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Rapid Thermal Decomposition of Precursors in
Solutions
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In the process rapid thermal decomposition of presursors in solutions
(RTDS), precursor material dissolved in water undergoes rapid hydrothermal
reactions in a near-critical and supercritical water environment.
Due to short residence times and extremely fast quenching rates, particle
growth in the reaction is limited. This method can also be used to
produce large quantities of ultra-small catalytic materials. The catalytic
performance of these materials far exceeds that of larger catalyst
particles made by conventional means. These new synthetic methods
can also be applied to the generation of a large range of nanophase
materials, composites and powders.
The RTDS process offers a method of generating both ultrafine
metal (hydroxy) oxides and mixed metal (hydroxy) oxides using inexpensive
water soluble precursors. The RTDS process is amenable to variations
in a number of processing parameters that, in turn, can affect the
particle characteristics. Among those readily adjusted are the precursor
salt and its concentration in solution, the processing temperature
and pressure, and the length of exposure to the heated region. Among
the nanocrystalline powder products which have been generated using
the RTDS process are a variety of iron (hydroxy) oxides and mixed
metal-iron oxides, titanium oxide, zirconium oxide, and nickel oxide.
| Examples of Nanocrystalline Powders Produced using
RTDS |
| Powder Product |
Precursor |
Grain Siz |
| a- Fe203 (Hematite) |
Fe(NO3)3 |
6-20 nm |
| Fe304 (Magnetite) |
FeSO4/urea |
<12 nm |
| NiFe2O4 (Trevorite) |
Fe(NO3)3/Ni(NO3)2/urea |
<10nm |
| NiO (Bunsenite) |
Ni(NO3)2 |
12 nm |
| NiO/ZrO2 |
Ni(NO3)2/ZrO(NO3)2 |
~10 nm |
| TiO2 (Anatase) |
K2TiO(C2O4)2 |
3 nm |
| ZnO (Zincite) |
Zn(NO3)2/urea |
34 nm |
The RTDS process involves
- high temperature hydrolysis of metal ions present in solution
to precipitate insoluble solid particles
- continuous flow of precursor-containing solution
- very brief exposure (seconds) of the metal-containing solution
to hydrothermal conditions resulting in particle nucleation
- abrupt thermal quench to halt crystallite growth.
The RTDS process features
- easily prepared feedstock solutions using water-soluble salts
- small volumes of pressurized fluids
- capacity for large volume powder production.
As it is used for the preparation of metal-bearing powders from aqueous
solutions, the RTDS technique involves rapid transport of a precursor-bearing
solution through a high temperature, high pressure vessel followed
by an abrupt transition to low temperature and ambient pressure. Precursors
typically include metal salts or water soluble metal-bearing complexes.
Solutions are pressurized using a high pressure reciprocating piston
pump and subsequently passed through a short length of high pressure
tubing which is heated by one of several possible methods. Temperatures
of the solutions at the downstream end of the heated tube are monitored
by a thermocouple mounted in the fluid flow. Typical fluid residence
times in the heated region of the apparatus were less than 2 seconds.
After passing through the heated tube, where hydrothermal reactions
between the metal precursor species and the high temperature water
occur, the solution is depressurized by passing it through a small
orifice. The resulting metal particle/water spray can be either directed
at a substrate for powder deposition or into a cooled collection vessel
where the powder is collected as an aqueous suspension. Powders can
be separated from the suspension by sedimentation, freeze drying,
or spray drying. Filtration is typically ineffective because of the
very small size of the suspended particles (<1 micrometer).
For information about supercritical fluid capabilities at PNNL,
please contact Clement Yonker, at (509) 372-4748, clem.yonker@pnl.gov. |
