCl₂?8H₂O) as the zirconium source, due to the constraints of mullite crystals. Therefore, Z2TM12 had both microcrack toughening and stress phase transformation toughening mechanisms. The flexural strength was increased from 162.40 MPa to 285.04 MPa, while the fracture toughness was improved to 3.55 MPa m^1/2 from 2.38 MPa m^1/2. Our achievement can be used as a reference to fabricate ZTM ceramics from coal gangue with high-value additions.     

Synthesis of Ti0.2Zr0.8B2 solid‐solution nanopowders by molten salt assisted borothermal reduction

High‐purity Ti_0.2Zr_0.8B₂ solid‐solution nanopowders were successfully synthesized via a molten salt assisted borothermal reduction at 1323‐1373 K using ZrO₂, TiO₂ and amorphous B as starting materials. The Ti_0.2Zr_0.8B₂ solid‐solution nanopowders synthesized at 1323 K show the largest specific surface area of 12.24 m²/g and the lowest equivalent average particle size of 86 nm. Meanwhile, they exhibit the high compositional uniformity and the good single‐crystal hexagonal structure. This study provides a new method to synthesize the high‐purity solid‐solution nanopowders of the transition‐metal borides.     

Chemical oxidation of residual carbon from ZnAl2O4 powders prepared by combustion synthesis

The removal of carbon residue from ZnAl₂O₄ nanopowders by annealing at 500–800 °C leads to a decrease of specific surface area from 228.1 m²/g to 47.6 m²/g. At the same time, the average crystallite size increased from 5.1 nm to 14.9 nm. In order to overcome these drawbacks, a new solution for removing the carbon residue has been suggested: chemical oxidation using hydrogen peroxide. In terms of carbon removal, a H₂O₂ treatment for 8 h at 107 °C proved to be equivalent to a heat treatment of 1 h at 600 °C. The benefits of chemical oxidation over thermal oxidation were obvious. The specific surface area was much larger (188.1 m²/g) in the case of the powder treated with H₂O₂. The average crystallite size (5.8 nm) of ZnAl₂O₄ powder treated with H₂O₂ was smaller than the crystallite size (8.2 nm) of the ZnAl₂O₄ powder annealed at 600 °C.     

Synthesis of Bi2Te3 nanopowders by vacuum arc plasma evaporation

Starting from elemental bismuth and tellurium, bismuth telluride (Bi₂Te₃) nanopowders were successfully prepared by vacuum arc plasma evaporation (VAPE) technique for the first time. The phase composition in the obtained nanopowders is closely related with the Bi:Te atomic ratio in starting precursor. The microstructure and morphology of the samples were characterized via X-ray diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). Compositional analysis was also carried out by energy dispersive analysis of X-rays (EDAX). The as-synthesized Bi₂Te₃ nanopowders have a rhombohedral crystal structure with lattice parameters a = 4.381 Å and c = 30.310 Å. The average particle size is about 35 nm obtained from TEM and confirmed from XRD results.     

Athermal effect of flash event on high-temperature plastic deformation in Y2O3-stabilized tetragonal ZrO2 polycrystal

This study experimentally demonstrates the athermal effect of a flash event under a DC electric field on high-temperature plastic deformation in a dense, fine-grained 3 mol% Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (3Y-TZP). A high-temperature three-point flexural test in the conventionally sintered 3Y-TZP body with an average grain size of 0.36 µm was conducted under a flash event with a current density of 67 mA·mm^?2 at a furnace temperature between 900 °C and 1300 °C. The flash event significantly enhanced the high-temperature plastic flow in the 3Y-TZP body, even though subtracting the effect of specimen temperature rise by Joule heating, while grain growth during the flexural tests was effectively suppressed under the examined conditions. The athermal effect on the plastic flow was attributed to the reduction in the activation energy for the high-temperature plastic flow.     

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.     

Relationship among the physicochemical properties, electrocatalytic performance and kinetics of carbon supported Pt catalyst for ethanol oxidation

A new carbon supported Pt (Pt/C(b)) catalyst was prepared by reducing H₂PtCl₆ in glycol solution using formic acid as a reducing agent, and has been found in this work to be highly active and stable for the electrochemical oxidation of ethanol. The preparation produces highly dispersed Pt particles, of 2.6 nm average size, and with high electrochemical surface area, 98 m²/g. The apparent activation energy of ethanol oxidation over the Pt/C(b) catalyst electrode is low, 10–14 kJ/mol, over the range of potentials from 0.3 to 0.6 V.     

