Mechanical properties and microstructure of Al2O3/TiAl in situ composites doped with Cr2O3

Al₂O₃/TiAl composites were successfully fabricated from powder mixtures of Ti, Al, TiO₂ and Cr₂O₃ by a hot-press-assisted exothermic dispersion method. The effect of the Cr₂O₃ addition on the microstructures and mechanical properties of Al₂O₃/TiAl composites was characterized, and the results showed that the Rockwell hardness, flexural strength and fracture toughness of the composites increased as the Cr₂O₃ content increased. When the Cr₂O₃ content was 2.5 wt%, the flexural strength and the fracture toughness attained peak values of 925 MPa and 8.55 MPa m^1/2, respectively. This improvement of mechanical properties was due to the more homogeneous and finer microstructure developed from the addition of Cr₂O₃ and an increase in the ratio of α₂-Ti₃Al to γ-TiAl matrix phases.     

Preparation and characterization of Nb2O5-Al2O3 composite oxide formed by cathodic electroplating and anodizing

Al foil was coated with niobium oxide by cathodic electroplating and anodized in a neutral boric acid solution to achieve high capacitance in a thin film capacitor. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) revealed the niobium oxide layer on Al to be a hydroxide-rich amorphous phase. The film was crystalline and had stoichiometric stability after annealing at temperatures up to 600 °C followed by anodizing at 500 V, and the specific capacitance of the Nb₂O₅-Al₂O₃ composite oxide was approximately 27% higher than that of Al₂O₃ without a Nb₂O₅ layer. The capacitance was quite stable to the resonance frequency. Overall, the Nb₂O₅-Al₂O₃ composite oxide film is a suitable material for thin film capacitors.     

In situ Al3Ti and Al2O3 nanoparticles reinforced Al composites produced by friction stir processing in an Al-TiO2 system

Al and TiO₂ powders were selected to fabricate in situ Al composites via multiple pass friction stir processing (FSP) based on the thermodynamic analysis. The microstructural investigations indicated FSP would induce reaction between Al and TiO₂. Al₃Ti and Al₂O₃ particles were formed after 4 pass FSP with 100% overlapping. The in situ particles were about 80 nm in size at various FSP conditions, and ultrafine matrix grains 602 nm in size were obtained when water cooling was applied during FSP. Tensile tests indicated that the in situ nanocomposites exhibited pronounced work hardening behavior and a good combination of strength and ductility.     

Preparation and properties of nano-SiC strengthening Al2O3 composite ceramics

The processing and mechanical behaviors of Al₂O₃-xwt.%SiC (x = 1, 2, 5, ASx) nano-composites prepared by the in situ synthesis of SiC from polycarbosilane (PCS) were investigated. The composites were densified by hot pressing. The microstructure and mechanical properties of the sintered composites were analyzed. The results showed that a fully dense structure was obtained when a few nano-SiC were doped and that the fracture toughness and strength were highly improved compared with those of monolithic Al₂O₃. The fracture toughness reached 5.1 MPa m^1/2 in AS2 composite. The maximum flexural strength was 516 MPa obtained in AS1 composite.     

Improved thermoelectric properties by adding Al for Zn in (ZnO)mIn2O3

The grain size and density of the sintered (Zn_1 − xAl_xO)_mIn₂O₃ bodies decreased with the small Al₂O₃ content (≤ 0.012), and then increased gradually by further increasing the Al₂O₃ content. The addition of Al for Zn in the (ZnO)_mIn₂O₃ led to an increase in both the electrical conductivity and the absolute value of the Seebeck coefficient. This indicates that the power factor was significantly enhanced by adding Al for Zn. The thermoelectric power factor was maximized to 1.67 × 10^− 3 W m^− 1 K^− 2 at 1073 K for the (Zn_0.992Al_0.008O)_mIn₂O₃ sample.     

