Processamento de nitreto de alumínio (AIN) em meio aquoso

Doutoramento em Ciência e Engenharia de MateriaisO presente trabalho teve como objectivo principal o estudo das condições experimentais conducentes à viabilização do processamento de pós de AlN em meio aquoso, e à sinterização das peças consolidadas a temperaturas inferiores (1750ºC) às normalmente usadas (1900ºC). As propriedades finais das amostras processadas em meio aquoso, designadamente a condutividade térmica, deveriam ser comparáveis às das amostras de AlN actualmente processadas em meio orgânico. Para o efeito, a superfície dos pós de AlN foi protegida contra as reacções de hidrólise usando duas substâncias de natureza hidrofílica: ácido fosfórico e di-hidrogenofosfato de alumínio. A ligação destas espécies químicas à superfície das partículas do AlN e a sua eficácia na protecção da hidrólise foi avaliada por espectroscopia de infravermelho, Ressonância Magnética Nuclear (RMN) e medição do pH de uma suspensão ao longo do tempo. Ambas as substâncias permitiram evitar as reacções de hidrólise, embora o di-hidrogenofosfato de alumínio, por possuir uma ligação à superfície mais forte, se tenha mostrado mais eficaz, sendo por isso a espécie adoptada para o tratamento do AlN e posterior prossecução do trabalho. Com o pó de AlN assim tratado, foi possível preparar suspensões estabilizadas contendo 50- vol.% de sólidos na presença de ligantes e plastificantes, que permitiram o processamento por várias técnicas coloidais: enchimento por barbotina (slip casting), granulação de pós por aspersão-congelamento-liofilização (freeze granulation) e preparação de cintas (tape casting). As peças obtidas foram posteriormente sinterizadas a 1750ºC, usando como aditivos de sinterização o CaF2 e o YF3. Testaram-se diferentes quantidades e proporções, tendo-se verificado que a mistura 3-wt% YF3 + 2-wt% CaF2 era aquela que, de um modo geral, permitia obter os valores de condutividade térmica mais elevados para as amostras preparadas pelas três técnicas de processamento. A microestrutura das amostras e as fases secundárias formadas foram avaliadas por Microscopia Electrónica de Varrimento (SEM), Microscopia Electrónica de Transmissão (TEM) e Difracção de raios-X (DRX). Os valores de condutividade térmica das amostras obtidas pelas diferentes técnicas de processamento e sinterizadas nas mesmas condições foram os seguintes: 115 W m-1 K-1 para as amostras obtidas por enchimento por barbotina, 140 W m-1 K-1 para as amostras prensadas a partir de grânulos e 200 W m-1 K-1 para as cintas. A presença de carbono, resultante da queima incompleta das substâncias orgânicas (ligantes e plastificantes) durante a pré-sinterização, e a distância a percorrer pelas espécies gasosas ricas em oxigénio são os principais factores que controlam a concentração final de oxigénio nas fronteiras de grão e na rede cristalina e, consequentemente, a condutividade térmica.The present work aims at processing AlN in aqueous media and sintering the obtained samples at lower temperatures (1750ºC) than those usually used (1900ºC), keeping or even improving the final properties, namely thermal conductivity, usually obtained from organic processing. In order to protect AlN powder against hydrolysis, two hydrophilic substances, phosphoric acid and aluminium di-hydrogenphosphate, were tested. The nature of the bonding of these species to the AlN particles surface and their ability to avoid hydrolysis reactions were evaluated by Fourier Transformed Infrared (FT-IR) and Nuclear Magnetic Resonance spectroscopy (RMN), and by monitoring the pH of the AlN suspension with time. Although both species allowed controlling hydrolysis reactions, aluminium di-hydrogenphosphate was selected for further progress of the work due to its stronger bonding to the AlN particles surface and consequently, to the higher stability of the astreated powder in aqueous media. It was possible to prepare stabilised AlN suspensions with 50-vol.% solids loading using AlN treated powder in presence of binders and plasticizers, which allows to process samples by different colloidal shaping techniques: slip casting, freeze powders granulation and tape casting. The obtained samples were then sintered at 1750ºC using CaF2 and YF3 as sintering aids. Different amounts and proportions were tested, with the highest thermal conductivity values being generally obtained for the mixture 3-wt% YF3 + 2-wt% CaF2 for the samples prepared by the different shaping techniques and sintered under the same conditions. The microstructural features and the secondary phases of the sintered samples were evaluated by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (DRX). The thermal conductivity of the samples prepared by slip casting, pressing of granules obtained by freeze granulation and tape casting are 115 W m-1 K-1, 140 W m-1 K-1 and 200 W m-1 K-1, respectively. The presence of carbon remaining from the burnout of the organic species (binder and plasticizer) and the diffusion path length that the oxygen rich species have to cross to be released from the grain boundaries were regarded as the main factors determining the final oxygen concentrations at the grain boundaries and crystalline lattice and, therefore, for the thermal conductivity values of the samples

