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  • 101. Pouchly, V.
    et al.
    Maca, K.
    Xiong, Yan
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Shen, J. Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Master Sintering Surface: A practical approach to its construction and utilization for Spark Plasma Sintering prediction2012In: Science of Sintering, ISSN 0350-820X, Vol. 44, no 2, p. 169-175Article in journal (Refereed)
    Abstract [en]

    The sintering is a complex thermally activated process, thus any prediction of sintering behaviour is very welcome not only for industrial purposes. Presented paper shows the possibility of densification prediction based on concept of Master Sintering Surface (MSS) for pressure assisted Spark Plasma Sintering (SPS). User friendly software for evaluation of the MSS is presented. The concept was used for densification prediction of alumina ceramics sintered by SPS.

  • 102. Pouchly, Vaclav
    et al.
    Maca, Karel
    Shen, Zhijian James
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Two-stage master sintering curve applied to two-step sintering of oxide ceramics2013In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 33, no 12, p. 2275-2283Article in journal (Refereed)
    Abstract [en]

    Tetragonal (3 mol% Y2O3) and two cubic zirconia (8 mol% Y2O3) as well as alumina green bodies were used for the construction of the Master Sintering Curve (MSC) created from sets of constant-rate-of-heating (CRH) sintering experiments. The activation energies calculated according to the MSC theory were 770 kJ/mol for Al2O3, 1270 kJ/mol for t-ZrO2, 620 kJ/mol and 750 kJ/mol for c-ZrO2. These values were verified by an alternative approach based on an analysis of the densification rate in the intermediate sintering stage. The MSCs established from the Two-Step Sintering (TSS) experiments showed at high densities a significant deflection from those constructed from the CRH experiments. This deflection was explained by lower sintering activation energy in the closed porosity stage. A new two-stage MSC model was developed to reflect the change in sintering activation energy and to describe TSS. The efficiency of TSS of four materials under investigation was correlated with their activation energies during the final sintering stage.

  • 103.
    Qian, Bin
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Forraiová, Lenka
    Institute of Materials Research of the Slovak Academy of Sciences.
    Saeidi, Kamran
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Changhong, Xiao
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Mazaheri, Mehdi
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Zhong, Yuan
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Lofaj, František
    bInstitute of Materials Research of the Slovak Academy of Sciences.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Defect-tolerant characteristics of laser sintered CoCrMo alloy ensured by strong structural hierarchiesManuscript (preprint) (Other academic)
    Abstract [en]

    Laser sintering of a CoCrMo alloy have been found to introduce intergrown structural features on several length scales from nano- to macro-levels, i.e. structural hierarchies. The quenching of the small laser formed melting pool, when the laser focus spot moves away, will preserve high temperature phases or microstructures. A micron-sized cellular microstructure will form with Mo being enriched in a nanometer thin zone at the cell boundaries and carbide formation is suppressed. Clusters of elongated cells (crystallites) join along one common crystal structure direction forming larger up to 100 µm bundles and these grow in different intercrossing directions in the microstructure. The interlocked framework prohibits mechanical creep or deformation by sliding along weak structure planes and consumes the energy of a propagating crack. A macro-scale “weld line” structure can be formed in 3-dimensions by the bottom-up approach; the sintering of layer by layer with a tailored scan track. The sintered alloy contained three types of defects comprising micron sized voids or cracks occurring at the cluster boundaries and larger cracks at the “weld line” boundaries. Unexpected good mechanical properties are achieved despite the observed microstructural defects and the laser sintered Co-Cr-Mo alloy seems very defect tolerant. This positive effect is ascribed to the strong structural hierarchies found in the laser sintered CoCrMo alloy. 

  • 104.
    Qian, Bin
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Saeidi, Kamran
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Kvetková, L.
    Lofaj, F.
    Xiao, Changhong
    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).
    Defects-tolerant Co-Cr-Mo dental alloys prepared by selective laser melting2015In: Dental Materials, ISSN 0109-5641, E-ISSN 1879-0097, Vol. 31, no 12, p. 1435-1444Article in journal (Refereed)
    Abstract [en]

    Objectives. CrCoMo alloy specimens were successfully fabricated using selective laser melting (SLM). The aim of this study was to carefully investigate microstructure of the SLM specimens in order to understand the influence of their structural features inter-grown on different length scales ranging from nano-to macro-levels on their mechanical properties.

    Methods. Two different sets of processing parameters developed for building the inner part( core) and the surface (skin) of dental prostheses were tested. Microstructures were characterized by SEM, EBSD and XRD analysis. The elemental distribution was assessed by EDS line profile analysis under TEM. The mechanical properties of the specimens were measured.

    Results. The microstructures of both specimens were characterized showing formation of grains comprised of columnar sub-grains with Mo-enrichment at the sub-grain boundaries. Clusters of columnar sub-grains grew coherently along one common crystallographic direction forming much larger single crystal grains which are intercrossing in different directions forming an overall dendrite-like microstructure. Three types of microstructural defects were occasionally observed; small voids (<10 mu m), fine cracks at grain boundaries (<10 mu m) and cracks at weld line boundaries (>10 mu m). Despite the presence of these defects, the yield and the ultimate tensile strength (UTS) were 870 and 430 MPa and 1300 MPa and 1160 MPa, respectively, for the skin and core specimens which are higher than casted dental alloy.

    Significance. Although the formation of microstructural defects is hard to be avoided during the SLM process, the SLM CoCrMo alloys can achieve improved mechanical properties than their casted counterparts, implying they are defect-tolerant.

  • 105.
    Qian, Bin
    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).
    Laser sintering of ceramics2013In: Journal of Asian Ceramic Societies, ISSN 2187-0764, Vol. 1, no 4, p. 315-321Article, review/survey (Refereed)
    Abstract [en]

    The progress of laser sintering of ceramics is reviewed by focusing on the principles and the potentials ofthis new technique as one member of a family of additive manufacturing technologies. After a brief intro-duction of the technology and the interaction between laser beam and ceramics, the characters of lasersintered ceramics are discussed in detail. The unique opportunities of achieving hierarchically structuredheterogeneities and non-equilibrium phase assemblages in bulk ceramics are illustrated. It reveals thatthe laser sintering is not only a feasible technology allowing net shape production of customized 3D partsin a single manufacturing operation but also a promising way for developing ceramics with structuresthat can hardly be made by conventional processing methods.

