Effect of niobium doping on the densification and grain growth in alumina

The effect of niobium doping on the densification and grain growth of nano-sized α-Al₂O₃ powders during sintering has been investigated. The dopant concentration added ranged from 0.1 to 0.5 mol%. It was observed that addition of niobium oxide could improve the densification of the pure alumina with a lower sintering temperature, a shorter sintering time. The effect is strengthened by increasing the amount of dopant. It also demonstrated that niobium dopant significantly promotes the grain growth of alumina during sintering and the grain size of alumina increases with increasing the amount of dopant in the added range.     

Fabrication of alumina-based ceramic foams utilizing superplasticity

Model ceramic foams were fabricated by expanding once-sintered dense shells utilizing the superplastic deformation of alumina dispersed with 3 mol% yttria-stabilized zirconia (3YSZ) or magnesia. The grain growth of alumina was suppressed by adding 3YSZ and the resultant grain size and amount of dispersion were closely related to the total porosity. Total porosity of the ceramic foam depended on the grain size and their distributions irrespective of the size of starting powders.     

Fabrication of near-infrared transparent Dy2Zr2O7 ceramic by controlled pre-calcination of powders

Fabrication of transparent or pore-free oxide ceramics usually requires sintering in a vacuum or reducing atmosphere. In the present paper, a near-infrared transparent Dy₂Zr₂O₇ ceramic was prepared by ordinary pressureless sintering under the air atmosphere, with a controlled pre-calcining of raw powders process. The key point of the process is to control the content of the phases by pre-calcination of the starting mixture of ZrO₂ and Dy₂O₃ powders. The influence of pre-calcining temperature on the content of phases, the particle sizes, on the sinterability of the mixture of Dy₂O₃ and ZrO₂ powders has been investigated. It was found that when the pre-calcination was conducted at 1200°C for 2 h, the powder contains 4 mol% Dy₂Zr₂O₇, 18 mol% m-ZrO₂, 30 mol% Dy₂O₃, and about 50 mol% t/c-ZrO₂ which is beneficial for the sintering of pore-free Dy₂Zr₂O₇ ceramics. The mechanism is discussed in the paper. The transparency of the sintered Dy₂Zr₂O₇ ceramic was measured, and their transmittance is about 63% at 1450 nm and 73% at 1900 nm. This work provides a simple and convenient way to prepare the near-infrared transparency ceramic without special sintering facilities.     

Fabrication and characterization of alumina and zirconia-toughened alumina porous structures

In the present study, alumina (Al₂O₃) and zirconia-toughened alumina (ZTA) porous structures (foams) were manufactured using the space holder technique. Al₂O₃ and ZTA foams with varying porosities from 20% to 69% were fabricated by adding different sizes (10, 20, and 40 μm) and different volume % of polystyrene beads (space holders) to Al₂O₃ and ZTA powders. All the fabricated foams were investigated under static conditions to assess the compressive behavior. It is observed that the compressive strength of these foams strongly depends on porosity, pore size, pore size distribution and pore wall thickness. Among all fabricated foams, Al₂O₃ foams with 20 vol% beads of 10 μm size showed a higher compressive strength of 700 MPa with low porosity (21%) and a higher pore wall thickness (2.8 μm). It is also observed that the pore wall thickness decreased with the increase in beads size and the volume % of the beads, resulting in a low compressive strength value of 8 MPa with a lower pore wall thickness of 1.75 μm at 80 vol% of 40 μm beads. All the foams, irrespective of pore size, showed a typical ceramic failure phenomenon up to 70 vol% of beads; after that, the failure behavior changed to complete open-cell fracture.     

Fabrication of novel BPO4 ceramic foams using the combination of the direct foaming method and freeze-drying techniques

In this study, ammonium phosphate monobasic and boric acid were used as the primary starting materials to produce BPO₄ powder by solid-state reaction. Using BPO₄ powders as the main raw material, BPO₄ ceramic foams were prepared for the first time using the direct foaming method and freeze-drying techniques. The effects of the additive content and solid loading on the slurry's rheological behavior were investigated, and the microstructures and properties of the as-prepared BPO₄ ceramic foams were examined. The results reveal that the porosity of the BPO₄ ceramic foams synthesized at 1223 K ranged from 84.2% to 90.4%, the compressive strength ranged from .12 MPa to .72 MPa, and the thermal conductivity ranged from .32 W/(m·K) to .74 W/(m·K) (298 K). The findings of this study have great significance for the development of new thermal insulation ceramic materials.     

