Osmotic drying of gelcast bodies in liquid desiccant

Osmotic drying of gelcast alumina bodies was carried out in water solutions of various polyethylene glycols (PEGs) with molecular weight ranging from 1000 to 80,000 g/mol. Up to 30% of the water content could be removed from the gelcast bodies immersed in a 43 wt% solution of PEG 80000. It was found that PEG 1000 was a less effective desiccant than the PEGs with higher molecular weights, even if the osmotic pressure was similar (4.9 MPa). The smaller PEG molecules penetrated the gelled bodies and reduced the dewatering. Moreover, the penetrated PEG molecules affected the pore structure of green bodies and in this way influenced the sintering behaviour. The time dependence of dewatering and the effect of the size of gelcast bodies were investigated and correlated with the green body structure and sintering behaviour in order to optimize the osmotic drying process.     

Influence of osmotic drying with an aqueous poly(ethylene glycol) liquid desiccant on alumina objects gelcast with gelatin

Gelcasting and liquid desiccant drying are novel forming and drying methods used to mitigate common issues associated with the fabrication of complex advanced ceramic objects. In this study, the molecular weight and osmotic pressure of aqueous poly(ethylene glycol) (PEG) desiccant solutions were simultaneously varied to understand their influence on the net mass loss rates of gelcast alumina samples prepared using gelatin as a gelling agent. Additionally, the amount of PEG diffusion and water diffusion to and from the ceramic samples after 150 min of immersion in the liquid desiccant was correlated to the solution properties as was the final bulk density of the sintered samples. Solutions with high molecular weight and low osmotic pressure resulted in low PEG gain and low water loss, while solutions with low molecular weight and high osmotic pressure resulted in high PEG gain and high water loss. In some cases, more than 40 wt% of the total water per sample was removed through the liquid desiccant drying process.     

New laser machining technology of Al2O3 ceramic with complex shape

A new method was developed for the fabrication of complex-shaped Al₂O₃ ceramic parts by combining laser machining and gelcasting technique. The unwanted ceramic powders parts were selectively removed by laser machining specified by a computer program, and the gelcast Al₂O₃ green bodies were machined to a designed shape by a CO₂ laser at a wavelength of 10.6 μm. The influences of solid loading, laser output power, scanning speed and nitrogen purge on the machining of green Al₂O₃ ceramic bodies were studied. The experimental parameters were optimized, the green Al₂O₃ bodies with solid loading of 40 vol% or below were easier to be machined, while the green bodies with solid loading of 45 vol% or above were hard to be further machined due to the surface sintering. Better machining quality and deeper machining depth could be obtained when the laser power is 30 W. The green Al₂O₃ bodies with complex shape were obtained by the laser machining.     

Formation Mechanism of Cracks During the Freeze Drying of Gelcast Ceramic Parts

The near‐net shaping of gelcast ceramic parts can be achieved using freeze‐drying technology. It was discovered that the cracks probably occurred inside the part during freeze drying due to the formation of liquid water. To explain the formation mechanism of cracks, the electrical resistance method was employed to measure the part's eutectic temperature which can be used to determine the freezing and melting state of the part. Affecting factors on cracking were investigated by testing the temperature and water loss of the part during the drying stage, and the cracks inside the part were detected through computed tomography (CT). It was found that when the temperature of the part was higher than the eutectic temperature, the ice crystals at the sublimation front would melt, resulting in the formation of wet green body. The cracks will occur inside the part because great capillary forces are formed in the wet green body. Cracks could be controlled through reducing heat supply by lowing shelf temperature, decreasing pressure of the drying chamber or lowering the dried layer resistance against water vapor by improving the freezing temperature or reducing the solid loading of the part.     

Aqueous Gelcasting of Fused Silica Using Colloidal Silica Binder

An aqueous‐based gelcasting of fused silica ceramics by using colloidal silica binder was developed. Fused silica slurries having different volume percentage of solid loading from 63 to 74 vol% in colloidal silica were made and the rheological properties were evaluated. It was found that the slurry with 73 vol% of solid loading with viscosity 0.70 Pa.s is suitable for this gelcasting system. The influence of solid loading on physical and mechanical properties of gelcast green and sintered bodies has been studied. The fabricated green body by using colloidal silica binder exhibited a flexural strength of 9 MPa and 88% of theoretical density with 2.2% of drying shrinkage while the sintered sample exhibited flexural strength of 60 MPa and 95% of theoretical density with 4.3% of sintering shrinkage. It was observed that, the nano silica particles from the colloidal silica binder is filling the interstitial positions in the consolidated fused silica green body and enhancing physical and mechanical properties.     