  • 106.
    Qian, Bin
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Taimisto, Lauri
    Lehti, Antti
    Heidi, Heidi
    Nyrhilä, Olli
    Salminen, Antti
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Monitoring of temperature profiles and surface morphologies during lasersintering of alumina ceramicsIn: Journal of Asian Ceramic Societies, ISSN 2187-0764Article in journal (Refereed)
    Abstract [en]

    Additive manufacturing of alumina by laser is a delicate process and small changes of processing parame-ters might cause less controlled and understood consequences. The real-time monitoring of temperatureprofiles, spectrum profiles and surface morphologies were evaluated in off-axial set-up for controllingthe laser sintering of alumina ceramics. The real-time spectrometer and pyrometer were used for rapidmonitoring of the thermal stability during the laser sintering process. An active illumination imagingsystem successfully recorded the high temperature melt pool and surrounding area simultaneously. Thecaptured images also showed how the defects form and progress during the laser sintering process. Allof these real-time monitoring methods have shown a great potential for on-line quality control duringlaser sintering of ceramics.

  • 107. Ren, Wen
    et al.
    Du, Faliang
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Wu, Zhe
    Yang, Xuechao
    Xu, Ge
    Guo, Lv-Hua
    The workflows of a novel self-glazed zirconia for dental prostheses fabrication: case reports2018In: Advances in Applied Ceramics: Structural, Functional and Bioceramics, ISSN 1743-6753, E-ISSN 1743-6761, Vol. 117, no 7, p. 406-413Article in journal (Refereed)
    Abstract [en]

    In the clinical application of ceramic prostheses, micro-leakage, porcelain chipping, low-treatment efficiency and quality uncertainty have appeared as the major problems that dentists encountered. However, the full-contour zirconia monolithic prostheses have the potentials for solving the problems. It appears that the full-contour zirconia monolithic prostheses produced through the fully digital workflow can ensure that the restorations can be closely aligned with the abutment and be easy to adjust and to wear, thus to assure the stability and accuracy of occlusal, which are crucial to the ultimate integration of the full-contour zirconia monolithic prostheses by avoiding unfavourable grinding. The newly developed full digital approach can greatly simplify the previous workflow that involved many manual operations. It improves not only the treatment efficiency but also the reliability of the prostheses by avoiding manual operational mistakes.

  • 108. Saeidi, K.
    et al.
    Neikter, M.
    Olsen, Jessica
    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).
    Akhtar, F.
    316L stainless steel designed to withstand intermediate temperature2017In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 135, p. 1-8Article in journal (Refereed)
    Abstract [en]

    Austenitic stainless steel 316L was fabricated for withstanding elevated temperature by selective laser melting (SLM). Tensile tests at 800 degrees C were carried out on laser melted 316L with two different strain rates of 0.05 S-1 and 0.25 S-1. The laser melted 316L showed tensile strength of approximately 400 MPa at 800 degrees C, which was superior to conventional 316L. Analysis of fracture surface showed that the 316L fractured in mixed mode, ductile and brittle fracture, with an elongation of 18% at 800 degrees C. In order to understand the mechanical response, laser melted 316L was thermally treated at 800 degrees C for microstructure and phase stability. X-ray diffraction (XRD) and Electron back scattered diffraction (EBSD) of 316L treated at 800 degrees C disclosed a textured material with single austenitic phase. SEM and EBSD showed that the characteristic and inherent microstructure of laser melted 316L, consisting of elongated grains with high angle grain boundaries containing subgrains with a smaller misorientation, remained similar to as-built SLM 316L during hot tensile test at 800 degrees C. The stable austenite phase and its stable hierarchical microstructure at 800 degrees C led to the superior mechanical response of laser melted 316L.

  • 109.
    Saeidi, Kamran
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Gao, X.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Lofaj, F.
    Kvetkova, L.
    Shen, Zhijian James
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Transformation of austenite to duplex austenite-ferrite assembly in annealed stainless steel 316L consolidated by laser melting2015In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 633, p. 463-469Article in journal (Refereed)
    Abstract [en]

    Laser melting (LM), with a focused Nd:YAG laser beam, was used to form solid bodies from 316L austenite stainless steel powder and the laser melted samples were heat treated at various temperatures. The phase changes in heat treated samples were characterized using X-ray diffraction (XRD). Samples heat treated at 800 degrees C and 900 degrees C remained single austenite while in samples heat treated at 1100 degrees C and 1400 degrees C a dual austenite-ferrite phase assembly was formed. The ferrite formation was further verified by electron back scattering diffraction (EBSD) and selective area diffraction (SAD). Microstructural changes were studied by scanning and transmission electron microscopy (SEM, TEM). In samples heat treated up to 900 degrees C, coalescence of the cellular-sub grains was noticed, whereas in sample heat treated at and above 1100 degrees C the formation of ferrite phase was observed. The correlation between the microstructure/phase assembly and the measured strength/microhardness were investigated, which indicated that the tensile strength of the laser melted material was significantly higher than that of the conventional 316L steel even after heat treatment whereas caution has to be taken when laser melted material will be exposed to an application temperature above 900 degrees C.

  • 110.
    Saeidi, Kamran
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Gao, X.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Zhong, Yuan
    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).
    Hardened austenite steel with columnar sub-grain structure formed by laser melting2015In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 625, p. 221-229Article in journal (Refereed)
    Abstract [en]

    Laser melting (LM), with a focused Nd: YAG laser beam, was used to form solid bodies from a 316L austenite stainless steel powder. The microstructure, phase content and texture of the LM stainless steel were characterized and compared with conventional 316L stainless steel. The crack-free LM samples achieved a relative density of 98.6 +/- 0.1%. The XRD pattern revealed a single phase Austenite with preferential crystallite growth along the (100) plane and an orientation degree of 0.84 on the building surface. A fine columnar sub-grain structure of size 0.5 mu m was observed inside each individual large grain of single-crystal nature and with grain sizes in the range of 10-100 mu m. Molybdenum was found to be enriched at the sub-grain boundaries accompanied with high dislocation concentrations. It was proposed that such a sub-grain structure is formed by the compositional fluctuation due to the slow kinetics of homogeneous alloying of large Mo atoms during rapid solidification. The local enrichment of misplaced Mo in the Austenite lattice induced a network of dislocation tangling, which would retard or even block the migration of newly formed dislocations under indentation force, turning otherwise a soft Austenite to hardened steel. In addition, local formation of spherical nano-inclusions of an amorphous chromium-containing silicate was observed. The origin and the implications of the formation of such oxide nano-inclusions were discussed.

