Starved Water Hydrolysis of Different Precursors and its Influence on the Properties of Precipitated Zirconia

The chemical composition of zirconia gels precipitated from methanol solutions with excess, stoichiometric or deficient amounts of water as well, as the phase composition of fine ZrO₂ powders obtained by thermal treatment of gels prepared by this method, were investigated. It was observed that both the stoichiometry and crystalline phase formation during thermal treatment of zirconia gels are strongly influenced by the amount of water added to the initial reaction mixture. Heating the hydrated zirconia gels in an inert oxygen-free atmosphere produced a black nonstoichiometric oxide. The degree of nonstoichiometry of zirconia and its microstructure are influenced by the initial conditions in the reaction mixture. The X-ray patterns of thermally treated samples prepared with a substoichiometric amount of water show power lines of monoclinic and tetragonal zirconia, while after the same thermal treatment to 700°C, those prepared with excess water in the initial methanol solution, show mainly tetragonal diffraction lines.     

Carbon Blacks Produced by Thermal Plasma: the Influence of the Reactor Geometry on the Product Morphology

Carbon black (CB) nanopowders were obtained by plasma decomposition of methane at various flow rates using inductively coupled thermal plasma torch system of 35 kW. Nitrogen was also introduced in some experiments along with the methane. Using a cylindrical shape reactor the obtained powders were composed mainly of spherical particles, non-uniform in terms of particles size with diameters between 30 and 150 nm. The shape and size of this reactor resulted in the presence of recirculation areas enabling the formation of large CB particles and other secondary volatile compounds. Changing the reactor to a conical geometry resulted in the production of CB powders showing a crystalline and flake-like morphology made of sheets having 6–16 graphitic planes. The conical shape avoids the presence of recirculation areas and promotes the formation of a uniform powder morphology throughout the reactor.     

Time‐Resolved Phase Transitions of the Nanocrystalline Cubic to Submicron Monoclinic Phase in Zirconia

The nanocrystalline cubic, tetragonal, and submicron monoclinic phases of pure zirconia were prepared by thermal decomposition of carbonate and hydroxide precursors. The crystallization and isothermal phase transformations of the oxide were studied using high temperature X‐ray diffraction, X‐ray diffraction and Raman spectra of quenched samples. Cubic zirconia formed first, and then progressively transformed to tetragonal and monoclinic phases at temperatures as low as 320°C. The cubic, tetragonal, and monoclinic phases for ZrO₂ were found to be distinct functions of crystallite size, indicating the nanocrystalline nature of these phases. They were found to exist within critical size ranges of 50 to 140 Å, 100 to 220 Å and 190 to 420 Å (±5 Å), respectively. Thus, as the crystallites grow during annealing, they first transform from cubic to tetragonal and then from tetragonal to monoclinic at critical sizes. The classical Avrami equation for nucleation and growth was applied to the tetragonal to monoclinic phase transition.     

Dehydroxylation and the Crystalline Phases in Sol-Gel Zirconia

The formation and evolution with temperature of the crystalline phases in sol-gel ZrO₂ was analyzed by using X-ray powder diffraction, refinement of the crystalline structures, ESR, and UV-Vis spectroscopy. The precursor phase of crystalline zirconia was amorphous Zr(OH)₄ with the same local order as the tetragonal crystalline phase. This amorphous phase dehydroxylated with temperature, generating nanocrystalline tetragonal zirconia, and producing point defects that stabilized the tetragonal structure, generated a paramagetic ESR signal with g values like the free electron, and had a light absorption band at 310 nm. When the sample was annealed at higher temperatures, it continued dehydroxilating, and the point defects disappeared, causing the transformation of the nanocrystalline tetragonal phase into nanocrystalline monoclinic zirconia. The two crystalline nanophases coexisted since the beginning of crystallization.     

紫外拉曼光谱研究焙烧气氛对氧化锆相变的影响

 以紫外拉曼光谱技术研究了在不同焙烧气氛中氧化锆样品的表面晶相结构及其转变过程．结果表明，在有氧气氛中焙烧的氧化锆样品易转变为单斜相，而在惰性气氛中焙烧可以有效地稳定样品体相和表面的四方相结构．在缺氧气氛中焙烧的样品暴露于空气中后，表面极易转变为单斜相，而其体相仍为四方相结构，这表明四方相氧化锆在样品表面是极不稳定的．     

