Rafael Salomão

Production of porous ceramic materialusing different sources of alumina and calcia

Numerous papers and publications report the use of microporous calcium hexaluminate (CaO.6Al2O3; CA6) as a key raw material for high temperature insulating materials. This material has unique properties with respect to chemical purity and mineral composition. Another important property of CA6 is its structure, which consists of platelet-shaped crystals that interlock. The free distance between the crystals defines the microporous structure. The low density in combination with the micropores hampers heat transfer by radiation at temperatures exceeding 1000 oC and results in a low thermal conductivity. Given the advantages presented by this material, it is necessary to understand the formation mechanism of CA6 grains in order to better develop the potential applications of this material. CA6 can be fabricated using organic binders to consolidate the Al2O3-CaCO3 powder mixture and to provide green strength so that a green body can be formed and retains the desired shape before heating. However, these organic binders must be completely thermally decomposed so that they do not remain in the sintered body as carbon or ash. Moreover, the use of organic binders releases large volumes of gases such as carbon dioxide from the green body during heating. Therefore, an eco-friendly ceramic fabrication process has been developed that employs an inorganic binder (hydraulic alumina). The aim of the present work was to study the synthesis of porous calcium-hexaluminate ceramics using calcined alumina or hydraulic alumina combined with different sources of calcia (CaCO3 and Ca(OH)2) at different temperatures. The materials produced were characterized by X-ray diffraction, scanning electron microscopy, apparent porosity and mercury intrusion porosimetry. The materials produced by hydraulic alumina presented higher porosity and larger pores compared to those produced from calcined alumina

A Comparison of Al(OH)3 and Mg(OH)2 as Inorganic Porogenic Agents for Alumina

Castable porous ceramics combine the high refractoriness of ceramics, the useful characteristics of porous materials and the straightforward installation of castable systems. In this study, particles of aluminium hydroxide (Al(OH)3) and magnesium hydroxide (Mg(OH)2) of similar average size were added separately to an alumina castable (up to 67 vol.-%). During thermal treatment (1100–1500°C), variations occurred in their porosity levels, mechanical properties, phase composition and microstructure. These were related to physical-chemical changes and Al2O3-MgO solid-state reactions. Both systems have potential to be technologically useful. AH-based structures showed intermediate levels of porosity (around 60%) and higher compression strength (above 10 MPa), which enable them to be employed as sintered lightweight aggregates for refractory insulating mortars. The MH-based castables, on the other hand, exhibited higher porosity levels (above 60%) and excellent dimensional stability. They can therefore be used as primary thermal insulators for long-life services at steelmaking, cement production and petrochemical plants.

Porous ceramics for high temperature applications

In order to reduce the energy consumption, the use of porous refractory ceramics as high-temperature insulating materials has grown significantly. Among the techniques employed in the production of these materials, such as the addition of foam and organic compounds, the generation of pores by phase transformation presents great technological interest, due to its easy processing route and lack of toxic volatiles. In the present work, this technique was employed to produce porous ceramics by the decomposition of aluminum and aluminum-magnesium hydroxides. The in-situ spinelization reaction reduces the densification at high temperatures. It was verified that the use of complex hydroxides generates greater porosity, mechanical strength and refractoriness.

Quelantes como aditivos anti-hidratação da magnésia

A magnesia confere aos concretos excelentes propriedades refratarias aliadas a uma elevada resistencia a escorias basicas. Entretanto, na presenca de agua, ela se hidrata em uma reacao expansiva formando o hidroxido de magnesio. A expansao gerada, se nao for controlada ou reduzida, danifica a estrutura do material podendo ate levar a sua completa desintegracao. Neste trabalho, aditivos conhecidos como quelantes foram adicionados a suspensao de sinter de magnesia, com e sem cimento de aluminato de calcio, a fim de evitar a reacao de hidratacao da magnesia. Sob o ponto de vista quimico, foram realizadas medidas de potencial zeta e grau de hidratacao, correlacionando a mudanca da carga superficial da magnesia gerada pela presenca dos aditivos a quantidade de hidroxido formada. Sob o ponto de vista mecânico e fisico, foram realizados o monitoramento da expansao volumetrica aparente e o registro de imagens das amostras. O melhor aditivo anti-hidratacao para as amostras com aluminato de calcio foi o acido citrico. A adicao de apenas 0,3%-p foi capaz de quase anular a quantidade de hidroxido formada e adicionalmente zerar a expansao.

Porogenic Behavior of Water in High-Alumina Castable Structures

Direct casting is a processing method that can produce large parts of complex geometry. It uses dispersions of ceramic particles in water and a hydraulic binder for consolidation. Studies on castable structures have reported the generation of a significant fraction of pores due to the presence of water. This effect occurs because drying preserves part of the interparticle pores originally occupied by water. The fraction of pores obtained in these cases can reach levels higher than 50%, affecting the properties of the structure. However, the porogenic behavior of water in these materials was not investigated in depth. In this work, aqueous suspensions of calcined alumina and hydratable alumina (hydraulic binder) of different water contents (40–60 vol.%) were prepared in a paddler mixer. For compositions of lower (5–35 vol.%) or higher (66–80 vol.%) water amounts, casting and curing steps were assisted by external pressure and a rotating device, respectively, to produce homogeneous microstructures. The total porosity after drying was similar to the initial volumetric content of water in the compositions when side effects, such as sedimentation and air entrapment, were prevented. Below water content of 25 vol.%, particles packing flaws became the main pore generator and the hydroxylation reaction of the binder no longer occurred effectively.

