Gelcasting Process Research Articles

Polycrystalline alumina ceramics doped with nanoparticles for increased transparency

Transparent alumina samples were prepared by the gelcasting process followed by pressureless presintering and hot isostatic pressing. High transparency was obtained by doping alumina powder with zirconia and spinel nanoparticles. The doping approach enabled accommodating 0.3 wt% zirconia and 0.175 wt% spinel nanoparticles without any light scattering. The doping combined with colloidal processing resulted in grain size refinement in a fully dense microstructure and adequate in-line transmission improvement. The maximum in-line transmission reached 70.4% at a wavelength of 632.8 nm and a sample thickness of 0.8 mm and was at the theoretical maximum in the infrared wavelength range between 2000 and 4000 nm. The presented processing approach featured near-net shape and scale-up capabilities.

Freeze gelcasting of naphthalene-in-aqueous alumina slurry emulsions for the preparation of macroporous alumina ceramics

An emulsion freeze gelcasting process has been studied for the preparation of macroporous alumina ceramics. The naphthalene-in-aqueous alumina slurry emulsions prepared at 90°C from a 30vol% alumina slurry undergo gelation on cooling in a mould due to the solidification of naphthalene and physical cross-linking of the carrageenan present in the aqueous slurry. The medium internal phase emulsions with naphthalene to alumina slurry volume ratios in the range of 1 to 1.86 show shear thinning flow behavior with low viscosity and yield stress, suitable for casting. The room temperature drying of the gelled bodies followed by naphthalene removal at 70°C and sintering at 1550°C produces macroporous alumina ceramics with porosity in the range of 65 to 78%. The macropore morphology depends on the emulsifying agent (sodium dodecyl sulphate) concentration and naphthalene to alumina slurry volume ratio. The average macropore size (35.1 to 4.9μm) decreases with an increase in the emulsifying agent concentration and naphthalene to alumina slurry volume ratio. The macroporous ceramics obtained have high compressive (7 to 28.75MPa) strength and Young’s modulus (553 to 1230MPa).

Research on Rheological Behavior of Si<sub>3</sub>N<sub>4</sub>-SiC Composites by Gelcasting

Using SiC particles and powder, the modified Si and α-Si3N4 powder as the main raw material, Si3N4-SiC composite materials have been fabricated by gelcasting. The Si3N4-SiC suspension with 75vol% (volume fraction) solid loading exhibits shear thinning rheological behavior under a shear rate from 10 to 160s-1, which meets the requirement of the gelcasting process. The bulk density of the green bodies after solidification is 2.57 g·cm-3 and the flexural strength 16 MPa, the green bodies have a dense and uniform structure. The bulk density and flexural strength of the Si3N4-SiC composites after nitriding are 2.67g·cm-3 and 48 MPa respectively; the strength is higher than that of other industrial products without gelcasting. Well-developed intertwined fibrous Si3N4 crystals grow on the surface of the SiC particles to form a tight microstructure in the Si3N4-SiC composites after firing.

Processing and Characterization of MMC Beads Based on Zirconia and TRIP Steel

A novel process for metal-matrix composite fabrication with the special focus on single beads and sintered bead structures is explored. The used gel-casting process by sodium alginate gelation is introduced, and various analyses with significant results are presented. The suspensions contained 16-7-3 steel and zirconia particles as well as sodium alginate and were subsequently added dropwise into water which contained solidifying agent for forming rubbery, substantially round beads. Sintered beads with adequate strength (~400 MPa) and perfect surface, homogeneous microstructure, and high energy absorption capability have been produced by this casting process. At lower strains (up to 15 pct), all zirconia reinforced steel beads obtain higher specific energy absorption (SEA) in comparison to pure steel beads. Especially the composition of 90 vol pct TRIP steel and 10 vol pct zirconia shows a significant improved energy absorption capability with 27.7 MJ/m3 at a strain of 15 pct. Pure steel only exhibits a SEA of 13.1 MJ/m3.

Fabrication of hierarchically porous Si3N4 foam ceramics with stamen-like cell structure

Inspired from the stamen structure in nature, hierarchically porous Si3N4 foam with stamen-like cell structure is fabricated by using egg white protein as pore forming agent in the gel casting process. Macropores with micro-interconnected pore on the wall are present which have average pore size of 0.36μm and 10–400μm. Graded rod-shaped Si3N4 grains, finer ones with aspect ratios of 10 on the wall and longer ones with aspect ratios of 20 in the interior of the macropores, are synthesized. Such porous Si3N4 exhibits good mechanical property, relatively high strength (11.9MPa) and high porosity (81.7%), indicating it is a potential high-temperature separation materials.

