Kong Ling Bing
Email Address
tslklb@nus.edu.sg
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Publication Reducing the size of monopole antennas using magneto-dielectric material loading(2012) Venkatarayalu, N.; Iddagoda, M.; Kong, L.B.; Ting, S.-K.; TEMASEK LABORATORIESEffect of loading a simple monopole antenna with magneto-dielectric material is investigated. The advantages in loading the antenna with magneto-dielectric material over loading with either dielectric or magnetic material for antenna miniaturization applications is demonstrated. To better compare the effects of different material loading, the shift in resonant frequency and the corresponding change in the Q-factor of the loaded antenna is analyzed. Investigations based on numerical simulations are validated by fabricating a prototype antenna with ferrite ceramic based composite material loading. Conclusions on the effect of the magneto-dielectric coating on other monopole configurations such as fat cylindrical monopoles are presented. © 2012 IEEE.Publication Structural and microwave properties of Fe-based nanopowders via mechanochemical synthesis(2007) Wu, J.-H.; Kong, L.-B.; Kim, Y.K.; TEMASEK LABORATORIESStructural and microwave properties of Fe-based nanoalloy powders, mechanochemically synthesized with a composition corresponding to Finemet (Fe73.5Si13.5B9Nb3Cu1), were investigated. The nanopowders, dominated by bcc-Fe (Si), consist of nanocrystallites and display high magnetization with low-coercivity. The microwave measurements show that the nanocomposites comprising the nanopowders possess high, broadband magnetic permeability.Publication Transitional metal-doped 8 mol% yttria-stabilized zirconia electrolytes(2009-10-05) Zhang, T.S.; Du, Z.H.; Li, S.; Kong, L.B.; Song, X.C.; Lu, J.; Ma, J.; TEMASEK LABORATORIESThe sintering, grain growth and ionic conductivities (especially the grain-boundary (GB) conductivity), of 8YSZ electrolytes with various silica levels (~ 30 ppm, ~ 500 ppm and ~ 3000 ppm), doped with 1 at% transitional metal oxides (TMOs), have been systematically investigated by means of dilatometer, electron microscopy and impedance analyzer. It is confirmed that small additions of TMOs (i.e., Fe, Mn, Co or Ni) promote the densification and grain growth of both the pure and Si-containing 8YSZ. The effect of TMOs on the ionic conductivities could be negative or positive, relying on the type of TMOs, sintered density and impurity level. For the dense and pure 8YSZ (with ~ 30 ppm SiO2), the addition of 1 at% TMOs led to a reduction in grain interior (GI) conductivity by ~ 25-33% with little effect on the GB conduction. For the impure 8YSZ (with ~ 500 ppm or 3000 ppm SiO2), except for FeO1.5, the other TMOs (i.e., Mn, Co or Ni) are extremely detrimental to the total conductivity by significantly reducing the GB conduction. Moreover, it is also found that the GB conductivity of the impure 8YSZ doped with Co or Ni is less sensitive to sintering temperature. FeO1.5 showed a scavenging effect on SiO2 in the impure 8YSZ, which is specially beneficial to the total conductivity of samples with higher silica levels and/or sintered at relatively low temperatures. © 2009 Elsevier B.V. All rights reserved.Publication Magnetodielectric Ni ferrite ceramics with Bi2O3 additive for potential antenna miniaturizations(2009-02) Liew, X.T.; Chan, K.C.; Kong, L.B.; TEMASEK LABORATORIESThis paper reports on the preparation and characterization of nickel ferrite (NiFe1.98O4) ceramics doped with Bi2O3 as sintering aid. Focus has been on the effects of concentration of Bi2O3 and sintering temperature on the densification, grain growth, dielectric, and magnetic properties of the NiFe1.98 O4 ceramics, with an aim at developing magnetodielectric properties, with almost equal real permeability and permittivity, as well as sufficiently low magnetic and dielectric loss tangents, over 3 to 30 MHz (high frequency or HF band). X-ray diffraction results indicated that there is no obvious reaction between NiFe1.98O4 and Bi2O3, at Bi2O3 levels of up to 7 wt% and temperatures up to 1150 °C. The addition of Bi2O3 facilitated