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  • 1. Adolfsson, E
    et al.
    Shen, Z
    Stockholm University.
    Densification of zirconia-hydroxyapatite ceramics without phase changes2006In: Bioceramics 18 PTS 1 and PTS 2, Vol. 309-311, 1141-1144 p.Article in journal (Refereed)
  • 2. Adolfsson, Erik
    et al.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Effects of granule density on strength and granule related defects in zirconia2012In: Journal of the European Ceramic Society, ISSN 0955-2219, Vol. 32, no 11, 2653-2659 p.Article 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.

  • 3. Ahmed, Istaq
    et al.
    Kinyanjui, Francis G.
    Steegstra, Patrick
    Shen, Zhijian J.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Eriksson, Sten-G.
    Nygren, Mats
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Improved Proton Conductivity in Spark-Plasma Sintered Dense Ceramic BaZr0.5In0.5O3-delta2010In: Electrochemical and solid-state letters, ISSN 1099-0062, Vol. 13, no 11, b130-B134 p.Article in journal (Refereed)
    Abstract [en]

    Spark-plasma sintering method was used to prepare dense proton conducting perovskite oxide BaZr0.5In0.5O3-delta. Analysis of X-ray powder diffraction data showed that the sample adopt the cubic crystal structure having the space group Pm3m. Thermogravimetric analysis of prehydrated samples showed significant mass losses beyond 300 degrees C due to loss of protons as water vapor. Scanning electron microscope images show that the grain size of the spark-plasma sintered dense sample was smaller than that of solid-state sintered porous sample. The highest total proton conductivity (2 X 10(-3) S cm(-1) at 450 degrees C) was found for dense spark-plasma sintered sample under wet H-2 than the samples prepared by other routes.

  • 4. An, Di
    et al.
    Li, Hezhen
    Xie, Zhipeng
    Zhu, Tianbin
    Luo, Xudong
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, China.
    Ma, Jing
    Additive manufacturing and characterization of complex Al2O3 parts based on a novel stereolithography method2017In: International Journal of Applied Ceramic Technology, ISSN 1546-542X, E-ISSN 1744-7402, Vol. 14, no 5, 836-844 p.Article in journal (Refereed)
    Abstract [en]

    In this work, we prepared Al2O3 ceramic green parts with complex geometry and architecture using an additive manufacturing process based on stereolithography. The rheological and thermal behavior of Al2O3 slurry was firstly examined and used to establish the conditions for molding and debinding. As opposed to previous researches that only focused on manufacture techniques, the sintering behavior and densification process were systematically investigated. In addition, special attentions were paid to the evolution of microstructure between green bodies and sintered parts. The results showed that debound parts were equipped with uniform particle packing and narrow pore size distribution. The dimensions of the Al2O3 parts changed anisotropically with the different processing steps. The densification process was greatly accelerated by the decrease in pore size and annihilating of interconnected pores in which significant grain growth was observed above 1450 degrees C. The sintered part also had a homogeneous microstructure and no interface between adjacent layers. High densification (relative density of 99.1%) and much desirable Vickers hardness (17.9 GPa) of Al2O3 parts were achieved at the sintering temperature of 1650 degrees C.

  • 5. Balazsi, C
    et al.
    Weber, F
    Kover, Z
    Shen, Z
    Stockholm University.
    Konya, Z
    Kasztovszky, Z
    Vertesy, Z
    Biro, LP
    Kiricsi, I
    Arato, P
    Application of carbon nanotubes to silicon nitride matrix reinforcements2006In: Current Applied Physics, Vol. 6, 124-130 p.Article in journal (Refereed)
  • 6. Buixaderas, E.
    et al.
    Bovtun, V.
    Kempa, M.
    Savinov, M.
    Nuzhnyy, D.
    Kadlec, F.
    Vanek, P.
    Petzelt, J.
    Eriksson, M.
    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).
    Broadband dielectric response and grain-size effect in K0.5Na0.5NbO3 ceramics2010In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 107, no 1, 14111- p.Article in journal (Refereed)
    Abstract [en]

    Dielectric spectra of two K0.5Na0.5NbO3 ceramics with different grain sizes (10 and 0.5 mu m) were measured from 10(2) to 10(14) Hz in a broad temperature range. The sequence of first-order phase transitions (cubic-tetragonal-orthorhombic-rhombohedral) was detected by differential scanning calorimetry, dielectric spectroscopy, and time-domain terahertz spectroscopy. The grain size affects all the phase transitions, which are more smeared in the small-grain sample. In the large-grain ceramics, two well-separated near-Debye relaxations are seen in the tetragonal phase, which suddenly merge on cooling across the tetragonal-orthorhombic transition, and on further cooling the lower-frequency relaxation strongly broadens. On reducing the grain size, the higher-frequency relaxation shifts from similar to 1 to similar to 20 GHz and the lower-frequency one strongly broadens. Without quantitative understanding, these effects could be assigned to domain-wall dynamics and its temperature and grain-size dependences. Similar to pure KNbO3, an overlapped central-mode-soft-mode type excitation was detected in the terahertz range related to the effective hopping and oscillations of the off-centered Nb ions in a multiwell potential.

  • 7.
    Cai, Yanbing
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Nygren, Mats
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Grins, Jekabs
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Esmaeilzadeh, Saeid
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Thermal Properties of Nitrogen-Rich Ca-α-Sialons2009In: Journal of the European Ceramic Society, ISSN 0955-2219, Vol. 29, no 16, 3409-3417 p.Article in journal (Refereed)
    Abstract [en]

    Nitrogen-rich Ca-α-Sialon (CaxSi12−2xAl2xN16 with x = 0.2, 0.4, and 0.8, 1.2 and 1.6) ceramics were prepared from the mixtures of Si3N4, AlN and CaH2 powders in a hot press at 1800 °C using a pressure of 35 MPa and a holding time of 4 h, and then were investigated with respect to reaction mechanism, phase stability and oxidation resistance. In addition the sample with x = 1.6 was prepared in the temperature range 600–1800 °C using a pressure of 35 MPa and a holding time of 2 h. The α-Sialon phase was first observed at 1400 °C but the α-Si3N4 and AlN phases were still present at 1700 °C. Phase pure Ca-α-Sialon ceramics could not be obtained until the sintering temperature reached 1800 °C. The phase pure nitrogen-rich Ca-α-Sialon exhibited no phase transformation in the temperature range 1400–1600 °C. In general, mixed α/β-Sialon showed better oxidation resistance than pure α-Sialon in the low temperature range (1250–1325 °C), while α-Sialons with compositions located at α/β-Sialon border-line showed significant weight gains over the entire temperature range tested (1250–1400 °C). The phases formed upon oxidation were characterized by X-ray, SEM and TEM studies.

