Preparation of a magnetic layer

Details Preparation of a magnetic layer Preparation of a magnetic layer

1999-04-30

2001-06-26

Pianalto, Bernard (Department: 1762)

Coating processes

Magnetic base or coating

Magnetic coating

C427S128000, C427S213300, C427S216000, C427S435000, C427S443100, C427S443200

Reexamination Certificate

active

06251475

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the preparation of particular types of magnetic layers and to their use in an anti-theft system.
BACKGROUND OF THE INVENTION
The lagging of magnetization behind the magnetizing force as the magnetic condition of a ferromagnetic material is changed, e.g. when applying an alternating external field, is called magnetic hysteresis. When a ferromagnetic sample that is initially demagnetized is subjected to an increasing external magnetic field H it reaches a particular flux density B
sat
at the maximum value of H. When the value of H is decreased again the decreasing flux density does not follow the path of increase but decreases at a rate less than that at which it rose. When H has reached zero again the value of B is not reduced to zero but to a value called the retentivity or remanence. The sample has retained a permanent magnetization. The value of B may be reduced to zero by reversing th magnetic field to negative and increasing its value to the so-called coercive force or coercivity. By further increasing H to negative values and then again reversing its direction a hysteresis loop as represented in
FIG. 1
is completed.
BRIEF AND DETAILED DESCRIPTION OF THE DRAWINGS
We define in the curve of FIG.
1
:
the saturation magnetization B
sat
which is proportional to the amount of material;
the coercive force C
M
which is dependent on the chemical composition, the particle size, the temperature, etc.;
the magnetic permeability (susceptibility or permitivity) P
M
which is dependent on the chemical composition, the degree of deformation of the material, etc.
A so-called soft ferromagnetic material shows a rather low coercive force; a so-called semi-soft ferromagnetic material shows a rather high coercive force. This properties are used in a special type of anti-theft labels, e.g. for preventing the theft from clothes out of shops, called EM-EAS labels (Electro Magnetic Electronic Article Surveillance). The principle works as follows. A label carrier is covered on one side with a soft magnetic layer having a coercive force of about 0.5 Oe, and on the other side with a semi-soft magnetic layer having a coercive force of about 100 Oe. The detection zone consists of a transmitter which transmits an alternating magnetic field with a force Z
M
positioned between 0.5 and 100 Oe, and of a receiver.
Under normal conditions the soft magnetic material will follow the alternating magnetic field. This is the case when the semi-soft layer is not magnetized (active situation). When one walks with this label through the detection zone the reversing of the magnetic dipole due to the high permeability (>40,000) will be detected by the receiver and as a consequence an alarm will go off. On the contrary, when the semi-soft layer is magnetized the soft material will be magnetized as well in the opposite sense. The transmitter is in this case not able to influence the soft magnetic material since the field strenght of the semi-soft material is larger than the strenght of the transmitted alternating field. As a consequence nothing is detected. The situations explained are briefly summarized in FIG.
2
and FIG.
3
.
For a particular brand of commercially available labels the semi-soft magnetic layer consists of a nickel mesh, and the soft magnetic layer consists of a complex alloy of Ni
a
Fe
b
Co
c
(MO)
d
B
e
. The problem with these magnetic layers is the fact that they are nowadays applied by means of sputtering in vacuo, a cumbersome and expensive technique.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a simple and cheap method for producing soft and semi-soft magnetic layers.
It is a further object of the invention to provide a use for such magnetic layers in the design of EM-EAS labels.
SUMMARY OF THE INVENTION
The above mentioned objects are realised by providing a process for the preparation of a magnetic element comprising a support and at least one magnetic layer, said process comprising the steps of:
(1) preparing an aqueous solution containing one or more type of metal ions including nickel ions,
(2) chemically reducing said one or more metal ions by means of a reducing agent thus forming an aqueous dispersion of metal particles including nickel,
(3) removing all superfluous ions from said aqueous dispersion by means of a washing step, preferably an ultrafiltration and/or diafiltration step, or by means of centrifugation,
(4) coating the resulting aqueous dispersion onto a support.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will now be explained on the hand of a preferred embodiment whereby the metal ions undergoing reduction are solely nickel(II) ions.
In a first step an aqueous solution of nickel(II) ions is prepared. A most suitable salt is Ni(NO
3
)
2
.6H
2
O. The solution is acidified with a small amount of nitric acid.
In a following step the nickel ions in the solution are reduced to highly dispersed metallic nickel particles of nanosize by means of the addition of a reducing agent. A preferred reducing agent is KBH
4
. The reducing agent can be added to the original nickel salt solution as a solid powder. More preferably, the reducing agent may be dissolved separately in a second aqueous medium and added to the nickel salt solution according to a single jet or a double jet procedure. Preferably, according to the double jet principle, the aqueous medium containing the nickel ions and the second solution containing the reducing agent are added together to a third aqueous medium.
The second aqueous solution comprising the reducing agent preferably also contains sulphite ions which strongly enhance the chemical stability of this solution.
In order to keep the nickel nanoparticles formed by reduction in colloidal dispersion a protective binder is preferably added to one or more of the three aqueous solutions involved. Preferably, this protective binder is added to the third aqueous medium wherein both others are jetted. A particularly preferred protective binder is carboxymethylcellulose (CMC). Other possible binders include gelatin, arabic gum, poly(acrylic acid), cellulose derivatives and other polysaccharides.
Preferably also a complexing agent is present in one of the three aqueous media described above. A preferred complexant is simply the well-known ethylenediaminetetraacetic acid (EDTA) or a homologous compound or a salt thereof. Another preferred one is citrate, e.g. triammonium citrate. Other suitable complexants include diethylenetriamine-pentaacetic acid (DTPA), trans-1, 2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CDTA), ethyleneglycol-O,O′-bis(2-aminoethyl)-N,N,N′,N′-tetraacetic acid (EGTA), N-(2-hydroxyethyl)ethylenediamine-N,N,N′-triacetic acid (HEDTA), etc. The complexing agent is preferably present in the third aqueous medium to which the other solutions are added according to the double jet principle.
In a following step 3 of the present invention the superfluous salts are first removed from the aqueous medium by a washing process, preferably involving ultrafiltration and/or diafiltration. Additionally or alternatively centrifugation can be used.
In any of the solutions involved in the preparation a so-called dispersing aid can be present. In a preferred embodiment this compound is added to the diafiltration liquid at the last stage of the preparation. Suitable dispersing aids in the case of nickel are phosphates, more particularly a hexametaphosphate such as sodium hexametaphosphate. Probably, the hexametaphosphate adsorbs to the surface of the alloy particles so that they become negatively charged. By electrostatic repulsion they are kept in dispersion. Also the phosphate inhibits further oxidation of the surface of the formed nanoparticles. In other words, the thin nickel oxide shell that will be formed inevitably around the nanoparticles since the reducing medium disappears during the washing step will be passivated by the hexametaphosphate. So in a preferred embodiment the nickel particles are ultrafiltrated e.g. th

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