Nanocrystalline Yttria-stabilized Zirconia Research Articles

Conventional sintering of nano-crystalline Yttria-Stabilized Zirconia enables high-strength, highly translucent and opalescent dental ceramics

Development of restorative materials capable of mimicking optical and mechanical performance of natural teeth is a quest in aesthetic density. Yttria-Stabilized Zirconia (YSZ) ceramics represent one of the most popular choices for dental restorations, owing to their biocompatibility, white colour, and the possibility to use CAD-CAM technologies. In particular, YSZ doped with 3 mol. % yttria (3YSZ) is popular because it presents high strength. Nonetheless, the limited light transmission of commercially available high strength 3YSZ does not meet the requirements of highly aesthetic cases. On the other side, YSZ presenting a larger portion of yttria are more translucent but exhibit modest strength. Here, we report on fabrication of dense zirconia nanostructures in bulk form via conventional pressure-less sintering at temperatures down to 1100–1200 °C, achieving highly translucent and strong 3YSZ with significant opalescent behaviour. Both Hall−Petch and inverse Hall-Petch relationship were observed in 3YSZ samples with average grain size in the range of 250 nm and 55 nm, demonstrating the importance of grain size control to enhance both optical and mechanical properties of zirconia ceramics, simultaneously. Maximum biaxial strength of 1980 ± 260 MPa, in-line light transmission of 38% in the visible spectrum and opalescence approaching that of enamel were obtained at optimum grain size of 80 ± 5 nm. The notable optical properties are linked to the miniaturization of the residual pores and refinement of grain size towards the nanoscale while the superior mechanical strength is justified by the activation of different energy dissipation processes at nano and macroscale.

Experimental and computational model approach to assess the photothermal effects in transparent nanocrystalline yttria stabilized zirconia cranial implant

Background and objectiveIn the last few years, we have been exploring the use of transparent nanocrystalline yttria-stabilized zirconia (nc-YSZ) ceramics as a biomedical transparent cranial implant, referred as the “Window to the Brain” (WttB). The WttB aims at providing chronical optical access to the brain for diagnostics and therapeutic procedures and it has shown to provide an effective means to obtain enhanced results from optical imaging techniques. The objective of this work is to explore the photothermal effects of the Wttb produced when it is irradiated by a laser source. MethodsWe make experimental and computer models. The thermal effects of laser irradiation on the nc-YSZ samples were evaluated upon registering the induced temperature changes by means of thermal imaging. The computer models try to mimic the experimental models using a similar geometry, reproducing the physical situation by a couple thermal-optical problem and adjusting the main parameters from the experimental results. ResultsExperimental and computational coincides in results: Temperatures at the bottom surface of the implant does not exceed those which produce thermal damage. The quantitative comparison between experimental and computational models show that differences in results are under a reasonable value of 5% and qualitatively we observe a similar behavior. The results provide optimum values for the thermal-optical nc-YSZ parameters considering a linear and exponential relationship with temperature for the absorption coefficient: The thermal conductivity is k = 2.13 W/m·K and the absorption coefficient α varies from 426 to 526 m−1 with the linear relationship, and k = 2.04 W/m·K and α ∈ [433,502] m−1 with the exponential. The reflection coefficient is R = 19% in both cases. ConclusionsThe temperatures achieved in the nc-YSZ during the laser irradiation are suitable for biomedical applications. The combination of experimental and computational models contributes to build a clinically oriented model with the thermal-optical parameters values stablished and to determine their influence in results. Specifically, the absorption coefficient of the nc-YSZ samples is the most influent parameter in the obtained temperatures. Moreover, this combination provides a method to evaluate the relevant thermal-optical parameters of nc-YSZ samples obtained with different manufacturing processes.

