Fabrication of CaSiO3 bioceramics with open and unidirectional macro-channels using an ice/fiber-templated method

Porous CaSiO₃ bioceramics with open and unidirectional macro-channels of pore size more than 200 μm are of particular interest for biomedical applications. An ice/fiber-templated method was employed for the fabrication of CaSiO₃ bioceramics with interconnected lamellar pores and macro-channels of pore size more than 200 μm. The pores formed by ice crystals transformed from cellular to lamellar, while the pores formed by fibers were aligned macro-channels, which were also in alignment with the lamellar pores. Keeping the initial slurry concentration constant and increasing the packing density of fibers, the volume fraction of macro-channels and open porosity increased, and the compressive strength decreased. Maintaining the packing density of fibers and increasing the initial slurry concentration, the pore sizes of lamellar pores and open porosity decreased, and the compressive strength increased. The results indicated that it was possible to manufacture porous CaSiO₃ bioceramics with the macro-channels of 250–350 μm, lamellae spacing of 50–100 μm, open porosity of 71.12–83.94% and compressive strength of 0.87–3.59 MPa, indicating the suitability for tissue engineering.     

Suspension stability and sintering influence on yttria-stabilized zirconia fabricated by colloidal processing

The stability of nano-zirconia 3YSZ powder in suspension was extensively studied by the colloidal method, and the optimum sintering temperature of the green sample fabricated through slip casting was determined. Zirconia suspensions with 10 vol% powder loading were prepared with distilled water, and HNO₃ was used to adjust the pH of the suspension to pH 1–6. All of the suspensions were subjected to sedimentation test, and the results showed that the suspensions adjusted to pH 2 had the lowest sediment volume. This finding indicates that a suspension with pH 2 produces higher packing density. Viscosity test was carried out for the suspensions added with dispersant ranging from 0.3 wt% to 0.7 wt% polyethyleneimine (PEI) with and without pH adjustment. The suspension containing 0.5 wt% PEI with pH 2 adjustment produced the lowest viscosity because of interparticle bond breakage in the aggregates, thus forming colloidally stable suspensions. The zirconia suspension containing 0.5 wt% PEI and whose pH was adjusted to pH 2 was chosen to be slip casted into cylindrical shape. Green samples were sintered at various sintering temperatures that ranged from 1100 °C to 1500 °C through a two-step sintering method. The sample sintered at 1500 °C was found to be porosite-free, and its highest relative density was 99.6% of the theoretical density. Morphological studies detected pores in the microstructure of the samples sintered at low sintering temperatures (1100 and 1200 °C). By contrast, the samples sintered at 1400 and 1500 °C were fully densified. However, the grain size of the sample sintered at 1500 °C was 230 nm, which indicated excessive grain growth. The Vickers hardness of the sample sintered at 1400 °C was found to be highest (12.9 GPa) and comparable to results found in the literature.     

Formation of rod-like Si3N4 grains in porous SRBSN bodies using 6Y2O3–2MgO sintering additives

The porous reaction-bonded silicon nitride (RBSN) bodies using (6 wt.% Y₂O₃–2 wt.% MgO) 6Y2M were fabricated by nitridation process at 1350 °C for 8 h. The porous gas pressure sintered (GPSed)-RBSN bodies post-sintered at 1550–1850 °C for 6 h show a microstructure with low aspect ratios having high porosity. The compressive strength of samples sintered at 1650 °C, 1750 °C and 1850 °C were about 146 MPa, 251 MPa and 285 MPa, respectively. The duration time for sintering had a significant effect on the microstructure and grain morphology of the GPSed-RBSN bodies. Even though the GPSed-RBSN was carried out at the comparatively low temperature (1550 °C) for 9 h, high aspect ratio of rod-like Si₃N₄ grains with about 9 was observed. The material properties of samples such as porosity, phase ratio (β/(α + β)) and compressive strength of sample sintered at 9 h were about 43.2%, 99% and 141 MPa, respectively.     

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.     