Mechanical and electrical properties of lithium stabilized sodium beta alumina solid electrolyte shaping by non-aqueous gelcasting

A novel non-aqueous gelcasting system based on the polymerization of 2-hydroxyethyl methacrylate (HEMA) with low toxicity is applied to the fabrication of lithium stabilized beta alumina solid electrolyte (Li-BASE). Triethanolamine and polyethylene glycol (MW. 1000) are used as dispersant and plasticizer, respectively. Effects of dispersant concentration, solids loading and plasticizer concentration on the rheological behavior of the suspensions are investigated and optimized. Cracking and warpage phenomena in the gelcast green body during drying process are discussed. Green body gelcast from the optimized suspension shows homogeneous microstructure with a relative density of 59.4 % and a bending strength of 17 MPa. Properties, including the relative density, phase composition, microstructure, bending strength and ionic conductivity, of Li-BASEs sintered from green body formed by gelcasting and cold isostatic press are evaluated. Results show that gelcasting forming technique performs well in preparing BASE ceramics with high Weibull modulus and ionic conductivity.     

Novel structure of ceramic tape for multilayer devices

To reduce the content of binders surrounding ceramic powders in a ceramic tape and the residual pore after burnout to a minimum, a ceramic tape with double layers was manufactured. One layer was comprised of only organic binder, which imparted sufficient strength and strong adhesive property to the green tape. The other one was a ceramic layer with a very small amount of binder. The binder content of the slurry for ceramic layer was less than 2 wt%, which could lower the viscosity, make the slurry well dispersed and considerably increase solid loading in the slurry. This higher solid loading led to higher green tape density, higher fired density of the product. The two-layer ceramic tape showed much better qualities than a common (or conventional) green tape especially in microstructure, laminatablilty, and tape density. In the multilayer structure made of the two-layer ceramic tape, the binder layer completely disappeared after binder-burnout and no defects from the two-layer structure were observed.     

Effect of monomers content in enhancing solid-state densification of silicon carbide ceramics by aqueous gelcasting and pressureless sintering

The possibility of obtaining solid-state sintered silicon carbide (SiC) through aqueous gelcasting using commercial SiC powders was demonstrated. Green bodies were prepared from thixotropic SiC slurries in aqueous medium with optimized pH and solid-loading. The monomer system in gelcasting provides strength to the green bodies through formation of a gel network by polymerization and the carbon from polymeric gel enhances the densification of SiC, thereby avoiding addition of carbon externally to the gelcasting batches. Maximum bulk density of 3.16 g/cm³ (98.4% of relative density) was achieved in gelcast SiC on sintering at 2150 °C in argon atmosphere. The effect of carbon on SiC densification is evinced from the changes in microstructure of sintered SiC with increase in carbon content. The density and microstructure of gelcast and sintered SiC was comparable to that obtained from dry pressing and sintering of additive mixed SiC powders.     

Binder jetting of ceramics: Powders,binders, printing parameters,equipment, and post-treatment

Binder jetting is expected to become the universal process for preparing ceramic parts because it can overcome multiple problems, such as the difficulty to prepare complex-shaped ceramic parts and the shrinkage of the sintering process, which appear in conventional ceramic preparation process. This paper introduces principles, steps, and applications of binder jetting printing ceramics. Furthermore, five key factors of binder jetting printing ceramics (powders, binders, printing parameters, equipment, and post-treatment process) have been investigated. Accordingly, effects of powders (including shape, particle size and distribution, and additives), binders (including binding method, droplet-formation mechanism, and droplet-infiltration kinetics), printing parameters (including layer thickness, saturation, solid binder, and printing orientation), equipment, and post-treatment (including de-powdering process, and densification process) on density, roughness, strength, accuracy, and resolution of ceramic parts have been discussed and summarized. This paper provides detailed analysis of techniques and mechanisms of binder jetting of ceramics, giving guidance on how to handle raw materials and select various processing parameters for achieving desired performance.     

