MODIFIED NI-ZN FERRITES FOR RADIOFREQUENCY APPLICATIONS

Abstract

Embodiments disclosed herein relate to using cobalt (Co) to fine tune the magnetic properties, such as permeability and magnetic loss, of nickel-zinc ferrites to improve the material performance in electronic applications. The method comprises replacing nickel (Ni) with sufficient Co.sup.+2 such that the relaxation peak associated with the Co.sup.+2 substitution and the relaxation peak associated with the nickel to zinc (Ni/Zn) ratio are into near coincidence. When the relaxation peaks overlap, the material permeability can be substantially maximized and magnetic loss substantially minimized. The resulting materials are useful and provide superior performance particularly for devices operating at the 13.56 MHz ISM band.

Claims

1. A composition represented by the formula
Ni.sub.1-w-x-y-zZn.sub.wCo.sub.xMn.sub.yCu.sub.zFe.sub.2-aO.sub.4-1.5a, w being greater than or equal to 0.2 and less than or equal to 0.6; x being greater than or equal to 0 and less than or equal to 0.2; y being greater than or equal to 0 and less than or equal to 0.2; z being greater than or equal to 0 and less than or equal to 0.2; and a being greater than or equal to 0 and less than or equal to 0.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 illustrates the variations in permeability (μ) of a nickel-zinc system at various levels of Ni and Zn content;

[0011] FIG. 2 shows a listing of FIGS. 2A-2L which illustrates a series of cobalt spectra showing the shift in frequency (x axis) of the first peak (lowest frequency) as well as complex permeability (y axis);

[0012] FIG. 3 illustrates that two relaxation peaks are observed in the magnetic permeability spectra of the material between 100 kHz and 1GHz;

[0013] FIG. 4 illustrates a method of tuning the magnetic properties of a Ni—Zn ferrite system according to one embodiment of the present disclosure;

[0014] FIG. 5 illustrates a method of manufacturing a cobalt doped nickel-zinc ferrite composition according to one embodiment of the present disclosure; and

[0015] FIG. 6 shows that in some embodiments, a wireless device can incorporate a material composition as described herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016] Disclosed herein are methods for fine tuning the magnetic properties of nickel zinc ferrites to improve the material performance in various electronic applications. Also disclosed herein are modified nickel zinc ferrite materials that are particularly suitable for use in various electronic devices operating at the 13.56 MHz ISM band. The modified nickel zinc ferrite material prepared according to embodiments described in the disclosure exhibits favorable magnetic properties such as increasing permeability and reducing magnetic loss.

[0017] Aspects and embodiments of the present invention are directed to improved materials for use in electronic devices. For example, these materials may be used to form an RF antenna for implantable medical devices, such as glucose sensors. These materials may also be used for other purposes, such as to form antennas for non-implantable devices, or other components of implantable or non-implantable devices. Advantageously, the materials have a combination of superior magnetic permeability and magnetic loss tangent at or about the 13.56 MHz industrial, medical and scientific band. In various embodiments, the materials are formed by fine tuning the permeability and magnetic loss of NiZn spinels with cobalt. As described in greater detail below, by bringing the relaxation peak associated with the Co.sup.2+ substitution and that associated with the Ni/Zn ratio into near coincidence, the permeability can be maximized and the magnetic loss minimized, such that permeability in excess of 100 can be achieved with Qs of the same order at 13.56 MHz. The same technique can be used to produce a series of NiZn plus Co materials with reducing Zn covering up to 200 MHz with permeability in excess of 10 and good Q.

Ni—Zn Systems

[0018] Nickel-zinc ferrites can be represented by the general formula Ni.sub.xZn.sub.1-xFe.sub.2O.sub.4 and are useful in electromagnetic applications that require high permeability. FIG. 1 illustrates the variations in permeability (μ) of a nickel-zinc ferrite system at various levels of Ni and Zn content. For example, the permeability decreases with decreasing zinc content at about 13.56 MHz. Variation of permeability suggests that low magnetic loss (high magnetic Q) material can be derived from the Ni—Zn system with low or zero zinc content in applications where the permeability is not so important. However, for certain RFID tags and sensors, the Ni—Zn system does not provide the optimum performance because either the permeability is too low for compositions with favorable Q, or that the Q is too low for compositions with high permeability.

