A. S. Edelstein

Nanoindentation study of the mechanical properties of copper‐nickel multilayered thin films

The mechanical properties of multilayered Cu‐Ni thin films with bilayer thicknesses of 1.6–12 nm were investigated by a nanoindentation technique. Force‐displacement curves generated during loading and unloading of a diamond tip indenter were used to determine the hardness and elastic properties of the films. No enhancement in the elastic properties (supermodulus effect) was seen, but an enhancement in the hardness was observed. It is suggested that the enhancement, which displayed a Hall–Petch‐type behavior, can be understood as owing to dislocation pinning at the interfaces analogous to the mechanism of grain boundary hardening.

Intermetallic phase formation during annealing of Al/Ni multilayers

The phase evolution during annealing of Al/Ni multilayer samples prepared by ion‐beam sputtering with composition modulation wavelengths Λ between 10 and 400 nm was determined using x‐ray diffraction and differential scanning calorimeter measurements. Samples with average compositions of Al0.40Ni0.60 and Al0.75Ni0.25 were investigated. For the Al0.40Ni0.60 samples the following results were obtained. A measure of the degree of periodicity and the sharpness of the interfaces in a sample with Λ=80 nm was the large number (over 20) of peaks observed in small‐angle x‐ray scattering measurements. A sample with Λ=10 nm was transformed by heat treatment directly to the AlNi phase. In the Λ=80 nm sample, the first phase formed after annealing was the metastable η phase. The η phase was identified as Al9Ni2. In the 400 nm wavelength sample, both the metastable η phase and the stable Al3Ni formed after the first exothermic reaction. For the Al0.75Ni0.25 samples two results were obtained. A Λ=11.4 nm sample transfor...

Diffusional reactions in composite synthesis

Abstract The thermal stability of advanced composites is dominated by the behavior of internal interfaces. In order to develop effective processing strategies and stable composite designs, it is essential to consider the relevant phase diagrams which are of ternary order or higher. In addition to phase diagram information, kinetic data such as the interdiffusion pathway and reaction rates are required to understand and control the possible interfacial chemical reactions. With this information, it may be possible to bias the reactions and to alter pathways. Often the initial nucleation stage of interfacial reactions has been neglected, but recent results indicate new kinetic behavior can develop during intermediate phase nucleation in a large concentration gradient. Multilayer thin film samples are well suited for probing the initial kinetic path and structural evolution during interdiffusion reaction and phase nucleation. In Al/Ni multilayer samples with compositional modulation wavelengths between 10 and 400 nm, thermal signal onsets due to phase nucleation have been examined to monitor the reaction kinetics and to probe the interdiffusion that precedes phase nucleation. The analysis of both bulk diffusion couple and multilayer sample behavior offers the basis for phase compatibility control that can be applied in developing stable composite structures by in situ reaction processing.

Reaction kinetics and biasing in Al/Ni multilayers

Abstract The phase evolution from annealing ion-beam-sputtered Al/Ni multilayers was studied using X-ray diffraction and differential scanning calorimetry measurements. The sequence of phase formation depends on the modulation wavelength Λ and the average composition 〈 c 〉. The initial phase formed may also depend on the composition gradient. Annealing short Λ samples with a composition of either Al 0.40 Ni 0.60 or Al 0.75 Ni 0.25 produced only AlNi or Al 3 Ni, i.e. the stable phase with the same 〈 c 〉. At intermediate Λ, Al 9 Ni 2 was often the initial phase formed. Experiments were performed on multilayer samples in which 20 nM AlNi layers, denoted as biasing layers, were placed between each Al and Ni layer. The AlNi modified the kinetics, increasing the nucleation temperatures and changing the amounts of the product phases. The AlNi layer appears to dissociate in the later stages of annealing. Composition gradients and biasing layers may be used to control the phases present in technological materials.

Alternative approach to nanocomposite synthesis by sputtering

A new method is presented which circumvents the usual thermodynamic limitations (alloying and compound formation) in fabricating phase‐separated materials. This opens whole new classes of materials that can be prepared as particulate composites.This method utilizes sputtering at high pressures (0.2–0.6 Torr) in a thermal gradient to produce nanoscale (<15 nm diameter) particles, which are then embedded in a matrix produced by normal sputtering. The microstructure and microhardness of 0.5‐μm‐thick composites of molybdenum particles (3–12 nm average particle size) in aluminum are presented as examples. This system cannot be prepared by the conventional phase separation technique of cosputtering.