Atomic-Scale Structure of Water-Based Zirconia Xerogels by X-Ray Diffraction

The structural evolution of zirconia thin films and gel powders has been evaluated by X-ray diffraction. Maxima (r ₁ and r ₂) of the experimental radial distribution function RDF and the bond angles were determined and correlated with TGA (thermogravimetric analysis), DTA (differential thermal analysis) and MS (mass spectrometry). The results indicate that the topological short-range structure (<5 Å) of amorphous zirconia thin films, independent of drying temperature, resembles that of crystalline tetragonal ZrO₂. In contrast, amorphous zirconia powder gels dried at temperatures below 120°C show atomic arrangements similar to that of tetragonal ZrO₂. The structure of these gels annealed at temperatures between 165–340°C resembles a distorted tetragonal ZrO₂, monoclinic-like structure. Zirconia powders and films contain crystalline tetragonal ZrO₂ at 400°C.     

CO2/H2在不同形态ZrO2上的吸附行为

利用傅立叶变换红外光谱技术(FT-IR)考察了CO2和CO2＋H2在不同形态氧化锆上的吸附和转化行为，结果表明,氧化锆的形态影响CO2的吸附形式和表面物种的生成.无定型氧化锆上主要生成碳酸氢盐和离子碳酸盐，单斜氧化锆上还出现了双齿碳酸盐，而在四方氧化锆上出现最强的线式吸附CO2，并生成聚碳酸盐.在氢气存在的条件下，单斜氧化锆上生成甲烷而在四方氧化锆上则生成甲酸盐.     

Parametric study of the decomposition of NH3 for an induction plasma reactor design

The present paper reports a study of the gas mixing and chemical transformation in an induction plasma reactor under atmospheric pressure, and its dependence on the plasma operating conditions. For this purpose, the thermal dissociation of ammonia into nitrogen and hydrogen was chosen because of the relative simplicity of the reactions involved and its use in a number of studies on plasma synthesis of ultrafine nitride ceramic powders using ammonia as nitriding agent. A hot-wall reactor configuration is investigated in which ammonia is injected radially through multiple orifices into the gases at the exit nozzle of an induction plasma torch. Concentration mapping in the mixing zone was carried out, using a VG-Micromass-PC 300 D quadrupole mass spectrometer, for different plasma power levels, in the range 13–24 kW. A 3-point injection mode is used with the injection ports oriented upstream at 45° to the torch axis. This allows uniform mixing of the injected gas in the plasma jet. A systematic study of the effects of plate power and ammonia and plasma gas flow rates on the mixing and dissociation of NH₃ in the reactor is reported. The results are analyzed and discussed from the viewpoint of their use for optimizing the design of induction plasma reactors, to he applied to the vapor-phase synthesis of ultrafine silicon nitride powders.     

不同形态ZrO2的制备及其表面性质研究

用ZrOCl2制备出具有单一形态的无定型、单斜和四方三种形态二氧化锆,采用SEM观察其表面形貌,FT-IR检测表面羟基形式,并通过NH3(CO2)-TPD和Py-FT-IR对其表面酸碱性进行研究.结果表明,不同形态二氧化锆的表面羟基种类具有较大差异:无定型二氧化锆表面存在氢键羟基,四方二氧化锆表面独有桥式羟基,而单斜二氧化锆表面具有两种类型的锥桥式羟基.同时不同形态二氧化锆表面酸碱性也具有较大差别.     

Phase Transformation of Nanosized Zirconia

1 INTRODUCTION Without introduction of any stabilizing reagent, mesoporous zirconia was prepared via solid state re- action-structure directed method, which broke through the traditional preparation route (liquid reaction)[1]. Using this novel method, the samples possess not only mesoporous structure, but also nano-size. Normal size zirconia has three kinds of crys- talline phases: monoclinic phase existing at room tem- perature, tetragonal phase stabilizing between 1170～2370 ℃ and cubi…     

Synthesis and Characterization of Yttria-Stabilized Zirconia (YSZ) Nano-Clusters for Thermal Barrier Coatings (TBCs) Applications

Yttria-stabilized zirconia (YSZ) nano-clusters were synthesized by a sol–gel process. The aim was to produce YSZ powders in order to prepare thick coatings for thermal barrier to be applied on gas turbine engine components. Yttrium nitrate hexahydrate and zirconium oxy-chloride octahydrate were used as a source of zirconium, citric acid was taken as a chelating agent, and ethylene glycol was used as a polysterification agent. The synthesized powders were characterized by X-ray diffraction, transmission electron microscopy, thermo-gravimetric analysis and differential scanning calorimetry, and Raman spectroscopy. Furthermore, parameters were critically analyzed in order to synthesize non-transformable (t′) tetragonal crystal structure, which is the best zirconia phase for high temperature thermal barrier coatings applications. In this regard, tetragonal YSZ nano-clusters were heated in an alumina crucible at a temperature of 1200 °C for 100 h.     

