Modified bismuth-substituted synthetic garnets for electronic applications

Abstract

Embodiments disclosed herein include methods of modifying synthetic garnets used in RF applications to reduce or eliminate Yttrium or other rare earth metals in the garnets without adversely affecting the magnetic properties of the material. Some embodiments include substituting Bismuth for some of the Yttrium on the dodecahedral sites and introducing one or more high valency ions to the octahedral and tetrahedral sites. Calcium may also be added to the dodecahedral sites for valency compensation induced by the high valency ions, which could effectively displace all or most of the Yttrium (Y) in microwave device garnets. The modified synthetic garnets with substituted Yttrium (Y) can be used in various microwave magnetic devices such as circulators, isolators and resonators.

Claims

1. (canceled)

2. A modified synthetic garnet having a composition represented by the formula Bi.sub.xY.sub.3-x-0.35Ca.sub.0.35Zr.sub.0.35Fe.sub.4.65O.sub.12, x being between 0.5 and 1.0, bismuth being substituted for yttrium on a dodecahedral site, zirconium being substituted for iron on an octahedral site, and calcium being added to a dodecahedral site to replace yttrium and balance charges with zirconium.

3. The modified synthetic garnet of claim 2 wherein x is between 0.6 and 0.8.

4. The modified synthetic garnet of claim 3 wherein x is 0.5.

5. A method of manufacturing a bismuth-modified synthetic garnet, the method comprising: providing a material include oxides, carbonates, or a combination thereof; forming a composition represented by the formula Bi.sub.xY.sub.3-x-0.35Ca.sub.0.35Zr.sub.0.35Fe.sub.4.65O.sub.12, x being between 0.5 and 1.0, bismuth being substituted for yttrium on a dodecahedral site, zirconium being substituted for iron on an octahedral site, and calcium being added to the dodecahedral site to replace yttrium and balance charges with zirconium.

6. The method of claim 5 wherein x is between 0.6 and 0.8.

7. The method of claim 5 wherein x is 0.5.

8. The method of claim 5 further including forming an electronic device from the composition.

9. The method of claim 5 further including: blending the material to form a mixture; drying the mixture; sieving the dried mixture; calcining the sieved mixture; milling the calcined material into particle sizes of about 0.5 micron to 10 micron; spray drying the milled material to form granules; pressing the granules; and calcining the pressed granules to form the composition.

10. The method of claim 9 wherein drying is at a temperature between 100-400 C.

11. The method of claim 9 wherein milling is performed in a vibratory mill, an attrition mill, or a jet mill.

12. The method of claim 9 wherein calcining the pressed granules is at a temperature between 850-1000 C.

13. The method of claim 9 wherein calcining the sieved mixture is at a temperature between 100-400 C.

14. The method of claim 13 wherein calcining the sieved mixture is at a temperature between 900-950 C.

15. An electronic device incorporating a synthetic garnet having a composition represented by the formula Bi.sub.xY.sub.3-x-0.35Ca.sub.0.35Zr.sub.0.35Fe.sub.4.65O.sub.12, x being between 0.5 and 1.0, bismuth being substituted for yttrium on a dodecahedral site, zirconium being substituted for iron on an octahedral site, and calcium being added to the dodecahedral site to replace yttrium and balance charges with zirconium.

16. The electronic device of claim 15 wherein x is between 0.6 and 0.8.

17. The electronic device of claim 15 wherein x is 0.5.

18. The electronic device of claim 15 wherein the device is an isolator.

19. The electronic device of claim 15 wherein the device is a circulator.

20. The electronic device of claim 15 wherein the device is configured for magnetic microwave applications.

21. The electronic device of claim 15 wherein the device is incorporated into a cellular base station.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 depicts a Yttrium based garnet crystal lattice structure;

[0019] FIG. 2 is a graph depicting variations of material properties versus varying levels of Vanadium in crystalline compositions represented by the formula Y.sub.2.15-2xBi.sub.0.5Ca.sub.0.35+2xZr.sub.0.35V.sub.xFe.sub.4.65-xO.sub.12, where x=0.1 to 0.8;

