Zirconia Powders Research Articles

Study of the densification and grain growth mechanisms occurring during spark plasma sintering of different submicronic yttria-stabilized zirconia powders

Densification and grain growth mechanisms of Yttria-Stabilized Zirconia sintered by Spark Plasma Sintering are investigated. Sintering trajectories of four commercial submicronic powders with different average particle sizes and yttria amounts have been established and sintering regimes determined. Densification mechanisms are determined in the regime where densification is occurring without grain growth using a model derived from hot-pressing. Grain growth mechanisms are determined using the conventional power law in the regime where ceramics are fully densified. Densification occurs by grain boundary sliding accommodated by an in-series interface-reaction/lattice diffusion of cations or by an overlapping of surface diffusion and grain boundary sliding mechanisms for tetragonal stabilized zirconia and by dislocation climbing for fully stabilized zirconia. A normal grain growth occurs for each ceramic, all composed of a single phase, contrary to the two-phased ceramics obtained in literature where grain growth occurs by segregation at grain boundaries.

Controllable synthesis of pure monodispersed zirconia nanopowders with tetragonal phase

AbstractNanopowders of pure zirconia have been synthesized using citric acid (CA)‐assisted lamellar liquid crystal template method. The microstructure of the zirconia powder prepared at the different mole ratios of CA to zirconium oxynitrate (ZN) was characterized by FT‐IR, X‐ray diffraction (XRD), laser particle size analyzer, Raman spectroscopy, and scanning electron microscope (SEM) methods. The phase structure of the sodium dodecyl sulfate (SDS)/C10H21OH/H2O system before and after adding mixing solution CA and ZN was determined by POM (Polarizing Optical Microscope). The results show that lamellar structure of the SDS/C10H21OH/H2O system after adding mixing solution CA and ZN is stable. The presence of CA inhibits agglomeration and growth of zirconia particle. The crystallite size of zirconia powders decreases and agglomerates lowly with addition of CA. Fourier transform infrared spectrometry (FI‐IR) analyses reveal that the structure of chelating organic complex is maintained in zirconia structure at high‐temperature calcination to cause oxygen vacancies which stabilizes the tetragonal phase of zirconia. The zirconia powders remained the single metastable tetragonal phase at the molar ratios of CA to ZN ranging from 1:3 to 5:1. The crystallite size of zirconia with spherical morphology varied from 32.2 to 20.1 nm with the increase of the molar ratio of CA to ZN in the range of 1:3 to 5:1.

Preparation of Nano-ZrO2 powder via a microwave-assisted hydrothermal method

ZrO2 nanocrystals were synthesised by a microwave-assisted hydrothermal method using zirconium oxychloride (ZrOCl2·8H2O), yttrium chloride (YCl3·6H2O), and liquor ammonia (NH3·H2O) as raw materials, triethanolamine (TEOA) as mineraliser, and polyethylene glycol (PEG) as dispersant. The obtained products were characterised with thermogravimetry-differential scanning calorimetry (TG-DSC), Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that the concentration of ZrOCl2·8H2O had little effect on the material properties, whereas the PEG molecular weight, microwave hydrothermal time and temperature, and the concentration of TEOA greatly influenced the dispersibility of the nano-sized zirconia powders. XRD and FT-IR analyses indicated that the ZrO2 nanocrystals synthesised by the microwave hydrothermal method had a tetragonal phase without any trace of monoclinic or cubic phases. The optimal parameters for preparing nano-zirconia powders with appreciable crystallinity and crystal forms included the use of PEG1000/PEG2000/PEG4000 dispersants, a microwave hydrothermal time of 30–50 min and a temperature of 200–240 °C, and a TEOA concentration of 0.3–0.5 M. Nano-ZrO2 powder prepared via our optimised microwave hydrothermal method contained mostly tetrahedral, spherically shaped, highly homogeneous, and well-dispersed 20–30 nm particles.

