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

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

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

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%

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

Hydrolysis Control of AlN Powders for the Aqueous Processing of Spherical AlN Granules

Spherical granules of aluminum nitride (AlN) with an average particle size of about 50m were produced from aqueous suspensions using an AlN powder surface treated against hydrolysis with aluminum dihydrogenphosphate [Al(H2PO4)3]. Two different amounts of Al(H2PO4)3 were tested and the effects of surface treatment and aging time were evaluated by various techniques (XRD, TG-DTA, zeta potential and pH measurements). The treated powder exhibited antihydrolytic property and good dispersing behavior, enabling the preparation of low-viscosity and high-concentration aqueous AlN slurries for freeze granulation. The spherical AlN granules were sintered in a boron nitride (BN) powder bed followed by ultrasonic washing of the AlN granulates/BN mixture to remove BN. The sintered spherical AlN granules present excellent crystallinity and high sphericity as observed from SEM micrographs

Influence of processing route on microstructure and mechanical properties of MgAl<SUB>2</SUB>O<SUB>4</SUB> spinel

This paper reports on process dependant microstructural and mechanical properties of MgAl2O4 spinel (MAS) ceramics. Two MAS powders with different chemical compositions were synthesized by solid-state reaction of alumina and calcined caustic magnesia at 1400 &#176;C for 1 h. The surface of the as obtained MAS powders was passivated against hydrolysis by coating it with H3PO4 and Al(H2PO4)3 species dissolved in ethanol at 80 &#176;C for 24 h. The as protected powders could then be dispersed in aqueous solutions of tetramethylammonium hydroxide (TMAH) and Duramax D-3005 as dispersing agents to obtain stable slurries with 45 vol.% solids loading. The stable aqueous MAS slurries were consolidated by slip casting (SC), gelcasting (GC), hydrolysis assisted solidification (HAS) and hydrolysis induced aqueous gelcasting (GCHAS) routes, fully dried and then sintered for 1 h at 1650 &#176;C. For comparison purposes, dense MAS ceramics were also prepared following a conventional dry-powder pressing (DP) and temperature induced gelation (TIG) routes. All the sintered MAS ceramics were thoroughly characterized for bulk density, apparent porosity, water absorption capacity, SEM microstructure, XRD phase, hardness, 3-point bend strength, and percentage of shrinkage to evaluate the suitability of the processing routes for fabricating defect free components with near-net shape. Among the various techniques employed, the GCHAS was found to be best for fabricating near-net shape MAS ceramics