Spray Pyrolysis‐Aerosol Deposition for the Production of Thick Yttria‐Stabilized Zirconia Coatings

Aerosol deposition (AD) is a coating technique wherein particles are impacted onto a target substrate at reduced pressures, and supersonic particle impact velocities lead to coating consolidation. The limiting step in AD application is often not supersonic deposition operation, but aerosolization of powder particles with the proper size distribution; the translational impact velocity is strongly size‐dependent. It is demonstrated that by directly synthesizing particles in the gas phase, size‐controlled ceramic particles can be injected into AD systems. This in situ formation step obviates the need for particle aerosolization. Ultrasonic spray pyrolysis (USP) is applied to produce yttria‐stabilized zirconia (YSZ), and USP is directly coupled with AD to produce consolidated, thick, YSZ coatings on metal substrates. USP‐AD yields YSZ coatings on stainless steel and aluminum substrates with porosities <0.20, which grow to thicknesses beyond 100 μm. Aerodynamic particle spectrometry and electron microscopy reveal that the depositing particles are 200 nm–1.2 μm in diameter, though each particle is composed of nanocrystalline YSZ. Supporting computational fluid dynamics calculations demonstrate that the YSZ particle impact speeds are above 300 m s−1. Thermal conductivity measurements demonstrate that USP‐AD coatings have conductivities consistent with those produced from high‐temperature processes.

Group Refractive Index of Nanocrystalline Yttria-Stabilized Zirconia Transparent Cranial Implants.

Transparent “Window to the Brain” (WttB) cranial implants made from a biocompatible ceramic, nanocrystalline Yttria-Stabilized Zirconia (nc-YSZ), were recently reported. These reports demonstrated chronic brain imaging across the implants in mice using optical coherence tomography (OCT) and laser speckle imaging. However, optical properties of these transparent cranial implants are neither completely characterized nor completely understood. In this study, we measure optical properties of the implant using a swept source OCT system with a spectral range of 136 nm centered at 1,300 nm to characterize the group refractive index of the nc-YSZ window, over a narrow range of temperatures at which the implant may be used during imaging or therapy (20–43°C). Group refractive index was found to be 2.1–2.2 for OCT imaging over this temperature range. Chromatic dispersion for this spectral range was observed to vary over the sample, sometimes flipping signs between normal and anomalous dispersion. These properties of nc-YSZ should be considered when designing optical systems and procedures that propagate light through the window, and when interpreting OCT brain images acquired across the window.

Enhancement of oxygen reduction reaction kinetics using infiltrated yttria-stabilized zirconia interlayers at the electrolyte/electrode interfaces of solid oxide fuel cells

We fabricate intermediate-temperature solid oxide fuel cells (SOFCs) with infiltrated film-like yttria-stabilized zirconia (YSZ) interlayers at the electrolyte/electrode interfaces, achieving improved electrochemical performances. A thin and conformal nanocrystalline YSZ interlayer is deposited at the interface between the infiltrated La0.8Sr0.2MnO3-δ nanoparticles and the YSZ scaffold using an infiltration process. The grain sizes of the infiltrated YSZ interlayers are controlled to be in the range of 8–40 nm with the sintering temperature for the infiltrated YSZ interlayer in the range of 800–1200 °C. The infiltrated YSZ interlayer provides 3.18 times reduced polarization resistance and 2.08 times increased maximum power density of 0.1 Ωcm2 and 1.54 Wcm−2 at 750 °C compared to those of an untreated cell, respectively. The substantially improved oxygen reduction reactions are attributable to the increased grain boundary densities in the infiltrated YSZ interlayer, which provide an increased number of reaction sites with a relatively low reaction barrier. Our results demonstrate that modifying the microstructures of the SOFC components at the electrolyte/electrode interfaces using conventional materials can improve electrode performances.

Chronic Brain Imaging Across a Transparent Nanocrystalline Yttria-Stabilized-Zirconia Cranial Implant.

