Sintering mechanism and dielectric properties of LiTaO3 matrix composites with the addition of Al2O3 particles

Highly densified Al₂O₃/LiTaO₃ (ALT) ceramic composites were fabricated by hot-pressing in a nitrogen atmosphere. The addition of Al₂O₃ particles could significantly improve the densification of LiTaO₃. Sintering mechanism of the LiTaO₃ ceramic incorporated with Al₂O₃ particles is proposed. Dielectric constant of 5 vol.% Al₂O₃/LiTaO₃ (5ALT) composite ceramic was slightly increased in the range from 30 kHz to 10⁶ Hz, but the dielectric loss was lowered in the whole range from 10³ Hz to 10⁶ Hz. Piezoelectric constant (d₃₃) of the 5ALT ceramic composite is about 50% of that of LiTaO₃ single crystal.     

Effect of CeO2 addition on densification and microstructure of Al2O3-YSZ composites

Alumina (Al₂O₃) and alumina-yttria stabilized zirconia (YSZ) composites containing 3 and 5 mass% ceria (CeO₂) were prepared by spark plasma sintering (SPS) at temperatures of 1350-1400 °C for 300 s under a pressure of 40 MPa. Densification, microstructure and mechanical properties of the Al₂O₃ based composites were investigated. Fully dense composites with a relative density of approximately 99% were obtained. The grain growth of alumina was inhibited significantly by the addition of 10 vol% zirconia, and formation of elongated CeAl₁₁O₁₈ grains was observed in the ceria containing composites sintered at 1400 °C. Al₂O₃-YSZ composites without CeO₂ had higher hardness than monolithic Al₂O₃ sintered body and the hardness of Al₂O₃-YSZ composites decreased from 20.3 GPa to 18.5 GPa when the content of ZrO₂ increased from 10 to 30 vol%. The fracture toughness of Al₂O₃ increased from 2.8 MPa m^1/2 to 5.6 MPa m^1/2 with the addition of 10 vol% YSZ, and further addition resulted in higher fracture toughness values. The highest value of fracture toughness, 6.2 MPa m^1/2, was achieved with the addition of 30 vol% YSZ.     

Investigation on mechanochemical synthesis of Al2O3/BN nanocomposite by aluminothermic reaction

Alpha-alumina–boron nitride (α-Al₂O₃–BN) nanocomposite was synthesized using mixtures of aluminum nitride, boron oxide and pure aluminum as raw materials via mechanochemical process under a low pressure of nitrogen gas (0.5 MPa). The phase transformation and structural evaluation during mechanochemical process were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential thermal analysis (DTA) techniques. The results indicated that high exothermic reaction of Al–B₂O₃ systems under the nitrogen pressure produced alumina, aluminum nitride (AlN), and aluminum oxynitride (Al₅O₆N) depending on the Al value and milling time, but no trace of boron nitride (BN) phases could be identified. On the other hand, AlN addition as a solid nitrogen source was effective in fabricating in-situ BN phase after 4 h milling process. In Al–B₂O₃–AlN system, the aluminothermic reaction provided sufficient heat for activating reaction between B₂O₃ and AlN to form BN compound. DTA analysis results showed that by increasing the activation time to 3 h, the temperature of both thermite and synthesis reactions significantly decreased and occurred as a one-step reaction. SEM and TEM observations confirmed that the range of particle size was within 100 nm.     

Development of input output relationships for self-healing Al2O3/SiC ceramic composites with Y2O3 additive using design of experiments

Al₂O₃/SiC ceramic composites with Y₂O₃ as an additive, was synthesized using the Taguchi method of design of experiments, so as to develop statistically sound input output relationships. The proportion of SiC was varied from 12 to 21 vol.% whereas that of Y₂O₃ was varied from 2.5 to 4 vol.%. The composites were sintered at 1500 °C for a soaking time period of 12 h in an air atmosphere. Cracks were induced on the composite surface using a Vickers indenter with a load varying between 20 and 40 kg. Fractographical analyses have been carried out using optical and/or scanning electron microscopy to investigate the surface crack propagation behavior. Thermal aging at 1300 °C in the time range of 0.5-12.5 h was applied to find optimal conditions for healing of the pre-cracked samples. The output parameters such as crack length, healed crack length, hardness and fracture toughness of the samples were correlated with appropriate inputs such as contents of SiC and Y₂O₃, crack-healing temperature, healing time, compaction pressure, indentation load using statistical analysis. Further, the extent of influence, exerted by pertinent input parameters on output parameters, was also identified.     