[0020] FIG. 3 is a graph depicting variations of material properties versus varying levels of (Zr, Nb) in crystalline compositions represented by the formula Bi.sub.0.9Ca.sub.0.9xY.sub.2.1-0.9x(Zr.sub.0.7Nb.sub.0.1).sub.xFe.sub.5-0.8xO.sub.12, where x=0.5 to 1.0;

[0021] FIGS. 4A-4G are graphs depicting the relationship between firing temperature and various properties at varying levels of Vanadium in crystalline compositions represented by the formula Bi.sub.0.9Ca.sub.0.9+2xZr.sub.0.7Nb.sub.0.1V.sub.xFe.sub.4.2-xO.sub.12 where x=00.6;

[0022] FIG. 5 is a graph depicting best linewidth versus composition of varying Vanadium content in crystalline compositions represented by the formula Bi.sub.0.9Ca.sub.0.9+2xZr.sub.0.7Nb.sub.0.1V.sub.xFe.sub.4.2-xO.sub.12 where x=00.6;

[0023] FIG. 6 is a graph illustrating the properties of crystal compositions represented by the formula Bi.sub.1.4Ca.sub.0.55+2xY.sub.1.05-2xZr.sub.0.55V.sub.xFe.sub.4.45-xO.sub.12, where x=00.525;

[0024] FIG. 7 illustrates a process flow for making a modified synthetic garnet of one preferred embodiment;

[0025] FIG. 8 is a schematic illustration of a circulator incorporating a modified synthetic garnet material of one preferred embodiment; and

[0026] FIG. 9 is a schematic illustration of a telecommunication system incorporating the isolator of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0027] Disclosed herein are methods of modifying synthetic garnet compositions, such as Yttrium Iron Garnet (YIG), to reduce or eliminate the use of rare earth metals in such compositions. Also disclosed herein are synthetic garnet materials having reduced or no rare earth metal content, methods of producing the materials, and the devices and systems incorporating such materials. The synthetic garnet materials prepared according to embodiments described in the disclosure exhibit favorable magnetic properties for microwave magnetic applications. These favorable properties include but are limited to low magnetic resonance line width, optimized density, saturation magnetization and dielectric loss tangent. Applicants have surprisingly found that when garnet compositions are doped with certain combinations of ions and prepared using certain processing techniques, a significant amount if not all of the rare earth elements can be substituted and yet still result in microwave magnetic crystalline materials with comparable, if not superior, performance characteristics as commercially available garnets containing Yttrium (Y) or other rare earth elements.

[0028] Synthetic garnets typically have the formula unit of A.sub.3B.sub.5O.sub.12, where A and B are trivalent metal ions. Yttrium Iron Garnet (YIG) is a synthetic garnet having the formula unit of Y.sub.3Fe.sub.5O.sub.12, which includes Yttrium (Y) in the 3+ oxidation state and Iron (Fe) in the 3+ oxidation state. The crystal structure of a YIG formula unit is depicted in FIG. 1. As shown in FIG. 1, YIG has a dodecahederal site, an octahedral site, and a tetrahedral site. The Y ions occupy the dodecahedral site while the Fe ions occupy the octahedral and tetrahedral sites. Each YIG unit cell, which is cubic in crystal classifications, has eight of these formula units.

[0029] The modified synthetic garnet compositions, in some embodiments, comprise substituting some or all of the Yttrium (Y) in Yttrium Iron Garnets (YIG) with a combination of other ions such that the resulting material maintains desirable magnetic properties for microwave applications. There have been past attempts toward doping YIG with different ions to modify the material properties. Some of these attempts, such as Bismuth (Bi) doped YIG, are described in Microwave Materials for Wireless Applications by D. B. Cruickshank, which is hereby incorporated by reference in its entirety. However, in practice ions used as substitutes may not behave predictably because of, for example, spin canting induced by the magnetic ion itself or by the effect of non-magnetic ions on the environment adjacent magnetic ions, reducing the degree alignment. Thus, the resulting magnetic properties cannot be predicted. Additionally, the amount of substitution is limited in some cases. Beyond a certain limit, the ion will not enter its preferred lattice site and either remains on the outside in a second phase compound or leaks into another site. Additionally, ion size and crystallographic orientation preferences may compete at high substitution levels, or substituting ions are influenced by the ion size and coordination of ions on other sites. As such, the assumption that the net magnetic behavior is the sum of independent sub-lattices or single ion anisotropy may not always apply in predicting magnetic properties.