Synthesis of Nb2O5 nanosheets and its electrochemical measurements

In this study, Nb₂O₅ nanosheets were first synthesized using NbO₂ particles as the precursor via a simple hydrothermal route. The synthesized Nb₂O₅ nanosheets are highly crystalline and their thicknesses are found to be ca. 3–5 nm. Based on the experimental results of XRD, SEM and TEM measurements, a possible mechanism for the formation of nanosheets was discussed. An electrode materials made of the product containing Nb₂O₅ nanosheets shows a larger capacity of 355 mAh g⁻¹ at a current density of 0.1 A g⁻¹. Cyclic measurements indicate that such an electrode exhibits a high reversible charge/discharge capacity and cycling stability. This might be attributed to the intrinsic characteristics of Nb₂O₅ nanosheets.     

Improvement in dielectric properties of Al2O3-doped Li0.30Cr0.02Ni0.68O ceramics

Li_0.30Cr_0.02Ni_0.68O giant dielectric ceramics doped with Al₂O₃ were prepared by solid-state reaction via sol-gel process. The sintered samples were characterized using X-ray powder diffraction and scanning electron microscopy, and dielectric properties were also investigated. All doped samples showed the single phase of cubic rock-salt structure NiO. With increasing Al₂O₃ content, the crystallite size and grain size decreased, possibly due to an occurrence of the secondary phases at grain boundaries which inhibit the grain growth. The sample with 0.2 wt.% Al₂O₃ showed nearly 7 times lower tanδ (2.37) and higher ε_r (7.25 × 10⁶) measured at 1 kHz and room temperature when compared to the pure sample.     

Microstructure characterization of fibrous HAp-(20 vol.% t-ZrO2)/Al2O3-(25 vol.% m-ZrO2) composites by multi-pass extrusion process

Core-shell structured HAp-(t-ZrO₂)/Al₂O₃-(m-ZrO₂) composites were fabricated using a multi extrusion process. The shell of Al₂O₃-(m-ZrO₂) phases was selected due to their excellent biocompatibility and mechanical properties and the core was designed with t-ZrO₂ dispersed in the HAp matrix. The t-ZrO₂ and m-ZrO₂ particles (< 400 nm) were homogeneously dispersed in the HAp and Al₂O₃ phases, respectively. In the HAp-(t-ZrO₂) core region, a heavy strain field contrast was observed due to the mismatch of their thermal expansion coefficients. The values of relative density, bending strength and Vickers hardness of the third pass fibrous HAp-(t-ZrO₂)/Al₂O₃-(m-ZrO₂) composites, which were sintered at 1400 °C, were about 93%, 169 MPa, and 792 Hv, respectively.     

Influence of excitation wavelengths on luminescent properties of Sr3Al2O6:Eu, Dy phosphors prepared by sol-gel-combustion processing

Eu²⁺ and Dy³⁺ ion co-doped Sr₃Al₂O₆ red-emitting long afterglow phosphor was synthesized by sol-gel-combustion methods using Sr(NO₃)₂, Al(NO₃)₃·9H₂O, Eu₂O₃, Dy₂O₃, H₃BO₃ and C₆H₈O₇·H₂O as raw materials. The crystalline structure of the phosphors were characterized by X-ray diffraction, luminescent properties of phosphors were analyzed by fluorescence spectrophotometer. The effect of excitation wavelengths on the luminescent properties of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphors was discussed. The emission peak of Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphor lays at 516 nm under the excitation of 360 nm, and at 612 nm under the excitation of 468 nm. The results reveal that the Sr₃Al₂O₆:Eu²⁺, Dy³⁺ phosphor will emit a yellow-green light upon UV illumination, and a bright red light upon visible light illumination. The emission mechanism was discussed according to the effect of nephelauxetic and crystal field on the 4f⁶5d¹ → 4f⁷ transition of the Eu²⁺ ions in Sr₃Al₂O₆. The afterglow time of (Sr_0.94Eu_0.03Dy_0.03)₃ Al₂O₆ phosphors lasts for over 600s after the excited source was cut off.     

