Grain-boundary cation diffusion in ceria tetragonal zirconia determined by constant-strain-rate deformation tests

Ceria tetragonal zirconia polycrystals with a content of 12 mol% ceria (CeTZP) have been tested in compression at constant strain rate between 1150 °C and 1300 °C. An accurate analysis of the stress–strain curves has permitted to determine the value of the grain boundary cation diffusion. The results are compared with those reported in literature for this alloy and yttria tetragonal zirconia polycrystals (YTZP). An isotopic effect is found to correlate both grain boundary diffusion coefficients.     

Influence of additives on the purity of tetragonal phase and grain size of ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP)

A comprehensive study on the influence of typical additives on zirconia (ZrO₂) crystallization was presented. Zirconium nitrate pentahydrate (Zr(NO₃)₄·5H₂O) and cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O) were employed as reagents, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) or glycerol were adopted as additives, and ammonia water was adopted as pH regulator. The ZrO₂ powders were prepared by hydrothermal method. The crystal phase purity, grain size and micro morphology of the ZrO₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) to investigate the influence of EDTA-2Na, glycerol and Ce⁴⁺ content on the purity of tetragonal phase and the grain size of ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP). It was found that EDTA-2Na could decrease the purity of tetragonal phase and alter the grain size of Ce-TZP nonlinearly, while glycerol could not decrease the purity of tetragonal phase and the grain size of Ce-TZP, and the grain size was not linear with the amount of glycerol; Doping Ce⁴⁺ could increase the purity of tetragonal phase of zirconia but could not decrease the grain size, and the grain size was not linear with the Ce⁴⁺ content; In addition, it was indicated that EDTA-2Na and glycerol could not improve the distribution uniformity of Ce⁴⁺. This study is expected to have provided a novel path to achieve tailored ZrO₂ crystals with reduced low-temperature degradation.     

Cyclic martensitic transformations and damage evolution in shape memory zirconia: Single crystals vs polycrystals

In zirconia-based shape memory ceramics (SMCs), cracking during the martensitic transformation can be avoided in structures that reduce the relative presence of grain boundaries where high levels of transformation mismatch stress develop. This approach has been well established in single crystals, but only for sample sizes below about 5 microns. In this paper, we extend the strategy of eliminating grain boundaries to bulk specimen scales by fabricating mm-scale single crystal SMCs via cold crucible induction melting. For 1.5 and 2.0 mol% Y₂O₃-ZrO₂, we study the cyclic martensitic transformation of both single crystal and polycrystal structures. Whereas single crystals have very repeatable transformation behavior in terms of hysteresis and strain amplitude, polycrystals degrade dramatically as they accumulate cracking damage with repeated cycling. As the polycrystal evolves from a pellet to granular packing of loose single crystals/grains, the energy dissipation converges with that of the single-crystal structure, and the energy spent on cracking throughout that process is captured by calorimetry analysis. These results verify that grain boundaries play a key role in damage evolution during martensitic transformation and that microstructural control can extend the size-scale of viable single crystal or oligocrystal SMCs from the micro- to the millimeter scale.     

Effects of twin boundaries and pre-existing defects on mechanical properties and deformation mechanisms of yttria-stabilized tetragonal zirconia

In annealing of yttria-stabilized tetragonal zirconia (YSTZ), {011}-specific twins and sub-surface defects are often observed, however their effects on the martensitic phase transformation and deformation behavior of YSTZ have never been investigated. In this work, the roles of twin boundaries (TBs) and pre-existing defects in determining the mechanical properties and subsequent deformation mechanisms of YSTZ nanopillars are studied. Using large-scale molecular dynamics simulations, we show that Young’s modulus and strength of YSTZ decrease with the increase of TB density, but the ductility of YSTZ pillars increases. Phase transformation behavior is found to be correlated to TB density. The sensitivity of mechanical responses of twinned structures to pre-existing defects is also studied. A competitive mechanism between TB-induced phase transformation and void-induced phase transformation is observed. When the diameter of a pre-existing void is smaller than a critical value, only TB-induced phase transformation occurs, which leads to void-insensitive mechanical properties.     