[0030] Considerations in selecting an effective substitution of rare earth metals in YIG for microwave magnetic applications include the optimization of the density, the magnetic resonance linewidth, the saturation magnetization, the Curie temperature, and the dielectric loss tangent in the resulting modified crystal structure. Magnetic resonance is derived from spinning electrons, which when excited by an appropriate radio frequency (RF) will show resonance proportional to an applied magnetic field and the frequency. The width of the resonance peak is usually defined at the half power points and is referred to as the magnetic resonance linewidth. It is generally desireable for the material to have a low linewidth because low linewidth manifests itself as low magnetic loss, which is required for all low insertion loss ferrite devices. The modified garnet compositions according to preferred embodiments of the present invention provide single crystal or polycrystalline materials with reduced Yttrium content and yet maintaining low linewidth and other desirable properties for microwave magnetic applications.

[0031] In some embodiments, a Yttrium based garnet is modified by substituting Bismuth (Bi.sup.3+) for some of the Yttrium (Y.sup.3+) on the dodecahedral sites of the garnet structure in combination with introducing one or more ions, such as divalent (+2), trivalent (+3), tetravalent (+4), pentavalent (+5) or hexavalent (+6) non-magnetic ions to the octahedral sites of the structure to replace at least some of the Iron (Fe.sup.3+). In a preferred implementation, one or more high valency non-magnetic ions such as Zirconium (Zr.sup.4+) or Niobium (Nb.sup.5+) can be introduced to the octahedral sites.

[0032] In some embodiments, a Yttrium based garnet is modified by introducing one or more high valency ions with an oxidation state greater than 3+ to the octahedral or tetrahedral sites of the garnet structure in combination with substituting Calcium (Ca.sup.2+) for Yttrium (Y.sup.3+) in the dodecahedral site of the structure for charge compensation induced by the high valency ions, hence reducing the Y.sup.3+ content. When non-trivalent ions are introduced, valency balance is maintained by introducing, for example, divalent Calcium (Ca.sup.2+) to balance the non-trivalent ions. For example, for each 4+ ion introduced to the octahedral or tetrahedral sites, one Y.sup.3+ ion is substituted with a Ca.sup.2+ ion. For each 5+ ion, two Y.sup.3+ ions are replaced by Ca.sup.2+ ions. For each 6+ ion, three Y.sup.3+ ions are replaced by Ca.sup.2+ ions. For each 6+ ion, three Y.sup.3 ions are replaced by Ca.sup.2+ ions. In one implementation, one or more high valence ions selected from the group consisting of Zr.sup.4+, Sn.sup.4+, T.sup.4+, Nb.sup.5+, Ta.sup.5+, Sb.sup.5+, W.sup.6+, and Mo.sup.6 is introduced to the octahedral or tetrahedral sites, and divalent Calcium (Ca.sup.2+) is used to balance the charges, which in turn reduces Y.sup.3+ content.

[0033] In some embodiments, a Yttrium based garnet is modified by introducing one or more high valency ions, such as Vanadium (V.sup.5+), to the tetrahedral site of the garnet structure to substitute for Fe.sup.3+ to further reduce the magnetic resonance linewidth of the resulting material. Without being bound by any theory, it is believed that the mechanism of ion substitution causes reduced magnetization of the tetrahedral site of the lattice, which results in higher net magnetization of the garnet, and by changing the magnetocrystalline environment of the ferric ions also reduces anisotropy and hence the ferromagnetic linewidth of the material.