Effect of carboxymethyl cellulose addition on the properties of Si3N4 ceramic foams

The effect of carboxymethyl cellulose (CMC) addition on the preparation of Si₃N₄ ceramic foam by the direct foaming method was investigated. The addition of CMC in the foam slurry can reduce the surface tension, increase the viscoelasticity of foams, and improve their stability and fluidity. The foam ceramics show low shrinkage during drying owing to the CMC and the gelation of acrylamide monomers. The surface structure of dried foam is uniform, and there are no macropores and cracks on the surface. The sintered Si₃N₄ foam ceramics have very uniform pore distribution with average pore size of about 16 μm; the flexure strength is as high as 3.8–77.2 MPa, and the porosity is about 60.6–82.1%.     

Microstructure and dielectric properties of Dy/Mn doped BaTiO3 ceramics

Dy/Mn doped BaTiO₃ with different Dy₂O₃ contents, ranging from 0.1 to 5.0 at% Dy, were investigated regarding their microstructural and dielectric characteristics. The content of 0.05 at% Mn was constant in all the investigated samples. The samples were prepared by the conventional solid state reaction and sintered at 1290°, and 1350 °C in air atmosphere for 2 h. The low doped samples (0.1 and 0.5 at% Dy) exhibit mainly fairly uniform and homogeneous microstructure with average grain sizes ranged from 0.3 μm to 3.0 μm. At 1350 °C, the appearance of secondary, abnormal, grains in the fine grain matrix and core–shell structure were observed in highly doped Dy/BaTiO₃. Dielectric measurements were carried out as a function of temperature up to 180 °C. The low doped samples sintered at 1350 °C, display the high value of dielectric permittivity at room temperature, 5600 for 0.1Dy/BaTiO₃. A nearly flat permittivity–temperature response was obtained in specimens with 2.0 and 5.0 at% additive content. Using a Curie–Weiss and modified Curie–Weiss low, the Curie constant (C), Curie like constant (C′), Curie temperature (T_C) and a critical exponent (γ) were calculated. The obtained values of γ pointed out the diffuse phase transformation in highly doped BaTiO₃ samples.     

Synthesis and characterization of diopside glass–ceramic matrix composite reinforced with aluminum titanate

Glass–ceramic composites in the SiO₂–CaO–MgO–(Na₂O) system, reinforced with 5, 10 and 20 wt.% aluminum titanate were synthesized by pressureless sintering. Optimum sintering temperatures with maximum relative density were determined for each composition. The composites were fired above the crystallization peak temperature of glass–ceramic. Mechanical properties of glass–ceramic and sintered composites, such as fracture toughness, flexural strength and Vickers microhardness, were investigated. The sintered composites were characterized by scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS) and X-ray diffraction (XRD). The results showed that the composite containing 10 wt.% aluminum titanate has desirable behavior in comparison to the base glass–ceramic and the other compositions. It seems that crack deflection by aluminum titanate particles is the prevalent mechanism for improving mechanical characteristics.     

TBA-based freeze/gel casting of porous hydroxyapatite scaffolds

Porous ceramic scaffolds with a controlled “designer” pore structure have been prepared by the freeze/gel casting route using a TBA-based hydroxyapatite slurry system with 20–40 wt.% solid content. The products were characterized in terms of sintered microstructure, together with physical and mechanical properties. After sintering at 1050–1250 °C, the advantages of freeze casting and gel casting appeared in the pore structure and compressive strength of the ceramics, i.e., unidirectional aligned macro-pore channels developed by controlling the solidification direction of the TBA solvent used in the freeze casting together with small diameter (micron sized) isolated pores formed in the dense outer walls of the pore channels when processed by gel casting. The sintered porosity and pore size generally resulted in a high solid loading giving low porosity and small pore size, this leading to higher compressive strengths. The scaffolds obtained exhibited an average porosity and compressive strength in the range 41.9–79.3% and 35.1–2.7 MPa, respectively, depending on the processing conditions used.     