Dewatering of spontaneous-coagulation-cast alumina ceramic gel by filtrating with low pressure

Water removal in colloidal processing is a tough and crucial problem because deformation and cracking of the wet body easily happen during the drying process. Inspired by conventional pressure filtration (PF), we combined spontaneous coagulation casting (SCC) with PF to partially remove water from the gelled sample before drying for the purpose of lessening the risk of cracking and shortening the drying period. The effect of pressure, holding time, and solid loading on the particle packing ratio of wet samples, the drying process, and the bulk density and homogeneity of green and sintered bodies was surveyed systemically. The PF process of 0.4 MPa for 2 h was optimized based on dewatering ratio from ceramic slurries. The particle packing ratio of the pressure-filtrated samples increased with the solid loading of ceramic slurries, and 61 vol% was obtained for the slurry with 56 vol% solid loading. The drying time of the pressure-filtrated samples was shortened by as much as 42%–53% compared with that of the gelled samples, and the linear shrinkage for the pressure-filtrated samples during drying was only approximately 0.5%, much smaller than that of the gelled samples (2.4%–4.6%). Furthermore, the bulk density of the pressure-filtrated green bodies was more than 2% higher than that of the gelled green bodies, and more homogeneous green bodies were obtained by PF than by SCC. The alumina ceramic with 98.8% relative density and a homogenous density distribution was obtained after being sintered at 1550 °C for 2 h.     

Gel-tape casting as a novel method for the production of flexible fine-grained alumina sheets

The paper reports on the development of a new aqueous gel-tape casting method for processing nanosized powders into thin fine-grained sheets with a high relative density after sintering. The method was based on casting a low-viscosity slurry into a tape, using the doctor-blade method and gelling the slurry after the casting. The gelled tape was removed from the tape carrier without any solvent evaporation and the drying of the gelled tape was performed later in a climate box under controlled temperature and humidity. The epoxy-based ceramic slurry could be prepared with a solid loading of 42 vol.%. Ceramic sheets with flat and smooth surfaces were prepared in a thickness range from 0.08 to 0.4 mm after sintering. The sintered sheets reached a relative density of 99.4%, with an average grain size of about 1.1 μm. The strength and flexibility of alumina sheets were investigated and discussed.     

Polymerization and Rheological Behavior of the Thermoresponsive Gelcasting System Based on N‐isopropylacrylamide

Recently, we developed an efficient method for rapid drying and improving relative density of the gelcast green body. In this work, polymerization and rheological behavior of the thermoresponsive gelcasting system based on N‐isopropylacrylamide were investigated. To initiate reaction of the system with 4 mL/L of catalyst, 0.25–0.5 mg/mL of initiator is considered optimal in the optimized temperature range 15–20°C. Suspensions with 40–55 vol.% solid loading still show shear‐thinning behavior, which is favorable for mixing, degassing, and casting. The final storage moduli of consolidated suspensions are approximately 10⁴ Pa, which is sufficiently high for gelcast green bodies.     

Preparation of high solid loading titania suspension in gelcasting using modified boiling rice extract (MBRE) as binder

Preparation of high solid loading homogeneous titania suspension using modified boiling rice extract (BRE) as consolidator (network-former)/binder for gelcasting application has been investigated. To achieve in situ consolidation forming of TiO₂ ceramic, the gel network formed by swelling and gelatinization of the modified BRE (MBRE) with 2-hydroxyethylmethacrylate was studied. The dispersion behaviour of the titania powder and rheology of the suspension under the influence of binder content, dispersant (Darvan 821A) concentration and pH of the dispersing media have been discussed. The present process of gel casting deals with 50-80 weight% solid loading of titania particles with MBRE (2-10 weight% to that of total solid loading) in presence of dispersant (ammonium salt of polyacrylic acid [(C₄H₅O₂-NH₄⁺)n]). The influence of BRE concentration and solid particle loading on rheological properties of aqueous titania suspensions has been analyzed under steady and oscillatory shear conditions. Thermogravimetry (TG) and differential thermal analysis (DTA) on gelcast green body has been evaluated and analyzed. The characterization of green and sintered body has been done with respect to density, porosity and microstructure.     

The role of plasticizer in optimizing the rheological behavior of ceramic pastes intended for stereolithography-based additive manufacturing

Adding plasticizer is an efficient way to regulate the rheological behavior of ceramic paste and quality of green body in stereolithography-based additive manufacturing. The type and content of plasticizers (polyethylene glycol (PEG) and dibutyl phthalate (DBP)) had substantial effects on the rheological behavior and solid loading of ceramic paste, leading to varied macro / micro structure and strength of the green and sintered parts. DBP significantly reduced the viscosity and increased solid loading, and could adjust the flexibility of the green body by reducing the crosslinking density of UV curing system. PEG could inhibit crack initiation to some extent, but it was less effective for preventing cracking than DBP on ceramic parts with large-sized cross sections. It was concluded that DBP was more suitable as a plasticizer in alumina paste for SL additive manufacturing to form dense parts without defects.     

