Advances in Applied Ceramics: Structural,Functional and Bioceramics

Ferromagnetism at room temperature has been observed at the interface between ZnO and MnO₂ in bulk samples. XANES experiments have been performed at the Mn K-edge. Coexistence of Mn³⁺ and Mn⁴⁺ is observed in the ferromagnetic samples, confirming the double exchange mechanism as the responsible for ferromagnetism at room temperature. Using ZnO previously calcined at 950°C as starting material, which has lower diffusion ability than normal ZnO, leads to higher saturation magnetisation values. This indicates that the kinetics of the diffusion of Zn into MnO₂ plays a crucial role in the magnetic properties of this system.

In this work, pure and Ca²⁺ and Sr²⁺ doped LaMnO₃ powders were obtained using polymerisable complex method from citrate precursors. La₂O₃, Mn(NO₃)₂ and citric acid were used for citrate formation, and ethylene glycol was later added as polymerisation agent. For the preparation of doped samples, solutions of Ca(CH₃COO)₂ and Sr(NO₃)₂ were used. Calcination was performed at 800°C for 6 h in oxygen atmosphere. X-ray diffraction analysis showed that single phase LaMnO₃, La_0·7Ca_0·3MnO₃, La_0·7Sr_0·3MnO₃ and La_0·7Ca_0·15Sr_0·15MnO₃ powders were obtained. Morphology of prepared samples was analysed using SEM analysis. Field cooled and zero field cooled magnetisations versus temperature and hysteresis loops of powders and sintered samples were analysed. Curie temperatures were in the range from 170 to 370 K, depending on composition. Measured magnetisations even reached 85 emu g^–1. The influence of dopants on magnetisation, Curie temperature and hysteresis loops were quantified and discussed.

The aim of the paper is to evidence the effect of the composition and oxygen stoichiometry on the thermodynamic, spin dynamics and magnetic properties of some micro- and nanostructured substituted manganites La_1–xA_xMn_1–yB_yO_3–δ (A=Ca; B=Co, Fe; x=0·33; y=0, 0·02 and 0·04). The partial molar thermodynamic properties of oxygen dissolution in the perovskite phase as well the equilibrium partial pressures of oxygen have been obtained using solid electrolyte electrochemical cells [electromotive force measurement (EMF)] method. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using the data obtained by a coulometric titration technique coupled with EMF measurements. Using electron paramagnetic resonance spectroscopy, the spin dynamics has been investigated. Particular attention was devoted to the discussion concerning the influence of doping on polaron activation energy E_a and exchange coupling integral J between Mn spins. The results were correlated with the variation of both saturation magnetisation M_s and Curie temperature T_c. The data were discussed in relation to significant changes in the overall concentration of defects. New features related to the modifications in properties connected with the nanocrystalline state were evidenced.

Z type Ba₃Co_1·4Fe_24·6O₄₁ for multilayer inductor applications at high frequency (up to 2 GHz) was prepared by a standard ceramic route. Single phase Z type ferrite is obtained at 1300°C. Dense samples sintered at 1300°C display a permeability of μ=30 throughout the megahertz range and a maximum μ″ at 0·5 GHz. The addition of Bi₂O₃ shifts the temperature of maximum shrinkage down to 900°C. Dense samples were obtained after sintering at 900°C; however, their permeability is only μ=5, but the maximum μ″ is at 2 GHz. It is shown that Co₂Z ferrite is not stable at T<1300°C and that partial decomposition causes a reduction of permeability. Ferrite tapes were prepared by tape casting, and multilayer structures were fabricated by screen printing, stacking, lamination and cofiring. Firing was performed at 1300, 1000 and 900°C, i.e. at high and low temperature ceramic cofiring conditions respectively. It is shown that Co₂Z hexagonal ferrites are suitable materials for multilayer inductors, although those prepared at 900°C suffer from lower permeability.

In this work, superconducting samples of (Bi_1.6Pb_0.4)Sr₂CaCu₂O_8+δ+3 wt.%Ag have been studied and compared with Ag free samples, (Bi_1.6Pb_0.4)Sr₂CaCu₂O_8+δ, and with Ag and Pb free ones, Bi₂Sr₂CaCu₂O_8+δ. It has been found that Pb doped samples present a worse texture when compared with the undoped ones, resulting in a drastic change on the microstructure. The results showed that electrical resistivity at room temperature, critical current as well as flexural strength are decreasing for the Pb doped samples. When Ag is added to these Pb doped samples, all the properties are improved even if they do not reach the same values as the undoped ones. On the other hand, it has been found that Pb doped samples showed E–I curves with very high sharpness values on the zone of the superconducting to normal transition, reaching n values (E∼Iⁿ) as high as 45 at 65 K, for samples with simultaneous Pb and Ag additions.

