Hua-Xin Peng

Microstructure of ceramic foams

This paper describes the preparation of ceramic foams by expansion of a ceramic suspension based on a polyurethane system. The microstructure and degree of reticulation of the foamed ceramic were examined and analysed with the help of a simple geometrical model. Like the porous ceramics prepared by the replica processing method, these foamed ceramics possess open cells in a nearly equiaxed shape but the cell size is much finer. The ratio of the window size to the cell size is a useful parameter for characterising the geometry of the foam and is related to the qualitative concept of degree of reticulation. For a face centred cubic array of cells it is related geometrically to the volume fraction of porosity and this relationship is tested using microstructural measurements for a range of ceramic foams.

Bi-continuous metal matrix composites

Bi-continuous alumina:aluminium composites were made by infiltrating an alumina preform which had the structure of a reticulated ceramic foam. The low density preforms were prepared from a polyurethane suspension of alumina powder which was pyrolysed and sintered after foaming. Higher density preforms consisted of ceramic foams with open cells. All these preforms were infiltrated with 6061 aluminium alloy using a modified squeeze caster fitted with a vacuum system and fine control of speed and pressure. The microstructure of the preform fitted an established relationship between the ratio of window diameter to cell diameter (k) and void volume fraction (Vp). Low k foams were infiltrated fully but on cooling below the solidus, interfacial debonding took place due to differential thermal contraction. This was overcome by modifying the processing conditions. High k foams which had high fractional porosity, retained sound interfacial bonding. The composites possess higher elastic modulus than conventional MMCs with a homogeneous reinforcement distribution at a given volume fraction. The loss of electrical conductivity is negligible in the lower volume fraction range because of the three dimensionally continuous aluminium phase. The experimental results are compared with a number of theoretical predictions.

Stress tunable properties of ferromagnetic microwires and their multifunctional composites

We report the results of a systematic study on stress tunable absorption of glass-coated amorphous Co68.7Fe4Ni1B13Si11Mo2.3 microwires and their composites. The magnetic microwires possess good stress-impedance properties and yield a stress dependence of absorption at gigahertz frequencies. The stress compensates the reverse effect of magnetic field on absorption. There exist strong stress dependences of the effective permittivity and transmission parameters. Composite failure due to the wire damage results in a dramatic change of the sign and magnitude of effective permittivity. The double peak is identified in the stress dependence of field tunability, in contrast to the single peak for the magnetic field tunability. All these results indicate that the present composites are very promising for detecting the ambient stress levels and interrogating the structural integrity.

Excellent magnetocaloric properties of melt-extracted Gd-based amorphous microwires

We report upon the excellent magnetocaloric properties of Gd53Al24Co20Zr3 amorphous microwires. In addition to obtaining the large magnetic entropy change (−ΔSM ∼ 10.3 J/kg K at TC ∼ 95 K), an extremely large value of refrigerant capacity (RC ∼ 733.4 J/kg) has been achieved for a field change of 5 T in an array of forty microwires arranged in parallel. This value of RC is about 79% and 103% larger than those of Gd (∼410 J/kg) and Gd5Si2Ge1.9Fe0.1 (∼360 J/kg) regardless of their magnetic ordering temperatures. The design and fabrication of a magnetic bed made of these parallel-arranged microwires would thus be a very promising approach for active magnetic refrigeration for nitrogen liquefaction. Since these microwires can easily be assembled as laminate structures, they have potential applications as a cooling device for micro electro mechanical systems and nano electro mechanical systems.

Exceptional electromagnetic interference shielding properties of ferromagnetic microwires enabled polymer composites

We present systematic studies of the electromagnetic interference (EMI) shielding and microwave properties of a new class of shielding material, i.e., the ferromagnetic microwires-embedded polymer composites. We show that at 1–2 GHz the shielding effectiveness (SE) of the continuous-wire composite reaches a high value of 18 dB (98.4% attenuation) for a very low filler loading of 0.024% and a thickness of 0.64 mm. The normalized SE of this new composite is about 70 times higher than that of the bucky paper-based composite and is two to four orders of magnitude higher than those of other shielding candidate materials. Complex permeability, permittivity, and impedance experiments reveal that the absorption of electromagnetic radiation is a dominant mechanism for EMI shielding of the studied composites. The advantages of high shielding efficiency, good physical integrity, low fabrication costs, and multifunctionalities make them an attractive candidate material for a variety of technological applications.

