Alumina gelcasting by using HEMA system

A low-toxicity gel system based on the polymerization of low-toxicity 2-hydroxyethyl methacrylate (HEMA) was successfully developed. In order to obtain high solid loading ceramic slurry with low viscosities, a polyelectrolyte dispersant was selected. The results based on alumina suggested that the polyelectrolyte dispersant was more effective for the new HEMA system than tri-ammonium citrate (TAC), which was widely used for dispersing alumina powders. The green alumina bodies derived through the HEMA exhibited a mechanical strength as high as 18 MPa. SEM photos revealed that the green body also had a high homogeneity. Dense complex-shaped ceramic parts were produced through the new gelcasting system.     

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.     

Gelcasting of silicon carbide ceramics using phenolic resin and furfuryl alcohol as the gel former

A new non-aqueous gelcasting system of phenolic resin and furfuryl alcohol combined with a curing catalyst was developed for casting of reaction bonded silicon carbide ceramics. This gelling system could be carried out in air, and the surface exfoliation phenomenon that seems inherent to the acrylamide gelcasting system could also be eliminated. Polymerization of the premix solutions and rheological properties of the non-aqueous silicon carbide suspensions were studied. After curing and subsequent pyrolysis of the concentrated silicon carbide suspension, homogenous silicon carbide/carbon green body with a relatively high strength of about 18 MPa could be formed. Dense complex-shaped SiC ceramic parts with flexure strength of 300±20 MPa and fracture toughness of 3.87±0.19 MPa m^1/2 can be successfully produced after reaction sintering at 1700 °C for 30 min under vacuum.     

α-Al2O3悬浮体的流变性及凝胶注模成型工艺的研究

陶瓷凝胶注模型工艺是一种新颖的成型工艺,具有很好的应用前景。它将高分子化学单体聚合的思路引入到陶瓷的成型工艺中,可制备高强度、高均匀性的陶瓷坯体。     

Elimination of Surface Spallation of Alumina Green Bodies Prepared by Acrylamide-Based Gelcasting via Poly(vinylpyrrolidone)

Surface spallation of gelcast alumina green bodies due to the exposure of the surface to air during gelation was successfully eliminated by adding a proper amount of poly(vinylpyrrolidone) (PVP) in aqueous acrylamide premix solution. The influences of PVP on the colloidal characteristics of alumina powder in aqueous solutions, the rheological properties of alumina slurries, the gelation process, and the properties and microstructures of the gelcast green bodies and the sintered samples were systematically investigated. Even though the addition of PVP to premix solution has slightly negative effects on the viscosity of alumina slurries, gelation and strength of green bodies, it has no obvious influence on the final structure and properties of alumina ceramics. Thus, PVP was considered an effective polymer to resolve the problem of the inhibition of gelation due to the presence of oxygen, which usually occurs in gelcasting during the formation of polymer network gel by in situ polymerization of monomers in air. The mechanism of PVP in eliminating the surface spallation problems of green bodies is also discussed.     

巯基链转移聚合法制备超支化聚甲基丙烯酸-β-羟乙酯

设计并合成新型含被保护巯基的苯乙烯型AB单体,并以其为支化单体,甲基丙烯酸-β-羟乙酯(HEMA)为聚合主单体,DMSO为聚合溶剂,AIBN为引发剂,自由基聚合法制备可溶于水的超支化聚甲基丙烯酸-β-羟乙酯(PHEMA),同时建立一种新型超支化聚合法——巯基链转移超支化聚合法。用凝胶渗透色谱、1HNMR、DSC等对AB支化单体、PHEMA进行一系列结构和性能表征。实验结果表明,随着AB支化单体加入量的增加,聚合物的重均分子量Mw逐渐增加,而其Tg则呈下降趋势,在水中溶解度增加,说明聚合物发生超支化。     

Gelcasting of alumina-chitosan beads

Several papers have reported the advantageous combination of chitosan and ceramic particles for such applications as biomimetic scaffolds, membranes, pollution remediation and gelcasting complex shapes. This work presents a novel gelcasting consolidation mechanism, based on the effects of pH changes on chitosan solubility and zeta potential of alumina particles. Unlike other chitosan-based gelcasting methods, it employs a small content of organic material (lower than 3 wt%) and does not require crosslinking agents (such as glutaraldehyde). With this new method alumina beads with 0.5-1 mm diameter could be produced, whose porosity and specific surface area could be tuned for various applications.     

