Modeling Growth of Au-Cu Nanocrystalliine Coatings

The electrodeposition process parameters of current density, pulse duration, and cell potential affect both the structure and composition of the foils. The mechanism for nucleation and growth as determined from current transients yield relationships for nucleus density and nucleation rate. To develop an understanding of the role of the process parameters on grain size--as a design structural parameter to control strength, for example, a formulation is presented to model the affects of the deposition energetics on grain size and morphology. An activation energy for the deposition process is modeled that reveals different growth mechanisms, wherein nucleation and diffusion effects are each dominant as dependent upon pulse duration. A diffusion coefficient common for each of the pulsed growth modes demarcates an observed transition in growth from smooth to rough surfaces. Empirical relationships are developed that relate the parameters of the deposition process to the morphology and grain size at the nanoscale. Regimes for nanocrystalline growth include a short and long pulse mode, each with distinct activation energies. The long pulse has the additional contribution of bulk-like diffusion whereas the short pulse is limited to surface diffusion and nucleation. For either pulse condition, a transition from a rough (or nodular) growth to a smooth surface results with an increase in the kinetics of diffusion

Sputter Deposition of Nanocones for Field Emission

Deposition into micron-sized holes is known to produce cone shapes as supported on substrates. Potential uses for the cones include field-forming devices as field ionizers and field emission cathodes. The application of such devices include flat panel displays and flash x-ray tubes. Process iterations to closely space arrays of sharp cones have been extensively documented during the past two decades using the physical vapor deposition method of evaporation. Sputter deposition is well known as a method to fill holes and trenches but has only recently been demonstrated as an alternative method to produce field emission cathodes. In a further reduction in size, we have been successful in demonstrating the ability to deposit a cone shape into a cavity with a 300nm diameter hole. Through comparison to the results of electron-beam evaporative deposition, a sputter deposited nanocone appears to be suitable for use as a field emission cathode

Metallic multilayers at the nanoscale

The development of multilayer structures has been driven by a wide range of commercial applications requiring enhanced material behaviors. Innovations in physical vapor deposition technologies, in particular magnetron sputtering, have enabled the synthesis of metallic-based structures with nanoscaled layer dimensions as small as one-to-two monolayers. Parameters used in the deposition process are paramount to the Formation of these small layer dimensions and the stability of the structure. Therefore, optimization of the desired material properties must be related to assessment of the actual microstructure. Characterization techniques as x-ray diffraction and high resolution microscopy are useful to reveal the interface and layer structure-whether ordered or disordered crystalline, amorphous, compositionally abrupt or graded, and/or lattice strained Techniques for the synthesis of metallic multilayers with subnanometric layers will be reviewed with applications based on enhancing material behaviors as reflectivity and magnetic anisotropy but with emphasis on experimental studies of mechanical properties

Modeling and the Sputter Deposition of Coatings Onto Spherical Capsules

The sputter deposition of coatings onto capsules of polymer and oxide shells as well as solid metal spheres is accomplished using a chambered substrate platform. Oxides and metal coatings are sputter deposited through a screen-aperture array onto a 0.3-1.2 mm diameter, solid spheres and hollow shells. Each shell is contained within its own individual chamber within a larger array. Ultrasonic vibration is the method used to produce a random bounce of each capsule within each chamber, in order to produce a coating with uniform thickness. Characterization of thin aluminum-oxide coated, platinum solid spheres and thicker copper-gold layer coated, hollow capsules (of both glass and polymer) show that uniform coatings can be produced using a screen-aperture chambered, substrate platform. Potential advantages of this approach compared to open-bounce pans include improved sample yield and reduced surface roughness from debris minimization. A process model for the coating growth on the capsules is developed to assess selection of the screen aperture based on the effects of sputter deposition parameters and the coating materials

Synthesis and characterization of nanophase face-centered-cubic titanium

Unalloyed Ti is sputter deposited in the formation of two trilayer films. Each layer within the combinations of Ni-Ti-Ni and Ti-Ni-Ti is less than 0.1 {mu}m thick. High resolution imaging and electron diffraction results are presented for the microstructural characterization of both trilayer films. Nanophase fcc Ti is initially found in crystalline layers grown on Ni whereas hcp Ti is found in layers grown without a Ni epilayer. Conditions are further examined under which the fcc to hcp transition occurs for Ti deposited on Ni

