Processing,microstructure and performance of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics prepared by laser floating zone melting with ultra-high temperature gradient

Directionally solidified Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary eutectic/off-eutectic composite ceramics with high density, homogeneous microstructures, well-oriented growth have been prepared by laser floating zone melting at different solidification rates from 4 to 400 µm/s. Uniform and stable melting zone is obtained by optimizing temperature field distribution to keep continuous and stable eutectic growth and prevent from cracks and defects. The as-solidified composite ceramic exhibits complexly irregular eutectic structure, in which the eutectic spacing is rapidly refined but dotted ZrO₂ number inside Al₂O₃ phase is decreased as increasing the solidification rate. The formation mechanism of ZrO₂ distributed inside Al₂O₃ matrix is revealed by examining the depression of solid/liquid interface. Furthermore, after heat exposure 1500 °C for 200 h, the eutectic microstructure only shows tiny coarsening, which indicates it has excellent microstructural stability. As increasing the ZrO₂ content, the fracture toughness can be improved up to 3.5 MPa m^1/2 at 20.6 mol% ZrO₂.     

Microstructure and cytotoxicity of Al2O3-ZrO2 eutectic bioceramics with high mechanical properties prepared by laser floating zone melting

Developing new generation of strong, tough and stable bioceramics used in dental filed has been highly desired for attaining the clinical requirement of secure and reliable therapy. In this paper, a novel Al₂O₃-ZrO₂ eutectic bioceramics with nearly fully density and extremely aesthetic luster was in-situ prepared by innovative laser floating zone melting (LFZM) method. The influence of solidification rates on microstructure evolution, mechanical properties and cytotoxicity was investigated. The eutectic bioceramics displayed a special three dimensional interpenetrating microstructure evolving with increasing the solidification rate. The eutectic colony structure occurred when solidification rate overpassed 8?µm/s, and lamellar spacing was below 1?µm when solidification rate exceeded 30?µm/s. The eutectic bioceramics solidified at 100?µm/s exhibited optimal mechanical properties with an average hardness of 16.53?GPa, fracture toughness of 6.5?MPa?m^1/2 and flexural strength of 1.37?GPa. The cytotoxicity of Al₂O₃-ZrO₂ eutectic bioceramics was evaluated by MTT methods according to ISO 10993-5 standard. Non-cytotoxic behavior was detected for the eutectic bioceramics, indicating this eutectic bioceramic could be used as promising dental restoration material.     

Investigation of the solidification behavior of Al2O3/YAG/YSZ ceramic in situ composite with off-eutectic composition

Directionally solidified Al₂O₃/YAG/YSZ ceramic in situ composite is an interesting candidate for the manufacture of turbine blade because of its excellent mechanical property. In the present study, two directionally solidified hypoeutectic and hypereutectic Al₂O₃/YAG/YSZ ceramic in situ composites are prepared by laser zone remelting, aiming to investigate the solidification behavior of the ternary composite with off-eutectic composition under high-temperature gradient. The results show that the composition and laser scanning rate significantly influence the solidification microstructure. The ternary in situ composite presents ultra-fine microstructure, and the eutectic interspacing is refined with the increase of the scanning rate. The Al₂O₃/YAG/YSZ hypoeutectic ceramic displays an irregular hypoeutectic network structure consisting of a primary Al₂O₃/YAG binary eutectic and fine Al₂O₃/YAG/YSZ ternary eutectic. Only at low scanning rate, homogeneous ternary eutectic-like microstructures are obtained in the hypoeutectic composition. Meanwhile, the Al₂O₃/YAG/YSZ hypereutectic ceramic shows homogeneous eutectic-like microstructure in most cases and the eutectic interspacing is finer than the ternary eutectic. Furthermore, the formation and evolution mechanism of the off-eutectic microstructure of the ternary composite are discussed.     

