Preparation, sintering and fracture behavior of Al2O3/LaAl11O18 ceramic composites

Al₂O₃/25 vol% LaAl₁₁O₁₈ composites were prepared by pressureless sintering at 1550°C with composite powders obtained by copercipiated method using La(NO₃) · 6H₂O and Al(NO₃)₃ · 9H₂O as starting materials. The enhanced reactive activity of Al₂O₃ and chemically homogeneous mixing of the constituents made LaAl₁₁O₁₈ phase to be formed at low temperature in composite powders. AlF₃ additive was used to reduce the transformation temperature of transition alumina. The LaAl₁₁O₁₈ grains in the composite powder obtained at 1500°C showed rodlike morphology distributed homogeneously in Al₂O₃ powder. The samples sintered at 1550°C for 4 h with CAS (CaO-Al₂O₃-SiO₂) sintering aid can obtain a high relative density. The effects of the sintering time on the grain growth of Al₂O₃ and the fracture toughness of the composites were studied and the results showed that LaAl₁₁O₁₈ grains reduced the growth of Al₂O₃ grains and the rodlike grains increased the fracture toughness. The improvement in fracture toughness of the composites was mainly attributed to the mechanism of crack deflection.     

Sintering, densification and crystallization of Ca–Al–B–Si–O glass/Al2O3 composites for LTCC application

Ca–Al–B–Si–O glass/Al₂O₃ composites were prepared based on the borosilicate glass powders (D₅₀ = 2.84) and Al₂O₃ ceramic powders (D₅₀ = 3.26), and the sintering, densification, crystallization of samples were investigated. The shrinkage of sample starts to have a sharp increase at 600 °C. The shrinkage of sample starts to have a further rapid increase after the glass softening temperature of about 713 °C. Glass/Al₂O₃ composites can be sintered at 875 °C/15 min and exhibit better properties of a relative density of 98.4 %, a λ value of 2.89 W/mK, a ε _r value of 7.82 and a tan δ value of 5.3 × 10^?4. The interface between glass and Al₂O₃ grains and the interface between anorthite and glass phase depicts a good compatibility according to transmission electron microcopy test. It is the low sintering temperature, high density and good compatibility with Ag electrodes that, guarantee borosilicate glass/Al₂O₃ composites suitable for low temperature co-fired ceramic materials.     

Effects of TiO<Subscript>2</Subscript>, ZrO<Subscript>2</Subscript> and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> dopants on the compressive strength of tricalcium phosphate

Tricalcium phosphate (TCP) powders synthesised using the Ca(NO₃)₂ and Ca(OH)₂ routes were doped with TiO₂, ZrO₂ and Al₂O₃ in order to increase their compressive strength. An ultimate compressive strength (UCS) of 255 ± 6 MPa was achieved for approximately 10 vol% TiO₂ doping compared to 30 ± 3 MPa for an un-doped control processed and tested in the same manner. Higher levels of TiO₂ doping resulted in smaller increases in UCS with 30 and 50 vol% achieving 213 ± 9 and 178 ± 15 MPa, respectively. Very small amounts of Al₂O₃ doping (< 0.5 vol%) also resulted in a stronger materials. However, under the processing conditions employed, higher levels of Al₂O₃ and ZrO₂ doping resulted in no beneficial effect on the UCS. Polyvinyl alcohol (PVA) was used as binding agent to facilitate processing. As expected, higher levels of PVA were associated with smaller increases in UCS. Powders synthesised using the Ca(OH)₂ route had smaller particle size and resulted in larger increases in UCS compared to the Ca(NO₃)₂-synthesised powders. Although some powders contained α and β-TCP phases, no other calcium phosphate, CaO, CaTiO₃ or CaZrO₃ phases were detected. In conclusion, a significant increase in the UCS of TCP was achieved by doping with approximately 10 vol% TiO₂ which is expected to have little or no effect on the bioactivity or bioresorbability of the material.     