  • 111.
    Saeidi, Kamran
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Kevetkova, Lenka
    Lofaj, Frantisk
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Novel ferritic stainless steel formed by laser melting from duplex stainless steel powder with advanced mechanical properties and high ductility2016In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 665, p. 59-65Article in journal (Refereed)
    Abstract [en]

    Stainless steel bodies with relative density of 99.5% (with the theoretical density being 7.8gr/cm3) were manufactured by laser melting (LM) of duplex 2507SAF steel powder. The crystalline phases of starting powder were fully ferrite with only a small trace of austenite. The chemical composition was unchanged during laser melting. A unique mosaic-type structure with mosaics of 100-150 μm size was formed after LM. Recrystallized grains with 1-5 μm was formed in between the mosaic boundaries. A great number of entangled dislocation loops resembling a loops with 100-200 nm size were also formed inside each of these mosaics and also within recrystallized micron size grains at the mosaic boundary zones. Nitrogen enriched areas and nitride phase were detected in the inner microstructure of the laser melted samples. The measured tensile strength, yield strength and microhardness were 1214 MPa, 1321 MPa and 450 HV, respectively, which is superior to that of conventional ferritic, austenitic and duplex stainless steels. The Enhanced mechanical properties are due to a number of nano- and microstructure factors such as the nano-sized dislocation loops restricting dislocation movements, different crystalline grain orientation of grains within the mosaics and boundary inclusions and precipitates that inhibit slip/slide effects. Despite of high strength and hardness, the laser melted ferritic steel was very ductile according to stress-strain curves and fracture analysis.

  • 112.
    Saeidi, Kamran
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Kvetkova, Lenka
    Lofajc, Frantisek
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Austenitic stainless steel strengthened by the in situ formation of oxide nanoinclusions2015In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 5, no 27, p. 20747-20750Article in journal (Refereed)
    Abstract [en]

    An austenitic stainless steel was prepared by laser melting. High resolution transmission electron microscopy with energy dispersive spectrometry confirmed the in situ formation of oxide nanoinclusions with average size less than 50 nm. Scanning electron microscopy examination revealed the homogeneous dispersion of the oxide nanoinclusions in the steel matrix. The tensile and yield strengths of the prepared specimens were 703 and 456 MPa respectively with high ductility which is significantly improved compared to its conventionally casted counterpart.

  • 113. Saeidi, Kamran
    et al.
    Olsén, Jon
    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).
    Scaled down microstructure hierarchy and local heterogeneity in laser melted 316L stainless steelIn: Materials Characterization, ISSN 1044-5803, E-ISSN 1873-4189Article in journal (Refereed)
  • 114.
    Saeidi, Kamran
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Olsén, Jon
    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).
    Unique bottom-up microstructure hierarchy and improved mechanical properties in stainless steel formed by selective laser meltingIn: Article in journal (Refereed)
    Abstract [en]

    Selective laser melting was used for production of 316L stainless steel. The SLM 316L steel shows unique structural hierarchies from the macro-scale melt pools down to the submicron/nano scale intragranular cellular sub-grains. The Structural hierarchies and heterogeneities and associated dislocations increased the mechanical strength and hardness of the laser melted stainless steel samples to 700 MPa and 325HV. The small sized confined intragranular subgrains and multi scale interlocking of the subgrains ensured high yield strength of 456 MPa and good ductility. The mechanical properties are considerably higher than a fully densified HIP316L and conventional 316L steel and comparable to dual phase stainless steel.

  • 115. Saeidi, Kamran
    et al.
    Zapata, Daniel Leon
    Lofaj, Frantisek
    Kvetkova, Lenka
    Olsén, Jon
    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).
    Akhtar, Farid
    Ultra-high strength martensitic 420 stainless steel with high ductility2019In: Additive manufacturing, ISSN 2214-8604, Vol. 29, article id 100803Article in journal (Refereed)
    Abstract [en]

    Martensitic 420 stainless steel was successfully fabricated by Selective laser melting (SLM) with > 99% relative density and high mechanical strength of 1670 MPa, yield strength of 600 MPa and elongation of 3.5%. X-ray diffraction (XRD) and scanning electron microscopy disclosed that the microstructure of SLM 420 consisted of colonies of 0.5-1 mu m sized cells and submicron martensitic needles with 11 wt. % austenite. Tempering of asbuilt SLM 420 stainless steel at 400 degrees C resulted in ultra-high strength material with high ductility. Ultimate tensile strength of 1800 MPa and yield strength of 1400 MPa were recorded with an elongation of 25%. Phase transformation analysis was carried out using Rietveld refinement of XRD data and electron backscattered diffraction (EBSD), which showed the transformation of martensite to austenite, and resulted in austenite content of 36 wt. % in tempered SLM 420 stainless steel. Transformation induced plasticity (TRIP), austenite formation and fine cellular substructure along with sub-micron martensite needles resulted in stainless steel with high tensile strength and ductility. The advanced mechanical properties were compared with conventionally made ultrahigh-strength steels, and the microstructure-properties relationships were disclosed.

  • 116.
    Salamon, David
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Eriksson, Mirva
    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).
    Shen, Zhijian James
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Can the use of pulsed direct current induce oscillation in the applied pressure during spark plasma sintering?2012In: Science and Technology of Advanced Materials, ISSN 1468-6996, E-ISSN 1878-5514, Vol. 13, no 1, p. 015005-Article in journal (Refereed)
    Abstract [en]

    The spark plasma sintering (SPS) process is known for its rapid densification of metals and ceramics. The mechanism behind this rapid densification has been discussed during the last few decades and is yet uncertain. During our SPS experiments we noticed oscillations in the applied pressure, related to a change in electric current. In this study, we investigated the effect of pulsed electrical current on the applied mechanical pressure and related changes in temperature. We eliminated the effect of sample shrinkage in the SPS setup and used a transparent quartz die allowing direct observation of the sample. We found that the use of pulsed direct electric current in our apparatus induces pressure oscillations with the amplitude depending on the current density. While sintering Ti samples we observed temperature oscillations resulting from pressure oscillations, which we attribute to magnetic forces generated within the SPS apparatus. The described current-pressure-temperature relations might increase understanding of the SPS process.

  • 117. Salamon, David
    et al.
    Kalousek, Radek
    Maca, Karel
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Rapid Grain Growth in 3Y-TZP Nanoceramics by Pressure-Assisted and Pressure-Less SPS2015In: Journal of The American Ceramic Society, ISSN 0002-7820, E-ISSN 1551-2916, Vol. 98, no 12, p. 3706-3712Article in journal (Refereed)
    Abstract [en]

    Pressure-less spark plasma sintering (SPS) is a new approach during which rapid densification of ceramic nanopowder green bodies is accompanied by rapid grain growth. Although the origin of this phenomenon has not yet been fully understood significant, difference in grain growth between pressure-less and pressure-assisted SPS was expected. In this work 3Y-TZP nanopowder with average particle size of 12 nm was consolidated using two-step approach: (1) at an intermediate temperature (600 degrees C to 1000 degrees C) SPS warm pressing followed by (2) high temperature (1400 degrees C to 1600 degrees C) pressure-less SPS. The standard one step pressure-assisted SPS experiments were quoted as references. Rapid grain growth was observed during both pressure-less and standard SPS. The samples prepared by both approaches at the same sintering temperature (1400 degrees C1600 degrees C) achieved identical grain size and grain size distribution, if large pores were eliminated in early stage by SPS warm pressing. The electric current, electromagnetic field, and mechanical pressure is proven to have a negligible direct influence on grain growth in 3Y-TZP ceramics at temperatures above 1000 degrees C under standard SPS conditions.

