Chunchun Li

Processing and bioactivity of 45S5 Bioglass®-graphene nanoplatelets composites

Well dispersed 45S5 Bioglass® (BG)-graphene nanoplatelets (GNP) composites were prepared after optimising the processing conditions. Fully dense BG nanocomposites with GNP loading of 1, 3 and 5xa0vol% were consolidated using Spark plasma sintering (SPS). SPS avoided any structural damage of GNP as confirmed using Raman spectroscopy. GNP increased the viscosity of BG-GNP composites resulting in an increase in the sintering temperature by ~50xa0°C compared to pure BG. Electrical conductivity of BG-GNP composites increased with increasing concentration of GNP. The highest conductivity of 13 S/m was observed for BG-GNP (5xa0vol%) composite which is ~9 orders of magnitude higher compared to pure BG. For both BG and BG-GNP composites, in vitro bioactivity testing was done using simulated body fluid for 1 and 3xa0days. XRD confirmed the formation of hydroxyapatite for BG and BG-GNP composites with cauliflower structures forming on top of the nano-composites surface. GNP increased the electrical conductivity of BG-GNP composites without affecting the bioactivity thus opening the possibility to fabricate bioactive and electrically conductive scaffolds for bone tissue engineering.

Microwave dielectric properties of CaO–La2O3–Nb2O5–TiO2 ceramics

A series of ceramics with a general formula Ca1+xLa4−xNbxTi5−xO17 (0xa0≤xa0xxa0≤xa04) were fabricated using the solid-state ceramic route. The phase, microstructure, and microwave dielectric properties varied distinctly with composition or the value of x. X-ray diffraction results showed that the two end member phases, CaLa4Ti5O17 and Ca5Nb4TiO17, crystallized into single phases with orthorhombic and monoclinic crystal structure, respectively. For intermediate compounds with xxa0=xa01, 2, and 3, mixture phases CaLa4Ti5O17 and Ca5Nb4TiO17 coexisted and a trace amount of second phase was detected. The ceramics showed high εr in the range of 45–52, relatively high quality factors with Qxa0×xa0f in the range of 9,870–15,680xa0GHz and τf value in the range between −38 and −126.4xa0ppm/°C. τf of CaLa4Ti5O17 can be tuned to a near-zero value by addition of suitable amount of TiO2.

Complex impedance analysis on a layered perovskite-like ceramic: La3Ti2TaO11

Ac-impedance analysis is a very convenient and powerful technique to study the electrical properties of polycrystalline materials and to correlate the electrical properties with their microstructure. One of the strengths of ac-impedance analysis is that it helps us to separate the contributions of grains from other impedance components, such as grain boundary, interfaces, electrodes, etc. [1–6]. The impedance measurements of a material give us data including real part and imaginary part components. Data may be analyzed in terms of four possible complex formalisms: the impedance Z*, the electric modulus M*, the admittance Y*, and the permittivity e*. These are interrelated in accordance with the relation:

Effects of zinc substitution on the dielectric properties of Ca5Nb4TiO17 microwave ceramics

Ca5-xZnxNb4TiO17 ceramics with 0 ≤ x ≤ 0.4 were prepared through a solid-state reaction method. Effects of zinc substitution on sintering behavior and microwave dielectric properties of Ca5Nb4TiO17 ceramics were investigated. The sintering temperature was significantly lowered from 1480°C for pure Ca5Nb4TiO17 to 1260°C for x = 0.4. The microwave dielectric properties are strongly correlated with the composition. It is worth noting that the temperature coefficient of resonant frequency (τf) displays a tendency toward positive value, ranging from −126.4 ppm/°C to −8.6 ppm/°C. A temperature stable microwave ceramic with dielectric constant of 52 and Q × f value of 9937 GHz is achieved at x = 0.4 and is a potential candidate for application as cores in dielectrically loaded antennas.

Study on properties of tantalum-doped La2Ti2O7 ferroelectric ceramics

A series of ceramics with a general formula, La2Ti2-xTaxO7, in which x = 0.05, 0.1, 0.2, and 0.3, were prepared by Spark Plasma Sintering (SPS). Effects of tantalum substitution for titanium on the structure, dielectric, and piezoelectric properties were studied. Results revealed that the structure changed gradually from 4-layer to 3-layer due to the higher valence of Ta. The solid solution limit of tantalum in La2Ti2O7 lattice was in the proximity of x = 0.2. The ferroelectric Curie temperature (Tc) decreased with increasing tantalum doping content. dc resistivity reached a maximum value at x = 0.2 with a value of ∼ 1.0 × 108Ω ⋅ cm at 600°C. The influence of texture on the piezoelectric properties of La2Ti2-xTaxO7 ceramics was also investigated. A maximum d33 value ∼2.1 pC/N was obtained at x = 0.2.

Reduced thermal conductivity by nanoscale intergrowths in perovskite like layered structure La2Ti2O7

The effect of substitution and oxidation-reduction on the thermal conductivity of perovskite-like layered structure (PLS) ceramics was investigated in relation to mass contrast and non-stoichiometry. Sr (acceptor) was substituted on the A site, while Ta (donor) was substituted on the B site of La2Ti2O7. Substitution in PLS materials creates atomic scale disorders to accommodate the non-stoichiometry. High resolution transmission electron microscopy and X ray diffraction revealed that acceptor substitution in La2Ti2O7 produced nanoscale intergrowths of nu2009=u20095 layered phase, while donor substitution produced nanoscale intergrowths of nu2009=u20093 layered phase. As a result of these nanoscale intergrowths, the thermal conductivity value reduced by as much as ∼20%. Pure La2Ti2O7 has a thermal conductivity value of ∼1.3u2009W/m K which dropped to a value of ∼1.12u2009W/m K for Sr doped La2Ti2O7 and ∼0.93u2009W/m K for Ta doped La2Ti2O7 at 573u2009K.

Dielectric and complex impedance analysis of Sr5Nb4TiO17 ceramic with perovskite-like structure

Dense Sr5Nb4TiO17 ceramic was prepared by a solid state reaction route. XRD and SEM results revealed the formation of single phase with perovskite-like structure and space group Pnnm at room temperature. Three sets of dielectric peaks were observed through dielectric measurement. A phase transition from orthorhombic Pnnm to Immm was obtained at ~596xa0°C. The combined Z″ and M″ plots confirmed the heterogeneous electrical microstructure of the Sr5Nb4TiO17 ceramic.