Ling Bing Kong

Recent progress in some composite materials and structures for specific electromagnetic applications

Abstract This review aims to summarise the progress in some materials and structures for electromagnetic applications, such as microwave absorption, electric shielding and antenna designs, which have been developed in recent years. Composites with spherical powders for microwave absorption focus mainly on those based on ferrites (especially hexagonal), carbonyl iron and related alloys and various newly emerged nanosized materials. Composites with long conductive fibres as fillers will be summarised, with speical attentions to prediction, measurment and evaluation of their performances. Metamaterials include structures for microwave absorbing applications, tunable materials or structures with reflection or transmission coefficients that are tunable by external magnetic or electric fields, and specially designed structures for microwave absorbing applications, with thickness much smaller than that of conventional composite materials and performances that can be optimised by the physical properties of substrates, and new metamaterials constructed with ferrite cores wound by metallic wire coils that exhibited unique magnetic properties, with extremely high real and imaginary permeability, which are adjustable or tunable by varying their configurations. Magnetodielectric materials, with matching permeability and permittivity, together with sufficiently low magnetic and dielectric loss tangents, with potential applications in antenna miniaturisation, will be discussed.

High microwave permittivity of multiwalled carbon nanotube composites

Complex permittivity spectra of the multiwalled carbon nanotube (MWNT)-epoxy composites with mass concentrations up to 25.9% were measured from 10 MHz to 20 GHz. The composites exhibit high real and imaginary relative permittivities over broad bandwidth, showing strong dependence on the MWNT loading. It is observed that distinct resonance takes place at ∼1.5 GHz, with the corresponding resonant frequency shifts downward as the MWNT concentration increases. The origin of the dielectric properties is explained in terms of the dielectric relaxation/resonance and electrical conduction behavior of the nanotube composites, substantiated by the agreement between simulation and experimental data.

Biomimetic processing of nanocrystallite bioactive apatite coating on titanium

Biomimetic processes have attracted huge attention in recent years due to their significant applications in biomedical areas such as bone tissue engineering. In the present study, a biomimetic process was employed to form a nanocrystallite apatite coating on metal. A thin bone-like apatite layer was coated onto titanium (Ti) metals via an alkali pre-treatment. This was followed by immersion in a simulated body fluid. Analysis of the coating by thin film x-ray diffraction and scanning electron microscope has shown that the apatite layer grown in this way exhibits nanostructure and has similar stoichiometry to that of natural bone. It is observed that the thickness of the apatite layer increases as the immersion period increases. The growth kinetics and mechanism are also discussed. A cross-sectional study has also shown that a uniform coating of carbonate-containing apatite (hydroxyapatite) is firmly adhered on the Ti metal. The adhesion of the apatite layer on the Ti substrate was further confirmed by a shear test, which has shown an average value of 9.5 MPa. The bioactivity of the coating was finally examined by cell culturing experiments. The results have shown that the nanocomposite prepared using the present method possesses good mechanical properties and bioactivity.

Improvement of dielectric loss tangent of Al2O3 doped Ba0.5Sr0.5TiO3 thin films for tunable microwave devices

Al2O3 doped Ba0.5Sr0.5TiO3 (BST) thin films, with different Al2O3 contents, were deposited on (100) LaAlO3 substrates by the pulsed laser deposition technique to develop agile thin films for tunable microwave device applications. The dielectric properties of Al2O3 doped BST films were determined with a nondestructive dual resonator near 7.7 GHz. We demonstrated that the Al2O3 doping plays a significant role in improving the dielectric properties of BST thin films. The Al2O3 doping successfully reduced the dielectric loss tangent (tan δ) from 0.03 (pure BST) to 0.011 (Al2O3 doped BST). Reduction in the loss tangent also leads to reduction in the dielectric constant and dielectric tunability. Our results showed that the BSTA4 film remains tunability=15.9%, which is sufficient for tunable microwave devices applications. Consequently, the Al2O3 doping improved the figure of merit (K) for the BST films from K=7.33 (pure BST) to K=14.45 (Al2O3 doped BST).

Preparation and characterization of Pb(Zr0.52Ti0.48)O3 ceramics from high-energy ball milling powders

Abstract Ultra-fine Pb(Zr0.52Ti0.48)O3 powder has been synthesized from the commercial PbO, TiO2 and ZrO2 powders using the high-energy ball milling technique in air at room temperature without any post-annealing. The synthesized powders milling for different hours are characterized using XRD, SEM, TEM and Raman spectra. XRD patterns and Raman spectra show that the perovskite phase of PZT can be formed from the mixture of the starting materials after milling for 20 h. The grain sizes of the powders have been estimated from the XRD patterns and TEM images to be ∼10 nm. The relationship between the microstructure of the PZT powders and the milling hours has been discussed. The PZT ceramics are derived for the synthesized powders by sintering the green pellets at 1100°C for 1 h. The dielectric properties of the PZT samples are comparable with those of the same composition PZT prepared by other methods, but the high-energy ball milling process is much easier than other reported methods. The high-energy ball milling method offers a very attractive advantage of avoiding the loss of volatile elements, such as Pb, Bi, in the procedure of synthesis of nano-powders with their uniform compositions.