Structure formation and properties of corundum ceramics based on metastable aluminium oxide doped with stabilized zirconium dioxide

The work presents a successful example of the use of YSZ binary systems for production of composite ceramic materials based on θ-Al₂O₃ with improved physical and mechanical characteristics which was previously considered an unpromising material. It was first obtained result of extreme nature of dependence of physical and mechanical properties of Al₂O₃+YSZ on the concentration of YSZ (ZrO₂–3mol% of Y₂O₃) additive. The sintering temperature was decreased on 250 °C (from 1800 to 1550 °C). The phase composition of powders and the structure of ceramics of the Al₂O₃ + YSZ system were investigated depending on the amount of YSZ dopant, the structure-properties relationship was established. It is shown that maximum of physical and mechanical characteristics achieved at YSZ concentration equal to 10% wt.     

LaCoO3 ceramics obtained from reactive powders

The aim of the present study was to establishing the correlation between the structure and properties of the LaCoO₃ powders obtained by aqueous sol–gel method with citric acid and their sintering behavior in order to obtain fully densified ceramics with perovskite structure. Two types of cobalt and lanthanum reagents were used in synthesis, namely nitrates and acetates. The sintering was realized at temperatures ranging between 800 and 1200 °C for 2 h. The sintered samples were investigated by classical ceramic methods (shrinkage, density, porosity) and by structural and morphological investigations: XRD, SEM, AFM and XPS. The electrical properties of the samples were determined by impedance spectroscopy. The ceramics obtained with powders starting with acetates have presented a lower sintering ability as compared with the samples obtained from powders starting with nitrates. LaCoO₃ ceramics with best properties was obtained from powders starting with nitrates sintered at 1100 °C.     

Improving the properties of ceramic foams by a vacuum infiltration process

Reticulated ceramic foams are widely used for industrial applications such as metal filtration, exhaust gas and air purification, catalyst support and others. In this work, the compression strength and specific surface area of reticulated foams have been improved, while at the same time maintaining a high level of permeability in the final foam structure. In particular, a vacuum infiltration step by using a suitable slurry, followed by a pre-sintering cycle was adopted for filling up the hollow struts, generated due to the burnout of the PU foam. Furthermore, various mixtures of fine and coarse-grained alumina as well as in combination with zirconia, were utilised with the aim of controlling the foam properties such as compression strength, specific surface area and permeability. The compression strength was improved by a factor of two for alumina foams by infiltrating the hollow struts, and by a factor of four when infiltrating the struts of ZTA foams, with the composition 70 mol% Al₂O₃ and 30 mol% ZrO₂. The weight gain resulting from the vacuum infiltration process was in the order of 10 wt%.     

Two-Step Sintering of oxide ceramics with various crystal structures

The influence of Two-Step Sintering (TSS) process on the final microstructure of oxide ceramic materials with three different crystal structures was studied. Two kinds of alumina (particle size 100 nm resp. 240 nm) as well as tetragonal zirconia (stabilized with 3 mol% Y₂O₃, particle size 60 nm) and cubic zirconia (8 mol% Y₂O₃, 140 nm) powders were cold isostatically pressed and pressurelessly sintered with different heating schedules. The microstructures achieved with TSS method were compared with microstructures achieved with conventional Single-Step Sintering (SSS) schedule. The results showed that the efficiency of the TSS of these oxide ceramics was more dependent on their crystal structure than on their particle size and green body microstructure. The method of TSS brought only negligible improvement of the microstructure of tetragonal zirconia and hexagonal alumina ceramics. On the other hand, TSS was successful in the sintering of cubic zirconia ceramics; it led to a decrease in grain size by a factor of 2.     