[0034] In some embodiments, Applicant has found that a combination of high Bismuth (Bi) doping combined with Vanadium (V) and Zirconium (Zr) induced Calcium (Ca) valency compensation could effectively displace all or most of the Yttrium (Y) in microwave device garnets. Applicants also have found that certain other high valency ions could also be used on the tetrahedral or octahedral sites and that a fairly high level of octahedral substitution in the garnet structure is preferred in order to obtain magnetic resonance linewidth in the 5 to 20 Oersted range. Moreover, Yttrium displacement is preferably accomplished by adding Calcium in addition to Bismuth to the dodecahedral site. Doping the octahedral or tetrahedral sites with higher valency ions, preferably greater than 3+, would allow more Calcium to be introduced to the dodecahedral site to compensate for the charges, which in turn would result in further reduction of Yttrium content.

Modified Synthetic Garnet Compositions

[0035] In one implementation, the modified synthetic garnet composition may be represented by general Formula I: Bi.sub.xCa.sub.y+2zY.sub.3-x-y-2zFe.sub.5-y-zZr.sub.yV.sub.zO.sub.12, where x=0 to 3, y=0 to 1, and z=0 to 1.5, more preferably x=0.5 to 1.4, y=0.3 to 0.55, and z=0 to 0.6. In a preferred implementation, 0.5 to 1.4 formula units of Bismuth (Bi) is substituted for some of the Yttrium (Y) on the dodecahedral site, 0.3 to 0.55 formula units of Zirconium (Zr) is substituted for some of the Iron (Fe) on the octahedral site. In some embodiments, up to 0.6 formula units of Vanadium (V) is substituted for some of the Iron (Fe) on the tetrahedral site. Charge balance is achieved by Calcium (Ca) substituting for some or all of the remaining Yttrium (Y). In some other embodiments, small amounts of Niobium (Nb) may be placed on the octahedral site and small amounts of Molybdenum (Mo) may be placed on the tetrahedral site.

[0036] In another implementation, the modified synthetic garnet composition may be represented by general Formula II: Bi.sub.xY.sub.3-x-0.35Ca.sub.0.35Zr.sub.0.35Fe.sub.4.65O.sub.12, where x=0.5 to 1.0, preferably x=0.6 to 0.8, more preferably x=0.5. In this implementation, 0.5 to 1.0 formula units of Bismuth (Bi) is substituted for some of the Yttrium (Y) on the dodecahedral site and Zirconium (Zr) is substituted for some of the Iron (Fe) on the octahedral site. Calcium (Ca.sup.2+) is added to the dodecahedral site to replace some of the remaining Y to balance the Zr charges. Bi content can be varied to achieve varying material properties while Zr is held fixed at Zr=0.35.

[0037] In another implementation, the modified garnet composition may be represented by general Formula III: Bi(Y,Ca).sub.2Fe.sub.4.2M.sup.I.sub.0.4M.sup.II.sub.0.4.O.sub.12, where M.sup.I is the octahedral substitution for Fe and can be selected from one or more of the following elements: In, Zn, Mg, Zr, Sn, Ta, Nb, Fe, Ti, and Sb, where M.sup.II is the tetrahedral substitution for Fe and can be selected from one or more of the following elements: Ga, W, Mo, Ge, V, Si.

[0038] In another implementation, the modified synthetic garnet composition may be represented by general Formula IV: Y.sub.2.15-2xBi.sub.0.5Ca.sub.0.35+2xZr.sub.0.35V.sub.xFe.sub.4.65-xO.sub.12, wherein x=0.1 to 0.8. In this implementation, 0.1 to 0.8 formula units of Vanadium (V) is added to the tetrahedral site to substitute for some of the Iron (Fe), and Calcium (Ca) is added to balance the V charges and replace some of the remaining Y while the levels of Bi and Zr remain fixed similar to Formula III. FIG. 2 illustrates variations of material properties in connection with varying levels of V. As shown in FIG. 2, the dielectric constant and density of the material remain largely constant with varying levels of V. Increasing levels of V reduces the 4PiMs by about 160 Gauss for each 0.1 of V. As further shown in FIG. 2, there are no appreciable changes in 3dB linewidth up to V=0.5.

