Gelcasting Process Research Articles

High performance and dense Si3N4 ceramic prepared by gelcasting combined with reaction re-sintering method

In this study, Si powder was used as raw material, Si3N4 powder as a diluent, Al2O3, MgO, Y2O3, and La2O3 as sintering additives to prepare Si3N4 ceramic with excellent properties using gelcasting and reaction re-sintering method. When the solid content of the slurry is 57.3 %, the viscosity is less than 1 Pa · s during the gelcasting process. After the reaction re-sintering, the sintered body of Si3N4 exhibited a bulk density of 3.15 ± 0.05 g/cm3, a relative density of 96 ± 1 %, open porosity of 5.7 %, bending strength of 610 ± 20 MPa, elastic modulus of 235 ± 10 GPa, thermal conductivity of 48 ± 5 W/(m·K), and a linear shrinkage rate of 12–13 %. These parameters indicate that Si3N4 ceramic possesses outstanding mechanical properties and thermal conductivity.

Gel-casting of mesoporous silica electrolyte membranes loaded with phosphoric acid for protonic electrolyte cells

Proton conducting membranes that operate at temperatures above 100 °C are highly sought after for use in energy devices such as fuel cells owing to the high catalytic activity of electrodes induced by elevated temperature and no requirement for water management. In the present study, novel mesoporous silica membranes were prepared by a gel-casting process using commercial colloidal silica and subsequent sintering. These membranes served as the scaffold for phosphoric acid-based electrolytes. Well-interconnected mesopores with uniform pore sizes was effective for hosting phosphoric acid (PA), and the tunable pore sizes (5.9–32.0 nm) were achieved by tuning the sintering temperature from 500 to 725 °C. High porosity (about 50%) and high hydrophilicity of silica membranes are beneficial to reach high phosphoric acid loading by an impregnation process. The PA-loaded membrane sintered at 700 °C achieved a high protonic conductivity of 0.11 S cm−1 and a peak power density of 287.3 mW cm−2 in hydrogen fuel cells at 160 °C. The conductivity is 1 order of magnitude higher than that of the pressed meso-silica membranes and comparable to that of PA-mesoporous silica spheres. This study has demonstrated a new method to construct high-temperature protonic electrolyte membranes.

A novel fast fabrication of crystal-oriented hydroxyapatite ceramics via gel-casting in high magnetic field

Hydroxyapatite ceramics with oriented crystal structures are successfully fabricated via a novel and fast gel-casting process in high magnetic field for the first time. Experimental conditions as direction and duration of gel-casting in magnetic field were altered for selective crystal orientation of a,b-/c-plane of hydroxyapatites. The effect of slurry preparation process as ball-milling and freeze-drying on orientation degree and grain growth behavior of hydroxyapatite particles is elucidated with detailed analysis of the size distribution of particles, viscosity of slurry, microstructure and crystal structure of sintered bodies with assistance of SEM, XRD and advanced mapping observation of EBSD patterns. In addition, the influence of sintering temperature on the crystal phase transformation of the oriented outermost surface is also clarified through Raman spectroscopy in depth. It was found that the freeze-drying processed slurry shows potential in enhancing the crystal orientation degree than the ball-milling case, during where a suppressed grain growth phenomenon and inhibition of crystal phase transformation on highly oriented hydroxyapatite was also observed.

Research on the gel casting process and flexural strength of high-stability alumina ceramics

Gel casting is an advanced wet-forming method for ceramics. In this paper, Isoabm 104 is selected as the dispersant and gelation agent for gel casting. The optimization of gel casting slurry and the preparation of the green and sintered alumina bodies are investigated. TAC reduces the viscosity of the slurry significantly due to electrostatic interaction. Isopropanol alcohol and vacuum tank eliminates the slurry’s air bubbles, thus improving the density of green bodies. A staged drying process is necessary to produce smooth and defect-free green bodies. After optimization of the slurry formulation and gel casting process, alumina ceramics with a Weibull modulus of 11.1 are successfully prepared.