Microstructure, growth mechanism and mechanical property of Al2O3-based eutectic ceramic in situ composites

Directionally solidified Al₂O₃-based eutectic ceramic in situ composites with inherently high melting point, low density, excellent microstructure stability, outstanding resistance to creep, corrosion and oxidation at elevated temperature, have attracted significant interest as promising candidate for high-temperature application. This paper reviews the recent research progress on Al₂O₃-based eutectic ceramic in situ composites in State Key Laboratory of Solidification Processing. Al₂O₃/YAG binary eutectic and Al₂O₃/YAG/ZrO₂ ternary eutectic ceramics are prepared by laser zone melting, electron beam floating zone melting and laser direct forming, respectively. The processing control, solidification characteristic, microstructure evolution, eutectic growth mechanism, phase interface structure, mechanical property and toughening mechanism are investigated. The high thermal gradient and cooling rate during solidification lead to the refined microstructure with minimum eutectic spacing of 100 nm. Besides the typical faceted/faceted eutectic growth manner, the faceted to non-faceted growth transition is found. The room-temperature hardness H_V and fracture toughness K_IC are measured with micro-indentation method. For Al₂O₃/YAG/ZrO₂, K_IC = 8.0 ± 2.0 MPa m^1/2 while for Al₂O₃/YAG, K_IC = 3.6 ± 0.4 MPa m^1/2. It is expectable that directionally solidified Al₂O₃-based eutectic ceramics are approaching practical application with the advancement of processing theory, technique and apparatus.     

Microstructures and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic by laser floating zone melting with high temperature gradient

Directionally solidified Al₂O₃/GdAlO₃ eutectic ceramic rods with high densities and low solidification defects are prepared by laser floating zone melting at solidification rate from 2 to 200 μm/s. The microstructure evolution, eutectic growth behavior and mechanical properties are investigated. At low solidification rates (<30 μm/s), the eutectic rods present a homogeneous irregular eutectic microstructure, whereas cellular microstructure containing regular lamella/rod structure is developed at higher solidification rates. The relationship is established between the eutectic interphase spacing and solidification rate, which follows the Magnin-Kurz eutectic model. The Vickers hardness (15.9–17.3 GPa) increases slightly with decreasing interphase spacing, but the fracture toughness (4.08 MPa m^1/2) shows little dependence with the solidification rate. Different crack propagation mechanisms are revealed among the indentation cracks. The flexural strength at ambient temperature reaches up to 1.14 GPa for the eutectic grown at 100 μm/s. The fracture surface analysis indicates that the surface defects are the main crack source.     

Directly fabricated Al2O3/GdAlO3 eutectic ceramic with large smooth surface by selective laser melting: Rapid solidification behavior and thermal field simulation

Selective laser melting (SLM), a novel approach for one-step melting and solidifying ceramic powder beds layer by layer without post-process of degreasing and sintering, has been developed to directly prepare highly dense (>95 %) Al₂O₃/GdAlO₃(GAP) eutectic composite ceramics with large smooth surfaces. Compact net-shaped plates with the maximum size of 73 × 24 × 5 mm³ are obtained by different strategies of laser pre-heating and multi-tracks’ deposition without any binders. Combined with the finite element thermodynamic coupling simulation results, it is proved that the stress between the substrate and depositions during SLM can be greatly reduced by the step-up preheating, and thus effectively improving the ceramic forming quality. The macro-morphology, microstructure evolution, rapid solidification behavior and mechanical properties of the SLM-ed eutectic ceramics are systematically investigated at different laser processing parameters. The microstructure transforms from ultra-fine irregular eutectic to complex regular eutectic with the increase of the scanning rate. The average eutectic spacing, and solidification rate has an approximately linear relationship consistent with the Jackson-Hunt (JH) model. The microhardness and fracture toughness can reach 17.1 ± 0.2 GPa and 4.5 ± 0.1 MPa·m^1/2, respectively. The results indicate that SLM method is a highly effective technique for fabricating high-performance net-shaped structural composite ceramics.     

Orientations and seed type effect on Al2O3-YAG-ZrO2 eutectic microstructure solidified from the melt by the micro-pulling down technique

Al₂O₃-YAG-ZrO₂ eutectic ceramic rods of 5 mm in diameter were grown by micro-pulling down technique. The seeding and the solidification rate affect microstructure, morphology, crystallography, and thermal stress of the solidified ceramics. The ternary eutectic grown through zirconia (111) seed had inhomogeneous and irregular cellular microstructures. At the stationary stable regime, the microstructure spacing (λ) depends on the pulling rate (v). Under solidification rate of 0.5 mm.min^?1, the rods grown by using eutectic poly-crystal, (100), (111) YAG, and c(0001), A(1?210), M(10?10) sapphire seeds, the YAG and ZrO₂ phases are oriented along the <100> direction parallel to the growth direction. The zirconia (111) seeding X-ray diagram eutectic presents additional peaks and the monoclinic ZrO₂ phase appears at the solidification rate of 1 mm.min-1. The rods grown through ZrO₂ seeding are more stressed than those solidified by using eutectic, YAG and sapphire seeds, respectively.     