Repeated non-diffuse optical imaging of the brain is difficult. This is due to the fact that the cranial bone is highly scattering and thus a strong optical barrier. Repeated craniotomies increase the risk of complications and may disrupt the biological systems being imaged. We previously introduced a potential solution in the form of a transparent ceramic cranial implant called the Window to the Brain (WttB) implant. This implant is made of nanocrystalline Yttria-Stabilized Zirconia (nc-YSZ), which possesses the requisite mechanical strength to serve as a permanent optical access window in human patients. In this present study, we demonstrate repeated brain imaging of n = 5 mice using both OCT and LSI across the WttB implant over 4 weeks. The main objectives are to determine if the WttB implant allows for chronic OCT imaging, and to shed further light on the question of whether optical access provided by the WttB implant remains stable over this duration in the body. The Window to the Brain implant allowed for stable repeated imaging of the mouse brain with Optical Coherence Tomography over 28 days, without loss of signal intensity. Repeated Laser Speckle Imaging was also possible over this timeframe, but signal to noise ratio and the sharpness of vessels in the images decreased with time. This can be partially explained by elevated blood flow during the first imaging session in response to trauma from the surgery, which was also detected by OCT flow imaging. These results are promising for long-term optical access through the WttB implant, making feasible chronic in vivo studies in multiple neurological models of brain disease.

Comparative Study of Yttria-Stabilized Zirconia Synthesis by Co-Precipitation and Solvothermal Methods

Yttria-stabilized zirconia (YSZ) at 8 mol.% is the most commonly used electrolyte material in the solid oxide fuel cell field. In this study, co-precipitation and solvothermal methods are used to synthesize nanocrystalline yttria-stabilized zirconia. The effects of the synthesis condition (calcination temperature) and different methods on the crystal phase, the microstructure and the electrical performance of YSZ are comparatively investigated and analyzed in detail. X-ray diffraction results reveal that both the co-precipitation and solvothermal methods can be used to successfully synthesize nano-YSZ ceramic powders with a pure crystal phase of fluorite cubic structure. The grain sizes of the YSZ samples prepared by these two methods are both smaller than that of the commercial Tosoh YSZ, with the sample from the solvothermal method revealing the smallest grain size. The electrochemical impedance spectra demonstrate that the YSZ prepared by co-precipitation with 500°C calcination reveals the smallest ohmic resistance among different calcination temperatures. It also shows a relatively lower ohmic resistance of 2.04 Ω cm2, while the YSZ sample synthesized by the solvothermal method has a higher ohmic resistance of 8.89 Ω cm2, with corresponding ionic conductivities of 0.0266 S cm−1 and 0.00562 S cm−1, respectively.

Characterization of ageing resistant transparent nanocrystalline yttria-stabilized zirconia implants.

The "Window to the Brain" is a transparent cranial implant under development, based on nanocrystalline yttria-stabilized zirconia (nc-YSZ) transparent ceramic material. Previous work has demonstrated the feasibility of this material to facilitate brain imaging over time, but the long-term stability of the material over decades in the body is unknown. In this study, the low-temperature degradation (LTD) of nc-YSZ of 3, 6, and 8 mol % yttria is compared before and after accelerated ageing treatments following ISO standards for assessing the ageing resistance of zirconia ceramics. After 100 hr of accelerated ageing (equivalent to many decades of ageing in the body), the samples do not show any signs of phase transformation to monoclinic by X-ray diffraction and micro-Raman spectroscopy. Moreover, the mechanical hardness of the samples did not decrease, and changes in optical transmittance from 500 to 1000 nm due to ageing treatments was minimal (below 3% for all samples), and unlikely to be due to phase transformation of surface crystals to monoclinic. These results indicate the nc-YSZ has excellent ageing resistance and can withstand long-term implantation conditions without exhibiting LTD.

A Strategy to Mitigate Grain Boundary Blocking in Nanocrystalline Zirconia

A major challenge in the application of nanostructured electrolytes in solid oxide electrochemical cells is grain boundary blocking originated from unsatisfied atomic bonding and coordination. The resulting increase in grain boundary resistivity works against the expected benefits from the enhanced ion exchange rates enabled by the extensive interfacial network in nanocrystalline materials. This study addresses this challenge by demonstrating that a reduction in the grain boundary excess energies increases the net ionic conductivity as directly measured by impedance electrical spectroscopy in nanocrystalline yttria-stabilized zirconia. The reduced grain boundary energy was designed by doping the system with lanthanum, leading to local excess energy reduction due to segregation of La to boundaries as observed by scanning transmission electron microscopy-based energy-dispersive spectroscopy. The results suggest rare-earth ions with favorable grain boundary segregation enthalpy can smooth out the energy land...