Combustion synthesis and properties of nanocrystalline zirconium oxide

Nanocrystalline tetragonal zirconia powders have been synthesized by aqueous combustion using glycine (Gly) as a fuel and zirconyl nitrate (ZN) as an oxidizer. The effect of the fuel-to-oxidant molar ratio on the structural and morphological properties of nanocrystalline zirconia powders was studied. Thermodynamic modeling of the combustion reaction showed that the increase in the Gly:ZN molar ratio leads to the increase in theoretical combustion temperature, heat of combustion and amount of produced gases. Powder properties were correlated with the nature of combustion and results of thermodynamic modelling. The increase in the Gly:ZN molar ratio produces more agglomerated powders characterized by a lower degree of uniformity, a lower specific surface area and a slightly bigger crystallite size. On the other hand, the presence of hard agglomerates suppresses the volume expansion, stabilizing tetragonal zirconia, as confirmed by Rietveld refinement. The absence of cubic zirconia was confirmed by FTIR and Raman Spectroscopy. The increase in the calcination temperature led to more agglomerated, compact and less uniform powders. The nanocrystalline nature of zirconia is the reason for the formation of bigger crystallites, the increase in the relative amount of monoclinic phase and sample sintering after calcination at high temperature. The highest measured specific surface area of zirconia was 45.8 m²·g⁻¹, obtained using a fuel-lean precursor.     

ZrO2 phase structure in coating films and powders obtained by sol-gel process

Zirconia coating film and powder obtained by the sol-gel route using zirconium n-propoxide as starting material and acid catalyst were investigated by the Perturbed Angular Correlations method, X-Ray Diffraction and Differential Scanning Calorimetry and Differential Thermal Analyses. The hyperfine interaction, measured after annealing the samples at increasing temperatures up to 1100°C, allowed to distinguish five different zirconium neighborhoods. Two of them describe rather disordered material which, on heating, crystallizes to the tetragonal phase and end finally in monoclinic zirconia. As compared with the powder, the film exhibits a minor fraction of an unidentified ordered form and a higher and more stable fraction of tetragonal phase. In addition, the tetragonal to monoclinic conversion takes place at higher temperatures and with a larger activation energy.Researcher for CONICET-ArgentinaResearcher for CICPBA-Argentina     

Structural and Morphological Control in the Preparation of High Surface Area Zirconia

The methods for the formation of zirconia including precipitation from aqueous salts, sol–gel synthesis from zirconium alkoxides, and the templated synthesis using surfactants are described in this review. The surface areas obtained vary widely but invariably decrease upon prolonged calcination. Digestion of hydrous zirconia and incorporation of dopants such as lanthanum, yttrium, or sulfate ions can increase the surface area and thermal stability. However, these methods also affect the crystal phase of zirconia. The transformation from the metastable tetragonal to the monoclinic phase occurs during the cooling phase of calcination. Mechanisms for the stabilization of the tetragonal phase are discussed. Zirconia with well-ordered mesopores or in the form of hollow spheres can be prepared but lack thermal stability, unless doped with phosphates, silicates or sulfates.     

Studies on atmospheric plasma abatement of PFCs

Microwave plasma at 2.45 GHz frequency operating at atmospheric pressure in synthetic gas mixtures containing N₂, CF₄, C₂F₆, CHF₃, and SF₆ were investigated experimentally for various gas mixture constituents and operating conditions, with respect to their ability to destroy perfluorocompounds. It was found that the destruction and removal efficiency (DRE) of the process is highly dependent on the total gas flow. DREs of up to 99.9% have been achieved using 1.8 kW of microwave power at 20 l/min total flow rate.     

Measurement of Spectral Emissivity and Thermal Conductivity of Zirconia by Thermal Radiation Calorimetry

The hemispherical total emissivity of partially stabilized zirconia has been obtained by a spectral analysis of the normal emission spectra between 400–850 K. Thermal conductivity was evaluated with this emissivity and temperature gradient in the sample. Effects of the scattering of the radiation in a ceramic sample are also investigated. In addition, specific heat capacity was measured by thermal radiation calorimetry. Current results are compared with those reported for tetragonal zirconia with the same yttria content and for monoclinic phase. This revised version was published online in July 2006 with corrections to the Cover Date.     