Fabrication of Fine‐Scale 1–3 Piezoelectric Arrays by Aqueous Gelcasting

Soft mold technique combined with a modified gelcasting process was developed to fabricate microscale piezoelectric pillar arrays. This technique involves the fabrication of high‐aspect‐ratio silicon master, generation of negative polydimethylsiloxane mold, and replica molding piezoelectric pillar array by gelcasting. A gelling system based on the polymerization of Hydantion epoxy resin and 3,3′‐Diaminodipropylamine (DPTA) was employed. Weibull analysis revealed that 20.0 wt% Hydantion epoxy resin was the optimized concentration while the highest Weibull modulus and characteristic flexural strength of 14.8 and 90.9 MPa were obtained for lead zirconate titanate sintered bodies, respectively. Excellent piezoelectric pillar arrays with lateral dimensions around 8 μm and aspect ratio higher than five were successfully fabricated.

Processamento e caracterização de espumas vitrocerâmicas do sistema Li2O-ZrO2-SiO2-Al2O3 (LZSA) produzidas por gelcasting

Neste trabalho, a técnica de viscosimetria foi utilizada para avaliar as características reológicas de suspensões preparadas com precursor vitrocerâmico do sistema LZSA (Li2O-ZrO2-SiO2-Al2O3). A partir da caracterização reológica, foi possível estabelecer condições composicionais adequadas de fração de sólidos, dispersante e monômeros orgânicos. A quantidade de agente espumante para produção de espumas vitrocerâmicas por gelcasting foi determinada pela variação volumétrica após agitação. As espumas resultantes foram submetidas a tratamentos térmicos no intervalo de temperatura compreendido entre 200 e 500ºC/60 min, para degradação da matéria orgânica e a 950ºC/60 min para sinterização e cristalização. Com os corpos cerâmicos porosos foram realizadas medidas de retração térmica, densidade aparente, análise microestrutural e resistência mecânica. A adequação das características reológicas do precursor LZSA por gelcasting permitiu a produção de espumas com elevada porosidade (> 90%) aberta e interconectada com boa estabilidade térmica e com resistência mecânica compatível com cerâmicas porosas.

Monoacryloyl esters of carbohydrates: Synthesis, polymerization and application in ceramic technology

The article presents the interdisciplinary research among organic synthesis, chemistry of polymers and ceramic technology. It presents the synthesis of monoacryloyl esters of fructose and glucose that is 1-O-acryloyl-d-fructose and 3-O-acryloyl-d-glucose, conditions of their polymerization and application in shaping of advanced ceramic powders by the so called gelcasting method. The paper presents the influence of carbohydrate esters on the viscosity of Al2O3 suspensions and microstructure of final ceramic samples. The results showed that synthesized esters of saccharides can play the role of organic monomers able to polymerize in situ and self-cross-linking compounds in gelcasting. The paper presents the proposed structure of polymeric network which is formed from acryloyl ester of glucose during gelcasting process. The paper describes rheological behaviour of slurries composed of synthesized substances and A2O3 powders, wetting angles of alumina substrate by synthesized compounds, differences in glass transition temperatures of polymers and the microstructure of obtained final ceramic samples.

Study on rheological behavior of WC slurry in gel-casting process and reactive infiltration of produced foam by molten Zr2Cu alloy

In the present work, rheological behavior of tungsten carbide (WC) slurries was evaluated to utilize it in gel-casting process, and effects of dispersant quantity and solids content on the viscosity of WC slurries were examined. It was found that tri-ammonium citrate to WC powder ratio of 0.25 was a preferable dispersant in such a non-toxic system. Maximum allowable WC powder was 25vol.% in the slurry. The slurry with 25vol.% WC powder and gelation additives was prepared, cast and dried at room temperature, then it was pre-sintered at 1450°C for 4h. Scanning electron microscope (SEM) micrograph of the pre-sintered WC foam showed a uniform porous microstructure. It had enough strength to be infiltrated with Zr2Cu molten. A reactive melt infiltration occurred during the practice, resulting in the formation of about 58vol.%W and 42vol.% ZrC in the final microstructure.

Comparison study of fabrication of ceramic rotor using various manufacturing methods

An alumina ceramic turbine rotor with both large volume (90mm in diameter and 20mm in height) and sub-millimeter geometry (0.42mm at the blade tip with curvature) was successfully prepared by using various manufacturing approaches. Polyjetting and computer numerical control (CNC) machining were respectively adopted to fabricate an integral sacrificial polymeric mold and a separable aluminum mold for the tailored gelcasting process, whereas selective laser sintering (SLS) was used to directly fabricate the turbine rotor by laser sintering the alumina and epoxy powders followed by liquid phase sintering in furnace. A series of experimental studies on these three rotor-forming routes were performed throughout the process. For the green rotor, the alumina slurry preparation, specific drying strategy and microstructure of the green part were presented and compared. For the sintered rotor, comparisons in terms of surface roughness, microstructure, mechanical properties and blade contour deformation were conducted. Finally, the most suitable method to fabricate the complex-shaped alumina rotor with the best performances was suggested.

