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  • 1. Dahl, P.
    et al.
    Kaus, I.
    Zhao, Z.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry, Inorganic Chemistry.
    Johnsson, Mats
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry, Inorganic Chemistry.
    Nygren, M.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry, Inorganic Chemistry.
    Wiik, K.
    Grande, T.
    Einarsrud, M. -A
    Densification and properties of zirconia prepared by three different sintering techniques2007In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 33, no 8, p. 1603-1610Article in journal (Refereed)
    Abstract [en]

    Densification of nanocrystalline yttria stabilized zirconia (YSZ) powder with 8 mol% Y2O3, prepared by a glycine/nitrate smoldering combustion method, was investigated by spark plasma sintering, hot pressing and conventional sintering. The spark plasma sintering technique was shown to be superior to the other methods giving dense materials (>= 96%) with uniform morphology at lower temperatures and shorter sintering time. The grain size of the materials was 0.21, 0.37 and 12 mu m after spark plasma sintering, hot pressing and conventional sintering, respectively. Total electrical conductivity of the materials showed no clear correlation with the grain size, but the activation energy for spark plasma sintered materials was slightly higher than for materials prepared by the two other densification methods. The hardness, measured by the Vickers indentation method, was found to be independent on grain size while fracture toughness, derived by the indentation method, was slightly decreasing with increasing grain size.

  • 2. Nunez-Gonzalez, Beatriz
    et al.
    Ortiz, Angel L.
    Guiberteau, Fernando
    Nygren, Mats
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Spark-plasma-sintering kinetics of ZrC-SiC powder mixtures subjected to high-energy co-ball-milling2013In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 39, no 8, p. 9691-9697Article in journal (Refereed)
    Abstract [en]

    The effect of SiC addition (5, 17.5, or 30 vol%) on the spark-plasma sinterability of ZrC ultra-high-temperature ceramics (UHTC) was investigated as a function of the intensity of high-energy co-ball-miffing. It was found that the spark-plasma-sintering (SPS) kinetics of the ZrC- SiC powder mixtures is enhanced progressively with increasing co-milling time, which is due to the progressive refinement of the crystallite sizes. It was also found that in practice the SiC addition is beneficial for the SPS kinetics of ZrC if the co-milling time is short enough so as to refine the SiC crystallite size only to the submicrometre range, although the improvement in sinterability does not correlate with the SiC content. On the contrary, the SiC addition is increasingly detrimental for those co-milling times that promote SiC refinement to the nanoscale. This unexpected trend is due to slower ZrC crystallite size refinement in the presence of SiC and, especially, to the complex role played during SPS by the passivating SiO2 films formed on the SiC particles, and is different from what has been observed in ZrB2-SiC. Finally, implications of interest for the UHTC community are discussed.

  • 3. Suarez, M.
    et al.
    Fernandez, A.
    Menendez, J. L.
    Nygren, Mats
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Torrecillas, R.
    Zhao, Zhe
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Effect of freeze-drying treatment on the optical properties of SPS-sintered alumina2013In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 39, no 6, p. 6669-6672Article in journal (Refereed)
    Abstract [en]

    This study looks at the influence of alumina powder processing on the preparation of transparent alumina by Spark Plasma Sintering (SPS). Zeta potential measurements were carried out on alumina suspensions in order to determine the best dispersion conditions. Stable slurries were submitted to a spray freeze drying process and their sintering behavior was compared with the corresponding non spray freeze dried powders. Transparent alumina samples were successfully prepared from alumina powders by Spark Plasma Sintering. An optical model considering pore and grain size distributions has been developed to obtain information about porosity in dense materials. It was found that the final density and, accordingly, the optical properties were improved when spray freeze dried starting powder was used.