The influence of annealing temperature on the structure and optical properties of Gd2Zr2O7 nanostructures prepared by chemical bath deposition method

Gadolinium zirconate (Gd2Zr2O7) powders were prepared for the first time using chemical bath deposition (CBD) method. In this study, the chemical bath deposition (CBD) method was employed due to its simplicity, cheap, low temperature, versatile and it gives better homogeneity. The effect of annealing temperature (Ta) on the thermal, structural, morphological, and optical properties were investigated. Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA) showed that the material is thermally stable between 400 and 900 °C. The X-ray diffraction (XRD) results confirmed the cubic Gd2Zr2O7 phase with hexagonal ZnO secondary peaks. Scanning electron microscopy (SEM) results revealed the transformation of irregular particles to hexagonal-like structure shape with an increase in Ta. Energy dispersive x-ray spectroscopy (EDS) analysis confirmed the presence of the expected elements. Transmission electron microscopy (TEM) confirmed the nano-nature of the prepared powders. Ultraviolet-visible spectroscopy (UV-Vis) spectra showed a slightly red shift with an increase in Ta. The photoluminescence (PL) results revealed that the emission intensity and peak position in UV region highly depends on the Ta. It was also noted that both secondary peaks from hexagonal ZnO and Ta regulates the defects emission. The international illumination (CIE) color chromaticity showed that the color shifts from blue to yellow-greenish with an increase in Ta. The approach in the present study opened a new way to tune band gap and luminescence intensity of Gd2Zr2O7.

Повышение свойств плазменных теплозащитных покрытий за счет напыления материалов, реагирующих с экзотермическими эффектами

The quality of heat-resistant coatings deposited by flame spraying is largely determined by the adhesion of the coating to the surface of the part. One of the ways to increase adhesion is to deposition intermediate layers of thermosetting powders between the base material and the coating. In this work, two versions of heat-protective coatings are investigated — a two-layer coating consisting of an Al – Ni sublayer (20 – 80 wt. %) and a main ZrO2 layer, and a single-layer coating sprayed from an aluminum-clad zirconium oxide powder (20 ZrO2 – 80 Al, wt. % ). The method of differential thermal analysis was used to determine the temperature ranges and values of the exothermic effects of oxidation and redox reactions during heating of Al – Ni and ZrO2 clad powders. A significant exothermic effect was found during oxidation of the aluminum cladding shell in the temperature range of 360 °C and a stronger thermal effect due to the redox reaction at a temperature of 920 °C. The microstructure and microhardness of the obtained coatings have been studied, and their thermal conductivity and adhesion have been assessed. The resistance of the coatings during thermal cycling tests has been determined. It has been established that thermal protective coatings made of aluminum-clad zirconium oxide powder have the best characteristics under thermal cycling conditions. A higher level of adhesion and thermal cyclic resistance of such coatings are due to an increase in the enthalpy of aluminum-clad ZrO2 powders due to exothermic reactions and the presence of a metal binder.

Evaluation of microstructure and mechanical properties of PMMA Matrix composites reinforced with residual YSZ from CAD/CAM Milling process

Background: Reinforcement of dental acrylics with fillers has yielded positive results. Different proportions of fillers have been added to strengthen the dental acrylics, but no consensus has been reached. Aim: The purpose of this study was to investigate the effects of the addition of different concentrations of yttria-stabilized zirconium (YSZ) obtained from the residues generated from the CAD-CAM milling of YSZ on the microstructure and mechanical properties of poly-methyl-methacrylate (PMMA)-YSZ composites. Materials and methods: Composite materials with different amounts (0.0 to 70.0% by weight) of recycled YSZ reinforced PMMA resin matrix were produced. Scanning electron microscope (SEM), energy dispersive electron spectrometer (EDS) and Fourier Transform Infrared Spectrometer (FTIR) were used for microstructural analysis. Among the mechanical properties, the Vickers microhardness test method for hardness, 2D profilometer for surface roughness and composite densities were evaluated by Archimedes method. Data were analyzed using a one-way analysis of variance (ANOVA) at a pre-set alpha of 0.05. Results: Microhardness and density increased until 60% by weight YSZ addition, while surface roughness remained unchanged but increased after 60% by weight YSZ addition. The addition of more than 60% by weight of YSZ caused agglomeration in the microstructures. The mechanical properties of poly-methyl-methacrylate decreased with more than 60% YSZ by weight. Conclusion: Reinforcement of PMMA with residue zirconia powder will increase the usage chance of residue YSZ powder and provide a safer use of PMMA.

Dynamic mechanical characters of PEG/zircon composites around PEG glassy region

Abstract Polyethylene glycol (PEG)/zircon (ZrSiO4) composites with various zircon sizes have been fabricated using a liquid method to study the effect of crystallite size on the thermo-mechanical properties of the composites around the glass temperature of PEG. The zircon powders were prepared by different heat treatment, i.e., 500, 1000, and 1200 °C, to acquire different crystallite size. The characterization included XRD, FTIR, and DMA. Results showed that the powders exhibited pure zircon with crystallite size of 226, 321, and 391 nm. Meanwhile, analyses of the FTIR and XRD data confirmed the successful synthesis of the composites. DMA analysis revealed the storage moduli, loss moduli, and tan delta characters of the PEG/zircon composites. The maximum storage moduli and glass transition temperature (Tg) were achieved by the 5 wt% zircon calcined at 500 °C (LPZ500), while the maximum loss moduli were achieved by the 5 wt% zircon calcined at 1200 °C (LPZ1200).