The Effect of Cobalt Doping

[0019] FIG. 2 shows a listing of FIGS. 2A-2L which illustrates a series of cobalt spectra showing the shift in frequency (x axis) of the first peak (lowest frequency) as well as complex permeability (y axis). The effect of cobalt on frequency begins to stall out at about 0.025 cobalt because the magnetocrystalline anisotropy eventually passes through a minimum, just as the complex permeability flattens out, then falls again. As shown in FIGS. 2A-2L, the cobalt driven first peak eventually merges with the second peak as the Co.sup.+2 concentration increases.

[0020] FIG. 3 shows that two relaxation peaks are observed in the magnetic permeability spectra of the material between 100 kHz and 1GHz. Without being bound to a particular theory, it is believed that the lower frequency peak corresponds to magnetic domain wall rotation and the higher frequency peak corresponds to magnetic domain wall bulging. It is also believed that the cobalt oxide may push the lower frequency relaxation peak associated with the magnetic loss to higher frequency values by reducing the magnetocrystalline anisotropy of the spinel material. These higher frequency values are, in some embodiments, higher than 13.56 MHz, which is a frequency often used in RFID tags and RF medical sensor applications. It is also believed that manganese may serve to prevent the iron from reducing from Fe.sup.3+ to Fe.sup.2+ state and therefore improves the dielectric loss of the material across the spectrum, and copper may serve as a sintering aid allowing the firing temperatures to be reduced, thus preventing Zn volatilization from the surface of a part formed from the material. Both the relaxation peaks referred to above may be adjusted to provide high permeability low-loss materials throughout a range of between about 1 MHz and about 200 MHz.

[0021] Without wishing to be bound by theory, it is believed that the second peak is determined by the Ni/Zn ratio and is therefore static for a fixed ratio. The Co.sup.2+ is lost as a distinguishing peak in the spectrum at higher Co.sup.2+ concentration. It is also believed that the first peak may be dominated by domain movement, and the second peak may be dominated by rotation and that the peaks can be merged at some Co.sup.2+ doping levels for a given Ni/Zn ratio, and that only the domain movement peak is strongly susceptible to Co.sup.2+.

[0022] Based on the inventors' theory that domain rotation is driven by the Ni/Zn ratio and is responsible for a characteristic peak frequency, and that wall movement (bulging via magnetorestriction) is driven by the Co.sup.2+ concentration which creates a second, independent characteristic peak, a combination of Co.sup.2+ and Ni/Zn can be selected to merge at a given frequency such that the slope of the absorption curve is a given frequency distance way to minimize magnetic losses. The optimum peak position can be selected depending on the desired permeability and loss. For some applications, the optimum peak position is about 100 MHz to give low losses but high permeability at 13.56 MHz.

Modified Ni—Zn Systems

[0023] Certain embodiments of the present disclosure provide a modified nickel-zinc ferrite material. The base nickel-zinc ferrite material preferably has a composition that is represented by the formula Ni.sub.0.5Zn.sub.0.5Fe.sub.2O.sub.4. The material has an iron deficiency of between 0.02 and 0.10 formula units, a cobalt content of between 0 and 0.05 formula units (substituting for Ni), and manganese (substituting for Fe) and copper (substituting for Ni) contents of between 0 and 0.03 formula units. Embodiments of the material can have a spinel crystal structure and can be single phase.

[0024] In some implementations, the modified Ni—Zn ferrite material can have a composition represented by the formula Ni.sub.1-w-x-y-zZn.sub.wCo.sub.xMn.sub.yCu.sub.zFe.sub.2O.sub.4 where w ranges from 0.2 to 0.6, and x, y, and z each range from 0 to 0.2, and a ranges from 0 to 0.2. In a preferred implementation, w=0.4725, x=0.0225, y=0.02, z=0, and a=0.08, which can result in a material that displays particularly desirable magnetic properties at 13.56 MHz. In another preferred implementation, w=0.4, x=0.0275, y=0.01, z=0, and a=0.08, which can result in a material that displays particularly desirable magnetic properties at 27 MHz. Table 1 below illustrates the effects of embodiments of Co substitution in a fully dense 5000 Gauss Ni.sub.1-x-yZn.sub.xCo.sub.yFe.sub.2O.sub.4 Spinel on Spectra.