Hydratable Alumina-Bonded Suspensions: Evolution of Microstructure and Physical Properties During First Heating

Hydratable alumina (HA) is a calcium-free and high-refractoriness binder for alumina-based suspensions. Although recent studies have improved its dispersion, mixing and drying behaviours, a drawback related to its loss of strength between 250 and 900°C remains unexplored. Pores generated after decomposition of HA curing products are usually an explanation for the effect; however, no experimental result has supported this hypothesis so far. This study investigated the effects of thermal treatment (120–1500°C) upon the microstructure and physical properties of calcined alumina suspensions containing different amounts of HA (10–40 vol.-%). Porosity, compression strength and flexural elastic modulus measurements, thermal linear variation and thermogravimetric analysis were compared with scanning electron microscopy and X-ray diffraction results. The average matrix particle size and amount of HA in the formulation play major roles in the types of curing products that are formed. The strength reduction observed during first heating was not directly associated with the increase in porosity.

Synthetisches amorphes SiO2 (SAS) für die keramische Industrie

KurzfassungSiliziumdoxid (SiO2) zählt zu den wichtigsten Eingangsmaterialien für die Lebensmittel- und Pharmaindustrie, Polymer-Verbundwerkstoffe sowie für die Tintenherstellung. In keramischen Werkstoffen werden feinkörnige Siliziumdioxidpartikel häufig als Pack- und Sinterhilfsmittel sowie zur Herstellung anderer Rohstoffe wie Mullit (3Al2O3·2SiO2) und Siliziumcarbid (SiC) eingesetzt. Da natürliches Siliziumdioxid einen relativ geringen Reinheitsgrad und inhomogene Eigenschaften aufweist, ist der Einsatz von synthetischem Siliziumdioxid in Feuerfestanwendungen und in der technischen Keramik notwendig. Diese Anwendungen erfordern eine verbesserte Kontrolle der Zusammensetzung und Mikrostruktur. Die Arbeit beschreibt einen systematischen Vergleich von synthetischem amorphem Siliziumdioxid (SAS) in vier verschiedenen Materialqualitäten. Die untersuchten SAS-Materialien wurden anhand verschiedener Verfahren formuliert (Natriumsilikat-Ausscheidungsprozess, SiCl4-Pyrolyse, Gewinnung aus Reishülsen und physikalische Abscheidung von gasförmigem Silizium). Unterschiede in der physikalisch-chemischen, thermischen und mikrostrukturellen Analyse jedes Stoffes stehen in Zusammenhang mit den jeweiligen Verfahrensweisen und Techniken des Herstellungsverfahrens. Die Arbeit bestätigte, dass die Synthesebedingungen einen bedeutenden Einfluss auf die Zusammensetzung und physikalischen Eigenschaften der getesteten SAS-Proben haben.AbstractSilica (SiO2) is one of the most important inputs for the food, pharmaceutics, polymer composite, and ink manufacturing industries. In ceramic materials, fine silica particles are widely used as a packing and sintering aid and to produce other raw materials like mullite (3Al2O3·2SiO2) and silicon carbide (SiC). As most of the silica sources found in nature have relatively low purity and nonhomogeneous properties, use of synthetic grades of silica is necessary in applications such as refractories and technical ceramics that require better control of product composition and microstructure. This paper describes a systematic comparison of four grades of synthetic amorphous silica (SAS) used in technical ceramics. The evaluated SAS materials were formulated by different methods (sodium silicate precipitation, SiCl4 pyrolysis, extraction from rice husks, and physical deposition of silicon vapour). Differences in the physico-chemical and thermal and microstructural characterization of each material are related to the principles and techniques involved in their manufacture. The study verified that synthesis conditions strongly influenced the composition and physical properties of the tested SAS samples.

Characterization of Synthetic Amorphous Silica (SAS) Used in the Ceramics Industry

Silica (SiO2) is one of the most important inputs for the food, pharmaceutics, polymer composite, and ink manufacturing industries. In ceramic materials, fine silica particles are widely used as a packing and sintering aid and to produce other raw materials like mullite (3Al2O3·2SiO2) and silicon carbide (SiC). As most of the silica sources found in nature have relatively low purity and nonhomogeneous properties, use of synthetic grades of silica is necessary in applications such as refractories and technical ceramics that require better control of product composition and microstructure. This paper describes a systematic comparison of four grades of synthetic amorphous silica (SAS) used in technical ceramics. The evaluated SAS materials were formulated by different methods (sodium silicate precipitation, SiCl4 pyrolysis, extraction from rice husks, and physical deposition of silicon vapour). Differences in the physicochemical and thermal and microstructural characterization of each material are related to the principles and techniques involved in their manufacture. The study verified that synthesis conditions strongly influenced the composition and physical properties of the tested SAS samples.