Jürgen G. Heinrich

Aqueous tape casting of silicon nitride

Abstract Slurries consisting of a low cost silicon nitride powder, sintering aids yttria and alumina, dispersants, binders, defoamers and water as a solvent were optimised for tape casting by electroacoustic and viscosity measurements and by casting experiments. The slurries exhibit shear-thinning behaviour due to the highly shear-thinning binder emulsion. Crack free tapes with a maximum thickness of approx. 250 μm and a binder content of 13–15 wt.% could be obtained. The green tapes with a high flexibility and green strength could be laminated easily by compression at room temperature. After sintering a dense microstructure developed. A significant shrinkage anisotropy parallel and perpendicular to the cast direction was observed.

Silicon Nitride Materials for Engine Applications

The various performance profiles of silicon nitride engine components lead to the development of different Si3N4-qualities for this application. Dense silicon nitride components can be produced by sintering Si3N4-powder compacts (SSN) or by post-densifying RBSN (HIP-RBSN). This paper first describes the correlation between processing parameters and the corresponding microstructure and second the optimization of mechanical properties of both, SSN and HIPRBSN, by using statistical methods.

Synthesis and thermal behaviour of β tricalcium phosphate precipitated from aqueous solutions

Abstract In the present study, β tricalcium phosphate (β-TCP) was prepared by precipitation from aqueous solutions. Calcium nitrate tetrahydrate Ca(NO3)2.4H2O and diammonium hydrogen phosphate (NH4)2HPO4 salts with an initial Ca/P molar ratio of 1·5 were dissolved in distilled water and mixed at 20°C and pH 10. Phase evolution of the as received precipitate was studied by X-ray diffraction and infrared spectroscopy before and after calcination in a dry air atmosphere at temperatures in the range 200-1400°C for 1 h. Thermal behaviour was investigated by simultaneous thermal analysis (DTA-TG). Calcium and phosphorus contents of the as received precipitate were determined by the inductively coupled plasma technique, and a Ca/P molar ratio of 1·49 ± 0·01 was found. The densities of the as received precipitate and of powder calcined at 800°C were determined by picnometery to be 2·43 and 3·01 ± 0·05 g cm-3 respectively. The experimental data suggest the formation of an amorphous phase corresponding to a TCP-like composition, which is calcium deficient and contains a very small amount of HPO42- groups.

Correlation between nitriding process and microstructure of reaction bonded aluminium nitride ceramics

Abstract Aluminium nitride samples were produced by a reaction bonding process using AlN and Al powders with flake-like and equiaxed morphologies as starting materials. Changing the particle sizes and morphologies of the aluminium starting powders led to changes in degree of reaction and microstructure of the resulting reaction bonded aluminium nitride ceramics. At 25 wt-%Al, the degree of reaction showed a maximum, decreasing at higher aluminium concentrations. The degree of reaction was increased by increasing nitrogen gas flowrate or nitriding temperature. It was also found to be increased by reducing the average particle size of the Al starting powder, the green bulk density, or the sample thickness, and also by choosing flakelike morphology. SEM and optical micrographs of reaction bonded aluminium nitride samples revealed the pore structure and morphology of primary and secondary aluminium nitride.

Investigation of microstructural evolution during nitridation of aluminium using simultaneous thermal analysis

Abstract Aluminium powders with different average particle sizes were synthesised into aluminium nitride in nitrogen atmosphere using simultaneous thermal analysis (TG/DTA). Experiments showed that reaction of Al powders started below the melting point of aluminium (660°C) in the solid–gas state, however most of the reaction took place above the melting point, in the liquid–gas state. Further conversion of Al to AlN took place up to 1200°C. The influence of particle size on the nitridation process was demonstrated by the use of different Al powders. Microstructural characterisation showed that two kinds of whisker were formed in the original Al powder bed, one with wool-like morphology and the other with needlelike morphology and sharp changes of direction.

