Nano-sized Zirconia Research Articles

Tetragonal Nanosized Zirconia: Hydrothermal Synthesis and Its Performance as a Promising Ceramic Reinforcement

In this study, we produced zirconia nanoparticles with a pure tetragonal phase, good dispersion, and an average particle size of approximately 7.3 nm using the modified hydrothermal method. Zirconium oxychloride (ZrOCl2-8H2O) was used as zirconium source, while propanetriol was used as an additive. The influence of propanetriol content, sonication time, hydrothermal temperature, and type of dispersant on the physical phase and dispersibility of zirconia nanoparticles was investigated. Monoclinic zirconia was found to completely transform into a tetragonal structure when the mass fraction of glycerol was increased to 5 wt%. With the increase in the mechanical stirring time under ultrasonic conditions, the size distribution range of the prepared particles became narrower and then wider, and the particle size became first smaller and then larger. Ultrasonic and mechanical stirring for 5 min had the best effect. When comparing the effects of different dispersants (PEG8000, PVP, and CTAB), it was found that the average particle size of zirconia nanoparticles prepared with 0.5 wt% PVP was the smallest. Furthermore, by adding different concentrations of pure tetragonal phase nanozirconia to 3Y-ZrO2 as reinforcement additives, the bending strength of the prepared ceramics increased first and then decreased with increasing addition amounts. When the amount of addition was 1 wt% and the ceramic was calcined at 1600 °C, the flexural strength of the ceramic increased significantly, which was about 1.6 times that of the unadded ceramic. The results are expected to provide a reference for the reinforcement of high-purity zirconia ceramics.

Expression of Interleukin-1β and Histological Changes of the Three-Dimensional Oral Mucosal Model in Response to Yttria-Stabilized Nanozirconia.

Over the years, advancement in ceramic-based dental restorative materials has led to the development of monolithic zirconia with increased translucency. The monolithic zirconia fabricated from nano-sized zirconia powders is shown to be superior in physical properties and more translucent for anterior dental restorations. Most in vitro studies on monolithic zirconia have focused mainly on the effect of surface treatment or the wear of the material, while the nanotoxicity of this material is yet to be explored. Hence, this research aimed to assess the biocompatibility of yttria-stabilized nanozirconia (3-YZP) on the three-dimensional oral mucosal models (3D-OMM). The 3D-OMMs were constructed using human gingival fibroblast (HGF) and immortalized human oral keratinocyte cell line (OKF6/TERT-2), co-cultured on an acellular dermal matrix. On day 12, the tissue models were exposed to 3-YZP (test) and inCoris TZI (IC) (reference material). The growth media were collected at 24 and 48 h of exposure to materials and assessed for IL-1β released. The 3D-OMMs were fixed with 10% formalin for the histopathological assessments. The concentration of the IL-1β was not statistically different between the two materials for 24 and 48 h of exposure (p = 0.892). Histologically, stratification of epithelial cells was formed without evidence of cytotoxic damage and the epithelial thickness measured was the same for all model tissues. The excellent biocompatibility of nanozirconia, as evidenced by the multiple endpoint analyses of the 3D-OMM, may indicate the potential of its clinical application as a restorative material.

Yttria-stabilized tetragonal zirconia prepared using gel casting and two-stage sintering

ABSTRACT Gel casting technology and a two-stage sintering process were used to produce yttria-stabilized tetragonal zirconia (YTZP) ceramics with excellent mechanical properties. A carbonic acid-based polyelectrolyte dispersant was used as a dispersant to prepare a well-dispersed nano-sized (94 nm) zirconia slurry with a high solid content (45 vol%) for gel casting. The functional carboxyl groups of the polyelectrolyte dispersant reacted with epoxy monomer (ethylene glycol diglycidyl ether, EGDGE) as it was added to the slurry, resulting in a decrease in gelling incubation time and an increase in viscosity, resulting in gel casting failure. Adding hydroquinone (HQ) to the slurry can cause polymerization to be delayed and the gel-casting working time to be extended. To control grain growth, a two-stage sintering process was used. The temperature for the first stage of sintering was set at 1300°C, then lowered to 1240°C and soaked for 24 h in the second stage to achieve a relative density of greater than 99%. The two-stage sintered sample has a Vickers hardness of 15.2 GPa, fracture toughness of 7.8 MPa.m1/2, and flexural strength of 771 MPa.