  • 8.
    Cai, Yanbing
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Grins, Jekabs
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Saeid, Esmaeilzadeh
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Sialon Ceramics Prepared by Using CaH2 as a Sintering Additive2008In: Journal of the American Ceramic Society, ISSN 0002-7820, Vol. 91, no 9, 2997-3004 p.Article in journal (Refereed)
    Abstract [en]

    In developing elongated a-sialon grains, a nitrogen-rich liquidphase sintering method was introduced by using CaH2 as asintering aid, so as to vary the N/O ratio of the liquid phaseformed in the sintering process while keeping the Si/Al ratiosconstant. With increasing addition the phase contents changedfrom single b-sialon to dual b/a-sialons and to single Ca-a-sialon.At low N/O ratios the microstructures contained mainlyequiaxed b-sialon grains, and at high N/O ratios well facetedelongated Ca-a-sialon grains. The improved toughness(KIC57.8 MPa .m1/2) and hardness (HV10517.5 GPa) propertiescan be attributed to the formation of interlocked microstructures.High-temperature compressive deformation testsindicated that the deformation onset temperature is determinedmainly by the Si/Al and N/O ratios, whereas the deformationrate is affected by the microstructure, i.e., the morphology andamounts of elongated a-sialon grains and residual glass phase,especially for the sialons with low N/O ratios.

  • 9. Candelario, Victor M.
    et al.
    Moreno, Rodrigo
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Guiberteau, Fernando
    Ortiz, Angel L.
    Liquid-phase assisted spark-plasma sintering of SiC nanoceramics and their nanocomposites with carbon nanotubes2017In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 37, no 5, 1929-1936 p.Article in journal (Refereed)
    Abstract [en]

    The appropriate conditions for liquid-phase assisted spark-plasma sintering (SPS) were identified for the fabrication of both SiC nanoceramics and their nanocomposites with carbon nanotubes (CNTs). A parametric study of the nanoceramics and nanocomposites with a given type of CNTs showed that the SPS temperature (as measured by the radial optical pyrometer) optimizing their densification, nanograin size, and mechanical properties is 1700 degrees C (soaking for a few minutes), below which there is incomplete densification, and above which there is obvious grain growth with no benefit in hardness or toughness in the case of the nanoceramics, and prejudicial to both properties in the case of the nanocomposites due to the CNT degradation. It was also shown that the nanocomposites have smaller nanograins than their nanoceramic counterparts, and are softer but tougher. Extension to nanocomposites with different types of CNTs confirmed these trends, and showed that the CNT features do not condition the densification, microstructure or mechanical properties of these nanocomposites.

  • 10. Candelario, Victor M.
    et al.
    Moreno, Rodrigo
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Ortiz, Angel L.
    Aqueous colloidal processing of nano-SiC and its nano-Y3Al5O12 liquid-phase sintering additives with carbon nanotubes2015In: Journal of the European Ceramic Society, ISSN 0955-2219, Vol. 35, no 13, 3363-3368 p.Article in journal (Refereed)
    Abstract [en]

    Carbon nanotubes (CNTs) have occasionally been observed to benefit the aqueous colloidal processing of nano-SiC with its nano-Y3Al5O12 liquid-phase-sintering additives. Experimental evidence is here presented for a broad set of CNTs with different morphology and/or surface functionalization confirming that CNTs (7 vol.% addition), regardless of their features, prevent the coagulation of these nanoceramic suspensions, whence it is inferred that aqueous colloidal processing is well-suited for the environmentally friendly preparation of the homogeneous mixtures of nanoceramic particles and CNTs required for the fabrication of CNT-reinforced ceramic matrix nanocomposites. Furthermore, it is shown that surface-functionalized CNTs seem to work better than deflocculated CNTs for the preparation of stable concentrated colloidal suspensions, whose rheological properties are in general very close, but with thinner CNTs being nonetheless preferable. Finally, the feasibility is demonstrated of fabricating SiC/CNT nanocomposites by aqueous colloidal processing followed by liquid-phase assisted spark-plasma sintering.

  • 11. Chaim, R.
    et al.
    Marder, R.
    Estournes, C.
    Shen, Zhijian James
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Densification and preservation of ceramic nanocrystalline character by spark plasma sintering2012In: Advances in Applied Ceramics: Structural, Functional and Bioceramics, ISSN 1743-6753, Vol. 111, no 5-6, 280-285 p.Article in journal (Refereed)
    Abstract [en]

    Spark plasma sintering is a hot pressing technique where rapid heating by dc electric pulses is used simultaneously with applied pressure. Thus, spark plasma sintering is highly suitable for rapid densification of ceramic nanoparticles and preservation of the final nanostructure. A considerable portion of the shrinkage during densification of the green compact of nanoparticles in the first and intermediate stages of sintering occurs during heating by particle rearrangement by sliding and rotation. Further densification to the final stage of sintering takes place by either plastic yield or diffusional processes. Full densification in the final stage of sintering is associated with diffusional processes only. Nanoparticle sliding and rotation during heating may also lead to grain coalescence, with much faster kinetics than normal grain growth at higher temperatures. Based on existing models for particle rearrangement and sliding, the contributions of these processes in conjunction with nanoparticle properties and process parameters were highlighted.

  • 12. Chaim, Rachman
    et al.
    Kalina, Michael
    Shen, James Z.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry, Inorganic Chemistry.
    Transparent yttrium aluminum garnet (YAG) ceramics by spark plasma sintering2007In: Journal of the European Ceramic Society, ISSN 0955-2219, Vol. 27, no 11, 3331-3337 p.Article in journal (Refereed)
    Abstract [en]

    Commercial nanocrystalline yttrium aluminum garnet (nc-YAG) powders were used for fabrication of dense and transparent YAG by spark plasma sintering (SPS). Spherical 34 nm size particles were densified by SPS between 1200 and 1500 degrees C using 50 and 100 MPa pressures for 3, 6, and 9 min durations. Fully dense and transparent polycrystalline cubic YAG with micrometer grain size were fabricated at very moderate SPS conditions, i.e. 1375 degrees C, 100 MPa for 3 min. Increase in the SPS duration and pressure significantly increased the density especially at the lower temperature range. The observed microstructure is in agreement with densification by nano-grain rotation and sliding at lower densities, followed by curvature driven grain boundary migration and normal grain growth at higher densities. Residual nanosize pores at the grain boundary junctions are an inherent microstructure feature due to the SPS process.

  • 13. Chaim, Rachman
    et al.
    Reshef, Ram
    Liu, Guanghua
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Low-temperature spark plasma sintering of NiO nanoparticles2011In: Materials Science & Engineering: A, ISSN 0921-5093, Vol. 528, no 6, 2936-2940 p.Article in journal (Refereed)
    Abstract [en]

    NiO nanoparticles of 20 nm in diameter were spark plasma sintered between 400 °C and 600 °C for 5 and 10 min durations. Application of 100 MPa pressure from room temperature resulted in densities between 75% and 92%. The final grain size was between 26 nm and 68 nm. Lower densities were recorded when 100 MPa was applied at the SPS temperature. Two shrinkage rate maxima of 3.4 × 10−3 s−1 and 2 × 10−3 s−1 were observed around 390 ± 10 °C and at the SPS temperature. The two shrinkage rate maxima were related to densification by particle sliding followed by diffusional grain boundary sliding during the heating. The strong effects of the surface and interfacial processes which are active during the SPS were highlighted.

