Gelcast Samples Research Articles

Preparation and optical properties of highly transparent MgAl1.9Ga0.1O4 ceramics via aqueous gel-casting method

The aqueous gel-casting technology has been widely used to prepare high-quality green body for various transparent ceramics with large dimension and complex shape. However, owing to the severe hydrolysis of MgAl2O4 powder, it is challenging to obtain thick aqueous slurry with high homogeneity and flowability. In this paper, the surface chemical state of MgAl2O4 powder was modified by introducing Ga3+, and stable MgAl1.9Ga0.1O4 aqueous slurry with high solid-phase loading (52 vol. %) and low viscosity (136 mPa·s, at a shear rate of 50 s-1) was successfully prepared. After pressureless presintering and hot isostatic pressing, the gel-cast sample exhibited much higher optical transmittance and more homogeneous microstructure than the dry-pressed sample, which is mainly derived from the improved homogeneity and densification of the green bodies and ceramics. The optical band gap, infrared cutoff wavelength, static refractive index and dispersion of both MgAl1.9Ga0.1O4 and MgAl2O4 transparent ceramics were systematically compared. It is indicated that the transparent MgAl1.9Ga0.1O4 ceramic has the increscent static refractive index of 1.695, the decrescent direct band gap energy of 6.15 eV and absorption coefficient of 0.49 cm-1 at 5 µm, which could be ascribable to the fact that Ga3+ has different electronic structure, higher electronic polarizability and larger ionic radius in comparison with Al3+. This work provides a dependable solution for preparation of spinel oxide ceramics with superior optical properties and large dimension.

Gelcasting and sintering of hydroxyapatite materials: Effect of particle size and Ca/P ratio on microstructural, mechanical and biological properties

The sintering behaviour and microstructural evolution of two batches of a commercial calcium-deficient hydroxyapatite powder were investigated. First, the sintered density as a function of the starting particle size distribution was studied, and the minimum particle size to get the desired target density was determined. Then, as the two batches were characterized by a slight difference in Ca/P ratio, the role of such ratio on phase and microstructural evolutions during sintering, as well as on mechanical and biological properties was investigated.It was observed that the powder with lower Ca/P ratio underwent significant hydroxyapatite (HA) to β-tricalcium phosphate (β-TCP) decomposition, with a simultaneous formation of tetracalcium phosphate (TTCP). The microstructure of sintered gelcast samples evolved during isothermal sintering at 1300 °C, moving from a starting homogeneous and narrow grain size distribution to a bimodal distribution after 3 h sintering. In fact, over time, large grains decomposed into smaller ones, finally providing a microstructure composed of coarse grains surrounded by plenty of ultra-fine grains. On the contrary, the powder with the higher Ca/P ratio provided a limited HA to β-TCP transformation, and normal grain growth by increasing the sintering time. Such differences lead to different mechanical properties for gelcast samples produced by the two powder batches, as the material with the lower Ca/P ratio affected by lower mechanical strength. Finally, sintered samples from both powders showed in-vitro bioactivity, with a larger surface coverage observed for the lower Ca/P ratio material. The morphology of the apatite layer seemed to be affected by the material composition, too, showing flake-like and needle-like morphologies depending on the Ca/P ratio of the starting powder.

Amalgamation and characterization of porous hydroxyapatite bio ceramics at two various temperatures

Hydroxyapatite (HAP) structures have a great significance in the medicinal area because of its presence in bone and teeth and it provides structural inflexibility to the medium. This study deals with the synthesis and characterization of HAP biomaterials. It is prepared through wet chemical precipitation method at two different ranges of temperatures, 25°C-35 °C and 35°C-45 °C and is denoted as HAP1 and HAP2 respectively. Porous HAP provides a powerful bond to the bones, which interconnect mechanically foremost to a hard fixation of the bone and teeth. Porous HAP processed from polymeric sponge process had 60–70% porosity with interconnected porous of diameter 100–400 μm range using poly vinyl alcohol (PVA). HAP is phase pure at temperature 1150 °C and gel cast sample of nano crystalline HAP porous materials have been sintered to high densities. This is characterized by XRD, SEM and the molar ratio calculated from EDX. The precipitated HAP basic crystalline size is found to be around 100 nm from XRD. Phosphate and hydroxide absorption peaks for green and sintered porous HAP scaffold materials are studied using FT-IR spectra. Thermogravimetric analysis for prepared material is studied out in air at a heating rate of 20 °C/min by using TGA between 35 °C and 700 °C.

