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  • 1.
    Adolfsson, Erik
    et al.
    RISE - Research Institutes of Sweden, Materials and Production, IVF.
    Shen, Z.
    Stockholm University.
    Effects of granule density on strength and granule related defects in zirconia2012In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 32, no 11, p. 2653-2659Article in journal (Refereed)
    Abstract [en]

    A suspension of zirconia powder (TZ3YSE) with a solids loading of 50. vol% was prepared by ball milling. Binders were added and some of the suspension was diluted to 40, 30 and 20. vol% before freeze granulation was performed. A spray dried material (TZ3YSEB) was used as a reference. The pore size distribution of the different granules was evaluated and from the microstructure it was shown that inhomogeneities were present in both the freeze granulated as well as in the spray dried granules. In addition, the density, microstructure as well as the strength of sintered materials prepared from the granules were studied. The results showed that a high green density or sintered density was not sufficient in order to achieve a high strength material. It was further shown that the strength was significantly influenced by the granule density and not by the inhomogeneities found in the granules. © 2012.

  • 2.
    Bergström, L
    et al.
    YKI – Ytkemiska institutet.
    Sjöström, E
    YKI – Ytkemiska institutet.
    Temperature induced flocculation of concentrated ceramic suspensions: rheological properties1999In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 19, p. 2117-2123Article in journal (Refereed)
    Abstract [en]

    Concentrated, sterically stabilised ceramic suspensions have been reversibly flocculated by changing the temperature. Using an amphiphilic polymer, Hypermer KD3, as dispersant for alumina and alumina-silicon carbide whisker composite mixtures in pentanol resulted in a transition from dispersed to flocculated state close to room temperature. The collapse of the adsorbed polymer layer with decreasing solvency (temperature) in the marginal solvent pentanol induces flocculation when the long-range van der Waals force overcomes the remaining steric repulsion. Temperature induced flocculation (TIF) has a drastic effect on the rheological properties. At high temperatures, T>30 °C, the suspensions have a low viscosity and neglible elasticity. When the temperature is lowered below 20 °C, the viscosity increases significantly and the viscoelastic behaviour becomes predominatly elastic. The elasticity and the associated particle network strength are sufficiently high for a highly concentrated gelled suspension to support its own weight. The potential use of TIF as a novel forming method was discussed with relation to other new shaping techniques.

  • 3.
    Carlborg, Markus
    et al.
    Umeå University, Sweden.
    Weiland, Fredrik
    RISE - Research Institutes of Sweden, Bioeconomy, ETC Energy Technology Center.
    Ma, Charlie
    Luleå University of Technology, Sweden.
    Backman, Rainer
    Umeå University, Sweden.
    Landälv, Ingvar
    Luleå University of Technology, Sweden.
    Winikka, Henrik
    RISE - Research Institutes of Sweden, Bioeconomy, ETC Energy Technology Center. Luleå University of Technology, Sweden.
    Exposure of refractory materials during high-temperature gasification of a woody biomass and peat mixture2018In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 38, no 2, p. 777-787Article in journal (Refereed)
    Abstract [en]

    Finding resilient refractory materials for slagging gasification systems have the potential to reduce costs and improve the overall plant availability by extending the service life. In this study, different refractory materials were evaluated under slagging gasification conditions. Refractory probes were continuously exposed for up to 27 h in an atmospheric, oxygen blown, entrained flow gasifier fired with a mixture of bark and peat powder. Slag infiltration depth and microstructure were studied using SEM EDS. Crystalline phases were identified with powder XRD. Increased levels of Al, originating from refractory materials, were seen in all slags. The fused cast materials were least affected, even though dissolution and slag penetration could still be observed. Thermodynamic equilibrium calculations were done for mixtures of refractory and slag, from which phase assemblages were predicted and viscosities for the liquid parts were estimated. © 2017 Elsevier Ltd

