Phase transformation kinetics of 3 mol% yttria partially stabilized zirconia (3Y-PSZ) nanopowders prepared by a non-isothermal process

A crystalline nanopowder of 3 mol% yttria-partially stabilized zirconia (3Y-PSZ) has been synthesized using ZrOCl₂ and Y(NO₃)₃ as raw materials throughout a co-precipitation process in an alcohol-water solution. The phase transformation kinetics of the 3Y-PSZ freeze dried precursor powders have been investigated by nonisothermal methods. Differential thermal and thermogravimetric analyses (DTA/TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) have been utilized to characterize the 3Y-PSZ nanocrystallites. When the 3Y-PSZ freeze dried powders are calcined in the range of 703-1073 K for 2 h, the crystal structure is composed of tetragonal and monoclinic ZrO₂. The BET specific surface area of the 3Y-PSZ freeze dried precursor powders calcined at 703 K for 2 h is 118.42 m²/g, which is equivalent to a crystallite size of 8.14 nm. The activation energy from tetragonal ZrO₂ converted to monoclinic ZrO₂ in the 3Y-PSZ freeze dried precursor powders was determined as 401.89 kJ/mol. The tetragonal (T) and monoclinic (M) ZrO₂ phases coexist with a spherical morphology, and based on TEM examination have a size distribution between 10 and 20 nm. When sintering green compacts of the 3Y-PSZ, a significant linear shrinkage of 8% is observed at about 1283 K. On sintering the densification cycle is complete at approximately 1623 K when a total shrinkage of 32% is observed and a final density above 99% of theoretical was achieved.     

Sintering behavior and mechanisms of NiO-doped 8 mol% yttria stabilized zirconia

Sintering behavior and densification mechanisms of NiO-doped YSZ were investigated by using a dilatometer, combined with XRD, SEM and HRTEM characterization. The solubility of NiO in YSZ is found to be 0.5-1 mol% at 1500 °C by XRD, and TEM reveals that, beyond solubility limit, the undissolved NiO exists in the form of nano and/or micro-sized crystals depending on the doping amount. The sintering model was used to address the enhanced sintering of YSZ as a result of small additions of NiO. Lattice diffusion is examined to be the rate-determining mechanism for the intermediate-stage sintering of both undoped and NiO-doped YSZ. However, the apparent activation energy for densification of YSZ is reduced by ∼70 kJ/mol upon NiO doping. It is concluded that the dissolved NiO contributes to the lowering of the activation energy and therefore the enhanced lattice diffusivity.     

Crystallization kinetics of lanthanum monoaluminate (LaAlO3) nanopowders prepared by co-precipitation process

Lanthanum monoaluminate (LaAlO₃) nanopowders were synthesized using La(NO₃)₃·6H₂O and Al(NO₃)₃·6H₂O as starting materials by a co-precipitation method. The crystallization kinetics of the LaAlO₃ nanopowders has been investigated by using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The XRD results and SAED patterns show that the rhombohedral LaAlO₃ nanopowders have been obtained when the precipitates as calcined at 1092 K for 10 min. The activation energy for the crystallization of the rhombohedral LaAlO₃ nanopowders is determined as 286.75 kJ/mol by a non-isothermal method. The TEM examination shows that the rhombohedral LaAlO₃ has a spherical morphology with the size ranging from 30 to 50 nm.     

Crystal structure and morphology of CeO2 doped stabilized zirconia ceramics under high-frequency microwave field sintering

Under high-frequency microwave irradiation, zirconia ceramics were prepared by sintering nano-CeO₂ (Ce = 7 mol%) doped zirconia powder. The different effects of temperature environment on the phase structure transformation, surface functional groups, microstructure, growth process, and density of doped zirconia were analyzed, and the optimized microwave sintering process for zirconia was determined. The experimental results reveal that the tetragonal phase of zirconia is positively correlated with the temperature when the temperature reaches about 1100 °C in the studied range. The reason is that the grain grows with the increase of sintering temperature, and the surface energy of grain decreases, which leads to the fluctuation of tetragonal phase content. The density of zirconia reaches 98.03% at 1300 °C, and the growth activation energy is 27.40 kJ/mol. There is no abnormal growth of zirconia particles, and the phase transition temperature decreases, which is attributed to the efficient heating of microwave and the incorporation of nano-ceria stabilizer.     