Combustion of metal powder with dinitrogen tetroxide

Here we present analysis of a novel reactive material system that employs dinitrogen tetroxide (N 2 O 4 ) as a liquid oxidizer with metal powder fuels. The oxidizer was added to micron scale aluminum and zirconium powders by a remote injection system. When ignited with a high voltage spark, the mixtures were observed to possess reactivity comparable to nanocomposite reactive materials, with open-tube flame expansion velocities from 500 to 1400 m/s depending on fuel/oxidizer ratio and tube diameter. Temperatures were observed to range from 3000 to 3500 K as measured with gray body fits to 16-channel time-resolved pyrometer and time-integrated spectrometer data. These values were significantly below calculated adiabatic flame temperatures, which we attribute to local deviations from stoichiometry and kinetic/energetic limitations similar to those observed in studies of particles burning in high pressures gaseous oxidizers. Al/N 2 O 4 reactivity was found to be most likely limited by the vaporization of the metal from the particle surface and Zr/N 2 O 4 was limited by slower burning and complex interactions involving the solubility of nitrogen and oxygen in the molten Zr. We also discuss the potential for these materials to be used to create an “on/off” reactive material, since N 2 O 4 can be added remotely and driven to evaporate via vacuum or purge of inert gases to return it to a safe condition.

In situ and tunable structuring of semiconductor-in-glass transparent composite.

SummarySemiconductor-in-glass composites are an exciting class of photonic materials for various fundamental applications. The significant challenge is the scalable elaboration of composite with the desirable combination of tunable structure, high semiconductor loading ratio, and excellent transparency. Here we report that the topological engineering strategy via hybridization of the glass network former enables to surmount the aforementioned challenge. It not only facilitates the in situ precipitation of (Ga2-xAlx)O3 domains with continuously tunable composition but also allows to simultaneously refine the grain size and enhance the crystallinity. In addition, the composites exhibit excellent transparency and can host various active dopants. We demonstrate the attractive broadband optical response of the composite and achieve the pulse laser operation in mid-infrared waveband. The findings are expected to provide a fundamental principle of in situ modification in hybrid system for generation of high-performance semiconductor-in-glass composites.

Manipulation of Lewis acid–base complexation in the Zr-centered quaternary system for monodisperse spherical zircons

Zircon (ZrSiO4) is a distinctive material widely investigated and applied for its superb physicochemical advantages. To date, great challenges remain in the synthesis of high-quality zircon powders for their stringent crystallization requirements at elevated temperature and inherent preferential growth along the [200] direction. Herein we, for the first time, present monodisperse spherical zircon (MSZ) powders synthesized via a facile one-step hydrothermolysis at 200 °C by rationally manipulating the Lewis acid–base complexation of zirconium with competitive ions (Lewis basic R ligands) in a Zr–OH–R–F quaternary system. Systematical investigations were revealed encircling the complexation of Zr4+ with the R ligands from various sources, which involve the adjusting acids (HR), Zr/Si reactants and halogen-containing mineralizers. Sufficient acidity from the adjusting acid and weak enough Zr–R affinity are confirmed as two paramount factors modulating the crystallization and growth of high-quality MSZ crystals, which undergo a unique metamorphic morphological transition from monolayered, to multilayered, to donut-shaped and ultimately to micrometer-sized spherical particles. The hydrothermal zircons are first confirmed as Zr-deficient (Zr/Si ratios <1.0) zirconium silicates as (ZrO4)1–xSi[(OH)1–y·Ry]4x, rather than the Si-deficient Zr(SiO4)1–x[(OH)·R]4x as initially proposed and hitherto generally accepted. The MSZ powders exhibit fair thermal stability and terrific monodispersity even after calcination at 1100 °C, superior photoreflectance and much lower thermal conductivity (0.18–0.48 W m−1 K−1) over non-spherical zircons. The novel MSZ powders provide arresting prospects to extend the applications of zircon in many traditional and emerging fields.