Microtransfer Molding of Gelcasting Suspensions to Fabricate Barrier Ribs for Plasma Display Panel

We have developed a new fabrication technique for the barrier ribs of a plasma display panel (PDP). The rib structure was formed on a glass plate by microtransfer molding a gelcasting suspension with a flexible soft mold. A well–dispersed gelcasting suspension of the glass frit was placed on the photolithographically patterned mold, followed by gelation and drying while pressed with a top glass plate. The rib structure replicated by micromolding was significantly influenced by the gelation kinetics, the green strength of the gelled body, and the mold release process. It was demonstrated that the box–type array of 1176 wells in the patterned area of 6.5 cm × 6.5 cm could be produced using the current method.     

A novel colloidal processing route to alumina ceramics

The fabrication of complex-shaped alumina ceramics following a new near-net shape technique based on hydrolysis induced aqueous gelcasting (GCHAS) is reported in this paper. Aqueous suspension containing 50 vol.% solids loading was prepared by dispersing alumina in an aqueous solution of methacrylamide and methlynebisacrylamide (17 wt.% in 6:1) using polycarboxylic acid as dispersing agent. Consolidation was accomplished by adding a polymerization initiator, a catalyst and AlN powder (4 wt.%). For comparison purposes, alumina ceramics were also consolidated by aqueous gelcasting (GC) and by hydrolysis assisted solidification (HAS) from the same concentrated suspensions, and by conventional dry pressing (DP) from freeze dried granules prepared from the same suspensions by freeze granulation. Among the four shaping techniques used, GCHAS was found to be best for consolidating near-net shape alumina components like thin wall radomes with the highest green strength ever reported for alumina ceramics. Green samples were sintered for 2 h at 1600 °C and then characterized for microstructure and mechanical properties.     

2-carboxyethyl acrylate as a new monomer preventing negative effect of oxygen inhibition in gelcasting of alumina

The paper presents the application of 2-carboxyethyl acrylate as a new monomer in gelcasting of alumina. The research showed that the use of 2-carboxyethyl has a positive influence on the properties of green bodies, limiting the negative effect of oxygen inhibition. Oxygen inhibition which hampers polymerization reaction is a well known negative phenomenon in gelcasting but still not fully overcome for samples formed in air atmosphere. As a reference monomer 2-hydroxyethyl acrylate was used in the research. Rheological behavior of alumina suspensions containing both monomers has been studied, as well as the properties of green and sintered bodies obtained by gelcasting have been measured. The differences in the rheological properties and thus ceramic-monomer interactions in the slurries has been analyzed. The high values of densities and mechanical strength, accompanied with the pictures of microstructures of sintered ceramic parts have been presented, as a result of a successfully completed gelcasting process of alumina powder.     

Synthesis of well‐defined responsive membranes with fixable solvent responsiveness