Microstructure evolution of Al2O3/Al2O3 joint brazed with Ag-Cu-Ti + B + TiH2 composite filler

Al₂O₃/Al₂O₃ joint was achieved using Ag-Cu-Ti + B + TiH₂ composite fillers at 900 °C for 10 min. The evolution mechanism of interface during brazing was discussed. Effects of Ti and B atoms content on microstructure of joints were investigated. Results show that a continuous and compact reaction layer Ti₃(Cu,Al)₃O forms at Al₂O₃/brazing alloy interface. Ti(Cu,Al) precipitates near Ti₃(Cu,Al)₃O layer. In situ synthesized TiB whiskers evenly distribute in Ag and Cu based solid solution. The higher content of B powders in composite fillers increases TiB whiskers content, but decreases the thickness of Ti₃(Cu,Al)₃O layer, while the higher TiH₂ powders content thickens Ti₃(Cu,Al)₃O layer. Ag and Cu based solid solutions become uniform and fine with the increasing of TiB whiskers content. Ti(Cu,Al) intermetallics content increase and they gradually distribute from Al₂O₃ side to the central of brazing alloy, but the content of Cu based solid solution decreases when the TiH₂ content increases.     

Relationship between fracture toughness characteristics and morphology of sintered Al2O3 ceramics

The influence of sintering temperature and soaking time on fracture toughness of Al₂O₃ ceramics has been investigated. The samples were prepared by solid state sintering at 1500, 1600 and 1700 °C for different soaking time periods. The fracture toughness of the sintered samples was determined by inducing cracks using Vickers indentation technique. Microstructural investigations on fracture surfaces obtained by three point bend test mode were made and correlated with fracture toughness. Crack deflection in the samples sintered at 1500 and 1600 °C for which ranges of fracture toughness are 5.2–5.4 and 5.0–5.6 MPa m^1/2 respectively, are found. The samples sintered at 1700 °C have lower fracture toughness ranging between 4.6 and 5.0 MPa m^1/2. These samples have larger grains and transgranular fracture mode is predominant. The crack deflection has further been revealed by SEM and AFM observations on fracture surface and fracture surface roughness respectively.     

Fabrication of novel multilayer Al2O3-(m-ZrO2)/t-ZrO2 fibrous ceramics composites

A novel layered microstructure in the Al₂O₃/ZrO₂ composites system was fabricated by the multipass extrusion method. The microstructure consisted with very fine alternate lamina of Al₂O₃-(m-ZrO₂) and t-ZrO₂. The composites were designed in such a way that a small group of 7 cylindrical alternate layers of Al₂O₃-(m-ZrO₂) and t-ZrO₂ made a concentric microgroup around 40 μm in diameter, with a common boundary layer between the adjacent groups. The thickness of both layers was around 2-3 μm. The microstructure was unidirectionally aligned throughout the composites. The composite microstructure was fibrous due to the unidirectional orientation of these microgroups. Detailed microstructure of the fabricated composites was characterized by SEM. The effect of the concentric layered microstructure on mechanical behavior was discussed. Material properties such as density, bending strength, Vickers hardness and fracture toughness were measured and evaluated depending on different sintering temperatures.     

Effect of Al2O3 addition on bioactivity,thermal and mechanical properties of some bioactive glasses

Three bio-phosphate glass-specimens with and without Al₂O₃ addition were prepared in order to shed light on their bioactivity behavior towards the simulated body fluid biological solution (SBF). The results revealed that Al₂O₃ has significant effect on the ability of bio-glass to form the hydroxycarbonate apatite layer on its surface. That layer was detected by FTIR spectra, SEM micrographs and EDAX pattern. Also, the effect of Al₂O₃ on the mechanical properties was studied by measuring the hardness of the glass samples, which increased by Al₂O₃ addition. The thermal expansion coefficient was decreased by increasing the Al₂O₃ percent in the bio-glass samples.     