Transparent high-strength nanosized yttria stabilized zirconia obtained by pressure-less sintering

This work describes the development of transparent high-strength Yttria-Stabilized Zirconia (YSZ) ceramics with ultra-fine grain size utilizing conventional pressure-less densification. Starting with nanoparticles with diameter < 10 nm, it was possible to achieve full densification (>99.5% of theoretical density) at a sintering temperature of 1100–1200 °C. The average grain size of the resulting dense ceramics was 75 nm in 3 mol. % YSZ and 85 nm in 8 mol. % YSZ, showing in-line light transmission of 38% and 51% at a wavelength of 800 nm and average biaxial strength (piston on three balls test on samples of diameter 12 mm and thickness 1 mm) of 1980 MPa and 680 MPa, respectively. The nano-grained structure, absence of color centers, and miniaturization of residual pores enable the excellent light transmission. The high biaxial strength is attributed to the refined microstructure, but also to the martensitic tetragonal-to-monoclinic phase transformation that remains active even in nano-sized zirconia grains.

Electron Momentum Density of Nanoparticles ZrO2: A Compton Profile Study

This work investigates the electronic momentum density (EMD) distribution in nanosize zirconia (ZrO2) using the technique of Compton scattering. The ZrO2 nanoparticles (11.2[Formula: see text]nm) are synthesized of mechanical milling and characterized by SEM, XRD and TEM probes. The Compton profile [Formula: see text] of nanoZrO2 is measured by Compton spectrometer 59.54[Formula: see text]KeV Gamma rays (Americium-241) source. The study finds out that EMD in nanoZrO2 is narrower comparing in case bulk ZrO2. This study adopts the ionic-model-based free atom [Formula: see text] calculation for many configurations (Zr)[Formula: see text](O[Formula: see text])2 ([Formula: see text]) to measure the charge transfer (CT) on the compound formation. According to this study’s findings, CT values in these materials are ranged from 1.5 to 1.0 electrons from Zr to O atom.

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.

Direct Observation of Enhanced Raman Scattering on Nano-Sized ZrO2 Substrate: Charge-Transfer Contribution

Direct observation of the surface-enhanced Raman scattering (SERS) of molecules adsorbed on nano-sized zirconia (ZrO2) substrates was first reported without the need for the addition of metal particles. It was found that ZrO2 nanoparticles can exhibit unprecedented Raman signal enhancements on the order of 103 for the probe molecule 4-mercaptobenzoic acid (4-MBA). The dramatic effect of the calcination temperature on the ZrO2 nanoparticles was also investigated. The ZrO2 nanoparticles with the particle diameter of 10.5 nm, which were prepared by calcination at a temperature of 500°C, have the highest SERS activity. A comparison between the experimental and calculation results indicates that charge transfer (CT) effects dominate the surface enhancement. The plentiful surface state of ZrO2 active substrate that is beneficial to CT resonance occurs between molecules and ZrO2 to produce a SERS effect. The CT process depends, to a large extent, on the intrinsic properties of the modifying molecules and the surface properties of the ZrO2. This is a new SERS phenomenon for ZrO2 that will expand the application of ZrO2 to microanalysis and is beneficial for studying the basic properties of both ZrO2 and SERS.

Pulsed Current Activated Sintering of Nanostructured ZrO₂ and 3YSZ and Their Mechanical Properties.

