Low-Temperature Processing and Mechanical Properties of Zirconia and Zirconia–Alumina Nanoceramics

The 1.5- to 3-mol%-Y₂O₃-stabilized tetragonal ZrO₂ (Y-TZP) and Al₂O₃/Y-TZP nanocomposite ceramics with 1 to 5 wt% of alumina were produced by a colloidal technique and low-temperature sintering. The influence of the ceramic processing conditions, resulting density, microstructure, and the alumina content on the hardness and toughness were determined. The densification of the zirconia (Y-TZP) ceramic at low temperatures was possible only when a highly uniform packing of the nanoaggregates was achieved in the green compacts. The bulk nanostructured 3-mol%-yttria-stabilized zirconia ceramic with an average grain size of 112 nm was shown to reach a hardness of 12.2 GPa and a fracture toughness of 9.3 MPa·m^1/2. The addition of alumina allowed the sintering process to be intensified. A nanograined bulk alumina/zirconia composite ceramic with an average grain size of 94 nm was obtained, and the hardness increased to 16.2 GPa. Nanograined tetragonal zirconia ceramics with a reduced yttria-stabilizer content were shown to reach fracture toughnesses between 12.6–14.8 MPa·m^1/2 (2Y-TZP) and 11.9–13.9 MPa·m^1/2 (1.5Y-TZP).     

Preparation and Characterization of Fine-Grained 3Y-TZP/BaFe12O19 In Situ Composites

Composites of BaFe₁₂O₁₉ particles dispersed throughout a 3-mol%-yttria-doped zirconia (3Y-TZP) matrix have been prepared using the pressureless reactive sintering of 3Y-TZP, BaCO₃, and γ-Fe₂O₃ powders. The reaction behavior of the mixed powder was studied with an in situ , high-temperature powder X-ray diffraction technique. The composite that was sintered at 1300°C consisted of submicrometer-sized 3Y-TZP grains and BaFe₁₂O₁₉ particles; the size of the 3Y-TZP grains was ∼100-300 nm, and the size of the BaFe₁₂O₁₉ particles was ∼200-500 nm. Based on the grain size, most of the BaFe₁₂O₁₉ grains presumably had a single-magnetic-domain structure. The 3Y-TZP/20-wt%-BaFe₁₂O₁₉ composite showed high magnetization and coercivity values, despite the low concentration of ferromagnetic phase. Preliminary mechanical tests revealed that the composite possessed moderately good mechanical properties.     

Preparation and Characterization of Platinum–Ceramic Composite Powders and Thin Films

Pt–ceramaic (TiO₂, ZrO₂, and ZrO₂–5 mol% Y₂O₃ (PSZ)) composite powders were prepared by liquid-phase reaction. These composite powders show highly functional properties such as the following. The sintered films of these powders are good metallic conductors. The resistivity of the Pt–PSZ film at room temperature is about 1.5 × 10⁻⁵Ω·cm, which is almost comparable to that of pure platinum (1.05 × 10⁻⁵Ω·cm). This film shows excellent cathodic oxygen reduction. The overpotential for the oxygen reduction at 750°C is less than 20 mV even at a high current density of 0.1 A/cm². The Pt–TiO₂ composite powders have highly catalytic activity for the reduction of NO gas.     

One-step hydrothermal synthesis of yttria-stabilized tetragonal zirconia polycrystalline nanopowders for blue-colored zirconia-cobalt aluminate spinel composite ceramics

A novel approach for the preparation of blue-color giving zirconia nanopowders by doping of 3?mol% Y₂O₃ through simple one-step hydrothermal process is proposed. A blue-color giving yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) powders were prepared by urea-based solution, zirconium acetate, CoCl₂·6H₂O and AlCl₃ precursors in hydrothermal vessel at 24?h, 150?°C and 20 bars. Based on the results, the synthesized blue-color giving Y-TZP nanopowders have entirely tetragonal structure as mono phase with 3.8?±?0.2?nm average grain size and (Y, Co, Al)_xZrO₂; x?≤?0.03?at. with chemical composition. Thermal treatment was also applied to synthesized Y-TZP powders at 1200?°C and 1450?°C to observe the color evolution. Only sharp blue was obtained in Y-TZP powders resulting the development of zirconia-cobalt aluminate spinel (ZrO₂-CoAl₂O₄) composite ceramic structure for both temperatures after heat-treatment. Herein, not only formation of CoAl₂O₄ but also incorporation of cobalt (Co) and aluminum (Al) into the Y-TZP grains plays a critical role on evolving of blue color. This synthesized Y-TZP nanopowders can be a good candidate for one-step production of blue-color sintered ZrO₂-CoAl₂O₄ spinel composite ceramics in numerous ceramic applications due to their superior structural and functional properties.     