  • 4. Suarez, M.
    et al.
    Fernandez, A.
    Menendez, J. L.
    Nygren, Mats
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Zhao, Zhe
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Torrecillas, R.
    Pablo, Acosta-Mora
    Haro-Gonzalez, P.
    Romero, J. J.
    Martin, I. R.
    Analysis of the upconversion processes of Nd3+ ions in transparent YAG ceramics2014In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 40, no 10, p. 15951-15956Article in journal (Refereed)
    Abstract [en]

    Optical properties of Nd3+ ions in YAG transparent ceramics have been analysed. The samples were prepared by a high temperature (1650 and 1680 degrees C) sintering followed by hot isostatic pressing (HIP) at 1650 or 1700 degrees C in order to obtain a well-homogeneous diffusion of the ions and functional transparency. The emission and luminescence decay curves from the F-4(3/2) metastable laser level of these samples are very similar to the ones obtained on crystal samples grown with the Czochralski method. However, excitation upconversion spectra obtained detecting at 590 nm for the transparent ceramic samples present important differences compared to the bulk YAG crystal, which can be explained by a change in the upconversion mechanism involved and the different arrangements of the Nd3+ ions in the YAG matrix. The analysis of the upconversion excitation spectra is shown to be a good method to detect the differences between the positions occupied by the Nd3+ ions in different matrices.

  • 5. Tafti, Mohsen Y.
    et al.
    Saleemi, Mohsin
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). KTH Royal Institute of Technology, Sweden.
    Han, Li
    Nong, Ngo V.
    Toprak, Muhammet S.
    On the chemical synthesis route to bulk-scale skutterudite materials2016In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 42, no 4, p. 5312-5318Article in journal (Refereed)
    Abstract [en]

    In this article an alternative high yield route for the synthesis of CoSb3-based unfilled skutterudites is presented. Using low-melting temperature salts of the constituents, melting and mixing them homogeneously in a hydrophobic liquid with postprocessing of the powders we achieve a more intimately mixed alloy compared to the conventional melting and metallurgical processes. The proposed method consists of a fast and low-temperature processing step followed by a thermochemical post-processing step, compared to the conventional methods of fabricating skutterudites, which require high temperatures and long processing times. Several structural characterization techniques were used to assess the mechanism of synthesis, verify the purity of the material as well as the reproducibility of the process. Detailed analysis and results are presented in support of the proposed process. Additionally, compaction of the powders with SPS technique provided a safe route to maintaining the nanopowder size and achieving low thermal conductivity (3 W/mK). The proposed method can easily be scaled up and adopted by the industry.

  • 6. Taheri, Mahdiar
    et al.
    Mazaheri, Mehdi
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Golestani-fard, Farhad
    Rezaie, Hamidreza
    Schaller, Robert
    High/room temperature mechanical properties of 3Y-TZP/CNTs composites2014In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 40, no 2, p. 3347-3352Article in journal (Refereed)
    Abstract [en]

    Spark Plasma Sintering (SPS) method was practiced, in the current investigation, to obtain fully dense materials with an excellent dispersion of MWCNTs (Multi-walled Carbon Nanotubes) within the matrix. Scanning and transmission electron microscopy micrographs as well as Raman Spectroscopy revealed no conspicuous signs of damages on the nanotubes' structure, as a result of applying appropriate processing measures. Measurements of mechanical properties' revealed the value of 15.2 +/- 0.4 MPa m(1/2) for indentation fracture toughness, impling our success to align transformation toughening (while the zirconia has the grain size around 1 mu m) with the effects of MWCNTs addition (5 wt%). High temperature mechanical properties of the composites have been investigated using Mechanical Spectroscopy (MS) up to 1600 K at frequency equal to 1 Hz. The MS results proved that CNTs could hinder grain-boundary sliding and subsequently lead to an enhanced creep resitance at high temperatures.

