N. M. van der Pers

Characterization of Al-Si-alloys rapidly quenched from the melt

Aluminium-silicon alloys with compositions in the range 0 at% to 33.9 at % Si were rapidly quenched from the melt at cooling rates between 106 and 107 K sec−1 using the melt-spinning technique. The resulting ribbons were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and X-ray diffraction methods. Metastable solid solubilities of silicon in aluminium were determined from lattice parameter and DSC data. The values found were strongly dependent on specimen thickness and a maximum of about 5 at % Si was reached for an alloy composition of 15 at % Sl (maximal equilibrium solid solubility of silicon in aluminium is 1.58 at % Si). Discrepancies between published values of metastable silicon solid solubities were related to the interpretation of the lattice parameter data. Alloy composition was shown to determine the lattice parameter of the silicon-rich phase. The crystallite sizes and the lattice distortions in the aluminium-rich and silicon-rich phases were determined by X-ray diffraction line profile analysis. From the aluminiumrich phase only strain broadening was observed whereas the silicon-rich phase gave rise to both size and strain broadening. The origin of the lattice strains was discussed. Changes in solidification behaviour are reflected in the structure parameters measured.

Thermodynamics and Long-Range Order of Interstitials in an h.c.p. Lattice: Nitrogen in ε-Fe2N1-z

Abstract The thermodynamics of nitrogen in the e-Fe 2 N 1 − z phase were evaluated. To this end absorption isotherms for nitrogen in e-Fe 2 N 1 − z , depicting the dependence of the nitrogen content on the applied chemical potential of nitrogen, were determined in the temperature range 673–823 K by equilibrating iron foils in gaseous ammonia/hydrogen mixtures. The absorption isotherms could be described very well by a Gorsky-Bragg-Williams approximation for long-range order (LRO) of nitrogen atoms on the hexagonal nitrogen sublattice, formed by the octahedral interstices of the h.c.p. sublattice of iron atoms. The actual occurrence of LRO was evidenced by diffraction analysis. Mossbauer spectra were interpreted as composed of spectra of iron atoms surrounded by 1–3 nitrogen atoms. The relative amounts of the iron environments as determined from the Mossbauer spectra agreed very well with the corresponding probabilities predicted by the LRO model.

Lattice changes of iron-nitrogen martensite on aging at room temperature

The aging behavior of iron-nitrogen martensite (5.5 at. pct N ≙5.8N/100Fe) at about 297 K was investigated by employing X-ray diffractometry, thereby following, in particular, the changes in the {002} and {200} line profiles. Martensitic specimens were prepared by gaseous nitriding of pure iron in a mixture of NH3 and H2, followed by quenching in brine and subsequently in liquid nitrogen. The aging process can be divided into two stages. First, a redistribution of nitrogen atoms in the martensite matrix occurs (aging time < about 40 hours) in three ways: segregation of nitrogen to lattice defects (about 0.07N/100Fe), transfer of a small amount of nitrogen (about 0.06N/100Fe) fromalb- toc-type octahedral interstices, and local enrichment in an ordered way of the majority of the nitrogen atoms (coherent α′’-Fe16N2 precipitates). Second, formation of incoherent α″-Fe16N2 takes place (aging time > about 40 hours). Within the range of aging times employed (up to 670 hours), the diffraction by the residual austenite did not change.

Stress control of piezoelectric ZnO films on silicon substrates

Piezoelectric zinc oxide (ZnO) films are widely used for the generation and detection of acoustic waves in non-piezoelectric substrates. The application of these films on silicon offers the possibility of realizing acoustic wave devices integrated with electronic circuits, e.g. sensors. It has been demonstrated that ZnO films can be combined with standard bipolar or CMOS IC processes. However, the stress in ZnO is still an important drawback and hinders the further development of silicon integrated acoustic wave devices, particularly if the ZnO layer is deposited on thin membranes, e.g. Lamb wave devices. In this paper we discuss aspects of stress in ZnO, methods to prevent or reduce the stress and we describe an IC-compatible method for the determination of the stress in ZnO films deposited on 100 mm diameter silicon wafers.

Ion beam synthesis of nitride layers in iron

Stoichiometric iron nitride layers have been synthesized by high dose, high energy nitrogen implantation into Fe using a two-step implantation process. First, a nitrogen preimplantation at ~100 °C is used to form nitride precipitates. A low temperature is necessary to restrict the nitrogen mobility. Second, 1 MeV implantation at ~300 °C leads to the formation of a stoichiometric γ′–Fe 4 N layer at the position of the preimplanted N atoms. Growth of this nitride layer proceeds by diffusion of the implanted N through either the α–Fe matrix (for 200 or 500 keV preimplantations) or the nitride layer itself (for 1 MeV preimplantation). During annealing above 350 °C the γ′ layers dissolve in a planar fashion, characterized by an activation energy of 2.3 eV. Phase formation during preimplantation and phase transformations during subsequent annealing or hot implantation can be understood from the thermodynamics for the Fe–N system, while the kinetics of layer growth is influenced by the beam-induced defects. The experiment and model suggest that γ′ is not a thermodynamically stable phase below 310 ± 10 °C and should decompose into α (ferrite) and ∊ nitride.