Microstructure and properties of directionally solidified LaB6 (100)-ZrB2 eutectic composite prepared by the optical zone melting method

Directionally solidified (DS) LaB₆ (001)-ZrB₂ eutectic composite is successfully prepared by optical zone melting technique. The effect of the solidification rate on the microstructure and properties is systemically investigated.With the increase in the solidification rate from 20 to 100 mm/h, the eutectic rods present a homogeneous eutectic microstructure. The relationship between the average eutectic spacing and the solidification rate is established. The fracture toughness shows an obvious anisotropy. Crack deflection and crack bridging play important roles in improving the fracture toughness of the material. As the solidification rate is increased from 20 to 300 mm/h, the fracture toughness, bending strength and current density firstly increase and then decrease. The highest fracture toughness of 5.16 MPa.m^1/2, bending strength of 809.04 MPa and current density of 36.24 A/cm² at 1873 K belong to the DS LaB₆ (001) - ZrB₂ eutectic composite obtained at V = 100 mm/h.     

Microstructure refinement approaches of melt-grown Al2O3/YAG/ZrO2 eutectic bulk

Microstructure developments of melt-grown Al₂O₃/YAG/ZrO₂ ceramic bulks were investigated by controlling composition, cooling rate, heterogeneous nucleation sites and melt superheating treatment. The solidification microstructure of sample with hypereutectic composition (ZrO₂ 20 mol%) is finer than that with hypoeutectic or eutectic ones. With increasing the cooling rate, microstructure of melt-grown samples develops from colony to dendrite and finally to cell. The microscopy and the components of samples vary with the melt superheating temperature and the type of heterogeneous nucleation sites. The microstructure evolutions of melt-grown Al₂O₃/YAG/ZrO₂ eutectic relate to the melt undercooling level and the solid–liquid interfaces stability.     

Study of the stability of the molten zone and the stresses induced during the growth of Al2O3-Y3Al5O12 eutectic composite by the laser floating zone technique

A series of analytical calculations is used to study both the effect of the thermal gradients and the stability of the molten zone in the laser floating zone growth of Al₂O₃-Y₃Al₅O₁₂ eutectic composite. The thermal gradients in the solidification interface have been calculated and the axial gradient compared with the experimental one of 4.5 × 10⁵ K/m. For these calculations the coefficients of heat transfer from the molten zone to the ambient at the solid-melt interface have been previously obtained. The thermal stresses generated by the high thermal gradients can induce crack formation during the cooling depending on the rod diameter. The theory predicts that it is possible to grow rods free of cracks up to R = 1.7 mm, at low rates (10 mm/h) in close agreement with the experimental critical radius of 1.6 mm.The dependence of the zone length on the input laser power used to carry out the growth is shown. The study of the floating zone profile allows determining the maximum stable zone length, verifying the stability criterion established by some authors.     

Effect of an abrupt change in pulling rate on microstructures of directionally solidified Al2O3-Er3Al5O12 eutectic and off-eutectic composite ceramics

Directionally solidified microstructures of Al₂O₃-Er₃Al₅O₁₂ eutectic and off-eutectic in situ composite ceramics were explored under abrupt-change pulling rate conditions. Corresponding temperature distributions and interface locations were studied. In eutectic composition, fluctuation of eutectic spacing occurred when the pulling rate increased abruptly. A gradually increase or abrupt increase in eutectic spacing was observed when the pulling rate decreased abruptly. In hypoeutectic and hypereutectic compositions, formation of the primary phases were suppressed when the pulling rate increased abruptly from 10?µm/s to 100?µm/s, while primary phases precipitated when the pulling rate decreased abruptly from 100?µm/s to 10?µm/s. The interface altitude decreased after the pulling rate increased abruptly, but increased after the pulling rate decreased abruptly. The liquid composition restriction (around the eutectic composition) at the eutectic interface plays an important role in the suppression of the primary dendrite and coupled eutectic oxides can be obtained in off-eutectic compositions even under higher solidification rate conditions.     