Development of Process for Producing Zirconium Powder of Nuclear Purity

Development of Process for Producing Zirconium Powder of Nuclear Purity

Ignition of zirconium powders placed near an electrostatic discharge

Abstract Electrostatic discharge (ESD) or spark produced by breakdown of a gap while discharging a capacitor is a common ignition stimulus for powders of reactive and energetic materials. It is a safety hazard, but is also used to initiate pyrotechnic formulations and as a characterization method of reactivity of energetic materials. In previous experimental studies, the powder served as one of the ESD electrodes. Ignition was found to be caused by Joule heating, with the main heat release by current passing through points of contact between particles. However, ignition may occur when the ESD does not pass directly through the powder. ESD generates a shock wave, which can lift the powder from a substrate even if the substrate is not an electrode. ESD also generates a plasma kernel heated to ca. 10,000 K, which exists for hundreds of µs, often much longer than the ESD current. Here, the effect of the ESD-induced shock and plasma on a reactive powder was studied experimentally for the first time. A thin layer of Zr powder was placed on a substrate located under a spark gap between two pin electrodes. Photo-sensors and high-speed video were used to document ignition and combustion events. The ESD energy and distance from the spark gap and powder were varied. At close distances, particles ignited consistently. Distance limits were determined, beyond which ignition was no longer observed. Higher ESD energies as well as longer ESD pulses led to ignition at greater distances. Ignition involved two distinct processes. First, submicron particles were lifted from the substrate on a time scale of single microseconds, interacted directly with the plasma kernel, and combusted. Optical emissions from this combustion lasted 100 – 600 µs, as expected for the burn time of submicron Zr particles in air. The second ignition event occurred several milliseconds later. It involved a flame propagating through Zr powder aerosol or combustion of individual lifted particles. This event was caused by coarser Zr particles radiatively heated during ESD. Importantly, the powder was aerosolized by convective flows caused by the rising plasma kernel making it possible for the self-heating of particles oxidizing in air to eventually lead to their ignition and combustion. Observed times for these delayed ignition and combustion events were in the order of tens of milliseconds.

Effect of magnesia rich spinel on densification and stabilization behavior of monoclinic zirconia

Fully cubic-stabilized zirconia ceramic composites have been successfully fabricated by conventional sintering technique using commercial monoclinic zirconia, Yttrium oxide and waste-derived magnesia-rich spinel (MMA) powder mixtures. In this study, effect of MMA content and sintering temperature on stabilization and densification properties of zirconia has been duly considered. The obtained results showed that m-ZrO2 in MMA-free Z0 reference specimen is partially stabilized upon temperature rising into tetragonal phase by Y3+ ions diffusion inside zirconia structure. MMA-free Z0 reference specimen sintered at 1600 oC showed m- and t-ZrO2 dual-phase structure with a relative density of 80.2%. Unlike, upon rising the sintering temperature, Z10–Z50 composites containing 10–50 wt% MMA demonstrated higher relative density of more than 99% and showed variant behavior, where their m-ZrO2 is transformed and stabilized into cubic form by diffusion of Y3+, Mg2+ and Al3+ ions inside zirconia lattice structure. The outcomes indicate that MMA has significantly improved both the densification and stabilization behavior of m-ZrO2 through facilitating Y3+ diffusion inside zirconia lattice structure.

YSZ-rGO composite ceramics by spark plasma sintering: The relation between thermal evolution of conductivity, microstructure and phase stability

Despite a high demand for green energy sources, low performance of solid oxide fuel cells (SOFC) hinders the potential for its practical application. Zirconia-graphene composites are promising candidates as solid electrolytes for SOFC enabling efficient heat transfer and effective ion charge carriers movement. Herein, we report the change in electrochemical behavior of spark plasma sintered zirconia-reduced graphene oxide (YSZ-rGO) composites. The conductivity is shown to be altered from fully ionic to mixed electronic-ionic mode upon a small amount of rGO addition. Mixture of freeze-dried nano-sized yttria stabilized cubic zirconia (YSZ) powder and microwave sintered reduced graphene oxide (rGO) was utilized for the fabrication of YSZ-rGO ceramic composites. According to SEM and EBSD data, addition of rGO resulted in a bi-modal grain size distribution. The material with 2 wt.% rGO has demonstrated a mixed electron-ion conductivity in a wide temperature range of 294–1073 K in thermal cycling experiments at oxygen partial pressure of 10−3–10−5 atm, where oxygen ion conductivity dominates over electronic one at elevated temperatures. The electrochemical stability of YSZ-2 wt.% rGO was tested by thermal cycling in nitrogen and argon atmospheres via impedance spectroscopy at 473–1073 K. The model to describe the conductivity evolution with temperature and thermal cycling has been suggested for the first time.

