FIELD OF THE INVENTION
This invention relates to a process for the preparation of
encapsulated metal nanoparticles.
BACKGROUND OF THE INVENTION
A metalloceramic gel is an important precursor for
processing fine ceramics and metalloceramics in a controlled size
as small as at a nanometer scale. Development of a metalloceramic
gel in a specific composition and its phase transformation to a
desired ceramic product has a lot of commercial interest in order
to (i) minimise the processing steps; and (ii) improve quality
and performance of product at an economic cost. The known
processes using sol—gel or alkoxide chemistry have so far been
performed with an activator of an organic chemical. The activator
used in these methods generates toxic gases during processing and
thermaldecomposition of gel to desired ceramics or
metalloceramics. It adds unwanted impurities.
Encapsulating small metal particles, of size of a nanometer
scale, in a corrosion resistant material, such as Al O , offers
2 3
an improved thermal stability of the product with superior wear
resistance and chemical stability. An Al 0 coating of metal
2 3
particles of iron, cobalt, or nickel is found to have
considerable improvements in their mechanical hardness,
electrical ( also thermal) resistivity and magnetic properties
useful for utility in cutting tools and magnetic devices. It
insulates the electrical conductors of such magnetic metal
particles to form a tunneling barrier so that they have a special
property of a tunnel magnetoresistance TMR (lioodera et al., Phys.
Rev. Lett.44 ( 1995) 3273 and Nassar et al. , Appl. Phys. Lett.73
(1998) 698). TMR offers specific applications of such magnets of
Al 0 encapsulated metal particles in magnetic sensors, non-
2 3
volatile memory devices, and other small devices and components.
OBJECTS OF THE INVENTION
An object of this invention is to propose a process for the
preparation of metalloceramic gel and encapsulated metal nano—
particles.
A further object of this invention is to propose a process
for the preparation of metalloceramic gel and encapsulated metal
nanoparticles which is simple and cost—effective.
Further objects and advantages of this invention will be
more apparent from the ensuing description.
BRIEF DESCRIPTION OF THE INVENTION
According to this invention there is provided a process for
the preparation of encapsulated metal nanoparticles comprising
dissolving aluminium metal with nascent surface prepared in a
manner such as herein described, into an aqueous solution of a
o
salt of a transition metal at a temperature in the range of 30
o
to 70 C;
cooling the solution to obtain a gel followed by washing the
gel and drying the same to obtain a solic metalloceramic gel,
heating the metalloceramic gel to obtain the encapsulated
metal nanoparticles.
According to this invention is further provided, a process
for the preparation of encapsulated metal nanoparticles
comprising heating the solid metalloceramic gel with hydrogen gas
o
at a temperature in the range of 500 to 850 C.
In accordance with this invention a self—induced natural
surface hydrolysis of metal cations dispersed in an aqueous
solution is explored for performing a metalloceramic gel at room
temperature. It involves a pure metal (which forms part of the
gel) to initiate and conduct the hydrolysis and gelation in a
single step. It yields an amorphous gel without using any other
additives. As a result, a highly pure ceramic or metalloceramic
powder appears after its decomposition C and reduction in the
case of the metalloceramics) at an elevated temperature. The
whole process is described as follows:
The chemical reaction in this process is carried out with
reactants of
(i) thin plates or chips of Al metal; and (ii) a M metal
salt.
The metal salt is a salt of a transition metal such as iron,
cobalt, nickel, copper, tin and silver. An aqueous solution of
M in a concentration of about 0.2 to 1 . 0M is obtained by
dissolving the salt in distilled water. A self-induced hydrolysis
reaction of M in order to form a metalloceramic gel in M ;
occurs as soon as thin plates or chips of Al-metal
with refreshed nascent surfaces are immersed in the M solution.
It is carried out by stirring the solution in a beaker on a
magnetic stirrer. This involves a self-induced spontaneous co-
reduction of M to an intermediate cation or metallic M
state followed by hydrolysis of latter into a hydroxyl gel in an
interconnected amorphous network structure in association with
in-situ hydrolysis of Al metal into A10(OH). H 0. Hydrolysis of
dispersed M cations in solution occurs much faster than of Al
metal itself into A.10 <0H.o(H 0 so that it permits M -M-M
hydrolysis through water in the solution. All this occurs at room
temperature and does not involve any activator to induce or
c on d u ct t h e r e a ct ions.
A usual Al-metal surface is always accompanied by a thin
indistinct inseparably oxide film of its own. That protects it
from further oxidation or hydrolysis in air or in an aqueous
medium. A nascent Al—surface efficient to induce action can be obtained by refining a refreshed surface
(obtained by decreasing and washing in distilled water to obtain
a fresh surface and then treating with about 0..1M of a mineral
acid such as HC1, for about 10 mins. followed by washing in water
again) and reacting with Hg -cations to obtain nascent surface.
In this process, the specimen is dipped in about aqueous
Hg solution for about 30 sees, and then rinsed properly with

distilled water to remove the excess Hg -cations. The Hg
cations react with Al atoms at the surface by getting reduced
2+
into Hg metal as per the reaction of3Hg +2A.l-*3Hg+2AL . It forms
in a thin amalgam film with nascent Al surface and that very
intimately adheres to the nascent Al surface. The amalgamation
induces the residual surface oxide film to peel off from the
mother surface and to segregate over amalgam a long with other-
byproduct impurities. This is removed easily alongwith excess
amalgam, if any, by washing in distilled water. The obtained
specimen has nascent surfaces. It conducts the desired hydrolysis
of dispersed M cations in an aqueous solution.
For example, the co-reduction and hydrolysis reactions
with Al-metal in an aqueous CoCI solution can be written as
3 CoCI +2 Al 3 Co + 2 A1C1
The obtained Co metal particles are extremely small in a few
nanometers in sire and get instantaneously hydrolysed into
Co(OH) by a reaction. Co + 2H 0 ->Co