Fabrication of Lu2O3:Eu transparent ceramics using powder consisting of mono-dispersed spheres

Mono-dispersed spherical Lu₂O₃:Eu (5 mol%) powders for transparent ceramics fabrication were synthesized by urea-based homogeneous precipitation technique. The effects of the doped-Eu³⁺ on the synthesis of Lu₂O₃:Eu particles were investigated in detail. The results show that the doping of Eu³⁺ ions into Lu system can significantly decrease the particle size of the resultant precursor spheres. Owing to the sequential precipitation in Lu/Eu system, there are compositional gradients within each of the resultant precursor spheres. Well dispersed, homogeneous and spherical/near spherical Lu₂O₃:Eu powders were obtained after calcination at 600–1000 °C for 4 h. The powder calcined at 600 °C shows better sintering behavior and can be densified into transparent ceramic after vacuum sintering at 1700 °C for 5 h. The luminescence properties of the obtained Lu₂O₃:Eu powder and transparent ceramic were also studied.     

The influence of the additive BaGeO3 on BaSnO3 ceramics

DTA, XRD and sintering investigations of the system BaSnO₃–BaGeO₃, prepared by a mixed-oxide method, are described herein. The melting temperature of this system is about 1270 ± 5 °C. We find a partial solubility of BaGeO₃ into BaSnO₃ of the order of 6–7 mol%. Up to 50 mol% BaGeO₃, the calcined powders (1150 °C) as well as the once-sintered samples consist of BaSnO₃ and orthorhombic BaGeO₃ at room temperature. A gradual appearance of hexagonal BaGeO₃ can be observed in calcined powders and once-sintered ceramics with a BaGeO₃ content above 50 mol%. After sintering at ≥1200 °C for more than 1 h all ceramic bodies consist of BaSnO₃ and orthorhombic BaGeO₃. The addition of BaGeO₃ leads to a considerable reduction of the sintering temperature and to a strong densification. Sintering at 1180 °C for 10 h and an addition of only 1 mol% BaGeO₃ leads to dense ceramic bodies with cubic-like grains.     

Highly Dense,Transparent α‐Al2O3 Ceramics From Ultrafine Nanoparticles Via a Standard SPS Sintering

Using discrete, ultrafine alumina, highly dense transparent (71% real in‐line transmission, RIT, λ = 640 nm) ceramics were achieved with grain size as small as 260 nm using standard SPS sintering. We show that use of La³⁺ as a dopant greatly reduces sensitivity to the sintering temperatures. Transparent alumina were achieved in a large range of sintering temperatures, 1140°C < T < 1200°C, thus providing better reliability and flexibility into the fabrication of large sintered transparent ceramic bodies.     

A novel process to high-strength and controlled-shrinkage Al2O3 foams with open-cell by Al powder hollowing technology

Ceramic foams with open-cell structures have attracted extensive attention due to their unique structure and superior properties. But these materials often exhibit the weakness of high sintered shrinkage and low strength at high porosity levels. In this work, novel ceramic foams with open-cell structures have been obtained using Al powder by combining direct foaming and gelation freezing (DF–GF). The foams are assembled by hollow Al₂O₃ particles resulting from the Kirkendall effect, in which expanded particles overcome the shrinkage of sintering. The influence of sintering temperature on the microstructure and properties of foams are investigated. The Al₂O₃ foams show near-zero-shrinkage at 1773 K after undergoing the process of first expansion and then shrinkage. Compared to other conventional open-cell foam, this foam displays relatively high compressive strength of 0.35–2.19 MPa at high porosity levels of 89.45%–94.45%, attributed to hierarchical pore structure and reaction bonding between Al and O₂. This method from pore structure design provides a novel route for the preparation of controlled shrinkage and high-compressive strength alumina foam with open-cell toward potential application.     

Properties and ballistic tests of strong B4C-TiB2 composites densified by gas pressure sintering

B₄C-TiB₂ composites with classical 75/25 vol ratio were sintered by pressureless sintering with and without gas pressure application in the final stage of densification, using a novel prototypal furnace. A small fraction of WC was introduced through high energy milling of the starting powders with WC-Co spheres. High energy milling facilitated the densification thanks to incorporation of WC impurities acting as sintering aid, and size reduction of the starting powders. Strength, stiffness and toughness of the ceramic densified at 2050 °C via gas pressure sintering were even better than hot pressed composites at 1900 °C. Depth of Penetration tests on plates with 3–5 mm thickness demonstrated that the gas pressure sintered material had a superior performance compared to the hot pressed one. This work also revealed that hardness was not the property spotting the best ballistic performance.     

