ICMCTF2006 Session H1: Nanostructured Coatings and Novel Deposition Strategies

Research activities on carbon nanotubes at NASA-Johnson Space Center include production, purification, characterization and their applications for human space flight. In-situ diagnostics during nanotube production by laser oven process resulted in a better understanding of the nanotube growth [1]. Details of the results from the â?oparametric study of the pulsed laser ablation process indicate the effect of production parameters including temperature, buffer gas, flow rate, pressure, and laser fluence [2]. A recently established NASA-JSC protocol [3] for SWCNT characterization is undergoing revision with feedback from nanotube community. Efforts at JSC over the past five years in composites have centered on structural polymer/nanotube systems. Recent activities broadened this focus to multifunctional materials, supercapaciotrs, fuel cells, regenerable CO₂ absorbers, electromagnetic shielding, radiation dosimetry and thermal management systems of interest for human space flight. Preliminary tests indicate improvement of performance in most of these applications because of the large surface area as well as high electrical and thermal conductivity exhibited by SWCNTs.

[1] Carl. D. Scott, Sivaram Arepalli, Pavel Nikolaev and R. E. Smalley, "Growth Mechanisms for Single-wall Carbon Nanotubes in a Laser Ablation Process", Applied Physics A, Vol. 72, May 2001, pp. 573-580 (2001)

[2] Arepalli S., Holmes W. A., Nikolaev P., Hadjiev V. G., and Scott C. D., "A Parametric Study of Single-Wall Carbon Nanotube Growth by Laser Ablation", J. Nanosci and Nanotech. Vol. 4, 737-747 (2004).

[3] Arepalli S., Nikolaev P., Gorelik O., Hadjiev V. G., Holmes W. A., Files B. S., and Yowell L., "Protocol for the Characterization of Single-Wall Carbon Nanotube Material Quality", Carbon, Vol. 42, p. 1783-1791 (2004).

SiC, TiN, and Si-Ti-N nanostructured coatings were deposited using a novel synthesis process by ballistic impaction of nanoparticles. The nanoparticles were synthesized by injecting chemical reactants into a thermal plasma that underwent a rapid expansion through a converging nozzle, resulting in gas-phase nucleation. The deposition rates of the coatings varied from 1 to 60 µm/min, depending on reactant flow rates while substrate temperatures during deposition ranged from 400 to 900⁰C. Molybdenum, polished silicon wafers, and two types of steel (high speed and aircraft grade steel) were used as substrate material. The reactants were silicon tetrachloride, and/or titanium tetrachloride, with methane additionally injected to produce the carbides, and ammonia to produce the nitrides. Experiments were conducted with a copper gettering system to reduce oxygen contamination, which is believed to have a detrimental effect on film hardness.

In-situ particle size distribution measurements were performed using a sampling probe interfaced to an extraction/dilution system in series with a scanning electrical mobility spectrometer. Measured distributions from the sampling probe show that particle size distributions peak between 10 to 20 nm under typical operating conditions.

Microstructural characterization of the films was performed using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Mechanical properties were evaluated using a nanoindenter and a tribometer. Chemical and elemental compositional profiles of the coatings were obtained using Auger electron spectroscopy (AES) depth profiles. Compositional profiles combined with other characterization techniques provided insight into the mechanism of coating formation and growth, enabling the optimization of operating conditions to achieve desired film properties.

The development of deposition methods to control the nanocrystalline grain structure promotes the enhancement of physical properties as strength and surface hardness. Recent work in pulsed electro-deposition enables the synthesis of AuCu alloys with grain sizes as small as 5 nm. The thermal stability of AuCu nanocrystalline structures, as subjected to isothermal anneal treatments, is now investigated to assess the fundamental difference between two dominant diffusion mechanisms - - bulk and grain growth. The mobility of grain boundaries and the resultant coarsening of grain size are distinguished from the process of bulk diffusion as assessed through microscopic changes in composition gradients. The use of x-ray diffraction reveals changes in the grain size through broadening of Bragg reflections as quantified using the Debye-Scherrer formulation. The assessment of bulk diffusion kinetics at low temperatures by x-ray diffraction is approached through the use of concentration waves and the microscopic theory of diffusion. This approach provides a low-temperature benchmark for reference to high-temperature isotope diffusion. Energy dispersive x-ray spectroscopy is used to spatially resolve the time-temperature dependence of artificially introduced composition gradients. Although the kinetics of bulk diffusion are slow at temperatures below 500 K, less than 10^-20 cm²-sec, the kinetics of grain boundary diffusion appear much faster at rates greater than 10^-16 cm²-sec. The present result suggests that the mechanical gains appreciated in as-deposited nanocrystalline AuCu are quite sensitive to low-temperature anneal treatments. A path to stabilize the nanostructure of the binary AuCu alloy is proposed through a ternary alloy addition.

This work was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.

In the present study, nanocrystalline TiO₂/Ag composite thin films were prepared by a sol-gel spin coating technique. By introducing polystyrene (PS) microshperes into the precursor solution, porous TiO₂/Ag thin films can be prepared after calcination at a temperature of 500°C for 2 hours. Three different sizes of PS microspheres will be used to prepare porous TiO₂ films. The as-prepared TiO₂ and TiO₂/Ag thin films were characterized by X-ray diffractometry, and scanning electron microscopy to reveal the structural and morphological differences. In addition, the photocatalytic properties of these films were investigated by degrading methylene blue under UV irradiation.

After calcination, the as-prepared TiO₂ films exhibited different porous structure when different size PS microspheres were introduced. XRD results showed that all TiO₂/Ag films exhibited a major anatase phase. The photodegradation of porous TiO₂ thin film prepared with 200 nm PS microspheres and doped with 1 % mol Ag exhibited the best photocatalytic efficiency where ~ 100% methylene blue can be decomposed after UV exposure for 8 hours.

Electrochromism have attracted many Research and Development (R&D) interests due to potential applications such as automobile and building glazings, energy saving, etc. Among the transition metal oxides possessing electrochromic properties, tungsten trioxide (WO₃) exhibits the best properties and being studied by many researchers. Molybdenum trioxide (MoO₃) also exhibits electrochromic properties that can change color from bluish (bleached state) to deep blue (colored state).

In the present study, MoO₃ thin films were prepared by a sol gel spin coating technique. The spin-coated thin films were amorphous and, after calcining, nanocrystalline MoO₃ thin films were prepared successfully. The effect of annealing temperature ranged from room temperature to 500 °C was investigated. Preliminary results showed that the 350°C heat-treated MoO₃ thin films exhibited the best electrochromic properties among the films examined in the present study.