Electrochemical Reduction of ZrO2 in Molten Electrolytes

The results of the electrochemical reduction of zirconium dioxide in molten electrolytes based on calcium or magnesium chloride on a liquid gallium cathode, bypassing the stage of granulation and sintering with carbohydrates, are presented. The liquid gallium cathode provides not only reliable contact with zirconium dioxide, but also favourable conditions for its reduction. The contact area of the gallium cathode with fine oxide powder is much larger than the contact area of the granulated and sintered zirconium dioxide with a solid conductor. This ensures a more uniform cathode polarization. Due to the lower specific mass, zirconium dioxide is located on the surface of gallium cathode, the convective movement of which provides more intense mass transfer at the interface of the phases and removal of recovery products from the zone of electrode reaction. Products of electrolysis under such conditions do not block neither the surface of zirconium dioxide nor the surface of the cathode. Zirconium, which is formed during the renewal, due to a larger specific mass, precipitates to the bottom of the electrolyser, and the layer of gallium protects it from interaction with the components of the molten electrolyte. In addition, due to the formation of alloys, the reduction of metal cations on liquid cathodes proceeds at more positive potentials than on solid indifferent cathodes, which reduces the specific energy consumption by electrolysis. The results of voltammetric studies confirm this conclusion.The reduction product is fine-grained zirconium powder with an average particle size of 1—3 microns, and purity of 99.9 %. As the density of the current increases, the value of the specific surface of the powder, the specific volume of the micropore and their average radius decrease. The degree of extraction depends on the composition of the electrolyte mixture and naturally decreases when replacing cations in the melts both on the basis of calcium chloride and on the basis of magnesium chloride in the following sequence Na+ > K+ > Li+. The melt based on compounds of calcium and sodium chloride provides the best performance. The removal degree of zirconium from such melt reaches 77 %.

Ferroelastic and plastic behaviors in pseudo-single crystal micropillars of nontransformable tetragonal zirconia

The orientation-dependent micromechanical properties of nontransformable tetragonal (t’) zirconia, which underwent a diffusionless transformation from the fluorite cubic phase and does not exhibit a stress-induced phase transformation, were characterized via pseudo-single crystal micropillar compression and electron microscopy. The t’ zirconia sample was obtained via atmospheric plasma spraying of 4.5 mol% yttria-stabilized zirconia (YSZ) powders into liquid nitrogen and consolidated into a bulk state via hot pressing at 1100°C. Dense and cylindrical micropillars were fabricated using a focused ion beam from pseudo-single crystalline regions, which exhibited a nanodomain microstructure of three t’ variants partitioned by {1 0 1}c twin boundaries with 90° symmetry. These micropillars were compressed using a flat-end diamond indenter. Near-<0 0 1>c compressions were attributed to ferroelastic domain switching and subsequent {1 0 1}c and/or {1 1 1}c hard slips. In ferroelastic deformation, a certain t’ variant diminished, and a binary domain microstructure developed with c axes perpendicular to the compressive direction. Near-<1 1 1>c compressions were governed by {0 0 1}c soft slips accompanied by strain hardening with negligible ferroelasticity, which resulted in buckling deformation with rotational kinking. In both the hard- and soft-slip orientations, ferroelastic toughening was observed with certain t’ variants awaken around the crack tips. Contrarily, cleavage fractures subsequent to yielding were observed in near-<1 0 1>c compressions. In the cubic counterpart with a domain-free microstructure (8.0 mol% YSZ), ferroelastic toughening was not observed. Hence, it is viewed as the origin of enhanced toughness in t’ zirconia.

Crystal defects and phase transitions of nanocrystalline yttria-stabilised zirconia induced by high-energy ball milling

Yttria-stabilised zirconia (YSZ) is a promising electrolyte for SOFCs and gas sensors. In this study, the particle size of a co-precipitated 5 mol% yttria-stabilised zirconia (5 YSZ) powder was refined from 10.47 μm to 130 nm via high-energy ball milling to improve its sinterability and ionic conductivity. The ball milling process increased the specific surface area of the 5 YSZ powder from approximately 11 to 22 m2 g−1. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) results indicated that the 5 YSZ crystallites decomposed into irregular shapes with the evolution of point, linear, and planar defects. An increase in the milling duration increased the number of oxygen defects in the 5 YSZ powder, as revealed by the X-ray photoelectron spectroscopy results. The tetragonal-to-monoclinic phase transformation occurring in the powder was investigated by X-ray diffraction, Raman spectroscopy, HRTEM, and selected-area electron diffraction pattern analyses. The ball-milled powders could be easily densified, but the presence of too many crystal defects and the large fraction of the m-ZrO2 phase were detrimental to the further densification of the 5 YSZ powders. In spite of the high sintering temperature (1500 °C) used in this study, the maximum relative density of 99.67% could be achieved for the powder ball-milled for 60 min at the rotor speed of 1500 rpm. Moreover, the ionic conductivity of 5 YSZ was improved significantly from 20.6 to 36.2 mS cm−1 (850 °C) after the high-energy ball milling process.