a liquid phase sintering mechanism for the densification of NiFe1.9804 ceramics. The addition of Bi2O3 not only improved the densification but also promoted the grain growth of NiFe1.98O4 ceramics. To achieve sufficiently low dielectric loss tangent, the concentration of Bi2O3 should not be less than 5 wt%. The low dielectric loss tangents of the samples doped with high concentrations of Bi2O3 can be attributed to the full densification of the ceramics. Magnetic properties of the NiFe1.98O4 ceramics, as a function of sintering temperature and Bi2O3 concentration, can be qualitatively explained by the Globus model. Promising magnetodielectric properties have been obtained in the sample doped with 5% Bi2O3 and sintered at 1050 °C for 2 h. The sample has almost equal values of permeability and permittivity of ∼12, together with low dielectric and magnetic loss tangents, over 3 to 30 MHz. This material might be useful for the miniaturization of HF (3 to 30 MHz) antennas. © 2009 Materials Research Society.Publication High-frequency properties and attenuation characteristics of WBa hexaferrite composites with doping of various oxides(2009-02) Li, Z.W.; Lin, G.Q.; Wu, Y.P.; Kong, L.B.; TEMASEK LABORATORIESDoping of various oxides can greatly modify the high-frequency magnetic and dielectric properties and, therefore, significantly influence the impedance matching and bandwidth characteristics of BaCoZnFe $-{16}$O $-{27}$ ferrite/epoxy composites. The experiments show that composites doped with V $-{2}$O $-{5}$ and IrO $-{2}+$V $-{2}$O $-{5}$ are potential candidates for use as EM attenuation materials with low reflectivity and broad bandwidth at S, C, and X microwave bands. The frequency bands for reflectivity $\vert R\vert \leq-10$ dB cover 3.613.7 GHz and 3.011.6 GHz at the thickness of 0.3 and 0.35 cm, and the relative bandwidth $W-{R}=3. 8$ and 3.9 is achieved, respectively, for composites doped with V$-{2}$O $-{5}$ and IrO $-{2}+$V $-{2}$O $-{5}$. © 2006 IEEE.Publication Greatly enhanced permeability and expanded bandwidth for spinel ferrite composites with flaky fillers(2010-11) Li, Z.W.; Yang, Z.H.; Huang, R.F.; Kong, L.B.; TEMASEK LABORATORIESSpinel ferrite flakes are prepared for use as fillers in electromagnetic (EM) composites. EM attenuation composites with the flaky fillers exhibit greatly enhanced high-frequency magnetic properties and attenuation properties. The complex permeability, μ′r,0 and μPrime;r, max, is enhanced by above 300%, and the relative attenuation bandwidth Wp is increased by 40%, as compared to those for composites with conventional spherical fillers. Wp achieves 80% of the theoretical maximum relative bandwidth. The composites with the flaky fillers are powerful and potential candidates as EM attenuation materials with broad bandwidth at microwave frequency. © 2006 IEEE.Publication Mullite phase formation and reaction sequences with the presence of pentoxides(2003-03-10) Kong, L.B.; Gan, Y.B.; Ma, J.; Zhang, T.S.; Boey, F.; Zhang, R.F.; TEMASEK LABORATORIESThe effects of pentavalent oxide (M2O5, M=V, Nb and Ta) on the phase formation of mullite, reaction sequence and microstructure development in the oxide mixtures have been investigated. It was found that V2O5 accelerated the mullite phase formation while Nb2O5 and Ta2O5 inhibited the mullitization. No reaction was observed between V2O5 and Al2O3 or SiO2, while a complicated reaction sequence was found in the mixtures doped with Nb2O5 and Ta2O5. V2O5 doping resulted in anisotropic grain growth, while equiaxed grains were grown from samples doped with Nb2O5 and Ta2O5. The three groups also demonstrated a different densification behavior. Samples doped with V2O5 were of a much lower density than the theoretical density of mullite, due to the anisotropic grain growth. The densification of the Nb2O5 and Ta2O5 groups was similar to the mixture of Al2O3 and SiO2 without dopants. These differences could be explained in terms of the role that the pentavalent oxides played during the formation of the SiO2-rich liquid phase in the mixtures. © 2002 Elsevier Science B.V. All rights reserved.Publication