  • 7. You, Qiangwei
    et al.
    Liu, Yongsheng
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Polytechnical University, Xi’an, Shanxi, PR China.
    Wan, Jiajia
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Li, Hongxia
    Yuan, Bo
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Sinosteel Luoyang Institute of Refractories Research Co., Ltd, PR China.
    Cheng, Laifei
    Wang, Gang
    Microstructure and properties of porous SiC ceramics by LPCVI technique regulation2017In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 43, no 15, p. 11855-11863Article in journal (Refereed)
    Abstract [en]

    A new gradient pore structure in porous SiC ceramics was fabricated by low pressure chemical vapor infiltration (LPCVI). Effects of deposition duration on the mechanical properties and permeability of porous SiC ceramics were investigated. Results demonstrated that pore diameter and shapes decreased from the surface to the interior along with LPCVI duration. Porous SiC ceramics with deposition duration of 160 h exhibited flexural strength of 48.05 MPa and fracture toughness of 1.30 MPa m(1/2), where 221% and 189% improvements were obtained compared to porous SiC ceramics without LPCVI, due to CVI-SiC layer strengthening effect. Additionally, at the same gas velocity, pressure drop increase rate was faster due to apparent porosity and pore size change.

  • 8. Yuan, Bo
    et al.
    Wang, Gang
    Li, Hongxia
    Liu, Leifeng
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Liu, Yongsheng
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Fabrication and microstructure of porous SiC ceramics with Al2O3 and CeO2 as sintering additives2016In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 42, no 11, p. 12613-12616Article in journal (Refereed)
    Abstract [en]

    In the present study, porous silicon carbide ceramics were prepared via spark plasma sintering at relatively low temperatures using Al2O3 and CeO2 as sintering additives. Sacrificial template was selected as the pore forming mechanism, and gelcasting was used to fix the slurry in a short time. The evolution process of the microstructures during different steps was observed by SEM. The influence of the sintering temperature and sintering additives on the shrinkage and porosity of the samples was studied. The microstructures of different samples were characterized, and the mechanical properties were also evaluated.

  • 9. Zamora, Victor
    et al.
    Nygren, Mats
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Guiberteau, Fernando
    Ortiz, Angel L.
    Effect of graphite addition on the spark-plasma sinterability of ZrB2 and ZrB2-SiC ultra-high-temperature ceramics2014In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 40, no 7, p. 11457-11464Article in journal (Refereed)
    Abstract [en]

    The effect of a 2 wt% addition of particulate graphite on the spark-plasma sintering (SPS) kinetics of both ZrB2 and ZrB2+30 vol% SiC was investigated using to that end two broad sets of powder mixtures prepared by high-energy co-ball-milling. It was observed that the particulate graphite addition enhances the SPS kinetics of both systems, and it was identified that, contrary to the case in conventional sintering, this improvement in sinterability is not due to the carbothermal reduction of oxides in situ during SPS but to the lubrication imposed by the graphite flakes inducing a greater green-body densification. Consequently, the graphite is not consumed during SPS but remains in the microstructure of the resulting ultra-high-temperature ceramics (UHTCs) as flaky particles dispersed homogeneously at grain boundaries. Implications of interest for the UHTC community are discussed.

  • 10. Zamora, Victor
    et al.
    Ortiz, Angel L.
    Guiberteau, Fernando
    Nygren, Mats
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    On the enhancement of the spark-plasma sintering kinetics of ZrB2-SiC powder mixtures subjected to high-energy co-ball-milling2013In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 39, no 4, p. 4191-4204Article in journal (Refereed)
    Abstract [en]