Utilização de polissacarídeos gelificantes no controlo reológico de suspensões cerâmicas e na conformação de corpos por gelcasting

O presente trabalho procura tirar partido das propriedades gelificantes de polissacarídeos para melhorar o controlo reológico de suspensões de materiais cerâmicos e de vidrados, e para conformar corpos por gelcasting. Para o efeito, lançou-se mão de polissacarídeos com poderes gelificantes diferentes, nomeadamente, capazes de formar geles fracos, quando o objectivo era melhorar o controlo reológico de suspensões, ou de originarem a formação de geles fortes, quer individualmente quer procurando efeitos sinérgicos de suas misturas, quando o fim pretendido era a gelificação de suspensões. A goma xantana foi adoptada para a preparação de geles fracos, estudando-se o efeito de diferentes adições no controlo reológico de suspensões de vidrados e de uma alumina com uma distribuição bimodal de tamanhos de partícula. Os vidrados são normalmente constituídos por partículas grossas que tendem a sedimentar na ausência de agentes suspensores. A alumina seleccionada, CT 530 SG (Alcoa) tem uma capacidade de empacotamento elevada permitindo a preparação de suspensões com elevadas concentrações de sólidos, as quais tendem a apresentar um carácter acentuadamente reo-espessante. A estabilidade das suspensões de vidrado ao longo do tempo de repouso foi avaliada através de testes de sedimentação, na presença de diferentes concentrações de goma xantana. Verificouse que adições crescentes de goma xantana tornavam gradualmente menos acentuado o comportamento reofluidificante da suspenção inicial, conferindo-lhe um carácter mais Newtoniano e, ao mesmo tempo, níveis de viscosidade crescentes. Por outro lado, os testes de sedimentação revelaram que era possível obter suspensões estáveis durante mais tempo e com viscosidades semelhantes, utilizando menores quantidades de polissacarídeo (0.1-wt%), quando comparadas com as quantidades requeridas (0.2-wt%) de uma mistura de duas carboximetilceluloses (CMC) usadas na indústria como agentes suspensores. Contrariamente ao observado nas suspensões de vidrado, a adição de goma xantana às suspensões concentradas de alumina (60-vol.%), acentuou o seu carácter reo-espessante, além do aumento esperado da viscosidade. O agar foi o polissacarídeo primeiramente seleccionado devido à sua reconhecida capacidade para formar geles fortes. O efeito sinérgico do agar e das galactomananas na incrementação da força do gel, frequentemente explorado na indústria alimentar, foi também estudado na gelificação de suspensões de alumina. Concluiu-se que uma mistura dos dois polissacarídeos na razão 80/20 agar/galactomananas era a que permitia um melhor compromisso entre a viscosidade baixa a temperatura inicial (60ºC), e a força do gel elevada à temperatura ambiente. Verificou-se ainda que 1-wt% desta mistura de polissacarídeos, relativamente à fracção de líquido, era suficiente para promover a consolidação das suspensões. A aplicação das condições experimentais de gelcasting com a alumina a pastas de porcelana deparava-se com a dificuldade inerente de preparar suspensões suficientemente concentradas para o efeito com formulações à base de argilominerais. Para ultrapassar esta dificuldade, foram propostas novas formulações em que o componente argiloso foi total ou parcialmente substituído por uma composição química semelhante mas na forma de meta-caulino, rejeitos de porcelana cozidos (caco), e uma mistura de silica e de alumina (pseudo-caulino). Esta aproximação permitiu a preparação de suspensões de porcelana com 60-vol.%, as quais puderam ser consolidadas por gel casting nas mesmas condições experimentais que a alumina. As características em cozido das diferentes formulações estudadas, foram avaliadas através de análises dilatométricas, microscopia electrónica de varrimento e difracção de Raios X. Verificou-se que as formulações preparadas com caco maturam a temperaturas inferiores às de uma pasta de porcelana com a mesma composição química mas preparadas a partir de matérias primas cruas. O efeito pode ser atribuído a uma diminuição da razão de aspecto das fibras de mulite devido à sua quebra durante a moagem do caco, e a uma continuada dissolução do quartzo na