Flexural Strength Of Green Bodies Research Articles

Controllable preparation and properties of gel-casting porous mullite ceramics via lignin fiber assisted pore formation method

The preparation of high-strength porous mullite via gel-casting has emerged as a hot topic, but balancing high strength and high porosity remains challenging. Herein, we successfully prepared porous mullite with uniform pore size and high strength using natural lignin fibers as the pore-forming agent via gel-casting. Moreover, we investigated the properties of mullite slurries, as well as green bodies and sintered ceramics in details. The strong water transport capacity of the lignin fibers enabled the homogeneous dispersion of gel system and improved the flowability of slurry. The flexural strength of green body was significantly increased to 16.07 MPa by optimizing the lignin fiber content. Porous mullite ceramics sintered at 1400 °C exhibited a homogeneous microstructure with average pore size of 5.62 μm. Furthermore, mullite ceramics had a porosity of 64.73% at a total solid loading of 55 wt% and a lignin fiber content of 10 wt%, whereas the compressive strength remains high at 11.32 MPa. Permeability tests confirmed the presence of interconnected pores, its coefficients k1 and k2 increases with the total porosity of ceramics. This work provides new insights into designing the preparation of porous mullite ceramics with high performance.

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.

Investigation of gelation mechanism and the effects of APS on AlN slurries with Isobam

A novel and simple gelling system based on a water-soluble copolymer of isobutylene and maleic anhydride (commercial name, Isobam) was adopted to manufacture various ceramics via spontaneous gelation. The gelation mechanism of AlN slurry was investigated using infrared spectroscopy. Results show that Isobam was hydrolyzed to produce the –COOH group, and the hydrogen bonding was formed between the –COOH and –CONH2 groups of Isobam during spontaneous gelation. Meanwhile, the gelling process of the Isobam system could be accelerated via enhancing hydrogen bonding through APS addition. With 1 wt% addition of APS, the gelation time reduced to 23 h, the viscosity of 50 vol% AlN slurry decreased to 0.21 Pa·s, and the flexural strength of green body improved by 48%. Therefore, the presence of hydrogen bonding in the Isobam gelation mechanism was confirmed, and it could be enhanced by introducing APS to accelerate gelation.

Effect of Processing Parameters on Gelcasting Process for Mo/Cu Powders

Abstract A non-aqueous gelcasting system of 2-hydroxyethyl methacrylate (HEMA)-1,6-hexanediol diacrylate (HDDA) was adopted to prepare concentrated slurry of Mo/Cu powders. The effects of dispersant dosage, monomer content and solid loading on rheological behavior of slurry were studied. The effects of processing parameters such as monomer content, monomer/cross-linker ratio, initiator dosage, and temperature on the gelation behavior and green bodies' flexural strength were studied. The results show that solid loading influences the viscosity of slurry the most, followed by dispersant dosage and monomer content. The flexural strength of green bodies is increased with the increasing of monomer content and the decreasing of monomer/cross-linker ratio, while the initiator dosage has a slightly effect. On the basis of them, the optimum processing parameters for Mo/Cu non-aqueous gelcasting are as follows: HEMA content is 25 vol%∼30 vol%, monomer/cross-linker ratio is 10:1∼15:1, initiator dosage is 1.5 vol%∼2.5 vol%, and gelation temperature is 60∼80 °C.

Microstructure and properties of SiCw/SiC composites prepared by gel-casting combined with precursor infiltration and pyrolysis

Near net-shaped SiCw/SiC composites with higher fracture toughness and isotropic mechanical properties were successfully fabricated by gel-casting combined with precursor infiltration and pyrolysis (PIP). In the gel-casting process, the effects of dispersant dosage, pH value, and solid loading on the rheology of slurries were investigated in detail. A well-dispersed slurry was obtained when the pH was 11 and dispersant dosage was 1.2wt%. As the solid loading increased from 28 to 40vol%, the flexural strength of green bodies and SiCw/SiC composites increased from 62.34 and 260.38 to 80.21MPa and 308.29MPa, respectively, and the fracture toughness of SiCw/SiC composites increased from 4.56 to 5.32MPam1/2. However, when the solid loading reached 44vol%, uneven pores were observed in the green bodies. The highest density and mechanical properties of composites were obtained when the solid loading was 40vol%. The toughening mechanisms involved were whisker bridging, whisker pull-out and crack deflection.