Potassium based bioactive glass for bone tissue engineering

A fundamental issue for the restoration of bone defects according to a tissue engineering approach is the development of highly porous bioactive scaffolds. The polymer burning out method is widely employed to fabricate bioceramic scaffolds because of its versatility, simplicity and low cost. However, the resulting scaffolds may suffer low porosity and non-interconnected pores. In the present contribution a new fabrication method is presented. Thanks to a recently developed potassium-based bioactive glass, which has the peculiarity to be sintered at a relatively low temperature (i.e. ∼750 °C), it was possible to use sodium chloride particles as pore generating agents, which helped to maintain the shape of the struts during the entire sintering process. The salt particles can be easily removed by immersing the scaffold in water, giving place to a structure that combines high porosity (in the 70–80 vol.% range) with interconnected pores and an appreciable mechanical behaviour (Young's modulus in the 3.4–3.7 MPa range according to compression tests).     

Sintering of clay-chamotte ceramic composites for refractory bricks

Clay-chamotte composites were realized for manufacturing refractory bricks. We used two kaolinitic refractory clays mined in Cameroon and two calcined clays (chamottes) with a large grain size (0.1–4 mm). Clay-chamotte composites containing various quantities of chamotte (0–50 wt%) were shaped and sintered at 1200–1350 °C. The structural characteristics of composites indicated the presence of quartz from the initial clay, cristobalite and mullite. SEM observations revealed very heterogeneous microstructures where porosity is weakly distributed and large pores are entrapped at the vicinity of large chamotte and quartz grains. In general, the global porosity increases with the chamotte content. A specific interpretation of the matrix role on the global sintering behaviour reveals that only a part of the matrix acts effectively. Since the most part of the global porosity is within the matrix, it is distributed in matrix zones, which participate effectively to sintering and in inert matrix zones where larger pores occur. The global mechanical strength is controlled by the matrix behaviour, but the high porosity of this phase is unfavourable to high strength values. Besides, the occurrence of larges pores and local cracks at large grain interfaces from thermal stresses are critical flaws, which reduce the mechanical strength.     

Sintering,microstructure and conductivity of La2NiO4+δ ceramic

Microstructure and electrical conducting properties of La₂NiO_4+δ ceramic were investigated in the sintering temperature range 1200–1400 °C. The results demonstrate that the microstructure and electrical conducting properties of La₂NiO_4+δ ceramic are sensitive to sintering temperature. Compared with a progressive densification development with sintering temperature from 1200 to 1300 °C along with an insignificant change in grain size, there is an exaggerated grain growth in the specimens sintered at higher temperatures. Increasing sintering temperature from 1200 to 1300 °C resulted in an enhancement of electrical conducting properties. Further increase of sintering temperature exceeding 1300 °C reduced the electrical conducting properties. A close relation between the microstructure and electrical conducting properties was suggested for La₂NiO_4+δ ceramic. With respect to the electrical conducting properties, the preferred sintering temperature of La₂NiO_4+δ ceramic was ascertained to be 1300 °C. The specimen sintered at 1300 °C exhibits a generally uniform microstructure together with electrical conductivities of 76–95 S cm⁻¹ at 600–800 °C.     

Pressureless sintered Al2O3–SiC nanocomposites

Al₂O₃ + 5 vol% SiC composite ceramics were prepared via a conventional powder processing route followed by pressureless sintering. Commercially available Al₂O₃ and SiC powders were milled together in an aqueous suspension. The slurry was freeze granulated, and green bodies were obtained by cold isostatic pressing of the granules. Pressureless sintering was carried out in a nitrogen atmosphere at 1750 and 1780 °C. Near full density (>99%) was achieved at 1780 °C. Densification at the lower sintering temperature was promoted by smaller additions of MgO. Vickers hardness and indentation fracture toughness varied around 18 GPa and 2.3 MPa m^1/2 after sintering at 1780 °C. Transmission electron microscopy revealed that the SiC particles were located predominantly to the interior of the matrix grains and well distributed throughout the composite microstructures. The intragranular particles had sizes in the range 50–200 nm while the intergranular particles were larger, typically 200–500 nm in diameter.     

