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  • 1. Herrmann, Mathias
    et al.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Schulz, Ingrid
    Hu, Jianfeng
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Jancar, Bostjan
    Silicon nitride nanoceramics densified by dynamic grain sliding2010In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 25, p. 2354-2361Article in journal (Refereed)
    Abstract [en]

    The densification behaviors of two silicon nitride nanopowder mixtures based respectively on a-Si3N4 and ß-Si3N4 as the major phase constituent were studied by spark plasma sintering. Sintering conditions were established where a low viscous liquid not in equilibrium with the main crystalline constituent(s) stimulated the grain sliding yet did not activate the reprecipitation mechanism that unavoidably yields grain growth. By this way of dynamic grain sliding full densification of silicon nitride nanoceramics was achieved with no noticeable involvement of a- to ß-Si3N4 phase transformation and grain growth. This processing principle opens the way toward flexible and precise tailoring of the microstructures and properties of Si3N4 ceramics. The obtained silicon nitride nanoceramics showed improved wear resistance, particularly under higher Hertzian stresses.

     

  • 2.
    Hu, Jianfeng
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Grain growth by Ordered Coalescence of crystallites in Ceramics: Grain Growth Mechanisms, Microstructure Evolution and Sintering2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Grain growth and densification process play the two most crucial roles on the microstructure evolution and the achieved performances during sintering of ceramics. In this thesis, the grain growth of SrTiO3, BaTiO3-SrTiO3 solid solutions and Si3N4 ceramics during spark plasma sintering (SPS) were investigated by electron microscopy.

    SrTiO3 ceramics starting from nanopowders were fabricated by SPS. A novel grain growth mechanism was discovered and named as ordered coalescence (OC) of nanocrystals. This mechanism involved nanocrystals as building blocks and is distinguished from atomic layer epitaxial growth (AEG) in classical sintering theory. The results also revealed that the dominant grain growth mechanism can be changed by varying heating rates. Low rate (10°C/min) gives AEG, whereas high rates (≥ 50°C/min) yields three-dimensional coalescence of nanocrystals, i.e. OC.

    BaTiO3-SrTiO3 sintered bodies were made by SPS of BaTiO3 and SrTiO3 nanopowders mixtures. A novel Sr1-xBaxTiO3 “solid solution” with mosaic-like single crystal structure was manufactured by OC of the precursor crystallites. This reveals a new path for preparation of solid solution grains or composites.  

    Si3N4 ceramics were prepared from α- or β-Si3N4 nanopowders at the same SPS conditions. The anisotropic OC of precipitated β-Si3N4 crystallites gives elongated β-Si3N4 grains at 1650°C using α-Si3N4 nanopowder. In contrast, AEG leads to the equi-axed β-Si3N4 grains using β-Si3N4 nanopowder. The metastable α- to β-Si3N4 phase transformation and OC accelerates anisotropic grain growth.

    Grain motions contribute to the densification process during pressureless sintered 3Y-ZrO2 (>87%TD) or SPS of SrTiO3 (>92%TD) ceramics. This extends the sintering range for active grain re-arrangement over that predicted by classical theory.

    In this thesis a new grain growth mechanism (OC) is proved by using SPS and nanopowders. By OC the microstructural evolution can be manipulated.

  • 3.
    Hu, Jianfeng
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Grain growth by multiple ordered coalescence of nanocrystals during spark plasma sintering of SrTiO3 nanopowders2012In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 60, no 18, p. 6405-6412Article in journal (Refereed)
    Abstract [en]

    Sintering is the most widely applied densification process for manufacturing polycrystalline materials in powder metallurgy and ceramic industries. Grain growth behavior during sintering has a crucial influence on the final microstructure and thus the achieved performance. So far, it has been accepted that grain growth, based on classic crystal growth theory, takes place via atomic diffusion driven by excess interfacial energy. This paper presents a novel grain growth mechanism resulting from multiple ordered coalescence of nanocrystals via the activation of rapid grain motions. In rapid solid-state sintering of a strontium titanate (SrTiO3) nanopowder, individual SrTiO3 nanocrystals can act as the building blocks and self-assemble to form larger grains. A quasi-liquid interfacial film achieved by surface melting of the nanocrystals plays an essential role in this new process by facilitating the grain motion and ordered coalescence of nanocrystals. The imperfect ordered coalescence of nanocrystals introduce deep structural heterogeneities characterized by the unique quasi-interfaces inside grown grains of single crystal signature. The quasi-interfaces consist of the vacancy arrays and/or aggregated line defects.