  • 118.
    Salamon, David
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Maca, Karel
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Rapid sintering of crack free zirconia ceramics by pressure less spark plasma sintering2012In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 66, no 11, p. 899-902Article in journal (Refereed)
    Abstract [en]

    Heating of a ceramic green body is a key step in sintering. We have created inside a spark plasma sintering apparatus pressure-less sintering conditions that allow homogeneous and extremely rapid heating. Dense and crack-free zirconia ceramic was sintered at heating rates of up to 500 degrees C min(-1) and dwell times of 2 min. This extremely fast sintering process is accompanied by extremely rapid grain growth, indicating a non classical sintering mechanism. No grain size gradients were observed inside the sintered zirconia ceramics.

  • 119. Schitco, Cristina
    et al.
    Turdean-Ionescu, Claudia
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Bazarjani, Mahdi Seifollahi
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Li, Duan
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Fasel, Claudia
    Donner, Wolfgang
    Shen, James
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Riedel, Ralf
    Gurlo, Aleksander
    Edén, Mattias
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Silicon oxycarbonitrides synthesized by ammonia-assisted thermolysis route from polymers: A total X-ray scattering, solid-state NMR, and TEM structural study2016In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 35, no 4, p. 979-989Article in journal (Refereed)
    Abstract [en]

    Solid-state nuclear magnetic resonance (NMR) spectroscopy, total X-ray scattering with a pair distribution function (PDF) analysis, and transmission electron microscopy (TEM) were employed to explore the structures of microporous and non-porous ceramics synthesized by an NH3-assisted thermolysis from polymers. Polysiloxane (SPR-212a, Starfire® Systems) and polysilazane (HTT-1800, KiON Speciality Polymers) polymers form microporous silicon oxycarbonitride ceramics with accessible and tailored micropores. 29Si magic-angle-spinning NMR showed that the introduction of nitrogen leads to structures incorporating considerable amounts of SiN4 and SiO2N2 building blocks. The samples derived from a polycarbosilane (SMP-10, Starfire® Systems) remained non-porous: for such a C-rich and N-bearing phase, the NMR, TEM, and PDF results suggested a Si network exhibiting domains dominated by either SiN or SiC bonds. 13C NMR revealed primarily “carbidic” CSi4 environments in the C-rich phases, as well as the formation of an amorphous sp2-hybridized carbon phase; both are believed to be detrimental for the micropore formation.

  • 120. Seifert, Martin
    et al.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Krenkel, Walter
    Motz, Guenter
    Nb(Si,C,N) composite materials densified by spark plasma sintering2015In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 35, no 12, p. 3319-3327Article in journal (Refereed)
    Abstract [en]

    Nb2N/Nb5Si3/Nb ceramic metal and Nb(C,N)/Nb5Si(3+x)Cx, ceramic like composites were densified by spark plasma sintering (SPS). The precursor multiphase powders were synthesized by solid-state reactions of a polysilazane precursor with niobium powder addition. By varying the amount of reactive species of both precursor and niobium the phase composition and structure of the synthesized multiphase powders can be tailored from metal dominated to mainly ceramic like. The results demonstrated that the SPS approach leads to highly dense samples at 1600 degrees C with an applied uniaxial pressure of 100 MPa. Furthermore, XRD measurements and EBSD analysis proofed that SPS is a feasible method to retain the original phase compositions and grain sizes within the multiphase powders during sintering. The evaluation of the Vickers indents revealed both a dependency of the measured values on phase composition and on residual porosity.

  • 121.
    Shen, Zhijian
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, China.
    Grüner, Daniel
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Eriksson, Mirva
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Belova, Lyubov M.
    Nan, Ce-Wen
    Yan, Haixue
    Ordered coalescence of nano-crystals in alkaline niobate ceramics with high remanent polarization2017In: Journal of Materiomics, ISSN 2352-8478, Vol. 3, no 4, p. 267-272Article in journal (Refereed)
    Abstract [en]

    Lead-free alkali niobates Na0.5K0.5NbO3 (NKN) ceramics, with significantly enhanced ferroelectric remanent polarization (P-r), were prepared using Spark Plasma Sintering (SPS). Three types of boundaries were observed in the ceramics, being grain boundaries between faceted grains, domain boundaries that separate ferroelectric domains inside individual grains, and nanoscale sub-grain boundaries that reveal the nano-scale mosaicity of individual grains. Part of the sub-grain boundaries were from initial powder particles. The other sub-grain boundaries were built by ordered coalescence of nano-crystals during rapid SPS process. It was worthwhile to emphasize that the ordered coalescence of nano-crystals in bulk ceramics during sintering takes place and completes within minutes. These sub-grain features would disappear at higher temperature by long time sintering. Rapid Spark Plasma Sintering allowed us to capture this transient microstructure. The significantly enhanced ferroelectric P-r of NKN was attributed to nanoscale sub-boundaries, which stimulated the dynamics of ferroelectric domain formation and switching.

  • 122.
    Shen, Zhijian
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Liu, Leifeng
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Xu, Xiqing
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Zhao, Jing
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Eriksson, Mirva
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Zhong, Yuan
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Adolfsson, Erik
    Liu, Yihong
    Kocjan, Andraz
    Fractography of self-glazed zirconia with improved reliability2017In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 37, no 14, p. 4339-4345Article in journal (Refereed)
    Abstract [en]

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

  • 123.
    Shen, Zhijian
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Nicula, R.
    Ishizaki, K.
    Stir, M.
    Vaucher, S.
    Rapid synthesis and densification of single-phase Al-Cu-Fe quasicrystals by spark plasma sintering or microwave heating2010In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 91, no 19-20, p. 1478-6443Article in journal (Refereed)
    Abstract [en]

    Quasicrystalline (QC) phases are often stable only within narrow composition domains. For this reason, the synthesis of larger amounts of single-phase quasicrystalline powders is difficult. Powder metallurgical approaches, based on mechanical milling followed by conventional heating, have been explored in the recent past. The manufacturing process for single-phase quasicrystals - either in the form of powders or as bulk parts - can be accelerated by orders of magnitude using rapid heating methods that involve pulsed electric currents and/or high-frequency electromagnetic fields. Prior knowledge of the phase transformation sequence and transformation kinetics, as revealed by in situ time-resolved synchrotron radiation experiments, is crucial in obtaining single-phase quasicrystals. We report on the simultaneous synthesis and densification of bulk single-phase Al-Cu-Fe QCs by spark plasma sintering (SPS) within minutes and on the ultrafast synthesis of single-phase Al-Cu-Fe quasicrystalline powders by microwave heating within seconds. The effect of electric current application in the rapid processing of pre-alloyed powders is discussed in relation to the faster diffusion and enhanced phase transformation kinetics.