Microstructure and mechanical properties of Al2O3/Er3Al5O12 eutectic rods grown by the laser-heated floating zone method

Eutectic rods of Al₂O₃-Er₃Al₅O₁₂ were grown by directional solidification using the laser-heated floating zone method at rates in the range 25-1500 mm/h. Their microstructure and mechanical properties (hardness, toughness and strength) were investigated as a function of the growth rate. A homogeneous and interpenetrated microstructure was found in most cases, and interphase spacing decreased with growth rate following the Hunt-Jackson law. Hardness increased slightly as the interphase spacing decreased while toughness was low and independent of the microstructure. The rods presented very high bending strength as a result of the homogeneous microstructure, and their strength increased rapidly as the interphase spacing decreased, reaching a maximum of 2.7 GPa for the rods grown at 750 mm/h. The bending strength remained constant up to 1300 K and decreased above this temperature. The relationship between the microstructure and the mechanical properties was established from the analysis of the microstructure and of the fracture mechanisms.     

Directionally solidified CeO2 (or GDC)/CoO eutectic ceramics as cermet precursors for SOFCs anodes: Microstructure cross-over

Rods of CeO₂ and gadolinium-doped CeO₂ (GDC)-CoO eutectics were prepared by directional solidification using a laser heated floating zone (LFZ) technique. The microstructure has been studied as a function of the growth rate from V = 10 to 750 mm/h. Regular eutectic microstructures are obtained except for the highest growth rate. The interspacing follows the λ²V = C law with C = 4.1(3) × 10⁻¹⁷ and 2.6(3) × 10⁻¹⁷ m³/s for CeO₂-CoO and GDC-CoO eutectics, respectively. A cross-over between fibrous and lamellar eutectic microstructures was observed depending on the growth rate. The crystallography of the eutectics was studied by Electron Backscatter Diffraction (EBSD). The growth directions [1 1 0]_GDC ∼ //[110]_CoO, and the interfacial planes (200)_GDC//(111)_CoO, were identified. Solubility of Co in the ceria matrix was determined by Energy Dispersive X-ray (EDX) Spectroscopy after Co was leached out from the matrix. Co solubility in ceria at 1650 °C was found to be less than 1 mol%.     

Effect of solidification path on the microstructure of Al2O3–Y2O3–ZrO2 ternary oxide eutectic ceramic system

The solidification path of the Al₂O₃–Y₂O₃–ZrO₂ ternary oxide eutectic composite ceramic is determined by a high temperature DTA and laser floating zone (LFZ) directional solidification method to investigate the effect of solidification path on the microstructure of the ternary oxide. The DTA and microstructure analyses show that the YAG or Al₂O₃ tends to form as primary phase under the unconstrained solidification conditions, and then the system enters ternary eutectic solidification during cooling from 1950 °C at rate of 20 °C/min. The as-solidified composite ceramic shows a divorced irregular eutectic structure consisting of Al₂O₃, YAG and ZrO₂ phases with a random distribution. The primary phases are however completely restrained at the directional solidification conditions with high temperature gradient, and the ternary composite by LFZ presents well coupled eutectic growth with ultra-fine microstructure and directional array. Furthermore, the eutectic transformation and growth mechanism of the composite ceramic under different solidification conditions are discussed.     

Silicon and its effect on the electrochemical properties of Li3V2(PO4)3 cathode material

In this study, silicon and its effect on the properties of Li₃V₂(PO₄)₃ were studied for lithium-ion battery applications. The composite material was synthesized and found to show enhanced capacity and cyclability. The presence of silicon in the composites was confirmed. Furthermore, large particles with rough, corroded-like structures formed, and these were distributed well with the silicon particles. The Li₃V₂(PO₄)₃-Si battery had good properties showing improved cyclability, an improved high performance rate, smaller impedance values and improved lithium-ion diffusion coefficients, as determined by cyclic voltammetry. Furthermore, the optimization of the silicon content led to a Li₃V₂(PO₄)₃-Si battery with a 2?wt% silicon loading that had a discharge capacity of 181?mA?h?g^?1. At 2?C, Li₃V₂(PO₄)₃-Si (2?wt%) still demonstrated a capacity of 111.8?mA?h?g^?1, which was 83.8% of its original capacity (compared with 70.3?mA?h?g^?1 and 63.8% for Li₃V₂(PO₄)₃) after 400 cycles.     

