Zirconia Grains Research Articles

Zirconia ageing is related to total hip arthroplasty aseptic loosening. A study of 45 retrieved zirconia heads

BackgroundY-TZP zirconia heads were recalled by the Food and Drug Administration (FDA) in 2001 and zirconia alone was no longer used in orthopedics. Tunnel furnace sintering was suspected of producing defects responsible for early material failure. As Zirconia Toughened Alumina (ZTA) matrices are widely used as bearing material and contain zirconia grains, there remains a need to better understand the in vivo ageing process of zirconia and its clinical implications when the material is produced by batch furnace sintering, the validated sintering process. Questions/objectivesIs there an association between the ageing of batch furnace produced zirconia and THA revision? Methods45 retrieved femoral heads, batch furnace sintered only, were analyzed. Roughness was measured by 3D profilometry, phase transfer by μRaman spectroscopy.Clinical data were compared with material characteristics. ResultsIrrespective of the cause of revision, all heads showed a crystallographic phase transition from tetragonal to monoclinic over 19.5%. A correlation was found between the phase change, roughness increase and aseptic loosening, with a threshold set at 24.5% of monoclinic phase. ConclusionsThe ageing process of zirconia may lead to aseptic loosening, which, in the absence of contrary evidence, prohibits its use as the sole component of orthopedic materials. ZTA matrices should be clinically monitored, especially in young patients, and better in vitro modelling needs to be performed. Level of evidenceIV; Case series.

Zirconia specimens printed by vat photopolymerization: Mechanical properties, fatigue properties, and fractography analysis.

The mechanical and fatigue properties of zirconia specimens printed by vat photopolymerization (VPP) were evaluated and compared with those of zirconia specimens milled by computer numerical control (CNC). Bar-shaped specimens were printed by stereolithography (SL) and digital light processing (DLP). CNC-milled specimens were used as control samples. The fracture toughness, hardness, and flexural strength properties of the zirconia specimens were evaluated via single edge V-notch beam tests, Vickers hardness tests, and 3-point bending tests. Dynamic fatigue tests were carried out in distilled water using a step-stress test. After static bending and dynamic step-stress testing, fractography analysis was performed. Statistical analysis was carried out to compare the fracture toughness, hardness, flexural strength, and fatigue cycle results of each group (α = 0.05). The fracture toughness values did not significantly differ among the groups (p>0.05). The flexural strength was 894.10MPa for SL, 831.46MPa for DLP, and 1140.39MPa for CNC. The flexural strength of CNC was greater than that of SL and DLP (p<0.01). The mean fatigue cycles were 23498.07 for SL, 19858.60 for DLP, and 31566.80 for CNC. The mean fatigue failure strength was 643.13MPa for SL, 530.63MPa for DLP, and 903.75MPa for CNC. The fatigue failure strength of CNC was greater than that of SL and DLP (p<0.05). Fractography analysis revealed material defects at the fracture origin for each group. A partially fused structure of the incompletely debonded resin could be observed in SL, and a porous region of incompletely sintered zirconia grains could be observed in CNC. The fracture toughness and hardness of zirconia printed by VPP are comparable to those of zirconia milled by CNC. However, zirconia milled by CNC has superior static flexural strength and dynamic fatigue resistance. Further studies are needed to explore the clinical applications of VPP-printed zirconia.

Effect of chewing simulation in different Ph solutions on flexural strength of monolithic zirconia

Abstract Aim In vitro study is to evaluate the effect of chewing simulation in different ph solutions on flexural strength of monolithic zirconia. Patients and methods Forty zirconia bars (N = 40) were cut off zirconia disc using IsoMet. Each bar was 2 mm thickness, 25 mm length and 5 mm width. Specimens were divided into four groups (n = 10). The first group (control) was ready for universal test of flexural strength. Second, third, and fourth group were ready for chewing simulation but in different PH solutions acidic, neutral, and alkaline respectively. These solutions were collected from the lab (acidic-PH = 4, alkaline PH = 10). Twenty extracted human premolars (divided vertically into two halves by IsoMet) were collected for study as antagonist material for chewing simulation. After chewing simulation of these groups, the flexural strength was measured for them and SEM. Results Group 1 (control) showed the highest flexural strength with the mean value (745.21 ± 69.47) followed by group 3 (neutral) (720.40 ± 92.47), then group 2 (acidic) (507.56 ± 111.99), and finally group 4 (alkaline) (497.26 ± 172.90) with the least flexure of strength. There was significance between group 1 (control) and group 2 (acidic) and highly significant with group 4 (alkaline). There was no significance with group 3 (neutral). SEM showed that no cracks in control group. Group (2) surfaces were very smooth with amorphous structure showing disperessed small pores. Group (3) air voids were found entrapped between zirconia grains. Group (4) the surface was rough showing large compact flacks’ fragmentation and branched cracks. Conclusions This study clearly illustrates the significance of flexural strength in different pH of an aqueous solution in terms of the chewing of zirconia ceramics. Cyclic loading decreases flexural strength of 4% yttrium zirconia in neutral but not significantly. In alkaline and in neutral there were significant decrease in flexural strength with chewing due to degradation of stabilizers and transformation tetragonal phase to monoclinic phase. This degradation was in alkaline group more than in acidic group.