As nanostructured materials possess high strength, high hardness, excellent ductility and toughness, undoubtedly, more attention has been paid for the application of nanomaterials. When conventional sintering processes are used to sinter nano-sized zirconia powders, concomitant grain growth leads to the destruction of the nanostructure. This focuses attention on consolidation methods in which grain growth can be eliminated or significantly reduced. To accomplish this, rapid sintering methods have been widely used to sinter nano-sized powders. Nanopowders of ZrO₂ and 3YSZ were fabricated and rapidly consolidated using high-energy ball milling and the pulsed current activated sintering (PCAS). The sintering behavior and crystallite size ZrO₂ and 3YSZ powders were evaluated. A nanostructured dense 3YSZ compact with a relative density of up to 99% was readily obtained within one min. The hardness and fracture toughness of ZrO₂ and 3YSZ sintered from powders milled for 10 h were 450 and 1160 kg/mm², 3 and 4.4 MPa·m1/2. Not only hardness but also fracture toughness of 3YSZ are higher than those of ZrO₂.

Reinforcement of dental resin composite via zirconium hydroxide coating and phosphate ester monomer conditioning of nano-zirconia fillers

ObjectivesThe present study aimed to evaluate effects of conditioning with the phosphate ester monomer 10-methacryloyloxydecyl dihydrogen phosphate (MDP), with and without precoating with zirconium hydroxide for nano-size zirconia fillers, on mechanical properties of dental resin composites. Materials and methodsNano-zirconia fillers coated with or without zirconium hydroxide [Zr(OH)4] were prepared. Transmission electron microscopy (TEM), Fourier infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to observe the coating and to characterize Zr(OH)4 coating on the zirconia filler surface. Zirconia fillers with or without Zr(OH)4 coating were conditioned with MDP and were subsequently used to prepare experimental resin composites. XPS was used to analyze the Zr-O-P bonds on the filler surface after MDP conditioning. Moreover, three-point bending strength and elastic modulus of prepared resin composites were measured, and Weibull analysis was performed. Resin composites without addition of zirconia fillers and the ones with addition of untreated or silane conditioned-zirconia fillers were set as controls. Cell counting kit (CCK)-8 was used to test cell cytotoxicity of these zirconia fillers-containing experimental resin composites. ResultsNano-zirconia fillers were coated with Zr(OH)4 through chemical deposition. FTIR and XPS analysis confirmed the increase of hydroxyl groups after Zr(OH)4 coating. XPS detected the highest contents of Zr-O-P bonds on MDP-conditioned zirconia fillers with pre-Zr(OH)4 coating, followed by MDP-conditioned zirconia fillers. Resin composite with added MDP-conditioned zirconia fillers with and without Zr(OH)4 coating exhibited greater three-point bending strength, elastic modulus values, and Weibull moduli. According to the cytotoxicity classification, resin composites containing experimental zirconia fillers were considered to have no significant cell cytotoxicity. ConclusionNano-zirconia fillers conditioned with MDP, with or without precoating with Zr(OH)4, improve the mechanical properties of resin composites, and are potentially safe for clinical use.

Theoretical analysis of experimental densification kinetics in final sintering stage of nano-sized zirconia

The experimental densification kinetics of 7.8 mol% Y2O3-stabilized zirconia was analyzed theoretically during isothermal sintering in the final stage. By taking concurrent grain growth into account, a possible value of the grain-size exponent n was examined. The Coble’s corner-pore model recognized widely was found not to be applicable for explaining the densification kinetics. The corner-pore model of n = 4 shows a significant divergence in the kinetics at different temperatures. Microstructural observation shows that most pores are not located at grain corners and have a size comparable to the surrounding grains. The observed pore structure is similar to the diffusive model where single pore is surrounded by dense body. The diffusive model combined with theoretical sintering stress predicts n = 1 or n = 2, which shows a good consistence to the measured densification kinetics. During sintering of nano-sized powder, it is found that the densification kinetics can be explained distinctively by the diffusive single-pore model.