Size effect and gas sensing characteristics of nanocrystalline xSnO2-(1−x)α-Fe2O3 ethanol sensors

Abstract Non-equilibrium nanocrystalline x SnO 2 -(1− x )α-Fe 2 O 3 powders have been prepared using the mechanical alloying technique. The thick film screen printing technology is then employed to fabricate these ethanol gas sensors. Their particle size and structural properties are systematically characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The gas sensing characteristics are also measured. Based on the experimental results, we have observed that particle size of the powders is drastically milled down to about 10 nm after 24 h of high-energy milling. A very high gas sensitivity value of 845 for 1000 ppm of ethanol gas in air has been obtained. Our proposed new structural model for these non-equilibrium nanocrystalline x SnO 2 -(1− x )α-Fe 2 O 3 materials explains both the lattice expansion of these high energy mechanically alloyed powders as well as the charge neutrality in terms of additional oxygen dangling bonds at the nano-sized particle surfaces. It is those enormous oxygen-dangling bonds at the particle surfaces that give rise to the high gas sensitivity. The sensors are found to be 32.5 times more selective to the ethanol gas compared to CO and H 2 gases.

Barium titanate derived from mechanochemically activated powders

Abstract A high-energy ball milling process was applied to a mixture with equal molar ratio of BaCO 3 and TiO 2 . X-ray diffraction data indicated that there was no reaction happening during the milling process although the particle size was greatly reduced. The effect of post-annealing on the phase formation and grain growth of BaTiO 3 was investigated by thermal analysis (DTA/TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). No reaction was observed in the sample annealed at 600°C. Ba 2 TiO 4 was found as the mixture annealed at 700°C. An 800°C-annealing led to single phase BaTiO 3 with a cubic structure. The temperature at which BaTiO 3 phase was derived is lower than required by the solid-state reaction process, which could be attributed to the refined grains/particles of the mixture as a result of high-energy ball milling. Tetragonal structured BaTiO 3 was formed at 1150°C together with a sharp increase in grain size.

Molten-salt-mediated synthesis of SiC nanowires for microwave absorption applications

Silicon carbide (SiC) nanowires were synthesized by a reaction of multiwall carbon nanotubes (MWCNTs) and silicon vapor from molten salt medium at 1250 °C. The phase, morphology, and microstructure of the nanowires were systemically characterized by X-ray diffraction, field emission scanning electron microscopy, and high resolution transmission electron microscopy. The results revealed that the nanowires were of single-crystalline β-SiC phase with the growth direction along [111] and had diameters of 20–80 nm and lengths up to several tens of micrometers. The molten salt introduced facilitated the evaporation of Si (vapor) onto MWCNTs (solid) and the growth of SiC nanowires followed the vapor–solid process. The investigation of microwave absorbability indicated that a minimum reflection loss of −17.4 dB at 11.2 GHz could be achieved with 30 wt% SiC nanowires as the filler in the silicone matrix. The attenuation of microwave could be attributed to the dielectric loss and a possible absorption mechanism was also discussed.

Preparation of Bi4Ti3O12 ceramics via a high-energy ball milling process

Abstract Nano-sized bismuth titanate (Bi4Ti3O12) powders were prepared by a high-energy ball milling process directly from their oxide mixture of Bi2O3 and TiO2. Bi4Ti3O12 phase can be formed from the oxide mixture after milling for 9 h. Almost single phase of Bi4Ti3O12 is obtained after milling for 15 h. With increasing milling time, the particle size of the mixture is gradually reduced. Bi4Ti3O12 ceramics were obtained by sintering the synthesized powders at temperatures ranging from 750°C to 950°C. The Bi4Ti3O12 ceramics sintered at 850°C for 1 h exhibited a density of 7.91 g/cm3, a dielectric of 243 and a dielectric loss of 0.017, and a remnant polarization of 24 μC/cm2 and a coercive field of 11 kV/cm, respectively. Piezoelectric parameters of the Bi4Ti3O12 ceramics are k33=56%, k31=58%. The Bi4Ti3O12 ceramics also possessed good pyroelectric properties. These results indicate that high-energy ball milling process is a promising way to prepare Bi4Ti3O12 ceramics.

Nanosized hydroxyapatite powders derived from coprecipitation process

Nanosized hydoxyapatite (Ca10(PO4)6(OH)2 or HA) powders were prepared by a coprecipitation process using calcium nitrate and phosphoric acid as starting materials. The synthesized powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) specific area measurment techniques. Single phase HA, with an average grain size of about 60 nm and a BET surface area of 62 m2/g, was obtained. No grain coarsening was observed when the HA powders were heated at 600°C for 4 hours. HA ceramics were obtained by sintering the powders at temperatures from 1000°C to 1200°C. Dense HA ceramics with a theoretical density of 98% and grain size of 6.5 μm were achieved after sintering the HA powders at 1200°C for 2 hours. HA phase was observed to decompose into tricalcium phosphate when sintered at 1300°C. The microstructure development of the sintered HA ceramics with sintering temperature was also characterized and discussed.