fase vítrea durante o recozimento. Ambos estes factores contribuem para o aumento do índice de deformação piroplástica.The present work takes advantage of the gelling properties of polysaccharides to improve the control of the rheological behaviour of aqueous suspensions of ceramic and glaze powders, and to consolidate ceramic bodies by gelcasting. To accomplish these aims, polysaccharides with different gelling powers were selected, namely, weak gels to improve the control of the rheological behaviour of aqueous suspensions, or strong gels used individually, or exploring the synergistic effects of their mixtures, when the objective pursued was the consolidation of the suspensions by gelcasting. The xanthan gum was adopted for the preparation of weak gels, the effects of different added amounts on the rheological behaviour of suspensions of a glaze or an alumina powder with a bimodal particle size distribution was studied. Glaze powders are usually constituted by coarse particles that tend to readily sediment in absence of suspending agents. The selected alumina, CT 530 SG (Alcoa), has a high packing ability enabling the preparation of high solids loading suspensions, which tend to exhibit accentuated shear-thickening behaviours. The stability of the glaze suspensions against settling was evaluated through sedimentation testes in the presence of different amounts of xanthan gum. It was observed that increasing amounts of xanthan gum gradually changed the accentuated shear-thinning behaviour of the starting suspension towards a Newtonian behaviour, while the viscosity was enhanced with increasing amounts of polysaccharide. On the other hand, the sedimentation testes revealed that a lower amount of polysaccharide (0.1-wt%) was required to obtain glaze suspensions of similar viscosities, which were stable for a longer time, when compared with other industrial glaze suspensions containing 0.2-wt% of a mixture of two carboxymethylcelluloses (CMC) as suspending agents. Contrarily to the observed with the glaze suspensions, the addition of different amounts of xanthan gum to a high concentrated (60-vol.%) alumina suspension accentuated the shear-thickening character of the starting suspension, accompanied by expected increases in viscosity with increasing amounts of polysaccharide. For gelcasting, agar was firstly selected due to its well-known capability to originate strong gels. The synergistic effect between agar and locust bean gum, i.e. galactomannan, to increment the gelling power, often explored in food industry, was also studied to consolidate alumina suspensions. It was concluded that a mixture of the two polysaccharides with a ratio of 80/20 agar/galactomanan enabled a better compromise to be established between the fluidity at the starting temperature (60ºC) and the gel strength at room temperature. It was further observed that only 1-wt% of the above mixture was required to promote the consolidation of the suspensions. For the transferring of gelcasting experiments from alumina to porcelain, the inherent difficulties of preparing clay-based suspensions containing solids volume fractions high enough for direct consolidation need to be overcome. Therefore, new formulations were proposed in which the clay component was totally or partially replaced by an equivalent chemical composition, but introduced as meta-kaolin, rejected milled pieces of porcelain (crook), or a mixture of silica and alumina (pseudo-kaolin). These approaches enabled the preparation of porcelain suspensions containing 60-vol.% solids, which could be consolidated by gelcasting like alumina suspensions do. The fired characteristics of different formulations were evaluated through dilatometric analysis scanning electron microscopy and X-ray diffraction. It was observed that the formulations containing crook matured at lower temperatures compared with the same raw composition. The effect could be attributed to a lower aspect ratio of the mullite fibres, which have been broken during the milling step, and to a continued dissolution of the free quartz into the glassy phase during the re-firing step. Both factors contribute to the increment of the pyroplastic deformation index.Mestrado em Ciência e Engenharia de Materiai

