|Title of Invention||
"A MAGNETIC CrO2 POLYMER COMPOSITION"
|Abstract||A magnetic CrO2 polymer composite blend comprising 90-95% by weight of the polymer preferably Low Density Poly Ethylene and 5-10% by weight of magnetic filler preferably CrO2 , prepared by the process of mixing the said polymer and said CrO2 in a mortar pestle to obtain a homogenous mixture , the said mixture is heated at a temperature of 95-100°C to obtain a blended melt, the said blended melt is then casted between two metal plates to obtain sheets by applying the pressure.|
|Full Text||The present invention relates to a magnetic Cr02 polymer Compsition blend for use in magnetic storage devices, such as in the audio and video tapes as magnetic read heads, magnetic field probes, or current voltage sensors in electrical devices and the process for the preparation of the same to provide a matrix for magnetic fillers.
The present invention also resides in the process for the preparation of a magnetic polymer composite blend comprising a polymer , preferably Low Density Poly Ethylene (LDPE) and the ferromagnetic filler preferably Chromium Oxide capable of exhibiting magnetic properties.
The embodiment of the invention resides in designing and fabrication of the conducting polymer blend which would mimic the properties of ferromagnetism and metallic conductivity of the bulk CrO2 and to characterize its electrical property in the presence of an external magnetic field.
Chromium (iv) oxide, CrO2 is known for its unique application as a tape material. It is a metal and a room temperature Ferro-magnet having a Curie Temperature of 385° K and at T=OK, it's a half metallic Ferro-magnet. This oxide is hydro-thermally prepared and has fine grained needles with acicular shape, a feature desired for magnetic storage applications.
Electrically conducting polymer composite materials exhibiting positive temperature co-efficient of resistance effect have been in use in resistance switching devices for many years. These materials are characterized by a switch temperature at which the material resistivity changes by orders of magnitude. The most studied polymer composite system which exhibits this effect consists of polyethylene loaded with carbon black . At temperature below 130° i.e. below the melting point of polyethylene there is an anomalous resistance which rises by orders of magnitude. This increase in resistance is believed to be due to the increased Carbon black particle separation which
forms a dis-continuous polyethylene phase expansion upon melting conducting polymer composite materials have been realized in co-polymer blends too, such as polyethylene-polystyrene copolymers wherein conducting fillers such as carbon black can bring about electrical percolation at doping levels below 3wt%. As a result, a co-polymer which otherwise shows resistance of the order of Ohms, shows below 100 Ohms when doped with conducting filler such as carbon black. This aspect in particular is used in sensor technology.
Polymer composite materials consisting of a random distribution of a filler throughout an insulating polymer are of interest for several applications. CrO2 being an important electronic material finds application as a recording material owing to its unique property of being ferromagnetic and metallic at room temperatures. Studies on CrO2 are limited owing to its thermal instability. The compound decomposes above 300°C which limits its application. The key tools in the design of a CrO2 based polymer blend are in appropriate polymer and selective localization of CrO2 grains in the blend. The best way to fabricate such a composite is to selectively disperse the CrO2 conducting chain with the filler in the insulating polymer matrix so that it forms a current conducting chain with the filler concentration as low as possible. An essential prerequisite for technological application is to retain the aciculate nature of the CrO2 grains during composite processing.
According to the present invention there is provided a magnetic CrO2 polymer composition for use in magnetic storage devices such as in the audio and video tapes as magnetic read heads, magnetic filed probes, or current voltage sensors in electrical devices comprising :
90 - 95% w/w of a polymer preferably Low Density Poly Ethylene; 5-10 w/w of magnetic filler preferably CrO2.
The present invention is a synergistic composition, the ingredients of which are not reacting chemically but interacting synergistically to produce unexpected results. The present composition is not a mere admixture.
The said polymer, preferably low density polyethylene is having a mesh size 200 micron and 99.9% purity and the said magnetic filler is in the form of powder.
The polymer composite blend is having the melting temperature of about 95°C .
The invention also resides in the process for the preparation of a magnetic Cr02 polymer composite blend, comprising
mixing the said polymer and said CrO2 into a mortar pestle to obtain a homogenous mixture
heating the said homogenous mixture at a temperature of 95-100° C to obtain a blended melt
casting the said melt between two metal plates to obtain sheets by applying the pressure.
The pressure applied to obtain the said sheet is in the range of from 4-5.5Kpa.