[0039] In another implementation, the modified synthetic garnet composition maybe represented by Formula V: Bi.sub.0.9Ca.sub.0.9xY.sub.2.1-0.9x(Zr.sub.0.7Nb.sub.0.1).sub.xFe.sub.5-0.8xO.sub.12, wherein x=0.5 to 1.0. In this implementation, the octahedral substitution is made with two high valency ions: Zr.sup.4+ and Nb.sup.5+ with Bi held constant at 0.9. FIG. 3 illustrates variations of material properties in connection with varying levels of (Zr, Nb). As shown in FIG. 3, the magnetic resonance linewidth decreased with higher octahedral substitutions. The magnetization also fell as the increase in total non-magnetic ions overcomes the higher non-magnetic octahedral substitutions.

[0040] In another implementation, the modified synthetic garnet composition may be represented by Formula VI: Bi.sub.0.9Ca.sub.0.9+2xY.sub.2.1-0.9-2xZr.sub.0.7Nb.sub.0.1V.sub.xFe.sub.4.2-xO.sub.12, where V=00.6. In this implementation, Vanadium is introduced to the octahedral site in addition to Zr and Nb. When V=0.6, Y is completely replaced. FIGS. 4A-4G illustrate the relationship between firing temperatures and various material properties as V level increases from 0 to 0.6. As illustrated, the 3 dB linewidth, measured in accordance with ASTM A883/A883M-01, tends to remain below 50 Oe at all V levels at firing temperatures below 1040 C. FIG. 5 illustrates the best linewidth at varying firing temperatures versus composition at varying levels of V of one preferred embodiment. In some implementations, the linewidth can be further reduced by annealing the material. The effect of annealing on linewidth of Bi.sub.0.9Ca.sub.0.9+2xY.sub.2.1-0.9-2xZr.sub.0.7Nb.sub.0.1V.sub.xFe.sub.4.2-xO.sub.12, where x=0.1 to 0.5 is illustrated in Table 2 below.

TABLE-US-00001 Linewidth (Oer) and Curie Temperature (degrees C.) Data Bi.sub.0.9Ca.sub.0.9+2xY.sub.2.10.92x(Zr,Nb).sub.0.8V.sub.xFe.sub.4.2xO.sub.12 Heat Treatment Heat Heat (Calcined Treatment 3 dB 3 dB Treatment 3 dB 3 dB Blend + 3 dB 3 dB after (Initial before after (Calcined before after Extended before extended Curie Formula Blend) anneal anneal Blend) anneal anneal Milling) anneal anneal Temp V = 0.5 1050 39 25 1030 38 20 1030 38 17 108 V = 0.4 1050 44 27 1030 48 18 1030 42 16 112 V = 0.3 1050 52 32 1030 46 19 1030 48 15 111 V = 0.2 1050 59 43 1030 55 21 1030 62 17 108 V = 0.1 1050 78 62 1030 61 24 1030 55 21 107

[0041] In another implementation, the modified synthetic garnet composition may be represented by Formula VI: Bi.sub.1.4Ca.sub.0.55+2xY.sub.1.05-2xZr.sub.0.55V.sub.xFe.sub.4.45-xO.sub.12, where x=00.525. In this implementation, the level of Bi doping is increased while the level of octahedral substitution is decreased. The material formed has higher Curie temperature and low linewidth. The Vanadium (V) content is varied from 0 to 0.525. When V=0.525, the composition is free of Yttrium. The resulting material achieved a linewidth of 20 Oe without subsequently heat treatment. FIG. 6 illustrates the properties of the material with varying amount of V. As shown in FIG. 6, V drops the dielectric constant rapidly, about 1 unit for each 0.1 of V in the formula unit, and drops the magnetization by about 80 Gauss for each 0.1 of V. Optimizing the processing parameters such as firing conditions have produced linewidth as low as 11 or V at or close to 0.525, which is free of Y. These values are comparable to commercially available Calcium Yttrium Zirconium Vanadium garnets of the same magnetization.