3D structure design of magnetic ferrite cores using gelcasting and pressure-less sintering process

Gelcasting is a well established process for ceramics manufacturing which recently has been proved to be successful for soft ferrites as well. This approach is particularly interesting for power electronics application in which the magnetic components (e.g. transformers and inductors) are three dimensionally integrated on the power module substrate. This paper proposes a gelcasting process adapted to make it more effective for 3D heterogeneous integration. The main novelties in this direction consist of low solid load (65wt%) and gelation without catalyst to improve casting and de-airing steps. The magnetic properties of gelcast samples are compared with commercial materials and correlated with the microstructure.

Magnetic field alignment in highly concentrated suspensions for gelcasting process

The aim of the current paper is to present the development of the method used for fabricating highly textured α-alumina elements. The experimental procedure described is based on a combination of two techniques: gelcasting is used in tandem with high magnetic field exposure. In the element shaping process, a new, low-toxic and environmentally friendly gelating system was implemented that is based on in situ polymerisation of an acryloyl derivative of galactose. Here, care was taken to achieve a highly textured structure of elements gelcast from the ceramic slurries of high solid contents (45–50vol%), which have not been processed successfully for magnetic field alignment before. A secondary aim of the study was to investigate the combined effect of magnetic field exposure duration and the idle time of polymerisation on the effectiveness of the alignment. In the course of the experiment, high degrees of crystalline orientation of the gelcast samples were obtained ranging from 0.92 to 0.96, developed after the subsequent sintering at 1600°C (the parameters were calculated on the basis of XRD pattern of the surface perpendicular to the magnetic field direction). The microstructures of the obtained elements are described in detail at the end of the paper, in a stereological analysis.

Fabrication of Barium Titanate/Hydroxyapatite with Aligned Pore Channels by Freeze Gelcasting

A novel and environmental-friendly route combining the conventional freeze casting and gel casting was proposed for the fabrication of aligned pore structures. The viscosity of the suspension increased with the increase of the addition of the Hydantion concentration. In comparison with the conventional freeze casting, the utilisation of a water-soluble Hydantion epoxy resin in the freeze casting promoted the gelation process and led to a higher compressive strength of the green body. After sintering, the freeze gelcast samples exhibited more than twice higher compressive strength and 11.5% higher porosity at the same time than those of conventional freeze cast sample. The improvements were attributed to the combination of gelation of Hydantion epoxy resin and the aligned ice growth, which led to the coexistence of aligned cylindrical and lamellar pore structures.

Diametral deformation behavior and machinability of methyl cellulose thermal gel cast processed alumina ceramics

Thermally induced gelation of methyl cellulose as the networking additive has been demonstrated for consolidating alumina from slurry of 70–72wt% solid loading. Concentration as low as 0.2wt% of methylcellulose had been found to be sufficient to form a uniform network in ceramics with enhanced machinability even with the conventional tools. Cast and machined samples could be sintered close to theoretical densities without any warpage or visible cracks with very fast heating rates of 3°C/min. Low failure stress and high compression distance exhibited by the thermal gel cast samples in comparison to the compacted samples during diametral tests compliments the observed machinability.

Freeze gelcasting of aqueous alumina suspensions for porous ceramics

A novel route combining the conventional freeze casting and gel casting was proposed for the fabrication of aligned pore structures. In comparison with the conventional freeze casting, the utilization of a water-soluble Hydantoin epoxy resin in the freeze casting promoted the gelation process and led to a higher compressive strength of the green body. After sintering, the freeze gelcast samples exhibited a twice higher compressive strength and 15.6% higher porosity at the same time than those of conventional freeze cast sample. The improvements were attributed to the combination of gelation of Hydantoin epoxy resin and the aligned ice growth, which led to the coexistence of aligned cylindrical and lamellar pore structures.

Gelcasting of Magnesium Aluminate Spinel Powder

A stoichiometric MgAl2O4 spinel (MAS) powder was processed in aqueous media and consolidated by gelcasting from suspensions containing 41–45 vol% solids loading. The MAS powder was first obtained by heat treating a compacted mixture of α‐Al2O3 and calcined caustic MgO at 1400°C for 1 h, followed by crushing and milling. Then, its surface was passivated against hydrolysis using an ethanol solution of H3PO4 and Al(H2PO4)3. The as‐treated surface MAS powder could then be dispersed in water using tetra methyl ammonium hydroxide and an ammonium salt of poly‐acrylic acid (Duramax D‐3005) as dispersing agents. The as‐obtained stable suspensions were gelcast, dried, and sintered at 1650°C for 1–3 h. For comparison purposes, the treated powder was also compacted by die pressing of freeze‐dried granules and sintered along with gelcast samples. Near‐net‐shape MAS components with 99.55% of the theoretical density could be fabricated by aqueous gelcasting upon sintering at 1650°C for 3 h. The MAS ceramics fabricated by gelcasting and die pressing exhibited comparable properties.