High frequency characteristics of the inductance (L) and the quality factor (Q) of square spiral type planar magnetic inductors prepared by direct write techniques were simulated using a Maxwell three-dimensional field simulator. The materials, cross-section shape, thickness and conductivity of the coil were varied to estimate the trend of the inductors’ performances. The area dimension of inductors utilised was 1·3 × 1·3 mm. Al₂O₃ was used as the substrate material. The used inductors with a permalloy (Ni₈₁Fe₁₉) core, silver (Ag) coils, trapezoid shaped cross-section of 30 μm thick coil and coil conductivity of 70% of Ag bulk value exhibit L=12·3 nH, maximum Q=33 near the frequency of 50 MHz and a self-resonant frequency of 0·78 GHz. The simulated high frequency data of inductors agreed well with those obtained from the equivalent circuit model of inductors.

Water debinding technique for ceramic injection moulding is competitive for its high debinding rate and environmental friendliness. In this research, the relationship between debinding ratio and debinding time at different temperatures was determined. The results showed that the water debinding process could be divided into dissolution controlled stage and diffusion controlled stage. The apparent interdiffusion coefficient was defined to investigate the impact of the porosity and pores tortuosity on the diffusion. A unified equation valid for samples with different dimensions was deduced. In addition, the microstructure evolution of the green body during water debinding was characterised by formation of the connected porosity and variation of the pore size distribution.

Nano Ti(C,N) powder was synthesised by mechanical activation assisted carbothermal reduction nitridation (MCRN) in this study. The starting powders of nano anatase/carbon black were activated first through high energy mechanical milling, then the milled powders were CRN treated at 1200 or 1250°C for 2 h, Ti(C,N) powder with average particle size of below 100 nm was obtained finally. Effects of main mechanical milling factors on anatase CRN reaction were investigated, including milling time, ball to powder weight ratio and milling speed. The mechanism of MCRN synthesising nano Ti(C,N) powder was also discussed in detail. The products were characterised by X-ray diffraction and scanning electron microscope. The results indicate that longer milling time, bigger ball to powder weight ratio and higher milling speed will be beneficial to MCRN reaction.

Porous ceramic materials have been thoroughly developed due to the wide number of applications in different areas, such as filters for environmental clean-up and reuse, bioreactors, gas or chemical sensors and support materials for catalysts or absorbents. One of the forming methods that can be selected for these applications is the freeze drying of suspensions. This method has received increased attention for manufacturing near net shaped parts with directional porosity. This work deals with the manufacture of porous alumina bodies by a freeze casting route. The different parameters of the process, such as the solids content of the suspension, the addition of cryoprotector (glycerol) and the freezing conditions (temperature and device) have been studied to determine their influence on the microstructure, density and porosity of the green and sintered pieces. Materials with different degrees of porosity and distribution of pores (aligned in the freezing direction or homogeneously distributed) have been obtained.

The preparation and mechanical properties of silicon infiltrated SiC–B₄C composite were studied in the present work. Gelcasting process was adopted to obtain green bodies with uniformity and compaction. The gelation behaviour of the slurry was controlled by introducing the polymerisation inhibitor mixture and the gelation reaction was delayed to about 10 min with increasing B₄C content from 20 to 40%. Mechanical properties of the SiC–B₄C sintered samples were tested and the results showed that increasing B₄C content led to an increase in bending strength, fracture toughness and hardness. Microstructure investigation revealed that the SiC–B₄C composites showed less content of free silicon and finer grains comparing to RBSC ceramics.

This paper investigates the fire barrier and heat insulation properties of newly developed low cost ceramic mouldable composites using a modern small scale testing method which exposes testing panels to the fire on one surface. At the same time, the temperature on the unexposed surface was measured and recorded constantly. Compared to glass/phenolic laminates and other polymer composites, it was observed that the low cost ceramic composites could withstand much longer time in the fire and were much more stable during fire testing without being structurally damaged under the severe thermal heat environment. The indications of the fire penetration give a promising applications future for these materials in construction, transportation, aerospace, etc. as high temperature thermal insulation and heat barriers.