Effect of recoating slurry compositions on the microstructure and properties of SiC reticulated porous ceramics

Silicon carbide reticulated porous ceramics (SiC RPCs) were fabricated by polymer sponge replica technique, followed by recoating with SiC slurries of two different sintering additives of MgO–Al₂O₃–SiO₂ (Slurry 1) and polycarbosilane (Slurry 2). The sintering temperature of SiC RPCs recoated with Slurry 2 was 1100 °C, which was 200 °C lower than that for one recoated with Slurry 1. The prepared SiC RPCs exhibited homogeneous microstructure and contained pores with different sizes which was entrapped in the strut of SiC RPCs, small pores with diameter lower than 4 μm and large pores with diameter higher than 10 μm. Bending strength of SiC RPCs recoated with Slurry 1 was two times higher than that for the non-recoated samples, which was 1.88 MPa and was a little higher than that for one recoated with slurry 2. At the same time, high thermal shock resistance and high refractoriness were achieved for SiC RPCs recoated with Slurry 2.     

Fabrication of sub-micrometer MgO transparent ceramics by spark plasma sintering

Transparent MgO ceramics were fabricated by spark plasma sintering (SPS) of the commercial MgO powder using LiF as the sintering additive. Effects of the additive amount and the SPS conditions (i.e., sintering temperature and heating rate) on the optical transparency and microstructure of the obtained MgO ceramics were investigated. The results showed that LiF facilitated rapid densification and grain growth. Thus, the MgO ceramics could be easily densified at a moderate temperature and under a low pressure. In addition, the transparency and microstructure of the MgO ceramics were found to be strongly dependent on the temperature and heating rate. For the MgO ceramics sintered at 900 °C for 5 min with the heating rate of 100 °C/min and the pressure of 30 MPa from the powders with 1 wt% LiF, the average in-line transmittance reached 85% in the range of 3 – 5 μm, and the average grain size is ∼0.7 μm.     

Enhancement of the optical transmittance of hot-pressed transparent yttria ceramics by a multi-step sintering process

In this work, we report the enhancement of the optical transmittance of yttria ceramics via a ‘pre-sintering/hot-pressing/hot-isostatic pressing’ multi-step process. The in-line transmittance reaches 78.0% at 400 nm and 83.2% at 1100 nm with an average grain size of approximately 1 µm. This sintering route is derived and upgraded from our previously reported one-step hot-pressing method for the fabrication of transparent yttria ceramics. Through the enhanced strength of the green body by pre-sintering, the formation of cracks and the breaking up of the as-hot-pressed samples can be effectively suppressed, leading to sound or crack-free but less transparent samples. After a post-HIPing treatment, the optical transmittance of the samples with a pre-sintering step was substantially improved to higher levels in the visible and the IR wavelengths compared to that of a sample which does not undergo pre-sintering. The corresponding mechanism was revealed by microstructural analyses.     

Application of hydrophilic ionic liquid treatment to the morphological observations of hydrated porous ceramic green bodies

In this study, a simple and convenient method for observing the surface morphology of hydrated porous ceramic green bodies is proposed. The porous hyrdoxyapatite (HAp) green body was prepared by a gelcasting process and was dried in a humid chamber from 90 to 50% relative humidity at 25 °C before subsequent treatment with a hydrophilic ionic liquid (IL). The surface morphology of the IL-treated porous HAp green body was observed using FE-SEM. The results showed that the pore morphology and microstructure of the HAp green body was readily observable without evidence of charging. The as-prepared sample showed pores approximately 300–600 μm in diameter, which gradually contracted to approximately 200–400 μm upon drying in the humid chamber. Following sintering at 1000 °C, the pores had further contracted to approximately 100–300 μm. The IL binds with the surrounding water to prevent the sample from drying in vacuum and acts as a conductive media, allowing the HAp ceramics to be observed in the electron microscope. In comparison to the micro-focused X-ray CT analysis, the fine pore structure (less than 100 μm) could only be observed using FE-SEM when the porous body had also been subjected to the IL treatment.     

Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3 ceramics

Commercial Y₂O₃ powder was used to fabricate Y₂O₃ ceramics sintered at 1600 °C and 1800 °C with concurrent addition of ZrO₂ and La₂O₃ as sintering aids. One group with different contents of La₂O₃ (0–10 mol%) with a fixed amount of 1 mol% ZrO₂ and another group with various contents of ZrO₂ (0–7 mol%) with a fixed amount of 10 mol% La₂O₃ were compared to investigate the effects of co-doping on the microstructural and optical properties of Y₂O₃ ceramics. At low sintering temperature of 1600 °C, the sample single doped with 10 mol% La₂O₃ exhibits much denser microstructure with a few small intragranular pores while the samples with ZrO₂ and La₂O₃ co-doping features a lot of large intergranular pores leading to lower density. When the sintering temperature increases to 1800 °C, samples using composite sintering aids exhibit finer microstructures and better optical properties than those of both ZrO₂ and La₂O₃ single-doped samples. It was proved that the grain growth suppression caused by ZrO₂ overwhelms the acceleration by La₂O₃. Meanwhile, 1 mol% ZrO₂ acts as a very important inflection point with regard to the influence of additive concentration on the transmittance, pore structure and grain size. The highest in-line transmittance of Y₂O₃ ceramic (1.2 mm in thickness) with 3 mol% of ZrO₂ and 10 mol% of La₂O₃ sintered at 1800 °C for 16 h is 81.9% at a wavelength of 1100 nm, with an average grain size of 11.2 µm.     

Highly transparent lutetium titanium oxide produced by spark plasma sintering

Transparent Lu₂Ti₂O₇ pyrochlore was fabricated by reactive sintering using spark plasma sintering at 1723 K for 45 min. The sintered body exhibited 72% transmittance at a wavelength of 2000 nm and 40% transmittance at 550 nm. The average grain size was 14.5 μm with uniform microstructure.     

Microstructure and densification of ZrB2–SiC composites prepared by spark plasma sintering

ZrB₂–SiC composites were prepared by spark plasma sintering (SPS) at temperatures of 1800–2100 °C for 180–300 s under a pressure of 20 MPa and at higher temperatures of above 2100 °C without a holding time under 10 MPa. Densification, microstructure and mechanical properties of ZrB₂–SiC composites were investigated. Fully dense ZrB₂–SiC composites containing 20–60 mass% SiC with a relative density of more than 99% were obtained at 2000 and 2100 °C for 180 s. Below 2120 °C, microstructures consisted of equiaxed ZrB₂ grains with a size of 2–5 μm and α-SiC grains with a size of 2–4 μm. Morphological change from equiaxed to elongated α-SiC grains was observed at higher temperatures. Vickers hardness of ZrB₂–SiC composites increased with increasing sintering temperature and SiC content up to 60 mass%, and ZrB₂–SiC composite containing 60 mass% SiC sintered at 2100 °C for 180 s had the highest value of 26.8 GPa. The highest fracture toughness was observed for ZrB₂–SiC composites containing 50 mass% SiC independent of sintering temperatures.     

Preparation of yttria nanopowders for use in transparent ceramics by dry ball-milling technique

To evaluate the effectiveness of dry ball-milling, yttria powders were ball-milled, and the optimal sample was spark plasma sintered to fabricated transparent yttria ceramics. The characterization of the powders and pellets were analyzed using XRD, FESEM, UV/VIS/NIR and FTIR spectrometer throughout the whole experimental process. The results indicated that ball-milling can significantly decrease yttria particle size and enhance the specifice surface area. The best duration is 20 h ball-milling with average particle size and specifice surface area of 64 nm and 20.4 m²/g, respectively. Transparent yttria ceramic was made up of grains in the size range of approximately 0.67 μm and the relative density was 99.8%. The in-line transmittance of the SPS-sintered yttria ceramic (2.5 mm thick) reached 83% in the infrared region, which is very close to the theoretical value of yttria.     