3D gel printing of alumina ceramics followed by efficient multi-step liquid desiccant drying

Dense alumina ceramics were additively manufactured efficiently through a 3D gel printing process. Hydroxyethyl cellulose (HEC) was applied to ensure the printability and rigid of the gel made from boehmite. A multi-step liquid desiccant drying method was implemented to improve the drying efficiency. The results showed that the solid loading and HEC addition were two useful parameters for adjusting the rheology properties of the gel to make it suitable for 3D printing. With polyethylene glycol(PEG) added as liquid desiccants, the printed bodies with section size of 10 mm could be dried within 26 h during which the deformation and crack formation was avoided despite a high linear shrinkage of 45 % was encountered. The successful preparation of dense monolithic alumna ceramics parts with an average grain size of 1 μm, 99 % of the theoretical density and a flexural strength of 380 ± 45 MPa indicated the potential of this process.     

High-precision green densities of thick films and their correlation with powder,ink, and film properties

A precise geometrical method employing optical profilometry for green density measurements of thick films is presented that provides a typical reproducibility of 0.1–0.2% theoretical density (TD) and a measurement uncertainty of 0.2–0.4% TD for layer thicknesses of around 50 μm. The procedure can be applied for all thick films with a dried thickness of 10 μm or greater. In a case study, the green densities of screen-printed zirconia layers were investigated as a function of the starting powders (grain sizes from 0.1 to 0.4 μm), the solid content, the chain length of ethyl cellulose as binder and its concentration, and two different dispersants and their concentration. Rheological ink properties, surface roughness, drying stresses from deflection measurements, the mechanical properties of green films, and the equivalent compaction pressure were measured and correlated with the green density data. Compressive binder forces and lubrication effects dominated the packing of the particles.     

水基SiC浆料的喷雾造粒特性

以水为液体介质,制备分散稳定的水基SiC料浆,并利用喷雾造粒技术对水基SiC浆料进行造粒,研究了碳化硅浆料固含量及喷雾干燥工艺对造粒粉特性的影响。研究结果表明:浆料固相含量对造粒粉体粒径分布影响明显,粉体粒径随固含量的增加而增大。在最佳的干燥工艺条件下,碳化硅粉体流动性得到较大改善,素坯密度增加,陶瓷力学性能提高。     

Rapid Prototyping of Piezoelectric Ceramics via Selective Laser Sintering and Gelcasting

This article presents a new lost mold rapid prototyping method which combines selective laser sintering (SLS) and gelcasting techniques for fabricating piezoelectric ceramics. SLS was used to fabricate sacrificial molds of the desired structure of the ceramic part. Then aqueous PZT (lead zirconate titanate) suspension was cast in the mold and solidified in situ through formation of a three-dimensional network gel. Because the polymer mold can be easily removed at the initial stage of sintering and the gelcast PZT body has a high green strength, the desired geometry of the PZT part can be completely retained after sintering of the ceramics. Complex-shaped PZT parts were successfully fabricated after using concentrated PZT suspension with low viscosity. Densities and electrical properties, such as the d ₃₃, the relative permittivity ε, the dielectric loss tg δ and the electromechanical coupling factor K _p of the gelcast PZT parts were also compared with those of the die-pressed PZT samples. The results indicated that the gel-forming process did not deteriorate the electrical properties of the samples, if proper dispersant was selected in developing concentrated ceramic slurry.     

Tape casting of nanocrystalline ceria gadolinia powder

A ceramic ceria gadolinia solid solution membrane for solid oxide fuel cells was fabricated by tape casting using a nanopowder of 37 nm average particle size. A novel combination of solvent and dispersant was used to disperse the nanoparticles. The polymer was added in a dilute stage to guarantee a homogeneous distribution. After casting a remarkable densification of the cast tape suspension from a solid loading of 20 up to 42 vol.% was observed during drying. The green tape was sintered to >92% theoretical density and was dense towards perfusion. The resulting grain size in the sintered specimen still was <200 nm.     

A Physical Model for the Drying of Gelcast Ceramics

Gelcasting is a promising new technology for manufactur-ing advanced structural ceramic components. The process involves drying of the "green" gelcast part before densifi-cation. The physical mechanisms that control this relatively long drying process are not well understood. In this study, several controlled experiments have been performed to elu-cidate the key mechanisms. A one-dimensional drying model has been formulated, based on evaporation and gas-eous diffusion through the part. Experimental data have been used to obtain correlations for model parameters. This model predicts the instantaneous moisture content of a gelcast sample with an accuracy of better than 10% when the dryer humidity, dryer temperature, and sample thick-ness are specified.