Synthesis of a novel langanite-type compound La3Al5.5Nb0.5O14 by the sol-gel process

Langanite (La₃Ga_5.5Nb_0.5O₁₄, LGN) and its isomorphs are a few piezoelectric materials which have the unique temperature compensation and piezoelectric properties. But the high Ga₂O₃ content makes them very expensive and limits their applications. We reported a new langanite-type compound La₃Al_5.5Nb_0.5O₁₄ (LAN) which has the advantage of no Ga₂O₃ content. Chemically homogeneous La₃Al_5.5Nb_0.5O₁₄ sol was synthesized using La (NO₃)₃·6H₂O, Al (NO₃)₃·9H₂O and niobium citrate as starting materials. Single-phased LAN powder was prepared by decomposition of a citrate polymer precursor and subsequent reactions at high temperatures. TG-DTA, XRD and FTIR were employed to investigate the transformation process of gel to LAN powder. The results showed that, after calcination at 900 °C, pure La₃Al_5.5Nb_0.5O₁₄ polycrystalline powder with a narrow particle size distribution was obtained, which has the same structure with La₃Ga_5.5Nb_0.5O₁₄.     

Effect of Nb and Nb2O5 additives on mechano-thermal processing of TiAl/Al2O3 nano-composite

In this research, TiAl matrix nano-composite with Al₂O₃ reinforcement was obtained by mechanical activation of TiO₂ and Al powder mixture and its subsequent heat treatment. Effect of Nb and/or Nb₂O₅ additions on the process was investigated. Structural changes and thermal behavior of the samples were evaluated by X-ray diffraction and differential thermal analysis, respectively. Moreover, the microstructure was characterized by transmission electron microscopy. The results confirmed the partial dissolution of Nb in Al during the milling stage in the Nb-added samples. The reaction mechanism during heat treatment in the sample without any additives was a two-stage process that was quite similar to the sample with Nb addition. However, Nb₂O₅ addition led to the progress of reaction through a single stage and with a higher rate. Both additives promoted formation of the Ti₃Al phase in the final products. The results confirmed the formation of nano-sized Al₂O₃ particles in a nano-crystalline Ti–Al matrix with a mean crystallite size of 30 nm.     

Synthesis, structure and exchange bias in Cr2O3/CrO2/Cr2O5 particles

We report on the synthesis, structure and magnetic properties of a novel exchange bias system with Cr₂O₃/CrO₂/Cr₂O₅ interfaces. Chromium oxide particles with mixed chromium valences were prepared by sintering CrO₃ in air. X-ray diffraction patterns show that CrO₃ lost its oxygen gradually with increasing temperature and time through Cr₃O₈, Cr₂O₅, CrO₂, and finally Cr₂O₃ at temperatures above 760 K. X-ray photoelectron spectra indicate a low CrO₂ content and a binding energy of 579.3 eV for Cr 2p_3/2 photoelectrons in Cr₂O₅. Chromium dioxide was found to stably coexist with Cr₂O₃ and Cr₂O₅ in the particles. Magnetic measurements show hysteresis loop shifts in the sample, indicating an exchange bias induced by antiferromagnetic Cr₂O₃/Cr₂O₅ in ferromagnetic CrO₂. An exchange bias of 9 mT at 5 K and a coercivity of 26.3 mT were observed in the chromium oxide particles containing CrO₂.     

Synthesis of Tb3Al5O12 (TAG) transparent ceramics for potential magneto-optical applications

Tb₃Al₅O₁₂ transparent ceramics have been prepared by solid state reaction and vacuum sintering. The optical quality and the microstructure of the samples were investigated. The sample sintered at 1650 °C possessed relatively good optical transparency from 400 nm to 1600 nm. The Verdet constant measured at 632.8 nm of the quasi-pore-free Tb₃Al₅O₁₂ transparent ceramic was −172.72 rad T⁻¹ m⁻¹, which was close to the counterpart of Tb₃Al₅O₁₂ single crystal. The thermal conductivity of the sample was also measured. To the best of our knowledge, this is the first time that Tb₃Al₅O₁₂ transparent ceramic with relatively good optical quality and magneto-optical property has been reported.     