  • 14. de Moraes, Elisangela Guzi
    et al.
    Li, Duan
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Colombo, Paolo
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Silicon nitride foams from emulsions sintered by rapid intense thermal radiation2015In: Journal of the European Ceramic Society, ISSN 0955-2219, Vol. 35, no 12, 3263-3272 p.Article in journal (Refereed)
    Abstract [en]

    Open cell Si3N4 foams were fabricated by an emulsification process and subsequent sintering. Emulsification took place in an aqueous environment by using octane as alkane phase and a surfactant as stabilizer. Effects of different octane concentrations (50 vol% and 70 vol%) and stirring velocities (700 rpm and 1000 rpm) on porosity and pore size distribution were investigated. The shaped foams were pressureless sintered at 1600 degrees C inside a modified SPS set-up. Si3N4 foams decorated with SiC nanowires and with an average cell size of 8-41 mu m and total porosity of 80-86 vol% were obtained. The compressive strength ranged from 2.0 MPa up to 9.9 MPa. Both the foaming and sintering processes contributed to the development of different microstructures.

  • 15. Dominguez-Rodriguez, Arturo
    et al.
    Gomez-Garcia, Diego
    Zapata-Solvas, Eugenio
    Shen, James Z.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry, Inorganic Chemistry.
    Chaim, Rachman
    Making ceramics ductile at low homologous temperatures2007In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 56, no 2, 89-91 p.Article in journal (Refereed)
    Abstract [en]

    Polycrystalline ceramics are brittle at room temperature but may deform by dislocation slip processes at higher temperatures, where T >= 0.5 T-M (T-M, the melting temperature), leading to strain hardening, i.e. the total strain to rupture is limited. We show that mutual sliding of nanometer grains and atomic transport along their boundaries enables low-temperature metal-like plasticity in nanocrystalline ceramics. Compression of fully dense MgO nanoceramics at temperatures as low as 700 degrees C (0.31 T-M) exhibited stress-strain curves with no strain hardening.

  • 16.
    Eden, Mattias
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry, Physical Chemistry.
    Grins, Jekabs
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry, Inorganic Chemistry.
    Jansson, Kjell
    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.
    Al and O substitutions in BaS-phases, Ba2AlxSi12-xN16-xO2+x: A TEM, XRD and solid-state NMR study2008In: Solid State Sciences, ISSN 1293-2558, Vol. 10, no 1, 50-60 p.Article in journal (Refereed)
    Abstract [en]

    In a series of Ba-based oxonitrido-silicate S-phases (Ba2AlxSi12-xN16-xO2+x) spanning a compositional range up to x approximate to 3, we examine the incorporation of Al and O by Si-29 and Al-27 magic-angle spinning (MAS) solid state nuclear magnetic resonance (NMR) and Al-27 triple-quantum MAS (3QMAS). The 3QMAS spectra reveal Al-27 signals from two distinct structural environments, assigned to AlN4 or AlN3O tetrahedra, respectively, and with their relative amounts depending on the S-phase substitution parameter x. Si-29 NMR show variable fractions of SiN4 and SiN3O environments. The NMR results accord overall with a structural substitution model for which O enters at one crystallographic position (occupied according to N4-xOx), in conjunction with a random Al for Si substitution at two distinct crystallographic positions. This leads to S-phase frameworks built from SiN4, SiN3O, AlN4 and AlN3O tetrahedra.

  • 17.
    Eriksson, Mirva
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Liu, Yi
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hu, Jiangfeng
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Gao, Lian
    Nygren, Mats
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Shen, Zhijian James
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Transparent hydroxyapatite ceramics with nanograin structure prepared by high pressure spark plasma sintering at the minimized sintering temperature2011In: Journal of the European Ceramic Society, ISSN 0955-2219, Vol. 31, no 9, 1533-1540 p.Article in journal (Refereed)
    Abstract [en]

    A hydrothermally processed bulky powder composed of loosely aggregated nano-sized rods was consolidated by spark plasma sintering. The use of a high pressure cell allows the application of pressure up to 500 MPa. It was found that applying of high pressure is beneficial for widening up the kinetic window for attaining dense HAp nanoceramics. The high transparency of HAp nanoceramics obtained in this study is ascribed to the high density and homogeneous nano-grained structure achieved besides the unique intrinsic optical properties of the HAp crystal itself, i.e. its low refractive index and very small birefringence. Achieving full densification at the minimized sintering temperature allows for the first time the preparation of transparent HAp nanoceramics with stoichiometric composition, i.e. avoiding the loss of structural water that commonly takes place during the conventional ways of sintering.

  • 18.
    Eriksson, Mirva
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Radwan, Mohamed
    Shen, Zhijian James
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Spark plasma sintering of wc, cemented carbide and functional graded materials2013In: International journal of refractory metals & hard materials, ISSN 0958-0611, ISSN 0263-4368, Vol. 36, 31-37 p.Article in journal (Refereed)
    Abstract [en]

    Spark plasma sintering (SPS) is an extremely fast solidification technique for compounds that are difficult to sinter within the material group's metals, ceramics, or composites thereof. SPS uses a uniaxial pressure and a very rapid heating cycle to consolidate these materials. The heating is generated by Joule effect when a strong, pulsed electric current passes the conductive graphite die and also through the sample, if conductive. Cemented carbides (hard metals) are mostly used for metal cutting and drilling, wood cutting or rock drilling tools and are consolidated either by pressureless sintering (PLS), hot pressing (HP), or hot isostatic pressing (HIP). With SPS the main benefit is the ability to control the WC grain size due to the short sintering times at high temperature. In addition, unwished reactions between WC and cobalt to form other phases are minimized. By SPS the amount of cobalt can be reduced towards zero in fully dense WC materials. With this technique it is easy to prepare gradient materials where a ductile weldable metal can be joined with the cemented carbide part.

