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.     

Microstructure and mechanical properties of Al2O3/Y3Al5O12/ZrO2 hypereutectic directionally solidified ceramic prepared by laser floating zone

Al₂O₃/Y₃Al₅O₁₂/ZrO₂ directionally solidified ceramic has been considered as a promising candidate for ultrahigh temperature structural materials due to its excellent performance even close to its melting point. In this work, laser floating zone (LFZ) solidification experiments were performed on Al₂O₃/Y₃Al₅O₁₂/ZrO₂ hypereutectic with the solidification rates between 2 μm/s and 30 μm/s. The full eutectic lamellar microstructure is obtained with hypereutectic composition. The solid/liquid interface morphology is investigated. The microstructure characteristic is discussed based on the solid/liquid interface. The variation of lamellar spacing with different compositions and solidification rates was reported and discussed by considering an irregular eutectic growth model. The maximum hardness and fracture toughness are 19.06 GPa and 3.8 MPa m^1/2, respectively. The toughening mechanism of ZrO₂ is discussed based on the scenario of the crack propagation pattern.     

Mechanical properties of highly textured porous Ni-YSZ and Co-YSZ cermets produced from directionally solidified eutectics

It is well known that several ceramic materials develop an usual; and sometimes unique; combination of properties as a result of mixing different phases with similar expansion coefficients. Sometimes they are elastically stiff, have low thermal expansion coefficients, and are resistant to chemical attack. As this paper will show, their mechanical properties are also enhanced.Nanoindentation is used to measure the mechanical properties for each phase of NiO-YSZ and CoO-YSZ eutectics produced by the laser floating zone technique, and also the analogues Ni-YSZ and Co-YSZ cermets produced by reduction from the eutectic precursors. The different tests have been performed at 100 nm, in order to obtain an imprint lower than the size of the secondary phase and extract the hardness and Young's modulus. Moreover, several tests have been performed at 2000 nm of indentation range to obtain the response of each material. The different imprints have been visualized by Atomic Force Microscopy.     

Preparation and microstructure of large-sized directionally solidified Al2O3/Y3Al5O12 eutectics with the seeding technique

A large-sized Al₂O₃/Y₃Al₅O₁₂ (YAG) eutectic single crystal is successfully prepared with the external seed by a modified Bridgman furnace. The microstructure, crystallography and interface structure of the large-sized Al₂O₃/YAG eutectic are well investigated. It is found that the longitudinal eutectic microstructure shows large length-to-width phase ratios. The crystallographic orientation relationship of the as-obtained large-sized Al₂O₃/YAG eutectic is consistent with that of the seed. The epitaxial solidification of the binary eutectic occurs, and the dominating of the seed is not lost in the long-range growth. The observed Al₂O₃/YAG interface structure is studied by near-coincidence site lattice (NCSL) theory. The volume strain for the NCSL is very low (0.02), which suggests that the interfaces have locally low interfacial energies. This small volume strain might be the reason for stable induced-growth along the seed.     

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.     

Ruby micro-piezospectroscopy in GdAlO3/Al2O3(/ZrO2), Er3Al5O12/Al2O3(/ZrO2) and Y3Al5O12/Al2O3(/ZrO2) binary and ternary directionally solidified eutectics

The fluorescence from (naturally present) Cr³⁺ impurities was used to measure the residual stress in the alumina phase of six melt-grown ceramic eutectic composites associating gadolinum aluminum perovskite (GAP), erbium aluminum garnet (EAG) or yttrium aluminum garnet (YAG) with α-alumina and cubic zirconia. Such measurements are reported for the first time in the GAP containing eutectics.In the usual hydrostatic assumption, we conclude to a residual compression in the range of ～70–400 MPa depending on the sample composition. The validity of the hydrostatic assumption is questioned when a microscope is used for the measurements.     

Mechanical properties of directionally solidified Al2O3/Y3Al5O12 eutectic ceramic prepared by optical floating zone technique

Al₂O₃/Y₃Al₅O₁₂(YAG) directionally solidified eutectic (DSE) crystal was prepared by optical floating zone technique. Al₂O₃/YAG DSE consists of continuous entangled Al₂O₃ and the YAG forming a three-dimensional networks structure. The volume fraction of porosity is ultra-low (0.013%) and the average equivalent diameters of most pores (>84%) are smaller than 4?μm. The Al₂O₃/YAG DSE shows excellent high-temperature elastic stiffness. The Young’s modulus at 1500?°C maintains more than 85% of the value at room temperature. Bending strength exhibits excellent retention up to high temperature as well. High-temperature ball indentation testing shows plastic deformation involving dislocations and twinning, which predominantly occur in Al₂O₃ phase, while the YAG phase is stable. Evaluation on H_v/E index predicts Al₂O₃/YAG DSE with moderate capability to accommodate damages. Our results highlight Al₂O₃/YAG DSE as excellent high-temperature structural materials.     