Enhanced toughness of zirconia ceramics with graphene platelets consolidated by spark plasma sintering

Graphene platelets reinforced zirconia (GPLs/ZrO₂) composites were prepared by spark plasma sintering in the present work. The effects of GPLs content on the densification route, microstructure feather, mechanical properties, and aging behaviors of such composites were investigated. In spite of the impeding effect of GPLs, high relative density of 98% was achieved for the composites owing to the uniform dispersion of GPLs. The addition of GPLs contributed to enhanced fracture toughness of the composites; when the added content was 1.0 wt.%, its fracture toughness reached up to 8.6 MPa·m^1/2. Also, aging behavior of the GPLs/ZrO₂ composites was investigated at 134°C for 24 hours. The monolithic ZrO₂ ceramic and GPLs/ZrO₂ composites presented residual ratio of 55% and 72% in fracture toughness, respectively. Thus, the incorporation of GPLs inhibited phase transformation from tetragonal phase to monoclinic phase of zirconia.     

Thermodynamic analysis and experimental verification of interface reactions of iron aluminide/tetragonal zirconia composite

Abstract

This paper described the thermodynamic analysis and experimental verification of interface reactions between iron aluminide intermetallic and tetragonal zirconia. Thermodynamic analysis confirmed that chemical reactions between Fe–Al intermetallic and ZrO₂ (3 mol.–%Y₂O₃ stabilised zirconia) mainly depended on the Al content in Fe–Al intermetallic. For ZrO₂(3Y)/Fe₃Al composite, the interface reactions to form Al₂O₃ and ZrAl₂ would take place when Al content was >40 at-% in Fe–Al intermetallic, while no interface reaction occurred when using Fe₃Al as toughening phase. ZrO₂(3Y)/Fe₃Al composite was synthesised by hot press sintering to further verify the thermodynamic analysis of interface reactions between iron aluminide intermetallic and tetragonal zirconia. The phase composition, morphology and interface structure of ZrO₂(3Y)/Fe₃Al were investigated by X-ray diffraction, SEM and TEM. The results show that Fe₃Al was thermodynamic stable in ZrO₂(3Y) matrix, which was in good agreement with thermodynamically analysis.     

Effect of Ultrasonication on the Microstructure and Tensile Elongation of Zirconia-Dispersed Alumina Ceramics Prepared by Colloidal Processing

To obtain dense, fine-grained ceramics, fine particles and advanced powder processing, such as colloidal processing, are needed. Al₂O₃ and ZrO₂ particles are dispersed in colloidal suspensions by electrosteric repulsion because of polyelectrolyte absorbed on their surfaces. However, additional redispersion treatment such as ultrasonication is required to obtain dispersed suspensions because fine particles tend to agglomerate. The results demonstrate that ultrasonication is effective in improving particle dispersion in suspensions and producing a homogeneous fine microstructure of sintered materials. Superplastic tensile ductility is improved by ultrasonication in preparing suspensions because of the dense and homogeneous fine microstructure.     

Microstructure and electrical properties of 3Y-TZP/Al2O3 composite obtained using the citrate gel method

3Y-TZP sinters with 1, 5, 10, and 15 mol% of Al₂O₃ were prepared using two different procedures from a 3-YSZ powder synthesized via the citrate process. In the first procedure, alumina was introduced during synthesis via the citrate method. In the second one, the 3-YSZ powder was impregnated with aluminum nitrate. All samples were sintered at 1773 K. The prepared composites were evaluated in terms of their microstructure, chemical and phase composition, and electrical properties. The total conductivity of the 3Y-TZP/Al₂O₃ composite material, which contained primarily the tetragonal phase, was found to increase with temperature, and to decrease for reduced concentrations of alumina in 3Y-TZP. In the case of the samples which had alumina introduced via impregnation, its higher content was not associated with significantly lower electrical conductivity. These samples generally exhibited higher conductivity than the samples to which alumina had been added via chemical synthesis.     