Thermal Stability and Mechanical Behavior of Ultra-Fine Bcc Ta and v Coatings

Ultra-refined microstructures of both tantalum (Ta) and vanadium (V) are produced using electron-beam evaporation and magnetron sputtering deposition. The thermal stability of the micron-to-submicron grain size foils is examined to quantify the kinetics and activation energy of diffusion, as well as identify the temperature transition in dominant mechanism from grain boundary to lattice diffusion. The activation energies for boundary diffusion in Ta and V determined from grain growth are 0.3 and 0.2 eV{center_dot}atom{sup -1}, respectively, versus lattice diffusion values of 4.3 and 3.2 eV{center_dot}atom{sup -1}, respectively. The mechanical behavior, as characterized by strength and hardness, is found to inversely scale with square-root grain size according to the Hall-Petch relationship. The strength of Ta and V increases two-fold from 400 MPa, as the grain size decreases from 2 to 0.75 {micro}m

Magnetic x-ray circular dichroism in nickel-gold multilayers

Magnetic circular dichroism in x-ray absorption is used to investigate the in-plane, remnant magnetization of well-characterized Ni{sub 0.48}/Au{sub 0.52} multilayers. Large superlattice strains are found in this multilayer system for samples with a 2nm layer pair spacing. A larger dichroism is found in the Ni 2p absorption edge for a 1.8 nm than for a 4.4 nm layer pair sample. The larger dichroism is consistent with a larger magnitude of in-plane strain for the Ni layers and a larger total magnetic anisotropy energy as previously shown from magnetization curves

Rapid Undercooling and Refreeze in Laser-Shock-Melted Bi(Zn)

We completed experiments in which we used a high-power laser to shock-melt a Bi(Zn) alloy and refreeze it in the shock release wave. We recovered the samples post shot for microscopic analysis and compared our results with the results from similar prior experiments with pure Bi. The targets in both sets of experiments were four-layer targets composed of BK7 glass, Al, the elemental Bi or Bi(Zn) alloy, and a transparent diagnostic window. There is conductive heating of the Bi through the Al layer from the hot plasma at the Al/BK7 boundary that depends on the Al thickness. Since the Bi(Zn) targets had a much thicker Al layer than did the Bi targets, the two sets of targets had somewhat different thermal histories even though they were driven to the same pressure. In this presentation we compare the resolidified Bi(Zn) microstructure to that of the Bi, accounting for the different thermal histories

Large Area Deposition of Field Emission Cathodes for Flat Panel Displays

The convention for field emission cathode (FEC) synthesis involves coating with a very-high tolerance in thickness uniformity using a planetary substrate fixture and a large source-to-substrate separation. New criteria for a deposition process must facilitate a reduction in the operating voltage by increasing the density of emitters through a reduction of cathode size and spacing. The objective of scaling the substrate size from small (less than 30 cm{sup 2}) to large (greater than 500 cm{sup 2}) areas further compounds manufacturing requirements to a point beyond that easily obtained by modifications to the convention for FEC deposition. A new patented approach to design, assemble, and operate a coating system enables FEC deposition over large areas through process control of source divergence coupled to incremental substrate positioning

The Thermal Stability of Nanocrystalline Au-Cu Alloys

Grain refinement to the nanocrystalline scale is known to enhance physical properties as strength and surface hardness. For the case of Au-Cu alloys, development of the pulsed electroplating has led to the functional control of nanocrystalline grain size in the as-deposited condition. The thermal aging of Au-Cu electrodeposits is now investigated to assess the stability of the nanocrystalline grain structure and the difference between two diffusion mechanisms. The mobility of grain boundaries, dominant at low temperatures, leads to coarsening of grain size whereas at high temperature the process of bulk diffusion dominates. Although the kinetics of bulk diffusion are slow below 500 K at 10{sup -20} cm{sup 2} {center_dot} sec, the kinetics of grain boundary diffusion are faster at 10{sup -16} cm{sup 2} {center_dot} sec. The diffusivity values indicate that the grain boundaries of the as-deposited nanocrystalline Au-Cu are mobile and sensitive to low-temperature anneal treatments affecting the grain size, hence the strength of the material