High optical quality Y2O3 transparent ceramics with fine grain size fabricated by low temperature air pre-sintering and post-HIP treatment

High optical quality Y₂O₃ transparent ceramics with fine grain size were successfully fabricated by air pre-sintering at various temperature ranging from 1500 to 1600 °C combined with a post-hot-isostatic pressing (HIP) treatment using co-precipitated powders as the starting material. The fully dense Y₂O₃ transparent ceramic with highest transparency was obtained by pre-sintered at 1550 °C for 4 h in air and post-HIPed at 1600 °C for 3 h (the pressure of HIP 200 MPa), and it had fine microstructure and the average grain size was 0.96 μm. In addition, the in-line transmittance of the ceramic reached 81.7% at 1064 nm (1 mm thickness). By this approach, the transparent Y₂O₃ ceramics with fine grain size (<1.6 μm) were elaborated without any sintering aid.     

Transparent Lu3NbO7 bodies prepared by reactive spark plasma sintering and their optical and mechanical properties

Transparent lutetium niobate (Lu₃NbO₇) bodies were prepared by reactive spark plasma sintering using Lu₂O₃ and Nb₂O₅ powders. Fully dense Lu₃NbO₇ bodies with density greater than 99.5% of the theoretical were obtained at 1300–1650 °C. The grains steadily grew from 0.1 to 0.6 μm with increasing sintering temperature from 1200 to 1450 °C and significant grain growth from 2.2 to 9.2 μm occurred at 1550–1650 °C. The Lu₃NbO₇ body sintered at 1450 °C showed the highest transmittance of 63% at 550 nm after heat treatment at 850 °C in air for 6 h. Fully dense, submicron-size transparent Lu₃NbO₇ exhibited Vickers hardness of ～13.0 GPa and indentation fracture toughness of 1.0 MPa m^1/2.     

Osseous differentiation on freeze casted 10CeTZP-Al2O3 structures

Three-dimensional structures with directionally oriented pore networks were fabricated from a 10 mol% ceria-stabilized zirconia and alumina composite (10CeTZP-Al₂O₃) via freeze casting. Ceramic suspensions of different concentrations (30, 40 and 50 wt% solids) were frozen at various rates (2, 5 and 10 °C/min) to obtain lamellar structures with aligned tubular pores of different characteristics: porosity (75–84%), pore dimensions (small diameter of the elliptical pores: 10–23 μm; large diameter of the elliptical pores: ∼200 ± 70 μm), lamella thickness (2.7–4 μm) and compression strength (1–12 MPa). In vitro assays confirmed the non-cytotoxic nature of the samples. Furthermore, specific osseous differentiation genes were quantified after incubating osteoblasts on different cross sections of the samples during 7 days in supplemented culture medium; results demonstrated that the freeze casted structures induce up to nine times more osseous gene expression than tissue culture polystyrene (TCPS), an advanced surface used for optimized in vitro cell growth.     

Fabrication and phase transition of La2−xLuxZr2O7 transparent ceramics

A series of transparent ceramics with the composition of La_2−xLu_xZr₂O₇ (x = 0−2.0) were prepared by solid-state reactive sintering in vacuum. With the increase of Lu content (x), phase transition from pyrochlore to defective fluorite occurred and a two-phase region existed in the range of x = 0.6−1.2. Grain sizes of the pyrochlore phase dominated samples (x < 0.5) were 11−14 μm, and that of the defective fluorite phase dominated samples were larger than 60 μm. However, grain sizes of the samples in the two-phase region were smaller than 3 μm. The La_0.8Lu_1.2Zr₂O₇ ceramic with the smallest grain size (∼2.5 μm) reached a highest in-line transmittance of 72.4% at 1100 nm among all the samples.     