  • 19.
    Eriksson, Mirva
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Department of Materials Chemistry.
    Salamon, David
    Nygren, Mats
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Department of Materials Chemistry.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Department of Materials Chemistry.
    Spark plasma sintering and deformation of Ti-TiB2 composites.2008In: Materials Science & Engineering, A: Structural Materials: Properties, Microstructure and Processing, ISSN 0921-5093, Vol. 1-2, no A475, 101-104 p.Article in journal (Refereed)
    Abstract [en]

    Spark plasma sintering (SPS) was used to investigate the densification and deformation behaviour of Ti–TiB2 composites. Fully densified samples were prepared with Ti addition larger than 5%. The prepared composites can be deformed under compression at 1700 °C to achieve a strain of 50% without cracking. At lower temperatures, cracks were initiated due to low ductility of TiB2 and low content of Ti. During the sintering and deformation, TiB is formed via a reaction between Ti and TiB2. To elucidate the formation mechanism of TiB in the SPS process, reactive sintering of TiB using element precursors was also performed. Fully dense samples were prepared but it was not possible to prepare pure uniphase TiB. The reactive sintering resulted in the formation of TiB and TiB2 mixtures at low temperatures and a mixture of TiB2 and Ti3B4 at high temperature

  • 20.
    Eriksson, Mirva
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Nygren, Mats
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Fast densification and deformation of titanium powder2005In: Powder Metallurgy, ISSN 0032-5899, Vol. 48, no 3, 231-236 p.Article in journal (Refereed)
    Abstract [en]

     A coarse titanium powder containing 0.2 wt-% oxygen with an average particle size of 45 micrometers was rapidly densified using the spark plasma sintering (SPS) technique. The fully consolidated specimens were also deformed in compression in the SPS unit and in a conventional hot pressing (HP) furnace. The densification and deformation behaviours were investigated by microstructural observation and examining the shrinkage and shrinkage rate recorded in real time. The efforts were focused on clarifying to what extent the possible spark discharging and intensive Joule heating at the particle contact points contribute to the densification, and what effect if any the pulsed electrical heating has on the densification. The results showed that what contributed to the densification were neither spark discharging nor anisotropic heating, but the particle deformation. Deformation occurs throughout the entire particle both in SPS and HP conditions, with a higher strain rate under SPS condition especially in the a phase region.

  • 21.
    Eriksson, Mirva
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Yan, Haixue
    School of Engineering and Materials Science, Queen Mary University of London,.
    Nygren, Mats
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Reece, Mike J
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Low temperature consolidated lead-free ferroelectric niobateceramics with improved electrical properties2010In: Journal of Materials Research, ISSN 0884-2914, Vol. 25, no 2, 240-247 p.Article in journal (Refereed)
    Abstract [en]

    There is a concerted effort to develop lead-free piezoelectric ceramics. ((Na0.5K0.5)NbO3 based ceramics have good electrical properties, and are a potential replacement material for lead zirconate titanate piezoelectric ceramics. In this work a commercial powder based on (Na0.5K0.5)NbO3 with an initial particle size of 260 nm was consolidated by plasma sintering (SPS). To avoid volatilization, high mechanical pressures were used to minimize the densification temperature. It was found that under a uniaxial pressure of 100 MPa, fully densified compacts can be prepared at 850. Ceramics densified at such a low temperature demonstrate an unusually high remanent polarization (30 mC/cm2) and high d33 (146 pC/N). The improved ferroelectric properties are ascribed to the homogeneous, dense, and submicron grained microstructure achieved.

  • 22.
    Eriksson, Mirva
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Department of Materials Chemistry.
    Yan, Haixue
    School of Engineering and Materials Science and Nanoforce Technology Ltd, Queen Mary University of London, London, United Kingdom.
    Viola, Giuseppe
    School of Engineering and Materials Science and Nanoforce Technology Ltd, Queen Mary University of London, London, United Kingdom.
    Ning, Huanpo
    School of Engineering and Materials Science and Nanoforce Technology Ltd, Queen Mary University of London, London, United Kingdom.
    Gruner, Daniel
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Department of Materials Chemistry.
    Nygren, Mats
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Department of Materials Chemistry.
    Reece, Mike
    School of Engineering and Materials Science and Nanoforce Technology Ltd, Queen Mary University of London, London, United Kingdom.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Department of Materials Chemistry.
    Effect of grain size on ferroelectric domain and electrical properties of submicron sized sodium potassium niobate ceramicsManuscript (preprint) (Other (popular science, discussion, etc.))
    Abstract [en]

    Piezoelectric ceramics of the composition Na0.5K0.5NbO3 (NKN) with grain sizes in the range of 0.2 - 1 mm were fabricated by Spark Plasma Sintering using normal pressure dies and a high pressure cell designed for pressures up to 500 MPa  with the purpose of investigating the effect of grain size on domain structures and electrical properties. Optimized processing conditions enabled ceramics of high densities (>99.5%TD) to be made at T≥850°C. It was found that domain size decreases with decreasing grain size and that non-180° ferroelectric domains walls were still visible in 200 nm sized grains. The room temperature dielectric constant firstly increased with decreasing grain size and then decreased in the low grain size regime. The materials with finer grain size displayed a broad ferro-paraelectric phase transition and a depression of the dielectric maximum at the Curie point. They also displayed an increase in the coercive field and approximately unchanged remnant polarization. The material sintered at 850°C represents a very good candidate for lead-free piezoelectric applications, because of its high piezoelectric constant (d33 = 160±2 pC/N).

  • 23.
    Eriksson, Mirva
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Yan, Haixue
    Viola, Giuseppe
    Ning, Huanpo
    Grunér, Daniel
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Nygren, Mats
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Reece, Mike J.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Ferroelectric Domain Structures and Electrical Properties of Fine-Grained Lead-Free Sodium Potassium Niobate Ceramics2011In: Journal of The American Ceramic Society, ISSN 0002-7820, E-ISSN 1551-2916, Vol. 94, no 10, 3391-3396 p.Article in journal (Refereed)
    Abstract [en]

    Piezoelectric ceramics of the composition Na(0.5)K(0.5)NbO(3) (NKN) with grain sizes in the range of 0.2-1 mu m were fabricated by spark plasma sintering. Ferroelectric domain size decreases with decreasing grain size and non-180 degrees ferroelectric domains walls were still visible in 200 nm sized grains. The Curie point of the ceramics was grain size independent. This suggests that the critical grain size for a single domain single grain structure for NKN is <200 nm. Optimized processing conditions enabled ceramics of high densities (>99.5% theoretical density) to be made at T >= 850 degrees C. For the dense ceramics (grain size >= 350 nm), the room temperature dielectric constant and coercive field increased with decreasing grain size. The remnant polarization was grain size independent. The material sintered at 850 degrees C is a very good candidate for lead-free piezoelectric applications because of its high piezoelectric constant (d(33) = 160 +/- 2 pC/N).

  • 24.
    Grins, Jekab
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Esmaeilzadeh, Saeid
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Svensson, Gunnar
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Shen, Zhijian James
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    High-resolution electron microscopy of a Sr-containing sialon polytypoid phase1999In: Journal of the European Ceramic Society, ISSN 0955-2219, Vol. 19, 2723-2730 p.Article in journal (Refereed)
    Abstract [en]

    A new type of Sr-containing sialon polytypoid phase with the structural formula SrSi10-xAl18+xN32-xOx (x approximate to l) has been found in the Sr-Si-Al-O-N system. The phase was characterised by X-ray powder diffraction (XRPD), and its structure was investigated by electron diffraction (ED) and high resolution electron microscopy (HREM). It is considerably disordered, but the average structure has a rhombohedral unit cell with a=5.335(5)approximate to root 3.a(AIN) and c= 79.1(1)Angstrom approximate to 30.c(AIN). The Sr atoms ave located in layers M-Sr-M, M=(Si/Al), at the origin of the unit cell with 12 X= (O,N) atoms around it, at distances of similar to 3 Angstrom, forming a cubo-octahedron. The X atoms that form a hexagon around the Sr atom in the ab plane are corner shared by M = (Si/Al) tetrahedra with opposite polarity in adjacent layers in which 2/3 of the tetrahedra are occupied. The M-Sr-M layers alternate with normally eight-layer-thick AIN type blocks, although the thickness of these blocks frequently varies. The structural model obtained from the HREM images includes a polarity reversal of the tetrahedra in the AIN blocks, similar to that proposed to occur in Si-Al-O-N polytypoid phases. The model with one Sr layer and 10 M = (Si,Al) layers per 1/3 of the repeat unit agrees with the composition of the phase and experimental HREM images.