Preparation of ZTA composite ceramics derived from sol–gel method by DIW printing

AbstractIn this study, the dense and fine‐grained zirconia toughened alumina (ZTA) composite ceramics were efficiently prepared by 3D gel printing technology. Zirconia powder, zirconium oxychloride, and zirconium sol were introduced into boehmite gel as zirconium sources, and their effects on rheological properties, drying characteristics, microstructure, and mechanical properties of ceramics were discussed. The results showed that all gels exhibit reversible shear thinning properties. The gel with zirconia powder has a relaxation phenomenon; zirconium sol forms another gel network in the boehmite gel network, and two linear viscoelastic regions are observed. The gel with zirconia powder has the highest solid loading, small drying shrinkage, and fewer drying defects. The grain growth of ZTA ceramics with zirconium sol was inhibited, and the zirconia grains were evenly distributed, with the highest bending strength of 518 ± 103 MPa. The new gel preparation method and gel drying process offer great possibilities for manufacturing optical glasses and functional ceramics with high‐performance geometric structures, which cannot be achieved by traditional manufacturing methods.

Microstructure and mechanical properties of a porous ceramic composite with needle‐like mullite and zirconia

AbstractPorous mullite ceramics have good properties for high‐temperature applications, but porosity gives place to ceramics with low mechanical strength, which restricts the service life in their potential applications. Therefore, performing modifications at the microscale to increase the mechanical strength has become a current challenge to expand its application fields. This work describes the properties of a porous mullite–zirconia composite produced by ceramic processing, using industrial kaolin and stabilized zirconia as raw materials. The growth of mullite needle‐like grains to reinforce the ceramic was promoted by the addition of a molybdenum oxide precursor. The effect of zirconia on the composite was analyzed through an experimental multi‐technique approach and considering a pure mullite sample, identically processed, as a reference. The novel composite has a porosity of about 50%, and presents a homogeneous microstructure, with interlocked mullite needle‐like grains and dispersed rounded zirconia grains. This morphology restricts the mullite tendency to shrink during sintering, giving the material a higher stiffness. In particular, the presence of zirconia in the composite improves both the flexural strength and the apparent Young modulus of the material (about 20% and up to 600%, respectively). These results encourage further investigations to establish this composite for different technological applications.

Modification effect of Mg(OH)2–2Al(OH)3 composite gels on Y-PSZ ceramics

A Mg(OH)2–2Al(OH)3 composite gel, synthesized via the sol-gel method, served as a modifier in the preparation of modified yttria partially stabilized zirconia (Y-PSZ) ceramics. The volume density, apparent porosity, compressive strength, thermal shock resistance, phase composition, and microstructure of the ceramics were systematically characterized. The findings reveal that, upon sintering, the Mg(OH)2–2Al(OH)3 composite gel decomposes into MgO–Al2O3 powder, which in turn in-situ generates a MgAl2O4 spinel reinforcing phase, characterized by its uniform size and distribution. The presence of this MgAl2O4 spinel applies compressive stress to the adjacent zirconia grains, effectively inhibiting their abnormal growth. By occupying the grain boundaries and establishing a mosaic structure, the compressive strength and thermal shock resistance of the Y-PSZ ceramics are significantly enhanced. Notably, an increase in the composite gel quantity correlates with a reduction in both volume density and apparent porosity of the Y-PSZ samples, whereas the compressive strength initially rises before diminishing, and the thermal shock resistance markedly improves. When the amount of Mg(OH)2–2Al(OH)3 composite gel introduced reaches 9 %, the comprehensive performance of the modified Y-PSZ ceramics is optimized. At this point, its compressive strength is increased by 1.26 times compared to the unmodified Y-PSZ ceramics, reaching 732 MPa; moreover, after undergoing 60 thermal shocks, its residual strength retention rate still remains as high as 59.15 %.

