Aqueous Ceramic Suspensions Research Articles

Carboxymethyl lignin as stabilizing agent in aqueous ceramic suspensions

Identifying new uses for residues of industries that process large quantities of biomass, as in bioethanol production, is essential for a sustainable development with reduced impact on the environment, which is the reason why many efforts have been devoted to find noble uses for lignins. In this study, a lignin obtained from sugarcane bagasse in a bioethanol producing plant was carboxymethylated to yield the water-soluble carboxymethyl lignin (CML), which was then used as stabilizing agent in aqueous alumina (Al2O3) suspensions. CML had a degree of substitution 0.46±0.01, in relation to the C9 unit of lignin, and behaved as a polyelectrolyte in a large pH range owing to the dissociation of carboxylic groups. The action of CML as stabilizing agent of alumina aqueous suspensions was investigated using viscometry, zeta potential, and photon correlation spectroscopy (PCS) measurements, mainly as a function of pH and time. Overall, the results showed that CML had a good performance as a deflocculating agent, because it led to dispersions with low viscosity and small change in particle size as a function of time. The positive effect from the addition of CML was confirmed in the morphological features of the material obtained from the alumina suspensions after elimination of water, as indicated by scanning electron microscopy. The stabilization of alumina suspensions afforded by CML opens the way for similar applications of modified lignins, whose electrical and structural properties may be tuned for specific uses in various industries, including the ceramic industry.

Comparison of micropatterning methods for ceramic surfaces

The fabrication of defined ceramic micropatterns smaller than 100 μm is due to the hardness and brittleness of ceramic materials still very challenging. However, in recent years, micropatterned ceramic surfaces have become highly interesting for biomedical applications or the fabrication of energy converting devices, such as solid oxide fuel or solar cells. In this study we evaluate six modern techniques for ceramic pattern fabrication with feature sizes ranging from 5 to 100 μm. Ceramic materials such as alumina, zirconia, silica and hydroxyapatite are discussed. Advantages and disadvantages for each process are highlighted and compared to the other techniques. Three of these techniques, namely microtransfer molding, modified micromolding and Aerosol-Jet ® printing generate patterns starting with aqueous ceramic suspensions. The other three techniques, micromachining and two different types of laser treatment produce micropatterns by material removal from solid ceramic substrates. The detailed analysis yields that properties such as the desired micropatterning size, shape or the production time are strongly dependant on the chosen technique.

Produção de cerâmicas celulares por emulsão seguida de gelificação

As cerâmicas celulares têm sido produzidas por diferentes métodos, resultando em uma ampla faixa de estruturas e propriedades. Este trabalho descreve uma nova rota para a produção de cerâmicas celulares, a qual se baseia na gelificação de emulsões constituídas de suspensões cerâmicas aquosas contendo monômeros orgânicos solúveis em água e uma fase líquida apolar emulsificada (querosene). Os efeitos do teor de sólidos e da fração de querosene nas propriedades reológicas das emulsões foram avaliados. A estrutura celular foi analisada por microscopia eletrônica de varredura e revelou poros isolados nas amostras de baixa porosidade, os quais mudaram para uma rede interconectada de poros com o aumento da porosidade. Compressão diametral foi usada para avaliar a resistência mecânica das amostras sinterizadas, a qual variou entre 2,0 e 48 MPa, dependendo da porosidade.

Red clay-based porous ceramic with pores created by yeast-based foaming technique

Red clay-based porous ceramic has been fabricated by using a biological foaming technique through reaction of yeast with starch in aqueous ceramic suspension. The starch plays a dual role as a growth substrate for gas bubble generation and a binder for ceramic particles. Starch-loaded ceramic slurries (55 wt% ceramic solid) were prepared, consisting of red clay, quartz, feldspar, and starch. Three types of starches were used, namely, cassava starch, corn starch, and rice flour. The rheological characterization of starch-loaded ceramic slurries showed shear thinning behavior. This resulted into a high foaming factor depending on the types of starches loaded and increased addition of yeast. Total fired porosities obtained were between 17 and 70%. These correspond to open porosities between 15% and 50%, and close porosities range from 6% to 35%. Microstructural investigation of the fired porous compacts showed irregular pore morphologies equipped with interconnecting channels that represent the open porosity.