  • 25.
    Gruner, Daniel
    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).
    Direct Scanning Electron Microscopy Imaging of Ferroelectric Domains After Ion Milling2010In: Journal of The American Ceramic Society, ISSN 0002-7820, E-ISSN 1551-2916, Vol. 93, no 1, 48-50 p.Article in journal (Refereed)
    Abstract [en]

    A method for directly observing the ferroelectric domain structure by scanning electron microscopy after argon ion milling has been established. Its advantages are exemplified by exposing the domain structure in three widely used ferroelectric ceramics, BaTiO3, (Na,K)NbO3, and Pb(Ti,Zr)O-3. Stable high-resolution images revealing domains with widths <30 nm have been obtained. The domain contrast is caused by electron channeling and is strongly dependent on the sample tilt angle. Owing to a strain- and defect-free surface generated by gentle ion milling, pronounced orientation contrast is observed.

  • 26.
    Grüner, Daniel
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Fäldt, Jenny
    Jansson, Kjell
    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).
    Argon Ion Beam Polishing: A Preparation Technique for Evaluating the Interface of Osseointegrated Implants with High Resolution2011In: International Journal of Oral & Maxillofacial Implants, ISSN 0882-2786, Vol. 26, no 3, 547-552 p.Article in journal (Refereed)
    Abstract [en]

    Purpose: The objective of this study was to assess the use of ion beam polishing for preparing cross sections suitable for high-resolution scanning electron microscope (SEM) investigation of dental implants with a brittle porous oxide layer and of bone/implant interfaces. Materials and Methods: Thirteen Nobel Biocare TiUnite implants were placed in minipigs. After 4 weeks, the implant and surrounding bone were removed en bloc and the implant was cut axially into two halves. The cross section was then polished with an argon ion beam. Additionally, ion beam-polished cross sections were prepared from four as-received implants. Ion beam-polished surfaces were studied with a field emission SEM (FE-SEM). Results: With FE-SEM, up to 1 mm along the interface of ion beam-polished implant surfaces can be studied with a resolution of a few nanometers. Filled and unfilled pores of the porous TiUnite coating can be distinguished, providing information on pore accessibility. Implant-bone interfaces can be analyzed using backscattered electron images, where titanium, the oxide layer, mineralized extracellular matrix, and osteocyte lacunae/resin/soft tissue can easily be distinguished as a result of atomic number contrast and the sharp boundaries between the different materials. Filled and unfilled pores can be distinguished. Characterization of local chemistry is possible with energy dispersive X-ray spectrometry, and bone growth into small pores (< 1 mu m) can be unambiguously confirmed. Conclusion: FE-SEM complements the established methods for the characterization of interfaces and bridges the wide gap in accessible length scale and resolution between the observations of mechanically polished interfaces by optical or scanning electron microscopes and the observation of focused ion beam-milled sections in a transmission electron microscope.

  • 27.
    Grüner, Daniel
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Shen, Zhijian James
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Ordered coalescence of nano-crystals during rapid solidification of ceramic melts2011In: CrysteEngComm, ISSN 1466-8033, Vol. 13, no 17, 5303-5305 p.Article in journal (Refereed)
    Abstract [en]

    Rapid cooling of an aluminosilicate-zirconia melt after laser sintering results in the formation of zirconia nano-crystals and dendritic zirconia crystals embedded in a glass matrix. The nano-crystals act as building blocks for the formation of non-faceted secondary crystals. Several secondary crystals built from two or a few nano-crystals with perfect structural coherence across the interface have been observed. Thus, the earliest states of these secondary crystals have been preserved by rapid cooling. Crystal growth can be described by a mechanism, ordered coalescence, which is characterized by self-assembly of nano-crystals combined with atom-by-atom growth and links classical and non-classical crystallization.

  • 28. Gu, Shijia
    et al.
    Zhang, Xin
    Wang, Lianjun
    Gan, Xuehui
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Jiang, Wan
    Direct indication of a higher central temperature achieved during spark plasma sintering process of a zeolite2015In: Journal of the European Ceramic Society, ISSN 0955-2219, Vol. 35, no 5, 1599-1603 p.Article in journal (Refereed)
    Abstract [en]

    The transition between transparent and opaque of as-sintered zeolite samples is employed to study the temperature distribution of non-conducting samples during spark plasma sintering (SPS) process in this paper. The results show that temperature gradients exist in both radial and axial directions, which provide direct evidence of temperature distribution in the SPS process. The sample prepared at 1325 degrees C is transparent in the center and opaque in the edge, which suggests that the temperature of the center is higher than the edge and the temperature differential is about 26 degrees C by the thermal analysis model based on the experimental data in ANSYS code. The axial temperature gradient is also present in the as-sintered sample obtained at 1315 degrees C. The transparent part of the upper surface is larger than the lower surface, which illustrates that the temperature of the upper surface was higher than the lower surface and the difference is about 5 degrees C.

  • 29.
    Hakeem, A. S.
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Daucé, R.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Leonova, E.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Edén, M.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Shen, Z.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Grins, J.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Esmaeilzadeh, S.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Silicate glasses with unprecedented high nitrogen and electropositive metal contents obtained by using metals as precursors2005In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 17, 2214-2216 p.Article in journal (Refereed)
    Abstract [en]

     

     

  • 30.
    Han, Guang
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Müller, Werner E. G.
    Wang, Xiaohong
    Lilja, Louise
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Porous titania surfaces on titanium with hierarchical macro- and mesoporosities for enhancing cell adhesion, proliferation and mineralization2015In: Materials science & engineering. C, biomimetic materials, sensors and systems, ISSN 0928-4931, Vol. 47, 376-383 p.Article in journal (Refereed)
    Abstract [en]

    Titanium received a macroporous titania surface layer by anodization, which contains open pores with average pore diameter around 5 gm. An additional mesoporous titania top layer following the contour of the macropores, of 100-200 nm thickness and with a pore diameter of 10 nm, was formed by using the evaporation-induced self-assembly (EISA) method with titanium (IV) tetraethoxide as the precursor. A coherent laminar titania surface layer was thus obtained, creating a hierarchical macro- and mesoporous surface that was characterized by high-resolution electron microscopy. The interfacial bonding between the surface layers and the titanium matrix was characterized by the scratch test that confirmed a stable and strong bonding of titania surface layers on titanium. The wettability to water and the effects on the osteosarcoma cell line (SaOS-2) proliferation and mineralization of the formed titania surface layers were studied systematically by cell culture and scanning electron microscopy. The results proved that the porous titania surface with hierarchical macro- and mesoporosities was hydrophilic that significantly promoted cell attachment and spreading. A synergistic role of the hierarchical macro- and mesoporosities was revealed in terms of enhancing cell adhesion, proliferation and mineralization, compared with the titania surface with solo scale topography.