Effect of inert glass on the physical and mechanical properties of zirconia dental ceramics fabricated by digital light processing

AbstractZirconia dental ceramics incorporated with inert dental glass (IDG) were fabricated by digital light processing (DLP). The effects of the amount of added IDG on the physical properties and microstructure of sintered samples were investigated. Highly dense zirconia dental ceramic samples with a relative density of 98.9% were obtained by adding different amounts of IDG sintered at 1500°C. With the addition of 1 wt% IDG, the Vickers hardness, fracture toughness, and wear resistance increased to 13.03 GPa, 6.33 MPa m1/2, and 0.03 mg/(mm2 min), respectively. However, with the incremental addition of IDG at 3 and 5 wt%, a slight reduction in Vickers hardness, fracture toughness, and wear resistance was observed. The values recorded were 12.68 GPa, 5.90 MPa m1/2, and 0.05 mg/(mm2 min) for the former, and 11.86 GPa, 5.48 MPa m1/2, and 0.06 mg/(mm2 min) for the latter. Scanning electron microscopy observation suggested that 1 wt% IDG incorporated into zirconia dental ceramic samples resulted in an optimal microstructure, with IDG existing at triple junctions of zirconia grains to form a discrete secondary phase. The broken surface revealed transgranular fracture features. This investigation indicates that adding a proper amount of IDG is an effective method to enhance the properties of DLP‐fabricated zirconia dental ceramics.

Ultra-high strength ZTA ceramic foams with large amounts of intragranular structures resulting from dual-phase sol

A creative way to overcome the general problem of high-porosity ceramic foams with low intensity has been proposed here. For the first time, zirconia toughened alumina (ZTA) ceramic foams with a large number of intragranular structures have been prepared by particle-stabilized foams using boehmite sol and ZrO2 sol as raw materials. The grain boundary mobility of alumina in ceramic foams is much higher than that of zirconia due to the extremely high sintering activity of the boehmite nanoparticles, and then a typical intragranular structure of alumina grains wrapping around zirconia grains is formed. Based on the strengthening effects of intragranular zirconia grains, the as-prepared ZTA foams have excellent mechanical properties, with ultra-high compressive strength of 10.3–88.0 MPa and high porosity of 91.0–79.1 %. This paper provides a novel strategy for the preparation of high-performance ceramic foams.

Effect of second phase addition of zirconia on the mechanical response of textured alumina ceramics

The effect of second phase addition of zirconia on the mechanical response of textured alumina was analysed. Highly textured monolithic tape-casted alumina was obtained through templated grain growth. Compositions containing 1, 2, 5 and 10 vol% of (i) non-stabilised and (ii) 3 mol% yttria-stabilised zirconia, respectively, were investigated. XRD analyses revealed that the texture degree decreased with increasing second phase content. Microstructural analysis showed zirconia grains inside the textured alumina grains for contents ≤ 5 vol%, affecting the mode of fracture. Fracture toughness of textured alumina significantly decreased with the addition of a second phase. In the case of non-stabilised zirconia, the constraint of the alumina matrix and the small grain size led to a lower fracture toughness in comparison to monolithic textured alumina ( K Ic = 5.1 MPa m 1/2 ). The fracture toughness of textured alumina with 3 mol% yttria-stabilised zirconia was comparable to equiaxed alumina, independent of the content ratio ( K Ic = 3.5 MPa m 1/2 ).

Ceria-stabilized zirconia-based composites printed by stereolithography: Impact of the processing method on the ductile behaviour and its transformation features

A commercial composite powder was used to develop a new paste for stereolithography processing (SLA) of dense 11Ce-TZP-based composites containing 8 vol.% of alumina and 8 vol.% of strontium aluminate. The same powder was used to prepare reference ceramics by conventional Cold Isostatic Pressing (CIP) and sintering. The use of SLA gave rise to a microstructure containing larger strontium aluminate clustering surrounded by zirconia grains devoid of alumina particles, which appeared embedded in a matrix of zirconia-alumina grains. This slightly more heterogeneous microstructure favored the formation of a greater number of thinner transformation bands, causing a larger plastic deformation in SLA samples (failure strain of 0.65 % instead of 0.35 % for CIP samples), without compromising characteristic strength ( σ 0 = 560 and 586 MPa respectively) . Thus, SLA does not affect the ductile behaviour that characterizes this type of ceramics and appears as a very promising method for the development of custom-made ceramic prostheses.