[0042] In another implementation, the modified synthetic garnet composition may be represented by Formula VII: Y.sub.2CaFe.sub.4.4Zr.sub.0.4Mo.sub.0.2O.sub.12. In this implementation, high valency ion Molybdenum (Mo) is added to the tetrahedral site to create a single phase crystal. In other implementations, the modified synthetic garnet compositions can be represented by a formula selected from the group consisting of: BiY.sub.2Fe.sub.4.6In.sub.0.4O.sub.12, BiCa.sub.0.4Y.sub.1.6Fe.sub.4.6Zr.sub.0.4O.sub.12, BiCa.sub.0.4Y1.6Fe.sub.4.6Ti..sub.0.4O.sub.12, BiCa.sub.0.8Y.sub.1.2Fe.sub.4.6Sb.sub.0.4O.sub.12, BiY.sub.2Fe.sub.4.6Ga.sub.0.4O.sub.12, BiCa.sub.1.2Y.sub.0.8Fe.sub.4.2In.sub.0.4Mo.sub.0.4O.sub.12, BiY.sub.1.2Ca.sub.0.8Fe4.2Zn.sub.0.4Mo..sub.0.4O.sub.12, BiY.sub.1.2Ca.sub.0.8Fe.sub.4.2Mg.sub.0.4Mo.sub.0.4O.sub.12, BiY.sub.0.4Ca.sub.1.6Fe.sub.4.2Zr.sub.0.4Mo.sub.0.4O.sub.12, BiY.sub.0.4Ca.sub.1.6Fe.sub.4.2Sn.sub.0.4Mo.sub.0.4O.sub.12, BiCa.sub.2Fe.sub.4.2Ta.sub.0.4Mo.sub.0.4O.sub.12, BiCa.sub.2Fe.sub.4.2Nb.sub.0.4Mo.sub.0.4O.sub.12, BiY.sub.0.8Ca.sub.1.2Fe.sub.4.6Mo.sub.0.4O.sub.12, and BiY.sub.0.4Ca.sub.1.6Fe.sub.4.2Ti.sub.0.4Mo.sub.0.4O.sub.12.

Preparation of the Modified Synthetic Garnet Compositions

[0043] The preparation of the modified synthetic garnet materials can be accomplished by using known ceramic techniques. A particular example of the process flow is illustrated in FIG. 7.

[0044] As shown in FIG. 7, the process begins with step 106 for weighing the raw material. The raw material may include oxides and carbonates such as Iron Oxide (Fe.sub.2O.sub.3), Bismuth Oxide (Bi.sub.2O.sub.3), Yttrium Oxide (Y.sub.2O.sub.3), Calcium Carbonate (CaCO.sub.3), Zirconium Oxide (ZrO.sub.2), Vanadium Pentoxide (V.sub.2O.sub.5), Yttrium Vanadate (YVO.sub.4), Bismuth Niobate (BiNbO.sub.4), Silica (SiO.sub.2), Niobium Pentoxide (Nb.sub.2O.sub.5), Antimony Oxide (Sb.sub.2O.sub.3), Molybdenum Oxide (MoO.sub.3), Indium Oxide (In.sub.2O.sub.3), or combinations thereof. In one embodiment, raw material consisting essentially of about 35-40 wt % Bismuth Oxide, more preferably about 38.61 wt %; about 10-12 wt % Calcium Oxide, more preferably about 10.62 wt %; about 35-40 wt % Iron Oxide, more preferably about 37 wt %, about 5-10 wt % Zirconium Oxide, more preferably about 8.02 wt %; about 4-6 wt % Vanadium Oxide, more preferably about 5.65 wt %. In addition, organic based materials may be used in a sol gel process for ethoxides and/or acrylates or citrate based techniques may be employed. Other known methods in the art such as co-precipitation of hydroxides may also be employed as a method to obtain the materials. The amount and selection of raw material depend on the specific formulation.

[0045] After the raw material is weighed, they are blended in Step 108 using methods consistent with the current state of the ceramic art, which can include aqueous blending using a mixing propeller, or aqueous blending using a vibratory mill with steel or zirconia media. In some embodiments, a glycine nitrate or spray pyrolysis technique may be used for blending and simultaneously reacting the raw materials.

[0046] The blended oxide is subsequently dried in Step 110, which can be accomplished by pouring the slurry into a pane and drying in an oven, preferably between 100-400 C. or by spray drying, or by other techniques known in the art.