  • 4. Greenwood, R
    et al.
    Bergström, L
    YKI – Ytkemiska institutet.
    Electroacoustic and rheological properties of aqueous Ce-ZrO2(Ce-TZP) suspensions1997In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 17, p. 537-548Article in journal (Refereed)
    Abstract [en]

    The submicron Ce-ZrO2 (13 mol% CeO2) powder was found to be slightly soluble at acidic conditions and the isoelectric point was determined to pHiep= 7.9 using electroacoustics in concentrated suspensions. The adsorption isotherms of two well characterised polyelectrolyte dispersants, Dispex A40 and Duramax D3007, could be related to the molecular structure. A combination of electroacoustic and rheological measurements showed that the viscosity is at a minimum at monolayer coverage of the dispersant when the covered particles are highly charged. The study illustrates how rheology and electroacoustics can be utilised in the investigation and optimisation of the properties of concentrated ceramic suspensions.

  • 5.
    Gustafsson, S.
    et al.
    Chalmers University of Technology.
    Falk, L.K.L.
    Chalmers University of Technology.
    Pitchford, J.E.
    University of Cambridge.
    Clegg, W.J.
    University of Cambridge.
    Lidén, Eva
    RISE - Research Institutes of Sweden, Swerea, Swerea IVF.
    Carlström, Elis
    RISE - Research Institutes of Sweden, Swerea, Swerea IVF.
    Development of microstructure during creep of polycrystalline mullite and a nanocomposite mullite/5 vol.% SiC2009In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 29, no 4, p. 539-550Article in journal (Refereed)
    Abstract [en]

    The microstructures of as-sintered and creep tested polycrystalline mullite and mullite reinforced with 5 vol.% nano-sized SiC particles have been characterized by scanning and transmission electron microscopy. The dislocation densities after tensile creep testing at 1300 and 1400 °C were virtually unchanged as compared to the as-sintered materials which indicates diffusion-controlled deformation. Mullite matrix grain boundaries bending around intergranular SiC particles suggest that grain boundary pinning, in addition to a reduced mullite grain size, contributed to the increased creep resistance of the mullite/5 vol.% SiC nanocomposite. Both materials showed pronounced cavitation at multi-grain junctions after creep testing at 1400 °C which suggests that unaccommodated grain boundary sliding, facilitated by softening of the intergranular glass, occurred at this temperature. This is consistent with the higher stress exponents at 1400 °C. © 2008 Elsevier Ltd. All rights reserved.

  • 6.
    Hellström, Kristina
    et al.
    RISE - Research Institutes of Sweden, Materials and Production, IVF.
    Ögren, Yngve
    RISE - Research Institutes of Sweden, Bioeconomy, ETC Energy Technology Center.
    da Silva, Melina
    RISE - Research Institutes of Sweden, Materials and Production, IVF.
    Cristea, Monica
    RISE - Research Institutes of Sweden, Materials and Production, IVF.
    Ström, Erik
    Kanthal AB, Sweden.
    Corrosion behavior of a Mo(Si, Al)2 composite at 1700°C in 95% N2 + 5% H22019In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619XArticle in journal (Refereed)
    Abstract [en]

    A Mo(Si, Al)2 based composite was pre-oxidized to establish an alumina scale on the material surface. Thereafter, the corrosion behavior of the composite was examined at 1700 °C for up to 24 h in 95% N2 + 5% H2. The weight change was followed by recording the material weight before and after exposure. The crystalline corrosion products were analyzed with X-ray diffraction (XRD) and the microstructure of the cross sectioned material was characterized using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). It was shown that AlN and Al5O6N layers developed on top of the pre-oxidized alumina layer and alumina threads develop out from the specimen surface. The accompanied aluminum consumption converts the substrate Mo(Si,Al)2 into Mo5Si3 immediately below the alumina scale to the extent that the Mo5Si3 becomes porous underneath the alumina scale. Corrosion mechanisms are discussed with the support of thermodynamic calculations.