Direct ink writing of 3Y-TZP ceramics using PEG-Laponite® as additive

This study investigated a new colloidal ink, containing Laponite® nanoclay as additive, to develop ZrO2-based ceramic prototypes by Direct Ink Writing (DIW). The ink developed contained 31 vol% 3Y-TZP as solid load and 69% v/v of gel containing 7.5% (w/w) Laponite®, polyethylene glycol (PEG), and dibutyl phthalate. The rheologyical behavior of this ceramic ink was analyzed and its extrudability was optimized. The samples were DIW 3D-printed at a speed of 10 mm/s with nozzle diameter of 0.41 mm. After drying for 24 h, the zirconia samples underwent debinding and were pre-sintered at 1100 °C under a heating rate of 1 °C/min. After pre-sintering, the samples were sintered at 1550 °C for 2 h. Subsequently, the sintered samples were characterized by Scanning Electron Microscopy and X-ray diffractometry. Fracture toughness was measured by Vickers indentation, whereas nano-hardness and Young’s modulus were assessed by dynamic Vickers nanoindentation (250–1960 mN loads). A relative density of 89.3 ± 0.5% was found. The following crystalline phases were observed: monoclinic-ZrO2 (34%), tetragonal-ZrO2 (31%), and cubic-ZrO2 (35%). This behavior is probably due to the segregation of Y+3 between the zirconia grains, favored by the presence of Laponite® nanoclay during the sintering process. The microstructure of the prototypes showed two distinct populations of ZrO2 grains with average particle sizes of 1 μm (monoclinic and tetragonal phases) and >5 μm (cubic phase). Vickers hardness (HV1000gF) was 952 ± 40 HV and fracture toughness was 4.6 ± 1.1 MPa m1/2. A Young’s modulus of ∼200 GPa and hardness values of 1837.9 and 1943.3 HV were found for loads of 250 and 1960 mN, respectively.

Mechanical performance and microstructure of 3Y‐TZP/Al2O3/GNPs medical ceramic surgical scalpel material prepared through SPS–HF dual sintering method

AbstractIn this study, 3 mol% yttria‐stabilized tetragonal zirconia polycrystal (3Y‐TZP)/Al2O3/graphene nanoplatelets (GNPs) medical ceramic materials for manufacturing surgical scalpels were sintered in vacuum in an SPS–625HF furnace. The mechanical performances and microstructures of the composites were investigated, and the influence mechanisms of the sintering temperature and amount of added GNPs were studied. During the sintering process at 1400°C and 30 MPa for 5 min, the added GNPs enhanced the mechanical properties of the 3Y‐TZP/Al2O3 composites. The results showed that the composite with .1 wt.% GNPs had 6.4% (910 ± 11 MPa) higher flexural strength than 3Y‐TZP/Al2O3. The composite with .4 wt.% GNPs had 38.7% (12.95 ± .22 MPa m1/2) greater fracture toughness than 3Y‐TZP/Al2O3. The main toughening mechanisms of 3Y‐TZP/Al2O3/GNPs were crack bridging, crack deflection, crack branching, GNPs bridging, transgranular fracture structures, and phase transformation of t‐ZrO1.95. The two‐stage densification displacement curve appeared at the optimal sintering temperature of the materials, and the 3Y‐TZP/Al2O3/GNPs composites with a two‐stage densification displacement curve had excellent mechanical properties. The added GNPs can inhibit the grain growth during the sintering process, thereby refining the zirconia grains. With the increase in GNPs content, the grain size and flexural strength of the composites decreased gradually. However, higher content of GNPs was beneficial to improve the relative density and thermal diffusivity of 3Y‐TZP/Al2O3/GNPs composite material.