Microstructure and Fracture Toughness of Yttria-Daped Tetragonal Zirconia Polycrystal/Mullite Composites Prepared by an in Situ Method

Yttria-daped tetragonal zirconia polycrystal (Y-TZP)/ mullite composites were prepared by three methods: in situ whisker growth (IS), physical mixing (PM) of zirconia powder and mullite whiskers, and reaction sintering (RS). Microstructures and fracture toughness values were compared. All the composites with 15 vol% of mullite could be densified to more than 95% relative density by firing at 1500° to 1500°C for 10 h. The fracture toughness of the composites as measured by the indentation method showed a clear enhancement compared with that of pure Y-TZP; the ranking was Y-TZP ≦ RS composite < PM composite < IS composite. Enhancement of the fracture toughness in composites was found to relte strongly to the aspect ratio of mullite particles.     

水热法合成羟磷灰石及其包覆二氧化锆粉体的制备

羟磷灰石(HA)单独作为生物陶瓷材料存在着强度和韧性的缺陷,加入二氧化锆可以在不影响其生物活性的前提下达到增强增韧的目的.首先研究了pH及水热时间对HA结晶的影响,得出了制备HA的较佳的水热条件.在此基础上,在二氧化锆水合物ZrO2·xH2O微粒上分步加入钙和磷酸根离子,并同时控制pH,使HA在ZrO2·xH2O表面沉积,从而制备出具有包覆结构的胶粒,然后进行水热处理.试验表明,HA与二氧化锆的水热结晶相互影响很大,被HA包覆的二氧化锆很难晶化,证明了包覆结构紧密.     

Microstructure and Bending Strength of 3Y-TZP/12Ce-TZP Ceramics Fabricated by Liquid-Phase Sintering at Low Temperature

With the addition of 1 wt% of MgO–Al₂O₃–SiO₂ glass as a sintering aid, 3Y-TZP/12Ce-TZP ceramics (composed from a mixture of 3Y-TZP and 12Ce-TZP powder) have been fabricated via liquid-phase sintering at 1250°–1400°C. In the sintered bodies, the grain growth of Y-TZP is almost unaffected, whereas that of Ce-TZP is inhibited. MgO·Al₂O₃ spinel and an amorphous phase that contains Al₂O₃ and SiO₂ (from the sintering aid) fully fill the grain junctions. The bending strength of 3Y-TZP/12Ce-TZP, when sintered at 1250°–1300°C, is ∼800–900 MPa, which is greater than that of 3Y-TZP ceramics without Ce-TZP particles. Ce-TZP grains and MgO·Al₂O₃ spinel in 3Y-TZP/12Ce-TZP ceramics may impede crack growth, and the bending strength is enhanced.     

Mechanical and electrochemical behavior of novel electrolytes based on partially stabilized zirconia for solid oxide fuel cells

In this study, 3 mol% yttria stabilized zirconia (3YSZ) is investigated as a SOFC electrolyte alternative to 8 mol% yttria stabilized zirconia (8YSZ). The mechanical and electrochemical properties of both materials are compared. The mechanical tests indicate that the thickness of 3YSZ can be reduced to half without sacrificing the strength compared to 8YSZ. By reducing the thickness of 3YSZ from 150 µm to 75 µm, the peak power density is shown to increase by around 80%. The performance is further enhanced by around 22% by designing of novel electrode structure with regular cut-off patterns previously optimized. However, the cell with novel designed 3YSZ electrolyte exhibits 30% lower maximum power density than that of the cell with 150 µm-thick standard 8YSZ electrolyte. Nevertheless, the loss in the performance may be tolerated by decreasing the fabrication cost revealing that 3YSZ electrolyte with cut-off patterns can be employed as SOFC electrolyte alternative to 8YSZ.     

