Produce absorbing coating by
using carbonyl iron powder
Absorbing material can be used in airplane coating for
reducing its reflector to radar and reducing radar cross
section, which can not only improve mission success rate, but
also can improve the survival probability of warplanes in
enemy range. Therefore, it will be of great significance in
developing new type absorbing materials.
According to loss mechanism, microwave absorption can be
divided into resistor type, dielectric type and magnetic
medium type. Based on resonance and magnetic hysteresis loss
principle, magnetic medium absorber can be used in reducing
electromagnetic wave, in which the representative materials
mainly include ferrite and micro metal powder. Micro metal
powder mainly includes carbonyl iron powder, carbonyl nickel
powder, permalloy, cobalt nickel alloy and so on. Among them,
carbonyl iron powder is a kind of typical magnetic medium
absorber, which has drawn more and more attentions as it has
wide absorption frequency band.
High quality absorbing
materials should meet the following requirements: a. it can
make sure that electromagnetic wave can deeply enter into
absorbing material when it is propagated to the material
surface, thus it will greatly reduce direct reflection of
electromagnetic wave; b. electromagnetic wave can be rapidly
reduced when it is spread inside this kind of material.
Experiment and theory both prove that in order to achieve
areflexion, the dielectric constant is required to be equal to
permeability. This is because dielectric constant is required
to be well matched with permeability in the high quality
absorbing materials. However, if we only use dielectric loss
absorber or magnetic loss absorber, it will cause mismatching
between complex dielectric constant and complex permeability.
By mixing carbonyl iron powder, the magnetic loss absorber,
with dielectric loss absorber, it can achieve good absorbing
effect, which can provide theory and experiment support for
the wide application of carbonyl iron powder.
In order to improve the microwave absorbing characteristics
and oxidation resistance of absorbing coating that uses
carbonyl iron powder as absorber, we can mix other types of
absorbers with carbonyl iron powder as well as can modified
carbonyl iron powders.
Carbonyl iron powder is a kind of iron powder that is obtained
after the decomposition of carbonyl iron. When iron
pentacarbonyl (Fe(CO)5) is heated to 70-80 degrees Celsius, it
will be resolved into Fe and CO as well as will be largely
decomposed at 155 degrees Celsius and will finally be
transformed into onion shape carbonyl iron powder that mainly
contains iron as well as carbon, oxygen and other impurities.
With the development of electromagnetic information and other
technologies, electromagnetic shielding under normal
temperature has already failed to satisfy market demands.
Although carbonyl iron powder has good temperature stability,
it will have low oxidation resistance when the oxidizing
temperature reaches about 200 degrees Celsius. Therefore,
antioxidant treatment is especially important for carbonyl
iron powder.
Experiments show that after reacting with other substances and
forming core-shell structure, the coated nano shell will be
beneficial for improving the thermal stability of carbonyl
iron, besides, the compactness of shell and shell content can
be regarded as the main factors that can influence the heat
resistance of core-shell composite particles. In addition,
Fe/SiO2 that forms the nucleus shell structure can also
inhibit eddy-current loss.
By using conducting polymer, it can also improve oxidation
resistance of carbonyl iron powder. For example, by coating a
layer of polyaniline resin on the carbonyl iron powder, it can
improve the oxidation resistance and corrosion resistance.
This is because Fe2O3 oxide layer will be formed between the
interface of polyaniline and carbonyl iron powder, thus it can
prevent the further oxidation of carbonyl iron powder and can
also increase dielectric constant.
In addition, research shows that when the other conditions are
remained unchanged, by only changing matrix type, the
reflectivity of absorbing materials will be changed obviously.
Therefore, it also can be regarded as one of the keys for the
design and preparation of absorbing coating by selecting
appropriate matrix and modifying.
Except for meeting various mechanical properties, absorbing
materials that used in the outside surface of airplane are
required to have good high temperature resistance at the same
time. As a result, epoxy resin has drawn more and more
attentions due to its good high temperature resistance.