     

  • 124.
    Shen, Zhijian
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Peng, Hong
    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.
    Formation of in-situ reinforced microstructure in α–sialon ceramics I: Stoichiometric oxygen-rich compositions2002In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 17, no 2, p. 336-342Article in journal (Refereed)
    Abstract [en]

    The abnormal grain growth in α–sialon ceramics was investigated. The preparations had stoichiometric compositions on the oxygen-rich phase boundary, and they were stabilized by Y, Nd, Sm, Dy, and Yb, respectively. Specimens were prepared from α–Si3N4 as precursor powder by applying conventional hot pressing and a novel rapid consolidation process, namely spark plasma sintering (SPS). Single-phase α–sialon ceramics with in situ reinforced bimodal microstructure, i.e., large elongated grains embedded in a matrix consisting of small equiaxed grains, were obtained above 1750 °C in all systems compacted by SPS and above 1800 °C in systems stabilized by Nd and Sm but not Dy, Y, or Yb by a two-step hot-pressing procedure. It was observed that the formation of abnormally grown α–sialon grains was strongly temperature-dependent, indicating that it was encouraged by the formation of a transient liquid phase that stimulated the dissolution of any remaining nitride precursors and early formed small α–sialon grains and sequentially facilitated supersaturation by the α–sialon constituents. The presence of elongated grains improves fracture resistance in the obtained materials.

  • 125.
    Shen, Zhijian
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Peng, Hong
    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.
    Formidable increase of superplasticity of ceramics in presence of an electric field2003In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 15, no 12, p. 1006-1009Article in journal (Refereed)
    Abstract [en]

    The ductility of silicon nitride-based ceramics is dramatically enhanced in the presence of a pulsed electric field/current that induces movement of the charged species present in the grain boundary glassy/liquid phase, and thereby promotes grain sliding along the grain boundaries. The Figure shows an α-sialon component before (right) and after (left) deformation.

  • 126.
    Shen, Zhijian
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Peng, Hong
    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.
    Rapid formation and deformation of Li-doped sialon ceramics2004In: Journal of The American Ceramic Society, ISSN 0002-7820, E-ISSN 1551-2916, Vol. 87, no 4, p. 727-729Article in journal (Refereed)
    Abstract [en]

    Two lithium-doped sialon ceramics were densified and superplastically deformed by spark plasma sintering (SPS). Rapid densification with linear shrinkage rates of approximately 5 × 10−3 s−1 were observed for samples heated at a rate of 100°C/min up to ∼1400°C under a uniaxial pressure of 40 MPa. Isothermal deformation by SPS-preprepared, fully densified ceramics performed at T≥ 1450°C yielded strain rates in the order of 10−2 s−2. It is suggested that a high heating rate promotes material transport via formation of a nonequilibrated oxygen-rich liquid of low viscosity. This finding most likely holds true for other liquid-phase sintered ceramics as well and has implications for cost-effective manufacturing of ceramic components.

  • 127.
    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.

  • 128.
    Shen, Zhijian
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Zhao, Zhe
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Peng, Hong
    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.
    Formation of tough interlocking microstructures in silicon nitride ceramics by dynamic ripening.2002In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 417, no 6886, p. 266-269Article in journal (Refereed)
    Abstract [en]

    Ceramics based on Si3N4 have been comprehensively studied and are widely used in structural applications1, 2. The development of an interlocking microstructure of elongated grains is vital to ensure that this family of ceramics have good damage tolerance3, 4. Until now this has been accomplished by heating the appropriate powder compacts to temperatures above 1,700 °C for extended periods. This procedure involves a necessary step of controlling the size and population of seeds—added ex situ or formed in situ—to ensure selective grain growth5, 6. Here we report the very fast (within minutes) in situ formation of a tough interlocking microstructure in Si3N4-based ceramics. The microstructures are obtained by a dynamic ripening mechanism, an anisotropic Ostwald ripening process that results from the rapid heating rate. The resulting microstructures are uniform and reproducible in terms of grain size distribution and mechanical properties, and are easily tailored by manipulating the kinetics. This process is very efficient and opens up new possibilities to optimize mechanical properties and cost-effectively manufacture ceramics.

  • 129. Shi, Andi
    et al.
    Wu, Zhe
    Huang, Jiangyong
    Liang, Qian
    Li, Qianqian
    Guo, Lvhua
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Wear performance of self-glazed zirconia crowns with different amount of occlusal adjustment after 6 months of clinical use2018In: Advances in Applied Ceramics: Structural, Functional and Bioceramics, ISSN 1743-6753, E-ISSN 1743-6761, Vol. 117, no 8, p. 445-451Article in journal (Refereed)
    Abstract [en]

    The amount of enamel wear on the antagonist occlusal surfaces caused by self-glazed zirconia crowns was compared with that caused by contralateral natural teeth. Thirteen self-glazed zirconia crowns were placed in situ. The impressions of self-glazed zirconia crowns, their antagonists and the control teeth were taken and scanned at baseline and 6-month follow-up. The patients were divided into two groups, the self-glazed crowns in one group were subjected to a large amount of grinding with well polishing (LaP group, n=7), while the other group required a little amount of grinding with well polishing (LiP group, n=6). Statistics were analysed by two-sided paired Student's t-tests to a significance level of p<.05. The results revealed that the maximum and mean enamel wear significantly different between the antagonists of self-glazed crowns and the control teeth (p<.05). Increased amount of enamel wear was found in LaP group (p<.05). The self-glazed zirconia crowns caused more enamel wear of antagonists than natural teeth after 6 months. Occlusal adjustment and polishing were considered as possible confounders which affected wear behaviour.

  • 130. Song, Lu
    et al.
    Ma, Jing
    Zhang, Qinghua
    Cao, Yidan
    Ran, Rui
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Unique redox properties in defective CeO2-x nanocrystallines synthesized by laser melting2018In: Science China Materials, ISSN 2095-8226, Vol. 61, no 8, p. 1078-1084Article in journal (Refereed)
    Abstract [en]

    Defects in cerium oxide, especially oxygen vacancies, play an essential role in its versatile applications and are efficiently preserved at ambient conditions in a non-equilibrium process. Herein, defective CeO2-x with heterogeneous structure was synthesized by high-energy laser melting, where a large amount of oxygen vacancies and Ce3+ could be introduced, leading to improved visible light absorption, narrowed bandgap and room temperature ferromagnetism. Moreover, this laser melted CeO2-x exhibits significantly enhanced low-temperature oxidation behaviors than the counterpart prepared by normal hydrogen-reduction. This unique redox performance could be attributed to the intragranular diffusion at the boundaries of assembled nanocrystallites. This method paves a new way for introducing unique multi-functions in oxide ceramics.

