Werner Riehemann

Synthesis and Characterization

As compared to bulk materials, magnetic nanoparticles possess distinct magnetic properties and attempts have been made to exploit their beneficial properties for technical and biomedical applications, e.g. for magnetic fluids, high-density magnetic recording, or biomedical diagnosis and therapy. Early magnetic fluids (MFs) were produced by grinding magnetite with heptane or long chain hydrocarbon and a grinding agent, e.g. oleic acid [152]. Later procedures for MFs precipitated Fe 3+/Fe 2+ of an aqueous solution with a base, coated the particles by oleic acid, and dispersed them in carrier liquid [161]. However, besides the elemental composition and crystal structure of the applied magnetic particles, particle size and particle size distribution determine the properties of the resulting MF. Many methods for nanoparticle synthesis including the preparation of metallic magnetic particles have been described in the literature. However, there still remain important questions, e.g. concerning control of particle size, shape, and monodispersity as well as their stability towards oxidation. Moreover, peptization by suitable surfactants or polymers into stable MFs is an important issue since each application in engineering or biomedicine needs special MFs with properties adjusted to the requirements of the system.

Dispersion hardening of metals by nanoscaled ceramic powders

Abstract Nanoscaled Al 2 O 3 -powders (n-Al 2 O 3 ) with a median particle diameter of 14 nm are distributed in microscaled copper powders by ball milling in a planetary mill followed by uniaxially hot pressing. The microstructures of the samples are analysed by SEM and EDX investigations and the hardness of the specimens is determined before and after heat treatments of the composites. The results on the produced Cu/n-Al 2 O 3 composites show that the nanoparticles are largely homogeneously distributed in the matrix grains, that the matrix grains are stabilised against growth even after heat treatments close to the melting temperature of Cu, and that the hardness of the sample is largely maintained after heat treatment. The results show that mechanical alloying of metal micropowders with oxide ceramic nanopowders can produce dispersion-strengthened materials. The mechanical alloying with n-powders can be easily applied to systems which cannot be dispersion-strengthened with n-oxide ceramics by other methods, e.g. internal oxidation. The last is shown by dispersion-strengthening of magnesium with n-Al 2 O 3 .

Electrodeposition of particle-strengthened nickel films

Abstract Dispersion-strengthened nickel films were obtained by co-deposition of Al 2 O 3 particles with a mean particle diameter of 14 nm followed by heat treatment at 850 °C. The co-deposition was achieved by addition of Al 2 O 3 powders in a typical Watts bath before electroplating of nickel on copper substrates. The Ni films are analyzed by light and transmission electron microscopy and by measurement of the coercivity and the microhardness. The particle-strengthened Ni films show a remarkable improvement of hardness due to grain stabilization and dispersion hardening of the nickel grains by alumina nanoparticles as observed by structural investigations of the samples. Reinforcement of electrodeposited nickel films by nanosized alumina particles is a simple process producing wear-resistant coatings.

Sintering of nanocrystalline ZrO2 and zirconia toughened alumina (ZTA)

Abstract Ultrafine oxide powders of ZrO 2 and MgO were produced by laser ablation. The pure atomised powders and mixtures thereof were heated up to temperatures between 600 and 1600°C. The tetragonal phase in pure ZrO 2 powder transformed continuously between approximately 400 and 1000°C to the monoclinic phase. On addition of 5 vol.% MgO the transformation temperature could be increased by about 400 K and grain growth suppressed. Dilatometer measurements on compressed powder specimens showed that the shrinkage curve for nanocrystalline ZrO 2 was depressed by 300 K relative to a microcrystalline ZrO 2 specimen. The ZrO 2 specimen with 5 vol.% MgO was isostatically pressed and sintered in the temperature range 1000–1500°C. Sintered specimens contained 20–50% of the tetragonal phase. The specimen sintered at 1250°C for 1 h achieved a relative density of 95%. Small increases above this were observed in specimens sintered between 1250 and 1500°C. The 90 vol.% Al 2 O 3 + 10 vol.% ZrO 2 (with 5 vol.% MgO) specimens were also sintered in the temperature range 1400–1600°C. The specimens achieved high density (98%), microhardness (17·8 GPa) and toughness (7·2 MPa m ) .

Mechanical spectroscopy of commercial AZ91 magnesium alloy

Abstract A damping peak at around 425 K for a frequency of about 1 Hz was found and it was correlated to the characteristic grain boundary peak of magnesium. An increase in the height of the grain boundary peak appears during the temperature cycles, controlled by the decrease of solute atoms in the matrix since the appearance of a precipitation process.

Laser-induced solid solution of the binary nanoparticle system Al2O3-ZrO2

Abstract Nanoparticles of the system Al2O3-ZrO2 were produced by simultaneous vaporization of Al2O3 and ZrO2 microparticles with the radiation of a 1000 W Nd:YAG-laser and subsequent condensation of the induced vapor in a controlled atmosphere. The nanoparticles were investigated by transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), electron diffraction (ED) and X-ray diffraction (XRD). The analysis of the as-prepared nanoparticles showed that the nanoparticles are single crystals having spherical shape with a median particle diameter of 14 nm. EDX and high resolution TEM investigations on single crystals showed the existence of a solid solution of the binary system Al2O3-ZrO2, which is formed during the particle nucleation and rapid quenching. XRD measurements of the powder reveal the presence of ZrO2 in the tetragonal structure (t-ZrO2) and of species of tetragonal structure with decreased lattice parameters compared to t-ZrO2. The last can be attributed to a metastable solid solution of the type Zr (1 − x) Al x O (2 − x 2 ) (0 , as also confirmed by ED measurements. When the powders were thermally treated at a temperature Ts = 700°C, the diffraction patterns assigned to the solid solution vanished in t he XRD and ED spectra due to decomposition of the particles as supported by EDX and high resolution TEM measurements on single nanoparticles. At Ts ≥ 1200 °C, enhanced grain growth of the particles was observed by TEM. XRD-investigation showed that the particles started transforming into their equilibrium phase assembly, m-ZrO2 and α-Al2O3.