Evaluation of sintering stresses of an Al2O3 powder with a self-loading apparatus

The sintering stress of an Al₂O₃ powder, σ_s, is evaluated from an equation σ_s = F_s/ρS_a, where F_s is the uniaxial tensile force necessary to just stop the sintering contraction, ρ is the relative density, and S_a is the cross-sectional area. During densification, σ_s increases to the maximum at a relative density of 85%, and then abruptly decreases. The variation of σ_s in the intermediate sintering stage suggests a simple cubic packing of particles. Densification in the final sintering stage is explained by shrinkage of the pores at the corners where four tetrakaidecahedra meet. The present surface tensions of the Al₂O₃ powder evaluated from the σ_s roughly equal a reported value.     

Effect of Y2O3 additive on conventional and microwave sintering of mullite

Mullite has become a strong candidate material for advanced structural and functional ceramics. Much interest has recently focused on sintering aids for mullite. The aim of this study was to evaluate the effect of Y₂O₃ as a sintering aid in the conventional and microwave sintering of mullite. To accomplish this study, a highly pure industrial mullite was used. Mullite with and without Y₂O₃ was pressed under a cold isostatic pressure of 200 MPa. Samples were sintered conventionally at 1400, 1450, 1500, 1550 and 1600 °C for 2 h and microwave-sintered for up to 40 min using a large range of power. The microstructure and physical properties of the microwave-sintered samples were compared to those of the conventionally sintered samples. The results showed that Y₂O₃ improved the densification of mullite bodies in the conventional and microwave sintering processes, but high densifications were achieved in just a few minutes when Y₂O₃ was used with microwave processing.     

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.     

Sintering kinetics of α-Al2O3 powder

The sintering kinetics of α-Al₂O₃ powder are reviewed in this paper. The initial sintering of α-Al₂O₃ micropowder and α-Al₂O₃ nanopowder is all controlled by grain boundary diffusion. The sintering kinetics dominate up to a relative density of 0.77, where the coarsening kinetics dominate during further densification. Herring's scaling law can be used to predict the approximate sintering temperature of α-Al₂O₃ powder and demonstrates that if the particle size can be reduced to <20 nm, sintering below 1000°C may be possible. ©     

Formation of TiO2 nano fibers on a micro-channeled Al2O3-ZrO2/TiO2 porous composite membrane for photocatalytic filtration

In this study, needle-shape TiO₂ fibers were successfully fabricated inside a micro-channeled Al₂O₃-ZrO₂ composite porous membrane system using sol-gel method. The micro-channeled Al₂O₃-ZrO₂ composite was fabricated using the fibrous monolithic (FM) process. Pure anatase phase TiO₂ was crystallized from the as-coated amorphous phase during calcination at 510 °C. The TiO₂ fibers grew on the surface frame of the micro-channeled Al₂O₃-ZrO₂ composite membrane and fully covered the inside of the micro-channeled pores. The specific surface area of the TiO₂ coated membrane system was dramatically increased by over 100 fold compared to that of the non-coated system. The photocatalytic activity of the membrane was also assessed and was shown to very effectively convert organic materials. Thus, this novel membrane holds promise for use as an advanced filtration system.     

Mechanical properties of Al2O3-Cr2O3/Cr3C2 nanocomposite fabricated by spark plasma sintering

The fine grains of Al₂O₃-Cr₂O₃/Cr-carbide nanocomposites were prepared by employing recently developed spark plasma sintering (SPS) technique. The initial materials were fabricated by a metal organic chemical vapor deposition (MOCVD) process, in which Cr(CO)₆ was used as a precursor and Al₂O₃ powders as matrix in a spouted chamber. The basic mechanical properties like hardness, fracture strength and toughness, and the nanoindentation characterization of nanocomposites such as Elastics modulus (E), elastic work (W_e) and plastic work (W_p) were analyzed. The microstructure of dislocation, transgranular and step-wise fracture surface were observed in the nanocomposites. The nanocomposites show fracture toughness of (4.8 MPa m^1/2) and facture strength (780 MPa), which is higher than monolithic alumina. The strengthening mechanism from the secondary phase and solid solution are also discussed in the present work. Nanoindentation characterization further illustrates the strengthening of nanocomposites.     