Tuneable Metacomposites Based on Functional Fillers

Metamaterials, traditionally in the form of artificial structures with surprising electromagnetic properties, have triggered unprecedented opportunities to achieve those fascinating applications that previously only exist in science-fiction works, for example, Harry Potter’s cloak. Nevertheless, their massive manufacturing costs incurred by their complicated structures restrict the scale-up and mass production. The ultimate properties are primarily (if not solely) determined by the intrinsic structures of metamaterials that make them merely ‘meta-structures’. In response to these issues, it is desirable to have a genuine engineering composite yet with metamaterial characteristics. Thus, ‘metacomposite’ has been proposed to account for a real piece of composite material. This has subsequently become a nascent area where metamaterial properties are attained under wider operating frequencies with certain tunability towards external magnetic fields or mechanical stresses. In this chapter, we start with an overview of metacomposites containing various dielectric and/or magnetic fillers following the fillers’ dimensions from 0D, 1D to 2D. We then critically discussed some progresses in metacomposites containing ferromagnetic microwires together with unparalleled advantages in microwave sensing and cloaking areas. Finally, the chapter is closed with an outlook of strategies for improving existing metacomposites and some future perspectives.

Giant magnetoimpedance effect in ultrasoft FeAlSiBCuNb nanocomposites for sensor applications

Fe73−xAlxSi14B8.5Cu1Nb3.5 (x=0,2) nanocomposite materials consisting of a nanocrystalline phase in an amorphous matrix were obtained by annealing their precursor amorphous ribbons, which were prepared by the melt-spinning technique, at different temperatures ranging between 350 and 650°C for 45min in vacuum. Investigation on their magnetic and magnetoimpedance properties indicates that the Al-containing sample (x=2) possesses superior magnetic softness and giant magnetoimpedance (GMI) effect over the Al-free counterpart. This can be likely ascribed to the increased magnetic permeability, decreased coercive force, and decreased resistivity. The increased magnetic permeability results from a reduction in magnetocrystalline anisotropy and saturation magnetostriction. The correlations between magnetic softness, electrical properties, and GMI behavior are discussed in the light of the skin effect model. These results indicate that the Al-containing Fe-based nanocomposite material can be ideally used for high-p...

Novel magnetic microwires-embedded composites for structural health monitoring applications

We report the results of a systematic study of the magnetic, mechanical, magnetoimpedance and field tunable properties of glass-coated amorphous Co68.7Fe4Ni1B13Si11Mo2.3 microwires and composites containing these microwires. The magnetic microwires possess good magnetic and mechanical properties. The magnetoimpedance ratio in the gigahertz range varies sensitively with applied fields below the anisotropy field but becomes unchanged for higher applied fields. The good mechanical properties are retained in the magnetic microwires-embedded composites. The strong field dependences of the effective permittivity and transmission parameters in the gigahertz range indicate that the present composites are very promising candidate materials for structural health monitoring and self-sensing applications.

Optimized giant magnetoimpedance effect in amorphous and nanocrystalline materials

This letter reports the giant magnetoimpedance (GMI) effect and its magnetic response in optimized Co70Fe5Si15Nb2.2Cu0.8B7 amorphous and Fe71Al2Si14B8.5Cu1Nb3.5 nanocrystalline ribbons. At a given frequency of 5 MHz, the largest GMI ratios of 513% and 640% were observed for the Co-based amorphous and Fe-based nanocrystalline samples, respectively. More interestingly, the magnetic response reached the largest value of 144%/Oe at the frequency of 4 MHz for the Co-based amorphous sample and of 40%/Oe at the frequency of 5 MHz for the Fe-based nanocrystalline sample. This is ideal for high-frequency and high-performance GMI-based sensor applications. The skin effect model was used to interpret the obtained results of GMI in connection with the magnetic-field and frequency dependences of the longitudinal permeability.

Microstructures and mechanical properties of engineered short fibre reinforced aluminium matrix composites

Aluminium matrix composites have been fabricated by squeeze casting into short fibre preforms modified on the basis of a phase contiguity model to improve mechanical performance. Fibre junctions were created in a planar random alumina fibre array by (a) sintering, (b) phosphoric acid treatment, (c) phosphoric acid/aluminium hydroxide (P/Al) treatment and (d) infiltration with alumina powder and sintering. The microstructures and mechanical properties of these composites were examined systematically. The results indicate that using phosphoric acid solution itself to create inter-fibre bonds in the preform gave rise to very low composite strength and ductility compared with that resulting from the as-received preform. This is mainly due to the severe damage to the fibres caused by chemical reaction. By employing the P/Al binder to create the cross-links, the composites reinforced with laboratory-made preforms yielded higher tensile strength relative to the uniform fibre reinforced composites without any sacrifice to the ductility and elastic modulus.