An in situ Raman study of dimethyl carbonate synthesis from carbon dioxide and methanol over zirconia

In situ Raman spectroscopy has been used to investigate the mechanism of dimethyl carbonate (DMC) synthesis via the reaction of methanol with carbon dioxide over zirconia. Methanol adsorption leads to the appearance of adsorbed methoxide groups, whereas CO₂ adsorption leads to the formation of carbonate species. Monomethyl carbonate species, (CH₃O)COO(Zr)₂, are formed by the reaction of methoxide and monodentate carbonate species and DMC is formed via the further reaction of monomethyl carbonate species with methanol. This sequence is supported by evidence that DMC decomposition on zirconia proceeds via the reverse of the proposed mechanism.     

预渗入法衬瓷层中氧化锆分布规律

针对表面涂层技术工艺复杂,容易剥落等问题,提出了预渗入法制备陶瓷涂层技术。即预先将坯体造孔,再用纳米陶瓷粉与合金粉通过特殊方式合成制成复合粉,将该复合粉涂于有孔坯体的表面,使粉料进入孔洞中;在气氛保护条件下烧结,使复合粉与基体材料同时烧结并形成冶金结合界面,有效提高了表面性能。利用扫描电镜观察了衬瓷层的显微组织,分析了纳米氧化锆陶瓷成分在衬瓷层中的分布规律;发现ZrO2含量为20%-40%时均可得到较为理想的衬瓷层;氧化锆陶瓷相从表面向基体呈递减趋势;决定衬瓷层的厚度的最重要原因是直孔洞的深度。     

Fabrication and properties of in situ reduced graphene oxide‐toughened zirconia composite ceramics

Graphene oxide and zirconia powders were mixed using a colloidal coating route. In situ reduced graphene oxide‐toughened zirconia ceramics were prepared by spark plasma sintering. Their microstructure, mechanical properties, and toughening mechanisms were investigated. The results show that graphene oxide can be easily reduced in situ during sintering and that it disperses homogeneously within the zirconia substrate. Compared with the toughness of 3 mol.% yttria‐stabilized zirconia, the fracture toughness of in situ reduced graphene oxide‐toughened zirconia increased by up to 175% (from ~6.07 to ~10.64 MPa·m^1/2) at 0.09 wt.% graphene oxide with a small increase in hardness. The improvement is more significant than that of prereduced graphene oxide‐toughened cases, and it is associated with the formation of a C‐O‐Zr bond at the interface in addition to conventional toughening mechanisms.     

High-efficiency shape memory copolymers of polycaprolactone/thermoplastic polyurethane fabricated via in situ ring-opening polymerization

Shape memory blends of polycaprolactone/thermoplastic polyurethane (PCL/TPU) were prepared by in situ ring-opening polymerization (ROP) of ε-caprolactone (CL) and thermoplastic polyurethane (TPU). Fourier infrared spectrometer and ¹H-NMR were used to characterize the chemical structure of PCL/TPU copolymers. The results show that TPU has been involved in the ROP of CL, leading to the formation of copolymers with homogeneous morphologies. Besides, pure PCL and all the blends exhibit an excellent shape fixation ratio of 100%, due to their high crystallinity. When a small amount of TPU is introduced, the crystallinity of PCL decreases, and as a result, the shape recovery ratio of the copolymer is enhanced compared with pure PCL. However, with the increased loading of TPU, the content of PCL as the reversible phase decreases and the storage modulus of the PCL/TPU blend declines, so the driving force for the blends to return from the temporary shape to the initial shape becomes smaller, leading to a decrease in the shape recovery ratio. Notably, when the amount of TPU is only 5%, the shape recovery ratio of the blend could reach 83.3%, which is 26% higher than that of pure PCL, and meanwhile, the tensile strength of the blend decreases slightly. This study provides a new strategy for the design of shape-memory materials with high shape-memory properties.     