Study on drying kinetics of calcium oxide doped zirconia by microwave-assisted drying

Ca–ZrO₂ is an essential structural and functional material, which is commonly used in refractories, electronic ceramics, and functional ceramics. The properties of Ca–ZrO₂ materials are depending on the quality of Ca–ZrO₂ powders. The main factors affecting the quality of powder are sintering temperature and the drying effect. This paper applied modern microwave drying technology to dry Ca–ZrO₂ powder. The impact of initial mass, microwave heating power, and initial moisture content on the drying of Ca–ZrO₂ were explored. The results showed that the average drying rate increased with the rise of initial mass, microwave heating power, and initial moisture content. Wang and Singh, Page, and Quadratic Model were applied to fit Ca–ZrO₂ with an initial moisture content of 5.6%, mass of 30 g, and microwave output power of 400 W. The results displayed that the Page model had a better fitting effect. It was also applicable to other different initial moisture content, original mass, and microwave heating power. The diffusion coefficient calculated by Fick's second law displayed that with the increase of initial mass, initial moisture content, and microwave heating power of Ca–ZrO₂, the effective diffusion coefficient increased first and then declined. When the Ca–ZrO₂ of microwave heating power was 640 W, mass was 30 g, and the moisture content was 5.65%, the effective diffusion coefficients of zirconia were 1.42533 × 10^?13, 2.91806 × 10^?13, 5.652.2471 × 10^?13 m²/s, respectively. To determine the activation energy of microwave dried zirconia, using the relationship between microwave power and activation energy, the activation energy of microwave dried zirconia was calculated to be ?23.39 g/W. This paper aims to rich experimental data for the industrial application of microwaves to strengthen dried zirconia and propose a theoretical basis.     

Synthesis and characterization of zirconia toughened alumina ceramics prepared by co-precipitation method

Undoped and 3 mol% yttrium doped ZrO₂–Al₂O₃ composite powders with partially stabilized ZrO₂ (PSZ) content varying from 0 to 30 wt% were prepared by a co-precipitation route using inorganic precursors Al(NO₃)₃, ZrOCl₂ and Y(NO₃)₃. The precipitates were characterized by DTA and subsequently calcined at 1200 °C for 4 h to achieve fine grained composite powders. The calcined powders were characterized by FTIR and XRD. In order to enhance the sinterability, the calcined powders were wet milled in a high energy ball mill. Powders were uniaxially pressed to form pellets and sintered at 1600 °C for 5 h to achieve greater than 96% relative density. Microstructural analysis of the sintered compacts revealed the uniform distribution of the zirconia particles among the alumina matrix. It was also observed that the faceted intergranular zirconia grains were present at the grain boundaries and junctions in the alumina matrix. Vickers indentation was carried out at 1 kgf load for hardness and 2 kgf load for estimating the critical stress concentration factor (K_c). Microscopic studies of the indented samples showed that cracks were propagating around the grain boundaries. Highest K_c ∼8.40 ± 0.4 MPa√m and hardness ∼16.31 ± 0.58 GPa was obtained for the 30 wt% PSZ-Al₂O₃ composite. The sintered density and critical stress intensity factor (K_c) achieved were compararble to that achieved earlier by hot press and SPS.     

Thermal and crystallisation behaviours of blends of polyamide 12 with styrene-ethylene/butylene-styrene rubbers

Polyamide 12 (PA12)/styrene-ethylene/butylene-styrene (SEBS) and PA12/maleic anhydride grafted SEBS (SEBS-g-MA) blends were prepared in a twin-screw extruder followed by injection moulding. Thermal and crystallisation behaviours of these blends were evaluated. Thermal properties and morphology of the blends were estimated using thermo gravimetric analysis (TGA) and scanning electron microscopy (SEM), respectively. The phase structure of the blends was interpreted by dynamic mechanical thermal analyser (DMTA). In terms of temperature at maximum rate of degradation (T_max) and integral procedural decomposition temperature (IPDT), it was found that PA12/SEBS-g-MA (PM) blends possessed greater thermal stability than PA12/SEBS (PS) blends. The kinetics of degradation process of PA12 and its blends were studied using Coats-Redfern (CR) method. It was found that there is no appreciable change in the thermal stability of PA12 in the presence of small amount of rubber phase. A good correlation was observed between the thermal properties and phase morphology of the blends. Melting and crystallisation behaviours of the blends were analysed by differential scanning calorimetry (DSC). These results showed that the melting and crystallisation behaviours of PA12 were not significantly affected by blending with rubbers. It was also observed that the functional group present in the rubber phase has little effect on the melting and crystallisation behaviours of PA12.     