A versatile method is described to synthesize a new family of solvent‐responsive membranes whose response states can be not only tunable but also fixable via ultraviolet (UV) irradiation induced crosslinking. The atom transfer radical polymerization (ATRP) initiator 2‐bromoisobutyryl bromide was first immobilized on the poly(ethylene terephthalate) (PET) track‐etched membrane followed by room‐temperature ATRP grafting of poly(2‐hydroxyethyl methacrylate) (PHEMA) and poly(2‐hydroxyethyl methacrylate‐co‐2‐(dimethylamino)ethyl methacrylate) (P(HEMA‐co‐DMAEMA)) respectively. The hydroxyl groups of PHEMA were further reacted with cinnamoyl chloride (a photosensitive monomer) to obtain photo‐crosslinkable PET‐g‐PHEMA/CA membrane and PET‐g‐P(HEMA/CA‐co‐DMAEMA) membrane. The length of grafted polymer chains was controllable by varying the polymerization time. X‐ray photoelectron spectroscopy, Fourier transform infrared spectroscopy in attenuated total reflection and thermogravimetric analysis were employed to characterize the resulting membranes. The various membrane surface morphologies resulting from different states of the grafted chains in water and dimethylformamide were characterized by scanning electron microscopy. It was demonstrated that the grafted P(HEMA/CA‐co‐DMAEMA) chains had more pronounced solvent responsivity than the grafted PHEMA/CA chains. The surface morphologies of the grafted membranes could be adjusted using different solvents and fixed by UV irradiation crosslinking. © 2014 Society of Chemical Industry     

Gelcasting of alumina suspension using gellan gum as gelling agent

As one of the excellent colloidal ceramic forming methods for fabricating complex shaped ceramic components, gelcasting has been extensively investigated in the past two decades. In this article, a novel nontoxic system for gelcasting of ceramics was investigated using gellan gum as gelling agent. The rheological and gelling properties of gellan gum solutions with different types of cations and different concentrations of divalent magnesium ions were studied. Influences of concentration of gellan gum and solid loading of alumina suspension on the rheological properties and gelling properties of alumina suspension were investigated. The rheological properties of concentrated alumina suspension mixing with gellan gum and magnesium chloride were evaluated. Dense ceramics prepared by gellan gum gelation showed homogeneous microstructure and good mechanical properties.     

Effect of Polyvinylpyrrolidone Additions on the Rheology of Aqueous,Highly Loaded Alumina Suspensions

The control of the rheological behavior of highly loaded ceramic/polymer suspensions affords the development of near‐net shape forming techniques. In this study, suspensions containing sub‐micrometer diameter alumina (up to 56 vol%) were fabricated using an anionic dispersant (≈4 vol%) and water‐soluble polyvinylpyrrolidone (PVP). The amount and ratio of molecular weights of PVP in the suspension were varied to influence flow behavior. The final pH of the system, ≈9.5, was higher than the isoelectric point (IEP) of alumina implying that the alumina powder possesses a negative surface charge. In the case of alumina at this pH, PVP does not adsorb onto the surface of the powder. The flow behavior of the PVP‐containing suspensions displayed yield‐pseudoplastic characteristics that closely agreed with the Herschel–Bulkley fluid model. The addition of PVP significantly changed the rheology of the system, increasing the shear yield stress and altering flow behavior. Interparticle interaction approximations of the suspensions were modeled to correlate with experimental observations.     

Use of HEMA in Gelcasting of Ceramics: A Case Study on Fused Silica

Gelcasting is a useful method for producing high-strength ceramic green parts. 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 material using a low-toxicity gel, 2-hydroxyethyl methacrylate (HEMA). The HEMA-based aqueous system was found to perform as well as AM-based systems in terms of the properties of gelcasting suspensions, cast green parts, and densified components. The new gelcasting system should be readily applicable to other ceramic materials.     

Poly(Vinylpyrrolidone)-Embedded Bismuth Ferrite—Poly(Vinylidene Fluoride-co-Hexafluoropropylene) Composites with Enhanced Dielectric Properties

The dielectric properties of poly(vinylpyrrolidone) (PVP)-modified bismuth ferrite (BFO) particles in the poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP) matrix were prepared through solution casting techniques. The composites showed enhanced dielectric constant (90) and reduced dielectric loss (<0.5) at 40?wt% of PVP-modified BFO particles. The dielectric constant of the resultant composites with PVP-modified BFO was much higher as compared to that of unmodified BFO-P(VDF-HFP) composites. The maximum remnant polarization reached (2Pr?～?1.12?µC?cm^?2) in the PVP-modified BFO-P(VDF-HFP) composites. The demonstrated approach might provide a route for using PVP-modified BFO particles to enhance the dielectric and ferroelectric performance of the composites.     