Amorphous precipitates in a crystalline matrix; precipitation of amorphous Si3N4 in α-Fe

Microstructural changes in solids due to phase transformations are induced in practice to bring about specific, desired properties. The present research project focused on the development of a kinetic model for the precipitation of silicon nitride in silicon containing {alpha}-Fe, more or less as performed earlier by the authors for the precipitation of CrN and AlN. In contrast with their earlier preliminary work from the past, a determined effort to reveal the structure of the nitrides was undertaken. It was found that the precipitates were amorphous, stoichiometric Si{sub 3}N{sub 4}. The evidence for this finding is presented here. Further, on the basis of recent work on the modeling of interface energies, it will be suggested here that the occurrence of initially amorphous precipitate particles may have a thermodynamic basis. The kinetic analysis will be presented elsewhere.

Structural inhomogeneities of AlSi alloys rapidly quenched from the melt

Hypo- and hyper-eutectic AlSi alloys were rapidly quenched from the melt using the melt-spinning technique with two spinning velocities. Structural differences between the wheel (chill) and upper sides of the melt-spun ribbons were investigated by optical and scanning electron microscopy and X-ray diffraction methods (texture- and size-strain analyses). The Al-rich phase of the hypo-eutectic alloys was textured. The textures observed from both sides of the ribbons were different; in neither case was it of fibre type. For the larger spinning velocity applied, the structural imperfection of the wheel side was larger than that of the upper side for both the Al-rich and the Si-rich phases.

Dielectric response of sputtered transition metal oxides

We have investigated the dielectric properties of thin layers of five oxides of transition metals (Ta2O5, HfO2, ZrO2, (ZrO2)0.91(Y2O3)0.09, and Sn0.2Zr0.2Ti0.6O2) sputtered from ceramic targets at different pressures. We find that layers deposited at low pressure behave as expected from literature, whereas layers deposited at high pressure all exhibit an anomalous dielectric response similar to that reported for the so-called “colossal” dielectric constant materials. The characterization of the thickness, frequency, and temperature dependence of the capacitance, as well as the comparison of film properties before and after annealing show that the anomalous dielectric response is due to quenched-in vacancies that act as dopants and cause the insulating layers to behave as semiconductors. An increase in quenched-in vacancies concentration with sputtering pressure results in a transition from normal to anomalous dielectric response and gradual increase in layer conductivity. In contrast, the refractive index...

Texture formation in sputter-deposited (Nb0.7,Ti0.3)N thin films

We studied the properties of (Nb0.7,Ti0.3)N films deposited by reactive magnetron sputtering in an atmosphere of argon and nitrogen at ambient substrate temperature, with a particular focus on the technological factors that determine film texture. The texture in the nitrides of transition metals determines many processes, including the wear resistance of tool coatings, diffusion in microelectronic devices, and the rate of chemical etching. Thus, since our goal is to use (Nb0.7,Ti0.3)N films in superconducting microelectronic devices, texture control is an essential element of our technology. We find that increasing the total gas pressure, while keeping the film chemical composition constant, results in a decrease in the ratio of the [200] and [111] x-ray diffraction (XRD) line intensities on Θ–2Θ Bragg–Brentano scans. Similar changes in XRD patterns are observed as the nitrogen injection increases for a constant sputtering pressure. In addition, XRD examination shows that some samples have in-plane textur...

Precipitation in liquid-quenched Al-Mg alloys; a study by use of X-ray diffraction line shift and line broadening

The precipitation process in liquid-quenched alloys of aluminium with 3.2, 10.5 and 16.7 at% Mg on ageing at 404, 450 and 492 K was investigated by X-ray diffraction analysis. It was found that only β′-phase precipitation occurred. During ageing, the average aluminium-matrix lattice parameter was determined from the line position, while a measure for the lattice spacing variations was deduced from line broadening. The β′-phase precipitation was found to occur inhomogeneously; for one alloy, splitting of diffraction lines was observed. The (asymmetrical) line-broadening changes observed during the β′-phase precipitation could consistently be conceived as dominated by the inhomogeneous nature of the β′-phase precipitation process. The (symmetrical) line broadening retained after completed precipitation was ascribed to the volume misfit between the precipitated β′-phase particles and the aluminium-matrix. Analysis of the aluminium-matrix lattice parameter changes with ageing time revealed that the β′-phase precipitation rate is governed by the magnesium volume diffusion.