Low loss dielectric materials for microwave applications

[0002] This invention relates to provision of new compositions of materials that have very low loss and relatively high dielectric permittivities at microwave frequencies.

[0003] An ideal solid state material for some microwave applications would have an associated loss, measured by the parameter tanδ, that is no larger than 0.001, would have an associated relative dielectric coefficient ∈_relthat is 100-1000, and would have a resonance frequency that is tunable over a range of 50 percent or more of a central frequency. It is unlikely that a material having all these characteristics exists, although workers in this field continue to search for “best possible” materials, based on relaxed criteria. The combined criteria set forth in the preceding are stronger than are needed in some applications. What is needed is a group of materials that comply with less daunting requirements that still permit fabrication and practical use of such materials in real world environments.

[0004] These needs are met by the invention, which provides a group of materials in which electrical conductivity is suppressed, providing very low associated loss coefficients tanδ and providing fairly high relative dielectric coefficients ∈. A general chemical composition for this group of materials is (Ba_ySr_1−y)(Ti_1−xM_x)O₃, with 0≦x≦1 and 0≦y≦1, and M is a substance or mixture including one or more of the elements Ta, Zr, Hf, V, Nb, Mo, W, Mn, Sc, Re, Al, Gd and Cr. Preferably, one or more of the elements represented by the symbol M has a valence of at least 4 in at least one oxidization state. The material may include one or more additives drawn from a second group including Mg, Ca, Sc, Y, La, Ce, Al, Ga, Cr and Nd.

[0005] FIGS. 1A and 1B graphically illustrate the respective tanδ and ∈_relcoefficients for the material (Ba_0.6Sr_0.4)(Ti_1−xW_x)O₃, for x in the range 0≦x≦0.35.

[0006] FIGS. 2A and 2B graphically illustrate the respective tanδ and ∈_relcoefficients for the material (Ba_0.6Sr_0.4)(Ti_1−xV_x)O₃, for x in the range 0≦x≦0.35.

[0007] FIGS. 3A and 3B graphically illustrate the respective tanδ and ∈_relcoefficients for the material (Ba_0.6Sr_0.4)(Ti_1−xTa_x)O₃, for x in the range 0≦x≦0.35.

[0008] FIGS. 4A and 4B graphically illustrate the respective tanδ and ∈_relcoefficients for the material (Ba_0.6Sr_0.4)(Ti_1−xNb_x)O₃, for x in the range 0≦x≦0.35.

[0009] FIGS. 5A and 5B graphically illustrate tunability of the respective tanδ and ∈_relcoefficients for the material (Ba_0.6Sr_0.4)(Ti_0.78W_0.22)O₃, as a function of bias electric field intensity.

[0010] FIGS. 6A and 6B graphically illustrate tunability of the respective tanδ and ∈ coefficients for the material (Ba_0.6Sr_0.4)(Ti_0.83V_0.17)O₃, as a function of bias electric field intensity.

[0011] FIG. 7 graphically illustrates tunability of the dielectric loss coefficient ∈_lossfor the substance or mixture (Ba_0.6Sr_0.4)(Ti_0.9Ta_0.1)O₃, as a function of voltage.

[0012] FIGS. 8 and 9 are spectra resulting from X-ray diffraction patterns of (Ba_0.6Sr_0.4)(Ti_0.7Ta_0.25W_0.05)O₃and (Ba_0.6Sr_0.4)(Ti_0.9Ta_0.1)O₃, prepared as thin film substances on a bulk ceramic and on MgO, respectively.

[0013] The invention includes solid state chemical compositions of the form

[0014] (Ba_ySr_1−y)(Ti_1−xM1_x)O₃,

[0015] with 0≦x≦1 and 0≦y≦1, where M is a substance or mixture including one or more of the elements Ta, Zr, Hf, V, Nb, Mo, W, Mn, Sc, Re, Al, Ga and Cr. The substance or mixture M preferably has a valence of at least 4 in a selected oxidization state.