The influence of ingredients of silica suspensions and laser exposure on UV curing behavior of aqueous ceramic suspensions in stereolithography

This paper focused on the influence of ingredients of silica suspensions and laser exposure on curing behavior of the highly concentrated aqueous silica suspensions. As a basic building unit, single cured lines were regarded as research objects to characterize the curing behavior of aqueous ceramic suspensions. The cured width and depth were characteristic parameters of single cured lines and measured with a digital optical microscope. The relationships between two characteristic parameters and ingredients of highly concentrated aqueous ceramic suspensions and laser exposure were investigated. The cured depth and width of single cured lines increased with the ceramic mean diameter and monomer concentrations. The cured width of single cured lines decreased with the solid content, but the cured depth increased with the solid content for silica suspensions. The cured depth and width of the single cured line all decreased with the laser scanning speed. The experimental results show that the ingredients of ceramic suspensions and laser exposure all have great influence on curing behavior of the highly concentrated silica suspensions, which indicates that the formula is an intrinsic factor on the curing behavior of ceramic suspensions and laser exposure is an exterior factor.

Macroporous Alumina Ceramics with Aligned Microporous Walls by Unidirectionally Freezing Foamed Aqueous Ceramic Suspensions

This paper reports a novel way of producing macroporous ceramics with aligned microporous walls by unidirectionally freezing foamed aqueous ceramic suspensions. The stability of the air bubbles introduced by magnetic stirring was improved remarkably using polyvinyl alcohol (PVA) as both a binder and emulsifying agent. Furthermore, the unidirectional freeze casting method could not only trap the air bubbles in the solidified sample successively but also induce the preferential growth of ice dendrites. This endowed the samples with uniform macropores surrounded by aligned microporous alumina walls after freeze drying and sintering. The sample produced with a PVA content of 8 wt% showed a high compressive strength of 4.6 ± 1.6 MPa at a high porosity of 91 vol%.

A novel aqueous ceramic suspension for ceramic stereolithography

Purpose – The purpose of this paper is to present a new ceramic suspension to fabricate complex ceramic parts by stereolithography (SL).

Low‐ and High‐temperature Processes and Mechanisms in the Preparation of Porous Ceramics via Starch Consolidation Casting

AbstractStarch consolidation casting (SCC) is a versatile and well established process for shaping ceramic green bodies by casting starch‐containing suspensions into impermeable molds. The process is based on the ability of starch to swell and absorb water from aqueous ceramic suspensions after heating to approx. 80°C. The swelling factor is highest for potato starch (3.7) and significantly lower for the other starch types (down to approx. 2). A simple mathematical model is presented that allows the minimum consolidation time to be estimated on the basis of the swelling kinetics of starch granules as measured via laser diffraction. The critical temperature range for starch burnout before sintering the ceramic is 300–500°C, as determined by thermal analysis (thermogravimetry and differential thermal analysis). In this temperature range gaseous products may ignite, leading to strongly exothermic thermal effects. If the bodies are large and the starch content high this may lead to defects in the (still unsintered) ceramic green bodies. Therefore this temperature range has to be carefully controlled in the production of ceramics prepared by SCC. The residual pores (remaining after starch burnout) are too large to be closed during the firing step (insufficient driving force for sintering due to too low surface curvature). Therefore they do not contribute to shrinkage, and shrinkage is determined by the ceramic matrix alone. Porosities higher than 50% (corresponding to the maximum nominal starch content) may be obtained by combining SCC with partial sintering (resulting in porosities up to 70%).

Processing and Properties of Graded Ceramic Filters

A new processing route for high‐performance graded ceramic filters is presented. The direct foaming process is based on the transition of a surfactant‐stabilized alkane dispersion in a stabilized aqueous ceramic powder suspension into mechanically stable ceramic foams with porosities up to 90%. The cell size distribution and, consequently, the permeability of the filters could be efficiently adjusted by the control of the high alkane phase‐emulsified suspension (HAPES) microstructure during emulsification. Furthermore, open porous graded structures are produced by combining HAPES layers with different droplet sizes. The tailored microstructural features, providing controlled permeabilities as well as high compressive strength of the cellular ceramics are of special interest for applications where fluid transport through ceramic microstructures is required. These include filters, in particular for aerosol filtration, temperature control membranes in autothermal reforming, catalytic supports including supports for batteries as well as matrices for immobilized microorganisms.