  • 7.
    Jurków, D.
    et al.
    Wroclaw University of Technology.
    Malecha, K.
    Wroclaw University of Technology.
    Stiernstedt, Johanna
    RISE, Swerea, Swerea IVF.
    Golonka, L.
    Wroclaw University of Technology.
    Influence of tapes' properties on the laser cutting process2011In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 31, no 9, p. 1589-1595Article in journal (Refereed)
    Abstract [en]

    Ceramic tapes are used to build 3-dimensional components and microsystems in layer manufacturing. The tapes are individually printed and structured before being stacked and laminated. The structuring process of the tapes affects the maximal resolution of fluidic channels, suspended bridges and beams, which in turn determines the scale of miniaturization of the produced components. The aim of this paper is to investigate if the tape composition can be optimized to improve the cutting resolution of laser cutting, which is a very flexible tool for micromachining. Using the Siemens star pattern, the laser cutting resolution was measured for alumina green tapes of different binder compositions with different laser settings. For all tapes the resolution was better the higher the laser beam velocity. At higher velocity though, a higher number of cutting cycles is necessary to cut the tape. The laser cutting resolution depends on the binder composition, but the laser parameters must also be optimized to achieve high cutting resolution. © 2011 Elsevier Ltd.

  • 8.
    Laarz, E
    et al.
    YKI – Ytkemiska institutet.
    Bergström, L
    YKI – Ytkemiska institutet.
    The effect of anionic polyelectrolytes on the properties of aqueous silicon nitride suspensions2000In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 20, p. 431-440Article in journal (Refereed)
    Abstract [en]

    The effect of anionic polyelectrolytes on the electrokinetic and rheological properties of concentrated Si3N4 suspensions was investigated experimentally. We have compared novel anionic methacrylic acid comb copolymer modifications with grafted poly(ethylene oxide) chains, PMMA-PEO, with poly(acrylic acid) (PAA); the rheological response as a function of pH was very similar, thus illustrating that the grafted PEO chains have a minor influence on the colloidal stability. The polyelectrolyte adsorption and, thus, colloidal stability at pH> is mainly governed by the surface charge of the solid phase. We found that the colloidal stability and the steady shear rheology of the Si3N4 suspensions with addition of anionic polyelectrolytes could be generalized using a division into four different pH regimes, following a previous study by Hackley. The viscosity minimum around pH 7 was attributed to electrosteric stabilization. The effect of excess addition of polyelectrolyte was also discussed; we attribute the significant increase in viscosity to the ionic strength increase caused by the release of the associated counterions.

  • 9.
    Laarz, E
    et al.
    YKI – Ytkemiska institutet.
    Carlsson, M
    Vivien, B
    YKI – Ytkemiska institutet.
    Johnsson, M
    Nygren, M
    Bergström, L
    YKI – Ytkemiska institutet.
    Colloidal processing of Al2O3-based composites reinforced with TiN and TiC particulates, whiskers and nanoparticles2001In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 21, p. 1027-1035Article in journal (Refereed)
    Abstract [en]

    A colloidal processing route has been developed for the preparation of dense and homogeneous Al2O3¯TiN/TiC composites. The dispersion and rheological properties of mixtures of TiN or TiC particulates and Al2O3 particles were investigated using electrokinetics and steady-shear rheology. We found that well-dispersed aqueous suspensions with low viscosity could be prepared by adding a poly(acrylic acid) dispersant and controlling pH in the alkaline range. This processing scheme was also suitable for preparation of whisker and nanoparticle composite suspensions. The alumina-based composite suspensions with a secondary-phase concentration of 25 vol.% were freeze-granulated and hot-pressed, and the resulting bodies were fully densified with well-dispersed secondary phases. Homogeneous Al2O3¯TiN nanoparticle composites could only be prepared with additions of up to 5 vol.% nanoparticles; higher additions resulted in agglomeration and subsequent grain growth of the nanoparticles.