Mechanical and Magnetic Properties of Novel Yttria-Stabilized Tetragonal Zirconia/Ni Nanocomposite Prepared by the Modified Internal Reduction Method

Dense ceramic/metal nanocomposite has been fabricated by internal reduction method, which includes a two-step process: sintering of ceramic–metal oxide solid solution and subsequent heat treatment in a reducing atmosphere to precipitate metal nanoparticles. This novel technique has been applied to yttria-stabilized tetragonal zirconia (Y-TZP) and nickel oxide (NiO) system to fabricate Y-TZP/Ni nanocomposite. Dense Y-TZP and 0.3 mol% NiO solid solution ceramic was successively prepared by the pressureless sintering, and Y-TZP/Ni was fabricated by the internal reduction treatment. The obtained Y-TZP/Ni nanocomposite possessed characteristic intragranular nanostructure with nano-sized metallic Ni particles of around 20 nm. Fracture toughness of both the solid solution and nanocomposite was remarkably improved because of the solid solution of NiO into Y-TZP and resultant destabilization of the tetragonal phase, and the Y-TZP/Ni nanocomposite was still destabilized by the remaining nickel solution after the reduction. The nanocomposite exhibited ferromagnetism, while the Y-TZP–NiO solid solution had diamagnetic nature. Comparison of saturation magnetization values revealed that 39.5 at.% of introduced nickel was reduced to metallic nanoparticle, proving the existence of residual NiO solute in zirconia that contributed to higher toughness value than the monolithic Y-TZP. It is concluded that the introduced internal reduction method is a suitable process to achieve multifunctional ZrO₂/Ni nanocomposite with high toughness and coexistent magnetic characteristic.     

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.     

Effect of in-situ formed whiskers diameter from tourmaline on microstructure and mechanical properties of 3Y-TZP ceramics

In this paper, in-situ whiskers reinforced 3 mol% Y₂O₃ stabilized tetragonal ZrO₂ (3Y-TZP) ceramics with different diameters were prepared using pressureless sintering by introducing tourmaline with different particle sizes into 3Y-TZP powders. The purpose of this research was to investigate the influence of in-situ formed whisker diameters on the densification, microstructure and mechanical properties of 3Y-TZP ceramics. The prepared ceramics were characterized by X-ray diffraction, scanning electron microscope and transmission electron microscope. Findings indicated that in-situ mullite whiskers formed by phase transformation of tourmaline particles can promote the densification of 3Y-TZP ceramics, and further improve the dispersion of mullite whiskers in the 3Y-TZP ceramics. More importantly, the average diameter of mullite whiskers can be controlled by altering the tourmaline particle size. When the average particle size of tourmaline is 500 nm, 3Y-TZP composites have a near-fully dense microstructure of 99.09%, with the ZrO₂ grain size of about 335 nm, the average diameter of mullite whiskers is 330 nm. Both the bending strength and fracture toughness reached optimal values of 836 ± 24 MPa and 10.6 ± 0.5 MPa m^0.5, respectively. This paper provides a new way to design of the microstructure and strength-toughness of zirconia composite ceramics.     

Manipulating microstructure and mechanical properties of CuO doped 3Y-TZP nano-ceramics using spark-plasma sintering

Nano-powder composites of 3Y-TZP doped with 8 mol% CuO were processed by spark-plasma sintering (SPS). A 96% dense composite ceramic with an average grain size of 70 nm was obtained by applying the SPS process at 1100 °C and 100 MPa for 1 min. In contrast to normal, pressureless, sintering during SPS reactions between CuO and 3Y-TZP were suppressed, the CuO phase was reduced to metallic Cu, while the 3Y-TZP phase remained almost purely tetragonal. Annealing after SPS results in grain growth and tetragonal to monoclinic zirconia phase transformation. The grain size and monoclinic zirconia phase content are strongly dependent on the annealing temperature. By combining the processing techniques studied in this work, including traditional pressureless sintering, properties of the composite ceramic can be tuned via manipulation of microstructure. Tuning the mechanical properties of dense 8 mol% CuO doped 3Y-TZP composite ceramic by utilising different processing techniques is given as an example.     