Effect of zirconia addition amount in glaze on mechanical properties of porcelain slabs

Applying toughened glaze layer on porcelain slabs can improve the fracture toughness of slabs and greatly reduce the production cost. In this study, porcelain slabs glaze with high toughness was fabricated by the processes of impregnation glazing and single firing method, using opaque frits, kaolin clay as the main raw materials, zirconia as an additive, and the effect of the addition amount of zirconia in glaze on fracture toughness of porcelain slabs was investigated. The results showed that the type and content of crystal phase of the glaze were greatly influenced by the addition amount of zirconia. Meanwhile, compared with the base glaze, the hardness and fracture toughness of the sample with zirconia glaze were significantly improved. Porcelain slabs with 10 wt% zirconia in glaze, sintering at 1200 °C, exhibited higher quality glaze and outstanding properties, including a water absorption of 1.95%, a Vickers hardness of 6.36 GPa, and a fracture toughness of 2.71 MPa m1/2. The toughening mechanism of the glaze layer was as follows: a large number of zirconium silicate grains with high hardness were generated by the reaction of added zirconia with silica in the glass phase, which increased the content of crystal phase and then prevented the propagation of cracks; moreover during the martensitic transformation of the tetragonal zirconia grains, the volume and shear strain were generated to offset the stress field generated by the crack tip, thus toughening the material.

Effect of Multiple Firings on the Microshear Bond Strength Between a Translucent Zirconia and a Resin Cement.

This study investigated the microshear bond strength between a resin cement and a translucent zirconia subjected to multiple characterization firings. 5Y-PSZ zirconia blocks (Katana UTML) were sliced, sintered (1550°C, 2 h), and randomly divided into six groups (n=10) according to the number of characterization firings (0, 1, 2, 3, 5, or 10) and aging (baseline or after thermocycling). Each characterization firing was performed at 750°C for 1 minute. The ceramic surfaces were all sandblasted with 50 μm Al2O3 and silanized. Then, cylinders of resin cement (0.96 mm diameter × 2 mm height) were bonded onto their surfaces. The baseline samples were immersed in distilled water for 24 hours before the microshear bond strength (μSBS) tests. The aged samples were tested after 5000 thermocycles in water (5°C-55°C). The failure modes were classified as adhesive, predominantly adhesive, or cohesive. Scanning electron microscope images of the failure modes and the ceramic surfaces after the firings were taken. The μSBS data were analyzed by two-way ANOVA and Tukey's test. The number of characterization firings and aging affected the bond strength. The highest bond strength values were observed from the 2-firing group at baseline. The μSBS results after 1, 2, or 3 characterization firings were similar at baseline and after aging. On the other hand, 0, 5, and 10 firings revealed the lowest bond strengths. The most frequent failures were adhesive and predominantly adhesive. Zirconia grains were not affected by the multiple firings. One to three characterization firings after sintering improve the bond strength of 5Y-PSZ to the resin cement when compared to none or several (five or ten) firing cycles.

Facile method for preparing lamellar structured alumina-zirconia ceramics with high toughness

This study describes a facile and effective method for fabricating alumina-zirconia composites with innovative lamellar structure as well as outstanding mechanical strength/toughness. To this end, the particle-stabilized foam method has been utilized to synthesize alumina ceramic foams built by single-layer pore wall firstly, providing the basic lamellar structure for the subsequently mixing with zirconia particles and hot-press sintering to achieve robust alumina-zirconia ceramics. The unique lamellar microstructure is composed of alumina sheets and zirconia grain layers alternated consistently, resulting in superior fracture toughness of 9.38 MPa m1/2, high flexural strength of 1.01 GPa and Vickers hardness of 17.04 GPa at the relative density of 99.63%. This approach opens up new possibilities for creating alumina-zirconia ceramics with exceptional mechanical performance.

Electric field-assisted in situ fabrication of carbon/zirconia nanocomposites with tunable conductivity for electromagnetic interference shielding applications

Carbon/ceramic nanocomposites have been considered as ideal candidates for electromagnetic interference shielding (EMI) applications in harsh environments. However, the conventional fabrication methods of carbon/ceramic composites are often complicated, costly and time-consuming. Herein, by applying a DC electric field during hot pressing sintering, we realized the direct growth of graphene-like carbon nanosheets (GCNs) in zirconia ceramics during sintering, achieving in situ fabrication of GCNs/zirconia nanocomposites with both high EMI shielding effectiveness (>40 dB) and mechanical strength (>1000 MPa) in one step. In particular, we found that applying a DC electric field could significantly enhance the carburization during sintering, and GCNs are in situ formed at zirconia grain boundaries. When the initial electric field is constant, the amount of in situ formed GCNs could be controlled by simply adjusting the current density, resulting in the tunable electrical conductivity. These findings will provide new opportunities for both experimental and theoretical studies on carbon/ceramic EMI shielding materials and the DC electric field-assisted sintering technique.