Direct fabrication of an integral ceramic mould by stereolithography

The purpose of this study was to develop a novel direct fabrication of an integral ceramic mould by stereolithography (SL). An aqueous ceramic suspension suitable for SL was prepared and its photosensitive parameters were measured to optimize its formula. For a thin-walled aerospace component, the whole process of designing, building, drying, binder removing, and sintering of an integral ceramic casting mould was carried out. Finally, to verify whether this casting mould could meet the requirements of the casting process, an aluminium casting part was cast with the anti-gravity casting method. Compared with traditional fabrication of a multi-layer ceramic shell for investment casting, this novel direct fabrication of an integral ceramic casting mould would meet the needs of not only research but also other small batch products with lower costs and shorter lead time.

New cellular ceramics from high alkane phase emulsified suspensions (HAPES)

Open porous mechanically stable ceramic foams are developed by a simple direct foaming process. The new processing route is based on the transition of a surfactant stabilized highly concentrated alkane phase homogeneously distributed in a stabilized aqueous ceramic powder suspension into high performance ceramic foams with porosities up to 90% and cell sizes ranging from 3 to 200 μm. The droplet size distribution of the high alkane phase emulsified suspensions (HAPES) is efficiently controlled by the stirring velocity during emulsification experimentally investigated for varying powder particle contents. Stable foams with tailored structural features can be prepared by adjusting the rheological characteristics of HAPES being dependent on the system and process parameters. The influence of the emulsification stirring velocity on the resulting HAPES droplet size is analysed on the basis of the Taylor model of mechanical shearing describing the stresses responsible for the fragmentation of the droplets.

Comb Polymer Architecture, Ionic Strength, and Particle Size Effects on the BaTiO3 Suspension Stability

We investigate the stability of aqueous barium titanate suspensions as a function of dispersant architecture, ionic strength, counterion valency, and particle size. Both pure polyelectrolytes, poly(acrylic acid) and poly(methacrylic acid) (PMAA), and comb polymer dispersants composed of a PMAA backbone with methoxy‐poly(ethylene oxide) (mPEO) teeth of varying molecular weights are studied. While each dispersant imparts stability to barium titanate suspensions at low ionic strength (<∼0.01M), only the PMAA–mPEO comb polymer with the longest teeth provides stability at higher ionic strengths independent of particle size and counterion valency. Our findings provide new insight into the design of comb polymer dispersants for stabilizing aqueous ceramic suspensions over a broad range of processing conditions.

Cellular Ceramics by Direct Foaming of Emulsified Ceramic Powder Suspensions

A new direct foaming method to produce macroporous cellular ceramics using surfactants as foam stabilizers is presented. The technology relies on the transition of a stabilized aqueous ceramic powder suspension containing a homogeneously dispersed alkane or air–alkane phase into cellular ceramics. The stabilization of the powder suspension and the emulsion is realized with particular emphasis on the interaction of both mechanisms providing enduring stability of the system up to high foaming degrees. Anionic, cationic, and nonionic surfactants were studied with their stabilization and foaming effects. The presence and influence of air bubbles was proved to be of negligible importance. Foaming is then provided by the evaporation of the emulsified alkane droplets, leading to the expansion of the emerging foam and giving rise to solids foams with cell sizes from 0.5 to 3 mm and porosities up to 97.5% after sintering. The microstructures of these filigree ceramics are stable and rigid with dense struts and uniform distributions of the solid phase and the porosity.

Production of Cellular Ceramics by Gel Casting Ceramic Emulsions

Cellular ceramics have been produced by several methods, giving a wide range of macrostructures and properties. This work describes a novel route to produce cellular ceramics based on the gel casting of emulsions consisting in an aqueous ceramic suspension containing water-soluble organic monomers and an emulsified insoluble liquid phase. The effects of solids loading and kerosene fraction on the rheological properties of emulsions were characterized. Samples with different kerosene additions (20, 30, 40 and 50 %vol.) were produced and their green and sintered densities were measured. The results have shown a good correlation between the volumetric fraction of kerosene into the emulsion and the porosity of samples. Diametrical compression was used to evaluate the strength of sintered samples, which varied with the porosity between 18 and 37 MPa. The cellular structure was analyzed by SEM and revealed isolated pores for samples with low porosity, which changed to an interconnected network of pores as the porosity increased.