Analysis of reactions during sintering of CuO-doped 3Y-TZP nano-powder composites

3Y-TZP (yttria-doped tetragonal zirconia) and CuO nano powders were prepared by co-precipitation and copper oxalate complexation–precipitation techniques, respectively. During sintering of powder compacts (8 mol% CuO-doped 3Y-TZP) of this two-phase system several solid-state reactions clearly influence densification behaviour. These reactions were analysed by several techniques like XPS, DSC/TGA and high-temperature XRD. A strong dissolution of CuO in the 3Y-TZP matrix occurs below 600 °C, resulting in significant enrichment of CuO in a 3Y-TZP grain-boundary layer with a thickness of several nanometres. This “transient” liquid phase strongly enhances densification. Around 860 °C a solid-state reaction between CuO and yttria as segregated to the 3Y-TZP grain boundaries occurs, forming Y₂Cu₂O₅. This solid-state reaction induces the formation of the thermodynamic stable monoclinic zirconia phase. The formation of this solid phase also retards densification. Using this knowledge of microstructural development during sintering it was possible to obtain a dense nano–nano composite with a grain size of only 120 nm after sintering at 960 °C.     

Design of High-Quality Pt–CeO2 Composite Anodes Supported by Carbon Black for Direct Methanol Fuel Cell Application

A Pt on nano-sized CeO₂ particles that in turn are supported on carbon black (CB) was synthesized using the co-impregnation method. This potential anode material for fuel cell applications was synthesized in a stepwise process. The pure CeO₂ was synthesized using an ammonium carbonate precipitation method, and the Pt particles dispersed on the CeO₂ in such a way that a uniform dispersion with the CB was obtained (Pt–CeO₂/CB). The electrochemical activity of the methanol (CH₃OH) oxidation reaction on the Pt–CeO₂/CB was investigated using cyclic voltammetry and chronoamperometry experimentation. The onset potential of CH₃OH oxidation reaction on the Pt–CeO₂/CB anode was shifted to a lower potential as compared with that on commercially available Pt–Ru/carbon (C) alloy anode. In addition, the activation energy of the Pt–CeO₂/CB anode was much lower than that of the Pt–Ru/C alloy anode. Moreover, the current density of the Pt–CeO₂/CB anode was much higher than that of the Pt–Ru/C alloy anode at temperatures between 28° and 60°C. These results suggest that the anode performance of the Pt–CeO₂/CB anode at the operating temperature of typical fuel cells (80°C) is superior to that of the more usual Pt–Ru/C alloy anode. Importantly, the rare metal, Ru, is not required in the present anode material and the amount of Pt required is also significantly reduced. As a consequence, we report a promising candidate Pt–CeO₂/CB composite anode for application in the development of direct methanol fuel cells.     

Synthesis,sintering and microstructure of 3Y-TZP/CuO nano-powder composites

Nanocrystalline 3Y-TZP and copper-oxide powders were prepared by co-precipitation of metal chlorides and copper oxalate precipitation respectively. CuO (0.8 mol%) doped 3Y-TZP powder compacts were prepared from the nanocrystalline powders. Dilatometer measurements on these compacts were performed to investigate the sintering behaviour. Microstructure investigations of the sintered compacts were conducted. It is found that additions of the copper-oxide powders in the nanocrystalline 3Y-TZP leads to an enhancement of densification, formation of monoclinic zirconia phase and significant zirconia grain growth during sintering.     

Cubic-Formation and Grain-Growth Mechanisms in Tetragonal Zirconia Polycrystal

The microstructure in Y₂O₃-stabilized tetragonal zirconia polycrystal (Y-TZP) sintered at 1300°–1500°C was examined to clarify the role of Y³⁺ ions on grain growth and the formation of cubic phase. The grain size and the fraction of the cubic phase in Y-TZP increased as the sintering temperature increased. Both the fraction of the tetragonal phase and the Y₂O₃ concentration within the tetragonal phase decreased with increasing fraction of the cubic phase. Scanning transmission electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) measurements revealed that cubic phase regions in grain interiors in Y-TZP generated as the sintering temperature increased. High-resolution electron microscopy and nanoprobe EDS measurements revealed that no amorphous layer or second phase existed along the grain-boundary faces in Y-TZP and Y³⁺ ions segregated at their grain boundaries over a width of ∼10 nm. Taking into account these results, it was clarified that cubic phase regions in grain interiors started to form from grain boundaries and the triple junctions in which Y³⁺ ions segregated. The cubic-formation and grain-growth mechanisms in Y-TZP can be explained using the grain boundary segregation-induced phase transformation model and the solute drag effect of Y³⁺ ions segregating along the grain boundary, respectively.     