Optimization of particle size,dispersity, and conductivity of 8 mol% Y2O3 doped tetragonal zirconia polycrystalline nanopowder prepared by modified sol-gel method via activated carbon absorption

8 mol% Y₂O₃ doped tetragonal zirconia polycrystalline (8Y-TZP) ceramic nanopowders were synthesized via a novel modified sol-gel method employing zirconium carbonate basic as zirconium resources. The activated carbon as a dispersant was added to the precursor solution during the formation of the sol. The phase behavior, thermal decomposition, microstructure morphology, and electrochemical performance of nanopowders with the addition of activated carbons were investigated by X-ray diffraction (XRD), differential thermal analysis (DTA), scanning electron microscopy (SEM), particles size distribution, and electrochemical impedance spectroscopy analysis (EIS). After adding the activated carbon, the average crystallite size of 8Y-TZP nanopowders decreased from about 53.16–33.51 nm when calcined at 900 ℃, and the 8Y-TZP nanopowders were produced loosely agglomerated. Meanwhile, compacts prepared by pressing the as-obtained 8Y-TZP nanopowders sintered to 98.8% relative density at 1600 ℃ and exhibited an average grain size of 0.89 µm, which brought a positive effect on ionic conductivity (0.079 S·cm^?1).     

Physical properties of 8 mol% Ceria doped yttria stabilised zirconia powder and ceramic and their behaviour during annealing and sintering

The thermal behaviour of 8 mol% Ceria doped 16 mol% yttria stabilised zirconia (Ce-YSZ) powder, synthesised by the wet co-precipitation method, has been investigated in details. The lattice parameter, mean crystallite size and lattice distortions have been determined as a function of calcination temperature. After grinding by attrition followed by fractionating in acetone, the powders were compacted and sintered at 1600 °C in air for 5 h. The sintering behaviour of pellets was studied by dilatometry, revealing different sintering behaviour depending on the calcination temperature of the powders and the type of agglomerates. The influence of these latter parameters on the microstructure of pellets has been investigated. The pellets produced from powders calcined at low-temperature, from 110 to 600 °C, present the highest densities and the best morphological structure.     

Study on the electrochemical effect of ZnO addition on flash sintering of 3 mol% yttria stabilized zirconia

Flash Sintering has shown tremendous promise in reducing the sintering time and temperature. However, significant heterogeneity in densification and microstructure was observed in the flash-sintered sample due to hotspot formation and electrochemical effects (electrochemical blackening). In this work, we have varied the extent of electrochemical reaction by (0.75–3 mol%) ZnO addition during flash sintering of 3 mol% yttria-stabilized Zirconia (3YSZ). A substantial increase in grain size was observed throughout with an increase in dopant concentration. With the addition of 0.75 mol% ZnO dopant in 3YSZ, a significant improvement in densification and a significant reduction in the presence of cracks were observed. The extent of electrochemical blackening was seen to be increasing with the increase in dopant concentration. The blackened phase was found to be distorted tetragonal Zirconia by Raman analysis. The ZnO doped 3YSZ showed ionic to electronic transition in conduction during flash sintering likely due to electrochemical effects.     

Investigation on Decomposition Kinetic and Thermal Stability of Metallocene Catalysts

Data on thermal stability of metallocene catalysts such as bis(n-butyl cyclopentadienyl) zirconium dichloride and bis(t-butyl cyclopentadienyl) zirconium dichloride is required because of their application in high temperature polymerization process. In the present study, the thermal stability of the bis(n-butyl cyclopentadienyl) zirconium dichloride and bis(t-butyl cyclopentadienyl) zirconium dichloride was determined by differential scanning calorimetry (DSC) and simultaneous thermogravimetry-differential thermal analysis (TG-DTA) techniques. The results of TG analysis revealed that the main thermal degradation for the bis(n-butyl cyclopentadienyl) zirconium dichloride and bis(t-butyl cyclopentadienyl) zirconium dichloride occurs in the temperature ranges of 194–360 °C and 195–350 °C, respectively. On the other hand, TG-DTA analysis indicated that bis(n-butyl cyclopentadienyl) zirconium dichloride melts (about 98.7 °C) before it decomposes. However, the thermal decomposition of the bis(t-butyl cyclopentadienyl) zirconium dichloride was started simultaneously with its melting. Also, the kinetic parameters such as activation energy and frequency factor for both compounds were obtained from the DSC data by non-isothermal methods proposed by Kissinger and Ozawa. Based on the values of activation energy obtained by Kissinger and Ozawa methods, the following order for the thermal stability was noticed: bis(t-butyl cyclopentadienyl) zirconium dichloride >bis(n-butyl cyclopentadienyl) zirconium dichloride. Finally, the values of ΔS^#, ΔH^# and ΔG^# of their decomposition reaction were calculated.     