Direct inkjet printing of Si 3N 4: Characterization of ink, green bodies and microstructure

Direct inkjet printing of aqueous ceramic suspensions with high solid content was carried out for generating ceramic layers as well as 3D-components. In this study, silicon nitride (LPS-Si 3N 4) was suspended with organic additives in an aqueous medium. Subsequently the suspension was adapted to this manufacturing technique concerning particle size and deflocculation. Thin layers and micro-scaled 3D-components, e.g. gearwheels and engineering parts, were generated and pressureless sintered. Mechanical properties (fracture toughness K Ic, hardness) and microstructure of printed LPS-Si 3N 4 were evaluated. Results show that dense structures with good mechanical properties were obtained. No delamination or other flaw-like textures were observed. The high potential for direct inkjet printing to manufacture high performance silicon nitride ceramics is thus demonstrated.

Estudo da produção de suspensões aquosas para serigrafia de anodos para PaCOS e introdução de modificadores de superfície

O presente trabalho tem como objetivo apresentar os resultados preliminares do desenvolvimento de suspensões cerâmicas aquosas para anodos de PaCOS. Estas suspensões foram utilizadas para a geração de eletrodos sobre eletrólitos planos de zircônia estabilizada com 8% mol de ítria (ZEI) em procedimentos compatíveis com a técnica de serigrafia, uma vez que a mesma leva a baixos custos de processamento e boas propriedades eletrocatalíticas dos componentes manufaturados dessa forma. Tais suspensões foram produzidas partindo-se de pós cerâmicos adquiridos comercialmente. Juntamente com coadjuvantes orgânicos de interesse, esse material foi processado através de moagem de alta energia, resultando em uma massa precursora da suspensão que, na fase final do procedimento, é hidratada com água destilada gerando uma pasta pronta para a deposição no eletrólito e posterior processamento cerâmico de queima e sinterização. Foi também estudada a formulação para anodo contendo, na água de intumescimento do polímero, íons precursores de óxidos com atividade eletrocatalítica (íons zircônio IV e ítrio III), visando a geração de uma estrutura auxiliar de ZEI após o processamento cerâmico, como uma maneira de otimizar a microestrutura do filme poroso formado. A utilização de dois passos de processamento, o primeiro de moagem a seco dos pós e o segundo de adição de solvente para hidratação (água), leva a uma vantagem de armazenamento, uma vez que a mistura precursora da suspensão pode ser mantida por longos períodos de tempo. A manufatura da pilha unitária para testes foi concluída pela utilização de suspensão comercial de manganita de lantânio dopada com estrôncio, utilizada como catodo. Análises da morfologia dos eletrodos fabricados, através de microscopia óptica e microscopia eletrônica de varredura, demonstraram a obtenção de estrutura porosa adequada e sua adesão ao substrato e testes de desempenho eletroquímico dos dispositivos prontos, evidenciaram uma melhora no desempenho, uma vez que houve aprimoramento do processamento com adição dos modificadores de superfície.

Influence of an Ultrasonic Treatment on the Structure and Flow Behaviour of Oxide Ceramic Masses

Abstract Oxide ceramic masses are used for catalysts and catalyst carriers. For a reliable processing hydrocolloids (e. g. cellulose) are usually added in order to decelerate demixing phenomena. Oxide ceramic masses react to simple shearing with hardening (peptisation: increase of the shear stress with the shear deformation) [1]. The present study analyses, if an ultrasonic treatment has also an impact on the structure, the shelf-life (in the green state), the correlated flow behaviour of oxide ceramic masses and presumably (not tested) the mechanical properties in the hardened, sintered state. The idea of using ultrasonic treatment is to change the microstructure (see below) and, therefore, to minimise or even give up the standard addition of stabilizers to minimize demixing in aqueous oxide ceramic suspensions. Besides the additional costs of an extra process unit, stabilisers cause often deteriorated mechanical properties (porosity, crack behaviour) of the ceramics in the hardened state after the sintering. Therefore, pump experiments (apparent viscosity), oscillatory (G´ and G´´) and steady shear experiments (h), particle-size analysis (particle-size distribution, agglomerate strength), light microscopy, decanting experiments and pH-determinations have been performed. The obtained results show, that the hardening of the apparent viscosity (derived from the flow) during pump experiments with simultaneous ultrasonic treatment in a flow cell is combined with an increase of the fine fraction, the formation of enlarged, but smoother agglomerates, the change of the pH-value and the evolution of a three-dimensional network (gelling). All these processes increase both the amount of bound/immobilised (chemically or physically bound by or onto the solid-surfaces) and of retained water (interior of agglomerates and/or the pores of the flowand ultrasonic-induced network). This means that the volume fraction of the rheologically "free" water decreases and simultaneously the effective solid volume fraction increases. With respect to the concept of the rheologically effective solid fraction this is combined with an increasing viscosity. At the same time the tendency of demixing decreases significantly. Thus, by an appropriate combination of shear flow and ultrasonic treatment, the aqueous oxide ceramic suspensions are stabilised and a reliable processing of the initially problematic solid/fluid mixtures can be realised without stabilisers (eluding their negative consequences with respect to the quality of the sintered state).