Synthesis and characterization of holmium doped lithium niobate powders

Holmium doped lithium niobate (LN:Ho) powders with initial concentration of holmium in the range 0–7 mol% were synthesized by the ceramic powder processing method. Niobium penta-oxide Nb₂O₅, lithium carboxide LiCO₃ and holmium oxide Ho₂O₃ with a purity of 99.99% were the starting materials. The phase content and lattice parameters of powders and ceramic pellets were characterized by X-ray diffraction (XRD). To further investigate the quality of the synthesized LN:Ho powders, the scanning electron microscopy (SEM) was used to determine the particle size and the morphology. The main results of this work point out the fact that the ceramic powder processing method is a well adapt method for obtaining high quality LN:Ho ceramics in the holmium concentration range analysed as the LiNbO₃ phase is lonely present in the ceramics at the end of the synthesized process and as their grain sizes are regular, with a maximum for the sample doped with 7 mol% of holmium.     

Fabrication of doped β-tricalcium phosphate bioceramics by Direct Ink Writing for bone repair applications

β-tricalcium phosphate (β-TCP, β-Ca₃(PO₄)₂) is one of the most attractive biomaterials for bone regeneration and β-TCP macroporous scaffolds are very promising for both cell proliferation and mechanical support. The Additive Manufacturing (AM) process called Direct Ink Writing (DIW), based on the extrusion of a concentrated ceramic slurry, is particularly adapted to resolve the main drawbacks associated with conventional shaping of ceramic scaffolds. In this work, co-doped β-TCP powders were synthetized and used to print macroporous scaffolds by DIW. Doped β-TCP powders have been proved to exhibit higher thermal stability, densification and mechanical properties compared to undoped β-TCP. Two co-doped compositions were produced via the aqueous precipitation technique combining magnesium, strontium, silver and copper cations: Mg-Sr (2.0–2.0 mol%) and Mg-Sr-Ag-Cu (2.0–2.0–0.1–0.1 mol%). DIW slurries were optimized with undoped and co-doped β-TCP with the use of a dispersant and a carboxymethylcellulose and polyethyleneimine mixture to obtain aqueous slurries filled with 42 vol% of powder. Complete rheological characterizations were realized to assess the suitability of the β-TCP slurries for the DIW process (shear-thinning and thixotropic behaviour). The whole processing chain including printing, osmotic drying (PEG 10000) and sintering (1100 °C, 3 h) was optimized to successfully print co-doped β-TCP macroporous scaffolds. Characterizations after sintering showed a reduction of macropores and microcracks using co-doped β-TCP powders as well as improved compressive strengths and densities compared to undoped β-TCP. A significant enhancement of compressive strength values was obtained compared to literature data.     

The role of diffusion behavior on the formation and evolution of the core-shell structure in BaTiO3-based ceramics

In this work, the influence of starting particle size and sintering conditions on the microstructures and dielectric properties of BaTiO₃-based ceramics coated with 0.3Bi(Zn_1/2Ti_1/2)O₃-0.7BaTiO₃ were investigated to reveal the core-shell structure by using high resolution transmission electron microscopy technique coupled with energy-dispersive spectrometer analysis. The ion-diffusion behavior plays a critical role in the formation and evolution of the core-shell structure and, therefore, significantly influences the dielectric properties. When using starting powders containing BaTiO₃ particles larger than 100 nm in size and sintering for shorter dwelling times (0.5-2.0 hours), a core-shell structure could be formed and retained owing to the limited diffusion behavior, enabling BaTiO₃-based ceramics to meet the X8R specification for multilayer ceramic capacitors applications at high temperatures. However, when using 80 nm BaTiO₃ nanopowders and further extending the dwelling time to 6.0 hours, more driving energy was provided to prompt ion diffusion, which led to the compositional inhomogeneity becoming homogenized.     