  • 31.
    Han, Guang
    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).
    Microscopic view of osseointegration and functional mechanisms of implant surfaces2015In: Materials science & engineering. C, biomimetic materials, sensors and systems, ISSN 0928-4931, Vol. 56, 380-385 p.Article in journal (Refereed)
    Abstract [en]

    Argon ion beam polishing technique was applied to prepare the cross sections of implants feasible for high resolution scanning electron microscope investigation. The interfacial microstructure between newly formed bone and implants with three modified surfaces retrieved after in vivo test using three different animal models was characterized. By this approach it has become possible to directly observe early bone formation, the increase of bone density, and the evolution of bone structure. The two bone growth mechanisms, distant osteogenesis and contact osteogenesis, can also be distinguished. These direct observations give, at microscopic level, a better view of osseointegration and expound the functional mechanisms of various implant surfaces for osseointegration.

  • 32.
    Han, Guang
    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).
    Quartz crystal microbalance titanium sensor with mesoporous poresManuscript (preprint) (Other academic)
    Abstract [en]

    The quartz crystal microbalance (QCM) is an effective tool for the surface study in real time. But mesoporous coating materials usually is thin layer and stick to the substrates, and its properties cannot be studied by conventional methods as other mesoporous materials. A new QCM sensor with mesoporous pores is expected. The new mesoporous QCM sensor has mesoporous titania coating with 200 nm thickness only on the active electrode side by the method with masking counter electrode side during dip-coating. The new mesoporous QCM sensor works well in Q-Sense E1 equipment.

  • 33. Han, Jianmin
    et al.
    Zhao, Jing
    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).
    Zirconia ceramics in metal-free implant dentistry2017In: Advances in Applied Ceramics: Structural, Functional and Bioceramics, ISSN 1743-6753, E-ISSN 1743-6761, Vol. 116, no 3, 138-150 p.Article, review/survey (Refereed)
    Abstract [en]

    Because of their outstanding mechanical properties, chemical stability, and biocompatibility, 3-mol % yttria-stabilised tetragonal zirconia polycrystals (3Y-TZP), known as zirconia ceramics in dentistry, are an important choice for various types of prosthesis. In addition to extensive use for crown and bridge construction, considerable interest has been generated for applications in implant dentistry, including full-contour zirconia crowns as supra-constructions, zirconia abutments, and novel zirconia implants. However, their use among dentist and researchers is controversial, especially compared with the well-established implants made of titanium alloys. As a latecomer, the merits and limitations of 3Y-TZP are awaiting careful investigation. Design, manufacturing, and clinical operation guidelines are urgently needed. The aim of this review was to address the present status of the application of zirconia ceramics related to implant dentistry by analysing the published data from both in vitro and in vivo studies. Suggestions are provided for potential improvements and suitable applications of zirconia ceramics in metal-free implant dentistry.

  • 34. Hedberg, Yolanda S.
    et al.
    Qian, Bin
    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).
    Virtanen, Sannakaisa
    Odnevall Wallinder, Inger
    In-vitro biocompatibility of CoCrMo dental alloys fabricated by selective laser melting2014In: Dental Materials, ISSN 0109-5641, E-ISSN 1879-0097, Vol. 30, no 5, 525-534 p.Article in journal (Refereed)
    Abstract [en]

    Objectives: Selective laser melting (SLM) is increasingly used for the fabrication of customized dental components made of metal alloys such as CoCrMo. The main aim of the present study is to elucidate the influence of the non-equilibrium microstructure obtained by SLM on corrosion susceptibility and extent of metal release (measure of biocompatibility).Methods: A multi-analytical approach has been employed by combining microscopic and bulk compositional tools with electrochemical techniques and chemical analyses of metals in biologically relevant fluids for three differently SLM fabricated CoCrMo alloys and one cast CoCrMo alloy used for comparison.Results: Rapid cooling and strong temperature gradients during laser melting resulted in the formation of a fine cellular structure with cell boundaries enriched in Mo (Co depleted), and suppression of carbide precipitation and formation of a martensitic ε (hcp) phase at the surface. These features were shown to improve the corrosion and metal release susceptibility of the SLM alloys compared with the cast alloy. Unique textures formed in the pattern of the melting pools of the three different laser melted CoCrMo alloys predominantly explain observed small, though significant, differences. The susceptibility for corrosion and metal release increased with an increased number (area) of laser melt pool boundaries.Significance: This study shows that integrative and interdisciplinary studies of microstructural characteristics, corrosion, and metal release are essential to combine and consider during the design and fabrication of CoCrMo dental components of optimal biocompatibility. The reason is that metal release from CoCrMo alloys is dependent on the fabrication procedure.

  • 35. 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, 2354-2361 p.Article 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.

     

  • 36.
    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, 6405-6412 p.Article 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.

  • 37.
    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)
  • 38.
    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).
    Intragranular heterojunctions formed by ordered coalescence of strontium and barium titanate nanocrystals2015In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 107, 14-17 p.Article in journal (Refereed)
    Abstract [en]

    Crystal growth by nanocrystal-assembly plays an important role in the synthesis and preparation of nanostructural materials. In most cases, this crystal-growth mechanism is reported to occur in unary nanocrystal systems and in solution environment. Here, we report a new observation of grain growth by ordered coalescence of nanocrystals occurring in SrTiO3-BaTiO3 binary system during solid-state sintering, which also results in unique oxide heterostructures inside coarsened grains in bulk polycrystalline materials.

  • 39.
    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)
  • 40.
    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, 270-273 p.Article 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.