Surface oxidation and dispersion behaviour of phenolic resin coated α-SiC powder in aqueous medium

The surface oxidation and dispersion behaviour of phenolic resin coated α-SiC powder in aqueous medium has been studied. For comparison purpose a pure α-SiC powder was also studied. Zeta potential and viscosity studies revealed that the phenolic resin coated SiC powder requires an electrostatic stabilisation, whereas, the pure SiC powder requires an electrosteric stabilisation in order to produce aqueous SiC slurries with high solids content. The decomposition behaviour of phenolic resin was assessed by differential scanning calorimetry and thermogravimetry techniques. The influence of aqueous processing on the surface characteristics of pure and the phenolic resin coated powders was assessed by means of Fourier transform infrared spectroscopy and by estimating the bulk density of their gelcast samples sintered at 2150°C for 1 h. Various characterisation results revealed that the phenolic resin coating slightly effects the dispersion behaviour of SiC powder in the aqueous medium but it protects powder from surface oxidation during aqueous processing. Sintered properties suggest that in order to obtain a dense SiC body with a bulk density of >3˙13 g cm–3 (∼97˙5% of the theoretical density) following an aqueous gelcasting technique, the phenolic resin (∼5%) coated SiC powder should possess an average particle/agglomerate size of <11˙0 μm, compact (green) density of >1˙67 g cm–3 and B content of >0˙5%.

Gel casting of aqueous suspensions of BaTiO3 nanopowders

Abstract Commercial nano-BaTiO3 powders have been formed into green bodies using colloidal forming routes. A study of the rheological behaviour of the suspensions as a function of dispersant concentration and homogenisation time was made in order to prepare stable concentrated suspensions of the nanopowders. Bulk components were then manufactured using aqueous slip and gel casting involving polysaccharides that gel on cooling, i.e. agar. The performance of theses consolidation techniques for obtaining dense green bodies from the BaTiO3 nanopowders was studied. It was possible to prepare relatively big gel cast samples with a similar density and microstructure and in a shorter time compared to those obtained by slip casting.

Evaluation of mechanical properties of gelcast lead zirconate titanate disks sintered at different temperatures

The mechanical properties of gelcast and sintered PZT disks were measured using a microindentation technique and compared with those of sintered cold isostatically pressed and die-pressed PZT disks. Microhardness increased with increasing density, whereas fracture toughness followed the reverse trend due to the effect of porosity. The hardness and modulus of elasticity values of the sintered gelcast samples were comparable with those of the sintered cold isostatically pressed and die-pressed samples.

Water-based gelcasting of lead zirconate titanate and evaluation of mechanical properties of the gelcast samples

Gelcast and sintered bodies obtained from ovalbumin-based slurry with 45% solid loading were crack free and showed density comparable to the cold isostatically pressed (CIP) and sintered lead zirconate titanate samples. The values of average hardness, modulus of elasticity and fracture toughness of a 1200 °C sintered gelcast sample obtained by microindentation are 5.9 GPa, 137.0 GPa and 0.96 MPa m1/2, respectively. The mechanical properties of the gelcast and sintered sample were comparable to that of the CIP-sintered sample.

Effects of monomer composition on the mechanical and machinability properties of gel-cast alumina green compacts

In the present work, machinable green gel-cast alumina compacts were prepared by using polyethyleneglycol (400) diacrylate (P4) and polyethyleneglycol (600) dimethacrylate (P6) together with the acrylamide (AAm) comonomer. The glass transition temperatures of copolymers decreased with the increasing of P4 or P6 amount in total copolymer. The green samples obtained in an aqueous system were mechanically analyzed by means of three-point bending. Flexural strength values increased, from 2 MPa to 25 MPa, as the weight ratio of P4 or P6 in total copolymer (AAm-P4 or AAm-P6) decreased from 90% to 3.5%, respectively. The green gel-cast samples prepared by using P4 or P6 were machined easily by using a lathe, drill and milling machine without damaging the samples, which have good surface finish. The binder removal was achieved at lower temperatures than those samples prepared by using only AAm.