  • 4.
    Hu, Jianfeng
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Grain growth kinetics determined by the heating rate during spark plasma sintering: 2Dnucleation versus ordered coalescence of nanocrystals2013Article in journal (Refereed)
  • 5.
    Hu, Jianfeng
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Mosaic-like structure in Barium Strotium Titanate solid solutionManuscript (preprint) (Other academic)
  • 6.
    Hu, Jianfeng
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Ordered coalescence of nano crystallites contributing to the rapid anisotropic grain growth in silicon nitride ceramics2013In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 69, no 3, p. 270-273Article in journal (Refereed)
    Abstract [en]

    Microstructural characterization is performed on two dense Si3N4 ceramic samples consolidated by spark plasma sintering (SPS): one fabricated using alpha-Si3N4 and the other using beta-Si3N4 as the starting powder. A novel mechanism is revealed where ordered coalescence of nano beta-crystallites accelerate the rapid beta-Si3N4 anisotropic grain growth. The rapid alpha- to beta-Si3N4 phase transformation via a high supersaturation of dissolved Si3N4 in the melt favors this mechanism. The high heating rate by SPS is essential for achieving such supersaturation.

  • 7.
    Li, Duan
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hu, Jianfeng
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Zhang, J. -Z
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Xi'An University of Architecture & Technology, Xian, China.
    Ma, J.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsing Hua University, Beijing, China.
    Densification as an exothermic process revealed by rapid high temperature consolidation of BaTiO3 nanopowder2014In: Advances in Applied Ceramics: Structural, Functional and Bioceramics, ISSN 1743-6753, E-ISSN 1743-6761, Vol. 113, no 4, p. 251-256Article in journal (Refereed)
    Abstract [en]

    Densification is an exothermic process according to the classical sintering theories; however, it has never been explored experimentally. In the present work, such heat release was successfully detected from nanosized BaTiO3 nanopowder compact, which was rapidly consolidated by spark plasma sintering. A reduction of total power consumption was observed immediately when rapid densification occurred. The effects of the deviation of overall electric resistance on total power consumption were analysed. The temperature at which a falling inflection point of the power supply was observed can be used as an indicator of the minimum temperature required for densification. This would be of help for defining the 'kinetic window' for processing of nanoceramics in sintering practice.

  • 8.
    Shen, Zhijian
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Yan, Haixue
    Grüner, Daniel
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Belova, Lyubov M.
    Sakamoto, Yasuhiro
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hu, Jianfeng
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Nan, Ce-Wen
    Hoeche, Thomas
    Reece, Michael J.
    Ferroelectric ceramics with enhanced remnant polarization by ordered coalescence of nano- crystals2012In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 22, no 44, p. 23547-23552Article in journal (Refereed)
    Abstract [en]

    An exceptional high ferroelectric remnant polarization (P-r) was observed in BaTiO3 ceramics owing to the formation of micron-sized grains possessing nano-scale mosaicity. Such a structural hierarchy was developed via a novel crystal-growth mechanism, namely ordered coalescence of nano-crystals achieved by synergetic atomic epitaxial growth and self-assembly of nano-crystals. The accommodating lattice defects in sub-grain boundaries due to the imperfect assembly of nano-crystals significantly contribute to the P-r enhancement by stimulating the dynamics of ferroelectric domain formation and switching. This finding defines a new approach to nanopowder sintering leading to enhanced properties sensitive to lattice defects.

  • 9.
    Xiong, Yan
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hu, Jianfeng
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Dynamic Pore Coalescence in Nanoceramic Consolidated by Two-Step Sintering Procedure2013In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 33, no 11, p. 2087-2092Article in journal (Refereed)
    Abstract [en]

    Two-step sintering (TSS) concept was adopted in the consolidation of 3 mol% yttria doped zirconia nanopowder. Partially densified bodies with 87% theoretical density (TD) were firstly prepared using high-pressure spark plasma sintering (SPS) technique and followed by second-step pressureless sintering. The samples achieved only 96% TD final density after 30 h soaking. It was found that the densification process was impeded by dynamic pore coalescence with a pore growth factor of 10. The phenomenon was explained by the coalescence of interconnected small pores generated by differential sintering of nanoceramic green compacts. Such pore coalescence was accompanied with particle movement, which resulted in sintering state deviating from the frozen state. Present results indicated that the active range for particle rearrangement was greatly extended during nanoceramic sintering and the efficiency of TSS approach was greatly dependent on the homogeneity of green bodies

  • 10.
    Xiong, Yan
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hu, Jianfeng
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Inorganic and Structural Chemistry.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Pouchly, Vaclav
    Maca, Karel
    Preparation of Transparent Nanoceramics by Suppressing Pore Coalescence2011In: Journal of The American Ceramic Society, ISSN 0002-7820, E-ISSN 1551-2916, Vol. 94, no 12, p. 4269-4273Article in journal (Refereed)
    Abstract [en]

    Microstructural developments in nanoceramics were investigated in 3Y-TZP compacts with relative density (RD) exceeding 93%. Special attentions were paid to the evolutions of pore structures. It was found that the densification process of nanoceramic compacts with apparently close porosity was greatly jeopardized by pore coalescence. This observation was interpreted by the coalescence of locally interconnected pores originated from inhomogeneous packing of particles. The pore coalescence can be suppressed by application of an external pressure. The processing principle was demonstrated by spark plasma sintering (SPS) with extended holding at a minimized sintering temperature. The highly dense 3Y-TZP nanoceramics containing no large pores became optically transparent.

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