  • 41. Istomin, S. Ya.
    et al.
    Antipov, E. V.
    Fedotov, Yu. S.
    Bredikhin, S. I.
    Lyskov, N. V.
    Shafeie, S.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Svensson, Gunnar
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Liu, Y.
    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).
    Crystal structure and high-temperature electrical conductivity of novel perovskite-related gallium and indium oxides2014In: Journal of Solid State Electrochemistry, ISSN 1432-8488, E-ISSN 1433-0768, Vol. 18, no 5, 1415-1423 p.Article in journal (Refereed)
    Abstract [en]

    Novel complex oxides Sr2Ga1+x In1-x O-5, x = 0.0-0.2 with brownmillerite-type structure were prepared in air at T = 1,273 K, 24 h. Study of the crystal structure of Sr2Ga1.1In0.9O5 refined using X-ray powder diffraction data (S.G. Icmm, a = 5.9694(1) , b = 15.2091(3) , c = 5.7122(1) , chi (2) = 2.48, R (F) (2) (=) 0.0504, R (p) = 0.0458) revealed ordering of Ga3+ and In3+ cations over tetrahedral and octahedral positions, respectively. A partial replacement of Sr2+ by La3+ according to formula Sr1-y La (y) Ga0.5In0.5O2.5+y/2, leads to the formation of a cubic perovskite (a = 4.0291(5) ) for y = 0.3. No ordering of oxygen vacancies or cations was observed in Sr0.7La0.3Ga0.5In0.5O2.65 as revealed by electron diffraction study. The trace diffusion coefficient (D (T)) of oxygen for cubic perovskite Sr0.7La0.3Ga0.5In0.5O2.65 is in the range 2.0 Au 10(-9)-6.3 Au 10(-8) cm(2)/s with activation energy 1.4(1) eV as determined by isotopic exchange depth profile technique using secondary ion mass spectrometry at 973-1,223 K. These values are close to those reported for Ca-doped ZrO2. High-temperature electrical conductivity of Sr0.7La0.3Ga0.5In0.5O2.65 studied by AC impedance was found to be nearly independent on oxygen partial pressure. Calculated values of activation energy at T < 1,073 K for hole and oxide-ion conductivities are 0.96 and 1.10 eV, respectively.

  • 42. Jiang, Q. H.
    et al.
    Liu, F. T.
    Nan, Ce-Wen
    Lin, Yuan-Hua
    Reece, M. J.
    Yan, H. X.
    Ning, H. P.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    High-temperature ferroelectric phase transition observed in multiferroic Bi0.91La0.05Tb0.04FeO32009In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 95, no 1, 12909- p.Article in journal (Refereed)
    Abstract [en]

    A single-phase Bi0.91La0.05Tb0.04FeO3 polycrystalline ceramic was fabricated by spark-plasma-sintering the precursor powder prepared by a sol-gel method. Temperature-dependent properties of polycrystalline Bi0.91La0.05Tb0.04FeO3 were characterized by x-ray diffraction, dielectric, and piezoelectric measurement. The x-ray diffraction results revealed a phase transition near 700 degrees C. Especially, temperature-dependent dielectric behavior demonstrated that there was a dielectric abnormal peak at about 697 degrees C, in addition to those two well-known dielectric abnormal peaks at 337 degrees C (Neel temperature) and 831 degrees C (Curie temperature). The observations, together with thermal depoled behavior, suggest a ferroelectric-ferroelectric phase transition from R(3)c to Pbnm at around 700 degrees C.

  • 43. Jiang, Q. H.
    et al.
    Shen, Z. J.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Zhou, J. P.
    Shi, Z.
    Nan, Ce-Wen
    Magnetoelectric composites of nickel ferrite and lead zirconnate titanate prepared by spark plasma sintering2007In: Journal of the European Ceramic Society, ISSN 0955-2219, Vol. 27, no 1, 279-284 p.Article in journal (Refereed)
    Abstract [en]

    Magnetoelectric (ME) bulk composites of ferrite and lead zirconnate titanate (PZT) were prepared by spark plasma sintering (SPS) of mechanically mixed ferrites. BaFe2O4 or NiFe2O4 and a soft lead zirconnate titanate, PZT-5A, powders. The feasibility of retarding possible reactions occurring between the ferrite and lead zirconnate titanate was approved by applying such a dynamic process as SPS. It was further revealed that nickel ferrite and PZT-5A is a more favorable combination that underwent no obvious reactions up to 1050 degrees C. Efforts were made to optimize the SPS processing parameters in order to produce immiscible composites with high electrical resistivity, low dielectric loss and better magnetoelectric response.

  • 44. Jiang, Qing-Hui
    et al.
    Mei, Ao
    Lin, Yuan-Hua
    Nan, Ce-Wen
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Ferroic properties of highly dense multiferroic Bi1-xLa0.05TbxFeO3 ceramics via sheltered spark plasma sintering2008In: Journal of The American Ceramic Society, ISSN 0002-7820, E-ISSN 1551-2916, Vol. 91, no 7, 2189-2194 p.Article in journal (Refereed)
    Abstract [en]

    Multiferroic Bi0.95-xLa0.05TbxFeO3 (BLTFO) ceramics were prepared by spark plasma sintering. The protection of CeO2 powders in the spark plasma sintering process can effectively restrain the valence fluctuation of iron ions and high-dense BLTFO ceramics with good dielectric and ferroelectric properties are fabricated. The BLTFO ceramics have low loss (tan delta similar to 5%) between 10(2) and 10(6) Hz. The doping of Tb can increase the dielectric and ferromagnetic properties, but decrease the ferroelectricity of BLTFO ceramics.

  • 45. Jing, Zhao
    et al.
    Ke, Zhang
    Yihong, Liu
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Effect of Multistep Processing Technique on the Formation of Micro-defects and Residual Stresses in Zirconia Dental Restorations2014In: Journal of Prosthodontics, ISSN 1059-941X, E-ISSN 1532-849X, Vol. 23, no 3, 206-212 p.Article in journal (Refereed)
    Abstract [en]

    Purpose The clinical failures of zirconia dental restorations are often caused by extrinsic artifacts introduced by processing. The aim of this study was to investigate the micro-defects and residual stresses generated during the multistep process of zirconia dental restorations. Materials and Methods Thermal spray granulated 3Y-TZP powders were dry pressed by two tools exhibiting distinctly different Young's moduli, cold isostatic pressed (CIP-ed), and pressure-less fully sintered. The green bodies pressed by a stiff tool were treated with different procedures: direct milling (green milling) followed by fully sintering; half-sintering and milling (raw milling) with or without fully sintering; and fully sintering followed by grinding. The fully sintered 3Y-TZP crowns were clinically adjusted using both a diamond bur and SiC bur, respectively. Phase composition and microstructure of the pressed, milled, and ground surfaces were studied by XRD and SEM. Results Tetragonal phase was the main phase of all detected 3Y-TZP specimens. Excessive residual stresses introduced by raw milling and grinding were confirmed by a strained T (111) peak, monoclinic phase, and obviously changed I(002)(t)/I(200)(t) ratio. The residual stresses would form a compressive stress layer, while it was too shallow to inhibit crack propagation even for ground specimens. Large voids with high-coordination numbers were the common packing micro-defects. Once formed, they were barely healed by CIP-ing and sintering. A stiff pressing tool was confirmed to be useful for reducing the surface packing voids. Milling removed the surface voids, but was no help for the interior ones. Raw milling introduced more serious chippings, most originating from the existing packing voids, than green milling due to its brittle failure and was less recommended for production. Grinding dense 3Y-TZP caused surface grain refinement and much more severe micro-defects, especially when clinical adjustment was applied by diamond bur compared to SiC bur. Conclusions Micro-defects and residual stresses are introduced and accumulated through the entire production chain and determine the final microstructure of zirconia dental restorations. Several procedural improvements are offered and expected to reduce processing micro-defects.