Evaluation of a transparent cranial implant as a permanent window for cerebral blood flow imaging.

Laser speckle imaging (LSI) of mouse cerebral blood flow was compared through a transparent nanocrystalline yttria-stabilized zirconia (nc-YSZ) cranial implant over time (at days 0, 14, and 28, n = 3 mice), and vs. LSI through native skull (at day 60, n = 1 mouse). The average sharpness of imaged vessels was found to remain stable, with relative change in sharpness under 7.69% ± 1.2% over 28 days. Through-implant images of vessels at day 60 appeared sharper and smaller on average, with microvessels clearly visible, compared to through-skull images where vessels appeared blurred and distorted. These results suggest that long-term imaging through this implant is feasible.

Atomistic modeling of La3+ doping segregation effect on nanocrystalline yttria-stabilized zirconia.

The effect of La3+ doping on the structure and ionic conductivity change in nanocrystalline yttria-stabilized zirconia (YSZ) was studied using a combination of Monte Carlo and molecular dynamics simulations. The simulation revealed the segregation of La3+ at eight tilt grain boundary (GB) structures and predicted an average grain boundary (GB) energy decrease of 0.25 J m-2, which is close to the experimental values reported in the literature. Cation stabilization was found to be the main reason for the GB energy decrease, and energy fluctuations near the grain boundary are smoothed out with La3+ segregation. Both dynamic and energetic analysis on the Σ13(510)/[001] GB structure revealed La3+ doping hinders O2- diffusion in the GB region, where the diffusion coefficient monotonically decreases with increasing La3+ doping concentration. The effect was attributed to the increase in the site-dependent migration barriers for O2- hopping caused by segregated La3+, which also leads to anisotropic diffusion at the GB.

Optimization of the Amount and Molecular Weight of Dispersing Agent PEG During the Co-Precipitation Preparation of Nano-Crystalline C-YSZ Powder.

Single-phase nano-crystalline yttria-stabilized zirconia (YSZ) powder was prepared by co-precipitation method using zirconium oxychloride (ZrOCl₂ · 8H₂O) and yttrium nitrate (Y(NO₃)₃ · 6H₂O) as raw materials and ammonium bicarbonate (NH₄HCO₃) and liquor ammonia (NH4OH) as the precipitators. In order to get the powder with favorable dispersibility, polyethylene glycol (PEG, HO(CH₂CH₂O) n H) with different molecular weights and amounts was used as a dispersant agent in the synthesis process and its effects on the YSZ powder was investigated. The scanning electron micrograph (SEM) images showed that the degree of agglomeration and the mean crystal size of YSZ powder varied with the amount and the molecular weight of PEG. The appropriate amount and molecular weight of PEG were given by comparing the dispersibility and crystal size distribution of powders. In the process, one also could find that the promotion of dispersion was insensitive to the molecular weight, and all PEG (400–2000) can exert an ability to reduce the agglomeration. The proper amount of PEG400, PEG600, PEG1000, PEG2000 was 3 wt%, 3 wt%, 2 wt%, 2 wt%, respectively. The transmission electric microscope studies in conjunction with the XRD analyzes verified the low degree of agglomeration and the size of YSZ powder varied between 10~20 nm.