Effect of annealing on grain growth and Y segregation behavior in tetragonal ZrO2 thin film

In order to fabricate tetragonal yttria stabilized zirconia samples with large grain size, 3 mol% Y₂O₃ doped zirconia thin films were grown on (0001) α-Al₂O₃ substrate by pulsed laser deposition (PLD) followed by subsequent high temperature annealing. The thin film samples were annealed at 1200°C, 1250°C, 1300°C, and 1350°C in order to obtain larger grain size without Y segregation. The microstructure and chemical composition of these annealed films were analyzed using atomic force microscopy, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The as-grown thin film was found to be composed of [111]-oriented grains of ∼100 nm connected with small-angle tilt boundaries. Based on analysis of annealed thin films, it was revealed that grain growth of tetragonal zirconia occurred anisotropically. Cross section scanning transmission electron microscopy observations revealed that such grain growth behavior is affected by the step-terrace structures of the sapphire substrate. Energy-dispersive X-ray spectroscopy showed that Y was found to distribute almost uniformly below 1300°C but to segregate at the grain boundaries at 1350°C. As a conclusion, the 1300°C-annealed sample shows the largest grain size with homogeneous Y distributions.     

Temperature-dependent mechanical and oxidation behavior of in situ formed ZrN/ZrO2-containing Si3N4-based composite

In this work, Si₃N₄ and Zr(NO₃)₄ were used as raw materials to prepare ZrN/ZrO₂-containing Si₃N₄-based ceramic composite. The processing, phase composition, and microstructure of the composite were investigated. Hardness and fracture toughness of the ceramics were evaluated via Vickers indentation in Ar at 25°C, 300°C, 600°C, and 900°C. During spark plasma sintering, Zr(NO₃)₄ was transformed into tetragonal ZrO₂, which further reacted with Si₃N₄, resulting in the formation of ZrN. The introduction of ZrN enhanced the high-temperature mechanical properties of the composite, and its hardness and fracture toughness reached 13.4 GPa and 6.1 MPa·m^1/2 at 900°C, respectively. The oxidation experiment was carried out in air at 1000°C, 1300°C, and 1500°C for 5 h. It was shown that high-temperature oxidation promoted the formation and growth of porous oxide layers. The microstructure and phase composition of the formed oxide layers were investigated in detail. Finally, it was identified that the obtained composite exhibited a higher thermal diffusivity than that of monolithic Si₃N₄ in the temperature range of 100°C–1000°C.     

Microstructure and mechanical properties of zirconia stabilized with increasing Y2O3, for use in dental restorations

The present in vitro study aims at characterizing dental zirconia ceramics, which are stabilized with a high amount of Y₂O₃. Two groups of specimens were fabricated by computer-aided design/computer-aided manufacturing technique. The specimens of each group were divided into two subgroups (SGs): SGs 1a and 2a contained a relatively low amount of Y₂O₃ (6–8 wt.%), whereas SGs 1b and 2b contained a higher amount of Y₂O₃ (8–10 wt.%). The influence of yttria content on their microstructure and mechanical properties was experimentally determined. The statistical significance of the differences in the mechanical properties between the SGs was evaluated by the t-test (p < 5% was considered statistically significant). Homogeneous and dense ceramics with fine nanostructure, comprising grains of yttria-stabilized tetragonal and cubic zirconia, sized between ∼160 and ∼800 nm, were produced. The increase of yttria content, which causes an increase in grain size, favors the formation of cubic zirconia, resulting in mechanical properties’ slight reduction; yet, the differences were not statistically significant. Consequently, the mechanical properties (HV 11.74–12.91 GPa, and K_IC 2.66–4.25 MPa m^0.5) and the good esthetics of the investigated zirconia ceramics stabilized with high yttria content qualify these zirconia materials for fabricating dental restorations, because they can approach the properties and the esthetics of dental hard tissues as well as the tooth structure.     

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

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