Transparent cubic-ZrO2 ceramics for application as optical lenses

Optical transparent polycrystalline ZrO₂ ceramics were fabricated by solid-state sintering process using first vacuum sintering followed by hot isostatic pressing. In the visible wavelength range (400–800 nm), the in-line transmittance of 5.6-mm thick samples reaches 68% at exemplary wavelength 600 nm (corresponding to an in-line absorbance based on 10 of A₁₀ = 0.08 cm^?1), which is approximately 90% of theoretical limit. The refractive indices of the ZrO₂ optoceramics at 630 nm (n_d) are varying between 2.10 and 2.20, depending on TiO₂ contents, the latter being used as sintering aid. The appearance of birefringence is strongly correlated to the addition of TiO₂ as sintering additive in the ceramic samples, whereas addition of TiO₂ and simultaneous increase in Y₂O₃ content resulted in a decrease of birefringence.     

Fabrication of transparent SiO2 glass by pressureless sintering and spark plasma sintering

Transparent SiO₂ bodies were prepared by pressureless sintering (PLS) and spark plasma sintering (SPS). The effects of sintering and annealing temperature on the transmittance of the SiO₂ bodies were investigated. The SiO₂ bodies sintered by SPS and PLS at 1073–1573 K were amorphous. With increasing the sintering temperature to 1673 K, the SiO₂ bodies sintered by PLS were crystallized while those sintered by SPS were still amorphous. The relative density of the SiO₂ bodies sintered by SPS was 98.5% at 1373 K and 100% at 1573 K, whereas that sintered by PLS was 92.6% at 1373 K and 98.9% at 1573 K. The transmittance was 91.0% and 81.5% at a wavelength (λ) of 2 μm for the SiO₂ sintered bodies by SPS and PLS, respectively. In the ultraviolet range, the transmittance of the SiO₂ bodies sintered by SPS at 1573 K was about 40% at λ = 200 nm and increased to 75% after annealing at 1073 K for 1 h, which was about three times of the transmittance of the SiO₂ bodies sintered by PLS (24.8%).     

Influence of the structure of MgO·nAl2O3 spinel lattices on transparent ceramics processing and properties

It is demonstrated that a complete elimination of pores on sintering is governed not only by the size of the ceramic powder particles and by the homogeneity of their mutual coordination but similarly strongly by the state of the crystal lattice: with different cation disorder at fixed stoichiometry (n = 1) the sintering temperatures may differ by as much as 200 °C at constant powder particle size and equal homogeneity of the green bodies. Additionally, the impact of stoichiometry was investigated over the range between n = 1 and n = 3 with retarded reactive sintering at moderately increased Al₂O₃ concentrations but promoted densification of alumina-rich compositions. Taking advantage of the observed effects, sintered spinel ceramics were derived by reactive sintering of undoped MgO/Al₂O₃ mixtures resulting in an in-line transmittance which equals spinel single crystals of similar composition from 200 nm wave length up to the IR range.     

SEM investigation of domain structure in (Ba,Ca,Pb)TiO3

In the present paper the microstructure and domain structure in modified BaTiO₃ with Pb and Ca as additives have been investigated using SEM technique. The (Ba,Pb)TiO₃ and (Ba,Ca,Pb)TiO₃ ceramics show a slight difference in grain size, being smaller in composites with Ca additives which acts as grain growth inhibitor. The domain configuration is almost the same. The small grain microstructure with tiny domains have been observed in specimen sintered at 1300°C and the average grain size is in the range 1–3 μm. For those specimens sintered at 1320°C the homogenous microstructure is also obtained with grain size around 2–4 μm. For both types of specimens, the single domain structure is associated with grain which size is lower than 2 μm. The banded domain structure could be observed in grains with size bigger than 3 μm. The bar shape grains and elongated grains together with some large region in microstructure are free of domain structure. The observed domain patterns reveal mainly the straight domain boundary lines with 90° domains walls. The wall thickness ranged from 0·03 μm to 0·15 μm, while the domain width is in the range of 0·1 μm–1 μm.     