Spray drying and sintering of zirconia based hollow powders

A study has been performed on both the spray drying of ceramic slurries to determine how the characteristics of the spray dried powder can be affected by the process parameters and the sintering of these agglomerated powders. Two materials have been investigated: yttria partially stabilized ziconia (PSZ) and zirconium silicate. Indeed spray drying enables the fabrication of powder by aggregation of different small particles using an appropriate organic binder. The rapid heat and mass transfer which occurs during drying combined with the presence of various slurry compounds result in dried granules having a large variety of shapes, from uniform solid elongated, pancake, donut-shaped, needlelike or hollow granules. First experiments have been focused on the slurry parameters: dispersant amount, binder type and pH. The colloidal stability of aqueous ceramic suspensions as a function of poly(acrylic) acid content, pH and binder nature has been investigated. The addition of a binder may modify the state of dispersion. Thanks to sedimentation tests, stable (dispersed) and unstable (flocculated) suspensions were defined. Then, dried granules have been produced by means of a single droplet drying method. Individual slurry droplets were suspended using a hypodermic syringe in a stream of heated air. The aim was to assess the effect of the state of dispersion and sedimentation stability of the slurry on the drying of a single droplet. It has been shown that granules prepared from a dispersed slurry (low Ratio Sediment Height—RSH < 50% for PSZ) result in hollow shapes that contain a single large open pore. Changing the nature of the binder from latex to poly(vinyl) alcohol (PVA) has an effect on the wall thickness of the hollow granules. Conversely, granules obtained from a flocculated slurry (high RSH > 60% for PSZ) result in solid granules. The sedimentation volume, which represents the state of dispersion of the suspension, is the major factor controlling the droplet drying mechanism. According to the former results, powders have been elaborated by the spray drying process using several slurries formulations. Image analyses have been performed on the granules to determine their morphologies. The shape of the agglomerated powder was mostly spherical and hollow. These results confirmed the morphologies predictions of the drying test. Finally, the evolution of the morphology of the PSZ spray dried hollow powder after sintering in an oxyacetylene flame or an atmospheric furnace was investigated. When the powder is sintered in a furnace, low shrinkage and approximately no shape variations were noticed, whereas using an oxyacetylene flame tends to densify the powder.

Aqueous Synthesis at 200oC of Sub-10 Nanometer Yttria Tetragonally Stabilized Zirconia Using a Metal-Ligand Approach

The objective of this study was to produce well-dispersed nanosized yttria tetragonally stabilized zirconia (Y-TZP) powder in aqueous suspension. The Y-TZP powder was produced by precipitation from homogeneous solution at 200°C under hydrothermal conditions. A homogeneous solution was created through the use of a complexing agent, which subsequently could be used in a dispersion scheme developed for the nanosized Y-TZP powder. Characterization of the Y-TZP powder was performed using X-ray diffraction, Raman spectroscopy, X-ray fluorescence, and high-resolution transmission electron microscope. The overall goal of this study is the eventual production of bulk ceramics from well-dispersed nanosized aqueous ceramic suspensions via wet processing techniques.

Analytical Electron Microscopy Study of Green Ceramics Formed from Aqueous Suspensions Using the Hydrolysis-Assisted Solidification Process

During the hydrolysis-assisted solidification (HAS) of aqueous ceramic suspensions, aluminum hydroxides are formed that bind the ceramic particles into a stiff wet body. Transmission electron microscopy investigations of HAS-processed Al2O3 and ZrO2 green parts after drying revealed that the secondary phase is amorphous and distributed uniformly around the host ceramic particles. The estimated thickness of this layer was 3–5 nm. Moreover, areas of a few tens of nanometers in size were found at three-particle junctions that contained an amorphous phase and individual nanocrystals of boehmite.