The non-isothermal kinetics analysis of the thermal decomposition of kaolinite by Effluent Gas Analysis technique

The thermal decomposition of kaolin with high-content of the medium ordered kaolinite was studied by Effluent Gas Analysis (EGA) under non-isothermal conditions. This technique enables to distinguish two overlaying processes during the thermal decomposition of kaolin: oxidation of organic compounds and dehydroxylation. The kinetic of non-isothermal dehydroxylation of kaolinite is controlled by the rate of the third-order reaction. For the given reaction mechanism, the overall activation energy (E_A) and pre-exponential (frequency) factor (A) values are 242 kJ mol⁻¹ and 2.21 × 10⁸ s⁻¹, respectively.     

Mechanism and kinetics of combustion-carbothermal synthesis of AlN nanopowders

In this work, the carbothermal reduction-nitridation process of low-temperature combustion synthesis (LCS) (Al₂O₃+C) precursor was investigated in detail. Compared with conventional precursor, the LCS precursor possesses many advantages such as amorphous structure, nanosized particles, homogeneous mixing at molecular level. The experimental results indicate that the methods for preparing precursor exert great influence on phase transformation of Al₂O₃, onset temperature of nitridation and reaction activity. During the calcination, the phase transformation of Al₂O₃ is hindered by a large amount of surrounding C particles rendering Al₂O₃ maintains high reactivity. Accordingly, the nitridation reaction initiates at 1300 °C and completes at 1500 °C for 2 h. Furthermore, the reaction mechanism was also discussed on the basis of experiments. More significantly, it is established that the activation energy of carbothermal reduction-nitridation reaction using LCS precursor is Eα=336 KJ/mol.     

Synthesis of nanocrystalline 8 mol% yttria stabilized zirconia powder from sucrose derived organic precursors

An organic precursor synthesis of 8 mol% yttria stabilized zirconia (YSZ) powder from Zr–Y composite nitrate solution and sucrose has been studied. Oxidation of sucrose in Zr–Y composite nitrate solution containing excess nitric acid in situ generates hydroxy carboxylic acids that forms a white sol which showed peaks at 1640 cm⁻¹ and 1363 cm⁻¹ in IR spectrum corresponding to hydroxy carboxylic acid complexes of Zr and Y. Precursor mass obtained by drying the sol on calcinations at 600 °C produced loosely agglomerated particles of cubic YSZ. Deagglomerated YSZ contain submicron particles with D₅₀ value of 0.5 μm and the particles are aggregates of nanocrystallites of nearly 10 nm size. Compacts prepared by pressing the YSZ powder sintered to 96.7% TD at 1450 °C. The sintered YSZ ceramic showed an average grain size of 2.2 μm.     

Synergetic effect of NiO and SiO2 on the sintering and properties of 8 mol% yttria-stabilized zirconia electrolytes

Yttria-doped zirconia electrolytes (e.g., 8 mol% yttria-stabilized ZrO₂, 8YSZ) have been considered to be the most promising candidates for applications in solid oxide fuel cells (SOFC). Due to the ubiquitous presence of SiO₂ impurities and wide use of Ni-containing anodes, it is therefore of great technical importance to understand the synergetic effect of NiO and SiO₂ on densification, grain growth and ionic conductivities (especially the grain boundary (GB) conduction) of zirconia electrolytes. In this study, three groups of 8YSZ ceramics, with Si contents of ∼30, ∼500 and ∼3000 ppm, have been designed. 1 at% NiO was added into these materials by a wet chemical method. The addition of SiO₂ has a negative effect on the sintering and densification, while the introduction of 1 at% NiO reduced the sintering temperature and promotes grain growth of the zirconia ceramics. However, the presence of small amount of NiO prevented full densification of 8YSZ ceramics. NiO also led to a decrease by ∼33% in grain interior (GI) conductivity, with little effect on the GB conduction of high-purity 8YSZ (∼30 ppm SiO₂). However, the coexistence of NiO and SiO₂ is extremely detrimental to total conductivity by significantly reducing the GB conduction. Moreover, it is observed that, unlike the 8YSZ-doped SiO₂ with only, whose GB conduction increases greatly with increasing sintering temperature, the GB conduction of the NiO and SiO₂ codoped samples is less sensitive to sintering temperature.     