Carbon Nano-Flakes Produced by an Inductively Coupled Thermal Plasma System for Catalyst Applications

Carbon material was produced using an inductively coupled thermal plasma torch system of 35 kW and a conical shape reactor. The carbon nanopowders were obtained by plasma decomposition of methane at various flow rates and show a uniform microstructure throughout the reactor. The product has a crystalline graphitic structure, with a stacking of between 6 and 16 planes and a nano-flake morphology with particles dimensions of approximately 100 nm long, 50 nm wide and 5 nm thick. Nitrogen was also introduced in some synthesis experiments along with the methane precursor using flow rates of 0.1 and 0.2 slpm. The resulting product has the same structural properties and the nitrogen is incorporated into the graphitic structure through pyridinic type bonds.     

Synthesis of Ozone at Atmospheric Pressure by a Quenched Induction-Coupled Plasma Torch

The technical feasibility of using an induction-coupled plasma (ICP) torch to synthesize ozone at atmospheric pressure is explored. Ozone concentrations up to ~250 ppm were achieved using a thermal plasma reactor system based on an ICP torch operating at 2.5 MHz and ~11 kVA with an argon/oxygen mixture as the plasma-forming gas. The corresponding production rate and yield were ~20 g ozone/hr and ~2g ozone/kWh, respectively. A gaseous oxygen quench formed ozone by rapid mixing of molecular oxygen with atomic oxygen produced by the torch. The ozone concentration in the reaction chamber was measured by Fourier Transform infrared (FTIR) spectroscopy over a wide range of experimental conditions and configurations. The geometry of the quench gas flow, the quench flow velocity, and the quench flow rate played important roles in determining the ozone concentration. The ozone concentration was sensitive to the torch RF power, but was insensitive to the torch gas flow rates. These observations are interpreted within the framework of a simple model of ozone synthesis.     

In-vitro hemolytic activity and free radical scavenging by sol-gel synthesized Fe3O4 stabilized ZrO2 nanoparticles

Zirconia ceramics have attained much consideration owing to the amazing mechanical strength and white color. These properties provide an opportunity for the use in biomedical applications. In the present study, an application oriented sol-gel route was adapted for synthesis of zirconia nanoparticles. ZrOCl₂·8H₂O was used as a precursor, iron oxide (Fe₃O₄) nanoparticles (pH 2 & pH 9) as a stabilizer and de-ionized water was used as a solvent. Sol-gel synthesized iron oxide stabilized zirconia nanoparticles were prepared by varying concentrations of iron oxide nanoparticles in the range of 2–10 wt%. X-ray diffraction results showed mixed phases at all wt% with acidic pH value, while pure tetragonal phase of zirconia was observed for stabilization with 6 wt% basic iron oxide. Maximum value of dielectric constant (~80 at log f = 4) and minimum value of tangent loss (~0.66 at log f = 4) were observed for zirconia stabilized with basic 6 wt% iron oxide. Maximum value of hardness (1410 ± 10 HV) along with high fracture toughness were observed with optimized stabilization. Very weak hemolytic activity and maximum scavenging (~76) antioxidant activity was observed under optimized conditions. Thus, it can be suggested that optimized nanoparticles, i.e. tetragonal zirconia stabilized with 6 wt% of basic Fe₃O₄, can be further useful for therapeutical and pharmaceutical applications.     

Thermal characteristics and analysis of pyrolysis effects during the mechanical alloying process of Astaloy CrM powders

Mechanical alloying processes are carried out usually with addition of some lubricants, referred to also as a process control agents (PCA). About 1–2 wt% of a PCA (usually stearic acid) is normally added to prevent excessive cold welding amongst the powder particles, especially when powders of ductile metals are milled. However, lubricants (maybe disintegrated after MA process) must be removed in the early stages of heating during the sintering process. The aim of the present study was to determine the effect of the MA process especially (i) milling time and (ii) influence of lubricant addition on thermal effects during heating of iron powder (Astaloy CrM) with the addition of 1 wt% stearic acid measured by differential scanning calorimeter (DSC) and thermogravimetry (TG) techniques. Removal of the lubricants during heating of the blended powders and exhaust gases from the furnace were monitored by quadruple mass spectrometry (QMS). Determination of the physical and technological properties of the powder mixture obtained from MA was performed with a laser particle sizer. Distribution of particle sizes, flow and apparent density were also measured. Furthermore, the shape and morphology of the powders were characterised by a light optical microscope (LOM) and scanning electron microscope (SEM).