[0016] FIGS. 1A and 1B graphically illustrate the respective loss and relative dielectric coefficients, tanδ and ∈ for a compound material (Ba_0.6Sr_0.4)(Ti_1−xW_x)O₃on a selected substrate LaAlO₃, with 0≦x≦0.35 and at a frequency f=1 GHz. As the index x increases to above x=0.015, the loss tangent tanδ drops sharply from 0.2-0.5 to values of the order of 0.001. As x increases from 0.0 to 0.35, the relative dielectric coefficient ∈_reldecreases, although not monotonically, from about 300 to values in the range 50-100.

[0017] FIGS. 2A and 2B graphically illustrate the respective loss and relative dielectric coefficients, tanδ and ∈ for a compound material (Ba_0.6Sr_0.4)(Ti_1−xV_x)O₃on a selected substrate LaAlO₃, with 0≦x≦0.35 and at a frequency f=1 GHz. As the index x increases from x=0.03 to x=0.35, the loss tangent tanδ drops slowly from 0.10 to values of the order of 0.01-0.05. As x increases from 0.0 to 0.35, the relative dielectric coefficient ∈_reldecreases, approximately monotonically, from about 600 to about 100.

[0018] FIGS. 3A and 3B graphically illustrate the respective loss and relative dielectric coefficients, tanδ and ∈ for a compound material (Ba_0.6Sr_0.4)(Ti_1−xTa_x)O₃on a selected substrate LaAlO₃, with 0≦x≦0.35 and at a frequency f=1 GHz. As the index x increases from 0.01 to 0.35, the loss tangent tanδ drops sharply from 0.37 to values of the order of 0.001-0.005. As x increases from 0.01 to 0.35, the relative dielectric coefficient ∈_relis approximately constant, in a range 100-200.

[0019] FIGS. 4A and 4B graphically illustrate the respective loss and relative dielectric coefficients, tanδ and ∈, for a compound material (Ba_0.6Sr_0.4)(Ti_1−xNb_x)O₃on a selected substrate LaAlO₃, with 0≦x≦0.35 and at a frequency f=1 GHz. As the index x increases from 0.03 to 0.35, the loss tangent tanδ varies from 0.4 to 0.05. As x increases from 0.01 to 0.35, the relative dielectric coefficient ∈_relvaries in a range 100-250.

[0020] FIGS. 5A and 5B graphically illustrate variation or tunability of the relative dielectric coefficient ∈_reland loss tangent tanδ, respectively, with variation of a bias electric field intensity E(bias) (10⁵volts/meter) for a particular material, (Ba_0.6Sr_0.4)(Ti_0.78W_0.22)O₃. Note that the value of ∈_rel(≈225) is highest near E(bias)=0 and declines approximately monotonically as the magnitude |E(bias)| increases; and ∈_relchanges by about 15% at E(bias)=4×10⁶volts/m.

[0021] FIGS. 6A and 6B graphically illustrate the variation or tunability of the relative dielectric coefficient ∈_reland loss tangent tanδ, respectively, with variation of a bias electric field intensity E(bias) (10⁵volts/m) for a particular material, (Ba_0.6Sr_0.4)(Ti_0.83V_0.17)O₃. The value of ∈_rel(≈225) is highest near E(bias)=0 and tends to decline as the magnitude |E(bias)| increases; and ∈_relchanges by less than 3% at E(bias)=4×10⁶volts/m.

[0022] FIG. 7 graphically illustrates variation or tunability of dielectric constant ∈_reland dielectric loss ∈_loss(>0) with applied voltage V for a thin film material, (Ba_0.6Sr_0.4)(Ti_0.9Ta_0.1)O₃, prepared on an MgO substrate. Note that C is highest for zero applied voltage and decreases approximately monotonically as |V| increases.

[0023] FIG. 8 is an intensity spectrum produced by a 2-theta scan or diffraction pattern of a thin film material, (Ba_0.6Sr_0.4)(Ti_0.9Ta_0.1)O₃, grown on an MgO substrate by ion beam sputtering. The cleanness of this spectrum indicates that the material is present in a single phase.

[0024] FIG. 9 is a spectrum produced by a 2-theta scan or diffraction pattern of a bulk ceramic material, (Ba_0.6Sr_0.4)(Ti_0.70Ta_0.25W_0.05)O₃. The cleanness of this spectrum also indicates that the material is present in a single phase.