Fabrication of nanostructure Al2O3/ZrO2 (Y2O3) eutectic by combustion synthesis melt-casting under ultra-high gravity

A novel method for preparing Al₂O₃/ZrO₂ (Y₂O₃) eutectic was developed by combining combustion synthesis with melt-casting under ultra-high gravity (CSMC-UHG). The application of UHG = 800 g resulted in a high relative density of 99.8%, and an orientation-growth along the UHG direction. The microstructure was composed of aligned growth regimes containing a triangular dispersion of orderly ZrO₂ rods in Al₂O₃ matrix with a spacing of 300 nm. The eutectic had a high fracture toughness up to 17.9 MPa·m^1/2, which was mainly attributed to the nanostructure and the elastic bridge effects of the aligned ZrO₂ rods.     

Reaction sintering fabrication of (AlN, TiN)-Al2O3 composite

The (AlN, TiN)-Al₂O₃ composites were fabricated by reaction sintering powder mixtures containing 10-30 wt.% (Al, Ti)-Al₂O₃ at 1420-1520°C in nitrogen. It was found that the densification and mechanical properties of the sintered composites depended strongly on the Al, Ti contents of the starting powder and hot pressing parameters. Reaction sintering 20 wt.% (Al, Ti)-Al₂O₃ powder in nitrogen in 1520°C for 30 min yields (AlN, TiN)-Al₂O₃ composites with the best mechanical properties, with a hardness HRA of 94.1, bending strength of 687 MPa, and fracture toughness of 6.5 MPa m^1/2. Microstructure analysis indicated that TiN is present as well dispersed particulates within a matrix of Al₂O₃. The AlN identified by XRD was not directly observed, but probably resides at the Al₂O₃ grain boundary. The fracture mode of these composites was observed to be transgranular.     

Molten salt synthesis of acicular sodium strontium niobate particles

Acicular particles of Na_0.5Sr_0.25NbO₃ and NaSr₂Nb₅O₁₅ were successfully synthesized in the KSr₂Nb₅O₁₅ + NaCl system and SrCO₃ + Nb₂O₅ + NaCl + KCl system, respectively. It was found that the anisometric particles could not be obtained in the SrCO₃ + Nb₂O₅ + NaCl system. A facile ion-exchange approach was utilized to prepare acicular Na_0.5Sr_0.25NbO₃ particles. Acicular NaSr₂Nb₅O₁₅ particles with large aspect ratio could be obtained at the weight ratio of salt-to-mixture of 5:1, which were ideal template for fabricating textured ceramics. The growth process was researched and the synthesis method was suggested to be a self-precursor templating route. The results will be propitious to guide the fabrication of other templates with anisotropic morphology which were hard to prepare directly.     

Lightweight geopolymer made of highly porous siliceous materials with various Na2O/Al2O3 and SiO2/Al2O3 ratios

The syntheses of lightweight geopolymeric materials from highly porous siliceous materials viz. diatomaceous earth (DE) and rice husk ash (RHA) with high starting SiO₂/Al₂O₃ ratios of 13.0-33.5 and Na₂O/Al₂O₃ ratios of 0.66-3.0 were studied. The effects of fineness and calcination temperature of DE, concentrations of NaOH and KOH, DE to RHA ratio; curing temperature and time on the mechanical properties and microstructures of the geopolymer pastes were investigated. The results indicated that the optimum calcination temperature of DE was 800 °C. Increasing fineness of DE and starting Na₂O/Al₂O₃ ratio resulted in an increase in compressive strength of geopolymer paste. Geopolymer pastes activated with NaOH gave higher compressive strengths than those with KOH. The optimum curing temperature and time were 75 °C and 5 days. The lightweight geopolymer material with mean bulk density of 0.88 g/cm³ and compressive strength of 15 kg/cm² was obtained. Incorporation of 40% RHA to increase starting SiO₂/Al₂O₃ and Na₂O/Al₂O₃ ratios to 22.5 and 1.7 and enhanced the compressive strength of geopolymer paste to 24 kg/cm² with only a marginal increase of bulk density to 1.01 g/cm³. However, the geopolymer materials with high Na₂O/Al₂O₃ (>1.5) were not stable in water submersion.     