Preparation and properties of lightweight, conformal, and load‐bearing TiC/Si3N4 absorbing composites by gel casting

AbstractIn view of the lack of conformal, load‐bearing, light‐weight, and high temperature‐resistant integrated wave‐absorbing composites suitable for extremely complex conditions, the gel‐casting process was successfully applied to porous Si3N4 wave‐absorbing composites in this work. A universal low‐slurry for gel casting was prepared by using two dispersants acting together (tetramethylammonium hydroxide solution and sodium lauryl sulfate), which ensured the smooth completion of the gel casting process. Combining the thermogravimetric and differential thermal gravity curves of the composite green body, reasonable parameters of the rubber discharge process are given to avoid the problem of the green body swelling and cracking. The porous TiC/Si3N4 composites were successfully prepared by holding them at 1650°C for 3 h. The bending strength, density, minimum reflection loss, absorption bandwidth, and matched thickness in the P‐band of the composites with 30 wt.% TiC addition were 54.08 ± 3.02 MPa, 1.906 g/cm3, ‐37.75 dB, 1.9 GHz, and 4.1 mm, respectively. The electromagnetic wave loss mechanisms of the composites are a combination of conductivity loss, multiple reflections and scattering, interfacial polarization, and defect polarization.

A novel synthesis approach of Si2N2O based wave-transparent ceramics combined with a gel-casting process

A novel synthesis approach of Si2N2O based wave-transparent ceramics combined with a gel-casting process

Microstructure and properties of aluminum nitride ceramics prepared by aqueous gelcasting method with nontoxic curdlan as gelling agent

Aluminum nitride ceramics were prepared by aqueous gelcasting method and pressureless sintering technique in N2 atmosphere using Y2O3 as sintering additives with nontoxic curdlan as gel system. The solidification mechanism of curdlan was studied. The effects of curdlan content and solid content on the microstructure, relative density and flexural strength of green bodies were investigated. The influences of Y2O3 content and sintering soaking time on the microstructure and properties of sintered bodies were also studied. The results show that, as the temperature increases to 80 °C, the ceramic powders solidify through three-dimensional gel networks of curdlan during gelling process. The green bodies can be successfully fabricated through aqueous gelcasting method with modified powder as original materials. Suitable curdlan content and solid content contribute to fabricating green body with uniform microstructures and high flexural strength. The relative density and flexural strength of sintered bodies enhance as the Y2O3 content and soaking time increase. The flexural strength and thermal conductivity are about 107.5∼172.3 MPa and 75.2∼112.5 W/(m·K), respectively. The sintered body with 4 wt% Y2O3 soaking for 3 h exhibits the highest thermal conductivity because of appropriate relative density, uniform microstructure and reasonable intergranular phase distribution. The mechanical property and thermal conductivity of sintered bodies can be improved by optimizing the gelcasting process parameter, Y2O3 content, and soaking time. The nontoxic gelling system will have wide application for aqueous gelcasting ceramic with complex shape.

Fully ceramic microencapsulated fuels with high TRISO particles loading capacity fabricated by gel-casting

A novel gel-casting process for fabrication of the fully ceramic microencapsulated (FCM) fuel with high TRISO particles loading capacity was proposed. In the gel-casting process, silicon carbide (SiC) ceramic was used as matrix, and low toxic N-methylol acrylamide was used as organic monomer with appropriate proportion of cross-linking agent, dispersant, and initiator to achieve the strength of green bodies. FCM fuel pellets with high TRISO particles loading capacity were successfully prepared by SPS process. By using the gel-casting method, the volume fraction of TRISO particles could reach 57%, and the average distance between TRISO particles in a plane was 52.4 μm, which could avoid TRISO particles damage introduced by direct contact between the particles. The structure of SiC matrix was dense and complete, and there was a coherent contact between the matrix and TRISO particles, which ensured the performance and reliability of FCM fuel.