Formation threshold and structural evolution of molybdenum nanocrystals with sputtering pressure

The evolution of the size and shape of molybdenum nanocrystals fabricated by sputtering in a thermal gradient has been studied as a function of the argon gas pressure, {ital p}. For 4{lt}{ital p}{lt}100 mTorr, continuous Mo films are deposited. At {ital p}=150 mTorr, isolated and well-faceted Mo nanocrystals of two sizes (20 and 5 nm average size) are formed. For 200{le}{ital p}{le}400 mTorr, the particle size decreases with increasing pressure and is about 7 nm at 400 mTorr. On increasing {ital p} further, larger particles start to form and at {ital p}=700 mTorr, particle agglomerates are observed. Possible mechanisms leading to these results are suggested.

Synthesis and characterization of magnetic nanocomposite films

Nanocomposite films were made consisting of Co nanoparticles, with coatings of amorphous Al2O3. Nanoparticles were deposited on room-temperature substrates by sputtering in 0.10–0.20 Torr Ar. Transmission electron microscope images showed average Co particle sizes between 70 and 223 A, for different sputtering rates and pressures. Selected-area electron diffraction and x-ray diffraction showed that the particles had a fcc structure for sizes ⩽122 A and a mixture of fcc and hcp phases for larger sizes. Magnetization measurements on a sample with 122 A mean-particle size showed significant hysteresis at room temperature, with a coercivity of 390 Oe. At 10 K, the coercivity increased to 3145 Oe, and a loop shift of 2020 Oe was measured when the sample was field cooled. This indicates a fraction of the Co was oxidized by reaction with the Al2O3 coating, resulting in a surface layer of antiferromagnetic Co oxide.

Enhanced magnetic anisotropy at the percolation threshold of Fe‐SiO2 composite thin films

Ferromagnetic resonance (FMR) measurements at 9.6 GHz and room temperature are reported for very thin (50 A) sputtered films of Fe‐SiO2 with volume fractions of Fe, f, spanning the percolation threshold at fc=0.57. It is inferred from the FMR linewidths that the magnetic anisotropy of the percolating composite of Fe particles peaks at fc. Possible mechanisms for this anisotropy enhancement are examined and it is argued that the shape anisotropy of the percolation clusters may be the principal contribution. These results and our interpretations provide a possible explanation for the peak in the coercivity near the percolation threshold of Fe‐SiO2 composite films recently reported by S. H. Liou and C. L. Chien [Appl. Phys. Lett. 52, 512 (1988)].

Self-arrangement of molybdenum particles into cubes

An unusual distribution of particle sizes has been observed following the formation of molybdenum particles by argon ion sputtering. Many of the molybdenum particles produced by sputtering at the threshold pressure for particle formation in the vapor appear to be single crystalline cubes. There are two prominent peaks in the edge length distribution of the cubes, one centered at 4.8 nanometers with a halfwidth of approximately 1.3 nanometers and the other at 17.5 nanometers. The peak for the larger cubes is approximately square and has a total width of 7.0 nanometers. Evidence is presented that the larger cubes are formed by a 3 by 3 by 3 self-arrangement of the smaller cubes, which contain approximately 7000 atoms. Self-arrangement in inorganic structures is normally only observed when the building blocks are atoms, molecules, or clusters of less than 100 atoms.

Reactions at amorphous SiC/Ni interfaces

When multilayer samples of polycrystalline Ni and amorphous SiC are heated, the sequence of phase formation initiates with a dissolution of Ni into the amorphous phase and is followed first by the formation of NiSi and then Ni2Si. Multilayer samples of a-SiC/Ni with modulation wavelengths of 83.9 nm and with the ratio of the thickness of the SiC layer to the Ni layer equal to 3.8 retain a multilayer structure even after they undergo two phase transitions. When annealing causes reactions to occur, the surface roughness, measured by atomic force microscopy (AFM), increases and was correlated with similar increases in the interface width. AFM measurements may provide a convenient way to observe reactions at buried interfaces.