Comparative study of Yb:Lu3Al5O12 and Yb:Lu2O3 laser ceramics produced from laser-ablated nanopowders

We present a comparative study of two Lu-based oxide ceramics doped with Yb³⁺ ions, namely Yb:Lu₃Al₅O₁₂ (garnet) and Yb:Lu₂O₃ (sesquioxide), promising for thin-disk lasers. The ceramics are fabricated using nanopowders of 3.6 at.% Yb:Lu₂O₃ and Al₂O₃ produced by laser ablation: Yb:Lu₃Al₅O₁₂ – by vacuum sintering at 1800 °C for 5 h with the addition of 1 wt% TEOS as a sintering aid, and Yb:Lu₂O₃ – by vacuum pre-sintering at 1250 °C for 2 h followed by Hot Isostatic Pressing at 1400 °C for 2 h under Ar gas pressure of 207 MPa. The comparison includes the structure, Raman spectra, transmission, optical spectroscopy and laser operation. The crystal-field splitting of Yb³⁺ multiplets is revealed for Lu₃Al₅O₁₂. A continuous-wave (CW) Yb:Lu₃Al₅O₁₂ ceramic microchip laser generates 5.65 W at 1031.1 nm with a slope efficiency of 67.2%. In the quasi-CW regime, the peak power is scaled up to 8.83 W. The power scaling for the Yb:Lu₂O₃ ceramic laser is limited by losses originating from residual coloration and inferior thermal behavior.     

Longitudinal cross-section microstructure of growth striation in Al2O3/Y3Al5O12/ZrO2 directionally solidified eutectic ceramic prepared by laser floating zone

Periodic growth striations with a width of 380 μm are observed in Al₂O₃/Y₃Al₅O₁₂/ZrO₂ ternary directionally solidified eutectic ceramic prepared by laser floating zone. It is found that the microstructure of the growth striations exhibits disparate growth characteristics. The direct experimental evidence of the formation of the microstructure of the growth striations is obtained through examining the solid–liquid interface. The lamellar coarsening of the growth striations is related to the nucleation of Al₂O₃ particles and their engulfment by the extended Y₃Al₅O₁₂ phase. The mechanism accounting for this phenomenon is explained by considering eutectic growth behavior under mutative condition induced by oscillatory convection.     

Transparent polycrystalline nanoceramics consisting of triclinic Al2SiO5 kyanite and Al2O3 corundum

Transparent polycrystalline nanoceramics consisting of triclinic Al₂SiO₅ kyanite (91.4 vol%) and Al₂O₃ corundum (8.6 vol%) were fabricated at 10 GPa and 1200‐1400°C. These materials were obtained by direct conversion from Al₂O₃‐SiO₂ glasses fabricated using the aerodynamic levitation technique. The material obtained at 10 GPa and 1200°C shows the highest optical transparency with a real in‐line transmission value of 78% at a wavelength of 645 nm and a sample‐thickness of 0.8 mm. This sample shows equigranular texture with an average grain size of 34 ± 13 nm. The optical transparency increases with decreasing mean grain size of the constituent phases. The relationship between real in‐line transmission and grain size is well explained by a grain‐boundary scattering model based on a classical theory.     

B2O3-La2O3-Ta2O5-ZrO2系统中新型ZLaF4玻璃的研制

研究了B2O3含量为20%(质量分数)时B2O3-La2O3-Ta2O5-ZrO2和B2O3-La2O3-Nb2O5-ZrO2系统的玻璃形成区域.在此基础上,逐一添加各种改善析晶性能的氧化物,研制出一种高折射率低色散的ZLaF4光学玻璃,并对其熔炼工艺、析晶性能作了一定的探讨,对一些性能进行了测试.     

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.     

Al2O3＋TiC陶瓷中Al2O3与TiC化学反应抑制的研究

本文通过Ｙ－盐溶液的形式加入到Ａｌ２Ｏ３粉料中,制备了Ｙ２Ｏ３表面固溶的Ａｌ２Ｏ３粉料,对其阻止Ａｌ２Ｏ３与ＴｉＣ之间的化学反应进行了研究。研究结果表明：采用Ｙ－盐溶液加入Ｙ２Ｏ３,Ｙ原子能均匀的分散在Ａｌ２Ｏ３的表面,高温时Ｙ２Ｏ３在Ａｌ２Ｏ３表面形成固溶体层,少量的Ｙ２Ｏ３加入量（０．３５ｗｔ％）,就能有效的阻止Ａｌ２Ｏ３与ＴｉＣ之间的化学反应。     

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.     