  • 10. Lidén, E
    et al.
    Bergström, L
    YKI – Ytkemiska institutet.
    Persson, M
    Carlsson, R
    Surface modification and dispersion of silicon nitride and silicon carbide powders1991In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 7, p. 361-368Article in journal (Refereed)
    Abstract [en]

    A surface modification technique using controlled hydrolysis and polymerization of Al-alkoxide is oresented. It was found by measuring the electrokinetic behaviour and the adsorption properties that a minimum amount of about 0.5 mg Al/m2 was necessary to give SiC and Si3N4 in cyclohexane using a commercial dispersant with an acidic headgroup.

  • 11.
    Lyckfeldt, Ola
    et al.
    RISE - Research Institutes of Sweden, Swerea, Swerea IVF.
    Palmqvist, Lisa
    RISE - Research Institutes of Sweden, Swerea, Swerea IVF.
    Carlström, Elis
    RISE - Research Institutes of Sweden, Swerea, Swerea IVF.
    Stabilization of alumina with polyelectrolyte and comb copolymer in solvent mixtures of water and alcohols2009In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 29, no 6, p. 1069-1076Article in journal (Refereed)
    Abstract [en]

    Solvent mixtures of water and ethanol and water and isopropanol have been evaluated for processing of concentrated alumina suspensions. The addition of alcohols may increase the long-term stability of suspensions with soluble ceramic species such as magnesia, which is added as a sintering aid. A poly(acrylic acid) and a hydrophilic comb copolymer were used as dispersants for the different solvent mixtures. The aim was to compare the stabilization efficiency at normal processing conditions, pH 9-10, through rheological measurements and to develop a robust system including magnesia with long-term stability. The electrostatic stabilization of the dispersants in the different solvent mixtures was studied by zeta potential measurements. Highly negative zeta potentials were observed for the poly(acrylic acid) at pH 9-10 in the solvent mixtures. A charge contribution was also seen from the adsorbed comb copolymer, however smaller than for the poly(acrylic acid). Low viscosity was obtained for suspensions stabilized with poly(acrylic acid) in solvent mixtures with either 25 vol% ethanol or isopropanol. Higher alcohol to water ratio led to flocculation of the suspension when poly(acrylic acid) was used as dispersant. Alumina suspensions with added magnesia in isopropanol:water 25:75 and poly(acrylic acid) as dispersant showed long-term stability. The viscosity remained almost constant during 4 days of aging. Suspensions stabilized with the comb copolymer dispersant gave stable systems with ethanol and isopropanol concentrations between 25 and 75 vol%. The superior dispersing efficiency of the comb copolymer at alcohol contents above 25 vol% was believed to originate from steric stabilization in combination with low effective particle size, giving low viscosity through lower apparent solid contents of the suspension. © 2008 Elsevier Ltd. All rights reserved.

  • 12.
    Shen, Zhijian
    et al.
    Stockholm University, Sweden.
    Liu, Leifeng
    Stockholm University, Sweden.
    Xu, Xiqing
    Stockholm University, Sweden.
    Zhao, Jing
    Stockholm University, Sweden.
    Eriksson, Mirva
    Stockholm University, Sweden.
    Zhong, Yuan
    Stockholm University, Sweden.
    Adolfsson, Erik
    RISE - Research Institutes of Sweden, Materials and Production, IVF.
    Liu, Yihong
    Peking University School and Hospital of Stomatology, China.
    Kocjan, Andraz
    Jozef Stefan Institute, Slovenia.
    Fractography of self-glazed zirconia with improved reliability2017In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, ISSN 09552219, Vol. 37, no 14, p. 4339-4345Article in journal (Refereed)
    Abstract [en]

    The fractography of a new grade of zirconia ceramics, known as self-glazed zirconia, was investigated. The as-sintered intact top surface was made with superior smoothness that mimicked the optical appearances of the natural teeth enamel. The beneath surface opposite to this was made hierarchically rough with microscopic pits of the size up to 60. μm together with grain-level roughness of about 2. μm. The three-point bending test of the samples made with the hierarchically rough surface being tensile one demonstrated an average bending strength of 1120. ±. 70. MPa and a Weibull modulus of as high as 18 ascribed to the improved structural homogeneity. Surface topography was found the main origins of crack initiation leading to fracture. The observed unusually predominant transgranular fracture mode of submicron-sized grains disclosed a possible toughening mechanism of disassembling of mesocrystalline grains that differs significantly from the commonly quoted phase transformation toughening of this category of ceramics.