[0025] The combinatorial material libraries in examples 1-4 were synthesized by ion beam sputtering, using a shadow mask for precision control. The multilayer precursors with a selected thickness profile for each layer were then annealed to form and form single phase material with an approximately uniform composition across a film thickness. Measurements of the relative dielectric coefficient values e were performed by a Microwave Scanning Microscope at a frequency f=1 GHz.

[0026] In the examples 1-4, a decrease of tanδ by about two orders of magnitude is observed, while the relative dielectric coefficient remains in the range 50-300. The loss tangent values are competitive with the best bulk ceramic values and are below the lower detection limit of the Microwave Scanning Microscope. The observed values for tanδ are consistent with the goal of impeding formation of Ti³⁺ ions, which can support an electronic conduction mechanism in octahedral sites and eddy currents, and encouraging formation of Ti⁴⁺ ions, using a reaction such as Ti³⁺→Ti⁴⁺+e⁻. In some previous work, Mn³⁺ ions were used successfully to suppress formation of Fe²⁺ ions in substances or mixtures such as NiFe³⁺_2−xMn³⁺_xO₄.

[0027] The measured data for the doped barium titanate are very similar to data for a modified spinel ferrite, with the loss tangents reaching a minimum in a composition range 0.02≦x≦0.05, with one difference: the tanδ data for the doped titanates become relatively flat for x larger than about 0.1. The fact that the present data are obtained using thin films, while the earlier spinel ferrite data are obtained from bulk ceramics, may explain part or all of the discrepancy.

[0028] The group of materials (Ba_ySr_1−y)(Ti_1−xM_x)O₃, provided by the invention have several applications that require relatively high values of ∈_reland very low values of tanδ. A first application is to dielectric filters and duplexer oscillators, for which ∈_rel=50-300 and tanδ<0.001 are desirable. A second application is to tunable filters, where ∈_rel=50-200 and tanδ<0.001, with Δ∈/∈≧10 percent are desirable. The dielectric coefficient ∈ can be adjusted by applying a variable magnetic field.

[0029] Ideally, the valence(s) of the element M in the composition

[0030] (Ba_ySr_1−y)(Ti_1−xM1_x)O₃,

[0031] should be 4+, 5+, 6+ or more, in order to suppress the motion of, or absorb, electrons in the field of such a composition. The best performance found thus far is for a mixture

[0032] (1−c1)·(Ba_0.6Sr_0.4)(TiO₃)+c1·(Ba_0.6Sr_0.4)(TaO₃),

[0033] where the fraction c1 satisfies 0≦c1≦0.3. X-ray diffraction data, such as those illustrated in FIGS. 8 and 9, indicate that this material is a single phase material with a measured lattice constant that increases from 3.965 Å to 3.990 Å as the fraction c1 of Ta increases from 0 to 0.3.

[0034] The substance or mixture (Ba_ySr_1−y)(Ti_1−xM_x)O₃, may also be fabricated as a thin film on a substrate of selected composition or as a bulk ceramic by powder sintering.

[0035] A microbeam X-ray diffractometer was used to measure the crystal structure as a function of composition in a material library. The results for (Ba_ySr_1−y)(Ti_1−xTa_x)O₃and (Ba_ySr_1−y)(Ti_1−zV_z) O₃are discussed in the following.

[0036] X-ray Diffraction of (Ba_ySr_1−y)(Ti_1−xTa_x)O₃. The (002) diffraction peak for this material moves toward lower angles, corresponding to increasing crystal lattice parameters, as the Ta fraction x increases from x=0 to x=0.18. The (002) diffraction peak then moves toward higher angles, corresponding to decreasing lattice parameters, as x increases beyond 0.18 up to x=0.3 (perovskite lattice structure). Due to the method of preparation of these thin films, the Ta ion will exit predominantly in the form Ta⁴⁺. The ionic radii of Ta⁴⁺ and Ti⁴⁺ are 0.68 Å and 0.61 Å, respectively. Because the Ta⁴⁺ radius is larger than the Ti⁴⁺ radius, replacement of Ti⁴⁺ ions by Ta⁴⁺ ions causes an increase of one or more of the crystal lattice parameters, for relatively small values of the Ta fraction x.