Mechanical properties of Al2O3 particle-Y-TZP matrix composite and its toughening mechanism

Dense Al₂O₃ particle-Y-TZP matrix (Al₂O₃<40 vol%) composite was prepared by pressureless sintering at 1550°C. Composites with 10–30 vol% Al₂O₃ particles showed enhanced fracture toughness, bending strength and Vicker's hardness as compared to single-phase Y-TZP. The highest strength (1150 MPa) and highest toughness (12.4 MPa m^1/2) were obtained for the composite containing 10 vol% Al₂O₃. It was found that, in addition to the contribution by the crack-deflection effect, the enhanced phase transformation from tetragonal to monoclinic during fracture was the main toughening mechanism in operation in the composites.     

Effect of cerium zirconate (Ce2Zr2O7) on microstructure and mechanical properties of Ce-ZTA

Powders for ZrO₂ toughened Al₂O₃ (ZTA) composites containing 8, 11, 13.8 and 16.5 vol% ZrO₂ (stabilized with 11.5 mol% CeO₂) are prepared by a hybrid sol-gel method using Al₂O₃ powders and a sol formed from Zr-alkoxide and cerium nitrate. Besides ZrO₂, a small amount of a Ce-zirconate phase (Ce₂Zr₂O₇) forms when the powders are calcined in air. The zirconate phase persists in the sintered specimens and its amount increases from surface to centre of the specimen. Presence of higher amount of Ce₂Zr₂O₇ promotes exaggerated grain growth of Al₂O₃. Particles of Zr rich phases are found to be trapped inside the Al₂O₃ grains. Composites exhibit higher fracture toughness despite lower transformability of t-ZrO₂ during fracture when they contain high amount of zirconate. Crack bridging is shown to be an important mechanism contributing to enhancement in fracture toughness in these composites.     

Effect of replacement of MgO by CaO on sintering,crystallization and properties of MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system glass-ceramics

The effects of replacement of MgO by CaO on the sintering and crystallization behavior of MgO–Al₂O₃–SiO₂ system glass-ceramics were investigated. The results show that with increasing CaO content, the glass transition temperature firstly increased and then decreased, the melting temperature was lowered and the crystallization temperature of the glass-ceramics shifted clearly towards higher temperatures. With the replacement of MgO by less than 3 wt.% CaO, the predominant crystalline phase in the glass-ceramics fired at 900 °C was found to be α-cordierite and the secondary crystalline phase to be μ-cordierite. When the replacement was increased to 10 wt.%, the predominant crystalline phase was found to be anorthite and the secondary phase to be α-cordierite. Both thermal expansion coefficient (TCE) and dielectric constant of samples increases with the replacement of MgO by CaO. The dielectric loss of sample with 5 wt.% CaO fired at 900 °C has the lowest value of 0.08%. Only the sample containing 5 wt.% and10 wt.% CaO (abbreviated as sample C5 and C10) can be fully sintered before 900 °C. Therefore, a dense and low dielectric loss glass-ceramic with predominant crystal phase of α-cordierite and some amount of anorthite was achieved by using fine glass powders (D₅₀ = 3 μm) fired at 875–900 °C. The as-sintered density approaches 98% theoretical density. The flexural strength of sample C5 firstly increases and then decreases with sintering temperature, which closely corresponds to its relative density. The TCE of sample C5 increases with increasing temperature. The dielectric property of sample C5 sintered at different temperatures depends on not only its relative density but also its crystalline phases. The dense and crystallized glass-ceramic C5 exhibits a low sintering temperature (≤900 °C), a fairly low dielectric constant (5.2–5.3), a low dielectric loss (≤10⁻³) at 1 MHz, a low TCE (4.0–4.25 × 10⁻⁶ K⁻¹), very close to that of Si (∼3.5 × 10⁻⁶ K⁻¹), and a higher flexural strength (≥134 MPa), suggesting that it would be a promising material in the electronic packaging field.     