  • 131. Song, Lu
    et al.
    Ma, Jing
    Zhang, Qinghua
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, China.
    Laser melted oxide ceramics: Multiscale structural evolution with non-equilibrium features2019In: Journal of Materiomics, ISSN 2352-8478, Vol. 5, no 3, p. 436-445Article in journal (Refereed)
    Abstract [en]

    Compared with the versatility in metal industry, application of laser on oxide ceramics is quite limited due to the intrinsic features of ceramics and limited understanding in laser-ceramic interaction mechanism, especially for high-energy laser that causes melting of materials. In this research, a study into general behaviors of several oxide ceramics melted by laser under inert atmosphere is presented. Key factors in determining state transformation, chemical reduction and phase structure are summarized, with further investigation into the evolution in microstructure at multiscale and the corresponding novelty and metastability. It is found that laser melting does show great potential in introducing deep reduction, unique microstructure, and notable increase in structure complexity and total entropy, and those features could contribute to some unconventional functional performance with brand-new structure-property relationship. 

  • 132. Song, Lu
    et al.
    Wang, Yafei
    Ma, Jing
    Zhang, Qinghua
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, China.
    Core/shell structured Zn/ZnO nanoparticles synthesized by gaseous laser ablation with enhanced photocatalysis efficiency2018In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 442, p. 101-105Article in journal (Refereed)
    Abstract [en]

    Zinc oxide (ZnO) is a competitive candidate in semiconductor photocatalysts, only if the efficiency could be fully optimized especially by tailored nanostructures. Here we report a kind of core/shell structured Zn/ZnO nanoparticles with enhanced photocatalysis efficiency, which were synthesized by a highly-productive gaseous laser ablation method. The nanodroplets generated by laser ablation would be reduced to zinc in the protective atmosphere, and further be oxidized at surface to form a specific core/shell structured Zn/ZnO nanoparticles within seconds. Thanks to the formation of this Zn-ZnO Schottky junction, the photocatalysis degradation efficiency of such core/shell Zn/ZnO nanostructure is significantly improved owing to the enhanced visible light absorption and inhibited carrier recombination by introducing the metallic zinc.

  • 133. Song, Lu
    et al.
    Zhang, Qinghua
    Ma, Jing
    Chen, Chonglin
    Xu, Ben
    Zhu, Mingke
    Xu, Xing
    Nan, Cewen
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). School of Materials Science and Engineering, China.
    Vacancy-ordered yttria stabilized zirconia as a low-temperature electronic conductor achieved by laser melting2019In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 39, no 4, p. 1374-1380Article in journal (Refereed)
    Abstract [en]

    Laser melting is known to be capable in initiating thorough evolution in microstructure and bringing novel functional performance in metals. But realization of this potential in ceramics only reaches a preliminary stage that needs further investigation. Here we demonstrate zirconia, traditionally an insulative ceramic at low temperature, could be transformed into an electronic conductor with the conductivity on order of 10(-3) S.cm(-1) at room temperature by a simple laser melting process without inducing metallic phases. Transmission electron microscopy and ab-initio simulation show that oversaturated oxygen vacancies, together with their ordered metastable distribution along < 001 >, are introduced during this non-equilibrium process, and result in a clear defect level significantly narrowing bandgap to less than 1 eV, leading to the considerable electronic conductivity. These results identify a strategy of utilizing this non-equilibrium method in oxide ceramics to realize some unconventional performances determined by metastable structure thoroughly altered down to atomic level.

  • 134.
    Sun, Tianyang
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Zhejiang University, People's Republic of China.
    Han, Guang
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Lindgren, Matteus
    Shen, Zhijian
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Stellenbosch University, South Africa.
    Adhesion of lactoferrin and bone morphogenetic protein-2 to a rutile surface: dependence on the surface hydrophobicity2014In: Biomaterials Science, ISSN 2047-4830, Vol. 2, no 8, p. 1090-1099Article in journal (Refereed)
    Abstract [en]

    Binding of the proteins human lactoferrin (LF) and human bone morphogenetic protein-2 (BMP2) to a hydroxylated TiO2 rutile (110) surface has been modeled using molecular dynamics (MD) simulations. In order to study the effect of the hydrophobicity of the rutile surface on the protein binding process, the rutile surface was made more hydrophilic or more hydrophobic by adjusting the rutile atomic charges. The binding of LF and BMP2 to the hydrophobic rutile surface occurred through direct contact between the protein and rutile via both hydrophobic and hydrophilic amino acids. This forced the proteins to undergo structural rearrangements, observed primarily in BMP2. Binding to the hydrophilic rutile surface was largely indirect via the hydration layer of water on the surface of rutile. Both LF and BMP2 had a higher binding strength to the hydrophobic rutile surfaces than to the hydrophilic surfaces, as seen in the larger amplitude of the binding energies.

  • 135. Svensson, Tomas
    et al.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laser spectroscopy of gas confined in nanoporous materials2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, no 2, p. 21107-Article in journal (Refereed)
    Abstract [en]

    We show that high-resolution laser spectroscopy can probe surface interactions of gas confined in nanocavities of porous materials. We report on strong line broadening and unfamiliar line shapes due to tight confinement, as well as signal enhancement due to multiple photon scattering. This new domain of laser spectroscopy constitute a challenge for the theory of collisions and spectroscopic line shapes, and open for new ways of analyzing porous materials and processes taking place therein.

  • 136.
    Thomas, Annu
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Andersson, Johanna
    Grüner, Daniel
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Osla, Fredrik
    Jansson, Kjell
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Fäldt, Jenny
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Direct observation of bone coherence with dental implants2012In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 32, no 11, p. 2607-2612Article in journal (Refereed)
    Abstract [en]

    A newly developed gentle ion beam polishing technique was established for preparing of cross sections of dental implants feasible for high resolution scanning electron microscope investigation. This approach was applied to investigate the interfacial microstructure between newly formed bone and dental implants with modified surfaces extracted after in vivo test in adult miniature pigs. The results obtained so far reveal that it has become possible to analyze the bone coherence to implants besides measuring the bone coverage. The amount and density of the mineralized extra cellular matrix has found to be different in different sub-microscopic regions around the implant. From our observations, it can be seen that new bone grows from the existing bone and advances towards the implant surface by in growth mechanism. The images also reveal that new bone is formed directly at the implant surface; we propose a deposition mechanism to explain this. Eventually the in grown and the deposited bone connect to give a good anchorage of the implant. This achievement bears implication for understanding osseointegration at microscopic level.