Brittle-flexible-brittle transition in nanocrystalline zirconia nanofibrous membranes

The study of nanocrystalline inorganic nanofibrous membranes (NINMs) with flexibility is one of the most active academic research areas in advanced functional nanofibrous materials. Nonetheless, the role of nanocrystallinity in flexibility has remained uninvestigated until now, according to typical Hall–Petch effects of nanocrystalline metals. Here, we show a surprising brittle-to-flexible-to-brittle transition of zirconia and yttria stabilized zirconia (YSZ) nanofibrous membranes as the grain size and pore size reduce below 26 and 13 nm, respectively. Moreover, classical and inverse Hall–Petch effects were revealed in nanocrystalline zirconia and YSZ nanofibrous systems, which correspond to intragranular and intergranular deformation mechanisms, respectively. Most importantly, the bending deformation mechanism of the flexible nanocrystalline YSZ nanofibrous membranes was proposed from macroscopical membranes to microscopic unit cells, including the slip of fibers, bending of fibers, grain activity, and dislocation motions, which was given for the first time in electrospun flexible NINM materials. Our discovery is fundamentally important for understanding deformation behavior and designing various flexible NINMs as promising candidates in the field of catalysis, supercapacitors, biomaterials, etc.

Obtaining highly dense YSZ nanoceramics by pressureless, unassisted sintering

For technological applications, zirconia is commonly blended with other oxides to stabilise the tetragonal and/or cubic phases at low temperature, being yttria the most frequently added dopant. It is generally desirable to obtain highly dense ceramics while maintaining grain sizes in the nanoscale ( < 100 nm). Small grains contribute to stabilise the tetragonal phase and to improve the toughness and flexural strength. Moreover, a higher ionic conductivity for cubic zirconia electrolytes is achieved with smaller grain size and lower thickness of the intergranular regions. The sintering onset temperatures required for nanometric particles are significantly reduced when compared to conventional micrometric powders. However, densification is generally accompanied by an undesirable grain coarsening. A Ramp and Hold Sintering (RHS) is the simplest densification schedule, consisting of heating up to the peak temperature followed by a holding time at that temperature. Another approach, called Two-Step Sintering (TSS) is based on the principle that the activation energy for grain growth is lower than the activation energy of densification. The key elements in this method are heating up to a high temperature to achieve a density >75% Theoretical Density (TD) to render the pores unstable, and then cooling down rapidly to a lower temperature to finish sintering and hinder grain growth. Alternatively, considerable efforts have been made in increasing the heating rate and/or reducing the hold time at the peak temperature during sintering cycles in processes that are generally referred to as ‘Fast Firing’ (FF) or ‘rapid sintering’. This review summarises the attempts in the literature for obtaining dense monolithic nanocrystalline Yttria-Stabilized Zirconia (YSZ) ceramics by pressureless sintering schedules carried out in conventional furnaces. RHS, FF and TSS schedules are reported only from YSZ as starting powders, i.e. without the aid of any additional dopant or grain growth inhibitor. For the sake of comparison, the discussion is focused mainly on YSZ nanoceramics with final densities >99% TD and average grain size < 100 nm. Powder and shaping effects on microstructure and properties of bulk nanoceramics are discussed. A comparison among sintering approaches is then made taking into account the microstructural development of the nanostructures and some key properties of the products.

Preparation of zirconia transparent ceramics by low temperature microwave sintering.

Nanocrystalline yttria-stabilized zirconia (ZrO2-8 mol%Y2O3, 8YSZ) was synthesized by a homogeneous precipitation process using urea as the precipitation agent. Zirconia (ZrO2) transparent ceramics samples have been successfully fabricated by a microwave sintering process at low temperature. The technologies of low-temperature microwave sintering and the relationships of the microstructures and properties of the specified samples have been investigated in detail. We have found out that the low-temperature microwave sintering has its obvious advantages over the other methods in manufacturing zirconia transparent ceramics.

Lattice expansion and contraction in nanocrystalline yttria-stabilized zirconia powders

Rietveld refinement of powder X-ray diffractograms from nanocrystalline yttria-stabilized zirconia (2, 3.5, 5.5 and 8 mol.% YO1.5) shows two tetragonal lattices. Transmission electron microscopy on powder with 8 mol.% YO1.5 reveals two types of particles: tetragonal phase agglomerates of crystallites up to 28 nm experience lattice expansion and unagglomerated primary particles of 6 ± 2 nm undergo lattice contraction. Lattice contraction is due to surface curvature. Expansion is due to grain boundary free volume and point defects.