Abnormal grain growth in alumina-doped hafnia ceramics

Hafnia (HfO₂) ceramics containing 0.0, 5.0, and 10.0 vol% Al₂O₃, respectively, were sintered at 1600°C for various periods from 2–24 h. Abnormal grain growth was found to occur in the Al₂O₃-containing compositions. Hafnia containing 5.0 vol% Al₂O₃ exhibits an average grain size of almost double that of the Al₂O₃-free hafnia matrix, coupled with a much wider grain-size distribution. The material containing 10.0 vol% Al₂O₃ shows a smaller average grain size than the composition containing 5.0 vol% Al₂O₃. However, its average grain size is still larger than that of the Al₂O₃-free hafnia on sintering at 1600°C for more than 8 h. Microstructural characterization, carried out using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with an energy dispersive analysis facility (EDX), indicated that there existed a continuous segregant layer at the grain boundaries and grain junctions in the Al₂O₃-free hafnia. Hafnia exhibits a low solubility in the segregant layer phase which inhibits the growth of the hafnia grains. The Al₂O₃ particles act as a scavenger for the silicon-rich glassy phase, damaging the continuous nature of the boundary segregant layer and promoting grain growth in the Al₂O₃-doped hafnia ceramics. The microstructural development at the sintering temperature is an overall result of the concurrent scavenger effect and grain pinning by the Al₂O₃ particles.     

The effect of grain boundary phase on contact damage resistance of alumina ceramics

The effect of grain boundary phase on contact damage behavior is investigated in alumina ceramics. Four types of aluminas doped with MgO, anorthite (CaO·Al₂O₃·2SiO₂), silica, and with both MgO and anorthite are prepared such that they have similar average grain size by adjusting sintering conditions. MgO-doped alumina composed of equiaxed grains shows brittle fracture behavior, and anorthite-doped alumina composed of elongated grains shows a quasi-plastic response under Hertzian sphere indentation. The co-doped alumina with MgO and anorthite, however, is damage tolerant even with its rounded grains, while silica-doped alumina with similar grain size and shape to anorthite-doped alumina shows abrupt strength degradation with low critical load for cone cracking. The damage behavior is discussed from the viewpoint of residual stress induced by thermal expansion mismatch between the grains and grain boundary phases. The damage tolerant behavior of alumina ceramics is significantly affected by the composition of grain boundary phase.     

Composition and Structure of Block-Type Ferrospheres Isolated from Calcium-Rich Power Plant Ash

Polished sections of individual ferrospheres 30 to 40 μm in size, with single-block and blocky structures and a variable glass phase content, have been studied using a scanning electron microscope equipped with an energy dispersive X-ray spectrometer system. The results demonstrate that the single-block globules consist of sintered magnetite crystallites containing Al₂O₃, MgO, and CaO as impurities and are formed from the pyrite of the initial coal. Characteristically, the ferrospheres with a variable glass phase content differ in the composition of local areas on polished sections of the globules, which attests to inhomogeneity of the melt droplets they formed from. We have identified groups of globules whose overall composition, as well as the composition of their local areas, meet general equations for the interrelation between the concentrations of their components: SiO₂ = f(FeO) and SiO₂ = f(Al₂O₃). Comparison of the coefficients of the SiO₂ = f(Al₂O₃) dependence for the globules with the silicate modulus (SiO₂/Al₂O₃) of the aluminosilicate mineral components of the coal indicates that the formation of this type of globules involves pyrite–anorthite or pyrite–albite associates containing quartz impurities. The composition of the spinel ferrite in the globules produced with the participation of anorthite comprises FeO, Al₂O₃, MgO, and CaO in concentrations of 85–96, 1.7–10, 0.1–1.8, and 0.3–2.8 wt %, respectively. In the albite-based globules, the respective concentrations are 81–92, 0.7–5.9, 1.0–5.7, and 2.2–5.6 wt %. The crystallite size and shape are determined by the size of the local melt areas where the total concentration of spinel-forming oxides exceeds 85 wt %.     