Effect of Speed Sintering on Low Temperature Degradation and Biaxial Flexural Strength of 5Y-TZP Zirconia.

Translucent zirconia is becoming the material of choice for the esthetic restorative material. We aimed to evaluate the surface structure, phase determination, translucency, and flexural strength of 5Y-TZP Zirconia (Katana STML Block and Disc) between the regular sintering and the speed sintering with and without low-temperature degradation (LTD). A total of 60 zirconia discs (30 per group; regular sintering and speed sintering) were used in this study. A CAM machine was used to mill cylinders out of the zirconia blanks and then cut into smaller discs. For the speed sintering, the zirconia blocks were milled into smaller discs. The zirconia discs were subjected to regular and speed sintering with and without LTD. Scanning electron microscopy was used to characterize the zirconia specimens and the zirconia grain size. Furthermore, the zirconia specimens were analyzed for elemental analysis using energy dispersive spectroscopy and phase identification using X-ray diffraction. The zirconia specimens were subjected to translucency measurements and biaxial flexural strength testing. The results of the zirconia specimens were compared among the groups. Statistical analysis was completed using SPSS version 20.0 to detect the statistically significant differences (p value = 0.05). A one-way ANOVA with multiple comparisons was performed using Scheffe analysis among the groups. The speed sintering presented smaller grain sizes. The zirconia specimens with and without LTD in regular and speed sintering presented a similar surface structure. Regular sintering showed more translucency compared to speed sintering. Multiple comparisons of the translucency parameter were a significant difference (p value < 0.05) between the various groups except for the comparison between speed sintering and speed sintering LTD. The regular sintering showed bigger gain sizes and slightly more translucency compared to speed sintering. The speed sintering showed higher biaxial flexural strengths compared to regular sintering. This shows that speed sintering can be considered a suitable method of sintering zirconia.

Zirconia-alumina multiphase ceramic fibers with exceptional thermal stability by melt-spinning from solid ceramic precursor

Zirconia-alumina multiphase ceramic fibers with 80 wt% (Z80A20 fiber) and 10 wt% (Z10A90 fiber) proportions of zirconia were prepared via melt-spinning and calcination from solid ceramic precursors synthesized by controllable hydrolysis of metallorganics. The zirconia-alumina multiphase fibers had a diameter of about 10 µm and were evenly distributed with alumina and zirconia grains. The Z80A20 and Z10A90 ceramic fibers had the highest filament tensile strength of 1.78 GPa and 1.87 GPa, respectively, with a peak value of 2.62 GPa and 2.71 GPa. The Z80A20 ceramic fiber has superior thermal stability compared to the Z10A90 ceramic fiber and a higher rate of filament strength retention due to the stability in grain size. After heat treatment at 1100 °C, 1200 °C, and 1300 °C for 1 h respectively, the filament tensile strength retention rate of Z80A20 ceramic fibers was 87 %, 80 %, and 40 %. While Z10A90 ceramic fiber was fragile after being heated at 1300 °C. The results showed that the high zirconia content facilitated the fiber's thermal stability.

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.

Space charge layer evolution at yttria-stabilized zirconia grain boundaries upon operation of solid oxide fuel cells

Long-lasting solid oxide fuel cells (SOFC) rely upon an intact ionically conducting network in both the electrolyte and electrode. Understanding the impact of the high-temperature electrochemical operations on the possible nanostructure evolutions of ionic conductors is indispensable for the development of durable SOFC. Here we present possibly the first experimental evidence of space charge layer evolution of grain boundaries (GBs) of ionic conducting yttria-stabilized zirconia (YSZ) in a Ni/YSZ composite anode of SOFCs. Cells with identical Ni/YSZ composite anodes are either thermally treated or electrochemically operated using either dry or humidified H2 fuel for up to 3,349 h. When operating under humidified H2 fuel, a discrete core-shell structure is found to develop exclusively along the YSZ/YSZ GB planes. The core-shell structure is elliptically shaped elongated along the GB planes. The core has a lower mass with a Y-depleted ZrO2-x with a cubic structure. The shell has a constant thickness of ∼1 nm and is Y-enriched. The nucleation and growth mechanisms of the core-shell structure are proposed in terms of the space charge layer evolution at YSZ GBs. The effect of the humidity in the fuel, the cell operating temperatures, and the current density on the formation of the core-shell structure is investigated.