A new Al2O3 porous ceramic prepared by addition of hollow spheres

A method for making porous ceramic prepared by adding hollow spheres was developed, and the resulting porous ceramic was named as hollow spheres ceramic. Water soluble epoxy resin was used as a gel former in the gelcasting process of the Al2O3 hollow sphere and Al2O3 powder, the porous ceramic porosity varies from 22.3 to 60.1 %. The influence of amount of Al2O3 hollow sphere and sintering temperature on the microstructure, compressive strength and thermal conductivity were investigated. With an increasing amount of hollow sphere in the matrix, the porosity increases, which leads to decreased bulk density, compressive strength and thermal conductivity. The compressive strength of the porous ceramics has a power law relation with the porosity, and the calculated power law index is 4.5. The equations of the relationship between porosity and thermal conductivity of porous ceramics are proposed. The thermal conductivity of samples with 60.1 % porosity is as low as 2.1 W/m k at room temperature.

Investigation of effect of acrylate gel maker parameters on properties of WC preforms for the production of W–ZrC composite

In this study, acrylate gel maker monomers (MAM and MBAM) were utilized in gel-casting process to fabricate tungsten carbide porous preform. In this research, by investigation of rheological behavior of the suspension, it was found that the optimum amount of dispersant was about 0.2wt.% of ceramic powder. Also, investigation of thermal behavior of polymer was carried out by means of DSC and TGA methods, and it was noticed that after reaching the temperature of 450°C, there would be no polymer present in the structure. The green bodies were dried and pre-sintered at 1450°C for 4h. The influence of the amount of monomers (MAM+MBAM) and also the ratio between monomers (MAM/MBAM) on the strength of dried green body and amount of porosity of pre-sintered body was evaluated.

Gelcasting of sol–gel derived mullite based on gelation of modified poly(isobutylene-alt-maleic anhydride)

Gelcasting is an attractive process of fabricating ceramics with complicated shapes. In the present work, modified poly(isobutylene-alt-maleic anhydride) (mPIAM), a low toxicity polymer, was used to form mullite green bodies. Poly(ethylene imine) (PEI) was used as a solidifying agent. The aqueous slurry containing 50vol% mullite ceramic powders, 3vol% mPIAM, and 0.15vol% PEI was prepared for the gelcasting process, and the minimum viscosity of 50vol% mullite slurries was obtained when 3.0vol% monomer was added. The slurry was stable at room temperature but solidified after being heated at 60°C for 25min. The green body was dried at room temperature for 48h, fired at 600°C for 1h, and pressureless sintered at 1500°C for 3h. Results showed that the produced green and sintered bodies both have homogenous structure. The relative density and flexural strength of the green body were 56.7%±0.8% and 12.5±0.6MPa, respectively, while those for the sintered body were 96.7%±0.5% and 376±82MPa, respectively.

YSZ fiber-reinforced porous YSZ ceramics with lowered thermal conductivity: Influence of the sintering temperature

Yttria-stabilized zirconia (YSZ) fiber was introduced to reinforce porous YSZ ceramics fabricated by tert-butyl alcohol (TBA)-based gel-casting process and pressureless sintering. The influence of sintering temperature on microstructure and properties was investigated systematically. Results showed that linear shrinkage of the green bodies during sintering increased and porosity decreased with increasing sintering temperature. Interfacial bonding can be adjusted easily through different sintering temperatures. Compressive strength of the porous YSZ ceramics was improved significantly with the introduction of fibers and the strengthening effect reached its optimum at the sintering temperature of 1500°C. Compared with the specimens without fiber addition, the compressive strength of fiber-reinforced specimens was almost doubled and the highest reached 45.0MPa with the porosity of 58.5%. In addition, the thermal conductivity was decreased with the introduction of fibers. The improved mechanical and thermal properties make porous YSZ ceramics more applicable as thermal insulating materials at high temperature.

Study on Gelcasting of Fused Silica Glass Using Glutinous Rice Flour as Binder

Gelcasting is a colloidal processing method for fabricating high‐strength and complex shape ceramic green bodies. However, industry has been reluctant to use the gelcasting technique because the most commonly used gel, acrylamide (AM), is a neurotoxin. Here, we report an attempt at the gelcasting of fused silica glass using a natural and nontoxic gel, glutinous rice flour (GRF) as binder. The GRF‐based aqueous system was found to behave excellently in the gelcasting process. Flexural strength of fused silica green bodies solidified with only 3 wt% GRF is up to 11.87 MPa. Bulk density and flexural strength of fused silica glass sintered at 1275 °C are 1.75 g/cm3 and 47.02 MPa, respectively.