Effect of a small amount of liquid-forming additives on the microstructure of Al2O3 ceramics

Sintering behaviour and microstructure of Al₂O₃ ceramics without additives and with 0.02–0.25 mol% CaO + SiO₂ (CaO/SiO₂ = 1) were investigated. When Al₂O₃ bodies were sintered at 1400 °C, the sinterability and the grain size decreased as the content of CaO + Si₂ increased. When Al₂O₃ ceramics with 0.05 – 0.25 mol% CaO + SiO₂ were sintered at higher sintering temperature, both CaO and SiO₂ reacted with Al₂O₃ to produce the liquid phase along grain boundaries, and exaggerated platelet Al₂O₃ grains, with an aspect ratio of about 4.5, were formed. Because the size of platelet grains decreased as the content of CaO + SiO₂ increased, the distribution of either SiO₂ particles or this intergranular phase of CaO – Al₂O₃ – SiO₂ might control the microstructure.     

Fabrication of new porcelain body using nonplastic raw materials by slip casting

New porcelain bodies using only nonplastic raw materials, such as volcanic glass, quartz, alumina and aluminous cement, were fabricated, and their properties were investigated. Green strength increased with increasing Al₂O₃ content at the constant amount of volcanic glass and aluminous cement, caused by the increase of green bulk density with small sized Al₂O₃ addition. The phases in the fired body were glass, -quartz, cristobalite, anorthite and -Al₂O₃. High flexural strength with 10 wt% Al₂O₃ addition was attributed to a strong residual stress induced by the large difference in the thermal expansion coefficient between the glass matrix and the quartz grains, and an additional prestress was induced by Al₂O₃ grains. Higher density and fewer fracture origin were also indicated as potential factors leading to the strengthening effect.     

The crystallization of diabase glass

The crystallization behaviour of diabase glass at elevated temperatures was studied in samples prepared by melting the diabase rock. DTA and X-ray analyses revealed the crystallization of diopside (CaO · MgO · 2SiO₂) at 865 C and anorthite (CaO · Al₂O₃ · 2SiO₂) at 1060 C. Further, the kinetics of crystallization of diopside were studied. The phenomenological Johnson-Mehl-Avrami equation was used and the exponentn=3/2 determined from the dependency of the volume fraction of the crystal phase (diopside) on time. The activation energy of crystallization of diabase glass 248 kJ mol^–1 was estimated on the basis of DTA measurements carried out at different heating rates and found to be in good agreement with literature data for similar glass.     

Preparation of Nanocrystalline Calcium Aluminate Powders

Nanocrystalline calcium aluminate (CaAl₂O₄and Ca₃Al₂O₆) powders have been prepared by pyrolysis of complex compound of aluminium and calcium with triethanolamine (TEA). The soluble metal ion-TEA complex forms the precursor material on complete dehydration of the mixtures of the complex of calcium-TEA and aluminium-TEA. The single-phase CaAl₂O₄and Ca₃Al₂O₆powders have resulted after heat treatment at 800° and 1000°C, respectively. The precursors and the heat-treated final powders have been characterized by X-ray diffractometry (XRD), differential thermal and thermogravimetric analysis TG/DTA, and transmission electron microscopy (TEM). The average particle sizes as measured by transmission electron microscopy studies are around 30–40 nm and 25–30 nm for CaAl₂O₄and Ca₃Al₂O₆powders, respectively. These nano-sized powders are very suitable for making various nano-composites with polymers and inorganic compounds.     