Kinetic effects of substitution Er3+ for Y3+ in (Y1-xErx)3Al5O12 garnet

The crystallization kinetics of erbium-substituted yttrium aluminum garnet (Er:YAG) from the amorphous phase was studied depending on the degree of substitution of yttrium by erbium: (Y_1-xEr_x)₃Al₅O₁₂, x = 0 – 0.9. The corresponding series of nanopowders were synthesized using the sol-gel method with binary sols as the starting materials. Crystallized single-phase Er:YAG powders were obtained at temperatures as low as 1370 K through the formation of intermediate phases. The crystallization kinetics of the Er:YAG precursors was analyzed using non-isothermal differential scanning calorimetry and X-ray diffraction analysis. The proposed unified crystallization model, based on which the kinetic parameters of the crystallization process of the Er:YAG from the amorphous phase were determined, depending on the content of erbium oxide. Time-temperature-transformation curves were developed from the obtained data to determine the optimal modes in the heat treatment and sintering of optical ceramics. Er:YAG transparent ceramics were obtained by vacuum sintering of sol-gel powders.     

Dielectric and microwave absorption properties of FeSiAl/Al2O3 composites containing FeSiAl particles of different sizes

Here in, the effects of FeSiAl particle size on the dielectric and microwave absorption properties of FeSiAl/Al₂O₃ composites were studied. FeSiAl/Al₂O₃ composites containing 18–25 μm, 25–48 μm, and 48–75 μm FeSiAl particles were prepared by hot-pressed sintering based on uniformly mixed FeSiAl and Al₂O₃ powders. Results show that the real permittivity and the imaginary permittivity are significantly promoted with increasing FeSiAl particle size, which is ascribed to the enhanced interfacial polarization and conductance loss. In addition, the favorable matching impedance and suitable attenuation coefficient enabled the composite containing 25–48 μm FeSiAl powder to show a minimum reflection loss of ?34.4 dB at 11.7 GHz and an effective absorption bandwidth (<-10 dB) of 1.4 GHz in 11.0–12.4 GHz, when the thickness is 1.1 mm. By adjusting the thickness to 1.4 mm, the effective absorption bandwidth of the composite reaches a maximum value of 2.0 GHz in the 8.3–10.3 GHz range, indicating tunable, strong, and highly efficient microwave absorption performance.     

Structural modification associated with Al2O3 addition in oxyfluoride glasses: Thermal and mechanical properties

In this work, the effect of gradual addition of Al₂O₃ substituting SiO₂ on the structural, thermal, and mechanical properties of SiO₂–BaF₂–K₂O–GdF₃–Sb₂O₃‐based oxyfluoride glasses have been studied. The X‐ray diffraction (XRD) patterns and differential scanning calorimetric (DSC) curves indicate that there is a distinct primary crystallization corresponding to BaGdF₅ phase formation in the samples without (0AlG) and with 5 mol% substitution of Al₂O₃ (5AlG) while the sample with 10 mol% of Al₂O₃ (10AlG) does not show such crystallization event. Further, the activation energy (E_a) for fluoride crystal formation is higher for the 5AlG in comparison to the 0AlG glass as determined by Kissinger, Augis‐Bennett and Ozawa models. Fourier transform infrared (FTIR) and Raman spectroscopy analysis confirmed the structural modification with the gradual addition of Al₂O₃ in the glass matrix revealing dominant presence of AlO₄ tetrahedral units in 10AlG sample unlike in 5AlG sample which exhibited the manifestation of AlO₆ units. Such structural variation has further been substantiated from the estimated elastic properties like Young's modulus (E), shear modulus (G), bulk modulus (K), longitudinal modulus (L), and mean ultrasonic velocity (U_m) by showing a decrease for 5AlG sample in comparison with 0AlG sample followed by subsequent increase for 10AlG sample.     

Enhanced dielectric and microwave absorption properties of Cr/Al2O3 coatings deposited by low‐power plasma spraying

Low‐power plasma‐sprayed Cr/Al₂O₃ coatings have been developed for their potential application as broad bandwidth, thin thickness, lightweight, and strong microwave‐absorbing materials. The dielectric and microwave absorption properties of the as‐sprayed coatings were studied in the X‐band (from 8.2 to 12.4 GHz). High complex permittivity of the coatings was obtained because of a large number of internal boundaries and the conductive networks. Meanwhile, a significant enhancement of microwave absorption properties of the coating was achieved due to the enhanced interfacial polarization and conductance loss. The reflection loss (RL) <?10 dB of the Al₂O₃–15Cr coating was obtained from 9.8 to 11.4 GHz by choosing an appropriate coating thickness, and an optimal minimum reflection loss (RL_min) of ?45.35 dB was achieved at 10.3 GHz with a thin thickness of 1.32 mm.