Microstructure and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic prepared with horizontal high-frequency zone melting

Directionally solidified eutectic oxide ceramics are very promising as a next-generation structural material for ultrahigh-temperature applications, above 1600?°C, owing to their outstanding properties of high corrosion resistance, oxidation resistance, high fracture strength and toughness, and high hardness. Herein, Al₂O₃/GdAlO₃ eutectic ceramic was prepared with horizontal high-frequency induction zone melting (HIZM), and the effects of the processing parameters on the eutectic microstructure and mechanical properties were investigated. The results indicated that the directionally solidified Al₂O₃/GdAlO₃ eutectic ceramic was composed only of the Al₂O₃ phase and GdAlO₃ phase penetrating mutually, and the Al₂O₃ phase was the substrate in which the GdAlO₃ phase was embedded. As the solidification rate increased from 1 to 5?mm/h, the eutectic microstructure underwent a transformation from an irregular pattern to a relatively regular “rod” or “lamellar” pattern, and the eutectic spacing constantly decreased, reaching a minimum value of 0.5?μm. The eutectic ceramic hardness and fracture toughness at room temperature increased continuously, reaching 23.36?GPa and 3.12?MPa?m^1/2, which were 2.3 times and 2.5 times those of the sintered ceramic with the same composition, respectively. Compared with the samples obtained from vertical high-frequency induction zone melting, the orientation of eutectic phases along the growth direction decreased significantly, and the size uniformity of the GdAlO₃ phase became poorer in the samples prepared with HIZM at the same solidification rate; nevertheless, the hardness and fracture toughness of the samples increased by 11% and 63%, respectively.     

Crystal structure,impedance and broadband dielectric spectra of ordered scheelite-structured Bi(Sc1/3Mo2/3)O4 ceramic

Bi(Sc_1/3Mo_2/3)O₄ ceramics were prepared via solid state reaction method. It crystallized with an ordered scheelite-related structure (a?=?16.9821(9)?Å, b?=?11.6097(3)?Å, c?=?5.3099(3)?Å and β?=?104.649(2)°) with a space group C12/C1, in which Bi³⁺, Sc³⁺ and Mo⁶⁺ are ?8, ?6 and ?4 coordinated, respectively. Bi(Sc_1/3Mo_2/3)O₄ ceramics were densifiedat 915?°C, giving a permittivity (ε_r) ～24.4, quality factor (Qf, Q?=?1/dielectric loss, f?=?resonant frequency) ～48, 100?GHz and temperature coefficient of resonant frequency (TCF)?～??68?ppm/°C. Impedance spectroscopy revealed that there was only a bulk response for conductivity with activation energy (E_a) ～0.97?eV, suggesting the compound is electrically and chemically homogeneous. Wide band dielectric spectra were employed to study the dielectric response of Bi(Sc_1/3Mo_2/3)O₄ from 20?Hz to 30?THz. ε_r was stable from 20?Hz to the GHz region, in which only ionic and electron displacive polarization contributed to the?ε_r.     

Laser machining of Al2O3-ZrO2 (3%Y2O3) eutectic composite

In this work we present the study of the interaction between NIR pulsed laser and Al₂O₃-ZrO₂ (3%Y₂O₃) eutectic composite. The effect produced by modifying the reference position as well as the working conditions and laser beam features has been studied when the samples are processed by means of pulse bursts.The samples were obtained by the laser floating zone technique using a CO₂ laser system. The laser machining was carried out with a Q-switched Nd:YAG laser at its fundamental wavelength of 1064 nm with pulse-widths in the nanosecond range.Geometric dimensions, i.e. ablated depth, machined width and removed volume as well as ablation yield of the resulting holes have been studied. We have described and discussed the morphology, composition and microstructure of the processed samples.