Flash sintering of yttria-stabilized zirconia powders coated with nanoscale films of alumina by atomic layer deposition

The addition of small quantities of aluminum oxide (Al₂O₃) to 8 mol% yttria-stabilized zirconia (8YSZ) benefits conventional sintering by acting as a sintering aid and altering grain growth behavior. However, it is uncertain if these benefits observed during conventional sintering extend to flash sintering. In this work, nanoscale films of Al₂O₃ are deposited on 8YSZ powders by particle atomic layer deposition (ALD). The ALD-coated powders were flash sintered using voltage-to-current control and current rate experiments. The sintering behavior, microstructural evolution, and ionic conductivities were characterized. The addition of Al₂O₃ films changed the conductivity of the starting powder, effectively moving the flash onset temperature. The grain size of the samples flashed with current rate experiments was ~65% smaller than that of conventionally sintered samples. Measurement of grain size and estimates of sample density as a function of temperature during flash sintering showed that small quantities of Al₂O₃ can enhance grain growth and sintering of 8YSZ. This suggests that Al₂O₃ dissolves into the 8YSZ grain boundaries during flash sintering to form complexions that enhance the diffusion of species controlling these processes.     

Characterization of 3Y-TZP/TiO2 hybrid experimental dental ceramics

The addition of titania to zirconia dental implants has been considered a promising choice to improve its bioactivity. This study aimed to evaluate the effect of different sintering conditions on the microstructure, density, optical properties and flexural strength of a 3Y-TZP/TiO₂ dental ceramic based on zirconia with two different titania contents (7.5 mol% and 12.5 mol%). 3Y-TZP/TiO₂ ceramic powders were synthesized by coprecipitation, uniaxially pressed and sintered at six different sintering conditions. Microstructural analysis of the sintered samples was performed by scanning electron microscopy and X-ray diffraction. Optical properties were measured using a spectrophotometer. The density was determined by Archimedes principle. Flexural strength was estimated by the biaxial flexure device. The microstructure and flexural strength of the 3Y-TZP/TiO₂ dental ceramic with 7.5% and 12.5 mol% were affected by the sintering conditions. Sintering the specimens at 1460 °C for 2 h increased the grain size and significantly decreased the flexural strength of 3Y-TZP/TiO₂ dental ceramic. The interaction (titania content x sintering conditions) affected the relative density and optical properties. A relative density greater than 98% was achieved for the T7.5 groups (sintered at 1260 °C/1 h, 1300 °C/1 h and 1300 °C/2 h) and for the T12.5 groups (sintered at 1260 °C/1 h, 1260 °C/4 h, 1300 °C/1 h and 1300 °C/2 h). The highest values of L*, a* and b* were respectively 87.2 (T7.5 group sintered at 1460 °C/2hs), 4.3 (T12.5 group sintered at 1300 °C/2hs) and 15.8 (T12.5 group sintered at 1300 °C/1 h). The material developed with 12.5 mol% of titania and sintered at 1300 °C/2 h showed high densification, flexural strength of 670 MPa and has good potential to be used in dentistry.     

Microstructure and mechanical properties of Al2O3–20 wt%Al2TiO5 composite prepared from alumina and titania nanopowders

In the present work, Al₂O₃–20 wt%Al₂TiO₅ composite was prepared from reaction sintering of alumina and titania nanopowders. The nano-sized raw powders were reconstituted into nanostructured particles by ball milling. Then, the nanostructured reconstituted powders were pressed and pressureless-sintered into bulk ceramics at 1300, 1400, 1500 °C for 2 h. The phase composition and microstructures of reconstituted powders and as-prepared ceramic composites were characterized by using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope and energy-dispersive spectrometer (EDS). The microstructural analysis of the ceramic showed that the average grain size of the alumina–aluminium titanate composite increases with increasing the temperature. Also, SEM proved the existence of a proper interface between Al₂TiO₅ and Al₂O₃ grains and preferential distribution of aluminium titanate particles in the grain boundaries. XRD analysis indicated the absence of rutile titania in the sintered composite ensuring complete formation of aluminium titanate. The hardness of the samples sintered at 1300, 1400, 1500 °C were 4.8, 6.2 and 8.5 GPa, respectively.