两步烧结法制备纳米氧化钇稳定的四方氧化锆陶瓷

采用共沉淀法制备纳米氧化钇稳定的四方氧化锆（yttria stabilized tetragonal zirconia,3Y-TZP）粉体。利用X射线衍射、N2吸附–脱附等温线,透射电子显微镜对3Y-TZP粉体的物理性能和化学性能进行表征。研究了纳米3Y-TZP粉体的烧结曲线,分析了3Y-TZP素坯在烧结过程中的致密化行为和显微结构,探讨了两步烧结工艺对3Y-TZP纳米陶瓷微观结构的影响。结果表明：采用共沉淀法,在600℃煅烧2h后,可获得晶粒尺寸为13nm、晶型发育良好、团聚较少的纳米3Y-TZP粉体;采用两步烧结法,将素坯升温至1200℃保温1min后,再降温到1050℃保温35h,可获得相对密度大于98%,晶粒尺寸约为100nm的3Y-TZP陶瓷。两步烧结法通过控制煅烧温度和保温时间,利用晶界扩散及其迁移动力学之间的差异,使晶粒生长受到抑制,样品烧结致密化得以维持,实现在晶粒无显著生长前提下完成致密化。     

Sintering behaviour and microstructure of 3Y-TZP + 8 mol% CuO nano-powder composite

Nanocrystalline 3Y-TZP and copper-oxide powders were prepared by co-precipitation of metal chlorides and copper oxalate complexation–precipitation, respectively. A significant enhancement in sintering activity of 3Y-TZP nano-powders, without presence of liquid phase, was achieved by addition of 8 mol% CuO nano-powder, resulting in an extremely fast densification between 750 and 900 °C. This enhancement in sintering activity was explained by an increase in grain-boundary mobility as caused by dissolution of CuO in the 3Y-TZP matrix. The nano-powder composite was densified to 96% by pressureless sintering at 1130 °C for 1 h. Considerable tetragonal to monoclinic phase transformation of the zirconia phase was observed by high temperature XRD analysis. This zirconia phase transformation is discussed in terms of reactions between CuO and yttria as segregated to the 3Y-TZP grain boundaries.     

The calcination temperature after platinum addition and its effect on Pt/WOx–ZrO2 properties

Platinum on tungsten oxide promoted zirconia was prepared using two different platinum precursors: hexachloroplatinic acid and tetraammine platinum nitrate. The catalysts were calcined at different temperatures after platinum addition. The materials were characterized by thermal-programmed reduction, transmission electron microscopy, hydrogen chemisorption at room temperature and they were used for n-hexane isomerization reaction. The calcination temperature after metal addition plays an important role on the catalyst properties. Calcination above 500 °C after platinum addition is not convenient because of the migration of zirconia species over Pt, thus decreasing the hydrogen dissociation capacity of Pt.     

玻璃渗透氧化锆全陶瓷牙科材料的制备和表征

开发了一种适合渗透多孔四方相氧化锆的玻璃,并通过玻璃渗透工艺制备了氧化锆-玻璃全陶瓷牙科材料. 研究表明,该渗透玻璃在渗透温度下(1100~1200℃)具有合适的粘度、良好的渗透性和化学相容性,且热膨胀系数与氧化锆匹配;熔融态玻璃通过毛细管作用力填充预烧后的多孔四方相氧化锆坯体的孔隙,形成氧化锆和玻璃相互交融的致密的三维网络结构,渗透过程中没有发生氧化锆从四方相到单斜相的转变. 该氧化锆-玻璃复合材料的弯曲强度和断裂韧性分别为400 MPa和5.5 MPa×m1/2,较氧化玻璃复合材料分别提高了32%和41%.     

Effect of Fe2O3 doping on colour and mechanical properties of Y-TZP ceramics

Yttria-stabilized zirconia (Y-TZP) samples with different Fe concentrations were prepared aiming to study the effects of Fe₂O₃ doping on colour and mechanical properties. Since colour is an important optical property for jewellery and watchmaking, the investigation of colour in zirconia ceramics has a great scientific and technological interest. An investigation of the mechanical and optical properties, specifically the colour, was developed starting from commercial partially yttria-stabilized zirconia (Y-TZP) powders produced by Emulsion Detonation Synthesis (EDS). Within the strategies to get colours, the use of colouring oxides such as iron oxide (Fe₂O₃) was the chosen approach. The addition of specific ions into the ZrO₂ matrix can be used to tune zirconia colour without compromising its outstanding mechanical properties. Doping with iron oxide has proved to be a suitable, reproducible and irreversible colouring mechanism, allowing the development of a chromatically beige stable material with respect to its use in different processing conditions such as different atmospheres and temperature ranges. XRD results suggested that iron ions dissolved into tetragonal zirconia phase are at interstitial positions since the unit-cell volume of the tetragonal zirconia increases with increasing iron content. The effect of dopant addition on the mechanical properties of Y-TZP ceramics was also assessed. Compared to the undoped samples, doped ones exhibit a similar Vickers hardness (>1200?MPa) and biaxial flexural strength (>1000?MPa). However, it was observed that Fe₂O₃ doping slightly decreased the fracture toughness of Y-TZP ceramics.     