Effects of glass additions on the dielectric properties and energy storage performance of Pb0.97La0.02(Zr0.56Sn0.35Ti0.09)O3 antiferroelectric ceramics

In this work, CdO–Bi₂O₃–PbO–ZnO–Al₂O₃–B₂O₃–SiO₂ low softening point glass powders were prepared and employed as sintering aid to improve the dielectric breakdown strength and reduce the sintering temperature of Pb_0.97La_0.02(Zr_0.56Sn_0.35Ti_0.09)O₃ antiferroelectric ceramics. The effects of glass content and sintering temperature on the densification, microstructure, dielectric properties and energy storage performance of Pb_0.97La_0.02(Zr_0.56Sn_0.35Ti_0.09)O₃ antiferroelectric ceramics have been investigated. With inclusion of glass, sintered densities comparable to those obtained by conventional sintering are achieved at only 1,050 °C. The breakdown strength of glass-added samples was notably improved due to the reduction of the grain size. The antiferroelectric to ferroelectric switching field and the ferroelectric to antiferroelectric field both increased with increasing glass content. The dielectric constant and dielectric loss decreased gradually with increasing glass content. As a result, the highest recoverable energy density of 3.3 J/cm³ with an energy efficiency of 80 % was achieved in 4 wt% glass-added sample sintered at 1,130 °C.     

Macroporous calcium phosphate glass-ceramic prepared by two-step pressing technique and using sucrose as a pore former

Macroporous calcium phosphate glass-ceramic with an initial glass composition of 60CaO ⋅ 30P₂O₅⋅ 3TiO₂⋅ 7Na₂O in mol% was successfully prepared by sintering the mixture compact consisting of calcium phosphate glass and sucrose powders, which was formed using a two-step pressing technique. After burning off the sucrose phase, a 3D interconnected macroporous structure was formed in the sintered body, in which the skeleton consisting of the calcium phosphate glass-ceramic (including β -calcium pyrophosphate and β -tricalcium phosphate as the crystalline phases) was transformed from the initial glass during the sintering. The macropores with several hundred microns in diameter and the large interconnection size (∼ 100 μ m), which result from the controllably large-sized sucrose particles and the hot-pressing at a little higher temperature than the sucrose’s melting point, are believed to meet the requirements for cell adhesion and bone tissue regeneration well. Moreover, in vitro dissolution behavior study indicates that the calcium phosphate glass-ceramic is soluble to an acetic acid solution of pH 5–7. These, together with the simplicity and feasibility of the innovative fabrication method itself, show that the formed porous glass-ceramic has a promising potential for application to a scaffold for bone tissue engineering.     

Processing of Al2O3/SiC ceramic cake preforms and their liquid Al metal infiltration

In order to prepare ceramic preforms, chemical processes were used rather than using mixing of ceramic powders to obtain porous Al₂O₃/SiC ceramic foams. A slurry was prepared by mixing aluminium sulphate and ammonium sulphate in the water, and silicon carbide powder was added into the slurry so that a uniform mixture of Al₂O₃/SiC cake could be produced. The resulting product was (NH₄)₂SO₄·Al₂(SO₄)₃·24H₂O plus silicon carbide particles (SiC_p) after dissolving chemicals in the water. This product was heated up in a ceramic crucible in the furnace. With the effect of heat it foamed and Al₂O₃/SiC cake was obtained. Resulting Al₂O₃ grains were arranged in a 3D honeycomb structure and the SiC particles were surrounded by the alumina grains. Consequently, homogeneous powder mixing and porosity distribution were obtained within the cake. The morphology of the powder connections was networking with flake like particles. These alumina particles resulted in large amounts of porosity which was desired for ceramic preforms to allow liquid metal flow during infiltration. The resulting high porous ceramic cake (preform) was placed in a sealed die and liquid aluminium was infiltrated by Ar pressure. The infiltration was achieved successfully and microstructures of the composites were examined.     