Influence of processing parameters on microstructure and biocompatibility of surface laser sintered hydroxyapatite-SiO2 composites.

Silica-doped hydroxyapatite (HA) is a promising material concerning biocompatibility to natural bone, bioactivity and osteoconductive characteristics. HA exhibits phase transformations during sintering which are attendant to the change in volume and thermal strain. To avoid cracks during sintering, the exact knowledge of the phase transition temperatures is necessary. The sintering behavior of HA can be improved by adding amorphous silica with a low coefficient of thermal expansion. Therefore, the phase transformations in the system HA-SiO2 were analyzed by using differential scanning calorimetry followed by quantitative phase analysis by X-ray diffraction with the Riedveld method. The maximum sintering temperature without reversible phase transformation was defined as 1265°C. In laser surface sintered (LSS) samples, amorphous SiO2 , HA, and Si-α-TCP (or α-TCP) were detected. By comparison, only crystalline phases, such as cristobalite, HA, β-TCP, and Si-α-TCP (or α-TCP), were determined after furnace sintering. Scanning electron microscopy micrographs of furnace sintered and LSS samples show the differences in the resulting microstructures. Biocompatibility was determined by measuring cell activity of osteoblasts cultivated on four laser-sintered materials in the HA-SiO2 system in comparison to normal cell culture plastic. Cell proliferation was similar on all surfaces. The level of the cell activity on day 8 varied depending on the composition of the material and increased linearly as the amorphous SiO2 content rose. Taken together a laser-based method to develop novel biocompatible HA-SiO2 ceramics with adjustable properties and possible applications as orthopedic bioceramics are discussed.

Production of superparagnetic nanospheres for hyperthermic therapy of surface (skin) cancer diseases

The idea of invention is that superparamagnetic nanoceramic particles are received in rotating cathode equipment. Microstructure of powder is studied by electronic microscopy and X-ray analysis. Also, powder homogeneity, particle size distribution and, respectively, stability and magnetic susceptibility are studied by magnetometric method. Average sizes of particles are 30-100 nm. The powder is homogeneous and in the process of analysis it was revealed its good stability. Research object is superparamagnetic powder. Relative value of its magnetic susceptibility is 1.00037. Sample magnetization measurements are done at cooling in zero magnetic field (ZFC) and in nonzero field (FC) modes, also magnetic hysteresis curves are measured at low and high temperatures compared to the mean blocking temperature (TB) for measured samples. From the state of maximum on the ZFC curve it was deduced that temperature of blocking in the given superparamagnetic subsystem of particles is TB ~ 60 K.

Growth of carbon nanotubes with different morphologies by pyrolising acetylene

Abstract Multiwalled carbon nanotubes (MWCNTs) with different morphologies have been prepared by pyrolysis of a mixture of acetylene-ferrocene over predeposited Co and Ni catalysts at 700°C. A high yield of carbon nanotubes with further purification have been obtained in the optimal conditions. The optimum synthesis parameters included synthesis temperature of 700°C, growth time of 30 min, flowrate of acetylene and hydrogen of 40 and 300 sccm respectively. Multiwall straight, curved, helically, coiled, planar-spiral and V-shaped nanotubes were found with diameters in the range of 10-70 nm and with lengths up to 5 μm. The morphology and structure features of the MWCNTs are characterised using scanning electron microscopy, transmission electron microscopy, energy dispersive spectra, Raman spectroscopy and thermogravimetry analyses.

The power of light: Self-organized formation of macroscopic amounts of silica melts controlled by laser light

CO2 laser systems with a power output of up to 12kW continuous wave have been employed for melting high purity amorphous silica (SiO2) powders. Under the intense light irradiation, the migration of matter on the silica sample has been observed. A net mass transport results in the formation of macroscopic structures in the liquid phase. Protrusions of up to 7mm height are formed against gravitational force and surface tension. For the first time, this work reports on the self-organized formation of macroscopic structures by viscous flow of a dielectric melt driven by laser light.