Thermal Properties of (Zr,TM)B2 Solid Solutions with TM = Ta,Mo, Re,V, and Cr

The thermal properties were investigated for hot‐pressed zirconium diboride containing solid solution additions of tantalum, molybdenum, rhenium, vanadium, and chromium. The nominal additions were equivalent to 3 at.% of each metal with respect to zirconium. Using 0.5 wt% carbon as a sintering aid, powders were hot‐pressed to near full density at 2150°C. Rietveld refinement of X‐ray diffraction data was used to measure lattice parameters and to ensure that the additives formed solid solutions. Thermal conductivities were calculated from measured thermal diffusivities and temperature‐dependent values for density and heat capacity. Thermal conductivities at 25°C ranged from 88 W·(m·K)^?1 for nominally pure ZrB₂ down to 28 W·(m·K)^?1 for (Zr,Cr)B₂. Electron contributions to thermal conductivity were calculated from electrical resistivity measurements using the Wiedemann–Franz law. Decreases in phonon and electron conduction correlated with the size of the metallic additive, indicating that changes in atom size in the Zr lattice positions reduced thermal transport.     

Effect of calcination temperature on structural,magnetic and optical properties of multiferroic YFeO3 nanopowders synthesized by a low temperature solid-state reaction

Nanosize multiferroic YFeO₃ powders have been synthesized via the low temperature solid-state reaction. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy all indicated that the phase-pure orthorhombic YFeO₃ powders were obtained at 800 °C with a size below 150 nm. X-ray photoelectron spectroscopy (XPS) showed the Fe³⁺ ions to be predominant. Magnetic hysteresis loops exhibited some ferromagnetic behaviour of the YFeO₃ nanopowders at ambient temperature. The maximum and remnant magnetizations of the powders were about 2.49 and 0.88 emu/g, respectively. Moreover, optical measurements demonstrated that the optical band gap of the nanopowders was around 2.4 eV, proving that they can strongly absorb visible light. So an easy and efficient way to synthesize YFeO₃ nanopowders with promising application in the magnetic and optical fields has been successfully developed.     

Densification of rare-earth (Lu, Gd, Nd)-doped alumina nanopowders obtained by a sol-gel route under seeding

Rare-earth (RE: Lu, Gd, Nd, 0.10 mol%)-doped alumina nanopowders were prepared by a new sol-gel route using polyhydroxoaluminum (PHA) and RECl₃ solutions under α-alumina (∼ 75 nm) seeding. Among the rare-earth dopants studied, Lu yields the most suitable nanopowders for low-temperature densification. The 0.10 mol% Lu-doped nanopowders, which were obtained at a calcination temperature of 900 °C under 5 mass% α-alumina seeding, consisted of ∼ 80-nm α-alumina particles and γ-alumina nanoparticles. Using these Lu-doped alumina nanopowders, fully densified alumina ceramics with a uniform microstructure composed of fine grains with an average size of 0.61 μm could be obtained at 1400 °C by pressureless sintering. Clearly, the Lu-doped nanopowders obtained here represent a viable option for fabricating dense, finer-grained alumina ceramics because an undoped sample with 5 mass% seeds gave a microstructure with an average grain size of 1.78 μm at 1400 °C.     

Effect of dopants and sintering temperature on microstructure and low temperature degradation of dental Y-TZP-zirconia

Accelerated ageing of dental TZP were investigated at 134 °C for 2 h under 2.3 bar water vapor pressure. The TZP blanks were sintered in the range from 1350 to 1580 °C. The average grain size of 1350 and 1400 °C sintered materials were <0.3 μm whereas higher sintering temperatures led to larger grain sizes. The grain size and dopants influence the stability of tetragonal phase of zirconia under LTD conditions. The Y-TZP with average grain sizes <0.3 μm did not reveal the martensitic tetragonal-monoclinic phase transformation after ageing, whereas zirconia with grain sizes larger 0.3 μm showed fractions of monoclinic phase. Alumina and Ceria stabilized grain size and Y-TZP against LTD. Y-TZP with low amounts of Fe₂O₃ (<0.15%) used for coloring did not show any detrimental effects under LTD conditions. As the Y-TZP ceramics with grain size larger than 0.3 μm are not stable under LTD conditions they are not recommended for long term use in moist environment.