A REVIEW OF THE DEVELOPMENT OF THE GEL CASTING METHOD FOR POROUS CERAMIC FABRICATION

The objective of this paper is to present an overview of the development of gel casting methods on synthesis techniques and the characteristics of gel casting porous ceramics and its applications. The gel casting method is a colloidal ceramic synthesis method that requires a short period for the ceramic body to develop. This review provides the latest developments in the gel casting process of synthesizing porous ceramics. At the beginning of the gel casting method development, the acrylamide (AM) system was the most commonly utilized system. Over time, the use of the AM system began to be minimized because of its toxicity. Researchers began to develop natural polymers as pore templates and natural minerals as raw materials in the gel casting method. In addition to polymers and raw materials, the gel casting process also requires additives such as gelling agents and dispersants. At the end of this review, a mapping of how to select the additives in the gel casting process, for AM systems and non-AM systems for the fabrication of porous ceramics is presented. The characteristic of porous ceramics produced by AM and non-AM systems are almost similar. Porous ceramic applications have been used as bone tissue supports, energy storage, filters, and catalyst support. This review will be a reference for future research related to the process of fabrication of advanced porous ceramics using the environmentally friendly gel casting method.

Piezoelectric porous α-quartz membrane by aqueous gel-casting with enhanced antifouling and mechanical properties

Porous α-quartz ceramics with improved mechanical strength were fabricated by using silica sol as a bonding agent in an aqueous gel-casting process. α-quartz dispersion was prepared with a solid load of 75 wt% and a viscosity of 1.66 Pa·s. α-Al2O3 particles were added to promote dispersion of the bonding agent. The optimum amounts of amorphous silica and α-Al2O3 were 10 % and 8 % of the mass of α-quartz powder, respectively. The thermal treatment of α-quartz green bodies was optimized for mechanical and permeation properties. Electrical fields up to 1 kV/mm were applied to the green bodies to accomplish α-quartz grains orientation and hence piezoelectric response. This caused an increase of piezo-acoustic response by 58 % at an excitation frequency of 200 kHz. In-situ mitigation of fouling was obtained by applying an alternating voltage of 60 V, during TiO2 suspension filtration, which resulted in an increase of the stationary flux by 23 %.

Mullite Ceramics Derived from Fly Ash Powder by Using Albumin as an Organic Gelling Agent

Mullite is a combination compound of alumina (Al2O3) and silica (SiO2). During the last two decades, mullite ceramics have become the crucial oxide material for both traditional and advanced applications due to their favorable properties, such as good strength at very high temperatures, low density, good thermal shock resistance, and chemically stable. Mullite is also known for its stoichiometry 3Al2O3.2SiO2, or sometimes it is called 3/2 mullite. In this present investigation, the authors attempt to fabricate mullite-based ceramic through a gel casting process by using an organic binder (egg white) to consolidate powder particles, followed by low-temperature sintering. Fly ash powder, china clay powder, and alumina powder were used as raw materials to make mullite ceramic. Green bodies were fabricated by taking various proportions of fly ash, china clay, and alumina, followed by sintering at 1200C, 1250C, and 1300C for 2 hours. The stability of slurries was studied by measuring zeta potential, and green sample fracture surfaces were analyzed by Field Emission Scanning Electron Microscopy (FESEM). Physical properties of sintered samples, such as linear shrinkage, density, porosity, and water absorption, were also calculated. Evidence of mullite formation was characterized by Field Emission Scanning Electron Microscopy (FESEM scanning electron microscopy (SEM) techniques. The samples containing 45 wt.% fly ash, 15 wt.% china clay, and 40 wt.% aluminas showed the best physical properties compared to other batch compositions and were well supported by the results obtained from FESEM results.

Defect-free drying of large fine-particle zirconia compacts prepared by gelcasting method

In this work, drying of bodies prepared by gelcasting fine submicrometre-sized zirconia particles was studied and a drying process for defect-free bodies with large cross-sections was proposed. It was found that the cracking of large bodies could be prevented by reducing the monomer content and using appropriate non-volatile cosolvents. Glycerol and polyethylene glycols with different molecular weights were used as non-volatile cosolvents in aqueous ceramic suspensions. The complex effects of the individual cosolvents on the gelcasting process and, in particular, on the drying step were investigated and explained. The applicability of individual cosolvents for the gelcasting process were discussed and their optimal use was indicated.