Phase stability, grain growth and photoluminescence property of nanocrystalline yttria-stabilized zirconia film under 500 keV Xe6+ ion irradiation

Nanocrystalline yttria stabilized zirconia (YSZ) film (with 8mol% Y2O3) was synthesized via a sol–gel method. Phase stability, grain-growth and defects of the YSZ film were investigated under 500keV Xe6+ ions irradiation to doses up to an average value of 22.2 displacements per atom (dpa). Atomic force microscope results show that the surface of the as-grown YSZ film is quite smooth and free of cracks. Transmission electron microscope results reveal that the average grain-size of the as-grown samples is around 12.6nm. With increasing irradiation does up to 22.2dpa, the average grain size increases, no phase transformation or amorphization is observed, which indicates that the phase structure of the nanocrystalline cubic YSZ film is quite stable. Two strong and broad emission peaks were detected in the YSZ film, and the emission intensity decreases obviously after irradiation. The possible mechanism of the grain growth was also discussed.

Atomic layer deposition of MgO films on yttria-stabilized zirconia microtubes

Deposition of MgO thin film on nanocrystalline yttria-stabilized zirconia microtubes was investigated. The microtubes were prepared by self-formation from threads drawn directly from zirconium butoxide [Zr(OBu)4] precursor and heat treated at 800°C. The tubes possessed 100% tetragonal phase, their typical outer diameter was 50μm, inner diameter 30μm and length 1cm. MgO films were deposited from β-diketonate-type precursor 2,2,6,6-tetramethyl-heptanedionato-3,5-magnesium(II) at 220°C by atomic layer deposition. Thickness of MgO film on microtubes was 15.8nm and growth rate 0.105Å/cycle.

Transparent nanocrystalline yttria-stabilized-zirconia calvarium prosthesis

Laser-based diagnostics and therapeutics show promise for many neurological disorders. However, the poor transparency of cranial bone (calvaria) limits the spatial resolution and interaction depth that can be achieved, thus constraining opportunity in this regard. Herein, we report preliminary results from efforts seeking to address this limitation through use of novel transparent cranial implants made from nanocrystalline yttria-stabilized zirconia (nc-YSZ). Using optical coherence tomography (OCT) imaging of underlying brain in an acute murine model, we show that signal strength is improved when imaging through nc-YSZ implants relative to native cranium. As such, this provides initial evidence supporting the feasibility of nc-YSZ as a transparent cranial implant material. Furthermore, it represents a crucial first step towards realization of an innovative new concept we are developing, which seeks to eventually provide a clinically-viable means for optically accessing the brain, on-demand, over large areas, and on a chronically-recurring basis, without need for repeated craniectomies. From the Clinical EditorIn this study, transparent nanocrystalline yttria-stabilized-zirconia is used as an experimental “cranium prosthesis” material, enabling the replacement of segments of cranial bone with a material that allows for optical access to the brain on a recurrent basis using optical imaging methods such as OCT.

XANES and EXAFS study of the local order in nanocrystalline yttria-stabilized zirconia

The local order around Zr and Y atoms of nanocrystalline yttria-stabilized zirconia (YSZ) powders with different grain sizes has been investigated by x-ray absorption spectroscopies. The samples were prepared by means of mechanical alloying with or without subsequent sintering treatment and also by milling commercial YSZ. Our study is motivated by the interest in the electrical properties of grain boundaries and the controversy about the level of disorder in the intergrain regions in nanocrystalline YSZ. The x-ray absorption near edge structure (XANES) analysis indicates that the local order of all the sintered samples is independent of the grain size. This is confirmed by the analysis of the extended x-ray absorption fine structure, which points out also that, in contrast to that found in sintered samples, the local order around the cation in the samples milled without further sintering treatment extends only to the first coordination shell. Finally, the results of ab initio Zr K-edge XANES calculations lead us to conclude that the observed changes of the shape of the white line are not related to a phase transformation but reflects the short-range order present in the as-milled samples.