Densification of fine-grained alumina ceramics doped by magnesia,yttria and zirconia evaluated by two different sintering models

The influence of various dopants (500 ppm MgO and Y₂O_3; 250 ppm ZrO₂) on sintering of fine-grained alumina ceramics was evaluated by high-temperature dilatometry. The apparent activation energy of sintering was estimated with the help of Master Sintering Curve and a model proposed by Wang and Raj. The densification kinetics was controlled by at least two mechanisms operating at low (higher activation energy) and high (lower activation energy) densities. Good agreement between the activation energies calculated with both models was observed for low as well as for high densities. The lowest value of activation energy exhibited undoped alumina; the addition of MgO resulted in slight increase of the activation energy. Y₂O₃ and ZrO₂ significantly inhibited the densification, which was reflected in the higher sintering activation energies. The low activation energies in the final sintering step indicates the importance of proper choice of sintering temperature, namely in the two-step sintering process.     

Microstructure and permeability of porous zirconia ceramic foams prepared via direct foaming with mixed surfactants

The permeability performance of porous ceramics from particle-stabilized foams is closely associated with the connectivity between bubble-evolved pores. In order to regulate the connectivity of pore structure, the zirconia ceramic foams were fabricated by direct foaming with mixed surfactants of cetyltrimethyl ammonium bromide (CTAB) and sodium N-lauroyl sarcosinate (SLS). Different solid loadings and CTAB:SLS mixing ratios were used in this study. The results indicate that the pore structures of zirconia ceramic foams were interconnected by open windows on the cell walls, and the porosity and average size of cell and cell windows could be tailored by adjusting the solid loading and CTAB:SLS mixing ratio. The decrease in solid loading and CTAB:SLS mixing ratio caused larger porosity and size of cells and cell windows, and thus resulted in the obvious augment of Darcian (k₁) and non-Darcian (k₂) permeability constants. The ranges of k₁ and k₂ of the as-fabricated zirconia ceramic foams are 6.92 × 10^?13-4.05 × 10^?10 m² and 2.09 × 10^?5-3.19 × 10^?9 m respectively.     

Synthesis,characterization and sinterablity of 10 mol% Sm2O3-doped CeO2 nanopowders via carbonate precipitation

A carbonate coprecipitation method has been used for the facile synthesis of highly reactive 10 mol% Sm₂O₃-doped CeO₂ (20SDC) nanopowders, employing nitrates as the starting salts and ammonium hydrogen carbonate (AHC) as the precipitant. The AHC/RE³⁺ (RE = Ce + Sm) molar ratio (R) and the reaction temperature (T) affect significantly the final yield and precursor properties, including chemical composition and particle morphology. Suitable processing conditions are T = 60 °C and R = 5.0–10, under which precipitation is complete and the resultant precursors show ultrafine particle size, spherical particle shape, and good dispersion. Thus, the processed precursors are rare-earth carbonates with an approximate formula of Ce_0.8Sm_0.2(CO₃)_1.5·1.8H₂O, which directly yield oxide solid-solutions upon thermal decomposition at a low temperature of ∼440 °C. The 20SDC solid solution powders calcined at 700 °C show excellent reactivity and have been densified to ∼99% of the theoretical via pressureless sintering at a very low temperature of 1200 °C for 4 h.     

Effect of DC and AC electric fields on crack healing behavior in 8 mol% yttria-stabilized cubic zirconia polycrystal

The effect of the flash event (FE) on microcrack healing behavior in 8 mol% yttria-stabilized zirconia was examined at healing temperatures of 1040 and 1230°C under the direct and alternating (DC and AC) electric fields. The crack healing behavior changed depending on the factors of the electric field, healing temperature, and crack length. Although the crack healing proceeded with the temperature, the healing rate increased with the crack length, suggesting that the external energy stored as crack surface energy would provide a driving force for the crack healing. Although the crack healing occurs even under the static annealing without the electric field, the healing rate was accelerated by FE significantly more under the AC field than under the DC field. The microcracks with a length of ≈20 μm were fully healed at 1230°C only for 10 min by the FE treatment under the AC field, and the flash healing behavior was four times faster than that of the static annealing. These results suggest that the enhanced healing behavior cannot be explained only by thermal effects, and the accelerated diffusivity caused additionally by nonthermal effect under FE might contribute to the enhanced healing behavior, especially in the AC electric field.     

Thermal degradation kinetics of polyethylene and silane‐crosslinked polyethylene

The thermal degradation of linear low‐density polyethylene (LLDPE) and linear low‐density silane‐crosslinked polyethylene (SXLPE) was studied. Kinetic evaluations were performed by model‐free kinetic analysis and multivariate nonlinear regression. Apparent kinetic parameters for the overall degradation were determined. The results show that the thermal stability of SXLPE was higher than that of LLDPE. Their decomposition reaction model was a single‐step process of an nth‐order reaction. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1172–1179, 2005