    The spark-plasma sintering (SPS) kinetics of ZrB2-SiC powder mixtures was investigated as a function of the degree of high-energy co-ball-milling and of the SiC content (5, 17.5, or 30 vol%). As in ZrB2 without SiC, it was found that the crystal size refinement induced by the continued milling progressively enhances the SPS kinetics of ZrB2-SiC, again only moderately if the refinement is to the ultra-fine range, but very marked if the refinement is to the nanoscale. It was also found that the SiC addition further enhances the SPS kinetics of ZrB2, although the improvement did not scale directly with the SiC content, and became less relevant with the refinement of the ZrB2 crystal sizes to the nanoscale. The improved kinetics induced by the SiC addition was identified as being due to the formation of amorphous borosilicate from the oxide passivating layers on the ZrB2 and SiC particles. This not only speeds up the interparticle diffusion, but also it is segregated under the application of pressure into the multi-grain joints, filling pores. The enhanced kinetics induced by the progressive milling is due to the continuous reduction of the diffusion distances and to the development of a greater density of grain boundaries available as faster diffusion paths, together with the greater formation of amorphous borosilicate. Implications of interest for the ultra-high-temperature ceramics community are discussed.

  • 11. Zhang, Ranran
    et al.
    Liu, Xujie
    Xiong, Zhiyuan
    Huang, Qianli
    Yang, Xing
    Yan, Hao
    Ma, Jing
    Feng, Qingling
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, China.
    Novel micro/nanostructured TiO2/ZnO coating with antibacterial capacity and cytocompatibility2018In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 44, no 8, p. 9711-9719Article in journal (Refereed)
    Abstract [en]

    Insufficient osseointegration between implants and bone, and peri-implant infection are the two most common factors contributing to implantation failure. As such, implant materials should ideally be designed with antibacterial capacity while maintaining cytocompatibility. Combining Zn, a known antibacterial agent, with micro/nanostructures on the implant surface to promote osseointegration, potentially reduces peri-implant infection. To this end, in the present study, we developed novel bifunctional micro/nanostructured titanium oxide/zinc oxide (TiO2/ZnO) coating by micro-arc oxidation (MAO) followed by hydrothermal treatment. The release rate of the Zn2+ ions was reduced by heat treatment which established a balance between antibacterial capacity and cytocompatibility. The results demonstrate a simple approach for developing a Zn-based material that balances antibacterial capacity and cytocompatibility as a promising coating material for Ti implants.

  • 12.
    Zhao, Jing
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Peking University.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsing Hua University .
    Si, Wenjie
    Wang, Xinzhi
    Bi-colored zirconia as dental restoration ceramics2013In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 39, no 8, p. 9277-9283Article in journal (Refereed)
    Abstract [en]

    Machinable zirconia blocks with gradient colors are fundamental for fabrication of full-contour esthetic dental restorations. The aim of this study was to process and evaluate bi-colored zirconia ceramics as a pilot dental material by using well-established techniques. Two commercially available partially stabilized zirconia granules, one undoped and one doped with 0.202 wt% Fe2O3, resulted in white and yellow colors after sintering, respectively. Bi-colored zirconia was fabricated by two-step dry pressing of both zirconia granules one above the other to form green bodies, followed by cold isostatic pressing (CIP) and, a two-step pressureless sintering finally at 1450 degrees C. The dilatometer results showed that the Fe2O3 doped zirconia sintered slightly rapid, but the difference of shrinkage between two powders was <1%. Sintered bars achieved full density, 6.018 g/cm(3) (similar to 99%TD), without cracks in the similar to 1 mm color gradient zone. The microstructures were characterized by scanning electron microscopy (SEM) and careful observation of both surface and interior provided no obvious structural difference of either grains or pores among the three distinct regions, comprising white, yellow and color gradient zone. Vickers hardness of hi-colored zirconia was similar to 13.1 GPa, with no obvious difference in the three regions. The four-point bending strength of the hi-colored zirconia bars was 745.5 +/- 159.6 MPa, which appeared noticeably lower than that of the single-colored references being above 1000 MPa. Fractographic analysis revealed that in most of the cases (60%) the fracture was initiated at the color gradient zone, where large voids with high coordination numbers, agglomerates with critical size and concentration of irregular grains with porous surfaces were observed. Above all, bi-colored zirconia ceramics prepared by the improved technique could meet the basic requirements of dental materials. The ways of minimizing the defects within hi-colored blocks should be developed for the production of esthetic zirconia ceramics of high strength and reliability.

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