Effect of particle size on non-aqueous gel-casting process for copper powder

In this article, Cu-based materials were prepared by non-aqueous gel casting. The influence of different size particles on the rheological properties in gel system and the microstructure of green bodies and sintered bodies were studied. The results show that with the increase in particle size, the apparent viscosity in gel system decreased, the inhomogeneity of green body increased, and the flexural strength of green body and the relative density of sintered body reduced. The higher flexural strength (39.93 MPa) of green body was attained from the sample with 29-μm Cu powder, which can completely meet the needs of machining.

New Low-Toxic PEG(200)DMA/AM Gel System for 3YSZ Gel-Casting

Gel-casting is promising in fabricating uniform and complex-shaped ceramic body, but it is confined in industrial application for the neurotoxic acrylamid (AM) gel system. Ether-contained oligomer poly(ethylene glycol)(200) dimethacrylate (PEG(200)DMA) with methacrylate end-group on either side of the long chain can act as cross-linking agent in free radical polymerization to form branchy gel network structure, which improves the green body strength. In this study, low-toxic PEG(200)DMA was chosen as the main binder and crosslinker and small dosage AM was applied to adjust the viscosity of the ceramic slurry to gel-cast 3YSZ (PEG(200)DMA: AM = 4:1, mass ratio). Influence of monomer concentration, dispersant concentration and solid loading on the rheological behavior of the slurry and the flexural strength of the green body were investigated using aqueous PEG(200)DMA/AM gel system to gel-cast 3YSZ. The results show that the flexural strength of green bodies notably increases with the increasing of monomer concentration and reaches high up to 14.4 MPa when gel-cast 3YSZ ceramics using PEG(200)DMA/AM gel system with appropriate amount of PAA-NH4, monomer and solid loading. This gel system obviously reduces the toxicity of the conventional AM system and, at the same time, ensures high flexural strength of green bodies.

A new aqueous PEGDMA/AM system for 3YSZ gelcasting

Poly(ethylene glycol)(400)dimethacrylate (PEG(400)DMA) was used as main binder and cross-linker to gelcast 3YSZ, while acrylamid (AM) as minor monomer. Influences of PEG(400)DMA and dispersant content on rheological property (including zeta potential and viscosity) of the slurry as well as on the flexural strength of green body were discussed. Effect of PEG(400)DMA on surface exfoliation of green body gelcast in air was also investigated. Zeta potential of the slurry was significantly affected by PEGDMA after substituting PEGDMA for AM. Improved rheological property and flexural strength of green body were obtained by adding 1.35wt% dispersant to slurry. Flexural strength of green body prepared by 45vol% solid loading slurry reached 18.7MPa at 6.5wt% monomer concentration. Surface exfoliation of green body was eliminated when slurry was solidified in air at 70°C with 3.5wt% PEG(400)DMA/AM in slurry.

Aqueous gelcasting of silica ceramics using DMAA

A low-toxicity and water-soluble monomer N, N-dimethylacrylamide (DMAA) was used as gelling agent in gelcasting of silica ceramics. In order to prepare high solid loading and low viscosity gelcasting slurries, effects of various parameters such as zeta potential, pH value, dispersant dosage and solid loading on rheological behaviors of the suspensions were investigated. The results suggest that the best conditions are pH 7.7 and dispersant dosage 0.1 wt%. The maximum flexural strength of green bodies with only 2 wt% organics in the slurry could be up to 15.3 MPa. The sintering shrinkage, bulk density, coefficient of linear expansion and flexural strength of sintered ceramics are 5.2%, 1.95 g/cm3, 0.58×10−6 k−1 (200 °C) and 67.4 MPa, respectively. The dielectric constant and loss (1 MHz) are 3.26 and 7.8×10−4, respectively.