[0037] It is unclear why a lattice parameter of the resulting material becomes larger as x increases beyond x=0.18. One possible reason is that some of the Ta⁴⁺ ions may become transformed into Ta⁵⁺ ions, because Ta⁵⁺ is a more stable form than is Ta⁴⁺, under normal conditions, and attraction between Ta⁵⁺ ions and O2⁻ ions is stronger than a corresponding attraction between Ta⁴⁺ ions and O²⁻ ions. This increased attraction causes a corresponding smaller Ta-O octahedron, with a correspondingly smaller lattice parameter. The difference between Ta⁴⁺ ionic radius and Ti⁴⁺ ionic radius, which may be the reason that (Ba_ySr_1−y)(Ti_1−xTa_x)O₃still possesses a perovskite structure for values of the Ta fraction x up to x=0.3.

[0038] X-ray Diffraction of (Ba_ySr_1−y)(Ti_1−zV_z)O₃. The (002) diffraction peak for this material moves toward higher angles, corresponding to decreasing crystal lattice parameters, as the V fraction z increases from z=0 to z=0.18. The (002) diffraction peak disappears for higher values of the fraction z. The V ion will exit predominantly in the form V⁴⁺. The ionic radii of V⁴⁺ and Ti⁴⁺ are 0.58 Å and 0.61 Å, respectively. Because the V⁴⁺ radius is larger than the Ti⁴⁺ radius, replacement of Ti⁴⁺ ions by V⁴⁺ ions causes an decrease of one or more of the crystal lattice parameters, for relatively small values of the V fraction z. When the V⁴⁺ ion displaces the Ti⁴⁺ ion and/or enters a void in the oxygen octahedron, the perovskite structure is progressively destroyed, and the (002) diffraction peak disappears.

[0039] It is well known that an oxygen vacancy and reduction of Ti⁴⁺ to Ti³⁺ occurs easily in Ba_ySr_1−yTiO₃. This change increases the electrical charges in the material and increases the tangent loss coefficient tanδ for the material. Addition of an element M⁵⁺ can prevent development of an oxygen vacancy and reduction of Ti⁴⁺ to Ti³⁺, which will reduce the loss tangent value. In many applications, the relative dielectric coefficient value ∈_relneeds to be several hundreds. Because of the lower polarization of Ta⁴⁺, relative to Ti⁴⁺, substitution of Ta for Ti will decrease the value ∈_rel.

[0040] When the fraction y in the material Ba_ySr_1−yTiO₃is greater than 0.5, the Curie temperature is at or above room temperature. Although the tunability is high in this range, the loss tangent is too high to be useful in most microwave work. Addition of Ta⁵⁺ can shift the Curie temperature to lower values. Although tunability will be affected to some extent by addition of Ta⁵⁺, the tangent loss value for the ceramic body will be lowered (and thus improved) dramatically, and a tunable material with relatively high Ba content will be obtained. Addition of Ta⁵⁺ is not, by itself, sufficient, in part because of an unbalanced replacement of Ti⁴⁺. Selective addition of trivalent ions into the material will balance the excess Ta⁵⁺ and may improve the tangent loss coefficient further. The size effect, distortion of a crystal cell and modest increase of the loss tangent, due to the added trivalent ions, also must be accounted for. Preferably, the radii of all ions substituted into the initial material Ba_ySr_1−yTiO₃should be as close as possible to the radii of Ti⁴⁺.

[0041] Based on results of combinatorial synthesis, we have prepared (Ba_0.6Sr_0.4)(Ti_1−xTa_x)O₃bulk ceramic materials. We observed a significant improvement in the tangent loss coefficient through partial substitution of Ta for Ti (x>0). However, the solid solution limit of Ta substitution in bulk ceramic corresponds to x≈0.1, which is much less than the limit (x≈0.3) for thin films. Substitution of trivalent elements, such as Al, Ga or Cr, plus Ta, for Ti will further improve the tangent loss coefficient for this material. The following are examples of the results.