Anti-crack machining of PZT ceramics for fabricating piezocomposites by using gelcasting technique

The high hardness and fragility of the PZT ceramics make it difficult for mechanical processing, especially in the processing of fine-scale composites. The present article applies gelcasting technique to the fabrication of ceramic frameworks for piezoelectric composites. Because of the excellent mechanical property of the gelcast green body, a green body dicing and later sintering way will successfully prevent the cracks inherently associated with the conventional ceramics dicing way. Homogeneous ceramic frameworks can be successfully fabricated by this novel method. The high density and strong piezoelectric effect of the gelcast sample also make it an especially suitable way for forming piezoelectric materials and piezocomposites.

Water based gelcasting of lead zirconate titanate

Gelcasting is a novel forming method for making high-quality ceramic parts by means of in situ polymerization where only a few percents of a polymerizable binder are needed. In this article the viscosities of lead zirconate titanate (PZT) suspensions with 15–52 vol.% solids loading were studied. After developing a concentrated PZT suspension with a low viscosity, gelcasting was successfully used to form PZT ceramic parts. Microstructures and piezoelectric properties of gelcast samples derived from suspensions with different solids loading were also investigated in comparison with those of die pressed ones. It was found that gelcast samples exhibited slightly stronger piezoelectric effect than the die pressed ones at the same sintering procedure. Based on the comparison of the density and pore structure results of the samples prepared by these two methods, their different piezoelectricity may be attributed to their microstructure difference.

Influence of forming methods on the microstructure of 3Y-TZP specimens

Nanosized 3Y-TZP powders with particle size of 10–40 nm were formed by gelcasting, dry pressing and cold isostatic pressing. The influence of particle size as well as forming methods on the microstructure and mechanical properties of specimens were investigated. Both SEM images and the analysis on pore size distribution reveal that the gelcast sample possessed more homogenous microstructure. 3Y-TZP powders with particle size of 10 nm have been gelcast with green density as 37.5% of theory and sintered density as high as 96% was achieved. Its fracture toughness was up to 11.9 MPa m 1/2 and the hardness value HV 10 is of 15.2 GPa. The difference of microstructure and mechanical properties is explained in terms of the differences in grain size and the forming methods.

Microwave sintering of nanosized hydroxyapatite powder compacts

Gel cast sample of nanocrystalline hydroxy apatite powder has been sintered to high densities by short-time microwave processing and is characterised by X-ray diffractometry (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SCM), density measurements and Vickers microhardness (Hv) measurements. The basic crystallite size of the precipitated HAP was found to be around 35 nm by XRD and TEM studies. The gel cast samples of the above powder could be sintered to above 95% by exposing to microwave radiation for a period of 5 min. The sintered grainsize of the sample was 200–300 nm having microhardness in the range of 5.25 GPa and the effect of microwave exposure time on the densification and grainsize is reported.

Strength and machining of gelcast SIC ceramics

The aqueous gelcasting technique was employed to form silicon carbide green samples for mechanical strength measurements and machining tests. The monomers used in this work were acrylamide (AM) and methylenebisacrylamide (MBAM). Polymerisation reaction was promoted by the addition of catalyst (N, N, N′, N′-tetramethyl ethylenediamine) and initiator (ammonium persulfate). Characterisation of the green, dried gelcast samples included measurements of density and bend strength (flexural strength). It was found that the bend strength of dry, green, gelcast samples depends critically on the slurry solid loading, the amount of monomers and the ratio of monomers contents (AM/MBAM). The highest strength value of 29 MPa was found with samples produced from the slurry with 25 vol% solid loading, the monomers content of ∼10 wt% and the ratio AM/MBAM of ∼16. An increase in solid loading from 25 to 35 vol% caused a decrease in the green sample strength to 20 MPa. Successful machining was achieved with all samples having bend strengths of over ∼6 MPa using standard machining equipment. In one set of samples, machining was done with cemented carbide (W-Co) tools and in the other set with ceramic cutting tools (Al2O3-TiC) using the same cutting speeds, feed rate and depths of cut. Ceramic cutting tools exhibited approximately three times longer life than the cemented carbide tools. The effect of solid loading and the concentration of monomers on the sintered density were also investigated.

PTCR characteristic of gelcast BaTiO 3 ceramic thermistor

This paper applied gelcasting, a novel forming method, to form BaTiO 3-based ceramic thermistor. The resistivity–temperature characteristics and microstructures of gelcast samples from suspensions with different solids loading were investigated in comparison with those of die-pressed ones. It is found that gelcast ceramics exhibit stronger PTCR effect than die-pressed ones at the same sintering systems and that the solids loading in the range of 42–50 vol.% has slight influence on the PTCR effect of gelcast ceramics. The two phenomena were explained.