  • 137. Tidehag, Per
    et al.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Digital dentistry calls the change of ceramics and ceramic processes2019In: Advances in Applied Ceramics: Structural, Functional and Bioceramics, ISSN 1743-6753, E-ISSN 1743-6761, Vol. 118, no 1-2, p. 83-90Article in journal (Refereed)
    Abstract [en]

    Since the start of the new digital impression-taking era which started around 2008 with the introduction of the 3M Lava COS, a slow transformation is gaining momentum and it looks as if this and next year will have a broad impact on the clinicians. From being merely a way of replacing impressions now the total digital workflow enters the industry. This means a whole new way of thinking and a new way to produce prosthetic work where a reproducible constant quality can be achieved. An obvious development of improved ceramic materials that simultaneously can satisfy the mechanical, bio- and aesthetic demands of the prostheses and the corresponding technologies for production that would work for the clinic digital workflow is out of the expertise of the dentists. This calls for a cross-disciplinary collaboration with the experts in the communities of ceramics and digital manufacturing.

  • 138. Trunec, Martin
    et al.
    Klimke, Jens
    Shen, Zhijian James
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Transparent alumina ceramics densified by a combinational approach of spark plasma sintering and hot isostatic pressing2016In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 36, no 16, p. 4333-4337Article in journal (Refereed)
    Abstract [en]

    In order to increase the in-line transmission of fine transparent alumina in visible light the grain growth during sintering of alumina ceramics was supressed using a combined densification process. This process combines presintering of a green body by spark plasma sintering with final hot isostatic pressing. The presintering by spark plasma sintering provided bodies with a substantially smaller grain size than pressureless presintering. It is shown that the fine-grained presintered microstructure could be retained during final hot isostatic pressing and alumina ceramics doped with spinel and zirconia nanoparticles in particular could be sintered to full density with only minor grain growth during final hot isostatic pressing. The novel combined densification process enhanced by the unique nanoparticle doping approach provided fully dense alumina ceramics with an average grain size of 237 nm and an in-line transmission of 76.2% at a wavelength of 632.8 nm and a sample thickness of 0.8 mm.

  • 139. Trunec, Martin
    et al.
    Maca, Karel
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Materials Chemistry.
    Warm pressing of zirconia nanoparticles by the spark plasma sintering technique2008In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 59, no 1, p. 23-26Article in journal (Refereed)
    Abstract [en]

    Zirconia nanoparticles were densified by pressureless sintering and spark plasma sintering (SPS). The evolution of relative density and pore size distribution in powder compacts during these sintering processes were compared. It was found that pore size increase was suppressed during SPS and that this facilitated the densification. The combined sintering process (low-temperature pre-sintering by SPS followed by pressureless sintering) was performed to confirm the advantage of the low-temperature SPS technique (referred to as SPS warm pressing). 

  • 140.
    Vasiliev, Petr O.
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Shen, Z. J.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Hodgkins, R. P.
    Bergström, L.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Meso/macroporous, mechanically stable silica monoliths of complex shape by controlled fusion of mesoporous spherical particles2006In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 18, p. 4933-4938Article in journal (Refereed)
  • 141. Viola, Giuseppe
    et al.
    Chong, Kok Boon
    Eriksson, Mirva
    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).
    Zeng, Jiangtao
    Yin, Qingrui
    Kan, Yanmei
    Wang, Peiling
    Ning, Huanpo
    Zhang, Hongtao
    Fitzpatrick, Michael E.
    Reece, Michael J.
    Yan, Haixue
    Effect of grain size on domain structures, dielectric and thermal depoling of Nd-substituted bismuth titanate ceramics2013In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, no 18, p. 182903-Article in journal (Refereed)
    Abstract [en]

    The microscopic origin of the grain size effects on the dielectric, piezoelectric, and thermal depoling properties of Aurivillius phase Bi3.15Nd0.85Ti3O12 was investigated. Using atomic force microscopy, domain walls were observed in micrometer grain size ceramics, but gradually disappeared with reducing grain size and were not found in ceramics with 90 nm grain size. In strain-electric field butterfly loops, the strain decreased with decreasing grain size indicating a decreasing contribution of non-180 degrees domain walls switching to the strain. Lattice distortion (a-b)/b decreased with decreasing grain size. The thermal depoling resistance decreased with decreasing grain size, due to increasing internal mechanical stresses.

  • 142. Wang, Dian-Zheng
    et al.
    Li, Kai-Lun
    Yu, Chen-Fan
    Ma, Jing
    Liu, Wei
    Shen, Zhi-Jian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, China.
    Cracking Behavior in Additively Manufactured Pure Tungsten2019In: Acta Metallurgica Sinica (English Letters), ISSN 1006-7191, E-ISSN 2194-1289, Vol. 32, no 1, p. 127-135Article in journal (Refereed)
    Abstract [en]

    In this study, near fully dense (96.5%) pure tungsten bulks were additively manufactured and the cracking behavior was investigated. A crack network with a spacing of similar to 100m was observed in the fabricated bulks. It was observed that the laser scanning strategy, which could tailor the microstructure, affected the crack distribution pattern in fabricated tungsten. The calculated surface temperature difference (7300K) was much higher than the cracking criterion (800K) of tungsten, indicating that cracking is almost inevitable in laser additive manufacturing of tungsten. It could be concluded that crack network formed because the cracks emerged in every laser molten track and then interconnected in the layer-by-layer building process.

  • 143. Wang, Dianzheng
    et al.
    Wang, Zhimin
    Li, Kailun
    Ma, Jing
    Liu, Wei
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, China.
    Cracking in laser additively manufactured W: Initiation mechanism and a suppression approach by alloying2019In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 162, p. 384-393Article in journal (Refereed)
    Abstract [en]

    Cracking represents the main challenge for exploiting tungsten in additive manufacturing. In this study, laser powder-bed-fusion technique was applied to additively manufacture tungsten. In the built bulks, the grain boundaries were found to be rich in nanoscale gas pores. On the basis of that, a nanopore segregation induced cracking initiation mechanism was proposed. In order to control cracks, W-6wt.%Ta alloy was produced and the cracking suppression mechanism was investigated. The W-6Ta alloy is characterized by a submicron intragranular cellular structure, which composed large amount of interlocked dislocations as revealed by transmission electron microscopy. Owing to the cellular structure, the nanopores were trapped inside grains, which can reduce the cracking possibility. Moreover, the W-Ta alloy possesses higher strength (by 17%) and higher energy dissipation rate (by 52%) than pure tungsten, which both are beneficial for crack reduction.