Utilization of Chemically Synthesized Fine Powders of SiC/Al2O3 Composites for Sintering

Fine powders of (Al₂O₃)_100–x(SiC)_x (0 ≤ x ≤ 50) composites were prepared by chemical route (named as pyrophoric technique) to achieve a uniform mixture of SiC in an alumina matrix. The chemically synthesized fine SiC/Al₂O₃ composite powders were sintered to form composites at 1450°C which is well below the sintering temperature of SiC. Sintering was performed in an argon atmosphere. Highly dense SiC/Al₂O₃ microstructures were achieved. An improvement in bulk density and hardness has been achieved for SiC/Al₂O₃ composites with 20 wt% of SiC. Hexagonal-shaped grains have been obtained in (Al₂O₃)₅₀(SiC)₅₀ composite with well-connected grain boundaries. The peak position of alumina in SiC/Al₂O₃ composites shifts toward lower wavenumbers in Fourier transform infrared spectroscopy and higher wavenumbers in Raman spectroscopy due to the incorporation of SiC in the composites. The optical band gap decreases with the addition of SiC and the composite behaves more like a semiconductor rather than an insulator. These properties make SiC/Al₂O₃ composites attractive for various industrial applications.     

Low-temperature sintering characteristics of nano-sized BaNd2Ti5O14 and Bi2O3–B2O3–ZnO–SiO2 glass powders prepared by gas-phase reactions

Nano-sized BaNd₂Ti₅O₁₄ (BNT) powders were prepared by spray pyrolysis from solutions containing ethylenediaminetetraacetic acid and citric acid. Treatment at temperatures ≥900 °C and subsequent milling resulted in nanoparticle powders with orthorhombic crystal structures. The mean particle size of the powder post-treated at 1000 °C was 160 nm. Nano-sized Bi₂O₃–B₂O₃–ZnO–SiO₂ glass powder with 33 nm average particle size was prepared by flame spray pyrolysis and used as a sintering agent for the BNT. BNT pellets sintered at 1100 °C without the glass had porous structures and fine grain sizes. Those similarly sintered with the glass had denser structures and larger grains.     

Low temperature synthesis of nanocrystalline calcium aluminate compounds with surfactant-assisted precipitation method

Nanocrystalline calcium aluminates with different CaO:Al₂O₃ and surfactant/metal ion molar ratios were prepared by wet chemical synthesis method using Poly (ethylene glycol)-block-poly(propylene glycol)-block poly(ethylene glycol) (PEG–PPG–PEG, MW:5800) as surfactant. X-ray diffraction (XRD) and N₂ adsorption–desorption results showed that the increase in CaO:Al₂O₃ ratio decreased the specific surface area and increased the particle sizes of prepared samples while the surfactant/metal ion molar ratios were kept constant. These analyses also declared that for the sample with CaO:Al₂O₃ = 1:2 (CA2) addition of polymeric surfactant increased the specific surface area and decreased the crystallite size. Scanning electron microscopy (SEM) results confirmed that size of particles for CaO:Al₂O₃ = 1:6 (CA6) sample are smaller than CA2. Transmission electron microscopy (TEM) revealed no particular particle shape for the CA2 sample but it showed the high degree of crystallinity and single phase for the prepared sample at 1100 °C.     

Strengthening of alumina by cerium-zirconate (Ce2Zr2O7)

Composite powders of Al₂O₃ and 0 to 30 vol% Ce₂Zr₂O₇ are prepared by a hybrid sol-gel method using Al₂O₃ powders and a sol formed from Zr-alkoxide and cerium nitrate. All the Zr from the sol goes to form the cerium zirconate phase when the powders are calcined in N₂. Pressureless sintering in air at 1500°C yields composites with high density (98%). Maximum values of fracture toughness and strength, 6.5 MPa and 620 MPa respectively, (e.g. 3.5 MPa and 350 MPa for pure Al₂O₃) are obtained in 10 vol% Ce₂Zr₂O₇ composite sintered in air. The dominant mechanism for enhancement in K _IC is believed to be crack bridging. Crack bridging activity in the 10 vol% composite is found to be maximum and extends upto 190 m from the crack tip.