Synergetic effect of NiO and SiO2 on the sintering and properties of 8 mol% yttria-stabilized zirconia electrolytes(2009-01-01) Zhang, T.S.; Chan, S.H.; Kong, L.B.; Sheng, P.T.; Ma, J.; TEMASEK LABORATORIESYttria-doped zirconia electrolytes (e.g., 8 mol% yttria-stabilized ZrO2, 8YSZ) have been considered to be the most promising candidates for applications in solid oxide fuel cells (SOFC). Due to the ubiquitous presence of SiO2 impurities and wide use of Ni-containing anodes, it is therefore of great technical importance to understand the synergetic effect of NiO and SiO2 on densification, grain growth and ionic conductivities (especially the grain boundary (GB) conduction) of zirconia electrolytes. In this study, three groups of 8YSZ ceramics, with Si contents of ∼30, ∼500 and ∼3000 ppm, have been designed. 1 at% NiO was added into these materials by a wet chemical method. The addition of SiO2 has a negative effect on the sintering and densification, while the introduction of 1 at% NiO reduced the sintering temperature and promotes grain growth of the zirconia ceramics. However, the presence of small amount of NiO prevented full densification of 8YSZ ceramics. NiO also led to a decrease by ∼33% in grain interior (GI) conductivity, with little effect on the GB conduction of high-purity 8YSZ (∼30 ppm SiO2). However, the coexistence of NiO and SiO2 is extremely detrimental to total conductivity by significantly reducing the GB conduction. Moreover, it is observed that, unlike the 8YSZ-doped SiO2 with only, whose GB conduction increases greatly with increasing sintering temperature, the GB conduction of the NiO and SiO2 codoped samples is less sensitive to sintering temperature. © 2008 Elsevier Ltd. All rights reserved.Publication Frequency dependence of effective permittivity of carbon nanotube composites(2007) Liu, L.; Matitsine, S.; Gan, Y.B.; Chen, L.F.; Kong, L.B.; Rozanov, K.N.; TEMASEK LABORATORIESDependence of the permittivity of single-walled and multiwalled carbon nanotube composites on frequency and concentration was investigated experimentally using the coaxial air-line method over 0.1-10 GHz. The results are in good agreement with that obtained using the impedance method. It is found that scaling law based on the percolation theory provides a good description of the frequency dependence of measured permittivity in carbon nanotube composites. Parameters of the scaling law and deviations from the percolation theory are also discussed. © 2007 American Institute of Physics.Publication Effect of Mn addition on the densification, grain growth and ionic conductivity of pure and SiO2-containing 8YSZ electrolytes(2009-02-16) Zhang, T.S.; Chan, S.H.; Wang, W.; Hbaieb, K.; Kong, L.B.; Ma, J.; TEMASEK LABORATORIES1 at.% Mn was introduced to highly pure (~ 30 ppm SiO2) and impure (~ 500 ppm SiO2) 8YSZ electrolytes via a wet chemical method. The densification behavior, sintering mechanism and ionic conductivity of the two groups of samples were systematically investigated. The addition of 1 at.% Mn enhanced the densification and promoted grain growth of both pure and impure samples, which is attributed to the fact that the presence of Mn significantly reduced the apparent activation energy, although it did not change the early-stage sintering mechanism (dominated by volume-diffusion). The addition of Mn decreased the grain interior (GI) conductivity, with almost little impact on the grain-boundary (GB) conduction for the pure samples. However, the combination of silica (a contaminant in impure 8YSZ) with Mn led to a significant deterioration to the GB effect compared with silica alone. Based on the estimated activation energies for GB conduction, it is found that the "constriction model" can be used to describe the GB behaviors of the samples more appropriately than other models. The GB coverage fraction decreased with increasing sintering temperature, leading to a continuous increase in the GB conduction, which is found to be associated with the dissolution of silica into zirconia lattice during sintering at high temperatures. © 2008 Elsevier B.V. All rights reserved.