No abstract

Ceramic composites with nanoparticles are intensively investigated due to their unique thermal, mechanic and electromagnetic properties. In this work, dielectric properties of phosphate ceramics with round silver nanoparticles of various sizes were studied in the wide frequency range of 20 Hz–40 GHz for microwave shielding applications. The percolation threshold in ceramics is close to 30 wt.% of Ag nanoparticles content and it is higher for bigger-sized nanoparticles. The microwave complex dielectric permittivity of ceramics above the percolation threshold is rather high (ε′ = 10 and ε″ = 10 at 30 GHz for ceramics with 50 wt.% inclusions of 30–50 nm size, it corresponds to almost 61% absorption of 2 mm-thickness plate) therefore these ceramics are suitable for microwave shielding applications. Moreover, the microwave absorption is bigger for ceramics with a larger concentration of fillers. In addition, it was demonstrated that the electrical transport in ceramics is thermally activated above room temperature and the potential barrier is almost independent of the concentration of nanoparticles. At very low temperature, the electrical transport in ceramics can be related to electron tunneling.

Composite materials with 83 wt.% of the 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 distributed in phosphate-bonded ceramics were prepared at three different pressures. A phosphate matrix comprises a mixture of an aluminum phosphate binder and melted periclase, MgO. All samples demonstrate a homogeneous distribution of the ferroelectric perovskite phase and are thermally stable up to 900 K. At higher temperatures, the pyrochlore cubic phase forms. It has been found that the density of the composites non-monotonously depends on the pressure. The dielectric permittivity and losses substantially increase with the density of the samples. The fabricated composites demonstrate diffused ferroelectric–paraelectric transition and prominent piezoelectric properties.

Thermostable heat-insulating composite materials based on solid magnesium phosphate and calcium phosphate binders, fly-ash and glass hollow microspheres have been developed and proposed. Thermal and phase transformations have been studied as well as strength characteristics of the initial composites and products of their thermal treatment up to 1000 °C have been determined. It is shown that the prepared phosphate composites are thermally stable in the temperature range of 20 –1000 °С, have a density 0.4 – 0.6 g/cm3 , thermal conductivity 0.11– 0.19 W/(m ⋅ K) and show sufficiently high strength properties (compressive strength 1.4 –3.2 MPa).

Thermostable composite materials based on solid magnesium phosphate and calcium phosphate, as well as hybrid calcium magnesium phosphate binders have been developed and investigated. Thermal and phase transformations of the phosphate composites have been studied. Strength characteristics of composite materials have been determined in the temperature range of 20–1000 °C. It is shown that the obtained phosphate composites have high strength properties (compressive strength reaches 120–130 MPa) and are characterised by high thermal stability in the temperature range up to 1000 °С. The low weight loss of the studied composites (no more than 10 %) and the absence of significant thermal effects indicate that they are promising for use as a thermostable matrix for obtaining functional composite materials.