Alternatively, in the process for the preparation of a magnetic polymer composite blend, the said polymer and said CrO2 are mixed to obtain a homogenous mixture , the said homogenous mixture is transferred in a die and pressure of 4.5-5.5Kpa is applied to obtain a pellet . The pellet so obtained is placed in a metallic mould and kept in a preheated hot press machine at a temperature of 90-95°C and at a pressure of 4 - 5.5Kpa to allow the said polymer to melt and flow into thick sheet . The said mould is then cooled to obtain a film having magnetic properties.
The said mould used in the subject invention is preferably made up of aluminum and said hot press machine is uni-axial hot press machine
The size of the said films so produced varies from 10mm to 25mm in diameter depending on the size of the powder compact of the mixture.
The subject application may better be understood with reference to accompanying drawings. However, the same should not be construed to restrict the scope of the application as they are for illustrative purposes only.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 depicts the X-ray diffractogram of the CrO2 polymer blend which shows magneto-resistive properties.
Figure 2 depicts the SEM micrograph of the CrO2-polymer blend showing the features of grains. The black regions along the grains are predominantly carbon rich.
Figure 3 depicts the resistance versus temperature plot.
DETAILED DESCRIPTION OF THE INVENTION
The invention describes a process in which a magnetic material is embedded in a highly insulating polymer.
The subject invention brings about a novel aspect of granular magneto-resistive system in which a polymer plays a role of a insulating matrix and the filler CrO2, which is ferromagnetic and a metal behaves just as a bulk CrO2, even with a composition of 10-20% by weight in 80-90% of the polymer.
The polymer used in the subject invention preferably is LDPE having commercial grade purity of 99.9% having a melting point of 130°C. To make a polymer blend , the physical mixture of the polymer and CrO2 is fed into a Bra bender to make a melt at 95-100° C. The melt is then casted between two metal plates and drawn a sheets by applying a pressure of 5Kpa.
In an another embodiment, the mixture of the polymer may be made as a powder compact by placing the same in a metallic mould having a diameter of 5mm and thickness of 3mm. The mould so used is a preferably made up of aluminum. The metallic mould is then kept in a uniaxial hot press machine maintained previously at a set temperature of 95° C. The polymer melts and flows into the thick sheet, on applying the desired pressure. The pressure so applied is up to 5Kpa. The mould is kept at the desired pressure for about 5 minutes and then the pressure is allowed to release. The mould then is removed from the said hot press and is allowed to cool. The film so obtained in the polymer blend which is black, glossy and about 8-11 mm in dia.
The thickness and the size of the blend can be varied depending upon the die used, amount of material taken and the size of the pellet taken.
The weight percentage composition can be varied as follows
The Cr02 grains tends to be fully spin polarized within each grain, meaning that the electrons of single grain have the same spin orientation. This polarization results in a ferromagnetic material Electrons within individual grains have the same spin orientation, but the spin orientation of the electrons in adjacent grain will typically be different. In order words the transfer integral,
T« * COS0M, where i and j are the grains aligned by an angle, theta 0.
The extent in intergrain electron transport in a material is highly dependent on the spin orientation of electrons in adjacent grains. It is relatively difficult for an electron of a certain spin orientation to move across the grains of a material when all the successive grains have electrons of different spin orientation. Upon application of the magnetic field to some spin polarized materials at least some of the grains will arrange themselves such their electrons can have the similar spins. The granular polarization makes electron transport across successive grains easier and thereby cause the resistivity of the material to decrease. This relationship between applied magnetic field and resistance is magneto-resistance.
In the subject invention, the presence of insulating polymer as a major phase in the polymer blend acts as the insulating barrier providing magnetic decoupling between adjacent grains of the CrO2.
The subject invention employs the ability of the polymer blend to respond by way of resistive changes so small changes in applied magnetic field.
Thick films of the said composite are made by uniaxial hot pressing procedure, in which a powder compact of the material includes polymer, magnetic filler and conducting additive polymer, by placing in a die maintained at 95°C.
Uniaxial hot pressing of the said compact into a thick film, can be achieved by maintaining the press at 95°C and applying pressures between 4-5 Tonnes.
The size of the said films can be varied from 10mm to 25mm in diameter depending on the size of the powder compact of the mixture.
The said composite shows characteristic X-ray diffractive features of the bulk CrO2 compound.
There is a unique drop in resistance of the polymer at low temperatures, 5K and at varying magnetic field up to 5 Tesla. A negative magneto-resistance ratio delta R/Rh up to 11% is achieved as a result of inter-grain tunneling of electrons in the presence of an external magnetic field.