  • 144. Wang, Dianzheng
    et al.
    Yu, Chenfan
    Ma, Jing
    Liu, Wei
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, China.
    Densification and crack suppression in selective laser melting of pure molybdenum2017In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 129, p. 44-52Article in journal (Refereed)
    Abstract [en]

    In this study, dense and crack-free pure Mo has been fabricated by selective laser melting. In order to obtain densification, the precursor powders were granulated and processed by plasma spheroidization. Parts of density of 10.16 g/cm(3) (99.1% of theoretical density) were obtained with spherical powders because of its increased laser absorptivity and packing density. The crack growth behaviors under various scanning strategies are analyzed by electron backscattered diffraction. The interlocking grain boundary structure, which increased the crack growth resistance and caused crack deviation, is formed under layer-wise rotated laser scanning. Cracks can be fully suppressed by applying the designed supporting structure. This study provides a novel route for the fabrication of complex Mo parts.

  • 145. Wang, Dianzheng
    et al.
    Yu, Chenfan
    Ma, Jing
    Liu, Wei
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, China.
    Reduced wear damage of carbon brushes via transfer layer upon W/Cu composite2018In: Materials Science and Technology, ISSN 0267-0836, E-ISSN 1743-2847, Vol. 34, no 2, p. 172-178Article in journal (Refereed)
    Abstract [en]

    An ultrafine (similar to 100 nm) W particle-reinforced Cu matrix composite was fabricated by spark plasma sintering. The tribological properties of the fabricated W/Cu composite were tested by pin-on-disc sliding experiments and compared with those of brass alloy. The wear rate of the carbon brush when sliding against the W/Cu composite (0.102mm(3) s(-1)) was much lower than that with brass (4.83mm(3) s(-1)). Simultaneously, the W/Cu composite possessed higher conductivities and greater surface hardness than brass. The surface roughness of the W/Cu composite was found to play a critical role in improving the robustness of the tribofilm and reducing the volume loss of the carbon brush. The W/Cu composite has promising engineering applications for the long-term wear protection of self-lubricating materials.

  • 146. Wang, Dianzheng
    et al.
    Yu, Chenfan
    Zhou, Xin
    Ma, Jing
    Liu, Wei
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, China.
    Dense Pure Tungsten Fabricated by Selective Laser Melting2017In: Applied Sciences, E-ISSN 2076-3417, Vol. 7, no 4, article id 430Article in journal (Refereed)
    Abstract [en]

    Additive manufacturing using tungsten, a brittle material, is difficult because of its high melting point, thermal conductivity, and oxidation tendency. In this study, pure tungsten parts with densities of up to 18.53 g/cm(3) (i.e., 96.0% of the theoretical density) were fabricated by selective laser melting. In order to minimize balling effects, the raw polyhedral tungsten powders underwent a spheroidization process before laser consolidation. Compared with polyhedral powders, the spherical powders showed increased laser absorptivity and packing density, which helped in the formation of a continuous molten track and promoted densification.

  • 147. Wang, Jing
    et al.
    Wang, Lianjun
    Liu, Guanghua
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry, Inorganic Chemistry.
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Substrate effect on the magnetoelectric behavior of Pb(Zr0.52Ti0.48)O3 film-On-CoFe2O4 bulk ceramic composites prepared by direct solution spin coating2009In: Journal of The American Ceramic Society, ISSN 0002-7820, E-ISSN 1551-2916, Vol. 92, no 11, p. 2654-2660Article in journal (Refereed)
    Abstract [en]

    Magnetoelectric (ME) composite structures of Pb(Zr0.52Ti0.48)O3 (PZT) and CoFe2O4 (CFO) were prepared by directly growing PZT films on highly dense CFO ceramics via a simple solution spin coating, rather than by conventional high-temperature cofiring. An obvious ME response, which had the same bias-dependent trend as the piezomagnetic coefficient of CFO ceramics, was observed in such film-on-bulk ceramic composites. It was found that the PZT films showed a good ferroelectric feature, and the ME response of the composites strongly depended on the resistivity of the CFO ceramics as both a substrate and a bottom electrode. The results suggest plenty of room for further enhancing the ME response of such films-on-ceramic substrate composites.

  • 148. Wang, Lianjun
    et al.
    Jiang, Wan
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Chen, Lidong
    Formation of a unique glass by spark plasma sintering of a zeolite2009In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 24, no 10, p. 3241-3245Article in journal (Refereed)
    Abstract [en]

    A simple approach, order–disorder transition (ODT), has been developed to synthesize a novel glass using ZSM-5 as starting materials. In this process, the ZSM-5 powders were pressed uniaxially in a graphite die and rapidly sintered using spark plasma sintering (SPS). High-resolution transmission electron microscopic images revealed that a few crystalline zeolite fragments were still preserved locally inside the SPS consolidated sample. Vickers microhardness and fracture toughness of this as-prepared transparent glass sample at room temperature reaches 7.3 ± 0.2 GPa and 2.0 ± 0.3MPa·m1/2, respectively. It is very interesting that these novel bulk transparent glasses exhibit ultraviolet photoluminescence (PL) properties at about ∼360 nm.

  • 149. Wang, Ling
    et al.
    Hu, Jianfeng
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Cheng, Yao
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Fu, Zhengyi
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Xiong, Yan
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Hubei University of Technology, China.
    Defect formation by order coalescence in vermicular grains during alumina phase transformation2015In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 107, p. 59-62Article in journal (Refereed)
    Abstract [en]

    Vermicular alpha-Al2O3 grains obtained from gamma- to alpha-phase transformation were investigated. X-ray diffraction patterns showed a left-shifting shoulder in all diffraction peaks of transformed alpha-Al2O3. High-resolution transmission electron microscopy results confirmed large amounts of defects inside the vermicular alpha-Al2O3 single-crystal. Combined with grain-growth discontinuity, the authors proposed that the defects inside vermicular grains resulted from assembly of nanosized alpha-crystallites by an ordered coalescence mechanism directly after phase transformation, by which the orientation alignment was thermodynamically favored in the solid-state sintering.

  • 150. Wang, Yafei
    et al.
    Xing, Leilei
    Li, Kailun
    Yu, Chenfan
    Ma, Jing
    Liu, Wei
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, P.R. China.
    Band-Like Distribution of Grains in Selective Laser Melting Track Under Keyhole Mode2019In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 50, no 2, p. 1035-1041Article in journal (Refereed)
    Abstract [en]

    Grain structure in the melt pool under keyhole mode is complex due to complicated solidified conditions, especially in consideration of the intense fluid flow. This paper aims to clarify grain structure and grain growth mechanism in melt pool under keyhole mode in selective laser melting. The microstructure from transverse cross section and longitudinal cross section near the centerline of 316L single tracks was characterized. In the longitudinal cross section, band-like grain structure formed. It is found that fluid flow can refine the grains, and large columnar grains grow towards crystallographic < 110 > direction.

1234 101 - 150 of 189
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