Formation of nano-sized cubic zirconia by aqueous sol–gel route

Nanocrystalline cubic zirconia particles were synthesized from [Zr{OPri}4] (A), [Zr{OPri}3{ONC9H6}] (1) and [Zr{OPri}2{ONC9H6}2] (2) {where, ONC9H6 = 8-hydroxyquinoline}. Formation of all the precursors was confirmed by elemental analysis, FT-IR and NMR (1H and 13C). The thermal stability of complexes (1) and (2) has been studied by thermal gravimetric analysis. Zirconia samples (a), (b), and (c) were formed by aqueous sol–gel transformations of (A), (1) and (2), respectively. All the oxide samples were characterized for its phase, crystallite size, and strain present by XRD and TEM. Surface morphologies of all the samples were investigated by SEM. Selected area diffraction (SAED) of (b) and (c) was done to confirm the results of XRD. The absorption spectra of oxide samples, (a), (b), and (c) show energy band gap of 3.17, 4.10, and 4.13 eV, respectively.

Modification of glass ionomer cement by incorporating nanozirconia-hydroxyapatite-silica nano-powder composite by the one-pot technique for hardness and aesthetics improvement

Abstract Nanosize zirconia was incorporated into nanosilicahydroxyapatite powder at different percentages by using the one-pot technique. The synthesized powder was spatulated by adding commercial conventional glass ionomer powder at different percentages. Vickers hardness and colour tests were carried out on the modified glass ionomer cement (GIC). Scanning electron images of the nanopowder revealed that the one-pot synthesis was able to produce nanopowder with less agglomeration and uniform distribution. Fourier transform infrared spectroscopy confirmed the presence of functional groups corresponding to each element. The incorporation of nanozirconia-silica-hydroxyapatite to the GIC resulted in significant improvements to the hardness of conventional glass ionomer cement; the highest hardness recorded was ~ 79 HV (5.07) which is a ~ 54% improvement over that for GIC. The aesthetic features of the new composite also improved.

An investigation on ZrO2 nano-particle incorporation, surface properties and electrochemical corrosion behaviour of PEO coating formed on Cp-Ti

Plasma Electrolytic Oxidation technique was used to produce nano-sized zirconia incorporated ceramic coating over commercially pure titanium using electrolytes with varying conductivities. Electrolytes with 5g/l trisodium orthophosphate, 4g/l monoclinic zirconia nano-particle and varying amount of potassium hydroxide were used. The interrelationship between electrolyte conductivity, particle incorporation, surface features, corrosion behaviour and scratch resistance of the coatings were studied. The phase composition, surface morphology and surface roughness were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical profilometer. The electrochemical corrosion behaviour of the samples was analysed from potentiodynamic polarization behaviour, electrochemical impedance spectroscopy and equivalent circuit modelling in a Kokubo 7.4pH simulated body fluid. The scratch resistance of the coatings was analysed by performing a scratch test using a Rockwell C diamond indenter with progressive loading upto 50N. The coatings exhibited both inert and reactive incorporation of zirconia nano-particles. The coating prepared with 6g/l KOH had lower inert incorporation zirconia, higher porosity, higher corrosion current (icorr=1.43×10−4mA/cm2), and exhibited gross spallation and poor scratch resistance. The coating prepared with 4g/l of KOH had the least corrosion current (icorr=5.18×10−8mA/cm2).