Producing Large Complex‐Shaped Ceramic Particle Stabilized Foams

Highly porous ceramic foams can be produced by combining particle stabilized foams and gelcasting concepts. Sulfonate‐type surfactants are selected to weakly hydrophobize the alumina surface and stabilize air bubbles in suspensions containing gelcasting additives, polyvinyl alcohol (PVA), and 2,5‐dimethoxy‐2,5‐dihydrofurane (DHF). The aim of this work was to prepare large complex‐shaped ceramic foam objects with homogeneous microstructure and high porosity. A key to avoiding drying cracks is to strengthen the wet green body via gelcasting. The influence of the amount of gelcasting additives on the mechanical strength of the ceramic foam green bodies is investigated as well as the effect of using cross‐linking agent versus the addition of just a binder. The presence of a cross‐linked polymeric network within the green body increases its mechanical strength and minimizes crack formation during drying.     

Adsorption of trypsin onto poly(2-hydroxyethyl methacrylate)/polystyrene composite microspheres and its enzymatic activity

Poly(2-hydroxyethyl methacrylate)/polystyrene (PHEMA/PS) composite microspheres were produced by emulsifier-free seeded emulsion polymerization for styrene in the presence of PHEMA seed particles. Effects of the surface characteristics of the PHEMA/PS composite microspheres on the adsorption immobilization of trypsin and on its enzymatic activity were discussed. Above 5 mol% of HEMA content, trypsin molecules adsorbed had high activity, 65–100% of the activity of free trypsin. The excellence of the composite microspheres as a carrier for trypsin seems to be closely related with the surface heterogeneity consisting of both hydrophilic and hydrophobic parts.     

Preparation of low-shrinkage and high-performance alumina ceramics via incorporation of pre-sintered alumina powder based on Isobam gelcasting

Mechanically robust alumina ceramics with low shrinkage were successfully prepared via gelcasting using Isobam as the gelling agent and pre-sintered alumina powder as the raw material for the first time. The influence of amount of pre-sintered alumina powder and powder's pre-sintering temperature on the viscosity of suspension and mechanical properties of low-shrinkage alumina ceramics were systematically investigated. The results showed that low-shrinkage alumina ceramics with flexural strength of 294.5?±?33.0?MPa, linear shrinkage of 7.79% and relative density of 91.5% can be obtained with the addition of 60?wt% alumina powder pre-sintered at 1600?°C. This convenient and facile approach is promising to fabricate low-shrinkage alumina ceramics with large sizes, high dimensional accuracy and complex shapes.     

Gas chromatographic determination of pyrolysis products from poly(2-hydroxyethyl methacrylate)

Summary Compared to poly(methyl methacrylate) (PMMA) or other poly(n-alkyl methacrylates), the thermal degradation of poly(2-hydroxyethyl methacrylate) (PHEMA) is rather complex, whereby the generated monomer (HEMA) is accompanied by a variety of other pyrolysis products. Besides HEMA, the dominating compounds were methacrylic acid and acetaldehyde, which could be established by gas chromatography.     

N-羟甲基丙烯酰胺聚合体系注凝成型纳米氮化硅陶瓷

注凝成型工艺中通常使用的单体丙烯酰胺是一种神经毒素,它阻止了注凝成型工艺的进一步发展,为此,通过实验研究了一种廉价、低毒N-羟甲基丙烯酰胺单体注凝成型体系及该体系形成凝胶时的用量、温度、预混液的pH值等对凝胶时间和凝胶强度的影响.结果表明:N-羟甲基丙烯酰胺聚合体系在无交联剂时仍能形成凝胶;在固相体积分数为40%纳米氮化硅陶瓷粉体的料浆中仅需质量分数的3%的N-羟甲基丙烯酰胺单体即可通过注凝成型得到可用于机械加工的坯体,完全可以代替丙烯酰胺应用于注凝成型工艺中.