AlN ceramics processed by aqueous slip casting

Fully dense aluminium nitride (AlN) ceramics consolidated by aqueous slip casting were obtained after sintering at 1750°C for 2h. The obtained samples were characterized by Vickers hardness (1000 Hv), flexural strength (200 MPa) and thermal conductivity (115 W/mK). YF3 and CaF2 were used as sintering additives in total amounts ranging from 5 to 7 wt% in YF3/CaF2 weight ratios of 1.25, 1.5, and 2. The compatibility between the thermochemically treated AlN powder and the sintering additives in the aqueous suspensions was investigated by rheological and zeta potential measurements. Highly concentrated (50 vol%) and well-dispersed suspensions could be prepared and used to consolidate homogeneous green bodies by slip casting. The phosphate species used to protect the surface AlN particles against hydrolysis could no longer be detected at temperatures higher than 1400°C. X-ray diffraction results revealed that all the new crystalline phases formed upon sintering belong to the system Al–Y–O–Ca, but their specific compositions as well as the thermal properties of the sintered materials were shown to depend on the total amount and ratio of sintering aids.The first author wishes to thank Fundação para a Ciência e Tecnologia of Portugal for the financial support under Grant No. SFRH/BD/8754/2002. Thanks are also due to Project PRAI-Centro for the financial support. The second author is in debt to Mcyt (Ministery of Science and Technology, Spain) Project No. MAT 2003-06147-C04-01.Peer reviewe

Development of translucent zirconia by robocasting

Partially translucent zirconia for dental applications was developed by robocasting using aqueous-based inks of zirconia doped with 5 mol.%Y2O3 as feedstock and a printing nozzle with 0.25 mm diameter. The manufactured samples were sintered at 1600 °C-2 h and characterized by crystalline phases, microstructure, mechanical properties and opacity. The sintered samples presented ≈94 % relative density, 12.7 ± 0.3 GPa as Vickers hardness, 3.91 MPa.m1/2 as fracture toughness and, 285 ± 57 MPa as flexural strength. The contrast ratio optical parameter varied in the range of 0.5 to 0.9, according to the material thickness. These optical outcomes are determined by the relative density and the microstructure, namely the presence of sub-micrometre tetragonal grains (26.4 %) and cubic phase grains (73.6 %) with sizes in the order of 1.5 μm, that justified the improved translucency obtained when compared to opaque tetragonal 3Y-TZP ceramics.publishe

Preventing hydrolysis of BaTiO3 powders during aqueous processing and of bulk ceramics after sintering

This present article addresses the reactive behaviour of barium titanate (BaTiO3, hereafter referred to as BT) with water either in powder form or as bulk ceramics. Chemisorbing a suitable phosphate-based protective agent at the surface of BT particles prevented hydrolysis reactions and made aqueous processing of BT a safe and viable route. Highly concentrated (50 vol.%) fluid and long term stable suspensions could be easily prepared from surface treated BT particles, contrasting with the poor stability and high viscosity of slurries derived from the as received powders. For aqueous suspensions, the course of hydrolysis reactions was monitored through changes in pH and concentrations of leached ionic species along ageing time. For bulk ceramics, the extent of hydrolysis reactions was assessed through the microstructural changes occurred at polished surfaces without and with surface treatment, which have been kept in contact with water for a certain time period. (C) 2015 Elsevier Ltd. All rights reserved

Is the ubiquitous presence of barium carbonate responsible for the poor aqueous processing ability of barium titanate?

The ubiquitous presence of barium carbonate (BaCO3 - BC) as an impurity in barium titanate (BaTiO3 - BT) has been pointed out as the main reason for the well-known difficulties found by many investigators when attempting to process BT powders in aqueous media. Different and controversial arguments have been put forward to justify the observed aqueous processing difficulties of BT, but a satisfactory explanation is still to be found. With this aim, a systematic study was here undertaken to shed further light on the solid/liquid interactions occurring at the surface of BC and BT particles and their impact on the dispersion ability of both powders, separately and mixed in certain proportions. Long term colloidal stability and high solid loadings (60 vol.%) were obtained for BC, while colloidal instability and a lower maximum content of solids (50 vol.%) could be achieved for BT. This responds to the question risen in the title. (C) 2013 Elsevier Ltd, All rights reserved