The preparation and properties of high-strength porous mullite ceramics by a novel non-toxic gelcasting process

A novel approach to fabricate porous mullite ceramics with homogeneous pore size and high-strength using green non-toxic and cost-effective poly-γ-glutamic acid (γ-PGA) gelling system was reported for the first time. Effect of γ-PGA addition, additive amount and solid loading on rheological behavior of the slurries, and microstructure and properties of samples were investigated systematically. By optimizing the solid loading of mullite samples, we are able to get the sample with small pores (< 200 µm) dominating (93.3% of the total pores), and compressive strength of the sample reaches up to 26.62 MPa. In addition, the mullite ceramics exhibited high porosity of 75.7% with low thermal conductivity of 0.279 W/(m·K) at room temperature. This study not only provides a green and non-toxic gelling system but also offers porous mullite ceramics with low thermal conductivity and excellent mechanical strength as an energy-saving thermal insulation material.

Resorcinol formaldehyde hydrogel: Synthesis, polymerization, and application in ceramic gel-casting

In the present study, resorcinol formaldehyde (RF) hydrogel was successfully synthesized and applied in the gel-casting process. The polymerization of the gel system and the effects of catalysts on the mechanical properties of the ceramic preform were investigated. Changing the type of catalyst to control the gel reaction rate enabled the in situ “immobilization” of the B4C particles, thereby enabling the preparation of B4C/C porous preforms with a uniform structure and controllable pore size. The results revealed that using Na2CO3, having a high polarizing power Na+ and a high degree of hydrolysis CO32–, as a catalyst, the B4C/C porous preforms with low porosity and best mechanical properties can be obtained. Further, the flexural strength and compressive strength could reach 21.2 MPa and 52.2 MPa, respectively.

Trace addition of cellulose nanofiber in gel-casting system for structurally controlled porous ceramics towards superior thermal-insulating property

In present work, structurally controlled porous hydroxyapatite ceramics are fabricated via a novel and facile gel-casting process with trace addition of cellulose nanofiber (CNF). The pore structure with respects to pore size, shape and open/close porosity is selectively controlled by altering the addition amount of CNF and surfactant, as well as forming time. The origin of pore structure controlling is systemically investigated in the point view of chemical/physical interaction between ceramic powder, CNF as well as dispersant, assisted by detailed characterizations as X-ray computer tomography observation, viscosity analysis, and z-potential analysis. Fabricated porous ceramic bodies exhibit superior low thermal conductivity which opens a new avenue for the wide applications of hydroxyapatite ceramics.

Effect Of Sintering Process On Micro-structure And Properties Of Mullite Porous Ceramics Containing magnesium oxide

Mullite porous ceramics with high porosity and good mechanical properties were prepared by foaming method combined with gel casting process using kaolin and alumina as main raw materials and sodium polyoxyethylene ether sulfate as foaming agent. The effects of sintering process and magnesium oxide content on the microstructure and properties of mullite porous ceramics were investigated. The studies show that: With the addition of magnesium oxide, the amount of mullite produced and the compactness of the sample will increase. At this time, the microscopic morphology of mullite is needle-like and prismatic, and the mechanical properties are better. and the pore size distribution gradually tends to be between 0.1-0.2mm and 0.2-0.3mm. The addition of magnesium oxide, The amount of mullite phase increased, the densification degree of the sample increased, and the acicular and prismatic mullite phases with good mechanical properties were mainly formed, and the porosity of mullite porous ceramics decreased and the compressive strength increased. The pore size distribution gradually tends to be between 0.1-0.2mm and 0.2-0.3mm. As the sintering temperature increases, the porosity of mullite porous ceramics decreases and the compressive strength increases. With the extension of the holding time, the amount of mullite phase generated increases, the porosity remains basically unchanged, and the compressive strength increases.

Enhanced electrical property of graphite/Al2O3 composite fabricated by reductive sintering of gel-casted body using cross-linked epoxy polymer

In the present study, graphite/alumina composites are fabricated via reductive sintering of gel-casted green bodies with structurally controlled cross-linked epoxy polymers for the first time. The cross-linking degrees of polymers are tuned by the amount ratio of epoxy monomer/polyvinyl alcohol cross-linker utilized in gel-casting process. Superior electrical properties with respect to 5-fold enhanced electrical conductivity and 2-fold higher carrier mobility are successfully achieved in graphite/alumina composite fabricated from cross-linked epoxy polymer, whose phenomenon is attributed to the excellent conductive path in ceramic matrix established by highly uniform network with improved graphitization degree.