Machining Behavior of Green RBSC Body Prepared by Gel-Casting

In this paper, the green RBSC was prepared by gel casting process. The influences of the monomer content (MBAM and AM) and monomer ratio (MBAM to AM) on the flexural strength were investigated. The results indicated the flexural strength of green body was up to 8MPa and the micro- structures were homogeneous. The green RBSC bodies were machined by CNC grinding machine with diamond-coated grinding tools. Machining tests showed the green component could be machined without appended defects. It was found that green machining as a ‘top-down’ approach was a potential alternative for the rapid fabrication of complex-shaped ceramics, especially for some one-of-a-kind products.

Preparation of RBSC Green Bodies by Gel-Casting

In this paper, green bodies of RBSC were prepared through the gel-casting process. The effects of monomer concentration and ratio of MBAM to AM on the strength and microstructure of the green bodies were studied. When the monomer content increased from 10wt% to 20wt %( relative to water in the slurry), the flexural strength increases efficiency. However, the flexural strength decreases when the monomer content was above 20wt%. Similarly, when the ratio of MBAM to AM increased from 1/19 to 1/9, it had the same effects on the flexural strength of green bodies. It was found that silicon carbide particles were packaged by the carbon particles with gel. When the monomer content was 20wt% and the ratio of MBAM to AM was 1/9, the green body showed the best homogeneous microstructures and its flexural strength achieved 8MPa.

Processing of High Solid Content Hydroxyapatite Slurries and Gel-Casting

An aqueous gel-casting of hydroxyapatite using a new polymeric system was investigated. The monomer, cross-linker, and initiator were dimethylacrylamide (DMAA), N,N′-methylenebisacrylamide (MBAM), and 2,2′-azobis[2-(2-imidazolin-2-yl)propane] (AZIP, respectively). The rheological properties of the gel-casting slurries and influencing factors were studied. The relative density, compressive strength, and flexural strength of green body were 57.5%, (37.0±6.5), and (13.8±1.0) MPa, respectively. After presureless sintering at 1250 ℃ for 2 h, the relative density of the final product is about 98.7%. Bending strength and elasticity modulus are (85.3±9.9) MPa and (133.0±10.2) GPa, respectively.

Gelcasting of Alumina Ceramic in Presence of Water-Soluble Polymer

A new ceramic gelcasting system was developed in this paper. The rheological behavior of alumina suspension with an addition of polymer was examined. Particular attention is given to the flexural strength of green body prepared from gelcasting in the presence of polymer.

Gelcasting of alumina with a mixed PVP–MAM system

A common and fatal problem of the low-toxicity gelcasting is the low flexural strength of green bodies. The objective of this study was to solve the problem by introducing water-soluble polyvinylpyrrolidone (PVP) polymer to the monomer system. The low-toxicity monomer and ceramic powder were selected as methacrylamide (MAM) and alumina, respectively. Flexural strength of green bodies was improved nearly by 30%, with no significant change in the property of sintered bodies when the addition amount of PVP is 2.8 wt.%. Influence of PVP content on rheological properties of alumina suspensions, their gelation time, and the microstructure of green and sintered bodies were investigated.

Gelcasting of alumina ceramic in mixed PVP–HEMA systems

AbstractAqueous gelcasting with a low toxicity monomer system has attracted increasing attention. The present study attempts to enhance the flexural strength of green bodies gelcast from a low toxicity hydroxyethyl methacrylate (HEMA) system by introducing an appropriate amount of the water soluble polymer polyvinylpyrrolidone (PVP). The rheological properties and gelling characteristics of PVP–HEMA/Al2O3 suspensions were examined. In addition, the flexural strength and microstructure of the gelcast green and sintered bodies are discussed.

Gelcasting of ceramic suspension in acrylamide/polyethylene glycol systems

A new polyethylene glycol/acrylamide-based gelcasting system has been developed. The new system performs at least as well as, and in some cases better than, the original acrylamide-based system. The development of this system is for the sake of eliminating the surface-exfoliation phenomenon of green bodies gelcast in air. This study concentrates attention on dispersion, rheological and gelation behavior in the new system and flexural strength of green body.