Al2O3-coated SnO2/TiO2 composite electrode for the dye-sensitized solar cell

A novel Al₂O₃-coated SnO₂/TiO₂ composite electrode has been applied to the dye-sensitized solar cell. In such an electrode, two kinds of energy barriers (SnO₂/TiO₂ and TiO₂/Al₂O₃) were designed to suppress the recombination processes of the photo-generated electrons and holes. After the SnO₂ was modified by colloid TiO₂, the photoelectric conversion efficiency of the SnO₂/TiO₂ composite cell increased to 2.08% by a factor of 2.8 comparing with that of the SnO₂ cell. The Al₂O₃ layer on the SnO₂/TiO₂ composite electrode further suppressed the generation of the dark current, resulting in 37% improvement in device performance comparing with the SnO₂/TiO₂ cell.     

Structural and optical properties of Tm:Y2O3 transparent ceramic with La2O3, ZrO2 as composite sintering aid

The paper reports the use of La₂O₃ and ZrO₂ co-doping as a composite sintering aid for the fabrication of Tm:Y₂O₃ transparent ceramics. Two groups of experiments were conducted for investigating the influences of composite sintering aids on the microstructures and the optical properties of Tm:Y₂O₃ transparent ceramics in contrast to single La³⁺ and single Zr⁴⁺ doped Tm:Y₂O₃. Samples with composite sintering aids could realize fine microstructures and good optical properties at relatively low sintering temperatures. Grain sizes around 10 μm and transmittances close to theoretical value at wavelength of 2 μm were achieved for the 9 at.% La³⁺, 3 at.% Zr⁴⁺ co-doped samples sintered at 1500-1600 °C. The influences of the composite sintering aids on the emission intensities and the phonon energies of Tm:Y₂O₃ ceramics were also investigated.     

Phase diagram of the Al2O3-HfO2-Y2O3 system

The phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system was first constructed in the temperature range 1200-2800 °C. The phase transformations in the system are completed in eutectic reactions. No ternary compounds or regions of appreciable solid solution were found in the components or binaries in this system. Four new ternary and three new quasibinary eutectics were found. The minimum melting temperature is 1755 °C and it corresponds to the ternary eutectic Al₂O₃ + HfO₂ + Y₃Al₅O₁₂. The solidus surface projection, the schematic of the alloy crystallization path and the vertical sections present the complete phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system.     

Corrosion mechanisms of Al2O3/MgAl2O4 by V2O5, NiO, Fe2O3 and vanadium slag

The effects of V₂O₅, NiO, Fe₂O₃ and vanadium slag on the corrosion of Al₂O₃ and MgAl₂O₄ have been investigated. The specimens of Al₂O₃ and MgAl₂O₄ with the respective oxides above mentioned were heated at 10 °C/min from room temperature up to three different temperatures: 1400, 1450 and 1500 °C. The corrosion mechanisms of each system were followed by XRD and SEM analyses. The results obtained showed that Al₂O₃ was less affected by the studied oxides than MgAl₂O₄. Alumina was only attacked by NiO forming NiAl₂O₄ spinel, while the MgAl₂O₄ spinel was attacked by V₂O₅ forming MgV₂O₆. It was also observed that Fe₂O₃ and Mg, Ni, V and Fe present in the vanadium slag diffused into Al₂O₃. On the other hand, the Fe₂O₃ and Ca, S, Si, Na, Mg, V and Fe diffused into the MgAl₂O₄ structure. Finally, the results obtained were compared with those predicted by the FactSage software.     

CVD synthesis of Al2O3 nanotubular structures using a powder source

Alumina (Al₂O₃) 1D nanotubular structures were synthesized by chemical vapor deposition (CVD) using aluminum (Al) and Al₂O₃ powder sources at a temperature of 1300 °C and a pressure of 100 Pa. At present, no research has been published regarding the synthesis of α-phase Al₂O₃ nanotubes using only a powder source. In this work, we attempted to grow α-phase Al₂O₃ nanotubes without catalysts. As-deposited nanotubular structures had an irregular and ragged surface morphology that was related to the boundary layer thickness. Moreover, complementary nanotubular structures can be obtained via an annealing process. A model to describe the irregular nanotubular structure formation was suggested, and the effect of the boundary layer thickness was demonstrated in our experimental conditions.