Processing of porous yttria-stabilized zirconia tapes: Influence of starch content and sintering temperature

The tape-casting process was used to produce porous yttria-stabilized zirconia (YSZ) substrates with volume fractions of porosity ranging from 28.9 to 53 vol.% by using starch as a fugitive additive. Concentrated aqueous YSZ slips with different amounts of starch and an acrylic latex binder were prepared. The influence of the volume fraction of starch and sintering temperature on the sintering behavior and final microstructure were investigated. The microstructure consisted of large pores created by the starch particles with lengths between 15 and 80 μm and smaller pores in the matrix with lengths between 0.6 and 3.8 μm. The pores in the matrix reduced the sinterability of the YSZ leading to the retention of closed porosity in the sintered tapes. The porosities were above those predicted for each of the starch contents. However, larger deviations from the predicted porosity were found as more starch was added. The open to total porosity ratio in the sintered tapes could be controlled by the volume fraction of added starch as well as by the sintering temperature. As the volume fraction of starch increased from 17.6 to 37.8 vol.% there was a gradual increase in the interconnectivity of the pore structure. The sintering shrinkage of the tapes at a given temperature could be directly related to the YSZ packing density in the matrix.     

Low-temperature sintering behavior of the nano-sized AlN powder achieved by super-fine grinding mill with Y2O3 and CaO additives

For low-temperature sintering, mixtures of AlN powder doped with 3.53 mass% Y₂O₃ and 0–2.0 mass% CaO as sintering additives were pulverized and dispersed in a vertical super-fine grinding mill with very small ZrO₂ beads. The particle sizes achieved ranged between 50 and 100 nm after grinding for 90 min. The mixtures were then fired at 1000–1500 °C for 0–6 h under nitrogen gas pressure of 0.1 MPa. All nano-sized powders showed pronounced densification from 1300 °C as revealed by shrinkage measurement. The larger amounts of sintering additives enhanced AlN sintering at lower temperatures. Densified AlN ceramics with very fine and uniform grains of 0.3–0.4 μm were obtained at a firing temperature of 1500 °C for 6 h.     

Preparation and properties of porous alumina ceramics with uni-directionally oriented pores by extrusion method using a plastic substance as a pore former

Porous alumina ceramics with uni-directionally aligned pores were prepared by an extrusion method using 0–40 vol.% poly (vinyl acetate) (PVAC) as the pore former. A paste was prepared by mixing 25 mass% distilled water, 4 mass% methylcellulose, 8 mass% oleic acid and 0.8 mass% ammonium poly (carboxylic acid). This paste was molded into a 10 mm Ø body using a ram-type extruder, dried at room temperature for 24 h, calcined at 600 °C for 1 h and sintered at 1500 °C for 2 h in air. The PVAC added to the paste was homogeneously dispersed and formed particles 0.1–150 μm in size which extended in the extrusion direction and were converted to through-hole pores after sintering. The resulting pore size distribution in the samples was bimodal, centered at about 0.4 μm with a broad peak at about 70 μm dia. The resulting porous alumina ceramics showed high gas permeability because of their uni-directionally oriented through-hole pore structure.     

Suppression of carbon contamination in SPSed CaF2 transparent ceramics by Mo foil

Transparent calcium fluoride (CaF₂) ceramics were fabricated by spark plasma sintering (SPS) using Mo foil to separate from graphite die, and the effect of Mo foil on transparency, microstructure and carbon contamination was investigated. The discoloration was removed and the transmittance increased from 8% to 54% at the wavelength of 300 nm and 63% to 86% at 1100 nm by using Mo foil. The average grain size of CaF₂ ceramics SPSed at 1100 °C was 16 μm by using graphite sheet, whereas it was 260 μm by using Mo foil, indicating that ceramics with large grain size are possible to reach high transmittance. Raman spectroscopy and X-ray photoelectron spectroscopy revealed that carbon contamination in the CaF₂ ceramics using Mo foil decreased 78% than that shielded by graphite sheet.