Determination of the formulation and curing conditions of thermosetting epoxy resins for optimizing their properties and future use in gelcasting process

AbstractThis work investigates the suitability of epoxy resins for possible use in an industrial gelcasting process. Two different formulations are studied using complementary experimental techniques. Both reactive formulations are based on a difunctional epoxy prepolymer (aliphatic BDGE or aromatic RDGE) mixed with a polyamine hardener (TEPA). The combination of analytical chemistry (NMR and FTIR spectroscopy) and calorimetric measurements (DSC) are used to determine the optimal epoxy/amine ratio for each reactive formulation. The gelation and vitrification times are investigated by dynamic rheometry through kinetic analyses over a large range of fixed temperature. It is found that the RDGE‐TEPA mixture is more reactive than BDGE‐TEPA with shorter curing starting times. The thermomechanical analyses performed on both optimized formulations show that the RDGE‐TEPA material presents better performances after curing (Tg close to 80°C) than the BDGE‐TEPA system (Tg lower than ambient temperature). Then, the effects of the cooling cycle on the thermomechanical profile of each polymer are investigated. The RDGE‐TEPA formulation is found to be much more subject to the formation of internal stresses created during the cross‐linking and cooling step than the BDGE‐TEPA material. Considering all these criteria, the BDGE‐TEPA system is ultimately more suitable for the gelcasting process than RDGE‐TEPA.

A Finite Element Analysis for Improvement of Shaping Process of Complex-Shaped Large-Size Silicon Carbide Mirrors.

In the gel-casting process, the proper selection of technological parameters is crucial for the final quality of a green body. In this work, the finite element method is used to investigate the mold characteristics in the gel-casting process, and the typical flow behaviors under different conditions are presented. Based on the distribution characteristics of temperature, pressure and flow field of gel polymer, the simulated results provide some possible reasons for the generation mechanisms of defects. Then, a series of simulations were performed to investigate the effect of process parameters on the molding quality of green gel-cast bodies. The results show that the decreasing loading speed can effectively reduce the number of defects and improve the molding quality. In addition, this paper presents a new technique by applying the exhaust hole to decrease the number of defects and, hence, improve structural integrity. The influence of the loading speed on the mold characteristics is well understood for the gating system with an exhaust hole, which suggests to us appropriate parameters for optimizing the molding design. This work provides a theoretical basis to explicate the generating mechanism of defects involved in the gel-casting process and acquires an optimized technique to produce a silicon carbide green body.

Transmittance enhancement of La0.4Gd1.6Zr2O7 transparent ceramic by aqueous AM gel-casting with pretreated powder

Via a powder pretreatment process, transparent La0.4Gd1.6Zr2O7 ceramics were successfully fabricated through the aqueous AM (acrylamide) gel-casting technique. Our research shows that the water absorption of La2O3 has significant influence on the viscosity of slurry, leading to low density of the green body and poor transparency of ceramic. By pretreating the mixed powder via a solid-state reaction, the viscosity and pH results have shown that the single-phase pretreated powder has a positive improvement on the gel-casting process. Compared with the mixed powder, the pretreating method has effectively improved the slurry with higher solid content and lower viscosity. Therefore, the pretreated powder combined with gel-casting method have a positive influence on the density of green body and the optical quality of obtained La0.4Gd1.6Zr2O7 ceramics. The polished ceramics with 40 mm in diameter (Ø = 40 mm) and 1.5 mm in thickness (δ = 1.5 mm) exhibited the in-line transmittance of 78% at 1100 nm and a maximum transmittance of about 80% in the infrared region, as well as a wide range of high stable transmittance among 0.6–6.0 µm wavelength. Our work opens a way for highly transparent ceramic preparation by aqueous AM gelling system with hygroscopic rare earth oxide as raw materials.