TABLE-US-00001 Cobalt 1.sup.st 2.sup.nd 3.sup.rd Initial Substitution Frequency Frequency Frequency Permeability (y) (MHz) (MHz) (MHz) at 1 MHz 0.00 16 merged merged 170 0.005 40 merged merged 92 0.01 50 70 150 68 0.0125 60 80 200 62 0.015 60 80 250 50 0.0175 60 85 250 55 0.02 70 100 250 45 0.0225 70 110 230 51 0.025 80 110 230 52 0.0275 100 120 230 54 0.03 100 130 230 50

Methods of Manufacturing Modified Ni—Zn Material

[0025] FIG. 4 illustrates a method of tuning the magnetic properties of a Ni—Zn ferrite system according to one embodiment of the present disclosure. The method begins with identifying a desired operational frequency in Step 100, followed by adjusting the nickel to zinc ratio in Step 110. Preferably, the Ni/Zn ratio is adjusted to provide a relaxation absorption peak at a desired frequency above a desired low magnetic loss frequency. The method further comprises adjusting the cobalt content in Step 120. Preferably, the cobalt content is adjusted to a level where the cobalt dominated relaxation peak merges into the low frequency end of the Ni/Zn ratio peak. The method can be followed by forming a part with the identified Ni/Zn ratio and Co concentration in Step 130. In some applications, it would be desirable to use excess cobalt because excess cobalt would likely reduce the magnetic permeability without increasing the magnetic loss. A desirable amount of cobalt can be determined by identifying an amount of cobalt that produces a relaxation absorption peak at a frequency that cannot be resolved from the Ni/Zn ratio peak by eye in an impedance analysis trace on the low frequency side. For example, if one were interested in an RF application at 27 MHz, a material could be synthesized with the composition Ni.sub.0.5725Co.sub.0.0275Zn.sub.0.4Fe.sub.2O.sub.4, which has a permeability of 54 and a magnetic Q greater than 100, wherein the magnetic Q is the ratio of the real permeability to the imaginary permeability at a specified frequency.

[0026] FIG. 5 illustrates a method of manufacturing a cobalt doped nickel-zinc ferrite composition according to one embodiment of the present disclosure. The method begins with Step 200 in which the raw oxides nickel oxide (NiO), iron oxide (Fe.sub.2O.sub.3), zinc oxide (ZnO), copper oxide (CuO, CuO.sub.2, Cu.sub.2O, and/or Cu.sub.2O.sub.3), cobalt oxide (CoO, Co.sub.2O.sub.3, and/or Co.sub.3O.sub.4), and manganese oxide (MnO, Mn.sub.2O.sub.3, Mn.sub.2O.sub.7, Mn.sub.3O.sub.4, MnO.sub.2, and/or MnO.sub.3), are blended by a shear mixing method such as a Cowles mixer or by vibratory mill blending. Step 210 indicates drying the material .The method further includes Step 220 in which the material is calcined at a temperature in the range of 900° C.-1,200° C. to react the components of the material and form the spinel phase, followed by Step 230 in which the material is milled to a particle size between 1 to 10 microns, and spray dried with added binder in Step 240. In some implementations, the method further comprises Step 250 in which the material is formed into a part by isostatic or hard die pressing and Step 260 in which the part is sintered to a temperature in the range of 1,100° C.-1400° C. in air or in oxygen.

Device and System Incorporating Modified Ni—Zn Material

[0027] The material compositions made in accordance with embodiments described herein can be used in a wide variety of applications including but not limited to antennas with high material content such as those useful for cellular phones, biomedical devices, and RFID sensors. FIG. 6 shows that in some embodiments, a wireless device 600 can incorporate a material composition as described herein. Such a device 600 can include a module 602, a battery 604, an interface 606, and an antenna 608. The antenna 608 can be configured to facilitate transmission and reception of RF signals, preferably in the 13.56 MHz range. The antenna 608 comprises a cobalt doped nickel zinc ferrite configured in such a manner that the cobalt content is adjusted to a level where the cobalt dominated relaxation peak merges into the low frequency end of the Ni/Zn ratio peak. In one implementation, at least a portion of the antenna 608 has a composition that can be represented by the formula Ni.sub.1-x-yZn.sub.wCo.sub.yFe.sub.2O.sub.4, where x=0.2 to 0.6, and 0≤y<0.2. In another implementation, at least a portion of the antenna 608 has a composition that can be represented by the formula Ni.sub.1-w-x-y-zZn.sub.wCo.sub.xMn.sub.yCu.sub.zFe.sub.2-aO.sub.4-x where 0.2≤w≤0.6; 0≤x≤0.2; 0≤y≤0.2; 0≤z≤0.2; and 0≤a≤0.2.

[0028] While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosure. Indeed, the novel compositions, methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the compositions, methods and systems described herein may be made without departing from the spirit of the disclosure. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosure.