Fernando dos Santos Ortega

Alternative gelling agents for the gelcasting of ceramic foams

Abstract In recent years, a new class of ceramic foams with porosity levels up to 90% was developed as a result of the association of the gelcasting process to the aeration of a suspension containing foaming agents. The gelation of foamed suspensions results from the in situ polymerization of water-soluble monomers. Nearly spherical pores and highly dense struts characterize the structure of these foams, which results in unprecedented properties, such as high permeability, low thermal conductivity, high specific surface area and high mechanical strength. The main drawbacks of this process are the inhibition of the polymerization in the presence of oxygen and the toxicity of the monomers. This work investigates two harmless alternative gelling agents that do not require atmosphere control to set foamed suspensions. The first route consists in the crosslinking of a previously dissolved polyvinyl-alcohol with the addition of an organotitanate. The other approach assessed the use of gelatin as a setting agent. This polymer gels the suspension due to changes in the structure of polypeptidic chains induced by temperature reduction. Gelatin-based systems presented higher storage modulus (G′) than the systems with crosslinked polyvinyl-alcohol chains. This characteristic prevented the formation of cracks during drying of gelatin-based samples, which constituted a limitation of PVAl-based systems. The results point out gelatin as a promising gelling agent to produce ceramic foams without the disadvantages of monomeric systems, such as the toxicity and the necessity of atmosphere control.

Monomer systems for the gelcasting of foams

The association of the gelcasting process with the aeration of ceramic suspensions allowed the development of a novel category of porous ceramics with unprecedented mechanical properties. One of the critical points in the processing of porous ceramics by this route involves the setting of the foams, which is based on a gelling reaction by the in situ polymerization of organic monomers dissolved in the liquid phase. A variety of monomeric systems that are known to be suitable gelling agents for setting ceramic suspensions into dense forms were investigated in this work, with a view to the production of ceramic foams. These systems, namely ammonium acrylate, N-hydroxymethylacrylamide, methacrylic acid, methacrylamide and methylene-bysacrylamide, were studied considering the main requirements to produce ceramic foams, which include short setting time and high wet green strength. The effects of monomer type on powder dispersion, reaction kinetics, and green strength of wet and dried gelled bodies were investigated. The results revealed that the chemical characteristics of each system can affect the dispersion and rheological properties of suspensions. Polymerization kinetics and wet and dry mechanical strength varied markedly, depending on the type of monomer used.

Desenvolvimento e aplicação de um software que automatiza o processo de combinação de matérias-primas na obtenção de produtos cerâmicos

Combining several raw materials to obtain a ceramic composition involves up to five different and simultaneous steps: a) control of particle size distribution; b) chemical composition; c) surface area; d) density; and e) cost. This work presents a software developed for Windows environment in partnership with Alcoa Aluminum S.A., which controls all five referred parameters. It also contains an optimized routine that allows automatically combination of an unlimited number of raw materials to achieve a designed distribution, based on the minimum square difference. Experimental verification of the routine involved preparation of alumina suspensions using nine different particle size ranges, combined both in a conventional and in an automated manner to provide designed distributions (Alfred and Andreasen, q = 0.37). The routines efficiency was verified by the high speed of the automated formulation process and the rheological results obtained. Close rheological behavior were found out, pointing out the accuracy of both approaches.

Produção de cerâmicas celulares por emulsão seguida de gelificação

Cellular ceramics have been produced by several methods, resulting in a wide range of structures and properties. This work describes a novel route to produce cellular ceramics based on the gel casting of emulsions consisting of an aqueous ceramic suspension containing water-soluble organic monomers and an emulsified insoluble liquid phase (kerosene). The effects of solids loading and kerosene content on the rheological properties of emulsions were evaluated. SEM of the cellular structure revealed isolated pores for samples with low porosity, which changed to an interconnected network of pores as the porosity increased. Diametrical compression was used to evaluate the strength of sintered samples, which varied in the range of 2.0 and 48 MPa, depending on the porosity.

Produção e caracterização de espumas de alumina pelo processo gelcasting sem controle atmosférico

The gelcasting process with the aeration of ceramic suspensions allows the production of ceramic foams with a wide range of porosity (50-90vol.%.). One of the critical steps of the process is the setting of the foam, which is based on a gelling reaction by the in-situ polymerization of monomers previously added in the ceramic suspension. This reaction is inhibited by the presence of oxygen, which requires atmospheric control for the stabilization of the foam. In this work, the feasibility of producing alumina foams by the gelcasting process without atmospheric control was studied. For the foams production, two different foaming agents were tested. The physical and mechanical properties of ceramic foams, as well the microstructure, were evaluated and compared with those produced with the same foaming agents but with atmospheric control. The results obtained using the two procedures were similar and with potential to be applied with technological purposes.

Li2O-ZrO2-SiO2 -Al2O3 glass-ceramic foams produced by the gelcasting process

Vitreous foams in the Li2O-ZrO2-SiO2-Al2O 3 (LZSA) system were produced by the gelcasting process with aeration of ceramic suspensions and without atmospheric control. By the addition of different concentrations of foaming agent (Fongraminox) it was possible to attain glass-ceramic foams in a narrow range of relative density (0.10-0.15). The glass-ceramic foams showed compressive strength of 2.5-3.7 MPa, which corresponds to porosity between 85-89%, and macrostructure with pores nearly spherical and interconnected, these characteristics make these materials attractive for the following applications technology: filters of molten metals and hot gas, and catalytic support.

Propriedades mecânicas de espumas cerâmicas produzidas via "gelcasting"

Porous ceramics have great technological importance, since they combine ceramics unique properties, such as refractoriness and chemical resistance, with high surface area, high permeability and low thermal conductivity. Nevertheless, increasing porosity generally deteriorates the mechanical behavior of ceramics. In this work, the mechanical properties of ceramic foams produced by the aeration of ceramic suspensions associated to the polymerization of previously added monomers (gelcasting process) is investigated. The macrostructure that results from this process is constituted of nearly spherical pores, a geometry that minimizes the concentration of mechanical stresses. Moreover the cell walls can be well densified during sintering, resulting in a homogeneous microstructure. The mechanical strength of ceramic foams with several densities was evaluated under 4-point bending test and compression. The results of flexural strength were used to calculate the Weilbull modulus of the samples. The Youngs modulus and shear modulus were measured using a mechanical resonance method. The results allowed a better comprehension of the main factors that affect the mechanical properties of this new material category and provided insights for its improvement.

Aspectos da Reologia e da Estabilidade de Suspensões Cerâmicas. Parte III: Mecanismo de Estabilização Eletroestérica de Suspensões com Alumina

The third and last part of this review about stabilization and rheological aspects of ceramic suspension gathers the knowledge in the two parts previously published. Here, the electrostatic and steric phenomena, related to the electrical double layer and polymeric molecules adsorption, respectively, are combined to explain the electrosteric stabilization mechanism of ceramic suspensions. The deflocculants used to afford such dispersion mechanism cover a specific kind of polymer, named polyelectrolytes, which are macromolecules that are ionizable in solution. The study of how the polyelectrolytes behave is justified due to the large use of this polymeric product in the ceramic industry. Polyacrylic acid (PAA) and polymethacrylic acid (PMAA) are examples of polyelectrolytes that are widely used in the processing of alumina based materials. The medium pH and ion concentration are strengthened, and the importance of the ionic strength is emphasized. As a practical application, the stability of alumina-PMAA system is presented. The viscosity and dispersion behaviour of the suspension are also reported. This study is concluded presenting results concerning the effect of molecular weigth on the suspension viscosity, and shows that it is not enough to select a polymer considering just its class. It is fundamental to specify the average molecular weight of the selected polymer.

Relação entre porosidade e os módulos elásticos de esponjas cerâmicas produzidas via "gelcasting"

Broad porosity range can be obtained in ceramic foams employing the gelcasting technique coupled to the aeration of ceramic suspensions. Alumina based foams were produced with porosity in the range of 62 to 87 volume-%, which corresponded to relative densities between 0.13 and 0.38. Fitting models after Mackenzie, Boccaccini and Gibson and Ashby, describing the dependency of elastic moduli on density or porosity, were applied. The bar resonance technique was used to determine the values of Youngs modulus, shear modulus and Poissons ratio of these cellular material. The fitting model that best describe the dependence of elastic moduli on porosity could be identified and discussed.

Efeito da técnica de aeração sobre a macroestrutura e a permeabilidade de espumas cerâmicas produzidas via "gelcasting"

The production of porous ceramics through the aeration of suspensions associated with the gelcasting process provides cellular porous ceramics with excellent properties and a wide range of densities. Nonetheless, the use of this material in applications such as filtering, thermal insulation, or catalyst supports requires a macrostructural tailoring in order to favor specific properties, such as permeability, thermal conductivity or specific surface area. These properties are determined by characteristics such as density, pore size distribution and connectivity between neighboring cells, which are defined during foam formation. The aim of this work was to assess the influence of the aeration process on the macrostructure of ceramic foams. A planetary mixer and a stirring vessel with a bubbling ring at the bottom surface were studied in this work. Alumina suspensions containing different foaming agent concentrations were submitted to mixing cycles in both foam generators. After gelling, samples were sintered and had their pore size distribution evaluated using image analysis and mercury porosimetry. The air permeability was also evaluated. The results showed that the aeration process may strongly influence the density and the mode pore size, having a significant effect on the materials permeability. It was observed that mercury porosimetry and image analysis lead to different pore size distributions. However, both results are useful, depending on the aim of the analysis.