  • 46. Kalousek, Radek
    et al.
    Spousta, Jiri
    Zlamal, Jakub
    Dub, Petr
    Sikola, Tomas
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Salamon, David
    Maca, Karel
    Rapid heating of zirconia nanoparticle-powder compacts by infrared radiation heat transfer2017In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 37, no 3, 1067-1072 p.Article in journal (Refereed)
    Abstract [en]

    Homogeneous rapid sintering of nanoparticle powder compacts of yttria-stabilized zirconia was achieved by the radiation heat transfer. Green bodies were prepared by cold isostatic pressing (CIP) at various pressures providing different porosity of samples before sintering. Pressure-less sintering was performed in air at a heating rate of 100 degrees C/min up to the 1500 degrees C/1 min. Scanning electron microscopy, mercury intrusion porosimetry, and Archimedes technique were used to characterize the microstructure and to determine the density of the green and sintered bodies. Contrary to expectations, our results reveal opposite dependence of the green- and sintered densities on the CIP pressure. Since the whole sintering process does not exceed 10 min, to propose what processes are responsible for observed results, our attention is focused on the radiation heat transfer from furnace heating elements into the ceramics. Our arguments are supported by numerical calculations of the electromagnetic field enhancement in/between particles.

  • 47. Kaluzhskikh, M. S.
    et al.
    Kazakov, S. M.
    Mazo, G. N.
    Istomin, S. Ya.
    Antipov, E. V.
    Gippius, A. A.
    Fedotov, Yu.
    Bredikhin, S. I.
    Liu, Yi
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Svensson, Gunnar
    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).
    High-temperature crystal structure and transport properties of the layered cuprates Ln(2)CuO(4), Ln=Pr, Nd and Sm2011In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 184, no 3, 698-704 p.Article in journal (Refereed)
    Abstract [en]

    High-temperature crystal structure of the layered cuprates Ln(2)CuO(4), Ln = Pr, Nd and Sm with tetragonal T'-structure was refined using X-ray powder diffraction data. Substantial anisotropy of the thermal expansion behavior was observed in their crystal structures with thermal expansion coefficients (TEC) along a- and c-axis changing from TEC(a)/TEC(c)approximate to 1.37 (Pr) to 0.89 (Nd) and 0.72 (Sm). Temperature dependence of the interatomic distances in Ln(2)CuO(4) shows significantly lower expansion rate of the chemical bond between Pr and oxygen atoms (O1) belonging to CuO(2)-planes (TEC(Pr-O1)= 11.7 ppm K(-1)) in comparison with other cuprates: TEC (Nd-O1)=15.2 ppm K(-1) and TEC (Sm-O1)= 15.1 ppm K(-1). High-temperature electrical conductivity of Pr(2)CuO(4) is the highest one in the whole studied temperature range (298-1173 K): 0.1-108 S/cm for Pr(2)CuO(4), 0.07-23 S/cm for Nd(2)CuO(4) and 2 X 10(-4)-9 S/cm for Sm(2)CuO(4). The trace diffusion coefficient (D(T)) of oxygen for Pr(2)CuO(4) determined by isotopic exchange depth profile (IEDP) technique using secondary ion mass spectrometry (SIMS) varies in the range 7.2 X 10(-13) Cm(2)/S (973 K) and 3.8 X 10(-10) Cm(2)/S (1173 K) which are in between those observed for the manganese and cobalt-based perovskites.

  • 48. Kamba, S.
    et al.
    Nuzhnyy, D.
    Vanek, P.
    Savinov, M.
    Knizek, K.
    Shen, Z.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Santava, E.
    Maca, K.
    Sadowski, M.
    Petzelt, J.
    Magnetodielectric effect and optic soft mode behaviour in quantum paraelectric EuTiO3 ceramics2007In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 80, no 2, 27002- p.Article in journal (Refereed)
    Abstract [en]

    Infrared reflectivity and time-domain terahertz transmission spectra of EuTiO3 ceramics revealed a polar optic phonon at 6-300K whose softening is fully responsible for the recently observed quantum paraelectric behaviour. Even if our EuTiO3 ceramics show lower permittivity than the single crystal due to a reduced density and/ or small amount of secondary pyrochlore Eu2Ti2O7 phase, we confirmed a magnetic field dependence of the permittivity, also slightly smaller than in single crystal. An attempt to reveal the soft phonon dependence at 1.8K on the magnetic field up to 13T remained below the accuracy of our infrared reflectivity experiment.

  • 49. Kocjan, Andraz
    et al.
    Pouchly, Vaclav
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Processing of zirconia nanoceramics from a coarse powder2015In: Journal of the European Ceramic Society, ISSN 0955-2219, Vol. 35, no 4, 1285-1295 p.Article in journal (Refereed)
    Abstract [en]

    We have merged the benefits of coarse-grained and nanocrystalline powders by the consolidation and sintering of coarse, yet nanostructured, mesoporous Y-TZP powder. The powder was composed of loosely aggregated, nanoscale crystallites, which can be seen as secondary particles or agglomerates. Their homogeneous, defect-free packing proved to be a viable pathway for the processing of zirconia nanoceramics. The powder consolidation yielded homogeneous green bodies with hierarchical heterogeneities in terms of intra- and inter-particle pore packing. The hierarchical heterogeneities had a pronounced effect on the densification and grain growth. The intra-particle pore coalescence along with a frozen interparticle porosity, prolonged the pore-pinning effect, separating the densification and grain-growth mechanisms. Increased heating rates promoted the grain-growth and densification via a competitive mechanism of primary crystallite ordered coalescence, while by applying pressure, the crystallite growth was completely prevented, making such a coarse powder suitable for the fabrication of zirconia nanoceramics.

  • 50.
    Kocjan, Andraz
    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).
    Colloidal processing and partial sintering of high-performance porous zirconia nanoceramics with hierarchical heterogeneities2013In: Journal of the European Ceramic Society, ISSN 0955-2219, Vol. 33, no 15-16, 3165-3176 p.Article in journal (Refereed)
    Abstract [en]

    High-performance, porous, Y-TZP nanoceramics with hierarchical heterogeneities originating from uniform, intra- and inter-particle packing were prepared by colloidal processing and partial sintering of a mesoporous powder. The powder consisted of 100-150-nanometer-sized secondary particles initially composed of smaller, loosely aggregated, primary, nanoscale crystallites. Green bodies were prepared by centrifugal slip casting of weakly flocculated suspensions. During the initial stage of sintering, necking between the secondary particles was accompanied by intraparticle pore coalescence, while the pores originating from the secondary particle packing remained intact. Such microstructures with porosity levels between 46 and 18.3% and pore areas between 18 and 4 m(2)/g led to attractive properties, i.e., much reduced thermal conductivities of 0.63-1.88W m(-1) K-1, high bending strengths of 70-540 MPa and lowered elastic moduli of 32-156 GPa, making them potentially ideal as thermal insulators and/or load-bearing porous biomaterials owing to the possibility of further impregnation with bioactive ingredients.

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