[0042] BaTiO₃powder (purity 99.9%), SrTiO₃powder (purity 99.9%) and Ta₂O₃powder (purity 99.9%) were used as staring materials and were combined to form (Ba_0.6Sr_0.4)(Ti_1−xTa_x)O_3+0.5x, with x=0.02, 0.05, 0.06, 0.07 and 0.08. The mass of each of these powders was 10-20 gm. Each formulated powder was mixed with ethanol (20-30 ml) and processed in a vibrating mill using agate jar and media for a time interval of 15-30 min, sieved at 200 mesh and dried.

[0043] Calcination of the powder was carried out in a zirconia crucible in air at a temperature T=1350° C. for 3 hours. The temperature elevation rate was +10° C./min, and the cooling rate was −10° C./min. After calcination, 0.1-0.2 gm organic binder (methyl cellulose and 20-30 ml ethanol were added to agate jar with the calcined powder and subjected to secondary pulverization in a vibration mill for 40-60 min.

[0044] The slurry was then dried and sieved at mesh 60 to further improve the homogeneity of the sample. The dry powder was molded into two cylinders of φ12.80×5.50 mm and φ05.00×2.50 m under a pressure of 100 MPa.

[0045] The molding product was degreased in air at T=700° C. for 3 hours, then sintered at T=1480° C. for 3 hours. The temperature elevation rate was 10° C./min, and the cooling rate was −5° C./min. The fired products were polished at both end faces to form dielectric specimens. For the smaller specimen of each of the samples, values of ∈_rel, f·Qu (GHz) and τ_fwere measured in a cavity type columnar resonator (TEO₀₁₁mode). The measuring frequency range was 2-10 GHz. The parameter τ_fwas measured over a temperature range T=25-50° C. and calculated according to τ_f=(f₅₀−f₂₅)/(f₂₅·ΔT), with Δt=25° C. For each of the larger specimens, a tunability value was measured. The values of ∈_rel, f·Qu and τ_ffor each of the composition fractions x are set forth in: Table 1.

[0046] From the results in Table 1, one verifies that ∈_reland τ_feach decrease approximately linearly with increasing values of x in the range 0.02≦x≦0.07. The decrease of ∈_relfor higher values of x is smaller. The value f·Qu reaches a maximum at or around x=0.07 and decreases for larger or smaller values of x.

[0047] Bulk ceramic substances or mixtures of composition Ba_0.6Sr_0.4Ti_0.92Ta_0.07M_0.01O₃, were prepared by the approach of Example 5, where the substance or mixture M includes one or more of the trivalent elements Al, Ga or Cr (each of purity 99.9%). The values of ∈_rel, f·Qu and τ_ffor each of the three compositions are set forth in Table 2.

[0048] The relative dielectric coefficient ∈_relcan be lowered by adding one of the trivalent cations Al, Ga or Cr, but the resulting values for f·Qu and τ_fare not changed much , relative to the corresponding values of the material Ba_0.6Sr_0.4Ti_0.93Ta_0.07O₃, using these compositions. The trivalent element Al was studied further.

[0049] Bulk ceramic materials of composition Ba_0.6Sr_0.4Ti_0.93−zTa_0.07Al_zO₃, were prepared by the approach of Example 5, with z=0, 0.01, 0.02, 0.03, 0.04 and 0.05.

[0050] The relative dielectric coefficient ∈_reland the value τ_fdecrease approximately linearly with increasing Al content (0≦z≦0.05). The value f·Qu reaches a maximum value at or around z=0.03 and decreases for larger or smaller values of z. From the results for f·Qu set forth in Tables 1, 2 and 3, the materials Ba_{0. 6}Sr_0.4Ti_0.90Ta_0.07Al_0.03O₃and Ba_0.6Sr_0.4Ti_0.89Ta_0.08Al_0.03O₃are preferred. For most microwave applications, the desired material properties are relatively high dielectric constant, which is tunable with varying bias electrical field, low tangent loss parameter, and low electrical conductivity (or low leakage current).