Influence of the de-waxing atmosphere on the properties of AlN ceramics processed from aqueous media

The influence of binder burnout atmosphere (air or N2) on surface chemistry of thermo-chemically treated AlN powders processed in aqueous media, and on the final properties of AlN ceramics was studied. The surface chemistry after de-waxing was accessed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD), SEM, high-resolution transmission electron microscopy (HR-TEM), were used to identify the phase assemblage and for microstructural analysis. The effects of the residual carbon and oxygen at the surface on the thermal conductivity and sintered density of AlN samples were investigated. The surface C/O ratios were observed to correlate with the sintering behaviour, the composition and distribution of secondary phases, and grain-boundary composition, as well as thermal conductivity of AlN samples. Thermal conductivities of about 140W/mK were obtained for the aqueous processed AlN samples de-waxed in nitrogen atmosphere and sintered for 2 h at 1750 ◦C in the presence of 4 wt.% YF3 + 2 wt.% CaF2 as sintering additives.Peer reviewe

Lead-free 0.5Ba(Zr0.2Ti0.8)O-3-0.5(Ba0.7Ca0.3)TiO3 powder surface treated against hydrolysis - a key for a successful aqueous processing

The properties of functional ceramics and their performance in industrial applications are strongly affected by the processing treatments. We report on long term stability of aqueous 0.5Ba(Zr0.2Ti0.8)O-3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) suspensions. The effects of aging time on pH and on the leaching extents of Ba, Zr and Ca elements were systematically investigated for the naked, and the surface treated particles against hydrolysis. Suspensions with solid loadings (Phi) up to 60 vol%, pseudoplastic flow behaviours and long term colloidal stability could be achieved from the surface treated powder. A fractal dimension (D) of similar to 1.97 to 1.98 was estimated for the BZT-BCT suspensions from the yield stress (tau(y)) versus (Phi) dependence, suggesting a reaction limited cluster aggregation (RLCA) type of particle aggregation with a substantial rearrangement occurring in the aggregates under shearing. The maximum solid loading (Phi(m)) approximate to 61 vol% was determined. The concentrated and stable suspensions represent a stride for successful processing of functional BZT-BCT ceramic components via various aqueous colloidal shaping methods (slip casting, gel-casting, etc.) for large scale industrial applications

An effective approach to reinforced closed-cell Al-alloy foams with multiwalled carbon nanotubes

Exploring the reinforcing role of carbon nanotubes to obtain materials (polymers, metals, ceramics) with enhanced properties has been often attempted, but the success is strongly limited by the dispersing degree of carbon nanotubes. Here we report on an innovative colloidal approach to disperse the carbon nanotubes in the powders mixture of the precursor materials in order to profit from their reinforcing potential and obtain a new class of closed-cell metal foams. The feasibility of the proposed approach was demonstrated for aluminium foams reinforced with multi-walled carbon nanotubes. These nanocomposite metal foams synergistically combine the remarkable properties of both metal foams and carbon nanotubes. The results indicate that the tubular structure of carbon nanotubes is preserved throughout the entire the process. The carbon nanotubes are individually dispersed, stretched and randomly aligned in the aluminium-matrix of these closed-cell foams, thus potentiating their homogeneous 3D reinforcing role. Accordingly, the Vickers micro-hardness of the closed-cell foams was greatly enhanced. (C) 2015 Elsevier Ltd. All rights reserved

A novel approach to prepare aluminium-alloy foams reinforced by carbon-nanotubes

This paper presents an innovative approach to prepare a new class of closed cell reinforced (nano composite) metal foams that synergistically explore stheremarkablepropertiesofbothmetalfoamsand carbon nanotubes. This approach combines colloidal processing (includingfreezegranulationlyophili sation) and powder technology, ensuring there tention of the tubular structureo fCOOH functionalised multiwall carbonnanotubes(MWCNTsCOOH) and their uniform distribution in themetalmatrix.The microstructural analysis reveal ednonagglomerated, welldispersed, stretchedanddirection allyaligned MWCNTswithintheAlmatrixof the foams. These conditions potentiate the reinforcingroleofMWCNTs with Vickersmicrohardnessincrements 4100%