Original Date: 03/08/1999
Revision Date: 01/18/2007
Information : Laser-Liquid-Solid Interaction Technique
The Applied Research Laboratory at the Pennsylvania State University (ARL Penn State) developed a novel process for synthesizing nanoparticles and nanotubes/nanorods. The process utilizes a laser-liquid-solid interaction (LLSI) which produces uniformly small particles from the precipitation of a solution. Applications are possible in many disciplines:
Biotechnology, as delivery vehicles for medications and genetic therapies Electronics, in solder pastes and small feature size metallizations
Structures, where single crystal microstructures are required
Primary attributes in the utilization of nanoparticles are a uniformly small size (less than 100 nanometers) and a reasonable production rate. The LLSI process accomplishes both of these characteristics. Previously available techniques have many shortfalls in producing nanoparticles including:
Mechanical Milling Produces micron-level particles but not nano-level ones, and creates a high level of contamination from ball bearings. Chemical Precipitation Difficult to control particle size and shape, and produces hazardous waste solutions.
Laser Ablation Has an extremely slow production rate of grams per day.
Thermal Spray Uses a slow process to produce micron-level particles, and some contamination exists.
Spray Pyrolysis Produces porous particles which are desirable only in ceramic applications.
Laser Pyrolysis Has a slow production rate, but is useful for iron oxides.
By contrast, the LLSI process provides uniform nano-level particles of good density and at a rate several orders of magnitude greater than most competing techniques. The synthesis of nanoparticles using the LLSI (Figure 3-5) is simple in principle. Pulsed laser energy is impinged on a rotating target substrate formed from a high-conductivity material such as copper. The target is heated, creating a localized plasma in the nearby solution, such as AgNO3 and water (a waste product of the photographic process). The localized plasma interacts with the solution, precipitating nanoparticles of metal (silver in this example). The figure also shows how a beam can be split into multiple beams, which speeds production and demonstrates a continuous flow process to enrich and re-circulate the solution.
The LLSI technique can be tailored to alter/control size and production rate by varying four key parameters: wavelength and energy of the laser; interaction time as determined by pulse length; composition and concentration of the pre-cursor solution; and thermal conductivity of the target substrate. This process also provides the ability to create nanotubes by using higher viscosity solutions, which prevents precipitated particles from washing away, thereby linking them to form tubes or rods in situ. The addition of a surfactant as a catalyst, such as ethylene glycol, increases formation and serves as a polymer coating to protect reactive materials such as aluminum from oxidizing. The inclusion of a second material in the solution makes its possible to form alloys of otherwise immiscible materials such as AgNi.
The ARL Penn State’s efforts show the clear advantage of the LLSI process over competing techniques. The LLSI process provides uniformly small nanoparticles from a variety of materials; promises a production rate two orders of magnitude beyond most competitors; enables particle production and coating to occur simultaneously; allows deposition to occur at room temperature; and is tailorable across a number of parameters. In addition, the process does not require a vacuum; provides nanotubes as well as particles; produces alloys of otherwise immiscible materials; and can be made continuous.
Figure 3-5. Synthesis of Nanoparticles by Laser-Liquid-Solid Interaction
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