CO2 selective hydrogenation to synthetic natural gas (SNG) over four nano-sized Ni/ZrO2 samples: ZrO2 crystalline phase & treatment impact

Two type zirconia (monoclinic and tetragonal phase ZrO2) carriers were synthesized via hydrothermal route, and nano-sized zirconia supported nickel catalysts were prepared by incipient impregnation then followed thermal treatment at 300°C to 500°C, for the CO2 selective hydrogenation to synthetic natural gas (SNG). The catalysts were characterized by XRD, CO2-TPD-MS, XPS, TPSR (CH4, CO2) techniques. For comparison, the catalyst NZ-W-400 (monoclinic) synthesized in water solvent exhibited a better catalytic activity than the catalyst NZ-M-400 (tetragonal) prepared in methanol solvent. The catalyst NZ-W-400 displayed more H2 absorbed sites, more basic sites and a lower temperature of initial CO2 activation. Then, the thermal treatment of monoclinic ZrO2 supported nickel precursor was manufactured at three temperature of 350, 400, 500°C. The TPSR experiments displayed that there were the lower temperature for CO2 activation and initial conversion (185°C) as well as the lower peak temperature of CH4 generation (318°C), for the catalyst calcined at 500°C. This sample contained the more basic sites and the higher catalytic activity, evidenced byCO2-TPD-MS and performance measurement. As for the NZ-W-350 sample, which exhibited the less basic sites and the lower catalytic activity, its initial temperature for CO2 activation and conversion was higher (214°C) as well as the higher peak temperature of CH4 formation (382°C).

Sintering and conductivity of nano-sized yttria-doped ZrO2 synthesized by a supercritical CO2-assisted sol-gel process

This study reports on the electrical characterization of nano-sized zirconia powders doped with 3 and 8mol% Yttria, referred to as 3Y-ZrO2 and 8Y-ZrO2, respectively. Powders were synthesized using a supercritical CO2 assisted sol-gel method to control the size and morphology of the powder particles. Spark plasma sintering (SPS) was used to sinter the powders and maintain a fine microstructure with small grain size. The microstructure and chemical composition of the resulting ceramics was evaluated by electron microscopy techniques, showing grain boundaries depleted of amorphous phase. Electric conductivity of the 8Y-ZrO2 pellets was determined by electrochemical impedance spectroscopy techniques, between 200 and 800°C, and correlated with the microstructure. The bulk electric response at 300°C is comparable to commercially available Y-ZrO2 compositions. Compared to conventionally prepared samples we obtain lower conductivity in the high temperature range, still values in this study are twice superior to 3Y-ZrO2 samples similarly prepared by SPS.

In vitro cell proliferation evaluation of porous nano-zirconia scaffolds with different porosity for bone tissue engineering

The selection of scaffold materials and the optimization of scaffold morphological and mechanical properties are critical for successful bone tissue engineering. We fabricated porous scaffolds of nano-sized zirconia using a replication technique. The study aimed to explore the relationship between porosity, pore size, mechanical strength, cell adhesion, and cell proliferation in the zirconia scaffolds. Macro- and micro-structures and compressive strength were comparatively tested. Beagle bone marrow stromal cells were seeded onto the scaffolds to evaluate cell seeding efficiency and cell proliferation profile over 14 d of incubation. The zirconia scaffolds presented a complex porous structure with good interconnectivity of pores. By increasing the sinter cycles, the porosity and pore size of the scaffolds decreased, with mean values ranging from 92.7–68.0% and 830–577 μm, respectively, accompanied by increased compressive strengths of 0.6–4.4 MPa. Cell seeding efficiency and cell proliferation over the first 7 d of incubation increased when the porosity decreased, with cell viability highest in the scaffold with a porosity of 75.2%. After 7 d of incubation, the cell proliferation increased when the porosity increased, highest in the scaffolds with a porosity of 92.7%. These results showed that the zirconia scaffold with a porosity of 75.2% possesses favorable mechanical and biological properties for future applications in bone tissue engineering.

Rheological characterization and shape control in gel-casting of nano-sized zirconia powders

In this study, a water soluble resin based gel system was developed incorporating nano-sized zirconia powders. The effects of the resin content and the solid loading on the rheological properties, density, mechanical properties and shape control have been investigated. All the slurries show typical shear-thinning behaviour. Higher resin-content leads to higher viscosity and larger green density. The density of the green component obtained from a slurry with 30wt% resin can reach 2.85g/cm3 (nearly 50% of the theoretical density). However, when the resin content is beyond 25wt%, the sintered density decreased. Higher density, solid-loading and applying external force are helpful to improve the shape control.