Influence of Process Parameters on the Aerosol Deposition (AD) of Yttria-Stabilized Zirconia Particles

Aerosol deposition (AD) is a novel deposition process for the fabrication of dense and rather thick oxide films at room temperature. The bonding of the deposited ceramic particles is based on a shock-loading consolidation, resulting from the impact of the ceramic particles on the substrate. However, the deposition mechanism is not fully understood. In addition, many technical challenges have been observed for achieving a successful deposition of the oxides with higher efficiency. In this work, the influence of different processing parameters on the properties of the deposited layer is studied. Proof of concept was done using 8 mol.% yttria-stabilized zirconia (8YSZ) powder as starting material. The window of deposition with respect to carrier gas flows for successful deposition was identified. The influence of this carrier gas flow, the substrate materials and the carrier gas species on the coating thickness, interface quality and coating microstructure was systematically investigated. The derived mechanical characteristics revealed an unexpected behavior related to a gradient microstructure. This study supports understanding of the mechanism of room-temperature impact consolidation and its effect on the mechanical properties of the deposited layer.

Synthesis and characterization of an experimental 3Y-TZP dental ceramic prepared by polymeric precursors method

The aim of the investigation was to synthesize 3 mol% yttria-stabilized zirconia (3Y-TZP) powders via polymeric precursor method (PPM). The precursor solution was preheated at 350ºC for 3h, subsequently thermally treated at 500ºC for 3h and 800ºC for 6h. The obtained materials were analyzed by Thermogravimetry-Derivative Thermogravimetry (TG/DTG), Differential Thermal Analysis (DTA), powder X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Two commercially available Y-TZP ceramic systems were chosen for comparison. XRD analysis of the synthesized 3Y-TZP powders revealed the crystallization of the tetragonal phase, while both commercial systems showed the coexistence of the monoclinic and tetragonal phases. SEM analysis showed that the powders thermally treated at 800°C consist of agglomerated spherical nanoparticles. Morphology of commercial systems also revealed nanosized spherical particles. Results revealed that the PPM led to ceramics with structural and morphological properties comparable to commercially available reinforced dental ceramics.

Effect of alumina added by mechanical mixing and co-doping on the densification mechanisms of zirconia nanoparticles at the initial stage of sintering

The comparative analysis of the mechanical activation and Al2O3 doping effect on the densification behavior of tetragonal 3Y-TZP nanopowders has been carried out. Two types of zirconia nanopowders doped by alumina have been studied. Namely, nanopowders obtained by the mechanical mixing and co-precipitation methods. The behavior of nanopowders at initial stage of sintering was examined by the constant rate of heating technique.It was found that the volume diffusion mechanism, which is realized by sintering the 3Y-TZP nanopowders undoped and doped with a small amount of Al2O3 nanopowders obtained by co-precipitation, ceases to function when the Al2O3 content increases to 2 wt% and the Al2O3 inclusions block the diffusion flux of Zr4+ cations from the volume of ZrO2 particles to the necks. Annealing of the structure defects caused by milling of the 3Y-TZP and Tosoh TZ-3Y powders during preliminary heating leads to the realization of the volume diffusion mechanism instead of the grain boundary diffusion mechanism at the initial sintering stage in milled nanopowders. Therefore, the GBD mechanism in the milled zirconia powders caused by the stressed and defective state of zirconia nanoparticles, and not by size effects.It was shown that the Al2O3 particles added to zirconia nanopowders by mechanical mixing did not change the volume diffusion mechanism at the initial stage of sintering of 3Y-TZP NPs until the Al2O3 concentration did not blocked the diffusion of cation to the necks.It was proposed to separate the effects of mechanical activation of nanopowders from the influence of doping on the diffusion mechanism at the initial stage of sintering of doped zirconia nanopowders. The simultaneous influence of both impacts (mechanical energy and chemical composition) can complicate and confuse reasoning.