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Porous BN/Si 3 N 4 composites with different Y 2 O 3 –MgO nanopowders have been fabricated by a pressureless sintering method. The densification, phase evolution, microstructure, mechanical properties and dielectric properties of the BN/Si 3 N 4 composites were investigated. The density and mechanical properties of the porous BN/Si 3 N 4 composite ceramics firstly increased with addition of Y 2 O 3 –MgO up to 20 wt% and then decreased, while the dielectric properties gently declined as the Y 2 O 3 –MgO contents increased. For the porous BN/Si 3 N 4 composites with 20 wt% Y 2 O 3 –MgO nanopowders, the porosity, flexural strength and dielectric constant reached 21.9%, 243.6 MPa and 5.5, respectively, which indicated that the composites could be one of the candidates for wave-transparent applications. [ABSTRACT FROM AUTHOR]

A series of nickel substituted copper ferrite nanocrystals (Ni x Cu 1-x Fe 2 O 4 , x = 0 , 0.1 , 0.3 , 0.5 , 0.7) were synthesized by co-precipitation technique. The prepared powders were calcined twice at 900 °C (pre-calcination) and 1100 °C (post-calcination) respectively. The structural, morphological and magnetic properties were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier transformed infrared spectroscopy (FT-IR) and vibrating sample magnetometer (VSM), respectively. XRD analysis shows that all the samples have a single-phase cubic spinel structure. The crystallite size was calculated by using Debye-Scherrer's formula. It was observed that the crystallite size decreases with the increasing nickel (Ni) content and increases after post-calcination. The saturation magnetization increased with the increasing Ni content, whereas the coercivity decreased. The saturation magnetization further increased after post-calcination. The parameters such as dielectric constant, dielectric loss and ac conductivity were also studied in the frequency range from 1 to 12 GHz. The complex dielectric permittivity of Ni x Cu 1-x Fe 2 O 4 nanocrystals was decreased with the increasing Ni contents and as well as after post calcination. Furthermore, the dielectric loss tangent and ac conductivity also decreases with the increasing Ni contents and as well as after post calcination. Image 1 • Co-precipitation technique is used to synthesis Ni x Cu 1-x Fe 2 O 4 (x = 0 , 0.1 , 0.3 , 0.5 , 0.7). • Structural and morphological investigation confirmed spinel structure of Ni x Cu 1-x Fe 2 O 4. • The particle size decreases with the increasing nickel content. • Magnetic properties improved with the addition of nickel contents and after post-calcination. • Complex permittivity decreases with increasing nickel contents and after post-calcination as well. [ABSTRACT FROM AUTHOR]

In this work, the epoxy-based composites filled with 3-Aminopropyltriethoxysilane (KH-550) modified binary filler of hexagonal boron nitride (h-BN) flakes and h-BN whiskers were fabricated, and the thermal and dielectric properties of composites were systematically investigated. Adding h-BN whiskers to the filler could effectively form heat conduction paths and improve the phonon transmission efficiency, thereby improving the thermal conductivity of the composites. According to our results, the thermal conductivity of composite with 27 wt% h-BN flakes and 3 wt% h-BN whiskers reached up to 0.819 W·m−1·K−1, which was 3.9 times that of pure epoxy resin. At the same time, the dielectric loss value and dielectric constant of this composite at 1 MHz were 0.02678 and 4.55, respectively, while pure epoxy resin were 0.03602 and 4.33. In addition, introducing h-BN whiskers into the matrix could effectively improve the thermal stability and heat dissipation performance of composites. [ABSTRACT FROM AUTHOR]

The SiC nanotubes were synthesized on porous Si 3 N 4 matrix by polysilazane immersion-pyrolysis. The β-SiC and free C contents increased with the increasing dipping times, which cause some clear vibration and loss peaks of the dielectric constant patterns emerged at around 12.9 GHz and 14.7 GHz in the samples of 20 wt% and 30 wt% benzoic acid, respectively. The reflectivity of the obtained ceramics were −7dB ∼ −10dB from the frequency of 8 GHz–18 GHz, which means that the amount of 80%–90% electromagnetic wave could be attenuated. The ceramics containing 40 wt% benzoic acid showed the high loss, low reflectivity and a wide absorbing band, indicating the high absorbing efficiency and good dielectric properties. Image 1 • The SiC NTs were synthesized. • 80%-90% EMI was attenuated for 10GHz. • The low R values means good absorbing ability. [ABSTRACT FROM AUTHOR]

Porous BN p /Si 3 N 4 ceramics were prepared by gas pressure sintering method as a candidate wave-transparent material. The effects of BN particle additions ranging from 0 wt% to 15 wt% on the phase compositions, microstructures, mechanical performance and dielectric properties were investigated. With the increase of BN content, the apparent porosity of the BN/Si 3 N 4 ceramics increased greatly from 2.3% to 21.2%. Although the mechanical properties were partially weaken, the dielectric performance of the porous BN/Si 3 N 4 was largely enhanced at the same time. For the 10% BN/Si 3 N 4 porous composite ceramics, the permittivity and dielectric loss was 33.5% and 62.2% lower than that of the porous Si 3 N 4 without BN content, respectively. Specifically, when 10% BN particles were added, the porous BN p /Si 3 N 4 has a porosity of 20.5% and the permittivity and dielectric loss were 5.14 and 0.0085 respectively with bending strength of 190.1 MPa and fracture toughness of 4.16 MPa m 1/2 , which can meet the required value of wave transparent materials. [ABSTRACT FROM AUTHOR]

Abstract: Cement-based 0-3 type piezoelectric composites, fabricated from white cement, lead zirconate titanate (PZT) powder and small amounts of carbon black, are exploited for potential applications in civil engineering. The nature of the aging and poling behavior of the carbon black/PZT/cement composites is investigated. It is shown that the use of a conductive carbon black phase can facilitate the poling process at room temperature. The influences of the carbon black on the piezoelectric and dielectric properties are studied as well. It is demonstrated that the piezoelectric sensitivities of the composites can be dramatically enhanced by incorporation of small amounts of carbon black, due to its conductive properties. With a poling field of 4kV/mm and poling time of 20min, the d 33, ɛ r , tan δ and K t values of the composite containing 70vol.% PZT and 1.0vol.% carbon black are 28.5pC/N, 202.6, 0.19 and 12.2%, respectively. [Copyright &y& Elsevier]

The SiCN precursor was prepared by cross-linking process using polysilazane (PSZ) as raw materials. The cross-linking behavior and dielectric properties of SiCN precursor were analyzed. The results indicated that the main reactions during cross-linking process were double bond addition reaction, hydrosilylation reaction, dehydrogenation coupling reaction and transamination reaction. The SiCN precursor was made up of Si-C-N network structure and the main chemical bonds were Si-C, Si-N and C-C. The appropriate cross-linking temperature of SiCN precursor was 600 °C. Furthermore, the increasing content and crystallization degree of free carbon could improve the dielectric properties of SiCN precursor. [ABSTRACT FROM AUTHOR]