[0047] The dried oxide blend is processed through a sieve in Step 112, which homogenizes the powder and breaks up soft agglomerates that may lead to dense particles after calcining.

[0048] The material is subsequently processed through a pre-sintering calcining in Step 114. Preferably, the material is loaded into a container such as an alumina or cordierite sagger and heat treated in the range of about 800-1000 C., more preferably about 900-950 C. Preferably, the firing temperature is low as higher firing temperatures have an adverse effect on linewidth.

[0049] After calcining, the material is milled in Step 116, preferably in a vibratory mill, an attrition mill, a jet mill or other standard comminution technique to reduce the median particle size into the range of about 0.5 micron to 10 microns. Milling is preferably done in a water based slurry but may also be done in ethyl alcohol or another organic based solvent.

[0050] The material is subsequently spray dried in Step 118. During the spray drying process, organic additives such as binders and plasticizers can be added to the slurry using techniques known in the art. The material is spray dried to provide granules amenable to pressing, preferably in the range of about 10 microns to 150 microns in size.

[0051] The spray dried granules are subsequently pressed in Step 120, preferably by uniaxial or isostatic pressing to achieve a pressed density to as close to 60% of the x-ray theoretical density as possible. In addition, other known methods such as tape casting, tape calendaring or extrusion may be employed as well to form the unfired body.

[0052] The pressed material is subsequently processed through a calcining process in Step 122. Preferably, the pressed material is placed on a setter plate made of material such as alumina which does not readily react with the garnet material. The setter plate is heated in a periodic kiln or a tunnel kiln in air or pressure oxygen in the range of between about 850 C. -1000 C. to obtain a dense ceramic compact. Other known treatment techniques such as induction heat may also be used in this step.

[0053] The dense ceramic compact is machined in the Step 124 to achieve dimensions suitable for the particular applications.

Devices and Systems Incorporating the Modified Synthetic Garnet Compositions

[0054] The modified synthetic garnet compositions made in accordance with the preferred embodiments in this disclosure can be utilized as a ferrite material in a number of different devices utilized in magnetic microwave applications, such as ferrite based isolators, circulators and resonators. Isolators and circulators are necessary in all cellular base stations to direct the RF energy and prevent the energy from flowing back and destroying circuit components. The modified synthetic garnet materials disclosed herein are designed to lower the magnetic resonance linewidth and raise the dielectric constant of the ferrite in circulators and isolators, thus allowing for desirable miniaturization of circulator components.

[0055] FIG. 8 schematically shows an example of a circulator 800 incorporating a modified synthetic garnet composition disclosed herein. As shown in FIG. 8, the circulator 800 has a pair of ferrite disks 802, 804 disposed between a pair of cylindrical magnets 806, 808. The ferrite disks 802, 804 are preferably made of modified synthetic garnet compositions whereby at least a portion of the Yttrium is replaced. In one embodiment, the composition can be represented by the formula Bi.sub.xCa.sub.y+2zY.sub.3-x-y-2zFe.sub.5-y-zZr.sub.yV.sub.zO.sub.12, wherein x is greater than or equal to 0.5 and less than or equal to 1.4; y is greater than or equal to 0.3 and less than or equal to 0.55; and z is greater than or equal to 0 and less than or equal to 0.6. The magnets 806, 808 can be arranged so as to yield generally axial field lines through the ferrite disks. Preferably, the ferrite disks have a magnetic resonance linewidth of 11 Oe or less.

[0056] FIG. 9 illustrates a telecommunication base station system 900 comprising a transceiver 902, a synthesizer 904, an RX filter 906, a TX filter 908, and magnetic isolators 910 and an antenna 912. The magnetic isolators 910 can be incorporated in a single channel PA and connectorized, integrated triplate or microstrip drop-in. In preferred implementations, the magnetic isolators 910 comprise a modified synthetic garnet material having at least some of the Yttrium substituted in accordance with certain embodiments described in this disclosure and having magnetic properties equivalent to conventional synthetic garnets.

[0057] 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.