Erosion behavior of SiC–WC composites

Dense silicon carbide (SiC) ceramics were prepared with 0, 10, 30 or 50 wt% WC particles by hot pressing powder mixtures of SiC, WC and oxide additives at 1800 °C for 1 h under a pressure of 40 MPa in an Ar atmosphere. Effects of alumina or SiC erodent particles and the WC content on the erosion performance of sintered SiC–WC composites were assessed. Microstructures of the sintered composites consisted of WC particles distributed in the equi-axed grain structure of SiC. Fracture surfaces showed a mixed mode of fracture, with a large extent of transgranular fracture observed in SiC ceramics prepared with 30 wt% WC. Crack bridging by WC enhanced toughening of the SiC ceramics. A maximum fracture toughness of 6.7 MPa*m^1/2 was observed for the SiC ceramics with 50 wt% WC, whereas a high hardness of 26 GPa was obtained for the SiC ceramics with 30 wt% WC. When eroded at normal incidence, two orders of magnitude less erosion occurred when SiC–WC composites were eroded by alumina particles than that eroded by SiC particles. The erosion rate of the composites increased with increasing angle of SiC particle impingement from 30° to 90°, and decreased with WC reinforcement up to 30 wt%. A minimum erosion wear rate of 6.6 mm³/kg was obtained for SiC–30 wt% WC composites. Effects of mechanical properties and microstructure on erosion of the sintered SiC–WC composites are discussed, and the dominant wear mechanisms are also elucidated.     

Gelcasting of alumina nanopowders based on gelation of sodium alginate

The alumina nanopowder was synthesized via the sol–gel method. θ-Alumina with crystallite size in the range of 25–110 nm was crystallized by calcination of the powder at 900 °C for 1 h. Sodium alginate, a natural innoxious polymer, was applied for in situ forming process of an Al₂O₃ green body, using calcium phosphate as a solidifier agent. Sodium hexa metaphosphate was also utilized as a chelator. Rheological and gelation behaviors of resultant slurry were analyzed. The viscosity of slurries with 15 vol.% alumina and 1.8 vol.% calcium phosphate dispersed by 1 wt.% sodium alginate solution, was less than 800 mPa s. The green bodies from the gelcasting process were dried at room temperature for 36 h and pressureless sintered at 1500 °C for 3 h. A uniform microstructure without huge grain growth was revealed by SEM.     

Preparation of boron carbide–aluminum composites by non-aqueous gelcasting

Gelcasting is an attractive forming process to fabricate ceramic parts with near-net-shape. In the present work, non-aqueous gelcasting of boron carbide (B₄C)–aluminum (Al) composites was studied. A stable B₄C–Al slurry with solids loading up to 55 vol.% for gelcasting was prepared. The slurry was solidified in situ to green body with the mean value of relative density of 64% and flexural strength of 21 MPa. The SEM images showed that powders in green body compact closely by the connection of polymer networks. B₄C–Al samples were also obtained by the process of gelcasting and sintering at 1300 °C for 1 h in 0.1 MPa Ar atmosphere. The average bulk density of sintered body was 2.05 g cm⁻³.     

Synthesis and characterization of MgAl2O4–ZrO2 composites

Different types of dense 5–97% ZrO₂–MgAl₂O₄ composites have been prepared using a MgAl₂O₄ spinel obtained by calcining a stoichiometric mixture of aluminium tri-hydroxide and caustic MgO at 1300 °C for 1 h, and a commercial yttria partially stabilized zirconia (YPSZ) powder as starting raw materials by sintering at various temperatures ranging from 1500 to 1650 °C for 2 h. The characteristics of the MgAl₂O₄ spinel, the YPSZ powder and the various sintered products were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface area, particle size analysis, Archimedes principle, and Vickers indentation method. Characterization results revealed that the YPSZ addition increases the sintering ability, fracture toughness and hardness of MgAl₂O₄ spinel, whereas, the MgAl₂O₄ spinel hampered the sintering ability of YPSZ when sintered at elevated temperatures. A 20-wt.% YPSZ was found to be sufficient to increase the hardness and fracture toughness of MgAl₂O₄ spinel from 406 to 1314 Hv and 2.5 to 3.45 MPa m^1/2, respectively, when sintered at 1600 °C for 2 h.     

Low temperature sintering and microwave dielectric properties of Li2ZnTi3O8 ceramics doped with ZnO–La2O3–B2O3 glass

Li₂ZnTi₃O₈ ceramics doped with ZnO–La₂O₃–B₂O₃ glass were prepared by the conventional solid-state ceramic route. The effects of the ZnO–La₂O₃–B₂O₃ glass on the sintering temperature, phase composition, microstructure and microwave dielectric properties of Li₂ZnTi₃O₈ ceramics were investigated. The addition of ZLB glass can reduce the sintering temperature of Li₂ZnTi₃O₈ ceramic from 1075 °C to 925 °C without obvious degradation of the microwave dielectric properties. Only a single phase Li₂ZnTi₃O₈ with cubic spinel structure is formed in Li₂ZnTi₃O₈ ceramic with ZLB addition sintered at 925 °C. Typically, 1.0 wt% ZLB-doped Li₂ZnTi₃O₈ ceramic sintered at 925 °C can reach a maximum relative density of 95.8% and exhibits good microwave dielectric properties of ε_r=24.34, Q×f=41,360 GHz and τ_f=−13.4 ppm/°C. Moreover, this material is compatible with Ag electrode, which makes it a promising candidate for LTCC application.     

Workability and setting parameters evaluation of colloidal silica bonded refractory suspensions

The efficiency of colloidal silica as a binder agent for castable matrix suspension in the presence of different setting agents and curing temperatures was evaluated. The tests were carried out trough rheometric techniques according to a systemic approach specifically developed for ceramic systems (oscillatory and normal force tests). Colloidal silica performed well as a binder agent for refractory suspensions when a suitable additive was selected. Among the additives analyzed, magnesium oxide was the most suitable for the evaluated systems. MgO addition in the range of 0.3–0.6 wt% and curing temperature of 25 °C were the suggested parameters for alumina and microsilica systems.     

Microstructure evaluation of sintered combustion-derived fine powder NiO–YSZ

Citrate–nitrate combustion synthesis was used for the preparation of NiO–YSZ. The main advantage of the preparation method used was reflected in the fact that after the synthesis both phases NiO and YSZ were randomly distributed on a nanometre level. The prepared NiO–YSZ powder composites were shaped, sintered and reduced to Ni–YSZ and subsequently submitted to microstructure investigations. Relative sintered densities higher than 90% were obtained at sintering temperatures as low as 1200 °C. A sintering temperature 1200 °C was also recognized as the preparation temperature that provided the smallest Ni grains in the final Ni–YSZ cermet with an average Ni-particle diameter as low as 0.27 μm.     

Effect of zirconia addition on pressureless sintering of boron carbide

This paper presents the results of experiments on pressureless sintering of boron carbide with varying addition of zirconia (ZrO₂: 0–30 wt.%). Green pellets were densified by sintering at 2275 °C in vacuum for 60 min and characterized by measurement of density, hardness, thermal conductivity and microstructure. Samples prepared with the addition of ≥5 wt.% ZrO₂ showed higher densities in the range of 93–96% ρth, compared to 86.63% ρth for boron carbide only. Addition of ZrO₂ was found to increase the hardness of sintered samples and regardless of ZrO₂ content, the hardness values ranged between 30 and 31.5 GPa. XRD of the sintered pellets showed the presence of ZrB₂. Optical microscope as well as electron probe microanalysis (EPMA) showed the presence of two phases, grey matrix with white precipitates. EPMA analysis of second phase revealed the presence of Zirconium in this phase. Fractography of boron carbide with 25% ZrO₂ showed the failure to be by mixed fracture (transgranular and intergranular). Thermal conductivity values of the samples measured in the temperature range of 400–1000 °C were marginally higher with the addition of ZrO₂.