  • 13. Skovgaard, Mette
    et al.
    Ahniyaz, Anwar
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Sveriges tekniska forskningsinstitut, YKI – Ytkemiska institutet.
    Sørensen, Bent F
    Almdal, Kristoffer
    van Lelieveld, Alexander
    Effect of microscale shear stresses on the martensitic phase transformation of nanocrystalline tetragonal zirconia powders2010In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 30, no 13, p. 2749-2755Article in journal (Refereed)
    Abstract [en]

    For the first time, the effect of microscale shear stress induced by both mechanical compression and ball-milling on the phase stability of nanocrystalline tetragonal zirconia (t-ZrO2) powders was studied in water free, inert atmosphere. It was found that nanocrystalline t-ZrO2 powders are extremely sensitive to both macroscopic uniaxial compressive strain and ball-milling induced shear stress and easily transform martensitically into the monoclinic phase. A linear relationship between applied compressive stress and the degree of tetragonal to monoclinic (t→m) phase transformation was observed. Ball-milling induced microscale stress has a similar effect on the t→m phase transformation. Furthermore, it was found that even very mild milling condition, such as 120 rpm, 1 h (0.5mm balls) was enough to induce phase transformation. Surfactant assisted ball-milling was found to be very effective in de-agglomeration of our nanocrystalline porous ZrO2 particles into discrete nanocrystals. However, the t→m phase transformation could not be avoided totally even at very mild milling condition. This suggests that the metastable t-ZrO2 is extreme sensitive to microscale shear stress induced by both mechanical compression and ball-milling. The findings presented in this work are very important in further understanding the stress-induced phase transformation of nanocrystalline t-ZrO2 powders in a water free atmosphere and their further stabilization in industrially relevant solvents.

  • 14. Uchida, N
    et al.
    Bergström, L
    YKI – Ytkemiska institutet.
    Quantification of the internal structure in ceramic green bodies using computer-assisted optical imaging1997In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 17, p. 1193-1200Article in journal (Refereed)
    Abstract [en]

    The internal structure of ceramic green bodies has been characterised by combining a computer controlled optical microscope with an image analysing system to provide statistically reliable data. The calcined porous alumina ceramic green body was made transparent by utilizing the immersion liquid technique which enabled the interior of the body to be observed. It was demonstrated how internal defects such as pores could be detected using this technique and an automated sequence of obtaining internal structure images and transferring and processing them using an image analysing software system has been developed. The technique proved to be a simple and rapid method of accurately determining the number distribution of pores in an alumina green body.

  • 15. Widegren, J
    et al.
    Bergström, L
    YKI – Ytkemiska institutet.
    The effect of acids and bases on the dispersion and stabilization of ceramic particles in ethanol2000In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 20, p. 659-665Article in journal (Refereed)
    Abstract [en]

    The dispersion and stability of alumina, titania, and silicon carbide powders in ethanolic medium have been investigated. An operational pH-scale, pH*, based on an ethanol-based reference electrode, was used to systematize the suspension properties. The electrokinetic behavior was determined as a function of pH*; we obtained isoelectric points in ethanol, pH*iep that differed significantly from the isoelectric points in aqueous media. For SiC, we obtained pH*iep=7.5; for the oxides, we obtained pH*iep=4.4 for Al2O3 and pH*iep=4.2 for TiO2. The isoelectric points were discussed in relation to the dissociation constants of the charge determining reactions at the powder surfaces. We have evaluated the long-term stability of the ethanolic dispersions through settling studies which showed that the primary particle size could be retained for extended times providing that the surface potential and ionic strength were optimized.

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