Preparation and thermoelectric properties of nc-Si:(Al2O3 + SiO2) composite film

A composite film of nanocrystalline Si (nc-Si) embedded in (Al₂O₃ + SiO₂) has been prepared on a quartz substrate by thermally evaporating a 400 nm thick Al film on a quartz substrate and annealing in air at 580 °C for 1 h. During annealing, the Al reacts with the SiO₂ of the quartz substrate and produces nc-Si, which is embedded in the (Al₂O₃ + SiO₂) film. The average size of nc-Si is ~ 22 nm and the thickness of the nc-Si:(Al₂O₃ + SiO₂) composite film is ~ 810 nm. It is found that the prepared film is thermoelectric with a Seebeck coefficient of − 624 μV/K at 293 K and − 225 μV/K at 413 K.     

Effect of Si3N4-particles addition in Ag-Cu-Ti filler alloy on Si3N4/TiAl brazed joint

A novel composite filler alloy was developed by introducing Si₃N_4p (p = particles) into Ag-Cu-Ti filler alloy. The brazing of Si₃N₄ ceramics and TiAl intermetallics was carried out using this composite filler alloy. The typical interfacial microstructure of brazed joints was: TiAl/AlCu₂Ti reaction layer/Ag_(s,s) + Al₄Cu₉ + Ti₅Si_3p + TiN_p/TiN + Ti₅Si₃ reaction layer/Si₃N₄. Effects of Si₃N_4p content in composite filler alloy on the interfacial microstructure and joining properties were investigated. The distribution of Ti₅Si_3p and TiN_p compounds in Ag-based solid solution led to the decrease of the mismatch of the coefficient of thermal expansion (CTE) and the Young's modulus between Si₃N₄ and TiAl substrate. The maximum shear strength of 115 MPa was obtained when 3 wt.% Si₃N_4p was added in the composite filler alloy. The fracture analysis showed that the addition of Si₃N_4p could improve the mechanical properties of the joint.     

Influences of post-annealing on structural and electrical properties of Bi1.5Zn1.0Nb1.5O7 thin films prepared by pulsed laser deposition

Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) thin films were prepared on Pt/TiO₂/SiO₂/Si(100) substrates at 650 °C under an oxygen pressure of 10 Pa by using pulsed laser deposition process. The crystallinity, microstructure and electrical properties of BZN thin films were investigated to verify the influences of post-annealing thermal process on them. The X-ray diffractometer (XRD) results indicate that all Bi_1.5Zn_1.0Nb_1.5O₇ thin films without post-annealing process or with post-annealing in situ vacuum chamber and in oxygen ambient exhibit a cubic pyrochlore structure. The improved crystallinity of BZN thin films through post-annealing was confirmed by XRD and scanning electron microscope (SEM) analysis. Dielectric constant and loss tangent of the as-deposited BZN thin films are 160 and 0.002 at 10 kHz, respectively. After annealing, dielectric properties of thin films are significantly improved. Dielectric constant and loss tangent of the in situ annealed films are 181 and 0.0005 at 10 kHz, respectively. But the films post-annealed in O₂ oven show the largest dielectric constant of 202 and the lowest loss tangent of 0.0002, which may attribute to the increase in grain size and the elimination of oxygen vacancies. Compared with the as-deposited BZN thin films, the post-annealed films also show the larger dielectric tunability and the lower leakage current density.