Bonding of alumina ceramics with nanoscaled alumina powders

Nanoscaled Al2O3-powders can be employed for diffusion bonding of alumina ceramics. In order to accomplish bonding of the ceramics, Al2O3-nanopowder with a median particle size of 14 nm in diameter is sandwiched between two commercial microcrystalline corund discs, followed by uniaxially hot compressing of the assembly in vacuum at 80 MPa and 1100 °C for 2 h. Scanning electron microscopic investigations reveal a nanocrystalline structure of the joint with a mean grain size of about 50 nm in diameter and extensive consolidation of the powder without substantial shrinkage void formation. Microhardness measurements across the interface yield a value of 200 HV. In order to achieve complete densification and strength enhancement of the joint material, the sample is subsequently sintered at 1500 °C and 1600 °C for several hours in air. It was found that the hardness of the joint depends strongly on the porosity content and/or grain size and that a hardness of 1700 HV is obtained when both a mean particle size of about 1 μm and complete densification of the joint is achieved. The results show for the first time that Al2O3-nanopowders are suitable for diffusion bonding of alumina ceramics. Possible mechanisms are discussed.

Thermoelastic damping of the low density metals AZ91 and DISPAL

Abstract Light metals have high thermoelastic internal friction due to high coefficients of thermal expansion which has to be considered for various structural damping investigations. The thermoelastic effect of a powder metallurgically produced aluminium composite DISPAL™ and a magnesium casting alloy AZ91 (9 wt.% Al, 1 wt.% Zn) was investigated by measurements of the logarithmic decrement of free vibrations of bending beams. Because of inhomogenous stress distribution, a temperature gradient occurs between the compressed and the extended surface of the specimen, which leads to a transverse heat flow. This causes damping that is sensitive to the frequency of vibration. When the time of stress reversal equals the time necessary for heat flow from the compressed to the extended regions, a maximum of the logarithmic decrement occurs. The damping was measured as a function of the specimen thickness and the frequency of vibration ranging from 10 to 130 Hz. The frequency range was realised by both, a variation of the thickness and the mass at the end of the bending beams. By drawing the logarithmic decrement versus the product of the frequency and the quadratic thickness, a Debye peak is obtained which overlays all other damping mechanisms. The thermal diffusivity was calculated by determining the position of the maximum of the peak. The height of the peak, Youngs modulus and the specific heat of each sample were used to calculate the coefficient of thermal expansion of the specimens. The values agree, within the limits of scatter, with values measured by a dilatometer and taken from the literature, respectively.

Dependence of internal friction of fibre-reinforced and unreinforced AZ91 on heat treatment

Abstract Damping measurements have been carried out on the widely used commercial magnesium alloy AZ91 both unreinforced and reinforced with δ-alumina short fibres by the melt infiltration technique. The damping was determined after isochronal heat treatments by the measurement of the logarithmic decrement of free bending beam vibrations at frequencies ranging from 65 to 85 Hz. The thermoelastic and external apparatus damping were subtracted to obtain the damping, which can be mostly attributed to the dislocation movement. A maximum hardness and internal friction as a function of the annealing temperature at about 260°C both for unreinforced and reinforced AZ91 was found. This could be correlated with the precipitation of Mg 17 Al 12 (β-phase). In comparison with the unreinforced AZ91 for the composite a general increase in damping was observed. All measurements of the logarithmic decrement up to an annealing temperature of about 413°C were explained by a model of Granato and Lucke. For higher temperatures a strong increase in damping could be observed and was correlated with the formation of cracks.

Study of relaxation of residual internal stress in Mg composites by internal friction

Abstract When a metal matrix composite is subjected to temperature changes, thermal stresses arise at the interfaces between the matrix and the reinforcement as a result of the considerable mismatch between the thermal expansion coefficients of the matrix and the reinforcement. Even moderate temperature changes can produce thermal stresses that exceed the matrix yield stress. Consequently, new dislocations are generated at the interfaces causing thermal fatigue (microstructural changes, matrix plastic deformation and irreversible shape changes). The microstructural changes in the matrix can be detected by damping and stress relaxation measurements. The logarithmic decrement and modulus defect were measured after thermal treatment at temperatures between room temperature and temperatures up to 400°C. The strain dependence of the logarithmic decrement can be divided into two regions. The values of the logarithmic decrement are also influenced by heat treatment and foreign atoms in the matrix. The results can be explained assuming that straining and thermomechanical treatment produce changes in the microstructure of the composites. The stress relaxation curves were analysed and the modulus defect, relaxation strength and activation energy for dislocation motion were estimated. It is very difficult to draw conclusions on the mechanisms responsible for the stress relaxation but part of the relaxation strength is due to reversible dislocation movement.