Tungsten oxides supported on nano-size zirconia for cyclic production of syngas and hydrogen by redox operations

For cyclic production of syngas and H2 by redox (methane reforming-water splitting) operations, samples of tungsten oxides supported on nano-size zirconia (WO3/n-ZrO2) were investigated at 1,223 and 1,273 K and compared with those on micron-size zirconia (WO3/µ-ZrO2). The reduction characteristics of WO3/n-ZrO2 observed in this study were consistent with those of WO3/µ-ZrO2 reported in the literature. Specifically, the reduction process comprised three stages, the syngas production rate decreased as WO3 content increased, and the overall degree of reduction gradually decreased with repeated cycles. However, there were differences due to the smaller particle size, namely, WO3/n-ZrO2 yielded a higher syngas production rate and a lower H2/(CO+CO2) ratio. In addition, the hydrogen yield by water splitting was significantly lower than the amount expected based on the overall degree of WO3 reduction. The H2/(CO+CO2) ratio also gradually decreased with repeated cycles. These results were mainly attributed to rapid sintering of WO3/n-ZrO2, which gradually began to resemble WO3/µ-ZrO2.

Effects of Mechanically Activated Milling and Calcination Process on the Phase Stability and Particle Morphology of Monoclinic Zirconia Synthesized by Hydrolysis of ZrOCl2 Solution

The purpose of this paper was to investigate the effect of a high-energy milling (HEM) process on the particle morphology and the correlation between a thermal treatment and tetragonal/monoclinic nanostructured zirconia powders obtained by a precipitation process. To eliminate chloride residue ions from hydrous zirconia, a modified washing method was used. It was found that the used washing method was effective in removing the chloride from the precipitated gel. In order to investigate the effect of a pre-milling process on the particle morphology of the precipitate, dried <TEX>$Zr(OH)_4$</TEX> was milled using a HEM machine with distilled water. The particle size of the <TEX>$Zr(OH)_4$</TEX> powder exposed to HEM reduced to 100~150 nm, whereas that of fresh <TEX>$Zr(OH)_4$</TEX> powder without a pre-milling process had a large and irregular size of 100 nm~1.5 <TEX>${\mu}m$</TEX>. Additionally, modified heat treatment process was proposed to achieve nano-sized zirconia having a pure monoclinic phase. It was evident that two-step calcining process was effective in perfectly eliminating the tetragonal phase, having a small average particle of ~100 nm with good uniformity compared to the sample calcined by a single-step process, showing a large average particle size of ~300 nm with an irregular particle shape and a broad particle size distribution. The modified method is considered to be a promising process for nano-sized zirconia having a fully monoclinic phase.

Sintering behavior and mechanical properties of mica-diopside glass–ceramic composites reinforced by nano and micro-sized zirconia particles

Primary glass–ceramic containing phlogopite and diopside crystals were prepared through sintering of 70wt.% mica bearing and 30wt.% wollastonite bearing base glasses.Different amounts of microsized and nanosized partially stabilized zirconia particles were added to the aforementioned primary composition to improve mechanical properties.Glass–ceramics and the relevant composites were sintered in a temperature interval of 950–1020°C at the heating rate of 10°C/min and soaking time of 3 hat maximum temperature.According to the obtained results, phlogopite and diopside were formed as the main crystalline phases during sintering of zirconia free specimens. However, dissolution of zirconia in the glass matrix led to the formation of the zircon phase in zirconia containing specimens. Based on the SEM observations, residual nano-sized zirconia was detectable as the agglomerates in the microstructures. Micro-sized zirconia enhanced bending strength of the relevant composites. On the other hand, toughness was promoted in the presence of nano-sized zirconia particles.