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  • 1.
    Kese, Kwadwo
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Li, Zhi-Cheng
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Semi-ellipse method for accounting for the pile-up contact area during nanoindentation with the Berkovich indenter2006In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 55, no 8, p. 699-702Article in journal (Refereed)
    Abstract [en]

    A method of approximating the pile-up contact perimeter as a semi-ellipse originally developed for soda-lime glass, has been investigated for its applicability to other materials including work-hardened copper, Zr-based bulk metallic glass, strontium fluoroapatite, tungsten carbide, silicon nitride and aluminium oxide. The study revealed that all the materials formed pile-ups during nanoindentation. The elastic modulus and hardness values obtained after correcting for the pile-up contact area using the semiellipse approximation were,consistent with those found in the literature.

  • 2.
    Kese, Kwadwo
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Li, Zhicheng
    KTH, Superseded Departments, Materials Science and Engineering.
    Bergman, Bill
    KTH, Superseded Departments, Materials Science and Engineering.
    Influence of residual stress on elastic modulus and hardness of soda-lime glass measured by nanoindentation2004In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 19, no 10, p. 3109-3119Article in journal (Refereed)
    Abstract [en]

    The influence of stress on the elastic modulus E and hardness H in soda-lime glass was studied in the Vickers residual stress field by nanoindentation. The Oliver-Pharr method of analysis first gave higher values of E and H, but after correcting for the pileup contact areas around the nanoindents, results consistent with literature values were obtained at regions in the stress field where the stresses were either low or close to zero. Determination of the pileup contact areas was made possible by the use of the atomic force microscope, which has facility for generating cross-section images of the indents. The elastic modulus was found to decrease with stress, which is explained with reference to the influence of applied stresses on the Si-O-Si bond angle. The hardness on the other hand did not depend on the stresses except in the region very close to the edge of the Vickers indent where the stresses are high.

  • 3.
    Kese, Kwadwo
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Ceramics.
    Li, Zhi-Cheng
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Ceramics.
    Bergman, Bill
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Ceramics.
    Method to account for true contact area in soda-lime glass during nanoindentation with the Berkovich tip2005In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 404, no 1-2, p. 1-8Article in journal (Refereed)
    Abstract [en]

    An. empirical relationship for estimating the pile-up contact area from the contact stiffness, S and the contact depth, h(c) has been developed. This was achieved first by using the atomic force microscope to image nanoindents made with the Berkovich indenter in soda-lime glass and approximating the pile-up contact perimeter as a semi-ellipse. Then, by determining the pile-up contact area for several peak indentation loads, a correlation was found between the pile-up contact area and the load used to generate it. The importance of this new method of determining the pile-up contact area is that the need for indent imaging is made completely redundant, since the contact stiffness is a quantity that is routinely obtained during nanoindentation data analysis. Elastic modulus of soda-lime glass of 70 +/- 1.5 GPa is measured with loads ranging from 20 to 500 mN. The hardness measured also falls within the range of values, 5.2-5.9 GPa, normally quoted in the literature for the glass.

  • 4.
    Li, Zhicheng
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Smuk, Lena
    KTH, Superseded Departments, Materials Science and Engineering.
    Bergman, Bill
    KTH, Superseded Departments, Materials Science and Engineering.
    Influence of AST additives on the stability of PTCR characteristics and microstructure in ferroelectric ceramics2004In: Journal of materials science. Materials in electronics, ISSN 0957-4522, E-ISSN 1573-482X, Vol. 15, no 8, p. 561-567Article in journal (Refereed)
    Abstract [en]

    (Ba0.69Pb0.31)TiO3 ceramics were prepared using Al2O3, SiO2, additives and excess of TiO2 (AST). The characteristics of positive temperature coefficient of resistivity (PTCR) was studied and the corresponding microstructures were investigated using atomic force microscopy and scanning electron microscopy. The results showed that the PTCR effect was related to the AST additives. The maximum value of resistivity in the ceramics with lower content of or without Al2O3 and SiO2 additives was much lower than in those with AST additives. Ceramics with low AST content, which were heated by electric field to a temperature much higher than their Curie temperature, lost the PTCR effect after the electric field stimulation. The microstructure observations revealed that re-crystallization took place in the ceramics with lower content of or without AST additives resulting in the loss of the PTCR effect.

  • 5.
    Li, Zhicheng
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Zhang, H.
    Bergman, Bill
    KTH, Superseded Departments, Materials Science and Engineering.
    Microstructure and PTCR effect of La-doped BaPbO3 ceramics2004In: Journal of materials science. Materials in electronics, ISSN 0957-4522, E-ISSN 1573-482X, Vol. 15, no 3, p. 183-186Article in journal (Refereed)
    Abstract [en]

    La-doped BaPbO3 ceramics were prepared by using BaCO3 and PbO. Electrical properties and microstructure of the ceramics were studied. The results show a thin surface layer with a very low resistivity, and the interior of the ceramics with the characteristics of positive temperature coefficient of resistivity (PTCR). The PTCR behavior was related to the La content. The investigation by scanning electron microscopy and transmission electron microscopy revealed that the low resistivity of the surface layer was due to formation of a nano-size BaPbO3 phase with metallic properties.

  • 6.
    Li, Zhicheng
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Zhang, H
    Liu, L
    Xu, Y B
    Growth and morphology of beta phase in an Mg-Y-Nd alloy2004In: Materials letters (General ed.), ISSN 0167-577X, E-ISSN 1873-4979, Vol. 58, no 24, p. 3021-3024Article in journal (Refereed)
    Abstract [en]

    The phase precipitated in an Mg-Y-Nd alloy during ageing was studied using electron microscopy. Observations were performed along the orientations paralleling to [0001] and [11 (2) over bar0] zone axes of the matrix. The results showed that the platelike p phase has three variants growing along three <1120> directions of the matrix, the beta phase grew faster along <0001>(alpha) and <1120>(alpha) directions than along the third direction (<1 (1) over bar 00>). The morphology of the beta phases was illustrated according to the analysis of the directional misfit between the beta phase and the matrix.

  • 7.
    Li, Zhicheng
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Zhang, H.
    Xu, Y. B.
    Direct observation of electron-beam-induced nucleation and growth in amorphous GaAs2004In: Materials Science in Semiconductor Processing, ISSN 1369-8001, E-ISSN 1873-4081, Vol. 7, no 1-2, p. 19-25Article in journal (Refereed)
    Abstract [en]

    In situ observation of the nucleation and growth stimulated by an electron beam in amorphous GaAs was performed using a high-resolution electron microscopy. The results showed that the crystallization was closely related to the current density of the electron beam. Crystallization could not take place when the current density was 50 pA/cm(2), nanocrystals with the random orientation formed under the 74 pA/cm(2) electron beam, large grains and twining structure formed during the crystallization induced by the 93 pA/cm(2) electron beam. Ionization process and electron-beam heating were suggested to be the possible mechanisms for the irradiation-induced crystallization.

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