Fabrication and characterization of mini alumina ceramic turbine rotor using a tailored gelcasting process

A mini alumina ceramic turbine rotor has been successfully fabricated using a tailored colloidal process based on gelcasting technology. A specialized forming, drying and sintering approach was developed for the complex-shaped component with both large volume (90mm in diameter and 20mm in height) and submillimeter geometry (0.42mm at the blade tip with curvature), to assure a highly dense product without warpage and cracks. The effects of solid content and temperature on the rheological behaviors of the prepared slurry were investigated to identify the best pourability. A novel approach using radius of curvature and overlapped area was adopted to compare the blade contour similarity under different sintering strategies and solid contents. In addition, the different physical and mechanical properties between main body and blade regions were discussed and their microstructures were analyzed. The preparation route for highly dense alumina rotor with minimum contour distortion was finally summarized.

Preparation of Al2O3–Ni Cermet Composites by Aqueous Gelcasting

The main objective of producing ceramic-metal composites is to improve the fracture toughness of the brittle ceramic matrix. Classical methods of powder metallurgy (such as isostatic or uniaxial pressing) are not capable of fabricating complex-shape elements. For this reason, colloidal processes have been applied recently in manufacturing of ceramic matrix composites. In the present work, Al2O3–3 vol.% Ni composites were prepared by aqueous gelcasting process and sintering in reducing atmosphere. The commercially available monomers 2-hydroxyethyl acrylate and N,N′-methylenebisacrylamide as cross-linking agents were used. The activator was N,N,N′,N′,-tetramethylenediamine (TEMED) and ammonium persulfate was added as an initiator of the polymerization process. Composite slurries were obtained from alumina powders (grain size 0.21 μm) and 3% (in respect to the total solid volume) of two nickel powders of different grain size (~10 and ~0.4 μm). The paper describes the rheological properties of the obtained composite slurries. The properties of green and sintered elements have also been presented. It was found that the presence of nickel particles had a significant influence on the slurry polymerization process. The green and sintered relative density was in every case higher than 57 and 97%, respectively.

A simple and effective method for gel casting of zirconia green bodies using phenolic resin as a binder

Abstract A simple, effective method for fabricating zirconia green bodies is described that utilizes phenolic resin as a binder in the gel casting process. Both the zeta potential of zirconia particles and the rheological behavior of the slurries were measured. To prepare stable, homogeneous, fluidic zirconia slurry with high solid loading, the zeta potential was adjusted by varying the phenolic resin content of the premixed solution. This represented a departure from normally adjusting the slurry pH by using an acids, alkali or dispersants. To promote gelation of the slurry, a curing agent was added. Gel casting a mixture of a 42 vol.% zirconia slurry containing 13 wt.% phenolic resin yielded an easily sintered, very homogeneous green body with the desired strength. The gelation time of the slurries and the mechanical strength of the green bodies were controlled by adjusting the quantity of the curing agent in the slurries.

Gelcasting of Alumina with Application of New Monomer Synthesised from Xylitol

Abstract A new monomer for application in the gelcasting process has been designed and synthesized, following the green chemistry trend to use non toxic and renewable compounds. Xylitol was the main substrate for the synthesis that included 3 stages. The final product, named 1-O-acryloylxylitol, was tested regarding its effectiveness as the processing agent for fabrication of alumina elements by gelcasting method. Rheological properties of the ceramic slurries were studied as well as the properties of shaped elements before and after sintering. The results are compared to those obtained by applying commercial monomer 2-hydroxyethyl acrylate.

Acoustic Absorption of a New Class of Alumina Foams with Various High-Porosity Levels

Abstract Recently, a new class of ceramic foams with porosity levels up to 90% has been developed as a result of the association of the gelcasting process and aeration of the ceramic suspension. This paper presents and discusses original results advertising sound absorbing capabilities of such foams. The authors man- ufactured three types of alumina foams in order to investigate three porosity levels, namely: 72, 88, and 90%. The microstructure of foams was examined and typical dimensions and average sizes of cells (pores) and cell-linking windows were found for each porosity case. Then, the acoustic absorption coefficient was measured in a wide frequency range for several samples of various thickness cut out from the foams. The results were discussed and compared with the acoustic absorption of typical polyurethane foams proving that the alumina foams with high porosity of 88-90% have excellent sound absorbing properties competitive with the quality of sound absorbing PU foams of higher porosity.