Effects of artificial aging on the biaxial flexural strength of Ce-TZP/Al2O3 and Y-TZP after various occlusal adjustments

The aim of this study was to determine the effect of aging on the biaxial flexural strength (BFS) of Ce-TZP/Al₂O₃ and Y-TZP after occlusal adjustment. NanoZr block (Ce-TZP/Al₂O₃ nanocomposite) and Katana zirconia block (Y-TZP) were prepared by milling with the aid of CAD/CAM into disk-shaped specimens. For each type of zirconia, 16 specimens were prepared without grinding for the control group (diameter of 16 mm and thickness of 1.20±0.05 mm, mean±SD), while 48 specimens were prepared for 3 experimental groups (n=16 each; 16 mm in diameter and 1.50±0.05 mm thick) with different types of surface grinding: superfine diamond bur (group I), zirconia stone bur (group II), and zirconia stone and fine polishing bur (group III). These specimens underwent an aging process in a steam autoclave for 5 h at 0.2 MPa and 134 °C, and then X-ray diffractometry was applied along with measurements of surface roughness and BFS. After occlusal adjustment, the monoclinic phase percentage increased in 3 experimental groups. Overall the increase was greater for Ce-TZP/Al₂O₃ than for Y-TZP. The Ra value showed similar changes for both types of zirconia. Following the aging process, Y-TZP showed a greater increase in the monoclinic phase percentage, but the change was not statistically significant. The Ra value showed similar changes in both types of zirconia, with no significant differences between before and after the aging process. The results of the BFS test showed that applying the aging process after grinding significantly increased the strength of both types of zirconia, with Ce-TZP/Al₂O₃ being significantly stronger than Y-TZP. The specimens treated by a superfine diamond bur exhibited the highest BFS in the four tested groups. Ce-TZP/Al₂O₃ had a higher BFS and greater resistance to low-temperature degradation than did Y-TZP.     

Fabrication of Dense Arrays of Platinum Nanowires on Silica, Alumina, Zirconia and Ceria Surfaces as 2-D Model Catalysts

High-density arrays of platinum nanowires with dimensions 20 nm × 5 nm × 12 μm (width × height × length) have been produced on planar oxide thin films of silica, alumina, zirconia, and ceria. In this multi-step fabrication process, sub-20 nm single crystalline silicon nanowires were fabricated by size reduction lithography. The Si nanowire patterns were then replicated to produce a high density of Pt nanowires by nanoimprint lithography. The width and height of the Pt nanowires are uniform and are controlled with nanometer precision. The Pt surface area is larger than 2 cm² on a 5 × 5 cm² oxide substrate. The catalytic oxidation of CO was carried out on zirconia-supported Pt nanowires. The reaction conditions (100 Torr O₂, 40 Torr CO, 513–593 K) and vacuum annealing (1023 K) did not change the nanowire structures.     

Gelcast zirconia ceramics for dental applications combining high strength and high translucency

Tetragonal zirconia polycrystals (TZP) represent a favorite material for monolithic ceramic dental restorations. However, all approaches employed so far to improve the translucency of dental zirconia ceramics are accompanied by a significant decline in strength. In this investigation, we developed dental 3Y-TZP ceramics that can provide excellent strength combined with enhanced translucency. The machinable tetragonal zirconia discs and blocks were prepared from fine mesostructured zirconia particles stabilized with 3 mol% of yttria using the gelcasting method. Zirconia ceramics with an average biaxial strength of 1184 MPa and translucency of 41.1% for a 1 mm thick sample were obtained. Due to its unique microstructure, this tetragonal ceramic provided a favorable combination of high translucency comparable to the high-translucent, tetragonal/cubic 4Y-TZP and very high strength achievable only in the pure tetragonal 3Y-TZP. The applicability and resistance to low-temperature degradation of the new dental ceramics was demonstrated.