The X-ray diffractogram of the CrO2 polymer blend is shown in figure 1 . The, first curve marked CB, 82468, denotes the CrO2-polymer blend. The other reflections shown in figure 1 are due to CrO2.
Figure 2 shows the scanning electron micro-graph of the as-prepared CrO2 polymer blend. The gray regions as shown in figure 2 are presumably the grains
Figure 3 shows the plot of normalized resistance versus temperature plot for the Cr02 polymer blend with and without the application of an external magnetic field. The sharp decrease in resistance at low temperatures below 10K . The drop in resistance arises out of inter-grain tunneling when two moments of neighboring grains are forced to align by applying an external magnetic field.
The subject application may better be explained with reference to examples, which are for illustrative purposes , hence, the same should not be construed to restrict the scope of the application.
20% of Cr02 and 80% of low density polyethylene were mixed together in a agate mortar and the intimately mixed powder was compacted together as a
pellet using a die of 5mm diameter. The powder was compacted to a thickness of about 4mm thickness by applying a pressure of about 3 Tonnes. The pellets so obtained were placed in a preheated die heated at 95° C . The die was closed and placed in a hydraulic press and a uni-axial pressure of about 5 Tonnes was applied . The polymer composite melts at the applied pressure and temperature to form a blend, which is circular in shape and having a thickness of about 1mm.
14% of CrO2 and 86% of low density polyethylene were mixed together in a agate mortar and the intimately mixed powder was compacted together as a pellet using a die of 5mm diameter. The powder was compacted to a thickness of about 3mm thickness by applying a pressure of about 4 Tonnes. The pellets so obtained were placed in a preheated die heated at 95° C . The die was closed and placed in a hydraulic press and a uni-axial pressure of 5 Tonnes was applied. The polymer composite melts at the applied pressure and temperature to form a blend, which is circular in shape and having a thickness of about 1.5mm, forming the basis for preparing the CrO2 blend with varying composition.
.A rectangular portion of the blend was employed for electrical resistivity studies. Four probe contacts were made with electrolytic grade silver paint and insulated copper wire. A He Cryostat, Oxford instrument was employed for studying the resistance versus temperature plot. The samples were cooled at 4.2K and the resistance as a function of applied magnetic field was measured at different temperatures.
The subject application is a mere statement of invention, where various modifications and alterations are possible without deviating from the scope of the invention, hence the present application should not be construed to restrict the scope of the invention.
1. A magnetic CrO2 polymer composition for use in magnetic storage devices such as
in the audio and video tapes as magnetic read heads, magnetic filed probes, or current voltage
sensors in electrical devices comprising :
90 - 95% w/w of a polymer preferably Low Density Poly Ethylene; 5-10 w/w of magnetic filler preferably CrO2.
2. A magnetic CrO2 polymer composition as claimed in claim 1, wherein the said
polymer is in the form of powder.
3. A magnetic CrO2 polymer composition as claimed in claim 1, wherein the said
magnetic filler is in the form of powder.
4. A magnetic CrO2 polymer composition as claimed in claim 1, wherein the said low
density polyethylene is having a mesh size 200 micron and 99.9% purity.
5. A magnetic CrO2 polymer composition as claimed in claim 1, wherein the melting
temperature of said polymer is 95°C.
6. A process for the preparation of a magnetic CrO2 polymer composition as claimed in
claim 1, wherein
mixing 90-95% of LDPE, and 5-10% of CrO2 in a mortar pestle to obtain a homogenous mixture
Heating the said homogenous mixture at a temperature of 95-100°C to obtain a blended melt
Casting the said melt between two metal plates to obtain the said composition in the form of sheets or in a metallic mould to obtain said composition in form of pellet by applying the pressure.
Optionally placing the pellet in a preheated mould and applying pressure to obtain a film having magnetic properties.
7. A process for the preparation of a magnetic CrO2 polymer composition as claimed in
claim 6, wherein the said pressure is applied to the metal plates to obtain said sheet is from 4-
8. A process for the preparation of a magnetic CrO2 polymer composition substantially
as herein before described with reference to the foregoing examples.
9. A magnetic CrO2 polymer composition substantially as herein before described with
reference to the foregoing examples.
|Indian Patent Application Number||933/DEL/2000|
|PG Journal Number||13/2008|
|Date of Filing||13-Oct-2000|
|Name of Patentee||INDIAN INSTITUTE OF TECHNOLOGY|
|Applicant Address||KANPUR - 208 016, UTTAR PRADESH, INDIA.|
|PCT International Classification Number||C08L 47/00|
|PCT International Application Number||N/A|
|PCT International Filing date|